J. Phys. Chem. B 2003, 107, 9175-9178
9175
Surface Synthesis of Zinc Oxide Nanoparticles on Silica Spheres: Preparation and Characterization Hai-Long Xia and Fang-Qiong Tang* Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China ReceiVed: May 21, 2002; In Final Form: March 29, 2003
A new method for the preparation of SiO2/ZnO composite nanoparticles by the simplified controlled doublejet precipitation technique is described. The transmission electron microscopic and scanning electron microscopic images of SiO2/ZnO show that ZnO is coated on the silica (SiO2) surface as a thin layer or single nanoparticle, dependent on the reactant conditions. The powder X-ray diffraction of the initial zinc oxide-silica powder yields diffraction peaks corresponding to ZnO phase. Energy-dispersive X-ray analysis shows the presence of Zn, O, and Si elements. Infrared spectroscopy illustrates the structural changes that occurred in the siloxane network and surface silanol groups of SiO2 upon the deposition of ZnO. The photoluminescence spectrum of SiO2/ZnO shows two emission peaks centered around 360 and 400 nm, respectively, which is similar to that of ZnO nanoparticles. We propose that the process takes place via formation of ZnO clusters on the silica surface modified by triethanolamine, followed by collision with other ZnO clusters in the solution, resulting in the precipitation of ZnO on the silica surface.
Introduction Semiconductor nanoparticles have been extensively studied from experimental and theoretical viewpoints, owing to their potential applications in solar energy conversion, photocatalysis, and the optoelectronic industry that stem from their sizedependent optical properties.1-5 This is associated with the quantum size effect and with the existence of a relatively large percentage of atoms at the surface. Nowadays, many synthetic routes have been developed to control the size and distribution of semiconductor nanoparticles. One of the most advanced and intriguing developments in the area of nanoparticles is the coating of semiconductor clusters on a solid support. This can be exploited to synthesize core/shell type materials with unique optical, electronic, magnetic, and catalytic properties that are more than the sum of their individual components. Although the technology for coating the nanoparticles on rather large substrates is well established, coating on very small substrates, such as submicron-sized particles, still remains a technical challenge. The coating of semiconductor nanoclusters has been accomplished using either of the two following strategies: (1) the transfer of solution phase synthesized semiconductor nanoclusters to a desired core substrate and further thermal treatment to obtain the core/shell material and (2) the surface or direct synthesis of semiconductor nanoclusters on a solid support. Previous investigations have demonstrated that inorganic particles dispersed in aqueous solutions can be coated with layers of various materials either by precipitation of the coating materials onto the cores or by direct surface reactions utilizing specific functional groups on the cores to induce coating. Homori and Matijevic have optimized the coating conditions and coated spindle-shaped hematite (R-Fe2O3) particles with silica layers by hydrolysis of the alkoxide tetraethoxysilane in 2-propanol.6,7 Liu and co-workers have electrodeposited zinc * Corresponding author. Telephone: 86-10-64888064. Fax: 86-1064862951. E-mail:
[email protected] or
[email protected].
oxide nanoparticles on single-crystal gold.8 Moreover, sonochemistry is also an alternative technique that can be employed for the production of coated particles. Dhas and co-workers have reported the synthesis of ZnS semiconductor nanoparticles on submicron-sized silica by ultrasound irradiation near room temperature.9 Currently, considerable efforts are being made to bind semiconductor nanoclusters to metal or inorganic surfaces using a self-assembled monolayer approach. Homola and Lorenz et al. have reported the coating of γ-Fe2O3 particles with preformed smaller silica particles by combining the particle mixtures under conditions where the two types of particles are oppositely charged.10 Pastoriza-Santos and Koktysh et al. have synthesized Ag/TiO2 core-shell nanoparticles by the layer-by-layer assembly.11 Nanocomposite multilayers can also be assembled on particle surface by using the layer-by-layer method based on colloidal templates. Keller and Johnson et al. have prepared alternating composite multilayers of exfoliated zirconium phosphate sheets and charged redox polymers on (3-aminopropyl)triethoxysilane-modified silica particles.12 Recently, the controlled doubled-jet precipitation method has been applied to synthesize the particles.13,14 In the present work we report for the first time the surface synthesis of ZnO semiconductor nanoparticles on submicron-sized SiO2 by the simplified controlled doubled-jet precipitation method. We have also focused our attention on the reaction mechanism of SiO2/ ZnO formation under the given conditions. Characterization was accomplished using various techniques, such as powder X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscopy, scanning electron microscopy, and infrared spectroscopy, and through the photoluminescence spectrum. Experimental Section Materials. Zinc acetate, tetraethyl orthosilicate, ammonium hydroxide, triethanolamine, and ethanol were purchased and
10.1021/jp0261511 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/23/2003
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Xia and Tang
Figure 1. Scheme for preparing SiO2/ZnO composite nanoparticle (TEOH represents triethanolamine, N(CH2CH2OH)3). The produced particle is heated at 700 °C to remove the triethanolamine, resulting in a core-shell material composed of a silica core and ZnO shell.
used without further purification. Doubly distilled water was used for the process. The reactants were introduced by a multichannel syringe pump (Cole-Parmer 74950) through the latex tubes. Monodipersed Silica Spheres. Monodispersed silica spheres were prepared with the procedure originally described by Sto¨ber et al.,15 i.e., hydrolysis of tetraethyl orthosilicate in an ethanol solution containing water and ammonia. In a typical experiment, a 5 mL ethanol solution of tetraethyl orthosilicate was added to a 35 mL ethanol solution of water and ammonia. The 40 mL mixture containing 0.3 mol/L tetraethyl orthosilicate, 2.3 mol/L H2O, and 1 mol/L NH3 was stirred at 30 °C for 5 h. The resulting silica spheres were centrifugally separated from the suspension and ultrasonically washed with ethanol. For analysis, the silica spheres were further washed with water. Surface Synthesis of ZnO on SiO2. Simultaneous introduction of triethanolamine and Zn(Ac)2 into the SiO2 ethanol aqueous solution yields ZnO-coated SiO2 composites. The schematic representation for the formation of SiO2/ZnO composite with different surface morphology is given in Figure 1. Typically, for the preparation of SiO2/ZnO, about 0.2 g of SiO2 was dispersed into a 30 mL ethanol-water (2:3 in volume) solution; then the solution containing SiO2 was heated to 90 °C. After 10 min, 1.6 mol/L triethanolamine and 0.02 mol/L Zn(Ac)2 were dropped simultaneously through latex tubes into the SiO2 ethanol aqueous solution at a constant flow rate, controlled by the multichannel syringe pump. The system was then continuously stirred for 1 h at 90 °C. The resulting white powders were recovered by centrifugation, washed repeatedly with doubly distilled water, and dried in a vacuum. Finally, the powders were sintered at 700 °C for 3 h. Characterization. The phase composition and nature of SiO2/ ZnO were determined by X-ray diffraction, using a Rigaku D/Max-RB Cu KR diffractometer (λ ) 1.5418 Å). The energydispersive X-ray analysis of SiO2/ZnO was carried out to ascertain the chemical composition, using a S-4200 electron microscope. The overall morphologies of SiO2 and SiO2/ZnO were obtained by transmission electron microscopy using a JEM-2000FX electron microscope and scanning electron mi-
croscopy using a S-4200 electron microscope. Samples for the transmission electron microscopic measurements were obtained by placing a drop of sample suspension in doubly distilled water on a Formvar copper grid, followed by air drying to remove the solvent. For the scanning electron microscopic measurements, a few drops of sample suspension were placed on aluminum foil. Infrared spectra were recorded using a Jasco FT-IR spectrometer, using transparent pellets of the compounds in KBr (A.R.) matrixes. KBr was used as the background file. The optical properties of SiO2/ZnO were measured with the photoluminescence spectrum recorded by an F-4500 fluorescence spectrophotometer. Results and Discussion X-ray Diffraction (XRD) and Energy-Dispersive X-ray Analysis. In Figure 2, the X-ray diffraction pattern of ZnO shows the presence of sharp peaks, corresponding to the zinc blende crystal structure. The X-ray diffraction profile of SiO2/ ZnO shows the presence of weak diffraction peaks, whose positions fully match those of the bare ZnO particles. The presence of weak X-ray diffraction peaks in the SiO2/ZnO sample can be attributed to the nature of ZnO and/or to the small amount of ZnO. The observed rising background in the lower angle side (2θ ∼ 21°, 24°) is due to the presence of the SiO2 core. The ZnO particle diameter t was calculated using the Debye-Scherer formula t ) 0.89λ/(β cos θ), where λ is the X-ray wavelength, θ is the Bragg diffraction angle, and β is the peak width at half-maximum. The XRD peaks at 36.25° and 56.42° in Figure 2 give ZnO diameter of 16.27 and 17.93 nm, respectively. The energy-dispersive X-ray profile of SiO2/ZnO shows the composites are composed of Si, Zn, and O elements. Transmission Electron Microscopy and Scanning Electron Microscopy. Compared with the bare SiO2 (Figure 3c), transmission electron microscopic images of SiO2/ZnO reveal the coating nature of the ZnO particles. It can be seen that ZnO is coated on the surface of SiO2 (170 nm diameter) as a thin layer or single nanoparticles. In Figure 3a,b, the central SiO2
Preparation of SiO2/ZnO Nanoparticles
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Figure 4. Scanning electron microscope micrographs of SiO2/ZnO composite nanoparticles. Left and right photos are taken at lower and higher magnification, respectively.
Figure 2. Powder X-ray diffraction patterns of (a) SiO2/ZnO and (b) ZnO.
Figure 5. FT-IR spectra of (a) SiO2 and (b) SiO2/ZnO composite.
Figure 3. Transmission electron microscope micrographs: (a) SiO2/ ZnO, (b) SiO2/ZnO, (c) SiO2, and (d) SiO2/ZnO. (a) and (b) are taken at lower and higher magnification, respectively. A little ammonia is added to adjust the pH of the solution to 9-10 before the injection of triethanolamine and zinc acetate in (d).
core is coated with a ZnO layer approximately 10-20 nm thickness on average, almost without any free bare zones. In Figure 3d, the SiO2 core is coated with ZnO nanoparticles, whose diameter is above 10 nm. Figure 3d clearly reveals that ZnO nanoparticles form and grow on the surface of SiO2. At the condition referred to above, we do not observe the presence of free SiO2 or ZnO particles. However, when the concentration of Zn(Ac)2 is increased beyond 0.05 mol/L, a collection of free ZnO particles has been observed. Scanning electron micrographs of SiO2/ZnO at different magnification are shown in Figure 4, which also indicate a ZnO coating on the surface of SiO2. Infrared Spectra. Infrared (IR) spectra of SiO2 and SiO2/ ZnO clearly show the formation of an interfacial bond between SiO2 and ZnO. The spectrum of SiO2 shows three absorption
bands in the region of 1600-400 cm-1, characteristic of the siloxane links. The absorption band at ∼460 cm-1 corresponds to the rocking mode, while the band at ∼810 cm-1 is due to the symmetric stretching of the Si-O-Si group. The observed broad doublet band in the wavenumber region of 1300-1000 cm-1 corresponds to the asymmetric stretching vibrational mode of the Si-O-Si bridge of the siloxane link. The sharp band at 1060 cm-1 corresponds to the characteristic oxygen asymmetric stretching mode. The splitting of the asymmetric stretching mode is probably due to the presence of strained siloxane links and surface silanols (disorder-induced coupling), as recognized earlier.16,17 The IR spectrum of SiO2/ZnO (Figure 5b) shows a significant change in the asymmetric stretching mode of the SiO2 core. The doublet of the SiO2 asymmetric stretching band is replaced by a sharp band near 1114 cm-1, corresponding to the asymmetric stretching mode and indicating a surface modification by the ZnO coating. A 54 cm-1 shift to a higher frequency is observed for the asymmetric stretching mode upon coating, probably associated with the bonding change around the [SiO4] tetrahedral. The splitting of the asymmetric stretching mode of SiO2 disappears upon ZnO coating on the SiO2 surface. However, a less intense small shoulder in the region of 1214 cm-1 could still be detected, typifying a low degree of disorder splitting, due to the depletion of strained siloxane networks and surface silanols of amorphous SiO2. Thus, the decreased intensity of the splitting of the asymmetric stretching mode reveals surface ordering by modifying the surface of SiO2 via triethanolamine. The above results demonstrate that the triethanolamine reactant appears to play a dual role, that is, in the
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Figure 6. Photoluminescence spectra of (a) SiO2/ZnO and (b) ZnO. The excitation wavelength for all spectra is 250 nm.
generation of ZnO and in the modification of the SiO2 surface for the coating. Photoluminescence Spectra. The photoluminescence spectrum of SiO2/ZnO in Figure 6 exhibits two emission peaks. One is at about 360 nm, which is well-known due to direct recombination of photogenerated electron-hole pairs between ZnO band gaps. The other is near 392 nm, while the photoluminescence spectrum of the sole ZnO shows an emission band centered at 398 nm. It is evident that the SiO2/ZnO emission feature is just from ZnO on the surface compared with sole ZnO and SiO2. In our experiment, the mole ratio of triethanolamine to Zn2+ is above 10:1, which indicates the access of base in the solution. Under high alkaline concentration, the band at 392 nm may originate from the molecular self-trapped excitons at surface centers, which may be complex and involve the triethanolamine group. The higher intensity of composite than that of ZnO may also be caused by this. Monticone and coworkers have also observed the blue band at λ ≈ 400 ( 30 nm and proposed a simple model to explain it,18 but this band identification is still in progress. Discussion. The formation of composite can be visualized below, as illustrated in Figure 1: The surface of the silica particles is predominantly covered by silanol (Si-OH) and siloxane (Si-O-Si) group,19-21 which is the “active” site of silica. In the system triethanolamine also functions as a base which will react with Zn(Ac)2 to produce ZnO.13,22 At the condition referred to above, triethanolamine molecules are absorbed onto the silica and further modify the silica surface by breaking the siloxane networks and surface silanols,21 resulting in modified silica (see eq 1 in Figure 1). Zn(Ac)2 in the solution will react with triethanolamine to form ZnO molecular cluster on the silica surface and in the solution simultaneously. ZnO cluster formed will be inclined to rapidly collide with other ZnO clusters to grow according to the ripening and aggregation theory.23,24 Additionally, the silica sphere has
Xia and Tang a larger surface area than a single ZnO cluster, which means ZnO cluster formed on the silica surface has a larger efficiency and probability to collide with other clusters in a appropriate concentration; as a result, ZnO will cover the silica surface. That is, ZnO prefers to grow on silica surface in our experiment. If the active sites on silica surface are partly covered by other molecules, such as ammonia, only the residual “active” sites can follow the former process, so some larger single ZnO nanoparticles will distribute on the silica surface (see Figure 3d). However, if the original Zn(Ac)2 concentration is increased enough, the number of ZnO clusters is so high that collision among ZnO clusters in the solution will be dominant. We also observed large ZnO particles and bare silica. This is testified to by our experiment when the Zn(Ac)2 concentration is increased beyond 0.05 mol/L. To gain more knowledge about the mechanism of the surface synthesis of ZnO on silica, we have carried out the above experiments with NaOH in place of triethanolamine. As a result, we cannot obtain the composite as before; furthermore, we have observed bare SiO2 and sole ZnO particles. Therefore, we believe that the reactant triethanolamine plays an important role in the process, which in turn strengthens the mechanism above. References and Notes (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Golan, Y.; Margulis, L.; Hodes, G.; Rubinstein, I.; Hutchison, J. L. Surf. Sci. 1994, 311, L633. (3) Kamat, P. V.; Shanghavi, B. J. Phys. Chem. 1997, 101, 7675. (4) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (5) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (6) Ohmori, M.; Matijevic, E. J. Colloid Interface Sci. 1993, 160, 288. (7) Ohmori, M.; Matijevic, E. J. Colloid Interface Sci. 1992, 150, 594. (8) Liu, R.; Vertegel, A. A.; Bohannan, E. W.; Sorenson, T. A.; Switzer, J. A. Chem. Mater. 2001, 13, 508. (9) Dhas, N. A.; Zaban, A.; Gedanken, A. Chem. Mater. 1999, 11, 806. (10) Homola, A. M.; Lorenz, M. R.; Sussner, H.; Rice, S. J. Appl. Phys. 1987, 61, 3898. (11) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2000, 16, 2731. (12) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (13) Zhong, Q. P.; Matijevic, E. J. Mater. Chem. 1996, 6, 443. (14) Her, Y. S.; Matijevic, E.; Chon, M. C. J. Mater. Res. 1995, 10, 3106. (15) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (16) Dhas, N. A.; Gedanken, A. Chem. Mater. 1997, 9, 3144. (17) Dhas, N. A.; Gedanken, A. Appl. Phys. Lett. 1998, 72, 2511. (18) Monticone, S.; Tufeu, R.; Kanaev, A. V. J. Phys. Chem. B 1998, 102, 2854. (19) Srinivasan, S.; Datye, A. K.; Smith, M. H.; Peden, C. H. F. J. Catal. 1994, 145, 565. (20) Armistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S. A.; Hockey, J. A. J. Phys. Chem. 1969, 73, 3947. (21) Iler, K. R. The Chemistry of Silica; John Wiley & Sons: New York, 1979; Chapter 6, p 622. (22) Chittofrati, A.; Matijevic, E. Colloid Surf. 1990, 48, 65. (23) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (24) Fisher, C. H.; Weller, H.; Katsikas, L.; Henglein, A. Langmuir 1989, 5, 429.
