Combined soft and hard X-ray ambient pressure photoelectron spectroscopy studies of semiconductor/electrolyte interfaces

Abstract

The development of solar fuel generating materials would greatly benefit from a molecular level understanding of the semiconductor/electrolyte interface and changes in the interface induced by an applied potential and illumination by solar light. Ambient pressure photoelectron spectroscopy techniques with both soft and hard X-rays, AP-XPS and AP-HAXPES respectively, have the potential to markedly contribute to this understanding. In this paper we initially provide two examples of current challenges in solar fuels material development that AP-XPS and AP-HAXPES can directly address. This will be followed by a brief description of the distinguishing and complementary characteristics of soft and hard X-ray AP-XPS and AP-HAXPES and best approaches to achieving monolayer sensitivity in solid/aqueous electrolyte studies. In particular we focus on the detection of adsorbed hydroxyl groups in the presence of aqueous hydroxyls in the electrolyte, a common situation when investigating photoanodes for solar fuel generating applications. The paper concludes by providing an example of a combined AP-XPS and AP-HAXPES study of a semiconductor/aqueous electrolyte interface currently used in water splitting devices specifically the BiVO 4 /aqueous potassium phosphate electrolyte interface.2

Interface Science Using Ambient Pressure Hard X-ray Photoelectron Spectroscopy

“…Currently, two different preparation and investigation approaches are used, which differ from each other for the side of the interface that is used for the X-ray incidence and electron detection [9].…”

“…On the other hand, the measurement of the interface layer properties is a challenging task. First, it requires operando or in situ experimental capabilities in order to capture the true makeup of the interfacial layer under operating or realistic working conditions, respectively [1,8,9]. Secondly, to characterize the true interfacial properties, one should use an experimental probe that limits the…”

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

“…Owing to the conformal nature of the investigated systems, as reported in our previous work (37), continuous layer stacks given by "thickness equivalents" were used to represent the catalyst structure throughout the whole investigation as a function of the applied electrochemical potential. This approach, which is similar to that used in our recent study (48,50), provides critical insights into changes of the layered catalyst structure, though quantitative thickness values must be taken as approximate and may be affected by material density and photoelectron inelastic mean free path (IMPF) differences in the different phases. Additional details of the approach can be found in the experimental section.…”

“…To perform a detailed surface and near-surface spectroscopic investigation, we have used ambient pressure X-ray photoelectron spectroscopy (APXPS) coupled with intermediate energy, or "tender", X-rays (hν = 4.0 keV). Taking advantage of the high photoelectron kinetic energy (49,50) it is possible to probe 10 -30 nm of a liquid phase and 2 -10 nm of a solid phase, while simultaneously undergoing electrochemical reactions (39, 42-44, 46, 48).…”

“…To perform a detailed surface and near-surface spectroscopic investigation, we have used ambient pressure X-ray photoelectron spectroscopy (APXPS) coupled with intermediate energy, or "tender", X-rays (hν = 4.0 keV). Taking advantage of the high photoelectron kinetic energy (49,50) it is possible to probe 10 -30 nm of a liquid phase and 2 -10 nm of a solid phase, while simultaneously undergoing electrochemical reactions (39, 42-44, 46, 48).Previously, we observed a significant improvement of OER electrocatalytic activity in a biphasic CoO x system, in which a disordered outer-surface layer of Co(OH) 2 is introduced atop a nanocrystalline spinel Co 3 O 4 phase via PE-ALD, relative to pure phase spinel Co 3 O 4 (37). However, the electrochemical and ex situ spectroscopic measurements utilized in that study were incapable of characterizing the extent of conversion from Co(OH) 2 to CoO(OH), the degree to which spinel Co 3 O 4 may undergo parallel chemical transformations, the composition of the film under real operational conditions, or the validity of the current mechanistic models for OER on CoO x formed via PE-ALD.…”