Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)

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Polar surface structures of the ZnO crystal

Compared with the non-polar surfaces ((101̅ 0) and (112̅ 0)), the polar surfaces of ZnO ((0001)-Zn and (0001̅ )-O surfaces) are more interesting from the scientific and technological points of view [32]. The hexagonal wurtzite structure of ZnO is represented as alternating planes made up of fourfold-coordinated Zn2+ and O2- ions along the c axis with alternating distances 1.99 Å and 0.69 Å, respectively [33]. Therefore, a (0001)-Zn terminated surface or a (0001̅ )-O terminated surface is created by cutting the ZnO crystal perpendicular to the c axis. If these two ZnO polar surfaces are not compensated, the electrostatic potential increases due to the divergence of the net diploe moment when increasing the thickness of ZnO crystal [32- 34], as shown in Figure 1.8. In order to reduce the electrostatic energy, three surface stabilization mechanisms are generally put forward [33]: (1) reduction of the charges of the Zn (O) surface by adsorption foreign molecules; (2) surface reconstruction by removal of surface atoms; (3) transfer of negative charge from the O plane to the Zn plane and creation of new surface states.

CO oxidation on the inverse ZnO/Pt(111) catalyst

Low temperature CO oxidation is considered as a prototype reaction on inverse oxide/Pt(111) catalysts, where 3d transition metal oxide (TMO) islands partly cover a platinum surface [46-48]. The attention of the researchers focused on the synergetic effect that could be observed at the platinum/TMO boundary [46, 48]. Indeed, the removal of pre-adsorbed CO by a steady-state flux of O2 (under a pressure in the 10-8 mbar range) was observed at room temperature, both for the FeO/Pt(111) system [46] and the NiO/Pt(111) system [47], which is naturally impossible on the pure Pt(111). It was shown that the reactivity of CO oxidation increases with the FeO1-x/Pt boundary length per surface unit [46], which supports the idea that the reaction takes place at the phase boundary. Further theoretical calculations addressing a series of 3d TMOs showed that the key reaction site is a coordinatively unsaturated (CUS) metal cation combined to nearby platinum atoms [48], as shown in Figure 1.18. Besides the TMO oxides (FeO and NiO) that were known experimentally to be efficient when combined with Pt(111), the theoretical calculations by Sun and coworkers [48], also explored the reactivity of a filled 3d band transition metal oxide ZnO. When compared to the pure Pt(111), the Pt-CUS Zn2+ ensemble favors the breaking of the O-O bond and lowers the barrier for CO oxidation by about 0.4 eV, according to calculations. Although the Pt-CUS Fe2+ ensemble presents a lower CO oxidation barrier than the Pt-CUS Zn2+ one, the single oxidation state of Zn (2+) is a tremendous advantage with respect to the former case, because the ferrous oxide can become ferric oxide under O2 rich conditions, which is highly detrimental to its efficiency [46].
Therefore, theoretical predictions make the ZnO/Pt(111) system worthy of interest, especially in oxygen-rich CO/O2 mixtures.

Theory of scanning tunneling microscopy

The concept of quantum tunneling effect plays an important role in the design and development of STM, as shown in Figure 2.3(a). In classical physics, an electron cannot penetrate or cross a barrier if its energy is smaller than the energy of the barrier. However, in quantum mechanics, an electron can tunnel through a barrier with some probability. Figure 2.3(b) shows the process of an electron crossing a potential barrier. For a rectangular barrier, the wave function of an electron in the direction z is described by the Schrodinger’s wave equation.

Description of scanning tunneling microscopy

Figure 2.4(a) shows the schematic of an STM setup. A tip is a significant ingredient of the STM setup, which is attached to a piezoelectric tube. The piezoelectric tube consists of three mutually perpendicular piezoelectric transducers, which are x, y and z, respectively. Once a voltage is applied, the piezoelectric transducers can expand or contract to make the tip move on the sample surface. Using the coarse approach and the subsequent fine approach, the tip can be brought to the sample surface within few Ångströms. In this case, the electron wavefunction of the tip is overlapped with the electron wavefunction of the sample. Upon applying a voltage, a tunneling current can be generated between the tip and the sample, which decays exponentially with the tip-sample distance. Therefore, the tunneling current is highly sensitive to adjust the tip-sample distance.
When the tip starts to scan the sample surface along the x and y directions, there are two scanning modes, which are the constant current mode and the constant height mode, shown in Figure 2.4(b) and (c), respectively. In the constant current mode, the tunneling current is converted to a voltage by the tunneling current amplifier, which is then compared with a reference value. The difference from the reference value is amplified to drive the z piezo to move on the sample surface: if the tunneling current is smaller than the reference value, the voltage then applied to the z piezo tends to extend the tip towards the sample surface, and vice versa. In this case, the feedback loop adjusts the distance z between the tip and the sample surface at each point of x and y for keeping the tunneling current constant during scanning. In the constant height mode, the tip-sample distance z is fixed during the two-dimensional scanning (x and y). Comparing to the constant current mode, the constant height mode is suitable for a flat sample surface, otherwise, the tip might have a high risk of crashing into the sample surface.
As the tip scans over the sample surface along the x and y direction, the equilibrium z positions, which represent a contour plot of the corresponding tunneling-current surface, is acquired and displayed in the computer. Generally, the bright spots in an STM image indicate high z values (e.g. protrusions) and the dark spots refer to the low z values (e.g. holes and pits).
For a more quantitative illustration of the topography of the sample, a line scan is often applied in an STM image, in which the unit of x or y is nanometer (nm) and the unit of z is picometer (pm).

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Table of contents :

Chapter 1: Introduction
1.1 Concerns of the environmental issues
1.2 Hydrogen fuel cells
1.3 Inverse oxide/metal catalysts
1.4 Zinc oxide structures
Polar surface structures of the ZnO crystal
Structures of ultrathin ZnO films
1.5 CO oxidation on the inverse ZnO/Pt(111) catalyst
Chapter 2: Experimental setup
2.1 Electron beam evaporation (e-beam evaporation)
2.2 Scanning tunneling microscopy (STM)
Theory of scanning tunneling microscopy
Description of scanning tunneling microscopy
Fabrication and cleaning of the tip for scanning tunneling microscopy
2.3 Scanning tunneling spectroscopy (STS)
2.4 Low-energy electron diffraction (LEED)
Description of low-energy electron diffraction
Theory of low-energy electron diffraction
Surface structures
2.5 Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)
Principle of X-ray photoelectron spectroscopy
Description of X-ray photoelectron spectroscopy
Description of near-ambient pressure X-ray photoelectron spectroscopy .
2.6 Quadrupole mass spectrometer (QMS)
Chapter 3: Fabrication of ZnO thin films on Pt(111)
3.1 Preparation of the Pt(111) substrate
3.2 Preparation of ZnO films on Pt(111)
3.3 Electronic properties of ZnO films on Pt(111)
HANG LIU – Thèse de doctorat – 2019
3.4 Summary and conclusions
Chapter 4: Morphology evolution of ZnO films on Pt(111) under CO oxidation conditions 
Summary and conclusions
Chapter 5: CO oxidation mechanisms at the ZnO/Pt(111) model catalyst
5.1 NAP-XPS setup as a flow reactor
5.2 QMS molar fractions for the plain Pt(111) and the ZnO/Pt(111) surfaces
5.3 CO oxidation reaction on the plain Pt(111) surface
5.4 CO oxidation reaction on the ZnO/Pt(111) surface
5.5 Carboxyl/formate species, reaction intermediates or spectators?
5.6 Summary and conclusions
Chapter 6: Conclusion and perspectives
6.1 Conclusions
6.2 Perspectives
Appendix: Synthesis of the large-diameter ZnTe crystal for THz emitting and detection 
Experimental setup
Results and discussion


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