Raj Ganesh S. Pala

Generic Process for Highly Stable Metallic Nanoparticle-Semiconductor Heterostructures via Click Chemistry for Electro/Photocatalytic Applications

Metallic nanoparticles (MNP) are utilized as electrocatalysts, cocatalysts, and photon absorbers in heterostructures that harvest solar energy. In such systems, the interface formed should be stable over a wide range of pH values and electrolytes. Many current nonthermal processing strategies rely on physical interactions to bind the MNP to the semiconductor. In this work, we demonstrate a generic chemical approach for fabricating highly stable electrochemically/photocatalytically active monolayers and tailored multilayered nanoparticle structures using azide/alkyne-modified Au, TiO2, and SiO2 nanoparticles on alkyne/azide-modified silicon, indium tin oxide, titania, stainless steel, and glass substrates via click chemistry. The stability, electrical, electrochemical, and photocatalytic properties of the interface are shown via electrochemical water splitting, methanol oxidation, and photocatalytic degradation of Rhodamine B (RhB) dye. The results suggest that the proposed approach can be extended for the large-scale fabrication of highly stable heterostructure materials for electrochemical and photoelectrocatalytic devices.

Determining the adsorptive and catalytic properties of strained metal surfaces using adsorption-induced stress

We demonstrate a model for determining the adsorptive and catalytic properties of strained metal surfaces based on linear elastic theory, using first-principles calculations of CO adsorption on Au and K surfaces and CO dissociation on Ru surface. The model involves a single calculation of the adsorption-induced surface stress on the unstrained metal surface, which determines quantitatively how adsorption energy changes with external strain. The model is generally applicable to both transition- and non-transition-metal surfaces, as well as to different adsorption sites on the same surface. Extending the model to both the reactant and transition state of surface reactions should allow determination of the effect of strain on surface reactivity.

Nature of reactive O2 and slow CO2 evolution kinetics in CO oxidation by TiO2 supported Au cluster

Recent experiments on CO oxidation reaction using seven-atom Au clusters deposited on TiO2 surface correlate CO2 formation with oxygen associated with Au clusters. We perform first principles calculations using a seven-atom Au cluster supported on a reduced TiO2 surface to explore potential candidates for the form of reactive oxygen. These calculations suggest a thermodynamically favorable path for O2 diffusion along the surface Ti row, resulting in its dissociated state bound to Au cluster and TiO2 surface. CO can approach along the same path and react with the O2 so dissociated to form CO2. The origin of the slow kinetic evolution of products observed in experiments is also investigated and is attributed to the strong binding of CO2 simultaneously to the Au cluster and the surface.

Sinter-resistant gold nanoparticles encapsulated by zeolite nanoshell for oxidation of cyclohexane

A large surface-to-volume ratio is a prerequisite for highly effective heterogeneous catalysts. Making catalysts in the form of nanoparticles provides a good way to realize this aim. However, agglomeration of such nanoparticles during the preparation and catalytic reaction remains a formidable problem. In the present work we have synthesized gold nanoparticles-coated with different zeolites, ZSM-5 and MCM-22, by hydrothermal route. The method adopted is generic where gold nanoparticles were firstly encapsulated by silica utilizing Stobers process and further these silica coated gold nanoparticles are transformed into Au@MCM-22 and Au@ZSM-5 by modified hydrothermal route. The sinter resistant gold nanoparticle core encapsulated by zeolitic nanoshell showed enhanced conversion for the test reaction of oxidation of cyclohexane to KA-oil, which is an important intermediate in the production of nylon-6 and nylon-6,6. The nano-capsules act as bifunctional catalyst, with the nanoparticles prevented from agglomeration during synthesis or catalytic applications, and the zeolitic-shell enhanced conversion and reusability of the nano-catalyst.

Volatility and Chain Length Interplay of Primary Amines: Mechanistic Investigation on the Stability and Reversibility of Ammonia Responsive Hybrid Perovskites.

Hybrid organic-inorganic perovskites possess promising signal transduction properties, which can be exploited in a variety of sensing applications. Interestingly, the highly polar nature of these materials, while being a bane in terms of stability, can be a boon for sensitivity when they are exposed to polar gases in a controlled atmosphere. However, signal transduction during sensing induces irreversible changes in the chemical and physical structure, which is one of the major lacuna preventing its utility in commercial applications. In the context of developing alkylammonium lead(II) iodide perovskite materials for sensing, here we address major issues such as reversibility of structure and properties, correlation between instability and properties of alkylamines, and relation between packing of alkyl chains inside the crystal lattice and the response time toward NH3 gas. The current investigation highlights that the vapor pressure of alkylamine formed in the presence of NH3 determines the reversibility and stability of the original perovskite lattice. In addition, close packing of alkyl chains inside the perovskite crystal lattice reduces the response toward NH3 gas. The mechanistic study addresses three important factors such as quick response, reversibility, and stability of perovskite materials in the presence of NH3 gas, which could lead to the design of stable and sensitive two-dimensional hybrid perovskite materials for developing sensors.

Hydroxylation induced stabilization of near-surface rocksalt nanostructure on wurtzite ZnO structure.

We present a density functional study of the structural behavior of zinc oxide nanostructures in basic growth condition which consequently leads to the formation of few layers of hydroxylated rocksalt structure over the wurtzite ZnO structure. We demonstrate the greater stability of the few layers of hydroxylated zinc oxide polar surface in rocksalt structure as compared to wurtzite structure. This coerces the near-surface layers of the nanostructure to acquire rocksalt structure giving rise to a trilayer structure consisting of a layer of hydroxyls on ZnO surface, rocksalt near-surface layers, and wurtzite bulk(or wurtzite sub-surface). The formation of coherent interface between rocksalt and wurtzite structure forces the hydroxylated trilayer structure to have lattice constant in between that of a rocksalt and wurtzite structure. Further, the hydroxylated rocksalt structure in the trilayer configuration is stable up to a critical size of the trilayer above which the increasing strain due to lattice mismatch between rocksalt and wurtzite structure overcomes the stabilizing effect of the hydroxylated rocksalt structure.

Dissolution induced self-selective Zn- and Ru-doped TiO2 structure for electrochemical generation of KClO3

We demonstrate an electrochemical approach to prepare a highly active and stable (Zn, Ru)-doped TiO2 (Ru0.26Ti0.73Zn0.01Ox) for electrochemical generation of KClO3. The essential ingredients of this approach consists of (1) co-electrodeposition of Ru, Ti, and Zn as a metallic alloy and then annealing it to form an oxide, (2) dissolution under electrochemical oxidative acidic environment of Ru and Zn-rich surface clusters having a high selectivity towards oxygen evolution reaction (OER) affording the Ti-enriched structure that is more selective to chlorine evolution reaction (CER), and (3) using this electrocatalyst for KClO3 production under near-neutral pH conditions. The dissolution of Ru- and Zn-enriched surfaces resulted in a porous structure with a higher electrochemical surface area and roughness. Furthermore, this electrochemical dissolution of the (Zn, Ru)-rich surface results in an electrocatalytic structure that is self-selective towards the KClO3 formation with a higher specific activity. The co-electrodeposition of Zn aids (1) the formation of the roughened and porous surface structure through surface dissolution of Zn- or ZnO-clusters and (2) the enhancement of conductivity of the electrocatalyst through the formation of oxygen vacancies. The obtained structure exhibited a higher specific activity and stability towards the KClO3 production in comparison to the untreated (Zn, Ru)-doped TiO2 or Ru-doped TiO2 or RuO2.

Poisson effect driven anomalous lattice expansion in metal nanoshells

Surface stress can have profound effects on nanoscale materials and can lead to a contraction of the lattice in nanoparticles to compensate for the under-coordination of the surface atoms. The effect of elastic properties like Poissons ratio can be accentuated in lower dimensional systems. The current study focuses on hollow metal nanoshells (MNSs), wherein there is interplay between the surface stresses existing in the inner and outer surfaces. Using a two scale computational method and transmission electron microscopy, we not only show a lattice expansion (in the radial direction) due to purely surface stress effects in a metallic system but also discover anomalous lattice expansion in the case of very thin walled MNSs. We argue that this effect, wherein the stress in the outer surface causes expansion in the radial lattice parameter (instead of compression), is a Poisson effect driven phenomenon. Although Ni nanoshells are used as an illustrative system for the studies, we generalize this effect for a...

A unified graphical formulation for synthesis of basic and subcolumn distillation sequences

ABSTRACT In contrast to the different approaches currently adopted for generating basic and side-split subcolumn distillation sequence for separating zeotropic multicomponent feed mixture, we present a unified graphical method applicable towards both basic and side-split subcolumn distillation sequence. For a given number of components in the feed mixture, we enforce constraints on a base graph to eliminate violations of conservation principles and to preclude distillation sequences that demand higher heat duty in all appraised practical scenarios. A compact set of algebraic constraints is transfixed using the graph counterpart for generating basic-only distillation configurations. These algebraic constraints utilize binary variables to quantify existence of submixture streams and this considerably reduces the number of variables in generating distillation sequences. We also suggest extension of the formulation to enable the exploration of thermally coupled configurations.

Brownian motion retarded polymer-encapsulated liquid crystal droplets anchored over a patterned substrate via click chemistry

Highly sensitive liquid crystalline (LC) materials are promising candidates for sensing applications. Anchoring of liquid crystal (LC) droplets over substrates can reduce the signal broadening due to retarded Brownian motion. In addition, anchoring over patterned substrates can facilitate the precise quantification of spatially resolved detection of analyte. To this end, we report a versatile approach to anchor LC droplets encapsulated within polymer capsules over a patterned substrate via a highly stable triazole linkage created through click chemistry. Confocal and polarized microcopy images confirm the anchoring of LC droplets over the glass substrates. Change in orientation (bipolar to radial) due to the binding of sodium dodecyl sulfate (SDS) surfactant and 1,2-dilauroyl-sn-glycero-3-phosphocholine (L-DLPC) at the LC interface suggests that the sensing capabilities of the Brownian motion-retarded encapsulated LC droplets is retained. It is also demonstrated that the so assembled systems are stable over six months, which makes them potential candidates for portable microfluidic sensors.