FT-IR and 1H NMR results
Figure 3 shows the FT-IR spectra of (a) HBPE, (b) HBP-SH, (c) Cu@HBP-SH, and (d) Cu@HBP-SH@Ag. The characteristic peaks of Cu particles coated by HBP-SH are marked. In Fig. 3c, the peak at 3420 cm−1 is assigned to O–H stretching vibration of HBP-SH, indicating presence of unesterified hydroxyl groups in HBP-SH. The band observed at around 2942 cm−1 is assigned to stretching vibration of –CH2 and –CH3. The peak seen at 2576 cm−1 is characteristic of –SH. The peak observed at around 2682 cm−1 corresponds to stretching vibration of S–CH2. The strong peak at 1720 cm−1 is assigned to carbonyl bond of ester groups. The peaks observed at 1154 cm−1 and 1048 cm−1 are assigned to symmetric stretching of C–O and asymmetric stretching of HBP-SH. The reaction of mercaptoacetic acid with HBPE was further confirmed by 1H-NMR. Figure 4 shows the 1H-NMR spectrum of HBP-SH. The chemical shift observed at 1.5 ppm (Fig. 4, peak a) indicates the presence of –SH group. Proton signals were also observed at 3.5 ppm to 4.0 ppm (–OOC–CH2–S– and –CH2–N<, Fig. 4, peaks b and e), 4.3 ppm (–C–CH2–O–, Fig. 4, peak c), 2.9 ppm (–N–CH2–, Fig. 4, peak d), and 2.5 ppm (–OOC–CH2, Fig. 4, peak f). These results confirm the coating of HBP-SH on the surface of the copper particles.16,17
Figure 5 shows the UV–Vis spectra of HBP-SH (a), Cu@HBP-SH (b), HBPE (c), and Cu@HBP-SH@Ag (d). It can be seen that the spectral profiles of HBP-SH (a) and HBPE (c) are similar. They all show sharp absorption bands at around 210 nm. The molecular structures of HBPE and HBP-SH contain ester groups with a conjugated carbon–oxygen double bond (C=O). These can produce π*–π* electronic transitions.18,19 Cu@HBP-SH (Fig. 5b) also shows a sharp absorption at around 205 nm, confirming that the surface of the Cu particles was coated with HBP-SH. After reaction with Ag, a new absorption peak appeared at around 425 nm for the Cu@HBP-SH@Ag (Fig. 5d) particles. This peak corresponds to the surface plasmon resonance absorption of the silver metal particles. As is known, Ag nanoparticles show an intense plasmon absorption band in this region. This analysis confirms that the Ag particles were coated on the copper.
Figure 6 shows the XRD patterns of Cu@HBP-SH (a) and Cu@HBP-SH@Ag (b) particles, clearly showing peaks characteristic of Cu particles: Cu (1 1 1) orientation at 43.2°, Cu (2 0 0) at 50.6°, and Cu (2 2 0) at 74.2°. After coating the Cu particles with Ag, the characteristic peaks of Ag were detected: Ag (1 1 1) orientation at 38.2°, Ag (2 0 0) at 44.4°, Ag (2 2 0) at 64.6°, and Ag (3 1 1) at 77.6° (Fig. 6b). These peaks are consistent with literature values.20,21 Because Ag was completely coated on the surface of Cu, the diffraction peaks were dominated by the Ag particles. These XRD results show that the Cu@HBP-SH@Ag particles were successfully synthesized.
Figure 7 shows the SEM-EDX images of the Cu@HBP-SH and Cu@HBP-SH@Ag particles. Figure 7a shows that the Cu@HBP-SH particles had a spherical structure with diameter of 1 μm to 2 μm. The elemental composition of the Cu@HBP-SH particles were measured by EDX (Fig. 7b), revealing signals for Cu, C, O, and S. These results indicate that the surface of copper was covered with HBP-SH. Figure 7c shows a SEM image of Cu@HBP-SH@Ag, revealing that Cu@HBP-SH@Ag particles with a smooth surface were obtained after Ag aggregation. Compared with Cu@HBP-SH, the particle size of Cu@HBP-SH@Ag was larger. All these results confirm the deposition of Ag on the surface of the Cu@HBP-SH particles. Figure 7d shows the EDX spectrum of Cu@HBP-SH@Ag. The EDX spectra confirmed the presence of elements Ag, Cu, C, O, and S, further indicating the formation of Cu@HBP-SH@Ag particles.22,23
The thermal properties of the samples were characterized by TGA in air atmosphere. Figure 8 shows the TGA curves of HBPE, HBP-SH, Cu@HBP-SH, and Cu@HBP-SH@Ag. It can be seen that the HBPE and HBP-SH showed similar TGA curves and all the samples showed two-step degradation profiles (Fig. 8A, a and b). The first weight loss of around 60% occurred between around 250°C and 350°C, being due to decomposition of outer terminal groups. The second weight loss of about 30% occurred in the temperature range from 350°C to 500°C. This can be attributed to decomposition of the core of the hyperbranched polymer. It can also be seen that the decomposition onset temperature of HBPE was higher than that of HBP-SH. As is known, the decomposition temperature of hyperbranched polymers depends on their terminal groups and molar mass. Although the molecular weight of HBP-SH is larger than that of HBP, the HBPE has a large number of OH groups, which favor the formation of strong hydrogen bonds between molecules. Therefore, HBPE shows better thermal stability than HBPE-SH. 24,25 Figure 8B shows the TGA curves of Cu@HBP-SH and Cu@HBP-SH@Ag particles, which show increasing parts for Cu@HBP-SH and Cu@HBP-SH@Ag because of the oxidation of copper. For Cu@HBP-SH and Cu@HBP-SH@Ag, weight loss occurs below 300°C, which can be attributed to decomposition of HBP-SH. Weight increases of 9% and 2% can also be observed for Cu@HBP-SH and Cu@HBP-SH@Ag, respectively (Fig. 8 B, c and d). This indicates that the Cu@HBP-SH@Ag particles showed better antioxidation properties than Cu.26