Ultraviolet emission of ZnO nano-polycrystalline films by modified successive ionic layer adsorption and reaction technique

Zinc oxide (ZnO) nano-polycrystalline films were successfully prepared by modified successive ionic layer adsorption and reaction at room temperature technique which was based on the alternate immersion of substrate in the alkaline zinc precursor and deionized water. ZnO films were formed through accumulation of ZnO crystal clusters. The synthesis, microstructure, and optical properties of ZnO nano-polycrystalline films were investigated. Prepared ZnO films exhibited wurtzite structure, with good surface morphology and optical properties. Ethanolamine was employed as a complex reagent, which improved the adsorption of zinc complex with substrate. Effects of technique parameters on the properties of ZnO nano-polycrystalline films were studied in detail. Some parameters, including cycle number of preparation, ratio of zinc to ethanolamine, pH value of precursor and zinc concentration, played key roles in the deposition of ZnO nano-polycrystalline films. Intensive and sharp ultraviolet emission peaks at about 400 nm could be observed in the photoluminescence spectra.     

In-situ investigation of ZnS deposition on mica by successive ionic layer adsorption and reaction method as studied with atomic force microscopy

The growth of ZnS thin films on a mica substrate in a Successive Ionic Layer Adsorption and Reaction (SILAR) process has been studied. The films were deposited in-situ using the commercial AFM liquid cell as flow-through reactor. Reactants and rinsing water were exchanged in the cell continuously by a computer controlled valve system. In this way the film growth could be observed directly in a time resolved sequence of images taken after 1 up to 100 cycles. The results show that the stability of the films increases between 1 and 30 cycles and that a full coverage of the surface is reached after approximately 50 cycles.     

Enhanced UV photosensing properties of ZnO nanowires prepared by electrodeposition and atomic layer deposition

A new process enabling the synthesis of zinc oxide (ZnO) and Al-doped ZnO nanowires (NWs) for photosensing applications is reported. By combining atomic layer deposition (ALD) for the seed layer preparation and electrodeposition for the NW growth, high-quality ZnO nanomaterials were prepared and tested as ultraviolet (UV) sensors. The obtained NWs are grown as arrays perpendicular to the substrate surface and present diameters between 70 and 130 nm depending on the Al doping, as seen from scanning electron microscopy (SEM) studies. Their hexagonal microstructure has been determined using X-ray diffraction and Raman spectroscopy. An excellent performance in UV sensing has been observed for the ZnO NWs with low Al doping, and a maximal photoresponse current of 11.1 mA has been measured. In addition, initial studies on the stability have shown that the NW photoresponse currents are stable, even after ten UV on/off cycles.     

用离子液前驱体合成高荧光性能的ZnO和CdS纳米材料

合成了含Cd和Zn的离子液,使其提供金属离子,与Na2S反应制备离子液包裹的半导体纳米材料．X射线衍射和透射电子显微镜分析结果显示：在Cd离子液与Na2S反应中,得到的是尺寸在5～6nm、离子液包裹CdS纳米晶,而在Zn的体系中,最终产物为尺寸在1μm左右六方盘形貌的ZnO颗粒．用热力学和结晶动力学详细讨论了造成Cd和Zn体系在反应条件相同时,产物的形貌和结构差别的原因．荧光性质的测试结果表明：该方法制备的ZnO和CdS纳米粒子具有很好的荧光性能．     

Highly luminescent ZnO and CdS nanostructures prepared by ionic liquid precursors

The ionic liquids containing Cd and Zn, which served as the metal-chalcogenides precursors, were synthesized and reacted with Na₂S to synthesize the ionic-liquid-capped semiconductors. The products were detected by XRD and TEM. The results demonstrated that the CdS was composed of 5–6 nm monodispersed nanocrystals. At the same time, the ZnO composed of 1 μm hexagonal-disk nanostructure was prepared under the same experimental condition. The difference of the morphology and structures between Zn and Cd systems was discussed by thermodynamics and crystallography. The fluorescence of as-prepared ZnO and CdS showed the excellent photoluminescence.     

The admittance of a simple electrocatalytic process coupled with an additional electrochemical adsorption reaction

The admittance of the process which is under activation control, is analysed in stationary conditions for the case of coupling in parallel with an additional electrochemical adsorption reaction which influences the electrode activity for the main process.The whole system has two time constants. As its equivalent circuit, a two-time constant electrical RC circuit of a particular structure is proposed; this structure consists of two subcircuits coupled in parallel.The use of a multi-time constant RC equivalent circuit for a faradaic process results in most cases in an ambiguous interpretation of the process, since the majority of structures of such circuits have multiple solutions. The choice of the correct interpretation needs reference to arguments outside the scope of ac measurements.It is indicated that the above ambiguity does not appear if one relinquishes the use of equivalent circuits in the modelling of complex faradaic systems.     

Monte Carlo modeling of chiral adsorption on nanostructured chiral surfaces and slit pores

Adsorptive separation of chiral molecules is a powerful technique that has long been used in the chemical and pharmaceutical industries. An important challenge in this field is to design and optimize new adsorbents to provide selective discrimination of enantiomers. In this article, we introduce an off-lattice model of chiral adsorption on nanostructured surfaces and slit pores with the aim of predicting their enantioslective properties. The concept presented here involves finding the optimal chiral pattern of active sites on the pore walls that maximizes the difference between the binding energies of the enantiomers. Our initial effort focuses on chiral molecules that do not have specific interactions with the pore surface. One candidate meeting this requirement is 1,2-dimethylcyclopropane (DMCP), a chiral hydrocarbon whose interaction with a model pore surface was described using the Lennard-Jones potential. To model the adsorption of DMCP, we used the Monte Carlo simulation method. It was demonstrated that the separation of the enantiomers of DMCP is hardly obtainable because of the smoothness of the potential energy surface for molecules physisorbed in the pore. However, the simulated results allowed the identification of key factors that influence the binding of the enantiomers of DMCP to the pore walls with a special distribution of active sites. This information will be useful in future considerations of the adsorption of more complex chiral molecules.     

Sintering behavior and apatite formation of diopside prepared by coprecipitation process

Sintering behavior and bioactivity of diopside, CaMgSi₂O₆, prepared by a coprecipitation process were examined for its biomedical applicability. As-prepared powder was synthesized by adding aqueous ammonia to an ethanol solution containing Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, and Si(OC₂H₅)₄ and characterized by means of TG–DTA, XRD, and TG–MS. The dried powder was X-ray amorphous and crystallized into diopside at 845.5 °C. The glass network formation by SiO₄ tetrahedra was almost completed below 800 °C. The bioactivity of the diopside prepared by sintering the compressed powder at 1100 °C for 2 h was evaluated by immersion of the sintered body in a simulated body fluid (SBF) at 36.5 °C. Leaf-like apatite particles were found to be formed on the surface of the sintered body and grew with passage of soaking time. This apatite-forming behavior in the SBF is related to the dissolution of Ca(II) ions from the sintered body in the early stage of immersion. Thus, diopside prepared by the coprecipitation process using the metal alkoxide and the metal salts was found to have an apatite-forming ability.     

Synthesis and field emission characteristics of bilayered ZnO nanorod array prepared by chemical reaction

The uniform, large-scale, and bilayered ZnO nanorod array on silicon substrate has been synthesized by a catalyst and template-free chemical reaction in a dilute solution. The effect of different precursor ZnO films on the morphology and size of the ZnO nanorod array has been investigated. Moreover, the morphology evolution of the ZnO nanorod array with the increase of reaction time indicates that the second growth is the reason for the decrease of the ZnO nanorod diameter and the formation of the bilayered ZnO nanorod array. Finally, the field emission from the ZnO nanorod array with different diameters is presented.     

Microcapsules with macroholes prepared by the competitive adsorption of surfactants on emulsion droplet surfaces

We demonstrate a simple, unique method for preparing microcapsules with holes in their shells. Cross-linked polymelamine microcapsules are prepared by the phase-separation method. The holey shell of each microcapsule is synthesized on the surface of an oil-in-water (O/W) emulsion droplet where a water-soluble polymeric surfactant and an oil-soluble surfactant are competitively adsorbed. The water-soluble polymeric surfactant provides a reaction site for shell formation. The oil-soluble surfactant molecules seem to self-assemble while the shells are being formed, so holes appear where they assemble. The critical degree of surface coverage of an emulsion droplet by the water-soluble polymeric surfactant needed to form the holey shells is determined to be 0.90 from theoretical calculations in which competitive adsorption is considered. Theoretical consideration suggests that the size and quantity of the holes in the microcapsule shells are controlled by the composition of the surfactants adsorbed on the surface of an emulsion droplet. This theoretical consideration is confirmed by experiments. The prepared microcapsule with controllable macroholes in its shell has the potential to be used for controlled release applications and can be used to fabricate a microcapsule that encapsulates hydrophilic compounds.     

Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction

Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.     

Enhanced photoelectrochemical activity of magnetically modified TiO2 prepared by a simple ex-situ route

Journal of Solid State Electrochemistry - Modified TiO2 nanocomposites have been recognized as attractive photocatalytic materials in solar energy conversion. The aim of this study is to enhance...     

Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

Synthesis and evolution of novel hollow ZnO urchins by a simple thermal evaporation process

Delicate hollow ZnO urchins have been fabricated by thermal evaporation of metallic zinc powders in a tube furnace without the use of additive, high temperature, or low pressure. The phase transformation, morphologies, and photoluminescence evolution of the ZnO products were carefully studied and investigated with X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectra. These studies indicated that the growth of hollow ZnO urchins involves the vaporization of Zn powder, solidification of liquid droplets, surface oxidation, sublimation, and self-catalytic growth of one-dimensional nanowires.     

Investigation of confined ionic liquid in nanostructured materials by a combination of SANS, contrast-matching SANS, and nitrogen adsorption

Small-angle neutron scattering (SANS), contrast-matching SANS, and nitrogen adsorption have been utilized to investigate the confined ionic liquid (IL) [bmim][PF(6)] phase in ordered mesoporous silica MCM-41 and SBA-15. The results suggest that the pores of SBA-15 are completely filled with IL whereas a small fraction of the pore volume, the pore "core", of MCM-41 is empty. The contrast-matching SANS measurements confirm the enhanced solubility of water in IL. In addition, they provide strong evidence that water does not enter the empty pore core of MCM-41, possibly because of the preferred orientation of the IL molecules in the adsorbed layer.     

Estimation of heat of adsorption from energy distribution on heterogeneous surfaces by a theoretical analysis of adsorption process

An attempt was made to calculate theq— function (experimental heat function) from the— function (energy level function) of a heterogeneous surface. In order to obtain the heat of adsorption, adsorption was assumed to take place on each energy level of the— function. In reversible adsorption, Langmuir's monolayer model was extended to determine the occupation of energy levels by molecules. On this basis, the heat of adsorption could be calculated. Irreversible adsorption was also examined on the basis of chemical kinetics. Molecules were supposed to be held on sites to which they first collided. These calculations were useful for estimating the extent of deviation of theq— function from the— function.     

Spectroscopic and electrical properties of SiO2 films prepared by simple and cost effective sol-gel process

Amorphous SiO2 thin films were prepared on glass and silicon substrates by cost effective sol-gel method. Tetra ethyl ortho silicate (TEOS) was used as the precursor material, ethanol as solvent and concentrated HCl as a catalyst. The films were characterized at different annealing temperatures. The optical transmittance was slightly increased with increase of annealing temperature. The refractive index was found to be 1.484 at 550 nm. The formation of SiO2 film was analyzed from FT-IR spectra. The MOS capacitors were designed using silicon (100) substrates. The current-voltage (I-V), capacitance-voltage (C-V) and dissipation-voltage (D-V) measurements were taken for all the annealed films deposited on Si (100). The variation of current density, resistivity and dielectric constant of SiO2 films with different annealing temperatures was investigated and discussed for its usage in applications like MOS capacitor. The results revealed the decrease of dielectric constant and increase of resistivity of SiO2 films with increasing annealing temperature.     

The gas—solid interactions in physical adsorption of simple gases by porous non-uniform solid surfaces

A new method is proposed, to characterize the gas—solid interactions in the adsorption of simple gases by porous non-uniform solid surfaces. The theoretical bases are developed using Pierotti's statistical treatment of physical adsorption.