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Enhancing the Output Charge Density of TENG via Building Longitudinal Paths of Electrostatic Charges in the contacting Layers Meihui Lai, Bolun Du, Hengyu Guo, Yi Xi, Huake Yang, Chenguo Hu, Jie Wang, and Zhong Lin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15238 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Applied Materials & Interfaces

3

Enhancing the Output Charge Density of TENG via Building Longitudinal Paths of Electrostatic Charges in the contacting Layers

4

Meihui Lai†, Bolun Du†, Hengyu Guo†, §, Yi Xi†, §, Huake Yang†, Chenguo Hu†, Jie

5

Wang*§, Zhong Lin Wang*§,

6

†

7

§

8

Beijing 100083, China

9

‖

1 2

‖

Department of Applied Physics Chongqing University Chongqing 400044, China Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences

School of Materials Science and Engineering, Georgia Institute of Technology

10

Atlanta, GA 30332-0245, USA

11

Key words: triboelectric nanogenerator (TENG), Tunnel effect, Longitudinal path,

12

Polydimethylsiloxane (PDMS), Inner space charges, current calculation

13

Abstract



Corresponding author.

Tel: +86 23 65678362, Fax: +86 23 65678362, E-mail: [email protected] (Y Xi) Tel: +86 1082854890, Fax: +86 1082854890, E-mail: [email protected] (J Wang) Tel: +86 1082854890, Fax:+86 1082854890, E-mail: [email protected] (ZL Wang) 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Surface charge density of tribo-layer is the most key-point parameter for developing

2

high performance triboelectric nanogenerator (TENG). Most of the previous works

3

were focus on the surface structural/chemical modification, nevertheless the internal

4

space of the tribo-layer and its mechanism exploration were less investigated. Herein,

5

in this paper, internal space charge zones are built through imbedding ravines and

6

gullies criss-cross gold layers in near-surface of tribo-layer, which leads to the high

7

output performance of TENG. As experimental results manifest, the transfer charge

8

density of gold-PDMS TENG (G-TENG) reaches to 168 μC m−2 . Through theoretical

9

analyses, gold layers act as the passageways and traps of the triboelectric charges when

10

drifting to the internal space of tribo-material. Moreover, the transport and storage

11

process of triboelectric charges in the frictional layer are investigated comprehensively

12

by quantum mechanics for the first time. The calculation method of output current of

13

TENG is proposed and the theoretical calculation results coincide with the test results

14

well. The results verify the application of the theoretical model and help with the

15

construction and development of theoretical system of TENG. Meanwhile, the relative

16

results can be directly attained by this new theoretical model and it is possible to make

17

full use of the theoretical analysis to achieve better performance of TENG. This study

18

paves an easy and novel way for enhancing the charge density of tribo-layer by internal

19

space construction and new underlying theoretical model.

20

1. Introduction

2 ACS Paragon Plus Environment

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Recently, the triboelectric nanogenerator (TENG) based on the coupling of electrostatic

2

induction and triboelectric effect has flourished for harvesting mechanical energy from

3

ambient environment 1. Since TENG can directly translate mechanical energy into

4

electrical energy and power the functional electronics instantly

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have been demonstrated for effectively harvesting mechanical energy around our living

6

conditions, for example, human motion

7

the developments of varied sensor devices based on triboelectric nanogenerators

8

(TENGs) arise

9

interaction

18-19

2, 13-14

5-7

2-4

, many approaches

, sea wave 8-9, wind 8, 10-12, and so on. With

, such as biomedicine

15-16

, electronic skin

17

and man-machine

, there is great expectation to enhance the output charge density of

10

biological-friendly TENGs 20. With the theoretical foundation perfecting increasingly

11

21-25

12

proceeding from theory.

13

Polydimethylsiloxane (PDMS, [Si(CH3)2O]n ), as a polymer electret, due to its

14

attractive properties such as plasticity, transparency, flexibility, high electronegativity

15

and biocompatibility 26, has been an outstanding candidate material for TENG 27. Most

16

of the previous works were focus on the surface structural/chemical modification 28-36,

17

however, the internal space of the tribo-layer was less investigated.

18

In this work, through imbedding ravines and gullies criss-cross gold layers in near-

19

surface of tribo-layer, the passageways of triboelectric charges drifting in the negative

20

frictional layer are built, which contributes to the forming of the internal space charge

21

zones in near-surface of negative frictional layer. In addition, the reason for choosing

, the output performance can be improved fundamentally with the help of



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37

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gold instead of other metals is that gold owns biological compatibility

. For the

2

existence of the internal-space-charge zones, the transfer charge density of gold-PDMS

3

TENG (G-TENG) reaches to 16.8 nC cm−2 , which is nearly 4 times the value of the

4

TENG based on pure PDMS, and the power density is up to 1 W m−2 at the load

5

resistance of ~8 MΩ. Through theoretical analyses, gold layers act as the passageways

6

and traps of the triboelectric charges when drifting to the internal space of tribo-material.

7

Moreover, the transport and storage process of triboelectric charges in the frictional

8

layer are investigated comprehensively. And the tunneling effect could exist inside

9

negative frictional layer, and the mechanism is analyzed by quantum mechanics for the

10

first time. Furthermore, the calculation method of output current of TENG is proposed,

11

and the theoretical calculation results coincide with the test results well. Meanwhile,

12

the relative results can be directly repeated by this new theoretical model. This study

13

paves a simple and novel way for enhancing the charge density of tribo-layer by internal

14

space construction.

15

2. Experimental section

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2.1. Fabrication of G-TENG devices

17

The fabrication process has been described in Fig. S1 in the Supporting Information

18

(SI). First, in order to fabricate the negative friction layer, a part of the TENG, a piece

19

of acrylic with side length of 4 cm was prepared as the substrate. Then the

20

corresponding size aluminum (Al) foil was adhered on the surface of this substrate with

21

doublesided kapton tape. After that, the PDMS solution (Sylgard 184, Dow Corning), 4 ACS Paragon Plus Environment

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which has been prepared through mixing elastomer and curing agents at the weight

2

percentage of 10/1 (w/w), was spin-coated on the surface of the aluminum foil. By

3

adjusting the spin speed of coating from 1000 rpm to 5000 rpm for 1 min, the thickness

4

of the friction layer can be controlled easily. Then, it was heated at 60 °C for 2 h on a

5

hot plate to solidify. Next, in order to obtain the gold@PDMS film, gold was coated

6

with magnetron sputtering deposition on the surface of PDMS@Al, and the time of

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magnetron sputtering deposition is within the range of 10 s to 60 s. Subsequently, the

8

gold@PDMS film was treated with plasma. Where after, gold PDMS composite film

9

(GPCF) as the negative friction layer formed after the last spin-coating of PDMS. To

10

enhance the interfacial effect between the PDMS and aluminum electrode/gold, the

11

PDMS@Al (PDMS@gold) has been put inside a vacuum chamber for 1 h after each

12

spin-coating process. A structure schematic of GPCF is shown in Fig. 1(a) and the field

13

emission scanning electron microscopy (FE-SEM) pictures of gold layer after plasma

14

treatment and the cross section of GPCF are presented in Fig. 1(b) and Fig. 1(c),

15

respectively. The photograph of GPCF is shown in the Fig. 1(d), which exhibits the

16

flexibility and transparency of it.

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To facilitate the comparison, all the different negative frictional layers share a single

18

positive friction layer part, which is composed of an acrylic substrate with a piece of 1

19

cm-side-length aluminum foil affixed to it. Finally, the negative friction layer and the

20

top positive friction layer are fabricated to be the G-TENG.

21

2.2. Characterization and Measurements 5 ACS Paragon Plus Environment


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The morphology and structure of the Gold-PDMS films were characterized by a field

2

emission scanning electron microscope (FE-SEM ， MIRA3 TESCAN). Relative

3

permittivity characterizations of gold PDMS composite films (GPCFs) were performed

4

by the broadband dielectric spectrometer (Germany NOVOCONTROL, Concept 40).

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A liner motor (42HBS48BJ4-TR0) was employed to render the negative friction layer

6

and the positive friction of G-TENG periodic contact separation, at a frequency of 2 Hz,

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with the model of simple harmonic vibration, to get the output signals. The output

8

signals generated from the TENG devices were gathered by the Data Acquisition Card

9

(NI PCI-6259). A Stanford low noise current preamplifier (model SR570) and a

10

Keithley voltage preamplifier (model 6514) were used to measure the output current

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and transfer charge (voltage) respectively.

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3. Results and discussion

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3.1Working Mechanism of TENG

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The contact-separation mode, a very basic and widely-used model of TENG, is

15

employed in this work for investigating the transfer charge of G-TENG. Figure 2(a-c)

16

schematically presents the basic working mechanism of the contact-separation mode of

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TENG which owns one dielectric with permittivity of 𝜀 and thickness of 𝑑, with the

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increase of the contact-separation cycles under the vertical compressive force. Once the

19

dielectric is driven to be in physical contact (Fig. 2(a)), charges are transferred to the

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surfaces of the dielectric from metal top electrode owing to the contact electrification

21

effect (Fig. 2(b)). Triboelectric charge density of frictional dielectric（𝜎𝑇 ） builds up 6 ACS Paragon Plus Environment

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due to a number of contacts between the dielectric and top electrode (Fig. 2(c)), and

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reaches saturation finally. In addition it is independent of the gap distance (𝑧) between

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the dielectric and top electrode. The electrostatic field built by the triboelectric charges

4

drives electrons to flow through the external load, resulting an accumulation of free

5

electrons in the electrode. The charge density of bottom electrode, σ𝐼 (𝑧, 𝑡)， which is

6

a function of the gap distance 𝑧(𝑡) between the dielectric and top electrode. Because

7

of law of conservation of charge, the charge density of top electrode is 𝜎𝑇 − 𝜎𝐼 (𝑧, 𝑡).

8

As shown in the Fig. 2(c), the electric field strength in the dielectric is 𝐸1 =

9

−𝜎𝐼 (𝑧, 𝑡)/𝜀, which can be obtained from the structure of plane-parallel capacitor, and

10

the electric field strength in the air gap is 𝐸2 = [𝜎𝑇 − 𝜎𝐼 (𝑧, 𝑡)]/𝜀0. The relative voltage

11

drop between the top electrode and the bottom electrode is

12

13 14

𝑉 = 𝐸1 ∙ 𝑑 + 𝐸2 ∙ 𝑧(𝑡) =

𝜎𝑇 −𝜎𝐼 (𝑧,𝑡) 𝜀0

𝑧(𝑡) −

𝜎𝐼 (𝑧,𝑡) 𝜀

𝑑

(1)

Under short-circuit condition, 𝑉 = 0, 𝜎 ∙𝜀∙𝑧(𝑡)

𝑇 𝜎𝐼 = 𝑧(𝑡)𝜀+𝑑𝜀

(2)

0

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This equation means that the transfer charge density is correlative to the charge

16

density on the dielectric surface and the permittivity of dielectric.

17

According to the previous work 21, the external current of TENG is Maxwell’s

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displacement current density which is

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𝐽𝐷 =

𝜕𝜎𝐼 𝜕𝑡

𝑑𝜀 𝜀

0 = 𝜎𝑇 𝑧 ′ (𝑡) [𝑧(𝑡)𝜀+𝑑𝜀

0]

2

(3) 7

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This equation means that the transfer charge is proportional to the charge density on the

2

dielectric surface, the permittivity of dielectric and the speed of top electrode.

3

Supposing the surface area of the dielectric is S, the corresponding current can be

4

attained,

5

𝐼 = 𝐽𝐷 𝑆 = 𝑆

𝜕𝜎𝐼 𝜕𝑡

𝑑𝜀 𝜀

0 = 𝑆𝜎𝑇 𝑧 ′ (𝑡) [𝑧(𝑡)𝜀+𝑑𝜀

0]

2

(4)

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In this work, the top electrode is driven via a liner motor, which is in situation of

7

simple harmonic vibration, and the distance 𝑧(𝑡) is

8 9 10 11

12

13 14 15 16

(5)

𝑧(𝑡) = 𝐴 − 𝐴𝑠𝑖𝑛(2𝜋𝑓𝑡) where the amplitude 𝐴 is set as 0.03 m and the frequency 𝑓 is 2 Hz. So the current can be rewritten as

𝐼 = −2𝜋𝑓𝑆𝐴𝜎𝑇 𝑐𝑜𝑠(2𝜋𝑓𝑡) {[𝐴−𝐴

𝑑𝜀0 𝜀 𝑠𝑖𝑛(2𝜋𝑓𝑡)]𝜀+𝑑𝜀0 }2

（6）

The tested current 𝐼𝑡𝑒𝑠𝑡 can be matched with the peak value of 𝐼 in Eq. (6), i.e. 𝐼𝑡𝑒𝑠𝑡 = 𝐼𝑚𝑎𝑥 = 𝐼(𝑡0 )

（7）

where 𝑡0 can be attained through solving the equation 𝑑𝐼 𝑑𝑡

（8）

=0

3.2 The influence of gold magnetron sputtering for captive structure of G-TENG


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From Eq. (2), it is obvious that the permittivity of dielectric and the charge producing

2

via triboelectrification on the dielectric play important roles in the transfer of charges.

3

Hence, the permittivity of dielectric friction layer in G-TENG is taken into

4

consideration. As we known, the thickness of gold layer is proportional to the gold

5

magnetron sputtering time. The transfer charge densities of gold-PDMS TENGs (G-

6

TENGs) with different gold magnetron sputtering time are shown in the Fig. 2(d), and

7

the illustration is the digital photograph of pure PDMS film and GPCFs with different

8

gold sputtering time. The transfer charge density of pure PDMS TENG is 5.3 nC cm−2,

9

and with the increase of sputtering time from 10 s to 60 s, the transfer charge densities

10

of G-TENGs keep around 10 nC cm−2 , which means not the thickness but the existence

11

of gold influences the transfer charge density. With the increase of gold magnetron

12

sputtering time, the thickness of gold layer also increases, but the further increase of

13

thickness (gold magnetron sputtering time) doesn’t contribute to the increase of transfer

14

charge density of G-TENGs. The same conclusion obtained in the capacitive structure

15

analysis. The experimental tests of relative permittivity of GPCFs with different

16

sputtering time are given in the Fig. 2(e), and more details can be found in Fig. S2. The

17

capacitive dielectric structure model of G-TENG is shown in the inset of Fig. 2(f), the

18

relative effective permittivity of GPCF 𝜀 is

19

𝜀 = 𝜀𝑃𝐷𝑀𝑆 + 𝑑

𝑑2

1 +𝑑3

𝜀𝑃𝐷𝑀𝑆

(9)

20

where 𝜀𝑃𝐷𝑀𝑆 is the relative permittivity of PDMS, 𝑑1 is the thickness of PDMS films

21

upon the magnetron sputtering gold layer (MSGL), 𝑑2 is the thickness of MSGL and 9 ACS Paragon Plus Environment


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𝑑3 is the thickness of PDMS films beneath MSGL. The theoretical derivation of 𝜀 is

2

presented in S3 in SI. As shown in the Fig. 2(f), the experimental data is consistent with

3

the theoretical results, and the variation of relative permittivity of GPCF is tiny. The

4

comparisons of the current in theory (𝐼𝑚𝑎𝑥 ) and the current in the experiments (𝐼𝑡𝑒𝑠𝑡 ) of

5

pure-PDMS TENG in different working duration are shown in the Table 1. The

6

theoretical calculation results coincide with the test results well, which verify the

7

application of the theoretical model. From the analysis above combined with Eq. (2),

8

we conclude that only 𝜎𝑇 has apparent influence on 𝜎𝐼 , so it is supposed to enlarge

9

the 𝜎𝑇 so as to achieve a larger 𝜎𝐼

10

3.3 Single gold layer

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The triboelectrification process can be further divided into three sub-processes: the

12

generation of the triboelectric charges, the dynamic motion of the triboelectric charges

13

in the frictional layer and the electrostatic induction of charges 22. There is a common

14

viewpoint about the generation of the triboelectric charges that extra mechanical energy,

15

coming from the process of the friction, converts to heat energy which stimulates the

16

bound state charged particles to overcome the work function and form free charges,

17

then these free charges redistribute according to the fermi level of the friction surface

18

38-39

19

(GPCF, the cyan part) capture electrons from positive friction layer (top aluminum, the

20

violet part) during the triboelectrification process. On account of the accumulation of

21

electrons on the frictional surface of GPCF, an electric field comes into being between

22

the frictional surface of GPCF and the bottom electrode, and the electric field direction

. As for the storage of the charges, shown in the Fig. 3(a), the negative friction layer


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1

is vertically upward. Hence, there are two electron transfer modes, one of which is a

2

diffusion process caused by the concentration gradient of electrodes and another is a

3

drift process caused by the electric field. In addition, since the actual electric field

4

intensity is large enough, the diffusion process can be neglected in principle 22. So the

5

drift process caused by the electric field is the main dynamic motion for triboelectric

6

charges in the frictional layer, and the drift process direction of triboelectric charges is

7

vertically downward. The z direction distribution of triboelectric charges density

8

decrease from top to bottom. Drift process of triboelectric charges decreases the charge

9

density of frictional layer surface, which contributes to the further accumulation of

10

triboelectric charges on the frictional layer surface. By reason of electrostatic induction

11

and charge conservation, with the movement of top electrode, there will be an alternate

12

electron flow in the external circuit between top electrode and bottom electrode. As Eq.

13

(2) shows, the quantity of flowing electrons in the external circuit is proportional to the

14

triboelectric charges in negative frictional layer. Therefore, increasing the amount of

15

triboelectric charges in the negative frictional layer is an extremely effective approach

16

to enhance the output performance of TENG.

17

As shown in the Fig. 3(b), when the triboelectric charges arrive in the interface between

18

PDMS and gold, owing to the strong electric field, the triboelectric charges will tunnel

19

from the PDMS to the gold. Then, the internal space charges zone forms. The energy

20

band diagram of PDMS and gold without electric field in real space is shown in Fig.

21

3(d-i). Thanks to the strong electric field (Fig. 3(d-ii)), the energy band will sloppe (Fig.

22

3(d-iii)), and the triboelectric charges will tunnel from PDMS to gold, and gold can trap 11 ACS Paragon Plus Environment


Page 12 of 33

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electrons more easily 40. The probability of tunnel (P) attained with the potential barrier

2

model in quantum mechanics is 41

3

𝑃 ∝ 𝐸𝑒𝑥𝑝 [−

𝜋2 ℏ

1

𝐸𝑔

(2𝑚𝐸𝑔 )2 (𝑞𝐸)]

(10)

4

Where 𝐸 is electric field strength in the interface between PDMS and gold, ℏ is the

5

reduced Plank constant, and 𝐸𝑔 is the height of potential barrier, i.e. the width of band

6

gap. Equation (10) exposes that with the increase of electric field, the probability of

7

tunnel increases apparently. It is obvious that the key point of probability of tunnel is

8

the electric field strength 𝐸. If 𝐸 in the interface between PDMS and gold vanishes,

9

the probability of tunnel will vanish too. So the limitation of the quantity of charges

10

that are stockpiled in the gold layer (the internal space charges zone) is equal to the

11

quantity of triboelectric charges in the PDMS upon gold layer. The FE-SEM picture of

12

gold layer is shown in the Fig. 3(c), it is clear that there are many ravines caused by the

13

plasma treatment in the gold layer, and the average width of ravines is about 15 nm.

14

The FE-SEM pictures of non-plasma treatment are provided in Fig. S4. Because of the

15

existence of ravines in the gold layer, there are many embossments similar to mountain

16

chain forming via PDMS, which is shown in Fig. 3(b). For a charged object, the charge

17

surface density is inversely proportional to the radius of curvature, in other words,

18

proportional to the curvature. The empaistic PDMS own lager curvature than flat PDMS,

19

so electrons will gather around empaistic PDMS. Many charges assemble in the

20

embossments, rendering the increase of local electric field strength, which leads to the

21

increase of the probability of tunnel. To verify the above mentioned suppose, G-TENGs 12 ACS Paragon Plus Environment

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1

with different gold magnetron sputtering time, different PDMS thickness upon gold

2

layers were fabricated respectively, and these G-TENGs were set to work for 30 min

3

under the same conditions. The transfer charge densities of G-TENGs are shown in the

4

Fig. 3(e), in which the total thickness of each GPCF is around 120 μm (Fig. S5), and

5

more details are provided in Fig. S6. The transfer charge densities of G-TENGs without

6

gold magnetron sputtering treatment are all 5.3 nC cm−2 , which is similar to the

7

previous experimental results. Consequently, the transfer charge densities of G-TENGs

8

are independent of the duration of gold magnetron sputtering. The transfer charge

9

densities of G-TENGs with plasma treatments, where the thickness of PDMS upon gold

10

layer is 10 μm, are around 9.8 nC cm−2 , which is close to double that of pure PDMS

11

TENGs’. The transfer charge densities of G-TENGs with plasma treatments, where the

12

thickness of PDMS upon gold layer is 20 μm, are around 9.5 nC cm−2 , which are

13

larger than 8.7 nC cm−2 and 6.4 nC cm−2 corresponding to the transfer charge

14

densities of G-TENGs without plasma treatments, where the thickness of PDMS upon

15

gold layer is 10 μm and 20 μm, respectively. According to the output performance, it

16

is conspicuous that the plasma treatments and the thickness of PDMS upon gold layer

17

are influential in the transfer charge densities of G-TENGs. The plasma treatment can

18

significantly enhance the transfer charge density, and thinner the PDMS upon gold layer

19

is, larger the transfer charge density will be. The plasma treatment leads to the increase

20

of local electric field strength in the interface between PDMS and gold, as a result of

21

which, the probability of tunnel rapidly increases. Meanwhile, the smaller thickness of

22

PDMS upon gold layer means the smaller drift depth of triboelectric charges, which 13 ACS Paragon Plus Environment


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1

leads to the increase of the quantity of triboelectric charges in the interface between

2

PDMS and gold. Thanks to the whole factors above, the quantity of charges in the

3

GPCF increases apparently. Note that the quantities of charges in the gold layer and

4

PDMS layer are identical.

5

3.4 Multi gold layers

6

To further investigate the triboelectric charges in GPCFs, the G-TENGs with multilayer

7

gold are fabricated. The transfer charge densities of G-TENGs are presented in the Fig.

8

4(a), and the FE-SEM insets clearly exhibit the number of gold layers. The transfer

9

charge densities are about 16.8 nC cm−2 , 16.7 nC cm−2 , 14.5 nC cm−2 , 9.7 nC cm−2

10

and 5.3 nC cm−2 for the number of gold layers from 4 to 0, respectively. With the

11

increase of number of gold layers, the transfer charge density increases and tends to be

12

steady around 16.8 nC cm−2 with the number of gold layers reaching 3. It means that

13

the charges can hardly further drift for being restricted by the depth of drift and

14

depressed by product of the probabilities of tunnel in each interface. As a result, there

15

is a limitation that the quantity of charges can reach. The probability that triboelectric

16

charges reach to the n-th gold layer (𝑃𝑛 ) can be expressed as 𝑃𝑛 = ∏𝑖=𝑛 𝑖=1 𝑃𝑖

17

(11)

18

where 𝑃𝑖 can be obtained from Eq. (10). Equation (11) indicates that the probability

19

that triboelectric charges reach to the nth gold layer decreases rapidly with the increase

20

of n.


Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

The phenomena observed in the experiments （Fig. 4(b), Fig. 5）are consistent with

2

theoretical analysis presented in the previous part:

3

(1) Though the behavior of transfer charges density of each G-TENG is not

4

synchronous, the transfer charges densities will reach their upper limit and tend to be

5

steady after sufficient duration. It should be emphasized that the more gold layers

6

there are, the more time is required to reach the upper limit

7

(2) The upper limit of transfer charges density of each G-TENG increases with the

8

number of gold layers, but it will be more and more slowly.

9

The triboelectric potential distributions of the G-TENGs are calculated by a finite

10

element analysis (FEA) method using COMSOL software in the scene that there are

11

equivalent charges in the gold layer and on the PDMS surface, as shown in the insets

12

of Fig. 4(b). The result of FEA matches the test result very well, which proves the idea

13

that there are surely charges in the layer again. The details of FEA process can be found

14

in Fig. S7.

15

Figure 6(a) displays the output voltage and current of 4-layer G-TENG as a function of

16

load resistance from 0 Ω to 200 MΩ. With the increase of the connected external

17

resistance, the output voltage increases while the output current decreases. The

18

instantaneous power outputs of the 4-layer G-TENG were characterized by the product

19

of the loads voltage and current measured of the resistors, the maximal electrical power

20

density is about 1 W m-2 at the load resistance of ~8 MΩ (Fig. 6(b)). The low impedance



Page 16 of 33

1

means a broad application of G-TENG. As shown in the Fig. 6(c), the 4-layer G-TENG

2

has been assembled to charge the commercial condenser.

3

The 4-layer G-TENG has been assembled into a self-powered electronic watch shown

4

in the Fig. 6(d), which converts mechanical energy from human beings to electric

5

energy to support the running of the electronic watch. And the charging management

6

circuit of the self-powered electronic watch is shown in the inset of Fig. 6(d). In addition,

7

the electric energy from the 4-layer G-TENG can also be used to light commercial light

8

emitting diodes (LEDs) without any extra power source (as shown in the inset of Fig.

9

6(d)), the circuit diagram of LEDs is shown in the Fig. S8. More details can be found

10

in the Video 1 and 2 respectively.

11

4. Conclusion

12

In summary, with the guidance of theory, a new G-TENG has been realized, which

13

takes high transfer charge density, biocompatibility and low impedance into

14

consideration simultaneously. In this paper, internal space charge zones built through

15

imbedding ravines and gullies criss-cross gold layers in near-surface of tribo-layer,

16

which leads to the high output performance of TENG. Through theoretical analyses,

17

gold layers act as the passageways and traps of the triboelectric charges when drifting

18

to the internal space of tribo-material. As experimental results shows, the transfer

19

charge density of G-TENG reaches to 168 μC m−2 , which is nearly 4 times the value

20

of the TENG based on pure PDMS. Moreover, the transport and storage process of

21

triboelectric charges in the frictional layer are investigated comprehensively for the first 16 ACS Paragon Plus Environment

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

time. Furthermore, the calculation method of output current of TENG is proposed, and

2

the theoretical calculation results coincide with the test results well. The results verify

3

the application of the theoretical model and help with the construction and development

4

of theoretical system of TENG. Meanwhile, the relative results can be directly attained

5

by this new theoretical model and it is possible to make full use of theoretical analysis

6

to achieve better performance of TENG. This study paves an easy and novel way for

7

enhancing the charge density of tribo-layer by internal space construction and new basic

8

theoretical model.

9

Acknowledgments

10

M.L. and B.D. contributed equally to this work. This work is supported by the NSFC

11

(51772036), NSFCQ (cstc2014jcyjA50030), the graduate scientific research and

12

innovation foundation of Chongqing, China (Grant NO. CYB17044), NSFC (51572040,

13

51402112), the Development Program (“863” Program) of China (2015AA034801),

14

the Fundamental Research Funds for the Central Universities (CQDXWL-2014-001,

15

CQDXWL-2013-012, 106112015CDJXY300004 and 106112017CDJXY300004),

16

National Key Research and Development Programs - Intergovernmental International

17

Cooperation

18

2016YFE0111500), the large-scale equipment sharing fund of Chongqing University.

19

ASSOCIATED CONTENT

20

Supporting information

in

Science

and

Technology Innovation

Project

(Grant

No.:



Page 18 of 33

1

The Supporting Information is available free of charge on the ACS Publications website

2

at DOI: XXXXXXX.

3

The fabrication process of G-TENGs, the test results of relative permittivity of GPCFs,

4

the theoretical derivation of the relative permittivity of GPCF, the FE-SEM pictures of

5

gold layers, FE-SEM cross-section pictures of GPCFs, the transfer charge density of G-

6

TENGs, the details of FEA process, the circuit diagram of G-TENG lights LEDs (PDF).

7

Video 1 and video 2 which demonstrate the work of self-power electronic watch and

8

G-TENG light LEDs, respectively (ZIP)

9

AUTHOR INFORMATION

10

*Corresponding Author

11

Tel: +86 23 65678362, Fax: +86 23 65678362, E-mail: [email protected] (Y Xi)

12

Tel: +86 1082854890, Fax: +86 1082854890, E-mail: [email protected] (J Wang)

13

Tel: +86 1082854890, Fax: +86 1082854890, E-mail: [email protected] (ZL Wang)

14

Notes

15

The authors declare no competing financial interest.

16


Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Page 26 of 33

1

Figure 1. (a) The schematic illustration of GPCF. (b) The FE-SEM picture of the

2

surface of gold layer which has been treated with plasma. (c) The FE-SEM cross-section

3

diagram of GPCF. (d) The digital image of GPCF.

4


Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Figure 2. (a-c) The working mechanism of contact-separation TENG. (d) The transfer

2

charge densities of G-TENGs with different gold magnetron sputtering time. (e) The

3

experimental test of relative permittivity of GPCFs with different sputtering time. (f)

4

The results of theoretical calculations and the experimental tests of relative

5

permittivity of GPCFs with different gold magnetron sputtering time.

6



Page 28 of 33

1

Figure 3. (a) Illustrations of electrons’ drift in G-TENG. (b) The schematic diagram of

2

electrons’ escape from PDMS to the gold. (c) The FE-SEM picture of surface of gold

3

after plasma treatment.

4

(d-i) The energy band of PDMS and gold without electric field in real space; (d-ii) the

5

electrostatic potential energy of electron; (d-iii) the slant energy band of PDMS and

6

gold in real space. (e) The charge transfer densities of plasma/non plasma G-TENGs

7

with different thickness of PDMS upon the gold layer.

(d) The energy band diagram of PDMS and gold in real space.

8


Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Figure 4. (a) The transfer charge density of G-TENGs with multilayer gold. (b) The

2

transfer charge densities of G-TENGs in different working duration. The insets are the

3

potential simulation diagrams of pure PDMS TENG and one-layer G-TENG.

4



Page 30 of 33

1

Figure 5. (a) The transfer charge density of pure PDMS TENG in different working

2

duration. (b) The transfer charge density of one-layer G-TENG in different working

3

duration. (c) The transfer charge density of two-layer G-TENG in different working

4

duration. (d) The transfer charge density of three-layer G-TENG in different working

5

duration.

6


Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Figure 6. (a) The current density and voltage under the outer variable resistance from

2

0 Ω to 200 MΩ. (b) The relationship between the instantaneous power outputs and load

3

resistance. And the effective power density harvested to 1 W m-2 at a load resistance of

4

8 MΩ. (c) Charging voltage as a function of the charging time for various capacitances.

5

(d) The self-powered electronic watch powered by G-TENG. The insets are the

6

charging management circuit system applied of self-powered watch and LEDs being lit

7

directly by G-TENG.

8 9



Page 32 of 33

1

Table 1. The comparisons of the current in theory (𝐼𝑚𝑎𝑥 ) and the current in the

2

experiments (𝐼𝑡𝑒𝑠𝑡 ) 𝐀 (cm2)

𝐭 𝟎 (𝐬)

𝐈𝐦𝐚𝐱 (𝛍𝐀)

2.70

1.00

60.12

0.65

0.76

14.47

2.70

1.00

300.12

0.86

1.01

15.15

2.70

1.00

600.12

0.93

1.10

15.45

2.70

1.00

900.12

0.95

1.12

15.17

𝛆𝐫

𝐈𝐭𝐞𝐬𝐭 (𝛍𝐀)

Relative standard deviation (%)

3

εr is the relative permittivity of pure PDMS film, S is the area of top electrode, S is

4

the contact area of top electrode and dielectric, t 0 is the working duration of pure PDMS

5

TENG, 𝐼𝑚𝑎𝑥 is the output current in theory, and 𝐼𝑡𝑒𝑠𝑡 is the output current in

6

experiment.

7


Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

TOC

2 3

Internal space charge zones built through imbedding ravines and gullies criss-cross gold

4

layers in near-surface of tribo-layer, which leads to the high output performance of

5

TENG. Moreover, the transport, storage process of triboelectric charges in the frictional

6

layer are investigated comprehensively by quantum mechanics for the first time.


Enhancing the Output Charge Density of TENG via ... - ACS Publications

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