Preparation and reactivity of epitaxially grown iron oxide films : a surface science investigation

• Main Author: Saipanya, Surin
• Published: Cardiff University 2007
• Subjects: 546
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.584200

The preparation and reactivity of epitaxtially grown iron oxides on a Cu(100) substrate have been studied. An Fe adlayer grew epitaxial ly on the substrate with 2 domains with a square unit cell of sides 2.55 A and 3.61 A. The oxide is prepared by oxidizing Fe multilayers at temperature 850 K using a high and low heating rate. A low heating rate gives a thick film (10-14 A) while a high heating rate gives a thinner (3-5 A) film. Both conditions give a well-ordered surface. However, the Cu substrate was not oxidized in this low-pressure/high-temperature oxidation treatment. Fe3C 4(100) and (111) surfaces are identified by the chemical composition of the surface from XP spectra and features of LEED and STM images such as the excellent long-range expitaxy, antiphase domain boundary strips and hexagonal superstructures. Au clusters were vapor-deposited on iron oxide Fe3I 4, oxidized Cu and clean Cu surfaces. It was found that the clusters grow in a 3D mode (Stranski-Krastanov) on the oxidized substrate but in a layer-by-layer mode (Frank van der Merwe) on clean Cu. On the iron oxide the Au nanoparticles have an average diameter of 30-50 A and are 10 A high. Annealing experiments show that the Au clusters are rather stable in the system and form well-defined hexagonal microcrystals. STS showed that the Au clusters on the oxide substrate show nonmetallic properties. LEED and STM show that Au grows epitaxial ly on clean Cu. Iron oxide (Fe3C4) and Au modified iron oxide surfaces were used for studies of the chemistry of acrylic acid, CCVrich CO2-O2 mixtures and NH 3-rich NH3-O2 mixtures at room temperature. The adsorption of acrylic acid on thin and thick iron oxide surfaces and Au modified thin and thick iron oxide surfaces resulted in carboxylate formation and vinyl adsorption together with the desorption of CO2 and ethylene at 500 K. Au nanoparticles on the oxide did not enhance the acid reaction on the surface. However, Au clusters on oxide films increased the surface reaction of small molecules such as CO2 (acid) and NH3 (base). STM results show the edges of the iron oxide islands and the top of Au nanoparticles are more reactive as their rims turn uneven after adsorption.