Graphite Oxide And Graphite Oxide-Based Composites : Physicochemical And Electrochemical Studies
One of the major directions of research in the area of materials science is to impart multifunctionalities to materials. Carbon stands on the top of the list to provide various multifunctional materials. It exists in all dimensions, zero (fullerene), one (carbon nanotube, CNT), two (graphene) and three (graphite) dimensions are very well-known for their versatility in various studies. They are also used in various applications in nanoelectronics, polymer composites, hydrogen production and storage, intercalation materials, drug delivery, sensing, catalysis, photovoltaics etc. Electrical conductivity of carbon can be tuned from insulator (diamond) to semiconductor (graphene) to conductor (graphite) with varying band gap. The main reason for this versatility and varied properties is that carbon can be involved in different hybridizations. Graphene, a single layer of graphite has fascinated the world during the last several years culminating in a Nobel prize for Physics in 2010. The present study is an attempt to understand the physicochemical and electrochemical properties of graphite oxide and its reduced form. Graphene oxide (GO) possesses oxygen containing functional groups such as carbonyl, carboxyl and epoxy groups distributed very randomly in the extended graphene sheet which makes it ionically conducting and electrically insulating. The AFM images of single layer of graphite (graphene) obtained from micromechanical cleavage method and that of EGO are shown in figure 1. EGO is a layered material similar to graphite and can form very stable aqueous colloids over a wide pH range of 2-11. The stability of the colloid is due to electrostatic repulsive interactions between the functional groups. EGO behaves like a molecule due to its thickness (~1 nm) and like a particle due to its two dimensional nature (lateral size can vary from nm to few microns). It behaves as amphiphilic molecule having both hydrophilic and hydrophobic nature. Figure 1d shows the STM image of EGO which clearly indicates oxidized and unoxidized regions which will impart hydrophilic and hydrophobic regions respectively. Figure 1: AFM image of (a) graphene (b) EGO. STM image of (c) HOPG and (d) EGO. The present work is related to exploring EGO as a multifunctional material. Both hydrophilic and amphiphilic nature is explored for various studies. Reduced GO (rGO) is synthesized from EGO by assembling at different interfaces (solid-liquid and liquid-air) followed by reduction. Since EGO is hydrophilic, it is brought to the air-water interface with the help of a surfactant (CTAB) through electrostatic interactions. It is reduced chemically by hydrazine vapour to rGO and electrochemically by assembling EGO on gold through electrostatic interactions between EGO and amine groups of cystamine (figure 2). The reduction process is followed by AFM, UV-Visible and in-situ Raman spectroelectrochemistry. Figure 2: Schematic of EGO self assembly, cyclic voltammogram showing electrochemical reduction and schematic for in-situ Raman spectroelectrochemistry. The next section deals with composites of EGO and polymers. EGO/polyaniline (PANI) composite is formed by electrochemical polymerization under applied surface pressure. The in-situ electrochemical polymerization of aniline in the sub-phase of Langmuir-Blodgett trough under applied surface pressure in presence of EGO at the air-water interface leads to preferential orientation of PANI in the polaronic form. This is followed by electrochemistry and Raman spectroscopy. Figure 3 shows differential pulse voltammograms of EGO/PANI obtained under two different conditions. Externally polymerized sample shows three redox peaks at 0.086/0.064 V (A/A‟), 0.390/0.430 V (B/B‟) and 0.520/0.560 mV (C/C‟) which correspond to leucoemaraldine/emaraldine, quinone/hydroquinone and emaraldine/pernigraniline redox states respectively. The peak at C/C‟ vanishes when aniline is polymerized in-trough under applied surface pressure. This implies that oxidation of emaraldine to pernigraniline becomes difficult when sample is prepared in-trough. The Raman spectroscopy clearly reveals the preferential orientation of PANI in planar polaronic structure. Figure 3. Differential pulse voltammograms for EGO/PANI complex obtained through external polymerization (black) and in-trough polymerization (red). In the next part, EGO is used as a proton conducting material for polymer electrolyte membrane fuel cell (PEMFC). EGO possesses hydrophilic and hydrophobic regions similar to nafion (sulfonated tetrafluoroethylene based fluoropolymer-copolymer) and hence it can act as a good ionically conducting membrane. EGO is incorporated in poly(vinyl alcohol) (PVA) matrix and used in the present studies. The ionic conductivity increases from 10 μS cm-1 to 370 μS cm-1 when EGO content is increased from 1wt% to 7wt% in PVA matrix. Power densities of 25 and 90 mW cm-2 are obtained for PVA and PVA/EGO membranes in H2-O2 fuel cell at 40 0C respectively. In the next section, EGO is used as receptor for simultaneous electrochemical detection of heavy metal ions such as Cd, Pb, Cu and Hg with detection limit of 5 μM, 1 pM, 5 μM and 5 μM respectively. During the process it is observed that the EGO/PbO composite can give rise to detection limit of 10 nM for arsenic. Along with detection, EGO can also be used as an effective adsorbent for inorganics (metal ions) as well as organics (dye molecules). EGO behaves as good adsorbent for heavy metal ions and cationic dyes and rGO adsorbs anionic dyes effectively. Spectroscopic techniques are used to understand the interactions between adsorbent and adsorbates. The thesis is presented as follows: Chapter 1 gives general introduction about graphene and graphite oxide with particular emphasis on the latter one. Chapter 2 gives details on the experimental methods followed, along with schematics for various adsorption processes. Chapter 3 focuses on assembling EGO at interfaces (solid-liquid and liquid-air) followed by reduction with chemical and electrochemical methods. Chapter 4 explores EGO as an amphiphilic material where EGO is assembled at air-water interface with anilinium and subsequent electropolymerization to EGO/PANI composites. EGO/PVA composite is used as electrolyte for PEMFC. Chapter 5 explores EGO as receptor for heavy metal ion detection (Cd, Pb, Cu and Hg). Chapter 6 deals with EGO as adsorbent for adsorption of inorganics (metal ions) as well as organics (dye molecules). This is followed by summary and conclusions. The appendix section gives details on the studies on preparation of exfoliated graphite with various metal ion intercalation. The covalent functionalization of EGO with metal phthalocyanines and its assembly at air-water interface forms second part of the appendix. (For figures pl see the abstract pdf file)
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