Effects of Surface Treatment and Carboxylic Acid ... - ACS Publications

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Effects of Surface Treatment and Carboxylic Acid and Anhydride Molecular Dipole Moments on the Volta Potential Values of Zinc Surfaces P. Taheri,†,‡ K. Pohl,§ G. Grundmeier,§ J. R. Flores,∥ F. Hannour,∥ J. H. W. de Wit,†,‡ J. M. C. Mol,‡ and H. Terryn*,†,⊥ †

Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands § Department of Technical and Macromolecular Chemistry, University of Paderborn, 33098 Paderborn, Germany ∥ Development and Technology, Tata Steel Research, PO Box 10.000, 1970 CA IJmuiden, The Netherlands ⊥ Department of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ‡

ABSTRACT: This study investigates the Volta potentials of the differently treated zinc surfaces and the interface dipole moments after adsorption of carboxylic acid and anhydride molecules on zinc surfaces by means of scanning Kelvin probe (SKP). The interfacial bonding properties of carbonaceous contamination as well as adsorbed succinic acid, myristic acid, and succinic anhydride molecules with zinc substrates have been investigated using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The variation of the Volta potential due to the treatments applied on zinc was evaluated by means of SKP. Moreover, the dipole moments of adsorbed carboxylic acid and anhydride molecules were measured and correlated to surface hydroxyl fractions and oxide electronic properties. The results clearly showed that the zinc Volta potential varies with the oxide composition, resistance, and the configuration of the molecules adsorbed. bonding to zinc oxide surfaces.12,13 In this case, acid−base interactions occur resulting in an interfacial dipole and a change of the distribution of the charge carriers. In addition to the metal surface properties, the charge distribution is altered by the adsorbate properties, e.g., molecular dipole, orientation, density, adsorption configuration, etc.14 It was shown that the adsorption mode of carboxylic acid and anhydride molecules varies with the molecular type and surface properties. In this case, as shown in Figure 1a, the adsorption of succinic acid molecules (COOH−(CH2)2−COOH) on zinc samples with a

1. INTRODUCTION The interaction of organic molecules with inorganic semiconductors can be conducted via a number of routes.1,2 Some investigations deal with self-assembling molecules (SAM) and zinc as an important element in the steel and catalysis industries.3−6 The oxide surface and bulk properties are crucial to determine the nature of the interfacial chemical interactions. A change of the density of mobile charge carriers and the electron affinity of the semiconductor is expected to change the self-assembly procedure.7−9 Accordingly, surfaces are treated in order to obtain beneficial oxide properties.10,11 Our recent prior studies showed that carboxylic acid and anhydride groups like succinic acid, myristic acid, and succinic anhydride molecules have the capability of coordinative © 2013 American Chemical Society

Received: September 27, 2012 Revised: January 8, 2013 Published: January 8, 2013 1712

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value of the electrochemical potential, μe, of an electron in an uncharged phase. The electrochemical potential μe can be written as18 μe = −ϕM − Fψ M

(2)

After connection of two metals, the electrochemical potential μe of the electrons within both phases will be identical. For solids, the electrochemical potential is equal to the Fermi level EF. This means that the initially different Fermi levels of the tip EKP and sample Esample are equalized after connection and that charging of one sample with respect to the other will occur causing a Volta potential difference between the probe tip and sample. This is given by ϕsample − ψ KP = (ϕKP − ϕsample)/F = Δψ sample

Therefore, for a given and constant work function of the reference metal the work function of the sample will be determined by measurement of the Volta potential difference.19,20 Grundmeier et al.21 proposed the effects of metal ion density present in the passive film on the Volta potential values according to the equation

Figure 1. Adsorption configurations of succinic acid molecules on zinc the samples with (a) a low hydroxyl fraction (OH < 20%), and (b) high hydroxyl (OH > 40%) fraction and adsorption configurations of (c) myristic acid and (d) ring-opening procedure of the succinic anhydride molecules.13

ΔΨ = (RT /F ) ln(aMe1/aMe2)

(4)

where ΔΨ is the Volta potential difference between the vibrating reference electrode and the oxide surface and aMe is the activity of the ion density which can be affected by the oxide structure and presence of adsorbates on the metal surface. Consequently, the measurement contact potential (ΔΨ) provides information about either the electronic properties of the surfaces or the molecular conformation at the metal− organic interface. In this study, the interfacial chemistry of the untreated and differently pretreated zinc samples as well as the bonding characteristics of modeled carboxylate functional groups are evaluated by means of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The electrochemical and semiconductor properties of the formed oxide layers are characterized by Mott−Schottky analysis. Additionally, the Volta potential shifts of zinc surfaces due to different treatments are measured to evaluate surface electronic properties and obtain reference Volta potential values for adsorption of the subsequent molecules. Volta potential variations due to adsorption of carboxylic acid and anhydride molecules are characterized and correlated to the interfacial bonding and oxide electronic properties.

low hydroxyl fraction (OH < 20%) was mainly conducted with one end, while they are capable to adsorb on zinc surfaces with both ends in case a high enough hydroxyl fraction (OH > 40%) is present on the surface (Figure 1b). Moreover, Figure 1c shows that the adsorption of myristic acid molecules (CH3− (CH2)12−COOH) on zinc samples is conducted with one end.13 Moreover, the adsorption of succinic anhydride molecules (C4H4O3) includes a ring-opening procedure and formation of succinic acid molecules as a transient product (Figure 1d). A variation of metal oxide electronic properties, e.g., Fermi level, as a result of surface treatment leads to a change of Volta potential value. Consequently studying the Volta potential variations versus treatments applied contributes to a better understanding the oxide electronic and semiconductor properties. Additionally, adsorption of molecules on metal surfaces alters charge distribution and substantially the work function of the metal substrate due to molecular dipole moments.15 As a result, Volta potential shift can be contributed to the charge transfer between metal and polymer phases, which in turn exhibits the metal and molecule reactivity. Kelvin probe has been used successfully in ambient atmosphere to measure the difference in work function between a conducting probe and conducting or semiconducting metals films.16,17 When an electron is transferred from infinity, where the vacuum potential is considered equal to zero, into the sample it gets a potential, which is equal to the Galvani potential, ϕ. The Galvani potential is the sum of two contributions: ϕ=χ+ψ

(3)

2. EXPERIMENTAL SECTION The substrate used in this work was commercially pure zinc sheet (99.95%) supplied by Goodfellow. The samples were mechanically grinded with SiC paper in subsequent steps and then polished to different grade diamond paste (9, 6, 3, 1, and 0.025 μm). Consequently, the samples were cleaned ultrasonically in ethanol and water for 2 min. Afterward, they were dried under a stream of compressed nitrogen gas. The zinc substrates were pretreated in different conditions according to the experimental parameters summarized in Table 1. Then, they were rinsed once with deionized water for 5 s and dried with compressed nitrogen gas. The adsorption process was conducted in tetrahydrofuran (THF) including 0.1 wt % of succinic acid, myristic acid, and succinic anhydride molecules. The molecules were adsorbed for 30 min, and then the samples were rinsed by THF for 5 s to remove the nonadsorbed

(1)

where χ is the surface potential and ψ the Volta potential. The Volta potential is the electrostatic potential an electron gets when it is transferred from infinity to a position just outside the sample. The surface potential is the energy needed to transfer the electron from just outside the sample, through the electrical double layer at the surface, into the sample reaching the bulk or Galvani potential. The work function can also be defined as the 1713

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EG&G 273 potentiostat. The reference was a saturated calomel electrode (SCE) connected to the main compartment through a salt bridge. A Pt wire coupled with a capacitance of 10 nF was employed parallel with the reference electrode to reduce the phase shift induced by the reference electrode in high frequencies.23 A flat platinum plate was utilized as counter electrode (CE), and the zinc sample was used as the working electrode. The measurements were performed in a borate buffer solution including 0.075 M Na2B4O7·10H2O + 0.3 M H3BO3, pH = 8.4, and the exposed area was ∼0.78 cm2. A 10 kHz−10 mHz frequency range with 10 frequency points per logarithmic decade and sinusoidal voltage of 10 mV RMS and the potential step size of 100 mV were applied. The experimental impedance data were fitted to an appropriate equivalent circuit using Zview software. Figure 2a shows the typical Bode plot of the pure zinc sample performed after 10 min of immersion in the borate buffer solution. Figure 2b shows the equivalent circuit used to fit the experimental data consisting of two parallel capacitance and resistance elements in series with the electrolyte resistance Re. CH/Rct elements represent the double layer in the electrolyte, which includes the charge transfer resistance (Rct) and Helmholtz double layer capacitance (CH).24 Csc/Rox elements correspond to the space charge capacitance (Csc) and oxide electrical resistance (Rox).25 Considering the nonideal behavior of the capacitive elements, they were assumed as constant phase elements (CPE) in the models. The resistive elements and the CPE parameters, Q and n, of the untreated pure zinc sample determined through a fitting procedure are presented in Table 2. These data are used to calculate space charge pseudocapacitance (Csc) proposed by Mansfeld26 and Helmholtz double layer pseudocapacitance (CH) according to the procedure explained elsewhere.27,28 CH and Csc values obtained from EIS data used to build up the Mott−Schottky curves.29−31 Figure 3 shows a typical Mott−Schottky curve of the zinc oxide when (1/Ctot)2 is plotted against the applied potential. The flatband potential (Vfb) is estimated from the extrapolation of the linear part the (1/Ctot)2 versus voltage curve where (1/Ctot)2 = 0.25 The local electrode potential of the untreated and differently treated zinc surfaces before and after molecular adsorption was measured by means of a custom-made height-regulating scanning Kelvin probe (SKP). The details of the height regulation and potential measurement are explained elsewhere.32 The measured electrode potential was correlated to the Volta potential difference between the substrate and the

Table 1. Experimental Parameters Used for the Pretreatments of the Zinc Substrate samplea

solution

pH

temp (°C)

1 2 3 4 5

0.05 M HCl deionized water 1 M Na2CO3 deionized water 0.2 M H3BO3 + 0.1 M NaOH

1.9 6.6 11.5 6.6 12.3

25 65 25 25 25

a

potential (V)

duration (min)

0.8

30 30 30 30 30

The polished untreated sample is named sample 0 in the text.

molecules. Bare oxides as the reference samples were also immersed in THF for 30 min to ensure that the surface changes due to the solvent effect during the immersion are considered and the measured changes are only due to the adsorbed molecules. All of the measurements discussed in this work were repeated 3−5 times to check the reproducibility of the results and obtain mean values and error bars. To evaluate the bonding characteristics of the adsorbed components on the metallic samples, a Thermo-nicolet Nexus FTIR apparatus was used equipped with a mercury−cadmium− telluride liquid nitrogen cooled detector and a nitrogen-purged measurement chamber. The FTIR measurements of the adsorbents were taken by reflection of the incident beam at an angle of incidence of 80° using p-polarized radiation. The FTIR measurement of bare samples was conducted versus the backgrounds collected from a clean gold substrate. To characterize the surface compositions, XPS analysis was conducted with a PHI 1600/3057 instrument using an incident X-ray radiation (Mg Kα1,2 = 1253.6 eV). The vacuum pressure was ∼5 × 10−9 Torr. Narrow multiplex scans were recorded with 29.35 eV pass energy and 0.1 eV step size. The measurements were done at 45° takeoff angles with respect to the sample surface. The spectra obtained on the oxide layers were shifted to set the C−C/C−H components of the C 1s peak at a binding energy of 284.8 eV to correct the sample charging.22 To evaluate the electronic properties of the oxides, Mott− Schottky analysis was performed at different dc potentials ranging from −1.0 to 0.8 VSCE. Mott−Schottky plots were built from parameters obtained from electrochemical impedance spectroscopy (EIS) data modeling. EIS measurements were conducted in a conventional three-electrode cell using an

Figure 2. (a) Typical Bode plot of the zinc oxide obtained at OCP. (b) Representative equivalent circuit of the zinc oxide/electrolyte system used for the EIS data fitting. 1714

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Table 2. EIS Fitting Data of the Untreated Pure Zinc Sample at Different Potentials E (VSCE)

Re (KΩ cm2)

Rct (KΩ cm2)

QH (10−6 sn Ω−1 cm−2)

nH

−1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 rel error (%)

1.43 1.43 1.49 1.49 1.49 1.49 1.49 1.48 1.48 1.48 0.2−0.8

24.0 9.2 17.3 10.7 12.8 14.0 14.2 15.5 7.9 9.6 8−13

78.5 14.6 10.4 12.1 10.3 90.0 72.9 60.4 3.6 1.8 0.6−4

0.89 0.91 0.92 0.92 0.92 0.99 0.93 0.94 1.00 1.00 0.02−0.1

CH (μF cm−2)

Rox (MΩ cm2)

Qsc (10−6 sn Ω−1 cm−2)

nsc

8.24 14.94 10.75 12.13 10.37 9.00 7.17 5.95 36.01 18.63

146.300 0.016 0.007 0.011 0.009 0.010 0.008 0.007 0.026 0.203 0.001−4

203.4 25.9 64.0 30.5 32.5 27.8 32.1 32.4 4.8 4.5 0.2−3

1.00 0.93 1.00 1.00 1.00 1.00 1.00 1.00 0.94 0.94 0.02−0.09

Csc (μF cm−2) 22.91 9.61 26.74 8.13 8.38 6.80 6.06 18.27 5.04 4.78

Figure 3. Typical Mott−Schottky plot of the zinc oxide.

reference electrode.33 The reference electrode used was a vibrating Cr/Ni wire with tip diameter of 100 μm and the potential of the reference electrode was calibrated against a Cu/ CuSO4 electrode in humid air (RH > 95%). Potential mappings were performed on the samples in the area of 5 mm × 5 mm in a dry inert atmosphere of N2 (RH < 10%). The potentials are given relative to a standard hydrogen electrode (SHE).

Figure 4. FTIR-RAS spectra of (a) untreated pure zinc substrate, (b) sample 2, and (c) sample 4 obtained with respect to a clean gold substrate.

3. RESULTS AND DISCUSSION 3.1. Surface Composition of the Untreated and Differently Pretreated Zinc Samples. Figure 4 shows infrared spectra of the differently pretreated zinc samples with respect to a clean gold substrate. This range is denoted as the fingerprint of the IR spectrum, which is comprised of specific vibrational modes of the organic compounds. The samples show a band around 910 cm−1, which is presumably the δ(OH) hydroxyl bending.34 Moreover, the broad bands around 3350 cm−1 can be ascribed to hydrogen stretching vibrations.35 Van den Brand et al.34 ascribed this band to the water in the oxide structure. On the other hand, the stretching vibration of water molecules is associated with the bending vibration δ(H2O) around 1650 cm−1 with a fixed 1:6 integrated peak intensity ratio.36−38 However, the FTIR spectra in Figure 4 do not show well-resolved H2O-related bands in this region. Consequently, it is probable that the bands around 3350 cm−1 originate from the hydroxyl fraction associated with the bands around 910 cm−1. The bands around 1620 cm−1 can be ascribed to the carbonyl (CO) stretching vibrations of carbonates. Other studies reported that the carbonyl vibration mainly appears around 1700 cm−1.39−41 Consequently, the observed shift to the lower wavenumbers of the carbonyl stretching vibration in this study

originates most likely from an interaction between zinc adatoms and the CO components resulting in a lower bond order.42 It is known that upon exposure of the metal substrates in the ambient air several organic contaminants form on the hydroxylated oxide surface, giving rise to a carbonaceous contamination.43 In order to assess the surface contamination and composition in detail, XPS measurements were performed on differently pretreated samples. Figure 5 shows the XPS, O 1s, and C 1s peak fittings of the untreated pure zinc sample. It can be seen that the O 1s peak is resolved into three subpeaks located around 530, 531.5, and 533 eV. They can be due to the contribution of O2−, OH−, and COx components, respectively.12 The presence of COx on zinc surfaces can be ascribed to surface carbonate and hydroxycarbonate species, which are immediately formed on zinc surfaces under atmospheric conditions by reactive adsorption of carbon dioxide and water.44,45 This phenomenon resulted in elimination of H2O and formation of COx on zinc surfaces. This is in an agreement with the FTIR results regarding the absence of water monolayers on Zn surfaces. 1715

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Figure 5. XPS (a) O 1s and (b) C 1s peak fittings of the untreated pure zinc substrate.

Table 3. Peak Deconvolution of C 1s Peaks Corresponding to C−C/C−H, C−COO C−O, CO, and O−CO/O−C−O− Components of the Untreated and Differently Pretreated Zinc Samples C−C/C−H

C−COO

C−O

CO

O−CO/O−C−O−

sample

BE (eV)

fwhm (eV)

BE (eV)

fwhm (eV)

BE (eV)

fwhm (eV)

BE (eV)

fwhm (eV)

BE (eV)

fwhm (eV)

peak area (c/s)

untreated pure zinc 1 2 3 4 5

284.80 284.74 284.77 284.73 284.74 284.72

1.70 1.44 1.48 1.50 1.66 1.55

285.30 285.34 285.37 285.33 285.34 285.32

2.00 1.74 1.78 1.80 1.96 1.85

286.40 286.44 286.47 286.43 286.44 286.42

1.70 1.34 1.38 1.40 1.56 1.45

287.70

1.90

287.77

1.78

291.00 288.34 288.37 288.33 288.34 288.32

1.80 1.54 1.58 1.60 1.76 1.65

890 493 369 806 797 733

illustrated by the absence of the CO subpeak. However, in the case of sample 2, the higher pretreatment temperature might lead to the reformation of carbonyl components upon exposure to the ambient air. Our previous study also showed that the surface pretreatments could result in some changes in the composition of the carboxylic components on zinc surfaces.12 Table 3 also shows that the O−CO/O−C−O− peak area of the untreated sample is higher than those of pretreated ones. This indicates a partial removal of the carboxylates from the surfaces by the pretreatments. Additionally, the O−CO/O− C−O− peak areas are the lowest for samples 1 and 2 compared to the other pretreated samples. This can be due to either the carboxylate-type contamination removal or the changes in metal surface composition after the pretreatments, resulting in lowering the interaction between the zinc surface and carbon dioxide from the atmosphere.12 It can also be seen that O−C O/O−C−O− peaks of the untreated zinc sample appear at higher binding energies as compared to the others, indicating the involvement of carboxyl components with hydrogen bondings on the untreated sample.51 This can be due to the higher portion of surface contamination on the untreated zinc sample, resulting in a molecular interaction between the carboxylate components.

The hydroxyl fraction derived from O 1s peak deconvolution not only contains contributions from the oxide matrix but also from the outermost layer of oxygen-containing organic carbon contamination, like C−O and O−CO/O−C−O− species.46,47 In order to obtain a reliable hydroxyl content, it is necessary to correct the O 1s photopeak. The correction of the hydroxyl fraction was done according to the procedure described by Wielant et al.48 C 1s peak deconvolution of the untreated and differently pretreated zinc samples is summarized in Table 3. The peaks located roughly at 284.8, 285.5, 286.6, 288.1, and 289.1 eV can be ascribed to C−C/C−H, C−COOX, C−O, CO, and O− CO/O−C−O− components, respectively.49,50 Table 3 presents two types of C 1s peak deconvolutions based on the presence of a CO subpeak. It is observed that the untreated zinc sample and the sample pretreated in hot water (65 °C) exhibit a CO subpeak. This supports the presence of carbonyl bands around 1620 cm−1 in FTIR spectra of these samples (Figure 4). The presence of a CO subpeak in the C 1s spectrum of the untreated zinc sample can be associated with the interaction of carbon dioxide (CO2) of atmosphere with the zinc surface upon exposure to the air and formation of carbonyl components on top. On the other hand, the pretreatments removed the carbonyl components form the substrates as 1716

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Figure 6. Volta potential values of the untreated and differently pretreated Zn samples upon the exposure to 10% RH atmosphere.

3.2. Volta Potential Values of the Untreated and Differently Pretreated Zinc Samples. Figure 6 shows the Volta potential line scans versus distance of the untreated and differently treated zinc samples after cleaning with THF upon the exposure to 10% RH atmosphere exhibiting a variation of the Volta potential values with the pretreatments applied. This can be due to several factors such as oxide thickness, oxide conductivity, chemical composition, and surface adsorbates.52,53 It is known that the Volta potential value reversely correlates to the band gap width. Furthermore, the change in the band gap width originates from different parameters, e.g., the (hydr)oxide fractions, oxide resistance, doping concentration, etc.54 The high Volta potential value of the untreated sample and sample 2 can be due to the presence of a higher amount of the carboxylic contamination as shown in Table 3, significantly contributing to the surface Volta potential value due to the relatively strong dipoles. On the other hand, the high Volta potential value of sample 5 can be correlated to the buffering effect of the treatments solution and the applied external potential removing the native Zn oxide. Figure 7a shows the average of the first line scan Volta potential values versus the hydroxyl fraction obtained for the untreated and differently treated zinc samples. Generally, a clear correlation between Volta potential values and hydroxyl fractions can hardly be distinguished. This indicates that oxide/hydroxyl fractions are not the only determining factors controlling Volta potential values. This can be correlated to other parameters being changed with treatments. Nevertheless, an increase in Volta potential values among samples 3−5 can be seen by hydroxyl fraction. This can be correlated to the increase of the surface basicity, which in turn results in a decrease of the band gap and consequently lower Fermi level position.55 In this case, as the oxide/hydroxyl fractions determine the ratios of the zinc to oxygen atoms, a change in acid−base properties can be considered as a factor affecting the Fermi level. As mentioned, the deviation of samples 1 and 2 from the observed trend shows the role of other factors affecting the Volta potential values. Since at the flatband potential no band bending occurs, it can be considered as a reference potential for semiconductor electrodes.56 Figure 7b exhibits Volta potential variations versus oxide resistance (Rox) of the untreated and differently treated zinc samples at the flatband potential. The Volta potential values increase gradually with the oxide resistance. This indicates an increase in the surface polarization degree by the oxide resistance (Rox) as an effective oxide electronic factor. Deviation of the untreated sample from the trend can be correlated to the presence of a higher amount of carbonaceous contamination as compared to the treated samples lowering Volta potential value. It is known that the oxide layer on zinc substrate is a n-type semiconductor with a flatband potential located above the conduction band position.29,57 In this case, the oxide resistance

Figure 7. Volta potential values versus (a) hydroxyl fraction and (b) oxide resistances (Rox) obtained for the untreated and differently treated zinc samples.

(Rox) is determined by dopant concentration (ND) and consequently flatband potential (Vfb).29 On the other hand, Hausbrand et al.54 showed that the position of the Volta potential would be identical to the flatband potential of the metal oxide. This explains the coherent relation between the Volta potential values and oxide resistance (Rox). The observed deviation from this rule might be due to the relatively high donor concentration existing in the zinc oxide layers setting the Fermi level near the valence band edge. 3.3. Volta Potential Shifts of the Untreated and Differently Pretreated Zinc Samples after Adsorption of the Carboxylic Acid and Anhydride Molecules. Carboxylic acid and anhydride molecules, i.e., succinic acid, myristic acid, and succinic anhydride molecules, were adsorbed 1717

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Figure 8. (a) Infrared spectra and (b) C 1s peak fittings of (1) succinic acid, (2) myristic acid, and (3) succinic anhydride molecules adsorbed on zinc substrate.

on the differently treated zinc samples. Figure 8a shows the infrared spectra of the molecules adsorbed on the zinc substrate. The presence of the bands around 1440 and 1580 cm−1, assigned to symmetric νs(COO−) and asymmetric νas(COO−) carboxylate stretching vibration bands, respectively, together with the absence of carbonyl (CO) at 1681 cm−1, indicate that the adsorption process resulted in the formation of carboxylate salts coordinatively bonded to the surfaces. Formation of the bonding with the oxide surface accompany with surface hydroxyls consumption.13,58 The adsorption of carboxylic groups to zinc substrate can be described by the equation R−COOH + MOH → R−COOM + H 2O

(5)

Adsorption of anhydride-type carboxylics, e.g., succinic anhydride, is accompanied by a ring-opening reaction prior to the adsorption. Our study showed that adsorption of carboxylic molecules is promoted by the hydroxyl fraction on the zinc surfaces.13 To evaluate the amount of carboxylates formed due to the adsorption of the molecules, XPS measurements are performed. Figure 8b shows C 1s peaks as measured for zinc after the adsorption of carboxylic molecules. It is obvious that C−C/C−H, C−COOX, C−O, and O−C−O− subpeaks contributed to the buildup of the C 1s peak of the carboxylates. The integrated peak area of O−C−O− components located roughly at 288 eV assigned to the formed carboxylates can be used to define the level of deprotonation.12 In order to subtract the equipment variations affecting the signal intensities obtained for different samples, the O−C−O− peak area was divided by the Zn 3p peak area. Figure 9 shows the first line scan Volta potential shifts versus the bare zinc surfaces due to the adsorption of succinic acid, succinic anhydride, and myristic acid molecules against the hydroxyl fraction. It is known that a singular carboxylic acid and

Figure 9. Volta potential shifts of the zinc surfaces due to the adsorption of succinic acid, succinic anhydride, and myristic acid molecules versus the hydroxyl fraction.

anhydride molecule possesses a neutral dipole moment.59 However, the Volta potential values clearly change after the molecular adsorption. Consequently, the observed potential change is expected to mainly originate from the interfacial bond and the corresponding rearrangement of electron density at the substrate−carboxylate interface. Clear anodic potential shifts due to the adsorption of the succinic acid and succinic anhydride molecules are observed for samples 2−4. Acid organic groups interacting with basic sites on the oxide surface result in a charge transfer from the basic groups to the acid sites, creating an interface dipole with its negative pole toward the organic film and positive pole toward the oxide surface. This is in agreement with the orientation of the dipole moment 1718

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the partial electron transfer between the substrate and the adsorbate resulted in an anodic Volta potential change, while a cathodic Volta potential shift is induced by the intrinsic dipole moment due to the molecular one-end adsorption. This cathodic Volta potential contribution may overcome the anodic Volta potential shift resulted in the cathodic overall Volta potential shift of the untreated sample and sample 1. Simonyan et al.67 and Yang et al.68 also showed that the adsorption of the molecules might have a significant intrinsic dipole moment that could considerably contribute to the overall Volta potential value. The cathodic Volta potential shifts due to myristic acid adsorption are observed in Figure 9. This presumably is related to formation of bands between the long adsorbed molecular chains (Figure 1c). The deviation of samples 2 and 3 from this rule may originate respectively from the presence of a rough surface and surface statues due to the treatment in an elevated temperature and a carbonate solution, as discussed in our previous work.29 These may result in a lower molecular ordering and consequently hydrogen bonding. Moreover, the results reveal that the Volta potential changes due to the myristic acid adsorption reaches a higher value than succinic acid and succinic anhydride adsorption. This can be correlated to the longer molecular chain of myristic acid molecules leading to a stronger proton donor and a higher van der Waals force shifting reaction 5 to the right side. The work function can be defined as the potential that an electron at the Fermi level overcomes to reach the level of zero kinetic energy away from a solid surface to infinity in a vacuum.18,69 When phases with different work function are brought into contact, electrons flow from the phase with lower work function to the one with a higher work function. This process continues until electron levels equilibrate and a contact potential drop forms between the phases by formation of the Schottky barrier which blocks further electron flow and creation of a potential drop.70,71 Consequently, the electronic properties of the metal oxide are expected to define the Schottky barrier and consequently the Volta potential shifts. Figure 10 shows the Volta potential shifts due to the adsorption of the molecules versus the flatband potentials (Vfb) of the untreated and differently treated zinc samples. A gradual increase of the Volta potential shifts versus flatband potentials

of these molecules, which has the negative pole facing from the surface upward.60,61 A comparison of the Volta potential shifts of the treated samples due to succinic acid and myristic acid adsorptions reveals no absolute increasing or decreasing trend versus the hydroxyl fraction. It can be seen that Volta potential shifts increase gradually in low hydroxyl fraction, reaching maximum values and subsequently decrease at higher hydroxyl fractions. This shows complex effects of molecular chemical and intrinsic dipole moments in Volta potential values. The increase of the Volta potential shifts can be correlated to a higher density of hydroxyls at the oxide surface resulting in a higher amount of Bronsted interactions (protonation) according to reaction 5. Consequently, a higher concentration of individual dipoles leads to a larger upward Volta potential shift. However, the decrease of Volta potential shifts at high hydroxyl fractions shows that the dipole moments are not necessarily proportional to the deprotonation level. As shown in Figure 1b, an increase in the hydroxyl fraction corresponds to an increase in the portion of the succinic acid molecules adsorbed with both ends. Our studies showed that the adsorption with both ends results in a weakening of the carboxylate bands.62 This may consequently results in the degradation of the molecular chemical dipole on the surface. Moreover, as shown in reaction 5, formation of carboxylates is accompanied by hydroxyl consumption. On the other hand, the results showed that the hydroxyl fraction results in an increase in the Volta potential shift. Consequently, surface dehydroxylation probably results in a cathodic Volta potential shift decreasing the overall Volta potential shifts of the samples with a high deprotonation level. In this case, a change in the isoelectric point (pHieps) of oxide is expected. Consequently, the ratio of the molecule dissociation constant (pKa) and the isoelectric point of the oxide (pHieps) changes. As a result, the charge separation caused by proton polarization in the electrical double layer of molecule−substrate changes.63 The potential shift for the untreated sample and sample 1 is cathodic due to succinic acid and succinic anhydride adsorptions. This shows that the Volta potential change of the samples subjected to the molecular adsorption is determined not only by the chemical dipole moment but also by the intrinsic dipole moment considering the fact that the Volta potential variation due to this intrinsic dipole moment can contribute in either anodic or cathodic directions.64−66 In this case, Nazarov et al.63 showed that the metal−monolayer interface can be simulated by the Helmholtz equation: ΔV = 4πNSμ cos θ /ε

(6)

where Ns is the surface coverage, μ the dipole moment for dipoles having the angle θ to the normal, and ε the dielectric permittivity. This shows that the orientation of dipoles influences the sign of the Volta potential. As a result, a change in the surface composition may affect the molecular dipole interactions and consequently the sign of the Volta potential. The intrinsic dipole moment is induced presumably by interactions between the adsorbates. Our studies showed that the scarcity of the hydroxyl fraction might cause a change in the configuration of the formed carboxylates due to the adsorption of succinic acid and succinic anhydride molecules.13 Figure 1a shows that the adsorption can be conducted with one end due to the lack of hydroxyl fraction that consequently results in formation of CH3 functionality. In these cases, the interface dipole originated from the chemical dipole potential created by

Figure 10. Volta potential values versus the flatband potentials (VFB) of the untreated and differently treated zinc samples. 1719

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(6) Yang, H.; Sun, Y.; Ji, J.; Song, W.; Zhu, X.; Yao, Y.; Zhang, Z. Corros. Sci. 2008, 50, 3160−3167. (7) Ho, J. C.; Yerushalmi, R.; Jacobson, Z. A.; Fan, Z.; Alley, R. L.; Javey, A. Nat. Mater. 2007, 7, 62−67. (8) Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121−128. (9) Anagaw, A.; Wolkow, R.; DiLabio, G. J. Phys. Chem. C 2008, 112, 3780−3784. (10) Wapner, K.; Stratmann, M.; Grundmeier, G. Silicon Chem. 2003, 2, 235−245. (11) Grundmeier, G.; Stratmann, M. Appl. Surf. Sci. 1999, 141, 43− 56. (12) Taheri, P.; Wielant, J.; Hauffman, T.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Mol, J. M. C.; Terryn, H. Electrochim. Acta 2011, 56, 1904−1911. (13) Taheri, P.; Hauffman, T.; Mol, J. M. C.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Terryn, H. J. Phys. Chem. C 2011, 115, 17054− 17067. (14) Cooper, A. J.; Keyvanfar, K.; Deberardinis, A.; Pu, L.; Bean, J. C. Appl. Surf. Sci. 2011, 257, 6138−6144. (15) Nazarov, A.; Thierry, D.; Volovitch, P.; Ogle, K. Surf. Interface Anal. 2011, 43, 1286−1298. (16) Kelvin, L. Philos. Mag. 1898, 46, 82−120. (17) Baikie, I. D.; Smith, P. J. S.; Porterfield, D. M.; Estrup, P. J. Rev. Sci. Instrum. 1999, 70, 1842−1850. (18) Hansen, D. C.; Hansen, K. M.; Ferrell, T. L.; Thundat, T. Langmuir 2003, 19, 7514−7520. (19) Janata, J.; Josowicz, M. Anal. Chem. 1997, 69, 293A−296A. (20) McNaught, A. D.; Wilkinson, A. IUPAC Compendium of Chemical Terminology, 2nd ed.; Blackwell Science Inc.: Malden, MA, 1997. (21) Grundmeier, G.; Stratmann, M. Appl. Surf. Sci. 1999, 141, 43. (22) Barr, T. L. Thin Solid Films 1994, 253, 277−284. (23) Larramona, G.; Gutierrez, C. J. Electrochem. Soc. 1989, 136, 2171−2178. (24) Gerischer, H. Electrochim. Acta 1990, 35, 1677−1699. (25) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (26) Hsu, C. H.; Mansfeld, F. Corrosion 2001, 57, 747−748. (27) Modiano, S.; Fugivara, C. S.; Benedetti, A. V. Corros. Sci. 2004, 46, 529−545. (28) Büchler, M.; Schmuki, P.; Bohni, H. J. Electrochem. Soc. 1998, 145, 609−614. (29) Taheri, P.; Hauffman, T.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Mol, J. M. C.; Terryn, H. Electrochim. Acta 2012, in press. (30) Wilson, H. W. J. Appl. Phys. 1977, 48, 4292. (31) Büchler, M.; Schmuki, P.; Bohni, H.; Stenberg, T.; Mantyla, T. J. Electrochem. Soc. 1998, 145, 378−385. (32) Wapner, K.; Schoenberger, B.; Stratmann, M.; Grundmeier, G. J. Electrochem. Soc. 2005, 152, E114−E122. (33) Stratmann, M.; Streckel, H. Corros. Sci. 1990, 30, 681−696. (34) van den Brand, J.; Blajiev, O.; Beentjes, P. C. J.; Terryn, H.; de Wit, J. H. W. Langmuir 2004, 20, 6308−6317. (35) Alwitt, R. S. The aluminium-water system. In Oxides and Oxide Films; Diggle, J. W., Ed.; Dekker: New York, 1976. (36) Vlaev, L.; Damyanov, D.; Mohamed, M. M. Colloids Surf. 1989, 36, 427−437. (37) van Gils, S.; Melendres, C. A.; Terryn, H. Surf. Interface Anal. 2003, 35, 387−394. (38) Melendres, C. A.; van Gils, S.; Terryn, H. Electrochem. Commun. 2001, 3, 737−741. (39) Thissen, P.; Wielant, J.; Köyer, M.; Toews, S.; Grundmeier, G. Surf. Coat. Technol. 2010, 204, 3578−3584. (40) Kataby, G.; Cojocaru, M.; Prozorov, R.; Gedanken, A. Langmuir 1999, 15, 1703−1708. (41) Shustak, G.; Domb, A.; Mandler, D. Langmuir 2004, 20, 7499− 7506. (42) Wang, M.; Teslova, T.; Xu, F.; Spataru, T.; Lombardi, J. R.; Birke, R. L. J. Phys. Chem. C 2007, 111, 3038−3043.

can be seen. This can be correlated to the fact that carboxylic molecules accept electrons from the Fermi level of the metal surface oxide, thus polarizing it anodically up to its own redox potential. Consequently, an increase in flatband potential results in higher amounts of electrons supplied by the Fermi level and consequently a higher anodic shift. The observed deviation of Volta potential shifts from this rule can be correlated to the variation of the molecular configuration changing the intrinsic dipole moments.

4. CONCLUSIONS This work presents the effects of oxide resistance, surface hydroxyl fraction, and adsorbates on the Volta potentials of zinc surfaces. The results showed that the Volta potential values increased with the oxide resistance and hydroxyl fraction present on zinc surfaces. Moreover, the presence of contamination on the zinc surface increases the Volta potential due to the dipole moments contributing to the total Volta potential values. It was shown that a higher hydroxyl content at the oxide surface resulted in a higher deprotonation level of the adsorbed carboxylic acid and anhydride molecules. This in turn strengthens the chemical dipole moment and consequently increases the Volta potential shifts. Moreover, a coherent relation between the Volta potential shift due to the molecular adsorption and the flatband potentials (Vfb) of the zinc samples was correlated to a higher amount of electrons supplied by the oxide Fermi level. On the other hand, the molecular rearrangement generated by the interaction between the molecules and the surface have an additional effect on the surface molecular dipoles, namely the intrinsic dipole moment. In this case, the one-end adsorption due to the adsorption of succinic acid and myristic acid molecules resulted in cathodic Volta potential shifts. Moreover, a decrease of the Volta potential shifts for the relatively high hydroxyl fractions was attributed to the degradation of the dipole orientation due to the both-end adsorption of succinic acid molecules as well as dehydroxylation the surfaces due to the hydroxyl consumption promoted by the molecular adsorption.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was carried out under the project number MC6.06254 in the framework of the Research Program of the Materials innovation institute M2i.

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