Susanta Sinha Roy

Role of graphene/metal oxide composites as photocatalysts, adsorbents and disinfectants in water treatment: a review

With a rapidly growing population, development of new materials, techniques and devices which can provide safe potable water continues to be one of the major research emphases of the scientific community. While the development of new metal oxide catalysts is progressing, albeit at a slower pace, the concurrent and rapid development of high surface area catalyst supports such as graphene and its functionalised derivatives has provided unprecedented promise in the development of multifunctional catalysts. Recent works have shown that metal oxide/graphene composites can perform multiple roles including (but not limited to): photocatalysts, adsorbents and antimicrobial agents making them an effective agent against all major water pollutants including organic molecules, heavy metal ions and water borne pathogens, respectively. This article presents a comprehensive review on the application of metal oxide/graphene composites in water treatment and their role as photocatalyst, adsorbent and disinfectant in water remediation. Through this review, we discuss the current state of the art in metal oxide/graphene composites for water purification and also provide a comprehensive analysis of the nature of interaction of these composites with various types of pollutants which dictates their photocatalytic, adsorptive and antimicrobial activities. The review concludes with a summary on the role of graphene based materials in removal of pollutants from water and some proposed strategies for designing of highly efficient multifunctional metal oxide/graphene composites for water remediation. A brief perspective on the challenges and new directions in the area is also provided for researchers interested in designing advanced water treatment strategies using graphene based advanced materials.

Nanocrystalline ruthenium oxide dispersed Few Layered Graphene (FLG) nanoflakes as supercapacitor electrodes

Significant enhancement in supercapacitor performance was achieved via the synthesis of nanocrystalline RuO2 on vertically aligned Few Layered Graphene (FLG) nanoflakes, synthesized on bare n-type heavily doped silicon substrates by microwave plasma chemical vapour deposition. The RuO2 nanoparticles (diameter <2 nm) were deposited using a combination of low base pressure radio frequency magnetron sputtering and subsequent electrochemical cycling in acidic media. The well-dispersed RuO2 nanoparticles on FLGs achieve a specific capacitance of the order of 650 F g−1. The specific capacitance of RuO2–FLGs is significantly higher than pristine sputtered RuO2 (∼320 F g−1) and FLGs (∼6 F g−1) indicative of the synergistic effect between the FLGs and RuO2. In addition, the fabricated RuO2–FLG supercapacitors show excellent cycling capability with approximately 70% retention of initial specific capacitance over 4000 cycles at high charging–discharging rates of 500 mV s−1. The superior electrochemical performance is attributed to the good electronic conductivity of the FLGs as well as high utilization of well-dispersed RuO2 nanoparticles on FLGs.

Spectroscopic analysis of a-C and a-CNx films prepared by ultrafast high repetition rate pulsed laser deposition

The effect of nitrogen partial pressure on amorphous carbon nitride (a-CNx) (0.0⩽x⩽0.17) and laser fluence on amorphous carbon (a-C) films prepared by ultrafast high repetition rate pulsed laser deposition has been studied. The chemical bonding structure of the films was investigated by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier transform infrared (FTIR) analysis. XPS studies of films revealed an increase in the CN sites at the expense of CC bonded carbon sites as nitrogen content in the films increased. Films (a-C) prepared as a function of laser fluence showed an increase in sp3-bonded carbon as laser fluence was increased from 0.36 to 1.7J∕cm2. The ID∕IG ratio and G peak position increased as a function of nitrogen whereas the full width at half maximum (FWHM) of the G peak decreased. This is indicative of either an increase in the size or number of the sp2 sites. Films prepared as a function of laser fluence revealed a decrease of the ID∕IG ratio and an increase in the FWH...

Recent advances in thermoelectric materials and solar thermoelectric generators – a critical review

Due to the fact that much of the worlds best solar resources are inversely correlated with population centers, significant motivation exists for developing technology which can deliver reliable and autonomous conversion of sunlight into electricity. Thermoelectric generators are gaining incremental ground in this area since they do not require moving parts and work well in remote locations. Thermoelectric materials have been extensively used in space satellites, automobiles, and, more recently, in solar thermal applications as power generators, known as solar thermoelectric generators (STEG). STEG systems are gaining significant interest in both concentrated and non-concentrated systems and have been employed in hybrid configurations with solar thermal and photovoltaic systems. In this article, the key developments in the field of thermoelectric materials and on-going research work on STEG design conducted by various researchers to date are critically reviewed. Finally, we highlight the strategic research directions being undertaken to make highly efficient thermoelectric materials for developing a cost-effective STEG system, which could serve to bring this technology towards commercial readiness.

Thinning of multilayer graphene to monolayer graphene in a plasma environment.

We present a facile approach to transform multilayer graphene to single-layer graphene in a gradual thinning process. Our technique is based upon gradual etching of multilayer graphene in a hydrogen and nitrogen plasma environment. High resolution transmission microscopy, selected area electron diffraction and Raman spectroscopy confirm the transformation of multilayer graphene to monolayer graphene at a substrate temperature of ∼ 400 °C. The shift in the position of the G-band peak shows a perfect linear dependence with substrate temperature, which indicates a controlled gradual etching process. Selected area electron diffraction also confirmed the removal of functional groups from the graphene surface due to the plasma treatment. We also show that plasma treatment can be used to engineer graphene nanomesh structures.

Temperature dependent evolution of the local electronic structure of atmospheric plasma treated carbon nanotubes: Near edge x-ray absorption fine structure study

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy has been employed to obtain the temperature dependent evolution of the electronic structure of acid treated carbon nanotubes, which were further modified by dielectric barrier discharge plasma processing in an ammonia atmosphere. The NEXAFS studies were performed from room temperature up to 900°C. The presence of oxygen and nitrogen containing functional groups was observed in C K edge, N K edge, and O K edge NEXAFS spectra of the multiwalled carbon nanotubes. The N K edge spectra revealed three types of π* features, the source of which was decisively identified by their temperature dependent evolution. It was established that these features are attributed to pyridinelike, NO, and graphitelike structures, respectively. The O K edge indicated that both carbonyl (CO), π*(CO), and ether C–O–C, σ*(CO), functionalities were present. Upon heating in a vacuum to 900°C the π*(CO) resonances disappeared while the σ*(CO) resonances were still present ...

Exploring the fundamental effects of deposition time on the microstructure of graphene nanoflakes by Raman scattering and X-ray diffraction

A systematic study is reported of the growth of vertically aligned few layered graphene (FLG) nanoflakes on Si (100) substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Asymmetric grazing incident angle X-ray diffraction (GIAXRD) studies revealed a structural transformation, from nanocrystalline graphite layers to FLG, with the increase of growth time. As the growth time increased we observed a preferred vertical orientation of FLGs accompanied by a sharp decrease in the d002 spacing. Transmission electron microscopy shows these structures have highly graphitized edge planes which terminate in a few layers (1–3) of graphene sheets. Detailed Raman studies not only support the structural transformation but also confirm that the process occurs via the sudden release of stress in nanocrystalline turbostratic graphite films. Graphical plot of all major Raman parameters (such as G peak position, ID/IG value, FWHM of D, G, and G′ peaks) vs.growth time shows a well defined trend. Using the graphical plots a tentative trajectory of the Raman parameters is proposed, which can be very useful in understanding structural transformation during growth process. Finally, a possible growth mechanism of FLGs is presented.

Graphene Supported Graphone/Graphane Bilayer Nanostructure Material for Spintronics

We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp2 to sp3 hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders.

Comparative in vitro cytotoxicity study of carbon nanotubes and titania nanostructures on human lung epithelial cells

The aim of this study is to assess in vitro cytotoxic effects of titania nanostructures and carbon nanotubes (CNTs) by exposing A549 lung epithelial cell line to these materials. Titania nanotubes (TiNTs) were grown by hydrothermal treatment of TiO(2) nanoparticles, followed by annealing them at 400°C. The titania nanostructures obtained on annealing (mixture of nanotubes and nanorods) were hollow and open ended, containing 3-5 layers of titania sheets, with an internal diameter ∼3-5 nm and external diameter ∼8-10 nm, and a specific surface area of 265 m(2)/g. As-supplied single walled (SWCNTs) and microwave plasma enhanced chemical vapour deposition (MPCVD) grown multi walled carbon nanotubes (MWCNTs) were used in this study. The lengths and diameters of the SWCNTs were 5-10nm and 0.5-3 nm respectively. The lengths and diameters of the MWCNTs were 25-30 μm and 10-30 nm respectively. The cell viability was evaluated using the MTT (3-(4,-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium) assay. No significant cytotoxic effects of titania nanostructures were observed over a period of a week of testing time, while the presence of CNTs in some cases demonstrated significant cytotoxic effects. Finally, possible reason of cytotoxicity is discussed in the light of microstructures of materials.

Electronic properties of a‐CNx thin films: An x-ray-absorption and photoemission spectroscopy study

The electronic properties of amorphous carbon nitride were studied by x-ray-absorption near-edge structure (XANES) and valence-band photoelectron spectroscopy (PES). The nitrogen incorporation was found to induce graphitization, as evidenced by an increase of the sp2 cluster in C and N K-edge XANES spectra. The structure is found to be similar to pyridine. Hybridized C–N bond lengths were determined from the position of the σ* resonance of XANES spectra and the obtained results suggest sp2 hybridization. A valence-band PES spectrum showed that the p‐π band became more intense than the p‐σ band upon higher at. % nitrogen addition, which confirmed the role played by the π bonds in controlling the electronic structure of a‐CNx films.