Author’s Accepted Manuscript Flexible in-plane graphene oxide moisture-electric converter for touchless interactive panel Huhu Cheng, Yaxin Huang, Liangti Qu, Qilong Cheng, Gaoquan Shi, Lan Jiang www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30807-8 https://doi.org/10.1016/j.nanoen.2017.12.033 NANOEN2414
To appear in: Nano Energy Received date: 30 October 2017 Revised date: 8 December 2017 Accepted date: 19 December 2017 Cite this article as: Huhu Cheng, Yaxin Huang, Liangti Qu, Qilong Cheng, Gaoquan Shi and Lan Jiang, Flexible in-plane graphene oxide moisture-electric converter for touchless interactive panel, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.12.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Flexible in-plane graphene oxide moisture-electric converter for touchless interactive panel Huhu Chenga,c, Yaxin Huanga, Liangti Qua,b,*, Qilong Chenga, Gaoquan Shic,*, Lan Jianga,d,* a Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China; State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China. b Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China. c Department of Chemistry, Tsinghua University, Beijing 100084, PR China. d Laser Micro-/Nano-Fabrication Laboratory, Beijing Institute of Technology, Beijing, 100081, P. R. China.
[email protected] [email protected] [email protected] *Corresponding authors.
Abstract Harvesting energy from nature have attracted much attentions especially the direct energy conversion from other power sources (e.g. light, heat, mechanical movement, moisture) into practical electricity. However, many of existing devices always rely heavily on external metal electrodes, and their rigid structure restrains further applications in portable touchless electronics or perceptual artificial skins. Herein, we demonstrate flexible in-plane moisture-electric converter (IPMEC) based on graphene oxide (GO) film for novel touchless interactive platform. An appreciable electric output voltage (~70 mV) and high electrical current density (12 mA cm -2) can be autonomously generated with ambient moisture variation on this IPMEC. The planar configuration and integrative laser reduced GO electrodes of IPMEC renders it highly flexibility, and greatly promotes the touchless interface formation between this device
and biological moisture sources (e.g., human finger). Based on this, diverse touchless devices have been developed including finger position annunciator for smart artificial skin, touchless switches and even handwriting panel. This work will provide a new pathway for the development of flexible moisture-electric converters and applications in touchless interactive platform for biomimetic or smart electronics. Graphical abstract
Flexible graphene oxide film touchless platform with in-plane moisture-electric conversion microdevices has been explored for human-machine
interactive
systems,
including
finger
position
annunciator for smart artificial skin, touchless switches and even handwriting panel.
Keywords graphene oxide, energy converter, moisture, touchless device, flexible
1. Introduction Touchless interaction devices, that can prevent the contact-induced contamination and damage, have attracted much attention due to their potential applications in future perceptual artificial skin and human-machine interaction systems.[1–4] Examples include radio frequency identification card, infrared temperature detector, and even
facial recognition system. Compared with traditional electronic equipment, these touchless devices removed the burdensome physical interactive system and made the interaction comfortable. However, the external bulky electric energy suppliers of current touchless devices restricted their applications in portable electronics or artificial skin. To solve these problems, diverse electrical generators based on advanced materials have been developed and incorporated into electronics.[5–10] For example, piezoelectric materials can convert human mechanical movements to electric power. Triboelectric devices can generate electricity under external friction when be touched, and the resulted electrical feedback can express the state of contact interface at the same time. Although the integrative power source and touch detectors have been integrated into many electronics by these ingenious materials. It is still difficult to achieve their applications in self-powered touchless devices by the limited access mode relying heavily on the contact between human and machine up to now. Because of the wide existence of moisture distribution around human bodies, converting moisture variation into other signals would provide opportunity for the construction of touchless interactive model. Recently developed moisture-electric energy transformation process based on graphene oxide (GO) can autonomously transfer moisture into electrical output, because the inner two-dimensional GO sheets has rich oxygen-related functional groups (e.g., –COOH) and large specific surface area, which can largely absorb ambient water molecules and then generate ionized protons.[11–23] However, it is still a big challenge to achieve a satisfactory ability of moisture-electric transformation for the construction of flexible devices because of the
rigid structure of GO assemblies in previous studies, restraining their applications in portable electronics at the same time. In addition, the utilization of additional metal electrodes that covered on the surface of active GO materials tremendously hindered them to be exposed to biological moisture source like human fingers or bodies as touchless interface. To address these problems, herein, we present a flexible in-plane moisture-electric converter (IPMEC) on GO assembled film for touchless interactive system. The IPMEC device is mainly composed of integrated and planar laser reduced graphene oxide (rGO) electrodes and GO with oxygen-containing groups gradient, making it can be fully exposed to moisture source. Water molecules in moisture can combine with GO and induce ion concentration gradient along with the planar direction of inner GO sheets, accordingly generating an appreciable electrical output (~70 mV, 12 mA cm-2) in external circuits and achieving a real-time feedback of moisture variations. As a result, diverse self-powered touchless panels including finger position sensor for smart artificial skin, touchless switches and even handwriting panel have been developed.
2. Experimental Section 2.1 Preparation of IPMEC GO dispersion (0.2 mg ml-1) was synthesized by modified Hummers’ method as we previously reported.[24–26] GO film was achieved using vacuum filtration of GO dispersion through a mixed cellulose membrane (0.22 μm pore size) and following peeling off. The laser writing reduction GO to rGO process was conducted with 458
nm laser under a preset program. The applied voltage was provided by a Keithley 2612 in the electric field polarization process to form gradient oxygen-containing groups in GO region of IPMEC device. 2.2 Characterization The morphology of as-prepared samples was examined by scanning electron microscope (SEM, FLexSEM 1000). X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D2 PHASER diffractometer with a Cu Kα irradiation source (λ = 1.54 Å). The optical photograph was obtained from Zeiss Axio Scope. A1 optical microscope. Mechanical property tests were conducted using an Instron 5943 universal testing machine with a strain rate of 0.5 mm min-1 for stretching. 2.3 Electrical measurements The output electrical signals of IPMEC were collected using a keithley 2612. The electrical output performance was measured in an enclosed container, and the relativity humidity is controlled by flow of dry nitrogen or nitrogen of high RH.
3. Results and discussion 3.1 Fabrication and characteristic of IPMEC Preparation of IPMEC on GO film is shown in Fig. 1. First, GO film (～1.5 μm thickness, Fig. 1a) was prepared by filtration of GO dispersion (0.2 mg mL-1). A following direct laser writing patterned pairs of electrical conductive rGO microelectrodes (～100 μm width, 50 μm spacing) on this GO film (Fig. 1b and c) through the in-situ laser reduction of GO. Inner GO sheets arranged uniformly along
with the horizontal direction between rGO electrodes in the sandwiched rGO/GO/rGO microstructure (Fig. 1d), which allowed the mechanical flexibility (Fig. 1c) and the integrity of the film. Second, a constant bias (5 V) was applied between two rGO electrodes in each IPMEC (Fig. 1e) within a container of high RH≈100% for about 150 s. Under the action of electric field polarization, the oxygen-containing groups on GO sheets distributed in a gradient way between rGO microelectrodes (Fig. 1f). Finally, a flexible GO film with IPMEC of planar rGO/GO/rGO structure was obtained.
Fig. 1. Schematic illustration of the preparation of IPMEC on flexible GO film. (a) An original GO film. (b) In-situ reduction of GO in the film by direct laser writing for arranging reduced GO (rGO) electrodes in pairs. (c) The obtained flexible GO film with tens of planar rGO electrodes. (d–f) The electrical chemistry polarization process between two rGO electrodes in one individual IPMEC, which is composed of one pair of rGO microelectrodes and GO in the middle part. (d) The original state of this device with planar rGO/GO/rGO configuration. (e) The polarization process when applying a biasing voltage between rGO electrodes under high relative humidity
(RH≈100%). (f) The final IPMEC with the gradient oxygen-containing groups on GO region between two rGO electrodes.
Fig. 2. Characteristics and electric output performance of IPMEC. (a) Photo of a GO film with pairs of rGO electrodes. (b) Photo of this flexible GO film. (c) Scanning electron microscope (SEM) image of the planar rGO/GO/rGO configuration on GO film. (d) Corresponding energy-dispersive X-ray spectra (EDX) line scans of C (horizontal red curve) and O elements (sloping green curve) on GO regions between rGO electrodes of one IPMEC device. (e) Schematic illustration of one moisture-electric conversion cycle. The V and A represented the induced voltage and current along the direction of arrows, respectively. (f) Current density and (g) Voltage output cycles of one IPMEC in response to the intermittent and periodic RH variation (ΔRH = 60%). Scale bars: a, 1 cm; b, 50 μm; c, 10 μm.
Fig. 2a displays a 15 cm2 GO film with 20 pairs of rGO microelectrodes prepared by the strategy above mentioned. Direct laser irradiation in-situ integrated black rGO interdigital microelectrodes in brown GO film, enabling it high strength (~170 MPa, Supporting Information, Fig. S1) and mechanical flexibility (Fig. 2b) compared with previous moisture-electric devices of rigid structure and additional metal electrodes.[11–15] SEM image in Fig. 2c can easily distinguish rGO and GO regions in the film because of their observing conductive discrepancy. Corresponding EDS (Supporting Information, Fig. S2) reveals the C/O atomic ratio for rGO is about 9:1 that is much higher than the original GO (2:1), indicating an effective reduction of GO after laser writing. The resulted favorable conductivity of rGO (~14 S cm -1) means it can be used as electrical electrodes for following construction of gradient oxygen-containing groups on GO region. In one individual rGO/GO/rGO unit, after applying a constant bias voltage (5V) between two rGO microelectrodes (Supporting Information, Fig. S3 and 4, detailed experimental data is in supporting information) under high RH (100%), the C/O atomic ratio gradually increases from rGO anode to cathode in GO region (Fig. 2d and S5) during the electric field polarization process. While the chemical composition of both rGO electrodes has negligible changes (Fig. S6). When this obtained rGO/GO/rGO IPMEC microdevice is exposed to high RH, protons are generated by the interactions between absorbed water and GO sheets,[11,12,16] forming a concentration gradient of protons under the influence of gradient oxygen-containing groups in GO region (Fig. 1e). Subsequently, the
spontaneous diffusion of protons under the drive of concentration gradient occurs, inducing potential and free electrons movement in the external circuit. 3.2. The electrical output performance of IPMEC triggered by moisture. As shown in Fig. 2f and g, a short-circuit current (Isc, ~12 mA cm-2) and open circuit voltage (Voc, ～70 mV) of one rGO/GO/rGO IPMEC was observed when the RH increased from 25% to 85% (ΔRH=60%) compared with the original state (Supporting Information, Fig. S7) without electrical polarization process. The electric output returned to the zero with a dynamic balance of the protons diffusion. Then, the reduction of RH from 85% to 25% induced a negative electric signal because that the protons drifted back to recombine with the negatively charged groups (e.g. –COO-) of GO sheets with the desorption of water molecules. The electrical signals of this IPMEC can be constantly exported when response to the variation of RH after hundreds of cycling tests (Supporting Information, Fig. S8), indicating the high stability of this device.
Fig. 3. The illustration and analysis of moisture-electric conversion process of IPMEC. (a) and (b) Schematic illustration of a moisture-electric conversion cycle with RH increase (a) and decrease (b). At high RH (a), the generated protons diffuse under the drive of concentration gradient through an enlarged channel induced by absorbed water molecules between GO sheets. At low RH (b), the protons turn back to recombine with the gradient oxygen groups in a narrow channel with amounts of water molecules desorption. (c) Experimental data of signal current output cycle of rGO/GO/rGO IPMEC. The RH maintained for about 0 s (up) and 15 s (down) at 85%, respectively, and then was reduced to 25%. On and off represents the RH changes from 25% to 85% and the reverse changes, respectively. (d) Serials of X-ray diffraction (XRD) results of GO at different RH.
Compared with previous moisture-electric energy converters, the current density of this rGO/GO/rGO IPMEC is up to a high value of about 12 mA cm2, which would benefit from the planar and integrative texture of active GO and rGO electrodes of this device. As illustrated in Fig. 3 and Supporting Information, Fig. S9, the lamellar structure of the GO sheets should provide unobstructed protons channels between GO
sheets along the horizontal direction. While the transport of protons must overcome the barrier from the stacking layers of GO in the vertical direction. This phenomenon has also been proved in many energy storage devices for a planar configuration.[27– 29] The electrochemical impedance spectroscopy (EIS) of planar GO at various humidity levels was further carried out to demonstrate the trend of generated current curve in the moisture-electric process (Supporting Information, Fig. S10). The reduction of the depressed semicircle in EIS curves revealed that amounts of absorbed water molecules on GO sheets dramatically improved the protons conductivity when RH increased. On the other hand, the interlayer spacing of GO sheets increases from 7.5 Å to be about 9.0 Å when the RH is about 85% (Fig. 3d) calculated from X-ray diffraction (XRD) results, highly enlarging the protons diffusion channels along the planar direction. Thus, the transformation process from moisture to output electric could be described as follows: an increase of RH (25% to 85%) induced the protons concentration gradient along with the horizontal direction of GO film under the influence of gradient oxygen-containing groups; Simultaneously, the amounts of absorbed water molecules and enlarged channels between GO sheets accelerated the protons diffusion driven by the gradient concentration, causing a sharp current peak of 12 mA cm-2 within about 1.5s (Fig. 3c). When protons concentration gradient faded away with the dynamic balance of protons diffusion under a constant high RH, free electron movement of external circuit was terminated and the current stabilized at the zero line (Fig. 3c). Subsequently, a decrease RH induced water molecules desorption and narrowed the protons diffusion channels between GO sheets (from 9.0 to 7.5Å),
making that protons drifted back to recombine with the gradient negatively charged groups (e.g., –COO) of GO sheets slowly. As shown in Fig. 3c, the negatively current density is about ~1.5 mA cm-2 and takes about 20 s for recovering to zero. Based on this inconsistency, the RH changes can be traced by the trend of generated current. As shown in Fig. 3c, the sharp positive peak of current means the surrounding RH is increasing and the stable current means the constant RH, while the relative blunt negative current implies a decrease of RH. In addition, the in-situ laser reduction process allowed the maintenance of a lamellar structure (Supporting Information, Fig. S11) in the whole body of rGO/GO/rGO structure, avoiding physical contact interface between additional electrodes and active layers and the use of heavy metals, providing an advantage to the applications in flexible and portable devices.
Fig. 4. The flexibility and sensitivity of IPMEC for humidity and human finger. (a) The generated Isc of one rGO/GO/rGO moisture-electric device under flat and bending states. (b) The durability test of this device undergoing the repeated flat-to-bending cycles. Current (c) and voltage (d) output of the device in response to the different ΔRH (50%, 35% and 25%). (e) The generated Isc with different objects and the changed position of finger (1 mm, 2 mm and 3 mm).
As expected, one rGO/GO/rGO IPMEC exhibited a similar Isc value (~12 mA cm-2) under both bending and flat states (Fig. 4a). During 500 cycles of bending test, this device showed a stable electric output triggered by moisture (Fig. 4b). Notably, the intensity of electric signals automatically regulated along with ΔRH changes. As shown in Fig. 4c and d, the Isc and Voc was about 5.8 mA cm−2 and 36 mV with ΔRH of 25%, respectively. A higher value of electric output (~9.2 mA cm−2 and 55 mV) was obtained with a higher ΔRH (40%), which can be easily explained by more adsorption capacity of water on GO under high RH. The performance of this device depends mainly on the change of humidity. Thus, this device can generate electrical voltage of about 32 mV and 26 mV (Fig. S12) with humidity increase (ΔRH~25%), when the initial ambient relative humidity is about 45% and 60%, respectively, indicating the excellent electricity generation of this moisture-electric converter under different humidity environment. Benefited from the excellent and stable moisture-electric responsive ability, the rGO/GO/rGO IPMEC is significant to detect the approaching finger with natural moisture distribution. When a finger was above the device with a distance of 1 mm (Fig. 4e), the water molecules diffused from the finger to the film and a current was generated. The generated electric signals were highly repeatable and the induced Isc is about 10 mA cm-2. A negative electric output revealed that the finger was far away, indicating a favorable touchless interaction between this rGO/GO/rGO IPMEC and human finger. This moisture-electric mode is also helpful to distinguish the organism from inorganic objects. A wood or tweezer at the same distance (1 mm) above the IPMEC can’t cause obvious electric signal
change (Fig. 4e) due to the lack of generated moisture. Meanwhile, the variable RH of finger at different position allowed that the detection of the distance between device and finger became possible. When this distance was about 2 mm, the generated Isc of IPMEC was about 8 mA cm-2 and reduced to be ~4.5 mA cm-2 at 3 mm (Fig. 4e). As a result, this rGO/GO/rGO IPMEC showed an excellent responsibility and recognition capability to the human finger and favorable position-recognition skills, which are important performance parameters for the construction of smart artificial skins and electric controllable equipment with advanced touchless interfaces.
Fig. 5. IPMEC device as finger position annunciator and artificial skin for smart robot. (a) Schematic illustration of a visible finger position annunciator. (b) The LED brightness of this annunciator changes with the position variation of touchless finger. (c) The IPMEC attached on a robot as artificial skin for the detection of an approaching finger.
3.3. Touchless devices as artificial skin and flexible panel. Transformation from moisture variation into electric signals of the rGO/GO/rGO IPMEC rendered it can be directly integrated into electronics for further applications. For example, an individual IPMEC that combined with light emitting diode (LED) and amplifiers (Supporting Information, Fig. S13) has been developed to convert
finger position variation into a more intuitive light density fluctuation (Fig. 5a and b), constructing a visible finger position annunciator. When a finger was approaching the device, LED was turned on. Because of the generated electric density changes above mentioned, the brightness of LED could reveal the finger position variation in the real time (Movie. S1). As shown in Fig. 5b, the LED was at off state at the beginning. When the finger was approaching to the device with a distance of 3 mm, the LED started to emit faint light (in the middle of Fig. 5b). The observing stronger light (in the bottom of Fig. 5b) implied that the finger is at a much closer distance (1 mm). Furthermore, the adequate mechanical property of this device applied it as perceptive artificial skin for smart machines or robots. Fig. 5c showed that a rGO/GO/rGO IPMEC was easily attached on the arm of a robot as a perceptual artificial skin. A bright LED pointed out that living organisms were invading to the skin induced by the moisture change.
Fig. 6. Touchless switches and the application in handwriting panel. (a) The rGO/GO/rGO moisture-electric device as touchless switch for a lamp control. (b) Schematic illustration of a flexible matrix handwriting panel composed of 3×3 individual rGO/GO/rGO IPMEC and external LED display screen. (c) Photo of this rGO/GO/rGO moisture-electric devices panel. Inset (c) shows the flexibility of this panel. (d) The performance of finger touchless interaction screen system. Inset (d) is the obtained “THU” when writing these words on this rGO/GO/rGO IPMEC handwriting panel with a touchless mode by finger.
Besides, the moisture-electric read-out schemes developed above present intriguing candidates for the control of electronics with a touchless interactive interfaces mode. The ON/OFF states of electronics is easily controllable by the movement of human finger when adding an electronic relay (Supporting Information, Fig. S14). As shown in Fig. 6a and Movie S2, an action of approaching finger will cause the output voltage above a set threshold value of electronic relay, impelling the switches on/off to
control the LED. Furthermore, in view of the promising finger controllable performance of this device and easily integrative fabrication process by laser direct writing, a flexible touchless handwriting panel was fabricated based on 3×3 individual rGO/GO/rGO IPMEC (Fig. 6b and c), in which rGO microelectrodes and wires were directly incorporated into the GO film. Fig. 6c shows the photograph of this flexible matrix sensing device. To demonstrate the touchless performance of the matrix device, one finger approached one individual pixel of flexible device and the corresponding region of external screen displayed (Fig. 6d). The motion trace of finger can also be showed in the screen. For example, when finger moved along with the trace of a capital letter “T”, the “T” displayed on the LED screen in the real time (Movie. S3). Finally, an abbreviated name “THU” of Tsinghua University was shown (Fig. 6d) when writing these words on the panel with a touchless mode by finger, respectively. As a result, the position of touchless finger above the flexible panel can be directly displayed on the connecting LED screen. It is predictable that the high-resolution pixels of touchless devices can be fabricated by increasing the density of the isolated moisture-electric device on the film and reducing the line width of microelectrodes.
4. Conclusion Flexible moisture-electric converter with planar rGO/GO/rGO structure on GO film has been explored for touchless interaction platform. The IPMEC device can output significant electrical signals (～12 mA cm-2) respond to moisture variation under both bending and flat states benefiting from the integrative and lamellar rGO/GO/rGO
structure. The regulated electrical output with humidity variations enables the IPMEC can trace the movement of approaching human finger, distinguishing organism with others. More impressively, the IPMEC can be directly integrated into the electronic devices to fabricate diverse touchless panels including finger position sensor for smart artificial skin, touchless switches and even handwriting screen.
Acknowledgements The authors declare no competing financial interest. We acknowledge the financial support from the National Key R&D Program of China (2017YFB1104300), NSFC (No. 21325415, 51673026, 51433005), Beijing Natural Science Foundation (2152028),
Beijing
Municipal
Science
and
Technology
Commission
(Z161100002116022) and China Postdoctoral Science Foundation (2016M600077, 2017T100062). Supplementary Movie caption Movie S1. Touchless finger position annunciator. Movie S2. Touchless switch controlled by approaching finger. Movie S3. Touchless handwriting panel. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at *********.
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Huhu Cheng received his Ph.D. degree in Chemistry from Beijing Institute of Technology in 2016. He is currently working as a postdoctoral researcher in Tsinghua University focusing on graphene-based materials for smart actuators and energy devices.
Yaxin Huang is currently a Ph.D. student under the supervision of Prof. Liangti Qu at Tsinghua University. His research mainly focuses on energy conversion/storage devices related to graphene-based materials.
Liangti Qu received a Ph.D. in Chemistry from Tsinghua University (China) in 2004. He is now a Changjiang professor with research focus on carbon-based/polymer-based advanced functional materials and energy-related devices.
Qilong Cheng received a B.S. in Mechanical Engineering from Tsinghua University (China) in 2017. He is currently a Ph.D. student at University of California, Berkeley. His research mainly focuses on smart materials and devices.
Gaoquan Shi received Ph. D degree in Nanjing University in 1992, and moved to Tsinghua University in 2000 as a Professor of Chemistry. His research interests are focused on syntheses and applications of conducting polymers and chemically modified graphene.
Lan Jiang received Ph.D. in Mechanical Engineering from Beijing Institute of Technology in 2000. He is now a Changjiang professor in Mechanical Engineering.
His
research
activities
are
focused
on
ultrafast
micro/nanofabrication.
Highlights
Flexible in-plane moisture-electric converter
High electrical current density (12 mA cm-2)
Touchless human-machine interactive systems, including finger position annunciator for smart artificial skin, touchless switches and even handwriting panel
laser
