A non-contact method for assessing the local geometrical thickness, porosity, and effective refractive index of synthetic opals using spectral optical coherence tomography is proposed. This approach was employed to investigate the diffusion of aqueous glycerol solutions with varying concentrations into the macropores of synthetic opals pre-filled with water. The kinetics of changes in the optical thickness and refractive index of synthetic opals during the optical clearing process were monitored. The obtained data were analyzed using the least-squares method, which enabled the determination of porosity 20–30

The transformation of shell structure of the core-shell particles of poly(methyl methacrylate)-silica and hollow SiO2 particles undergoing heat treatment was studied by infrared (IR) spectroscopy using deconvolution of absorption bands in the wavenumber range 400–650 cm–1. It was revealed that the changes in the IR absorption spectra during annealing were caused by structural transformation of the particles’ material due to its restructuring, including the formation of silica ring clusters of different sizes and changes of their ratio during annealing. The used method of the spectra deconvolution made it possible to consider in more detail the parameters and contributions of the vibration modes of such clusters to the absorption spectra of SiO2. A modified structure of a polysiloxane, synthesized through hydrolysis of trimethoxy(vinyl)silane, which represents a polymer chain of repeating fragments containing four- and three-fold (Si–O) rings linked by a bridging oxygen atom, is proposed. It has been found that skeletal vibrations of 6-fold (Si–O) rings combined with transverse optical and longitudinal optic vibration modes of the Si–O–Si bonds make the principal contribution to total IR absorption of the studied samples both in amorphous and in crystal silica structures obtained after annealing.

Prospective materials for hydrogen storage should have a high hydrogen content, high adsorption/desorption rates, and consist of abundant elements. Simultaneous implementation of these properties in one material poses a great challenge for researchers. To solve this problem, placement of hydrogen in hollow silicate glass microspheres with a diameter greater than 5 mu m was previously proposed as one of the possible ways for hydrogen storage. Additionally, various deuterium-containing spheres were proposed as fuel targets in laser-initiated thermonuclear reactions. In this study, opal matrices consisting of hollow silica nanospheres with an outer diameter of 289 nm and a shell thickness of 25 nm were hydrogenated to X = 0.94 mol H2 per mole SiO2 at a pressure of 7.5 GPa and a temperature of 413 K. This highest hydrogen content of silicates achieved to date dropped to X = 0.8 after keeping the sample in liquid nitrogen at ambient pressure for three days, and then stopped changing. Scanning electron microscopy showed that hydrogenation did not damage the shape of the nanospheres. Raman spectroscopy demonstrated that hydrogen molecules formed a gas in the cavities inside the spherical SiO2 shells and a solid solution in the shells. The density of the hydrogen gas inside the cavities estimated from the intensity of the H2 vibrational mode was about 0.016 g/cm3, which is 52 times greater than its density at the same temperature and normal pressure.

Submicron hollow spherical silica particles with a shell thickness of 16-93 nm were obtained by the template method using polymethylmethacrylate templates. The colorimetric characteristics (lightness L* and chroma C*) of their dry powders and dispersions in glycerol-water solutions were investigated as a function of the geometric parameters of the particles and their concentration in the suspension. The dry powders of the hollow silica particles exhibited maximum values of lightness (>98.5) irrespective of the particle shell thicknesses. The chroma values for all the dispersions of the samples were less than 3, which, in combination with high lightness values (85-95), makes these particles promising candidates to be used as a white pigment in liquid media, particularly as a replacement for titanium dioxide in food and pharmaceutical products.

We deposited self-assembled films of a few monolayers of silica spheres from the water-based colloid on vertically moving hydrophilic single crystal diamond substrates. The use of acoustic agitation of the water suspension with the SiO2 nanospheres significantly improved the long-range order of the films with opal structure. The study examines the evolution of the structure, specifically the size of single crystal domains, with the assistance of sound frequency (0-2500 Hz) and intensity (0-125 dB). Optimal (resonance) parameters are determined based on scanning electron microscopy and optical reflection spectroscopy of the films. The production of high-quality photonic crystals, including templates for diamond-based ordered composites and inverse opal structures, is a promising application of acoustic crystallization of opals on diamond substrates.

The kinetics of the synthesis of silica nanoparticles (<50 nm) has been studied under the conditions of heterogeneous hydrolysis of tetraethoxysilane (TEOS) using L-arginine as an alkaline catalyst. The rates of silica formation have been determined in a temperature range of 10–95°C at catalyst concentrations of 6–150 mM. It has been shown that the activation energy of the process depends on catalyst concentration and varies in a range of 21.5–13.9 kJ/mol, while decreasing linearly with increasing concentration of L-arginine in the system. The criterion of maintaining the monodispersity has been estimated for SiO2 particles being grown “onto seeds.” The density of submicron-sized silica particles has been experimentally determined as depending on the annealing temperature. Within a temperature range of 200–1000°C, the particle density varies from 2.04 to 2.20 g/cm3.

The chemical vapor deposition synthesis of periodic structures in the form of single-crystal diamond–SiO2 nanosphere composites, which exhibit the properties of photonic crystals in the visible spectral range, are reported.

The absorption and desorption of hydrogen at high pressure and temperature by opal matrices formed by amorphous silica spheres with a diameter of 0.235 and 1.6 mm have been studied. The Raman spectra of hydrogen-saturated opal matrices measured at a temperature of 80 K and ambient pressure show that hydrogen molecules are adsorbed in two different ways, directly into silica spheres and mesopores between them. The kinetics of hydrogen desorption was studied in-situ from a change in the relative intensity of rotational modes in the Raman spectra under annealing at 163-213 K. The hydrogen content decreases exponentially under isothermal heating, while the exponential decay time constant T increases with a decreasing temperature showing the activation nature of desorption. The data for various temperatures are well described by the Arrhenius dependence t(T) = A x exp(EA/kBT) with the activation energy EA=(162 +/- 13) meV and time (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

We report on synthesis of periodical structures by chemical vapor deposition in form of a composite “single crystal diamond – SiO2 nanospheres” which shows photon crystal properties in the visible.

Hollow submicrometer-sized SiO 2 particles are synthesized, and changes in the structure and morphology of their shells during heat treatment are studied. The dependences of the shrinkage of silica shells on the temperature of annealing of particles are studied. It is found the shells of hollow particles are pore-free and impervious to liquids after annealing at 600°C.

Hollow SiO2 particles of submicron size were synthesized and changes in the structures and morphology of their shells during heat treatment were investigated. The dependences of the shrinkage of silica shells on the annealing temperature of the particles were studied. It has been found that after annealing at 600°C, shells of hollow particles become non-porous and impermeable to liquid media.

Terahertz (THz) technology offers a variety of applications in medical spectroscopy and imaging. In such applications, tissues are commonly assumed to be optically homogeneous at the THz-wavelength scale, while the THz-wave-tissue interactions are described (in a simplified manner) within the effective medium theory (EMT). Meanwhile, recent studies of tissues from human, animal, and plants involving emerging modalities of superresolution (beyond the Abbe diffraction limit) THz microscopy, have found tissue inhomogeneities with dimensions comparable to the THz wavelength (approximately lambda), which can lead to THz-wave scattering effects. This poses a problem of studying an interplay between the THz-wave absorption and the scattering phenomena in soft turbid tissues. To mitigate this challenge, in this paper, a tissue-mimicking phantom is developed that has the form of a gelatin slab, as a highly absorbing hydrated matrix, into which silicon dioxide (SiO2) microparticles are embedded, with their lower refractive index and loss and subwavelength or mesoscale diameters. The analytical methods of Lorenz-Mie scattering theory have predicted a nonisotropic differential extinction cross section for such scatterers, which results in the non-Rayleigh scattering regime and casts doubt on the applicability of EMT for such tissues. Surprisingly, we have found theoretically, using the radiative transfer theory (RTT), and confirmed experimentally, using THz pulsed spectroscopy, that the effective optical properties of the proposed phantom are still determined by EMT over wide ranges of the diameters (d <= 0.47 lambda) and volume fractions (f(v)<= 0.2) of the scatterers. This effect was attributed to the strong THz-wave loss in a host medium and, therefore, should be general for a variety of soft tissues in the THz range. Thus, our findings hopefully broaden the applicability of EMT for describing the interactions between THz radiation and soft turbid tissues.

Artificial opals fabricated by sedimentation and self-assembly of colloidal SiO2 nanoparticles and annealed at different temperatures were recently considered favorable terahertz (THz) optical materials with manageable optical properties. However, interactions between such a porous material and water vapour in a humid atmosphere can hamper their THz applications due to the related changes in the material parameters and additional power loss. To quantify such an effect, in this paper, moisture adsorption by artificial SiO2 opals is studied using THz pulsed spectroscopy. Particularly, opals of two kinds were sedimented from the colloidal suspension of 300-nm-diameter SiO2 nanoparticles with different intraglobular structures and porosity. They were annealed at temperatures of 200–800°C aimed at changing their internal structure, porosity, and THz optical properties. Opals were dehydrated in a vacuum and then exposed to a humid atmosphere with 82.0 ± 2.0% relative humidity, while their THz complex dielectric permittivity was evaluated in situ in the 0.5–2.5 THz range. The observed changes in the THz dielectric curves were analyzed using the sum rule and the adsorption kinetics models. Our findings reveal a strong dependence of the THz dielectric response, amount of adsorbed water, and adsorption time constant on the opal type and annealing conditions. This effect has a general character: it can hamper real-live applications of a variety of porous THz optical materials and, thus, should be taken into account during their synthesis.

We report on synthesis of periodical structures by chemical vapor deposition in form of a composite “single crystal diamond – SiO2 nanospheres” which shows photon crystal properties in the visible.

Changes in the morphology and structure of the core-shell particles of polymethyl methacrylate-silicon dioxide and hollow SiO2 particles during their heat treatment were studied by electron microscopy, infrared spectroscopy, and X-ray diffraction. The polymeric core of the PMMA-SiO2 hybrid particle was found to undergo an unusual transformation when exposed to the electron microscope beam: its shrinkage occurs through the formation of a spherical cavity. It was shown that the process of silica-shell formation occurs in the temperature range of 200–600 °C and is accompanied by the loss of vinyl- and OH-groups. It was determined by the method of X-ray diffraction, that in the place of the interaction of PMMA and the shell, the degree of ordering of the polymer is higher than that in the volume of the polymer core. It was shown that the frequency of the TO3-vibrational mode (asymmetric stretching vibrations of the Si–O–Si bonds) increases with an increase in the annealing temperature, which is associated with the densification of the silicon dioxide shell.

Opal-like structures are proposed as a promising new material for terahertz (THz) optics. Opal matrices deposited from SiO2 globules with a diameter of 300 nm with subsequent annealing in the temperature range 200–1500°C are investigated. It is experimentally shown that the THz optical properties of such a material can vary over a wide range with an increase in the annealing temperature (refractive index from 1.6 to 1.95, amplitude absorption coefficient of the material from 7 to 0.4 cm–1). On the basis of effective medium theory, namely the Bruggemann equation, a model is proposed that makes it possible to predict the optical properties of the material under study depending on the annealing temperature. To demonstrate the possibility of manufacturing an optical THz component, a plane-convex cylindrical lens is fabricated from the material under study. The possibility of varying the optical properties of the opal matrix in a wide range, a low absorption coefficient, and the absence of dispersion in the THz spectral region show the prospects of opal-like materials for THz applications.