Photocatalytic activity and photoelectric performance enhancement for ZnWO4 by fluorine substitution
Graphical abstract
ZnWO4−xF2x presented high photocatalytic activity for the MB degradation. The high activity of ZnWO4−xF2x photocatalyst comes from higher density of surface hydroxyl groups which induced OH radicals to improve photocatalytic reaction, and higher donor density and efficiency of charge separation.
Highlights
► The formation of fluorine substitution can be attained by a two step process. ► Comparing with ZnWO4, the activity of ZnWO4−xF2x almost doubled. ► Enhanced activity was attributed to higher density hydroxyl of ZnWO4−xF2x. ► ZnWO4−xF2x possessed higher donor density and efficiency of charge separation.
Introduction
Semiconductor photocatalytic processes have been widely applied as techniques of destruction of organic pollutants in wastewater [1], [2]. Recently, some tungstates such as Bi2WO6 and ZnWO4 have presented high photoactivities for the pollutant degradation [3], [4], [5]. Their unique combination of physical and chemical properties, in terms of molecular and electronic versatility, reactivity, and stability, make us believe they are fine photocatalytic candidate materials. Therefore, further improving their photocatalytic activity for practical application is significant and meaningful.
In these years, fluorine ion acting as promising dopants in TiO2 had attracted a great deal of attention [6], [7], [8], [9], [10]. TiO2 photocatalyst after being doped by fluorine exhibit greater enhanced photocatalytic activity than before. The mechanisms of enhanced photocatalytic activities follow several ways: (1) Doping fluorine ion convert Ti4+ to Ti3+ by charge compensation and presence of a certain amount of Ti3+ reduce the electron-hole recombination rate [6]. (2) Fluorine doping induce homogeneous free OH radicals to promote photocatalytic reaction [7], [8]. (3) Fluorine doping result in the increase in anatase crystallinity [9], etc.
Very recently, a few fluorine doped non-TiO2 catalysts also exhibited a similar enhancement of photocatalytic activities. Fluorine doped SrTiO3 showed about three times the photocatalytic activity compared with that of undoped SrTiO3 [11]. The enhancement of photocatalytic activity was mainly ascribable to the formation of oxygen vacancies and increase of effective electron mobility. Additionally, enhanced photocatalytic activity on Bi2WO6 and ZnWO4 by fluorine introduction was reported by our and the other groups [12], [13], [14], [15]. Our preliminary experiments demonstrated that the improved photocatalytic activity by interstitial F impurity was ascribed to an increase of the transfer-rate of photogenerated electrons to the surface of photocatalyst [13]. In this work, we want to know whether the photocatalytic mechanism is different between F substitution and F interstitial doped ZnWO4.
We reported herein the photocatalytic performance of fluorine substituted ZnWO4 (ZnWO4−xF2x) prepared by a two-step process. To our knowledge, the effects of fluorine substitution and heat treatment on the photocatalytic activity of ZnWO4 have not been reported. In this study, we investigated the effects of fluorine on photoelectric behavior and photocatalytic reaction of ZnWO4. The results gained in this study are helpful for understanding the mechanism in the F substituted O-doped ZnWO4, which also show promising photocatalytic activity.
Section snippets
Synthesis of substituted sample
ZnWO4−xF2x samples were prepared by a two-step process. All chemicals used were analytic grade reagents, without further purification. The starting materials of 0.01 mol Na2WO4 and 30 mL hydrofluoric acid were stirred and heated, thus, forming Na2WO2F4 compound (see Supporting Information Fig. S1). In what followed, the Na2WO2F4 and Zn (NO3)2 were soaked in water in a stoichiometric ratio, refluxed on a mantle heater for 24 h. The products were washed with water and filtered. The obtained
Chemical states of fluorine
Fig. 1 shows XRD pattern of ZnWO4−xF2x obtained at different annealing temperature. It illustrates that fluorine substitution did not result in the development of new crystal orientations or changes in preferential orientations. This result is agreement with the quantum mechanical study by Fan groups, which confirmed that the whole structural variations following O replacement with F were found to be slight [16]. Hence, independently of presence or absence of fluorine, the samples appeared to
Conclusions
Fluorine substitution ZnWO4 (ZnWO4−xF2x) sample was prepared by a two-step process. Fluorine substitution affected not only the photoelectric properties but also the photocatalytic activity of ZnWO4. The enhancement of photocatalytic activity could be attributed to the increasing in number of surface hydroxyl groups, and higher donor density and efficiency of charge separation caused by fluorine substitution. The photodegradation of MB in ZnWO4−xF2x system mainly proceeded via indirect reaction
Acknowledgements
This work is supported by National Natural Science Foundation of China (20925725 and 50972070) and National Basic Research Program of China (2007CB613303).
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