Study On Soft Chemical Synthesis And Photocatalytic Performances Of Niobates

Niobates are typical functional materials that have attracted broad attention because of their excellent photocatalytic, piezoelectric, dielectric and ferroelectric, nonlinear optical, ionic conductivity, pyro-electric and photorefractive properties. In addition, niobates possess excellent properties in photocatalytic degradation of organic pollution and photocatalytic splitting water into H2 and O2. However, metal niobates are usually synthesized by the solid-state or molten-salt method at high temperature, where the particle size and morphology are difficult to be controlled.Soft chemical process can overcome the potential barriers of the solid-state reactions via the moderate and slow reaction processes. Soft chemical process can prepare the novel materials through the controllable chemical reactions and it has been widely used in the controllable synthesis of functional materials. Thus, in this thesis, the two-dimensional Ag2Nb4O11 mesocrystals, one-dimensional AgNbO3, ZnNb2O6, ZnNb2O6/KNbO3 nanostructures were synthesized via the soft chemical process using layered or tunnel structural potassium niobates as precursors. The transformation mechanisms from precursor to final products were revealed via tracing the evolution of intermediate products. The photocatalytic properties of the samples were also explored.(1) The platelike Ag2Nb4O11 mesocrystals were prepared via the soft chemical topotactic reaction using the layered structure K4Nb6O17 platelike particles as precursor. The formation mechanism was investigated through tracing the evolution of structure and morphology of intermediate products during the reaction, and it contains two processes. One is the ion-exchange reaction with the interlaminar ions of layered precursor. The other is in situ topotactic structure transformation reaction of the platelike Ag3HNb6O17 into platelike Ag2Nb4O11 mesocrystals during the heat-treatment process. Moreover, the degradation of RhB over the platelike Ag2Nb4O11 mesocrystals can reach to 95.3% after the irradiation of 20 min. The enhancement of photocatalytic performance of Ag2Nb4O11 mesocrystals can be attributed to its unique morphology and exposed facet, which affect the band gap and energy band structure.(2) A topotactic soft chemical process was used for the synthesis of one-dimensional AgNbO3 nanostructures. Firstly, using a tunnel structure potassium niobate K2Nb2O6·nH2O filiform crystal as precursor, the K+ in K2Nb2O6·nH2O were exchanged by Ag+ in the AgNO3 water solution, forming Ag+ ion-exchanged sample. Secondly, Ag+ ion-exchanged sample was heat-treated to obtain the perovskite AgNbO3 by in situ topotactic structure transform reaction. The one-dimensional nanostructures AgNbO3 was constructed from nanocrystals arranged in [011]-crystal axis and predominantly exposed (100) facet. The samples were able to degrade MB under ultraviolet irradiation and catalyze water-splitting in aqueous solution under illumination of visible light.(3) The one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and one-dimensional ZnNb2O6 nanostructures were synthesized via in situ topotactic structural transformation reaction using the tunnel structure K2Nb206 filiform crystal as precursor. Firstly, Zn2+ions intercalate into K2Nb2O6 crystal by the exchange K+ ions of K2Nb206 crystal with Zn2+ in Zn(NO3)2 or Zn(CH3COO)2 aqueous solution, to form two different Zn2+-exchanged samples, and then these Zn2+-exchanged samples topotacticly transformed into one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and ZnNb2O6 nanostructures during heat-treatment, respectively. Photocatalytic experiments showed that one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and ZnNb2O6 nanostructures have excellent photocatalytic performance for the degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO). The synergistic effect makes the hetero-nanostructures display the greatest photocatalytic activity.