1
Ultrathin and ordered stacking of silica nanoparticles
2
via spin-assisted layer-by-layer assembly under
3
dehydrated
4
ultrafiltration membranes
5
Daisuke Saeki and Hideto Matsuyama*
6
Center for Membrane and Film Technology, Department of Chemical Science and Engineering,
7
Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
8
E-mail address of Daisuke Saeki:
[email protected]
9
*
conditions
for
the
fabrication
of
Corresponding author. E-mail:
[email protected]; Tel.: +81-78-803-6180; Fax: +81-78-803-6180.
10
11
1
1
ABSTRACT
2
Ultrafiltration (UF) membranes composed of an ultrathin and ordered layer of silica nanoparticles
3
(NPs) was fabricated via spin-assisted layer-by-layer (LbL) assembly in dehydrated conditions.
4
Anionic silica NPs and cationic sacrificial polymers were alternately stacked on an anodic
5
aluminum oxide support membrane via electrostatic interactions. The assembly was then
6
calcinated to remove the sacrificial polymers and form a stacked-NP membrane. The structure of
7
the NP layers was controlled by altering the number of stacked-NP layers formed during the LbL
8
assembly processes and performing an ethanol-based dehydration treatment. Increasing the
9
number of NP layers increased the total thickness of the stacked-NP layer and decreased the water
10
permeability. The rejection performance against dextran was improved by increasing the number
11
of NP layers, indicating a decrease in the number of defects. The ethanol-based dehydration
12
treatment successfully decreased the total thickness of the stacked-NP layer and improved the
13
rejection performance against dextran. The surface morphology of the NP layers became smoother
14
by the ethanol-based treatment. These results indicate that the ethanol-based treatment produces
15
dense NP layers with an ordered structure by removing water molecules adsorbed on the sacrificial
16
polymers and NPs. The performance of the stacked-NP membranes was superior to that of
17
commercial UF membranes.
18
Keywords: layer-by-layer assembly; stacked-nanoparticle membrane; ultrafiltration membrane,
19
dehydration.
2
1
1. INTRODUCTION
2
Membrane separation has attracted much attention for water purification applications
3
because of the high space and energy efficiencies compared to heat-based processes, such as
4
distillation. Ultrafiltration (UF) is widely applied to separate dispersed materials, such as particles
5
and colloids, from a solvent. The uniformity of the pores and low film thickness of an active
6
separation layer are important factors for improving the solvent permeability and selective
7
rejection of targets. Polymeric UF membranes are used because of their low fabrication costs and
8
easy fabrication processes. Such membranes are fabricated from various polymers via phase
9
separation methods, which is induced by a change in temperature or the addition of non-solvents
10
[1]. Ceramic UF membranes are also used because of their high chemical and thermal stability,
11
and are mainly fabricated via sol-gel methods [2, 3].
12
Layer-by-layer (LbL) assembly methods have been used to easily fabricate well-controlled,
13
thin, polymeric films on various substrates through molecular interactions, such as electrostatic
14
interactions [4], hydrophobic interactions [5], hydrogen bonding [6, 7], and covalent bonding [8,
15
9]. In the case of LbL assembly by electrostatic interactions, a substrate is alternately immersed in
16
aqueous solutions/dispersions of oppositely charged materials, such as polyelectrolytes, and an
17
extremely thin film is obtained. The film thickness can be controlled at the nanometer scale. LbL
18
assembly has been applied to the preparation of microcapsules [10], surface coatings [11], and
19
membranes for liquid [12-16] and gas separation [17]. LbL assembly of nanotubes [18-21] and
20
nanosheets [22-26] is also attracted for the fabrication of functional thin structures.
21
Recently, structures composed of stacked nanoparticles (NPs) have been applied to the
22
fabrication of both thin films for functional coatings [27-32] and porous membranes for liquid
3
1
separation [33-38]. The well-packed structure of the NPs produces a uniform surface structure with
2
uniform pores between the NPs. Stacked-NP structure in the previous studies are formed by
3
vacuum filtration [33, 34, 36, 37], dip- or rod-coating [35, 39], LbL assembly [28-30, 32, 38], and
4
spin-assisted LbL assembly [15, 27, 31, 40-42], etc. In particular, spin-assisted LbL assembly is
5
effective at controlling the uniformity and density of stacked structures. This is because the
6
centrifugal force caused by the spinning process removes adsorbed water molecules from the
7
materials stacked on a substrate, which results in the densification of the stacked materials [27].
8
Therefore, stacked-NP structures formed via spin-assisted LbL assembly have great potential for
9
the fabrication of porous membranes with well-controlled pore diameters and a thin active
10
separation layer for water purification applications.
11
In this study, we fabricated UF membranes composed of an extremely thin stacked-NP
12
layer via spin-assisted LbL assembly. Anionic silica NPs were alternately stacked with cationic
13
sacrificial polymers on a porous support membrane via spin-assisted LbL assembly and
14
electrostatic interactions. After a calcination process, a stacked-NP layer was formed on the
15
support membrane. The performance of the stacked-NP layer as a UF membrane was evaluated
16
based on its water permeability and rejection of various molecular weights of dextran. On LbL
17
assembly processes in aqueous conditions, polyelectrolytes have a large free volume caused by the
18
adsorption of water molecules, and prevent well packing of NPs. For further improvement of the
19
packing density of the stacked-NP layer and membrane performance, we examined the dehydration
20
of the sacrificial polymers by performing an ethanol-based treatment to remove water molecules
21
adsorbed on the sacrificial polymers and NPs during the LbL assembly processes.
22
4
1
2. EXPERIMENTAL
2
2.1. Materials
3
All chemicals were purchased from Wako Pure Chemical Industries (Osaka, Japan) unless
4
otherwise specified, and used as received. Aqueous solutions were prepared with Milli-Q water (>
5
18.2 MΩ⋅cm, Merck Millipore, Billerica, MA, USA). An anodic aluminum oxide membrane
6
(Anodisc, pore diameter = 0.02 μm, Whatman, Maidstone, England) was used as a support
7
membrane.
8
Polydiallyldimethylammonium chloride (PDDA; molecular weight (Mw) = 400–500 kDa) was
9
purchased from Sigma-Aldrich (St. Louis, MO, USA), and used to form sacrificial layers as
10
polycations. Dextran was purchased from Nakarai Tesque (20 kDa; Kyoto, Japan), and Sigma-
11
Aldrich (6, 40, 100, and 500 kDa) to evaluate the rejection performance and pore diameter of the
12
membranes. Ethanol was used to dehydrate membrane surfaces because it is a water-miscible,
13
volatile solvent and is a non-solvent for silica NPs and PDDA. The chemical structures of TEOS
14
and PDDA, and the surface morphology of an anodic aluminum oxide support membrane are
15
shown in Fig. 1.
Tetraethyl orthosilicate (TEOS) was used
to
prepare the silica NPs.
5
1 2
Fig. 1. Chemical structures of (A) TEOS and (B) PDDA. (C) Field emission scanning electron
3
microscopy (FE-SEM) image of the surface of an anodic aluminum oxide support membrane.
4 5
2.2. Preparation of silica NPs
6
A suspension of silica NPs was prepared by a sol-gel method [29]. TEOS (2.23 mL) was
7
added to 992.5 mL of Milli-Q water and magnetically stirred at 400 rpm for 2 h. Then, 5.63 mL of
8
aqueous sodium hydroxide (0.05 mol) was added to the TEOS solution and stirred for 2 days. The
9
prepared suspension was filtered through a MILLEX-AP syringe filter (pore diameter = 2.0 µm,
10
Merck Millipore, Billerica, MA, USA) to remove large aggregates. The diameter of the as-
11
prepared NPs was evaluated via dynamic light scattering (DLS; ELSZ-1000ZS, Otsuka
12
Electronics, Osaka, Japan). The morphology of the as-prepared NPs was observed with field
13
emission scanning electron microscopy (FE-SEM; JSF-7500F, JEOL, Tokyo, Japan). For the FE-
14
SEM observations, the samples were freeze-dried under vacuum conditions (FDU-1200; Tokyo
15
Rikakikai, Tokyo, Japan) and then coated with osmium (Neoc; Meiwafosis, Tokyo, Japan).
6
1
2.3. Fabrication of stacked silica-NP membranes
2
Stacked silica-NP membranes were prepared via spin-assisted LbL assembly with or
3
without a dehydration process (Fig. 2). First, an anodic aluminum oxide membrane was placed on
4
the stage of the spin-coater (MS-A100, Mikasa, Tokyo, Japan). A sacrificial cationic PDDA layer
5
was then formed on the surface of the support membrane. An aqueous solution of PDDA (0.25
6
wt.%) was dropped on the support membrane and spun at 3000 rpm for 15 s. Then, Milli-Q water
7
was dropped and spun to wash the membrane surface ten times. For some of the membranes
8
fabricated, ethanol was dropped and spun to dehydrate the membrane surface ten times. Next, a
9
layer of stacked silica NPs was formed on the sacrificial PDDA layer. The silica-NP suspension
10
was dropped, spun, and washed under the same conditions as the PDDA layer. This stacking cycle
11
was repeated 3, 5, or 7 times. The PDDA/stacked-NP membranes were calcinated at 500 οC for 5
12
h to remove the sacrificial PDDA layers and bind the NPs each other [2, 29], and the membranes
13
composed of 3, 5, or 7 layers of stacked NPs were obtained.
14 15
Fig. 2. Schematic diagram of the spin-assisted layer-by-layer (LbL) method.
7
1
2.4. Characterization of the surface properties of the fabricated membranes
2
The surface morphology of the fabricated membranes was observed via FE-SEM and
3
atomic force microscopy (AFM; SPA-400, Hitachi High-Tech Science, Tokyo, Japan). The total
4
thickness of the NP layer was measured from cross-sectional FE-SEM images. The AFM
5
observations were carried out with a SI-DF40P2 cantilever (Hitachi High-Tech Science) in tapping
6
mode. The ζ-potentials of the silica-NP and PDDA surfaces were measured with an electrophoretic
7
light-scattering apparatus (ELSZ-1000ZS) using a flat quartz cell in aqueous sodium chloride (10
8
mmol) at a pH of 6.7. The chemical composition of the membrane surfaces was analyzed with X-
9
ray photoelectron spectroscopy (XPS; JPS-9010, JEOL). For the AFM observations, FE-SEM
10
observations, and XPS analyses, the samples were freeze-dried under vacuum conditions.
11 12
2.5. Membrane performance
13
The membrane performance was evaluated with a cross-flow membrane filtration system,
14
the details of which were previously reported by our group [43]. The effective diameter of each
15
membrane sample was 10.0 mm. The water permeability of the fabricated membranes was
16
evaluated using Milli-Q water as a feed solution, and calculated from the weight gain of the
17
permeate solution. Aqueous solutions containing 1.0 g/L of one of five molecular weights of
18
dextran (6, 20, 40, 100, and 500 kDa) and 1.0 g/L of ethylene glycol as a reference material were
19
used as the feed solutions to evaluate the rejection performance of the membranes. The feed
20
solution was pressurized to 2.5 bar. The dextran concentration in the permeate solution was
21
analyzed via gel permeation chromatography (GPC) at 1.0 mL/min and 40 οC with a refractive
22
index detector (RID-10A, Shimadzu, Kyoto, Japan) and a Shodex SB-805HQ column (Showa
8
1
Denko, Tokyo, Japan). The rejection of dextran, R, was calculated from the peak-area ratios with
2
the following equation: R = 1 − (AP,d/AP,r)/(AF,d/AF,r), where AP,d, AP,r, AF,d, and AF,r indicate the
3
peak areas of dextran and ethylene glycol in the permeate and feed solutions, respectively. The
4
standard deviations were calculated from the measurements for at least three samples
5
independently fabricated. For comparison purposes, the performance of commercial organic UF
6
membranes (polyethersulfone membrane, molecular weight cut-off = 30 kDa; regenerated
7
cellulose membranes, molecular weight cut-off = 30, 100, and 300 kDa; Millipore) and ceramic
8
UF membranes (molecular weight cut-off = 15, 50, 150, and 300 kDa; Sterlitech, Kent, WA, USA)
9
was also evaluated.
10 11
3. RESULTS AND DISCUSSION
12
3.1. Stacked silica-NP membranes fabricated via spin-assisted LbL assembly
13
The average diameter of the as-prepared silica NPs was approximately 8.2 nm (standard
14
deviation (S.D.) = 0.7 nm), which was measured by DLS. An FE-SEM image of the NPs (Fig. S1
15
of the Supplementary Material) also shows that the diameter of the NPs is approximately 10 nm.
16
The ζ-potentials of the silica-NP and PDDA surfaces were −17.5 and 21.2 mV, respectively. Table
17
1 shows the chemical compositions of the membrane surfaces, which were determined via XPS
18
analysis. When the surface of the support membrane is coated with PDDA, carbon, nitrogen, and
19
chlorine are detected. The subsequent coating of the membrane with silica NPs increases and
20
decreases the relative concentrations of silicon and aluminum, respectively. Nitrogen was not
21
detected in the membrane surface after the calcination process because the sacrificial PDDA layers
22
decompose during this process. The FE-SEM images (Fig. S2 of the Supplementary Material)
9
1
show the same trends, i.e., a large number of silica NPs are present on the support membrane
2
coated with NPs after the sacrificial PDDA layer was formed, while only a few NPs are present
3
without the PDDA layer. This result indicates that the silica NPs interact with the PDDA-coated
4
support membrane via electrostatic interactions, and LbL assembly enables the alternate stacking
5
of layers of silica NPs and sacrificial PDDA on the support membrane.
6 7
Table 1. Chemical compositions of the surfaces of the stacked-NP membranes, which were
8
determined via XPS analysis (all values in atomic %).
9
a
Membranes
Al
O
Si
C
N
Cl
Pristine membrane
30.43
69.57
-a
-
-
-
PDDA-coated membrane
28.40
57.47
-
10.34
0.82
2.97
PDDA- and NP-coated membrane (3 layer)
8.42
61.12
24.59
4.68
1.19
-
PDDA- and NP-coated membrane (3 layer) after calcination
9.11
60.59
27.60
2.70
-
-
- = element was not detected.
10 11
Fig. 3 shows FE-SEM images of the membranes coated with different numbers of NP
12
layers after calcination. For the fabrication of these membranes, the ethanol-based dehydration
13
treatment was not carried out. The surfaces of all the fabricated membranes are not flat, and there
14
is a large amount of aggregates that are greater than 20 nm in diameter (Figs. 3(A−C)). Therefore,
15
the stacking of the NPs on the support membranes is not well-ordered. The surface of the support
10
1
membrane coated with 3 layers of NPs is not fully covered with the NPs (Fig. 3(A)). On the other
2
hand, the surfaces of the membranes coated with 5 and 7 layers are completely covered (Figs. 3(B)
3
and (C), respectively). In addition, the total thickness of the stacked-NP layer increases as the
4
number of NP layers increases (Figs. 3(D−F)), indicating that the NPs were stacked by the LbL
5
assembly processes. The surface morphology did not change before and after the membrane
6
performance evaluations.
7 8
Fig. 3. FE-SEM images of the membranes coated with different numbers of NP layers and without
9
the ethanol-based dehydration treatment. (A−C) Surface views. (D−F) Cross-sectional views.
10 11
Fig. 4 shows the performance of the membranes coated with different numbers of NP
12
layers. The support membrane did not reject any molecular weight of dextran, and its water
13
permeability was determined to be 4.6 × 103 L/(m2⋅h⋅bar). The water permeability of the stacked-
14
NP membranes is drastically lower than that of the pristine support membrane, and it decreases as
15
the number of NP layers increases. The increase of the water permeability did not occur during the
16
permeation experiments, indicating the high stability of the fabricated membranes. The rejection
11
1
of the 3-layer stacked-NP membrane for 500-kDa dextran is approximately 50%. This result could
2
be explained by the fact that the pores of this membrane are not completely covered with NPs,
3
which agrees with the FE-SEM image shown in Fig. 3(A). The rejection of the 5-layer stacked-NP
4
membrane for 500-kDa dextran is over 90%, while that of the 7-layer stacked-NP membrane for
5
100-kDa dextran is nearly 90%. Therefore, the rejection performance is improved by increasing
6
the number of NP layers because of the more complete coverage of the pores of the support
7
membrane with NPs.
8 9
Fig. 4. Performance of the stacked-NP membranes. (A) Water permeability of the membranes and
10
total thickness of the stacked-NP layer as functions of the number of NP layers. The filled and
11
empty symbols indicate the water permeability and total thickness of the stacked-NP layer,
12
1
respectively. (B) Rejection of the membranes against different molecular weights of dextran. The
2
diamonds, squares, and triangles indicate the 3-, 5-, and 7-layer stacked-NP membranes,
3
respectively.
4 5
3.2. Effects of the ethanol-based dehydration treatment on the membrane performance
6
In conventional LbL assembly methods that use electrostatic interactions in aqueous
7
conditions, cationic and anionic polymers are stacked in a hydrated state, i.e., the polymers have a
8
large free volume because of the presence of water molecules. In this study, to achieve a higher
9
packing density of the NP layers, we examined the dehydration of the stacked polymers by
10
performing an ethanol-based treatment between the alternate stacking of the NP layers and
11
sacrificial polymeric layers. Fig. 5 shows FE-SEM images of the stacked-NP membranes that were
12
subjected to the ethanol-based dehydration treatment. The membrane surfaces are completely
13
covered with the NPs under all stacking conditions, and are flatter than the corresponding untreated
14
membranes (Figs. 3(A−C)). In addition, the total thickness of the stacked-NP layer increases as
15
the number of NP layers increases (Figs. 5(D−F)), which indicates that the ethanol-based treatment
16
is applicable to the LbL assembly processes of the stacked-NP layers.
13
1 2
Fig. 5. FE-SEM images of the membranes coated with different numbers of NP layers and
3
subjected to the ethanol-based dehydration treatment. (A−C) Surface views. (D−F) Cross-sectional
4
views.
5 6
The performance of the stacked-NP membranes subjected to the ethanol-based treatment
7
is shown in Fig. 6. The water permeability decreases as the number of NP layers increases. In
8
addition, the water permeability of each ethanol-treated membrane is lower than that of the
9
corresponding untreated membrane (Fig. 4(A)). The dextran rejection of the ethanol-treated
10
membranes is much higher than that of the untreated membranes, and the molecular-weight cut-
11
off curves of the ethanol-treated membranes are very sharp, as shown in Fig. 6(B). The dextran
12
rejection of the ethanol-treated, 3-layer, stacked-NP membrane is the same as that of the untreated,
13
7-layer, stacked-NP membrane. Furthermore, the rejection of the ethanol-treated, 7-layer, stacked-
14
NP membrane for 6-kDa dextran is nearly 90%. These results indicate that the NPs are more
15
densely packed after the ethanol-based dehydration treatment, and the pores between the NPs are
16
more uniform.
14
1 2
Fig. 6. Performance of the stacked-NP membranes subjected to the ethanol-based dehydration
3
treatment. (A) Water permeability of the membranes and total thickness of the stacked-NP layer
4
as functions of the number of NP layers. The filled and empty symbols indicate the water
5
permeability and total thickness of the stacked-NP layer, respectively. (B) Rejection of the
6
membranes against different molecular weights of dextran. The diamonds, squares, and triangles
7
indicate the 3-, 5-, and 7-layer stacked-NP membranes.
8 9
We compared the performance of the fabricated stacked-NP membranes to that of
10
commercial organic and inorganic membranes. Fig. 7 shows the relationship between the water
11
permeability of the membranes and rejection against 100-kDa dextran. The upper right region of
15
1
Fig. 7 indicates a high membrane performance, with both a high water permeability and high
2
rejection. The stacked-NP membranes fabricated in this study have higher water permeabilities
3
compared to the commercial membranes at the same rejection value. In addition, the ethanol-based
4
dehydration treatment improves the membrane performance. The thickness of the active separation
5
layer of commercial UF membranes is generally several hundred nanometers to several
6
micrometers. The stacked layers of NPs on the fabricated membranes are extremely thin, i.e.,
7
several tens of nanometers, compared to the active separation layer of commercial membranes.
8
The controlled thickness of the active separation layer and uniform pores between the NPs are
9
responsible for the superior performance of the stacked-NP membranes. The stacked-NP
10
membranes have advantages of high water permeability, precisely-controlled pore size, and high
11
chemical and thermal stability as inorganic membranes over commercial membranes, although
12
they require multi-step processes for fabricating.
13 14
Fig. 7. Relationship between the water permeability and rejection against 100-kDa dextran for the
15
commercial and fabricated membranes. The filled and empty diamonds, circles, and squares
16
1
indicate the fabricated stacked-NP membranes without and with the ethanol-based dehydration
2
treatment, commercial organic membranes, and commercial inorganic membranes, respectively.
3 4
3.3. Effects of the ethanol-based dehydration treatment on the membrane structure
5
To investigate the effects of the ethanol-based dehydration treatment on the membrane
6
structure, we observed the surface structure of the membranes via AFM (Fig. 8). The average
7
surface roughness (Ra) of the untreated and ethanol-treated, 3-layer, stacked-NP membranes is
8
19.0 and 4.5 nm, respectively. Thus, the surface of the stacked-NP membrane is smoothed by the
9
ethanol-based treatment. The surface roughness of the untreated stacked-NP membrane is much
10
larger than the average NP radius, while that of the ethanol-treated membrane is nearly equal to
11
the average NP radius of 4.1 nm. This indicates that the stacking of NPs on the membrane becomes
12
more ordered by the ethanol-based dehydration treatment.
13
17
1
Fig. 8. AFM images of the surfaces of the (A) untreated and (B) ethanol-treated, 3-layer, stacked-
2
NP membranes. The average surface roughness (Ra) of (A) and (B) is 19.0 and 4.5 nm,
3
respectively.
4 5
Table 2 shows a summary of the total thickness and pore diameter of the NP layer of the
6
fabricated membranes. The pore diameter was calculated from the molecular-weight cut-off for
7
dextran, i.e., the molecular weight of dextran at which 90% of the solute was retained by the
8
membrane, and the hydrodynamic diameter of dextran (Ddextran), which was calculated with the
9
following equation: Ddextran = 0.066 × Mw0.463 [44]. The theoretical values for the NP-layer
10
thickness and pore diameter were estimated from the NP diameter when assuming a simple cubic
11
packing: Single NP layers are stacked onto each other keeping their two-dimensional structure,
12
when sacrificial polymeric layers are removed by the calcination (Fig. S3). Increasing the number
13
of NP layers decreases the pore diameter, the values of which are close to the theoretical values
14
because of the decreased number of defects. The defects are caused by areas of the support
15
membrane that are not covered with NPs. The ethanol-based dehydration treatment clearly
16
decreases the total thickness and pore diameter of the NP layer, and the values of the
17
aforementioned parameters for the ethanol-treated membranes are close to the theoretical values.
18
From these results, the ethanol-based treatment is effective at enhancing the density and uniformity
19
of the NP layers. The improved membrane performance shown in Fig. 7 is considered to be due to
20
structural changes in the stacked-NP layers that result in a more ordered structure. The solutes and
21
solvent were separated by the uniform pores between the NPs. The stacked-NP membranes have
22
been reported for UF and NF membranes by controlling the particle diameter [33]. The membrane
23
performance could be controlled by altering the diameter of the NPs.
18
1
Table 2. Total thickness and pore diameter of the NP layer of the fabricated membranes. Number of NP layers
2
a
3
b
4
c
3
5
7
Untreated membranes
Total thicknessa [nm]
42.3
71.4
85.8
Pore diameterb [nm]
-
28.7
13.6
Ethanol-treated membranes
Total thicknessa [nm]
36.7
47.6
63.5
Pore diameterb [nm]
28.7
8.9
3.7
Theoretical valuesc
Total thickness [nm]
24.6
41.0
57.4
Pore diameter [nm]
3.4
Evaluated from FE-SEM images. Calculated from the molecular-weight cut-off for dextran.
Estimated for a simple cubic packing.
5 6
When the sacrificial polymers and NPs are stacked on the support membrane by LbL
7
assembly, the sacrificial polymeric layers retain a large amount of water molecules, and thus, have
8
a large free volume, which causes the loose packing of the NPs (Fig. 9(A)). Chiarelli et al. [40]
9
reported that performing a heat treatment on the LbL assembly of oppositely charged
10
polyelectrolytes removes surface water molecules and results in a uniform surface. In this study,
11
the ethanol-based dehydration treatment also removed surface water molecules from the sacrificial
12
polymeric layers (Fig. 9(B)). As a result, the NPs were densely stacked and well-ordered, and the
13
membrane performance was improved (Fig. 7). In addition, the ethanol-based dehydration method
14
is milder than a heat treatment, and it has minimal effects on the structure of the NPs and sacrificial
15
polymers. The formation of uniform, thin, well-packed NP layers onto porous substrates is
16
important for fabricating membranes with high permeability and selectivity. The previously
17
reported techniques like as vacuum filtration are generally difficult to expand the membrane size,
19
1
although they can easily form well-packed structures onto porous substrates [33, 34, 36, 37]. Spin-
2
assisted LbL assembly could also barely expands the membrane size. The ethanol-based
3
dehydration treatment is effective to traditional dip-LbL assembly to obtain more thin and dense
4
stacked-NP layers. LbL assembly under dehydrated conditions would be able to form stacked-NP
5
layers onto various inorganic substrates including flat sheets, hollow fibers, and monolith
6
substrates for the expansion of the membrane size. Furthermore, the technique presented in this
7
paper is applicable to the stacking of functional nanomaterials, such as nanotubes and nanosheets
8
[21, 23, 45-47], for the membrane fabrication with highly controlled structures.
9 10
Fig. 9. Schematic diagram showing a possible mechanism for the effect of the ethanol-based
11
dehydration treatment on the stacking of NPs on porous substrates by spin-assisted LbL assembly.
12
The cationic polyelectrolyte PDDA was dehydrated by the ethanol-based treatment.
13
20
1
4. CONCLUSIONS
2
In this study, we developed a fabrication technique of UF membranes that use NPs as the
3
separating elements via spin-assisted LbL assembly and a subsequent dehydration process. Silica
4
NPs were alternately stacked with cationic sacrificial polymers on an anodic aluminum oxide
5
support membrane. The total thickness of the stacked-NP layer increased as the number of NP
6
layers increased, and it was controlled to be less than 100 nm. The rejection performance against
7
dextran was also improved as the number of NP layers increased. The surface roughness of the
8
stacked-NP layer on the support membrane was larger than the NP diameter because of the low
9
packing density. To control the packing density of the NPs, we dehydrated the sacrificial polymer
10
layers during the LbL assembly processes with an ethanol-based treatment. While the total
11
thickness and water permeability of the stacked-NP layer decreased by the ethanol-based
12
treatment, the rejection performance was greatly increased. AFM measurements showed that the
13
surface of the stacked-NP layer was more uniform and smoother by the ethanol-based treatment,
14
indicating that the stacking of the NPs was more orderly. The membrane performance of the as-
15
fabricated membranes was superior to that of commercial membranes because of their extremely
16
thin active separation layer and well-ordered stacked-NP structure.
17 18
SUPPLEMENTARY MATERIAL
19
The supplementary material includes FE-SEM images of the prepared silica NPs (Fig. S1), the
20
surfaces of support membranes coated with silica NPs (Fig. S2(A)) and silica NPs after a sacrificial
21
PDDA layer was formed (Fig. S2(B)), and schematic images of the theoretical total thickness and
22
pore diameter of the stacked-NP membranes when assuming a simple cubic packing (Fig. S3).
21
1
ACKNOWLEDGMENTS
2
This work was financially supported by a research grant from the Organization for Membrane and
3
Film Technology, Japan. We thank Mr. Yusuke Nakasuji for his help of XPS measurements and
4
Mr. Shinji Kawada, Tsuyoshi Yamashita, and Fumiya Sako for their encouragement.
5 6
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