Cast-and-Use Super Black Coating Based on Polymer-Derived Hierarchical Porous Carbon Spheres

Abstract

Super black materials with extremely low reflectance and high absorption of incident light are crucial in various applications with strict optical requirements. However, the existing super black materials suffer from technical complexity in practical use whereas the underlying physics of their ultralow reflectance is still unclear. Herein, hierarchical porous carbon spheres are prepared using poly­(vinylidene chloride-co-vinyl chloride) as a precursor. Dehydrochlorination treatment has proved to be very crucial for the morphology robustness of the porous polymer spheres during carbonization process. Coatings cast from the dispersion of these carbon spheres exhibit a hemispherical reflectance of <0.2% in the visible region and <0.3% in the range of 300–2000 nm, and the lowest reflectance of 0.14% can be reached. This super black coating has great advantages in its easy availability of starting materials, low equipment requirement, and high adaptability onto various substrates. Besides, a theoretical investigation suggests that a small scatterer size and low volume-filling ratio are the two most essential factors in realizing ultralow reflectance, which also offers an instructive guidance for the rational design of super black materials.

Carbon Vesicles: A Symmetry‐Breaking Strategy for Wide‐Band and Solvent‐Processable Ultrablack Coating Materials

“…This simple model predicts the reflectance of coatings of randomly positioned uniform particles, which also applies to the situations of uniform hollow absorbing spheres. In brief, hemispherical reflectance R hemi is a combined result of interfacial reflection and the back scattering of light by inner structures, which can be expressed by [ 24 ] R interf denotes the interfacial reflection and can be calculated by the well‐known Fresnel equation, while R diff describes the reflection contributed by back scattering, and can be estimated by Hartal's multiple scattering theory. [ 35 ] For coatings composed of uniform hollow absorbing spheres, their reflectance can be predicted utilizing this model when the composing spheres are treated as independent scattering units with random spatial distribution.…”

“…In addition, when TEOS dosage increases, the interlayer will become nano‐porous, and the outer layer will become thinner, which will lead to a strong small‐conductor effect. [ 17,24 ] The enhanced damping of electron vibration will result in an enhanced light‐absorption ability, further reducing the coating reflectance.…”

“…As an improvement, sprayable carbon‐sphere‐based ultrablack coating material with the lowest reflectance of 0.14% has been developed very recently. [ 24 ] However, the design and fabrication of solvent‐processable ultrablack materials with comparable performance with VA‐CNT arrays still face up to a great challenge.…”

“…[ 13–17 ] Low filling ratio and small scatterer size (at least in one dimension) are necessary for realizing ultralow reflectance. [ 24 ] Notably, HCS with a very thin shell is a fairly simple solution to fulfill these two requirements simultaneously. However, colloidal spheres usually exhibit structural colors due to the short‐range order of the composing spheres.…”

“…Structural colors exhibited by different HCS coatings are analyzed on the basis of the model we proposed previously. [ 24 ] To develop wide‐band ultrablack coating material free of structural colors, CVs are fabricated through a templated co‐condensation of silica and resorcinol formaldehyde (RF) resin, followed by the subsequent carbonization and template removal. The as‐fabricated CV shows a very thin inner layer, a rough outer layer, as well as a nano‐porous interlayer.…”