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.