Unravelling local environments in mixed TiO2–SiO2 thin films by XPS and ab initio calculations

Pavel Ondračka,David Nečas,Michèle Carette,Stéphane Elisabeth,David Holec,Agnès Granier,Antoine Goullet,Lenka Zajíčková,Mireille Richard-Plouet

doi:10.1016/j.apsusc.2019.145056

Abstract

Mixed TixSi1−xO2 oxide can exhibit a partial phase separation of the TiO2 and SiO2 phases at the atomic level. The quantification of TiO2–SiO2 mixing in the amorphous material is complicated and was so far done mostly by infrared spectroscopy. We developed a new approach to the fitting of X-ray photoelectron spectroscopy data for the quantification of partial phase separation in amorphous TixSi1−xO2 thin films deposited by plasma enhanced chemical vapour deposition. Several fitting constraints reducing the total number of degrees of freedom in the fits and thus the fit uncertainty were obtained by using core electron binding energies predicted by density functional theory calculations on TixSi1−xO2 amorphous supercells. Consequently, a decomposition of the O1s peak into TiO2, SiO2 and mixed components was possible. The component areas ratios were compared with the ratios predicted by older theoretical models based on the atomic environment statistics and we also developed several new models corresponding to more realistic atomic structure and partial mixing. Based on the comparison we conclude that the studied films are mostly disordered, with only a moderate phase separation.

Highlights

The mixed TiO2–SiO2 oxides have attracted considerable attention in the area of photocatalysis as it has been shown that they are more active than pure TiO2 [1,2,3]
Ab initio modelling techniques were employed to test if a decomposition of the O 1s peak into just the TiO2, mixed and SiO2 parts in a real three dimensional structure can be justified and to reveal any trends which could be used to reduce the uncertainty during the experimental fits
An oxygen atom can have two, three or four neighbours with different number of Ti or Si atoms – see Fig. 2a for the environments found in structures with x = 0.5

Summary

Introduction

The mixed TiO2–SiO2 oxides have attracted considerable attention in the area of photocatalysis as it has been shown that they are more active than pure TiO2 [1,2,3]. The atomic structure of mixed TiO2–SiO2 films has been studied quite extensively as it influences both the optical and catalytic activity. There are some contradictory reports mentioning dominantly 6-coordinated Ti atoms even at low TiO2 concentrations [20,21]. It has been established, both experimentally and theoretically, that there are no direct Ti–Ti, Si–Si or Ti–Si bonds, and the Ti and Si atoms are always interconnected through the bridging oxygen atoms [16,22,23]

Methods

Results

Conclusion