Summary of Academician Wang Zhonglin, mater. Today: the root of contact electrification

Author: triboelectrification is the most common and basic electrical phenomenon in the research group of Academician Wang Zhonglin Its scientific name is contact electrification (CE), that is, the charge transfer phenomenon at the interface of two substances after contact Although the human record of contact electrification can be traced back to 2600 years ago, and people are also familiar with this phenomenon, but for the root of contact electrification, the scientific community has not been conclusive What is the carrier of charge transfer in contact electrification? Electrons, ions or matter? Recently, Academician Wang Zhonglin of Beijing nano energy and system research institute was invited to publish a review article entitled "the root of contact electrification" in the international authoritative journal materials today, which clearly answered the puzzle of human beings for thousands of years (z.l Wang and A.C Wang, on the origin of contact electrification Materials today, 2019 DOI: 10.1016/j.mattod.2019.05.016 ）。 According to Academician Wang Zhonglin and his collaborators, the root of contact electrification is electronic transfer In 2012, Academician Wang Zhonglin team invented triboelectric nanogenerators (Teng), which are used to convert disordered, low-frequency and distributed mechanical energy into electrical energy, with a power density of up to 500 W / M 2 In order to improve the performance of Teng, the mechanism of contact electrification was studied Kelvin probe force microscope (KPFM), developed from atomic force microscope (AFM), is an important tool to study this problem In Figure 1, KPFM can clearly show the potential distribution of SiO2 surface after AFM metal probe repeatedly sweeps the surface of SiO2 After 8 times of friction, the static charge accumulated on the insulator surface reaches saturation Fig 1 The potential distribution generated by AFM metal probe rubbing the surface of SiO2 for many times As shown in Fig 2, Academician Wang Zhonglin's team obtained the relationship between the distance between probe and sample (ASP) and the surface potential difference (Δ V) by adjusting the amplitude (A0) of AFM probe, and found that only in the repulsion area can contact electrification occur This means that only when the distance between two atoms belonging to different materials is less than the equilibrium distance (or bond length), the electron cloud will overlap, and then the charge transfer will occur This is the electron cloud potential well model proposed by Academician Wang Zhonglin (Fig 3) The potential well model of electron cloud may be a universal model to reveal the mechanism of contact electrification According to this, Academician Wang Zhonglin predicted that the electron transfer caused by the contact of two kinds of insulator may release energy in the form of photon emission, isovibrator excitation or light excitation In addition, the contact electrification between the same materials may be attributed to the difference of surface curvature (surface energy) Fig 2 The relationship between the amplitude (a 0), the distance between the probe and the sample (a SP) and the surface potential difference (Δ V) of AFM probe Fig 3 The electron cloud potential well model has rarely been reported in the past about the effect of temperature on contact initiation Recently, Academician Wang Zhonglin's team used Teng and KPFM and other means to regulate the temperature of the two materials, and observed the contact electrification and charge decay It was found that: electrons tend to transfer from the hot end to the cold end; at high temperature, the contact electrification phenomenon will weaken or even disappear (figure 4) These experimental results are consistent with the thermionic emission model It is proved that the source of contact electrification is electron transfer Fig 4 The influence of temperature on the surface charge attenuation of Ti SiO 2 friction nano generator On the other hand, the light also has an effect on the surface static charge In Figure 5, the team of Academician Wang Zhonglin irradiates the surface of the insulator rubbed by AFM probe with ultraviolet light The results show that the shorter the wavelength of ultraviolet light, or the higher the power, the faster the attenuation of the surface friction static charge The experimental results are consistent with the photoemission model It is further proved that the source of contact electrification is electron transfer Fig 5 It is necessary but difficult to quantitatively characterize the contact electrification of SiO2 and PVC under different wavelength and different power of UV irradiation Recently, Academician Wang Zhonglin's team selected liquid metals as reference materials and measured the triboelectric sequences of more than 50 metals and polymer materials This effective technology can be extended to almost all materials, including ceramics and semiconductors In conclusion, there are five reasons why the contact electrification is caused by electron rather than ion transfer: 1 The release process of surface charge follows the electron thermal emission model; if it is ion transfer, according to the Boltzmann distribution, the amount of friction charge is larger at high temperature, which is strictly inconsistent with the experimental results 2 According to the mechanism of ion transfer, water molecules play an important role; however, the experimental results show that the lower the environmental humidity is, the greater the amount of transferred charge is, and the contact electrification phenomenon also exists in the oil 3 At 623k, there are almost no water related ions on the solid surface, but there is still contact electrification 4 The contact capacity of 10-6 Torr in vacuum environment is 5 times of that under normal pressure 5 Metal insulator semiconductor point contact system can produce continuous tunneling current This shows that water molecules are not necessary for contact electrification, which is a charge transfer process dominated by electrons When the distance between atoms is in the repulsive region, the electron cloud will overlap, and the electron will transfer between the two substances The surface discharge phenomenon caused by temperature rise follows the electron thermal emission model By applying an electric field (bias), the electrical properties of the transferred charge can be reversed Further research shows that the work required to separate the two charged surfaces is equivalent to the fracture energy of the material, which indicates that the contact electrification is related to the interaction of the electron cloud, that is, there is a strong correlation between the contact electrification and the overlap of the electron cloud, thus denying the ion transfer mechanism Finally, the contact behavior between any surface is related to the formation and fracture of the bond, thus externalizing into contact electrification For the phenomenon of solid-liquid contact electrification, Academician Wang Zhonglin proposed that electron transfer rather than double electric layer is the root of solid-liquid contact electrification As shown in Figure 6, both liquid and solid surfaces are uncharged in the original phase The liquid then flows under external drive The liquid molecules adjacent to the solid surface and the atoms on the solid surface produce an electron cloud which overlaps to realize the electron transfer (one atom in every 30000 surface atoms may participate in the electron transfer), making the solid surface charged After that, the liquid molecules will form a double layer structure on the charged solid surface In other words, the solid-liquid electron transfer at the initial stage is the root cause of the formation of the double layer Figure 6 The two-step process of the formation of a double electric layer at the solid-liquid interface Finally, according to the theory and the existing experimental results, Academician Wang Zhonglin concluded that the root cause of all (solid / liquid / gas) substances contacting and electrifying is the superposition of electron clouds Raindrops carry a negative power source to electrons transferred from air molecules to water droplets The droplet can be made into Teng by passing through the suspension film In addition, when P-and n-type semiconductors contact, the electrons in the surface state of n-type semiconductors will transfer to the holes of p-type semiconductors, making p-type semiconductors negatively charged and n-type semiconductors positively charged Although human beings have known electric contact for 2600 years, their understanding is still immature This review summarizes the latest development of contact electrification and concludes that the root of contact electrification is electron transfer Ion transfer is the result of initial electron transfer, which is the root cause of the formation of double electric layers at the solid-liquid interface The electron transfer caused by the overlap of electron clouds is the source of contact electrification between solid, liquid and gas The purpose of studying contact electrification mechanism is to build high-performance Teng, which can be used in micro nano energy devices, self driving sensors and blue energy, so as to deeply influence the development of Internet of things, biomedical, robotics and artificial intelligence This series of work has been strongly supported by the Chinese Academy of Sciences.