V. Ramakrishnan

Synthesis of few layer graphene by direct exfoliation of graphite and a Raman spectroscopic study

The exfoliation of graphene from pristine graphite in a liquid phase was achieved successfully via sonication followed by centrifugation method. Ultraviolet–visible (UV–vis) spectra of the obtained graphene dispersions at different exfoliation time indicated that the concentration of graphene dispersion increased markedly with increasing exfoliation time. The sheet-like morphology of the exfoliated graphene was revealed by Scanning Electron Microscopy (SEM) image. Further, the morphological change in different exfoliation time was investigated by Atomic Force Microscopy (AFM). A complete structural and defect characterization was probed using micro-Raman spectroscopic technique. The shape and position of the 2D band of Raman spectra revealed the formation of bilayer to few layer graphene. Also, Raman mapping confirmed the presence of uniformly distributed bilayer graphene sheets on the substrate.

Nonlinear optical absorption in silver nanosol

Nonlinear optical absorption in silver nanosol was investigated at selected wavelengths (456 nm, 477 nm and 532 nm) using open aperture Z-scan technique. It was observed that nature of nonlinear absorption is sensitively dependent on input fluence as well as on excitation wavelength. Besides, the present sample was found to exhibit reverse saturable absorption (RSA) and saturable absorption (SA) at these wavelengths depending on excitation fluence. RSA is attributed to enhanced absorption resulting from photochemical changes. SA observed for fluence values lower and higher than those corresponding to RSA are, respectively, attributed to plasmon bleach and saturation of RSA.

Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties

Zinc oxide/Graphene (GZ) composites with different concentrations of ZnO were successfully synthesized through simple chemical precipitation method. The X-ray diffraction pattern and the micro-Raman spectroscopic technique revealed the formation of GZ composite, and the energy dispersive X-ray spectrometry analysis showed the purity of the prepared samples. The ZnO nanoparticles decorated graphene sheets were clearly visible in the field emission scanning electron micrograph. Raman mapping was employed to analyze the homogeneity of the prepared samples. The diffuse-reflectance spectra clearly indicated that the formation of GZ composites promoted the absorption in the visible region also. The photocatalytic activity of ZnO and GZ composites was studied by the photodegradation of Methylene blue dye. The results revealed that the GZ composites exhibited a higher photocatalytic activity than pristine ZnO. Hence, we proposed a simple wet chemical method to synthesize GZ composite and its application on photocatalysis was demonstrated.

Preparation and characterization of ZnO/graphene nanocomposite for improved photovoltaic performance

Zinc oxide (ZnO) nanoparticles and ZnO/graphene (ZG) nanocomposite were synthesized via simple chemical route and its application as a photoanode for dye-sensitized solar cell (DSSC) was demonstrated. The prepared ZnO and ZG were structurally characterized by X-ray diffraction and micro-Raman techniques. The scanning electron micrograph of ZG revealed the spherical-shaped ZnO nanoparticles of particle size ~160xa0nm was anchored on the two-dimensional graphene sheets. UV–Vis absorption spectroscopy showed that the ZG nanocomposite has enriched visible light absorption. The DSSCs were fabricated using the synthesized ZnO and ZG nanocomposite as photoanode and the effect of low-cost organic dyesxa0on the photovoltaic performances of the solar cells were investigated. Comprehensive performances of ZG are better than that of ZnO-based DSSCs. The ZG DSSCs show power conversion efficiency (PCE) of 1.5 and 0.98xa0% for RB and EY sensitized electrodes, respectively. Moreover, the ZG dominates in many aspects due to the presence of graphene.

Synthesis of the graphene-ZnTiO3 nanocomposite for solar light assisted photodegradation of methylene blue

Cubic and hexagonal phase zinc titanate (ZT) nanoparticles were synthesized via simple chemical precipitation method. The graphene-zinc titanate (GZT) nanocomposites were prepared by using the synthesized ZT nanoparticles and graphene oxide as precursors. The synthesized materials were characterized by various spectroscopic techniques. The agglomerated ZT nanoparticles anchored on graphene sheets are clearly visible in the field emission scanning electron micrograph (FE-SEM) image. Raman mapping of the GZT nanocomposites revealed the homogeneity and distribution of ZT nanoparticles on the surface of graphene. The UV–visible absorption and photoluminescence spectra of the samples suggest that the GZT nanocomposites can be used as efficient photocatalysts to remove organic dye from water. The photocatalytic activity of the synthesized photocatalysts was evaluated by the photodegradation of methylene blue dye under sunlight irradiation. The enhanced absorption in the visible region of the GZT samples compared to the ZT samples played a vital role during the photocatalysis. The hexagonal phase GZT nanocomposite displayed remarkable photocatalytic activity compared to the bare ZT nanoparticles. The possible electron transfer mechanism for graphene-ZT interface during the photocatalysis process is also proposed. Furthermore, the reusability and stability tests for the prepared photocatalysts were made and reported.

Facile microwave-assisted synthesis of titanium dioxide decorated graphene nanocomposite for photodegradation of organic dyes

An efficient and facile method was adopted to prepare TiO2-graphene (TG) nanocomposites with TiO2 nanoparticles uniformly distributed on graphene. By adjusting the amount of TiO2 precursor, both high and low dense TiO2 nanoparticles on graphene were effectively attained via electrostatic attraction between graphene oxide sheets and TiO2 nanoparticles. The prepared nanocomposites were characterized by various characterization techniques. The TG nanocomposite showed an excellent activity for the photodegradation of the organic dyes such as methylene blue (MB) and rose bengal (RB) under ultra violet (UV) light irradiation. The TG nanocomposite of TG 2.5 showed better photocatalytic performance than bare TiO2 nanoparticles and other composites. The enhanced activity of the composite material is attributed to the reduction in charge recombination and interaction of organic dyes with graphene. The decrease in charge recombination was evidenced from the photoluminescence (PL) spectra. The observed results sugges...

Synthesis and characterization of Ni/Ag nanocomposite for surface enhanced Raman scattering measurement

Ni nanoflowers were successfully synthesized by wet chemical method using hydrazine hydrate as reducing agent. Ni/Ag nanocomposite was prepared by simple redox-transmetalation reaction by using the synthesized Ni nanoflowers and silver nitrate as precursors. The x-ray diffraction pattern revealed the formation of face centered cubic crystal structured Ni nanostructure and Ni/Ag nanocomposite. Scanning electron micrograph was used to study the surface morphology of Ni nanoflowers and Ni/Ag nanocomposite. The energy dispersive x-ray spectrometry analysis showed the purity of the prepared samples. The synthesized Ni/Ag nanocomposite was made into film and used as surface enhanced Raman scattering (SERS) substrate for probing the SERS signal of methylene blue molecule. The Ni/Ag nanocomposite showed significantly stronger Raman signal than the bare glass substrate.

Yellow and warm white light emitting Zn doped Y2O3 for near UV excitable phosphor converted WLED

Optical properties of Zn doped Y2O3 microsheets prepared by sol–gel combustion method have been investigated and their application in phosphor converted white LED has been examined. The formation of single phase, well crystalline cubic Y2O3 is confirmed from powder XRD results. Effective substitution of Zn in Y2O3 crystal lattice is inferred from shifting of diffraction peaks. SEM images have showed that undoped as well as Zn doped Y2O3 formed as microsheets. Doping of Zn enhanced the growth of the sheets and its length increased from 1.5 to 19xa0µm. Development of structural disorder in Y2O3 crystal structure after Zn doping and confirmation of the conserved cubic structure of Zn doped Y2O3 without any secondary phase have been revealed from micro-Raman spectra. The optical band gap of Y2O3 has been altered after Zn doping and it is found to be decreased from 5.6 to 5.22xa0eV as increasing Zn concentration. Both undoped and Zn doped Y2O3 showed a broad visible emission from blue to green region due to various defects and impurities present in it. Broad PL excitation spectrum inferred the possibility to attain the visible emission under the excitation of light with wide range of wavelength from near UV to blue region. Excitation of pure Y2O3 under near UV (375xa0nm) LED chip lead to the emission of yellow light whereas Zn doped Y2O3 emitted warm white light with color coordinate of (0.42, 0.35), colour rendering index of 77.6 and correlated color temperature (CCT) of 2840xa0K. Hence, Zn doped Y2O3 discussed in the present work can be a better replacement for various rare earth doped phosphors in the application of phosphor converted WLED (pc-WLED).

Exfoliation of high-quality graphene in volatile and nonvolatile solvents

High-quality few-layer to multilayer graphene sheets were exfoliated in N,N-dimethylformamide (DMF) and 1-propanol by simple liquid-phase exfoliation method. Effect of sonication time on the yield and quality of graphene exfoliated in DMF has been studied. The graphene synthesized in 1-propanol and DMF was characterized and compared with one another. The graphene concentration achieved after 8xa0h of sonication in DMF and 1-propanol was 115 and 69xa0µg/ml, respectively. The graphene dispersion in DMF and 1-propanol consisted of flakes with thickness of ~2.7 and ~4xa0nm, respectively. Structural quality of the graphene synthesized in both the solvents was further investigated by Raman spectroscopy and Raman intensity mapping. Graphene sheets exfoliated in DMF and 1-propanol showed the electrical conductivity of 1172 and 224xa0S/m, respectively, at room temperature. These results revealed that defect-free bilayer graphene (BLG) to few-layer graphene (FLG) and multilayer graphene (MLG) were produced in DMF and 1-propanol, respectively.