Rational manufacture of yolk-shell and core-shell metal oxide double layers from silicalated coordination polymer double layers

Several benefits derived from the structural features of yolk-shell-type materials are well acknowledged and therefore the rational manufacture of well-designed yolk-shell materials is of great concern in material development. Herein, we report a convenient method for the selective construction of yolk-shell or core-shell-type hybrid metal oxide double layers (silica@MxOy@M'x'Oy'; M and M' = Er, Y, Tb, or Gd) via calcining silicalated coordination polymer (CP) double layers (silica@MCP@M'CP), which are prepared via two-step CP growth on carboxylic acid-terminated silica microspheres. The different production of a yolk-shell or core-shell structure from similar silica@MCP@M'CP is found to be governed by the thermal decomposition properties of the two CP layers within silica@MCP@M'CP. One-step decomposition of the two CP layers results in a core-shell metal oxide double layer, whereas two-step sequential decomposition of the two CP layers with a sufficient time interval between the first and second decompositions results in a yolk-shell metal oxide double layer. Moreover, the two-step sequential decomposition process during the formation of a yolk-shell turns out to occur in two different sequences, such as out-to-in and in-to-out, depending on the decomposition properties of the inner and outer CP layers. The space between the yolk and shell metal oxides is also modulated by altering the thickness of the inner MCP layer in silica@MCP@M'CP.