Ag multicoated microspheres

Applied Surface Science 254 (2008) 1942–1946 www.elsevier.com/locate/apsusc

Preparation and characterization of SiO2/ZrO2/Ag multicoated microspheres Xiaoyun Ye, Yuming Zhou *, Yanqing Sun, Jing Chen, Zhiqiang Wang School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China Received 11 June 2007; received in revised form 30 July 2007; accepted 1 August 2007 Available online 15 August 2007

Abstract A new type of multicoated silica/zirconia/silver (SiO2/ZrO2/Ag) core-shell composite microspheres is synthesized in this paper. In the process, ZrO2-decorated silica (SiO2/ZrO2) core-shell composites were firstly fabricated by the modification of zirconia on silica microspheres through the hydrolysis of zirconium precursor. Subsequently, on SiO2/ZrO2 composite cores, silver nanoparticles were introduced via ultrasonic irradiation and acted as ‘‘Ag seeds’’ for the formation of integrate silver shell by further reduction of silver ions using formaldehyde as reducer. The resulting samples were characterized by transmission electron microscopy, X-ray diffraction, Fourier-transform infrared, energy-dispersive X-ray, and UV– vis spectroscopy, indicating that zirconia and silver layers were successfully coated on the surfaces of silica microspheres. # 2007 Elsevier B.V. All rights reserved. Keywords: SiO2/ZrO2; Silver; Composite; Transmission electron microscopy; X-ray diffractometry

1. Introduction Core-shell composite particles with various morphologies and controlled chemical composition have become one of the most effective sources for advanced materials, because of their potential applications in areas such as coatings, catalysis, electromagnetism materials and photonic crystals [1–10]. Furthermore, the structure, size, and composition of the particles can be altered in a controlled way over a broad range to tailor a variety of novel properties (e.g., optical, electrical, catalytic, magnetic, and mechanical). There have been employed several techniques to achieve the coating of core particles with organic or inorganic layers [11,12]. The methods developed to produce polymer-coated particles include aqueous dispersion polymerization [13], heterophase polymerization [14], and miniemulsion polymerization [15–19]. Moreover, inorganic and hybrid coatings have been commonly prepared by surface functionalization deposition [12,20,21], sono-chemical synthesis [22,23], and inverse micelle precipitation [24] to form the coatings on the cores. Recently, the silica/silver core-shell nanocomposite particles

* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617. E-mail address: [email protected] (Y. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.024

have been prepared in our group based on the ultrasonic irradiation method, which helps facilitate the surface modification of the particles by the deposition of silver nanoparticles [25]. In addition, great efforts on the preparation of core-shell particles have been focused on various multilayer composite materials, consisting of multicomposition coatings such as organic, inorganic and metal with remarkably controlled properties, e.g., SiO2/PS/TiO2, Fe3O4/PSt/TiO2, Ni/PSt/TiO2, and SiO2/Ni/TiO2. At present, a good variety of composite particles have been synthesized with the help of several procedures, especially for metal coatings (e.g., Au, Ag) on the inorganic cores of silica particles [21,22,25–27]. But only one kind of inorganic materials was used as the core among them. In this paper, we chose silica/zirconia core-shell colloid particles as the core for the preparation of silica/zirconia/silver (SiO2/ZrO2/Ag) multilayer core-shell composite particles with the shell of metal Ag as the outermost layer. Herein, silica microspheres were firstly modified with zirconia layers by the hydrolysis of the zirconium precursor. These colloid particles were mixed with silver ions under ultrasonic irradiation to induce the oxidation– reduction reaction in DMF in the presence of PVP, which was used as a protective and complex agent for the fabrication of silver nanoparticles (served as ‘‘Ag seeds’’). This was followed by the growth of silver shell based on the nucleation sites of Ag

X. Ye et al. / Applied Surface Science 254 (2008) 1942–1946

seeds, using formaldehyde as reducer. To the best of our knowledge, we should believe that no previous study has been reported and other multilayer composite particles with various inorganic/metal materials can also be considered. 2. Experimental

1943

Ultrasound irradiation was accomplished using a high-intensity ultrasonic probe (Xinzhi Co., China, Ti-horn, 20 kHz, 800 W/ cm2) immersed directly in the reaction solution. When the reaction was finished, a dark brown precipitation was observed. The mixture, which was signed as SiO2/ZrO2/Ag(sd), was centrifugally separated from the suspension and ultrasonically washed with water.

2.1. Materials Zirconium propoxide solution (70% in propanol) was purchased from Fluka (USA), tetraethoxysilane (TEOS, 99%) was obtained from Shanghai Chemical Reagent Co. (China) and distilled under reduced pressure prior to use. Silver nitrate (AgNO3, 99.5%), poly(vinylpyrrolidone)(PVP, K30, polymerization degree 360), N,N-dimethylformamide (DMF), aqueous ammonia (28%) and formaldehyde (37%), also from Shanghai Chemical Reagent Co. (China), were used as received without further purification. In all preparations ethanol (99.7%), isopropanol (99.7%) and deionized water were used.

2.2.4. The growth of silver shell To achieve the further growth of a silver shell, the following process was repeated three times. Ten milliliters of the seeded colloid particle aqueous solution (0.1 wt%) was mixed with 10 mL of AgNO3 solution (0.15 mM). And then 25 mL of formaldehyde was added, immediately followed by 25 mL of concentrated ammonia. The obtained composite microspheres were signed as SiO2/ZrO2/Ag, and the chemical scheme for the formation of its multicoated microspheres is shown in Scheme 1. 2.3. Characterization

2.2. Methods 2.2.1. Monodisperse silica microspheres Amorphous silica submicrospheres in the size range from 350 to 375 nm were synthesized through base-catalyzed hydrolysis of TEOS, as described by Sto¨ber et al. [28]. The obtained silica spheres were washed thoroughly with water in a centrifuge and then dried under vacuum at room temperature. 2.2.2. Zirconia coating of silica microspheres Zirconia was coated on monodisperse silica spheres by using a similar procedure reported by Hanprasopwattana et al. [29] and Guo and Dong [1]. Typically, silica spheres dispersed in isopropanol were mixed with a certain amount of zirconium propoxide and water. The concentration of zirconium propoxide and water was kept at 0.01 M and 0.32 M, respectively. The mixture was stirred and refluxed for 1.5 h. Subsequently, the resulting zirconia-decorated silica spheres were centrifugally separated and washed thoroughly. The above procedure was repeated several times in order to obtain the complete coating. The zirconia-decorated silica spheres after five-time depositions were calcined at varied temperatures of 400 8C, 600 8C, 800 8C and 1000 8C. The sample dealt at 600 8C was chosen as the substrate for the following silver deposition. 2.2.3. Sonochemical deposition of silver nanoparticles on SiO2/ZrO2 microspheres Circular deposition was performed for the fabrication of silver nanoparticles with desired density. The initial deposition of silver nanoparticles on the surfaces of SiO2/ZrO2 spheres was performed following an ultrasonic procedure processed as follows. Five milliliters of AgNO3 (0.8 mM) aqueous solution was added to a 45 mL PVP solution in DMF ([AgNO3]/ [PVP] = 0.1), following the addition of as-synthesized SiO2/ ZrO2 composite particles (0.02 wt%). The mixture was exposed to high-intensity ultrasound irradiation for 0.5 h at 28 8C.

Transmission electron microscopy (TEM) was performed with a Hitachi H-600 microscope operating at 120 kV. Samples were prepared by placing drops of the colloids dispersion on a Cu grid (200 mesh; placed onto filter paper to remove excess solvent), and allowing the solvent to evaporate at room temperature. Energy-dispersive X-ray spectra (EDX) were obtained on the microscope of Sirion 200 (Holand). Fouriertransform infrared spectroscopy (FT-IR) was carried out on a Nicolet Magna-IR 750 spectrometer (USA) in the 4000– 400 cm 1 wave-number range using KBr pellets. X-ray diffractometry (XRD) was performed with an X-ray diffractometer (XD-3A) operating at 40 kV and 30 mA with Cu Ka radiation. UV–vis absorption spectra were measured at room temperature on a Shimadzu UV-2201 spectrophotometer, using quartz cell with a 1-cm optical path length. All the samples were diluted with water before taking spectra.

Scheme 1. Reaction scheme for the formation of SiO2/ZrO2/Ag multicoated microspheres.

1944

X. Ye et al. / Applied Surface Science 254 (2008) 1942–1946

3. Results and discussion

Table 1 Element composition of the samples

The SiO2/ZrO2/Ag multicoated microspheres suffering several procedures can be visualized by transmission electron microscopy (TEM) as illustrated in Fig. 1. The pure silica particles shown in Fig. 1a are spherical in shape and relatively homogeneous. Observed from the images, it is obvious that the surface uniformity of zirconia is improved by circular zirconia coatings on the surfaces of silica spheres. A heterogeneous surface is obtained after the first deposition of zirconia as revealed in Fig. 1b. Successive deposition of zirconia led to a further uniform surface of zirconia (Fig. 1c and d). Five deposition cycles later, a thin layer of zirconia round the outside

Elements

O Si Zr

Weight (%) SiO2/ZrO2-1

SiO2/ZrO2-3

SiO2/ZrO2-5

48.47 46.95 4.58

46.63 37.70 15.67

44.19 19.93 35.88

of the composites can be observed. EDX was performed to examine the zirconium composition of the multicoated microspheres. The corresponding data of element composition (O, Si, Zr) are listed in Table 1. As can be seen from Table 1, the

Fig. 1. TEM micrographs of silica spheres during the successive deposition steps: (a) bare silica; (b–d) the first, third and fifth deposition of zirconia on the silica surface; (e) the deposition of silver nanoparticles on the SiO2/ZrO2 spheres; and (f) the growth of silver shell on the SiO2/ZrO2 spheres.

X. Ye et al. / Applied Surface Science 254 (2008) 1942–1946

content of Zr is increased with the repeating deposition of zirconia, indicating the formation of thicker zirconia shell on the surfaces of the silica microspheres. The presence of the loaded Ag nanoparticles and Ag shell on the SiO2/ZrO2 spheres clearly resulted in the dark black appearance of the composite microspheres and increased surface roughness (Fig. 1e and f). As shown in Fig. 1e, an innumerability of Ag nanoparticles is formed with a diameter of around 20 nm on the surfaces of SiO2/ZrO2 composite microspheres and acted as ‘‘seeds’’ for the further growth of Ag shell. After the reduction of additional AgNO3 with formaldehyde in the presence of ammonia, the silver shell is fabricated and completely covered on the surfaces of the composites with a high density of silver nanoparticles as announced in Fig. 1f. Fig. 2 shows the XRD patterns of the SiO2/ZrO2/Ag multicoated microspheres and the SiO2/ZrO2 composite microspheres calcined at different temperatures ranging from 400 to 1000 8C. As can be seen from Fig. 2a–d, a broad peak of amorphous silica around 208 appears. There are other peaks besides that of pure silica, which correspond to the zirconia phase. Upon heating the composites to 400 8C for 3 h, four

Fig. 2. XRD patterns of SiO2/ZrO2 microspheres calcined at (a) 400 8C; (b) 600 8C; (c) 800 8C; (d) 1000 8C and coated by (e) silver shell.

1945

strong peaks corresponding to the (1 0 1), (0 0 2, 1 1 0), (1 1 2, 2 0 0), and (1 0 3, 2 1 1) appeared, which is the characteristic of tetragonal zirconia with good crystallinity [30], manifesting the pattern of mixture of tetragonal zirconia and silica. Meanwhile, upon heating the composites to 800 8C, the peaks remained the same. However, when the temperature of the composites was further raised to 1000 8C, the zirconia layer on the surfaces of silica spheres started to transform the crystalline phase from tetragonal zirconia to a monoclinic one [31]. This further confirmed the presence of zirconia on the silica spheres and the formation of SiO2/ZrO2 composite microspheres. Besides, a typical pattern of the face-centered cubic (fcc) structure of metallic silver is observed after the growth of Ag shell on the SiO2/ZrO2 composites as shown in Fig. 2e, indicating the formation of pure silver of high crystallinity (JCPDS file, No. 4783). Fig. 3 shows the UV–vis spectra of the SiO2/ZrO2 colloids before and after silver deposition. According to Mie scattering theory [32], particles of dielectric or semiconducting cores coated with a metallic shell manifest a strong optical resonance by virtue of the relative thickness of the core and its metallic shell. Although an absorption peak attributed to the surface plasmon resonance for Ag nanoparticles is generally observed at around 400 nm, no remarkable peaks were found for the SiO2/ZrO2/Ag(sd) colloid (Fig. 3b). It is clear that the surface plasmon band of Ag is screened by the strong scattering from the ZrO2-modified silica colloids. Due to Mie plasmon resonance excitations from the metal nanoparticles, a peak at 430 nm appears with the increased thickness of the metallic shell on the surface of the SiO2/ZrO2 colloid, indicating the formation of a silver nanoshell. The IR spectra of silica, SiO2/ZrO2 composites, SiO2/ZrO2/ Ag(sd) composites, and SiO2/ZrO2/Ag multicoated microspheres are shown in Fig. 4. It clearly illustrates that all IR spectra show broad peaks corresponding to –OH (n  3430 cm 1) and to adsorbed molecular water (dOH = 1630 cm 1). Besides, all these spectra show strong intensity of the peaks in the broad range from 1000 to 1300 cm 1. This range corresponds to the asymmetric stretching (AS) vibration mode of the Si–O–Si bridge of the

Fig. 3. UV–vis absorption spectra of SiO2/ZrO2 microspheres during the successive deposition steps: (a) SiO2/ZrO2 microspheres; (b) SiO2/ZrO2/Ag(sd) composites; and (c) SiO2/ZrO2/Ag multicoated microspheres.

1946

X. Ye et al. / Applied Surface Science 254 (2008) 1942–1946

Acknowledgements We thank the Six Top Talents of Jiangsu Province of China (06-A-033), the New Century Talents Program of the Ministry of Education of China (NCET-04-0482), and the National Nature Science Foundation of China (50377005) for financial support of this research. We are also grateful to Mr. Aiqun Xu from the Analysis and Testing Centre of Southeast University for his kind help in the measurements. References Fig. 4. IR spectra of (a) silica microspheres; (b) SiO2/ZrO2 microspheres; (c) SiO2/ZrO2/Ag(sd) composites; and (d) SiO2/ZrO2/Ag multicoated microspheres.

siloxane link [33]. In addition, two relatively strong peaks at 800– 1000 cm 1 corresponding to the symmetric stretching of the Si– O–Si group can be observed in Fig. 4a, while one of the peaks is not found in the spectra of surface-modified silica microspheres (Fig. 4b–d). This result is similar to the report on silver-coated silica spheres, the peak of which completely disappeared after the deposition of silver on silica spheres [22]. There are no significant differences between the IR spectra of samples with varied coatings on the surface of silica. The result indicates that the introduction of silver nanoparticles has little influence on the IR characteristics of SiO2/ZrO2 composites. A similar kind of phenomenon was also observed in other system with modification of zirconia by Fe [34].

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusions It is demonstrated that the SiO2/ZrO2/Ag core-shell composite microspheres were obtained by the combination of zirconia modification on the surfaces of silica microspheres and subsequent silver deposition. Zirconia coatings formed by the hydrolysis and condensation of zirconium precursor were modified on the surfaces of silica particles. Nanoparticles of silver, 20 nm in size, were deposited later by ultrasound on the surfaces of SiO2/ZrO2 composites, followed by complete covering of silver shell, resulting in the formation of SiO2/ ZrO2/Ag core-shell composite microspheres. The repeating deposition process could control the amount of zirconia and get complete coating with silver. Electron microscopy, absorption spectroscopy, X-ray diffraction, and infrared spectroscopy were used to reveal various properties of the materials, indicating the zirconia and silver shells successfully covered on the surfaces of the silica to fabricate multicoated microspheres. This work opens up new possibilities for the deposition of other metal such as gold on the tailored substrates with various inorganic composites.

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

X.C. Guo, P. Dong, Langmuir 15 (1999) 5535. X.C. Guo, X.P. Zhao, H.L. Guo, Q. Zhao, Langmuir 19 (2003) 9799. H.X. Guo, X.P. Zhao, Opt. Mater. 22 (2003) 39. H.X. Guo, X.P. Zhao, G.H. Ning, G.Q. Liu, Langmuir 19 (2003) 4884. Y. Xia, B. Gates, Z.Y. Li, Adv. Mater. 13 (2001) 409. F. Caruso, Adv. Mater. 13 (2001) 11. R.A. Caruso, A. Susha, F. Caruso, Chem. Mater. 13 (2001) 400. P. Wang, D. Chen, F.Q. Tang, Langmuir 22 (2006) 4832. J.B. Jackson, N.J. Halas, J. Phys. Chem. B 105 (2001) 2743. J. Li, H.C. Zeng, Angew. Chem. Int. Ed. 44 (2005) 4342. F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. R.A. Caruso, M. Antonietti, Chem. Mater. 13 (2001) 3272. M.J. Percy, V. Michailidou, S.P. Armes, Langmuir 19 (2003) 2072. S. Reculusa, C. Poncet-Legrand, S. Ravaine, C. Mingotaud, E. Duguet, E. Bourgeat-Lami, Chem. Mater. 14 (2002) 2354. K. Landfester, Adv. Mater. 13 (2001) 765. F. Tiarks, K. Landfester, M. Antonietti, Langmuir 17 (2001) 5775. M. Zhang, G. Gao, C. Li, F. Liu, Langmuir 20 (2004) 1420. J. Zhou, M. Chen, X.G. Qiao, L.M. Wu, Langmuir 22 (2006) 10175. R. Palkovits, H. Althues, A. Rumplecker, B. Tesche, A. Dreier, U. Holle, G. Fink, C.H. Cheng, D.F. Shantz, S. Kaskel, Langmuir 21 (2005) 6048. T. Pham, J.B. Jackson, N.J. Halas, T.R. Lee, Langmuir 18 (2002) 4915. Z.J. Jiang, C.Y. Liu, J. Phys. Chem. B 107 (2003) 12411. V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Langmuir 18 (2002) 3352. V.G. Pol, A. Gedanken, J. Calderon-Moreno, Chem. Mater. 15 (2003) 1111. P. Lianos, J.K. Thomas, J. Colloid Interface Sci. 117 (1987) 505. X.Y. Ye, Y.M. Zhou, J. Chen, Y.Q. Sun, Appl. Surf. Sci. 253 (2007) 6264. S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 243. Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. A. Hanprasopwattana, S. Srinivasan, A.G. Sault, A.K. Datye, Langmuir 12 (1996) 3173. H.H. Liang, Z.X. Deng, X. Jiang, F.L. Li, Y.D. Li, Inorg. Chem. 41 (2002) 3602. S.B. Xie, E. Iglesia, A.T. Bell, Chem. Mater. 12 (2000) 2442. S.J. Oldenburg, G.D. Hale, J.B. Jackson, N.J. Halas, Appl. Phys. Lett. 75 (1999) 1063. C.T. Kirk, Phys. Rev. B 38 (1988) 1255. J.A. Navio, M.C. Hidalgo, G. Colon, S.G. Botta, M.I. Litter, Langmuir 17 (2001) 202.