Comparing photoelectrochemical water oxidation, recombination kinetics and charge trapping in the three polymorphs of TiO2

Benjamin Moss,James Durrant,Savio Moniz,Kee Kean Lim,Paolo Fornasiero,Andreas Kafizas,Piers Barnes,Junwang Tang,Alessandro Beltram

doi:10.1038/s41598-017-03065-5

Abstract

In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.

Highlights

Many first row transition metal oxides (TMOs) show great potential for use as photoelectrodes for the generation solar fuels due to their abundance, stability and low toxicity[1,2,3]
Particle shape and size varied between the polymorphs, the specific surface area determined through BET analysis (Supplementary Fig. S6) was commensurate in all phases - with anatase presenting around twice the surface area of brookite or rutile
UV-visible spectroscopy shows the absorptance of anatase and

Summary

Introduction

Many first row transition metal oxides (TMOs) show great potential for use as photoelectrodes for the generation solar fuels due to their abundance, stability and low toxicity[1,2,3]. A technique capable of measuring the density of occupied mid-gap trap states (DOTS), known as transient infrared absorption - energy scanning spectroscopy (TRIRA-ESS), was employed by Weng and co-workers to investigate the polymorphs of TiO2. Subsequent work using TRIRA-ESS has shown that this feature is absent in rutile, where only deep traps are observed, but present in brookite. Most recently, this line of investigation has suggested that differences in the DOTS between the phases may be related to their differing capacities for overall water splitting[26, 27]

Methods

Results

Conclusion