Enhanced photocatalytic activity in anodized WO3-loaded TiO2 nanotubes
Introduction
The development of photocatalysis has been the focus of considerable attention in recent years with photocatalysis being used in a variety of products across a broad range of research areas, including especially environmental and energy-related fields [1]. Of the many different photocatalysts, TiO2 is one of the most important known photocatalyst materials able to efficiently decompose almost any kind of environmental pollutants due to the position of its valence and conduction bands [2]. TiO2 possesses many advantages and unique functional properties compared with other semiconducting materials, such as low production cost, non-toxicity, self-cleaning property, ready availability, strong photocatalytic activity, and exceptional photo corrosion resistance [3], [4].
It is well known that there are many factors which can exert significant influence on photocatalytic performance, including the size, specific surface area, pore volume, pore structure, crystalline phase, and the exposed surface facets. The construction of TiO2 nano- or micro-structures with interesting morphologies and properties has recently attracted considerable attention [1]. TiO2 materials with one-dimensional structures, such as tubes, possess unique properties and advantages for photocatalytic reactions. In tubes, the higher surface-to-volume ratio enables a reduction in the hole–electron recombination rate and a high interfacial charge carrier transfer rate, with both of these effects being favorable for photocatalytic reactions [1], [5]. Different methods have been developed for synthesizing TiO2 nanotubes. These include sol–gel, microwave irradiation, hydrothermal processing, template synthesis and electrochemical oxidation. Among these, electrochemical anodization is a simple, cost effective and powerful technique that is widely used because it is controllable and reproducible. In addition, this method produces strongly adherent nanoporous TiO2 layer on the substrate, which is more desirable for many practical applications [6], [7], [8]. Following the anodization process, TiO2 nanotube arrays are formed on the foil surface and have straight channels against the foil [1].
However, one of the hindrances to the widespread use of TiO2 as a photoelectrode is the rapid recombination of photogenerated electron–hole pairs [3], [9]. The drawback cannot be overcome by only optimizing the dimensional features of TiO2 nanotubes [10]. It is well known that coupling TiO2 with metal oxides is an approach to improve the photocatalytic activity of TiO2 [11]. Because coupling two semiconductors with different redox energy levels can increase the charge separation for their corresponding conduction and valence bands [12]. Among the different metal oxides, tungsten trioxide (WO3) with the band gap of 2.8 eV, is one of the most important materials to be coupled with TiO2 for the enhancement of photocatalytic activity [11]. The suitable conduction band potential of WO3 allows the transfer of photo-induced electrons from TiO2 by facilitating effective charge separation [10].
Many studies highlighted that the coupling mechanism between WO3 and TiO2 could facilitate better charge separation. However, most of these studies involved WO3–TiO2 photocatalysts in the form of particles or spheres or thin films, and there are only a few studies focused on growth of the 1D highly ordered WO3–TiO2 nanotube arrays. Very recently, WO3–TiO2 nanotubes system have gained considerable attention, and various method such as sol–gel [9], hydrothermal [13], anodization [14] have been followed to incorporate WO3 on TiO2 nanotubes. Lai et al. [3], [10] synthesized TiO2 nanotubes by simple anodization and after that loaded WO3 on nanotubes via wet impregnation and radio frequency sputtering methods. These composite nanotube photo electrodes significantly enhanced their photo electrochemical (PEC) water-splitting performances compared with pure TiO2 nanotube photo electrodes. However, almost all methods using to incorporate WO3 are complicated, and long time is needed for their process. To the best of our knowledge, there is only one literature that synthesized TiO2–WO3 nanotubular composite via single-step anodization, which is done by Smith and coworkers [14]. They reported that the composite material was evaluated for photo electrochemical water splitting and demonstrated a 46% increase in conversion efficiency by incorporating WO3 compared to TiO2 nanotubes prepared under similar conditions. Therefore, more and detail studies are needed to investigate simple process for synthesizing WO3–TiO2 nanotubes and also their photocatalytic performance.
In this study, we report the formation of pure TiO2 and WO3–TiO2 nanotubular composites through a single-step anodization of titanium that exhibit enhanced photocatalytic properties. The samples are characterized in detail employing XRD, XPS, EDX, SEM, and UV–Vis spectrophotometry methods. A paradigm between structure and photocatalytic characteristics of the samples is established.
Section snippets
Experimental
Commercially pure grade II titanium foils were used as substrate. Prior to anodization, substrates were cleaned via a multi-step process including mechanical polishing, chemical etching in diluted HF solution (HF:H2O = 1:20 vol.%) at room temperature for 30 s, and ultrasonic cleaning for 15 min in acetone and ethanol, respectively. The substrates were washed by distilled water in between the steps. As-cleaned samples were clamped on a clip and immersed in the electrolyte and anodized at 20 V for 1 h
Morphology and structure
Fig. 1 shows the top and cross-section views of both pure and WO3 incorporated TiO2 samples where the formation of nanotubes is evident. It can be seen that (Fig. 1(a)) well-aligned TiO2 nanotube arrays with a 75 nm diameter and a 300 nm length have formed. The tubular structure provides superior photocatalytic decomposition of organic pollutant because it provides a large surface area, facilitates the diffusion of organic pollutions into the TiO2 nanotubes, and enhances the absorption yield of
Conclusions
- 1.
Pure TiO2 and WO3–TiO2 nanotubes with a diameter of 75 nm and a length of 300 nm were successfully synthesized by single-step anodization of Ti foils.
- 2.
It was observed that WO3–TiO2 nanotubes grown in low concentration of tungstate salt had a nanotubular structure like pure TiO2 nanotubes. However, at high concentrations of tungstate salt, the microstructure changed and a worm-like morphology was observed.
- 3.
XPS analysis demonstrated that both WO3 and Ti–O–W bonds were formed. Therefore, WO3 not only
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