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Electrochimica Acta 107 (2013) 313– 319

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

ffect of water and annealing temperature of anodized TiO2anotubes on hydrogen production in photoelectrochemical cell

ongkun Lia,b, Hongmei Yua,∗, Changkun Zhanga,b, Wei Songa, Guangfu Lia,b,higang Shaoa, Baolian Yia

Laboratory of Fuel Cells, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, ChinaGraduate University of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

rticle history:eceived 26 January 2013eceived in revised form 14 May 2013ccepted 19 May 2013vailable online xxx

eywords:

a b s t r a c t

An efficient and economical technology for hydrogen production via solar water splitting in a newlydesigned photoelectrochemical (PEC) cell is reported. The core of the PEC cell is a membrane electrodeassembly (MEA) that consisted of a TiO2 nanotube photoanode, a Pt/C cathode and alkaline membrane.The TiO2 nanotube arrays (NTs) were prepared by electrochemical anodization of titanium mesh in amixed electrolyte solution of glycol and NH4F and then calcined at different temperature to transformthe amorphous structure into crystalline. Effect of water content on the morphology of the TiO2NTs is

hotoelectrochemical celliO2 nanotubesorphology

nnealing treatmentydrogen production.

investigated, and the optimal amount of water in the electrolyte is between 10 wt% and 80 wt%. Emphasiswas the effect of the annealing temperature of anodized TiO2NTs on the hydrogen production in thePEC cell. The results indicate that the crystal phase and morphology of TiO2NTs are stable at 450 ◦C,which exhibits the best photocatalytic activity. Photocurrent generation of 1.55 mA/cm2 under UV-lightirradiation under applied bias (0.6 V vs. Normal Hydrogen Electrode, NHE) shows good performance onhydrogen production.

. Introduction

Photoelectrochemical splitting of water into hydrogen andxygen by the direct use of sunlight is an ideal, renewable methodf hydrogen production. Since the first report of solar-drivenhotoelectrochemical energy conversion in 1972 by Fujishima andonda, TiO2 has been widely investigated as the most promisingaterial for photocatalytic applications due to its high photocat-

lytic activity and photochemical stability [1–7]. Compared withther TiO2 films prepared by chemical vapor deposition (CVD)8,9], liquid-phase deposition (LPD) [10,11], sol–gel synthesis [12]nd magnetron sputtering deposition methods [13], the electro-hemical anodization method is regarded as one of the relativelyimple techniques to synthesize TiO2 nanotubes with large surfacerea [14–16]. Moreover, the one-dimensional and highly orderedanotube architecture offers an excellent electrical channel forectorial charge transfer so that photoinduced electron-holes pairsan be effectively separated, resulting in an obvious improvement

n the photoelectochemical performance. And the uniformly stabletructures show unique physical and chemical properties becausehe TiO2NTs have much more free spaces in their interior as well

∗ Corresponding author. Tel.: +86 411 84379051; fax: +86 411 84379185.E-mail address: hmyu@dicp.ac.cn (H. Yu).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.05.090

© 2013 Elsevier Ltd. All rights reserved.

as outer space that can be filled with active materials whichgive them an essential predominance over the TiO2 powders. Butthe TiO2NTs can be easily distorted during the electrochemicalanodization, which would affect their photoelectron properties. Soit is important to synthesize highly ordered TiO2NTs with tunablemorphology, which possess good uniformity and conformability,and has good performance.

Besides the photoelectrode, the PECs’ structure is also an impor-tant factor for hydrogen generation in the photoelectrolysis ofwater. Polymer membrane electrode assembly consisting of a P-25TiO2 photoanode, a Pt cathode, and a proton exchange membranefor the hydrogen production is reported [17–19], while the pho-tocatalyst casted onto the carbon cloth or carbon paper will bepeeled easily after long time run. Besides, proton exchange mem-brane is not a good choice for the PEC cell due to ion (Na+, K+)exchange when using alkaline solution as the electrolyte, whereOH− can scavenge the holes better than pristine water and enhancethe electron-holes separation. In our early report [20], we designeda PEC cell that based the asbestos as the separator for the evolvedgases, but the asbestos is harmful to human beings and envi-ronmental unfriendly that can’t be used widely. Therefore, it is

necessary to choose a suitable membrane that can be substitutefor the asbestos in the PEC cell.

In this present study, a newly designed PEC cell based on mem-brane electrode assembly with two-compartment is proposed and

dx.doi.org/10.1016/j.electacta.2013.05.090http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactahttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.electacta.2013.05.090&domain=pdfmailto:hmyu@dicp.ac.cndx.doi.org/10.1016/j.electacta.2013.05.090

314 Y. Li et al. / Electrochimica Acta 107 (2013) 313– 319

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Fig. 1. (a) Schematic diagram of the membrane electrode assem

nvestigated. The core of the PEC cell is an MEA incorporated withlkaline anion exchange membrane as the membrane and TiO2/Tiesh as the anode and Pt/C casted onto carbon paper as the cath-

de. The MEA serves as a compact but effective reactor for waterplitting as well as an effective gas separator. The water content onhe morphology of anodized TiO2NTs is investigated and discussed.he smoothness of the top surface and the high orderliness of theTs presented the improvement of PEC performance. Besides, the

mpact of annealing temperature on the hydrogen production ofiO2NTs in the proposed PEC cell is investigated in detail.

. Experimental

.1. Preparation of TiO2 composite NTs

Prior to the anodization, titanium meshes (99.7% purity) wereltrasonically cleaned with acetone followed by deionized waterinse. All the anodization was carried out in a two-electrode elec-rochemical cell with titanium mesh as the anode and platinum foils the cathode under a constant voltage 30 V at room temperature.he distance between the two electrodes is kept at 4 cm in all exper-ments. A direct current power supply (HY 1791-20S, YaGuanglectronics Co. Ltd.) was used as the voltage source to drive thenodization. The Ti foil was anodized in the electrolyte containing

mixture of ethylene glycol, H2O and NH4F. In order to achieveifferent morphologies of nanotubular array layers, the water con-ent in the electrolyte ranged from 0 wt% to 100 wt% kept constanturing the 6 h anodization. After electrochemical anodization, thes-anodized TiO2NTs were properly rinsed with deionized water toemove the occluded ions, and then dried in an air oven. Crystal-ized samples were obtained by annealing treatment as-anodizedmorphous at various temperatures (350–750 ◦C) each for 2 h.

.2. Morphology and crystal characterization

The morphology of the TiO2NTs was obtained by a field emis-ion scanning electron microscope (FESEM; Hitachi, S-4800) withn acceleration voltage 5.0 kV. The crystal phases were analyzed bysing an X’ pert pro (PAN-analytical) with a Cu K� tube.

.3. MEA fabrication and characterization

MEA was fabricated with the process similar to our previous

ork [20]. Fig. 1a portrays the schematic diagram of the membrane

lectrode assembly. Toray carbon paper coated with Pt/C particlesPt loading of 0.4 mg/cm2) is used as the cathode. Ti meshes withiO2NTs annealed at various temperatures served as the anodes.

nd (b) the photoelectrochemical cell for hydrogen generation.

A201 membrane (thickness 28 �m; Tokuyama) without any pre-treatment was used as the anion exchange membrane. With theanode (TiO2NTs) and the cathode (Pt/C carbon paper) at two sidesof the A201 membrane, the membrane electrode assembly waspressed between two stainless steel flow-field plates (Fig. 1b).There was parallel flow at the cathode side, and open chamberwith a 24 cm2 quartz window at the anode side, which allowedthe photocatalyst surface exposed to the light. Rubber rings werepositioned in the cell as the sealing.

The photoelectrochemical water splitting measurement wascarried out in our designed PEC cell using an electrochemical work-ing station (PARStat 2273, Princeton). The PEC cell was describedas Fig. 1b that the MEA was pressed between two stainless steelflow-field plates. A 300 W Xenon lamp served as the light sourceand optical filter kept the UV-light (365 ± 15 nm) intensity at100 mW/cm2. A peristaltic pump circulated 1 mol/L KOH solutionfrom a reservoir to the reactor chamber. The product gas was ana-lyzed by a gas chromatography (GC-14C, Shimadzu).

3. Results and discussion

3.1. Characterization of the TiO2 NTs

Fig. 2 shows the FESEM image of the self-organized, highlyordered titania nanotube arrays prepared by the electrochemicalanodization in an electrolytic solution consisting of different watercontents and 0.5 wt% NH4F in ethylene glycol.

As shown in the Fig. 2, the morphology of the anodized nano-tubes changed much with the increasing of water content in theelectrolyte at 30 V for 6 h. The structure of the 10 wt% water con-tent sample (Fig. 2c) is highly ordered with circular shape, whereas,when decreasing the water content to 2 wt% and keeping con-centration of F− constant and other conditions unchanged, thenanowires/belt appear on the top of the nanotube arrays (Fig. 2b),especially when the water content was nearly to 0% (Fig. 2a) thatthe nanowires twist with each other like a mass of cotton. The twistcan be easily peeled off from the Ti foil when dried with flow dryer.Therefore, it can be used as a technique for pretreated Ti foil toobtain a clean surface. By increasing the water content to 80 wt%and above (Fig. 2e and f) in the electrolyte, the nanowire/belt struc-ture does not appear and the preexisted TiO2NTs structure changesto TiO2 nanofoam. There are a lot of caves in the wall of the nano-tubes that made the TiO2NTs like a sponge. The TiO2 nanofoam

is strong enough that its structure changed little during the heattreatment at 450 ◦C.

On the basis of the above observations, the growth mechanismcan be deduced as follows. Under a high applied potential and the

Y. Li et al. / Electrochimica Acta 107 (2013) 313– 319 315

Fig. 2. FESEM images of TiO2NTs prepared by electrochemical anodization in (0 wt%, a; 2 wt%, b; 10 wt%, c; 40 wt%, d; 80 wt%, e; 100 wt%, f;) water + 0.5 wt% NH4F in ethyleneg

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lycol at 30 V for 6 h.

resence of water molecules in the electrolyte, Ti was oxidized toorm a thin barrier oxide layer of TiO2 [11,21].

H2O → O2 + 4e + 4H+(field assisted) (1)

i + O2 → TiO2(field assisted) (2)he presence of the fluoride ion, F−, and the high applied potentialed to TiO2 substance dissolution.

iO2 + 6F−+ 4H+ → TiF62− + 2H2O(field and chemical dissolution)(3)

Individual TiO2NTs formed on the Ti surface for the combinedrocesses in the above three reactions. In the first and two-step,he electric field and solution diffusion rate in the Ti surface wereot consistent, which caused the NTs formed with random spatialistribution in the initial growth process (Fig. 3a). Despite of theccurrence of the autocatalytic phenomenon, only the NTs under

ptimal starting conditions were allowed to grow continuously,nd the random growth negatively influenced the formation ofrdered NTs. In the third step, the Ti surface morphology exhib-ted a regular structure, which resulted in a uniform electric field

and an even solution diffusion rate. The smoothness and order-liness of the NTs were significantly improved in this process. Inaddition, the length of the NTs did not increase because of theequilibrium between the electrochemical etching and chemical dis-solution, which limited the formation of long NTs. From the Fig. 3b,we can seen that the inside diameter of the nanotube changed muchfrom the top to the bottom, the diameter at the top was more thanthe bottom and the corresponding tube wall thickness decreased alot. Fig. 3c shows the possible reason of the above three structures,and the nanograss forming process has been discussed in our earlyreport [20]. F− transfers faster in water than in ethylene glycol, andthere are more F− on the surface of the TiO2 arrays in water thanthat in ethylene glycol, which leads to the faster corrosive speed.Compared with the effect of the nanotube wall nonuniformity, theformation of nanograss’s structure is more sensitive to the watercontent in the electrolyte. When the water content was 80 wt%and above, the F− enter into the inside of the NTs easily and theF− assaults the weak point of the NTs intensively, that the first-

formed NTs were corroded completely and further F− etch leadingthe lower part of the NTs to turn into the porous structure. Theheight of the nanofoam was less than 500 nm after 6 h anodizationthat confirmed the above viewpoint.

316 Y. Li et al. / Electrochimica Acta 107 (2013) 313– 319

F l anodo 2 nanF r + 0.5

pifiaawtetfcta(na

Fi

ig. 3. (a) FESEM image of bottom view of the TiO2NTs prepared by electrochemicaf arrangement could be observed (see text for details); (b) the TEM of a single TiOESEM images of TiO2NTs prepared by electrochemical anodization in 10 wt% wate

Fig. 4 shows the morphology of the titania nanotubular arraysrepared in 10 wt% water electrolyte and changed with the anodiz-

ng potential and time. When we decrease the anodizing potentialrom 30 V to 20 V, the nanograss begins to appear when the anodiz-ng time last for 10 h. Similar phenomena were repeated when thenodization were operated at 50 V for 4 h and 60 V for 2 h (Fig. 3dnd e). It indicates that the nanograss was more easily to come outhen increasing the anodizing potential in the same electrolyte. As

he anodizing potential increasing, the time to the electrochemicaltching and chemical dissolution equilibrium would be shortenedhat the nanograss was more easily to come out. It is also observedrom the Fig. 3b that the pore diameter and the tube wall thicknesshanges much with anodization time. The pore diameter (d) ofhe TiO2 nanotubes increases from 60 nm to 100 nm with thenodization time increasing that the d2 much more than the d1

Fig. 4). On the other hand, the tube wall thicknesses of the titaniaanotubes decreases from 62 nm to 27 nm with an increase innodization time, which is easy for the nanograss forming. The

ig. 4. Morphology of the TiO2NTs changed with the anodizing potential and timen 10 wt% water electrolyte.

ization in 10 wt% water + 0.5 wt% NH4F in ethylene glycol at 60 V for 1 h, four typesotube; (c) the growth mechanism of different morphologies of TiO2NRs; (d and e)

wt%NH4F in ethylene glycol at (d) 50 V for 4 h and (e) 60 V for 2 h.

above discussion shows that this process is effective to producehighly ordered TiO2NTs with tunable morphology.

3.2. Photoelectrochemical characterization

In order to avoid the nanowire and nanofoam appearance, allthe samples were prepared in the electrolytes with 0.5 wt% NH4Fand 10 wt% H2O in an ethylene glycol solution. Fig. 5a–d shows theFESEM images of the as-anodized highly ordered TiO2NTs. It canbe seen that the structure of the NTs is highly ordered with cir-cular shape, and the length of the NTs varies with the anodizationtime, which increases as the anodization time changes from 1 hto 4 h. The nanotube arrays prepared at 30 V for 4 h are selectedfor the further investigation. Morphologies of the TiO2NTs samplesannealed from 350 ◦C to 650 ◦C are shown in Fig. 5e–h. The figuresshow clearly that the morphology of the TiO2NTs structure is sig-nificantly affected by the annealing temperature. There is no greatchange in the pore diameter and tube length after annealing for 2 hbelow 450 ◦C (Fig. 5e and f). However, the pore diameter changesobviously after the annealing above 550 ◦C, especially the morphol-ogy of the NTs at top. When the annealing temperature is 650 ◦C, thetop of the nanotube arrays collapse completely and grown togetherthat formed TiO2 nanoparticles (Fig. 5g and h). The nanoparticlesaggregate at the surface of the NTs and block the light entering.

Fig. 6 shows the XRD patterns of as-anodized and the annealedsamples at different temperatures from 350 ◦C to 750 ◦C, in whichamorphous regions were gradually crystallized to form anatase andfinally rutile phases. As shown in the figure, the XRD patterns of theas-anodized TiO2NTs before annealing treatment shows only theamorphous structure in nature, which indicates that as-anodizedTiO2 is amorphous. The sample annealed at 450 ◦C shows two obvi-ous peaks at (25.1) and (48.0), corresponding to anatase TiO2 (101)and (200). The surface of the sample changed from khaki to blue.However, a signal diffraction peak at (27.6) of rutile (110) beginsto appear with annealing temperature at 550 ◦C, indicating thatthe anatase phase starts to transform into rutile phase during theprocess of heat treatment, and the sample appears gray. With fur-

ther increase in temperature up to 750 ◦C, the intensity of rutileincreases sharply while the intensity of anatase diminishes. It indi-cates that the anatase has transformed into rutile completely athigh temperature.

Y. Li et al. / Electrochimica Acta 107 (2013) 313– 319 317

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ig. 5. FESEM images of TiO2NTs prepared by electrochemical anodization in 2 wt%mages (the top surface views) of TiO2NTs prepared by electrochemical anodization50 ◦C.

The photocatalytic activity of TiO2NTs was evaluated by measur-ng the photocurrent under illumination with an external appliedoltage 0.6 V. The applied potential can assist the electron trans-erring from TiO2 to Pt particles rapidly through the outsideircuits, as well as reducing the recombination of electrons andoles.

The transient photocurrent response during the light on-offycles of the PEC cell shown in Fig. 7a indicates that the PECell has good photo-electricity properties. It demonstrates that

iO2NTs can be effectively applied for hydrogen production in theEC cell. The anode with TiO2NTs annealed at 450 ◦C shows theighest photocurrent density, which has more charge carrier underV-light irradiation. This result can be attributed to the excellent

r + 0.5 wt% NH4F in ethylene glycol at 30 V for (a) 1 h, (b) 2 h, (c) 3 h, (d) 4 h; FESEMV for 4 h and annealed at different temperatures (e) 350 ◦C, (f) 450 ◦C, (g) 550 ◦C, (h)

crystallization and the well-ordered nanotubular structures, whichare beneficial for vectorial charge transfer at TiO2/electrolyte inter-faces. The efficiency of photon-to-hydrogen generation (�) shouldbe the ratio of the power used in water splitting to the input lightpower, which can be calculated using the equation [3].

� = I0(1.23 − Vbias)Jlight

where I0 is the photocurrent density (A/m2), 1.23 V is the theoret-ical potential required for water splitting, Vbias (V) is the appliedexternal potential, Jlight (W/m2) is the light irradiance, and A isthe irradiated area (m2). With the equation, � of the photoanode

318 Y. Li et al. / Electrochimica Act

Fig. 6. XRD pattern of the TiO2NTs prepared at 30 V for 4 h and annealed at differenttemperatures.

Fig. 7. (a) Potentiostatic (I-t) curve obtained from the PEC cell containing TiO2nanotube as an anode and Pt/C carbon paper as a cathode at a potential of 0.6 Vvs. NHE; (b) Time-dependent photocurrent density of the PEC cell under UV-lightillumination, measured in 1 M KOH solution.

a 107 (2013) 313– 319

using UV spectrum can be calculated as 1.13% (at +0.6 V vs. NHE,I0=1.8 mA/cm2, Jlight=100 mW/cm2).

A 4 h experiment is performed under UV-light illumination withan external bias potential of 0.6 V for the PEC cell for hydrogenproduction. Deaerated KOH solution (1 mol/L) is cycled by a pumpwith constant flow rate through both sides of the compartmentsin the PEC cell. The trapped gas is analyzed by gas chromatog-raphy and proves the gas is hydrogen. The photocurrent keepsconstant during the entire 4 h test (Fig. 7b). It indicates that theTiO2/Ti mesh electrode is stable to withstand the mechanical stressthat shows no loss of photoactivity. The PEC cell with photoactivearea 2.8 cm2 generates nearly 5.6 ml hydrogen and 2.6 ml oxygenduring the 4 h experiment under the steady-state condition, andthe molar ratio of the evolved H2/O2 is about 2:1. The hydrogenevolution rate of 0.57 ml/h cm2 can be detected at the average cur-rent of 1.55 mA/cm2, which is 76% of the theoretic rate. The smalldiscrepancy between the theoretical and observed yield probablyoriginates from the H2 collecting.

4. Conclusions

In the present work, we showed that the water content inthe electrolyte had a significant effect on the morphology of self-organized TiO2 nanotubes. To achieve highly smooth and orderedTiO2NTs, optimal water content in the electrolyte was necessary.Using the highly ordered TiO2NTs as the photoanode, alkalinemembrane as the anion exchange membrane, Pt/C carbon paper asthe cathode to form MEA can direct photoelectrolyse water underUV-light illumination. The TiO2NTs annealed under 450 ◦C exhibitthe highest photocurrent density, which is about 1.55 mA/cm2 witha H2 evolution rate up to 0.57 ml/h cm2. Most important, the alka-line membrane used in this PEC cell is feasible to separate theevolved gases, and the OH− transferred easily through it which candecrease the ohmic loss existed in ion transmit. Compared with thePEC cell based on the asbestos, the � of the new designed PEC cellis equivalent, while the anion exchange membrane is much betterthan the asbestos due to its environmental friendly and its com-pactness. Pt/C at carbon paper as the cathode rather than Pt foilminimized the precious catalyst loading.

Acknowledgments

This work was financially supported by the National NaturalScience Foundations of China (No.21176234/B0609).

References

[1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting,Chemical Society Reviews 38 (2009) 253.

[2] T.T. Isimjan, S. Rohani, A.K. Ray, Photoelectrochemical water splitting for hydro-gen generation on highly ordered TiO2 nanotubes fabricated by using Ti ascathode, International Journal of Hydrogen Energy 37 (2012) 103.

[3] R. Marschall, C. Klaysom, A. Mukherji, M. Wark, G. Lu, L. Wang, Compositeproton-conducting polymer membranes for clean hydrogen production withsolar light in a simple photoelectrochemical compartment cell, InternationalJournal of Hydrogen Energy 37 (2012) 4012.

[4] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductorelectrode, Nature 238 (1972) 37.

[5] C.L. Tseng, C.J. Tseng, K.Y. Cheng, L.W. Hourng, J.C. Chen, L.C. Weng, S.K. Wu,Numerical analysis of the solar reactor design for a photoelectrochemicalhydrogen production system, International Journal of Hydrogen Energy 37(2012) 13053.

[6] N. Naseri, P. Sangpour, A.Z. Moshfegh, Visible light active Au:TiO2 nanocom-posite photoanodes for water splitting: Sol-gel vs. Sputtering, ElectrochimicaActa 56 (2011) 1150.

[7] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splittingby a chemically modified n-TiO2, Science 297 (2002) 2243.

[8] J.H. Jung, H. Kobayashi, K.J.C. van Bommel, S. Shinkai, T. Shimizu, Creation ofnovel helical ribbon and double-layered nanotube TiO2 structures using anorganogel template, Chemistry of Materials 14 (2002) 1445.

http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0005http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0010http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0015http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0020http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0025http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0030http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.com/S0013-4686(13)01013-X/sbref0035http://refhub.elsevier.c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Y. Li et al. / Electrochimi

[9] M. Matsuoka, M. Kitano, M. Takeuchi, M. Anpo, J.M. Thomas, Photocatalyticwater splitting on visible light-responsive TiO2 thin films prepared by aRF magnetron sputtering deposition method, Topics in Catalysis 35 (2005)305.

10] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with highaspect ratios for efficient solar water splitting, Nano Letters 6 (2006) 24.

11] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, A novel method for thesynthesis of titania nanotubes using sonoelectrochemical method and its appli-cation for photoelectrochemical splitting of water, Journal of Catalysis 246(2007) 362.

12] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, Design of a highly efficientphotoelectrolytic cell for hydrogen generation by water splitting: Applicationof TiO(2−x)C(x) nanotubes as a photoanode and Pt/TiO2 nanotubes as a cathode,Journal of Physical Chemistry. C 111 (2007) 8677.

13] P. Roy, C. Das, K. Lee, R. Hahn, T. Ruff, M. Moll, P. Schmuki, Oxide nanotubeson Ti Ru alloys: strongly enhanced and stable photoelectrochemical activ-

ity for water splitting, Journal of the American Chemical Society 113 (2011)5629.

14] Z. Liu, W. Hou, P. Pavaskar, M. Aykol, S.B. Cronin, Plasmon resonant enhance-ment of photocatalytic water splitting under visible illumination, Nano Letters11 (2011) 1111.

[

a 107 (2013) 313– 319 319

15] Z. Zhang, M.F. Hossain, T. Takahashi, Photoelectrochemical water splitting onhighly smooth and ordered TiO2 nanotube arrays for hydrogen generation,International Journal of Hydrogen Energy 35 (2010) 8528.

16] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Enhanced pho-tocleavage of water using titania nanotube arrays, Nano Letters 5 (2005)191.

17] B. Seger, P.V. Kamat, Fuel cell geared in reverse: photocatalytic hydrogen pro-duction using a TiO2/Nafion/Pt membrane assembly with no applied bias,Journal of Physical Chemistry. C 113 (2009) 18946.

18] L.J. Minggu, W.R.W. Daud, M.B. Kassim, An overview of photocells and pho-toreactors for photoelectrochemical water splitting, International Journal ofHydrogen Energy 35 (2010) 5233.

19] K.T. Jeng, Y.C. Liu, Y.F. Leu, Y.Z. Zeng, J.C. Chung, T.Y. Wei, Membrane electrodeassembly-based photoelectrochemical cell for hydrogen generation, Interna-tional Journal of Hydrogen Energy 35 (2010) 10890.

20] Y. Li, H. Yu, W. Song, G. Li, B. Yi, Z. Shao, A novel photoelectrochemical cell

with self-organized TiO2 nanotubes as photoanodes for hydrogen generation,International Journal of Hydrogen Energy 36 (2011) 14374.

21] J. Gong, Y. Lai, C. Lin, Electrochemically multi-anodized TiO2 nanotube arraysfor enhancing hydrogen generation by photoelectrocatalytic water splitting,Electrochimica Acta 55 (2010) 4776.

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Effect of water and annealing temperature of anodized TiO2 nanotubes on hydrogen production in photoelectrochemical cell1 Introduction2 Experimental2.1 Preparation of TiO2 composite NTs2.2 Morphology and crystal characterization2.3 MEA fabrication and characterization

3 Results and discussion3.1 Characterization of the TiO2 NTs3.2 Photoelectrochemical characterization

4 ConclusionsAcknowledgmentsReferences