Microstructure Studies on Hexagonal Layered Ni-S Nanocrystals
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Microstructure Studies on Hexagonal Layered Ni-S Nanocrystals Xiang-Cheng Sun1,*, J. Reyes-Gasga2 and J. A.Toledo3 1
Center for Material for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487-0209. *Tel: 205-348-280,*Fax: 205-348-2346,*Email:[email protected] 2 Institute of Physics, National University of Mexico, 04250, D.F. Mexico 3 Molecular Engineer, Instituto Mexicano del Petroleo, 07730, D.F. Mexico ABSTRACT: Layered nickel-sulfides (Ni-S) nanocrystals had been successfully prepared by lowtemperature hydrothermal approach. Rietveld X-ray diffraction (XRD) analysis revealed that these layered Ni-S materials have two different hexagonal structure phases, NiS1.03 and NiS, both phases are NiAs-type with space group of P63/mmc. X-ray energy dispersive spectra (XEDS) analysis and TEM observations also confirmed these two nanocrystals of NiS1.03 and NiS. High-resolution transmission electron microscopy (HREM) lattice images indicated moiré fringes with rotational or translational patterns were observed in these layered Ni-S nanophases materials. In particular, it was found that all the simulated HREM images were in good agreement with the observed HREM lattice images. Especially, the simulated SAED patterns obtained from the hexagonal Ni-S crystallographic model showed that the experimental SAED patterns taken along [11.0], [00.1] and [10.0] zone axes presented reflections produced by double diffraction effect. INTRODUCTION Nickel sulfides (Ni-S) have been the subject of consideration interest for several years because it exhibited metal-insulator and magnetic phases transition [1-3]. In addition, novel Ni-S catalytic materials can be used as a hydrosulfurization catalyst [4] and in solar storage to absorb solar energy [5]. As well known, stoichiometric NiS crystallizes in two different phases [6]; a rhombohedral phase (β, millerite) and a hexagonal phase (α, NiAs type). The β-α phase transformation can occur at a certain temperature or under pressure. On the other hand, a series of non-stoichimometric Ni-S compounds were synthesized by using various preparation methods [7]. Especially, the non-stoichiometric NiS1.03 represented by the hexagonal NiAs type can be prepared by the element reaction between nickel and sulfur in a strongly coordinating solvent [8]. Hexagonal NiAs type NiS1.03 catalysts showed very good catalytic properties, and it was found that the formation of this NiAs-like hexagonal NiS1.03 structure could be related to the formation of nickel vacancies [9]. In the present study, layered Ni-S nanocrystals with two NiAs-type structures have been synthesized using a hydrothermal method at lower temperature (200oC). The aim of our study was to characterize the detailed structure of these Ni-S nanocrystals using x-ray diffraction (XRD), electron microscopy (TEM, HREM) in combination with simulated HREM images and, electron diffraction (SAED) EXPERIMENTALS The hydrothermal preparation of Ni-S nanocrystals is described in the following: at first, both aqueous solutions of ni
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