XPS guide: Charge neutralization and binding energy referencing for insulating samples

Abstract

This guide deals with methods to control surface charging during XPS analysis of insulating samples and approaches to extracting useful binding energy information. The guide summarizes the causes of surface charging, how to recognize when it occurs, approaches to minimize charge buildup, and methods used to adjust or correct XPS photoelectron binding energies when charge control systems are used. There are multiple ways to control surface charge buildup during XPS measurements and examples of systems on advanced XPS instruments are described. There is no single, simple, and foolproof way to extract binding energies on insulating material, but advantages and limitations of several approaches are described. Because of the variety of approaches and limitations of each it is critical for researchers to accurately describe the procedures that have been applied in research reports and publications.

Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

“…†† †† The problem of ionization-induced charging is well-known in solid insulator studies and is usually sufficiently counteracted using neutralization instrumentation such as electron flood guns. 95 Notably, the charging of the surface of a volatile, flowing aqueous solution in a low-vacuum environment cannot be compensated in this way. In order to accurately and generally perform the E F referencing procedure, the electrons from the metal sample would also need to be detected following traversal of the solution–vacuum interface, for example using a PES-compatible solution-on-metal sample system incorporating a continuous solution flow (to avoid sample contamination and cumulative photo-induced degradation).…”

“… †† The problem of ionization-induced charging is well-known in solid insulator studies and is usually sufficiently counteracted using neutralization instrumentation such as electron flood guns. 95 Notably, the charging of the surface of a volatile, flowing aqueous solution in a low-vacuum environment cannot be compensated in this way. …”

Controlling surface chemistry and mechanical properties of metal ionogels through Lewis acidity and basicity

“…differential surface charging). 27 The XPS derived N/O ratios for [omim]Cl ionogels showed a large decrease from 16.6 to 6.5 when increasing the PEGDA content from 5-40 wt%. However, there was only a small amount of oxygen at the surface-1.8 at% for 40 wt% PEGDAwhich demonstrated that [omim]Cl ionogels surfaces were ionic liquid rich.…”

Ambient Temperature CO Oxidation Using Palladium–Platinum Bimetallic Catalysts Supported on Tin Oxide/Alumina

“…1.6 eV lower than that of metallic Pt, and whilst such anomalous values have been observed for PtAu alloys of varying composition and attributed to localisation of s-d bands [39], equally low values have also been observed for Pt supported on Al2O3 [40], indicating a possible preferential interaction with the alumina substrate and bimetallic PtSn/Al2O3 catalysts. However, given the Pt(4d5/2) value was consistent with that for metallic Pt, we considered this to be an effect of two distinct charge states, which we attributed to regions of different thicknesses (or island sizes) of SnO2 as implied by the Sn(3d5/2) binding energy discussed previously [41][42][43]. Assuming an ideal Pt/Pd ratio of 0.25 based on the metal loadings, then no direct correlation with Pt/Pd ratio and activity was For all catalysts, the percentage of Sn added to the alumina was the same; however, it is clear from Figure 10 that the Sn(3d) signal intensity changed markedly, which is related to the dispersion of Sn, with the most intense signal coming from the lowest-weight loading (Figure 10e), and is attributed to the wetting of the alumina beads by the SnO 2 .…”