Ultra-black coatings is the new black

I’ve been 40 years discovering that the queen of all colors was black. (Pierre-Auguste Renoir)

The quest to create the perfect light absorber – the ultra-black – has been a subject of fascination since the inception of the ‘black body’ theory by Kirchhoff and the formulation of the law of black body radiation by Planck. The concept of ultra-black materials, capable of absorbing an extraordinary amount of incident light across a wide spectral range, goes beyond scholarly interest and has practical applications such as in the suppression of stray light and reflection in optical systems. The need for surfaces with extremely low reflectance, often referred to as ‘ultra-black’ surfaces is seen in applications involving high-performance optical devices and solar energy collectors.

The production of ultra-black materials has been, as always, inspired by nature. Deep-sea fishes hold the distinction of being the darkest creatures on Earth, making them the first aquatic ultra-black animals known. The skin of these fishes contains ellipsoidal melanosomes, organelles filled with melanin, arranged in a random yet continuous layer (Figure 1).

Figure 1. Skin ultrastructure and melanosome geometry of deep-sea ultra-black fishes, Oneirodes sp. (A) and Idiacanthus antrostomas (B–F). Image reproduced without modification from [1].

Ultra-black male cattleheart butterfly wings, set against black construction paper, appear even darker due to an optical illusion caused by their 3-D wing scale structures, not an excess of melanin. High-resolution scanning electron microscopy of these wings has shown the presence of deep and thick parallel ridges and pillars on their scales. These structures create a mesh-like surface that scatters and absorbs light, giving these butterflies their ultra-black appearance (Figure 2). Likewise, male birds of paradise possess ‘super black’ feathers that can absorb up to 99.95% of light. This effect results from the microscopic structure of their feathers which exhibit tightly packed filaments that bend upward, leaving deep cavities between them [2].

Figure 2. SEM structure of the ridges and pillars in the SEM images of the cattlebeart butterfly wing. Image adapted from [3].

Several methods are used to create black surfaces, including surface texturing [4], optical grating [5], etching [6], and applying coatings, the last being the simplest of methods to produce ultra-black surfaces.

Carbon, often associated with blackness (‘as black as coal’), has become the focal point of research in developing ultra-black coatings, and numerous forms of carbon have been explored for this purpose. The light absorption (and thereby the blackness) of carbon has long been known to depend upon its allotropic form [7]. Diamond, with its sp³ hybridized orbitals, has a larger bandgap than graphitic structures that have an sp² molecular structure (Figure 3). As a result, graphite possesses the capacity to absorb electromagnetic radiation across a wide range of wavelengths, making it a promising candidate for the development of ultra-black coatings. Various types of carbon structures such as nanopores, microcavities, carbon nanotubes, and forest-like graphenes have been designed to absorb an exceptionally high ratio of light energy.

Figure 3. The energy gap between the highest occupied molecular orbital (HOMO) and the least unoccupied molecular orbital (LUMO) as a function of sp² hybridization degree. Image reproduced without modification from [8].

Carbon nanotubes, essentially sheets of graphene rolled into a cylinder, have been extensively used as ultra-black materials in premium optical applications. In particular, vertically aligned carbon nanotubes (VACNT) achieve reflectance as low as 0.012% for red light [9]. It must be mentioned that the ultra-blackness of VACNT arises from both the intrinsic electromagnetic absorption of carbon nanotube (CNT) [10] and the nanostructure, which results in lower reflection at the interphase (Figure 4).

Figure 4. A schematic of the reflection and refraction of light at the interface between air and VACNT forests. In scenario (A), some light reflects, and some refracts into the higher-index materials. In (B), reflection occurs off a nanotube forest with a low density, making its effective index of refraction closer to that of air (n1). Image reproduced from [9].

In 2016, Surrey Nanosystems, a spin-out company from the University of Surrey in the UK, granted Turner Prize-winning artist Anish Kapoor exclusive rights to Vantablack, hailed as the world's ‘blackest black.’ Made of VACNT arrays, Vantablack was initially developed through the chemical vapor deposition (CVD) of CNTs on aluminum foils (Figure 5). The spacing of the tubes is such that virtually all of the light arriving at the surface enters the spaces between the tubes and is absorbed after multiple reflections between neighboring tubes. Originally intended for enhancing the performance of electro-optical imaging systems in satellites, Vantablack quickly found applications in a wide range of fields, including optics, sensing technologies, art, and design. Kapoor showcased sculptural artworks using Vantablack at the 2022 Venice Biennale, further demonstrating the material's versatility and impact on the world of art and technology.

Figure 5. SEM image of Vantablack® coating at magnifications of 2500x [11].

MIT engineers have reported on VACNT-based coatings, resembling a fuzzy forest of microscopic carbon filaments, grown on a chlorine-etched aluminum foil surface. To showcase this development, the artists and engineers at MIT collaborated on an artwork featuring a 16.78-carat natural yellow diamond from LJ West Diamonds, valued at $2 million, coated with the ultra-black CNT material. The visual impact of this innovation is striking, transforming the brilliantly faceted gem into a captivating flat, black void (Figure 6).

Figure 6. The uncoated diamond and diamond coated with VACNT [12].

Highly light-absorbing coatings based on hollow carbon nanospheres have been produced via a simple, high-performing air-spraying process with a tailored paint formulation containing the nanospheres as an absorbing pigment and a fluororesin as a binder [13]. The excellent solar absorptance of the obtained coatings was shown to result from their hierarchical nano- and microscale surface morphology, providing a refractive index gradient on the air-coating interface and 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 hollow carbon spheres (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 (Figure 7).

Figure 7. (a) Schematic of the synthesis of HCS. (b) Series of SEM images showing the surface morphology of the HCS-based coatings after their reactive ion etching of 10 min (scale bar: 500 nm).

Carbon aerogels have been studied for ultra-black coatings. Carbon aerogels are commonly synthesized from resorcinol-formaldehyde resin (RF resin) followed by CO₂ activation. Such aerogels have been found to have reflectance values less than 0.4% in a wide wavelength range. The reflectance of different coatings, which using carbon aerogels as functional pigments, ranged from 1.8% to 4.3% in the visible light region (400−760 nm), while it ranged from 1.9% to 4.2% in the near-infrared region (760–1100 nm) [14].

Apart from carbon, diverse textured structures, incorporating columns, cones, and holes, have been fabricated on Si, SiO₂, metal oxides, and polymers [15].

In a 2018 study, Si surfaces were textured using a non-cryogenic reactive ion etch with a plasma to result in a reflectance of 3% or lower in the wavelength range 300–1000 nm after an etch time of 2 min [16]. More recently, researchers have reported the use of ion beam etching to prepare wafer-scale ultra-black crystalline Si (c-Si) featuring nano/micro hybrid structures. These structures combine the advantages of nanowires and micropillars in photovoltaic cell applications, resulting in a remarkably low reflectance of less than 1% within the 600–1000 nm wavelength range. Moreover, they exhibit an impressive light absorption rate of 98.82% in the visible and near-infrared regions, spanning from 400 to 1100 nm, under AM 1.5 G illumination [17].

Super-black coatings have applications in diverse fields, from high-performance optical devices and solar energy collectors to art and design. NASA, for example, tested super-black nanotechnology on the International Space Station. Developed over six years, the super-black CNT coating was incredibly thin and uniform, made of pure carbon, and aimed at enhancing sensitive detectors’ performance in a smaller and more affordable solar coronagraph.

In a recent study, researchers aimed to create an efficient solar absorber for photothermal desalination. They utilized a novel ultra-black paint, Black 3.0, in hot-pressed melamine foam networks to develop an ultra-black evaporation device. This device exhibited exceptional solar absorption and salt-rejection capabilities, leading to a high freshwater evaporation rate of 2.48 kg m⁻² h⁻¹ under one sun (1 kW m⁻²). The interfacial solar evaporator produced 2.8 kg m⁻² of drinkable water daily, even in cloudy winter weather, and demonstrated stability in water with a pH range of 1–14 [18].

As research progresses, the potential of ultra-black materials seems endless. Future advancements may unlock even more applications, from environmental and energy solutions to space exploration and beyond. However, challenges remain, and ongoing research will be critical to address scalability, cost-effectiveness, and environmental impact. As the pursuit of ultra-black materials continues, collaboration between scientists, engineers, and even artists will be key to unlocking the full potential of these extraordinary materials.