Converting Wood to Graphene by Laser Scribing

Graphene has proved its supremacy in a wide variety of applications due to its high electrical conductivity, and excellent chemical and mechanical properties. The conventional method of synthesis of graphene by chemical vapor deposition (CVD) has certain limitations such as need for high temperature, type of substrates, requirement for post-treatments such as etching and critical point drying, or aerogel formation. Laser-induced graphene (LIG) can be formed on commercial polyimide (PI) films, which is considered as an alternate method for CVD to prepare graphene. Researchers at Rice University, USA and Beihang University, China have demonstrated that LIG can be formed on the surface of wood with a high electrical conductivity. Moreover, the graphene layer can be further modified by electrodeposition of polyaniline (PANI) to make it as a supercapacitor or plate it with Co-P and Ni-Fe hydroxides to catalyze hydrogen evolution and oxygen evolution reactions (OERs).

The 3D porous LIG graphene was formed on the surface of pine wood by irradiating it with a 10.6 μm CO₂ laser under Ar or H₂ atmosphere (Fig. 1(a)). The LIG graphene is formed only on the area of the pine wood, which is scribed by the laser whereas other areas remain unchanged (inset of Fig. 1(a)). This attribute helps patterning of LIG with different shapes (Fig. 1(b)). The inert atmosphere helps in the development of graphene with a stable structure while ablation of wood in air has lead to decomposition of the lignocellulose structure. The LIGs were prepared from pine wood using 10% (1.6 W), 30% (4.1 W), 50% (6.3 W), 70% (7.8 W), and 90% (8.6 W). The LIGs prepared using pine, oak and birch woods are designated as P-LIG-x, O-LIG-x and B-LIG-x, respectively, where x signifies the power percentage of the laser.

Fig. 1 (a) Schematic illustration of the formation of LIG using CO₂ laser under Ar or H₂ atmosphere; (b) Photograph of LIG patterned with letter ‘R’ on wood.

The morphology, chemical composition, structure, porosity, crystallite size, electrical properties and I_D/I_G ratio of P-LIG show a strong dependence on the laser power. Morphological features of the pine wood, laser scribed at varying laser powers indicate the evolution of a porous structure due to the liberation of gas during laser irradiation. The pore size of the P-LIGs is decreased with an increase in laser power from 10% to 70% (Figs. 2). The thermal stability and electrical conductivity of P-LIGs are increased with an increase in laser power.

Fig. 2 Morphological features of the laser-scribed pine wood at varying powers: (a) 30%; (b) 50%; and (f) 70%.

The chemical composition of the P-LIG is changed during laser irradiation; an increase in laser power leads to a decrease in C-O content and an increase an in C-C and carboxyl contents (Fig. 3(a)). The structural changes of P-LIG with laser power are confirmed by Raman spectra and TEM. At 10% laser power, no apparent Raman signal is detected while at 30% formation of amorphous carbon is evident by the broad D peak at ≈1350 cm⁻¹ and the weak 2D peak at ≈2700 cm⁻¹. As the laser power is increased from 50% to 90%, sharpening of the D and G peaks along with an enhancement in  the intensity of 2D peak indicate the formation of the graphene structure (Fig. 3(b)).

Fig. 3 (a) Change in chemical composition derived from XPS; and (b) Raman spectra of P-LIG as a function of laser power

The predominance of amorphous carbon without a clear graphene lattice at 30% laser power and graphene carbon over 50% laser power is confirmed by TEM (Fig. 4). The crystalline size of P-LIG reach a maximum at 70% power. The large I_2D/I_G ratio indicate that P-LIG-70 has a stacking of fewest-layered graphene. An increase in laser power, in general, has resulted in the formation of graphene with desired characteristics. Nevertheless, at 90% laser power, overheating of the pine wood has lead to an inferior LIG structure.

Fig. 4 TEM images of (a) P-LIG-30; (b) P-LIG-50; and (c) P-LIG-70

Similar to pine wood, 3D porous LIG is also formed using oak and birch woods. A comparison of the characteristics of LIGs formed using pine, oak and birch woods at a laser power of 70% indicates a lower I_D/I_G ratio of 0.48 for O-LIG-70, followed by at 0.73 and 0.85 for B-LIG-70 and P-LIG-70, respectively. Since aliphatic carbon moieties are more reactive than aromatic carbon, the hemicellulose and cellulose of oak and birch woods are easily decomposed during laser irradiation, thus resulting in O-LIG-70 and B-LIG-70 with more defects. In contrast, a higher aromatic lignin content favours the  generation of P-LIG-70 with a lower degree of defects. In spite of the lignin component, the formation of LIG also depends on the inherent composite structure of wood consisting of cellulose, hemicelluloses, and lignin.

The P-LIG is converted to a supercapacitor by electrodepositing polyaniline (P-LIG-PANI). The cyclic voltammetry curves of P-LIG-PANI in the potential window of −0.2 to 0.8 V indicate two characteristic pairs of redox peaks that correspond to leucoemeraldine/emeraldine and emeraldine/ pernigraniline transition of PANI, thus confirming the pseudocapacitive characteristics of PANI upon P-LIG (Fig. 5(a)). The galvanostatic charge-discharge curves of P-LIG-PANI indicate a specific areal capacitance of ≈780 and ≈320 mF/cm² at 1 and 10 mA/cm², respectively (Fig. 5(b)).

Fig. 5 (a) CV of P-LIG-PANI and P-LIG in 1 M H₂SO₄ at a scan rate of 20 mV/s; and (b) Galvanostatic charge–discharge curves of P-LIG-PANI at varying current densities.

Electrodeposition of Co-P on P-LIG-70 (P-LIG-Co-P) shows that if can be tuned to catalyze the hydrogen and oxygen evolution reaction (HER and OER). The polarization curves of P-LIG-Co-P in 1 M KOH delivers a HER current density of ≈62 mA/cm² at 200 mV overpotential and an OER current density of ≈20 mA/cm² at 400 mV overpotential over an area of ≈0.5 cm² (Fig. 6(a)). The Tafel slopes extrapolated from the polarization curves are ≈35 and 280 mV/decade for HER and OER, respectively (Fig. 6(b)). The OER performance can be improved by electrodepositing NiFe hydroxides on P-LIG (P-LIG-NiFe). Photographic image of P-LIG-Co-P as cathode and P-LIG-NiFe as anode, powered by two 1.5 V batteries in series are shown in Fig. 6(c)).

Fig. 6 (a) HER and OER windows of P-LIG-Co-P and P-LIG-NiFe in 1 M KOH; (b) HER and OER Tafel slopes of P-LIG-Co-P and P-LIG-NiFe; and (c) Photograph of P-LIG-Co-P and P-LIG-NiFe are powered by two 1.5 V batteries in series.

By a suitable choice of materials that can be deposited on LIG, it can be tailored to suit diverse applications such as supercapacitors and water splitting systems. The methodology opens up new avenues to engineer woods surfaces for diverse electronic applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ruquan Ye et al., Laser-Induced Graphene Formation on Wood, Adv. Mater. 2017, 1702211, DOI: 10.1002/adma.201702211

Self-folding 3D graphene

Graphene finds widespread application in energy storage, sensors and flexible electronics. For most of the applications, a planar geometry of graphene would suffice while applications such as wearable electronics, biological or dispersible sensors, and actuators demand a curved and folded architectures. Since pristine graphene is highly chemically inert, achieving controlled self-folding in response to external environmental stimuli is very difficult.

Researchers at the Johns Hopkins University and Massachusetts Institute of Technology, USA have developed a method to fold and unfold monolayer graphene into ordered 3D structures so that they can be designed and fabricated in accordance with a predictable shape (Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084). The processing involves various stages including surface functionalization of graphene, transfer of the functionalized graphene on patterned Al coated on Si, shape design by photolithography, removal of unwanted graphene by oxygen plasma etching, removal of Al by chemical etching and folding of the functionalized graphene by an increase in solution temperature.

Surface functionalization of graphene

Immersion of monolayer graphene in a dilute aqueous solution of dopamine (2.0 mg/ml) at pH of 8.5 (10 mM tris-HCl) for 2 – 4 h promoted self-polymerization of dopamine, that lead to the formation of a thin layer (~5 nm) of polydopamine (PD) on the surface of graphene. Subsequently, the PD-coated graphene was immersed in a dilute aqueous solution containing amine-terminated poly(N-isopropylacrylamide) (PNIPAM) (2.0 mg/ml) at pH 8.5 (10 mM tris-HCl) at 60 °C for 3 h (Fig. 1(a)). The PD served as an intermediate active layer to graft PNIPAM on PD-coated graphene. The thermoresponsive properties of PNIPAM enables the surface functionalized graphene to behave as an ultrathin shape-changing material.

Fabrication of self-folding microstructures

A patterned sacrificial Al layer was deposited on Si. Subsequently, the PD-PNIPAM functionalized graphene was transferred onto the substrate. The functionalized graphene was patterned into various shapes by photolithography, and the graphene in unwanted areas was removed by oxygen plasma etching. The underlying Al layer was dissolved using 5 mM NaOH + 3 mM sodium dodecyl sulphate. Folding of the functionalized graphene was induced by heating the solution to 45 °C using a hot plate. Selective pinning prevented the folded structures from being washed away (Fig. 1(b)).

Fig. 1 Schematic illustration of surface functionalization, patterning, fabrication and folding process of graphene microstructures

Grafting of the thermoresponsive PNIPAM to the surface of graphene is necessary for folding; neither the pristine graphene nor the PD-graphene exhibit the self-folding behavior with an increase in temperature. Different 3D shapes, including flower, dumbbell, and box can be obtained after folding (Fig. 2(a-c)). The reversible switching behavior of PNIPAM also helps to unfold the structure by reversing the temperature from 45 °C to 25 °C (Fig. 2(d-f). Addition of a rigid polymer layer to the petals (increase in thickness up to 100 nm) reduces the adhesion between the petals and favours easy reversibility (Fig. 2(g-i)).

The flower shaped folding tends to fold its free petals toward the center and go from an open to a closed state, it is possible to encapsulate live cells within the self-folding flower. The temperature employed for cell culture (37 °C) is sufficient to induce folding of the functionalized graphene flowers to encapsulate the cells inside its petals. It is confirmed that the cells are alive after encapsulation, which suggests that the self-folding process is biocompatible and can be used to capture biological cargo.

The process is highly tunable and offer control over the self-folding nature of graphene. The extent of folding is increased with an increase in temperature from 25 °C to 45 °C while reversing the temperature enabled unfolding of the structure. Different 3D shapes such as flower, dumbbell, box, etc., can be achieved. A variety of applications such as encapsulation and delivery of cells, design and fabrication of novel electrical devices and field-effect transistors, formation of a variety of Origami and Kirigami shape-changing structures, etc. are envisioned.

Fig. 2 (a-c) Snapshots of temperature-induced self-folding of ultrathin graphene microstructures with a flower geometry; (d-i) Reversibility of the temperature-induced self-folding. The sequence of images (d, e f) shows folding and unfolding of functionalized graphene flower; The sequence of images (g, h, i) shows folding and unfolding of a flower with rigid SU8 polymer petals, with better stability and reversibility but with increased thickness (100 nm). Scale bars: (a-c) 100 µm; and (d-i) 50 µm.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084