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

Rapid charging of your smart phones – Are we getting closer to reality?

Supercapacitors are used as an alternative power source for rechargeable batteries due to their efficient operation at high power density, long cycle life and improved safety. Nevertheless, the limited energy density, typically of the order of 5-8 Wh/L, limits their widespread use for many practical applications. Boosting capacitance and extending window of cell voltage are the available options to impart further improvement in their energy density. Researchers at University of Waterloo, Canada and Jain University, Bangalore, India have proposed a novel approach towards the development of high voltage super capacitors with high energy density (ACS Nano, 2017, 11 (10), pp 10077–10087).

Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and Tween 20 (nonionic surfactant) were mixed together to obtain a stable microemulsion with nanometer sized particles. Upon mixing it with graphene oxide (GO), the surfactant stabilized microemulsion spontaneously adsorbs on the surface of GO. This dispersion was directly casted onto copper with the formation of a dense nanocomposite film of GO/IL/Tween 20. Subsequent thermal treatment leads to the removal of IL by evaporation and reduction of GO to reduced graphene oxide (rGO). The resultant electrode is referred as IL-mediated reduced graphene oxide (IM-rGO).

Fig. 1 Schematic of fabrication of IM-rGO electrode assembly: (a) spontaneous adsorption of surfactant stabilized microemulsion particles on the surface of GO; (b) enlarged view of EMImTFSI/Tween 20/H₂O microemulsion particle; (c) film structure after drop-casting and water evaporation; and (d) film structure after evaporation of Tween 20 following thermal reduction

The surface morphology of the nanocomposite film reveals the presence of macropores (Fig. 2(a)) due to evaporation of water and Tween 20 during thermal treatment. Morphology at the cross-section indicates a layered structure (Fig. 2(b)), in which the sheets lay parallel to the current collector, thus providing a relatively high bulk density.

Fig. 2 Morphology of the IM-rGO film fabricated using 60% IL: (a) at the surface; and (b) at the cross-section

The electrochemical performance of the IM-rGO electrode fabricated using 60 wt% of IL at RT is depicted in Fig. 3. The formation of a dense film enabled a CV of 218 F/cm³. This electrode offered a maximum energy density of 45 Wh/L at a power density of 571.4 W/L and maintained a high energy density of 21.7 Wh/L at a power density as high as 6.04 kW/L at RT.

Fig. 3 Electrochemical performance of 60% IL electrodes at RT: (a) CVs and (b) GCDs for IM-rGO at RT; (c) specific capacitance at varying current density

Eliminating the macropores of the film still remains a challenge and elimination of macropores would help to achieve even higher bulk density. The easy adoptability of the proposed methodology provides new avenues for the manufacturing of large-scale supercapacitors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Zimin She et al., ACS Nano, 2017, 11 (10), pp 10077–10087