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Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts

Abstract

Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.

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Fig. 1: The colour of doped and undoped SrTiO3 powders and the morphology of the photocatalyst sheets.
Fig. 2: Connecting the oxidation of Rh to the charge carrier dynamics in (La,)Rh:SrTiO3 photocatalyst sheets.
Fig. 3: Effect of Rh and La doping on theoretical and experimental band structure of SrTiO3.
Fig. 4: A simplified electronic structure model.
Fig. 5: A simple surface depletion model that explains the in situ charge carrier dynamics of (La,)Rh:SrTiO3 photocatalyst sheets.
Fig. 6: The role of the Rh oxidation state and La co-doping in determining the performance of a Mo:BiVO4/(La),Rh:SrTiO3 Z-scheme device.

Data availability

The data presented in the main body of this paper are available in csv format on Zenodo at https://doi.org/10.5281/zenodo.4063942 and source data are available in opj format on Zenodo at https://doi.org/10.5281/zenodo.4071556. Both can be used under the Creative Commons Attribution licence 4.0.

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Acknowledgements

B.M. thanks the EPSRC for a doctoral training partnership and the UK Solar Fuels Network for an exchange scholarship to the group of K.D. B.M. also thanks R. Palgrave for discussion of his previous work on Rh-doped TiO2 and doped oxides and D. H. K. Murthy for advice on sample choice and handling. S.S. thanks the EPSRC for a doctoral training partnership. L.S. and J.R.D. acknowledge funding from the European Research Council (H2020-MSCA-IF-2016, Grant no. 749231 and AdG Intersolar, Grant no. 291482, respectively). Funding was also obtained from the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization. A.R. acknowledges the support from the Analytical Chemistry Trust Fund for her CAMS-UK Fellowship and from Imperial College London for her Imperial College Research Fellowship. K.D. thanks the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO) for support. R.G. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for operational funding (Grant no. RGPIN-2019-05521). A.K. thanks Imperial College for a Junior Research Fellowship, the EPSRC for a Capital Award Emphasising Support for Early Career Researchers and the Royal Society for an Equipment Grant (RSG\R1\180434).

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B.M. carried out all the optical, SEM and XPS measurements and wrote the manuscript with help from L.S. and S.S. XPS measurements and interpretation of the results were supervised by A.R. and D.J.P. Q.W. synthesized all the materials, fabricated all the devices and performed the X-ray diffraction. K.T.B. performed all the calculations of doped, co-doped and undoped SrTiO3, with the exception of the DFT study of Rh-doping concentration, which was performed by R.G.-C. L.S. supervised this work, guided the SEM, EDX and SEC measurements, oversaw data interpretation and manuscript preparation and conceptualized Figs. 4–6. R.G. and A.K. trained B.M. and supervised the optical measurements. T.H. co-supervised this work and K.D. and J.R.D. directed the research. All the authors commented on the manuscript.

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Correspondence to Ludmilla Steier.

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Supplementary Methods, Table 1, Figs. 0–7 and associated brief discussions.

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Moss, B., Wang, Q., Butler, K.T. et al. Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts. Nat. Mater. 20, 511–517 (2021). https://doi.org/10.1038/s41563-020-00868-2

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