Investigation of defect formation and control in poly-crystalline anatase titanium dioxide

titanium dioxide (tio2);titania;defect engineering;short range order;carrier concentration

Barlaz, David ; Seebauer ; Edmund G.

关键词: titanium dioxide (tio2);    titania;    defect engineering;    short range order;    carrier concentration;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46852/David_Barlaz.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Titanium dioxide (TiO2) has become one of the most prominent materials in the research efforts related to the clean production of energy and value added chemicals.TiO2 has been praised for the catalytic activity of its anatase phase, its abundance in nature, low toxicity, stability in aqueous environments and ease of processing.Among its drawbacks is a high native carrier concentration likely due to the formation of oxygen vacancies and their clustering at grain boundaries which impede hole mediated photocatalytic reactions.Previous efforts to defect engineer atomic layer (ALD) grown TiO2 were successful by varying film thickness, with the explanation that films differentially compacted during annealing according to thickness leading to less area at grain boundaries for defects to aggregate. We sought to further that success by several methods.Lowering growth rate and increasing synthesis temperature sought to lower carrier concentration by increasing surface mobility of deposited atoms increasing the density of unannealed films.Varying the ramp rate used in annealing was also performed to test for variations in crystallite formation and growth with the goal of producing larger grains with reduced net interfacial area for defects to aggregate.The methods of increasing surface mobility each had the effect of lowering carrier concentration by about ½ an order of magnitude.Short range order in the form of subcritical nuclei was suggested as an explanation.The presence of such order had already been observed during surface diffusion experiments on amorphous titanium dioxide.Subcritical nuclei present in the bulk of an amorphous film are the first to grow upon annealing possibly crowding out newly seeded nuclei thereby increasing the average crystallite size and film density.Their increased production by increased surface mobility and increased growth time (thickness in the previous work) led to denser films and lower carrier concentrations.The critical point in the annealing process for their growth and for the formation of new nuclei was not able to be determined as variations in ramp rate led to no appreciable variation in carrier concentration.This suggested that the critical temperatures are close to the annealing temperature or that the time scale for this production is shorter than that accessed by rapid thermal annealing.