Synthesizing Mixed Phase Titania Nanocomposites with EnhancedPublic Deposited
Recent work points out the importance of the solid-solid interface in explaining the high photoactivity of mixed phase TiO2 catalysts. The goal of this research was to probe the synthesis-structure-function relationships of the solid-solid interfaces created by the reactive direct current (DC) magnetron sputtering of titanium dioxide. I hypothesize that the reactive DC magnetron sputtering is a useful method for synthesizing photo- catalysts with unique structure including solid-solid interfaces and surface defects that are associated with enhanced photoreactivity as well as a photoresponse shifted to longer wavelengths of light. I showed that sputter deposition provides excellent control of the phase and interface formation as well as the stoichiometry of the films. I explored the effects exerted by the process parameters of pressure, oxygen partial pressure, target power, substrate bias (RF), deposition incidence angle, and post annealing treatment on the structural and functional characteristics of the catalysts. I have successfully made pure and mixed phase TiO2films. These films were characterized with UV-Vis, XPS, AFM, SEM, TEM, XRD and EPR. to determine optical properties, elemental stoichiometry, surface morphology, phase distribution and chemical coordination. Bundles of anatase-rutile nano-columns having high densities of dual-scale of interfaces among and within the columns are fabricated. Photocatalytic performance of the sputtered films as measured by the oxidation of the pol- lutant, acetaldehyde, and the reduction of CO2 for fuel (CH4) production was compared (normalized for surface area) to that of mixed phase TiO2 fabricated by other methods, including flame hydrolysis powders, and solgel deposited TiO2 films. The sputtered mixed phase materials were far superior to the commercial standard (Degussa P25) and solgel TiO2 based on gas phase reaction of acetaldehyde oxidation under UV light and CO2 reduction under both UV and visible illuminations. The sputtered films also displayed a light response strongly shifted into the visible range. This is explained by the creation of non-stoichiometric titania films having unique features that we can tailor to the solar energy harvest. By further studying the non-stoichiometric titania, I observed an optimal non-stoichiometry for the titania films in terms of methane yield from CO2 reduction. On one hand, the oxygen vacancies, which are mostly produced at the solid-solid interfaces, are associated with a redshift photoresponse and served as trapping sites or/and adsorp- tion sites to increase the photocatlytic efficiency. On the other hand, excessive oxygen vacancies might also serve as recombination centers to hinder the reactivity. These two competing effects explained the fact that there is an optimum non-stoichiometry. In addi- tion, I studied the influence of adding other reactive gases such as nitrogen and hydrogen during the sputtering as well as deposition angles (normal, low and glancing angles), on the structures and reactivities of titania based nanocomposites. Our work illustrates the feasibility of reactive DC magnetron sputtering as both a powerful reserach tool and potentially practical technique for manufacturing highly active nanostructured TiO2 photocatalysts tailored for solar applications.