Abstract:
Empowered by crystalline ordered structures and homogeneous
fabrication techniques, covalent organic frameworks (COFs) have been realized with
uniform morphologies and isotropic properties. However, such homogeneity often
hinders various surface-dependent properties observed in asymmetric nanostructures.
The challenge remains to induce heterogeneity in COFs by creating an asymmetric
superstructure such as a Janus thin film. In this regard, we propose a versatile yet
straightforward interfacial layer-grafting strategy to fabricate free-standing Janus-type
COF-graphene thin films. Herein, two-dimensional graphene sheets were utilized as the
suitable grafter due to the possibility of noncovalent interactions between the layers. The
versatility of the approach was demonstrated by fabricating two distinct Janus-type films, with the COF surface interwoven with
nanofibers and nanospheres. The Janus-type films showcase opposing surface morphologies originating from graphene sheets and
COF nanofibers or nanospheres, preserving the porosity (552−600 m2 g−1
). The unique surface chemistries of the constituent layers
further endow the films with orthogonal mechanical properties, as confirmed by the nanoindentation technique. Interestingly, the
graphene sheets favor the Janus-type assembly of COF nanofibers over the nanospheres. This is reflected in the better
nanomechanical properties of COFfiber-graphene films (Egraphene = 300−1200 MPa; ECOF = 15−60 MPa) compared to the COFspheregraphene films (Egraphene = 11−14 MPa; ECOF = 2−5 MPa). These results indicate a direct relationship between the mechanical
properties and homo/heterogeneity of Janus-type COF films.
