Fusing frameworks

Heterostructures are composed of two or more materials with distinct properties and are ubiquitous in electronic devices.¹ When the underlying lattices are coherently linked, the interface can controllably modify charge transport, unlocking functionalities absent in parent structures. Beyond electrical characteristics, crystalline heterostructures have been engineered for customized heat and mass transport properties.^2,3

A common approach for the synthesis of heterostructures is epitaxial growth; typically, crystalline layers are deposited onto a substrate whose lattice orientation guides the alignment of the overlying lattice along one or more crystallographic directions. In porous crystalline frameworks, epitaxy enables an additional degree of control – the orientation of the pore network – thus enabling systematic studies of host–guest processes with crystal-engineering precision.⁴ Technologically relevant host–guest interactions in frameworks with macroscopically aligned crystals include enhanced molecular sieves,⁵ anisotropic optical emissions⁶ and improved sensing properties.⁷

MOF heterostructures were first reported over a decade ago. These pioneering examples, inspired by semiconductor superlattices, demonstrated that the modular chemistry of MOFs can be combined with the rigor of epitaxy, establishing the field of porous framework interfaces. The earliest studies showed that MOF-on-MOF and MOF-on-ceramic architectures could be obtained by matching lattice parameters.^6,8

This crystal engineering heteroepitaxy method was initially explored on powdery crystals but the lack of control in the orientation of the particles was unsuitable for device fabrication. Subsequently, MOF-on-MOF deposition methods (e.g. layer-by-layer) were applied to surface-mounted MOF films, affording heteroepitaxial crystalline coatings whose lattice normal is perpendicular to the substrate.⁹ Then, oriented ceramic films have been used as sacrificial templates to direct the growth of iso-oriented MOF lattices, leading to the fabrication of centimetre-scale, 3D-oriented MOF films and micropatterns that display anisotropic optoelectronic properties (Fig. 1).¹⁰

Fig. 1 Fabrication of 3D-oriented MOF films and micropatterns.

Fusing distinct framework materials (e.g. MOFs and COFs) while preserving their crystal orientation in a heteroepitaxial relationship is a more demanding challenge, because the linkages are covalent in COFs and coordinative in MOFs. Li and co-workers¹¹ (Fig. 2) employed a “concentration–acid co-regulation” strategy to confine the growth of a TpPa COF to the surface of the zirconium-based MOF UiO-66, producing MOF@COF hybrids under ambient conditions. Formation of the MOF/COF heterojunction resulted in a 2.5-fold enhancement in CO₂ photoreduction relative to the parent frameworks, an effect attributed to improved charge separation and broadened light absorption.

Fig. 2 Facile construction of highly efficient NUT in air. Reproduced from ref. 11 with permission; copyright 2025.

Beyond photocatalysis, such bicontinuous architectures merit investigation for tandem separations, cascade catalysis and chemoresponsive membranes, as they combine orthogonal pore chemistries in a single solid. The crystal engineering of MOF@COF heterostructures with intimate, coherent interfaces using a straightforward protocol will accelerate advances in framework chemistry. In the near term, these materials are expected to impact gas adsorption and separation science, where hierarchical porosity and chemically distinct pore environments are advantageous. Continued progress in high-resolution electron microscopy will permit quantitative analysis of interfacial coherence, supporting the rational design of crystalline hybrids from the nano- to the macro-scale. In the longer term, patterned, 3D-oriented MOF@COF domains may be integrated into photonic circuits and sensing platforms.

References

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