Investigation On The Fabrication Of Si/TiO₂ Nanostructure For Photoelectrochemical Hydrogen Generation

The growing energy demand and increasing concern on climate change have aroused much attention on photoelectrochemical (PEC) water splitting. TiO2 is regarded as one of the most promising materials for photoelectrode, considering its low cost, high chemical stability and nontoxicity. However, the poor sunlight absorption and low electron mobility strictly limited the further application. As we realized the controlled fabrication of silicon nanowires and explored the properties modification method for TiO2, in this dissertation, we intensively investigate the fabrication of Si/TiO2 nano heterostructure and its properties modification for PEC water splitting. The specific works are described below.(1) The fabrication of silicon nanowires. We achieved the silicon nanowires by metal assisted chemical etching. However, the size and distribution of the nanowires are disordered. The nanosphere lithography technology then was introduced. Monolayer nanosphere array self-assembled on silicon wafer was used as etching mask. The size of the nanospheres can be tuned by O2 RIE, thus the diameter and distribution of the nanowires can be well controlled. At last we obtained the vertical aligned silicon nanowires with smooth surface and high aspect ratio (exceeding 80:1). To achieve large area nanowire array, the nano imprint lithography was introduced to fabricate the etching mask. The nanowire array was formed by ICP-Bosch process. The area of the obtained array is same as imprint template (2.5cm X 2.5cm).(2) The modification of TiO2 nanorods for PEC water splitting. We achieved the Sn-doped TiO2 nanorods by adding SnCl4 into reaction solution and investigated the effect of Sn doping on the photocatalytic activity of the nanorods with different doping levels. The measurements reveal that photocurrent is increased about 50% as the value of Sn/Ti closed to 1%. Besides, we explored the effect of hydrogen annealing. The experiments demonstrate that hydrogen annealing can introduce tremendous oxygen vacancies in the surface layer of the nanorods. Its conductivity and solar absorption is increased significantly and lead to the enhancement of photocatalytic activity. Moreover, we proposed an IrO2 nanoparticles modified CdSe/CdS co-sensitized TiO2 photoanode and investigated the effect of IrO2 nanoparticles on the properties of sensitized nanorods. Results reveal that after the modification the onset potential and saturated potential shows obvious negative shift. The photocurrent is enhanced significantly. As the photogenerated holes are successively scavenging from the quantum dots, the photocorrosion is efficiently suppressed, which leads to the substantial enhancement of stability.(3) The fabrication and modification of Si/TiO2 nanostructure for PEC water splitting. The TiO2 nanorods were hydrothermally grown on the surface of the silicon nanowires to achieve the heterostructure. After that the hydrogen annealing is applied to the heterostructure, which is then proved as an effective method in the experiments. The stability of the hydrogenated Si/TiO2 heterostructure is also amazing as there is no obvious decay of photocurrent over 5h. Besides, we proposed a Ni film modified Si/TiO2 heterostructure photoanode and studied the influence of Ni film on the PEC performance of the heterostructure. Results reveal that Ni film modification substantially increases the saturation photocurrent of the photoanode. Moreover, we proposed and fabricated a PEC tandem cell composed of TiO2 photoanode and perovskite solar cell, which can realize the water splitting independently without any assistant bias potential. At last, we demonstrated an integrated PEC device with water splitting and photo-pseudocapacitor functions based on TiO2/NiO heterostructure which achieved the immediate solar energy conversion and storage. Both the reductive and oxidative energies are utilized leading to the substantial increase of solar utilization efficiency.