As sketched in Figure 6.1, soft X-rays and electrons interact with the electronic structure of a PEC candidate material in numerous ways. In photoelectron spectroscopy (PES), one of the fundamental techniques of surface science [4,5], an incoming soft X-ray photon excites an electron, either from a core level or the valence band. If excited with X-rays, then PES is often called "X-ray photoelectron spectroscopy" (XPS), while PES with UV excitation is often also called "UV photoelectron spectroscopy" (UPS). In PES, the emitted electron and its kinetic energy are detected, giving rise to a spectrum that shows the number of electrons emitted as a function of kinetic energy. The energy axis can be easily converted from "kinetic energy" to "binding energy," taking the excitation (photon) energy and the work function of the electron analyzer into account. While core-level spectra are element-specific and chemically sensitive, valence-band spectra give detailed insight into the band structure, in our context, in particular, the energy position of the VBM with respect to the Fermi energy (EF). The information derived from PES is representative of the electronic structure at the surface of a sample, since the detected electrons have an inelastic mean free path in the nanometer range [6,7]. A photoemission spectrum can also be used to derive the work function of a sample surface (i.e., the minimal energy required to remove an electron from the sample to the vacuum level in front of the sample surface) by evaluating the secondary electron cut-off at low kinetic energies.
The inverted photoemission process, in which an incoming electron is placed into an unoccupied state above the vacuum level and subsequently decays into a lower unoccupied state, is used for inverse photoemission spectroscopy (IPES ). By detecting emitted UV photons as a function of electron energy, it is possible to extract a spectrum of the conduction band, in our context, in particular, to derive the energy position of the CBM with respect to EF.
As in the case of PES, IPES is a surface-sensitive technique due to the small inelastic mean free path of the incoming electrons. Thus, by combining PES and IPES, it is possible to derive a very detailed picture of the electronic structure at the surface of a sample, including the energies of VBM, CBM and the vacuum level with respect to the Fermi energy, as will be discussed in Section 6.3 using the example of WO3. Note that the determination of the VBM and CBM energies also implies a determination of the electronic surface bandgap. As mentioned in the introduction, this electronic surface bandgap can deviate significantly from the optical bulk bandgap that is responsible for light absorption in a PEC candidate material.
In X-ray emission spectroscopy (XES [9,10]), a core electron is removed from the system, and one of the two possible channels for the subsequent decay of this core hole (namely the emission of a soft X-ray photon) is monitored. An XES spectrum thus depicts the number of emitted photons as a function of photon energy, which represents a partial and local density of states "partial" because of the need to obey dipole selection rules, and "local" because of the local character of the core hole that initiates the X-ray emission process. An XES spectrum of a decay process that involves valence electrons thus gives additional insights into the valence-band structure, as will be demonstrated in Section 6.4. Furthermore, the localized character of the core hole can be used to probe the local chemical environment of the core-excited atom and thus allows XES to also reveal information about the chemical bonding.
In X-ray absorption spectroscopy (XAS or NEXAFS near-edge X-ray absorption fine structure ), a core electron is excited into an unoccupied state and the ensuing core-hole decay process (Auger electron emission or X-ray fluorescence) is monitored, either in the electron channel (electron yield) and/or in the fluorescence channel (fluorescence yield FY). XAS is thus sensitive to both chemical shifts in the ionized core level and the conduction-band structure into which the core electron is excited.
It should be noted that all of the here-described spectroscopies are not only governed by the initial state configuration of the specific technique (i.e., the ground state for PES or a core-hole state for XES), but are more precisely described by transition elements that take both initial and final states (and an operator describing the photon field) into account. In the case of XAS, for example, a determination of the energy position of the conduction-band minimum from the leading edge of the XAS spectrum can be obscured by the potential presence of a core exciton, that is, an electron hole pair that is bound by Coulomb interaction and that is formed between the hole in the ionized core level and the electron excited into the conduction band. If a core exciton is present, the onset of the XAS spectrum is shifted to lower excitation energies, approximately by the binding energy of the core exciton.
In contrast to PES and IPES, which involve electrons in the detection or excitation process, respectively, XES and FY XAS use photon-in photon-out processes. Consequently, the information depth is defined by the attenuation length of the employed soft X-ray photons, which is typically on the order of a few tens to a few hundred nanometers . Thus, the information derived from XES and FY XAS spectra describes the near-surface bulk of the probed sample. Furthermore, the increased information depth forms the basis of exciting new developments in in situ XES and XAS spectroscopy using suitably designed in situ cells (see, e.g., [13 18]). While soft X-rays typically require an ultra-high-vacuum environment, these cells allow the use of XES and FY XAS of samples in non-vacuum environments, such as a PEC electrolyte. A brief outlook of such an approach is given in Section 6.5.
To perform such spectroscopies, sophisticated equipment is required. For IPES, a high-flux, low-energy electron source with a narrow thermal broadening is needed, combined with an efficient UV band-pass detection system. Such set-ups are rare and typically located in stationary ultra-high vacuum systems in a single-investigator lab environment.
PES set-ups, in contrast, have seen a remarkable commercial development and are frequently found in lab environments, as well as at synchrotron radiation sources. While the lab environment allows constant access, the synchrotron radiation environment has experimental advantages, in particular the tunability of the photon energy and the (potentially) higher-energy resolution compared to lab-based X-ray sources.
XES requires a high-flux excitation source and thus is best performed with high-brilliance synchrotron radiation from a third-generation synchrotron light source. For XES, the tunability of the excitation source is very important to optimize photoionization cross-sections. This is particularly true in the soft X-ray regime, since the competing relaxation process (i.e., Auger electron emission) is substantially faster and thus overwhelmingly dominant. Consequently, XES is a "photon-hungry" experiment, requiring high-flux excitation, as well as efficient detection. In order to extract detailed information about the electronic structure, however, this detection also needs to be performed with high-energy resolution (i.e., on the scale of a few tenths of an eV), which in turn calls for highly sophisticated soft X-ray spectrometers [19 22].
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