3.1 Gelation of hydrophilic polymer bearing metal-coordination unit with Au(III) ion
HPMC was synthesized by the reaction of poly(vinyl alcohol) with methyl isothiocyanate according to our reported procedure.27 To obtain the porous gels, dispersed aqueous solutions (0.3 mL, pH 1) of different concentrations of HPMC (5, 10, 15, 20, 25, and 30 wt%) were added dropwise to aqueous solutions of NaAuCl4 (20 mL, pH 1) in a test tube, and the mixtures were allowed to stand at room temperature for 5 h. When the dispersed aqueous solution of HPMC with 5 wt% concentration was added to Au(III) aqueous solution, gelation did not occur and the HPMC solution fell to the bottom of the test tube (Fig. 1a). A 10 wt% concentration of HPMC resulted in less than 1% yield (Fig. 1b). These results indicate that low concentration of HPMC did not promote the cross-linking. On the other hand, gelation of 30 wt% HPMC with Au(III) occurred immediately after the dropwise addition of the HPMC solution. Instant separation between aqueous phases of HPMC and Au(III) was seen due to the higher hydrophobicity of the HPMC phase than that of the Au(III) phase and the fast cross-linking at the interface. The cross-linking with Au(III) proceeded from the surface to the inside of the droplets of HPMC solution to afford aggregated gels. When the concentration of HPMC was decreased to 25 wt%, the formed gels were slightly elongated (Fig. 1e). Finally, the gelation of HPMC (15 and 20 wt%) with Au(III) provided elongated and fibrous gels (Figs. 1c and 1d). Thus, the gelation behavior is explained as follows. In the gelation of 5 and 10 wt% HPMC, the low concentration of metal-coordination units led to poor cross-linking ability, and little gelation occurred. In contrast, the cross-linking ability was improved by the use of a high concentration HPMC (25 and 30 wt%). Consequently, cross-linking occurred immediately after dropwise addition of HPMC solution, resulting in the formation of aggregated gels on the surface of the Au(III) aqueous solution. For 15 and 20 wt% HPMC, Au(III) gradually cross-linked with HPMC at the appropriate cross-linking rate to provide elongated and fibrous gels.
Table 1 summarizes the degree of cross-linking and the yield of the gels. In order to determine the degree of cross-linking of Au(III), the concentration of the supernatant solution after gelation was measured by AAS. The degree of cross-linking of Au(III) in the gels (number moles of Au adsorbed per gram of polymer) was calculated using:
Degree of cross-linking (mmolAu/gHPMC) = amount of cross-linked Au(III) (mmol) / weight of HPMC (g) (1)
To determine the yields, the obtained gels were washed with distilled water and the unreacted HPMC and NaAuCl4 were removed by Soxhlet extraction with distilled water. The gels were dried to a constant weight at 70 oC in vacuo to obtain the dry gel.
The degree of cross-linking for gels obtained from 10 wt% HPMC dispersed aqueous solution was low due to the little gelation occurring as described above (Table 1, entry 1). The degree of cross-linking for 25 and 30 wt% HPMC were lower (entries 4 and 5) due to the low surface area of the formed aggregated gels. In contrast, the degree of cross-linking for 15 and 20 wt% HPMC were higher (entries 2 and 3) due to the higher surface area (Figs. 1c and 1d), as seen in the SEM images of the gels (vide infra). The yield increased as the degree of cross-linking increased, suggesting that the cross-linking density increased with increasing the degree of cross-linking. The yields were calculated using:
Yield (%) = Wgel / (WHPMC + WAu(III)) (2)
where Wgel is the weight of the obtained dry gel, and WHPMC, and WAu(III) are the weights of HPMC and Au(III), respectively, used in the reaction.
Table 1
Gelation behavior of HPMC with Au(III).a)
entry | HPMC concentration (wt %) | cross-linking amount (mmol) | degree of cross-linking (mmolAu/gHPMC) | yield (%) |
1 | 10 | 0.018 | 6.31 | < 1 |
2 | 15 | 0.027 | 9.47 | 12 |
3 | 20 | 0.019 | 46 | 11 |
4 | 25 | 0.012 | 50 | 7 |
5 | 30 | 0.010 | 16 | 9 |
Conditions: Dispersed aqueous solution of HPMC (0.3 mL, pH 1); aqueous solutions of NaAuCl4 (4.00 mmol/L, 20.0 mL, pH 1); room temperature; rection time = 5 h.
To determine the morphology of the obtained gels, an SEM analysis was conducted (Fig. 2). Whereas the porosity was low in the gel obtained from the 25 wt% HPMC dispersed aqueous solution (Fig. 2a), porous gels were obtained at 15 and 20 wt% (Figs. 2b and 2c). The porous gel obtained at 15 wt% was used for structural characterization. To confirm the Au coordination sites, IR spectroscopy was performed before and after gelation. Figure 3a shows the IR spectra of HPMC and the gel. The absorption peak at 1550 cm− 1 assignable to C = S stretching vibrations becomes smaller after gelation. This indicates that the thiocarbonyl groups coordinated to Au. The narrow-scan XPS spectrum shown in Fig. 3b (bold black line) exhibits Au4f5/2 and Au4f7/2 peaks. Because the peaks for Au4f5/2 and Au4f7/2 had shoulders on their high-bonding-energy side, they were fitted using a Gaussian Lorentzian (G-L) function. The peaks at 84.5 and 88.1 eV, and at 85.8 and 90.1 eV, were assigned to Au(0) and Au(III), respectively (broken black and solid red lines).27,29–31 The ratio of the Au(0) to Au(III) was 88:12. The TEM image in Fig. 3 shows that the Au nanoparticles (black dots) are homogeneously dispersed in the gel. These results suggest that almost Au(III) ions were reduced to Au(0) during the gelation process and the formation of Au nanoparticles occurred simultaneously, because Au(III) is a strong oxidant.32–34
The proposed mechanism for formation of Au(0) nanoparticles in the gels is shown in Scheme 2. The structure of HPMC suggests tautomerism between thiocarbonyl- and thioenol-type structures. Thiol groups of the thioenol structure are oxidized by tetrachloroaurate ions (AuCl4−) due to the high oxidizability of Au(III),32–34 resulting in the formation of Au-S cross-links and gold clusters (step (i)). The reduction of AuCl4− is catalyzed on the surface of the clusters, allowing the gold nanoparticles to grow and become stable on the porous gels (step (ii)).
3.2 Synthesis of porous Au
The synthesis of porous Au utilizing the porous gels as templates was next examined. To determine the calcination temperature for the polymer parts in the gels, TG analysis of the porous gel obtained using 15 wt% HPMC was carried out. TG curve showed some degradation steps but no weight loss was observed between 500 and 600 oC, indicating that no polymer remained in that temperature range (see Supporting Information, Figure S1). Consequently, we determined the calcination temperature for the gels to be 550 oC. The porous gels obtained with 15, 20, and 25 wt% HPMC were heated at 5 oC/min and calcined at 550 oC for 2 h. SEM analysis showed that few pores were present in the gel with 25 wt% HPMC (Fig. 4a). In contrast, SEM analysis of the gels obtained with 15 and 20 wt% HPMC revealed the formation of porous Au containing micrometer size pores (Figs. 4b and 4c). The morphology of the porous Au reflects that of the gel before calcination; therefore, the porous gel acts as a template for the synthesis of porous Au. The XPS spectrum of the porous Au showed Au 4f5/2 and Au 4f7/2 peaks at 87.4 and 83.7 eV, respectively, which are typical of Au(0) species (Fig. 4d).27,29–31 All Au(III) ions in for the gels were reduced during calcination under an Ar atmosphere. The TG analysis results the porous Au indicated almost no weight loss, and the purity of the porous Au obtained from gels produced with 15 and 20 wt% HPMC was very high (96 and 99%, respectively). The corresponding electrical conductivity was 0.85 and 0.40 S/m, respectively, while the gels alone indicated no conductivity.
The morphology of the porous Au reflects that of the porous gel, indicating that the porous gel acts as a template. TEM analysis of the porous gels showed that Au nanoparticles were homogeneously dispersed in the gels. The proposed formation mechanism for the porous Au is that calcination of the porous gels causes the polymer parts to degrade and become volatilized. Self-assembly of the remaining Au nanoparticles then gives rise to a three-dimensional micropore structure.