First principle study of Nb defects in anatase (101) TiO 2 surface Ardeshir Baktash a , Alireza Sasani b , Azadeh Alavizargar c , Kavoos Mirabbaszadeh d,n a Institute of Nano Science and NanoTechnology, University of Kashan, Kashan P.O. Box 87317-51167, Iran b Department of Science, Karaj Islamic Azad University, Karaj, Alborz P.O. Box 31485-313, Iran c School of Nano-Sciences, Institute for Research in Fundamental Sciences, Tehran P.O. Box 19395-5531, Iran d Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran P. O. Box 15875-4413, Iran article info Article history: Received 26 November 2015 Received in revised form 31 December 2015 Accepted 23 January 2016 Keywords: Niobium doped Anatase TiO 2 Electronic structure Crystal structure Density functional theory abstract In this work, effects of Niobium (Nb) defects on TiO 2 surface using density functional theory (DFT) are investigated. Based on formation energy of the defects, their occurrences in two different extreme conditions, O-rich and O-poor conditions, are evaluated. Effects of Nb defects on surface and its electronic structure are studied and it is demonstrated that Nb doping widens valence band in deep energy level leaving the band gap without any change and it also lowers oxygen vacancy defect concentration due to the stronger bonding of Nb sub defect with oxygen atoms specially bridging oxygen (most probable defect site for Oxygen vacancy). Higher density of Nb substitutional defects (Nb sub ) are examined and it is shown that higher density doping of TiO 2 surface leads to uniform distribution of defects over the anatase structure as a result of interaction of Nb defects when they are close and this fact prevents segregation of Nb atoms in Nb-doped TiO 2 . & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Among the semiconductor photo-catalysts, Titanium dioxide (TiO 2 ), or Titania, has been extensively studied both experimen- tally and theoretically because of its characteristics like low cost, non-toxicity, high chemical and thermal stability and catalytic activity. These distinguishing qualities of TiO 2 made it very sui- table for a wide range of environmental applications [1,2] and inexpensive Dye Synthesized Solar Cells [3,4]. Several research groups have examined Niobium (Nb) doped TiO 2 films with dif- ferent doping levels [5–10]. Recently, it has been discovered that a doping level of about 6% of Nb into the anatase TiO 2 leads to ex- cellent conductivity and transparency [5,11–22]. Lee et al. used 6% of Nb doped TiO 2 as a compact layer in dye sensitized solar cells (DSCs). They found that the use of Nb-doped TiO 2 increased the photocurrent density (J SC ) [13]. Hasin et al. used Pt supported Nb- doped TiO 2 as counter electrodes for DSCs and they found lower charge transfer resistance and larger exchange current density [23]. Huang and coworkers prepared nanocrystals of Nb-doped TiO 2 for DSCs and they observed that the photoelectric conversion efficiency of DSCs with Nb-doped TiO 2 is higher compared to DSCs with pure TiO 2 [7]. While incorporation of many transition metals in TiO 2 leads to narrowing its band gap and influence the gap states, Nb doping of anatase TiO 2 does not lead to significant gap narrowing and does not affect the gap states [10]. Moreover, Nb doping in TiO 2 anatase is able to positively affect carrier density (up to 10 21 cm À2 ) and consequently electrical conductivity (over 10 3 –10 4 Ω À1 cm À2 ) [24]. For Highly Nb-doped TiO 2 , however, metal-like behavior was observed [5], and makes it unfavorable for electrochemical applications as a result of the increased electron recombination rate [25]. Studying the effect of doping Niobium on the band structure of TiO 2 shows that a d-natured conduction band forms because the 4d orbitals of Niobium strongly hybridize with the 3d orbitals of the Ti which results in metallic behavior in Nb doped TiO 2 [26]. Characteristics such as high stability, main- tainable resources, high electrical conductivity and transparency of Nb-doped TiO 2 , comparability with widely used conducting oxide (TCO), and the tin-doped In 2 O 3 (ITO) [24] make the Nb-doped TiO 2 as a perfect replacement for expensive and rare Indium in appli- cations like photovoltaic cells. Furthermore, recently low Nb doped TiO 2 was used as an electrode to control the junction characteristics and enhance the charge transport in dye synthe- sized solar cells at the interface with dye molecules and the electrolytes [10]. Although different experimental and theoretical studies have been done on Niobium doped anatase TiO 2 , there are few reports on scientific reasons for surface and electronic structure of the Nb- doped TiO 2 [10,27]. In this work, we studied the effect of Niobium on the stability and electronic structure of anatase TiO 2 . For comparison purposes and finding the most probable defect mode, we simulated different possible doping configurations including niobium interstitial defect and niobium substitutional defect on Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing http://dx.doi.org/10.1016/j.mssp.2016.01.017 1369-8001/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: mirabbas@aut.ac.ir (K. Mirabbaszadeh). Materials Science in Semiconductor Processing 45 (2016) 45–50