Dinesh Rangappa

Ultrathin Nanosheets of Li2MSiO4 (M = Fe, Mn) as High-Capacity Li-Ion Battery Electrode

Novel ultrathin Li(2)MnSiO(4) nanosheets have been prepared in a rapid one pot supercritical fluid synthesis method. Nanosheets structured cathode material exhibits a discharge capacity of ~340 mAh/g at 45 ± 5 °C. This result shows two lithium extraction/insertion performances with good cycle ability without any structural instability up to 20 cycles. The two-dimensional nanosheets structure enables us to overcome structural instability problem in the lithium metal silicate based cathode materials and allows successful insertion/extraction of two complete lithium ions.

Superhydrophilic Graphene-Loaded TiO2 Thin Film for Self-Cleaning Applications

We develop a simple approach to fabricate graphene-loaded TiO(2) thin films on glass substrates by the spin-coating technique. Our graphene-loaded TiO(2) films were highly conductive and transparent and showed enhanced photocatalytic activities. More significantly, graphene/TiO(2) films displayed superhydrophilicity within a short time even under a white fluorescent light bulb, as compared to a pure TiO(2) film. The enhanced photocatalytic activity of graphene/TiO(2) films is attributed to its efficient charge separation, owing to electrons injection from the conduction band of TiO(2) to graphene. The electroconductivity of the graphene-loaded TiO(2) thin film also contributes to the self-cleaning function by its antifouling effect against particulate contaminants. The present study reveals the ability of graphene as a low cost cocatalyst instead of expensive noble metals (Pt, Pd), and further shows its capability for the application of self-cleaning coatings with transparency. The promising characteristics of (inexpensive, transparent, conductive, superhydrophilic, and highly photocatalytically active) graphene-loaded TiO(2) films may have the potential use in various indoor applications.

Efficient reduced graphene oxide grafted porous Fe3O4 composite as a high performance anode material for Li-ion batteries

Here, we report facile fabrication of Fe3O4-reduced graphene oxide (Fe3O4-RGO) composite by a novel approach, i.e., microwave assisted combustion synthesis of porous Fe3O4 particles followed by decoration of Fe3O4 by RGO. The characterization studies of Fe3O4-RGO composite demonstrate formation of face centered cubic hexagonal crystalline Fe3O4, and homogeneous grafting of Fe3O4 particles by RGO. The nitrogen adsorption-desorption isotherm shows presence of a porous structure with a surface area and a pore volume of 81.67 m(2) g(-1), and 0.106 cm(3) g(-1) respectively. Raman spectroscopic studies of Fe3O4-RGO composite confirm the existence of graphitic carbon. Electrochemical studies reveal that the composite exhibits high reversible Li-ion storage capacity with enhanced cycle life and high coulombic efficiency. The Fe3O4-RGO composite showed a reversible capacity ∼612, 543, and ∼446 mA h g(-1) at current rates of 1 C, 3 C and 5 C, respectively, with a coulombic efficiency of 98% after 50 cycles, which is higher than graphite, and Fe3O4-carbon composite. The cyclic voltammetry experiment reveals the irreversible and reversible Li-ion storage in Fe3O4-RGO composite during the starting and subsequent cycles. The results emphasize the importance of our strategy which exhibited promising electrochemical performance in terms of high capacity retention and good cycling stability. The synergistic properties, (i) improved ionic diffusion by porous Fe3O4 particles with a high surface area and pore volume, and (ii) increased electronic conductivity by RGO grafting attributed to the excellent electrochemical performance of Fe3O4, which make this material attractive to use as anode materials for lithium ion storage.

Size and shape controlled LiMnPO4 nanocrystals by a supercritical ethanol process and their electrochemical properties

In this paper, we report the preparation of systematically size and shape controlled LiMnPO4 nanocrystals under supercritical fluid conditions. The effect of different reaction conditions such as the reaction time, temperature, surfactant and precursor concentration on the size and shape of the LiMnPO4 nanocrystals was studied in detail. It was noticed that shorter reaction time and lower reaction temperature facilitated the formation of crystalline LiMnPO4 nanocrystals with size ∼10 nm. The nanocrystals ranging from 7 to 24 nm were obtained by controlling different reaction conditions. The formation mechanism for the LiMnPO4 nanocrystals is proposed based on the obtained results. The effect of nanosize on the electrochemical properties of LiMnPO4 nanocrystals was studied. Improved electrochemical performance was observed for ∼20 nm sized LiMnPO4 after conductive carbon coating. This study indicates the importance of LiMnPO4 nanocrystals below 50 nm size in improving the electrochemical performance of LiMnPO4 cathodes.

Rapid one-pot synthesis of LiMPO4 (M = Fe, Mn) colloidal nanocrystals by supercritical ethanol process

We report a rapid one-pot supercritical fluid approach to prepare the desired size and morphology controlled LiMPO(4) nanocrystals, using oleylamine as both capping and reducing agent.

Direct preparation of 1-PSA modified graphene nanosheets by supercritical fluidic exfoliation and its electrochemical properties

In this paper, we report on 1-pyrene sulfonic acid sodium salt (1-PSA) modified graphene nanosheets (imGNS) prepared by a novel one-pot in situ supercritical fluid exfoliation and modification reaction. The 1-PSA molecules on the surface of the graphene layers acted as electron withdrawing groups resulting in an electron transfer from the GNS surface to the 1-PSA molecules. This was confirmed by a red shift of 1572∼1576 cm−1, in the graphitic ‘G’ band of the Raman spectra. This novel approach led to an increase in the yield of mono to double layer graphene sheets (about 60%). The charge–discharge measurement shows improved Li-ion charge–discharge properties of the imGNS with an increase in the concentration of 1-PSA molecules in comparison with starting graphite materials. This report provides a new direction for graphene exfoliation and surface modification for technological applications.

Synthesis and organic modification of CoAl2O4 nanocrystals under supercritical water conditions

The synthesis and in situ organic surface modification of cobalt aluminate nanoparticles were carried out to change the hydrophilic surface of pigment nanoparticles to hydrophobic by using organic reagents such as R-COOH and R-NH2 along with the respective metal hydroxide as starting materials at supercritical temperature, under 30–40 MPa pressure at a rapid reaction rate. The organic ligand capping could effectively inhibit the particle growth and also control the size of the nanocrystals. The resultant nanoparticles are dispersed in a binary solvent system, where organic modified particles are dispersed well in an upper organic solvent medium.

Low-Temperature Direct Conversion of Cu–In Films to CuInSe2 via Selenization Reaction in Supercritical Fluid

In this study, we achieve the direct conversion of metallic Cu-In films to compound semiconductor CuInSe(2) films, at quite low temperature around 300 °C using less hazardous metalorganic selenium source in supercritical fluid (SCF). We investigated the effects of temperature and fluid (ethanol) density, and found that supercritical ethanol plays a crucial role in this low-temperature selenization reaction. Such SCF-assisted direct conversion reactions can facilitate large-scale, low-temperature, and rapid synthesis of CuInSe(2) films, which can potentially lead to the low-cost production of solar cells.

Supercritical Fluid Processing of Graphene and Graphene Oxide

Graphene which is single or few atomic layer carbon atoms in a hexagonal network is emerging as a leader among the 2D nanoscale materials [1]. Structurally, graphene is free of defects, with all the carbon atoms linked together by strong and flexible bonds. This has made graphene to have unusual electronic, optical, mechanical and thermal properties. The outstanding properties of graphene reported so far includes Young’s modulus (~1100Gpa), fracture strength (125 Gpa), high thermal conductivity (~ 5000 W m-1K-1), quantum Hall effect at room temperature [2–4], an ambipolar electric field effect along with ballistic conduction of charge carriers [5], tunable band gap and so on. Many efforts have been made on the preparation of the graphene via a number of physical and chemical methods. Some of these methods provides high quality graphene and has opened up new possible routes to address the challenges in preparation and molecular engineering of high quality processable graphene at low cost and large scale [4]. In spite of this progress, the standard procedure to make the high quality graphene is micromechanical cleavage method, which is suitable for only fundamental studies [3]. Alternatively, selective epitaxial growth of graphene on metal or nonmetal substrates using chemical vapor deposition or by thermal decomposition of SiC was developed [6]. The graphene-type carbon materials have been produced by substrate free CVD and radio-frequency plasma-enhanced CVD and so on [7]. On the other hand, the chemical routes are widely considered as a promising approach for large scale production [8]. This approach provides processable graphene, that can be easily cast into various structures or integrate graphene with other materials to form nanocomposites [8, 9]. Currently, there are two most popular chemical approaches to obtain graphene: dispersion and exfoliation of graphite/graphene oxide/graphite intercalation compounds and its reduction after exfoliation or direct exfoliation without chemical modification in suitable organic solvents or surfactants [8]. Unlike the direct exfoliation approach, the chemical modification method results in considerable destruction of graphene electronic structure, thus compromising its unique properties [10, 11]. In addition, these methods involve several steps and need 3-5 days to allow the intercalants and organic solvents to fully insert into the graphitic layers [12]. In contrast to this oxidation and reduction method, some people have developed direct graphite exfoliation method using suitable organic solvent. A high quality single layer graphene sheets (GS) have been prepared by the chemical solution process in which graphite was directly exfoliated in an organic solvent such as N-methylpyrrolidone (NMP)

Preparation of aqueous dispersible styrene–maleic amide encapsulated CoAl2O4 nanocrystals using supercritical water flow type apparatus

Abstract Styrene–maleic amide (SMA) encapsulated cobalt blue nanocrystals were synthesised by the supercritical water method using a flow type reactor. A monodisperse, water dispersible nanocrystal blue pigment with the particle size of 5 nm was obtained by SMA encapsulation. Influence of different reaction conditions such as polymer concentration, pH, encapsulation temperature, etc., on the encapsulation and optical property of the cobalt blue nanocrystals is studied in detail. About 6–8 wt-% SMA concentration and the encapsulation temperature between 270 and 300°C were found to be favourable conditions for successful SMA encapsulation. This novel polymer encapsulation process could be used for mass production of different polymer inorganic hybrid materials that can be used in a variety of applications such as metallic finishing, contrast enhancing luminescent pigments, paint production, electronics and high end optical filters.