DOI: 10.1002/cctc.201300440 Photocatalytic Activity and Selectivity of ZnO Materials in the Decomposition of Organic Compounds Feng Lin,* [a, e] Bogdan Cojocaru, [b] Chia-Ling Chou, [a] Christopher A. Cadigan, [a] Yazhou Ji, [a] Dennis Nordlund, [c] Tsu-Chien Weng, [c] Zhifeng Zheng, [a, d] Vasile I. Pârvulescu,* [b] and Ryan M. Richards* [a] Introduction ZnO is a particularly interesting semiconductor with a wide direct band-gap and a large electron–hole binding energy. [1] Nanostructured ZnO has been intensively investigated owing to its versatile and tunable morphologies and properties. [2, 3] The hybridization of ZnO materials with conventional dopants or nanoparticles has been considered as a feasible method of modifying its electronic structure and properties. [4–8] Specifical- ly, efforts have been devoted to achieving p-type ZnO through elemental doping. [9] However, the synthesis of p-type ZnO re- mains challenging if alkali metals, such as Li and Na, are used as dopants, because the interstitial sites in wurtzite ZnO are more favorable for alkali ions, owing to their small ionic sizes, thus resulting in n-type semiconductive ZnO. [10, 11] Furthermore, the relocation of lithium ions in ZnO lattices was observed in recent studies and the ratio between Li Zn (i.e., Li in the Zn sites) and Li i · (i.e., Li in the interstitial sites) is temperature and process dependent. [11] The intrinsic mobility of Li doping usual- ly leads to the complexity of defect states in ZnO, including those at the surface, [12] which then influence electron–hole separation under photon excitation, such as UV irradiation. [12–14] Thus, an understanding of the surface-defect states is relevant to many properties of ZnO materials, including photocatalytic activity and selectivity. [14–17] The photocatalytic degradation of pollutants in the gas or liquid phases is an efficient process for environmental remedia- tion and water decontamination. [18] One advantage of photoca- talytic degradation over adsorbent methods is the facile recy- clability of the photocatalysts. [19] Typically, thermal combustion is necessary for the removal of pollutants from adsorbents; [20, 21] however, this process requires high temperatures and, thus, is energy intensive. ZnO-based materials are efficient photocata- lysts for the degradation of pollutants (e.g., organic dyes) under UV irradiation. [5] Dopants and nanocomposite materials have been investigated for improving the photocatalytic activi- ty of ZnO materials. [7, 22, 23] This improved photocatalytic activity has mainly been ascribed to a modified band structure and/or improved electron–hole separation. [22] Interestingly, several studies have reported decreased photocatalytic activity ZnO and Li-doped ZnO photocatalysts were prepared by using a solvothermal method, aided by a supercritical drying tech- nique. The structure and morphology of the photocatalysts were investigated by using SEM, X-ray diffraction (XRD), UV/Vis and Raman spectroscopy. The photocatalytic activity and selec- tivity were investigated in the aqueous-phase photodegrada- tion of methylene blue and phenol as model reactions. Herein, it is reported for the first time that Li doping can lead to signif- icant deactivation of the photocatalytic activity (i.e., decreased oxidization capability) of ZnO materials. The distribution of in- termediate products (i.e., selectivity) was also significantly modified in the decomposition of phenol catalyzed by Li-doped ZnO compared to that catalyzed by ZnO. Photolumi- nescence (PL) and soft X-ray absorption spectroscopy (XAS) studies suggested that dopant-induced surface-defect states acted as electron–hole combination centers and changed the adsorbate/surface binding, thus causing the deactivation of photocatalytic activity and altering the photocatalytic selectivi- ty in Li-doped ZnO materials. [a] Dr. F. Lin, C.-L. Chou, Dr. C. A. Cadigan, Y. Ji, Prof. Dr. Z. Zheng, Prof. Dr. R. M. Richards Department of Chemistry and Geochemistry Materials Science Colorado School of Mines Golden, CO 80401 (USA) E-mail : rrichard@mines.edu [b] Dr. B. Cojocaru, Prof. Dr. V. I. Pârvulescu Department of Organic Chemistry Biochemistry and Catalysis Faculty of Chemistry University of Bucharest Bdul Regina Elisabeta, 4-12, Bucharest 030016 (Romania) E-mail : vasile.parvulescu@g.unibuc.ro [c] Dr. D. Nordlund, Dr. T.-C. Weng Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park, CA 94025 (USA) [d] Prof. Dr. Z. Zheng College of Materials Engineering Southwest Forestry University Kunming 650224 (China) [e] Dr. F. Lin Current Address: Environmental Energy Technologies Division Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail : flin@lbl.gov # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3841 – 3846 3841 CHEMCATCHEM FULL PAPERS