Morphology evolution of ZnO films on Pt(111) under CO oxidation conditions 

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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.

Electron beam evaporation (e-beam evaporation)

Zinc evaporation is performed using e-beam evaporation in UHV conditions method. Figure 2.1 shows the e-beam evaporator and the schematic of e-beam evaporation process, respectively. Zn wires (diameter = 2 mm, purity = 99.5%) are contained in a Mo crucible in the evaporator. When the evaporator works, the crucible is bombarded with electron beams from a tungsten filament controlled by a power supply. Consequently, Zn wires are heated in the crucible above the melting temperature (692.68 K). Zn atoms in the vapor phase precipitate and then form a thin film onto the Pt(111) single crystal.

Scanning tunneling microscopy (STM)

Scanning tunneling microscopy was invented by Gerd Binnig and Heinrich Rohrer [52-54] in 1982, who were awarded the 1986 Nobel physics prize in physics. STM has proven to be a powerful technique in surface nanoscience and nanotechnology over the past three decades, which provides access to investigate the surface topography with atomic resolution, the local density of states in samples ranging from metal, semiconductors to superconductors, and manipulate individual atoms and molecules on the sample surface.

Fabrication and cleaning of the tip for scanning tunneling microscopy

The tip quality is important because it determines the performance of the scanning tunneling microscope. Specifically, the tip shape (mechanical rigidness), geometry (sharpness) and chemical composition (cleanness) are three key elements for acquiring a high resolution STM image [58]. Tungsten (W) tip are widely used in the UHV conditions because of its high melting temperature (3695 K), the great mechanical strength and the simple fabrication using an electrochemical etching method. Figure 2.5(a) shows the schematic of the electrochemical etching method for a W tip fabrication, which is called the drop-off technique of the electrochemical etching [59]. During the etching process, the highest etching rate of the W wire (Ф = 0.25 mm) occurs at the interface between air and an electrolyte (2M NaOH), which gradually causes the necking and dropping of the bottom part of the W wire in NaOH, eventually results in a STM tip formation, as shown in Figure 2.5(b) and (c). The tip sharpness mainly affected by the dropping part (immersed part) of the W wire in NaOH. For example, the big weight of the dropping part may cause the tip bending and recoil, making the apex of the tip deformative and blunt. The small weight of the dropping part may cause the immersed W wire completely dissolved in NaOH. In a word, the proper immersed length of the W tip in NaOH is very significant for making a sharp tip, however the suitable parameter of the immersed length is usually obtained empirically.
Once the W tip is fabricated after the electrochemical etching process under the ambient conditions, the tip is inevitably covered by a thin residual layer of tungsten oxide and various contaminants, such as residuals and hydrocarbon from the electrolyte and ambient conditions [58]. Therefore, an annealing procedure of the tip in the UHV chamber is very mandatory to remove the surface oxide and contaminants of the tip. Moreover, the annealing process can reduce the plastic deformation and other stress-induced defects of the tip caused by the etching process [60].

Low-energy electron diffraction (LEED)

Low-energy electron diffraction was discovered by Davisson and Germer in 1927 [61, 62] and did not become a popular tool for surface analysis until 1960 due to the inadequate vacuum techniques and the slow detection methods. LEED is a science tool which is very sensitive to determine the surface structure. The basic reason of the high surface sensitivity of LEED is that the mean free path of the electron is very low (5-10 Å) in solid materials based on the energy range of low-energy electrons (20-200 eV). In this case, a few atomic layers of the sample are detected by the electron beam and consequently the scattered electrons originate specifically from the sample surface.

READ  Electron Energy Loss Spectroscopy

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

XPS is a surface-sensitive quantitative analysis technique. With the development of a high-resolution XPS by Kai Siegbahn and his research group in Uppsala University in Sweden [64], XPS allowed to measure accurate binding energy of photoelectron peaks, in which the investigation of electronic structure has been realized. The work of Kai Siegbahn was awarded by the physics Nobel prize in 1981.

Principle of X-ray photoelectron spectroscopy

An XPS measurement is operated by irradiating the sample surface with a beam of monochromatic X-rays. If the photon energy is high enough, electrons from the sample atoms can be excited and create the photoelectrons, which are then emitted from the sample surface. Figure 2.9 is the schematic of a photoemission process. In this process, the electrons on a given orbital are first promoted from a bound state to an excited state by absorbing certain photon energy ℎ . Before escaping to vacuum, the photoelectrons travel some distance inside the sample, in which the photoelectrons collide elastically or inelastically with the lattice atoms of the sample. The elastically scattered electrons escape from the sample surface without any kinetic energy loss, which will form the main XPS core level peak (also called adiabatic peak). On the contrary, the inelastically scattered electrons escape from the sample surface with losing a part of the kinetic energy, which will create the background of the XPS spectrum. Therefore, the surface sensitivity of XPS is determined by the electron inelastic mean free path (IMFP), which is the characteristic length that an electron on average travels through before suffering an inelastic scattering. After reaching the sample surface, the photoelectrons still need to overcome a barrier potential which exist at the sample surface in order to get to the vacuum level. That barrier potential refers to the work function of the sample.

Description of near-ambient pressure X-ray photoelectron spectroscopy

Due to the exponential decrease of the photoelectron signal in gas phase, NAP-XPS technique is designed to partially overcome the pressure gap in surface science. Unlike the ordinary XPS, NAP-XPS allows the sample to be exposed in high pressures in order of millibars, which can study the dynamics of catalytic reactions. The most significant change for NAP-XPS is the introduction of the differential pumping in the electron analyzer system which dramatically decreases photoelectron attenuation between the sample and the analyzer. The early design of the differential pumping system is shown in Figure 2.12 (a). In this design, the photoelectrons travel through differential pumping regions before reaching the analyzer, which are unfocused between the apertures. The unfocused photoelectrons would cause a serious loss in collection efficiency. Fortunately, the new design of the differential pumping system (Figure 2.12 (b)) is combined with a series of electrostatic lenses which greatly increase the electron collection rate. Different from the laboratory X-ray source (Al/Mg source) used in the XPS, the synchrotron-based light is applied onto the NAP-XPS, which can produce a high flux of photons and a more focalized size at the sample generating more photoelectrons. Nowadays, a number of synchrotron-based NAP-XPS setups have arisen all over the world, such as SOLEIL, DIAMOND, BESSY, ALS and SSRL [66].

Table of contents :

Acknowledgements
Abstract
Abbreviations
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)
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
Introduction
Experimental setup
Results and discussion
Conclusions
References

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