Hydrothermal Synthesis And Photocatalytic Properties Of Tantalate Powders

A novel tantalate photocatalyst was non-Ti system photocatalyst and it hada special crystal structure and band structure. It had a great advantage in watersplitting and the degradation of organic compounds, and had more expansiveapplication prospects compared with the traditional TiO2catalyst. In thesephotocatalysts, NaTaO₃photocatalyst with perovskite structure had highstructure tolerande, and it benefited the ion exchange. The NaTaO₃crystal wasformed by the connection of the common angle of TaO6octahedron. The bondangle of Ta-O-Ta was163°, and the band gap was4.0eV. It had a highphotocatalytic activity under the ultraviolet light.In this paper, the NaTaO₃photocatalyst was prepared by hydrothermalmethod. The effects of the reaction temperatures and the alkali concentration tothe powder synthesis were studied. Using NH4F as a dopant, the N/F-NaTaO₃powder with high catalytic activity was prepared, and its photocatalytic propertyunder visible light was improved. The composition, morphology andmicrostructure of NaTaO₃powder were characterized by XRD, SEM and TEM.The elements composition and the valence state were characterized by X-rayphotoelectron spectroscopy analysis. Using rhodamine B （RhB） and methylorange as degradation products, the photocatalytic activity of the powders underthe UV and visible light was studied, and the mechanism to the higherphotocatalytic activity by non-metal ion doping was analysed. In order to avoidthe secondary pollution of the photocatalytic powders, NaTaO₃films wereprepared at different annealing temperatures, and its photocatalytic activityunder the UV light was studied in this paper.First, the effects of the different reaction temperatures and the alkaliconcentration to the pure phase NaTaO₃synthesis were discussed. The resultsshowed that the orthorhombic bulk NaTaO₃crystal was obtained at the different temperatures and alkali concentrations. The anionic coordination polyhedronwas formed by Ta₂O₅and OH-which dissolved in the solution, and thendewatered to form NaTaO₃crystal nucleus. The grain size was between200-720nm. The average band gap was3.9eV. It had a great absorption to UVlight. The specific surface area was about2.0m2/g. The powders were preparedat140℃, NaOH1.0mol/L had the biggest specific surface area, and itsphotocatalytic activity was the best. The degradation to RhB was about94.7%irradiated by the UV light after210min.Because of the high energy gap and low utilization factor to the light,non-metallic doping was used to improve the photocatalytic activity in thispaper. NH4F was used for a dopant. The effect of the reactant concentration ratioand reaction temperature on NaTaO₃powders was discussed. The N/F-NaTaO₃photocatalyst was obtained. The as-prepared powders were orthorhombic bulkstructure. The XRD pattern showed the main phase was NaTaO₃, but thediffraction peak red-shifted. The grain size was increased evidently comparedwith pure phase NaTaO₃and was about350-900nm. The band gap had no greatchange. The XPS analysis showed there were Ta-N bond and Ta-F bond at402.030eV and684.584eV. It proved that N and F elements entered into thecrystal lattice. The non-metal doped crystal appeared the defect of the lattice,which enhanced the effective electronic activity, prevented the composite of theelectron and hole pairs, and increased the photocatalytic activity. TheN/F-NaTaO₃photocatalyst which was prepared at240℃with NaOH/NH4F of2.0/1.0had a high photocatalytic activity. The degradation rate of RhB solutionwas96%under the UV light after120min.Because the powders were difficult to collect, and could cause secondarypollution, NaTaO₃films were prepared in this experiment. Using TaCl5,CH3COONa and citric acid as raw materials, the precursor solution of NaTaO₃films was synthesized by sol-gel method, and the films were prepared bydip-pulling method on the glass substrates. The effects of annealing temperatureon the composition, morphology and photocatalytic activity were disscussed.The as-prepared films were orthorhombic NaTaO₃. The surface morphology ofthe films was even and compact. The crystallinity of the films was best underthe annealing temperature of610℃and the pore phase appeared. The degradation rate of RhB solution was51%under the UV light after100min.