Tuhin Kumar Maji

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.

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.

Achieving tunable doping of MoSe2 based devices using GO@MoSe2 heterostructure

Doping nature of MoSe2, one of the promising Graphene analogous device material, can be tuned by controlling the concentration of functional groups in Graphene oxide (GO)@MoSe2 heterostructure. In this study, by first-principles simulation, we have observed that GO can be used as a carrier injection layer for MoSe2, where n or p type carriers are introduced within MoSe2 layer depending on the type and concentration of functional moieties in it. Both n and p-type Schottky barrier height modulations are investigated for different modeled configurations of the heterostructure. This combinatorial heterostructure can be a promising material for future electronic device application.

Designing Hybrids of Graphene Oxide and Gold Nanoparticles for Nonlinear Optical Response

Nonlinear optical absorption of light by materials are weak due to its perturbative nature, although a strong nonlinear response is of crucial importance to applications in optical limiting and switching. Here we demonstrate experimentally and theoretically an extremely efficient scheme of excited state absorption by charge transfer between donor and acceptor materials as the new method to enhance the nonlinear absorption by orders of magnitude. With this idea, we have demonstrated strong excited state absorption (ESA) in reduced graphene oxide that otherwise shows increased transparency at high fluence and enhancement of ESA by one orders of magnitude in graphene oxide by attaching gold nanoparticles (AuNP) in the tandem configuration that acts as an efficient charge transfer pair when excited at the plasmonic wavelength. To explain the unprecedented enhancement, we have developed a five-level rate equation model based on the charge transfer between the two materials and numerically simulated the results. To understand the correlation of interfacial charge-transfer with the concentration and type of the functional ligands attached to the graphene oxide sheet, we have investigated the AuNP-graphene oxide interface with various possible ligand configurations from first-principles calculations. By using the strong ESA of our hybrid materials, we have fabricated liquid cell-based high-performance optical limiters with important device parameters better than that of the benchmark optical limiters.

Hole-doping and contact induced spin-polarization in Weyl semimetal TaAs

TaAs is an ideal Weyl semi-metal consisting of conducting surface states composed of massless Weyl fermions. We have investigated the effects of Nb doping in bulk TaAs. For TaAs/Au and TaAs/Ag interfaces, we have observed huge hole doping to the TaAs system. In addition, the introduced carriers impart a large spin polarization within otherwise non-spin-polarized TaAs.

Development of a photo-catalytic converter for potential use in the detoxification of Cr(VI) metal in water from natural resources

Porphyrin dye sensitized nanomaterials (nanohybrids) are advantageous for application in photocatalysis due to their additional absorption band that matches the solar irradiance spectrum. Here we have impregnated protoporphyrin IX with copper(II) ions and sensitized porous TiO2 microspheres (∼1.5 μm in diameter) in order to synthesize a functional nanohybrid for application in visible light photocatalysis. While electron microscopy studies (FESEM and HRTEM) confirm the inorganic porous structure of the microspheres, Fourier transform infrared (FTIR), electronic spectroscopy and picosecond resolved fluorescence studies on the sensitizing PP molecules reveal the formation of the functional nanohybrid. Hexavalent chromium (Cr(VI)) in the aquatic environment is a commonly identified proven carcinogenic heavy metal pollutant, whereas Cr(III) is nontoxic and considered to be an essential nutrient for living organisms. We have applied the nanohybrid for an efficient reduction of toxic Cr(VI) metal ions to non-toxic Cr(III) in water under visible light illumination. The advantage of the Cu ion impregnation into the sensitizing PP dye is evident from the intact efficacy in the photocatalytic reduction in the presence of water dissolved metal ions (Cr3+ and Fe3+) is confirmed from our study. On the other hand, the nanohybrid without the Cu(II) suffers from interference from other dissolved metal ions during the photocatalytic reduction process. For a prototype application, we have developed an active filter (having physical and chemical filtering capability) by depositing the nanohybrid on an extended surface of a stainless-steel metal mesh (size 2 cm × 2 cm, pore size 150 μm × 200 μm). The prototype active filter exhibits significant potential for chemical filtering of toxic Cr(VI) ions (photoreduction to Cr(III)) along with physical cleaning of suspended particulates present in water.

Doping induced carrier and band-gap modulation in bulk versus nano for topological insulators: A test case of Stibnite

We aim at comparing the electronic properties of topological insulator Sb2S3 in bulk and Nanorod using density-functional scheme and investigating the effects of Se-doping at chalcogen-site. While going from bulk to nano, there is a drastic change in the band gap due to surface-induced strain. However, the trend of band gap modulation with increased Se doping is more prominent in bulk. Interestingly, Se-doping introduces different type of carriers in bulk and nano.

Theoretical and experimental study of multiferroic BiFeO3

Nano-crystalline BiFeO3 was synthesized by chemical combustion route. Magnetic study revealed a ferromagnetic behavior at room temperature with magnetic coercivity of ~ 808 Oe. First principles GGA + U investigations for bulk systems indicate that for collinear orientation of magnetic moments, there are intra-plane ferromagnetic and inter-plane anti-ferromagnetic coupling. However, after incorporation of spin-orbital (SO) coupling, the system stabilizes into a canted spiral spin structure. Maintaining resemblance with experimental scenario, we have investigated nano-clusters of the same material, which under collinear orientation, follow similar magnetic order as bulk. With SO-coupling and the consequential non-collinear canting of the surface Fe ions, experimentally observed magnetic behavior could be quantitatively reproduced along with additional prediction interesting attributes like half-metallic nature.