Unnikrishnan Manju

Reversible Lithium Storage in Manganese 1,3,5-Benzenetricarboxylate Metal–Organic Framework with High Capacity and Rate Performance

Metal organic frameworks (MOFs) with diverse structural chemistry are being projected as futuristic electrode materials for Li-ion batteries. In this work, we report synthesis of Mn-1,3,5-benzenetricarboxylate MOF by a simple solvothermal method and its application as an anode material for the first time. Scanning electron microscopy of the synthesized MOF shows a bar shaped morphology where these bars, about 1 μm wide and of varied lengths between 2 and 20 μm, are made of porous sheets containing mesoporous walls and macroporous channels. The MOF anode, when examined in the potential window of 0.01-2.0 V versus Li/Li(+), shows high specific capacities of 694 and 400 mAh g(-1) at current densities of 0.1 and 1.0 A g(-1) along with good cyclability, retention of capacity, and sustenance of the MOF network. Ex situ X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy studies on the electrode material at different states of charge suggest that the usual conversion reaction for Li storage might not be applicable in this case. Conjugated carboxylates being weakly electron withdrawing ligands with a stronger π-π interaction, a probable alternative Li storage mechanism has been proposed that involves the organic moiety. The present results show promise for applying Mn-1,3,5-benzenetricarboxylate MOF as high performance <2 V anode.

Defect engineered d0 ferromagnetism in tin-doped indium oxide nanostructures and nanocrystalline thin-films

Origin of unexpected defect engineered room-temperature ferromagnetism observed in tin-doped indium oxide (ITO) nanostructures (Nanowires, Nano-combs) and nanocrystalline thin films fabricated by pulsed laser deposition has been investigated. It is found that the ITO nanostructures prepared under argon environment exhibit strongest ferromagnetic signature as compared to that nanocrystalline thin films grown at oxygen. The evidence of singly ionized oxygen vacancy (V0+) defects, obtained from various spectroscopic measurements, suggests that such V0+ defects are mainly responsible for the intrinsic ferromagnetic ordering. The exchange interaction of the defects provides extensive opportunity to tune the room-temperature d0 ferromagnetism and optical properties of ITOs.

Giant magnetocaloric effect in GdAlO3 and a comparative study with GdMnO3

The magnetic properties and magnetocaloric effect of polycrystalline GdAlO3 and GdMnO3 have been investigated to assess their potential usage as magnetic refrigerants at cryogenic temperatures. These compounds undergo antiferromagnetic transitions at low temperatures which are associated with the giant magnetic entropy change effect () ~40.9 J Kg K−1 under a field change of 0–9 T for GdAlO3 while the moderate effect of 18 J Kg K−1 is observed for polycrystalline GdMnO3. Though the relative cooling power of both the stated materials is similar however, the absence of magnetic and thermal hysteresis makes GdAlO3 a more efficient magnetic refrigerant than GdMnO3.

Extremely fast Au–Ag alloy–dealloy associated reversible plasmonic modifications in SiO2 films

We report a unique alloy–dealloy phenomenon of Au–Ag nanoparticles inside SiO2 films with clear plasmonic modifications between the absorptions of Ag (∼415 nm) and Au (∼524 nm). An Au–Ag (1 : 1) alloy nanoparticles (average size: 4.5 nm)-incorporated transparent SiO2 film is prepared on a glass substrate using mercaptosuccinic acid capped Au nanoparticles and Ag+ co-doped hybrid sol. The Au–Ag (1 : 1) alloy-originated plasmon band (465 ± 1 nm) is gradually red-shifted with increasing temperature (50 to 400 °C) due to the partial oxidation of Ag, causing a systematic modification of the alloy composition. The 1 : 1 alloy, however, reverted very quickly, showing its original plasmon band in the presence of a small amount of H2 due to re-reduction of the oxidized Ag and instantaneous re-alloying. During the Ag oxidation, the Si–OH groups associated with the embedded SiO2 matrix exchange Ag+ to form Ag–O–Si linkages; they subsequently release Ag very quickly in H2 and dissolve again into the parent alloy. As a result, the films exhibit reversible and rapid optical changes while cycling in 0.1% to 1% H2 (balance Ar) and air in the temperature range from 50 to 400 °C. This unique reversible alloy–dealloy phenomenon clearly demonstrates the mechanism of plasmonic modification associated with Au–Ag nanoparticles embedded in the sol–gel SiO2 film matrix.

Local inversion symmetry breaking and spin-phonon coupling in the perovskite GdCrO3

Our detailed temperature dependent synchrotron powder x-ray diffraction studies along with first-principles density functional perturbation theory calculations, enable us to shed light on the origin of ferroelectricity in GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar distortion is driven by local symmetry breaking and by local distortions dominated by Gd off-centering. Our study reveals an intimate analogy between GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less displacive compared to Y, which results in an orthorhombic P na21 structure in GdCrO3 in contrast to monoclinic structure in YCrO3 and consequently, decreases its polar property. This is due to the subtle forces involving Gd-4f electrons either directly or indirectly. A strong magneto-electric coupling is revealed using Raman measurements based analysis in the system below Cr-ordering temperature, indicating their relevance to ferroelectric modulation.

Complex magnetic behavior in GdCrO3

Magnetic interactions in Gadolinium orthochromites (GdCrO3) are quite complex. It shows strong temperature dependency related to Gd3+-Gd3+, Gd3+-Cr3+ and Cr3+-Cr3+ interactions, resulting in exotic phenomena like spin flipping and spin reorientation. These behaviors are successfully explained by considering Cr 3d-Gd 4f magnetic coupling. The nearest neighbor symmetric and antisymmetric exchange coupling in Cr-sublattice was found to be Je = 11.058 K and D = 2.64 K from modified Curie-Weiss law modeled by Moriya, while positive Zeeman energy between net moments and the applied external magnetic fields was found to drive spin flipping.Magnetic interactions in Gadolinium orthochromites (GdCrO3) are quite complex. It shows strong temperature dependency related to Gd3+-Gd3+, Gd3+-Cr3+ and Cr3+-Cr3+ interactions, resulting in exotic phenomena like spin flipping and spin reorientation. These behaviors are successfully explained by considering Cr 3d-Gd 4f magnetic coupling. The nearest neighbor symmetric and antisymmetric exchange coupling in Cr-sublattice was found to be Je = 11.058 K and D = 2.64 K from modified Curie-Weiss law modeled by Moriya, while positive Zeeman energy between net moments and the applied external magnetic fields was found to drive spin flipping.

Carboxylic acid terminated, solution exfoliated graphite by organic acylation and its application in drug delivery

AbstractGraphite nanosheets are considered as a promising material for a range of applications from flexible electronics to functional nanodevices such as biosensors, intelligent coatings and drug delivery. Chemical functionalization of graphite nanosheets with organic/inorganic materials offers an alternative approach to control the electronic properties of graphene, which is a zero band gap semiconductor in pristine form. In this paper, we report the aromatic electrophilic substitution of solution exfoliated graphite nanosheets (SEGn). The highly conjugated π-electronic system of graphite nanosheets enable it to have an amphiphilic characteristic in aromatic substitution reactions. The substitution was achieved through Friedel–Crafts (FC) acylation reaction under mild conditions using succinic anhydride as acylating agent and anhydrous aluminum chloride as Lewis acid. Such reaction renders towards the carboxylic acid terminated graphite nanosheets (SEGn–FC) that usually requires harsh reaction conditions. The product thus obtained was characterized using various spectroscopic and microscopic techniques. Highly stable water-dispersed sodium salt of carboxylic acid terminated graphite nanosheets (SEGn–FC-Na) was also prepared. A comparative sheet-resistance measurements of SEGn, SEGn–FC and SEGn–FC-Na were also done. Finally, the anticancer drug doxorubicin (DOX) was loaded on water dispersible SEGn–FC-Na with a loading capacity of 0.266 mg mg−1 of SEGn–FC-Na and the release of DOX from this water-soluble DOX-loaded SEGn–FC-Na at two different temperatures was found to be strongly pH dependent. Graphical AbstractSelective carboxylic acid terminated solution exfoliated graphite nanosheets were achieved through Friedel–Crafts acylation reaction and dispersed in water by making Na-salt of the same. Anticancer drug doxorubicin was successfully loaded onto this highly water dispersible Na-salt and the drug release was found to be pH dependant. The low cost and efficient drug release make it a potential carrier for targeted drug delivery.

Self-immobilized Pd nanowires as an excellent platform for a continuous flow reactor: Efficiency, stability and regeneration

Despite extensive use of Pd nanocrystals as catalysts, the realization of a Pd-based continuous flow reactor remains a challenge. Difficulties arise due to ill-defined anchoring of the nanocrystals on a substrate and reactivity of the substrate under different reaction conditions. We demonstrate the first metal (Pd) nanowire-based catalytic flow reactor that can be used across different filtration platforms, wherein, reactants flow through a porous network of nanowires (10-1000 nm pore sizes) and the product can be collected as filtrate. Controlling the growth parameters and obtaining high aspect ratio of the nanowires (diameter = ∼13 nm and length > 8000 nm) is necessary for successful fabrication of this flow reactor. The reactor performance is similar to a conventional reactor, but without requiring energy-expensive mechanical stirring. Synchrotron-based EXAFS studies were used to examine the catalyst microstructure and Operando FT-IR spectroscopic studies were used to devise a regenerative strategy. We show that after prolonged use, the catalyst performance can be regenerated up to 99% by a simple wash-off process without disturbing the catalyst bed. Thus, collection, regeneration and redispersion processes of the catalyst in conventional industrial reactors can be avoided. Another important advantage is avoiding specific catalyst-anchoring substrates, which are not only expensive, but also non-universal in nature.

Optical and low temperature magnetic properties study on sol-gel derived misfit calcium cobaltite

Phase pure Calcium cobaltite is prepared by sol-gel method using citric acid as complexing agent. Optical properties are characterized by FTIR and UV-vis diffuse reflectance spectroscopy. Magnetic measurement at 5-390 shows presence of ferromagnetic to paramagnetic transition around 20 K.