Sustainable wood electronics by iron-catalyzed laser-induced graphitization for large-scale applications

The work addresses the need for scalable, durable, and truly sustainable electronics for applications such as smart buildings and IoT. State-of-the-art “green electronics” often rely on reassembled (nano)cellulose substrates and small disposable devices, raising concerns over energy- and chemical-intensive processing and limited durability. Wood is abundant, renewable, mechanically robust, and aesthetically valuable, but lacks intrinsic electrical conductivity and suffers damage under conventional laser-induced graphitization (LIG). The research question is whether wood surfaces can be efficiently and uniformly converted into conductive, graphitic-like structures at scale, in ambient conditions, without compromising mechanical integrity, to enable practical wood-based electronic devices.

Prior approaches to conductive wood include coatings with metal nanoparticles or carbon inks and bulk impregnation; these often compromise sustainability and/or mechanical integrity, yielding low conductivities (≤0.5 S m⁻¹). Conventional LIG on wood and cellulose requires inert atmospheres, fire retardants (e.g., boric acid), and multiple laser passes, yet still causes high ablation and cracks, with inhomogeneous structures. Femtosecond lasers can reduce thermal damage but at very slow writing speeds and still considerable ablation. Transition-metal-assisted graphitization is known to promote carbonization/graphitization in biomass. These limitations motivate a more sustainable, efficient, single-step ambient LIG process suitable for complex, anisotropic wood surfaces.

An iron-catalyzed LIG (IC-LIG) process was developed using a water-based iron–tannic acid ink inspired by iron-gall ink. Ink formulation: tannic acid (33 g) dissolved in 72 g DI water at 60 °C with stirring; glycerol (5 g) added as crack suppressant; iron(III) citrate (7 g) added as Fe source; gum arabic included for suspension stabilization. The ink was brush-applied in two thin layers to wood veneers and paper; penetration formed a 20–80 μm coating (substrate dependent), smoothing surface irregularities; samples equilibrated 12 h at 20 °C, 65% RH. Laser treatment used a commercial 10.6 μm CO₂ laser (Speedy 300, 80 W max, 35 m s⁻¹ max scan). Typical parameters: 15–30% power (≈13 W), scan speeds 150–350 mm s⁻¹ (e.g., 200–270 mm s⁻¹), image density 1000 ppi, defocus ~50 mm; single pass in ambient air. Electrical characterization used four-point probe and contactless eddy-current sheet resistance mapping over large areas (up to 100 cm²). Structural and chemical analyses included Raman spectroscopy (532 nm), optical/SEM/TEM imaging, wide-angle X-ray diffraction (WAXD), and X-ray photoelectron spectroscopy (XPS). Mechanical properties were assessed via tensile tests on native vs IC-LIG-treated veneers. Control experiments used iron-free ink (tannic acid + gum arabic + glycerol) to assess iron’s role (FTIR absorption at 10.6 μm; required multiple passes; morphology; WAXD). Devices fabricated included: strain sensors on spruce/beech veneers; flexible electrodes on thin wild cherry veneers (~450 μm); a capacitive touch panel (wild cherry veneer with patterned IC-LIG buttons connected to an Arduino + MPR121 controller); and an electroluminescent (EL) device with an IC-LIG back electrode (20×20 mm²) plus dielectric and PEDOT:PSS top electrode.

- IC-LIG achieved highly conductive graphitic-like patterns on wood and paper with a single laser pass in ambient air; conductivities up to ≥2500 S m⁻¹ reported. Large (≈100 cm²) areas showed uniform sheet resistivity by eddy-current mapping and agreed with four-point probe measurements. No significant resistivity anisotropy relative to wood fibers or scanning direction. - The iron–tannic ink protected substrates from ablation/thermal damage and enhanced absorption at the laser wavelength (10.6 μm), enabling high-quality graphitization even on thin veneers (~450 μm) without compromising mechanical properties (tensile strength retained). - Raman spectroscopy exhibited prominent G peaks (1570–1580 cm⁻¹) and characteristic D/2D features indicative of graphitic carbon; spectral features resembled materials produced at very high temperatures, despite modest laser power, consistent with localized high temperatures during lasing. - Morphology: a highly interconnected 3D porous carbon network with a distinct, denser top surface; SEM/TEM showed nanoscale features and iron-rich nanoparticles embedded in carbon. XPS/WAXD indicated presence of graphitic carbon with iron species (Fe, Fe oxides, and Fe₃C signatures) consistent with iron-catalyzed graphitization. - Mechanistic insight: iron is crucial; iron-free ink required ≥2 passes, yielded amorphous carbon foam with lower conductivity (sheet resistivity ≈60–70 Ω·cm⁻¹) and lacked nanostructures; with iron, catalyzed conversion proceeds via amorphous carbon and iron oxides to Fe₃C nanoparticles that promote graphitization. - Devices: • Strain sensors on wood showed reproducible resistance–strain correlation and durability over extensive cycling (reported beyond ~69,000 cycles) with minimal performance loss. • Flexible IC-LIG wood electrodes on ~450 μm wild cherry veneers maintained conductivity after repeated bending/twisting; robust adhesion outperformed commercial carbon inks under ultrasonic stress. • Capacitive touch panel on a decorative veneer reliably switched LEDs via self-capacitance sensing while preserving wood haptics and flexibility. • EL device using IC-LIG back electrode formed a thin (~60 μm) flexible panel with homogeneous emission; achieved up to ~85% of the brightness of an otherwise identical device using a copper-foil back electrode. Operated at 110 V, 7.75 kHz (emission onset); at 325 V and 75 Hz, emission became more uniform and shifted color from blue to light turquoise. - Metal loading was low (~5.6 wt% Fe), substantially less than typical thermo-catalytic graphitization approaches (up to ~30 wt%).

The study demonstrates that iron-catalyzed laser-induced graphitization enables single-step, ambient-atmosphere conversion of wood surfaces into uniform, highly conductive, graphitic-like carbon without degrading bulk mechanical properties. By coupling a bio-based iron–tannin ink with conventional CO₂ laser processing, the approach overcomes key drawbacks of prior LIG on wood (inert atmospheres, fire retardants, multiple passes, high ablation/cracking) while achieving large-area homogeneity and high throughput. The electrical, structural, and morphological characterizations substantiate efficient graphitization driven by iron catalysis, with porous architectures advantageous for flexible electronics. The successful fabrication of durable strain sensors, robust flexible electrodes, capacitive touch interfaces, and an EL device underscores the method’s relevance for sustainable electronics in building-scale applications and beyond, where wood’s mechanical and aesthetic properties are valued and metal substitution by biomass-derived conductors is desirable.

IC-LIG provides a sustainable, scalable route to fabricate large-area, highly conductive graphitic patterns directly on wood and paper via a single ambient laser pass using a bio-based iron–tannin ink. The process yields homogeneous, mechanically compatible conductive layers suitable for practical devices, demonstrated by durable strain sensors, flexible interconnects, capacitive touch panels, and a functional EL device. The work advances green electronics by leveraging wood as both substrate and component, minimizing processing complexity and hazardous inputs. Future research should clarify laser–matter interaction mechanisms in IC-LIG, optimize processing for higher resolution and tailored porosity, extend to diverse wood species and thicknesses, and explore integration in energy storage, structural health monitoring, and large-area human–machine interfaces.

- Mechanistic understanding remains partial; the interplay of rapid laser heating, iron speciation (Fe, Fe oxides, Fe₃C), and graphitization kinetics needs deeper in situ studies. - Some device demonstrations (e.g., EL) require relatively high operating voltages; optimizing electrode morphology and dielectric stacks could reduce driving fields. - Spatial resolution may be constrained by defocusing used to protect substrates; trade-offs between resolution and processing speed warrant further optimization. - Generalizability across all wood species, veneer thicknesses, and surface treatments requires broader validation. - Iron volatilization and distribution gradients under high local temperatures were observed; controlling metal retention and uniformity could improve consistency. - Reported cycle durability varies in text (≥20,000 vs >69,000); standardized long-term testing under varied environmental conditions (humidity, temperature) is needed for reliability assessment.