Template- and Magnetic Field-Free Chemical Reduction of Ni Nanochains for Ni–Al₂O₃ Cermet Films: Growth Control, Characterization, and Application

Abstract

Cermet-based solar absorbers containing nickel (Ni) in a nanochain (NC) structure embedded in an aluminum oxide (Al 2 O 3 ) film demonstrated a high absorptance of more than 90% of the solar spectrum. In this work, Ni NCs were successfully prepared by a simple chemical reduction method without the assistance of a template or magnetic field. The formation of Ni nanoparticles (NPs) in different configurations was controlled by adjusting the NaOH:NiCl 2 molar ratio. Not only was NaOH used to adjust the solution pH but it also induced the reduction reaction to be faster and so resulted in a larger number of Ni nuclei. Together with the intrinsic magnetic property of Ni, Ni NPs tended to orient in a chain-like manner to form Ni NCs that remained stable throughout the reaction. Increasing the NaOH:NiCl 2 molar ratio up to 8 led to a uniform morphology of Ni NCs. However, at higher molar ratios (above 8), the NCs were likely to collapse at the end of the reaction, forming near-globular particles. With its unique structure, metallic Ni NCs were employed by incorporating them into a ceramic layer of Al 2 O 3 , which can be used as efficient cermet materials. Compared to a conventional cermet with embedded spherical Ni NPs, a 16.4% increase in solar absorptance was observed with the Ni NCs due to their enhanced absorption and scattering in the solar spectrum. Moreover, increasing the Ni NC content in the Al 2 O 3 layer gradually enhanced the solar absorptance to 0.91 and so was a good solar absorber.