Fabrication of Robust Reference Tips and Reference Electrodes for

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Fabrication of Robust Reference Tips and Reference Electrodes for Kelvin Probe Applications in Changing Atmospheres Matthias Uebel, Ashokanand Vimalanandan, Abdelaziz Laaboudi, Stefan Evers, Martin Stratmann, Detlef Diesing, and Michael Rohwerder Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02533 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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Fabrication of Robust Reference Tips and Reference Electrodes for Kelvin Probe Applications in Changing Atmospheres M. Uebel1, A. Vimalanandan1, A. Laaboudi1, S. Evers1, M. Stratmann1, D. Diesing2 and M. Rohwerder*,1 1

Max-Planck-Institut für Eisenforschung GmbH, Department of Interface Chemistry and

Surface Engineering, Max-Planck-Str. 1, 40237 Düsseldorf, Germany 2

University of Duisburg-Essen, Faculty of Chemistry, Universitätsstr. 5, 45141 Essen,

Germany KEYWORDS.

scanning

Kelvin

probe (SKP),

work

function,

self-assembled

monolayer (SAM), dipole moment, humidity

ABSTRACT. The scanning Kelvin probe (SKP) is a versatile method for the measurement of the Volta potential difference between a sample and the SKP-tip ( ). Based on

suitable calibration, this technique is highly suited for the application in corrosion science due to its ability to serve as a very sensitive non-contact and non-destructive method for determining the electrode potential, even at buried interfaces beneath coatings or on surfaces covered by ultra-thin electrolyte layers, which are not accessible by standard reference electrodes. However, the potential of the reference (i.e. the SKP-tip) will be influenced by variations of the surrounding atmosphere, resulting in errors of the electrode potential

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referred to the sample. The objective of this work is to provide a stable SKP-tip which can be used in different or changing atmosphere, e.g. within a wide range of relative humidity (approx. 0 %-rh – 99 %-rh) or variing O2 partial pressure, without showing a change of its potential (Note that work function measured in non UHV atmospheres are electrochemical in nature1 and hence in the following we will refer to the potential of the SKP-tip instead of its work function). In that regard, the SKP-tip is in a first approach modified with self-assembled monolayers (SAMs) in order to create a hydrophobic barrier between the metallic surface and the surrounding atmosphere. The changes in potential upon varying relative humidity (∆ ) of different bare metallic substrates are quantified and it is shown that these potential differences cannot be minimized by SAMs. On the contrary, the ∆ increases for every examined material system modified with SAMs. The major explanation for this observation is the dipole layer at the interface metal | SAM, causing an interfacial adsorption of water molecules even in a preferred orientation of their dipole moments, which leads to a changed work function and consequently to the correlated electrode potential. However, thin paraffin coatings were found to lead to a strongly reduced ∆ , finally validated with novel robust Ag/Ag+ reference electrodes. It is also shown, that nickel as SKP-tip material is seemingly more stable in varying atmospheric conditions compared to widely used Ni/Cr, stainless steel or gold as SKP-tip material.

1. Introduction The scanning Kelvin probe (SKP) is able to measure very precisely the Volta potential difference, between the SKP-tip and a given sample (∆Ψ ) – even insulating ones, if

charging is avoided.2, 3 This can be interpreted as the electrode potential difference.1 Provided that the SKP-tip is calibrated accurately, this provides the potential of the sample with respect to the standard hydrogen electrode (E vs. SHE)1. Detailed information about the principles of


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SKP can be found elsewhere4, 5. It has to be taken into account, that this final value

(∆Ψ ) is comprised of both contributions – the one of the sample of interest and the one of the SKP-tip itself. If measurements have to be carried out in different environments, an inert SKP-tip material has to be used so that changes of the measured ∆Ψ can be

mainly referred to the changes of the sample. However, the work function of the SKP-tip may vary with changing atmospheric conditions such as humidity, O2 partial pressure or temperature.6 Kelvin probe measurements under gas change conditions were e.g. carried out by Senöz et al. to investigate filiform corrosion, more specifically the visualization of the underlying process that is the galvanic coupling of active heads and intermetallic particles7, or by Salgin et al. who monitored the local work function changes on aluminum oxide, induced by the displacement of mobile surface ions upon changed ambient atmosphere.8 However, while in these works relative differences can be safely interpreted, concerning the absolute values of potential changes on a sample requires certainty that the potential of the SKP-tip itself remains constant. Rohwerder and Evers used the onset of binary phase formation of α-Pd-H as reference point for the work function during hydrogen permeation experiments, where absolute values are important, since a defined calibration under the prevalent dry nitrogen atmosphere was not feasible with the common reference electrodes based on liquid/solid redox couples.9 Scully and Schaller have calibrated a SKP as a function of diffusible hydrogen concentrations for investigations of pitting corrosion on ultra-high-strength steels at constant humidity of 57 %-rh10. However, such calibration may critically depend on humidity. Another important application of robust reference tips and reference electrodes is the use in mobile field Kelvin probes (FKP) where e.g. the humidity will vary in dependency of the application site11. It should be also mentioned, that the calibration of a SKP is usually realized with a reference electrode comprised of a solid/liquid redox couple, such as Cu in contact with


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CuSO4 saturated in water, i.e. the calibration of the SKP occurs in very humid atmosphere as the SKP-tip is positioned directly over the liquid phase.12, 13, 14 Despite the assumed high local humidity at the SKP-tip positioned close to the liquid phase, Hassel et al measured a humidity induced change in electrode potential of approx. 30 mV for a Cu/CuSO4 reference electrode exposed to air with a relative humidity within the SKP-chamber ranging from 30 – 90 %15. Subsequent measurements will then just be correct if performed under equal atmospheric conditions. Furthermore, the response of the SKP-tip upon changing atmospheric conditions may be similar to the one of the investigated samples and hence its change will not be measurable, at least not correctly. So, a SKP-tip which work function is as stable as possible is especially strongly of interest for measurements performed in different atmosphere16,

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and/or when very small changes in potential have to be reliably detected. Ehahoun et al. have described the fabrication of a Ag/AgCl/KCl SKP-tip18, extending the earlier work of Atanasoski et al.19 The presented reference tips for the use in Kelvin probe applications are based on Ag/AgCl and especially insensitive for O2 adsorption (see 18) and have been evaluated for relative humidity (rh) between 40 % and ~100 % (see 19). This paper first describes the fabrication of plane-ended SKP-tips based on Ni(80)/Cr(20) in detail and finally how to easily modify them to a humidity- and O2-insensitive reference electrode in the relative humidity range between ~0% - ~95% and for oxygen content between ~0 % - ~20 % in dry atmosphere. Besides, the impact of self-assembled monolayers on interfacial water adsorption and the general fabrication of reference electrodes applicable in dry environment is described. The latter aspect might be of great interest for novel Kelvin probe applications, such as hydrogen permeation measurements which have to be carried out in dry nitrogen atmosphere9, 20, 21, or determination of the oxygen reduction reaction at buried interfaces by Kelvin probe22 and for FKP applications.11


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2. Materials and methods 2.1. Fabrication of tips for Kelvin probe measurements For the fabrication of plane-ended SKP-tips a Ni80/Cr20 wire, purchased from Goodfellow GmbH, with an initial diameter of 500 ± 5 µm was used. The wire was straightened and cut to desired length of about 25 mm. The straightening of the wire was realized by pulling them until fracture. The tensile strength is according to the manufacturer of 650 – 1100 MPa. The pieces of wire were then cleaned with acetone (technical grade, AppliChem GmbH) in an ultrasonic bath for 5 min. and etched to appropriate diameter by potentiostatic etching. For this purpose the pieces of wire were used as a working electrode and connected to a DC voltage source (Straton Gerätetechnik GmbH, model 2223.13) with a platinum net as counter electrode. The electrodes were immersed in an acidic etching solution composed of 20 vol.-% ultrapure water (type 1 prepared with USF ELGA water puriﬁcation system with a conductivity of less than 0.055 µS cm-1), 15 vol.-% conc. sulfuric acid (supplied from VWR KGaA), 65 vol.-% conc. o-phosphoric acid (supplied from AppliChem GmbH) and a voltage of 4 V was applied and measured directly at the electrodes with a multimeter (model 7150 plus digital multimeter, Schlumberger Messgeräte GmbH). A second multimeter (model 175 True RMS multimeter, Fluke Corporation) was used to measure the current flow due to the anodic dissolution of the Ni/Cr wire. In order to etch wires with certain diameters the etching process was calibrated independent from the etching time and the immersion depth. To achieve this, ten calibration samples were etched to different diameters and for each the current at the beginning of the etching process I0, the current at the end of the etching process IE, the initial diameter d0 and the final diameter dE were noted. By plotting the respective ratios I0/IE over d0/dE a time-independent calibration equation could have been obtained. After etching the wires were again cleaned with acetone in an ultrasonic bath for 5 minutes,


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subsequently rinsed with ultrapure water, dried in an air stream and embedded in an acetone soluble epoxy resin (Technovit 5071, Heraeus Kulzer GmbH) with the use of a homemade wire holder. The tips of the embedded wire were then ground with SiC grinding paper (grain size P80/120/220/400/600/800/1000/2500/4000 WS Flex 18 C SK, Hermes Schleifmittel GmbH & Co. KG) and subsequently polished with a diamond paste of 3 µm and 1 µm grain size respectively to receive plane-ended SKP-tips. Finally, the epoxy resin was removed in acetone and the shape and dimension of the prepared SKP-tips was proven with a field emission scanning electron microscope (SEM) from ZEISS (Model LEO 1550 VP, GEMINI) using predominantly an acceleration voltage of 15 kV and a working distance of 6-9 mm. Additionally with choice of a homemade multi wire holder (n = 9) the whole preparation procedure could be performed simultaneously for 9 wires resulting in a narrow distribution of the diameters (± 1.8 µm) and fast production. In this work, comparatively large SKP-tip diameters (~ 420 µm) were used in order to ensure a good signal/noise ratio. The diameter of the SKP-tips was measured with the SEM using the software SmartSEM v5.06.

2.2. Samples and sample modification with self-assembled monolayers Preparation of selected bare metal samples

All selected metals should be resistant against corrosion in humid atmosphere. Furthermore, they should be suitable for a modification with self-assembled monolayers (SAMs). For that reason, ultrapure aluminum (99.999 %), stainless steel (X2CrNiMo17-12-2) both purchased from GoodFellow GmbH, pure nickel foil (99.995 %, 1 mm thick) purchased from Alfa Aesar and gold samples (physical-vapor-deposited gold layer (thickness: 100 nm) with a chromium interlayer (thickness: ~5 nm) as adhesion promoter on microscopy glass (purchased from Hirschmann Laborgeräte GmbH & Co. KG)) were used. Chromium was


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evaporated on Si wafers (purchased from Siegert Wafer GmbH, 4”, 0.9 mm thickness, onesided polished, cut to dimensions of 20 x 20 mm). The aluminum samples (dimensions: 1.5 × 20 × 20 (mm), analysis (ppm): Al-matrix, Cr 0.035, Fe 0.748, Cu < 0.4, Mg 1.17, Mn 0.041, Ni 0.011, Si 1.18, Ti 0.18, Zr 0.005) were cleaned for 3 min. in a solution consisting of 69 vol.-% distilled water, 20 vol.-% hydrochlorid acid (conc. 37 %, p.a., supplied by VWR Int.), 10 vol.-% nitric acid (conc. 69 %, p.a., supplied by VWR Int.) and 1 vol.-% fluoric acid (conc. 40 %, p.a., supplied byAppliChem GmbH) at 60 °C and rinsed with ultrapure water and ethanol (purity p.a., supplied by VWR Int.) afterwards. The stainless steel samples (dimensions: 2 × 20 × 20 (mm), typical analysis (wt.-%): C ≤ 0.03, Cr 16.5-18.5, Ni 10.5-13, Mo 2-2.5) and nickel samples were cleaned with ethanol and acetone. Nickel, aluminum and stainless steel samples were ground with SiC grinding paper up to grit P4000, polished with a diamond paste with an average grain size of 3 µm and 1 µm respectively, re-purified in an ultrasonic bath with ethanol for 3 min. and dried under an air stream. The gold samples were prepared in the following way: The microscopy glass slides were cut to dimensions of 25 × 25 mm (1 mm thickness) and carefully cleaned before PVD, i.e. they were first pre-cleaned by rinsing in ultrapure water followed by subsequent treatment in an ultrasonic bath with water and ethanol, respectively. Afterwards they were immersed in freshly prepared piranha solution (50 vol.-% hydrogen peroxide, conc.: 30 %, purity p.a. and 50 vol.-% sulfuric acid, conc. 95-97 %, purity p.a., both supplied by Merck KGaA) – Caution: piranha solution is a high oxidant solution that reacts violently with organic compounds. After emerging the glass pieces from the piranha solution they were intensively rinsed with water and ethanol and dried under a nitrogen stream and subsequently put in the physical vapor deposition device (Model Univex 450, Leybold AG). First chromium was


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evaporated (deposition rate: 5 Å/s, thickness: approx. 10 nm) and then gold (deposition rate: 2.5 Å/s, thickness: approx. 100 nm), using an electron beam evaporator typically at 10-6 mbar chamber pressure. For pure chromium samples, chromium was deposited onto Si wafers to a final thickness of 500 nm using a deposition rate of 2.5 Å/s. Samples that were stored at high humidity, were put into a closed chamber containing beakers filled with K2SO4(sat. in H2O, purity: ≥ 99 %) purchased from Sigma-Aldrich Corp. The equilibrium relative humidity of saturated potassium sulfate is 97.038 ± 0.38923. The humidity within the humidity chamber was confirmed to be 97 %-rh by the use of a standard hygrometer.

Modification of metallic substrates with SAMs

Aluminium-octadecylphosphonic acid (Al-ODPA): The freshly polished aluminum samples were sonicated in tetrahydrofuran (purity: p.a., purchased from Merck KGaA) and ethanol (purity: p.a., purchased from VWR Int.), each for 3 minutes. The samples were rinsed first with the used solvent and afterwards with deionized water when changing the solvent. In order to increase the number of hydroxyl groups on the aluminum surface the samples were immersed in 0.1 M sodium hydroxide solution (99.7 % NaOH, purchased from VWR Int.) for 30 seconds, rinsed with deionized water and dried under a nitrogen stream. Thus pretreated samples were then immersed for 6 minutes in a 1 mM octadecylphosphonic acid (purity: 97 %, purchased from Sigma-Aldrich Corp.) solution (solvent: tetrahydrofuran) to initiate the formation of a SAM. Afterwards the sample was removed out of the solution and put in an oven at 120 °C for 45 minutes, rinsed with absolute ethanol and deionized water and finally dried under a nitrogen stream.


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Aluminium-octadecyltrimethoxysilane (Al-OTS): The pretreatment of the aluminum samples before initiating the solution based selfassembly is identical to the one described before. For the preparation of the octadecyltrimethoxysilane solution methanol and water was first placed in a beaker and the pH was adjusted to 5 using acetic acid. Then the desired amount of octadecyltrimethoxysilane was added under stirring, the pH was controlled again and the solution was filled up with the solvent up to the targeted volume. The solution was then kept on stirring overnight. Then the sample was covered with a prepared octadecyltrimethoxysilane solution consisting of 8 vol.-% octadecyltrimethoxysilane (purity: 90 %, ρ = 0.883 g/cm³, purchased from SigmaAldrich Corp., warmed to 30 °C before use in order to decrease viscosity), 86 vol.-% methanol (purity p.a., purchased from Merck KGaA) and 6 vol.-% ultrapure water. After a reaction time of 2 minutes the silane solution was removed by moving the sample carefully in absolute ethanol. Subsequently the sample was annealed for 20 minutes at 110 °C. After cooling down to room temperature the modified aluminum sample was intensively rinsed with absolute ethanol and deionized water in order to remove physisorbed molecules from the surface and it was finally dried under a nitrogen stream.

Stainless steel-octadecylphosphonic acid (1.4404-ODPA): The freshly polished stainless steel samples were sonicated in tetrahydrofuran (purity: p.a., purchased from Merck KGaA) and ethanol (purity: p.a., purchased from VWR Int.) each for 3 minutes. The samples were rinsed first with the used solvent and afterwards with deionized water when changing the solvent. The stainless steel samples were immersed in a 0.1 M sodium hydroxide solution for 2 minutes immediately after annealing them at 110 °C for 20 minutes. After rinsing in absolute ethanol and deionized water they were dried in a nitrogen stream. This way prepared samples were then put in an ethanol based 1 mM


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octadecylphosphonic acid solution for 6 minutes and stored for 45 minutes at 110 °C afterwards. After cooling down to room temperature the modified stainless steel sample was intensively rinsed with absolute ethanol and deionized water in order to remove physisorbed molecules from the surface and it was finally dried under a nitrogen stream.

Stainless steel-octadecyltrimethoxysilane (1.4404-OTMS): The procedure is the same as described for the modification of aluminum substrates with octadecyltrimethoxysilane but the immersion time in 0.1 M sodium hydroxide was 2 minutes instead of 30 seconds and it was additionally annealed at 110 °C for 20 minutes immediately before.

Gold-1-decanethiol (Au-DT): The as described before prepared gold samples were put overnight (12 h) in a freshly prepared ethanol based 1 mM 1-decanethiol (purity: 99 %, ρ = 0.841 g/ml, M = 174.35 g/mol, purchased from Sigma-Aldrich Corp.) solution. After emersion the modified samples were rinsed carefully in absolute ethanol and dried under a nitrogen stream.

All samples modified with self-assembled monolayers were characterized or measured directly after preparation.

Coating of metallic substrates and SKP-tip with paraffin

In order to create a hydrophobic barrier without including polar bonds at the interface metal | coating, thin paraffin (Pastilles pure Ph. Eur., mp 53 °C, sulfate < 0.015 %, purchased from AppliChem GmbH) coatings were applied on the sample and on the SKP-tip as well.


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The stainless steel (1.4404) samples prepared as described before were annealed for 20 minutes at 120 °C and immediately spin-coated at 2000 rpm for 30 seconds using a spincoater (Model P6700, Speciality Coating Systems Inc.) with the paraffin heated to 95 °C. Then the coated sample was again heated to 150 °C and spinned at 3000 rpm for 30 seconds without adding more paraffin in order to decrease the thickness of the paraffin film. The SKP-tip was coated with paraffin by dip-coating. Therefore, first the prepared SKP-tip was sonicated in absolute ethanol and tetrahydrofuran each for 10 min. and the SKP-tip, fixed vertically in a produced holder for SKP-tips where the SKP-tips flat bottom is directed to the top, was annealed for 30 minutes at 110 °C. Immediately the SKP-tip was dipped once in a stirred paraffin bath heated to 150 °C and removed after 1 minute. Then the SKP-tip was put in an oven at 110 °C with the flat bottom directed vertically to the ground. After a while the excess paraffin accumulated at the flat bottom of the SKP-tip was carefully dabbed off with a precision wipe.

Preparation of reference electrodes applicable in dry atmosphere

Two different types of reference electrodes based on Ag/Ag+ redox couple were prepared: Ag/AgCl/KClcrystal and Ag/AgCl/KClcrystal/Paraffin. Si wafers (4”, 1 mm thickness, double-sided polished, purchased from Siegert Wafer GmbH) were cut to pieces of 15 mm by 25 mm used as substrates for the samples. Then silver was evaporated using the physical vapor deposition device (Model Univex 450, Leybold AG). First chromium was evaporated (deposition rate: 5 Å/s, thickness: approx. 10 nm) and then silver (deposition rate: 7.5 Å/s, thickness: approx. 1 µm) using an electron beam evaporator typically at 2*10-7 mbar chamber pressure. Then AgCl was deposited electrochemically onto the samples using a 3 electrode setup consisting of a commercial


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Ag/AgCl reference electrode in 3 M KCl (Order number: 6.0726.107, E vs. SHE = 207 mV (25 °C), purchased from Metrohm GmbH), a platinum foil as counter electrode and the sample as working electrode with almost full exposure (5 mm of sample outside) to the electrolyte (1 M KCl). The samples were polarized first at -0.1 V vs. Ag/AgCl for 30 s and subsequently at + 0.1 V vs. Ag/AgCl for 120 s using a Solatron Schlumberger 1286 potentiostat. The samples polarized in 1 M KCl electrolyte solution were just dried carefully under a nitrogen stream after removal in order to form KCl crystals on the surface (according to 18). The paraffin coatings were produced as described for the stainless steel samples but the Ag surface was connected with a piece of copper tape (one-sided sticky, purchased from Plano GmbH) before.

2.3. Surface characterization of gold samples To detect the thiol bound to gold, X-ray photoelectron spectroscopy (XPS, Quantum 2000 from Physical Electronics Inc.) was performed at a takeoff angle of 45°, with a monochromatic Al Kα1 source (1486.7 eV) operating at 15 kV and 25 W on a spot size of 100 µm by 100 µm upon a basis pressure smaller than 5 × 10-9 mbar. All binding energies were calibrated relative to the Au 4f7/2 position at 84.0 eV. The software CASA XPS was used for processing the data. Since the signal intensity of Au-S is very small within the SAM the average Au-S signal intensity from 3 measurements were used to improve the signal/noise ratio thus pronouncing the relevant peak. The electrochemical current-voltage characteristic was analyzed in order to measure the inhibition of the oxygen reduction reaction on 1-decanethiol modified (SAMs) gold surfaces compared to bare gold surfaces. The measurements were realized using a 3 electrode setup built in a homemade cell, consisting of a commercial Ag/AgCl reference electrode in 3 M KCl (Order number: 6.0726.107, E vs SHE = 207 mV (25 °C), purchased from Metrohm


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GmbH), a platinum foil as counter electrode and the sample as working electrode with a contact area of 0.1963 cm² exposed to the electrolyte (0.1 M NaOH, purged gently with O2 gas). A potential ranging from 0.2 V - -1.4 V vs. Ag/AgCl(3 M KCl) was passed through in ® cathodic direction with a scan rate of 10 mV/s using a potentiostat VoltaLab PST050 from

Radiometer Analytical SAS and the software Voltmaster 4 was used.

2.4. Scanning Kelvin probe measurements Scanning Kelvin probe (SKP) measurements were performed with a custom made SKP device operated in nulling technique. The SKP device has been equipped with a software controlled and heatable humidity system that allows a relatively quick change of humidity (air or nitrogen atmosphere) ranging from 2 %-rh to 93 %-rh within about 20 minutes. The applied 14.5 h lasting humidity program (air) for each measurement consists of a 30 minutes lasting initial phase in 93 %-rh, followed by 3 wet (93 %-rh) to dry (2 %-rh) transitions (3 times dry and 2 times wet atmosphere each 2 h) and 4 h without active humidity control system at about 80 %-rh. The potential was measured by a surface scan (2000 µm by 2000 µm with 200,200 (x,y / µm) step size resulting in 100 measurement points) and is given as the mean potential with standard deviation. Figure 1 shows the applied humidity and measurement management. The prepared SKP-tips described in paragraph 2.1 have been used with or without varying kinds of modifications with an oscillation frequency that was found to be the individual resonance frequency and throughout the experiments a voltage of 1.5 V was applied to generate the oscillation amplitude of the SKP-tip. Since the used SKP is not equipped with a height regulation system, the distance SKP-tip to sample was estimated based on light microscopy and adjusted with the signal intensity displayed on an oscilloscope. Before each measurement, the system was calibrated against a Cu/CuSO4 (sat. in H2O)


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reference electrode and the subsequent measured potentials are given with respect to the standard hydrogen electrode (SHE).

Figure 1. Measurement conditions: relative humidity (rh) profile during a measurement time of 14.5 hours a), surface scan of 2000 µm by 2000 µm with 200,200 (x,y) step size measured within 100 measurement points b) The validation measurements were carried out with a commercial SKP system with height regulation equipped with a heatable humidification system (approx. realizable humidity range: 0.02 < rh < 95) and an air and nitrogen gas connection from KM Soft Control. A 99.2 µm Ni/Cr tip (bare or coated with paraffin) operated at an oscillation frequency of 934 Hz and a voltage of 2.1 V was applied to generate the oscillation amplitude. The calibration of the SKP was done with a Cu/CuSO4 (sat. in H2O) reference electrode and all potentials are referred to the standard hydrogen electrode (SHE). Repetitive point measurements of 3 min. were performed and the respective average and standard deviation are plotted.

3. Results and discussion Fabrication of plane-ended SKP-tips

First, a calibration of the etching process was realized as described in the experimental part. In Figure 2 the obtained calibration equation is shown as it was received based on the etching of 10 calibration samples. The calibration equation for such an etching process allows one to


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calculate the current IE needed for obtaining a targeted diameter dE starting from a known initial diameter d0 and associated current I0. I0 is noted 15 seconds after applying the voltage. For evaluating 4 control samples with different targeted diameters 100, 200, 300 and 400 µm were etched based on the calculated current IE, whereas completely different immersion depths were used. The average derivation from the targeted diameter to the reached diameter is 15 µm, thus proving the suitability of this time- and immersion depth independent etching process. R2= 0,99

3

I I

0 e

= 0.6 d 0 + 0.43

d

e

I0 / Ie

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2

calibration sample control sample linear regression

1 1

2

3

4

5

d0 / de

Figure 2. Experimental investigation of the current based time and immersion depth independent calibration equation by etching 10 calibration samples for different etching times. The plot shows the ratio of the current at the beginning of the etching process (I0) and the one at the end (Ie) as a function of the ratio of the initial Ni/Cr wire diameter (d0) and the Ni/Cr wire diameter after etching (de). The 4 control samples proof the suitability of the obtained calibration equation. For the simultaneous etching and grinding/polishing of numerous SKP-tips a home-made 9 spot wire holder was used, and for this study, relatively large SKP-tip diameter of 420 µm were targeted, thus ensuring a good signal to noise ratio. The application of the evaluated


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etching process with the multi spot tip holder led to 9 SKP-tips with an average diameter of 419,8 µm ± 1,8 µm (see one example in Figure 3).

Figure 3. SEM micrographs of etched and polished plane-ended SKP-tip (left) and transition from non-etched to etched area (right). The average diameter of the 9 prepared SKP tips is 419.8 µm ± 1.8 µm. (SEM parameters: working distance (WD) = 6 – 8 mm, magnification (M) = 150x, acceleration voltage (EHT) = 15 kV) Characterization of surface modified samples

For the self-assembled monolayer (SAM) of 1-decanethiol (C10SH) on X-ray photoelectron spectroscopy was used to detect the relevant gold-thiolate (Au-S) bonding with a typical binding energy S 2p3/2 of 162.1 eV 24. Since the concentration of sulphur within this SAM is very low and furthermore superimposed by the signal of the substrate, the XP spectra of three measurements were averaged thus emphasizing the relevant Au-S signal. The XP spectra shown in Figure 4 proofs the existence of the Au-S bonding. The S 2p3/2,S 2p1/2 doublet is fitted with a splitting of 1.2 eV and the intensity ratio is 2:1. This corresponds to a thiolate bonding onto a non-oxidized gold surface24.


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6400

intensity / CPS

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6200

S 2p 3/2 (BE=162.1 eV) S 2p 1/2 (BE=163.3 eV) Background Fit Envelope

6000

168

166

164

162

160

158

binding energy / eV

Figure 4. S 2p3/2 and S 2p1/2 XP spectra after adsorption of self-assembled monolayer of 1-decanethiol on gold. The shown spectra are averaged from 3 measurement spots on one sample and were recorded after 12 h of adsorption from 1 mM 1-decanethiol in ethanol solution. The binding energies are calibrated on Au 4f7/2 with a binding energy of 84 eV. The solution based self-assembled monolayer of C10SH on gold is known to form dense packed monolayers with highly ordered alkyl chains25. So this system should be a good candidate to proof the general applicability of the concept which is to avoid humidity induced changes in work function of the SKP-tip - and subsequently parasitic error of the measured Volta potential difference4 due to adsorption of water molecules by providing a ultra-thin hydrophobic barrier onto the SKP-tip. The SAM of C10SH on gold has a thickness in the range of 1 nm26. Any coating or layer onto the SKP-tip should vary within the range of minimum thickness required to gain the desired barrier properties against interfacial water adsorption and the maximum thickness still enabling the very close arrangement of SKP-tip and sample parallel to each other whereby charging effects have to be prevented. The current-voltage characteristics (I-U curves) of gold and a SAM of C10SH on gold in aerated alkaline media demonstrate the presence of a densely packed SAM - the onset of oxygen reduction potential for the gold modified sample is shifted by about 300 mV in cathodic direction compared to the bare gold (see Figure 5).This was also demonstrated to be


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present for neutral media by Vago et al.27. The desorption of SAM of 1-decanethiol on gold in alkaline media proceeds at a potential of approx. 1 V (Ag/AgCl(sat.))28. However, the monolayer preserves at least partially its structure and steps close to the surface29 (like partial hovering of the SAM), which explains the different onset of the hydrogen evolution potential. 0.1 M NaOH, pH = 13, 10 mV/s

0 O2 reduction

i / (µA*cm-2)

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-100

-200 H2 evolution

-300

Au Au-DT

-1.5

-1.0

-0.5

0.0

U / V vs. Ag|AgCl(3 M KCl)

Figure 5. I-U curves of gold (Au) and SAM of 1-decanethiol on gold (Au-DT) in aerated 0.1 M NaOH solution with an initial bulk pH of 13. The results obtained by XPS and I-U measurements confirm the formation of a chemisorbed and densely packed self-assembled monolayer of 1-decanethiol on gold.

Scanning Kelvin probe measurements in wet to dry cycling atmosphere

Since the Volta potential difference depends on both, the condition of SKP-tip and sample , these two contributions to the Volta potential difference cannot be measured independently 4. Hence, first all measurements are performed with a bare Ni/Cr (80/20) SKP-tip in order to work out the effectiveness of the different surface modifications on the sample. Then the most effective surface modification was selected to be applied onto the SKP-tip itself. A reliable calibration of the SKP-tip vs. Cu/CuSO4 is only possible in humid atmosphere. Hence, in the following the measured values are stated as electrode potential of the sample referred to SHE. However, upon changing the atmospheric conditions, for instance towards


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lower humidity, this reference might be lost due to changes mainly in the dipole potential of the surface of the SKP-tip, which is neglected in every former work. It should be noted here that even re-calibration of the SKP-tip over a Cu/CuSO4 reference electrode will not be reliable, as over the surface of that reference a high humidity prevails and when the SKP-tip is moved away from that reference the potential of the SKP-tip will change again. The electrode potential E (average of surface scan like described in the experimental part) of clean gold often inclines to drift like displayed in Figure 6 a), but the measured initial electrode potential within the first wet period of about 400 mV vs SHE is reliable as it was also measured with SKP in other works, e.g.30 , and also the tendency to show a significant drift of potential within Kelvin probe measurements was observed elsewhere19. A gold surface absolutely free of contamination does not provide surface groups with strong dipole moments (e.g. hydroxy groups), so that water molecules from the gas phase adsorb with a negligible preferred orientation of their dipole moments. Contamination, e.g. by organic molecules containing dipole moments adsorbed onto the gold surface or the migration of Cr from the adhesion supporting layer31 (less likely as it is negligible if the Cr layer is sufficiently thin, then even no Cr migration occurs during flame annealing32,

33

), could

explain the tendency of a decreasing electrode potential with time and also the distinct drop of potential upon change from dry to humid atmosphere. These aspects and the impact of the past treatment of the gold surface on its behavior point to irreproducible potential trajectories upon changing atmospheres.19, 31 The humidity induced electrode potential difference ∆ of clean gold was observed to be ranging from 30 – 80 mV (50 mV indicated in Figure 6a)). Surprisingly even more considerable is the determined trend of E for the gold sample modified with 1-decanethiol (SAM). First of all, the initial E is shifted towards lower potentials by about 100 mV compared to the non-modified clean gold surface which is against expectation since the electrical field at the interface Au | SAM is oriented from


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positive to negative34 which should cause an increase of work function and subsequently an shift of E to more positive potentials. The explanation might lie in the presence of the above mentioned contamination of the initial gold surface that is displace by the decanethiol SAM. Moreover, the humidity induced electrode potential difference ∆ for the gold surface modified with 1-decanethiol (SAM) is - against intention - much more pronounced (about 100 mV) compared to the non-modified gold surface. The electrode potential within the wet cycles is lower, hence water molecules obviously adsorb with the oxygen atom pointing towards the positively charged gold surface, thus decreasing the work function. It is suggested that this oriented adsorption of water molecules is a direct consequence of the polarized interface metal | SAM (with defects).


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Figure 6. Electrode potential on clean gold (Au) and SAM of 1-decanethiol on gold (Au-DT) a), on bare aluminum (Al), SAM of octadecylphosphonic acid on aluminum (Al-ODPA) and SAM of octadecyltrimethoxysilane on aluminum (Al-OTS) b) and on bare stainless steel (1.4404), SAM of octadecylphosphonic acid on stainless steel (1.4404-ODPA) and SAM of octadecyltrimethoxysilane on stainless steel (1.4404-OTS) c) monitored by scanning Kelvin


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probe during performing wet (rh ≈ 93 %) and dry air atmosphere (rh ≈ 2 %). The dashed lines indicate the points of switching the atmosphere from dry to wet and vice versa. The inset scales give the typical values for the respective magnitude of humidity induced potential difference ∆ . The data points represent the average potential and standard derivation of surface scans (see Figure 1 b)). The homemade SKP was operated at the resonance frequency of the SKP-tip (diameter 400 µm) of ω = 835 Hz. The results obtained from the most promising system, gold modified with 1-decanethiol (SAM), already indicate that the chosen approach - the use of self-assembled monolayers as a hydrophobic barrier against interfacial adsorption of water molecules - might not be successful. This is confirmed by the results of the measurements on aluminum and modified aluminum surfaces shown in Figure 6 b). The initial E of the bare aluminum surface in wet air is about – 900 mV vs SHE wich for aged aluminium (oxide) corresponds to data reported in the literature7 (note that for freshly polished aluminium values of about -1 V vs SHE and below are reported1 and the ∆ is in the range of 80-100 mV. The greater ∆ for Al compared to Au should arise from the presence of more polar surface groups and thus from a more pronounced susceptibility for oriented water adsorption onto Al surface. The electrode potential of aluminum modified with octadecylphosphonic acid is shifted to more positive potentials by about 200 mV, which now is consistent with the assumption of a polarized interface metal | SAM expected to be oriented from positive (metal) to negative (reactive headgroups within the SAM)35, 36 thus increasing the work function and the related electrode potential. The aluminum modified with octadecyltrimethoxysilane (SAM) is shifted to more positive potentials by another ~380 mV which might be due to the stronger interfacial polarization caused by crosslinked S-O bondings. Likewise the greater standard derivation of the electrode potential for the Al-OTS possibly indicates a more inhomogeneous arrangement


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of the SAM. However, a clear humidity dependent trajectory of E can be seen for both modified Al surfaces, and again, both ∆ are more intense (∆,- ≈ 150 ,

∆,- ≈ 200 ) compared to the bare Al surface (where the change moreover is in the opposite direction). The E within the cycles in humid atmosphere is then lower which reflects the adsorption of water molecules with its oxygen atoms pointing towards the positively charged Al interface. The prompt undulation of E upon chainging the humidity condition is most likely contributed by an associated pressure drop within the SKP chamber, and the fact that this homemade SKP system is working just precisely in resonance-mode, and hence must be handled as artefacts. Moreover it is of course also affected by adsorbed water. The latter contribution to prompt undulation of E upon changed humidity conditions explains why this effect is less pronounced for all modified material systems. In this case the interfacial water desorption is retarded. The E characteristics of bare Al, which is partly out of sequence with respect to the trend upon wet/dry changes, could be caused by transitions from aluminumoxide to -hydroxide and the other way around thus changed polarity conditions. The results obtained for stainless steel samples also attest, as now expected, the impracticality of SAMs used as hydrophobic barriers against interfacial water adsorption (Figure 6 c)). It is remarkable that even the bare stainless steel sample shows a strong humidity-dependent potential behavior (∆,".$$%$ ≈ 150 ), but anyway the respective electrode potential measured in humid (~100 mV vs SHE) as well as dry (~200 mV vs SHE) atmosphere match quite good with data of comparable systems reported in literature2. It has to be mentioned that this large ∆,".$$%$ was not detected for every measurement but it was always in the range of 75 - 100 mV. The shift of E to more positive values upon modification with ODPA is similar to that one observed for aluminum. Contrary to that, the modification


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of ss316L with OTS (1.4404-OTS) led to a less intense bonding at the interface 316L | SAM as this is reflected by the smaller anodic shift of E, with respect to the same substrate modified with ODPA and especially to the Al sample modified with OTS as well. This may be confirmed by the significantly greater standard deviation within the surface potential distribution of scans and might additionally also indicate a misalignment of the alkyl chains and/or the presence of multilayers37.

In the following once more the effect of hydrophobic barriers against interfacial adsorption of water will be analyzed, but this time a high polarity at the 1.4404 | hydrophobic coating interface should be avoided. For this the surfaces of the substrates were modified with less polar molecules: thin layers of paraffin were deposited onto the sample (stainless steel 1.4404) and onto the SKP-tip, either by spin coating or dip coating, respectively. It can be taken from Figure 7 a) that this approach is effective since the ∆ is now reduced to a remarkable value of approx. 40 mV upon humidity conditions changing from ~ 2 %-rh to ~ 93 %-rh. The still remaining ∆ (involving both the coated sample and SKP-tip) is most likely based at least partly due to the resonance working mode of the used homemade SKP setup (see also Figure 7 b)). This homemade SKP setup gives very precise values if its running in resonance mode (the oscillation frequency is equal to the resonance frequency of the SKP-tip). The change of gas flow from humid air to dry air is linked to a pressure drop within the SKP chamber, which disturbs the oscillation frequency of the SKP-tip. This changing of gas flow is indicated in Figure 7 b) by the temperature in the humid gas line (solid red curve) – the temperature increases while the humid gas flow through the heated humidity system is active and it decreases again after switching it off. However, the temperature within the SKP chamber is almost constant. The moments of switching the gas


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flow through the humidity system correlate with the measured electrode potential E (indicated with the dotted parables). Especially freshly prepared paraffin coatings on the SKP-tip generates an electrostatic charging which dissipates after some time (see higher electrode potential at the beginning of measurements in Figure 7).

Figure 7. Electrode potential monitored by scanning Kelvin probe during performing wet (rh ≈ 93 %) and dry (rh ≈ 2 %) cycles in air of stainless steel (1.4404) sample covered with a thin Paraffin coating measured with SKP tip coated with Paraffin as well a), and KP point measurement of a Cu/Cu2+ reference electrode in humid atmosphere (average rh = 91 %) with a small tolerance level (rh ± 0.1%) resulting in switching on and off the humid air line regularly as indicated by the temperature measured in this gas line (solid red line). The electrode potential correlates with this switching points as pointed out by the inserted dotted parables b). The homemade SKP was operated at the resonance frequency of the SKP-tip (diameter 400 µm, coated with paraffin) of ω = 832 Hz. Electrode potential stability of metals chosen as SKP-tip material

As mentioned before, the ∆ of bare stainless steel 1.4404 (316L) was found to vary in the range of 75 – 150 mV (see most pronounced case displayed in Figure 6 c)). This finding implies the assumption, that firstly Ni80/Cr20 might not be the ideal SKP-tip material, and


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secondly that the dipole moment structure of the formed oxide changes with time. To unravel the contributions of the metals involved the most in forming the inner and outer oxide layer38, nickel, chromium and stainless steel 1.4404 were measured separated. The metals were measured freshly polished and again after storage at 97 %-rh for 12 days. The measurements were performed in dry (rh < 0.02 %) and humid (rh ≈ 95 %) atmosphere with a commercial SKP system equipped with a paraffin coated Ni/Cr (80/20) SKP-tip and the results are displayed in Figure 8.

Figure 8. Electrode potential monitored by commercial SKP with height regulation system during performing wet (rh ≈ 95 %) and dry (rh ≈ 0.02 %) cycles in air of freshly polished/prepared stainless steel (1.4404), nickel (99.5 %) and chromium (500 nm by PVD onto glass) samples a), and after storage of the samples for 12 days at rh = 97-% b). The


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measurements were carried out with a paraffin coated Ni/Cr SKP-tip (100 µm) at an oscillation frequency of 934 Hz. The initial electrode potential E of freshly prepared Ni, Cr and 1.4404 (Figure 8 a)) are lower compared to the aged ones (Figure 8 b)). That indicates a change of oxide structure and/or thickness. This explains the mentioned different magnitude of ∆ observed for 1.4404, which might furthermore be influenced by the grinding depth applied during sample preparation. The latter assumption is also based on the very different behavior observed for pure Cr and Ni upon changed humidity conditions. Interestingly, chromium exhibits a very strong ∆ of about 200 mV that is almost independent from the sample history, whereas there is a very small ∆ for freshly polished nickel. On the other hand the ∆ of nickel is not

constant,

but

it

increases

with

time

(∆,&,' ≈ 10-15 mV,

∆,&,"( )*@,- % ≈ 50 mV). Additionally, the electrode potential of freshly prepared nickel shifts by about 100 mV to a value of about 200 mV vs. SHE after ~ 120 hours measurement time (data not shown in Figure 8 a)). An increase of ∆ after storage at high humidity for 12 days is also present for stainless steel (∆,".$$%$,' ≈ 75 mV,

∆,".$$%$,"( )*@,- % ≈ 100 mV). Conclusively, the data presented in Figure 8 clearly demonstrates, that the potential difference upon changed humidity conditions of stainless steel 1.4404 mainly origins from its chromium content. Accordingly, it can be concluded that nickel as SKP-tip material may result in a more stable SKP-signal under cycling atmospheric conditions, although it cannot replace reference electrodes for calibration purposes under varying atmospheric conditions. The electrode potential stability of nickel oxide surface is also confirmed by other researcher, as nickel was introduced as suitable reference material by Frankel et al. for SKPFM measurements39.


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Validation measurements with reference electrodes usable in dry atmosphere

For validation issues a commercial SKP system working in non-resonance mode with a Ni/Cr SKP-tip (99.2 µm diameter) was used to investigate the stability of different types of reference systems (based on Cu/Cu2+ and Ag/Ag+ redox couple) in humid air (rh ≈ 95 %), dry air (rh < 0.02 %) and dry nitrogen (rh < 0.02 %, O2 < 0.01 %) atmosphere. Initially the SKP was commonly calibrated against a Cu/Cu2+ reference electrode in humid air (rh ≈ 95 %). Then the 3 reference electrodes, Cu/CuSO4, Ag/AgCl/KCl (referring to

18

) and

Ag/AgCl/KCl/Paraffin were measured alternatingly in a long-term measurement while changing the atmospheric conditions. First the measurements were realized with a bare SKP-tip (Figure 9 a)-c)), then the same samples were measured at the same locations with a paraffin coated SKP-tip (Figure 9 d)-e)).It is practically not possible to measure the Cu/Cu2+ reference electrode in dry atmosphere, so this reference electrode could just be measured in humid air. For the measurements with the bare SKP-tip (Figure 9 a)-c)) there is a huge dip of E present for both Ag/Ag+ reference electrodes upon suddenly switching from humid to dry atmosphere (Figure 9 b)). The progressions of these potential dips are similar to the ones observed for some other systems, e.g. bare aluminum surfaces analyzed with a bare SKP-tip (Figure 6 b)). However, it is not well understood yet for this system, where exactly this phenomenon origins from. It could origin from the insulating paraffin coating onto the Ag/Ag+ reference that might cause electrostatic charging upon fast change of the atmosphere (and the correlated larger gas flow, possibly causing the charging). On the aluminum samples (Figure 6 b)) the insulating oxide layer might play a role here. Note, that the samples were measured alternatingly within one experiment and electrostatic charging is a long-ranged force, so for this reason the SKP-tip might be also affected even if it is placed in the vicinity of electrostatic charging. Furthermore electrostatic charges require relatively much time to


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dissipate and cannot be compensated by the SKP backing potential.40 However, the potentials measured for the Ag/Ag+ based reference electrodes with SKP in humid air are in quite good agreement with the expected value of E(Ag/AgCl(sat.

KCl))

= 0.199 V vs SHE known from

commercial bulk solution electrochemistry. The potential of the Ag/AgCl/KCl reference electrode starts to fluctuate strongly after about 45 hours exposure to dry atmosphere (Figure 9 a), starting from t > 65 h) which might be due to structural changes. It is obvious that a SKP-tip designed of this material system would undergo the same disturbance. For the Ag/AgCl/KCl/Paraffin reference electrode this fluctuation sets in approx. 10 hours later compared to the uncoated one and it is much less pronounced, i.e. the paraffin coating provides a certain protection against such a degradation. Astonishingly, the stable electrode potential within dry air atmosphere (~35 < t < 60) is greater for the paraffin coated Ag/Ag+ reference electrode (i.e. Ag coverd by a precipitated AgCl film, see experimental part) compared to the non-coated one. So, the ∆ is approx. 120 mV and 45 mV for Ag/AgCl/KCl/Paraffin and Ag/AgCl/KCl, respectively. Also this huge ∆ for Ag/AgCl/KCl/Paraffin might be due to electrostatic charges at the paraffin surface, that are more persistent in dry atmosphere, in combination with the small tip to sample distance set for the 100 µm tip (see 41). However, the potentials approximate to a final difference of about 25 mV after long term exposure in this dry environment. An only very small O2-sensitivity was detected in dry atmosphere upon changing the atmosphere from air to N2 for both reference electrodes, whereby it is negligible after initial small dip of E for the paraffin coated Ag/Ag+ and in the order of approx. 3-5 mV for the uncoated one (Figure 9 c)). Ehahoun et al. also measured an O2-sensitivity of approx. 3 mV on HOPG with an Ag/AgCl/KCl SKP-tip in humid atmosphere (typ. 85-95 %-rh)18, which indicates the presence of an ultrathin water layer at the AgCl/KCl(crystal) surface even in dry atmosphere, that finally grants the formation of sat. KCl solution onto the Ag/Ag+ substrate.


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Figure 9. Electrode potential of different reference electrodes monitored by a commercial scanning Kelvin probe during changing the atmospheric conditions from humid air (rh ≈ 95 %) to dry air (rh < 0.02 %) and to dry N2 (rh < 0.02 %, O2 < 0.01%). The measurements were carried out either with a bare Ni/Cr SKP-tip (a-c) or with a paraffin coated SKP-tip (d-f) at an oscillation frequency of 934 Hz. The same samples were measured afterwards also with the same SKP-tip, which was now covered with a thin paraffin coating (Figure 9 d)-f)). The electrode potential of the copper


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reference electrode in humid air is very similar to the value measured with the bare SKP-tip under the same atmospheric conditions, which clearly confirms no negative influence of the paraffin coating onto the SKP-tip on the SKP measurement routine itself – the signal stability is obviously even slightly improved for the measurement with the paraffin coated SKP-tip. The electrode potentials of the bare Ag/AgCl/KCl reference electrode again starts to fluctuate after about 5 h exposure to humid air (Figure 9 d)), which indicates structural changes at the interface induced by the humidity transition from dry to humid atmosphere (Note, the samples were exposed to dry atmosphere before for about 100 h during the last periods shown in Figure 9 a)). However, the potential stabilizes again at t ≈ 15 h (Figure 9 e)). This fluctuation is not present for the paraffin coated Ag/AgCl/KCl reference electrode, demonstrating again a certain protection for the interface, i.e. it is more robust. The reaction of the Ag/Ag+ based reference electrodes upon drastically changed humidity conditions is of utmost interest: now with the paraffin coated SKP-tip the initial dip of E is clearly reduced and of similar magnitude for both samples, indicating that the coating of the tip clearly has a beneficial effect. The time needed to stabilize to a constant potential is also reduced to about 2.5 hours and the stable potentials of both electrodes are quite similar. The ∆ is approx. 61 mV and 21 mV for the Ag/AgCl/KCl and Ag/AgCl/KCl/Paraffin reference electrode, respectively. Both reference electrodes exhibit a negligible response to oxygen removal (Figure 9 f)). As an important remark it should be mentioned, that both Ag/Ag+ based reference electrodes were fabricated about 2 months before the measurements shown in Figure 9 were carried out, thus the reference electrodes do not suffer fast ageing processes that strongly affects the electrode potential in an unpredictable manner.

4. Conclusion


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A reliable and quick method for fabricating plane-ended Ni/Cr(80/20) SKP-tips with a desired diameter was demonstrated. It was found that the use of SAMs with hydrophobic alkyl chains is not suitable to provide humidity-insensitive SKP tips. Instead, a metal modified with a SAM shows a stronger dependency of the electrode potential measured with the SKP technique on humidity. This is suggested to be due water molecules diffusing from the gas phase to the polarized interface and then adsorbing with a preferred orientation of their dipole moment in the oriented dipole layer at the interface metal | SAM. Defects in the SAM, such as at domain boundaries caused by misalignment of the alkyl chains within the SAM, may play a role in determining the extent of oriented water adsorption at the interface. The use of hydrophobic coatings without a chemical bonding to the substrate, hence avoiding the formation of a charged interface, was found to be an effective and easy to realize approach for the preparation of humidity-insensitive SKP-tips. Further developments of the hydrophobic coating, e.g. decrease of thickness and/or choice of material, will be carried out to improve the signal stability upon drastic changes of the surrounding atmosphere conditions. Besides, the preparation of robust reference electrodes (especially the Ag/AgCl/KCl/paraffin system) applicable also in dry environment was evaluated. Additionally, pure nickel used as SKP-tip material was identified to be a promising candidate for a more stable Kelvin probe signal irrespective of the atmospheric conditions. The stability of SKP-tip and reference electrodes is of special importance for various SKP applications, especially for those that are carried out in various atmospheres, e.g. hydrogen permeation measurements that has to be performed in dry nitrogen atmosphere or determination of oxygen reduction reaction at buried interfaces using Kelvin probes. Another important application is the use in mobile field Kelvin probes (FKP).11

AUTHOR INFORMATION


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*Corresponding author. Tel.: +49 211 6792 442; fax: +49 211 6792 218 E-mail address: [email protected] (PD Dr. Michael Rohwerder) Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by the Max Planck Society. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Mr. Bernd Schönberger and Mr. Ivan Gonzales are acknowledged for their contributions during the re-activation of the homemade scanning Kelvin probe system and the modification of the SKP software with respect to logged humidity data in tandem with the potential data. Dr. Sergiy Merzlikin is acknowledged for fitting the XPS data, Mr. Eberhard Heinen for doing the PVD and Mr. Mariusz Wicinski for sharing his strong experience with SKP systems. REFERENCES 1. Hausbrand, R.; Stratmann, M.; Rohwerder, M. The physical meaning of electrode potentials at metal surfaces and polymer/metal interfaces: Consequences for delamination. J Electrochem Soc 2008, 155 (7), C369-C379. 2. Nazarov, A. P.; Thierry, D. Scanning Kelvin probe study of metal/polymer interfaces. Electrochim Acta 2004, 49 (17-18), 2955-2964. 3. Huang, S. M.; Atanasoski, R. T.; Oriani, R. A. Detection of Effects of Low ElectricFields with the Kelvin Probe. J Electrochem Soc 1993, 140 (4), 1065-1067. 4. Stratmann, M. The Investigation of the Corrosion Properties of Metals, Covered with Adsorbed Electrolyte Layers - a New Experimental-Technique. Corros Sci 1987, 27 (8), 869872. 5. Baikie, I. D.; Estrup, P. J. Low cost PC based scanning Kelvin probe. Rev Sci Instrum 1998, 69 (11), 3902-3907. 6. Burgstaller, W.; Schimo, G.; Walkner, S.; Hassel, A. W. Scanning Kelvin Probe System and Applications. Xxxiv. Moderni Elektrochemicke Metody 2014, 28-30.


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7. Senoz, C.; Borodin, S.; Stratmann, M.; Rohwerder, M. In situ detection of differences in the electrochemical activity of Al2Cu IMPs and investigation of their effect on FFC by scanning Kelvin probe force microscopy. Corros Sci 2012, 58, 307-314. 8. Salgin, B.; Hamou, R. F.; Rohwerder, M. Monitoring surface ion mobility on aluminum oxide: Effect of chemical pretreatments. Electrochim Acta 2013, 110, 526-533. 9. Evers, S.; Senoz, C.; Rohwerder, M. Hydrogen detection in metals: a review and introduction of a Kelvin probe approach. Sci Technol Adv Mat 2013, 14 (1). 10. Schaller, R. F.; Scully, J. R. Spatial determination of diffusible hydrogen concentrations proximate to pits in a Fe-Cr-Ni-Mo steel using the Scanning Kelvin Probe. Electrochem Commun 2016, 63, 5-9. 11. Christian Michelson Research. http://cmr.no/projects/10425/field-kelvin-probe-fkp/. (accessed 12.07.2017). 12. Leng, A.; Streckel, H.; Stratmann, M. The delamination of polymeric coatings from steel. Part 1. Calibration of the Kelvinprobe and basic delamination mechanism. Corros Sci 1999, 41 (3), 547-578. 13. Cook, A. B.; Barrett, Z.; Lyon, S. B.; McMurray, H. N.; Walton, J.; Williams, G. Calibration of the scanning Kelvin probe force microscope under controlled environmental conditions. Electrochim Acta 2012, 66, 100-105. 14. Sababi, M.; Terryn, H.; Mol, J. M. C. The influence of a Zr-based conversion treatment on interfacial bonding strength and stability of epoxy coated carbon steel. Prog Org Coat 2017, 105, 29-36. 15. Burgstaller, W.; Schimo, G.; Hassel, A. W. Challenges in hydrogen quantification using Kelvin probe technique at different levels of relative humidity. J Solid State Electr 2017, 21 (6), 1785-1796. 16. Ozkanat, O.; Salgin, B.; Rohwerder, M.; Mol, J. M. C.; de Wit, H.; Terryn, H. Scanning Kelvin Probe Study of (Oxyhydr)oxide Surface of Aluminum Alloy. J Phys Chem C 2012, 116 (2), 1805-1811. 17. Nazarov, A.; Thierry, D.; Prosek, T. Formation of Galvanic Cells and Localized Corrosion of Zinc and Zinc Alloys Under Atmospheric Conditions. Corrosion 2017, 73 (1), 77-86. 18. Ehahoun, H.; Stratmann, M.; Rohwerder, M. Ag/AgCl/KC1 micro-electrodes as O-2insensitive reference tips for dynamic scanning Kelvin probe measurement. Electrochim Acta 2005, 50 (13), 2667-2674. 19. Atanasoski, R. T.; Huang, S. M.; Albani, O.; Oriani, R. A. A Dry Redox Couple Employed as the Reference Material in Work Function and Corrosion Potential Measurements by the Kelvin Technique. Corros Sci 1994, 36 (9), 1513-1521. 20. Evers, S.; Rohwerder, M. The hydrogen electrode in the "dry": A Kelvin probe approach to measuring hydrogen in metals. Electrochem Commun 2012, 24, 85-88. 21. Schimo, G.; Burgstaller, W.; Hassel, A. W. Rolling Direction Dependent Diffusion Coefficients of Hydrogen in Ferritic Steel by SDCM Charging and SKP Probing. Isij Int 2016, 56 (3), 487-491. 22. Vijayshankar, D.; Altin, A.; Merola, C.; Bashir, A.; Heinen, E.; Rohwerder, M. Probing the Buried Metal-Organic Coating Interfacial Reaction Kinetic Mechanisms by a Hydrogen Permeation Based Potentiometric Approach. J Electrochem Soc 2016, 163 (13), C778-C783. 23. Greenspan, L. Humidity Fixed-Points of Binary Saturated Aqueous-Solutions. J Res Nbs a Phys Ch 1977, 81 (1), 89-96. 24. Yan, C.; Golzhauser, A.; Grunze, M.; Woll, C. Formation of alkanethiolate selfassembled monolayers on oxidized gold surfaces. Langmuir 1999, 15 (7), 2414-2419. 25. Ulman, A. Ultrathin Organic Films; Academic Press: Boston San Diego, 1991.


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26. Bethencourt, M. I.; Srisombat, L. O.; Chinwangso, P.; Lee, T. R. SAMs on Gold Derived from the Direct Adsorption of Alkanethioacetates Are Inferior to Those Derived from the Direct Adsorption of Alkanethiols. Langmuir 2009, 25 (3), 1265-1271. 27. Vago, E. R.; Deweldige, K.; Rohwerder, M.; Stratmann, M. Electroreduction of Oxygen on Octadecylmercaptan Self-Assembled Monolayers. Fresen J Anal Chem 1995, 353 (3-4), 316-319. 28. Sumi, T.; Wano, H.; Uosaki, K. Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer of decanethiol on the Au(111) surface in KOH plus ethanol solution. J Electroanal Chem 2003, 550, 321-325. 29. Muglali, M. I.; Erbe, A.; Chen, Y.; Barth, C.; Koelsch, P.; Rohwerder, M. Modulation of electrochemical hydrogen evolution rate by araliphatic thiol monolayers on gold. Electrochim Acta 2013, 90, 17-26. 30. Senoz, C.; Evers, S.; Stratmann, M.; Rohwerder, M. Scanning Kelvin Probe as a highly sensitive tool for detecting hydrogen permeation with high local resolution. Electrochem Commun 2011, 13 (12), 1542-1545. 31. Holloway, P. H. Gold/chromium metallizations for electronic devices. Gold Bulletin 1979, 12 (3), 99-106. 32. Haiss, W.; Lackey, D.; Sass, J. K.; Besocke, K. H. Atomic resolution scanning tunneling microscopy images of Au(111) surfaces in air and polar organic solvents. The Journal of Chemical Physics 1991, 95 (3), 2193-2196. 33. Rohwerder, M.; de Weldige, K.; Stratmann, M. Potential dependence of the kinetics of thiol self-organization on Au(111). J Solid State Electr 1998, 2 (2), 88-93. 34. Whitesides, G. M.; Laibinis, P. E. Wet Chemical Approaches to the Characterization of Organic-Surfaces - Self-Assembled Monolayers, Wetting, and the Physical OrganicChemistry of the Solid Liquid Interface. Langmuir 1990, 6 (1), 87-96. 35. Maege, I.; Jaehne, E.; Henke, A.; Adler, H. J. P.; Bram, C.; Jung, C.; Stratmann, M. Self-assembling adhesion promoters for corrosion resistant metal polymer interfaces. Prog Org Coat 1998, 34 (1-4), 1-12. 36. Thissen, P.; Valtiner, M.; Grundmeier, G. Stability of Phosphonic Acid SelfAssembled Monolayers on Amorphous and Single-Crystalline Aluminum Oxide Surfaces in Aqueous Solution. Langmuir 2010, 26 (1), 156-164. 37. Houssiau, L.; Bertrand, P. TOF-SIMS study of organosilane adsorption on model hydroxyl terminated surfaces. Appl Surf Sci 2003, 203, 580-585. 38. Ohmi, T.; Nakagawa, Y.; Nakamura, M.; Ohki, A.; Koyama, T. Formation of chromium oxide on 316L austenitic stainless steel. J Vac Sci Technol A 1996, 14 (4), 25052510. 39. Guillaumin, V.; Schmutz, P.; Frankel, G. S. Characterization of corrosion interfaces by the scanning Kelvin probe force microscopy technique. J Electrochem Soc 2001, 148 (5), B163-B173. 40. Frankel, G. S.; Stratmann, M.; Rohwerder, M.; Michalik, A.; Maier, B.; Dora, J.; Wicinski, M. Potential control under thin aqueous layers using a Kelvin Probe. Corros Sci 2007, 49 (4), 2021-2036. 41. Wicinski, M.; Burgstaller, W.; Hassel, A. W. Lateral resolution in scanning Kelvin probe microscopy. Corros Sci 2016, 104, 1-8.


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Fabrication of Robust Reference Tips and Reference Electrodes for

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