Design and synthesis of ordered, metal-free layered materials is intrinsically difficult due to the limitations of vapor deposition processes that are used in their making. Mixed-dimensional (2D/3D) metal-free van der Waals (vdW) heterostructures based on triazine (C N ) linkers grow as large area, transparent yellow-orange membranes on copper surfaces from solution. The membranes have an indirect band gap (E = 1.91 eV, E = 1.84 eV) and are moderately porous (124 m g ). The material consists of a crystalline 2D phase that is fully sp hybridized and provides structural stability, and an amorphous, porous phase with mixed sp -sp hybridization. Interestingly, this 2D/3D vdW heterostructure grows in a twinned mechanism from a one-pot reaction mixture: unprecedented for metal-free frameworks and a direct consequence of on-catalyst synthesis. Thanks to the efficient type I heterojunction, electron transfer processes are fundamentally improved and hence, the material is capable of metal-free, light-induced hydrogen evolution from water without the need for a noble metal cocatalyst (34 µmol h g without Pt). The results highlight that twinned growth mechanisms are observed in the realm of "wet" chemistry, and that they can be used to fabricate otherwise challenging 2D/3D vdW heterostructures with composite properties.

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