Computational study on oxynitride perovskites for CO 2 photoreduction Ahmed M. Hafez a , Abdallah F. Zedan b , Siham Y. AlQaradawi b , Noha M. Salem a , Nageh K. Allam a,⇑ a Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt b Department of Chemistry and Earth Sciences, Qatar University, P.O. Box 110003, Doha, Qatar article info Article history: Received 18 March 2016 Received in revised form 23 May 2016 Accepted 24 May 2016 Keywords: Perovskite Carbon dioxide Reduction DFT Effective mass abstract The photocatalytic conversion of CO 2 into chemical fuels is an attractive route for recycling this green- house gas. However, the large scale application of such approach is limited by the low selectivity and activity of the currently used photocatalysts. Using first principles calculations, we report on the selection of optimum oxynitride perovskites as photocatalysts for photoelectrochemical CO 2 reduction. The results revealed six perovskites that perfectly straddle the carbon dioxide redox potential; namely, BaTaO 2 N, SrTaO 2 N, CaTaO 2 N, LaTiO 2 N, BaNbO 2 N, and SrNbO 2 N. The electronic structure and the effective mass of the selected candidates are discussed in details, the partial and total density of states illustrated the orbi- tal hybridization and the contribution of each element in the valence and conduction band minima. The effect of cation size in the ABO 2 N perovskites on the band gap is investigated and discussed. The optical properties of the selected perovskites are calculated to account for their photoactivity. Moreover, the effect of W doping on improving the selectivity of perovskites toward specific hydrocarbon product (methane) is discussed in details. This study reveals the promising optical and structural properties of oxynitride perovskite candidates for CO 2 photoreduction. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction More energy from sunlight strikes the earth in one hour than all of the energy consumed on the planet in one year. Thus, the chal- lenge modern society faces is not one of identifying a sustainable energy source, but rather one of capitalizing on the vast solar resource base. Of the various storage solutions considered in recent years, solar fuels, in which sunlight is converted into chemical energy, are particularly attractive because of the long shelf-life of such fuels and their ready integration with the existing fuel storage and delivery infrastructure [1–6]. Recently, there has been a significant interest in recycling the undesirable by-products of fossil fuel combustion; especially CO 2 . Conversion of CO 2 into a high-energy content, storable and porta- ble fuel, suitable for use in the current hydrocarbon-based energy infrastructure, is an attractive pathway [7–12]. However, this pro- cess is energy intense and will only be useful if a renewable energy source is used for that purpose [9]. A possible sustainable avenue is to use photocatalysts for the conversion of CO 2 into useful hydro- carbons with the help of solar energy [13]. A useful carbon cycle involving only H 2 O and CO 2 is very promising in this regard. This is simply to mimic Nature that has provided us with an amazing strategy, photosynthesis, to utilize CO 2 while replenishing oxygen and providing fuels. One great advantage of having water in the proposed artificial photosynthesis process is that water is as an electron donor [14], which can provide electrons and protons to facilitate reactions (1)–(5) below: CO 2 þ 2H þ þ 2e  ! HCOOH ð1Þ HCOOH þ 2H þ þ 2e  ! HCHO þ H 2 O ð2Þ HCHO þ 2H þ þ 2e  ! CH 3 OH ð3Þ CH 3 OH þ 2H þ þ 2e  ! CH 4 þ H 2 O ð4Þ 2H þ þ 2e  ! H 2 ð5Þ However, the artificial photosynthetic conversion of CO 2 is hin- dered by some factors [15–17], such as the thermochemically unfa- vorable one-electron reduction potential of CO 2 (E° = 1.9 V NHE ), the low solubility of CO 2 in water (33 mM at 298 K and 1 atm), the strong oxidation power of photogenerated holes (or OH radi- cals), during the photocatalytic process, that can react with inter- mediates conversion products, and the competition with H 2 generation (H + /H 2 O potential is 0.41 V NHE whereas CO/CO 2 potential is 0.52 V NHE at pH 7). Various approaches [14,18–23] have been attempted to surpass the above mentioned limitations. However, the electrode material is still playing the major role in determining the nature of the resulting conversion products and their selectivity. The surface properties (e.g., roughness, porosity, http://dx.doi.org/10.1016/j.enconman.2016.05.069 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: nageh.allam@aucegypt.edu (N.K. Allam). Energy Conversion and Management 122 (2016) 207–214 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman