Nanoporous SiO2 Research Articles

Synthesis of Nanoporous SiO2-TiO2-ZrO2 Ceramics Using Sol-gel Technology

The aim of research is to develop an area of nanoporous ceramics of ternary systems. Nanoporous ceramic system SiO 2 -TiO 2 -ZrO 2 has been synthesized via sol-gel technology by hydrolysis of tetraethylorthosilicate (TEOS), zirconia and titanium alcoxides solutions. The sols have been polymerized at room temperature to obtain gels and dried at 100 o C, then milled for 1 or 6 hours for particle homogenization, pressed into samples, and then sintered at 800 o C or 1000 o C in air. The samples have been characterized by XRD, particle size distribution, crystallite size distribution, compressive strength. Porosity of ceramic samples has been determined by nitrogen adsorption-desorption isotherms.

Sol–gel preparation and characterization of SiO2 coated VO2 films with enhanced transmittance and high thermochromic performance

Vanadium dioxide (VO2) films prepared at low-temperature with a low cost are considerable for energy-saving applications. Here, SiO2 coated VO2 films with clearly enhanced visible transmittance by introducing antireflection coatings (ARCs) and excellent thermochromic performance were present. The VO2 films have been prepared via a stable and low-cost sol–gel synthesis route using vanadium pentaoxide powder as precursor, and their structural, morphological, optical and electrical properties and thermochromic performance were systemically characterized. The resistance of VO2 films varies by 4 orders of magnitude and the transmittance changes from 11.8% to 69.3% at 2500nm while no significant deviation appears in the visible region during metal–insulator transition (MIT). Nanoporous SiO2 coating with good optical transparency was coated on the surface of VO2 film via sol–gel dip-coating technique to enhance its optical transmittance, and the visible transmittance is increased by 14.6% due to the significantly decreased reflectance. The critical transition temperature (63°C) and infrared switching properties of VO2 films are not much deteriorated by applying SiO2 layer. The synergistic effect of antireflection and thermochromism on SiO2 coated VO2 films was investigated.

Growth and characterization of horizontal GaN wires on silicon

We report the growth of in-plane GaN wires on silicon by metalorganic chemical vapor deposition. Triangular-shaped GaN microwires with semi-polar sidewalls are observed to grow on top of a GaN/Si template patterned with nano-porous SiO2. With a length-to-thickness ratio ∼200, the GaN wires are well aligned along the three equivalent 〈112¯0〉 directions. Micro-Raman measurements indicate negligible stress and a low defect density inside the wires. Stacking faults were found to be the only defect type in the GaN wire by cross-sectional transmission electron microscopy. The GaN wires exhibited high conductivity, and the resistivity was 20–30 mΩ cm, regardless of the wire thickness. With proper heterostructure and doping design, these highly aligned GaN wires are promising for photonic and electronic applications monolithically integrated on silicon.

Core@dual‐Shell Nanoporous SiO2–TiO2 Composite Fibers with High Flexibility and Its Photocatalytic Activity

Structural design is of great importance to the performance of photocatalysts in environmental remediation. Therefore, micro/nanofibrous morphology and nanoporous local structures have been found to be beneficial to improve the photocatalytic activity. In this investigation, we report the design and fabrication of flexible and thermal stable nanoporous SiO2–TiO2 composite fibers as efficient photocatalysts. Combining electrospinning and modified Stöber techniques, core‐shell and mesoporous SiO2 fibers with high flexibility were fabricated and employed as the scaffold for supporting TiO2 nanoparticles. A nanoporous shell of TiO2 nanoparticles was then muffled over the SiO2 fibers to form core@dual‐shell SiO2–TiO2 composite fibers with hierarchically porous structure, which were conveniently patterned into a nonwoven, recyclable film. This nonwoven film exhibits better photocatalytic activity for Rhodamine B degradation under UV irradiation compared with some other TiO2‐based materials reported in recent years.

Dielectric multilayer-based fiber optic sensor enabling simultaneous measurement of humidity and temperature.

A multilayer-based fiber optic sensor enabling simultaneous measurement of humidity and temperature is proposed and demonstrated. The sensitive elements were multilayer coatings consisting of nano-porous TiO(2) and SiO(2) films, which were deposited on fiber end-face to form a Fabry-Perot (F-P) filter structure. Relative-humidity (RH) sensing is correlated with the shift of interference fringe due to the change of effective refractive index of porous coatings when exposed to different RH environments. The sensor is sealed in a glass tube in case of temperature measurement. Experimental results show that the average sensitivity are 0.43nm/%RH and 0.63nm/°C respectively when environmental RH changes from 1.8%RH to 74.7%RH and temperature changes from 21.4°C to 38.8°C. The proposed sensors present high repeatability, and especially highly sensitive to lower moisture measure.

Thin Ionic Liquid Membranes Based on Inorganic Supports with Different Pore Sizes

Nanoporous TiO2 and SiO2–ZrO2 membranes with controlled pore sizes were prepared by sol–gel processing and combined with a high CO2 solubility ionic liquid, 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), by hand-coating the ionic liquid onto the membrane surface, and then by a spray-coating procedure. The CO2 permeation through SILMs based on a support with 20 nm pores (SILM-20), PCO2 = 2.28 ± 0.16 × 10–8 mol/(m2·s·Pa), was 62% higher than that permeance obtained for SILMs with 1 nm pore sizes (SILM-1). Besides, the activation energy for permeation increased for membranes with smaller pore sizes, which suggested that the properties of the ionic liquid were influenced by the interaction with the membrane pore surface. The membranes prepared by spray coating revealed a CO2 permeance as high as PCO2 = 4.31 ± 0.13 × 10–8 mol/(m2·s·Pa), with an ideal CO2/N2 selectivity of α(CO2/N2) = 31.18 ± 1.34, which outperformed the current state-of-the-art materials for CO2/N2 separation.

Functionalized Nanoporous Gyroid SiO2 with Double-Stimuli-Responsive Properties as Environment-Selective Delivery Systems

Herein, we aim to fabricate nanoporous gyroid SiO2 from templated sol–gel reaction using degradable block copolymer with gyroid-forming nanostructure as a template and then to functionalize the nanoporous materials using “smart” polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), brushes via the “grafting from” method to give double-stimuli-responsive properties. By taking advantage of the responses to environmental stimuli, both thermal and pH, the pore features can be well-defined by the stretching and recoiling of PDMAEMA brushes because of their adjustable chain conformations with reversible character. The responsive properties with respect to environmental stimuli can be successfully traced by temperature-resolved small-angle X-ray scattering (SAXS) in aqueous environment. Owing to the high specific surface area and porosity, 3D pore network, biocompatibility, and environmental responses, the functionalized nanoporous gyroid SiO2 is further demonstrated as a stimuli-responsive controlled re...

Spectral properties of nanoporous SiO2 matrices with polymethine dye molecules

Spectral properties of nanoporous SiO2 matrices with polymethine dye molecules

Ab initio calculations of the effect of impurity iron ions on the interaction of CH4 and NH3 with nanoporous silicates

The interaction of CH4 and NH3 molecules with the surface of pure and iron-doped nanoporous SiO2 has been simulated by ab initio calculations. The energetically favorable positions for adsorbate molecules have been found and the variation of the local atomic structure of SiO2 has been studied in detail. It was shown that the adsorption energy of CH4 and NH3 molecules on the surface of nanoporous SiO2 was 0.16 and 0.51 eV, respectively.

Adsorption Configuration of CH<sub>4</sub> on Fe/SiO<sub>2</sub> Surface: First-Principles Calculations

In the present work the effects of the isomorphic substitution of silicon with iron on the structure of a nanoporous SiO2 surface and its interaction with methane were studied within a density functional theory framework. We predicted the structures and detailed energetics for the adsorption of CH4 on the surface. We found that the local atomic structure of the Fe/SiO2 nanocomposites and the CH4 molecules changed because of the interactions.

Composite materials based on nanoporous SiO2 matrices and squarylium dye

Sol–gel SiO2 matrices with different contents of HF acid were synthesized. The matrices were annealed at 600°C and impregnated with squarylium dye SqD1 in methylmethacrylate (MMA) solvent. Xerogel nanoporous structure of SiO2 matrices was found to contain 5–12nm and 60–600nm pores. The size of large pores increases with the concentration of HF acid. For the synthesis of SiO2 matrices the ratio n(HF/TEOS)=0.25–0.38 is the most optimal. At other HF concentrations the formed matrices are prone to cracking at drying, annealing and impregnation with the dye solution in MMA. The location of the maxima of the absorption and luminescence bands of SqD1 dye in the composite SiO2:SqD1:ММА is found to be influenced by the surface state of SiO2 xerogel depending on the concentration of HF acid at the synthesis of the matrices.

An ultrabroad band omni-directional anti-reflective coating with quasi-gradient refractive index distribution based on Si–SiOxNy–SiO2 materials system

An ultrabroad band omni-directional antireflective coating was fabricated using both reactive magnetron sputtering and glancing angled electron beam evaporation methods. Gradient index amorphous Si, SiNx, and SiOxNy thin films were deposited by tuning the flow rate of the reactive gases, while the gradient index distribution of the nanoporous SiO2 stacks was obtained by rotating the angle of the substrate. A low average reflectivity of less than 2% at normal incidence in the wavelength range 280 to 3300 nm was achieved, and the average reflectivity over the angle range 15 to 89° was 3.7% for the wavelength between 300 and 1700 nm.

Glucose oxidase and catalase co-immobilization on biosynthesized nanoporous SiO2 for removal of dissolved oxygen in water: Corrosion controlling of boilers

Nanoporous SiO2 was synthesized by sol–gel method using dry yeast cells as a template. Additionally, characterization of the biosynthesized nanoporous SiO2 was investigated. Bio-synthesized nanoporous SiO2 has the specific surface area of 460m2/g and total pore volume of 0.36cm3/g. Co-immobilization of glucose oxidase and catalase by adsorption method was done on biosynthesized nanoporous SiO2 and immobilized enzymes were used for removal of water dissolved oxygen. Additionally, modeling and optimization for removal of water dissolved oxygen was performed using rotatable response surface methodology (RSM). Experimental values were in good agreement with the predicated ones and the model was highly significant.

Fabrication of hollow SiO2 and Au (core)–SiO2 (shell) nanostructures of different shapes by CdS template dissolution

Core–shell silica (SiO2) coated CdS nanorods (NR) and nanospheres (NS) were prepared (SiO2@CdS) by deposition of a Si–O–Si amorphous layer over the CdS surface through the hydrolysis of 3-mercaptopropyltrimethoxysilane and tetraethylorthosilicate. Nanoporous SiO2 matrix (NPSM), hollow SiO2 nanotubes (HSNT) and nanospheres (HSNS) useful for efficient adsorption and catalytic processes were prepared by chemical dissolution of CdS–NS (size: 9–10 nm) and CdS–NR (length: 116–128 nm and width: 6–11 nm) template from SiO2@CdS with 2 M HNO3. These SiO2 nanostructures were characterized by optical absorption, TEM, EDX, SAED and BET surface area analysis. TEM images revealed the fabrication of slightly distorted HSNS (size: 9–12 nm) and closed HSNT (length: 30–45 nm and diameter: 9–14 nm) of shorter dimensions than the CdS–NR template used. The BET surface area (112–134 m2 g−1) of NPSM and HSNS is found to be larger than the surface area (29–51 m2 g−1) of SiO2@CdS composites indicating hollow SiO2 morphology. Silica coated Au (SiO2@Au) composites formed by CdS dissolution from Au (2 wt%) deposited CdS–NR core-encapsulated into SiO2 shell (SiO2@Au–CdS–NR) exhibited a surface plasmon band at 550 nm and displayed high catalytic activity for 4-nitrophenol reduction by Au nanoparticle.

Roles of plasma-generated vacuum-ultraviolet photons and oxygen radicals in damaging nanoporous low-k films

One important class of low-k materials used as interconnect dielectrics employs methyl groups added to nanoporous SiO2 matrices. These carbon-doped oxide materials are known to be susceptible to damage from plasma species during various stages of plasma processing. Two key active species generated in O2 plasma are oxygen (O) radicals and vacuum-ultraviolet (VUV) photons. These species are known to cause carbon loss, resulting in damaging increases in dielectric constant throughout the film. However, the mechanisms through which this damage is incurred are poorly understood. By capping the substrate in different ways during plasma exposure, it is possible to expose films to either photons alone or O atoms alone. The authors report measurements of damage induced by VUV photons only, O radicals only, and the combination of O radicals and photons. Through HF stripping, they note that carbon extraction from photons and from radicals yields different outcomes; the profile of carbon concentration within the modified region is different for each case. Damage from photons alone can be modeled and model predictions are in good agreement with measurements. Damage from O atoms alone can only be modeled if it is assumed that the near-surface region has a significantly reduced diffusivity compared to the bulk of the film. Experiment and model agree that both photons alone and O radicals alone damage the material by removing carbon. When radicals and photons are present simultaneously during plasma exposure, however, more C removal appears to be occurring in the model than experimentally observed. Remarkably, if only radicals are exposed to the film after short (10–30 s) plasma exposures, very little additional damage is incurred during this radical-only exposure. The most straightforward interpretation of these results appears to be that photons combine synergistically with radicals in the pores to narrow the pores, thereby reducing film diffusivity in the C-poor, plasma-damaged regions.

Cost-effective nanoporous SiO2–TiO2 coatings on glass substrates with antireflective and self-cleaning properties

Simultaneous antireflective and self-cleaning properties have been realized by depositing double-layered SiO2–TiO2 coatings onto glass substrates using a combined sol–gel dip-coating process in which the two oxide layers were deposited in succession. The first layer is composed of a hybrid methyl-functionalized nanoporous SiO2 material that provides on average a 6% anti-reflection gain. The degree of antireflectivity can be controlled by adjusting the thickness and refractive index by selecting suitable solvents and pore-forming agents in the coating mixture. The second layer is an ultrathin TiO2 nanoporous layer deposited on top of antireflective layer. The high hardness of the TiO2 top layer acts as a protection barrier toward mechanical damage and ensures the surface is self-cleaning. An average anti-reflectivity of 3.4% achieved over a spectrum range of 400–800nm for the optimal SiO2–TiO2 bilayers is obtained. It is found that a homogeneous pore size distribution in the SiO2 layer introduced by using isopropanol as the solvent can compensate for the TiO2 band edge absorption based on the optical interference that occurs between the two layers. These systems are easy to produce on a large scale at low cost and exhibit high mechanical and chemical durability. These materials are thus suitable for use as bifunctional antireflective and self-cleaning coatings for use as large substrates for solar cells.

Double and Single Network Phases in Polystyrene-block-poly(l-lactide) Diblock Copolymers

Here, the phase behaviors of polystyrene-block-poly(l-lactide) (PS–PLLA) diblock copolymers are studied, and network phases can be found. In addition to conventional double gyroid phase with double networks, a single network phase (an Fddd phase with orthorhombic network) can be found, as evidenced by small-angle X-ray scattering and transmission electron microscopy (TEM). The single network texture with trivalent connectors of the Fddd can be directly visualized by using 3D TEM (i.e., electron tomography), and the observation under polarized light microscopy shows its birefringence character. The formation of stable Fddd is attributed to intrinsic inhomogeneity in the chain conformations of the chiral block (i.e., PLLA). Subsequently, nanoporous PS with continuous nanochannel networks can be obtained by hydrolysis of the PLLA block in the PS–PLLA and then used as a template for sol–gel reaction of tetraethyl orthosilicate. Consequently, nanoporous SiO2 with free-standing, well-defined network texture can be fabricated after removal of the PS template to give an open-cell porous ceramic material with high specific surface area (as high as 1170 m2/g) due to its nanostructured texture and high porosity.

Rational designing of nanoporous nanopattern arrays of Au, Pt and SiO2: synthesis using lithography, sputtering and selective dissolution

Nanoporous materials are of great importance in a variety of applications such as heterogeneous catalysts because of their large specific surface area and tuneable porosity. Herein, we proposed an approach for rationally designing nanoporous materials on a substrate by lithography, co-sputtering and selective dissolution. The physical processes (i.e., lithography and sputtering) were used to design shapes and arrangements of nanostructures, and the chemical dissolution process was used to form nanopores inside the nano-patterns. In order to investigate the compatibility of the sputtering and selective etching with the lithography process, we formed patterned nanoporous Au by co-sputtering of AuCu followed by selective dissolution of Cu with acid. Interestingly, using this process, dish-shaped Au nanostructure arrays formed through lithography processes were successfully decorated with nanoporous Au, resulting in the enhancement of SERS intensity by 10 times. Then, in order to show the versatility of our approach, we synthesized nanoporous SiO2 patterns. In this case, nanoporous SiO2 was synthesized by co-sputtering of SiO2 with Cu followed by selective dissolution of Cu with acid. Because of removal of the thermal process, this SiO2 synthesis appears to be compatible with a variety of nanofabrication procedures. Finally, in order to investigate further merits of the use of sputtering, we demonstrated the synthesis of dendritic Pt nanoporous particle arrays by co-sputtering and acid treatment. Interestingly, nanoporous particle arrays, i.e., hierarchical structure, were formed from PtCu crystal grains without coagulation, showing a possibility that crystal grains can play a role in the formation of hierarchical nanostructures.

Optical coatings on f iber for relative-humidity sensing applications

A fiber-optic relative-humidity (RH) sensor composed of multilayer of porous dielectric films is proposed. The transducer deposited on fiber end-face is multilayer coating consisting of nano-porous TiO2 and SiO2 films, which forms a low-fineness Fabry-Perot (F-P) filter with one of minimum reflections at about 1 350-nm wavelength. The dielectric thin films realized by e-beam evaporation without ion-source assistance have columnar and porous structures, which exhibit sensitivity to RH change of environments. When the sensor is exposed to an environment of RH change from 10.9% to 92.8%, experimental results demonstrate 77.9-nm shift of characteristic wavelength.

Optical and morphological characterisation of low refractive index materials for coatings on solar collector glazing

Nanostructured coatings based on the elements Si, O, Mg, and F have been deposited as thin films by sol–gel dip-coating in a particle-free atmosphere. The refractive index of the prepared SiO2 and quaternary Mg–F–Si–O thin films has been determined from spectrophotometric and ellipsometric data. The morphology of those thin films has been observed by TEM.Nanoporous SiO2 coatings with a pore size smaller than 3 nm (TEM) and a pore volume fraction of 30% (as inferred from ellipsometric measurements) have been achieved. They are characterised by significantly lower refractive index values (approx. 1.32 at 550 nm) than compact SiO2 (approx. 1.46).Quaternary Mg–F–Si–O thin films are characterised by a surprisingly low refractive index (approx. 1.26 at 550 nm), even lower than that of dense MgF2 coatings (approx. 1.38). Preliminary results of transmission electron microscopy suggest that these films are of nanocomposite nature.In both cases, highly transparent samples have been produced in a single dip-coating step followed by simple thermal annealing in air. Broad spectral transmittance maxima are observed exceeding values of 98.5% (nanoporous SiO2) and 99.5% (quaternary Mg–F–Si–O films).The quaternary films might exhibit a higher ageing stability than porous SiO2 films with respect to pore-filling and could therefore be a promising alternative for single-layered anti-reflection coatings as well as for multi-layered coloured coatings on solar collector glazing.