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Tunability of photogenerated charge carrier density on semiconductors by in-situ electrochemical treatments Liu, Xu


With fossil fuels increasingly being exhausted and environment pollutions getting worse, it is necessary for our generation to look for sustainable, renewable and environmentally safe alternative energy sources. Solar energy is so far the most available resource, with around 120,000 TW of solar energy striking the surface of the earth. However, sunlight is just available in the daytime, can change within hours or seasons, and it is spread over low-density collection areas. An efficient way of energy storage is required for the utilization of solar energy. So far, one of the most practical ways to store a significant amount of energy is through a chemical energy carrier. Hydrogen fuel is one of the prime candidates as a future energy carrier which is environmentally friendly during its production, delivery, and consumption. Hydrogen production by photoelectrochemical water splitting using a semiconductor catalyst could be one of the most promising ways to harvest solar energy. In this thesis, an in-situ potentiodynamic approach (cyclic voltammetry) was used to modify the photoelectrocatalytic properties of nanostructured electrodes in different media. The effect of the morphology was studied by comparing TiO₂ nanotube and nanorod. Also, the influence of the modification on WO₃, ZnO materials was evaluated. The photogenerated charge carrier separation was studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and Mott-Schottky plots. The morphologies of the samples were tested by scanning electron microscope (SEM). X-ray diffraction (XRD) was used to analyze crystallinity. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemical composition. The experimental data proved that electronic properties could be changed by the self-doping process, hence, improving the optical absorption properties and increase charge transfer rates. In this way, the photoelectrocatalytic activity of semiconductors was enhanced. The observed behaviors from electrochemical measurements suggested that morphology has a vital role in the capacitive properties. A semiconductor tailored via band structure modification indicates that the electrochemical treatment can be a systematic and straightforward technique for developing novel photoelectrocatalysts with enhanced performances under visible light.

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