Silver Exchange of Layered Metal Oxides and Their Photocatalytic Activities

Abstract

Layered Dion–Jacobson phases RbLaNb2O7 and RbA2Nb3O10 (A = Ca, Sr) and the Ruddlesden–Popper phase Rb2La2Ti3O10 were prepared by solid-state methods at a reaction time of 50 h and a temperature of 1100 °C. The products were silver-exchanged within a AgNO3 flux at a reaction time of 24 h and a temperature of 250 °C. Substitution of silver cations into the interlayer spacing of the layered structures is found to decrease the optical bandgap sizes on average by ∼0.5 to ∼1.0 eV. The products were found by scanning electron microscopy (SEM) to exhibit irregularly shaped platelet morphologies with an average size of ∼1–5 μm across their lateral dimensions and stepped edges ranging from ∼20 to ∼300 nm in height. Significant increases in photocatalytic hydrogen production rates for all silver-exchanged products were observed. The silver-exchanged RbA2Nb3O10 layered structures exhibited the highest photocatalytic hydrogen formation rates under ultraviolet and visible irradiation (∼13,616 μmol H2·g–1·h–1). These rates were 10 times higher than prior to silver exchange (∼1,418 μmol H2·g–1·h–1). However, photocatalytic activity under only visible light irradiation is not observed. It is also found that the silver cations located at the surfaces are reduced to Ag(s) after prolonged UV and visible light exposure in solution, which functions to increase their activity under UV irradiation. Electronic-structure calculations based on density functional theory show that the highest-energy valence band states are composed of Ag 4d-orbital and O 2p-orbital contributions within the interlayer spacing of the structure. The lowest-energy conduction band states arise from the Nb/Ti d-orbital and O 2p-orbital contributions that are confined to the two-dimensional niobate/titanate sheets within the structures and along which the excited-electrons can preferentially migrate.