Direct Work Function Measurement by Gas Phase Photoelectron Spectroscopy and Its Application on PbS Nanoparticles

Abstract

Work function is a fundamental property of a material's surface. It is playing an ever more important role in engineering new energy materials and efficient energy devices, especially in the field of photovoltaic devices, catalysis, semiconductor heterojunctions, nanotechnology, and electrochemistry. Using ambient pressure X-ray photoelectron spectroscopy (APXPS), we have measured the binding energies of core level photoelectrons of Ar gas in the vicinity of several reference materials with known work functions (Au(111), Pt(111), graphite) and PbS nanoparticles. We demonstrate an unambiguously negative correlation between the work functions of reference samples and the binding energies of Ar 2p core level photoelectrons detected from the Ar gas near the sample surface region. Using this experimentally determined linear relationship between the surface work function and Ar gas core level photoelectron binding energy, we can measure the surface work function of different materials under different gas environments. To demonstrate the potential applications of this ambient pressure XPS technique in nanotechnology and solar energy research, we investigate the work functions of PbS nanoparticles with various capping ligands: methoxide, mercaptopropionic acid, and ethanedithiol. Significant Fermi level position changes are observed for PbS nanoparticles when the nanoparticle size and capping ligands are varied. The corresponding changes in the valence band maximum illustrate that an efficient quantum dot solar cell design has to take into account the electrochemical effect of the capping ligand as well.

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni–CeO2(111) catalysts: an in situ study of C–C and O–H bond scission

“…The changes of the gas phase peak has been observed previously, 35 and it is related to several factors including the changes in concentration of reactants and products, the work function at the near surface of the sample as well as variations of partial pressures of each gas. Therefore, considering the complexity of the gas mixture (reactants and products), we represent the gas phase feature with only one single broad peak.…”

Ambient-Pressure XPS Study of a Ni–Fe Electrocatalyst for the Oxygen Evolution Reaction

“…8 These molecules near the surface have core level ionization energies that are constant with respect to the vacuum level but will appear with a peak shift in XPS when the work function is changed because the XPS binding energy scale is referenced to the Fermi level of the electrode. 18 In addition to the applied external potential, the surface work function is also sensitive to the chemical and structural changes of the surface, which may cause deviation from the 1:1 relation between the applied external potential and the detected binding energy shift of the gas phase molecules. The gas phase peak position is also affected by the electrostatic potential at the site where the ionization takes place, the effect of which was minimized here by grounding the entrance nozzle of the analyzer.…”

Comparison of AP-XPS and PLIF Measurements During CO Oxidation Over Pd Single Crystals

“…Furthermore, it can also be seen that the CO 2 peak is shifting towards higher energy as the surface goes from partly to fully active. The reason is the change in the surface work function [26,27] as the CO desorbs fully from the surface and the oxygen fully adsorb on the surface. Decreasing the total pressure even further to 0.13 mbar (Fig.…”