| Since the discovery of photocatalytic water splitting of TiO2 electrode by Fujishima and Honda, enormous efforts have been spent on the study of oxide semiconductors (i.e. SrTiO3 and TiO2) due to their important potential applications on photovoltaics and photocatalysis. Many oxide semiconductors have excellent photocatalytic properties, but they generally only response to ultraviolet (UV) light for a wide band gap, while UV light accounts for only a small portion of solar energy (<10%). The modulation of band gap to enable the absorption edge to move from the UV region to visible light one will enhance the performance of the oxide semiconductor. On the other hand, in the process of photocatalysis the efficiency of employing electron hole pairs is gernally low; most of the photo-generated carriers recombine and dissipate as heat. To prevent the recombination of photo-generated carriers is also one of the main researches on photocatalysis.This thesis focused on above two points, and the enhancement of photocatalysis was studied through the hydrogen and/or ammonia heat treatment method on the SrTiO3 substrate.First, we studied the effect of hydrogen heat treatment (hydrogenation) on the photocatalytic activity of SrTiO3.Hydrogenation of SrTiO3 was carried out by annealing in the hydrogen forming gas (H25%/N2), rapidly cooling after annealing. We evaluated the photocatalytic activity of SrTiO3 by measuring absorption of methylene blue (MB) with different catalysts loaded after illumination for a certain time. Photocatalytic experiments showed that SrTiO3 hydrogen annealed (SrTiO3-H2) under temperatures exceeding～900℃showed a significantly enhanced UV photocatalysis compared to as received SrTiO3, with a maximum increase of 5 times. However, SrTiO3-H2 had no visible light photocatalysis, like the received SrTiO3. XPS measurements suggested the Fermi level of SrTiO3-H2 (>～900℃) was significantly higher than as received one. For SrTiO3 hydrogenated at 1000℃, the Fermi level increased by more than 1.6 eV. Fermi level would increase the band bending in the internal surface of SrTiO3 when it contacts with the solution. This will contribute to enhanced UV photocatalysis because the steep band bending will help separate photo-generated carriers. The hydrogenated SrTiO3 was also characterized by UV-visible absorption spectroscopy and electron paramagnetic resonance (EPR). UV-visible absorption spectra showed the absorption edge of hydrogenated SrTiO3 did not shift compared with the received SrTiO3, though there were two absorption peaks at 430 and 520 nm and a strong absorption band at the region exceeding 600 nm. EPR showed Fe3+ impurities existed in as received SrTiO3 which is common for commercial SrTiO3. Al and Fe were considered to be present in oxides at levels typically no less than 10-100 ppma. After hydrogenation (>～880℃) the Fe3+ particles disappeared, which was a sidelight of free electron increase and could contributed to the saturation effect of free electrons, like dangling bond saturation happened in hydrogenated Si semiconductor.We then studied the effect of ammonia heat treatment (ammoniation) on visible light photocatalysis of SrTiO3. Ammoniation was aimed at nitrogen ion doping through substitution of lattice oxygen in SrTiO3 by nitrogen. Nitrogen incorporation can induce several doping levels above the valence band maximum, which enables the visible light photocatalytic reaction. Ammoniation was conducted in ammonia annealing, naturally cooling after annealing. It was found that annealing temperature and ammonia flow rate affected the effect of N incorporation in SrTiO3. Visible light photocatalytic experiments showed that ammoniated SrTiO3 (SrTiO3-NH3) had a visible light photocatalytic activity, and ammoniation at 800-900℃was most effective. UV-visible absorption spectra showed a red shift of the absorption edge in SrTiO3-NH3. XPS measurements showed that the N/O ratio in the surface of SrTiO3-NH3 (900℃,～30L/h) was 0.06. Fermi level in SrTiO3-NH3 had also undergone a shift (～0.7eV), but far from significant as SrTiO3-H2 (～1.6eV).Finally, we studied the synergistic effects of hydrogenation and ammoniation in the SrTiO3. We found that visible light photocatalysis in the SrTiO3 hydrogenated followed by ammoniation (SrTiO3-H2-NH3) was close to that of SrTiO3-NH3, while SrTiO3-NH3-H2 (ammoniated and then hydrogenation) exhibited a significantly enhanced visible light photocatalysis than SrTiO3-NH3. XPS measurements showd that Fermi level in SrTiO3-H2-NH3 (～0.4eV) was close to that of SrTiO3-NH3 (～0.7eV), while Fermi level in SrTiO3-NH3-H2 (～1.4eV) was significantly more than SrTiO3-NH3, and close to that of SrTiO3-H2 (～1.6eV). Comparison to the observation in hydrogenated SrTiO3, it can be concluded in SrTiO3-NH3-H2 the synergistic effects of ammoniation (nitrogen doping) and hydrogenation is achieved, and together cause an enhanced visible light photocatalysis. |
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