Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Effects of Ni-doping on the photo-catalytic activity of TiO 2 anatase and rutile: Simulation and experiment Mohammad Reza Elahifard a, ⁎ , Seyedsaeid Ahmadvand b , Amir Mirzanejad b a Chemical Engineering Department, Faculty of Engineering, Ardakan University, Ardakan, Iran b Department of Chemistry, University of Nevada, Reno, 1664 North Virginia Street, Reno, NV 89557-0216, USA ARTICLE INFO Keywords: Band gap Ni-doping TiO 2 Photo-catalytic activity Density functional theory ABSTRACT The Ni impurity has an inconsistent impact on the photo-catalytic activity of TiO 2 in different regions of elec- tromagnetic radiation. In this work, the effect of different concentrations of Ni doping into anatase and rutile TiO 2 structures is investigated theoretically and experimentally. Doubling the concentration of doped Ni does not change the photo-catalytic activity of TiO 2 significantly according to photo-degradation of Acid Blue 92 (AB 92) under ultra violate and visible (UV–Vis) lights. However, increment of the dopant concentration enhances the thermodynamic yield of TiO 2 crystalline structure (i.e. rutile) at low temperature calcination of TiO 2 . Density functional theory (DFT) calculations also confirm the impact of Ni impurity on the higher stability of rutile phase. Computational geometry optimization favors a heterogeneous distribution of Ni atoms in 12.5 at% Ni- TiO 2 , that is verified by a broad impurity peak inside the band gap of TiO 2 in UV–Vis diffuse reflectance spectrum (UV–Vis DRS). The DRS and DFT results denote a negligible change in the band gap energy of TiO 2 compared to Ni-TiO 2 . Based on DFT results, generation of defect states gives rise to photo-catalytic activities of Ni-TiO 2 in the invisible region. However, adding Ni to anatase TiO 2 changes the type of the band gap from indirect to direct and reduces its photo-efficiency in the degradation of AB 92 under UV irradiation. In addition, a positive shift of the valance and conduction band edges of TiO 2 occurs after Ni doping, which reduces the photo-oxidation activity of TiO 2 . 1. Introduction TiO 2 has drawn a wide interest owing to its low cost, high photo- catalytic activity, stability, and friendly environmental features [1–5]. The photo-catalytic activity of TiO 2 in the UV region, with a band-gap of 3.0–3.2 eV, could be extended to other regions via band-gap en- gineering [6–9]. For instance, metal doping can shift the absorption edge of TiO 2 from UV-region to lower energies giving rise to photo- catalytic efficiency in the visible region [10–15]. However, the effi- ciency of metal doping on the photo-catalytic activity of TiO 2 in the UV region is disputable yet [16–18]. Formation of defect states in the band- gap domain of TiO 2 by metal ions, facilitates the photo-excitation of TiO 2 in the visible region [19–21]. Several experimental and theoretical studies have been done on the impacts of nickel-doping on the photo- catalytic efficiency of TiO 2 [22–32]. Energetically favorable replace- ment of Ti with Ni (with similar ionic radii) in TiO 2 lattice and stability in photo-catalytic activity of Ni-TiO 2 make Ni an appealing dopant among other transition metals [33]. Efficient degradation of Azo-dyes, main pollutants (50–70%) of dyestuffs industrial sewages, is very critical for natural environments. Different concentrations of Ni dopant in TiO 2 lattice result in different photo-catalytic activity in degradation of Azo-dyes under the solar light. [18,23,27,34,35]. Ni-generated defect states in the middle of the band-gap domain would reduce the required energy for electronic transitions correlated with the catalytic activity. Nevertheless, re- combination of electron-hole (e-h) pairs in higher concentrations of metal dopants might lower the photo-efficiency of the catalyst. Optimal impurity values (0.5–10%) exhibit more efficient photo-catalytic ac- tivities in the experiment, among which 6.25% and 12.5 at% Ni-TiO 2 are subject to many theoretical studies as well [33,35–39]. Among different possible structures for TiO 2 , anatase and rutile phase have shown the highest photo-catalytic efficiency and stability, respectively [40–42]. Woll et al. [43] have demonstrated that the type of the band- gap plays predominant role in the photo-catalytic activity of different TiO 2 phases. The longer lifetime for photo-generated e-h pairs and thus the higher photo-catalytic activity in anatase phase in comparison with rutile is due to indirect band-gap in anatase phase vs. direct one in rutile https://doi.org/10.1016/j.mssp.2018.05.001 Received 6 October 2017; Received in revised form 17 April 2018; Accepted 2 May 2018 ⁎ Corresponding author. E-mail address: mrelahifard@ardakan.ac.ir (M.R. Elahifard). Materials Science in Semiconductor Processing 84 (2018) 10–16 1369-8001/ © 2018 Published by Elsevier Ltd. T