Publications

Transformation of the metastable cubic form of ZrO2 to the monoclinic form in xerogels and metal-ZrO2 composites: a kinetic study

Pressure-induced phase transformation and structural resilience of single-wall carbon nanotube bundles

We report here an in situ x-ray diffraction investigation of the structural changes in carbon single-wall nanotube bundles under quasihydrostatic pressures up to 13 GPa. In contrast with a recent study [Phys. Rev. Lett. 85, 1887 (2000)] our results show that the triangular lattice of the carbon nanotube bundles continues to persist up to \ensuremath{\sim}10 GPa. The lattice is seen to relax just before the phase transformation that is observed at \ensuremath{\sim}10 GPa. Further, our results display the reversibility of the two-dimensional lattice symmetry even after compression up to 13 GPa well beyond the 5 GPa value observed recently. These experimental results explicitly validate the predicted remarkable mechanical resilience of the nanotubes.

Two‐Dimensional Materials and their Hetero‐Superlattices for Photocatalytic Hydrogen Evolution Reaction

Since the discovery of graphene, there has been intensive research on other two‐dimensional (2D) analogues. Some of them are elemental 2D materials such as phosphorene and bismuthene. Others are binary compounds fascinating in terms of energy applications are transition metal dichalcogenides (TMDCs: MoS 2 , MoSe 2 ) and MXenes (Ti 3 C 2 , Nb 2 C 3 ). 1T‐forms of MoS 2 and MoSe 2 are probably the best materials reported for dye‐sensitized photocatalytic hydrogen evolution reaction (HER). Here, we first highlight the phase engineering in 2D‐TMDCs to enhance hydrogen evolution activity. Generation of hetero‐superlattices of TMDCs with other HER active materials such as graphene, carbon nitride and borocarbonitrides, by utilizing coupling and electrostatic restacking strategies, appears to be advantageous for photocatalytic application. These hetero‐superlattices provide a counterpoint to the van der Waals heterostructures by means of covalent bonds. Ladder‐like networks of heterolayers generated due to cross‐linking enhance the interfacial area and charge‐transfer interactions, thereby improving HER activity. The use of 2D phosphorene as a photocatalytic material is limited by its ambient instability. Covalent functionalization of phosphorene surface with tris(pentafluorophenyl) borane and benzyl group increases ambient stability and dispersibility. The functionalized surface can be further utilized to cross‐link with amine or acid‐modified TMDCs. The HER activity obtained with phosphorene‐MoS 2 superlattices is the highest reported among the phosphorene‐based systems, thus setting a new example for metal‐free catalysis. The ambient instability of MXenes and other related 2d materials under photocatalytic conditions can be resolved either by functionalization or by forming hetero‐superlattices.

Bismuth-tungsten oxide bronzes: a study of intergrowth phases and related aspects

Reaction of bismuth metal with WO 3 in the absence of oxygen yields interesting bronze-like phases. From analytical electron microscopy and X-ray photoelectron spectroscopy, the product phases are found to have the general composition Bi x WO 3 with bismuth in the 3+ state. Structural investigations made with high resolution electron micrscopy and cognate techniques reveal that when x < 0.02, a perovskite bronze is formed. When x ≽ 0.02, however, intergrowth tungsten bronzes (i. t. b.) containing varying widths of the WO 3 slab are formed, the lattice periodicity being in the range 2.3-5.1 nm in a direction perpendicular to the WO 3 slabs. Image-matching studies indicate that the bismuth atoms are in the tunnels of the hexagonal tungsten bronze (h. t. b.) strips and the h. t. b. strips always remain one-tunnel wide. Annealed samples show a satellite structure around the superlattice spots in the electron diffraction patterns, possibly owing to ordering of the bismuth atoms in the tunnels. The i. t. b. phases show recurrent intergrowths extending up to 100 nm in several crystals. The periodicity varies considerably within the same crystal wherever there is disordered intergrowth, but unit cell dimensions can be assigned from X-ray and electron diffraction patterns. The maximum value of x in the i. t. b. phases is ca . 0.07 and there is no evidence for the i. t. b. phase progressively giving way to the h. t. b. phase with increase in x . Hexagonal tungsten bronzes that contain bismuth with x up to 0.02 can be formed by starting from hexagonal WO 3 , but the h. t. b. phase seems to be metastable. Optical, magnetic and electron transport properties of the i. t. b. phases have been measured and it appears that the electrons become itinerant when x > 0.05.

Photoacoustic spectroscopy of solids and surfaces

No abstract is provided for this article.

Binding of DNA Nucleobases and Nucleosides with Graphene

Interaction of two different samples of graphene with DNA nucleobases and nucleosides is investigated by isothermal titration calorimetry. The relative interaction energies of the nucleobases decrease in the order guanine (G)>adenine (A)>cytosine (C)>thymine (T) in aqueous solutions, although the positions of C and T seem to be interchangeable. The same trend is found with the nucleosides. Interaction energies of the A-T and G-C pairs are somewhere between those of the constituent bases. Theoretical calculations including van der Waals interaction and solvation energies give the trend G>A approximately T>C. The magnitudes of the interaction energies of the nucleobases with graphene are similar to those found with single-walled carbon nanotubes.

Superconducting borate derivatives of 123 cuprates

Incorporation of BO3- 3 ion stabilizes YSr2Cu3O7, but the borate derivative is non superconducting. Superconductivity is not obtained by the partial substitution of Y by Ca in YSr2Cu3−x(BO3)x0z. Superconductivity has however been found in YSr2−yBayCu3−x(BO3)x)z as well as Y0.5Ca0.5Sr2−yBayCu3−x(BO3)xOz (y=1, x=0.25). The borate ion is coordinated to copper reducing its point group symmetry. Observation of superconductivity in borate derivatives prepared under ambient conditions is noteworthy.

Structure and superconducting properties of Tl1−yPbyY1−xCaxSr2Cu2O7

Structure and superconducting properties of Tl1−y Pb y Y1−x Ca x Sr2Cu2O7 (y=0.0, 0.1, 0.25 and 0.5) , derived from the parent insulator TlYSr2Cu2O7, have been investigated for different values of x. XANES studies show Pb to be in the 4+ state while Tl is in the 3+ state, suggesting thereby that in this series, (x−y) approximately corresponds to the hole concentration. The in-plane CuO distance decreases with increase in x for all values of y. The apical CuO distance as well as the Tl(Pb)O(2) distance show anomalies at x=0.5 in the series with y=0.25; at this composition, T c also reaches a maximum. In general, the composition (value of x) at which the T c of Tl1−y Pb y Y1−x Ca x Sr2Cu2O7 reaches a maximum depends on the Pb content and the maximum T c itself increases with increase in y, reaching a value of 105 K at y=0.5. More interestingly, a maximum T c occurs at an (x−y) value of ∼0.25, which is close to the hole concentration at maximum T c in other cuprate superconductors containing two CuO2 layers.

A 105K superconductor with three CuO layers in the bismuth cuprate system : Bi1.5Pb0.5Ca2.5Sr1.5Cu3O10+δ

The n=3 member of the Bi1.5Pb0.5 (Ca, Sr) n+1CunO2n+4+δ system has been prepared and characterized by X-ray diffraction and electron microscopy. High-Tc superconductivity in the n=3 member has been established by resistivity, AC susceptibility and microwave absorption measurements. It has a Tc of ∼ 105K compared to a Tc of ∼ 82K of the corresponding n=2 member.

Synthesis of Pb-based cuprate superconductors by the sol-gel method

The sol-gel method provides a convenient means for preparing lead cuprates of the types Pb2Sr2−y Ba y Ln 1−x Ca x Cu3O8 (2213 type) with Ln=Y or rare earth and Pb1−z Cu z Sr2−y Ba y Y1−x Ca x Cu2O7(1212 type). Superconducting compositions of these two families showing sharp transitions can be prepared in a reproducible manner by this method.

ChemInform Abstract: Phase Transitions in Organic Solids

No abstract is provided for this article.

Polymorphic Transitions in Anhydrous Sodium Sulphate

No abstract is provided for this article.

Superconductivity of La2−ySryCu1−xNixO4+δ (y = 0.1, 0.2; 0⩽x⩽0.5) The hole-compensating effect of Ni

Magnetization measurements below 50 K on ceramic La2−y Sr y Cu1−x Ni x O4+δ (y = 0.1, 0.2; 0⩽x⩽0.5) show a progressive diminution of superconducting properties with increasing x. The larger suppressive action of Ni in the y=0.1 system than that for y=0.2 is attributed to the hole-compensating effect of Ni3+. The assumption that nickel is in the 3+ state satisfactorily explains: (1) the reduction in hole concentration, (2) a right-shift of the T c versus y curve with x, and (3) the low magnetic moment carried by Ni atoms, in the La2−ySryCu1−xNixO4+δ system.

Formation of single-crystalline CuS at the organic–aqueous interface

We report here the results of a study to understand the formation mechanism of single crystals of the transition metal chalcogenide, CuS, at the water–toluene interface through an interfacial reaction. Systematic measurements carried out using synchrotron x-ray scattering, electron microscopy, atomic force microscopy and calorimetric techniques clearly show that nano-crystallites of CuS form within a few minutes at the interface as the reagents are brought from the organic (upper) and aqueous (lower) layers to the interface, then crystallization of CuS proceeds over a few hours only by reorganization, despite the large excess available in both upper and lower liquid phases. The interface confinement and passivation by organics is critical here in the formation of single crystals having sizes of 6 and 200 nm along the normal and in-plane directions of the liquid–liquid interface.

An investigation of well-characterized small gold clusters by photoelectron spectroscopy, tunneling spectroscopy, and cognate techniques

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAn investigation of well-characterized small gold clusters by photoelectron spectroscopy, tunneling spectroscopy, and cognate techniquesC. N. R. Rao, V. Vijayakrishnan, Hemantkumar N. Aiyer, G. U. Kulkarni, and G. N. SubbannaCite this: J. Phys. Chem. 1993, 97, 43, 11157–11160Publication Date (Print):October 1, 1993Publication History Published online1 May 2002Published inissue 1 October 1993https://doi.org/10.1021/j100145a006RIGHTS & PERMISSIONSArticle Views296Altmetric-Citations53LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Hydrophobic channels in crystals of an α-aminoisobutyric acid pentapeptide

The crystal structure of the pentapeptide p-toluene-sulfonyl-(α-aminoisobutyryl)5-methyl ester (Tosyl-(Aib)5-OMe) has been determined in the space group P I . Pentapeptide molecules are folded in the 310 helical conformation and packed together, so as to yield a hydrophobic channel with a minimim diameter of 5.2 Å. The channel contains crystallographically disordered material. This structure provides a model for channel formation by hydrophobic peptide aggregates and should prove useful in studies of alamethicin, suzukacillin and related Aib containing membrane channels. Triclinic (P I ) crystals of the pentapeptide are obtained in the presence of LiClO4 in aqueous methanol, whereas crystallization from methanol alone yields crystals in the space group Pbca. The conformations of the peptide in the two crystal forms are very similar and only the molecular packing is dramatically different.

GRAPHENE: SYNTHESIS, FUNCTIONALIZATION AND PROPERTIES

Graphenes with varying number of layers can be synthesized by different strategies. Thus, single-layer graphene is obtained by the reduction of single layer graphene oxide, CVD and other methods besides micromechanical cleavage. Few-layer graphenes are prepared by the conversion of nanodiamond, arcdischarge of graphite and other means. We briefly present the various methods of synthesis and the nature of graphenes obtained. We then discuss the various properties of graphenes. The remarkable property of graphene of quenching fluorescence of aromatic molecules is shown to be associated with photo-induced electron transfer, on the basis of fluorescence decay and time-resolved transient absorption spectroscopic measurements. The interaction of electron donor and acceptor molecules with few-layer graphene samples has been discussed. Decoration of metal nano-particles on graphene sheets and the resulting changes in electronic structure are examined. Few-layer graphenes exhibit ferromagnetic features along with antiferromagnetic properties, independent of the method of preparation. Graphene-like MoS 2 and WS 2 have been prepared by chemical methods, and the materials are characterized by electron microscopy, atomic force microscopy (AFM) and other methods. Boron nitride analogues of graphene have been obtained by a simple chemical procedure starting with boric acid and urea and have been characterized by various techniques.

Direct conversion of calcium carbonate to C1–C3 hydrocarbons

With the objective of investigating the direct conversion of inorganic carbonates such as CaCO3 to hydrocarbons, assisted by transition metal ions, we have carried out studies on CaCO3 in an intimate admixture with iron oxides (FeCaCO) with a wide range of Fe/Ca mole ratios (x), prepared by co-precipitation. The hydrogen reduction of FeCaCO at 673 K gives up to 23% yield of the hydrocarbons CH4, C2H4, C2H6 and C3H8, leaving solid iron residues in the form of iron metal, oxides and carbide particles. The yield of hydrocarbons increases with x and the conversion of hydrocarbons occurs through the formation of CO. While the total yield of hydrocarbons obtained by us is comparable to that in the Fischer–Tropsch synthesis, the selectivity for C2–C3 hydrocarbons reported here is noteworthy.