Study on tunable fabrication of the ultra-black Ni–P film and its blacking mechanism

This paper reports that novel method to tune the reflection properties of the ultra-black nickel–phosphorus (Ni–P) film by anodization of the Ni–P films in non-oxide acid electrolyte. The Ni–P films (x%, x stands for mass percentage of phosphorus), which possess both crystalline and amorphous structure with a thickness of 10 μm confirmed by XRD and SEM, were obtained by electroless deposited method on Al substrates. The blacking process of the Ni–P film is first divided definitely into two parts involving acid etching and oxidation testified by XPS and SEM. The mechanism study on blacking process demonstrates the acid etching introduces the change in morphology inducing the presence in conical cavities with minute hairlike structures and decreasing the reflectance; oxidation brings on the change in chemical composition enhancing the absorption capacity. The ultra-black Ni–P film with an etched pore depth of 5 μm obtained by anodization method demonstrates easiness of control and operation, strong adhesion and low reflectance (0.45%). Therefore, this work provides a facile approach for the tunable fabrication of the ultra-black Ni–P film based on anodization method in non-oxidizing acid electrolyte, which can be practical applied in fields of black coating.