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Controllable Synthesis of Undoped and Doped Calcium Niobate Nanocrystals for Tailored Structural, Electronic, and Luminescent Properties

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journal contribution
posted on 22.02.2016, 08:53 by Yiguo Su, Liman Peng, Chunfang Du, Xiaojing Wang
In this work, we report on the phase and particle size control of Ca2Nb2O7 nanocrystals with an aim of tailoring structural, electronic, and luminescent properties. Using citric acid as the capping agent, the as-prepared nanocrystals exhibited a phase transformation from orthorhombic CaNb2O6 to cubic Ca2Nb2O7. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, Fourier transformed infrared spectroscopy, UV–vis diffuse reflectance spectroscopy, and luminescence spectroscopy. It is found that Ca2Nb2O7 showed particle sizes ranging from 14.6 to 26.9 nm by regulating the pH value under hydrothermal conditions. Contrary to the theoretical predictions of the quantum size effect, Ca2Nb2O7 showed an abnormal band gap narrowing with particle size reduction, which can be well-defined as a function of lattice volumes, surface defects, and the surface dipole layer. As for the Eu3+-doped samples, it is shown that Eu3+ and K+ were simultaneously substituted at Ca2+ sites in the Ca2Nb2O7 host lattice, which allows one to vary the local symmetry surrounding Eu3+ for an enhanced luminescence property. As the consequence of the particle size effect and variation of the local symmetry, a maximum quantum yield of 28% was observed for 19.2 nm Ca2Nb2O7.