Abstract
Described in this thesis are studies of selected reaction on gold single crystal surface utilizing UHV surface science techniques and electrochemical method. Described in Chapter 2 is a report of synchrotron-based, high resolution core level spectroscopy study of 1-decanethiol thin film on Au(111) single crystal prepared from both gas-phase dosing and solution immersion to vary the coverage in a wider range. S 2p core level exhibits a single, well-characterized spin-orbit doublet with S 2p2/3 at 162.0 eV, irrespective of the preparation methods for the films, indicating the identical chemical interaction between sulfur and gold atoms. However, C 1s core level starts from 284.0 eV at low coverage, develops a high-binding shoulder at intermediate coverage and eventually becomes a single peak at 285.0 eV for solution phase preparation. Angle-resolved XPS measurements of substrate core level signals provide a measure of the film thickness. The C 1s peak at 284.0 eV for the low coverage phase is associated with the film of thinner dimension of 4.0 A, presumably due to the stripe phase of the film. The C 1s peak at 285.0 eV is associated with a thicker film of 12.6 A in thickness, corresponding to a fully-extended, trans molecular thin film at high coverage. The difference of C 1s binding energy is attributed to the incomplete final-state relaxation of the carbon atoms in a fully extended configuration. Moreover, the observation of a clear, well-defined, single-peak S 2p signal does not collaborate with the sulfur dimer model proposed based on x-ray standing wave and glazing incidence x-ray scattering results. Described in Chapter 3 is the adsorption and thermal reaction of decanethiol on clean Au(111) studied by using thermal desorption spectra (TDS) and synchrotron- based high-resolution XPS. The results show that gas-phase dosed decanethiol chemisorbs on Au(111) at below 250 K. A further S-H bond breaking leads to the formation of decanethiolate at below 350 K. Binding energy of S 2p3/2 core level for different species is assigned as 163.2 eV for physisorbed decanethiol, 161.7~161.9 eV for chemisorbed decanethiol, 162.1 eV for thiolate formation and 161.0 eV for atomic sulfur. Decanethiolate is stable up to 350 K, and starts to decompose upon further annealing to above 400 K, releasing decene, decanethiol, H-2S into gas phase and leaving adsorbed sulfur atom on surface. The reaction mechanism was proposed to be initiated by direct disproportionation reaction between adsorbed decanethiolate. The lying-down configuration is essential for long chain alkane thiolate decomposition and decanethiolate in the upright configuration in the dense c(4 x 2) phase initially desorbed as decanethiol or decyl disulfide, and then follow the same decomposed reaction. In Chapter 4, The adsorption of molecular S-2 on Au(111) surface has been studied by thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), synchrotron radiation based x-ray photoelectron spectroscopy (XPS) and sulfur K-edge near-edge x-ray absorption fine structure spectroscopy (NEXAFS). For sulfur multilayer, sulfur allotropes of composition Sn (n = 2 to 8) are formed on the Au(111) surface and thermal desorption, occurring between 200 and 400 K, produces various sulfur species. Chemisorbed sulfur atoms form different adsorption structures at different coverages: a complex adsorption structure at a coverage between 0.36 and 0.47 ML as well as a (O3 x O3) R30° structure at a coverage of 0.33 ML. The desorption of chemisorbed sulfur from Au(111) surface results in the evolution of S2 and S exclusively. The XPS data give evidence of the third sulfur species locating on top of chemisorbed layer at coverages between 0.47 and 0.85. The species is claimed to be a chemisorbed S2 species. The respective S 2p3/2 core level binding energies are 162.90 ± 0.05 eV for physisorption multilayer, 161.82~ 162.05 eV for chemisorbed S2 and 161.13 ± 0.05 eV for atomically adsorbed sulfur. The binding energy of atomic sulfur to Au(111) surface is calculated as 139 kj/mol. Chapter 5 reports the electroreduction of nitrobenzene studied on Au single crystals of (111), (110), (100), and (210) orientations and on Au polycrystal in dilute aqueous perchlorate solution using cyclic voltammetry. The voltammograms were found to be sensitively dependent on surface orientation. Notably, the reduction on -(110), -(210), and -(poly) electrodes can lead to a stably adsorbed redox intermediate, not reported before, at about 0.33 V vs. SCE and the intermediate is deduced as phenyldihydroxylamine and phenyldihydroxylamine cation redox couple. The unusual stability of the redox intermediate is found to correlate with the orientation of adsorbed water molecule that can be intimately controlled by an applied potential in relationship to the potential of zero charge on the surfaces.