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Different nanocomposites between reduced graphene oxide (rGO) and Ni(OH)2 nanoparticles were synthesized through modifications in the polyol method (starting from graphene oxide (GO) dispersion in ethylene glycol and nickel acetate), processed as thin films through the liquid-liquid interfacial route, homogeneously deposited over transparent electrodes and spectroscopically, microscopically and electrochemically characterized. The thin and transparent nanocomposite films (112 to 513 nm thickness, 62.6 to 19.9% transmittance at 550 nm) consist of α-Ni(OH)2 nanoparticles (mean diameter of 4.9 nm) homogeneously decorating the rGO sheets. As a control sample, neat Ni(OH)2 was prepared in the same way, consisting of porous nanoparticles with diameter ranging from 30 to 80 nm. The nanocomposite thin films present multifunctionality and they were applied as electrodes to alkaline batteries, as electrochromic material and as active component to electrochemical sensor to glycerol. In all the cases the nanocomposite films presented better performances when compared to the neat Ni(OH)2 nanoparticles, showing energy and power of 43.7 W h kg−1 and 4.8 kW kg−1 (8.24 A g−1) respectively, electrochromic efficiency reaching 70 cm2 C−1 and limit of detection as low as 15.4 ± 1.2 μmol L−1.

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Hexagonal close packed (hcp) nickel nanoparticles stabilized by polyvinylpyrrolidone (PVP) were synthesized through the thermal treatment of face centered cubic (fcc) nickel nanoparticles. Controlling both the temperature of the heat treatment and the amount of PVP was possible the control of the hcp/fcc rate in the samples, where the higher Ni/PVP ratio produces only the hcp-nickel phase (average size of 8.9 nm) highly stable in air. The crystalline structure, the presence of PVP, the size of the nanoparticles and the stability of the hcp-nickel were confirmed using X-ray diffractometry, Fourier transform infrared spectroscopy, transmission electron microscopy, Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis. Thin films of hcp and fcc nickel nanoparticles were prepared through a biphasic system and deposited over indium-doped tin oxide (ITO) substrates, which were electrochemically characterized and applied as glycerol amperometric sensors in NaOH medium. Parameters as the number of cycles applied and the scan rate were evaluated and indicate that hcp nickel nanoparticles are more reactive to form Ni(OH)2 and lead to more electroactive Ni(OH)2 structure. The hcp nickel nanoparticles-modified electrode showed the best sensitivity (0.258 μA L μmol(-1)) and detection limit (2.4 μmol L(-1)) toward glycerol.

High values of maximum rates of hydrogen production are obtained using four thin film nanocomposites that showed recycling features.

Nanocomposites between nickel nanoparticles and graphene were obtained starting from nickel cations and graphenide solutions (negatively charged graphene layers) as both reducing agent to nickel cations and graphene source. Different nanomaterials were obtained in two different solvents, N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF), with different nickel/graphene ratios. The nanomaterials were characterized by UV-Vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). All the samples consist of large graphene layers highly decorated with crystalline nickel nanoparticles, of size ranging from 2 to 10 nm. Thin films of the samples were deposited on indium-tin oxide (ITO) substrates and electrochemically characterized in alkaline medium, leading to Ni(OH)2/NiOOH redox pair, where the increase of the nickel proportion in the nanocomposites resulted in higher peak currents. The samples obtained in NMP showed the best performance with a fivefold increase of the peak currents, consistent with the lower charge transfer resistance as seen by electrochemical impedance spectroscopy (EIS).

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