galvanic-cell-based synthesis and photovoltaic performance of zno-cds core-shell nanorod arrays for...
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Galvanic-cell-based Synthesis and Photovoltaic Performance of ZnO-CdS core-shell Nanorod Arrays for Quantum Dots Sensitized Solar
Cells
Le Ha Chi1, a *, Pham Duy Long 1,b, Hoang Vu Chung 1,c, Do Thi Phuong1,d,
Do Xuan Mai1,e, Nguyen Thi Tu Oanh1,f, Thach Thi Dao Lien2,j
and Le Van Trung3,h 1Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc
Viet Street, Cau Giay District, Hanoi, Viet Nam
2Yen Vien Highschool, Yen Vien town, Gia Lam district, Hanoi, Viet Nam
3 University of Science and Technology of Hanoi, 18 Hoang Quoc Viet street, Cau Giay District, Hanoi, Viet Nam
a*[email protected], [email protected], [email protected], [email protected], [email protected], [email protected],
[email protected], [email protected]
Keywords: Galvanic-cell-based synthesis, ZnO-CdS, Core-shell, Nanorod arrays, Quantum dots sensitized solar cells.
Abstract. Zinc oxide (ZnO) is recognized as one of the most attractive metal oxides because of its
direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), which make it promising
for various applications in solar cells, gas sensors, photocatalysis and so on. Here, we report a facile
synthesis to grow well-aligned ZnO nanorod arrays on SnO2: F (FTO) glass substrates without the
ZnO seed layer using a Galvanic-cell-based method at low temperature (<100oC). CdS quantum dot
thin films were then deposited on the nanorod arrays in turn by an effective successive ionic layer
adsorption and reaction (SILAR) process to form a ZnO/CdS core-shell structure electrode.
Structural, morphological and optical properties of the ZnO/CdS nanorod heterojunctions were
investigated. The results indicate that CdS quantum dot thin films were uniformly deposited on the
ZnO nanorods and the thickness of the CdS shell can be controlled by varying the number of the
adsorption and reaction cycles. The number of quantum dots layers affects on photovoltaic
performance of the ZnO/CdS core-shell nanorod arrays has been investigated as photoanodes in
quantum dots sensitized solar cells.
Introduction
Nanostructured ZnO materials, especially one-dimensional (1D) zinc oxide (ZnO) nanostructures
such as rods, wires and tubes have received broad attention due to their distinguished performance in
electronics, optics and photonicsZnO is a key technological material due to its unique
semiconducting and piezoelectric properties with a direct wide band gap of 3.37 eV, a large exciton
binding energy of 60 meV at room temperature as well as low cost and non-toxicity[1]. With
reduction in size, novel electrical, optical and chemical properties are introduced, which are largely
believed to be the result of surface and quantum confinement effects. One-dimensional (1D) ZnO
nanostructures have been synthesized by a wide range of techniques, such as wet chemical methods
[2], sputtering [3], electrodeposition [4] and solution-based growth methods [5]. Among them,
solution-based methods are especially attractive for industrial applications because of the low-cost,
low-processing temperature and ease of morphology control. However, the growth of ZnO nanorods
on various substrates is usually required a seed layer, ultra-thin layer of packed ZnO nanocrystals
acting as homoepitaxial nucleation sites, to improve the density and vertical alignment of the
nanorods. Recently, Junling Wang et al. described a novel galvanic-cell-based approach towards the
direct growth of ZnO nanorod arrays on various conducting substrates at low temperature without the
Applied Mechanics and Materials Vol. 618 (2014) pp 64-68 Submitted: 17.06.2014Online available since 2014/Aug/18 at www.scientific.net Accepted: 17.06.2014© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.618.64
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seed layer [6]. This approach is simple and the growth mechanism is use of the contact potential
between different materials as the driving force for ZnO growth on conducting substrates. However,
in this presentation, to attach Al on cathode substrate, Al was selectively deposited on the cathode
substrate by pulse laser deposition (PLD), which could be the disadvantage due to high cost and high
vacuum growth methods.
In this paper, we will focus on controlling the growth of ZnO nanorod arrays on conducting
substrates using a Galvanic-cell-based method at low temperature. For the large-scale applications,
Al foil was simply used as sacrificing anode substrate and fluorine-doped tin oxide (FTO) substrate
was used for the growth of ZnO nanorods. The changes in orientation, diameter, density, and length
of ZnO nanorods via different synthesized conditions of temperature and growth time were
examined. Then CdS quantum dot thin films were sequentially deposited on the ZnO nanorod arrays
by an effective successive ionic layer adsorption and reaction (SILAR) process to form a ZnO/CdS
core-shell structure electrode. The number of CdS quantum dots layers affects on photovoltaic
performance of the ZnO/CdS core-shell nanorod photoanodes has been investigated for quantum dots
sensitized solar cell application.
Experimental
Galvanic-cell-based Synthesis of ZnO Nanorod Arrays
Growth of ZnO nanorod arrays on conducting substrates using a Galvanic-cell-based structure as
shown in figure 1 was carried out by suspending Al foil as the sacrificing anode and ZnO growth
occurs on the fluorine-doped tin oxide (FTO) coating on glass substrates (Aldrich, sheet resistance of
about 7 Ω.-1) in an aqueous solution containing 25 mM zinc nitrate hexahydrate (Zn(NO3)2.6H2O)
and 25 mM hexamethylenetetramine (HMTA C6H12N4) at 70oC. All the chemicals are of highest
purity and commercially available. FTO substrates were cleaned ultrasonically with ethanol, acetone
and de-ionized water for 10 min, respectively, and then dried before ZnO growth. The samples were
then removed from solution, rinsed with deionized water, and dried at room temperature.
Synthesis of ZnO/CdS core–shell Nanorods
The CdS QDs was deposited on the ZnO nanorod arrays by a successive ionic layer adsorption and
reaction (SILAR) technique. The samples were successively dipped in two different aqueous
solutions for 1 minute. One is containing Cd2+
cations (0.1 M Cd(NO3)2) and the other is containing
S2-
anions (0.1 M Na2S). Between each immersion step, the samples were rinsed with de-ionized
water for 10 seconds to remove excess ions that were weakly bound to the samples surfaces. The
two-step dipping procedure is termed as one SILAR cycle. After several cycles, the color of
ZnO/FTO film changed from white to yellow which implies the formation of CdS on the ZnO surface.
Finally, the samples were thoroughly washed with ethanol and deionized water and then dried at
room temperature. In this work we deposited five, ten and fifteen layers of CdS QDs on ZnO nanorod
arrays, namely ZnO/CdS(5), ZnO/CdS(10) and ZnO/CdS(15) core/shell nanorod arrays. The effect of
SILAR cycles on the device performance has been carefully studied.
Device Fabrication
To evaluate their photovoltaic performance, the CdS QDs sensitized ZnO nanorods on the FTO
substrate were sandwiched together with Pt counter electrode, using front-side illumination
conditions. Pt counter electrodes were fabricated by thermal depositing H2PtCl6 onto the FTO coated
glass. The internal space of the cell was filled with the electrolyte including the mixture of 1 M Na2S,
1 M S and 0.2 M KCl in a methanol and water (7 : 3) solution.
Characterization
To investigate the effect of applied voltage, temperature and growth time on the growth of ZnO
nanorods during hydrothermal synthesis via a Galvanic-cell-based structure, Autolab PGSTAT-30
machine and Keithley 2000 multimeter were used. The morphology of samples was investigated by
using a “Hitachi S-4800” Field Emission Scanning Electron Microscopy (FE-SEM). The crystalline
Applied Mechanics and Materials Vol. 618 65
phase was identified by X-ray diffraction (XRD) using a D8 Advance Bruker powder X-ray
diffractometer (Cu Kα as radiation source, λ=0.15406 nm).
The samples (ZnO or ZnO/CdS nanorod arrays) were characterized by transmission electron
microscopy (TEM, Jeol JEM1010). The UV-Visible (UV–vis) absorption spectra of the samples were
measured by a Jasco V-670 UV-visible spectrophotometer equipped with an integrating sphere.
Photoluminescence spectra (PL) were carried out by using a Microspec-2356 spectrophotometer with
a He–Cd laser as an excitation source (λ=325 nm). The current–voltage performance was measured
using an Auto-Lab Potentiostat PGS-30 under the illumination of a tungsten–halogen lamp (20
mW/cm2). The illuminated area on the electrode surface was about 1.0 cm
2.
Results and Discussion
Morphology Analysis
Fig. 1. (a) Top-view and (b) Cross-sectional FESEM images of ZnO nanorod arrays synthesized on FTO substrates using
a Galvanic-cell-based method at 70oC for 1 hour.
The Galvanic-cell-based ZnO growth mechanism is clearly demonstrated in [13]. The work function
difference between Al and FTO which being the substrate for ZnO growth, creates a bias that drives
the reactions. Al will lose electrons to develop a positive charge, and the electron will transfer to the
surface of cathode substrate. Then reduction reaction of dissolved oxygen occur on the cathode
substrate followed by the formation of Zn(OH)2 and dehydration to form ZnO. Fig. 1. shows top-view
and cross-sectional images of an ZnO nanorod arrays taken by a field emission scanning electron
microscope. The SEM images show that well-aligned ZnO nanorods synthesized by a
Galvanic-cell-based method at 70oC for 1 hour are directly grown on the FTO substrate without the
seed layer. The average length of ZnO nanorods is about 1 µm and diameter around 150 nm. At lower
temperature, as grown ZnO nanorods are smaller diameter and lower density distribution. When the
reaction temperature and growth time increases to 90oC and 4 h, the diameter of ZnO increases to 500
nm while the length of nanorods increases significantly.
Fig. 2. (a) FESEM and (b) TEM images of the ZnO/CdS(5) core/shell nanorod after the deposition of five layers of CdS
QDs on ZnO nanorod arrays
Fig. 2. a and 2b are depicted the ZnO/CdS(5) core/shell nanorods taken by a field emission
scanning electron microscope (FESEM) and transmission electron microscopy (TEM)
correspondingly. It is seen that the entire ZnO nanorod core is completely covered layer-by-layer by
CdS quantum dots shell. The CdS layer thickness is estimated about 5.7 ± 0.8 nm and only slight
nanoparticle aggregations on the top. The results in figure 2 indicate that CdS quantum dot thin films
were uniformly deposited on the ZnO nanorods and the thickness of the CdS shell can be controlled
by varying the number of SILAR cycles.
Crystalline Structure Analysis
XRD measurement was carried out to study the crystalline structure of the samples. Fig. 3.a shows the
XRD pattern of ZnO nanorod arrays grown on FTO substrate with majority diffraction peaks of the
hexagonal phase of ZnO (JCPDS: 36-1451) besides some SnO2 peaks corresponding to the substrates
(FTO glass). The XRD pattern of as prepared ZnO/CdS core–shell nanorod arrays in fig. 3.b were not
detected any peaks of CdS. However, the XRD pattern of ZnO/CdS after heat treatment at 300oC for
66 Materials, Machines and Development of Technologies for IndustrialProduction
60 min in Argon ambient (Fig. 4c) demonstrates that the CdS with a hexagonal structure (JCPDS:
65-3414) was successfully observed after SILAR process. The results indicate that formation of very
small CdS quantum dots on the ZnO surface and after calcination of ZnO/CdS sample, the intensity of
ZnO and CdS peaks was significantly stronger than those from curve b, indicating the crystallinity
was improved via the calcination.
20 30 40 50 60 70
* FTO
Zn
O(1
12)
Zn
O(1
03
)
Zn
O(1
10
)
Zn
O(1
02
)
Cd
S(1
10
)
Zn
O(1
01)
Zn
O(1
00
)
Cd
S(1
01
)
Zn
O(0
02
)
Cd
S(1
00
)
**
*
**
*
*
2θθθθ (degree)
Inte
nsi
ty (
a.u
.)
c
b
a
Fig. 3. (a) XRD patterns of ZnO nanorods grown on FTO substrate, (b) as prepared ZnO/CdS core–shell nanorods and (c)
ZnO/CdS core–shell nanorods annealed at 300oC for 60 min in Argon ambient.
Optical Properties of ZnO/CdS Core-shell Nanorod Arrays
To investigate the optical properties of ZnO/CdS core-shell nanorods samples, their UV–vis
absorption spectra were measured within the wavelength range of 300–800 nm. As can be seen in
figure 4a, an absorption edge of the ZnO film at about 400 nm is observed. Comparing with the pure
ZnO film in white color, the color of ZnO/CdS nanocomposite film turned into yellow corresponding
with the red shift to about 550 nm of the ZnO/CdS optical absorption edge. Fig. 4.(b) shows the room
temperature photoluminescence (PL) spectra of ZnO nanorod arrays and different ZnO/CdS
core–shell nanorod arrays samples. ZnO nanorod arrays film exhibits a near band edge emission at
384 nm and a wide peak centered at 555 nm for the deep level emission. The deep level emission has
been attributed to several intraband defects in crystals, such as oxygen and zinc vacancies [7]. The PL
spectra of ZnO/CdS samples show similar emission profiles to those of the pristine ZnO in the
measured range from 380 to 900 nm, which suggests that the remaining luminescence in the
ZnO-CdS samples is due to the radiative decay of the excitons from ZnO. The results imply an
effective electron transfer from excited CdS shell to ZnO nanorod due to the favorable staggered band
alignment between ZnO and CdS. The emissions of CdS nanoparticles are only observed in the
ZnO/CdS (15) samples. The enhancement in the visible luminescence is attributed to the well known
yellow emission from surface CdS originating from its point defects such as cadmium interstitial and
sulfur vacancies.
Fig. 4. UV–vis absorption spectra (a) and Photoluminescence (PL) spectra (b) of ZnO/CdS core–shell nanorod arrays with
different CdS number layers (a) ZnO, (b) ZnO/CdS (5), (c) ZnO/CdS (10) and (d) ZnO/CdS (15).
Photovoltaic Performance of ZnO/CdS Core-shell Nanorod Arrays Fig. 5. shows schematic illustration of the structure of the as-assembled QDSSCs based on ZnO/CdS
core-shell nanorod arrays photoelectrode in the mixture of 1 M Na2S, 1 M S and 0.2 M KCl in a
methanol and water (7 : 3) solution and Pt as counter electrode. Under illumination, all electrodes
showed a photoresponse, in that ZnO/CdS(10) core/shell nanorod arrays exhibited a enhanced
photocurrent density (Jsc = 0.65 mA.cm−2
at a potential of 0 V) as seen in figure 6. This can be
attributed to the improved visible light absorption by the CdS nanocrystals,. In the meanwhile,
ZnO/CdS(15) core/shell nanorod arrays samples show the highest Voc=0.67 V. The scheme of figure
7 depicts electron–hole pair generation by incident photons, electron injection from the excited CdS
nanocrystal shell into the ZnO nanorod core (interconduction band transfer). Holes, on the other hand,
are transferred through the polysulfide electrolyte and collected in the Pt electrode. The 1D-ZnO
nanorod architecture provides a direct pathway for electron transport from ZnO to the FTO substrate.
300 400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 d
c
b
a
Wavelength (nm)
Ab
so
rba
nce (
a.u
.)
ZnO
ZnO/CdS(5)
ZnO/CdS(10)
ZnO/CdS(15)
400 500 600 700 800 900
d
c
b
a
Wavelength (nm)
PL
in
ten
sit
y (
a.u
.)
ZnO
ZnO/CdS(5)
ZnO/CdS(10)
ZnO/CdS(15)
Applied Mechanics and Materials Vol. 618 67
Fig. 5. Schematic illustration of the structure of a quantum dots sensitized solar cell (QDSSC) and a scheme illustrating
the principle of charge transfer processes from CdS quantum dots into a ZnO nanorod
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ZnO/CdS(5)
ZnO/CdS(10)
ZnO/CdS(15)
Cu
rren
t d
ensi
ty (
mA
/cm
2)
Voltage (V)
Fig. 6. J–V curves of the QDSSCs fabricated with the different CdS QDs number layers on nanorod arrays photoanode
Summary
QDSSCs based on ZnO/CdS core–shell nanorod arrays show promising developments for the next
generation of solar cells. ZnO is a good alternative to TiO2 due to their energy band structure and
physical properties and high electronic mobility. In addition, ZnO is easy to form anisotropic
structures at low temparature, which presents unique electronic and optical properties. Furthermore,
ZnO nanorod photoelectrode film is advantageous for the distribution of QDs. However, the
efficiency of ZnO-based QDSSCs is still low, which is likely due to the high surface charge
recombination in ZnO. The high surface charge recombination can be attributed to many defects of
the ZnO surface. Besides, the chemical unstability of ZnO makes it easy for ZnO to react with the
electrolyte. With the recent advances in the using of CdS QDs coverage, we expect an efficient
improvement in developing QDSSCs in the future.
Acknowledgments
This work was supported by National key laboratory for electronic materials and devices, Institute of
Materials Science, Vietnam Academy of Science and Technology.
References
[1] L. Yang, Z.Q. Zhang, Z. Wang, Y. Sun, M. Gao, J. Yang, Y. S. Yan, ZnO nanotubes: Controllable
synthesis and tunable UV emission modulated by the wall thickness, Physica E 54 (2013) 53–58.
[2] B.Weintraub, Y. Deng, Z.L. Wang, Position-controlled seedless growth of ZnO nanorod arrays on
a polymer substrate via wet chemical synthesis, J. Phys. Chem. C 111 (2007) 10162–10165.
[3] W. T. Chiou, W. Y. Wu, J. M. Ting, Growth of single crystal ZnO nanowires using sputter
deposition. Diam. Relat. Mater., 12 (2003) 1841–1844.
[4] S. P. Anthony, J. I. Lee, J. K. Kim, Tuning optical band gap of vertically aligned ZnO nanowire
arrays grown by homoepitaxial electrodeposition. Appl. Phys. Lett. 90 (2007) 103107–103109.
[5] L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, P. Yang, General
route to vertical ZnO nanowire arrays using textured ZnO seeds, Nano Lett. 5 (2005) 1231–1236.
[6] Z. Zheng, Z. S. Lim, Y. Peng, L. You, L. Chen, J. Wang, General Route to ZnO Nanorod Arrays
on Conducting Substrates via Galvanic-cell-based approach, Sci. Rep. 3 (2013) 2434.
[7] Q. Cui, C. Liu, F. Wu, W. Yue, Z. Qiu, H. Zhang, F. Gao, W. Shen, M. Wang, Performance
Improvement in Polymer/ZnO Nanoarray Hybrid Solar Cells by Formation of ZnO/CdS-Core/Shell
Heterostructures, J. Phys. Chem. C 117 (2013) 5626−5637.
68 Materials, Machines and Development of Technologies for IndustrialProduction
Materials, Machines and Development of Technologies for Industrial Production 10.4028/www.scientific.net/AMM.618 Galvanic-Cell-Based Synthesis and Photovoltaic Performance of ZnO-CdS Core-Shell Nanorod Arrays
for Quantum Dots Sensitized Solar Cells 10.4028/www.scientific.net/AMM.618.64
DOI References
[2] B. Weintraub, Y. Deng, Z.L. Wang, Position-controlled seedless growth of ZnO nanorod arrays on a
polymer substrate via wet chemical synthesis, J. Phys. Chem. C 111 (2007) 10162-10165.
http://dx.doi.org/10.1021/jp073806v [5] L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, P. Yang, General route to
vertical ZnO nanowire arrays using textured ZnO seeds, Nano Lett. 5 (2005) 1231-1236.
http://dx.doi.org/10.1021/nl050788p [7] Q. Cui, C. Liu, F. Wu, W. Yue, Z. Qiu, H. Zhang, F. Gao, W. Shen, M. Wang, Performance Improvement
in Polymer/ZnO Nanoarray Hybrid Solar Cells by Formation of ZnO/CdS-Core/Shell Heterostructures, J.
Phys. Chem. C 117 (2013) 5626−5637.
http://dx.doi.org/10.1021/jp312728t