A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals

Abstract

(2016). A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals. Physical Chemistry Chemical Physics, 18(46) Abstract: Doped BiVO4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of x-ray spectroscopic methods to probe the nature of localized electron states in W:BiVO4 and variations in the electronic structure along the crystal axes related to the materials conductivity anisotropy. We verify that tungsten substitutes as W 6+ for V 5+ in BiVO4. This results in the presence of inter-band gap states related to electrons at V 4+ sites of e symmetry. The energetic position of the states in the 2 band gap suggest that they are highly localized and may act as recombination centres.Polarization dependent x-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO4.

Chemical, Structural, and Electronic Characterization of the (010) Surface of Single Crystalline Bismuth Vanadate

“…whereas Mo 6+ substitutes V 5+ sites in the bulk. 33 On the other hand, similar results have recently been reported by Jovic et al 22,35 As we will discuss in the next section, the local coordination of Mo has a significant influence on the electronic properties of the Mo(1 at.%):BiVO 4 (010)…”

“…59 The prominent peak at 534.1 eV is related to O 2p and Bi 6p hybridized states. 20 Finally, a broad feature centered at a tetrahedral crystal field, as previously reported for Mo-doped BiVO 4 35 and similar to that observed for Na 2 MoO 4 . 60 We compared our measured values to those obtained using the multiplet approach in the CTM4XAS program.…”

“…A similar spectral feature was observed by Smith et al when performing ResPES along the shallow 3p → 3d transition on both Ti 2 O 3 and V 2 O 3 and attributed to resonance occurring with highly localized metal d-states. 72, 64 Jovic et al recently observed a similar feature above VBM in a ResPES study of W and Mo doped single crystalline BiVO 4 22,35. These authors attributed this feature to highly localized V 4+ states of e symmetry as a result of Mo or W doping and stated that it is consistent with DFT calculations showing small polaron localization in V 3dz 2 states 32.…”

The Fast-Track Water Oxidation Channel on BiVO4 Opened by Nitrogen Treatment

“…The VB of BiVO 4 shown in Figure c has primarily an O 2p character that is observed as a broad peak centered at ∼5 eV . For BiVO 4 , the VB maximum is found around 1.5 eV, while a new feature spanning between 1.5 and 0.4 eV is obtained after nitrogen treatment, which could be attributed to the states due to V O . V O are not sufficient to change the bandgap of BiVO 4 , supported by our UV–vis spectral measurements that do not indicate a significant band gap narrowing (Figure S8c).…”

“…49 For BiVO 4 , the VB maximum is found around 1.5 eV, 50 while a new feature spanning between 1.5 and 0.4 eV is obtained after nitrogen treatment, which could be attributed to the states due to V O . 51 V O are not sufficient to change the bandgap of BiVO 4 , 22 supported by our UV−vis spectral measurements that do not indicate a significant band gap narrowing (Figure S8c). However, we cannot fully rule out the formation of V O in the bulk of BiVO 4 despite the fact that the Raman and UV−vis spectra show no clear evidence.…”