The subtle usage of NH4BF4: Facile and economical fabrication of Ti black surface for enhanced optical absorption performance

The Ti black surface has been widely used in aerospace and photocatalysis for many years. However, the preparation technique for the Ti black surface has several disadvantages, such as a complex process, high cost and safety concerns. In this study, a new chemical immersion method using the mixed aqueous solution of 0.715 mol/L NH4BF4 and 0.004 mol/L HF was proposed to prepare a uniform Ti black surface, which is very facile, economical and efficient. Due to the etching of HF, the light-trapping nanostructure was formed on Ti surface. Grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) observations indicated that the black surface was covered by the film mainly composed of TiOx, Ti(OH)x, and TiH2. The diffuse reflectance spectra demonstrated that the black surface exhibited great optical absorption performance with the maximum reflectance of 7.6 % to the UV–visible light and 35.3 % to the near-infrared light. The as-fabricated TiOx film with narrower bandgap could be contributed to the doping of Ti3+ and F− ions. The presence of F− ions in the film prevented the Ti3+ ions in the bulk of TiOx from being oxidized. The combined contribution of light-trapping nanostructure and TiOx film enhanced the optical absorption performance of Ti surface.