A Strategy towards Light-Absorbing Coatings Based on Optically Black Nanoporous Alumina with Tailored Disorder.

Mikhail Pashchanka,Gennady Cherkashinin

doi:10.3390/ma14195827

Abstract

This work provides a conceptually new way of thinking about the light-absorbing mechanism in additive-free black porous anodic alumina (black PAA, or b-PAA) layers obtained via “burning” anodizing regime. The new insight into the controllable photonic effects in PAA allows the implementation of the optical blackening method based on the deliberate randomization of the initially well-ordered nanopore arrangement. The proposed black coloration mechanism rests solely on the destructive interference of light after its multiple scattering. Similar effects have been earlier considered for some natural or artificially created biomimetic structures (e.g., the so-called “moth eye effect”, or the coloration mechanism in the Neurothemis tullia dragonfly wings). Comprehensive analysis confirmed that the chemical composition of b-PAA has only a minor influence on the color changes and the optical density increase, and that the light-absorbing properties most likely result from the structural effects. The new functional 2D materials exhibit strong adhesion to aluminum surface, are cost-effective and suitable for application under harsh thermal or UV-light conditions. They are potentially useful for manufacturing of optical devices or heat-resistant coatings in aerospace technologies, as well as solid supports for biological filtration and fluorescence imaging.

Highlights

Optical and photonic properties of porous anodic alumina (PAA) layers have gained considerable attention of late [1]
This difference between the optical properties of photonic crystals and disordered photonic materials follows from the photonic band gap formation mechanism: for certain wavelengths of radiation, refractive indices, and spacing between the light scattering elements, complete cancellation of the waves reflected from different interfaces can occur in all directions, so that particular wavelengths are not transmitted by the layer [6]
We have developed a firm scientific and technological basis for electrochemical manufacturing of homogeneous optically black pigment-free alumina coatings

Summary

Introduction

Optical and photonic properties of porous anodic alumina (PAA) layers have gained considerable attention of late [1]. While the long-range ordered photonic crystals exhibit a low density of optical modes within a narrow frequency window (formation of the well-defined photonic band gap in naturally occurring PAA [4,5]), the random systems could hypothetically show a strong multiple scattering and absorb in a broader wavelengths interval (light entrapping) This difference between the optical properties of photonic crystals and disordered photonic materials follows from the photonic band gap formation mechanism: for certain wavelengths of radiation, refractive indices, and spacing between the light scattering elements, complete cancellation of the waves (destructive interference) reflected from different interfaces can occur in all directions, so that particular wavelengths are not transmitted by the layer [6]. We demonstrate that uniform large-area black coatings can be achieved using pigment-free deliberately randomized nanoporous alumina These films exhibit strong adhesion to the Al surface, are mechanically robust, cost-effective and suitable for application under harsh conditions such as elevated temperatures or irradiation by UV-light. Numerous analytical methods confirm that the light absorbing properties of b-PAA very likely arise from the photonic effects, but not from the variable non-stoichiometric composition or occasional admixture, such as carbon, sulfur, copper-related phases, or fine-dispersed inclusions of metallic aluminum which could possibly be impregnated into the growing alumina under high current densities

Materials and Methods

Findings

Conclusions