| Alkaline niobates are very important perovskite materials due to their piezoelectricity, ferroelectricity, electro-optic, nonlinear optical response and photocatalytic properties. Recent years, in the area of piezoelectric ceramic, people are trying to use lead-free piezoelectric ceramics to replace the wide-used lead-based piezoelectric ceramics Pb(ZrxTi1-x)O3(PZT). Potassium sodium niobate [(K,Na)NbO3] is considered as the most promising lead-free alternative because of the exceptional piezoelectric response. As the end members of this group, sodium niobate (NaNbO3) and potassium niobate (KNbO3) has attracted great attentions. With the development of nanotechnology, NaNbO3 and KNbO3 nanocrystals have been widely studied and find applications in the area of photocatalysis, nanogenerator, nano-electromechanical and nano-photonics. One-dimensional nanostructures are expected to play an important role as both interconnects and functional units in the fabrication of electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions. Therefore, it’s quite meaningful to prepare one-dimensional alkaline niobates nanostructures and study their properties.Hydrothermal method is an efficient approach to prepare nanomaterials. In this thesis, Nb foil was used as the Nb-source, NaOH and H2O2 were used as the starting reagent. One-dimensional NaNbO3 nanowire was prepared by hydrothermal method. XRD, IR spectra and SEM were used to characterize the variation of phase and morphology during the hydrothermal process. Na2Nb2O6-H2O nanowire was firstly prepared under hydrothermal condition and it is a metastable phase. With the increasing of hydrothermal time, Na2Nb2O6-H2O loses combined water and transform into NaNbO3 gradually. At last, NaNbO3 microcubes were prepared. Using Na2Nb2O6-H2O as the precursor, NaNbO3 nanowires can be obtained by annealing the precursor in air at 500℃.Considering the polymorphism of NaNbO3, XRD, Raman spectroscopy was adopted to analyze the crystalline structure of NaNbO3 nanowire and NaNbO3 microcube. It’s surprising to find that these two kinds of NaNbO3 possess different crystalline structure. NaNbO3 nanowire is a ferroelectric phase with space group Pmc21, while NaNbO3 microcube is an antiferroelectric phase with space group Pbma.Piezoelectric force microscopy (PFM) was used to characterize the piezoelectric and ferroelectric property of NaNbO3 nanowire. It’s proved that NaNbO3 nanowire has the piezoelectric response. And more importantly, the standard ferroelectric butterfly amplitude curve and phase curve were obtained on NaNbO3 nanowire. The ferroelectric behavior of NaNbO3 nanowire confirms its ferroelectric crystalline structure directly. The generalized gradient approximation (GGA) electronic calculations were used to reveal the electronic structure of NaNbO3 in Pbma and Pmc21 space group. However, their density of states and band structure are almost the same except the type of band gap. Pmc21 NaNbO3 has the direct band gap while Pbma NaNbO3 has the indirect band gap.KNbO3 one-dimensional nanostructure was prepared by two-step hydrothermal method. The first step hydrothermal treatment was to obtain the transparent solution containing Nb ionic. By adding concentrated alkali, white powder can precipitate. Then the second hydrothermal treatment was adopted to crystalize the precipitation. During the second hydrothermal process, a metastable phase was prepared firstly. And with the increasing of hydrothermal time, the metastable phase transformed into KNbO3 at last. PFM was also utilized to characterize the piezoelectric property of KNbO3 nanowire. KNbO3 nanowire showed obvious piezoelectric response. The piezoelectric amplitude showed good linear relationship with applied Ac voltage and the piezoelectric coefficient d33 is calculated to be 14.1pm/V. |
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