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► IR spectra of Pt/ceria model catalysts monitor various surface sites. ► Regular and defect sites are identified. ► Metal-support interactions and oxygen spillover is probed.
...Vibrational spectroscopy of adsorbed CO is commonly used for the characterization of oxide-supported metal catalysts, in spite of persisting ambiguities concerning the band assignment. Here, we present reference data on well-defined Pt/CeO2(111)/Cu(111) model catalysts aiming at clear identification of the related features. Growth and morphology of the Pt particles are investigated by scanning tunneling microscopy (STM), the adsorption properties are probed by temperature programmed desorption (TPD), and the electronic structure has previously been studied by photoelectron spectroscopy (PES) and resonant photoelectron spectroscopy (RPES). Combining molecular beam (MB) methods and infrared reflection absorption spectroscopy (IRAS), we correlate this information to the interaction with CO.
Two types of defect adsorption sites are identified on the ceria support (2097cm−1, 2120cm−1), which are assigned to structural defects and reduced Ce3+ centers. The relative abundance of these sites changes upon thermal treatment. On the Pt nanoparticles, CO first adsorbs on-top at step and edge sites (2066cm−1), before adsorption on the (111) facets occurs in bridging (1875cm−1) and on-top (2080–2097cm−1) geometry. For the first time, we identify vibrational CO features, which arise from electronic metal-support interactions and from oxygen reverse spillover. On reduced CeO2−x(111), a red-shifted band at 2053cm−1 appears, which is attributed to increased π-backbonding from Pt to CO, as a result of a change in electronic metal-oxide interaction. Thermal treatment also activates oxygen reverse spillover from the support to the Pt nanoparticle, which gives rise to a blue-shifted feature in the CO spectrum (2090–2105cm−1) due to coadsorbed oxygen next to CO sites.
•A simple gas sensor with nanostructured SnO2 sensitive layer was studied in-situ by NAP-XPS at conditions, which are close to working conditions of the gas sensor.•The chemisorbed oxygen on the tin ...oxide surface was not detected while exposing to oxygen atmosphere, however, the shift of XPS peaks caused by the band bending indirectly confirmed it.•Exposure of the surface to mixture of O2/EtOH resulted in disappearance of the band bending effect in the XPS spectra.•Different carbonaceous groups including ethoxy and acetate groups were observed on the sensor surface by NAP-XPS that supported the existing theory about the chemical interaction of ethanol with the chemisorbed oxygen.
In this article, we present the results of an investigation into chemical processes which take place at the surface of SnO2-based chemiresistor in various atmospheres (1 mbar of argon, 1 mbar of oxygen, 0.1 mbar of ethanol, 1 mbar of oxygen + 0.1 mbar of ethanol mixture) at common working temperatures (450 and 573 K). The key method for nanoscale analysis was the Near Ambient Pressure X-ray Photoelectron Spectroscopy. In parallel the resistance and DC-responses of SnO2 layer were in-situ monitored providing information about macroscale processes during gas sensing. The change in the sensor resistance after exposure to the ethanol-containing atmospheres together with the disappearance of the band bending effect and observation of different carbonaceous groups including ethoxy groups and acetaldehyde molecules on the sensor surface in the XPS spectra supported the theory of chemical interaction of ethanol with the chemisorbed oxygen. The NAP-XPS spectra also showed that the nanostructured tin oxide is partially reduced even after being exposed to pure oxygen at 573 K. Exposing this surface to the mixture of O2/EtOH did not significantly increase the surface reduction probably due to slow kinetics of the ethanol reduction process and fast kinetics of the oxygen re-oxidation process. However, it was demonstrated that the surface of sensor is slowly getting contaminated by carbon.
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Reduced ceria can be reoxidized by CO2 at room temperature in complete absence of any noble metal co-catalysts, surface hydroxyl groups, or water on its surface. The activation of CO2 on ceria-based ...catalysts opens chemical routes to the use of CO2 as an oxidizing agent. Toward a better understanding of the underlying surface reactions, we have performed a model study on well-defined CeO2- x thin films on Cu(111) under ultra-high-vacuum conditions. Resonant photoelectron spectroscopy is used as a highly sensitive tool to monitor changes in the oxidation state of cerium as a function of CO2 exposure and thermal treatment. It is shown that partial reoxidation of CeO2- x by CO2 occurs with high reaction probabilities (i) even at temperatures as low as room temperature, (ii) on pure CeO2- x in the complete absence of any noble metal co-catalysts, and (iii) in the absence of surface hydroxyl groups or water. In contrast to previous studies on ceria powders, the findings clearly demonstrate the high activity of pure hydroxyl- and metal-free ceria surfaces for CO 2 activation, even at low reaction temperatures. PUBLICATION ABSTRACT
Adjustable morphology and degree of reduction represent desirable properties of model oxide substrates for heterogeneous catalysis. We investigate these properties in CeO2 (ceria) thin films on ...Cu(111) using scanning tunneling microscopy and photoelectron spectroscopy. We identify growth mechanisms of ceria on Cu(111): formation of incomplete oxide interfacial layer and formation of three-dimensional ceria pyramids by stacking of monolayer-high islands. Using these mechanisms, we control the coverage, the number of open monolayers, and the step density of ceria thin films on Cu(111). Annealing in vacuum allows us to control besides the morphology also the degree of ceria surface reduction. We find a correlation between surface reduction and morphological stability in annealed ceria layers. Oriented and stoichiometric thin films of ceria on Cu(111) can be prepared at temperatures as low as 150 and 250 °C. Both the morphology and the surface reduction of these films readily change with increasing temperature, which must be accounted for in considering temperature-programmed experiments with ceria on Cu(111).
Abstract
This work reports the effect of different processing parameters on the structural and morphological characteristics of MoSe
2
layers grown by chemical vapour deposition (CVD), using MoO
3
...and Se powders as solid precursors. It shows the strong dependence of the size, shape and thickness of the MoSe
2
layers on the processing parameters. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM) and the thickness of the deposited layers was determined by atomic force microscopy (AFM). Raman and photoluminescence (PL) spectroscopies were used to confirm the high quality of the MoSe
2
layers. Surface composition was examined by photoelectron spectroscopy (XPS). Moreover, the MoSe
2
/SiO
x
/Si heterojunctions exhibit diode behaviour, with a rectification ratio of 10, measured at ±2.0 V, which is due to the p-i-n heterojunctions formed at the p-Si/SiO
x
/MoSe
2
interface. A photovoltaic effect was observed with a short circuit current density (
J
sc
), open circuit voltage (
V
OC
) and efficiency of −0.80 mA/cm
2
, 1.55 V and 0.5%, respectively. These results provide a guide for the preparation of p-i-n heterojunctions based on few-layer MoSe
2
with improved photovoltaic response.
Interaction of water with fully oxidized and partially reduced CeO₂(111) thin film model catalyst grown on a Cu(111) surface was investigated by photoelectron spectroscopy (PES), scanning tunneling ...microscopy (STM) and temperature programmed desorption (TPD). On the stoichiometric surface water adsorbs molecularly at low temperatures (≤120K) while on the reduced surface the adsorption is partially dissociative with formation of OH groups. STM indicates no morphology variation and a very sensitive Ce 4d–4f resonant photoelectron spectroscopy (RPES) no noticeable change of the oxidation state of ceria upon water adsorption and subsequent complete desorption. Formation of co-adsorbed phase of residual water with OH during molecular water desorption, on the other hand, leads to a substantial resonance of the Ce³⁺ photoemission state around 170K. We propose that this behaviour indicating ceria reduction is in this case an electronic effect of the Ce 4f charge accumulation and Ce 5d charge depletion.
•We compare the Ce 3d XPS levels of fluorine-free and fluorine doped CeOX layers.•The presence of fluorine greatly influences the overall shape of Ce 3d XPS spectra.•Fluorine doublets are ...characteristic for the Ce–F interaction.•CeOXFY layers gradually regain the CeO2 character with excess of oxygen.•Applied fitting procedure allows to evaluate the fluorine concentration in the layer.
Fluorine atoms have been reported as an impurity in cerium oxide; however, their influence on the electronic structure and chemical properties is not well understood yet. We show the changes in Ce 3d photoelectron spectra features induced by fluorine. We introduce a fitting procedure which allows to evaluate the concentration of fluorine in the cerium oxide layer. A correlation between the fluorine concentration and the characteristic Ce–F interaction doublets added to Ce 3d spectra fits is presented in this Letter. We studied also the interaction of the prepared layers with small doses of CO and O2 gases at elevated temperatures.
Adsorption and desorption of methanol on a CeO2(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy ...of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce4+ - > Ce3+ transition and an increase of the Ce 4f electronic state occupancy.
The reactive vacuum deposition of CeO
2 on Cu(1
1
1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, ...and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1
1
1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO
2(1
1
1) structure corresponding to the (1.5
×
1.5) structure. The CeO
2–Cu(1
1
1) films exhibited mixed valence states and temperature dependent CeO
2–Ce
2O
3 transition above 900
K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.
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