Direct Mapping of Band Positions in Doped and Undoped Hematite during Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical water splitting is a promising pathway for the direct conversion of renewable solar energy to easy to store and use chemical energy. The performance of a photoelectrochemical device is determined in large part by the heterogeneous interface between the photoanode and the electrolyte, which we here characterize directly under operating conditions using interface-specific probes. Utilizing X-ray photoelectron spectroscopy as a noncontact probe of local electrical potentials, we demonstrate direct measurements of the band alignment at the semiconductor/electrolyte interface of an operating hematite/KOH photoelectrochemical cell as a function of solar illumination, applied potential, and doping. We provide evidence for the absence of in-gap states in this system, which is contrary to previous measurements using indirect methods, and give a comprehensive description of shifts in the band positions and limiting processes during the photoelectrochemical reaction.

Interface Science Using Ambient Pressure Hard X-ray Photoelectron Spectroscopy

“…∆t is found to be about 12 × 10 3 s (~3.3 h). This limitation in the ionic diffusion rate is confirmed by a recent study of Shavorskiy et al, where the authors observe a significant IR drop in the liquid film at the hematite/liquid electrolyte interface under PEC conditions (for applied potentials above~1.2 V vs. RHE) [17]. This mass transport limitation has also an important effect on the achievable current densities within the liquid layer.…”

“…To estimate the diffusion time scale of the hydroxyl groups from the liquid meniscus through the liquid layer, we can use Fick's first law of The "dip and pull" method can be also utilized to perform in situ (photo)electrochemical and photoemission measurements. Two additional electrodes can be mounted on the sample holder, leading to an effective three-electrode electrochemical cell [11][12][13][14][15][16][17]. The bottom part of the electrodes (∆), being kept in the bulk electrolyte, provides the electrochemical continuity to apply a potential to the thin electrolyte layer on the sample (working electrode) surface, thus enabling the investigation of solid/liquid electrified interfaces [11][12][13][14][15][16][17].…”

“…Two additional electrodes can be mounted on the sample holder, leading to an effective three-electrode electrochemical cell [11][12][13][14][15][16][17]. The bottom part of the electrodes (∆), being kept in the bulk electrolyte, provides the electrochemical continuity to apply a potential to the thin electrolyte layer on the sample (working electrode) surface, thus enabling the investigation of solid/liquid electrified interfaces [11][12][13][14][15][16][17]. A drawback of this method (and, in general, of the "free-surface" liquid layer approach) is constituted by the mass transport limitation in the electrolyte layer, since the latter is effectively static in the direction parallel to the solid surface (with the exception of liquid flow due to eventual differences in the temperature between the electrolyte reservoir and the measurement spot above the liquid meniscus, causing thermo-capillary or Bénard-Marangoni convection).…”

“…This poses a constraint on the thickness (t) of the investigated semiconductor, where t~λ CC . In addition, such approach is applicable to fundamental investigations at interfaces such as the probing of the electrical potential distribution simultaneously within the solid (i.e., band-bending) and the liquid side of the junction (the double/diffuse layer) [13,17].…”

“…Therefore, several spectroscopic methods based on photon in/photon out or photon in/electron out approaches have been applied to investigate solid/liquid interfaces. Among them, synchrotron-based techniques such as surface X-ray diffraction, X-ray absorption/emission and photoelectron spectroscopy, and "laboratory-based" techniques, such as infrared, Raman and non-linear optical spectroscopies have been recently developed [10].In this context, X-ray photoelectron spectroscopy (XPS) stands as an excellent characterization tool, since it offers elemental and chemical sensitivity, simultaneously measuring local built-in electrical potentials via the detection of rigid photoelectron kinetic energy shifts in both core and valence levels [11][12][13][14][15][16][17]. Due to the high vapor pressure of many liquids of interest, differential pumping schemes have to be used in photoelectron analyzers to minimize the elastic and inelastic scattering of electrons in the gas phase above the liquid side of the junction [18][19][20].…”