Electrochemical Methods to Core-Shell Nanostructures

Underpotential deposition (UPD) is one useful way to the fabrication of metal mono-layer on metal nanoparticles [57]. UPD has been widely used in the deposition of metal atoms electrochemically onto another metal substrate at a potential slightly lower than the equilibrium potential for its reduction to metal [58]. This deposition is self-limiting, and atoms can only deposit up to single monolayer. When Pt is used as the core metal, several metals including Ru, Pd, and Ag can be directly deposited as monolayer via UPD [59]. 

However, UPD method cannot be used to prepare Pt monolayer on other metals readily because of the mismatched redox potentials. Adzic and coworkers developed a method to deposit Pt monolayer on metal substrates indirectly [59c]. In this approach, a sacrificial copper monolayer is first deposited on a selected metal using the UPD method, followed by a galvanic replacement reaction by Pt ions (Fig. 2.13) [60]. This approach successfully broadens the possible metal elements that can be directly or indirectly deposited by this technique. 

Yang and coworkers reported an electrochemical method to control the surface and bulk compositions, size, and morphology of Ag-Pt heterogeneous nanostructures by potential cycle in perchloric acid (HCIO ) aqueous solution (Fig. 2.14) [61]. When the PtigAg nanoparticles were treated at 0.6V or lower, which is below the 

FIGURE 2.13 Schematic illustration of the synthesis of Pt monolayer on metal nanoparticles using UPD followed by a galvanic replacement reaction. Reprinted with permission from Ref. [21]. Elsevier B.V. 

FIGURE 2.14 Schematic illustration of compositional and strucmral changes of AgPt alloy nanoparticles made by controlled dissolution of Ag. Reprinted with permission from Ref. [61]. Wiley-VCH Verlag GmbH Co. KGaA. (See insert for color representation of the figure.) 

---