LaF3, CeF3, CeF3 : Tb3+, and CeF3 : Tb3+@LaF3 (core-shell) nanoplates: Hydrothermal synthesis and luminescence properties

LaF3. CeF3, CeF3:Tb3+, and CeF3:Tb3+ @LaF3 (core-shell) 2D nanoplates have been successfully synthesized by a facile and effective hydrothermal process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. The experimental results indicate that the organic additive, trisodium citrate (Cit(3-)), as a shape modifier has the dynamic effect by adjusting the growth rate of different crystal facets, resulting in forming the anisotropic geometries of the final products. The possible formation mechanisms for different products have been presented. The CeF3, CeF3:Tb3+, and CeF3:Tb3+ @LaF3 (core/shell) nanoplates show characteristic emission of Ce3+ (5d-4f) and Tb3+ (f-f), respectively.

Highly uniform and monodisperse beta-NaYF4 : Ln(3+) (Ln = Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprism crystals: Hydrothermal synthesis and luminescent properties

beta-NaYF4:Ln(3+) (Ln = Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprisms with remarkably uniform morphology and size have been synthesized via a facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. It is found that sodium citrate as a shape modifier introduced into the reaction system plays a critical role in the shape evolution of the final products. Furthermore, the shape and size of the products can be further manipulated by adjusting the molar ratio of citrate/RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Eu3+, Tb3+, Yb3+/Er3+, or Yb3+/Tm3+). Under the excitation of 397 nm ultraviolet light, NaYF4:xEu(3+) (x = 1.5, 5%) shows the emission lines of Eu3+ corresponding to D-5(0-3) -> F-7(J) (J = 0-4) transitions from 400 to 700 nm (whole visible spectral region) with different intensity, resulting in yellow and red down-conversion (DC) light emissions, respectively.

Oriented growth and luminescence of ZnO : Eu films prepared by sol-gel process

A series of Eu3+-doped ZnO films have been prepared by a sol-gel method. These films were characterized by X-ray diffraction (XRD) and photoluminecent spectra (PL). Effects of synthetic parameters, such as annealing atmosphere, temperature and concentration of doped ions, on the highly oriented crystal growth were studied in detail. The crystalline structures of films annealed in vacuum have a wurtzite symmetry with highly c-axis orientation. A characteristic D-5(0) -> F-7(J)(J = 1, 2, 3 and 4) red emission is observed due to energy transfer from the ZnO host to the doped Eu3+ in the c-oriented ZnO films.

Synthesis of nanowires, nanorods and nanoparticles of ZnO through modulating the ratio of water to methanol by using a mild and simple solution method

ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol have been synthesized by using a mild and simple solution method. The as-prepared ZnO nanostructures have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to play an important role in the formation of ZnO nanostructures. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio.

Tunable photoluminescent and cathodoluminescent properties of ZnO and ZnO : Zn phosphors

ZnO and ZnO: Zn powder phosphors were prepared by the polyol-method followed by annealing in air and reducing gas, respectively. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectra (XPS), electron paramagnetic resonance (EPR), and photoluminescence (PL) and cathodoluminescence ( CL) spectra, respectively. The results indicate that all samples are in agreement with the hexagonal structure of the ZnO phase and the particle sizes are in the range of 1-2 mu m. The PL and CL spectra of ZnO powders annealed at 950 degrees C in air consist of a weak ultraviolet emission band ( similar to 390 nm) and a broad emission band centered at about 527 nm, exhibiting yellow emission color to the naked eyes. When the sample was reduced at the temperatures from 500 to 1050 degrees C, the yellow emission decreased gradually and disappeared completely at 800 degrees C, whereas the ultraviolet emission band became the strongest. Above this temperature, the green emission ( similar to 500 nm) appeared and increased with increasing of reducing temperatures.

ZnO-based hollow microspheres: Biopolymer-assisted assemblies from ZnO nanorods

Many efforts have been made in fabricating three-dimensional (3D) ordered zinc oxide (ZnO) nanostructures due to their growing applications in separations, sensors, catalysis, bioscience, and photonics. Here, we developed a new synthetic route to 3D ZnO-based hollow microspheres by a facile solution-based method through a water-soluble biopolymer (sodium alginate) assisted assembly from ZnO nanorods. The products were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy. Raman and photoluminescence spectra of the ZnO-based hollow microspheres were obtained at room temperature to investigate their optical properties. The hollow microspheres exhibit exciting emission features with a wide band covering nearly all the visible region. The calculated CIE (Commission Internationale d'Eclairage) coordinates are 0.24 and 0.31, which fall at the edge of the white region (the 1931 CIE diagram). A possible growth mechanism of the 3D ZnO superstructures based on typical biopolymer-crystal interactions in aqueous solution is tentatively proposed, which might be really interesting because of the participation of the biopolymer.

Langmuir-Blodgett assembly of colloidal crystals combined with controlled infiltration of conducting metal oxides

Colloidal photonic crystals (PhCs) possess a periodic dielectric structure which gives rise to a photonic band gap (PBG) and offer great potential in the ability to modify or control light at visible wavelengths. Although the refractive index contrast between the void or infill and the matrix material is paramount for photonics applications, integration into real optoelectronics devices will require a range of added functionalities such as conductivity. As such, colloidal PhCs can be used as templates to direct infiltration of other functional materials using a range of deposition strategies. The work in this thesis seeks to address two challenges; first to develop a reproducible strategy based on Langmuir-Blodgett (LB) deposition to assemble high quality colloidal PhCs based on silica with precise film thickness as most other assembly methods suffer from a lack of reproducibility thickness control. The second is to investigate the use of LBdeposited colloidal PhCs as templates for infiltration with conducting metal oxide materials using vapor phase deposition techniques. Part of this work describes the synthesis and assembly of colloidal silica spheres with different surface chemical functionalities at the air-water interface in preparation for LB deposition. Modification of surface funtionality conferred varying levels of hydrophobicity upon the particles. The behaviour of silica monolayer films at the air-water interface was characterised by Brewster Angle Microscopy and surface pressure isotherms with a view to optimising the parameters for LB deposition of multilayer colloidal PhC films. Optical characterisation of LB-fabricated colloidal PhCs indicated high quality photonic behaviour, exhibiting a pseudo PBG with a sharp Bragg diffraction peak in the visible region and reflectance intensities greater than 60%. Finally the atomic layer deposition (ALD) of nominally undoped ZnO and aluminium “doped” ZnO (Al-doped ZnO) inside the pores of a colloidal PhC assembled by the LB technique was carried out. ALD growth in this study was performed using trimethyl aluminium (TMA) and water as precursors for the alumina and diethyl zinc (DEZn) and water for the ZnO. The ZnO:Al films were grown in a laminate mode, where DEZn pulses were substituted for TMA pulses in the sequences with a Zn:Al ratio 19:1. The ALD growth of ZnO and ZnO:Al in colloidal PhCs was shown to be highly conformal, tuneable and reproducible whilst maintaining excellent photonic character. Furthermore, at high levels of infiltration the opal composite films demonstrated significant conductivity.

Ultrasound-assisted green synthesis of nanocrystalline ZnO in the ionic liquid [hmim][NTf2]

For the first time, zinc oxide nanoparticles have been synthesized by the sonochemical method in an ionic liquid, 1-hexyl-3-methylimidazolium his (trifluoromethylsulfonyl) imide, liquid [hmim][NTf2] as a solvent. The morphology and structure of ZnO nanoparticles have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A possible mechanism is proposed to explain the formation of ZnO nanostructures. (C) 2008 Elsevier B.V. All rights reserved.

ZnO nanofluids: Green synthesis, characterization, and antibacterial activity

Zinc oxide nanoparticles have been synthesized by microwave decomposition of zinc acetate precursor using an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [bmim][NTf2] as a green solvent. The structure and morphology of ZnO nanoparticles have been characterized using X-ray diffraction and transmission electron microscopy. The ZnO nanofluids have been prepared by dispersing ZnO nanoparticles in glycerol as a base fluid in the presence of ammonium citrate as a dispersant. The antibacterial activity of suspensions of ZnO nanofluids against (E. coli) has been evaluated by estimating the reduction ratio of the bacteria treated with ZnO. Survival ratio of bacteria decreases with increasing the concentrations of ZnO nanofluids and time. The results show that an increase in the concentrations of ZnO nanofluids produces strong antibacterial activity toward E. coli. (C) 2010 Elsevier B.V. All rights reserved.

The transformation of cyclohexane on Zn/H-ZSM-5 zeolites

Samples of Zn/H-ZSM-5 zeolite were prepared by impregnation of the parent zeolite with Zn(NO3)(2). The state of zinc in the samples was analyzed by XPS measurements, (ald the degree of reduction for the zinc oxide on the ZSM-5 zeolite surface in hydrogen atmosphere was determined, as well as the influence of this reducing treatment upon the activity and selectivity for aromatics of zeolites in aromatization of cyclohexane. It resulted that the degree of reduction depends on the concentration of zinc in the zeolite and is influenced by the presence of alumina binder. The results of the activity and selectivity to aromatics were correlated with the reduction of zinc oxide.

One-step synthesis of ZnO nanosheets: a blue-white fluorophore

Zinc oxide is synthesised at low temperature (80A degrees C) in nanosheet geometry using a substrate-free, single-step, wet-chemical method and is found to act as a blue-white fluorophore. Investigation by atomic force microscopy, electron microscopy, and X-ray diffraction confirms zinc oxide material of nanosheet morphology where the individual nanosheets are polycrystalline in nature with the crystalline structure being of wurtzite character. Raman spectroscopy indicates the presence of various defects, while photoluminescence measurements show intense green (centre wavelength approximately 515 nm) blue (approximately 450 nm), and less dominant red (approximately 640 nm) emissions due to a variety of vacancy and interstitial defects, mostly associated with surfaces or grain boundaries. The resulting colour coordinate on the CIE-1931 standard is (0.23, 0.33), demonstrating potential for use as a blue-white fluorescent coating in conjunction with ultraviolet emitting LEDs. Although the defects are often treated as draw-backs of ZnO, here we demonstrate useful broadband visible fluorescence properties in as-prepared ZnO. 

Dual interfacial modifications of hierarchically structured iodine-doped ZnO photoanodes for high-efficiency dye-sensitized solar cells

A nanocomposite porous electrode structure consisting of hierarchical iodine-doped zinc oxide (I-ZnO) aggregates combined with the two simple solution-processed interfacial modifications i.e. a ZnO compact layer (CL) and a TiO2 protective layer (PL) has been developed in order to understand electron transport and recombination in the photoanode matrix, together with boosting the conversion efficiency of I-ZnO based dye-sensitized solar cells (DSCs). Electrochemical impedance spectra demonstrate that ZnO CL pre-treatment and TiO2 PL post-treatment synergistically reduce charge-transfer resistance and suppress electron recombination. Furthermore, the electron lifetime in two combined modifications of IZnO + CL + PL photoelectrode is the longest in comparison with the other three photoelectrodes. As a consequence, the overall conversion efficiency of I-ZnO + CL + PL DSC is significantly enhanced to 6.79%, with a 36% enhancement compared with unmodified I-ZnO DSC. Moreover, the stability of I-ZnO + CL + PL cell is improved as compared to I-ZnO one. The mechanism of electron transfer and recombination upon the introduction of ZnO CL and TiO2 PL is also proposed in this work.

Fully transparent ZnO thin-film transistor produced at room temperature

Advanced Materials, Vol. 17, nº 5

Estudo de problemas de corrosão em cabos de aço zincado

As cablagens metálicas são aplicadas abundantemente na montagem dos automóveis, para accionamento de diversos dispositivos, tais como a abertura de portas, abertura da mala ou da tampa do motor, abertura do tampão do depósito de combustível, accionamento do travão de mão, etc. Nas cablagens metálicas utilizadas na indústria automóvel é comummente utilizado arame entrançado de aço zincado, o qual foi já estudado resistir à agressividade das condições em que habitualmente trabalham. No entanto, para que esta estabilidade de funcionamento exista, torna-se necessário que a qualidade do arame entrançado seja a mais adequada. O presente trabalho surgiu da necessidade de averiguar quais as razões que estariam por detrás do aparecimento de quantidades relativamente abundantes de Óxido de Zinco (ZnO) em cablagens metálicas utilizadas na indústria automóvel, poucas semanas depois do seu fabrico. O estudo foi levado a cabo com vista a satisfazer as necessidades de investigação deste problema por parte de uma empresa de cablagens metálicas instalada no nosso país, a FicoCables, estabelecendo quais as causas-raiz deste problema, com vista a poder evitálo no futuro. O trabalho foi concluído com sucesso, tendo-se averiguado que existiam lacunas graves de limpeza do cabo entre as operações de conformação do arame e de zincagem, com deposição de elementos potenciadores do Óxido de Zinco na interface, os quais viriam a revelar-se cruciais à posteriori, para a nucleação e desenvolvimento do Óxido de Zinco na superfície do cabo.