Facet-preserving conversion of Pd@Pt core-shell rhombic dodecahedra into Pt nanocages with extended {110} facets.

Hollow nanostructures with well-defined facets are attractive for improving the utilization of active metals while maintaining facet-dependent surface properties. However, preserving such facets during the transformation of core-shell nanocrystals into hollow architectures remains a major synthetic challenge. Here, shape-retentive rhombic dodecahedral Pt nanocages exposing Pt{110} facets were constructed using Pd rhombic dodecahedra as removable templates. Synthetic control experiments identified the halide environment during Pt growth and the Pt shell thickness as key parameters governing successful hollow structure formation. Bromide-assisted growth produced a sufficiently continuous Pt shell, while a thickness of about 2 nm provided an effective balance between structural stability and accessibility of the Pd core, enabling reproducible formation of shape-retentive Pt nanocages. Core removal increased the electrochemically active surface area by 2.1-fold and the Pt mass activity for the oxygen reduction reaction by 2.2-fold, while the specific activity remained nearly unchanged. Moreover, the activity normalized to the total metal mass increased by more than 10-fold. These results demonstrate that the hollow architecture enhances active metal utilization while preserving the intrinsic activity of the Pt{110} facet, providing a rational strategy for constructing facet-controlled hollow nanostructures.