Femtosecond Charge Density Modulations in Photoexcited CuWO₄

Abstract

Copper tungstate (CuWO 4 ) is an important semiconductor with a sophisticated and debatable electronic structure that has a direct impact on its chemistry. Using the PAL-XFEL source, we study the electronic dynamics of photoexcited CuWO 4 . The Cu L 3 X-ray absorption spectrum shifts to lower energy upon photoexcitation, which implies that the photoexcitation process from the oxygen valence band to the tungsten conduction band effectively increases the charge density on the Cu atoms. The decay time of this spectral change is 400 fs indicating that the increased charge density exists only for a very short time and relaxes electronically. The initial increased charge density gives rise to a structural change on a time scale longer than 200 ps.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

“…To do so, we tested the CuWO 4 :580 electrode with an electrolyte containing H 2 O 2 as an electron donor. Hydrogen peroxide is much more easily oxidizable than water (+0.68 V SHE vs 1.23 V SHE for water) and has a higher reaction rate (H 2 O 2 to H 2 O is a 2-hole process, while H 2 O to O 2 is a 4-hole process) . The current doubled to 0.67 mA cm –2 at 1.23 V SHE in the presence of H 2 O 2 (Figure C).…”

“…Furthermore, the CuWO 4 band gap can be narrowed by substituting tungsten with molybdenum in quaternary CuW x Mo y O 4 . For example, CuW 0.5 Mo 0.5 O 4 has a band gap of 2.0 eV, corresponding to an absorption edge of 650 nm. − This tungstate family is stable in neutral and slightly basic pH because of the overlap between Cu 3d orbitals with O 2p orbitals. , This hybridization is also responsible for the negative shift of the CuWO 4 valence band compared to that of binary WO 3 . Moreover, CuWO 4 is suitable to assemble heterojunction systems with BiVO 4 , , likewise WO 3 . − …”

“… 20 − 22 This tungstate family is stable in neutral and slightly basic pH because of the overlap between Cu 3d orbitals with O 2p orbitals. 23 , 24 This hybridization is also responsible for the negative shift of the CuWO 4 valence band compared to that of binary WO 3 . 25 Moreover, CuWO 4 is suitable to assemble heterojunction systems with BiVO 4 , 26 , 27 likewise WO 3 .…”

Electronic Structure and Ultrafast Electron Dynamics in CuO Photocatalysts Probed by Surface Sensitive Femtosecond X-ray Absorption Near-Edge Structure Spectroscopy

“…For example, in CuWO 4 , it has been reported that unoccupied Cu 3d orbitals act as a spectator state during photoexcitation. 48 If the excited electron occupies primarily the Cu 4s orbital, the charge density around the Cu atoms increases; however, the newly formed Cu 1+ would remain as a 3d 9 system. Because the d orbital occupancy remains largely unchanged, we expect no significant change in multiple structures in the XANES spectrum.…”

“…There is precedence for photoexcitation into Cu 4s conduction band states. For example, in CuWO 4 , it has been reported that unoccupied Cu 3d orbitals act as a spectator state during photoexcitation . If the excited electron occupies primarily the Cu 4s orbital, the charge density around the Cu atoms increases; however, the newly formed Cu 1+ would remain as a 3d 9 system.…”