3.1. Preparation of Ni(OH)2@GO composite nanosheets
The ultrathin Ni(OH)2 nanosheets were prepared according to our previous work.25,26 The 2-aminoethanol (3.0 mM) aqueous solution quickly mixed with an equivalent volume of Ni(NO3)2 (2.0 mM) aqueous solution and then aged for 30 mins at 20 °C. Clearly, the formed Ni(OH)2 nanosheets have the thickness of about 1.2 nm, and uniform width in the range of 13 to 50 nm with the average of 25 nm (Figure S1, Supporting Information). Subsequently, the Ni(OH)2 nanosheets were mixed into the aqueous GO nanosheets dispersion to produce the Ni(OH)2@GO composite nanosheets. The GO nanosheets have typical monolayer structure with the thickness of about 1.0 nm, as displayed by TEM and AFM (Figure 1b, Figure S2, Supporting Information). Moreover, the GO nanosheets have the uniform width that ranges from 500 to 1300 nm and fits Gauss distribution well with the average of 800 nm (Figure 1b). It is known that the GO nanosheets have many carboxyl and hydroxy on their surface, and thus often are used to adsorb positively charged chemicals due to the electrostatic interaction.27 As well, the Ni(OH)2 nanosheets are exactly positively charged and will assemble on the GO nanosheets in their mixture, leading to the formation of Ni(OH)2@GO composite nanosheets (Figure 1a).
Figure 1c shows an AFM image of Ni(OH)2@GO composite nanosheets, in which the dark background is a silicon wafer and bright layers are the GO nanosheets. Clearly, there are many bright spots on the GO nanosheets, which are attributed to the adsorbed Ni(OH)2 nanosheets. The height of Ni(OH)2@GO composite nanosheets at positions 1 and 2 was also measured on the AFM image and shown in Figure 1d. The total height of composite nanosheets is about 2.2 nm, composed of 1 nm GO and 1.2 nm Ni(OH)2, in which the flat “substrates” below 1 nm are GO nanosheets while the raised “peaks” over 1 nm are the Ni(OH)2 nanosheets adsorbed on the GO nanosheets. This result was also revealed by TEM (Figure S3a, Supporting Information). Comparing to the GO nanosheets, the Ni(OH)2@GO composite nanosheets have greater width with a broader distribution and the average of 3000 nm due to assembling of Ni(OH)2nanosheets on the GO nanosheets (Figure S3b, Supporting Information). From these, the prepared Ni(OH)2@GO composite nanosheets mono-dispersed in water should form the laminar membrane on the porous substrate by the conventional filtration stacking process.