Studies On The Photocatalytic Performance And Mechanism Of Metal Sulfides And Their Modified TiO₂

Metal sulfides have been widely studied and applied in the field of photocatalysis. However, the present semiconductor theory which has been generally accepted in photocatalysis seems difficult to explain the diverse photocatalytic performances of different metal sulfide photocatalysts (e.g., SnS2and CdS etc.), especially for the photocatalytic degradation of those organic compounds with different functional groups. This thesis focuses on the cleaning wastewater by photocatalysis over SnS2. The adsorption and photocatalytic performances for various organic compounds with different functional groups have been examined and compared with other metal sulfide photocatalysts. Based on the determination of adsorption and reaction kinetics including intermediates, evaluation of the activation of metal sulfides and the oxidizing species, the difference in reaction mechanism and its correlation to the photocatalytic activity is elucidated. Then, the SnS2-modified TiO2in single-crystal with exposed (001) facets has been synthesized and employed in photocatalytic degradation of organic pollutants in wastewater. Finally, the mesoporous SnS2/TiO2with core-shell structure has also been fabricated and used in photocatalytic reduction of Cr(VI) in aqueous solution. On one hand, the mechanisms for both the formation of core-shell structure and the crystal growth have been explored. On the other hand, the correlation of the photocatalytic performance to the structural characteristics has been discussed based on the investigation on the adsorption and photocatalytic reaction mechanisms, the single-crystalline structure of TiO2including dominant facets, the dispersion degree of SnS2, the adsorption and diffusion of reactants, and the activation of photocatalysts by light irradiation etc. The research work could be described in the following four parts:1. The photocatalytic performances of the SnS2and the CdS under the irradiation of visible light were investigated and compared by using different organic dyes as reactants. The photodegradation of organic dyes containing N=N double bond on the SnS2followed a reduction mechanism with photoelectrons as active species while the photodegradation of organic dyes containing N=N double bond on the CdS and the photodegradation of organic dyes without N=N double bond on either the SnS? or the CdS followed an oxidation mechanism with·O2and·OH radicals as active species. The SnS2exhibited much higher activity than the CdS during the photocatalytic degradation of organic dyes containing N=N double bond, since the reduction of reactant molecules on the SnS2surface was much faster than the migration of either the·O2-or the·OH radicals. However, the SnS2displayed even lower activity than the CdS in the photocatalytic degradation of other organic dyes without N=N double bond since all these reactions followed the oxidation mechanism and the SnS2displayed a lower efficiency than the CdS in producing·O2-or·OH radicals due to the presence of the Sn4+/Sn2+transition.2. The SnS2nanosheets in hexagonal crystal phase with large specific surface area were synthesized by a simple hydrothermal method at different temperatures. The as-made SnS2exhibited high adsorption for RhB and other organic dyes with the molecular structure similar to RhB, but displayed very low adsorption for methyl orange (MO), showing the selective adsorption of SnS2for different organic compounds. The XPS analysis demonstrated the active sites of SnS2for adsorption were unsaturated Sn(IV) on the surface, which favored the adsorption of RhB-like molecules owing to the coordination between the positively charged N atoms in RhB-like molecules and the unsaturated Sn(Ⅳ) sites on SnS2-Furthermore, the SnS2nanosheets were employed to remove RhB pollutant with high concentration in wastewater based on cooperative adsorption and photocatalytic degradation, which showed good potential in practical application.3. Pure and F-modified anatase TiO2with dominant (001) and (101) facets were synthesized by hydrolysis of Ti(OBu)4under hydrothermal conditions, followed by treating in HF or NaOH solution. The TiO2with dominant (001) facets exhibited higher activity than that with dominant (101) facets in the photocatalytic degradation of RhB but showed lower activity in the photocatalytic degradation of MO. Meanwhile, F-modification on the (001) facets promoted photocatalytic degradation of either RhB or MO but F-modification on the (101) facets was unfavorable for the photocatalytic degradation of MO. Based on structural characterizations and kinetic studies, the difference in the photocatalytic performance was discussed by considering different active species for photocatalytic oxidation of RhB and MO. Subsequently, the SnS2doped single-crystalline TiO2with mesoporous structure (SnS2/TiO2) was synthesized by co-alcoholysis of SnCl4and TiOSO4in tert-butyl alcohol, followed by ion-exchange with thioacetamide (TAA) under hydrothermal conditions. The SnSi/TiO2contained rutile phase and its content was dependent on the amount of modified SnS2. The as-prepared SnS2/TiO2exhibited different activities in photocatalytic degradation of RhB and MO, possibly due to the different adsorption and/or photocatalytic reaction mechanisms.4. Core-shell SnS2/TiO2photocatalysts with mesoporous structure and high specific surface area were successfully synthesized by consecutive solvothermal and hydrothermal treatments. These SnS2/TiO2photocatalysts contained SnS2nanoparticles uniformly dispersed onto both the inner and the outer shells, which were favorable for the diffusion and adsorption of reactant molecules. The formation mechanism of the core-shell structure was explored based on the detailed characterizations including FESEM, TEM, TG-DTA, FTIR, XPS and XRD etc. During photocatalytic reduction of Cr(VI) into Cr(Ⅲ), the as-prepared SnSi/TiO2exhibited high activity owing to the high exposure of SnS2nanoparticles, which, besides the promotion on adsorption and activation of reactant molecules, could efficiently harvest lights and generate photoinduced carriers. Moreover, the mesoporous channels on the inner and the outer shells also facilitated the diffusion and adsorption of reactant molecules. The product Cr(III) resulting from photocatalytic reduction of Cr(VI) could be simultaneously adsorbed by the core-shell SnS2/TiO2, which could be easily separated from water, showing good potential in removing heavy metallic ions from wastewater.