Wood-based superblack

Abstract

Light is a powerful and sustainable resource, but it can be detrimental to the performance and longevity of optical devices. Materials with near-zero light reflectance, i.e. superblack materials, are sought to improve the performance of several light-centered technologies. Here we report a simple top-down strategy, guided by computational methods, to develop robust superblack materials following metal-free wood delignification and carbonization (1500 °C). Subwavelength severed cells evolve under shrinkage stresses, yielding vertically aligned carbon microfiber arrays with a thickness of ~100 µm and light reflectance as low as 0.36% and independent of the incidence angle. The formation of such structures is rationalized based on delignification method, lignin content, carbonization temperature and wood density. Moreover, our measurements indicate a laser beam reflectivity lower than commercial light stoppers in current use. Overall, the wood-based superblack material is introduced as a mechanically robust surrogate for microfabricated carbon nanotube arrays.

Fig. 1. Fabrication of superblack wood.

a–c Schematic illustration showing the structure of low-density wood (W), delignified wood (DW) and our superblack wood. d Schematic illustration showing the light reflection in bulk carbon, a cross-section of wood carbonized below 1100 °C, and superblack wood. e Our superblack wood surface under intense light illumination (~10,000 lx). The side surface gets whiteout, in contrast to the low reflection of the superblack wood cross section. f Measured light reflectance of non-processed wood carbon and our superblack wood.

Fig. 2. Morphology of traditional carbonized wood and FE simulation on their effect on light interactions.

Scanning electron microscope (SEM) images of cross section of (a) wood (W) and (b, c) carbonized wood at 1100 °C, cW. a, b The ray cells (shown in green color) are collapsed into a nearly monolithic structure. d 3D visualization of microCT scans of carbonized balsa wood. e Idealized procedure adopted to investigate carbonized wood-light interactions by FE modelling (BC: boundary conditions). Simulated light reflectance on (f) 2D vertical cylinder array with varied cell end tilt angle (g) displaying the distribution of the electric field norm for cell end tilt angle at 0° and 30°, as well as (h) cell wall thickness, (i) lumen width, and (j) fiber length.

Fig. 3. Formation of bandsaw-like carbon structures in wood.

a Photographs showing wood (W) and carbonized wood (cW) at 1500 °C. b Proposed mechanism for the formation of bandsaw-like microarrays in cW. c–g SEM images of the morphology of cW obtained at 1500 °C. c SEM image showing bandsaw-like microarrays were formed in wood cell walls, while ray cells were converted into flat structures. d Side-view SEM image showing vertically aligned bandsaw-like microarrays in wood. e, f Top-view SEM image showing the morphology and dimension of bandsaw-like microarrays. g Top-view SEM image showing carbonized vessels and ray cells. h Measured total reflectance (TR) of cW obtained at 800–1500 °C. i Simulated light reflectance of cW with flat cell walls and bandsaw-like microarrays. j Raman spectra of cW obtained at 800–1500 °C.

Fig. 4. Formation of sharp microfiber carbon array in delignified wood.

a Photographs showing delignified wood (DW) and DW carbonized at 1500 °C (cDW). b Mechanism for the formation of vertically aligned microfiber arrays in cDW. c Total light reflectance of cW and cDW. SEM images of (d) DW and (e) cDW obtained at 1500 °C. Ray cells, lumens and vessels were colored in green, indigo, and cyan, respectively. f Top-view SEM image showing the cross-section of DW carbonized at 1500 °C. g, h Side-view SEM images showing the morphology and dimension of vertically aligned carbon microfiber arrays. i Total light reflectance (at λ = 700 nm) of cDW and lignin content of DW as a function of delignification time. j Simulated light reflectance of cW compared to cDW microfiber arrays with varied inter-fiber porosity. k Total light reflectance (TR, at λ = 700 nm) of cW and cDW as a function of balsa wood density.

Fig. 5. Opacity, angle dependency and practical applications of superblack wood.

a Total transmittance of cDW through the longitudinal direction (tree-growth direction) with thicknesses of ~3 and 7 mm. b Specular reflectance of cDW using illumination from 0 to 65° with respect to the normal direction. c, d Directional reflection properties of cDW using normal illumination and observation at 90° (i), 45° (ii) and near-edge ~0° (iii). e Film of carbon nanoparticles with diameter of (i) 900 nm (3% reflection), (ii) 200 nm (4% reflection), and (iii) Superblack wood photographed under intense light illumination (~10,000 lx) in high dynamic range (HDR). f Low light scattering from superblack wood under laser (power of 100 and 1 mW). g Photographs of superblack wood with bare finger contact and immersed in water.