FIRST PRINCIPLES COMPUTATIONS Properties of amorphous and crystalline titanium dioxide from first principles

We used first-principles methods to generate amorphous TiO2 (a-TiO2) models and our simulations lead to chemically ordered amorphous networks. We analyzed the structural, electronic, and optical properties of the resulting structures and compared with crystalline phases. We propose that two peaks found in the Ti–Ti pair correlation correspond to edge-sharing and corner-sharing Ti–Ti pairs. Resulting coordination numbers for Ti (*6) and O (*3) and the corresponding angle distributions suggest that local structural features of bulk crystalline TiO2 are retained in a-TiO2. The electronic density of states and the inverse participation ratio reveal that highly localized tail states at the valence band edge are due to the displacement of O atoms from the plane containing three neighboring Ti atoms; whereas, the tail states at the conduction band edge are localized on over-coordinated Ti atoms. The C-point electronic gap of *2.2 eV is comparable to calculated results for bulk crystalline TiO2 despite the presence of topological disorder in the amorphous network. The calculated dielectric functions suggest that the amorphous phase of TiO2 has isotropic optical properties in contrast to those of tetragonal rutile and anatase phases. The average static dielectric constant and the fundamental absorption edge for a-TiO2 are comparable to those of the crystalline phases. Introduction The discovery of titanium dioxide’s ability to split water by photocatalysis under ultraviolet light by Fujishima and Honda [1] has led to enormous work on the material (see Ref. [2] for a recent review). The hope is that titanium dioxide (TiO2), widely used as a pigment in white paint and in sunscreen, may prove to be an economical material for use in photovoltaic, photocatalytic, and sensing applications [2]. The majority of studies on titania are based upon three crystalline phases (anatase, rutile, and brookite), as well as in multiple forms (bulk, nanoparticle, thin film, etc.). However, titania is naturally obtained as powder consisting of a mixture of crystalline and amorphous phases. Various methods have been employed to enhance the crystalline quality of titania (e.g., Ref. [3]). However, recent research, including the results presented herein, has focused on understanding the structural and electronic properties of amorphous titania (a-TiO2) with the hope that the desirable properties of TiO2 can be found in this less processed, thus cheaper, form of the material [4–9]. For example, a-TiO2 has been synthesized as a tinted or enhanced photocatalyst [10, 11], used to purify dye-polluted water [12], and applied to resistive random access memory applications [13]. As more synthesis techniques, like those of Battiston et al. [14] and Zhao et al. [15], are developed to create a-TiO2 thin films, a-TiO2 will be used in applications traditionally reserved for crystalline TiO2 or other more expensive amorphous films. Through our research, we aim for a deeper understanding of the energetic and electronic properties of a-TiO2 while confirming structural properties to aid in the development of these materials as a viable solution to current energy and environmental issues. B. Prasai B. Cai D. A. Drabold (&) Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA e-mail: [email protected] M. K. Underwood J. P. Lewis Department of Physics, West Virginia University, Morgantown, WV 26506, USA