Low-temperature solution process for preparing flexible ... · carbon nanotube, Congo red, flexible electronics, supercapacitors ABSTRACT Single-walled carbon nanotubes (SWNT) are

Nano Res

1

Low-temperature solution process for preparing

flexible transparent carbon nanotubes film and its

application in flexible supercapacitors

Ashok K. Sundramoorthy1 (), Yi-Cheng Wang

1, and Sundaram Gunasekaran

1 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0880-1

http://www.thenanoresearch.com on August 17, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research

DOI 10.1007/s12274-015-0880-1

TABLE OF CONTENTS (TOC)




Ashok K. Sundramoorthy*, Yi-Cheng Wang and

Sundaram Gunasekaran*

Department of Biological Systems Engineering,

University of Wisconsin-Madison, 460 Henry Mall,

Madison, WI 53706, United States

A diazo dye (Congo red)-based simple solution process has been

developed to prepare highly transparent flexible carbon nanotubes

film with low sheet resistance (34±6.6 Ω/ ) and high

transmittance (81% at 550 nm), which is also shown suitable as a

potential flexible supercapacitor electrode material.




Ashok K. Sundramoorthy1(), Yi-Cheng Wang

1, and Sundaram Gunasekaran

1()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

transparent electrode,

manganese dioxide,

carbon nanotube,

Congo red,

flexible electronics,

supercapacitors

ABSTRACT

Single-walled carbon nanotubes (SWNT) are known for their high conductivity,

mechanical strength, transparency, and flexibility, making them suitable for use

in flexible electronics, transparent electrodes, and energy-storage and

energy-harvesting applications. However, to exploit these properties, SWNT

should be de-bundled in a surfactant solution for further processing and use.

We report a new method to prepare SWNT-based transparent conducting film

(TCF) using a diazo dye 3, 3'- ([1,1'-biphenyl] -4,4'-diyl) bis (4-amino

naphthalene-1-sulfonic acid), commonly known as Congo red (CR), as a

dispersant. About 20-nm-thick uniform TCFs were prepared on rigid glass and

flexible polyethylene terephthalate (PET) substrates. The CR-SWNT dispersion

and the CR-SWNT TCFs were characterized via UV-Vis-NIR, Raman

spectroscopy, FT-IR spectroscopy, transmission electron microscopy (TEM),

field-emission scanning electron microscopy (FE-SEM) and dynamic light

scattering (DLS) measurements. The sheet resistivity of the CR-SWNT TCFs

was about 34±6.6 Ω/ at a transmittance of 81% (at 550 nm), which is

comparable to that of indium tin oxide (ITO)-based films. Unlike SWNT

dispersions prepared in common surfactants, such as sodium dodecyl sulfate

(SDS), sodium cholate (SC), and Triton X-100, the CR-SWNT dispersion is

amenable to forming TCF by drop coating. The CR-SWNT TCF is also very

stable and maintains its very low sheet resistivity even after 1,000 consecutive

bending cycles of 8 mm bending radius. Further, manganese dioxide (MnO2)

was electrochemically deposited on the CR-SWNT-PET film. The as-prepared

MnO2-CR-SWNT-PET film exhibited high specific capacitance and bendability,

which make it a promising candidate as an electrode material for flexible

supercapacitors.

1 Introduction

There is a growing demand for high quality

transparent conducting films (TCFs) because of

their use in flat-panel displays, electrochromic

windows, photovoltaics, polymer solar cells,

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to S. Gunasekaran, [email protected]; A.K. Sundramoorthy, [email protected]

Research Article

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

hand-held devices, energy technologies, and

biosensors[1]. Currently, indium tin oxide (ITO)

film-coated transparent conducting materials are

commonly used in applications where TCFs are

needed. The ITO-based materials offer relatively

low sheet resistance (~20 Ω/) and fairly high

transmittance (>80% at 550 nm)[2]. However, the

cost of indium is on the rise due to increasing

demand and limited availability[2]. Furthermore,

ITO-based materials are unsuitable for

high-flexibility applications, because they become

brittle after a few bending cycles[3]. The market for

transparent conductive film and glass is estimated

to reach $6.3 billion in 2024[4, 5]. Therefore, other

materials such as conducting polymers[6], metal

wire/nanomesh[7, 8], nanofibers[9], single-walled

carbon nanotubes (SWNT)[10-14], SWNT-metal

hybrids[5, 15-17], and graphene[18] are being

investigated as potential alternatives to ITO.

Although conducting polymers such as polyaniline,

polypyrrole and poly(3,4-ethylenedioxythiophene)

exhibit conductivity similar to that of metal

nanowires (copper and silver), their conductivity is

sensitive to air due to redox doping process and

will degrade significantly with time[19]. Further,

conducting polymers absorb light in the visible

range, giving a distinct color to the TCFs[19]. Metal

nanowire/nanomesh-based TCFs offer high

conductivity, but their transparency is usually a

trade-off with sheet resistivity compared to ITO[20].

The high cost of metal (silver, copper etc.) is not

conducive to large-scale cost-effective production of

metal nanowire-based TCFs; moreover, their

flexibility is also limited. Thus among various

alternatives to ITO, SWNT-based TCFs are expected

to gain significant market share because of their

high mobility (100,000 cm2/Vs)[21], current carrying

capacity (109 A/cm2)[22], flexibility (without

fracture)[23], and transparency in the visible

range[5, 12].

SWNT-based TCFs can be prepared by various

methods such as spin coating[24], dip coating[25],

spray coating[26], bar coater[27] and by vacuum

filtration[28]. However, since as-synthesized SWNT

are highly hydrophobic bundles, they must be

dispersed in a common surfactant such as sodium

dodecyl sulfate (SDS), sodium cholate (SC), and

Triton X-100 to take advantage of their numerous

unique properties[29]. The sheet resistivity of TCFs

prepared using SWNT dispersed in common

surfactants is rather high. The electric conduction of

SWNT film (at SWNT–SWNT junctions) is

dominated by tunneling[30]. The thin insulating

surfactant (or polymer) layer surrounding the

nanotubes prevents direct contact between the

SWNT[31]. Since the adhering surfactants could not

easily be rinsed away[32], the SWNT films were

annealed at a high temperature or acid-washed to

remove the adhering surfactant and reduce the

sheet resistance[33-35]. Both annealing and acid

washing will damage the structure of SWNT, which

also affects the conductivity of nanotubes[36].

Furthermore, the requirement of high-temperature

annealing treatment precludes using flexible

substrates such as polyethylene terephthalate (PET)

and polydimethylsiloxane (PDMS). Therefore, the

SWNT-based films were first prepared on a solid

substrate and annealed before being transferred

onto a flexible substrate[11, 37, 38].

By considering the potentials of SWNT and the

need for ITO-free TCFs, it is necessary to develop

an effective solution-based process that can be used

for large-scale production of flexible SWNT film

with low sheet resistivity and high transparency.

Herein, we present such a method using

3,3'-([1,1'-biphenyl]-4,4'-diyl)bis(4-aminonaphthalen

e-1-sulfonic acid), commonly known as Congo red

(CR), to disperse SWNT. CR is a symmetrical linear

molecule (Fig. 1(a)) soluble in both aqueous and

organic solvents, such as dimethylformamide (DMF)

and ethanol. Due to the strong hydrophobic

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

polycyclic aromatic functional groups with

ionizable sulfonic groups, CR was expected to

disperse SWNT in aqueous solution[39]. We

extensively studied the ability of CR as a dispersant

for SWNT by comparing it with that of SDS, SC,

and Triton X-100.

We used SWNT dispersed in CR (CR-SWNT) to

form a thin-transparent film with very low sheet

resistivity (34±6.6 Ω/) and high transparency

(transmittance ~81%). CR-SWNT could be used to

prepare TCFs by drop coating or spin coating on

rigid and flexible substrates. It is worth mentioning

that drop coating is not suitable for SWNT

dispersed in SDS (SDS-SWNT), SC (SC-SWNT), and

Triton X-100 (Triton-SWNT). Our CR-SWNT-based

TCFs are viable alternatives to ITO-based materials

or carbon-based flexible conducting films,

especially for applications requiring flexible

substrates. To our knowledge, this is the first

extensive study on using CR as a dispersant for

preparing SWNT-based TCFs.

Flexible energy storage devices are important for

a wide range of applications such as flexible

electronic displays, wearable devices, and flexible

solid-state supercapacitors[40-43]. Specifically,

transparent electrodes play a major role in

preparing entirely transparent flexible organic

light-emitting diodes, field-effect transistors,

energy-harvesting and flexible power

sources[44-46]. Supercapacitors with flexible power

source are required to realize flexible

energy-storage devices that can function under

considerable physical deformation and stress[41,

47]. Ge et al. reported preparing transparent and

flexible electrodes and supercapacitors using

polyaniline/SWNT composite thin films with

specific capacitance of 55 F.g-1 at a current density

of 2.6 A.g-1 [48]. Due to the exceptional

pseudo-capacitance properties of manganese

dioxide (MnO2), it is popularly used in the

preparation of supercapacitors. Recently, ultralight

and flexible MnO2/carbon foam composites[49],

manganese ferrite (MnFe2O4)/graphene-based

flexible supercapacitors[50], MnO2/stainless

steel-based mesh-supercapacitor[51],

electrochemically grown large-area alpha-MnO2

nanoflower arrays on flexible graphite paper[52]

and solvothermal method to fabricate

MnFe2O4/graphene hybrids on flexible graphite

sheets have been demonstrated[50].

In this work, we electrochemically deposited

MnO2 onto our CR-SWNT-PET flexible electrode to

demonstrate the potential use of our TCFs in

flexible energy-storage applications such as

supercapacitors. We also investigated specific

capacitance, galvanostatic charging-discharging,

bendability, and stability of the as-prepared

MnO2-CR-SWNT-PET electrode.

2 Experimental

2.1 Materials

Pristine P2-SWNT (arc-discharge) was purchased

from Carbon Solutions, lnc (Riverside, CA, USA).

CR was purchased from MP Biomedicals, LLC.

(Solon, OH, USA). Sodium cholate hydrate (SC) and

Triton™ X-100 were received from Alfa Aesar

(Ward Hill, MA, USA). Manganese (II) acetate,

ethanol and dimethylformamide (DMF) were from

Sigma-Aldrich (St. Louis, MO USA). Sodium

dodecyl sulfate (SDS) was received from

AMRESCO® LLC (Solon, OH, USA). Sodium sulfate

(Na2SO4) was from FisherChemicals (Fair Lawn, NJ,

USA). All other reagents were of analytical grade

and used without further purification. In all

experiments, deionized water with resistivity of 18

MΩcm was used.

2.2 Preparation of CR-SWNT dispersion

P2-SWNT (5 mg) were mixed in 10 mL of 1 mM CR


4 Nano Res.

solution and bath-sonicated for 10 min. The mixture

of CR and SWNT was further ultrasonicated in a

130-W Ultrasonic processor for one hour using ice

bath to control temperature of the mixture. Finally,

the CR-SWNT was centrifuged at 12,000 rpm for

one hour and the supernatant was collected and

used. For comparison, supernatants of pristine

P2-SWNT dispersed in 1% SDS, 1% SC, and 1%

Triton X-100 were similarly collected.

2.3 Preparation of CR-SWNT- based TCFs

SWNT films were prepared on rigid glass, flexible

PET, or PDMS substrates by spin coating or drop

coating of CR-SWNT dispersion. For spin coating,

CR-SWNT was dropped onto the flat substrate,

which was spun at 2000 rpm. The CR-SWNT-coated

substrate was dried at 60 °C for 30 min to 1 h and

washed with DMF and ethanol multiple times, and

then using deionized water (it is not recommended

to directly wash with water before DMF/ethanol

washing). Finally, the CR-SWNT-coated substrates

were again dried at 60 °C for 30 min. For drop

coating, CR-SWNT, SDS-SWNT, or Triton-SWNT

were pipetted and allowed to uniformly spread on

the substrate for 2 min before drying at 60 °C in an

air oven.

2.4 Characterization of CR-SWNT

The films were characterized using FE-SEM (Leo

1530 Field Emission SEM), LabRAM Aramis

(Horiba JobinYvon) confocal Raman microscope,

and Bruker’s AFM microscope. DLS analysis was

performed using 90 Plus Particle size analyzer

(Brookhaven Instruments). A UV-Vis-NIR

spectrophotometer (Lambda 25, PerkinElmer) and a

Fourier transform-infrared (FT-IR) spectrometer

with universal attenuated total reflectance (ATR)

sampling accessory (Spectrum 100, PerkinElmer)

were used for characterizing CR-SWNT and TCFs.

Transmittance of the films was measured after

baseline subtraction with bare glass or PET

substrate. Thus, %transmittance reported is solely

due to the SWNT films. A spin coater (6800 Spin

Coater Series, Specialty Coating Systems,

Indianapolis, Indiana, USA) was used to spin coat

nanotubes. CR-SWNT was centrifuged using

Eppendorf centrifuge 5415C. Contact angles were

measured using Dataphysics OCA 15 unit. The

Hewlett Packard 4142B Modular DC

source/monitor connected HP 34401A multimeter

with a contact four-point probe station (Cascade

Microtech Inc., Beaverton, Oregon, USA) was used

to measure the sheet resistivity of the film.

Electrochemical measurements were performed

using an electrochemical analyzer (CHI 660C, CH

Instrument Inc., Austin, TX, USA).

2.5. Preparation of MnO2-CR-SWNT-PET electrode

To prepare a flexible, indium-free supercapacitor,

MnO2 was potentiodynamically deposited onto the

CR-SWNT-PET substrate through cyclic

voltammetry (CV) at 50 mV.s−1 by potential

sweeping between 0.0 and 1.4 V for 10 cycles.

Electrochemical deposition of MnO2 was achieved

using CR-SWNT-PET as a working electrode in a

three-electrode cell setup with 50 mM manganese(II)

acetate and 100 mM Na2SO4 solution. Platinum wire

and silver/silver chloride (Ag/AgCl) (in 3 M KCl)

were used as counter and reference electrodes,

respectively. An alligator clip was connected with a

copper foil to the CR-SWNT-PET film to ensure

good electrical contact. After successful

electrochemical deposition, MnO2-CR-SWNT-PET

was thoroughly washed with deionized water and

dried in an air oven at 60 °C for 30 min. Surface

morphology of the MnO2 film was characterized via

FE-SEM. To study the electrochemical properties of

the MnO2-CR-SWNT-PET, Nafion® (5% w/w)

solution (Fuel Cell Earth Llc., Woburn, MA, USA)

was placed on the top of the film[53]. After solvent

evaporation, Nafion layer was covered the film and


5 Nano Res.

served as the separator. In addition,

MnO2-CR-SWNT-PET was soaked in distilled water

for 30 min, and then electrochemical studies were

performed in 100 mM Na2SO4 solution. We used a

three-electrode system used for CV measurements

described above. The unstable first cycle of CV

measurements was discarded.

Electrochemical stability of electrode materials is

important for flexible supercapacitor applications.

Our CR-SWNT-PET film had superior flexible

properties. To verify if the MnO2/CR-SWNT-PET

film was stable when flexed, test was performed by

bending it back-and-forth manually for 100 times,

with a bending radius of 8 mm.

3 Results and discussion

3.1 Preparation of CR-SWNT-TCF

Our scheme for preparing CR-SWNT-based TCFs

on a rigid (glass) and flexible (PET or PDMS)

substrates by drop coating or spin coating is

depicted in Figure 1(b). As shown in Figure 1(c),

TCFs prepared using CR-SWNT formed a uniform

thin film after oven drying for 30 min at 60 C. After

DMF/ethanol washing, CR-SWNT film was very

stable and strongly adhered to the substrate even

after repeated washings with water. However, films

could not be prepared similarly using SDS-SWNT

and Triton-SWNT, as they became aggregated and

partly sloughed off from the substrate after washing

with DMF, ethanol, and water (Figs. S1(a), and (b)).

Our observation was similar to those of others who

reported pre-functionalization or treatment of the

substrate (with 3-aminopropyltrimeoxysilane) was

required to form stable SWNT films using common

surfactant dispersions[17].

Since CR is soluble in DMF, we also

attempted to disperse SWNT in both DMF and DMF

with 1 mM CR (CR-DMF) solutions. Stable

dispersions of SWNT were obtained both in DMF

and CR-DMF; however, the dispersibility of SWNT

was higher in CR-DMF than in DMF. Furthermore,

films were easily formed on PDMS substrate by

drop coating of SWNT-CR-DMF dispersion (Fig.

S2(a)), which was not the case with the SWNT-DMF

dispersion (Fig. S2(b)). This indicates that the

addition of CR affords the formation SWNT film

and its adherence on the PDMS substrate.

3.2 Adhesion properties of CR-SWNT dispersion

The consistencies of the CR solution and all

SWNT dispersions were similar. However, the CR

aqueous solution and the CR-SWNT tended to

adhere to the walls of 1.5-mL microcentrifuge tube,

which was evidenced by inverting the tubes (Fig.

2(a), a, a’ and b, b’). While SDS-SWNT and

Triton-SWNT descended (Fig. 2(a), c, c’ and d, d’),

the CR solution and CR-SWNT did not (Fig. 2(a)).

This suggests that CR has an inherent ability to

form a matrix structure with certain short-range

binding affinity to the hydrophobic surface.

Presumably, the hydrophobic backbone of CR

molecules interacts with hydrophobic surface and

position self-assembly of SWNT on the substrate.

This could be the reason why we are able to form

TCFs on substrates with CR-SWNT, unlike with

SDS-SWNT or Triton-SWNT (Fig. 2(a) and Fig. S2).

3.3 UV-Vis-NIR, Raman and FT-IR

characterization

The UV-Vis-NIR spectra of the CR-SWNT exhibited

a red-shift in the absorption peak of CR, from 497

nm to 502 nm (Fig. 2(b)), suggesting a strong charge

transfer between CR and SWNT in aqueous

solution. UV-Vis-NIR spectra of SC-SWNT show

two distinct semiconducting bands (S22, S33) and

one metallic band (M11), which are characteristics

of SWNT prepared by arc-discharge (Fig. 2(b),

curve c)[54]. However, S22 and M11 bands are

suppressed for CR-SWNT due to strong adsorption

of CR on the nanotubes (Fig. 2(b), curve b).


6 Nano Res.

To verify the nature of interaction between SWNT

and CR, Raman spectra of CR-SWNT film, CR

powder (control) and pristine SWNT were obtained

(Fig. 3(a)). Comparing the spectra of CR-SWNT film

and pristine SWNT (Fig. 3(a)), we notice that the G+

band had up-shifted from 1591 to 1596 cm-1 and that

the intensity of G- band decreased substantially, due

to the strong electron charge transfer from CR to

SWNT (Fig. 3(a) and Fig. S3(a))[55-57]. Raman

peaks of CR were also present in the spectra of

CR-SWNT film even after repeated washing with

DMF and ethanol, which further confirmed strong

non-covalent binding between CR and the

nanotubes (Fig. 3(a) curve ii). However, Raman

peaks of CR disappeared when the CR-SWNT film

on glass substrate was thermally annealed at 500 C

in a vacuum furnace for 10 min, but the disorder (D)

band (at 1312 cm-1) slightly increased (Fig. S3(b),

curve ii). This is attributed to covalent reaction

happening on the nanotubes by radicals generated

from CR at high temperature. Thermal annealing

step decreased the inter-tube contact resistance,

consequently the SWNT films prepared on a glass

substrate showed sheet resistivity of 366±138Ω/ at

a transmittance of 87 % (at 550 nm) (Fig. 3(b)).

The interaction between CR and SWNT was also

confirmed by FT-IR. The spectrum for pristine

SWNT (Fig. 4) were similar to those previously

reported[58], without any distinct features in the

range of 600 to 4000 cm-1 possibly due to the high

quality of the P2-SWNT. The FT-IR spectrum for CR

showed a broad and strong absorption band at 3464

cm-1 (N–H bonds), with several other peaks at 1041,

1176 (the stretching vibration of S=O due to -SO3-),

1597 (assigned to stretching vibration of –N=N–

bond), 1352 and 1222 cm-1 (the stretching vibrations

of =C–N= group adjacent to aromatic ring). Other

bands located at 904, 832, 749 and 697 cm-1 were

assigned to aromatic rings (C–H) of CR (Fig. 4(a),

(b)). Significant new bands were observed in the

spectrum for CR-SWNT film (Fig. 4(a) and (b)),

confirming that some functional groups are

attached to the nanotubes. The S=O stretching

vibration observed in the CR spectrum was present

in the CR-SWNT spectrum, but red-shifted to a

lower wavenumber of 1030 cm-1, perhaps due to the

π-π stacking between SWNT and CR molecules.

3.4 SEM, AFM, TEM and DLS measurements

We further characterized the CR-SWNT-TCF using

FE-SEM and AFM. The FE-SEM micrographs (Figs.

5(a), (b)) show a dense network of SWNT forming a

20-nm-thick film on the substrate. The CR-SWNT

film and SC-SWNT film on copper grid were also

determined by measuring the diameters of SWNT

and uniformity of films using transmission electron

microscopy (TEM) (Fig. 5(c), (d)). The average

diameters of individual SWNT and nanotube

bundles in the CR-SWNT film were in the range of

1.2 to 7 nm (Fig. 5(c)), which was smaller than those

in the SDS-SWNT film (~1.2 to 30 nm; individual

tubes to bundles) (Fig. 5(d)). The diameter of

arc-discharge SWNT was in the range of 1.2 to 1.6

nm[59]. Thus, the presence of up to 7 nm diameter

entities in our film clearly indicates that bundles of

up to three to four nanotubes exist. However, a

highly dense film network was observed for

CR-SWNT film compared to SC-SWNT film.

To ascertain why CR-SWNT could form uniform

nanotube network film on rigid and flexible

substrates without any surface treatment, we

investigated further. The CR solution and

SDS-SWNT and SC-SWNT formed concave

meniscus in a plastic cuvette (Fig. S4). Adhesion is

responsible for meniscus formation, and this has to

do in part with fairly high surface tension of water.

When CR, SDS and SC were dispersed in water, the

surface tension decreased; further, due to poor

interaction between the dispersions and the cuvette,

a concave meniscus was formed (Fig. S4). However,

with CR-SWNT the concavity of the meniscus was

not obvious, which we believe was due to the


7 Nano Res.

attraction of CR molecules to the hydrophobic

molecules on the cuvette surface (Fig. S4 (b)). Sa et

al. reported that SDS-SWNT would form micelles in

the solution[60]. When SWNT dispersed in

surfactant solution, the individual or bundles of

SWNT were wrapped and separated completely by

the surfactant molecules[60]. Charged SWNT were

stabilized in water due to the repulsive forces of

negatively charged polar head groups of SDS. Due

to hydrophobicity, SWNT were lifted above the

air-water interface[61]. In our case, with the

addition of SWNT in CR solution, hydrophobic part

of CR (both biphenyl and naphthalene rings) would

interact with SWNT (via hydrophobic

interaction)[62]. As shown in Fig. S4(b), almost a

flat meniscus was observed due to the strong

molecular repulsion between SWNT. We assume

that ionized polar head group of CR (SO3-) stays

dipped in water, while the hydrophobic CR-SWNT

part tends to escape projecting toward the air-water

interface. That is why an almost flat air-water

interface was observed with the CR-SWNT (Fig.

S4(b)).

Apparently, as shown in Figure S5, at lower

concentrations (2.9 and 28.6 µM) the CR molecules

do not interact and adhere sufficiently to the walls

of the microcentrifuge tube that was observed at

higher concentrations (≥57.1 µM). This clearly

shows strong adhesion between CR molecules and

hydrophobic surfaces[63]. In addition, a drop of CR

and SC solutions and CR-SWNT, SDS-SWNT, and

SC-SWNT were placed on PET substrate to

demonstrate strong adhesion of CR on PET. Both

CR solution and CR-SWNT formed an almost

uniform film upon drying (Table S1, (a), (b)). In

contrast, SC, SC-SWNT, and SDS-SWNT became

aggregated (Table S1 (c), (d), (e)). We noticed that

after removing free CR molecules by washing with

DMF and water, a very thin layer of CR remained,

which showed that CR binds with PET. This was

confirmed by obtaining UV-Vis spectra of bare PET,

PET after applying a thin film of CR (CR-PET), and

CR-PET after washing with DMF. For CR-PET,

before DMF washing, a strong broad absorption

peak was observed from 400 to 600 nm (centered at

500 nm) (Fig. S6), and after DMF washing, about 2%

decrease in transmittance was observed, which

indicates that a thin-layer of CR is still attached to

PET (Fig. S6). The amine and sulfonic acid groups

of CR apparently interacted with the substrate and

formed a uniform CR-SWNT film after drying

(without any additional surface treatment).

The distribution of hydrodynamic radius of

single or bundles of nanotubes present in

CR-SWNT, SC-SWNT, and SDS-SWNT were

measured by DLS, which showed that the mean

diameter of nanotube bundle was larger in

CR-SWNT (104 nm) than in SC-SWNT and

SDS-SWNT (70 and 75 nm, respectively) (Fig. 6).

These values represent the hydrodynamic diameter

of SWNT bundles plus any adhering surfactant

molecules. The presence of nanotube bundles in

CR-SWNT indicate that they might be wrapped

with the supramolecular structure of CR molecules.

3.5 Sheet resistance, flexibility and contact-angle

measurements

The SWNT films prepared on a PET substrate (30

mm × 30 mm) by spin coating of CR-SWNT is

shown in Figure 7(a). This as-prepared

CR-SWNT-PET film is conductive with a sheet

resistivity of 183.6±19.7 Ω/ at 71% transmittance

(at 550 nm) (Fig. S7 and Fig. 7(b), red curve). The

effect of SWNT density on the conductivity of the

film was studied by AFM (Fig. 7(c)). SWNT films

with lower density of nanotubes were also prepared

by adjusting the volume of CR-SWNT for

comparison (Fig. S8). The sheet resistivity of the

lower-density (16 nanotubes/µm2) film (1.56 kΩ/

at 90% transmittance) was much higher than that of

the higher-density (>50 nanotubes/µm2) film (183.6

Ω/ at 71% transmittance).


8 Nano Res.

The hydrophobicity of the CR-SWNT-PET film

was studied after drying under flow of nitrogen gas.

The highest contact angle value of the as-prepared

film was ~100 (Fig. S9). The hydrophobicity of the

film may arise due to the synergistic effect of long

aromatic backbone of CR and SWNT, which may be

preferable for flexible electronic display

applications[28].

The sheet resistivity of the CR-SWNT-PET film

may be further lowered by treating with nitric acid

(HNO3)[64]. We immersed the film in HNO3 for 30

min at room temperature. The UV-Vis-NIR

transmittance spectra showed that after the HNO3

treatment and following washing with water, the

CR molecules were removed (Fig. 7(b), blue curve).

The absorption intensity of the inter-band energetic

transition of S22, S33 and M11 of SWNT were

almost bleached off, which shows the acid-doping

effect of nanotubes[65]. The sheet resistivity of the

film after HNO3 treatment was 34±6.6 Ω/ (at a

transmittance of 81% at 550 nm) (Fig. 7(d)), which is

several folds better than that of the untreated film

(183.6 ±19.7 Ω/) (Fig. S7). Furthermore, the sheet

resistivity of the film remained virtually unchanged

even after 1,000 bending cycles at bending radius of

8 mm, confirming the high flexibility and stability of

CR-SWNT-PET TCFs (Fig. 7(d)).

To demonstrate the potential application of CR in

preparing TCFs, we attempted to prepare TCF films

by drop coating of 500 µL CR-SWNT, Triton-SWNT,

and SDS-SWNT on PET substrates (as shown in Fig.

1(b) and experimental section 2.3). While a uniform

TCF film was obtained with CR-SWNT, both

Triton-SWNT and SDS-SWNT became aggregated

during drying at 60 °C for 30 min. Sheet resistance

and transparency of these films showed that the

lowest sheet resistance of 57.69±23.50 Ω/ at

transparency of 73.95±6.12 % was obtained for

CR-SWNT-PET (Table S1 and Fig. S10). These

results confirm that CR is the best dispersant,

among those we studied to prepare SWNT-based

TCFs. In addition, estimated costs of preparing

TCFs reveal the cost-effectiveness of our

CR-SWNT-PET (Table S2). Hence our TCF is

suitable for economical large-scale production of

TCF for industrial applications, without requiring

processes such as vacuum deposition, thermal

annealing etc.

3.6 CR-SWNT TCF in Flexible Supercapacitors

The CR-SWNT TCF (width 1 cm × length 3 cm)

acquired yellow color after electrodeposition of

MnO2 (Fig. 8). As shown in Figure 9(a), during first

anodic scan, two anodic peaks were observed at

0.81 and 1.1 V, which correspond to the two distinct

electrochemical oxidation mechanisms of

manganese and deposition of MnO2 on

CR-SWNT-PET film (Equations 1-3)[66, 67]. With

the increasing cycle number, both anodic (at 1.1 V)

and cathodic (at 0.3 V) peaks increased, which

indicated successful deposition of MnO2 on the

CR-SWNT-PET film according to the following

equations (Equations 1-3) (Fig. 9(a)).

Mn2+ Mn3+ + e- (1)

Mn3+ + 2H2O MnOOH + 3H+ (2)

MnOOH MnO2 + H+ + e- (3)

FE-SEM image of MnO2-SWNT-PET showed

homogeneous and compact layer of MnO2 particles

(Fig. 8(c)), which imply high electrical conductivity

of our CR-SWNT-PET film. The CVs of the

CR-SWNT-PET film before and after MnO2

deposition in 100 mM Na2SO4 solution at 10 mV/s

are compared in Figure 9(b). From the CV data, the

specific capacitance (Cs, F.g-1) of the

MnO2-CR-SWNT-PET film at different scan rates

was calculated using equation 4 [68]:

(4)


9 Nano Res.

Where, m is the mass of MnO2 (1.5 mg), v is the

scan rate, (Va-Vc) is the potential window of CV,

and I(V)d(V) is half of the integrated area under the

CV curve.

Electrochemical impedance spectroscopy (EIS)

was further used to study changes in conductivity

of CR-SWNT-PET electrodes after MnO2 deposition.

EIS was performed in 0.1 M KCl solution containing

5 mM [Fe(CN)6]3−/4− at 0.2 V vs. Ag/AgCl for the

CR-SWNT-PET and MnO2-CR-SWNT-PET

electrodes. The Nyquist plots of the impedance

spectra obtained are presented in Fig. 8(d). An

intercept at the Z'(real axis) in high-frequency

region corresponds to the ohmic electrolyte

resistance (Rs = ~65 Ω). In addition, EIS spectra of

both electrodes contain a semicircle in the high

frequency region (due to the charge-transfer process,

Rct) and a straight sloping line in the low frequency

region (due to the diffusion of potassium ions (Rsf)

into active material). As expected, the calculated Rct

value of CR-SWNT-PET increased from 427 Ω to

611 Ω after MnO2 coating[69].

Further, after MnO2 coating on CR-SWNT-PET

electrode, a uniform layer of polymer electrolyte

was formed by drop coating of Nafion solution (see

details in experimental section 2.5). Finally,

Nafion/MnO2-CR-SWNT-PET electrode was used to

test its electrochemical properties. A substantial

increase in the Cs value was obtained after MnO2

deposition (34.53 F.g-1 compared to 1.6 F.g-1) (Fig.

9(b)).

3.7 Effect of CR-SWNT film thickness on MnO2

deposition

To ascertain the effect of CR-SWNT film thickness

on MnO2 deposition, we prepared three

CR-SWNT-PET electrodes with thicknesses of 30, 60

and 140 nm. The film thickness was measured using

AFM (Fig. S11, (a), (b), (c)). Then, CR-SWNT-PET

(width 1 cm × length 1 cm) was exposed to

electrolyte solution containing 50 mM manganese

acetate and 100 mM Na2SO4 at 50 mV/s scan rate for

10 cycles (Fig. 9(a)). After successful

electrodeposition of MnO2 on each CR-SWNT-PET

electrode, CVs were recorded individually in 100

mM Na2SO4 solution at a scan rate of 10 mV/s and

compared (Fig. S11 (d), curves (a), (b), (c)). The Cs

values of 30-, 60-, and 140-nm thick

MnO2/CR-SWNT-PET were calculated as 3.76, 10.98,

and 7.13 F.g-1, respectively. Thus, Cs appears to

reach a peak in between 60 and 140 nm substrate

film thickness. The decrease in Cs value after

reaching a peak may be due to the complete

coverage and saturation of MnO2 layer on the

CR-SWNT film. We used 60-nm-thick

CR-SWNT-PET film for further electrochemical

studies.

3.8 Effect of scan rate and galvanostatic

charge/discharge studies

The effect of scan rate (from 10 to 250 mV/s) on the

CVs of Nafion/MnO2-SWNT-PET electrode showed

rectangular (at lower scan rates) and

quasi-rectangular (at higher scan rates) shapes

without any peaks, which indicated an ideal

electrical double layer capacitance and fast

charging/discharging of the

Nafion/MnO2-CR-SWNT-PET electrode (Fig. 9(c)).

As shown in Figure 9(d), the highest Cs was

recorded (34.53 F g-1) at a low scan rate of 10 mV/s,

with Cs declining with increasing scan rate due to

direct impact on the diffusion time of cations into

the matrix, which leads to a sharp decrease in the

available capacity (Fig. 9(d))[70].

Furthermore, the CVs recorded before and after

consecutive 100 bending cycles (Fig. S12) of

MnO2-CR-SWNT-PET electrode in 100 mM Na2SO4

solution showed no significant changes in the

capacitance (Fig. 10(a)). Figure 10(b) shows the

galvanostatic charge/discharge curve of


10 Nano Res.

Nafion/MnO2-CR-SWNT-PET in 100 mM Na2SO4

between 0 and 0.8 V at various current densities.

The charging curves were almost symmetric to their

corresponding discharging curves at current

densities from 140 to 700 mA.g-1, which is indicative

of good capacitive behavior of the flexible

Nafion/MnO2-CR-SWNT-PET electrode. In addition,

the symmetrical linear triangle shaped

charging/discharging curves indicate good

reversibility and increased Faraday redox reaction

of the electrodeposited MnO2 (Fig. 10(c)). The

electrochemical stability of

Nafion/MnO2-CR-SWNT-PET electrode was

investigated by potential cycling in the range of 0 to

0.8 V in 100 mM Na2SO4 aqueous solution (Fig.

10(d)). We found that the

Nafion/MnO2-CR-SWNT-PET retained about 80% of

the initial capacitance after 600 cycles. Thus, our

CR-SWNT-PET is highly conductive and flexible,

and suitable for manufacturing flexible

supercapacitors.

4. Conclusions

We described a fairly simple and inexpensive

method to prepare SWNT-based TCF using CR as a

dispersant. Our method allows casting films on

rigid and flexible substrates without surface

treatment by either drop coating or spin coating.

The sheet resistivity and transparency of our SWNT

film (34±6.6 Ω/ at a transmittance of 81% (at 550

nm)) are much better than those reported in the

literature (see Table S3), and indeed comparable to

those of ITO-based films. The CR-SWNT-PET is

useful as a highly conducting electrode in

electrochemical system, and can be used as a

potential alternative to ITO-free electrode. The

potential use of our TCF for energy-storage

application was demonstrated by preparing and

testing a highly flexible supercapacitor electrode by

electrodepositing MnO2 on the CR-SWNT-PET

substrate.

Acknowledgments

The authors acknowledge the support of the

Wisconsin Alumni Research Foundation.

Electronic Supplementary Material:

Supplementary material (SWNT-TCF images of SDS,

SC, Triton-X-100 and DMF, Raman spectra,

photograph showing meniscus, different

concentrations of CR in water, sheet resistivity of

CR-SWNT-PET, AFM results, contact angle

measurements, CR-SWNT, SDS-SWNT and

Triton-SWNT TCFs, CR-SWNT film thickness

measurements, flexibility testing, cost estimation

table and sheet resistivity comparison) is available

in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

References

[1] Ellmer, K. Past Ahievements and Future Challenges

in the Development of Optically Transparent

Electrodes. Nat. Photonics 2012, 6, 808-816.

[2] Angmo, D.; Krebs, F. C. Flexible ITO-Free Polymer

Solar Cells. J. Appl. Polym. Sci. 2013, 129, 1-14.

[3] Lewis, J. Material Challenge for Flexible Organic

Devices. Mater. Today 2006, 9, 38-45.

[4] Ghaffarzadeh;, K.; Das, R. Transparent Conductive

Films (TCF) 2014-2024: Forecasts, Markets,

Technologies. DTechEx, 2014.

[5] Hecht, D. S.;Hu, L. B.; Irvin, G. Emerging

Transparent Electrodes Based on Thin Films of

Carbon Nanotubes, Graphene, and Metallic

Nanostructures. Adv. Mater. 2011, 23, 1482-1513.

[6] Xia, Y. J.;Sun, K.; Ouyang, J. Y. Solution-Processed

Metallic Conducting Polymer Films as Transparent

Electrode of Optoelectronic Devices. Adv. Mater.

2012, 24, 2436-2440.

[7] Kiruthika, S.;Gupta, R.;Rao, K. D. M.;Chakraborty,

S.;Padmavathy, N.; Kulkarni, G. U. Large Area

Solution Processed Transparent Conducting Electrode

Based on Highly Interconnected Cu Wire Network. J.

Mater. Chem. C 2014, 2, 2089-2094.

[8] Guo, C. F.;Sun, T. Y.;Liu, Q. H.;Suo, Z. G.; Ren, Z.

F. Highly Stretchable and Transparent Nanomesh

http://dx.doi.org/10.1007/s12274-***-****-*


11 Nano Res.

Electrodes Made by Grain Boundary Lithography.

Nat. Commun. 2014, 5, 3121.

[9] Hsu, P.-C.;Wu, H.;Carney, T. J.;McDowell, M.

T.;Yang, Y.;Garnett, E. C.;Li, M.;Hu, L.; Cui, Y.

Passivation Coating on Electrospun Copper

Nanofibers for Stable Transparent Electrodes. ACS

Nano 2012, 6, 5150-5156.

[10] Liu, Q. F.;Fujigaya, T.;Cheng, H. M.; Nakashima, N.

Free-Standing Highly Conductive Transparent

Ultrathin Single-Walled Carbon Nanotube Films. J.

Am. Chem. Soc. 2010, 132, 16581-16586.

[11] Ho, X. N.; Wei, J. Films of Carbon Nanomaterials for

Transparent Conductors. Materials 2013, 6,

2155-2181.

[12] De Volder, M. F. L.;Tawfick, S. H.;Baughman, R. H.;

Hart, A. J. Carbon Nanotubes: Present and Future

Commercial Applications. Science 2013, 339,

535-539.

[13] Rosner, B.;Guldi, D. M.;Chen, J.;Minett, A. I.; Fink,

R. H. Dispersion and Caracterization of Arc

Discharge Single-walled Carbon Nanotubes -

Towards Conducting Transparent Films. Nanoscale

2014, 6, 3695-3703.

[14] Mistry, K. S.;Larsen, B. A.;Bergeson, J. D.;Barnes, T.

M.;Teeter, G.;Engtrakul, C.; Blackburn, J. L. n-Type

Transparent Conducting Films of Small Molecule and

Polymer Amine Doped Single-Walled Carbon

Nanotubes. ACS Nano 2011, 5, 3714-3723.

[15] Wang, J.;Zhang, J.;Sundramoorthy, A. K.;Chen, P.;

Chan-Park, M. B. Solution-processed flexible

transparent conductors based on carbon nanotubes

and silver grid hybrid films. Nanoscale 2014, 6,

4560-4565.

[16] Yang, S. B.;Kong, B. S.;Jung, D. H.;Baek, Y. K.;Han,

C. S.;Oh, S. K.; Jung, H. T. Recent advances in

hybrids of carbon nanotube network films and

nanomaterials for their potential applications as

transparent conducting films. Nanoscale 2011, 3,

1361-1373.

[17] Park, S.;Vosguerichian, M.; Bao, Z. A. A Review of

Fabrication and Applications of Carbon Nanotube

Film-based Flexible Electronics. Nanoscale 2013, 5,

1727-1752.

[18] Wang, X. H.;Tao, L.;Hao, Y. F.;Liu, Z. H.;Chou,

H.;Kholmanov, I.;Chen, S. S.;Tan, C.;Jayant, N.;Yu,

Q. K.;Akinwande, D.; Ruoff, R. S. Direct

Delamination of Graphene for High-Performance

Plastic Electronics. Small 2014, 10, 694-698.

[19] Wang, P.-C.;Liu, L.-H.;Alemu Mengistie, D.;Li,

K.-H.;Wen, B.-J.;Liu, T.-S.; Chu, C.-W. Transparent

Electrodes Based on Conducting Polymers for

Display Applications. Displays 2013, 34, 301-314.

[20] Hu, L.;Kim, H. S.;Lee, J.-Y.;Peumans, P.; Cui, Y.

Scalable Coating and Properties of Transparent,

Flexible, Silver Nanowire Electrodes. ACS Nano

2010, 4, 2955-2963.

[21] Dürkop, T.;Getty, S. A.;Cobas, E.; Fuhrer, M. S.

Extraordinary Mobility in Semiconducting Carbon

Nanotubes. Nano Lett. 2003, 4, 35-39.

[22] Yao, Z.;Kane, C. L.; Dekker, C. High-Field Electrical

Transport in Single-Wall Carbon Nanotubes. Phys.

Rev. Lett. 2000, 84, 2941-2944.

[23] Zhang, D. H.;Ryu, K.;Liu, X. L.;Polikarpov, E.;Ly,

J.;Tompson, M. E.; Zhou, C. W. Transparent,

Conductive, and Flexible Carbon Nanotube Films and

Their Application in Organic Light-emitting Diodes.

Nano Lett. 2006, 6, 1880-1886.

[24] Yim, J. H.;Kim, Y. S.;Koh, K. H.; Lee, S. Fabrication

of Transparent Single Wall Carbon Nanotube Films

with Low Sheet Resistance. J. Vac. Sci. Technol. B

2008, 26, 851-855.

[25] Tyler, T. P.;Brock, R. E.;Karmel, H. J.;Marks, T. J.;

Hersam, M. C. Electronically Monodisperse

Single-Walled Carbon Nanotube Thin Films as

Transparent Conducting Anodes in Organic

Photovoltaic Devices. Adv. Energy Mater. 2011, 1,

785-791.

[26] Tenent, R. C.;Barnes, T. M.;Bergeson, J. D.;Ferguson,

A. J.;To, B.;Gedvilas, L. M.;Heben, M. J.; Blackburn,

J. L. Ultrasmooth, Large-Area, High-Uniformity,

Conductive Transparent

Single-Walled-Carbon-Nanotube Films for

Photovoltaics Produced by Ultrasonic Spraying. Adv.

Mater. 2009, 21, 3210-3216.

[27] Jung, H.;Yu, J. S.;Lee, H. P.;Kim, J. M.;Park, J. Y.;

Kim, D. A scalable fabrication of highly transparent

and conductive thin films using

fluorosurfactant-assisted single-walled carbon

nanotube dispersions. Carbon 2013, 52, 259-266.

[28] Yang, S. B.;Kong, B. S.; Jung, H. T. Multistep

Deposition of Gold Nanoparticles on Single-Walled

Carbon Nanotubes for High-Performance Transparent

Conducting Films. J. Phys. Chem. C 2012, 116,

25581-25587.

[29] Tkalya, E. E.;Ghislandi, M.;de With, G.; Koning, C.

E. The Use of Surfactants for Dispersing Carbon

Nanotubes and Graphene to Make Conductive

Nanocomposites. Curr. Opin. Colloid Interface Sci.

2012, 17, 225-231.

[30] Kymakis, E.; Amaratunga, G. A. J. Electrical

Properties of Single-Wall Carbon Nanotube-Polymer

Composite Films. J. Appl. Phys. 2006, 99, 084302.

[31] Rahman, R.; Servati, P. Effects of Inter-Tube

Distance and Alignment on Tunnelling Resistance

and Strain Sensitivity of Nanotube/Polymer

Composite Films. Nanotechnology 2012, 23, 055703.

[32] Geng, H.-Z.;Kim, K. K.;So, K. P.;Lee, Y. S.;Chang,

Y.; Lee, Y. H. Effect of Acid Treatment on Carbon


12 Nano Res.

Nanotube-Based Flexible Transparent Conducting

Films. J. Am. Chem. Soc. 2007, 129, 7758-7759.

[33] Jin, R.;Zhou, Z. X.;Mandrus, D.;Ivanov, I. N.;Eres,

G.;Howe, J. Y.;Puretzky, A. A.; Geohegan, D. B. The

Effect of Annealing on the Electrical and Thermal

Transport Properties of Macroscopic Bundles of Long

Multi-Wall Carbon Nanotubes. Physica B 2007, 388,

326-330.

[34] Zhang, Q.;Vichchulada, P.;Shivareddy, S.; Lay, M.

Reducing Electrical Resistance in Single-Walled

Carbon Nanotube Networks: Effect of the Location of

Metal Contacts and Low-Temperature Annealing. J.

Mater. Sci. 2012, 47, 3233-3240.

[35] Guo, H.-L.;Wang, X.-F.;Qian, Q.-Y.;Wang, F.-B.;

Xia, X.-H. A Green Approach to the Synthesis of

Graphene Nanosheets. ACS Nano 2009, 3,

2653-2659.

[36] Dumitrescu, I.;Wilson, N. R.; Macpherson, J. V.

Functionalizing Single-Walled Carbon Nanotube

Networks: Effect on Electrical and Electrochemical

Properties. J. Phys. Chem. C 2007, 111,

12944-12953.

[37] Lobez, J. M.;Han, S.-J.;Afzali, A.; Hannon, J. B.

Surface Selective One-Step Fabrication of Carbon

Nanotube Thin Films with High Density. ACS Nano

2014, 8, 4954-4960.

[38] Nirmalraj, P. N.;Lyons, P. E.;De, S.;Coleman, J. N.;

Boland, J. J. Electrical Connectivity in Single-Walled

Carbon Nanotube Networks. Nano. Letters 2009, 9,

3890-3895.

[39] Hu, C.;Chen, Z.;Shen, A.;Shen, X.;Li, J.; Hu, S.

Water-Soluble Single-Walled Carbon Nanotubes via

Noncovalent Functionalization by a Rigid, Planar and

Conjugated Diazo Dye. Carbon 2006, 44, 428-434.

[40] Lu, X. H.;Yu, M. H.;Wang, G. M.;Tong, Y. X.; Li, Y.

Flexible Solid-State Supercapacitors: Design,

Fabrication and Applications. Energy Environ. Sci.

2014, 7, 2160-2181.

[41] Fei, H.;Yang, C.;Bao, H.; Wang, G. Flexible

All-Solid-State Supercapacitors Based on

Graphene/Carbon Black Nanoparticle Film

Electrodes and Cross-Linked Poly(Vinyl

Alcohol)–H2SO4 Porous Gel Electrolytes. J. Power

Sources 2014, 266, 488-495.

[42] Peng, L.;Peng, X.;Liu, B.;Wu, C.;Xie, Y.; Yu, G.

Ultrathin Two-Dimensional MnO2/Graphene Hybrid

Nanostructures for High-Performance, Flexible

Planar Supercapacitors. Nano Lett. 2013, 13,

2151-2157.

[43] Wu, C.;Lu, X.;Peng, L.;Xu, K.;Peng, X.;Huang, J.;Yu,

G.; Xie, Y. Two-dimensional vanadyl phosphate

ultrathin nanosheets for high energy density and

flexible pseudocapacitors. Nat Commun 2013, 4,

2431.

[44] Liu, F.;Song, S.;Xue, D.; Zhang, H. Folded

Structured Graphene Paper for High Performance

Electrode Materials. Adv. Mater. 2012, 24,

1089-1094.

[45] Nyholm, L.;Nyström, G.;Mihranyan, A.; Strømme, M.

Toward Flexible Polymer and Paper-Based Energy

Storage Devices. Adv. Mater. 2011, 23, 3751-3769.

[46] Li, H.;Zhao, Q.;Wang, W.;Dong, H.;Xu, D.;Zou,

G.;Duan, H.; Yu, D. Novel Planar-Structure

Electrochemical Devices for Highly Flexible

Semitransparent Power Generation/Storage Sources.

Nano Lett. 2013, 13, 1271-1277.

[47] Miller, J. R. Valuing Reversible Energy Storage.

Science 2012, 335, 1312-1313.

[48] Ge, J.;Cheng, G. H.; Chen, L. W. Transparent and

Flexible Electrodes and Supercapacitors using

Polyaniline/Single-Walled Carbon Nanotube

Composite Thin Films. Nanoscale 2011, 3,

3084-3088.

[49] He, S.; Chen, W. High Performance Supercapacitors

Based on Three-Dimensional Ultralight Flexible

Manganese Oxide Nanosheets/Carbon Foam

Composites. J. Power Sources 2014, 262, 391-400.

[50] Cai, W.;Lai, T.;Dai, W.; Ye, J. A Facile Approach to

Fabricate Flexible All-Solid-State Supercapacitors

Based on MnFe2O4/Graphene Hybrids. J. Power

Sources 2014, 255, 170-178.

[51] Shi, C.;Zhao, Q.;Li, H.;Liao, Z.-M.; Yu, D. Low Cost

and Flexible Mesh-Based Supercapacitors for

Promising Large-Area Flexible/Wearable Energy

Storage. Nano Energy 2014, 6, 82-91.

[52] Li, W. Y.;Xu, K. B.;Li, B.;Sun, J. Q.;Jiang, F. R.;Yu,

Z. S.;Zou, R. J.;Chen, Z. G.; Hu, J. Q. MnO2

Nanoflower Arrays with High Rate Capability for

Flexible Supercapacitors. Chemelectrochem 2014, 1,

1003-1008.

[53] Cole, D. P.;Reddy, A. L. M.;Hahm, M. G.;McCotter,

R.;Hart, A. H. C.;Vajtai, R.;Ajayan, P. M.;Karna, S.

P.; Bundy, M. L. Electromechanical Properties of

Polymer Electrolyte-Based Stretchable

Supercapacitors. Adv. Energy Mater. 2014, 4,

1300844.

[54] Seo, J. W. T.;Yoder, N. L.;Shastry, T. A.;Humes, J.

J.;Johns, J. E.;Green, A. A.; Hersam, M. C. Diameter

Refinement of Semiconducting Arc Discharge

Single-Walled Carbon Nanotubes via Density

Gradient Ultracentrifugation. J. Phys. Chem. Lett.

2013, 4, 2805-2810.

[55] Sundramoorthy, A. K.;Mesgari, S.;Wang, J.;Kumar,

R.;Sk, M. A.;Yeap, S. H.;Zhang, Q.;Sze, S. K.;Lim,

K. H.; Chan-Park, M. B. Scalable and Effective

Enrichment of Semiconducting Single-Walled

Carbon Nanotubes by a Dual Selective


13 Nano Res.

Naphthalene-Based Azo Dispersant. J. Am. Chem.

Soc. 2013, 135, 5569-5581.

[56] Li, J.;Huang, Y.;Chen, P.; Chan-Park, M. B. In Situ

Charge-Transfer-Induced Transition from Metallic to

Semiconducting Single-Walled Carbon Nanotubes.

Chem. Mat. 2013, 25, 4464-4470.

[57] Mesgari, S.;Sundramoorthy, A. K.;Loo, L. S.;

Chan-Park, M. B. Gel electrophoresis using a

selective radical for the separation of single-walled

carbon nanotubes. Faraday Discuss. 2014, 173,

351-363.

[58] Zhang, W.; Silva, S. R. P. Raman and FT-IR Studies

on Dye-Assisted Dispersion and Flocculation of

Single Walled Carbon Nanotubes. Spectroc. Acta Pt.

A-Molec. 2010, 77, 175-178.

[59] Shin, H.-J.;Kim, S. M.;Yoon, S.-M.;Benayad, A.;Kim,

K. K.;Kim, S. J.;Park, H. K.;Choi, J.-Y.; Lee, Y. H.

Tailoring Electronic Structures of Carbon Nanotubes

by Solvent with Electron-Donating and -Withdrawing

Groups. J. Am. Chem. Soc. 2008, 130, 2062-2066.

[60] Sa, V.; Kornev, K. G. Analysis of Stability of

Nanotube Dispersions Using Surface Tension

Isotherms. Langmuir 2011, 27, 13451-13460.

[61] Matarredona, O.;Rhoads, H.;Li, Z.;Harwell, J.

H.;Balzano, L.; Resasco, D. E. Dispersion of

Single-Walled Carbon Nanotubes in Aqueous

Solutions of the Anionic Surfactant NaDDBS. J. Phys.

Chem. B 2003, 107, 13357-13367.

[62] Li, F.;Bao, Y.;Chai, J.;Zhang, Q.;Han, D.; Niu, L.

Synthesis and Application of Widely Soluble

Graphene Sheets. Langmuir 2010, 26, 12314-12320.

[63] Frid, P.;Anisimov, S. V.; Popovic, N. Congo Red and

Protein Aggregation in Neurodegenerative Diseases.

Brain Res. Rev. 2007, 53, 135-160.

[64] Mirri, F.;Ma, A. W. K.;Hsu, T. T.;Behabtu,

N.;Eichmann, S. L.;Young, C. C.;Tsentalovich, D. E.;

Pasquali, M. High-Performance Carbon Nanotube

Transparent Conductive Films by Scalable Dip

Coating. ACS Nano 2012, 6, 9737-9744.

[65] Gao, H. J.;Izquierdo, R.; Truong, V. V. Chemical

Vapor Doping of Transparent and Conductive Films

of Carbon Nanotubes. Chem. Phys. Lett. 2012, 546,

109-114.

[66] Dupont, M. F.; Donne, S. W. Nucleation and Growth

of Electrodeposited Manganese Dioxide for

Electrochemical Capacitors. Electrochim. Acta 2014,

120, 219-225.

[67] Le, W.-Z.;Liu, Y.-Q.; Hu, G.-Q. Preparation of

Manganese Dioxide Modified Glassy Carbon

Electrode By A Novel Film Plating/Cyclic

Voltammetry Method For H2o2 Detection. J. Chil.

Chem. Soc. 2009, 54, 366-371.

[68] Aboutalebi, S. H.;Chidembo, A. T.;Salari,

M.;Konstantinov, K.;Wexler, D.;Liu, H. K.; Dou, S.

X. Comparison of GO, GO/MWCNTs Composite and

MWCNTs as Potential Electrode Materials for

Supercapacitors. Energy Environ. Sci. 2011, 4,

1855-1865.

[69] Feng, L.;Xuan, Z.;Zhao, H.;Bai, Y.;Guo, J.;Su, C.-w.;

Chen, X. MnO2 prepared by hydrothermal method

and electrochemical performance as anode for

lithium-ion battery. Nanoscale Res Lett 2014, 9, 1-8.

[70] Xu, C.;Li, B.;Du, H.;Kang, F.; Zeng, Y.

Electrochemical Properties of Nanosized Hydrous

Manganese Dioxide Synthesized by a Self-Reacting

Microemulsion Method. J. Power Sources 2008, 180,

664-670.


Nano Res.

Figures

Figure 1 (a) Chemical structure of CR, (b) scheme for preparing SWNT-based TCFs using CR-SWNT dispersion on rigid glass and

flexible PET substrates, and (c) CR-SWNT TCF preparation by drop coating.


Nano Res.

Figure 2. (a) CR solution (a, a'), and dispersions of CR-SWNT (b, b'), SDS-SWNT (c, c'), and Triton-SWNT (d, d'). (b) UV-Vis-NIR

spectra of (a) CR solution and dispersions of (b) CR-SWNT and (c) SC-SWNT.


Nano Res.

Figure 3 (a) Raman spectra of (i) CR powder (control), (ii) CR-SWNT and (iii) pristine-SWNT samples (using 633 nm excitation

laser). (b) Transmittance (measured after baseline subtraction with bare glass) of CR-SWNT-film-coated glass substrate.


Nano Res.

Figure 4 (a) FT-IR spectra of CR powder (control), pristine P2-SWNT and CR-SWNT film. (b) Enlarged view of the curves in (a)

from 600 to 2000 cm-1.


Nano Res.

Figure 5 FE-SEM (a, b) images of as-prepared CR-SWNT films on glass substrate and TEM images of (c) CR-SWNT and (d)

SC-SWNT films on TEM copper grid.


Nano Res.

Figure 6 Hydrodynamic radius (size) measurement of CR-SWNT, SC-SWNT, and SDS-SWNT dispersions.


Nano Res.

Figure 7 (a) CR-SWNT-film-coated PET substrate. (b) Transmittance measured after baseline subtraction with bare PET of

CR-SWNT-PET before and after washing in nitric acid solution. (c) AFM image of CR-SWNT-PET (with height measurement curve

superimposed). (d) Sheet resistivity of acid-treated CR-SWNT-PET as a function of number of bending cycles (bending radius = 8

mm).


Nano Res.

Figure 8 CR-SWNT-PET electrode (a) before and (b) after MnO2 deposition and (c) SEM image of deposited MnO2 (b). (d)

Electrochemical impedance spectroscopy of CR-SWNT-PET and MnO2-CR-SWNT-PET in 0.1 M KCl solution containing 5 mM

[Fe(CN)6]3−/4−. Applied potential was 0.2 V vs. Ag/AgCl with frequency range from 0.01 to 100,000 Hz and amplitude of 5 mV.


Nano Res.

Figure 9 (a) Electrochemical deposition of MnO2 onto a CR-SWNT-PET film from a solution containing 50 mM manganese acetate

and 100 mM Na2SO4 at 50 mV/s scan rate. (b) Comparative cyclic voltammograms for Nafion/MnO2-CR-SWNT-PET and

CR-SWNT-PET in 100 mM Na2SO4 solution at 10 mV/s. (c) Effect of scan rate for Nafion/MnO2-CR-SWNT-PET film in 100 mM

Na2SO4 solution at (from inner to outer) 10, 20, 50, 100, 150, 200, and 250 mV/s. (d) Variation of specific capacitance with scan rate

for Nafion/MnO2-CR-SWNT-PET electrode.


Nano Res.

Figure 10 (a) Comparison of cyclic voltammograms of MnO2-CR-SWNT-PET electrode before (blue curve) and after (red curve) 100

bending cycles (bending radius= 8 mm), Scan rate = 10 mV/s using 100 mM Na2SO4 as electrolyte. (b) Galvanostatic

charge/discharge curves of Nafion/MnO2-CR-SWNT-PET electrode at (a) 700, (b) 350, (c) 210, and (d) 140 mA·g-1. (c) Galvanostatic

charge–discharge curves (10 cycles) for Nafion/MnO2-CR-SWNT-PET electrode at a current density of 140 mA.g-1. (d) Cycle life of

Nafion/MnO2-CR-SWNT-PET electrode in 100 mM Na2SO4 solution.

Low-temperature solution process for preparing flexible ... · carbon nanotube, Congo red, flexible electronics, supercapacitors ABSTRACT Single-walled carbon nanotubes (SWNT) are

Documents