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.