Tunable Ti3+-Mediated Charge Carrier Dynamics of Atomic Layer Deposition-Grown Amorphous TiO2

Jesse Saari,Harri Ali-Löytty,Markku Hannula,Minttu Maria Kauppinen,Henrik Grönbeck,Mika Valden,Kimmo Lahtonen,Antti Tukiainen,Nikolai V Tkachenko,Lauri Palmolahti,Ramsha Khan

doi:10.1021/acs.jpcc.1c10919

Jesse Saari, Harri Ali-Löytty + Show 9 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c10919

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Abstract

Amorphous titania (am.-TiO2) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and electrical properties compared to crystalline TiO2. Here, we study the effects of intrinsic Ti3+ and nitrogen defects in am.-TiO2 thin films via the atomic layer deposition (ALD) chemistry of tetrakis(dimethylamido)titanium(IV) (TDMAT) and H2O precursors at growth temperatures of 100–200 °C. X-ray photoelectron spectroscopy (XPS) and computational analysis allow us to identify structural disorder-induced penta- and heptacoordinated Ti4+ ions (Ti5/7c4+), which are related to the formation of Ti3+ defects in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric composition, which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti3+ 3d in-gap states increases with the ALD growth temperature, inducing both visible-light absorption and electrical conductivity via the polaron hopping mechanism. At 200 °C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO2 that are essential to the performance of protective photoelectrode coatings in photoelectrochemical solar fuel reactors.

Highlights

Since the discovery of photoelectrochemical (PEC) water splitting introduced first by Fujishima and Honda in 1972 using n-type rutile titanium dioxide (TiO2), photocatalysts based on crystalline TiO2 have been widely studied materials.[1]
To investigate the effect of the atomic layer deposition (ALD) growth temperature on the chemical and electronic structures of am.-TiO2, the 30 nm thick ALD am.-TiO2 grown at 100, 150, 175, and 200 °C was measured by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)
Annealing the sample grown at 100 °C in air at 500 °C resulted in anatase TiO2 (Figure S1), which served as a reference for the six-coordinated Ti4+ (Ti6c4+).[3,4]

Summary

■ INTRODUCTION

Since the discovery of photoelectrochemical (PEC) water splitting introduced first by Fujishima and Honda in 1972 using n-type rutile titanium dioxide (TiO2), photocatalysts based on crystalline TiO2 have been widely studied materials.[1]. PES valence band (VB) spectra, electron trap states of TiO2 are commonly located around 0.2−1.2 eV below the Fermi level.[21,24,26,28,31] Transient absorption spectroscopy, instead, provides a method to study the dynamics of charge carriers, and determine, e.g., carrier lifetime that is a critical factor to determine the photocatalytic activity of a material.[32−35]. Previous computational studies have employed molecular dynamics to generate structural models for am.TiO2 using the melt-and-quench method.[5,38] As the structural models for am.-TiO2 necessarily have large unit cells containing >200 atoms, sampling the core-level shifts (CLSs) of all atoms for many structures becomes unfeasible It is not clear how representative the melt-and-quench structures are for the experimental am.-TiO2 structures. The lifetime decay of the samples was measured up to 5 ns for all the samples

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES