RESUMO
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 origin and nature of localized electron states in W:BiVO4. Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W6+ for V5+ in BiVO4. This is shown to result in the presence of inter-band gap states related to electrons at V4+ sites of e symmetry. The energetic position of the states in the 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.
RESUMO
The addition of a metal overlayer to a semiconductor photocatalyst is a frequently used synthetic route to passivate the surface and, via the formation of a Schottky barrier, to enhance catalytic activity of the photocatalyst material. While it is known that Schottky junctions decrease recombination by charge separation, measurements of the depletion region dynamics have remained elusive. Here, we use ultrafast pump-probe transient photoelectron spectroscopy to measure material-specific dynamics of the Zn/n-GaP(100) system. Through photoemission measurements the Schottky barrier height is determined to be 2.1 ± 0.1 eV at 10 monolayers of total Zn deposition. Transient photoemission measurements utilizing a 400 nm pump pulse show that, after excitation, holes are transferred from n-GaP(100) to the Zn overlayer within a few ps, as evidenced by shifts of the Zn 3d and Ga 3d core levels to higher binding energies. Within the timescale of the experiment (130 ps) no carrier recombination is observed in the junction. Furthermore, a long-lived surface photovoltage signal is observed at times >1 ms after photoexcitation. This work further exemplifies the potential of transient extreme ultraviolet photoelectron spectroscopy as a material-specific technique for the study of heterojunctions.