Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci Tailoring the structural properties and electronic structure of anatase, brookite and rutile phase TiO 2 nanoparticles: DFTB calculations Hasan Kurban a,b , Mehmet Dalkilic a , Selçuk Temiz c , Mustafa Kurban d, ⁎ a Computer Science Department, Indiana University, Bloomington 47405 IN, USA b Computer Engineering Department, Siirt University, 56100 Siirt, Turkey c Department of Metallurgical and Materials Engineering, Eskişehir Osmangazi University, 26040 Eskişehir, Turkey d Department of Electronics and Automation, Kırşehir Ahi Evran University, 40100 Kırşehir, Turkey ARTICLE INFO Keywords: Nanoparticles TiO 2 Segregation phenomena DFTB ABSTRACT In this study, we perform a theoretical investigation using the density functional tight-binding (DFTB) approach for the structural analysis and electronic structure of anatase, brookite and rutile phase TiO 2 nanoparticles (NPs). Our results show that the number of Ti-O bonds is greater than that of O-O, while the number of Ti-Ti bonds is fewer. Thus, large amounts of O atoms prefer to connect to Ti atoms. The increase in the temperature of the NPs contributes to an increase in the interaction of Ti–O bonding, but a decrease in the O-O bonding. The segregation of Ti and O atoms shows that Ti atoms tend to co-locate at the center, while O atoms tend to reside on the surface. Increasing temperature causes a decrease of the bandgap from 3.59 to 2.62 eV for the brookite phase, which is much more energetically favorable compared to the bulk, while it could increase the bandgap from 3.15 to 3.61 eV for anatase phase. For three-phase TiO 2 NPs, LUMO and Fermi levels decrease. The HOMO level of anatase phase NP decreases, but it increases for brookite and rutile phase TiO 2 nanoparticles. An increase in the temperature contributes to the stabilization of anatase phase TiO 2 NP due to a decrease in the HOMO energies. 1. Introduction Titanium dioxide (TiO 2 ) has attracted intense scrutiny as a photo- catalyst in water splitting pigments, gas sensors, in hydrogen gas evo- lution, self-cleaning surfaces, solar cells, etc., [1] owing to its catalytic properties that provide good stability, non-toxicity, and low-cost pro- duction [2–6]. TiO 2 is known to naturally occur in mineral forms—the most abundant being rutile and as other rarer polymorphs anatase and brookite [7]. These various polymorphs exhibit different phase char- acteristics due to the deviation in the lattice arrangements that can be exploited for various applications. For example, anatase demonstrates the highest photocatalytic activity [8–10] compared to either brookite or rutile, whereas the photocatalytic activity of defective brookite is better than either anatase or rutile [11]. Observing the bandgap of bulk TiO 2 is 3.2, 3.1 and 3.0 eV for anatase, brookite and rutile, respectively [12–14], a large bandgap restricts its use, however, only to the narrow light-response range of ultraviolet (only about 3–5% of solar spectrum) [15]. The bandgap is significantly affected by factors like phase struc- ture, temperature, and crystal size. TiO 2 nanoparticles (NPs) have re- cently been receiving more attention in organic synthesis [16–18], in different inorganic and organic reactions [19–22], and in the preparation of derivations of molecules [23–26], because of their useful and unique properties. Additionally, TiO 2 NPs are used in health and the environment e.g., environmental remediation, self-cleaning and self- disinfection [27,28], and in novel biomedical applications [29,30]. There has also been prior work using DFTB on TiO 2 and TiN nano- particles that have shown that these systems typically are intermixed with each other (i.e., TiO 2 readily mixes with TiN and vice versa) in realistic environments [31]. Improving the performance of NP materials can be achieved using different approaches, for example, by adding an atom, applying pres- sure, increasing the temperature, and modifying crystallite size that change their morphologies [32–35]. By applying this general principle, this work presents the structural and electronic properties of TiO 2 NPs by using the density functional based tight binding (DFTB) method to study the influence of the temperature on the three different phases, anatase, brookite and rutile of TiO 2 NPs. The performance of DFTB calculations has been shown on NPs in the previous studies [36,37]. In this study, we analyzed the HOMO, LUMO and the frontier molecular orbital energy gap (E g ), total energy, density of states (DOS), radial distribution functions (RDFs), order parameter (R) to analyze the seg- regation phenomena of Ti and O atoms and the number of bonds https://doi.org/10.1016/j.commatsci.2020.109843 Received 11 April 2020; Received in revised form 25 May 2020; Accepted 26 May 2020 ⁎ Corresponding author. E-mail addresses: mkurbanphys@gmail.com, mkurban@ahievran.edu.tr (M. Kurban). Computational Materials Science 183 (2020) 109843 0927-0256/ © 2020 Elsevier B.V. All rights reserved. T