The Electrochemical Surface Forces Apparatus: The Effect of

Article pubs.acs.org/Langmuir

The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions Markus Valtiner,†,∥ Xavier Banquy,† Kai Kristiansen,† George W. Greene,‡ and Jacob N. Israelachvili*,† †

Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5080, United States Institute of Frontier Materials, Deakin University, 221 Burwood Hwy, Burwood, 3125, Australia

‡

ABSTRACT: We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with Å resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force−distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with Å-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal−ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg−Healy− Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force−distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)3 phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.

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INTRODUCTION The ability to control surface charges and electrochemical surface potentials can provide an effective lever for the design of targeted properties of interfacial structures and dynamics, as well as a means to moderate interfacial chemical reactions. For instance, controlling the surface charge and the related electric double layer structure can be utilized to modulate colloidal stability.1,2 Furthermore, the control of surface charges and the ion distributions across biological interfaces can steer, mediate, and/or activate complex biological processes such as ion and molecular transport across membranes, membrane and bacterial adhesion,3−6 biofouling,7 joint lubrication and friction, as well as drug-delivery systems.8 Likewise, the electrochemical surface potential can influence properties and function of devices such as batteries,9,10 microelectromechanical (MEMS) systems,11 microfluidic devices,12,13 as well as meso-structured14 and tribological materials.15−18 For example, the tribological behavior of water-based lubrication systems19 and electrowetting phenomena20 depend on surface © 2012 American Chemical Society

potentials. Electrochemical surface potentials and interfacial charge distributions influence technologically important processes such as electrochemical and electrocatalytic synthesis,21,22 and conversely, corrosive degradation,23,24 dissolution of inorganic surfaces,25 and the passivation of metals and alloys.26,27 Both anodic oxide growth and metal dissolution pose a critical hurdle that limits lifetime and performance of catalytic nanoparticles28,29 and nuclear waste storage containers.30 Consequently, measurement and control of interfacial charge distributions and chemical reactions at electrode interfaces play a critical role in advancing toward higher performance materials, devices, and tribological systems. One unique path to controlling and manipulating both interaction forces and interfacial electrochemical reactions at material interfaces is through controlling and measuring the electrochemical surface Received: May 3, 2012 Revised: August 3, 2012 Published: August 9, 2012 13080

dx.doi.org/10.1021/la3018216 | Langmuir 2012, 28, 13080−13093

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Figure 1. Schematic of the electrochemical three-electrode setup used in the EC-SFA. In this setup, we apply an external electrochemical potential to the gold electrode and measure forces and distances between apposing surfaces using the procedure outlined in the experimental section (cf. text for details). Using ΔU = U − UPZC, the applied electrochemical potential, U, is referenced to the so-called potential of zero charge UPZC for which U = UPZC. At UPZC, the effective gold surface potential ΨAu(UPZC), as measured by means of force distance data, is zero. UPZC depends on the surface orientation and roughness.

potentials.41 Additionally, one of the surfaces can be mounted on a 3D sensor-actuator attachment,42,43 which enables independent and simultaneous force measurements and displacements of a surface in three orthogonal directions, allowing one to measure and correlate friction forces and applied electrochemical potentials. For this particular setup, the three-electrode setup is implemented in a miniature bath, allowing EC-SFA measurements in small liquid volumes of about 1−2 mL (which is convenient when using expensive liquids and solutions). The combined capabilities of the EC-SFA to simultaneously measure and control surface potentials, interaction and friction forces, and absolute surface separation distances and visualize the contact topography44 allows for direct investigation of the effects and correlations of electrochemical surface potentials and nanoscale surface roughness, thin film growth and dissolution, as well as the competing long and short-range attractive and repulsive interaction forces, and friction forces. Here the EC-SFA is used to measure (1) interaction forces between apposing dissimilar smooth and rough surfaces, (2) the growth of a passivating oxide film on gold, and (3) electroviscous effects at sliding gold−ceramic contacts. The results allow for a deeper understanding of electrochemically polarized surfaces and illuminate the potential applications of the EC-SFA attachments.

potentials and electrochemical currents at confined interfaces. Various electrochemical atomic force microscope31,32 (ECAFM) and electrochemical scanning tunneling microscope33,34 (EC-STM) setups have been designed, and interfacial forces, structures, and reaction dynamics were probed with these scanning probe microscopes (SPM). Hillier et al. provided a comprehensive analysis of the interaction forces between a gold electrode and silica spheres using an EC-AFM.35 Frechette and Vanderlick36 designed the first electrochemical surface forces apparatus setup and studied interfacial forces across mica|gold interfaces. In this report, we describe experiments performed using two newly designed electrochemical surface forces apparatus attachments (EC-SFA) that enable the measurement and control of both interaction forces as well as friction between similar and dissimilar apposing surfaces in situ in an electrolyte (as well as in weakly conducting and dielectric media37). The newly designed EC-SFA attachments have a significantly improved electrochemical performance and stability and are simpler to implement compared to the previous design. The EC-SFA designs consist of a three-electrode setup, using a Pt-counter electrode (CE), both atomically smooth38−40 and rough metal working electrodes (in this case Au electrodes were used), and a specif ically designed Ag|AgCl (in 3 N KCl) microreference electrode that provides stable and accurate reference 13081


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Figure 2. Typical cyclic voltamogram (CV) of a typical gold-electrode recorded with a linear sweep rate of 50 mV/s in 1 mM HNO3 at pH =3 using the newly developed setup shown in Figure 1.

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(2) atomically smooth back-silvered mica modified with a molecularly smooth self-assembly monolayer of (3-aminopropyl)-triethylsilane (APTES) deposited by means of vapor deposition. Both surfaces are atomically smooth and have a rms (root mean square) roughnesses σrms ≤ 3 Å. The thickness of the vapor deposited APTES layer is typically about 3 Å as measured with the SFA. APTES was chosen mainly because it belongs to a family of end functionalized reactive silanes commonly used for self-assembly on various substrates (such as nanoparticles, colloidal particles, steel sheets, etc.), where they function as adhesion promotors or stabilizers. At pH values below the amine pKA (pKA = 9.6), the amine head groups are fully protonated, giving rise to a positively charged surface with an effective surface potential of 55 mV that remains constant during experiments.45 AFM topography scans were done in tapping mode using an MFP-3D atomic force microscope from Asylum Research.

METHODS AND MATERIALS

EC-SFA and Interaction Force Measurements. The electrochemical three-electrode attachment consists of the gold working electrodes (WE), a platinum counter electrode (CE), as well as an Ag| AgCl (3 N KCl) microreference electrode,41 to which the external potential, U, is referenced during the experiment. The electrochemical potential was controlled using a Gamry potentiostat (Reference 600 Series). The two newly designed experimental EC-SFA setups are described in detail below. All force measurements were performed in aqueous HNO3 solutions as previously described using an SFA-2000 setup42 with the interacting surfaces in crossed-cylinder geometry, which is equivalent to a sphere-on-flat geometry. Measurements were done at 21 °C. White light multiple beam interferometry allows for the measurement of the separation distance between the two surfaces (with 0.1 nm resolution) and the surface geometry (with micrometer resolution) using the so-called fringes of equal chromatic order. For details on the SFA 2000 setup, the reader is referred to our earlier work.41 3D Sensor-Actuator and Friction Measurements. The 3D sensor-actuator attachment to the SFA2000 can simultaneously and independently drive the upper surface and measure forces on this surface in all three orthogonal spatial directions,42,43 as well as employ the multiple beam interferometry as described above. Two piezostacks push each a lever system that drives the stage of the sample holder in-plane motion. The lever system contains a couple of leaf springs that secure a decoupled motion in either the X- or Y-direction. Both the measured off-axis signal when applying a load on the sensor and the displacement orthogonal to the applied actuator direction are less than 5%. The procedure of normal displacement is the same as that for regular SFA. The distance control is in the ∼0.1 nm range in the normal direction and μm range in the plane of the surfaces. Four spring rods with strain gauges detect the in-plane motion, and load springs detect the normal force. The friction forces measuring springs on the 3D sensor-actuator have a lateral spring constant of 350 N/m with a force resolution in the nN range. Chemical and Materials. Solutions were prepared from titration standards for HNO3 (0.2 mol/L). Before experiment, solutions were filtered and deaerated by refluxing the solution for at least 2 h in argon atmosphere. Water used for the experiments was deionized using a Milli-Q filtration system. We used three distinctly different goldelectrode surface preparations: atomically smooth gold films (Au-1) were prepared by template stripping from an atomically smooth mica surface;38,40 rough electrode surfaces were prepared by PVD depositing first a 2 nm thick titanium adhesion promoting layer followed by a 45 nm thick gold film (Au-2). Further roughening of a PVD gold film surface was achieved electrochemically by applying oxidation−reduction cyclic voltamograms from −200 mV vs Ag|AgCl to +1400 mV vs Ag|AgCl at a high rate of 3 V/s (Au-3). As apposing surfaces, we used both (1) atomically smooth back-silvered mica and

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THE ELECTROCHEMICAL SFA SETUP Figure 1 shows a schematic of the electrochemical attachment that was designed for the SFA 2000.42 The attachment provides a three-electrode setup consisting of: (1) a working electrode (WE)-that defines the studied interface; (2) a counter electrode (CE)-that supplies the electrochemical current required by the WE; (3) a reference electrode (RE) that maintains at a constant reference potential. During experiments, the applied electrochemical potential U of the working electrode (WE) is both measured and controlled with respect to the Ag|AgCl reference electrode (RE) using a potentiostat. The in-house prepared Ag| AgCl reference microelectrode contains a silver wire prepared with an electrochemically deposited layer of AgCl. The Ag|AgCl wire is placed inside a tubing that was filled with solidified (3% agarosis) 3 N KCl solution.41 This tubing was then glued into a so-called “Luggin-capillary”, which provides a sensing point as close as possible to the WE in order to minimize any ohmic drop caused by the solution resistance. An additional Teflon tube was glued into the Luggin-capillary in order to allow withdrawing of electrolyte through the capillary to avoid bubbles. Figure 2 shows a typical cyclic voltamogram (CV) measured with the EC-SFA attachment. For measuring a CV, a potential sweep is imposed on the working electrode. The applied potential sweep is a triangular wave between chosen negative and positive limits. The current that passes in response to the potential sweep is recorded and displayed as shown in Figure 2. This particular CV was measured with a sweep-rate of 50 mV/s. The upper potential axis shows the applied potential with reference to UNHE, the “normal hydrogen electrode” (according 13082


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Figure 3. AFM scans showing the surface topography of the three different gold electrodes used in the EC-SFA experiments: (Au 1) Atomically smooth gold surface prepared by templating from mica;38−40 (Au 2) moderately rough gold surfaces prepared by PVD deposition; and (Au 3) electrochemically roughened gold surfaces. Also shown are the line traces and rms-roughness σrms obtained from the AFM data. The indicated surface labels for the respective surfaces are used throughout the text.

to UNHE = U + 197 mV), which is a common practice in electrochemistry. When interpreting and comparing forcemeasurements as a function of the applied electrochemical potential, it is more convenient to use a scale that refers to the potential of zero charge UPZC as defined in Figure 1 (i.e., the electrochemically applied potential were the surface charge on the working electrode is zero). The lower potential axis is shifted accordingly to the potential of zero charge, which varies as a function of the pH, electrolyte composition, surface orientation, as well as surface roughness and is thus different for every gold surface. There are three regions and five peaks indicated in the CV. The position and the sweep rate dependent shape of the peaks in a CV provide an “electrochemical spectrum” of the studied interface. In region A, the current is associated with charging of the electric double layer and may often include specific adsorption of ions onto the electrode surface. The peak (I) is for example interpreted as chemisorption of nitrate and hydroxide ions from the solution onto the surface.46 In region B, surface oxidation processes (Au oxidation in this case) occur and in response a faradaic current (i.e., a current generated by an oxidation/reduction reaction at an electrochemical interface) flows through the interface. Figure 2 shows three oxidation peaks II, III, and IV at 1330, 1400, and 1650 mV measured against the NHE, respectively. These are associated with the formation of a passivating oxide layer on the gold surface, and provide valuable information about the mechanism of the oxide film formation (for an in-depth discussion, see refs 46−48). When reversing the potential, the gold surface reduces again as reflected by the negative current peak (V) at 1025 mV vs NHE. In region C, electrochemical processes such as reduction of solvated protons or solvated traces of oxygen at the electrode surface can be observed. In summary, the possible interfacial processes that can be studied in arbitrary electrolytes using the newly designed ECSFA setup range from in situ variation of the electric double layer charging, to specific ion adsorption, surface oxidation/ reduction, deposition of thin films (a noble metal oxide layer in this case), as well as electrochemical reduction/oxidation of solution species (ranging from oxygen and protons to organic

molecules). Also, distribution, adhesion, and adsorption of nanoparticle additives in aqueous solutions can be studied close to an electrochemically adjustable surface. Thus, the EC-SFA provides a powerful tool to study interfacial interaction forces and distances, while simultaneously interfacial chemistries, compositions, and ion-distributions can be controllably modulated using electrochemical potential control. In this work, we focus on electrochemical potential control (i.e., potentiostatic and potential-step experiments). It is worth mentioning that the EC-SFA also provides the possibility to work under current control; that is, a desired current can be set and a targeted reaction can be studied under such constant current conditions.

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RESULTS AND DISCUSSION The Effect of Surface Roughness and Electrochemical Surface Potentials on Interaction Forces between Dissimilar Surfaces. Using the experimental EC-SFA system shown in Figure 1, we measured quasi-static (i.e., with very slow approach speeds of 1−2 nm/s) force−distance profiles between gold electrode surfaces (WE) with varying degrees of nanoscale roughness and an atomically smooth mica surface modified with a self-assembled (3-aminoproyl)-triethylsilane monolayer (APTES-SAM) in 1.0 mM aqueous solutions of HNO3 at pH = 3. Three different gold working electrodes with distinctly different surface morphologies were used: (Au-1) Atomically smooth gold surface prepared by templating from mica;38−40 (Au-2) rough gold surface prepared by physical vapor deposition (PVD); and a (Au-3) gold surface that was electrochemically roughened by applying several fast oxidation/ reduction cycles (3 V/s). Figure 3 illustrates representative AFM topography scans showing the characteristic morphology of each of the prepared gold surfaces as well as respective height cross sections. The Au-1 surface, which was templated from atomically smooth mica, has virtually no topographic features and appears to be atomically smooth on a large scale (≫10 μm). The Au-2 surface shows smooth grains with diameters ranging from 50 to 100 nm and has an rms-roughness of 12 Å, which is typical for gold surfaces prepared by PVD. The electrochemically 13083


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roughened Au-3 surface shows a similar granular structure; however, the grains are covered with additional smaller asperities that are formed due to the fast oxidation/reduction cycles applied during the electrochemical roughening. The rmsroughness of the Au-3 surface is about 17 Å, which is only slightly higher compared to the Au-2-surface. Nevertheless, the high number density of small asperities significantly alters the surface morphology, as these asperities extend further out and have a higher radius of curvature compared to the smooth grains of the Au-2 surface. Figure 4 shows the force−distance profiles of the three differently rough gold surfaces facing an atomically smooth mica surface coated with a self-assembled APTES monolayer. These force distance profiles display typical data measured with the EC-SFA during approach and under different (positive and negative) externally applied surface potentials for the three electrode surfaces displaying varying degrees of nanoroughness. The contact distance D = 0 in Figure 4 is defined as the distance measured between the gold electrode and the SAM layer in dry N2 under high load and with no externally applied potential. The experimental “pull-off” force profiles measured during separation were discussed extensively in a previous communication.40 Figure 4A shows typical force distance profiles measured during the approach of an atomically smooth gold electrode and an APTES coated molecularly smooth mica surface in 1 mM HNO3 at pH = 3. The approach curves show two distinctly different force regimes: At separation distance, D, ranging from 40 nm to about 4 nm, the data shows a long-range electric double layer force which is attractive for electrochemical potentials below the PZC and repulsive above the PZC. This interfacial force profile is consistent with the surface chemistry at the apposing interfaces. The surface terminating amine head group of the APTES coated mica surface is charged positively at the experimental pH value of pH = 3, while the surface potential of the gold electrode was modulated in situ using the electrochemical setup. If the electrochemical potential of the gold surface is shifted toward negative values (or in electrochemical phrasing: a cathodic potential shift is applied), electrons are transferred into the gold electrode, leading to a negative charging of the interface. Thus, at negative potentials, below the potential of zero charge, PZC, the interfaces are oppositely charged and the resulting electric double layer forces are attractive. Stepping the applied electrochemical potential to more positive values (i.e., toward anodic potentials) by withdrawing electrons from the gold electrode, results in a positive charging and ultimately oxidation of the interface. Consequently, the electric double layer force between the two positively charged apposing surfaces is repulsive at positive applied electrochemical potentials. Figure 5A shows that DLVO theory (i.e., a superposition of electric double layer forces and attractive van der Waals forces) fits the data very well using constant potential boundary conditions for dissimilar interacting surfaces49,50 down to separation distances of about D = 4 nm. At separation distance, D, below 4 nm, the force runs in Figure 4A indicate an additional exponential repulsive force contribution due to hydration forces arising from the confinement of hydrated ions and water between the two apposing surfaces.51,52 Figure 5A shows that the hydration forces can be fitted well using an empirical exponential force law.18,53 The fitting model, equations, and details of the data fitting are discussed below (see theoretical analysis). Interestingly, the

Figure 4. Typical force−distance profiles measured during approach of the apposing gold and APTES coated mica surfaces measured as function of the externally applied electrochemical potentials (ΔU = U − UPZC) for an atomically smooth (Au-1), moderately rough (Au-2), and electrochemically roughened gold electrode surfaces (Au-3) shown in Figure 3. The external applied potentials ΔU are color-coded, and positions of the hard walls are indicated.

hydration forces show a distinct correlation with the applied electrochemical potential as well. At potentials above the PZC, where the apposing surfaces are both positively charged, the hydration forces have an exponential decay with a characteristic decay length, λHYD, of 1.14 nm and the apparent hard wall (i.e., 13084


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Figure 5. Data fitting to the extended DLVO theory: (A) For the atomically smooth Au-1 surfaces, the data can be fitted well using DVLO contributions (electric double layer, EDL, and van der Waals forces, VDW).57 And an additional term for the repulsive exponential hydration forces:52,54 the fitted curve (red) is split up into the individual contributions (black, equation numbers are indicated). (B) For the rough surfaces an additional repulsive exponential force term was introduced according to eq 7, and the onset of the individual force contributions is shifted (cf. text for details). The fitted surface potentials, ΨAu, for all three surfaces are given in Table 1. The typical experimental error of the measured data points is about 0.01 mN/m. The R2 values of the fitting procedure (we use a Levenberg−Marquart algorithm) are typically on the order of 0.94−0.96, indicating a quantitative and predictive quality of our model.

the distance, D, of the closest approach where the interfacial force shows a sharp increase) is shifted out to about 2 nm. The characteristic decay length is smaller compared to typical values measured for cations confined between two negatively charged surfaces.52,54 This is consistent with the interpretation of hydration forces, which are due to the steric confinement of the ions and their hydration shells between apposing surfaces. Typically, the hydration shells of cations are larger and bind water molecules stronger compared to anions, thus giving rise to higher hydration forces with larger characteristic decay length. At potentials below the PZC, the characteristic decay length decreases to λHYD = 0.65 nm and the hard wall shifts inward to about 3−5 Å. At these potentials the apposing surfaces are oppositely charged. The APTES coated mica surface is still positively charged and the electric double layer is characterized by an increased hydrated anion concentration at the interfaces. In contrast, at potentials ΔU below the PZC (or ΔU < 0), the gold surface is charged positive, having an electric double layer made up entirely from protons in the chosen experimental 1 mM HNO3 solution. The fitted decay length is on the order of the size of the confined hydrated anion (i.e., 0.79 nm55), while the protons, which have no hydration shell, do not give rise to a hydration force, which is consistent with the decreased characteristic decay length. Also, the observed inward shift of the hard wall is consistent with fewer confined hydrated ions trapped between the apposing surfaces. The ensuing force profiles shown in Figure 4B and C reveal various aspects of the effect of the nanoscale roughness at the gold electrode. First, for both rough surfaces Au-2 and Au-3, the hard walls are shifted further out by about 2 nm compared to force profiles measured for the smooth Au-1 gold electrode. The shift of the hard walls is on the order of the rms-roughness measured with AFM (see Figure 2). Second, at distances, D, ranging from 40 nm to about 18 nm, the data indicates longrange DLVO forces, which show a similar trend as discussed

above as a function of the applied electrochemical potential. The electric double layer forces switch from attractive to repulsive when the potential is stepped from negative toward positive potentials. The electric double layer forces are also fitted well using EDL theory (see theoretical analysis below). It is noteworthy that the data fitting, as shown in detail for the Au-3 surface in Figure 5B, indicates that the onset of the electric double layer force is similarly shifted outward by about the rms-roughness of the gold surfaces. Moreover, the fitted effective electric double layer potentials obtained for positive gold surface potentials are considerably weaker by about a factor of 2−3, compared to EDL forces measured and fitted for atomically smooth gold electrode surfaces (see Figure 5A). Also, this behavior is directly visible by comparing the magnitude of the EDL forces between D = 40 nm and D = 18 nm in Figure 4. The rough surfaces present a larger adsorption area and thus more favorable adsorption positions for adsorbing anions at these potentials, which in turn lead to stronger screening of the electrode and lower effective surface potentials of the rough surfaces compared to the smooth Au-1 surface (see also ref 40). Barten et al.56 described a similar behavior in colloidal probe AFM measurements between silica spheres and gold electrodes, where they found effective surface potentials in the range from +40 mV to −40 mV. They ascribed the reason for lower surface potentials “compared to the local diffuse double layer potential” to the surface roughness of the colloidal probe, which is typically on the order of σrms = 1−2 nm and comparable to rms roughness observed in this work. Third, at distances D between 18 nm and the hard wall distance at 2−6 nm, the ensuing force profiles measured for the nanoscale rough Au-2 and Au-3 gold electrodes show an additional, exponentially repulsive force component, which is superimposed on the force profiles, leading to significant deviation from DLVO theory at much greater distances (D ≤ 18 nm) compared to the smooth Au-1-surface (D ≤ 4 nm). We measured multiple force runs at the same position (typically 13085


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three force runs with no waiting time between them) and find that the repulsive steric forces on gold appear elastic up to the maximum applied compression forces (F/Rmax = 15 mN/m) as observed by the perfect superimposition of the force runs measured consecutively. Only for the electrochemically roughened surfaces (Au-3), the very first approach at a given contact is significantly different from the subsequent ones due to signs of plastic deformations. After the first approach, no further plastic deformations were measured (regardless of the applied potential). Thus, this additional repulsive force is due to an additional steric repulsion force arising from the elastic deformation of the outermost asperities at the rough electrode surfaces. These asperities can extend out to up to 10−15 nm.40 As displayed in Figure 5B, the effect of this additional steric repulsion can be fitted well by introducing an additional repulsive exponential force term with a characteristic decay length of 2−3 times the rms-roughness σrms. A similar empirical fitting equation works very well for describing interaction forces between rough polymer surfaces in dry air and oil.53 Figures 5 and 6 show how the experimental data are fitted to an extended

the following, we will give all relevant equations in detail and show experimental data with theoretical fits for force distance curves obtained with gold electrode surfaces of varying degree of nanoscale roughness. van der Waals Forces. The nonretarded VDW interaction between dissimilar surfaces 1 and 1′ with an adsorbed layer 2′ of thickness T′ and a roughness layer 2 of thickness T across a medium 3 can be approximated by the expression:57−59 E VDW (D) = − +

A1 ′ 31A32 ′ 3 A1 ′ 21A323 1 ⎡ A 232 ′ ⎢ 2 − − 2 12π ⎢⎣ D (D + T ) (D + T ′)2 A1 ′ 2 ′ 1 ′A121 ⎤ ⎥ (D + T + T ′)2 ⎥⎦

(1)

where Axyz represents the Hamaker constant for the interaction of material x and material z across the medium y. Thus, the effective Hamaker constant should vary with the separation distance D. For symmetrical systems, this force is always attractive, but for dissimilar surfaces, as in our experiments, it can become repulsive or attractive, depending on the properties of the interacting media. At small separation distances, the equation for the VDW force should tend toward A232’ which in the system in this study is the Hamaker constant for the interaction of the roughness layer and the thin film. At large separation distance the equation for the VDW force should asymptote toward A1’31 which is the Hamaker constant for the two interacting semi-infinite bulk materials. This description fits the rule of thumb on VDW force, that this force is dominated by the bulk properties at large separations (i.e., at large separation the effective Hamaker constant is A1’31) and by the properties (i.e., effective Hamaker constant A232’) of the adlayers at separation distances smaller than the thickness of these adlayers. To employ the full VDW force function as shown in eq 1, the knowledge and determination of all relevant Hamaker constants is necessary, which for real systems can often be very complicated and even erroneous.57,58 To overcome this complication, one simple and elegant approximation is to simply shift the onset of the VDW interaction inward by a distance D0 on the order of the roughness (or thickness of ad layers) of the electrode surface σrms. This will result in a monotonic decrease of the effective VDW force toward the expected value at close separations. The idea to shift the plane of origin was first suggested by Czarnecki et al.,60 following this argument the VDW force for the interaction across crossed cylinders with a radius of curvature R is then given by the following approximate equation:

Figure 6. Comparison of force−distance curves in approach and theoretical fit according to eq 8 for the electrochemically roughened (Au-3) surfaces. Note that the scale on the force-axis is logarithmic above zero. The theoretical model accurately fits the data and can reproduce the main features of the force runs. For these fitting the EDL onset was shifted out by the rms-roughness and the VDW force was shifted inward to simulate a decreasing effective Hamaker constant. The fitted effective gold surface potentials are indicated (see also Table 1) and the used fitting parameters are summarized in Table 2.

FVDW (D) = −

AHR 6(D − D0)2

(2)

Using Lifshitz theory, the nonretarded Hamaker constant of a dielectric (nonconducting) and a metal (conducting) medium interacting across an aqueous solution can be calculated the range of interest using the following approximate equation:57,58

DLVO model. The model is derived and discussed in detail in the following section. Theoretical Analysis of Interactions between Dissimilar Rough and Smooth Surfaces in Aqueous Electrolytes. The interaction forces between two atomically smooth surfaces (with no polymers adsorbed to them) in aqueous electrolytes include the van der Waals (VDW), electric double layer (EDL), hydration, and oscillatory (between smooth surfaces only) forces.57 Surface roughness will modify the equation for the VDW, EDL, and hydration forces as well as induce an additional repulsive steric force, which arises from compression of asperities between the interacting surfaces. In

Av>0 =

2 2 hv1v2 3 ⎛ n1 − n3 ⎞ ⎜ 2 ⎟ 2 ⎡ ⎤ 8 2 ⎝ n1 + n3 ⎠ v2 ⎢v1 + 2 2 ⎥ n1 − n3 ⎦ ⎣

(3)

where n1 and ν1 refer to the refractive index and absorption frequency of the dieletric material, respectively, ν2 to the absorption frequency of the metal, and n3 to the refractive index 13086


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have been proposed.57 It is noteworthy that the double layer force between dissimilar apposing surfaces can change sign depending on the particular boundary conditions. At close distance, the constant charge boundary condition predicts repulsive forces for like-charged surfaces. In contrast, as the surfaces come into close contact (D→0), the interaction with constant potential boundary condition between dissimilar surfaces becomes attractive for surfaces with potentials of like sign.49,57 In Figure 5A, this behavior of the constant potential boundary condition can be seen in the theoretical EDL curve, which perfectly fits the experimental data. Constant charge predictions did not fit our data. The total interaction given by superimposing the VDW and EDL forces is described in the DLVO theory.57 Non-DLVO Forces. At distances D ≤ λD the analysis included two non-DLVO force terms in order to describe the presented experimental data. First, repulsive hydration forces are expected in electrolyte solutions at distances D ≤ 2−3 nm. In the present system, the electrolyte is composed of protons and anions. As discussed above, depending on the (externally controlled) surface potential, anions (in particular nitrates) can specifically adsorb to the gold surface with their hydration shell, forming the so-called inner Helmholtz plane of the electric double layer. Hydrated ions bound to a surface give rise to a repulsive hydration force, which is believed to be due to the energy needed to dehydrate the specifically bound ions.52,54 Specifically bound protons do not give rise to a hydration force as they have no hydration shell. This difference in specifically bound ions gives rise to the potential dependent effective hard wall distance shift observed in Figure 4. The repulsion due to hydration shows an exponential behavior given by the simple exponential equation:

of the medium (aqueous solution in our case). Note that this “dispersion” contribution becomes retarded starting at 10−20 nm and should be fully retarded by about 50 nm (i.e., the onset of the Casimir force regime). The “zero-frequency” contribution to AH is given by Av=0 =

1 ⎛ ε1 − ε3 ⎞⎛ ε2 − ε3 ⎞ kT ⎜ ⎟⎜ ⎟ 4 ⎝ ε1 + ε3 ⎠⎝ ε2 + ε3 ⎠

(4)

where εn are the static dielectric constants of the three media. However, this contribution gets screened by the diffusive electric double layer and is about 50% less at the Debye length λD. Using eqs 3 and 4 (with ν1 = 3.0 × 1015 s−1 and ν2 = 4.0 × 1015 s−1), the effective Hamaker constant in the range of interest (up to 20 nm) is calculated to be AH = 5 × 10−20 J. The effective Hamaker constant at a close distance within the range of the roughness and SAM layer thickness is estimated to decrease by about a factor of 2 by eq 1 and can be modeled well using the simplified VDW force in eq 2. More rigorous treatments are possible;61,62 however, this equation allows a very effective yet simple qualitative and quantitative treatment of roughness as well as adlayer (such as SAM layers) effects. Electric Double Layer Forces. The electrostatic double layer (EDL) interaction for the interaction of asymmetric systems is very complex for two surfaces at close distance.63 The electrochemical potential of the gold surface in the present system was kept constant by means of a potentiostat, while the surface charge of the amino-terminated APTES was constant and determined by the environmental conditions (i.e., pH and ionic strength). Thus, a behavior between constant charge and constant potential may be expected for the whole system, and/ or charge regulation mechanism may be effective. Such models have been extensively discussed in the literature.64−68 These different combinations of boundary conditions can have a pronounced effect on the interaction forces at separation distances below one Debye-length, while the EDL interactions are expected to dominate at separations D ≥ λD. As shown in detailed below, at separations smaller than one Debye length, other force contributions dominate. At larger separations, the EDL forces can be fitted well with an approximate equation of the exact nonlinear solution of the Poisson−Boltzmann equation for the electric double layer interaction between surface A with a radius of curvature R, and a flat surface B with constant potential boundary conditions. The equation we used can be derived from the so-called Hogg−Healy−Fuerstenau (HHF) equation49,57 according to FEDL(D) =

WHYD(D) = +W0 e−D / λHYD

(6)

where the typical decay length λHYD is about 0.6−1.1 nm for specifically adsorbed cations in 1:1 electrolytes.54 For hydration forces arising from specifically bound cations W0 was found to be typically below 3−30 mJ/m2. The second non-DLVO force included in this analysis is due to surface roughness. Surface roughness dominated the force distance profiles at distance D ≈ 2−4 σrms in two distinct ways. First, for the rough surfaces Au-2 and Au-3, we have to include an additional repulsive interaction taking into account the steric effects of surface roughness. For rough polymer surfaces interacting across air or oil, steric repulsion was successfully modeled using an exponential function:

2πRε0ε [2ϕA ϕB e−D / λD − (ϕA 2 + ϕB 2) e−2D / λD] λD

WST(D) = +S0 e−D / λST

(5)

(7)

where the typical decay length λSR is expected to be in the range of 1−2 times the rms-roughness σrms of the rough surface.53 Likewise, in the limit of elastic deformations (i.e., at low maximum applied compression forces of Fmax < 15 mN/m), this equation is believed to be valid for a rough gold electrode apposing a SAM layer on mica; however, the decay length is expected to be higher in electrolyte solutions. Second, the surface roughness will effectively shift the onset of the electric double layer and hydration forces to larger distances by about the rms roughness. In our modeling we shift the onset of the EDL forces to σrms. The force−distance profile data is fitted to our extended model, which includes the DLVO interactions and two additive

where λD is the Debye length. This equation is a limiting form of the HHF equation for D ≥ λD and provides an excellent approximation up to effective surface potentials of 60 mV in absolute value. For absolute values above 60 mV, the accuracy of determining accurate surface potentials ϕ from experimental data decreases, but still data can be fit qualitatively in many cases surprisingly well. For our data, we find that the data is fitted well by eq 5 up to surface potentials of 140−150 mV. Predictions of the EDL force between dissimilar surfaces with constant charge boundary conditions differ significantly from the predictions for constant potential. Approximate expressions for constant charge are more complicated and several models 13087


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Table 1. Comparison of Fitted Effective Surface Potentialsa and Externally Applied Electrochemical Potentials for the Three Different Gold Electrodes applied electrochemical potential, ΔU (mV) Au-1 surface, ΨAu (mV) Au-2 Surface, ΨAu (mV) Au-3 surface, ΨAu (mV) a

−300

−100

0

−35 ± 2 −47 ± 3

−19 ± 2 −12 ± 1 −25 ± 2

0±2 0±2 0±2

100

200

300

400

39 ± 3 14 ± 1 13 ± 2

500

105 ± 5 34 ± 2 27 ± 2

600

700

130 ± 10 34 ± 2 35 ± 2

45 ± 2 32 ± 2

For the apposing APTES coated mica surface, a surface potential of +55 mV was used.45

Table 2. Typical Values for the Parameters of eq 8 Used for Fitting the Experimental Data for Positive and Negative Surface Potentials of the Differently Rough Gold Surfacesa

Au-1 (ϕAu < 0) Au-2 (ϕAu < 0) Au-3 (ϕAu < 0) Au-1 (ϕAu > 0) Au-2 (ϕAu > 0) Au-3 (ϕAu > 0)

decay length of hydration forces, λHYD [nm]b

prefactor hydration forces, W0 [mJ/m2]b

0.65 0.65 0.65 1.14 1.14 1.14

9−11 13−15 14−15 10−12 14−16 15−18

normalized decay length of steric repulsion, λSR/σrmsc

prefactor steric repulsions, S0 [mJ/m2]

2.7 2.5

18−23 18−23

2.5 2.7

25−30 25−30

For the fitting, the Debye length λD was always set to the experimental value of 9.6 nm. The Hamaker constant Abulk was set to the value for mica/ gold interactions of Abulk = 5 × 10−20 J. For the modeling of the VDW forces, D0 was set to 0.2 nm and onset of the hydration forces and the EDL forces was shifted by the rms-roughness of the respective surfaces, for the two rough surfaces. bTypical values for λHYD reported in the literature for cations range from λHYD = 0.6 nm to λHYD = 1.1 nm.54 W0 is typically found to be between 3 and 30 mJ/m2. cThe typical range of the decay length λSR of exponential steric forces measured between rough polymer surfaces in air is found to be 1−2 times the rms-roughness. In oil, the decay length increases significantly.53 a

terminated mica surface, giving rise to a hydration force with a shorter decay length of λHYD = 0.65 nm, which is close to the radius of hydrated nitrate ions. In conclusion, we would like to note that even though this model contains a number of parameters, the different force contributions can be fitted independently from each other. In particular, each of the parameters is effective at different ranges (e.g., hydration forces only within 1−2 nm; EDL forces between 10 and 40 nm; and surface roughness at distances from 2 to 4 times σrms). Utilizing these different force regimes, we can obtain the various fitting parameters independently and unambiguously. The model is certainly simplifying the problem to some extent, but in many cases this is a very convenient and powerful model that captures the overall force distance characteristics of complex rough and smooth surface interactions over large potential ranges. Anodic Oxide Growth on Gold Electrodes. Figure 7 shows ensuing force runs measured at high positive (anodic) applied electrochemical potentials during approach of both the atomically smooth Au-1 surface and the electrochemically roughened Au-3 surface facing an APTES coated molecularly smooth mica surface in 1 mM HNO3 solutions of pH = 3. For these measurements, the gold surfaces were polarized at electrochemical potentials ΔU above the oxidation potential ΔUOX where gold electrode surfaces oxidize, forming a passive oxide adlayer. The force runs where recorded during an electrochemical potential step experiment, where the gold electrode was polarized from 0 mV directly to 1650 mV with respect to the NHE electrode. In such an experiment, also the current that passes through the gold electrode can be measured as a function of time, providing important details about the electrochemical reaction at the interface (e.g., charge density I/A consumed at the interface). The force measurements were performed when the current density of the oxidation reaction decayed to a constant low value of about 10−20 nA/cm2,

repulsive forces arising from hydration forces and steric roughness: F(D) = FVDW (D − D0) + FEDL(D + σRMS) + FST(D , σRMS) + FHYD(D + σRMS)

(8)

With only a few fitting parameters that are effective at quite different ranges, this semiempirical model can fit the experimental data surprisingly accurately for both the flat electrode surface (Au-1) and rough electrode surfaces (Au-2 and Au-3) apposing the SAM layer on atomically smooth mica as can be seen from Figures 5 and 6. In Table 2, the typical values of the parameters of this extended DLVO model are presented. Table 2 displays the decay length of the hydration forces λHYD, the decay length of the steric repulsion normalized by the rms-roughness λSR/σRM as well as the exponential prefactor for the hydration forces W0 and the steric force S0. Interestingly, the steric repulsion does not depend on the electrochemically applied potential, while both the characteristic decay length as well as the magnitude of the hydration forces vary as a function of the applied electrochemical potential. The variation of the hydration forces is due to the potential dependent adsorption/desorption of ions at the gold electrode interface. The magnitude of the hydration forces increases with the applied electrochemical potential. In particular, at positive applied electrochemical potentials (where ϕAu > 0), anions can specifically adsorb on the gold electrode, giving rise to higher hydration force (i.e., reflected in the increased magnitude of the prefactor). Also, the decay length of the hydration forces of λHYD = 1.14 nm at positive electrochemically applied potentials is about a factor of 2 larger compared to negative applied potentials because specifically adsorbed anions on both apposing surfaces can contribute to the hydration forces. At negative applied electrochemical potentials (where ϕAu < 0), anions only adsorb to the amine13088


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Figure 7. Typical force−distance profiles measured during approach of the gold and APTES coated mica surfaces measured at high (anodic) externally applied electrochemical potentials that are higher than the oxidation potential, UOX, where the gold surface oxidizes (ΔU ≫ ΔUOX). Force−distance profiles (A) of the smooth gold surface Au-1 and (B) of the electrochemically roughened Au-3 gold surface. The force runs and hard walls are compared to force runs with similar effective gold surface potentials where there was no oxide layer present. A clear shift of the hard wall, which is correlated with the oxide thickness, ΔTOX, is observed (cf. Table 3).

chemical AFM measurements using Si3N4 tips (in situ grown oxide)70 or air bubbles (ex situ grown oxide)71 as probes. A second important detail can be seen by comparing the force runs measured at high positive applied potentials (gray curves, Figure 7) to force runs measured at negative potentials which show similar effective surface potentials (red curves, Figure 7). The hard-wall distances measured for the two oxidized gold surfaces are shifted significantly outward by 4−6 nm. We would like to emphasize that, in contrast to an ECAFM, an EC-SFA can directly measure the thickness of a growing oxide in situ as the SFA measures the absolute distance independently from the force measurement. It appears that the observed hard-wall shift directly correlates with the thickness ΔTOX of the passivating gold oxide film that was deposited on the gold electrode. In contrast to other noble metals such as platinum, or iridium where only monomolecular layers up to 10 Å are formed,69,72 gold electrodes are wellknown to form thicker oxide layers of up to 100 Å.73 While electrochemical methods provide measures of both charge as well as oxide layer capacitances,72 a direct in situ measure of the oxide layer thickness and chemistry has not been possible before. The absolute distance control provided by the EC-SFA allows the direct in situ measurements of the oxide layer thicknesses, which allows further insight into the oxide film chemistry in situ under applied electrochemical potentials. Table 3 shows the measured hard wall shifts and the theoretical oxide film thicknesses calculated from the measured charge passed through the gold electrode surface. For the calculation of the theoretical oxide film thicknesses the following electrochemical oxidation reactions are considered:

indicating the formation of a fully covering adlayer providing passivation with very slow anodic dissolution rates of the electrode. The force profiles shown in Figure 7 reveal interesting details of the anodic formation of a passivating noble-metal oxide-film formed on the smooth (Au-1) and rough (Au-3) gold surfaces (Au-2 surfaces behave like Au-3 surfaces and are thus not shown here). First, the change of the surface chemistry from a bare metal electrode toward a metal-oxide covered electrode is reflected in the force profiles recorded at high anodic potentials (red curves, Figure 7). The measurements revealed attractive longrange electric double layer forces, indicating that the formed gold oxide surfaces are negatively charged even at the low experimental pH of 3 (the apposing amine-terminated APTES coated mica surface are charged positively). The fitted effective surface potential of −25 mV of the gold oxide is due to the surface chemistry of the in situ formed gold oxide (i.e., adsorbed hydroxide ions lead to a net negative surface charge). The electrochemical potential drop across the oxide does not directly reflect in the fitted electric double layer potential. This is consistent with the widely accepted oxide growth mechanism on gold electrodes that involves hydroxide adsorption and subsequent “turnover” and incorporation of the oxygen atoms into the bulk metal (i.e., a place exchange of adsorbed oxygen atoms and bulk gold atoms).46−48,69 The potential drop of the electrochemical potential across the growing oxide is the driving force for the oxygen migration from the oxide|solution interface toward the gold|oxide interface. If the equilibrium thickness of the oxide at a given potential is reached, the electrochemical potential drops almost entirely across the oxide and the electrochemical activity becomes very low (i.e., an ongoing anodic dissolution with 10−20 nA/cm2 in this work). The metal oxide is then “passivating” (i.e., the chemical reactivity is minimized) and the measured electric double layer potential is mainly influenced by the gold oxide chemistry. Similar effects due to the changed surface chemistry of the oxide-covered gold electrode have been observed by electro-

2Au + 3H 2O → Au 2O3 + 6e− + 6H+

(reaction 1)

Au + 3H 2O → Au(OH)3 + 3e− + 3H+

(reaction 2)

It is thus assumed that either the gold oxide consists of Au2O3 (density ρ = 11.34 g/cm3) or it is hydrated consisting of Au(OH)3 (density ρ = 4 g/cm3). The thickness of the oxide 13089


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and within the error margin of the method. This is consistent with data from Barnett73 who showed that the thick gold oxide is composed of Au(OH)3. For the two rough surfaces an apparently thicker oxide film of about 60 Å was estimated from the EC-SFA data, which is about twice the expected oxide film thickness. It appears that oxide film formed on a rough surface is not homogeneous, and it is possible that either the film is less dense or surface roughness leads to a faster and thicker oxide growth at asperities, where the electric fields are considerably larger. Relationship between Friction Forces, Adhesion, Electric Double Layers, and Electrochemical Potentials at Gold−Mica Contacts. Figure 8 shows an electrochemical SFA setup equipped with the 3D sensor-actuator device previously developed in our lab.42,43 In addition to measurements performed with the setup shown in the previous section, this novel setup allows for measurements of frictional properties of liquids and thin films confined between a working electrode (in this case again a gold surface) and an apposing surface (in this case mica without an APTES film). Therefore, it becomes possible to measure and quantify interaction forces in three orthogonal directions simultaneously during any kind of standard electrochemical experiment. The electrochemical cell described in Figure 8 is composed of a small cup (1−2 mL total volume) made with a ceramic or glass base and a cylindrical glass wall. The base is coated with a

Table 3. Comparison of (i) the Thickness of the Anodic Oxide (i.e. oxide grown at high positive potentials) That Was Grown on Gold Measured Using SFA Data and (ii) Theoretical Thickness Calculated Using eq 9

Au 1 Au 2 Au 3

charge extracted from the WE, QOX (C)

calculated thickness assuming Au2O3, ΔTOX (Å)

calculated thickness assuming Au(OH)3, ΔTOX (Å)

measured thickness using SFA data, ΔTOX (Å)

1.47 1.42 1.43

9.4 9.1 9.2

31.4 30.2 30.4

32.7 ± 3.3 61 ± 2.5 60 ± 2.5

layer can then be estimated using the following equation (derived from Faraday's law): ΔTOX =

Q (t ) MW [m] zF ρ

(9)

where Q(t) is the measured charge density (I(t)/(area of the electrode)) consumed by the electrode according to Q(t) = 1/A∫ I(t) dt, the Faraday constant, F = 96 485.3 As/mol, and the density and molecular weight MW of the respective oxide film. Interestingly, the data shown in Table 3 indicates that the gold layer formed on the atomically smooth Au-1 electrode consists of a hydrated Au(OH)3, since the calculated and measured oxide thicknesses of 31.4 and 32.7 Å are comparable

Figure 8. Experimental setup used to study tribological properties of metal on mica contacts in acidic solution. The upper surface is held by the 3D sensor-actuator recently developed for the SFA 2000 which allows for displacement and force measurements in three orthogonal directions simultaneously. The lower surface is immersed in a miniature bath designed to host the electrochemical electrodes. 13090


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250 nm thick layer of platinum using PVD and forms the counter electrode (CE). The Ag|AgCl reference electrode (RE) is guided through the stainless steel housing of the sensoractuator. The RE consists of a flexible PTFE-tube filled with a solidified 3 N KCl solution (3 wt % agarosis) and a silver wire coated with AgCl (see Methods and Materials). The sensoractuator device allows for the upper surface to be translated in three orthogonal directions at a speed rate ranging from 0.1 nm/s to 100 μm/s, while simultaneously shear forces can be measured independently in three orthogonal directions. It is important to note that, during the motion of the surface, the RE does not move and does not interfere with any of the surfaces. In order to illustrate the advantages of this new experimental setup, we performed two experiments using atomically smooth mica and an atomically smooth gold surface, both immersed in a 10 mM HNO3 solution at pH = 2. At these solution conditions, the atomically smooth mica surface is charged positively, while the surface charge of the gold electrode can be modulated from positive to negative using electrochemical potential control as explained above. Normal and lateral interaction forces were measured at different applied potentials to the gold electrode. As shown in Figure 9A, the normal interaction force, F, depends again strongly on the surface potential ΔU (referenced to the potential of zero charge of the gold surface). At zero surface charge (ΔU = 0 at the gold surface), the interaction forces are purely repulsive on the approach and well described by DLVO theory of dissimilar surfaces (eqs 1−5). The experimental decay length of the interaction force profile measured at separation distances greater than 6 nm is equal to 2.5 nm which is in very good agreement to the theoretical value of the Debye length λD = 3.5 nm. At separation distances below 5 nm, the surfaces jump into adhesive contact due to the van der Waals attraction. At separation distances smaller than 2 nm, the force profile presents a strong repulsion regime of characteristic decay length of 2.2 nm, which is indicative of strong repulsive hydration forces between the mica and the gold interfaces. The data also show that the hydrated layers are weakly bound to the surfaces since the separation distance between them reaches less than 2 Å at a normal force of 10 mN/m. These results are in good agreement with previously reported experiments36 on a similar system under different solution conditions. On separation, a van der Waals adhesive minimum was observed at F/R = −1 mN/m, corresponding to an interfacial energy of E = 0.16 mJ/m2 . As the gold surface potential is increased to ΔU = +400 mV, the ensuing force profiles are repulsive, show no jump in, and are clearly shifted toward larger separation distances. The decay length of the force curve is unchanged at separation distances larger than 5 nm and corresponds to the experimental Debye length, λD. At smaller distances, a second purely repulsive regime is observed with characteristic decay length of 1 nm, much less than the value measured at zero surface charge. This change in the force law is also accompanied by a thickening of the hydration layer. Since no jump in is observed during the approach of the surfaces, we can conclude that the water molecules and the ions present in the solution are bound much more strongly to the surfaces, especially to the gold surface. Upon separation of the surfaces, a small adhesive minimum of about 0.2 mN/m was measured (corresponding to a surface energy E = 0.03 mJ/m2). Using the sensor-actuator device, we measured the frictional forces between the mica and gold surfaces at the two studied

Figure 9. (A) Experimental interaction forces between mica and flat gold surface in 10 mM HNO3 at two different gold surface potential ΔU. (B) Evolution of the friction forces between mica and gold in the same conditions as (A) during reciprocal (back and forth) motion of the upper surface. Upon abruptly increasing the gold surface potential, the friction forces Fs and Fk experience a similarly abrupt jump. Since the average separation distance between the surfaces is not changing during this process (C), this sudden change in friction forces is due to an increase of the normal force.

potentials, namely, ΔU = 0 and +400 mV. In Figure 9B, the time evolution of the friction force (also known as friction trace) is shown during reciprocal motion of the upper surface at a constant applied normal force of F = 1.7 mN and sliding velocity of v = 500 nm/s. The friction trace observed at zero surface charge (ΔU = 0 mV) present smooth plateau indicative of smooth sliding. The kinetic friction force Fk measured at these plateaus was Fk = 0.31 mN. As soon as the surface potential of the gold surface is increased to ΔU = 400 mV, the friction force abruptly increases to reach a significantly higher value of Fk = 0.41 mN that increases transiently and stabilizes to 13091


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accuracy on rough and smooth surfaces. The EC-SFA provides a very useful tool not only for the study of oxide films grown on noble metals, but it may also prove useful for studying a large variety of electrochemical deposition reactions such as cathodic metal deposition (at negative potentials),78,79 dissolution of passive films,26,27 as well as various oxidation/reduction reactions of rheological and/or organic thin films adsorbed on electrochemically active interfaces.23 Finally, the potential dependent friction forces measurements at gold−mica contacts revealed a dramatically increased shear viscosity at electrified gold interfaces, which is likely due to migration and redistribution of ions at the interface and restructuring of water into solidlike layers. The EC-SFA provides a means to simultaneously correlate thin film rheological properties, interaction forces (adhesion and friction), and interfacial electrochemical reactions at electrified interfaces. The presented experiments illuminate the capabilities of the EC-SFA setup, which will potentially be useful for the study of various biological, geological, and materials interfaces.

0.43 mN after a couple of sliding cycles. Surprisingly, during this process, the separation distance did not vary significantly (D = 2.2−2.3 nm), which indicates that the changes observed in the friction forces were essentially due to an electroviscous effect. From the force curve presented in Figure 9A, it is expected that, at 2.3 nm of separation distance, the two double layers of the surfaces overlap strongly giving rise to a drastic increase of the thin film viscosity of the fluid. The effective shear viscosity of the fluid between the surfaces can be estimated using the equation: Fk =

⎛ 2R ⎞ 16 πRην log⎜ ⎟ ⎝D⎠ 5

for

D≪R

(10)

with R being the curvature of the surfaces (2 cm), v the sliding speed (0.5 μm/s), and η the effective viscosity of the confined fluid. Using eq 10, the viscosity of the thin film of water at ΔU = 0 mV was estimated to 190 Pa·s and increases to 250 Pa·s at ΔU = 400 mV. These values are 5 orders of magnitude higher than bulk water viscosity. Such large values of water viscosity have been reported for thin films confining 3−4 layers of water molecules, that is, for a film thickness below 1.5 nm, and have been ascribed to the freezing of the water molecules between the surfaces.74 Also, dynamically measured AFM force− distance measurements recorded at high approach speed of 500 nm/s (i.e., comparable to the sliding speeds reported in this work) found evidence of a significantly increased thin film viscosity of the interfacial water at electrified gold electrodes.75 Interestingly, the dynamic friction coefficient μk = Fk/F measured at both potentials is found to vary between 0.18 and 0.25 which is close to the values reported for ice at similar driving speeds.76 Therefore, the transient increase in kinetic friction upon changing the gold surface potential is mainly due to migration and redistribution of ions at the interface and restructuration of the water layers to a more solidlike structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, D-40237, Germany. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Division of Materials Sciences under Award No. DE-FG0287ER-45331 (development of instrumentation). M.V. acknowledges financial support through a Marie Curie International Outgoing Fellowship within the seventh European Community Framework Program under Award No. IOF-253079. We also thank the McCutchen Foundation for support.

■

CONCLUSIONS We presented two newly designed electrochemical SFA attachments, which provide in situ control of surface potentials and interfacial electrochemical reactions and a simultaneous measurement of normal interaction forces, friction forces, distances, and surface separations as well as contact shapes between dissimilar apposing surfaces. Using the EC-SFA, setup we measured force−distance profiles, using atomically smooth and nanoscale rough gold electrode surfaces. Nanoscale roughness significantly altered the effective counterion distribution and measured force profiles at electrified interfaces, which potentially contributes to the large discrepancies between classical electro-kinetic theory and microfludic device experiments.77 The measured force profiles between rough and smooth surfaces were modeled accurately by an extended DLVO model, where the onsets of both VDW and EDL forces are shifted by the rms-roughness, and two additional exponentially repulsive contributions due to steric and hydration forces are introduced. This semiempirical but powerful model allows understanding of interface forces between rough and smooth surfaces, and provides the means for device designer to predict and controllably modulate the interaction forces in applications such as MEMS, microfluidic devices, and adhesive and rheological systems. The combined capabilities of the EC-SFA and the absolute distance control allowed for the in situ study of electrochemically growing (or modif ied) thin f ilms. The thickness of anodically growing oxide films was measured in situ with Å

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REFERENCES

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The Electrochemical Surface Forces Apparatus: The Effect of

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