Prasenjit Kar

Facile synthesis of Pd nanostructures in hexagonal mesophases as a promising electrocatalyst for ethanol oxidation

One of the significant challenges for the commercialization of direct ethanol fuel cells (DEFCs) is the preparation of active, robust, and low-cost catalysts. In this work, a facile and reproducible method is demonstrated for the synthesis of Pd assembled nanostructures in a hexagonal mesophase formed by a quaternary system (Pd-doped water, surfactant, oil, and cosurfactant) via photoirradiation. The formation of Pd nanostructures in the confined region of hexagonal mesophases was further supported by water relaxation dynamics study using a solvation probe. The mesophases can be doped with high concentrations of a palladium salt (0.1 M) without any disturbance to the structure of the mesophases which results in a high yield and facilitates the clean synthesis of Pd nanostructures without using any toxic chemicals. Electrochemical measurement confirms that the as-prepared catalysts exhibit significant electrocatalytic activity for ethanol oxidation in alkaline solution. Additionally, we present an alternative strategy using reduced graphene oxide nanosheets in combination with Nafion (a proton conducting phase) as a support, revealing the pronounced impact on dramatically enhanced electrocatalytic activity and stability of Pd nanostructures compared to Nafion alone. This unique combination allowed the effective dispersion of the Pd nanostructures that is responsible for the enhancement of the catalytic activity. Our approach paves the way towards the rational design of practically relevant catalysts with both enhanced activity and durability for fuel cell applications.

Microwave-assisted synthesis of porous Mn2O3 nanoballs as bifunctional electrocatalyst for oxygen reduction and evolution reaction

Technological hurdles that still prevent the commercialization of fuel cell technologies necessitate designing low-cost, efficient and non-precious metals. These could serve as alternatives to high-cost Pt-based materials. Herein, a facile and effective microwave-assisted route has been developed to synthesize structurally uniform and electrochemically active pure and transition metal-doped manganese oxide nanoballs (Mn2O3 NBs) for fuel cell applications. The average diameter of pure and doped Mn2O3 NBs was found to be ~610 nm and ~650 nm, respectively, as estimated using transmission electron microscopy (TEM). The nanoparticles possess a good degree of crystallinity as evident from the lattice fringes in high-resolution transmission electron microscopy (HRTEM). The cubic crystal phase was ascertained using X-ray diffraction (XRD). The energy dispersive spectroscopic (EDS) elemental mapping confirms the formation of copper-doped Mn2O3 NBs. The experimental parameter using trioctylphosphine oxide (TOPO) as the chelating agent to control the nanostructure growth has been adequately addressed using scanning electron microscopy (SEM). The solid NBs were formed by the self-assembly of very small Mn2O3 nanoparticles as evident from the SEM image. Moreover, the concentration of TOPO was found to be the key factor whose subtle variation can effectively control the size of the as-prepared Mn2O3 NBs. The cyclic voltammetry and galvanostatic charge/discharge studies demonstrated enhanced electrochemical performance for copper-doped Mn2O3 NBs which is supported by a 5.2 times higher electrochemically active surface area (EASA) in comparison with pure Mn2O3 NBs. Electrochemical investigations indicate that both pure and copper-doped Mn2O3 NBs exhibit a bifunctional catalytic activity toward the four-electron electrochemical reduction as well the evolution of oxygen in alkaline media. Copper doping in Mn2O3 NBs revealed its pronounced impact on the electrocatalytic activity with a high current density for the electrochemical oxygen reduction and evolution reaction. The synthetic approach provides a general platform for fabricating well-defined porous metal oxide nanostructures with prospective applications as low-cost catalysts for alkaline fuel cells.

Enhanced Charge Separation and FRET at Heterojunctions between Semiconductor Nanoparticles and Conducting Polymer Nanofibers for Efficient Solar Light Harvesting

Energy harvesting from solar light employing nanostructured materials offer an economic way to resolve energy and environmental issues. We have developed an efficient light harvesting heterostructure based on poly(diphenylbutadiyne) (PDPB) nanofibers and ZnO nanoparticles (NPs) via a solution phase synthetic route. ZnO NPs (~20 nm) were homogeneously loaded onto the PDPB nanofibers as evident from several analytical and spectroscopic techniques. The photoinduced electron transfer from PDPB nanofibers to ZnO NPs has been confirmed by steady state and picosecond-resolved photoluminescence studies. The co-sensitization for multiple photon harvesting (with different energies) at the heterojunction has been achieved via a systematic extension of conjugation from monomeric to polymeric diphenyl butadiyne moiety in the proximity of the ZnO NPs. On the other hand, energy transfer from the surface defects of ZnO NPs (~5 nm) to PDPB nanofibers through Förster Resonance Energy Transfer (FRET) confirms the close proximity with molecular resolution. The manifestation of efficient charge separation has been realized with ~5 fold increase in photocatalytic degradation of organic pollutants in comparison to polymer nanofibers counterpart under visible light irradiation. Our results provide a novel approach for the development of nanoheterojunctions for efficient light harvesting which will be helpful in designing future solar devices.

Nano surface engineering of Mn2O3 for potential light-harvesting application

Manganese oxides are well known applied materials including their use as efficient catalysts for various environmental applications. Multiple oxidation states and their change due to various experimental conditions are concluded to be responsible for their multifaceted functionality. Here we demonstrate that the interaction of a small organic ligand with one of the oxide varieties induces completely new optical properties and functionalities (photocatalysis). We have synthesized Mn2O3 microspheres via a hydrothermal route and characterized them using scanning electron microscopy (SEM), X-ray diffraction (XRD) and elemental mapping (EDAX). When the microspheres are allowed to interact with the biologically important small ligand citrate, nanometer-sized surface functionalized Mn2O3 (NPs) are formed. Raman and Fourier transformed infrared spectroscopy confirm the covalent attachment of the citrate ligand to the dangling bond of Mn at the material surface. While cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) analysis confirm multiple surface charge states after the citrate functionalization of the Mn2O3 NPs, new optical properties of the surface engineered nanomaterials in terms of absorption and emission emerge consequently. The engineered material offers a novel photocatalytic functionality to the model water contaminant methylene blue (MB). The effect of doping other metal ions including Fe3+ and Cu2+ on the optical and catalytic properties is also investigated. In order to prepare a prototype for potential environmental application of water decontamination, we have synthesized and duly functionalized the material on the extended surface of a stainless steel metal mesh (size 2 cm × 1.5 cm, pore size 150 μm × 200 μm). We demonstrate that the functionalized mesh always works as a “physical” filter of suspended particulates. However, it works as a “chemical” filter (photocatalyst) for the potential water soluble contaminant (MB) in the presence of solar light.

Impact of Metal Ions in Porphyrin-Based Applied Materials for Visible-Light Photocatalysis: Key Information from Ultrafast Electronic Spectroscopy

Protoporphyrin IX-zinc oxide (PP-ZnO) nanohybrids have been synthesized for applications in photocatalytic devices. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and steady-state infrared, absorption, and emission spectroscopies have been used to analyze the structural details and optical properties of these nanohybrids. Time-resolved fluorescence and transient absorption techniques have been applied to study the ultrafast dynamic events that are key to photocatalytic activities. The photocatalytic efficiency under visible-light irradiation in the presence of naturally abundant iron(III) and copper(II) ions has been found to be significantly retarded in the former case, but enhanced in the latter case. More importantly, femtosecond (fs) transient absorption data have clearly demonstrated that the residence of photoexcited electrons from the sensitizer PP in the centrally located iron moiety hinders ground-state bleach recovery of the sensitizer, affecting the overall photocatalytic rate of the nanohybrid. The presence of copper(II) ions, on the other hand, offers additional stability against photobleaching and eventually enhances the efficiency of photocatalysis. In addition, we have also explored the role of UV light in the efficiency of photocatalysis and have rationalized our observations from femtosecond- to picosecond-resolved studies.

Facile synthesis of reduced graphene oxide–gold nanohybrid for potential use in industrial waste-water treatment

Abstract Here, we report a facile approach, by the photochemical reduction technique, for in situ synthesis of Au-reduced graphene oxide (Au-RGO) nanohybrids, which demonstrate excellent adsorption capacities and recyclability for a broad range of dyes. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) data confirm the successful synthesis of Au-RGO nanohybrids. The effect of several experimental parameters (temperature and pH) variation can effectively control the dye adsorption capability. Furthermore, kinetic adsorption data reveal that the adsorption process follows a pseudo second-order model. The negative value of Gibbs free energy (ΔG0) confirms spontaneity while the positive enthalpy (ΔH0) indicates the endothermic nature of the adsorption process. Picosecond resolved fluorescence technique unravels the excited state dynamical processes of dye molecules adsorbed on the Au-RGO surface. Time resolved fluorescence quenching of Rh123 after adsorption on Au-RGO nanohybrids indicates efficient energy transfer from Rh123 to Au nanoparticles. A prototype device has been fabricated using Au-RGO nanohybrids on a syringe filter (pore size: 0.220 μm) and the experimental data indicate efficient removal of dyes from waste water with high recyclability. The application of this nanohybrid may lead to the development of an efficient reusable adsorbent in portable water purification.

In-Situ Hydrothermal Synthesis of Bi–Bi2O2CO3 Heterojunction Photocatalyst with Enhanced Visible Light Photocatalytic Activity

AbstractBismuth containing nanomaterials recently received increasing attention with respect to environmental applications because of their low cost, high stability and nontoxicity. In this work, Bi–Bi2O2CO3 heterojunctions were fabricated by in-situ decoration of Bi nanoparticles on Bi2O2CO3 nanosheets via a simple hydrothermal synthesis approach. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to confirm the morphology of the nanosheet-like heterostructure of the Bi–Bi2O2CO3 composite. Detailed ultrafast electronic spectroscopy reveals that the in-situ decoration of Bi nanoparticles on Bi2O2CO3 nanosheets exhibit a dramatically enhanced electron-hole pair separation rate, which results in an extraordinarily high photocatalytic activity for the degradation of a model organic dye, methylene blue (MB) under visible light illumination. Cycling experiments revealed a good photochemical stability of the Bi–Bi2O2CO3 heterojunction under repeated irradiation. Photocurrent measurements further indicated that the heterojunction incredibly enhanced the charge generation and suppressed the charge recombination of photogenerated electron-hole pairs.

Enhanced photovoltage in DSSCs: synergistic combination of a silver modified TiO2 photoanode and a low cost counter electrode

In this study, we have tailored both the active electrode with silver modified TiO2 (Ag–TiO2) as well as the counter electrode (CE) with Pt–reduced graphene oxide (Pt@RGO) nanocomposites to realize efficient and low cost devices. The synergistic combination of both modified electrodes leads to an improved light to electrical energy conversion with an overall efficiency of 8%. An increase in the photovoltage (VOC) of ∼16% (0.74 to 0.86 V) is achieved using Ag–TiO2 in comparison to the bare TiO2. This can be attributed to the shifting of the quasi-Fermi level of the TiO2 photoanode close to the conduction band in the presence of Ag nanoparticles (NPs) due to the formation of the Schottky barrier. On the other hand, the facile synthesis of Pt NPs on RGO nanosheets by a photo-reduction method without using chemical reducing or stabilizing agents demonstrates a higher efficiency than Pt as a CE due to the cooperation of the catalytic activity of Pt and the high electron conductivity of the RGO as a stable supporting material having more interfacial active sites. The quantity of Pt in the Pt@RGO nanocomposites is 10 times lower than in the Pt CE which reduces the cost and makes it viable for large scale commercial utilization.

Direct observation of electronic transition–plasmon coupling for enhanced electron injection in dye-sensitized solar cells

We illustrate experimental evidence of the effect of surface plasmon resonance (SPR) of a noble metal on the ultrafast-electron injection efficiencies of a sensitizing dye in proximity of a wide band gap semiconductor. We have compared the effect of Au with Al nanoparticles as the former have a strong SPR band (peak 560 nm) at the emission (∼600 nm) of the model dye protoporphyrin IX (PP) in the proximity of mesoporous TiO2 nanoparticles in a model dye sensitized solar cell (DSSC). We have used detailed electron microscopic procedures for the characterization of Au/Al nanoparticle-embedded TiO2, the host of PP. Picosecond resolved emission spectroscopy on the model dye reveals an ultrafast component consistent with photoinduced electron transfer (PET) from the dye to the TiO2 matrix in the presence of Au nanoparticles. In order to investigate the dipolar separation of PP from the Au nanoparticle surface, we have employed a Forster Resonance Energy Transfer (FRET) strategy in the PP–Au nanoparticle system in the absence of TiO2. Although the time scale of FRET and PET were found to be similar, the plasmon induced enhanced electron transfer in the case of Au nanoparticles is found to be clear from various device parameters of the plasmonic solar cell (DSSC) designed from the materials. We have also fabricated a DSSC with the developed materials consisting of Al–Au nanoparticles with N719 dye as sensitizer. The fabricated DSSC exhibits a much higher power conversion efficiency of (7.1 ± 0.1)% compared to that with TiO2 alone (5.63 ± 0.13)%. The outstanding performance of DSSC based on plasmonic nanoparticles was attributed to the plasmonic coupling and scattering effect for enhanced electron injection efficiencies.

A novel nanohybrid for cancer theranostics: folate sensitized Fe2O3 nanoparticles for colorectal cancer diagnosis and photodynamic therapy

Organic-inorganic nanohybrids are becoming popular for their potential biological applications, including diagnosis and treatment of cancerous cells. The motive of this study is to synthesise a nanohybrid for the diagnosis and therapy of colorectal cancer. Here we have developed a facile and cost-effective synthesis of folic acid (FA) templated Fe2O3 nanoparticles with excellent colloidal stability in water using a hydrothermal method for the theranostics applications. The attachment of FA to Fe2O3 was confirmed using various spectroscopic techniques including FTIR and picosecond resolved fluorescence studies. The nanohybrid (FA-Fe2O3) is a combination of two nontoxic ingredients FA and Fe2O3, showing remarkable photodynamic therapeutic (PDT) activity in human colorectal carcinoma cell lines (HCT 116) via generation of intracellular ROS. The light induced enhanced ROS activity of the nanohybrid causes significant nuclear DNA damage, as confirmed from the comet assay. Assessment of p53, Bax, Bcl2, cytochrome c (cyt c) protein expression and caspase 9/3 activity provides vivid evidence for cell death via an apoptotic pathway. In vitro magnetic resonance imaging (MRI) experiments in folate receptor (FR) overexpressed cancer cells (HCT 116) and FR deficient human embryonic kidney cells (HEK 293) reveal the target specificity of the nanohybrid towards cancer cells, and are thus pronounced MRI contrasting agents for the diagnosis of colorectal cancer.