Preparation and characterization of Zr doped TiO₂ nanotube arrays on the titanium sheet and their enhanced photocatalytic activity

Abstract

This paper described a new method for the preparation of Zr doped TiO₂ nanotube arrays by electrochemical method. TiO₂ nanotube arrays were prepared by anodization with titanium anode and platinum cathode. Afterwards, the formed TiO₂ nanotube arrays and Pt were used as cathode and anode, respectively, for preparation of Zr/TiO₂ nanotube arrays in the electrolyte of 0.1 M Zr(NO₃)₄ with different voltage and post-calcination process. The nanotube arrays were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV–Vis diffusion reflection spectra (DRS). The photocatalytic activities of these nanotubes were investigated with Rhodamine B as the model pollutant and the results demonstrated that the photocatalytic efficiency of Zr doped TiO₂ nanotubes was much better than that of TiO₂ nanotubes under UV irradiation. Zr/TiO₂ nanotube arrays doped at 7 V and calcined at 600 °C (denoted as TiO₂-7 V-600) achieved the best photocatalytic efficiency and the most optimal doping ratio was 0.047 (Zr/Ti). TiO₂-7 V-600 could be reused for more than 20 times and maintained good photocatalytic activities.

Introduction

TiO₂ nanotube have received considerable attention in recent years because of its higher surface area, better adsorption ability and higher photocatalytic activity in comparison with TiO₂ powders [1]. Hydro-thermal method has been widely used to prepare TiO₂ nanotube [2], [3]. However, TiO₂ nanotubes prepared with such method are difficult to be recovered and reused and also are limited in the applications. Nowadays there is a creative way to solve this problem, which is to prepare oriented nanotubes on substrates and create ordered arrays. Many developed methods have been reported [4], [5] and anodic oxidation is a simple, cheap and straight-forward method to prepare highly ordered TiO₂ nanotube arrays on Ti substrate [6], [7]. It has been found that the morphology of TiO₂ nanotube arrays prepared by anodic oxidation is affected by the anodization voltage and time, electrolyte composition and temperature, etc. [4], [8], [9], [10], [11]. Owing to their tight coherence to the Ti substrate and recyclable property, TiO₂ nanotube arrays prepared by anodization have been widely studied in photoelectrocatalysis [12], dye-sensitized solar cells [13], and water splitting [14], [15] in recent years.

During the procedure for improving the photocatalytic activity of TiO₂, various methods such as element doping [16], [17], [18], [19], [20], [21], noble metal deposition [22], [23], surface modification [24] and semiconductor composition [25] have been carried out, among them, doping element into TiO₂ lattice is an effective way [16], [17], [18], [19], [20], [21]. It has been found that some metals such as transitional metals Zn, Cr and Fe doping can improve the separation rate of photo-induced electrons and holes and enhance the photocatalytic activity of TiO₂ [19], [21], [26]. Nonmetal doping such as Si, F, C, N can either increase the photocatalytic efficiency [27] or extend the absorb band to visible light regions [28], [29], [30].

Zr and Ti are both IV B elements and have the similar atom semidiameters (Ti, 2 Å; Zr 2.16 Å) and the same valent state (+4). Both oxides TiO₂ and ZrO₂ are n-type semiconductors with similar physicochemical properties. Therefore, it is possible for Zr atom to enter into the TiO₂ lattice and substitute for Ti atom. Both of our previous work [31], [32] and others’ research [33], [34] have demonstrated that incorporating Zr⁴⁺ into TiO₂ can introduce lattice defects and lead to higher photocatalytic activity than that of pure TiO₂.

The goal of present study is to prepare Zr doped TiO₂ nanotube arrays on Ti sheet and further investigate their photocatalytic efficiencies.