Atomikerroskasvatusmenetelmällä kasvatetun titaanidioksidikalvon ominaisuudet valosähkökemiallisessa veden hajottamisessa

Photoelectrochemical water splitting is one of the potential ways of utilizing solar energy. A major issue for the method and for renewable energy production would be the development of an efficient and a cost-effective semiconductor photoanode. In this Master of Science Thesis properties of atomic layer deposited (ALD) titanium dioxide film (TiO₂), such as a chemical composition, a crystal structure and the ability to absorb sunlight, are studied in as-deposited conditions and after oxidative annealing. By understanding thoroughly the properties of atomic layer deposited titanium dioxide and the effects of post-annealing in air, titanium dioxide can be better utilized in semiconductor photoanodes used in photoelectrochemical water splitting.

Titanium dioxide films examined in this study were grown on n-type phosphorus-doped silicon (n-Si(100)(P)) and fused quartz used as substrates by atomic layer deposition at 200 °C using tetrakis(dimethylamido)titanium (TDMAT) and deionized water as precursors. The annealing of some of the samples at 500 °C in air was made in a tube furnace. The concentrations of the elements and chemical states of the atomic layer deposited titanium dioxide films were studied by X-ray photoelectron spectroscopy (XPS). The film thickness and refractive index were determined by ellipsometer and the absorption properties of the titanium dioxide film were measured by UV/Vis/NIR spectrophotometer. In addition, crystallographic results from X-ray diffraction (XRD) and Raman spectroscopy were also utilized, as well as the results of the photoelectrochemical cell and solar simulator on titanium dioxide photoanode performance.

Based on the results, at 200 °C atomic layer deposited titanium dioxide is amorphous and absorbs visible light as ”black” TiO₂. At the oxidative annealing at 500 °C titanium dioxide crystallizes into rutile and becomes ”white” TiO₂ that absorbs less visible light. Both titanium dioxide films contain the lower Ti^3+/2+ oxidation states of titanium that may indicate oxygen vacancies. Nitrogen is found only in as-deposited titanium dioxide. The annealed titanium dioxide is stable in electrolyte, achieving 0.20 % ABPE for water splitting reaction.