Neena S. John

Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers

Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.

Hybrid films of reduced graphene oxide with noble metal nanoparticles generated at a liquid/liquid interface for applications in catalysis

Free-standing ultra-thin hybrid films of reduced graphene oxide (rGO) with Au, Ag and Pd nanoparticles are generated at an aqueous/organic interface by in situ chemical reduction and spontaneous assembly. The reduction is initiated at a ‘bare’ interface or a ‘modified’ interface in a single step or two-step synthetic strategy. The hybrid materials are characterized by UV-visible, infra-red and Raman spectroscopies, X-ray diffraction, scanning electron (SEM), transmission electron (TEM) and atomic force microscopies (AFM). UV-visible spectra confirm the presence of isolated metal nanoparticles grafted on to rGO layers and Raman spectra signal a charge transfer across the constituent metal nanoparticles and rGO in the hybrid material. SEM and AFM studies show that the morphology of the hybrid films constitutes a homogeneous dispersion of metal nanoparticles and rGO for reduction at the ‘bare’ interface, and a random grafting of metal nanoparticles on rGO for reduction at the ‘modified’ interface. A mechanism for the formation of the films is proposed that involves a simultaneous transport and reduction of GO sheets and metal precursor at the interface or a directed reduction of metal precursor on rGO surface, facilitated by external aids. The utility of these hybrid films as catalysts is exemplified in p-nitrophenol reduction. Our method provides a fast, simple and inexpensive route to obtain free-standing hybrid films of rGO with metal nanoparticles for various applications.

Dip-pen nanolithography with magnetic Fe2O3 nanocrystals

Dip-pen nanolithography has been employed to obtain magnetic nanopatterns of γ-Fe2O3 nanocrystals on mica and silicon substrates. The chemical and magnetic nature of the patterns have been characterized employing low-energy electron microscopy, x-ray photoemission electron microscopy, and magnetic force microscopy measurements.

Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study

The quenching of the fluorescence of 1-aminopyrene (1-Ap) by reduced graphene oxide (rGO) has been investigated using spectroscopic techniques. In spite of the upward curvature in the Stern-Volmer plot, the unchanged spectral signature of the absorption of 1-Ap in the presence of rGO and the decrease in fluorescence lifetime with increasing rGO concentration point toward the dynamic nature of the quenching. Detailed analysis of steady state and time-resolved spectroscopic data has shown that the quenching arises due to the photoinduced electron transfer from 1-Ap to rGO. This is again supported by estimating the Gibbs free energy change for the ground as well as excited state electron transfer. Ab initio calculations under the density functional theory (DFT) formalism reveal that the possibility of π-π stacking is very slim in the 1-Ap-rGO system and the electron density resides completely on 1-Ap in the highest occupied molecular orbital (HOMO) and on graphene in the lowest unoccupied molecular orbital (LUMO), supporting the experimental findings of the intermolecular electron transfer between 1-Ap and rGO in the excited state.

Evolution of Surface Morphology with Introduction of Stacking Faults in Zeolites

This paper sets out to try to determine some of the nanoscopic details of template action in zeolites. The problem has been addressed by monitoring the effects of competitive templating using, in particular, atomic force microscopy and high-resolution scanning electron microscopy. Using these techniques, it is possible to determine the subtle crystal growth changes that occur as a result of altering the concentration of these competitive templating agents. This work concerns the two important intergrowth systems MFI-MEL and FAU-EMT. It was found that some organic templating agents provide much greater structure-directing specificity. So much so in the case of the MFI-MEL system that a 2 mol% doping with the highly specific tetrapropylammonium cation drastically changes the fundamental growth processes. Furthermore, the effect of template crowding is shown to reduce specificity. This work shows how extensive frustrated intergrowth structures can still be accommodated within a nominal zeolite single crystal.

Electrical characteristics of layered palladium alkanethiolates by conducting atomic force microscopy

Current-voltage measurements on individual Pd(II) alkanethiolate nanostructures of varying bilayer thicknesses (hexyl to hexadecyl) employing conducting atomic force microscopy have shown the presence of a low current region near zero bias, the width of which increases with the bilayer thickness. The resistance in this region varies exponentially with the bilayer thickness with a low decay parameter value of 0.2±0.04A−1 indicating a long-range nonresonant tunneling through the alkyl chains. The changeover from low current to high current with increasing bias is accompanied by a negative differential resistance feature, which arises due to Pd–S charge transfer.

Formation of a rectifier with gold nanoclusters : X-ray reflectivity and atomic force microscopy measurements

Gold nanoclusters encapsulated with organic molecules are of great interest for its possible applications in the fields of molecular electronics, catalysis and medical science. Here we demonstrate that monolayer and bilayer films of thiol-capped gold nanoclusters can exhibit diode-like properties provided controlled spatial asymmetry exist between two tunnel junctions used to connect a thiol capped gold nanoclusters. Current-voltage characteristics of this rectifier were obtained from conducting probe atomic force microscopy measurements and also from conventional two probe resistance measurements. Systematic x-ray reflectivity and atomic force microscopy measurements were carried out to characterize the spatial asymmetry introduced by a monolayer of fatty acid salt gadolinium stearate used to deposit thiol-capped gold nanocluster molecules on hydrophilic SiO2-Si(001) substrate by Langmuir Blodgett technique. This information was used to explain prominent rectification observed in these nano-structured films.

Cellular network formation of hydrophobic alkanethiol capped gold nanoparticles on mica surface mediated by water islands.

Dendritic and cellular networks of nanoparticles are known to form commonly either by random diffusion-limited aggregation or by solvent evaporation dynamics. Using alkanethiol capped gold nanoparticles deposited on mica imaged under ambient and controlled water vapor conditions by atomic force microscope and in situ scanning electron microscope, respectively, we show a third mechanism in action. The cellular network consisting of open and closed polygons is formed by the nucleation and lateral growth of adsorbed water islands, the contact lines of which push the randomly distributed hydrophobic nanoparticles along the growth directions, eventually leading to the polygonal structure formation as the boundaries of the growing islands meet. Such nanoparticle displacement has been possible due to the weakly adhering nature of the hydrophilic substrate, mica. These results demonstrate an important but hitherto neglected effect of adsorbed water in the structure formation on hydrophilic substrates and provide a facile tool for the fabrication of nanoparticle networks without specific particle or substrate modifications and without a tight control on particle deposition conditions during the solvent evaporation.

High-Resolution scanning electron and atomic force microscopies: observation of nanometer features on zeolite Surfaces

It is now possible to observe nanometer features on the surfaces of zeolitic materials using high-resolution scanning electron microscopy. By taking ibidem measurements in combination with atomic force microscopy we are able to illustrate the strengths and weaknesses of both techniques and judge respective resolving power.

Nanoscale photocurrent distribution in ultra-thin films of zinc oxide nanoparticles and their hybrid with reduced graphene oxide

The electrical properties of ultra-thin films of ZnO nanoparticles and their hybrid with reduced graphene oxide (rGO), prepared at a liquid/liquid interface, are investigated. The photocurrent distribution in ZnO and rGO–ZnO films at nanoscale under UV irradiation is explored using conducting atomic force microscopy (C-AFM) and compared with the bulk electrical measurements. At the rGO–ZnO interface, ohmic contact is achieved rather than the Schottky junction encountered in bare ZnO with a metal electrode. Enhancement of the photocurrent is observed in both cases and the photoresponse mapping by C-AFM reveals an inhomogeneous current distribution at the nanoscale that is associated with various ZnO nanostructures in the film. While a small population of the nanostructures contributes higher photocurrents in bare ZnO film, the majority of the photoresponsive ZnO nanostructures provide high photoresponse in rGO–ZnO. This nanoscale electrical study gives insights into the local current contribution from individual nanostructures toward bulk electrical properties and can aid in understanding photovoltaic device performances.