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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Electric conduction and excess charge in silicate glass/air interfaces Victor T. C. Paiva, Leandra P. Santos, Douglas S. da Silva, Thiago A. L. Burgo, and Fernando Galembeck Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00606 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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Electric conduction and excess charge in silicate glass/air interfaces Victor T. C. Paivaa, Leandra P. Santosa, Douglas S. da Silvaa, Thiago A. L. Burgob and Fernando Galembecka,* aInstitute

of Chemistry, University of Campinas, SP, Brazil 13083–970

bDepartment

of Physics, Federal University of Santa Maria, RS, Brazil 97105–900

* Corresponding author: [email protected]

ABSTRACT

The glass/air interface shows electrical properties that are unexpected, for a widely used electrical insulator. The mobility of interfacial charge carriers under 80% relative humidity (RH) is 4.81 × 10–5 m2 s–1 V–1, three orders of magnitude higher than the electrophoretic mobility of simple ions in water and less than two orders of magnitude lower than electron mobility in copper metal. This allows the glass/air interface to reach the same potential as a biased contacting metal, quickly. The interfacial surface resistance R increases by more than five orders of magnitude when RH decreases from 80% to 2%, following an S-shaped curve with small hysteresis. Moreover, the biased surfaces store charge, as shown by Kelvin potential measurements. Applying an electric field parallel to the surface produces RHdependent results: under low humidity, the interface behaves as expected for an ideal 2-

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dimensional parallel-plate capacitor, while under high RH it acquires and maintains excess negative charge, that is lost under low RH. The glass surface morphology and potential distribution change on the glass/air interface changes under high RH and applied potential, including the extensive elimination of non-glass contaminating particles and potential levelling. All these surprising results are explained by using a protonic charge transfer mechanism: mobile protons dissociated from silanol groups migrate rapidly along a fieldoriented adsorbed water layer, while the matrix-bound silicate anions remain immobile. Glass may thus be classified as the ionic analogue of a topological insulator but based on structural features and charge transfer mechanisms different from the chalcogenides that have been receiving great attention in the literature.

Keywords: Kelvin probe, glass surface potential, electrostatic patterns, excess charge, surface conductance, adsorbed water.

INTRODUCTION Glass and silica have been widely used in electrical insulators, e.g. in high-voltage power lines and in electronic circuitry. Failure of glass or glazed insulators in a moist atmosphere has been known since the times of early telegraph pole lines and is assigned to the formation of a surface water film. Knowledge of the electrical properties of glass/air and glass/water interfaces has received inputs from different areas other than glass insulators, since the early 20th century. Important examples are the glass electrodes for pH and ion concentration measurements,1 gas sensors based on soda-lime glass2 and glass powder pre-treatment in solid electrolyte fabrication.3


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The kinetic studies of H2O vapor adsorption and desorption in related model systems contribute information on the amounts and arrangement of interfacial water structure. Glass surfaces cooled in ambient air start to absorb water at 1–2% relative humidity (RH),4 and a clean glass surface is covered with adsorbed water sufficient to form a monolayer at an RH of around 30–50% at 20 °C,5 but H2O multilayer formation at θ ≥ 0.3 monolayer has been inferred from infrared spectra. Thus, the glass surface is covered by water islands, under low coverage. Sum-frequency-generation (SFG) vibration spectra of water at the glass/air interface are drastically different from those of the water layers on a silica glass surface and the liquid water/air interface. A sharp peak at 3200 cm–1 is assigned to hydronium ions, and its intensity varies with the RH, evidencing the equilibrium between interfacial and gas-phase water.6 Water vapor desorption rates increase at higher H2O coverages, showing a decrease in water cohesion and the appearance of repulsive forces within the interfacial layer.7 The effect of humidity on water/glass contact angles has been known for many years and is used to reveal glass surface modification under different conditions, including surface cleanliness.8 Dielectric loss and surface conductivity of non-alkali glass, soda glass, fused silica and two polymer substrates in contact with water vapor increase rapidly to maximum peaks then gradually decrease, showing that the surfaces undergo important modifications and suggesting that the adsorbed water undergoes a phase change.9 AC surface conductivity of silica nanoparticles increases with adsorbed water film thickness, following an empirical power law.10 The dependence of the conductivity of silica gel on surface hydroxyl ion concentration and on the adsorption densities of water and other polar molecules11 has been interpreted in terms of a protonic conductivity model involving a variable donor dissociation energy dependent on the dielectric constant of the silica–


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adsorbate system.12 The protonic conduction mechanism is supported by other results on the glass/water interface conductivity.13,14 Excess electric charge was unexpectedly observed in a silica insulator separating microfabricated electrodes in SOW (silica-on-wafer) devices, under high RH,15,16 and this was recently confirmed in a study of gas sensors.2 Charge acquisition under high humidity is intriguing because it diverges from the expected negative role of humidity in observed electrostatic phenomena that is familiar to researchers in this area and is taught to STEM students.17 Silica charging under increasing humidity was explained15 by considering silica surface chemistry, including the reversible hydrolysis of the silanoxide (Si–O–Si) group to silanol (Si–OH), which is a weak Brønsted acid.18 The inverse (silanol condensation) reaction takes place on dry glass surfaces, but full dehydration of silanol-coated surfaces requires temperatures as high as 400 °C,19,20 or rinsing with dry volatile liquids.21 Another important factor is sodium ion leaching from the surface, leaving silicate/silicic acid sites. Thus, glass interfaces with water or moist air have a polyelectrolyte character, analogous to a conductive polymer membrane.22,23,24 Charge build-up on microscopic silica insulators was among the first examples of electrostatic charging at a solid/gas interface increasing under high humidity, which is currently referred to as hygroelectricity 16,25,26,27 and is being explored in energy scavenging.28,29,30,31,32,33,34 It may also explain hysteresis or memory effects to microelectromechanical (MEMS) electrostatic actuators made from silicon wafers.35,36 The present work describes electric conduction and electric charge build-up on macroscopic silicate glass surfaces, under dc voltages.


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EXPERIMENTAL SECTION Materials Soda-lime glass sheets and microscope glass slides measuring 76 mm × 25 mm (Cral, Precision Glass Line) were cleaned with distilled water followed by ethanol (Synth) spray and dried at room temperature under 50–60% RH prior to use. Silanization37 was performed by dipping glass sheets in NaOH/H2O/ethanol 1:2:2 (w/w) solution for 60 min, followed by rinsing with ethanol and drying at 65 ºC for 12 h. After drying, the glass sheets were immersed in a 10% (w/w) trimethylchlorosilane (Sigma-Aldrich) ethanolic solution for 8 h, rinsed with ethanol, dried and kept in a desiccator. Slides used in resistance measurements were partly coated with a thin evaporated gold film, leaving a 10 mm gap between the gold electrodes over the whole 2.5-cm slide width. Copper tape strips (3M, 1181) were laid on the glass surface or over the gold coating film to form the electrical connections. DC resistance measurements DC resistance of the glass surfaces was measured by connecting the evaporated gold electrodes in series with a small capacitor and a Keithley 2410 power supply delivering a square wave with 1/120 Hz frequency and 0 and 2 V or 20 V limits to the RC pair. The voltage across the capacitor was measured with 1/20 Hz frequency on a Keithley model 6514 electrometer while changing the RH. Different capacitors were used to change the time constant of the RC series, thus allowing resistance measurement over a broad range. The output of the Keithley electrometer was recorded using an NI USB-6009 acquisition board with 100 ms steps.


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Macroscopic electric potential scanning Glass surface charging under one or two electrodes was carried out by connecting the copper strips to a dc voltage power supply (Spellman, CZE 1000R). Surface potentials were scanned horizontally with a high-resolution Kelvin electrode (Trek, 6000B-7C, 1.32 mm aperture diameter) connected to an electrostatic voltmeter (Trek 347), whose output was recorded using an NI USB-6009 board. This system can measure potential within the ±3300 V range (with 1 V resolution) across an x-y plane, while keeping 2 mm distance from the tested surface, as recommended by the electrode manufacturer. Another setup for monitoring charge build-up and dissipation was an insulated glass slide connected through an overlaid stainless-steel sheet, copper strip and a switch to a dc power supply. Two independent Kelvin electrodes were held 2 mm above the centers of the bare glass surface and the metal plate, 38 mm from the metal glass junction. Glass sheet and electrode were supported on polyethylene (PE) foam. Samples and Kelvin electrodes were always held within a Faraday cage. RH inside the box was controlled by bubbling compressed N2 through deionized water, while the temperature was kept at 23 ± 2 ºC. Temperature and humidity were continuously monitored using a digital thermo-hygrometer (Minipa, MTH-1380). Prior to Kelvin measurements, all samples were allowed to equilibrate to zero potential, and they were not touched or moved during the experiments. Charge density calculation Surface charge density on each pixel was calculated following a previously described procedure.38,39 Pixels on the surface map are squares with 3 mm sides each and the map is a 256 × 256-pixel matrix, where virtual charges are placed. The electrostatic potential (VT)


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measured 2 mm from the matrix plane is generated by all charges (qi) weighted by the distance r from the charge to the measuring point and can be calculated using a C++ code for the equation of the principle of superposition defined as follows: 𝑛

1

𝑛

𝑞𝑖

𝑉𝑇 = ∑𝑖 = 1𝑉 = 4𝜋𝜀0∑𝑖 = 1𝑟𝑖

(1)

The number of excess charge per pixel is adjusted by trial and error, until the calculated and measured potentials match, within experimental error. Charge density was also confirmed by using Equation 2, which is the solution to Poisson’s equation for a uniform surface charge distribution in a circle: 𝜎

𝑉 = 2𝜖

(

0

𝑅2 + 𝑟2 ― 𝑟)

(2)

where R is the radius of the circle, r is the distance from the surface to the Kelvin probe and

 is the charge density (Coulomb m2). Atomic force microscopy Atomic force microscopy (AFM) was performed in a Park NX10 (Park Systems, Suwon – Korea) instrument equipped with a SmartScan software version 1.0. RTM 11a. Microscope glass slides were used as received. Parallel copper tape strips were laid on the glass surface, 5.8 cm apart and connected to a dc power supply (3B Scientific model U33010) at  1 kV, under 80% RH for 24 hours, when the surface potential measured with the Kelvin electrode was -1 kV. The slides (before and after applying  1 kV potential) were placed on the AFM sample holder and imaged in tapping mode. The measurements were made using a high frequency rotated monolithic silicon probe (TAP300-G BudgetSensors, Bulgaria) with a 300 kHz nominal resonance frequency of and 40 N m-1 force constant. All measurements were made at room temperature (23 ± 5 °C) and a relative humidity of 80 ± 5%, with a 0.35


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Hz scanning rate. Images were treated offline using XEI software version 4.3.4 Build 22. RTM 1. Also, XEI software was used to calculate average surface roughness (Ra) and fractal dimension (D) carried at three different regions of each sample. Additional AFM/KFM experiments are in Supplementary Information.

RESULTS AND DISCUSSION Experimental results are grouped as follows: 1) dc resistance of glass slide surfaces, as a function of RH and voltage; 2) electric potential on a glass surface contacting a biased electrode; 3) electric potential on a glass surface fitted with two parallel electrodes; 4) AFM glass surface examination. Resistance measurements show that the conductivity of glass interfaces under high humidity is unexpectedly high, suggesting that glass charging may take place when this surface is biased. This was observed under two different electrode configurations. AFM microscopy showed morphological change of the electrified water film, compatible with the detected change in the electrical properties. DC resistance of a glass slide surface. The resulting plots are represented in Figure 1 and show the capacitor voltage rising under the voltage applied to the RC series (during the first minute of the wave period) and decreasing again during the next half-period. Capacitor voltages measured using this setup are shown in Figure 1 as a function of time, together with the RH. Each plot presents two main regions: under low humidity, the capacitor voltage increases and decreases linearly by only a fraction of the applied potential, while under high humidity the plot is approximately represented by a square wave with maximum and minimum voltages very close to the values delivered by the power supply.


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Figure 1. Plots of measured capacitor voltage during successive charge and discharge cycles, lasting for 1 min each. Increasing (left) or decreasing (right) RH is also plotted. Plots a-d, eh were obtained using respectively 0.3 nF and 110 nF capacitors. Applied voltages were 2V (c,d), 20 (a,b,e,f) and -20V (g,h).

This difference is due to the change in the time constant for capacitor charge/discharge (τ = RC): under low humidity, R is high and the current reaching the capacitor is low, while R


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decreases under high humidity, when the capacitor is quickly charged or discharged. There is also a transition zone at 43–50% RH, where capacitor voltage changes approach a sharkstooth shape: charging and discharging are fast but incomplete, under the 1-minute time allotted for each step. Detailed examination of the capacitor voltage rise-and-decay curves acquired under higher humidity shows very good fit with the simple exponential function expected in capacitor charge and discharge in an RC series, as in the examples shown in Figure S1. The glass surface resistance is thus constant during each charge/discharge cycle, showing that the measurements are not affected by electrode polarization, modifications in the conductive medium or electrochemical reactions that often prevent the use of dc current in resistance measurements. In this case, each exponential curve yields a time constant that is combined with the capacitance, thus yielding the glass surface resistance. However, under low humidity the measured voltages cover only a fraction of the applied voltage and the calculation of the time constant from a small interval within the whole voltage span is not reliable. In this case, the capacitor voltage change is used to calculate successively the charging current and the glass surface resistance, using the simple Equations 3–5, for every time t: 𝑉1(𝑡) = 𝑉 ― 𝑉2(𝑡) ∆𝑉2

Δ𝑞

𝑖(𝑡) = Δ𝑡 (𝑡) = 𝐶 ∆𝑡 (𝑡) 𝑅(𝑡) =

𝑉1(𝑡) 𝑖(𝑡)

(3) (4) (5)

where V is the source voltage (2 or 20 V), i(t) is the charging current passing through the glass surface, q is the charge carried by the passing current, C is the capacitance, V2 is the voltage measured across the capacitor and V1 is the voltage across the two electrodes mounted


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on the glass surface. This is a simple and rigorous procedure that proved convenient to measure high resistance and also low capacitance. Glass surface resistance calculated from the results in Figure 1 and other similar plots are shown in Figure 2.

Figure 2. Glass surface resistance as a function of RH. Different symbols refer to increasing or decreasing RH and capacitor charging or discharging step. Blue: decreasing RH. Red: increasing RH.

Figure 2 shows two smooth inverted sigmoid curves: one for resistance calculated from data acquired while humidity was increasing, the other when it was decreasing. The two curves are well superimposed in both the higher and lower humidity range but not in the middle range. This type of hysteresis is expected, considering that glass surface resistance depends on the amount of water on the glass surface, coupled to surface water vapor sorption and desorption, which are relatively slow processes.40


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The conductivity of the interfacial water layer under 80 % RH can be estimated using equation 6, where the resistance R = 108 Ω, the interelectrode distance (l) is 10 mm, the width of the conducting water layer is 2.5 cm and its thickness is ca.1 nm, yielding a cross-section area (A) equal to 2.5 x 10-11 m2:

𝜎=

𝑙 = 4 𝑆 𝑚 ―1 𝑅𝐴

(6)

This is six orders of magnitude higher than pure water conductivity,41 surprisingly. Assuming that conduction is protonic,11,12 the ionic conductance of the glass/moist air interfacial layer can be calculated using equation 7: Λ𝑜𝑚 =

𝑙 𝑅𝐴𝑐

(7)

Considering the literature data for pKa1 of the acidic silanol groups42,43,44 and [H+] = 100–10–1 mol m–3, the molar conductivity is within 4–40 S m2 mol–1. This is surprisingly high, since λ𝑜𝑚 = 0.035 S m2 mol-1 for H3O+ ions in water. 45 Measurements made on a glass slide prewashed with an alkaline solution yielded negligible values of V2; thus, the capacitor did not acquire charge at all, showing that the alkali treatment changed the glass surface profoundly, bringing its resistance to values above the upper limit of Figure 2. Electric potential in an isolated glass surface, contacting a biased electrode. Electrostatic potential was measured using a Kelvin (non-contact) electrode, on glass slides fitted with a metal strip and lying on an insulating polyethylene foam, within a grounded metal box, as shown in Figure 3a. The samples (electrodes and glass) were scanned with the Kelvin electrode with an initial potential of 0 ± 2 V, when the metal strip was grounded. However,


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biasing the strip under an RH of 50% or higher (Figure 3b) imparts positive potential to the sheet, reaching uniform values close to the dc voltage applied.

Figure 3. a) Experimental setup used to measure the surface potential on glass in contact with a single electrode at 300 V. b) Map obtained immediately after biasing the electrode under 50% RH. c) Map obtained immediately after biasing under 1% RH and d) 150 s after acquiring the map in c). Pixels on the maps are 10 mm × 10 mm, with 1.0 s measurement interval between pixels. Scan direction is left-to-right in the bottom line, then right-to-left in the adjacent line and so on, as indicated schematically. Dashed yellow lines show the electrode position.

Biasing the metal strip under 1% RH produces a slow increase of the glass surface potential. This is seen in Figure 3c, which shows the potential map recorded with the scanning Kelvin electrode, where the pixels at the bottom were recorded first, at shorter times after biasing the electrode, while the pixels at the top were recorded at the end of the surface scan. The


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potential gradient along the bottom line is thus much larger than along the upper line. After 150 s (Figure 3d), the surface potential of the glass under 1% RH is the same as the bias potential, within 5 V (5%). Analogous behavior was found using negative dc bias and also when the charged glass sheet was discharged by grounding (Figure S2). Thus, charge spreads fast across the glass surface but with a strong dependence on the surrounding humidity. This is markedly different from the behavior of thermoplastics and other hydrophobic surfaces,46 which always show pronounced and durable potential gradients across small areas. A similar setup, shown in Figure 4a, was used to measure the potential over a fixed point over the center of the glass slide as a function of time but with higher time resolution. The metal electrode was initially grounded, then biased (±300 V) and the electrostatic potential was recorded for 100 s. Two Kelvin electrodes were used, one over glass and the other over the electrode. Under high RH, the glass surface potential quickly reaches the same value as the electrode (Figures 4b,c), with symmetrical charge and discharge plots. Charge mobility under 80% RH may be estimated as follows: the 38 mm distance between the metal edge and the pixel under the electrode was travelled in less than 0.1 s, under the electric field that is initially equal to 300 V per 38 mm. Charge mobility is calculated as  = vi / E, where the particle speed vi > 0.38 m s-1 and the electric field E = 300 V/ 0.038 m. Thus,  > 4.8 × 10–5 m2 s–1 V–1 and the molar conductivity calculated from this figure is 4.6 S m2 mol –1, which is included in the conductivity range obtained using the resistance data, 4-40 S m2 mol –1. This is in good agreement, considering that all the assumptions made are based on experimental data obtained in this work or from the literature,42,43,44 without any adjustable parameter.


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Figure 4. a) Schematic setup for measuring potential at a fixed point over a glass slide in contact with a biased metal electrode, under controlled RH. b) Potential on glass and metal surfaces, following electrode bias (±300 V). c) Potential on the same surfaces after grounding the electrodes.

The protonic mobility on glass surface is thus more than three orders of magnitude higher than the electrophoretic mobility of simple ions in water (for instance, 4.81 × 10–8 m2 s–1 V–1


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under 10 mmol L–1 concentration),45 and less than two orders of magnitude lower than electron mobility in copper metal (44.2 × 10–4 m2 s–1 V–1). This explains the ability of a glass surface to store charge that has been observed under different circumstances38,47,48,49 and is a novel singularity of interfacial water. Excess charge in a glass surface mounted as the insulator in a 2D parallel-plate capacitor. The setup shown in Figure 5a is a 2-dimensional approximation of a parallel-plate capacitor that is expected to display a uniform and stable potential gradient, provided the dielectric layer is also uniform and stable. This is observed under low RH (3%), when the two electrodes are biased at 450 V dc, as shown in Figure 5b. Some equipotential lines are not exactly parallel to the electrodes, probably due to glass surface heterogeneity, but line scans normal to the electrodes show only small deviations from linearity, within a few volts (See Supporting Information Figure S3). However, when the relative humidity is raised to 80% RH under the 450 V dc bias, the electrostatic potential map changes pronouncedly with time, as shown in Figure 5c. The initial scan at 80% RH already shows non-uniform potential gradient, and the whole surface presents a negative potential 90 min later (Figure 5d), even near the positive electrode. Thus, the macroscopic glass sheet acquires excess negative charge with uniform density, analogous to SOW charging at the micro-scale.15 Grounding the electrodes under 80% RH restores uniform zero potential within a few seconds, showing that excess surface charge is dissipated 4–5 orders of magnitude faster than it is accumulated. When the sample is kept untouched under the same conditions and the electrodes are biased again, the glass surface acquires negative charge again but much faster than in the first run,


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as shown in Figure 6. It then took only 9 min to reach the same negative potentials that were initially reached after 10 h.

Figure 5. Electrostatic potential measurements on glass surface, made with a scanning Kelvin probe. a) Under 450 V applied to the two electrodes. b) Steady potential map under 3% RH. c) Initial potential map under 80% RH. d) The same but measured 90 min later. Time required for each map acquisition: 28 min. The position of the copper electrodes is shown in the plots.


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Figure 6. Electrostatic potential of glass sheet under 450 V and 80% RH. a) Potential versus interelectrode distance (for a line along the center of the glass sheet) as a function of biasing time. b) Potential vs distance measured in a second run, after a) was completed and the electrodes were grounded for 15 min.

This suggests that the glass surface underwent some sort of modification during the experiment reported in Figure 6, allowing faster charge acquisition in the second and subsequent runs. Repeating the bias/electrode grounding cycles consecutively, the glass surface repeatedly acquires negative potential rapidly, showing that the initial change is irreversible but that excess charge formation is highly reproducible, following the initial


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exposure of the glass to the electric field under high humidity, analogous to the observation made during surface resistance measurements. This was further examined by AFM microscopy and will be presented ahead. In another experiment, glass surface potential was measured while RH was changed, and a dc bias applied to the electrodes (Figure 7a). The same glass sheet used in Figures 5 and 6 was initially electrified until the whole surface showed negative potential, then it was grounded and further biased while RH changed within the sample container, as described in the caption of Figure 7. It is then possible to observe the dependence of glass surface charging state when RH is raised or lowered under a constant electric field. Sample negative charge acquired under 80% RH decreases under lower RH (50%) to disappear at 3% RH, showing that water desorption carries charge away from the surface. Similar sets of experiments were performed using several other glass plates, showing reproducible results provided the sample was first electrified under high humidity. The effect of RH disappears in a glass slide coated with silane (Figure 7b) where the hydrophilic silanol sites are replaced by hydrophobic methoxy groups that do not undergo ionic dissociation. Thus, contact with one or two electrodes produces different excess charge patterns on the glass surface when one or two electrodes are applied. A single biased electrode produces the same result as observed earlier for pure water50 while a dc potential difference always produces excess negative charge.


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Figure 7. a) Effect of changing the RH on electrostatic potential maps on glass, following this protocol: 1 h at 3% humidity, increase humidity to 80% (30 min), 1 h at 80% humidity, decrease humidity to 50% (15 min), 1 h at 50% humidity, decrease humidity to 3% (30 min), 1 h at 3%, followed by grounding. b) Potential measured on glass coated with silane, under 3% and 85% RH.

Fast surface charging under high humidity agrees with observations made on other hydrophilic surfaces, 9,27 but surface silanization or washing with alkali eliminate the ability of glass to acquire charge. This sensitivity to prior treatment and to the history of the glass surface is likely to be the reason of the dependence of the conditioning step (voltage applied under high humidity), to achieve reproducible results. This suggests that the glass surface


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undergoes some sort of modification that affects its interaction with water vapor and may be related to the complex structure and reactivity of the silicate glass surfaces, or perhaps to contamination under the atmosphere, akin to metals and other high-surface-energy solids.51 On the other hand, the reproducibility of the results obtained following the pre-treatment suggests that this type of surface conditioning may by useful under other practical situations. Excess charge on glass surface was calculated using the C++ code and/or the Poisson equation, both neglecting the roughness of the glass surface. A 350 V potential across the glass surface corresponds to 63 ions µm–2 of the geometric surface, or 9.42 × 10−16 mol of ions in each pixel of the electrostatic map (9 mm2). Since hydroxyl group concentration on the surface of amorphous silica52 is about 5 OH groups nm−2, the charge density on the glass surface corresponds to 0.0013% SiOH groups ionized to SiO−, or 13 ppm. A more realistic estimate, introducing glass surface roughness, would lead to a still lower fraction of ionized groups. Since the glass surface is covered by six molecular layers of water under 80% RH,4 the depletion of H+ ions is too small to affect the pH of the adsorbed water under the voltages used in this work. Thus, the significant excess voltage on the glass surface corresponds to a hardly detectable change in stoichiometric balance. So far, these low concentrations of charged species did not allow their detection by spectroscopic techniques that can be used under atmospheric conditions. However, operating under much higher voltage could produce enough amounts of ions (the calculated fraction of silanol groups necessary to exceed the dielectric breakdown of dry air, when the glass plate is 1 m away from a grounded metal sheet, is ca. 10.7%). IR and Raman spectral changes in biased water have been recently detected but using much higher voltage53 or a very special enhanced Raman technique.54 When a single electrode is in contact with the glass slide under high humidity, its surface acquires the same potential as the electrode. This is analogous to what happens when bulk ACS Paragon Plus Environment

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water acquires excess charge, which was examined in great detail in a previous paper from this group50 and in related works:55 When the two electrodes contact the glass surface and a potential difference is applied, mobile surface ions (H+ from silanol dissociation and water self-ionization, which also produces OH–) migrate to the electrodes of opposite sign. Ionization and ion migration depend on the water film adsorbed on the surface and do not take place on a dry glass surface. H+ and OH– migrate rapidly16 and are converted to neutral molecules at the electrodes. SiO− anions are also formed, but they are covalently bonded to the glass matrix; therefore, they are immobile, holding the fixed negative charge on the glass surface. When the electrodes are grounded, silicate groups bind H+ from adsorbed water, while excess negative charge is transferred to the ground at the positive electrode or to the atmosphere. Glass surface modification: AFM results. During this work, different experiments showed a change in the behavior of glass surfaces, when the initial runs were compared to those made after the slides were subjected to an electric field, in the presence of high humidity. Moreover, the pronounced effect of humidity on glass surface conductance and charging behavior suggests that the water surface pattern changes under increasing humidity. To better understand these effects, glass slide surfaces were examined by AFM microscopy, both before and after the application of high voltage under high RH. The micrograph in Figure 8a shows a large number of nanoparticles sitting on the clean but untreated glass surface, under the atmosphere of the laboratory. After applying 34 kV m-1 (2 kV across 5.8 cm interelectrode distance) parallel to the surface, under 80% RH for 24 hours (within a container outside the microscope) and imaging again under the atmosphere, the micrographs show a lower number of larger particles, as shown in Figure 8b. Moreover, both micrographs show a very thin surface layer, whose thickness matches the expected thickness for the adsorbed water layer.


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Figure 8. AFM topography images of a glass slide before (a) and after (b) applying a dc potential of ±1 kV, under 80 % RH for 24 hours. (c) and (d) are the same as (a) and (b) but showing intensified contrast and brightness.

This appears initially (Figure 8a) as scattered islands mixed with particle clusters, but it changes following the application of the electric field (Figure 8b), under high RH. Figure 8 c, d are the same as a and b, but with increased contrast and brightness, to allow a better observation of the thin water layer. Film alignment provides paths for ion conduction that persist unless the humidity is lowered beneath the percolation threshold. This is expected


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considering known characteristics of wetting/dewetting behavior of glass and other surfaces.56 Additional results from AFM/KFM imaging are in the Figures S4, S5, S6 and Table S1. The glass electrification phenomena described in this work may evolve into procedures to make electrets, if suitable encapsulation procedures are devised.57 The present results may also contribute to improved insulators used in high-voltage transmission lines,58 decreasing flashover due to condensed moisture, dust deposition and charge build-up.59 The arguments used in this work and in previous related literature are probably applicable to other hydrophilic solid surfaces (lignocellulosic materials, sugar, cellulose pulp, oxides, clays, metals coated with oxides, and related systems). Thus, these findings could help to improve safety conditions in the handling and storage of powders. Last but not least, there is currently interest in the wide possibilities opened by topological insulators. Following Moore: ‘The easiest way to describe a topological insulator is as an insulator that always has a metallic boundary when placed next to a vacuum or an “ordinary” insulator.’60 Soda-lime glass with conductive surfaces is non-metallic and is structurally, electronically and functionally different from known topological insulators, but both classes of materials share a common feature: a large difference between bulk and surface conductivity. The surface resistance of glass is sensitive to the environment and is easily controlled, opening several possibilities for creating and applying new materials and devices. For instance, glass surfaces can be used as intrinsically safe high-voltage probes, since they carry the signal quickly but under very high resistance that can still be modulated by the air moisture.


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CONCLUSIONS Increasing humidity in the 2–85% range decreases the electrical resistance of silicate glass/air interfaces by more than five orders of magnitude. Interfacial water films align along the applied electric field, providing a path for ion conduction, depending on the amount of adsorbed water and thus on the RH. This is due to protonic conduction with a mobility that is orders of magnitude higher than in bulk water and only two orders of magnitude lower than electronic mobility in copper metal. Water ion mobility and discharge at biased metal electrodes allow glass surfaces to acquire and store excess electric charge under high humidity, reaching many hundreds of volts (and perhaps much higher voltages), like pure water contacting biased electrodes. The excess charge depends on the electrode arrangement: a potential difference between two parallel electrodes produces excess negative charge only, independent of the absolute applied potentials, due to proton discharge accompanied by matrix-bound silicate ion formation. Thus, glass/air interfaces display unique electrostatic and electrochemical properties, showing some similarity to bulk water, but with important differences. The combination of glass bulk high resistance with a surface conductivity that is modulated by atmospheric water vapor may prove useful in sensors and various devices.

ASSOCIATED CONTENT

Supplementary data (Figures S1-S6 and Table 1).


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ACKNOWLEDGEMENTS

This work was supported by Brazilian agencies MCTIC/CNPq (465452/2014–0) and FAPESP (2014/50906–9) through INCT/INOMAT (National Institute for Complex Functional Materials). It was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) - Finance Code 001. LPS thanks CAPES (88887.284776/2018–00). The authors thank Théo da Mota dos Santos for performing part of the calculations.

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Plots of measured capacitor voltage during successive charge and discharge cycles, lasting for 1 min each. Increasing (left) or decreasing (right) RH is also plotted. Plots a-d, e-h were obtained using respectively 0.3 nF and 110 nF capacitors. Applied voltages were 2V (c,d), 20 (a,b,e,f) and -20V (g,h). 119x156mm (300 x 300 DPI)


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Glass surface resistance as a function of RH. Different symbols refer to increasing or decreasing RH and capacitor charging or discharging step. Blue: decreasing RH. Red: increasing RH. 85x54mm (300 x 300 DPI)


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a) Experimental setup used to measure the surface potential on glass in contact with a single electrode at 300 V. b) Map obtained immediately after biasing the electrode under 50% RH. c) Map obtained immediately after biasing under 1% RH and d) 150 s after acquiring the map in c). Pixels on the maps are 10 mm × 10 mm, with 1.0 s measurement interval between pixels. Scan direction is left-to-right in the bottom line, then right-to-left in the adjacent line and so on, as indicated schematically. Dashed yellow lines show the electrode position. 110x82mm (300 x 300 DPI)


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a) Schematic setup for measuring potential at a fixed point over a glass slide in contact with a biased metal electrode, under controlled RH. b) Potential on glass and metal surfaces, following electrode bias (±300 V). c) Potential on the same surfaces after grounding the electrodes. 135x146mm (300 x 300 DPI)


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Electrostatic potential measurements on glass surface, made with a scanning Kelvin probe. a) Under ±450 V applied to the two electrodes. b) Steady potential map under 3% RH. c) Initial potential map under 80% RH. d) The same but measured 90 min later. Time required for each map acquisition: 28 min. The position of the copper electrodes is shown in the plots. 140x104mm (300 x 300 DPI)


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Electrostatic potential of glass sheet under ±450 V and 80% RH. a) Potential versus interelectrode distance (for a line along the center of the glass sheet) as a function of biasing time. b) Potential vs distance measured in a second run, after a) was completed and the electrodes were grounded for 15 min. 71x122mm (300 x 300 DPI)


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a) Effect of changing the RH on electrostatic potential maps on glass, following this protocol: 1 h at 3% humidity, increase humidity to 80% (30 min), 1 h at 80% humidity, decrease humidity to 50% (15 min), 1 h at 50% humidity, decrease humidity to 3% (30 min), 1 h at 3%, followed by grounding. b) Potential measured on glass coated with silane, under 3% and 85% RH. 69x114mm (300 x 300 DPI)


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AFM topography images of a glass slide before (a) and after (b) applying a dc potential of ±1 kV, under 80 % RH for 24 hours. (c) and (d) are the same as (a) and (b) but showing intensified contrast and brightness. 170x145mm (300 x 300 DPI)


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air interfaces

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