Effects of pH on hydrothermal synthesis and characterization of visible-light-driven BiVO4 photocatalyst
Graphical abstract
Visible-light-driven BiVO4 photocatalyst was hydrothermally synthesized at different pHs and exhibited changed morphologies, components and photocatalysis regularly. It reveals that monoclinic BiVO4 shows better photocatalytic activities than tetragonal phase when evaluated by the decolorization of methyl orange in aqueous solution under visible light irradiation; and dispersive particles also represent higher activities than coagulate ones regarding of their photocatalysis.
Introduction
Since photocatalysis is regarded world-wide as one of the most promising solutions to solve the severe problems of energy shortage and environmental crisis, the development of visible-light-driven photocatalysts has been attracting much attention [1], [2], [3]. In the past decades, the interests of researchers in the application of photocatalysts have grown dramatically, which mainly involved the selective synthesis of efficient visible-light-driven photocatalysts, the photocatalysis mechanism and the effect factors about the degradation of organic contaminants, etc. [4], [5], [6], [7].
As one of the non-titania based semiconductor photocatalysts, bismuth vanadate (BiVO4) has recently attracted considerable attention for its strong photocatalysis for water splitting and pollutant decomposing under visible light irradiation [5]. Up until now, many methods for the preparation of BiVO4, such as solid-state reaction, sol–gel method, coprecipitation and metalorganic decomposition, etc., have been reported [8], [9], [10]. There are three crystalline phases reported for synthetic BiVO4, that is, the monoclinic sheelite-type, the tetragonal sheelite-type and the tetragonal zircon-type [9]. All the three crystal structures are well formed and are found to have tetrahedral V, which is coordinated by four oxygens, with Bi eightfold coordinated [11]. Although their photocatalytic properties are strongly related to their crystal structures and morphology [12], only limited researches have been done on the phase selectivity and associated catalytic activities under different synthetic conditions.
This study reported a hydrothermal route for selective synthesis of the highly crystalline BiVO4 powders with high efficiency. For comparison, the pHs of the precursor mixtures were pre-set to different values. All the synthetic samples, which showed different photocatalytic activities, were characterized and compared using XRD, SEM, DRS, nitrogen adsorption and Raman techniques.
Section snippets
Materials and synthesis of bismuth vanadate
Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and Ammonium metavanadate (NH4VO3) supplied from Beijing Chemical Company were used directly without any further purification. Other chemicals used were all analytical grade. Solutions were prepared using deionized water. In a typical preparation, 0.02 mol Bi(NO3)3·5H2O and 0.02 mol NH4VO3 were dissolved in 20 mL of 65% (w/w) HNO3 and 20 mL 6 mol/L NaOH solutions separately, and each stirred for 2 h at room temperature. After that, these two mixtures were
Powder formation
The hydrothermal treatment for the precursor mixtures always leads to highly crystalline samples. Fig. 1 shows the XRD patterns of the prepared samples as a function of pH values. For the samples synthesized at pH = 11.5 and 8.8, the diffraction data obtained match well with those for the monoclinic BiVO4 (JCPDS card no. 14-0688). As can be seen from Fig. 1f and e, no peaks of any other phases or impurities were detected. As the pH decreases to 5.3 (Fig. 1c), the peaks for the tetragonal BiVO4
Conclusion
A facile hydrothermal route for the synthesis of the visible-light-driven BiVO4 photocatalyst at different pH conditions has been studied. The results showed that pH values of precursors strongly affected the structure and morphology of powders. The powder prepared at low pH contained mixed phase including monoclinic and tetragonal BiVO4. With the increase of pH, a pure phase monoclinic type BiVO4 could be synthesized selectively. Meanwhile, different pH values led to different morphologies of
Acknowledgements
We acknowledge the financial support from the Scientific Research Foundation of North China University of Technology (NCUT) and the Natural Science Foundation of Beijing.
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