Recent Approaches for Bridging the Pressure Gap in Photoelectron Microspectroscopy

Abstract

Ambient-pressure photoelectron spectroscopy (APPES) and microscopy are at the frontier of modern chemical analysis at liquid-gas, solid-liquid and solid-gas interfaces, bridging science and engineering of functional materials. Complementing the current state-of-the art of the instruments, we survey in this short review several alternative APPES approaches, developed recently in the scanning photoelectron microscope (SPEM) at the Elettra laboratory. In particular, we report on experimental setups for dynamic near-ambient pressure environment, using pulsed-gas injection in the vicinity of samples or reaction cells with very small apertures, allowing for experiments without introducing additional differential pumping stages. The major part of the review is dedicated to the construction and performance of novel environmental cells using ultrathin electron-transparent but molecularly impermeable membranes to isolate the gas or liquid ambient from the electron detector operating in ultra-high vacuum (UHV). We demonstrate that two dimensional materials, such as graphene and derivatives, are mechanically robust to withstand atmospheric - UHV pressure differences and are sufficiently transparent for the photoelectrons emitted from samples immersed in the liquid or gaseous media. There are many unique opportunities for APPES using X-rays over a wide energy range. We show representative results that illustrate the potential of these ‘ambient-pressure’ approaches. Combined with the ca 100 nm lateral resolution of SPEM, they can overcome the pressure gap challenges and address the evolution of chemical composition and electronic structure at surface and interfaces under realistic operation conditions with unprecedented lateral and spectral resolution.

Ambient Pressure Photoelectron Spectroscopy: Opportunities in Catalysis from Solids to Liquids and Introducing Time Resolution

“…In a few recent studies, photoemission of samples enclosed with graphene oxide and graphene membranes were performed. A detailed overview regarding the preparation method, photoelectron permeability test of prepared graphene oxide and graphene films, and the synthetic procedure of the graphene membranes is available in . A route to synthesize high quality monolayer graphene is by chemical vapor deposition on metal substrates, of which copper is preferred due to its low carbon solubility.…”

“…Presently, photoemission measurements around tens of mbar pressures are routinely possible with the commercially available differentially pumped electron analyzers. In lieu of present APXPS arrangements of a small reaction cell or a comparatively large UHV chamber, a modified setup can be designed with a photoelectron semi‐transparent membrane covered cell, which can reach atmospheric and even higher pressures without affecting the vacuum of the analyzer . This will be further discussed below.…”

Needs and Gaps for Catalysis in Addressing Transitions in Chemistry and Energy from a Sustainability Perspective

“…[25] Under thesec onditions, photoemission from the gas phase may overcome surface-bound contributions. [26] Operations up to 800 8C have been demonstrated by using an infrared laser beam with aq uadrupole mass spectrometer positioned in the first differential pumping stage to monitorgas composition. [25] Conversely,c ells with ap inhole or with ag raphene membrane combinedw ith ap ulsed gas jet can be used (to increasethe pressureonthe surface of the catalyst without increasing the background pressure) for gasphase/catalyst operando studies.…”

LEEM and PEEM as Probing Tools to Address Questions in Catalysis

“…The idea behind this approach is to isolate the imaging components of LEEM/PEEM microscopes from the high pressure environment at which the samples are submitted in order to avoid technical problems with the high voltages typically used (15-20 kV). In this sense, graphene-based windows have proven to be quite robust, in terms of stability towards reactive gases and high pressures, and transparent to electrons [70][71][72]. In the case of the microarray, the principle is the same, i.e., the liquid phase is trapped in the pores of porous materials that are covered with graphene.…”