Elsevier

Applied Surface Science

Volume 257, Issue 17, 15 June 2011, Pages 7758-7762
Applied Surface Science

Hydrothermal synthesis and photoelectric properties of BiVO4 with different morphologies: An efficient visible-light photocatalyst

https://doi.org/10.1016/j.apsusc.2011.04.025Get rights and content

Abstract

Different morphologies of monoclinic BiVO4 with smaller size were hydrothermal synthesized by simply adjusting the amount of surfactant (polyvinyl pyrrolidone PVP K30) added. The detailed field emission scanning electron microscope (FESEM) analysis revealed that the amount of PVP added could significantly affect the morphology and size of BiVO4. Their photocatalytic activities were evaluated by the decolorization of methylene blue (MB) aqueous solution under visible-light irradiation (λ > 400 nm), and the as-prepared sample with well-assembled flower-like morphology showed a much higher photocatalytic activity due to larger specific surface area and higher separation efficiency of photo-induced carriers. The relationship between the behavior of photo-induced carriers and photocatalytic activity was studied using the surface photovoltage spectroscopy (SPS) and corresponding phase spectra.

Graphical abstract

BiVO4 with smaller size were synthesized by adjusting the amount of PVP, the relationship between the separation of photo-induced carriers and vis-photocatalytic activity was studied using the surface photovoltage technology.

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Highlights

► A method simply adjusting the amount of PVP added. ► The synthesized BiVO4 has a smaller size and a larger specific surface area. ► The relationship between the separation of photo-induced carriers and photocatalytic activity was studied using the surface photovoltage technical and corresponding phase spectra.

Introduction

Semiconductor photocatalysis, an ideal “green” technology, is attracting considerable attention for the environmental cleaning and H-energy production, where the active photocatalytic material is definitely an important key. In most cases, photocatalytic degradation is conducted over the TiO2 owing to its peculiarities of chemical inertness, no-photocorrosion, low cost, and nontoxicity [1], [2], [3]. However, its wide band gap (∼3.2 eV) greatly hinder their practical application. In view of the efficient utilization of visible light, the largest proportion of the solar spectrum and artificial light sources, the development of visible-light driven photocatalyst with high activity is indispensable.

A feasible approach to realize visible-light photocatalysis is to develop a new photocatalytic material independent of TiO2. Recently, monoclinic bismuth vanadate (BiVO4) has gained much attention due to its narrow band gap (2.4 eV) and excellent photocatalytic performance under visible light illumination [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. It is well known that BiVO4 mainly exists in three crystalline phases: monoclinic scheelite (s-m) structure, tetragonal zircon (z-t) structure, and tetragonal scheelite (s-t) structure [14]. Peng et al. synthesized microspheric and lamellar BiVO4 powders through a hydrothermal process using cetyltrimethyl ammonium bromide (CTAB) as a template-directing reagent. Experimental results indicate that microspheric BiVO4 with particle sizes in the range of 7-12 μm with a mixed crystal consisting of tetragonal and monoclinic phases can be derived from a relatively low hydrothermal temperature (<160 °C), whereas lamellar BiVO4 with a pure monoclinic phase can be obtained at a higher hydrothermal temperature (200 °C) and BiVO4 (s-m) possesses higher photocatalytic activity than BiVO4 (z-t) under visible light [15]. Shao et al. reported the synthesis of an active monoclinic bismuth vanadate visible photocatalyst with nanoribbons structure. The as-prepared BiVO4 nanoribbons were up to hundreds of micrometers in length, 60–80 nm in width, 15–20 nm in thickness, and grew along the (0 1 0) direction [16]. Zhao and co-workers prepared hyperbranched monoclinic BiVO4 on a large scale and with good uniformity by a surfactant-free hydrothermal route. As-synthesized monoclinic BiVO4 exhibits excellent photocatalytic ability in the photodegradation reaction of an aqueous solution of RB under visible light [17]. Although many works have been reported on the synthesis of highly efficient BiVO4 catalyst, most of them have a larger size (about 5 μm), thus, the specific surface areas are relatively lower. Until now, only a few works have concentrated on the exact effect of charge transfer properties on the photocatalytic properties of BiVO4 catalysts. The photocatalytic activity is mainly affected by three factors including absorbance efficiency of photons, specific surface area and the photo-carrier transport properties [18]. Therefore, the synthesis of BiVO4 catalyst with larger specific surface areas and studying the behavior of photo-generated charge carriers are necessary. And a better understanding of this information will provide greater insight into the intrinsic reasons of the enhancement in photocatalytic activity.

In this paper, we report the preparation of monoclinic BiVO4 with different morphologies via a facile surfactant-assisted hydrothermal method. The advantage of the surfactant added lies in the fact that it can change the micro-environment of reaction and adjust the growth habit of particles, thus, leading to the formation of products with a final desirable morphology and relatively larger surface area. Further as a well-established contactless and non-destructive technique, surface photovoltage technique studies were carried out to investigate the correlation between the photo-induced charge transfer properties and photocatalytic activity of the samples. It relies on analyzing illumination-induced changes in the surface voltage, especially when combined with the electric field-modified technique, which can provide a rapid and direct investigation of the band-gap transition [19].

Section snippets

Preparation of the BiVO4 samples

All reagents were of AR grades and used without further purification. In a typical synthesis, 0.24 g Bi(NO3)3·5H2O and a certain amount of PVP (PVP K30) were dissolved into 90 mL H2O under vigorous stirring to form a transparent solution. Then 0.4 g Na3VO4·12H2O was added. After stirring for 30 min, the pH of the resulting solution was adjusted to 1 with HNO3 (65–68 wt%) solution. After stirring for another 30 min, the suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave.

Results and discussion

Fig. 1 shows the XRD patterns of BiVO4 with different molar ratios of PVP/VO43− calcined at 500 °C for 2 h. All BiVO4 samples exhibited a similar XRD pattern that can be indexed to monoclinic BiVO4 (JCPDS card No. 14-0688). There are no peaks of any other phases or impurities. And the peak intensity of S20 is much weaker than the other three samples indicating its poor crystallinity. The relative diffraction intensity ratio about the (0 4 0)/(1 2 1) planes of S10 is much higher than the corresponding

Conclusion

A series of BiVO4 photocatalysts with different morphologies, smaller size and larger special surface area were synthesized via a surfactant-assisted hydrothermal method by regulating the amount of PVP. It was found that the amount of PVP played an important role in determining the morphology of BiVO4 samples. Their photocatalytic activities were evaluated by the decolorization of MB in aqueous solution under visible light irradiation and the flower-like BiVO4 sample (S30) exhibits a high

Acknowledgments

This work was supported by National Basic Research Program of China (973 Program) (No. 2007CB613303) and the National Natural Science Foundation of China (Nos. 20703020 and 20873053).

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