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.