Hydrothermal SiO2 Nanopowders: Obtaining Them and Their Characteristics.

The technological mode of obtaining amorphous SiO2 nanopowders based on hydrothermal solutions is proposed in this study. Polycondensation of orthosilicic acid as well as ultrafiltration membrane separation, and cryochemical vacuum sublimation were used. The characteristics of nanopowders were determined by tunneling electron microscopy, low-temperature nitrogen adsorption, X-ray diffraction, and small-angle X-ray scattering. The scheme allows to adjust density, particle diameters of nanopowders, specific surface area, as well as diameters, area and volume of the pore. Thus, the structure of nanopowders is regulated—the volume fraction of the packing of spherical particles in aggregates and agglomerates, the size of agglomerates, and the number of particles in agglomerates. The pour densities of the nanopowders depend on the SiO2 content in sols, which were 0.02 to 0.3 g/cm3. Nanoparticles specific surface area was brought to 500 m2/g by low temperature polycondensation. Nanoparticle aggregates specific pore volume (0.2–0.3 g/cm3) weakly depend on powders density. The volume fraction of the packing of SiO2 nanoparticles in aggregates was 0.6–0.7. Solid samples of compacted nanopowders had a compressive strength of up to 337 MPa. Possible applications of hydrothermal SiO2 nanopowders are considered.


Introduction
To date, a wide range of methods for producing various types of powders of amorphous dioxide silicon are known. At the same time, the need for SiO 2 nanopowders-particles which have a high specific surface area up to 1000 m 2 /g and significant chemical activity-is increasing. Cheap sources of such materials and low-cost technologies for their production are needed.
Traditional applications of SiO 2 nanopowders are known for the production of ceramics, glass, catalyst supports, sorbents, rubber fillers, polymeric materials, paper, abrasive materials, and medical preparations [1]. In the large-scale production of pyrogenic SiO 2 nanopowders, the flame hydrolysis of SiCl 4 in an atmosphere (H 2 -O 2 ) is used [1]. The flame temperature, flow rate and volumetric proportions of SiCl 4 , H 2 , O 2 gases, control the size and specific surface area of the nanoparticles. Another major production is the production of silica fume by condensation of gases in ferroalloy furnaces (condensed silica fume).
Another group of methods is based on the preparation of SiO 2 particles from the liquid phase using a sol-gel transition. This group includes the preparation of SiO 2 silicogels using a sol-gel transition followed by subcritical or supercritical gel drying [2,3]. In this case, the hydrolysis and polycondensation of molecules and the preparation of sols of colloidal particles of SiO 2 are used Nanomaterials 2020, 10 at the first stage of the process. The precursors of SiO 2 sols are metal alkoxides and chlorides, tetraethoxysilane and alkali metal silicates (Na, K, Li). At the gel stage, acid treatment with formamide is used to control the porous structure [4,5], and one of the most important parameters is the pH of the medium. The sol-gel method has produced a large number of mesoporous materials with a wide range of applications [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. To obtain mesoporous materials, different variants of the Stober synthesis are used with the use of template additives of surfactants, water-soluble polymers, and previously obtained dense particles of sols [20]. Various forms of surfactant micellar solutions are used to synthesize mesoporous SiO 2 particles [21][22][23][24][25][26][27][28]. SiO 2 mesospheres are also synthesized with preliminary coagulation of the sol with electrolytes and subsequent polymer addition to separate the aggregates and prevent them from sticking together during drying [29]. There are methods for the synthesis of mixed oxides, hollow spheres, and objects of the core-mesoporous shell type [30,31]. SiO 2 is one of the most common components for producing nanopowders, optical elements, medical preparations, thin films, fibers, nanotubes, nanowires, additives to hard films to increase tensile strength, hardness of hybrid coatings, and porous composite ceramics, SiO 2 -Me x O y nanocomposites [32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The possibility of obtaining colloidal SiO 2 based on cheap waste of glass powder was shown in [46]. The production of SiO 2 powders from rice husk has been developed as well [47].
Hydrothermal solutions are a new raw material source for the production of SiO 2 nanopowders. For its development, it is necessary to develop a technology for producing SiO 2 nanopowders taking into account the parameters of the hydrothermal medium: temperature, pH, mineralization, ionic strength, polycondensation kinetics of orthosilicic acid, sizes and concentration of SiO 2 particles, and stability of SiO 2 nanoparticles in an aqueous medium.
The objectives of this article were: -Create a technological route for the production of SiO 2 nanopowders based on a hydrothermal solution with specific surface area up to 500 m 2 /g using the methods of ultrafiltration membrane separation and cryochemical vacuum sublimation.
-Create regulation parameters of the structure of the nanopowders: the diameters of SiO 2 nanoparticles, specific surface area of nanopowders, diameters and specific pore volume, pour density, volume fraction of spherical particles in aggregates and agglomerates, sizes of agglomerates and number of particles in agglomerates.
-Assessment of possible applications of the obtained nanopowders.

Methods for Producing Nanopowders
Silica is formed in a hydrothermal solution from molecules of orthosilicic acid (OSA), which comes from the chemical interaction of water of a hydrothermal solution with aluminosilicate minerals of rocks (orthoclase, microcline K(AlSi 3 O 8 ), albite Na(AlSi 3 O 8 ), anorthite Ca(Al 2 Si 2 O 8 , etc.) in the bowels of hydrothermal deposits at high pressures (10-25 MPa and above) and temperatures (250-300 • C and above). As the solution rises to the surface through the productive wells of geothermal power plants (GeoPP), temperature and pressure decrease, and the solution becomes supersaturated with respect to the solubility of C e amorphous silica. In the solution, polycondensation and nucleation of OSA molecules occur, leading to the formation of spherical silica nanoparticles with a diameter of 5 to 100 nm. In addition to silica, other components are in solution, the concentrations of which are given in Table 1. Silica is in solution in two states: solid (SiO 2 particles) and dissolved (OSA molecules). At the first stage of the process, OSA polycondensation and the growth of SiO 2 nanoparticles were carried out at a certain temperature and pH of the hydrothermal solution. The final particle sizes of silica depend primarily on the temperature and pH at which the polycondensation of OSA molecules takes place. An increase in the polycondensation temperature and a decrease in pH slow down the reaction and increase the final particle size.
At the polycondensation stage, the temperature ranged from 20 to 90 • C (by preliminary cooling in heat exchangers), pH = 8.0-9.3. The range of silica concentrations in the initial solution is Ct = 400-800 mg/kg (t indicates the total silica content equal to the sum of the concentrations of the colloidal phase and dissolved Cs). The nucleation rate of silicic acid in an aqueous solution (nucl/(kg · s)) is described by Equation (1) [48][49][50]: where Q LP = 3.34 × 10 25 kg −1 -the Lohse-Pound factor; k B -the Boltzmann constant; M Si -the molar mass of SiO 2 ; N A -the Avogadro number; T-the absolute temperature, K; in free energy associated with the formation of a nucleus of critical radius R c ; ρ-density of amorphous silica, kg/m 3 ; σ sw -surface tension at the silica-water interface, J/m 2 ; Z-Zeldovich factor.
where n cr = (4 × π/3) × (ρ × N A/ M Si ) × R c 3 -number of SiO 2 molecules in the nucleus of critical size; × T) 0.5 ; R MD -the rate of molecular deposition of silicic acid (g·cm 2 ·min −1 ), which determines the particle growth rate: where k OH (T), F(pH, pH nom ), f f (S a )-auxiliary functions depending on temperature, pH, ionic strength I s and supersaturation S m . The characteristic polycondensation time-the temperature at which the supersaturation value decreased e = 2.71 times from the initial one-was at 20 • C and pH = 8.5, τ p = 118.8 min, and at 50 • C, τ p = 240.0 min.
With a decrease in the polycondensation temperature and an increase in the initial supersaturation S m , the nucleation I N rate increased and, accordingly, the final average diameter d m of SiO 2 nanoparticles decreased, and the polycondensation of OSA passed faster. At pH = 8.0-9.3 and temperatures of 65-90 • C, the d m values were 59-90 nm, at 40-65 • C, d m = 40-60 nm, and at 20-40 • C, d m = 5-40 nm.
After completion of the polycondensation of OSA and the growth of SiO 2 nanoparticles, concentrated aqueous sols were obtained by three-stage ultrafiltration membrane concentration. At the first stage, the SiO 2 content in the sol was increased from 0.05 to 0.3-0.4 wt.%, at the second stage it increased up to 10 wt.%, on the third it increased up to 20% to 40 wt.%. The capillary type ultrafiltration membrane cartridge had an internal capillary diameter of 0.8 mm, a filter surface area of 55 m 2 , a minimum mass weight cut off parameter MWCO = 10-100 kD, a pressure drop across the membrane layer of 0.025-0.4 MPa, and permeability membranes (0.025-0.8) m 3 /m 2 ·h·MPa. The final SiO 2 content in sols was brought to 100.0-600.0 g/dm 3 = 10-40 wt.%, salinity TDS = 800-2000 mg/dm 3 , Nanomaterials 2020, 10, 624 4 of 28 specific conductivity 0.8-1.56 mS/cm, and dynamic viscosity 1-120 MPa·s (20 • C). The choice of pore sizes of polymer ultrafiltration membranes (MWCO = 10-100 kD) can provide high selectivity for SiO 2 nanoparticles and low selectivity for ions of dissolved salts. Therefore, the parameter m s = [SiO 2 ]/TDS continuously increases with increasing SiO 2 content (up to 300 and higher), the inverse parameter (1/m s ) decreases to 0.003 and lower, and there was no accumulation of ions in the concentrate. As a result, the value of the zeta potential of the surface of nanoparticles in concentrated sols fell in the range from −56 to −25 mV, which ensured the stability of particles to aggregation due to electrostatic repulsion without forced incorporation of stabilizers with SiO 2 content up to 62.5 wt.%. Figure 1 shows the results of dynamic light scattering determination of the diameter distribution of particle volume for a sol sample with a content of SiO 2 = 178 g/dm 3 , pH = 9.0, the average diameter of SiO 2 particles in volume d m = 8.5 nm. The average value of the zeta potential of the particle surface found by the method electrophoretic light scattering was ξ m = −42.0 mV (Zetasizer, Malvern, UK). Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 29 ultrafiltration membrane cartridge had an internal capillary diameter of 0.8 mm, a filter surface area of 55 m 2 , a minimum mass weight cut off parameter MWCO = 10-100 kD, a pressure drop across the membrane layer of 0.025-0.4 MPa, and permeability membranes (0.025-0.8) m 3 /m 2 • h •MPa. The final SiO2 content in sols was brought to 100.0-600.0 g/dm 3 = 10-40 wt.%, salinity TDS = 800-2000 mg/dm 3 , specific conductivity 0.8-1.56 mS/cm, and dynamic viscosity 1-120 MPa • s (20 °C). The choice of pore sizes of polymer ultrafiltration membranes (MWCO = 10-100 kD) can provide high selectivity for SiO2 nanoparticles and low selectivity for ions of dissolved salts. Therefore, the parameter ms = [SiO2]/TDS continuously increases with increasing SiO2 content (up to 300 and higher), the inverse parameter (1/ms) decreases to 0.003 and lower, and there was no accumulation of ions in the concentrate. As a result, the value of the zeta potential of the surface of nanoparticles in concentrated sols fell in the range from -56 to -25 mV, which ensured the stability of particles to aggregation due to electrostatic repulsion without forced incorporation of stabilizers with SiO2 content up to 62.5 wt.%. Figure 1 shows the results of dynamic light scattering determination of the diameter distribution of particle volume for a sol sample with a content of SiO2 = 178 g/dm 3 , pH = 9.0, the average diameter of SiO2 particles in volume dm = 8.5 nm. The average value of the zeta potential of   SiO 2 nanopowders were obtained using cryochemical vacuum sublimation of sols. Cryochemical technology includes a sequence of main stages: (1) Dispersion of the sol and cryocrystallization of droplets of a dispersed medium; (2) sublimation of the solvent from the cryogranulate obtained in the previous step; (3) desublimation of the solvent. The cryochemical setup is shown in Figures 2 and 3. Before sublimation in a vacuum chamber, silica sols were dispersed using a nozzle, the droplets were solidified in liquid nitrogen at a temperature of 77 K, and cryogranules were obtained. After dispersion, the droplet size was 20 to 100 µm, the corresponding average droplet cooling rate was about 125 K/s, and the crystallization rate was 0.26 mm/s. The small size of the sol droplets and the high heat transfer surface made it possible to achieve rapid hardening of the droplets and the absence of particle adhesion. The particle sizes in the powders did not exceed the particle sizes in the sols. Vacuum sublimation took place at pressures from 0.02 to 0.05 mm Hg without fragments of droplet moisture and particles sticking together ( Figure 4). To accelerate sublimation, heating was used. The temperature range of the heating surfaces in different parts of the vacuum chamber as it was heated during sublimation ranged from −80 to +25 • C ( Figure 5). Productivity of the unit with a power consumption from 3 to 5 kW is 0.15-0.20 L/h. The residual water content in nanopowders was adjusted to 0.2 wt.% Figure 2. Scheme of the cryochemical vacuum sublimation setup for producing SiO2 nanopowder: 1-apparatus for the preparation of an aqueous sol; 2-metering pump; 3-cryogranulator; 4-tanker with liquid nitrogen; 5-capacity for storing cryogranules; 6-refrigerator; 7-sublimation apparatus; 8-box for storing nanopowder samples. Before sublimation in a vacuum chamber, silica sols were dispersed using a nozzle, the droplets were solidified in liquid nitrogen at a temperature of 77 K, and cryogranules were obtained. After dispersion, the droplet size was 20 to 100 μm, the corresponding average droplet cooling rate was about 125 K/s, and the crystallization rate was 0.26 mm/s. The small size of the sol droplets and the high heat transfer surface made it possible to achieve rapid hardening of the droplets and the absence of particle adhesion. The particle sizes in the powders did not exceed the particle sizes in the sols. Vacuum sublimation took place at pressures from 0.02 to 0.05 mm Hg without fragments of droplet moisture and particles sticking together ( Figure 4). To accelerate sublimation, heating was used. The temperature range of the heating surfaces in different parts of the vacuum chamber as it was heated during sublimation ranged from −80 to +25 °C ( Figure 5). Productivity of the unit with a power consumption from 3 to 5 kW is 0.15-0.20 L/h. The residual water content in nanopowders was adjusted to 0.2 wt.%  Before sublimation in a vacuum chamber, silica sols were dispersed using a nozzle, the droplets were solidified in liquid nitrogen at a temperature of 77 K, and cryogranules were obtained. After dispersion, the droplet size was 20 to 100 μm, the corresponding average droplet cooling rate was about 125 K/s, and the crystallization rate was 0.26 mm/s. The small size of the sol droplets and the high heat transfer surface made it possible to achieve rapid hardening of the droplets and the absence of particle adhesion. The particle sizes in the powders did not exceed the particle sizes in the sols. Vacuum sublimation took place at pressures from 0.02 to 0.05 mm Hg without fragments of droplet moisture and particles sticking together ( Figure 4). To accelerate sublimation, heating was used. The temperature range of the heating surfaces in different parts of the vacuum chamber as it was heated during sublimation ranged from −80 to +25 °C ( Figure 5). Productivity of the unit with a power consumption from 3 to 5 kW is 0.15-0.20 L/h. The residual water content in nanopowders was adjusted to 0.2 wt.%

Research Methods
The spherical shape of nanopowder particles was confirmed by TEM images obtained with a transmission electron microscope JEM-100CX, JEOL, Hiroshima, Tokyo, Japan.
Pour density of uncompacted nanopowders were measured using a Scott PT-SV100 volume meter with a system of alternating inclined shelves for "transfusion" of samples in the volume for weighing along a S-shaped path, which ensured the uniform distribution of nanopowders. By the method of low-temperature nitrogen adsorption (ASAP-2010, Micromeritics Instrument Corporation, Norcross, GA, USA), adsorption-desorption curves were obtained. According to the adsorption-desorption curves for samples of nanopowders calculated the: -BET area; -specific surface areas, specific volumes, average diameters at one point and along the curves of adsorption and desorption of the area (BJH method); -differential and integral distribution of area and volume over pore diameters; -specific areas and volumes of micropores were found (with a diameter of less than 2 nm).
The amorphous structure of nanopowders was established by X-ray diffraction analysis (ARL X'TRA, Thermo Scientific, Switzerland).
Using small-angle X-ray scattering (SAXS), the dependences of the scattered radiation intensity function on the wave scattering vector were established (RigakuUltima IV, Rigaku Americas Corporation, Woodlands, TX, USA, rotating Cu anode, X-ray wavelength -1.54 Å).
To determine the concentration of impurity components in nanopowders, a S4 PIONEER X-ray fluorescence spectrometer (Bruker, GmbH, Germany) was used. Thermogravimetric analysis and estimation of mass losses during heating of the nanopowder were performed on a Pyris Diamond TG/DTA derivatograph (PerkinElmer LLC, Norwalk, CT, USA).

Research Methods
The spherical shape of nanopowder particles was confirmed by TEM images obtained with a transmission electron microscope JEM-100CX, JEOL, Hiroshima, Tokyo, Japan.
Pour density of uncompacted nanopowders were measured using a PT-SV100 Scott volumeter (Pharma Test Apparatebau AG, Germany) with a system of alternating inclined shelves for "transfusion" of samples in the volume for weighing along a S-shaped path, which ensured the uniform distribution of nanopowders.
By the method of low-temperature nitrogen adsorption (ASAP-2010, Micromeritics Instrument Corporation, Norcross, GA, USA), adsorption-desorption curves were obtained. According to the adsorption-desorption curves for samples of nanopowders calculated the: -BET area; -specific surface areas, specific volumes, average diameters at one point and along the curves of adsorption and desorption of the area (BJH method); -differential and integral distribution of area and volume over pore diameters; -specific areas and volumes of micropores were found (with a diameter of less than 2 nm). The amorphous structure of nanopowders was established by X-ray diffraction analysis (ARL X'TRA, Thermo Scientific, Basel, Switzerland).
Using small-angle X-ray scattering (SAXS), the dependences of the scattered radiation intensity function on the wave scattering vector were established (RigakuUltima IV, Rigaku Americas Corporation, Woodlands, TX, USA, rotating Cu anode, X-ray wavelength -1.54 Å).
To determine the concentration of impurity components in nanopowders, a S4 PIONEER X-ray fluorescence spectrometer (Bruker, GmbH, Bremen, Germany) was used. Thermogravimetric analysis and estimation of mass losses during heating of the nanopowder were performed on a Pyris Diamond TG/DTA derivatograph (PerkinElmer LLC, Norwalk, CT, USA).
For determining compressive strength of the samples of compacted nanopowders servohydravlic machine Shimadzu AGS-X (Shimadzu Corporation, Japan) was used.

SEM Images
According to scanning electron microscopy images with magnification factors equal to 250-7000, the sizes of the structures formed because of vacuum sublimation cryogranules of sols were within 20.0 to 100.0 µm. Figure 6 shows images of powder structures after sublimation of the solvent with a successively increasing coefficient of increase of 247, 500, 800, 1000, 2500, and 7000 times. After removal of the solvent, a porous-mesh structure of powder particles remains, preserving the features of a spherical shape and the size of solid cryogranule. Cavities formed inside the residual structures in their central part after solvent removal, which indicates the hardening mechanism of a sol drop of water removal from solid cryogranules. With light exposure, the residual structures were destroyed, forming flakes with a thickness of 0.1 to 0.2 µm.
According to scanning electron microscopy images with magnification factors equal to 250-7000, the sizes of the structures formed because of vacuum sublimation cryogranules of sols were within 20.0 to 100.0 μm. Figure 6 shows images of powder structures after sublimation of the solvent with a successively increasing coefficient of increase of 247, 500, 800, 1000, 2500, and 7000 times. After removal of the solvent, a porous-mesh structure of powder particles remains, preserving the features of a spherical shape and the size of solid cryogranule. Cavities formed inside the residual structures in their central part after solvent removal, which indicates the hardening mechanism of a sol drop of water removal from solid cryogranules. With light exposure, the residual structures were destroyed, forming flakes with a thickness of 0.1 to 0.2 μm.     Table 2 and Figure 8 show the dependence of the pour density of powders ρ p on the content of [SiO 2 ] in sols. When the content of [SiO 2 ] in sols was from 2.4 to 90 g/dm 3 , the density ρ p after sublimation of water molecules from cryogranules and replacing them with air molecules was higher than the content of SiO 2 in sols. Accordingly, after the sublimation of water molecules, the volume concentration of SiO 2 nanoparticles in the nanopowder increased compared to the sol, and the average distance between the particles decreased. Therefore, at [SiO 2 ] = 2.4 g/dm 3 , ρ p was 20.5 g/dm 3 . When the content of [SiO 2 ] in sols was higher than 90 g/dm 3 after sublimation of water molecules, the concentration of SiO 2 nanoparticles in the volume with air molecules decreased, and ρ p became lower than [SiO 2 ]: at [SiO 2 ] = 520 g/dm 3 , ρ p = 274 g/dm 3 . Accordingly, after sublimation of water molecules, the volume concentration of SiO 2 nanoparticles decreased, and the average distance between particles increased. The ρ p /[SiO 2 ] ratio decreased with increasing [SiO 2 ] content in sols from 8.5 to 0.53,  Table 2 and Figure 8 show the dependence of the pour density of powders ρp on the content of [SiO2] in sols. When the content of [SiO2] in sols was from 2.4 to 90 g/dm 3 , the density ρp after sublimation of water molecules from cryogranules and replacing them with air molecules was higher than the content of SiO2 in sols. Accordingly, after the sublimation of water molecules, the volume concentration of SiO2 nanoparticles in the nanopowder increased compared to the sol, and the average distance between the particles decreased. Therefore, at [SiO2] = 2.4 g/dm 3 , ρp was 20.5 g/dm 3 . When the content of [SiO2] in sols was higher than 90 g/dm 3 after sublimation of water molecules, the concentration of SiO2 nanoparticles in the volume with air molecules decreased, and ρp became lower than [SiO2]: at [SiO2] = 520 g/dm 3 , ρp = 274 g/dm 3 . Accordingly, after sublimation of water molecules, the volume concentration of SiO2 nanoparticles decreased, and the average distance between particles increased. The ρp/[SiO2] ratio decreased with increasing [SiO2] content in sols from 8.5 to 0.53, while in the range of [SiO2] = 100-520 g/dm 3 it was changing relatively little: from 0.75 to 0.53. In the range of [SiO2] contents in sols from 100 to 520 g/dm 3 , the ρp ([SiO2]) dependence was close to linear.   Table 3 shows the characteristics of the pores of powder samples established by low-temperature nitrogen adsorption. The characteristics of the samples are given in order of increasing values of their BET area S BET . Figures 9-11, for five of the samples in Table 3, in an ascending order of S BET , show graphs of nitrogen adsorption-desorption isotherms, differential and integral distributions of pore area and volume over diameters.    Table 3 shows the characteristics of the pores of powder samples established by low-temperature nitrogen adsorption. The characteristics of the samples are given in order of increasing values of their BET area SBET. Figures 9-11, for five of the samples in Table 3, in an    . Pore characteristics of the UF-3-8 sample obtained by the low-temperature nitrogen adsorption method: (a) adsorption-desorption curves (p/p 0 -relative nitrogen pressure, p 0 -nitrogen saturation pressure at a temperature of 77 K); (b) differential distribution of area over pore diameter; (c) integral distribution of the area along the pore diameter; (d) xifferential distribution of volume over pore diameter; (e) integral distribution of volume by pore diameter.

Pore Characteristics of Nanopowders Obtained by Cryochemical Vacuum Sublimation of SiO2 Sols
Nitrogen sorption-desorption isotherms are of type IV and have a hysteresis loop characteristic of mesopores with diameters from 2 to 50 nm and allow one to estimate the pore size distribution. Hysteresis on the isotherm graph allows us to conclude that nanopowders are a globular system consisting of spherical particles, each of which is in contact with two or more neighboring particles. By lowering the temperature of the hydrothermal solution at the OSA polycondensation stage from 90 to 20 • C, a decrease in the sizes of SiO 2 particles was achieved. Additionally, there was an increase in their specific surface area and a decrease in the average pore diameter; Figure 10. Pore characteristics of the UF-6-26 sample obtained by the low-temperature nitrogen adsorption method: (a) adsorption-desorption curves (p/p 0 -relative nitrogen pressure, p 0 -nitrogen saturation pressure at a temperature of 77 K); (b) differential distribution of area over pore diameter; (c) integral distribution of the area along the pore diameter; (d) differential distribution of volume over pore diameter; (e) integral distribution of volume by pore diameter.
With a temperature decrease at the OSA polycondensation to 20 • C, the BET-nanopowder area was regulated and increased to 500 m 2/ g. In this case, the specific pore volume V p was in a narrow range of 0.20 to 0.30 cm 3 /g and the average pore size decreased to 2.7 nm ( Table 3). The specific pore volume V p depended weakly on the density of nanopowders.
The specific pore volume V p = 0.20-0.30 cm 3 /g showed that spherical SiO 2 particles form aggregates with a high-volume fraction. The volume fraction of SiO 2 particles with a density of 2.2 g/cm 3 in aggregates at V p = 0.20-0.30 cm 3 /g was V s /V aggr = 0.7-0.6 (V s is the volume of SiO 2 particles occupied in the aggregate, V aggr is the aggregate volume), the density of the substance in aggregates was 1.32 to 1.54 g/cm 3 . The density of the substance in the aggregates was much higher than the density of nanopowders ρ p = 0.02-0.274 g/cm 3 . Figure 11. Pore characteristics of the UF-12-6 sample obtained by the low-temperature nitrogen adsorption method: (a) adsorption-desorption curves (p/p 0 -relative nitrogen pressure, p 0 -nitrogen saturation pressure at a temperature of 77 K); (b) differential distribution of area over pore diameter; (c) integral distribution of the area along the pore diameter; (d) differential distribution of volume over pore diameter; (e) integral distribution of volume by pore diameter.
The ratio of the average pore diameter d p to the average surface particle diameter d BET for most of the nanopowder samples ranged from 0.3 to 0.43, to 0.5 (Table 3), which also testified to the high-volume density of the packing of SiO 2 particles in the aggregates. The differential distributions of the pore area and volume over the diameters are rather narrow and are characterized by a relatively small width. The fraction of micropore area in the studied nanopowders is no more than 10% to 15%, and the proportion of micropore volume is not more than 1% to 3% ( Table 3).
The samples NM-200, 201, 204 of SiO 2 nanopowders were produced by precipitation from precursor Na 2 SiO 3 and the samples of pyrogenic SiO 2 nanopowders were produced by the flame hydrolysis of SiCl 4 [1]. Nitrogen sorption-desorption isotherms of precipitated samples NM-200, NM-201 and of pyrogenic SiO 2 nanopowder NM-202 were another type then of hydrothermal nanosilica powdes (Figures 9-12). Pore characteristics of pyrogenic and precipitated SiO 2 nanopowders established by BET-method are in Table 4. The form of the hysteresis loop of NM-200, 201, and 202 samples differs from the form of loop of hydrothermal samples, and the structure of SiO 2 particles aggregates and agglomerates differs in precipitated and pyrogenic samples from the hydrothermal samples. The specific pore volume V p = 0.499-0.513 cm 3  The specific pore volume Vp = 0.20-0.30 cm 3 /g showed that spherical SiO2 particles form aggregates with a high-volume fraction. The volume fraction of SiO2 particles with a density of 2.2 g/cm 3 in aggregates at Vp = 0.20-0.30 cm 3 /g was Vs/Vaggr = 0.7-0.6 (Vs is the volume of SiO2 particles occupied in the aggregate, Vaggr is the aggregate volume), the density of the substance in aggregates was 1.32 to 1.54 g/cm 3 . The density of the substance in the aggregates was much higher than the density of nanopowders ρp = 0.02-0.274 g/cm 3 .
The ratio of the average pore diameter dp to the average surface particle diameter dBET for most of the nanopowder samples ranged from 0.3 to 0.43, to 0.5 (Table 3), which also testified to the high-volume density of the packing of SiO2 particles in the aggregates. The differential distributions of the pore area and volume over the diameters are rather narrow and are characterized by a relatively small width. The fraction of micropore area in the studied nanopowders is no more than 10% to 15%, and the proportion of micropore volume is not more than 1% to 3% ( Table 3).
The samples NM-200, 201, 204 of SiO2 nanopowders were produced by precipitation from precursor Na2SiO3 and the samples of pyrogenic SiO2 nanopowders were produced by the flame hydrolysis of SiCl4 [1]. Nitrogen sorption-desorption isotherms of precipitated samples NM-200, NM-201 and of pyrogenic SiO2 nanopowder NM-202 were another type then of hydrothermal nanosilica powdes (Figures 9-12). Pore characteristics of pyrogenic and precipitated SiO2 nanopowders established by BET-method are in Table 4

The XRD Data and Small Angle X-ray Scattering
Samples of nanopowders had an amorphous structure without the presence of crystalline phases (Figure 13a). After calcination at 1200 • C for 2 h, cristobalite peaks appeared in the diffractogram of the samples (Figure 13b). In the X-Ray data of all samples NM-200, 201, 204 precipitated from Na 2 SiO 3 precursor the presence of Na 3 SO 4 crystalline impurities at 2Ө= 32, 34 degrees and crystalline impurities of Boehmite (γ-AlO(OH)) were observed [1]. In the pyrogenic samples NM-202 and 203, the presence of Boehmite was detected by XRD [1].

The XRD Data and Small Angle X-ray Scattering
Samples of nanopowders had an amorphous structure without the presence of crystalline phases (Figure 13a). After calcination at 1200 °C for 2 h, cristobalite peaks appeared in the diffractogram of the samples (Figure 13b). In the X-Ray data of all samples NM-200, 201, 204 precipitated from Na2SiO3 precursor the presence of Na3SO4 crystalline impurities at 2Ө = 32, 34 degrees and crystalline impurities of Boehmite (γ-AlO(OH)) were observed [1]. In the pyrogenic samples NM-202 and 203, the presence of Boehmite was detected by XRD [1].
(a) (b) Figure 13. The XRD data of the nanopowder: (a) before calcination; (b) after calcination; Ө is the angle between the plane of the sample and the direction of radiation incidence. ARL X'TRA device (CuKa radiation, wavelength: 1.54 Å).
Samples of SiO2 nanopowders isolated from sols were studied by small angle X-ray scattering (SAXS) (Figure 14). The dependences of the intensity of the scattered electromagnetic radiation ISR (q) on the wave vector q = 4π × sin (Ө)/λ (Ө is half the scattering angle and λ is the X-ray wavelength) were obtained for five different samples of SiO2 nanopowders in logarithmic coordinates. Sample 1: nanopowder obtained by cryochemical vacuum sublimation of the sol with a SiO2 content of 100 g/dm 3 (precursor: hydrothermal solution). Samples 2 and 3: for nanopowders obtained by sol-gel and cryochemical vacuum sublimation of gels, the precursor is an aqueous solution of sodium silicate, the SiO2 content in the sol is 100 g/dm 3 . Sample 4: for nanopowder obtained by sol-gel transition and cryochemical vacuum sublimation of the gel, the precursor is an hydrothermal solution, SiO2 content in the sol 100 g/dm 3 . Sample 5: for nanopowder obtained by sol-gel transition and gel drying, the precursor is tetraethoxysilane. Figure 13. The XRD data of the nanopowder: (a) before calcination; (b) after calcination; Өis the angle between the plane of the sample and the direction of radiation incidence. ARL X'TRA device (CuKa radiation, wavelength: 1.54 Å).
Samples of SiO 2 nanopowders isolated from sols were studied by small angle X-ray scattering (SAXS) (Figure 14). The dependences of the intensity of the scattered electromagnetic radiation I SR (q) on the wave vector q = 4π × sin (Ө)/λ (Ө is half the scattering angle and λ is the X-ray wavelength) were obtained for five different samples of SiO 2 nanopowders in logarithmic coordinates. Sample 1: nanopowder obtained by cryochemical vacuum sublimation of the sol with a SiO 2 content of 100 g/dm 3 (precursor: hydrothermal solution). Samples 2 and 3: for nanopowders obtained by sol-gel and cryochemical vacuum sublimation of gels, the precursor is an aqueous solution of sodium silicate, the SiO 2 content in the sol is 100 g/dm 3 . Sample 4: for nanopowder obtained by sol-gel transition and cryochemical vacuum sublimation of the gel, the precursor is an hydrothermal solution, SiO 2 content in the sol 100 g/dm 3 . Sample 5: for nanopowder obtained by sol-gel transition and gel drying, the precursor is tetraethoxysilane. According to Figure 14, only for sample 1 graph logISR(q) -log (q) in the range q = 0.21 to 0 nm −1 was close to linear, which indicates the mode of scattering by fractal agglomerates [51][52][53][54][55][56][57]: where Df is the fractal dimension. According to the slope of the dependence logISR(q)-log(q), the dimension Df for the nanopowder of sample 1 was 2.21. In the range q = 1.0 to 3.0 nm −1 for sample 1, the modulus of the slope of the logISR(q)-log(q) dependence was 4.05, in the region q = 0.08 to 0.2 nm −1 , it was 3.97, which corresponds to Porod's scattering regime. For sample 1, approximation of the dependence logISR(q) by the Guinier's function is ISR(q) = exp (-Rg 2 ×q 2 /3). Here, Rg is the gyration radius, for the ranges q = 1.0 to 3.0 nm −1 and q = 0.08 -to 0.2 nm −1 , respectively, where the primary particle size is 2Rg1 = 4.92 nm and the gyration radius of agglomerates is 2Rg2 = 24.4 nm. The relation between gyration radius Rg2 and outer diameter of agglomerates outer diameter Dagglom is Dagglom = ((Df +2)/Df) 0.5 ×2Rg2 = 33.7 nm. The number Nagglom of primary SiO2 nanoparticles with a diameter of 2Rg1, which are in the fractal agglomerate of size Dagglom, can estimated as [54,55]: According to Figure 14, only for sample 1 graph logI SR (q) -log (q) in the range q = 0.21 to 0 nm −1 was close to linear, which indicates the mode of scattering by fractal agglomerates [51][52][53][54][55][56][57]: where D f is the fractal dimension. According to the slope of the dependence logI SR (q)-log(q), the dimension D f for the nanopowder of sample 1 was 2.21. In the range q = 1.0 to 3.0 nm −1 for sample 1, the modulus of the slope of the logI SR (q)-log(q) dependence was 4.05, in the region q = 0.08 to 0.2 nm −1 , it was 3.97, which corresponds to Porod's scattering regime. For sample 1, approximation of the dependence logI SR (q) by the Guinier's function is I SR (q) = exp (-R g 2 ×q 2 /3). Here, R g is the gyration radius, for the ranges q = 1.0 to 3.0 nm −1 and q = 0.08 -to 0.2 nm −1 , respectively, where the primary particle size is 2R g1 = 4.92 nm and the gyration radius of agglomerates is 2R g2 = 24.4 nm. The relation between gyration radius R g2 and outer diameter of agglomerates outer diameter D agglom is D agglom = ((D f +2)/D f ) 0.5 ×2R g2 = 33.7 nm. The number N agglom of primary SiO 2 nanoparticles with a diameter of 2R g1 , which are in the fractal agglomerate of size D agglom , can estimated as [54,55]: The average volume fraction of SiO 2 primary particles in agglomerate is Fractal dimension of the hydrothermal nanopowders samples was lower than of the precipitated and pyrogenic samples [1,52,54,57,58]. Physicochemical and biophysicochemical interactions of nanoparticles with cells are in strong dependence from fractal dimension D f and parameters of the structure of agglomerates 2×R g1 , D agglom , N agglom , as from particles shape, surface electric charge and morphology [59][60][61][62][63][64]. Table 5 shows the concentrations of impurity components in the silica nanopowder obtained by cryochemical vacuum sublimation of the sol at a SiO 2 content of 500 g/dm 3 in the sol. The total content of impurities with respect to SiO 2 does not exceed 0.3 wt.%.  Table 6 shows the dependence of the mass of the nanopowder sample (wt.%) on temperature, according to thermogravimetric analysis. Taking into account the specific surface area of silica S BET (m 2 /g) and the mass loss ∆mH 2 O (wt.%) due to the removal of water and OH-groups during thermogravimetric analysis, one can find the total concentration δ OH (OH/nm 2 ) of all silanol groups. These groups are located both on the surface and in the volume of silica conventionally assigned to the specific surface of the nanopowder sample [65]:

Evaluation of the Density of Surface Silanol Groups of Si-OH
Having taken S BET = 300 m 2 /g for the sample, the final temperature at which all silanol groups are completely removed is equal to 1000 • C, and taking into account the data in Table 5, the values of the total δ OH (on the surface and inside the volume) were obtained. These values conventionally calculated per unit surface area of the sample for different temperatures (Table 7). Note. Symbol T, • C-temperature of sample pretreatment in vacuum. δ OH is the total water loss obtained by thermogravimetric analysis when the sample was calcined to high temperatures and expressed as the number of OH-groups, referred to the surface unit of SiO 2 . α OH is the averaged total true concentration of silanols on the SiO 2 surface depending on the pretreatment temperature obtained by Zhuravlev according to the method of deutero-exchange [66]. γ OH is the content of internal silanols per unit surface area of SiO 2 , obtained as the difference between the corresponding δ OH and α OH values at the same fixed temperature (this value is also formally expressed as the number of OH groups per unit surface area of SiO 2 (γ OH , OH/nm 2 )).

Experiments with Compacted SiO 2 Nanopowders
Samples of SiO 2 nanopowder were compacted on a hydraulic press at pressures of 1000 to 2000 MPa for 2 to 24 h; then, after hardening, they calcined at temperatures of 700, 800, 1000, and 1100 • C for 2 to 4 h. After compaction and calcination, the mechanical characteristics of solid samples were determined using the Shimadzu complex with registration of the force-strain curves (Figures 15 and 16, Table 8). Table 8 shows the values of compressive strength in the range 135-337 MPa. This indicates a high specific surface and high surface energy of SiO 2 nanoparticles. determined using the Shimadzu complex with registration of the force-strain curves (Figures 15 and  16, Table 8). Table 8 shows the values of compressive strength in the range 135-337 MPa. This indicates a high specific surface and high surface energy of SiO2 nanoparticles.
Sizes of the sample 5 (thickness × width × height), mm: 5.0 × 11.9 × 2.9; sample density 1.7 g/cm 3 ; indentation speed1 mm/min; maximum power 20057.6 N; maximum strain 337.1 N/mm 2 ; amplitude of the stroke, 1.681 mm; maximum elongation, 1.681 mm; maximum deformation 57.88%; maximum time 100.95 s.  true concentration of silanols on the SiO2 surface depending on the pretreatment temperature obtained by Zhuravlev according to the method of deutero-exchange [66]. γOH is the content of internal silanols per unit surface area of SiO2, obtained as the difference between the corresponding δOH and αOH values at the same fixed temperature (this value is also formally expressed as the number of OH groups per unit surface area of SiO2 (γOH, OH/nm 2 )).

Experiments with Compacted SiO2 Nanopowders.
Samples of SiO2 nanopowder were compacted on a hydraulic press at pressures of 1000 to 2000 MPa for 2 to 24 h; then, after hardening, they calcined at temperatures of 700, 800, 1000, and 1100 °C for 2 to 4 h. After compaction and calcination, the mechanical characteristics of solid samples were determined using the Shimadzu complex with registration of the force-strain curves (Figures 15 and  16, Table 8). Table 8 shows the values of compressive strength in the range 135-337 MPa. This indicates a high specific surface and high surface energy of SiO2 nanoparticles.

Prospects for Research and Applications of Hydrothermal Nanopowders SiO 2
Further studies of the characteristics and possible applications of hydrothermal SiO 2 nanopowders can be continued in the following areas: -production of silicates of metals [67][68][69]; -receiving glasses; -obtaining silicon carbide SiC; -formation of ceramic forms based on SiO 2 nanopowders; -obtaining heat insulators; -determination of the sorption capacity of nanopowders and obtaining sorbents for water purification and sorbents for gas chromatography; -studies of the possibility of using nanopowders as catalyst supports. Using SiO 2 nanoparticles, which have a high and chemically active surface, one can purposefully influence [68][69][70][71][72][73][74][75][76][77][78][79]: -the kinetics of hydration of the basic cement minerals C 3 S, C 2 S, C 3 A, C 4 AF and increasing the rate of CSH gel formation up to 20% and polymerisation [68,69,77,78]; -reducing the size and shape of the particles of the gel of the hydrates of calcium silicate C-S-H, increasing the density of their volume packaging; -reducing content of Ca(OH) 2 up to 20% to 30% and, thus, increasing content of CSH gel in hardened concrete because of rapid kinetics of pozzolanic reaction of SiO 2 nanoparticles with Ca(OH) 2 [74,75]; hydrothermal SiO 2 nanoparticles with great specific surface area up to 500 m 2 /g and high chemical activity due the surface density of Si-OH groups up to 4.9 nm −2 , which significantly accelerates the kinetics of pozzolanic reaction [68,77,78]; -increase the volume fraction of C-S-H gel phases with greater elasticity and hardness, Ca/Si relation due to modification of nanostructure of hardened concrete, and, as a result, increase the compressive and bending strength of concrete, reduce pore volume, increase water resistance, frost resistance, chemical resistance, and, as a result, the durability of concrete.
Nanosilica obtained based on a hydrothermal solution is applicable as an effective nanomodifier of concrete and is used [77][78][79]: (1) to accelerate hardening; (2) increasing the compressive strength of concrete at the early age up to 120% and about 40% in the age of 28 days; increasing of the concrete's compressive strength with additive of hydrothermal nanosilica was 10% higher than with additive of colloid nanosilica based on Na 2 SiO 3 precursor [72]; (3) reduction of Portland cement consumption up to 30%. A sufficiently developed application of hydrothermal SiO 2 nanoparticles is the intensification of photosynthesis in chloroplasts of plant cells due to the photoluminescent radiation of SiO 2 nanoparticles. SiO 2 nanoparticles due their optical properties can absorb solar radiation in ultraviolet region with a wave-length of 200 to 360 nm and emit of luminescent radiation in visible region with a wave length of 400 to 500 nm, in which the efficiency to absorb radiation by photosynthetic pigments and carotenoids is high [80][81][82]. An increase in the proportion of photosynthetic pigments of chlorophylls a (62%) and b (79.3%) [82][83][84], as a result, an increase in the growth rate, biochemical and biometric indicators at all stages of plant growth and development, a significant increase crop yields of agricultural plants from 9% to 60% [82][83][84][85], increase of contents of carotenoids-14.5%, B 2 -130%, B 5 -60%, B 6 -230%, B 9 -230% and C-14.4% vitamins [82][83][84] and rising biological activity of raw plant's mass with respect to cultures of Daphnia magna-352% and Paramecium caudatum-90.5% [82][83][84][85][86]. Hydrothermal SiO 2 nanoparticles have great inhibition ability on microflora (Leveilluia taurica, Ocidiopsis sicula) [87].

1.
A technological route proposed that allows one to obtain amorphous SiO 2 nanopowders based on a hydrothermal solution. The scheme includes the OSA polycondensation processes, ultrafiltration membrane separation, and cryochemical vacuum sublimation. The route allows to regulate parameters of the structure of the powder: the pour density, the diameters of the particle, specific surface area, diameters, pore area and volume, volume fraction of spherical particles in aggregates and agglomerates, sizes of agglomerates and number of particles in agglomerates, and fractal dimension. The parameters of the structure of hydrothermal nanosilica powders (ρ p , d BET , S BET , V p , V s /V aggr , 2×R g1 , D agglom , N agglom , D f ) differs from precipitated and pyrogenic samples. The structure parameters determine physical and chemical activity and applications of nanopowders. The interactions between SiO 2 nanoparticles, surface properties, parameters of double electric layer and stability of SiO 2 nanoparticles differs in hydrothermal sols and nanopowders from interactions in sols produced from Na 2 SiO 3 solutions or in precipited and pirogenic SiO 2 nanopowders. The difference in interactions of SiO 2 nanoparticles arised from the ion concentrations, ionic strength of hydrothermal solution and kinetics of OSA's polycondencation. The difference in nanoparticles interactions leads to the difference in structure parameters of nanopowders. The structure parameters determines physical and chemical activity of SiO 2 nanopowders and it's applications.