Band bending and dipole effect at interface of metal-nanoparticles and TiO₂ directly observed by angular-resolved hard X-ray photoemission spectroscopy

Abstract

This paper describes the observation of band bending and band edge shifts at the interfaces between nanoscale metals and TiO2 film over a wide depth range by angular-resolved hard X-ray photoemission spectroscopy (HAXPES). The HAXPES results indicate strong electrostatic interactions between the TiO2 semiconductor and metal nanoparticles, while density functional theory (DFT) calculations suggest that these interactions are primarily associated with charge transfer leading to electric dipole moments at the interface in the ground state. The effects of these dipole moments are not limited to the surface but also occur deep in the bulk of the semiconductor, and are highly dependent on the coverage of the metal nanoparticles on the semiconductor species.

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…Since 2013 a considerable number of studies performed at the beamline using an X-ray energy of 7.94 keV have provided insights into predominantly structured, multilayer samples for device applications. [68,[239][240][241] AR-HAXPES was identified as a useful technique to study such multilayer systems from the beginning, including the first HAXPES paper at SPring-8 which explored the θ dependence of the Si 1s spectra of a HfO 2 /SiO 2 /Si system. [29] Of particular interest in such studies are changes to the band positions at interfaces, including band bending and rigid band edge shifts, which can be inferred from core level shifts as a function of distance from a surface or interface.…”

“…explored the interaction of nanoscale metal particles, including Pt, Au and Rh, with TiO . [239] In combination with density functional theory calculations they were able to identify electrostatic interactions taking place, which influenced not only the surface and interfaces, but the bulk properties of TiO 2 . This study showcases the crucial information AR-HAXPES can provide on device-relevant structures.…”

Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible‐Light‐Driven Overall Water Splitting

“…In the case of conventional TiO 2 electrodes, it was reported that loading of nanoscale metals Rh, Pt, and Au on TiO 2 electrode surfaces changes their electronic states such as surface charges and band‐bending characteristics . Rh showed the largest negative shift of Ti 2p, indicating an upward band bending toward the TiO 2 surface as a result of a surface‐charge equilibrium that induces a surface dipole moment.…”

“…Rh showed the largest negative shift of Ti 2p, indicating an upward band bending toward the TiO 2 surface as a result of a surface‐charge equilibrium that induces a surface dipole moment. DFT calculations explained this tendency nicely, attributing it to the ionization energy values for these metals . The lower ionization energy for metals generally increases the charge transfer in the vicinity of the oxygen in TiO 2 .…”

“…HAXPES showed a clear downward band bending of 0.4 eV from the bulk to the surface of the bare N‐Ta 2 O 5 film, whereas the magnitude of the band bending was reduced for Au/N‐Ta 2 O 5 , Pt/N‐Ta 2 O 5 , and Rh/N‐Ta 2 O 5 with particle sizes of 2–3 nm, which spontaneously aggregated after deposition on the N‐Ta 2 O 5 surface . This implies that these nanoscale metallic cocatalysts available for proton reduction induce strong electronic interactions with the N‐Ta 2 O 5 semiconductor.…”