STUDY ON CHEMICAL REDUCTION OF GRAPHENE OXIDE INTO GRAPHENE – THEIR PHYSICOCHEMICAL BEHAVIOR

Abstract

Over the last two decades nanoscience and nanotechnology have grown up in both fundamental research and emerging technologies. An enormous development in these areas has made intense societal impact. The physicochemical properties of nanomaterials viz. ceramics, semiconductors and metals, can be modulated due to quantum confinement and surface effects. When the size of the material is reduced either less than or equal to the magnitude of de Broglie wavelength, the quantum confinement effect starts controlling their physicochemical properties. The physicochemical properties of the nanomaterials could also be manipulated/enhanced extensively by modification/passivation of their surface. These effects have been extensively explored for their applications in wide ranging multidisciplinary areas viz. electronics, energy, environment and health. All these factors allowed fabricating nanomaterials at the atomic or molecular level to develop new nanostructures with tunable properties. These advancements have been further strengthened by the discoveries of variety of carbon nanomaterials such as fullerenes, carbon nanotubes, and graphene. In this area, a spectacular growth has taken place, contributed by various factors such as varied bonding configurations, dimensionalities, aspect ratios, chirality, tunable band gaps, and presence of surface defects/edge chemistry. After the ground breaking experiment in 2004 by K. S. Novoselov and A. K. Geim on the discovery of graphene, which fetched them to the noble prize of physics in 2010, the research on graphene has attracted global attention. This material due to its remarkable properties and tremendous potential for wide ranging applications has generated a wave of excitement among the researchers in the 21st century. It has made carbon based research as one of the most enthralling topics in the materials research. ii Graphene due to its unique optical, electrical, electronic, mechanical, thermal and electrochemical properties finds its diversified applications in sensing, field effect transistors, catalysis, light emitting diodes, NEMS, transparent conductive electrode, Li-ion batteries and supercapacitors. A number of protocols are being pursued for enhancing its physicochemical properties such as changing their morphology, doping and making its nanocomposites with metals, semiconductors and chalcogenides. A number of physical and chemical methods have been adopted for the synthesis of graphene. Chemical methods have been widely employed due to the ease of synthesis, a better control on their size and shape by carrying out desired functionalization. Among different chemical methods, chemical reduction of graphene oxide (GO) for the production of graphene has been considered to be one of the most effective approaches because of its being economical, easy manipulation and bulk production. The literature survey on these systems revealed that a large number of reducing agents have been employed to perform the reduction of GO to yield graphene. But several of these are of corrosive nature. Thus it required to develop protocols which make use of mild/environmental friendly reducing agents. It also needs to carry out a systematic exploration of the effect of different parameters for controlling their physicochemical behavior. In view of the above gaps, the present thesis work explores relatively mild/environmental friendly reducing agent(s) for the reduction of GO to produce graphene. The effect of various parameters such as concentrations of precursor(s), pH of the media considering its effect on the nucleophilicity of reducing agent(s) and heating time have been investigated systematically by performing the reduction of GO at relatively lower temperature. Based on the analysis of the physicochemical behavior, their possible applications have been suggested. iii The entire work presented in this thesis has been divided into six chapters. A brief account of the contents of these chapters has been furnished below: The first chapter presents a brief introduction on variety of nanomaterials investigated over last two decades. These nanomaterials have been categorized into ceramics, semiconductors, metallic and carbonaceous nanomaterials. A brief description of ceramics, metallic and semiconductor nanostructures about their present status has been discussed. A specific emphasis has been made on carbon nanostructures and these have been classified on the basis of their dimensionalities with a brief description of each of the categories. In the later sub section of carbon nanomaterials, graphene and its various factors that leads to the change in their physicochemical properties have been discussed more elaborately. This chapter also enumerates the objectives of the present work along with their brief outcome. The second chapter furnish experimental details of the used materials/reagents, equipment and techniques. The as-synthesized graphene nanostructures have been characterized in terms of their optical, structure, morphology, electrical and electrochemical properties by making use of advanced analytical techniques such as Ultraviolet-Visible (UV-Vis), Raman, X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), 13C solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), current-voltage (I-V) and cyclic voltammetry (CV). A brief account of the methodologies used for performing their characterization, different equations and formulae used for the data analyses have been provided. It also includes the experimental procedure used for the synthesis of GO and general procedure used for the modification of working electrode. iv The third chapter presents the synthesis of ultrathin graphene sheets (GRH-MA) employing malonic acid as a reducing agent under mild experimental conditions. Optical, infrared and Raman spectroscopy, increased C/O ratio in FESEM and TEM analyses indicate the effective reduction of GO. This observation is also supported by the XPS and 13C solid-state NMR spectra which exhibited an increase in the graphitic character. AFM analysis gives their thickness to be 0.41 ± 0.03 nm. For GRH-MA, I-V measurements showed about four orders of magnitude higher conductivity (4.4 S/cm) as compared to that of GO (3.05 x 10-4 S/cm). Under similar experimental conditions, the reduction of GO by oxalic acid (GRH-Ox) was observed to take relatively much longer heating time (9 h) and it was also indicated to be less efficient by optical, Raman spectroscopy, and I-V measurements. The efficient reduction of GO by malonic acid is thus understood by the presence of active methylene group, which makes it as an effective nucleophile. XRD of GRH-MA sheets exhibited the „d‟ spacing of 0.36 nm and annealing of this sample at a mild temperature of 300 °C (GRH-MA300) reduced the interlayer spacing to 0.35 nm suggesting the increased ordering of graphene sheet upon annealing. This observation is also supported by HRTEM analysis of GRH-MA300 which showed a similar „d‟ spacing of 0.35 ± 0.01 nm with hexagonal structure. IR analysis of this sample exhibited a significant reduction in the oxygen functionalities and I-V measurement showed more than 4-fold increase in conductivity (18.1 S/cm) as compared to that of GRH-MA (4.4 S/cm). Galvanostatic chargedischarge measurements of GRH-MA shows the high specific capacitance (Cs) value of 220 F/g at 1 A/g which is much higher to that of GO (2.5 F/g at 1 A/g). Moreover, it also shows fairly good cyclic stability for 1000 cycles of charge-discharge at 10 A/g. It also exhibited the energy density of 15.35 Wh/kg at a power density of 3947 W/kg having fairly high coulombic efficiency of 100-101%. These features clearly demonstrate its potential for energy storage applications. v The fourth chapter presents a novel one-step wet chemical approach to synthesize graphene nanoribbons (GNRs) by the reduction of GO using malonic acid as a reducing agent. The optical, XRD, HRTEM, Raman, IR, XPS and 13C NMR demonstrated the effective reduction of GO. FESEM analysis indicated the formation of curled and entangled graphene nanoribbons which is found to be 150-300 nm wide by TEM measurements. The average thickness of GNRs has been estimated by AFM at 3.3 ± 0.2 nm, which is reduced significantly to 1.1 ± 0.5 nm upon its annealing at 300 °C (GNRs-300). In the process of nucleation and growth, the intermediate(s) formed between the malonic acid and GO undergo twisting/folding involving supramolecular interactions. In the process of self assembly it yields ~ 0.15 to 1 mm long curled GNRs. 13C NMR demonstrates a significant increase in sp2 character of nanoribbons following the order: GO < GNRs < GNRs-300 as was also evidenced by the conductivity measurements which also followed the same order. GNRs exhibited high Cs value of 301 F/g at 1 A/g with good cyclic stability for 4000 charge-discharge cycles at 15 A/g, and high energy density/power density (16.84 Wh/kg/5944 W/kg) having coulombic efficiency of 100% in aqueous electrolyte demonstrating their tremendous potential as electrode material for energy storage applications. The fifth chapter has been divided into two sections. Section A reports the one-pot synthesis of N-doped graphene-Ag nanocomposites (GRH-GlyAg) involving in-situ generation of Ag nanoparticles (NPs). The simultaneous reduction of GO and Ag+ to produce GRH-GlyAg has been achieved under mild reaction conditions using environmental benign reducing agent, glycine, in aqueous medium without adding any external stabilizer. XRD and selected area diffraction (SAED) analyses revealed the presence of Ag in facecentered cubic (fcc) structure. HRTEM analysis shows the „d‟ spacing of 0.236 nm corresponding to the highest intense (111) plane of Ag matching to the fcc structure. The Nvi doping of graphene and its uniform decoration by Ag NPs (with an av. dia. of 17.5) in GRHGlyAg with relatively low surface atomic % of Ag (0.309) is evidenced by TEM and XPS analyses, respectively. Raman spectroscopy also revealed the decoration of GRH-Gly with Ag NPs resulted in the enhancement of the D and G bands by about 365%. The presence of Ag in GRH-GlyAg prevents the folding of graphene and is assigned due to the supramolecular interactions of Ag with different moieties of N. It was further evidenced by IR, 13C NMR and XPS analyses of GRH-GlyAg, which resulted in the enhancement of its surface area and electrical conductivity as compared to that of GRH-Gly. The presence of Ag NPs on GRH-Gly increased the current response in cyclic voltammetry by more than seven-fold as compared to that of GRH-Gly. These nanocomposites exhibited fairly high SERS activity for 4-aminothiophenol, employed as the probe molecule and allowed its detection at 50 nM concentration even for the fairly small sized Ag NPs used in the present work. Section B of this chapter describes the 2-aminoisobutyric acid (AIB), a derivative of glycine, mediated functionalization and reduction of GO using an environmental benign protocol for producing N-doped graphene. The reduction of GO by AIB occurs efficiently in both acidic and mild basic pH conditions. Relatively faster reduction by AIB at pH 10.5 (3 h) compared to that at pH 4.5 (7 h) has been attributed to the increased nucleophilicity of amino and carboxyl groups. The reduction of GO by AIB was also found to be more efficient than to that of glycine at both high and low pH(s). The reduction of GO to graphene by using AIB as a reducing agent at pH 10.5 is indicated by the optical and IR studies, which is further evidenced by 13C solid-state MAS NMR and XPS spectrocopy. AFM, FESEM and TEM studies exhibit the formation of graphene sheets. SAED and HRTEM analyses suggest crystalline nature of these sheets and its „d‟ spacing was estimated to be 0.38 nm, respectively. The as-synthesized N-doped graphene exhibits high vii conductivity (6.3 S/cm) as compared to that of GO (2.7 x 10-4 S/cm) suggesting the restoration of graphitic character. IR, Raman, 13C NMR and XPS suggested the functionalization of graphene with N. An increase in ID/IG ratio for GRH-AIB (1.02) compared to GO (0.89) suggests an increase in the number of smaller sp2 domains. GRHAIB exhibited high Cs value of 228 F/g at 1 A/g with fairly good cyclic stability for 1000 cycles at 10 A/g. It also exhibited high energy density value of 20.26 Wh/kg at a power density of 400 W/kg. These features suggest its potential for energy storage applications. A mechanism for the functionalization and reduction of GO by amino acids is discussed. The sixth chapter presents the conclusions arrived from third, fourth and fifth chapters. A comparison of the effect of precursors and reaction conditions on the morphology and physicochemical properties of the as-synthesized graphene nanostructures will be presented. Based on these properties their applications for energy storage devices and sensing have been suggested. The future directions for this type of work have also been proposed.