Hydrothermal SiO₂ Nanopowders: Obtaining Them and Their Characteristics

Abstract

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.

1. 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 SiO₂ nanopowders—particles which have a high specific surface area up to 1000 m²/g and significant chemical activity—is increasing. Cheap sources of such materials and low-cost technologies for their production are needed.

Traditional applications of SiO₂ 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 SiO₂ nanopowders, the flame hydrolysis of SiCl₄ in an atmosphere (H₂-O₂) is used [1]. The flame temperature, flow rate and volumetric proportions of SiCl₄, H₂, O₂ 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 SiO₂ particles from the liquid phase using a sol-gel transition. This group includes the preparation of SiO₂ 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 SiO₂ are used at the first stage of the process. The precursors of SiO₂ 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 SiO₂ particles [21,22,23,24,25,26,27,28]. SiO₂ 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]. SiO₂ 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, SiO₂-Me_xO_ynanocomposites [32,33,34,35,36,37,38,39,40,41,42,43,44,45]. The possibility of obtaining colloidal SiO₂ based on cheap waste of glass powder was shown in [46]. The production of SiO₂ powders from rice husk has been developed as well [47].

Hydrothermal solutions are a new raw material source for the production of SiO₂ nanopowders. For its development, it is necessary to develop a technology for producing SiO₂ nanopowders taking into account the parameters of the hydrothermal medium: temperature, pH, mineralization, ionic strength, polycondensation kinetics of orthosilicic acid, sizes and concentration of SiO₂ particles, and stability of SiO₂ nanoparticles in an aqueous medium.

The objectives of this article were:

- Create a technological route for the production of SiO₂ nanopowders based on a hydrothermal solution with specific surface area up to 500 m²/g using the methods of ultrafiltration membrane separation and cryochemical vacuum sublimation.

- Create regulation parameters of the structure of the nanopowders: the diameters of SiO₂ 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.

2. Materials and Methods

2.1. 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(AlSi₃O₈), albite Na(AlSi₃O₈), anorthite Ca(Al₂Si₂O₈, 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. The concentration of the main components of the initial hydrothermal solution.

Component	Na+	K+	Li+	Ca2+	Mg2+	Fe2+, 3+	Al3+	Cl–	SO42	HCO3–	CO32–	H3BO3	SiO2 total
Concentration, mg/dm3	282	48.1	1.5	2.8	4.7	< 0.1	< 0.1	251.8	220.9	45.2	61.8	91.8	780

Ionic strength of the solution I_s = 14.218 mmol/kg, electrical conductivity σ_el = 1.1–1.3 mS/cm, pH = 9.2.

. Silica is in solution in two states: solid (SiO₂ particles) and dissolved (OSA molecules).

At the first stage of the process, OSA polycondensation and the growth of SiO₂ 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]:

$$I_N=Q_{LP}\times Z\times R_{MD}\times A_{cr}\times N_A\times M_{Si}^{-1}\times e^{-\Delta F_{cr}/k_B\times T},$$

(1)

where Q_LP = 3.34 × 10²⁵ kg⁻¹—the Lohse-Pound factor; k_B—the Boltzmann constant; M_Si—the molar mass of SiO₂; N_A—the Avogadro number; T—the absolute temperature, K; A_cr = 4⋅π × R_c²—critical nucleus surface area, m²; ΔF_cr = σ_s × A_cr/3 = (16 × π/3) × σ_sw³(M_SI/ρ × N_A × k_B×T × lnS_m)²—change in free energy associated with the formation of a nucleus of critical radius R_c; ρ—density of amorphous silica, kg/m³; σ_sw—surface tension at the silica-water interface, J/m²; Z—Zeldovich factor.

$$Z=\sqrt{-\frac{{\partial }^2\Delta F_{cr}/\partial n_{cr}^2}{2\times \pi \times k_B\times T}},$$

(2)

where n_cr = (4 × π/3) × (ρ × N_A/M_Si) × R_c³—number of SiO₂ molecules in the nucleus of critical size; R_c = 2 × σ_sw × M_Si/(ρ × N_A × k_B × T × lnS_m)—critical radius; Z = (2/3) × (3/(4π × ρ × n_cr²))^1/3 × (σ_sw/k_B × T)^0.5; R_MD—the rate of molecular deposition of silicic acid (g⋅cm²⋅min⁻¹), which determines the particle growth rate:

R_MD = F(pH, pH_nom) × k_OH(T) × f_f(S_a) × (1 − S_N⁻¹),

(3)

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 SiO₂ 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 SiO₂ nanoparticles, concentrated aqueous sols were obtained by three-stage ultrafiltration membrane concentration. At the first stage, the SiO₂ 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 m², 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) m³/m²·h·MPa. The final SiO₂ content in sols was brought to 100.0–600.0 g/dm³ = 10–40 wt.%, salinity TDS = 800–2000 mg/dm³, 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 m_s = [SiO₂]/TDS continuously increases with increasing SiO₂ 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 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/dm³, pH = 9.0, the average diameter of SiO₂ 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).

SiO₂ 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 Figure 2 and Figure 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.%

2.2. 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.

3. Results

3.1. 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.

Figure 7 shows the images of nanopowder particles obtained by transmission electron microscopy (TEM) (the content of [SiO₂] in the sol before sublimation is 100 g/dm³).

3.2. Pour Density of SiO₂ Nanopowders

Table 2. Pour density of nanopowders, ρ_p, depending on the SiO₂ content in the sol.

[SiO2],g/dm3	2.4	5.2	6.93	10.4	17.56	32	65.85	120	131.7	160	260	520
ρp, g/dm3	20.5	29	35	43.8	55	55	84	91	100	117	168	274
ρp/[SiO2]	8.5	5.6	5.05	4.21	3.13	1.71	1.27	0.76	0.76	0.73	0.65	0.53

and Figure 8 show the dependence of the pour density of powders ρ_p on the content of [SiO₂] in sols. When the content of [SiO₂] in sols was from 2.4 to 90 g/dm³, the density ρ_p 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/dm³, ρ_p was 20.5 g/dm³. When the content of [SiO₂] in sols was higher than 90 g/dm³ after sublimation of water molecules, the concentration of SiO₂ nanoparticles in the volume with air molecules decreased, and ρ_p became lower than [SiO₂]: at [SiO₂] = 520 g/dm³, ρ_p = 274 g/dm³. Accordingly, after sublimation of water molecules, the volume concentration of SiO₂ nanoparticles decreased, and the average distance between particles increased. The ρ_p/[SiO₂] ratio decreased with increasing [SiO₂] content in sols from 8.5 to 0.53, while in the range of [SiO₂] = 100–520 g/dm³ it was changing relatively little: from 0.75 to 0.53. In the range of [SiO₂] contents in sols from 100 to 520 g/dm³, the ρ_p ([SiO₂]) dependence was close to linear.

3.3. Pore Characteristics of Nanopowders Obtained by Cryochemical Vacuum Sublimation of SiO₂ Sols

Table 3. Pore characteristics of powders established by low-temperature nitrogen adsorption.

Sample ID	[SiO2], g/dm3	ρp, g/dm3	SBET, m2/g	Pore area by adsorption curve (BJH), SBET,,m2/g	Pore area by desorption curve (BJH), m2/g	Single point pore volume, vp,cm3/g	Pore volume by adsorption curve (BJH), cm3/g	Pore volume by desorption curve (BJH), cm3/g	dBET,nm	Average pore diameter, dp, nm	Average pore diameter by adsorption curve, nm	Average pore diameter by desorption curve, nm	Area of micropores, m2/g	Volume of micropores, cm3/g
UF-1-9	128.0	86	45.4	35.6	37.7	0.10	0.23	0.237	60	9.4	26.4	25.1	2.18	d.n.
UF-2-32	233.8	229	56.8	47.0	51.8	0.15	0.19	0.19	48.0	10.9	16.6	15.2	5.6	0.001
UF-3-8	24.4	52	62.0	48.3	58.0	0.19	0.24	0.25	44.0	12.6	20.5	17.3	11.4	0.004
UF-4-34	586.9	344	74.0	63.9	69.7	0.18	0.19	0.20	36.8	10.0	12.4	11.6	5.0	0.001
UF-5-25	108.9	52	97.7	78.8	90.4	0.22	0.26	0.27	27.9	9.4	13.5	11.9	13.9	0.005
UF-6-26	114.5	90	120.4	111.4	121.2	0.21	0.22	0.23	22.6	7.0	8.2	7.6	8.9	0.002
UF-7-17	28.0	35	166.5	151.4	162.1	0.25	0.28	0.28	16.4	6.2	7.5	7.1	8.2	0.001
UF-8-21	14.0	15.7	200.8	158.1	166.6	0.20	0.22	0.23	13.6	4.0	5.8	5.5	10.8	0.001
UF-9-43	170.9	231.7	209.9	199.6	239.1	0.21	0.20	0.22	13.0	4.0	4.0	4.0	0.1	d.n.
UF-10-3	82.5	90.0	316.0	272.1	289.9	0.243	0.216	0.221	8.6	3.0	3.2	3.0	d.n.	d.n.
UF-11-20	33.2	58	360.4	256.9	280.8	0.301	0.280	0.290	7.56	3.3	4.2	4.1	33.8	0.010
UF-12-16	66.0	86	476.3	354.3	367.1	0.32	0.26	0.27	5.72	2.70	3.0	2.94	0.1	d.n.

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. Figure 9, Figure 10 and Figure 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.

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 SiO₂ particles was achieved. Additionally, there was an increase in their specific surface area and a decrease in the average pore diameter;

d_p = 4 × V_p/S_BET

(4)

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 cm³/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 cm³/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/cm³ in aggregates at V_p = 0.20–0.30 cm³/g was V_s/V_aggr = 0.7–0.6 (V_s is the volume of SiO₂ 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/cm³. The density of the substance in the aggregates was much higher than the density of nanopowders ρ_p = 0.02–0.274 g/cm³.

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 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 (Figure 9, Figure 10, Figure 11 and Figure 12). Pore characteristics of pyrogenic and precipitated SiO₂ nanopowders established by BET-method are in

Table 4. Pore characteristics of pyrogenic and precipitated SiO₂ nanopowders [1] established by the BET(Brunauer–Emmett–Teller)-method.

Sample ID	ρp, g/dm3	SBET, m2/g	Pore Volume, cm3/g	Area of Micropores, m2/g	Volume of Micropores, cm3/g
NM-200	120.0	189.1	0.79	30.0	0.01181
NM-201	280.0	140.4	0.581	23.1	0.00916
NM-202	130.0	204.1	0.513	8.26	0.00084
NM-203	30.0	203.9	0.499	5.3	0.0
NM-204	160.0	136.6	0.50	17.48	0.00666

. 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 SiO₂ particles aggregates and agglomerates differs in precipitated and pyrogenic samples from the hydrothermal samples.

The specific pore volume V_p = 0.499–0.513 cm³/g of pyrogenic SiO₂ nanopowder NM-202, 203 showed that the volume fraction V_s/V_aggr of SiO₂ particles in aggregates was about 0.5. In the samples of precipitated nanopowders NM-200, 201, 204 with V_p = 0.79, 0.581, 0.50 volume fraction was V_s/V_aggr = 0.364, 0.438, 0.475. The volume fraction V_s/V_agg in the samples NM-(200-204) was lower than in the UF samples of hydrothermal nanopowders and indicated another structure of aggregates. The fraction of the area of the micropore and volume were the same as in UF samples.

3.4. 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 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].

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 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 SiO₂ nanopowders in logarithmic coordinates. Sample 1: nanopowder obtained by cryochemical vacuum sublimation of the sol with a SiO₂ content of 100 g/dm³ (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/dm³. 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/dm³. 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 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]:

I_{SR ~} q^-Df,

(5)

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_g²×q²/3). 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 SiO₂ nanoparticles with a diameter of 2R_g1, which are in the fractal agglomerate of size D_agglom, can estimated as [54,55]:

N_agglom = (D_agglom/2×R_g1)^D_f = 69.3 ~ 69-70.

(6)

The average volume fraction of SiO₂ primary particles in agglomerate is (D_agglom/2 × R_g1)^D_f ⁻³ = 0.218.

For sample 4, the scattering mode on I_SR ~ q^-D_f fractals was realized in the range q = 0.2 to 0.5 nm⁻¹. The fractal dimension, determined by the slope of the dependence logI_SR(q)– log(q), is D_f = 2.33.

Nanopowders obtained by cryochemical vacuum sublimation of sols based on a hydrothermal solution were characterized by a fractal dimension in the range D_f = 2.04 to 2.21. Sodium silicate and tetraethoxysilane nanopowders were characterized by D_f = 2.2 to 2.3.

Sample NM-202 of pyrogenic SiO₂ powder obtained by the flame hydrolysis of SiCl₄ was characterized by SAXS, TEM, DLS and BET methods in [1]: D_f = 2.5, 2×R_g1 = 16 nm, 2×R_g2 = 100 nm, N_agglom = 200. Two samples, NM-200 and NM-201, were produced by precipitation from the precursor solution of Na₂SiO₃ were characterized by parameters: NM-200—D_f = 2.45, 2×R_g1 = 18 nm, 2×R_g2 = 440 nm, N_agglom = 3500; M-201—D_f = 2.45, 2×R_g1 = 20 nm, 2×R_g2 = 80 nm, N_agglom = 457. 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].

3.5. The Limits of the Content of Impurity Components in Nanopowders

Table 5. The concentration of the chemical components of silica nanopowder (X-ray fluorescence spectrometer “S4 PIONEER”).

Oxides	Concentration, wt.%
SiO2	99.7
TiO2	0.00
Al2O3	0.173
FeO	0.00
Cr2O3	0.00
MgO	0.00
CaO	0.034
Na2O	0.034
K2O	0.069
MnO	0.00
NiO	0.00
ZnO	0.00
Total	100.0

shows the concentrations of impurity components in the silica nanopowder obtained by cryochemical vacuum sublimation of the sol at a SiO₂ content of 500 g/dm³ in the sol. The total content of impurities with respect to SiO₂ does not exceed 0.3 wt.%.

3.6. Evaluation of the Density of Surface Silanol Groups of Si-OH

Table 6. Dependence of the mass of the nanopowder sample (wt.%) on temperature.

22.6 °C	100 °C	200 °C	300 °C	400 °C	500 °C	600 °C	700 °C	800 °C	900 °C	1000 °C	1100 °C
100%	94.65%	92.81%	92.10%	91.30%	90.58%	90.09%	89.76%	89.49%	89.27%	89.09%	88.61%

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 (m²/g) and the mass loss ΔmH₂O (wt.%) due to the removal of water and OH-groups during thermogravimetric analysis, one can find the total concentration δ_OH(OH/nm²) 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]:

δ_OH = (Δm_H2O 2 × 6.02 × 10³)/(18 × S_BET).

(7)

Having taken S_BET = 300 m²/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. Distribution of OH-groups between surface and volume for hydrothermal silica sample.

T, °C	200	300	400	500	600	700	800	900
δOH, OH/nm2	8.29	6.71	4.92	3.33	2.23	1.49	0.89	0.40
αOH, OH/nm2	4.90	3.56	2.33	1.84	1.52	1.30	0.70	0.40
γOH, OH/nm2	3.39	3.15	2.59	1.49	0.71	0.19	0.19	0.0

).

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 SiO₂. α_OH is the averaged total true concentration of silanols on the SiO₂ 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 SiO₂, 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 SiO₂ (γ_OH, OH/nm²)).

3.7. 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 (Figure 15 and Figure 16,

Table 8. Characteristics of compacted SiO₂ nanopowder samples during compressive strength tests.

Sample ID	Speed, mm/min	Shape	Dimensions (Thickness × Width × Height), mm	Maximum Force, N	Maximum Strain, N/mm2	Amplitude of the Stroke, mm	Maximum Elongation, %	Maximum Elongation, mm	Maximum Time, s
1	1	plane	4.9 × 11.9 × 3.3	11735.4	201.259	0.42831	12.9792	0.42831	25.7
2	1	plane	4.9 × 11.9 × 3.6	17145.4	294.040	0.65710	18.2529	0.65710	39.46
3	1	plane	5.5 × 13.5 × 3.1	10032.2	135.114	1.80619	58.2641	1.80619	108.370
4	1	plane	5.1 × 12.0 × 3.5	18897.6	308.784	1.81967	51.9905	1.81967	109.170
5	1	plane	5.0 × 11.9 × 2.9	20057.6	337.102	1.68196	57.9986	1.68196	100.950

). 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/cm³; indentation speed1 mm/min; maximum power 20057.6 N; maximum strain 337.1 N/mm²; amplitude of the stroke, 1.681 mm; maximum elongation, 1.681 mm; maximum deformation 57.88%; maximum time 100.95 s.

4. Prospects for Research and Applications of Hydrothermal Nanopowders SiO₂

Further studies of the characteristics and possible applications of hydrothermal SiO₂ 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 SiO₂ 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 SiO₂ 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 C₃S, C₂S, C₃A, C₄AF 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 SiO₂ nanoparticles with Ca(OH)₂ [74,75]; hydrothermal SiO₂ nanoparticles with great specific surface area up to 500 m²/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 Na₂SiO₃ precursor [72];

(3) reduction of Portland cement consumption up to 30%.

A sufficiently developed application of hydrothermal SiO₂ nanoparticles is the intensification of photosynthesis in chloroplasts of plant cells due to the photoluminescent radiation of SiO₂ nanoparticles. SiO₂ 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%, B₂—130%, B₅—60%, B₆—230%, B₉—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 SiO₂ nanoparticles have great inhibition ability on microflora (Leveilluia taurica, Ocidiopsis sicula) [87].

Hydrothermal nanosilica used as a feed additive that increases the productivity of farm animals (8% to 10%), rate of mass growth (10% to 40%), strength of the bone (17%), blood characteristics (Ca/P relation) and immunity (25% rising of the proportion of big forms of lymphocytes) [88,89,90,91].

Non-toxic [86,87,88,89,90,91] hydrothermal SiO₂ nanopowders can be the basis for the creation of medical preparations:

- enterosorbents,

- drugs that improve the structure of bone, strength and plasticity of the articular-bone tissue and reduce Ca leaking.

5. Conclusions

1. A technological route proposed that allows one to obtain amorphous SiO₂ 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 SiO₂ nanoparticles, surface properties, parameters of double electric layer and stability of SiO₂ nanoparticles differs in hydrothermal sols and nanopowders from interactions in sols produced from Na₂SiO₃ solutions or in precipited and pirogenic SiO₂ nanopowders. The difference in interactions of SiO₂ 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 SiO2 nanopowders and it’s applications.

The interactions between SiO₂ nanoparticles, surface properties, parameters of double electric layer and stability of SiO₂ nanoparticles differs in hydrothermal sols and nanopowders from interactions in sols produced from Na₂SiO₃ solutions or in precipited and pirogenic SiO₂ nanopowders. The difference in interactions of SiO₂ nanoparticles arised from the ion concentractions, 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 SiO₂ nanopowders and it’s applications.

2. The values of the average particle diameter of SiO₂ in sols, according to the data of dynamic light scattering, ranged from 5 to 100 nm. The average particle diameter of SiO₂ in powders, according to tunnel electron microscopy and the BET method, was in the same range of 5 to 100 nm.

3. The pour density of nanopowders ρ_p depended on the content of [SiO₂] in the sol and, therefore, on the concentration of particles and the average distance between them. When the content of [SiO₂] in sols ranged from 2.4 to 90 g/dm³, the pour density was higher than [SiO₂], respectively SiO₂ particles came together after sublimation of water molecules. At a content of [SiO₂] above 90 g/dm³, the pour density was lower than [SiO₂] and the average distance between SiO₂ particles, respectively, increased. In the range of [SiO₂] contents in sols from 100 to 520 g/dm³, the ρ_p ([SiO₂]) dependence was close to linear.

4. By lowering the temperature of the hydrothermal solution at the OSA polycondensation stage from 90 to 20 °C, we achieved a decrease in the size of SiO₂ particles and, accordingly, an increase in their specific surface to 500 m^2/g and a decrease in pore diameter from 15 to 2.7 nm. The specific pore volume was in the range of 0.20 to 0.30 cm³/g and varied little depending on the specific surface area and density of nanopowders. Spherical particles of SiO₂ in nanopowders form aggregates with a high-volume fraction of 0.7 to 0.6. The density of matter in aggregates, 1.32–1.54 g/cm³, was significantly higher than the density of nanopowders, which was 0.02–0.274 g/cm³. The ratio of the average pore diameter d_p to the average surface particle diameter d_BET from 0.3–0.43 to 0.5 also indicated a high-volume fraction of the packing of SiO₂ particles in the aggregates. According to the SAXS data, aggregates of SiO₂ nanoparticles form agglomerates with a fractal dimension of 2.04–2.21.

5. The content of impurity components in nanopowders can be brought up to 0.3 wt.% due to ultrafiltration membrane separation of SiO₂ nanoparticles and ions of dissolved salts, an increase in the SiO₂ content in the sol, and an increase in the ratio m_s = [SiO₂]/TDS to 300 and higher.

6. Tests of compacted SiO₂ nanopowders showed values of compressive strength in the range 135–337 MPa. This indicates a high specific surface area and high surface energy of SiO₂ nanoparticles.

7. Nanopowders obtained by the proposed technology have prospects for the use in the production of glass, silicon carbide, ceramics, concrete nanomodifiers, sorbents, plant growth stimulants, feed additives for agricultural animals, and medicines.