Enhanced photoelectrocatalytic performance of Zn-doped WO₃ photocatalysts for nitrite ions degradation under visible light

Abstract

WO₃ and Zn-doped WO₃ thin films were prepared on indium-tin oxide glass by a dip-coating. The composite films were characterized by UV–Vis absorption spectra, X-ray diffraction and scanning electron microscope. The effect of preparation conditions (concentration of Zn, annealing temperature, number of layers) on the photocurrent was studied. It was found that the photocurrent under visible light displayed the highest value for 2% Zn–WO₃ films annealed at 400 °C. The photocatalytic activity of the Zn-doped WO₃ was evaluated in terms of decay rate of nitrite ions under visible light. The influence of applied potential, initial pH and nitrite concentration on the reaction rate was studied. The experiments demonstrated that ${\text{NO}}_2^{-}$ could be efficiently degraded on the doped photoanode that showed a higher activity than the undoped WO₃ especially under high anodic potential (>0.7 V). The rate of degradation was enhanced in aqueous NaCl solutions. Furthermore, it was demonstrated that the photodegradation mechanism of ${\text{NO}}_2^{-}$ proceeded mainly indirectly via OH radicals. The possible reason of enhancement of reaction rate was also discussed.

Introduction

Numerous studies of photocatalytic degradation of pollutants as a method of purifying water have been carried out for more than 20 year (Hoffmann et al., 1995, Andrew and Stephen, 1997). The photoelectrochemical (PEC) degradation of pollutants is an extension of heterogeneous photocatalytic progress, in which the catalyst is placed on an electrode that is controlled potentiostatically (Leng et al., 2000, Li and Li, 2003). In this way, the photogenerated electron-hole pairs are separated by means of the externally applied electric filed. Several semiconductor materials have been reported to offer high photocatalytic activity. Among these semiconductors, TiO₂, due to the excellent stability over a wide pH (0–14) and high photoactivity under ultraviolet light, has been employed as photocatalysts in most investigations (Choi et al., 1994, Andrew and Stephen, 1997). However, TiO₂ has a large bandgap of about 3.2 eV and hence is photoexited only by UV light, which occupies only 5% of the solar spectra (Choi et al., 1994, Hoffmann et al., 1995). Therefore, semiconductor electrodes that can exhibit photoresponse to the visible light are strongly demanded.

Tungsten trioxide is also an important photoactive material with a bandgap energy of 2.5 eV and can thus absorb the blue part of solar spectrum up to ca. 500 nm (Spichiger-Ulmann and Augustynski, 1983). The PEC behavior of WO₃ films was extensively studied for electrochromic applications (Shiyanovskaya and Hepel, 1998), solar energy conversion (Shiyanovskaya and Hepel, 1999) and photodegradation of pollutants (Luo and Hepel, 2001, Hepel and Hazelton, 2005, Solarska et al., 2005). However, the position of its conduction band (CB) is too positive to allow oxygen reduction, rendering the photocatalytic degradation of pollution under open-circuit conditions practically impossible (Santato et al., 2001, Solarska et al., 2005). Solarska et al. (2005) found that WO₃ film electrode could be used as a photoanode for visible light assisted PEC degradation of several organic pollutants. A higher photoelectrocatalytic activity for the degradation of naphthol blue black diazo dye has been observed for WO₃ film electrodes, prepared by electrodeposition, than for TiO₂ nanoparticulate film electrodes (Luo and Hepel, 2001). Recently, WO₃ was used as a photoanode to degrade Remazol Black B dye under visible light (Hepel and Hazelton, 2005).

Several techniques for improving the photoresponse of oxide electrodes, which have led to higher sunlight conversion efficiency, have already been proposed (Choi et al., 1994, Hoffmann et al., 1995, Andrew and Stephen, 1997). For example, combining WO₃ with other materials, such as TiO₂ (Tada et al., 2004), could improve its PEC performance. WO₃ doped with different metallic ions, such as Mg²⁺, Al³⁺, In³⁺, Fe³⁺, Zr⁴⁺, was investigated and showed much better photophysical properties than pure WO₃ (Tang et al., 2003). The effect of different transition metals (Fe, Co, Ni, Cu and Zn), at different concentrations, on the photocatalytic activity of WO₃ for splitting of water into hydrogen and oxygen was studied but only under UV laser irradiation (Hameed et al., 2004). Improvement of light absorption spectra has been observed by incorporation of acceptor-type ions in crystal lattice of TiO₂ (Shi et al., 2006). Taking into account that Ti⁴⁺ ions form acceptor-type centers in a crystal lattice of WO₃, Radecka et al. (2005) found that an improvement in the PEC properties of Ti-doped WO₃ with respect to the undoped one. However, they did not investigate its photoelectrocatalytic performance. Considering that Zn²⁺ (0.074 nm) and Ti⁴⁺ (0.075 nm) ions have almost the same ionic radius and that both match the ionic radius of W⁶⁺, making it possible to be incorporated into bulk WO₃ lattice, we may expect a similar improvement in the PEC performance of Zn-doped WO₃ to that observed in the case of Ti doped.

Usually an organic pollutant (mostly a dye) was selected as the target chemical in the previous photodegradation experiments on WO₃ film electrode (Luo and Hepel, 2001, Hepel and Hazelton, 2005, Solarska et al., 2005). However, the dye itself can absorb visible light and the degradation mechanism is complicated and thus uncertain. Nitrite ions, a common inorganic pollutant in the environment, can be formed in water-supply systems as a result of nitrifying bacteria activity. Most photocatalytic experiments of nitrate were carried out under the illumination of UV-light (Mills and Domènech, 1993), rarely under visible light (Shi et al., 2006) or solar light (Chen and Cao, 2002). Here we will choose nitrite ions as the target pollutant, their electrochemically assisted photochemical oxidation (PCO) in a Na₂SO₄ aqueous solution using the Zn-doped WO₃ and undoped electrode under visible light is systematically studied and the relevant mechanisms are discussed.