Optical microscope (Figure ) and SEM images (Figure ) of the graphene surface exposed after removing Cu reveal the absence of any visible cracks. The reproducibility of this transfer strategy was probed by cutting a ~8 × 12 cm 2 graphene coupon (Figure ) into 24 pieces of 2 × 2 cm 2 coupons (Figure ) and fabricating membranes from each coupon (Figure ). An unsuccessful transfer typically leaves yellow residues on the white PES support because the Cu etchant (FeCl3) penetrates the broken graphene (Figure ). This is a useful and quick method to check the success of the transfer. We did not observe the yellow stain in these samples, indicating a successful transfer. Selective gas permeation from all membranes confirmed that graphene transfer's success rate was 100% (Table ). The variation in CO2 permeance in these samples is attributed to non-optimized O3 velocity for these samples (see the next section).