Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 12362-12368
pubs.acs.org/IECR
Preparation of SiS and SiO2 Nanospheres Guo-Dong Sun,† Guo-Hua Zhang,*,† Kuo-Chih Chou,† and An-Ping Dong*,‡ †
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China The Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, Shanghai Jiao Tong University, Shanghai 200240, China
‡
S Supporting Information *
ABSTRACT: SiS vapor is generated as a byproduct in the process of preparing metal silicides by silicothermic reduction of their metal disulfides, and the vapor will cause severe pollution without rational utilization. In this paper, an efficient method was proposed to prepare ultrafine SiS spheres and silica spheres. In the process of synthesizing molybdenum disilicide, yellow-orange SiS nanospheres with different sizes were prepared by rapidly condensing SiS vapor in different temperature zones. After SiS powders were roasted at high temperatures in air atmosphere, silica nanospheres were obtained. The shape and size of the obtained SiO2 particles remained almost the same as that of the SiS particles. X-ray diffraction, Fourier transform Raman spectrometry, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, dynamic light scattering measurement, etc. were used to characterize the products.
1. INTRODUCTION Nanomaterials with a controlled size and particle size distribution are applied in a wide range of fields because of their novel physical and chemical properties. For instance, ultrafine silica has already found many applications such as fillers in rubbers and paints, plastics, lightweight structural materials, fine precision equipment for manufacturing nanocomposite materials, adsorbents, catalyst support and drug delivery in biomedical engineering, and environmental applications.1−12 Many methods have been reported to synthesize silica nanoparticles including wet and vapor phase processes. Generally, wet-phase methods involve simultaneous hydrolysis and condensation, such as sol−gel technique, which is a common method for silica synthesis, chemical precipitation, hydrothermal technique, and pressurized carbonation.3,6−9 In vapor-phase synthesis of nanoparticles, suitable conditions, such as the temperature and degree of supersaturation, need to be created.2 The flame oxidation method using tetrachloride (SiCl4) hydrolyzed in a hydrogen−oxygen flame to synthesize fumed silica was developed in the 1960s.3,4,13 Recently, Yan3 and Park4 reported a vapor-phase method by hydrolysis of silicon tetrachloride vapor with water vapor. Compared with the wet-phase methods, the vapor-phase method can produce dry silica directly, eliminating the processes of filtration, © 2017 American Chemical Society
washing, drying, and calcining again, and can result in a prodcut with a higher purity.3,4,14 SiS vapor is generated as a byproduct in the preparation process of metal silicides by silicothermic reduction of their metal disulfides. For instance, in recent papers,15 a novel method was proposed to prepare molybdenum silicide by silicothermic reduction of MoS2. At above 1100 °C, SiS vapor is generated and separated completely from molybdenum silicide, which could cause severe pollution without rational utilization. Therefore, effective ways to utilize SiS vapor is an urgent need. However, there are few studies focusing on the utilization of SiS. In this paper, a study on the preparation of differently sized SiS nanospheres at different temperature zones by the vapor deposition method was conducted in the silicothermic reduction process of molybdenum disulfide. The mechanisms of SiS vapor deposition and size control have been discussed in detail. Then the obtained SiS nanospheres were calcined in air to synthesize silica nanospheres. Received: Revised: Accepted: Published: 12362
August 9, 2017 October 7, 2017 October 12, 2017 October 12, 2017 DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
Article
Industrial & Engineering Chemistry Research
Figure 1. Schematic diagram of the apparatus: 1, alumina tube; 2, electrical furnace; 3, constant temperature zone; 4, thermocouple; 5, zone I; 6, zone II; 7, zone III; 8, zone IV; 9, alumina crucible; 10, sample.
Figure 2. Photos of the yellow-orange SiS powder obtained from (a) zone I, (b) zone II, (c) zone III, and (d) zone IV.
2. EXPERIMENTAL SECTION The synthesis of molybdenum silicide by silicothermic reduction of MoS2 was described in detail in the literature.12 The raw materials used were molybdenum disulfide (MoS2) with a purity of 98% and silicon (Si) with a purity of 99.99%. In the present study, a simple apparatus, shown in Figure 1, was employed. The raw mixtures of MoS2 and Si with a MoS2/Si molar ratio of 1:4 were thoroughly mixed and then cold pressed into a pellet with a diameter of 18 mm and height of 7.5 mm (about 5 g). The alumina crucible with the sample was placed into the constant temperature zone of the electric heating furnace and calcined at 1400 °C with the protection of flowing argon (400 mL/min). A yellow-orange SiS powder was deposited in the low-temperature zone. The yellow-orange powder was collected and calcined in an air atmosphere at different temperatures (800, 900, and 1000 °C) for 2 h with a heating rate of 5 °C/min. The off-gas was treated by Ca(OH)2 slurry. The characteristics of samples were determined by X-ray diffraction (XRD) (SmartLab, Japan), Fourier transfrom (FT)Raman spectrometer, Fourier transform infrared (FT-IR) spectroscopy (Nicolet-490), and Field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS). Dynamic light scattering (DLS) measurement (Horiba, Japan) was used to determine the particle zeta potential and size histograms comprising dispersion indexes.
different zones, from which it can be seen that the color gradually becomes lighter from zone I to zone IV. 3.1.2. Characteristics of SiS Nanospheres. On the basis of the conclusions of Byerley et al.,16 solid SiS is amorphous; it exists in various forms from a yellow-orange fluffy material to a hard glassy reddish-black solid, which could be obtained only by condensing gaseous SiS formed at high temperature. The color of powders obtained in this work from zone I to zone IV matches the color of the described yellow-orange fluffy material. According to the thermodynamic calculation of MoS2−Si system, SiS is generated based on reaction 1.15 In addition, it was found that the actual weight loss rate at 1400 °C (43.9%) is very close to the theoretical value (44.12%), which is calculated according to reaction 1 provided that SiS escaped completely from the sample. Therefore, according to the above analyses and the previous study,15 the yellow powder can be proven indirectly to be SiS. MoS2 + 4Si = MoSi 2 + 2SiS
(1)
The XRD patterns of SiS nanospheres also indicated that the obtained yellow SiS powder is amorphous, as shown in Figure 3 (zone II). However, the confirmation of this cannot be done by Raman spectrometry because SiS was unstable and decomposed when the Raman laser irradiated it. Considering that a long heat treatment time is needed for SiS to be decomposed to SiS2 and Si at low temperature and the feature color of SiS2 is white,16,17 the yellow-orange SiS was heat-treated under the protection of argon at 700 °C for 24 h. It was found that the annealed SiS sample lost the original characteristic color and changed obviously from yellow-range to white. The XRD patterns of the white powder are presented in Figure 4a, from which it can be obviously seen that there are characteristic peaks of SiS2 and Si. In addition, the Raman patterns of the white powder are shown in Figure 4b, and the theoretical and experimental Raman frequencies of solid SiS2 are shown in Table 1. This result can also prove that the white powder is composed of SiS2 and Si. In addition, the temperature range for
3. RESULTS AND DISCUSSION 3.1. Preparation and Characteristics of SiS Nanospheres. 3.1.1. Preparation of SiS Nanospheres. It has been known that SiS could be in the gas phase when the temperature is over 1100 °C.15 In the present study, the SiS vapor was generated in the high-temperature zone (1400 °C) and was instantly carried out by the flowing argon and deposited in the low-temperature zones. It was found that the collected yellow powder was mainly deposited in zones I−IV, as shown in Figure 1. Figure 2 shows the photos of the powders collected at 12363
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
Article
Industrial & Engineering Chemistry Research
creation of a supersaturated vapor is necessary, in which the vapor phase is thermodynamically unstable in contrast to forming the solid material.2 The Gibbs free energy for forming a spherical particle can be expressed as the sum of the volume energy and the surface energy.21,22 The particle is energetically unfavorable and cannot exist stably until it reaches a critical radius (r0). Thus, the nucleation barrier is the amount of energy required to form a particle with the critical radius (r0).21 This free-energy barrier is mainly determined by the degree of supersaturation (S) (see the Supporting Information). Furthermore, if steady-state cluster formation and distribution are considered, the nucleation rate depends on the supersaturation of vapor19 (see the Supporting Information). According to the temperature dependence of vapor pressure of SiS solid,23 it is possible to estimate the supersaturation of the vapor. The equilibrium vapor pressure of SiS and its supersaturation under different actual partial pressures were calculated (Figure S2). It can be found that the equilibrium vapor pressure of SiS decreases significantly with the reduction of the temperature, leading to the rapid growth of supersaturation of the SiS vapor at the same time. Once particles are formed in the vapor phase, it could be easier for molecules to deposit on these particles rather than to form new nuclei. Therefore, particle growth can be performed by both condensation and Brownian coagulation19 (see the Supporting Information). If the initial supersaturation is limited, the nucleation rate would be slow, leading to lower initial nuclei concentrations, in which case Brownian coagulation would be limited and particles would be grown to larger size mainly by condensation; the distribution of the nanoparticle size could be narrow24 (see the Supporting Information). However, in some systems, a pretty high initial supersaturation can lead to the formation of very large number of particles, which is beneficial to Brownian coagulation. In this condition, the particles can grow by Brownian coagulation and coalesce. When the temperature is sufficiently high, particles could coalesce quickly and dense sphere particles can be obtained. In contrast, at lower temperature, the particles could coagulate to lose agglomerates owing to the slow coalescence.2,19 Therefore, the processes of nucleation, particle growth, particle coagulation, and coalescence can have great influences on the sizes of nanoparticles resulting from vapor deposition.
Figure 3. XRD patterns of the yellow-orange SiS powder and the silica substrate.
the decomposition of SiS is from 627 to 1036 °C (see the Supporting Information). Therefore, the temperature of deposited zones should not be higher than 627 °C in order to avoid the decomposition of SiS. In the present study, the generated SiS vapor was carried out instantly to the lowtemperature zones (below 615 °C) to generate the solid yellow-orange SiS powder. 3.1.3. FE-SEM Analyses of SiS Nanospheres. Typical FESEM micrographs of SiS powder obtained at different zones are shown in Figure 5a−d. In order to obtain the particle size distributions of the SiS nanospheres, typical FE-SEM micrographs of SiS powder obtained at different zones were employed. According to the size of no less than 250 spheres, the average size of the SiS spheres from zone I to zone IV are 259, 211, 174, and 105 nm, respectively, from which it can be seen that the size decreases gradually from zone I to zone IV. The size distributions of SiS nanospheres for different zones are depicted in Figure 5e. It can be seen that the SiS spheres in zones II and III have a narrower size distribution than those in zones I and IV. In addition, it also can be found from Figure 5a that the spheres in zone I have a more obviously sintering phenomenon than other zones. The mechanism of vapor-phase synthesis of nanoparticles includes nucleation, particle growth, particle coagulation, and coalescence.2,19,20 In the process of nucleation and growth,
Figure 4. (a) XRD patterns and (b) Raman spectrum patterns of the white powder obtained after heat treatment of the yellow-orange SiS powder. 12364
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
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Industrial & Engineering Chemistry Research Table 1. Theoretical and Experimental Raman Frequencies of Solid SiS2 theoretical values (NP-SiS2) (cm−1)18 experimental values in the literature (cm−1)18 this experiment (cm−1)
138.9 141.6 139.7
166.7 177.2 178.6
180.3 185.0 185.8
comb. 219.2 218.8
347.0 353.2 353.3
444.8 433.9 432.7
453.8 449.1 449.7
646.0 629.1 624.9
Figure 5. FE-SEM micrographs of SiS nanospheres obtained at different temperature zones: (a) zone I, (b) zone II, (c) zone III, and (d) zone IV; (e) size distribution curves of SiS nanospheres for different zones.
Figure 6. (a) White SiO2 powder obtained by calcining SiS powder (zone II) in air at 1000 °C; (b) XRD patterns of SiO2 nanospheres prepared at different temperatures; (c) FTIR spectra of the white SiO2 powder prepared at 1000 °C.
the degree of supersaturation in this zone is pretty low, resulting in the critical radius (r0) and the free energy barrier being dramatically high. Therefore, it was difficult for SiS vapor
In the present study, even though the generated SiS vapor had a high partial pressure in the high-temperature zone, there was also a high equilibrium vapor pressure of SiS. Therefore, 12365
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
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Industrial & Engineering Chemistry Research
Figure 7. FE-SEM micrographs of the raw SiS nanospheres and SiO2 nanospheres calcined at different temperatures: (a) raw SiS nanospheres (zone II), (b) 800 °C, (c) 900 °C, and (d) 1000 °C; (e) EDS spectrum patterns of the SiO2 nanospheres in panel d.
to nucleate. However, when the SiS vapor arrived at zone I, the high degree of supersaturation led to the formation of larger numbers of particles, and the high concentration of particles resulted in a high rate of Brownian coagulation; because of the sufficient temperature in this zone, the agglomerated particles coalesced quickly to grow to dense spherical particles. Therefore, in this condition, narrow particle size distribution was not obtained because of the dominant growth mechanism of coagulation and coalescence. Although the partial pressure of the remaining SiS vapor reduced in zone I, the equilibrium partial pressure of SiS vapor also decreased dramatically from zone I to zone II. As a result, a relatively high supersaturation could still be created in zone II to have a relatively high concentration of nuclei. However, it could be lower compared to that of zone II because of the lower temperature and SiS vapor. Therefore, the rate for Brownian coagulation and the coalescence of particles could be relatively slower, while the particle growth by condensation could be increased, leading to the smaller average diameter of the nanospheres and the narrower distribution of nanospheres. In addition, in zone III, a
smaller size and a narrower distribution of nanospheres were also obtained. However, compared to zones I to III, because of the fairly low temperature and remaining SiS vapor in zone IV, the coalescence rate was very slow and the growth of particles by accretion was limited; then the particles grew by coagulation, and loose structures were formed. Therefore, from zone I to zone IV, because of the decreasing temperature and partial pressure of SiS vapor, nucleation, particle growth, particle coagulation, and coalescence changed gradually, resulting in the differences of product size and morphology. 3.2. Preparation of SiO2 Nanospheres by Oxidation of SiS Nanospheres. The oxidation reaction of SiS is represented by reaction 2, and the change of standard Gibbs free energy is shown by eq 3, as calculated by Factsage 7.0. It is obvious that reaction 2 is thermodynamically feasible from 0 to 1300 °C. Besides, it is an exothermic reaction, and little energy will be consumed during the oxidation process. However, considering that the oxidation reaction rate of SiS is slow at low temperatures, there will be evaporation loss of SiS at temperatures that are too high; therefore, the oxidation 12366
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
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Industrial & Engineering Chemistry Research
Figure 8. Particle size distribution curves of (a) SiO2 prepared at 900 °C (zone II) and (b) SiO2 prepared at 1000 °C (zone II); (c) zeta potential of SiO2 prepared at 900 and 1000 °C.
Figure 9. Concept of the proposed method.
roasting was conducted in the temperature range of 800 to 1000 °C in the air conditions with a slow heating rate of 5 °C/ min. After oxidation roasting, the color of the powder was changed obviously from yellow to white, as shown in Figure 6a. SiS + 2O2 = SiO2 + SO2
(2)
Δr G θ = −1044.02 + 0.1733T (kJ·mol−1)
(3)
In Figure 7a,b it can be seen that both the shape and size of the SiO2 nanospheres prepared at various temperatures were almost unchanged compared to the SiS nanospheres. This phenomenon may result from the SiS nanospheres being able to be oxidized in the process of raising temperature, and a protective oxide layer can be formed, which could enable the nanospheres to keep the morphology at high temperature (800−1000 °C). Figure 7c shows the EDS spectrum of SiO2 nanospheres calcined at 1000 °C, indicating that only O and Si elements were detected (C and Au were from C film and sprayed Au, respectively). When these results are combined with the XRD results, it can be concluded that the sample is pure amorphous SiO2. Panels a and b of Figure 8 show the size distribution curves of the SiO2 prepared at 900 and 1000 °C (zone II), respectively. The average particle sizes of the SiO2 nanospheres prepared at 900 and 1000 °C were measured to be 326.1 and 371.0 nm, respectively, which are relatively higher compared with the average diameter of nanospheres obtained by FE-SEM observation (about 211 nm). The reason may be the agglomeration of nanospheres, and it was reported that the size obtained by DLS is usually greater than that measured with electron microscopy.25 In addition, the polydispersity indexes (PDIs) of the SiO2 nanospheres prepared at 900 and 1000 °C, which is an important parameter for particle-size distribution, were 0.24 and 0.285, respectively, indicating a fairly uniform particle size. Zeta potential is the most important parameter
Figure 6b shows the XRD patterns of silica obtained at different temperatures. It can be seen that all products exhibit a broad and intense diffraction peak at about 22° [21.83° (1000 °C), 22.36° (900 °C), and 22.79° (800 °C)], which is extraordinary similar to the XRD patterns of the silica in the amorphous state.3,9,14 In Figure 6b, it can also be observed that the broad peaks tend to sharpen and shift slightly with increasing calcination temperature. Wang et al.14 indicated that the reason for the shift of diffraction peak is that the samples were gradually converted to a crystalline state. The FTIR analysis was applied to investigate their chemical bonds. Figure 6c shows the FTIR spectra of the white SiO2 powder calcined at 1000 °C (zone II). It can be seen that there are three sharp absorption bands at 1133, 807, and 485 cm−1, which is related to the stretching and bonding vibration of the silicon−oxygen bond and can be attributed to the asymmetric vibrations of Si− O−Si (1133 cm−1), the symmetric stretching vibrations of O− Si−O (807 cm−1), and the bending vibrations of Si−O (485 cm−1).9 12367
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
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Industrial & Engineering Chemistry Research that defines surface properties of solids in aqueous solutions.26 One of the most popular uses of the zeta potential data is to relate it with colloid stability. Nanoparticle dispersions with zeta potential values of ±0−10 mV, ±10−20 mV, ±20−30 mV, and > ±30 mV are guidelines classifying particles as highly unstable, relatively stable, moderately stable, and highly stable, respectively.25 Therefore, the zeta potential of SiO2 nanospheres prepared at 900 and 1000 °C was measured, and the results are shown in Figure 8 c, from which it can be seen that their zeta potential values are −64.8 mv and −60.8 mv, respectively, indicating that they are highly stable. As discussed above, the concept of the proposed method can be described as in Figure 9. By this process, the byproduct SiS during the preparation of molybdenum disilicide could obtain efficient utilization. What is more, SiS nanospheres may have great potential for preparing other materials.17
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4. CONCLUSIONS This paper proposed an effective method to utilize SiS vapor. SiS nanospheres with different average sizes from about 105 to 259 nm were successfully prepared by rapidly condensing SiS vapor, and the particle size could be controlled at different temperature zones. After the SiS powder was roasted at high temperatures in air atmosphere, SiO2 nanospheres were successfully prepared. The final product of SiO2 maintained almost the same morphology of SiS after being roasted in the temperature range of 800 to 1000 °C.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03292. Decomposition reaction of SiS and detailed mechanism analyses of vapor deposition (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 1062333703. Fax: +86 1062332570. E-mail:
[email protected]. *Tel.: +86 1054742661. Fax: +86 1054742661. E-mail:
[email protected]. ORCID
Guo-Hua Zhang: 0000-0003-1786-035X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Thanks are given for the financial support from the National Natural Science Foundation of China (U1460201).
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REFERENCES
(1) Wang, L.; Zhang, G. H.; Chou, K. C. Synthesis of nanocrystalline molybdenum by hydrogen reduction of industrial grade MoO3. Int. J. Refract. Hard Met. 2016, 59, 100−104. (2) Swihart, M. T. Vapor-phase synthesis of nanoparticles. Curr. Opin. Colloid Interface Sci. 2003, 8, 127−133. (3) Yan, F.; Jiang, J. G.; Chen, X. J.; Tian, S. C.; Li, K. M. Synthesis and Characterization of Silica Nanoparticles Preparing by LowTemperature Vapor-Phase Hydrolysis of SiCl4. Ind. Eng. Chem. Res. 2014, 53, 11884−11890. 12368
DOI: 10.1021/acs.iecr.7b03292 Ind. Eng. Chem. Res. 2017, 56, 12362−12368
