Figure 6.3a shows the aggregates of Fe3O4 nanoparticle chains and nanoparticles. Y-junctions made up of Fe3O4 nanoparticles (average diameter of 10 nm) are observed in the enlarged TEM image (indicated by arrows in Figure 6.3b). It is difficult to obtain a good TEM picture of more isolated Y-junctions because of the thermal motion experienced at ambient temperatures. The presence of many individual nanopar-ticles indicates that a snapshot of dynamic clusters was observed, which is consistent with that reported by Butter et al. [70].
The presence of Fe3O4 is identified from the electron diffraction pattern and the x-ray diffraction pattern (Figure 6.4). The patterns obtained confirm that the nanoparticles prepared in this study are Fe3O4 nanoparticles [71]. The Bragg reflection peaks are all relatively broad because of the extremely small dimensions of the Fe3O4 nanoparticles.
FIGURE 6.2 Schematic formation of PANI nanorods and nanotubes with Y-junctions. (Xia, H., Cheng, D., Xiao, C., Chan, H.S.O. J. Mater. Chem., 2005, 15, 4161.Reproduced by permission of The Royal Society of Chemistry.)
FIGURE 6.3 TEM images of assemblies of Fe3O4 nanoparticles and aniline: (a) before polymerization, (b) an enlargement of a part of (a). (Xia, H.; Cheng, D.; Xiao, C.; Chan, H.S.O. J. Mater. Chem., 2005, 15, 4161. Reproduced by permission of The Royal Society of Chemistry.)
APS Addition APS Addition
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6.7 Morphology
In this section some important issues of morphology as related to PANI products are presented.
6.7.1 Effect of Aniline Concentration on Morphology of PANI Products The TEM picture of PANI nanorods obtained at an optimal [An] of 0.10 mol/L is shown in Figure 6.5.
The average diameter of the PANI nanorods is about 80 nm (Figure 6.5a). One clear case of a single Y-junction PANI nanorod is shown in Figure 6.5b. Figure 6.5c shows how multiple Y-junctions can FIGURE 6.4 (a) Selected areaelectron diffraction pattern of Fe3O4 nanoparticles deposited from dispersion, (b) x-ray diffraction pattern of Fe3O4 nanoparticles on glass substrate. (Xia, H.; Cheng, D.; Xiao, C.; Chan, H.S.O.
J. Mater. Chem., 2005, 15, 4161. Reproduced by permission of The Royal Society of Chemistry.)
FIGURE 6.5 TEM photos of PANI nanorods with Y-junctions ([An] = 0.10 mol/L, [An]: [APS] = 1, reaction time
= 18 h, temperature = 2.5°C, initial pH = 8). (Xia, H.; Cheng, D.; Xiao, C.; Chan, H.S.O. J. Mater. Chem., 2005, 15, 4161. Reproduced by permission of The Royal Society of Chemistry.)
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coexist, which is consistent with the results of the self-assembly of Fe3O4 nanoparticles reported by Butter [70] and works based on computer simulations [72,73]. In addition, it is interesting to note that some X-junctions are also present due to the connection of the Y-junction to another nanorod. This result confirms that higher levels of magnetic particle organization are possible, and some of the PANI chains may form long, ringlike structures or connect to form an extended network [70,73,74]. About 30% of the products are well-fined Y-junction structures.
The morphology of PANI nanorods prepared at other concentrations of aniline is shown in Figure 6.6.
When [An] was at a low of 0.05 mol/L, the dendritic structures of the PANI nanorods obtained (Figure 6.6a) are very similar to those predicted by analytical simulations [72,73]. When [An] is increased from 0.10 to 0.20 mol/L, the diameter of PANI nanorods increases only a little compared to the increase in length (shown in Figure 6.6b), but there are fewer Y-junctions. The increase in [An] may inhibit the formation of the Fe3O4 templates, and only normal PANI nanorods are formed because the polymeriza-tion of cylindrical micelles formed by aniline monomers in the reacpolymeriza-tion system is likely to dominate [68].
Accordingly, PANI nanotubes with Y-junctions were prepared at an initial pH of about 5 as compared to 8 for the nanorods. No Y-junction PANI nanotubes or nanorods can be obtained if the initial pH of reaction system is below 3. This is because the template is dissolved in this low pH before polymerization.
Similar to the PANI nanorods, the morphology of PANI Y-junction nanotubes is also affected by the aniline concentration (Figure 6.7). TEM image (Figure 6.7a) shows that the Y-junction PANI nanotube has an outer diameter of around 70 nm and an inner diameter of about 10 nm, which is equal to the average diameter of the Fe3O4 nanoparticles. The angle θ enclosed between the lower arms of the Y-junction nanotube is about 60°. Figure 6.7a shows that some Fe3O4 nanoparticles are still present, and perfectly hollow PANI nanotubes can be formed (Figure 6.7b) by reducing pH of the sample solution to about 1 and kept for 24 h before characterization.
When [An] is increased from 0.13 to 0.26 mol/L, for example, the outer and inner diameters of PANI nanotubes changed significantly, but the wall thickness only changed slightly (Figure 6.7c). As found in the PANI nanorods, Y-junction nanotubes disappeared (Figure 6.7d) when [An] is too high, for the same reasons outlined for the PANI nanorods.
6.7.2 Influence of Organic Solvent on Morphology of PANI Products
As previously mentioned, self-assembled flexible chains of magnetic particles are determined by a thermo-dynamic balance of forces [75,76]. If the self-assembled flexible chains of Fe3O4 nanoparticles acted FIGURE 6.6 TEM images of PANI nanorods at other concentrations of aniline: (a) 0.20 mol/L, (b) 0.05 mol/L ([An]: [APS] = 1, reaction time = 18 h, temperature = 2.5°C, initial pH = 8). (Xia, H., Cheng, D., Xiao, C., Chan, H.S.O. J. Mater. Chem., 2005, 15, 4161. Reproduced by permission of The Royal Society of Chemistry.)
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as the templates in the formation of Y-junction PANI nanorods, the presence of an organic solvent is expected to have some influence on the shape of the PANI products. When an organic solvent, ethanol, is added into the reaction system (ethanol/Fe3O4 solution = 1:10 by volume), the Y-junction PANI nanorods are able to form rings (Figure 6.8a), and the chosen overlapping region of PANI nanorods is enlarged and clearly shown in Figure 6.8b.
FIGURE 6.7 TEM images of PANI products at different concentrations of aniline: (a) 0.13 mol/L, (b) 0.13 mol/L after treatment in HCl (pH = 1) solution, (c) 0.20 mol/L, (d) 0.26 mol/L ([An] : [APS] = 1, reaction time = 18 h, temperature = 2.5°C, initial pH = 5).
FIGURE 6.8 (a) TEM images of PANI rings with Y-junctions, (b) an enlargement of a part of (a) ([An] = 0.10 mol/L, [An ] : [APS] = 1, reaction time = 18 h, temperature = 2.5°C, initial pH = 8, ethanol/Fe3O4 solution = 1:10 by volume). (Xia, H., Cheng, D., Xiao, C., Chan, H.S.O. J. Mater. Chem., 2005, 15, 4161. Reproduced by permission of The Royal Society of Chemistry.)
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Alternatively, aggregates of PANI-Fe3O4 composite are obtained when an organic solvent, toluene, immis-cible with water, is added into the reaction system (toluene/Fe3O4 solution = 1:10 by volume). That is because the toluene solvent destroys the surfactant layer of the water-soluble Fe3O4 nanoparticles (Figure 6.9a), and then makes Fe3O4 nanoparticles hydrophobic. Consequently, the normal PANI-Fe3O4 composite is obtained (Figure 6.9b).
Controlled experiments were also carried out. Normal PANI nanostructures (rods or tubes) are obtained if aniline is polymerized under the same conditions without the Fe3O4 nanoparticles [68]. The incorporation of NP5 and CD produced nanoparticles only (Figure 6.10). These results confirm the important contribution of the Fe3O4 nanoparticles as the template. In addition, our results are significantly different from polyaniline-Fe3O4 composites [77,78], in which Fe3O4 nanoparticles are hydrophobic and there are no surfactant layers. When the concentration of Fe3O4 nanoparticles is very low (Figure 6.11) in our system, PANI nanofibers are obtained. This result is consistent with those reported by Huang et al.
[79]. Under this condition, the Fe3O4 nanoparticles cannot act as a template, and aniline is polymerized into a fibril structure.
FIGURE 6.9 TEM images of Fe3O4 nanoparticles and aniline monomers in the reaction system (toluene/Fe3O4 solution = 1:10 by volume): (a) before polymerization, (b) PANI-Fe3O4 composite after polymerization.
FIGURE 6.10 TEM image of PANI particles obtained in presence of NP5 and CD.
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