Elucidating the electronic structures of β-Ag2MoO4 and Ag2O nanocrystals via theoretical and experimental approaches towards electrochemical water splitting and CO2 reduction

In this paper, we demonstrate a combined theoretical and experimental study on the electronic structure, and the optical and electrochemical properties of β-Ag2MoO4 and Ag2O. These crystals were synthesized using the hydrothermal method and were characterized using X-ray diffraction (XRD), Rietveld refinement, and TEM techniques. XRD and Rietveld results confirmed that β-Ag2MoO4 has a spinel-type cubic structure. The optical properties were investigated by UV-Vis spectroscopy. DFT+U formalism, via on-site Coulomb corrections for the d orbital electrons of Ag and Mo atoms (Ud) and the 2p orbital electrons of O atoms (Up) provided an improved band gap for β-Ag2MoO4. Examination of the density of states revealed the energy states in the valence and conduction bands of the β-Ag2MoO4 and Ag2O. The theoretical band structure indicated an indirect band gap of approximately 3.41 eV. Furthermore, CO2 electroreduction, and hydrogen and oxygen evolution reactions on the surface of β-Ag2MoO4 and Ag2O were studied and a comparative investigation on molybdate-derived silver and oxide-derived silver was performed. The electrochemical results demonstrate that β-Ag2MoO4 and Ag2O can be good electrocatalysts for water splitting and CO2 reduction. The CO2 electroreduction results also indicate that CO2 reduction intermediates adsorbed strongly on the surface of Ag2O, which increased the overpotential for the hydrogen evolution reaction on the surface of Ag2O by as much as 0.68 V against the value of 0.6 V for Ag2MoO4, at a current density of −1.0 mA cm−2.