Abstract
This paper presents a study on pulsed plasma-chemical synthesis of fluorine- and gold-doped silicon oxide nanopowder. The gold- and fluorine-containing precursors were gold chloride (AuCl3) and sulphur hexafluoride (SF6). Pulsed plasma-chemical synthesis is realized on the laboratory stand, including a plasma-chemical reactor and TEA-500 electron accelerator. The parameters of the electron beam are as follows: 400–450 keV electron energy, 60 ns half-amplitude pulse duration, up to 200 J pulse energy, and 5 cm beam diameter. We confirmed the composite structure of SixOy@Au by using transmission electron microscopy and energy-dispersive spectroscopy. We determined the chemical composition and morphology of synthesized SixOy@Au and SixOy@F nanocomposites. The material contained a SixOy@Au carrier with an average size of 50–150 nm and a shell of fine particles with an average size of 5–10 nm.
1. Introduction
Silicon dioxide (SiO2) nanopowder is widely used in various industries. It is used as a filler for polymeric paint and lacquer materials, improving the abrasion and durability of paints [1, 2]. SiO2 is often used as a food additive with the intention of avoiding the clumping and caking of food [3, 4]. It is also used in the manufacture of toothpastes and medicines [5, 6]. SiO2 is one of the main components in the production of glass, abrasives, ceramics, and concrete [7, 8]. Silicon dioxide is used in radio electronics, in particular, the production of microcircuits and fibre optic cables [9, 10]. At present, the properties of nanocomposite structures, which are used to create materials with preassigned properties, are being actively studied in modern solid-state physics [11–14]. Production of a composite based on SiO2 with various chemical elements (C, Au, and F) enables the improvement of the physical and chemical properties of the synthesized composite and expands its scope of use. Among such composites, SiO2@Au and SiO2@F composites deserve special attention. Nanoparticles of noble metals attract much attention because of their unique properties and many applications: chemical analysis, medical diagnostics and treatments, sensors, bactericidal materials, surface-enhanced Raman spectroscopy (SERS), enhancement of the fluorescence of organic dyes, etc. [15–19]. To obtain the nanoscale composites of SiO2@Au and SiO2@F, the liquid-phase method, sol-gel method, classical chlorine process, and flame synthesis are used, among other methods [20–27].
In [21], a mesoporous SiO2@Au composite with a specific surface area of 650 m2/g was obtained using the sol-gel method. The following silicon- and gold-containing precursors were used for the synthesis: tetraethoxysilane (C2H5O)4Si of high purity (99%) and sodium tetrachloroaurate (AuCl4Nа). The obtained composite was stabilized by annealing in air at 600°C. The composite would be useful as a standard for studying the optical and catalytic properties of particles of noble metals. The SiO2@Au composite was synthesized in [22] using a combination of the sol-gel method and calcination processes. In studies of the physical and chemical properties of composites, it was found that the calcination temperature has an obvious effect on the morphology and structure of the sample. The catalytic activity of the SiO2@Au nanocomposite was studied, and it was found that the synthesized SiO2@Au composites possess high catalytic activity. In [23], the SiO2@Au composite was synthesized by the sol-gel method. The synthesis process included four steps: (a) preparation of silicon dioxide; (b) grafting gold nanoparticles over a SiO2 shell; (c) priming of the silica-coated gold nanoparticles with 2 to 10 nm gold colloids; and finally (d) formation of the complete shell. The average size of the synthesized particles was 170 nm with a standard deviation of 18 nm. The SiO2@Au composite was obtained in [27] via the Stöber chemical method. The composite consisted of two particles of SiO2 and Au. The average particle size for SiO2 was about 300 nm and 100 nm for Au. The SiO2@Au nanocomposites possessed enhanced electrical and mechanical properties compared with silicon dioxide nanoparticles.
At this stage of research in the field of obtaining nanocomposites based on silicon dioxide doped with fluorine or gold particles, the possibility of organizing a pulsed plasma-chemical process for producing such nanomaterials has not been studied. This work is devoted to experimental research in this direction.
2. Experimental
The ultrafine silicon dioxide was synthesized using a TEA-500 pulsed electron accelerator [28–32]. Silicon tetrachloride, hydrogen, and oxygen were used in the experiments. The reaction chamber was pumped out to a pressure of ∼7.6 Pa before introducing the mixture of gases. Figure 1 shows the experimental setup.
To obtain SixOy@Au nanocomposites, we used gold chloride (AuCl3). The gold-containing precursor was obtained from jewellers’ gold by dissolution followed by evaporation. The composition of jewellers’ gold contains 58.5% gold, copper 33.5%, and silver 8%. The plasma-chemical reactor was preevacuated to 5 Torr, while SiCl4, O2, H2, and AuCl3 were simultaneously introduced; after that, the reactor was heated to 180°C. As a result of heating, gold trichloride was decomposed into gold monochloride and molecular chlorine.
Next, a pulsed electron beam was injected into the reactor, initiating reactions to obtain a nanosized silica powder as described in [30]. One of the by-products of plasma-chemical synthesis is water vapour. The synthesis process had a chain character and proceeded with significant heat generation. Two factors, namely, elevated temperature and moisture, contributed to the decomposition of gold monochloride (AuCl) into Au and AuCl3.
As a result, SixOy@Au nanopowder was produced (the mass of the obtained powder was about 1.2 g per one act of pulsed electron beam impact on the initial reagents).
To obtain the SixOy@F composite, the following reagents were used: sulphur hexafluoride, silicon tetrachloride, hydrogen, and oxygen. The initial reagents were injected into the plasma-chemical reactor, followed by the injection of the electron beam. As a result of the action of a pulsed electron beam on a gas mixture (SF6 + SiCl4 + H2 + O2), several parallel chemical reactions were initiated. These reactions stimulated the interaction of chlorine and hydrogen, the oxidation of hydrogen, and the oxidation of sulphur hexafluoride with the formation of oxofluorides (OF, OF2, etc.), which participated in the synthesis of the final product of SixOy@F. Synthesized SixOy@F powders were white. During the action of the pulsed electron beam, the mass of the obtained powder was about 0.8 g. Pulsed plasma-chemical synthesis of the SixOy@Au and SixOy@F nanocomposites was implemented in one step; all reagents were mixed in advance, and the synthesis process was implemented in one pulse. Moreover, no additional technological operations were required such as hardening and drying.
3. Results and Discussion
The chemical composition of the SixOy@Au nanocomposite, obtained using the pulsed plasma-chemical method, was determined using an Oxford Instruments ED2000 energy-dispersive X-ray fluorescence spectrometer. The X-ray fluorescence spectrum of the synthesized composite is shown in Figure 2.
As can be seen from Figure 2, in addition to gold, the material contains a significant amount of impurities of copper, zinc, and nickel, which is likely because gold of gem quality was used to create the initial gold-containing precursor. Moreover, the iron content was noticeable, which can be explained by the participation of the metal parts of the reactor in the synthesis process during heating.
The Brunauer–Emmett–Teller (BET) method was used to study the specific surface area for all synthesized SixOy@Au samples. The specific surface area for the synthesized SixOy@Au samples ranged from 140 to 220 m2/g.
The morphology of the SixOy@Au nanocomposites was determined using a JEOL-II-100 (Jeol Ltd., Japan) transmission electron microscope. The morphology of the SixOy@Au nanocomposites is shown in Figure 3.
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(b)
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The TEM images show that the synthesized particles had an irregular shape evenly covered with fine particles (Figures 3(a) and 3(b)). Powder morphology is represented by globules consisting of spherical- or oval-shaped fused nanoparticles. Two regions are noticeable in the images: dark (Au particles) and light (SixOy particles), which indicates that the composite particles consisted of two components. The morphology of the composite was characterized by clusters of particles amalgamated into a single structure. The morphology of small particles was rounded (Figure 3(c)). Small particles of gold are evenly distributed on the surface of large particles (Figure 3(d)). The average size of the small particles (Au) ranged from 5 to 10 nm, and the diameter of the SixOy particles ranged from 60 to 150 nm. Figure 4 presents the histogram of the particle size distribution. Histograms were constructed with a sample of over 1000 particles.
The silicon oxide in the obtained nanocomposites was amorphous. The synthesized SiO2@Au composites were studied using an energy-dispersive X-ray spectroscopy (EDX method). The relative content of oxygen and silicon in the SixOy@Au composite according to EDX-spectra was 26 wt.% and 57 wt.%, respectively. Mass content of gold in the composite was 17%.
Figure 5 shows micrographs of the synthesized SixOy@F nanomaterial.
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The nanomaterial had an amorphous structure, as evidenced by the absence of reflections on the microdiffractogram. The size of the SixOy@F composite powders ranged from 20–45 nm (Figure 6).
Using the energy-dispersive X-ray spectroscopy (EDS) method, particles of the SixOy@F nanocomposite were analysed to estimate the fluorine content in the synthesized samples. The spectrum was taken at 20 points. Fluorine was rather evenly distributed throughout the nanomaterial particles. The average content reached 48 at.%, which indicates that fluorine is mainly located on the surface.
Figure 7 shows the characteristic infrared absorption spectra of the SixOy@F composite powders in the range from 400 to 4000 cm−1 (Nicolet 5700, Thermo Fisher Scientific, USA). The studied powder was premixed with KBr and pressed into a tablet. The reflection spectrum of pure KBr was subtracted from the reflection spectrum of the mixture.
The IR absorption spectrum shows absorption bands typical for the SixOy material. The 1090 and 815 сm−1 peaks responsible for Si–O–Si bond fluctuation were typical for the samples under study. The 460 сm−1 peak is responsible for the bond oscillations in the Si–O–Si group. The band with a center of ∼940 cm–1 corresponds to the stretching vibrations of the internal OH-bond of SiOH. In addition, we recorded the bands at 1500–2000 cm−1 in the spectrum, which correspond to the H–O–H group. The stretching vibrations of hydroxyl groups and water molecules, νoн, form an intense band in the region of 3200–3600 cm−1 [33, 34]. This fact suggests that, in addition to physically adsorbed fluorine, the surface of the nanoparticles probably contains largely silanol groups (SiOH), which adsorb water molecules [35]. Comparing the reflection spectra of the composite SixOy@F and the SiO2 nanopowder obtained by the pulsed plasma-chemical method [36], an absolutely identical picture can be seen. Addition of sulphur hexafluoride to the initial mixture did not lead to the formation of chemical bonds that could be fixed.
4. Conclusion
We have shown that it is possible to synthesize nanosized silicon dioxide doped with gold particles and fluorine by using a pulsed plasma-chemical process. The SixOy@Au and SixOy@F composite powders, which consist of particles of irregular shape with a diameter of 20 to 100 nm, were synthesized. Silicon oxide in the obtained nanocomposites was amorphous. In the TEM images of SixOy@Au nanocomposites, fine Au particles uniformly distributed on the surface of larger particles are visible.
Synthesized composites have a high potential for use as catalysts. The use of SixOy@Au as a filler for conductive inks is also possible. The SixOy@F-synthesized composites have a high potential as an additive for paints, as it is known from the literature that particles with a fluorine-containing surface have hydrophobic properties. In addition, the synthesized composite can be of interest to additive technologies. Due to the fluorine content in the surface layer of the nanoparticles, it is potentially possible to use the nanoparticles in order to reduce the temperature at which the 3D printing process takes place.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was partially supported by the Russian Science Foundation (research project no. 17-73-10269).