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Preparation and properties of silica nanoparticles from oil shale ash
Powder Technology 191 (2009) 47–51
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Preparation and properties of silica nanoparticles from oil shale ash Gui-Mei Gao a, Hai-Feng Zou a, Shu-Cai Gan a,⁎, Zhao-Jun Liu b, Bai-Chao An a, Ji-Jing Xu a, Guang-Huan Li a a b
College of Chemistry, Jilin University, 6 Ximinzhu street, Changchun 130026, PR China College of Geoscience, Jilin University, 6 Ximinzhu street, Changchun 130026, PR China
a r t i c l e
i n f o
Article history: Received 10 June 2008 Received in revised form 7 September 2008 Accepted 16 September 2008 Available online 20 September 2008 Keywords: Oil shale ash Azeotropic distillation Ultrasonic technique
a b s t r a c t The method of preparing spherical silica nanoparticles from the oil shale ash (OSA) via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation, was reported. The effects of ultrasonic and azeotropic distillation on the particle size and distribution have been investigated. Further, the morphology and properties of the silica particles were examined. The X-ray fluorescence spectroscopy (XRF) and Brunauer Emmett Teller (BET) analysis confirmed that the powders consist of silica nanoparticles with high purity, 99.90% and specific surface area of 697 m2/g. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis indicate that the silica nanoparticles are amorphous and that the surface of silica nanoparticles is modified by the organics. The transmission electron microscopy (TEM) images of the sample show that good dispersion and uniform silica particles with an average diameter of about 10 nm are prepared. The results obtained in the mentioned method prove that the oil shale ash (OSA) can be used for production of silica nanoparticles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction A by-product of oil shale processing is ash, which is considered a serious environmental problem. Therefore, there is a need for a proper strategy for ash handling, disposal and utilization. Recent studies have demonstrated that the OSA can be used for production of cement and concrete [1–3], sorbents [4–8], tobermorites [9–11], soil treatment for agricultural purpose [12], while the large portion is dumped in landfills. It is found that the OSA is mixture of inorganic and organic components, whose major chemical compositions are silicon dioxide (SiO2) and alumina (Al2O3) etc., and whose major mineral compositions are quartz (SiO2), illite [K(Al,Fe)2AlSi3O10(OH)2·H2O], kaolinite [Al4(SiO10)(OH)6], chlorite [(Mg,Fe)5(Al,Si)5O10(OH)8] and calcite [CaCO3] [5]. So, an alternative manner is the conversion of this ash into a high-grade silica product, which is considered an environmental friendly product. However, there is little available information about extraction of silica from OSA. Ultrafine silica (SiO2) powder has considerable potential for a wide range application including catalysts, adsorbents, light-weight structural materials, humidity sensors, colloidal damper in the field of mechanical engineering, and other fine precision equipment [13–15]. However, the large scale commercial production of silica nanoparticles has been limited for mainly two reasons: (i) the use of high-cost alkoxides [tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS)] in the sol–gel or W/O emulsion synthesis, and (ii) nanoparticles ⁎ Corresponding author. Tel.: +86 431 87652736. E-mail address: [email protected] (S.-C. Gan). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.09.006
aggregating into large particles during dryness. An important topic in silica nanoparticles research area is to explore cheap raw material for silica nanoparticles production and to find a conformable drying method. Sonochemistry arises from acoustic cavitation phenomenon, that is the formation, growth and implosive collapse of bubbles in a liquid medium [16]. The extremely high temperatures (N5000 K) and pressure (N20 Mpa) and very high cooling rates (N107 K s− 1) attained during acoustic cavitation lead to many unique properties in the irradiated solution. This method is a simple and effective route for preparing ultrafine powders on a nanometer scale and with homogeneous particle size distribution [17,18]. In the recent years many kinds of nanomaterials have been prepared by this method, such as ZnO, MnO, SBA-15 [19–21]. Incidentally, so far, no attempt has been made to prepare silica nanoparticles through sonochemical route. In the present work, the spherical silica nanoparticles were synthesized using OSA as silica source via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation. The effects of ultrasonic and azeotropic distillation on the particle size and distribution were investigated. 2. Experiment 2.1. Materials and reagents The OSA was used as silica source for synthesis of silica nanoparticles. It was taken from oil retorting factory of Jilin province. The chemical compositions of the OSA are shown in Table 1, and it indicates that the OSA consists of a wide variety of acidic, basic,
48
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
Table 1 Chemical compositions of the OSA before and after the pretreatment and the silica powders Component Raw oil shale ash (wt.%)
Calcined and leached oil shale ash (wt.%)
Silica powders (wt.%)
SiO2 Al2O3 TFe2O3 P2O5 CaO MgO TiO2 Na2O K2O MnO2 LOI
86.8561 8.1541 1.8061 0.3163 0.0781 0.3032 0.7010 0.4501 0.6511 0.0381 0.6458
99.9034 0.0128 0.0084 0.0155 0.0049 0.0083 0.0107 0.0111 0.0054 0.0010 0.0035
64.7232 15.8514 8.6109 0.5117 0.9907 1.8221 1.0411 0.7046 1.1733 0.1701 3.4237
amphoteric oxides and incorporated hydrocarbons. All chemical reagents were analytical grade supplied by Beijing Chemical Reagent Research Institute. 2.2. Experimental procedure 2.2.1. Pretreatment of the OSA The pretreatment of the OSA consisted of the mechanical, thermal, acid treatments. The OSA was crushed to about 0.1 mm, and burned in a muffle furnace at 550 °C for 2 h to remove all incorporated hydrocarbons. An acid washing step was used to remove the small quantities of minerals prior to silica extraction from OSA in the following manner. The calcined OSA (10 g) was acid-leached with 30 wt.% sulfuric acid solution (50 ml) at 100 °C for 2 h in a pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle. Then the slurry was filtered and washed with distilled water for several times until the pH value equaled 7. 2.2.2. Preparation of sodium silicate solution 30 wt.% sodium hydroxide solution (50 ml) was added to the pretreated OSA and boiled for 5 h in a pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle to dissolve the silica, and to produce a sodium silicate solution. The solution was filtered and washed with boiling distilled water. The filtrate and washing were allowed to cool to room temperature. 2.2.3. Synthesis of silica nanoparticles Firstly, 10 g of polyethylene glycol was dissolved in the water (90 ml). subsequently, OSA derived sodium silicate was slowly added to the polyethylene glycol water solution that was being sonicated at 50 °C. Then, 0.5 mol/L sulfuric acid solution was added gradually into the solution in order to initiate the hydrolysis-condensation reaction and sonication was continued 0.5 h. The sol was adjusted until the pH value equaled 4. The resulting gel mixture was aged at 50 °C for more than 8 h. The silica gel aged was filtered and washed by distilled water for several times. The filtration cake was distilled with n-butanol, and then calcined at 550 °C for 2 h in atmospheric condition to remove the surfactant. The silica nanoparticles were obtained finally.
(SiO2) in the OSA is not attacked by H2SO4 solution in the acid treatment. The silica nanoparticles are obtained by the following reactions: NaOH þ SiO2 →Na2 SiO3 þ H2 O
ð1Þ
Na2 SiO3 þ H2 SO4 →SiO2 þ Na2 SO4 þ H2 O:
ð2Þ
The sodium sulfate (Na2SO4) in the silica gel can be washed out easily due to its high solubility in water. Table 1 shows that the contents of the extraneous elements are reduced to a low level during the preparation process. The purity of silica nanoparticles is higher than 99.90%. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku Rotaflex diffractometer equipped with a rotating anode using Cu Kα radiation. The X-ray diffraction scanning angle is from 0° to 80° and rate is 0.01°/s. Fig.1 shows the XRD pattern of the calcined silica particles. The typical silica characteristic is observed at a broad peak centered at 2θ = 22.5°, which indicates the sample is amorphous [22]. The result is accord with selected area electron diffraction. BET analysis was used to determine the total specific surface of silica nanoparticles (Sample was dried at 120 °C for 12 h). BET data indicates that the specific surface area of silica nanoparticles prepared by this method is as high as 697 m2/g. In order to investigate the action of ultrasonic cavitation during the formation of silica particles another experiment was performed at the same conditions except for ultrasonic irradiation. To find the effect of the sonication time continued, the experiments were performed varying the sonication time continued at a rang 0.5–1.5 h. Transmission electron microscopy images of silica nanoparticles were obtained with a JEM-2000EX microscope. Silica nanoparticles were dispersed ultrasonically in ethanol, and a drop of suspension was deposited on a carbon coated copper grid. TEM micrographs of silica nanoparticles obtained from the rigorous magnetic stirring (a), and the ultrasonic technique at hydrolysis-condensation periods of 0.5 h (b), 1 h (c) and 1.5 h (d) were exhibited in Fig. 2. Among them, the smallest spherical silica particle size with narrow uniform distribution can be observed in Fig. 2-b, and the average size of particles is about 10 nm. Obviously, the average particle size decreases from 60 nm to 5 nm and the powders are less agglomerated when the rigorous magnetic stirring is replaced by ultrasonic cavitation. However, when ultrasonic periods were increased, serious aggregate silica particles with the larger particle size are obtained (Fig. 2-c,d). Schematic representation of the proposed mechanism for silica nanoparticles formation using ultrasonic is shown in Fig. 3. Comparing with traditional stirring, ultrasonic cavitation is easily achieving microscopic uniform mixing, eliminating partial uneven concentration,
3. Results and discussion The chemical compositions of the OSA before and after the pretreatment and the silica powders were determined by X-ray fluorescence spectroscopy, and the results are presented in Table 1. Unlike conventional organic silicon compounds, the OSA is a industrial waste, which contains several main extraneous components. The thermal and acid treatments are efficient, resulting in a material with a high reduction in the Fe2O3, Al2O3, CaO and MgO content. The silica
Fig. 1. XRD pattern of the silica nanoparticles.
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
49
Fig. 2. TEM micrographs of silica nanoparticles obtained from the rigorous magnetic stirring (a); and the ultrasonic technique at hydrolysis-condensation periods of 0.5 h (b), 1 h (c) and 1.5 h (d).
stimulating formation of new phase, and crushing the aggregates. It can be seen from Fig. 3, the silica particles of new generation were possibly surrounded by solvents and residual surfactants, and easily agglomerate into a large particles. Ultrasonic cavitation can crush the silica particles which have grown up to small particles through mechanical method. The silica particles were uniformly dispersed in the solvents. These results indicated that ultrasonic is a key factor to the formation of small, uniform nanoparticles and an optimum hydrolysis-condensation period of the ultrasonic cavitation is 0.5 h. Fig. 4 shows TEM micrographs of silica nanoparticles synthesized by the oven drying and azeotropic distillation technique. Both particles exhibit the same average size of 10 nm only in the case of oven drying the particles are aggregated. It can been seen that the azeotropic distillation evidently restrains agglomerating. The small size and good dispersion are ascribed to the azeotropic distillation process. The purpose of this method is to remove the water contained in the wet colloid in the form of azeotropic substance, so it can prevent the formation of hard agglomerates during the upcoming processes of drying and calcinations. Usually, in wet colloid, the excess water molecules interact with the free hydroxyls on the surface of the colloid particles through hydrogen bonds. When particles get close, this molecules will draw neighboring particles together. These bridging water molecules can be removed when the colloid begins to dry and the hydrogen bonds between hydroxyls on the surface of two neighboring particles will draw them closer. Further drying process will cause the formation of strong chemical bonds between neighboring particles, and then the hard agglomerates appears: Si–OH þ HO–Si→Si–O–Si þ H2 O:
surface of the particles were replaced by –OC4H9 group (Fig. 5). Then the possibility for the formation of chemical bonds was greatly eliminated. Possible mechanism of azeotropic distillation can be explained by the following facts. (1), the excess n-butanol molecules can not draw particles together through the formation of hydrogen bonds. (2), during the following solvent removal process, no hydrogen bond can be formed among neighboring particles because the hydroxyl groups on the particle surface are replaced by butoxy ones. (3), the butoxy group has steric hindrance that can prevent the approach of particles. As a result, the azeotropic distillation dramatically reduces the possibility of the formation of chemical bonds and prevents the formation of hard agglomerates. Thermogravimetric-differential thermal analysis curves of the samples were recorded on a Rigaku TG-8120 instrument at a heating rate of 5 °C min− 1 and using α-alumina as the standard materials. TGDTA curves of the silica nanoparticles synthesized by the oven drying and azeotropic distillation technique are shown in Fig. 6. In the case of the oven drying technique (Fig. 6-a), the weight loss is complete within two steps and a resultant weight loss of 20.4% is found and beyond 500 °C, there was particularly no weight loss. In the DTA curve,
ð3Þ
However, after the azeotropic distillation process, the excess water molecules in the colloid were removed and the hydroxyl groups on the
Fig. 3. Schematic representation of the proposed mechanism for silica nanoparticles formation using ultrasonic.
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G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
Fig. 6. TG-DTA curves of the silica nanoparticles synthesized by the oven drying (a) and azeotropic distillation technique (b). Fig. 4. TEM micrographs of silica nanoparticles synthesized by the oven drying (a) and azeotropic distillation technique (b).
an obvious endothermal peak was noticed at around 80–110 °C, which is due to the evaporation of water adsorbed on the powder surface, and about 9.6% mass loss is observed in the TG curve correspondingly. The exothermal peak around 280 °C shows that surfactant is decomposed, and about 10.8% mass loss is observed in the TG curve correspondingly [23]. The observed TGA curve for the azeotropic distillation technique prepared powders (Fig. 6-b) shows only one step weight loss and a resultant weight loss of 17.50% is found. The weight loss is complete 200–450 °C. A sharp exothermal peak around
280 °C shows that surfactant is decomposed, the exothermal peaks around 360 °C and 390 °C show that n-butanol are carbonized and burned, respectively. No obvious endothermal peak and little mass loss are observed before 210 °C. This can be attributed to the removal of the adsorbing water after n-butanol azeotropic distillation. 3.1. FT-IR spectroscopy studies Infrared spectra of the samples were recorded on a Shimadzu FT-IR 8200PC Fourier transform infrared spectrophotometer by KBr disk method. The FT-IR spectra of the silica nanoparticles dried at 100 °C is
Fig. 5. Possible mechanism of azeotropic distillation.
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
51
References
Fig. 7. FT-IR spectra of the nanoparticles.
shown in Fig. 7. The peaks at 1093, 788, and 466 cm− 1 are due to the asymmetric, symmetric and the bending modes of SiO2 respectively [24]. The broad absorption band at 3440 cm− 1 and the peak at 1645 cm− 1 for the sample are due to the –OH groups. The absorption bands observed at 3132 and 1402 cm− 1 are due to stretching and bending of C–H bonds. The FT-IR spectra shows C–H peaks 3132 and 1402 cm− 1, clearly indicating the organic modification of the nanoparticles surface. 4. Conclusions A new synthetic method for spherical silica nanoparticles using the OSA as the silica source and the PEG as surfactant via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation is developed. This method is a simple and effective route for preparing ultrafine powders on a nanometer scale and with homogeneous particle size distribution. The specific surface area is as high as 697 m2/g. It leads to the low cost production of silica nanoparticles for practical applications. Furthermore, it provides a new way to solve the problem of OSA pollution. Acknowledgements This work was supported by foundation from the scientific research program No: 20051015, Development Program of China (863 Program, Grant 2007AA06Z202) and No: 20070405.
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Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Preparation and properties of silica nanoparticles from oil shale ash Gui-Mei Gao a, Hai-Feng Zou a, Shu-Cai Gan a,⁎, Zhao-Jun Liu b, Bai-Chao An a, Ji-Jing Xu a, Guang-Huan Li a a b
College of Chemistry, Jilin University, 6 Ximinzhu street, Changchun 130026, PR China College of Geoscience, Jilin University, 6 Ximinzhu street, Changchun 130026, PR China
a r t i c l e
i n f o
Article history: Received 10 June 2008 Received in revised form 7 September 2008 Accepted 16 September 2008 Available online 20 September 2008 Keywords: Oil shale ash Azeotropic distillation Ultrasonic technique
a b s t r a c t The method of preparing spherical silica nanoparticles from the oil shale ash (OSA) via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation, was reported. The effects of ultrasonic and azeotropic distillation on the particle size and distribution have been investigated. Further, the morphology and properties of the silica particles were examined. The X-ray fluorescence spectroscopy (XRF) and Brunauer Emmett Teller (BET) analysis confirmed that the powders consist of silica nanoparticles with high purity, 99.90% and specific surface area of 697 m2/g. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis indicate that the silica nanoparticles are amorphous and that the surface of silica nanoparticles is modified by the organics. The transmission electron microscopy (TEM) images of the sample show that good dispersion and uniform silica particles with an average diameter of about 10 nm are prepared. The results obtained in the mentioned method prove that the oil shale ash (OSA) can be used for production of silica nanoparticles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction A by-product of oil shale processing is ash, which is considered a serious environmental problem. Therefore, there is a need for a proper strategy for ash handling, disposal and utilization. Recent studies have demonstrated that the OSA can be used for production of cement and concrete [1–3], sorbents [4–8], tobermorites [9–11], soil treatment for agricultural purpose [12], while the large portion is dumped in landfills. It is found that the OSA is mixture of inorganic and organic components, whose major chemical compositions are silicon dioxide (SiO2) and alumina (Al2O3) etc., and whose major mineral compositions are quartz (SiO2), illite [K(Al,Fe)2AlSi3O10(OH)2·H2O], kaolinite [Al4(SiO10)(OH)6], chlorite [(Mg,Fe)5(Al,Si)5O10(OH)8] and calcite [CaCO3] [5]. So, an alternative manner is the conversion of this ash into a high-grade silica product, which is considered an environmental friendly product. However, there is little available information about extraction of silica from OSA. Ultrafine silica (SiO2) powder has considerable potential for a wide range application including catalysts, adsorbents, light-weight structural materials, humidity sensors, colloidal damper in the field of mechanical engineering, and other fine precision equipment [13–15]. However, the large scale commercial production of silica nanoparticles has been limited for mainly two reasons: (i) the use of high-cost alkoxides [tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS)] in the sol–gel or W/O emulsion synthesis, and (ii) nanoparticles ⁎ Corresponding author. Tel.: +86 431 87652736. E-mail address: [email protected] (S.-C. Gan). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.09.006
aggregating into large particles during dryness. An important topic in silica nanoparticles research area is to explore cheap raw material for silica nanoparticles production and to find a conformable drying method. Sonochemistry arises from acoustic cavitation phenomenon, that is the formation, growth and implosive collapse of bubbles in a liquid medium [16]. The extremely high temperatures (N5000 K) and pressure (N20 Mpa) and very high cooling rates (N107 K s− 1) attained during acoustic cavitation lead to many unique properties in the irradiated solution. This method is a simple and effective route for preparing ultrafine powders on a nanometer scale and with homogeneous particle size distribution [17,18]. In the recent years many kinds of nanomaterials have been prepared by this method, such as ZnO, MnO, SBA-15 [19–21]. Incidentally, so far, no attempt has been made to prepare silica nanoparticles through sonochemical route. In the present work, the spherical silica nanoparticles were synthesized using OSA as silica source via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation. The effects of ultrasonic and azeotropic distillation on the particle size and distribution were investigated. 2. Experiment 2.1. Materials and reagents The OSA was used as silica source for synthesis of silica nanoparticles. It was taken from oil retorting factory of Jilin province. The chemical compositions of the OSA are shown in Table 1, and it indicates that the OSA consists of a wide variety of acidic, basic,
48
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
Table 1 Chemical compositions of the OSA before and after the pretreatment and the silica powders Component Raw oil shale ash (wt.%)
Calcined and leached oil shale ash (wt.%)
Silica powders (wt.%)
SiO2 Al2O3 TFe2O3 P2O5 CaO MgO TiO2 Na2O K2O MnO2 LOI
86.8561 8.1541 1.8061 0.3163 0.0781 0.3032 0.7010 0.4501 0.6511 0.0381 0.6458
99.9034 0.0128 0.0084 0.0155 0.0049 0.0083 0.0107 0.0111 0.0054 0.0010 0.0035
64.7232 15.8514 8.6109 0.5117 0.9907 1.8221 1.0411 0.7046 1.1733 0.1701 3.4237
amphoteric oxides and incorporated hydrocarbons. All chemical reagents were analytical grade supplied by Beijing Chemical Reagent Research Institute. 2.2. Experimental procedure 2.2.1. Pretreatment of the OSA The pretreatment of the OSA consisted of the mechanical, thermal, acid treatments. The OSA was crushed to about 0.1 mm, and burned in a muffle furnace at 550 °C for 2 h to remove all incorporated hydrocarbons. An acid washing step was used to remove the small quantities of minerals prior to silica extraction from OSA in the following manner. The calcined OSA (10 g) was acid-leached with 30 wt.% sulfuric acid solution (50 ml) at 100 °C for 2 h in a pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle. Then the slurry was filtered and washed with distilled water for several times until the pH value equaled 7. 2.2.2. Preparation of sodium silicate solution 30 wt.% sodium hydroxide solution (50 ml) was added to the pretreated OSA and boiled for 5 h in a pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle to dissolve the silica, and to produce a sodium silicate solution. The solution was filtered and washed with boiling distilled water. The filtrate and washing were allowed to cool to room temperature. 2.2.3. Synthesis of silica nanoparticles Firstly, 10 g of polyethylene glycol was dissolved in the water (90 ml). subsequently, OSA derived sodium silicate was slowly added to the polyethylene glycol water solution that was being sonicated at 50 °C. Then, 0.5 mol/L sulfuric acid solution was added gradually into the solution in order to initiate the hydrolysis-condensation reaction and sonication was continued 0.5 h. The sol was adjusted until the pH value equaled 4. The resulting gel mixture was aged at 50 °C for more than 8 h. The silica gel aged was filtered and washed by distilled water for several times. The filtration cake was distilled with n-butanol, and then calcined at 550 °C for 2 h in atmospheric condition to remove the surfactant. The silica nanoparticles were obtained finally.
(SiO2) in the OSA is not attacked by H2SO4 solution in the acid treatment. The silica nanoparticles are obtained by the following reactions: NaOH þ SiO2 →Na2 SiO3 þ H2 O
ð1Þ
Na2 SiO3 þ H2 SO4 →SiO2 þ Na2 SO4 þ H2 O:
ð2Þ
The sodium sulfate (Na2SO4) in the silica gel can be washed out easily due to its high solubility in water. Table 1 shows that the contents of the extraneous elements are reduced to a low level during the preparation process. The purity of silica nanoparticles is higher than 99.90%. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku Rotaflex diffractometer equipped with a rotating anode using Cu Kα radiation. The X-ray diffraction scanning angle is from 0° to 80° and rate is 0.01°/s. Fig.1 shows the XRD pattern of the calcined silica particles. The typical silica characteristic is observed at a broad peak centered at 2θ = 22.5°, which indicates the sample is amorphous [22]. The result is accord with selected area electron diffraction. BET analysis was used to determine the total specific surface of silica nanoparticles (Sample was dried at 120 °C for 12 h). BET data indicates that the specific surface area of silica nanoparticles prepared by this method is as high as 697 m2/g. In order to investigate the action of ultrasonic cavitation during the formation of silica particles another experiment was performed at the same conditions except for ultrasonic irradiation. To find the effect of the sonication time continued, the experiments were performed varying the sonication time continued at a rang 0.5–1.5 h. Transmission electron microscopy images of silica nanoparticles were obtained with a JEM-2000EX microscope. Silica nanoparticles were dispersed ultrasonically in ethanol, and a drop of suspension was deposited on a carbon coated copper grid. TEM micrographs of silica nanoparticles obtained from the rigorous magnetic stirring (a), and the ultrasonic technique at hydrolysis-condensation periods of 0.5 h (b), 1 h (c) and 1.5 h (d) were exhibited in Fig. 2. Among them, the smallest spherical silica particle size with narrow uniform distribution can be observed in Fig. 2-b, and the average size of particles is about 10 nm. Obviously, the average particle size decreases from 60 nm to 5 nm and the powders are less agglomerated when the rigorous magnetic stirring is replaced by ultrasonic cavitation. However, when ultrasonic periods were increased, serious aggregate silica particles with the larger particle size are obtained (Fig. 2-c,d). Schematic representation of the proposed mechanism for silica nanoparticles formation using ultrasonic is shown in Fig. 3. Comparing with traditional stirring, ultrasonic cavitation is easily achieving microscopic uniform mixing, eliminating partial uneven concentration,
3. Results and discussion The chemical compositions of the OSA before and after the pretreatment and the silica powders were determined by X-ray fluorescence spectroscopy, and the results are presented in Table 1. Unlike conventional organic silicon compounds, the OSA is a industrial waste, which contains several main extraneous components. The thermal and acid treatments are efficient, resulting in a material with a high reduction in the Fe2O3, Al2O3, CaO and MgO content. The silica
Fig. 1. XRD pattern of the silica nanoparticles.
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
49
Fig. 2. TEM micrographs of silica nanoparticles obtained from the rigorous magnetic stirring (a); and the ultrasonic technique at hydrolysis-condensation periods of 0.5 h (b), 1 h (c) and 1.5 h (d).
stimulating formation of new phase, and crushing the aggregates. It can be seen from Fig. 3, the silica particles of new generation were possibly surrounded by solvents and residual surfactants, and easily agglomerate into a large particles. Ultrasonic cavitation can crush the silica particles which have grown up to small particles through mechanical method. The silica particles were uniformly dispersed in the solvents. These results indicated that ultrasonic is a key factor to the formation of small, uniform nanoparticles and an optimum hydrolysis-condensation period of the ultrasonic cavitation is 0.5 h. Fig. 4 shows TEM micrographs of silica nanoparticles synthesized by the oven drying and azeotropic distillation technique. Both particles exhibit the same average size of 10 nm only in the case of oven drying the particles are aggregated. It can been seen that the azeotropic distillation evidently restrains agglomerating. The small size and good dispersion are ascribed to the azeotropic distillation process. The purpose of this method is to remove the water contained in the wet colloid in the form of azeotropic substance, so it can prevent the formation of hard agglomerates during the upcoming processes of drying and calcinations. Usually, in wet colloid, the excess water molecules interact with the free hydroxyls on the surface of the colloid particles through hydrogen bonds. When particles get close, this molecules will draw neighboring particles together. These bridging water molecules can be removed when the colloid begins to dry and the hydrogen bonds between hydroxyls on the surface of two neighboring particles will draw them closer. Further drying process will cause the formation of strong chemical bonds between neighboring particles, and then the hard agglomerates appears: Si–OH þ HO–Si→Si–O–Si þ H2 O:
surface of the particles were replaced by –OC4H9 group (Fig. 5). Then the possibility for the formation of chemical bonds was greatly eliminated. Possible mechanism of azeotropic distillation can be explained by the following facts. (1), the excess n-butanol molecules can not draw particles together through the formation of hydrogen bonds. (2), during the following solvent removal process, no hydrogen bond can be formed among neighboring particles because the hydroxyl groups on the particle surface are replaced by butoxy ones. (3), the butoxy group has steric hindrance that can prevent the approach of particles. As a result, the azeotropic distillation dramatically reduces the possibility of the formation of chemical bonds and prevents the formation of hard agglomerates. Thermogravimetric-differential thermal analysis curves of the samples were recorded on a Rigaku TG-8120 instrument at a heating rate of 5 °C min− 1 and using α-alumina as the standard materials. TGDTA curves of the silica nanoparticles synthesized by the oven drying and azeotropic distillation technique are shown in Fig. 6. In the case of the oven drying technique (Fig. 6-a), the weight loss is complete within two steps and a resultant weight loss of 20.4% is found and beyond 500 °C, there was particularly no weight loss. In the DTA curve,
ð3Þ
However, after the azeotropic distillation process, the excess water molecules in the colloid were removed and the hydroxyl groups on the
Fig. 3. Schematic representation of the proposed mechanism for silica nanoparticles formation using ultrasonic.
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Fig. 6. TG-DTA curves of the silica nanoparticles synthesized by the oven drying (a) and azeotropic distillation technique (b). Fig. 4. TEM micrographs of silica nanoparticles synthesized by the oven drying (a) and azeotropic distillation technique (b).
an obvious endothermal peak was noticed at around 80–110 °C, which is due to the evaporation of water adsorbed on the powder surface, and about 9.6% mass loss is observed in the TG curve correspondingly. The exothermal peak around 280 °C shows that surfactant is decomposed, and about 10.8% mass loss is observed in the TG curve correspondingly [23]. The observed TGA curve for the azeotropic distillation technique prepared powders (Fig. 6-b) shows only one step weight loss and a resultant weight loss of 17.50% is found. The weight loss is complete 200–450 °C. A sharp exothermal peak around
280 °C shows that surfactant is decomposed, the exothermal peaks around 360 °C and 390 °C show that n-butanol are carbonized and burned, respectively. No obvious endothermal peak and little mass loss are observed before 210 °C. This can be attributed to the removal of the adsorbing water after n-butanol azeotropic distillation. 3.1. FT-IR spectroscopy studies Infrared spectra of the samples were recorded on a Shimadzu FT-IR 8200PC Fourier transform infrared spectrophotometer by KBr disk method. The FT-IR spectra of the silica nanoparticles dried at 100 °C is
Fig. 5. Possible mechanism of azeotropic distillation.
G.-M. Gao et al. / Powder Technology 191 (2009) 47–51
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References
Fig. 7. FT-IR spectra of the nanoparticles.
shown in Fig. 7. The peaks at 1093, 788, and 466 cm− 1 are due to the asymmetric, symmetric and the bending modes of SiO2 respectively [24]. The broad absorption band at 3440 cm− 1 and the peak at 1645 cm− 1 for the sample are due to the –OH groups. The absorption bands observed at 3132 and 1402 cm− 1 are due to stretching and bending of C–H bonds. The FT-IR spectra shows C–H peaks 3132 and 1402 cm− 1, clearly indicating the organic modification of the nanoparticles surface. 4. Conclusions A new synthetic method for spherical silica nanoparticles using the OSA as the silica source and the PEG as surfactant via the ultrasonic technique in the hydrolysis-condensation stage followed by azeotropic distillation is developed. This method is a simple and effective route for preparing ultrafine powders on a nanometer scale and with homogeneous particle size distribution. The specific surface area is as high as 697 m2/g. It leads to the low cost production of silica nanoparticles for practical applications. Furthermore, it provides a new way to solve the problem of OSA pollution. Acknowledgements This work was supported by foundation from the scientific research program No: 20051015, Development Program of China (863 Program, Grant 2007AA06Z202) and No: 20070405.
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