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.