Adarsh Kaniyoor

Graphene synthesis via hydrogen induced low temperature exfoliation of graphite oxide

A novel and facile technique for the rapid synthesis of few layered graphene sheets via hydrogen induced reduction–exfoliation of graphite oxide at a low temperature of ∼200 °C is reported.

A Raman spectroscopic investigation of graphite oxide derived graphene

Graphene sheets that are now routinely obtained by the exfoliation/reduction of graphite oxide exhibit Raman spectra unlike traditional graphene systems. The general attributes of the Raman spectra of these ‘wrinkled graphene’ are first reaffirmed by evaluating the spectra of samples prepared by seven different exfoliation-reduction methods. These graphene sheets exhibit highly broadened D and G Raman bands and in addition, have a modulated bump in place of the conventional 2D (G′) band. It is shown that the high wavenumber ‘bump’ can be resolved into the conventional 2D band and several defect activated peaks such as G*, D+D′ and 2D′. The broad G band could also be deconvoluted into the actual G band and the D′ band, thereby attributing the broadening in the G band to the presence of this defect activated band. Two additional modes, named as D* at 1190 cm-1 and D** at ∼1500 cm-1 could be identified. These peculiar features in the Raman spectrum of ‘graphene’ are attributed to the highly disordered and wr...

Thermally exfoliated graphene based counter electrode for low cost dye sensitized solar cells

Graphene obtained from thermal exfoliation of graphite oxide are highly wrinkled and have large surface area. Their wrinkled nature is expected to give them excellent catalytic activity. Herein, we demonstrate the use of thermally exfoliated graphene (TEG) as cost effective electrocatalyst for the tri-iodide reduction in dye sensitized solar cells (DSSCs). X-ray diffraction, Raman and Infra red spectroscopy and electron microscopy studies confirm the defective and wrinkled nature of TEG. BET surface area measurement show a large surface area of ∼ 470 m2/g. The counter electrode was fabricated by drop casting a slurry of TEG dispersed in a Nafion:Ethanol solution on fluorine doped tin oxide (FTO) substrates. The use of Nafion prevented film “peel off,” thus ensuring a good substrate adhesion. Electrochemical impedance spectroscopy reveals that TEG had a catalytic performance comparable to that of Pt, suggesting its use as counter electrode material. As expected, the DSSC fabricated with Nafion solubilized ...

Enhanced optical limiting in functionalized hydrogen exfoliated graphene and its metal hybrids

We present the mechanism and performance of optical limiting (OL) in hydrogen exfoliated graphene (HEG), functionalized HEG (f-HEG) and its metal hybrids. At the wavelengths used, the mechanism of nonlinear absorption (NLA) involves two-photon absorption and excited state absorption in the nanosecond excitation regime, and saturable absorption in combination with two-photon absorption in the femtosecond (ultrafast) excitation regime. The role of defects in the OL performance of HEG and f-HEG is investigated with the help of their Raman spectra. OL efficiency of f-HEG is found to improve with Pt and Pd nanoparticle decoration due to an enhanced NLA, which arises mainly from interband transitions between the d band and the s–p conduction band in the metal NPs, and charge transfer between f-HEG and metal NPs. Thermally induced light scattering is negligible in these water dispersed systems.

Soft functionalization of graphene for enhanced tri-iodide reduction in dye sensitized solar cells

Anchoring functional moieties on the surface of graphene is an important step in the processing of graphene for numerous applications. In this article, the effects of two different functionalization methods on graphene viz. the conventional method involving ultrasonication in acids and a novel polyelectrolyte based method are systematically investigated. While the conventional acid treatment method makes graphene highly hydrophilic, it also causes severe damage to the graphene sheets and leads to disordered restacking. In contrast, the soft functionalization method based on polyelectrolytes preserves the morphology to a significant extent. Moreover, the presence of charges on the surface of polyelectrolyte modified graphene causes mutual repulsion between the sheets, thereby assisting in the fabrication of optically transparent films. The presence of charges also affects the electrochemical behaviour towards tri-iodide reduction, which is investigated via electrochemical impedance spectroscopy. New equivalent circuits are proposed to interpret the observed response.

Enhanced optical limiting and carrier dynamics in metal oxide-hydrogen exfoliated graphene hybrids

Hydrogen exfoliated graphene (HEG) is an interesting class of few-layer graphene, which is synthesized via hydrogen induced simultaneous exfoliation-reduction of graphite oxide. HEG exhibits strong optical limiting (OL) due to defect states arising from a large number of structural defects as well as oxygen functionalities present on its surface. Recently, we have shown that OL in HEG can be improved by simple acid functionalization, as it results in an increased number of defects. In the present study, we demonstrate that the OL performance of functionalized HEG (f-HEG) can be further improved, in both the short-pulse (nanosecond) and ultrafast (femtosecond) laser excitation regimes, using hybrids of f-HEG with transition metal oxide nanoparticles (NPs) such as CuO. The enhancement in the OL efficiency of the hybrid arises from strong nonlinear absorption in CuO NPs, which is determined mostly by interband and intraband transitions. The presence of defect states in the samples is confirmed using ultrafast pump–probe measurements, which reveal a delayed carrier relaxation due to carrier trapping by these states. Furthermore, we show that the occurrence of induced thermal scattering is minimal in these water dispersed systems, such that OL occurs predominantly due to nonlinear absorption.

Synthesis and characterization of gold graphene composite with dyes as model substrates for decolorization: a surfactant free laser ablation approach.

A facile surfactant free laser ablation mediated synthesis (LAMS) of gold-graphene composite is reported here. The material was characterized using transmission electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, powdered X-ray diffraction, Raman spectroscopy, Zeta potential measurements and UV-Visible spectroscopic techniques. The as-synthesized gold-graphene composite was effectively utilized as catalyst for decolorization of 4 important textile and laser dyes. The integration of gold nanoparticles (AuNPs) with high surface area graphene has enhanced the catalytic activity of AuNPs. This enhanced activity is attributed to the synergistic interplay of pristine golds electronic relay and π-π stacking of graphene with the dyes. This is evident when the Rhodamine B (RB) reduction rate of the composite is nearly twice faster than that of commercial citrate capped AuNPs of similar size. In case of Methylene blue (MB) the rate of reduction is 17,000 times faster than uncatalyzed reaction. This synthetic method opens door to laser ablation based fabrication of metal catalysts on graphene for improved performance without the aid of linkers and surfactants.

Hydrogen Exfoliated Graphene As Counter Electrode for Dye Sensitized Solar Cells

In the present work, we demonstrate the use of few layered Graphene sheets (HEG) synthesized by hydrogen induced exfoliation technique as a novel counter electrode material for dye sensitized solar cells (DSSCs). The presence of wrinkles and low oxygen content gives a high surface area and good electrical conductivity to HEG, making it superior to other graphenes. Electrochemical impedance spectroscopy studies were carried out to determine the catalytic activity of HEG towards tri-iodide reduction. The effect of annealing temperature on HEG counter electrode was also studied. The DSSC fabricated with HEG as counter electrode shows excellent current density, comparable to that of Pt counter electrode and exhibits a power conversion efficiency of ~ 3.6 % under one sun illumination. Keywords-Graphene; Hydrogen exfoliation; dye sensitized solar cells; counter electrode; catalytic activity

Dye Sensitized Solar Cells Based on Mesoporous Titanate and Titania Nanoparticles

Herein, we report the synthesis of mesoporous titanate and titania nanoparticles and their application in Dye sensitized solar cells (DSSCs). The samples were adequately characterized. DSSCs fabricated with titanate and titania particles had a maximum efficiency of ∼0.8% and 2.1%, respectively. The variation in solar cell parameters with increasing thickness was also investigated and the results have been discussed.