Elsevier

Desalination

Volume 249, Issue 2, 15 December 2009, Pages 736-741
Desalination

Preparation of Ti/SnO2–Sb2O4 photoanode by electrodeposition and dip coating for PEC oxidations

https://doi.org/10.1016/j.desal.2009.01.035Get rights and content

Abstract

Thin films of antimony-doped SnO2 on titanium substrate with a doping range of 1.5–8 mol% were prepared by an electrodeposition and dip coating method. The prepared Ti/SnO2–Sb2O4 thin films were tested as a photoanode in the photoelectrocatalytic(PEC) experiments to degrade phenol in aqueous solution in order to evaluate their PEC performance. The photocatalytic (PC), electrocatalytic (EC) and PEC activity of Ti/SnO2–Sb2O4 thin films was compared in the degradation processes. And the effect of annealing temperature on their PEC activity was also investigated. The experimental results confirmed that the Sb-doped Ti/SnO2 thin films enhanced the phenol degradation and the Ti/SnO2–Sb2O4 film containing 6 mol% of Sb calcinated at 450 °C achieved the best performance for phenol degradation. The degradation experiments also demonstrated that the Ti/SnO2–Sb2O4 film achieved faster degradation of phenol in the PEC process than in the PC and EC processes. Compared with Ti/TiO2 and Ti/SnO2 photoanodes, the Ti/SnO2–Sb2O4 photoanode showed higher activity.

Introduction

Pure SnO2 is a semiconductor with a wide band gap (Eg = 3.5–3.8 eV), which is transparent and chemically and thermally stable. Some metal-doped SnO2 such as antimony-doped SnO2 shows metal-like properties, and a changeable property of light absorption. Therefore it has been used as electrodes in solar cells and electrochemical cells, liquid crystal displays, gas sensors, etc. [1], [2], [3], [4], [5].

Many reports have described the preparations and properties of antimony-doped SnO2 thin film and their applications [6], [7], [8], [9]. As so far in water treatment, antimony-doped SnO2 electrode is almost applied on electrocatalytic processes due to its good properties such as extremely high overpotential for oxygen-evolution, good electrical conductivity and catalytic activity. However, there was a lack of the investigation of its application in PEC degradation of organics in water.

From the nature properties of materials [10] Sb-doped SnO2 is still a n-type semiconductor, but a photocatalyst with the higher band gap than TiO2.

Thus a UV light (λ = 326–355 nm) needs to be used to excite the SnO2 in the PEC application. On the other hand, it should be noticed that the valence band level of SnO2 is lower (or more positive) than that of TiO2 as shown in Fig. 1. Compared with the TiO2, the oxidizing potential of photogenerated holes of SnO2 is higher and the oxidizing power of these holes should be stronger. Hence these holes can more easily oxide H2O or OH to form. OH due to the bigger difference between their redox potentials. Furthermore, the conduction band level of SnO2 is also lower than that of TiO2, which may not be negative enough to reduce water to hydrogen, dioxygen to superoxide, or to hydrogen peroxide. However, with the help of an external bias, i.e., electro-assistance photocatalysis, the oxidation/reduction reaction could be easily achieved. Therefore the external bias will play an important role in the SnO2 PEC process, even an indispensable role. Because of this reason, a new catalyst of Sb-doped SnO2 as a novel photoanode was introduced into a PEC process in this study. The aim of this work is at integrating the electrochemical (EC) reaction process and electro-assisted PC reaction process in one reaction system in order to obtain an enhanced oxidation rate for organic removal.

The scope of this work included (i) to prepare a Sb-doped SnO2 thin film photoelectrode using a simple procedure of electrodeposition and dip coating, (ii) to conduct a series of experiments to confirm the effects of Sb-doping levels, heat treatment temperature, and the structure of the thin films on the properties of this photoanode, (iii) furthermore to clarify the feasibility of this novel photoanode in the PEC application of organic removal via the degradation of phenol in water.

Section snippets

Preparation of Ti/SnO2–Sn2O4 electrode

A Sb-doped SnO2 thin film was prepared on the surface of the Ti plate substrate by electrodeposition to form a prime layer and by dip coating to form a final layer, according to the following procedure: (i) a piece of raw Ti plate (70 mm × 10 mm and effective area = 6.0 cm2) was burnished cleanly using an abrasive paper, and etched in 40% NaOH solution and subsequently in 10% oxalic acid solution at about 95 °C for 4 h for surface pretreatment, (ii) the pretreated Ti plate was then fully rinsed with

XRD analysis of Sb-doped SnO2 film

The typical XRD pattern of the Sb-doped SnO2 electrode (6% Sb) calcined at 450 °C is shown in Fig. 3. This pattern shows clearly that the surface of electrode was mainly SnO2 crystal with rutile structure, it is in good agreement with X-ray powder data file of ASTM card. Meanwhile, the peaks for antimony oxides were difficult to be observed, this may be related to the similar phase's lattice structure of antimony oxide with SnO2 or little content of antimony in mixtures. Wang et al. [13] studied

Conclusions

From the above study we may conclude that this novel electrode of Sb-doped SnO2 thin film on titanium substrate was prepared successfully. The nature of Sb-doped SnO2 material makes it feasible as a photoelectrode for the application of organic removal from water by PEC technique due to its structure of band gap and metal-like property. The performance of the photoanode is strongly dependent on the preparation conditions, particularly the Sb-doping level in SnO2 and the annealing temperature.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (Grant No. 20476070) and Natural Science Foundation of Shanxi Province (20041020). The authors thank Prof. X. Z. Li (Department of Civil & Structural Engineering, the Hong Kong Polytechnic University) for useful discussions and the revision of English language.

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