Breath Figure Micromolding Approach for Regulating the

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Breath Figure Micromolding Approach for Regulating the Microstructures of Polymeric Films for Triboelectric Nanogenerators Jianliang Gong, Bingang Xu, and Xiaoming Tao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14729 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Breath Figure Micromolding Approach for Regulating the Microstructures of Polymeric Films for Triboelectric Nanogenerators

Jianliang Gong, Bingang Xu,* Xiaoming Tao

Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China *To whom correspondence should be addressed. E-mail: [email protected]; Tel: +852-2766 4544

ACKNOWLEDGEMENTS The authors acknowledge The Hong Kong Polytechnic University for funding supports of this work.

Gong Jianliang would also like to thank The Hong Kong

Polytechnic University for providing him with a postgraduate scholarship.

KEYWORDS: triboelectric nanogenerators, breath figure molds, micro lens arrays, adjustable surface microstructures, mechanical energy, human body motion.

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Graphical Abstract A simple and efficient breath figure micromolding approach has been developed to rapidly regulate the microstructures of polydimethylsiloxane films for the assembly of triboelectric nanogenerators to harvest mechanical energy.


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Abstract Triboelectric nanogenerator (TENG) is an innovative kind of energy harvester recently developed based on organic materials for converting mechanical energy into electricity through the conjunction use of triboelectric effect and electrostatic induction.

Polymeric materials and their microstructures play key roles in the

generation, accumulation, and retainment of triboelectric charges, which decisively determines the final electric performance of TENGs.

Herein we report a simple and

efficient breath figure micromolding approach to rapidly regulate the surface microstructures of polymeric films for the assembly of TENGs.

Honeycomb porous

films (HPFs) with adjustable pore size and dimensional architectures were firstly prepared by the BF technique through simply adjusting the concentration of polymer solution.

They were then used as negative molds for straightforward synthesis of

polydimethylsiloxane (PDMS) films with different micro lens arrays (MLAs) and lens sizes, which were further assembled for TENGs to investigate the influence of film microstructures.

All MLA-based TENGs were found to have an obviously enhanced

electric performance in comparison with flat PDMS film-based TENG. Specifically, up to three times of improvement in the electric performance can be achieved by the MLA-based TENG with optimal surface microstructures over flat PDMS film-based TENG under the same triggering conditions.

It was further successfully used to

harvest the waste mechanical energy generated by different human body motions, including the finger tapping, hand clapping and walking with a frequency ranging from 0.5 Hz to 5.5 Hz.



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Introduction

With the popularization of wearable mobile electronics, seeking sustainable portable energy sources for powering these electronics is increasingly urgent and significant to replace conventional energy storage units with limited lifetime.1-3

Currently the

most potential alternatives are energy harvesters that can directly convert ubiquitous ambient solar, mechanical, or thermal energy into electricity, which have been developed based on photovoltaic, piezoelectric, pyroelectric and triboelectric effects.4-8

Among them, an innovative kind of energy harvesters with the

conjunction use of triboelectric effect and electrostatic induction, namely triboelectric nanogenerator (TENG), has recently attracted great attention owing to its easy accessibility, scalability, versatility, and promising electric performance.2, 9-11

TENG is also named organic nanogenerator because its most used materials are polymers.11

Its generation of triboelectric electricity can be simply caused by a

physical process of contact friction and separation between two different materials under an externally applied force.

Polymeric materials play key roles in the

generation, accumulation, and retainment of triboelectric charges, which determines the electric performance of TENGs.2, 9-12

Constructing surface microstructures on

the surface of materials can increase the effective contact area during the friction process, thereby producing more triboelectric charges for further performance enhancement of TENGs.13-15

It is noted that the microstructures of triboelectic

materials for TENGs are often created by using either patterned substrates as reusable templates or removable bricks as sacrificial templates, such as patterned silicon 4 ACS Paragon Plus Environment

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wafers, colloidal crystals and microphase-separated block copolymers.13-15

They are

valuable and possess different advantages, but the former strategy has to undergo painstaking work on engraving desired pattern on rigid template beforehand, while the latter strategy usually requires additional steps of synthesizing and removing the sacrificial bricks.

In other words, these techniques often suffer from the high cost of

lithography facilities or time-consuming multistep procedures.

Moreover, once the

template is chosen, the morphology and size of replica microstructures are generally fixed.

However, to obtain the maximum performance of a TENG, the morphology

of used polymer films usually requires to be adjusted frequently for the optimal microstructure.

If adopting the conventional templating strategy for the

microstructures of polymer films, time-consuming, high-cost and exhaustive fabrication procedures for the fabrication of multitudinous templates cannot be avoided.

Therefore, the development of simple, rapid and efficient approaches for

the microstructures of triboelectric materials is very desired and significant.

Breath Figures (BFs) are figures of water droplets that can be generated by exhaling aqueous vapor onto a cold surface, which are known as annoying fog and dew phenomena in nature.16-18

Through the evaporative cooling of solvent, drying the

polymer solution film on a substrate under humid environment is a rapid method of generating BFs, which can reversely act as a templating role for fabricating the porous microstructures of films.

BFs are actually natural templating medium because of

their nontoxic, easily available and dynamic characteristics.

And this BF templating

technique avoids complicated work on either preparing or removing templates 5 ACS Paragon Plus Environment


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because all the formation, assembly and removal of aqueous droplets are spontaneous. It has been rapidly developed for straightforward fabrication of honeycomb-like films with the pore size ranging from several hundred nanometers to dozens of micrometers based on different polymers and other materials.19-21

Beyond the typical honeycomb

surface feature, the BF technique can also be used for the fabrication of polymer films with tunable pore shapes, symmetrical nano-/micro-particles and asymmetrical microspherical caps.22-25

These one-step assembled polymer films with adjustable

morphology can be either directly used for various applications,18, 26-28 or further act as economical templates for the fabrication of similar/complementary microstructures with other materials by the combination of different techniques, such as soft lithography, cross-linkage, in situ growth, pyrolysis, sputter-coating and reactive ion etching.29-33

Herein, we present a simple and efficient breath figure micromolding approach for rapid preparation of polymer films with tunable surface microstructures to assemble TENGs for harvesting mechanical energy.

Firstly, honeycomb porous films (HPFs)

with adjustable pore size and dimensional architectures are prepared by the BF technique through simply adjusting the concentration of a silicon-containing graft copolymer solution.

The pore arrays of HPFs then act as negative micromolds for

straightforward preparation of PDMS films with micro lens arrays (MLAs) which are further applied for the assembly of TENGs.

The influence of PDMS microstructures

on the electric performance of TENGs is systematically investigated and evaluated based on a vertical contact-separation mode under different triggering conditions. 6 ACS Paragon Plus Environment

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Finally, the most promising TENG is used to harvest the mechanical energy generated by different human body motions.

2

Experimental

2.1 Materials A silicon-containing graft copolymer poly(dimethylsiloxane)-graft-polyacrylates (PDMS-g-PAs) with a number-average molecular weight of ~26,000 was purchased from Sigma-Aldrich, Ltd.

And its molecular weight distribution determined by gel

permeation chromatography (GPC) was 1.88. approximately 80 wt%.

PDMS precursors were purchased from Momentive

Performance Materials Japan LLC. brought from 3M Corp.

Its PDMS composition was

Conductive and flexible Cu/Ni fabric (CNF) was

CHCl3 (anhydrous, ≥99.9%) was also obtained from

Sigma-Aldrich Chemical Company.

All materials were used as received without

further purification unless stated otherwise.

2.2 Preparation of HPFs via BF technique The BF process was operated in a sealed glass bottle with a cap at ambient temperature (23-25 oC).

An approximately saturated humidity (the relative humidity

is 99%) in the vessel was achieved by adding a small amount of distilled water into the bottle beforehand. A piece of glass substrate was adhered onto the top of a plastic stand with a double-sided tape and placed into the glass vessel. was 1 cm higher than the liquid level.

The substrate

Polymer solutions with concentration ranging

from 1.5 to 60 mg mL-1 were prepared by dissolving PDMS-g-PAs in CHCl3.


A


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liquid film was prepared by casting polymer solution onto the substrate with a microsyringe.

With organic solvent volatilization, the transparent solution surface

became turbid.

After complete evaporation of residual solvent and water, an opaque

film was obtained on the substrate.

2.3 Templating synthesis of MLAs with HPFs as templates The as-prepared PDMS-g-PAs HPFs by the BF technique acted as a negative mold. PDMS precursors were homogeneously mixed and then cast on the HPF mold according to the mass-to-surface-area ratio of 0.048 g cm-2 by a doctor-blading method.

Then it was shifted to a vacuum oven for degassing at room temperature to

obtain PDMS liquid film without air bubbles.

After the liquid film becoming

transparent, the temperature was increased to 80 oC for cross-linking reaction, which was held for three hours.

The cross-linked PDMS film can be directly peeled off

from the HPF mold carefully.

The resultant PDMS film was robust and

self-supported, which possesses a convex MLA pattern (See Supporting Information (SI) for details, Fig. S1).

PDMS film with flat surface was prepared on a planar

substrate by the same method.

The obtained film samples possessed the similar

thickness of approximately 510 µm.

2.4 Assembly of PDMS film-based TENGs The assembly of TENGs based on the PDMS films involves the following procedures. Firstly, two polypropylene (PP) plates were used as top and bottom substrates respectively.

Then an aluminum (Al) sheet was adhered to the inner surface of top


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substrate to act as dual roles of top electrode and contact (friction) surface.

And a

conductive adhesive film was adhered to the inner surface of another substrate to act as bottom electrode.

Thirdly, PDMS film was fixed by adhering its smooth surface

to the conductive adhesive, while its surface patterned with MLAs was used as the frictional surface.

Finally, a polyurethane (PU) sponge with good elasticity was used

as insulation spacer and placed between the two substrates for producing reciprocating motion under external forces.

When TENG was used for harvesting

the mechanical energy generated by different human body motions, the Al sheet was replaced by flexible conductive CNF.

The final TENG after packaging possessed the

size of 6 cm x 6 cm x 0.8 cm, which is freestanding, bendable, lightweight (less than 10 g), and portable (Fig. S2).

It can be either placed on desk for harvesting finger

tapping energy, held for harvesting the hand clapping energy, or built inside shoe sole for harvesting the mechanical energy generated by walking. 2.5 Characterization PDMS-g-PAs films and PDMS replicas were coated with a thin layer of gold (around 2 nm) for imaging by a scanning electron microscope (SEM, JEOL Model JSM-6490). A 10 keV electron beam was used for the observation at a working distance of 10 mm. For the cross sections, the PDMS-g-PAs films were broken in liquid nitrogen, while PDMS replicas were the fracture surfaces obtained by rapidly tearing.

Molecular

weight distribution was determined by GPC with a Waters 1515 pump and Waters 1515 differential refractive index detector (set at 30 °C).

It used a series of three

linear Styragel columns (HT2, HT4, and HT5) at an oven temperature of 45 °C. The



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eluent was tetrahydrofuran at a ﬂow rate of 1.0 mL min-1.

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A series of

low-polydispersity polystyrene standards were employed for the GPC calibration. The electric performance of TENGs based on PDMS films with different MLAs was firstly evaluated by utilizing a mechanical compression system at a frequency of 3.3 Hz.

During the compressing process, their open-circuit voltages (Voc) were

monitored by an oscilloscope of 1 MΩ internal resistance (Keisight DSO-X3014A) with a high voltage probe of 8 MΩ internal resistance (N2790A, maximum 1400V). For all TENG samples, their voltages were found to increase with the impacting time initially, and then stabilize at a maximum value with a small fluctuation in several minutes.

The Voc signals of each sample were recorded after impacting for 10

minutes.

The output performance of TENGs was evaluated by connecting different

pure resistors as an external load.

An optimal load resistance was fixed to further

investigate the influence of triggering conditions on the output performance of TENG. For a quantitative evaluation without the influence of springs or elastic spacers, a modified free fall experiment device was developed for testing the output performance of TENG based on operation mode of vertical contact-separation mode of TENG.

The falling weight was fixed at 400 g, which was equivalent to the impact

force of 4 N (See SI for more details, Fig. S3).

3

Results & Discussion

3.1 Preparation of HPFs for templating synthesis of MLAs In our previous study, PDMS-g-PAs has been explored and demonstrated as a good candidate for the preparation of highly ordered HPFs by the BF technique because of 10 ACS Paragon Plus Environment

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its unique chemical structures.34

Herein the influence of solution concentration on

the formation of HPFs was studied.

It was encouragingly found that highly ordered

honeycomb porous microstructures can be obtained in a wide range of solution concentration from a very low content 0.50 mg mL-1 to 45 mg mL-1, as demonstrated in Fig. 1b-i.

Actually, the typical honeycomb features on film surface could be

found until the solution concentration was reduced to 0.15 mg mL-1 (Fig. S4).

And

when the solution concentration was over 60 mg mL-1, the resultant HPF became less regular, displaying large cell-like structures decorated with smaller pores (Fig. 1a). More specifically, a single layer of hexagonally arranged holes was observed for the film samples prepared at relatively low solution concentrations (Fig. 1h and i).

As

the solution concentration reaching at 1.50 mg mL-1, the existence of pores inside the film was found (Fig. 1g). With the continuing increase of solution concentration, the pores in the second layer were observed clearly through the top surface of the film, as demonstrated in Fig. 1b-f.



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Figure 1. Morphology evolution of film surfaces revealed by SEM with the decrease of solution concentration.

The films are prepared by the BF technique from

PDMS-g-PAs/CHCl3 solutions with different concentrations: (a) 60 mg mL-1; (b) 45 mg mL-1, (c) 30 mg mL-1, (d) 15 mg mL-1, (e) 7.5 mg mL-1, (f) 3.0 mg mL-1, (g) 1.50 mg mL-1, (h) 0.75 mg mL-1, and (i) 0.50 mg mL-1

The cross-sectional views of HPFs were further examined by SEM.

As shown in Fig.

2, the revealed inner structures of HPFs show an obvious evolution of dimensional architecture with the decrease of solution concentration.

Except the films cast from

the solution with extremely low concentration (Fig. 2i), porous structures throughout the whole film were observed clearly.

Especially, single-layer porous structures

were obtained when the casting solution is below 1.5 mg mL (Fig. 2g and h), while


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multiple porous layers were found over 3.0 mg mL-1 respectively (Fig. 2a-f). Besides multiple porous layers, a dense polymer stratum was found at the film bottom with the concentration of 15 mg mL-1 (Fig. 2d).

The thickness of dense stratum

roughly increases gradually with the rising concentration of casting solution while the thickness of porous layers decreases slightly (Fig.2a-c).

Figure 2. Cross-sectional views of HPFs cast from different solution concentrations: (a) 60 mg mL-1; (b) 45 mg mL-1, (c) 30 mg mL-1, (d) 15 mg mL-1, (e) 7.5 mg mL-1, (f) 3.0 mg mL-1, (g) 1.50 mg mL-1, (h) 0.75 mg mL-1, and (i) 0.50 mg mL-1

PDMS-g-PAs HPFs prepared from 0.5 mg mL-1, 0.75 mg mL-1, 1.5 mg mL-1, 3.0 mg mL-1, 7.5 mg mL-1, 15 mg mL-1, 30 mg mL-1, 45 mg mL-1, and 60 mg mL-1 solutions by the BF technique above were used as negative molds for templating synthesis of



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PDMS films with MLAs respectively.

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Their resultant PDMS MLAs were

correspondingly named MLA-0.5, MLA-0.75, MLA-1.5, MLA-3.0, MLA-7.5, MLA-15, MLA-30, MLA-45 and MLA-60.

Most of them possess similar surface

morphologies with their HPF molds under optical microscopy (See SI, Fig. S5). They were further revealed by SEM at different magnifications.

The surface

morphology of MLA-60 shows irregular lens-like microstructures with the size of dozens of micrometers (Fig. 3a and the inset). For all the other PDMS film samples, hexagonally ordered lens-like microstructures with homogeneous size were found in large scale.

And the lens sizes were drastically decreased to several micrometers, as

shown in Fig. 3b-i and their insets.

Figure 3. Top views of MLA films prepared with different HPFs as negative molds: (a) MLA-60, (b) MLA-45, (c) MLA-30, (d) MLA-15, (e) MLA-7.5, (f) MLA-3.0, (g)


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MLA-1.50, (h) MLA-0.75, and (i) MLA-0.50.

The corresponding insets are their

magnified views, respectively

The corresponding cross-sectional views in Fig. 4 demonstrate that the lens-like microstructures above are convex lens arrays. and mono-layered (Fig. 4a). µm.

The lenses of MLA-60 are large-size

Its MLA region is at the thickness of approximately 5.7

For MLA-45 and MLA-30, multiple layers of small lenses were clearly

observed (Fig. 4b and c); while stacked micro lens can only be found in some regions for MLA-15 and MLA-7.5. and e).

They both possess a monolayer in most regions (Fig. 4d

The measured thicknesses of MLA region (highlighted in light orange) for

these samples are approximately 8.7 µm, 7.6 µm, 5.4 µm and 4.0 µm, respectively. The MLA region of MLA-3.0 starts to be not obvious (Fig. 4f).

And for other MLA

samples including MLA-1.5, MLA-0.75 and MLA-0.5, their MLA regions become very indiscernible gradually, as shown in Fig. 4g-i.

Generally, the thickness of MLA

region possesses an obviously decreasing trend with the decrease of solution concentration.

This can be explained that the concentration of casting solution plays

a determined role on the pore layers and thickness of HPF molds.



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Figure 4. Cross-sectional views of different MLA films: (a) MLA-60, (b) MLA-45, (c) MLA-30, (d) MLA-15, (e) MLA-7.5, (f) MLA-3.0, (g) MLA-1.50, (h) MLA-0.75, and (i) MLA-0.50.

The region of MLAs in (a)-(f) are colored with light orange

The dependences of the pore size of HPFs and lens size of MLAs on the solution concentration were compared and plotted in Fig. 5a, while Fig. 5b shows the influences of solution concentration on the dimensional microstructures of HPFs.

It

is obvious that the average size of pores on film surface fluctuates with the solution concentration (Fig. 5a, hexagons in navy blue), while the film thickness shows an upward trend (Fig. 5b).

Specifically, the thickness of dense stratum emerging at 7.5

mg mL-1 drastically increases from about 0.6 µm to 9.0 µm at 45 mg mL-1, while the thickness of porous layers possesses a decrease trend with the increase of solution


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concentration after a steady rise to 6.6 µm at 7.5 mg from approximately 0.4 µm at 0.5 mg mL-1.

Porous microstructured films without dense stratums can be classified into

two types: monolayer HPFs and multilayer HPFs.

The pore sizes of the monolayer

HPFs obtained at low concentrations (< 1.5 mg mL-1) exhibit downward trends, while the sizes of multilayer HPF obtained at intermediate concentrations show an increasing trend.

HPFs with multilayer porous structures on a dense stratum were

fabricated after increasing to 15 mg mL-1, and their pore sizes decline again.

The

size variation trend of MLAs with the increase of solution concentration is basically coincident with the pore size variation of HPFs with solution concentration (Fig. 5a, spheres in wine red).

The average micro lens sizes of these samples roughly range

from 2.4 µm at 45 mg mL-1 to 3.9 µm at 1.5 mg mL-1.

The scale bars in Fig. 5a

represent the distribution of pore/lens sizes rather than the experimental errors.

The

short bar lengths indicate that both HPFs and MLAs possess a narrow size distribution. It is noted that all the resultant micro lens sizes are larger than the pore size of their corresponding HPF molds. positions.

This can be ascribed to their different measurement

If regarding the pore shape of HPFs as an air sphere, the measured pore

size of HPFs is actually the diameter of sphere at a latitudinal position, while the lens size is equivalent to the equator diameter of sphere, as indicated in Fig. S1.



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Figure 5. (a) Plot and comparison of the dependence of lens size of MLAs (spheres in wine red) and pore size of HPFs (hexagons in navy blue) on solution concentrations; (b) Graphical comparison of the thickness of porous layers and dense stratum in the films cast from different solution concentrations

3.2 Assembly and evaluation of MLA film-based TENGs A vertical contact-separation mode-based TENG usually comprises four main parts: (1) two dissimilar materials for generating triboelectric charges by friction (2) two electrodes for producing and transferring induced electrons, (3) two insulation substrates for packaging, and (4) springs or elastic spacer allowing the reciprocal motion of substrates.

Herein an Al sheet was used as conductive layer to act as dual

roles of top electrode and contact (friction) surface.

The main procedures for the

assembly of TENGs based on the PDMS films prepared above were illustrated in Fig. 6 (See experimental section for more details).


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Figure 6. Schematic of assembling a TENG based on PDMS film, (a) preparing two insulation substrates, (b) introducing a conductive layer on the inner surface for each substrate, (c) fixing a PDMS film on the conductive area of bottom substrate, and (d) adding a hollow insulation spacer between the two substrates

It is generally accepted that the operation principle of the TENG working in vertical contact-separation mode is based on the coupling of triboelectric effect and electrostatic effect.10-11, 35-36

Under open-circuit conditions (i.e. the top electrode is

not connected with the bottom electrode), no charge is generated in the original separation state (Fig. 7a-i).

When the top metal electrode is brought to contact with

PDMS film by pressing, close contact enables the electrons, ions and/or molecules to transfer between the inner surfaces of two materials, leading to the creation of triboelectric charges at the contact area.

Because PDMS possesses much higher

negative polarity than metal materials, electrons and ions are usually transferred from metal material.37

This leads to the creation of net positive charges at the metal

surface and the same amount of net negative charges retained at the PDMS surface, respectively (Fig. 7a-ii).

An electric potential difference will be established when

PDMS film and metal sheet become separated again.

Such a potential difference can



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be gradually increased to a maximum value by repeatedly impacting.

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This is

because repetitive friction can accumulate the triboelectric charges at the dielectric PDMS film until achieving a saturation state.38

When a load (such as pure resistance R) is electrically connected between the top electrode and bottom electrode, free electrons induced by the triboelectric charges on PDMS film will be driven to flow back and forth between the two electrodes for building an opposite potential to balance the electrostatic field generated during the separating and contacting processes, thereby leading to the generation of electric energy (current).35-36

Specifically, when metal sheet (top electrode) is brought to

completely contact with PDMS film (at contact state) after certain times of impact, the positive triboelectric charges of top electrode are equal to the negative charges retained on the PDMS film (Fig. 7b-i).

Once the top electrode is separated, the

bottom electrode possesses a lower electric potential than the top electrode, leading to the production of an electric potential difference.

Such potential difference will

drive the electrons induced on the bottom electrode to flow through the external load for screening the positive triboelectric charges on the top electrode (Fig. 7b-ii). When the top electrode is separated at a distance of d, positive triboelectric charges are completely screened on the top electrode, leaving an equal amount of induced positive charges on the bottom electrode (cause by the loss of electrons) (Fig. 7b-iii). During the subsequent contacting process, an electric potential difference with reversed polarity will be produced with the decrease of separation distance. Correspondingly, the electrons are driven to flow in a reversed direction (Fig. 7b-iv). 20 ACS Paragon Plus Environment

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They will keep screening the inductive positive charges on the bottom electrode until the top electrode is brought to completely contact with PDMS film again.

An

alternative voltage and current can be measured during the separating and contacting process, which are usually used to characterize the performance of TENG.

Figure 7. (a) Schematic of charging the surface of PDMS film under open-circuit conditions by friction cause by repeatedly impacting: (i) no charge generated in the original separation state, (ii) surface charges generated and accumulated on the surface of PDMS films after repeatedly impacting.

(b) Schematics of working

principle of PDMS film-based TENG connected with a resistance load: (i) At contact state, positive triboelectric charges on top electrode are equal to the negative charges on PDMS film; (ii) during the separating process, the potential difference drives electrons induced on the bottom electrode to flow through the external load for screening the positive triboelectric charges on the top electrode; (iii) at the separation distance of d, positive triboelectric charges on the top electrode are completely



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screened, while an equal amount of induced charges are produced on the bottom electrode; (iv) During the subsequent contacting process, electrons are driven back for screening the induced positive charges on the bottom electrode

MLA-3.0, MLA-7.5, MLA-15, MLA-30, MLA-45, and MLA-60 PDMS films were used for the assembly of TENGs.

And their electric performances were firstly

evaluated by utilizing a mechanical compression system with a frequency of 3.3 Hz. For comparison, a TENG based on PDMS film with flat surface was used as control sample.

The generated typical Voc pulse signals were compared in Fig. 8.

It was

found that all TENGs based on MLA samples possess larger peak voltages than the flat film-based TENG (Fig. 8a). Voc.

And MLA-15 based TENG has the largest peaks of

Fig. 8b and c show that each impact generates a pair of positive voltage pulse

and negative voltage pulse.

The positive peak voltage and negative peak voltage

were produced during the contacting and separating process respectively, while the Voc at zero corresponds to the contact position.

The different performance of

TENGs can be ascribed to the different surface microstructures of used PDMS films. It has been commonly accepted that the triboelectric charge density on the surfaces of dielectric materials mainly determines the performance of TENGs,10 while the generation of surface charges in contact electrification is a complex process that involves a combination of bond cleavage, chemical changes, and material transfer.39 This process is influenced by many variables, including material types, surface morphology and triggering conditions.

The generation of triboelectric charges for

flat PDMS surface is mainly dependent on the elastic deformation of whole film, 22 ACS Paragon Plus Environment

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while PDMS films with MLA surfaces can produce triboelectric through the elastic deformation of both numerous isolated micro lenses and whole film.

When an

impact force was applied, the strain of isolated micro lens occurs more easily and largely than the whole film, which would lead to the production of more triboelectric charges under the same contact area.

Therefore, MLA patterned film-based TENGs

can possess better electric performance than the TENG based on PDMS film with flat surface under specific triggering conditions.

Peak-to-peak volate Figure 8. Electric performance of TENGs based on PDMS films with different MLAs, (a) Typical open-circuit voltage signals generated by different TENGs, (b) magnified 23 ACS Paragon Plus Environment


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voltage signals of MLA-15 based TENG, and (c) its enlarged view of several voltage pulses

To more explicitly compare the electric performance of different TENGs, the peak values of voltage pulses generated by each impact were collected for calculating the average value of peak-to-peak voltage. shown in Fig. 9.

The calculated results are compared and

Specifically, the average voltages of TENGs based on MLA-3.0,

MLA-7.5, MLA-15, MLA-30, MLA-45 and MLA-60 are 19.40 V, 20.00 V, 40.80 V, 32.30 V, 19.34 V, and 23.22 V, respectively, while flat PDMS film-based TENG has only an average voltage of 13.83 V under the same impacting conditions.

This

clearly indicates that all MLA-based TENGs have a better electric performance than flat PDMS film-based TENG.

However, it is noted that there are no obvious rules

between the performance improvement of TENGs and the lens size of MLAs.

This

can be ascribed to the different thickness of MLA region that changes reversely with the size variation.

The decrease of MLA size generally accompanies with the

increasing thickness of MLA region (which is comprised of three-dimensionally stacked micro lens).

The decrease of MLA size may increase the number density of

micro lens, thereby increasing the specific surface area for larger contact area to produce triboelectric charges.

And the multiple layers of MLA region with smaller

surface lens size tend to deform more easily under the impact of external force. However, some vacant space among sparsely stacked lenses may be left.

This

phenomenon will decrease the effective area of PDMS film for contact electrification, which finally results in decreasing the production of triboelectric charges. 24 ACS Paragon Plus Environment

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Nevertheless, for MLA-15 based TENG, an enhanced electric performance of approximately two times larger than that of flat PDMS film-based TENG was achieved.

It should be pointed out that the measured voltages are much less than

those reported in the previous literatures.40-41

Based on a theoretical model for

contact-mode TENGs developed by Wang et al.,42 the main reasons can be ascribed to the different external resistance and the loading conditions such as the distance and velocity of the contact/separation between triboelectric surfaces.

Particularly, the

peak value of voltage possesses a sharp increase with the increase of resistance in the range of 107 to 109 Ω

42

.

In this study, the obtained voltages were measured by a

voltage probe with an internal resistance of 8 MΩ, which is actually in two or three orders lower than the resistance used for the measurement of open-circuit voltage.

Figure 9. Comparison of electric performance of TENGs based on PDMS films with different MLAs and a flat surface

The output performance of TENGs with an external load was evaluated based on different pure resistors.

Fig. 10a shows the typical current pulse signals generated 25 ACS Paragon Plus Environment


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by MLA-15 based TENG under the load resistance of 20 MΩ, 10 MΩ, 5.1 MΩ, 1.0 MΩ and 0.51 MΩ respectively.

Their peak currents generally possess a decreasing

trend with the increase of load resistance (Fig. 10b), which is due to Ohmic loss.35 To find the matched external resistance that can generate maximum electric performance, the output current was used to calculate the instantaneous output power (P) based on the following equation.

=

(1)

Where P, Ipeak and R represent the instantaneous power, the peak current and external resistance, respectively.

The dependence of instantaneous powers on the load resistance was shown in Fig. 10c. It is clearly found that at the load resistance of 5.1 MΩ, the instantaneous power output of MLA-15 based TENG reaches the maximum.


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Figure 10. Output performance of MLA-15 based TENG with different pure resistors as external load, (a) typical output current signals, (b) the dependence of average peak-to-peak current on the load resistance, and (c) the dependence of instantaneous power on the load resistance

The load resistance was then fixed at 5.1 MΩ to further investigate the influence of triggering conditions on the output performance of MLA-15 based TENG.

For a

quantitative evaluation without the influence of springs or elastic spacers, a modified free fall experiment device (see SI for more details, Fig. S3) was developed for testing the output performance of TENG based on operation mode of vertical



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contact-separation mode of TENG.

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Different from the repetitive signals of TENGs

generated by the mechanical compression system mentioned before, the electrical signals triggered by the falling weight display an attenuated wavy curve.

The typical

output voltages of MLA-15 based TENG triggered by the falling weight at 1 cm, 3 cm and 5 cm are shown in Fig. 6.11a-c, respectively.

The clearly attenuated peak

signals well reflect the vibration attenuation process of the falling weight caused by rebounding.

Basically, every collision can generate a pair of positive voltage peak

and negative voltage peak.

The peak voltages generated by the first collision were

measured for the calculation of output voltage.

The obtained voltages at 1 cm, 3 cm

and 5 cm are 49.45 V, 79.50 V and 115.17 V, respectively. against the falling height, as shown in Fig. 6.11d.

They were further plotted

It shows that the output voltage

generally possesses an increasing trend with the increase of falling height.

This

indicates that increasing the impact force can effectively promote the electric performance of PDMS film-based TENGs.


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Figure 11. Typical output voltage signals of MLA-15 based TENG triggered by the falling weight at different heights, (a) 1 cm, (b) 3 cm and (c) 5 cm, and (d) the dependence of output voltage on the falling height

3.3 Applications of MLA film-based TENGs for harvesting mechanical energy generated by different body motions The MLA-15 based TENG is highly sensitive to external force, and can even make an immediate response to the finger tapping.

Fig. 12a shows the repetitive and sharp

output voltage signals generated at the load resistance of 5.1 MΩ by the rapid finger tapping with a frequency of approximately 4.0 Hz.

Because the external load was

pure resistance, the electric energy was equal to the Joule heating energy.

Therefore,

its generated electric energy (E) can be calculated based on time-integral of the output



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voltages with the following equation

= =

(2)

where E is the electric energy generated by TENG, Q is the Joule heating energy, U is the output voltage, R is the load resistance, and t1 and t2 represent the start time and end time of a single contact, respectively.

Fig. 12b shows a pair of positive voltage peak and negative voltage peak triggered by a finger tap.

Its maximum peak voltage can be up to about 7.6 V.

The total electric

energy generated by this light tap is about 0.078 µJ.

Figure 12. (a) The electric performance of MLA-15 based TENG by rapid tapping with a finger, (b) a voltage pulse output produced by one finger tap

The output voltages of TENG generated by handclapping at different frequencies are shown in Fig. 13a.

It shows that the generated peak voltages generally possess an

increasing trend with the increase of clapping frequency. peak voltage is up to 146.1 V (Fig. 13b).

At 5.5 Hz, the maximum

Even when the TENG was clapped with

hand at a normal frequency (about 1.2 Hz), a maximum peak voltage of 93.6 V can be obtained (Fig. 13c).

And the electric energy generated by one clap is 9.12 µJ at 5.5 30 ACS Paragon Plus Environment

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Hz and 4.16 µJ at 1.2 Hz, respectively.

Figure 13. (a) The electric performance of MLA-15 based TENG by handclapping at different frequencies, (b) a voltage pulse output produced by rapid handclapping (~5.5 Hz) and (c) a voltage pulse output produced by normal handclapping (~1.2 Hz) for the calculation of electric energy

The MLA-15 based TENG was also built under the shoe sole for harvesting the mechanical energy produced by a 60 Kg adult.

The output voltages generated by

human foot with the frequency from 0.5 Hz to 2.5 Hz were shown in Fig. 14a.

It

indicates that the output voltages generally show an increasing trend with the increase of pace frequency.

The maximum peak voltages at 0.5 Hz (strolling, Fig. 14d), 1.5

Hz (normal walking, Fig.14c), and 2.5 Hz (jogging, Fig. 14b) can be up to 4.9 V, 14.5 V and 60.4 V, respectively.

The corresponding generated electric energies by one

pace are 0.10 µJ, 0.29 µJ, and 1.93 µJ, respectively.

Benefiting from the excellent



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mechanical properties of PDMS, the MLA-15 based TENG is very stable and durable for practical applications.

When it was continuously impacted at 3.5 Hz for

approximately 72 h, no performance degradation was found.

Actually, our previous

study has demonstrated that the PDMS film-based TENGs can be continuously operated at least one million cycles without the degradation of electric performance.43 Considering the waste mechanical energy produced by various human body motions in daily life, the accumulative electric energy converted by the robust TENG is quite attractive.

Figure 14. (a) The electric performance of MLA-15 based TENG triggered by human foot at different frequencies, (b) a voltage pulse output produced by jogging (~2.5 Hz) and (c) a voltage pulse output produced by walking (~1.5 Hz), and (d) a voltage pulse


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output produced by strolling (~0.5 Hz)

4

Conclusion

In summary, based on the HPFs prepared by the BF technique, a simple, straightforward and efficient micromolding approach has been developed for the synthesis of PDMS films with adjustable MLAs.

The resultant PDMS films were

successfully employed as triboelectric material for the assembly of vertical contact-separation based TENGs to harvest mechanical energy.

Compared with flat

PDMS film-based TENG, all MLA-based TENGs have an obviously enhanced electric performance.

Particularly for MLA-15 based TENG, up to three times of

improvement in the electric performance of flat PDMS film-based TENG can be obtained under the same triggering conditions. It was further successfully used to harvest the waste mechanical energy generated by different human body motions, including the finger tapping, handclapping and walking with a frequency ranging from 0.5 Hz to 5.5 Hz. The harvesting of energy generated by these frequent human activities in daily life is considerably attractive.

This study shows the promising

applications of MLAs derived from HPFs in TENGs for converting the mechanical energy generated by human body motions into electricity.

Supporting Information Schematic of molding synthesis of MLAs, digital photographs of final TENG device, schematic of the modified free fall experiment test, SEM images of HPFs prepared from low solution concentrations, optical microscopy images of MLAs.


The


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Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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Breath Figure Micromolding Approach for Regulating the

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