A Crucial Role of Bond Covalency Competition in Determining the Bandgap and Photocatalytic Performance of Silver Oxosalts

The optical bandgaps, the surface charges, and the photocatalytic activities of the silver oxosalts Ag3AsO4, Ag2CO3, Ag3PO4, Ag2SO4, and Ag2SeO4 are systematically investigated with several experimental techniques and first principles density functional theory calculations. The trends in the optical bandgaps and the surface charges of these silver oxosalts, Agx(XOy)z, are analyzed by considering how the X–O bond covalency affects the charge on the terminal oxygen atoms and the Ag–O bond covalency. The optical bandgaps of Agx(XOy)z are well-described by the bond-covalency competition in the Ag–O–X linkages because an increase in the overlap between the O 2s/2p and X ns/np orbitals decreases the overlap between the Ag 4d and O 2s/2p orbitals. The optical bandgap increases linearly with increasing the Z/r ratio of the atom X, a simple measure of the X–O bond covalency. In the photodegradation of charged molecules, the surface charge of Agx(XOy)z plays a prominent role and decreases with increasing the Z/r ratio. As expected from the present theoretical predictions, newly investigated Ag2SeO4 exhibits a promising photocatalytic activity under visible light. The Z/r ratio of the central atom X provides an effective measure for predicting the photocatalyst performance and the optical bandgap of silver oxosalts Agx(XOy)z.