Gazi A. Ahmed

Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study.

HYPOTHESIS Undoped TiO2 nanoparticles are considered as a poor photocatalytic candidate in visible light due to the wide band gap. Incorporation of Gd ions is expected to modulate the electronic structure of the material and thereby enhance the photocatalytic properties of the material. EXPERIMENTS Gadolinium doped TiO2 nanoparticles were fabricated via a simple sol-gel method. FINDINGS The surface area of Gd doped TiO2 (225m(2)/g) nanoparticles is much higher than that of undoped TiO2 (95m(2)/g). Doping of Gadolinium enhances the visible light absorption property of TiO2 nanoparticles. Photoluminescence intensity increases at 0.03 and 0.05mol and thereafter reduces at 0.07mol. The photocatalytic efficiency of these nanoparticles is evaluated by observing degradation of phenol in aqueous solution under visible light. The doped nanoparticles are found to exhibit better photocatalytic activity. This enhancement has been attributed to the introduction of the Gd 4f energy levels in the band gap of TiO2. The presence of these states has been further confirmed by theoretical study based on density functional theory (DFT). It is speculated that the 4f states of Gd act as efficient electron trap centers. These 4f states facilitate electron migration to the surface making available free carriers to take part in photocatalysis.

An efficient quasi solid state dye sensitized solar cell based on polyethylene glycol/graphene nanosheet gel electrolytes

A poly(ethylene glycol)/graphene nanosheet quasi solid gel electrolyte was synthesized using an in situ polymerization technique for dye-sensitized solar cells (DSSCs). Fabrication of the DSSCs was carried out by sandwiching the poly(ethylene glycol)/graphene nanosheet (PEG/graphene nanosheet) gel electrolyte in between the dye sensitized TiO2 nanoflower photoanode and platinum based counter electrode using a spacer of thickness 25 μm. However, the graphene nanosheets form a network with PEG matrix channels in the gel electrolyte which enhances charge transportation. The PEG/graphene nanosheet gel electrolyte was characterized using Fourier transform infrared spectroscopy, cyclic voltammetry, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, atomic force microscopy and electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy results demonstrate the reduction of charge transfer resistance (Rct) with the incorporation of graphene nanosheets which promotes charge transportation through the gel electrolyte. The reduction of (Rct) enhances the device efficiency which was observed in the current density vs. voltage (J–V) measurements and thereby the incident photon to converted electron (IPCE) curves. The maximum photovoltaic conversion efficiency of 5.16% was achieved.

Mie scattering computation of spherical particles with very large size parameters using an improved program with variable speed and accuracy

The electromagnetic radiation scattering patterns were computed using an improved C program to study variations in the patterns with changes in the size distribution, size parameters and refractive index of small particles in a volume element. The particle size distributions considered were gamma, normal and lognormal. The program is stable for computation of the theoretical values of the non-zero elements of the scattering matrix, efficiency factors, single scattering albedo, radiation pressure and asymmetry parameter for particles ranging from very small to very large size parameters. One of the significant features of the program is that it incorporates two methods for the determination of the optimal number of terms required for the computation of Mie series with the added benefit of having the option of either going for computational speed or accuracy. After a comparison of the C program with other reported benchmark results, it has been found that the program is very accurate and reliable for electromagnetic scattering computations.

Detector array incorporated optical scattering instrument for nephelometric measurements on small particles

The design and fabrication of a laser-based laboratory light scattering instrument that uses an array of 16 static Si photodetectors and can be operated at three different incident wavelengths (543.5 nm, 594.5 nm and 632.8 nm) are described. The instrument can measure scattered light signals from 10° to 170° in steps of 1°. The accuracy and the reliability of the setup were verified by conducting measurements on the light scattering properties of polystyrene spheres suspended in water and comparing the results with theoretical Mie calculations. The measurements using this instrument on the light scattering properties of spheroidal rutile (TiO2) particles of average diameter 250 nm are presented.

Two Photon Emission and Nonlinear Optical Imaging of Acetonitrile-Treated Quasi-Spherical Nanoscale PbS Systems

The fabrication and nonlinear optical imaging of ~ 5-nm-sized lead sulfide (PbS) nanoscale particles prepared in a water-soluble polymer matrix is reported. The two photon excitation fluorescence (TPEF) studies (using Ti:Sapphire laser, λex ~ 706 nm, P = 400 mW, pulse ~ 80 fs) clearly show the foremost band-edge and asymmetric trap-related emissions. The chain-like organization of PbS nanoparticles was evident from the electron microscopy studies, and the impression from a cluster of nanopatterns was observable through laser scanning confocal microscopy. We have observed adequate correspondence between the TPEF spectra-based nonlinear optical images and spots corresponding to PbS nanoparticle assemblies. In addition, the PbS nanoparticles that are capable of producing strong second harmonic signals were efficiently imaged along the forward direction of the nonlinear microscopy. Investigation of two photon absorption processes along with the second-harmonic-generated imaging indicates the possibility of using the nanoscale particles in single molecule fluorescence spectroscopy, biomolecular labeling, and/or imaging applications.

Surface Plasmon Resonance-Based Protein Bio-Sensing Using a Kretschmann Configured Double Prism Arrangement

In this paper, we demonstrate bio-sensing response of bio-medically relevant proteins by employing a surface plasmon resonance (SPR)-based experimental setup that uses a set of double prisms in Kretschmann configuration. The SPR signal of nanoscale Ag-films of varying thicknesses was first monitored experimentally and then verified through computational simulation. The SPR signal was found to be substantially modulated at the metal-dielectric interface when the nanoscale Ag-films are exposed to bio-molecules, such as catalase, papain, bovine serum albumin, trypsin, and lysozyme, which were evaluated independently.

Adverse effect of Mn doping on the magnetic ordering in Mn doped TiO2 nanoparticles

Herein, we have examined the role played by non-magnetic Mn on the magnetic properties of TiO2 nanoparticles. The samples display complete paramagnetic behavior at room temperature as well as at 10 K. Paramagnetism might be associated with the presence of isolated magnetic spins of Mn2+ in the system. Observation of negative Curie–Weiss temperature indicates presence of antiferromagnetic interaction in the system. Direct exchange interaction of Mn2+–Mn2+ and antiferromagnetic superexchange interaction of Mn2+ ions via lattice O2− ions contribute to antiferromagnetism. Surprisingly, ferromagnetism appears in pure and Mn doped (0.07 mol) TiO2 nanoparticles on vacuum calcination. Although Mn doped TiO2 displays ferromagnetism, there is a loss in magnetization as compared to the undoped TiO2. It is speculated that low growth temperature during vacuum calcinations might have resulted in amorphous MnO phase separation. This could result in competing ferromagnetic–antiferromagnetic interactions and overall reduction in magnetization. From the results it is evident that TiO2 could be made ferromagnetic by introducing sufficient concentration of oxygen vacancies in the system. Mn doping, however, has an adverse effect on the overall ferromagnetic ordering, as there is possibility of antiferromagnetic d–d exchange interaction of Mn2+ ions as well as possibility of antiferromagnetic manganese oxide phases separation.

Laboratory measurements of the light scattering properties of bentonite clay particles embedded in a cylindrical polymer matrix

The volume scattering function and degree of linear polarization of randomly oriented bentonite clay particles were investigated as a function of scattering angle at 543.5 nm, 594.5 nm and 632.8 nm incident laser wavelengths by using a detector array-incorporated laboratory light scattering setup. Readings were taken in steps of 1° from an angle of 10° to 170° and each detector was separated from the next one by an angle of 10°. A transparent cylindrical polymer matrix made of cycloaliphatic amine-cured thermosetting epoxy resin was used to hold the scattering samples in front of the laser beam. For background correction the measurements were taken in differential mode.

Structural and optical characterization of fresh water diatoms (Cyclotella sp.): Nature's nanoporous silica manufacturing plant

Siliceous frustules were extracted from a representative fresh water diatom species (Cyclotella sp.) by treating with aqueous hydrochloric (HCl) acid. The structural characterizations of cleaned frustules were examined by scanning electron microscope (SEM). The microscopy images showed that the diatoms have a regular circular shape and are of almost equal size (average length is 9μm and average width is 3 μm). From energy dispersive X –ray spectroscopy (SEM-EDS) spot analysis it was confirmed that the frustules isolated from diatoms are composed mainly of silicon in the form of amorphous silica (SiO2). The bond information of chemical substances of diatom frustules was carried out at ambient temperature by means of Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectrum as recorded in transmittance mode showed the characteristic peaks for diatom biosilica, including for Si-O-Si stretching vibration at 1057 and 776 cm-1. Photoluminescence (PL) measurements of diatom frustules were performed at room temperature and it was observed that they emitted strong blue PL centered at 440nm when excited with ultraviolet (UV) radiation.

Visible Light Harvesting Titania-Coated Diatom Frustules with Superior Photocatalytic Activity

Porous silica frustules of diatoms are naturally occurring microcellular, solar light harvesting hierarchical structures. In this work, titania-deposited diatom frustule is presented as a heterogeneous catalyst having high efficiency for photodegradation of major pollutants under visible light irradiation. The heterogeneous catalyst is synthesized by depositing titania nanoparticles in the pores and surface of diatom frustules by solgel method and is characterized by XRD, UV–visible absorption spectroscopy, SEM, and TEM. Increase in crystallite size with the transformation of titania phase at high-temperature synthesis is confirmed by XRD. UV–visible spectra reveal that synthesized catalysts exhibit absorption in the visible light range and are able to perform catalytic activity for the degradation of azo-dyes. The effect of photocatalyst on dye concentration in aqueous solution and presence of anatase–rutile titania phases are also presented. As-prepared catalysts responsive to visible light could be a promising candidate in application of environment remediation by harvesting solar energy with the use of the morphology of diatom frustules and presence of nano titania phases in it.