Carbonaceous nickel oxide composite films with homogenously dispersed metallic Ni nanoparticles by a facile sol–gel method

A composite of Ni/C/NiO thin films, in which the metallic nickel nanoparticles (MNNs) are homogeneously dispersed, has been prepared by a facile one-step sol–gel technique as a solar-selective absorber (SSA). Comprehensive analysis of the as-prepared composite films has been carried out employing various characterization techniques. The results reveal that both metallic Ni and NiO nanoparticles are surrounded by the resident carbon and the MNNs emerge from the NiO precursor reduced by the resident carbon. The size of the homogeneously dispersed MNNs increases with the annealing temperature from ~4 to ~7 nm. A HCP–FCC phase transition of MNNs was found at about 400–450 °C, while the contents of Ni and Ni2+ are almost equal at the same time. The solar-selective absorptance properties of Ni/C/NiO composite films were studied by measuring the reflectance in the UV-Vis and infrared wavelength range. The results indicate that the smaller MNNs and the thinner films attenuate the absorptance but enhance the emittance. The one-layer Ni/C/NiO composite film on copper substrate annealed at 450 °C is preferred in the present work, exhibiting an absorptance of about 0.86 and an emittance of about 0.11, and capable of a good candidate as SSA for the low–middle temperature applications.