Sprayable Ultrablack Coating Based on Hollow Carbon Nanospheres

Inexpensive, easy-to-implement coatings exhibiting extremely low reflectance within a broad spectral range, and good adhesion to substrates are in high demand for high-precision optical instruments and solar energy harvesting. Herein, we demonstrate a highly absorbing coating based on hollow carbon nanospheres (HCSs). The coatings are formed via a simple and high-performing air-spraying process with a tailored paint formulation containing HCSs as an absorbing pigment and a fluororesin as a binder. By optimizing the pigment/binder mass ratio (P/B), we produce functional coatings that exhibit solar absorptance up to 0.985 and good adhesion to aluminum sheets of grade 2 (according to the ISO 2409 standard). The excellent solar absorptance of the obtained coatings results from their hierarchical nano- and microscale surface morphology, providing a refractive index gradient on the air–coating interface as well as remarkable light trapping performance. The former is due to the hollow structure in carbon spheres, which is preserved after the addition of the binder because the size of binder particles is larger than the holes on the shell of the HCSs. The latter is attributed to the micronodules and micropits of the coating surface formed by the agglomeration of the HCSs, which enhances absorption by multiple scattering.