Tuning electronic structure of ZnO nanowires via 3d transition metal dopants for improved photo-electrochemical water splitting: An ab initio study

Abstract

ZnO nanowires have been proposed as potential photo-anode materials for photo-electrochemical water splitting due to their low toxicity, simple synthesis and easy modification routes. However, ZnO suffers from low PEC activity and photo-corrosion eff ;ects, and therefore, application of ZnO nanowires in PEC water splitting still awaits development of effective design and synthesis strategies to improve its PEC efficiencies to commercially viable levels. Here, we present ab initio Density Functional Theory calculations considering 3d transition metal doping as a potential route towards attainment of ZnO nanowires with superior PEC activity. Our results show that the stability of 3d transition metal dopants in ZnO NWs is dependent on the d character of the transition metal dopant as well as their concentration and doping site, with most transition metal atoms being energetically most favorable at the Zn substitutional site both in O-rich and Zn-rich conditions considered. Specifically, we find all 3d transition metal dopants in ZnO NW under O-rich conditions as well as Sc, Ti and V under Zn-rich conditions have negative formation energies at the considered dopant concentrations of 1−6 atm. %, indicating that these dopants can readily be incorporated into ZnO NWs at thermodynamic equilibrium conditions. The electronic properties of Ti and V at 2% and 4% dopant concentration, respectively, yield a staggered band-structure configuration, while Sc, Cr, Mn, Co, Ni, and Cu dopants in ZnO NWs induce band-edge states. In addition, 3d TM dopants induces significant red-shift of the absorption edge of ZnO NW due to reduction in band gap, and are projected to improve visual light harvesting capabilities. Finally, the band alignment relative to the redox potential of water revealed that the valence band maximum of Sc, V, Ni and Cu doped ZnO NWs remains strongly positive above the oxidation potential of O2/H2O, while their reduction potential remain negative below the reduction potential of H+/H2, favouring PEC applications.