The Synthesis, Characterization And Photocatalytic Activity Of Alkali Metal Niobates

Perovskite-type niobate have attracted much attention due to the particular ferroelectric, piezoelectricity, dielectric and nonlinear optical properties. As the candidate material of lead-free piezoelectricity ceramics, alkali metal niobates, MNbO3(M=Li, Na, K), become the important value.The thermal decomposition kinetics of as-synthesized ammonium niobium oxalate was investigated in this paper. The molecular formula of synthesized ammonium niobium oxalate was confirmed as NH4[NbO(C2O4)2(H2O)2]-3H2O by elemental analysis, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis. The decomposition of the complex proceeds through three steps. The decomposition products are high active ammonium niobium hydroxide at330℃and niobium pentoxide at640℃. The corresponding apparent activation energies of three steps were calculated by OFW, Friedman and iteration methods. The values are90,140and200kJ/mol, respectively. The most probable kinetic models of the first two decomposition steps of the complex have been estimated by Coats-Redfern integral and Achar-Bridly-Sharp differential methods. The most probable kinetic model of the first dehydration step is described as Avrami-Erofeev (n=2/3), while the second one is fitted by Avrami-Erofeev (n=3). Ammonium niobium oxalate will be selected as niobium source to prepare niobate. Niobate would be prepared by the reaction of high active ammonium niobium hydroxide obtained by the decomposition of ammonium niobium oxalate with alkali metal compounds.LiNbO3and NaNbO3nanocrystalline were prepared using high active alkali metal compounds and different niobium source (ammonium niobium oxalate, ammonium niobium hydroxide or niobium pentoxide) as raw materials by improved solid-state method. The influence of grinding method of raw materials on products has been discussed. After a group of raw materials was mixed uniformly, some water or ethanol were added to produce slurry. The slurry was pestled and then dried. The precursor obtained by this manner has two advantages. On the one hand, it can be mixed uniformly. On the other hand, an ion-exchange reaction occurred between the raw materials after adding some water, which made the final reaction occur easily. XRD results show that LiNbO3and NaNbO3nanocrystalline can be prepared at400℃with average particle size of30nm when using ammonium niobium oxalate as niobium source. The average band-gap energy of LiNbO3and NaNbO3obtained at different temperatures is3.95eV and3.30eV, respectively.Potassium niobate was prepared using Nb2O5and K2CO3as raw materials by traditional solid-state. Single phase KNbO3can be obtained at800℃. It can be found from XRD, FTIR and thermogravimetry (TG) analysis, an irreversible ion-exchange reaction between NH4+and K+occurs when high active ammonium niobium oxalate reacted with potassium acetate. Single-phase KNbO3can be obtained when calcining the produced intermediate at500℃for3h. About80%of these particles have an average particle size of20-50nm by XRD, scanning electron microscope (SEM) and transmission electron microscopy (TEM) analysis. Stoichimetric K0.5Na0.5NbO3was synthesized by using potassium sodium tartrate and ammonium niobium oxalate as raw materials. Stoichimetric K0.5Na0.5NbO3with average size of68nm was prepared at500℃by XRD and X-ray fluorescence (XRF) analysis.The catalytic degradation of organic dye was investigated using as-synthesized MNbO3(M=Li, Na, K) and K0.5Na0.5NbO3as photocatalyst. Based on the screening experiments, the ability of catalytic degradation of organic dye (methylene blue, acid red G or methyl orange) for every as-synthesized alkali metal niobates was studied under high pressure mercury lamp. The degradation mechanism of photocatalysis was preliminarily studied. The degradation of methylene blue dry was discussed for potassium niobate (the calcined product at500℃) with photocatalytic properties. The effects of the dosage of catalyst and initial concentration of methylene blue solution were investigated respectively. From the consideration of processing cost and catalytic separation, the best dosage is0.75g L-1.