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Structural evolution of titanium dioxide during reduction in high-pressure hydrogen

Nature Materials volume 17, pages 923–928 (2018)Cite this article

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Abstract

The excellent photocatalytic properties of titanium oxide (TiO2) under ultraviolet light have long motivated the search for doping strategies capable of extending its photoactivity to the visible part of the spectrum. One approach is high-pressure and high-temperature hydrogenation, which results in reduced ‘black TiO2’ nanoparticles with a crystalline core and a disordered shell that absorbs visible light. Here we elucidate the formation mechanism and structural features of black TiO2 using first-principles-validated reactive force field molecular dynamics simulations of anatase TiO2 surfaces and nanoparticles at high temperature and under high hydrogen pressures. Simulations reveal that surface oxygen vacancies created upon reaction of H2 with surface oxygen atoms diffuse towards the bulk material but encounter a high barrier for subsurface migration on {001} facets of the nanoparticles, which initiates surface disordering. Besides confirming that the hydrogenated amorphous shell has a key role in the photoactivity of black TiO2, our results provide insight into the properties of the disordered surface layers that are observed on regular anatase nanocrystals under photocatalytic water-splitting conditions.

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Fig. 1: Surface stability in H2 and mixed H2/H2O atmosphere from ab initio thermodynamics calculations.
Fig. 2: Time evolution of the reduction process in slabs exposing different surfaces.
Fig. 3: Diffusion and distribution of oxygen vacancies in slabs exposing different surfaces.
Fig. 4: Reduction and disordering of a spherical nanoparticle during and after high-pressure hydrogenation.
Fig. 5: Spatial distribution of oxygen vacancies and surface disordering after hydrogenation.
Fig. 6: Effects of hydrogenation on the electronic DOS.

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Acknowledgements

This work was supported by DoE-BES, the Division of Chemical Sciences, Geosciences and Biosciences under award DE-FG02-12ER16286. We used the resources of the National Energy Research Scientific Computing Center (DoE contract number DE-AC02-05CH11231). We also acknowledge use of the TIGRESS High Performance Computer Center at Princeton University.

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  1. Department of Chemistry, Princeton University, Princeton, NJ, USA

    Sencer Selcuk, Xunhua Zhao & Annabella Selloni

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  1. Sencer Selcuk
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Contributions

A.S. initiated and supervised this research project. S.S. designed the models and the computational approaches, and performed the simulations, their analysis and visualization. X.Z. performed and analysed the DFT cluster calculations of the electronic properties. All authors contributed to discussions and writing the manuscript.

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Correspondence to Sencer Selcuk.

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Supplementary information

Supplementary Information

Supplementary Sections 1–5, Supplementary Figures 1–18, Supplementary References 1–9

Supplementary Video 1

Surface Reduction

Supplementary Video 2

VO dynamics

Supplementary Video 3

Amorphization Dynamics

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Selcuk, S., Zhao, X. & Selloni, A. Structural evolution of titanium dioxide during reduction in high-pressure hydrogen. Nature Mater 17, 923–928 (2018). https://doi.org/10.1038/s41563-018-0135-0

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