| Strontium titanate (SrTiO3, or STO) is a typical perovskite-type oxide photocatalyst. It possesses a wide band gap of 3.2 eV similar to most of other transition metal oxides. Only ultraviloet (UV) lights can be absorbed for its wide band gap, which account for a small part of solar radiation.An enhanced performance will be expected if the band gap narrowed. On the other hand, to suppress the recombination of carriers is another key task of photocatalytic studies, since photogenerated electrons and holes tend to recombine and dissipate as heat.In this work, the band structruce is modulated in STO aimed to enhanced phocatalysis for wide spectrum. STO(100) wafers were chosen for the study in order to faciliate the study of the mechanism.Firstly, the effects of 1.5-keV nitrogen ion beam bombardment on the STO surface composition, structure and photocatalytic reactivity were systematically studied. Optimal conditions for nitrogen substituted STO sample preparation were determined. The photocatalytic reactivity strongly depended on the preparation temperature and implantation dose. High substrate temperature needed for a high catalytic reactivity. High temperature improved the crystallinity of bombarded STO, but expelled the nitrogen doping. An optimum substrate temperature was determined as about 1000 K and the optimum dose is about 5.6×1018/cm2. Under those conditions, the nitrogen doping concentration is 12.6 at.%. High resolved high-resolution transition electron microgragh showed the doped STO maintained the perovskite structure. The absorption edge was shifted to about 540 nm. Photoelectron spectroscopy and contact angle analysis showed that surface defects were introduced after the ion bombardment. The significant wide spectrum photocatalysis was due to substituted nitrogen doping and the introduction of surface defects.Secondly, significantly enhanced photcatalytic reactivity under UV lights was found in STO after annealed in hydrogen forming gas (H2:N2= 5%:95%) and quenched. The mechanism was clarified by a variety of analytical tools, including UV-visible absorption spectroscopy, photoluminescence, electron spin resonance, photoelectron spectroscopy, atomic force microscopy and contact angle measurements. The key of hydrogenation is passivation of the radiative recombination centers and active defects with unpaired electrons. A small amount of oxygen vacancy defects was produced in the hydrogenation, but no significant change was found in the adsorption. We also studied the high-pressure hydrogenation of STO, and found no redshift of the absorption edge and visible light photocatalysis in spite of a slightly increased UV light photocatalysis. Our results suggested that the black TiO2 by hydrogenation was not an universal phenomenon.Finally, plasmonic Ag/STO composite photocatalyst was prepared by the chemical deposition of Ag nanoparticles on STO. Ag/STO showed a broad absorption at 450-650 nm. The surface plasmon resonance enhanced visible light catalysis was observed in Ag/STO. Proper deposition of Ag nanoparticles also enhanced the UV light photocatalysis. A multiphoton absorption model was proposed on surface plasmon resonance in Ag/STO.Main points of this paper are as follows:1. Systematically studied the effect of the nitrogen ion beam bombardment on the composition, structure and photocatalytic activity in STO. The optimal conditions for STO-N sample preparation were determined. The STO-N exhibited enhanced wide spectrum photocatalysis.2. Devoloped a hydrogenation-and-quench method for enhanced UV light photocatalysis in STO. The mechanism was clarified through experimental analysis. The key of the hydrogenation is passivation of the radiative recombination centers and active defects with unpaired electrons.3. Prepared plasmonic Ag/STO composite photocatalyst, which showed an enhanced and broad spectrum photocatasis. A new model of multiphoton absorption was proposed on plasmonic Ag/STO. |
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