Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modified by Reverse Micelles

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Article Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modiﬁed by Reverse Micelles

Miriam C. Rodríguez González, Maximina Luis-Sunga, Ricardo M. Souto , Alberto Hernández Creus, Elena Pastor and Gonzalo García *

Instituto de Materiales y Nanotecnología, Departamento de Química, Universidad de La Laguna, PO Box 456, 38200 La Laguna, Spain; [email protected] (M.C.R.G.); [email protected] (M.L.-S.); [email protected] (R.M.S.); [email protected] (A.H.C.); [email protected] (E.P.) * Correspondence: [email protected]

Abstract: The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efﬁciency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modiﬁed by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a

monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in

Citation: Rodríguez González, M.C.; conditions of long-term storage but not in the operation of devices with such contacts. In this regard, Luis-Sunga, M.; Souto, R.M.; the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim Hernández Creus, A.; Pastor, E.; of the present work is to understand the reactions occurring at the surfactant/metal interface, which García, G. Spectroelectrochemical may help to improve the fabrication of novel electrodes. Behavior of Polycrystalline Gold Electrode Modiﬁed by Reverse Keywords: gold; adsorption; surfactant; micelles; AFM; EIS; electron tunneling Micelles. Molecules 2021, 26, 471. https://doi.org/10.3390/molecules 26020471 1. Introduction Academic Editor: Jacek Ryl Electrical contact is deﬁned as the interface between the current-carrying members Received: 2 December 2020 of an electrical/electronic device. Its goal is to allow a continuous passage of electrical Accepted: 14 January 2021 Published: 18 January 2021 current through the interface, which can only be achieved if it is established a good metal-to-metal contact [1]. Electronic connectors and many other contacts must function

Publisher’s Note: MDPI stays neutral in chemically aggressive environments [2]. Therefore, the need to ensure and maintain with regard to jurisdictional claims in a long lifetime of the electronic connections under severe operation conditions has been published maps and institutional affil- emphasized. The preferred contact materials for electronic connectors, which are used iations. primarily as electrodeposits and coatings, are precious metals such as gold, palladium and their alloys [1]. In this regard, gold materials have received increasing interest in many fields of research due to their excellent physical and chemical properties [3]. Gold is a noble metal and plays an important role in nanoscience and nanotechnology because of its high stability Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. at the nanoscale [4]. Corrosion of gold can only take place in the presence of strong oxidants This article is an open access article agents and complexing ions. Gold may suffer corrosion and even dissolve in solutions distributed under the terms and containing complexing agents [3]. This phenomenon is relevant in the electronics industry conditions of the Creative Commons due to its use in thin-film applications and integrated-circuit technology [5]. Also, the Attribution (CC BY) license (https:// addition of copper, palladium, silver or platinum obtaining binary and ternary alloys may creativecommons.org/licenses/by/ improve hardness without losing tarnish resistance but its use is restricted to low current 4.0/). applications [1]. Among the plating materials, gold has the highest standard electrode

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potential (E0 = 1.38 V). When thin, precious metal finishes, most commonly gold, are used the corrosion is originated by intrinsic porosity, wear tracks and occasionally defects caused by forming and handling during the manufacturing process [2]. A perfect gold coating does not present corrosion problems; however, due to the high cost of this material, the gold plating is usually so thin that the final coating is not free of pores. These pores cause corrosion of electrically conductive surfaces with protective coatings [6]. One way of protecting against the corrosion of conductive surfaces consists of passivation. In this process, the metal surface is changed creating resistance to environmental agents. Metal surfaces can be passivated by their own oxide layer or by a non-reactive layer such as self-organizing nanoparticles. In addition to resisting corrosion, the conductive surfaces must meet two important requirements: (i) that the passivation layer must not significantly increase or destabilize the electrical resistance and (ii) the passivation layer must withstand the operating temperature (which will be the sum of the resistive heating and ambient temperature) [6]. The use of corrosion inhibitors is one of the most effective methods for protection against corrosion in an acid medium. A typical inhibitor should: (i) remove water from the metal surface, (ii) interact with the cathodic or anodic reaction sites to inhibit the corresponding redox reaction and (iii) impede the transport of water and corrosive species to the metal surface [6,7]. Organic compounds are widely used in industry as inhibitors for acidic media. This type of compounds may contain multiple bonds with nitrogen, sulfur and oxygen atoms through which they can be adsorbed on the metal surface [7]. It is expected to found efficient inhibitors that can function in a wide range of parameters, considering that the effectiveness of the inhibition depends on the conditions of the system (temperature, pH, the composition of the material and duration) and on the structure of the inhibitor compound [8]. In this regard, surfactant inhibitors have great advantages such as high inhibition efficiency, low cost, easy production and low toxicity. Besides, the study of surfactant substances adsorbed on metallic surfaces is important for electrochemical studies such as the inhibition of corrosion and adhesion phenomena. It is necessary that the surfactant functional group is adsorbed onto the metal surface to achieve corrosion inhibition. The degree of adsorption depends on the surface structure, the nature of the metal, the chemical nature of the inhibitor, the adsorption mode and the type of media. Metal corrosion will be inhibited by adsorbing organic molecules or ions on the surface forming a protective layer that will reduce or prevent metallic oxidation [8,9]. Hence, adsorption is critical and its capacity is generally related to the ability to aggregate to form micelles [8]. One of the better established ways to obtain micelles is the microemulsion technique. In this process, small amounts of water are added to a surfactant/oil solution, obtaining a mixture known as a water-in-oil microemulsion [10]. The molecules of surfactants are amphiphilic, with a hydrophilic head and a hydrophobic tail and therefore they are easily adsorbed at interfaces. At low concentrations, they mainly exist as individual molecules but as the concentration increases and the critical micellar concentration (CMC) is reached, the surfactant molecules create the so-called micelles. In aqueous solution, the hydrophobic tails are found within the micelle and only the hydrophilic part is exposed to the aqueous phase [11]. The promising potential application of micelles as corrosion inhibitors can lead to a better understanding of the relationship between the adsorption of the surfactant molecules and the metal surface helping to improve the corrosion inhibition, which would be a step forward to the improvement in the protection of electrical contacts.

2. Results and Discussion 2.1. Atomic Force Microscopy (AFM) Characterization Figure1 depicts the AFM analysis for the samples prepared. Figure1a,d show the bare substrate prior to any modiﬁcation. A surface formed by gold grains of around 50 nm can be found. Multilayer of micelles with diameter close to 20 nm was formed on the gold surface after deposition of 20 µL of micelles-containing solution (Figure1c,f). After that, Molecules 2021, 26, 471 3 of 9

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that,the the sample sample was was copiously copiously rinsed rinsed with with acetone acetone and and ultra-pure ultra-pure water water and and a monolayera monolayer of ofcircular circular features features of of ca. ca. 20 20 nm nm in diameterin diamet adsorbeder adsorbed on theon the gold gold grains grains previously previously observed observedin Figure in Figure1a,d are 1a,d discerned are discerned (Figure 1(Figureb,e). The 1b latter,e). The indicates latter indicates strong adsorption strong adsorption strength strengthof a monolayer of a monolayer of micelles of micelles onto the onto gold the substrate gold substrate and that and the that interaction the interaction energy betweenenergy betweenthe gold the surface gold surface and the and micelles the micelles is higher is thanhigher that than between that between water, heptanewater, heptane and acetone and acetonemolecules molecules and the and gold the substrate. gold substrate.

FigureFigure 1. 1. AtomicAtomic force force microscopy microscopy (AFM) (AFM) topographic topographic images images of of 2 2µmµm ×× 2 2µmµm (a (–ac–)c )and and 0.6 0.6 µmµm ×× 0.60.6 µmµm (d (–df–)f )for for bare bare polycrystallinepolycrystalline gold gold substrate substrate (a (a,d,d),), monolayer monolayer of of micelles micelles (b (b,e,e) )and and multilayer multilayer of of micelles micelles (c (c,f,f).).

TheThe previous previous is is supported supported by by DLS DLS measuremen measurementsts that that indicate indicate a amicelle micelle average average size size ofof ca. ca. 14 14 nm nm in in solution solution (Figure (Figure S1), S1), which which in increasescreases to to 20 20 nm nm in in diameter diameter with with 1.4 1.4 nm nm in in thicknessthickness (Figure (Figure S2) S2) after after micelles micelles deposition deposition onto onto gold, gold, that that is, is, micelles micelles flatten flatten in in contact contact withwith gold gold due due to to the the strong strong interaction interaction between between them. them. In In this this sense, sense, it it is is remarkable remarkable the the stabilitystability of of the the adsorbedadsorbed monolayermonolayer of of micelles micelles on on the the gold gold surface surface that remainsthat remains unchanged un- changedafter long after exposure long exposure (~7 days) (~7 into days) acidic into (1 acidic M H 2(1SO M4), H alkaline2SO4), alkaline (1 M NaOH) (1 M NaOH) and organic and organic(heptane) (heptane) media. media. TheThe observations observations in in Figure Figure 11 can can be be easily easily explained explained by by the the critical critical micelle micelle concentra- concentra- tiontion (CMC), (CMC), which which is is a a key key factor factor in determiningdetermining thethe effectiveness effectiveness of of a a corrosion corrosion inhibitor inhibitor [8 ]. [8].Above Above the the CMC CMC the goldthe gold surface surface is covered is covered with a with monolayer a monolayer of micelles of micelles and the additional and the additionalmolecules molecules combine tocombine form micelles to form ormicelles multiple or layersmultiple (Figure layers1c,f) (Figure [ 8]. The1c,f) additional [8]. The additionalorganic molecules organic aremolecules easily removed are easily by removed washing withby washing acetone andwith water, acetone leaving and awater, strong leavingadsorbed a strong monolayer adsorbed film monolayer of micelles film on the of micelles gold surface on the (Figure gold1 surfaceb,e). (Figure 1b,e). SurfaceSurface roughness roughness (RMS) (RMS) data data for for small-scale small-scale images images (Figure (Figure 1d–f)1d–f) confirm confirm the the gold gold surfacesurface coverage coverage by bymicelles. micelles. Indeed, Indeed, RMS RMS values values of 2.819, of 2.819, 2.556 and 2.556 1.739 and nm 1.739 were nm found were forfound the bare for the substrate, bare substrate, a monolayer a monolayer and multilayer and multilayer of micelles, of micelles, respectively. respectively. The last The indi- last catesindicates the formation the formation of smooth of smooth mono/multilayer mono/multilayer of micelles of micelles onto the onto gold the substrate gold substrate with a subsequentwith a subsequent decrease decrease in roughness in roughness of the surface of the upon surface adsorption. upon adsorption. SinceSince multilayer multilayer of of micelles micelles are are not not strongly strongly adsorbed adsorbed on on the the gold gold substrate, substrate, electro- electro- chemicalchemical and and EIS EIS experiments experiments of of bare bare gold gold and and gold-covered gold-covered by by a amonolayer monolayer of of micelles micelles areare described, described, compared compared and and analyzed analyzed in in the the following following subsections. subsections. 2.2. Electrochemical Characterization of Micelle Monolayer Formed on Gold 2.2. Electrochemical Characterization of Micelle Monolayer Formed on Gold A comparative analysis of the electrochemical behavior of the micelle-coated gold sur- A comparative analysis of the electrochemical behavior of the micelle-coated gold face and the pristine electrode was performed by considering simple one-electron transfer surface and the pristine electrode was performed by considering simple one-electron transfer reactions involving a change in the oxidation state of iron, namely hexacyanoferrate(III), [Fe(CN)6]3−, and ferrocenemethanol, FcMeOH, in 0.1 M KCl solution. The iron

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reactions involving a change in the oxidation state of iron, namely hexacyanoferrate(III), 3− Molecules 2021, 26, x [Fe(CN)6] , and ferrocenemethanol, FcMeOH, in 0.1 M KCl solution. The iron complexes4 of 9 were chosen as to consider both an outer-sphere redox system, FcMeOH1+/0, and an inner- 3−/4− spherecomplexes redox were pair, chosen [Fe(CN) as6 ]to consider. Measurements both an outer-sphere were initiated redox after system, 60 min FcMeOH immersion1+/0, inand the an solution inner-sphere to attain redox their pair, corresponding [Fe(CN)6]3−/4 open−. Measurements circuit potentials: were initiated +0.165 and after +0.090 60 min V Molecules 2021, 26, x vs.immersion SCE, respectively. in the solution No significantto attain their potential corresponding differences open4 (namely,of 10 circuit lesspotentials: than 3 +0.165 mV) were and complexesobserved+0.090 Vwere withinvs. chosen SCE, theas torespectively. OCPconsider values both an No displayedouter-sphere significant redox by the system,potential pristine FcMeOH differences and1+/0, the modified (namely, gold less surfaces than 3 andformV) an each inner-spherewere given observed redox redox pair, mediatorwithin [Fe(CN) 6]the3−/4 system.−. MeasurementsOCP values were di initiatedsplayed after by 60 minthe pristine and the modified immersion in the solution to attain their corresponding open circuit potentials: +0.165 and +0.090gold V Figuressurfaces vs. SCE, respectively.2 forand each3 display No given significant theredox cyclicpotential mediator voltammogramsdifferences system. (namely, less recorded than 3 for pristine gold and mV)micelle-modified wereFigures observed 2 within and goldthe 3 OCPdisplay electrodes values the displayed cyclic immersed by voltammogramsthe pristine in and 0.1 the M modified KCl recorded solution for containingpristine gold 1 mMand gold surfaces for each given redox mediator system. concentrationmicelle-modifiedFigures 2 and 3 display of gold either the cyclicelectrodes ferrocenemethanol voltammograms immersed recorded inor for 0.1 hexacyanoferrate(III).pristine M KCl gold solution and containing A well-defined 1 mM con- micelle-modified(quasi-)reversiblecentration of gold either electrodes system ferrocenemethanol immersed is observed in 0.1 M KCl or in solution hexacyanoferrate(III). all cases, containing although 1 mM con- the A well-defined attenuation effect(quasi-)re- due centrationto surface of either modification ferrocenemethanol by or micelles hexacyanoferrate(III). greatly A depended well-defined (quasi-)re- on the nature of the redox media- versibleversible system system is observed is observedin all cases, althoughin all cases,the attenuation although effect duethe to attenuation surface effect due to surface modificationtormodification species. by micelles by micellesgreatly depended greatly on th dependede nature of the onredox th mediatore nature species. of the redox mediator species.

FigureFigureFigure 2. Cyclic 2.2. Cyclic voltammogram voltammogramvoltammogram for pristine and forfor micelle-coated pristinepristine andgold and electrodes micelle-coated micelle-coated in 0.1 M KCl gold gold+ 1 electrodes electrodes in in 0.1 0.1 M M KCl KCl + + 1 1 mM mM ferrocenemethanol. Scan rate: 20 mV s−1; Area: 0.5 cm2. ferrocenemethanol.mM ferrocenemethanol. Scan Scan rate: rate: 20 mV 20 smV−1; Area:s−1; Area: 0.5 cm0.52 cm. 2. As expected, voltammetric profiles of both electrodes in 0.1 M KCl solution containing ferrocenemethanolAs expected, are voltammetric approximately equal proﬁles (Figure 2). of On both the electrodesother hand, the in pres- 0.1 M KCl solution containing ence of aAs monolayer expected, of micelles voltammetric on the gold electrode profiles in 0.1of Mboth KCl solutionelectrodes containing in 0.1 M KCl solution contain- hexacyanoferrate(III)ferrocenemethanoling ferrocenemethanol increases are the approximatelyare irreversibility approximately of the equalsystems equal (Figure(i.e., faradaic(Figure2). peaks On 2). thebe- On other the other hand, hand, the presence the pres- comeofence a more monolayer of separated)a monolayer and of micellesslightly of micellesdecreases on the the on goldactive the surface electrodegold area electrode (Figure in 0.1 3). in MIndeed, 0.1 KCl M solution KCl solution containing containing hexa- the active surface area decreases about 10% after modification of the gold electrode by a monolayercyanoferrate(III)hexacyanoferrate(III) of micelles. Interestingly, increases increases the the latter irreversibility the is inirreversibility agreement ofwith the AFM of systems theanalysis, syst in (i.e.,ems faradaic(i.e., faradaic peaks peaks become be- whichmorecome a surface separated)more roughness separated) and (RMS) slightly reductionand slightly decreases of 10% was decreases thediscerned active (see the surface Section active 2.1). area surface (Figure area3). (Figure Indeed, 3). the Indeed, active surfacethe active area surface decreases area aboutdecreases 10% about after modiﬁcation 10% after modification of the gold of electrode the gold by electrode a monolayer by a ofmonolayer micelles. Interestingly,of micelles. Interestingly, the latter is in the agreement latter is within agreement AFM analysis, with inAFM which analysis, a surface in roughnesswhich a surface (RMS) roughness reduction (RMS) of 10% reduction was discerned of 10% (see was Section discerned 2.1). (see Section 2.1).

Figure 3. Cyclic voltammogram for pristine and micelle modified gold in 0.1 M KCl + 1 mM K3Fe(CN)6. Scan rate: 20 mV s−1; Area: 0.5 cm2.

FigureFigure 3. CyclicCyclic voltammogram voltammogram for for pristine pristine and and micelle micelle modified modiﬁed gold gold in 0.1 in M 0.1 KCl M + KCl 1 mM + 1 mM K3Fe(CN)6. Scan rate: 20 mV s−−1;1 Area: 0.5 cm2. 2 K3Fe(CN)6. Scan rate: 20 mV s ; Area: 0.5 cm .

2.3. Electrochemical Impedance Spectra of Micelle Monolayer Formed on Gold The assessment of the different electrochemical reactivity of the micelle-coated gold surface for each redox mediator is more readily observable using electrochemical impedance spectroscopy (EIS). Figure 4 depicts the Nyquist and Bode impedance plots of pristine and micelle-coated gold for the redox conversion of ferrocenemethanol. As can be

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2.3. Electrochemical Impedance Spectra of Micelle Monolayer Formed on Gold The assessment of the different electrochemical reactivity of the micelle-coated gold Molecules 2021, 26, x surface for each redox mediator is more readily observable using electrochemical impedance5 of 9 spectroscopy (EIS). Figure4 depicts the Nyquist and Bode impedance plots of pristine seenand micelle-coatedfrom the Nyquist gold plots for shown the redox in Figure conversion 4a, similar of ferrocenemethanol. impedance spectra Aswere can obtained be seen infrom both the cases. Nyquist They plots display shown one in time Figure constant4a, similar and they impedance can be spectrasatisfactorily were obtainedfitted using in theboth Randles-Ershler cases. They display equivalent one time circuit constant shown and in Figure they can 5, where be satisfactorily Rs relates to ﬁtted the resistance using the ofRandles-Ershler the solutions, R equivalentct corresponds circuit to the shown electron in Figure transfer5, where reaction,Rs relatesQdl to the to thecapacitive resistance re- sponse of the double layer and W accounts for the diffusion of the soluble redox species. of the solutions, Rct corresponds to the electron transfer reaction, Qdl to the capacitive Oneresponse constant of the phase double element layer and (CPE)W accountswas employ for theed diffusioninstead of of a the capacitor, soluble redoxowing species. to re- portedOne constant dispersion phase effects element produced (CPE) was by employedmicroscopic instead roughness of a capacitor, of the metal owing surface to reported [12]. Therefore,dispersion the effects redox produced process byis kinetically microscopic controlled roughness in ofthe the high metal frequency surface range, [12]. Therefore, whereas thethe Warburg redox process impedance is kinetically dominates controlled at lower in the fr highequencies. frequency The range,system whereas shows no the apparent Warburg heterogeneitiesimpedance dominates and the at redox lower reaction frequencies. can Thebe described system shows to occur no apparent homogeneously heterogeneities on the surfacesand the redoxof both reaction pristine can and be describedmicelle-coated to occur gold homogeneously electrodes of apparently on the surfaces equal of areas. both Thispristine result and agrees micelle-coated well with goldthe cyclic electrodes voltammetry of apparently study equal(see Section areas. This2.2) and result with agrees the assumptionwell with the that cyclic an outer-sphere voltammetry system study (see does Section not involve 2.2) and specific with thesurface assumption adsorption that for an theouter-sphere electron transfer system process does not to involve occur [13]. speciﬁc Therefore, surface adsorptionsimilar behaviors for the are electron recorded transfer for theprocess pristine to occur and [the13]. micelle-coated Therefore, similar gold behaviors electrodes are for recorded the redox for conversion the pristine of and ferro- the cenemethanol.micelle-coated gold electrodes for the redox conversion of ferrocenemethanol.

FigureFigure 4. 4. (a(a) )Nyquist Nyquist and and (b (,bc),c Bode) Bode diagrams diagrams of ofthe the impedance impedance spectra spectra of pristine of pristine and andmicelle- micelle- modifiedmodiﬁed gold gold in in 0.1 0.1 M M KCl KCl + 1 + mM 1 mM ferrocenemeth ferrocenemethanol.anol. The The solid solid lines lines and andthe discrete the discrete points points correspondcorrespond to to the the fitted ﬁtted and the measured data, respectively.

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FigureFigure 5.5. EquivalentEquivalent electrical circuit employed to fi ﬁtt the electrochemical impedance impedance spectroscopy spectroscopy (EIS)(EIS) spectra.spectra.

WhenWhen hexacyanoferratehexacyanoferrate waswas usedused asas redoxredox mediatormediator thethe heterogeneousheterogeneous electronelectron transfertransfer occurredoccurred significantlysignificantly fasterfaster thanthan inin thethe casecase ofof ferrocenemethanolferrocenemethanol asas shownshown byby thethe impedanceimpedance spectraspectra inin FigureFigure6 6.. In In addition, addition, the the Nyquist Nyquist and and Bode Bode plots plots for for this this redox redox mediatormediator werewere observedobserved toto clearlyclearly separateseparate betweenbetween themthem forfor thethe pristinepristine and the micelle- coatedcoated goldgold electrodes,electrodes, duedue toto markedmarked decreasedecrease inin thethe reversibilityreversibility ofof thethe heterogeneousheterogeneous electronelectron transfertransfer (HET)(HET) duedue toto thethe occurrenceoccurrence ofof thethe surfacesurface filmfilm formedformed byby thethe micellemicelle layerlayer on the the surface surface of of gold gold (Figure (Figure 6).6 ).Yet, Yet, the thespectra spectra could could be satisfactorily be satisfactorily fitted fittedby the byRandles-Ershler the Randles-Ershler equivalent equivalent circuit for circuit both forthe bothpristine the and pristine the micelle-coated and the micelle-coated gold elec- goldtrodes electrodes as described as described by the equivalent by the equivalent circuit shown circuit in shown Figure in 5 Figure and the5 and obtained the obtained imped- impedanceance parameters parameters are also are included also included in Table in Table 1. This1. This feature feature suggests suggests that that the the main main effect effect of ofthe the micelle micelle layer layer on on the the HET HET process process is is to to promote promote some some degree of irreversibility inin thethe otherwiseotherwise fastfast electron electron transfer transfer reaction reaction associated associated to the to reduction the reduction of hexacyanoferrate(III). of hexacyanoferrate(III).Table 1 lists the impedance parameters used in the modelling of the impedance spectra givenTable in Figures 1 lists4 andthe 6impedance. It is interesting parameters to notice used that in a the 3-fold modelling increase of of the the impedance charge transfer spec- resistancetra given inRct Figuresoccurred 4 and when 6. It the is HETinteresting for the to reduction notice that of hexacyanoferrate(III)a 3-fold increase of the occurred charge on the micelle-coated Au surface compared to the pristine metal condition, whereas all transfer resistance Rct occurred when the HET for the reduction of hexacyanoferrate(III) theoccurred remaining on the parameters micelle-coated showed Au onlysurface minor compared variations to betweenthe pristine the metal two conditions, condition, R evenwhereas for theall thect values remaining determined parameters for the showed ferrocenemethanol only minor variations redox system. between In summary, the two electrochemical impedance data agree well with the observations previously derived from conditions, even for the Rct values determined for the ferrocenemethanol redox system. In the electrochemical characterization using cyclic voltammetry and the surface analytical summary, electrochemical impedance data agree well with the observations previously findings revealed by AFM imaging. derived from the electrochemical characterization using cyclic voltammetry and the surface analytical findings revealed by AFM imaging. Table 1. Electrochemical parameters obtained from EIS data measured for pristine and micelle- modified gold for the heterogeneous electron transfer (HET) of ferrocenemethanol and hexacyanofer- Table 1. Electrochemical parameters obtained from EIS data measured for pristine and micelle- rate(III).modified The gold meaning for the ofheterogeneous the electrochemical electron parameters transfer (HET) is given of ferrocenemethanol by the equivalent circuitand hexacy- shown in Figureanoferrate(III).5. The meaning of the electrochemical parameters is given by the equivalent circuit shown in Figure 5. Redox Mediator Impedance Ferrocenemethanol Hexacyanoferrate(III) Parameter Redox Mediator Impedance Pristine Au FerrocenemethanolMicelle-Coated Au PristineHexacyanoferrate(III) Au Micelle-Coated Au 2 Rs,ParameterΩ cm 15.36 16.30Micelle- 11.12Pristine Micelle-Coated 13.22 −2 n Pristine Au Qdl, mS cm s 0.3661 0.4016Coated Au 0.2805Au 0.2009Au n 2 0.6953 0.6769 0.7507 0.7939 Rdls, Ω cm 15.36 16.30 11.12 13.22 2 1372 1005 19.01 58.62 RQctdl, Ω, mScm cm−2 sn 0.3661 0.4016 0.2805 0.2009 2 −1 × −5 × −4 × −4 × −4 W, Ω cm nsdl 9.612 100.6953 1.087 100.6769 2.0480.750710 2.0160.793910 Rct, Ω cm2 1372 1005 19.01 58.62 W, Ω cm2 s−1 9.612 × 10−5 1.087 × 10−4 2.048 × 10−4 2.016 × 10−4

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Figure 6. ((aa)) Nyquist Nyquist and and (b (,bc,)c Bode) Bode diagrams diagrams of ofthe the impedance impedance spectra spectra of pristine of pristine and andmicelle- micelle- modiﬁedmodified goldgold inin 0.10.1 MM KClKCl ++ 11 mMmM KK33Fe(CN)6.. The The solid solid lines lines and and the discrete pointspoints correspondcorrespond to theto the ﬁtted fitted and and the the measured measured data, data, respectively. respectively.

3. Materials and Methods 3.1. Preparation of the Material Reverse micelles were obtained by the microemulsion method method previously previously described described inin [10[10,14,15].,14,15]. Water/oil Water/oil microemulsion microemulsion comprised comprised of water/polyethyleneglycol-dodecylether of water/polyethyleneglycol-do- (BRIJdecylether 30)/n (BRIJ-heptane 30)/n was-heptane used towas obtain used theto obtain reverse the micelles. reverse The micelles. size of The the size micelles of the is determinedmicelles is determined by the molar by ratio the molar (R) of ratio water (R) to of surfactant water to surfactant that is six inthat the is current six in the work. current The sizework. of The the size micelles of the is micelles corroborated is corroborat by dynamiced by dynamic light scattering light scattering (DLS) measurements (DLS) measure- on aments Malvern on a Zetasizer Malvern NanoZetasizer S (Malvern Nano S Instruments(Malvern Instruments Ltd., Malvern, Ltd., Malvern, UK) with UK) a detection with a ◦ ◦ angledetection of 173 angle. All of measurements173°. All measurements in this study in th wereis study taken were at a taken temperature at a temperature of 25 C and of 25 at least°C and 3 repeat at least measurements 3 repeat measurements were taken were to check taken for to result check repeatability. for result repeatability. The intensity The size distributionsintensity size weredistributions obtained were from obtained analysis from of the analysis correlation of the functions correlation using functions the algorithm using inthe the algorithm instrument in the software. instrument Figure software. S1 reveals Figu a monodispersere S1 reveals a samplemonodisperse with micelle sample average with sizemicelle close average to 14 nm. size close to 14 nm. For the preparation ofof thethe modiﬁedmodified goldgold surface,surface, polycrystalline polycrystalline Au Au plates plates (Geometric (Geomet- 2 arearic area ~0.5 ~0.5 cm cm; Arrandee,2; Arrandee, Werther, Werther, Germany) Germany) were were cleaned cleaned by the by ultrasoundthe ultrasound method method (ﬁrst with(first acetonewith acetone and thenand then with with ultra-pure ultra-pure water) water) and and used used as theas the substrate. substrate. Then, Then, 20 20µL µL of theof the micellar micellar solution solution is is drop-casted drop-casted on on the the clean clean polycrystalline polycrystalline goldgold substrate. In In order order toto obtain a a monolayer monolayer of of micelles micelles containing containing water water onto onto the thegold gold surface, surface, the sample the sample was

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was thoroughly rinsed with acetone and ultra-pure water to remove the excess of the organic material.

3.2. Atomic Force Microscopy (AFM) Analysis Topographic atomic force microscopy (AFM) images were acquired in Peak-Force and Tapping modes using a multimode microscope with a Nanoscope V control unit from Bruker. Scan rates of 0.5–1.2 Hz were used. Measurements were done with Scan-Asyst HR (50–100 kHz and 1–5 N m−1) tips (Bruker). AFM images were analyzed with Gwyddion software [16].

3.3. Electrochemical Characterization of the Material The electrochemical tests were conducted using an AUTOLAB PGSTAT302N electrochemical workstation (Metrohm Autolab bv, Utrecht, The Netherlands) in the standard three electrode conﬁguration, wherein a saturated calomel electrode (SCE) and a gold grid were employed as the reference and the counter electrodes, respectively. The pristine gold electrode or micelle-coated gold electrode served as the working electrode (surface area, 0.5 cm2). Before each experiment, the working electrode was immersed in a 0.1 M KCl test solution containing 1 mM concentration of a reversible redox mediator for 60 min to attain a steady state open circuit potential (OCP). Potassium hexacyanoferrate (III) (99%, Sigma-Aldrich; St. Louis, MO, USA) and ferrocenemethanol (97%, Sigma-Aldrich; St. Louis, MO, USA) were employed as redox mediators to characterize the reactivity of the working electrode towards the electron transfer reaction. Solutions were prepared using pro analysis reactants and ultrapure deionized water (resistivity = 18.2 MΩ cm, Milli-Q; Millipore, Burlington, MA, USA). Tests were performed in the de-aerated test solutions at the laboratory temperature (20 ± 2 ◦C). Electrochemical impedance spectra (EIS) were obtained using a perturbation ampli- tude of ±10 mV (vs. OCP) and a frequency interval from 100 kHz to 100 mHz. EIS data were analyzed using Yeum’s ZsimpWin 2.00 software and ﬁtted to electrical equivalent circuits. The impedance data were represented in terms of both Nyquist (imaginary component of the impedance as a function of the real component) and Bode (logarithm of the impedance modulus, |Z| and phase angle (ϕ) as a function of the logarithm of the frequency (f ) plots. Cyclic voltammograms (CV) were recorded in the range from −0.25 to +0.70 V vs. SCE with a scan rate of 20 mV s−1.

4. Conclusions In this study, the influence of reverse micelles (water/polyethyleneglycol-dodecylether/ n-heptane; R = 6) on the corrosion behavior of polycrystalline gold in 0.1 M KCl solution was investigated using atomic force microscopy (AFM), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques. AFM images showed a smooth monolayer of micelles strongly adsorbed on the gold surface, which decreases the surface roughness (RMS) of the bare electrode about 10%. In this sense, electrochemical experiments performed with the inner-sphere redox pair species revealed that the active surface area also decreases around 10% after gold modification by a monolayer of micelles, although a decrease in the reversibility of the redox system was discerned. The latter was confirmed by EIS experiments in which the charge transfer resistance rises after gold modification by a monolayer of micelles. Furthermore, EIS results indicated the Randles-Ershler equivalent circuit as the operative in the presence and the absence of micelles on the gold electrode, which indicates a thin film of micelles covering the gold surface. All in all, the outcomes suggest a compact thin film (1.4 nm thickness) of micelles strongly adsorbed on the gold electrode that only allows electron tunneling through the organic barrier. This consequence would be more beneficial for metal protection in conditions of long-term storage, since molecular/metal species are impeded to pass through the physical barrier but electron tunneling conduction is still possible. Molecules 2021, 26, 471 9 of 9

Supplementary Materials: The following are available online, Figure S1: Size distribution by intensity of reverse micelles in n-heptane solution. Average size = 13.96 nm. Figure S2. 600 × 200 nm2 AFM image showing the topography of a micelle layer on an Au(111) surface bearing a 400 × 100 nm2 scratch made in a smooth Au(111) terrace (a). Depth profile histogram exhibiting the depth value distributions related (calculated from the region marked by the white-dashed rectangle) to bare gold, blue line and the micelles, red (b). From the height difference between the later, the thickness of the film, that is 1.4 nm, can be obtained. Author Contributions: Conceptualization, G.G.; methodology, M.C.R.G. and M.L.-S.; software, R.M.S. and M.C.R.G.; validation, G.G., R.M.S. and A.H.C.; formal analysis, G.G., A.H.C., M.C.R.G. and R.M.S.; resources, E.P. and A.H.C.; data curation, M.C.R.G. and R.M.S.; writing—original draft preparation, R.M.S., M.C.R.G., M.L.-S. and G.G.; writing—review and editing, G.G., R.M.S., E.P. and A.H.C.; funding acquisition, E.P. and G.G. All authors have read and agreed to the published version of the manuscript. Funding: The Spanish Ministry of Science and Innovation (MICINN) has supported this work under project ENE2017-83976-C2-2-R (co-funded by FEDER). The APC was funded by MDPI. Data Availability Statement: The data presented in this study are available in the current work. Acknowledgments: Authors acknowledge the use of SEGAI—ULL facilities. G.G. acknowledges the “Viera y Clavijo” program (ACIISI & ULL), NANOtec, INTech, Cabildo de Tenerife and ULL for laboratory facilities. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the micelles are available from the authors.

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