11786
J. Phys. Chem. C 2008, 112, 11786–11790
Surface Modification of Silicon Carbide Nanoparticles by Azo Radical Initiators Motoyuki Iijima* and Hidehiro Kamiya* Institute of Symbiotic Science and Technology, Tokyo UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ReceiVed: October 1, 2007; ReVised Manuscript ReceiVed: May 8, 2008
The surface of silicon carbide (SiC) nanoparticles was modified with three types of azo radical initiators: 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AMPA), and 2,2′azobis[N-(2-carboxyethyl)-2-methylpropionamidine)n-hydrate (ACMPA). The radical species generated from the azo initiators successfully reacted with the unsaturated hydrocarbons on the surface of SiC nanoparticles. Consequently, the hydrophobic SiC surface became hydrophilic, and the dispersion stability of SiC nanoparticles in aqueous solution improved significantly. Further, it was found that the stability of SiC nanoparticles in aqueous solution under various pH values could be controlled by the structure of the azo radical initiators. When SiC nanoparticles reacted with AIBN and were further hydrolyzed by NaOH aqueous solution, since carboxyl groups were introduced on their surface, their stability improved at pH g5. Contrarily, when SiC nanoparticles were modified with AMPA, amine groups were formed on the SiC surface so that SiC nanoparticles were stabilized at pH e3. When SiC nanoparticles were modified by ACMPA, which consists of both amine groups and carboxyl groups, the modified SiC nanoparticles possessed amphoteric properties; the SiC nanoparticles were stabilized at pH 3 and 11. It was deduced that modifying SiC nanoparticles with azo radical initiators is a significant method for tuning the surface properties of SiC nanoparticles. Introduction Siliconcarbide (SiC) particles are one of the most useful raw materials for high-temperature structural materials due to their significant hardness, great strength, and high oxidation resistance.1–4 They are widely applied to ceramic materials for hightemperature uses and to additives for increasing the strength or thermal properties of composite materials. With regard to fabricating SiC ceramics or SiC-dispersed composite materials, controlling the dispersion stability of SiC particles in aqueous solution is the most essential strategy to effectively control the microstructure of a green body. This control can reduce defects in the material and improve the properties and reliability of ceramic or composite materials. Recently, many researchers have paid close attention to increasing the dispersion stability of SiC particles in an aqueous solution by modifying their surfaces. The use of polymeric dispersants such as poly(ethylene imine) (PEI), poly(acrylic acid), and poly(vinyl alcohol) is one the most widely accepted methods to improve the SiC surface and stability of SiC in water.5–9 For example, the presence of PEI in an acidic SiC suspension leads to the improvement of the dispersion stability of SiC particles due to the generation of steric repulsive forces by the effective adsorption of protonated PEI on negatively charged SiC particles.5 However, the optimum additive condition of the polymeric dispersant to increase the stability of the suspension is very restricted. Many factors such as the chemical composition of the particle, particle size, particle concentration, and additive amount of the dispersant must be carefully considered to obtain a stabilized suspension.9–15 Polymeric dispersants must be designed so that they can effectively adsorb on the particle surface at the required pH values. Further, they should be able to form polymer chain structures that are suitable * Corresponding authors. E-mail: (M.I.)
[email protected] and (K.H.)
[email protected]; tel. and fax: 81-42-388-7068.
enough to create effective steric repulsive forces between particles but not form interparticle bridges, which accelerate particle agglomeration. Although the addition of polymeric dispersants is the conventional method, it is quite difficult to determine the optimum additive condition to stabilize particles under various required conditions. The chemical modification of SiC particles is another method to design a SiC surface and improve the stability of SiC particles in an aqueous solution. Thus far, several modification techniques have been reported, such as oxidation of the SiC surface by strong acids16 or photocatalytic reaction with TiO2,17 polymerization of aniline on the surface of SiC particles,18,19 and coating with other inorganic materials such as Al2O3.20 Indeed, by using each of these surface modification techniques, the property of SiC surfaces, such as chemical composition and wettability, do change. However, these techniques cannot be used to tune the properties of a SiC particle surface, such as for controlling surface potential and dispersion stability under various required pH values. The aim of this study was to establish a new surface modification technique for SiC particles that can easily tune the properties of the particle surface, such as surface potentials, as per requirement. Havel and Colomban21 reported that the Gand D-bands, which are typical Raman peaks for graphitestructured carbon rings and defects of graphite-structured carbon rings, respectively, can be detected in SiC-based fibers. This suggests the existence of unsaturated hydrocarbons on the surface of SiC particles. In this study, our challenge was to modify the SiC particle surface by reacting the unsaturated hydrocarbons on SiC with radical species generated from azo radical initiators. The surface potential of SiC particles and their dispersion stability in aqueous solution with various pH values were tuned by selecting the chemical structure of azo radical initiators. To our knowledge, this is the first report on the modification of SiC particles with azo radical initiators.
10.1021/jp709608p CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
Surface Modification of SiC Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11787 SCHEME 1: Reaction Scheme for Surface Modification of SiC-A Nanoparticles with AIBN
Figure 1. Molecular structures of (a) AIBN, (b) AMPA, and (c) AMCPA.
Experimental Procedures Materials. All chemicals were used without further purifications. Two types of β-SiC nanoparticles with an average particle diameter of ∼30 nm (SiC-A, BET surface area: 49 m2/g) and ∼50 nm (SiC-B, BET surface area: 43 m2/g) were obtained from Sumitomo Osaka Cement, Osaka, Japan and the Institute of Energy Science and Technology Co. Ltd., respectively. 2,2′Azobisisobutyronitrile (AIBN, 98%), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AMPA, 95%), and 2,2′-azobis[N(2-carboxyethyl)-2-methylpropionamidine]n-hydrate (ACMPA, 95%) were purchased from Wako Pure Chemical Industry, Ltd. Sodium hydroxide (97%), toluene (99.5%), and methanol (99%) were purchased from Kanto Chemical Co., Ltd. Surface Modification of SiC Nanoparticles with Azo Radical Initiators. SiC nanoparticles (1.0 g) were dispersed in 40 mL of toluene with sonication and stirred for 30 min under nitrogen gas flow. Then, a mixture of 30 mL of toluene and 15 mmol of AIBN (Figure 1a) was added to the SiC nanoparticle solution and stirred for 3 h at 75 °C under nitrogen gas flow. The modified SiC nanoparticles were washed with toluene and dispersed into a solution that contained 30 mL of 10 N sodium hydroxide aqueous solution and 30 mL of methanol. The solution with SiC nanoparticles was then stirred for 1 day at 60 °C to hydrolyze the cyano groups to carboxyl groups. Finally, the SiC nanoparticles were washed twice with water and collected after drying overnight at 120 °C. For modifying SiC nanoparticles with AMPA (Figure 1b) or ACMPA (Figure 1c), the nanoparticles were dispersed in methanol and reacted with the corresponding azo radical initiator for 8 h. Then, the particles were washed twice with 50% methanol aqueous solution and dried overnight at 120 °C. Characterization. The changes in surface characteristics by surface modification were measured by Raman spectroscopy (excitation laser line: 532 nm) and FT-IR, which were performed on Nicolet Almega XR (Thermo Electron Co., Ltd.) and Nicolet Nexus 470 (Thermo Electron Co., Ltd.) instruments, respectively. The amount of azo radical initiators reacted on SiC nanoparticles was determined by an organic element analyzer (JM-10 Microcoder, J Science Laboratory. Co., Ltd.), in which a known amount of SiC nanoparticles was injected and combusted at 900 °C and the nitrogen content was measured as N2 by using a differential thermal conductivity detector (calibrated by analytical grade p-nitroaniline) after flowing though a reduction chamber. The measured values were cor-
rected by using the backgrounds determined from a blank powder-free air sample. The surface potential of SiC nanoparticles at various pH values was measured using a Z-analyzer (Nihon lufuto, Co. Ltd.). The stability of SiC nanoparticles in water was determined by observing the sedimentation behavior of a 1 wt % SiC/water solution, and their aggregated size in water was measured by dynamic light scattering (HPP5001, Malvern Instruments, Ltd.) using a 0.01 wt % SiC/water solution, respectively. Results and Discussion Surface Modification of SiC Nanoparticles (SiC-A) with Azo Radical Initiators. Scheme 1 shows the estimated reaction scheme of radical initiators on the surface of SiC nanoparticles. Since it is reported that polymers can be grafted onto unsaturated hydrocarbons at the surface of carbon blacks by radical reactions,22 it is expected that the radical species formed from radical initiators also can react with unsaturated hydrocarbons on the surface of SiC nanoparticles. By this reaction, new organic functional groups from azo radical initiators, such as cyano groups in the case of modification with AIBN, can be introduced on the surface of SiC nanoparticles. Figure 2 shows the Raman spectra of SiC-A nanoparticles before and after surface modification in the 400-2000 cm-1 range. In all spectra, two typical bands were clearly observed at 1347 and 1581 cm-1. These two bands are quite similar to the D- and G-bands observed in carbon-related materials such as carbon blacks and carbon nanotubes. The D-band at 1347 cm-1 can be attributed to the presence of disorder or defects in the sp2 graphitic structure, while the G-band corresponds to ordered graphite.23 The detection of the D-band from SiC-A nanoparticles is strong evidence for the existence of unsaturated hydrocarbons on the surface of SiC-A nanoparticles. From Figure 2, the intensity ratios between the D-band and the G-band (D/G) were 1.42, 1.15, 1.30, and 1.28 in the case of raw SiC and SiC modified
Figure 2. Raman spectra of SiC-A nanoparticles (a) before and after surface modification with (b) AIBN, (c) AMPA, and (d) AMCPA.
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Iijima and Kamiya
Figure 5. Surface potential of SiC-A nanoparticles in water with various pH values before and after surface modification with AIBN, AMPA, and AMCPA.
Figure 3. FT-IR spectra of SiC-A nanoparticles (a) before and after surface modification with (b) AIBN, (c) AMPA, and (d) AMCPA.
Figure 4. Reacted amounts of azo radical initiators on SiC-A nanoparticles.
with AIBN, AMPA, and AMCPA, respectively. We can see that the values of the intensity ratio D/G reduced with surface modification. This indicates a decrease in unsaturated hydrocarbons by their reactions with azo radical initiators. Figure 3 shows the FT-IR spectrum of SiC-A nanoparticles before and after surface modification. The bands at 1630, 1567, and 1420 cm-1 can be attributed to CdN bonds of imine groups, N-H bending vibrations, and asymmetric vibration of COO-, respectively.24–26 As compared to raw SiC-A nanoparticles, the band corresponding to the COO- groups increased when the particles were modified with AIBN and then hydrolyzed with NaOH aqueous solution. Further, the band corresponding to the N-H and CdN groups increased when the particles were modified with AMPA. When SiC-A nanoparticles were modified by ACMPA, an increase in the intensity of the band corresponding to COO-, N-H, and CdN was observed. The organic functional groups related to each azo radical initiator (e.g., COO- groups for AIBN, CdN and N-H groups for AMPA, and COO-, CdN, and N-H groups for AMCPA) were introduced on the surface of SiC nanoparticles by a radical reaction, as shown in Scheme 1. Figure 4 shows the reacted amount of azo radical initiators on SiC-A nanoparticles; 5.4, 4.1, and 2.1 µmol of azo radical initiators were reacted on 1 m2 of SiC surface when they were modified with AIBN, AMPA, and AMCPA, respectively. The effect of surface modification with each radical initiator on the surface potential of SiC-A nanoparticles in water at various pH values is shown in Figure 5. In the case of raw SiC-A nanoparticles, the surface is negatively charged at a pH >3. A
decrease in the surface potential at a pH >5 due to the dissociation of the carboxyl group that originally existed on raw SiC-A also was observed. In comparison to raw particles, when SiC-A nanoparticles were modified with AIBN, the surface potential decreased by ∼10 mV at a pH >5, while it did not change at pH 3. Because the density of the carboxyl group was successfully increased by surface modification with AIBN, the surface potential decreased when the groups were dissociated at a pH >5. SiC-A nanoparticles modified with AMPA showed different properties as compared to those modified with AIBN. In the case of the former, while the surface potential did not change at pH 11, in comparison to raw particles, it increased significantly as the pH of the solution decreased. This was because the protonation of amine and imine groups, which were introduced by surface modification with AMPA, occurred at low pH values. Because the surface of SiC-A nanoparticles modified with AMCPA consisted of both anionic groups (carboxyl groups) and cationic groups (amine and imine groups), the surface potential exhibited amphoteric properties. At pH 3, since the carboxyl groups did not dissociate while amine and imine groups were protonated, the surface potential significantly increased as compared to the case of raw SiC-A nanoparticles. At pH 5-9, carboxyl groups and cationic groups dissociated and protonated, respectively. In AMCPA, since the number of cationic groups is greater than that of anionic groups (Figure 1c), the surface potential of SiC-A nanoparticles modified with AMCPA slightly increased in this pH region. At pH 11, since the cationic groups did not protonate while carboxyl groups dissociated, the surface potential significantly decreased, as compared to the case of raw particles. Figure 6 shows the sedimentation behavior of SiC-A nanoparticles modified by azo radical initiators in an aqueous medium that was allowed to settle for 6 h. The average aggregated size of SiC-A nanoparticles in aqueous medium after surface modification is shown in Figure 7. In Figure 7, the aggregated size of raw SiC-A nanoparticles in methanol, which was the solvent during the surface modification procedure, also is shown. When raw SiC-A nanoparticles were dispersed into water, they rapidly formed agglomerates and sediments (Figure 6a); the average aggregate size was determined as ∼520 nm at pH 3 and decreased to ∼440 nm (Figure 7) due to the slight increase of electrostatic repulsive force of dissociated carboxyl groups (Figure 5). As shown in Figures 6 and 7, the dispersion stability of SiC-A nanoparticles greatly improved by surface modification with azo radical initiators due to the generation of hydrophilic groups on SiC-A nanoparticles. The pH value at which the dispersion stability of a modified SiC nanoparticles/water suspension improves can be controlled by the chemical structure
Surface Modification of SiC Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11789
Figure 8. Stability of SiC-B nanoparticles in water at various pH values. SiC-B nanoparticles (a) before modification and (b) after modification with AMCPA. The suspension was allowed to settle for 6 h.
Figure 6. Stability of SiC-A nanoparticles in water at various pH values: SiC-A nanoparticles (a) before surface modification and after modification with (b) AIBN, (c) AMPA, and (d) AMCPA. The suspension was allowed to settle for (a) 10 min and (b-d) 6 h.
Figure 9. Surface potential of SiC-B nanoparticles in water with various pH values (a) before and (b) after surface modification with AMCPA.
Figure 7. Average aggregated size of SiC-A nanoparticles in water with various pH values before and after surface modification with AIBN, AMPA, and AMCPA.
of azo radical initiators. When SiC-A nanoparticles were modified by AIBN, they showed an anionic property: good dispersion stability at pH >5 where the generated carboxyl groups dissociated. While the average aggregated size was measured as ∼460 nm at pH 3, they reduced to ∼340 nm at pH >5. Contrarily, when SiC-A nanoparticles were modified with AMPA, they showed a cationic property: a significant improvement in the dispersion stability at pH 3, the IEP of this raw SiC-A nanoparticle was below pH 3; this suggests the partial oxidation of SiC-A nanoparticles. To investigate the effect of surface structure of SiC nanoparticles on their dispersion stability in aqueous medium and the degree of surface modification by azo radical initiators, we modified the surface of SiC-B nanoparticles that were from a different vender. These particles were characterized to have a nearly stoichiometric C/Si composition (C/Si ) 0.96) from the whole SiC nanoparticles but have extra carbon and oxygen species at the surface that were due to the silicon oxycarbide phase.27 Figures 8 and 9 show the sedimentation behavior of SiC-B nanoparticles in an aqueous medium that were allowed to settle for 6 h and the surface potential of SiC-B nanoparticles in water with various pH values, respectively. Since these SiC-B nanoparticles were hydrophilic with less free carbon species and rich silicon oxycarbides at the surface, they were stable in aqueous medium when the pH was >5 even before surface modification. From Figure 9, we observe that the IEP of SiC-B nanoparticles was nearly pH 4, although extra oxygen species were detected at the surface. As already mentioned in the case of SiC-A nanoparticles, if the surfaces of SiC nanoparticles were oxidized and silanol structures formed,
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Iijima and Kamiya by the structure of azo radical initiators. Thus, reactions with azo radical initiators could be a strategic method to design the surface of SiC nanoparticles. References and Notes
Figure 10. Average aggregated size of SiC-B nanoparticles in water with various pH values (a) before and (b) after surface modification with AMCPA.
the IEP should shift to lower pH values.28 By taking into account the IEP of SiC-B nanoparticles, we can characterize that SiC-B nanoparticles have a rich oxycarbide phase and a less oxidized phase at the surface. When the pH value was 3, the SiC-B nanoparticles aggregated due to the small value of the surface potential. By modifying their surface by ACMPA, the surface potential of SiC nanoparticles slightly increased at pH 3 and decreased at pH >5 (Figure 9), which resulted in the improvement of dispersion stability (Figures 8 and 10). The surface modification of SiC nanoparticles by an azo radical initiator can also be occurred with less free carbon and rich silicon oxycarbide species on their surface. However, the reacted amounts of azo radical initiators were small (0.9 µmol/m2) as compared to the case of modifying SiC-A nanoparticles with free carbon species. It is expected that the existence of free carbon species on SiC nanoparticles can enhance their surface modification with azo radical initiators. Conclusion The presence of unsaturated hydrocarbon species on the surface of SiC nanoparticles was detected by Raman spectroscopy. We successfully reacted azo radical initiators with such unsaturated hydrocarbons and increased the density of hydrophilic groups such as amines, imines, and carboxyl groups on the surface of SiC nanoparticles. By the surface modification of SiC nanoparticles with azo radical initiators, the dispersion stability of SiC nanoparticles in water significantly improved at certain pH values. The pH values where the stability of the surface-modified SiC nanoparticles improved can be controlled
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