Porous SiO2 Research Articles

Pore structure analysis of ionic liquid-templated porous silica using positron annihilation lifetime spectroscopy

Abstract Fundamental understanding of the pore structure of porous materials is essential for designing materials and controlling their properties to realize various applications. In this study, we quantified the structures of micropores as well as the mesopores in ionic liquid (IL)-templated porous silica using positron annihilation lifetime spectroscopy (PALS). Our PALS analyses distinguished three types of open spaces in IL-templated porous silica: (i) Vacancy clusters and/or microvoids, (ii) micropores between primary silica particles (1.0–1.4 nm in size), and (iii) mesopores (4–7 nm in size). Upon increasing the IL/SiO2 ratio, the size of the micropore between the primary particles decreased from 1.4 nm to 1.0 nm, implying that the sizes of the silica particles decreased because of the small volume of SiO2 compared with that of IL. In contrast, the sizes of the mesopores increased with the IL/SiO2 ratio, indicating that the open space expands according to the increase in the IL volume. In conclusion, the PALS method allowed comprehensive evaluation of the pore structure of the IL-templated porous SiO2. The relationship between pore structure and IL content will be used for optimization of the mechanical properties and chemical selectivity of porous SiO2.

Effect of moisture adsorption on the broadband dielectric response of SiO2-based nanoporous glass

Atmospheric water adsorbed by porous materials may significantly change its electrodynamic response in a wide frequency range. Mechanical and chemical stabilities of silicon dioxide along with a proven manufacturing method of porous SiO2 samples made it convenient for studying the effects of moisture adsorption on various parameters of the porous media. We report the dielectric properties of SiO2-based nanoporous glass in the frequency range of 20 Hz–400 THz at ambient atmospheric conditions and at a low residual pressure of ≤1 mbar. We observed a significant low-frequency dispersion of the complex dielectric permittivity and enhancement of dielectric loss for the porous sample exposed to the ambient moisture. In the terahertz range, the change in dielectric response is smaller and correlates with the moisture saturation of the sample.

Preventing Anion Exchange between Perovskite Nanocrystals by Confinement in Porous SiO2 Nanobeads.

All-inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) are highly attractive due to their outstanding optical and electrical properties. However, poor stability and easy anion exchanges between CsPbX3 nanocrystals with different halides limit their applications in light-emitting diodes (LEDs). To solve the problems, we developed an approach to in situ synthesize CsPbX3 NCs into porous silica colloidal spheres, which can effectively prevent anion exchange and increase photo stability. Based on our results, we first proved that the anion exchange between CsPbX3 nanocrystals is mainly driven by physical collision of the nanocrystals, not requiring a bridge such as a solvent. We subsequently used an optimized ratio of green, red, and blue SiO2/CsPbX3 composites as solid-state luminescent materials to fabricate single-layer white light-emitting diodes (WLEDs). No anion exchanges have been observed in the LED fabrication and lighting process.

Preparation of Superhydrophobic Porous SiO₂ Aerogel Using Methyl Trimethoxy Silane Single Precursor and Superhydrophobic Cotton Fabric Coating from It.

Superhydrophobic cotton fabrics were prepared by simultaneous incorporation of SiO₂ aerogel particles and polydimethylsiloxane (PDMS). The SiO₂ aerogels were synthesized via acid-base catalyzed sol-gel reaction with methyl trimethoxy silane (MTMS) as the single precursor and oxalic acid and ammonium hydroxide as the catalyst in methanol (MeOH) solution by drying under ambient pressure. The preparation parameters (e.g., MTMS/MeOH molar ratio, oxalic acid/MTMS molar ratio, gelation pH value, and gelation temperature) had great influences on the density and porosity of the SiO₂ aerogel. The obtained SiO₂ aerogel had low density, high porosity and high specific surface area, showing the typical rough mesoporous structure. The prepared bulk SiO₂ aerogel displayed excellent superhydrophobicity with a water contact angle (WCA) of 151.0 ± 0.8°. Superhydrophobic cotton fabric with a WCA of 155.6 ± 0.9° for a 5 μL water droplet was successfully obtained by simply coating the PDMS/SiO₂ aerogel composite solution via dip-pad-cure process. This could be attributed to the combination of SiO₂ aerogel particles with porous rough microstructure, high specific surface area and PDMS adhesive layer with low surface energy. The effect of PDMS/SiO₂ aerogel coating treatment on the mechanical strength properties of the cotton fabrics was negligible. This simple approach may pave the potential way for practical applications.

Magnetic Fe3O4@SiO2-Pt and Fe3O4@SiO2-Pt@SiO2 Structures for HDN of Indole.

The effect of a second porous SiO2 shell in the activity and selectivity of the Fe3O4@SiO2–Pt catalyst in the hydrodenitrogenation of indole is reported. The double Fe3O4@SiO2–Pt@SiO2 structure was prepared by coating Fe3O4 nanoparticles with tetraethyl orthosilicate (TEOS) with a further impregnation of 1.0 wt.% of Pt on the (3-aminopropyl)triethoxysilane functionalized Fe3O4@SiO2 structures. The second porous SiO2 shell, obtained by using a hexadecyltrimethylammonium bromide (CTAB) template, covered the Fe3O4@SiO2–Pt catalyst with a well-defined and narrow pore-sized distribution. The full characterization by TEM, inductively coupled plasma-optical emission spectroscopy (ICP-OES), XRD, and N2 adsorption isotherm at 77 K and vibrating sample magnetometry (VSM) of the catalysts indicates homogeneous core@shell structures with a controlled nano-size of metallic Pt. A significant effect of the double SiO2 shell in the catalytic performance was demonstrated by both a higher activity to eliminate the nitrogen atom of the indole molecule present in model liquid fuel and the improvement of the catalytic stability reaching four consecutive reaction cycles with only a slight conversion level decrease.

Core‐Shell Nanoparticles with a Redox Polymer Core and a Silica Porous Shell as High‐Performance Cathode Material for Lithium‐Ion Batteries

A facile and novel method for the fabrication of core‐shell nanoparticles (PTMA@SiO2) based on a poly(2,2,6,6‐tetramethylpiperidinyloxy‐4‐yl methacrylate) (PTMA) core and a porous SiO2 shell is reported. The core‐shell nanoparticles are further self‐assembled with negatively charged multi‐walled carbon nanotubes (MWCNTs), which results in the formation of a free‐standing cathode electrode. The porous SiO2 shell not only effectively improves the stability of the linear PTMA redox polymer with low molar mass in organic electrolytes but also leads to the uniform dispersion of PTMA active units in the MWCNTs conductive network. The PTMA@SiO2@MWCNT composite electrode exhibits a specific capacity as high as 73.8 mAh g at 1 C and only 0.11% capacity loss per cycle at a rate of 2 C.

Thermal behavior and effect of SiO2 and PVA-SiO2 matrix on formation of Ni–Zn ferrite nanoparticles

The paper presents the synthesis of ZnFe2O4/SiO2, NiFe2O4/SiO2, Ni0.4Zn0.6Fe2O4/SiO2 and Ni0.4Zn0.6Fe2O4/PVA-SiO2 nanocomposites by a modified sol–gel method. The influence of matrix type on the formation of Ni-, Zn- and Fe-succinate precursors and their decomposition into ferrites within the pores of SiO2 or PVA-SiO2 matrices was investigated by thermal analysis. The decomposition processes, formation of crystalline phases and mass changes during samples annealing at different temperatures were also assessed by thermal analysis. The formation and decomposition of succinates and the formation of the matrix were studied by Fourier transform infrared spectroscopy, while the formation of main (ZnFe2O4, NiFe2O4) and secondary phases (SiO2) was studied by X-ray diffraction analysis. The influence of matrix type on the formation of the crystalline phases was also investigated. Transmission electron microscopy revealed spherical-shaped ferrite particles with particle size of 4–50 nm. The particle size increases with increasing Zn content and annealing temperature. The coloring properties and the potential ceramic applications of the obtained dark-brown–black powders were tested by their incorporation in opaque tile glazes and application on ceramic tile.

Atomic Layer Deposition ZnO Over-Coated Cu/SiO2 Catalysts for Methanol Synthesis from CO2 Hydrogenation

Cu-ZnO-based catalysts are of importance for CO2 utilization to synthesize methanol. However, the mechanisms of CO2 activation, the split of the C=O double bond, and the formation of C-H and O-H bonds are still debatable. To understand this mechanism and to improve the selectivity of methanol formation, the combination of strong electronic adsorption (SEA) and atomic layer deposition (ALD) was used to form catalysts with Cu nanoparticles surrounded by a non-uniform ZnO layer, uniform atomic layer of ZnO, or multiple layers of ZnO on porous SiO2. N2 adsorption, H2 temperature-programmed reduction (H2-TPR) X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX), CO-chemisorption, CO2 temperature-programmed desorption (CO2-TPD), X-ray adsorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were used to characterize the catalysts. The catalyst activity was correlated to the number of metallic sites. The catalyst of 5 wt% Cu over-coated with a single atomic layer of ZnO exhibited higher methanol selectivity. This catalyst has comparatively more metallic sites (smaller Cu particles with good distribution) and basic site (uniform ZnO layer) formation, and a stronger interaction between them, which provided necessary synergy for the CO2 activation and hydrogenation to form methanol.

Regulation of Osteogenic Markers at Late Stage of Osteoblast Differentiation in Silicon and Zinc Doped Porous TCP.

Calcium phosphates (CaPs) are one of the most widely used synthetic materials for bone grafting applications in the orthopedic industry. Recent trends in synthetic bone graft applications have shifted towards the incorporation of metal trace elements that extend the performance of CaPs to have osteoinductive properties. The objective of this study is to investigate the effects of silicon (Si) and zinc (Zn) dopants in highly porous tricalcium phosphate (TCP) scaffolds on late-stage osteoblast cell differentiation markers. In this study, an oil emulsion method is utilized to fabricate highly porous SiO2 doped β-TCP (Si-TCP) and ZnO doped β-TCP (Zn-TCP) scaffolds through the incorporation of 0.5 wt.% SiO2 and 0.25 wt.% ZnO, respectively, to the β-TCP scaffold. Reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) is utilized to analyze the mRNA expression of osteoprotegerin (OPG), receptor activator of nuclear kappa beta ligand (RANKL), bone morphogenetic protein 2 (BMP2), and runt-related transcription factor 2 (Runx2) at the later stage of osteoblast differentiation, day 21 and day 28. Results show that the addition of Si and Zn to the β-TCP structure inhibited the β to α-TCP phase transformation and enhance the density without affecting the dissolution properties. Normal BMP-2 and Runx2 transcriptions are observed in both Si-TCP and Zn-TCP scaffolds at the initial time point, as demonstrated by RT-qPCR. Moreover, the addition of both Si and Zn positively regulate the osteoprotegerin: receptor activator of nuclear factor k-β ligand (OPG:RANKL) ratio at 21-days for Si-TCP and Zn-TCP scaffolds. These results demonstrate the effects of Si and Zn doped porous β-TCP scaffolds on the upregulation of osteoblast marker gene expression including OPG, RANKL, BMP-2, and Runx2, indicating the role of trace elements on the effective regulation of late-stage osteoblast cell differentiation markers.

Resistive switching of CuO nanofibers embedded into hollow channels of SiO2 layer

In this work, we have formed CuO memristive array embedded into porous SiO2 layer. Such an approach would create synaptic system on silicon substrate using standard integrated electronics operations. It also provides control over the geometric parameters of artificial synapses, which determines their synaptic weights. The results of synaptic behavior have shown that the value of synaptic weights and currents are dependent on the thickness of the CuO filled SiO2 porous layer.

Synthesis of β-SiC powders by the carbothermal reduction of porous SiO2–C hybrid precursors with controlled surface area

SiO2–C precursors with various surface areas were derived from tetraethyl orthosilicate and phenolic resin as Si and C sources, respectively, by a modified sol–gel process using the in situ precipitation of phenol resin in a prepared wet gel. The surface area of the SiO2–C precursors was varied from 20 to 175 m2/g by changing the C/Si molar ratio in the preform. β-SiC powders were synthesized using carbothermal reduction in vacuum at the temperature range of 1200–1600 °C. The effects of the temperature and heat treatment time as well as that of the surface area of the preform on the formation of β-SiC powders were studied. It was determined that the formation of β-SiC started at 1200 °C and was considerably promoted as the heat treatment temperature and time further increased during the carbothermal reduction of SiO2–C preforms with high surface area. When high surface area SiO2–C preforms were used, highly crystalline SiC powders were synthesized at 1600 °C in vacuum with a high yield of 85%.

Porous SiO2 nanoparticles containing ruthenium or sulfur compounds: Sonochemical producing and sonoluminescence in aqueous suspensions.

Aqueous suspensions of silicon dioxide porous nanoparticles (average size 10-30 nm, average pore size 5.8 nm) were obtained via ultrasonic dispersing. As was shown through recording SiO molecular lines in a moving single-bubble sonoluminescence spectrum, these nanoparticles penetrate into the bubble and then undergo decay. Similarly, suspensions of SiO2 nanoparticles, the pores of which were saturated with ruthenium dodecacarbonyl or elemental sulfur, were obtained by impregnation of the initial powder with solutions of these reagents in chloroform followed by evaporation of the solvent. Single-bubble sonoluminescence spectra of these suspensions contain more intense lines of Ru or S and Sn+ as compared with the SiO lines. This also proves the involvement of water insoluble ruthenium and sulfur compounds into bubble sonoluminescent reactions in the heterogenic aqueous medium. Using the method of comparing the experimentally obtained and computer simulated luminescent spectra, we determined the effective electronic temperature TeRu, which was 9000 ± 500 K, in non-equilibrium plasma of a bubble levitating in the ultrasonic field.

Porous SiO2 coated AlxFeyZr1-x-yO2 solid superacid nanoparticles with negative charge for polyvinylidene fluoride (PVDF) membrane: Cleaning and partial desalinating seawater

In this work, porous SiO2 coated AlxFeyZr1-x-yO2 solid superacid nanoparticles with negative charge (CS-SAFZr) were synthesized via hydrolysis, sulfation and sulfonation, and characterized by SEM, TEM, XRD, BET and so on. The results show that the size of CS-SAFZr nanoparticles prepared under the optimum preparation conditions is around 80 nm, thickness of the porous SiO2 shell is about 20 nm, Hammett acidity is -16.197 and ion exchange capacity (IEC) is 0.98 mmol·g−1. Correspondingly, ferrum (Fe) and aluminum (Al) elements are successfully doped into the ZrO2 lattice and the doped nanoparticles present a specific surface area of 396.2 m2 g−1 with abundant hydroxyl and sulfonic acid groups on the surface. To investigate the properties of the nanoparticles as the filler, polyvinylidene fluoride (PVDF) was used as a candidate to prepare CS-SAFZr/PVDF ultrafiltration (UF) composite membranes and the performance were characterized via cleaning and desalinating seawater. Results indicate that the CS-SAFZr nanoparticles strengthen their compatibility with the membrane via hydrogen bonds and improve performances of PVDF membrane. The suspended solid and conductivity decline ratio of permeate seawater was 1.8 mg L−1 and 13.20% respectively, indicating that CS-SAFZr/PVDF membrane performs seawater cleaning and partial desalination. Therefore, CS-SAFZr nanoparticles can be a promising candidate to modify PVDF membrane for cleaning and desalinating seawater.

A green route for pyrolysis poly-generation of typical high ash biomass, rice husk: Effects on simultaneous production of carbonic oxide-rich syngas, phenol-abundant bio-oil, high-adsorption porous carbon and amorphous silicon dioxide

Rice husk is a widespread agriculture waste in rice-farming country. High silica content in rice husk prevent its efficient utilization. So in this work, concept of poly-generation was introduced to improve its utilization value. This study provided CO-rich syngas, phenol-abundant bio-oil, high-adsorption porous carbon and amorphous SiO2 as four end products for first time via combination of acid washing and activated carbon catalyst. Specifically, acid washing effectively decreasedsoluble ash, which altered pyrolysis paths, increased volatiles release and reduced impurities in bio-char. After catalytic pyrolysis, phenol content of 65.56% and CO of 56.09 vol% were detected in bio-oil and syngas from AWRH. For solid products, acid washing benefited both bio-char and silica. A low-cost porous carbon with developed pores and rich surface functional groups was prepared for water absorption. And high purity amorphous SiO2 was recycled from alkali etching solution. Finally, a green process with no waste emission was proposed.

Morphology enhancement of SiO2 aerogel films grown on Si substrate using dense SiO2 buffer layer

Porous SiO2 film has been widely studied due to its extensive applications in many fields. This paper presents a newly produced porous SiO2 film made by traditional sol–gel method. Bare Si and Si with SiO2 buffer layer were used as substrate. The SiO2 buffer layer was 500 nm in thickness and was grown by thermal oxidization. The structural properties of SiO2 aerogel films spin-coated on both materials were observed by optical microscope (OM) and scanning electron microscope (SEM). Results reveal that the surface of SiO2 aerogel films on bare Si is rough and discontinuous. While flat and smooth surface is observed on sample with SiO2 buffer layer. This indicates that by inserting SiO2 buffer layer, the structural property of SiO2 aerogel film deposited on Si is improved.

Influence of Si and Fe/Cr oxides as intermediate layers in the fabrication of supported Pd membranes

This paper presents for the first time relevant insights on the influence of tuning porous stainless steel (PSS) supports for the preparation of metallic membranes by the Electroless Pore-Plated (ELP-PP) method. Two types of oxides, used as intermediate layer between the support and the Pd layer, were deposited on raw PSS supports for reducing porosity and surface roughness. Both independent and combined routes were considered: (i) Fe2O3-Cr2O3 oxides obtained by direct calcination of the metallic support and (ii) dense and porous SiO2 deposited by dip-coating (DC) technique. Membranes morphology (porosity and surface homogeneity) was characterized by SEM and EDX analysis and gravimetric analysis was used to calculate the total amount of Pd deposited on the external selective layer. Preliminary screening tests suggested that the combined route (calcination followed by DC of SiO2) did not show evident beneficial effects for the preparation of Pd films by ELP-PP. Based on those results, four tubular membranes were fabricated using independent routes: one by calcination and other three by DC, including 1, 2 and 3 layers of SiO2. The first one, including a Fe2O3-Cr2O3 oxides intermediate layer, maintained its mechanical integrity when tested up to ΔP = 4 bar. On the other hand, it was observed that the use of SiO2 intermediate layers noticeably affects the ELP-PP method. Extensive characterization of fresh and used samples has been performed in order to understand the influence of the oxides in the final preparation of a continuous and thin layer of Pd. Results obtained evidenced that the most adequate sample was the support prepared with 2 layers of SiO2 and studies continue to improve membrane stability and to reduce thickness.

Ag2O-Ag/CAC/SiO2 composite for visible light photocatalytic degradation of cumene hydroperoxide in water

A visible light-active composite, Ag2O-Ag/CAC/SiO2, was produced by loading an aqueous silver salt solution onto the layer of hydrated calcium aluminate cement coating on porous SiO2 sphere. The mild cement hydration condition induced the formation of in situ Ag2O precipitates on the cement surface which were partially decomposed to metallic Ag after drying and appropriate heat treatment at 200°C to enhance both decomposition and crystallinity of Ag2O. The coexistence of Ag2O and Ag in the composite was clearly confirmed by XRD and XPS, and the mechanisms for the formation of Ag2O particles on the hydrated cement layer and for the photocatalytic degradation of CHP were also suggested. The photocatalytic activity of Ag2O-Ag/CAC/SiO2 composite (calcined at 200°C), evaluated under UV and visible lights irradiation through the photodecomposition of CHP in water quantified by HPLC, was two times higher than that of Ag2O-Ag/CAC/SiO2 composite (dried at 45°C), and able to completely decompose CHP in 5h. Additionally, the photostability of Ag2O-Ag/CAC/SiO2 photocatalyst remained unchanged for 5 recycling tests.

Fabrication of Si–SiO2@Fe/NC composite from industrial waste AlSiFe powders as high stability anodes for lithium ion batteries

Industrial waste AlSiFe powders are employed to provide polyporous SiO2 frameworks by alkali etching and Si nanoparticles are embedded in the gaps of the porous SiO2 frameworks via ultrasonic stirring. The nitrogen doped carbon layers are generated on the surface of the porous frameworks to yield the Si–SiO2@Fe/NC composite by polymerization of dopamine and subsequent carbonization process, in which the polyporous SiO2 frameworks can not only relieve huge volume changes of the active Si particles, but also can provide abundant charge transfer channels for electrons and lithium ions. Moreover, the nitrogen doped carbon layers significantly promote the electronic conductivity of the composite. Benefitting from these coordinating effects, the Si–SiO2@Fe/NC anodes demonstrate outstanding cycle performance with a charge specific capacity of 1394.7 mAh g−1 and up to 100% capacity retention rate after 100 cycles under test current of 200 mA g−1, excellent rate performance with a charge specific capacity of 653.1 mAh g−1 under test current of 3000 mA g−1 and enhanced lithium ion diffusion rate of 4.161 E−13 cm2 s−1 in the 100th cycle. As a consequence, the comprehensive fabrication of the Si–SiO2@Fe/NC composite from industrial wastes could provide a promising idea to develop novel anodes for lithium ion batteries.

Construction of High-Performance, High-Temperature Proton Exchange Membranes through Incorporating SiO2 Nanoparticles into Novel Cross-linked Polybenzimidazole Networks.

The practical applications of phosphoric acid-doped polybenzimidazole (PA-PBI) as high-temperature proton exchange membranes (HT-PEMs) are mainly limited by their poor dimensional-mechanical stability at high acid doping levels (ADLs) and the leaching of PA from membranes during fuel cell operation. In this work, to overcome these issues, we fabricated novel cross-linked PBI networks with additional imidazole groups by employing a newly synthesized bibenzimidazole-containing dichloro compound as cross-linker and an arylether-type Ph-PBI as matrix. Ph-PBI featured by good solubility under high molecular weight offers satisfactory film-forming ability and mechanical strength using for the matrix. Importantly, the additional imidazole moieties in BIM-2Cl endow the cross-linked PBI membranes improved dimensional-mechanical stability with simultaneously enhanced ADLs and proton conductivity. Furthermore, superior acid retention capability is obtained by incorporating porous polyhydroxy SiO2 nanoparticles into these cross-linked networks. As a result, the SiO2/cross-linked PBI composite membranes are suitable to manufacture membrane electrode assemblies (MEAs), and an excellent H2/O2 cell performance with a peak power density of 497 mW cm-2 at 160 °C under anhydrous conditions can be achieved.

Harnessing the Aqueous Chemistry of Silicon: Self-Assembling Porous Silicon/Silica Microribbons

The synthesis of microribbons based on the assembly of porous silicon nanoparticles (pSiNPs) in a silica matrix is reported. The formation of these structures is driven by dissolution and reprecipitation of silica derived from the NPs upon drying of an aqueous colloidal dispersion. The process generates composite films that fracture into filaments due to geometric stresses associated with drying of the film on a curved surface. By controlling NP concentration, solvent, and temperature during the evaporation process, well-defined microribbons with a rectangular cross section of ∼25 × 100 microns and lengths on the order of 1 cm are formed. Partial thermal oxidation of the ribbons generates luminescent Si-SiO2 core-shell composites, and complete oxidation generates porous SiO2 ribbons with retention of the mesoporous nanostructure. The pores can be infiltrated with daunorubicin as a model drug, and the resulting material shows sustained release of the chemotherapeutic for more than 70 days.