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

Abstract

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 (U_d) and the 2p orbital electrons of O atoms (U_p) 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.