Surface engineering and on-site charge neutralization for the regulation of contact electrification
Youbin Zheng a,1, Shaochen Ma a,b,1, Enrico Benassi a,f,1, Yange Feng a, Shiwei Xu a,b, Ning Luo a,e, Ying Liu b, Li Cheng c, Yong Qin c, Miaomiao Yuan c, Zuankai Wang d, Daoai Wang a,e,*, Feng Zhou a
a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China c Institute of Nanoscience and Nanotechnology, Lanzhou University, Lanzhou 730000, China d Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 999077, Hong Kong, China e Qingdao Center of Resource Chemistry and New Materials, Qingdao 266100, China f Public Technical Service Center, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Electrostatic charges can accumulate on insulator surfaces under contact electrification, resulting in hazardous conditions. Despite significant progress in eliminating charges of contact electrification, there are still several limitations, including the need to dope other materials, which can alter their original properties, and the diffi-culties of fabrication. Here, a new post-treatment antistatic strategy is demonstrated to significantly reduce the accumulation of static charge by controlling the spatial distribution of tribopositive and tribonegative regions. On-site interface charge neutralization between tribopositive and tribonegative regions leads to rapid charge decay, without conductive spraying or grounding, which is especially useful in some extreme scenarios, such as the aerospace industry and the electronics industry. By using this surface engineering strategy, finished materials can be easily retrofit into antistatic materials, which will open up promising possibilities for antistatic polymers in a wide range of applications.
1. Introduction
Contact electrification, [1–3] a natural phenomenon associated with the transfer of charge when two different surfaces contact and subse-quently separate, plays an important role in science and industry. Although contact electrification has been widely used in many appli-cations, such as electrophotography [4,5] and triboelectric nano-generator (TENG) [6–10], the accumulation of static charges on insulator surfaces usually causes slight but annoying consequences in many activities, for example, the adhesion of dust particles on surfaces. Moreover, electrical discharge derived from the accumulated charge may potentially result in huge personal health loss or significant societal costs, such as serious fire and explosion [11–14]. To overcome this problem, researchers are constantly seeking novel and cost-effective antistatic strategies that may eliminate the frictional charges gener-ated from contact electrification or quickly dissipate charges away from
insulator surfaces [15–17], especially in the fields of electronic industry and aerospace. A lot of effort has been made to eliminate static charge, including fabricating polymeric composite materials [18,19], doping [20,21], and adding antistatic agents [22]. Although these are widely used methods in current industry, most of them suffer from multiple limitations, such as complex, high cost, and doping other material may unfavourably change their original properties, reducing their mechani-cal strength, even toxic. Therefore, it remains challenging but tempting to develop an easy and non-destructive antistatic strategy without grounding or conductive spraying, especially in some extreme scenarios, such as the aerospace industry and the electronics industry.
During the contact electrification process, there is a dynamic equi-librium between frictional charge generation and neutralization [23]. Charge generation and charge neutralization are two crucial factors for the accumulation of static charges on insulator surfaces. To reduce charge generation, X. Zhang et al. [24] proposed a strategy to fabricate
* Corresponding author at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail address: [email protected] (D. Wang).
1 These authors contributed equally to this work.
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Nano Energy
journal homepage: www.elsevier.com/locate/nanoen
https://doi.org/10.1016/j.nanoen.2021.106687 Received 23 June 2021; Received in revised form 30 October 2021; Accepted 1 November 2021
antistatic polymers that resists charging when rubbing against another material, through copolymerizing an appropriate ratio of molecules that tend to charge positively or negatively after contact electrification. To promote charge neutralization, H. T. Baytekin et al. [25] demonstrated an approach to discharge a polymer by doping free radical scavengers to remove free radicals that co-localized with static charges. These research efforts have proved that either reducing charge generation or promoting charge neutralization can significantly prevent the accumu-lation of static charges on insulator surfaces. However, both copoly-merization and doping process should be done during manufacturing and can’t be used for retrofitting existing polymers/insulators, which restrict their practical applications. Considering contact electrification occurs on the surface of the friction pair materials and both electron and ions can diffuse laterally on insulators [26–28], it is possible to neutralize the friction charges in situ at the friction interface by surface engineering, which might be a new antistatic strategy merging of two above-mentioned strategies. Until now, few studies have been con-ducted to assess the feasibility of eliminating the frictional charges by this strategy. If it can be successfully used in existing polymer-s/insulators, it should open up a wide field for the practical applications of antistatic polymers.
Here, based on the above hypotheses, we provided a new surface engineering anti-static strategy, which is different from the previous methods with the advantages of simple processes low cost, and retrofit ability for finished materials with chemical modification technologies. Based on the electrification nature of the material, the anti-static prop-erties of the material can be realized by controlling the spatial distri-bution of tribopositive and tribonegative regions, without conductive spraying or grounding, which is especially useful in some extreme sce-narios, such as the aerospace industry (it is impossible to ground an aircraft in space) and the electronics industry (conductive spraying cannot be used in some precision electronics). By modulating the ratio of tribopositive and tribonegative regions, charge generation is inhibited and the static charge density reduced by 99.53%. Moreover, with the help of spatial distribution of tribopositive and tribonegative regions, charge neutralization is promoted and the charge decay rate increased by 61.14%. Benefiting from the decrease of static charge density and the increase of charge decay, we fabricated an antistatic surface with good performance for preventing electrostatic adhesion. Therefore, this controllable surface engineering method is an attractive strategy to develop high performance antistatic surface with a wide range of alternative existing products and mature chemical modification tech-nologies, which will open up promising possibilities for antistatic polymers in a wide range of applications.
2. Experimental Section
2.1. Materials
Nylon-11(98%, pellets) was obtained from Tianjin Heowns Biochemical Technology Co., Ltd. Trichloro(1H,1H,2H,2H- perfluorooctyl)silane (PFTS, 97%) was obtained from Sigma-Aldrich. PI films (thickness of 50 µm) were obtained from Shenzhen Runsea electronics Co., Ltd. Aluminium foil tape and PET film are purchased from local market. All other chemicals were of analytical-grade reagents and used without further treatment.
2.2. Preparation of nylon (NY) and fluorinated nylon (NY-F) membrane
0.8 g NY pellets were hot pressed to 50 µm by a plane mould in 200 ◦C and then cooled down to room temperature to obtain the NY membrane. Subsequently, NY membrane was rinsed with ethanol, deionized water, and dried by N2 flow. To obtain NY-F membranes, the as-prepared nylon membranes were treated with oxygen plasma and then put into a desiccator with 20 μL PFTS [29]. The desiccator was evacuated for 5 min and then stopped to wait for 20 min. This process
was repeated three times, and the PFTS modified membranes were dried in an oven at 90 ◦C for 2 h to obtain the NY-F membranes.
2.3. Preparation of polymer surface with different area ratio and spatial distribution of NY and NY-F
The antistatic polymers with different area ratio and contact inter-face number are prepared as shown in Fig. S1. Teflon tape masks with different area ratio and spatial distribution are used to cover on the surface of NY substrate. After vacuum modification of PFTS, removing the Teflon tape mask and subsequently drying at 90 ◦C for 2 h, a uniform and patterned antistatic polymer with different area ratio and spatial distribution can be obtained. Micro-structured NY-F/NY films were prepared by a simplified hot processing technique using different grit size sandpapers (100 grit, 1000 grit, 2000 grit) as templates.
2.4. Computational Details
In order to compute the dielectric properties of the two materials (NY and NY-F), mechanical calculations were performed on the monomers and short oligomers. The molecular geometries were fully optimized both in the gas phase at Density Functional Theory (DFT) level. The vibrational frequencies and thermochemicals were computed in har-monic approximation at T = 298.15 K and p = 1 atm, and no imaginary frequencies were found. The calculations were performed by using M06–2X [30] DFT functionals in combination with 6–311 + +G* * [31–35]. The static and frequency-dependent molecular polarizabilities were computed as suggested in reference [36]. The dielectric constant and refractive index were computed by mean of the Mossotti-Clausius and Lorenz-Lorentz formulae, respectively, using the experimental values of density at 25 ◦C. Integration grid for the electronic density was set to 250 radial shells and 974 angular points. Convergence criteria of Self-Consistent Field were set to 10− 12 for root mean square (RMS) change in density matrix and 10− 10 for maximum change in density matrix. Convergence criteria for optimizations were set to 2 × 10− 6 a.u. for maximum force, 1 × 10− 6 a.u. for RMS force, 6 × 10− 6 a.u. for maximum displacement and 4 × 10− 6 a.u. for RMS displacement. All calculations were performed using GAUSSIAN G09. D01 package [37].
2.5. Characterization
For measurement of tribocharges, the contact-separation mode TENG was used and the polymer friction layer composed of NY and NY-F was cut into square pieces and attached with aluminium foil tape to form one of the tribo-layer. The aluminium foil was used as another tribo-electric material. The device was driven by a commercial mechanical motor (IVCL17–56) with 5 Hz contact frequency and 5800 µm ampli-tude. The short-circuit current was measured by low noise current amplifier (Stanford Research Systems, SR570), while the electric charge was measured by 6517B electrometer. The experimental data were collected through LabView program. All experiments were performed at a relative humidity of 25% at room temperature (25 ◦C) (except where otherwise indicated). Elemental mapping of the NY-F/NY polymer was performed by a scanning electron microscopy with energy dispersive X- ray spectrometry (SEM-EDS, Phenom Pro Desktop SEM). The tribo-electric potential of polymer friction layer was performed using a commercial atomic force microscopy (Dimension Icon, Bruker) with KPFM mode. The conductive tips of Co/Cr coating with the force con-stant of 3 N/m were used in KPFM mode.
3. Results and discussion
3.1. Design and fabrication of antistatic polymer
Inspired by equipotential distribution on surface of metal, we introduce a strategy to neutralize the friction charges in situ at the
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friction interface by surface engineering, including chemical modifica-tion and spatial distribution, which is shown in Fig. 1a. According to the triboelectric series [38–40], we chose Nylon-11 (NY) as the substrate because it tends to be positively tribocharged. By chemically modifying NY substrate with Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFTS), the fluorinated Nylon-11 (NY-F) layer could act as the nega-tive friction material which tends to be negatively tribocharged. When contacting with aluminium, the antistatic polymer NY-F/NY will lose electrons to aluminium in NY part, and gain electrons from aluminium in NY-F part during the triboelectrification process. In metal equipo-tential layer (aluminium), positive charges and negative charges will be neutralized rapidly when aluminium is separated from NY-F/NY poly-mer. Meanwhile, on the surface of NY-F/NY polymer, although most positive charges and negative charges cannot be neutralized because of polymer’s electrical insulating property, a few of positive and negative charges at the contact interface between triboelectric positive surface (NY) and triboelectric negative surface (NY-F) will be neutralized. Elemental mapping by SEM-EDS and triboelectric potential by KPFM of the NY-F/NY polymer were performed to verify the chemical composi-tion and surface potential after rubbing with Al. SEM image (Fig. 1b) and elemental mapping (Fig. 1c,d) of the junction area of NY-F and NY clearly show that NY-F was located in the left side while NY was located in the right side. After rubbing with Al, the surface potential of the selected areas of I(NY-F), II(NYF/NY) and III(NY) were performed and shown in Fig. 1d-f. NY-F shows negative potential and NY shows positive potential, which is consistent with the triboelectric series. It should also be noted that there is a colour gradient from dark brown to light brown in NY-F/NY area, which indicates the neutralization of positive charges
and negative charges. To further confirm our inference, Keyence surface potential probe (KEYENCE, Japan) was used to scan the surface poten-tial of a larger sample (4 cm × 4 cm). As shown in Fig. S2a, the com-posite film was placed on an iron plate with a grounded flat surface, and the distance between the surface potential probe and the developed composite film was 2 cm. The surface potential was recorded when the probe was moving from one side of the film to another side. Fig. S2b shows the surface potential line scanning result of the tribo-charged composite film. The results show that the NY part was positive charged while the NY-F part was negative charged after the composite film rubbing with Al. When the probe went across the interface between NY and NY-F, there is a potential drop from + 20 V to − 40 V, which indicates the neutralization of positive charges and negative charges. This result is consistent with the result of surface potential test in Fig. 1f. It should be note that the surface potential value is much larger than that of KPFM because of larger sample size. Together these results show that there is the neutralization of positive charges and negative charges at NY-F/NY interface. Therefore, it would be a potential strategy to elim-inate static charge by modulating surface chemical composition (inhibiting charge generation) and spatial distribution (promoting charge neutralization).
3.2. Chemical composition modulation strategy to control net triboelectric charge
To investigate the contact electrification process of NY-F/NY poly-mer, we fabricated the contact-separation mode TENG based on the NY- F/NY polymer with different area ratio. As shown in Fig. 2a, the area
Fig. 1. Design and experimental KPFM surface potential maps of antistatic polymer. (a) Schematic mechanism of antistatic NY-F/NY polymer surfaces by interface neutralization of tribocharges. SEM image (b) and elemental mapping (c, d) of the NY-F/NY polymer. The surface potentials of the selected areas, I(e), II(f) and III (g).
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ratio of NY-F to NY is modulated as 7:1, 3:1, 1:1, 1:3, 1:5, 1:7, 1:9, respectively. To simplify the discussion, the composition of NY-F/NY friction materials is defined as Φ NY-F + (1-Φ) NY, where Φ is the fluorination ratio of NY-F/NY friction materials, varied from 0 (pure NY) to 1 (pure NY-F). The top part of the sample is friction pair of aluminium triboelectrode (Fig. 2b), which is ranked between NY and NY-F in the triboelectric series. The bottom part of the sample is the NY-F/NY polymer attached with aluminium electrode as the back-conducting layer.
In order to compare the generated triboelectric charges, various TENGs based on NY, NY-F and NY-F/NY composite polymer layers under different Φ were assemble. By adjusting the fluorinated area of NY membrane (Φ), it is feasible to tune the net charge on the polymer surface generated by contact electrification and alter the charge polarity of the total NY-F/NY friction material. Electrometer was used to dynamically measure the static charge on NY-F/NY polymer surface. We keep rubbing NY-F/NY polymer with Al until triboelectric charges rea-ches the stable and maximum value. Fig. 2c and d demonstrate the output current and charge density of the NY-F/NY polymer with different Φ after rubbing with Al. The NY-F layer is charged negatively against Al material, while the NY material is positively charged. As Φ increases from 0% to 16.7%, the short-circuit current gradually de-creases from 2.603 μA to 0.403 μA and the accumulated charge density gradually decreases from the initial 4.231 μC/m2 (the charge density of NY) to 0.125 μC/m2, which is reduced by 97.05%. During this process, the positive charge generated by NY frictional electrification dominates, and the negative charge generated by NY-F frictional electrification
gradually offsets the positive charge of NY, resulting in a decrease in the total charge accumulation. When Φ is 16.7%, the negative charge generated by NY-F can almost completely offset the positive charge generated by NY, and the total charge density is 0.125 μC/m2. This result indicates that we can regulate the net triboelectric charge on the surface of the NY-F/NY polymer by using this chemical composition modulation strategy to prevent static charge. As Φ was increased from 16.7% to 1 (pure NY-F), the negative charge generated by NY-F dominates and completely offsets the positive charge of NY. NY-F continues to generate more negative charge and the total charge density gradually changing from 0.125 μC/m2 to − 26.719 μC/m2 (the charge density of NY-F). And the short-circuit current gradually changed from 0.407 μA to − 15.708 μA. The NY-F/NY layer (Φ = 16.7%) produces 99.53% less charge than pure NY-F, which means the NY/NY-F material (Φ = 16.67%) did not almost charge by contact electrification. This result is approximately similar to the result of the fitted curve (Fig. 2e) with the zero point when Φ=15.61%. As Φ increases proportionally, the accumulated charge in-creases approximately linearly. Consequently, we demonstrated that a non-charging polymer can be fabricated with appropriate Φ. Therefore, it is an effective way to achieve a non-charging polymer material by surface chemical modification of part surface of the material. This post- treatment strategy can be used for surface electrostatic protection of many finished materials without changing the properties and functions of the materials themselves.
To further study the effect of area fraction of NY-F to NY on the total charge accumulation, the short-circuit current and charge accumulation process of pure NY-F (Φ = 100%) and NY-F/NY (Φ = 16.7%) were
Fig. 2. The charge tendency of the polymers with chemical composition modulation. (a) Schematic illustration of the polymer layers with different area ratio of NY-F to NY (Φ). (b) Structure of the contact-separation mode TENG based on NY-F/NY polymer (Φ = 16.7%). The output current (c) and the charge density (d) of TENGs with different Φ. (e) Experiment results and curve fitting of charge density. Short-circuit current (f) and charge accumulation process (g) of pure NY-F and NY-F/NY (Φ = 16.7%) friction materials.
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compared in detail. The currents of pure NY-F based TENG and NY-F/NY (Φ = 16.7%) based TENG in the first 5 s are shown in Fig. 2f. The current with pure NY-F layer reached a higher value and it has a significant increasing tendency, while the TENG with NY-F/NY (Φ = 16.7%) fric-tion layer has a lower current value and shorter charge accumulation process. With the continuation of the contact-separate processes, the current of the TENG with pure NY-F has an obvious increase stage while the current of the TENG with NY-F/NY (Φ = 16.7%) remains stable as shown in Fig. 2g, indicating the long-term stability of the antistatic strategy by adjusting the surface components and area ratio of polymers.
3.3. Theory and Calculations
In order to clarify the neutralization mechanism and the relationship between triboelectric charges and the area ratio of opposite triboelectric charging material, we adopted a theoretical model described in our previous work [41] to calculate the surface charge density as a function of the composition of the system (see Supporting Information Sections S1: Calculation). We considered the composite ΦNY-F + (1–Φ) NY. To understand the effect of Φ on the surface charge density, we calculated
the surface charge density as a function of the composition of the system. The theoretical machinery allows to compute the desired property σ once the geometrical parameters, the contacting speed and dielectric properties of the material (in particular ε and n2) are known. The geometrical parameters were taken from the experiments (viz., 4 cm × 4 cm), as well as the contacting speed ν0 = 11.6 cm/s. The dielectric quantities could be either experimental or obtained from calculations. (For the sake of simplicity, we assumed isotropic values in the calculations; this seemed to be reasonable to us due to the nature of the chemical system.) In this study, the experimental dielectric proper-ties of NY and NY-F were undetermined and thus we evoked quantum chemistry (in particular, DFT methods) to simulate these values, starting from the static and frequency-dependent isotropic values of the electric dipole polarizabilities, that were computed within the CPHF framework, and the experimental values of the materials’ densities (see computa-tional details in Methods). The results are depicted in Fig. 3a. There is a monotonic behaviour of the charge density, increasing as a function of Φ. The two extremal values for Φ = 0 and 1 are 4.432 μC/m2 and − 26.09 μC/m2, respectively. The zero of the function is predicted at Φ ~ 0.15, which is rather close to the value of 15.61% in Fig. 2e. The
Fig. 3. Theoretical calculations and experimental results of on-site surface neutralization. (a) Comparison between experimental (red circles) and theoretical surface charge density (unfilled black circles) as a function of the different area ratio of NY-F and NY (Φ); least-squares linear fit of the experimental data is also depicted (intercept = (4.8 ± 0.5) μC/m2; slope = (− 31 ± 1) %− 1; Residual Sum of Squares = 4.92912; Pearson’s r = − 0.99628; Adj. R2 = 0.99152). (b) Schematic illustration of the polymer layers with different N. (c) Illustration of antistatic structure of NY-F and NY (N = 2). The insert shows schematic diagram of the neutralization of positive and negative charges from two materials with opposite triboelectric polarity. Theoretical calculations of charge density (d) and charge decay (e) of the polymer layer with different N. (f) Line chart of theory charge decay rate of polymer layers with different N. As the number increases, the rate of charge discharging increases. Experimental results of charge density (g), normalized charge decay curves (h) and line chart of charge decay rate (i). All error bars are based on at least three independent experiments for each material.
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central part of the graph reflects an approximatively linear behaviour. Deviation from the linearity is due to the fact that this model accounts for some effects (mainly border, field penetration and back-polarization effects), usually neglected, but that increase in importance when the dimensionalities thickness and depth are comparable. In the simulated system these effects are not huge, since the dielectric constants of the two materials are not that different. It is possible to note that these ef-fects are more remarkable in case of rather different materials.
In addition to the area ratio, the distribution of the two materials (NY and NY-F) may also have an important effect on the triboelectrification. Under the premise of Φ = 50%, we further calculated the evolution of the percentage of surface charge density as a function of time by dividing the composite NY/NY-F layer with equal area into two equal parts, four equal parts, six equal parts, eight equal parts which were marked as N = 1, N = 2, N = 3 and N = 4 (Fig. 3b-c), respectively. N indicates the number of “stripes” of each type of material. Fig. 3d shows the theo-retical charge density with different number of the contact interfaces of NY-F and NY after rubbing with Al, which indicates that the position of the positive and negative materials has no significant effect on the accumulation of charge. The simulations depicted in Fig. 3e show the charge decay as a function of time corresponding to a double- exponential behaviour. The mechanism of the phenomenon has been described by a model that accounts for two steps and each step has a different kinetics. In particular, the time decay can formalize as follows:
σ%(t) = σ%(∞)+ σ%1e− 1/τ1 + σ%2e− 1/τ2 (1)
where the two characteristic times, τ1 and τ2, refer to the two processes. The kinetic parameters of Eq. (1) are collected in Table S1. One process is faster than the other, and the characteristic times decrease as a function of N. The weight of each process (mathematically described by σ%1 and σ%2) also changes as a function of N.
The off-set (σ%(∞)) indicates the residual charge density at infinite time. This quantity does not linearly depend on N as shown in Fig. 3f, and this is reasonable because when N tends to infinity (a situation that we may chemically interpret as a “mixture” of the two materials), this quantity has to tend to a “saturation” value (namely, σ%(∞)), and not to decrease linearly (to minus infinity). We can derive:
σ%(∞) = σ%(sat)(∞)+Ae− kN (2)
where σ%(sat)(∞)= 25.3%, A= 31.26%, and k ≅0.356. So, in principle,
with the increase of the number of stripes, it would be a more efficient device. Of course, if the NY-F/NY layer is divided into an “infinite” number of strips, then other phenomena will occur close to no friction charge left. We did not calculate cases with big N for reasons of time and computational cost. It should be noted that σ%1 and σ%2 in Eq. (1) (green symbols in Fig. 3f) correspond to decay rate.
3.4. On-site interface neutralization
To verify the theoretical calculation of charge density and charge decay, we explored the neutralization of positive and negative tribo-electric charges at contact interface. The friction layers with different N of positive and negative material are obtained by changing the degree of refinement of the alternation of NY and NY-F. It should be noted that the use of Φ = 50% instead of Φ = 16.7% is to avoid experimental errors caused by a smaller friction area with too little charge. We checked the current and charge density of friction layer with different N of NY-F/NY after rubbing with Al. The short-circuit currents of the N = 1, N = 2, N = 3 and N = 4 friction materials were − 4.516 μA, − 3.227 μA, − 3.559 μA, − 3.693 μA respectively with a standard deviation of only 0.547 (Fig. S3). The charge densities were − 9.618 μC/m2, − 6.675 μC/ m2, − 7.513 μC/m2, − 7.906 μC/m2 respectively and the standard de-viation was 1.238 (Fig. 3g), which could be attributed to the experi-mental error. Compared with the short-circuit currents and charge
density of the pure NY-F friction material (− 15.708 μA and − 26.719 μC/ m2), the floating value of these sub-division results is much smaller. In other words, when positive (NY) and negative (NY-F) friction materials have the same area ratio and different number of interfaces, the maximum charge accumulation is almost the same. This result is consistent with the theoretical calculation of charge density, which means the area ratio of positive and negative electrical materials dom-inates the accumulated maximum charge.
To further investigate the effect of contact interfaces of NY and NY-F on the triboelectric charge, we measured the variation of surface charge as a function of time. NY-F/NY layer keeps rubbing with Al until we get stable and maximum value of triboelectric charge. This charge value was marked as the initial charge accumulation value. Then, we measured the charge values every one minute in the first 5 min and every five minutes in last 35 min (Figs. 3h and S4). A general tendency shows that all the friction materials have the similar behaviour that the charge decrease rapidly in the first 5 min and then obtain a steady state with low decrease state. The charge of N = 1 friction material with 1 contact interface decreased by 52.75% within 40 min, while the charge of N = 4 with 7 contact interfaces decreased by 67.97%. The charge of N = 2 and N = 3 which have 3 and 5 contact interfaces decreased by 58.26% and 62.61% respectively, which are faster than that of N = 1. Compared with NY-F (42.18%), the charge decay rate of N = 4 friction material is increased about 61.14% (Fig. 3i). More contact interfaces between positive insulating material and negative insulating material results in faster charge decay. In other words, when the friction surface has the same area ratio of positive and negative insulating materials, the charge decay rate may be simply dependent on the number of the contact in-terfaces, in which, there is non-negligible neutralization of positive and negative charges at the contact interface. And the number of interfaces of NY-F/NY only has a slight influence on the accumulated maximum charge amounts output of the composite materials but significantly changes the rate of charge decay, which is theoretically proven as shown in Fig. 3d–f. In addition, the effects of surface roughness and relative humidity on the triboelectric charge decay were evaluated as shown in Fig. S5. Both flat (Fig. S5a) and micro-structured (Fig. S5b-d) NY-F/NY films exhibited a similar trend that the charge gradually decreased with prolonged time (Fig. S5e). The charge decay occurs faster on flat film than that on micro-structured samples and the order of charge decay rate is: Flat > 100 Grit > 1000 Grit > 2000 Grit. Furthermore, the charge decay was much slower at 40% relative humidity than at 60% or 80% relative humidity (Fig. S5f).
In the contact-separation triboelectrification process, triboelectric positive material (NY) and triboelectric negative material (NY-F) show positive and negative potential, respectively. As a result, there is charge neutralization at the interface, and charge can be released in a fast manner. When the charge is accumulated to a stable value, the material reaches its maximum charge storage capacity. Notably, such a charge neutralization at the contact interfaces is “linear neutralization” by na-ture, which is weaker than the charge generation in all of contact sur-face. That explains why the maximum triboelectric charge value achieved in the charge accumulation process has a significant linear relationship with the area ratio while is not necessarily related to the number of interfaces. The triboelectric potential distribution of NY-F/ NY contact interfaces was measured by KPFM to verify it. Fig. 4a and c show the variation in topography and the surface potential of the NY- F/NY/NY-F friction layer which has two contact interfaces. Fig. 4b and d are the cross-section height and potential information of selected line (white) in Fig. 4a and c, respectively (3D topography and surface po-tential images are shown in Fig. S6). Clearly, after rubbing with Al, the areas of NY-F are negatively charged with a negative potential (dark areas on both sides), and the NY is positively charged with a positive potential (bright area in middle). There is obvious surface potential difference between NY-F and NY areas with the potential changing from − 0.3 V to + 0.9 V and the NY/NY-F contact interface from + 0.9 V to − 0.25 V. In the contact interface area of NY and NY-F, there are regions
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with lower potential and even to zero, where is neutralization of positive and negative charges between triboelectric positive material (NY) and triboelectric negative material (NY-F), in accordance with the charge neutralization mechanism in Figs. 1a and 3c.
To further clarify the mechanism and universality of the surface neutralization strategy, we fabricated other TENGs based on the friction layers made of two different physically stitching materials (Figs. S7a and S8). We chose Kapton (PI) as the triboelectric negative material, which also tends to become negatively charged when contacting with Al. Here NY and PI layers are physically stitching together and there are micro- gaps between PI and NY, which means no contact interface exists. PI charged negatively when contacting with Al, while NY charged posi-tively. As the PI area increases proportionally, the accumulated charge changes approximately linearly (Fig. S7b-c). When the area ratio of PI to NY is at around 1: 2 (33.34% PI), the short-circuit current and the triboelectric charge density generated by contact electrification is 0.109 μA and − 0.427 μC/m2, which reduce 96.03% and 92.98% respectively, comparing with that of pure PI material (2.745 μA, − 6.085 μC/m2). The charge density will be around zero when the area ratio of PI to NY is 35.24% (Fig. S7d). These results show good adjustability and univer-sality of this chemical composition modulation strategy for fabricating antistatic polymer by controlling the area ratio of the positive insulating material versus negative insulating material.
The influence of the number of the physically splice interfaces of PI and NY on triboelectric charge density and charge decay rate is also investigated. There is almost no significant change in the total charge accumulation with the increase of the degree of refinement (Fig. S7e), which means the charge density of the friction materials is independent of the degree of refinement. This result is similar to the case of NY-F/NY composite friction layer prepared by chemical modification method. However, unlike the result in Fig. 3h, the degree of refinement of the friction materials splicing PI and NY does not have a significant influ-ence on the charge decay rate (Fig. S7f), owing to the difficulty of charge transfer on the surface of insulating polymer which cannot be
neutralized through these micro-gaps between PI and NY. Meanwhile, the charge decay rate for friction materials (NY-F/NY layer) which have the contact interfaces formed by chemical modification increases with increasing the degree of refinement (red point, Fig. 5a), owing to the positive and negative friction charges could be neutralized on-site at the contact interface. Further analysis with surface conductivity has been carried out. For NY-F, the surface conductivity is 0.5848 × 10− 10 S and for N = 1, 2, 3 and 4, it is 0.5451, 0.5478, 0.57716, 0.5847 × 10− 10 S, respectively (Fig. 5b). These values are highly consistent with pure NY, whose surface conductivity is 0.561 × 10− 10 S. Low surface conductivity indicates that all of these polymers are insulators and it’s difficult to dissipate charge from their surfaces. Hence, the primary contribution to the charge decay is from the on-site charge neutralization at the contact interfaces between two materials with opposite triboelectric properties. This result indicates on-site interface charge neutralization plays an important role to design static-dissipative materials. Furthermore, this spatial distribution modulation and on-site interface neutralization strategy can be applied to large-scale retrofit existing polymers/in-sulators into antistatic materials.
3.5. Practical application
In order to demonstrate the power of our surface engineering strat-egy for retrofitting existing polymers/insulators into antistatic mate-rials, we conducted adhesion experiments which were shown in Fig. 6 and Supplementary Videos. All the samples are discharged by using an ionizing air blower before experiments. Subsequently, NY-F, NY and NY- F/NY (Φ = 16.7%) polymer films are rubbed with aluminium foil both in sliding mode (Fig. 6a and Video S1) and contacting mode (Fig. 6b and Video S2). NY-F and NY polymer surfaces gain significantly charge and a lot of polystyrene foam balls are adhered onto their surfaces (Fig. 6b-c and f-g). For NY-F/NY (Φ = 16.7%), only a few polystyrene foam balls adhered to the surface (Fig. 6d and h). Thus, this surface after post- treatment demonstrates good performance on preventing the
Fig. 4. The KPFM measurement results. (a) Topography and (b) cross-section profile images of NY-F/NY layer, (c) surface potential image and (d) cross-section profile images of NY-F/NY layer after rubbing with Al.
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Nano Energy 91 (2022) 106687
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electrostatic adhesion. Without being limited to a NY layer, many other finished materials can be retrofit into antistatic materials by using this post-treatment strategy.
4. Conclusions
In summary, we have demonstrated a novel post-treatment strategy for dramatically eliminating accumulation of static charge on polymer surface by modulating surface chemical composition and spatial distri-bution. More importantly, we demonstrate that on-site interface charge neutralization between triboelectric positive insulating surface and triboelectric negative insulating surface made the major contribution to charge decay. Modulating surface chemical composition and spatial distribution can effectively inhibit charge generation and promote charge neutralization. By using this surface engineering strategy, finished materials can be easily retrofit into antistatic materials, which will open up promising avenues for the practical applications of anti-static polymers.
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.
Acknowledgements
The authors thank Prof. Z. Zhang and Dr. S. Pan in Shanghai Jiao Tong University for fruitful discussions on the mechanism calculations. Thanks for the financial support of the Program for Taishan Scholars of Shandong Province (No. ts20190965), the National Key Research and Development Program of China (2020YFF0304600), the National Nat-ural Science Foundation of China (No. 51905518, 21603242) and the Innovation Leading Talents Program of Qingdao (19-3-2-23-zhc) in China. The Authors are grateful to the Siberian Supercomputer Centre of Institute of Computational Mathematics and Mathematical Geophysics (Russian Academy of Sciences, Novosibirsk, Russian Federation) for kindly providing the computational resources; the technical staff of the Institute is also thanked for the assistance.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2021.106687.
Fig. 5. Charge decay and conductivities of as-prepared layers. (a) Line chart of charge decay rate of NY-F/NY with the contact interface and PI/NY without the contact interface. (b) Surface conductivities of the NY-F, NY and NY-F/NY with different N.
Fig. 6. Resisting the adhesion of polystyrene foam ball on the composite polymer surface even after rubbing. (a) Schematic of the sliding mode. Polymer film is moving horizontally on the aluminium foil. (b-d) Images of the surfaces of different polymers adhering with different amounts of the polystyrene foam balls after rubbing with aluminium in sliding mode: (b) NY-F, (c) NY, (d) NY-F/NY (Φ = 16.7%). (e) Schematic of the contacting mode. Polymer film is moving vertically upon the aluminium foil. (f-h) Images of the surfaces of different polymers adhering with different amounts of the polystyrene foam balls after rubbing with aluminium in contacting mode: (f) NY-F, (g) NY, (h) NY-F/NY (Φ = 16.7%).
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Youbin Zheng received his B.S. degree in Electronic Device and Materials Engineering (2010) and Ph.D. degree in Material Physics and Chemistry (2015), supervised by Prof. Yong Qin from Lanzhou University, China. During 2015–2018, he served as a research assistant at Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. He is currently a postdoctoral fellow at Technion-Israel Institute of Technology, supervised by Prof. Hossam Haick. His research interests include tribo-electrification, chemical/bio-sensors and self-powered health monitoring systems.
Shaochen Ma received her B. S. degree in Materials Science and Engineering from University of Jinan (2016) and Master degree in Materials Engineering, supervised by Prof. Ying Liu and Daoai Wang from Ocean University of China (2019). Her research interests include triboelectrification and anti-static materials.
Enrico Benassi received his Ph.D. degree at the University of Modena and Reggio Emilia, Italy (2010). He worked in Italy, Russia, China, Kazakhstan and United States of America. His research fields include molecular physics, quantum mechanical calculations, theoretical chemistry, theoretical spectroscopies, nonlinear optical properties, intra- and inter-molecular in-teractions, solvent and surface effects.
Yange Feng received his B. S. (2013) in Chemistry from Qingdao University of Science and Technology and Ph.D. (2018) in Physical Chemistry in Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS). He is currently a research assistant in State Key Laboratory of Solid Lubrication, LICP, CAS. He mainly focuses on the research of green triboelectricity and nano devices based on triboelectricity.
Shiwei Xu received his B.S. (2017) in Luoyang Institute of Science and Technology and Master degree (2021) at Institute of Materials Science and Engineering, Ocean University of China. He mainly focuses on the research of green triboelec-tricity and nano devices based on triboelectricity.
Ning Luo received his B.S. (2015) in Northwestern Poly-technical University. He is currently pursuing the Ph.D degree in State Key Laboratory of Solidification Processing, North-western Polytechnical University. His research mainly focuses on the mechanism of triboelectrification and the control strategy of static charge in polymer.
Ying Liu received her Ph.D. in Lanzhou Institute of Chemical Physics, CAS. From 2011 to 2013, she worked as a Postdoc in Ocean University of China. She is currently an associate pro-fessor in School of Materials Science and Engineering, Ocean University of China. Her research interests include nano ma-terials, triboelectrification, physics and chemistry in tribology.
Li Cheng received his B.S. (2011) in Physics and Ph.D. (2016) in Material Physics and Chemistry from Lanzhou University. Now he is an associate professor in School of Materials and Energy of Lanzhou University at Institute of Nanoscience and Nanotechnology. His research mainly focuses on nano-generators and self-powered nanodevices.
Yong Qin received his B.S. (1999) in Material Physics and Ph. D. (2004) in Material Physics and Chemistry from Lanzhou University. From 2007 to 2009, he worked as a visiting scholar and Postdoc in Professor Zhong Lin Wang’s group at Georgia Institute of Technology. Currently, he is a professor at the Institute of Nanoscience and Nanotechnology, Lanzhou Uni-versity, where he holds a Cheung Kong Chair Professorship. His research interests include nanoenergy technology, functional nanodevice and self-powered nanosystem. Details can be found at: http://www.yqin.lzu.edu.cn.
Miaomiao Yuan received her Ph.D. degree in Lanzhou Uni-versity (2016). From 2017 to 2019, she worked as an associate professor in the Southern Medical University, P.R. China. And she is now an associate professor supported by ‘‘Top Hundred Talents’’ Program of Sun Yat-sen University. Her research in-terests include nanomaterials for cancer diagnosis and therapy, and self-powered health monitoring systems.
Zuankai Wang received his B.S. in Mechanical Engineering at Jilin University in China in 2000, M.S. in Microelectronics at Chinese Academy of Sciences in 2003 and Ph.D. in Mechanical Engineering at Rensselaer Polytechnic Institute in 2008. He joined the Department of Mechanical and Biomedical Engi-neering at the City University of Hong Kong as an assistant professor in October 2009 and became an associate professor in July 2014. In July 2018, he became a professor in Department of Mechanical Engineering, City University of Hong Kong. His research interests are focused on the nature-inspired topolog-ical mechanical systems and bio-inspired materials.
Daoai Wang received his Ph.D. in Lanzhou Institute of Chemical Physics, CAS. From 2009 to 2010, he worked as a Postdoc in Max Planck Institute of Microstructure Physics. From 2010 to 2013, he worked as a JSPS researcher in the University of Tokyo. He is currently a professor and the deputy director in State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. His research interests include triboelectrification, anti-static, physics and chemistry in tribology.
Feng Zhou got his Ph.D. in 2004 in Lanzhou Institute of Chemical Physics. He spent three years (2005–2008) in the department of Chemistry, University of Cambridge as a post-doctoral research associate. And he is currently a professor and deputy director in State Key Lab of Solid Lubrication, LICP, CAS. His research interests are the micro/nanostructured sur-faces for lubrication, drag/noise reduction and anti-biofouling applications, high performance lubricants. Details can be found at: http://www.licp.cas.cn/zfz/.
