rXXXX American Chemical Society A dx.doi.org/10.1021/jp1116118 | J. Phys. Chem. C XXXX, XXX, 000–000 ARTICLE pubs.acs.org/JPCC Effect of Annealing Temperature on the Hydrogen Production of TiO 2 Nanotube Arrays in a Two-Compartment Photoelectrochemical Cell Yan Sun, † Kangping Yan,* ,† Guixin Wang, † Wei Guo, ‡ and Tingli Ma* ,‡ † School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China 1. INTRODUCTION Due to the depletion and pollution of fossil fuel, extensive studies have been carried out on clean and renewable energy. Therefore, hydrogen obtained from renewable sources such as sunlight has attracted many researchers’ attention. Since Fujishima and Honda 1 have demonstrated that water could be decomposed into H 2 and O 2 on a TiO 2 electrode in a photoelectrochemical (PEC) cell, photocatalytic splitting of water using semiconductor has been regarded as an ideal method for converting solar energy into hydrogen energy. TiO 2 has become the most promising material for photocatalytic applications due to its low cost, nontoxicity, and photostability. In fact, the powder suspension system has been employed intensively for water splitting. 2À4 However, several problems have limited the practical application of powder photocatalysts. (1) It is difficult for the separation and recovery of photocatalysts in the aqueous solution. (2) The photogenerated electrons (e À ) and holes (h þ ) easily recombined in the suspen- sion. The rate of recombination could be restrained by loading Pt on the catalyst surface or adding various sacrificial agents such as methanol, ethanol, and formaldehyde, 5 which apparently raise the cost of hydrogen production. (3) The system yields a mixture of H 2 and O 2 for the proximity of the redox sites. 6 Thus, the immobilization of TiO 2 photocatalysts in the form of films has been widely investigated for hydrogen production utilizing solar energy in photoelectrochemical cells. 7À10 Compared with other TiO 2 films prepared by chemical vapor deposition (CVD), liquid-phase deposition (LPD), solÀgel, and magnetron sputtering deposition methods, 11À14 nanotubular structure shows large surface area without increasing the geo- metric area due to the external and internal surfaces. Moreover, the one-dimensional highly ordered nanotube architecture offers an excellent electrical channel for vectorial electron transfer. 15 Therefore, TiO 2 nanotubes (NTs) have been intensively studied and fabricated with various methods, 16À19 including electroche- mical anodization, hydrothermal treatment, and template synth- esis. Among these methods, electrochemical anodization is con- sidered as a relative simple technique to synthesize self-organized TiO 2 NTs discovered by Grimes group. 20 The pore size, wall thickness, and tube length can be precisely controlled by vary- ing the electrochemical conditions consisting of anodization voltage, anodization time, the concentration of electrolyte, and heat treatment, etc. 21À23 As we know, the heat treatment of TiO 2 has a great effect on its grain size, surface morphology, crystalline phase composition, and photoelectrochemical properties. 24À26 Therefore, it is necessary to further study the effect of annealing temperature on TiO 2 NTs. 27À30 To the best of our knowledge, Received: December 7, 2010 Revised: March 24, 2011 ABSTRACT: Due to the energy crisis, it is necessary to develop clean and renewable energy sources. In this study, we report an efficient and economical technology to produce hydrogen from solar energy by splitting water in a two- compartment photoelectrochemical (PEC) cell without any external applied voltage. To enhance the solar conversion efficiency, highly ordered TiO 2 nanotube arrays with 4 μm in length were synthesized by a rapid anodization process in ethylene glycol electrolyte. Crystal phase and morphology of the TiO 2 nanotubes (NTs) samples annealed at various temperatures were characterized by XRD and FESEM. Transient photocurrent response and linear sweep voltammetry curves were measured using electrochemical working station under solar light illumination. The photocatalytic activity was evaluated by the hydrogen production in the PEC cell. The results indicated that the crystal phase and morphology of TiO 2 NTs had no great changes at low annealing temperatures. Anatase phase and tubular structure of TiO 2 NTs were stable up to 450 °C. With further increase in temperature, the crystallization transformation from anatase to rutile phase appeared, accompanied by the destruction of tubular structures. Due to the excellent crystallization and the maintenance of tubular structures, TiO 2 NTs annealed at 450 °C exhibited the highest photoconversion efficiency of 4.49% and maximum hydrogen production rate of 122 μmol/(h 3 cm 2 ), which is superior to most of those reported so far.