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

1

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

1Department of Science and Technology, Campus Norrköping, Linköping University,

Norrköping, SE-601 74 Norrköping, Sweden

2Printed 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: eiman.satti.osman@liu.se

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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

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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

4

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

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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

6

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 Zn-

O 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

7

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 [46-

49].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

8

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..

9

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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.

12

Figure 1:

(a) (b)

(c) (d)

13

Figure 2:

(a) (b)

(c) (d)

14

Figure 3:

0 10 20 30 40 50 60 70-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Vo

ltag

e (

mV

)

Time (Seconds)

Voltage (Low)

Voltage (High)

(a)

0 10 20 30 40 50 60 70-6

-4

-2

0

2

4

6

Voltage (High)

Voltage (Low)

Time (Seconds)

Vo

ltag

e (

V)

-2

0

2

Vo

ltag

e (V

)

(b)

15

(c)

0 10 20 30 40 50 60 70-15

-10

-5

0

5

10

15

Current (High)

Current (Low)

Time (Seconds)

Cu

rren

t (m

A)

-4

-3

-2

-1

0

1

2

3

4

Cu

rren

t (mA

)

(d )

16

-10 0 10 20 30 40 50 60 70

-0.2

-0.1

0.0

0.1

0.2

Vo

ltag

e (

V)

Time (Seconds)

(e )

17

Figure 4:

(a)

(b) (c)

Dielectric

No handwriting pressure

Dielectric Dielectric

With handwriting pressure

_

+

+

+

_

_ _

+

18

Figure 5:

0 10 20 30 40 50 60 70-6

-4

-2

0

2

4

6 Paper Voltage (High)

PEDOT:PSS Voltage (High)

V

olt

ag

e (

v)

Time (Seconds)

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

V

olt

ag

e (

V)

(a)

(b)