Enhanced photocatalytic activity in anodized WO₃-loaded TiO₂ nanotubes

Highlights

• TiO₂ and WO₃-grafted TiO₂ nanotubes were grown via single step anodizing of titanium foils.

• We were able to control morphology and growth mode of TiO₂ nanotubes by changing the electrolyte composition.

• The grafting TiO₂ nanotubes with WO₃ enhances the photocatalytic activity.

Abstract

In this work, TiO₂ and WO₃-grafted TiO₂ nanotubes were grown via anodizing of titanium substrates in tungstate containing electrolytes. The samples were characterized in detail by XRD, XPS, SEM, EDX, and UV–Vis spectrophotometry techniques. Besides, photocatalytic characteristics were evaluated through measuring the degradation rate of 4-chlorophenol to establish a correlation between structure and photochemical properties. We were able to control morphology and growth mode of nanotubes from a tubular to a worm-like structure by changing the electrolyte composition. The samples possessed an anatase–rutile matrix where the anatase/rutile ratio was found to increase with the concentration of tungstate in the electrolyte. We attributed this observation to change in electrical conductivity of the electrolyte and the heat generated on the substrates. It was unambiguously revealed that a composite of WO₃ and TiO₂ forms and, in parallel, tungsten is doped into the crystalline lattice of TiO₂. The maximum photocatalytic reaction rate constant for TiO₂ and WO₃–TiO₂ samples was determined to be 0.0131 and 0.0174 min⁻¹ respectively. The grafting TiO₂ nanotubes with WO₃ enhances the photocatalytic activity mainly due to the hindrance of charge carrier recombination and the formation of a more acidic surface. We established a correlation between structure, stoichiometry, and photocatalytic characteristics of 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.