Electrochimica Acta 136 (2014) 404–411 Contents lists available at ScienceDirect Electrochimica Acta j our na l ho me pa g e: www.elsevier.com/locate/electacta Preparation and Characterization of TiO 2 Nanotube Arrays in Ionic Liquid for Water Splitting R.G. Freitas, M.A. Santanna, E.C. Pereira ∗ NANOFAEL – LIEC – Departamento de Química. Universidade Federal de São Carlos – C.P.: 676 – CEP: 13565-905, São Carlos, SP, Brazil a r t i c l e i n f o Article history: Received 27 March 2014 Accepted 18 May 2014 Available online 27 May 2014 Keywords: TiO2 nanotubes water splitting electron lifetime photocurrent transients a b s t r a c t In the present work, it is described the fabrication and characterization of TiO 2 nantotubes prepared potentiostatically using 1.0 or 5.0 vol.% ionic liquid electrolyte medium at 10 or 20 ◦ C. These experimental conditions led to TiO 2 nanotubes with 63-77 nm pore diameter, crystallite sizes between 27-33 nm, and band-gap values in the 3.1-3.2 eV range. Although the morphology and band-gap values were similar, the water splitting reaction showed differences in terms of photocurrent, up to 300%. Besides, we report also important changes in those parameters related with electronic defects, such as carrier density (N D ), width of depletion layer (w) and Debye length ( D ). Then, the enhancement on photoelectrochemical properties could be related to a decrease in the rate of surface recombination processes. Finally, these proposition are in agreement with the open-circuit voltage-decay, where was observed a higher electron lifetime for TiO 2 NT obtained at higher vol.% of ionic liquid. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Efficient splitting of water into H 2 /O 2 , photocatalytically acti- vated, is probably the most desired pathway to convert abundant solar energy into “hydrogen”, the proposed clean energy carrier of the future. The total amount of solar energy impinging on the earth‘s surface in one year is about 3x10 24 J, or approximately 10 4 times the worldwide yearly consumption of energy [1]. The search for efficient conversion of solar energy into other useful forms is, in view of the increasing anxiety over the exhaustion of fossil energy resources and attendant global warming, one of the most important challenges for future. However, there are three major important tasks for the solar production of hydrogen: (1) Stability. Metal oxides are the most photochemically stable semiconductors in aqueous solution, but their band-gaps are either too large (≈3.0 eV) to absorb a significant fraction of incident solar energy, or their semiconductor charac- teristics (e.g. charge transport) are poor [2]. (2) Band-gap values. Considering the water splitting energy of 1.23 eV and overpotential losses, the semiconductor(s) should have a band-gap greater than 1.7 eV. However, semiconductors with relative low band-gaps have been found to lack stability during water splitting [2]. (3) Energy level edge position. Even though a semiconductor may generate suf- ficient energy to drive an electrochemical reaction, the band edge ∗ Corresponding author. Tel.: +55 16 3351 8214; fax: +55 16 3351 8214. E-mail addresses: ernesto@ufscar.br, ernestopereira51@gmail.com (E.C. Pereira). position may prevent it from react leading to water splitting [2]. For spontaneous water splitting, water oxidation and reduction potential must lie between the valence and conduction band edges. Among inorganic oxides, TiO 2 is an important candidate for efficiently water splitting reaction. Several papers in the litera- ture [3,4] have demonstrated that besides the microstructure, the morphology is important to optimize the oxide properties. Then, TiO 2 nanotubes (TiO 2 NT) seem to be a promise material as pho- toanode to water splitting application, once these nanostructures provide unique electronic properties, such as high electron mobil- ity [5], appropriated band edge positions [6], quantum confinement effects [7], high specific surface area [8], high mechanical strength [9] and photocorrosion resistance [10]. In this scenario, TiO 2 NTs seem to be a versatile compound and one of the most investigated nanostructure in materials science. Among the factors that affect the TiO 2 NT properties, elec- trochemically obtained, the non-aqueous electrolyte can be regarded as a very important variable. In this context, ionic liquid (IL) have many applications, such as powerful solvents and electrically conducting fluids (electrolytes). Specifically dur- ing titanium anodization, the use of a fluor salt which is liquid at ambient temperature, such as 1-butyl-3-methyl-imidazolium- tetrafluoroborate (BMIM-BF 4 ), is important as fluoride precursors source which determine the quality, morphology and density of the nanotubes. The use of ionic liquid as electrolyte during the TiO 2 NTs obtainment by electrochemical means is quite recent, and just few papers have reported it. Schmuki et al. [11] observed the formation of well-ordered self-organized TiO 2 nanotube layers in BMIN-BF 4 http://dx.doi.org/10.1016/j.electacta.2014.05.097 0013-4686/© 2014 Elsevier Ltd. All rights reserved.