- Email: [email protected]
SiO2 nanocomposites emitting specific luminescence colors
Optical Materials 26 (2004) 95–100 www.elsevier.com/locate/optmat
Synthesis of ZnO/SiO2 nanocomposites emitting specific luminescence colors Mikrajuddin Abdullah
a,1
, Shinji Shibamoto b, Kikuo Okuyama
a,*
a
b
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan Hiroshima Joint Research Center for Nanotechnology Particle Project, Japan Chemical Innovation Institute, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Received 17 September 2003; received in revised form 9 January 2004; accepted 11 January 2004 Available online 2 March 2004
Abstract Composites of zinc oxide (ZnO) nanoparticles in silica matrix can be produced by initially synthesizing a ZnO colloid (containing ZnO nanoparticles in ethanol solution), mixed with tetraethoxysilane (TEOS), followed by spray drying, to produce powder comprised of submicrometer sized particles. The initial size of ZnO nanoparticles in the colloid is about 3 nm when the precursors are first mixed, but the size increases with aging, following an approximate equation D / ta , a ¼ constant [J. Phys. Chem. B 102 (1998) 2854]. By using ZnO colloids that have been aged for different times, composites containing a specific size of ZnO can be produced. Since the excitation and emission luminescence spectral positions of ZnO are dependent on particle size (shift to blue region with reducing particle size), composites that emit a specific color can be produced using this method. We have been able to produce composites that emit colors from blue (460 nm) up to yellow-green (550 nm). 2004 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is frequently used as a reinforcing filler for elastomers and is used in pharmaceuticals, cosmetics, raw materials for varistor, phosphor and ferrite, catalyst, sensors, etc. [1]. Doping the ZnO with In, Al, B, Ga, or F results in the production of transparent conductors [2–7], which are important in photovoltaic applications. Doping with rare earth ions, such as erbium, results in high optical gain amplifiers [8]. ZnO emits a broad luminescence emission in the green-yellow region, and, as a result, it is a potential material for use in white light sources. When the particle size is reduced below 6 nm, a shift in the emission spectrum to shorter wavelengths due to the quantum confinement of electrons and holes in the particles occurs. The peak for the emission spectrum is located in *
Corresponding author. Tel.: +81-824-24-7716; fax: +81-824-247850. E-mail address: [email protected] (K. Okuyama). 1 Permanent address: Department of Physics, Bandung Institute of Technology, Jl. Genaca 10 Bandung 40132, Indonesia. 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.01.006
the blue region for particle sizes smaller than about 3 nm. Very large ZnO particles show an emission spectrum that peaks at a wavelength of around 550 nm, i.e., a green-yellow color. If ZnO nanoparticles could be produced in a range of sizes from about 3 nm up to around 10 nm, a variety of emission peaks could be produced. One simple method for producing ZnO nanoparticles is a hydrolysis method, first described by Spanhel and Anderson [9]. In this method, a ZnO colloid is produced in an ethanol solution with an initial particle size (just after mixing the precursors) as small as 3 nm. The particle size then continues to grow, or age, after synthesis, even if it is stored at 0 C. After about 5 days of aging, the particle size reaches a size of about 5.5 nm. Monticone et al., reported that the time dependence of ZnO particle size produced by this method can be fitted to the expression D ¼ 2Ata , with D the particle diameter (nm), A ¼ 2:6 0:3 nm daya , and a ¼ 0:33 0:03, and t, the time in days [10]. The visible luminescence on ZnO is also dependent on particle size. Spanhel and Anderson observed that the luminescence peak of about 500 nm for fresh colloid shifted to 560 nm after 5 days of aging
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[9]. The shift in the luminescence peak resulted in the change of the energy band gap opening due to the change in crystalline size, which can be roughly predicted using the Brush equation [11], Eg ðRÞ ¼ Eg ð1Þ þ h2 =8lR2 1:8e2 =4pje0 R. Here, R denotes the particle radius, Eg ð1Þ the band gap of bulk ZnO, l the reduced mass, satisfying 1=l ¼ 1=me þ 1=mh with me and mh the effective masses of electron and hole, respectively, me the free mass of electron, and j the dielectric constant. It has been proposed that, the visible luminescence is caused by the breakage of excitons and an electron then jumps to a state located near the center of the gap [12]. The increase in band gap by reduction in the particle size, shifts the position of the luminescence spectra to shorter wavelengths. If the time evolution of the luminescence spectra of a ZnO colloid prepared by Spanhel and Anderson’s method is observed, the fresh colloid emits a nearly blue color, changing to green, and finally reaching a yellow-green color. This definitive change is very interesting. If the size of particles in a ZnO colloid could be stopped at various aging times, ZnO particles that produce a variety of colors, spreading from nearly blue to nearly yellow, could be produced. The objective of this work was to prepare ZnO composites emitting various colors, using a simple method. After t time aging of a ZnO colloid prepared by the Spanhel and Anderson method [9], a sample coated with silica. Initially, the ZnO colloid and silica precursor were mixed and then quickly spray dried to obtain particle composite of amorphous silica containing nanoparticles of ZnO. Depending on the aging time of the colloid, the color of the powder can be varied. The synthesis of such ZnO/SiO2 composites has also been reported by us previously [13] using a ZnO colloid and a colloid of silica nanoparticles as precursors and Tani et al. using a flame spray pyrolysis method [14]. In our previous work [13], no report on the control of the color of the ZnO composite was presented. In the Tani et al. work [14], no report on the luminescence spectra of the sample was presented.
ture of around 0 C followed by extensive stirring for several minutes. Fig. 1 shows the experimental setup used to produce the ZnO/SiO2 nanocomposites. It consists of three sections: (i) a spray generator, (ii) a vertical tubular furnace, and (iii) a particle collector. As the precursors for producing a submicron size of silica particles containing nanoparticles of ZnO, 100 ml of ZnO colloid, aged for different times, was mixed with 1 or 2 g of tetraethoxysilane (TEOS, Kanto Chemicals, Japan). The spray generator used was an ultrasonic spray nebulizer with a 1.75 MHz resonator (Omron Corp.). The droplets were carried by a stream of nitrogen gas into the tubular reactor. The reactor size has a diameter of 13 mm and a length of 600 mm, and was divided into three arbitrary temperature controllable zones (each 200 mm length). The setting temperatures from the inlet were 250, 450, and 450 C. The prepared particles were collected in an electrostatic precipitator fixed at a DC voltage of )7 kV, and maintained at 150 C to prevent the condensation of water. The morphology of the prepared particles was observed using a Hitachi S-5000 field emission scanning electron microscope (FE-SEM, Hitachi Ltd.). TEM
2. Experimental Initially we prepared a ZnO colloid using the standard Spanhel and Anderson method [9]. Zn(CH3 COO)2 Æ 2H2 O (Wako Pure Chemicals, Japan) 0.1 M in 200 ml of ethanol was distilled at 80 C for approximately 3 h to produce 80 ml of a hygroscopic solution and 120 ml of unused distillate. LiOH Æ H2 O (Wako Pure Chemicals, Japan) was dissolved in 120 ml ethanol to give concentration of 0.23 M. An extensive mixing process (usually longer than 1 h) was performed since the LiOH Æ H2 O powder is difficult to dissolve in ethanol [15,16]. The hygroscopic solution, 80 ml, and 120 ml of the LiOH solution were mixed at a tempera-
Fig. 1. Schematic of the spray drier reactor used to produce ZnO/SiO2 particle composites using ZnO colloid and TEOS as precursors.
M. Abdullah et al. / Optical Materials 26 (2004) 95–100
images were recorded using a JEOL JEM-2010 instrument. XRD patterns were detected using a Rigaku Denki RINT2000 instrument, Cu Ka1 source, and excitation and luminescence spectra were obtained using a Shimazu RF-5300PC spectrophotometer equipped with a Xe source.
3. Results Fig. 2 shows SEM pictures of samples that had been spray dried from a zero-aged ZnO colloid: (a) prepared without the addition of TEOS and (b) with the addition of 1 g TEOS. The particle sizes were mostly below 1 lm. The variation in the particle size was caused by variations in the droplet size generated by the atomizer. The particle shape was nearly spherical, both for the sample containing ZnO nanoparticles only or the composite of ZnO nanoparticles/silica.
Fig. 2. SEM pictures of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without the addition of TEOS and (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS. Insets are enlarged views of selected particles.
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Fig. 3 shows a TEM image of a sample prepared using a zero-hour-aged ZnO colloid that had been mixed with 1 g TEOS. The picture size is 80 · 80 nm2 . The composite contains ZnO nanoparticles around 3 nm in size (an example is indicated by an arrow). XRD patterns of samples prepared using a zerohour-aged ZnO colloid without TEOS (a) and zerohour-aged ZnO colloid that had been mixed with 1 g TEOS (b) are shown in Fig. 4. The familiar peaks corresponding to wurtzite ZnO are clearly observed. The peak intensities are weaker in sample containing silica, possibly the result of the presence of silica in an amorphous state around the ZnO particles as well as a reduction in the ZnO number concentration in that particular sample. The luminescence spectra of samples prepared at various conditions are shown in Fig. 5. The sample obtained by simply spray drying a zero-hour-aged ZnO colloid has a luminescence spectrum that peaks at around 523 nm. By adding 1 g TEOS into the zero-houraged ZnO colloid before spraying, the luminescence peak is shifted to a position around 460 nm, and the intensity improved as well. For a sample prepared using a 12 h-aged ZnO colloid or a one-week-aged colloid, each of which had been mixed with 1 g of TEOS, the luminescence peaks are located at positions very close to that of the zero-hour-aged ZnO colloid without TEOS. The peak positions were located at around 516 and 529 nm, for samples prepared using 12 h and one-week-aged ZnO colloids, respectively. We also prepared samples by mixing different aged colloids followed by mixing with TEOS, before spray drying, as shown in Fig. 6. Sample prepared by mixing a
Fig. 3. TEM picture of a sample prepared using a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS. The picture size is 80 · 80 nm2 .
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Fig. 6. Luminescence emission of ZnO/SiO2 particle composites prepared using: (a) mixing a ZnO colloid aged for zero hours (50 ml), a ZnO colloid aged for one-week (50 ml) and 1 g TEOS and (b) mixing a ZnO colloid aged for zero hours (17 ml), a ZnO colloid aged for oneweek (83 ml) and 1 g TEOS. Solid curves are the measured data and dashed curves were calculated using Eq. (1), with a1 =a2 ¼ 1 for the left curve and a1 =a2 ¼ 1=15 for the right one.
2θ [degree] Fig. 4. XRD patterns of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without the addition of TEOS and (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS.
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Fig. 5. Luminescence emission of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without addition of TEOS, (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS, (c) a ZnO colloid aged for 12 h (100 ml) with the addition of 1 g TEOS, and (d) a ZnO colloid aged for one-week (100 ml) with the addition of 1 g TEOS.
zero-hour colloid and a one-week colloid in a ratio of 1/ 1 vol./vol. exhibited a peak position near that of the sample prepared using the zero-hour-aged colloid that has been mixed only with 1 g of TEOS. Only a small enhancement in the intensity was observed at a wavelength that coincided with the position of the luminescence peak of a sample prepared using a one-week
colloid mixed 1 g of TEOS. The use of similar volumes resulted in the number concentration of ZnO particles in the 12 h-aged colloid being smaller than that for the zero-hour sample. The fraction in the number concentration of nanoparticles in the zero-hour colloid was about 2.5 times larger than that for the one-week colloid. If a larger volume of one-week colloid so that the ratio of zero-hour colloid and one-week colloid was 1/5 vol./vol, curve (d) is obtained. The peak is located near the peak for the sample prepared using a one-week colloid. This approach appears to be promising for controlling the ‘‘color’’ of ZnO composites. Fig. 7 shows the color of composites prepared at different conditions. Fig. 7(a)–(d) represent pictures of samples prepared using ZnO colloids aged for different times: (a) 0 h, (b) 12 h, (c), 1 d, and (d) one-week, each was mixed with 1 g TEOS and dried. The samples were illuminated using a handy ultraviolet source 365 nm in wavelength. Fig. 7(e) shows the sample prepared using a ZnO colloid aged for zero hours, and then dried without the addition of TEOS. Sample 7(f) shows sample (b) 1 week after the sample preparation. Compared with sample (b), no change in color was observed, indicating that the luminescence color of the sample was stable (time independent). Sample (g) was prepared by mixing 50 ml of fresh ZnO colloid and 50 ml of a one-week-aged ZnO colloid and 1 g TEOS. The luminescence spectrum corresponds to the left curve in Fig. 6. Sample (h) was prepared by mixing 100/6 ml fresh ZnO colloid and 500/6 ml oneweek-aged ZnO colloid and 1 g TEOS. The corresponding luminescence curve is shown in the right curve in Fig. 6.
M. Abdullah et al. / Optical Materials 26 (2004) 95–100
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particles is between 140 nm and 1.5 lm. This corresponds to the observed sizes of the particles in this range. The presence of elapsed time before mixing the ZnO colloid with TEOS, caused the particle size to grow larger and the band gap to be right shifted as well as the luminescence spectra. An increase in particle size reduced the particle number concentration, which in turn weakened the luminescence intensity, as reported previously [15,16]. The present report is in agreement with this scenario. The total intensity of a composite prepared by mixing colloids aged at different times can be written as IT ðkÞ ¼ a1 I1 ðkÞ þ a2 I2 ðkÞ;
Fig. 7. Digital pictures of samples prepared at various conditions when excited using a handy UV lamp with a wavelength of 365 nm: (a) ZnO colloid aged for zero hours (100 ml) with 1 g TEOS, (b) a ZnO colloid aged for 12 h (100 ml) with 1 g TEOS, (c) a ZnO colloid aged for day (100 ml) with 1 g TEOS, (d) a ZnO colloid aged for one-week (100 ml) with 1 g TEOS, (e) a ZnO colloid aged for zero hours (100 ml) without TEOS, (f) is sample (b) observed one-week later, (g) mixing a ZnO colloid aged for zero hours (50 ml), a ZnO colloid aged for one-week (50 ml) and 1 g TEOS, and (h) mixing a ZnO colloid aged for zero hours (17 ml), a ZnO colloid aged for one-week (83 ml) and 1 g TEOS.
4. Discussion The origin of the variation in particle composite size can be briefly summarized as follows. Using a similar atomizer, the atomized droplet size was measured for water as a precursor using a light-scattering particle size analyzer (Malvern Instruments Corp.). The particle size was in the range between 1 and 11 lm, and the mean particle size was around 4.5 lm. The size of powder particles can be estimated based on the concentration of precursors. The molarities of ZnO and silicon in 100 ml of the precursor were about 0.0048 and 0.01 mol, respectively. Assuming a homogenous distribution of precursors, in droplets with diameters of 1 and 11 lm, the molarity of ZnO and silica are about (1.52 · 1017 , 5.23 · 1017 mol) and (3.35 · 1014 , 6.97 · 1014 mol), respectively. Using a figure for the density of silica of 2.2 g/cm3 and ZnO of 5.6 g/cm3 , the diameter of the powder
ð1Þ
with I1 and I2 the intensities of samples prepared using colloid 1 with 1 g of TEOS and using colloid 2 with 1 g TEOS, where the volume of these colloids are similar, a1 and a2 are proportional coefficients. Assume index 1 is associated with sample prepared using the zero-hour colloid and index 2 is associated with the sample prepared using 12 h colloid. Assuming that a is proportional to the volume of colloid, i.e., a1 =a2 ¼ 1 and a1 =a2 ¼ 1=5, a reasonable fit is obtained for the left curve in Fig. 6, but the right curve does not fit. Instead, the right curve can be well fitted using a1 =a2 ¼ 1=15, i.e., three times smaller than the volume fraction of colloid used to prepare the sample, i.e., 1/5. This suggests that the intensity is not a linear function of particle number concentration.
5. Conclusion Composites of ZnO nanoparticles dispersed in amorphous silica particles were produced using a combination of a sol–gel process and spray drying method. Using ZnO colloids aged at specified times, composites emitting a specific luminescence color can be produced. We produced composites emitting specific colors from blue to yellow-green using this method. This method can be easily transferred to other colloids in which the optical (or general physical properties) varies with aging time. In the composite, the optical property does not change further, since the particles are trapped and protected in a solid matrix.
Acknowledgements Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for M.A. is gratefully acknowledged. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO)’s Nanotechnology Materials Program––Nanotechnology Particle Project based on fund
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provided by the Ministry of Economy, Trade, and Industry (METI), Japan. We thank Keisuke Kondo and Takanori Nakayu for assistantship and Dr. I. Wuled Lenggoro (Hiroshima University) for valuable discussions.
References [1] T. Tani, L. Madler, S.E. Prastinis, J. Nanoprt. Res. 4 (2002) 337. [2] S. Major, A. Banerjee, K.L. Chopra, Thin Solid Films 122 (1984) 31. [3] T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) L280. [4] S.N. Qiu, C.X. Qiu, I. Shih, Sol. Energy Mater. 15 (1987) 261. [5] P.S. Vijayakumar, K.A. Blaker, R.D. Weiting, B. Wong, A.T. Halani, C. Park, US Patent No. 4,751,149, 1988.
[6] B.H. Choi, H.B. Im, J.S. Song, K.H. Yoon, Thin Solid Films 193 (1990) 712. [7] J. Hu, R.G. Gordon, Sol. Cells 30 (1991) 437; J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880, 72 (1992) 5381. [8] M. Kohls, T. Schmidt, H. Katschorek, L. Spanhel, G. Muller, N. Mais, A. Wolf, A. Forchel, Adv. Mater. 11 (1999) 288. [9] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826. [10] S. Monticone, R. Tufeu, A.V. Kanaev, J. Phys. Chem. B 102 (1998) 2854. [11] L. Brus, J. Phys. Chem. 90 (1986) 2555. [12] M. Abdullah, T. Morimoto, K. Okuyama, Adv. Funct. Mater. 13 (2003) 800. [13] Mikrajuddin, F. Iskandar, F.G. Shi, K. Okuyama, J. Appl. Phys. 89 (2001) 6431. [14] T. Tani, L. Madler, S.E. Pratsinis, J. Mater. Sci. 37 (2002) 4627. [15] M. Abdullah, I.W. Lenggoro, K. Okuyama, F.G. Shi, J. Phys. Chem. B 107 (2003) 1957. [16] Mikrajuddin, I.W. Lenggoro, K. Okuyama, F.G. Shi, J. Electrochem. Soc. 149 (2002) H107.
Synthesis of ZnO/SiO2 nanocomposites emitting specific luminescence colors Mikrajuddin Abdullah
a,1
, Shinji Shibamoto b, Kikuo Okuyama
a,*
a
b
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan Hiroshima Joint Research Center for Nanotechnology Particle Project, Japan Chemical Innovation Institute, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Received 17 September 2003; received in revised form 9 January 2004; accepted 11 January 2004 Available online 2 March 2004
Abstract Composites of zinc oxide (ZnO) nanoparticles in silica matrix can be produced by initially synthesizing a ZnO colloid (containing ZnO nanoparticles in ethanol solution), mixed with tetraethoxysilane (TEOS), followed by spray drying, to produce powder comprised of submicrometer sized particles. The initial size of ZnO nanoparticles in the colloid is about 3 nm when the precursors are first mixed, but the size increases with aging, following an approximate equation D / ta , a ¼ constant [J. Phys. Chem. B 102 (1998) 2854]. By using ZnO colloids that have been aged for different times, composites containing a specific size of ZnO can be produced. Since the excitation and emission luminescence spectral positions of ZnO are dependent on particle size (shift to blue region with reducing particle size), composites that emit a specific color can be produced using this method. We have been able to produce composites that emit colors from blue (460 nm) up to yellow-green (550 nm). 2004 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is frequently used as a reinforcing filler for elastomers and is used in pharmaceuticals, cosmetics, raw materials for varistor, phosphor and ferrite, catalyst, sensors, etc. [1]. Doping the ZnO with In, Al, B, Ga, or F results in the production of transparent conductors [2–7], which are important in photovoltaic applications. Doping with rare earth ions, such as erbium, results in high optical gain amplifiers [8]. ZnO emits a broad luminescence emission in the green-yellow region, and, as a result, it is a potential material for use in white light sources. When the particle size is reduced below 6 nm, a shift in the emission spectrum to shorter wavelengths due to the quantum confinement of electrons and holes in the particles occurs. The peak for the emission spectrum is located in *
Corresponding author. Tel.: +81-824-24-7716; fax: +81-824-247850. E-mail address: [email protected] (K. Okuyama). 1 Permanent address: Department of Physics, Bandung Institute of Technology, Jl. Genaca 10 Bandung 40132, Indonesia. 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.01.006
the blue region for particle sizes smaller than about 3 nm. Very large ZnO particles show an emission spectrum that peaks at a wavelength of around 550 nm, i.e., a green-yellow color. If ZnO nanoparticles could be produced in a range of sizes from about 3 nm up to around 10 nm, a variety of emission peaks could be produced. One simple method for producing ZnO nanoparticles is a hydrolysis method, first described by Spanhel and Anderson [9]. In this method, a ZnO colloid is produced in an ethanol solution with an initial particle size (just after mixing the precursors) as small as 3 nm. The particle size then continues to grow, or age, after synthesis, even if it is stored at 0 C. After about 5 days of aging, the particle size reaches a size of about 5.5 nm. Monticone et al., reported that the time dependence of ZnO particle size produced by this method can be fitted to the expression D ¼ 2Ata , with D the particle diameter (nm), A ¼ 2:6 0:3 nm daya , and a ¼ 0:33 0:03, and t, the time in days [10]. The visible luminescence on ZnO is also dependent on particle size. Spanhel and Anderson observed that the luminescence peak of about 500 nm for fresh colloid shifted to 560 nm after 5 days of aging
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M. Abdullah et al. / Optical Materials 26 (2004) 95–100
[9]. The shift in the luminescence peak resulted in the change of the energy band gap opening due to the change in crystalline size, which can be roughly predicted using the Brush equation [11], Eg ðRÞ ¼ Eg ð1Þ þ h2 =8lR2 1:8e2 =4pje0 R. Here, R denotes the particle radius, Eg ð1Þ the band gap of bulk ZnO, l the reduced mass, satisfying 1=l ¼ 1=me þ 1=mh with me and mh the effective masses of electron and hole, respectively, me the free mass of electron, and j the dielectric constant. It has been proposed that, the visible luminescence is caused by the breakage of excitons and an electron then jumps to a state located near the center of the gap [12]. The increase in band gap by reduction in the particle size, shifts the position of the luminescence spectra to shorter wavelengths. If the time evolution of the luminescence spectra of a ZnO colloid prepared by Spanhel and Anderson’s method is observed, the fresh colloid emits a nearly blue color, changing to green, and finally reaching a yellow-green color. This definitive change is very interesting. If the size of particles in a ZnO colloid could be stopped at various aging times, ZnO particles that produce a variety of colors, spreading from nearly blue to nearly yellow, could be produced. The objective of this work was to prepare ZnO composites emitting various colors, using a simple method. After t time aging of a ZnO colloid prepared by the Spanhel and Anderson method [9], a sample coated with silica. Initially, the ZnO colloid and silica precursor were mixed and then quickly spray dried to obtain particle composite of amorphous silica containing nanoparticles of ZnO. Depending on the aging time of the colloid, the color of the powder can be varied. The synthesis of such ZnO/SiO2 composites has also been reported by us previously [13] using a ZnO colloid and a colloid of silica nanoparticles as precursors and Tani et al. using a flame spray pyrolysis method [14]. In our previous work [13], no report on the control of the color of the ZnO composite was presented. In the Tani et al. work [14], no report on the luminescence spectra of the sample was presented.
ture of around 0 C followed by extensive stirring for several minutes. Fig. 1 shows the experimental setup used to produce the ZnO/SiO2 nanocomposites. It consists of three sections: (i) a spray generator, (ii) a vertical tubular furnace, and (iii) a particle collector. As the precursors for producing a submicron size of silica particles containing nanoparticles of ZnO, 100 ml of ZnO colloid, aged for different times, was mixed with 1 or 2 g of tetraethoxysilane (TEOS, Kanto Chemicals, Japan). The spray generator used was an ultrasonic spray nebulizer with a 1.75 MHz resonator (Omron Corp.). The droplets were carried by a stream of nitrogen gas into the tubular reactor. The reactor size has a diameter of 13 mm and a length of 600 mm, and was divided into three arbitrary temperature controllable zones (each 200 mm length). The setting temperatures from the inlet were 250, 450, and 450 C. The prepared particles were collected in an electrostatic precipitator fixed at a DC voltage of )7 kV, and maintained at 150 C to prevent the condensation of water. The morphology of the prepared particles was observed using a Hitachi S-5000 field emission scanning electron microscope (FE-SEM, Hitachi Ltd.). TEM
2. Experimental Initially we prepared a ZnO colloid using the standard Spanhel and Anderson method [9]. Zn(CH3 COO)2 Æ 2H2 O (Wako Pure Chemicals, Japan) 0.1 M in 200 ml of ethanol was distilled at 80 C for approximately 3 h to produce 80 ml of a hygroscopic solution and 120 ml of unused distillate. LiOH Æ H2 O (Wako Pure Chemicals, Japan) was dissolved in 120 ml ethanol to give concentration of 0.23 M. An extensive mixing process (usually longer than 1 h) was performed since the LiOH Æ H2 O powder is difficult to dissolve in ethanol [15,16]. The hygroscopic solution, 80 ml, and 120 ml of the LiOH solution were mixed at a tempera-
Fig. 1. Schematic of the spray drier reactor used to produce ZnO/SiO2 particle composites using ZnO colloid and TEOS as precursors.
M. Abdullah et al. / Optical Materials 26 (2004) 95–100
images were recorded using a JEOL JEM-2010 instrument. XRD patterns were detected using a Rigaku Denki RINT2000 instrument, Cu Ka1 source, and excitation and luminescence spectra were obtained using a Shimazu RF-5300PC spectrophotometer equipped with a Xe source.
3. Results Fig. 2 shows SEM pictures of samples that had been spray dried from a zero-aged ZnO colloid: (a) prepared without the addition of TEOS and (b) with the addition of 1 g TEOS. The particle sizes were mostly below 1 lm. The variation in the particle size was caused by variations in the droplet size generated by the atomizer. The particle shape was nearly spherical, both for the sample containing ZnO nanoparticles only or the composite of ZnO nanoparticles/silica.
Fig. 2. SEM pictures of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without the addition of TEOS and (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS. Insets are enlarged views of selected particles.
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Fig. 3 shows a TEM image of a sample prepared using a zero-hour-aged ZnO colloid that had been mixed with 1 g TEOS. The picture size is 80 · 80 nm2 . The composite contains ZnO nanoparticles around 3 nm in size (an example is indicated by an arrow). XRD patterns of samples prepared using a zerohour-aged ZnO colloid without TEOS (a) and zerohour-aged ZnO colloid that had been mixed with 1 g TEOS (b) are shown in Fig. 4. The familiar peaks corresponding to wurtzite ZnO are clearly observed. The peak intensities are weaker in sample containing silica, possibly the result of the presence of silica in an amorphous state around the ZnO particles as well as a reduction in the ZnO number concentration in that particular sample. The luminescence spectra of samples prepared at various conditions are shown in Fig. 5. The sample obtained by simply spray drying a zero-hour-aged ZnO colloid has a luminescence spectrum that peaks at around 523 nm. By adding 1 g TEOS into the zero-houraged ZnO colloid before spraying, the luminescence peak is shifted to a position around 460 nm, and the intensity improved as well. For a sample prepared using a 12 h-aged ZnO colloid or a one-week-aged colloid, each of which had been mixed with 1 g of TEOS, the luminescence peaks are located at positions very close to that of the zero-hour-aged ZnO colloid without TEOS. The peak positions were located at around 516 and 529 nm, for samples prepared using 12 h and one-week-aged ZnO colloids, respectively. We also prepared samples by mixing different aged colloids followed by mixing with TEOS, before spray drying, as shown in Fig. 6. Sample prepared by mixing a
Fig. 3. TEM picture of a sample prepared using a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS. The picture size is 80 · 80 nm2 .
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Fig. 6. Luminescence emission of ZnO/SiO2 particle composites prepared using: (a) mixing a ZnO colloid aged for zero hours (50 ml), a ZnO colloid aged for one-week (50 ml) and 1 g TEOS and (b) mixing a ZnO colloid aged for zero hours (17 ml), a ZnO colloid aged for oneweek (83 ml) and 1 g TEOS. Solid curves are the measured data and dashed curves were calculated using Eq. (1), with a1 =a2 ¼ 1 for the left curve and a1 =a2 ¼ 1=15 for the right one.
2θ [degree] Fig. 4. XRD patterns of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without the addition of TEOS and (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS.
160
(b)
Intensity [au]
120
80
(c)
(a)
40
(d) 0 400
450
500
550
600
650
Wavelength [nm]
Fig. 5. Luminescence emission of ZnO/SiO2 particle composites prepared using (a) a ZnO colloid aged for zero hours without addition of TEOS, (b) a ZnO colloid aged for zero hours (100 ml) with the addition of 1 g TEOS, (c) a ZnO colloid aged for 12 h (100 ml) with the addition of 1 g TEOS, and (d) a ZnO colloid aged for one-week (100 ml) with the addition of 1 g TEOS.
zero-hour colloid and a one-week colloid in a ratio of 1/ 1 vol./vol. exhibited a peak position near that of the sample prepared using the zero-hour-aged colloid that has been mixed only with 1 g of TEOS. Only a small enhancement in the intensity was observed at a wavelength that coincided with the position of the luminescence peak of a sample prepared using a one-week
colloid mixed 1 g of TEOS. The use of similar volumes resulted in the number concentration of ZnO particles in the 12 h-aged colloid being smaller than that for the zero-hour sample. The fraction in the number concentration of nanoparticles in the zero-hour colloid was about 2.5 times larger than that for the one-week colloid. If a larger volume of one-week colloid so that the ratio of zero-hour colloid and one-week colloid was 1/5 vol./vol, curve (d) is obtained. The peak is located near the peak for the sample prepared using a one-week colloid. This approach appears to be promising for controlling the ‘‘color’’ of ZnO composites. Fig. 7 shows the color of composites prepared at different conditions. Fig. 7(a)–(d) represent pictures of samples prepared using ZnO colloids aged for different times: (a) 0 h, (b) 12 h, (c), 1 d, and (d) one-week, each was mixed with 1 g TEOS and dried. The samples were illuminated using a handy ultraviolet source 365 nm in wavelength. Fig. 7(e) shows the sample prepared using a ZnO colloid aged for zero hours, and then dried without the addition of TEOS. Sample 7(f) shows sample (b) 1 week after the sample preparation. Compared with sample (b), no change in color was observed, indicating that the luminescence color of the sample was stable (time independent). Sample (g) was prepared by mixing 50 ml of fresh ZnO colloid and 50 ml of a one-week-aged ZnO colloid and 1 g TEOS. The luminescence spectrum corresponds to the left curve in Fig. 6. Sample (h) was prepared by mixing 100/6 ml fresh ZnO colloid and 500/6 ml oneweek-aged ZnO colloid and 1 g TEOS. The corresponding luminescence curve is shown in the right curve in Fig. 6.
M. Abdullah et al. / Optical Materials 26 (2004) 95–100
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particles is between 140 nm and 1.5 lm. This corresponds to the observed sizes of the particles in this range. The presence of elapsed time before mixing the ZnO colloid with TEOS, caused the particle size to grow larger and the band gap to be right shifted as well as the luminescence spectra. An increase in particle size reduced the particle number concentration, which in turn weakened the luminescence intensity, as reported previously [15,16]. The present report is in agreement with this scenario. The total intensity of a composite prepared by mixing colloids aged at different times can be written as IT ðkÞ ¼ a1 I1 ðkÞ þ a2 I2 ðkÞ;
Fig. 7. Digital pictures of samples prepared at various conditions when excited using a handy UV lamp with a wavelength of 365 nm: (a) ZnO colloid aged for zero hours (100 ml) with 1 g TEOS, (b) a ZnO colloid aged for 12 h (100 ml) with 1 g TEOS, (c) a ZnO colloid aged for day (100 ml) with 1 g TEOS, (d) a ZnO colloid aged for one-week (100 ml) with 1 g TEOS, (e) a ZnO colloid aged for zero hours (100 ml) without TEOS, (f) is sample (b) observed one-week later, (g) mixing a ZnO colloid aged for zero hours (50 ml), a ZnO colloid aged for one-week (50 ml) and 1 g TEOS, and (h) mixing a ZnO colloid aged for zero hours (17 ml), a ZnO colloid aged for one-week (83 ml) and 1 g TEOS.
4. Discussion The origin of the variation in particle composite size can be briefly summarized as follows. Using a similar atomizer, the atomized droplet size was measured for water as a precursor using a light-scattering particle size analyzer (Malvern Instruments Corp.). The particle size was in the range between 1 and 11 lm, and the mean particle size was around 4.5 lm. The size of powder particles can be estimated based on the concentration of precursors. The molarities of ZnO and silicon in 100 ml of the precursor were about 0.0048 and 0.01 mol, respectively. Assuming a homogenous distribution of precursors, in droplets with diameters of 1 and 11 lm, the molarity of ZnO and silica are about (1.52 · 1017 , 5.23 · 1017 mol) and (3.35 · 1014 , 6.97 · 1014 mol), respectively. Using a figure for the density of silica of 2.2 g/cm3 and ZnO of 5.6 g/cm3 , the diameter of the powder
ð1Þ
with I1 and I2 the intensities of samples prepared using colloid 1 with 1 g of TEOS and using colloid 2 with 1 g TEOS, where the volume of these colloids are similar, a1 and a2 are proportional coefficients. Assume index 1 is associated with sample prepared using the zero-hour colloid and index 2 is associated with the sample prepared using 12 h colloid. Assuming that a is proportional to the volume of colloid, i.e., a1 =a2 ¼ 1 and a1 =a2 ¼ 1=5, a reasonable fit is obtained for the left curve in Fig. 6, but the right curve does not fit. Instead, the right curve can be well fitted using a1 =a2 ¼ 1=15, i.e., three times smaller than the volume fraction of colloid used to prepare the sample, i.e., 1/5. This suggests that the intensity is not a linear function of particle number concentration.
5. Conclusion Composites of ZnO nanoparticles dispersed in amorphous silica particles were produced using a combination of a sol–gel process and spray drying method. Using ZnO colloids aged at specified times, composites emitting a specific luminescence color can be produced. We produced composites emitting specific colors from blue to yellow-green using this method. This method can be easily transferred to other colloids in which the optical (or general physical properties) varies with aging time. In the composite, the optical property does not change further, since the particles are trapped and protected in a solid matrix.
Acknowledgements Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for M.A. is gratefully acknowledged. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO)’s Nanotechnology Materials Program––Nanotechnology Particle Project based on fund
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provided by the Ministry of Economy, Trade, and Industry (METI), Japan. We thank Keisuke Kondo and Takanori Nakayu for assistantship and Dr. I. Wuled Lenggoro (Hiroshima University) for valuable discussions.
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