3.2. Formation and nanosheet-templated channels of NST-GO
membranes
Figure 1a shows the fabrication procedure for making the NST-GO
membranes. Briefly, (i ) the Ni(OH)2 nanosheet
dispersion was mixed with the GO dispersion to form
Ni(OH)2@GO composite nanosheets. (ii ) The
resulting composite nanosheet dispersion was filtered to form the
Ni(OH)2@GO nanosheet membrane across a microfiltration
filter. (iii ) The Ni(OH)2@GO membrane was reduced
by the hydrazine reduction process.13,28 (iv )
Finally, the NST-GO membrane was obtained by removal of
Ni(OH)2 nanosheets using dilute hydrochloric acid
solution. The membrane structure was adjusted by controlling volume of
Ni(OH)2 nanosheet dispersion. Compared with the pure GO
membrane, the Ni(OH)2@GO nanosheet membrane has rougher
surfaces after reduction due to the intercalation of
Ni(OH)2 nanosheets, as shown in Figure 2a and Figure 2b.
Excitingly, the formed NST-GO membrane has many wrinkles on the surfaces
resulted from removal of Ni(OH)2 nanosheets (Figure 2c).
That is, this membrane has higher surface porosity than the pure GO
membrane, which are favorable to permeance of water molecules into the
membranes.
Elemental composition (C, O and Ni) of GO-based membranes prepared in
this work was determined by EDS. The results are showed in Figure S4a
(Supporting Information). The Ni(OH)2@GO membrane has
three kinds of elements (C, O and Ni), in which the O content was
decreased markedly after reduction and further reduced largely after
removal of Ni(OH)2 nanosheets. Meanwhile, the Ni was
disappeared completely in the NST-GO membrane. These results reveal the
Ni(OH)2 nanosheets were dissolved successfully by acid
treatment. Figure 2d and Figure 2e show cross-sectional SEM images with
elemental mapping images of the reduced Ni(OH)2@GO
membrane before and after removal of Ni(OH)2 nanosheets.
The prepared membranes have typical lamellar structure of GO-based
membranes.29,30 It is found that the red spots (Ni)
intersperse among blue background (C) in the mapping image, meaning that
Ni(OH)2 nanosheets were dispersed uniformly among large
GO nanosheets. After filtering the HCl solution, the red spots
disappeared and replaced by black spots, which are our expected
nanosheets-templated channels in the work.
XRD spectra of GO-based membranes were characterized and showed in
Figure S4b (Supporting Information). The characteristic peaks are
located at 9.84°, 7.11°, 7.56° and 9.06° for the reduced GO membrane,
reduced Ni(OH)2@GO membrane, NST-GO membrane and reduced
NST-GO membrane, respectively. Accordingly, their d -spacing are
0.88, 1.21, 1.14 and 0.95 nm respectively, as listed in Figure 2f. It
can be found that the d -spacing of reduced
Ni(OH)2@GO membrane is 1.21 nm, which is equate to the
thickness of Ni(OH)2 nanosheets. That is, most of
Ni(OH)2 nanosheets were mono-intercalated between two GO
nanosheets. The d -spacing decreases slightly to 1.14 nm after
removal of Ni(OH)2 nanosheets, suggesting the
nanosheet-templated channels are broadened from 0.88 to 1.14 nm,
contributing to improvement of water permeance. The d -spacing
would be back to 0.95 nm after reduction again. This is because the
unreduced regions, which had adsorbed Ni(OH)2nanosheets, were reduced at this time. From above evidence, the
hydrophilic nanosheet-templated channels were fabricated successfully in
the GO membrane.