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SiO2 nanocomposites obtained by impregnation of mesoporous silica
Composites Science and Technology 63 (2003) 1187–1191 www.elsevier.com/locate/compscitech
ZnO/SiO2 nanocomposites obtained by impregnation of mesoporous silica C. Cannas, M. Mainas, A. Musinu*, G. Piccaluga Dipartimento Scienze Chimiche, Cittadella Universitaria, S.S 554 Km 4.5, 09042, Monserrato (CA), Italy
Abstract The synthesis of ZnO–SiO2 nanocomposites through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions is reported. During thermal treatment the samples evolve toward the formation of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica stable up to the temperature of 700 C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution is obtained using ethanolic instead of aqueous solutions. At higher temperatures the samples evolve towards the formation of a zinc silicate phase. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: 61.43.G; 61.16.B; 61.46
1. Introduction ZnO-based nanocomposites are of considerable interest in ceramics technology for applications as varistors, sensor elements and photoluminescent materials. In particular, varistors used in high voltage applications require a small grain size in order to keep the varistor volume low, while the overall properties of varistors are greatly improved by the reduction of the zinc oxide particles to nanometer range. A potential application in the optoelectronic industry stems from the size-dependent optical properties, associated with the quantum size effect and with the existence of a high percentage of atoms at the nanoparticle surface. Several preparation methods, like sol-gel [1–3], impregnation [4,5] and molecular capping [6,7], have been proposed for the dispersion of ZnO nanoparticles in silica [1–5] or polymeric [6,7] matrices in order to avoid the tendency of nanoparticles to aggregate. This study is focused on the synthesis and characterization of nanocomposites in which ZnO nanocrystals are incorporated into commercial mesoporous silica through impregnation with zinc nitrate aqueous or ethanolic solutions, and submitted to the appropriate thermal treatments. The effect of the solvent, time and temperature of impregnation is investigated. Structural evolution of the samples towards thermal treatments * Corresponding author. E-mail address: [email protected] (A. Musinu).
and chemical modifications of the precursor deposited onto silica are studied by BET, XRD and TEM techniques.
2. Experimental Four samples were prepared by impregnating 1 g of silica (Aldrich, surface area=300 m2/g, average porosity=150 A˚) with 50 ml of aqueous or ethanolic solutions of zinc nitrate 1.6 M. Impregnation was performed in three steps in which two aliquots of 15 ml and one of 20 ml of the solution were taken into contact with silica under continuous stirring. At the end of each step a cycle of centrifugation was carried out to separate solid by the impregnation liquid. In the final cycle, solid was washed with a zinc free solution having the same pH of the final suspension that varies in dependence of the different experimental conditions. After drying at 60 C for 48 h, TG measurements were carried out on a Mettler Toledo 851 under oxygen flow (heating rate 10 K min 1). The data indicate that the weight loss is almost complete at T > 450 C, therefore the solids were submitted to five consecutive steps of calcination at the temperature of 500, 600, 700, 800 and 900 C, leaving the samples at each temperature for 1 h up to 700 C and 30 min up to 900 C. Chemical analyses, carried out with a plasma ICP Perkin Elmer 2000, are reported in Table 1, together with the experimental preparation conditions and the acronyms for the samples which will
0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00040-X
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C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Table 1 Preparation conditions and compositions of the ZnO–SiO2 compositesa Sample
Solvent
Impregnation time (h)
Temperature ( C)
ZnO (wt.%)
Z1 Z2 Z3 Z4
Water Water Water Ethanol
24 24 1 1
24 60 60 60
4 9 6 6
a
Estimated errors on compositions are within 2%.
be used in the following. Also silica reference samples were obtained with the same impregnation conditions, but using a zinc nitrate-free solution. N2 physisorption measurements were performed on a Sorpomatic 1990 System (CE Instruments) by determining the N2 ad/desorption isotherms at 77 K. Before analysis, the samples were outgassed at 400 C for 15 h. The surface area (SBET) was estimated by the BET method [8] and the pore size distribution by the Dollimore-Heal method. [9] XRD patterns were acquired by using a –2 conventional equipment (Siemens D-500) at Mo Ka wavelength. Electron micrographs were acquired using a JEOL 200CX transmission electron microscope operating at 200 KV; electron diffraction micrographs were obtained with the selected area diffraction mode (SAD).
Fig. 1. XRD patterns of the Z2 nanocomposite at the various thermal treatments.
3. Results and discussion All the four composites exhibit the same behaviour as a function of thermal treatment. This is however more clearly observable in the Z2 sample because of the highest concentration of ZnO, according to the values reported in Table 1. Therefore, in Fig. 1 the XRD patterns of the Z2 sample at drying and calcination temperatures are reported. The pattern of the dried sample exhibits a series of crystalline peaks, superimposed to the band of amorphous silica at about 2=10 , most of which can be ascribed to the ZnNO3(OH)*H2O phase (PDF CARD No. 27-1491). The evolution from this crystalline precursor to ZnO zincite phase (PDF Card No. 36-1451) promoted by the thermal treatment is evidenced in the patterns of the calcined samples from 500 to 700 C. Peak widths and intensities remain unmodified in this range of temperature, roughly suggesting a constancy of ZnO particle sizes. In the spectrum of the sample treated at 800 C, a new peak appears at about 2=11.8 , accompanied with a sharpening of the band at about 2=10 , which can be ascribed to the incipient formation of a zinc silicate phase, while the series of peaks ascribed to zincite phase slightly broaden. At 900 C the presence of zinc silicate is further confirmed by the series of peaks ascribable to b-Zn2SiO4 phase
Fig. 2. XRD patterns of the four nanocomposites treated at 700 C (zincite phase).
C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
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Fig. 3. TEM bright field micrograph (a) and pertinent SAD (b) of the Z2 nanocomposite treated at 700 C.
(PDF CARD No. 14-0653). This result implies that at T5800 C a solid state reaction between zinc oxide and silica matrix takes place. Since the temperature of 700 C represents the highest at which the ZnO–SiO2 nanocomposites are stable, a comparison among the four composites treated at this temperature is carried out. XRD patterns of the samples are reported in Fig. 2. The series of peaks ascribed to the zincite phase are present, superimposed to that of silica matrix, with a sensible variation in intensity among the various preparations, which is consistent with the different ZnO concentration. TEM bright field images of the samples treated at 700 C show the presence of some portions of still naked porous silica and other parts where the porous structure is partially covered by quite large particles (Fig. 3a). However, dark field observations show that these particles resulted to be aggregates of nanocrystals belonging to zincite phase, as confirmed by SAD reported in Fig. 3b and in agreement with XRD patterns. Moreover, dark field images show considerable amounts of particles filling the matrix pores in all the samples, which are too small (< 3 nm) to be clearly evidenced in bright field mode, but countable in the dark field micrographs as it is possible to observe in Fig. 4. Particle size distributions, calculated on about 300 particles for Z1, Z2 and Z4 samples, including the ones in the pores and those on the silica surface, are reported in Fig. 5. The sizes range from 2.5 to about 40 nm, and exhibit a maximum at 7.5 nm in the Z1 and Z4 samples, while in the Z2 sample two maxima are present respectively at 2.5 and 17.5 nm suggesting the presence of a ‘‘double’’ distribution. The other samples exhibit on the whole a rather narrow distribution, as evidenced by TEM, which appears slightly narrower and therefore more advantageous in the Z4 sample. This result can be ascribed to the lower viscosity of the ethanolic solution respect to the aqueous ones, which easily enters the
matrix pores therefore making the use of ethanol as impregnation solvent more favourable. In order to estimate the degree of surface coating and pore filling, surface area and pore diameter of Z1 and Z2 composites treated at 700 C are compared in Table 2 with the values obtained for a silica reference sample. The analysis evidences a decrease in specific surface area, going from the reference to the impregnated samples and more evident for the more concentrated Z2 sample. This suggests that the impregnation process involves larger areas of the support when the dispersing phase is in higher amount, implying that the dispersing phase is spread over the surface in a quite homogeneous way. The percentage of pore with diameter larger than 15 nm is lower in nanocomposites than in the reference sample and still this difference is higher for the more concentrated sample, suggesting that the impregnation process involves a filling of pores in the range 15–20 nm. This is consistent with the particle size distribution obtained by TEM results. In fact, this is actually a size double of that of
Fig. 4. Dark field micrograph of the Z2 nanocomposite treated at 700 C.
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C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Fig. 5. Particle size distribution of the Z1, Z2 and Z4 nanocomposites treated at 700 C. Table 2 BET results for silica reference sample (R) and Z1 and Z2 nanocomposites treated at 700 Ca Sample*
R Z1 Z2 a
Specific surface area (m2/g)
304 255 228
Rel. vol%
Pore size (mean value) (nm)
Average pore diameter >20 nm
Average pore diameter 20–15 nm
Average pore diameter 15–10 nm
Average pore diameter <10 nm
8 6 5
33 29 27
48 55 54
13 10 11
14.1 14.4 14.6
Estimated error on surface area and on pore diameter is within 3%.
the mean particles (7.5 nm), but it has to be considered that the ZnNO3(OH)*H2O particle precursor is obviously larger than the zinc oxide particle in the final material. However, though the pore size imposes constraints on the impregnation, in some of the samples, and particularly for Z2, also a rough covering of the silica surface takes place. In this last sample, the peculiar process parameters of the preparation allow to obtain composites with high ZnO content, but strongly affect the spread of the size distribution. In fact, while the size of the nanoparticles encapsulated in the pores remains almost constant, the long time and high temperature of impregnation cause an inhomogeneous growth of the particles dispersed over the surface.
4. Conclusions ZnO–SiO2 nanocomposites were synthesized through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions. The samples treated at 700 C exhibit the presence of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica at 700 C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution of particles with mean sizes of about 7.5 nm is obtained using ethanolic instead of aqueous solutions. At higher temperatures the system evolves towards the formation of a zinc silicate phase.
C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Acknowledgements The financial support of MIUR, CNR PF MSTA is gratefully acknowledged.
[4]
[5]
References [6] [1] Cannas C, Casu M, Musinu A, Lai A, Piccaluga G. XRD, TEM and 29Si MAS NMR study of sol-gel ZnO/SiO2 nanocomposites. J Mater Chem 1999;9:1765. [2] Lu S, Zhang L, Yao X. Structure and properties of ZnS/SiO2 nanocomposites. Chin Sci Bull 1996;41:1923. [3] Lorenz C, Emmerling A, Fricke J, Schmidt T, Hilgendorff M, Spanhel L, Muller G. Aerogels containing strongly photo-
[7]
[8] [9]
1191
luminescing zinc oxide nanocrystals. J Non-Cryst Solids 1998: 238. Zhang WH, Shi JL, Wang LZ, Yan DS. Preparation and characterization of ZnO clusters inside mesoporous silica. Chem Mater 2000;12:1408. Khouchaf L, Tullier MH, Wark M, Paillaud JJ, Soulard M. Structure of zinc oxide particles in anopores of microporous material. J Physique IV 1997;7:C-267. Oner M, Norwig J, Meyer WH, Wegner G. Control of ZnO crystallization by PEO-b-PMAA dibloch copolymer. Chem Mater 1998;10:460. Guo L, Yang S, Yang C, Yu P, Wang J, Ge W, et al. Synthesis and characterization of poly(vinylpyrrolidone)-modified zinc oxide nanoparticles. Chem Mater 2000;12:2268. Brunauer S, Emmet LS, Teller E, Amer J. Chem Soc 1938;60:309. Dollimore D, Heal GR. J Appl Chem 1964;14:109.
ZnO/SiO2 nanocomposites obtained by impregnation of mesoporous silica C. Cannas, M. Mainas, A. Musinu*, G. Piccaluga Dipartimento Scienze Chimiche, Cittadella Universitaria, S.S 554 Km 4.5, 09042, Monserrato (CA), Italy
Abstract The synthesis of ZnO–SiO2 nanocomposites through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions is reported. During thermal treatment the samples evolve toward the formation of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica stable up to the temperature of 700 C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution is obtained using ethanolic instead of aqueous solutions. At higher temperatures the samples evolve towards the formation of a zinc silicate phase. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: 61.43.G; 61.16.B; 61.46
1. Introduction ZnO-based nanocomposites are of considerable interest in ceramics technology for applications as varistors, sensor elements and photoluminescent materials. In particular, varistors used in high voltage applications require a small grain size in order to keep the varistor volume low, while the overall properties of varistors are greatly improved by the reduction of the zinc oxide particles to nanometer range. A potential application in the optoelectronic industry stems from the size-dependent optical properties, associated with the quantum size effect and with the existence of a high percentage of atoms at the nanoparticle surface. Several preparation methods, like sol-gel [1–3], impregnation [4,5] and molecular capping [6,7], have been proposed for the dispersion of ZnO nanoparticles in silica [1–5] or polymeric [6,7] matrices in order to avoid the tendency of nanoparticles to aggregate. This study is focused on the synthesis and characterization of nanocomposites in which ZnO nanocrystals are incorporated into commercial mesoporous silica through impregnation with zinc nitrate aqueous or ethanolic solutions, and submitted to the appropriate thermal treatments. The effect of the solvent, time and temperature of impregnation is investigated. Structural evolution of the samples towards thermal treatments * Corresponding author. E-mail address: [email protected] (A. Musinu).
and chemical modifications of the precursor deposited onto silica are studied by BET, XRD and TEM techniques.
2. Experimental Four samples were prepared by impregnating 1 g of silica (Aldrich, surface area=300 m2/g, average porosity=150 A˚) with 50 ml of aqueous or ethanolic solutions of zinc nitrate 1.6 M. Impregnation was performed in three steps in which two aliquots of 15 ml and one of 20 ml of the solution were taken into contact with silica under continuous stirring. At the end of each step a cycle of centrifugation was carried out to separate solid by the impregnation liquid. In the final cycle, solid was washed with a zinc free solution having the same pH of the final suspension that varies in dependence of the different experimental conditions. After drying at 60 C for 48 h, TG measurements were carried out on a Mettler Toledo 851 under oxygen flow (heating rate 10 K min 1). The data indicate that the weight loss is almost complete at T > 450 C, therefore the solids were submitted to five consecutive steps of calcination at the temperature of 500, 600, 700, 800 and 900 C, leaving the samples at each temperature for 1 h up to 700 C and 30 min up to 900 C. Chemical analyses, carried out with a plasma ICP Perkin Elmer 2000, are reported in Table 1, together with the experimental preparation conditions and the acronyms for the samples which will
0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00040-X
1188
C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Table 1 Preparation conditions and compositions of the ZnO–SiO2 compositesa Sample
Solvent
Impregnation time (h)
Temperature ( C)
ZnO (wt.%)
Z1 Z2 Z3 Z4
Water Water Water Ethanol
24 24 1 1
24 60 60 60
4 9 6 6
a
Estimated errors on compositions are within 2%.
be used in the following. Also silica reference samples were obtained with the same impregnation conditions, but using a zinc nitrate-free solution. N2 physisorption measurements were performed on a Sorpomatic 1990 System (CE Instruments) by determining the N2 ad/desorption isotherms at 77 K. Before analysis, the samples were outgassed at 400 C for 15 h. The surface area (SBET) was estimated by the BET method [8] and the pore size distribution by the Dollimore-Heal method. [9] XRD patterns were acquired by using a –2 conventional equipment (Siemens D-500) at Mo Ka wavelength. Electron micrographs were acquired using a JEOL 200CX transmission electron microscope operating at 200 KV; electron diffraction micrographs were obtained with the selected area diffraction mode (SAD).
Fig. 1. XRD patterns of the Z2 nanocomposite at the various thermal treatments.
3. Results and discussion All the four composites exhibit the same behaviour as a function of thermal treatment. This is however more clearly observable in the Z2 sample because of the highest concentration of ZnO, according to the values reported in Table 1. Therefore, in Fig. 1 the XRD patterns of the Z2 sample at drying and calcination temperatures are reported. The pattern of the dried sample exhibits a series of crystalline peaks, superimposed to the band of amorphous silica at about 2=10 , most of which can be ascribed to the ZnNO3(OH)*H2O phase (PDF CARD No. 27-1491). The evolution from this crystalline precursor to ZnO zincite phase (PDF Card No. 36-1451) promoted by the thermal treatment is evidenced in the patterns of the calcined samples from 500 to 700 C. Peak widths and intensities remain unmodified in this range of temperature, roughly suggesting a constancy of ZnO particle sizes. In the spectrum of the sample treated at 800 C, a new peak appears at about 2=11.8 , accompanied with a sharpening of the band at about 2=10 , which can be ascribed to the incipient formation of a zinc silicate phase, while the series of peaks ascribed to zincite phase slightly broaden. At 900 C the presence of zinc silicate is further confirmed by the series of peaks ascribable to b-Zn2SiO4 phase
Fig. 2. XRD patterns of the four nanocomposites treated at 700 C (zincite phase).
C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
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Fig. 3. TEM bright field micrograph (a) and pertinent SAD (b) of the Z2 nanocomposite treated at 700 C.
(PDF CARD No. 14-0653). This result implies that at T5800 C a solid state reaction between zinc oxide and silica matrix takes place. Since the temperature of 700 C represents the highest at which the ZnO–SiO2 nanocomposites are stable, a comparison among the four composites treated at this temperature is carried out. XRD patterns of the samples are reported in Fig. 2. The series of peaks ascribed to the zincite phase are present, superimposed to that of silica matrix, with a sensible variation in intensity among the various preparations, which is consistent with the different ZnO concentration. TEM bright field images of the samples treated at 700 C show the presence of some portions of still naked porous silica and other parts where the porous structure is partially covered by quite large particles (Fig. 3a). However, dark field observations show that these particles resulted to be aggregates of nanocrystals belonging to zincite phase, as confirmed by SAD reported in Fig. 3b and in agreement with XRD patterns. Moreover, dark field images show considerable amounts of particles filling the matrix pores in all the samples, which are too small (< 3 nm) to be clearly evidenced in bright field mode, but countable in the dark field micrographs as it is possible to observe in Fig. 4. Particle size distributions, calculated on about 300 particles for Z1, Z2 and Z4 samples, including the ones in the pores and those on the silica surface, are reported in Fig. 5. The sizes range from 2.5 to about 40 nm, and exhibit a maximum at 7.5 nm in the Z1 and Z4 samples, while in the Z2 sample two maxima are present respectively at 2.5 and 17.5 nm suggesting the presence of a ‘‘double’’ distribution. The other samples exhibit on the whole a rather narrow distribution, as evidenced by TEM, which appears slightly narrower and therefore more advantageous in the Z4 sample. This result can be ascribed to the lower viscosity of the ethanolic solution respect to the aqueous ones, which easily enters the
matrix pores therefore making the use of ethanol as impregnation solvent more favourable. In order to estimate the degree of surface coating and pore filling, surface area and pore diameter of Z1 and Z2 composites treated at 700 C are compared in Table 2 with the values obtained for a silica reference sample. The analysis evidences a decrease in specific surface area, going from the reference to the impregnated samples and more evident for the more concentrated Z2 sample. This suggests that the impregnation process involves larger areas of the support when the dispersing phase is in higher amount, implying that the dispersing phase is spread over the surface in a quite homogeneous way. The percentage of pore with diameter larger than 15 nm is lower in nanocomposites than in the reference sample and still this difference is higher for the more concentrated sample, suggesting that the impregnation process involves a filling of pores in the range 15–20 nm. This is consistent with the particle size distribution obtained by TEM results. In fact, this is actually a size double of that of
Fig. 4. Dark field micrograph of the Z2 nanocomposite treated at 700 C.
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C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Fig. 5. Particle size distribution of the Z1, Z2 and Z4 nanocomposites treated at 700 C. Table 2 BET results for silica reference sample (R) and Z1 and Z2 nanocomposites treated at 700 Ca Sample*
R Z1 Z2 a
Specific surface area (m2/g)
304 255 228
Rel. vol%
Pore size (mean value) (nm)
Average pore diameter >20 nm
Average pore diameter 20–15 nm
Average pore diameter 15–10 nm
Average pore diameter <10 nm
8 6 5
33 29 27
48 55 54
13 10 11
14.1 14.4 14.6
Estimated error on surface area and on pore diameter is within 3%.
the mean particles (7.5 nm), but it has to be considered that the ZnNO3(OH)*H2O particle precursor is obviously larger than the zinc oxide particle in the final material. However, though the pore size imposes constraints on the impregnation, in some of the samples, and particularly for Z2, also a rough covering of the silica surface takes place. In this last sample, the peculiar process parameters of the preparation allow to obtain composites with high ZnO content, but strongly affect the spread of the size distribution. In fact, while the size of the nanoparticles encapsulated in the pores remains almost constant, the long time and high temperature of impregnation cause an inhomogeneous growth of the particles dispersed over the surface.
4. Conclusions ZnO–SiO2 nanocomposites were synthesized through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions. The samples treated at 700 C exhibit the presence of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica at 700 C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution of particles with mean sizes of about 7.5 nm is obtained using ethanolic instead of aqueous solutions. At higher temperatures the system evolves towards the formation of a zinc silicate phase.
C. Cannas et al. / Composites Science and Technology 63 (2003) 1187–1191
Acknowledgements The financial support of MIUR, CNR PF MSTA is gratefully acknowledged.
[4]
[5]
References [6] [1] Cannas C, Casu M, Musinu A, Lai A, Piccaluga G. XRD, TEM and 29Si MAS NMR study of sol-gel ZnO/SiO2 nanocomposites. J Mater Chem 1999;9:1765. [2] Lu S, Zhang L, Yao X. Structure and properties of ZnS/SiO2 nanocomposites. Chin Sci Bull 1996;41:1923. [3] Lorenz C, Emmerling A, Fricke J, Schmidt T, Hilgendorff M, Spanhel L, Muller G. Aerogels containing strongly photo-
[7]
[8] [9]
1191
luminescing zinc oxide nanocrystals. J Non-Cryst Solids 1998: 238. Zhang WH, Shi JL, Wang LZ, Yan DS. Preparation and characterization of ZnO clusters inside mesoporous silica. Chem Mater 2000;12:1408. Khouchaf L, Tullier MH, Wark M, Paillaud JJ, Soulard M. Structure of zinc oxide particles in anopores of microporous material. J Physique IV 1997;7:C-267. Oner M, Norwig J, Meyer WH, Wegner G. Control of ZnO crystallization by PEO-b-PMAA dibloch copolymer. Chem Mater 1998;10:460. Guo L, Yang S, Yang C, Yu P, Wang J, Ge W, et al. Synthesis and characterization of poly(vinylpyrrolidone)-modified zinc oxide nanoparticles. Chem Mater 2000;12:2268. Brunauer S, Emmet LS, Teller E, Amer J. Chem Soc 1938;60:309. Dollimore D, Heal GR. J Appl Chem 1964;14:109.