Preparation And Modification Of Novel Pyrochlore-Type Photocatalytic Materials And Their Photocatalytic Activity For Water Splitting Under Visible Light Irradiation

With regard to a subject of great interest that the photocatalytic water splitting using solar energy generates clean energy hydrogen, this thesis focuses on preparation and modification of visible-light-response pyrochlore-type (A2BB'O7) photocatalytic materials and the investigation of their photocatalytic activity for water splitting. To explore new photocatalytic materials, a novel visible-light-response photocatalyst Sm2InNbO7 with 4f-d10-d0 configuration was successfully prepared using compounds containing Sm3+ with 4f orbital electrons, compounds containing In3+ with d10 configuration in p-block and compounds containing Nb5+ with d0 configuration in d-block. By substitution of A site and B'site and doping of B site and O site, a series of complex (sulf) oxides were synthesized and their photocatalytic activities of water splitting were also concerned. In addition, structure-property relationship between substitution or doping and photocatalytic activities was discussed in close conjunction with their electronic structure and crystal structure.1. The novel visible-light-response pyrochlore-type compound Sm2InNbO7 with 4f-d10-d0 configuration was synthesized by a solid-state reaction method. Atom coordinates, bond lengths and bond angles were obtained by Rietveld refinement of structure. Band structure and state density were calculated by Castep based on density functional theory (DFT). The result of calculation indicated that conduction band and valence band of Sm2InNbO7 were composed of In5s5p+Nb4d and Sm4f, respectively. The photocatalytic activity of Sm2InNbO7 was investigated under visible light (λ>400 nm) irradiation. It was found that Sm2InNbO7 can decompose water into H2 and O2 in the presence of appropriate sacrificial reagents, and the activity can be efficiently enhanced by Pt deposition. As for pure water, only H2 evolution was observed, and the activity was remarkably improved when NiOx was loaded on Sm2InNbO7.2. The photocatalyst Sm2InNbO7 was synthesized through calcining the precursors prepared by a Sol-gel method. In the calcining process, it can be seen that characteristic peak of Sm2InNb07 appeared initially at 973K, pyrochlore-type Sm2InNb07was completely formed at 1073K, and its crystallization was gradually enhanced with increasing calcination temperature. At 1173K, the photocatalytic activity of Sm2InNb07 showed the highest, better than that of the one prepared by solid state method. This may be due to higher surface areas and narrower band gap of catalyst prepared by sol-gel method.3. Two other new pyrochlore-type compounds Sm2InTa07 and Nd2InTa07 were prepared by the substitution of Ta by Nb at B'sites and Nd by Sm at A sites. It was found that the structures of both compounds were the same as that of Sm2InNb07 by Rietveld refinement of structure, and their conduction band and valence band were composed of In5s5p +Ta5d and Sm4f or Nd4f, respectively, through electronic structure calculation. The order of their photocatalytic activities was as following: Sm2InNb07< Sm2InTa07< Nd2InTa07, which was in close correlation with their lattice distortion and the M-01-M (M=In, Ta, Nb) angle between the corner-linked octahedral.4. Sm2In1-xCrxNb07 was obtained by doping with Cr at B sites for Sm2InNb07. Although the structure was unchanged, the photocatalytic activity of water splitting was improved in appropriate doping concentration (about 2 mol%). Compared Sm2In0.98Cr0.02NbO7 with Sm2InNb07, the rate of H2 evolution increased 2.9 times for CH3OH aqueous solution and 1.4 times for pure water under visible light irradiation. Deficient crytal lattice arised by the doping of In3+by Cr6+ was helpful to produce electron, and Cr 3d orbitals formed a new valence band above the filled Sm4f orbitals and thus made the narrower band gap of the photocatalyst, which were responsible for the improvement activity of photocatalyst Sm2In0.98Cr0.02NbO7.5. Platelike Sm2InNbS2O5 with layered perovskite structure was obtained by sulfurizing (1123K for 1 h) amorphous Sm2InNb07, which was prepared though calcinating (773K for 12 h) the precursor synthesized by sol-gel method. Compared with Sm2InNbO7, absorption of oxysulfide occurred red-shift and resulted in narrower band gap of oxysulfide. Simulant calculation of the electronic structure showed that valence band of Sm2InNbS2O5 was composed of S3p and Sm4f, and its conduction band was of S3p, Sm4f, In5sIn5p and Nb4d. The H2 evolution activity of Sm2InNbS2O5 was about 6 times higher than that of Sm2InNb07. The improvement of the activity for Sm2InNbS2O5 was owing to the fact that its M-O-M angle between octahedral was closer to 180°, the band gap was narrower and the site of the conduction band was more negative.