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Self-cleaning triboelectric nanogenerator based on TiO2 photocatalysis
Journal Pre-proof Self-cleaning Triboelectric nanogenerator based on TiO2 photocatalysis Hui Liu, Yawei Feng, Jiajia Shao, Yao Chen, Zhong Lin Wang, Hexing Li, Xiangyu Chen, Zhenfeng Bian PII:
S2211-2855(20)30056-2
DOI:
https://doi.org/10.1016/j.nanoen.2020.104499
Reference:
NANOEN 104499
To appear in:
Nano Energy
Received Date: 28 October 2019 Revised Date:
16 December 2019
Accepted Date: 13 January 2020
Please cite this article as: H. Liu, Y. Feng, J. Shao, Y. Chen, Z.L. Wang, H. Li, X. Chen, Z. Bian, Self-cleaning Triboelectric nanogenerator based on TiO2 photocatalysis, Nano Energy, https:// doi.org/10.1016/j.nanoen.2020.104499. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Graphical abstract
Self-cleaning Triboelectric nanogenerator based on TiO2 photocatalysis
Hui Liu,a,
§
Yawei Feng,b,c,
§
Jiajia Shao,b,c Yao Chen,a Zhong Lin Wangb,c,e Hexing Li,a,d Xiangyu
Chen,b,c,* Zhenfeng Bian,a,*
a
The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key Laboratory of
Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, PR China b
CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and
Sensor Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, PR China c
School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing
100049, PR China d
School of Mathematics and Physics, Shanghai Key Laboratory of Materials Protection and
Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, PR China e
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA
30332, USA Email: X. C., [email protected]; Z. B., [email protected];
§ Hui Liu and Yawei Feng contributed equally to this work.
Abstract: Dielectric polymer materials are indispensable for the fabrication of self-powered nanosystem and electronic skin. However, the performance of the device is often seriously degraded due to the contamination of the surface. It is very important to keep the clean of the surface of the device, especially for the application of wearable devices. In this work, triboelectric nanogenerator (TENG) integrated with TiO2 photocatalysis is developed for realizing the dual functions of energy generation and self-cleaning interface. The friction layer of TENG is sponged polydimethylsiloxane (PDMS) film which is prepared by sacrificial template method with the pore size varied from hundreds of microns to several millimeters. When the interface of TENG is contaminated by organic pollutants, the open circuit voltage (VOC) of the device decrease from 125 V to 88 V. Under the solar illumination for 35 min, the self-cleaning interface can degrade the contaminants attached on the tribo-surface, while VOC raises from 88 V to 115 V after illumination, indicating that 92% of the TENG’s output performance can be fully recovered. The photocatalysis-based self-cleaning effect on polymer tribo-material can contribute to the application of TENG towards e-skin and wearable devices.
Keywords: triboelectric nanogenerator, dielectric polymer, frictional layer, self-cleaning, photocatalysis
Introduction Power generation by harvesting ambient mechanical energy to operate micro- or nanodevices is considered as one promising solution for internet of things and wearable devices.[1-5] Triboelectric nanogenerator (TENG) can efficiently convert ambient mechanical energy, such as human motion, wind blow, water wave or other kinds of mechanical vibration, into electricity based on the contact electrification and electrostatic induction. Distinct from the mechanism of electromagnetic generator, the foundational principle of TENG can be found in Maxwell displacement current, which is related to the changing of the polarization of the dielectrics.[6-12] As a new type of energy collector, the application of TENGs has been extended into various field, such as self-powered nanosystem, self-powered electrochemistry, human-machine interaction, far beyond the original energy harvesting device.[13-20] Polymers are the basic dielectric materials for the fabrication of the self-powered nanosystem and flexible wearable devices.[21-26] As the friction layer of TENG, the intrinsic properties of polymer are very important to its electrical output performance. Emerging polymers with high permittivity or modified functional groups are designed to suppress the discharge, improve the surface charge density and applied in some special environment.[27-32] However, the triboelectric layer can be polluted in the specific condition, such as on-skin sensing and wearable device.[33-37] The contaminated devices may not only affect their normal operation, but also influence the wearing comfort, and potentially harm human health. Thus, self-cleaning of the devices is indispensable for the real application of TENG technique.[38-40] Photocatalysis has been demonstrated as a sustainable technology for organic pollutants degradation by harvesting solar energy.[41-43] Titanium dioxide nanoparticles (TiO2 NPs) are recognized as one of the most applied non-toxic photocatalysts due to their high activity, light stability and low cost[44-46]. By the excitation of UV light, free electrons (e-) are excited to the conduction band (CB) of TiO2, leaving holes (h+) in the valence band (VB). The generated charge carriers with high energy state, along with the derived free radicals, are active for the initiation of organic compounds degradation into small nontoxic molecules of H2O and CO2.[47-50] Therefore, if the photocatalytic degradation function is combined with the TENG, it can produce an effective self-cleaning effect and the recovery of device performance.
In this work, self-cleaning function is applied on the tribo-interface of TENG, which can achieve dual function of contact electrification and pollution degradation. The widely used PDMS dielectric is selected as the friction layer of TENG device for its high electronegativity and flexibility. Sponged PDMS film is prepared with the pore size varied from hundreds of micrometers to several millimeters. TiO2 NPs is loaded into the pore structures of the PDMS surface. The contaminated interface of TENG results in the decrease of both open circuit voltage (VOC) and short-circuit current (Isc) of the device. Under the solar light, the contaminations attached on the tribo surface are cleaned by the photocatalytic material and the electrical performance of the constructed TENG device can be recovered. The photocatalysis-based self-cleaning effect on polymer surface proposed can contribute to TENG performance recovery and further promote its application in e-skin and wearable devices.
2. Experimental section 2.1 Fabrication of sponged PDMS The fabrication of sponge PDMS film follows the reported process.[51] PDMS glue (Sylgard 184, Dow Corning) is mixed by elastomer and curing agent at a mass ratio of 10:1. After stirring for 30 minutes, the bubbles are removed in vacuum condition. Three templates with volume of 60 mm × 60 mm × 2 mm, 60 mm × 60 mm × 4 mm and 60 mm × 60 mm × 6 mm were prepared using acrylic plate. Then 8g, 14g, 20g of sugar cubes with average length of 2 mm, as show in Figure S1a-b, are placed into three templates accordingly. The PDMS mixtures are poured into templates and degassed under vacuum. Due to capillary forces, PDMS glue can penetrate into the intergranular space of the sugar template. After 2 hours of curing process at 80 °C and the subsequent hot water (90-95°C) treatment to dissolve the sugar particles, sponged PDMS friction layers with different thickness are obtained. For the fabrication of TENG, all sponges are cut into size of 4 cm×4 cm with different thickness.
2.2 Preparation of TiO2/PDMS sponge Different concentrations of suspension are prepared by dispersing TiO2 NPs (Degussa P25) into deionized water. 5 mL of the above photocatalyst suspensions is sprayed onto the surface of the
sponged PDMS layers to obtain the TiO2/PDMS with different loading amounts. After the residual deionized water in the sponges evaporated at room temperature, TiO2/PDMS sponge layers are prepared. The adopted photocatalyst concentrations in this fabrication process are 0.1wt%, 0.05wt%, 0.01wt%, 0.005wt% and 0.001wt%, respectively. The morphology is characterized using scanning electron microscopy (SEM, Nova NanoSEM 450 and Hitachi SU8020) equipped with an energy dispersive X-ray spectroscopy (EDS, Bruker quantax 400).
2.3 TENG fabrication and measurements A vertical contact–separation mode TENG (CS-TENG) is fabricated, where the effective area is kept to be 4 cm × 4 cm. The upper part of the TENG is constructed by attaching an aluminum (Al) foil to an acrylic board substrate (4 cm×4 cm × 3 cm). The TiO2/PDMS layer stacked on an acrylic board substrate (4 cm×4 cm × 3 cm) adhered with a same area of Al foil is act as the bottom layer. Cu wires are connected to the two metal layers for electrical measurements.
2.4 Self-cleaning experiments Rhodamine B (RhB) and methylene blue (MB) steams are used as contaminant source to simulate the possible contamination happened during the operation of TENG. Here, 5 mL of 5 ppm RhB and 5 mL of 5 ppm MB are sprayed to the surface of PDMS sponge, while the performance change of the TENG before and after contamination are studied. A solar simulator (Zolix, LCSS500-NMNY, 11.2 mW·cm-2) with a 400 nm UV cut-off filter is vertically irradiated to PDMS surface. The surface absorption spectrum is measured with a spectrophotometer (Shimadzu UV 3600) at intervals.
3. Results and discussion 3.1 Working Mechanism of TENG Figure 1a illustrates the basic working mechanism of the CS-TENG under the action of a vertical compressive force. When contacted with metal material, polymer material is negatively charged according to the triboelectric series.[52] After friction, the dielectric polymer surface and metal electrode possess the equivalent charges with opposite types. Defining a special state when the top electrode is far infinitely away from the dielectric polymer (state i) as the initial state, the bottom
metal electrode is positively charged due to the electrostatic induction, while the top electrode with no charges. When an external force is applied (state ii-iii), positive charges are induced on the top electrode, and the extra electrons flow to the bottom electrode through the loading circuit, resulting in a unidirectional current pulse, which can be indicated by the waveform in Figure 1b. As for the release process (state iii-iv), the situation is reversed. The reduced charge density on the top electrode leads to the backward of electrons, and an inversive current pulse signal appears (Figure 1b). During the periodic press and release of TENG, the electrons flow back and forth through the external circuit, resulting in the alternating current and voltage. The charge density (σ) on dielectric surface is critical to the electrical performance of a TENG device. In the configuration of vertical contact–separation mode, the metal acts both as an upper electrode and as a friction material. The traditional capacitance model for output performance analysis is inaccurate because of the neglecting of edge effect.[53, 54] Thus, we construct a three-dimensional (3D) mathematical model (Figure 1c) for the quantitative performance analysis.[55] For the construction of mathematical model, a virtual charged plane with the same size (length of a, width of b) of metal electrode is set. Due to this plane is upon the mental electrode and in air, the permittivity can be set to be the value of air (ε0). The charge on this plane is resulted from the friction with dielectric polymer (ε2), thus the surface of this virtual plane is the same charge density (+σT) as dielectric layer (-σT), but an opposite charge type. In a particular state, the charge density on the bottom electrode is +σu, thus the charge density on top electrode is -σu. The analysis is start from Leibniz integral rule, thus the potential at the top electrode z1 is described as ф
, ,
=−
+
+
−
(1) the potential at the bottom electrode z4 is described as ф
, ,
=−
+
+
−
(2) where = arctan
# $
%
!" &
'
&
(3)
When a resistance R is connected to the electrodes, the voltage over R is ,-
∆) = *+ = −* ,. = −*/
,
,.
(4)
Where A is the area of electrode, Q is the transferred charge, and Kirchhoffs' law states −*/
,
,.
=ф
, ,
−ф
, ,
(5)
For open-circuit (OC) conditions, Q is 0 without charge transfer, the height values of z1 and z2 are approximate because of the virtual plane. Therefore, the VOC can be calculated by −)01 =
−
(6)
From Eq.6, the properties of dielectric layer, such as the permittivity constant ε2 and surface charge density σT, are essential for the output performance of CS-TENG.
3.2 Characterization of the TiO2/PDMS sponge layer Figure 2a shows the preparation process of the TiO2/PDMS sponge layer. The main steps include raw materials mixing, curing, cutting, washing, and TiO2 photocatalyst NPs loading. PDMS sponge with the thickness of 6 mm, 4 mm, 2 mm are fabricated firstly, where the porous structures of PDMS layer are formed by sacrificial template method. Optical photographs of the enlarged sponger surface in Figure S2 in the Supporting Information clearly exhibit the pores varying from hundreds of microns to several millimeters. The PDMS sponge serves as a three-dimensional framework for TiO2 NPs loading due to the abundant macro-pore structures. The photos in Figure 2b show the sponger-like appearances and opalescent color of the PDMS layer loaded with photocatalyst TiO2 NPs. Though there are no notable differences for naked eyes, the micro-domains of PDMS sponge layers before and after TiO2 NPs (using 0.05 wt% of the suspension) loading are distinct, as shown in Figure 2c-e. Agglomerated TiO2 NPs are clearly distributed on the surface of photocatalyst-loaded PDMS (Figure 2d,e), while the untreated PDMS surface shows smooth profile (Figure 2c). The SEM images and EDS of the TiO2/sponge PDMS in Figure S3 reveal that photocatalyst nanoparticles almost fully cover the polymer surface. After attaching the Al foils and acrylic substrates, a CS-TENG can be fabricated. The detailed information of the fabricated TENG device can be seen in the diagram of Figure 2f. Sponged
PDMS with or without TiO2 NPs (factual contact area of 16 cm2) is act as the dielectric material for electrification, while Al foils have the dual functions of output electrode and the tribo-layer for contact with the sponged PDMS.
3.3 Influence of surface pollution For the electrical property testing, a linear motor with a working frequency of 1 Hz is applied to drive the TENG, and the VOC, ISC and maximum transferred charges (Qsc) are measured by a Keithley 6514 system electrometer. Figure 3a shows the setup for electrical property testing, and Figure 3b~3d shows the performance of the TENG devices fabricated by pure PDMS spongy with different thicknesses. It shows no obvious difference between the sponge-like PDMS with a thickness of 6 mm and 4mm, both has a VOC value of about 105 V, ISC value of 2.0 µA, and transferred charge of 35 nC. For the TENG fabricated by 2 mm of PDMS spongy, the VOC is about 125 V, the ISC was 2.9 µA, and the maximum transferred charge was 40 nC. PDMS sponge with thickness of 6 and 4 mm displayed similar performances but higher performance for the PDMS sponge with 2 mm. The quality and grain size of cube sugar can decide the effective thickness of the PDMS sponge under compression. Meanwhile, the effective thickness of the dielectric film can influence the output performance. For the PDMS with a thickness of 2 mm, the length of PDMS holes produced by sugar agent is close to the thickness of the sponge PDMS. Hence, during the contact motion the effective thickness of the PDMS film under the compression can be much smaller than 2 mm, which leads to a sudden increase of the output performance in comparison with the other two samples. The polymer-based friction layers are vulnerable to organic contaminant in some unsealed application scenario. For some self-powered on-skin sensor and wearable devices, the contact to skin or motion movement may cause skin secretions adhere to the polymer surface, which decreases the surface charge density, disturbs operations, reduces wear comfort, even shorten their work lifetime. The pollution effect is simulated with colored dye RhB steam. Figure 3e-g shows the performance results of the polluted TENG by this organic contaminant, where both the VOC, Qsc and ISC show significant decrease at the work frequency of 1 Hz. For the worst case, the VOC and Qsc from the polluted device using 4 mm of PDMS spongy almost drop to 60% of the original
value, while the ISC drops to about 78%. Actually, for the sample with the thickness of 4 mm and 6 mm, the output performance and their performance change after being polluted are quite comparable with each other. However, the sample with thickness of 2 mm shows a quite different performance in comparison with the other samples. We believe the reason is attributed to the detailed structure of the PDMS sponge. The length of sacrificial sugar agent is 2 mm, which is close to the thickness of the sponge PDMS. Hence, during the contact motion the effective thickness of the PDMS film under the compression can be much smaller than 2 mm. Meanwhile, there are more penetrating channels existing inside this 2 mm samples. All these factors may influence the performance of this PDMS during the experiments. Hence, if the similar amount of the contaminated material is retained on the surface, the reduction of surface charge on PDMS sponges with different thicknesses can be different. Though the parameters output from the polluted TENG using 2 mm of PDMS spongy decrease not so obviously as the others, it is still irreversible for their attenuation phenomena.
3.4 Self-cleaning and recovery of TENG performance To restrain the performance degradation caused by surface contamination, self-cleaning effect is introduced by semiconductor photocatalytic technology. Different amount TiO2 NPs are loaded onto the porous surface of PDMS spongy by spraying of photocatalyst suspension with different concentrations. Besides the self-cleaning effect under solar illumination, TiO2 NPs can change the surface charge density and permittivity of PDMS layer, thus regulating the electrical output of TENG.[30, 56] Figure 4a-c exhibit the performance of the TENG fabricated by 2 mm of PDMS spongy loaded with TiO2 NPs, where the performance of all the samples are at similar level but with slight differences. It is revealed that the electrical output can be influenced slightly by the loaded TiO2 mass ratio. For the lower loading amount using lower concentration of suspension (0.001 – 0.05 wt%), VOC, Q and ISC are slightly increased, which may be attributed to the increased permittivity by semiconductor NPs loading. However, if we further increase the loading ratio, the electrical performances decline accordingly. The aggregation of excessive TiO2 NPs on the surface of PDMS may reduce the effective contact area between PDMS and Al foil, resulting in the suppression of the output performance. The effects of loading ratio on other PDMS layers
with different thickness are also investigated. Similar changing trend is represented on these thicker friction layers, as shown in Figure S4. The output power - resistance profiles of the device fabricated by 2 mm PDMS layer loaded with 0.05 wt% TiO2 NPs are shown in Figure 4d. It displays explosive increase at high loading resistance at hundreds of megaohms, and reaches to its maximum instantaneous of 0.54 mW power at 210 MΩ, corresponding to the power density of 0.34 W·cm-2. Figure 4e shows the capability of charging a capacitor for the above TENG device. At the operation frequency of 1 Hz, the voltage of the 4.7, 22, 47 µF capacitor can reach 2.97, 1.23, 0.77 V, respectively. We have also prepared more experiments to show the possible application of this self-cleaning PDMS in the field of self-powered sensor. Here, a pressure sensor based on this porous PDMS with TiO2 is prepared (Figure S5a), where the porous PDMS with the thickness of 2 mm is applied as the dielectric layer and the TiO2 NPs of 0.05 wt% are loaded on the surface. Meanwhile, this dielectric layer is kept to be a cylinder with a diameter of 1.5 cm. The voltage output from this TENG with different applied pressure has been further studied to reveal its ability to detect normal force. Figure S5b shows the dynamic output signals from this self-cleaning TENG with different applied pressures on the device. The summarized change relationship is shown in Figure S5c. Here, the output voltage value increases linearly with the increase of pressure and thus, it can be used to quantitatively measure the amplitude of pressure. Maximum sensing pressure can reach 25.8 kPa and sensitivity is 1.43 kPa V-1. This TENG can also be applied as keyboard sensors to detect typing force (Figure. S5d). In addition, the durability are also performed by repeating compression and release action. The parameters ISC shows ignorable changing after nearly 2000
times motion cycles (Figure S6a) and after 4 weeks (Figure S6b), indicating the stability of the TENG fabricated by sponged PDMS. Meanwhile, In order to evaluate the mechanical properties of the TENG devices based on sponged PDMS with TiO2 photocatalyst, the uniaxial tensile test was studied, as can be seen in Figure S7. The ultimate stress of 0.36 MPa at a strain of 1.9 (mm/mm) can be achieved for the TENG sample, and the young’s module of 0.189 MPa. However, this polluted PDMS surface loaded with TiO2 NPs can also significantly affect the performance of TENG, leading to the largely decrease of Voc and Isc (Figure 4f).
Photocatalysis can remove the surface contaminant by organic compound degradation. Three steps are included in this process, which is shown in Figure 5. First, the photocatalyst TiO2 NPs absorb the photons whose energy larger than its band gap. Then free electrons (e-) are excited to the CB of TiO2, and the same number of holes (h+) are generated in VB. After the generated charge pair separates and migrates to the surface of NPs, redox reactions are induced at the interface of NPs and external chemical circumstance. The separated electrons and holes can also generate hydroxyl groups (·OH) and superoxide radicals (·O2-) based on the reaction with the adsorbed water (H2O) and oxygen (O2), as illustrated by the equations in Figure 5. Eventually, holes, as well as hydroxyl radicals and superoxide radicals, directly participate in the oxidation reaction for the contaminant degradation. This is the mechanism of self-cleaning proposed in this work. The self-cleaning effect upon the PDMS surface was demonstrated using 2 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs. Due to existence of surficial RhB molecules, it shows maximum absorption intensity at 552 nm for the polluted PDMS layer, which can be seen in Figure 6a. Under the irradiation of solar simulator, most RhB molecules decomposed, as evidenced by the gradually reduced absorption intensity of PDMS surface. The decolorization process is also recorded by digital photos visualized in Figure 6b at an interval of 5 min. After irradiation for 35 min, only less than 11% of dye molecules remains on the surface, implying the robust ability for contaminants destroying. By removing the surface contaminant, the electrical performance of TENG can be recovered to a certain extent. Figure 6c-6e shows VOC, Q and ISC of the original, polluted and self-cleaned devices. After 35 min of solar illumination, the VOC raises from 88 V to 115 V. Meanwhile, the transferred charge Qsc returns to 38 nC from the polluted value of 29 nC, which is larger than 90% of its original intensity, and the ISC it raises to 2.7 µA from 2.2 µA after contamination removal. Figure 6f shows the repeatability of the electrical output performance of sponge PDMS after degradation of contaminants. After three self-cleaning cycles (contaminating-cleaning process), the voltage output can still reach about 110 V. As for the Q and ISC, the values can reach to 35 nC and 2.8 µA, as shown in Figure S8a-b. The electrical output performance of the TENG prepared using TiO2 NPs-loaded PDMS as friction material shows no distinct decline, suggesting its robust durability. The persistent pollutant removal ability is also
studied by repeated tests. After repeating this contaminating-cleaning test for three cycles (Figure S9), the contaminate removal efficiency can also maintain at 80 %.The EDS of TiO2/sponge
PDMS before and after contaminants removal, as show in Figure S10, imply the good mechanical stability of photocatalyst-loaded dielectric surface. We further studied photo-assistance performance of this porous PDMS with TiO2 photocatalyst. Here, a solar simulator with a 400 nm cut-off filter was employed to illuminate the PDMS surface loaded with TiO2 photocatalyst, while, the similar contact-electrification measurements have been performed under the illumination. The results can be seen in Figure S11 and no significant change in electrical output performance can be observed with and without illumination. These results demonstrated that this self-cleaning surface can work stably with various conditions. As a fact, the contamination removal efficiency is rather depending on the photocatalyst loading amounts. For the loading suspension concentration varies from 0.001 wt% to 0.1 wt% (Figure S12), the dye decompose rate increases from 63% to 90% within the treatment duration of 35 min. Besides, the self-cleaning effect upon the PDMS surface is demonstrated using 6 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs and the red dye on the dielectric layer surface faded after 35 min of solar irradiation, as shown in Figure 6h. In addition, MB dye was also adopted as the simulation pollutants. As visualized in Figure 6i, blue-colored MB fades upon the photocatalyst-loaded PDMS surface, hinting the application universality of self-cleaning. This work is only TENG involved, however, we believe the self-cleaning strategy for device performance recovery, long-time operation is easy to be extended to other fields, as the selection diversity of photocatalysts and dielectric materials.
Conclusions In summary, TENG integrated with self-cleaning function is developed for the electrical performance recovery of the polluted one. By loading TiO2 NPs photocatalyst onto the porous PDMS polymer, simulated pollutant degrades and the surface of frictional layer becomes clean, along with the recovery of electrical performance of the constructed TENG device under UV light irradiation. Based on selecting 2 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs,
the value of Voc decreases from the 125 V to 88 V, when the friction layer is contaminated by organic pollutants. Under the solar illumination for 35 min, the self-cleaning interface can remove the contaminants attached on the tribo-surface, while VOC increases from 88 V to 115 V, indicating that 92% of the performance can be fully recovered. While the repeatability of the electrical output performance of sponge PDMS after degradation of contaminants is pretty good. After three cycles, the voltage output can still reach about 110 V, suggesting that the electrical output of the TENG can be remain 90% of its original intensity for a long-time operation. The proposed strategy based on self-cleaning effect of semiconductor photocatalysis upon polymer surface in this work, not only promotes the TENG electrical output slightly, but also contributes to the performance recovery of contaminated device. Due to the selection diversity of photocatalysts and dielectric materials, we believe this strategy is very easy to be expanded into other fields, especially the application in self-powered nanosystem, on-skin sensors and wearable devices.
Supporting Information Supporting Information is available online or from the author. Acknowledgements Hui Liu and Yawei Feng contributed equally to this work. This work was supported by the National Key R & D Project from Minister of Science and Technology (2016YFA0202704), the National Natural Science Foundation of China (Grant Nos. 21876114, 51775049, 21761142011, 51572174), Shanghai Government (19160712900), Beijing Natural Science Foundation (4192069), International Joint Laboratory on Resource Chemistry (IJLRC), and Ministry of Education of China (PCSIRT_IRT_16R49). Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Beijing Municipal Science & Technology Commission (Z171100000317001), Young Top-Notch Talents Program of Beijing Excellent Talents Funding (2017000021223ZK03) and Shanghai Engineering Research Center of Green Energy Chemical Engineering (18DZ2254200).
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Figure 1. Schematic diagram of the CS-TENG. (a) The basic working mechanism of the CS-TENG under the action of a vertical compressive force. (b) The current waveform of CS-TENG during one contact-separation cycle. (c) The 3D model for the quantitative analysis of CS-TENG output performance.
Figure 2. (a) A schematic shows the preparation process of the TiO2/PDMS sponge layer. (b) Digital photographs of different thickness of the spongy PDMS layer loaded with photocatalyst TiO2 NPs. SEM images of the microstructure of (c) PDMS and (d-e) TiO2/PDMS sponge. (f) Schematic of the CS-TENG with photocatalyst-loaded PDMS spongy as friction layer.
Figure 3. (a) The setup for electrical property testing, which including a Keithley 6514 system electrometer and linear motor. (b-d) The CS-TENG performance fabricated by pure PDMS spongy with different thicknesses. (e-g) The electrical performance comparison before and after the surface polluted by RhB contaminant.
Figure 4. The performance of the CS-TENG fabricated by 2 mm of PDMS spongy loaded with TiO2 NPs. (a) VOC, (b) Q, (c) ISC, and (d) the instantaneous power - resistance profiles and (e) the capability of charging a capacitor of the CS-TENG. (f) The electrical properties of the polluted CS-TENG loaded with TiO2 NPs.
Figure 5. Schematic of organic containment degradation by photocatalyst NPs in the presence of solar light.
Figure 6. Self-cleaning and performance recovery of the device fabricated using TiO2/PDMS. (a) The absorption spectra change and (b) digital photographs of the polluted surface recorded at an interval of 5 min under solar light irradiation. (c-e) The electrical performance of the original, polluted and self-cleaned devices and (f) the VOC change after three cycles of contaminants degradation. (g) Photocatalytic performance of RhB degradation on PDMS layer with different TiO2 loadings under the irradiation of solar simulator. Self-cleaning of (h) RhB dye and (i) MB dye on the surface of TiO2/PDMS for 35 min of solar light treatment.
AUTHOR INFORMATION
Hui Liu received her B.S. degree in Applied Chemistry from Huainan Normal University in 2017, and he is currently pursuing a M.S. degree in Shanghai Normal University. His research interests are mainly focused on the coupling of piezotronics/piezo-phototronics and catalysis.
Yawei Feng received his master degree in Industrial Catalysis from Shanghai Normal University in 2018, and he is currently pursuing his Ph.D. in Beijing Institute of Nanoenergy and Nanosystems (BINN), Chinese Academy of Sciences. His research interests are focused on the nanogenerator, self-powered chemistry and the coupling of piezotronics/piezo-phototronics and catalysis.
Jiajia Shao received his Ph.D. degree from The National Center for Nanoscience and Technology in 2019. Now he is a postdoctoral researcher in Prof. Zhong Lin Wang's group at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His research interests include theoretical and experimental studies on: mechanical energy harvesting by triboelectric nanogenerators and energy storage, self-powered systems, nanogenerator-based sensors.
Yao Chen received her B.S. degree in Composite Materials Science and Engineering from Fujian Normal University in 2017, and she is currently pursuing a M.S. degree in Shanghai Normal University. Her research interests are mainly focused on the TiO2-based nanomaterials for environmental and energy photocatalysis.
Prof. Zhong Lin (Z. L.) Wang is the Hightower Chair in Materials Science and Engineering and Regents’ Professor at Georgia Tech, the chief scientist and director of the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His discovery and breakthroughs in developing nanogenerators and
self-powered nanosystems establish the principle and technological road map for harvesting mechanical energy from environmental and biological systems for powering personal electronics and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics
Prof. Hexing Li got doctor degree from Fudan University. Now, he is working as a vice-director of Chinese National Photocatalysis Committee and an Associated Editor of Appl. Catal. B Environ. He is also working as editorial board members of ACS Appl. Mater. Interface, Catal. Commun. and Current Green Chem. etc. His research interest in environmental catalysis including photocatalysis for removing pollutants and thermocatalysis for green chemistry. More than 200 papers and 2 monographs have been published.
Prof. Xiangyu Chen received his B.S. degree in Electrical Engineering from Tsinghua University in 2007 and hisPh.D. in Electronics Physics from Tokyo Institute ofTechnology in 2013. Now he is a professor in BeijingInstitute of nanoenergy and nanosystems, ChineseAcademic of Sciences. His main research interests have been focused on the field of functional dielectric materials,self-powered nano energy system and the nonlinear opticallaser system for characterizing the electrical properties of the devices.
Prof. Zhenfeng Bian received his PhD in Environmental chemistry from the Shanghai Normal University in 2010. He has been a JSPS Postdoctoral Fellow in the lab of Professor Tetsuro Majima during 2010-2013. He became a Professor in the Department of Chemistry at Shanghai Normal University in 2013. He research interests are focused on the design and synthesis of TiO2-based nanomaterials for environmental and energy photocatalysis.
i
b
ii +
- - - - - - - - - + + + + + + + + + +
+
+
+
Current
a
iii +
+
+
Electrode
+
- - - - - - - - - + + + + + +
i
ii
+ + + + + - - - - - - - - - + + + + +
Time
c
iv i
- - - - - - - - - + + + + + +
Dielectrc
iv
iii
a
Sugar cubes
PDMS
Photocatalyst
Curing
Washing
Loading
f
2 mm
4 mm
6 mm
b
c
Arcylic
d
Al
PDMS
e
Photocatalyst
a
b
6 mm 2 mm
120
c
4 mm
6 mm 2 mm
-40
d
4 mm
6mm 4mm 2mm
3
90 60 30
Current (μA)
Charge (nC)
Voltage (V)
2 -30
-20 -10 0
0 0
5
10
15
20
25
30
0
5
10
15
20
25
-2
30
0
5
-40
90 60 30
10
15
20
25
Time (s)
g
-50 Orginal Polluted
3
Current (μA)
f
Orginal Polluted
Charge (nC)
Voltage (V)
-1
Time (s)
150 120
0
-3
Time (s)
e
1
-30 -20
Original Polluted
2
1
-10
0
0
0
6mm
4mm
2mm
6mm
4mm
2mm
6mm
4mm
2mm
30
0.1wt% 0.005wt%
0.05wt% 0.001wt%
b
0.01wt% 0wt%
Charge (nC)
Voltage (V)
120 90 60
30
-50
0.1wt% 0.005wt%
0.05wt% 0.001wt%
-40
2
-30
1
-20 -10
0
10
20
30
40
50
0
60
e
0.6
10
20
1.0
0.2
0.5
0.1
0.0 10M
40
50
-1 -2
0
60
10
100M
Resistance (Ω)
0.0 1G
20
30
40
50
60
Voltage Current
3
Time (s)
f 120
Voltage (V)
0.3
1.5
Voltage (V)
0.4
2.0
Power (mW)
Current (μA)
2.5
1M
30
4.7 μF 22 μF 47 μF
3
0.5
0.01wt% 0wt%
0
Time (s)
3.0
0.05wt% 0.001wt%
-3
Time (s)
100k
0.1wt% 0.005wt%
3
0
0
d
c
0.01wt% 0wt%
2
1
80
2
40
1
0
0
0 0
30
60
90
Time (s)
120
Original
Polluted
Current (μA)
150
Current (μA)
a
O2 e-
CB
·O2-
Excitation
Organic containment
h+ VB H2O
·OH Photocatalyst NPs
Self-cleaning effect refreshes the surface
CO2, H2O
a
0 min 5 min 10 mim 15 mim 20 mim 25 mim 30 mim 35 mim
0.05wt% 0.20
Abs. (a.u.)
b
0.25
0.15 0.10
2 cm
0 min
5 min
10 min
15 min
20 min
25 min
30 min
35 min
0.05 0.00
400
500
600
700
Wavelength (nm) 150
Original
Charge (nC)
120
Voltage (V)
d
Polluted Self-cleaned
90 60
-50
-20
0
0 10
15
20
25
30
35
0
1st cycle
3rd cycle
2nd cycle
g RhB concentration (%)
Voltage (V)
120 90 60 30 0
0
10
20
Time (s)
30
40
Polluted Self-cleaned
1 0
-1 -2 -3
10
Time (s)
f
Original
3 2
-30
-10
5
e
Polluted Self-cleaned
-40
30
0
Original
Current (μA)
c
20
0
30
Time (s)
5
10
15
20
25
30
Time (s)
h 0.001 wt% 0.005 wt% 0.01 wt% 0.05 wt% 0.1 wt%
100 80 60
2 cm
0 min
35 min
2 cm
0 min
35 min
i
40 20
0 0
10
20
Time (min)
30
40
Research Highlights： 1. TENG integrated with self-cleaning function is proposed for the electrical performance recovery of the polluted one. 2. By loading TiO2 photocatalyst onto the porous dielectric polymer, the polluted surface of TENG device can be refreshed and the performances recover to 90% of its original level after solar light treatment. 3. Developed strategy is very easy to be expanded into self-powered nanosystem, on-skin sensors and wearable devices due to the selection diversity of photocatalysts and dielectric materials.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2211-2855(20)30056-2
DOI:
https://doi.org/10.1016/j.nanoen.2020.104499
Reference:
NANOEN 104499
To appear in:
Nano Energy
Received Date: 28 October 2019 Revised Date:
16 December 2019
Accepted Date: 13 January 2020
Please cite this article as: H. Liu, Y. Feng, J. Shao, Y. Chen, Z.L. Wang, H. Li, X. Chen, Z. Bian, Self-cleaning Triboelectric nanogenerator based on TiO2 photocatalysis, Nano Energy, https:// doi.org/10.1016/j.nanoen.2020.104499. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Graphical abstract
Self-cleaning Triboelectric nanogenerator based on TiO2 photocatalysis
Hui Liu,a,
§
Yawei Feng,b,c,
§
Jiajia Shao,b,c Yao Chen,a Zhong Lin Wangb,c,e Hexing Li,a,d Xiangyu
Chen,b,c,* Zhenfeng Bian,a,*
a
The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key Laboratory of
Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, PR China b
CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and
Sensor Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, PR China c
School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing
100049, PR China d
School of Mathematics and Physics, Shanghai Key Laboratory of Materials Protection and
Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, PR China e
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA
30332, USA Email: X. C., [email protected]; Z. B., [email protected];
§ Hui Liu and Yawei Feng contributed equally to this work.
Abstract: Dielectric polymer materials are indispensable for the fabrication of self-powered nanosystem and electronic skin. However, the performance of the device is often seriously degraded due to the contamination of the surface. It is very important to keep the clean of the surface of the device, especially for the application of wearable devices. In this work, triboelectric nanogenerator (TENG) integrated with TiO2 photocatalysis is developed for realizing the dual functions of energy generation and self-cleaning interface. The friction layer of TENG is sponged polydimethylsiloxane (PDMS) film which is prepared by sacrificial template method with the pore size varied from hundreds of microns to several millimeters. When the interface of TENG is contaminated by organic pollutants, the open circuit voltage (VOC) of the device decrease from 125 V to 88 V. Under the solar illumination for 35 min, the self-cleaning interface can degrade the contaminants attached on the tribo-surface, while VOC raises from 88 V to 115 V after illumination, indicating that 92% of the TENG’s output performance can be fully recovered. The photocatalysis-based self-cleaning effect on polymer tribo-material can contribute to the application of TENG towards e-skin and wearable devices.
Keywords: triboelectric nanogenerator, dielectric polymer, frictional layer, self-cleaning, photocatalysis
Introduction Power generation by harvesting ambient mechanical energy to operate micro- or nanodevices is considered as one promising solution for internet of things and wearable devices.[1-5] Triboelectric nanogenerator (TENG) can efficiently convert ambient mechanical energy, such as human motion, wind blow, water wave or other kinds of mechanical vibration, into electricity based on the contact electrification and electrostatic induction. Distinct from the mechanism of electromagnetic generator, the foundational principle of TENG can be found in Maxwell displacement current, which is related to the changing of the polarization of the dielectrics.[6-12] As a new type of energy collector, the application of TENGs has been extended into various field, such as self-powered nanosystem, self-powered electrochemistry, human-machine interaction, far beyond the original energy harvesting device.[13-20] Polymers are the basic dielectric materials for the fabrication of the self-powered nanosystem and flexible wearable devices.[21-26] As the friction layer of TENG, the intrinsic properties of polymer are very important to its electrical output performance. Emerging polymers with high permittivity or modified functional groups are designed to suppress the discharge, improve the surface charge density and applied in some special environment.[27-32] However, the triboelectric layer can be polluted in the specific condition, such as on-skin sensing and wearable device.[33-37] The contaminated devices may not only affect their normal operation, but also influence the wearing comfort, and potentially harm human health. Thus, self-cleaning of the devices is indispensable for the real application of TENG technique.[38-40] Photocatalysis has been demonstrated as a sustainable technology for organic pollutants degradation by harvesting solar energy.[41-43] Titanium dioxide nanoparticles (TiO2 NPs) are recognized as one of the most applied non-toxic photocatalysts due to their high activity, light stability and low cost[44-46]. By the excitation of UV light, free electrons (e-) are excited to the conduction band (CB) of TiO2, leaving holes (h+) in the valence band (VB). The generated charge carriers with high energy state, along with the derived free radicals, are active for the initiation of organic compounds degradation into small nontoxic molecules of H2O and CO2.[47-50] Therefore, if the photocatalytic degradation function is combined with the TENG, it can produce an effective self-cleaning effect and the recovery of device performance.
In this work, self-cleaning function is applied on the tribo-interface of TENG, which can achieve dual function of contact electrification and pollution degradation. The widely used PDMS dielectric is selected as the friction layer of TENG device for its high electronegativity and flexibility. Sponged PDMS film is prepared with the pore size varied from hundreds of micrometers to several millimeters. TiO2 NPs is loaded into the pore structures of the PDMS surface. The contaminated interface of TENG results in the decrease of both open circuit voltage (VOC) and short-circuit current (Isc) of the device. Under the solar light, the contaminations attached on the tribo surface are cleaned by the photocatalytic material and the electrical performance of the constructed TENG device can be recovered. The photocatalysis-based self-cleaning effect on polymer surface proposed can contribute to TENG performance recovery and further promote its application in e-skin and wearable devices.
2. Experimental section 2.1 Fabrication of sponged PDMS The fabrication of sponge PDMS film follows the reported process.[51] PDMS glue (Sylgard 184, Dow Corning) is mixed by elastomer and curing agent at a mass ratio of 10:1. After stirring for 30 minutes, the bubbles are removed in vacuum condition. Three templates with volume of 60 mm × 60 mm × 2 mm, 60 mm × 60 mm × 4 mm and 60 mm × 60 mm × 6 mm were prepared using acrylic plate. Then 8g, 14g, 20g of sugar cubes with average length of 2 mm, as show in Figure S1a-b, are placed into three templates accordingly. The PDMS mixtures are poured into templates and degassed under vacuum. Due to capillary forces, PDMS glue can penetrate into the intergranular space of the sugar template. After 2 hours of curing process at 80 °C and the subsequent hot water (90-95°C) treatment to dissolve the sugar particles, sponged PDMS friction layers with different thickness are obtained. For the fabrication of TENG, all sponges are cut into size of 4 cm×4 cm with different thickness.
2.2 Preparation of TiO2/PDMS sponge Different concentrations of suspension are prepared by dispersing TiO2 NPs (Degussa P25) into deionized water. 5 mL of the above photocatalyst suspensions is sprayed onto the surface of the
sponged PDMS layers to obtain the TiO2/PDMS with different loading amounts. After the residual deionized water in the sponges evaporated at room temperature, TiO2/PDMS sponge layers are prepared. The adopted photocatalyst concentrations in this fabrication process are 0.1wt%, 0.05wt%, 0.01wt%, 0.005wt% and 0.001wt%, respectively. The morphology is characterized using scanning electron microscopy (SEM, Nova NanoSEM 450 and Hitachi SU8020) equipped with an energy dispersive X-ray spectroscopy (EDS, Bruker quantax 400).
2.3 TENG fabrication and measurements A vertical contact–separation mode TENG (CS-TENG) is fabricated, where the effective area is kept to be 4 cm × 4 cm. The upper part of the TENG is constructed by attaching an aluminum (Al) foil to an acrylic board substrate (4 cm×4 cm × 3 cm). The TiO2/PDMS layer stacked on an acrylic board substrate (4 cm×4 cm × 3 cm) adhered with a same area of Al foil is act as the bottom layer. Cu wires are connected to the two metal layers for electrical measurements.
2.4 Self-cleaning experiments Rhodamine B (RhB) and methylene blue (MB) steams are used as contaminant source to simulate the possible contamination happened during the operation of TENG. Here, 5 mL of 5 ppm RhB and 5 mL of 5 ppm MB are sprayed to the surface of PDMS sponge, while the performance change of the TENG before and after contamination are studied. A solar simulator (Zolix, LCSS500-NMNY, 11.2 mW·cm-2) with a 400 nm UV cut-off filter is vertically irradiated to PDMS surface. The surface absorption spectrum is measured with a spectrophotometer (Shimadzu UV 3600) at intervals.
3. Results and discussion 3.1 Working Mechanism of TENG Figure 1a illustrates the basic working mechanism of the CS-TENG under the action of a vertical compressive force. When contacted with metal material, polymer material is negatively charged according to the triboelectric series.[52] After friction, the dielectric polymer surface and metal electrode possess the equivalent charges with opposite types. Defining a special state when the top electrode is far infinitely away from the dielectric polymer (state i) as the initial state, the bottom
metal electrode is positively charged due to the electrostatic induction, while the top electrode with no charges. When an external force is applied (state ii-iii), positive charges are induced on the top electrode, and the extra electrons flow to the bottom electrode through the loading circuit, resulting in a unidirectional current pulse, which can be indicated by the waveform in Figure 1b. As for the release process (state iii-iv), the situation is reversed. The reduced charge density on the top electrode leads to the backward of electrons, and an inversive current pulse signal appears (Figure 1b). During the periodic press and release of TENG, the electrons flow back and forth through the external circuit, resulting in the alternating current and voltage. The charge density (σ) on dielectric surface is critical to the electrical performance of a TENG device. In the configuration of vertical contact–separation mode, the metal acts both as an upper electrode and as a friction material. The traditional capacitance model for output performance analysis is inaccurate because of the neglecting of edge effect.[53, 54] Thus, we construct a three-dimensional (3D) mathematical model (Figure 1c) for the quantitative performance analysis.[55] For the construction of mathematical model, a virtual charged plane with the same size (length of a, width of b) of metal electrode is set. Due to this plane is upon the mental electrode and in air, the permittivity can be set to be the value of air (ε0). The charge on this plane is resulted from the friction with dielectric polymer (ε2), thus the surface of this virtual plane is the same charge density (+σT) as dielectric layer (-σT), but an opposite charge type. In a particular state, the charge density on the bottom electrode is +σu, thus the charge density on top electrode is -σu. The analysis is start from Leibniz integral rule, thus the potential at the top electrode z1 is described as ф
, ,
=−
+
+
−
(1) the potential at the bottom electrode z4 is described as ф
, ,
=−
+
+
−
(2) where = arctan
# $
%
!" &
'
&
(3)
When a resistance R is connected to the electrodes, the voltage over R is ,-
∆) = *+ = −* ,. = −*/
,
,.
(4)
Where A is the area of electrode, Q is the transferred charge, and Kirchhoffs' law states −*/
,
,.
=ф
, ,
−ф
, ,
(5)
For open-circuit (OC) conditions, Q is 0 without charge transfer, the height values of z1 and z2 are approximate because of the virtual plane. Therefore, the VOC can be calculated by −)01 =
−
(6)
From Eq.6, the properties of dielectric layer, such as the permittivity constant ε2 and surface charge density σT, are essential for the output performance of CS-TENG.
3.2 Characterization of the TiO2/PDMS sponge layer Figure 2a shows the preparation process of the TiO2/PDMS sponge layer. The main steps include raw materials mixing, curing, cutting, washing, and TiO2 photocatalyst NPs loading. PDMS sponge with the thickness of 6 mm, 4 mm, 2 mm are fabricated firstly, where the porous structures of PDMS layer are formed by sacrificial template method. Optical photographs of the enlarged sponger surface in Figure S2 in the Supporting Information clearly exhibit the pores varying from hundreds of microns to several millimeters. The PDMS sponge serves as a three-dimensional framework for TiO2 NPs loading due to the abundant macro-pore structures. The photos in Figure 2b show the sponger-like appearances and opalescent color of the PDMS layer loaded with photocatalyst TiO2 NPs. Though there are no notable differences for naked eyes, the micro-domains of PDMS sponge layers before and after TiO2 NPs (using 0.05 wt% of the suspension) loading are distinct, as shown in Figure 2c-e. Agglomerated TiO2 NPs are clearly distributed on the surface of photocatalyst-loaded PDMS (Figure 2d,e), while the untreated PDMS surface shows smooth profile (Figure 2c). The SEM images and EDS of the TiO2/sponge PDMS in Figure S3 reveal that photocatalyst nanoparticles almost fully cover the polymer surface. After attaching the Al foils and acrylic substrates, a CS-TENG can be fabricated. The detailed information of the fabricated TENG device can be seen in the diagram of Figure 2f. Sponged
PDMS with or without TiO2 NPs (factual contact area of 16 cm2) is act as the dielectric material for electrification, while Al foils have the dual functions of output electrode and the tribo-layer for contact with the sponged PDMS.
3.3 Influence of surface pollution For the electrical property testing, a linear motor with a working frequency of 1 Hz is applied to drive the TENG, and the VOC, ISC and maximum transferred charges (Qsc) are measured by a Keithley 6514 system electrometer. Figure 3a shows the setup for electrical property testing, and Figure 3b~3d shows the performance of the TENG devices fabricated by pure PDMS spongy with different thicknesses. It shows no obvious difference between the sponge-like PDMS with a thickness of 6 mm and 4mm, both has a VOC value of about 105 V, ISC value of 2.0 µA, and transferred charge of 35 nC. For the TENG fabricated by 2 mm of PDMS spongy, the VOC is about 125 V, the ISC was 2.9 µA, and the maximum transferred charge was 40 nC. PDMS sponge with thickness of 6 and 4 mm displayed similar performances but higher performance for the PDMS sponge with 2 mm. The quality and grain size of cube sugar can decide the effective thickness of the PDMS sponge under compression. Meanwhile, the effective thickness of the dielectric film can influence the output performance. For the PDMS with a thickness of 2 mm, the length of PDMS holes produced by sugar agent is close to the thickness of the sponge PDMS. Hence, during the contact motion the effective thickness of the PDMS film under the compression can be much smaller than 2 mm, which leads to a sudden increase of the output performance in comparison with the other two samples. The polymer-based friction layers are vulnerable to organic contaminant in some unsealed application scenario. For some self-powered on-skin sensor and wearable devices, the contact to skin or motion movement may cause skin secretions adhere to the polymer surface, which decreases the surface charge density, disturbs operations, reduces wear comfort, even shorten their work lifetime. The pollution effect is simulated with colored dye RhB steam. Figure 3e-g shows the performance results of the polluted TENG by this organic contaminant, where both the VOC, Qsc and ISC show significant decrease at the work frequency of 1 Hz. For the worst case, the VOC and Qsc from the polluted device using 4 mm of PDMS spongy almost drop to 60% of the original
value, while the ISC drops to about 78%. Actually, for the sample with the thickness of 4 mm and 6 mm, the output performance and their performance change after being polluted are quite comparable with each other. However, the sample with thickness of 2 mm shows a quite different performance in comparison with the other samples. We believe the reason is attributed to the detailed structure of the PDMS sponge. The length of sacrificial sugar agent is 2 mm, which is close to the thickness of the sponge PDMS. Hence, during the contact motion the effective thickness of the PDMS film under the compression can be much smaller than 2 mm. Meanwhile, there are more penetrating channels existing inside this 2 mm samples. All these factors may influence the performance of this PDMS during the experiments. Hence, if the similar amount of the contaminated material is retained on the surface, the reduction of surface charge on PDMS sponges with different thicknesses can be different. Though the parameters output from the polluted TENG using 2 mm of PDMS spongy decrease not so obviously as the others, it is still irreversible for their attenuation phenomena.
3.4 Self-cleaning and recovery of TENG performance To restrain the performance degradation caused by surface contamination, self-cleaning effect is introduced by semiconductor photocatalytic technology. Different amount TiO2 NPs are loaded onto the porous surface of PDMS spongy by spraying of photocatalyst suspension with different concentrations. Besides the self-cleaning effect under solar illumination, TiO2 NPs can change the surface charge density and permittivity of PDMS layer, thus regulating the electrical output of TENG.[30, 56] Figure 4a-c exhibit the performance of the TENG fabricated by 2 mm of PDMS spongy loaded with TiO2 NPs, where the performance of all the samples are at similar level but with slight differences. It is revealed that the electrical output can be influenced slightly by the loaded TiO2 mass ratio. For the lower loading amount using lower concentration of suspension (0.001 – 0.05 wt%), VOC, Q and ISC are slightly increased, which may be attributed to the increased permittivity by semiconductor NPs loading. However, if we further increase the loading ratio, the electrical performances decline accordingly. The aggregation of excessive TiO2 NPs on the surface of PDMS may reduce the effective contact area between PDMS and Al foil, resulting in the suppression of the output performance. The effects of loading ratio on other PDMS layers
with different thickness are also investigated. Similar changing trend is represented on these thicker friction layers, as shown in Figure S4. The output power - resistance profiles of the device fabricated by 2 mm PDMS layer loaded with 0.05 wt% TiO2 NPs are shown in Figure 4d. It displays explosive increase at high loading resistance at hundreds of megaohms, and reaches to its maximum instantaneous of 0.54 mW power at 210 MΩ, corresponding to the power density of 0.34 W·cm-2. Figure 4e shows the capability of charging a capacitor for the above TENG device. At the operation frequency of 1 Hz, the voltage of the 4.7, 22, 47 µF capacitor can reach 2.97, 1.23, 0.77 V, respectively. We have also prepared more experiments to show the possible application of this self-cleaning PDMS in the field of self-powered sensor. Here, a pressure sensor based on this porous PDMS with TiO2 is prepared (Figure S5a), where the porous PDMS with the thickness of 2 mm is applied as the dielectric layer and the TiO2 NPs of 0.05 wt% are loaded on the surface. Meanwhile, this dielectric layer is kept to be a cylinder with a diameter of 1.5 cm. The voltage output from this TENG with different applied pressure has been further studied to reveal its ability to detect normal force. Figure S5b shows the dynamic output signals from this self-cleaning TENG with different applied pressures on the device. The summarized change relationship is shown in Figure S5c. Here, the output voltage value increases linearly with the increase of pressure and thus, it can be used to quantitatively measure the amplitude of pressure. Maximum sensing pressure can reach 25.8 kPa and sensitivity is 1.43 kPa V-1. This TENG can also be applied as keyboard sensors to detect typing force (Figure. S5d). In addition, the durability are also performed by repeating compression and release action. The parameters ISC shows ignorable changing after nearly 2000
times motion cycles (Figure S6a) and after 4 weeks (Figure S6b), indicating the stability of the TENG fabricated by sponged PDMS. Meanwhile, In order to evaluate the mechanical properties of the TENG devices based on sponged PDMS with TiO2 photocatalyst, the uniaxial tensile test was studied, as can be seen in Figure S7. The ultimate stress of 0.36 MPa at a strain of 1.9 (mm/mm) can be achieved for the TENG sample, and the young’s module of 0.189 MPa. However, this polluted PDMS surface loaded with TiO2 NPs can also significantly affect the performance of TENG, leading to the largely decrease of Voc and Isc (Figure 4f).
Photocatalysis can remove the surface contaminant by organic compound degradation. Three steps are included in this process, which is shown in Figure 5. First, the photocatalyst TiO2 NPs absorb the photons whose energy larger than its band gap. Then free electrons (e-) are excited to the CB of TiO2, and the same number of holes (h+) are generated in VB. After the generated charge pair separates and migrates to the surface of NPs, redox reactions are induced at the interface of NPs and external chemical circumstance. The separated electrons and holes can also generate hydroxyl groups (·OH) and superoxide radicals (·O2-) based on the reaction with the adsorbed water (H2O) and oxygen (O2), as illustrated by the equations in Figure 5. Eventually, holes, as well as hydroxyl radicals and superoxide radicals, directly participate in the oxidation reaction for the contaminant degradation. This is the mechanism of self-cleaning proposed in this work. The self-cleaning effect upon the PDMS surface was demonstrated using 2 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs. Due to existence of surficial RhB molecules, it shows maximum absorption intensity at 552 nm for the polluted PDMS layer, which can be seen in Figure 6a. Under the irradiation of solar simulator, most RhB molecules decomposed, as evidenced by the gradually reduced absorption intensity of PDMS surface. The decolorization process is also recorded by digital photos visualized in Figure 6b at an interval of 5 min. After irradiation for 35 min, only less than 11% of dye molecules remains on the surface, implying the robust ability for contaminants destroying. By removing the surface contaminant, the electrical performance of TENG can be recovered to a certain extent. Figure 6c-6e shows VOC, Q and ISC of the original, polluted and self-cleaned devices. After 35 min of solar illumination, the VOC raises from 88 V to 115 V. Meanwhile, the transferred charge Qsc returns to 38 nC from the polluted value of 29 nC, which is larger than 90% of its original intensity, and the ISC it raises to 2.7 µA from 2.2 µA after contamination removal. Figure 6f shows the repeatability of the electrical output performance of sponge PDMS after degradation of contaminants. After three self-cleaning cycles (contaminating-cleaning process), the voltage output can still reach about 110 V. As for the Q and ISC, the values can reach to 35 nC and 2.8 µA, as shown in Figure S8a-b. The electrical output performance of the TENG prepared using TiO2 NPs-loaded PDMS as friction material shows no distinct decline, suggesting its robust durability. The persistent pollutant removal ability is also
studied by repeated tests. After repeating this contaminating-cleaning test for three cycles (Figure S9), the contaminate removal efficiency can also maintain at 80 %.The EDS of TiO2/sponge
PDMS before and after contaminants removal, as show in Figure S10, imply the good mechanical stability of photocatalyst-loaded dielectric surface. We further studied photo-assistance performance of this porous PDMS with TiO2 photocatalyst. Here, a solar simulator with a 400 nm cut-off filter was employed to illuminate the PDMS surface loaded with TiO2 photocatalyst, while, the similar contact-electrification measurements have been performed under the illumination. The results can be seen in Figure S11 and no significant change in electrical output performance can be observed with and without illumination. These results demonstrated that this self-cleaning surface can work stably with various conditions. As a fact, the contamination removal efficiency is rather depending on the photocatalyst loading amounts. For the loading suspension concentration varies from 0.001 wt% to 0.1 wt% (Figure S12), the dye decompose rate increases from 63% to 90% within the treatment duration of 35 min. Besides, the self-cleaning effect upon the PDMS surface is demonstrated using 6 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs and the red dye on the dielectric layer surface faded after 35 min of solar irradiation, as shown in Figure 6h. In addition, MB dye was also adopted as the simulation pollutants. As visualized in Figure 6i, blue-colored MB fades upon the photocatalyst-loaded PDMS surface, hinting the application universality of self-cleaning. This work is only TENG involved, however, we believe the self-cleaning strategy for device performance recovery, long-time operation is easy to be extended to other fields, as the selection diversity of photocatalysts and dielectric materials.
Conclusions In summary, TENG integrated with self-cleaning function is developed for the electrical performance recovery of the polluted one. By loading TiO2 NPs photocatalyst onto the porous PDMS polymer, simulated pollutant degrades and the surface of frictional layer becomes clean, along with the recovery of electrical performance of the constructed TENG device under UV light irradiation. Based on selecting 2 mm of dielectric layer loaded with 0.05 wt% photocatalyst NPs,
the value of Voc decreases from the 125 V to 88 V, when the friction layer is contaminated by organic pollutants. Under the solar illumination for 35 min, the self-cleaning interface can remove the contaminants attached on the tribo-surface, while VOC increases from 88 V to 115 V, indicating that 92% of the performance can be fully recovered. While the repeatability of the electrical output performance of sponge PDMS after degradation of contaminants is pretty good. After three cycles, the voltage output can still reach about 110 V, suggesting that the electrical output of the TENG can be remain 90% of its original intensity for a long-time operation. The proposed strategy based on self-cleaning effect of semiconductor photocatalysis upon polymer surface in this work, not only promotes the TENG electrical output slightly, but also contributes to the performance recovery of contaminated device. Due to the selection diversity of photocatalysts and dielectric materials, we believe this strategy is very easy to be expanded into other fields, especially the application in self-powered nanosystem, on-skin sensors and wearable devices.
Supporting Information Supporting Information is available online or from the author. Acknowledgements Hui Liu and Yawei Feng contributed equally to this work. This work was supported by the National Key R & D Project from Minister of Science and Technology (2016YFA0202704), the National Natural Science Foundation of China (Grant Nos. 21876114, 51775049, 21761142011, 51572174), Shanghai Government (19160712900), Beijing Natural Science Foundation (4192069), International Joint Laboratory on Resource Chemistry (IJLRC), and Ministry of Education of China (PCSIRT_IRT_16R49). Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Beijing Municipal Science & Technology Commission (Z171100000317001), Young Top-Notch Talents Program of Beijing Excellent Talents Funding (2017000021223ZK03) and Shanghai Engineering Research Center of Green Energy Chemical Engineering (18DZ2254200).
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Figure 1. Schematic diagram of the CS-TENG. (a) The basic working mechanism of the CS-TENG under the action of a vertical compressive force. (b) The current waveform of CS-TENG during one contact-separation cycle. (c) The 3D model for the quantitative analysis of CS-TENG output performance.
Figure 2. (a) A schematic shows the preparation process of the TiO2/PDMS sponge layer. (b) Digital photographs of different thickness of the spongy PDMS layer loaded with photocatalyst TiO2 NPs. SEM images of the microstructure of (c) PDMS and (d-e) TiO2/PDMS sponge. (f) Schematic of the CS-TENG with photocatalyst-loaded PDMS spongy as friction layer.
Figure 3. (a) The setup for electrical property testing, which including a Keithley 6514 system electrometer and linear motor. (b-d) The CS-TENG performance fabricated by pure PDMS spongy with different thicknesses. (e-g) The electrical performance comparison before and after the surface polluted by RhB contaminant.
Figure 4. The performance of the CS-TENG fabricated by 2 mm of PDMS spongy loaded with TiO2 NPs. (a) VOC, (b) Q, (c) ISC, and (d) the instantaneous power - resistance profiles and (e) the capability of charging a capacitor of the CS-TENG. (f) The electrical properties of the polluted CS-TENG loaded with TiO2 NPs.
Figure 5. Schematic of organic containment degradation by photocatalyst NPs in the presence of solar light.
Figure 6. Self-cleaning and performance recovery of the device fabricated using TiO2/PDMS. (a) The absorption spectra change and (b) digital photographs of the polluted surface recorded at an interval of 5 min under solar light irradiation. (c-e) The electrical performance of the original, polluted and self-cleaned devices and (f) the VOC change after three cycles of contaminants degradation. (g) Photocatalytic performance of RhB degradation on PDMS layer with different TiO2 loadings under the irradiation of solar simulator. Self-cleaning of (h) RhB dye and (i) MB dye on the surface of TiO2/PDMS for 35 min of solar light treatment.
AUTHOR INFORMATION
Hui Liu received her B.S. degree in Applied Chemistry from Huainan Normal University in 2017, and he is currently pursuing a M.S. degree in Shanghai Normal University. His research interests are mainly focused on the coupling of piezotronics/piezo-phototronics and catalysis.
Yawei Feng received his master degree in Industrial Catalysis from Shanghai Normal University in 2018, and he is currently pursuing his Ph.D. in Beijing Institute of Nanoenergy and Nanosystems (BINN), Chinese Academy of Sciences. His research interests are focused on the nanogenerator, self-powered chemistry and the coupling of piezotronics/piezo-phototronics and catalysis.
Jiajia Shao received his Ph.D. degree from The National Center for Nanoscience and Technology in 2019. Now he is a postdoctoral researcher in Prof. Zhong Lin Wang's group at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His research interests include theoretical and experimental studies on: mechanical energy harvesting by triboelectric nanogenerators and energy storage, self-powered systems, nanogenerator-based sensors.
Yao Chen received her B.S. degree in Composite Materials Science and Engineering from Fujian Normal University in 2017, and she is currently pursuing a M.S. degree in Shanghai Normal University. Her research interests are mainly focused on the TiO2-based nanomaterials for environmental and energy photocatalysis.
Prof. Zhong Lin (Z. L.) Wang is the Hightower Chair in Materials Science and Engineering and Regents’ Professor at Georgia Tech, the chief scientist and director of the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His discovery and breakthroughs in developing nanogenerators and
self-powered nanosystems establish the principle and technological road map for harvesting mechanical energy from environmental and biological systems for powering personal electronics and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics
Prof. Hexing Li got doctor degree from Fudan University. Now, he is working as a vice-director of Chinese National Photocatalysis Committee and an Associated Editor of Appl. Catal. B Environ. He is also working as editorial board members of ACS Appl. Mater. Interface, Catal. Commun. and Current Green Chem. etc. His research interest in environmental catalysis including photocatalysis for removing pollutants and thermocatalysis for green chemistry. More than 200 papers and 2 monographs have been published.
Prof. Xiangyu Chen received his B.S. degree in Electrical Engineering from Tsinghua University in 2007 and hisPh.D. in Electronics Physics from Tokyo Institute ofTechnology in 2013. Now he is a professor in BeijingInstitute of nanoenergy and nanosystems, ChineseAcademic of Sciences. His main research interests have been focused on the field of functional dielectric materials,self-powered nano energy system and the nonlinear opticallaser system for characterizing the electrical properties of the devices.
Prof. Zhenfeng Bian received his PhD in Environmental chemistry from the Shanghai Normal University in 2010. He has been a JSPS Postdoctoral Fellow in the lab of Professor Tetsuro Majima during 2010-2013. He became a Professor in the Department of Chemistry at Shanghai Normal University in 2013. He research interests are focused on the design and synthesis of TiO2-based nanomaterials for environmental and energy photocatalysis.
i
b
ii +
- - - - - - - - - + + + + + + + + + +
+
+
+
Current
a
iii +
+
+
Electrode
+
- - - - - - - - - + + + + + +
i
ii
+ + + + + - - - - - - - - - + + + + +
Time
c
iv i
- - - - - - - - - + + + + + +
Dielectrc
iv
iii
a
Sugar cubes
PDMS
Photocatalyst
Curing
Washing
Loading
f
2 mm
4 mm
6 mm
b
c
Arcylic
d
Al
PDMS
e
Photocatalyst
a
b
6 mm 2 mm
120
c
4 mm
6 mm 2 mm
-40
d
4 mm
6mm 4mm 2mm
3
90 60 30
Current (μA)
Charge (nC)
Voltage (V)
2 -30
-20 -10 0
0 0
5
10
15
20
25
30
0
5
10
15
20
25
-2
30
0
5
-40
90 60 30
10
15
20
25
Time (s)
g
-50 Orginal Polluted
3
Current (μA)
f
Orginal Polluted
Charge (nC)
Voltage (V)
-1
Time (s)
150 120
0
-3
Time (s)
e
1
-30 -20
Original Polluted
2
1
-10
0
0
0
6mm
4mm
2mm
6mm
4mm
2mm
6mm
4mm
2mm
30
0.1wt% 0.005wt%
0.05wt% 0.001wt%
b
0.01wt% 0wt%
Charge (nC)
Voltage (V)
120 90 60
30
-50
0.1wt% 0.005wt%
0.05wt% 0.001wt%
-40
2
-30
1
-20 -10
0
10
20
30
40
50
0
60
e
0.6
10
20
1.0
0.2
0.5
0.1
0.0 10M
40
50
-1 -2
0
60
10
100M
Resistance (Ω)
0.0 1G
20
30
40
50
60
Voltage Current
3
Time (s)
f 120
Voltage (V)
0.3
1.5
Voltage (V)
0.4
2.0
Power (mW)
Current (μA)
2.5
1M
30
4.7 μF 22 μF 47 μF
3
0.5
0.01wt% 0wt%
0
Time (s)
3.0
0.05wt% 0.001wt%
-3
Time (s)
100k
0.1wt% 0.005wt%
3
0
0
d
c
0.01wt% 0wt%
2
1
80
2
40
1
0
0
0 0
30
60
90
Time (s)
120
Original
Polluted
Current (μA)
150
Current (μA)
a
O2 e-
CB
·O2-
Excitation
Organic containment
h+ VB H2O
·OH Photocatalyst NPs
Self-cleaning effect refreshes the surface
CO2, H2O
a
0 min 5 min 10 mim 15 mim 20 mim 25 mim 30 mim 35 mim
0.05wt% 0.20
Abs. (a.u.)
b
0.25
0.15 0.10
2 cm
0 min
5 min
10 min
15 min
20 min
25 min
30 min
35 min
0.05 0.00
400
500
600
700
Wavelength (nm) 150
Original
Charge (nC)
120
Voltage (V)
d
Polluted Self-cleaned
90 60
-50
-20
0
0 10
15
20
25
30
35
0
1st cycle
3rd cycle
2nd cycle
g RhB concentration (%)
Voltage (V)
120 90 60 30 0
0
10
20
Time (s)
30
40
Polluted Self-cleaned
1 0
-1 -2 -3
10
Time (s)
f
Original
3 2
-30
-10
5
e
Polluted Self-cleaned
-40
30
0
Original
Current (μA)
c
20
0
30
Time (s)
5
10
15
20
25
30
Time (s)
h 0.001 wt% 0.005 wt% 0.01 wt% 0.05 wt% 0.1 wt%
100 80 60
2 cm
0 min
35 min
2 cm
0 min
35 min
i
40 20
0 0
10
20
Time (min)
30
40
Research Highlights： 1. TENG integrated with self-cleaning function is proposed for the electrical performance recovery of the polluted one. 2. By loading TiO2 photocatalyst onto the porous dielectric polymer, the polluted surface of TENG device can be refreshed and the performances recover to 90% of its original level after solar light treatment. 3. Developed strategy is very easy to be expanded into self-powered nanosystem, on-skin sensors and wearable devices due to the selection diversity of photocatalysts and dielectric materials.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: