Nano Res 

1

Flexible piezoelectric nanogenerators based on fiber/

ZnO nanowires/ paper hybrid structure for energy

harvesting Qingliang Liao1, Zheng Zhang1, Xiaohui Zhang1, Markus Mohr2, Yue Zhang1 (), and Hans-Jörg Fecht2

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0453-8

http://www.thenanoresearch.com on March 17, 2014

© Tsinghua University Press 2014

Just Accepted  

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Nano Research  DOI 10.1007/s12274‐014‐0453‐8 

1

TABLE OF CONTENTS (TOC)

Flexible piezoelectric nanogenerators based on fiber/

ZnO nanowires/ paper hybrid structure for energy

harvesting

Qingliang Liao1, Zheng Zhang1, Xiaohui Zhang1,

Markus Mohr2, Yue Zhang1, *, and Hans-Jörg Fecht2

1 Department of Materials Physics and Chemistry, State

Key Laboratory for Advanced Metals and Materials,

University of Science and Technology Beijing, Beijing

100083, China. 2 Institute of Micro and Nanomaterials, Ulm University,

Ulm 89081, Germany

We present a novel approach to fabricate flexible piezoelectric

nanogenerators (NGs) consisting of ZnO nanowires (NWs) on carbon

fibers and foldable Au-coated ZnO NWs/ paper. The electric output of

the NGs can be controlled by increasing the fiber number, adjusting

the strain rate and connection modes. The developed NGs can be used

for smart textile structures and wearable nanodevices.

Provide the authors’ webside if possible.

Author 1, webside 1

Author 2, webside 2

2

Flexible piezoelectric nanogenerators based on fiber/ ZnO nanowires/ paper hybrid structure for energy harvesting

Qingliang Liao1, Zheng Zhang1, Xiaohui Zhang1, Markus Mohr2, Yue Zhang1(), and Hans-Jörg Fecht2 1 Department of Materials Physics and Chemistry, State Key Laboratory for Advanced Metals and Materials, University of Science

and Technology Beijing, Beijing 100083, China. 2 Institute of Micro and Nanomaterials, Ulm University, Ulm 89081, Germany

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

 

ABSTRACT   We present a novel, low-cost approach to fabricate flexible piezoelectric nanogenerators (NGs) consisting of

ZnO nanowires (NWs) on carbon fibers and foldable Au-coated ZnO NWs on paper. By using such designed

structure of the NGs, the radial ZnO NWs on a cylindrical fiber can be utilized fully and the electric output of

NG is improved. The electric output behavior of the NGs can be optionally controlled by increasing the fiber

number, adjusting the strain rate and connection modes. For the single-fiber based NGs, the output voltage is

17 mV and the current density is about 0.09 μAcm-2, and the electric output is enhanced greatly compared to

that of previous similar micro-fiber based NGs. Compared with the single-fiber based NGs, the output current

of the multi-fiber based NGs made of 200 carbon fibers increased by 100 times. The output voltage of 18 mV

and current of 35 nA are generated from the multi-fiber based NGs. The electric energy generated by the NGs is

enough to power a practical device. The developed novel NGs can be used for smart textile structures,

wearable and self-powered nanodevices.

KEYWORDS ZnO, nanowires, hybrid structure, flexible nanogenerators, piezotronic

1 Introduction

Energy harvesting from our living environment

to power small electronic devices and systems is a

critical issue for sustainable development and

attracts increasing attention [1, 2]. Mechanical

energy is one of the most abundant and popular

energies in our daily life, which can range from

wind energy to mechanical vibration [3],

sonic/ultrasonic waves [4], flowing air [5], muscle

stretching [6], and more. Both the piezoelectric and

triboelectric nanogenerators (NGs) can convert the

mechanical energy into electricity, but they have

different working mechanisms. The work of

piezoelectric NGs depends on the piezoelectric

3

effect of nanostructures [3, 4]. A power density of

1–780 mW/cm3 has been achieved from piezoelectric

NGs. The energy conversion of triboelectric NGs is

achieved by coupling between the triboelectric

effect and the electrostatic induction. Now, the area

power density of triboelectric NGs reaches 313

W/m2 and the NGs can light up hundreds of LED

bulbs [7]. The triboelectric NGs have high energy

collection efficiency which is much larger than the

piezoelectric NGs. The piezoelectric polarization

charges of the piezoelectric NGs can be created at

the end of each nanostructure by applying strain,

pressure or force. Therefore, the piezoelectric NGs

have excellent sensitivity to external strain, which is

a noteworthy advantage over triboelectric NGs. In

addition, the output power of piezoelectric NGs can

be calculated and predicted based on the

piezoelectric coefficient and external strain and is

enough to power the micro/nano electronic devices

[8, 9, 10]. As the first-generation NGs, the

piezoelectric NGs have special advantages over

other type NGs and were applied in the fields of

accurate sensor and self-powered system.

By utilizing the coupled semiconducting and

piezoelectric properties, considerable attentions

have been focused on exploiting zinc oxide (ZnO)

piezoelectric NGs [11, 12]. To enhance the

conversion efficiency and the excellent adaptability

of the NGs, the fabrication of ZnO piezoelectric

NGs on flexible substrate has become one of the

most attractive research topics. Remarkable efforts

have also been invested in the fabrication of ZnO

piezoelectric NGs on many flexible substrates [5, 8,

13]. Due to its lightweight, inexpensive, and

foldable character, fibers were demonstrated to be

an efficient substrate for flexible NGs [14-16]. These

fiber-based NGs have an added advantage of being

flexible and foldable power sources which is ideal

for applications such as implantable biomedical

sensors. For the ZnO piezoelectric NGs based on

fibers, the unique cylindrical surface of fiber has an

unavoidable drawback, that is the small effective

contact area between two electrodes [14]. High

output power and flexibility are very important in

the development of NGs, especially for its potential

applications in flexible and wearable electronics.

ZnO nanowires (NWs) grown radially around the

fibers in the fiber-based NGs, thus designing an

all-round electrode which can utilize all ZnO NWs

on fiber is an effective way to increase the contact

area and improve the energy output. Paper has

been used as a substrate to invent new electronic

devices and NGs [17-19]. The paper substrate could

be folded in any angle and is a perfect candidate of

all-round electrode. Construction of fiber-based

NGs using the foldable outer paper electrode is a

feasible way to enhance the output of NGs.

Here, we demonstrate a novel, low-cost approach

to fabricate a flexible hybrid nanogenerator (NG)

consisting of central ZnO NWs on carbon fibers and

outer Au coated ZnO NWs on papers. The ZnO

NWs grown radially around carbon fibers were

wrapped with the folded Au coated paper-ZnO.

Owing to a coupled piezoelectric-semiconducting

process, an alternating current (AC) output was

achieved from the fabricated NG driven by the

external pressure. The electric output of the

single-fiber based NG was improved greatly

compared to that of previous similar micro-fiber

based NGs. The generated electric energy from the

NGs was stored by capacitors and it was used to

light up a light emitting diode (LED). In addition,

the voltage and current outputs of the NG were

controlled by changing the NG structure, external

strain rate, and connection mode, respectively. This

work establishes a new method to fabricate flexible,

foldable, wearable and robust power sources in any

shape that converts low-frequency mechanical

movements in our daily life into electricity.

2 Experimental

2.1 Synthesis of ZnO NWs on carbon fibers and

papers

4

The ZnO NWs were grown on carbon fibers and

paper substrates using a two-step hydrothermal

growth approach [20]. Firstly, a ZnO seed layer film

was coated on surface of substrates. Then the

seeded substrates were placed in an aqueous

solution to grow ZnO NWs. The seed layers

fabricated on two kinds of substrate are also by

coating and thermally decomposing. The carbon

fibers and papers were cleaned ultrasonically in

acetone, ethanol and deionized water. The carbon

fibers were immersed in the seed solution of 0.05 M

zinc acetate in ethanol for 2 min and pulled up.

Then ZnO seeds were deposited on carbon fibers by

thermally decomposing zinc acetate at 350 ℃ for

15 min in air [21]. This step was repeated 3~4 times

and the carbon fiber was completely covered by

ZnO seed layer. For the fabrication of seed layer on

papers, paper substrate was immersed in the seed

solution (0.08 M zinc acetate in ethanol) and the

solution spread across paper [18]. Then the paper

was dried completely for 10 minutes at 100 ℃in air.

This step was repeated 5~6 times to ensure

complete coverage of the paper fibers with a ZnO

seed layer. Finally, the paper was annealed at

160 ℃ for 6 h in air, which made zinc acetate

convert to the ZnO completely.

The ZnO NWs were grown on seed-coated

carbon fibers and papers via a hydrothermal

growth method. The growth solution was prepared

by dissolving equal volume (25 mM) zinc nitrate

hexahydrate and hexamine in deionized water. The

seeded carbon fiber grew at 90℃ for 4 h. The

seeded paper was floated into the growth solution

and the growth was carried out for 6 h at 90 ℃.

2.2 Fabrication of the flexible fiber-based NGs

The fiber-based hybrid NG is composed of central

as-grown ZnO NWs on fibers and the outer ZnO

NWs on papers. For the fabrication of the flexible

NG, the ZnO NWs at one side of carbon fiber were

etched off locally by a HCl solution to expose the

fiber electrode for making contact. The carbon fiber

serves not only as the substrate for the growth of

ZnO NWs, but also as an electrode. The Au layer

was coated on the paper-based ZnO nanowire (NW)

film by a magnetron sputtering system (Shenyang

Huiyu Company). The deposition vacuum is 1 Pa

and the deposition current is 80 mA. A fairly

uniform Au coating was achieved on the paper

surface and the thickness of the Au coating was

about 200 nm, which was read by a thin film

deposition controller inside the sputtering system.

Then, the Au-coated ZnO-paper was conductive

and can be used as a foldable electrode. The ZnO

NW coated carbon fiber was all wrapped up with a

flexible Au-coated ZnO-paper. The outer paper was

pressed and form steady contact between the Au

coated ZnO NWs and ZnO NWs on fibers. The

schematic of a single fiber-based hybrid NG is

shown in Fig. 1(a). The ZnO NWs on the central

carbon fiber can be used fully for converting the

mechanical energy to electric energy. The carbon

fibers were around 10 μm in diameter and about 15

mm in length. The effective area of each ZnO coated

fiber is about 10 μm*15 mm. By increasing the

number of the central carbon fiber, multi-fiber

based NGs were also constructed. The

corresponding schematic for a multi-fiber based NG

is shown in Fig. 1(b). The multi-fiber based NGs

were constructed by aligning the carbon fibers

covered by ZnO NWs in parallel between two

Au-coated ZnO-papers. The whole NG was fixed on

a flexible substrate. The exposed carbon fibers

without ZnO NWs were connected by Ag paste.

During the current generating process, one

electrode was the central exposed carbon fibers and

another electrode was the Au-coated ZnO NWs on

papers.

2.3 Characterization of the fabricated NG

The morphologies of the as-grown ZnO NWs on

different substrates were characterized by scanning

electron microscopy (SEM) (Leo 1550FE Zeiss). A

high-resolution transmission electron microscopy

(HRTEM) (FEI Tecnai-G2-F20) was used to

characterize the microstructure of the fabricated

5

ZnO NWs. The electrical outputs of the NG were

investigated when the NGs were under the external

press force. The open circuit voltage was measured

by a low noise preamplifier (model SR560, Stanford

Research Systems), and the short circuit current was

measured by a low noise current amplifier

(DHPCA-100, FEMTO). The I-V characteristics of

the NG were measured using a Keithley 4200

semiconductor characterization system.

3 Results and discussion

The fiber-based hybrid NGs are shown in Fig. 1.

Figures 1(a) and 1(b) show the schematic of fiber

based NG. Typical SEM images of the fabricated

ZnO NWs coated carbon fiber structure are shown

in Figs. 1(c) 1(d), and 1(e). A carbon fiber covered

by ZnO NWs is shown in Fig. 1(c), showing that all

carbon fibers are well covered by ZnO NWs. Along

the entire length of the fiber, ZnO NWs grow

radially and exhibited a very uniform coverage and

well cylindrical shape (Figs. S1(a) and S1(b)). The

diameter of whole ZnO NW coated carbon fiber is

about 11 μm. Based on the cut-away view SEM

image of the ZnO NW/fiber (shown in Fig. 1(d)), the

typical length of the ZnO NW is about 2 μm. The

NWs’ tips are separated from each other with small

tilting angles, but their bottom ends are bonded

together tightly through the ZnO seed layer. A

high-resolution SEM in Fig. 1(e) indicates that the

diameter of ZnO NWs is about 100 nm and all ZnO

NWs stand almost vertical to the carbon fiber. The

ZnO NWs have a hexagonal cross-section and their

top and side surfaces are smooth and clean. The

grown ZnO NWs are able to form the reliable

metal-semiconductor junctions with the metal

electrode. The space between the NWs is about

several hundreds of nanometres, which is large

enough for them to be bent to generate the

piezoelectric potential [14, 22].

Figure 1. Design of a flexible fiber-based hybrid NG and the SEM images of synthesized ZnO NWs on the carbon fiber. (a) The schematic diagrams of single-fiber based NG consisting of the central ZnO NWs on carbon fibers and outer foldable Au-coated ZnO NWs on paper, the central exposed carbon fiber and the outer Au film are the two contact electrodes. (b) The schematic diagrams of multi-fiber based NG, the carbon fibers covered by ZnO NWs were aligned in parallel between two Au-coated ZnO-papers and the whole NG was fixed on a flexible substrate. The central exposed carbon fibers and the Au-coated ZnO-papers are the two contact electeodes. (c) The SEM images of ZnO NW densely grown around carbon fibers, ZnO NWs grow radially and exhibited a very uniform coverage. (d) The cut-away view SEM image of the ZnO NW structure after removing the fiber, which shows that the length of the ZnO NW is about 2 μm. (e) The enlarged SEM image of the surface of the carbon fibers covered by ZnO NWs, and the ZnO NWs with a diameter of 100 nm distribute on the carbon fiber uiniformly.

6

The SEM images shown in Figs. S1(c) and S1(d)

clearly reveal the large area and uniform coating of

ZnO NWs on the paper substrate. The cellulose

fibers are quite smooth before hydrothermal growth,

but become very rough after growth of ZnO. Figure

S1(d) shows that the ZnO NWs grow densely on the

surfaces of the fibers in a radial direction. Typical

SEM and TEM images of ZnO NWs before and after

coating of Au film are shown in Fig. 2. Figs. 2 (a),

2(b) and 2(c) show the results of the as-grown ZnO

NWs. Fig. 2(a) shows the SEM image of the

as-grown ZnO NW array, and the ZnO NWs

distribute on the paper substrate uniformly. The

TEM image shows that the diameter and the length

of the ZnO NW are about 80 nm and 1 μm

respectively. The HRTEM image from an individual

ZnO NW shows that the interplanar spacing is

about 0.26 nm, which corresponds to �the (002)

crystal plane, and the growth direction for ZnO

NWs is the [0001] orientation [23]. Figs. 2 (d), 2(e)

and 2(f) show the results of the ZnO NWs after

coating of Au film. The SEM image of the Au coated

ZnO NWs on paper substrate is shown in Fig. 2(d).

After coating of 200 nm Au film, the ZnO NWs

maintain the original cross-section. The TEM image

of ZnO NWs is shown in Fig. 2(e), and the diameter

of Au-coated ZnO NW increases to about 180 nm.

The whole outer surface of ZnO NWs has been

coated by Au particles uniformly, and the Au films

consisting of particles can be used as the effective

contact electrodes. The HRTEM image shows the

diameter of Au particle is about 10 nm. To ensure

the good conductivity of the Au-coated ZnO-paper,

the sheet resistances of Au-coated ZnO/paper were

measured by four point probe measurement

(Electronic Supplementary Material). The results

show that Au-coated ZnO-papers have good

conductivity and can be used as foldable contact

electrodes in NGs. The ZnO NWs on papers have a

smaller length and similar diameter compare to the

ZnO NWs on fibers. The Au coated ZnO NWs can

act as an array of driving metal tips that deflect the

ZnO NWs on the fiber [3, 14]. The fabricated NGs in

our research are highly flexible and foldable.

Figure 2. The SEM and TEM images for the as-grown and Au-coated ZnO NWs on paper substrates. (a) The SEM image of as-grown ZnO NW array, (b) A low-magnification TEM image of a single ZnO NW, and (c) The HRTEM image of the ZnO NW before coating of Au film. The diameter of the ZnO NWs is about 80 nm and the growth direction is the [0001] orientation. (d) The SEM image of ZnO NW array, (e) A low-magnification TEM image of ZnO NWs, and (f) The HRTEM image of the ZnO NW after coating of Au film. The ZnO NWs were uniformly coated by Au particle with a diameter of 10 nm.

7

When a pressure is periodically applied to the

single-fiber based NG, the performance of the NG is

characterized by measuring the short-circuit current

and the open-circuit voltage. The single-fiber NG

generates an AC output, which is shown in Fig. 3. A

positive voltage/current pulse for fast press (FP) of

the outer ZnO-paper and a corresponding negative

pulse for fast release (FR) were recorded (where

‘fast’ means an press period is about 0.3 s). For a

ZnO NW coated fiber with a diameter of 10 μm and

a length of 15 mm, the maximum output voltage

reaches 17 mV and the output current is 400-520 pA.

The corresponding current density of the designed

NG is 0.09 μAcm-2. The output power per unit

contact area is about 51 μWm-2.

Figure 3. Electrical output of a single-fiber based NG. (a) and (b) are the open-circuit voltage (top) and short-circuit current (bottom) of a NG when subject to repeated cycles of fast press (FP) and fast release (FR) when forward-connected to the measurement system.

Accordingly, a multi-fiber based NG made of 200

carbon fibers was constructed, and the schematic of

the NG is shown in Fig. 1(f). The output voltage and

current of NG being subjected to repeated cycles of

FP and FR are shown in Fig. 4. Figures 4(a) and 4(b)

show the output voltage and current of a typical

multi-fiber based NG when the current meter was

forward connected to the NG. Just like the

single-fiber based NG, an AC output was achieved

from the multi-fiber based NG. The output current

of the multi-fiber based NG increases compare to

the single-fiber based NG and approaches 35 nA.

Compared with the single-fiber based NG, the

output current of the multi-fiber based NG

increased by about 100 times. According to the

output current of the single-fiber NG, the current of

200 parallel fibers can be calculated and it is about

80 nA, which is much larger than the measured

value. When 200 fibers are arranged between two

layer paper electrodes, the entire outer surface of all

fibers cannot contact the outer paper electrodes. The

effective average contact area of one fiber decreases

compared to the single-fiber NG. Therefore, the

measured current is only half as much as the

expected current. However, the output current of

NG can be increased effectively by increasing the

number of fibers. The output voltage of the

multi-fiber NG is about 18 mV, which is determined

by the difference between the Fermi energies for top

and the bottom electrode and maintains the similar

value as the single-fiber NGs [24]. The maximum

output power of the multi-fiber NG is about 2.1 nW.

Figure 4. Electrical output of a multi-fiber based NG. (a) and (b) are the output voltage and current of a NG subjected to repeated cycles of FP and FR under forward-connected mode. (c) and (d) are the output voltage and current of a NG subjected to repeated cycles of FP and FR under reverse-connected mode.

Comparisons on the device performance of this

work with other previous similar reports are listed

8

in Table 1. The compared NGs are mainly the fiber

or paper based flexible piezoelectric type. As a

piezoelectric NG with a Schottky contact, the

designed fiber-based NG has lower output current

and voltage than the piezoelectric NG with a thin

insulating layer [10]. But the output performance of

the designed NG is much better than the previous

similar micro-fiber based NGs [14]. Especially, the

output power density is higher than that of other

piezoelectric fiber or paper NGs [15, 17]. The results

demonstrate that the design method is a feasible

way to fabricate flexible fiber-based NGs. Moreover,

the output current of the NGs could be greatly

enhanced by increasing the number of the ZnO

NWs coated fiber.

Table 1．Comparisons on the output performance of different piezoelectric NGs.

Type of NGs Structure Output voltage Output current Output power Reference

Fiber-based NGs 200 ZnO NW coated fibers 18mV 35 nA

(0.09 μA cm-2)

51 μW m-2 Our work

Multi-fiber hybrid NGs Three ZnO NW coated fibers 3mV 4nA 2.4 mW m-2 [14]

Hybrid-fiber NGs A PVDF layer on the

ZnO-NW grown fiber.

32 mV 2.1 nA cm-2 16 μW cm-3 [15]

Fiber-based hybrid NGs Textured ZnO NW film

grown on carbon fibers, 1000

carbon fibers.

3V 200nA

(0.06 mAcm-2)-- [16]

Flexible NGs ZnO-cellulose nanocomposite 80 mV 1.25 μA 50 nW cm -2 [17]

Flexible fiber NGs Textured ZnO NW film

grown on carbon fibers, 100

carbon fibers

3.2 V 0.15 μA cm -2 -- [5]

Integrated NGs ZnO NW arrays covered by

PMMA layer 58 V 134 μA 0.78 W cm-3 [10]

The difference among the heights of the current

peaks for the press and release might because of

different straining rate [13]. When connection mode

of the current meter was reverse, the output voltage

and current pulses of the NG are shown in Figs. 4(c)

and 4(d). It can be seen that the sign of the output

signal of the reverse connection is just opposite to

that of the forward connection. The output of the

NG after switching the polarity has a small decrease

compare to that before the polarity, and the small

decrease is caused by the bias current in the

measurement system [13]. Therefore, the results

demonstrate that the measured signal indeed came

from NGs rather than from the measurement

system or environmental noise [25].

The electrical output of the NG also depends on

the strain rate applied to the NG [13, 19, 26].

Usually, the voltage/current output for a high strain

rate is significantly higher than the voltage/current

output for a low strain rate. Figure 5 shows the

current outputs of a NG at different strain rates.

Figure 5(a) shows the current output of the NG

under FP and FR. An AC-type output current is

obtained, which is consistent with the previous

results. Moreover, the peak values of the positive

and negative current pulses are almost the same.

When the NG is subjected to FP and slow release

(SS) (where ‘slow’ means an press period is about 1

s), the output current is shown in Fig. 5(b). A

periodic high positive current and low negative

current is generated from the NG due to the change

9

of strain rate. The values of output current decrease

with the increasing of strain rate. On the contrary,

the current output is shown in Fig. 5(c) when the

NG is subjected to slow press (SP) and FR. Similarly,

a periodic high negative current output is detected.

When the NG is subjected to SP and SR, the low

positive and negative current pulses are shown in

Fig. 5(d). By comparison of results, it is very clear

that the output current of the NG depends on the

strain rates directly. For the voltage output of NG

under different strain rates, similar results can also

be obtained (Fig. S2). Obviously, the desired electric

output of NGs can be achieved simply by

controlling the strain rate. The results are consistent

with the theory of previous reports. The number of

net charges is mainly determined by the established

speeds of the built-in piezoelectric potential and the

external circuitry [26]. When the NG is subjected to

FP or FR, the net charges increase quickly, resulting

in a fast electrical pulse. However, a slow press or

release generates a lower output signal over a

longer period of time. The total charges transported

in a press-release cycle remain the same. Increasing

the straining rate significantly increases the

electrical output.

Figure 5. Current outputs of a multi-fiber based NG with different strain rates under a constant applied strain. (a) Current output generated from the NG under FP and FR. (b) Current output generated from the NG under FP and slow release (SR). (c) Current output generated from the NG under slow press (SP) and FR. (d) Current output generated from the NG under SP and SR.

In order to verify the output signal, it is possible

to measure the output voltages and currents for two

NGs connected by ‘linear superposition’. There are

two different multi-fiber based NGs made of

different number of fiber. NG 1 was made of about

200 fibers and NG 2 was made of about 300 fibers.

The output voltage when the two NGs are in serial

is shown in Fig. 6(a). The output voltages of two

NGs are 15 mV and 18 mV respectively. The total

output voltage of NG 1+2 is approximately the sum

of the output voltages of the individual NG. The

output voltage of NGs can be enhanced by

connecting them in series. The output current when

the two NGs are in parallel is shown in Figure 6(b).

NG 2 has an output current of 52 nA, which is

larger than that of NG 1. The output current can be

modulated by the controlling the number of fiber.

The total output current of NG 1+2 is also the sum

of the output currents of the individual NG. The

results demonstrate that the output current could

be added up when the two NGs were connected in

parallel. The output voltage and current could be

greatly enhanced by linearly integrating a number

of NGs.

The output of the NG can be controlled by

increasing the fiber number, adjusting the strain

rate and connection modes. Therefore, a multi-fiber

based NG 3 with about 600 fibers was fabricated

and the output current is shown in Fig. 6(c). The

current is about 110 nA, which is higher than that of

NG 1 and NG 2. In order to accomplish the real

application of the fiber based NG, a

charging-discharging circuit with two consecutive

steps was used to store the generated energy and

drive devices [8]. Firstly, the output energy was

stored by charging capacitors. Upon finishing

charging, the stored energy by the capacitors

release and power a white LED. The capacitors

were simultaneously charged by parallel connection,

but discharged by series connection. The image of

the LED in dim background at the moment when it

10

was lit up is shown in Fig. 6(d). The output power is

enough to power some practical devices.

Figure 6. The ‘linear superposition’ test of outputs for fiber based NGs and application of the electric energy generated by the NGs to drive a LED. (a) The output voltage of the two NGs (NG 1 + NG 2) is the sum of two NGs when they were connected in series. (b) The output current is the sum of the two NGs (NG 1 + NG 2) when they were connected in parallel. (c) The output current from a multi-fiber NG 3 with 600 fibers, whose energy was stored and drive a LED. (d) Image of the white LED in dim background at the moment when it was lit up by the energy generated from the NG 3.

The generation of current in the NG can be

explained using the energy band diagram as shown

in Figure 7. The ZnO NWs used in our experiments

were grown on the carbon fiber and their top ends

were contacted by Au electrode. Au and carbon

have a work function of 4.8 and 5.0 eV respectively

[16, 27, 28], which are larger than the electron

affinity of ZnO (4.5 eV) [29]. Thus, two Schottky

contacts at the two ends of ZnO NW are formed as

soon as the Au-coated ZnO NW is in contact with

the ZnO NWs. I-V characteristic of the NG shows

that the Schottky contacts have distinctly different

barrier heights (Fig. S3). Figure 7(a) shows that a

strain-free ZnO NW usually has non-symmetric

Schottky contacts at its two ends, and two Schottky

barrier heights are noted as Ф1 and Ф2, respectively.

There are different contact types at Au-ZnO

interface, as shown in Fig. S3. I, II, and III are the

three possible types of contact between ZnO NWs

and an Au/ZnO NW electrode [3, 4, 30]. For the

piezoelectric NG, the relative moving process

between ZnO and Au electrode determines the

electric output [3, 4, 31]. The external force

generates different relative movements. In this

study, the external force moves the outer Au coated

ZnO NW only back and down but no side-to-side

with respect to the ZnO NWs on carbon fiber. Types

I and II in Fig. S4 are the dominant contact types

and the piezopotential was generated in the same

direction by the external force. When the NG is

pressed by external force, a piezoelectric field is

created in the ZnO NW due to polarization of ions

in the crystal. The compressed side of the ZnO NWs

at the Au electrode has a negative potential (V-),

and its stretched side electrode has a positive

potential (V+).

11

Figure 7. Proposed mechanism for the generation of current in NGs, Schematic diagram (top) of contact between ZnO NWs and the Au/ZnO NW electrode and corresponding energy band diagram (bottom). (a) A strain-free ZnO has two Schottky contacts with a metal electrode on the outer surface (Au) and a carbon fiber at the core, Ф1 and Ф2 are the SBHs at the two contacts, respectively. (b) When the NG is subjected to an external force, the conduction band and the Fermi level of Au electrode side are raised by △Фp with respect to the C electrode. (c) A constant force is applied to the NG and the system reaches an equilibrium. The electrons flow from Au to C results in a shift of the Fermi levels by △Ф. (d) When the external force applied to the FNG released, the piezopotential disappears and there is a relative drop in Fermi level by △Фp which drives the electrons flow back to reach equilibrium.

In our study, the negative side (V-) forms at the

surface Au electrode side, as shown in Fig. 7(b). The

negative piezopotential gives a rise in the

conduction band and the Fermi level of the Au

electrode side, which is raised by Фp with respect

to the C electrode [13, 16]. Electrons will flow from

the right-hand side Au electrode to the left-hand

side C electrode through an external load, showing

a sharp peak in the measured current. Because of

the Schottky barrier on the left-hand side, these

electrons are accumulated around the interfacial

region between the carbon fiber electrode and the

NW until the Fermi levels of the two electrodes

reach a new equilibrium. The new Schottky barrier

of Au electrode side is Ф2’’= Ф2’+ Ф, as shown in

Fig. 7(c). When the external press force is released,

the piezoelectric potential inside the ZnO NW

disappear immediately. The Fermi level of the

right-hand electrode drops Фp and the electrons

flow back from the left-hand electrode through the

external circuit to the right-hand electrode (shown

in Fig. 7(d)). The electrons accumulated at the

left-hand C electrode flow back via the external

circuit, and a current peak in the opposite direction

is created. The process ends when the system

returns to its original state. The role of the Schottky

barrier is to prevent those mobile charges from

passing through the metal-semiconductor interface

12

[5, 16, 32]. A cycled strain induced in the ZnO NW

by alternating the externally applied pressure

results in an AC output. When the Fermi levels of

the two sides reach equilibrium again, the

generating process ends. This is the whole working

principle of one time AC output. When the cycled

strain applied onto the NG, continuous AC outputs

generate and power other nanodevices.

4 Conclusions

In summary, we have demonstrated a fiber-based

hybrid NG consisting of ZnO NWs/carbon fibers

and Au coated ZnO-papers, which can be used in

converting mechanical energy into electricity. By

utilizing the foldable paper as the electrode, the

effective working ZnO NWs of the NG can be

increased greatly and the corresponding current

density reached to 0.09 μAcm-2. The output energy

of the designed NG was enhanced greatly

compared to that of previous micro-fiber based

NGs. The electric output of the NG can be

improved by increasing the number of the fibers.

Compared with the single-fiber based NG, the

output current of the multi-fiber based NG made of

200 carbon fibers increased by 100 times. An output

current with a peak value of 35 nA was generated.

The desired electric output can be obtained by

adjusting the external strain rate and the amount of

devices connected in parallel or in series. The

electric energy generated by the multi-fiber based

NG was stored and used to light up a LED. The

reported fiber-based NG can be applied for

implantable devices, self-powered nano/micro

devices, and smart wearable systems. This work

provides a new method to fabricate flexible,

foldable, and adjustable power sources in any shape,

even in textile structures and clothes.

Acknowledgements

This work was supported by the National Major

Research Program of China (2013CB932602), the

Major Project of International Cooperation and

Exchanges (2012DFA50990), NSFC (51172022,

51232001, and 51372020), the Fundamental Research

Funds for Central Universities, Program for New

Century Excellent Talents in University, Beijing

Higher Education Young Elite Teacher Project, the

Programme of Introducing Talents of Discipline to

Universities, and Program for Changjiang Scholars

and Innovative Research Team in University.

Electronic Supplementary Material: Supplementary

material ( (1) SEM images of ZnO NWs/carbon fiber

structure and ZnO-coated paper, (2) voltage outputs

of a multi-fiber based NG with different strain rates

under a constant applied strain, (3) a typical I–V

characteristic of a working multi-fiber based NG, (4)

the interface and possible contact types between ZnO

NWs and the Au/ZnO NW electrode. (5) sheet

resistance measurement of the Au-coated ZnO-paper)

is available in the online version of this article at

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

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