Metal deposition on TiO2(110) via thermal evaporation, photolysis and thermolysis: a combined FT-RAIRS and XPS study

Abstract

Model systems combined with surface science techniques provide a powerful tool for the study of oxide supported metal overlayers. This thesis describes the synthesis and characterisation of Cu, Mo and Au overlayers on TiO2(110) deposited using a number of methods.
Cu overlayers were deposited on the TiO2(110) surface via Physical Vapour Deposition (PVD) at 295 K. Using CO as a probe adsorbate molecule, FT-RAIRS revealed that Cu islands with sizes less than ca. 3.5 nm exhibited predominantly high Miller Index surfaces, whereas larger islands exhibited (110), (100) and (111) facets. By comparing XPS and FT-RAIRS results collected before and after annealing, a complex growth mode is proposed which resembles a combination of Volmer-Weber and Stranski-Krastanov types. The reactivity between O2 and pre-adsorbed CO appeared highest for the particles exhibiting the high Miller Index facets. Finally, FT-RAIRS allowed for the identification of a threshold Cu coverage, 4.48 MLE, above which the influence of the oxide support to the local dielectric field seized, and metallic behaviour was exhibited.
The photolysis of Mo(CO)6 adsorbed on the TiO2(110) surface at 130 K was studied as an alternative to PVD deposition of Mo. Photo-dissociation of 1.53 L of exposed Mo(CO)6 resulted in a single photoproduct, Mo(CO)3, which adopted a geometry perpendicular to the surface. The tricarbonyl was the main photoproduct following the dissociation of 2.73 L of exposed Mo(CO)6, although higher carbonyls were also observed with FT-RAIRS. Photolysis of adsorbed multilayers resulted in the production of Mo(CO)5 and traces of Mo(CO)4. The dependence between the initial coverage of the Mo(CO)6 and the photoproducts provides direct evidence of a substrate mediated decomposition path on TiO2(110).
The deposition of (CH3)2Au(acac) on TiO2(110) at different temperatures was examined to establish its potential application as an OrganoMetallic Chemical Vapour Deposition (OMCVD) precursor of highly dispersed Au particles. At 130 K, oxidised Au overlayers were prepared, whereas at 295 K a mixture of metallic and oxidised gold overlayers were observed, indicating the formation of an intermediate decomposition product of the precursor. In both cases, annealing above 700 K resulted in metallic Au. Deposition at 550 K resulted directly in the formation of metallic gold overlayers, but with some contamination by residual graphitic carbon.

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ORCID for Brian Hayden: orcid.org/0000-0002-7762-1812

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