Designing And Preparation Of Novel Visible-light Active Photocatalysts And Their Photocatalytic Performance And Mechanisms

Environmental pollution and energy crisis has become the two critical issues for the sustainable development of human beings. Semiconductor-based photocatalysis technology has been regarded as a green and ideal method to deal with the environmental pollution and energy crisis because this technology not only can use solar energy to eliminate environmental pollution, but also can directly convert solar energy into fuels. The key to accelerate the practical applications of photocatalysis is efficient and robust photocatalysts with wide spectral response to solar light. Although numerous semiconductors have been explored as photocatalytic materials, fewphotocatalysts can meet all the above criteria as well as the requirments for practical applications. Therefore, it is of great siginifance to develop novel high-performance photocatalysts. Among those visible-light active photocatalysts, silver phosphate(Ag3PO4) and graphite carbon nitride(g-C3N4) have been paid more and more attention owing to their excellent properties and prospective applications. However, Ag3PO4 suffers from self-solution and self-photo-degradation, thus limiting its practical applications.g-C3N4 exhibits low photcatalytic activity due to its narrow visible-light reponse and the conjugation effect between π electrons. In order to accelerate pratical applications of the photocatalysis technology, this dissertation aims at developing novel high-performance silver phosphate- and carbon nitride-based composite photocatalysts, and the following researchs have been carried out:1. Novel high-efficiency visible-light-driven Ag3PO4/Ag I systems have been prepared via facile precipitation-ion exchange process. The as-prepared Ag3PO4/Ag I hybrids exhibit excellent photocatalytic activity on the degradation of MO, which is superior over those of pure Ag3PO4 and Ag I under visible light(>420 nm). Ag NPs easily form on the surface of Ag3PO4/Ag I in the early stage of the photocatalytic oxidation process and then act as the Z scheme charge transmission bridge in the photocatalytic oxidation process. The excellent photocatalytic activity of Ag3PO4/Ag I may originate from the efficient separation of photogenerated electron-hole pairs through the Z-scheme system composed of Ag3PO4, Ag,and Ag I. Moreover, Ag3PO4/Ag I exhibited good stability in the photocatalytic oxidation process under visible light irradiation. On the basis of their efficient and stable photocatalytic activity, Ag3PO4/Ag Isystems show promise for use in environmental purification of organic pollutants.2. Novel high-efficiency visible-light-driven Ag3PO4/Ag/Si C Zsystems have been successfully prepared via a facile in situ precipitation process at room temperature. The Ag3PO4/Ag/Si C photocatalysts showed obviously superior photocatalytic activity over that of Ag3PO4/Ag and Si C/Ag for the photo-degradation of MO and phenol under visible light irradiation. The superior photocatalytic activity of Ag3PO4/Ag/Si C may originate from the efficient separation of photogenerated electron–hole pairs through the Z-scheme system composed of Ag3PO4, Ag and Si C. The Z-scheme photocatalytic process of the hybrid photocatalysts was also supported by active species trapping and PL experiment results. Furthermore, Ag3PO4/Ag/Si C possessed good stability in the photocatalytic oxidation process under visible light irradiation. It is expected that the Ag3PO4/Ag/Si C with high photocatalytic activities will greatly promote their practical application to eliminate organic pollutants.3. Pyridine-doped g-C3N4 photocatalysts have been successfully synthesized by copolymerizing 2,6-diaminopyridine with dicyandiamide and systematically investigated by theoretical calculations and experiment. Both textural structure and electronic structure of g-C3N4 photocatalyst can be greatly controlled via molecular doping of its conjugated frameworks with π-deficient pyridine rings. Integrating π-deficient pyridine ring into the g-C3N4 network by modification with 2,6-diaminopyridine does not alter the crystal structures or the core chemical skeleton of g-C3N4 but the texture and morphology. The incorporation of pyridine in g-C3N4 skeleton can effectively extend and delocalize the aromatic π-conjugated system, adjusting its intrinsic semiconductor properties, such as engineering theband structure with tunable bandgap and facilitating the migration and separation of photo-generating electron–hole pairs. Hence, asa result of pyridine doping, an overall enhanced photocatalyticactivity is achieved. The O2?-/?OOH radical is the major oxidation species in the photocatalytic oxidation process. The modulated g-C3N4 incorporating π-deficient aromatic systems possess a higher reduction potential because the extension of optical absorption of g-C3N4 mostly results from the up-shift of HOMO, which is favorable for the photocatalytic degradation of organic pollutant. This work may provide a common and simple route along with an insight into modulating the structure and properties of g-C3N4 via incorporation conjugated functional groups with π-deficient aromatic systems into g-C3N4 frameworks for improving its photocatalytic activity.4. High-visible-light-active g-C3N4/Ni S hybrid photocatalysts are successfully prepared by a simple ion-exchange method. Ni S has been proven to be an effective and stable cocatalyst to enhance photocatalytic H2-production activity of g-C3N4. The optimal loading content of Ni S was determined to be 1.5 mol % for g-C3N4, and the corresponding H2-production rate was 44.77 μmol h-1 under visible-light irradiation, which approaches to the optimal photocatalytic H2-production of Pt/g-C3N4(2.0 wt % Pt/g-C3N4). The highly effective photocatalytic. H2-production of g-C3N4/Ni S hybrid photocatalysts was ascribed to Ni S acting as electron-trapping center and active sites for H2-production, and then preventing charge recombination. This work demonstrates that Ni S is a promising cocatalyst for developing high efficiency photocatalysts and the ion-exchange method is an effective and economic method for depositing cocatalyst for g-C3N4.