FIRST PRINCIPLES STUDY OF ELECTRONIC AND OPTICAL PROPERTIES OF PRISTINE AND X ( Cu , Ag AND Au ) DOPED BiOBr

The electronic structures and optical properties of pristine BiOBr and Cu, Ag and Au doped BiOBr have been analyzed by using a standard density functional theory based ab-initio approach employing generalized gradient approximation through revised Perdew Burke Ernzerhoff type parameterization. The calculation shows that both the doped and pristine BiOBr have indirect band gap, the band gap of the pristine BiOBr found 2.22eV, whereas band gap significantly reduced after doping Cu, Ag and Au on BiOBr. The band gap of Cu, Ag and Au doped BiOBr are 1.2eV, 0.9eV and 1.76eV respectively. The optical properties have been studied through dielectric function, both pure and doped BiOBr shows anisotropic nature.


INTRODUCTION
In the past few year, semiconductor based photocatalysts have developed as an effective compound for removing organic and inorganic component from water and air (Mills et al., 1993;Hoffmann et al.,1995;Oliveros et al., 1993;Carey et al., 1976).Among semiconductor photocatalysts TiO 2 has received much focused , because of its strong oxidizing power, nontoxicity and cheapness (Ao et al., 2004), however, very low amount of light is absorbed for photo degradation process due to its wide band gap (3.2 eV) (Liqiang et al., 2002).Therefore, to increase the maximum utilization of electromagnetic wave (visible light) in order to boost photocatalytic activities, the improvement of semiconductor photocatalysts has developed as a vital issue in current photocatalysis research.Pristine (Huang, 2009;Zhang, et al., 2006;Wang et al., 2009), doped (Miguel et al., 2013;Jiahui et al. 2013) and co-doped (Jiang et al., 2014(Jiang et al., & 2015) ) bismuth oxyhalides preformed excellent photocatalytic activities because of its narrow band gap and outstanding photocatalytic performance and have become worthwhile candidate for research.Recently, BiOBr has appeared as new kind of visible light responsive photocatalysts and received much more interest from researcher, also made it as potential photocatalysts.Experimentally and theoretically large number of strategies have been developed to narrowing the band gap and boost the optical absorption of BiOBr, which includes doping metal (Li et al., 2015) or co-doping of (Jiang et al., 2014(Jiang et al., & 2015)), doping non-metal elements (Jiang et al., 2014).As doping species, variable-valance metallic ions have been successive methods to narrowing band gap and enhancing the photocatalytic properties (Liu et al., 2014).BiOBr has lamellar-structure and a ptype intrinsic semiconductor with indirect band gap to be in rang of 2.77-2.9eV experimentally (Jiang et al., 2010;Wang et al., 2008) and 2.36 eV DFT calculation (Huang et al., 2009).The wider band gap (~2.9 eV) of BiOBr indicate that, only a part of the visible light could be absorb.The recent experimental work shows that, doping transition metal Ag, Al, Fe (Jiang et al., 2013) and Ti (Wang et al., 2012) on BiOBr narrow the band gap and increase the wavelength response rang to the visible region.The present work focuses to engineer band structure of pristine BiOBr by doping transition metal X (Cu, Ag, and Au) and enhance the optical absorption near visible light range also examine the optical property through dielectric function analysis.Organization of this paper is as follows, next section discusses the method and computational details, followed by result and discussion section and conclusion of the work.

ATK-VNL is based on the methodology, model, and algorithms development in academic code
TransSIESTA and is further development of TransSIESTA-C (Stokbro et al., 2003 andin part, McDcal (Taylor et al., 2001), using localized basis sets as developed in SIESTA (Soler et al., 2002).The generalized gradient approximation with revised Perdew Burke Eenernoff (Ying et al., 2011;Perdew et al., 1996) parameterization used as exchange correlation function.Double-ζ-doublepolarized (DZDP) basis set used to describe valence electrons with localized pseudo atomic orbitals (PAOs) (Bachelet et al., 1982).A mesh cut off of 75 Hartree is applied in entire calculation with maximum force tolerance set to 0.05eV/A and kpoint sampling of 5×5×5.Before proceeding to further computational analysis, all the samples were freely optimized.

RESULTS AND DISCUSSION Crystal Structure Analysis
In this section we discusses the crystal structure of pure BiOBr and X (Cu, Ag, Au) doped BiOBr before and after optimization.Here, we substituted X element at the place of Br on Pristine BiOBr.The optimized structures are given below.As the transition metals are doped the bond angle increased for Bi-O-Bi.We can see that the bond angle of O-X-O is decreases with the growing X atomic number.

Electronic Band Structure
The energy difference on conduction band and valance band gives the energy band gap, here, on pure BiOBr valance band is due to the presence of O (2s, 2p) and Bi (6s), Br (4s, 4p) states whereas, conduction band consist mainly Bi (5d, 6p) and Br (4d) states.
Our calculation shows an indirect band gap of 2.21 eV on BiOBr, which is agreeable with the earlier computational work (Huang et al., 2009) and lower then experimental analysis (Wang et al., 2008;Jiang et al., 2010).Again the computed band gap of doped BiOBr-Cu is 1.2 eV, BiOBr-Ag is 0.9 eV and BiOBr-Au 1.76eV which are all indirect in nature.The band gap of pristine and doped BiOBr are given below.

Optical spectrum
The complex dielectric function ε(ω) described the optical properties of matter, by observing the response of the system to electromagnetic wave (light).It can be expressed as ε(ω)=ε 1 (ω) + iε 2 (ω) (1) Where ε 1 (ω) is the real and ε 2 (ω) is the imaginary part and can be analyzed through the Kramers-Krining dispersion relation.The imaginary part of dielectric function can be assigned the absorption peaks.As the photon energy is increased, the first peak links to the transition from the valence band to the conduction band.Here, the X elements introduced the dorbitals, has a significant contribution on valence band.The peaks appeared on optical spectrum are similar to the peaks of DOS, on the presence of X element, the peaks got higher value in optical spectrum at low photon energy.The BiOBr-Ag structure has highest peak around 3.6 with lowest photon energy 1.60 eV and pure BiOBr has lowest peak at highest photon energy.

CONCLUSION
Electronic band structures, density of state and optical spectrum of pure and X doped BiOBr have been calculated, by using DFT based abinitio approach.The electronic properties were studied via band structure and density of state.
Wider band gap of BiOBr can be narrowed by doping X (Cu, Ag, and Au) elements.The doped structure claim themselves as a potential photocatalyst material due to their narrow band gap, among them BiOBr-Ag has narrow band gap and suitable candidate for photocatayst.The optical properties examined through dielectric function analysis and found that the maximum peaks appeared at BiOBr-Ag doped structure at lowest photon energy and lowest peaks appeared on BiOBr structure at highest photon energy.
Both the pure and doped structures are anisotropic in nature.