Controlling particle size, shape, nucleation, and selfassembly on surfaces are some of the main challenges facing electronic device fabrication. In this work, growth of gold nanoparticles over a wide range of sizes was investigated by using a novel wet chemical method, where potassium iodide is used as the reducing solution and gold chloride as the metal precursor, on silicon substrates. Four parameters were studied: soaking time, solution temperature, concentration of the solution of gold chloride, and surface pre-treatment of the substrate. Synthesized nanoparticles were then characterized using scanning electron microscopy (SEM). The precise control of the location and order of the grown gold overlayer was achieved by using focused ion beam (FIB) patterning of a silicon surface, pre-treated with potassium iodide. By varying the soaking time and temperature, different particle sizes and shapes were obtained. Flat geometrical shapes and spherical shapes were observed. We believe, that the method described in this work is potentially a straightforward and efficient way to fabricate gold contacts for microelectronics.
The reactions of ethanol have been investigated over anatase TiO2(101) and rutile TiO2 (110) single crystals by STM and on-line mass spectrometry to determine the adsorbate species in the dark and post UV illumination, in the presence and absence of O2 or Au nanoclusters, in order to extract initial reaction parameters under photo-excitation. On anatase (101) TiO2 single crystal the reaction rate for the photo-oxidation of ethanol to acetaldehyde is found to be strongly dependent on O2 partial pressures and surface coverage with an order of the reaction for O2 close to 0.15. Carbon-carbon bond dissociation leading to CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of TiO2 rutile (110) single crystal, as previously reported by our group and others. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. Upon UV exposure at (and above) 3×10-8 mbar O2, a third species is identified as a reaction end-product, which can be tentatively attributed to acetate/formate species, in line with XPS C1s measurements. The room temperature photo-oxidation of ethanol has also been investigated over a rutile TiO2(110) single crystal by STM and on-line mass spectrometry, in the presence of O2 to determine adsorbate species remaining post UV exposure and emitted gas phase products. In addition to acetaldehyde, methyl radical was detected in the mass spectrometry resulting from the photo-fragmentation of an acetaldehyde-O complex. Formic acid species bound to Ti5c in a bi-dentate mode were identified by STM on the surface after the reaction. The effect of O2 partial pressure on the reaction selectivity demonstrated a dominance of the photo-fragmentation process with increasing pressure unlike the case of anatase TiO2(101) single crystal. In addition the effect of Au particles on the reaction of the TiO2(110 rutile has been investigated for hydrogen production under photo-irradiation in UHV conditions. It was found that particle size in the range 4 to 8 nm do not change the reaction rate yet particle density does. Detailed reaction mechanism for both reduction and oxidation reactions are addressed based on structural and kinetic information from the UHV systems and compared to those of their corresponding powder forms in liquid-solid photocatalytic reactions conditions.
Unlike thermally driven catalytic reactions by metals, the reaction rates in photo-catalysis do not scale with neither the amount of metals nor with their size. Because of the complexity of multi-component photo-catalysts in powder forms, this phenomenon that has been routinely observed for over three decades, has so far no fundamental explanations. In order to probe into this, hydrogen production rates from ethanol over Au clusters with different sizes deposited on TiO2(110) rutile single crystal, were studied by scanning tunneling microscopy (STM) and online mass spectrometry. A non-linear increase of the rate of hydrogen with increasing surface coverage of gold was observed. While Au particles with sizes ranging from 4 to 8 Å, marginally affected the reaction rate, the inter-particle distance was found to be crucial. Increasing the separation distance resulted in increasing the normalized reaction rate. These results are explained in terms of competition between particles for excited electrons to reduce hydrogen ions of surface hydroxyls to molecular hydrogen. The reason for nonlinearity is postulated to be due to two considerably different time scale, the picosecond scale (associated with Debye length) of charge transfer at the interface Au/TiO2 and the much slower time scale of electron transfer in chemical reactions.
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