Naveen Chandrasekaran

One-Pot Synthesis of Interpenetrating Inorganic/Organic Networks of CuO/Resorcinol-Formaldehyde Aerogels: Nanostructured Energetic Materials

For many applications ranging from catalysis to sensors to energetic materials, it is desirable to produce intimate mixtures of nanoparticles. For instance, to improve the reaction rates of energetic materials, the oxidizing agent and the fuel need to be mixed as intimately as possible, ideally at the nanoscopic level. In this context, the acidity of a hydrated CuCl(2) solution reacting toward a network of CuO nanoparticles (a good oxidant) is used to induce one-pot cogelation of a nanostructured network of a resorcinol-formaldehyde resin (RF, the fuel). The resulting wet gels are dried to aerogels, and upon pyrolysis under Ar, the interpenetrating CuO/RF network undergoes a smelting reaction toward metallic Cu. Upon ignition in the open air, pure RF aerogels do not burn, while CuO/RF composites, even with substoichiometric CuO, sustain combustion, burning completely leaving only a solid residue of CuO whose role then has been that of a redox mediator through the smelting reaction.

Smelting in the age of nano: iron aerogels

Smelting of interpenetrating networks of resorcinol-formaldehyde (RF) and iron oxide (FeOx) aerogels yields porous ferromagnetic and superparamagnetic materials in monolithic form with compositions closely resembling that of pig iron.

The effect of compactness on the carbothermal conversion of interpenetrating metal oxide/resorcinol-formaldehyde nanoparticle networks to porous metals and carbides

This study establishes that the necessary and sufficient condition for efficient reaction between nanoparticles includes both high surface-to-volume ratios and high compactness. For this, a wide range of interpenetrating networks of resorcinol-formaldehyde (RF) and metal oxide (MOx, M: Fe, Co, Ni, Sn, Cu, Cr, Ti, Hf, Y, Dy) nanoparticles were synthesized via a simple one-pot process using the acidity of gelling solutions of hydrated metal ions to catalyze gelation of RF. The compactness of the nanoparticles in the dry composites is controlled by the drying method: supercritical fluid (SCF) CO2 drying affords aerogels with open skeletal frameworks, while drying under ambient pressure yields much more compact xerogels. A second independent method to impart compactness is by crosslinking the framework nanoparticles with a conformal polyurea (PUA) coating followed by drying with SCF CO2: although those materials (X-aerogels) have an open aerogel-like structure, upon heating in the 200 °C range, the conformal PUA coating melts and causes local structural collapse of the underlying framework creating macropores defined by xerogel-like walls. Depending on the chemical identity of the metal ion, pyrolysis at higher temperatures sets off carbothermal processes yielding pure metal monolithic nanostructures (up to 800 °C; cases of M; Fe, Co, Ni, Sn, Cu) or carbides (up to 1400 °C; cases of M: Cr, Ti, Hf). Irrespective of the specific chemical processes responsible for those transformations, the rate determining factor is the innate compactness of the xerogels, or the induced skeletal compactness in X-aerogels: both kind of materials react at as much as 400 °C lower temperatures than their corresponding native aerogels. By comparison, bulk (micron size) mixtures of the corresponding oxides and carbon black remained practically unreacted in the entire temperature range used for the nanoparticle networks. In addition to the significance of the RF-MOx interpenetrating networks in the design of new materials (mesoporous and macroporous monolithic metals and carbides), the effect of compactness on the activation of the carbothermal processes has important implications for process-design engineering.

Robust PEDOT films by covalent bonding to substrates using in tandem sol–gel, surface initiated free-radical and redox polymerization

Poly(3,4-ethylenedioxythiophene), PEDOT, films are used as antistatic coatings on electrically insulating substrates such as plastic and glass. A novel method for the synthesis of conducting PEDOT films on insulators relies on sol–gel chemistry to attach a di-Si(OEt)3 functionalized free radical initiator (AIBN) on oxidized surfaces, followed by: (a) attachment of 3,4-(vinylenedioxy)thiophene (VDOT: an analogue to EDOT susceptible to radical addition through its vinylenedioxy group); and, (b) oxidative (with FeCl3) co-polymerization of surface-confined VDOT with 3,4-ethylenedioxythiophene (EDOT). In conjunction with classical photolithography, the method yields thin (∼150 nm) yet dense, pinhole-free (confirmed electrochemically), hard (>6H), extremely adhesive (5B), patterned, highly conducting (52 mho cm−1) films. The process is applied mainly on glass but it works equally well on oxidized metal surfaces (aluminum, steel, Pt). Control studies related to “grafting from” with surface-confined AIBN were conducted by growing inexpensive poly(styrene) and poly(methylmethacrylate) films.

Delocalization of electronic states in graphene oxide stabilised mesoporous silica nanoparticles revealed using photoluminescence

Mesoporous silica aerogel obtained through a sol–gel process when surface treated with 2-D graphene oxide sheets reveals many fascinating features particularly, leading to multiple coordination complexes based on amide linkages. These could arise from reaction of amine groups present in the silica network and carboxylic acid groups in the edge planes of 2-D GO sheets as demonstrated and probed using photoluminescence spectra. New centres arising from the GO networking with silica nanoparticles seem to be more covalent (strong delocalization) as reflected from the asymmetric green luminescence bands.