One-pot synthesis of Au-M@SiO2 (M = Rh, Pd, Ir, Pt) core–shell nanoparticles as highly efficient catalysts for the reduction of 4-nitrophenol

The conversion of p-nitrophenol (4-NP) to p-aminophenol (4-AP) is of great significance for pharmaceutical and material manufacturing. In this work, Au-M@SiO2 (M = Rh, Pd, Ir, Pt) nanoparticles (NPs) with core–shell structures, which are expected to be excellent catalysts for the transformation of 4-NP to 4-AP, were synthesized by a facile one-pot one-step method. The structure and composition of the NPs were characterized through transmission electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Au-M@SiO2 (M = Rh, Pd, Ir, Pt) core–shell NPs showed excellent catalytic activity in the reduction of 4-NP, which is superior to most catalysts reported in the previous literature. The enhanced catalytic activity of Au-M@SiO2 core–shell NPs is presumably related to the bimetallic synergistic effect. This study provides a simple strategy to synthesize core–shell bimetallic NPs for catalytic applications.


Results and discussion
Here, Au-Rh@SiO 2 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. 1a,b, the as-prepared sample possesses core-shell structure and good monodispersity. The elemental mapping analysis of the sample in Fig. 1d-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@SiO 2 NPs with core-shell structures were synthesized successfully by this one-pot method. Au-Rh@SiO 2 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 Rh 39 . 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 mass loading of Au-Rh in Au-Rh@SiO 2 core-shell NPs by chemical method. It is assumed that the precursors HAuCl 4 , RhCl 3 and TEOS are completely converted into Au, Rh and SiO 2 , respectively. The experimental mass loading of Au-Rh in Au-Rh@SiO 2 core-shell NPs determined by their mass ratio is 7.9%. Au-Rh NPs was obtained by dissolving the silica shell with excess concentrated hydrofluoric acid. The experimental value is slightly smaller than the theoretical value (8.45%) because of the presence of C TAB in the silica layer. The crystal structure of Au-Rh@SiO 2 core-shell NPs was obtained by XRD. As shown in Fig. 2a 40 . These peaks are in accordance with the peak positions of Au@SiO 2 in Fig. S2, indicating the formation of a non-alloyed 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. In addition, a broad peak stemming from the SiO 2 shell is actually seen at around 2θ ≈ 20°. The elemental composition and states of Au-Rh@SiO 2 core-shell NPs were further analyzed and characterized by XPS, as shown in Fig. 2b. 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 3d 5/2 and Rh 3d 3/2 of Rh (0), and those with binding energies of 306.5 eV and 312.2 eV can be related to Rh 3d 5/2 and Rh 3d 3/2 of Rh (III) 41,42 . Compared with the standard values, the peaks of Rh species shift to higher binding energies 43 . 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 shell 44 . 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 www.nature.com/scientificreports/ 1.0 by the calculation of XPS peak areas, which is consistent with that of ICP-MS and the molar ratio of their precursors. In addition, the two peaks of Au (0) shift to the higher binding energy, which is mainly due to the electron interaction between Au and Rh 45 . 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@SiO 2 core-shell and Au-Rh NPs, the ratio of Rh (0) and Rh (III) decreased from 1.45 to 0.75. These observations indicate that Rh NPs are oxidized 41,43 or the alloy phase generates at the interface between Au and Rh during the XPS measurement [14][15][16] . This further revealed that the existence of silica shell could improve their stability.
The formation mechanism of Au-Rh@SiO 2 core-shell NPs was investigated by acquiring TEM images of the samples taken at different reaction times. Here, Au-Rh@SiO 2 core-shell NPs were synthesized by a one-pot one-step method developed by our research group and Zhao's group 46,47 . HAuCl 4 and RhCl 3 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. 3a. In Fig. 3b, 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@SiO 2 core-shell NPs initially formed after 20 min of reaction, as shown in Fig. 3c. 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 SiO 2 shells on the surface and Au-Rh NPs 48 . As the consumption of CTAB, the spherical micelles adopted may result in the isotropic growth of SiO 2 shells. As shown in Fig. 3d, Au-Rh@SiO 2 core-shell NPs finally formed at 60 min. The forming process mechanism of Au-Rh@SiO 2 core-shell NPs is illustrated in Fig. 3e.  www.nature.com/scientificreports/ 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 Au 1 -Rh 2 @SiO 2 and Au 2 -Rh 1 @SiO 2 core-shell NPs, which were synthesized with HAuCl 4 to RhCl 3 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 Rh NPs were encapsulated in the center of silica for the Au 1 -Rh 2 @SiO 2 core-shell NPs in Fig. 4a. For Au 2 -Rh 1 @SiO 2 core-shell NPs in Fig. 4b, 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 Au 1 -Rh 2 and Au 2 -Rh 1 bimetallic NPs without silica shells synthesized in the absence of TEOS by XPS. As shown in Figs. S5 and S6, the molar ratios of Au to Rh of Au 1 -Rh 2 and Au 2 -Rh 1 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 5a,b,c shows the TEM image, HAADF-STEM image and elemental mapping of Au-Pd@SiO 2 , Au-Ir@SiO 2 and Au-Pt@SiO 2 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 Figs. S7 and S8, Au-Pd@SiO 2 , Au-Ir@SiO 2 and Au-Pt@SiO 2 core-shell NPs can be successfully synthesized using this one-pot one-step method simply by changing the corresponding metallic precursor.  Fig. S9, respectively. The UV-vis spectra of Au-Rh@SiO 2 core-shell NPs as a catalyst in Fig. 6a show that the reduction reaction of 4-NP finished within 5 min, indicating its good catalytic performance. The catalytic capability of Rh@SiO 2 (Fig. S10a), Au@SiO 2 , and the mixture was further measured. Distinctly, in Fig. 6b-d, in the presence of Rh@SiO 2 , Au@SiO 2 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@SiO 2 core-shell NPs feature superior catalytic performance over their corresponding single component catalysts.
In addition, the investigation on kinetics of the catalysts were further analyzed. If the ratio of C t /C 0 (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 constant 49 . The corresponding results are shown in Fig. 7. The first-order kinetic constants of Au-Rh@SiO 2 core-shell NPs, Rh@SiO 2 , Au@SiO 2 and the mixture of Rh@SiO 2 and Au@SiO 2 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@SiO 2 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 literature 1,12,50-53 , revealing its excellent catalytic activity.
According to the results of TEM and XPS characterization of the Au-Rh@SiO 2 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@SiO 2 core-shell NPs with different molar ratios of Au and Rh in Figs. S12 and S13 show UV-vis spectra at different reaction times in the presence of Au 1 -Rh 2 @SiO 2 core-shell NPs and Au 2 -Rh 1 @SiO 2 core-shell NPs. Figure 8 plots ln (C t /C 0 ) versus time in the presence of Au-Rh@SiO 2 core-shell NPs with different molar ratios of Au and Rh. Compared with the kinetic constant of Au-Rh@SiO 2 core-shell NPs, Au 2 -Rh 1 @SiO 2 core-shell NPs exhibited the lowest catalytic performance due to the low content of Rh. However, for the sample of Au 1 -Rh 2 @SiO 2 core-shell NPs, its catalytic performance was still lower than that of Au-Rh@SiO 2 core-shell NPs. As observed in Fig. 3a, a thicker layer of Rh NPs was formed and prevented the synergistic effect of Au and Rh. In conclusion, the Au-Rh@SiO 2 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. Figs. S14, S15, S16 and S17 show the catalytic performance of Au-M@SiO 2 (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@SiO 2 (M = Rh, Pd, Ir, Pt) core-shell NPs as catalysts for the reduction of 4-NP. Au-Rh@SiO 2 core-shell NPs exhibit the highest catalytic activity  www.nature.com/scientificreports/ for the reduction of 4-NP in the excess of NaBH 4, which is due to the higher adsorption energy of 4-NP on Rh surfaces and more efficient interfacial electron transfer between Au and Rh. The recyclability of Au-Rh@SiO 2 core-shell NPs shown in Fig. 10 demonstrates that the conversion rate is still around 90% after 5 cycles, indicating its high stability. As shown in Fig. S18, the first-order kinetic constant of Au-Rh NPs to the reduction reaction is 0.807 min −1 , which is slightly lower than that (0.826 min −1 ) of Au-Rh@ SiO 2 core-shell NPs. As observed in Fig. S19, Au-Rh bimetallic NPs occurs obvious agglomeration after catalysis three times. The conversion rate of Au-Rh bimetallic NPs after catalysis three times reduced to 79%. On the  www.nature.com/scientificreports/ contrary, Au-Rh@SiO 2 core-shell NPs still can keep the original morphology and structure after five cycles of catalysis. It illustrates that SiO 2 shell layers can improve the stability. Therefore, the highly efficient catalytic performance of Au-Rh@SiO 2 core-shell NPs mainly comes from the synergistic effect of Au-Rh and core-shell structure. The catalytic performance of Au-Rh@SiO 2 core-shell NPs is superior to that of the corresponding monometallic@SiO 2 NPs or their mixture. TEM and XPS characterizations indicate the formation of heterojunction structures at the boundary of Au-Rh and their electron interaction. The electron interaction is insufficient as decreasing the molar ratio of Rh/Au and is hindered as increasing the molar ratio of Rh/Au. It sufficiently demonstrates the synergistic effect of Au-Rh results in the highly efficient catalytic performance for the reduction of 4-NP. The SiO 2 shell layer improves their high catalytic cycle stability and recovery.

Conclusions
In summary, Au-M@SiO 2 (M = Rh, Pd, Ir, Pt) NPs with core-shell structures were successfully synthesized by a facile one-pot one-step method. The morphology, crystal structure, elemental composition and formation mechanism of these core-shell NPs were investigated. Au-M@SiO 2 (M = Rh, Pd, Ir, Pt) core-shell NPs showed excellent catalytic activity in the reduction of 4-NP, which is superior to most catalysts reported in the previous literature. The enhanced catalytic activity of Au-M@SiO 2 core-shell NPs is presumably related to the bimetallic synergistic effect. This study provides a simple strategy to synthesize core-shell bimetallic NPs for catalytic applications.

Methods
Materials. Chloroauric  Synthesis. Au@SiO 2 core-shell NPs were synthesized by a facile one-pot one-step method using formaldehyde as a reducing agent 46,47 . Similarly, M@SiO 2 (M = Rh, Pd, Ir and Pt) core-shell NPs were prepared by changing the corresponding metal precursors.
The above method is still suitable for the synthesis of bimetallic Au-M@SiO 2 (M = Rh, Pd, Ir and Pt) core-shell NPs. Taking Au-Rh@SiO 2 core-shell NPs as an example, in a typical synthesis, 38 mL of deionized water, 6 mL of ethanol, 0.07 g of CTAB, 1 mL of HCHO, 1 mL of NaOH (0.5 mol/L), 1 mL of 8.14 mmol/L HAuCl 4 , 1 mL of 8.14 mmol/L RhCl 3 and 0.10 mL of TEOS were mixed in a 100 mL of beaker under ultrasonication. The beaker was left in an oil bath at 80 °C for 60 min under stirring. After cooling to room temperature naturally, the product was centrifuged at 11,000 rpm for 20 min and washed two times with water to remove the surfactant and unreacted chemicals. Finally, the precipitate was dissolved in 3 mL of water for further characterization.