3.4. d-spacing control of NST-GO membranes
It has been reported that the amount of intercalator between GO layers may influence the d -spacing of GO membranes. Therefore, thed -spacing of NST-GO membranes was measured by XRD and the spectra are displayed in Figure 4a. Clearly, the characteristic peak progressively shifts from 8.00° to 6.93°, as Ni(OH)2volume increases from 1 mL to 7 mL. It indicates that thed -spacing is extended from 1.07 nm to 1.25 nm after Ni(OH)2 intercalation, as shown in Figure 4b. Typically, the d-spacing of the NST-GO membrane prepared from 5 mL Ni(OH)2 nanosheet dispersion is 1.14 nm, which is 30% larger than that of reduced GO membrane (0.88 nm) but still smaller than the molecular size of most dyes (Figure S8, Supporting Information). This means improvement of the membrane permeance without sacrifice of the dye rejection.
Notably, the increase of d -spacing of NST-GO membranes in Figure 4b can be separated for three stages, indicating three forms of NST-GO membranes. As shown in Figure 4c, the three stages are named sub-saturation, saturation and over-saturation, respectively. In sub-saturation stage, when added Ni(OH)2 volume is below 3 mL, GO nanosheets begin to be intercalated in most places but it is insufficient for these Ni(OH)2 nanosheets to full the whole interlayer. Therefore, the d -spacing is increased continuously in the sub-saturation stage. Then, in the saturation stage, it can be found a region between Ni(OH)2 volume of 3 mL and 5 mL, where thed -spacing is increased slowly. In this stage, the Ni(OH)2 nanosheets fill up interlayers totally to replenish unfilled areas of sub-saturation stage and finally form a whole lay of Ni(OH)2 nanosheets. Therefore, thed -spacing in the saturation stage finally becomes at the value of the thickness of single Ni(OH)2 nanosheet. Afterwards, as the Ni(OH)2 volume continually increases, the Ni(OH)2 nanosheets are intercalated as multilayers instead of monolayers between two GO nanosheets, resulting in amorphous water channels and increasing the d -spacing. In this stage, thed -spacing may increase to 1.25 nm or more, which is not favor of keeping the high rejection of those small dyes. Overall, thed -spacing of NST-GO membranes can be controlled by using different volumes of Ni(OH)2 dispersion and the membrane prepared from 5 mL Ni(OH)2 dispersion is the optimal membrane for dye separation.
In retrospect of Figure 3b, it can be easily found these three stages as well. The thickness is increased rapidly as Ni(OH)2volume increasing in the sub-saturation stage because of the enlargedd -spacing. Then, the thickness shows a decelerated growth in the saturation stage due to the decelerated growth of d -spacing, followed by another rapid increase in the over-saturation stage. Hence, the microstructure of NST-GO membranes is directly influenced by thed -spacing and can be also controlled by added the Ni(OH)2 volume indirectly.