Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 239 – 242 30th Eurosensors Conference, EUROSENSORS 2016 Fabrication and characterization of self-powered active hydrogen sensor based on triboelectric nanogenerator A.S.M Iftekhar, Gwiy-Sang Chung* School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea Abstract In this study, triboelectric nanogenerator based self-powered active flexible hydrogen (H2) sensor (TEHS) has been investigated using a micropatterned PDMS and Pd nanoparticles coated vertical ZnO nanorods (Pd-ZnO NRs) Adsorption-desorption of H2 on the Pd-ZnO surface during contact electrification with the micropatterned PDMS film due to applied mechanical force was studied in terms of triboelectric outputs In air, TEHS showed the peak-to-peak open-circuit voltage and short-circuit current of around 5.2 V and 80 nA, respectively, while the output voltages varied with different H gas concentrations The output voltage and response of the sensor were found to be nearly 1.1 V and 373% to vol% ppm H2, respectively, and the response time was measured to be around 100 sec These results of the proposed approach suggest the possibility for developing a practical selfpowered system for gas sensing applications © 2016 2016The TheAuthors Authors Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd This (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Triboelectric; palladium nanoparticles; ZnO nanorods; PDMS; self-powered active hydrogen sensor Introduction Hydrogen (H2) gas has widespread applications in various fields for long [1] However, the possible hazards from the leakage of H2 over a wide range of concentrations are enormous Hence, the reliable detection of H along with high sensitivity and selectivity is crucial in order to maintain the environmental and personal safety Huge number of research related to H2 sensor based on conductivity, optical, thermal, etc including flexibility have been reported in the literature till now However, the expected performance from the reported works has not been realized adequately * Corresponding author Tel.: +82-52-259-1248; fax: +82-52-259-1686 E-mail address: gschung@ulsan.ac.kr 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.171 240 A.S.M Iftekhar and Gwiy-Sang Chung / Procedia Engineering 168 (2016) 239 – 242 in practice due to the need of external power supply for their operation, operating temperature requirement, and the lack of sufficient portability and flexibility In order to realize the above mentioned criteria and to meet the current demand, piezoelectric nanogenerator based self-powered active sensors have been proposed [2-5] However, the proposed works were not fully wellsuited for practical applications due to their compact and completely sealed device structures In recent years, extensive works have been focused on the development of triboelectric-based self-powered active chemical/gas sensors [6-8] In comparison to piezoelectric, triboelectric-based sensors work simply by the formation of a dipole layer after triboelectric contact and static separation between two materials of the triboelectric-series In the current work, we presented a triboelectric-based self-powered active H2 sensor (TEHS) and investigated its sensing properties systematically in a contact-separation mood at various H2 gas concentrations The as-fabricated device was composed of a layer of Pd nanoparticles decorated ZnO nanorods (Pd NPs/ZnO NRs) array (as sensing surface) and a layer of surface modified PDMS (micropyramids) (for triboelectric operation) It was believed that the major advantages and enhanced sensing properties of the proposed sensor will open up feasibility to develop self-powered active H2 sensors for practical applications in the near future Experimental Section A thin layer of gold (Au) with a thickness of 400 nm was deposited on the PET film through radio-frequency magnetron sputtering method Pd NPs/ZnO NRs were then grown on the Au/PET film in a similar way as done in our previous work [9] On the other hand, surface modified PDMS film was prepared by using a Si micropyramid template In a typical process, arrays of inverted pyramid shapes were patterned on a × cm2 p-type Si wafer using photolithography and wet chemical etching The PDMS solution with curing agents (ratio 10:1) was then spincoated on the as-prepared patterned Si mold A piece of aluminum (Al) layer was laminated on the smoothed side of the PDMS film, gently peeled-off, and transferred onto a PET film The TEHS was fabricated by compiling the Pd NPs/ZnO NRs/Au/PET film and the PDMS/Al/PET film separated by two spacers at the edges The schematic of the fabricated device is depicted in Fig 1a The Pd NPs/ZnO NRs/Au/PET film at the top part of the device was used as the main sensing surface, while the PDMS/Al/PET film at the bottom part was used to contribute the triboelectrification; Al and Au layer were used as the electrode Gas sensing properties of the device were carried out in a lab-made gas chamber, composed of two inlets for gas supply and vacuum pressure, respectively, and one outlet for air passage The triboelectric outputs of the TEHS were measured using a functional oscilloscope (Lecroy Wave Runner 610) and a low noise output characterization system (Keithley 4200-SCS, with a plug in preamplifier Keithley 4200-PA-1) Results and Discussion Fig 1a and 1b show the schematic of the as-prepared TEHS device and the FESEM image of the as-synthesized pyramid micropatterns on the PDMS film, respectively Fig 1b presents that the micropatterned PDMS film was uniformly covered by orderly arrayed micropyramids with an edge length of 20 μm and height of 20 μm The figure clearly shows the formation of smooth edged micropyramids with sharp tips Fig (a) Schematic of the as-fabricated TEHS and (b) FESEM image of the as-synthesized pyramid micropatterns on PDMS surface A.S.M Iftekhar and Gwiy-Sang Chung / Procedia Engineering 168 (2016) 239 – 242 241 Fig (a) A single pressing and releasing peak of the TEHS (b) In-situ variation of triboelectric output voltage with and without H exposure Fig (a) Output voltages and (b) the corresponding response values of the TEHS with various H2 concentrations (c) Response time characteristics of the TEHS (d) Output voltage variation with various relative humidity Fig 2a shows a single pressing and releasing peak in air after imposing a contact force of 5.3 N, which reveals that the contact force on the TEHS device was sufficient to generate triboelectric output voltage A maximum voltage of 5.2 V can be obtained The higher pressing peak compared to the releasing peak was attributed to the slower separation rate between top and bottom layers due to the restoring force and adhesiveness of the PDMS film and the spacers The as-synthesized micropyramid arrays played a vital role in enhancing the contact area, which induced large triboelectric charge density on the films’ surface When the ZnO NRs surface was brought into contact with the micropyramid PDMS film due to the applied external force, the negative and positive charges were distributed at the PDMS and ZnO surfaces, respectively These charges caused electron flow through external loads from the bottom electrode (Al) to the top electrode (Au) and produced triboelectric outputs Fig 2b shows the triboelectric output voltage variation of the TEHS in dry air and under various concentrations of H at room temperature It is observed that upon exposure to 0.01, 0.05, 0.1, 0.5, and vol% H2, the Vpk-pk value of the TEHS 242 A.S.M Iftekhar and Gwiy-Sang Chung / Procedia Engineering 168 (2016) 239 – 242 reduced to about 3.9, 3.2, 2.7, 1.7, and 1.1 V, respectively When TEHS was exposed to H at a certain concentration, the chemisorbed oxygen ions on the Pd-ZnO surface reacted with the H2 molecules and released the electrons flowing back into the sensing surface The surface free electrons then screened the triboelectric signals, thus reduced the output voltage [5] Fig 3a and 3b show the output voltages and the corresponding response values of the TEHS in air and under various H2 concentrations, respectively Sensor response was calculated using the following equation, (Va-Vg)/Vg × 100, where Va and Vg represent the open-circuit voltage of the TEHS in air and at a certain concentration of H 2, respectively It is clearly observed that the device can effectively detect H2 down to 10 ppm (Fig 3a) Moreover, the TEHS showed a maximum response of 373% to vol% gas concentration (Fig 3b) Zero percent (0%) response denotes no response in air (0 ppm H2) The response time characteristics of the sensor is shown in Fig 3c, where the response time was defined as the time to reach 90% of the total output voltage change A relatively slower response time of 100 sec was measured during the exposure of 0.5 vol% H2 (almost similar time was observed for 0.01 to vol% gas exposure) TEHS device performance was evaluated under moist environment in order to investigate the humidity effect on the device as the triboelectric device surface is highly influenced by humidity and the results are shown in Fig 3d As expected, the output voltages decrease with increasing RH concentration (RH: 30-90%) A negligible voltage variation was observed up to 45% RH; however, the output voltage degradation rate at higher RH concentration was significant, which might be attributed to the intrinsic deleterious humidity influence property of the triboelectric materials Conclusions In summary, we have successfully fabricated a self-powered active H2 sensor based on triboelectric effect using a layer of Pd NPs/ZnO NRs/Au/PET and a micropyramid PDMS film In air, the device showed a maximum triboelectric voltage of 5.2 V and a current of 80 nA Upon exposure to vol% H2 gas concentration, the device showed a maximum sensitivity (sensor response) of about ~ 373% and a relatively slow response time of 100 sec The device can effectively detect H2 down to 10 ppm without showing any significant response degradation up to the relative humidity of 45% We hope that the aforementioned features will open possible means to encourage further studies in self-powered technology, especially for gas sensing applications Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded in 2014 by the Ministry of Science, ICT and Future Planning (NRF-2014R1A2A2A01002668) References [1] T Hubert, L.B.- Brett, G Black, U Banach, Hydrogen sensors- A review, Sens Actuators B 157 (2011) 329-352 [2] Y Lin, P Deng, Y Nie, Y Hu, L Xing, Y Zhang, X Xue, Room-temperature self-powered ethanol sensing of a Pd/ZnO nanoarray nanogenerator driven by human finger movement, Nanoscale (2014) 4604-4610 [3] R Yu, C Pan, J Chen, G Zhu, Z.L Wang, Enhanced performance of a ZnO nanowire-based self-powered glucose sensor by piezotronic effect, Adv Funct Mater 23 (2013) 5868-5874 [4] B Saravanakumar, S Soyoon, S.-J Kim, Self-powered pH sensor based on a flexible organic-inorganic hybrid composite nanogenerator, ACS Appl Mater Inter (2014) 13716-13723 [5] Y Fu, W Zang, P Wang, L Xing, X Xue, Y Zhang, Portable room-temperature self-powered/ active H2 sensor driven by human motion through piezoelectric screening effect, Nano Energy (2014) 34-43 [6] Z.-H Lin, Y Xie, Y Yang, S Wang, G Zhu, Z.L Wang, Enhanced triboelectric nanogenerators and triboelectric nanosensor using chemically modified TiO2 nanomaterials, ACS Nano (2013) 4554-4560 [7] Z.-H Lin, G Zhu, Y.S Zhou, Y Yang, P Bai, J Chen, Z.L Wang, A self-powered triboelectric nanosensor for mercury ion detection, Angew Chem Int Ed 52 (2013) 5065-5069 [8] Z.-H Lin, G Cheng, W Wu, K.C Pradel, Z.L Wang, Dual-mode triboelectric nanogenerator for harvesting water energy and as a selfpowered ethanol nanosensor, ACS Nano (2014) 6440-6448 [9] T.-R Rashid, D.-T Phan, G.-S Chung, A flexible hydrogen sensor based on Pd nanoparticles decorated ZnO nanorods grown on polyimide tape, Sens Actuators B 185 (2013) 777-784