Nano Technology - Theses

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Browsing Nano Technology - Theses by Supervisor "Srikanth, V.V.S.S"
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    Few layered graphene and its composites as anode materials in Li-ion batteries and EMI shielding materials
    (University of Hyderabad, 2017-06-01) Sandeep Kumar, Marka ; Srikanth, V.V.S.S
    In this thesis, few layered graphene (FLG) and its composites are synthesized using easy and novel synthesis routes. The synthesized composites are tested as anode materials in Li ion batteries and as materials for electromagnetic interference (EMI) shielding. Two novel graphene based composites namely Co2Mo3O8/reduced graphene oxide (Co2Mo3O8/rGO) and Germanium/reduced graphene oxide (Ge/rGO) composites are synthesized and tested as anode materials in Li ion batteries while two other novel composites namely few layered graphene (FLG)/poly vinyl alcohol (PVA) and graphene wrapped magnesium oxide (G@MgO)/PVA composites are synthesized and tested as EMI shielding materials. Co2Mo3O8/rGO composite is constituted with hierarchical and submicron sized Co2Mo3O8 hexagonal nanoplatelets (50 nm thick) entrenched in between thin graphene layers. When cycled in the voltage range of 0.005-3.0 V at a current density of 60 mA/g, Co2Mo3O8/rGO composite delivered an excellent reversible specific capacity of ~954 mA h/g that corresponds to 82% capacity retention at the end of the 60th cycle which is higher than the theoretical capacity of both Co2Mo3O8 and graphene. Co2Mo3O8/rGO composite also showed excellent rate capability. Electrochemical conversion and electrochemical adsorption and desorption are found to be the primary reasons for Li storage in Co2Mo3O8/rGO composite. Excellent synergy between Co2Mo3O8 and rGO in the composite is attributed for the observed high specific capacity, long cycle life and good rate capability. When Ge/rGO composite was cycled in the voltage range of 0.005-3.0 V at a current density of 160 mA/g, it showed an excellent reversible capacity of ~539 mA h/g that corresponds to 98% capacity retention at the end of the 100th cycle. Ge/rGO composite also showed high reversible capacity retention, good rate capability, and excellent cycle life. Alloying, de-alloying and electrochemical adsorption and desorption type reaction mechanism are the primary reasons for lithium storage in germanium and graphene, respectively. In the case of EMI shielding materials, 1 mm thick FLG/PVA composite sheets containing 0.2 and 0.6 wt.% of FLG as filler are prepared using a simple casting process. A maximum electromagnetic interference (EMI) shielding effectiveness (SE) of ~19.5 dB (in the X-band, 8.2–12.4 GHz) was obtained in case of the FLG/PVA composite with 0.6 wt.% of FLG. Absorption is found to be the dominant mechanism for EMI shielding. The high EMI SE is attributed to the network-like features formed by FLG in PVA matrix. In another work, temperature (30-120 °C) dependent complex permittivity of 1 mm thick G@MgO/PVA_Coag (0.4, 0.6, 0.8, 1.0, 3.0, 10.0, and 15.0wt.%) and G@MgO/PVA_ac (0.4, and 3.0wt.% as-casted) composite samples in low frequency (20 Hz - 2 MHz), and high frequency (i.e., 8.2-12.4 GHz)ranges are measured. In both the frequency ranges, G@MgO/PVA_Coag composites performed superior dielectric properties than PVA, and G@MgO/PVA_ac composites. A strong interfacial polarization (from M” vs. log (f) plots) is observed in coagulated, and as-casted G@MgO/PVA composites. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both the composites, however, it is predominant in G@MgO/PVA_Coag composites than in G@MgO/PVA_ac composites. In the X-band, EMI SE of the G@MgO/PVA_Coag composites is greater than that of the G@MgO/PVA_ac composites at a particular weight fraction of G@MgO. At 30 and 120 °C, the average EMI SE values of ~14.2 and ~18.9 dB, respectively, are measured for 10.0 G@MgO/PVA_Coag while 15.0G@MgO/PVA_Coag composite showed EMI SE of ~20.6 and ~27.5 dB, respectively. In the case of G@MgO/PVA_Coag and G@MgO/PVA_ac composites too absorption is found to be the dominating mechanism for EMI SE.
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    Integration of solution processed novel graphenaceous materials as functional layers in dye sensitized and polymer solar cells
    (University of Hyderabad, 2019-04-01) Charanadhar, Nagavolu ; Srikanth, V.V.S.S
    Harnessing renewable solar energy through different technologies is greatly dependent on the advancement of solar grade materials’ science and engineering. Worldwide, scientists and engineers are focusing on developing novel solar cell designs which can be easily manufactured at low cost. In this context, 3rd generation (3G) solar energy technologies namely Gratzel cells or Dye Sensitized Solar Cells (DSSCs) and Bulk heterojunction organic photovoltaics (BHJ OPV) are expected to challenge the performance of Si based solar cells and compete for a significant market share in the field of next generation solar cells. These technologies gained prominence due to their low cost, light weight construction and printable nature over large area flexible substrates. This thesis work demonstrates an integration of inexpensive novel Graphenaceous Materials solution, for the above mentioned solar technologies energy harvesting, explore selection of suitable material for their energy efficient utilization and fabrication method. Initially, Graphene oxide (GO) was synthesized using a modified Hummers method and was reduced by using focused sunlight to obtain solar reduced graphene oxide (SRGO). GO and SRGO are then used as Pt free counter electrode materials in dye sensitized solar cells (DSSCs). GO and SRGO counter electrodes were prepared by a simple spray coating method to produce homogeneous electrode layers. The DSSCs with GO and SRGO counter electrodes exhibited an overall power conversion efficiencies of ~3.4 and ~4%, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy reveal that the DSSC with SRGO counter electrode exhibits higher electro-catalytic activity and lower charge transfer resistance at the electrode/electrolyte interface (in comparison to the DSSC with GO) resulting in higher conversion efficiency. Moreover, the microstructural features of SRGO are found to be suitable for its improved interaction with the liquid electrolyte and the enhanced electro-catalytic activity at its surface.
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    Synthesis and dielectric behavior of various novel metal oxide decorated reduced-graphene oxide composites
    (University of Hyderabad, 2017-04-01) Rama Krishna, Jammula ; Srikanth, V.V.S.S
    Developing novel dielectric materials has always been an interesting area in material’s research owing to their importance especially in capacitive energy storage. In this context, at the time of problem formulation for this thesis work (i.e., during the year 2012), the dielectric behavior of graphene filled polymers has attracted good attention. However, at that time there were no reported works on “dielectric behavior” of metal oxides (time tested dielectric materials) and graphene containing nanocomposites. Therefore in this thesis work elucidation of various aspects of dielectric behavior of reduced-graphene oxide (r-GO) and metal oxide containing nanocomposites has been taken up. Owing to the presence of residual oxygen functional groups and defects in r-GO, it is hypothesized that it will have a unique influence on the dielectric behavior of metal oxides and r-GO containing nanocomposites. As anticipated, a unique and strong interfacial polarization (Maxwell-Wagner polarization) was observed in metal oxide decorated r-GO nanocomposites synthesized by molecular level mixing technique which resulted in homogenous distribution of metal oxide particles on the surfaces of r-GO sheets. In this thesis work, CuO/r-GO, ZnO/r-GO, MgO/r-GO and NiO/r- GO nanocomposites were synthesized and their dielectric behavior in correlation with their morphology, crystallinity/phase and composition has been elucidated. This thesis also provides a comprehensive treatment to understand the dielectric behavior, especially the dielectric relaxation in metal oxide and r-GO containing nanocomposites. The treatment involves fitting experimental results with suitable theoretical models (for example, Havriliak- Negami relaxation model) that enable the intricate examination of physical mechanisms that controlled the dielectric behavior of the nanocomposites under consideration. Percolation effect on the dielectric permittivity was elucidated in the case of NiO/r-GO nanocomposite which exhibited a giant dielectric permittivity of 3688. This work will pave a way to understand and control the possible physical mechanisms that might take place at a very small length scales and in turn will be useful to control the dielectric behavior of graphene based nanocomposites in particular and nanocomposites, in general