Venkata Krishnan

Electrochemical-Coupling Layer-by-Layer (ECC–LbL) Assembly

Electrochemical-coupling layer-by-layer (ECC-LbL) assembly is introduced as a novel fabrication methodology for preparing layered thin films. This method allows us to covalently immobilize functional units (e.g., porphyrin, fullerene, and fluorene) into thin films having desired thicknesses and designable sequences for both homo- and heteroassemblies while ensuring efficient layer-to-layer electronic interactions. Films were prepared using a conventional electrochemical setup by a simple and inexpensive process from which various layering sequences can be obtained, and the photovoltaic functions of a prototype p/n heterojunction device were demonstrated.

Nanoporous Carbon Sensor with Cage-in-Fiber Structure: Highly Selective Aniline Adsorbent toward Cancer Risk Management

Carbon nanocage-embedded nanofibrous film works as a highly selective adsorbent of carcinogen aromatic amines. By using quartz crystal microbalance techniques, even ppm levels of aniline can be repetitively detected, while other chemical compounds such as water, ammonia, and benzene give negligible responses. This technique should be applicable for high-throughput cancer risk management.

Vortex-Aligned Fullerene Nanowhiskers as a Scaffold for Orienting Cell Growth

A versatile method for the rapid fabrication of aligned fullerene C60 nanowhiskers (C60NWs) at the air-water interface is presented. This method is based on the vortex motion of a subphase (water), which directs floating C60NWs to align on the water surface according to the direction of rotational flow. Aligned C60NWs could be transferred onto many different flat substrates, and, in this case, aligned C60NWs on glass substrates were employed as a scaffold for cell culture. Bone forming human osteoblast MG63 cells adhered well to the C60NWs, and their growth was found to be oriented with the axis of the aligned C60NWs. Cells grown on aligned C60NWs were more highly oriented with the axis of alignment than when grown on randomly oriented nanowhiskers. A study of cell proliferation on the C60NWs revealed their low toxicity, indicating their potential for use in biomedical applications.

Synergetic effect of MoS2–RGO doping to enhance the photocatalytic performance of ZnO nanoparticles

The sunlight driven photocatalytic activity of semiconductor based nanostructures has attracted widespread attention in recent years for environmental remediation and energy applications. Numerous good semiconductors, including ZnO, have wide bandgaps and are active only under ultraviolet light, which comprises only 5% of sunlight. Several strategies, such as noble metal doping, non-metal doping, etc., have been adapted to make ZnO heterostructures active in the visible light region. One other strategy is to dope ZnO with narrow bandgap semiconductors like MoS2. In addition, co-doping with graphene as a support material can enhance pollutant adsorption and can aid in electron transport, thereby leading to pollutant degradation. In this work, we report our investigations on the synergetic role played by MoS2–RGO doping to enhance the photocatalytic activity of ZnO nanoparticles, and especially to utilize both the UV and visible light regions of the solar spectrum. The ZnO–MoS2–RGO heterostructures, having different levels of doping, were prepared by a facile hydrothermal method and were characterized thoroughly using different spectroscopy and microscopy techniques. The photocatalytic performance was evaluated by studying the degradation of methylene blue, a model dye pollutant, and carbendazim, a colorless hazardous fungicide, under natural sunlight irradiation. The results reveal that doping of ZnO nanoparticles with 1 wt% MoS2–RGO was optimal and possessed the highest photocatalytic activity among all the investigated samples. A possible mechanism is proposed and discussed in detail.

Zr and Hf oxoclusters as building blocks for the preparation of nanostructured hybrid materials and binary oxides MO2–SiO2(M = Hf, Zr)

Silica materials embedding ZrO2 or HfO2 were prepared by copolymerisation of organically modified oxozirconium or oxohafnium clusters M4O2(OMc)12 (M = Zr, Hf and OMc = methacrylate) with (methacryloxymethyl)triethoxysilane or (methacryloxypropyl)trimethoxysilane. Free radical copolymerisation of the oxoclusters bearing 12 methacrylate groups with the methacrylate-functionalized siloxanes allows stable anchoring of the clusters to the silica network formed by the hydrolysis and condensation of the alkoxy groups. This route represents a valuable strategy to yield a very homogeneous dispersion of the MO2 precursors inside the silica matrix. The composition and the microstructural features of the starting hybrid gels were studied by solid state 13C and 29Si NMR spectroscopy and FT IR transmission spectroscopy. Their evolution upon mild heating (up to 180 °C) was followed by FT-IR Attenuated Total Reflectance spectroscopy (ATR), while their thermal behaviour was studied by thermogravimetric analysis (TGA). The covalent incorporation of the clusters into the silica hybrid matrix was studied at several temperatures. Through X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) it is demonstrated that temperatures above 800 °C yield binary oxides MO2–SiO2 (M = Zr, Hf). Delayed crystallisation of HfO2 and tetragonal ZrO2 from 450 °C to at least 800 °C was detected, which is ascribed to the presence of a homogeneous dispersion of the guest oxide in the silica matrix.

N-doped ZnO–MoS2 binary heterojunctions: the dual role of 2D MoS2 in the enhancement of photostability and photocatalytic activity under visible light irradiation for tetracycline degradation

In this work, we report the fabrication of binary semiconductor heterojunctions comprising N-doped ZnO nanorods loaded with two-dimensional MoS2 nanoflowers in varying amounts, using a facile hydrothermal synthesis method. These semiconductor heterojunctions have been demonstrated to be highly efficient photocatalysts with enhanced performance under visible light irradiation for the degradation of a pharmaceutical pollutant, tetracycline. The superior photocatalytic activity of the heterojunctions can be attributed to the synergistic effect of N-doping of ZnO and loading of MoS2 leading to higher absorption of visible light, efficient separation of photogenerated charge carriers and rapid charge transfer to reaction sites, as per the conduction band potentials of both N-doped ZnO and MoS2. In addition, the two-dimensional nanoflower morphology of MoS2 provides more reaction sites for the adsorption of pollutants, due to its large surface area. Furthermore, the transfer of holes from the valence band of N-doped ZnO to the valence band of MoS2 prevents the photocorrosion of N-doped ZnO resulting in enhanced photostability of the catalyst during the reaction.

EXAFS spectroscopy: Fundamentals, measurement techniques, data evaluation and applications in the field of phthalocyanines

Abstract EXAFS spectroscopy is a useful method for determining the local structure around a specific atom in disordered systems. This technique provides information about the coordination number, the nature of the scattering atoms surrounding a particular absorbing atom, the interatomic distance between the absorbing atom and the backscattering atoms, and Debye–Waller factor. The measurements are done with high energy X-rays, which are normally generated by synchrotron radiation sources. The data analysis is facilitated by specially developed program packages suitable for evaluation purposes. EXAFS spectroscopy is employed in several fields for a variety of applications. Here the structural characterization of a series of amorphous μ-oxo-bridged metallophthalocyanine dimers is presented. It is found that the phthalocyanine macrocycle has significant influence in the spectra and the results obtained are in agreement with the well-known structure of phthalocyanine complexes.

Efficient Electron Transfer across ZnO-MoS2-RGO Heterojunction for Remarkably Enhanced Sunlight Driven Photocatalytic Hydrogen Evolution

The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS2 -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H2 evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S2- and SO32- ions) exhibits an enhanced rate of H2 evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS2 -RGO nanosheets gives the highest photocatalytic H2 production of 28.616 mmol h-1  gcat-1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS2 cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H2 generation.

Controlled synthesis, bioimaging and toxicity assessments in strong red emitting Mn2+ doped NaYF4:Yb3+/Ho3+ nanophosphors

Rare earth, Yb3+/Ho3+ doped NaYF4 nanophosphors showed augmentation of visible green and red light emission by the introduction of Mn2+ as a co-dopant. Nanophosphors were characterized for their morphology and upconversion (UC) fluorescence, as a function of co-dopant concentration. The effect of Mn2+ doping has been investigated to explore the UC mechanism of the phosphors. With the introduction of Mn2+, the intensity of all the emission bands was increased, in particular the red band, which was most significant at 40 mol% doping. A possible mechanism for the enhancement of the red emission has been proposed. The prepared UC nanoparticles were functionalized with polyethylene glycol–polyethylene imine co-polymer to make them water dispersible and successfully applied for cancer cell imaging.

Highly Directional 1D Supramolecular Assembly of New Diketopyrrolopyrrole-Based Gel for Organic Solar Cell Applications.

A new thermoreversible organogel based on diketopyrrolopyrrole dye (DPP-NCO) is reported for the first time and evolved as a new building block for the fabrication of 1D supramolecular assembly. AFM analysis illustrated that its gel state is composed of different sized 1D rods. DPP-NCO gel used as an additive in organic solar cells yields high efficiency of 7.9% owing to better nanophase separation of its active layer.