Synthesis And Characterization Of Nano-crystalline Niobates

Using semiconductor photocatalysts for water splitting into H2 and O2 in a stoichiometric ratio under solar light irradiation is considered to be a feasible method that could solve the problems of environmental contamination and energy crisis. In recent years, the perovskite niobate compounds with outstanding stability, strong metal support interaction show highly photocatalytic activity for hydrogen production from water. Moreover, niobate compounds are accelerator agents and catalysis supports in the process of photocatalysis.In this thesis, Nb₂O₅·nH₂O gel prepared by melting Nb₂O₅ in KOH is chosen as niobium source. Sheet-like KNbO₃ powders are prepared with using a wet chemical method in the process of templated crystallization without adding any organic reagent. In the hydrothermal process, the hydrothermal system consists of the precursor mixture of metal chloride or nitrate and Nb₂O₅·nH₂O gel in KOH solution with proper concentrations. Phase composition and particle morphology are effectively controlled by adjusting the reaction temperature, reaction time and KOH concentration, etc. The initial investigation of their optical properties indicates that the prepared powders exhibit good light absorption properties in the ultraviolet light region and can be used as a highly active photocatalyst to decompose water.(1) Nb₂O₅·nH₂O gel prepared by melting Nb₂O₅ in KOH is chosen as niobium source. KNbO₃ nanosheets and aggregated crystallites are successfully synthesized by a wet chemical method with using KNO₃ and K2CO₃ as potassium sources, respectively. The powders obtained are characterized by XRD, SEM, TEM and UV-Vis. Templated crystallization is realized by using KNO₃ as potassium source. The appearance of intermediate crystalline phase of K4Nb6O17 is considered to be the key factor that determined the phase composition and morphology of the final products. The prepared powders exhibit good light absorption properties in the ultraviolet light region. (2) Nanocrystalline strontium niobate powders (Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅) were successfully synthesized in the hydrothermal system of Sr²⁺-Nb₂O₅·nH₂O-KOH. The phase composition and morphology of the final products are characterized by means of XRD and TEM. Nb⁵⁺/Sr²⁺ ratio of 1: 1 favors the formation of Sr₂Nb₂O₇ nanorods, while Sr₅Nb₄O₁₅ nanoplates could be obtained as Nb⁵⁺/Sr²⁺ ratio was 1: 4. The band gap energy of Sr₂Nb₂O₇ estimated from UV-Vis DRS is 3.97 eV, which is larger than that of the bulk materials (3.9 eV)(3) The pure phase of Sr₂Nb₂O₇ can be prepared in 0.3-5M KOH solutions in a hydrothermal system of Sr²⁺-Nb₂O₅·nH₂O-KOH. The effects of the KOH concentration on crystal structures and morphologies of the final products are investigated. With the increase of KOH concentration, Sr₂Nb₂O₇ crystallites in morphology can be changed from nanoneedles to nanorods, nanosheets, and finally into nano-sized flakes, and their aspect ratios gradually decreased. No obvious difference could be detected from two UV-Vis DRS curves except a little blue-shift of nanosheets relative to nanoneedles.(4) Nano-crystalline Ca2Nb₂O₇ powders with a diameter of 20-30nm can be prepared in a hydrothermal system of Ca²⁺-Nb₂O₅·nH₂O-KOH. The effects of the KOH concentration on crystal structures and morphologies of the final products are investigated. The pure phase of Ca2Nb₂O₇ can be prepared in 0.1-3M KOH solutions. The optimal KOH concentration is 0.1M. The nanosized Ca2Nb₂O₇ powders exhibit good light absorption properties in the ultraviolet light region.