Development of anion-doped metal oxides for solar -to -hydrogen conversion
Abstract
Photoelectrochemical water splitting using solar energy has drawn considerable attention due to the importance of using hydrogen as a clean and renewable energy. Wide band-gap semiconductors are the most promising materials for this purpose due to their good stability and catalytic activity, but their poor visible light absorption represents a major problem. The solar absorption of TiO2 and In2O3 were improved by doping with anions of several main group elements (C, N, and P). Analytical techniques, including nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), X-ray Photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), elemental analysis, Raman and UV-Vis spectroscopy were employed to characterize these materials. The photoelectrochemical activity for water splitting was evaluated using a photoelectrochemical cell. All anion-doped TiO2 electrodes investigated increased the visible absorption (λ > 400 nm) of n-TiO2, leading to higher photoactivity under solar light. In addition, a new visible-light absorbing photocatalyst, N-doped In2O3, was developed. N-doping of In2O 3 reduces the band gap with N substituting for oxygen in the lattice or present in interstitial sites, depending on the N source used. In-depth characterization using NMR, XPS and EPR showed that inert amino- and nitrate-type species adsorbed on the surface were produced from N sources, and count towards the N atomic percent but do not increase the activity of In2O 3. However, a paramagnetic nitrate-type species in interstitial sites was identified as the origin of the photoelectrochemical improvement of N-doped In2O3 prepared using NH4Cl as the dopant source. At optimal conditions, N-doped In2O3 materials show an obvious absorption in the visible region and exhibit significantly higher photoelectrochemical activity for water splitting than undoped materials, with observed photocurrent densities as high as 1 mA/cm2. The analyses presented in this thesis provide much needed insight towards understanding the structure and improving the efficiency for photoelectrochemical water splitting of these promising materials.
Degree
Ph.D.
Advisors
Raftery, Purdue University.
Subject Area
Analytical chemistry
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