Complete Prevention of Contact Electrification by Molecular Engineering

Abstract

•Facile fabrication of antistatic coatings on surfaces with arbitrary shapes•Chemically heterogeneous but electrostatically homogeneous antistatic surfaces•Suppression of charge generation on antistatic surfaces during contact electrification Electrostatic charges caused by contact electrification are ubiquitous in nature, and their accumulation on surfaces could cause many undesirable consequences. Despite notable progress, existing antistatic approaches are heavily dependent on either the modification of the bulk materials or the delicate control of surface patterning. Taking advantage of the fact that contact electrification originates from electron transfer dictated by surface potential difference, herein, we report the rational design of chemically heterogeneous but electrostatically homogeneous coatings through the molecular engineering of the surface potential to suppress electron transfer so as to completely prevent electrostatic charge generation during contact electrification. Our strategy is general, endowing the simple fabrication of robust antistatic coatings on arbitrary surfaces. Electrostatic charges are easily generated on surfaces during contact electrification. Although these invisible charges have emerged as a new dimension in mediating the functions of surfaces, such as energy conversion, liquid transport, reactivity, and adsorbability, the accumulation of charges on surfaces can also pose many undesirable consequences. Despite notable progress, existing approaches in engineering antistatic surfaces suffer from limitations such as the need to modify bulk materials or for delicate control of patterning on surfaces that rely on the neutralization of generated charges. Herein, we report a general toolbox for designing antistatic coatings by leveraging on chemically heterogeneous components with electron-donating and electron-accepting functions, i.e., N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and 1H,1H,2H,2H-perfluorooctyl trimethoxysilane, to molecularly engineer the surface potential to achieve an electrostatic homogeneity and completely prevent charge generation. Our approach is general, which allows the facile fabrication of antistatic coatings on various materials, even flexible and curved, with good re-writability and transparency. Electrostatic charges are easily generated on surfaces during contact electrification. Although these invisible charges have emerged as a new dimension in mediating the functions of surfaces, such as energy conversion, liquid transport, reactivity, and adsorbability, the accumulation of charges on surfaces can also pose many undesirable consequences. Despite notable progress, existing approaches in engineering antistatic surfaces suffer from limitations such as the need to modify bulk materials or for delicate control of patterning on surfaces that rely on the neutralization of generated charges. Herein, we report a general toolbox for designing antistatic coatings by leveraging on chemically heterogeneous components with electron-donating and electron-accepting functions, i.e., N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and 1H,1H,2H,2H-perfluorooctyl trimethoxysilane, to molecularly engineer the surface potential to achieve an electrostatic homogeneity and completely prevent charge generation. Our approach is general, which allows the facile fabrication of antistatic coatings on various materials, even flexible and curved, with good re-writability and transparency. Developing antistatic surfaces that reduce the generation and accumulation of electrostatic charges caused by contact electrification is of fundamental interest and practical importance in science and industry.1Lowell J. Rose-Innes A. Contact electrification.Adv. Phys. 1980; 29: 947-1023Crossref Scopus (667) Google Scholar,2Lacks D.J. Shinbrot T. Long-standing and unresolved issues in triboelectric charging.Nat. Rev. Chem. 2019; 3: 465-476Crossref Scopus (82) Google Scholar With sizes several orders of magnitude smaller than that of the surface topography, surface charges are capable of modulating the functions of surfaces, such as power generation,3Xu W. Zheng H. Liu Y. Zhou X. Zhang C. Song Y. Deng X. Leung M. Yang Z. Xu R.X. et al.A droplet-based electricity generator with high instantaneous power density.Nature. 2020; 578: 392-396Crossref PubMed Scopus (259) Google Scholar,4Xu W. Zhou X. Hao C. Zheng H. Liu Y. Yan X. Yang Z. Leung M. Zeng X.C. Xu R.X. et al.SLIPS-TENG: robust triboelectric nanogenerator with optical and charge transparency using a slippery interface.Natl. Sci. Rev. 2019; 6: 540-550Crossref PubMed Scopus (40) Google Scholar electrophotography,5Pai D.M. Springett B.E. Physics of electrophotography.Rev. Mod. Phys. 1993; 65: 163-211Crossref Scopus (285) Google Scholar particle separation,6McCarty L.S. Whitesides G.M. Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets.Angew. Chem. Int. Ed. 2008; 47: 2188-2207Crossref PubMed Scopus (623) Google Scholar droplet manipulation,7Sun Q. Wang D. Li Y. Zhang J. Ye S. Cui J. Chen L. Wang Z. Butt H.J. Vollmer D. Deng X. Surface charge printing for programmed droplet transport.Nat. Mater. 2019; 18: 936-941Crossref PubMed Scopus (151) Google Scholar and chemical synthesis,8Liu C.Y. Bard A.J. Electrostatic electrochemistry at insulators.Nat. Mater. 2008; 7: 505-509Crossref PubMed Scopus (190) Google Scholar but the presence of ubiquitous, invisible electrostatic charges is also devastating in some cases. For example, electric discharging owing to the excess accumulation of electrostatic charges on highly insulating surfaces may destroy electronic equipment and even ignite flammable substances.9Baytekin H.T. Baytekin B. Hermans T.M. Kowalczyk B. Grzybowski B.A. Control of surface charges by radicals as a principle of antistatic polymers protecting electronic circuitry.Science. 2013; 341: 1368-1371Crossref PubMed Scopus (95) Google Scholar, 10Ohsawa A. Brush and propagating brush discharges on insulating sheets in contact with a grounded conductor.J. Electrostat. 2017; 88: 171-176Crossref Scopus (5) Google Scholar, 11Tamminen P. Ukkonen L. Sydanheimo L. Correlation of component human body model and charged device model qualification levels with electrical failures in electronics assembly.J. Electrostat. 2016; 79: 38-44Crossref Scopus (7) Google Scholar In addition, in the pharmaceutical industry, where precise medicine is favored, the electrostatic charges generated on drug ingredients with low conductivity could lead to non-uniform dosages and other unexpected outcomes.12Wong J. Kwok P.C.L. Chan H.K. Electrostatics in pharmaceutical solids.Chem. Eng. Sci. 2015; 125: 225-237Crossref Scopus (31) Google Scholar Generally, antistatic surfaces can be achieved either through the timely dissipation of generated electrostatic charges or by reducing, even preventing, their generation. The former strategy is the most widely used because of its simplicity, whereby charges could be dissipated by earthing,13Paasi J. Nurmi S. Vuorinen R. Strengell S. Maijala P. Performance of ESD protective materials at low relative humidity.J. Electrostat. 2001; 51: 429-434Crossref Scopus (45) Google Scholar increasing conductivity, or adding radical-scavenging molecules.5Pai D.M. Springett B.E. Physics of electrophotography.Rev. Mod. Phys. 1993; 65: 163-211Crossref Scopus (285) Google Scholar,9Baytekin H.T. Baytekin B. Hermans T.M. Kowalczyk B. Grzybowski B.A. Control of surface charges by radicals as a principle of antistatic polymers protecting electronic circuitry.Science. 2013; 341: 1368-1371Crossref PubMed Scopus (95) Google Scholar,14Li K. Zhang C. Du Z. Li H. Zou W. Preparation of humidity-responsive antistatic carbon nanotube/PEI nanocomposites.Synth. Met. 2012; 162: 2010-2015Crossref Scopus (19) Google Scholar, 15Li X. Zhang L. Feng Y. Zhang X. Wang D. Zhou F. Solid–liquid triboelectrification control and antistatic materials design based on interface wettability control.Adv. Funct. Mater. 2019; 29: 1903587Crossref Scopus (33) Google Scholar, 16Krupa I. Mikova G. Novak I. Janigova I. Nogellova Z. Lednicky F. Prokes J. Electrically conductive composites of polyethylene filled with polyamide particles coated with silver.Eur. Polym. J. 2007; 43: 2401-2413Crossref Scopus (52) Google Scholar, 17Chen K. Xiong C.X. Li L.B. Zhou L. Lei Y.A. Dong L.J. Conductive mechanism of antistatic poly(ethylene terephthalate)/ZnOw composites.Polym. Compos. 2009; 30: 226-231Crossref Scopus (35) Google Scholar Despite their efficacy in the prevention of charge accumulation, these methods cannot fundamentally prevent the generation of charges. More promising approaches that could reduce, even completely prevent, electrostatic charge generation during contact electrification can be made through chemically heterogeneous modification, such as copolymerizing monomers that tend to be positively charged with those that tend to be negatively charged.18Zhang X. Huang X. Kwok S.W. Soh S. Designing non-charging surfaces from non-conductive polymers.Adv. Mater. 2016; 28: 3024-3029Crossref PubMed Scopus (21) Google Scholar Nevertheless, bulk modification is suitable for only a limited sort of materials with strong charging abilities.19Zhang X. Ao C.K. Soh S. Nonconductive noncharging composites: tunable and stretchable materials for adaptive prevention of charging by contact electrification.ACS Appl. Mater. Interfaces. 2020; 12: 5274-5285Crossref PubMed Scopus (4) Google Scholar Instead, taking advantage of the fact that contact electrification is an interfacial phenomenon, the modification of physicochemical properties of surfaces using chemically heterogeneous coatings has emerged as a more general method amenable to a broad spectrum of materials.15Li X. Zhang L. Feng Y. Zhang X. Wang D. Zhou F. Solid–liquid triboelectrification control and antistatic materials design based on interface wettability control.Adv. Funct. Mater. 2019; 29: 1903587Crossref Scopus (33) Google Scholar,20Baytekin H. Patashinski A. Branicki M. Baytekin B. Soh S. Grzybowski B.A. The mosaic of surface charge in contact electrification.Science. 2011; 333: 308-312Crossref PubMed Scopus (489) Google Scholar, 21Byun K.E. Cho Y. Seol M. Kim S. Kim S.W. Shin H.J. Park S. Hwang S. Control of triboelectrification by engineering surface dipole and surface electronic state.ACS Appl. Mater. Interfaces. 2016; 8: 18519-18525Crossref PubMed Scopus (56) Google Scholar, 22Shin S.H. Bae Y.E. Moon H.K. Kim J. Choi S.H. Kim Y. Yoon H.J. Lee M.H. Nah J. Formation of triboelectric series via atomic-level surface functionalization for triboelectric energy harvesting.ACS Nano. 2017; 11: 6131-6138Crossref PubMed Scopus (91) Google Scholar, 23Chen L. Shi Q. Sun Y. Nguyen T. Lee C. Soh S. Controlling surface charge generated by contact electrification: strategies and applications.Adv. Mater. 2018; 30: 1802405Crossref Scopus (69) Google Scholar These chemically heterogeneous coatings are spatially patterned with distinctive chemical components that tend to gain positive and negative charges, achieving a collective charge neutralization and electrostatic heterogeneity on the whole surface after contact electrification.24Soh S. Chen X. Vella S.J. Choi W. Gong J.L. Whitesides G.M. Layer-by-layer films for tunable and rewritable control of contact electrification.Soft Matter. 2013; 9: 10233-10238Crossref Scopus (14) Google Scholar, 25Gumbley P. Thomas 3rd, S.W. Reversible photochemical tuning of net charge separation from contact electrification.ACS Appl. Mater. Interfaces. 2014; 6: 8754-8761Crossref PubMed Scopus (11) Google Scholar, 26Thomas 3rd, S.W. Vella S.J. Dickey M.D. Kaufman G.K. Whitesides G.M. Controlling the kinetics of contact electrification with patterned surfaces.J. Am. Chem. Soc. 2009; 131: 8746-8747Crossref PubMed Scopus (27) Google Scholar, 27Thomas 3rd, S.W. Vella S.J. Kaufman G.K. Whitesides G.M. Patterns of electrostatic charge and discharge in contact electrification.Angew. Chem. Int. Ed. 2008; 47: 6654-6656Crossref PubMed Scopus (45) Google Scholar However, these electrostatically heterogeneous coatings are susceptible to several drawbacks. First, the patterns should be constructed using distinctive chemical components that possess strong charging capability to gain positive and negative charges. The neutralization of generated electrostatic charges demands delicate control of the spatial arrangement and size of individual patterns to ensure the generation of equivalent amounts of positive and negative charges simultaneously. Without proper control, the localized surface maintains a strong charging ability and the antistatic effect breaks down owing to the imbalance between the positive and negative charges. Second, achieving preferred patterns requires sophisticated fabrication, and it becomes more challenging to fabricate patterns based on non-planar surfaces. More importantly, a carefully designed patterning on the antistatic surface is suitable for only a specific counter surface, and thus re-designing and manufacturing of the patterns are required when a new counter surface is encountered, which is laborious in process and time consuming. As a result, in striking contrast to conventional electrostatically heterogeneous patterning approaches that rely on the stringent control of charge neutralization, a general, facile, and robust antistatic strategy is desired. In this study, we develop a new molecularly engineered coating that fundamentally overcomes the inherent challenges encountered in conventional coatings by completely preventing the electrostatic charge generation and transfer on any spatial location. Our strategy mainly relies on molecularly engineering two chemically heterogeneous groups with strong charging capability to modulate the surface potential for the achievement of electrostatically homogeneous antistatic surfaces with complete prevention of charge generation during contact electrification. We also demonstrate that compared with the conventional electrostatic heterogeneous surfaces, such a molecularly engineered antistatic coating exhibits excellent advantages in terms of fabrication and antistatic performance. The antistatic coating is designed by choice of two kinds of ethanol-soluble silane molecules, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (APTS) and 1H,1H,2H,2H-perfluorooctyl trimethoxysilane (FOTS), because of their specific natures (shown in Figures 1A and 1B ). First, both APTS and FOTS possess silane groups, which can easily bond with a wide spectrum of easily hydroxylated materials, such as polymers (polyethylene [PE], polypropylene [PP], polycarbonate [PC], polyoxymethylene [POM], polyamide [PA6]), metals (copper), inorganic non-metals (glass, SiO2), fabrics (nylon), other materials (polydimethylsiloxane [PDMS], paper), and so on. The existence of such silane-anchored coatings is demonstrated by the change in wettability (in Figure S1), and we also used X-ray photoelectron spectroscopy (XPS) spectra to confirm the evidence of ATPS and FOTS on SiO2 (in Figure 1C). Second, both APTS and FOTS show strong charging ability due to their specific functional groups. APTS is easily positively charged owing to the electron-donating nature of amino groups, whereas FOTS, consisting of electron-accepting fluorine atoms, is easily charged negatively.22Shin S.H. Bae Y.E. Moon H.K. Kim J. Choi S.H. Kim Y. Yoon H.J. Lee M.H. Nah J. Formation of triboelectric series via atomic-level surface functionalization for triboelectric energy harvesting.ACS Nano. 2017; 11: 6131-6138Crossref PubMed Scopus (91) Google Scholar As shown in Table S1, APTS surfaces on any substrate are always positively charged, while FOTS surfaces on any substrate are negatively charged, after contact with any uncoated substrate used in our experiments. Based on the charging ability of uncoated substrates shown in Table S2, we determined a new triboelectric series containing APTS and FOTS surfaces on substrates (Figure 1A). In this series, FOTS and APTS have strong but opposite charging abilities, which is crucial to the design of ideal antistatic coating. Guided by the distinctive charging ability of APTS and FOTS, we fabricated the antistatic coatings using a simple dip-coating process by properly tailoring the molar fraction of APTS (λ) or FOTS (1 − λ). In our experiment, pre-determined substrates are first hydroxylated by oxygen plasma and then dipped into the 5 mM mixed ethanol solution comprising both APTS and FOTS for 3 h. After that, the coated substrates were rinsed with ethanol to remove the possible silane molecule residues, followed by drying by a nitrogen stream. Unlike chemically homogeneous coatings with APTS or FOTS alone, the obtained coatings are chemically heterogeneous and the surface energy can be easily regulated by tailoring λ. As reflected by the static contact angles of water, with an increase in λ, the contact angles of coatings decrease linearly, indicating the increase in their surface energy (Figure 1D), which is also confirmed by the dynamic contact angles of water in Table S3. We then prepared coatings with various λ based on elastic PDMS substrates to investigate how λ affects the charging ability of the coatings using the setup shown in Figures 2A and S2. The utility of soft and flat PDMS material (with a compressive modulus of ~3.13 MPa in Figure S3 and roughness of ~0.239 nm in Figure S4) as the substrate eliminates the unwanted point contact encountered by rigid substrates. Before measurement, we used an ionizing air blower to remove possible pre-existing charges on surfaces. When the coating is vertically contacted by the counter substrate, electrostatic charges with equal quantity but opposite polarity are generated at the interface of the two surfaces owing to contact electrification.28Sutka A. Malnieks K. Lapcinskis L. Kaufelde P. Linarts A. Berzina A. Zabels R. Jurkans V. Gornevs I. Blums J. Knite M. The role of intermolecular forces in contact electrification on polymer surfaces and triboelectric nanogenerators.Energy Environ. Sci. 2019; 12: 2417-2421Crossref Google Scholar,29Zou H. Zhang Y. Guo L. Wang P. He X. Dai G. Zheng H. Chen C. Wang A.C. Xu C. Wang Z.L. Quantifying the triboelectric series.Nat. Commun. 2019; 10: 1427Crossref PubMed Scopus (421) Google Scholar In such contact mode, there is no charge transfer between two electrodes. As the two surfaces are then vertically separated, an electric potential difference across these two surfaces is built, driving the flow of the charges between the two back electrodes, which is evidenced by the charge variation in Figure 2B. The amount of transferred charges in separate mode is equal to the generated electrostatic charges on the coating29Zou H. Zhang Y. Guo L. Wang P. He X. Dai G. Zheng H. Chen C. Wang A.C. Xu C. Wang Z.L. Quantifying the triboelectric series.Nat. Commun. 2019; 10: 1427Crossref PubMed Scopus (421) Google Scholar and is dependent on λ. The maximum negative and positive electrostatic charges occur on chemically homogeneous coatings with λ = 0 or λ = 1, respectively, and the crossover in the polarity of the electrostatic charges happens when λ lies between 0.6 and 0.8. When the coating contacts the counter substrate again, a new contact-separation cycle starts (the whole process is shown in Video S1). The amount of electrostatic charges in the separate mode is stable even after 2,000 contact-separation cycles (Figure S5), indicating the robustness and stability of the coatings during the contact electrification. https://www.cell.com/cms/asset/5dc2329b-6bcd-43ca-93b8-b7e689b0268e/mmc2.mp4Loading ... Download .mp4 (1.45 MB) Help with .mp4 files Video S1. The Measurement of Generated Electrostatic Charges and Schematic Drawing of Electron Transfer during the Contact-Separation Cycles The critical λ to manifest a complete non-charging surface, defined as λNCS, can also be theoretically determined by considering the charge density (σ) as a function of λ (Figure 2C), which is fitted by a linear equation expressed as:σ=λσAPTS+(1−λ)σFOTS,(Equation 1) where σFOTS and σAPTS are the charge densities on FOTS (λ = 0) and APTS (λ = 1) surfaces, respectively. For the coatings based on PDMS after contact with copper, σFOTS and σAPTS are approximately −21.5 and 10.6 μC/m2, respectively, and the λNCS is calculated to be 0.67, as denoted by the red star in Figure 2C. For simplicity, we chose 0.67 as the pre-determined value of λNCS in the subsequent measurement and characterization. The macroscopic antistatic effect relevant to practical applications can be reflected by comparing the electrostatic attraction of polystyrene (PS) spheres on surfaces without and with antistatic coating. As shown in Figures 2D and 2E and Video S2, repeatedly immersing two surfaces into and out of a chamber full of PS spheres with an average diameter of ~3 mm leads to totally different scenarios in the expression of attractive coulombic force between two objects. The pristine PDMS without coating is covered by numerous PS spheres, leading to a large coverage rate of PS spheres that is the result of the area covered by PS spheres divided by the entire surface area (right y axis of Figure 2F). In contrast, for the antistatic PDMS, the coverage rate of PS spheres is near zero. The difference can also be reflected by characterizing the evolution of electrostatic charges on these two surfaces (left y axis of Figure 2F). For the pristine PDMS, with an increase in contact cycles, the electrostatic charges increase gradually and eventually reach a stable value after 20 contact cycles. In contrast, the accumulation of electrostatic charges on antistatic PDMS is negligible. Further, we also examined the stability of the antistatic effect under different relative humidity conditions (Figure 2G). It is well known that electrostatic charges are more easily generated under dry conditions, which is proved by the coverage rate of PS spheres on the pristine surface. As the relative humidity decreases, the pristine PDMS adheres to more spheres. However, the antistatic PDMS still shows a stable antistatic function under lower relative humidity conditions, which is not changed even if the surface is stored for 120 days. https://www.cell.com/cms/asset/f247dbf7-b8b6-43bd-9459-b64a878aa05a/mmc3.mp4Loading ... Download .mp4 (5.75 MB) Help with .mp4 files Video S2. Comparison of Adsorption of PS Spheres on Pristine and Antistatic PDMS Surfaces The antistatic effect can also be quantified from the perspective of surface potential, an inherent property of a material that reflects the direction of charge transfer.21Byun K.E. Cho Y. Seol M. Kim S. Kim S.W. Shin H.J. Park S. Hwang S. Control of triboelectrification by engineering surface dipole and surface electronic state.ACS Appl. Mater. Interfaces. 2016; 8: 18519-18525Crossref PubMed Scopus (56) Google Scholar Generally, the surface potential of a coating can be derived by measuring its contact potential difference (VCPD) from a fixed tip using Kelvin probe force microscopy.30Melitz W. Shen J. Kummel A.C. Lee S. Kelvin probe force microscopy and its application.Surf. Sci. Rep. 2011; 66: 1-27Crossref Scopus (872) Google Scholar We prepared coatings with various λ on the silica substrate because of its ability to accurately map surface potential, and we chose PDMS as the counter surface. Figure 3A shows the VCPD images of these surfaces before (top) and after contact (bottom), and their corresponding values are shown in the upper part of Figure 3B. Clearly, VCPD is positively proportional to λ, suggesting the effect of the molecularly engineered coating in mediating surface potential. Further, the change in VCPD on each sample after relative to before contact, i.e., ΔVCPD (lower part of Figure 3B), reflects the charging ability of the coatings. We found that ΔVCPD on the coating with λNCS is 3 orders of magnitude smaller than that of uncoated SiO2, suggesting the apparent antistatic effect of the coating with λNCS. In contrast, without proper control of the ratio of FOTS and APTS, the coating with FOTS alone obtains the most negative charge, whereas the coating with APTS alone gains the most positive charge. Importantly, careful inspection of VCPD images reveals that the molecularly engineered surface potentials of all the samples were homogeneous regardless of before or after contact, reminiscent of a signature of electrostatic homogeneity. Such an electrostatic homogeneity manifested on the uncoated, FOTS, and APTS surfaces can be easily understood because of their chemical homogeneity. For chemically heterogeneous coating with λNCS, its electrostatic homogeneity could be validated by the range of the VCPD before and after contact. As shown in Figure S6, the highest and lowest values of VCPD on the coating with λNCS are much lower and higher than those of the VCPD on APTS and FOTS surfaces, respectively, regardless of before or after contact, which demonstrates the collective effect of FOTS and APTS in regulating the VCPD of the coating with λNCS. Such cooperation could further be deduced by comparing the density of one charge and a silane molecule. The density of one charge is equal to charge density (σ) divided by the elementary charge e (1.60 × 10−19 C). Based on the maximum charge density σ (~20.1 μC/m2) measured on the FOTS surface, the maximum density of one charge is calculated to be ~1/8,000 nm−2. Since the density of one silane molecule is on the order of 1 nm−2,31Grabbe A. Double-layer interactions between silylated silica surfaces.Langmuir. 1993; 9: 797-801Crossref Scopus (58) Google Scholar one charge is roughly occupied by about 8,000 uniformly distributed silane molecules (defined as an electrostatic patch). Such an electrostatic patch is also applicable to the coating with λNCS, in which each electrostatic patch generates one charge in contact electrification; therefore, the entire surface is electrostatically homogeneous, as illustrated in Figure 3C. After understanding the effect of λ on surface potential, we further considered the antistatic mechanism of coatings. Despite extensive research, the roles of electron transfer, ion transfer, and materials transfer in contact electrification are still in debate. Considering that the ion transfer mechanism is more applicable for ionic polymers8Liu C.Y. Bard A.J. Electrostatic electrochemistry at insulators.Nat. Mater. 2008; 7: 505-509Crossref PubMed Scopus (190) Google Scholar and the material transfer mechanism is mainly limited to materials with low Young's modulus or the surfaces with low cohesive energy,32Baytekin H.T. Baytekin B. Incorvati J.T. Grzybowski B.A. Material transfer and polarity reversal in contact charging.Angew. Chem. Int. Ed. 2012; 51: 4843-4847Crossref PubMed Scopus (105) Google Scholar in this work, we tend to choose the electron transfer mechanism to explain our experimental results. Briefly, according to the widely used electron-cloud-potential well model,33Xu C. Zi Y. Wang A.C. Zou H. Dai Y. He X. Wang P. Wang Y.C. Feng P. Li D. Wang Z.L. On the electron-transfer mechanism in the contact-electrification effect.Adv. Mater. 2018; 30: 1706790Crossref PubMed Scopus (249) Google Scholar, 34Xu C. Wang A.C. Zou H. Zhang B. Zhang C. Zi Y. Pan L. Wang P. Feng P. Lin Z. et al.Raising the working temperature of a triboelectric nanogenerator by quenching down electron thermionic emission in contact-electrification.Adv. Mater. 2018; 30: 1803968Crossref Scopus (96) Google Scholar, 35Wang Z.L. Wang A.C. On the origin of contact-electrification.Mater. Today. 2019; 30: 34-51Crossref Scopus (318) Google Scholar during the contact of two surfaces with different surface potentials, the electron clouds overlap, and the energy barrier lowers, resulting in electron transfer from the surface with a higher surface potential to the surface with a lower surface potential. After separation, transferred electrons remain on the surface with a lower surface potential, resulting in the two surfaces obtaining positive and negative charges. As shown in Figures 3D and 3E, FOTS surface (λ0.0), with a lower surface potential, obtains the electrons from the counter surface and becomes negatively charged after contact, while the APTS surface (λ1.0) becomes positively charged owing to its loss of electrons to the counter surface. However, by molecularly engineering λNCS and hence the same surface potential between the coating and the counter surface, the electron transfer between these two surfaces can be completely suppressed, thereby preventing the generation of charges. Further, it is anticipated that other combinations of molecules with strong negative and positive charging ability, such as halogen-terminated molecules and aminated molecules, could also be used to fabricate antistatic coating by engineering surface potential.22Shin S.H. Bae Y.E. Moon H.K. Kim J. Choi S.H. Kim Y. Yoon H.J. Lee M.H. Nah J. Formation of triboelectric series via atomic-level surface functionalization for triboelectric energy harvesting.ACS Nano. 2017; 11: 6131-6138Crossref PubMed Scopus (91) Google Scholar We further demonstrated the generality of our molecularly engineered antistatic coatings, which is independent of counter substrates and also applicable to a wide range of substrates. Figure 4A shows the charge densities on as-prepared PDMS surfaces with various λ after contact with different counter substrates (including PP, PE, PC, POM, and PA6). Regardless of the types of counter substrates, the charge densities on coatings are always linearly proportional to λ, which means their λNCS could also be calculated using Equation 1. In addition, the continuous tailoring in both polarity and amount of charges could also be obtained by tuning λ based on Equation 1. The generality of our antistatic surface is further confirmed when these five kinds of substrates are also coated. We compared the charge densities on these uncoated and coated substrates, as shown in Figure 4B. On one hand, APTS and FOTS surfaces still show strong charging ability. On the other hand, the charge densities on the coating with λNCS are much lower than those on pristine substrates, suggesting the efficacy in preventing the generation of electrostatic charges. Our electrostatically homogeneous antistatic surface shows pronounced advantages over existing electrostatically heterogeneous antistatic surfaces.24Soh S. Chen X. Vella S.J. Choi W. Gong J.L. Whitesides G.M. Layer-by-layer films for tunable and rewritable control of contact electrification.Soft Matter. 2013; 9: 10233-10238Crossref Scopus (14) Google Scholar, 25Gumbley P. Thomas 3rd, S.W. Reversible photochemical tuning of net charge separation from contact electrification.ACS Appl. Mater. Interfaces. 2014; 6: 8754-8761Crossref PubMed Scopus (11) Google Scholar, 26Thomas 3rd, S.W. Vella S.J. Dickey M.D. Kaufman G.K. Whitesides G.M. Controlling the kinetics of contact electrification with patterned surfaces.J. Am. Chem. Soc. 2009; 131: 8746-8747Crossref PubMed Scopus (27) Google Scholar, 27Thomas 3rd, S.W. Vella S.J. Kaufman G.K. Whitesides G.M. Patterns of electrostatic charge and discharge in contact electrification.Angew. Chem. Int. Ed. 2008; 47: 6654-6656Crossref PubMed Scopus (45) Google Scholar Existing antistatic surfaces rely on careful control of the size and spatial distribution of patterns so as to achieve neutralization of the positive and negative charges (in Figure 4C). In this case, the antistatic effect in localized areas may break down owing to the mismatch in the amount of positive and negative charges (in Figure 4C). In striking contrast, even when our electrostatically heterogeneous coatings with various λ are cut to different fractions (Figure 4D), the charge densities remain stable (Figure 4E), demonstrating the efficacy of the electrostatic homogeneity in maintaining a sustained antistatic effect. Such an electrostatic homogeneity also allows for engineering an antistatic surface on arbitrary substrates of any shape by simply using standard processes, such as dip coating, spin coating, or spray coating. Figure 4F shows as-fabricated antistatic coatings on curved substrates, including the outside of a PDMS ring and the inside of a rubber tube with a length of ~9 cm. The charge density on these antistatic surfaces is much lower than on a pristine surface, validating the efficacy of our electrostatically homogeneous antistatic surface in preventing the generation of electrostatic charges. Such antistatic surfaces engineered on a curved morphology would provide additional flexibility and dimension for industrially antistatic applications, such as in non-metal petroleum pipelines. Two more appealing features of our antistatic coatings are high transparency and excellent re-writability. As confirmed by the optical photographs and the transmission of light recorded by UV-Vis spectroscopy (Figure S7), the samples with and without coatings exhibit identical transparency, indicating that the thin (less than 2 nm, in Figure S8) and smooth (~0.264 nm, in Figure S4) coatings do not affect the optical transmission of the samples. In application scenarios, an antistatic coating is inevitably subject to different counter materials, and thus it is important to dynamically re-write the coating to sustain the preferred antistatic property. The conventional antistatic coating with pre-determined patterning is suitable for only a specific counter surface, and thus the variation in the counter surfaces inevitably poses re-designing and laborious manufacturing of the coating. In contrast, our antistatic coating can be easily re-coated based on different counter surfaces by merely changing the fraction ratio of APTS and FOTS in ethanol solution, which is easier and less time consuming. Briefly, we first used oxygen plasma to etch away the original coating, followed by rinsing with ethanol to remove the silane molecule residues. The new coating with any desired λ was then re-coated on the original location using the same method as before. As shown in Figure 4G, a FOTS surface on a PDMS substrate could be first changed to APTS and then converted to FOTS again. Notably, in such a re-writing process, the charge density and wettability of the surface remain unchanged. In summary, we have demonstrated a novel, general approach for engineering antistatic surfaces that completely prevents electrostatic charge generation by molecularly modulating the surface potential. Our antistatic coatings are chemically heterogeneous but electrostatically homogeneous, imparting various advantages, such as simple and re-writable fabrication, strong robustness, high transparency, and a wide range of substrate and morphology choices. Moreover, in addition to the choice of electron-donating and electron-accepting molecules reported in this work, the antistatic surfaces that completely prevent unwanted electrostatic charges through the molecular engineering surface potential could also be constructed based on other molecules, thus extending the choice of chemical components as well. Furthermore, the combination of chemical components with strong but opposite charging abilities also provides a meaningful platform to obtain the desired amount and polarity of charges on surfaces by contact electrification, which is important in fundamental scientific research and practical applications such as in molecular mass spectroscopy.36Li A. Zi Y. Guo H. Wang Z.L. Fernandez F.M. Triboelectric nanogenerators for sensitive nano-coulomb molecular mass spectrometry.Nat. Nanotechnol. 2017; 12: 481-487Crossref PubMed Scopus (159) Google Scholar