Electrochemically Controlled Synthesis of Few-Layer Graphene Nanosheets and its Evaluation for Applications

Abstract

In this study, few-layer graphene nanosheets (FLGNSs) have been synthesized by electrochemical intercalation followed by exfoliation technique. Three different protic electrolytes such as aq. H2SO4, HClO4 and HNO3 have been used separately. The major intercalants are 2 4 SO  , 4 ClO and 3 NO anions of different sizes, where the rate of impact to the pyrolytic graphite sheet has been monitored by varying the concentration of the electrolytes to 0.5, 1.0, 1.5 and 2.0 M in each case. The effects of sizes of the intercalants and its rate of intercalations on the as-synthesized FLGNSs have been studied. From the in-situ analyses, the exfoliation rates have been significantly increased with the increase in size as well as the concentration of the intercalants. Various physicochemical analyses on the electrochemically exfoliated FLGNSs have been performed in the colloidal as well as solid state. From the colloidal state, the exfoliated FLGNSs dimensions as well as its conductivity have been measured. The thermal stability and yield of FLGNSs flakes have been measured by TGA. The structural properties like phase, lattice spacing, dis-orderness and crystallite sizes of the as-synthesized FLGNSs have been analyzed by XRD and Raman spectroscopy. The (002) and (001) lattice planes of graphene and graphene oxide has been observed at around 24.5° and 11° (2θ) from the XRD spectra respectively. Again, the characteristics peaks at around 1345, 1590 and 2700 cm-1 corresponds to D, G and 2D bands of the FLGNSs in the Raman spectra respectively. This shows the mixture of sp2 and sp3 contents in the electrochemically exfoliated FLGNSs. The qualitative as well as quantitative analyses of the functional endowment on the FLGNSs have been performed by FTIR, XPS and UV-visible spectroscopy. The FTIR analysis depicts the presence of various hydroxylation, carboxylation and aromatic carbon structures in the FLGNSs. The quantification of the functional groups and sp2 content in the FLGNSs has been analyzed by XPS. The UV-visible spectra show the electronic transitions of π-π* and n-π* due to the presence of C=C bond in the aromatic structure and C=O, carbonyl functional groups respectively. The optical band gaps also have been measured from the Tauc plots. The morphological as well as topographical analyses have been performed by FESEM, TEM and AFM. From the FESEM, the domain sizes, agglomerations, curliness at the edges and stratified nature of FLGNSs have been shown. From the TEM analyses, the number of layers in the graphene sheets measured from the lattice fringe analysis. Again the number of layers has been analyzed by the topographic analysis performed by AFM and it varies in between 3-8 layers.
The functional application as supercapacitive performance of the as-synthesized FLGNSs have been performed by Swagelok type configured two electrode potentiostat. From the cyclic voltammetry (CV) and charge-discharge (CD) measurements, the FLGNSs synthesized from 1.5 M H2SO4 (S3), 2.0 M HClO4 (C4) and 1.0 M HNO3 (N2) electrolytic conditioned shows maximum capacitance in the respective categories. The Ragone plot shows maximum energy density of 12.35 Wh kg-1 and maximum power density of 3.01 kW kg-1 by S3 and N2 FLGNSs respectively. From the 5000 cycle CD test, it has been observed that the FLGNSs shows ~100 % stability in delivering the power performance. It attributes to the non-faradic EDLC reactions of the materials.
The FLGNSs obtained from the extreme electrolytic conditions such as 2.0 M of H2SO4 (S4), HClO4 (C4), and HNO3 (N4) are used as nano-filler in the epoxy (EF) and glass fiber/epoxy (GEF) matrixed polymer composite structures. The nano-fillers have been used 0.1 and 0.3 wt.% in the composite structures. The functional groups present in the FLGNSs act as anchoring agent to the epoxy polymer for enhancement in the mechanical properties. It has been observed that the N4 FLGNSs nano-filler show the maximum enhancement of 42.6 and 28.2 % of flexural strength in EF and GEF polymer composite structures. Similarly, the modulus has been increased to 33.5 and 57.7 % in the EF and GEF polymer composite structures. The fact is attributed to the high extent of the carboxyl-functional endowment in the N4 FLGNSs than S4 and C4 FLGNSs. Again at high concentration of nano-filler to 0.3 wt.%, the mechanical properties of the composites have been drastically reduced. The fact depicts the agglomeration of the FLGNSs in the epoxy matrix, which inhibits the homogeneous bonding in the polymer structures.