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Mutually-dependent kinetics and energetics of photocatalyst/co-catalyst/two-redox liquid junctions

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Water-splitting by photocatalyst particles has attracted much attention recently for its potential to produce renewable H2 at scale. However, the correlation between the energetics at photocatalyst/co-catalyst/water interfaces and their interfacial charge-transfer kinetics is still elusive, especially when the energetics are expected to vary spatially along the liquid-junction interface. First, we derived a kinetic model for photocatalyst particles in contact with two-redox potentials, H+/H2 and O2/H2O, i.e., a semiconductor/two-redox liquid junction. We adopted the principle of detailed balance proven for one-redox liquid junctions and extended this principle to a locally out-of-equilibrium electrolyte containing multiple redox potentials, the condition typical for photocatalysts. To validate the model, we established a characterization framework to simulate photocatalyst operation by using photoelectrodes. The open-circuit conditions mimicked operating photocatalyst surfaces; and the (quasi-) Fermi levels, probed by ohmic back contacts, indicated charge-separation efficiency. Quantitative data fitting further validated the two-redox kinetic model. These characterizations correlated local energetics with multi-electron charge-transfer kinetics, which exhibit tuneable branching ratios controlled by H2-and-O2 gas-mixture compositions and co-catalyst selectivity. Unlike the conventional photoelectrode/electrolyte interfaces, SrTiO3 model particles decorated with Pt co-catalysts were found to bear liquid-junction interfaces of spatially varying energetics with designated reductive and oxidative sites. It is shown that, uniquely for photocatalysts, the local kinetic-controlled energetics vary spatially across photocatalyst/co-catalyst/water interfaces of individual particles, and affect charge-separation efficiency sensitively. The mutually dependent behaviour between local kinetics and spatially varying energetics were confirmed for two practical photocatalytic systems, Al-doped SrTiO3 and Ta3N5. This study exemplified and elucidated the design principles for developing efficient photocatalysts.

Graphical abstract: Mutually-dependent kinetics and energetics of photocatalyst/co-catalyst/two-redox liquid junctions

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The article was received on 10 Sep 2019, accepted on 21 Oct 2019 and first published on 21 Oct 2019

Article type: Paper
DOI: 10.1039/C9EE02910A
Energy Environ. Sci., 2020, Advance Article

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    Mutually-dependent kinetics and energetics of photocatalyst/co-catalyst/two-redox liquid junctions

    Z. Pan, R. Yanagi, Q. Wang, X. Shen, Q. Zhu, Y. Xue, J. A. Röhr, T. Hisatomi, K. Domen and S. Hu, Energy Environ. Sci., 2020, Advance Article , DOI: 10.1039/C9EE02910A

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