Influence of Hydrothermal Treatment on Crystalline Form of SiO2 Synthesized by Sol-Gel Method

The hydrothermal method and traditional sol-gel process for the synthesis of SiO2 particles were comparatively studied. For the synthesis of SiO2, tetraethyl orthosilicate (tetraethoxysilane or TEOS) and ammonium chloride (NH4OH) (as a catalyst) were used. SiO2 was prepared by the TEOS hydrolysis reaction at room temperature and hydrothermal treatment. Morphology and structure of the final products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR) and scanning electron micrograph (SEM). It was found that SiO2 product synthesized by the TEOS hydrolysis reaction at room temperature has an amorphous phase. XRD analysis shows the presence of an amorphous halo - a strong peak at Bragg angle 21 °-22 °. The hydrothermal treatment leads to crystallization of the material.


Introduction
In recent years, silicon dioxide (SiO 2 ) has been widely used in various industries, such as mechanical engineering telecommunication, micro-electronics, and construction. Silicon dioxide has unique electrical magnetic and optical properties, thermo stability and low flammability [1][2][3][4]. Studies have shown that the use of silicon dioxide as filler in composite materials, even in small quantities, significantly improves their original properties [5][6][7]. SiO 2 exists in many crystalline forms, the most popular ones being quartz, cristobalite, tridymite, stishovite, and coesite, but its best-known form is amorphous silicon dioxide. The SiO 2 nanoparticles are commonly synthesized by means of various approaches, such as precipitation [8,9], sol-gel [10,11], and hydrothermal synthesis [12,13]. Other methods that can be used include a plasmachemical method [14], an electron beam method, and a mechanochemical method [15].
Generally, amorphous SiO 2 can be produced by mixing aqueous solutions of sodium metasilicate with acid. For example, Musić, Filipović-Vinceković and Sekovanić [16] precipitated silica by the reaction of sodium silicate solution neutralization with H 2 SO 4 solution. Primary SiO 2 particles were ~ 15 to ~ 30 nm in size, and they are aggregated into bigger particles. Amorphous SiO 2 particles showed a specific surface area up to 130 m 2 g -1 depending on the parameters of the precipitation process. Wardiyati, Adi and Deswita [17] synthesized SiO 2 by the sol-gel method using a precursor of Na 2 SiO 3 and catalyst of H 2 SO 4 at a sintering temperature of ≤ 600 °C. The size of the obtained particles was around 20 nm. Meanwhile, the surface area was 298.31 m 2 /g. Qisti, Indurate and Suprihatin [18] used the hydrothermal method in synthesizing SiO 2 to produce nanosilica that was uniform and homogeneous.
The present paper is aimed to investigate influence of hydrothermal treatment on the crystalline form of SiO 2 synthesized by the sol-gel method.

Materials and methods 2.1. Materials
All reagents used were of analytical grade purity. In this synthesis procedure, tetraethyl orthosilicate (tetraethoxysilane or TEOS) was used. It was purchased from Sofex Silicone, Moscow, Russia (99.9 % purity). TEOS is the chemical compound with the formula Si(OC 2 H 5 ) 4 . TEOS it is a colorless liquid that degrades in water. TEOS is the ethyl ester of orthosilicic acid, Si(OH) 4 .
Ammonium chloride (NH 4 OH) used as catalyst was purchased from Sofex Silicone, Moscow, Russia (99.9 % purity). To carry out the TEOS hydrolysis reaction, distilled water by GOST 6709-72 "Distilled water -Technical specifications" (pH=6.6) was used. The TEOS hydrolysis reaction (synthesis of SiO 2 ) was carried out according to the scheme:

Hydrothermal synthesis
To carry out TEOS hydrolysis under hydrothermal conditions, the GSA-0.3 high-pressure reactor was used ( fig.1). The reactor is designed for scientific and experimental research and chemical reactions in supercritical conditions of high pressure and temperature. Hydrolysis of TEOS in an alkaline medium at room temperature was carried out for 3 days, followed by drying at a temperature of 110-120° C. Hydrolysis of TEOS in an alkaline medium under hydrothermal treatment was carried out in a high-pressure reactor (temperature 300 ° C, pressure 55 MPa) for 7 hours, followed by drying at a temperature of 110-120 ° C.

Research Methods
The research was performed with the use of high-tech equipment from High Technologies Center of V.G. Shukhov BSTU (Belgorod, Russia). X-ray phase analysis was performed with the use of the DRON-3 diffract meter (CuKα radiation, Ni filter) according to the standard method. The tube voltage amounted to 20 kV, the anode current was 20 mA, and the detector rotation rate was 2.4 degrees per minute with 1° angular pitch. The measurement rate limit reached 1000-4000 pulses per second. Data analysis and preliminary processing was performed with PDWin software (DrWin, Qual) using the PDF JCPDS database (version 2.02 1999).
Scanning (raster) electronic microscopy was used for the surface morphology identification and microanalysis of the surface layer of the derived bismuth compound. Microphotography was performed using electronic microscope Tescan Mira 3 LMU.
The Fourier transform IR spectrum was recorded using a IR spectrometer VERTEX 70. The specimen was pressed with a spectroscopically pure KBr matrix.

Result and discussion
X-Ray diffractometer (XRD) analysis was carried out in the 2θ range of 0°-35°.The result of X-Ray diffraction pattern of SiO 2 synthesized by the TEOS hydrolysis reaction at room temperature and under hydrothermal treatment is shown in Figure 2. Analysis of Figure 2 (a) shows the presence of an amorphous halo -a strong peak at Bragg angle 21 °-22 °. This result indicates that SiO 2 product synthesized by the TEOS hydrolysis reaction at room temperature has an amorphous phase. The diffraction pattern in Figure 2(a) showed only one peak which indicatesthat the SiO 2 product has a high degree of purity.The XRD peak of SiO 2 synthesized by the TEOS hydrolysis reaction at hydrothermal treatment (fig 2,b) becomes sharper at room temperature. It can be concluded that hydrothermal treatment leads to crystallization of the material. A b Figure 2. XRD analysis of SiO 2 synthesized by the TEOS hydrolysis reaction at room temperature (a) and hydrothermal treatment (b) Figure 3 shows the microstructure of SiO 2 synthesized by the TEOS hydrolysis reaction at room temperature (a) and hydrothermal treatment (b). Figure 3a shows images of the amorphous SiO 2 particles. As can be seen, primary particles show a tendency to form bigger particles (aggregates). Particle size does not exceed 2µm. Microphotography shows that the powder obtained has high levels of porosity. Analysis of micrographs of silica powder obtained using hydrothermal treatment (fig 3,b) also shows that the received substance has a crystalline structure (the crystal size in transverse dimension reaches 20 µm). The analysis of electron microscopy confirms the results of X-ray phase analysis in terms of the crystallinity of the obtained substances. However, the microphotography (fig 3,b) shows a significant amount of the substance with an amorphous structure. For a complete polymorphic transformation into β-quartz, higher temperature of 573 °C and pressure of at least 1 bar (0.1 MPa) is required. a b Figure 3. Microphotography of SiO 2 synthesized by the TEOS hydrolysis reaction at room temperature (a) and hydrothermal treatment (b) Figure 4 shows the IR-spectrum of SiO 2 synthesized by the TEOS hydrolysis reaction at hydrothermal treatment. As presented in Figure 4, band corresponding to 3460 cm -1 is the stretching vibrational absorption of silicon hydroxyl and physic adsorptive water (H 2 O), and 1620 cm -1 corresponds to the bending vibrational absorption of physic adsorptive water (-OH bending vibrations of H 2 O molecules). A very strong and predominant absorbance peak corresponding to 1080 cm -1 can be assigned to Si-O-Si asymmetric stretching vibrations. The IR band corresponding to 457 cm -1 is due to O-Si-O bending vibrations. The IR band corresponding to 788 cm -1 is assigned to Si-OH group. Presence of physic adsorptive water indicates that the material has an amorphous phase, i.e. that complete crystallization of SiO 2 during hydrothermal treatment did not occur.

Conclusion
SiO 2 particles have been synthesized successfully both by hydrothermal method and traditional sol-gel process. It was found that SiO 2 product synthesized by TEOS hydrolysis reaction at room temperature has an amorphous phase. XRD analysis shows the presence of an amorphous halo -a strong peak at Bragg angle 21 °-22 °. The XRD peak of SiO 2 synthesized by the TEOS hydrolysis reaction during hydrothermal treatment becomes sharper at room temperature. It can be concluded that hydrothermal treatment leads to crystallization of the material.
The microphotography of SiO 2 synthesized by TEOS hydrolysis reaction at room temperature also shows images of the amorphous SiO 2 particles. The microphotography of SiO 2 synthesized by the TEOS hydrolysis reactionat hydrothermal treatment shows that the obtained substance has a crystalline structure. However, it shows a significant amount of the substance with an amorphous structure. For a complete polymorphic transformation into β-quartz, higher temperature and pressure are required.

Acknowledgments
The reported study was funded by the Russian Science Foundation (project No. 17-79-10075).