Synthesis and characterisation of bismuth(III) vanadate

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Abstract

Modified hydrothermal and ‘wet’ precipitation routes at room temperature were employed to synthesise pure monoclinic BiVO4 powders of varying particle morphologies. Monoclinic BiVO4 powder was also prepared by a solid-state reaction at 700°C. Depending on the synthesis conditions, the colour of BiVO4 varies from inhomogeneously yellow–brown to homogeneously and intensive lemon yellow. BiVO4 prepared by solid-state reaction consisted of large compact particles about 15 μm in size and of irregular shape. At higher magnification, the formation of domains with smooth terrace-like surfaces was observed. These domains ended with well-defined walls, and the edges of these walls were relatively very sharp. BiVO4 synthesised by the hydrothermal and an aqueous process at RT consisted of smaller particles (0.3–1.2 μm) and of much bigger regular crystals with a well-defined crystal habit. A continuous shift of the most intense Raman band to lower wavenumbers reveals that the average short range symmetry of the VO4 tetrahedra becomes more regular. The values of FWHM for the same Raman band increase from sample prepared by solid-state reaction to sample obtained at RT. The Raman results suggested that a sample prepared at high temperature consisted of less symmetric VO4 tetrahedra than samples prepared at low temperature and that the high-temperature sample showed better crystallinity with less defects than the samples prepared by an aqueous process under mild conditions. FT-IR spectra showed main features typical of the vanadates of other metal(3+) cations. However, the position of IR bands recorded for BiVO4 depended on the synthesis route.

Introduction

Bismuth(III) vanadate (BiVO4) shows ferroelastic and pyrooptical properties. BiVO4 is a possible candidate for use as electrolyte or cathode material in solid oxide fuel cells. In addition, BiVO4 shows catalytic and photocatalytic properties. It is commonly used as catalyst in oxidative dehydrogenation, e.g. converting ethyl benzene to styrene. BiVO4 is a very good photocatalyst, as shown by Kudo et al. [1], [2], [3], who reported a high activity for the O2 evolution from an aqueous silver nitrate solution under visible light irradiation. BiVO4 is also a commercially available high-performance pigment that could eliminate the more expensive and less stable organic pigment. It is classified as non-toxic and could replace toxic pigments such as cadmium and lead-based paints. However, it has been proven difficult to control the pigmentary colours of BiVO4. Colour intensity depends on many factors including phase composition, stoichiometry, particle size and morphology. Also, photophysical and photocatalytic properties of BiVO4 are strongly influenced by the way of preparation and its crystal structure. For example, tetragonal structure of BiVO4 absorbs in the ultraviolet region (band gap 2.9 eV), whereas monoclinic structure of BiVO4 with a 2.4 eV band gap has a characteristic visible light absorption in addition to the UV band [2]. The photocatalytic activities of tetragonal and monoclinic BiVO4 differ markedly. Also, the photocatalytic activity of the monoclinic BiVO4 prepared by the aqueous process at room temperature was much higher than that of monoclinic BiVO4 prepared by a conventional solid-state reaction even in the same crystal structure.

Three main crystal forms of BiVO4 are known; tetragonal (zircon-type structure), monoclinic (distorted scheelite structure, fergusonite structure) and tetragonal scheelite structure (high-temperature phase). The phase transition between monoclinic scheelite structure and tetragonal scheelite structure of BiVO4 reversibly occurs at about 255 °C (ferroelastic to paraelastic transition), whereas the irreversible transition from tetragonal zircon-type structure to monoclinic BiVO4 occurs after heat treatment at 400–500 °C and cooling to room temperature.

Generally, monoclinic BiVO4 is usually obtained by the high-temperature process, whereas tetragonal BiVO4 with a zircon-type structure is prepared in aqueous media by the low-temperature process. Bhattacharya et al. [4] reported a co-precipitation method from a Bi(NO3)3 nitric acid solution and an aqueous NH4VO3 at room temperature to produce fully crystalline zircon-type tetragonal BiVO4. Hirota et al. [5] synthesised monoclinic BiVO4 by the hydrolysis of bismuth and vanadyl double alkoxide (sol–gel method). Monoclinic BiVO4 powders were also prepared by a mild hydrothermal method (140–200 °C) using an aqueous solution of bismuth nitrate and two different vanadium sources (V2O5 and NaVO3) [6]. The preparation of BiVO4 as a monolayer on silica surfaces [7] or as a thin film on glass substrate obtained by the metalorganic decomposition technique has also been reported [8]. Wood and Glasser [9] prepared the pigmentary grade BiVO4 by precipitation from an aqueous solution. Depending on the respective precipitation condition, either tetragonal or monoclinic structured phase were obtained.

In this work, we present a modified hydrothermal and RT synthesis routes used to obtain monoclinic BiVO4 powders of various particle morphologies. Instead of the conventional inorganic alkali, in the present preparation of BiVO4 a strong organic alkali, tetramethylammonium hydroxide (TMAH) capable of reaching pH values near ∼14 in aqueous medium, was used. In a previous work, this organic alkali was employed in the synthesis of several metallic oxide such as Fe2O3, ZnO and RuO2 [10], [11], [12]. Synthesis of monoclinic BiVO4 powder was also performed starting from the highly acid aqueous solution under hydrothermal condition. For comparison, monoclinic scheelite BiVO4 powder was also prepared by a solid-state reaction at 700°C. Bismuth vanadate powders were characterised by using XRD, Raman, FT-IR and TEM/SEM techniques.

Section snippets

Experimental

Bi(NO3)3·5H2O (p.a., Kemika), NH4VO3 (p.a., Merck), tetramethylammonium hydroxide, TMAH, (CH3)4NOH, 25% (w/w) aqueous solution, electronic grade, 99.999% (metal basis) (Alfa Aeser®) and doubly distilled water were used. Samples were synthesised under the experimental conditions summarised in Table 1.

Sample BiV-1 was synthesised using solid-state reaction by mixing stoichiometric amounts of Bi(NO3)3·5H2O and NH4VO3. During the mixing, the white colour of powders changes immediately to intensive

Results and discussion

Fig. 2 shows the characteristic part of the X-ray powder diffraction patterns of samples BiV-1 to BiV-4. All these samples contained monoclinic BiVO4 identified according to diffraction data in ICDD Powder Diffraction File (PDF) [13], e.g. card No. 83-1700, 83-1698, 14-0688 (monoclinic symmetry, space group I2/a, unit-cell parameters a=5.195, b=11.701, c=5.092 Å, β=90.38°, mineral name: clino-bisvanite). Diffraction lines of BiVO4 were rather sharp for all investigated samples. XRD patterns of

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