Can Static Electricity on a Conductor Drive a Redox Reaction: Contact

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Can Static Electricity on Conductor Drive Redox Reaction?: Contact Electrification of Au by PDMS, Charge Inversion in Water, and Redox Reaction Changsuk Yun, Seung-Hoon Lee, Jehyeok Ryu, Kyungsoon Park, Jae-Won Jang, Juhyoun Kwak, and Seongpil Hwang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07297 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Journal of the American Chemical Society

Can Static Electricity on Conductor Drive Redox Reaction?: Contact Electrification of Au by PDMS, Charge Inversion in Water, and Redox Reaction

Changsuk Yun,†,⊥ Seung-Hoon Lee,‡,⊥ Jehyeok Ryu,‡ Kyungsoon Park, § Jae-Won Jang,‡,* Juhyoun Kwak,†,* and Seongpil Hwang§,*

† Department

of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon

34141, Korea ‡Department

§

of Physics, Pukyong National University, Busan, 48513, Korea

Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea

⊥equal

contribution

* Corresponding Authors: Prof. Seongpil Hwang, Prof. Juhyoun Kwak, Prof. Jae-Won Jang

E-mail: [email protected]; [email protected]; [email protected] 1

ACS Paragon Plus Environment

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ABSTRACT

We investigated the static charge generation by contact electrification between Au and polydimethylsiloxane (PDMS) and the redox reaction by the static charge in aqueous phase, to reveal the mechanism of contact electrification and redox reaction which may be applied to mechanical-to-chemical energy harvesting. First, the static charge distribution on the equipotential Au was probed through Kelvin probe force microscopy (KPFM) in air after the contact with patterned PDMS. Positive charges are localized on the contact areas indicating the ion migration while the polarity becomes negative after water contact. Second, the redox reaction by the charged Au was electrochemically monitored using open circuit potential (OCP), stripping voltammetry, and copper underpotential deposition (UPD). All electrochemical experiments consistently resulted in the reduction of reactant by the charged Au within highly dielectric water media. We concluded that the reduction is not driven by the discharge of static charge on Au but by reducing radicals.

Keywords: Static Charge, Energy harvesting, Mechanochemistry, Kelvin Probe Force Microscopy, Open-circuit potential, Underpotential deposition

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INTRODUCTION Contact electrification was the starting point to current electricity and electrochemistry since the Greek philosopher of Thales showed it by rubbing amber against wool more than 2500 years ago. It is the charging process by the contact and subsequent separation between two solid surfaces, such as metal-metal, metal-insulator, and insulator-insulator.1-3 In spite of long history, there are two controversial mechanisms on this charge transfer between two solid.4-7 The first hypothesis is the electron transfer between two solids having different work functions.7-8 The second hypothesis is the ion transfer between two solids.9-11 The former is suitable to explain contact electrification between two conductors while the latter is favored for the case between two insulators. The recent reports make explanation on the contact electrification more controversial. Grzybowski group investigated the formation of static charge by the contact between homogenous dielectric materials (polydimethylsiloxane, PDMS) and found the mosaic of surface charge where both positive and negative region exists.9 The radical chemicals are also formed along with the mosaic, which was suggested as a reducing agent for redox reaction.12 These results on the contact electrification from Grzybowski group can be summarized as following; (1) The contact electrification between dielectrics are originated from ion migration but it is not one-way. The charged surface contains both of polarities (+ and -) but one of them is excess. (2) The radical chemicals by mechanochemical activation are also formed during the contact electrification. This radical can induce the redox chemistry within the solution. Redox chemistry of the static charge has drawn interest as a method for mechanical-tochemical energy conversion and storage,13-14 and electrochemistry at insulator in order to harvest 3



waste energy of human motion or store the energy from triboelectric generator.7 Bard group investigated the redox reaction by static charge on polymers, in which polymers were charged by rubbing and then the immersion into the electrolyte for subsequent redox chemistry.7-8 In contrast to Gryzbowski group, Bard group concluded that electrons on the charged insulator can invoke various redox reactions including hydrogen generation, metal reduction, ferricyanide reduction, and chemiluminescence in aqueous phase. They called these electrons cryptoelectrons, whose energy is different from molecular states or energy band in materials. The charge density of the static charge (up to 1015 electrons/cm2 corresponding to 160 C/cm2)7 are five orders of magnitude higher than the values reported in air (nC/cm2).9 This hypothesis was not supported by recent report,15 in which adsorption of reactant on insulator may mislead the conclusion in cryptoelectron model. Because previous reports have focused at static charge on dielectric materials, the direct observation of the potential in aqueous phase as a driving force of redox reaction is impossible. Instead, indirect measurement based on concentration changes induced by redox reactions at insulator has been served as indicators for the amount of redox reaction by charged insulator at equilibrium. Thus, the identification between adsorption and reduction of reactants is not clear, generating the controversial results. Herein, we investigate the electrostatic and electrochemical behavior of conductive Au charged by the contact with the insulator of PDMS because (1) the static charge on the conducting phase of Au may reveal the dominant mechanism (cryptoelectron or ion transfer), (2) the measurement of interfacial potential difference between the charged Au and electrolyte by Nernst equation give us the direct information on driving force for redox reaction by static charge, and (3) the precise quantification of the charge and adsorbates from stripping voltammetry and surface4


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sensitive electrochemistry may give us the insight on the redox reaction by contact electrification. We hoped that this investigation reveals the answer to the following two question: What is the mechanism for contact electrification between metallic conductor and insulator? Can the excess charge on conductor drive the redox reaction in aqueous phase? For these purposes, Au was charged by the simple contact and separation with PDMS as shown in Scheme 1a. Note that the elastomer of PDMS has been widely used as stamp in soft lithography and microfluidics due to very low surface free energy and inertness to chemicals,16 indicating the low possibility of chemical interaction between PDMS and Au. The static charge on Au was monitored through surface potential measured with Kelvin probe force microscopy (KPFM) in air. After the contact with water, initial positive potential on charged Au showed the charge inversion into negative. Then, electrochemistry on the charged Au were investigated in term of (1) open circuit potential (OCP) to estimate interfacial potential difference or Nernst potential, (2) stripping voltammetry after redox reaction with the charged Au and the electrolyte containing Ag+ cations, and (3) Cu UPD to measure the surface coverage of adsorbates by contact electrification. We focused at the charged Au instead of the charged PDMS9-10 to utilize the ability of conducting phase (equipotential surface) for direct electrical/electrochemical measurements. The results show that the local charge on conductor supports ion migration mechanism for contact electrification and this charge is insufficient to drive the reduction reaction caused by the radical species on Au surface.

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Scheme 1. Illustration of (a) electrification of Au by the contact with PDMS and (b) charge distribution of the charged Au depending on the proposed models. Charges spread evenly on entire equipotential surface in the case of electron transfer while charges bound to molecular orbitals are localized on the contact area in the case of ion transfer.

EXPERIMENTAL SECTION Preparation The gold wafers were manufactured by thermal evaporation 4 nm of Ti and 150 nm of Au on the Si wafers (1 angstrom per second). The Au wafers were cleaned by piranha solution (3:1 mixture of H2SO4 and 30% H2O2, volume to volume ratio) for 3 min and followed by rinsing with 6


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distilled water and N2 blowing. The PDMS (Dow Corning, Sylgard 184) was prepared by mixing elastomer base and curing agent (10:1, weight to weight ratio) followed by removing bubbles of prepolymer in vacuum chamber. The prepolymer was poured onto a Si wafer to obtain flat surface of the PDMS and annealed at 65 C for 2 h. The PDMS was cut in the small pieces (1 cm × 2 cm) and rinsed with ethanol and N2. The dot-patterned PDMS (dot size: 4 μm, pitch: 10 μm) was made with the same method of flat PDMS except template. A PDMS piece was contacted manually with an Au substrate, followed by separation for charging. Characterization Surface potential and phase measurement. Contact potential difference (VCPD) and phase images were obtained by amplitude-modulated KPFM (Dimension Icon, Bruker). In our KPFM measurement, positive (or negative) change of VCPD of sample represents a decrease (or increase) of electrons.17-18 A Pt/Ir coated tip (SCM-PIT, Bruker) is used during the KPFM measurement with lift height of 10 nm and applying AC bias of 500 mV to a tip. Scan size is 25 μm × 25 μm and scan rate is 0.6 Hz with a pixel resolution of 256 × 256. To investigate electrification of Au by the contact with PDMS, VCPD of the Au film before and after contact of flat and dot patterned PDMS were measured, respectively. Zoomed VCPD images (4.5 μm × 4.5 μm) of contacted and non-contacted areas by the dot-patterned PDMS were obtained with a scan rate of 0.55 Hz and a pixel resolution of 512 × 512. After contact of the dot-patterned PDMS, VCPD of the contacted area was measured at various intervals of time (denoted as elapsed time) to measure time-dependence of VCPD at the contact area. Electrostatic charge on the charged Au under aqueous solution was indirectly studied due 7



to limited operation of KPFM in air. 20 μl of 18.2 MΩ deionized water (Milli-Q integral 3, Millipore SAS) was dropped to the charged area of the Au film. After 20 s, water droplet was gently removed by a blower and then VCPD images were obtained in the air (not in solution). Phase and VCPD images were analyzed with using NanoScope Analysis program. VCPD of the Au film was set as a baseline with carrying out “plane fit” and “flatten” functions of the software. To remove background noise, maximum twice low pass filtering was carried out for all images in the manuscript. Optical image of the dot-patterned PDMS was obtained with bright field mode of an optical microscope (Axio Scope A1, Zeiss). Electrochemistry. CHI900B potentiostat was used for the electrochemical measurement with the three-electrode configuration consisted of the Au working electrode, a Pt wire counter electrode, and Hg/Hg2SO4 reference electrode (MSE) except potentiometry. The open circuit potential (OCP) was measured in 1 mM potassium ferricyanide (Fe(CN)63-)/potassium ferrocyanide (Fe(CN)64-) at 1:1 molar ratio (ref: bare Au wafer, supporting electrolyte: 10 mM K2SO4). The other Au wafer connected with working electrode terminal was contacted with PDMS (during 5 s) and subsequently separated, and then immersed in electrolyte. For quantification of redox reaction on charged Au, the charged Au was incubated 10 mM Ag2SO4 under dark condition to minimize photoreduction for 5 min and washed with distilled water. The deposited Ag was monitored by linear sweep voltammetry (LSV) where the electrolyte was 0.5 M H2SO4 (electro active area: 0.502 cm2). The Cu UPD was monitored by cyclic voltammogram (CV) where electrolyte solution was 10 mM CuSO4 in 0.05 M H2SO4 (electro active area: 0.502 cm2). All reagents for electrochemistry was purchased from Sigma Aldrich.

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RESULTS AND DISCUSSION The static charge on Au by contact electrification was monitored by KPFM. We use the patterned PDMS to identify the charging mechanism. If the electron transfer causes the static charge, charges are homogeneously distributed over entire Au surface because of equipotential of conducting Au (Scheme 1b). In contrast, localized charges are observed in the case of ion migration so that VCPD image may be the replication of the pattern in PDMS. Figure 1b shows the VCPD images of the charged Au by the electrification with the patterned PDMS shown in Figure 1a. The contacted regions were positively charged indicating the ion migration. During the acquisition of KPFM, both VCPD and phase images were simultaneously obtained. Phase image of Figure 1c shows the pattern which is another evidence of molecular transportation to Au. Thus, we can conclude the transportation of molecules from PDMS to Au but VCPD images give us just difference in VCPD of specific region compared to background. Figure 1b may imply either (1) cations on contact area with neutral Au or (2) neutral molecules on contact area with negatively charged Au by electron transfer, which can mislead the mechanism of static charging. To check the polarity of entire surface, VCPD was measured on Au with the flat PDMS without patterns. The charged Au became ca. 100 mV higher than that without electrification and showed more positive values after repeated contact electrification (Figure S1). Surface charge density (σ) on Au by contact electrification can be calculated using the simply capacitor model19; σ=

𝜀0∆𝑉𝐶𝑃𝐷 ℎ

(eq. 1)

where ε0 is the vacuum permittivity (8.85410-12 F/m), ∆VCPD (100 mV) is the VCPD change of Au before and after PDMS contact, and h is the lift height (10 nm) between a tip and Au film during 9



KPFM measurement. In the results, σ is 8.8 nC/cm2 on Au by contact electrification and shows similar levels with previously reported value in air.9 For the contact electrification between PDMSs, mosaic pattern from mixture of both positive and negative charge was reported.9 In our experiments, however, we cannot find the evidence for mosaic from low roughness factor (Ra) of charged region which is similar to Ra of non-charged region (Ra is the arithmetic average of |VCPD|). Meanwhile, Ra of the opposite PDMS side (Figure S2) also supports the absence of mosaic. The non-mosaic pattern is rational result in that heterogeneity of charge have not appeared on metal contacted with dielectric.19 Note that neutral species such as radicals can exist along with positive species. Thus, we concluded that contact electrification between Au and PDMS in air induces the localized positive charge on Au side by ion migration.

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Figure 1. (a) Optical image of dot-patterned PDMS. Diameter of the dot is 4.03 μm and center to center distance is 10.07 μm. (b) VCPD image of Au after contact of the dot-patterned PDMS with zoom images of contacted and non-contacted areas marked by white dotted and blue rectangle lines. Averaged VCPD of contacted areas is 13.4 ± 2.4 mV. No distinct VCPD is observed in noncontacted areas. In zoom images, similar roughness factor (Ra) values are calculated in contacted and non-contacted areas. (c) Phase image of Au after contact of the dot-patterned PDMS. Contacted areas are faintly observed as dark contrasted circles.

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The relaxation behavior of charged Au in air before and after water treatment was investigated to confirm the static charge for subsequent redox chemistry in aqueous phase. Figure 2 shows the time-dependent VCPD images of charged Au in air before water treatment. VCPD on the spot decreased gradually with staying time in air, whose relaxation curve is similar to decay process of conventional RC circuit; 𝑡

𝑉𝐶𝑃𝐷 = 𝑉0𝑒

―𝜏

+ 𝑦0

(eq. 2)

Where V0 is the voltage at time = 0,  is time constant and y0 is the offset potential when the elapsed time passes enough. Time constant is ca. 170 minutes. All static charges were dissipated after ca. 700 minutes. It is reasonable that most static charge on Au is not lost during the transfer from air to water from relatively long time constant. Next, the charged Au was treated by deionized water. Dielectric constant of water is 80 times higher than that of air. It is highly expected that the interfacial potential difference between Au and water is just a few mV compared to ca.100 mV in air, which is too small to drive the redox reaction. From the ion-migration point of view, charge is bound to molecular orbital of ion whose energy is strongly affected in aqueous phase such as solvation and ion-pairing. Water can react or solvate the ion species on Au. Thus, we want to observe VCPD of charged Au in water but KPFM cannot be operated in water because of the exposure of electrified tip to aqueous phase without proper insulation. Instead of direct measurement, we tried to acquire Nernst potential for the interfacial potential difference in water (vide infra) and observe VCPD of charged Au after water treatment to check the reaction or solvation as shown in Figure 3. Interestingly, the positively charged spots in Figure 3b become oppositely charged indicating either the reaction or solvation of charged species (Figure 3d). In the case of 12


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reaction, cations should experience the reduction to anions concomitant with the oxidation of water molecules to induce the charge inversion. Considering the large time constant for the discharge process in air containing humidity, this reaction is not favorable. For solvation, the binding of neutral water molecule will not make the opposite polarity. The binding of multivalent anion or polyelectrolyte is necessary to induce the charge inversion similar to previous reports for DNA.20-21 Except charged species, Au may contain neutral radical species by homolytic cleavage of covalent bond of PDMS.22 The stability upon the contact time with water given in the supporting information, however, imply that the reaction of radicals with water is not dominant for charge inversion. We speculate that multivalent anions at very low concentration in deionized water or inevitable contamination in laboratory environment may electrostatically bind to the positively charged adsorbate or may adhere to adsorbate through chemical interaction. Although the mechanism of charge inversion is inconclusive, this interesting result is reproducible in KPFM. As a summary, positively charged Au is relatively stable in air and the polarity is inversed by the contact of water although the mechanism is not clear.

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Figure 2. (a-d) Representative VCPD images after contact of the dot-patterned PDMS at irregular intervals of time. (e) Time-dependent VCPD with single exponential fitting with y0 of 3.86 mV, V0 of 22.07 mV, τ of 169.95 min. We assume that y0 is an offset potential generated by transferred mass during the contact, which can contribute to a nonzero VCPD value for a long elapsed time. White dotted lines in (a-d) indicate where VCPD values are measured.

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Figure 3. (a) Phase and (b) VCPD images before water exposure. Averaged VCPD of the contacted areas is 25.9 ± 3.1 mV. (c) Phase and (d) VCPD images after water exposure (for 20 s). Averaged VCPD of the contacted areas is -13.9 ± 1.3 mV. In phase images (a, c), contacted areas are observed as the dark colored circles. (e) Line profile of the VCPD adapted from red and blue dotted lines in (b, d).

What happens when the positively charged Au is immersed into the electrolyte containing redox species? Can the positive static charges oxidize the chemical species by either (1) sufficient surface charge density even in high dielectric medium or (2) sufficient lowest unoccupied molecular orbital (LUMO) of cation under aqueous phase? Or (3) can the inverted charge/radicals 15



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reduces the chemicals instead of oxidation? The conducting Au enables the direct measurement of potential called OCP during the transfer from air to water as shown in Figure 4a. At first, Au connected to potentiostat was charged by PDMS in air and then it was immersed in the electrolyte containing redox pairs of Fe(CN)63-/Fe(CN)64- (1:1 molar ratio). The outer-sphere electron transfer between Au and this pair is so fast that Fermi level of Au becomes equal to electrochemical potential of the pair (). Namely, electrode potential is the same to well-known Nernst equation written as follows; 𝐸𝑐ℎ𝑎𝑟𝑔𝑒𝑑 𝐴𝑢 = 𝐸𝐹𝑒(𝐶𝑁)36 ― /𝐹𝑒(𝐶𝑁)46 ― = 𝐸0𝐹𝑒(𝐶𝑁)36 ― /𝐹𝑒(𝐶𝑁)46 ― ―

𝑅𝑇

𝑎𝐹𝑒(𝐶𝑁)4 ―

𝐹 𝑙𝑛𝑎

6

― 𝐹𝑒(𝐶𝑁)3 6

(eq. 3)

E0 is the standard reduction potential, R is gas constant, F is Faraday constant, and a is activity of chemical species on the charged Au. Note that activity is the local concentration on electrode surface not in bulk solution. We used Au wafer as a reference electrode whose potential is dependent on activity of the redox couples by Nernst equation. This is simple null test so that the effect of charged Au is easily recognized as written in eq. 4. 𝑉𝑂𝐶𝑃 = 𝐸𝑐ℎ𝑎𝑟𝑔𝑒𝑑 𝐴𝑢 ― 𝐸𝑏𝑎𝑟𝑒 𝐴𝑢 = ―

𝑅𝑇 𝐹

𝑎𝐹𝑒(𝐶𝑁)4 ―

𝑙𝑛𝑎

6

― 𝐹𝑒(𝐶𝑁)3 6

(eq. 4)

If the charged Au drives oxidation, OCP will increase to positive value due to the logarithmic term. In the case of reduction, OCP will decrease to negative value while OCP will keep zero without redox reaction. Figure 4b shows the time versus OCP curve during the immersion into the electrolyte. Before immersion, the potential of charged Au is floating in air (Figure S3). When the electrode was immersed in electrolyte, OCP dramatically decreased due to the reduction of Fe(CN)63- followed by gradual increase of OCP. The value ca. -55 mV is corresponding to [Fe(CN)64-]/[Fe(CN)63-]=1/8.5 from eq. 4. Then OCP finally approaches to 0 V indicating the same 16


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Nernst potential on bare Au. Immersion of bare Au without contact electrification as a control shows the fast approach to 0 V. This behavior seems to be complex but is very reasonable because involved chemical species experience the mass transportation such as diffusion. The product of reduction reaction will diffuse out to bulk side decreasing the concentration at electrode surface. OCP result shows that charged Au has more negative potential than bare Au and reduction is favorable by the charge transfer between chemicals on Au and Fe(CN)63-. This demonstrates that the charged Au induce reduction from the inverted charge/radicals instead of the oxidation. The inverted charge density (1.2 nC/cm2, based on eq. 1) from -13.9 mV in air in Figure 3e is not sufficient to reduce chemicals because this charge density on Au is corresponding to only -0.17 mV under highly dielectric medium of water considering the relative permittivity of water (the value of 80.1). Thus, we speculate that radicals itself or their anionic derivatives in Figure 3 may induce the reduction. In both cases, the redox reaction is not originated from the static charge (cations) but from radicals or their derivatives (anion). In other words, the static charge itself is hard to drive redox reaction but radicals can do.

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Figure 4. (a) Illustration of process for measuring potential difference between charge Au and bare Au wafer (reference: bare Au). (b) The plot of the OCP vs. time (red: charged Au by contacted with PDMS, black: bare Au). Inset shows zoom image of 0-5 s range.

Then, how many charges are involved in reduction chemistry? The accumulation of product by redox reaction on the charged Au offer the more precise quantity than the diffusional product as shown in Figure 4.7-8 We chose the silver reduction because of the high standard reduction potential guarantee the spontaneous reduction with the sensitive stripping voltammetry.23 The LSVs on the Au after soaking in the silver solution demonstrates the stripping of silver near 0 V in Figure 5. It is worth to mention that the anodic peaks at 0.4 V are corresponding to Ag UPD formed by recently reported anti-galvanic reduction (Figure S4).24 The charge by anti-galvanic reduction seems to be different in batch so that the peak near 0 V was selected to estimate the charge. The charge density of reduction chemistry was ca.15 C/cm2, which is ca. 10 times lower density than the value on Teflon (160 C/cm2) by electrochemistry,7 18


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while it is 104 times higher than ca. 0.5-4.0 nC/cm2 for PDMS-PDMS contact25 and a few nC/cm2 on a commercial Scotch tape by physical probe.12 It should be mentioned that a few nC/cm2 for the contact electrification in air were in good agreement with the previous reports for various insulators with different rubbing methods and an upper limit of contact electrification is up to ca. 100 nC/cm2.5 The charge density did not depend on the number of contacts (Figure S5). Thus, the number of electrons for Ag reduction is much larger than the number of electron by contact electrification, indicating that the reduction might be caused by radicals not by positive static charge. Namely, this density shows not the density of static charge but the density of radicals on Au with higher highest occupied molecular orbital (HOMO) level than atomic orbital of Ag. We believe that 10 times lower density between Au and PDMS contact compared to that between Teflon and polymethylmetacrylate is reasonable because the transfer of radical species may be strongly affected by chemical nature of polymers.

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Figure 5. (a) Illustration of the silver deposition and stripping process. (b) LSV for stripping of Ag on the Au (1, 10, 30 times contacted with PDMS) and on bare Au (black solid line) in 0.5 M H2SO4 after immersion in Ag+ solution for 5 min.

In addition to stripping voltammetry, we investigated Cu underpotential deposition (UPD) on Au with and without contact electrification (Figure 6) to investigate the coverage of adsorbates on the charged Au. The Cu UPD caused by the strong interaction between adatom of Cu and underlying Au is very sensitive to the surface state of Au. The CV on the evaporated Au showed the well-known 2 peaks for Cu UPD before the overpotential deposition of Cu, where the first peak is corresponding to a (3  3)R30 honeycomb structure and the second is corresponding to a (1  1).26 The inset of Figure 6 shows both the first cathodic (bottom) and anodic (top) UPD peak on the bare Au and the charged Au. The anodic peak on the charged Au decreased from 56.2 A to 48.1 A with increment in the shoulder around -0.16 V, while cathodic peak decreased - 32.5 A 20


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to -17.0 A. The anodic first peak in CV shows the similar peak shape where only ca. 6.2 % decrease in the charge was observed. These reveal that adsorbates by the contact electrification hinder the Cu UPD process. Ca. 0.47 ML of adsorbates was estimated from decrease in the first cathodic peak because adsorbates seem to be detached slightly at first cathodic potential as shown in the recovery of the first anodic peak (Figure S8). The identity of adsorbates may not be a single small molecule but broken polymer chains. Therefore, 0.47 ML is just hindered area not the number of adsorbates, which can have the multiple footprints for UPD sites. Compared to 0.07 ML from Ag stripping considering 222 C/cm2 for Ag monolayer on Au(111)26 and 8.5 × 10-5 ML from static charge, 0.47 ML indicate that either (1) only small portion of adsorbates are reactive to Ag reduction assuming a single small molecule or (2) a broken polymer chain with one radical adhere to ca. 6.7 UPD sites assuming that all adsorbates contain one reducing electron. When PDMS were immersed in various organic solvent to remove the residual uncured monomer, the reducing power is very similar (Figure S9). Therefore, we speculate the large broken polymer chains with one electron as a radical covers the multiple sites, which requires future investigation on the adsorbate. We concluded that the contact electrification between Au and PDMS forms 0.47 ML of adsorbates (mixture of cations and radicals) and radicals have the higher HOMO lever than Ag with minute ions. Scheme 2 demonstrates the summary of our finding. Contact electrification between Au and PDMS transfer adsorbates including cations and neutral radical, to Au making local positive charge on Au. When it is immersed in the aqueous solution, the positive charge in cations is inverted into negative charge which is stabilized by solvation of highly dielectric water. Meanwhile, some portion of radicals or their derivatives with sufficiently high energy HOMO can drive the reduction reaction. The other portion of radicals are not involved in the reduction 21



chemistry but the relaxation process of radicals is still unclear.

Figure 6. CV of Cu UPD reaction on charged Au (red) and bare Au (black) in 10 mM CuSO4 (0.05 M H2SO4). The inset shows zoom images of each anodic (top) and cathodic (bottom) position at ca. -0.16 V vs. MSE.

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Scheme 2. Illustration of proposed mechanism for electrification and redox reaction in aqueous phase. The charge identities were formed on the surface Au after the contact with PDMS. When the charged Au was immersed in aqueous electrolyte, the charge identity had effect on electrochemistry and was monitored by OCP difference, Ag+ stripping, and Cu UPD blocking. As a result, the charge density from Ag reducing radicals and electrostatic cations was low compared with Cu UPD blocking effect. This fact may imply that larger broken polymer with one electron as a radical adhere to multiple UPD sites.

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CONCLUSION We investigated the static charge by contact electrification between Au and PDMS and the redox reaction by the static charge in aqueous phase to reveal the mechanism of static charge and of redox reaction. First, the mapping of electrostatic charge on the equipotential Au was probed through KPFM in air along with the patterned PDMS. The VCPD and phase images on charged Au demonstrated positive potential on the Au compared with non-contact area, concluding that cation migration from PDMS to charged Au. The relaxation behavior of static charge in air shows 170 minutes as a time constant guaranteeing the stability of charge before exposure to aqueous phase. When the charged Au was contacted with water, charge inversion from positive to negative is observed. Second, the redox reaction by the charged Au in aqueous phase was electrochemically investigated. Measurement of OCP, stripping voltammetry, and Cu UPD shows the reduction chemistry, the quantification of charge, and the coverage of adsorbates, respectively. The charge density from experiments implied that the charged Au contains the mixture of cations for static electricity in air and reducing radicals for electrochemistry which cover multiple UPD sites for surface passivation. Therefore, the mechanochemistry, not triboelectricity, is dominant to drive the reduction reaction from contact between Au and PDMS. Our report implies that mechanical-tochemical conversion as an energy harvesting should focus at radicals by mechanochemistry instead of triboelectricity.

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ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications website: Investigation of VCPD images by KPFM, electrochemical analysis by potentiometry and voltammetry, characterization of radical and Ag formation by UV-vis, SEM, and XPS, and measurement of mass change by QCM during the contact electrification (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected]

ACKNOWLEDGEMENTS J.K. acknowledges support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MOE: Ministry of Education) (No. 2017R1D1A1B03031806). S.H. acknowledges the support from Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. NRF2017R1A2B4012056) and J.-W.J. acknowledges that this research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1B07045244). 25



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20. Besteman, K.; Van Eijk, K.; Lemay, S. G., Charge inversion accompanies DNA condensation by multivalent ions. Nat. Phys. 2007, 3, 641-644. 21. Wang, Q., Charge Inversion by Flexible Polyelectrolytes on Flat Surfaces from SelfConsistent Field Calculations. Macromolecules 2005, 38, 8911-8922. 22. Baytekin, 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, 1368-1371. 23. Hwang, S.; Kim, E.; Kwak, J., Electrochemical detection of DNA hybridization using biometallization. Anal. chem. 2005, 77, 579-584. 24. Kang, H.; Kim, B.-G.; Na, H. B.; Hwang, S., Anti-galvanic reduction of silver ion on gold and its role in anisotropic growth of gold nanomaterials. J. Phys. Chem. C 2015, 119, 2597425982. 25. Baytekin, H. T.; Baytekin, B.; Soh, S.; Grzybowski, B. A., Is Water Necessary for Contact Electrification? Angew. Chem. Int. Ed. 2011, 50, 6766-6770. 26. Herrero, E.; Buller, L. J.; Abruña, H. D., Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. 2001, 101, 1897-1930.

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TABLE OF CONTENTS (ToC)

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Scheme 1. Illustration of (a) electrification of Au by the contact with PDMS and (b) charge distribution of the charged Au depending on the proposed models. Charges spread evenly on entire equipotential surface in the case of electron transfer while charges bound to molecular orbitals are localized on the contact area in the case of ion transfer. 151x96mm (300 x 300 DPI)


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Scheme 2. Illustration of proposed mechanism for electrification and redox reaction in aqueous phase. The charge identities were formed on the surface Au after the contact with PDMS. When the charged Au was immersed in aqueous electrolyte, the charge identity had effect on electrochemistry and was monitored by OCP difference, Ag+ stripping, and Cu UPD blocking. As a result, the charge density from Ag reducing radicals and electrostatic cations was low compared with Cu UPD blocking effect. This fact may imply that larger broken polymer with one electron as a radical adhere to multiple UPD sites. 127x107mm (300 x 300 DPI)



Figure 1. (a) Optical image of dot-patterned PDMS. Diameter of the dot is 4.03 μm and center to center distance is 10.07 μm. (b) VCPD image of Au after contact of the dot-patterned PDMS with zoom images of contacted and non-contacted areas marked by white dotted and blue rectangle lines. Averaged VCPD of contacted areas is 13.4 ± 2.4 mV. No distinct VCPD is observed in non-contacted areas. In zoom images, similar roughness factor (Ra) values are calculated in contacted and non-contacted areas. (c) Phase image of Au after contact of the dot-patterned PDMS. Contacted areas are faintly observed as dark contrasted circles. 152x127mm (300 x 300 DPI)


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Figure 2. (a-d) Representative VCPD images after contact of the dot-patterned PDMS at irregular intervals of time. (e) Time-dependent VCPD with single exponential fitting with y0 of 3.86 mV, V0 of 22.07 mV, τ of 169.95 min. We assume that y0 is an offset potential generated by transferred mass during the contact, which can contribute to a nonzero VCPD value for a long elapsed time. White dotted lines in (a-d) indicate where VCPD values are measured. 177x87mm (300 x 300 DPI)



Figure 3. (a) Phase and (b) VCPD images before water exposure. Averaged VCPD of the contacted areas is 25.9 ± 3.1 mV. (c) Phase and (d) VCPD images after water exposure (for 20 s). Averaged VCPD of the contacted areas is -13.9 ± 1.3 mV. In phase images (a, c), contacted areas are observed as the dark colored circles. (e) Line profile of the VCPD adapted from red and blue dotted lines in (b, d). 177x106mm (300 x 300 DPI)


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Figure 4. (a) Illustration of process for measuring potential difference between charge Au and bare Au wafer (reference: bare Au). (b) The plot of the OCP vs. time (red: charged Au by contacted with PDMS, black: bare Au). Inset shows zoom image of 0-5 s range. 177x75mm (300 x 300 DPI)



Figure 5. (a) Illustration of the silver deposition and stripping process. (b) LSV for stripping of Ag on the Au (1, 10, 30 times contacted with PDMS) and on bare Au (black solid line) in 0.5 M H2SO4 after immersion in Ag+ solution for 5 min. 177x78mm (300 x 300 DPI)


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Figure 6. CV of Cu UPD reaction on charged Au (red) and bare Au (black) in 10 mM CuSO4 (0.05 M H2SO4). The inset shows zoom images of each anodic (top) and cathodic (bottom) position at ca. -0.16 V vs. MSE. 177x127mm (300 x 300 DPI)



Graphical Table of Contents 82x43mm (300 x 300 DPI)


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Can Static Electricity on a Conductor Drive a Redox Reaction: Contact

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