Electronic Structure of Lanthanide-Doped Bismuth Vanadates: A Systematic Study by X-ray Photoelectron and Optical Spectroscopies

Abstract

Monoclinic BiVO4 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 H2O/H2 reduction. As a consequence, BiVO4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO4 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 BiVO4 quaternary oxides of stoichiometry Bi0.5Ln0.5VO4 (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 Bi3+ and the Ln3+ 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 BiVO4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO4, 4f-related states appear above the top of the main valence band of BiVO4 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.

Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO3

“…The optical spectra of PrInO 3 , NdInO 3 , and SmInO 3 are considerably more complicated than that of LaInO 3 , with sharp peaks associated with localized 4f → 4f transitions to the low energy of the interband onsets. It is beyond the scope of the current paper to discuss assignment of this structure in detail, but in general terms, the 4f → 4f bands are very similar to those for the simple Ln 2 O 3 oxides or more complex doped materials such as the Bi 0.5 Ln 0.5 VO 4 vanadates …”

“…It is beyond the scope of the current paper to discuss assignment of this structure in detail, but in general terms, the 4f → 4f bands are very similar to those for the simple Ln 2 O 3 oxides or more complex doped materials such as the Bi 0.5 Ln 0.5 VO 4 vanadates. 66 The band structure calculations show that the band gaps for all four perovskites are either direct (LaInO 3 and NdInO 3 ) or that the energy difference between a lower energy indirect gap and the direct gap is so small (<0.01 eV) as to be negligible (PrInO 3 and SmInO 3 ). It is therefore appropriate to estimate the position of the interband absorption onset by plotting (F(R)hν) 2 against photon energy hν.…”

The Local and Electronic Structure Study of LuxGd1−xVO4 (0 ≤ x ≤ 1) Solid Solution Nanocrystals

“…The predominant peak with a bump on its lower-binding-energy side, ranging from 2.5 eV to 12.5 eV, is the valence band (VB). The strongest splitting peaks located at ~9.0 eV and ~7.8 eV, in the middle of the VB, should be assigned to Lu 4 f 5/2 and 4 f 7/2 doublets [ 17 ]. Meanwhile, the Gd 4 f state presents as an asymmetric peak with much lower intensity located at ~8.2 eV, which is overlapped with the Lu 4 f doublet and a bump (broad and very low intensity) of the hybridized states between the bonding O 2 p and V 3 d .…”

“…In order to further expand the optical applications of REVO 4 -series materials, the influence of mixed-crystal hosts on the luminescence properties has also been explored. For example, Robinson et al proved that Bi 1− x Dy x VO 4 showed broader XRD peaks compared to DyVO 4 , and a progressive decrease in the bandgap with the increase in Bi concentration and crystal field effects should not be negligible for the 4 f -4 f transitions [ 17 ]. Kang et al confirmed that the adjustment of the cation fractions could tune the excitation tail and emission band maximum of Bi 3+ ions through bandgap modulation in the (Y,Sc)VO 4 host [ 18 ].…”