Synthesis and catalysis properties of NiO flower-like spheres and nanosheets: Water-induced phase transformation of nick
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Junjia Xiao, Ye Gou, and Biaohua Chena) Faculty of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, China (Received 4 August 2011; accepted 17 October 2011)
An oleylamine-assisted solvothermal approach has been developed for the synthesis of nickel hydroxide nanomaterials. a-Ni(OH)2 flower-like spheres and b-Ni(OH)2 hexagonal sheets were obtained by tuning the water volume in the synthesis system. The water-induced phase and morphology evolution from a-Ni(OH)2 spheres to b-Ni(OH)2 sheets were investigated in detail by controlled experiments based on their crystal structures. Moreover, NiO spheres and sheets were obtained by direct thermal decomposition of corresponding nickel hydroxides. N2 adsorption/ desorption, temperature-programmed reduction with H2, and catalytic activity tests reveal that NiO spheres exhibit higher surface area, larger pore volume, higher reducibility, and better catalysis activities for CO oxidation than NiO sheets.
I. INTRODUCTION
NiO nanomaterials, which have at least one dimension within nanoscale, have aroused world-wide interests due to their unique chemical and physical properties that are different from bulk counterparts and due to their widerange application as electrode materials for lithium-ion batteries,1 catalysts,2,3 electrochemical supercapacitors,4,5 magnetic materials,6 and gas sensors.7 As the properties of NiO nanomaterials are greatly size- and shapedependent, synthesis of NiO nanomaterials with controllable size and morphology is significant for designing, preparing, and selecting NiO nanomaterials with desirable properties. Various NiO nanostructures, such as nanowires,8 nanosheets,9 nanoflowers,10 nanotubes,11 nanorings,12,13 and nanospanners,14 have been synthesized by precursor decomposition,15 hydrothermal method,16 sol– gel method, and template method.17,18 Among the numerous methods, the direct decomposition of nickel hydroxide is the most common and facile way for the synthesis of NiO. Studies have shown that NiO materials could be obtained with unchanged morphology by direct thermal decomposition of nickel hydroxides.19 Thus, nickel hydroxides were given considerable attention for the crucial application as the most important precursors for the synthesis of NiO, as well as a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.375 J. Mater. Res., Vol. 26, No. 24, Dec 28, 2011
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electrode material in alkaline rechargeable batteries.20,21 Nickel hydroxides usually adopt two well-documented polymorphs: a-Ni(OH)2 and b-Ni(OH)2. Both a-Ni(OH)2 and b-Ni(OH)2 have layered structure. b-Ni(OH)2 adopts the chemical formula Ni(OH)2 and crystallizes in a hexagonal brucite-type structure.15 The ordered “NiO2” slabs stack closely along the c-axis, and the interlayer distance is about 4.6 Å.22 Compared with b-Ni(OH)2, a-Ni(OH)2 is a poorly ordered phase with a larger interlayer spacing of 7.6 Å, in which anions were intercalated in the positively
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