Surface morphology of the original PS particles to serve as cores for the coating fabricated by emulsion polymerization is shown in Figure a. It can be seen that the obtained PS particles are spherical in shape with smooth surface. The average diameter and potential of PS particles as measured by ZS Nano Essentials were 450 nm and −34.5 mV, respectively. The core particles were monodispersed. The as-fabricated, monodispersed PS latex is essential for the fabrication of high-quality PS/Ag CSSNs.
SEM images. PS core particles as fabricated (a) and after PEI surface modification (b).
Since these PS particles are covalently bonded whereas the nature of bonds in Ag is metallic, it is difficult to bond Ag atoms directly onto the PS surface. Therefore, surface modification with functional groups is essential for the purpose. Thus, cationic polyelectrolyte PEI was used to attach functional group of NH on the PS surface as shown in Figure b. The functional group of NH here mainly acts as a linker that easily coordinates with Ag ions in the solution. The polyelectrolyte PEI adsorption on the PS particles occurs spontaneously due to the driving force provided by electrostatic attraction, rendering a reverse of the surface charge of the PS particles. After the surface modification, the Zeta potential of the PEI-modified PS particles was measured to be 25.1 mV.
Figure a,b shows the EDS analysis for chemical compositions of the PS particles and the PEI-modified PS particles. As shown in Figure a, except the peak of Si from the supporting substrate, there is only a strong absorption peak of carbon in the PS particles where the PS particles contain only carbon and hydrogen, and the hydrogen cannot be detected by EDS. Compared with Figure a, a weak peak for nitrogen appeared in the PEI-modified PS particles as given in Figure b, which illustrates the component of the functional group of NH in the PEI. Figure c,d gives a comparison between the infrared (IR) transmittance spectra of the PS particles and the PEI-modified PS particles. The absorption band at 3,440 cm−1
is due to the presence of hydroxyl group on the PS spheres, while the other absorption bands at 3,028, 2,925, 1,608, 1,497, 1,454, 760, and 700 cm−1
are due to the asymmetric, symmetric, and deformed vibrations of the -CH2
group of the benzene ring. It is obvious that the broad absorption peak at 3,412 cm−1
is typical of the N-H bond stretching vibrations, and peaks at 1,601, 1,498, and 1,450 cm−1
stem from stretching vibrations of the C-C bonds in the benzene ring [22
]. The IR spectra indicate as well that the PEI has been attached on the PS particles.
EDS and FTIR spectra. EDS spectra of PS particles (a) and PEI-modified PS particles (b) on Si substrate; FTIR spectra of PS particles (c) and PEI-modified PS particles (d).
Figure shows the SEM images of the PEI-modified PS particles coated with Ag at different stages. Figure a shows that during the seeding stage after the PEI modification, the Ag seeds uniformly coated on the surface of the PEI-modified PS colloids. However, the colloids adhered together due to the presence of the PEI (the case is the same for the adhesion of the PEI-modified PS colloids as shown in Figure b). It should be noted that although the PEI herein adsorbed on the PS core surface can bond with Ag ions via the immense interaction between the NH groups and the Ag ions, the PEI can also act as a reductant in the fabrication of Ag nanoparticles [20
]. Figure b shows the final products of the PS/Ag core-shell-shaped nanostructures as obtained by reduction of the Ag+
ions with the sodium citrate. The products were washed by deionized water where the excess PEI was removed, and thus, the isolated core-shell structure can be obtained. We can see that the obtained PS/Ag CSSNs are well-dispersed, uniformly coated with a good coverage of Ag where the coated metallic Ag is not oxidized as confirmed in Figure .
SEM images of PS/Ag CSSNs at different stages of fabrication. After seeding where Ag ions were coated on the surface of PEI-modified PS particles at 100°C (a); after reducing where Ag ions were reduced at 80°C (b).
Our study shows that different fabrication parameters have obvious influence on the structure of the PS/Ag CSSNs. For this reason, we studied systematically crucial influences of the key parameters on the structure of the PS/Ag CSSNs. First, the influence of temperature was investigated, and the results are shown in Figure . In order to avoid any obvious glass transition of the PS, we studied the processing temperature not higher than 100°C. The AgNO3 was added into the PEI-modified PS colloids, stirred or seeded at different temperatures (60°C, 80°C, and 100°C) for 1 h, and subsequently reduced by sodium citrate for another 30 min at 80°C as shown in Figure a,b,c. We can see that the coverage of Ag nanoparticles was increased with the seeding temperature, which can be explained by crystal nucleation mechanism. It is well known that Ag was deposited on the surface of the PEI-modified PS cores by the reducibility of functional groups of NH. For this reason, Ag ions were reduced and formed crystal nuclei. However, only those nuclei which overcomed critical nucleation energy could become stable and able to grow. As we have known, the critical nucleation energy was decreased with the temperature. Therefore, the higher the temperature is, the more silver nanoparticles will be formed. Obviously, the seeding temperatures of 60°C and 80°C were too low for the seeding of the Ag nanoparticles onto the PS colloids, whereas the best seeding temperature is 100°C at which the PS cores were well coated.
Figure 6 SEM images of PS/Ag CSSNs fabricated at different temperatures of seeding and reduction. (a) 60°C and 80°C, (b) 80°C and 80°C, (c) 100°C and 80°C, (d) 100°C and 70°C, (e) 100°C and (more ...)
Figure d,e,f shows the SEM images of PS/Ag CSSNs seeded with the same nucleation temperature at 100°C for 1 h but reduced or grew for 30 min with the sodium citrate at different temperatures (70°C, 90°C, and 100°C). We can see that the size of the Ag particles gradually increases with the reducing temperature. It indicates that the key factor affecting the crystal growth via reduction is the diffusion of ions in the solution. With the increase in temperature, the diffusion process becomes facile, and the size of Ag nanoparticles becomes larger. Nonetheless, the size of Ag nanoparticle is too large as reduced at temperature higher than 90°C which effects homogeneity of the PS/Ag CSSNs. As a consequence, the best coating process is seeding at 100°C and then reducing at 80°C.
The seeding time is another important factor in the fabrication, which was found to have a particular effect on the Ag coverage of the PS/Ag CSSNs. We can see this obviously from Figure . In Figure a, few Ag nanoparticles seeded, but they barely covered the PS cores in 20 min. As the time increased, more Ag nanoparticles nucleated, and the nucleated Ag nanoparticles further grew up. Thus, PS cores become more covered with the Ag nanoparticles as shown in Figure b,c. Finally, the change of the PS/Ag CSSNs' Ag coverage is not so obvious after 1 h later as seen in Figure d. The observation indicates that there were more Ag+ ions in the solution which could transport to the Ag crystal nuclei as the time increased. However, the system seemed to reach the state of saturation with the PEI after the modification in a certain time later. As a result, the best time for the Ag seeding for the advanced fabrication of the PS/Ag CSSNs should be longer than 1 h.
SEM images of PS/Ag CSSNs. Seeded at 100°C for (a) 20 min, (b) 40 min, (c) 1 h, and (d) 1.5 h.
The environment of the Ag+
in solution in many cases can be critically important for the rational control over surface and interfacial modifications of the molecular absorption process. The commonly used polyelectrolyte binders are very sensitive to their local ionic environment [23
]. Our experiments demonstrated that when silver ammonia solution instead of silver nitrate was added into the PEI-modified PS colloids as shown by the comparison of Figure b,a, the core-shell-shaped structures easily adhered together and were not well coated. Therefore, Ag ions hydrolyzed from AgNO3
were used widely in the latest papers. The detailed mechanism for the difference is under a further exploration.
SEM images of PS/Ag CSSNs. Fabricated via seeding at 100°C for 1 h by different Ag ions: (a) AgNO3 and (b) silver ammonia solution.