Effects of carbon content on the photocatalytic activity of C/BiVO4 composites under visible light irradiation

doi:10.1016/j.matchemphys.2009.08.028

Materials Chemistry and Physics

Volume 119, Issues 1–2, 15 January 2010, Pages 106-111

https://doi.org/10.1016/j.matchemphys.2009.08.028 Get rights and content

Abstract

Carbon-loaded BiVO₄ composite photocatalysts were prepared using an impregnation method, and their ability to photocatalytically degrade Rhodamine B dye solution under visible light irradiation was investigated. The prepared composite photocatalysts were characterized by X-ray diffraction (XRD), field-emission electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET) surface area measurements, and UV–vis diffuse reflectance spectra. We found that the carbon was well-dispersed on the surface of BiVO₄. The photocatalytic activity of the composite photocatalysts for the degradation of Rhodamine B (RhB) in aqueous solution under visible light irradiation (>420 nm) was higher than that of pure BiVO₄. Moreover, the degradation efficiency increased as the carbon content increased up to 3 wt%. The mechanism of enhanced photocatalytic activity is discussed with reference to surface area, optical absorption properties, and charge separation.

Introduction

Photocatalysis is a clean technology that is employed to treat various types of contaminants. In particular, photocatalysis with solar energy can be used in wastewater purification to remove organic contaminants by oxidative decomposition of volatile organic species [1]. The development of visible light-active photocatalysts has attracted considerable attention in the quest to utilize solar energy effectively [2], [3], [4]. Recently, a large number of undoped, single-phase semiconductor photocatalysts with particular absorption abilities in the visible light range have been developed; these include BiVO₄ [5], [6], [7], Bi₂WO₆ [8], [9], and CaBi₂O₄ [10]. Among these photocatalysts, BiVO₄ has been demonstrated to be an efficient photocatalyst under visible light irradiation. However, the photocatalytic activity of pure BiVO₄ is limited due to its poor adsorption abilities and the fast recombination rates of photo-induced charge carriers [11]. To enhance the photocatalytic activity of BiVO₄, the particle size needs to be reduced or the powder morphology needs to be modified using a chemical-based synthesis route. However, chemical-based methods are expensive, toxic, and complicated.

It has been reported that loading a small amount of a noble metal or metal oxide such as Pt [12], [13], NiO [14], RuO₂ [15], or carbon [16], [17] on the surface of a photocatalyst can enhance the photocatalytic activity of the bulk photocatalyst through rapid transfer or separation of photo-induced charge carriers from the bulk region to the surface. Thus far, several materials have been loaded on BiVO₄ to enhance photocatalytic activity. Kohtani et al. [18] reported that Ag-BiVO₄ showed high photocatalytic activity for decomposing long-chain alkyl-phenols and polycyclic aromatic hydrocarbons under visible light. Long et al. [11] synthesized a Co₃O₄/BiVO₄ composite with improved photocatalytic activity due to the formation of nano-dimensional p–n hetero-junctions with ohmic contacts. Ge [19] also studied the photocatalytic activity of Pd/BiVO₄ and demonstrated that it degraded methyl orange efficiently under visible light irradiation. However, the processes required to load noble metals on BiVO₄ are expensive and complicated. Therefore, development of a simple loading process using low cost materials is needed.

In the present study, carbon-loaded BiVO₄ composite photocatalysts were prepared using a simple impregnation method, and their photocatalytic ability to degrade Rhodamine B dye under visible light irradiation was investigated. Moreover, the mechanisms underlying the enhanced photocatalytic activity of these photocatalysts were also investigated.

Section snippets

Preparation of catalysts

All chemicals were reagent grade and used without further purification. The BiVO₄ powders were synthesized using Bi₂O₃ (99.9%, High Purity Chemicals, Japan) and V₂O₅ (99.9%, High Purity Chemicals, Japan). Stoichiometric mixtures of starting materials were ball-milled in a polyethylene bottle containing ZrO₂ media for 24 h using absolute ethanol. The mixture was then rapidly dried and calcined at 800–900 °C for 2 h. The calcined powders were ball-milled again for 12 h.

The composites with carbon were

Crystal structure and microstructure

It has been reported that BiVO₄ has three main crystal forms: a tetragonal zircon structure (band-gap energy (E_g) = 2.9 eV), a monoclinic scheelite structure (E_g = 2.4 eV), and a tetragonal scheelite structure (E_g = 2.34 eV) [6]. Among these polymorphs, only the monoclinic scheelite structure of BiVO₄ exhibits higher photocatalytic performance under visible light irradiation due to the distortion of the Bi–O polyhedron by the 6s² lone pairs of the Bi³⁺ ion [6]. Therefore, it is imperative that the

Conclusions

Carbon-loaded BiVO₄ composites were prepared by an impregnation method and their photocatalytic activity was investigated. Carbon nano-islands were deposited on the surface of BiVO₄ without agglomeration. As the carbon content increased, the surface area of the BiVO₄ composite powder increased. Moreover, a decrease in band-gap, as well as broad background absorption in the visible light region were observed. Compared to pure (un-loaded) BiVO₄, the carbon-loaded BiVO₄ composites exhibited higher

Acknowledgment

This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MOST) (R01-2007-000-11075-0).

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Effects of carbon content on the photocatalytic activity of C/BiVO4 composites under visible light irradiation

Abstract

Introduction

Section snippets

Preparation of catalysts

Crystal structure and microstructure

Conclusions

Acknowledgment

Appl. Catal. B

Mater. Chem. Phys.

J. Mol. Catal. A: Chem.

Int. J. Hydrogen Energy

Catal. Commun.

Mater. Chem. Phys.

Chem. Rev.

Angew. Chem., Int. Ed.

ChemPhyschem.

Catal. Lett.

Chem. Mater.

Chem. Lett.

Effects of carbon content on the photocatalytic activity of C/BiVO₄ composites under visible light irradiation