Rajnish Dhiman

SiC nanocrystals as Pt catalyst supports for fuel cell applications

A robust catalyst support is pivotal to Proton Exchange Membrane Fuel Cells (PEMFCs) to overcome challenges such as catalyst support corrosion, low catalyst utilization and overall capital cost. SiC is a promising candidate material which could be applied as a catalyst support in PEMFCs. SiC nanocrystals are here synthesized using nano-porous carbon black (Vulcan® XC-72) as a template using two different reactions, which result in particle sizes in the ranges of 50–150 nm (SiC-SPR) and 25–35 nm (SiC-NS). Pt nano-catalysts of size 5–8 nm and 4–5 nm have successfully been uniformly deposited on the nanocrystals of SiC-SPR and SiC-NS by the polyol method. The SiC substrates are subjected to an acid treatment to introduce the surface groups, which help to anchor the Pt nano-catalysts. These SiC based catalysts have been found to have a higher electrochemical activity than commercially available Vulcan based catalysts (BASF & HISPEC). These promising results signal a new era of SiC based catalysts for fuel cell applications.

Oxygen reduction and methanol oxidation behaviour of SiC based Pt nanocatalysts for proton exchange membrane fuel cells

Research with proton exchange membrane fuel cells has demonstrated their potential as important providers of clean energy. The commercialization of this type of fuel cell needs a breakthrough in the electrocatalyst technology to reduce the relatively large amount of noble metal platinum used with the present carbon based substrates. We have recently examined suitably sized silicon carbide (SiC) particles as catalyst supports for fuel cells based on the stable chemical and mechanical properties of this material. In the present study, we have continued our work with studies of the oxygen reduction and methanol oxidation reactions of SiC supported catalysts and measured them against commercially available carbon based catalysts. The deconvolution of the hydrogen desorption signals in CV cycles shows a higher contribution of Pt (110) and Pt (111) peaks compared to Pt (100) for SiC based supports than for carbon based commercial catalysts, when HClO4 is used as an electrolyte. The Pt (110) and Pt (111) facets are shown to have higher electrochemical activities than Pt (100) facets. To the best of our knowledge, methanol oxidation studies and the comparison of peak deconvolutions of the H desorption region in CV cyclic studies are reported here for the first time for SiC based catalysts. The reaction kinetics for the oxygen reduction and for methanol oxidation with Pt/SiC are observed to be similar to the carbon based catalysts. The SiC based catalyst shows a higher specific surface activity than BASF (Pt/C) for methanol oxidation and oxygen reduction while the mass activity values are comparable.

Electrochemical Stability and Postmortem Studies of Pt/SiC Catalysts for Polymer Electrolyte Membrane Fuel Cells

In the presented work, the electrochemical stability of platinized silicon carbide is studied. Postmortem transmission electron microscopy and X-ray photoelectron spectroscopy were used to document the change in the morphology and structure upon potential cycling of Pt/SiC catalysts. Two different potential cycle aging tests were used in order to accelerate the support corrosion, simulating start-up/shutdown and load cycling. On the basis of the results, we draw two main conclusions. First, platinized silicon carbide exhibits improved electrochemical stability over platinized active carbons. Second, silicon carbide undergoes at least mild oxidation if not even silicon leaching.

Growth of SiC Nano-Whiskers on Powdered SiC

SiC nano-whiskers may become important functional elements of future micro-electro-mechanical-systems (MEMS) components and gas sensors. We have developed some relatively easy procedures to form such SiC nanostructures from saw dust. The whiskers have been characterized by Raman mcroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Growth of aluminum oxide on silicon carbide with an atomically sharp interface

The development of SiC wafers with properties suitable for electronic device fabrication is now well established commercially. A critical issue for developing metal–oxide–semiconductor field effect transistor devices of SiC is the choice of dielectric materials for surface passivation and insulating coatings. Although SiO2 grown thermally on SiC is a possibility for the gate dielectric, this system has a number of problems related to the higher band gap of SiC, which energetically favors more interface states than for SiO2 on Si, and the low dielectric constant of SiO2 leading to 2.5× higher electric fields across the oxide than in the surface of SiC, and to a premature breakdown at the higher fields and higher temperatures that SiC devices are designed to operate under. As a replacement for SiO2, amorphous Al2O3 thin film coatings have some strong advocates, both for n- and p-type SiC, due to the value of its band gap and the position of its band edges with respect to the band edges of the underlying sem...

Nanostructured Materials in Different Dimensions for Sensing Applications

Future sensing elements should be more specific, more sensitive, more reversible, and faster than today’s elements. These future sensing devices will either be integrated with suitable signal detection circuitry, typically based on Si microelectronics, or with optical signal detection, and finally interfaced to relevant state-of-the-art signal recognition hard- and software. Some of the more critical uses of sensors are in the dynamic surveillance of system parameters in complex machinery or in biological systems, such as our own bodies. Most of these demands are likely to be met by the continued rapid development of functional nanomaterials including bio-nanomaterials and biocompatible nanomaterials. A strong and increasing trend, also clear at this NATO-ASI, is the focus on using Au-dots deposited on various substrates for optical field enhancements and for other synergistic effects on electronic properties such as sheet conductivity, when deposited on polymer films or on metal oxide surfaces. Gas sensing with metal oxide surfaces is another very active area of development, where the high surface to volume ratio of thin films or nano-crystalline objects are in focus. In this report we demonstrate examples of the processing of silicon surfaces, aluminum surfaces and wooden saw dust powders to create nanostructured materials with interesting functional properties in novel types of self-limiting and self-organizing growths of one-, two- and three dimensional nano-template (i.e. nano-building block) systems, with a range of functionalities, as-formed, or after further integration. However, the focus in this report is on the growth processes and further treatments, as these are relatively new, and thus not widely known, but highly relevant for the functional properties of the resulting nanostructures, and for integration of the structures with silicon or in more complex systems.

Study of structural modification of PVA by incorporating Ag nanoparticles

Nanocomposites of PVA with Ag nanoparticles dispersed in it were synthesized using solution casting method. The morphology and size distribution of Ag nanoparticles embedded in PVA matrix were obtained by transmission electron microscopy (TEM) and Field emission scanning electron microscopy (FE-SEM). Raman spectroscopy was used to examine structural changes taking place inside polyvinyl alcohol (PVA) matrix due to incorporation of Ag nanoparticle. Raman analysis indicates that Ag nanoparticles interact with PVA through H-bonding.

Cu nanoparticles induced modification in optical and structural properties of PVA

Cu nanoparticles were synthesized in PVA matrix by chemical reduction method. Structural characterization of synthesized Cu-PVA nano composite was carried out using UV-Visible Spectroscopy, Transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Appearance of characteristic SPR peak of Cu nanoparticles at 591 nm in absorption spectra of Cu-PVA colloidal solution confirms the formation of Cu nanoparticles. Size of the Cu nanoparticles comes out to be 16 nm from TEM images. Morphology of Cu-PVA nano composite was further confirmed using SEM. With the insertion of Cu nanoparticles in PVA matrix a significant increase in the value of Urbach energy (disorder content) from 0.39 eV to 0.96 eV and a decline in optical band gap from 4.59 eV to 2.10 eV have been observed.