Embed Size (px)
Citation preview
Nanoscale
PAPER
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article OnlineView Journal | View Issue
aSchool of Materials Science and Engineerin
Technology, Shanghai 200093, China. E-m
edu.cn; Fax: +86 21 5527 0632; Tel: +86 21bDepartment of Chemistry and Biochemistr
University of California, Los Angeles (UCL
USA. E-mail: [email protected]; Fax: +1cInstitute of Materials Chemistry, College of
University, 1239 Si-Ping Road, Shanghai 20
[email protected] of Civil and Environmental Eng
Sustainability, and California NanoSystem
Angeles (UCLA), Los Angeles, California 9009eDepartment of Applied Chemistry, Universi
Africa
† Electronic supplementary informationshis/transmittances of ATR/FT-IR spcomposite membranes under different nube seen in Table S1. See DOI: 10.1039/c3n
Cite this: Nanoscale, 2013, 5, 3856
Received 25th January 2013Accepted 26th February 2013
DOI: 10.1039/c3nr00441d
www.rsc.org/nanoscale
3856 | Nanoscale, 2013, 5, 3856–386
Carbon nanotube-templated polyaniline nanofibers:synthesis, flash welding and ultrafiltration membranes†
Yaozu Liao,*abc Deng-Guang Yu,a Xia Wang,*a Wei Chain,a Xin-Gui Li,c
Eric M. V. Hoekde and Richard B. Kaner*b
Electro-active switchable ultrafiltration membranes are of great interest due to the possibility of external
control over permeability, selectivity, anti-fouling and cleaning. Here, we report on hybrid single-walled
carbon nanotube (SWCNT)–polyaniline (PANi) nanofibers synthesized by in situ polymerization of aniline
in the presence of oxidized SWCNTs. The composite nanofibers exhibit unique morphology of core–shell
(SWCNT–PANi) structures with average total diameters of 60 nm with 10 to 30 nm thick PANi coatings.
The composite nanofibers are easily dispersed in polar aprotic solvents and cast into asymmetric
membranes via a nonsolvent induced phase separation. The hybrid SWCNT–PANi membranes are
electrically conductive at neutral pH and exhibit ultrafiltration-like permeability and selectivity when
filtering aqueous suspensions of 6 nm diameter bovine serum albumin and 48 nm diameter silica
particles. A novel flash welding technique is utilized to tune the morphology, porosity, conductivity,
permeability and nanoparticle rejection of the SWCNT–PANi composite ultrafiltration membranes. Upon
flash welding, both conductivity and pure water permeability of the membranes improves by nearly a
factor of 10, while maintaining silica nanoparticle rejection levels above 90%. Flash welding of SWCNT–
PANi composite membranes holds promise for formation of electrochemically tunable membranes.
Introduction
Asymmetric ultraltration (UF) membranes are used in manyapplications involving chemical, environmental, biological andmedical engineering due to their unique separation capability, easyto scale-up possibilities, low energy consumption and recover-ability.1,2 However, separation properties of UF membranes arelimited and specic to only certain applications. Therefore thedevelopment of functionalized UF membranes with electrochemi-cally switchable separation properties could be of great interest.3 In
g, University of Shanghai for Science and
ail: [email protected]; wangxia@usst.
5527 4069
y and California NanoSystems Institute,
A), Los Angeles, California, 90095-1569,
(310) 206-4038; Tel: +1 (310) 825-5346
Materials Science and Engineering, Tongji
0092, China. E-mail: [email protected];
ineering, Institute of the Environment and
s Institute, University of California, Los
5-1969, USA. E-mail: [email protected]
ty of Johannesburg, Johannesburg, South
(ESI) available: Characteristic chemicalectral bands for SWCNT–PANi EBmbers of ash welds at full power canr00441d
2
the caseof electrically conductingmembranes, an externally appliedelectrical potential could contribute to the charge density and totalelectrical potential of the membranes. Therefore, the electricalpotential of a conducting membrane could be tuned simply byadjusting the potential applied by an external power source, andthereby, charged solute transport (i.e., selectivity) in theory could becontrolled. However, such successful electroactivemembranes haveso far only been demonstrated with relatively expensive, heavy andrigid metal or ceramic semiconductor membranes.4–6 Therefore,simple and scalable routes to light, low-cost and exible UFmembranes with tunable electroactivites are clearly desirable.
Single-walled carbon nanotubes (SWCNTs) possess excellentmechanical and electronic properties, e.g., they can carry an elec-trical current density up to 4 � 109 A cm�2, which is 3 orders ofmagnitude higher than typical metals such as aluminum orcopper.7 The Young's modulus of a SWCNT ranges from 0.32 to1.47 TPawith strengths between 10 and 52GPa and a toughness of770 J g�1.8 SWCNTs usually exhibit diameters of 1–2 nm andlengths of a fewmicrometers or more. Carbon nanotubes are thusgood candidates to serve as both mechanical and electrical rein-forcements for light-weight composites with a low percolationthreshold,9 especially for hybrid separation membranes.10,11
Polyaniline (PANi), a prototypical conjugated polymer, has beenextensively studied for potential applications in electronic devicesdue to its facile synthesis, environmental stability, unique elec-tronic properties and simple acid/base doping/dedoping chem-istry.12,13 PANi nanobers have demonstrated enhanced
This journal is ª The Royal Society of Chemistry 2013
Paper Nanoscale
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
performance in applications such as chemical sensors,14 super-capacitors,15 catalyst supports16 and membrane devices17 due totheir high surface area, porous nature and hydrophilicity. Inparticular, the PANi nanobers possess a unique property amongthe conducting polymers in that lms can be patterned by bothash and laser welding.18,19 During welding, the nanobernetworksareexposed toshortburstsofhigh intensity light, alteringtheir morphology, conductivity, doping and spectroscopic prop-erties. The ash welding technique exploits the photothermalresponse of PANi nanobers, thus opening new avenues for pro-cessing and patterning polymer-basedmaterials that could lead toelectronic devices and asymmetric membranes.18–20
By integrating the excellent properties of PANi and SWCNTs, itmay be possible to combine the advantages of both for the devel-opment of new nanotechnologies, electronics and separationdevices. For PANi, polymer molecules could orient along thecarbon nanotubes producing high surface area materials. ForSWCNTs, the addition of polymers could be helpful for improvingthe processing of composites. However, it is hard to get homoge-neous or miscible blends since the solubility or dispersibility ofSWCNTs and PANi is quite different. Despite a variety of in situpolymerization techniques that have been tried for creating suchhomogeneous composites,21–25 the synthesis of SWCNT–PANicomposite materials with bulk one-dimensional (1D) nano-structures, good dispersibility, high conductivity and low SWCNTloadinghave so farmetwithonly limited success.Recently,wehavereported an initiator-assisted chemical processing technique thatproduces nanobers of PANi coated on SWCNTs.26 The resultingmaterials exhibit exceptional conductivity and mobility and areremarkably easy to prepare in surfactant-free water dispersions.27
In this work, we report a novel technique to synthesize 1Dnanocomposites comprised of SWCNT-templated PANi nano-bers in the absence of an initiator. In order to obtain morereaction sites and higher miscibility between the two compo-nents, the SWCNTs are oxidized by strong acids (H2SO4 andHNO3) to introduce active carboxyl groups, and then applied astemplates for coating uniform PANi layers. The SWCNT-tem-plated PANi nanobers are further solvent-processed therebyproducing neutral-conductive and asymmetric UF membranesusing a simple nonsolvent induced phase separation (NIPS)technique. Additionally, ash welding is utilized to tune themorphology, conductivity, permeability and nanoparticlerejection of the SWCNT–PANi composite UF membranes. Theeffects of varying the percent power and number of ash weldson the conductivity as well as separation properties includingpure water ux, silica (SiO2) and bovine serum albumin (BSA)nanoparticle rejection of the electrically conducting SWCNT–PANi UF membranes are studied. The combination of highelectrical conductivity, porosity, hydrophilicity, permeabilityand selectivity of the SWCNT–PANi composite membranesshow promise for cross-ow electro-ltration applications.
Fig. 1 Schematic representation of synthetic routes to (a) SWCNT-templatedPANi nanofibers and (b) flash welded ultrafiltration membranes, a clear contrastbetween the welded (uncovered) and unwelded (covered) parts can be seen inthe top-right picture.
ExperimentalMaterials
Raw single-walled carbon nanotubes (SWCNTs, 60–70%) with anarrow diameter distribution peaked at 1.4 nm were purchased
This journal is ª The Royal Society of Chemistry 2013
from Carbon Solution, Inc. (USA). They were synthesized using aNi/Y catalyst by chemical vapor deposition. Silica nanoparticles(SiO2, average diameter of 48 nm) were supplied from NissanChemical Corp. (USA). All other chemical reagents includinganiline (99.5%), ammonium peroxydisulfate (APS, 98%), bovineserum albumin (BSA, 96%, average diameter of 6 nm), nitricacid (HNO3, 68–70%), sulfuric acid (H2SO4, 95–98%), methanol(MeOH, 99.6%), N-methylpyrrolidinone (NMP, 99%) andsodium hydroxide (NaOH, 98%) were purchased from Sigma-Aldrich and used as received.
Oxidation of SWCNTs
In order to improve the purity and dispersibility of SWCNTs,raw SWCNTs were oxidized using a strong acid mixture ofHNO3/H2SO4 (1/3, v/v) (Fig. 1a). Typically, 1.5 g of raw SWCNTswere added into the 100 mL mixture of concentrated HNO3/H2SO4 (1/3, v/v) at 60 �C for 6 h, then cooled to room tempera-ture and poured into 1 L of cold de-ionized (DI) water. Theproduct was repeatedly centrifuged and washed with DI waterand MeOH until the centrifuged solution reached a neutral pH.Finally, 50 mL of carbon nanotubes as a DI water suspensioncontaining �1.0 g of oxidized SWCNTs was obtained. Thesuspension was horn sonicated for 30 min before using it as atemplate for the synthesis of PANi nanobers.
SWCNT-templated synthesis of PANi nanobers
The SWCNT-templated PANi nanobers were synthesized byin situ chemical oxidative polymerization of aniline in theoxidized SWCNT suspension (Fig. 1a). Typically, a monomersolution composed of aniline (20 mL, 0.2194 mol) dissolved inaqueous H2SO4 (0.5 M, 1.0 L) was mixed at room temperaturewith the above oxidized SWCNT suspension and then magnet-ically stirred for 4 h. An oxidant solution composed of APS (12 g,0.0526 mol) dissolved in aqueous H2SO4 (0.5 M, 1.0 L) was thenpoured into the above mixture at room temperature. The
Nanoscale, 2013, 5, 3856–3862 | 3857
Fig. 2 SEM images of (a) purified SWCNTs and (a, inset) raw SWCNTs; (b)SWCNT-templated PANi nanofibers and (c and d) TEM images of SWCNT-tem-plated PANi nanofibers at (c) low and (d) high magnifications.
Nanoscale Paper
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
reaction was rapidly stirred for 1 h to evenly distribute theoxidant, monomer and SWCNTs and then le standing for 23 h.The product was recovered by microltration (0.45 mm Dura-pore� membrane, Millipore) and washed with NaOH (1.0 M,1.0 L) followed by DI water (1.0 L) and MeOH (1.0 L) to producethe dedoped emeraldine oxidation state of PANi. A very smallamount of the product as DI water dispersion was used forcharacterization by microscopy. A dried powder (2.51 g) ofSWCNT-templated PANi emeraldine base (EB) nanobers wasobtained by drying the samples in vacuum at 50 �C for two days.For comparison, 1.52 g of pure PANi EB was synthesized in theabsence of oxidized SWCNTs by following the same technique.Therefore, the oxidized SWCNT content in the nal dedopedcomposite nanobers is estimated to be approximately 40%,with the assumption that the presence of SWCNTs has anegligible effect on the aniline polymerization yield.
Preparation and ash welding of SWCNT–PANi compositemembranes
The SWCNT–PANi composite membranes were created by anonsolvent induced phase separation (NIPS) technique using DIwater as the coagulation bath (Fig. 1b). In this process, 0.55 g ofSWCNT–PANi EB composite was slowly added into 9.9 g of NMPand magnetically stirred for one day; then another 0.55 g ofcomposite was slowly added and the mixture was stirred for24 h, forming a homogeneous casting solution at a concentra-tion of 10%. The solution was cast on a commercial nonwovenpolyester support fabric (NanoH2O Inc., Los Angeles, California)and then immersed in 18 MU laboratory DI water. Porousmembranes of �100 mm in thickness were obtained aer thesolvent/nonsolvent (NMP/water) exchange and induced precip-itation of SWCNTs and PANi. To investigate the effect of ashwelding on the conductivity, permeability and nanoparticlerejection, dried SWCNT–PANi composite membranes were ashwelded by holding the devices 5 cm above the surface of themembranes and applying a full power (640 W) ash. The powerof the ash welding set-up (Alienbees B1600 640Ws purchasedfrom Digital Photography Solutions, USA) can be adjusted from0 to 100% of full power.
Chemical composition, morphology characterization andproperty measurements
The chemical bonding in the raw SWCNTs, oxidized SWCNTsand SWCNT–PANi EB composite membranes were character-ized by using Attenuated Total Reection/Fourier transforminfrared (ATR/FT-IR, JASCO 620) spectroscopy. The morphol-ogies of the samples were imaged using eld emission scanningelectron microscopy (SEM, JEOL JSM-6700) and transmissionelectron microscopy (TEM, PHILIPS CM120). The conductivitiesof the SWCNT–PANi EB composite membranes were obtainedby using a two-probe technique as previously reported.28 Theresistances in ohms per square cm (U cm�2) of the asymmetricmembranes were measured using a two-point probe setup bypainting two silver lines of the same length and sequencedistance onto the surface. Initial permeability (i.e., pure waterux) and selectivity (i.e., nanoparticle rejection) measurements
3858 | Nanoscale, 2013, 5, 3856–3862
were conducted in a dead-end ultraltration stirred-cell appa-ratus under a trans-membrane pressure of �10 psi at 25 �C.Silica (SiO2) and bovine serum albumin (BSA) nanoparticleswith average diameters of 48 and 6 nm, respectively, were usedto evaluate the selectivities of the membranes. The concentra-tions of SiO2 and BSA were measured using a Hach 2100NTurbidimeter and a HP 8453 UV-vis spectrophotometer,respectively. Particle rejections (R) were calculated fromeqn (1):29
R ¼ 1 � Cp/Cf (1)
where Cp and Cf are nanoparticle concentrations in thepermeate and feed streams, respectively. The permeability andrejection data points were calculated based on ve separatemeasurements for different membrane samples cast ondifferent days. The characteristic pore size (r) of eachmembrane was determined from eqn (2):29
r ¼ [l(2 � l)]2 (2)
where l ¼ rs/rp, rs is the solute particle radius and rp is themembrane pore radius.
Results and discussionMorphology and chemical composition of oxidized SWCNTs
An SEM image of raw SWCNTs shows nanobrillar morphologytogether with large aggregates of carbon (Fig. 2a, inset). Aerpurication with strong acids (HNO3/H2SO4), the oxidizedSWCNTs exhibit clear nanobundles with diameters between 5and 20 nm, as shown in Fig. 2a.
An ATR/FT-IR spectrum of raw SWCNTs displays two bandsnear 1590 and 1250 cm�1 that can be attributed to the C]Cstretching of the polyaromatic backbone of the nanotubes(Fig. 3a). The oxidized SWCNTs exhibit an additional band near1730 cm�1 (Fig. 3b), indicating the formation of carboxylic acidfunctionalities on the sidewalls of the SWCNTs.30 The presenceof carboxylic acid groups offers opportunities for furtherderivatization reactions as well as better processability. The
This journal is ª The Royal Society of Chemistry 2013
Fig. 3 ATR/FT-IR spectra of (a) raw and (b) oxidized SWCNTs.
Paper Nanoscale
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
oxidization enables SWCNTs to form well-dispersed electro-statically stabilized colloids in water and ethanol as reportedpreviously.31,32
Morphology, chemical composition and ash welding ofSWCNT–PANi composites
Carboxylic groups on the nanotubes can bind aniline monomerthrough hydrogen bonds and/or ionic bonds. Such specicinteractions have proven to be effective in improving themiscibility of traditional polymer composites, and similarsuccess is expected for SWCNT–PANi composites. Both SEMand TEM images of SWCNT–PANi composites show uniquecore–shell (SWCNT–PANi) 1D nanostructures with averagediameters of 60 nm, where oxidized SWCNTs were uniformlysheathed by 10–30 nm thick PANi coatings (Fig. 2b–d). Thecomposite dispersions with a 10% weight fraction have beencreated by dispersing the core–shell structured SWCNT-tem-plated PANi EB nanobers in NMP. By adopting the NIPStechnique, ultraltration membranes processed from suchdispersions exhibit smooth surfaces with an average porediameter of 60 nm (Fig. 4a). It appears that the core–shell 1Dnanostructures of SWCNT–PANi composites survived the NIPS.However, the surface coatings of PANi became smooth sincepart of the dedoped PANi may have been dissolved by the
Fig. 4 SEM images of SWCNT–PANi composite membrane surfaces of (a) orig-inal and (b) flash welded at 100% intensity; and (c) cross-sectional and (d) sub-structural SEM images of the flash-welded membrane.
This journal is ª The Royal Society of Chemistry 2013
solvents. The surviving SWCNT-templated PANi nanobersenable the membranes created to be welded using a ash.
Aer exposure of the composite membrane to ash weldingat full power, a large contrast can be readily seen between thewelded (uncovered) and unwelded (covered) parts (Fig. 1b, top-right). The striking morphological effects of the compositemembranes resulting from ash welding can be further imagedby SEM. The membranes show a distinct change in surfaceroughness, becoming smoother and shinier at sites exposed tothe ash (Fig. 4b). The cross-sectional SEM image reveals thatwelded sites increase from bottom (opposite to the ash expo-sure) to top (facing the ash exposure), leading to asymmetri-cally structured membranes (Fig. 4c). Careful SEM analyses ofthe cross-section indicate that a large amount of the oxidizedSWCNTs become exposed (Fig. 4d, arrows), in stark contrast towhen all the nanotubes were sheathed by PANi before ashwelding. The SWCNTs survived the ash welding, indicating thecarbon nanotubes have no changes on the ash welding of thePANi nanobers. Therefore, combination of the SWCNTs andPANi nanobers gives an ideal composite where one contributesto an excellent conductivity and the other contributes theunique property of ash welding.
Apparently, to obtain complete welding of the membrane,the ash must penetrate the entire membrane which can beaccomplished either by decreasing the thickness of themembrane or by increasing the intensity of the ash. Since theash is impaired during penetration of the composite, thishelps to explain why asymmetric membranes are created. Uponadjusting the ash intensity from 6.25 to 12.5% (Fig. 5a and b),the surface morphology of the composite membrane changeslittle until the power intensity is further increased to 25%(Fig. 5c). By increasing the intensity to 75% of full power, themembranes exhibit their smoothest and shiniest surface(Fig. 5d–f).
The reason may be that low ash intensity is insufficient toweld the membranes, while excessive ash intensity causesuneven welding. In addition, upon increasing the number ofash welds, the pore size of the composite membranes increase(Fig. 6), likely due to more welded sites.
Previous studies have shown a chemical cross-linkingprocess occurs in PANi nanobers when they are laser welded19
or heat treated.33 ATR/FT-IR spectroscopy is used to verify the
Fig. 5 SEM images of SWCNT–PANi composite membrane surfaces flash weldedat the following percent intensities: (a) 6.25, (b) 12.5, (c) 25, (d) 50, (e) 75 and (f)100%. Scale bar: 1000 nm.
Nanoscale, 2013, 5, 3856–3862 | 3859
Fig. 6 SEM images of SWCNT–PANi composite membrane surfaces flash weldedthe following number of times: (a) 0, (b) 1, (c) 2, (d) 3, (e) 4 and (f) 5 at full powerintensity.
Nanoscale Paper
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
occurrence of the chemical reactions generated by the ashwelding, as displayed in Fig. 7a and b. Unwelded SWCNT–PANicomposite membranes exhibit four characteristic bands at1590, 1496, 1300 and 1165 cm�1 that can be attributed toquinoid, benzenoid, C–N aromatic amine and –N]quinoid]N– (electron-like band) stretching modes, respectively of PANiEB nanobers;26 an additional band at 827 cm�1 can beassigned to the CAr–H bending modes in para-disubstitutedaromatic rings.34 Once the membranes are ash welded, thebands at 1590 an 1165 cm�1 blue-shi to 1602 and 1171 cm�1,respectively, implying that a chemical conversion of quinoidrings into benzenoid rings likely occurs during the ash weld-ing process. Moreover, the band at 1300 cm�1 red-shis to 1282cm�1 and the band at 827 cm�1 red-shis to 811 cm�1 (TableS1†), revealing that new bands such as tertiary amine nitrogenand trisubstituted benzene groups are generated by ashwelding. Note that gas evolution can be observed during ash
Fig. 7 (a and b) Effect on the number of flash welds on the ATR/FT-IR spectra of SW2, 3, 4 and 5 at a full power intensity in (a) the high and (b) the low frequency rangenanofibers in the composite.
3860 | Nanoscale, 2013, 5, 3856–3862
welding, probably due to dehydrogenation and/or additionreactions of CAr–H groups with C–N aromatic amine andquinoid groups (Fig. 7c). Thus, we conclude that the “melted”appearance of the ash welded membranes is likely caused by achemical cross-linking reaction. This is consistent withprevious results of cross-linking processes induced by laserwelding19 and heat treatment.33 With an increasing number ofash welds, the bands have no further chemical shis; however,the percent transmittance increases (Table S1†) indicating thatthe cross-linking density continues to increase.
Conductivity, permeability and rejection of SWCNT–PANicomposite membranes
We have shown that the morphology and chemical compositionof the SWNCT–PANi composite membranes can be readilychanged by controlling the ash welding process. This tech-nique can thus be utilized to tune the conductivity, permeabilityand rejection of SWCNT–PANi composite membranes. Theinuence of ash intensity and number of ash welds on theconductivity of the SWCNT–PANimembranes is shown in Fig. 8.
Unwelded SWCNT–PANi composite membranes exhibit asheet resistivity of �5.6 MU,�1 as compared to >100 MU,�1
for PANi EB membranes. Aer increasing the exposure intensityfrom 25 to 50 to 75 and then to 100% of full power, the sheetresistivity decreases to 4.5, 3.5, 1.4 and 0.55 MU ,�1, respec-tively. The improvements in conductivity of the compositemembranes that result from ash welding at rst appear toconict with decreases in conductivity reported for pure PANinanobers.18,19 However, here the PANi nanobers used arededoped and therefore electrically insulating. Since the dedo-ped PANi EB nanobers don't contribute to the conductivity, the
CNT–PANi composite membrane flash welded the following number of times: 0, 1,s; (c) a schematic illustration of the proposed cross-linking process for the PANi EB
This journal is ª The Royal Society of Chemistry 2013
Fig. 8 Effect of the number of flash welds and the flash intensity on the sheetresistance of the SWCNT–PANi EB composite membranes.
Paper Nanoscale
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
SWCNTs are primarily responsible for electronic transportwithin the composite membranes. As indicated by the analysesof morphology and chemical composition above, the sheathedSWCNTs become exposed due to “melting” of the PANi nano-bers. This actually leads to a lower percolation threshold forconductivity in the SWCNT–PANi composite membranes.Additionally, ash welding may reduce the oxidized SWCNTs inan effect similar to that observed with oxidized graphene35 andlaser-scribed graphene,36 thus enhancing the conductivity of theSWCNTs. Both reasons help explain why the conductivities ofthe composite membranes increase aer ash welding. Sincethe welding reaction occurs instantaneously, increasing thenumber of ash welds has a negligible effect on the chemicalcomposition (Table S1†). The SWCNT–PANi compositemembranes therefore show stable conductive properties(Fig. 8).
The inuence of the ash welding intensity on the perme-ability (i.e., pure water ux) and SiO2 and BSA nanoparticlerejections (i.e., selectivities) is shown in Fig. 9. UnweldedSWCNT–PANi composite membranes reject 95% of SiO2 and28% of BSA nanoparticles. When the percent ash intensity isincreased from 12.5 to 25, 50, 75 and 100%, the rejection of SiO2
Fig. 9 Effect of the flash welding intensity on the (a) SiO2 nanoparticle rejection,(b) BSA nanoparticle rejection and (c) permeability (i.e., pure water flux) of theSWCNT–PANi composite membranes.
This journal is ª The Royal Society of Chemistry 2013
is essentially unaffected (Fig. 9a); however, the rejection of BSAdecreases to 18.5, 14.5, 6.6, 2.2 and 1.4% (Fig. 9b), respectively.These observations point to a strong dependence of membranerejection on the type of the foulants involved, or more speci-cally, on their size. This implies that the asymmetricmembranes exhibit excellent size-selective rejection properties,enabling particle size selective separations. Unweldedmembranes display 54.7 gfd psi�1 of water ux. This valueincreases signicantly to 208 and then to 266, 381, 398 and 415gfd psi�1 when the percent ash intensity is increased from 12.5to 25, 50, 75 and 100%, respectively (Fig. 9c). The changes insurface roughness, pore size, porosity as well as bulkmorphology upon the ash welding appear to explain theimprovements in the water ux.
Note that, relatively high rejection is maintained for thelarger SiO2 nanoparticles (48 nm), but low rejection is observedfor the smaller size of BSA nanoparticles (6 nm) in bothunwelded and welded membranes, classifying them as “loose”UF membranes. From the BSA rejection data we estimate thecharacteristic “average pore diameter” (i.e., the membrane poreradius, rp) of these membranes to be 19, 24, 28, 42, 80 and112 nm for ash welding intensities of 0, 12.5, 25, 50, 75 and100%, respectively. The properties found here are amenable tocross-ow electro-ultraltration where the ltering membranesfunction as electrodes. Thus adjusting the physicochemicalfeatures of ultraltration membranes and molecules to beselectively transported, including charge, chemical interactionsand molecular size, can be exploited to enhance transportselectivity.37,38 Considering that these SWCNT–PANi asymmetricmembranes operate at neutral pH, we therefore believe that therejection of the charged BSA molecules could be enhanced byapplying an electrical current through the membranes.
Conclusions
We have successfully synthesized SWCNT-templated PANinanobers by in situ chemical oxidative polymerization ofaniline in an oxidized SWCNT suspension. The 1D compositenanobers that are produced exhibit a unique core–shell(SWCNT–PANi) structure with an average diameter of 60 nmand thicknesses of 10 to 30 nm for the PANi coating layer. Goodprocessability and conductivity of the composite nanobersenable formation of SWCNT–PANi ultraltration membranesthat are conductive at neutral pH using well-established non-solvent induced phase separation. Flash welding can be used totune the morphology, porosity, conductivity, permeability andnanoparticle selectivity of the SWCNT–PANi asymmetric ultra-ltration membranes. Upon ash welding, both the conduc-tivity and pure water permeability of the membranes improvedby a factor of 10, while the SiO2 nanoparticle selectivityremained constant and above 95%. However, the BSA selectivitydecreased due to the increase in pore size from ash welding.Thus, phase inverted SWCNT–PANi asymmetric membranesoffer new degrees of freedom for making novel ultraltrationmembranes and ash welding enables further ne-tuning of theelectrical conductivity, hydrophilicity, water permeability andparticle selectivity.
Nanoscale, 2013, 5, 3856–3862 | 3861
Nanoscale Paper
Dow
nloa
ded
by U
nive
rsity
of
Illin
ois
- U
rban
a on
26/
04/2
013
11:1
4:26
. Pu
blis
hed
on 2
7 Fe
brua
ry 2
013
on h
ttp://
pubs
.rsc
.org
| do
i:10.
1039
/C3N
R00
441D
View Article Online
Acknowledgements
The authors thank the National Natural Science Foundations ofChina (51203090, Y.Z.L.), the Innovation Program of ShanghaiMunicipal Education Commission (13YZ074, Y.Z.L.), NaturalScience Foundation of Shanghai (12ZR1446700, Y.Z.L.) andAbraxis Bioscience (R.B.K. and E.M.V.H.) for nancial support.The authors also thank Ms. Yue Wang for the TEMmeasurements.
Notes and references
1 R. Ghosh, J. Chromatogr., A, 2002, 952, 13.2 M. Feins and K. K. Sirkar, Biotechnol. Bioeng., 2004, 86, 603.3 C. Weidlich and K. M. Mangold, Electrochim. Acta, 2011, 56,3481.
4 M. Nishizawa, V. P. Menon and C. R. Martin, Science, 1995,268, 700.
5 I. Vlassiouk, P. Y. Apel, S. N. Dmitriev, K. Healy andZ. S. Siwy, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 21039.
6 N. A. Luechinger, S. G. Walt and W. Stark, J. Chem. Eng.Mater. Sci., 2010, 22, 4980.
7 S. H. Hong and S. Myung, Nat. Nanotechnol., 2007, 2, 207.8 M. F. Yu, B. S. Files, S. Arepalli and R. S. Ruoff, Phys. Rev.Lett., 2000, 84, 5552.
9 G. Nechifor, S. I. Voicu, A. C. Nechifor and S. Garea,Desalination, 2009, 241, 342.
10 L. J. Sweetman, L. Nghiem, I. Chironi, G. Triani, M. in hetPanhuis and S. F. Ralph, J. Mater. Chem., 2012, 22, 13800.
11 T.-H. Weng, H.-H. Tseng and M.-Y. Wey, Int. J. HydrogenEnergy, 2009, 34, 8707.
12 D. Li, J. X. Huang and R. B. Kaner, Acc. Chem. Res., 2009, 42,135.
13 H. D. Tran, D. Li and R. B. Kaner, Adv. Mater., 2009, 21, 1487.14 J. X. Huang, S. Virji, B. H. Weiller and R. B. Kaner, J. Am.
Chem. Soc., 2003, 125, 314.15 Q. Wu, Y. X. Xu, Z. Y. Yao, A. R. Liu and G. Q. Shi, ACS Nano,
2010, 4, 1963.16 J. Han, L. Y. Li and R. Guo, Macromolecules, 2010, 43, 10636.17 Z. F. Fan, Z. Wang, N. Sun, J. X. Wang and S. C. Wang, J.
Membr. Sci., 2008, 320, 363.
3862 | Nanoscale, 2013, 5, 3856–3862
18 J. X. Huang and R. B. Kaner, Nat. Mater., 2004, 3, 783.19 V. Strong, Y. Wang, A. Patatanyan, P. G. Whitten,
G. M. Spinks, G. G. Wallace and R. B. Kaner, Nano Lett.,2011, 11, 3128.
20 D. Li and Y. N. Xia, Nat. Mater., 2004, 3, 753.21 Z. X. Wei, M. X. Wan, T. Lin and L. M. Dai, Adv. Mater., 2003,
15, 136.22 W. R. Small, F. Masdarolomoor, G. G. Wallace and M. int het
Panhuis, J. Mater. Chem., 2007, 17, 4359.23 M. Ginic-Markovic, J. G. Matisons, R. Cervini, G. P. Simon
and P. M. Fredericks, Chem. Mater., 2006, 18, 6258.24 R. V. Salvatierra, M. M. Oliveira and A. J. G. Zarbin, Chem.
Mater., 2010, 22, 5222.25 Q. Yao, L. D. Chen, W. Q. Zhang, S. C. Liufu and X. H. Chen,
ACS Nano, 2010, 4, 2445.26 Y. Z. Liao, C. Zhang, Y. Zhang, V. Strong, J. S. Tang, X. G. Li,
E. M. V. Hoek, K. L. Wang and R. B. Kaner, Nano Lett., 2011,11, 954.
27 Y. Z. Liao, C. Zhang, X. Wang, X. G. Li, S. J. Ippolito,K. Kalantar-zadeh and R. B. Kaner, J. Phys. Chem. C, 2011,115, 16187.
28 Y. Z. Liao, X. G. Li and R. B. Kaner, ACS Nano, 2010, 4, 5193.29 G. R. Guillen, T. P. Farrell, R. B. Kaner and E. M. V. Hoek, J.
Mater. Chem., 2010, 20, 4621.30 K. J. MacKenzie, O. M. Dunens, M. J. Hanus and A. T. Harris,
Carbon, 2011, 49, 4179.31 M. S. P. Shaffer, X. Fan and A. H. Windle, Carbon, 1998, 36,
1603.32 L. B. Hu, D. S. Hecht and G. Gruner, Chem. Rev., 2010, 110,
5790.33 S. Bhadra and D. Khastgir, Polym. Degrad. Stab., 2008, 93,
1094.34 R. Mathew, B. R. Mattes and M. P. Espe, Synth. Met., 2002,
131, 141.35 L. J. Cote, R. Cruz-Silva and J. X. Huang, J. Am. Chem. Soc.,
2009, 131, 11027.36 M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science,
2012, 335, 1326.37 K. B. Jirage, J. C. Hulteen and C. R. Martin, Science, 1997,
278, 655.38 W. D. Comper and E. F. Glasgow, Kidney Int., 1995, 47, 1242.
This journal is ª The Royal Society of Chemistry 2013
