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Efficient solar water-splitting using a nanocrystalline CoO photocatalyst

Abstract

The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously1,2,3,4,5,6,7,8, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation1,2,3,4. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1−xZnx)(N1−xOx)) do not exhibit good activity beyond 440 nm (refs 1,2,9) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%6,7. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.

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Figure 1: Structural and chemical characterizations of CoO nanoparticles.
Figure 2: Characterizations of hydrogen and oxygen evolutions with gas chromatography and mass spectrometry.
Figure 3: Bandgaps and band-edge positions of CoO nanoparticles and micropowders.

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Acknowledgements

Financial support from the Robert A. Welch Foundation (E-1728), National Science Foundation (DMR 0907336) and Department of Energy (DE-FG02-13ER46917) is acknowledged. We thank J. Shan and P. Tian for assistance with the purchase of the femtosecond laser, M. Shen, J. Deng, R. Thummel, R. Zhong, H. Fang, Z. Tang, D. Achey, X. Ni, J. Sun, X. Ren, Y. Qi, S Pei, J. Zhao, P. Ruchhoeft, Q. Zhang and J. Allen for help and valuable discussions, and Z. Liu, X. Gong, P. Peng, P. Zhu and M. Fang for various contributions.

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Contributions

L.L. and J.M.B. conceived, designed and performed the experiments, and analysed the data. D.W. and F.R-H. performed TEM studies, Q.Z. and Y.L. performed ultraviolet–visible measurements, S.B. and X.C. helped design and fabricate reaction chambers, and assisted in electrochemical studies, Q.Y, J.Z. and G.F. assisted in laser ablation, Z.R. and H.F. assisted in ball-milling synthesis, Z.S., Z.Z., Y.W. and X.L. assisted in mass spectrometry, gas chromatography, SEM studies and the solar-simulator set-up, J.M.B. wrote the manuscript and prepared the figures. All authors discussed the results and commented on the manuscript. J.M.B. supervised the project.

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Correspondence to Jiming Bao.

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Liao, L., Zhang, Q., Su, Z. et al. Efficient solar water-splitting using a nanocrystalline CoO photocatalyst. Nature Nanotech 9, 69–73 (2014). https://doi.org/10.1038/nnano.2013.272

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