Macropore Arrays Research Articles

A molecularly imprinted photonic polymer sensor with high selectivity for tetracyclines analysis in food

A molecularly imprinted photonic polymer (MIPP) sensor for respective detection of tetracycline, oxytetracycline and chlortetracycline is developed based on the combination of a colloidal crystal templating method and a molecular imprinting technique. Colloidal crystal templates are prepared from monodisperse polystyrene colloids. The molecularly imprinted polymer, which is embodied in the colloidal crystal templates, is synthesized with acrylic acid and acrylamide as monomers, N,N'-methylene bisacrylamide as a cross-linker and tetracyclines (TCs) as imprinting template molecules. After removal of the colloidal crystal template and the molecularly imprinted template, the resulted MIPP consists of a three-dimensional, highly ordered and interconnected macroporous array with a thin hydrogel wall, where nanocavities complementary to analytes in shape and binding sites are distributed. The response of MIPP to TCs stimulants in aqueous solution is detected through a readable Bragg diffraction red-shift, which is due to the lattice change of MIPP structures responding to their rebinding to the target TCs molecules. A linear relationship was found between the Δλ and the concentration of TCs in the range from 0.04 μM to 0.24 μM. With this sensory system, direct and selective detection of TCs has been achieved without using label techniques and expensive instruments. The developed method has been applied successfully to detect tetracycline in milk and honey samples.

Highly Stable and Reusable Multimodal Zeolite TS‐1 Based Catalysts with Hierarchically Interconnected Three‐Level Micro–Meso–Macroporous Structure

Microporous titanosilicates, such as TS-1, are a class of important catalysts with high activities and selectivities coupled with environmentally benign catalytic performance, and play a vital role in a series of catalytic oxidation reactions with H2O2. [1] However, an important drawback of these titanosilicate catalysts is that their pores are too small to be accessed by bulky reactants, and this hinders their use in the fine-chemical and pharmaceutical industries. Nanosized TS1 materials were initially considered as a potential approach to improving the accessibility of such catalysts because, owing to their larger external surface areas, they have more active sites than conventional zeolites. However, complex processes for their separation from reaction products and the ease of aggregation of the nanosized zeolites during synthesis and catalytic reactions limit seriously their development. Recent progress in the field has seen the incorporation of titanium ions into the framework of mesoporous materials and grafting of a titanocene complex onto mesoporous silica. These ordered mesoporous titanosilicates have pore diameters of 2–8 nm and exhibit catalytic properties for the oxidation of bulky reactants under mild conditions. Unfortunately, when compared with TS-1, their catalytic activity, for example, that of Ti-MCM-41, is relatively low. This is attributed to the difference in the titanium coordination environment (amorphous nature of the mesoporous wall). A series of ordered mesoporous titanosilicates have been synthesized by assembly of preformed titanosilicate zeolite precursors with triblock copolymers, and showed good activity in the oxidation of small molecules such as phenol and styrene as well as bulkier molecules like trimethylphenol. However, calcination leads to a significant reduction in catalytic activity towards both small and bulky molecules, due to the relatively low stability of catalytically active fourcoordinate titanium sites in these materials compared to those in TS-1. The relatively low stabilities of both the titanium species and the structure in catalytic processes may be related to imperfectly condensed mesoporous walls. Possibly, the degree of crystallization of the mesoporous walls should be enhanced. Therefore, mesoporous titanosilicates with an fully crystalline structure are highly desirable. Novel 3D crystalline metallosilicates with expanded pores were recently synthesized from 2D Ti-MWW (MWW-type titanosilicate) precursors, according to a strategy of inserting a monomeric Si source into the interlayer spaces. The resultant materials showed expanded pore apertures, high crystallinity, and outstanding redox catalytic properties towards bulky molecules. Consequently, increasing the pore size has been one of the goals of structural control, to permit the penetration of large molecules into the host porous structure. Macroporous titanosilicates with crystalline structure are particularly interesting, due to their improved transport properties. Well-defined macroporous arrays should show optimal fluxes, whereby diffusion is not a limiting issue. Therefore more efficient catalysts could be targeted through the controlled design of hierarchically meso–macroporous titanosilicates with crystalline structure, principally by introducing the multipore system evenly throughout the framework. The ideal hierarchically porous structure in efficient titanosilicate catalysts should contain a macropore system to enhance mass transport, mesopores for precise selectivity, and microporous zeolitic structure to provide the catalytically active sites. More attractive applications could be developed if new titanosilicate catalysts could be constructed with hierarchical micro–meso–macropore systems yet still be composed of the same highly active [*] Dr. L.-H. Chen , Dr. Y. Li, Dr. X.-Y. Yang , Prof. B.-L. Su State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology Luoshi Road 122, Wuhan 430070 (China) E-mail: xyyang@whut.edu.cn baoliansu@whut.edu.cn

Fabrication of Ordered or Disordered Macroporous Structures with Various Ceramic Materials from Metal Oxide Nanoparticles or Precursors

Two different schemes were adopted to fabricate ordered macroporous structures with face centered cubic lattice of air spheres. Monodisperse polymeric latex suspension, which was synthesized by emulsifier-free emulsion polymerization, was mixed with metal oxide ceramic nanoparticles, followed by evaporation-induced self-assembly of the mixed hetero-colloidal particles. After calcination, inverse opal was generated during burning out the organic nanospheres. Inverse opals made of silica or iron oxide were fabricated according to this procedure. Other approach, which utilizes ceramic precursors instead of nanoparticles was adopted successfully to prepare ordered macroporous structure of titania with skeleton structures as well as lithium niobate inverted structures. Similarly, two different schemes were utilized to obtain disordered macroporous structures with random arrays of macropores. Disordered macroporous structure made of indium tin oxide (ITO) was obtained by fabricating colloidal glass of polystyrene microspheres with low monodispersity and subsequent infiltration of the ITO nanoparticles followed by heat treatment at high temperature for burning out the organic microspheres. Similar random structure of titania was also fabricated by mixing polystyrene building block particles with titania nanoparticles having large particle size followed by the calcinations of the samples.

Introduction of a planar defect in a molecularly imprinted photonic crystal sensor for the detection of bisphenol A

This paper reports the preparation of a molecularly imprinted inverse opal hydrogel containing a 2D defect layer, by combining the Langmuir–Blodgett technique and the photonic crystal template method. By coupling the exceptional characteristics of molecularly imprinted polymers, sensitive to the presence of a target molecule, and those of photonic crystals in a single device, we could obtain a defect-embedded imprinted photonic polymer consisting in a three-dimensional, highly-ordered and interconnected macroporous array, where nanocavities complementary to analytes in shape and binding sites are distributed. As a proof of concept, we prepared a three-dimensional macroporous array of poly(methacrylic acid) (PMAA) containing molecular imprints of bisphenol A (BPA) and a planar defect layer consisting in macropores of different size. The optical properties of the resulting inverse opal were investigated using reflection spectroscopy. The defect layer was shown to enhance the sensitivity of the photonic crystal material, opening new possibilities towards the development smart optical sensing devices.

MIM in 3D: Dream or reality? (invited)

Last decades great effort has been put in the development of 3D capacitors. These capacitors are used for RF decoupling and should therefore have a high capacitance density associated with a sufficient breakdown voltage. Increased capacitance densities have been achieved by exploring the use of the third dimension in silicon, e.g. pores and trenches and considering dielectric layers with a higher dielectric permittivity, so-called higher k dielectrics formed by alternative deposition techniques, e.g. Atomic Layer Deposition (ALD). Starting with the formation of wide pores using the “ Bosch” process, we eventually developed high aspect ratio macropore arrays that have been used as the carrier substrate for the capacitors. These arrays have been filled by conventional LPCVD MOS layers with ONO-dielectrics and in situ doped polycrystalline silicon, initially as single layer stack, but also as a double stack capacitor (MOSOS/MIMIM). Further, higher k materials, such as Al 2O 3, Ta 2O 5, HfO 2 and even rare earth materials with nanoclusters have been considered in our attempt to achieve ultimate capacitance densities. Our current record capacitance density has been realized using a multiple capacitor structure with ALD electrodes and high k dielectric layers, the so-called MIMIMIM capacitor.

Optical and electrochemical properties of ordered macroporous gold array on the ITO surface

The electrochemical and optical properties of transparent, two-dimensional macroporous gold film were investigated. Colloidal crystal templates were assembled onto indium-doped tin oxide (ITO) glass surface through vertical depositing method. Following gold electrodeposition, they were removed by dissolution with tetrahydrofuran (THF). The highly ordered macroporous gold array was achieved. It was characterized by scanning electron microscope (SEM) and ultraviolet visible (UV–vis) spectrophotometry. The optical transparency of the gold film was near 25% and fairly constant between 300 and 900 nm. The macroporous gold film electrode was mounted into a thin-layer transmission cell. The electrochemical response was evaluated by thin-layer cyclic voltammograms (CV) of the Fe (CN) 6 3−/Fe (CN) 6 4− couple. Thin-layer cell exhibits good shape of waves and nearly symmetric cathodic and anodic waves. E 0′ value and n of TMPD + /TMPD (TMPD is acronyms for N,N,N′,N′-tetramethyl-p-phenylenediamine, and TMPD + is its mono-cation radical) couple were determined. Furthermore, results demonstrated electrolytic equilibrium was faster reached in macroporous gold film than ITO electrode.

Macroporous microelectrode arrays for measurements with reduced noise

In order to understand the dynamics of large neuronal networks, one of the challenges of nowadays neuroscience is to reach the possibility to record large ensembles of neurons simultaneously. In this context, microelectrode arrays (MEA) are gaining a rapidly growing interest. However, among several major problems, increased thermal noise becomes a limiting factor when the size of the microelectrodes becomes small and weak electrical activity has to be measured. In this work, we try to overcome this limitation by using porous microelectrode arrays. In order to combine the advantages of an overall small geometric surface with an increased active surface, standard MEAs are covered with a porous metal overlayer, obtained via the colloidal crystal templating method. Using this approach, we were able to obtain first promising results with respect to a reduction of the noise level.

Photonic crystal pH sensor containing a planar defect for fast and enhanced response

This paper describes the introduction of a planar defect layer within inverse opal hydrogels for the elaboration of tunable photonic crystal pH sensors with enhanced optical properties. We demonstrate a mechanically robust, highly sensitive and fast response photonic crystal fabricated by a stepwise strategy combining the Langmuir–Blodgett technique and the photonic crystal template method. The resulting material consists of a three-dimensional, highly-ordered and interconnected macroporous array of poly(methacrylic acid), which is a hydrogel sensitive to pH. The optical properties of these inverse opals were investigated using reflection spectroscopy. The defect layer was shown to enhance both the sensitivity and the response time of the photonic crystal sensing materials.

Three novel ways of making thin-film crystalline-silicon layers on glass for solar cell applications

Monolithic solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. This paper describes three different approaches that we are currently investigating to address the challenge to form high-quality crystalline-silicon layers on glass substrates. The SLiM-Cut approach is a wafering technique that results in 50-micron thick layers with a minimum of material loss. We show that crack initiation can be used as a means to better control the lift-off process. The epifree approach involves the lift-off of ultra-thin monocrystalline films formed by reorganization of cylindrical macropore arrays in silicon upon annealing. So far, films with a thickness of around 1 μm and very simple cells with an efficiency of up to 4.1% have been achieved. Finally, a seed layer approach is presented based on the epitaxial thickening by thermal CVD of monocrystalline silicon layers bonded on glass-ceramic substrates. Very promising cell efficiencies of 11% and Voc values of up to 610 mV have been achieved using a very simple and non-optimized cell structure.

Epitaxy‐free monocrystalline silicon thin film: first steps beyond proof‐of‐concept solar cells

AbstractThe “Epifree” process involves the lift‐off of a high‐quality monocrystalline film formed by reorganization upon annealing of cylindrical macropore arrays in silicon, and can thus provide high‐quality silicon films without resorting to costly epitaxy. The challenge of this new process lies in etching controlled and regular pores in silicon in a cost‐efficient way, and in developing a process compatible with the difficulty of handling a micron‐thin material. Proof‐of‐concept cells have previously been achieved and this paper presents the latest progress, with a first development of thicker films and the inclusion of rear‐side passivation. The energy‐conversion efficiency of 1‐µm‐thin Epifree cells was improved from 2.6 to 4.1% by depositing a stack of amorphous silicon (a‐Si) layers as rear‐side passivation. The increase in Voc was, however, limited and bound to a drop in Jsc. The choice of a‐Si was revealed to be unsuitable because of the thinness of the film and the presence of a full aluminum rear contact. The thinness of the film leads to a decrease in rear‐side reflectivity by the a‐Si absorption, and the aluminum, although not leading to crystallization, partly migrates inside the a‐Si stack upon anodic bonding as shown by TEM. These factors indicate that an alternative surface passivation should be developed. In parallel to process developments, the material was thickened by modifying the macropore array dimensions, leading to a 2.4‐µm‐thick material over 1 cm × 1 cm areas. The efficiency of the next cells is expected to increase with this thicker material. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of voltage on photo-electrochemical etching of n-type macroporous silicon arrays

The influence of voltage on photo-electrochemical etching (PEC) of macroporous silicon arrays (MSA) was researched. According to the theory of the space charge region, I—V scan curves and the reaction mechanism of the n-type silicon anodic oxidation in HF solution under different current densities, the pore morphology influenced by the working voltage were studied and analyzed in detail. The results show that increasing the etching voltage will lead to distortion of the pore morphology, decreasing etching voltage will result in an increase in the blind porosity, and the constant etching voltage for a long time will cause gradual bifurcation. Through the optimization of the process parameters, the perfect MSA structure with a pore depth of 317 μm, a pore size of 3 μm and an aspect ratio of 105 was obtained.

Synthesis of a Silica Material with Surface Macroporous Arrays by a Template Method

Materials with defined porous structures have drawn considerate attention for decades. Most of these materials display similar porous structure within the entity. Sometimes materials with different surface and internal structures would be appreciated. In the present study, a microporous silica material with surface macroporous arrays was prepared by a sol-gel technique combined with a template method. The sol was prepared via hydrolysis of tetraethoxylsilane by hydrochloric acid. To this sol, monodispersed, hydrophobic polystyrene spheres were added. By ultrasonication, the spheres floated and assembled to regular arrays on the aqueous sol surface. At low temperature, the sol was gradually gelated, which froze the spheres in-situ. By washing with toluene, the polystyrene spheres were dissolved, leaving aligned arrays on the surface of the gel. The macropores can be easily controlled by selecting particles with suitable sizes. The material would find application in catalysis, sensor and adsorption.

Photoelectrochemical etching of uniform macropore array on full 5-inch silicon wafers

We analyze the two main factors causing non-uniformity of the etched macropore array first, and then a novel photoelectrochemical etching setup for large area silicon wafers is described. This etching setup refined typical etching setups by a water cooling system and a shower-head shaped electrolyte circulator. Experimental results showed that the uniform macropore array on full 5-inch n-type silicon wafers could be fabricated by this etching setup. The morphology of the macropore array can be controlled by adjusting the corresponding etching parameters.

Influence of etching current density on the morphology of macroporous silicon arrays by photo-electrochemical etching

Macroporous silicon arrays (MSA) have attracted much attention for their potential applications in photonic crystals, silicon microchannel plates, MEMS devices and so on. In order to fabricate perfect MSA structure, photo-electrochemical (PEC) etching of MSA and the influence of etching current on the pore morphology were studied in detail. The current-voltage curve of a polished n-type silicon wafer was presented in aqueous HF using back-side illumination. The critical current density JPS was discussed and the basic condition of etching current density for steady MSA growth was proposed. An indirect method was presented to measure the relation of JPS at the pore tip and etching time. MSA growth was realized with the pore diameter constant by changing the etching current density according to the measuring result of JPS. MSA with 295 μm of depth and 98 of aspect ratio was obtained.

A capacitive humidity sensor based on ordered macroporous silicon with thin film surface coating

In this paper, we report a new approach for humidity sensing. The sensor is based on ordered macroporous silicon with a Ta 2O 5 thin film coating. The ordered macroporous silicon array has perfectly aligned pores and uniform pore size (4 μm). The 95 nm Ta 2O 5 thin film is uniformly deposited on the pore surface by atomic layer deposition (ALD), which acts as an adsorption enhancement layer. The sensor's capacitance is measured to RH changes. The sensor shows very high sensitivity and small hysteresis, especially at high RH levels. It also shows very good repeatability and long term stability.

Fabrication of Flexible Thin Film with Pattern Structure and Macroporous Array Consisting of Nanoparticles by Electrophoretic Deposition

The electrophoretic deposition (EPD) possessing a low working temperature is used to fabricate functional thin films with microstructures onto a flexible substrate. We easily fabricate a flexible thin film in A4 size consisting of TiO2 nanoparticles onto an indium–tin-oxide/poly(ethylene terephthalate) (ITO/PET). The nanoparticle behavior during the EPD process is described and discussed according to different deposition time. The functional thin film can be applied as a flexible electrode layer of electrochromic displays and dye-sensitized solar cells. Furthermore, owing to the controllable deposition region of EPD, a pattern layer consisting of CdSe nanoparticles can be formed onto the flexible substrate within 1 min by EPD. Finally, by colloidal-template-mediated EPD, a nanostructure of a macroporous array onto flexible substrate can also be obtained. The result shows that the fabrication of nanostructures onto a flexible substrate is easy by EPD.

Formation of Macropore and Three-Dimensional Nanorod Array in p-Type Silicon

We carried out a study on the change in pore wall thickness depending on the current density in p-type silicon. We attempted the formation of a uniform macropore or nanorod array with a high aspect ratio in p-type silicon by electrochemical etching through the optimization of the hydrogen fluoride (HF)/organic electrolyte composition and the design of the mask pattern. The electrochemical etching of p-type silicon in the HF: dimethylsulfoxide (DMSO): deionized (DI) water = 1:5:5 electrolyte can control the velocity of a reaction between an electrolyte and a hole necessary for the electrochemical etching of silicon through the mixing of the protic property of DI water and the aprotic property of DMSO. In this study, we fabricated a p-type silicon nanorod array of three-dimensional structures with an approximately 350 nm diameter from macroporous Si by applying two-step currents (40 mA, 200 s + 38 mA, 1600 s) to a 1.8 cm2 circular area using an optimized HF: DMSO: DI water = 1:5:5 electrolyte composition.

Atomic layer deposition ZnO film as seed layer for hydrothermal growth of ZnO nanorods

AbstractAtomic layer deposition (ALD) ZnO film as seed layer for growing aligned ZnO nanorods arrays is demonstrated. The effects of the deposition temperature and film thickness to the morphology of the ZnO nanorods are studied. The ALD is found to have its advantage over the conventional dip-coating method when being applied to three-dimensional (3D) substrates, as exemplified by the macroporous Si adn CNT arrays. As one example, the CNT-ZnO 3D hybrid nanostructures are obtained which might be useful for energy-related applications.

Large-area monocrystalline silicon thin films by annealing of macroporous arrays: Understanding and tackling defects in the material

A concept that could provide a thin monocrystalline-silicon absorber layer without resorting to the expensive step of epitaxy would be very appealing for reducing the cost of solar cells. The empty-space-in-silicon technique by which thin films of silicon can be formed by reorganization of regular arrays of cylindrical voids at high temperature may be such a concept if the high quality of the thin film could be ensured on centimeter-large areas. While previous works mainly investigated the influence of the porous array on the final structure, this work focuses on the practical aspects of the high-temperature step and its application to large areas. An insight into the defects that may form is given and the origin of these defects is discussed, providing recommendations on how to avoid them. Surface roughening, pitting, formation of holes, and silicon pillars could be attributed to the nonuniform reactions between Si, SiO2, and SiO. Hydrogen atmospheres are therefore preferred for reorganization of macroporous arrays. Argon atmospheres, however, may provide high-quality silicon thin films as well, possibly even more easily transferable, as long as annealing is performed in controlled, clean, and oxygen-free conditions. Our experiments on large areas also highlight the importance of kinetics, which had not been considered up to now and which will require further understanding to ensure a complete reorganization over any wafer area.

Three-dimensional Macropore Arrays in $p$-type Silicon Fabricated by Electrochemical Etching

Ordered macropores formed in p-type silicon substrates (10 ∼ 20 Ω·cm) by using electrochemical anodization in various HF-containing electrolytes was investigated under different operating conditions. The effect of electrolyte composition and etch pits on controlling the formation of macropores on periodically arranged pores is reported. The results revealed that the natures of the constituents of the solution play very important roles in determining pore formation and morphology and that stable ordered macropore growth is not possible without an inverse pyramid notch. By using a Si/SiGe/Si/SiGe/p-type silicon structure, we were able to fabricate ordered pillar structures without an etch pit formation process. A possible role of two sets of Si/SiGe layers in pillar formation is proposed.