Color construction of multi-colored carbon fibers using glucose

The study addresses the long-standing challenge of imparting vivid, tunable colors to carbon fibers (CFs), which are typically black due to high crystallinity and low chemical affinity. Conventional dyeing is ineffective for CFs, necessitating alternative, structure-based coloration. Structural colors arise from light–matter interactions in micro/nanostructures and are widespread in nature. Prior approaches have used ordered photonic crystals or chemically grown periodic nanostructures, but often involve complex, solvent-dependent assembly processes and are sensitive to disorder, which lowers color saturation. This work proposes a simple, low-cost, bioinspired strategy: in-situ self-growth of carbon spheres (CSs) on CF surfaces via hydrothermal carbonization (HTC) of glucose, enabling angle-independent, robust structural coloration. The central hypothesis is that disordered, hemisphere-attached CSs with controlled average diameters can produce vivid, tunable colors through Mie scattering while forming strong interfacial adhesion to CFs for durability.

The paper reviews structural color generation using natural and synthetic materials, including SiO2, single-crystal Cu2O, ZIF-8, SnO2 inverse opals, cellulosic films, polystyrene beads, block copolymers, and acrylic copolymers, highlighting scalability and tunability. Fabrication methods such as spray synthesis, casting, dip- and spin-coating with UV polymerization, nonequilibrium assembly, electrophoretic deposition, atomic layer deposition, and optical lithography commonly rely on solvent evaporation to assemble particles. Ordered photonic crystals offer high spectral selectivity but are hampered by unavoidable disorder, which broadens spectra and reduces saturation. Inspired by natural disordered systems (random PS aggregation, geometric disorder in plasmonic nanostructures, amorphous photonic structures), researchers have leveraged controlled disorder, sometimes adding broadband absorbers (graphene nanosheets with quantum dots, polydopamine shells, eumelanin, cuttlefish ink) to suppress incoherent scattering and enhance saturation. The authors position hydrothermal, biopolymer-derived structural colors as environmentally friendly alternatives, with glucose-derived carbon as a promising, low-cost route.

Color construction was achieved via one-step hydrothermal carbonization (HTC) of glucose to form carbon spheres (CSs) in situ on CF surfaces. Key steps: (1) Pretreatment of CF fabric in acetone:ethanol:deionized water (1:1:2 v/v) for 48 h; drying at 80 °C. (2) Immersion in glucose solutions of controlled concentration: typically 7 g glucose in 70 mL water (denoted 7CC), with variants from 4 to 17 g per 70 mL (4CC–17CC). (3) Hydrothermal reaction in a 100 mL reactor: heat to 250 °C for 2 h at 3 °C min−1; natural cooling. (4) Post-treatment: immersion in aqueous ammonia (28% NH3·H2O) for 60 min, washing, and drying at 80 °C. The same protocol was applied to CF yarns (5.8 g mass). Residual solids and supernatants were separated by centrifugation; solids were washed and dried. Mechanism studies: The HTC of glucose proceeds via dehydration to 5-hydroxymethylfurfural (HMF), acid-catalyzed polymerization/condensation to polyfuranic species, and subsequent aromatization leading to nucleation and growth of carbon-rich CSs (core–shell) that adhere hemispherically to CFs. Characterization: SEM and TEM assessed CS morphology (front and side views) and core–shell features; XRD showed broad peaks (~20.6° 2θ) indicating disordered, low-graphitized CSs; FTIR identified aromatic, furan, and carbonyl functionalities; solid and solution 13C NMR tracked evolution from polyfuranic chains (110–120 and 140–150 ppm) to aromatized structures (125–130 ppm); XPS monitored increasing C/O ratios and formation of O–C–O and C=O during reaction; MALDI-TOF detected oligomeric species (m/z ~318–573). Elemental analysis indicated ~67.17 wt% carbon in CSs at 250 °C, 2 h. Optical evaluation: Reflectance spectra and K/S measurements determined characteristic wavelengths and color depth; CIE chromaticity mapping; 2D Fourier transforms of SEM images verified disorder (central bright spot without rings). Angle-resolved reflectance (30°–150°) probed angle independence. Stability tests: (i) Mechanical rubbing (ISO 105-X12) at 50 kPa for 10 cycles; extended 30 cycles for additional verification. (ii) Soaking/washing in 50 wt% acetic acid under 60 vibrations per min for 120 min. (iii) Accelerated light aging under xenon lamp at 38 °C, 47% RH, irradiance ~1.271×10^4 W m−2. Electrical surface contact resistance and tensile strength of CF bundles were measured. Simulations: FDTD with wavelength-dependent refractive index of CSs simulated reflectance for random ensembles of spheres with average diameters matching experiments (212.0, 258.7, 282.9, 308.6, 350.5 nm). Simulations assessed the effect of random size distributions (“chaos”), showing hue determined by average diameter and robustness to distribution variations, consistent with Mie scattering from individual spheres. Weaving demonstration: Multi-colored CF yarns were woven on an industrial weaving machine (plain weave) to produce a 15×15 cm fabric, validating mechanical robustness during processing.

• Multi-color CF fabrics achieved via in-situ growth of CSs from glucose by HTC; color controlled by glucose concentration through resulting CS diameter distributions. Major colors and corresponding optical features: blue (4CC), yellow (7CC), orange-red (10CC), purple (13CC), green (17CC). Characteristic reflectance wavelengths observed at ~400, 430, 470, 540, and 570 nm, respectively. - Average CS diameters correlated with color hue: ~212.0 nm (blue), 258.7 nm (yellow), 282.9 nm (orange-red), 308.6 nm (purple), 350.0 nm (green). A clear relationship between characteristic wavelength and average CS diameter was established. - Angle-independent coloration: angle-resolved reflectance showed consistent peaks as detection angle varied from 30° to 150°. - Disordered CS arrangement confirmed by 2D Fourier transform patterns (bright central spot, no rings), indicating loss of order; optical response dominated by single-particle Mie resonances. - FDTD simulations reproduced experimental reflectance trends for random ensembles, confirming Mie-scattering-driven hue determined by average sphere diameter; characteristic wavelengths remained unchanged under different random size distributions at fixed average size. - Strong interfacial adhesion: SEM side views showed hemispherical attachment of CSs to CFs; HTC processing promoted robust CS–CF interfaces. - Color homogeneity: K/S spectra measured at three positions per sample showed minimal variation, with stable K/S values at characteristic wavelengths. - Durability: After 10 cycles of mechanical rubbing at 50 kPa, subsequent soaking/washing in 50 wt% acetic acid for 120 min at 60 vib/min, and accelerated light aging at 38 °C, 47% RH, ~1.271×10^4 W m−2, K/S spectra and characteristic K/S values showed no obvious change across colors (4CC, 7CC, 10CC, 13CC, 17CC). Extended 30-cycle rubbing also maintained color stability. - Mechanical/electrical impacts: Colored CF bundles exhibited increased tensile strength and increased surface contact resistance due to CS–CF contacts. - Materials/processing advantages: Glucose served as the sole, low-cost feedstock; color variation controlled by glucose concentration under fixed time/temperature, suggesting scalability. - Chemical structure evolution verified: FTIR, NMR, XPS, and MALDI-TOF supported a mechanism from glucose through HMF and polyfuranic intermediates to aromatized carbonaceous CSs with core–shell features.

The results validate the hypothesis that disordered, in-situ-grown carbon spheres on CFs can generate vivid, angle-independent structural colors via Mie scattering, with hue dictated by the average CS diameter. By modulating glucose concentration in a one-step HTC process, CS size distributions are controlled, enabling predictable color tuning without pigments or complex templating. The hemispherical attachment and chemical evolution during HTC yield strong CS–CF interfacial interactions, conferring excellent resistance to mechanical abrasion, acidic environments, and intense light exposure. The optical modeling corroborates the experimental mechanism: single-particle resonances persist in random ensembles, making the color robust to disorder inherent in scalable fabrication. This reconciles the typical drawbacks of disorder in photonic systems by leveraging it to achieve non-iridescent, saturated colors on otherwise dye-inert CFs. The approach’s simplicity, low cost (glucose feedstock), and compatibility with industrial weaving highlight its potential relevance for colored CF-based textiles, composites, and functional devices.

This work introduces a simple, low-cost, and scalable strategy to endow carbon fibers with multi-color, angle-independent structural coloration by in-situ self-growth of carbon spheres via glucose hydrothermal carbonization. Color tuning is achieved by controlling CS average diameter through glucose concentration, with experimentally and computationally verified Mie-scattering-driven mechanisms. The colored CFs exhibit high color homogeneity and robustness against mechanical rubbing, acid immersion, and accelerated light aging, and can be processed into multi-colored fabrics on industrial weaving machines. The approach holds promise for industrial color construction of CFs and may be extendable to other high-performance fibers in future studies.