plasma and super black wood

Colour plays a huge role in the world around us. For flora and fauna, colour can serve as a tool to attract attention or to repel. In science, colour is also used in various fields, from optics to engineering. Some types of colour are much more valuable than others due to their properties. Scientists from the University of British Columbia (Vancouver, Canada) conducted experiments with plasma in an attempt to make wood less permeable to water, but their work led to an unexpected result – an ultra-black material that absorbs almost all light in the ultraviolet and visible spectrum. How exactly did this accidental discovery happen, what properties does the resulting material have, and where can it be used? We will find the answers to these questions in the scientists' report.

Research basis

Super black materials have very low reflectivity due to structural absorption of light. They have attracted considerable scientific and industrial attention due to their important applications in many fields: astronomy, photoelectronics, optical science, etc. In these applications, super black materials minimize unwanted light reflection, allowing devices to operate more accurately or efficiently. In other fields, such as art and design, the appeal of super black materials lies in their ability to create bizarre visual effects due to the enormous contrast between black and adjacent colored objects or surfaces.

In nature, this effect is also quite common, especially among birds. In birds, super-black areas have been defined as those that have less than 2% directional reflectance under normal light incidence. Reflectance values ​​of super-black spots in 32 bird species ranged from 0.045 to 1.97% with an average of 0.94% (300–700 nm).

In attempts to simulate ultra-black materials, good results have been achieved with materials containing aligned carbon nanotubes (CNTs from aligned carbon nanotube), for example, low density CNT array (0.045%), coating Vantablack (0.035%) and CNT metal foil (0.005%). The current record holder for low reflectivity material (< 0.0002%) is ion-track microtextured polymer with an anti-backscatter matrix.

The low reflectivity of materials such as Vantablack is due to the high light absorption of graphene and the ability of vertical CNT arrays to reduce surface reflectance. In the case of the low-density CNT array, its low reflectivity has been attributed to its random surface profile and the presence of a loose network of entangled nanotubes, in addition to vertically aligned nanotubes. Other structures can also be used to reduce the reflectivity of synthetic materials, including nanopores and microcavities. Even more diverse structures have been found in natural super-black materials, including the complex microstructures of bird barbs, cuticular microlens arrays in peacock spiders, and the polydisperse honeycomb configurations in butterfly wings. Structural features of butterfly wings have been used as biomimetic models for the creation of super-black polymer films. As the scientists note, this biomimetic route to creating super-black materials has the advantage that the films are thinner than known alternatives and can be produced at lower temperatures using plasma-enhanced chemical vapor deposition, rather than being grown from CNTs.

Biomimicry of natural structural material, as described above, is used everywhere. However, not all natural materials are suitable for implementing the effect of super-black color. For example, wood is used to create lightweight, rigid and durable composites, but is not a model for creating super-black materials, because even the darkest types of wood, such as ebony (Diospyros spp.) or African blackwood (Dalbergia melanoxylon) do not have structural features that reduce reflectivity.

However, there is interest in using wood in applications where blackness is beneficial, such as solar steam generation and water desalination, because wood is widely available, inexpensive, and sustainable. In these applications, wood is carbonized and retains its porous microstructure, creating a black material with a reflectivity of 3%. Creating additional porosity by micro-drilling the wood before carbonization further reduces reflectivity to 2%.

The scientists admit that they created the super-black wood by chance during undirected research into using plasma etching to “process” new microstructures onto the surfaces of basswood (Tilia americana). Scientists named this material Nxylon, a neologism created from Nyx (the Greek goddess of the night) and xylon (Greek for “woody material”).

Research results

Image #1

The first indication that it might be possible to create an ultra-black material by plasma modification of basswood surfaces was when the scientists investigated the effect of applied plasma energy on the color of the transverse surfaces, which expose the cut ends of the wood's porous cellular elements. Photographs of basswood samples illustrate a positive correlation between the applied plasma energy and the blackness of the modified wood (pictures above). At higher plasma energies (400 and 500 W), the transverse surfaces of the basswood were dark black, velvety, and much blacker than the similarly modified longitudinal surfaces.

Image #2

These visual observations are confirmed by the reflectance measurements converted to CIE lightness values ​​of the plasma-modified surfaces (graph above). The CIE lightness values ​​of the blackest cross-sections (400 or 500 W) are an order of magnitude lower (blacker) than those of the darkest wood species, ebony (21.9) and African blackwood (22.5), as well as the resulting carbonized basswood sample (17.0). In the blackest sample (500 W), subtle light lines representing thicker-walled latewood at the growth ring boundaries can be seen. These lighter bands were included in the CIE lightness measurements because the spectrophotometer spot size was 5 mm. Including the lighter latewood bands in the CIE color measurements inflated the CIE values. For example, the CIE value for an area with 2 latewood bands was 0.88, whereas for an area containing 4 growth rings it was 2.37.

Image #3

Reflectivity measurements of the 500W sample showed that it had very low UV/VIS reflectivity, averaging 0.68% (300 to 700 nm) (graph above), comparable to reflectivity values ​​reported for super black spots on 32 bird species. The graph also compares the reflectivity of plasma-modified super black wood to the reflectivity of two materials used commercially to control UV/VIS reflections:

• super black paper (Flock 55) created by depositing carbon fibers (flock) onto an adhesive base;
• Low reflective black aluminum.

The reflectivity of Super Blackwood is lower than that of aluminum black sheet steel and similar to that of Flock 55 paper, except in the range from 300 to 400 nm, where the reflectivity of Super Black Basswood is lower. Another low-reflectivity flocked paper (Flock 65) has almost the same reflectivity as the created plasma-modified (500 W) Super Blackwood. Reflectivity measurements using a dual-beam UV–Visible–NIR spectrophotometer avoided areas containing thicker latewood cells that were lighter than adjacent earlywood. However, the reflectivity measurements included beams that were also lighter, possibly overestimating the reflectivity values.

The black color and low reflectivity of plasma-modified basswood appear to be directionally dependent, which is consistent with other super-black materials that exhibit columnar structure. Further studies are needed to quantify the directionality dependence of reflectivity of plasma-modified basswood.

Image #4

The microstructure of plasma-modified basswood cross-sections was examined using scanning electron microscopy (SEM). The wood is invariably gold-plated prior to SEM to make it electrically conductive. After gold-vanadium alloy coating, the cross-section of plasma-treated (500 W) basswood retained its black color, whereas the unmodified control was golden in color (images above). The retention of black coloration in the super-black breast feather of the bird-of-paradise (Ptiloris paradiseus Swainson) after gold coating has been used as strong evidence for structural light absorption. However, the 500 W plasma-modified wood sample exhibits faint light lines aligned diagonally from right to left. These lines represent rays composed of ray parenchyma cells oriented in the radial direction (4b). The cut ends of the ray cells are present on the tangential surfaces, but not on the transverse surfaces as is the case for fibers, vessels, and vertically oriented parenchyma. Two other faint lines running from left to right are also present in the plasma-modified gold-coated wood sample (4b). The latter are rows of thick-walled, radially flattened cells that delimit the annual rings in linden trees (5a, 5b).

Image #5

The vertical orientation of the hollow cellular elements of the basswood was preserved after plasma treatment, but significant modification of the various cellular elements of the wood occurred. The plasma etched the cell walls that separate the clusters of large water-conducting pores, creating large voids on the wood surface. These large voids were separated by a network of hollow fibrous elements. The cell walls of the fibers became thinner and separated, forming conical columns. These conical columns were sometimes so thin that they became entangled on the wood surface (5e). These microstructures were absent in the wood sample carbonized at 350 °C, although carbonization made the fiber walls thinner (5f).

Image #6

Much higher carbonization temperatures (1500 °C) would be required to create the conical cell wall columns in wood. The origin of the vertically oriented columns in plasma-modified basswood is not clear from SEM, but observations in a structurally simpler and more homogeneous wood, New Zealand white pine (Dacrycarpus dacrydioides), after plasma treatment, suggest how they form (pictures above). Cross-sections of unmodified New Zealand white pine show longitudinal tracheids separated by a lignin-rich middle lamella that is thickest at the cell corners (6a, 6b). After plasma modification, thin conical columns are formed at each of the cell corners, suggesting that the columns represent lignin-rich remnants of cell corners after differential plasma etching of cellulose and lignin in cell walls (6c, 6d).

Image #7

X-ray micro-CT confirmed many of the SEM observations above. X-ray micro-CT has a larger field of view due to its resolution than SEM, however, in the 3D renderings it is possible to see how vertical columns form on the plasma-modified surfaces by comparing the cell wall morphology of the thick-walled fibers at the tree-ring boundaries with the thin-walled fibers (images above). The larger field of view provided by X-ray micro-CT allowed us to see that the microstructure of the rays and thick-walled fibers at the tree-ring boundaries was preserved to a greater extent than the thin-walled fibers and vessels (7a). High magnification 3D image of the layer shows vascular pits and fibrous columns in the low-density (0 to 0.7 mm) superficial surface area of ​​super black lime (7b).

Image #8

X-ray CT data were used to create a density profile through the plasma-modified super black wood. The results show that the modified zone extends to a depth of ≈ 1 mm, and the densities within this zone at depths of 0.1, 0.15, 0.2 mm were 0.035, 0.1, and 0.195 g/cm³ respectively (graph above). In comparison, low-density foams such as silica aerogels typically have densities in the range of 0.003 to 0.5 g/cm³.

Image #9

FTIR spectra (from Fourier-transform infrared spectroscopyi.e. Fourier transform infrared spectroscopy) of untreated and plasma-treated linden are shown above. Peaks at 898 cm^-1 (frequency of group C1 in cellulose and hemicellulose), 1160 cm^-1 (asymmetric C─O─C band in cellulose and hemicellulose), 1235 cm^-1 (syringyl nuclei in lignin and C═O in xylan), 1370 cm^-1 (CH bend₂ in cellulose and hemicellulose) and 1735 cm^-1 (C═O stretching in xylan) decreased as a result of plasma treatment. In contrast, the peak at 1600 cm^-1 (stretching of the benzene ring in lignin) became larger, and peaks at 1460 cm^-1 (CH deformation in lignin and CH bending in xylan) and 1505 cm^-1 (stretching of the benzene ring in lignin) became more distinct. These observations are consistent with previous studies showing that lignin in wood cell walls is more resistant to plasma etching than cellulose and hemicellulose.

For a more detailed look at the nuances of the study, I recommend taking a look at scientists' report.

Epilogue

In the work reviewed today, the scientists showed that plasma etching of linden cellular elements and polymer components formed a low-density wood surface with an ultra-black structural color and a reflectivity of less than 1% in the UV-Visible range. Analyses and tests showed how the ultra-black coloration was related to the applied plasma energy and the etching of the vertical fibrous elements of the linden into deep pits, lignin-rich columns, and tangled fibrils. The ultra-black surfaces retain subtle grain features resulting from partial plasma etching of thick-walled fibers or the presence of horizontally aligned ray tissues.

This method of creating super-black material by plasma modification of linden does not require a preliminary lithography stage and does not create liquid waste. In principle, super-black wood can be created by plasma modification of other diffuse-porous deciduous species of medium and low density. Scientists are confident that the method they have developed will become a new stage in the life of wood, which has become a material of the past in the opinion of many. Moreover, a cheaper and more environmentally friendly method of obtaining super-black materials will be extremely beneficial for various applications, from optics to electronics.

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