## 3 Results and Discussion
### Membrane Characterization  
In agreement with previous studies [39; 43], no essential change in surface morphology of the membrane was evident upon the addition of AG in the casting solutions (see Figure 4). The same can be said for the addition of GO, where the surface morphology remained unchanged when compared to the neat membrane (M1). Interestingly, an increase in surface porosity can be seen upon the incorporation of AG and GO (see M5 and M8 in Figure 4) compared to the neat membrane (M1). Importantly, no defects were observed upon surveying the surface structures of all the fabricated membranes. Figure 3: (**a**) TGA (Thermogravimetric analysis) (in blue) and DTGA (derivative) (in red) thermograms for GO. The area under the different segments (area 1, area 2 and area 3) of the graph and the total area under the graph are indicated as % in the table (see inset of image (**a**). (**b**) Shows an overlay of the TGA results for graphite and GO. The SEM cross-section images in Figure 4 reveal the typical asymmetrical structures that are expected as a result of the instantaneous demixing between solvent and non-solvent in the phase inversion membrane preparation method. The result of such rapid demixing are the exaggerated large macrovoid structures that are seen in all membranes imaged. The finger-like projections above the macrovoids add to the porosity of the membrane and, therefore, improve the membrane flux [44]. The introduction of hydrophilic GO to the dope solution leads to its thermodynamic instability, with a faster mixing rate between solvent and non-solvent in the coagulation bath during membrane casting [33] that appears to make obvious the asymmetric cross-sectional structure of the membrane. Specifically, a dense skin layer at the top complemented by a thick porous layer with protruding finger-like structures extending downwards within a sponge like matrix. In agreement with other studies, the increase in GO content within membranes causes an increase in the number of pores, whilst a reduction in wall thickness is observed [45]. This allows for increased water permeation. Interlocking channels are also seen in the SEM images for membranes with the highest GO loading. These features, along with the vast number of pores, work together to increase water permeability. In line with this notion, Figure 5a shows the results for the average total porosity for each membrane where we see the total porosity increasing as the loading concentration of GO increases. There is a 33% increase in porosity from M1 to M8 samples which have the highest nanofiller loading. Figure 4: FESEM images of the membrane surfaces and cross-section view of fractured membranes M1, M5 and M8. Figure 5: Average total porosity of PES membranes M1 to M9 (**a**), Average pore size for PES membranes M1 to M9 (**b**). As seen in Figure 5a, a marked increase in membrane porosity is found when AG is introduced into the PES membrane (see porosities for M2 and M1 samples), in accordance with previous studies [39; 46]. There is also a clear trend of increased membrane porosity with the GO loading (from M2 to M8). However, when we compare the porosity values for the M2 (0.1 wt.% AG without GO) and M9 (0.3 wt.% GO without AG) samples, it is seen that the M9 sample is more porous. This finding suggests that GO plays a more influential role in terms of pore formation and, hence, total membrane porosity compared to AG. In a similar trend to total porosity, the average pore size was observed to increase to a maximum at M8 and then decrease at M9 (see Figure 5b). The average pore size increased by 1.8 fold from 25.5 nm for M1 samples (neat membrane) to 66.4 nm for M8 membrane, which was cast at 5 wt. % GO and 1 wt. % AG in the dope solution. This increase in membrane pore size is a characteristic manifestation when nanofillers are incorporated into polymeric membranes.