Photoelectron spectroscopy of metal surfaces for potential heterogeneous catalysis

Georg Held

DOI: 10.1039/9781849732833-00001

1 Introduction

The "Photoelectric Effect" was discovered in 1888 by Wilhelm Hallwachs and famously explained by Albert Einstein in 1905 as the annihilation of a photon by transferring all its energy to the excitation of a bound electron into vacuum. Einstein's work, together with Planck's law for black-body radiation, introduced the concept of photons as quanta of electromagnetic radiation and was pivotal for the development of quantum theory in general, however it was the development of precise electron energy spectrometers in the 1950's by Kai Siegbahn and coworkers, which turned X-ray induced photoemission into X-ray Photoelectron Spectroscopy (XPS), one of the most powerful tools of surface analysis available to date. The availability of synchrotron radiation for "parasitic users" since the mid 1960's and particularly the improved beam qualities of purpose-built third generation synchrotrons that started to go online in the 1990's has marked another step-change, now enabling spectroscopic measurements at the time scale of seconds and at an energy resolution that is essentially determined by the natural line width of the samples under investigation. These improvements have led to developments of new variants of XPS, which are especially useful to research in the field of catalysis. A recent issue of the Journal "Nuclear Instruments and Methods in Physics Research", which is dedicated to Kai Siegbahn (volume 601, issues 1–2) provides an excellent overview over recent developments in XPS. The present review aims at providing a summary of new experimental techniques and other methods related to XPS, which are relevant to the field of heterogeneous metal catalysts and their application, roughly covering the last decade since 2000.

1.1 Physical principles

The kinetic energy of a photoelectron emitted from a solid is

[MATHEMATICAL EXPRESSION OMITTED] (1)

hv is the photon energy, ΦSpectr the workfunction of the spectrometer (typically around 5 eV), and EB the electron binding energy (positive, with respect to the Fermi energy). When a monochromated UV or X-ray source and a calibrated electron spectrometer are used the kinetic energy spectrum can be converted directly into binding energies using Equation (1). The spectral region of the valence electrons (binding energies below 20 eV) is affected strongly by the chemical bonds of the emitter atoms and/or the crystal structure of a solid and cannot, therefore, be used for elemental analysis. The binding energies of core-level electrons, however, are characteristic for the chemical identities of the emitter atoms. Their photoionisation cross section depends on the photon energy and polarisation and on the nature of the core orbital but not on the chemical environment of the emitter atom if diffraction effects can be ignored (see below). This enables quantitative elemental analysis. The strong interaction between electrons and solid matter leads to a short escape depth of photoelectrons of only around 1 nm if their kinetic energy is below 1000eV. This makes XPS very surface sensitive.

The remaining core and valence electrons will react to the creation of the core hole at the same timescale as the photo-ionisation process. This relaxed electron system partially screens the core hole and leads to a reduction of the measured binding energy, EB, by several eV with respect to the binding energy of the core electron before ionisation, which is often referred to as the initial state or Koopmans energy. The relaxation energy depends on the configuration of the system after the photoionisation has taken place and is, therefore, a "final state effect".

Eventually, the core hole is filled with an electron from a higher-lying orbital shell. The difference in binding energies is either used to create a photon or transferred to another electron which is excited into vacuum. The former core-hole decay mechanism is called X-ray fluorescence and leads to the creation of a new electron hole in the upper orbital. The latter mechanism, Auger electron emission, leads to a final state with two electron holes and to additional well-defined peaks in the electron energy spectrum, which are also characteristic for chemical nature of the emitter atom. For a particular orbital the ratio between X-ray fluorescence and Auger electron emission increases with increasing atomic number. The newly created electron holes lead to a cascade of fluorescence and Auger emission with decreasing energies.

Other final state effects include satellite peaks due to electronic excitations or incomplete relaxation of the electron system, broadening due to vibrational excitations induced by the photoionisation process and peak broadening due to short core-hole lifetimes. The latter effect is a direct consequence of Heisenberg's uncertainty principle and is particularly noticeable for core holes that can be filled via fast Auger processes, which include electronic states from the same shell (Coster-Kronig process).

1.2 Chemical shifts

Both initial state energy and final state relaxation, are affected by changes in the valence electronic structure of the emitter atom. This leads to XPS binding energy differences of up to several eV for the same element in different chemical environments. Such chemical shifts provide the basis for analysing the chemical composition of a sample in much more detail than a simple elemental analysis.

1.2.1 Oxidation state, surface core level shifts (SCLS). Two types of chemical shifts are particularly useful to characterise metal catalyst surfaces or nanoparticles, namely shifts due to the metal oxidation state and surface core level shifts. Both are related to a change in the valence electron density near the emitter atom.

For low lying core levels of transition metals an increase in the oxidation state leads to a positive shift in the binding energy, which has been used frequently to determine the chemical state of model catalysts surfaces and nanoparticles. This shift is commonly considered as initial state effect: the reduced number of valence electrons in the metal cation leads to a lower degree of repulsion...