Photoelectrochemical oxidation of p-nitrophenol on an expanded graphite-TiO2 electrode

In the quest for more efficient photoanodes in the photoelectrochemical oxidation processes for organic pollutant degradation and mineralisation in water treatment, we present the synthesis, characterisation and photoelectrochemical application of expanded graphite-TiO2 composite (EG-TiO2) prepared using the sol-gel method with organically modified silicate. The Brunauer-Emmett-Teller surface area analyser, ultraviolet-visible diffuse reflectance, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry, Raman spectrometry and X-ray photoelectron spectroscopy were employed for the characterisation of the composites. The applicability of the EG-TiO2 as photoanode material was investigated by the photoelectrochemical degradation of p-nitrophenol as a target pollutant in a 0.1 M Na2SO4 (pH 7) solution at a current density of 5 mA cm(-2). After optimising the TiO2 loading, initial p-nitrophenol concentration, pH and current density, a removal efficiency of 62% with an apparent kinetic rate constant of 10.4 x 10(-3) min(-1) was obtained for the photoelectrochemical process as compared to electrochemical oxidation and photolysis, where removal efficiencies of 6% and 24% were obtained respectively after 90 min. Furthermore, the EG-TiO2 electrode was able to withstand high current density due to its high stability. The EG-TiO2 electrode was also used to degrade 0.3 x 10(-4) M methylene blue and 0.1 x 10(-4) M Eosin Yellowish, leading to 94% and 47% removal efficiency within 120 reaction time. This confirms the suitability of the EG-TiO2 electrode to degrade other organic pollutants.

Carbon nanotube-ZnO nanowire hybrid architectures as multifunctional devices

We report on multifunctional devices based on CNT arrays-ZnO nanowires hybrid architectures. The hybrid structure exhibit excellent high current Schottky like behavior with ZnO as p-type and an ideality factor close to the ideal value. Further the CNT-ZnO hybrid structures can be used as high current p-type field effect transistors that can deliver currents of the order of milliamperes and also can be used as ultraviolet detectors with controllable current on-off ratio and response time. The p-type nature of ZnO and possible mechanism for the rectifying characteristics of CNT-ZnO has been presented.

Carbon nanotube coated fiber Bragg grating for photomechanical optic modulator

We have demonstrated novel concept of utilizing the photomechanical actuation in carbon nanotubes (CNTs) to tune and reversibly switch the Bragg wavelength. When fiber Bragg grating coated with CNTs (CNT-FBG) is exposed externally to a wide range of optical wavelengths, e. g., ultraviolet to infrared (0.2-200 mu m), a strain is induced in the CNTs which alters the grating pitch and refractive index in the CNT-FBG system resulting in a shift in the Bragg wavelength. This novel approach will find applications in telecommunication, sensors and actuators, and also for real time monitoring of the photomechanical actuation in nanoscale materials. (C) 2013 AIP Publishing LLC.

Renewable Energy via Photocatalysis

The generation of renewable energy through photocatalysis is an attractive option to utilize the abundantly available solar radiation for a sustainable future. Photocatalysis refers to charge-carrier, i.e. electron and hole, mediated reactions occurring on a semiconductor surface in presence of ultraviolet or visible light radiation. Photocatalysis is a well established advanced oxidation technique for the decontamination of toxic organic pollutants to CO2 and H2O. However, the generation of energy in the form of hydrogen, hydrocarbon fuels and electricity via photocatalysis is an upcoming field with great many technical challenges towards practical implementation. This review will describe the fundamental reaction mechanism of (i) photocatalytic water splitting, (ii) photocatalytic H-2 generation in presence of different sacrificial agents, (iii) H-2 and electricity generation in a photofuel cell, (iv) photocatalytic reduction of CO2 to hydrocarbons and useful chemicals, and (v) photocatalytic water-gas shift reaction. A historic and recent perspective of the above conversion techniques, especially with regard to the development of TiO2-based and non-TiO2 materials is provided. The activity of different materials for the above reactions based on quantifiers like reaction rate, quantum yield and incident-photon-to-current efficiency is compared, and key design considerations of the ``best'' photocatalyst or photoelectrode is outlined. An overall assessment of the research area indicates that the presently achieved quantum efficiencies for the above reactions are rather moderate in the visible region, and the goal is to develop a catalyst that absorbs visible radiation, provides good charge-carrier separation, and exhibits high stability for long periods of usage.

Facile synthesis of Cu2CoSnS4 nanoparticles exhibiting red-edge-effect: Application in hybrid photonic devices

Cu2CoSnS4 (CCTS) quaternary semiconducting nanoparticles with size distribution from 20 nm to 60 nm were synthesized by one-pot low temperature time and surfactant dependent hydrothermal route. Nanoparticles were characterized structurally and optically. Excitation dependent fluorescence exhibited a dynamic stoke shift referring to the Red-Edge-Effect with peak shifting by a greater magnitude (>100 nm) towards red side, in all the samples. Hybrid devices, fabricated from CCTS nanoparticle inorganic counterparts benefitting from the conjugation of organic P3HT polymer matrix, were demonstrated for photodetection under infra-red and A. M 1.5 solar light illuminations. Faster rise and decay constants of 37 ms and 166 ms, with one order photocurrent amplification from 1.6 x 10(-6) A in the dark to 6.55 x 10(-5) A, upon the 18.50 mW cm(-2) IR lamp illumination, make CCTS a potential candidate for photodetector and photovoltaic applications. (C) 2013 AIP Publishing LLC.

Contrast in luminescence characteristics (intense UV to bright visible light) of ZnO nanostructures with the variation in microstructure

We report the controlled variation of luminescence of ZnO nanostructures from intense ultraviolet to bright visible light. Deliberate addition of surfactants in the reaction medium not only leads to growth anisotropy of ZnO, but also alters the luminescence property. ZnO nanoclusters comprising of very fine particles with crystallite sizes approximate to 15-22nm were prepared in a non-aqueous medium, either from a single alcohol or from their mixtures. Introduction of the aqueous solution of the surfactant helps in altering the microstructure of ZnO nanostructure to nanorods, nanodumb-bells as well as the luminescence property. The as-prepared powder material is found to be well crystallized. Defects introduced by the surfactant in aqueous medium play an important role in substantial transition in the optical luminescence. Chromaticity coordinates were found to lie in the yellow region of color space. This gives an impression of white light emission from ZnO nanocrystals, when excited by a blue laser. Oxygen vacancy is described as the major defect responsible for visible light emission as quantified by X-ray photoelectron spectroscopy and Raman analysis.

ENTANGLEMENT ENTROPY FROM SURFACE TERMS IN GENERAL RELATIVITY

Entanglement entropy in local quantum field theories is typically ultraviolet divergent due to short distance effects in the neighborhood of the entangling region. In the context of gauge/gravity duality, we show that surface terms in general relativity are able to capture this entanglement entropy. In particular, we demonstrate that for 1+1-dimensional (1 + 1d) conformal field theories (CFTs) at finite temperature whose gravity dual is Banados-Teitelboim-Zanelli (BTZ) black hole, the Gibbons-Hawking-York term precisely reproduces the entanglement entropy which can be computed independently in the field theory.

Self propagating combustion synthesis and luminescent properties of nanocrystalline CeO2: Tb3+ (1-10 mol%) phosphors

Undoped and Tb3+ (1-10 mol%) doped CeO2 nanophosphors were synthesized by low temperature solution combustion method. The combustion derived products were well studied by Powder X-ray diffraction (PXRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and Ultraviolet visible (UV-Vis) characterizations. The thermoluminescence (TL) glow curves of CeO2: Tb3+ (1-10 mol%) nanophosphors exposed to c source (60Co) for various doses were discussed for the first time. Two TL glow peaks recorded at 182 and 262 degrees C respectively. The TL intensity at 262 degrees C peak increases linearly in the dose range 0.5-7 kGy. Further, this peak was well defined, intense and glow peak structure does not change with c-dose as a result, it was quite useful in TL dosimetry of ionizing radiations. The kinetic parameters associated with the glow peak were estimated using Chen's half width method. The photoluminescence emission (PLE) spectra consists of characteristic peaks at 544 and 655 nm which were attributed to D-5(4) -> F-7(5) and D-5(4) -> F-7(2) transitions of Tb3+ ions. The optimal concentration of Tb3+ ions was found to be 7 mol%. The color co-ordinates of CeO2: Tb3+ (1-10 mol%) located in green region. Hence, this phosphor was quite useful for display applications. (C) 2013 Elsevier B. V. All rights reserved.

Low temperature synthesis of pure cubic ZrO2 nanopowder: Structural and luminescence studies

Pure cubic zirconia (ZrO2) nanopowder is prepared for the first time by simple low temperature solution combustion method without calcination. The product is characterized by Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infra Red spectroscopy (FTIR) and Ultraviolet-Visible spectroscopy (UV-Vis). The PXRD showed the formation of pure stable cubic ZrO2 nanopowders with average crystallite size ranging from 6 to 12 nm. The lattice parameters were calculated from Rietveld refinement method. SEM micrograph shows fluffy, mesoporous, agglomerated particles with large number of voids. TEM micrograph shows honey comb like arrangement of particles with particle size similar to 10 nm. The PL emission spectrum excited at 210 nm and 240 nm consists of intense bands centered at similar to 365 and similar to 390 nm. Both the samples show shoulder peak at 420 nm, along with four weak emission bands at similar to 484, similar to 528, similar to 614 and similar to 726 nm. TL studies were carried out pre-irradiating samples with gamma-rays ranging from 1 to 5 KGy at room temperature. A well resolved glow peak at 377 degrees C is recorded which can be ascribed to deep traps. With increase in gamma radiation there is linear increase in TL intensity which shows the possible use of ZrO2 as dosimetric material. (C) 2013 Elsevier B.V. All rights reserved.

Facile hydrothermal synthesis of carbon nanoparticles and possible application as white light phosphors and catalysts for the reduction of nitrophenol

We report a simple hydrothermal synthesis of highly reproducible carbon nanoparticles in a size range between 2 and 7 nmfroma single precursor sucrose without either surface passivating agents or acids and bases. The carbon nanoparticles can be used as white light phosphors, especially for ultraviolet light emitting diodes and metal-free catalyst for the reduction of nitrophenol.

Near-infrared photoactive Cu3BiS3 thin films by co-evaporation

Semiconducting Cu3BiS3 (CBS) thin films were deposited by co-evaporation of Cu, Bi elemental metallic precursors, with in situ sulphurisation, using a quartz effusion cell. Cu3BiS3 thin films were structurally characterized by XRD and FE-SEM. The chemical bonding of the ions was examined by XPS. As deposited films were demonstrated for metal-semiconductor-metal near IR photodectection under lamp and laser illuminations. The photo current amplified to three orders and two orders of magnitude upon the IR lamp and 60 m W cm(-2) 1064 nm IR laser illuminations, respectively. Larger grains, made up of nano needle bunches aided the transport of carriers. Transport properties were explained based on the trap assisted space charge conduction mechanism. Steady state detector parameters like responsivity varied from 1.04 AW(-1) at 60 m Wcm(-2) to 0.22 AW(-1) at 20 m Wcm(-2). Detector sensitivity of 295 was found to be promising and further could be tuned for better responsivity and efficiency in utilization of near infra-red photodetector. (C) 2014 AIP Publishing LLC.

Defect-Induced Enhancement and Quenching Control of Photocurrent in Few-Layer Graphene Photodetectors

A novel approach is presented for achieving an enhanced photo-response in a few layer graphene (FLG) based photodetector that is realized by introducing defect sites in the FLG. Fabrication induced wrinkle formation in graphene presented a four-fold enhancement in the photocurrent when compared to unfold PLC. Interestingly, it was observed that the addition of few multiwalled carbon nanotubes to an FLG improves the photocurrent by two-fold along with a highly stable response as compared to FLG alone.

Metal-metal charge transfer and interfacial charge transfer mechanism for the visible light photocatalytic activity of cerium and nitrogen co-doped TiO2

Nanosized cerium and nitrogen co-doped TiO2 (Ce-TiO2-xNx) was synthesized by sol gel method and characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), FESEM, Fourier transform infrared, N-2 adsorption and desorption methods, photoluminescence and ultraviolet-visible (UV-vis) DRS techniques. PXRD analysis shows the dopant decreases the crystallite sizes and slows the crystallization of the titania matrix. XPS confirm the existence of cerium ion in +3 or +4 state, and nitrogen in -3 state in Ce-TiO2-xNx. The modified surface of TiO2 provides highly active sites for the dyes at the periphery of the Ce-O-Ti interface and also inhibits Ce particles from sintering. UV-visible DRS studies show that the metal-metal charge transfer (MMCT) of Ti/Ce assembly (Ti4+/Ce3+ -> Ti3+/Ce4+) is responsible for the visible light photocatalytic activity. Photoluminescence was used to determine the effect of cerium ion on the electron-hole pair separation between the two interfaces Ce-TiO2-xNx and Ce2O3. This separation increases with the increase of cerium and nitrogen ion concentrations of doped samples. The degradation kinetics of methylene blue and methyl violet dyes in the presence of sol gel TiO2, Ce-TiO2-xNx and commercial Degussa P25 was determined. The higher visible light activity of Ce-TiO2-xNx was due to the participation of MMCT and interfacial charge transfer mechanism.

Structural refinement, optical and electrical properties of Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 ceramics

Samarium doped barium zirconate titanate ceramics with general formula Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 x = 0, 0.01, 0.02, and 0.03] were prepared by high energy ball milling method. X-ray diffraction patterns and micro-Raman spectroscopy confirmed that these ceramics have a single phase with a tetragonal structure. Rietveld refinement data were employed to model BaO12], SmO12], ZrO6], and TiO6] clusters in the lattice. Scanning electron microscopy shows a reduction in average grain size with the increase of Sm3+ ions into lattice. Temperature-dependent dielectric studies indicate a ferroelectric phase transition and the transition temperature decreases with an increase in Sm3+ ion content. The nature of the transition was investigated by the Curie-Weiss law and it is observed that the diffusivity increases with Sm3+ ion content. The ferroelectric hysteresis loop illustrates that the remnant polarization and coercive field increase with an increase in Sm3+ ions content. Optical properties of the ceramics were studied using ultraviolet-visible diffuse reflectance spectroscopy.

Solution Processed Cu2CoSnS4 Thin Films for Photovoltaic Applications

Earth abundant alternative chalcopyrite Cu2CoSnS4 (CCTS) thin films were deposited by a facile sol-gel process onto larger substrates. Temperature dependence of the process control of deposition and desired phase formations was studied in detail. Films were analyzed for complete transformation from amorphous to polycrystalline, with textured structures for stannite phase, as reflected from the X-ray diffraction and with nearly stoichiometric compositions of Cu:Co:Sn:S = 2:0:1:0:1:0:4:0 from EDAX analysis. Morphological investigations revealed that the CCTS films with larger grains, on the order of its thickness, were synthesized at higher temperature of 500 degrees C. The optimal band gap for application in photovoltaics was estimated to be 1.4 eV. Devices with SLG/CCTS/Al geometry were fabricated for real time demonstration of photoconductivity under A.M 1.5 G solar and 1064 rim infrared laser illuminations. A photodetector showed one order current amplification from similar to 1.9 X 10(-6) A in the dark to 2.2 x 10(-5) A and 9.8 X 10(-6) A under A.M 1.5 G illumination and 50 mW cm(-2) IR laser, respectively. Detector sensitivity, responsivity, external quantum efficiency, and gain were estimated as 4.2, 0.12 A/W, 14.74% and 14.77%, respectively, at 50 mW cm(-2) laser illuminations. An ON and OFF ratio of 2.5 proved that CCTS can be considered as a potential absorber in low cost photovoltaics applications.