Architecture of Pd–Au Bimetallic Nanoparticles in Sodium Bis(2-ethylhexyl)sulfosuccinate Reverse Micelles As Investigated by X-ray Absorption Spectroscopy

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Abstract
In this study, we demonstrate the unique application of X-ray absorption spectroscopy (XAS) as a fundamental characterization tool to help in designing and controlling the architecture of Pd–Au bimetallic nanoparticles within a water-in-oil microemulsion system of water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane. Structural insights obtained from the in situ XAS measurements recorded at each step during the formation process revealed that Pd–Au bimetallic clusters with various Pd–Au atomic stackings are formed by properly performing hydrazine reduction and redox transmetalation reactions sequentially within water-in-oil microemulsions. A structural model is provided to explain reasonably each reaction step and to give detailed insight into the nucleation and growth mechanism of Pd–Au bimetallic clusters. The combination of in situ XAS analysis at both the Pd K-edge and the Au LIII-edge and UV–vis absorption spectral features confirms that the formation of Pd–Au bimetallic clusters follows a (Pdnuclei–Austack)–Pdsurf stacking. This result further implies that the thickness of Austack and Pdsurf layers may be modulated by varying the dosage of the Au precursor and hydrazine, respectively. In addition, a bimetallic (Pd–Au)alloy nanocluster with a (Pdnuclei–Austack)–(Pd–Aualloy)surf stacking was also designed and synthesized in order to check the feasibility of Pdsurf layer modification. The result reveals that the Pdsurf layer of the stacked (Pdnuclei–Au)stack bimetallic clusters can be successfully modified to form a (Au–Pd alloy)surf layer by a co-reduction of Pd and Au ions by hydrazine. Further, we demonstrate the alloying extent or atomic distribution of Pd and Au in Pd–Au bimetallic nanoparticles from the derived XAS structural parameters. The complete XAS-based methodology, demonstrated here on the Pd–Au bimetallic system, can easily be extended to design and control the alloying extent or atomic distribution, atomic stacking, and electronic structure to construct many other types of bimetallic systems for interesting applications.

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