Photocatalytic water splitting with a quantum efficiency of almost unity

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Abstract

Overall water splitting, evolving hydrogen and oxygen in a 2:1 stoichiometric ratio,  using particulate photocatalysts is a potential means of achieving scalable and economically viable solar hydrogen production. To obtain high solar energy conversion efficiency, the quantum efficiency of the photocatalytic reaction must be increased over a wide range of wavelengths and semiconductors with narrow bandgaps need to be designed. However, the quantum efficiency associated with overall water splitting using existing photocatalysts is typically lower than ten per cent1,2. Thus, whether a particulate photocatalyst can enable a quantum efficiency of 100 per cent for the greatly endergonic water-splitting reaction remains an open question. Here we demonstrate overall water splitting at an external quantum efficiency of up to 96 per cent at wavelengths between 350 and 360 nanometres, which is equivalent to an internal quantum efficiency of almost unity, using a modified aluminium-doped strontium titanate (SrTiO3:Al) photocatalyst3,4. By selectively photodepositing the cocatalysts Rh/Cr2O3 (ref. 5) and CoOOH (refs. 3,6) for the hydrogen and oxygen evolution reactions, respectively, on different crystal facets of the semiconductor particles using anisotropic charge transport, the hydrogen and oxygen evolution reactions could be promoted separately. This enabled multiple consecutive forward charge transfers without backward charge transfer, reaching the upper limit of quantum efficiency for overall water splitting. Our work demonstrates the feasibility of overall water splitting free from charge recombination losses and introduces an ideal cocatalyst/photocatalyst structure for efficient water splitting.

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Fig. 1: Photocatalytic water-splitting activities.
Fig. 2: Location of cocatalysts.
Fig. 3: Transmission electron microscopy.
Fig. 4: Simulations of photocarrier distributions in SrTiO3:Al particles.

Data availability

Supporting data are available at the Shinshu University Institutional Repository at http://hdl.handle.net/10091/00021822.

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Acknowledgements

This work was primarily supported by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO) and partly supported by JSPS KAKENHI grant number JP19K05669, JP16K06862. A part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (grant number JPMXP09A-19-UT-0023). We are grateful to M. Yamaguchi and Y. Kuromiya at the University of Tokyo for the preparation and evaluation of photocatalysts.

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Authors

Contributions

T.T. conceived the photocatalyst design and performed the photocatalytic reactions. T.H. synthesized Al-doped SrTiO3. J.J. and Y.S. performed quantum efficiency measurements. M.N. and N.S. performed electron microscopy measurements. V.N. and K.S. performed electrical simulations. K.D. supervised the project. T.T., T.H. and K.D. wrote the manuscript with input from all authors.

Corresponding author

Correspondence to Kazunari Domen.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks David Tilley, Martijn Zwijnenburg and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Dependence of water-splitting activities on the amounts and components of cocatalysts.

a, b, Dependence of water-splitting activity on the amount of Cr2O3 added to Rh (0.1 wt%)-loaded SrTiO3:Al (a) and CoOOH added to Rh (0.1 wt%)/Cr2O3(0.05 wt%)-loaded SrTiO3:Al (b). c, Time course of H2 and O2 evolution over SrTiO3:Al unloaded (i) and loaded with Rh (0.1 wt%) (ii), Rh (0.1 wt%)/CoOOH (0.05 wt%) (iii), CoOOH (0.05 wt%) (iv) and CoOOH (0.05 wt%)/Cr2O3 (0.05 wt%) (v).

Extended Data Fig. 2 Reproducibility and stability of water-splitting activity of SrTiO3:Al loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%).

a, Variation of water-splitting activity of SrTiO3:Al loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%) (25 samples) from different batches (B) and lots (L). Batches and lots represent SrTiO3:Al samples and the cocatalysts synthesized and loaded at different timings, respectively. The averages (μ) of the H2 and O2 evolution rates were 3.54 mmol h−1 and 1.78 mmol h−1 and their standard deviations (σ) were 0.26 mmol h−1 and 0.12 mmol h−1, respectively. All the gas evolution rates were within μ ± 2σ. b, Stability test of SrTiO3:Al loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%) during photocatalytic water splitting. The produced H2 and O2 accumulated in the reaction system were evacuated every 2.5 h. The inner wall of the reactor window was cleaned before every evacuation.

Extended Data Fig. 3

Time course of H2 and O2 evolution over SrTiO3:Al photodeposited with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%) under simulated sunlight irradiation.

Extended Data Fig. 4

Effect of initial background pressure on the photocatalytic water-splitting activity of SrTiO3:Al photodeposited with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%).

Extended Data Fig. 5 Simulations of graded band energy values and resulting anisotropic charge separation based on various differences in work function between the {110} and {100} facets.

a, Maps of conduction band energy and logarithms of the electron and hole densities (n and p, respectively). b, c, Energy band diagrams (b) and electron and hole densities (c) as functions of position. See Methods for further discussion.

Extended Data Fig. 6

Power spectrum of the Xe lamp used in the study.

Extended Data Fig. 7

XRD pattern of the synthesized SrTiO3:Al.

Extended Data Fig. 8 EQE measurement.

a, Photographs of the devices used in the measurement. Left, side view of the measurement system. Middle left, top view of the measurement system. Middle right, arrangement of the lamp and reactor. Right, arrangement of the Si photodiode. b, c, Illustrations of water-splitting reactor and photon-counting system (b) and illumination unit (c). d, Example of light intensity distribution (365-nm bandpass filter and 0.1-optical-density neutral density filter; left) and model used to calculate the number of incident photons (right).

Extended Data Table 1 Results of EQE measurements
Extended Data Table 2 Parameters used in the calculations of the energy band diagrams and electron and hole densities for the SrTiO3:Al particulate system

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Takata, T., Jiang, J., Sakata, Y. et al. Photocatalytic water splitting with a quantum efficiency of almost unity. Nature 581, 411–414 (2020). https://doi.org/10.1038/s41586-020-2278-9

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