Formation of Hierarchical NiO Coatings on the Surface of Al 2 O 3 Substrates under Hydrothermal Conditions
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HESIS AND PROPERTIES OF INORGANIC COMPOUNDS
Formation of Hierarchical NiO Coatings on the Surface of Al2O3 Substrates under Hydrothermal Conditions T. L. Simonenkoa, *, V. A. Bocharovaa, b, Ph. Yu. Gorobtsova, N. P. Simonenkoa, A. G. Muradovab, E. P. Simonenkoa, V. G. Sevastyanova, and N. T. Kuznetsova aKurnakov
Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia b Mendeleev University of Chemical Technology of Russia, Moscow, 125047 Russia *e-mail: [email protected] Received April 10, 2020; revised April 26, 2020; accepted April 27, 2020
Abstract—A hierarchical nickel oxide film was grown on the surface of a polycrystalline Al2O3 substrate by hydrothermal synthesis. The microstructure of the obtained coating was studied by scanning electron microscopy and atomic force microscopy, which showed that this coating comprises porous nanosheets (about 7 nm thick) arranged at various angles to each other and to the surface of the substrate. The lateral sizes of these nanosheets were in the range 3–5 μm. The local electrophysical characteristics of the obtained oxide coating were investigated by scanning capacitance microscopy and Kelvin probe force microscopy; using the obtained results, the maps of the distributions of the surface potential and capacitance contrast over the surface of the NiO film were constructed, and the work function of the surface of this film was calculated. Keywords: hydrothermal synthesis, hierarchical structures, nanosheets, coating, nickel oxide, NiO, electrode, fuel cell DOI: 10.1134/S0036023620090193
INTRODUCTION Nickel oxide NiO is a quite widely occurring wideband-gap (3.5–4.0-eV) p-type semiconductor, one of the most popular transition metal compounds owing to a set of its physicochemical characteristics: high chemical and thermal stability, catalytic activity, specific capacitance (to 3750 F/g), low toxicity, and also commercial availability caused by its wide occurrence [1, 2]. Numerous studies have been made to date to produce materials based on nickel oxide as powders, thin and thick films, and ceramics by a wide range physical and chemical synthesis methods for various practical applications: laser ablation [3], atomic layer deposition [4], coprecipitation [5, 6], sol–gel method [7, 8], spray pyrolysis [9, 10], electrodeposition [11, 12], synthesis in microemulsions [13, 14], hydrothermal and solvothermal synthesis [15, 16], and also template synthesis [17, 18]. Note also that the optimization of conventional methods and the development of new techniques for manufacturing nanosized semiconductor materials of various, including complex, composition with the desired characteristics superior to the existing ones [19–21] is one of the most important problems of modern materials science. In this context, there has recently been a new trend in producing various types of nanomaterials, in particular, nanopowders and coatings with a hierarchical microstruc-
ture, which are efficiently used to create alternative energy devices (fuel cells,
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