posted on 2021-12-29, 20:35authored byShibani Mohata, Kaushik Dey, Surojit Bhunia, Neethu Thomas, E. Bhoje Gowd, Thalasseril G. Ajithkumar, C. Malla Reddy, Rahul Banerjee
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 COFsphere-graphene 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.