Wood-based superblack

The study addresses how to create sustainable, mechanically robust superblack materials (reflectance <0.4%) to improve light harvesting and management in applications such as optical baffles, blackbody cavities, solar energy harvesting, and detectors. Existing superblack materials, including etched Ni–P coatings, vertically aligned carbon nanotube (VACNT) arrays, and ultralow-density carbon aerogels, can achieve very low reflectance but often require energy-intensive fabrication, rely on synthetic or metallic precursors, or suffer from fragility and cohesion issues. Wood, a renewable, hierarchically structured material of vertically aligned tubular cells with high mechanical integrity, has been explored for antireflective surfaces; however, prior wood-derived carbons generally exhibit reflectance in the 1–3% range due to microscale feature dimensions and preserved anatomical elements that promote backscattering. The research hypothesizes that targeted top-down deconstruction of wood cell walls—through metal-free delignification followed by high-temperature carbonization—can convert tubular micrometric structures into subwavelength, vertically aligned microfiber arrays that enhance multiple internal reflections, suppress backscattering, and achieve superblack performance while retaining robustness and sustainability advantages.

Prior work on superblack materials includes: (i) etched electroless Ni–P microcavity surfaces achieving ~0.2–0.5% reflectance; (ii) VACNT arrays with reflectance <0.03%, but grown by catalyst-assisted CVD on metal substrates and prone to fragility and moisture sensitivity; (iii) isotropic carbon aerogels with ultralow reflectance but poor cohesion and mechanical robustness; and (iv) polymer-embedded microcavity textures attaining hemispherical reflectance down to ~0.02%, still relying on synthetic precursors. Bioinspired superblack structures occur in nature (e.g., fish, butterfly, birds) via combined pigmentary absorption and structural light trapping. Wood-based black materials leveraging the inherent cellular architecture have achieved 97–99% absorption but typically retain 1–3% reflectance due to microscale features and intact reflective anatomical elements. The distinction between black and superblack is variably defined; herein, superblack denotes reflectance <0.4%. This work aims to push wood-derived materials beyond prior limits by engineering subwavelength features via delignification and high-temperature carbonization.

Overview: A top-down process combining wood delignification and high-temperature carbonization was used, guided by finite element (FE) simulations to optimize anatomical and subwavelength features for light trapping. Materials: Balsa wood (Ochroma pyramidale) with densities 90, 105, 160, 205, and 340 mg/cm³; pine (400 mg/cm³) and cedar (560 mg/cm³). Reagents included hydrogen peroxide (35 wt%), glacial acetic acid, acetone, sodium chlorite (≥80 wt%). Finite Element (FE) optical modeling: Using COMSOL Multiphysics (Wave Optics), idealized 2D arrays of straight rectangular or trapezoidal pillars modeled graphite-like optical properties. Geometric parameters represented wood anatomical elements: fiber length, cell wall thickness, lumen width, and cell end tilt. Periodic boundary conditions and plane wave illumination (350–800 nm) were used. Scenarios included variations in tilt (0–60°), wall thickness (0.7–6 µm), lumen (7.5–60 µm), and fiber length (20–200 µm). Subwavelength features were modeled with trapezoidal pillars (short base 0.2 µm, long base 1.5 µm) and heights of 30 µm (bandsaw arrays) or 100 µm (microfiber arrays), including interfiber porosity to emulate blackbody-like units. Delignification (metal-free): Wood blocks (40 × 10 × 17 mm; radial × longitudinal × tangential) were infiltrated overnight at 25 °C with an equal-volume mixture of H2O2 (35 wt%) and glacial acetic acid, then delignified at 80 °C for prescribed times. Controls used 1 wt% NaClO2 at pH 4.6 (acetic acid) at 80 °C. Post-treatment, samples were dialyzed to pH ~5, clamped to retain structure, then frozen (−20 °C or liquid N2) and freeze-dried. Surface preparation: Cross-sections were smoothed by microtome. Samples were acetone-extracted overnight to remove lipophilic extractives, rinsed with DI water, frozen (−20 °C), and microtome-smoothed to clear, flat cross-sections. Thermostabilization and carbonization: Dried at 60 °C (24 h), then oxidatively thermostabilized at 250 °C for 2 h under N2 (1 °C/min). Carbonization under N2 at 800–1500 °C (5 °C/min ramp, 1 h hold). Post-carbonization rinse: 1 M HCl, then DI water to neutral pH; final dry at 105 °C. Characterization: Lignin content by NREL/TP-510-42618 protocol (72% H2SO4 at 30 °C 1 h, dilute to ~4% and autoclave at 121 °C 1 h; UV at 205 nm for soluble fraction; ash at 525 °C 5 h). SEM (Zeiss Sigma VP, 5 kV; Ir 4 nm coating) with EDS for semi-quant; Raman (Horiba LabRAM HR, 633 nm) for D (~1350 cm⁻¹) and G (~1590 cm⁻¹) bands. X-ray micro-CT (Xradia MicroXCT-400, 1.15 µm pixel; 40 kV, 4 W; 1861 projections, 15 s exp) with reconstruction using pi2. Reflectance: Shimadzu UV-2600 with 60 mm integrating sphere at 8° incidence/hemispherical detection, BaSO4 standard. Transmittance: Shimadzu UV-2600 ISR-2600 Plus integrating sphere on ~3 and 7 mm samples. Angle-varying reflectance: halogen source with controlled incidence; collection via collimating optics into Ocean Optics spectrometer; relative spectral shapes (θ–2θ geometry). Mechanical testing: uniaxial compression parallel to grain at 0.1 mm/s after conditioning (23 °C, 50% RH). Durability: finger press (~7 kPa), compressed air (~50 kPa), tape peel (~7 kPa), linear abrasion (P2000 sandpaper, ~7 kPa). Laser beam stopper test: 515 nm, ~200 fs pulses at ~752 kHz, 1–100 mW; compared against commercial beam blocks LB1/M and LB2/M; transmission through 7 mm samples measured with power meter after beam splitter. IR imaging for photothermal response under 1 sun illumination (Abet SunLite simulator), monitored by IR camera (8–14 µm). Process logic: Step 1—Elevate carbonization temperature (>1300 °C) on untreated wood to induce bandsaw-like subwavelength arrays (sharp edges, graded RI) and reduce reflectance to ~0.9%. Step 2—Apply metal-free delignification to loosen and partially isolate cellulose microfibrils, then carbonize at 1500 °C to form vertically aligned, sparse carbon microfiber arrays (~100 µm height, nanometric tips), achieving superblack reflectance (~0.36%). FE modeling guided design choices (cell end tilt, wall thickness, lumen width, microfiber porosity/geometry).

• Elevating carbonization temperature from ≤1100 °C to 1300–1500 °C transforms wood cell walls into bandsaw-like microarrays with sharp, submicron edges, lowering cross-sectional total reflectance from ~3% to ~0.9% at 1300 °C; FE simulations corroborate the role of sharp edges and graded refractive index in reducing reflectance to <1%.
• Metal-free delignification (H2O2:CH3COOH) prior to 1500 °C carbonization yields vertically aligned carbon microfiber arrays (~100 µm height; nanometric tips), achieving superblack reflectance of ~0.36% (≈99.64% absorption), with minimal angle dependence (specular reflectance <0.5% up to 55° incidence).
• FE modeling identified key anatomical parameters: increased cell-end tilt (up to ~30°) enhances absorption; thinner walls and larger lumina reduce scattering; fiber lengths >50 µm are needed for effective multiple internal reflections.
• Chemical composition (graphitization degree by Raman D/G ratio) changes minimally from 800 to 1500 °C; improved blackness at high T mainly arises from physical nanostructuring, not chemistry.
• Near-complete delignification is critical: residual lignin >3% hinders microfibril isolation and microfiber formation, increasing reflectance; low-porosity/aggregated fibril bundles reflect more than porous, sparse assemblies (FE results).
• Wood density affects outcomes: best reflectance at ~160 kg/m³ balsa; lighter (<90 kg/m³) and denser (>340 kg/m³) balsa yield higher reflectance (~0.61–0.64%). Pine and cedar (denser, thicker cell walls) yield minima ~0.77% and ~0.85%.
• Opacity: total transmittance through ~3 mm cDW <0.02%; through 7 mm below detector noise; longitudinal structure hinders light passage.
• Laser performance: under 100 mW, scattered reflection ~30 ppm—lower than LB1/M and ~3× lower than LB2/M; at 12 mW, transmitted power along longitudinal direction <0.1%.
• Mechanical robustness: superblack wood shows compressive strength ~140 kPa (vs >450 kPa for conventional wood carbons); withstands water washing, 105 °C drying, long-term storage (23 °C/50% RH), air blow (~50 kPa), tape peeling, and finger touch (~5–7 kPa) without loss of blackness; damaged by sandpaper abrasion and high compressive loads.
• Angle independence and sustained darkness under intense illumination (~10,000 lx); absence of edge shininess unlike VACNTs.

The work resolves a key limitation of prior wood-derived black materials—microscale features and preserved reflective anatomical elements—by converting wood cell walls into subwavelength, vertically aligned carbon microfiber arrays. FE simulations established the optical roles of cellular parameters (cell-end tilt, wall thickness, lumen width, and fiber length) and guided the structural targets for effective multiple internal reflections and reduced backscattering. Experimentally, raising carbonization temperature alone induces bandsaw-like nanofeatures and graded refractive index to reduce reflectance to ~0.9%. Achieving superblack requires further deconstruction via metal-free delignification to isolate microfibrils; subsequent carbonization at 1500 °C splits and aligns them into sparse, sharp-tipped microfibers (~100 µm), dramatically suppressing reflectance to 0.36% with minimal angular dependence and negligible transmittance. The optical improvements stem primarily from physical structuring rather than increased graphitization, as evidenced by nearly constant Raman D/G ratios. The approach preserves mechanical integrity compared with ultralow-density aerogels and offers better robustness than VACNT forests while maintaining superior optical performance for practical uses such as laser beam trapping. The methodology is adaptable across wood densities and species, though optimal results were achieved with balsa around 160 kg/m³. The results highlight sustainable pathways for high-performance, angle-independent optical absorbers leveraging natural hierarchical architectures, with implications for optical instrumentation, metrology, and solar energy harvesting.

This study introduces a sustainable, mechanically robust superblack material derived from wood by coupling metal-free delignification with high-temperature carbonization. Guided by FE modeling, the process converts wood’s tubular micrometric anatomy into subwavelength, vertically aligned carbon microfiber arrays (~100 µm) that enable multiple internal reflections and minimize backscattering, achieving angle-insensitive reflectance as low as 0.36% and near-zero transmittance. The approach surpasses prior wood-based black materials and rivals state-of-the-art superblack systems while offering simpler, scalable processing and bio-based feedstocks. Future work should explore: (i) surface or near-surface carbonization to enhance yield and structural integrity while reducing bulk processing; (ii) optimization across diverse wood species and densities, with improved delignification strategies for denser woods; (iii) tailoring microfiber geometry and porosity for spectral targeting (e.g., NIR, broadband); (iv) durability enhancements against abrasion without compromising optical performance; and (v) integration into complex 3D shapes using mature wood machining for application-specific optical components.

• Abrasion sensitivity: Sandpaper abrasion and high compressive loads degrade superblackness, indicating limited resistance to severe mechanical wear.
• Species/density dependence: Optimal reflectance achieved with specific balsa densities (~160 kg/m³); denser species (pine, cedar) show higher minima (~0.77–0.85%) due to thicker walls and smaller lumina; delignification is more challenging in dense woods.
• Process sensitivity to residuals: NaClO2-based delignification leaves sodium residues that inhibit deconstruction and maintain higher reflectance (~3%), highlighting the need for metal-free processing.
• Reduced compressive strength vs conventional wood carbons: Superblack microfibers lower compressive strength (~140 kPa) compared to common wood carbons (>450 kPa), potentially limiting load-bearing applications.
• Chemical handling and processing time: Delignification requires acid/peroxide handling, temperature control, and extended washing/dialysis, which may impact scalability unless optimized.