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.