Defect states contributed nanoscale contact electrification at ZnO nanowires packed film surfaces
Graphical abstract
Efficient conversion of mechanical energy in our surrounding environment into electric power has become a promising strategy for meeting the ever-increasing energy consumption of small and distributed electronics. The contact-electrification-based triboelectric nanogenerators are one of the emerging devices to achieve such energy conversion. However, conventional contact electrifications between two insulators are limited by their low current density and alternating current output. Here we report a nanoscale contact electrification induced direct current output based on the flow of electrons from the defect states of the ZnO nanowires-packed film to the contact sliding conductive AFM tip. Combining experimental materials characterization and density functional theory (DFT) calculations, the direct current output is closely related to the concentration of oxygen vacancy defect states on the surface of ZnO nanowires: the higher the oxygen vacancy concentration, the higher the current output. Under optimized conditions, we obtain an ultrahigh current density of ~108 A m-2, which is several orders of magnitude higher than that of the conventional contact electrification and other effects. This work provides a new route of utilizing defect states contributed contact electrification for realizing nanoscale mechanical energy scavenging.
Introduction
The ever-increasing energy consumption has become one of the greatest challenges of the development of the modern society [1,2]. Scavenging mechanical energy from our surrounding environment [[3], [4], [5], [6]] has attracted tremendous scientific efforts. The triboelectric nanogenerator (TENG) [[7], [8], [9], [10], [11], [12]] is one of such promising devices to convert mechanical energy into electric power. Using TENG, various environmental mechanical energy sources such as human motion [7], rotating motion [8], vibration [9], water drop [10], sea wave [11], wind [12], etc., can be scavenged effectively. In conventional TENGs based on typical contact electrification, a pulsed AC output can be obtained by the periodical vertical/horizontal separation of two dielectric materials with opposite electrostatic charges. Although these devices generate a high output voltage, its output current density (J) is relatively low because of its high impedance. In order to obtain a DC output, some rectification methods, such as power management circuits [13] and rectifier bridge [14], are needed, resulting in poor portability and difficulties in the high-efficient utilization of energy. These characteristics block the widespread applications of these devices in wearable electronics and self-powered sensors, etc.
With the aim to convert mechanical energy directly into DC output, dielectric breakdown has been used to realize a continuous DC output by sliding a metal electrode on the surface of polymer materials [15], which needs a large breakdown electric field of larger than 3 kV/mm, inducing a high impedance of these devices [16]. To obtain a DC output with low impedance, the metal electrodes have been utilized to directly slide on the surfaces of semiconducting materials [[17], [18], [19]]. The DC output has been found by sliding a metal tip on MoS2 multi-layers due to that the electrons in MoS2 may be excited from the valence band and the surface states to the conduction band, and then transmit into the metal side [17]. However, it is necessary to understand where the electrons come from: the valance band or the surface states. Moreover, if the electrons were excited into the conduction band, it is difficult to be unaffected by the built-in electric field in the depletion layer of Schottky junction, which will drive electrons to flow along the semiconductor side. By sliding a metal tip on the surface of n-type Si materials, an DC output can be observed due to the tribovoltaic effect, which is about the excitation of the electron and hole pairs in the conduction and valence band, respectively, by the energy released due to the formation of new bond at friction interface [18].
Inspired by the above progress, we used conductive atomic force microscopy (AFM) to demonstrate a DC output while sliding on a ZnO nanowires-packed film surface. Specifically, we propose defect states contributed nanoscale contact electrification between the conductive AFM tip and the ZnO nanowires-packed film, which is dominated for driving electrons to flow from defect states in ZnO nanowires to metal tip. The ZnO sample used here is literally a film that eliminates the transverse deflection of the nanowires composing the film during AFM scanning. This is different from the piezoelectric nanogenerator case of contact scanning AFM tip across spaced grown nanowires so that each nanowire can be deflected freely without any space restriction in the transverse directions [4,5]. Due to the tip-enhanced electrical field, the DC output exhibits an ultrahigh high current density in the order of 108 A m-2, which is several orders of magnitude higher than that based on typical contact electrification and other effects. We systematically investigate the influence of various working conditions for the performances of the DC output and reveal the current response enhancement at surfaces of ZnO nanowires with higher concentrations of oxygen vacancy defect states, using larger contact force as well as slower scan rate. We also found that repeated scanning at the same location can attenuate the current output signal, but the attenuated signal can recover by itself with the recovery of oxygen vacancy density. This work provides an effective strategy of using the electrons in defect states of some nanomaterials to obtain DC output when the metal tip is sliding on the surfaces of these nanomaterials, offering a new method for nanoscale mechanical energy scavenging.
Section snippets
Experimental section
Synthesis. In a typical procedure, the ITO-coated glass substrates were cleaned ultrasonically with several times in glass clean agent, ethanol and deionized water, and then dried by high purity compressed air. Subsequently, a layer of ZnO seeds with thickness of about 150 nm was deposited on the cleaned ITO glasses by radio frequency (RF) magnetron sputtering (Ar atmosphere, room temperature) under a needed time and operating power. Then 2.9748 g zinc nitrate (ZnNO3·6H2O) and 1.402 g
Results and discussion
A schematic illustration of the DC generation has been exhibited in Fig. 1a, showing its basic configuration. The defect states contributed nanoscale contact electrification is demonstrated by sliding a Pt/Ir-coated silicon AFM tip on ZnO nanowires-packed film. When the AFM tip is rubbing on the ZnO sample, electrons can be transferred from the ZnO surface to the tip spontaneously and then flow to the ground through an external circuit, resulting in a negative DC signal. Fig. 1b depicts the
Conclusion
In summary, the DC output of nanoscale contact electrification between an AFM tip and ZnO nanowires-packed film has been investigated. Due to the tip-enhanced electrical field at nanoscale, the obtained DC output exhibits an ultrahigh high current density of 108 A m-2, which is much higher than those of previous strategies based on typical contact electrification and other effects. Our combined experimental (XPS, PL and KPFM) and theoretical (DFT calculations) analysis reveals that the
CRediT author statement
Yiding Song: contributed equally, fabricated the materials and carried out the performance measurement, Formal analysis, analyzed the data and co-wrote the manuscript. All authors contributed to the final version of the manuscript. Nan Wang: contributed equally, fabricated the materials and carried out the performance measurement. Mohamed M. Fadlallah: contributed equally, performed DFT calculations, Formal analysis, analyzed the data and co-wrote the manuscript. All authors contributed to the
Declaration of competing interest
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.
Acknowledgments
Y.S., N.W. and M.F. contributed equally. This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0202701), the National Natural Science Foundation of China (Grant No. 51472055), External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121411KYS820150028), the 2015 Annual Beijing Talents Fund (Grant No. 2015000021223ZK32), Qingdao National Laboratory for Marine Science and Technology (No. 2017ASKJ01), and the University of Chinese Academy of Sciences
Yiding Song is currently a master degree candidate in the research group of Prof. Ya Yang at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences (CAS). His recent research interests are new energy harvesting strategy and atomic force microscopy.
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Yiding Song is currently a master degree candidate in the research group of Prof. Ya Yang at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences (CAS). His recent research interests are new energy harvesting strategy and atomic force microscopy.
Nan Wang is a postdoc in the research group of Prof. Ya Yang at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences (CAS). She received her Ph.D. in electroanalytical chemistry, from Changchun Institute of Applied Chemistry, CAS. Her research interests focus on direct-current triboelectric nanogenerator and mechanical properties of 2D materials.
Dr. Mohamed M. Fadlallah received his Ph.D. in Physics from the Institute of Physics, Augsburg University, Augsburg, Germany in 2010. Currently, he is an associate professor in the Faculty of Science, Physics Department, Benha University, Benha, Egypt. He was a visiting researcher and post-doctor at Augsburg University, Augsburg, Germany, and Center for Computational Energy Research, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, the Netherlands. His area of expertise is computational materials science with a focus on the electronic, transport, and photocatalytic properties of metal oxides, one-, two-, and three-dimensional structures.
Dr. Shuxia Tao received her PhD of Computational Materials Science from Eindhoven University of Technology (TU/e), the Netherlands. After three years as post-doctoral researcher at NIKHEF, she currently is an Assistant Professor at TU/e. With two prestigious personal grants, CSER tenure track and NWO START-UP, she currently leads a young research group Computational Materials Physics. Her research interests lie in the development and application of Density Functional Theory and multiscale computational methods in the area of novel energy conversion and storage technologies. More details of her research can be found at: https://www.shuxiatao.com/.
Prof. Ya Yang received his Ph.D. in Materials Science and Engineering from University of Science and Technology Beijing, China. He is currently a professor at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, China. He has developed various new hybridized and multi-effects coupled devices, opening up the new principles of the device design and coupled effects, and the new approaches of improving output performances of energy-related devices. His main research interests focus on the field of hybridized and coupled devices for energy conversion, self-powered sensing, and some new physical effects. Details can be found at: http://www.researcherid.com/rid/A-7219-2016.
Prof. Zhong Lin (ZL) Wang received his Ph.D. from Arizona State University in physics. He now is the Hightower Chair in Materials Science and Engineering, Regents’ Professor, Engineering Distinguished Professor and Director, Center for Nanostructure Characterization, at Georgia Tech. Dr. Wang has made original and innovative contributions to the synthesis, discovery, characterization and understanding of fundamental physical properties of oxide nanobelts and nanowires, as well as applications of nanowires in energy sciences, electronics, optoelectronics and biological science. His discovery and breakthroughs in developing nanogenerators established the principle and technological road map for harvesting mechanical energy from environment and biological systems for powering personal electronics. His research on self-powered nanosystems has inspired the worldwide effort in academia and industry for studying energy for micro-nano-systems, which is now a distinct disciplinary in energy research and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics by introducing piezoelectric potential gated charge transport process in fabricating new electronic and optoelectronic devices. Details can be found at: http://www.nanoscience.gatech.edu.