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Covalent Fixation of Surface Oxygen Atoms on Hematite Photoanode for Enhanced Water Oxidation Zhuofeng Hu, Zhurui Shen, and Jimmy C Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04058 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015
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Chemistry of Materials
Covalent Fixation of Surface Oxygen Atoms on Hematite Photoanode for Enhanced Water Oxidation Zhuofeng Hu,†,§ Zhurui Shen*,†,‡,§ and Jimmy C. Yu*,†,§ Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, PR China.
†
Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China
‡
§
Shenzhen Research Institute, The Chinese University of Hong Kong, Shenzhen, PR China
ABSTRACT: Suppression of surface states is one of the general issues for metal oxide photoanodes in water oxidation. For hematite (α-Fe2O3), the surface states are mainly attributed to Fe3+/Fe2+ redox couples in oxygen deficient regions (surface oxygen vacancies). To date, most of the passivation overlayers against surface states are metal oxides. However, oxygen vacancies are prevalent for most metal oxides. This is because their formation in metal oxides is often thermodynamically favorable. In contrast, the formation of oxygen vacancies is more energy-consuming when oxygen atoms are covalently bonded. Based on this understanding, we propose a new strategy to transform the surface of Fe2O3 into amorphous iron phosphate (denoted “Fe-Pi”), where the oxygen atoms are “covalently fixed” in phosphate (PO43-). As a result, the oxygen vacancies are decreased and the surface states are effectively suppressed. The onset potential of corresponding photoanode shifts negatively by 0.15 V and the photocurrent density increases by 4.2 (simulated sunlight) and 4.1 (visible light) times. The suppression of surface states by amorphous Fe-Pi overlayer is then confirmed by series of electrochemical analysis. This work is expected to create new opportunities for optimizing the performance of Fe2O3 and other metal oxide photoanodes.
1. Introduction Hematite has been extensively studied as a promising photoanode for photoelectrochemical (PEC) water oxidation due to its low toxicity, sufficient positive valance band edge position for water oxidation and high chemical stability in alkaline electrolytes.1-8 With a band gap of 2.2 eV, Fe2O3 has a theoretical solar-to-hydrogen efficiency of 16.8 % under AM 1.5G irradiation.9 However, it is difficult to reach the theoretical value due to several limitations. Sluggish water oxidation kinetics shifts the onset potential positively by 0.4-0.6 V.10 This problem can be overcome by integrating Fe2O3 with oxygen evolution cocatalysts like cobalt-phosphate (Co-Pi)11-13 and iridium oxide (IrO2).14-17 Moreover, the charge transfer in αFe2O3 is slow due to low carrier mobility (
