Article pubs.acs.org/JPCC
Integration of Quartz Crystal Microbalance with Vibrational Sum Frequency Spectroscopy−Quantification of the Initial Oxidation of Alkanethiol-Covered Copper Saman Hosseinpour,† Markus Schwind,‡ Bengt Kasemo,‡ Christofer Leygraf,† and C. Magnus Johnson*,† †
Department of Chemistry, Division of Surface and Corrosion Science, KTH Royal Institute of Technology, Drottning Kristinas v. 51, SE-100 44 Stockholm, Sweden ‡ Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *
ABSTRACT: We report the first integration of the interface sensitive technique vibrational sum frequency spectroscopy (VSFS) and the mass sensitive technique quartz crystal microbalance (QCM). VSFS−QCM has been applied in-situ to follow the formation of a thin Cu2O-like oxide on octadecanethiol-covered copper in dry air at ambient pressure conditions. We observed significant changes and an evolution of the VSF spectra caused by alterations in the electronic properties of the metal surface, and simultaneous shifts in the QCM resonance frequency due to a mass change during the formation of the oxide. QCM and VSFS exhibit a resolution corresponding to the formation of around 2% and 5% of an ideal monolayer of Cu2O, respectively. The successful integration of QCM increases the versatility of VSFS in numerous applications, where simultaneous in situ mass and spectroscopic information is desirable.
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INTRODUCTION The detection of interfacial reactions and adsorbed surface species is of paramount importance in most surface-related phenomena observed in, e.g., protein adsorption, electrochemistry, heterogeneous catalysis, cell membrane reactions, corrosion, and oxidation. The current study forms part of a broader program with the aim to develop a molecular foundation for atmospheric corrosion, a highly complex form of corrosion involving three phases, the metal, the aqueous adlayer, and the gas phase, and two interfaces, the solid/liquid and the liquid/gas interfaces.1 As part of this effort, vibrational sum frequency spectroscopy (VSFS), an interface sensitive tool that allows studies of the structure and dynamics of surface processes on a molecular level,2 has been used to obtain molecular information about critical issues in atmospheric corrosion. These investigations include the liquid/gas interface3 and also the ligands formed at the solid/liquid interface.4 In contrast to the linear techniques infrared and Raman spectroscopy, VSFS possesses an inherent surface sensitivity due to the second-order nonlinear interaction of light with matter5 and the lack of inversion symmetry at surfaces and interfaces. VSFS has several important advantages, such as a submonolayer detection limit, inherent surface sensitivity, and its capability to deduce orientational information about the surface molecules. However, a limitation of VSFS is the difficulty to quantify the data in order to deduce the number of scattering species or their surface coverage, for example. The reason is the multitude and complexity of different contributions to the total VSFS signal. The VSFS signal depends on © 2012 American Chemical Society
such diverse parameters as the number density and the average orientation of interfacial species,6 the phase and amplitude of the contribution from the metal substrate compared to that of the adsorbate,7 the Fresnel factors,8 and possible resonances of the visible beam frequency with optical transitions in the interfacial regime extending from the metal to the adsorbate.9,10 Another factor that may complicate the quantification of VSFS signals is the possible enhancement of the VSFS signal due to surface plasmons, similar to surface-enhanced Raman scattering11 on structured surfaces.12 The overall conclusion is that in order to calibrate the VSFS signal intensity with respect to surface coverage or amount of scattering species, it is necessary to introduce another, independently acting, surface sensitive technique in parallel with VSFS.2 Alkanethiol-based self-assembled monolayers (SAMs) on metallic surfaces have been extensively studied with VSFS, which has allowed the conformation and orientation of the SAMs to be determined.13 Thiols as possible candidates for metal corrosion inhibition have also been explored,14 and the ability of the thiols to reduce the corrosion rate has been attributed to the hydrophobic hydrocarbon chains and their possibility to retain an all-trans configuration in contact with water.15 However, these layers are permeable to corrosive species such as O2 or H2O, most probably through structural defects in the close-packed layer.16 Recently, the oxidation of octadecanethiol (ODT)-covered polycrystalline copper under Received: July 4, 2012 Revised: October 24, 2012 Published: October 26, 2012 24549
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correlation of QCM and VSFS, we also discuss the direct spectroscopic information obtained by VSFS. In this first effort to integrate VSFS and QCM, we have chosen a metal−adsorbate system that is well-known to us, rather than exploring a completely new system. The reason is the many possibilities in which the quantification of VSFS data otherwise could have been misinterpreted. Hence, the present paper is a natural extension of the previously reported one on ODT-covered copper exposed to dry air.17 In contrast to those semiquantitative measurements of the oxide film growth, the new experiments yield information about the oxidation process on an absolute mass scale. In addition, a more thorough discussion on the physical interpretation of the presented VSFS data is given. To put the issue of quantitative in situ measurements of ultrathin oxide film formation into a broader perspective, this study should also be viewed in relation to another study, in which a third in situ optical technique, indirect nanoplasmonic sensing, has been introduced together with VSFS and QCM and applied to Cu-ODT exposed to dry air.30 The successful integration of QCM with VSFS is expected to increase the versatility of VSFS in numerous fields. Examples of improved possibilities, just to mention a few, are studies of the kinetics of adsorbate formation (where changes in ordering during increased adsorbate packing are followed by VSFS, while QCM follows the simultaneous mass increase),31 the interaction between polymer films and humidity in the environment (where VSFS can reveal changes in the molecular structure of the polymer films induced by humidity while QCM can monitor concomitant mass changes),32 the corrosion of polymer electrolyte membrane fuel cells (where VSFS can monitor the structural changes of the polymer during degradation and QCM of the simultaneous mass loss),33 and the biodurability of potentially toxic nanoparticles. Further, the solid/liquid interface of QCM crystals can potentially be investigated by VSFS if liquids not absorbing IR radiation in the spectral region of interest are used.34 This will allow real time examinations of protein and surfactant (e.g., thiolization) adsorption from the liquid state on metal surfaces, for example.
dry air exposure conditions was studied with VSFS and complemented with infrared reflection/absorption spectroscopy (IRAS) as well as cathodic reduction investigations.17 The VSFS data provided evidence that oxygen molecules from air penetrate through the ODT layer and react with the underlying copper atoms, resulting in the formation of an ultrathin oxide layer on the copper surface. The thickness of this oxide was below the detection limit of IRAS, while the upper limit of the oxide thickness could be determined with cathodic reduction to be approximately 1 nm after 19 h of exposure. In this article we report the first integration of the quartz crystal microbalance (QCM), possessing a submonolayer mass sensitivity,18,19 with VSFS. This integration of the two techniques enables a real time quantification of the VSFS signal in terms of a mass change obtained under in situ conditions. An effort has been made to correlate the changes of two of the main parameters of VSFS, the phase and the amplitude of the nonresonant part, to the mass changes detected by QCM. QCM possesses a great versatility and has been applied to explore, e.g., the kinetics of various corrosion processes under in situ conditions, for instance, in combination with electrochemical techniques to study reaction rates of surfaces in contact with solutions20,21 or after integration with IRAS, enabling the in situ chemical characterization of corrosion products and their quantification during exposure of copper or zinc to humidified air containing carboxylic acids.22,23 The combination of QCM and nanoplasmonic sensing, a novel optical technique following the changes of localized surface plasmon resonances in metal nanoparticles, has also been used to study the corrosion of aluminum nanoparticles in water.24 Other important areas where QCM has been successfully applied are in protein adsorption on metal surfaces,25 in determinations of the kinetics in nanoparticle adsorption as well as the layer thickness in, for example, layerby-layer assemblies,25 and studies of antibody−antigen interactions.26 Combining QCM with VSFS in-situ will in addition to the mass information obtained by QCM simultaneously provide spectroscopic information that can reveal changes in the molecular structure of the adsorbate without the introduction of possible contaminants during the sample handling in the ambient laboratory atmosphere. While VSFS and QCM have been used to study metal−adsorbate systems in separate experiments,27−29 the techniques have to the authors’ knowledge never been fully integrated for real time in situ measurements on the same system. A few efforts were made previously in the authors’ laboratory to integrate QCM and VSFS. They were, however, not successful as the signal changes of the QCM resonance frequency during the experiments were smaller than the changes due to the temperature instability in the applied QCM system. An effective temperature control of the QCM quartz crystal was, thus, the key to the successful integration of QCM and VSFS for the detection of submonolayer changes in the present work. In spite of the excellent temperature stability in the QCM system applied in this study, it is still necessary to compensate for temperature changes in the quartz crystal induced by the impinging visible and infrared laser beams during the VSFS measurements. One part of this paper, therefore, discusses this compensation. In the main part of the paper, the mass changes measured by QCM are correlated with the nonresonant signal obtained with VSFS during a corrosion measurement. As a model system, octadecanethiol (ODT)-covered copper exposed for 10 hours to dry air has been chosen. Besides the possible
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EXPERIMENTAL SECTION Vibrational Sum Frequency Spectroscopy (VSFS). The experimental VSFS setup has been described in detail in a previous publication, and here only the most essential concepts are discussed.17 An Ekspla Nd:YAG picosecond laser (PL2251A-20) with a 27 ps pulse length and a repetition rate of 20 Hz was used to generate the fundamental output at 1064 nm. In these measurements the output energy was adjusted to be 25 mJ on average. To generate the tunable IR and the 532 nm (“visible”) beam, a Laservision OPG/OPA with nonlinear crystals was employed. The output IR beam with an average energy of 300 μJ was tuned in the spectral range 2750−3050 cm−1 with a scan rate of 1 cm−1/s. To overlap the IR and visible beams, a copropagating geometry was used with angles of incidence of 63° and 55° with respect to the surface normal, respectively. To account for fluctuations in the intensity of the laser beams, the VSF spectra were normalized by the IR and visible energies. The intensities of the IR and visible laser beams were measured by reflecting parts of the beams to energy meters during the measurements. All VSF spectra shown were recorded with the VSF, visible, and IR beams polarized in the plane of incidence, denoted as the PPP polarization combination. 24550
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VSFS Principles. For a vibration to give rise to a VSFS signal it needs to be both IR and Raman active. The VSFS signal is enhanced when the frequency of the IR beam coincides with a vibrational transition frequency of the surface molecules. The intensity of the VSFS signal (IVSF) is dependent on the intensity of the incoming beams (IIR and Ivis) and the effective 35,36 second-order susceptibility (χ(2) eff ) according to eq 1:
shifts upon the mass changes on the sensor surface in real time according to the Sauerbrey equation Δm = −C Δf /n
where Δm is the added mass, n is the order of harmonic oscillation, Δf is the shift in the resonance frequency, and C is the sensitivity constant related to the crystal properties (17.7 ng·Hz−1). Sample Preparation. Copper-coated AT-cut quartz crystals with a thickness of 300 nm copper (5 MHz, Q-Sense Sensors) and with a diameter of 13 mm were used as the substrates. The samples were first sonicated in absolute ethanol (Merck, Germany, min 99.9%) for 5 min. After sonication, the samples were immersed in a solution of 5 wt % amidosulfonic acid in Milli-Q water for 60 s for the oxide layer on the samples to be removed.40 The octadecanethiol (ODT, CH3(CH2)17SH, 98%, Aldrich) deposition was performed for 2 h in a 1 mM ODT/ethanol solution purged with a stream of nitrogen (N2) gas 30 min before and also during the deposition. The samples were then dried with nitrogen gas and immediately put into the exposure cell. To avoid any contamination all the glassware was kept in dilute Deconex 11 Universal (Borer Chemie AG, Zuchwil, Switzerland) for at least 24 h and then rinsed with a vast amount of Milli-Q water (18.2 MΩcm). Exposure Chamber. The Q-Sense window module 401 was used as the exposure chamber. Having a transparent sapphire window over the QCM crystal allowed the laser beams to overlap on the surface while the chamber was purged with dry nitrogen or dry air. Although the birefringent property of the sapphire window results in mixed S and P polarized fields at the surface, this effect is insignificant.41 The internal chamber volume over the sample was 100 μL. The dry nitrogen used here had an oxygen content of less than 100 ppb (ppb = volume parts per billion). Filtered, dried, and CO2 reduced (less than 20 ppm, volume parts per million) air was used in all the measurements. The temperature of the incoming air/ nitrogen was kept at 21 ± 0.25 °C using a thermostatic bath described elsewhere.42 The temperature of the crystal surface was actively controlled with a built-in temperature control in the QCM module, allowing the temperature to be controlled within ±0.02 °C. The integration of a mass sensitive technique, QCM, and a surface sensitive spectroscopic technique, VSFS, was used to identify and also quantify the oxide formation and its growth after exposure of ODT-covered copper to dry air in a closed cell under atmospheric pressure conditions. A schematic of the QCM cell integrated with VSFS is shown in Figure 1. The in situ, real time conditions entailed that the QCM data were acquired while the visible and tunable IR beams were
2
(2) IVSF ∝ χeff I visIIR
(1)
(2) χeff is a third-rank tensor with 27 elements containing information about the surface molecules through the secondorder susceptibility χ(2) as well as the Fresnel factors. χ(2) contains contributions from two components: a nonresonant (2) part (χ(2) NR) and a resonant part (χR ) according to eq 2
(2) χ (2) = χNR +
∑ χR,(2)n
(2)
n
where n is the number of resonant vibrations contributing to the VSFS signal. The nonresonant part originates from the substrate and substrate/adsorbate interactions, and the resonant parts originate from molecular vibrations of molecules residing at the surface.37 The nonresonant term will change upon formation of an adsorbate layer and can, as described in this article, be used to follow the oxidation of copper to copper oxide. The VSF spectral shape is determined by the relative phase and amplitude of the nonresonant and resonant susceptibilities according to (2) IVSF ∝ χNR + χR(2) (2) = χNR
2
2
+ χR(2)
2
(2) + 2 χNR χR(2) cos(ε − δ(ω))
(3)
with ε and δ(ω) being the phase of the nonresonant and resonant signal, respectively. Note that only the difference between these two phases can be determined in these experiments. The obtained VSF spectrum can then be fitted to the following equation 2
ISFG(ωIR ) ∝
(2) χNR
+
∑
χR(2)
n (2) ∝ χNR,eff +
∑ n
An ωn − ωIR + i Γn
(5)
2
(4)
with the An, ωn, and Γn representing the amplitude, frequency, and damping constant of the nth vibrational mode when the frequency of the IR beam ωIR is tuned in the corresponding (2) range and χNR,eff representing the effective nonresonant susceptibility. The interference between resonant and nonresonant parts of the VSFS signal generates spectra with different shapes ranging from resonances observed as dips, derivative shapes, and peaks at the resonance frequencies. For metals, the contribution of the nonresonant part of the signal is usually large and needs to be considered carefully in analyzing and interpreting the VSFS results. A frequency independent nonresonant background is assumed in all spectra since the spectral range is very narrow.38,39 Quartz Crystal Microbalance (QCM). The Q-Sense E1 module with an accompanying electrical interface was used to measure the fundamental and harmonic resonant frequency
Figure 1. Integrated QCM/VSFS system. 24551
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RESULTS AND DISCUSSION The in situ VSF spectra of copper-coated quartz crystals covered with ODT are presented in Figure 3. The spectra were
hitting the QCM sample. It was noticed that both beams had an effect on the frequency response obtained by the QCM. Since the intensity of the visible beam was basically constant during the experiments, its effect can be neglected on the overall frequency response. However, it should be noted that the tiny fluctuations of the visible beam intensity increased the noise level of the QCM response compared to QCM measurements with the laser turned off, as shown in the inset of Figure 2. In
Figure 3. Evolution of the VSF spectra in the CH stretching region during exposure to dry air. The spectra are offset for clarity. Experimental data and fitted curves are shown as empty squares and black lines, respectively. For visual clarification the main peaks related to the CH3 symmetric stretch (∼2875 cm−1), CH3 Fermi resonance (∼2938 cm−1), and CH3 antisymmetric stretch (∼2965 cm−1) are marked with vertical dashed lines. The resonance frequency is constant, but it appears as if it is changing due to the continuously changing phase difference between the resonant and nonresonant contributions.
Figure 2. Simultaneous recording of the changes in the frequency response of the QCM sensor crystal (black) and the intensity of the tunable IR beam (red). A scan from 2750 to 3050 cm−1 starts at time zero and ends at 300 s. The subsequent instantaneous drop in the QCM frequency is due to the IR beam being blocked during 80 s. After that period of time a new VSFS scan starts. The inset shows the different noise levels in the QCM signal with and without IR and visible laser beams incident on the surface.
acquired in the CH stretching region from 2750 to 3050 cm−1, while each sample was exposed to a flow of dry air in the exposure cell. In a previous study,17 it was shown that an immersion of copper in amidosulfonic acid removes the thin oxide layer formed on the polished copper surface during exposure in ambient air. Upon immediate deposition of ODT, a self-assembled monolayer was formed on the copper sample, as seen with VSFS. The evolution of the VSF spectra during exposure to dry air for up to 10 h is shown in Figure 3. The initial spectrum, denoted as “before exposure”, was acquired in a flow of pure N2. This spectrum is considered to be representative of nonoxidized copper. However, since all measurements were performed in N2 at atmospheric pressure with traces of oxygen gas present and since it takes some time (∼15 s) to insert the sample in the exposure cell, the possibility of formation of low amounts of copper oxide cannot be excluded. However, a good reproducibility in the results was obtained. Previous reference experiments have shown that the VSF spectrum of a similar sample remained unchanged after prolonged exposure to N2 up to 24 h.17 The complete assignment of the resonance peaks in this region has previously been dealt with by Lu et al.,43 MacPhail et al.,44 and Jennings et al.,45 for example. Briefly, the CH region contains the CH2 symmetric stretching vibration at ∼2850 cm−1, the CH3 symmetric stretching vibration at ∼2875 cm−1, the CH2 antisymmetric stretching vibration at ∼2912 cm−1, a Fermi resonance between a CH3 bending overtone and the CH3 symmetric stretching vibration at ∼2938 cm−1, and the CH3 antisymmetric stretching vibration at ∼2965 cm−1. However, due to the Cs symmetry of the terminating methyl group in this system, two antisymmetric peaks at 2951 and 2967 cm−1 were used in the fit, as proposed by Bain et al.39 Previous fitting of the same system was performed with only five peaks17 and
contrast to the fixed visible beam, the IR beam was scanned during the in situ VSFS measurements from 2750 to 3050 cm−1, resulting in changes in the IR energy. These changes are directly reflected in the frequency of the QCM crystals with around 0.2 Hz change in this region. After each scan in the mentioned range, the IR beam was blocked for ∼80 s for the motors connected to nonlinear crystals in the OPG/OPA and the monochromator to return to their initial positions, and also for a background spectrum to be acquired to account for the noise in the system. During this period, during which the IR beam was blocked, the frequency obtained from the QCM dropped, and after the IR beam was unblocked and the new VSFS measurement was started, the QCM signal returned to a value close to the frequency for the starting point of the previous VSFS scan. The difference between the QCM frequencies at the beginning of two concomitant VSFS measurements represents the real frequency change due to the mass change on the crystal within this period. In Figure 2, the correlation between the QCM frequency and the IR beam intensity recorded in the corresponding time is shown. For the values from these two independent measurements to be comparable, an arbitrary factor is used to match the QCM frequency with the IR intensity. As clearly seen, the QCM response follows the IR beam energy. Having compensated for the IR beam influence, the remaining changes of the resonance frequency are due to oxide formation and the influence of the visible beam. However, the intensity of the visible beam is constant as mentioned above, and therefore the remaining frequency change is due to the oxide formation. 24552
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resulted in slightly larger fitting errors, but the resulting phases and amplitudes were very similar to what is presented here. The VSF spectrum obtained under these oxide-free conditions clearly shows resonance features assigned to CH2 and CH3 groups, the peaks from the former originating from gauche defects present in the ODT chain. For an all-trans configuration in long hydrocarbon chains, the CH2 groups reside in a centrosymmetric environment with an inversion center between each pair of CH2 groups and are thus VSFS inactive. In the presence of gauche defects, the inversion symmetry is broken and the CH2 vibrations appear in the VSF spectrum.46 As evident from Figure 3, the shape of the spectra continuously changes during the exposure to dry air, and the resonant features are transformed from dips to peaks during the sample oxidation. Thus, a clear indication of the ongoing oxidation process is provided by the VSFS spectral evolution in the CH stretching region. The evolution of the VSF spectra when the oxide film is formed is due to the fact that copper and cuprite have different nonresonant backgrounds (amplitude and phase), resulting in a time dependent constructive and destructive interference between the resonant and the nonresonant VSF signals.17 All the fits were performed with the same or opposite phase for the resonant vibrations, as opposed to the use of different phases for different vibrational modes reported in some studies.47,48 However, since the fitting with the same (or opposite) phases matches the experimental data very well, the use of different phases was neglected. Next, we consider the interpretation of the VSFS data by extracting the fitted phase and amplitude of the nonresonant part of the VSFS signal according to eqs 3 and 4. In Figure 4,
ously in the system studied here. Rather, the fact that the nonresonant background varies throughout the exposure reflects that a number of different surface-related properties change continuously during the formation of the thin oxide layer. A detailed discussion on this issue has been given for selfassembled n-alkanethiols adsorbed on gold.37 Some critical factors that can change the spectrum and complicate the interpretations are: the overall shape of the VSF spectrum and its dependence on the surface coverage of adsorbed species that can give rise to both constructive and destructive interference between the resonant and nonresonant contributions to the signal, the change in adsorbate orientation, conformation or degree of ordering, variation of phase shift between different resonant vibrations,9,47 substrate−adsorbate interactions, possible resonances of the visible beam with optical transitions in the metal−adsorbate interfacial regime,9 Fresnel factors,8 or changes in VSFS signal yields caused by plasmon enhancements.12 However, the phase and the amplitude still constitute important parameters for qualitatively following the oxidation in VSFS studies alone.17 Figure 5a displays the mass changes detected by QCM simultaneously to the acquisition of the VSF spectra during exposure for up to 10 h of ODT-covered copper to dry air. The mass data were obtained in situ by monitoring the change in
Figure 4. Changes in the phase (solid symbols) and amplitude (open symbols) of the nonresonant part of the VSFS signal during the exposure to dry air. Three separate measurements are shown to illustrate the reproducibility of the obtained results. Since the absolute VSFS amplitudes are irreproducible between different measurements for experimental reasons, the amplitudes have been normalized to have the same value at the starting time.
these parameters have been plotted versus exposure time. As evident from Figure 4, both the amplitude of the nonresonant part of the efficient second-order susceptibility, χNR,eff, and the phase shift between the resonant and nonresonant parts start to change when the exposure starts and change continuously for 10 h. While the amplitude of χNR,eff and the phase shift between the resonant and nonresonant terms can be obtained from the fits, the physical meaning of χNR,eff cannot be defined unambigu-
Figure 5. Calculated mass changes obtained from (a, top) three independent QCM measurements integrated with VSFS when ODTcovered copper crystals were exposed to dry N2 gas (t < 0 h) and then to dry air (t > 0 h). The zero point on the x-axis is set to match the time for the onset of the exposure to dry air. The zero point on the yaxis corresponds to zero mass gain at the beginning of the exposure to air. (b, bottom) Long-term QCM measurement (black) and a linear fit to the experimental results (red). 24553
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formation of an inhomogeneous oxide layer or the formation of oxide as localized islands could not be excluded with the current methods. This could also lead to an essentially constant appearance of gauche defects. As a comparison of the capability of VSFS in detecting the initial oxide formation on ODT-covered copper surfaces, cathodic reduction measurements resulted in an estimated oxide thickness with an upper limit of 1 nm after 19 h of exposure in dry air.17 The kinetics of the mass gain obtained from QCM appears to indicate an oxide growth rate which is fast in the beginning and then levels off (Figure 5a) during the first 10 h. Because of the extremely small mass gains monitored and the temperature sensitivity of the QCM, it turned out that the temperature variations of the incoming air (±0.25 °C) in long time measurements (>10 h) obstructed the results and gave misleading mass gain results. For this reason the QCM data in Figure 5a after longer exposure times than 10 h had to be excluded, since the temperature stability of the incoming air could not be maintained for longer times than 10 h (see the Supporting Information). It should be noted that the total mass change in these measurements is extremely small (i.e., in the nanogram range) and therefore the sensitivity toward changes in temperature is higher than in measurements dealing with greater mass changes. In Figure 5a, two regions with different slopes in the mass gain can be identified. The slope in the first region, up to ∼3.5 h, which corresponds to a more rapid mass gain of ∼5 ng/ (cm2·h), is followed by a slower mass gain rate of ∼0.6−0.8 ng/ (cm2·h). This can tentatively be explained by the initial penetration of oxygen molecules through more easily accessible and permeable pathways of the ODT layer, most probably connected with gauche defects of the layer, followed by the transport through less accessible and permeable pathways of the ODT-layer after a few hours of exposure and onward. QCM measurements were additionally performed during dry air exposures of the same system for up to 10 days, Figure 5b, which gave an average mass gain rate due to oxide formation of ∼0.8 ng/(cm2·h), with the temperature fluctuations on the smaller time scale being negligible. Hence, this average mass gain rate of a 10 day exposure is comparable to the average mass gain rate observed in the second region (3.5−10 h) associated with the slower mass gain rate during the first 10 h, according to Figures 5a and 5b, and suggests that the mass increase due to the very slow oxide formation under the current conditions is nearly linear in time. The total mass gain reached after 10 days is around 200 ng/cm2. The mass gain for an ideal monolayer of Cu2O is around 28 ng/cm2, assuming a unit cell size of cubic Cu2O of 0.43 nm51 and a density of around 6 g/ cm3, which assumes that the film properties are similar to a bulk sample. This is probably not reflecting the real conditions but serves as an estimate of the thickness. With an average observed mass gain rate of 0.8 ng/(cm2·h), the equivalent mass of an ideal monolayer of Cu2O is obtained after around 35 h of exposure, and after 10 days about 7 monolayers of Cu2O are formed. Under current exposure conditions VSFS can detect significant changes within 2 h, which when related to the mass gain detected by QCM corresponds to about 5% of an ideal monolayer of Cu2O, while the QCM signal shows an immediate change after the exposure to dry air. As mentioned before QCM holds a superior time resolution and detection limit compared to VSFS. However, based on the temperature variation detected within the first 10 h of exposure
QCM resonance frequency and using eq 5. As mentioned above, the exposure cell was purged with N2 to obtain reference conditions for the VSF spectrum and the corresponding baseline for the QCM frequency of the oxide-free copper surface. Since the exposure cell needs to be opened during mounting of the sample, small amounts of humidity were introduced into the exposure chamber despite the continuous flow of N2. Therefore, during the mounting procedure, which took less than 15 s, an unknown number of water monolayers were adsorbed on the copper surface. The evaporation of these monolayers can partly be responsible for the mass loss seen upon closure of the cell and continued exposure to dry N2. A further contribution to the mass loss observed by QCM may be the evaporation of remaining solvent molecules adsorbed during the ODT deposition. The latter contribution is assumed to be a more likely reason for the mass loss, considering that water probably would not adsorb to a large extent on the hydrophobic ODT layer. After the QCM signal reached a stable frequency within ±0.05 Hz, the flowing gas was changed from N2 to dry air. This moment is observed in Figure 5a as a sudden change in the QCM response due to a change in the gas pressure inside the cell caused by a pressure change in the connected gas tubes. The frequency change and mass gain at the time corresponding to this moment were arbitrarily set to zero to show the onset of dry air exposure more clearly. The increase in the corresponding mass was then assumed to be caused by the mass gain due to the formation of a cuprite (Cu2O)-like42 layer with a mixed crystalline and amorphous structure.49 X-ray photoelectron spectroscopy measurements of a similar sample after 20 h of exposure to dry air confirmed that the oxide formed on the surface is composed of Cu(I) oxide, whereas no Cu(II) oxide could be detected, as shown in the Supporting Information. Figure 5a displays three independent QCM mass gain curves to illustrate the reproducibility. The mass gain was the result of the formation of Cu2O, and the total mass of Cu2O formed was calculated by multiplying the mass gain extracted from QCM by 8.94, which is the molar weight of Cu2O (143.1 g·mol−1) divided by the molar weight of oxygen (16 g·mol−1). After 10 h of exposure to dry air, the resulting mass gain was around 11 ± 3 ng/cm2. The measurement error is based on the scatter in results between the three curves after 10 h of exposure. Assuming the formation of a homogeneous Cu2O-layer with an expected density of around 6 g/cm3,50 an average oxide film thickness of 0.15 ± 0.05 nm (equivalent to ∼0.35% of a monolayer) was obtained after 10 h. Although the assumption of a homogeneous oxide layer may not be entirely correct, it is possible to trust the absolute mass gain due to the oxidation. Since the lattice parameters are larger for cuprite than bare copper, the ODT chains are expected to become more disordered upon oxide formation, under the assumption that the ODT molecules still are bonded to the copper atoms. As mentioned above, an increased chain disorder would be revealed by an enhanced CH2 to CH3 amplitude ratio. However, such a change was not observed and could be due to the fact that the changes are within the experimental and fitting error in our analysis. The observation may also be explained by the presence of an ultrathin oxide layer which is formed instantaneously and unavoidably during the very short time in atmospheric pressure conditions before the ODT deposition. Hence, the initial lattice parameter of the surface layer probably is more similar to that of bulky Cu2O than of metallic Cu. Further, it should be noted that the possibility of a 24554
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The Journal of Physical Chemistry C
Article
(±0.02 °C) and its effect on the frequency measured by QCM, which is about 2 Hz change per 1 °C change in temperature, the mass change detection limit of QCM under the present conditions is limited to 0.7 ng/cm2 or less than 2% of an ideal monolayer of Cu2O. The oxide formation rates under highly benign and dry oxidation conditions usually follow the so-called Cabrera−Mott theory,49,52 characterized by an oxidation rate that gradually decreases due to the highly protective properties of the oxide film under these benign exposure conditions. However, the Cabrera−Mott theory did not fit to our results. Rather, the fact that the oxide growth rate is nearly constant throughout the investigated 10 day exposure suggests that the rate-limiting step is transport of oxygen molecules through the ODT layer toward the reacting copper surface. However, a more detailed discussion on possible mechanisms for the present oxide formation is outside the scope of the article. It should be noted that the measured corrosion rates under this condition with and without laser beams were equivalent, and no detectable effect of the laser beams on the oxidation rate was observed. In Figure 5b the sharp frequency/mass changes in the beginning of the measurement and also after around 140 h are due to the laser which was turned on and then off to collect VSF spectra at these exposure times (see also the discussion about the laser effect in the Experimental Section and Figure 2). It should also be noted that the small QCM signal fluctuations discussed in Figure 5a still are present in Figure 5b but are hardly seen due to the compressed scale here. In the next step we combined the information obtained from VSFS (nonresonant amplitude and phase) and QCM (mass) to examine if the results of the two techniques can be correlated to each other, thus enabling a quantification of the VSFS results. Although a linear relationship between the increased mass due to the oxide formation on copper and corresponding changes in the amplitude or phase of the nonresonant part of the VSFS signal cannot be assumed a priori, because of the possible mutual electronic interactions between molecules at high coverage and also because of the complex nature of susceptibilities,53 it can be seen in Figure 6 that both the fitted amplitude plotted on an inverted y-axis and the phase of the nonresonant part of the VSFS signal show a good correlation with the mass gain measured by QCM. The amplitude was plotted on an inverted y-axis in Figure 6 since the value of the amplitude decreases upon oxidation (Figure 4), whereas the
mass increases. Thus, despite a significant number of contributing factors to the change in the nonresonant background as discussed above, it appears as the combined effect has a linear correlation with the mass changes detected by QCM up to 8 h, where the oxide thickness is
