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Binary molecular-semiconductor p–n junctions for photoelectrocatalytic CO2 reduction

Abstract

In one approach to solar energy conversion, light-harvesting sensitizers absorb and convert photons into electron–hole pairs to drive water splitting or CO2 reduction to produce fuels. Despite recent progress in photoelectrocatalytic cells, experimental realization of a high-performance photocathode for solar-driven CO2 reduction has proven difficult. Here, we use a binary p–n junction strategy to prepare a series of photocathodes that convert sunlight into high-energy electrons for efficient CO2 reduction to formate. The photocathodes integrate a semiconductor p–n junction comprising GaN nanowire arrays on silicon, with molecular p–n junctions self-assembled on the semiconductor surface. Solar irradiation of the photocathodes generates redox-separated states that interact to form an intermediate state with remotely separated electrons and holes at the catalyst and semiconductor, respectively. The photocathodes reduce CO2 to formate at stable photocurrent densities of around −1.1 mA cm−2 during 20 h of irradiation with Faradaic efficiencies of up to 64%.

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Fig. 1: Molecular and electronic structures.
Fig. 2: Photoelectrocatalytic currents and products.
Fig. 3: Photocathodes with diluted surface coverages of the assembly, and photoelectrocatalytic performances.
Fig. 4: Photoinduced electron transfer in the molecular assemblies characterized by transient absorption spectroscopy.
Fig. 5: Photoinduced electron transfer steps towards CO2 reduction.
Fig. 6: Reductive quenching and energy profiles for the NPhN derivatives.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The research on molecular assemblies and the design and fabrication of photocathodes and photoelectrocatalysis was supported by the US Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, under award number DE-SC0015739 (to B.S., T.-T.L. and T.J.M.). The design and synthesis of Si | n-GaN photocathodes was supported by the HydroGEN Advanced Water Splitting Materials Consortium, which was established as part of the Energy Materials Network under the US DOE Office of Energy Efficiency and Renewable Energy Fuel Cell Technologies Office under award number DE-EE0008086 (S.V. and Z.M.). The experiments with nanosecond transient absorption and using the fluorimeter and solar simulator were performed with the instruments within the AMPED EFRC Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels, an Energy Frontier Research Center funded by the US DOE Office of Science, Office of Basic Energy Sciences under award DE-SC0001011. The ALD experiments were performed at the Chapel Hill Analytical and Nanofabrication Laboratory—a member of the North Carolina Research Triangle Nanotechnology Network supported by the National Science Foundation (grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure.

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T.J.M. and B.S. conceived the research. T.J.M., B.S., Z.M. and S.V. designed the experiments. B.S., S.V., T.-T.L., L.T.-G. and M.K.B. performed the experiments and measurements. B.S. synthesized NPhN, RuCt and the surface diluent. L.T.-G. synthesized Ru(CP)22+. S.V. and Z.M. synthesized the Si | n-GaN electrode. T.J.M. and B.S. wrote the paper with input from all authors.

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Correspondence to Thomas J. Meyer.

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Shan, B., Vanka, S., Li, TT. et al. Binary molecular-semiconductor p–n junctions for photoelectrocatalytic CO2 reduction. Nat Energy 4, 290–299 (2019). https://doi.org/10.1038/s41560-019-0345-y

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