Metal–Organic Framework–Derived “Ship-in-Bottle” Method: Heterogeneous Yolk@Shell Metal Oxides for Heterogeneous Sensing

Heterogeneous yolk@shell (YS) metal oxides (MOs) with tailorable chemical compositions and spatial locations have great potential in sensors and heterogeneous catalysis. However, achieving the one-step synthesis of heterogeneous YS MOs, with a spinel oxide shell and rock salt-structured oxide yolk, remains a challenging task. Herein, we present an inhomogeneous metal–organic framework (MOF)-derived “ship-in-bottle” strategy for preparing YS NiO@NiFe₂O₄ heterostructure nanospheres. The methodology relies on a kinetically controlled reaction via the Kirkendall effect, during which the synchronous etching of Ni-MOF and framework cation substitution take place simultaneously, forming an inhomogeneous double-shelled MOF precursor with an inner shell of Ni-MOF and an outer shell of Fe/Ni-MOF. Subsequently, adopting a thermal contraction strategy for further MOF precursor derivatization contributes to the interface separation between the inner and outer shells and then induces voids to in situ form YS heterostructure nanospheres. Accordingly, the resultant heterogeneous YS NiO@NiFe₂O₄ is applied in gas sensors, exhibiting regional reaction and shell catalytic filter effects, which stably and selectively detect traces of p-xylene (6.9 ppb) in a highly discriminative manner (S_p-xylene/S_toluene = 4.0) under high humidity (90% RH). This work paves a path for the elaborate design of different functional YS nanomaterials for use in sensors and catalysis.