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 (Al2O3) 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:NiCl2 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:NiCl2 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 Al2O3, 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 Al2O3 layer gradually enhanced the solar absorptance to 0.91 and so was a good solar absorber.