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Handwriting enabled harvested piezoelectric power using ZnO nanowires/polymer composite on paper substrate

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Handwriting enabled harvested piezoelectric power using ZnO nanowires/polymer composite on paper substrate
Handwriting enabled harvested piezoelectric
power using ZnO nanowires/polymer composite
on paper substrate
Eiman Satti Osman, Mats Sandberg, Magnus Willander and Omer Nur
Linköping University Post Print
N.B.: When citing this work, cite the original article.
Original Publication:
Eiman Satti Osman, Mats Sandberg, Magnus Willander and Omer Nur, Handwriting enabled
harvested piezoelectric power using ZnO nanowires/polymer composite on paper substrate,
2014, NANO ENERGY, (9), 221-228.
http://dx.doi.org/10.1016/j.nanoen.2014.07.014
Copyright: Elsevier
http://www.elsevier.com/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-112837
Handwriting enabled harvested piezoelectric power using
ZnO nanowires/polymer composite on paper substrate
E. S. Nour1,*, M. O. Sandberg1,2, M. Willander1, and O. Nur1
1
Department of Science and Technology, Campus Norrköping, Linköping University,
Norrköping, SE-601 74 Norrköping, Sweden
2
Printed Electronics, Acreo AB, P.O. Box 787, 60117
Norrköping, Sweden
Abstract
We here, present a flexible handwriting driven nanogenerator (NG) based on zinc oxide
(ZnO) nanowires (NWs)/polymer composite grown/deposited on paper substrate. The targeted
configuration is composed of ZnO NWs/PVDF polymer ink pasted and sandwiched between
two pieces of paper with ZnO NWs grown chemically on one side of each piece of paper.
Other configurations utilizing a ZnO/PVDF ink with different ZnO morphology on paper
platform and others on plastic platform were fabricated for comparison. The mechanical
pressure exerted on the paper platform while handwriting is then harvested by the ZnO
NWs/polymer based NG to deliver electrical energy. Two handwriting modes were tested;
these were slow (low pressure) and fast (high pressure) handwriting. The maximum achieved
harvested open circuit voltage was 4.8 V. While an out power density as high as 1.3 mW/mm2
was estimated when connecting the NG to a 100 Ω load resistor. The observed results were
stable and reproducible. The present NG provides a low cost and scalable approach with many
potential applications, like e.g. programmable paper for signature verification.
Key words: Zinc oxide nanowires, polymers, paper substrate, piezoelectricity, energy
harvesting, nanogenerators.
*
Corresponding author: [email protected]
1
Introduction
Self-powered systems are a new emerging technology, which allows the use of a system
or a device that gives energy without the need for external power like a battery or any other
type of source [1]. This technology can for example use harvested energy from sources around
us such as ambient mechanical vibrations, noise, and human movement and converted it to
electric energy using the piezoelectric effect [1].
For nanoscale devices, the size of traditional batteries is not suitable and will lead to
loss of the concept of ‘’nano’’. This is due to the large size and the relatively large magnitude
of the delivered power from traditional source. The development of a nanogenerator to
convert energy from the environment into electric energy would facilitate the development of
self-powered systems relying on nanodevices. In recent years, various articles have reported
the utilization of the piezoelectric properties of zinc oxide (ZnO) nanostructures, but most of
these efforts have focused on studying single ZnO nanowire (NW) [3, 4]. A few investigations
have discussed ZnO nanowire arrays with glass, sapphire, Si, or other hard substrates [3-5].
Other research groups have exploited the piezoelectric effect of various flexible substrates,
such as paper, zinc foil, PET (polyethylene terephthalate), and some more [2, 6], because it is
difficult to enhance the piezoelectric effect by transferring the mechanical energy into
electrical energy using hard substrates such as glass, sapphire, Si, etc.
Zinc oxide is a II-VI wide direct bandgap (3.7 eV) semiconductor and has excellent
piezoelectric properties. Hence it exhibits potential for novel applications since we can couple
the fields of electronics, photonics and piezoelectricity [7, 8]. This is one of the reasons why
ZnO has attracted a lot of interest for developing nanogenerators [1]. The synthesis of aligned
ZnO NW arrays has been studied [9, 10] during the past several years due to the various
potential applications of aligned ZnO NW arrays in light emitting diodes [11-13], lasers
[14,15], solar cells [16-18], nanogenerators [19-21] and piezotronics [22, 23], etc. Various
methods have been reported for synthesizing ZnO NW arrays, mainly including physical
vapor phase transport and deposition [24-26], metal organic chemical vapor deposition
(MOCVD) [27, 28] and the low temperature hydrothermal approach [29-31]. Nevertheless, the
hydrothermal is advantageous when compared to other methods because the growth
temperature is suitable for soft substrates like e.g. paper substrate. Furthermore, the
hydrothermal method is low cost and can be adopted for mass production [32]. Recently, ZnO
NWs have received more attention for their ability to convert mechanical energy to
2
harvestable electrical energy [33–36]. In addition to the development of systems depending
only on ZnO NW arrays, ZnO composite structures have been suggested. As a polymer
piezoelectric material and owing to the good optical transparency and mechanical flexibility,
poly (vinylidene fluoride- trifluoroethylene) (P(VDF-TrFE)), has been exploited [37, 38].
In the present letter, we demonstrate a handwriting driven piezoelectric nanogenerator
(NG) based on ZnO NWs/PVDF polymer hybrid structure grown on paper substrate. We
demonstrate the applications of P(VDF-TrFE)/ZnO NWs ink sandwiched between two pieces
of paper coated with vertically aligned ZnO NWs grown on silver electrodes to act as an
actuator. Two different P(VDF-TrFE)/ZnO NWs ink configurations have been fabricated. The
first is based on hydrothermal ZnO NWs filtered from growth solutions and the other is based
on ZnO tetrapods. After processing of the NG, pressure from handwriting is used to harvest
electrical energy. The demonstrated NG exhibited good mechanical durability, high
sensitivity, and provides scalable simple low cost approach.
Experimental section
In the fabrication of the handwriting enabled piezoelectric nanogenerators, basically
two different configurations of ZnO NWs/polymer composite grown on a paper substrate
were used. A third sample was prepared on a PEDOT:PSS coated plastic substrate for
comparison. In addition another comparison fourth sample was prepared using pure PVDF ink
without ZnO nano powder. The choice of paper as a platform in this work is basically due to
two reasons. The first is the possibility to introduce devices with low cost, while the second is
due to the softness and porous nature of the paper that allow efficient transfer of pressure
forces and consequently the possibility of efficiently harvesting mechanical energy into
electrical energy.
In all fabricated configurations used ZnO NWs were first grown on paper substrate.
The paper substrates used in our experiments were cut from a large piece of common packing
paper with high flexibility (Invercote G from Holmen AB, Sweden). After being cleaned
ultrasonically in acetone and ethanol, a 10/50nm layer of chrome/silver was evaporated on the
paper substrate to act as a contact. In a second step we grow ZnO NWs on this paper substrate
using the low-temperature chemical growth technique. Usually after two steps, well aligned
ZnO NWs can be grown using this method. The details of this growth can be found in [39,
40]. The growth was terminated after 6 hrs. The paper sample was cleaned with deionized
water and left to dry in an ordinary laboratory oven set at 80 oC for ten minutes. The growth
3
nutrient solution which was transparent when mixed becomes blurry and full of ZnO
nanostructures after the termination of the growth. This solution was kept and filtered and the
collected powder was used later. The resulting white powder was composed of ZnO NWs
[39]. This powder was kept and used later to fabricated a ZnO/polymer composite NGs.
Depending on the nutrient growth solution concentration and the duration of the growth
period, different sizes and amounts of NWs can be obtained by filtering the growth nutrients
solution [41]. For the extraction of the filtered ZnO NWs we have performed many
experiments with nutrient solutions having different concentrations. From these experiments
we have achieved ZnO powder in a range between 0.1 mg/mL and up to 20.0 mg/mL. This
chemically grown ZnO NWs powder was used for the first piezoelectric NG configuration.
While for the second configuration another commercial ZnO tetrapod material prepared by
physical method was used [42].
The electrical harvested energy measurement was performed using a Keithley 2400
source meter. The Keithley 2400 is also interfaced to a computer to plot the data. For
estimating the open circuit output voltage, the two end contacts of the nanogenerator were
connected to the source meter. The mode of measurement used was voltage versus time. For
the case of the current measurement (short circuit current), a load resistor is connected in
parallel to the nanogenerator and the voltage across this load resistance versus time was
measured. To measure and estimate the exerted pressure when handwriting is performed a
simple setup is arranged. In this set up the paper substrate is fixed on top of a balance in such
a way that the exerted pressure is only transferred to the paper/balance table i.e. the rest of the
hand supported parts are not in contact to the paper/balance table. The weight exerted during
handwriting is then transferred to pressure by using the surface area of the pen tip. This was
performed for both fast and slow handwriting modes, and was repeated several times and the
mean value was used.
Result and discussion
The structural properties of the grown nanostructures were performed using scanning
electron microscopy (SEM) and powder X-ray diffraction (XRD). Figure 1a shows a low
magnification SEM micrograph of the ZnO NWs grown on paper substrate. As clearly seen
dense ZnO NWs arrays were achieved. Figure 1b shows a high magnification SEM image
indicating a hexagonal NWs with a diameter ranging between 0.8 – 1.5 µm. X-ray diffraction
pattern from this sample is shown in Figure 1c. As can be seen only diffraction peaks from
4
ZnO and Ag were observed. While Figure 1d shows a SEM image of ZnO NWs grown on the
PEDOT:PSS coated plastic and used as mentioned above for comparison purpose.
The fabrication of the present NG proceeded as follows: 0.01g of the filtered ZnO NWs
powder (or from the commercial ZnO tetrapods) was added to 1 mL of P(VDF-TrFE) (70:30).
The mixture was stirred at 60°C for 6 hours to form a homogenous ink. The addition of the
ZnO nano powder to the PVDF-TrFE will lead to enhance the stability of the piezoelectric
polymer [43, 44]. A small drop from the ink was applied on top of the paper substrate with
chemically grown ZnO NWs (Stencil print method). Then the paper was dried for 15 min. at
room temperature and then heated at 80 °C for 10 minutes. Typically different pieces of paper
of a size of around 4.2 cm x 2.0 cm were used. After this two similar paper pieces were
attached face to face and electrical wires were connected from both faces and handwriting
was performed on one face while monitoring the electrical output on a computer screen.
Figure 2a shows a digital photograph of the final fabricated NG. While in Figure 2b a
schematic diagram displaying the structure of the general configuration used for the
piezoelectric NG is shown. Finally Figure 2c-d show photographs during the measurement.
The electrical characterization of the NG grown on paper using ZnO NWs/PVDF polymer
was performed using a Keithley 2400 source meter and the measurement data was transferred
to a computer (as shown in Figure 2c). The measurement was performed for two modes; fast
and slow handwriting modes as different mechanical energy are applied for these two
different modes. The handwriting speed and the corresponding pressure vary from person to
person and depend on the type of pen used. The slow mode corresponds to low applied
pressure, while the fast handwriting mode corresponds to high applied pressure. The
handwriting was performed using a ball pen with a tip spatial area of about 0.16 cm2. In our
experiments the handwriting speeds were estimated to be about 100-120 letters/minute and
about 200-240 letters/minute for the slow and fast handwriting modes, respectively. The
corresponding pressures were measured to 8.2 N/cm2 and 28.4 N/cm2, respectively. The
harvested electrical voltage and current were recorded for a handwriting period of around 1
minute.
Figure 3 (a) shows the open circuit output voltage of the NG fabricated using the ZnO
tetrapods/P(VDF-TrFE) ink pasted on ZnO NWs grown on paper substrate. The maximum
harvested output voltage achieved was 1.8 mV for fast speed handwriting and was down to
around 0.3 mV for slow speed handwriting. The corresponding electrical open circuit output
voltage for the other configuration using the chemically grown ZnO NWs is shown in Figure
3b. As it is clearly seen the output voltage is increased to a maximum value of 2.0 V for slow
5
speed handwriting and reached a maximum of 4.8 V for fast speed handwriting. This indicates
an enhancement when using ZnO NWs grown chemically compared to commercial ZnO
tetrapods grown using high temperature physical approach [42]. To estimate the short circuit
current delivered from this NG, a 100 Ω load resistance is connected as shown in Figure 3c.
The voltage across this 100 Ω load resistance was measured and the current is then estimated.
As shown in figure 3d, a maximum current of 14.4 mA was measured for high speed
handwriting while the corresponding value was 2.8 mA for low speed handwriting. These
values are higher than results published from configurations having ZnO NWs with pure
PVDF grown on fiber [42]. and even higher than NG fabricated from ink of a mixture of polar
ZnO NWs with PMMA grown on flexible polystyrene plastic substrate [44].This high value is
attributed to the relatively high pressure exerted by the pen tip. To separate the effect of the
ink mixture (Figure 3a, b) from that of a pure polymer ink with no ZnO nano powder, a
sample with only pure P(VDF-TrFE) sandwiched between two pieces of paper with ZnO
NWs grown chemically on their surface is fabricated. This configuration showed relatively
lower output voltage compared to the results of Figure 3a-b. This is shown in Figure 3e where
a maximum of 0.2 V was achieved when performing high speed handwriting. To investigate
the effect of the substrate, we have also fabricated a NG using our ink sandwiched between
two ZnO NWs grown by the chemical methods on PEDOT:PSS coated plastic (see below).
According to theoretical calculations, the output voltage from a nanowire is linearly
proportional to the magnitude of the deformation caused by the external applied force or
pressure [45]. Hence for maximizing the harvested mechanical energy an efficient transfer of
the applied pressure would be preferable. While the ZnO NWs have a positive value of the
piezoelectric coefficient d33, the value for the PVDF gives a negative voltage when polarized
[44]. Nevertheless, in a tri-layer based configuration similar to the one presented here (ZnO
NWs/PVDF/ZnO NWs) the PVDF was poled to achieve maximum harvested output power
[44]. In our case the PVDF is not poled and hence it acts as a dielectric layer. The
crystallographic alignment of the ZnO NWs indicates their piezoelectric response to external
pressure which is in our case due to handwriting. Generally when a ZnO NW is bent (due to
external force) a separation of the static ionic charge centers in the tetrahedral coordinated ZnO system results in a piezoelectric potential gradient along the c-axis. Because the c-axes of
all the NWs are aligned parallel to one another, the piezoelectric potentials created along each
NW would have the same tendency of distribution, leading to an enhanced macroscopic
behavior [45]. Based on this, the working principle of the handwriting enables NG is as
6
follows: Figure 4 illustrates all possible bending situations of the two ZnO NWs layers (on the
top or the bottom paper) with or without handwriting. When no handwriting is applied (Figure
4a) i.e. no bending, no c-axis potential gradient along the ZnO NWs will be observed.
Nevertheless, when the NWs are bent, there will be two possible distinct situations. These two
situations are illustrated in Figure 4b-c. In the situation of Figure 4b, the bending of the NWs
leads to create a piezoelectric potential gradient in such a way that the top paper contact will
have a positive potential while the bottom paper contact will have a negative potential. A
situation similar to this but with reversed voltage polarity can also exist. In that case the
harvested potential will be in the opposite direction to that of figure 4b. The last situation,
shown in Figure 4c, is when similar voltage polarity exists at both top and bottom contacts.
Assuming that the ZnO NWs are identical and upon handwriting they bent similarly, the
harvested voltage from the top and bottom paper will cancel each other.
The efficient transfer of the applied external mechanical energy will also depend on the
platform hosting the NG. A NG on plastic was fabricated for comparison with paper and
hence it was having a configuration similar to that of the device of Figure 3b, i.e. polymer ink
with ZnO NWs powder grown chemically and sandwiched between two pieces of paper
having ZnO NWs on a surface grown over Ag/Cr thin layer. The ZnO NWs used for this
plastic based NG are shown in Figure 1d. The result of the output voltage of the plastic
platform based ZnO NWs NG is shown in Figure 5a together with the output voltage of the
device of Figure 3b. As it is clearly seen, the delivered open circuit output voltage from this
plastic based NG has reached a maximum value of around 0.04 V. This value is relatively
much lower than the highest value obtained from similar ZnO NWs/polymer ink stacking
configuration NG based on paper platform (4.8 V). This shows that the choice of the porous
and soft substrate, i.e. paper, has led to an improvement of the output voltage by a factor of
100 compared to the case when using plastic as a platform. This is believed to be due to the
more efficient transfer of handwriting pressure to the nanowires rather than being absorbed
through bending of the whole platform.
To estimate the output power delivered from NGs different circuits can be used [4649].The basic test is to connect the NG to a load resistance (RL). By measuring the maximum
voltage across this load resistance (VR) the output power (P) can be estimated as [46].
We have connected our NG that delivered the highest open circuit output voltage to
different load resistors varying between 100 Ω and up to 3 kΩ. Then the pen tip spatial area
7
while handwriting is used to estimate the output power density. Figure 5b displays the output
power density obtained from the ZnO NWs grown on paper platform and for a configuration
with ZnO NWs/PVDF polymer ink pasted and sandwiched between the two pieces of paper
with ZnO NWs grown chemically (configuration of figure 3b). As can be seen from Figure
5b, the maximum observed output power density has reached about 1.3 mW/mm2 when a load
resistor of 100 Ω is connected to the NG. This NG was then tested to operate a commercial
light emitting diode (LEDs). The insert in Figure 5b shows a commercial red LED operated
using the NG of figure 3b. From the presented results, it seems that the stacking ink layer
(layer of PVDF with filtered ZnO NWs) in the middle contributes to increase the harvested
output power. In the configuration with PVDF with the filtered ZnO NWs sandwiched
between two pieces of paper covered with vertical ZnO NWs when handwriting pressure is
applied on the surface of the NG we achieved the highest output due to two contributions. The
first effect is from the bending of the vertical ZnO NWs grown on the paper coated with
Cr/Ag. The second contribution is due to the filtered ZnO NWs which forms a composite with
the PVDF polymer. More detailed study on the individual contribution, together with the
estimation of the efficiency of harvesting mechanical pressure when handwriting in
connection to the stacking sequence is under present investigations.
Conclusion
In summary, in this letter we demonstrate a ZnO NWs/PVDF polymer based hybrid
handwriting driven NG fabricated on soft porous paper platform. From the different
configurations studied, it was concluded that the highest harvested electrical output power
was achieved when using a ZnO NWs/ PVDF polymer ink pasted and sandwiched between
two pieces of paper with ZnO NWs grown chemically on the side of each piece of paper. This
stacking configuration has delivered a voltage as high as 4.8 V and a current as high as 14.4
mA. A maximum spatial output power density of 1.3 mW/mm2 was achieved for fast
handwriting mode (200-240 letters/minute). The present approach is promising, scalable, and
the principle of harvesting mechanical vibration on paper platform can be utilized from any
machineries. The present piezoelectric NG can be used for many applications, like e.g.
development of signature verification programmable smart paper, handwriting in dark paper
when integrating light emitting diodes on the same paper etc..
8
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Figure Captions
Figure 1: (a) shows a low magnification SEM micrograph of the ZnO NWs grown on paper
substrate, (b) shows a high magnification SEM image indicating hexagonal NWs, (c) shows
x-ray diffraction pattern from this sample, and (d) shows a SEM image of ZnO NWs grown
on the PEDOT: PSS coated plastic and used for comparison purpose.
Figure 2: The setup of measurement (a) a digital photograph of the final fabricated NG is
shown, and (b) shows a schematic diagram displaying the structure of the general
configuration used for the present piezoelectric NG, (c) and (d) photographs during the
measurement using the handwriting enabled harvested electrical energy, respectively.
Figure 3: The performance of ZnO NWs/PVDF paper based NG, (a) the output voltage
achieved by using commercial ZnO tetrapod powder as a function of time for slow and high
speed handwriting. (b) The output voltage achieved using ZnO NWs filtered powder as a
function of time at slow and high speed handwriting. (c) Schematic diagram of the circuit
used to estimate the short circuit current through a load resistance. (d) The short circuit output
current through a 100 load resistor measured for the structure of (b) as function of time for
fast and low speed handwriting. Finally in (e) the open circuit output voltage as a function of
time of a NG fabricated by pure polymer ink sandwiched between two ZnO NWs grown on
paper is shown.
Figure 4: Schematic diagram illustrating the different possibilities of the ZnO NWs bending
due to pressure exerted while handwriting is applied. The expected voltage polarity on the
paper contact (top and bottom) is also shown.
Figure 5: (a) The performance of ZnO NWs/PVDF NG fabricated on paper and on
PEDOT:PSS plastic platforms for comparison, and finally in (b) the maximum output power
density as function of load resistance for NG based on ZnO NWs/PVDF polymer ink pasted
and sandwiched between two pieces of paper with ZnO NWs grown chemically on one side of
each piece of paper is shown. The inset is a digital photograph showing a light emitting diodes
operated by handwriting harvested power from the handwriting enabled paper nanogenerator.
11
Figure 1:
(a)
(b)
(c)
(d)
12
Figure 2:
(a)
(b)
(c)
(d)
13
Figure 3:
2.2
Voltage (Low)
Voltage (High)
2.0
Voltage (mV)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0
10
20
30
40
50
60
70
Time (Seconds)
(a)
Voltage (High)
Voltage (Low)
6
2
2
0
0
-2
-2
-4
-6
0
10
20
30
40
Time (Seconds)
(b)
14
50
60
70
Voltage (V)
Voltage (V)
4
(c)
4
Current (High)
Current (Low)
3
10
2
5
1
0
0
-1
-5
-2
-10
-15
-3
0
10
20
30
40
50
Time (Seconds)
)d(
15
60
70
-4
Current (mA)
Current (mA)
15
Voltage (V)
0.2
0.1
0.0
-0.1
-0.2
-10
0
10
20
30
40
50
Time (Seconds)
)e(
16
60
70
Figure 4:
No handwriting pressure
Dielectric
(a)
With handwriting pressure
+
_
+
_
Dielectric
+
Dielectric
_
_
+
(b)
(c)
17
Figure 5:
6
Paper Voltage (High)
PEDOT:PSS Voltage (High)
0.04
2
0.03
0.02
0
0.01
-2
Voltage (V)
Voltage (v)
4
0.05
0.00
-4
-6
-0.01
0
10
20
30
40
50
Time (Seconds)
(a)
(b)
18
60
-0.02
70
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