Au–Ag and Pt–Ag bimetallic nanoparticles@halloysite nanotubes: morphological modulation, improvement of thermal stability and catalytic performance

In this study, Au–Ag and Pt–Ag bimetallic nanocages were loaded on natural halloysite nanotubes (HNTs) via galvanic exchange based on Ag@HNT. By changing the ratio of Au to Ag or Pt to Ag in exchange processes, Au–Ag@HNT and Pt–Ag@HNT with different nanostructures were generated. Both Au–Ag@HNT and Pt–Ag@HNT systems showed significantly improved efficiency as peroxidase-like catalysts in the oxidation of o-phenylenediamine compared with monometallic Au@HNT and Pt@HNT, although inert Ag is dominant in the composition of both Au–Ag and Pt–Ag nanocages. On the other hand, loading on HNTs enhanced the thermal stability for every system, whether monometallic Ag nanoparticles, bimetallic Au–Ag or Pt–Ag nanocages. Ag@HNT sustained thermal treatment at 400 °C in nitrogen with improved catalytic performance, while Au–Ag@HNT and Pt–Ag@HNT maintained or even had slightly enhanced catalytic efficiency after thermal treatment at 200 °C in nitrogen. This study demonstrated that natural halloysite nanotubes are a good support for various metallic nanoparticles, improving their catalytic efficiency and thermal stability.


Contents
For the synthesis of Au@HNT, 10 mM HAuCl 4 aqueous solution (0.8 mL) and 8 mg HNT were added to PVP aqueous solution (0.5 mg mL -1 , 40 mL) in 50 o C water bath.
Then, AA aqueous solution (100 mM, 0.16 mL) was slowly dropped into the above solution under vigorously magnetic stirring.The color of the solution was changed from light yellow to purple within several seconds, indicating the formation of Au@HNT.The reaction mixture was stirred for 10 min and centrifuged at 9000 rpm for 15 min.The as-prepared Au NPs and Au@HNT were washed twice with deionized water, and dried at 65 o C in oven overnight.TEM images of Au NPs and Au@HNT were shown in Fig. S10.

Preparation of Pt NPs and Pt@HNT
10 mM H 2 PtCl 6 •6H 2 O aqueous solution (2 mL) was added to deionized water (40 mL) at room temperature.Then, NaBH 4 aqueous solution (100 mM, 0.4 mL) was slowly dropped into the above solution under vigorously magnetic stirring.The color of the solution was changed from light yellow to brown within several seconds due to the formation of Pt NPs.The reaction mixture was stirred for 30 min and centrifuged at 12000 rpm for 15 min.
Then, NaBH 4 aqueous solution (100 mM, 0.4 mL) was slowly dropped into the above solution under vigorously magnetic stirring.The color of the solution was changed from light yellow to dark brown within several seconds, indicating the formation of Pt@HNT.The reaction mixture was stirred for 30 min and centrifuged at 9000 rpm for 15 min.
The as-prepared Pt NPs and Pt@HNT were washed twice with deionized water and ethanol, respectively, and dried at 65 o C in oven overnight.TEM images of Pt NPs and Pt@HNT were shown in Fig. S10.

Fig. S2 .
Fig. S2.SEM image of Ag NPs prepared at 40 o C without HNT.

Fig. S4 .
Fig. S4.The content of Ag and Au(Pt) in Au(Pt)-Ag@HNT with different adding Au(Pt):Ag atomic ratios measured by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).

Fig. S8 .
Fig. S8.Peroxidase-like catalytic performance of Au-Ag@HNT and Pt-Ag@HNT generated in different atomic ratios.Absorbance at 418 nm as a function of time was measured in the presence of Au-Ag@HNT (a) and Pt-Ag@HNT (b).The purple line marks the value of the control experiment.Reaction conditions: H 2 O 2 (0.3 M), OPD (0.3 mM), and catalyst (0.2 mg) at 40 o C.

Table S1 .
The experimental parameters in the synthesis of bimetallic nanoparticles@HNTs.*

Table S2 .
The 4-NP reduction rate constant k* in the presence of Ag@HNT prepared at 40 o C, 60 o C, and 80 o C before and after heating at 400 o C under nitrogen.

Table S3 .
The comparison of DAP formation rate constant k* (normalized based on the mole of active elements) in our catalytic systems and that reported in literatures.Au(Pt)-Ag@HNT was prepared with the adding atomic ratio Au(Pt):Ag of 0.1.