Crystallization characteristics of zinc oxide under electric field and Raman spectrum analysis of polarized products
Authors: Li Yan , Zhang Lin-Bin , Li Jiao , Lian Xiao-Xue , Zhu Jun-Wu ,   College of Science, Civil Aviation University of China, Tianjin 300300, China,   Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

It is of great theoretical and practical significance to study the regulation of the structure, morphology and properties of nanomaterials by using high voltage electric field in the field of functional materials. Here, ZnO nanocrystalline powders are synthesized under the condition of high voltage electric field. The effect of electric field on the structure, point defect and Raman spectrum of ZnO is studied.The structure, Raman shift and defect distribution of the product are characterized by (XRD), scanning electron microscope (SEM) and Raman spectroscopy (Raman spectroscopy).The results show that the complete crystallization time and temperature of zinc oxide under high voltage electric field are longer and higher than those without electric field. The direct current electric field can significantly promote the nucleation of zinc oxide in the precursor and reduce the rate of crystallization.The morphologies of ZnO obtained under different electric field intensities are obviously different. At a lower electric field intensity, ZnO presents lamellar or stripy morphology that is formed by many 50 nm-diameter nanoparticles. At a higher electric field intensity, ZnO exhibits short conical particles. It can be inferred that the high voltage electric field inhibits the growth of zinc oxide along the c axis (the strongest polar direction).The Raman spectra of the cathode surface and the anode surface showing obvious difference after the nano-ZnO powder has been polarized in the DC electric field.The intensity of the second-order optical phonon mode <i>A</i><sub>1</sub>(LO) on the anode surface at 1050 cm<sup>–1</sup> increases significantly under the condition of obvious leakage current, and the ratio (<i>I</i><sub>1</sub>/<i>I</i><sub>2</sub>) of Raman intensity (<i>I</i><sub>1</sub> = 438 cm<sup>–1</sup> and <i>I</i><sub>2</sub> = 1050 cm<sup>–1</sup>) is linearly related to the field strength of the polarized electric field.When the positive and negative sides of the sample disc turn over, the 1050 cm<sup>–1</sup> peak increases on the anode surface and tends to disappear on cathode surface.The zinc vacancies with negative charge move toward the anode and the concentration of zinc vacancies on one side of the anode increases significantly, which makes the surface of zinc oxide nanoparticles in the local area of the anode surface exhibit obvious negative electric properties, and increases the local electric field significantly to form a double Shaw base barrier.The Raman shift of 1050 cm<sup>–1</sup> belongs to the second order optical phonon <i>A</i><sub>1</sub> (LO) vibrational mode, which is usually in inactive or silent state. When the current passes through, the grain boundary double Schottky barrier is established, which enhances the vibration of the <i>A</i><sub>1</sub> (LO) phonon and increases its Raman frequency shift.It can be concluded that the enhancement of the 1050 cm<sup>–1</sup> Raman peak on the anode surface is related to the redistribution of defects in ZnO grains and the double Schottky barrier.