AENL – Alternative Energy and Nanotechnology

Carbon Nanotubes

At IIT Madras, we synthesise multiwalled carbon nanotubes (MWNTs), by a novel technique using catalytic decomposition of acetylene over alloy-hydride catalysts. These catalysts having high catalytic activity are prepared by hydrogen decrepitation technique.Various Zr based AB₂ and Mm (Misch metal) based AB₂/AB₃/AB₅ alloy hydrides are being used as catalysts for the growth of MWNTs. Purification of these as-grown MWNTs are being carried out by air oxidation and acid treatment, which removes the undesirable products. XRD, BET surface area measurements, TGA, SEM, TEM, and Raman spectroscopy are being employed to characterize these MWNTs.

High entropy oxides (HEOs) or multicomponent entropy stabilized oxides is a name given to a new class of metal oxides composed of five or more principal metal in equal or near equal atomic ratio. Due to high configurational entropy effect, they adopt single-phase cubic structure instead of forming multiple intermetallic phases. In the recent years, HEOs have grabbed considerable attention in the research community owing to their unique physicochemical properties such as high mechanical and thermal stability, excellent electrical and magnetic properties, resistance to oxidation, corrosion and wear etc. and these properties improve greatly when size is reduced

In this regard, we have synthesized HEO nanoparticles (Fe-, Ni-, Co-, Cr- and Al-) based in our laboratory adopting simple sol-gel autocombustion technique. We could achieve particle size of ~17 nm. For practical application of HEO nanoparticles in supercapacitors, a unique carbon-metal oxide nanohybrid is prepared by utilizing HEO nanoparticles as a cost-effective catalyst for the growth of carbon nanotubes (HEO-CNTs).

A well known chemical vapor deposition (CVD) technique is followed for growing HEO-CNTs nanocomposite and a very high yield (~15 times to catalyst) is achieved. A two-electrode supercapacitor based on HEO-CNTs symmetric electrodes and ionic liquid-based electrolyte with wide voltage window of 2.5 V is assembled and a high specific capacitance of 286 F g^-1 is attained. At the same time, high energy and power density of ~217 W h kg^-1 and ~25 kW kg^-1 is realized. Moreover, a charged supercapacitor is able to light up a commercial red-light emitting diode (LED) for more than 250 s demonstrating great potential in real world applications.

We have synthesized etched carbon nanotubes (ECNTs) using solid state pyrolysis method in our lab. Due to the etching of the tubes and increased defect density in the carbon structure it appears to be the promising material for different catalytic applications. TEM micrograph confirms the etching of the tubes shown in figure (a). Currently, we are using this material as an electrocatalyst and catalytically active support for hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) for fuel cells. Also, we are currently investigating Vitamin D sensing and lithium ion battery performance using ECNT as an active material.

Graphene

In our lab, we synthesize Graphene, a two dimensional carbon allotrope, from Griphite oxide (GO) either by a novel technique or by thermal exfoliation. GO is made by Hammer’s method. Functionalization of Graphene for different applications is being carried out. Nitrogen doping by Nitrogen plasma treatment, in-situ nitrogen doping by thermal as well as chemical exfoliation are also carried out on graphene. Hybrid Nanocomposites, like, Graphene/Metal oxide, Graphene/polymer etc. are synthesized by different novel techniques. A few layer Graphene stacks, called Graphite Nano Platelets, are synthesized by simple acid treatment. Different techniques like XRD, SEM, HRTEM, BET Surface area measurements, Raman spectroscopy, TGA /DSC and NMR are being used for characterization of Graphene as well as functionalized Graphene. Having challenging applications in Energy, Environmental and Biomedical research fields, research on its cost effective synthesis is also going on by our group.

Large scale synthesis of graphene quantum dots are prepared by chemical vapour deposition technique. Boron-doped and nitrogen-doped graphene quantum dots are prepared at low temperature using graphite oxide without the use of dialysis bag. The electrochemical lithium and sodium ion storage properties of as prepared GQD and heteroatom doped GQD are investigated. B-GQD exhibits high volumetric energy density of 537 Ah L^-1 and 214 Ah L^-1 with an average voltage of 0.43 V and 0.57 respectively for lithium ion and sodium ion batteries.

2D Atomic Layers: Beyond Graphene

Magnesium diboride (MgB₂), a binary inorganic compound composed of covalently bonded boron atoms arranged in a hexagonally packed structure with alternating intercalation of magnesium cations. A simple reaction of MgB₂ with water leads to the formation of hydrophilic boron-based nanosheets (HBNs). These nanosheets have a high affinity towards the polar solvent (water, ethylene glycol, propylene glycol, etc) which are widely used as conventional heat transfer fluids. In the field of nanofluids, material development research is mainly focussed on graphene-based nanocomposites for nanofluids preparation. However, preparing a stable nanofluid using graphene requires additional surface modifications to improve its affinity towards the polar solvent. Stable dispersion of HBNs into deionized water and ethylene glycol without any surface modification leads to a significant enhancement of 26% in thermal conductivity as compared to these base fluids.

High Entropy Alloy & High Entropy oxides

High entropy materials are the “multi-metallic cocktails” in metallurgy, which are having five or more than five metals, all in equal proportions. Due to the high configurational entropy arising from such conditions, these types of materials form a simple single-phase structure. The high entropy alloys are high strength, corrosion resistant materials which find application in several industries. However, the exploration of the functional properties of the nanostructured high entropy alloys and high entropy oxides have caught the attention of the researchers only recently. High entropy alloy nanoparticles can have multifunctional properties which can have potential applications in several technologies. A novel bottom-up approach has been developed to produce CoCrNiAlFe alloy and oxide nanoparticles with different phases. Various physical and electrochemical properties are extensively studied for applications in oxygen electrocatalysis, high energy Li-O₂ batteries, supercapacitors. Such materials are also demonstrated to be useful in the high yield growth of carbon nanotubes.

Electrospun Carbon Nanofibers

The application of electrospun nanofibers ranges from textile industry to biomedical industry. Because of the scalability and simplicity of the process, this technique is widely utilized in materials research. Electrospinning technique can be utilized to synthesize various 1D fibrous structured materials and composites with multifunctional properties and can be applied in various fields like energy storage, sensing, biomedical materials, electronics etc. In our lab, we are utilizing electrospinning technique to synthesize various metal oxide/carbon nanofiber composites by carbonizing polymer nanofibers. Electrospinning is one of the most fascinating fiber production methods. In this process, polymer fibers are produced by applying a high electric field (of the order of ~15kV) between a syringe containing polymer solution and a collector plate. Because of the high electric field, static charges will be produced on the surface of the polymer solution and those charges are attracted towards the collector plate. When the polymer solution is pushed by a syringe pump, the droplet at the tip of the syringe will travel towards the collector plate in a whipping motion, producing fibers of nanoscale. Hence the name electrostatic spinning or electrospinning. Electrospinning produces polymer fibers of diameters in micron and sub-micron range. This method is very simple to set up and scalable.

Metal Oxide Nanostructures

In our lab, we synthesis large quantities of metal oxide nanostructures by chemical vapor deposition, hydrothermal, solvothermal method etc. The method gives the feasibility of making large quantities of nanostructures. Also we synthesis the nanocomposites of carbon nanomaterials and metal oxide nanostructures by various methods like reduction method, in-situ reduction method, hydrothermal etc.

Field Emission from Carbon Nano Materials

Electron sources play an essential role in information display. They mostly use the thermionic emission mechanism, where electrons are emitted from heated filaments ( hot cathodes ). Field emission is an alternative mechanism to extract electrons. It is a quantum effect where under a sufficiently high external electric field, electrons near the Fermi level can tunnel through the energy barrier and escape to the vacuum level. A large field enhancement factor, high electrical conductivity and environmental stability are prerequisites for an efficient field emitter. For this reason, carbon nanotubes have been considered as one of the best field emitters due to their unique properties such as high aspect ratio, chemical inertness, high mechanical strength and high electrical conductivity. Several applications based on CNT emitters have been proposed in last few years including flat-panel displays, microwave tubes and lamps. Presently, we are involved in the field emission studies of the different types of the carbon nanotubes (SWNTs, MWNTs and nanocoils) grown over varried types of substrates. Figures below show the Field emission study setup which is to be housed in vacuum for measurements. The second figure shows the field emission from MWNTs.

Carbon Nano Materials for EMI Shielding

In recent years electronics field has diversified in telecommunication systems, cellular phones, high speed communication systems, military devices, wireless devices etc. Due to the increase in use of high operating frequency and band width in electronic systems, there are concerns and more chances of deterioration of the radio wave environment known as electromagnetic interference (EMI). This EMI has adverse effects on electronic equipments such as false operation due to unwanted electromagnetic waves and leakage of information in wireless telecommunications. Recently, conductive polymer nano-composites have attracted a great deal of academic and industrial interest due to their potential applications in many areas including EMI shielding.