Amorphous Silica Layer Research Articles

Physiochemical and Optical Properties of GdF3:Pr@LaF3@SiO2 Microspheres

The polyol-based co-precipitation process was employed for synthesis of GdF3:Pr (core) and GdF3:Pr@LaF3 (core-shell) microspheres (MSs). Subsequently, an amorphous silica layer was deposited surrounding the core-shell MSs, which was verified from high-resolution transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and FTIR results. The absorption spectral results revealed the high solubility with good colloidal stability in aqueous solvents. The detailed structural and morphological analysis, as well as crystallinity of the samples, was investigated through X-ray diffraction, TEM and band gap energy results. The experimentally calculated band gap energy was found to decrease after gradually coating insulating layers of LaF3 and amorphous silica over the surface, because of an effective increase in particle size. The Pr3+-doped GdF3 shows sharp 4f15d1→4f2 emission bands (260-480 nm) as well as typical 4f2→ 4f2 emission lines (460-800 nm) of Pr3+ under 4f2 →4f15d1 excitation. After surface coating, comparative photoluminescence properties of the MSs were investigated by excitation and emission spectra. The origin of the different types of emission transitions were analyzed in details.

Effect of Surface Functionalization on Structural and Optical Properties of Luminescent LaF₃:Sm Nanoparticles.

Samarium (Sm3+)-doped LaF3 nanoparticles (NPs) subsequently encapsulated with inert crystalline LaF3 and amorphous silica layers were prepared by polyol and sol-gel chemical process, respectively. These surface modified core/shell/SiO2-nanostructured were characterized by X-ray diffraction (XRD), FE-transmission electron microscopy (TEM), thermal analysis, FTIR, UV/Vis absorption, bang gap energy and photoluminescence spectroscopy. The FETEM, EDX and FTIR spectral studies clearly revealed that the silica layer has been formed surrounding the core-NPs. Comparative spectral analysis indicated that core/shell/SiO2-NPs revealed high solubility in aqueous and non-aqueous solvents. The decrease in band gap energy after surface growth of an inert LaF3 and silica shells is directly correlated to the increase in grain size. On comparing the emission intensity, a significant enhancement was observed after inert layer coating, whereas, it suppress after silica encapsulation due to the non-radiative transitions. The increase luminescent intensity after inert shell growth indicates that a significant amount of non-radiative centers existing on the surface of core/shell nanoparticles can be eliminated by the shielding effect of LaF3 shells. These observed results indicate that the as-prepared core/shell/SiO2-NPs could be highly useful in broad photonic based applications such as optical sensor/optical bio-probe and light emitting diode.

Wollastonite carbonation in water-bearing supercritical CO2: Effects of water saturation conditions, temperature, and pressure

Abstract The carbonation of silicates in water-bearing supercritical CO2 (scCO2) can affect the performance of CO2-enhanced subsurface operation. This study investigated the effects of water, temperature, and pressure on the carbonation of wollastonite (CaSiO3) in water-bearing scCO2 at 35–93 °C, 25–125 bar, and from dry conditions to 140 times more water than needed for saturation (140 × Sw). The extent of reaction increased with more water, higher pressure, and interestingly, lower temperature. In addition, for the first time, we report on water-bearing scCO2–silicate reactions for samples with varying prior histories of water saturation conditions, temperature, and pressure. For a sample initially reacted at 100% water saturation (1 × Sw) and then with 45 × Sw, the final reaction extent was smaller than the sample reacted only with 45 × Sw. Similarly, for a sample initially reacted at 60 °C and then at 35 °C, the reaction extent was smaller than for a sample reacted only at 35 °C. In contrast, for a sample initially reacted at 50 bar and then at 100 bar, the reaction extent was the same as for a sample reacted only at 100 bar. The amorphous silica layers formed on the particle surfaces are more permeable with more water and lower temperature, and are not affected by pressure. These findings provide new insights into the extent to which water-bearing scCO2–silicates reactions depend on environmental conditions and the reactions' impacts on subsurface CO2 injection.

Магнитные нанокомпозиты MgFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=-/SiO-=SUB=-2-=/SUB=- типа ядро/оболочка для биомедицинских применений: синтез и свойства

AbstractMagnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe_2O_4) is a core, and an amorphous silicon dioxide (silica SiO_2) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe_2O_4/SiO_2 MNC has the core/shell structure formed by the Fe‒O–Si chemical bond. After coating with silica, the MgFe_2O_4/SiO_2 MNC saturation magnetization significantly decreases in comparison with MgFe_2O_4 particles without a SiO_2 shell. Spherical particles agglomerated from MgFe_2O_4 nanocrystallites ∼9.6 and ∼11.5 nm in size function as cores coated with SiO_2 shells ∼30 and ∼50 nm thick, respectively. The total size of obtained CS MNCs is ∼200 and 300 nm, respectively. Synthesized CS MgFe_2O_4/SiO_2 MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.

In-vitro cytotoxicity evaluation of surface design luminescent lanthanide core/shell nanocrystals

Lanthanide nanocrystals (NCs) are the most promising luminescent materials for bioapplications, but their use is hindered by difficulties in obtaining biocompatible and photoluminescence lanthanide NCs. To solve this problem, a simple and versatile strategy was developed for improving the luminescence efficiency with the hydrophilicity of the lanthanide NCs. In this study, the effects of shell formation on structural, morphological, and optical properties (optical absorption, band-gap energy, excitation, emission, and luminescent decay time) were evaluated. To improve the luminescence efficiency and aqueous dispersion, luminescent core-NCs were encapsulated with inert NaGdF4 and amorphous silica layers. These surface coating layers significantly improved the luminescence efficiency and dispersion of the core/shell NCs in which the silica surface provides a negatively charged surface to the NCs at physiological pH. Optical properties of these NCs strongly depend on the external change of NCs, demonstrating the impact of coating in improving the luminescence efficiency. The outcomes can be ascribed to the development of surface chemical bonds betweencore/shell and noncrystalline SiO2shell via GdOSi bridges, activating the ‘dormant’ Ce3+ and Tb3+ions on the surface of NCs. An intensive emission and good hydrophilic property from the active functional groups in solutions show a great potential for applications such as multi-analyte fluorescent biolabeling, optical biosensing, staining, display, and other optical technologies. The core/shell/SiO2 NCs showed higher nontoxicity and biocompatibility with respect to the core NCs because of biocompatible silica surface modification, facilitating entry into the living cells. Therefore, this developed synthesis approach might advance the field of biomolecule-based nanotechnology in near future.

Structural effects on dissolution of silica polymorphs in various solutions

The dissolution and precipitation of silica minerals in rocks, soils, and sediments are essential processes of material transformations near the Earth’s surface. In this study, the dissolution of α-quartz and α-cristobalite are investigated at 25 °C in HNO3, NaOH, KCl, and MgCl2 solutions. The amounts of silicon release from α-quartz in HNO3 and electrolyte solutions are larger than those from α-cristobalite, and the circumstance is in consistence with the density of surface silanols. In NaOH solutions, the amounts of silicon release increase significantly and the maximum amount is about 30 times higher than that in acid and electrolyte solutions. Moreover, the maximum silicon release is inversely proportional to the density of surface silanols, resulting in a lower silicon release from α-quartz in comparison to that from α-cristobalite. It could be deduced that there is a possible correlation with structural defects, for instance, oxygen vacancies, demonstrated by the electron paramagnetic resonance (EPR) spectrum. By analyzing the surface composition of samples after dissolution, the result of X-ray photoelectron spectroscopy (XPS) confirms that no stable amorphous silica layer form on the surface of α-quartz and α-cristobalite under the current experimental conditions, along with the verification of high-resolution transmission electron microscope (TEM). Furthermore, the surface species of α-quartz and α-cristobalite after reacting with different solutions are also qualitatively determined by XPS.

Towards an optimum silicon heterojunction solar cell configuration for high temperature and high light intensity environment

We report on the performance of Silicon Heterojunction (SHJ) solar cell under high operating temperature and varying irradiance conditions typical to desert environment. In order to define the best solar cell configuration that resist high operating temperature conditions, two different intrinsic passivation layers were tested, namely, an intrinsic amorphous silicon a-SiOx:H with CO2/SiH4 ratio of 0.4 and a-SiOx:H with CO2/SiH4 ratio of 0.8, and the obtained performance were compared with those of a standard SHJ cell configuration having a-Si:H passivation layer. Our results showed how the short circuit current density Jsc, and fill factor FF temperature-dependency are impacted by the cell’s configuration. While the short circuit current density Jsc for cells with a-SiOx:H layers was found to improve as compared with that of standard a-Si:H layer, introducing the intrinsic amorphous silicon oxide (a-SiOx:H) layer with CO2/SiH4 ratio of 0.8 has resulted in a reduction of the FF at room temperature due to hindering the carrier transport by the band structure. Besides, this FF was found to improve as the temperature increases from 15 to 45 °C, thus, a positive FF temperature coefficient.

Silica-modified luminescent LaPO4 :Eu@LaPO4 @SiO2 core/shell nanorods: Synthesis, structural and luminescent properties.

Monoclinic-type tetragonal LaPO4 :Eu (core) and LaPO4 :Eu@LaPO4 (core/shell) nanorods (NRs) were successfully prepared using a urea-based co-precipitation process under ambient conditions. An amorphous silica layer was coated around the luminescent core/shell NRs via the sol-gel process to improve their solubility and colloidal stability in aqueous and non-aqueous media. The prepared nano-products were systematically characterized by X-ray diffraction pattern, transmission electron microscopy, energy dispersive X-ray analysis, and FTIR, UV/Vis, and photoluminescence spectroscopy to examine their phase purity, crystal phase, surface chemistry, solubility and luminescence characteristics. The length and diameter of the nano-products were in the range 80-120nm and 10-15nm, respectively. High solubility of the silica-modified core/shell/Si NRs was found for the aqueous medium. The luminescent core NRs exhibited characteristic excitation and emission transitions in the visible region that were greatly affected by surface growth of insulating LaPO4 and silica layers due to the multiphonon relaxation rate. Our luminescence spectral results clearly show a distinct difference in intensities for core, core/shell, and core/shell/Si NRs. Highly luminescent NRs with good solubility could be useful candidates for a variety of photonic-based biomedical applications.

A Thermodynamic Consideration on the Mechanism of Ultrasensitive Moisture Sensing by Amorphous Silica

We detected the change in the phase velocity of surface acoustic waves propagating along the amorphous silica layer on a spherical single-crystal quartz due to the very small amount of moisture at less than 1~upmu hbox {mol}{cdot } hbox {mol}^{-1} in the ambient hbox {N}_{2} gas. This was made possible for the first time because this system, called a ball SAW moisture sensor, was extremely sensitive. The measured phase velocity changed as a function of moisture density and temperature and was fitted strikingly well with a thermodynamic model assuming that dissociated hbox {H}^{+} and hbox {OH}^{-} dissolved into the amorphous silica layer at room temperature.

Band Offset Reduction at Defect-Rich p/i Interface Through a Wide Bandgap a-SiO:H Buffer Layer

In single junction p-i-n solar cells, the optical losses can be mitigated by inserting the wide band gap amorphous silicon oxide layer at the defect-rich p/i interface. In this paper, a simulation and experimental study on the performance of p-i-n solar cells by inserting the intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) buffer layer at the p/i interface is reported. The i-a-SiO:H film has been deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) (13.6 MHz) at a low substrate temperature of approximately 230°C. The p/i interface is crucial to solar cell performance because the first few nanometers of the intrinsic layer are a defect-rich layer, having the band gap discontinuity, resulting in band offset. Thus, the carrier recombination probability increases in the vicinity of p/i interface because of high defect density and short carrier lifetime. Aided by optically calibrated simulations and with the support of experimental results, this study shows that a wide band gap thin undoped a-SiO:H buffer layer with higher photoconductivity reduces the band-gap offset and minimizes the recombination of photogenerated charge carriers at the defect-rich p/i interface. It has also been found that the a-SiO:H buffer layer augmented the electric field inside the device. As a result, the overall performance of the a-Si:H-based single junction solar cell has significantly improved. By employing a ∼5 nm thick a-SiO:H buffer layer, the blue response of the cell has been improved, resulting in 7.34% and 18.62% enhancement in fill factor (FF) and power conversion efficiency ( η ), respectively, as compared to the buffer-less cell.

Passivation mechanism in silicon heterojunction solar cells with intrinsic hydrogenated amorphous silicon oxide layers

In this work, we use intrinsic hydrogenated amorphous silicon oxide layers (a-SiOx:H) with varying oxygen content (cO) but similar hydrogen content to passivate the crystalline silicon wafers. Using our deposition conditions, we obtain an effective lifetime (τeff) above 5 ms for cO ≤ 6 at. % for passivation layers with a thickness of 36 ± 2 nm. We subsequently reduce the thickness of the layers using an accurate wet etching method to ∼7 nm and deposit p- and n-type doped layers fabricating a device structure. After the deposition of the doped layers, τeff appears to be predominantly determined by the doped layers themselves and is less dependent on the cO of the a-SiOx:H layers. The results suggest that τeff is determined by the field-effect rather than by chemical passivation.

In vitro characterisation of a sol–gel derived in situ silica-coated silicate and carbonate co-doped hydroxyapatite nanopowder for bone grafting

Design and synthesis of materials with better properties and performance are essential requirements in the field of biomaterials science that would directly improve patient quality of life. For this purpose, in situ silica-coated silicate and carbonate co-doped hydroxyapatite (Sc/S.C.HA) nanopowder was synthesized via the sol–gel method. Characterisation of the prepared nanopowder was carried out by XRD, FTIR, TEM, SEM, EDX, ICP, zeta potential, acid dissolution test, and cell culture test. The substitution of the silicate and carbonate ions into hydroxyapatite structure was confirmed by FTIR analysis. XRD analysis showed that silica is an amorphous phase, which played a role in covering the surface of the S.C.HA nanoparticles as confirmed by acid dissolution test. Low thickness and low integrity of the amorphous silica surface layer facilitated ions release from S.C.HA nanoparticles into physiological saline solution. Zeta potential of the prepared nanopowder suspended in physiological saline solution was −27.3±0.2mV at pH7.4. This negatively charged surface, due to the presence of amorphous silica layer upon the S.C.HA nanoparticles, not only had an accelerating effect on in vitro biomineralization of apatite, but also had a positive effect on cell attachment.

Spatially-variable carbonation reactions in polycrystalline olivine

Carbon dioxide (CO2) injection into olivine-rich mafic and ultramafic rocks is expected to result in the precipitation of divalent metal carbonate minerals, permanently storing the CO2 underground. Previous experiments that used unconsolidated forsterite (Mg2SiO4) particles in experimental investigations of reactions with water and carbon dioxide have been useful for determining the identity, rates of formation, and spatial location of carbonate mineral reaction products. However there remains a need for information regarding the influence of the internal pore structure and grain boundary surfaces on the extent and locations of these reactions in dense aggregates. We conducted several experiments at 100°C and 100bar CO2 using sintered San Carlos olivine (Fo90) and pure forsterite (Fo100) cylinders, and we documented the type and spatial distribution of the reaction products. Timing of carbonation was measured using in-situ 13C NMR spectroscopy without removing the sample from the reactor. Ex-situ solid-state NMR spectroscopy, Raman spectroscopy, and electron microscopy were used to examine reacted samples and precipitates. Within 15days, magnesite is observed only on the surface of Fo90. After 53 and 102days of reaction, magnesite and amorphous silica are observed as a crust around the entire Fo100 cylinder and as isolated layers within the sample. The spatial transition from an amorphous silica layer to the host Fo100 indicates that the development of amorphous silica did not impede further forsterite dissolution. While earlier studies documented localized reactions at the grain scale, the development of distinct zones of magnesite and amorphous silica suggest that divalent metal cations are mobile during carbonation of olivine. Grain boundaries, pore structure, and geochemical gradients strongly influence the locations of silicate mineral dissolution and carbonate mineral precipitation even in the absence of advective transport or confinement. The clear spatial distribution of precipitates observed in this study indicates that carbonation in heterogeneous reservoirs may be strongly affected by grain-scale microstructure.

Zeolite A-Carbon Membranes: Possibilities in H2 Purification and how to Overcome their Shortcomings

This work describes the modification of zeolite Na-LTA membranes supported on macroporous carbon materials, prepared by a combination of secondary hydrothermal treatment followed by different alternative post-synthesis procedures, which aim at improving the permeance properties of the as-synthesized Na-LTA membranes with a simulated reformer mixture (H2, CO, CO2 and H2O) towards their use in a hydrogen purification device.These post-synthetic treatments include the deposition of a thin layer of amorphous silica formed by the hydrolysis of a silicon alcoxide, the coating with a thin metallic film by electroless plating, and the deposition of noble metal nanoparticles. Our results indicate that some of these treatments, which may be performed very quickly compared to other treatments which are generally used in order to improve the quality of the membranes, result in membranes which may effectively separate H2 from CO under simulated reformer conditions. Considering the simple approach employed in some of the cases described in this study, the potential benefits should be considered highly interesting in fields such as membrane recovery and membrane selectivity control.

Beneficial Effect of a Pre-ceramic Polymer Coating on the Protection at 900 °C of a Commercial AISI 304 Stainless Steel

An effective and low-cost method for applying polysilazane-based barrier coating on stainless steel is presented. A SiN (PHPS) precursor has been used for applications of polymeric and ceramic-like coating material. With precursor solution in dibutyl ether, AISI 304 stainless steel substrates were coated by means of a simple dip coating technique. After pyrolysis stage at 800 °C, the efficiency of the PHPS-based coating has been proved, and oxidation tests were done at 900 °C in air. By measuring the weight gain, and by using analytical techniques as in situ X-Ray Diffraction and Scanning Electron Microscopy, it has been shown that an amorphous silica layer formed from the initial PHPS-based coating and acts as a protective glass which provides complete separation of the AISI 304 steel surface from the oxidizing environment. This protective silica layer appears to be a very good diffusion barrier by preventing iron-containing oxide formation.

Synthesis of SiO2-Coated Fe3O4 Nanoparticles Using Ultrasound and Its Application in DNA Extraction from Formalin-Fixed, Paraffin-Embedded Human Cancer Tissues

SiO2-coated Fe3O4 nanoparticles (Fe3O4@SiO2 NPs) were successfully synthesized using ultrasound in order to extract DNA from cancer tissues for application in diagnostics. The core 10.7-nm-diameter Fe3O4 nanoparticles were synthesized by co-precipitation of Fe3+ and Fe2+ as reaction substrates and NH4OH as precipitant, then coated with a thin layer of amorphous silica by a modified Stober method. Further SiO2 coating using alkaline hydrolysis of tetraethyl orthosilicate in ethanol and water mixture was accelerated in the presence of a 37-kHz ultrasound, resulting in the NPs having different sizes of 14.5 nm (version M1), 24.4 nm (version M2), and 34.9 nm (version M3) with saturation magnetization values of 50.2 emu/g, 18.6 emu/g, 10.3 emu/g, respectively. Among the three Fe3O4@SiO2 NPs versions, the M1 NPs allowed extraction of DNAs from 10 mg formalin-fixed and paraffin-embedded (FFPE) tissues of nasopharyngeal carcinoma patients with the highest recovery of about 100–500 ng/μl and good purity (A260/A280: 1.8–1.9). The extracted DNAs could be used as templates for downstream amplification of 252-bp sequencing specifically for the Braf cancer biomarker gene using polymerase chain reaction (PCR), as well as detection of the pathogenic Epstein–Barr virus (EBV) and the human papilloma-virus (HPV) using real-time PCR. DNA extraction recoveries of both EBV and HPV using Fe3O4@SiO2 NPs M1 were significantly better that those using commercialized Fe3O4@SiO2 microbeads, as indicated by lower threshold cycles of all fluorescent signals including fluorescein amidite (FAM) dye representative for EBV infection, hexachlorofluorescein (HEX) dye representative for β-globin (internal control), and SYBR Green dye representative for HPV infection in tested clinical samples from patients with nasopharyngeal carcinoma (NPC).

Impact of silica environment on hyperfine interactions in 𝜖-Fe2O3 nanoparticles

Magnetic nanoparticles have found broad applications in medicine, especially for cell targeting and transport, and as contrast agents in MRI. Our samples of 𝜖-Fe2O3 nanoparticles were prepared by annealing in silica matrix, which was leached off and the bare particles were then coated with amorphous silica layers of various thicknesses. The distribution of particle sizes was determined from the TEM pictures giving the average size ∼20 nm and the thickness of silica coating ∼5; 8; 12; 19 nm. The particles were further characterized by the XRPD and DC magnetic measurements. The nanoparticles consisted mainly of 𝜖-Fe2O3 with admixtures of ∼1 % of the α phase and less than 1 % of the γ phase. The hysteresis loops displayed coercivities of ∼2 T at room temperature. The parameters of hyperfine interactions were derived from transmission Mossbauer spectra. Observed differences of hyperfine fields for nanoparticles in the matrix and the bare ones are ascribed to strains produced during cooling of the composite. This interpretation is supported by slight changes of their lattice parameters and increase of the elementary cell volume deduced from XRD. The temperature dependence of the magnetization indicated a two-step magnetic transition of the 𝜖-Fe2O3 nanoparticles spread between ∼85 K and ∼150 K, which is slightly modified by remanent tensile stresses in the case of nanoparticles in the matrix. The subsequent coating of the bare particles by silica produced no further change in hyperfine parameters, which indicates that this procedure does not modify magnetic properties of nanoparticles.

Nano-Tomography of Porous Geological Materials Using Focused Ion Beam-Scanning Electron Microscopy

Tomographic analysis using focused ion beam-scanning electron microscopy (FIB-SEM) provides three-dimensional information about solid materials with a resolution of a few nanometres and thus bridges the gap between X-ray and transmission electron microscopic tomography techniques. This contribution serves as an introduction and overview of FIB-SEM tomography applied to porous materials. Using two different porous Earth materials, a diatomite specimen, and an experimentally produced amorphous silica layer on olivine, we discuss the experimental setup of FIB-SEM tomography. We then focus on image processing procedures, including image alignment, correction, and segmentation to finally result in a three-dimensional, quantified pore network representation of the two example materials. To each image processing step we consider potential issues, such as imaging the back of pore walls, and the generation of image artefacts through the application of processing algorithms. We conclude that there is no single image processing recipe; processing steps need to be decided on a case-by-case study.

Oxide-Coated Carbon Black for Cathode Catalyst Support

Carbon black is an essential material for PEFC. Due to its specific surface area, conductivity, pore-making structure and stability for acidic environment, carbon black is so far the best choice as the catalyst support for cathode and anode. However, carbon black is being one of the critical elements which limit the lifetime of PEFC. Especially in cathode, oxidative degradation of carbon black support is resulting in the depression of catalyst activity. In order to prevent carbon black support from its oxidative degradation, surface coating with silicon oxide (silica) on acetylene black was tried using sol-gel method. Nanometer-thick amorphous silica layer was formed on almost the whole surface on the acetylene black. Its effect on resistivity against oxidation, Pt supporting function and total behavior of the catalyst is to be discussed.

Oxide-Coated Carbon Black for Cathode Catalyst Support

Carbon black is an essential material for PEFC; on the other hand, carbon black is concerned as the critical element which limits the lifetime of PEFC. Especially in the cathode, oxidative degradation depressess the catalyst activity. In order to prevent carbon black support from its oxidative degradation, surface coating with silicon oxide (silica) on acetylene black was carried out using “sol-gel method”. Thin amorphous silica layer was formed at the nanometer thickness on the almost surface on acetylene black. Its effect on the anti-oxidation, Pt supporting function and total behavior of the catalyst are discussed.