Electronic structure of lanthanide-doped bismuth vanadates: A systematic study by x-ray photoelectron and optical spectroscopies

Abstract

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials.