Figure 5. (a) Free energy barrier for gas molecules passing
through Ti4C3 nanopores with differentd. (b) PMF curves for He and CH4 molecules
passing through the 2.92 Å
Ti4C3 nanopore.
In order to study the influence of the functional groups, -O were
removed from the 3.02 Å
Ti3C2O2 nanopore to
produce artificial nanopores, named as
Ti3C2-O2 nanopore withd of 3.32 Å (the -O2 suffix is to distinguish it
from the ‘real’ Ti3C2 model shown inFigure 1b , whose structure is different). Simulation results
show that, -O functional groups reduce the pore diameter, resulting in
reduced permeance for both CH4 and He (Figure
6a ). The change of CH4 permeance
(Ti3C2O2 vs.
Ti3C2-O2) is much more
significant than He, as the pore diameter is larger than He yet smaller
than CH4 (see previous discussion). Consequently,S He/CH4 increases from 5.9
(Ti3C2-O2) to 22
(Ti3C2O2). Further,
Ti3C2-O2’s PMF barrier
for CH4 or He is lower than that of
Ti3C2 (Figure 6b ), and itsS He/CH4 calculated from the TST agrees with that
from permeance simulations, as other ~3 Å MXene
nanopores do (Figure S8 ). All these results indicate that the
effect of functional groups on gas permeance andS He/CH4 through the MXene nanopores can well be
explained in terms of pore diameter and PMF.