Unusual reactivity of visible-light-responsive AgBr–BiOBr heterojunction photocatalysts Liang Kong a , Zheng Jiang a,b,⇑ , Henry H. Lai a , Rebecca J. Nicholls c , Tiancun Xiao a , Martin O. Jones d , Peter P. Edwards a,⇑ a Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, UK b Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK c Department of Materials, University of Oxford, Oxford OX1 3PH, UK d STFC, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton OX11 0QX, UK article info Article history: Received 13 February 2012 Revised 14 June 2012 Accepted 15 June 2012 Available online 3 August 2012 Keywords: AgBr–BiOBr Heterojunction Photocatalysis Visible-light Rhodamine B Deactivation abstract AgBr–BiOBr heterojunction photocatalysts with varying loadings of AgBr (<5.0 wt% AgBr) were synthe- sized through an effective co-precipitation method and used for the photodegradation of Rhodamine B under visible-light irradiation. Superior photocatalytic activities relative to that of pure BiOBr were observed on the AgBr–BiOBr catalysts with low AgBr loading (up to 0.5 wt%). Higher AgBr loadings (>0.5 wt%) lead to isolated AgBr species and reduced photocatalytic activity. Among such catalysts, the AgBr (0.2 wt%)–BiOBr exhibits the highest visible-light-responsive photoactivity, which can decolorize Rhodamine B within 30 min. However, these AgBr–BiOBr materials gradually lost their photoactivity in the cycling photocatalytic tests. Possible mechanisms for both the enhanced photocatalytic activity and deactivation of the AgBr–BiOBr heterojunctions were proposed on basis of theoretical speculation and experimental observations. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Increasing concerns of fossil-fuel depletion and environment deterioration have stimulated great progresses in the potential photocatalytic utilization of solar energy for both the environmen- tal remediation and harvesting solar power [1,2]. However, most of oxide semiconductor photocatalysts (i.e., TiO 2 , SnO x , ZnO, etc.) can mainly harvest UV light, which accounts for less than 5% of the to- tal energy in the solar spectrum [3]. Great efforts have been made in exploring new visible-light-responsive photocatalysts in order to utilize a larger portion of solar energy, particularly extending to the visible-light region [4]. Recently, bismuth oxybromide (BiO- Br), a simple p-type semiconductor, has emerged as a potential vis- ible-light-responsive photocatalyst with excellent photocatalytic activity and stability. Unfortunately, its absorption of visible sun- light is limited by the relatively large electronic bandgap (E g = 2.90 eV) of BiOBr [5]. A variety of strategies, such as impurity doping [6], surface dec- oration (metal deposition or heterojunction) [4,7] and photo-sensi- tization [7], have been developed to modify the photocatalysts in order to improve their absorption to visible light and photocatalytic performance. Among the modified photocatalysts, semiconductor heterojunctions, particularly p–n heterojunctions, offer great prom- ise due to their tunable light absorption, low cost, and additional synergism between their closely contacted p-type and n-type semi- conductor constituents [8]. In practice, an effective heterojunction should have the matched band structure, requisite bandgap energy, suitable molar ratio and close contact between the hybrid semicon- ductors [7,9], which enable increased light absorption, efficient charge separation, and accelerated transfer of photogenerated e  – h + through the interfaces for subsequent photoreactions [10,11]. Most recently, enhanced visible-light-responsive photocatalytic activity has been realized over some BiOCl/Bi 2 O 3 [12], BiOBr/ ZnFe 2 O 4 [13], and AgI/BiOI [14] hybrid p–n heterojunction photocatalysts, in which the bismuth oxyhalide (BiOX, X = Cl, Br, I) semiconductors play key roles. Excellent visible-light-driven photo- catalytic activity and stability were also observed over AgBr-based n–n typed heterojunctions, such as AgBr/TiO 2 , AgBr/ZnO, and AgBr/SiO 2 [15–17], on which a significant amount of AgBr loaded on n-type UV-responsive semiconductors. However, the working mechanism currently remains uncertain since the high AgBr loading amount would hinder the identification of the key roles of the underlying heterojunctions. AgBr (E g = 2.69 eV) [18] has a smaller bandgap and its anionic component might favor a close interface 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.06.011 ⇑ Corresponding authors. Address: Department of Chemistry, Inorganic Chemis- try Laboratory, University of Oxford, Oxford OX1 3QR, UK (Z. Jiang). E-mail addresses: zhjiang76@hotmail.com (Z. Jiang), peter.edwards@chem.ox. ac.uk (P.P. Edwards). Journal of Catalysis 293 (2012) 116–125 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat