Reactive DC Magnetron Sputtering for the Deposition of Semiconductor Thin Films and their Use in Photoelectrochemical and Photocatalytic Applications

The optimization of semiconductor thin films is of paramount importance for advancing photoelectrochemical (PEC) and photocatalytic applications. This thesis investigates titanium dioxide (TiO2) layers as an n-type semiconductor and copper oxide (CuO) thin films, a p-type semiconductor, deposited via reactive direct current (DC) magnetron sputtering (MS), focusing on enhancing their functional performance through innovative deposition and post processing techniques. For TiO2 photoanodes, a titanium (Ti) interlayer was introduced on the substrate to improve charge transfer and preserve anatase crystallinity during annealing, thereby enhancing PEC performance. The interlayer facilitated the formation of the anatase phase even at elevated annealing temperatures (e.g., 800 °C), suppressing the transformation to rutile. The formation of anatase results in improved crystallinity and favorable electronic structure, supporting efficient charge separation and transfer. In particular, its band edge positions are thermodynamically well-aligned with the water redox potentials, enabling effective carrier extraction during PEC operation. Photocurrent response and wavelength-dependent IPCE were systematically analyzed as indicators of charge transfer efficiency and overall PEC performance. The optimized TiO2 layers were further studied for photocatalytic hydrogen evolution reaction (HER) activity, where platinum (Pt) single-atom (SA) co-catalysts were deposited onto the surface. This significantly improved HER performance, and the relationship between Pt SA loading and photocatalytic efficiency was quantitatively assessed. Beyond co-catalyst effects, the light absorption properties—governed by film thickness, annealing-induced crystallinity, and excitation wavelength—were comprehensively examined to maximize photon-to-H2 conversion. For CuO photocathodes, the oxygen partial pressure during sputter deposition was systematically varied to produce distinct copper oxide phases, including CuO, Cu2O, and Cu4O3. The phase formation is governed by the oxygen chemical potential, reflecting the thermodynamic stability ranges of these oxides under reactive sputtering conditions. Among the as-deposited films, CuO exhibited superior photoelectrochemical properties due to its narrow band gap (1.3–1.7 eV), strong visible light absorption, and favorable band alignment for water reduction. Post-deposition air annealing was then employed to convert all phases into phase-pure CuO, enhancing crystallinity and carrier mobility and leading to significantly improved photocurrent generation. However, annealing at excessively high temperatures induced Sn diffusion from the FTO substrate into the CuO layer, which introduced recombination centers and deteriorated PEC performance. The incident photon-to-current efficiency (IPCE) of the annealed films was evaluated to assess spectral photoresponse, and further optimization of annealing temperature and film thickness was carried out to balance light absorption with carrier transport, maximizing photoelectrochemical efficiency while minimizing recombination and interfacial degradation. This work highlights the versatility of reactive DC magnetron sputtering as a powerful technique for fabricating high-performance semiconductor thin films of TiO2 and CuO. The ability to precisely control key deposition parameters—such as thin metal interlayers, oxygen partial pressure, and sputtering duration—enables thermodynamic control over phase formation and the tuning of crystallinity, thickness, and optical absorption. These features support optimization of the electronic structure and interfacial properties, leading to improved charge separation and transfer. When combined with post-deposition strategies such as annealing and co-catalyst integration, this approach supports the development of efficient and stable materials for both photoelectrochemical and photocatalytic applications. Overall, the integration of reactive sputtering and targeted post-treatment provides a scalable and controllable route toward high-efficiency thin film systems for solar-driven energy conversion.