Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously been limited to the chemical transformation of metallic nanostructures. We demonstrated galvanic replacement reactions in metal oxide nanocrystals as well. When manganese oxide (Mn 3 O 4 ) nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn 3 O 4 /γ-Fe 2 O 3 ("nanoboxes") were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of γ-Fe 2 O 3 ("nanocages"). Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries. The generality of this approach was demonstrated with other metal pairs, including Co 3 O 4 /SnO 2 and Mn 3 O 4 /SnO 2 .
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