Michael Rajamathi

Oxide and chalcogenide nanoparticles from hydrothermal/solvothermal reactions

We review recent reports of solvothermal or hydrothermal procedures for the preparation of isolated nanoparticles of some important oxide and chalcogenide materials. The synthetic procedures listed here have the advantages of being relatively inexpensive in terms of the solvents used, arguably green (when water is the solvent) and amenable to scale-up. Handling or processing under inert conditions are rarely called for. We include descriptions of work involving the preparation of capped quantum dots using solvothermal techniques as well as microwave-hydrothermal routes and flow-hydrothermal routes that allow continuous and rapid processing of nanoparticulate materials.

N-Doped Graphene–VO2(B) Nanosheet-Built 3D Flower Hybrid for Lithium Ion Battery

Recently, we have shown that the graphene-VO2(B) nanotube hybrid is a promising lithium ion battery cathode material (Nethravathi et al. Carbon, 2012, 50, 4839-4846). Though the observed capacity of this material was quite satisfactory, the rate capability was not. To improve the rate capability we wanted to prepare a graphene-VO2(B) hybrid in which the VO2(B) would be built on 2D nanosheets that would enable better electrode-electrolyte contact. Such a material, a N-doped graphene-VO2(B) nanosheet-built 3D flower hybrid, is fabricated by a single-step hydrothermal reaction within a mixture of ammonium vanadate and colloidal dispersion of graphite oxide. The 3D VO2(B) flowers which are uniformly distributed on N-doped graphene are composed of ultrathin 2D nanosheets. When used in lithium ion batteries, this material exhibits a large capacity, high rate capability, and excellent cycling stability. The enhanced performance results from its unique features: excellent electronic conductivity associated with the N-doped graphene, short transportation length for lithium ions related to ultrathin nanosheets, and improved charge transfer due to the anchoring of the VO2(B) flowers to N-doped graphene.

Functionalized-graphene modified graphite electrode for the selective determination of dopamine in presence of uric acid and ascorbic acid

Graphene is chemically synthesized by solvothermal reduction of colloidal dispersions of graphite oxide. Graphite electrode is modified with functionalized-graphene for electrochemical applications. Electrochemical characterization of functionalized-graphene modified graphite electrode (FGGE) is carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The behavior of FGGE towards ascorbic acid (AA), dopamine (DA) and uric acid (UA) has been investigated by CV, differential pulse voltammetry (DPV) and chronoamperommetry (CA). The FGGE showed excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to that of the bare graphite electrode. The electrochemical oxidation signals of AA, DA and UA are well separated into three distinct peaks with peak potential separation of 193mv, 172mv and 264mV between AA-DA, DA-UA and AA-UA respectively in CV studies and the corresponding peak potential separations in DPV mode are 204mv, 141mv and 345mv. The FGGE is successfully used for the simultaneous detection of AA, DA and UA in their ternary mixture and DA in serum and pharmaceutical samples. The excellent electrocatalytic behavior of FGGE may lead to new applications in electrochemical analysis.

Polymorphism in nickel hydroxide: role of interstratification

In addition to the well-known α and β modifications, nickel hydroxide is shown to exist in a number of poorly crystalline forms—clearly distinguishable by their signature X-ray powder diffraction patterns. DIFFaX simulations combined with compositional analysis and IR spectral data indicate that these are interstratified phases consisting of α- and β-type structural motifs intermixed in varying proportions.

Graphite Oxide-Intercalated Anionic Clay and Its Decomposition to Graphene-Inorganic Material Nanocomposites

A graphite oxide-intercalated anionic clay (nickel zinc hydroxysalt) has been prepared using the aqueous colloidal dispersions of negatively charged graphite oxide sheets and aminobenzoate-intercalated anionic clay layers as precursors. When the two colloidal dispersions are reacted, the interlayer aminobenzoate ions are displaced from the anionic clay and the negatively charged graphite oxide sheets are intercalated between the positively charged layers of the anionic clay. The thermal decomposition of the intercalated solid at different temperatures yields graphene-metal oxide/metal nanocomposites. Electron microscopic analysis of the composites indicates that the nanoparticles are intercalated between the layers of graphite in many regions of these solids although the graphite layers are largely exfoliated and not stacked well together.

Cobalt Hydroxide/Oxide Hexagonal Ring–Graphene Hybrids through Chemical Etching of Metal Hydroxide Platelets by Graphene Oxide: Energy Storage Applications

The reaction of β-Co(OH)2 hexagonal platelets with graphite oxide in an aqueous colloidal dispersion results in the formation of β-Co(OH)2 hexagonal rings anchored to graphene oxide layers. The interaction between the basic hydroxide layers and the acidic groups on graphene oxide induces chemical etching of the hexagonal platelets, forming β-Co(OH)2 hexagonal rings. On heating in air or N2, the hydroxide hybrid is morphotactically converted to porous Co3O4/CoO hexagonal ring-graphene hybrids. Porous NiCo2O4 hexagonal ring-graphene hybrid is also obtained through a similar process starting from β-Ni0.33Co0.67(OH)2 platelets. As electrode materials for supercapacitors or lithium-ion batteries, these materials exhibit a large capacity, high rate capability, and excellent cycling stability.

Chemical synthesis of α-cobalt hydroxide

Abstract Precipitation reactions using ammonia yield a novel cobalt hydroxide phase that is structurally and compositionally similar to α-nickel hydroxide. The use of other synthetic methods yields the well-known β-Co(OH) 2 . The slab composition, mode of anion inclusion, and thermal behavior of the hydroxides obtained by ammonia precipitation are similar to those of α-nickel hydroxide; however, the materials are poorly ordered. A DIFFaX simulation of the powder X-ray diffraction patterns offers the best visual match with the observed patterns for a 50% stacking disorder and a disc radius between 100 and 1000 A.

A solvothermal route to capped nanoparticles of γ-Fe2O3 and CoFe2O4

The decomposition of single or multiple transition metal cupferron complexes in organic solvents under solvothermal conditions and in the presence of long chain amines yields the corresponding oxide nanoparticles. The examples presented here are maghemite γ-Fe2O3 nanoparticles from an FeIII–cupferron complex and spinel CoFe2O4 nanoparticles starting from CoII–cupferron complex and FeIII–cupferron complex taken in suitable proportions. The nanoparticles are capped with n-octylamine or n-dodecylamine. The presence of amine in the reaction is found to be essential for the formation of the product. The magnetic behavior of pressed pellets of these nanoparticles is presented.

On the relationship between α-nickel hydroxide and the basic salts of nickel

Abstract α -Nickel hydroxide is isostructural with the hydroxide-rich basic salts of nickel. The anions in the basic salts are strongly grafted to the nickel hydroxide sheets, but in α -nickel hydroxide, the anions are loosely intercalated between the hydrated nickel hydroxide sheets, [Ni(OH) 2 − x (H 2 O) x ] x+ . The loose intercalation causes the sheets to become disordered and leads to a turbostratic structure. In contrast, the nickel hydroxy-anion sheets in the basic salts are stacked together in a highly ordered fashion. The basic salts of nickel transform into the α -hydroxide upon ageing in a tartarate buffer, into β -hydroxide at pH 4, and into β bc (bc: badly crystallized) phase in 1 M KOH.

On the existence of a nickel hydroxide phase which is neither α nor β

A novel phase of nickel hydroxide with an average interlayer spacing 5.4–5.6 A has been synthesized which is neither α nor β type but is an interstratification of both. It ages to the β form in strong alkali. These observations have implications on the dissolution–reprecipitation mechanism suggested for the α→β transformation of nickel hydroxide.