A Novel Method to Maintain the Sample Position and Pressure in Differentially Pumped Systems Below the Resolution Limit of Optical Microscopy Techniques

We present a new method to maintain constant gas pressure over a sample during in situ measurements. The example shown here is a differentially pumped high-pressure X-ray photoelectron spectroscopy system, but this technique could be applied to many in situ instruments. By using the pressure of the differential stage as a feedback source to change the sample position, a new level of consistency has been achieved. Depending on the absolute value of the sample-to-aperture distance, this technique allows one to maintain the distance within several hundred nanometers, which is below the limit of typical optical microscopy systems. We show that this method is well suited to compensate for thermal drift. Thus, X-ray photoelectron spectroscopy data can be acquired continuously while the sample is heated and maintaining constant pressure over the sample. By implementing a precise manipulator feedback system, pressure variations of less than 5% were reached while the temperature was varied by 400 ℃. The system is also shown to be highly stable under significant changes in gas flow. After changing the flow by a factor of two, the pressure returned to the set value within 60 s

Role of Oxidation–Reduction Dynamics in the Application of Cu/ZnO-Based Catalysts

We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain anatomistic picture of the catalyst’s structure under in situ oxidizing and reducing conditions.The Cu/ZnO model catalysts were investigated at elevated gas pressures by highenergy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectronspectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu$_2$Ounder atmospheric conditions at room temperature. As the nanoparticles swell duringoxidation, they maintain their epitaxy on basal ZnO (000±1) surfaces, whereas on thevicinal ZnO (10$\bar{14}$) surface, the nanoparticles undergo a coherent tilt. We find thatthe oxidation process is fully reversible under H$_2$ flow at 500 K, resulting in predominantlywell-aligned nanoparticles on the basal surfaces, whereas the orientation of CuNPs on vicinal ZnO was only partially restored. The analysis of the substrate crystaltruncation rods evidences the stability of basal ZnO surfaces under all gas conditions.No Cu-Zn bulk alloy formation is observed. Under CO$_2$ flow, no diffraction signalfrom the nanoparticles is detected, pointing to their completely disordered state. TheAP-XPS results are in line with the formation of CuO. Scanning electron microscopyimages show that massive mass transport has set in, leading to the formation of largeragglomerates

Hard x-ray photoelectron spectroscopy : a snapshot of the state-of-the-art in 2020

Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities

A high-pressure x-ray photoelectron spectroscopy instrument for studies of industrially relevant catalytic reactions at pressures of several bars

We present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditionsat pressures of multiple bars. The instrument builds around the novel concept of a “virtual cell” in which a gas flow onto the sample surfacecreates a localized high-pressure pillow. This allows the instrument to be operated with a low pressure of a few millibar in the main chamber,while simultaneously a local pressure exceeding 1 bar can be supplied at the sample surface. Synchrotron based hard x-ray excitation is usedto increase the electron mean free path in the gas region between sample and analyzer while grazing incidence <5○ close to total externalrefection conditions enhances surface sensitivity. The aperture separating the high-pressure region from the differential pumping of theelectron spectrometer consists of multiple, evenly spaced, micrometer sized holes matching the footprint of the x-ray beam on the sample.The resulting signal is highly dependent on the sample-to-aperture distance because photoemitted electrons are subject to strong scatteringin the gas phase. Therefore, high precision control of the sample-to-aperture distance is crucial. A fully integrated manipulator allows forsample movement with step sizes of 10 nm between 0 and −5 mm with very low vibrational amplitude and also for sample heating up to500 ○C under reaction conditions. We demonstrate the performance of this novel instrument with bulk 2p spectra of a copper single crystal atHe pressures of up to 2.5 bars and C1s spectra measured in gas mixtures of CO + H2 at pressures of up to 790 mbar. The capability to detectemitted photoelectrons at several bars opens the prospect for studies of catalytic reactions under industrially relevant operando conditions