Synthesis and photocatalytic performances of BiVO4 by ammonia co-precipitation process
Abstract
BiVO4 was prepared by a co-precipitation process using aqueous ammonia solution, followed by heating treatment at various temperatures. The crystalline structure and crystallization process, and their influences on photocatalytic O2 evolution and organic pollutants degradation were investigated. It demonstrated that the crystalline structure is still the vital factor for the activities of both reactions. However, the crystallinity of BiVO4 gives a major influence on the activity of O2 evolution, whereas the surface area, plays an important role for photocatalytic MB decomposition.
Introduction
The photocatalytic splitting of water into hydrogen and oxygen, and the photocatalytic degradation of organic pollutants are promising reactions for solving urgent energy and environmental issues that confront mankind today. Since the photoelectrochemical water splitting (the Honda–Fujishima effect) was reported in 1972, [1] great progresses have been made on the research and application of photocatalysis both in energy and environmental fields. To date, these researches focused mainly on two directions: one of which is to design and develop visible-light-responsive photocatalysts, because the utilization of visible-light, which accounts for more than half of the solar spectrum, is significant. Another direction is to improve the photocatalytic reactivity and efficiency by optimization of experimental conditions, by synthesizing new types of photocatalysts, or by chemical and physical modifications of known photocatalysts.
Bismuth vanadate (BiVO4) belongs to the group of ternary bismuth oxide compound, Bi–M–O (MMo, W, V, Nb and Ta), which exhibits unique physical and chemical properties. BiVO4 has been explored as ferroelastic, [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] acousto-optical, [3], [4] ion conductive [5] and pigmentary [20], [21] materials. More interestingly, BiVO4 synthesized by an aqueous process showed excellent photocatalytic O2 evolution property from an aqueous AgNO3 solution under visible-light irradiation [22], [23], [24], [25], [26], [27], [28]. This highly oxidative reactivity of BiVO4 induced by the visible-light irradiation promotes its applications in oxidative decomposition of environmental pollutions. However, up to now the extensive application in environmental remediation is not got along well, because the small surface area of bulk BiVO4 is unfavorable for the adsorption/desorption kinetics of organic pollutants onto the surface of photocatalyst, which affect significantly on their photocatalytic oxidation and decomposition. Therefore, it is necessary to find strategies to improve the visible-light photocatalytic reactivity and efficiency of BiVO4 for organic pollutants decomposition. In this aspect, one effective approach is the preparation of extremely small-sized photocatalyst either by nano-techniques or by dispersing photocatalysts onto a support (SiO2, Al2O3, zeolites). For example, TiO2 nano-sized particles of less than 10 nm show significant enhancement in photocatalytic reactivity attributed to the quantum-size effect, [29], [30], [31], [32], [33] due to the electronic state variation of TiO2 as well as to the short distance required for the photogenerated electron–hole pairs to reach the surface. Another effective approach is the direct synthesis of microporous or mesoporous photocatalysts. This approach may provide the photocatalyst a large surface area, which is favorable for kinetic adsorption/desorption of organic pollutants, and therefore enhance the photocatalytic reactivity and efficiency [34].
For BiVO4, many advance approaches, including solid-state or melting reaction, [35], [36], [37] aqueous, [22], [23], [24], [25], [26], [38] sol–gel, [39], [40], [41], [42], [43], [44] and hydrothermal processes, [27], [28], [45], [46], [47], [48] had been developed to prepare BiVO4. Each of these approaches led to a characteristic crystalline structure, which thus showed significant differences in the photocatalytic performances. We have found previously that an amorphous BiVO4 may be synthesized by ammonia co-precipitation method [28]. This method is a facile and inexpensive process, and may be adopted to disperse BiVO4 onto a support, or to form a porous BiVO4. In this paper, we first report the photocatalytic performances of the crystalline BiVO4 samples prepared by calcinations of the amorphous precursor at various temperatures. The structure and crystallization of the prepared samples were characterized by several physico-chemical techniques in order to make a full understand for the materials.
Section snippets
Preparations
Twelve millimole of ammonium metavanadate (NH4VO3, Kanto Chemical, purity: 99.5%) and 12.0 mmol of Bi(NO3)3·5H2O were dissolved separately into 50 ml of 2.0 mol L−1 nitric acid solution, then mix them together to obtain a stable and yellow homogeneous solution. Increase the pH of the above mix solution by dropwise titration of ammonia solution (Kanto Chemical, 28–30% NH3) under stirring, orange precipitates were obtained. Followed by adjusting the pH to 9 for an entire precipitation, the orange
The crystalline structure of BiVO4
Fig. 1 shows the X-ray diffraction patterns of samples prepared by ammonia co-precipitation. The patterns of the solid-state prepared sample (BiVO4(s)) and zircon-structure (BiVO4(z)) were also included for comparison. The sample prepared by solid-state reaction at 973 K for 8 h (BiVO4(s)) showed a pure phase monoclinic scheelite BiVO4. For the synthesized samples, the diffraction peaks that index zircon-type BiVO4 were not observed clearly, instead, all the peaks match well to the monoclinic
Conclusion
A simple method to synthesize nanocrystalline BiVO4 has been established. By adding aqueous ammonium solution into the mix solution of Bi(NO3)3 and NH4VO3 in HNO3 under room temperature, an amorphous BiVO4 was first formed. By heat-treatment of the amorphous BiVO4 at various temperatures, BiVO4 with the grain size from nanoparticles to bulk materials can be formed. All the crystalline samples were monoclinic scheelite phase. For photocatalytic O2 evolution reaction, the crystallinity of BiVO4
Acknowledgment
This research was supported by CREST/JST and a Grant-in-Aid (No. 14050090) for the Priority Area Research (No. 417) from MEXT, Japan, and Nissan Science Foundation.
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