Enhanced Photocatalytic Activity Of New Generation Photocatalysts By Surface/Interface Engineering

Semiconductor photocatalysis has attracted great interest because it provides a promising pathway for solving energy supply and environmental pollution problems. The basic concept of semiconductor photocatalysis involves the generation of electrons in the conduction band and holes in the valence band within semiconductor upon light irradiation at the energy equal or higher than the band gap of the semiconductor. Subsequently, the utilization of photoexcited charge carriers to initiate redox reactions with suitable substrates on and/or near the semiconductor surface.On the one hand, the fast recombination of photogenerated charge carriers in semiconductor is unavoidable. The competition between the transferring and the recombination process of photogenerated charge carriers is thereby an important factor to determine the photocatalytic performance of semiconductor. On the other hand, high performance of photocatalyst requires high oxidation power of the holes in the valence band (VB) and enough reduction power of the electrons in the conduction band (CB), which could be offered by wide-band-gap semiconductors. Unfortunately, the poor response to visible light for the wide-band-gap semiconductors unavoidably leads to their low utilization efficiencies in the full solar spectrum. So far intensive attempts have dedicated to band-structure engineering to overcome this drawback via foreign nonmetal or metal element doping. However, foreign element doping would bring with the undesirable thermal instability and increased carrier trapping of the doped semiconductors as well as reduced redox reactivity.Therefore, the ideas of this dissertation are the fabrication of the highly active photocatalysts with controllable surface/interface to achieve the enhanced photocatalytic performance.1. Plasmonic photocatalyst AgCl/Ag with controlled size and shape was prepared by a rapid microwave nonaqueous strategy. By rationally varying the reaction temperature and the microwave irradiation time, we study the formation mechanism of the AgCl/Ag pyramids. The surface plasmon resonance (SPR) properties of the as-prepared AgCl/Ag are found to be somewhat relevant to the size, morphology and constituent. The as-prepared AgCl/Ag exhibits highly photocatalytic activity and good reusability for decomposing organic pollutants (such as MO, RhB and PCP) under indoor artificial daylight illumination. We also find that the AgCl/Ag have a more powerful ability to harvest diffuse indoor daylight, which could complete the degradation of 10 mg/L MO within 15 min. The active species trapped experiments show that the photocatalytic degradation of organic pollutes in the AgCl/Ag system may proceed through direct hole transfer.2. Plasmonic photocatalyst AgBr/Ag also can be synthesized by a rapid microwave nonaqueous strategy. It has been demonstrated that noble metal NPs such as Ag NPs function as visible-light harvesting and electron-generating center during the daylight photocatalysis of AgBr/Ag. The novel Ag plasmonic photocatalysis could cooperate with the conventional AgBr semiconductor photocatalysis to enhance the overall daylight activity of AgBr/Ag greatly because of an interesting synergistic effect. After systematically investigating daylight photocatalysis mechanism of AgBr/Ag, we attribute the synergistic effect to SPR induced local electric field enhancement on Ag, which could accelerate the generation of e+/h+pairs in the AgBr, producing more electrons on the conduction band of AgBr under daylight irradiation.3. The self-doped BiOCl with plasmonic silver modification can be synthesized by a facile one-pot nonaqueous protocol. The in situ self-doping could extend the photoresponse of BiOCl from UV to visible light region, leading to remarkable visible light photocatalytic activity on the degradation of organic pollutants. Furthermore, the plasmonic Ag modification of self-doped BiOCl could greatly enhance their photocatalytic activity because of the SPR effect of Ag nanoparticles in the visible light region, evidenced by the photocurrent action spectra.4. ZnO/BiOI heterostructures were synthesized by a facile chemical bath method at a low temperature. The controlled morphology and constituent of the ZnO/BiOI heterostructures were achieved via simply tuning the Bi/Zn molar ratios. The resulting ZnO/BiOI heterostructures exhibited highly photocatalytic activity on the degradation of methyl orange under visible light irradiation. The behavior of photogenerated charges in the ZnO/BiOI heterostructures was investigated by surface photovoltage spectroscopy and transient photovoltage measurements. It was found that photoinduced charge transfer property of p-type BiOI could be improved greatly by coupling with n-type ZnO. The heterojunction at interface between BiOI and ZnO could efficiently reduce the recombination of electron-hole pairs and thus increase the lifetime of charge carriers by 15 times. The enhanced photocatalytic activity of ZnO/BiOI heterostructures is attributed to high surface area and heterojunction effect.5. BiOCl single-crystalline nanosheets with exposed{001} and{010} facets can be selectively synthesized by a facile hydrothermal approach. By rationally controlling the concentration of the hydroxyl ions in reaction system, easily manipulating surface properties of BiOCl is achieved. The photocatalytic performance of bismuth oxychlorides was systematically investigated by the photodegradation of organic pollutes such as methyl orange (MO) and salicylic acid (SA), which was found to be facet-dependent. The BiOCl single-crystalline nanosheets with exposed{001} facets exhibited higher activity on direct semiconductor photoexcitation pollutant degradation under UV light because of the cooperative effect between surface atomic structure and suitable internal electric fields, but the counterpart with exposed{010} facets possessed superior activity on indirect dye photosensitization degradation under visible light owing to its larger surface area and open channel characteristic.