Electrochemical characterization of TiO2/WOx nanotubes for photocatalytic application
© Vedarajan et al.; licensee Springer. 2014
Received: 21 July 2014
Accepted: 2 October 2014
Published: 14 October 2014
TiO2/WOx nanotubes have unique photo-energy retention properties that have gathered scientific interest. Herein, we report the synthesis, morphological characterization, and the electrochemical characterization of TiO2/WOx nanotubes compared with pure TiO2 nanotubes, prepared by anodization technique. Significant structural differences were not observed in TiO2/WOx nanotubes as observed by using scanning electron microscopy and transmission electron microscopy. The charge transfer resistance of TiO2/WOx before and after photo irradiation determined by using electrochemical impedance spectroscopy proves the inherent energy retention property which was not observed in pure TiO2 nanotubes.
KeywordsTiO2/WOx nanotubes Photoelectrochemical water splitting Photocatalytic application
Solar energy is clean, safe, and limitless; hence, tapping solar energy would be beneficial at global scale. In order to realize a solar-driven energy conversion device, semiconducting materials are required that absorb sunlight and accomplish an unhindered electron injection from valence band to conduction band, enabling performance of electric work in the circuit. Photoelectrochemical (PEC) water splitting to generate hydrogen for its use as fuel is considered to be a feasible alternative and sustainable energy system. Ideal materials for photoelectrochemical water splitting are semiconductors, which Fujishima and Honda first demonstrated using TiO2. As a result, this field of research has gathered significant attention by the research fraternity to achieve a highly efficient system producing hydrogen and oxygen by splitting up of water using most of the solar spectrum supplemented by a little or no electrical energy. Metal oxides, in particular, TiO2 and WO3, possess congenial electronic structure leading to good photoactivity and chemical stability. Further, their low cost and availability in abundance make them the chosen material for photoanodic reactions in aqueous electrolytes . TiO2 has been a material extensively studied for water photooxidation . More recently, TiO2 nanotubes (TNT) have gained much attention due to its simple synthesis  and enhanced photoelectrochemical performance over its nanoparticle counterpart. Despite the improved performance of TiO2 nanotubes, enhancing the visible light activity as well as reducing the charge recombination losses is required to increase water-splitting efficiency if practical applications are to be realized. On the other hand, tungsten trioxide (WO3) is a visible light photoactive material  with a bandgap of approximately 2.7 eV. Although it is a promising material by itself, coupling it with TiO2 has shown to be beneficial in many applications . The formation of heterojunctions of two semiconductors is an appealing method to increase visible light activity while maintaining the properties of each component. Moreover, the incorporation of WO3 is, in particular, ambient due to the nearly similar ionic radius of W+6 to that of Ti4+. As a result of which WO3 can be easily coupled into the TiO2 lattice during anodization process. Titania nanotubes incorporated with tungsten oxide have been reported to possess enhanced optical and electronic properties compared to its pure form [6–14]. Further, this mixed oxide composite nanotube shows a unique photon energy retention feature, i.e., the photon to electrical energy conversion process does not cease immediately after curbing the photon influx but stops gradually. This feature in TiO2 + WOx nanotubes has not been studied in depth. Investigations angled from the energy retention viewpoint of this metal oxide composite and its effect over dye-sensitized solar cell and photoelectrochemical water splitting was conceptualized in this endeavor.
Hence, in this present work, we have attempted to employ electrochemical impedance spectroscopy in evaluating TiO2/WOx nanotubes as a candidate material for water splitting. The TiO2/WO3 nanotubular composite was prepared through a single-step anodization of titanium in an aqueous bath of NH4F and H3PW12O40 (phosphotungstic acid (PTA)). The morphology of the nanotubes was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) analysis. The nanotubes were tested for its charge transfer resistance in an electrolyte at pH approximately 6 under air mass (AM) 1.5 simulated solar irradiation. A time-oriented electrochemical impedance spectroscopy was carried out to assess the charge retention property of the mixed oxide nanotube and was compared with its pure counterpart.
Synthesis of TiO2/WOx nanotubes
Synthesis of TiO2 nanotubes and TiO2/WOx was carried out similar to previously reported procedure. In short, cleaned, polished titanium metal strips were anodized at DC voltage of 50 V for two and a half hour in an aqueous solution containing 0.5 wt.% NH4F for TNT synthesis and 0.5 wt.% NH4F with 2.4 wt.% PTA (Kanto Chemicals, Tokyo, Japan) for TNT-WOx synthesis. Preliminary studies indicated an amount of 2.4 wt.% of PTA to be the optimal loading .
Characterization of TNT and TNT/WOx
Firstly, the synthesized TiO2 and TiO2/WOx nanotubes were characterized by using scanning electron microscope (SEM; Hitachi Model H-4600, Tokyo, Japan) and transmission electron microscope (TEM; Hitachi Model H-7100, Tokyo, Japan) for its morphology. Secondly, the elemental analysis was carried out using energy dispersive X-ray (EDX; Horiba, Kyoto, Japan) analysis. Finally, the electrochemical experiments were carried out in a conventional three-electrode setup in which the TiO2 or TiO2/WOx served as the photoanode, Pt as the cathode, and Ag/AgCl as the reference electrode. The electrolyte used was 0.1 M NaOH (Kanto chemicals, Tokyo, Japan). A computer-controlled potentiostat coupled with a frequency response analyzer was used to control the potential and record the electrochemical impedance spectra (VersaStat-3, Princeton Applied Research, Oak Ridge, TN, USA). The photoanodes were illuminated by a 300 W solar simulator with an AM 1.5 at one sun intensity (Peccell Technologies, Inc., Yokohama, Japan) (approximately 87 mW/cm2).
Results and discussion
Morphological and elemental analysis
The morphological and elemental analyses were carried out by scanning electron microscopy and transmission electron microscopy. No significant structural difference was observed in TiO2/WOx nanotubes compared to TiO2 nanotubes revealing honeycomb structure. Electrochemical impedance spectroscopy exhibited a high photoactivity from TiO2/WOx nanotubes compared to TiO2 nanotubes. A time-dependent impedance analysis evinced the charge retention property of the mixed oxide, indicating its possible use in many photocatalytic applications.
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