Designing visible-light-responsive La2Ti2O7 photocatalysts by surface co-doping of nonmetal and metal combinations

Abstract

The large band gap of La₂Ti₂O₇ has hindered its potential application in photocatalytic water splitting under visible-light irradiation. In this study, we have utilized a passivated approach to tailor the band edges of La₂Ti₂O₇ by combining nonmetals X (X = N and C) with metals M (M = V, Nb, Ta, Mo and W) as dopants to substitute surface O and Ti sites, respectively. The formation energies show that the synthesis of co-doped systems might be easier under O-rich conditions than Ti-rich conditions. The calculated band edges of all the passivated co-doped systems well match with visible light absorption and the oxidation or reduction potential of water. Among them, Mo and W as dopants have more remarkable influence on the narrowing of the band gap and the enhancement of the optical absorption in the visible light region. The hydrogen evolution reaction (HER) on each studied La₂Ti₂O₇ surface is endothermic, and tends to occur on the surface O site far away from the metal dopant. The values of ΔG_H suggest that (N+V)-, (2N+Mo)- and (2N+W)-doped systems exhibit higher catalytic activities toward the HER than the pure surface. The active site of the oxygen evolution reaction (OER) is the O site binding to the metal dopant. The difference in the active sites of HER and OER on the co-doped La₂Ti₂O₇ surface is in favor of the separation of photoinduced electrons and holes. Co-doping combinations of (N+Ta), (C+Mo), and (2N+Mo) have comparable and even lower overpotential of OER than the pristine surface. Through the design, we find that (2N+Mo)-doped La₂Ti₂O₇ may become a visible-light-responsive photocatalyst for overall water splitting based on its superior electronic, optical and photocatalytic properties.