In recent years single-walled carbon nanotubes (SWNTs),in the form of freestanding films called bucky papers (BPs) oras micrometer-thick films deposited on a substrate, have found
Enhanced Field Emission and Improved Supercapacitor Obtained from Plasma-Modified Bucky Paper
Soumyendu Roy , Reeti Bajpai , Navneet Soin , Preeti Bajpai , Kiran S. Hazra ,
Neha Kulshrestha , Susanta Sinha Roy , James A. McLaughlin , and D. S. Misra*
several applications[ 1 , 2 ] such as lithium-ion batteries, fuel cells,hydrogen storage, chemical sensors,[ 3 ] actuators,[ 4 ] thermal heatsinks,[ 5 ] and lightweight, robust, and flexible fibers.[ 6 ] Herein, weconcentrate on two applications of BP, namely supercapacitorelectrodes[ 2 , 7 , 8 ] and field emitters.[ 9 , 10 ] Although macroscopicin nature these films retain most of the properties of the 1Dnanotubes. They are easy to handle and because of statistical
averaging over an ensemble of a large number of tubes thesefilms have reproducible and controllable characteristics, unlikedevices based on a single nanotube. Electrochemical capaci-tors with extremely high values of capacitance (several thou-sand farads) are popularly called supercapacitors. They haveseveral desirable characteristics, such as high power density(much higher than batteries and fuel cells), portability, and alife extending over several cycles of charging–discharging. Theyare used alongside batteries in applications like power backup,mobile devices, pacemakers, air bags, electrical vehicles, etc. Fur-ther developments in supercapacitors are aimed at improvingthe energy density and hence the specific capacitance. Carbon-nanotube (CNT) films have a large accessible surface area,DOI: 10.1002/smll.201002330
S. Roy, R. Bajpai, K. S. Hazra, N. Kulshrestha, Prof. D. S. Misra
Nanotechnology and Integrated Bioengineering Centre
University of Ulster at Jordanstown
Newtownabbey BT37 0QB, Northern Ireland, UK
P. Bajpai
Department of Metallurgical Engineering and Materials Science
Indian Institute of Technology Bombay
Mumbai 400 076, India
T he surface morphology of bucky papers (BPs) made from single-walled carbonnanotubes (CNTs) is modified by plasma treatment resulting in the formation of vertical microstructures on the surface. The shapes of these structures are either
pillarlike or conelike depending on whether the gas used during plasma treatment is Ar or CH 4 . A complex interplay between different factors, such as the electric fieldwithin the plasma sheath, polarization of the CNT, intertubular cohesive forces,
and ion bombardment, result in the formation of these structures. The roles playedby these factors are quantitatively and qualitatively analyzed. The final material is
flexible, substrate-free, composite-free, made only of CNTs, and has discrete verticallyaligned structures on its surface. It shows enhanced field emission and electrochemical charge-storage capabilities. The field enhancement factor is increased by 6.8 times, andthe turn-on field drops by 3.5 times from an initial value of 0.35 to 0.1 V μ m− 1 as aresult of the treatment. The increase in Brunauer–Emmett–Teller surface area resultsin about a fourfold improvement in the specific capacitance of the BP electrodes.Capacitance values before and after the treatments are 75 and 290 F g− 1 , respectively.
It is predicted that this controlled surface modification technique could be put to good
use in several applications based on macroscopic CNT films.
Bucky Paper
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the highest conductivity per unit mass, and electrochemicalinertness. These properties make them an attractive electrodematerial for supercapacitors. Electrodes based on CNTs orcomposites of CNTs with polyacrylonitrile and transition-metaloxides such as Ru2 O3 , In2 O3 , and MnO2 have been demon-strated successfully.[ 11–13 ] There have been numerous studiesexploring the field-emission (FE) properties of CNT films.
CNTs have several advantages over other materials, includinghigh aspect ratio, high electron mobility and conductivity, andnear-ballistic transport resulting in low operational tempera-tures. They are also resistant to chemical poisoning and physicalsputtering due to the bombardment of positive ions during FE.BP is particularly well suited for forming flexible devices ascompared to other complicated techniques.[ 14 ]
Plasma is a unique medium for enabling reactions andmodifications of nanostructures. It has been used to pro-duce Spindt-type conical CNT microstructures for FE, syn-thesis, surface functionalization, doping, and purification of CNTs.[ 15 , 16 ] In this article we demonstrate that plasma treat-ment can convert a BP surface consisting of densely packed
horizontal SWNTs into an array of vertical microstructureshaving shapes that resemble cones and pillars. Vertical arraysof CNTs can be easily grown by chemical vapor deposition(CVD) on lithographically patterned catalyst. However, fac-tors like the presence of metal catalysts, weak adherence of CNTs to the substrate, and presence of the substrate itself canlead to undesirable effects in several applications. The plasma-modified BP presented in this study is a freestanding, all-CNTmaterial in which the tubes on the surface are vertically alignedand formed into discrete shapes. Additionally the plasmaprocessing technique is decoupled from the synthesis processand can be implemented easily and cost effectively (because of the absence of lithography). We used mixtures of H2 + Ar and
H2 + CH4 gases for plasma treatment. BP retains its flexibilityafter the modification and can be easily integrated with anydevice for a multitude of applications. We demonstrate the use-fulness of this technique by showing that the FE property of BP and the charge-storage capability of supercapacitors madefrom BP electrodes improve significantly after plasma modifi-cation. The field enhancement factor (β ) increases by 6.8 timeswhile the turn-on field (V T ) decreases by 3.5 times after treat-ment. The lowest V T obtained was ≈ 0.1 V μ m− 1 . The specificcapacitance of BP electrodes could be enhanced by 3.9 timesto 290 F g− 1 . We show that by controlling the characteristicsof the plasma one can control the shape of the microstruc-tures to a certain extent. We also provide a rough quantitative
analysis of the different factors involved in the modificationprocess, such as the plasma sheath potential, dipole model of CNTs, intertubular cohesive energies, and ion bombardment.A complex interplay between these different factors results inthe formation of the microstructures on the BP surface leadingto the subsequent enhancement in properties.
2. Results and Discussion
Figure 1 shows electron microscopy and optical imagesof as-synthesized SWNTs and BP. The images in Figure 2
compare the surface of BP before and after plasma treatment.
It is evident that during plasma modification, layers of CNTmaterial first stood up in the form of walls and then thosewalls were etched by the plasma to produce vertical pillarsor cones on the BP surface. There are a few conspicuous dif-ferences between the etching effects of Ar and CH4 plasmas.Firstly, Ar plasma etches much more rapidly than CH4 plasma. Secondly, under the same conditions of temperatureand pressure the Ar plasma ball appears to be larger in sizeto the naked eye than the CH4 plasma ball. The Ar plasma
ball partially engulfs the BP and the substrate holder whilethe CH4 ball hovers on top of the BP (see insets in Figure 2eand g). This could enable the Ar plasma to etch both the topand the bottom of the vertical walls formed on the BP at asimilar rate, thus resulting in pillar-type microstructures withblunt tops. The CH4 plasma appears to be concentrated nearthe top of the walls, and etches the top more rapidly than thebottom resulting in cone-type structures with sharper tips.Also, after 4 h of treatment with CH4 plasma CNT bundlesappear to protrude from the otherwise solid-looking cones.The significance of these observations lies in the fact that theshape and size of the microstructures formed on the BP maybe controlled by varying the properties of the plasma pro-
ducing it. Further optimization is required in this direction.In an earlier study,[ 17 ] characteristics of the plasma
formed in our microwave plasma chemical vapor deposition(MPCVD) system had been investigated. At a pressure of 70 Torr (1 Torr = 133.322 Pa) and a temperature of 925 ° Cthe mixture of hydrogen and methane gases (with 99.2%H2 ) yielded the following temperature and density of elec-trons: T eV ≈ 10 eV and n ≈ 1018 m− 3 . A plasma sheath havingan extent equal to the Debye length of electrons (λ )[ 18 ] isformed surrounding the BP resulting in an electric field per-pendicular to the BP surface. The sheath potential or self-potential of plasma[ 18 ] is T eV ln
√ mi/4B me
= 38.8 V, where
m i and m e are the masses of the methane ions and electrons,
Figure 1. a,b) Transmission electron microscopy (TEM) images of
as-synthesized SWNT material; (b) is taken at higher magnification.
c) Scanning electron microscopy (SEM) images showing bundles
of SWNTs disentangled from the BP matrix and emerging out from a
damaged portion. d) Optical images demonstrating the flexibility of BP.It can be folded and unfolded without causing any visible damage.
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respectively. The Debye length is given by 8 = 7.4 × 103
T eVn
meters = 20 μ m. Hence the electric field existing due to theplasma is ≈ 2 V μ m− 1 . CNTs will be polarized when subjectedto this external electric field and behave as electric dipoles[ 19–22 ] with induced moments that are directly proportional to thefield (E ). The decrease in potential energy per unit length (PE)
as the tube orientation changes from horizontal to vertical isgiven by 12α E
2 ,[ 23 ] where α is the component of polarizabilityparallel to the tube axis. The component perpendicular to thetube axis is much smaller and has been neglected here. [ 22 ]
Owing to strong van der Waals forces of attraction,SWNTs have a tendency to agglomerate or coalesce andform hexagonally packed bundles called “ropes”. The SWNTswithin a bundle are strongly bound to one another with cohe-sive energies that depend on their diameters.[ 24 ] In some recentexperiments it was reported that SWNTs can form very densesolid materials under the influence of the surface tension of liq-uids.[ 25 ] Using UV–visible absorption spectroscopy (Figure 3)and a Kataura plot,[ 26 ] we found that the majority of SWNTs
Figure 2. Plasma modification of BP. a) High-magnification SEM image of the surface of as-synthesized BP. b–d) Progressive evolution of the BP
surface with CH4 + H2 plasma treatment. b) After 1 h, vertical wall-like features start to appear. Slowly these convert into cones. c) After 2 h. d) After 4 h.
e) Magnified image of (d). The inset shows schematically the relative position of the CH4 plasma ball (circle) with respect to the BP (rectangle).
f) After 1 h of Ar + H2 plasma treatment. g) After 2 h. The inset shows the Ar plasma ball engulfing the BP. The difference in the position of the plasmaball probably gives rise to the difference in the shape of the microstructures from conelike to pillarlike. h) Raman spectra before and after plasma
treatments. The spectra have been normalized with respect to the G bands. The strong radial breathing modes and weak D bands [ 32 ] suggest
that the crystallinity of the SWNTs remains intact after the treatments. i) X-ray photoelectron spectroscopy (XPS) shows the absence of doping or
functionalization. The hump at 286 eV is indicative of OH groups present on oxidized SWNTs[ 31 ] (which is generally a side effect of acid purification)
and it disappears after the plasma treatments.
Figure 3. Optical absorption spectrum in the visible and near-infrared
region of purified SWNT material. The large peak at about 1930 nm
corresponds to S11 electronic transitions of semiconducting SWNTs. The
possible diameter of these tubes (1.35 nm) was estimated by using a
simple Kataura plot.[ 26 ]
400 800 1200 1600 2000
0.0
0.2
0.4
0.6
0.8
A b s o r b a n c e
wavelength (nm)
1930
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in our sample had a diameter around1.35 nm for which the cohesive energy is≈ 0.4 eV Å− 1 .[ 24 , 27 ] If this cohesive energyis less than the PE, then it is energeticallyfavorable for the tubes to become vertical.Approximating the CNT as a metallic cyl-inder with length (l ) > > radius (R ), simple
classical electrostatic theory gives[
21
,
22
]
α = 2/ [24(ln (l l /R)−1) . For tubes witha diameter of 1.35 nm and a length of 1 μ m, PE turns out to be 0.001 eV Å− 1 ,which is two orders of magnitude lessthan the cohesive energy. For tubes on thelonger side with l = 5 μ m, PE = 0.02 eV Å− 1 ,which is still one order of magnitudeless. For semiconducting SWNTs (whichmost of the tubes in our samples are), thevalue of PE will be even less,[ 21 ] about1000 times smaller than that of metallictubes of the same size.[ 23 ] However, the
temperature of the plasma is very high(T ≈ 900 ° C) so that kT ≈ 0.1 eV (wherek is the Boltzmann constant), ≈ 33% of thebandgap[ 23 ] of tubes with a diameter of 1.35 nm. Hence at such an elevated tem-perature there may be lots of free elec-trons and a semiconductor can be treatedas a metal.[ 20 ] However, from the abovecalculations it is clear that whether thetubes are metallic or semiconducting, of moderate or signifi-cant length, the electric field arising due to the plasma is notstrong enough to overcome the intertubular attractive forcesand align the tubes vertically.
Further evidence of this fact can be drawn from theFE experiments where a field of ≈ 1 V μ m− 1 exists; how-ever, no alignment or modification on the BP surface wasobserved. This implies that the ion and electron bombard-ment taking place on the BP surface due to the plasma notonly etches away the BP and CNTs but also plays a criticalrole in alignment. These collisions are energetic enough totransfer sufficient momentum to the CNTs to weaken thecohesive forces. The CNTs can then under the influence of the sheath potential align themselves vertically. Anotherinteresting observation in these experiments is the factthat it is not the individual tubes that become aligned butlayers of CNT material. Lennard–Jones potential[ 24 ] is gen-
erally used to describe the van der Waals force betweentwo molecules or atoms. It drops off extremely rapidly asthe separation increases. BP is formed by deposition of layers or chunks of SWNT material on top of one another.We believe that within BP there will be regions or domainswhere SWNTs will exist as strongly bound ropes with inter-tube separations that will be close to the ideal equilibriumvalues. However, the distance between two such domainswill be much larger, thus making these the structural weakpoints much like grain boundaries in polycrystalline solids.It is from these points that the CNT layers are pulled upfrom the matrix of BP by the plasma and then etched toform pillars and cones.
Improvement in the FE characteristics with plasmatreatment is obvious from Figure 4. Fowler–Nordheim(FN) theory[ 28 ] was employed to extract the fieldenhancement factor (β ) from the data. Briefly it states that
J ∝$ 2E2
n A exp− B
n 3/2
$ E
, where J is current density, E is appliedelectric field, A is the area of the emitter, B is a constant withvalue 6.83 × 109 V eV− 3/2 m− 1 , and ϕ is the work function of theCNT (5 eV). β is basically the ratio of the local field at the sur-face of the emitter to the macroscopic applied field. Anotherparameter used to compare the performance of differentemitters is turn-on field (V T ), defined as the macroscopicfield required to produce a current density of 10 μ A cm− 2 .Changes in the values of β (at low electric field regime) andV T with plasma treatment are depicted in Figure 4c. Thesevalues compare well with other reported experiments.[ 10 , 29 , 30 ] Four hours of CH4 plasma treatment increased β from 3568to 24 250 and brought down V T from 0.35 V μ m− 1 to as low
as 0.1 V μ m− 1 . This turn-on field is among the lowest reportedvalues in the literature. It is worthwhile noting that in theseexperiments we have avoided extensive functionalization,doping, and defect creation, as is evident from XPS [ 31 ] andRaman spectra[ 32 ] (Figure 2h,i). Hence the improvement isentirely due to the modification of the BP surface geometry.The protruding conical shape with possibly just a few nano-tubes at its tip will have very high local fields and reducedelectrostatic screening.
The difference in the microstructures formed on the BPsurface after treatment with CH4 and Ar plasma is reflectedin the different degrees of improvement in the FE parameters.Figure 5 shows the results of cyclic voltammetry (CV) and
Figure 4. FE properties of BP. a) Plot of current density versus applied field for as-produced
BP and BP treated with CH4 and Ar plasmas for different time intervals. b) Corresponding FN
plots. c) Variation of the two important device parameters of the BP field emitters with plasma
treatment. d) Time stability measurements of the emitters. After plasma treatment the currents
get slightly noisier. The values of standard deviations ( σ , in μ A) in the recorded currents are
shown. There is no significant loss in current after 5 h of operation.
0.2 0.4 0.6 0 . 0
0 0 0
0 . 0
0 0 4
0 . 0
0 0 8
0 . 0
0 1 2
J ( A / c m
2 )
C H
4 -
4 h
r
E (V/um)
B P
A r - 1 h
r
C H
4 - 2
h r
A r - 2 h r(a)
2 4 6 8 10 12
-15
-12
-9
-6
-3
B P
A r - 1 h r
C H 4 - 2
h r
A r - 2 h r
CH4-4hr
l n ( J / E 2 )
1/E (um/V)
(b)
0
8 0 0 0
1 6 0 0 0
2 4 0 0 0
f i e l d e n h a n c e m e n t f a c t o r
BP Ar-1hr CH4-2hr Ar-2hr CH
4-4hr
(c)
0 . 0
8
0 . 1
6
0 . 2
4
0 . 3
2
field enhancement factor
turn on field
t u r n o n f i e l d ( V / u m )
0 1 2 3 4 5
15
20
25
30
35
40
c u r r e n t ( μ A )
time (hr)
BP (σ = 0.90)
Ar-2hr (σ = 1.80)
CH4-4hr (σ = 1.65)(d)
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galvanostatic charging–discharging (CD) experiments. Spe-cific capacitance (C sp ) has been evaluated from both the CVand CD curves using the relation:[ 33 ] C sp = I
dV /dt
1mA+ 1
mB
,
where I is the current, dV /dt is the scan rate in the case of CV
and slope of the curves in CD, and m A and m B are the massesof the two BP electrodes. The energy density (U ) of the deviceis obtained by the formula U = 1
2CV
2 with C being the ratioof the total capacitance to the total mass of the electrodesand V the working voltage. Since the electrochemical doublelayer capacitors formed at the two electrodes are in a seriescombination, C = C sp /4.[ 13 ] The maximum power density (P )of the supercapacitor is calculated using P = V 2 /(4RM ),[ 7 , 12 , 13 ] where M is the total mass of BP electrodes and R is theequivalent series resistance obtained from the ohmic IRdrops in the CD curves. The results are presented in Table 1.The capacitor made with BP electrodes had a C sp of 75 F g− 1 ,U of 8.5 W h kg− 1 , and P of 2 kW kg− 1 . These results are
in good agreement with previously published values.[ 8 , 11 , 12 ] After treatment with Ar plasma for 2 h the values of C sp andP are increased to 290 F g− 1 and 7 kW kg− 1 , respectively.
In these experiments the electrical contacts were formedby simply attaching BP to crocodile clips with flattened jaws.This resulted in high values of R (a few hundred ohms). Thereis significant scope for optimizing the contact resistance andimproving P . For a detailed description of the current state-of-the-art carbon-based supercapacitors, readers are advised tocheck the review articles.[ 34 , 35 ] There have been some reportsabout increasing the capacitance of CNT-based electrodesby plasma treatment, but these have concentrated on func-tionalization by oxygen-containing groups.[ 36 , 37 ] Contrary to
these experiments the improvement in our case can be attrib-uted to purely geometrical factors. Formation of the vertical
microstructures on the BP surface increases the electrode areaavailable to ionic species within the electrolyte. The Brunauer–Emmett–Teller (BET) surface area of BP before and afterAr plasma modification, as measured by the N2 adsorption–
desorption technique, was found to be 328 and 969 m2 g− 1 ,respectively. The increase in area is commensurate with theincrease in capacitance. The reported values of the specific sur-face area of CNTs are generally in the range 120–500 m2 g− 1 ,[ 34 ] with some exceptions.[ 25 ] It is remarkable that our methodcan enhance the specific capacitance of CNT electrodes by afactor comparable to that observed in experiments involvingaddition of faradaic pseudocapacitive materials to CNTs.[ 11 , 13 ] Finally, we would like to stress that it is not the exact values of the parameters, but rather the changes in them produced asa result of the modification of surface morphology by plasmathat are more fundamental to this study. In principle one couldstart with SWNT material having better crystallinity or purity,
or functionalize the nanotubes, or form composites of it withother materials and produce better device performance byapplying the plasma modification technique.
3. Conclusion
We have demonstrated that plasma-assisted modificationcould be a viable route towards improving the applicabilityof SWNT papers. Freestanding vertical CNT microstructurescould be formed on the BP surface. The shape of these struc-tures depended on the characteristics of the plasma treatmentperformed. We have analyzed the different phenomena that
had a role in the formation of these structures. It was seen thatthe self-potential of plasma is not strong enough to overcomethe cohesive forces between SWNTs and the extra energyhas to come from ion bombardment. Plasma modificationimproved the FE properties and increased the specific capaci-tance of BP electrodes by modifying the surface geometry andmicrostructure. The turn-on field could be reduced by 3.5 times,the BET surface area increased by almost three times, and spe-cific capacitance increased by about four times after plasmamodification. In principle several other applications based onCNT films, for example hydrophobic and self-cleaning con-ducting surfaces, electrochemical sensors, fuel cells, and Li-ionbatteries could also benefit immensely from this technique.
Figure 5. Characteristics of the supercapacitors fabricated with BP electrodes. a) CV curves at a scan rate of 200 mV s − 1 obtained with BP and
Ar-plasma-treated BP. Currents have been normalized by the masses of the electrodes for better comparison. Vertical widths of the boxes are
proportional to the capacitances. b) CD curves obtained with 1 mA current. The ohmic potential drops are shown as IR. c) Cyclic stability of the
capacitors measured at 200 mV s− 1 . There is a minor drop in capacitance of about 3 to 4% after 1000 cycles.
-0.4 0.0 0.4 0.8
-0.04
-0.02
0.00
0.02
0.04
BP
A r - 2 h r
s
p e c i f i c c u r r e n t ( A / m g )
voltage (V)
(a)
100 200 300
0.0
0.5
1.0
1.5
2.0
2.5
IR
v o l t a g e ( V )
time (s)
IR
BP
(b)
20 40 60 80
0.0
0.5
1.0
1.5
2.0Ar-2hr
0 200 400 600 800 1000215
220
225
230
235 Ar-2hr
BP
cycle number
S p
e c i f i c c a p a c i t a n c e ( F / g )
60.0
61.5
63.0
64.5
66.0
S
p e c i f i c c a p a c i t a n c e ( F / g )(c)
Table 1. The main device parameters associated with the super-capacitors formed from BP and Ar-plasma-treated BP.
Parameter a) C sp [F g− 1 ] U [W h kg− 1 ] P [kW kg− 1 ]
BP 75 and 66b) 8.5 2
Ar-2 h 290 and 230b) 17 7
a) C sp , U , and P are the specific capacitance, energy density, and maximum power density of
the supercapacitors; b) These two values are obtained from cyclic voltammetry while the rest
are obtained from charging–discharging experiments.
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