Unidirectionally aligned diphenylalanine nanotube/ microtube arrays with excellent supercapacitive performance
Jinlei Zhang, Xinglong Wu (), Zhixing Gan, Xiaobin Zhu, and Yamin Jin National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
inorganic ones such as ordered carbon NTs from the
perspective of biocompatibility [11, 12], piezoelectricity
and hydrophilicity. In order to fully realize their
potential, a new self-assembly requiring only mild
conditions must be developed to obtain unidirectionally
aligned NT/MT arrays [2, 6, 12, 16].
Previous investigations have revealed opposite
charges on the two ends of the NTs, and large MTs are
formed by connecting the NTs together by the dipole
interaction between NTs/MTs [21–23]. This suggests
that aligned micro-forests can be produced in solution
at room temperature by introducing an external
electric field. In this work, the fabrication process is
demonstrated and the resulting FF MT micro-forests
have a supercapacitance of 1,000 μF·cm–2. This is
much higher than the values reported previously in
peptide NT (PNT) arrays [12] and is close to the value
predicted by a theoretical calculation according to the
microstructure of PNT [24–26].
2 Experimental
2.1 Materials
The lyophilized form of FF was purchased from
Bechem Company. The experiments were carried out
in a sealed chamber with the relative humidity (RH)
controlled precisely to 1 and the temperature to 22 °C.
The fresh FF solution was prepared by dissolving FF
monomers in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP,
Aladdin) at a concentration of 100 mg·mL–1. To avoid
aggregation, the FF solution was prepared freshly
before the experiments.
2.2 Apparatus
Scanning electron microscopy (SEM) images: The
samples were observed by a 5350 NE Dawson Creek
Derive SEM. The images were taken from the top view
or side view at specific stages. Atomic force microscopy
(AFM) and electric force microscopy (EFM) images:
AFM and EFM were conducted at the same time
and location on a NanoScope IV NS4-1 instrument.
Electrochemical measurements: The tests were
conducted in a three-electrode system comprising a
working electrode made of the substrate, platinum
wire counter electrode, and reference electrode made
of a silver/silver chloride electrode. The electrolyte was
0.05 M KH2PO4 and 0.5 M KCl. The capacitance is
derived from the cyclic voltammetry results and the
working electrode area was 0.125 cm2. The capacitance
was estimated from the experimental graph of the
charge–discharge process using the simple expression
C = I/(dV/dt) (where I is the applied constant current
density in μA·cm–2).
2.3 Preparation of vertically aligned FF MTs under
the influence of an external electric field
A cleaned fluorine-doped tin oxide coated glass (FTO,
Nanjing Chemical Reagents) substrate fixed between
two metal plates with an interval of no more than
3.5 mm connected to a direct current power supply
outside the chamber through conducting wires was
put horizontally in a hermetical chamber (Fig. 1(a)).
100 μL of the FF solution was dropped onto the surface
of the FTO after the direct current power supply
stabilized at +150 V.
3 Results and discussion
The FF monomers were dissolved in HFP and placed
on an FTO substrate. The experiments were conducted
in a cylindrical chamber in which the water activity
Figure 1 (a) Experimental setup to fabricate the unidirectionally aligned NT/MT array. (b) Schematic illustrating the FF self- assembling process in the presence of an upward external electric field of 150 V. (c) Structural schematic of the unidirectionally grown FF NT/MT array. (d) Schematic of the FF self-assembled process with a downward external electric field of 150 V.
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931 Nano Res. 2014, 7(6): 929–937
was precisely controlled. An external electric field
of 150 V was applied by putting the FTO substrate
horizontally between two metal plates connected to a
direct current power supply. The initial orientation of
the external field was vertically upward (Fig. 1(a)). The
temperature and relative humidity were maintained
at 22 °C and 1, respectively. The top electrode had a
small hole 0.5 mm in diameter through which the HFP
solution was dropped onto the FTO substrate via a
medical needle. After rapid evaporation of the HFP,
the FTO surface was covered with a thin layer. The SEM
image of the top view in Fig. 2(a) shows micro-forests
unidirectionally aligned perpendicular to the surface
covering the entire surface. The enlarged image
(Fig. 2(b)) reveals that almost all the tubes have open
ends and the tube walls are composed of multi-layers
with a total thickness up to 1.5 μm. They are thicker
than other MTs produced under similar same solutions
but without an external field (left side of Fig. 2(c)).
Furthermore, the spaces among the tubes are filled by
thinner NTs (the red circle in Fig. 2(b)).
Figure 2 (a) SEM image (top view) of the unidirectionally aligned NT/MT array. (b) Locally enlarged SEM image. The red circle shows some thinner NTs. The inset shows the top SEM image of an enlarged MT with some small NTs connecting to the top end of the large MT. (c) Morphology of the FF NTs/MTs formed on a FTO substrate coated with acidic 3C-SiC NPs. (d) Comparison between the surface morphologies of the MTs formed on a FTO substrate with acidic 3C-SiC NPs (right side) and without 3C-SiC NPs (left side). All the scale bars are 50 m.
FF molecules in the HFP solution self-assemble only
in the presence of water [27, 28]. As a result, some of
the FF monomers nucleate on the substrate on the
bottom of the FF/HFP solution to form small NTs. The
NTs/MTs grown from the substrate (positive charge)
grow upwards in the presence of an external field, with
the negatively charged ends contacting the substrate
thus having a downward polar electric field (Fig. 1(b)).
At the same time, many small NTs are formed at the
liquid–air interface due to the large supply of water
molecules [24–29]. These small NTs also have a down-
ward polarization field under the external electric field
[21–23] and serve as the building blocks for further
self-assembly. They sink by virtue of the charge inter-
action finally growing into large MTs. Since the MTs
grown from the substrate and air interface have the
same polarization direction, they will connect vertically
(inset of Fig. 2(b)). The X-ray diffraction results indicate
that the MTs have a single crystalline structure with
multiple channels in the tube walls (Fig. S1 in the
Electronic Supplementary Material (ESM)). Due to the
uniform positive charge distribution on the substrate
surface, the NTs nucleate with high density, resulting
in the formation of large and dense MTs [30, 31]. The
scattered MT array in Fig. 2(c) can thus be attributed to
an insufficiently compact positive charge distribution
on the substrate. Hence, we can control the growth
behavior of the MT nanoforests via the surface charge
distribution as shown in Fig. 2(d).
Since partial NTs on the substrate can integrate with
other smaller NTs in the initial stage, they grow faster
and have bigger diameters. The NTs formed later
have smaller diameters and only stand straight in the
compressed space due to strong electrostatic repulsion
from neighboring MTs (Fig. 2(b)) (the cylindrical
surface of MT is positively charged [32, 33]). As a
result, the MTs will arrange along the direction of the
external electric field with the help of electrostatic
repulsion of the cylindrical tube surfaces finally for-
ming a densely unidirectional FF MT array (Figs. 1(c)
and 2(a)). Thus the presence of a high density of
nucleation sites is responsible for the observed
unidirectional dense FF MT array. Our experiments
show that the ratio of ambient RH to the FF
concentration (RH:FF ratio) plays an important role
in the unidirectional growth of the FF micro-forests.
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932 Nano Res. 2014, 7(6): 929–937
In the presence of the upward electric field at RH = 1
and volume of HFP = 100 μL, self-organization of
the well-aligned micro-forests occurs only when the
concentration of the FF monomers is between 80 and
140 mg/mL (Figs. S2 and S3 in the ESM). At lower
concentrations, clusters of micro-forests are found
randomly on the surface and they do not connect to
form a NT/MT array because of a lack of subsequent
nuclei. At higher concentrations, only an amorphous
FF film is formed due to the lack of water molecules
[34–36]. When the electric field direction coincides with
that of gravity, FF micro-forests cannot be observed.
Here, the substrate surface is negatively charged. Since
the cylindrical surfaces of the MTs are positively
charged, the NTs formed initially are attracted to the
substrate forming oblique MTs (Fig. 1(d)). As a result,
the MTs are stacked and the morphology of the MTs
is similar to that formed without an electric field
(left side of Fig. 2(d)). In this case, the FF monomers
are easily converted into an amorphous film (Figs. S4
and S5 in the ESM). Here we would like to stress that
these MTs/NTs are rather stable and no spontaneous
transformation from MTs/NTs to vesicles occurs
because a relatively high FF concentration is used
during their growth [28] and the electrostatic repulsion
is not a main factor for every MT/NT formation [37].
In addition, since MT/NT formation depends upon
many factors, different experimental conditions can
lead to different lengths. In our experiments, the
MT/NT lengths are near 140 μm (Fig. 4(b)) under a
+150 V applied electric field and those triggered by
acidic 3C-SiC nanoparticles are near 220 μm (Fig. S3
in the ESM).
EFM was conducted to evaluate the surface charge
on the MT array in order to elucidate the mechanism
of formation. EFM is a dual scanning technique in
which a topographical profile can be obtained from
the first line scan using the tapping mode. The phase,
frequency, and amplitude associated with resonant
oscillation of the tip can be obtained from the second
scan. The tip oscillates during the second pass at a
fixed distance above the sample surface guided by
the topography data acquired from the first one. The
changes on the sample surface, such as the fixed
charges and dielectric constant of the materials, can
be derived from the recorded information in Fig. 3(a).
Here, the EFM data measured during the retraced scan
is recorded as ΔΦ. If a voltage is applied between the
tip and sample in the presence of an electrostatic force
F, the phase shift of the tip cantilever is given by Eq. (1)
[38, 39], where z is the tip-to-surface distance (fixed at
60 nm), Q is the quality factor, and k is the spring
constant of the cantilever. To relate to the charge
density, we need to incorporate the gradient of the
force F. Here, van der Waals forces are neglected and
so F is totally electrostatic. To calculate the tip–surface
force, we treat the tip and surface as point charges and
assume the charges to be limited only on the tube
surface. Thus, the tip–electrode force caused by the
capacitor can be written as Eq. (2) [40, 41], where C is
the capacitance of the tip–sample system, ε0 is the
permittivity of vacuum, and Qs is the surface charge on
the sample. The tip–sample system acts as a parallel
plate capacitor (the tip is treated as a circular disk of
radius R) and is used to identify the sign of the surface
charge. Here, C is calculated by Eq. (3), where h is the
thickness of the sample and εP is the permittivity of
the sample. The magnitude of the surface charge is
given by Eq. (4) [42, 43]. Accordingly, the sign of surface
charges can be deduced from the EFM data. Eq. (4)
is quadratic with Vtip, and tan(ΔΦ) versus Vtip turns
out to be a parabolic concave downwards curve with
a negative y-intercept. The parabola should have a
vertex with coordinate x at Eq. (5), with its sign only
depending on the sign of Qs. As a result, by plotting
tan (ΔΦ) versus Vtip and noting the Vtip coordinate of
the vertex, Qs can be confirmed. When the surface of
the sample is uncharged (Qs = 0), the parabola has its
vertex at Vtip = 0 and is symmetric about the y-axis. On
a charged surface, if the charge is positive (Qs > 0), the
parabola has its vertex Vtip > 0 and a negative surface
charge leads to the opposite vertex Vtip < 0. Figures 3(b)
and 3(c) respectively depict the AFM topography and
phase shifts of a single FF MT. Phase shifts of the up-
left and down-right parts in Fig. 3(c) are obtained at
Vtip = 5 V and +5 V, respectively. The line profiles (blue
lines) showing the morphology according to the up-left
and down-right line scans are presented in the insets
of Fig. 3(c). The corresponding EFM data (phase shifts
along the two line scans) are also shown in the insets
(red lines). The opposite phase shift at negative and
positive voltages is obviously a result of the interaction
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933 Nano Res. 2014, 7(6): 929–937
between the MT surface charge and the tip charge.
Figure 3(d) shows the line scan image of the profiles
during intermittent contact between the tip and the
same MT surface as the tip voltage is varied from 4 V
to 10 V. The data are taken from the central part of the
EFM image on the MT exterior surface. The parabola
of tan(ΔΦ) against the tip bias voltage is plotted in
Fig. 3(e). Its vertex Vtip is larger than 0, meaning that
the surface charge on the MT exterior is positive. The
surface charge of the sample is: Qs = 2.52751 × 10–18 C,
according to the intercept of the parabola at Vtip = 0.
Taking into consideration the tip–surface system, the
effective area of the tip as a circular disk is R2 and
the charge density is estimated to be about 5.02840 ×
10–4 C/m2. Therefore, when the direction of the external
electric field is downwards, the attraction between
the MT surface and substrate leads to oblique growth
of the MTs.
tan( )
Q F
k z (1)
2
s tip s2
tip 2
0
1( )
2 4
CQ V QCF z V
z z
(2)
2
0 p
p
2 RC
z h
(3)
Figure 3 (a) Illustration of the EFM measurement technique. Firstly a topographic line scan is made and then the tip is lifted to a fixedheight of 60 nm from the surface and the reverse line scan is made based on the potential difference between the AFM tip and thedifferent charge distribution on the surface. (b) AFM micrograph of a single MT. (c) Associated EFM phase response of the same MT. The line profiles of both the morphology (blue line) and EFM phase shift (red line) indicated by the line scans 1 and 2 are plotted together in the insets. (d) Line scan image of the profiles resulting from intermittent contact between the tip and the same MT surface as the tipvoltage is varied from –4 to 10 V. (e) Quadratic fitted line of the EFM phase shifts as a function of the tip bias voltage. (f) 3D top-end morphology (AFM image) and (g) the corresponding output voltage (EFM image taken under amplitude mode) of the unidirectionallyaligned MTs. (h) Line profiles of the output voltage (red) and topography image (black) on a single MT.
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934 Nano Res. 2014, 7(6): 929–937
2
22tip s2 s
tip2 3
00
1tan( )
2 2 22
V Q Q QQQ C C CV
k z zz k zk z
(4)
stip 2
p0
3
4 22
Q hV
zz
(5)
3C-SiC nanoparticles (NPs) are positively charged in
an acidic environment and can be negatively charged
in an alkaline medium [44, 45]. Positively charged
3C-SiC NPs were deposited on FTO and changes in
the surface charge monitored by EFM as described
above [46–49]. AFM images of the 3C-SiC NP film and
the corresponding output voltage, which represents the
distribution of electric field, are depicted in Fig. S6 (in
the ESM). They were simultaneously recorded using
the tapping mode at a tip distance of up to 60 nm.
During EFM, the FTO was grounded. As shown in
Figs. S6(b) and S6(d) (in the ESM), most of the 3C-SiC
film surface has a positive potential which is consistent
with our conclusion [44]. The SEM image of the FF
NT/MT array formed on the acidic 3C-SiC NP modified
FTO with no applied external electric field is shown
in Fig. 2(c). Since the distribution of positively charged
NPs on the substrate is not completely homogenous,
oblique MTs were formed.
The 3D morphology of the MT array fabricated in
the presence of the upward external field and corres-
ponding output voltage are displayed in Figs. 3(f)
and 3(g), respectively. The top of the NT/MT array is
positively charged. Fig. 3(h) shows the height profile
of the top side of a single MT and corresponding
output voltage. The output voltage is positive and
almost constant, indicating that the top side is positively
charged.
A larger area containing aligned FF MTs can be
fabricated on charged substrates (such as FTO) by this
technique as compared with thermal approaches [2, 5].
The micro-forests have a denser MT arrangement and
the MTs do not have closed ends [7, 8, 14, 19]. The MT
array also retains multiple hydrophilic channels which
determine the wettability properties of the peptide
microtubes (PMTs) [50]. Hence, the capacitance of our
MT array is expected to be larger. Figure 4(a) exhibits
Figure 4 (a) Cyclic voltammetry measurements of the FTO electrode coated with unidirectionally aligned NTs/MTs (red line). The insetillustrates the porous wall of the NT/MTs on the larger specific area. (b) Stability of the unidirectionally aligned FF NT/MT array in capacitance after repeated cycles. The inset shows the SEM image of the unidirectionally aligned NT/MT array after soaking in the electrolytefor a long time (>3 days). Scale bar is 50 μm. (c) Charge–discharge curves of an MT-coated FTO electrode at various constant current densities. (d) Cyclic voltammograms of the FTO electrode with the MT array at three different scanning rates of 10, 50 and 100 mV·s–1.
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935 Nano Res. 2014, 7(6): 929–937
the cyclic voltammetry (CV) characteristics of the
NT/MT array. It has a nearly rectangular shape
representative of an ideal electrode (Fig. 1(c)) and
behaves as an electrochemical double-layer super-
capacitor. The capacitance can be calculated from
the current–voltage curve by Eq. (6), where i is the
current (A), V is the potential window, v is the scan
rate (mV·s–1), and S is the working surface area of
the electrode (cm2). The modified electrode shows a
higher electric capacity of up to 1,000 μF·cm–2 at a
scanning rate of 50 mV·s–1, which is more than
120 times larger than that of the pure FTO electrode
of around 8 μF·cm–2 determined electrochemically.
Figure 4(b) indicates that the NT/MT array is stable
after hundreds of cycles. This can be further verified
via the SEM observation. The inset of Fig. 4(b) shows
the side view of the aligned MTs immersed in the
electrolyte for more than three days and excellent
stability is demonstrated.
idVC
vSV
(6)
Typical galvanostatic charge–discharge curves of the
MT-array-modified electrodes are shown in Fig. 4(c).
The potential–time (V–t) responses were measured
under different charge–discharge rates (current
densities). A similar triangular form of the charge–
discharge curve could be found even at 24 μA·cm–2
and at this current density, the capacitance is about
1,170 μF·cm–2 from the linear portion of the discharge
curve. The capacitance decreases with increasing
discharge current density, consistent with the cyclic
voltammograms obtained at different scanning rates
displayed in Fig. 4(d). Even at a large scanning rate of
100 mV·s–1, the capacitive system still shows an almost
rectangular shape and large current response.
4 Conclusions
Room-temperature controllable fabrication of unidirec-
tionally aligned micro-forests with centimeter scale
area on conductive charged substrate has been
demonstrated by applying an external electric field.
This controllable fabrication process can be expected
to lead to wide applications of the NT/MT arrays
in nanogenerators and information storage devices.
In this work, we reveal that the self-assembled
biomicrostructures possess desirable properties, for
instance, supercapacitance of 1,000 μF·cm–2.
Acknowledgements
This work was supported by the National Basic
Research Programs of China under Grants Nos.
2011CB922102 and 2013CB932901 and the National
Natural Science Foundation of China (Nos. 11374141
and 21203098 ).
Electronic Supplementary Material: Supplementary
material (XRD results, EFM images, AFM and EFM
characterization, electrochemical measurements) is
available in the online version of this article at
http://dx.doi.org/10.1007/s12274-014-0455-6.
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