Showing papers by "Thalappil Pradeep published in 2003"

Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects

Abstract: Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10–20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at a loading of 0.000 26 (by volume). The effect was 7%–14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.

Freely Dispersible Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 Core−Shell Nanoparticles: One-Step Synthesis, Characterization, Spectroscopy, and Optical Limiting Properties

Abstract: We report a one-step route for the synthesis of Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 particles in nanometer dimensions, with controllable shell thickness. This scalable procedure leads to stable and freely dispersible particles, and bulk nanocomposite materials have been made this way. The procedure leads to particles of various morphologies, with a crystalline core in the size range of 30−60 nm diameter and an amorphous shell of ∼3 nm thickness in a typical synthesis. The core diameter and shell thickness (in the range of 1−10 nm) can be varied, leading to different absorption maxima. The material has been characterized with microscopic, diffraction, and spectroscopic techniques. The metal particle growth occurs by the carbamic acid reduction route followed by hydrolysis of the metal oxide precursor, resulting in the oxide cover. The particles could be precipitated and redispersed. The shell, upon thermal treatment, gets converted to crystalline oxides. Cyclic voltammetric studies confirm the core−shel...

Solvothermal synthesis of silver nanoparticles from thiolates

Abstract: A new solvothermal method for the synthesis of thiol-protected silver nanoparticles starting from silver thiolates is reported. The method has been tried with thiols of different chain length, such as octane and octadecane thiols, and the particle size was found to be nearly the same for both molecules. The synthesis was dependent on heating conditions and the best results were obtained when the temperature was between 180 and 200 °C. Addition of complexation agents such as acetyl acetone or triethylamine to the solvent did not change the product distribution significantly.

Current understanding of the structure, phase transitions and dynamics of self-assembled monolayers on two- and three-dimensional surfaces

Abstract: Self-assembled monolayers (SAMs) have been the preferred material systems to investigate specific properties in several disciplines for nearly 15 years. Advances in various techniques and their application to the study of SAMs have dramatically improved our understanding of these systems. Adaptation of this molecular architecture into three dimensions in the form of monolayer-protected clusters enabled the use of bulk techniques for the study of monolayers. Here we review recent developments in the structural properties and temperature-induced phase transitions of both these SAM structures. While alkanethiols on Au (111) and Ag (111) are taken as archetypal systems to discuss the properties of SAMs on two-dimensional surfaces, their assemblies on Au and Ag cluster surfaces are taken as examples for SAMs on three-dimensional surfaces. A comparison of these two SAMs is provided.

Halocarbon mineralization and catalytic destruction by metal nanoparticles

Abstract: Halocarbons undergo catalytic destruction and mineralization with silver and gold nanoparticles in solution forming metal halides and amorphous carbon. The reaction, studied for several halocarbons and one chlorofluorocarbon, is efficient and complete destruction occurs within several hours at room temperature. The methodology can be applied for detection, destruction and removal of halocarbons with complete recovery of the products, implying possible applications.

Detection and extraction of endosulfan by metal nanoparticles

Abstract: One of the most common pesticides in the developing world, endosulfan, can be detected in ppm levels using gold nanoparticles. Endosulfan adsorbs on the nanoparticle surface and upon interaction for a long time, the nanoparticles precipitate from the solution. Interaction with silver is weak, yet adsorption occurs leading to removal of endosulfan from the solution. A multilayer assembly of gold nanoparticles prepared on a glass substrate shows excellent spectrophotometric response suggesting potential applications.

ZrO2 bubbles from core–shell nanoparticles

Abstract: Selective removal of metal cores from core–shell Ag@ZrO2 (ZrO2 coated Ag) and Au@ZrO2 (ZrO2 coated Au) nanoparticles result in stable and freely suspendable oxide nanobubbles of varying dimensions, both in thickness and in diameter. The metal core is removed by a newly found reaction in which halocarbons, generally chlorides, oxidise the metal core and leach out the metal ions. Reduction in the surface plasmon excitation intensity and decrease in the voltammetric current during metal core removal were used to study the process.

Absence of the rotator phase and evolution of dynamical motions in cluster monolayers

Abstract: Alkyl chain dynamics in monolayer protected metal cluster systems has been studied by the quasielastic neutron scattering technique, using two different instruments having very different energy windows. Long chain thiolate protected clusters such as Au–ODT (Au–SC18H37) are rotationally frozen at room temperature (RT) in a wide time scale of 10−9 to 10−12 s. The rotator phase is absent at RT even in much smaller chain length systems, Au–OT (Au–SC8H17) and Au–HT (Au–SC6H13). Dynamics was found to evolve upon increase in temperature. Alkyl chains in a metal cluster superlattice such as Ag–ODT (Ag–SC18H37) are also dynamically frozen at RT in the whole time scale range. Evolution of dynamics with temperature is found to be different in the superlattice and isolated cluster systems. In the former, the chains participating in the inter-cluster interaction were found to be dynamically different from those without. On heating just above the chain melting temperature (Tcm), chains not involved in the inter-cluster...

Ciprofloxacin@SiO2: Fluorescent nanobubbles

Abstract: We report a new nanomaterial in which ciprofloxacin molecules are incorporated inside silica nanobubbles, denoted as ciprofloxacin@SiO2. The material has been characterised using UV/Vis absorption spectroscopy, transmission electron microscopy, cyclic voltammetry, and emission spectroscopy. The material is stable and the freestanding particles can be precipitated and redispersed in several solvents. Confinement of the molecule is complete as leaching through the shell is minimal. The material behaves like free ciprofloxacin in solution; however, effects of confinement are manifested. Energy transfer reaction between ciprofloxacin@SiO2 and Tb3+ was monitored by emission spectroscopy. The emission intensity decreased with metal ion exposure indicating selective electronic interaction.