Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory.

John J Carey,James A Quirk,Keith P Mckenna

doi:10.1021/acs.jpcc.1c03136

John J Carey, James A Quirk + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c03136

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Abstract

Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO2-B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO2-B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO2 in photocatalysis and photoelectrochemistry, which we discuss.

Highlights

Titanium dioxide (TiO2) is a non-toxic and earth-abundant semiconductor that finds a number of photocatalytic and renewable energy applications, for example, as an electron transport layer in solar cell devices,[1] a photocatalyst for removing harmful NOx species from the environment, and a water splitting catalyst for hydrogen production.[2]
For photochemical applications, electrons and holes that are photogenerated in the bulk need to diffuse efficiently to surfaces where they can interact with adsorbates to facilitate chemical reactions.[3,4]
Complex features in temperature-dependent photoluminescence spectra can be interpreted in terms of carrier dynamics, where evidence suggests that in general holes are less mobile than electrons and the determining factor for recombination.[20,21]

Summary

Introduction

Titanium dioxide (TiO2) is a non-toxic and earth-abundant semiconductor that finds a number of photocatalytic and renewable energy applications, for example, as an electron transport layer in solar cell devices,[1] a photocatalyst for removing harmful NOx species from the environment, and a water splitting catalyst for hydrogen production.[2]. Even if small polarons can form, it does not necessarily mean the mechanism of long range carrier diffusion is purely polaronic (i.e., hopping of polarons between neighboring lattice sites) Instead, it may be more favorable for the small polaron to thermally ionize before propagating as a band-like carrier and subsequently retrapping.[11−13] It is challenging to probe such processes experimentally and a deep understanding is currently missing. The dynamics of photoinduced holes are critical in nanostructured TiO2 photoelectrodes for water splitting.[18,19] Complex features in temperature-dependent photoluminescence spectra can be interpreted in terms of carrier dynamics, where evidence suggests that in general holes are less mobile than electrons and the determining factor for recombination.[20,21] The examples given above provide clear evidence of the importance of hole polaron migration in TiO2 and need for deeper atomistic insights

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