A novel method for the assembly of nano-zeolite crystals on porous stainless steel microchannel and then zeolite film growth

A new method that is simple, efficient, and clean was developed for seeding surfaces with a monolayer of covalently bonded zeolite seeds. This method was used to seed the microchannels fabricated on porous stainless steel with NaA nano-zeolites enabling the deposition and growth of defect-free zeolite film. The NaA nano-zeolites were attached to the surface of the stainless steel via alkoxysilane linkers (e.g., 3-chloropropyltrimethoxysilane and 3-aminopropyltrimethoxysilane) grafted on the stainless steel. The NaA zeolite grown on the microchannel by hydrothermal synthesis consists of intergrown, (1 1 1)-oriented pyramidal NaA crystals that completely clad the stainless steel grains. The zeolite cladding the grains grew uniformly until the zeolite layers of neighboring grain impinges, then intergrew to fully bridge the gaps between the grains forming a defect-free film layer. A separation factor of 10,000 and a flux of 0.04 kg m⁻² h⁻¹ were obtained for water pervaporation from a 3 wt% water-benzaldehyde solution at room temperature.     

Synthesis and characterization of (h 0 h)-oriented silicalite-1 films on α-Al2O3 substrates

A simple and well-designed synthesis procedure is proposed to fabricate silicalite-1 films on porous α-Al₂O₃ substrates on purpose of preventing the aluminum leaching. The continuous and 2 μm thick seed layer of silicalite-1 crystals is fabricated by using a spin coater. The first-time seeded growth is performed to synthesize a thin layer of intergrown ZSM-5 crystals on the silicalite-1 seed layer, where the use of low alkalinity and short synthesis time is to reduce the aluminum leaching. The intergrown layer of ZSM-5 crystals serves as a barrier to block the aluminum leaching from porous α-Al₂O₃ substrates in the second-time seeded growth, leading to the formation of ca. 11 μm thick intergrown and oriented silicalite-1 films with an extremely high Si/Al ratio. According to SEM images and XRD measurements, the as-synthesized silicalite-1 film is dense, continuous, and (1 0 1)-oriented. The electron probe microanalysis (EPMA) of the resulting film demonstrates that there is no aluminum leaching in the second-time seeded growth. The leaking tests confirm that non-zeolitic pores in the silicalite-1 film are negligible.     

Growth of crystalline quartz films with AT-cut plane by means of catalyst-enhanced vapor-phase epitaxy under atmospheric pressure

Crystalline quartz films with an AT-cut plane have been grown by catalyst-enhanced vapor-phase epitaxy, at atmospheric pressure, using two quartz buffer layers on a sapphire (110) substrate. In this method, the first quartz buffer layer was deposited on the sapphire (110) substrate at 773 K. After annealing at 823 K, the second buffer layer was deposited at 723 K. The crystal quartz epitaxial layer was then grown at 843 K. The X-ray full-width-at-half-maximum (FWHM) value of the crystalline quartz film obtained was smaller than that of crystalline quartz prepared using a hydrothermal process. The crystalline quality of the quartz films was dependent on the thickness of the buffer layers. Furthermore, it was found that angle control of the cut plane depended on the film thickness of the second buffer layer. The quartz films grown by vapor phase epitaxy show good oscillation characteristics at room temperature.     

High-quality oriented ZnO films grown by sol-gel process assisted with ZnO seed layer

High-quality oriented ZnO films were prepared on silicon and quartz glass by sol-gel, assisted with a ZnO seed layer. The effects of the seed layer on the orientation, morphology and optical properties of ZnO films were investigated. Results show that the seed layer can effectively induce the growth of high-quality oriented ZnO films on two substrates, and the effectiveness of the seed layer strongly depends on preparation conditions, i.e., the spin-coating layer number and the preheating temperature. ZnO films with five layers on the seed layer preheated at 500 °C exhibit the single (0 0 2) orientation, which is much stronger than that on the flat substrate. Additionally, ZnO films on the seed layer show a denser internal structure and higher optical quality than that on the flat substrate. At ten layers, however, ZnO films on the seed layer show the multiple-orientation, which is similar to that on the flat substrate. Finally, the physical mechanism underlying the growth behavior of ZnO films assisted with the seed layer was discussed.     

Membranes and films of zeolite and zeolite-like materials

An ideal structure of zeolite membrane should be a slice of a perfect zeolite crystal attached on a porous metal or ceramic support. To maximize the throughput, the zeolite layer must be very thin, limited only by the cell dimension of zeolite. Separation of a mixture may then be achieved based on the molecular sieving ability of zeolite, which allows only molecules smaller than a critical size to pass through. A variety of methods have been reported for the preparation of zeolite membranes, but so far a perfect epitaxial zeolite layer is still out of reach and only a polycrystalline zeolite membrane can be obtained. The first part of this review focuses on the permeation of gases and vapors through a polycrystalline zeolite membrane as a separation means. The effect of microstructure on permeance will also be discussed, as well as the preparation methods leading to different microstructures. In addition to the usage as a shape-selective membrane, thin films of zeolite and zeolite-like molecular sieves can also serve as hosts for the encapsulation and orientation of guest atoms and molecules and their clusters. In the second part of this review, the production of layers of aligned microporous molecular sieve crystals on supports and the fabrication of supported thin zeolite-like nanoporous silica films as well as their potential applications on the preparation of advanced materials are discussed.     

Electrochemical and hydrothermal deposition of ZnO on silicon: from continuous films to nanocrystals

This article presents the study of the electrochemical deposition of zinc oxide from the non-aqueous solution based on dimethyl sulfoxide and zinc chloride into the porous silicon matrix. The features of the deposition process depending on the thickness of the porous silicon layer are presented. It is shown that after deposition process the porous silicon matrix is filled with zinc oxide nanocrystals with a diameter of 10–50 nm. The electrochemically deposited zinc oxide layers on top of porous silicon are shown to have a crystalline structure. It is also shown that zinc oxide crystals formed by hydrothermal method on the surface of electrochemically deposited zinc oxide film demonstrate ultra-violet luminescence. The effect of the porous silicon layer thickness on the morphology of the zinc oxide is shown. The structures obtained demonstrated two luminescence bands peaking at the 375 and 600 nm wavelengths. Possible applications of ZnO nanostructures, porous and continuous polycrystalline ZnO films such as gas sensors, light-emitting diodes, photovoltaic devices, and nanopiezo energy generators are considered. Aspects of integration with conventional silicon technology are also discussed.     

Molecular dynamics simulation of pervaporation in zeolite membranes

The pervaporation separation of liquid mixtures of water/ethanol and water/methanol using three zeolite (Silicalite, NaA and Chabazite) membranes has been examined using the method of molecular dynamics. The main goal of this study was to identify intermolecular interactions between water, methanol, ethanol and the membrane surface that play a critical role in the separations. This would then allow better membranes to be designed more efficiently and systematically than the trial-and-error procedures often being used. Our simulations correctly exhibited all the qualitative experimental observations for these systems, including the hydrophobic or hydrophilic behaviour of zeolite membranes. The simulations showed that, for Silicalite zeolite, the separation is strongly influenced by the selective adsorption of ethanol. The separation factor, as a consequence, increases almost exponentially as the ethanol composition decreases. For ethanol dehydration in NaA and Chabazite, pore size was found to play a very important role in the separation; very high separation factors were therefore possible. Simulations were also used to investigate the effect of pore structure, feed compositions and operating conditions on the pervaporation efficiency. Finally, our simulations also demonstrated that molecular simulations could serve as a useful screening tool to determine the suitability of a membrane for potential pervaporation separation applications. Simulations can cost only a small fraction of an experiment, and can therefore be used to design experiments most likely to be successful.     

Effect of seed layer on the growth of well-aligned ZnO nanowires

We demonstrate that vertical well-aligned crystalline ZnO nanowire arrays were grown on ZnO/glass substrates by a low-temperature solution method. Different thicknesses of ZnO seed layers on glass substrates were prepared by radio-frequency sputtering. In this work it was found that the morphology of ZnO nanowires strongly depends on the thickness of ZnO seed layers. The average diameter of nanowires is increased from 50 to 130 nm and the nanowire density is decreased from 110 to 60 μm⁻² while the seed layer thickness is varied from 20 to 1000 nm. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced carrier collection of hybrid polymer photovoltaic devices based on ZnO.     

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31-36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2-3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300-400 nm thickness.     

Ceramic membrane by tape casting and sol-gel coating for microfiltration and ultrafiltration application

Alumina membrane filters in the form of thin (0.3-0.8 mm) discs of 25-30 mm diameter suitable for microfiltration application have been fabricated by tape-casting technique. Further using this microfiltration membrane as substrate, boehmite sol coating was applied on it and ultrafiltration membrane with very small thickness was formed. The pore size of the microfiltration membrane could be varied in the range of 0.1-0.7 μm through optimisation of experimental parameter. In addition, each membrane shows a very narrow pore size distribution. The most important factor, which determines the pore size of the membrane, is the initial particle size and its distribution of the ceramic powder. The top thin ultrafiltration, boehmite layer was prepared by sol-gel method, with a thickness of 0.5 μm. Particle size of the sol was approximately 30-40 nm. The structure and formation of the layer was analysed through TEM. At 550 °C formation of the top layer was completed. The pore size of the ultrafiltration membrane measured from TEM micrograph was almost 10 nm. Results of microbial (Escherichia coli—smallest-sized water-borne bacteria) test confirm the possibility of separation through this membrane     

Vanadium carbide coatings on die steel deposited by the thermo-reactive diffusion technique

Thermal reactive diffusion coating of vanadium carbide on DIN 1.2367 die steel substrate was performed in a powder mixture consisting of ferro-vanadium, ammonium chloride, alumina and naphthalene at 950, 1050 and 1150 °C for 1-5 h. The carbide layers were characterized by means of microstructure, microhardness, X-ray diffraction and chemical analysis. Depending on the coating process time and temperature, the thickness of the vanadium carbide layer formed on the substrate ranged from 2.3 to 23.2 μm. The hardness of vanadium carbide layers was about 2487 HV. Dry wear tests for uncoated and coated DIN 1.2367 die steel were carried out on pin-on-disk configuration and at a sliding speed of 0.13 m/s. The results showed superior wear properties of the coated samples. The kinetics of vanadium carbide coating by the pack method was also studied and the activation energy for the thermo-reactive diffusion process was estimated to be 173.2 kJ/mol.     

Highly porous nickel oxide thin films prepared by a hydrothermal synthesis method for electrochromic application

We report NiO nanowall thin films prepared by a facile hydrothermal synthesis method and their electrochromic application. The as-prepared porous nanowall NiO thin films show a highly porous structure built up by many interconnected nanoflakes with a thickness of about 30 nm. The electrochromic performances of the NiO films are characterized by means of UV–vis spectroscopy and cyclic voltammetry (CV) measurements. The effect of the annealing temperature on electrochromic properties is discussed. The NiO nanowall film annealed at 300 °C exhibits much better electrochromic performance than those counterparts annealed at higher temperature. The film annealed at 300 °C exhibits a noticeable electrochromism with reversible color changes from transparent to brown dark and presents a transmittance variation with 77% at 550 nm. The NiO nanowall film also shows good reaction kinetics with fast switching speed, and the coloration and bleaching times are 3 s and 4 s, respectively. The improved electrochromic performances are due to the porous morphological characteristics with fast ion and electron transfer resulting in fast reaction kinetics and high color contrast.     

Porous PVdF-HFP/P123 electrolyte membrane containing flexible quasi-solid-state dye-sensitized solar cells produced by the compression method

Flexible quasi-solid-state dye-sensitized solar cells (DSSCs) with porous poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)/polyethylene oxide-co-polypropylene oxide-co-polyethylene oxide (P123) electrolyte membranes were fabricated and their photocurrent–voltage (I–V) characteristics are studied. Flexible TiO₂ photoelectrodes were prepared using the compression method and porous PVdF-HFP/P123 membranes, by the nonsolvent-induced phase inversion technique. To activate the electrolyte membrane, the membrane was immersed in liquid-state electrolyte. Increased compression pressure improved the interconnection between TiO₂ nanoparticles, enhancing the photovoltaic performances of the flexible liquid-state DSSCs to a maximum of 3.92% efficiency. Meanwhile, the overall pore structure of the PVdF-HFP/P123 membranes was controlled by varying the blend ratio of P123 to PVdF-HFP. Membranes higher in P123 content gave larger pores and pore volume, increasing the electrolyte uptake of the porous membrane, and thus the ionic conductivity of the electrolyte membrane as well. The photovoltaic characteristics of the flexible quasi-solid-state DSSCs containing a porous PVdF-HFP/P123 electrolyte membrane showed a maximum at 50 wt% P123 content, which gave a short-circuit current density (J_sc) value of 7.28 mA/cm², an open-circuit voltage (V_oc) of 0.67 V, a fill factor (FF) of 0.61 and an energy conversion efficiency (η) of 2.98%. Furthermore, the device designed in this study showed good durability compared to those based on liquid-state electrolyte.     

Oxygen transport in La2NiO4 + δ: Assessment of surface limitations and multilayer membrane architectures

The steady-state oxygen permeation through dense La₂NiO_4 + δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La₂NiO_4 + δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in the case when one dense layer, ~ 60 μm in thickness, is sandwiched between relatively thin (< 150 μm) porous layers. Whilst the permeability of such membranes is still affected by surface-exchange kinetics, increasing thickness of the porous supporting components leads to gas diffusion limitations.     

The effect of pre-annealing of sputtered ZnO seed layers on growth of ZnO nanorods through a hydrothermal method

Oriented ZnO nanorods were grown on ion-beam-sputtered ZnO seed layers through a hydrothermal approach without any metal catalyst. The sputtered ZnO seed layers were pre-annealed at different temperatures before the growth of ZnO nanorods. The effects of pre-annealing of the ZnO seed layers on the growth rate, crystallinity and optical properties of ZnO nanorods thereon were studied. The obtained ZnO nanorods had a wurtzite structure and grew along the preferential [0001] orientation with a normal direction to the substrates. Results show that the growth rate and density of the ZnO nanorods strongly depend on the pre-treatment conditions of the ZnO seed layer. With higher pre-treatment temperature, the crystallinity and surface characteristics of the ZnO seed layer were improved and thereafter the growth rate of ZnO nanorods thereon increased. Photoluminescence spectroscopy results show that the UV emission also becomes stronger and sharper with increasing annealing temperature of the ZnO seed layer.     

Alignment-controlled hydrothermal growth of well-aligned ZnO nanorod arrays

ZnO nanorod arrays (ZNAs) were prepared via a two-step seeding and solution hydrothermal growth process. Effects of preparing parameters such as seed layer, colloid concentration, substrate and precursor concentration, on the alignment control of ZNAs were systematically investigated. The deviation angle of ZnO nanorods was measured to evaluate the alignment of arrays. Results show that seed layer not only controls the vertical orientation of ZNAs, but also the compactness of ZNAs. Altering colloid concentration and substrate can influence the microstructure of ZnO seed layer and affect the ordered alignment of ZNAs. The precursor concentration has an insignificant effect on the alignment of ZNAs but has great impact on the morphology of ZNAs. Alignment-controlled and well-aligned ZnO nanorods with different diameter and aspect ratio can be obtained by properly controlling the preparing parameters. A growth mechanism was proposed for the growth of ZnO nanorods.     

Thickness dependence on thermal stability of sputtered Ag nanolayer on Ti/Si(1 0 0)

Thermal stability of Ag layer on Ti coated Si substrate for different thicknesses of the Ag layer have been studied. To do this, after sputter-deposition of a 10 nm Ti buffer layer on the Si(1 0 0) substrate, an Ag layer with different thicknesses (150-5 nm) was sputtered on the buffer layer. Post annealing process of the samples was performed in an N₂ ambient at a flow rate of 200 ml/min in a temperature range from 500 to 700 °C for 30 min. The electrical property of the heat-treated multilayer with the different thicknesses of Ag layer was examined by four-point-probe sheet resistance measurement at the room temperature. Phase formation and crystallographic orientation of the silver layers were studied by θ-2θ X-ray diffraction analysis. The surface topography and morphology of the heat-treated films were determined by atomic force microscopy, and also, scanning electron microscopy. Four-point- probe electrical measurement showed no considerable variation of sheet resistance by reducing the thickness of the annealed Ag films down to 25 nm. Surface roughness of the Ag films with (1 1 1) preferred crystallographic orientation was much smaller than the film thickness, which is a necessary condition for nanometric contact layers. Therefore, we have shown that the Ag layers with suitable nano-thicknesses sputtered on 10 nm Ti buffer layer were thermally stable up to 700 °C.     

Template-free synthesis of TS-1 zeolite film on tubular mullite support

The TS-1 film on tubular mullite support was prepared by secondary growth via template-free route using tetraethyl orthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) as silica and titanium sources. The as-made films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and ultraviolet-visible spectroscopy (UV-vis). Continuous TS-1 seed layer was bonded tightly to the mullite substrates. After secondary growth in the template-free synthesis solution, intergrown TS-1 zeolite film with the typical MFI-type structure was formed on the outer surface of support. The Si atoms in zeolitic lattice were found to be isomorphously substituted by Ti atoms which existed only in tetrahedral coordination. The thickness of the obtained TS-1 zeolite film was less than 20 μm.     

A layer of reduced switchable polarisation in (Pb,La)TiO3 films leading to a thickness dependence of the ferroelectric parameters

Two sets of ferroelectric (Pb,La)TiO₃ thin films have been prepared by a diol-based sol-gel technique by varying the heating rate to the crystallisation temperature of 650°C. Films of increasing thickness were obtained by repeating the solution deposition from 1 to 5 times. The switchable polarisation was evaluated from hysteresis loops and from the integration of the switching current transients during pulse tests, and was found to significantly depend on film thickness. Measurements of the thickness dependence of the reciprocal capacitance and Rutherford backscattering spectrometry results indicate that a layer with different dielectric permittivity, and composition for one set of films, existed next to the bottom electrode. In the one set of films it originated because of interdiffusion between the film and the substrate, while in the other set, it was linked to the tensile stress at the film/substrate interface. These layers had a reduced switchable polarisation, which caused the observed dependence of their properties on thickness.     

Experimental IR study and ab initio modelling of ethylene adsorption in a MFI-type host zeolite

Different ab initio methods and experimental results are used to investigate the effect of the adsorption of one ethylene molecule on silicalite-1, a MFI-type zeolite. We used simplified models to simulate a portion of a straight or sinusoidal channel of silicalite-1 at a quantum level. The calculated absorption spectra of the models are qualitatively in good agreement with the experimental FTIR spectrum of silicalite-1. Additionally we simulate the FTIR spectrum of the isolated ethylene molecule and that of an ethylene molecule in interaction with the above-mentioned zeolite models. Results are discussed depending on the method and specific basis set and compared with experiments and previous molecular dynamics simulations.