DOI: 10.1002/chem.201202602 Exploring the Reactivity of Multicomponent Photocatalysts: Insight into the Complex Valence Band of BiOBr Yan-Fen Fang, [a] Wan-Hong Ma, [a] Ying-Ping Huang,* [a] and Gen-Wei Cheng [b] Introduction Since Asahi and co-workers reported in 2001 that N-doped TiO 2 responds to visible light, [1] the difference in oxidative reactivity between visible- and UV-induced activation of multicomponent semiconductor photocatalysts has remained unclear. [2] Bismuth oxybromide (BiOBr), which has two kinds of anions (O 2 and Br  ) and responds to both UV and visible irradiation, [3] is an ideal system to investigate the band structure of multicomponent semiconductors and their reactivity under different wavelength regions of irradiation. BiOBr may have two plausible valence-band structures : 1) one valence band derived from the hybridization of O 2p and Br 4p orbitals, the activation of which by different wavelengths of light would lead to a hole (h vb + ) with the same reactivity; and 2) two discrete valence bands construct- ed respectively by O 2p and Br 4p orbitals, which are excited by different wavelengths of light (O 2p to Bi 6p and Br 4p to Bi 6p) and have h vb + with different oxidation potentials (h O2p + and h Bi 6p + ). [4] Even though the catalyst has a band structure of the second kind, the interband relaxation of the h vb + (h O2p + to h Br 4p + ) would also lead to a single h vb + type (h Br 4p + ) under both UV and visible irradiation (Figure 1), which makes the determination of the band structure very difficult. In photocatalytic reactions, COH is the most important radical intermediate. It is formed by the oxidation of H 2 O Abstract: The band structure of multi- component semiconductor photocata- lysts, as well as their reactivity distinc- tion under different wavelengths of light, is still unclear. BiOBr, which is a typical multicomponent semiconductor, may have two possible valence-band structures, that is, two discrete valence bands constructed respectively from O 2p and Br 4p orbitals, or one valence band derived from the hybridization of these orbitals. In this work, aqueous photocatalytic hydroxylation is applied as the probe reaction to investigate the nature and reactions of photogenerated holes in BiOBr. Three organic com- pounds (microcystin-LR, aniline, and benzoic acid) with different oxidation potentials were selected as substrates. Isotope labeling (H 2 18 O as the solvent) was used to determine the source of the O atom in the hydroxyl group of the products, which distinguishes the contribution of different hydroxylation pathways. Furthermore, a spin-trapping ESR method was used to quantify the reactive oxygen species (COH and COOH) formed in the reaction system. The different isotope abundances of the hydroxyl O atom of the products formed, as well as the reverse trend of the COH/COOH ratio with the oxidative resistance of the substrate under UV and visible irradiation, reveal that BiOBr has two separate valence bands, which have different oxidation ability and respond to UV and visible light, re- spectively. This study shows that the band structure of semiconductor photo- catalysts can be reliably analyzed with an isotope labeling method. Keywords: ESR spectroscopy · ACHTUNGTRENNUNGhydroxylation · isotopic labeling · photocatalysis · semiconductors [a] Dr. Y.-F. Fang, Prof. W.-H. Ma, Prof. Y.-P. Huang Engineering Research Centre of Eco-environment in Three Gorges Reservoir Region, Ministry of Education China Three Gorges University, Hubei 443002 (P.R. China) Fax: (+ 86) 717-639-7488 E-mail : huangyp@ctgu.edu.cn [b] Prof. G.-W. Cheng Institute of Mountain Hazards and Environment of Chinese Academy of Sciences Chengdu 610041 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201202602. Figure 1. Potentials (versus normal hydrogen electrode, NHE) for the single-electron transfer of H 2 O and O 2 and substrates used in this investi- gation, with corresponding reactions on the conduction band (CB) and valence band (VB) of activated BiOBr. No assumptions are made con- cerning the type of charge transfer involved. ROS = reactive oxygen spe- cies, MC-LR = microcystin-LR.  2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 3224 – 3229 3224