processing and applications of carbon nanotubes …both carbon nanotubes (cnts) and graphene are low...
TRANSCRIPT
PROCESSING AND APPLICATIONS OF CARBON
NANOTUBES AND GRAPHENE
MEI XIAOGUANG
(B.Eng., Tsinghua University, China)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF MATERIALS SCIENCE AND
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
i
Acknowledgements
Firstly, I would like to express my earnest gratitude to my supervisor Asst. Prof.
Ouyang Jianyong for his supervision and guidance during my Ph. D candidature. His
invaluable advice and enthusiastic encouragement never fail to inspire me. I am
indebted to him more than he knows.
I am equally thankful to my group members Mr. Fan Benhu, Mr. Wu Zhonglian,
Ms. Zhang Hongmei, Mr. Li Aiyuan, Ms. Zhou Dan, Ms. Xia Yijie, Mr. Sun Kuan,
Ms. Cho Swee Jen and Mr. Neo Chin Yong for their devoted help and cooperation in
my research work. Special thanks go to Ms. Cho Swee Jen for her contribution in the
fabricating and characterization of the dye sensitized solar cells.
I am grateful to the lab technicians in the department of materials science and
engineering for their trainings and technical support.
I am also deeply indebted to my family for their unconditional love and endless
support in the past years.
Finally, I want to express my thanks to the National University of Singapore and
Singapore Ministry of Education for their financial support.
ii
Table of contents
Acknowledgements ...................................................................................................... i
Table of contents ......................................................................................................... ii
Summary ...................................................................................................................... v
List of Tables ............................................................................................................. vii
List of Figures ........................................................................................................... viii
List of publications ................................................................................................... xiii
Chapter 1. Introduction ............................................................................................. 1
1.1. Structure of carbon nanotubes ........................................................................... 1
1.2. Properties of carbon nanotubes ......................................................................... 3
1.2.1. Electronic and electrical properties ........................................................... 3
1.2.2. Optical properties ....................................................................................... 5
1.2.3. Mechanical properties ................................................................................ 7
1.2.4. Thermal properties...................................................................................... 8
1.3. Dispersion of carbon nanotubes ........................................................................ 8
1.4. Applications of carbon nanotubes ................................................................... 10
1.5. Structure and morphology of graphene ........................................................... 12
1.6. Properties of graphene .................................................................................... 14
1.6.1. Electronic and electrical properties ......................................................... 14
1.6.2. Optical properties ..................................................................................... 15
1.6.3. Mechanical properties .............................................................................. 16
1.6.4. Thermal properties.................................................................................... 16
1.7. Synthesis of graphene ...................................................................................... 17
1.7.1. Mechanical exfoliation of graphite ........................................................... 17
1.7.2. Direct growth ............................................................................................ 18
1.7.3. Reduction of graphene oxide .................................................................... 18
iii
1.8. Applications of graphene ................................................................................. 20
Chapter 2. Experimental .......................................................................................... 22
2.1. Materials .......................................................................................................... 22
2.2. Experimental techniques .................................................................................. 23
2.2.1. Preparation of CNT gels by mechanical grinding .................................... 23
2.2.2. Preparation of SWCNT films by gel coating and spray coating ............... 23
2.2.3. Preparation of CNT Counter electrode .................................................... 24
2.2.4. Fabrication of DSCs ................................................................................. 25
2.2.5. Oxidization of graphite ............................................................................. 25
2.2.6. Exfoliation and reduction of GO............................................................... 26
2.2.7. Re-dispersion of rGO ................................................................................ 27
2.2.8. Preparation of GO films by spin coating .................................................. 28
2.2.9. Preparation of transparent rGO films by the reduction of GO films ....... 28
2.3. Characterization .............................................................................................. 29
2.3.1. Dynamic mechanical characterization ..................................................... 29
2.3.2. Conductivity measurement ........................................................................ 30
2.3.3. Morphology characterization ................................................................... 31
2.3.4. Thermal analyses ...................................................................................... 31
2.3.5. Optical spectroscopy ................................................................................. 31
2.3.6. Photovoltaic performance characterization ............................................. 32
2.3.7. Electrochemical characterization ............................................................. 33
2.3.8. X-ray photoelectron spectroscopy ............................................................ 34
Chapter 3. Gels of carbon nanotubes and a nonionic surfactant prepared by
mechanical grinding.................................................................................................. 35
3.1. Introduction...................................................................................................... 35
3.2. Results and discussion ..................................................................................... 37
3.2.1. Gels of MWCNTs and POETE .................................................................. 37
3.2.2. Gels of SWCNTs and POETE ................................................................... 41
3.3. Conclusion ....................................................................................................... 44
Chapter 4. Highly conductive and transparent single-walled carbon nanotube
thin films fabricated by gel coating ......................................................................... 45
4.1. Introduction...................................................................................................... 45
iv
4.2. Results and discussion ..................................................................................... 46
4.2.1. Fabrication of SWCNT films by gel coating ............................................. 46
4.2.2. Optical and electrical properties of SWCNT films ................................... 53
4.3. Conclusion ....................................................................................................... 61
Chapter 5. High-performance dye-sensitized solar cells with gel-coated binder-
free carbon nanotube films as counter electrode ................................................... 63
5.1. Introduction...................................................................................................... 63
5.2. Results and discussion ..................................................................................... 65
5.2.1. DSCs with SWCNT films as counter electrode ......................................... 65
5.2.2. DSCs with MWCMT films as counter electrode ....................................... 75
5.2.3. DSCs with SWCNT/CMC and MWCNT/CMC films as counter electrode 78
5.3. Conclusion ....................................................................................................... 81
Chapter 6. Ultrasonication-assisted ultrafast reduction of graphene oxide by zinc
powder at room temperature ................................................................................... 82
6.1. Introduction...................................................................................................... 82
6.2. Results and discussion ..................................................................................... 82
6.2.1. GO Reduction with Zn Powder ................................................................. 82
6.2.2. Structure and Properties of Zn-rGO ......................................................... 87
6.3. Conclusion ....................................................................................................... 94
Chapter 7. Metal reduced graphene thin films as transparent and flexible
electrodes ................................................................................................................... 95
7.1. Introduction...................................................................................................... 95
7.2. Results and discussion ..................................................................................... 96
7.2.1. Reduction of spin coated GO films ........................................................... 96
7.2.2. Properties of the rGO films....................................................................... 99
7.3. Conclusion ..................................................................................................... 103
Bibliography ............................................................................................................ 109
v
Summary
Both carbon nanotubes (CNTs) and graphene are low dimensional carbon isotopes
with graphitic structures. They exhibit extraordinary electrical properties arising from
their quantum size effects in nanometer scale and have been considered as building
blocks of the next generation electronic devices. However, their practical applications
have been severely impeded by lack of effective processing technique.
CNTs have a strong tendency to aggregate due to their large surface area and high
aspect ratio. Although random networks or oriented arrays of individual CNTs can be
directly grown by chemical vapor deposition (CVD), their fabrication costs are too
high for mass production. Currently, commercially available CNTs are generally in
the form of large agglomerates. Thus, the CNT aggregates must be debundled and
dispersed into liquid medium for most of their applications. The conventional way is
to disperse CNTs in organic solvents or water dispersed with surfactant. But the
concentration of CNTs in solutions is lower than 0.1 wt%, so that copious solvents
are used. A novel approach was developed to dispersing CNTs directly into
surfactants without any solvent. The CNTs form gels with the surfactants by
mechanical grinding or ultrasonication. The concentration of CNTs can be up to 1
wt% in the gels. CNT films can be prepared on various substrates by coating the
CNT/surfactant gels and subsequently heating. Highly conductive and transparent
CNT thin films can be prepared by controlling the thickness. They can have a sheet
resistance of the films of 100 Ω/ and a transmittance of 70% at 550 nm. They have
vi
potential application as transparent electrode of optoelectronic devices. Thick CNT
films can be readily obtained from the CNT/surfactant gels. They have good adhesion
to substrates, and excellent catalysis for some electrochemical reaction. They were
used as the counter electrode of high-performance dye-sensitized solar cells.
Like CNTs, the large scale processing techniques are also very important for
graphene. Solution processing techniques have advantages over the dry processing
techniques in terms of the low cost and preparation of graphene films in a large area.
One key step in the solution processing techniques is the reduction of graphene oxide
(GO) into graphene. During my PhD program, we developed an ultrafast green
method to reduce GO with Zn powder in mild acid solutions under ultrasonication.
The reduction is complete within 1 min, and the reduced graphene can have a
conductivity of 15000 S/m. The reduced graphene was processed into conductive and
transparent films. These graphene films are promising candidate as the transparent
electrode of optoelectronic devices as well.
vii
List of Tables
Table 5.1. Photovoltaic parameters of DSCs with Pt and a SWCNT film of 6 m in
thickness as the counter electrode. .............................................................................. 68
Table 5.2. Photovoltaic stability of a DSC with Pt and a SWCNT film of 6 m in
thickness as the counter electrode. .............................................................................. 73
Table 5.3. Photovoltaic parameters of DSCs with SWCNT films of different
thicknesses as the counter electrode. .......................................................................... 74
Table 5.4. Photovoltaic stability of a DSC with a 6 m-thick MWCNT film as the
counter electrode. ........................................................................................................ 77
Table 5.5. Photovoltaic parameters of DSCs with SWCNT/CMC and MWCNT/CMC
films as the counter electrode. The thickness of the CNT/CMC films was 10 µm. ... 80
Table 6.1. Times required for GO reduction by different methods reported in
literature. ..................................................................................................................... 84
viii
List of Figures
Figure 1.1. Schematic structures of (a) a SWCNT and (b) a MWCNT (Pictures from
internet). ........................................................................................................................ 1
Figure 1.2. Graphene sheet segment showing indexed lattice points and the rolling
vector [3]. ...................................................................................................................... 2
Figure 1.3. Density of electronic states for a semiconducting SWCNT. Solid arrows
depict the optical excitation and emission transitions; dashed arrows denote
nonradioactive relaxation of electrons and holes before emission [3]. ......................... 4
Figure 1.4. Colors of SWCNTs with different chiralities [13]. .................................... 6
Figure 1.5. The photoluminescence emission spectrum and the optical absorption
spectrum of individual SWCNTs suspended in a Sodium dodecyl sulfate (SDS)
solution [14]. ................................................................................................................. 7
Figure 1.6. Schematic presentation of a functionalization process of oxidized CNTs
[25]. ............................................................................................................................. 10
Figure 1.7. Graphene as the parent of all graphitic forms [71]. .................................. 13
Figure 1.8. The schematic illustration of corrugated graphene [75]. .......................... 13
Figure 1.9. Electronic dispersion in the honeycomb lattice [78]. The magnified the
image shows the linear relationship between E and k near one Dirac point. .............. 14
Figure 1.10. (a) A Photograph of a 50-mm aperture partially covered by graphene and
its bilayer. The inset shows the sample design: A 20-mm-thick metal support
structure has several apertures of 20, 30, and 50 mm in diameter with graphene
crystallites placed over them [84]. (b) Excitation processes responsible for absorption
of light in graphene [84]. ............................................................................................ 16
Figure 1.11. Chemical structural models of (a) pristine graphene, (b) GO and (c) rGO
[104]. ........................................................................................................................... 20
Figure 3.1. Photographs of a MWCNT/POETE gel. (a) Phase-separation behavior of
the gel (upper phase) and excess POETE (lower phase), observed after centrifugation
of a ground mixture of MWCNTs and POETE. The dotted line indicates the surface
of the liquid phase. (b) Extrusion of the gel from a needle. (c) Electrical conduction of
the gel with the four-point probe technique. ............................................................... 38
Figure 3.2. SEM images of (a) as-received MWCNTs and (b) a MWCNT/POETE gel.
..................................................................................................................................... 39
ix
Figure 3.3. TEM image of MWCNTs. The sample was preparedby dispersing a
MWCNT/POETE gel in water. ................................................................................... 39
Figure 3.4. Angular frequency (ω) dependencies of dynamic storage (G’, solid
symbols) and loss moduli (G”, open symbols) of a MWCNT/POE gel at (a) 25 oC and
(b) 125 oC. Applied strain amplitudes (c) were 1% (circles) and 5% (triangles). ...... 40
Figure 3.5. SEM images of (a) as-received SWCNTs and (b) a SWCNT/POETE gel.
..................................................................................................................................... 42
Figure 3.6. Raman spectra of (a) as-received SWCNTs and (b) a SWCNT/POETE gel.
..................................................................................................................................... 43
Figure 3.7. Angular frequency (ω) dependencies of dynamic storage (G’, solid
symbols) and loss moduli (G”, open symbols) of a SWCNT/POETE gel at 25 oC.
Applied strain amplitudes (c) were 1% (circles) and 5% (triangles). ......................... 43
Figure 4.1. TGA curves of (a) POETE, (b) a SWCNT:POETE gel, and (c) pure
SWCNTs in air. ........................................................................................................... 47
Figure 4.2. Photographs of (a) a SWCNT:POETE layer on a glass substrate coated by
doctor blade, (b) the SWCNT film obtained from the SWCNT:POETE layer by
heating at 250 oC for 15 min and then at 500
oC for 20 min, (c) the SWCNT film
transferred onto a flexible PET substrate. Picture (d) shows the flexibility of the
SWCNT film on PET. ................................................................................................. 49
Figure 4.3. SEM images of an SWCNT film (a) after the heat treatment and (b) after
heat and HNO3 treatments. (c) EDX of the particles in image (a). ............................. 50
Figure 4.4. UV-vis-IR transmittance spectra of a SWCNT film (a) before and (b)
immediately after the HNO3 treatment. The inset shows the corresponding absorption
spectra. ........................................................................................................................ 52
Figure 4.5. (a) Raman spectra of pristine SWCNTs (solid curve) and a SWCNT film
(dashed curve). (b) Raman intensity ratios of the D band to G band (ID/IG) of (i) as-
received SWCNTs and a SWCNT:POETE gel after successively heating at (ii) 300 oC,
(iii) 400 oC, (iv) 500
oC and (v) after subsequent HNO3 treatment. The heating time
for each heat treatment is 20 min. ............................................................................... 53
Figure 4.6. Sheet resistance-transmittance curves of SWCNT films prepared from
SWCNT:POETE gels which were formed by dispersing SWCNTs in POETE under
ultrasonication of 15 W for different periods. ............................................................ 54
Figure 4.7. SEM images of SWCNTs prepared from SWCNT:POETE gels which
were prepared under ultrasonication at 15 W for (a) 10 min, (b) 20 min, (c) 40 min
and (d) 80 min. (e) Average bundle lengths of SWCNT in the SWCNT/POETE gels
after ultrasonication treatment for different periods. .................................................. 55
Figure 4.8. TEM images SWCNTs after ultrasonication treatment at 15 W for (a) 10
min, (b) 20 min, (c) 40 min and (d) 80 min. ............................................................... 56
x
Figure 4.9. The distribution of SWCNT bundle length (a) and diameter (b) after 20
min of ultrasonication. ................................................................................................ 57
Figure 4.10. Sheet resistance-transmittance curves of SWCNT films prepared from
SWCNT:POETE gels which were formed by dispersing SWCNTs in POETE under
ultrasonication of different powers for 20 min. .......................................................... 58
Figure 4.11. Variations of the sheet resistance and the transmittance at 550 nm of
SWCNT films with (a) the heating temperature and (b) the heating time. The heating
temperature in (b) is 480 oC. ....................................................................................... 59
Figure 4.12. (a) Sheet resistance-transmittance curves of SWCNT films fabricated by
gel coating (solid squares), spray coating with (open circles) and without heat
treatment (solid triangles). (b) SEM image of SWCNTs prepared by drop casting a
CMC aqueous solution dispersed with SWCNTs and subsequently heating at 500 oC
for 20min. .................................................................................................................... 61
Figure 5.1. Working mechanism of DSCs [214]. ....................................................... 64
Figure 5.2. FTIR spectra of PEG, SWCNT/PEG gel and SWCNT film prepared by
gel coating. .................................................................................................................. 66
Figure 5.3. SEM images of SWCNT films on FTO glass. Image (a) has a low
magnification, while images (b), (c) and (d) have a high magnification. The
thicknesses of the SWCNT films were: (a) and (b) 2 µm, (c) 6 µm, and (d) 10 µm. . 67
Figure 5.4. Current-voltage curves of DSCs with Pt and a SWCNT film of 6 m in
thickness as the counter electrode. The tests were performed under AM 1.5
illumination. ................................................................................................................ 68
Figure 5.5. Nyquist plots of ac impedance spectroscopy of symmetrical structures
with Pt as the two electrodes and two SWCNT films of 6 m in thickness as the two
electrodes. ................................................................................................................... 70
Figure 5.6. Cyclic voltammograms of I2/I3- with Pt and a SWCNT film of 6 μm in
thickness as the working electrode. The potential sweep rate of is 10 mV S-1
. The four
peaks in the cyclic voltammograms correspond to four redox reactions: (1) I3- + 2e
-→
3I-, (2) 3I
- → I3
- + 2e
-, (3) 2I
- → I2+2e
-, (4) I2+2e
-→2I-. ............................................ 71
Figure 5.7. Current-voltage curves of DSCs with (a) a SWCNT film and (b) Pt as the
counter electrode. The DSCs were tested immediately, one week and 4 weeks after
device fabrication. The thickness of the SWCNT film was 6 m, and the tests were
carried out under AM 1.5 illumination. ...................................................................... 72
Figure 5.8. Cyclic voltammograms of the 1st and 100
th cycle with (a) a SWCNT film
of 6 μm in thickness and (b) Pt as the working electrode. The electrolyte is an
acetonitrile solution consisting of 5 mM LiI, 0.25 mM I2 and 0.1 M (C4H9)4NPF6. The
scan rate is 10 mV s-1
. ................................................................................................. 74
xi
Figure 5.9. Current-voltage curve of a DSC with a MWCNT film of 6 m in
thickness as the counter electrode. The device was tested under AM 1.5 illumination.
..................................................................................................................................... 75
Figure 5.10. Nyquist plot of ac impedance spectroscopy of a symmetrical structure
with two MWCNT films of 6 m in thickness as the two electrodes. ........................ 76
Figure 5.11. Current-voltage curves of a DSC with a MWCNT film of 6 m in
thickness as the counter electrode. The DSCs were tested immediately, one week and
4 weeks after device fabrication. The tests were carried out under AM 1.5
illumination. ................................................................................................................ 76
Figure 5.12. Current-voltage curves of DSCs with a SWCNT/CMC film and a
MWCNT/CMC film as the counter electrode. The thickness of the SWCNT/CMC and
MWCNT/CMC films was 10 µm. The tests were performed under AM 1.5
illumination. ................................................................................................................ 79
Figure 5.13. Raman spectra of binder-free (a) SWCNT and (b) MWCNT films
prepared by gel coating. The Raman intensities were normalized to the corresponding
G band intensities. The wavelength of the laser source is 514.5 nm. ......................... 81
Figure 6.1. (a) Photographs of a GO solution (A) before and (B) after reduction with
Zn powder. (b) Photographs of Zn-rGO dispersed in solutions of (C) Brij 30, (D)
SDBS, (E) cetrimonium bromide, (F) poly(sodium 4-styrenesulfonate) acid and (G)
CMC. (c) Photographs of hydrazine-rGO dispersed in (H) water and (I) a SDBS
solution. ....................................................................................................................... 83
Figure 6.2. SEM images of (a) GO and (b) Zn-rGO, AFM images of (c) GO and (d)
Zn-rGO, (e) TEM image of a free standing Zn-rGO sheet suspended on a lacey
carbon TEM grid and (f) selected area electron diffraction (SEAD) pattern of the Zn-
rGO sheet in (e).. ......................................................................................................... 86
Figure 6.3. FTIR spectra of (a) GO and (b) Zn-rGO. The absorbance of Zn-rGO was
magnified by 10 times. ................................................................................................ 88
Figure 6.4. Normalized UV-vis absorption spectra of a diluted GO solution (solid
curve) and a diluted Zn-rGO/carboxymethyl cellulose sodium dispersion (dash curve).
Zn-rGO was prepared by reducing GO at a Zn-to-GO weight ratio of 2. (b)
Dependence of the →* transition peak position on the Zn-to-GO weight ratio.
Carboxymethyl cellulose sodium was chosen as the dispersion agent due to its weak
absorption in the UV-vis range. .................................................................................. 88
Figure 6.5. (a) Raman spectra of graphite, GO, hydrazine-rGO and Zn-rGO. (b)
Variation of the ID/IG ratio of Zn rGO with the Zn-to-GO weight ratio. .................... 90
Figure 6.6. TGA curves of GO, Zn-rGO and hydrazine-rGO in air. .......................... 91
Figure 6.7. (a) XPS survey scans of graphite, GO and Zn-rGO. (b) Deconvolution of
C1s XPS of GO. (c) C1s peaks of graphite (solid curve) and Zn-rGO (dash curve). ... 92
xii
Figure 6.8. Photograph of a rGO film transferred onto a glass substrate. .................. 93
Figure 7.1. SEM images of GO nanosheets deposited on Si substrate by spin coating
at 3000 rpm. Concentrations of GO suspensions are (a) 0.2 mg/ml and (b) 2 mg/ml. (c)
SEM image of a rGO film. .......................................................................................... 97
Figure 7.2. (a) The photograph of a 5 nm-thick GO (left) film and a 30 nm-thick GO
film (right) before and after reduction with Al. (b) The photograph of a rGO film on
PET substrate. ............................................................................................................. 99
Figure 7.3. (a) UV-vis absorption of a 30 nm GO film before and after reduction with
30 nm Al. (b) The evolution of the → * transition peaks and the 550 nm
transmittance of rGO films with respect to increasing thickness of Al. ................... 100
Figure 7.4. (a) Raman spectra of a 30 nm GO film before and after reduction and (b)
ID/IG ratios of GO films reduced by Al with different thickness. ............................. 100
Figure 7.5. (a) XPS surveys of a GO film and a rGO film, and high resolution C1s
peaks of (b) a GO film and (c) a rGO film. .............................................................. 101
Figure 7.6. (a) Sheet resistance vs. transmittance of rGO films with different
thickness. (b) Dependence of the thickness and transmittance of rGO films on the
number of spin coating cycles. The rGO films were reduced by 30 nm-thick Al. ... 103
xiii
List of publications
1. Mei XG, Zheng HQ, Ouyang JY. Ultrafast reduction of graphene oxide with Zn
powder in neutral and alkaline solutions at room temperature promoted by the
formation of metal complexes. J. Mater. Chem., 2012, accepted, DOI:
10.1039/c2jm30552f.
2. Mei XG, Ouyang JY. Ultrasonication-Assisted Ultrafast Reduction of Graphene
Oxide by Zinc Powder at Room Temperature. Carbon, 2011, 49, 5389-5397.
3. Mei XG, Ouyang JY. Highly Conductive and Transparent Single-Walled Carbon
Nanotube Thin Films Fabricated by Gel Coating. J. Mater. Chem., 2011, 21,
17842-17849.
4. Mei XG, Ouyang JY. Electronically and Ionically Conductive Gels of Ionic
Liquids and Charge-Transfer Tetrathiafulvalene-Tetracyanoquinodimethane
(TTF-TCNQ). Langmuir, 2011, 27, 10953-10961.
5. Zhou D, Mei XG, Ouyang JY. Ionic Conductivity Enhancement of Polyethylene
Oxide-LiClO4 Solid Polymer Electrolyte by Adding Functionalized Multi-Walled
Carbon Nanotubes. J. Phys. Chem., 2011, 115, 16688–16694. (co first author)
6. Mei XG, Cho SJ, Fan BH, Ouyang JY. High-Performance Dye-Sensitized Solar
Cells with Gel-Coated Binder-Free Carbon Nanotube Films as Counter Electrode.
Nanotechnology, 2010, 21, 395202.
7. Mei XG, Ouyang JY. Gels of Carbon Nanotubes and a Nonionic Surfactant
Prepared by Mechanical Grinding. Carbon, 2010, 48, 293-299.
xiv
8. Fan BH, Mei XG, Sun K, Ouyang JY. Conducting Polymer/Carbon Nanotube
Composite as Counter Electrode of Dye-Sensitized Solar Cells. Appl. Phys. Lett.,
2008, 93, 143103.
9. Fan BH, Mei XG, Ouyang JY. Significant Conductivity Enhancement of
Conductive Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Films by
Adding Anionic Surfactants into Polymer Solution. Macromolecules, 2008, 41,
5971-5973.
10. Cho SJ, Mei XG, Ouyang JY. Templateless and Surfactantless Deposition of
Hierarchical Nanoporous Platinum on Substrates with Good Adhesion through
Chemical Reduction of a Platinum Precursor in Solid State. Submitted.
Provisional patent:
Ouyang JY, Cho SJ, Mei XG. New Invention Disclosure; ILO Ref: 11238N; Title:
Preparation of Nanostructured and Porous Metals on Substrates through Chemical
Reduction of Metal Precursors in Solid State. NUS ILO Reference No: 11238N.
1
Chapter 1. Introduction
1.1. Structure of carbon nanotubes
Carbon nanotubes (CNTs) were firstly discovered in 1991 by Sumio Iijima as a
byproduct of C60 [1]. As members of fullerene family, CNTs can be considered as
rolling of graphite sheets into seamless cylinders (Fig. 1.1). They can be classified
into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes
(DWCNTs) and multi-walled carbon nanotubes (MWCNT) according to the number
of graphene sheet layers.
Figure 1.1. Schematic structures of (a) a SWCNT and (b) a MWCNT (Pictures from internet).
A SWCNT has a cylinder shape with a typical diameter of around 1 nm and is made
of a single layer of graphene sheet [2]. The SWCNT structure is related to the rolling
direction of the graphene sheet, which is called the chirality [3]. As shown in Fig. 1.2,
a pair of indices (n, m) called the chiral vectors are usually used to indicate the
chirality (Ch) of a SWCNT
Ch = na1 + ma2 (1)
( a ) ( b )
2
where n and m are two integers, and a1 and a2 are unit vectors of the graphene lattice.
When m = 0, the CNTs have a "zigzag" structure, while they have an "armchair"
structure when n = m. "Chiral" structure is used for a general CNT structure with m
0 and n m. The diameter of a SWCNT is determined by its chiral vectors:
(2)
with a = 0.246 nm
Figure 1.2. Graphene sheet segment showing indexed lattice points and the rolling vector [3].
DWCNTs and MWCNTs can be visualized as concentric tubes wrapped by two or
multiple layers of graphene sheets. Distance between two cylinder layers in DWCNTs
and MWCNTs is approximately 0.340 nm [4], slightly larger than that in graphite,
which is 0.335 nm [4]. Each graphene layer in a DWCNT or a MWCNT has its own
chirality, thus the electronic structures of DWCNTs or MWCNTs are much more
complex than SWCNTs. The overall diameters of MWCNTs range from a few
nanometers to about one hundred nanometers, depending on the number of graphene
layers [4].
3
1.2. Properties of carbon nanotubes
The perfect lattice structure aligned along the tube axis endows CNTs with the in-
plane properties of graphite, such as high conductivity, excellent mechanical strength
and stiffness, and chemical inertness. In addition, a lot of new properties originated
from quantum effects have been discovered for CNTs due to their unique one
dimensional (1D) nanostructure.
1.2.1. Electronic and electrical properties
The carbon atoms are sp2 hybridized in CNTs. Their Pz orbitals form a conjugated
system. The electronic state density of a SWCNT is composed of a series of
discontinuous spikes like other 1D semiconducting materials (Fig. 1.3). These spikes
are called van Hove singularities. The electronic structure of a SWCNT strongly
depends on its chirality [5,6]. For example, a SWCNT with the armchair chirality (n =
m) has no band gap between the conduction band and the valence band, so it is
metallic. A SWCNT with n - m = 3q (q is an integer) has a quite small band gap. It
thus exhibits quasi-metallic behavior. In contrast, a SWCNT with other chirality has a
sufficiently wide band gap, so that it behaviors as a semiconductor. The band gap of a
semiconducting SWCNT is approximately inversely proportional to its diameter [6,7]:
– (4)
where γ0 is the C–C tight-binding overlap energy, ac–c the nearest neighbor C–C
distance (0.142 nm) and d the diameter. This theory also implies that a graphene sheet
can be regarded as a SWCNT with infinite diameter and has no band gap.
4
Both metallic and semiconducting SWCNTs usually coexist in the products
synthesized by chemical vapor deposition (CVD) or other growth methods. The
metallic to semiconducting ratio of SWCNTs in commercially available products is
~1/2 [4]. The electrical conduction through semiconducting SWCNTs is dependent
on the relative position of the Fermi level with respect to the band edges. Thus, the
conductivity of the semiconducting SWCNTs can be significantly enhanced via
chemical or electrostatic doping. The highly doped semiconducting SWCNTs could
be as conductive as the metallic SWCNTs [4].
Figure 1.3. Density of electronic states for a semiconducting SWCNT. Solid arrows depict the optical
excitation and emission transitions; dashed arrows denote nonradioactive relaxation of electrons and
holes before emission [3].
5
SWCNTs can be regarded as quantum wires due to their nanoscale diameters. The
electronic conduction through a SWCNT undergoes a ballistic process, which means
that electrons can flow in SWCNTs over a relatively long distance without any
impediment [8]. The mean free path of charge carries in SWCNTs can be as long as
several micrometers [9], nearly 2 orders higher than that in a metal. The mechanism
of ballistic conduction is different from that of superconduction. Even when there is
no scattering, the electrons in the ballistic conduction experience the quantum
resistance, which is a constant given by [10]:
Rq = h/c2 = 26 kΩ (5)
where h = 6.63 × 10-34
m2 kg/s is the Planck's constant and c = 3 × 10
8 is the speed of
light.
The Ballistic conduction leads to very high conductivities. A SWCNT can have a
conductivity in the order of 104 to 10
5 S/cm [11] and a current density of 10
9 A/cm
[12], which is three orders higher than that of copper.
The electronic structures for MWCNTs are more complicate. Similar to graphite,
the inter layer interaction of MWCNTs is very week. Each cylinder layer of a
MWCNT behaves as an isolated SWCNT. Since the metallic layers and
semiconducting layers are connected in parallel, the overall electrical property for
MWCNTs is metallic.
1.2.2. Optical properties
The discontinuous energy band structures of SWCNTs also lead to unique optical
properties. As illustrated in Fig. 1.3, both the valence band and conduction band of a
6
SWCNT are composed of several individual Van Hove peaks. Electrons transitions
can take place between the Van Hove peaks in the different bands. Such transitions
are always associated with the absorption and emission of photons. Fig. 1.4 shows
that SWCNTs with different chiralities have different colors [13], corresponding to
their specific electronic structures. However, SWCNTs with mixed chiralities only
exhibit a black color.
Figure 1.4. Colors of SWCNTs with different chiralities [13].
While the van Hove transitions from low energy bands to high energy bands leads
to optical absorption, the transitions towards the opposite direction can induce the
photoluminescence emission of SWCNTs. As presented in Fig. 1.5, the optical
absorption spectra of SWCNTs exhibit a series of separated absorption peaks in the
vis-NIR range [ 14 ]. Well resolved characteristic absorption peaks can only be
observed from individually dispersed SWCNT samples, since the side-by-side van
der Waals contact of SWCNTs can cause the broadening of the absorption peaks
[14,15]. The photoluminescence emission spectra of SWCNTs have similar patterns
as their absorption spectra (Fig. 1.5). However, though all SWCNT species contribute
to the photon absorption, only the semiconducting tubes contribute to the
photoluminescence. The photoluminescence intensity is dramatically reduced by the
7
bundling of SWCNTs, since the presence of one metallic tube in a bundle will quench
the electronic excitation on adjacent semiconducting tubes [14]. No emission or
absorption spectra can be observed in MWCNTs due to the coexistence of metallic
layers and semiconducting layers.
Figure 1.5. The photoluminescence emission spectrum and the optical absorption spectrum of
individual SWCNTs suspended in a Sodium dodecyl sulfate (SDS) solution [14].
1.2.3. Mechanical properties
CNTs are among the stiffest and strongest materials that have been discovered.
Their Young's modulus is about 1 TPa [16,17], roughly five times greater than steel,
and their yield strength can be higher than 100 GPa [17,18], which is 100 times
greater than steel. In addition, CNTs are highly flexible. They can restore their
original shape after bending or buckling. The exceptional mechanical properties of
CNTs can be explained by their perfect lattice structure and the high strength of
carbon-carbon bonds.
8
1.2.4. Thermal properties
The strong covalent bonds in CNTs give rise to a high group velocity of the
phonons. Moreover, the perfect lattice structure and the high rigidity of CNTs
eliminates elastic scattering from defects, resulting into a large mean free path for the
phonons. Therefore, CNTs are excellent thermal conductors. Berber at el. reported
that the thermal conductivity of an isolated SWCNT can be as high as 6600 W m-1
K-1
at room temperature [19], and Kim et al. fount that the thermal conductivity of an
isolated MWCNT is about 3000 W m-1
K-1
at room temperature [20]. Bundling can
significantly reduce the thermal conductivities of CNTs. For example, the thermal
conductivity of aligned SWCNT bundles is only around 200 W m-1
K-1
[21]. CNTs
are also thermally stable. The decomposition temperature of CNTs is estimated to be
~2800 oC in vacuum and ~750
oC in air [22].
1.3. Dispersion of carbon nanotubes
SWCNTs have smooth surfaces and large specific surface areas. As a result, the
van der Waals interaction between two parallelly aligned SWCNTs is very strong
[10]. SWCNTs usually arrange into bundles with the diameter between 5~100
nanometers and form hexagonal lattices within the bundles. These bundles can further
entangle into large aggregates. MWCNTs generally exist as individual tubes and do
not form bundles like SWCNTs, but they can still entangle into aggregates. Bundling
and agglomeration undermine the extraordinary properties of CNTs, so the separation
and processing of CNTs is extremely important.
9
CNTs can be directly dispersed into certain organic solvents, such as
dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP) and
hexamethylphosphoramide (HMPA), by ultrasonication or high shear mixing [23,24].
These solvents have surface energies matching that of CNTs, so they can wet the
surface of CNTs and prevent their re-aggregation after dispersion. CNTs are not
water soluble, but they can be suspended in aqueous solutions of surfactants or
polymers [25-29], since the surfactant and polymer molecules can wrap around CNTs
to form hydrophilic shells.
It has been found that the solubility of CNTs in organic solvents or surfactant
solutions can be predicted by the Hansen solubility parameters [30]:
(6)
where δd, δp, and δh are the dispersive, polar, and hydrogen-bonding components
respectively. These parameters successfully illustrate the miscibility of hydrophobic
polymers or the dispersion state of organoclays in organic solvents [ 31 , 32 ].
Systematic study shows that the dispersive component δd, rather than the total
solubility parameter δt, has the most significant effect on the dispersion of CNTs [30].
Namely, CNTs can be stably suspended in solutions with δd between 17 to 18 MPa1/2
.
It has also been observed that CNTs precipitate in solutions with high δp, because the
CNT surface is nonpolar and hydrophobic.
Another approach to disperse CNTs is to improve their chemical compatibility with
solvents by chemical functionalization [ 33 ]. The most common fuctionalization
treatment is the oxidization of CNTs with strong acids like H2SO4, HNO3 or their
mixtures, at an elevated temperature [34]. This treatment leads to the formation of
10
carboxylic moieties, firstly on the ends and then on the side walls of CNTs. These
carboxylic moieties serve as the active sites for further derivatization [ 35 - 37 ].
However, aggressive chemical functionalization can introduce structural defects on
CNTs, resulting into inferior conductivities and mechanical strength. Fig. 1.6
schematically illustrates a typical functionalization treatment of CNTs.
Figure 1.6. Schematic presentation of a functionalization process of oxidized CNTs [25].
1.4. Applications of carbon nanotubes
CNTs have a lot of attractive applications based on their unique structures and
properties. For example, the 1D structure of CNTs gives rise to applications such as
AFM tips [ 38 ] and quantum wires [ 39 ], while their hollow structure leads to
applications like hydrogen storage [40,41]. The semiconducting CNTs can be used as
thin film transistors (TFTs) [42,43], field-effect transistors (FET) [44] and field
emitters [ 45 ]. CNTs are also important biomaterials because of their good
biocompatibility. The bio-applications of CNTs include drug delivery and
bioelectronics [46].
11
One of the most popular applications for CNTs is to use them as electrode materials.
While individual CNTs are difficult to handle due to their nanoscale dimensions, the
studies on CNT electrodes mainly focus on macroscopic CNT films. The CNT films
with thickness less than 100 nm are highly transparent due to the low coverage of 2D
CNT networks. They are promising candidate for transparent electrodes in place of
indium tin oxide (ITO). The most important advantage of CNT thin films over ITO is
their excellent flexibility and mechanical durability. CNT films coated on
polyethylene terephthalate (PET) substrates can be bended repeatedly for over 20000
cycles without showing obvious signs of fatigue [47]. The transparent CNT electrodes
can be used in a wide range of flexible optoelectronic devices including solar cells
[48,49], flat panel displays [50], touch panels [51] and organic light emitting diodes
(LEDs) [52]. On the other hand, CNT thick films with nanoporous structures can be
used as high performance electrode in various energy devices, such as supercapacitors
[53], fuel cells [54] and batteries [55], due to their large specific surface area and high
conductivity.
Another important application of CNTs is as the counter electrode of dye sensitized
solar cells (DSCs) [56]. The counter electrode plays an important role in DSCs by
reducing the iodide/triiionided based electrolyte. The reduction of iodide and triiodide
has a low overpotential on CNTs. Both SWCNTs and MWCNTs were reported as the
counter electrode of DSCs in literature [57-64]. Two types of methods, dry and wet
processing methods, were exploited to fabricate the CNT films on a conductive
substrate, which is usually fluorine tin oxide (FTO) glass. The dry methods are to
directly grow CNT films on conductive substrate, whereas in the wet methods CNTs
12
are dispersed in a solvent under assistant of a surfactant or a polymer and the CNT
films are fabricated by solution processing, such as spraying or vacuum filtration or
doctor blade. The wet processing method is more convenient and effective to make
CNT films in a large area. However, the CNT films prepared by the wet methods
usually have a poor adhesion to substrates [65-69]. They detach from the substrates
when in electrolyte. Hence, binders, such as epoxy and other polymers, are usually
used to stabilize CNT films on substrates. But these inert binders can lower the
catalytic and conductive capabilities of the CNT films since they reduce the contact
area between CNTs and electrolyte and block the inter-nanotube charge transport [70].
1.5. Structure and morphology of graphene
Graphene is composed of a single-layer of sp2 hybridized carbon atoms arranging in
a strictly two dimensional (2D) honeycomb lattice. The distance between two
adjacent carbon atoms is about 1.42 Å. Each carbon atom donates an electron from
the π orbital to the delocalized electron network to form a conjugated system. The
arrangement of atoms in graphene is the same as that in fullerenes, CNTs and
graphite. Thus graphene can be considered as the 2D building material of all the
carbon allotropes with graphitic lattice [71] (Fig. 1.7).
Before the discovery of graphene [72,73], it has been argued that strictly 2D
crystals could not exist, since they are thermodynamically unstable. Mermin proved
that the melting temperature of thin films drops rapidly with decreasing thickness and
the films become unstable at a thickness of dozens of atomic layers [74]. This
conception was overthrown only after 2004, when monolayer graphene was
13
exfoliated from bulk materials for the first time [72]. Further investigation shows that
the existence of graphene does not contradict with the laws of thermodynamics, since
the free standing graphene adopted a corrugated morphology to minimized the total
free energy (Fig. 1.8) [75 ,76 ]. The corrugations on free standing graphene are
estimated to have a lateral dimension of about 8 to 10 nm and a vertical displacement
of about 0.7 to 1 nm [77]. Graphene has a theoretical specific surface area lager than
2000 m2/g
Figure 1.7. Graphene as the parent of all graphitic forms [71].
Figure 1.8. The schematic illustration of corrugated graphene [75].
14
1.6. Properties of graphene
1.6.1. Electronic and electrical properties
Fig. 1.9 shows the energy dispersion in the honeycomb lattice [78]. The π and π*
bands in graphene have asymmetric structures. The two bands are connected only at
the six corners of the first Brillouin zone of the honeycomb lattice. Electrons and
holes near these corners behave like relativistic particles described by the Dirac
equation for spin 1/2 particles [79]. The electrons and holes are referred as Dirac
fermions, and the six corners are called Dirac points. The Fermi level of graphene
crosses the Dirac points. For Dirac fermions, E is linear with k as described by [79]:
(7)
where kx and ky are lattice vectors in the reciprocal lattice of graphene, vF ~ 106 m/s is
the Fermi velocity, and ħ = 1.0546×10−34
J s is the reduced Plank constant. Dirac
fermions in graphene have zero effective mass and a velocity about 300 times slower
than that of light.
Figure 1.9. Electronic dispersion in the honeycomb lattice [78]. The magnified the image shows the
linear relationship between E and k near one Dirac point.
15
According to the energy band structure, graphene is a semimetal or a
semiconductor with zero band gap [80]. Although the charge carrier density of
graphene is nearly zero at the Fermi level, it still exhibited conductivity in the order
of 4e2/h without doping [71]. It has been suggested that the rippling of the graphene
sheet or the ionized impurities may lead to local puddles of carriers and contribute to
the conduction. Like CNTs, the electrical conductance in graphene is also achieved
by ballistic transport on the sub-micron scale [81]. As a result, electrons can travel
long distances without scattering, which leads to high charge carrier mobilities. Under
ambient conditions, the charge carrier mobility in graphene is higher than 15000 cm2
V-1
s-1
[72,73], and can be increased to 200000 cm2 V
-1 s
-1 by minimizing impurity
scattering [82]. The mobility values depend weakly on temperature [83]. Graphene
also exhibits remarkable ambipolar electric field effect. Its charge carriers can be
tuned from electrons to holes in concentrations as high as 1013
cm-2
upon doping [71].
1.6.2. Optical properties
As an atomic monolayer, graphene has a relatively high opacity. It can absorb 2.3%
light in the whole visible range (Fig. 1.10a) [84]. The light absorption of graphene
can be attributed to the electron excitation near the Dirac points as shown in Fig.
1.10b. The light absorption is independent with the photon energy in the visible range.
Theoretically, the transmittance of graphene is determined solely by the fine structure
constant α = e2/ c, which is a universal constant describing the coupling between
light and relativistic electrons (Dirac fermions). The transmittance of monolayer
graphene in the visible range is given by:
16
T ≡ (1 + 0.5πα)-2
= 97.7% (8)
This value is in good agreement with the experimental result [84]. The transmittance
of muti-layer graphene nanosheets or films decreases linear with the increasing
number of graphene layers.
Figure 1.10. (a) A Photograph of a 50-mm aperture partially covered by graphene and its bilayer. The
inset shows the sample design: A 20-mm-thick metal support structure has several apertures of 20, 30,
and 50 mm in diameter with graphene crystallites placed over them [84]. (b) Excitation processes
responsible for absorption of light in graphene [84].
1.6.3. Mechanical properties
Since graphene and CNTs share the same lattice structure, they have comparable
mechanical properties. Young’s modulus and fracture strength of pristine graphene
are estimated to be as high as 1 TPa and 130 GPa respectively [85]. Graphene also
has high rigidity as a single atomic layer material. Its spring constant is in the range
of 1~5 N/m [86].
1.6.4. Thermal properties
At room temperature, the thermal conductivity of graphene is phonon dominated
because the electron density at its Fermi level is very low. The theoretical thermal
( a ) ( b )
17
conductivity of graphene is around 6000 W m-1
K-1
[19], close to that of CNTs. But
the conductivity values derived from experiments are more or less lower due to the
presence of defects. Balandin et al. reported that the room temperature thermal
conductivity of suspended graphene prepared by micromechanically exfoliation is
around 5000 W m-1
K-1
[87]. Cai et al. reported that the graphene grown by CVD has
a thermal conductivity of around 2500 W m-1
K-1
at 350 K [88].
1.7. Synthesis of graphene
1.7.1. Mechanical exfoliation of graphite
Single layer graphene sheets were firstly exfoliated from bulk graphite by the
simple micromechanical cleavage method [72,73]. In this method, graphite (either
natural graphite or highly ordered pyrolyitc graphite) is rubbed against a solid
substrate like SiO2, leaving a lot of thin flakes adhering on it. Although most of the
flakes are multilayer graphite nanosheets, graphene monolayers are also among the
products [72,73]. Graphene sheets with lateral dimensions exceeding 100 µm can be
obtained using this method [84].
Graphene can also be prepared by exfoliating graphite in certain organic solvents or
surfactant solutions via ultrasonication [89,90]. Most of the organic solvents used in
the dispersion of CNTs, like DMF and NMP, can be used for the exfoliation of
graphite. The lateral dimensions of graphene nanosheets prepared by this method are
about 1 µm. The fractions of graphene monolayers in the products are usually lower
than a few percents [89,90].
18
1.7.2. Direct growth
In the CVD methods, graphene films are usually grown on transition metal
substrates at the temperature of ~1000 oC [91-96]. Thickness of the CVD grown
graphene films can be accurately controlled. For example, Ruoff et al. [93] reported a
CVD method to grow large-area and continuous single layer graphene films on
copper foils using mixed methane and hydrogen as precursors. The overlapping area
on as synthesized graphene film is less than 5%.
The epitaxial growth of graphene is carried out by heating single crystal SiC
substrates at a temperature of 1200~1600 oC in vacuum or inert gas atmosphere.
Upon heating, silicon in SiC evaporated faster than carbon due to its lower
sublimation temperature, and the excess carbon atoms on the surface finally
rearranged into graphene structures [97-100].
1.7.3. Reduction of graphene oxide
One important derivative of graphite is graphite oxide, which can be prepared by
oxidizing graphite with mixtures of strong oxidants at an elevated temperature [101].
Graphite oxide contains considerable amount functional groups, such as epoxide,
hydroxyl, carbonyl and carboxyl groups [102-104]. These functional groups increase
the distance between graphite layers and weaken the inter-plane van der Waals
interactions. Thus the graphite oxide can be easily exfoliated into monolayer
nanosheets called graphene oxide (GO) (Fig. 1.11b). Unlike graphene, GO has very
high solubility in water due to the presence of hydrophilic functional groups.
19
The reduction of GO is one of the most promising techniques for the large scale
synthesis of graphene at low cost. A lot of approaches, including high temperature
annealing in oxygen free environment [105], hydrogen plasma reduction [106],
electrochemical reduction [107] and chemical reduction [108-117], can be used to
reduce GO. Among these approaches, the chemical reduction is mostly convenient
and has the lowest requirement on facilities. Stankovich et al. reported the the first
chemical reduction of graphene by heating GO in a hydrazine solution at 100 oC for
24h [108,109]. This method can remove most of the functional groups from GO and
partially restore its electrical conductivity. The reduced GO (rGO) had a conductivity
of ~2420 S/m, five orders higher than that of GO before reduction. Other resultants
such as dimethylhydrazine [110], hydroquinone [111], NaBH4 [112.113] and HI [114]
have also been used in the reduction of GO and similar results were obtained.
However, these chemicals are either toxic or extremely corrosive. Recently, several
groups developed “green” approaches to reduce GO using less aggressive chemicals.
Pei et al. reported the rapid deoxygenation of GO in alkaline solutions [115].
However, the oxygen content in the rGO was still quite high after reduction. Dreyer et
al. reported the reduction of GO with alcohols [116]. They found that GO reduced by
benzyl alcohol at 100 oC for 24 h had a conductivity of 4600 S/m and a carbon to
oxygen ratio of 30:1. Fan et al. reported the effective reduction of GO with Fe
powder in HCl solutions [117]. The reduction took about 6h and the final
conductivity of the rGO was around 2300 S/m.
The oxidization of graphite is an irreversible process. Thus even after reduction, the
resulted products are not true graphene (Fig. 1.11a). As shown in Fig. 1.11c, there are
20
a lot of vacancies left on the basal plane of rGO, and some functional groups on the
plane edges cannot be completely removed.
Figure 1.11. Chemical structural models of (a) pristine graphene, (b) GO and (c) rGO [104].
1.8. Applications of graphene
Graphene has many similar applications as CNTs due to their similar properties.
Individual graphene monolayers can be used as TFT channels [118,119]. The field-
effect mobility of graphene is one order higher than that of Si [120]. By tailoring the
morphology of graphene, the Ion/Ioff ratios of graphene based TFT can be increased to
107 [121]. Graphene can also be used as high performance chemical sensors [122-125]
or biosensors [126,127], because the conductivity of monolayer graphene has a strong
dependence on surface absorption. The sensibility of graphene toward certain
molecules can be greatly enhanced by specific chemical modification.
Graphene is also an important electrode material due to its large specific surface
area, high electrical conductivity, excellent flexibility and good stability. Thick and
porous graphene films can be used as electrode in clean energy devices such as Li
batteries [128] and ultracapacitors [129,130]. They are also idea supporting materials
for metal catalysts. The graphene/metal nanoparticle composites have been used as
21
catalytic electrode in high performance fuel cells [80]. Thin films of graphene are
promising substitutes for ITO as transparent electrode in optoelectronic devices due
to their high conductivity and high transparency. Bae et al. reported the fabrication of
30-inch graphene films by CVD [96]. As synthesized graphene films can be entirely
transferred onto flexible substrates using a roll-to-roll lamination technique. A 90%
transparent graphene film doped with HNO3 exhibits a sheet resistance of ~30 Ω/
[96], which is close to the sheet resistance of commercially available ITO. Although
the CVD methods provide graphene films with highest conductivity, they are not
suitable for large-scale production due to the low throughput and the high
requirement on facilities. A more convenient and economical way to fabricate large
area graphene films is to reduce solution processed GO films with reductants such as
hydrazine hydrate or NaBH4. However, the conductivity of rGO films is usually much
lower than that of the CVD grown graphene films [108,109].
22
Chapter 2. Experimental
2.1. Materials
SWCNTs and MWCNTs produced by chemical vapor CVD were purchased from
Chengdu Organic Chemicals Co. Ltd in China. The SWCNTs (Product Code :TNS)
have a purity of > 90%, outer diameter of 1-2 nm, inner diameter of 0.8-1.6 nm,
length of 5 ~ 30 µm and specific surface area of > 380 m2/g. The MWCNTs (Product
Code :TNM2) had a purity of > 95%, outer diameter of 8-15 nm, inner diameter of 3-
5 nm, length of 5~30 µm and specific surface area of > 233 m2/g. Both SWCNTs and
MWCNTs are not functionalized.
The TiO2 pastes are purchased from Solaronix (Ti-Nanoxide T) and Dyesol (DSL
18NR-T and WER4-O Reflector). The dye, cis-diisothiocyanato-bis(2,2’-bipyridyl-
4,4’-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719), was also
purchased from Solaronix.
High-purity natural graphite powder (SP-1graphite, purity > 99.99%) with an
average particle size of ~30 μm was purchased from the Bay Carbon Inc. Zn powder
with average diameters less than 30 μm was obtained from Riedel-de Haën. Other
chemicals, including KMnO4, NaNO3, H2O2, hydrazine hydrate, polyethylene glycol
dodecyl ether (Brij 30), cetrimonium bromide, sodium dodecylbenzene sulfonate
(SDBS), CMC, poly(sodium 4-styrenesulfonate) acid, poly(ethylene glycol) (PEG,
molecular weight 200 g mol-1
, melting point = -65 oC), Polyoxyethylene (12)
tridecyl ether [POETE, C13H27(OCH2CH2)nOH, n =12] H2SO4, HNO3, HCl and Al,
were supplied by Sigma-Aldrich. All the chemicals were used without further
purification.
23
2.2. Experimental techniques
2.2.1. Preparation of CNT gels by mechanical grinding
CNTs (10 mg) were mixed with 2 ml POETE in an agate mortar. The mixture was
ground for 1 h, and it turned into a sticky paste during the grinding. The paste was
centrifuged with a Profuge 14D at 10000 rpm (force equal to 9200 g) for 30 min. The
mixture separated into two phases. The top transparent liquid phase was decanted
from the containers after centrifugation. The bottom phase was collected, after the
container with the bottom phase was put upside down for 1 h to let the excess POETE
flow out under gravity.
2.2.2. Preparation of SWCNT films by gel coating and spray coating
A gel was formed by mechanically grinding a mixture of SWCNTs and POETE in
an agate mortar for 30 min and subsequently ultrasonicating the mixture with a
Sonics Vibracell VCX130PB ultrasonic disperser (maximum energy output 130 W) at
100% amplitude for various periods. The typical ratio of SWCNTs to POETE was 5
mg SWCNTs in 2 ml POETE before mixing. The excess POETE in the gel was
removed by centrifugation with a Profuge 14D centrifuge at 10000 rpm (force equal
to 9200 g) for 30 min.
The SWCNT:POETE gels were coated on glass substrates by doctor blade. The
thickness was controlled with a 50 µm-thick Scotch tape as a gap between the
substrate and the blade. The coated SWCNT:POETE layers were heated at 250 oC for
15 min and successively at a temperature between 480 and 520 oC for another 20 min
in air. After cooling down to room temperature in ambient environment, The SWCNT
24
films were immersed in acetone or ethanol for 1 min and successively soaked in 65%
HNO3 solution for 1 h to remove the impurities left by POETE. Finally, the films
were dried in air at room temperature.
The transfer of an SWCNT film from a glass substrate to a polymer substrate was
carried out by slowly immersing a dried film into water at room temperature. The
film detached from the glass substrate and floated on water. A PET substrate was
used to pick up the film from water.
SWCNT films were also prepared by spray coating for comparison. The spray
coating was carried out according to reference [131]. 2 mg SWCNTs were dispersed
in 20 ml aqueous solution of 0.5 wt% sodium carboxymethyl cellulose (CMC) by
ultrasonication for 10 min. Large bundles and particles were removed from the
suspension by centrifuging at 12000 rpm for 60 min. The SWCNT/CMC dispersion
was spray coated onto glass substrates with a Sealey air brush. The temperature of the
substrates was 100 oC during spray coating. After the evaporation of the solvent, the
films were separated into two groups. One group were directly immersed in 14 M
HNO3 solution overnight to remove CMC. The other group were heated at 500 oC for
20 min and then soaked in 14 M HNO3 overnight. All the SWCNT films were finally
dried under nitrogen flow at room temperature.
2.2.3. Preparation of CNT Counter electrode
10 mg CNTs (SWCNTs or MWCNTs) were mixed with 2 ml PEG. The mixture
was ground in an agate mortar for 10 min and subsequently sonicated for another 10
min with a Sonics Vibracell VCX130PB sonicator. The mixture was centrifuged with
25
a Profuge 14D centrifuge at 10 krpm for 30 min to remove the excess PEG, so that a
black CNT/PEG gel was obtained. CNT films were fabricated by coating the
CNT/PEG gels on FTO glass substrates by the doctor blade method, and removing
PEG by heating at 250 oC for 10 min and subsequently at 450
oC for 30 min in air.
2.2.4. Fabrication of DSCs
DSCs were fabricated through the conventional process [70]. The TiO2 working
electrode consists of a mesoporous TiO2 layer of 15 μm in thickness (Solaronix Ti-
Nanoxide T of 10 μm in thickness and Dyesol DSL 18NR-T of 5 μm in thickness)
and a scattering TiO2 layer of 5 μm in thickness (Dyesol WER4-O Reflector paste).
The sintered TiO2 electrodes were immersed in a 0.5 mM N719 dye solution in
acetonitrile/tert-butanol (volume ratio: 1:1) for 24 h. The dye-impregnated TiO2
electrode was assembled and sealed with a counter electrode with a Solaronix sealing
foil of 60 m in thickness. The cells were filled with a liquid electrolyte comprising
0.6 M 1-methyl-3-propylimidazolium iodine, 0.03 M I2, 0.1 M guanidine thiocyanate,
and 0.5 M 4-tert-butyl pyridine in acetonitrile/valeronitrile (volume ratio = 85:15).
The CNT films on FTO glass were used as the counter electrode of DSCs. Control
DSCs were fabricated using Pt as the counter electrode, which was prepared by
pyrolysis of ethanol solution of 0.2 M H2PtCl6 at 400 oC for 15 min.
2.2.5. Oxidization of graphite
Graphite was oxidized through a modified Hummers’ process [114]. 1 g graphite
powder was mixed with 1 g NaNO3 in 48 ml 98% H2SO4. 6 g KMnO4 was slowly
26
added into the mixture under stirring. The mixture was stirred in an ice bath for 90
min and subsequently in a water bath of 35 oC for 2 h. Then, 40 ml H2O was slowly
dropped into the mixture over a period of 30 min. Temperature of the mixture rose to
around 90 oC during the water addition. During the oxidization process, concentrated
nitric acid is produced by mixing NaNO3 and H2SO4 with H2O. HNO3 is a common
oxidizing agent known to react strongly with aromatic carbon surfaces. The mixing of
KMnO4 and H2SO4 produces dimanganeseheptoxide (Mn2O7), which can selectively
oxidize unsaturated aliphatic double bonds over aromatic double bonds. In synergy,
these chemicals can break the conjugated C=C bond in graphite and introduce various
functional groups such as epoxide, hydroxyl, carbonyl and carboxyl groups. Finally,
100 ml H2O was added into the mixture, followed by the addition of 10 ml 30% H2O2
solution to reduce the residual permanganate and manganese dioxide to colorless
soluble manganese sulfate. The sediment in the mixture was collected by
centrifugation. It was repeatedly rinsed with 5% HCl solution and successively with
de-ionized water. Finally, the oxidized graphite was frozen in liquid nitrogen and
dried with a Labconco Freezone freeze drier for two days.
2.2.6. Exfoliation and reduction of GO
50 mg oxidized graphite was dispersed in 50 ml H2O under ultrasonication with a
Sonics & Materials Vibracell VC 505 ultrasonic processor. A homogeneous brown
GO solution was obtained, and no particle was observed with naked eyes. After the
pH value of the solution was adjusted to 2 by the addition of 37% HCl solution, 100
mg Zn powder was added into the GO solution. The mixture was immediately
27
sonicated with the ultrasonic processor for another 1 min. The top part of the brown
solution turned into transparent, and black precipitates appeared at the bottom after
the sonication. The excessive Zn powder was dissolved by adding 5 ml 37% HCl.
rGO was collected by vacuum filtration. It was rinsed with excessive water and
successively desiccated with the freeze-drier for two days.
For comparison, GO was also reduced by hydrazine hydrate according to the well-
established method reported in literature [109]. 0.5 ml hydrazine hydrate was added
into 50 ml 0.1 wt% GO aqueous solution. The solution was stirred in a water bath of
95 oC for 24 h. Black precipitates appeared during the reduction. The rGO product
was collected by vacuum filtration and desiccated with the freeze-drier for two days.
2.2.7. Re-dispersion of rGO
An rGO suspension was directly mixed with an aqueous solution of 1 wt%
surfactant in 1:1 volume ratio. Various surfactants, including Brij 30, cetrimonium
bromide, 4-dodecylbenzenesulfonic acid (SBDS), CMC, poly(sodium 4-
styrenesulfonate), were used. The mixtures were sonicated with the ultrasonic
processor for 2 min and then centrifuged with a Hettich Rotina 35 tabletop centrifuge
at 5000 rpm for 10 min to remove the excessive Zn powder and large aggregates.
The rGO films were prepared by the vacuum filtration of SDBS solutions dispersed
with rGO through a mixed cellulose ester (MCE) membrane. After rinsing with
excessive water for 10 times, the rGO films together with the MCE membranes were
put onto a glass substrate with the films in touch with the substrates. The MCE
membranes were dissolved by immersing the films into acetone for 2 days. In order to
28
completely remove the residual surfactant, the rGO films were then soaked in 37%
HCl solution for 4 days and subsequently dried in ambient environment at 120 oC for
30 min. The films were cut into strips for the conductivity measurement.
2.2.8. Preparation of GO films by spin coating
GO were prepared and purified using the method described in Chapter 5. The
purified GO was dispersed in a 1:5 H2O/ethanol mixture by 10 min of bath
ultrasonication and subsequently centrifuged at 3000 rpm for 10 min to remove any
large aggregates. Concentration of GO in the final suspension was determined to be
~2 mg/ml by measuring the weight of desiccated GO. GO films were prepared by
spin coating the GO suspension on various substrates, including quartz, Si and PET,
at a spin rate of 3000 rpm. The quartz and Si substrates were used for the optical and
scanning electron microscopy (SEM) characterizations respectively. To accelerate the
solvent evaporation, the substrates were blown with N2 flow during the spin coating.
Thickness of the GO films can be controlled by varying the repeating cycles of the
spin coating process. The GO films were then baked on a hotplate at 100 oC for 30
min to remove the residual H2O.
2.2.9. Preparation of transparent rGO films by the reduction of GO films
30 nm thick Al was deposited onto the GO films at a deposition rate of 2 nm/s
using an Edwards Auto 306 thermal evaporator. The Al coated GO films were then
soaked into 4M HCl solution for 2 hours. Al was etched away during the acid
29
treatment, and the GO films were reduced at the same time. Finally, the rGO films
were rinsed repeatedly with H2O and dried with N2 flow in ambient environment.
2.3. Characterization
2.3.1. Dynamic mechanical characterization
The rheological properties of the CNT gels were studied using a Rheometric
Scientific Advanced Rheometric Expansion System (ARES) with the cone-to-plate
configuration in the dynamic mode. The diameter of the cone was 2.5 cm, the cone
angle was 0.04 rad, and the gap between cone and plate was 0.5 mm.
During measurement, the gels were sandwiched between the cone and plate. A
periodic shear strain:
γ = γ0sin(tω) (9)
was applied to the cone, and the stress of gel was measured. The stress is also
periodic as described by the following equation:
τ = τ0sin(tω + φ) (10)
where φ is the phase lag between stress and strain and ω is the oscillation frequency
of strain. For perfect elastic materials φ= 0, and for perfect inelastic materials φ= 90o.
The dynamic mechanical behavior of a material can be characterized by two
parameters, the storage modulus G’ representing the elastic portion, and the loss
modulus G” representing the viscous portion. The two moduli are defined by the
following equation:
(11)
(12)
30
Gels are large amount of fluids trapped inside 3D solid networks. Although the main
components of gels are liquid, their dynamical mechanical properties are similar to
those of elastic solids. As a result, the storage modulus G’ of a gel must be much
larger than its loss modulus G”, and G’ should be independent of the oscillation
frequency ω.
2.3.2. Conductivity measurement
The conductivities of materials were measured by the four-point probe technique
with a Keithley 2400 source/meter.
The conductivity of the CNT/POETE gel was measured by placing the gel on a
glass slide attached with four copper foils which served as the four probes. The
thickness of the gel was controlled by placing another glass substrate on the top of the
gel, that is, the CNT/POETE gel was sandwiched between two glass substrates. The
thickness of the gels for the conductivity study was measured by a micrometer caliper.
The conductivities of CNT and graphene thin films were measured with the van der
Pauw configuration. The thin film samples were cut into square shape, and four
indium electrodes were pressed onto the four corners of the films, serving as contact
electrode for the measurement. The thicknesses of the films were determined with a
Veeco NanoScope IV Multi-Mode atomic force microscopy (AFM) by scanning
across the film edges.
31
2.3.3. Morphology characterization
SEM images and energy dispersive X-ray (EDX) spectra of CNTs, CNT gels and
CNT films were obtained with a Hitachi S-4100 scanning electron microscope. SEM
images of GO and rGO were taken with a Zeiss Supra 40 FE scanning electron
microscope. TEM images were taken with a JOEL JEM 3010F transmission electron
microscope equipped with a field emission gun. TEM samples were prepared by
dispersing the materials, such as CNT gels, GO and rGO, into deionized water, and
then dropped the suspensions on on lacey carbon TEM grids.
2.3.4. Thermal analyses
The thermal gravimetric analyses (TGA) were carried out in air with a Du Pont
2950 TGA. The POETE, MWCNTs, and SWCNTs samples were put at 120 oC for 10
min and then heated to 700 oC at a heating rate of 10
oC/min under a constant air flow
of 60 cc/min. The GO and rGO samples were directly heated to 700 oC at a heating
rate of 5 oC/min under a constant air flow of 60 cc/min.
2.3.5. Optical spectroscopy
Raman spectroscopy was used to probe the lattice structure of graphene and CNTs.
The spectra were acquired in a back scattering configuration using a Jobin Yvon
Horiba LabRam HR800 Raman system with a 514.5 nm Ar+ laser as the excitation
source. The laser power was 2mW, and the resolution was 2.4 cm-1
. For CNT gels,
the accumulation time was 900 s. For other samples, including CNTs, GO and rGO,
the accumulation time was 120 s.
32
Raman spectra of SWCNTs have three bands in 120 cm-1
to 1800 cm-1
range,
corresponding to three vibration modes. The radial breathing mode (RBM) at 120 ~
300 nm is caused by the collective vibration of carbon atoms along the radial
direction. The D band at around 1350 cm-1
is usually caused by the defects in
graphitic lattice, and the G ban is caused by the in plane oscillation of the sp2
hybridized carbon in graphitic lattice. The Raman spectrum of graphene is very
similar to that of SWCNTs, except that there is no RBM band. For pristine graphene,
the D bands are almost negligibly due to the absence of defects. However, the D
bands for rGO are usually intense due to the existence of numerous sp2 domains.
The UV-vis-IR spectroscopy was used to investigate the transparency and energy
band structures of materials. The absorption and transmittance spectra were acquired
with a Varian Cary 5000 UV-vis-NIR spectrometer. The thin film samples were
coated onto quartz substrates for the optical measurement. And the liquid samples
were prepared by dispersing the materials into deionized water and were measured in
quartz cuvettes.
FTIR spectroscopy was used to study the chemical structure of materials. FTIR
spectra were acquired with a Varian 3100 FT-IR spectrometer and the samples were
prepared by dispersing the materials in KBr pellets.
2.3.6. Photovoltaic performance characterization
The photovoltaic performance of the DSC was measured by applying a scanning
potential bias V to the cell and measuring the generated photocurrent I with a
computer-programmed Keithley 2400 source/meter. The voltage step of the scanning
33
voltage was 10 mV. During the measurement, the TiO2 working electrode of the DSC,
which is covered by a circular mask with a diameter of 5.2 mm, was illuminated with
a AM1.5 Newport’s Oriel class A solar simulator (100 mW cm-2
) certified by the JIS
C 8912 standard. The current density J of the DSC is calculated by normalizing the
photocurrent with the area of the circular mask. The bias at which the photocurrent is
zero is indicated as the open circuit voltage Voc, while the current density measured at
zero bias was indicated as the short circuit current density Jsc. The energy conversion
efficiency μ of the DSCs is the maximum product of V and J. The fill facter FF of the
DSCs is determined by the following equation:
(13)
2.3.7. Electrochemical characterization
Cyclic voltammetry (CV) was carried out in an acetonitrile solution comprising 5
mM LiI, 0.25 mM I2 and 0.1 M tetrabutylammonium hexafluorophosphate with an
Autolab PGSTAT302N+FRA2 electrochemical system. CNT films and Pt on FTO
glass substrates, which were used as the counter electrode in the DSCs, were used as
the working electrode in the CVs. A Pt wire and an Ag/Ag+ reference electrode were
used as the counter electrode and reference electrode, respectively. The scan rate is 10
mV s-1
. Ac impedance spectroscopy was performed on symmetric cells, which had
the same electrolyte as in the CVs between two identical Pt or CNT electrodes. The
frequency range was from 0.05 to 105 Hz, and the oscillation amplitude was 10 mV.
34
2.3.8. X-ray photoelectron spectroscopy
The X-ray photoelectron spectroscopy (XPS) was used the analyze the atomic
composition of GO and rGO. The XPS spectra were taken using an Axis Ultra DLD
X-ray photoelectron spectrometer equipped with an Al Ka X-ray source (1486.6 eV).
The energy step size of the XPS was 1 eV for survery scans and 0.05 eV for the fine
scans. The CasaXPS version 2.3.14 program was used to subtract the Shirley
background, analyze the composition and deconvolute the XPS peaks. The
deconvolutions were performed by a combination of Gaussian (80%) and Lorentzian
(20%) distributions. The survey and fine scan XPS spectra were calibrated with the
sp2 C1s band at 284.5 eV and the deconvoluted peak of sp
2 hybridized carbon at 284.5
eV, respectively.
35
Chapter 3. Gels of carbon nanotubes and a
nonionic surfactant prepared by mechanical
grinding
3.1. Introduction
CNTs have been attracting strong attention owning to their fascinating 1D structure
and unique mechanical, electrical and thermal properties [16,39,69,70,132-136]. They
are promising in various important applications, including in composite materials,
electronic devices, and biology. Though great progress has been made in the
fundamental study of CNTs, their application is severely impeded by their poor
processibility, since they do not melt and usually have quite low solubility in solvents
[137,138]. Many methods have been developed to improve the dispersion capability
of CNTs in solvents, including covalent sidewall modification, polymer wrapping,
modification through π-π interaction with aromatic molecules, and surfactants
[33,137-142]. However, these methods require high power ultrasonication treatments
or chemical modifications, which could bring severe defects to CNTs and alter their
physical properties. Moreover, these methods are still far away from the large scale
processing of CNTs, because the concentration of CNTs dispersed in solutions is
usually very low. To efficiently process CNTs is still an important research topic. A
simple and efficient method which can disperse CNTs at high concentration is
required. The method reported by Fukushima et al. on gels of CNTs and ionic liquids
opens a new possibility to process CNTs in large scale [143,144]. Those gels are also
36
called bucky gels of ionic liquids, and they can be prepared by simple mechanical
grinding. This method was also studied by other laboratories due to its significance in
processing CNTs [145-148]. Debundling of CNTs takes place during the mechanical
grinding. The gel behavior was confirmed by the rheological characterization. The gel
formation of CNTs with ionic liquids is related to the cation-π or the normal van der
Waals interactions between them. Applications using CNTs dispersed by this method
were demonstrated, such as in actuators, electrochemistry, biology, and dispersion of
CNTs in water [149-157]. Moreover, Sekitani et al. prepared CNT/ionic liquid gels
with high concentration of CNTs through jet-milling homogenizer [158,159]. They
found that the mixtures of the gels with rubber could be processed into stretchable
conductive films. In parallel to the research on the gels of CNTs and ionic liquids, it
was discovered that suspensions of CNTs and surfactants, obtained by
ultracentrifugation of solutions dispersed with surfactants and CNTs, also behave as
gels [160-162]. Obviously, the method reported by Fukushima et al. is more efficient
and simpler.
This chapter describes the preparation of gels of CNTs and a liquid nonionic
surfactant, which is a fluid at room temperature, by mechanical grinding. The gel
formation can be attributed to the CNT networks mediated by the van der Waals
interaction between CNTs and the nonionic surfactant. Compared with ionic liquids,
nonionic surfactants have a lot of advantages such as low price, non-toxic, non-
corrosive and low boiling point. They can be easily removed away from the
composite gel by simple heat treatment.
37
3.2. Results and discussion
3.2.1. Gels of MWCNTs and POETE
Surfactants have been frequently used to disperse CNTs in solvents, particularly in
water [142]. POETE as a nonionic surfactant exhibited good wettability for
MWCNTs, when they were mixed together. The mixture of MWCNTs and POETE
turned into a soft, black, and sticky paste after mechanically ground for about 1 h.
Excess POETE was used to have a better dispersion of CNTs. The excess POETE
was removed by the subsequent centrifugation of the paste. The paste separated into
two phases in the centrifuge tube after the centrifugation at 10 krpm for 30 min. The
bottom phase was a black gel, while the top was transparent POETE. Fig. 3.1a shows
a picture of an upside-down centrifuge tube containing the two phases. The black gel
remained on the bottom of the centrifuge tube and did not fall down under gravity.
Since the gels of CNTs and ionic liquids are called bucky gels of ionic liquids [143],
the gels of CNTs and a nonionic surfactant can be named as bucky gels of the
nonionic surfactant as well. The weight percentage of MWCNTs in the
MWCNT/POETE gel was determined after removing the excess POETE. It was 1.5%
by weight, that is, the maximum gelling efficiency was 66 g POETE for 1 g
MWCNTs. The MWCNT/POETE gel could be extruded from a needle and did not
break down under gravity (Fig. 3.1b).
The MWCNT/POETE gels were stable at room temperature. They did not show
any remarkable change after several weeks. The gels still kept the solid-like shape
under heating up to 200 oC. They also did not shrivel even in vacuum of 10
-4 mbar.
The stability under heating or in vacuum is related to the low volatility of POETE.
38
The gels were electrically conductive. Their conductivity was about 4 × 10-3
S cm-1
as
determined by the four-point probe technique (Fig. 3.1c). The electrical conduction
can be attributed to the formation of continuous MWCNT network in the gel since the
liquid component POETE is insulating.
Figure 3.1. Photographs of a MWCNT/POETE gel. (a) Phase-separation behavior of the gel (upper
phase) and excess POETE (lower phase), observed after centrifugation of a ground mixture of
MWCNTs and POETE. The dotted line indicates the surface of the liquid phase. (b) Extrusion of the
gel from a needle. (c) Electrical conduction of the gel with the four-point probe technique.
Fig. 3.2 shows the SEM images of the as-received MWCNTs and a
MWCNT/POETE gel. These SEM images indicate significant change of MWCNTs
during the mechanical grinding. The as-received MWCNTs were heavily entangled,
and some of them bundled together. They became separated and showed less bundles
in the gel. This indicates that the MWCNTs debundled during the grinding, just
similar to the grinding of CNTs in ionic liquid [143]. Individual MWCNTs could be
observed by TEM after dispersing the MWCNT/POETE gel in water (Fig. 3.3). The
average diameter of the MWCNTs was about 15 nm, consistent with the specification
provided by the manufacturer.
39
Figure 3.2. SEM images of (a) as-received MWCNTs and (b) a MWCNT/POETE gel.
Figure 3.3. TEM image of MWCNTs. The sample was preparedby dispersing a MWCNT/POETE gel
in water.
The rheological properties of the MWCNT/POETE gels were characterized as well
(Fig. 3.4a). The dynamic storage modulus (G’) is almost flat over a wide range of the
angular frequency (ω) from 0.1 to 100 rad s-1
at the amplitude of applied strain (γ)
being 1% or 5%, while the loss modulus (G”) does not show any sign of relaxation.
G’ is always higher than G” at a certain angular frequency. This dynamic mechanical
behavior is similar to that of CNT/ionic liquid gels or CNT/surfactant suspensions
obtained by ultracentrifugation of solution dispersed with surfactant and CNTs [143-
40
145,160-162]. These results indicate that the MWCNT/POETE gel prepared by the
mechanical grinding has permanent networks and therefore behaves as a gel. The
MWCNT/POETE mixtures are stable and behave as gels at high temperature. G’ is
flat in the angular frequency from 0.1 to 100 rad s-1
, and it is much higher than G” at
125 oC (Fig. 3.4b).
Figure 3.4. Angular frequency (ω) dependencies of dynamic storage (G’, solid symbols) and loss
moduli (G”, open symbols) of a MWCNT/POE gel at (a) 25 oC and (b) 125
oC. Applied strain
amplitudes (c) were 1% (circles) and 5% (triangles).
The dispersion of MWCNTs in POETE is related to the van der Waals interaction
between MWCNTs and POETE, since POETE is a nonionic surfactant, and
surfactants have been frequently used to disperse CNTs in solvents [142]. This
interaction helps MWCNTs debundle when ground, and it stabilizes MWCNTs in
POETE. Thus, the networks in the MWCNT/POETE gel are formed by MWCNTs
and mediated by the van der Waals force between MWCNTs and POETE. The
conductive property of the MWCNT/POETE gel is due to the MWCNT networks in
the gel. The formation of the MWCNT/POETE gel is related to the length of the
MWCNTs. Short MWCNTs, which were about 1 µm in length, were also used for the
41
gel preparation through a similar process. The maximum gelling efficiency was down
to 32.3 g POETE for 1 g short MWCNTs. The lower gelling efficiency of the short
MWCNTs compared with long MWCNTs also implies that the networks in the
MWCNT/POTET gels are formed by the MWCNTs. More short MWCNTs are
needed to form the networks. Besides average CNT length, other factors such as CNT
diameters, surface defects and impurities can also affect the gelling efficiency of
CNTs with surfactants.
3.2.2. Gels of SWCNTs and POETE
Similar gels could be prepared when SWCNTs were used to replace MWCNTs
through a similar process. Fig. 3.5a is the SEM image of as-received SWCNTs. They
had an average length of 5~30 µm terms of the information from the manufacturer.
Some impurities, including metal nanoparticles and amouphous carbon, could be
observed as well, while such impurities were not observed in the as-received
MWCNTs. These impurities could not be completely removed by the conventional
purification processes, such as heating and acid treatment of CNTs.
The SWCNTs were mixed with POETE, and the mixture turned into a sticky paste
after grinding for about 1 h. The maximum gelling efficiency was 37.5 g POETE for
1 g SWCNT. This gelling efficiency is lower than that for long MWCNTs. The
reason may be related to the impurities in the SWCNTs. Fukushima et al. also
observed that the impurity could lower the gelling efficiency of CNTs for ionic
liquids [143,144]. Fewer bundles were observed, and the bundles become smaller in
the SWCNT/POETE gel compared with the as-received SWCNTs (Fig. 3.5b). Thus,
42
the debundling of SWCNTs occurred as well when the mixture of SWCNTs and
POETE was ground.
Figure 3.5. SEM images of (a) as-received SWCNTs and (b) a SWCNT/POETE gel.
The dispersion of SWCNTs in POETE was characterized by the Raman
spectroscopy as well (Fig. 3.6). The bands at the range of 1500 to 1600 cm-1
are the
tangential modes of SWCNTs, that are, the G modes of CNTs, and the band at 1348
cm-1
is from the carbon nanoparticles or SWCNT defects [163,164]. No significant
change was observed in the position of the Raman bands, while the intensity ratio of
the G modes at 1587 cm-1
to that at 1348 cm-1
became slightly higher for the
SWCNT/POETE gel than for the as-received SWCNTs. This indicates that some
carbon nanoparticles may be removed after the centrifugation of the mixture. This is
consistent with the observation by Yu et al. [164].
The rheological properties of the SWCNT/POE gel are quite similar to that of the
MWCNT/POETE gel (Fig. 3.7). G’ exhibited a plateau over the angular frequency
range from 100 to 0.1 rad s-1
, while G” does not exhibits any relaxation. G’ is always
higher than G”. These dynamic mechanical results also indicate that the
SWCNT/POETE mixture prepared by grinding behaviors as a gel.
43
Figure 3.6. Raman spectra of (a) as-received SWCNTs and (b) a SWCNT/POETE gel.
Figure 3.7. Angular frequency (ω) dependencies of dynamic storage (G’, solid symbols) and loss
moduli (G”, open symbols) of a SWCNT/POETE gel at 25 oC. Applied strain amplitudes (c) were 1%
(circles) and 5% (triangles).
44
Similar to the MWCNT/POETE gels, the SWCNT/POETE gels were electrically
conductive, and the efficiency for SWCNTs to gelate POETE decreased when short
SWCNTs were used. The maximum gelling efficiency was only 16.5 g POETE for 1
g SWCNTs with average length of 1 µm. These results combined with the rheological
properties of the SWCNT/POETE gels suggest that the permanent networks in the
SWCNT/POETE gel are formed by SWCNTs.
3.3. Conclusion
Bucky gels were formed by mechanically grinding mixtures of CNTs and POETE,
which was a nonionic surfactant and a fluid at room temperature. The CNTs debundle
during the grinding and form networks through the physical cross-linking in the
CNT/POETE gels. The gels have good stability up to 200 oC or in vacuum.
Finally, it is worth to point out that our method to disperse CNTs in POETE by
mechanical grinding is different from the conventional methods to disperse CNTs in
solution of surfactants, in which an ultrasonication process is needed [142,165-167].
No solvent is used in our method, and our method can disperse CNTs in a much
higher concentration than the conventional ways.
45
Chapter 4. Highly conductive and transparent
single-walled carbon nanotube thin films
fabricated by gel coating
4.1. Introduction
Optoelectronic devices, including liquid-crystal displays (LCDs), LEDs, solar cells,
touch-panel displays, electrochromic display and lasers, have been attracting strong
attention. Their market is huge and has been rapidly expanding. At least one electrode
of an optoelectronic device is required to be transparent for light transport. ITO is
conventionally the most popular material as the transparent electrode. However, ITO
has problems of scarce indium source in earth that results into the sky-rocketing of
the indium price and mechanical brittleness that makes it unsuitable in the flexible
electronic devices. New materials are urgently needed to substitute ITO as the
transparent electrode in optoelectronic devices. Besides conducting polymers [168-
170 ] and graphene [ 171 , 172 ], SWCNTs are also very promising as the next-
generation transparent electrode materials because of their high conductivity, high
transparency, high mechanical flexibility, high thermal stability and chemical
inertness [134,173]. One of the challenges for the application of SWCNTs lies in the
difficulty in efficiently processing SWCNTs on a large scale. Though films can be
prepared by direct SWCNT growth [174-179] or solid-state drawing [135,180 ],
solution processing is regarded as the most efficient way to fabricate SWCNT films
on substrates. Several solution-processing techniques have been reported to fabricate
46
SWCNT films, including vacuum filtration [69,133,181,182], spray coating [131,183],
ink-jet printing [ 184 ], spin coating [ 185 ,186 ], drop casting [ 187 ], dip coating
[188,189], quasi-Langmuir Blodgett deposition [190], and bar coating [165,191]. A
common step in these methods is the dispersion of SWCNTs in organic solvents or
surfactant solutions. Copious solvent is needed for the dispersion of a small amount
of SWCNTs.
This chapter reports a novel method to fabricate highly transparent and conductive
SWCNT films on substrates by gel coating. SWCNTs can form gels with some liquid
nonionic surfactants such as POETE [192] or low molecular weight polyethylene
glycol (PEG) [56]. The gels can be coated and converted into SWCNT films by
heating to remove the surfactants. The films become very thin and transparent after
heating at a temperature between 480 and 520 oC. They can have a sheet resistance of
100 Ω/ at 70% transmittance or a sheet resistance of 250 Ω/ at 80% transmittance.
The films detach from glass substrates and float on water when they are immersed
into water. They can be then transferred to other substrates like flexible polymer
sheets.
4.2. Results and discussion
4.2.1. Fabrication of SWCNT films by gel coating
The gels formed by dispersing SWCNTs in POETE can have a SWCNT
concentration of more than 1 wt%, much higher than the SWCNT concentration
dispersed in solvents or solutions which is usually less than 0.1 wt% [193]. The bucky
47
gels were physical gels. They were coated on substrates by doctor blade. They
converted into SWCNT films after the removal of the surfactant through heating.
100 200 300 400 500 600 7000.01
0.1
1
10
100
(c)
(b)
We
igh
t p
erc
en
tag
e (
%)
Temperature (oC)
(a)
Figure 4.1. TGA curves of (a) POETE, (b) a SWCNT:POETE gel, and (c) pure SWCNTs in air.
As suggested by the TGA studies (Fig. 4.1), POETE starts the weight loss at about
220 oC and loses more than 99.5% weight at 400
oC. The weight loss is due to the
vaporization, oxidation and decomposition of POETE. Two weight loss processes
could be observed for the SWCNT:POETE gel. More than 98% weight loss occurs at
temperature below 400 oC, which can be attributed to the evaporation, decomposition
and oxidation of POETE. Another weight loss takes place at temperature above 450
oC. Less than 0.05% weight remains till 580
oC. The second weight loss is mainly due
to the thermal oxidation of SWCNTs [194-197]. Though it is difficult to identify the
onset temperature for the thermal oxidation of SWCNTs in the gel because the
SWCNT amount is much less than POETE, the weight loss in the temperature range
of above 450 oC becomes more remarkable than in the temperature range of 350 to
48
450 oC. By comparing the weight loss of the SWCNT:POETE gel with that of
SWCNTs, we can conclude that the thermal oxidation of SWCNTs in the gel occurs
at lower temperature. The weight loss terminates at around 580 oC for the gel, while it
goes up to 700 oC for pure SWCNTs. The lower thermal stability of SWCNTs in the
gel may be related to their dispersion, since separated SWCNTs have higher specific
areas than heavily bundled SWCNTs.
The films obtained from the SWCNT:POETE gels were black and thick, after they
were heated at 450 oC for 20 min. However, if the heating temperature is higher than
480 oC, the films become thinner with time, and they finally become transparent. The
uniformity of the films is affected by the heating rate. When a gel-coated
SWCNT:POETE layer was heated at a temperature higher than 300 oC immediately
after coating, POETE vigorously boiled and rapidly vaporized, giving rise to a non-
uniform film. A two-step heat treatment was designed to make highly uniform films.
The first step was to heat the gel-coated SWCNT:POETE layers at 250 oC for 15 min.
POETE vaporized gently at this temperature. The SWCNT:POETE layers turned into
black solid films as the result of the partial POETE removal. The second step was to
further heat the SWCNT:POETE films at a temperature between 480 and 520 oC for
20 min. During the second heating the SWCNT films become thin and transparent.
Fig. 4.2a and Fig. 4.2b present the pictures of a SWCNT:POETE layer on glass
before and after the heat treatment, respectively. A transparent SWCNT film was
obtained after the heat treatment.
Free-standing SWCNT films were obtained by slowly immersing dried SWCNT
films into water as the result of the detachment of the hydrophobic films from glass
49
substrate. The film floated on water after detaching from the glass substrate, and it
can be picked up by a hydrophobic PET substrate. Similar transfer techniques have
also been reported by other labs [198,199]. The films transferred by this method had
good adhesion to PET. Fig. 4.2c and Fig. 4.2d are photographs of a SWCNT film on
PET prepared by such a transfer process.
Figure 4.2. Photographs of (a) a SWCNT:POETE layer on a glass substrate coated by doctor blade, (b)
the SWCNT film obtained from the SWCNT:POETE layer by heating at 250 oC for 15 min and then at
500 oC for 20 min, (c) the SWCNT film transferred onto a flexible PET substrate. Picture (d) shows the
flexibility of the SWCNT film on PET.
The uniformity of the films after the heat treatment was studied by SEM (Fig. 4.3a).
Besides SWCNTs, lots of particles in submicrometer size could be observed. EDX
analysis indicates that the main elements of these particles are carbon, potassium and
oxygen (Fig 4.3c). In addition, the particles can be dissolved in acid. They may thus
(b) (a)
(d) (c)
50
be K2CO3 or/and KOH originated from POETE, as KOH is usually used as the
catalyst in the synthesis of poly(ethylene oxide)-based nonionic surfactants [200,201].
0 1 2 3 4
0.0
0.2
0.4
0.6
0.8
1.0
KO
No
rma
lize
d C
ou
nts
Energy (keV)
C (c)
Figure 4.3. SEM images of an SWCNT film (a) after the heat treatment and (b) after heat and HNO3
treatments. (c) EDX of the particles in image (a).
These impurities could be completely removed by soaking the films in 65 wt%
nitric acid. As mentioned above, the hydrophobic SWCNT films readily detached
from the glass substrates, when they were directly immersed into the water. This
detachment problem was solved by immersing the films in a polar organic solvent
like ethanol or acetone for 1 min prior to the immersion into the HNO3 solution. Fig.
4.3b is the SEM image of a film after the ethanol and HNO3 treatments. The film is
uniform and has no particle.
(a) (b)
51
The removal of the particles makes the films more transparent from the UV to
infrared range as shown in Fig. 4.4. The gain in the transparency can be attributed to
the elimination of the light scattering by the particles. The inset in Fig. 4.4 shows the
corresponding absorption spectra of the same film before and immediately after the
HNO3 treatment. The characteristic absorption bands of SWCNTs corresponding to
the van Hove singularity transitions [14] can be clearly observed in the absorption
spectrum of the SWCNT film prior to the HNO3 treatment. These bands become
weak immediately after the HNO3 treatment. This can be attributed to the p-type
(holes) doping of SWCNTs by HNO3 treatment, which induces the depletion of
electrons from the van Hove singularities [133,202].
Fig. 4.5a shows the Raman spectra of the as-received SWCNTs and an SWCNT
film after the heat and HNO3 treatments. The Raman bands in the range of 200 to 300
cm-1
are the radical breathing modes (RBMs) corresponding to the collective
oscillation of carbon atoms along the radial direction [203]. The positions of these
bands are sensitive to the diameter of SWCNTs [204]. RBMs of the pristine SWCNTs
are in the range of 200 to 275 cm-1
, suggesting the diameters of SWCNTs are below
1.2 nm according to the relationship between the RBM positions and the SWCNT
diameter [204]. There is no remarkable change on the RBMs after the heat and HNO3
treatments. Thus, those treatments did not affect the diameter of SWCNTs.
The band at 1687 cm-1
is the G band corresponding to the in-plane vibration of the
sp2 hybridized carbon atoms on the side wall of SWCNTs, and the band at 1350 cm
-1
is the D band arising from the defects in the graphitic structures [203-206]. The
decrease in the intensity ratio of the D band to G band (ID/IG) indicates that the heat
52
and HNO3 treatments substantially reduce the impurities while do not damage the
aromatic structure of SWCNTs.
400 600 800 1000 1200 1400 1600 18000
20
40
60
80
100
(b)
(a)
600 800 1000 1200 1400 16000.0
0.1
0.2
0.3
0.4
Ab
so
rpti
on
Wave length/nm
(b)
Tra
ns
mit
tan
ce
/%
Wave length/nm
(a)
Figure 4.4. UV-vis-IR transmittance spectra of a SWCNT film (a) before and (b) immediately after the
HNO3 treatment. The inset shows the corresponding absorption spectra.
Raman spectroscopy was carried out to monitor the change of the carbonaceous
impurities by successively heating an SWCNT:POETE gel at three different
temperatures, 300, 400 and 500 oC (Fig. 4.5b). It was also used to study the final
HNO3 treatment of the SWCNT film. The intensity ratio ID/IG of the film decreases
dramatically after heating at 400 oC, indicating that most of the carbonaceous
impurities are removed from SWCNTs during heating at temperature below 400 oC.
Moreover, there is no remarkable change on the Raman spectrum after the HNO3
treatment, which confirms that the particles observed in Fig. 4.3a are not
carbonaceous impurities.
53
150 200 250 1300 1400 1500 1600 17000.0
0.2
0.4
0.6
0.8
1.0
RBM
G band
No
rma
lize
d i
nte
ns
ity
Raman shift (cm-1)
D band
(a)
0.00
0.05
0.10
(i)
(v)(iv)(iii)
(ii)
I D/I
G
(b)
Figure 4.5. (a) Raman spectra of pristine SWCNTs (solid curve) and a SWCNT film (dashed curve).
(b) Raman intensity ratios of the D band to G band (ID/IG) of (i) as-received SWCNTs and a
SWCNT:POETE gel after successively heating at (ii) 300 oC, (iii) 400
oC, (iv) 500
oC and (v) after
subsequent HNO3 treatment. The heating time for each heat treatment is 20 min.
4.2.2. Optical and electrical properties of SWCNT films
The SWCNT films were transparent and conductive. The optical and electrical
properties were affected by the experimental conditions during the film fabrication.
At first, the effect of the ultrasonication time during the SWCNT dispersion in
POETE was investigated. Films processed at the power of 15 W with five different
ultrasonication times, 10, 20, 30, 40 and 80 min, were characterized. As shown in Fig.
4.6, 20~30 min is the optimal ultrasonication time to achieve a high conductivity for
the films. The films processed at the ultrasonication time of 20 min have a sheet
resistance of 250 / at 80% transmittance. Though ultrasonication helps the
debundling and the dispersion of SWCNTs in POETE, extensive ultrasonication
shortens SWCNT bundles. The better SWCNT dispersion can increase while
shortening of SWCNT bundles can reduce the conductivity of the films [131, 207-
209].
54
50 60 70 80 90
100
1000
80 min
40 min
30 min
Sh
ee
t re
sis
tan
ce
(S
q)
Transmittance at 550 nm (%)
20 min
10 min
(b)
Figure 4.6. Sheet resistance-transmittance curves of SWCNT films prepared from SWCNT:POETE
gels which were formed by dispersing SWCNTs in POETE under ultrasonication of 15 W for different
periods.
The effect of the ultrasonication on the length of the SWCNT bundles were studied
by SEM. Figs. 4.7a-d presents the SEM images of SWCNTs obtained from
SWCNT:POETE gels which were prepared under ultrasonication for different periods.
The samples were prepared by dispersing the SWCNT/POETE gels into water, and
then drop cast the solutions onto glass substrates. The samples were subsequently
heated at 500 oC for 20 min to remove the surfactant. A thin layer (10 nm) of gold
was sputtered on to the glass substrates to enhance the contrast of the SEM images.
Fig. 4.7a shows that SWCNTs still entangled together when the gel was
ultrasonicated for 10 min. The SWCNTs by the ultrasonication of 20 min are well
disentangled, and their average length is close to 4 μm (Fig. 4.7b). Further increase in
the ultrasonication time results into shorter SWCNT bundles as depicted in Fig. 4.7e.
55
0 10 20 30 40 50 60 70 80 901
2
3
4
5
Av
era
ge
bu
nd
le l
en
gth
(m
)
Ultrasonication time (min)
(e)
Figure 4.7. SEM images of SWCNTs prepared from SWCNT:POETE gels which were prepared under
ultrasonication at 15 W for (a) 10 min, (b) 20 min, (c) 40 min and (d) 80 min. (e) Average bundle
lengths of SWCNT in the SWCNT/POETE gels after ultrasonication treatment for different periods.
(a) (b)
(c) (d)
56
The effect of the ultrasonication on the SWCNT bundle sizes was investigated by
TEM (Fig. 4.8). After 10 min of ultrasonication, the SWCNT bundles are still
entangled. After SWCNTs are ultrasonicated for 20 min, the average diameter of
SWCNT bundles decreases to about 20 nm. The SWCNT bundle diameter further
decreases to 15 nm after 80 min of ultrasonication. Since the decrease in SWCNT
bundle length by ultrasonication is more significant than the decrease in bundle size,
ultrasonication for more than 20 min gives rise to the increase in the sheet resistance
of the SWCNT films. The distribution of SWCNT bundle length and diameter after
20 min of ultrasonication is illustrated in Fig. 4.9.
Figure 4.8. TEM images SWCNTs after ultrasonication treatment at 15 W for (a) 10 min, (b) 20 min,
(c) 40 min and (d) 80 min.
(a) (b)
(c) (d)
57
1 2 3 4 5 6 70
2
4
6
8
10
12
14(a)
Nu
mb
er
of
bu
nd
les
Bundle length (m)
5 10 15 20 25 30 35 400
2
4
6
8
10
12
14
16
18
20
Nu
mb
er
of
bu
nd
les
Bundle diameter (nm)
(b)
Figure 4.9. The distribution of SWCNT bundle length (a) and diameter (b) after 20 min of
ultrasonication.
The power of the ultrasonication has similar effect on the performance of the
SWCNT films. Fig. 4.10 shows the effect of ultrasonication power on the sheet
resistance and transmittance of the SWCNT films. The films processed with an
ultrasonication power of 15 W exhibited a lower sheet resistance at a same
58
transmittance than with a power of 45 W and without an ultrasonication process.
These results can also be understood in terms of the effect of the ultrasonication on
the debundling and the length of SWCNTs.
50 60 70 80 90
100
1000
Transmittance at 550 nm (%)
Sh
ee
t re
sis
tan
ce
(S
q)
15 W
45 W
w/o ultrasonication
Figure 4.10. Sheet resistance-transmittance curves of SWCNT films prepared from SWCNT:POETE
gels which were formed by dispersing SWCNTs in POETE under ultrasonication of different powers
for 20 min.
The transmittance through a SWCNT film is also related to the heating conditions
during the second heating step. Heating can reduce the thickness of the films, which
results into the increase in both the transmittance and the sheet resistance. Fig. 4.11a
illustrates the variations of the transmittance and the sheet resistance of the films with
the heating temperature during the second heating step. The first heating step was the
same for all the films, that is, at 250 oC for 15 min. The heating time during the
second heating step was 20 min for all the films, but the heating temperature was
varied between 480 and 520 oC. Both the transmittance and sheet resistance increase
with the increase in the heating temperature. These variations are attributed to the
59
heating effect on the thickness of the films. The thicknesses of SWCNT films were
measured by AFM. A SWCNT film of 75 nm thick had a transmittance of 47.2%. The
transmittance increased to 70.2% and 85.3% when the film thickness was reduced to
60 and 35 nm by heating, respectively. The reduction in the thickness of SWCNT
films is caused by the decomposition of SWCNTs during heat treatment. This result is
in agreement with the TGA curves shown in Fig. 4.1.
480 490 500 510 52050
60
70
80
90
0
100
200
300
400
500
600
700
Sh
ee
t re
sis
tan
ce
(s
q)
Tra
ns
mit
tan
ce
(%
)
Heating temperature (o
C)
(a)
10 20 30 40 50 60 7040
50
60
70
80
90
0
100
200
300
400
500
600
700
Sh
ee
t re
sis
tan
ce
(s
q)
Tra
ns
mit
tan
ce
(%
)
Heating time (min)
(b)
Figure 4.11. Variations of the sheet resistance and the transmittance at 550 nm of SWCNT films with
(a) the heating temperature and (b) the heating time. The heating temperature in (b) is 480 oC.
60
Fig. 4.11b shows the variations of the transmittance and the sheet resistance of the
SWCNT films with the heating time at 480 oC. The changes in the transmittance and
the sheet resistance are similar to that by the heating temperature. Both the
transmittance and the sheet resistance increase with the increase of the heating time.
These are also due to the effect of heating on the thickness of the films.
SWCNT films were also prepared by spray coating using the same pristine
SWCNTs for comparison. The curve of the sheet resistance versus the transmittance
is presented together with that of the SWCNT films by get coating (Fig. 4.12a). The
sheet resistance of the gel-coated SWCNT films is always lower than that by the
spray-coated SWCNT films at a same transmittance. The effect of heat treatment on
the transmittance and sheet resistance of the spray-coated SWCNT films were also
studied. After heating at 500 oC for 20 min, the sheet resistance of the spray coated
films decrease slightly at the same transmittance. Hence, the low sheet resistance of
the gel-coated SWCNT films is not due to the heat treatment at 480-520 oC.
Presumably, the high transmittance-to-sheet resistance ratio observed on the gel-
coated SWCNT films is due to the long SWCNT bundles dispersed in the gels. The
SWCNT:POETE gels are formed as the result of the three-dimensional SWCNT solid
networks in POETE, and long SWCNTs can facilitate the formation of the three-
dimensional networks [56,192]. The gel-coating process may be particularly
important to process long SWCNTs for fabricating highly conductive SWCNT films,
because long SWCNTs can be dispersed in the gels. Other solution processing
techniques use SWCNTs dispersed in aqueous solution of a surfactant or an organic
solvent, in which SWCNT bundles must be short to have a stable dispersion. We
61
studied the length of SWCNT bundles which were dispersed in CMC aqueous
solution. The SEM image indicates that their average length is about 2 µm (Fig.
4.12b), which is shorter than average SWCNT length in the SWCNT:POETE gel.
This difference can account for the different resistances by the gel coating and spray
coating.
50 60 70 80 9010
100
1000
(a)
Sh
ee
t re
sis
tan
ce
(
/Sq
)
Transmittance at 550 nm (%)
Figure 4.12. (a) Sheet resistance-transmittance curves of SWCNT films fabricated by gel coating
(solid squares), spray coating with (open circles) and without heat treatment (solid triangles). (b) SEM
image of SWCNTs prepared by drop casting a CMC aqueous solution dispersed with SWCNTs and
subsequently heating at 500 oC for 20min.
The sheet resistance of the gel-coated films may be further lowered if even longer
SWCNTs are used. Joshi et al. demonstrated that gel coating can be use also to
process very long carbon nanofibers [210]. Though ultra-long SWCNTs prepared by
CVD were recently reported [211,212], they are not commercially available at this
moment.
4.3. Conclusion
Transparent and conductive SWCNT films were fabricated by the gel formation of
SWCNTs with POETE, gel coating and removal of the surfactant through heating.
(b)
62
Heating at a temperature between 480 and 520 oC can give rise to thin and transparent
SWCNT films. These films become more transparent after the further treatment with
HNO3 to remove the particles originated from POETE. They can have a high
transmittance-to-sheet resistance ratio, which is attributed to the long SWCNTs
dispersed in the gels. SWCNT films with a sheet resistance of around 250 Ω/ at 80%
transmittance were obtained. The films can be transferred from rigid substrates to
flexible polymer substrates.
63
Chapter 5. High-performance dye-sensitized
solar cells with gel-coated binder-free carbon
nanotube films as counter electrode
5.1. Introduction
Dye-sensitized solar cells (DSCs) are promising to be the next-generation solar
cells arising from their high light-to-electricity conversion efficiency and low
fabrication cost [213,214]. The working mechanism of DSCs is illustrated in Fig. 5.1.
A photon absorbed by a dye molecule stimulates an electron transition from the
HOMO (HOMO: highest occupied molecular orbital) to LUMO (LUMO: lowest
unoccupied molecular orbital) of the dye molecule attached to the TiO2 working
electrode. The electron on LUMO transfers to the TiO2 working electrode, and the
dye molecule is regenerated by taking an electron from iodide, 2dye+ + 3I
- dye
0 +
I3-. The potential offset between the conduction band of TiO2 and the reduction
potential of the electrolyte determines the open circuit voltage of DSCs. The
regeneration of iodide from triiodide occurs at the counter electrode, I3- + 2e 3I
-.
Thus, the counter electrode has a strong effect on the photovoltaic performance of
DSCs. The requirements on the counter electrode include high conductivity and
excellent catalysis for the reduction of triiodide. Platinum (Pt) is the popular material
as the counter electrode of DSCs, owing to its high conductivity and excellent
catalysis [215-219]. Pt can be deposited on a conductive substrate by several methods,
such as pyrolysis of Pt precursors, magnetron sputtering, electrochemical deposition
64
and electroless deposition. The most efficient Pt counter electrode is fabricated by
pyrolysis of Pt precursors at a temperature high than 380 oC. Such a high-temperature
process prevents the Pt deposition on flexible substrates like polymers. Recently, it
has been discovered that Pt deposited by polyol reduction of a Pt precursor at a
temperature below 200 oC could be as efficient as that by pyrolysis [219]. But there
are still some other problems related to Pt as the counter electrode of DSCs. The
noble Pt significantly increases the cost of DSCs. It also degrades over time in the
electrolyte of DSCs [217-220]. Therefore, great effort has been made in search for
new materials as the counter electrode of DSCs. Several Pt-free materials were
reported as the counter electrode of DSCs, including carbons [221-228], CNTs [57-
64], conducting polymers [229-233], composites of conducting polymer and CNTs
[70].
Figure 5.1. Working mechanism of DSCs [214].
Among the Pt-free materials, CNTs are promising as the counter electrode of DSCs,
because they can have good catalysis and high conductivity. Recently, we discovered
65
that CNTs could form gels with some liquid organic compounds and binder-free CNT
films could be fabricated by coating the CNT gels on various substrates and
subsequently removing the organic compounds through heating [192]. Those CNT
films have good adhesion to substrates. This chapter reports the application of gel-
coated binder-free films of CNTs, including SWCNTs and MWCNTs, as the counter
electrode of DSCs. Power conversion efficiency over 8% and excellent stability over
time were achieved on DSCs with a SWCNT film as the counter electrode.
5.2. Results and discussion
5.2.1. DSCs with SWCNT films as counter electrode
SWCNT/PEG gels were prepared by mechanically grinding SWCNTs with PEG
and subsequent ultrasonication. The dispersion was similar to the dispersion of
SWCNTs in POETE as described in chapter 3 [192]. But there were more CNT
bindles in the SWCNT/PEG gels than in the SWCNT/POETE gels. We found that
these CNT bindles were helpful for the application of SWCNT films as the counter
electrode of DSCs, since DSCs with bundled SWCNT electrode exhibited higher
photovoltaic performance and better stability. Besides, PEG has lower boiling
temperature than POETE, thus the removal of PEG by heat treatment is easier,
especially when the gel films are thick. The SWCNT films were fabricated from the
SWCNT/PEG gel by the doctor blade method and subsequent removal of PEG by
heating.
The removal of PEG was confirmed by FTIR spectroscopy (Fig. 5.2). No
vibrational band of PEG was observed in the FTIR spectra of the samples after
66
heating. The SWCNT films had a quite rough surface as observed on the SEM image
of low magnification (Fig. 5.3a). SWCNT bundles could be observed on the SEM
images of high magnification (Fig. 5.3b-d). The morphology did not remarkably
change with the variation of the SWCNT film thickness.
4000 3500 3000 2500 2000 1500 1000 500
SWCNT film
Wave number (cm-1
)
SWCNT/PEG gel
Ab
so
rpti
on
PEG
Figure 5.2. FTIR spectra of PEG, SWCNT/PEG gel and SWCNT film prepared by gel coating.
The SWCNT films prepared from the SWCNT/PEG gel had good adhesion to the
FTO substrate. They were used as the counter electrode of DSCs. Fig. 5.4 presents
the current-voltage curves of a DSC with a SWCNT film of 6 m in thickness as the
counter electrode and a control DSC with the conventional Pt counter electrode. The
DSCs were tested immediately after the device fabrication. The short-circuit current
density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion
efficiency () are listed in Table 5.1 for both devices. The two DSCs exhibited
comparable photovoltaic performance. The differences in the values (Jsc, Voc and FF)
for them are basically in experimental error. The DSC with the SWCNT counter
67
electrode exhibited photovoltaic efficiency of 7.81% under AM1.5 illumination (100
mW cm-2
).
Figure 5.3. SEM images of SWCNT films on FTO glass. Image (a) has a low magnification, while
images (b), (c) and (d) have a high magnification. The thicknesses of the SWCNT films were: (a) and
(b) 2 µm, (c) 6 µm, and (d) 10 µm.
The DSCs with the SWCNT counter electrode exhibited good repeatability in the
photovoltaic performance. More than 50 DSCs with SWCNT counter electrode were
fabricated and tested. The variations in the values of Jsc, Voc, FF and were 0.46
mA cm-2
, 0.011 V, 0.006 and 0.16%, respectively.
The photovoltaic efficiency of our DSCs with the SWCNT counter electrode is
significantly higher than those reported by other laboratories [57,58]. Their DSCs
( b ) ( a )
( c ) ( d )
68
with SWCNT counter electrode exhibited photovoltaic efficiencies lower than 5%.
The possible reason is that binders were used in their SWCNT films. Those binders
reduce the effective contact between SWCNT and electrolyte, and block the charge
transport among the CNTs. In contrast, the redox species can penetrate into our
binder-free SWCNT films. In addition, SWCNTs with different electrochemical
catalyses might be used in different labs.
0.0 0.2 0.4 0.6 0.80
5
10
15
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
Voltage (V)
SWCNT
Pt
Figure 5.4. Current-voltage curves of DSCs with Pt and a SWCNT film of 6 m in thickness as the
counter electrode. The tests were performed under AM 1.5 illumination.
Table 5.1. Photovoltaic parameters of DSCs with Pt and a SWCNT film of 6 m in thickness as the
counter electrode.
Counter
electrode
Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
Pt 0.746 14.63 0.73 7.92
SWCNT 0.742 14.25 0.74 7.81
69
To understand the photovoltaic performance of our DSCs with the SWCNT counter
electrode, the charge transfer resistance (RCT) during the redox of iodide/triiodide pair
on a SWCNT film was studied on symmetrical cells filled with the same electrolyte
used in the DSCs by the ac impedance spectroscopy (Fig. 5.5). The semicircle at the
high frequency range corresponds to the charge transfer process between the
electrolyte and electrode, while the semicircle at the lower frequency range
corresponds to the Warburg diffusion [215,216]. The inset of Fig. 5.5 is the
equivalent circuit of the symmetric cells, in which Cd and RS are the dielectric
capacitance and charge transfer resistance at the electrode/electrolyte interfaces,
respectively, and ZD is the Warburg diffusion impedance. The charge transfer
resistances of the SWCNT electrode and Pt electrode, which are 0.6 Ω cm2 and 1.6 Ω
cm2 respectively, were obtained by fitting the first semicircles of the impedance
spectra. RS on the SWCNT electrode is even lower than on Pt. This can be attributed
to the large surface area of the SWCNT electrode. The symmetric cells with SWCNT
electrodes also show larger Warburg diffusion impedance, which can be attributed to
the larger diffusion length of electrolyte in the porous SWCNT electrode.
The electrochemical catalysis of SWCNTs on the reduction of triiodide was further
investigated by CV (Fig. 5.6). A SWCNT film of 6 m in thickness was used as the
working electrode. The cyclic voltammogram is quite similar to that with Pt as the
working electrode. Two oxidation/reduction pairs were observed in the CVs. The pair
in the potential range of 0.1 to 0.5 V vs Ag/Ag+ corresponds to the redox of iodine
and triiodide, 3I2 + 2e → 2I3-, and the other pair in the potential range of -0.3 to 0.1 V
vs Ag/Ag+ is the redox of iodide and triiodide, I3
- + 2e → 3I
- [228] . The species in
70
the latter are the redox species in DSCs. The peak potential for the reduction of
triiodide is the same for the two working electrodes, while the peak current with the
SWCNT working electrode is even higher than that with the Pt working electrode.
This indicates that the SWCNT film can effectively catalyze the redox of iodide and
triiodide as Pt. The capacitance current with the SWCNT working electrode was
remarkably higher than that with the Pt working electrode. This is caused by the large
surface area of the SWCNT electrode.
14 16 18 200
-1
-2
-3
-4
RS
1/2Cd
ZD2RCT
RS
1/2Cd
ZD2RCT
high frequency
Pt
Z" (
cm
2)
Z' ( cm2)
SWCNT
low frequency
Figure 5.5. Nyquist plots of ac impedance spectroscopy of symmetrical structures with Pt as the two
electrodes and two SWCNT films of 6 m in thickness as the two electrodes.
Electrochemical catalysis has been reported on both SWCNTs and MWCNTs [234-
241]. They can catalyze oxidation or reduction of many species, though the reason
remains unknown. However, more research was carried out on MWCNTs than on
SWCNTs as the counter electrode of DSCs, and much lower photovoltaic efficiencies
were reported on DSCs with a SWCNT counter electrode. The high photovoltaic
performance of our DSCs with SWCNT counter electrode can be attributed to the
71
high catalysis and electrical conductivity of our SWCNT films due to the absence of
binders.
-0.4 -0.2 0.0 0.2 0.4-1.0
-0.5
0.0
0.5
1.0(4)
(3)
(2)
Pt
SWCNT
Cu
rre
nt
de
ns
ity
(m
A/c
m-2
)
Potential (V vs Ag/Ag+
)
(1)
Figure 5.6. Cyclic voltammograms of I2/I3- with Pt and a SWCNT film of 6 μm in thickness as the
working electrode. The potential sweep rate of is 10 mV S-1
. The four peaks in the cyclic
voltammograms correspond to four redox reactions: (1) I3- + 2e
-→ 3I
-, (2) 3I
- → I3
- + 2e
-, (3) 2I
- →
I2+2e-, (4) I2+2e
-→2I
-.
The DSCs with the SWCNT counter electrode exhibited good stability. No
detachment of the SWCNT films from FTO glass was observed after one month. The
photovoltaic stability was studied by testing the DSCs immediately, one week, two
weeks, three weeks and four weeks after the device fabrication. Fig. 5.7a and Fig.
5.7b shows the I-V curves of a DSC with a SWCNT counter electrode and a control
DSC with Pt counter electrode tested immediately, one week and four weeks after the
device fabrication, respectively. Only three I-V curves were presented for each DSC
in the figure for clarity. The DSCs exhibited photovoltaic efficiencies a little different
from the ones presented in Fig. 5.4, but the trend of the photovoltaic performance
with time was similar for them.
72
The photovoltaic performances are listed in Table 5.2. Voc and FF for the DSC with
the SWCNT counter electrode increased while Jsc decreased with time. Voc increased
from 0.742 to 0.804 V after one week and to 0.827 V after four weeks. The gain in
Voc yielded to the efficiency from 7.81% to 8.56% after one week and to 8.17%
after four weeks. did not decrease after four weeks. In comparison, the control DSC
exhibited remarkable decrease in the efficiency. It dropped from 7.92% to 7.51%
after four weeks. The different changes in the efficiencies of the two DSCs can be
attributed to the time dependence of the Voc values. The Voc of the control DSC
increased from 0.746 V immediately after device fabrication to only 0.784 V after
four weeks. The increase of Voc over time at room temperature was also observed by
others [62], and can be explained by the shift of TiO2 conduction band edge toward
more negative position upon aging [62, 242]. The lower Voc of the DSC with Pt
electrode might be attributed to the diffusion of Pt into FTO glass [243]. The decrease
of Jsc over time is caused by the degradation of the device.
0.0 0.2 0.4 0.6 0.80
5
10
15
( a )
Voltage (V)
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
Immediately
after 1 week
after 4 weeks
0.0 0.2 0.4 0.6 0.80
5
10
15
Immediately
1 week later
4 week later
Voltage (V)
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
( b )
Figure 5.7. Current-voltage curves of DSCs with (a) a SWCNT film and (b) Pt as the counter electrode.
The DSCs were tested immediately, one week and 4 weeks after device fabrication. The thickness of
the SWCNT film was 6 m, and the tests were carried out under AM 1.5 illumination.
73
The stability of SWCNT films in the electrolyte during the electrochemical process
was further investigated by CV with a SWCNT film of 6 m in thickness as the
working electrode. The CV curve almost did not change after 100 electrochemical
cycles (Fig. 5.8a). Pt prepared on FTO glass by pyrolysis was used as the working
electrode for the CV study for comparison (Fig. 5.8b). The redox current significantly
decreased after 100 cycles. The decrease in the redox current is attributed to the
detachment of Pt from the substrate. Thus, SWCNT is even more stable than Pt in the
electrolyte. The CV stability is consistent with the stability in photovoltaic
performance of the DSCs.
Table 5.2. Photovoltaic stability of a DSC with Pt and a SWCNT film of 6 m in thickness as the
counter electrode.
Counter
electrode
Time Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
SWCNT Immediately 0.742 14.25 0.74 7.81
After 1 week 0.804 14.21 0.75 8.56
After 2 weeks 0.820 13.75 0.74 8.39
After 3 weeks 0.826 13.20 0.76 8.24
After 4 weeks 0.827 12.87 0.77 8.17
Pt Immediately 0.746 14.63 0.73 7.92
After 1 week 0.778 13.62 0.75 7.94
After 2 weeks 0.781 13.19 0.76 7.81
After 3 weeks 0.780 12.68 0.77 7.57
After 4 weeks 0.784 12.54 0.76 7.51
74
-0.4 -0.2 0.0 0.2 0.4-1.0
-0.5
0.0
0.5
1.0
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
Potential (V vs Ag/Ag+)
1st cycle
100th cycle
( a )
-0.4 -0.2 0.0 0.2 0.4-1.0
-0.5
0.0
0.5
1.0
100th cycle
Potential (V vs Ag/Ag+)
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
( b )
1st cycle
Figure 5.8. Cyclic voltammograms of the 1st and 100
th cycle with (a) a SWCNT film of 6 μm in
thickness and (b) Pt as the working electrode. The electrolyte is an acetonitrile solution consisting of 5
mM LiI, 0.25 mM I2 and 0.1 M (C4H9)4NPF6. The scan rate is 10 mV s-1
.
SWCNT films with thicknesses from 2 to 10 m were used as the counter electrode
of DSCs to investigate the effect of SWCNT film thickness on the performance of
DSCs (Table 5.3). Voc slight increases, Jsc is almost unchanged, and FF increases
with the thickness of the SWCNT films. The DSC with a SWCNT film of 8 m in
thickness exhibited the highest photovoltaic efficiency of 7.91%.
Table 5.3. Photovoltaic parameters of DSCs with SWCNT films of different thicknesses as the counter
electrode.
Thickness Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
2 μm 0.724 14.13 0.70 7.15
4 μm 0.730 14.58 0.71 7.58
6 μm 0.735 14.39 0.73 7.71
8 μm 0.737 14.71 0.73 7.91
10 μm 0.733 14.42 0.74 7.79
75
5.2.2. DSCs with MWCMT films as counter electrode
Binder-free MWCNT films were also fabricated from MWCNT/PEG gels through a
similar process as SWCNT films. Fig. 5.9 shows the current-voltage curve of a DSC
with a MWCNT film of 6 m in thickness as the counter electrode. The photovoltaic
performance of this device was: Jsc = 14.13 mA cm-2
, Voc = 0.766 V, FF = 70.72 and
= 7.65%. The photovoltaic efficiency is comparable to the control DSC with the
conventional Pt counter electrode.
0.0 0.2 0.4 0.6 0.80
5
10
15
Cu
rre
nt
de
ns
ity
(m
A c
m-2)
Voltage (V)
Figure 5.9. Current-voltage curve of a DSC with a MWCNT film of 6 m in thickness as the counter
electrode. The device was tested under AM 1.5 illumination.
The charge-transfer resistance RCT of the redox of the iodide/triiodide pair on a
MWCNT film was studied by the ac impedance spectroscopy (Fig. 5.10). It was 0.75
cm2, which is higher than that on the SWCNT film. This can be attributed to the
higher surface area for the SWCNT film than the MWCNT film. The Warburg
76
diffusion impedance of MWCNT film is slightly smaller than that of SWCNT film,
which means electrolyte has higher diffusion length in the SWCNT film.
15 16 17 180.0
-0.2
-0.4
-0.6
-0.8
-1.0
Z" (c
m2)
Z' ( cm2)
Figure 5.10. Nyquist plot of ac impedance spectroscopy of a symmetrical structure with two MWCNT
films of 6 m in thickness as the two electrodes.
0.0 0.2 0.4 0.6 0.80
5
10
15
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
Voltage (V)
immediately
after 1 week
after 4 weeks
Figure 5.11. Current-voltage curves of a DSC with a MWCNT film of 6 m in thickness as the
counter electrode. The DSCs were tested immediately, one week and 4 weeks after device fabrication.
The tests were carried out under AM 1.5 illumination.
77
Table 5.4. Photovoltaic stability of a DSC with a 6 m-thick MWCNT film as the counter electrode.
Time Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
Immediately 0.747 14.49 0.71 7.63
After 1 week 0.785 13.48 0.69 7.34
After 2 weeks 0.789 12.60 0.69 6.89
After 3 weeks 0.785 12.47 0.69 6.77
After 4 weeks 0.790 12.26 0.69 6.63
The stability of the current-voltage curves over time was investigated for the DSCs
with a MWCNT counter electrode (Fig. 5.11). The photovoltaic performances are
listed in Table 5.4. Voc increased from 0.747 to 0.790 V, and the photovoltaic
efficiency decreased from 7.63% to 6.63% after 4 weeks. This stability in the
photovoltaic efficiency is similar to the control DSC with the Pt counter electrode
while worse than the DSC with the SWCNT counter electrode. This can be attributed
to the time dependences of the Voc values. The Voc for the DSC with the MWCNT
counter electrode increased with time but less significantly than that of the DSCs with
the SWCNT counter electrode. Increase of Voc over time on DSCs with a CNT
counter electrode has also been reported by other laboratories but the reason remains
unknown [58,62,64]. Probably, this is also related to the surface area of the counter
electrode. The SWCNT counter electrode has a higher surface area so that Voc
increases more significantly with time.
78
Our experimental results indicate the DSCs with a SWCNT counter electrode
exhibit photovoltaic performance even higher than the DSCs with a MWCNT counter
electrode. This is different from that reported in literature [57-64]. Higher
photovoltaic performance was reported on DSCs with a MWCNT counter electrode
than that with a SWCNT counter electrode. We believe that the poor photovoltaic
performance of the DSCs with a SWCNT counter electrode observed by other labs is
related to the SWCNT processing. In fact, SWCNTs and MWCNTs exhibited similar
catalysis on the reduction or oxidation of other systems [234-241]. Moreover, an
SWCNT film always has electrical conductivity saliently higher than an MWCNT
film [131,244]. The high conductivity favors the application of the SWCNT films as
the counter electrode of DSCs.
5.2.3. DSCs with SWCNT/CMC and MWCNT/CMC films as counter electrode
A problem for the CNT films by solution processing is the poor adhesion to
substrates. Various materials, such as conducting polymers and CMC, were used as
the binders to enhance the adhesion [62,70]. Lee et al. reported DSCs using
MWCNT/CMC films prepared from a MWCNT/CMC paste as the counter electrode
[62]. To further understand the binder effect on the photovoltaic performance, control
DSCs using SWCNT/CMC and MWCNT/CMC films as the counter electrode were
also fabricated and characterized. The pastes of SWCNT/CMC and MWCNT/CMC
were prepared in terms of the method reported by Lee et al. [62]. It was difficult to
obtain thin SWCNT/CMC and MWCNT/CMC films from these pastes due to the
79
high CNT concentrations in the pastes. The thinnest SWCNT/CMC and
MWCNT/CMC films were about 10 µm in thickness.
0.0 0.2 0.4 0.6 0.80
5
10
15
Cu
rre
nt
de
ns
ity
(m
A c
m-2
)
Voltage (V)
MWCNT/CMC
SWCNT/CMC
Figure 5.12. Current-voltage curves of DSCs with a SWCNT/CMC film and a MWCNT/CMC film as
the counter electrode. The thickness of the SWCNT/CMC and MWCNT/CMC films was 10 µm. The
tests were performed under AM 1.5 illumination.
Fig. 5.12 shows the current-voltage curves of these control DSCs under AM1.5
illumination. Both SWCNT/CMC and MWCNT/CMC films were used as the counter
electrode. The thickness of these films was 10 µm. The photovoltaic parameters are
summarized in Table 5.5. The DSC with the SWCNT/CMC counter electrode was
6.81%, remarkably lower that with binder-free SWCNT counter electrode. As listed
in Table 5.3, the DSC with a binder-free SWCNT film of 10 µm in thickness
exhibited an efficiency of 7.79%. Comparing the photovoltaic performance of the two
DSCs with the SWCNT and SWCNT/CMC counter electrodes, it is concluded that
the binder does not affect the Voc value while it saliently lowers the Jsc and FF values.
The lowering Jsc value can be attributed to the decreasing surface area by the binder
80
that may wrap the SWCNTs, because the electrochemical catalysis is proportional to
the contact area between the electrode and the electrolyte. The lowering FF value can
be due to the increase in the series resistance by the insulating binder.
Table 5.5. Photovoltaic parameters of DSCs with SWCNT/CMC and MWCNT/CMC films as the
counter electrode. The thickness of the CNT/CMC films was 10 µm.
Counter
electrode
Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
SWCNT/CMC 0.738 13.92 0.663 6.81
MWCNT/CMC 0.746 14.54 0.661 7.17
The CMC binder also affects the photovoltaic efficiency of the DSC with the
MWCNT counter electrode but its effect is less significant than that with the SWCNT
counter electrode. The DSC with the MWCNT/CMC counter electrode exhibited a
photovoltaic efficiency of 7.17%. This efficiency is comparable to that reported by
Lee et al. [62]. Comparing the photovoltaic performances of the DSCs with the
MWCNT and MWCNT/CMC counter electrodes, the principal effect by the CMC
binder is on the FF value. Obviously, the lowering FF value by CMC can be
attributed to the increase in the series resistance induced by the insulator binder.
An interesting observation is that CMC has different effects on the Jsc values of
DSCs with the SWCNT and MWCNT counter electrodes. This may be related to the
different electrochemical catalyses of MWCNTs and SWCNTs. The electrochemical
catalysis of CNTs probably originates from the defects [62]. The defects of the
binder-free SWCNT and MWCNT films prepared by gel coating were studied by
Raman spectroscopy (Fig. 5.13). The MWCNTs have much higher relative D-band
81
intensity with respect to the G-band than the SWCNTs, that is, MWCNTs have much
higher defect density than SWCNTs. The high defect density causes the insignificant
binder effect on the Jsc value of the DSCs with the MWCNT/CMC counter electrode.
1300 1400 1500 1600 1700
0.0
0.2
0.4
0.6
0.8
1.0
D
No
rma
lize
d i
nte
ns
ity
Raman shift (cm-1)
( a )D
1300 1400 1500 1600 1700
0.0
0.2
0.4
0.6
0.8
1.0
Raman shift (cm-1)
GD
N
orm
ali
ze
d i
nte
ns
ity
( b )
Figure 5.13. Raman spectra of binder-free (a) SWCNT and (b) MWCNT films prepared by gel coating.
The Raman intensities were normalized to the corresponding G band intensities. The wavelength of the
laser source is 514.5 nm.
5.3. Conclusion
Binder-free SWCNT and MWCNT films with good adhesion to FTO glass were
fabricated by coating CNT/PEG gels and subsequently removing PEG through
heating. These CNT films were rough and could be used as the counter electrode of
high-performance DSCs. The DSCs with a CNT counter electrode exhibited a light-
to-electricity conversion efficiency comparable to DSCs with the conventional Pt
counter electrode. The DSCs with the SWCNT counter electrode had better stability
in the photovoltaic performance than that with the conventional Pt or MWCNT
counter electrode. The electrochemical characterizations indicate that CNTs can
effectively catalyze the redox of iodide and triiodide. It may be related to the high
surface area of the CNT films.
82
Chapter 6. Ultrasonication-assisted ultrafast
reduction of graphene oxide by zinc powder at
room temperature
6.1. Introduction
This chapter reports the efficient reduction of GO with Zn powder under
ultrasonication in mild acid solutions. The reduction is complete within 1 min at room
temperature. It is significantly faster than that with Fe powder [117]. The ultrafast
reduction is attributed to the low reduction potential of Zn/Zn2+
and the acceleration
effect by ultrasonication [245]. The Zn-reduced GO (Zn-rGO) has a high quality. It
has a C/O ratio of 33.5 and a conductivity up to 15000 S/m.
6.2. Results and discussion
6.2.1. GO Reduction with Zn Powder
The reduction of GO was carried out mixing GO and Zn powder in an acidic
solution under ultrasonication. The typical weight of the Zn powder is twice of that of
GO. The ultrasonication can significantly accelerate the reduction. The color of the
GO suspension remained unchanged after the addition of Zn powder, but turned into
black within 1 min after the ultrasonication was applied. Fig. 6.1a presents the
photograph of a GO solution before and after the reduction.
The ultrasonication-assisted Zn reduction of GO is much faster than most of the
methods reported in literature. Table 6.1 lists the times required for the GO reduction
83
by other methods in literature，most of which take hours or even days to complete
the reduction. Recently Pei et al. reported the reduction of GO with concentrated HI
solution at 100 oC [114]. The reduction can be completed in 1 min for GO thin films
less than 100 nm in thickness. But a longer reduction time is needed for a thicker GO
film. For example, the reduction takes 45-60 min for 5 m-thick GO films.
Figure 6.1. (a) Photographs of a GO solution (A) before and (B) after reduction with Zn powder. (b)
Photographs of Zn-rGO dispersed in solutions of (C) Brij 30, (D) SDBS, (E) cetrimonium bromide, (F)
poly(sodium 4-styrenesulfonate) acid and (G) CMC. (c) Photographs of hydrazine-rGO dispersed in (H)
water and (I) a SDBS solution.
The reduction can be achieved only when the Zn powder is in direct contact with
GO. When a surfactant like SDBS was present in the GO solution, no color change
was observed even after the solution was sonicated for 10 min. It is attributed to the
separation of Zn and GO by the surfactant. It was reported that the chemically
reduced GO undergoes irreversible aggregation and is very difficult to be re-dispersed
in aqueous solutions [108]. But we found that the Zn-rGO can be dispersed into
solutions of surfactants, such as Brij 30, sodium dodecylbenzene sulfonate (SDBS),
cetrimonium bromide, poly(sodium 4-styrenesulfonate) acid and carboxymethyl
cellulose sodium, after sonication for merely 2 min. Fig. 6.1b illustrates the
photograph of various Zn-rGO dispersions. The big particles were removed by
( a ) ( b ) ( c )
84
centrifugation at 5 krpm for 10 min. The maximum concentration of Zn-rGO can be
higher than 1 mg/ml in some surfactant solutions. The Zn-rGO dispersions are very
stable, no obvious precipitation or re-aggregation was observed even after one month.
The good dispersibility of Zn-rGO can be attributed to its low degree of aggregation.
While the initial concentration of GO is only 0.1 wt%, the Zn-rGO precipitate swells
into 80% of the solution volume. Fig. 6.1c is the photograph of hydrazine reduced
GO (hydrazine-rGO) and the hydrazine-rGO/SDBS dispersion after the centrifugation.
The apparent volume of the hydrazine-rGO is much smaller than that of the Zn-rGO.
Accordingly, the concentration of hydrazine-rGO dispersed in a surfactant solution is
also much lower.
Table 6.1. Times required for GO reduction by different methods reported in literature.
Method Reduction time
Reduction with H2 at 450 oC [105] 2h
Anneal in vacuum at 1100 oC [172] 6h
Reduction with hydrazine vapor at 40 oC and subsequently
anneal at 400 oC [172]
22 h
Reduction in hydrazine solution at 100 oC [108] 24h
Reduction in 98% hydrazine hydrate [246] One week
Reduction with hydroquinone [111] 20h
Reduction with dimethylhydrazine at 80 oC [110] 24h
Multi-step reduction with NaBH4, aryl diazonium salt of
sulfanilic acid and hydrazine [112]
27h
Reduction with NaBH4 solution [113] 2h (thin film)
Reduction with 55% HI at 100 oC [114] 45~60 min (5 µm films)
< 1min (100 nm films)
Reduction with Fe powder [117] 6h
Hydrothermal at 180 oC [247] 6h
85
Reduction with benzyl alcohol at 100 oC [116] 24h
Reduction in NaOH or KOH solution at 50~90 oC [115] A few minutes
The size and morphology of Zn-rGO were studied by SEM, AFM and TEM. Fig.
6.2a and Fig. 6.2b show the SEM images of GO and Zn-rGO. The samples were
prepared by drop casting a dilute GO solution and a dilute Zn-rGO/SDBS dispersion
onto silicon substrates. The lateral dimensions of the Zn-rGO sheets range from a few
hundreds of nanometers to 1 m, similar to those of GO sheets before reduction.
These results suggest that Zn-rGO may not be damaged by the ultrasonication during
reduction and dispersion, probably because the time of ultrasonication treatment is
very short [248]. The thickness of GO can be determined according to the AFM
images (Fig. 6.2c and Fig. 6.2d). It is 1 nm and decreases to 0.55 nm after the Zn
reduction. The decrease in the thickness is mainly due to the removal of functional
group from the basal plane of graphene [246,249]. The TEM image shows that the
Zn-rGO has a crumpled morphology (Fig. 6.2e), thanks to the high surface energy of
free standing graphene [109,117].
The electron diffraction pattern of Zn-rGO (Fig. 6.2f) includes two well defined
circles with d spacings of 1.2 Å and 2.1 Å, respectively. They correspond to the
2110 and 1100 crystal planes of graphene [75,250]. The circle-shaped diffraction
pattern is due to the polycrystalline structure. This result is consistent with the
observation by Gomez-Navarro et al. that a rGO sheet has a lot of highly crystallized
graphene domains with an average dimension of a few nanometers [ 251 ]. The
absence of any sharp diffraction spot suggests that these domains randomly orient in a
long range.
86
Figure 6.2. SEM images of (a) GO and (b) Zn-rGO, AFM images of (c) GO and (d) Zn-rGO, (e) TEM
image of a free standing Zn-rGO sheet suspended on a lacey carbon TEM grid and (f) selected area
electron diffraction (SEAD) pattern of the Zn-rGO sheet in (e)..
(a) (b)
(d) (c)
(e) (f)
87
6.2.2. Structure and Properties of Zn-rGO
The reduction of GO includes the removal of functional groups and the restoration
of conjugated π systems. The chemical structures of GO before and after the
reduction were studied by FTIR spectroscopy (Fig. 6.3). Characteristic peaks
corresponding to various functional groups, including C-O (1060 cm-1
), C-OH (1226
cm-1
), O-H (1412 cm-1
) and C=O (1733 cm-1
), can be observed in the FTIR spectrum
of GO [112,252]. The assignment of the intense band at 1627 cm-1
is still under
debate. It might be attributed either to the oxygen containing groups like esters,
absorbed H2O or the un-oxidized graphitic domains [253,254]. After the reduction,
most oxygen containing chemical groups were removed as shown in the FTIR
spectrum of rGO. However, very weak peaks at around 1400 cm-1
and 1200 cm-1
can
still be observed, indicating that some hydroxyl groups still remains even after
reduction.
The recovery of conjugated C=C bonds of the Zn-rGO was investigated by UV-vis
absorption spectroscopy. There is a strong absorption peak at around 230 nm for the
GO solution (Fig. 6.4a). This band corresponds to the →* transition [106,247,255].
GO has very weak absorption in the visible range due to the destruction of conjugated
π system. After reduction, the →* transition band shifts to red, from 230 to 272 nm,
and the absorbance in the whole visible range increases dramatically. These results
confirm the restoration of conjugated structure of graphene [256,257].
The UV-vis absorption of rGO depends on the amount of Zn powder. Fig. 6.4b
shows the dependence of the peak position of the π→π* band on the amount of Zn
powder. GO is partially reduced, so that the π→π* transition peak appears between
88
230 and 272 nm, when the weight ratio of Zn to GO is below 2. When the weight
ratio is increased to more than 2, the peak position locates at 272 nm and does not
exhibit any further shift with the increasing Zn amount.
2000 1800 1600 1400 1200 1000
(b) x 10
O-H
C-OHC-O
Ab
so
rba
nc
e
Wave number (cm-1)
C=O
(a)
Figure 6.3. FTIR spectra of (a) GO and (b) Zn-rGO. The absorbance of Zn-rGO was magnified by 10
times.
200 300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
No
rma
lize
d a
bs
orp
tio
n
Wave length (nm)
(a)
0 2 4 6 8 10
230
240
250
260
270
280
Pe
ak
po
sit
ion
(c
m-1
)
Zn/GO (wt/wt)
(b)
Figure 6.4. Normalized UV-vis absorption spectra of a diluted GO solution (solid curve) and a diluted
Zn-rGO/carboxymethyl cellulose sodium dispersion (dash curve). Zn-rGO was prepared by reducing
GO at a Zn-to-GO weight ratio of 2. (b) Dependence of the →* transition peak position on the Zn-
to-GO weight ratio. Carboxymethyl cellulose sodium was chosen as the dispersion agent due to its
weak absorption in the UV-vis range.
89
The structural change of GO during the reduction was investigated by the Raman
spectroscopy (Fig. 6.5a). Raman spectra of graphite and hydrazine-rGO were also
presented for comparison. There are two bands in the range from 1200 to 1800 cm-1
in
the Raman spectrum of graphite. The intense band at 1580 cm-1
is the G band
corresponding to the in-plain stretching of graphite lattice [249,258], while the band
at 1350 cm-1
is the D band arising from the small domain sized graphitic regions
[258,259]. The D band of pristine graphite is almost indiscernible due the low density
of defects. Upon oxidization, both the G and D band undergo significantly changes as
the result of the conversion from sp2 to sp
3 hybridization of some carbon atoms. The
G band shifts from 1580 to 1600 cm-1
, because the isolated double bonds formed
during the oxidization has higher resonant vibration frequencies than the conjugated
double bonds in graphite [249]. After the reduction of GO by Zn, the G band shifts
back to 1580 cm-1
. This indicates the restoration of the conjugated π systems. The G-
band of hydrazine-rGO also shifts to red, but the shift is less. It appears at 1590 cm-1
.
Therefore, Zn can reduce GO more effectively than hydrazine hydrate.
The oxidization of graphite also leads to the significant increase in the relative D
band intensity. After the reduction by either Zn or hydrazine hydrate, the ratio of the
D to G band intensity (ID/IG) further increased. This result can be ascribed to the
restoration of numerous graphitic domains from amorphous regions of GO, which
gives rise to stronger D band signal [258,259]. The increasing ID/IG ratio was usually
observed after the chemical reduction of graphene oxide into graphene [109,113,257].
Fig. 6.5b presents the variation of the ID/IG ratio with the Zn-to-GO weight ratio for
90
Zn-rGO. ID/IG increased with the increasing Zn amount when the Zn/GO ratio is
below 2. It becomes almost constant after the Zn/GO ratio is higher than 2.
1000 1200 1400 1600 1800
Zn-rGO
Hydrazine-rGO
GO
In
ten
sit
y
Raman shift (cm-1)
1580 cm-1
Graphite
(a)
0 2 4 6 8 101.0
1.2
1.4
1.6
1.8 (b)
I D/I
G
Zn/GO (wt/wt)
Figure 6.5. (a) Raman spectra of graphite, GO, hydrazine-rGO and Zn-rGO. (b) Variation of the ID/IG
ratio of Zn rGO with the Zn-to-GO weight ratio.
The recovery of graphene structure can improve the thermal stability of rGO. Fig.
6.6 shows the TGA curves of GO, Zn-rGO and hydrazine-rGO. There are two weight
loss processes for GO. At the first process, GO loses about 30% weight at around 200
oC, which is caused by the pyrolysis of oxygen containing functional groups
[109,117]. The second process take place at around 460 oC in air. It is the oxidization
of carbon atoms. The first process becomes less significant for the hydrazine-rGO.
The weight loss is only about 5% up to 200 oC, since most of the functional groups
are removed during the reduction. The oxidization temperature of the hydrazine-rGO
is higher than that of the GO by 50 oC. The first process becomes invisible for Zn-
rGO. There is no obvious weight loss up to 200 oC. The oxidization takes place at 590
oC, much higher than that of both GO and hydrazine-rGO. These results indicate that
the open defects in the original GO were well “patched” by the newly recovered sp2
domains.
91
100 200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0 Zn-rGO
Hydrazine rGO
460 oC
510 oC
No
rma
lize
d w
eig
ht
Temperature (oC)
590 oC
GO
Figure 6.6. TGA curves of GO, Zn-rGO and hydrazine-rGO in air.
The different weight losses suggest that GO and Zn-rGO have different oxygen
amounts. The compositions of GO and Zn-rGO were investigated by XPS. Fig. 6.7a
shows the XPS surveys of GO and Zn-rGO. Zn is hardly detected in the XPS of Zn-
rGO, which implies that Zn does not form any complex with GO. This is different
from rGO reduced by reductants like hydrazine hydrate or NaBH4 [109,113]. The
C/O atomic ratio was estimated from the XPS surveys. It is 2.58 for GO and increases
to 33.5 for Zn-rGO. The C/O atomic ratio of Zn-rGO is higher than that reduced by
other chemicals, like hydrazine, NaBH4, alkalis, alcohols and Fe [109,113,115-117].
The C1s XPS of GO can be deconvoluted into 4 XPS bands with peak positions at
284.5, 286.7, 287.8 and 288.8 eV (Fig. 6.7b). They can be assigned to sp2 hybridized
carbon and carbon atoms in C-O, C=O and COOH groups, respectively
[109,113,117,258]. The C1s XPS of Zn-rGO is similar to that of the pristine graphite
but saliently different from that of GO (Fig. 6.7c). These XPS data confirm the
92
effective removal of functional groups and restoration of conjugated C=C bonds after
the reduction of GO by Zn.
200 400 600 800 1000 1200
O 1s
GO
Zn-rGO
Inte
ns
ity
Binding Energy (eV)
Graphite
C 1s
(a)
290 288 286 284 282
Inte
ns
ity
Binding Energy (eV)
GO
sp2
-C-O
-C=O
-COOH
(b)
294 292 290 288 286 284 282
Inte
ns
ity
Binding Energy (eV)
(c)
Figure 6.7. (a) XPS survey scans of graphite, GO and Zn-rGO. (b) Deconvolution of C1s XPS of GO.
(c) C1s peaks of graphite (solid curve) and Zn-rGO (dash curve).
The GO reduction gives rise to the restoration of the electrical properties of
graphene. The electrical conductivity of rGO is also one of the most critical criteria to
evaluate the reduction techniques. Conductivity of the Zn-rGO was investigated by
measuring the resistance of Zn-rGO films prepared by vacuum filtration. Fig. 6.8 is
the photograph of a Zn-rGO film transferred onto a glass substrate. The film has a
shinning surface with metallic luster. Its conductivity is 15000 S/m. This conductivity
93
is close to that of rGO reduced through the high temperature annealing [260], and it is
much higher than that of rGOs reduced by other chemicals in solution [109,111-
113,115-117]. Conductivities of the Zn-rGO films depend on the degree of reduction.
When the Zn/GO weight ratio is higher than 2, the conductivities of the Zn-rGO films
are around 15000 S/m. However, when the Zn/GO weight ratio is 1, the conductivity
of the corresponding Zn-rGO films is only about 10 S/m.
Figure 6.8. Photograph of a rGO film transferred onto a glass substrate.
The reaction between Zn and GO in acid solution can be expressed by two half
reactions:
Zn → Zn2+
+ 2e-, and
GO + aH+ + be
- → rGO + cH2O [107,117].
The standard reduction potential of Zn/Zn2+
is -0.76 V vs SHE, whereas the reduction
potential of the GO reduction is about –0.40 V vs SHE in solution of pH = 4
according to Zhou et al. [107]. The solutions used to reduce GO in our experiments
94
have a pH value of 2, so that the reduction potential of GO should be higher than -
0.40 V vs SHE. These potential values indicate that Zn can effectively reduce GO.
The reduction of GO by Zn under ultrasonication is much faster than that by Fe
powder [117]. This can be attributed to the acceleration by the ultrasonication and the
higher reduction capability of Zn than Fe. The reduction potential for Fe → Fe2+
+ 2e-
is -0.44 V vs SHE, remarkably lower than the Zn/Zn2+
reduction potential. Because of
the higher activity of Zn, the reduction of GO by Zn can take place in a weak acidic
solution, while the reduction by Fe requires a strong acidic solution.
6.3. Conclusion
This chapter reports an effective and ultrafast method to reduced GO by Zn at room
temperature. The reduction was carried out by mixing GO with Zn powder in solution
under ultrasonication. It completed within 1 min, significantly shorter than by other
chemical reduction techniques. rGO can be readily re-dispersed into solutions of
various surfactants. FTIR, UV, Raman and XPS studies confirms the removal of
oxygen-containing functional groups and the restoration of conjugated π systems. The
C/O ratio of Zn-rGO is 33.5, higher than rGO reduced by other reductants. The Zn-
rGO also has good thermal stability and high electrical conductivity. The excellent
reduction can be attributed to the low reduction potential of Zn and the acceleration
by the ultrasonication.
95
Chapter 7. Metal reduced graphene thin films
as transparent and flexible electrodes
7.1. Introduction
Besides conducting polymers [168-170] and 2D CNT networks [174-191], thin
films of graphene have also been regarded as promising candidates for transparent
electrodes to replace ITO. In spite of the inferior conductivity, graphene electrodes
have a lot of advantages over ITO, such as low material cost and high flexibility. The
application of transparent graphene electrode in various flexible electronic devices
such as transparent field emission devices [261], solar cells [262], touch screens and
flat panel displays [96] has been demonstrated recently. Highly conductive graphene
films can be prepared by direct CVD growth [93,96,171] or high temperature
reduction of GO films [105,172]. However, in these methods, graphene films can
only be deposited on inorganic substrates with melting point higher than 1000 oC.
Additional post treatments are required in order to transfer the graphene films onto
other flexible substrates [96,263]. Besides, the high temperature process significantly
increases the fabrication cost, thus is unsuitable for the large-scale production.
Alternatively, flexible graphene films can be prepared at temperatures lower than 100
oC by reducing GO films with reductive chemicals [113,172,257,264]. Since GO has
high solubility in water and many polar organic solvents, it can be easily deposited
into thin films by solution processing techniques such as self-assembly [265], spin
coating [172,266], spray coating [113], dip coating [250] and vacuum filtration [267].
The graphene films can be reduced with a wide range of reductants, such as hydrazine
96
hydrate [264], NaBH4 [113] or HI [114]. But these chemicals are generally harmful to
human and environment.
This chapter demonstrates an eco-friendly and facile route to reduce spin coated
GO films on flexible substrates at low temperature. The reduction was achieved by
depositing a thin layer of Al onto the GO films by thermal evaporation and
subsequently etching away Al with diluted acid solutions. As prepared rGO thin films
exhibit a conductivity as high as 20000 S/m, comparable with that of GO films
reduced by other strong reductants such as hydrazine hydrate or NaBH4 [113,172,
256].
7.2. Results and discussion
7.2.1. Reduction of spin coated GO films
Spin coating is a convenient and reliable technique to deposit thin films on
substrates. It has been reported that uniform GO films with diameter as large as 300
mm can be easily deposited onto SiO2/Si wafer via spin coating [ 268 ]. By
maintaining the concentration and volume of the GO suspension dropped on the
substrates, the spreading time of suspension, and the rotating speed of the spin coater,
thickness of the GO films can be controlled in nearly 100% precision [268]. The
uniformity of the spin coated films strongly depends on the wetability of solutions on
the substrates [264,266,268]. Since common substrates like Si, SiO2, glass and PET
are hydrophobic, aqueous suspensions of GO cannot form uniform films on these
substrates. In this work, GO was dissolved in the H2O/ethanol mixture instead of pure
H2O, because polar organic solvents like ethanol and methanol can significantly
97
enhance the wetbility of H2O on hydrophobic substrates by lowing the surface tension
of water [266,268].
Concentration of the GO suspension is important for the full coverage of GO on the
substrates. Conventionally, the concentration of GO suspension used for spin coating
should be in the range between 0.5 mg/ml to 3.0 mg/ml [264]. Fig. 7.1 are SEM
images of GO nanosheets deposited by spin coating of GO suspensions with different
concentrations. At lower concentration, the GO sheets just distributed randomly on
the substrate. A continuous GO film formed when the concentration of GO reaches 2
mg/ml. The graphene films are almost completely transparent (Fig. 7.2a).
Figure 7.1. SEM images of GO nanosheets deposited on Si substrate by spin coating at 3000 rpm.
Concentrations of GO suspensions are (a) 0.2 mg/ml and (b) 2 mg/ml. (c) SEM image of a rGO film.
(a) (b)
(c)
98
After annealing on a hot plate at 100 oC for 30 min, a thin layer of Al was deposited
onto the surface of GO by thermal evaporation at high vacuum (< 1× 10-6
mBar) to
minimize the formation of Al2O3 at the GO/Al interface. In this experiment, Al was
chosen as the reducing agent for GO instead of Zn, mainly because the boiling point
of Zn is too low, so the thermal evaporation of Zn is difficult to control. It must be
noted that the color of GO films did not change after the deposition of Al, which
means Al alone cannot reduce GO. The Al coated GO films were then immersed into
4M HCl solution. The transparent GO films turned dark almost immediately when the
etching of Al began. This phenomenon indicates that H+ is necessary for the reduction
of GO. The rGO film shows a generally flat morphology with narrow wrinkles (Fig.
7.1c).
The redox reaction between metals and GO has been discussed in detail in the
previous chapter. When Al contacts GO in an acid solution, Al is oxidized and
electrons transfer from Al to GO due to the lower reduction potential of Al. The
electron enriched GO further reacts with H+ to produce graphene and H2O [107,117].
The process can be described by the following reactions:
Al → Al3+
+ 3e-, and
GO + aH+ + be
- → rGO + cH2O
H+ plays an important role in the reactions. It can remove the oxide layer on the
surface of Al, and also increase reduction potential of rGO/GO. H+ can also react with
Al to form H2. However, this process is not helpful to the reduction of GO as proved
by Fan et al. [269].
99
Fig. 7.2a shows the photograph of a 5 nm GO film and a 30 nm GO film before and
after reduction. The transmittance of the thicker rGO film is obviously lower than the
thinner one, that is to say, the entire GO films rather than topmost layers are reduced
by Al. Since the reduction is carried out in room temperature, the rGO films can be
directly deposited on flexible substrates (Fig. 7.2b).
Figure 7.2. (a) The photograph of a 5 nm-thick GO (left) film and a 30 nm-thick GO film (right)
before and after reduction with Al. (b) The photograph of a rGO film on PET substrate.
7.2.2. Properties of the rGO films
The reduction of the GO films was investigated by UV-vis spectroscopy. Fig. 7.3a
shows the absorption spectra of a 30 nm thick GO film before and after reduction by
30 nm-thick Al. The absorption spectra of the GO films are similar to that of GO
suspensions presented in the previous chapter. The prominent peak at 233 nm
corresponds to the →* transition of aromatic C=C bonds, while the shoulder at
around 300 nm is be attributed to the n→* transition of the C=O bond [255]. After
reduction, the n→* transition peak disappeared due to the removal of functional
groups [255] and the →* transition peak shifts to 272 nm due to the restoration of
conjugated structures [256,270]. Meanwhile, the absorption in the whole visible range
also increases dramatically. The reduction of rGO films strongly depends on the
(a) (b)
100
thickness of the Al layers. With increasing Al thickness, the →* transition peak
exhibits stronger red-shift, and it final rests at 272 nm when the thickness of the Al
layer exceeds 30 nm. The transmittance of rGO films in visible range shows similar
trend of change (Fig. 7.3b). These results suggest that 30 nm is the minimum required
thickness of Al to completely reduce a 30 nm thick GO film.
200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0
rGO film
No
rma
lize
d i
nte
ns
ity
Wave length (nm)
GO film
(a)
0 10 20 30 40 50280
270
260
250
240
230
220
Re
lati
ve
ab
so
rpti
on
at
55
0 n
m
Po
sit
ion
of *
tra
ns
itio
n p
ea
k
Al thickness (nm)
(b)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Figure 7.3. (a) UV-vis absorption of a 30 nm GO film before and after reduction with 30 nm Al. (b)
The evolution of the →* transition peaks and the 550 nm transmittance of rGO films with respect to
increasing thickness of Al.
1000 1200 1400 1600 18000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1588 cm-1
GO film
No
rma
lize
d i
nte
ns
ity
Raman shift (cm-1)
rGO film
1600 cm-1
(a)
0 10 20 30 40 501.0
1.2
1.4
1.6
1.8
2.0
I D/I
G r
ati
o
Al thickness (nm)
(b)
Figure 7.4. (a) Raman spectra of a 30 nm GO film before and after reduction and (b) ID/IG ratios of GO
films reduced by Al with different thickness.
Fig. 7.4a is the Raman spectra of a GO film and a rGO film. As discussed in the
previous chapter, the G band of the GO film redshifts while the ID/IG ratio increases,
101
which confirm the restoration of small graphene domains from the amorphous regions
on the GO [249]. The ID/IG ratio of the rGO film is 1.97, even higher than that of rGO
reduced by Zn powder in solutions, which shows Al can reduce GO even more
effectively than Zn. This result is plausible since the standard reduction potential of
Al (-1.66 V) is much lower than that of Zn (-0.76 V). Fig. 7.4b shows dependence of
ID/IG ratio on the thickness of the Al layers. The ID/IG ratio is almost linearly with the
Al thickness and finally rests at the maximum value of 1.97 when Al layer is
thickener than 30 nm. The Raman results give the same conclusion as the UV-vis
absorption spectra.
100 200 300 400 500 600 700 800 900 1000 1100 1200
O1s
rGO film
Inte
ns
ity
Binding energy (eV)
GO film
C1s
(a)
290 289 288 287 286 285 284 283 2820
1000
2000
3000
4000
5000
6000
Binding Energy (eV)
In
ten
sit
y
GO
sp2
-C-O
-C=O
-COO
(b)
290 289 288 287 286 285 284 283 282
Inte
ns
ity
Binding Energy (eV)
rGO
sp2
sp3
-C-O
-C=O
-COO-
(c)
Figure 7.5. (a) XPS surveys of a GO film and a rGO film, and high resolution C1s peaks of (b) a GO
film and (c) a rGO film.
102
The composition of the GO and rGO films were analyzed with XPS. The XPS
surveys scan of GO film (Fig. 7.5a) shows prominent C1s and O1s. After reduction,
the intensity of the O1s peak decreases dramatically and C/O ratio of the film
increases from 2.68 to 45.3. This value is much higher than those of rGO reduced by
other chemicals [109,113,115-117]. The C1s XPS of GO can be deconvoluted into 4
peaks corresponding to sp2 hybridized carbon atoms (284.5 eV), C-O (286.5 eV),
C=O (287.8 eV) and COOH (288.8 eV) (Fig. 7.5b) [109,113,114,271,272]. After
reduction the intensities of all these peaks related to the oxygen containing functional
groups decrease dramatically while the intensity of the sp2 band increases. A band
corresponding to sp3 hybridized carbon (285.3) eV can be deconvoluted from the C1s
XPS of rGO [271,272]. The appearance of this new band shows that even after the
removal of oxygen containing functional groups, the π bonds in rGO cannot be
completely recovered.
The restoration of the conjugated π system leads to the recovery of electrical
conductivity. Fig. 7.6a shows the sheet resistance and transmittance of rGO films on
PET substrate. The rGO films have a sheet resistance of ~7 kΩ/ at a transmittance of
80%. Thickness of the rGO films was controlled by tuning the number of spin coating
cycles as shown in Fig. 7.6b. Both the opacity and the thickness of the rGO films
exhibit a linear relationship with the number of spin coating cycles. The rGO film
deposited by 1cycle of spin coating has a transmittance of 83% and a thickness of 5
nm. After spin coating for 5 cycles, the film transmittance dropped to about 61%, and
the film thickness increases to about 23 nm. The conductivity of the graphene thin
103
films is ~20 000 S/m by calculation, comparable with that of GO films reduced by
strong reductants such as hydrazine hydrate and NaBH4 [113,172,256].
Apart from the reduction techniques, the conductivity of the rGO films also
depends on other factors such as the dimension of the rGO sheets, the uniformity of
the films [265] and chemical doping [113]. For example, Zhao JP reported that the
conductivity the of rGO films was increased by about 1 order when the average area
of the rGO sheet was increased from 100~300 µm2 to 7000 µm
2. Cote LJ et al.
reported that the conductivity of rGO films doubled after doping with AuCl3. Thus
the performance of our Al reduced GO films might be further improved by
optimizing these experimental parameters.
60 65 70 75 80 850
2
4
6
8
10
12
Sh
ee
t re
sis
tan
ce
k
Transmittance at 550 nm (%)
(a)
1 2 3 4 50
5
10
15
20
25
Tra
ns
mit
tan
ce
at
55
0 n
m (
%)
Th
ick
ne
ss
(n
m)
Number of spins
60
65
70
75
80
85
90
(b)
Figure 7.6. (a) Sheet resistance vs. transmittance of rGO films with different thickness. (b)
Dependence of the thickness and transmittance of rGO films on the number of spin coating cycles. The
rGO films were reduced by 30 nm-thick Al.
7.3. Conclusion
This chapter describes a facile and eco-friendly technique to deposit transparent and
conductive graphene thin films on flexible substrates. The graphene films were
prepared by reducing spin coated GO films with a thin layer of Al in acid solutions at
room temperature. The quality of the rGO films strongly depends on the thickness of
104
the Al layer deposited on GO. The UV-vis spectroscopy and Raman spectroscopy
characterization show that at least 30 nm-thick Al is required for complete reduction
of the rGO thin films. The C/O ratio of the GO films reduced under optimized
condition is as high as 45.3, much higher than that of rGO reduced by other chemical
methods. Conductivity of the rGO films is around 20 000 S/cm, comparable with that
of rGO films reduced by hydrazine hydrate. This conductivity might be further
improved by chemical doping or by further increasing the dimension of GO sheets.
105
Chapter 8. Conclusion and future works
CNTs and graphene have a lot of important properties such as high electrical
conductivity, high charge carrier mobility, excellent catalytic ability, large specific
surface area and good flexibility. Thus they have a lot of potential applications in
future electronic devices. However, the processing of CNTs and graphene is
extremely difficult, since both materials are insoluble in any solvents and easy to
aggregate. As a result, the practical application of CNTs and graphene are challenged
by their poor processibilities. This thesis focuses on the large scale processing
techniques as well as the practical application of CNTs and graphene.
Although the synthesis techniques of CNTs are well established after 20 years’
development and high quality CNTs can be easily procured from the market at low
prices, there still lacks an efficient technique to disentangle the commercially
available CNTs at a large quantity. In chapter 2 of this thesis, a novel gel phase
processing technique of CNTs is demonstrated. The CNT composite gels were
prepared by mixing CNTs and a liquid nonionic surfactant, POETE, with mechanical
grinding. The heavily entangled CNTs debundled during the grinding and dispersed
into individual tubes or fine bundles in POETE. Dynamic mechanical measurements
and SEM characterization suggest that the formation of the CNT/POETE gels is the
result of the physical-crosslinking CNT networks, mediated by the van der Waals
interaction between CNTs and the nonionic surfactant. The gels were stable from
room temperature up to 200 oC and did not shrivel even in vacuum. Concentration of
CNTs in the gels can be as high as 1 wt%, several orders higher than the CNT
106
concentration in solutions. That means the gel phase dispersion of CNTs is much
more efficient than the traditional solution dispersion techniques.
The following two chapters report the application of gel processed CNT films as
electrode materials. Chapter 4 demonstrates the preparation of transparent SWCNT
electrode via gel coating. SWCNT films were obtained by coating the
SWCNT/POETE composite gels on glass substrates and subsequently removing the
surfactant through heat treatment. The SWCNT films became very thin and
transparent after heating at a temperature between 480 and 520 oC. They exhibited
good uniformity and a high transmittance-to-sheet resistance ratio which is attributed
to the long SWCNTs dispersed in the gels. Films with a sheet resistance of around
100 Ω/ at 70% transmittance were obtained. The films can be easily transferred
from rigid substrates like glass to flexible polymer substrates like PET. These
transparent SWCNT electrodes have potential applications in flexible optoelectronic
devices in place of ITO.
Chapter 5 demonstrates the application of thick CNT films as counter electrode of
DSCs. Both SWCNT and MWCNT counter electrodes were fabricated using a similar
gel coating technique described in chapter 4, except that the dispersion agent was
changed from POETE in PEG and the heating temperature was lowered to 450 oC.
The DSCs with SWCNT or MWCNT counter electrode exhibited a light-to-electricity
conversion efficiency comparable with the DSCs with the conventional Pt counter
electrode, when the devices were tested immediately after fabrication. Moreover, The
DSCs with SWCNT counter electrode exhibited better stability in the photovoltaic
performance than the DSCs with Pt counter electrode. The high photovoltaic
107
performance of these DSCs is related to the excellent electrochemical catalysis of
CNTs on the redox of iodide/triiodide pair, as revealed by the cyclic voltammetry and
ac impedance spectroscopy.
Our study on graphene focuses on the efficient production of graphene. Currently,
chemical reduction of GO has been considered as one of the most promising
approaches to produce graphene in large quantities. But conventional chemical
reduction techniques usually use harmful chemicals and are time consuming. Chapter
6 reports a green and rapid method to reduce GO at a large quantity. The reduction
was achieved by mixing GO and Zn powder in mild acid solution under
ultrasonication. The reduction can be completed within 1 min. The C/O atomic ratio
of GO increased from 2.58 to 33.5 after the reduction as determined by XPS. The
rGO can have a conductivity of 15000 S/m. It has been reported that GO usually
undergoes irreversibly aggregation after chemical reduction. But our Zn-rGO can be
readily re-dispersed into solutions of various surfactants at concentrations higher than
0.1 wt%.
Chapter 7 reports the fabrication of transparent and flexible graphene films using
the metal reduction technique. The GO films were coated onto flexible PET
substrates using spin coating. A thin layer of Al was then deposited onto the GO films
by thermal evaporation. Finally, the Al coated GO films were soaked into acid
solutions to complete the reduction and Al was removed simultaneously. The
transmittance and sheet resistance of the films can be tuned in a wide range by
controlling the spin coating cycles. The rGO films exhibit a sheet resistance of 4
108
kΩ/ at the transmittance of 70%, corresponding to a conductivity of 20000 S/m. The
high degree of reduction can be attributed to the low reduction potential of Al.
Future works can be motivated and derived from this thesis. The following ideas
are proposed for further study:
1. This thesis only discussed the gel phase processing of CNTs. However, since
graphene has similar structure as CNTs, it is reasonable to assume that graphene can
form gels with nonionic surfactants as well. Thus research on the gel phase
processing of graphene will be carried out in the future.
2. At present, only two groups of materials, ionic liquids and surfactants, have
been reported to be able to from gels with pristine CNTs. In the future, we will study
the gel formation between CNTs with other liquids such as various biofluids. This
proposal is based on two concerns. Firstly, some biofluids, like fat, has certain
structural similarities with the nonionic surfactants, thus might be able to disperse
CNTs. Secondly, the good compatibility of CNTs with biofluids is critical for their
applications in medicine and drug delivery.
3. This thesis demonstrated the fabrication of transparent CNT and graphene
electrodes. In future works, we will try to integrate these electrodes into
optoelectronics such as organic solar cells and LEDs.
4. The application of gel coated CNT and graphene thick films on energy devices,
such as lithium batteries, supercapacitors and fuel cells, will also be investigated.
109
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