Nanosheet-templated graphene oxide membranes for fast molecule
separation
Zhen Lin1,2,
Chuan Hu1, Qinglin Liu1, Qiugen
Zhang1,*
1Department of Chemical and Biochemical Engineering,
College of Chemistry & Chemical Engineering, Xiamen University, Xiamen
361005, China.
2Department of Applied Physics,
School of Science, Aalto University,
P.O. Box 15100, FI-00076 Espoo, Finland
*E-mail: qgzhang@xmu.edu.cn (Zhang); Tel: +86-592-2188072; Fax:
+86-592-2184822
Abstr act
Intercalated laminar membrane with controllable interlayer spacing
(d -spacing) is one of the most effective membranes for fast
molecule separation. In this work, we demonstrate a versatile strategy
to create nanosheet-templated water channels in laminar graphene oxide
(GO) membranes. The 1.2 nm-thick
nickel hydroxide nanosheets as
sacrificed intercalators provide a chance to control thed -spacing and simultaneously retain hydrophilicity. The resultant
membranes have controllable channels and exhibit over 6 times higher
water permeance than the unintercalated membrane. The 880 nm-thick
nanosheet-templated GO (NST-GO) membrane has accurate d -spacing
of about 1.14 nm and shows high water permeance of 120.3 L
m−2 h−1 bar−1 and
good molecule separation property,
reflecting
in high rejection for larger dyes (90.1% for erythrosine b (EB)), while
low rejection for smaller dyes (58.3% for methylene blue (MB)).
Furthermore, this strategy of intercalating and sacrificing nanosheets
has higher potential than traditional intercalation in controllingd -spacing of laminar membranes.
Topical Heading: Separations: Materials, Devices and Processes
Key Words: molecule separation; laminar membrane; sacrificed
intercalator; nanosheet-templated membrane; d -spacing control
1. Introduction
Molecule separation membrane has attracted increasing attention over the
past few decades because of its accuracy and
efficiency.1,2 Typically, isoporous membranes have
pores with uniform size which are applicable for molecule
separation.3,4 There are many methods to prepare the
isoporous membranes, such as interference
lithography,5 breath figure,6copolymer self-assembly,7 etc. Among them, controllingd -spacing is an effective way to prepare the isoporous membranes
with laminar nanochannels, such as GO membranes, MoS2membranes, WS2 membranes, etc.8,9These membranes have alterable d -spacing because the
two-dimensional nanosheets can be slid or
intercalated.10 Therefore, it is flexible to adjust
different d -spacing and make suitable laminar nanochannels of
these membranes for the separation of target molecules.
The GO membranes are one of the most common membranes that have laminar
structure.11,12 The d -spacing between two GO
nanosheets is about 0.9 nm, which is smaller than most of dye molecules
but much larger than water molecules.13,14 This means
the water molecules can penetrate easily while the dye molecules are
rejected effectively. As a result, the GO membranes often have excellent
dye rejection. However, the GO membranes also have unsatisfactory
permeance because of their narrow water channels resulting from smalld -spacing.15 Therefore, one of the most
effective ways to improve the water permeance without compromising
rejection is increasing d -spacing but controlling at an
appropriate value.
Recently, intercalating nanomaterials into the interlayers of GO
nanosheets for d -spacing control has attracted broad
attention.16,17 The most nanomaterials used are
zero-dimensional spheres. For example, Chen et al. reported a
kind of pyrrolidinium-functionalized fullerene intercalated GO
membranes.18 The membranes have broadenedd -spacing of about 1.46 nm, which results in the improvement of
water permeance from 4 L m−2 h−1bar−1 to 34 L m−2h−1 bar−1. Another zero-dimensional
sphere are ions. Chen et al. intercalated ions, such as
K+, Na+, Ca2+,
Li+ or Mg2+, into GO interlayers to
decrease the d -spacing and fabricated the cation-controlled
membranes for desalination.19 The KCl-controlled
membrane shows the ion rejection of more than 99%. One-dimensional
nanowires or nanotubes are sometimes considered as well. For example,
Zeng et al. used the modified carbon nanotubes as intercalators
to control the d -spacing at about 0.85 nm.20The membranes have the enhanced water permeance of 34.4 L
m−2 h−1 bar−1 with
over 90% rejection for dyes. Different from zero-dimensional and
one-dimensional nanomaterials, two-dimensional nanosheets are more
facile as intercalators because of their large surface area and lamellar
structure like GO nanosheets.21 However, they are
seldom reported because only the thin and uniform nanosheets can be used
for intercalation. For example, Liao et al. used ultrathin
zwitterionic MXene nanosheets to intercalate between GO
nanosheets.22 The resulting membranes have the high
water permeance of 110 L m−2 h−1bar−1 and good rejection for molecules whose size is
larger than 2 nm.
Although the intercalators are various, almost of them are designed
remaining in the membranes. These intercalators occupy water channels,
increase filtration resistance, hinder water transmission, and finally
cause unnecessary loss of water permeance. Therefore, a novel way to
control the d -spacing but prevent remaining of the intercalators
in the membranes should be more effective to improve the permeance of GO
membranes. Inorganic hydroxide nanomaterials are potential candidates.
They not only regulate d -spacing by their shapes but also can be
easily removed by acid dissolution.23 That is, thed -spacing of GO membranes can be controlled accurately, and
meanwhile the water channels can be even unimpeded than traditional
intercalators after sacrificing the inorganic hydroxides. For example,
Huang et al. reported nanostrands-channeled GO membranes by
sacrificing copper hydroxide nanostrands as intercalators, which
increases d -spacing from 0.85 nm to 0.99 nm.24The membranes have the high rejection of 99% for large molecules like
tetrakis (1-methylpyridinium-4-yl) porphyrin p-toluenesulfonate (TMPyP)
while low rejection of 36% for small molecules like
[Fe(CN)6]3-.
The Ni(OH)2 nanosheets, prepared by a facile way in our
previous work, show great potential as
intercalators.25,26 The nanosheets are desired
two-dimensional nanomaterials which have uniform thickness and
controllable size determined by aging time. For example, the size of 0.5
hour-aged Ni(OH)2 nanosheets is about 50×50
nm2 with the thickness of about 1.2 nm. Therefore, if
the nanosheets are used as intercalators, the d -spacing of GO
membranes can be easily controlled at about 1.2 nm that is a proper size
for dye separation. In this paper, we demonstrate a facile strategy to
construct hydrophilic laminar channels in the GO membranes for fast
molecular separation via the templates of ultrathin
Ni(OH)2 nanosheets. As shown in Figure 1a, the 1.2
nm-thick Ni(OH)2 nanosheets with abundant positive
charges assembled on the surface of negatively-charged GO nanosheets and
formed Ni(OH)2@GO composite nanosheets, which are used
to fabricate NST-GO membranes via filtering, reducing and
template removing in turn. The Ni(OH)2 nanosheets
provide a chance to control the d -spacing between GO nanosheets
and simultaneously retain their hydrophilicity after reducing.
Amazingly, the resulting NST-GO membranes exhibit over 6 times higher
water permeance than the
unintercalated membrane, showing a great potential in fast molecular
separation.