Twinned Growth of Metal-Free, Triazine-Based Photocatalyst Films as Mixed-Dimensional (2D/3D) van der Waals Heterostructures

This is the pre-peer reviewed version of the following article: Schwarz, D.; Noda, Y.; Klouda, J.; Schwarzová-Pecková, K.; Tarábek, J.; Rybáček, J.; Janoušek, J.; Simon, F.; Opanasenko, M. V.; Čejka, J.; Acharjya, A.; Schmidt, J.; Selve, S.; Reiter-Scherer, V.; Severin, N.; Rabe, J. P.; Ecorchard, P.; He, J.; Polozij, M.; Nachtigall, P.; Bojdys,* M. J. Adv. Mater. 2017, DOI: 10.1002/adma.201703399.

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 (C₃N₃) linkers grow as large area, transparent yellow-orange membranes on copper surfaces from solution. The membranes have an indirect band gap (E_g,opt = 1.91 eV, E_g,elec = 1.84 eV) and are moderately porous (124 m² g⁻¹). The material consists of a crystalline 2D phase that is fully sp² hybridized and provides structural stability, and an amorphous, porous phase with mixed sp²–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⁻¹ g⁻¹ 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.