Nucleation, the initial process in vapor condensation, crystal nucleation, melting, and boiling, is the localized emergence of a distinct thermodynamic phase at the nanoscale that macroscopically ...grows in size with the attachment of growth units. These phase changes are the result of atomistic events driven by thermal fluctuations. The occurrence of atomistic level events with the length scales on the order of 10–10 m and time scales of 10–13 S equivalent to the vibrational frequency of atoms makes the nucleation a very complicated phenomenon to study. Even though abundant literature is available about fundamental aspects of nucleation, the knowledge on these phenomena is far from complete. The classical pathway to nucleation which was once considered to have general applicability to all nucleating systems is gradually giving way to a nonclassical pathway which is now considered as a dominating mechanism in solution crystallization and other systems. In this review, an attempt is made to compare underlying physical principles involved in various nucleating systems and their theoretical treatment based on classical nucleation theory, and other important theories such as a density functional approach and diffuse interface theory. The limitations of classical theory, the gradual evolution of a nonclassical two-step pathway to nucleation, and the questions that have to be addressed in the future are discussed systematically.
A novel approach to efficient bifunctional catalytic electrodes for water splitting is developed, based on a counterintuitive choice of an insulating hydrogel polymer (chitosan, CS)–Prussian blue ...analogue (PBA, KCoFe) nanocomposite thin film on nickel foam. The polymer matrix in KCoFe-CS enables the formation of framelike structures of the non-noble metal-based catalyst nanocrystals, in addition to improving their stability. An optimized cycling protocol leads to a substantial enhancement of the electrocatalytic efficiency for oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER), achieving relatively low overpotentials of 272 and 320 mV (@ 10 and 20 mA cm–2) and 146 mV (@ 10 mA cm–2), respectively, reduced Tafel slopes, and increased Faradaic efficiencies of 98 and 96%; the overpotentials estimated based on the electrochemically active surface area show similar trends. The polymer encapsulation and the cycling protocol are key to the realization of the desirable combination of enhanced efficiency and stability demonstrated up to 50 h for both OER and HER. Detailed characterizations of the postcycling catalytic electrode show that favorable morphological changes of the polymer matrix with concomitant reduction in the PBA nanocrystal size lead to the enhanced activity. The bifunctional activity of the catalytic electrode is demonstrated by the stable water splitting achieved with a 20 mA cm–2 current density at 1.55 V. The present study unravels the utility of hydrogel polymer matrices (without the use of binders like Nafion) in realizing sustainable water splitting electrocatalysts with high stability and efficiency, through the combined effect of confining the electrolyte within and favorably modifying the catalyst nanoparticles and the nanocomposite morphology.
Molecules exhibiting enhanced fluorescence emission in the aggregated/solid state are of immense interest in various functional materials applications. While most traditional fluorophores suffer ...fluorescence quenching upon aggregation, select molecules show fluorescence efficiency enhancement from the molecular to the materials state. An overview of these systems and the popular 'aggregation-induced emission' phenomenon is presented, followed by a critical appraisal of the related models largely focused on intramolecular structural motions. The enhanced fluorescence of crystals, nanocrystals, ultrathin films and amorphous particles of the family of diaminodicyanoquinodimethanes is discussed subsequently. Fluorescence switching accompanying amorphous-crystalline transformations in these materials is described; insights gained into two-step nucleation, crystallinity as a tool for the hierarchical assembly of molecular materials, and the new domain of functional molecular phase change materials, are highlighted. An emerging model that emphasizes the relevance of 'specifically oriented aggregation' in molecular assemblies and intra/intermolecular effects, with consequences for new materials designs is discussed in the final part.
Systems, mechanisms and unique phenomena associated with fluorescence enhancement from molecules to materials are reviewed, highlighting the critical role of molecular assembly.
Molecular materials have emerged as a major theme in contemporary science and technology, thanks to the synergetic confluence of the power of synthetic chemistry, the predictive and analytical ...capability of condensed matter physics, and the versatility of materials science and engineering. The stringent demands in terms of molecular design and supramolecular assembly to be met in the fabrication of nonlinear optical (NLO) materials for applications such as optical second harmonic generation (SHG) make it a unique arena covering fascinating explorations in chemical synthesis, molecular and materials characterization, theoretical modeling, NLO studies, and photonics technology. Our search for conceptually novel and practically simple but effective approaches to the design of molecular building blocks for crystals exhibiting efficient SHG has led us to the avenues described in this Account. The focus has been on the incorporation of structural, symmetry, and shape features in dipolar NLO-phores to realize noncentrosymmetric or polar molecular assemblies suitable to elicit appreciable SHG responses. Attachment of n-alkyl chains of optimal length and remote functional groups were shown to be effective methods to steer the assembly of achiral NLO-phore units into SHG-active noncentrosymmetric lattices. Computational modeling of molecules, molecular clusters, and molecular assemblies in crystals provides valuable insight into the observed structure–function correlations. A systematic exploration of the impact of the placement of stereogenic centers in strongly zwitterionic NLO-phores on the molecular organization led us to the effective exploitation of C 2-symmetric units to form helical superstructures capable of efficient SHG. New materials developed are based on organic and metal−organic molecules as well as coordination polymers. The potential utility of molecular shape could be demonstrated through the realization of perfectly polar organization in a family of screw-shaped dipolar molecules. Combination of optical transparency, thermal stability, and feasibility of fabricating thin films with orientationally ordered crystallites are important aspects of these materials. Several examples presented in this Account highlight the significance of molecules-in-materials by illustrating that not only the individual molecules and their organization in the crystal lattice but also the intermolecular interactions exert critical impact on the nonlinear optical response of the materials. The role of cooperative interactions in some of the cases is pointed out. This Account projects a range of design strategies for molecular SHG crystals and avenues for expanding further on the present observations; the need to address and exploit the contribution of intermolecular interactions is specially noted. The different examples presented illustrate not only the fabrication of new families of materials based on interesting models proposed earlier but also the emergence of new models from the novel materials developed.
•Electricity production from distillery wastewater was investigated in the MFC.•Mixed bacterial culture isolated from distillery wastewater and used as biocatalyst.•Cyclic voltammogram confirmed the ...exoelectrogenic activity of isolated culture.•The power density increased with increase in the anolyte pH from 6 to 8.•Performance of MFC increased with increasing COD concentration and conductivity.
The effect of various system parameters such as wastewater Chemical Oxygen Demand (COD) concentration, pH, conductivity, membrane size and thickness on efficient energy production using mixed isolated culture from the distillery wastewater in the MFC was studied. The power density increased with increase in the anolyte pH from 6 to 8. The peak power density and COD removal efficiency was observed as 63.8±0.65mW/m2 and 63.5±1.5% at pH 8, respectively. The MFC performance increased with increasing COD concentration (800–3200mg/l), conductivity (1.1–9.7mS/cm) and membrane area (8–24cm2). The MFC operating with wastewater COD concentration of 3200mg/l and its conductivity of 9.7mS/cm produced the highest power density of 202±6mW/m2 with a corresponding current density of 412±12mA/m2. The results showed that the efficient electricity generation and simultaneous treatment of distillery wastewater can be attained in the MFC.
Molecular crystals with unusual morphologies characteristic of “hopper crystals” have rarely been explored; such structures are potentially useful for eliciting specific responses to external fields ...or stimuli. Upon simple reprecipitation and controlled growth, select members of a family of strongly zwitterionic molecules, assembling in non-centrosymmetric lattices, are shown to form microcrystals with novel “hopper” morphology. The molecular aggregation is monitored by their characteristic fluorescence enhancement; X-ray diffraction, microscopy, and surface potential mapping provide insight into the development of the unique morphology. Under optimized conditions of electron beam irradiation in a scanning electron microscope, the prototype microcrystal is found to exhibit smooth, prominent, and reversible actuation. Even though electrically triggered macromolecular actuators as well as mechanically responsive and photo/thermosalient molecular crystals are known, controlled bending/folding induced by electron beams are rare and have been demonstrated only in specialized nanostructures. The current observations with a simple small-molecule-based hopper microcrystal are analyzed by a detailed examination of the crystal lattice structure and asymmetric dipole distribution, together with the simulations of the electron beam interactions. An empirical model developed for the responses in the local electrostatic field provides a mechanistic understanding of the actuation process.
Hallmarks of a successful catalyst include simplicity of design and low cost of fabrication, high efficiency, facile recovery and extensive reusability, amenability to monitoring between reuses, and ...ease of scale up. Even though the number of palladium nanoparticle based catalysts reported for the Suzuki–Miyaura reaction has grown exponentially in recent years, the aforesaid criteria are rarely met in a single system. We present a palladium nanoparticle-embedded polymer thin film which functions as a highly efficient and reusable “dip catalyst” for the Suzuki–Miyaura reaction. The multilayer free-standing nanocomposite thin film is fabricated using a simple in situ process through thermal annealing of a spin-coated film of poly(vinyl alcohol) (PVA) containing the palladium precursor. Fabrication parameters of the Pd-PVA film are optimized for enhanced catalyst efficiency. The catalyst is shown to produce very high yield, turn over number and turn over frequency in the prototypical reaction of iodobenzene with phenylboronic acid. The “dip catalyst” film is easily retrieved from the reaction system and reintroduced in successive batches; the high efficiency is retained beyond 30 cycles. The thin film structure enabled convenient catalyst monitoring by spectroscopy and microscopy between reruns. Efficient use of the catalyst up to 5 mmol scale reaction is demonstrated. A simple figure-of-merit is formulated to quantify the catalyst performance, and the present catalyst is evaluated in the context of those reported earlier. Preliminary exploration of the utility of the thin film catalyst in the Suzuki–Miyaura reaction with several substrates as well as in the Heck and Sonogashira coupling reactions is carried out.
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•NiMoO4 was used as novel cathode nanocatalyst for biohydrogen production in MEC.•Nanocatalyst enhanced H2 production from Sugar industry effluent vs. Phosphate buffer.•NiMoO4/Ni-Foam ...exhibited better MEC performance and H2 recoveries compared to control.•Modified Cathode produced 0.12 ± 0.002 L L−1 D−1 of H2 at favorable operating conditions.
Biohydrogen production in Microbial Electrolysis Cell (MEC) had inspired the researchers to overcome the challenges associated towards sustainability. Despite microbial community and various substrates, economical cathode catalyst development is most significant factor for enhancing hydrogen production in the MEC. Hence, in this study, the performance of MEC was investigated with a sugar industry effluent (COD 4200 ± 20 mg/L) with graphite anode and modified Nickel foam (NF) cathode. Nickel molybdate (NiMoO4) coated NF achieved a higher hydrogen production rate 0.12 ± 0.01 L.L−1D−1 as compared to control under favorable conditions. Electrochemical characterizations demonstrated that the improved catalytic activity of novel nanocatalyst with lower impedance favoring faster hydrogen evolution kinetics. The MEC with the novel catalyst performed with 58.2% coloumbic efficiency, 20.36% cathodic hydrogen recovery, 11.96% overall hydrogen recovery and 54.38% COD removal efficiency for a 250 mL substrate during 5 days’ batch cycle. Hence, the potentiality of modified cathode was established with the real time industrial effluent highlighting the waste to wealth bio-electrochemical technology.
Metal nanoparticle-polymer composites are versatile materials which not only combine the unique characteristics of the components, but also manifest mutualistic effects between the two. Embedding ...inside polymer thin films facilitates immobilization and organization of the metal nanoparticles and tuning of their electronic and optical responses by the dielectric environment. The embedded metal nanoparticles in turn can impact upon the various material attributes of the polymer matrix. Some of the most convenient and attractive routes to the fabrication of metal nanoparticle-embedded polymer thin films involve in situ generation of the nanoparticles through reduction or decomposition of appropriate precursors inside the solid film. In this tutorial review we present an overview of the different methodologies developed using this general concept and describe the environment-friendly protocol we have optimized for the fabrication of noble metal nanostructures inside polymer thin films, using aqueous media for the synthesis and deploying the polymer itself as the reducing as well as stabilizing agent. A variety of techniques that have been exploited to characterize the precursor to product transformation inside the polymer film are discussed. The unique control provided by the in situ fabrication route on the size, shape and distribution of the nanostructures, and application of the polymer thin films with the in situ generated metal nanoparticles in areas such as nonlinear optics, surface enhanced Raman scattering, e-beam lithography, microwave absorption, non-volatile memory devices and random lasers, illustrate the versatility of these materials. A brief appraisal of the avenues for future developments in this area is presented.
A major part of contemporary nanomaterials research is focused on metal and semiconductor nanoparticles, constituted of extended lattices of atoms or ions. Molecular nanoparticles assembled from ...small molecules through non-covalent interactions are relatively less explored but equally fascinating materials. Their unique and versatile characteristics have attracted considerable attention in recent years, establishing their identity and status as a novel class of nanomaterials. Optical characteristics of molecular nanoparticles capture the essence of their nanoscale features and form the basis of a variety of applications. This review describes the advances made in the field of fabrication of molecular nanoparticles, the wide spectrum of their optical and nonlinear optical characteristics and explorations of the potential applications that exploit their unique optical attributes.
An overview of the fabrication methodologies, unique nanoscale optical responses and related applications of small molecule based nanoparticles is presented.
