pubs.acs.org/Langmuir © 2009 American Chemical Society
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Tailored Growth of In(OH)3 Shell on Functionalized Polystyrene Beads Mukesh Agrawal,*,† Andrij Pich,‡,§ Smrati Gupta,†, Nikolaos E. Zafeiropoulos,†,^ Petr Formanek, † Dieter Jehnichen, † and Manfred Stamm*,† Leibniz-Institut f€ ur Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany, and ‡Institut f€ ur Makromolekulare Chemie, Technische Universit€ at Dresden, Zellescher Weg 19, 01069 Dresden, Germany. § Current address: DWI an der RWTH Aachen e.V., Pauwelsstr. 8, 52056 Aachen, Germany. Current address: Institut f€ ur Makromolekulare Chemie, Technische Universit€ at Dresden, Zellescher Weg 19, 01069 Dresden, Germany. ^ Current address: Department of Materials Science & Engineering, University of Ioannina, Greece. )
†
Received June 17, 2009. Revised Manuscript Received September 17, 2009 Fabrication of organic-inorganic composite particles with tailored size, shape, and morphology has been attracting great attention from researchers because of their fascinating properties and applications in a variety of potential fields. In this study, we report on the fabrication of PS-In(OH)3 (polystyrene-indium hydroxide) composite particles by hydrolyzing the In(OC3H7)3 (indium isopropoxide) salt in the presence of β-diketone functionalized PS colloidal particles. A systematic investigation of the employed reaction conditions allowed us to tune the morphology, size, and In(OH)3 content of the PS-In(OH)3 composite particles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results illustrate that variation in the employed concentration of the In(OC3H7)3 salt in reaction media can effectively tune the morphology of resulting composite particles between “core-shell” and “raspberry-like”. X-ray diffraction (XRD) analysis confirms the phase purity of In(OH)3 nanoparticles precipitated on the surface of PS beads. Colloidal stability of the composite particles has been found to be reduced with increasing the deposited amount of In(OH)3 nanoparticles. Thermogravimetric analysis (TGA) suggests a continuous increase in the deposited amount of In(OH)3 nanoparticles with increasing concentration of In(OC3H7)3 salt in reaction media. The resulting PS-In(OH)3 composite particles are envisioned to be used in a myriad of potential applications including fabrication of optoelectronic devices, absorption/separation supporting material, catalysts, and hydrophobic surfaces.
Introduction Recently, there has been an immense interest surrounding the fabrication of nanoscale composite particles with tailored structural, optical, and surface properties.1 Facile and flexible strategies that can afford fine control over the synthesis and modification of particles have been realized as being of paramount importance in building new classes of colloids.2 The interest in these materials arises from the fact that they exhibit the properties which are substantially different from those of the cores in terms of their surface chemical composition, higher surface area, increased stability, and magnetic and optical properties. So far, a number of approaches have been employed for the fabrication of these nanostructured materials. The first protocol involves direct precipitation of the desired coating material onto the template surface. For example, Hanprasopwattana et al.3 reported the coating of submicrometer sized silica particles with a titania layer via the hydrolysis of titanium alkoxide precursor. A second approach involves surface reactions utilizing specific functional groups on the cores to induce the coating process.4 Akashi and co-workers described coating of the
poly(N-isopropylacrylamide) functionalized polystyrene (PS) microspheres with platinum nanoparticles.4a Similarly, Liz-Marzan et al.4b reported the coating of gold colloids with silica shell exploiting the silane coupling agent (3-aminopropyl)-trimethoxysilane. Alternatively, the deposition of the preformed inorganic particles on the template surfaces has also been reported to achieve composite particles.5 Apart from direct coating, polyelectrolytes have been employed to facilitate the deposition of nanoparticles on PS beads. For example, Caruso et al.6,7 described the deposition of magnetic and silica nanoparticles on PS microspheres through a layer-by-layer (LBL) deposition technique using polyelectrolytes. In most of the cases, PS beads have been used as a core for the fabrication of composite particles because of their easy synthesis, modulation in size, and surface functionalization. A variety of inorganic materials including CdS and CdSe/CdS,8 TiO2,9 Fe3O4,10 ZnS,11 zeolites,12 and so on have been coated on PS beads to achieve the organic-inorganic composite particles. To the best of our knowledge, fabrication of PS colloidal particles coated with In(OH)3 nanoparticles has never been reported. Indium hydroxide (In(OH)3) is an important semiconductor material which shows size, shape, and morphology dependent
*To whom correspondence should be addressed. E-mail: agrawal@ipfdd. de (M.A.);
[email protected] (M.S.).
(5) Agrawal, M.; Rubio-Retama, J.; Zafeiropoulos, N. E.; Gaponik, N.; Gupta, S.; Cimrova, V.; Lesnyak, V.; Lopez-Cabarcos, E.; Tzavalas, S.; Rojas-Reyna, R.; Eychm€uller, A.; Stamm, M. Langmuir 2008, 24, 9820. (6) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109. (7) Caruso, F.; Caruso, R. A.; Mohwald, H. Chem. Mater. 1999, 11, 3309. (8) Sherman, R. L.; Ford, W. T. Langmuir 2005, 21, 5218. (9) (a) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Stamm, M. Colloid Polym. Sci. 2008, 286, 593. (b) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827. (10) Huang, Z.; Tang, F.; Zhang, L. Thin Solid Films 2005, 471, 105. (11) Pich, A.; Hain, J.; Prots, Yu.; Adler, H.-J. P. Polymer 2005, 46, 7931. (12) Valtchev, V. Chem. Mater. 2002, 14, 956.
(1) (a) Matijevic, E. In Fine Particle Science and Technology; Pelizetti, E., Ed.; Kluwer Academic Publishers: Dordrecht, 1996; pp 1-16. (b) Partch, R. In Materials Synthesis and Characterization; Perry, D., Ed.; Plenum Press: New York, 1997; pp 1-17. (2) Caruso, F. Adv. Mater. 2001, 13, 11. (3) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173. (4) (a) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (b) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (c) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Rubio-Retama, J.; Simon, F.; Stamm, M. J. Mater. Chem. 2008, 18, 2581.
526 DOI: 10.1021/la9021933
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exceptional physical and chemical properties.13-15 Being a semiconductor, it can be used in a wide range of potential applications.16 It has already gained much attention because of its outstanding properties such as wide band gap (∼5.15 eV),13 electrical conductivity (10-7-10-3 S cm-1),14 and photocatalytic/catalytic activities15 and hence is being widely used in a broad spectrum of important applications including the fabrication of nanoscale optoelectronic devices, photonic crystals, solar cells, optical sensitizers, UV detectors, industrial photocatalysts, precursor for In2O3, and additive of alkaline batteries.17 A number of synthetic strategies including the precipitation in a surfactant solution,18 hydrothermal method,19 and controlled double-jet precipitation20 have been proposed for the synthesis of In(OH)3 nanoparticles with effective control on their shape, size, and morphology. Recently, Chen et al.21 reported a solvothermal process to synthesize the In(OH)3 nanoparticles with a variety of interesting morphologies such as cottonlike, burrlike, and podlike. Nevertheless, a controlled precipitation of In(OH)3 nanoparticles on the surface of organic/inorganic cores to achieve the core-shell composite particles has never been explored. Owing to the excellent properties of In(OH)3 and their manipulation with size and morphology as well as considering the templating as an effective mean of the nanoparticles’ stabilization, we believe that it is deemed reasonable to explore the possibilities of templating the In(OH)3 nanoparticles against a suitable template. In the present study, to the best of our knowledge, for the first time we report on a facile and versatile approach to the fabrication of such PS-In(OH)3 composite particles. The motivation arises from the versatility of the system to produce the PSIn(OH)3 composite particles with “core-shell” and “raspberrylike” morphologies. We found that variation in the employed concentration of In(OH)3 precursor in reaction media can effectively manipulate the morphology of the resulting composite particles. Needless to say, controlling the morphological and structural properties of composite materials at the nano/microscale is of pivotal importance, as these structural characteristics strongly influence their performance. One can find a number of studies in the literature which report on the fabrication of the organic-inorganic composite particles with either core-shell22 or raspberry-like morphology.23 However, only few studies have been reported on the preparation of composite particles with controlled morphology, as investigation of the reaction parameters responsible for the deposition of the inorganic nanoparticles on an organic core in a tailored fashion is far from straightforward. Recently, we reported a tailored deposition of (13) Avivi, S.; Mastai, Y.; Gedanken, A. Chem. Mater. 2000, 12, 1229. (14) Ishida, T.; Kuwabara, K.; Koumoto, K. J. Ceram. Soc. Jpn. 1998, 106, 381. (15) Onishi, Y.; Ogawa, D.; Yasuda, M.; Baba, A. J. Am. Chem. Soc. 2002, 124, 13690. (16) (a) Park, W.; King, J. S.; Neff, C. W.; Liddell, C.; Summ ers, C. J. Phys. Status Solidi B 2002, 229, 949. (b) H€orner, G.; Johne, P.; K€unneth, R.; Twardzik, G.; Roth, H.; Clark, T.; Kirsch, H. Chem.;Eur. J. 1999, 5, 208. (c) Gaetzel, M. Platinum Met. Rev. 1994, 38, 151. (17) (a) Duquenne, C.; Gillet, J. P.; Kervennal, J.; Ruppin, C.; Vaultier, M. WO Patent, No. 2004074229(A1), Sept. 2, 2004. (b) Jin, B. T.; Qi, L. CN Patent, No. 1354530, June 19, 2002. (c) Li, B.; Xie, Y.; Jing, M.; Rong, G.; Tang, Y.; Zhang, G. Langmuir 2006, 22, 9380. (d) Lei, Z. B.; Ma, G. J.; Liu, M. Y.; You, W. S.; Yan, H. J.; Wu, G. P.; Takata, T.; Hara, M.; Domen, K.; Li, C. J. Catal. 2006, 237, 322. (18) Zhang, X. H.; Xie, S. Y.; Ni, Z. M.; Zhang, X.; Jiang, Z. Y.; Xie, Z. X.; Huang, R. B.; Zheng, L. S. Inorg. Chem. Commun. 2003, 6, 1445. (19) Zhu, H. L.; Wang, Y.; Wang, N. Y.; Li, Y.; Yang, J. Mater. Lett. 2004, 58, 2631. (20) Wang, L.; Perez-Maqueda, L. A.; Matijevic, E. Colloid Polym. Sci. 1998, 276, 847. (21) Chen, S. G.; Huang, Y. F.; Cheng, Y.; Xia, Q.; Liao, H. W.; Long, C. G. Mater. Lett. 2008, 62, 1634. (22) (a) Imhof, A. Langmuir 2001, 17, 3579. (b) Wang, P.; Chen, D.; Tang, F. Q. Langmuir 2006, 22, 4832. (23) Chen, M.; Wu, L.; Zhou, S.; You, B. Macromolecules 2004, 37, 9613.
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ZnO nanoparticles on PS colloidal particles and investigated the effect of various reaction parameters on the morphology of composite particles.24 Owing to the nanoscale dimension, simple synthesis, and effective control over structural parameters, fabricated nanostructured materials can be used in a variety of applications including building blocks for photonic crystals,25 photocatalysts,26 superhydrophobic materials,27 functional fine particles, absorption/separation supporting materials,2 as well as heterogeneous catalysts.15 Due to versatility, simplicity, high efficiency, low cost, and suitability for environmentally friendly large-scale production, the described approach seems quite promising as a synthetic route for the fabrication of PS-In(OH)3 composite particles.
Experimental Section Materials. Styrene (ST) and acetoacetoxyethylmethacrylate (AAEM) (97%) were purchased from Fluka and Aldrich, respectively, and passed through an inhibitor removal column followed by vacuum distillation before use. Indium(III) isopropoxide [In(OC3H7)3] (5% w/v in 2-propanol) was purchased from Alfa Aesar (Germany) and used as received. Triethanolamine (TEA) (98%), sodium peroxydisulfate (SPDS) (97%), and 2-propanol (99.5%) were obtained from Aldrich and used without additional purification. Ultrapure Millipore water was used throughout the experiments. Synthesis of Polystyrene Particles. PS beads with an average diameter of 540 nm were synthesized by a surfactant free emulsion polymerization as reported in our previous study. In a typical process, a double-wall glass reactor equipped with mechanical stirrer, reflux condenser, nitrogen inlet, and temperature controller was charged with 170 g of water, 19 g of styrene, and 1 g of AAEM. After deoxygenating the reaction mixture via passing the nitrogen gas for 30 min, the temperature was increased to 70 C and aqueous SPDS solution (0.3 g in 10 g water) was added into it to start the polymerization process. The reaction was allowed to proceed for another 24 h, and PS latex particles were obtained as a stable dispersion in water with ca. 10 wt % solid content. Synthesis of Polystyrene-In(OH)3 Composite Particles. To prepare the In(OH)3 coated PS particles, a controlled hydrolysis of In(OC3H7)3 salt has been carried out in the presence of β-diketone functionalized PS beads at 30 C in 2-propanol. In three different reaction sets, 0.08, 0.17, and 0.34 mM indium isopropoxide (0.5, 1, and 2 mL of the 5% w/V indium isopropoxide in 2-propanol) were placed into the round-bottom flask equipped with a magnetic stirrer and water condenser and subsequently diluted with 7 mL of extra pure 2-propanol. In order to stabilize the indium isopropoxide, TEA (one mole per mole of indium isopropoxide) was added into each reaction mixture and the temperature was increased to 30 C. After stirring for 10 min, 1 g of latex (containing 10 wt % PS particles) particles mixed with 5 mL of 2-propanol were added into each reaction media. After reacting for 24 h at 30 C, PS-In(OH)3 composite particles were cleaned by two centrifugation/redispersion cycles in each 2-propanol and water, respectively. Characterization Methods. Colloidal stability measurements were performed with a LUMiFuge 114 analyzer (LUM GmbH, Germany). Measurements were performed in glass tubes at 3000 rpm rotor speed. Transmission electron microscopy (TEM) images were recorded on a Zeiss Omega 912 microscope at 160 kV. High-resolution TEM (HR-TEM) images were recorded with a Philips CM200 microscope operated at 200 kV. (24) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Gupta, S.; Pionteck, J.; Simon, F.; Stamm, M. Chem. Mater. 2007, 19, 1845. (25) Park, S. H.; Gates, B.; Xia, Y. Adv. Mater. 1999, 11, 462. (26) Yan, T.; Long, J.; Chen, Y.; Wang, X.; Li, D.; Fu, X. C. R. Chim. 2008, 11, 101. (27) Synytska, A.; Ionov, L.; Grundke, K.; Stamm, M. Langmuir 2009, 25, 3132.
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Figure 1. SEM images of (a) PS template particles (540 nm size) and PS-In(OH)3 composite particles prepared at (b) 0.08 mM (700 nm size), (c) 0.17 mM (730 nm size), and (d) 0.34 mM (750 nm size) concentrations of In(OC3H7)3 salt. Samples were prepared via drying a drop of the water dispersion on a carbon coated copper grid. Scanning electron microscopy (SEM) images were taken on a Gemini microscope (Zeiss, Germany) at an accelerating voltage of 4 kV. Samples were prepared by drying a few drops of water dispersion on an aluminum support at room temperature. In order to increase the contrast and quality of images, the samples were coated with a thin Au/Pd layer prior to analysis. Thermogravimetric analysis (TGA) was performed on a TGA 7 (Perkin-Elmer) analyzer. Before the measurement, samples were dried under vacuum for ca. 48 h. Subsequently, the samples were heated in platinum crucibles in temperature range between 25 and 700 C with nitrogen as carrier gas at 5 K/min heating rate. X-ray scattering patterns were scanned by analyzing the powdery samples on a Seifert XRD 3003 T/T diffractometer using a Cu KR monochromatic beam (1.54 A˚).
Results and Discussion Polystyrene Core Particles. A surfactant free emulsion copolymerization process of the styrene (ST) and acetoacetoxyethylmethacrylate (AAEM) has been employed for the preparation of β-diketone functionalized PS template particles.28 We believe that AAEM locates predominantly on the surface of emulsion droplets during the copolymerization process due to its hydrophilic character and hence stabilizes the colloidal system (28) Pich, A.; Bhattacharya, S.; Adler, H.-J. P. Polymer 2005, 46, 1077.
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as well as provides the β-diketone functionality to the PS beads. In addition, we found that the size of these PS colloidal particles can be effectively controlled by varying the amount of AAEM comonomer in the reaction media.28 Figure 1a shows an SEM image of PS beads employed as templates in the present study indicating that these particles are nearly monodisperse with the mean diameter of 540 nm and exhibit quite smooth surfaces. Polystyrene-In(OH)3 Composite Particles. PS-In(OH)3 composite particles have been fabricated by carrying out the hydrolysis of In(OC3H7)3 salt in the presence of β-diketone functionalized PS beads. Table 1 depicts the variation in structural parameters such as size, morphology, and In(OH)3 contents of the fabricated PS-In(OH)3 composite particles as a function of the employed concentration of In(OC3H7)3 salt in reaction media. Figure 1b-d illustrates SEM images of PS-In(OH)3 composite particles prepared at different concentrations of In(OC3H7)3 salt. One can observe that coated particles are monodisperse in size and spherical in shape similar to the neat PS beads. Figure 1b reveals the presence of a complete, continuous, and smooth layer of In(OH)3 nanoparticles on the PS core, indicating a typical core-shell morphology of composite particles. Notably, particle size has increased from 540 nm (neat PS) to 700 nm after the coating process, indicating the In(OH)3 shell thickness as 80 nm (thickness of the deposited In(OH)3 shell is half of the increment in total diameter of the particles after the coating process). In order to further confirm the presence of the In(OH)3 shell in the Langmuir 2010, 26(1), 526–532
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Table 1. Variation in Structural Parameters of PS-In(OH)3 Composite Particles as a Function of the Employed Concentration of In[OC3H7]3 Salt in Reaction Media employed amount of In[OC3H7]3 salt PS-In(OH)3 sample 1 2 3
size (nm) 700 730 750
In(OH)3 content (%)
morphology
mM
mg
employed amount of PS (mg)
core-shell raspberry raspberry
0.08 0.17 0.34
23.3 49.6 99.3
100 100 100
theoretical
experimental
13.23 28.16 56.38
4.32 6.70 9.88
Figure 2. TEM images of PS-In(OH)3 composite particles prepared at (a) 0.08 mM and (b) 0.34 mM In(OC3H7)3 concentrations. (c) HRTEM image and (d) FFT pattern of the In(OH)3 nanoparticles deposited on the surface of a PS core of the PS-In(OH)3 composite particles.
hybrid particles, shown in Figure 1b, we calculated the required amount of In(OH)3 nanoparticles to get the microscopically (SEM/TEM) observed increase in the size of PS beads after the coating process and compared it with that observed by TGA. These calculations reveal that, in order to cover the 540 nm size PS beads with an 80 nm thick In(OH)3 shell, the resulting PS-In(OH)3 sample should have an In(OH)3/PS ratio of approximately 0.0049, which is in agreement with the In(OH)3 content of these PS-In(OH)3 particles, that is, 0.0043 as evaluated by TGA. These calculations succinctly demonstrate the presence of In(OH)3 shells on the PS beads. It should be noted that TGA results show the In2O3 contents of composite particles, which were converted into the In(OH)3 contents for the above-mentioned calculation, taking into consideration the conversion of In(OH)3 into In2O3. Interestingly, when the In(OC3H7)3 concentration is increased from 0.08 to 0.17 and 0.34 mM in reaction media, obtained PS-In(OH)3 composite particles shift gradually from core-shell to raspberry-like morphology. Figure 1c and d illustrates SEM images of PS-In(OH)3 composite particles prepared at 0.17 and 0.34 mM In(OC3H7)3 concentrations. One can observe that these particles possess a rough and discontinuous corona of the Langmuir 2010, 26(1), 526–532
In(OH)3 nanoparticles on the surface of PS beads. A closer look of these images further reveals that the composite particles are also as homogeneous in size and spherical in shape as the neat PS beads. Moreover, these SEM results show that raspberry-like particles prepared at 0.17 and 0.34 mM In(OC3H7)3 concentrations have the mean diameters of 730 and 750 nm, respectively, which are higher than those of the core-shell particles (700 nm) prepared at 0.08 mM In(OC3H7)3 concentration. This could be attributed to the increase in the concentration of In(OH)3 nanoparticle precursors in the reaction media with increasing employed concentration of In(OC3H7)3 salt during coating process. For many applications, this modulation in size and morphology of the composite particles might be considered of paramount interest because these structural parameters may have a tremendous effect upon the mechanical, electronic, magnetic, and optical properties of these nanostructures and thus might play a fundamental role in determining the performance of the final materials. Figure 2a and b illustrates TEM images of PS-In(OH)3 composite particles prepared at 0.08 mM and 0.34 mM In(OC3H7)3 concentrations, respectively. In agreement with SEM DOI: 10.1021/la9021933
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results, these images also confirm the modulation in morphology and size of PS-In(OH)3 composite particles with an increase in employed concentration of In(OC3H7)3 salt in reaction media. It is worth mentioning here that, owing to the high thickness of the deposited In(OH)3 shell (80 nm) on PS beads, no difference has been observed in the gray scales of the PS core and inorganic shell in the TEM image of PS-In(OH)3 core-shell particles (Figure 2a). Similar results (in terms of the appearance of coreshell particles in TEM images) have been observed by Wang et al.22b for PS-TiO2 and Liu29 for PS-ZnO core-shell particles. Figure 2c shows a typical HR-TEM image of In(OH)3 nanocrystals from the shell of PS-In(OH)3 composite particles prepared at 0.17 mM In(OC3H7)3 concentration. The lattice fringes from the In(OH)3 nanocrystal can be clearly observed in this micrograph. The distance between two adjacent lattice fringes has been found to be (4.1 ( 0.2) A˚ which is within the measurement error consistent with the separation of 3.92 A˚ of (200) planes of the body centered cubic crystalline phase of In(OH)3. In addition, the fast Fourier transform (FFT) image (shown in Figure 2d) indicates the electron diffraction patterns attributable to the In(OH)3 crystalline phase. Apparently, reflections from (200), (220), (222), (400), and (420) lattice planes can clearly be observed.30 It is well-known that preparation of metal hydroxide nanoparticles from metal alkoxides,that is, M(OR)x involves replacement of the alkoxide groups (-OR) through the nucleophilic attack of the oxygen atom of a water molecule accompanied by the release of alcohol.31 Scheme 1 shows chemical reactions involved in the hydrolysis of In(OC3H7)3 salt in aqueous media. Being highly polar in nature, it is likely for β-diketone groups to develop physical interactions with In(OH)3 precursors, thus directing deposition of the In(OH)3 nanoparticles on the surface of PS beads. It is well-known that β-diketone compounds are good chelating agents, and hence, they have been used for the synthesis of metal oxide nanoparticles in the past, owing to their strong affinity to form complexes with metal oxide precursors.32 The schematic presentation of the mechanism involved in the formation of PS-In(OH)3 composite particles is shown in Scheme 2. It illustrates a PS bead surrounded by the In(OH)3 precursors in reaction media. A cross section on the surface of this PS bead reveals a gray colored core enriched with PS chains and a thin shell of black colored AAEM polymer chains. As mentioned earlier, this AAEM comonomer provides the β-diketone functionality to the particle surface to interact with hydroxyl groups of In(OH)3 precursors. One can see that PS-In(OH)3 composite particles prepared at lower concentration of In(OC3H7)3 salt (0.08 mM) possess a core-shell morphology. While an increase in indium salt concentration to 0.17 and 0.34 mM leads to the preparation of raspberry-like composite particles. This manipulation in morphology of the resulting PS-In(OH)3 particles could be attributed to the variation in the rate of hydrolysis with the alteration in the employed concentration of In(OC3H7)3 salt in the reaction media. It is well-known that the hydrolysis rate of metal alkoxides is governed by a variety of reaction parameters such as temperature, pH, and nature of metal alkoxides.31 Nevertheless, the main parameter that affects the course of the hydrolysis reaction is the molar ratio of reactants (i.e., the metal alkoxide (29) Liu, P. Colloids Surf., A 2006, 291, 155. (30) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y. Cryst. Growth Des. 2005, 5, 147. (31) Turova, N. Y.; Turevskaya, E. P.; Kessler, V. G.; Yanovskaya, M. I. The chemistry of metal alkoxides ; Kluwer Academic Publishers: New York, 2002; Chapter 9, p 107. (32) Niederberger, M.; Garnweitner, G. Chem.;Eur. J. 2006, 12, 7282.
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Agrawal et al. Scheme 1. Reaction Scheme for Hydrolysis of In(OC3H7)3 Salt
to water ratio).31 This ratio determines the composition and the properties of the resulting metal hydroxides and oxide nanoparticles and thus allows them to be obtained in different forms of powders, films, glasses, and fibers. It is believed that preparation of metal oxide/hydroxide nanoparticles is composed of two stages, namely, nucleation and growth processes.31 All the reaction parameters, which affect the rate of these two processes, greatly influence the morphology of the resulting nanoparticles. For example, Yang et al.33 reported that an increase in the pH of the reaction media during the preparation of In(OH)3 nanoparticles in a hydrothermal process leads to the change in morphology of resulting nanoparticles due to the acceleration in nucleation and growth processes. In the initial induction stage of the hydrolysis, solutes are formed to yield a supersaturated solution leading to the nucleation of nanoparticles. We presume that a controlled hydrolysis rate at lower In(OC3H7)3 salt concentration leads to the slow nucleation of the In(OH)3 nanoparticles which preferentially takes place on the template surface due to the physical interaction between β-diketone groups and nanoparticle precursors (as shown in Scheme 2). Subsequently, a controlled growth process results in the formation of a well-defined, complete, and smooth In(OH)3 shell enveloping the template surface. On the contrary, a fast and uncontrolled hydrolysis at higher In(OC3H7)3 salt concentrations accelerates the nucleation process, leading to the generation of a large number of nuclei in reaction media. When all the produced nuclei are not consumed by the template surface due to their fast rate of generation, they try to agglomerate with each other in reaction media. The impetus for the aggregation of these nuclei is to minimize the surface energy. In such conditions, heterogeneous precipitation (on template surface) of nanoparticles is dominated by homogeneous precipitation (in reaction media). Once nucleation is established, the nuclei grow rapidly and simultaneously in the same direction (oriented attachment) to form rodlike structures such the ones as reported by Yang et al.,33 rendering a raspberry-like morphology to the composite particles. From Table 1, one can observe that as the employed concentration of In(OC3H7)3 salt increases the ratio between theoretically calculated and experimentally observed amounts of In(OH)3 increases, which further confirms the above-mentioned mechanism. An increase in the concentration of In(OC3H7)3 salt in reaction media causes the increase in the rate of hydrolysis, which in turn leads to the higher degree of nucleation of the In(OH)3 nanoparticles into the solution. Consequently, a higher amount of the In(OH)3 nanoparticles is generated in solution through the homogeneous nucleation and ratio between amounts of nanoparticles deposited on PS and those in solution increases. All the (33) Yang, J.; Lin, C.; Wang, Z.; Lin, J. Inorg. Chem. 2006, 45, 8973.
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Scheme 2. Schematic Presentation of the Preparation of PS-In(OH)3 Composite Particles with Tailored Morphology
In(OH)3 nanoparticles generated in solution are removed via centrifugation during the cleaning process. Colloidal stability of PS-In(OH)3 composite particles has been investigated by a sedimentation method developed by Lerche and Sobisch.34 In a special centrifuge, an integrated optoelectronic sensor system allows spatial and temporal changes of light transmission through the aqueous dispersion of particles under centrifugation force. Throughout the measurement, transmission profiles are recorded and the sedimentation process can be depicted as a function of the time with the transmission through the sample. Figure 3a illustrates that transmission of the light through the solution increases with time, indicating that particles precipitate due to the action of centrifugal force. One can observe from these results that composite particles show different sedimentation behavior from those of neat PS beads. Figure 3b illustrates sedimentation velocities of neat PS beads and PS-In(OH)3 composite particles calculated from corresponding transmission-time plots. Deposition of inorganic material of PS beads induces faster particle sedimentation at similar experimental conditions. It can be concluded that colloidal stability of the PS beads decreases after the deposition of In(OH)3 nanoparticles on the surfaces. However, variation in inorganic contents of PS-In(OH)3 particles from 3.54 to 8.11 wt % has not been observed to affect the colloidal stability of PS-In(OH)3 composite particles significantly. It is worth mentioning here that fabricated composite particles do not completely settle down even after centrifuging for 2500 s, which attributes to their high colloidal stability due to the hydrogen bonding between In(OH)3 and water molecules. Figure 4a shows TGA scans of uncoated PS beads as well as PS-In(OH)3 composite particles prepared at different In(OC3H7)3 salt concentrations. In all cases, weight loss below 200 C can be ascribed to the desorption of physically bonded water and removal of organic residue from the samples. In the case of composite particles, a slight weight loss in the range of 210 C 240 C can be attributed to the transformation of In(OH)3 into the InOOH.35 A subsequent sharp decrease in weight in all samples in the range of 280-450 C seems to be associated with the decomposition of the PS core. In the case of composite particles, transformation of InOOH into In2O3, which takes place at 435 C, also contributes to this stage of weight loss.35 These results are in agreement with the previously reported studies on the thermal degradation of In(OH)3.35 For example, Zhu et al.36
reported the fabrication of the In(OH)3 nanoarchitectures and found the In(OH)3 to In2O3 conversion in the range of 210700 C. In addition, recently, Liu et al.37 reported fabrication of microtubes of In2O3 through thermal decomposition of In(OH)3 above 400 C. However, some of the earlier studies38 have reported the In(OOH) to In2O3 transformation between 200 and 250 C, which has been attributed to the size of the resulting In(OH)3 nanoparticles. For example, Avivi et al.13 reported In(OOH) to In2O3 transformation in the range of 250 C and
(34) Sobisch, T.; Lerche, D. Colloid Polym. Sci. 2000, 278, 369. (35) Roy, R.; Shafer, M. W. J. Phys. Chem. 1954, 58, 372. (36) Zhu, H.; Wang, X.; Yang, F.; Yang, X. Cryst. Growth Des. 2008, 8, 950.
(37) Liu, X.; Zhou, L.; Yi, R.; Zhang, N.; Shi, R.; Gao, G.; Qiu, G. J. Phys. Chem. C 2008, 112, 18426. (38) Pramanik, N. C.; Das, S.; Biswas, P. K. Mater. Lett. 2002, 56, 671.
Langmuir 2010, 26(1), 526–532
Figure 3. (a) Transmission-time curves and (b) sedimentation velocities of PS beads and PS-In(OH)3 composite particles coated with different amount of In(OH)3 nanoparticles.
DOI: 10.1021/la9021933
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from the inset in Figure 4a that the final residue of PS-In(OH)3 composite particles increases with an increase in the employed concentration of In(OC3H7)3 salt in the reaction media. PSIn(OH)3 composite particles prepared at 0.08, 0.17, and 0.34 mM In(OC3H7)3 concentrations exhibit final solid contents of 3.54%, 5.50%, and 8.11%, respectively. The phase purity of as-prepared PS-In(OH)3 composite particles has been evidenced with X-ray diffraction, and the results are shown in Figure 4b. All the reflections from presented XRD patterns could be readily indexed to a pure body centered cubic phase of In(OH)3 (JCPDS 85-1338).30,36 One can observe that the XRD patterns of these composite particles possess a noticeable background, attributable to the presence of amorphous PS in the sample. In addition, restricted/imperfect crystallization of In(OH)3 nanoparticles in the presence of template particles in reaction media may also contribute to this amorphous background. We have observed a similar effect of the restricted crystallization of Ta2O5 nanoparticles in the presence of organic residue in our previous study.39
Conclusions
Figure 4. (a) TGA analysis of (1) neat PS particles and PSIn(OH)3 composite particles prepared at (2) 0.08 mM, (3) 0.17 mM, and (4) 0.34 mM concentrations of In(OC3H7)3 salt. Inset shows the variation in solid content of composite particles as a function of the employed concentration of In(OC3H7)3 salt in reaction media. (b) XRD pattern of PS-In(OH)3 composite particles prepared at (1) 0.08 mM, (2) 0.17 mM, and (c) 0.34 mM In(OC3H7)3 concentrations.
they mentioned that this shift in phase transformation temperature from 435 to 250 C can be related to the nanometric scale of the products, whose large surface area makes them more reactive with smaller activation energies. Additionally, one can observe
532 DOI: 10.1021/la9021933
In summary, we demonstrated a simple and versatile approach to the fabrication of PS-In(OH)3 composite particles. The described method allows us to tune the size, morphology, and In(OH)3 content of composite particles by manipulating the employed concentration of In(OC3H7)3 salt in reaction media. In addition, we believe that the core size of composite particles can also be tuned by employing the PS beads of desired diameters as a template during the coating process. We presume that, being a semiconductor material, In(OH)3 can render a wide range of applicability to the obtained nanostructured materials. Acknowledgment. The authors are thankful to Prof. H. Lichte for providing the HR-TEM facility at Triebenberg Laboratory, Technische Universit€at Dresden. In addition, we acknowledge Mrs. Ellen Kern, Mr. Alex Mensch, and Dr. Rudiger H€assler for helping out in SEM, TEM, and TGA measurements, respectively. (39) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Simon, P.; Stamm, M. Langmuir 2008, 24, 1013.
Langmuir 2010, 26(1), 526–532
