A sonochemical route to visible-light-driven high-activity BiVO4 photocatalyst

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Abstract

Visible-light-induced BiVO4 photocatalyst with monoclinic scheelite structure has been successfully synthesized via a facile sonochemical method. The as-prepared BiVO4 photocatalyst exhibited relatively high surface areas, consisting of primary nanocrystals with average size of ca. 50 nm. The BiVO4 nanocrystals showed a strong absorption in the visible light region and the band gap was estimated to be ca. 2.45 eV, representing an obvious blue-shift compared with that of the bulk sample. The photocatalytic activities were also evaluated by decolorization of methyl orange under visible light (λ > 400 nm). The results indicate that the photodegradation rate of as-prepared BiVO4 nanocrystals is quite high, up to 90% in 30 min, which is much better than that of the reference sample prepared by solid-state reaction (ca. 8%) and the standard photocatalyst P25 (ca. 6%) under the same conditions.

Introduction

Recently, the decomposition of harmful organic and inorganic pollutants using photosensitized semiconductors as catalyst has attracted increasing attention [1], [2]. Particularly, TiO2 has been well known as an effective photocatalyst and its photocatalytic behavior has been extensively studied [3], [4], [5], [6]. However, TiO2 only respond to UV light which occupies only 4% of the whole solar energy, while the visible light accounting for 43% is open to exploiture. The development of visible-light-driven photocatalysts, therefore, has become one of the most challenging topics recently.

Bismuth vanadate (BiVO4) has been recently recognized as a strong photocatalyst for water splitting and pollutant decomposing under visible light irradiation [7], [8], [9]. BiVO4 exists in three phases, monoclinic sheelite, tetragonal zircon and tetragonal sheelite [10]. The photocatalytic properties of BiVO4 are strongly related to its crystal phase, for example, the photocatalytic activity of monoclinic phase was much higher than that of the other two [11]. Several methods have been reported for the preparation of BiVO4, such as solid-state reaction, co-precipitation, hydrothermal treatment and metalorganic decomposition [12], [13], [14], [15]. However, most of these methods require high reaction temperature, or long reaction time, and the particle size of products is generally rather big. On the other hand, sonochemical techniques have been recently developed for the fast synthesis of nanosized functional inorganic materials [16], [17], [18], [19]. The unique chemical effects of ultrasound arise from acoustic cavitation, that is, the ultrasonic vibrations produce microscopic bubbles (cavities), which expand and implode violently, creating millions of shock waves. During the collapse of the bubble, very high temperatures (>5000 K), pressures (>20 MPa) and cooling rates (>1010 K/s) can be achieved [20]. It is expected that sonochemical approach may create inorganic materials with smaller crystal size and higher surface area, which is recognized to be beneficial to the photocatalytic activities [19]. However, to the best of our knowledge, the sonochemical approach for BiVO4 photocatalyst has not yet been reported.

In this study, we report a facile sonochemical route for the synthesis of visible-light-driven BiVO4 photocatalyst with high efficiency. The composition and microstructure of as-prepared products were investigated. The photocatalytic activity was also evaluated, in comparison with that of reference samples prepared by the solid-state reaction and that of standard photocatalyst P25.

Section snippets

Synthesis

All the reagents used in our experiments were of analytical purity and were used as received from Shanghai Chemical Company. In a typical preparation, aqueous solutions of Bi(NO3)3·5H2O and NH4VO3 were mixed together in 1:1 molar ratio, the mixture was then stirred for 1 h at room temperature. Afterward, the mixture was exposed to high-intensity ultrasound irradiation for 60 min. The yellow precipitates were centrifuged, washed with de-ionized water and absolute ethanol, and then dried at 353 K in

Formation of the BiVO4 crystals

The composition and phase transformation process of BiVO4 products according to the ultrasonic reaction time was investigated using XRD measurement. The XRD patterns of BiVO4 samples obtained after ultrasonic irradiation for 30 and 60 min are shown in Fig. 1(a and b), respectively. When ultrasonic irradiation time was 30 min, as shown in Fig. 1(a), the diffraction pattern indexed to monoclinic BiVO4 (JCPDS No.: 14-0688) was detected, along with tetragonal BiVO4 (JCPDS No.: 14-0133). The

Conclusion

A facile sonochemical approach has been developed for the synthesis of visible-light-driven BiVO4 photocatalyst. The average crystal size of as-prepared BiVO4 particles is ca. 50 nm. Furthermore, the primary crystals that construct the as-prepared BiVO4 particles share the same orientation along [1 1 0] direction. The as-prepared samples exhibited relatively high surface areas of ca. 4.16 m2/g, which is about 16 times higher than that of the reference sample by solid-state reaction. The as-prepared

Acknowledgements

We acknowledge the financial support from Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program).

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