Navneet Soin

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.

Novel “3-D spacer” all fibre piezoelectric textiles for energy harvesting applications

The piezoelectric effect in poly(vinylidene fluoride), PVDF, was discovered over four decades ago and since then, significant work has been carried out aiming at the production of high β-phase fibres and their integration into fabric structures for energy harvesting. However, little work has been done in the area of production of “true piezoelectric fabric structures” based on flexible polymeric materials such as PVDF. In this work, we demonstrate “3D spacer” technology based all-fibre piezoelectric fabrics as power generators and energy harvesters. The knitted single-structure piezoelectric generator consists of high β-phase (∼80%) piezoelectric PVDF monofilaments as the spacer yarn interconnected between silver (Ag) coated polyamide multifilament yarn layers acting as the top and bottom electrodes. The novel and unique textile structure provides an output power density in the range of 1.10–5.10 μW cm−2 at applied impact pressures in the range of 0.02–0.10 MPa, thus providing significantly higher power outputs and efficiencies over the existing 2D woven and nonwoven piezoelectric structures. The high energy efficiency, mechanical durability and comfort of the soft, flexible and all-fibre based power generator are highly attractive for a variety of potential applications such as wearable electronic systems and energy harvesters charged from the ambient environment or by human movement.

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.

Enhanced Field Emission and Improved Supercapacitor Obtained from Plasma‐Modified Bucky Paper

The surface morphology of bucky papers (BPs) made from single-walled carbon nanotubes (CNTs) is modified by plasma treatment resulting in the formation of vertical microstructures on the surface. The shapes of these structures are either pillarlike or conelike depending on whether the gas used during plasma treatment is Ar or CH(4) . A complex interplay between different factors, such as the electric field within the plasma sheath, polarization of the CNT, intertubular cohesive forces, and ion bombardment, result in the formation of these structures. The roles played by these factors are quantitatively and qualitatively analyzed. The final material is flexible, substrate-free, composite-free, made only of CNTs, and has discrete vertically aligned structures on its surface. It shows enhanced field emission and electrochemical charge-storage capabilities. The field enhancement factor is increased by 6.8 times, and the turn-on field drops by 3.5 times from an initial value of 0.35 to 0.1 V μm(-1) as a result of the treatment. The increase in Brunauer-Emmett-Teller surface area results in about a fourfold improvement in the specific capacitance of the BP electrodes. Capacitance values before and after the treatments are 75 and 290 F g(-1) , respectively. It is predicted that this controlled surface modification technique could be put to good use in several applications based on macroscopic CNT films.

Sedimentation behaviour in electrorheological fluids based on suspensions of zeolite particles in silicone oil.

Sedimentation is a known and expected shortcoming of electrorheological fluids (ERFs) due to the inherent difference in the constituent densities. The long-term sedimentation causes loss of the electrorheological phenomenon and the exploitable electromechanical and viscoelastic properties despite the presence of the stimulating electric field. In this work, we report the effect of temperature and surfactant concentration on the stability of ERFs prepared from zeolite particles and silicone oil with primary focus on the sedimentation of the particles in the ERF. As the temperature stability of the ERFs is fundamentally important, we have studied three different ERF suspensions composed of different zeolite particles, in silicone oil. These ERFs have been comparatively evaluated for their sedimentation over time, across a wide range of temperatures (-40°C to +60°C). The influence of surfactant concentration on the colloidal stability of the ERFs has also been investigated. A novel method of acoustic stirring (kHz range) on the homogenisation of the ERFs has been proposed and its effect on the sedimentation process evaluated. These results are useful for assessment of alternative suspension methods for specific applications.

Electromagnetic wave absorption properties of ternary poly(vinylidene fluoride)/magnetite nanocomposites with carbon nanotubes and graphene

Ternary nanocomposite systems of poly(vinylidene fluoride)/magnetite/carbon nanotube (PVDF/Fe3O4/CNT) and poly(vinylidene fluoride)/magnetite/graphene (PVDF/Fe3O4/GN), were prepared using high shear twin screw compounding followed by compression moulding. The electromagnetic (EM) microwave absorption properties of the nanocomposites were investigated in the frequency range of 3–10 GHz. PVDF/Fe3O4/CNT samples with the thickness d = 0.7 mm present a minimum reflection loss (RL) of −28.8 dB at 5.6 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB. PVDF/Fe3O4/GN with a thickness of 0.9 mm, presents a minimum RL of −22.6 dB at 5.4 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB as well. The excellent microwave absorption properties of both nanocomposites, in terms of minimum RL value and broad absorption bandwidth, are mainly due to the enhanced magnetic losses. The results indicate that the ternary nanocomposites studied here, can be used as an attractive candidate for EM absorption materials in diverse fields of various technological applications, not only in the frequency range 3–10 GHz, but also at frequencies 10 GHz for PVDF/Fe3O4/GN with a realistic thickness of close to 1 mm.

Low-Pressure H2, NH3 Microwave Plasma Treatment of Polytetrafluoroethylene (PTFE) Powders: Chemical, Thermal and Wettability Analysis

Functionalization of Polytetrafluoroethylene (PTFE) powders of ~6 μm particle size is carried out using low-pressure 2.45 GHz H2, NH3 microwave plasmas for various durations (2.5, 10 h) to chemically modify their surface and alter their surface energy. The X-ray Photoelectron Spectroscopy (XPS) analyses reveal that plasma treatment leads to significant defluorination (F/C atomic ratio of 1.13 and 1.30 for 10 h NH3 and H2 plasma treatments, respectively vs. 1.86 for pristine PTFE), along with the incorporation of functional polar moieties on the surface, resulting in enhanced wettability. Analysis of temperature dependent XPS revealed a loss of surface moieties above 200 °C, however, the functional groups are not completely removable even at higher temperatures (>300 °C), thus enabling the use of plasma treated PTFE powders as potential tribological fillers in high temperature engineering polymers. Ageing studies carried over a period of 12 months revealed that while the surface changes degenerate over time, again, they are not completely reversible. These functionalised PTFE powders can be further used for applications into smart, high performance materials such as tribological fillers for engineering polymers and bio-medical, bio-material applications.

Novel π-conjugated iron oxide/reduced graphene oxide nanocomposites for high performance electrochemical supercapacitors

A novel nanocomposite consisting of π-conjugated 2-aminoterephthalic acid (ATA) coated iron oxide (Fe3O4) nanoparticles and reduced graphene oxide (RGO) has been synthesized using a facile combination of wet-chemistry and low-power sonication. The ATA–Fe3O4/RGO nanocomposites exhibited a high specific capacitance of the order of 576 F g−1; significantly higher than that of pristine Fe3O4 (132 F g−1) and RGO (60 F g−1) counterparts, indicative of a synergistic effect between the ATA–Fe3O4 and RGO components. Furthermore, the maximum energy storage density was calculated to be 75 W h kg−1 (at a current density of 6 A g−1). The charging–discharging analysis showed promising long-term stability with nearly 86% retention of the capacitance after 5000 cycles. The superior capacitive behaviour of these ATA–Fe3O4/RGO nanocomposites is attributed to the synergistic effect of the π-conjugated ATA coating on Fe3O4 which enhances the pseudo-capacitive charge transfer process of Fe3O4 and works in conjunction with the surface functional groups (such as carboxylic, amino and amide) present on the RGO surface, providing enhanced double layer capacitance. Thus, the current system exploits the advantages of both the double layer capacitors and pseudocapacitors in a hybrid structure.