Hydrothermal preparation of BiVO4 powders

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Abstract

Monoclinic bismuth vanadate (BiVO4) powders were prepared by a mild hydrothermal method, using an aqueous solution of bismuth nitrate and two different vanadium sources (V2O5 and NaVO3). The characterization of as-prepared BiVO4 was carried out by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The results revealed that in the present work the temperature of 200 and 140 °C are sufficient to prepare phase pure BiVO4 powders from V2O5 and NaVO3, respectively. The BiVO4 sample prepared from V2O5 consists of an agglomeration of small rod-like particles. When BiVO4 was prepared from NaVO3, monoclinic BiVO4 powder with a plate form is obtained.

Introduction

Bismuth vanadate (BiVO4) has recently attracted considerable attention for it exhibits interesting technological properties, such as ferroelasticity [1], [2], [3] and ionic conductivity [4]. BiVO4 can also be used to oxygen separation, acting as a photocatalyst under the action of visible light [5]. BiVO4 based yellow pigments are potential substitutes for lead and cadmium based paints, due to their high performance and problem-free ecotoxicological behavior [6]. There are three crystalline phases reported for synthetic BiVO4, the monoclinic fergusonite-type, the tetragonal sheelite-type and the tetragonal zircon-type [2], [7]. At 255 °C, the phase transition between monoclinic phase and the tetragonal sheelite-type phase is reversible. This phase transition leads to a pronounced thermochromic behavior.

Some synthetic methods are mainly employed for the preparation of bulk BiVO4. Monoclinic BiVO4 is usually prepared by solid-state reaction starting from the metallic oxides at temperature above 600 °C [8]. Tetragonal BiVO4 with a zircon-type structure is prepared by a precipitation method at room temperature [7]. Monoclinic BiVO4 is also prepared by aqueous process from layered vanadates at room temperature [5]. Thus, the obtained crystal form depends on the preparation method.

For the preparation of inorganic solids, a variety of methods have been proposed, including solid-state reactions, sol–gel method, hydrolysis of metal alkoxides, etc. However, these methods have disadvantages, such as complex operating procedure, the high price of raw material. Furthermore, they generally need high-temperature treatment. On the other hand, hydrothermal method has frequently been used for preparing ceramic powders for a variety of applications [9], [10] due to its advantage over other methods. First, precipitation from supersaturated solutions can be controlled. Thus, size, size distribution and particle morphology can be controlled. Furthermore, hydrothermal method facilitates the preparation of powders in a single step without any post-preparation process, such as calcinations or milling. As far as we know, there is no data on hydrothermal preparation of BiVO4 ceramics so far. In this paper, a simple hydrothermal route has been employed to prepare crystalline powders of BiVO4 at low temperature.

Section snippets

Experimental

Two vanadium compounds, vanadium pentoxide (V2O5) and sodium metavanadate (NaVO3) stock solution, were used as vanadium sources in this study. The NaVO3 stock solution was prepared by stirring stoichiometric amounts of V2O5 and sodium hydroxide (NaOH) in distilled water for 12 h. Aqueous solutions of bismuth nitrate (Bi(NO3)3·5H2O) and V2O5 or NaVO3 stock solution were mixed together according to the Bi/V ratio of 1, where the pH value was about 2.0 and 5.0, respectively. One molar NaOH solution

Results and discussion

Fig. 1 shows the XRD patterns of BiVO4 samples prepared from V2O5 at 160, 180 and 200 °C for 12 h. At 160 and 180 °C, some amounts of impurities are observed. For the samples treated at 200 °C, no signal from secondary phase has been detected, all peaks can be indexed to monoclinic BiVO4 (JCPD file no: 14-688). The XRD results clearly show that 200 °C is a critical temperature for the preparation of single phase of monoclinic BiVO4 under the present experimental conditions.

Fig. 2 shows the XRD

Conclusion

The combined powder XRD, Raman and FTIR spectroscopy suggest that monoclinic BiVO4 have been successfully synthesized by the hydrothermal method. It is observed that starting material has great effect on the morphology of samples and the temperature required to obtain a single phase. A temperature of 140 °C is sufficient for preparing phase pure BiVO4 from NaVO3, which is lower than that required using V2O5 as starting material. To our knowledge, the reaction temperature for crystallizing

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