3.3. Microstructure and hydrophilicity of NST-GO membranes
Figure S5 (Supporting Information) shows surface SEM images of reduced
GO membrane and NST-GO membrane under the low magnification. Obviously,
both of them have the characteristic surface morphology of GO-based
membranes.31 However, the later has many wrinkles
appeared on its surface due to the removal of Ni(OH)2nanosheets that were intercalated between GO nanosheets. That is, the
membrane surface has many nanosheet-templated channels that benefit
water transport into the membrane.
To study effect of content of Ni(OH)2 nanosheets on the
membrane formation, a series of NST-GO membranes were prepared by mixing
5 mL GO dispersion and x mL Ni(OH)2 nanosheet dispersion
(x = 0, 1, 2, 3, 4, 5, 6 and 7). Figure 3a and Figure S6 (Supporting
Information) show cross-sectional SEM images of the prepared NST-GO
membranes. All of the membranes have the lamellar microstructure as the
most GO membranes.32 Moreover, the membrane thickness
increases gradually from 0.33 to 2.06 μm with the volume of
Ni(OH)2 nanosheet dispersion from 0 to 7 mL (Figure 3b).
This suggests the d -spacing between GO nanosheets were enlarged
continually with the Ni(OH)2 nanosheet loading. It is
noted that the membrane thickness increases quickly at a low
Ni(OH)2 nanosheet loading, slowly at appropriate loading
and quickly again at high loading. This phenomenon should have the great
influence on the formation of nanosheets-templated channels and will be
discussed later.
In general, the GO membranes are not stable in aqueous solution and thus
need further reduction for practical applications. However, excessive
reduction will cause decrease of water permeance. Thus, keeping good
hydrophilicity after reduction is essential for improving water
permeance of GO membranes. Figure 3c and Figure S7 (Supporting
Information) show static water contact angle of NST-GO membranes. It is
found that contact angle is decreased with Ni(OH)2volume increasing. Typically, the contact angle of reduced GO membrane
is 78.4° while that of NST-GO membrane prepared from 7 mL
Ni(OH)2 nanosheet dispersion is as low as 39.1°. This is
because more Ni(OH)2 nanosheets adsorb on GO nanosheets
and protect more areas from reduction when immersing hydrazine.
Therefore, the membranes with more sacrificed Ni(OH)2nanosheets have smaller contact angle, resulting in better
hydrophilicity.