Objective Due to the advantage of high electrical conductivity, thermal conductivity, and superior surface area ratio, graphene has become the current research focus in the application of flexible energy storage and sensor. Comparing the chemical vapor deposition, mechanical exfoliation, and epitaxial growth, the direct reduction of graphene oxide (GO) can satisfy the demand for graphene production in the industrial field. Currently, the methods of GO reduction are chemical, thermal, and photon reductions. Based on reduction efficiency and cost-benefit, laser irradiation is an efficient way to remove the surface oxygen group for GO reduction without special physical and chemical conditions. Thus, laser reduction can be considered a highly effective method for graphene production. Some study has focused on different methods of GO through laser reduction, such as KrF excimer, ultraviolet, and femtosecond laser. Despite these investigations on GO reduction, simultaneous reduction and nanopattern of GO through laser irradiation are still challenging. To further investigate the morphology and structural properties of reduced GO, this study compares the morphology of the reduced GO with different nanostructures through femtosecond laser irradiation with 1030 nm and 257 nm. Besides, the influence of different laser-induced nanostructures on the electrochemical impendence will be discussed.

Methods GO can be obtained using Hummers methods. Different from graphene, surface oxygen-containing groups located at the surface and margin of GO nanosheets improve the hydrophilia property. By the preparation of GO dispersion, spray coating was used to form a uniform GO film on the polyethylene terephthalate (PET) substrate. After that, a femtosecond laser with 1030 nm and 257 nm was irradiated on the GO surface to construct the nanostructure. The morphology and characteristics of nanostructure were compared to show the difference of GO through femtosecond laser irradiation. The all-solid-state interdigital micro-supercapacitors were constructed with the assistance of PVA/H₂SO₄ to obtain the electrochemical performance of GO by femtosecond laser.

Results and Discussions The surface ablated morphology of GO using femtosecond laser irradiation was observed. The comparison results showed that the morphology evolution in GO has not followed the linear change with an increase in the incident laser energy and pulses number (Figs. 3 and 5). Under the 1030 nm laser irradiation, the ablated region of GO occurred in the layered annular structure, resulting from energy deposition and thermal diffusion. However, a large number of nanosheets located at the ablated margin of GO were obtained by 257 nm laser irradiation. The photochemical effect plays a significant role in laser irradiation. Two surface laser-induced nanostructures were further investigated to obtain the mechanism in the morphology evolution of GO (Fig. 7). Femtosecond laser-induced periodic surface structures (LIPSSs) with a high and low spatial frequency contributes to the surface plasmon polaritons (SPPs) on the GO surface. The coupling effect of SPPs and LIPSSs can result in the formation of nanostructure by 1030 nm femtosecond laser irradiation. The photomechanical effect induced by photochemical action is the main reason for the groove nanostructure’s formation by 257 nm laser irradiation. Combined with the results of the Raman spectrum of GO (Fig. 8), the ratio of the intensity of D and 2D peaks relative to that of G peak was calculated. Thus, the 1030 nm laser irradiation is essential for improving the transformation of graphite structure from sp ³ to sp ² and removing surface oxygen-containing groups. Through the electrochemical impendence spectra (Fig. 9), the impendence spectra of different nanostructures induced by laser irradiation with 1030 nm and 257 nm display apparent distinct. The ohmic resistance value nanostructure with LIPSSs or stripe is 40 Ω, which is lower than that of the nanostructure with groove morphology. According to the test data fitting, the nanostructure with LIPSSs or stripe morphology demonstrates the process of charge transformation at the high frequency and ion diffusion at the low frequency. The results suggested that the nanostructure by femtosecond laser irradiation with 1030 nm can improve the electrochemical action of micro-supercapacitors.

Conclusions In this study, the morphology and characteristics of GO nanostructures were investigated using femtosecond laser irradiation. Under the 1030 nm laser irradiation, the interference effect of SPPs and incident laser results in the formation of stripe nanostructure with the period of subwavelength. The groove nanostructure by 257 nm laser irradiation contributes to the photochemical effect. Based on the analysis of the Raman spectra of GO by femtosecond laser irradiation, the GO reduction level by 1030 nm femtosecond laser irradiation is higher than that of GO by 257 nm laser irradiation. Compared with the results of electrochemical impendence of different nanostructures by femtosecond laser irradiation, the GO nanostructure by 1030 nm laser irradiation improves the rate of ion diffusion of electrodes and decreases the ohmic resistance. This study will strengthen the practical application of simultaneous reduction and nanopatterning of GO by femtosecond laser in microelectronic devices.