Dispersion of Defects in TiO2 Semiconductor: Oxygen Vacancies in the Bulk and Surface of Rutile and Anatase

Mohammadreza Elahifard,Amir Mirzanejad,Reza Behjatmanesh-Ardakani,Seyedsaeid Ahmadvand,Mohammad Reza Sadrian

doi:10.3390/catal10040397

Mohammadreza Elahifard, Amir Mirzanejad + Show 3 more

Open Access

https://doi.org/10.3390/catal10040397

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Abstract

Oxygen deficiency (O-vacancy) contributes to the photoefficiency of TiO2 semiconductors by generating electron rich active sites. In this paper, the dispersion of O-vacancies in both bulk and surface of anatase and rutile phases was computationally investigated. The results showed that the O-vacancies dispersed in single- and double-cluster forms in the anatase and rutile phases, respectively, in both bulk and surface. The distribution of the O-vacancies was (roughly) homogeneous in anatase, and heterogenous in rutile bulk. The O-vacancy formation energy, width of defect band, and charge distribution indicated the overlap of the defect states in the rutile phase and thus eased the formation of clusters. Removal of the first and the second oxygen atoms from the rutile surface took less energy than the anatase one, which resulted in a higher deficiency concentration on the rutile surface. However, these deficiencies formed one active site per unit cell of rutile. On the other hand, the first O-vacancy formed on the surface and the second one formed in the subsurface of anatase (per unit cell). Supported by previous studies, we argue that this distribution of O-vacancies in anatase (surface and subsurface) could potentially create more active sites on its surface.

Highlights

Nonstoichiometric titanium dioxide (TiO2 ) has drawn broad interest due to its important role in photocatalysis, solar cells, and environmental procedures [1,2,3,4,5,6,7,8,9]
Having a looser and more flexible structure, anatase has been reported to have higher O-vacancy concentration and higher photocatalytic activity based on ultraviolet photoemission spectroscopy (UPS) results [16]
Modeling was performed for different defective structures to study the size effects (Ti8 O16, Ti16 O32, Ti32 O64, and Ti64 O128 ), and eventually the primitive cells Ti16 O32, Ti16 O31 and Ti16 O30 were used to simulate the perfect and single/double O-vacancy structures, respectively

Summary

Introduction

Nonstoichiometric titanium dioxide (TiO2 ) has drawn broad interest due to its important role in photocatalysis, solar cells, and environmental procedures [1,2,3,4,5,6,7,8,9]. Having a looser and more flexible structure, anatase has been reported to have higher O-vacancy concentration and higher photocatalytic activity based on ultraviolet photoemission spectroscopy (UPS) results [16]. Many other experimental results have indicated a lower concentration of O-vacancy on the surface of anatase compared to rutile [17,18]. A majority of experimental studies, suggest a higher surface/bulk ratio of O-vacancy in rutile [10,20]. To shed more light on the existing controversy, this study investigated the dispersion of O-vacancies in the bulk and surface of anatase and rutile. To this end, single O-vacancies and their effects were computationally studied and compared with those of double O-vacancies. All geometrically unique arrangements were simulated for single/double O-vacancy structures using full potential DFT calculations

Computational Details

O-Vacancy in the Bulk Structure of Rutile and Anatase

O-Vacancy on the Surface of Rutile and Anatase

Conclusions