On the Two-Step Mechanism for Synthesis of Transition-Metal Nanoparticles

The two-step particle synthesis mechanism, also known as the Finke-Watzky (1997) mechanism, has emerged as a significant development in the field of nanoparticle synthesis. It explains a characteristic feature of the synthesis of transition metal nanoparticles, an induction period in precursor concentration followed by its rapid sigmoidal decrease. The classical LaMer theory (1950) of particle formation fails to capture this behavior. The two-step mechanism considers slow continuous nucleation and autocatalytic growth of particles directly from precursor as its two kinetic steps. In the present work, we test the two-step mechanism rigorously using population balance models. We find that it explains precursor consumption very well, but fails to explain particle synthesis. The effect of continued nucleation on particle synthesis is not suppressed sufficiently by the rapid autocatalytic growth of particles. The nucleation continues to increase breadth of size distributions to unexpectedly large values as compared to those observed experimentally. A number of variations of the original mechanism with additional reaction steps are investigated next. The simulations show that continued nucleation from the beginning of the synthesis leads to formation of highly polydisperse particles in all of the tested cases. A short nucleation window, realized with delayed onset of nucleation and its suppression soon after in one of the variations, appears as one way to explain all of the known experimental observations. The present investigations clearly establish the need to revisit the two-step particle synthesis mechanism.

Effect of the nature of a transition metal dopant in BaTiO3 perovskite on the catalytic reduction of nitrobenzene

In the present study, we have synthesized Fe, Co and Ni doped BaTiO3 catalyst by a wet chemical synthesis method using oxalic acid as a chelating agent. The concentration of the metal dopant varies from 0 to 5 mol% in the catalysts. The physical and chemical properties of doped BaTiO3 catalysts were studied using various analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), BET surface area and Transmission electron microscopy (TEM). The acidic strength of the catalysts was measured using a n-butylamine potentiometric titration method. The bulk BaTiO3 catalyst exhibits a tetragonal phase with the P4mm space group. A structural transition from tetrahedral to cubic phase was observed for Fe, Co and Ni doped BaTiO3 catalysts with an increase in doped metal concentration from 1 to 5 mol%. The particle sizes of the catalysts were calculated from TEM images and are in the range of 30-80 nm. All the catalysts were tested for the catalytic reduction of nitrobenzene to azoxybenzene. The BaTiO3 catalyst was found to be highly active and less selective compared to the doped catalysts which are active and highly selective towards azoxybenzene. The increase in selectivity towards azoxybenzene is due to an increase in acidic strength and reduction ability of the doped metal. It was also observed that the nature of the metal dopant and their content at the B-site has an impact on the catalytic reduction of nitrobenzene. The Co doped BaTiO3 catalyst showed better activity with only 0.5 mol% doping than Fe and Ni doped BaTiO3 catalysts with maximum nitrobenzene conversion of 91% with 78% selectivity to azoxybenzene. An optimum Fe loading of 2.5 mol% in BaTiO3 is required to achieve 100% conversion with 93% selectivity whereas Ni with 5 mol% showed a conversion of 93% and a azoxybenzene selectivity of 84%.

Transition metal-free Minisci reaction promoted by NCS, and TBHP: acylation of heteroarenes

A method for acylation for heteroarenes under metal-free conditions has been described using NCS as an additive and TBHP as an oxidant. This method has been successfully employed in acylation of a variety of aldehyde with heteroarenes. The application of the method has been illustrated in synthesizing isoquinoline derived natural products. This strategy provides an efficient, mild and inexpensive method for acylation of heteroarenes. (C) 2015 Elsevier Ltd. All rights reserved.

High temperature thermoelectric properties of Zr and Hf based transition metal dichalcogenides: A first principles study

We investigate the electronic and thermal transport properties of bulk MX2 compounds (M = Zr, Hf and X = S, Se) by first-principles calculations and semi-classical Boltzmann transport theory. The band structure shows the confinement of heavy and light bands along the out of plane and in-plane directions, respectively. This results in high electrical conductivity (sigma) and large thermopower leading to a high power factor (S-2 sigma) for moderate n-type doping. The phonon dispersion demonstrates low frequency flat acoustical modes, which results in low group velocities (v(g)). Consequently, lowering the lattice thermal conductivity (kappa(latt)) below 2 W/m K. Low kappa(latt) combined with high power factor results in ZT > 0.8 for all the bulk MX2 compounds at high temperature of 1200 K. In particular, the ZT(max) of HfSe2 exceeds 1 at 1400 K. Our results show that Hf/Zr based dichalcogenides are very promising for high temperature thermoelectric application. (C) 2015 AIP Publishing LLC.

Percolative switching in transition metal dichalcogenide field-effect transistors at room temperature

We have addressed the microscopic transport mechanism at the switching or `on-off' transition in transition metal dichalcogenide (TMDC) field-effect transistors (FETs), which has been a controversial topic in TMDC electronics, especially at room temperature. With simultaneous measurement of channel conductivity and its slow time-dependent fluctuation (or noise) in ultrathin WSe2 and MoS2 FETs on insulating SiO2 substrates where noise arises from McWhorter-type carrier number fluctuations, we establish that the switching in conventional backgated TMDC FETs is a classical percolation transition in a medium of inhomogeneous carrier density distribution. From the experimentally observed exponents in the scaling of noise magnitude with conductivity, we observe unambiguous signatures of percolation in a random resistor network, particularly, in WSe2 FETs close to switching, which crosses over to continuum percolation at a higher doping level. We demonstrate a powerful experimental probe to the microscopic nature of near-threshold electrical transport in TMDC FETs, irrespective of the material detail, device geometry, or carrier mobility, which can be extended to other classes of 2D material-based devices as well.

Study of electron states of solids by techniques of electron spectroscopy

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal-ligand overlap. Use of photoelectron spectroscopy in investigating metal-insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.

Four decades of research in solid state chemistry

Solid state chemistry was in its infancy when the author got interested in the subject. In this article, the author outlines the manner in which the subject has grown over the last four decades, citing representative examples from his own contributions to the different facets of the subject. The various aspects covered include synthesis, structure, defects, phase transitions, transition metal oxides, catalysts, superconductors, metal clusters and fullerenes. In an effort to demonstrate the breadth and vitality of the subject, the author shares his own experiences and aspirations and gives expression to the agony and ecstacy in carrying out experimental research in such a frontier area in India.

Novel materials, materials design and synthetic strategies: recent advances and new directions

There have been major advances in solid state and materials chemistry in the last two decades and the subject is growing rapidly. In this account, a few of the important aspects of materials chemistry of interest to the author are presented. Accordingly, transition metal oxides, which constitute the most fascinating class of inorganic materials, receive greater attention, Metal-insulator transitions in oxides, high temperature superconductivity in cuprates and colossal magnetoresistance in manganates are discussed at some length and the outstanding problems indicated, We then discuss certain other important classes of materials which include molecular materials, biomolecular materials and porous solids. Recent developments in synthetic strategies for inorganic materials are reviewed. Some results on metal nanoparticles and nanotubes are briefly presented. The overview, which is essentially intended to provide a flavour of the subject and show how it works, lists references to many crucial reviews in the recent literature.

Preparation of a thermally sensitive ammonium perchlorate

The effect of ultrasound on the thermal decomposition behaviour of ammonium perchlorate (AP) has been investigated. It was observed that significant changes in the thermal behaviour of A-P, followed using differential thermal analysis and thermogravimetry, was observed when AP was subjected to power ultrasound in water saturated with oxygen-nitrogen mixture. The decomposition temperature of AP was found to have been lowered by nearly 25degreesC. A similar thermal sensitization was observed in AP when subjected to sonication in the presence of transition metal oxides. Kinetic parameters were calculated for AP, modified AP and catalyzed AP decomposition using non-isothermal kinetics. The activation energy for the decomposition of the sonicated AP samples were found to be lower than normal AP.

Large nonlinear refraction and two photon absorption in ferroelectric Bi2VO5.5 thin films

Ferroelectric c-oriented Bi2VO5.5 (BVO) thin films (thickness approximate to 300 nm) were fabricated by pulsed laser deposition on corning glass substrates. Nonlinear refractive index (n(2)) and two photon absorption coefficient (beta) were measured by Z-scan technique at 532 nm wavelength delivering pulses with 10 ns duration. Relatively large values of n(2) = 2.05 +/- 0.2 x 10(-10) cm(2)/W and beta = 9.36 +/- 0.3 cm/MW were obtained for BVO thin films. Origin of the large optical nonlinearities in BVO thin films was discussed based on bond-orbital theory of transition metal oxides. (c) 2012 Elsevier B.V. All rights reserved.

C. N. R. Rao and the Growth of Solid State and Materials Chemistry as a Central Domain of Research

In celebrating Professor C. N. R. Rao's 80th birthday, this article recalls his singular contributions to solid state and materials chemistry for about sixty years. In so doing, the article also traces the growth of the field as a central domain of research in chemical sciences from its early origins in Europe. Although Rao's major work lies in solid state and materials chemistry - a field which he started and nurtured in India while its importance was being recognized internationally - his contributions to other areas of chemistry (and physics), viz., molecular spectroscopy, phase transitions, fullerenes, graphene, nanomaterials and multiferroics are equally significant. Illustrative examples of his work devoted to rare earth and transition metal oxides, defects and nonstoichiometry, metal-insulator transitions, investigation of crystal and electronic structures of a variety of solids by means of electron microscopies and photoelectron spectroscopy, superconducting cuprates, magnetoresistive manganites, multiferroic metal oxides of various structures and, last but not the least, development of new strategies for chemical synthesis of a wide variety of solids including nanomaterials and framework solids in different dimensionalities, are highlighted. The article also captures his exemplary role as a science teacher, science educationist and institution builder in post-Independence India.

Copper oxide nanoparticles: an antidermatophytic agent for Trichophyton spp.

Copper oxide (CuO) is one of the most important transition metal oxides due to its unique properties. It is used in various technological applications such as high critical temperature, superconductors, gas sensors, in photoconductive applications and so on. Recently, it has been used as an antimicrobial agent against various pathogenic bacteria. In the present investigation, we studied the structural and antidermatophytic properties of CuO nanoparticles (NPs) synthesized by a precipitation technique. Copper sulfate was used as a precursor and sodium hydroxide as a reducing agent. Scanning electron microscopy (SEM) showed flower-shaped CuO NPs and X-ray diffraction (XRD) pattern showed the crystalline nature of CuO NPs. These NPs were evaluated against two prevalent species of dermatophytes, i.e. Trichophyton rubrum and T. mentagrophytes by using the broth microdilution technique. Further, the NPs activity was also compared with synthetic sertaconazole. Although better antidermatophytic activity was exhibited with sertaconazole as compared to NPs, being synthetic, sertaconazole may not be preferred, as it shows different adverse effects. Trichophyton mentagrophytes is more susceptible to NPs than T. rubrum. A phylogenetic approach was applied for predicting differences in susceptibility of pathogens.

Insulator-metal transition

A theory of the insulator-metal transition in transition-metal compounds is developed in terms of the collapse of the effective energy gap which is a function of the thermally excited electron-hole pairs. This dependence is shown to arise from the hole-lattice interaction. The reaction of the lattice is found to be equivalent to generating an internal positive pressure (strain). Estimates show that the observed typical behaviour of the conductivity jump and the change of volume at the transition temperature can be explained by the present theory.

Electronic phase separation in correlated oxides: The phenomenon, its present status and future prospects

Many transition metal oxide materials of high chemical purity are not necessarily monophasic. Thus, single crystals of chemically pure rare earth manganites and cobaltates of the general formula Ln1-xAxMO3 (Ln=rare earth metal, A=alkaline earth metal, M=Mn, Co) exhibit the phenomenon of electronic phase separation wherein phases of different electronic and magnetic properties coexist. Such phase separation, the length scale of which can vary anywhere between a few nanometers to microns, gives distinct signatures in X-ray and neutron diffraction patterns, electrical and magnetic properties, as well as in NMR and other spectroscopies. While the probe one employs to investigate electronic phase separation depends on the length scale, it is noteworthy that direct imaging of the inhomogeneities has been accomplished. Some understanding of this phenomenon has been possible on the basis of some of the theoretical models, but we are far from unraveling the varied aspects of this new phenomenon. Herein, we present the highlights of experimental techniques and theoretical approaches, and comment on the future outlook for this fascinating phenomenon

Manganese-mediated ferromagnetism in La2Fe1-xMn2xCr1-xO6 perovskite oxides

We have investigated the structure and magnetic properties of the perovskite oxides of the formula La2Fe1-xMn2xCr1-xO6 (0 < x < 1.0). For 0 < x <= 0.5, the members adopt the orthorhombic (Pbnm) structure, where the transition metal atoms are disordered at the 4b sites and the MO6 (M = Fe, Mn, Cr) octahedra become increasingly distorted with increasing x. For 0.65 <= x < 1.0, the members adopt the rhombohedral (R-3c) structure that is similar to LaMnO3+delta (delta >= 0.1) where the MO6 octahedra are undistorted. While the magnetic properties of the latter series are largely similar to the parent LaMnO3+delta arising from the double-exchange (DE) between mixed valent Mn-III/Mn-IV, the magnetic properties of the orthorhombic members show a distinct (albeit weak) ferromagnetism (T-C similar to 200 K) that seems to arise from a Mn-III-mediated superexchange (SE) between Fe-III/Cr-III in the disordered perovskite structure containing Fe-III, Mn-III and Cr-III.