Heske, C.; Weinhardt, L.; Bär, M.: X-Ray and Electron Spectroscopy Studies of Oxide Semiconductors for Photoelectrochemical Hydrogen Production. In: Vayssieres, L. [Ed.] : On solar hydrogen & nanotechnologySingapore: Wiley, 2009. - ISBN 978-0-470-82397-2, p. 143-161
Abstract:
As outlined in various chapters throughout this book, a successful implementation of photoelectrochemical hydrogen production using sunlight (PEC) requires significant material science breakthroughs. A material needs to be found that simultaneously fulfils several requirements, among them an optimized bulk bandgap for efficient utilization of the incoming solar photon flux and its spectral distribution, an optimized electronic structure at the interface between the material and the surrounding electrolyte, and a sufficient chemical stability (lifetime) of the material under the conditions of a very high or a very low pH value in the electrolyte.While no single such material is in hand today, significant advances have been made with a variety of materials that fulfill at least one of these requirements, as outlined in this book. How can the search for the optimal (“holy grail”) material be facilitated? Ultimately, of course, a PEC candidate material will be judged by its ability to split water and to produce hydrogen in a cost-effective way. To reach this goal, however, individual properties of particular materials need to be understood and optimized, and, in particular, fundamental barriers in one (or more) of the requirements need to be identified. It is thus crucially important to be able to characterize candidate materials with respect to each material requirement individually, that is, independent of the other requirements. It is the purpose of this chapter to demonstrate how soft X-ray- and electron-based spectroscopic methods are a powerful and evolving “tool chest” to do just that: to focus on a specific materials requirement and to collect information about the fundamental properties of a candidate material that are of direct relevance to the ultimate performance in a PEC cell. We will focus on two particular requirements: the understanding of the electronic structure of the material surface (and, ultimately, its interface with the electrolyte), as well as the chemical structure relevant for chemical stability, by discussing two examples. The first example is based on the need of a PEC material to exhibit suitable positions of the conduction band minimum (CBM) at the surface of the hydrogen electrode and of the valence band maximum (VBM) at the surface of the oxygen electrode, respectively. A detailed knowledge of these levels is of large importance for the choice and optimization of an electrode material. However, a direct determination of these levels – especially at the surface of the material – is not straightforward. In most studies related to this topic, one of the energy levels (the VBM for p-type systems or the CBM for n-type systems) is determined by electrochemical methods. These techniques require specific sets of assumptions about the possibility of achieving flat-band conditions, and the position of the other band edge (i.e., the CBM for p-type systems or the VBM for n-type systems) is generally inferred from optically determined bulk bandgaps. However, in general, electronic bandgaps and band edge positions at the surface of compound semiconductors (and thus also at the interface with the electrolyte) are different from optical bulk gaps and bulk band edge positions (see, for example, [1–3] for respective studies on chalcopyrite compound semiconductors). For a correct description it is thus necessary to measure band edge positions and gaps directly with surface-sensitive techniques. This will be demonstrated in Section 6.3. In terms of chemical stability, in particular of a multicomponent material system, a detailed understanding of the chemical composition at and near the surface is crucial. With such understanding, chemical changes during operation (or, as in our case, air exposure) can be monitored precisely, giving detailed insight into the fundamental chemical behavior of a PEC candidate material. Of particular interest is the ability to derive bond-specific composition information, that is, not