WO2022124948A1

WO2022124948A1 - Method of silicon recycling: synthesis of silica nanoparticles in an aqueous solution

Info

Publication number: WO2022124948A1
Application number: PCT/RU2021/050430
Authority: WO
Inventors: Julia Vladimirovna BONDAREVA; Timur Flyurovich ASLYAMOV; Iskander Shaukatovich AKHATOV; Stanislav Aleksandrovich EVLASHIN
Original assignee: Autonomous Non-Profit Organization For Higher Education “Skolkovo Institute Of Science And Technology”
Priority date: 2020-12-11
Filing date: 2021-12-10
Publication date: 2022-06-16
Also published as: RU2758782C1

Abstract

The invention relates to a method of top-down one- stage synthesis of silicon dioxide nanoparticles from bulk silicon. Bulk silicon is HR-, N-, P-silicon wafers. HR-, N-, P-silicon wafers are converted into monodisperse silicon dioxide nanoparticles at pH≥5 of an aqueous solution by hydrothermal synthesis at the temperature range of 297.15-453.15 K for 2-96 hours with the ability to control the average size of silicon dioxide nanoparticles.

Description

METHOD OF SILICON RECYCLING: SYNTHESIS OF SILICA NANOPARTICLES IN AN AQUEOUS SOLUTION

The field of invention.

The invention relates to the field of nanoparticle production, namely, nanoparticles of silicon dioxide from the source of bulk silicon.

State of the art.

In general, all approaches of nanoparticle synthesis can be divided into two groups - bottom-up and top-down. The methods of "growing" nanoparticles from individual atoms or molecules - condensation methods (bottom-up); methods of obtaining nanoparticles by grinding a conventional macrosample - dispersion methods (top-down).

As for nanoparticles of silicon dioxide (silica, SiOs), there are examples of their preparation using the Stober bottom-up approach or using an alternative bottom-up method for creating a microemulsion, for which tetraethylorthosilicate (tetraethyl silicate, ethyl silicate, tetraethoxysilane ((CsHsO^Si (TEOS), tetramethoxysilane) and inorganic salts (sodium silicate) serve as sources of silicon.

The multistage Stober method is based on the hydrolysis of silicon alkoxides in an aqueous- alcoholic solution in the presence of ammonium hydroxide as a catalyst. This method allows one to control the particle size. [Stober, W .; Fink, A .; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. Journal of Colloid and Interface Science 1968, 26, 62- 69.] Silicon alkoxides can be hydrolyzed in both alkaline and acidic media, resulting in polycondensation and the formation of siloxane bonds. As a result of this procedure, a colloidal solution of nanoparticles is formed. The nanoparticles obtained by the Stober method use an ammonium solution as a catalyst, which allows an optimal reaction rate, at which the synthesis time is shortened, to be obtained. The Stober method involves the condensation reaction of tetraethyl orthosilicate in a mixture of ethanol (ethyl alcohol) and water at room temperature under alkaline conditions.

The main disadvantages of the Stober method are the impossibility of creating SiOs nanoparticles with a narrow size distribution not exceeding 100 nm, the use of a large amount of ethanol, and a long synthesis time. Bogush and Zukoski reduced the average particle size to 40 nm, by changing the synthesis conditions: ammonium concentration, reaction temperature, and solvent choice [Bogush, G.H .; Tracy, M.A .; Zukoski, C.F., IV Preparation of Monodisperse Silica Particles: Control of Size and Mass Fraction. Journal of Non-Crystalline Solids 1988, 104, 95- 106.]. T. Yokoi and his team have developed a method for the synthesis of spherical particles with an average size of 12-23 nm by hydrolysis of TEOS in a solution of cyclohexane in the presence of amino acids [Yokoi, T .; Sakamoto, Y .; Terasaki, O .; Kubota, Y .; Okubo, T .; Tatsumi, T. Periodic Arrangement of Silica Nanospheres Assisted by Amino Acids. Journal of American Chemical Society 2006, 128, 13664-13665.]. The use of amino acids makes it possible to control the particle size and obtain particles of less than 50 nm in size with a high degree of monodispersity.

An alternative to the Stober’s is the microemulsion method. In this approach, surfactant molecules are dissolved in organic solvents and form spherical micelles [Wang, W .; Fu, X. an .; Tang, J. an .; Jiang, L. Preparation of Submicron Spherical Particles of Silica by the Water-in-Oil Microemulsion Method. Colloids and Surfaces A, 1993, 81 , 177-180. Chang, C.L .; Fogler, H.S. Controlled Formation of Silica Particles from Tetraethyl Orthosilicate in Nonionic Water-in-Oil Microemulsions. Langmuir 1997,13, 3295-3307], In aqueous solutions, polar groups form cavities, in which the synthesis of nanoparticles occurs at certain ratios of the silicon precursor to the catalyst. When using the reverse microemulsion method, water droplets in the oil play a role of a nanoreactor for the subsequent synthesis of silicon dioxide nanoparticles.

However, the inverse microemulsion method uses a large amount of surfactant, which can be difficult to remove from the system. In addition, the synthesis of silicon dioxide nanoparticles in large volumes is difficult due to the nature of the inverse microemulsion method. The main disadvantage of this approach is the high cost, complexity, and the lack of environmental friendliness of the process associated with the purification of the final product from surfactants.

Using hydrolysis in aqueous and aqueous ammonia solutions a number of improvements to these two basic methods have been patented in the following patents in China and Korea.

In three slightly differing from each other (with a single basic principle) inventions in the patents of China of one patent holder and one team of authors CN 107720760 (A), CN 107285325 (A), CN 108751208 (A), similar modifications of the TEOS are considered, the highlighted merits of which are environmental friendliness and simplicity. The method is easy to operate, cheap, and environmentally friendly. The prepared SiOs nanoparticles have a uniform spherical morphology and uniform particle size, as well as universal applicability and potential for large-scale production. The invention relates to methods of obtaining monodisperse nanoparticles of silicon dioxide, obtained using different types of microemulsions that do not contain surfactants. The formation of SiOs nanoparticles in a microemulsion system without surfactants has been systematically studied. A new type of surfactant-free microemulsion system is being formed. The principle of the invention is to first prepare a template of a surfactant-free microemulsion using water and other starting materials, and then add the reagent to the system, use ultrasound or stirring to completely dissolve the reagent in the oil phase of the surfactant-free microemulsion system.

The closest to the proposed in this patent invention is the method for synthesis of nanoparticles described in the invention CN 107720760 (A) - 2018-02-23 METHOD FOR PREPARATION OF SILICON DIOXIDE NANO-PARTICLES OF DIFFERENT SIZES BY REGULATING AND CONTROLLING AMOUNT OF ADDED AMMONIUM HYDROXIDE AND SILICATE ESTER, which noted the possibility of regulating the size of nanoparticles by regulating and controlling the added amount of ammonium hydroxide and silicic acid esters. The patent uses dichloromethane as the oil phase and ethanol as solvent to create a surfactant-free water/ethanol/dichloromethane microemulsion system. Different volumes of silicate (methyl orthosilicate, ethyl orthosilicate, or propyl orthosilicate) are completely dissolved in the spherical "oil core" of the prepared microemulsion without surfactants, under the action of ultrasonic or magnetic stirring, and then are catalyzed by different amounts of ammonia water. Conditions for hydrolysis and polycondensation of silicate: exposure for 1 -24 hours in a water bath at 25-35 ° C. After completion of the reaction, the white precipitate at the bottom is collected by separation in a centrifuge and washed several times with a polar solvent to obtain monodisperse SiOs nanoparticles. Ammonia is also slowly added to the microemulsion system with dissolved silicate therein, and after the addition is complete, stirring is continued for a certain period of time and then transferred to a water bath at 25 ° C for settling over 24 hours. Ethanol and methylene chloride contained in the liquid component after centrifugal separation of this invention can be recovered by distillation. The nanoparticle size is uniform and can be controlled over a wide range. The method has universal applicability and is valuable for large-scale production. In this case, the size of the resulting SiOs nanoparticles is 35-420 nm (much larger than in the newly proposed method).

In close to the previous invention under the patent CN 107285325 (A) - 2017-10-24 NOVEL GREEN SINGLE-DISPERSED SILICA NANOPARTICLE PREPARATION METHOD, the method also includes preparation of a water/ethanol/dichloromethane microemulsion that does not contain surfactants, complete dissolution of TEOS in the oil phase of the microemulsion prepared in the first stage, under the action of ultrasound and stirring, subjecting TEOS to hydrolytic polycondensation and centrifugal separation after the reaction; repeated washing of solid components with a polar solvent to obtain monodisperse SiOs nanoparticles and distillation of the remaining liquid components to collect ethanol and dichloromethane. Ethanol is used as a solvent and dichloromethane is used as an oil phase. These two solvents are not only cheap but also, due to their low boiling point, can be obtained by simple distillation after preparing SiOs nanoparticles. It can be recycled to meet the recycling goal. The conditions for hydrolysis and polycondensation are as follows: exposure for 1 -24 hours in a water bath at 25-35 ° C. The particle size of silica nanospheres is 275-785 nm.

In the invention under the patent CN 108751208 (A) - 2018-1 1 -06 [Monodisperse silicon dioxide nanosphere prepared by surfactant-free microemulsion and preparation method thereof], a surfactant-free microemulsion system prepared from ethyl acetate, isopropyl alcohols, and water is created; ethyl acetate as the oil phase in the system, and isopropyl alcohols are used as solvents. The method is characterized by obtaining nanospheres of silicon dioxide with a uniform and also controlled size by adjusting the amount of ethyl acetate content. The particle size of silicon dioxide is in the range of 200-246 nm while having a good morphology and clean surface.

It can be noted that in comparison to the newly proposed method for producing nanoparticles of silicon dioxide, in the descriptions of these Chinese inventions, the pH parameter of the aqueous solution, which is essential for the new method, is not mentioned.

Also, two Korean patent applications: KR 20140098625 (A) and KR 20080063007 (A), are important for state of art characterization.

In the invention KR 20140098625 (A) - 2014-08-08 PROCESS FOR PREPARING WATER DISPERSIBLE SILICA NANOPARTICLE (WO 20141 19913 (A1 ) - 2014-08-07) METHOD FOR MANUFACTURING SILICA NANOPARTICLES WITH EXCELLENT WATER DISPERSION PROPERTIES). WO 2014119913 (A1 ) uses a TEOS method comprising the step of reacting base materials and a precursor (precursor) of silicon dioxide to obtain a solution with polyvinylpyrrolidone (PVP) in a C1 -C5 alcohol and water mixture. The C1 -C5 alcohol is selected from the group consisting of methanol, ethanol, propanol, isopropanol, butanol, isobutanol, pentanol, and isopentanol. The base material is selected from the group consisting of ammonia, sodium hydroxide, and potassium hydroxide. The silica precursor is selected from the group consisting of tetraethylorthosilicate (TEOS), tetraethyl orthosilicate, tetraethylorthosilicate, tetrachlorosilane, and sodium silicate. Using a simple one-step synthesis process, the silicon dioxide nanoparticles of a size from 20 to 100 nm are obtained, are well dispersed in water, and remain water-dispersible without agglomeration for a long period of time without surface modification of the prepared silicon dioxide nanoparticles. In the invention KR 20080063007 (A) - 2008-07-03 A SILICA NANO PARTICLE AND METHOD FOR MANUFACTURING THEREOF the most popular Stober method is modified by proposing a new solution. A method for producing silicon dioxide nanoparticles is proposed, which makes it easy to obtain a large amount of silicon dioxide nanoparticles at room temperature by improving the existing Stober method with the addition of an anionic polymer. A method for production of silica nanoparticles includes a step (S10) of mixing and mixing the anionic polymer with distilled water or a buffer solution; a step (S20) of adding alcohol to the mixed solution to mix the alcohol with the mixed solution; a step (S30) of mixing ammonia and TEOS with a mixed solution with the addition of alcohol; and a step (S40) of reacting the mixed solution with ammonia and TEOS through a mixing process to form silica nanoparticles and purifying the silica nanoparticles with alcohol. The anionic polymer is a water-soluble anionic functional polymer-based on polyacrylic acid, polymeth acrylic acid, polytriopheneacetic acid, polysulfonate styrene, or a combination thereof. After adding the anionic polymer to distilled water or a pH-adjusted buffer solution (note: the specific pH value is not indicated in the invention, but there is a reference in the prior art to the US patent US 5597512, which describes the mixing process of an aqueous solution containing an active silicic acid solution, in which the pH is adjusted to 2-5) the anionic polymer is agitated and agitated under predetermined conditions. Stirring is preferably carried out at 500-1500 rpm for 10-60 minutes. After adding the anionic polymer to distilled water or pH adjusted buffer and stirring, alcohols such as ethanol or methanol are added to the stirred mixed solution and stirred (S20). Nanoparticles have sizes from 90 nm to 350 nm (note: in the new proposed invention, the size of the nanoparticles is even smaller). The reaction temperature during stirring is 40 ° C to 80 ° C (Note: in the new proposed invention, the temperature values may be slightly higher). After mixing ammonia and TEOS in a solution to which alcohols are added, the mixture is reacted by means of a stirring process to form silicon dioxide nanoparticles of various shapes. In this case, the mixing time is 10 hours. It is preferable to operate at 1500 ~ 2500 rpm for ~ 15 hours. The silica nanoparticles formed by washing can be sized and checked for uniformity using an Otsuka Electronics particle size analyzer and a scanning electron microscope (SEM). When silica nanoparticles are formed after mixing distilled water and anionic polymer at a suitable weight ratio, the particle size distribution is uniform regardless of the amount of anionic polymer.

A common feature of all the abovementioned inventions is that, in principle, all these methods of obtaining nanoparticles of silicon dioxide are bottom-up, and do not use bulk silicon (for example, silicon solar panels) as a source of silicon for the synthesis of SiOs nanoparticles.

Disclosure of invention

In contrast to all known methods, the prior art proposes a one-step method of top-down conversion of bulk silicon (as a source) into silicon dioxide nanoparticles. The choice of bulk silicon as a raw material for the creation of silicon oxide nanoparticles is explained by its availability and widespread use in the production of semiconductor materials and solar cells. Converting silicon to silicon oxide nanoparticles is an important environmental concern as it is a way to recycle silicon waste. The proposed synthesis method has great potential for the processing and disposal of silicon waste, which are transformed into nanoparticles suitable for reuse. The method is scalable and does not use toxic reagents. An efficient way of converting bulk silicon, for example, recycling bulk silicon waste, including degraded silicon wafers of solar cells, into silicon dioxide nanoparticles will give a second life to silicon waste in the fields of material processing, optics, photonics, pharmaceutical, and cosmetic applications, rubber and plastic products. In addition to the environmental and transformative potential of the proposed method, it is also easy to use, reliable, scalable, and allows one to control the size of nanoparticles of silicon dioxide. By controlling the temperature and time of hydrolysis, it is possible to change both the average size of nanoparticles and the distribution width.

Silica (silicon dioxide) nanoparticles are obtained by dissolving bulk silicon plates in aqueous solutions with a pH of at least 5 (hydrolysis and hydrothermal synthesis). Variations in the temperature (from 20 ° C to 300 ° C) and time (from 2 h to 96 h) of synthesis make it possible to control the average particle size in the range from 8 to 50 nm.

Thus, we propose a method of top-down one-stage synthesis of nanoparticles of silicon dioxide, which by the means of hydrolysis or hydrothermal synthesis at pH > 5 processes an aqueous solution of bulk silicon macroparticles into monodisperse nanoparticles of silicon dioxide and provides regulation of the average size of nanoparticles in the range from 8 to 50 nm at temperatures from 20 ° C to 300 ° C (from 300 to 453 K) and synthesis times from 2 h to 96 h (on average 10 h).

The method can use ammonium hydroxide (aqueous solution) or organic bases (CH₃NH₂, (C₂H₅)₃N, N₂H₄, (CH₃)₄NOH, etc.) as additional catalysts, and inorganic alkalis (LiOH, NaOH, KOH, etc.), inorganic salts (K₂CO₃, NaNO₂, NaCI, NaHCO₃, etc.) as solutions.

The method preferably uses a reactor and a heating element to control the temperature above 20 ⁰ C; an autoclave or a container or vessel made of metal, T eflon, or ceramics is used as a reactor.

List of figures.

FIG. 1 . Photo block diagram of the process of converting a bulk silicon wafer into silicon dioxide nanoparticles.

FIG. 2. Computer graphic block diagram of the processing of a bulk silicon wafer into silicon dioxide nanoparticles.

FIG. 3. Dependences of the reaction time (dissolution time of 0.3 g of bulk silicon) on the pH value in the absence or presence of various catalysts.

FIG. 4. Dependence of the average size of SiO₂ nanoparticles on: a) hydrolysis time (at T = 453.15 K); b) temperature (with ammonium concentration C ammonia = const = 1 %).

FIG. 5. SEM image of silicon dioxide SiO₂ nanoparticles obtained in a solution of distilled water for a week.

FIG. 6. SEM images of SiO₂ nanoparticles obtained at different process times.

FIG. 7. SEM images of SiO₂ nanoparticles obtained at different temperatures.

FIG. 8. SEM images of SiO₂ nanoparticles and histograms of particle size distribution: under conditions of 48 hours of synthesis and 453.15 K for (a), (c); 96 hours and 453.15 K for (b), (d). FIG. 9. TEM (transmission electron microscopy) images of silicon oxide nanoparticles SiOs synthesized at different temperatures: a) 353.15 K b) 453.15 K. Insets show scattered electron diffraction ring images. High-resolution SEM images are shown for temperatures: c) 353.15 K d) 453.15 K.

Implementation of the invention.

Figures 1 and 2 show a pictorial diagram and a computer graphic diagram of the process of conversion of a bulk silicon wafer into silica nanoparticles.

The positions in FIG. 1 ,2 are indicated in the sequence of technological stages of the method: 1 - etching (purification) of the original macroparticle 6 of bulk silicon, 2 - loading the purified macroparticle 7 into a sealed autoclave 8; 3 - hydrothermal synthesis of nanoparticles 9 of silica in an autoclave 8; 4 - unloading nanoparticles 9 of silicon dioxide from the autoclave 8; 5 - visualization of the obtained nanoparticles 9 to study their properties. Hydrothermal synthesis is carried out taking into account the pH values on a scale of 10 and a temperature of 11 .

The surface of silicon wafers (in the example in Fig. 1 , 3 cm in size) were pre-cleaned by etching, then placed in an autoclave in an aqueous solution or in an ammonium solution, are converted into nanoparticles of silicon dioxide at different temperatures and times of synthesis (hydrothermal synthesis or decomposition). The synthesized silica particles are spherical and have the same size order. Silicon dioxide nanoparticles can be obtained without the use of ammonium, in pure water. Silica nanoparticles were obtained by dissolving silicon wafers in aqueous solutions with a pH of at least 5 (Fig. 3 shows the dependences of the reaction time (dissolution time of 0.3 g of bulk silicon macroparticles) on the pH value in the absence or presence of various catalysts: position 12 - in pure water with pH = 5, 13 - in pure water with pH = 6, 14 - in raw tap water (tap water) with pH = 7; 15 - in an aqueous solution of NaCI with pH = 8; 16 - in an aqueous solution K2CO3 and / or NasNOs with pH = 9; 17 - in an aqueous solution of ammonium and / or hydrazine with pH = 10; 18 - in an aqueous solution of LiOH and / or KOH and / or CH3NH2 with pH = 1 1 .

The reaction temperature and time are the main parameters affecting the size and shape of the nanoparticle size distribution. The described method allows the synthesis of particles with a controlled average size in the range from 8 nm to 50 nm.

Since the resulting nanoparticles tend to form aggregates, an increase in the synthesis time leads to an increase in the average particle size and the manifestation of a bimodal form of particle size distribution.

Examples of implementation of the invention

HR-, N-, and P-silicon wafers were used as a source of bulk silicon.

The synthesis of silicon oxide nanoparticles was carried out in an aqueous solution without the addition of salts or bases. But since the process in distilled water is rather slow, bases, acting as catalysts, were also predominantly added to the system. Base as a catalyst: ammonium hydroxide (25% aqueous solution manufactured by SigmaTek), deionized water (>18 kQ cm-1 ) to create a reaction medium. An aqueous solution of ammonium was used to speed up the particle creation process. Silicon was immersed in a 57 ml ammonium solution with a concentration ranging from 0.53 M to 13.16 M. These solutions were transferred into sealed stainless steel autoclaves with a Teflon coating and kept at various temperatures in the range from 297.15 K to 453.15 K at different synthesis times from 2 to 96 hours at autogenous pressure. After thermal treatment, the autoclave was removed from the thermal chamber and cooled down at room temperature. The internal pressure of the autoclave also varied depending on the temperature and filling of the autoclave (57, 47, 37, and 27 ml).

The effect of variation of ammonium concentration did not have a significant effect on the process; for five different concentrations, the particle size remained at about 40 nm. A decrease in the amount of ammonium led to the formation of a narrower particle size distribution, no peaks in the range of 80-140 nm were observed, which indicates high monodispersity of the resulting nanoparticles.

A similar procedure was carried out for the synthesis of silicon dioxide nanoparticles in alternative organic and inorganic bases and salts, the concentration of alternative solvents varied from 0.1 M to 1 M. Organic bases (methylamine, triethylamine, hydrazine, tetramethylammonium hydroxide), inorganic alkalis (LiOH, NaOH, KOH), inorganic salts (K2CO3, NaNOs, NaCI, NaHCOs), acids (hydrochloric acid and hydrofluoric acid) were tested.

FIG. 3 presents information on the influence of pH of the solution of the alternative bases, which were used to obtain particles of silicon dioxide. It is possible to obtain nanoparticles of silica using various organic and inorganic bases with pH values greater than 9, as well as aqueous solutions of inorganic salts, which provide a weak alkaline reaction.

The morphology and size of silicon oxide particles were studied using a Carl Zeiss Supra 40 system scanning electron microscope (SEM) and a JEM 21 OOF (UHR / Cs) transmission electron microscope (TEM) with an accelerating voltage of 200 kV.

Particle size statistics were obtained after processing SEM images of particles in LabView software, which allows to determine statistics on particle size and distribution to be accumulated for a set of images. The number of particles used to generate statistics reaches 500. The shape of the distribution and the average particle size for one process were obtained for each set of experimental parameters of the process.

Fourier transform infrared spectra in the range 1500-400 cm^-1 were recorded using a Bruker Vertex 70V spectrophotometer. The spectra with an accuracy of 0.5 cm-1 were obtained as a result of the transmission of films deposited on silicon, which is transparent in the infrared range. The crystal structures were analyzed using a Huber G670 diffractometer with CoKa radiation (wavelength = 1.78892).

For the theoretical confirmation of the obtained physicochemical results, a computer simulation of the method was also carried out. The presence of surface Si-H bonds can be seen from the simulation results. A similar atomic configuration is popular for studies of the oxidation of silicon crystals, which were initially acid etched to form Si - H bonds on the silicon surface. It is assumed that the OH concentration is significant and determined by pH; the formation of OH bonds can remove Si monomers (SiHx (OH) y, where x + y = 4) from the surface. These silicon monomers initiate the self-assembly of silicon oxide into nanoparticles. Thus, the pH value significantly affects the rate of nanoparticle formation, which coincides with practical experimental data. Also, impurities of P (phosphorus), B (boron), and molecular defects on the silicon surface lead to an acceleration of the reaction rate.

The reaction time affects the particle size. The numerical dependence (graph (19)) of the average particle size on the reaction time is shown in Fig. 4a. The average particle size initially increases sharply and then becomes constant. Thus, the particle formation process exhibits a characteristic saturation time of the order of 24 hours. FIG. 4b shows a graph (20) of changes in the average sizes of nanoparticles obtained at saturation times (24 hours or more) at various temperatures from the range 323-453 K. Analysis of the particle size by SEM images (Fig. 5 - SEM image of silica nanoparticles SiOs in a solution of distilled water, for a week; Fig. 6 - SEM images of SiOs nanoparticles obtained over different times; Fig. 7 - SEM images of SiOs nanoparticles obtained at different temperatures) demonstrates the monotonic increase of the average particle size with increasing temperature ( Fig. 7 and Fig. 4b). These experimental results are consistent with Arrhenius's law.

In this case, the volume of loading/pressure inside the vessel of the autoclave has practically no effect on the average size of the formed particles. For three different autoclave feed values, the particle size remained at about 30 nm. Thus, the temperature and reaction time are the main parameters affecting the size of the formed particles. The selection of the reaction time and temperature makes it possible to obtain the smallest particles with a uniform monodisperse size distribution (Fig. 8 - SEM images of SiOs nanoparticles and a histogram (21 , 22), a monodisperse particle size distribution is sufficient: under conditions of 48 hours of synthesis and 453.15 K for (a), (c - histogram (21 )); 96 hours and 453.15 K for (b), (d - histogram (22)).

For a more complete visualization of the obtained particles, studies were carried out using TEM (transmission electron microscopy). The combination of SEM and TEM provides insight into the size and shape of individual particles, the morphology of particle aggregations, and the overall internal structure of particles. The synthesized silica particles are spherical and have the same size order. FIG. 9 shows TEM images of silica nanoparticles SiOs synthesized at different temperatures: a) 353.15 K b) 453.15 K. Insets show scattered electron diffraction ring images. High-resolution TEM images are also shown for temperatures: c) 353.15 K d) 453.15 K. From the TEM images it can be concluded that nanoparticles have a tendency to form aggregates.

Thus, a top-down method for the hydrothermal synthesis of silicon oxide nanoparticles has been developed and tested. The described approach makes it possible to convert silicon wafers placed in an autoclave with an aqueous solution into silica nanoparticles at different temperatures and times of synthesis. The analysis showed that the temperature and reaction time are the main parameters influencing the size and shape of the particle size distribution. The described method allows the synthesis of particles with controlled average sizes in the range from 8 nm to 50 nm. Silicon dioxide nanoparticles can be obtained in pure water, without using ammonium as a catalyst. The demonstrated synthesis method has great potential for recycling silicon waste, which is transformed into nanoparticles suitable for reuse. The method is scalable and does not use toxic reagents. In addition to the experimental importance of the method, it can serve as the basis for the formation of further research on the processing of bulk silicon.

Claims

The invention formula

1 . Method of top-down one-stage synthesis of silica nanoparticles from bulk silicon. HR-, N-, P- silicon wafers are converted into monodisperse silicon dioxide nanoparticles at pH>5 of an aqueous solution by hydrothermal synthesis at the temperature range of 297.15-453.15 K for 2- 96 hours with the ability to control the average size of silica nanoparticles.

2. The method according to claim 1 , uses ammonium hydroxide (aqueous solution) or organic bases (CH₃NH₂, (C₂H₅)3N, N₂H₄, (CHs^NOH, etc.) as catalysts, inorganic alkalis (LiOH, NaOH, KOH, etc. .), inorganic salts (K₂COs, NaNO₂, NaCI, NaHCOs, etc.) as solutions.

3. The method according to claim 1 or 2, uses a reactor and a heating element to control the temperature of hydrothermal synthesis of nanoparticles of silicon dioxide from bulk silicon above 20 ° C; an autoclave, or a container, or a vessel made of metal, Teflon, or ceramics is used as a reactor.

4. The method according to claim 1 or 2 provides the size of the synthesized nanoparticles of silica in the range from 8 nm to 50 nm.

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