Here, Au-Rh@SiO2 core-shell NPs are used as an example to illustrate the one-pot synthesis of silica-coated bimetallic Au-M (M = Rh, Pd, Ir and Pt) NPs. The morphology, structure and size of the samples were characterized by TEM. As shown in Fig. 1(a) and 1(b), the as-prepared sample possesses core-shell structure and good monodispersity. The elemental mapping analysis of the sample in Fig. 1(d-f) demonstrates that Rh is distributed uniformly in the side of Au, and Au-Rh NPs were coated by silica shell, which further affirms the formation of core-shell structure. It is concluded that Au-Rh@SiO2 NPs with core-shell structures were synthesized successfully by this one-pot method. Au-Rh@SiO2 NPs have a core-shell structure. To further clarify the crystalline structure between Au and Rh, Au-Rh NPs synthesized without TEOS were characterized by TEM and high-resolution TEM. As shown in Fig. S1 in the ESM (Electronic Supplementary Material), Au-Rh NPs with core-satellite structure are similar to some Rh “planets” around an Au core. The formation of this unique structure is mainly because of the immiscibility and lattice mismatch of Au and Rh36. There are some heterojunction structures at the boundary between Au and Rh, which could contribute to their electron interaction and further improve their catalytic performance.
The crystal structure of Au-Rh@SiO2 core-shell NPs was obtained by XRD. As shown in Fig. 2(a), the main peaks at 38.6°, 45.0°, 65.2°, 78.1°, and 82.8° can be ascribed to the (111), (200), (220), (311), and (222) planes of Au (JCPDS: 04-0784)37. These peaks are in accordance with the peak positions of Au@SiO2 in Fig. S2, indicating the formation of a nonalloyed structure. However, there are no characteristic peaks of Rh. This might be attributed to the fact that the particle size of Rh is too small to be obtained. The elemental composition and states of Au-Rh@SiO2 core-shell NPs were further analyzed and characterized by XPS, as shown in Fig. 2(b). The main peaks of the Rh 3d spectrum can be deconvoluted into four peaks. The two peaks at 304.6 eV and 308.2 eV can be related to Rh 3d5/2 and Rh 3d3/2 of Rh (0), and those with binding energies of 306.5 eV and 312.2 eV can be related to Rh 3d5/2 and Rh 3d3/2 of Rh (III)38,39. Compared with the standard values, the peaks of Rh species shift to higher binding energies40. Moreover, through peak area integral calculation, the ratio of Rh (0) and Rh (III) was found to be 1.45. The peak of Au 4f is too weak to clearly observe in the survey-level XPS spectrum (Fig. S3), which is mainly because the Au-Rh “core-satellite” structure is coated by a silica shell41. This further confirmed the formation of a heterojunction structure rather than alloy, which is in agreement with the XRD result. The sample of Au-Rh NPs (Fig. S1) synthesized without TEOS was also characterized by XPS. As shown in Fig. S4, Au 4f core-level and Rh 3d core-level signals are present due to the absence of a silica shell. The molar ratio of Au and Rh is 1.0 by the calculation of XPS peak areas, which is consistent with the molar ratio of their precursors. In addition, the two peaks of Au (0) shift to the lower binding energy, which is mainly due to the electron interaction between Au and Rh32. The electronegativity of Au is higher than that of Rh, which results in an increased electron density of Au and a reduction of electron binding energy. Comparing the XPS spectra of Au-Rh@SiO2 core-shell and Au-Rh NPs, the ratio of Rh (0) and Rh (III) decreased from 1.45 to 0.75. This further revealed that the existence of silica shell could improve their stability.
The formation mechanism of Au-Rh@SiO2 core-shell NPs was investigated by acquiring TEM images of the samples taken at different reaction times. Here, Au-Rh@SiO2 core-shell NPs were synthesized by a one-pot one-step method developed by our research group and Zhao’s group42,43. HAuCl4 and RhCl3 were reduced by formaldehyde to result in the formation of Au-Rh core-satellite NPs after the reacted solution was heated at 80°C for 5 min. At the same time, a thin layer of silica formed on the surface by base-catalyzed hydrolysis and polymerization of TEOS, as shown in Fig. 3(a). In Fig. 3(b), most of the Au-Rh NPs coated with a thin silica layer silica are on the outer edge of the silica spheres. The structures of the Au-Rh@SiO2 core-shell NPs initially formed after 20 min of reaction, as shown in Fig. 3(c). However, Au-Rh NPs are not at the center of the silica spheres. At higher concentrations of CTAB, the lamellar micelles formed could induce anisotropic growth of SiO2 shells on the surface and Au-Rh NPs44. As the consumption of CTAB, the spherical micelles adopted may result in the isotropic growth of SiO2 shells. As shown in Fig. 3(d), Au-Rh@SiO2 core-shell NPs finally formed at 60 min. The forming process mechanism of Au-Rh@SiO2 core-shell NPs is illustrated in Fig. 3(e).
We also investigated the effect of different precursor ratios (1:2 and 2:1) of Au and Rh on the resulting NPs. The samples were defined as Au1-Rh2@SiO2 and Au2-Rh1@SiO2 core-shell NPs, which were synthesized with HAuCl4 to RhCl3 molar ratios of 1:2 and 2:1, respectively. As demonstrated in Fig. 4, the two samples are with core-shell structures, the core is Au-Rh bimetallic NPs, and the shell with similar thickness is silica. When we carefully checked the structure of Au-Rh bimetallic NPs, it could be clearly seen that Au-Rh bimetallic NPs with core-satellite structure and isolated Ru NPs were encapsulated in the center of silica for the Au1-Rh2@SiO2 core-shell NPs in Fig. 4(a). For Au2-Rh1@SiO2 core-shell NPs in Fig. 4(b), Au is not coated well with Rh particles and Au-Rh core-satellite structure is not formed in the center of the silica shell. Similarly, we also examined the molar ratio of Au and Rh in Au1-Rh2 and Au2-Rh1 bimetallic NPs without silica shells synthesized in the absence of TEOS by XPS. As shown in Fig. S5 and S6, the molar ratios of Au to Rh of Au1-Rh2 and Au2-Rh1 bimetallic NPs are 0.56 and 1.68 according to the calculation of the corresponding XPS peak areas, respectively.
We also employed this method to synthesize silica-coated bimetallic NPs consisting of Au and other platinum group metals (Pd, Ir, Pt). Figure 5(a, b, c) shows the TEM image, HAADF-STEM image and elemental mapping of Au-Pd@SiO2, Au-Ir@SiO2 and Au-Pt@SiO2 core-shell NPs, respectively. Combined with the results of TEM and XRD characterizations of Au-Pd, Au-Ir and Au-Pt bimetallic NPs shown in Fig. S7 and S8, Au-Pd@SiO2, Au-Ir@SiO2 and Au-Pt@SiO2 core-shell NPs can be successfully synthesized using this one-pot one-step method simply by changing the corresponding metallic precursor.
To explore the catalytic reduction performance of 4-NP to 4-AP, UV–vis spectra in the presence of NaBH4 during different reaction times were monitored. The UV–vis absorption peaks of 4-NP and 4-AP are located at 400 nm and 300 nm in Fig. S9, respectively. The UV–vis spectra of Au-Rh@SiO2 core-shell NPs as a catalyst in Fig. 6(a) show that the reduction reaction of 4-NP finished within 5 min, indicating its good catalytic performance. The catalytic capability of Rh@SiO2 (Fig. S10a), Au@SiO2, and the mixture was further measured. Distinctly, in Fig. 6(b-d), in the presence of Rh@SiO2, Au@SiO2 and the mixture as a catalyst, this reaction finished within 8 min, 60 min and 13 min, respectively. The broad peak at approximately 500–550 nm comes from the surface resonance plasmon resonance of Au NPs (Fig. S11). This phenomenon exhibits that Au-Rh@SiO2 core-shell NPs feature superior catalytic performance over their corresponding single component catalysts.
Table 1
Comparison of catalytic performance of bimetallic nanocatalysts for the reduction of 4-NP
Catalyst
|
Mass
|
Kinetic constant
|
References
|
MOF-supported Au@Ag core-shell NPs
|
—
|
0.298 min− 1
|
12
|
Au@Pd core-shell nanocubes
|
1 mL of 4.2×1010 particle/mL
|
2.05 ± 0.12 min− 1
|
46
|
Ni–Pt NPs
|
1 mL of 0.1 mg/mL
|
0.116 min− 1
|
47
|
Pd/Au(3:1)@g-C3N4-N
|
—
|
0.5310 min− 1
|
48
|
Au/Pd core/shell NPs
|
75 µL of 2.0 mM
|
0.32 min− 1
|
1
|
Cu/Ag alloy NPs
|
2 mg
|
0.237 min− 1
|
49
|
Au-Rh@SiO2 core-shell structure
|
1 mL of 0.019 mg/mL
|
0.826 min− 1
|
This work
|
In addition, the investigation on kinetics of the catalysts were further analyzed. If the ratio of Ct/C0 (4-NP concentration at t minutes and 0 min) is proportional to time, this reduction reaction could be seen as first order kinetic, and the slope of the fitting line is a first order kinetic constant45. The corresponding results are shown in Fig. 7. The first-order kinetic constants of Au-Rh@SiO2 core-shell NPs, Rh@SiO2, Au@SiO2 and the mixture of Rh@SiO2 and Au@SiO2 NPs as catalysts are 0.826 min− 1, 0.276 min− 1, 0.037 min− 1 and 0.228 min− 1, respectively. Clearly, Au-Rh@SiO2 core-shell NPs have the maximum kinetic constant, also suggesting that this material exhibits the best catalytic performance. In addition, it can be observed from Table 1 that this catalyst is also superior to other bimetallic nanocatalysts reported in the previous literature1,12,48−49, revealing its excellent catalytic activity.
According to the results of TEM and XPS characterization of the Au-Rh@SiO2 core-shell NPs, Au-Rh bimetallic NPs exhibit core-satellite structures. There are some heterojunction structures at the boundary between Au and Rh, which result in their electron interaction and could improve their catalytic performance. To verify our hypothesis, the catalytic performance of Au-Rh@SiO2 core-shell NPs with different molar ratios of Au and Rh in Fig. S12 and S13 show UV–vis spectra at different reaction times in the presence of Au1-Rh2@SiO2 core-shell NPs and Au2-Rh1@SiO2 core-shell NPs. Figure 8 plots ln (Ct/C0) vs. time in the presence of Au-Rh@SiO2 core-shell NPs with different molar ratios of Au and Rh. Compared with the kinetic constant of Au-Rh@SiO2 core-shell NPs, Au2-Rh1@SiO2 core-shell NPs exhibited the lowest catalytic performance due to the low content of Rh. However, for the sample of Au1-Rh2@SiO2 core-shell NPs, its catalytic performance was still lower than that of Au-Rh@SiO2 core-shell NPs. As observed in Fig. 3(a), a thicker layer of Rh NPs was formed and prevented the synergistic effect of Au and Rh. In conclusion, the Au-Rh@SiO2 core-shell NPs as highly efficient catalysts for the reduction of 4-NP could be attributed to the synergistic effect coming from their unique core-shell structure and electronic interaction of Au and Rh.
The synergistic effect was also confirmed by other silica-coated Au-based PGMs bimetallic NPs. Fig. S14-17 show the catalytic performance of Au-M@SiO2 (M = Pd, Ir, Pt) core-shell NPs for the reduction of 4-NP. They exhibit highly efficient catalytic activity with first-order kinetic constants of 0.359 min− 1, 0.100 min− 1 and 0.135 min− 1, respectively. As shown in Fig. 9, we summarized the Au-M@SiO2 (M = Rh, Pd, Ir, Pt) core-shell NPs as catalysts for the reduction of 4-NP. Au-Rh@SiO2 core-shell NPs exhibit the highest catalytic activity for the reduction of 4-NP in the excess of NaBH4, which is due to the higher adsorption energy of 4-NP on Rh surfaces and more efficient interfacial electron transfer between Au and Rh.