Synthesis and Structure of Titania Nanotubes for Hydrogen Generation

Sangworn Wantawee1,a,Pacharee Krongkitsiri2,b, Tippawan Saipin3,c, Buagun Samran1,d and Udom Tipparach1,*e

1Department of Physics, UbonRatchathani University, UbonRatchathani 34190 Thailand

2Department Department of Science and Mathematics, Faculty of Industry and Technology, Rajamangala University of Technology Isan, Phangkon, Sakonnakon, 47160, Thailand

3Department of Arts and Sciences, UbonRatchathani Technical College, Mueang District,UbonRatchathani Province 34000 Thailand

asangworn77@hotmail.com,bpacharee_kr@hotmail.com, ctipss_2@hotmail.comdbuagun@hotmail.com, e*udomt@hotmail.com

Keywords:Titania nanotubes, TiO2, anodization, Hydrogen generation, Renewable energy

Abstract.Titania nanotubes (TiO2 NTs) working electrodes for hydrogen production by

photoelectrocatalytic water splitting were synthesized by means of anodization method. The

electrolytes were the mixtures of oxalic acid (H2C2O4), ammonium fluoride (NH4F), and sodium

sulphate(VI) (Na2SO4)with different pHs. A constant dc power supply at 20 V was used as anodic

voltage. The samples were annealed at 450 °C for 2 hrs. Scanning Electron Microscopy (SEM) and

X-ray diffraction (XRD) were used to characterized TiO2 NTs microstructure. TiO2 NTs with

diameter of 100 nm were obtained when pH 3 electrolyte consisting of 0.08 M oxalic acid, 0.5 wt%

NH4F, and 1.0 wt% Na2SO4 was used. Without external applied potential, the maximum photocurrent

density was 2.8 mA/cm2 under illumination of 100 mW/cm

2. Hydrogen was generated at an overall

photoconversion efficiency of 3.4 %.

Introduction

Conversion of sunlight to chemical energy in the form of hydrogen is a target of scientific and

technological interests which solutions may be feasible importance as a renewable source of

sustainable and environmentally energy for next generations. Photoelectrolysis of water to produce

hydrogen (H2) has attracted attention due to the depletion of fossil fuels and global warming. The

photocatalytic evolution of H2 and O2 from wateroccurs when a photocatalytic agent with appropriate

band gap is illuminated by sufficiently energetic light. Honda and Fujishima [1] demonstrated the

process of electrochemical photolysis of water using semiconductor TiO2 as a working anode.Since

then, a large number of semiconductor materials have been employed for photoanodes, also called

working anodes for hydrogen production. All of semiconductor materials, TiO2 and modified

structure TiO2 are widely used because they are highly photocatalytic, stable, abundant, and

environmentally safe [2].

The scientific principles for the hydrogen generation were established byBockris and Uosaki in

Bockris and Uosaki in 1976 [3], Bockris in 1980 [4], Bockris in 2003 [5], Bockris et al. in 1981 [6],

Gerischer in 1977 [7], and Chandra in 1985 [8] and reviewed by numerous literatures e.g., by

Linsebigler et al.in 1995 [9],Bak et al. in 2002 [10], and Nowotny et al. in 2007 [11]. Recently, most

of efforts have been focused on nanostructuredtitania such as nanoparticles [12], nanoflims [13], and

nanotubes [14-15] to increase water photo-splitting efficiency due to their high surface-to-volume

ratios and size-dependent properties in hoping to increase energy conversion efficiecy.

Nanostructured Titania may be synthesized by many methods [12-16] such as sol-gel, and

anodization. Anodization has been one of the most popular in making Titania nanotubes (TiO2 NTs).

We have recently prepared mesoporous of TiO2 electrodesby anodization [17]. It is believed that

ability to control the structures of TiO2 NTscan be expected to positively impact a variety of

importanttechnologies and the structures of TiO2 NTs depend largely on many preparation

Advanced Materials Research Vol. 741 (2013) pp 84-89Online available since 2013/Aug/30 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.741.84

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parameters such as temperature, electrolytes, and anodic voltages. In this paper, we present the effect

of electrolytes on the structures and photoelectrochemicalproperties of TiO2 NTs for hydrogen

production.

Experimental

TiO2 NTs were grown by anodization method at room temperature. Titanium sheet with 0.25 mm

thick, 99.7% purity purchased from Sigma Aldrich were firstly polished by various abrasive papers.

After polishing, polished Ti substrates were ultrasonically cleaned in the mixture of acetone and

ethanol. The mixed electrolytes consisting of 1/12 M C2O4H2�2H2O, 0.5wt% NH4F, and 1.0wt%

Na2SO4 with different pH values (3, 5, and 7) were used. The pH of the mixed electrolytes were

adjusted by addingappropriate amount of 1 M H2SO4, or 1 M NaOH.Titanium substrate samples were

anodized usinghome-made housing for the substrate. The system consists of a two-electrode

configuration with apiece of highly pure platinum counter electrode. The configuration of the set up

is shown in Fig. 1. This set up allows only one face of titaniun substrates contact with the electrolyte.

The anodization process was carried on under a constant dc potential 20 V for 2 hrs. Then, anodized

Ti substrates were rained by distilled water and dried in the flow of N2gas. All substrates were

annealed at 450 ˚C for 2 hrs to obtain anatase crystallinephases of TiO2 [13-16,]. The current and

anodization time data were collected computer-controlled apparatus equipped by LabVIEW

programming.To investigate the surface morphology and microstructure of TiO2NTs, all samples

were characterized by SEM and XRD techniques. The determine optical bandgap and

photocatalytic activity for water splitting of TiO2NTs the sample were characterized by UV-Vis and

I-V curve measurement.

Figure 1. Experimental equipment diagram of one-faceanodization for TiO2NTs

Advanced Materials Research Vol. 741 85

Results and Discussion

Fig. 1 shows XRD patterns of Ti sheet and post annealed TiO2 NTs. The XRD patterns show that the

phases of TiO2 NTs are anatase. The anatase (101) peak shows prominently when the anodization

Figure 2. XRD patterns of post annealed TiO2 NTs grown in the electrolytes with different pHs

Figure 3. SEM images of TiO2 NTs grown in the different pHelectrolytes (a) pH 5 and (b) pH 3.

0

500

1000

1500

2000

2500

20 30 40 50 60 70

Inte

nsi

ty (

A.U

.)

2Θ

Ti metal foil

TiO2 NTs pH 3

TiO2 NTs pH 7

TiO2 NTs pH 5A

(101)

A (

101)

A (

101

)

86 Materials and Nanotechnology in Engineering

Figure 4. Photocurrent densities of working electrodes made by TiO2 NTs grown in the different pH

electrolytes compared with the electrode made by Ti0.975 Fe0.025 O2sol-gel dip-coating film.

Figure 5. Photoconversion curves of working electrodes made by TiO2 NTs grown in the different

pH electrolytes compared with the electrode made by Ti0.975 Fe0.025 O2sol-gel dip-coating film.

was carried out in the electrolyte with pH 3. The SEM images of post-annealed TiO2 NTs grown in

the different pH electrolytes. The morphology of the TiO2 was found to be influenced by pH of the

anodizing electrolyte. The prepared samples with the anodization in pH 5 and pH 7 electrolytes were

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

pH7

pH5pH3

Fe 2.5 %

Pho

tocu

rren

t d

ensi

ty (

mA

/cm

2)

Applied Potential (V)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

pH7

pH5pH3Fe 2.5 %

Photo

con

ver

sion E

ffic

ien

cy (

%)

Applied Potential (V)

Advanced Materials Research Vol. 741 87

the titania films with anatase phase. The TiO2 NTs were not observed while the samples with

anodization in pH 3 electrolyte showed clearly titania nanotubes (TiO2 NTs) with diameters of about

100 nm.

Photoconversionefficiency of TiO2 NTs was examined by a three-electrode cell at room

temperature under the illuminationof100 mW/cm2in a 1 M KOH solution. The photocurrent density

and applied potential (J-V) curves were shown in Fig. 4. The photoconversion efficiency of water

electrolysis was calculated based on the following relations [17-18],

η�%� = J� × �E� �� − �E�����

I�× 100%,

whereJph is the photocurrent density in mA/cm2,E� �� is the standard reversible potential which is 1.23

V, E��� is the applied potential obtained from equation, E��� =E� �� − E���, Emeasis the electrode

potential (vs. a reference electrode) of photoanode at which photocurrent is measured, Eaoc is the

electrode potential (vs. a reference electrode) of the same photoanode at open-circuit condition under

the same illumination and the sane electrolyte solution. I0 is the intensity of incident light of100

mW/cm2. The variation of the photoconversion efficiency curves as a function applied potential are

shown in Fig. 5. A maximum photoconversionefficiency of 3.4 % was obtained for TiO2 NTs

anodized in the electrolyte with pH 3. This photoconversion efficiency (3.4 %) of TiO2 NTs

photoanode is larger than that (0.8%) of nano-TiO2 film photoanode made by sol-gel dip coating [13,

19]. The increase of photoconversion efficiency may be resulted from the fact that the surface areas of

TiO2 NTs are larger than those ofnano-TiO2 films due to the agglomeration of the small spherical

crystals of nanoTiO2 crystals by nature [13]. Another reason may be due to the reduction of TiO2 NTs

optical bandgapenergy because nitrogen from NH4F used for making the electrolyte may incorporate

the structure of TiO2 NTs. It is well known that N-doped TiO2 lowers the bandgap energy of TiO2[20,

21] leading to increase the utilization of sunlight spectrums and also increase photoactivity. This may

be possible to identify the existence of nitrogen in the structure of TiO2 NTs by X-ray photoelectron

spectroscopy (XPS).

Summary

TiO2 NTs photoanodeshave been synthesized on titanium foils by anodization method in the mixtures

of oxalic acid (H2C2O4), ammonium fluoride (NH4F), and sodium sulphate(VI) (Na2SO4) with

different pHs. The post anodization process was carried out by anneaing in air at 450 °C for 2 hrs. The

best condition for growing TiO2 NTs was the electrolyte with pH 3. In this condition, the diameter of

prepared TiO2 NTs was about 100 nm. The phase of prepared TiO2 NTs was predominantly anatase.

The hydrogen production occurred with overall the photoconversion efficiency of 3.4 %.

Acknowledgements

The authors thank National Research Council of Thailand (NRCT) and Thailand Research Fund

(TRF)for funding. Sangworn Wantaweeand BuagunSamran would like to thank Faculty of Science

UbonRatchathani University for partially financial supports. One of the authors,

PachareeKrongkitsiri, would like to thank Department of Science and Mathematics, Faculty of

Industry and Technology, Rajamangala University of Technology Isan,Phangkon,

SakonnakonProvince for financial support and scholarship.

88 Materials and Nanotechnology in Engineering

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