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Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) studies of steel surface corrosion
Journal of Electroanalytical Chemistry 853 (2019) 113559
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Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) studies of steel surface corrosion Burke C. Barlow, Bao Guo, Arthur Situm, Andrew P. Grosvenor **, Ian J. Burgess * Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: SHINERS Tin (IV) oxide shells Alkaline conditions Steel Corrosion Benzotriazole
Electrochemical shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) in alkaline environments using Au@SnO2 core-shell particles is reported. Au nanoparticles covered by an approximately 4 nm thick SnO2 shell are shown to be pinhole free and provide strong surface enhanced Raman scattering of a pyridine derivative adsorbed on smooth gold surfaces. The Au@SnO2 core-shell particles are used to demonstrate the first electrochemical SHINERS spectra of corrosion products on 304 stainless steel and carbon steel. Unlike Au@SiO2 particles, the SnO2 shells are not degraded by the local high pH created by electrolysis reactions. The SHINERS spectra for 304 stainless steel are indicative of amorphous or microcrystalline Fe(OH)2 with a small contribution from Cr(VI) oxide at high overpotentials. In the presence of KCl, a band attributed to γ-FeOOH is found in the spectra. For carbon steel the SHINERS spectra did not show any evidence of Fe oxides, hydroxides or oxyhydroxides but rather the formation of the Fe-water complex, [Fe(H2O)6]nþ (n ¼ 2,3) at the electrode surface. SHINERS data for carbon steel in the presence of an organic inhibitor, benzotriazole (BTA), indicate that BTA inhibits the formation of Fe-water complexes at the surface of the electrode that form in its absence.
1. Introduction Surface enhanced Raman spectroscopy (SERS) has been used to study the corrosion products formed in situ on iron and steel surfaces since the late 1980s [1]. In most of these studies, a discontinuous silver layer was deposited on the iron or steel substrate, providing enhancement of the Raman signal for corrosion products near the boundary of the corroding substrate and the noble metal [2–5]. Similarly, SERS spectra of corrosion products, including Fe3O4 and Fe2O3 have been obtained by laser ablation of these materials onto a SERS active Ag substrate [6]. A major limitation of this SERS strategy is that the thermodynamic potential window for Ag limits the positive potential range of interest for iron and steels. Another limitation is that many solution species, such as organic corrosion inhibitors also readily adsorb on noble metal surfaces. Since the noble metal surface is electrically connected to the substrate metal, potential dependent SERS spectra may be a convolution of responses from the substrate metal as well as the discontinuous Ag coating. The development of shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) has allowed the isolation, both chemically and electrically, of the field-enhancing noble metal and the substrate of
interest. In a SHINERS experiment metal colloids that can support localized surface plasmon polariton (LSPP) resonances are coated with a thin shell (typically less than 5 nm) of non-metallic material, most commonly SiO2, to create shell-isolated nanoparticles (SHINs) [7–9]. Upon excitation with light resonant with the LSPP modes, the metal cores generate intense, highly localized electric fields that extend beyond the non-metallic shell. The shell acts as an electrical insulator and isolates the noble metal core, which typically has a strong affinity for organic molecules, from the electrolyte solution. SHINERS has been highly advantageous in fundamental studies of molecular films on single crystals [10–13]. The technique also has been used in applied studies. Honesty and Gewirth [12] used SHINERS to determine that small, benzotriazole (BTA) oligomers on grain boundaries are more susceptible to cathodic degradation on polycrystalline copper surfaces. Gewirth and co-workers subsequently used the SHINERS technique to study the role of molecular additives in copper electroplating [14] and the transpassive behaviour of Ni alloys [15]. Similarly, in a series of reports, Lipkowski and co-workers used SHINERS to study the molecular species involved in gold leaching baths [16–18]. A significant limitation of SHINERS is the susceptibility of the silica
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A.P. Grosvenor), [email protected] (I.J. Burgess). https://doi.org/10.1016/j.jelechem.2019.113559 Received 11 May 2019; Received in revised form 3 October 2019; Accepted 8 October 2019 Available online 10 October 2019 1572-6657/© 2019 Published by Elsevier B.V.
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shell to dissolve in a matter of minutes when exposed to high pH solutions [19]. Such high pH conditions can also be created electrochemically in unbuffered electrolytes when oxygen is reduced or when hydrogen is evolved at sufficiently negative applied potentials. Dissolution of the silica shell leaves the noble metal nanoparticle in direct electrical contact with the underlying noble metal substrate and the subsequent potential dependent Raman signal can be dominated by signal originating from the noble metal core of the nanoparticle. Changing the shell material to other metal oxides such as Al2O3 can increase the range of compatible pH conditions [9,20]. Herein we demonstrate the use of alkaline stable, tin oxide encapsulated gold nanoparticles (Au@SnO2) for SHINERS studies of electrified surfaces. We report for the first time SHINERS measurements on carbon steel and 304 stainless steel under potential control. In the case of 304 stainless steel, the addition of KCl to the electrolyte was investigated while for carbon steel, the role of the corrosion inhibitor benzotriazole (BTA) was investigated.
A similar procedure was followed for cleaning the carbon steel samples except that the water was dried from the surface with a stream of nitrogen. SHINs were deposited on the sample surfaces by drop casting 500 μL of the acetone suspension (ACS grade) and allowing the solvent to dry under ambient conditions. Raman data was collected on a Renishaw Invia Reflex Raman Microscope using a 785 nm laser diode and 1200 line/mm grating. Spectra were collected using 10% laser power and a 5 objective lens for a power of 16.9 mW at focus as determined in a separate experiment. The exposure time for each spectrum was 10 s. Reported Raman shifts were calibrated using the 520 cm1 band of an internal Si element. Electrochemistry was performed using an Autolab PGStat 302 N potentiostat, a Pt coil auxiliary electrode, a saturated calomel electrode (SCE) reference and 102 M KClO4 as the supporting electrolyte. No effort was made to remove dissolved oxygen from the electrolytes for either the electrochemical or spectroelectrochemical measurements.
2. Experimental section
3. Results and discussion
SnO2 SHINs preparation Citrate stabilized gold nanoparticles were synthesized according to an established procedure [21]. Briefly, 100 mL of 0.01% wt aqueous HAuCl4 solution was brought to a boil and 700 μL of 1% wt. Aqueous sodium citrate tribasic solution was quickly added. The resulting suspension was stirred at a boil for an additional 30 min and then cooled to room temperature. The SnO2 coating procedure has also been described previously [19] where 40 mL of citrate stabilized Au nanoparticle suspension was heated to 80 C. 1.33 mL of freshly prepared aqueous 5 103 M Na2SnO3 solution was added to the heated suspension and the resulting SHIN particles were immediately removed from the heating stage and transferred to centrifuge tubes. The suspension was centrifuged at 6000 r.p.m. for 30 min. The supernatant was then decanted and the SHINs were re-suspended in Millipore water. The washing and centrifuging steps were repeated two additional times. The particles were suspended in acetone (ACS grade) for storage and use after the final washing. Transmission Electron Microscopy SHINs suspensions were diluted with acetone (ACS grade) and deposited on 200 mesh Cu TEM grids (Ted Pella Inc.). TEM images were collected on a Hitachi HT7700 microscope operating at an accelerating voltage of 100.0 keV in high resolution observational mode. The reported average Au core size and SnO2 shell thickness were determined by measuring 16 particles (55 shell thickness measurements) in the TEM images using Gwyddion image analysis software. SHINs Pinhole Test To ensure the as synthesized SHINs were free of defects, a pinhole test was done using gold and silicon wafer substrates as described previously [21]. Gold substrates were prepared by RF magnetron sputtering approximately 200 nm of Au at 0.1 nm s1 on the conductive side of indium tin oxide (ITO) coated glass. The sputtering unit was operated at 20 W with a base pressure of 105 Torr and an Argon pressure of 2 103 Torr. Equal volumes (~0.5 mL) of SHINs suspension in acetone were deposited on the Si and Au substrates. These substrates were then placed in 102 M 4-methoxypyridine (MOP) solution and their Raman spectra were recorded. The presence of MOP peaks for SHINS on the Si substrate indicates a significant number of pinholes in the SnO2 shell, while their absence indicates a pinhole free shell. Electrochemical SHINERS Electrochemical SHINERS measurements were carried out using a custom made in situ Raman cell fitted with ports for auxiliary and reference electrodes. The working electrodes were cut from either low carbon steel sheets or 304 stainless steel sheets (McMaster-Carr). The stated composition of the low carbon steel was 98.8–99.3% Fe with carbon content between 0.18% and 0.26%. Sulfur and phosphorous content was 0.05%. The steel samples were abraded using 600 grit sandpaper, followed by fine polishing using 15 and 6 μm diamond suspension pastes (PACE technologies) on micropad2 polishing pads (PACE technologies). The stainless steel surfaces were then rinsed with ethanol and copious amounts of Millipore water before drying in air.
3.1. Characterization of Au@SnO2 SHINs Physical characterization of Au@SnO2 SHINs was done by observing the particle size and shell thickness using TEM. Fig. 1 shows representative TEM images of the particles. The thin SnO2 shell is visible as a lighter area around the dark Au core. Generally, the SHINs are found in clusters of between 3 and 8 particles and are interconnected either by their Au cores or by their SnO2 shells. The SnO2 shell appears to be continuous around the clusters of particles and have an average thickness of 3.7 nm with a standard deviation of 1.2 nm based on 55 measurements of the shell thicknesses. The gold cores are spheroids, having a long axis that is on average 40.2 nm with a standard deviation of 4.3 nm and a
Fig. 1. TEM images of Au@SnO2 SHINS. 2
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short axis of 30.2 nm with a standard deviation of 2.5 nm. The optical extinction of freely dispersed Au@SnO2 SHINs exhibits a maximum at 535 nm with a long wavelength tail that extends for several hundred nanometers. Although the localized surface plasmon polariton (LSPP) is not resonant with the selected Raman laser excitation wavelength, the deposition of the SHINs on solid supports clearly leads to partial particle aggregation which is known to shift the LSPP resonance to longer wavelengths. Alternatively, the diameter of the Au cores could potentially be tuned in an attempt to maximize the resonance between the Raman laser line and the deposited particles. However, this approach was not pursued in this work. 3.2. SHINERS and pinhole test of SHINs using 4-methoxypyridine Fig. 2 contains spectra collected in 102 M MOP solution for (a) SHINs on a Au substrate, (b) a Au substrate with no SHINs and (c) SHINs on a Si substrate. By comparison of Fig. 2 (a) and (b), it is evident that the Au@SnO2 SHINs enhance the MOP signal as peaks are only present in the spectrum of the Au substrate when SHINs are present on the surface. However, this experiment doesn’t differentiate between MOP molecules adsorbed on the underling gold surface or on the Au cores of the SHINS if defects in the SnO2 shells expose the Au core to the MOP containing solution. To rule out the latter, the SERS intensity was also measured using Au@SnO2 SHINs deposited on a flat Si wafer in place of the Au substrate. Comparison of Fig. 2 (a) and (c) gives evidence that the SHINs are free of pinholes. Since MOP does not adsorb on Si or SnO2, any MOP signals appearing in the spectrum for SHINs on the Si substrate must arise from MOP adsorbed on the Au cores of the SHINs through pinhole defects in the shells. SHINs are considered to be suitably pinhole free if the MOP signal originating from MOP adsorbed on the flat substrate is very large compared to the SERS signal originating from MOP adsorbed on Au nanoparticle cores. This condition is clearly satisfied and the MOP spectrum in Fig. 2a relative to that shown in Fig. 2c is indicative of pinhole free Au@SnO2 SHINs.
Fig. 3. Cyclic voltammograms (10 mV/s scan rate) for 304 stainless steel in 102 M KClO4 in the presence (a) and absence (b) of 102 M KCl. Trace c) is the same as trace b) but for a 304 s s working electrode decorated with Au@SnO2 SHINs. The crosshair on each voltammogram represents the origin.
b
scan is shown for each sample. In general, the same features are present in Fig. 3 (b) and (c) and their similarity is evidence of both the insulating nature of the particle shells and the fact that the Au@SnO2 are electrochemically inert. For electrolyte free of KCl (Fig. 3b and c), a feature in the positive going potential scan is observed at 0.46 V. This peak is often attributed to iron oxidation and the formation of either magnetite (Fe3O4) or FeOOH, through the intermediate Fe(OH)2 [22–24]. The current density drops as the potential is scanned further positive, reaching a small but stable value for potentials in the approximate range of 0.20 V–0.20 V. This corresponds to the 304 stainless steel passive state. A broad feature appears in the positive potential scan with a local current maximum at 0.55 V and an increasingly positive current is observed at potentials greater than 0.85 V. Both features are attributed to the formation of oxides of increasingly higher oxidation states of chromium [22,24,25]. In the negative going potential scan there are two main features, a small peak around 0.0 V which is attributed to the reduction of Cr(VI) to Cr(III) [22,24,25] and a broad reduction feature between 0.65 V and 1.0 V that we interpret to be the convolution of reduction peaks for molecular oxygen and iron oxides. The voltammetry in the presence of 102 M KCl was quite similar except for the larger current density of the peak observed in the positive potential scan at around 0.61 V. Depending on how quickly the local pH near the electrode drops after the most negative polarizations, the peak at 0.61 V corresponds to either active iron oxidation and the dissolution of Fe2þ species or oxidation and the precipitation of a layer of Fe(OH)2 [22]. In the case of the former, the increased current density of the voltammetric feature at 0.61 V in Fig. 3a may suggest that a thicker Fe(OH)2 layer is formed when chloride is present in the electrolyte. Alternatively, if the pH is low enough to prevent the formation of an insoluble layer, the presence of chloride likely accelerates the active dissolution of the iron.
c
3.4. Electrochemical SHINERS on stainless steel
3.3. Electrochemistry of stainless steel substrates
Raman intensity / a.u.
Fig. 3 contains cyclic voltammograms for grade 304 stainless steel (304 s s) in 102 M KClO4 with (a) 102 M KCl, (b) no KCl and (c) no KCl where the substrate is decorated with Au@SnO2 SHINs. It was found that the voltammetry changed during the first 2–3 potential cycles but became largely invariant with further cycling. Thus, the fourth potential
a
800
900
wavenumber / cm
1000
Prior to conducting the SHINERS experiments, the potential of the working electrode was cycled several times between 1.0 V and þ1.0 V vs. SCE until the current-potential traces became largely invariant with scan number. As described below, this also provided a mechanism to demonstrate the inertness of Au@SnO2 to alkaline conditions. As the unbuffered electrolyte was not de-oxygenated, the application of negative potentials results in significant pH increase near the steel electrode due to the reduction of oxygen. The increase in pH had a deleterious
1100
-1
Fig. 2. Results of pinhole testing used to determine the presence of defects in the SnO2 shells. Raman spectra of a 102 M 4-methoxypyridine (MOP) solution covering different substrates were measured. The substrates were a) Au@SnO2 SHINs on a smooth gold Au substrate b) a smooth Au substrate with no SHINs and c) Au@SnO2 SHINs on a Si substrate. 3
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remains relatively constant in the negative potential scan between 1.0 V and approximately 0.0 V. However, this band does become more intense between 0.0 V and 0.60 V. At further negative potentials the intensity of the 560 cm1 band decreases until it disappears at 1.0 V. The band position also shifts to lower wavenumbers during negative potential stepping and returns to 540 cm1 at 0.8 V. Fig. 4b contains analogous potential dependent spectra for 304 s s in 102 M KClO4 with the addition of 102 M KCl to the electrolyte. Fig. 5b contains the corresponding peak integrals and voltammetry. The presence of chloride gives rise to several changes in the spectra. During the positive potential sweep, the appearance of the main band at 540 cm1 requires a more positive potential before reaching its plateau intensity compared to the chloride-free experiment. The voltammetric peak at ca. 0.61 V is unique to the chloride containing electrolyte but is not correlated with any obvious new SHINERS signals. However, the main Raman band centered at 540 cm1 and appearing at E 0.60 V is a much broader feature in Fig. 4b compared to Fig. 4a. This may imply the presence of unresolved chloride species in the iron oxidation products that are formed prior to the onset of the 304 s s passive regime. A band at 265 cm1, which is not observed in the chloride free spectra, is evident at E þ0.40 V in the positive potential scan direction. This band is present in the negative potential scan direction for E 0.60 V (see Fig. 5b). A third difference in the spectra for the chloride containing electrolyte is the delayed appearance of the 795 cm1 peak. For the chloride free electrolyte this peak is evident around þ1.0 V in the positive potential scan while in the presence of chloride this band is not noticeable until ~0.60 V in the negative potential scan direction. The broad peak centered between 540 and 560 cm1 has been reported in SERS experiments with potentiostatically grown oxide films on Fe in solutions of borate[5], carbonate [4], nitrate [4], and sulfate [3]. In the work of Oblonsky and Devine [5], the passive film on Fe was grown by polarization at 0.1 V vs SCE for 1 h and then reduced by an initiating a negative potential scan to 0.90 V. At wavenumbers below 800 cm1, their spectra are similar in appearance to those presented in this work. Specifically, one broad band in the range of 540–560 cm1 was observed and found to disappear when the applied potential was more negative than 0.70 V. This band was attributed to the symmetric stretch (A1 mode) of amorphous or microcrystalline Fe(OH)2 although the authors noted that the spectra were also indicative of small amounts of either Fe3O4 or Fe2O3. Therefore, we interpret the broad peak centered between 540 and 560 cm1 in our electrochemical SHINERS data to be the symmetric stretch of amorphous Fe(OH)2[26]. The band appearing at 265 cm1 at 0.40 V in the presence of chloride likely arises from the formation of lepidocrocite also known as γ-FeOOH [4,27]. Chloride ions are known to stabilize this phase and a strong Raman band has been reported at ~255 cm1 [4,5]. The intermediate species in the conversion between Fe(OH)2 and γ-FeOOH is green rust, which can take several forms depending on the exact environmental conditions [2,28]. Previous in situ Raman studies of green rust showed an intense band at 665 cm1 which is notably absent in our SHINERS results [29]. In addition, the absence of strong bands in the 660-680 cm1 range also rules out any significant contribution from Fe3O4[5]. The band at 795 cm1 cannot be attributed with any degree of certainty to iron oxides as the closest reported bands are 745 cm1 for β-FeOOH and 710-740 cm1 for γ-Fe2O3 [6]. The possibility that this band is related to γ-Fe2O3 is inconsistent with the literature as additional intense bands associated with this phase should be found in the range of 645–670 cm1 [29–31]. Previous studies have attributed an 827 cm1 peak to Cr(III) oxides, specifically Cr2O3 [5, 32]. However, the appearance of the 795 cm1 band is also coincident with features in the voltammetry that are attributed to the oxidation of Cr(III) to Cr(VI). In situ thermal oxidation of Cr coupons has shown a band at 826 cm1 which was assigned to Cr(VI) incorporated into a mixed valence Cr2O3-like structure [33]. Ex situ work with Cr3O8 and Cr2O5 found that the Cr(VI)–O–Cr(VI) asymmetric stretch falls in the range of 719–848 cm1 and is likely the origin of the 795 cm1 band. This assignment is bolstered by the fact that the peak completely disappears at
effect on the stability of the shell of Au@SiO2 nanoparticles whereas Au@SnO2 nanoparticles provided stable responses during repeated potential cycling. Electrochemical SHINERS spectra of the 304 stainless steel in 102 M KClO4 are presented in Fig. 4a and are separated into positive (top) and negative (bottom) scan directions. Peak integrals for the major features are presented in Fig. 5a along with the corresponding voltammetry. The potential was first held at 1.0 V for 120s and a spectrum was collected. The potential was then stepped in the positive direction in 0.20 V intervals up to 1.0 V with a SHINERS spectrum collected at each potential step position. The working electrode potential was then stepped in the reverse direction and spectra were collected. At the initial bias of 1.0 V the spectra are essentially featureless indicating that any Raman active components of the oxide layer are completely reduced at this potential. As the potential is stepped in the positive direction a broad peak at 540 cm1 becomes evident in the spectrum acquired at 0.60 V. This peak grows in intensity, and shifts to higher wavenumbers over the next several hundred millivolts until the intensity plateaus at a potential of ~0.0 V. The intensity of this band remains more or less constant throughout the rest of the positive potential scan (see Fig. 5a). At 1.0 V, an additional weak band is observed around 795 cm1. This band becomes more prominent during the negative potential scan, reaching a maximum around 0.0 V, before decreasing in intensity and eventually disappearing around 0.60 V. The low frequency band
Fig. 4. SHINERS spectra of 304 stainless steel in 102 M KClO4 as a function of applied potential in the absence (a) and presence (b) of 102 M KCl. Both sets of spectra are divided into two panels top: spectra acquired during a series of positive-going potential steps and bottom: spectra acquired during a subsequent series of negative-going potential steps. 4
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Fig. 5. Integrated Raman intensities from Fig. 4 as a function of applied potential (top) and a linearized cyclic voltammogram (bottom) for 304 s s in 102 M KClO4 in the absence (a) and presence (b) of 102 M KCl.
potentials more negative than 0.6 V in the negative potential scan which is consistent with the fact that Cr(VI) is reduced to Cr(III) in this potential range [22,24,25].
evidenced by the SHINERS data for this system (vide infra). The reduction feature in curves (b) and (c) is likely the convolution of the reduction of surface oxides, double layer [Fe(H2O)6]3þ and molecular oxygen in the electrolyte. When 102 M BTA is present in the electrolyte (Fig. 6a), the voltammetry is quite different compared to the CV obtained in additive free electrolyte. In general, in the presence of BTA the current density is higher in the passive region and there is a small anodic feature at approximately 0.0 V in the positive potential scan. The main anodic feature observed at ~0.9 V when BTA is absent, although a significant anodic current is observed at the positive limits of the applied potential scan. The new feature is not associated with the formation of [Fe(H2O)6]3þ as will be shown in the SHINERS data and is likely related to oxidation of BTA.
3.5. Electrochemistry of carbon steel substrates Fig. 6 contains cyclic voltammetry traces for carbon steel in (a) 102 M KClO4 with 102 M BTA, (b) 102 M KClO4, and (c) Au@SnO2 SHIN decorated surface in 102 M KClO4. The similarity of curves (b) and (c) again provides evidence that the SHINs do no alter the electrochemistry of the substrate steel nor do they appear to be electroactive. There is a slight decrease of current density when the carbon steel surface is decorated with SHINs likely caused by an effective decrease in electrode surface area due to the presence of the SHINs. In the absence of BTA, the curves show only two features, an anodic peak is observed at ~0.9 V in the positive going potential scan and a cathodic peak appears at approximately 0.7 V in the negative potential scan. We interpret the anodic feature to arise from the formation of [Fe(H2O)6]þ3 as will be
3.6. Electrochemical SHINERS on carbon steel Electrochemical SHINERS spectra for carbon steel in 102 M KClO4 solution are presented in Fig. 7a, separated into positive and negative potential step directions (top and bottom respectively). A spectrum was first collected at OCP (0.20 V) followed by applied potentials stepped in the positive direction up to 1.2 V, then descending steps in the negative direction. The weak band at 931 cm1 is present in all spectra for carbon steel even in the absence of SHINs. This, combined with the fact that the intensity of the band is largely completely invariant with potential, indicates that this is not a SHINERS response. This peak can be assigned to the Cl–O stretching of the solution perchlorate ions [34]. The normal Raman perchlorate peak is also present in the 304 s s spectra. However it is not on scale with the much higher intensity SHINERS signals observed in those spectra. As the electrode potential is stepped positive from 0.20 V in 0.20 V intervals, the peak corresponding to the perchlorate stretch remains relatively unchanged while the peak observed at approximately 304 cm1 disappears above 0.40 V which is likely associated with the formation of a thin layer of Fe(II) corrosion products. At 0.80 V a new, relatively intense peak, is observed at 454 cm1 with a lower wavenumber shoulder around 400 cm1. The intensity of this peak continues to grow throughout the remaining positive potential steps and persists during the negative-going potential steps until a maximum intensity is reached at ~0.40 V on the return cycle. The peak intensity remains relatively constant as the potential is further decreased to 0.20 V before becoming completely absent at around 0.80 V. The
Fig. 6. Cyclic voltammograms (10 mV/s scan rate) for carbon steel in 102 M KClO4 in the presence (a) and absence (b) of 102 M BTA. Trace c) is the same as trace b) but for a carbon steel working electrode decorated with Au@SnO2 SHINs. The crosshair on each voltammogram represents the origin. 5
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Fig. 7. SHINERS spectra of carbon steel in 102 M KClO4 as a function of applied potential in the absence (a) and presence (b) of 102 M BTA. Both sets of spectra are divided into two panels top: spectra acquired during a series of positive-going potential steps and bottom: spectra acquired during a subsequent series of negativegoing potential steps.
454 cm1 peak demonstrates the sensitivity advantage of SHINERS as compared to normal Raman spectroscopy as the potential dependent spectra in the absence of SHINs did not show this band. The 454 cm1 band does not correspond to any known iron oxides or oxyhydroxides but has been attributed to Fe–OH2 stretching modes in Fe(III) water complexes such as [Fe(OH2)6]3þ [35,36]. The shoulder around 400 cm1 has also been documented and has been assigned to arise due to stretching modes associated with Fe–OH or Fe–OH2 bonding in solution species of [Fe(OH2)6]2þ [35,36]. The SHINERS spectra of carbon steel are quite different when the electrolyte contains 102 M BTA (Fig. 7b). At 0.20 V (OCP) the peak at 304 cm1 is absent, however, there are three additional weak peaks that can be assigned to BTA at 782 cm1, 1012 cm1 and 1387 cm1. The band at 1387 cm1 is assigned to the triazole ring stretching mode with some contributions from C–C stretching and C–H out of plane bending. The band at 1012 cm1 is assigned to the trigonal ring breathing mode while the peak at 782 cm1 can be assigned to a C–C in-plane stretch [37, 38]. It should be noted that in a separate experiment in the absence of SHINs, the bands at 782 cm1 and 1012 cm1 were clearly visible, indicating that these bands do not arise from SHINERS and correspond to BTA in solution. The band at 1387 cm1 on the other hand, was only present when SHINs were on the surface. Enhancement of this band suggests an end-on orientation of the BTA, meaning the BTA is coordinated via the nitrogen atoms in the ring [37,38]. It is also interesting to note that other bands which are expected to show enhancement based on surface selection rules for this end-on orientation are absent from the spectrum. These are the 1576 cm1 band associated with the C–C in-plane stretching mode as well as the 1290 cm1 band associated with CH in-plane bending and CC in-plane stretching [37,38]. The main feature at 454 cm1 is absent from the SHINERS spectra when the electrolyte contained BTA. In addition, despite the increased passive current density and features in the voltammetry, no peaks associated with Fe corrosion products are found in the spectra obtained at positive potentials. This supports the idea that BTA enhances the dissolution of Fe
through a very thin passive film, however it inhibits the formation of the Fe-water complexes at the surface of the electrode that form in its absence. 4. Conclusions There have been no previous demonstrations of the use of SnO2 protected gold nanoparticles for surface enhanced Raman studies despite previous reports in the literature of their synthesis. Herein, we have shown for the first time that SHINERS experiments are possible using Au@SnO2 SHINs. The synthesis procedure yielded clusters of ~35 nm diameter particles, encased in SnO2 with an average thickness of 3.7 nm. SHINERS data was gathered for a pyridine derivative on a gold substrate as a proof of concept followed by the first demonstration of electrochemical SHINERS on 304 stainless steel and carbon steel substrates. For 304 stainless steel, the SHINERS spectra were dominated by a broad feature centered around 550 cm1, which was taken to be mostly amorphous Fe(OH)2. An additional SHINERS peak at ~795 cm1 is thought to correspond to Cr(VI)–O bonds in a mixed valence (Cr(III)/ Cr(VI)) oxide. When the electrolyte contained 102 M KCl, the SHINERS spectra also had some contribution from γ-FeOOH which was formed at E 0.4 V during positive-going potential increments. This peak persisted when the potential was decreased until the applied potential was less than 0.4 V. We found no evidence for green rust, the intermediate species in the formation of γ-FeOOH from Fe(OH)2, in our SHINERS data. For carbon steel, the only feature in the SHINERS spectra was a broad feature centered at 454 cm1, which also contained a shoulder at approximately 400 cm1. These two features had been previously reported and are not due to iron oxides, hydroxides or oxyhydroxides, but rather iron water complexes present at the surface of the electrode. When the corrosion inhibitor BTA was added to the electrolyte, these features were not observed at any potential and there were no features in the Raman data attributable to Fe species, thus demonstrating the ability of BTA to inhibit the oxidation of steel. We did not observe a film of 6
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corrosion products forming on the electrode surfaces for either substrate using optical microscopy, nor did we observe any regular Raman signal from Fe species. This indicates that the Raman signals reported in this work arise from very thin films and demonstrates that enhancement from the Au@SnO2 particles is critical.
[15] N.R. Honesty, A.A. Gewirth, Corros. Sci. 67 (2013) 67–74. [16] S.R. Smith, J.J. Leitch, C. Zhou, J. Mirza, S.-B. Li, X.-D. Tian, Y.-F. Huang, Z.Q. Tian, J.Y. Baron, Y. Choi, J. Lipkowski, Anal. Chem. 87 (2015) 3791–3799. [17] S.R. Smith, C. Zhou, J.Y. Baron, Y. Choi, J. Lipkowski, Electrochim. Acta 210 (2016) 925–934. [18] S.R. Smith, J.Y. Baron, Y. Choi, J. Lipkowski, J. Raman Spectrosc. 48 (2017) 197–203. [19] S.H. Lee, I. Rusakova, D.M. Hoffman, A.J. Jacobson, T.R. Lee, ACS Appl. Mater. Interfaces 5 (2013) 2479–2484. [20] X.-D. Lin, J.-F. Li, Y.-F. Huang, X.-D. Tian, V. Uzayisenga, S.-B. Li, B. Ren, Z.-Q. Tian, J. Electroanal. Chem. 688 (2013) 5–11. [21] J.F. Li, X.D. Tian, S.B. Li, J.R. Anema, Z.L. Yang, Y. Ding, Y.F. Wu, Y.M. Zeng, Q.Z. Chen, B. Ren, Z.L. Wang, Z.Q. Tian, Nat. Protoc. 8 (2012) 52. [22] T.L.S.L. Wijesinghe, D.J. Blackwood, Appl. Surf. Sci. 253 (2006) 1006–1009. [23] L. Freire, M.A. Catarino, M.I. Godinho, M.J. Ferreira, M.G.S. Ferreira, A.M.P. Sim~ oes, M.F. Montemor, Cement Concr. Compos. 34 (2012) 1075–1081. Donik, M. Jenko, Corros. Sci. 49 (2007) 2083–2098. [24] A. Kocijan, C. [25] B. Zhang, S. Hao, J. Wu, X. Li, C. Li, X. Di, Y. Huang, Mater. Char. 131 (2017) 168–174. [26] M.G.S. Ferreira, T. Moura e Silva, A. Catarino, M. Pankuch, C.A. Melendres, J. Electrochem. Soc. 139 (1992) 3146–3151. [27] X. Zhang, K. Xiao, C. Dong, J. Wu, X. Li, Y. Huang, Eng. Fail. Anal. 18 (2011) 1981–1989. [28] S. Grousset, F. Kergourlay, D. Neff, E. Foy, J.L. Gallias, S. Reguer, P. Dillmann, A. Noumowe, J. Anal. Atomic Spectrom. 30 (2015) 721–729. [29] N. Boucherit, A. Hugot-Le Goff, S. Joiret, Corros. Sci. 32 (1991) 497–507. [30] T. Ohtsuka, K. Kubo, N. Sato, Corrosion 42 (1986) 476–481. [31] D. Thierry, D. Persson, C. Leygraf, N. Boucherit, A. Hugot-le Goff, Corros. Sci. 32 (1991) 273–284. [32] R. S, K. D, N. G, U.K. Mudali, Appl. Surf. Sci. 428 (2018) 1106–1118. [33] J.E. Maslar, W.S. Hurst, W.J. Bowers, J.H. Hendricks, M.I. Aquino, I. Levin, Appl. Surf. Sci. 180 (2001) 102–118. [34] M.I.S. Sastry, S. Singh, Can. J. Chem. 63 (1985) 1351–1356. [35] P.M.L. Bonin, W. Jędral, M.S. Odziemkowski, R.W. Gillham, Corros. Sci. 42 (2000) 1921–1939. [36] P.M.L. Bonin, M.S. Odziemkowski, R.W. Gillham, Corros. Sci. 40 (1998) 1391–1409. [37] J.L. Yao, B. Ren, Z.F. Huang, P.G. Cao, R.A. Gu, Z.-Q. Tian, Electrochim. Acta 48 (2003) 1263–1271. [38] S. Thomas, S. Venkateswaran, S. Kapoor, R. D’Cunha, T. Mukherjee, Spectrochim. Acta A Mol. Biomol. Spectrosc. 60 (2004) 25–29.
Acknowledgements The International Mineral Innovation Institute (IMII) and the Collaborative Research and Development Grants Program of the Natural Sciences and Engineering Research Council of Canada (NSERC) are thanked for funding this research. Also thanked are the potash mining companies Nutrient, Mosaic, and BHP for funding provided to the IMII. References [1] J.C. Rubim, J. Dünnwald, J. Electroanal. Chem. Interfacial Electrochem. 258 (1989) 327–344. [2] J. Gui, T.M. Devine, Corros. Sci. 32 (1991) 1105–1124. [3] J. Gui, T.M. Devine, Corros. Sci. 36 (1994) 441–462. [4] J. Gui, T.M. Devine, Corros. Sci. 37 (1995) 1177–1189. [5] L.J. Oblonsky, T.M. Devine, Corros. Sci. 37 (1995) 17–41. [6] L.J. Oblonsky, S. Virtanen, V. Schroeder, T.M. Devine, J. Electrochem. Soc. 144 (1997) 1604–1609. [7] D. Graham, Angew. Chem. Int. Ed. 49 (2010) 9325–9327. [8] J.R. Anema, J.-F. Li, Z.-L. Yang, B. Ren, Z.-Q. Tian, Annu. Rev. Anal. Chem. 4 (2011) 129–150. [9] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Nature 464 (2010) 392. [10] J.-F. Li, S.-B. Li, J.R. Anema, Z.-L. Yang, Y.-F. Huang, Y. Ding, Y.-F. Wu, X.-S. Zhou, D.-Y. Wu, B. Ren, Z.-L. Wang, Z.-Q. Tian, Appl. Spectrosc. 65 (2011) 620–626. [11] J.-F. Li, J.R. Anema, Y.-C. Yu, Z.-L. Yang, Y.-F. Huang, X.-S. Zhou, B. Ren, Z.-Q. Tian, Chem. Commun. 47 (2011) 2023–2025. [12] N.R. Honesty, A.A. Gewirth, J. Raman Spectrosc. 43 (2012) 46–50. [13] J.-F. Li, S.-Y. Ding, Z.-L. Yang, M.-L. Bai, J.R. Anema, X. Wang, A. Wang, D.-Y. Wu, B. Ren, S.-M. Hou, T. Wandlowski, Z.-Q. Tian, J. Am. Chem. Soc. 133 (2011) 15922–15925. [14] K.G. Schmitt, R. Schmidt, H.F. von-Horsten, G. Vazhenin, A.A. Gewirth, J. Phys. Chem. C 119 (2015) 23453–23462.
7
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) studies of steel surface corrosion Burke C. Barlow, Bao Guo, Arthur Situm, Andrew P. Grosvenor **, Ian J. Burgess * Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: SHINERS Tin (IV) oxide shells Alkaline conditions Steel Corrosion Benzotriazole
Electrochemical shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) in alkaline environments using Au@SnO2 core-shell particles is reported. Au nanoparticles covered by an approximately 4 nm thick SnO2 shell are shown to be pinhole free and provide strong surface enhanced Raman scattering of a pyridine derivative adsorbed on smooth gold surfaces. The Au@SnO2 core-shell particles are used to demonstrate the first electrochemical SHINERS spectra of corrosion products on 304 stainless steel and carbon steel. Unlike Au@SiO2 particles, the SnO2 shells are not degraded by the local high pH created by electrolysis reactions. The SHINERS spectra for 304 stainless steel are indicative of amorphous or microcrystalline Fe(OH)2 with a small contribution from Cr(VI) oxide at high overpotentials. In the presence of KCl, a band attributed to γ-FeOOH is found in the spectra. For carbon steel the SHINERS spectra did not show any evidence of Fe oxides, hydroxides or oxyhydroxides but rather the formation of the Fe-water complex, [Fe(H2O)6]nþ (n ¼ 2,3) at the electrode surface. SHINERS data for carbon steel in the presence of an organic inhibitor, benzotriazole (BTA), indicate that BTA inhibits the formation of Fe-water complexes at the surface of the electrode that form in its absence.
1. Introduction Surface enhanced Raman spectroscopy (SERS) has been used to study the corrosion products formed in situ on iron and steel surfaces since the late 1980s [1]. In most of these studies, a discontinuous silver layer was deposited on the iron or steel substrate, providing enhancement of the Raman signal for corrosion products near the boundary of the corroding substrate and the noble metal [2–5]. Similarly, SERS spectra of corrosion products, including Fe3O4 and Fe2O3 have been obtained by laser ablation of these materials onto a SERS active Ag substrate [6]. A major limitation of this SERS strategy is that the thermodynamic potential window for Ag limits the positive potential range of interest for iron and steels. Another limitation is that many solution species, such as organic corrosion inhibitors also readily adsorb on noble metal surfaces. Since the noble metal surface is electrically connected to the substrate metal, potential dependent SERS spectra may be a convolution of responses from the substrate metal as well as the discontinuous Ag coating. The development of shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) has allowed the isolation, both chemically and electrically, of the field-enhancing noble metal and the substrate of
interest. In a SHINERS experiment metal colloids that can support localized surface plasmon polariton (LSPP) resonances are coated with a thin shell (typically less than 5 nm) of non-metallic material, most commonly SiO2, to create shell-isolated nanoparticles (SHINs) [7–9]. Upon excitation with light resonant with the LSPP modes, the metal cores generate intense, highly localized electric fields that extend beyond the non-metallic shell. The shell acts as an electrical insulator and isolates the noble metal core, which typically has a strong affinity for organic molecules, from the electrolyte solution. SHINERS has been highly advantageous in fundamental studies of molecular films on single crystals [10–13]. The technique also has been used in applied studies. Honesty and Gewirth [12] used SHINERS to determine that small, benzotriazole (BTA) oligomers on grain boundaries are more susceptible to cathodic degradation on polycrystalline copper surfaces. Gewirth and co-workers subsequently used the SHINERS technique to study the role of molecular additives in copper electroplating [14] and the transpassive behaviour of Ni alloys [15]. Similarly, in a series of reports, Lipkowski and co-workers used SHINERS to study the molecular species involved in gold leaching baths [16–18]. A significant limitation of SHINERS is the susceptibility of the silica
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A.P. Grosvenor), [email protected] (I.J. Burgess). https://doi.org/10.1016/j.jelechem.2019.113559 Received 11 May 2019; Received in revised form 3 October 2019; Accepted 8 October 2019 Available online 10 October 2019 1572-6657/© 2019 Published by Elsevier B.V.
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shell to dissolve in a matter of minutes when exposed to high pH solutions [19]. Such high pH conditions can also be created electrochemically in unbuffered electrolytes when oxygen is reduced or when hydrogen is evolved at sufficiently negative applied potentials. Dissolution of the silica shell leaves the noble metal nanoparticle in direct electrical contact with the underlying noble metal substrate and the subsequent potential dependent Raman signal can be dominated by signal originating from the noble metal core of the nanoparticle. Changing the shell material to other metal oxides such as Al2O3 can increase the range of compatible pH conditions [9,20]. Herein we demonstrate the use of alkaline stable, tin oxide encapsulated gold nanoparticles (Au@SnO2) for SHINERS studies of electrified surfaces. We report for the first time SHINERS measurements on carbon steel and 304 stainless steel under potential control. In the case of 304 stainless steel, the addition of KCl to the electrolyte was investigated while for carbon steel, the role of the corrosion inhibitor benzotriazole (BTA) was investigated.
A similar procedure was followed for cleaning the carbon steel samples except that the water was dried from the surface with a stream of nitrogen. SHINs were deposited on the sample surfaces by drop casting 500 μL of the acetone suspension (ACS grade) and allowing the solvent to dry under ambient conditions. Raman data was collected on a Renishaw Invia Reflex Raman Microscope using a 785 nm laser diode and 1200 line/mm grating. Spectra were collected using 10% laser power and a 5 objective lens for a power of 16.9 mW at focus as determined in a separate experiment. The exposure time for each spectrum was 10 s. Reported Raman shifts were calibrated using the 520 cm1 band of an internal Si element. Electrochemistry was performed using an Autolab PGStat 302 N potentiostat, a Pt coil auxiliary electrode, a saturated calomel electrode (SCE) reference and 102 M KClO4 as the supporting electrolyte. No effort was made to remove dissolved oxygen from the electrolytes for either the electrochemical or spectroelectrochemical measurements.
2. Experimental section
3. Results and discussion
SnO2 SHINs preparation Citrate stabilized gold nanoparticles were synthesized according to an established procedure [21]. Briefly, 100 mL of 0.01% wt aqueous HAuCl4 solution was brought to a boil and 700 μL of 1% wt. Aqueous sodium citrate tribasic solution was quickly added. The resulting suspension was stirred at a boil for an additional 30 min and then cooled to room temperature. The SnO2 coating procedure has also been described previously [19] where 40 mL of citrate stabilized Au nanoparticle suspension was heated to 80 C. 1.33 mL of freshly prepared aqueous 5 103 M Na2SnO3 solution was added to the heated suspension and the resulting SHIN particles were immediately removed from the heating stage and transferred to centrifuge tubes. The suspension was centrifuged at 6000 r.p.m. for 30 min. The supernatant was then decanted and the SHINs were re-suspended in Millipore water. The washing and centrifuging steps were repeated two additional times. The particles were suspended in acetone (ACS grade) for storage and use after the final washing. Transmission Electron Microscopy SHINs suspensions were diluted with acetone (ACS grade) and deposited on 200 mesh Cu TEM grids (Ted Pella Inc.). TEM images were collected on a Hitachi HT7700 microscope operating at an accelerating voltage of 100.0 keV in high resolution observational mode. The reported average Au core size and SnO2 shell thickness were determined by measuring 16 particles (55 shell thickness measurements) in the TEM images using Gwyddion image analysis software. SHINs Pinhole Test To ensure the as synthesized SHINs were free of defects, a pinhole test was done using gold and silicon wafer substrates as described previously [21]. Gold substrates were prepared by RF magnetron sputtering approximately 200 nm of Au at 0.1 nm s1 on the conductive side of indium tin oxide (ITO) coated glass. The sputtering unit was operated at 20 W with a base pressure of 105 Torr and an Argon pressure of 2 103 Torr. Equal volumes (~0.5 mL) of SHINs suspension in acetone were deposited on the Si and Au substrates. These substrates were then placed in 102 M 4-methoxypyridine (MOP) solution and their Raman spectra were recorded. The presence of MOP peaks for SHINS on the Si substrate indicates a significant number of pinholes in the SnO2 shell, while their absence indicates a pinhole free shell. Electrochemical SHINERS Electrochemical SHINERS measurements were carried out using a custom made in situ Raman cell fitted with ports for auxiliary and reference electrodes. The working electrodes were cut from either low carbon steel sheets or 304 stainless steel sheets (McMaster-Carr). The stated composition of the low carbon steel was 98.8–99.3% Fe with carbon content between 0.18% and 0.26%. Sulfur and phosphorous content was 0.05%. The steel samples were abraded using 600 grit sandpaper, followed by fine polishing using 15 and 6 μm diamond suspension pastes (PACE technologies) on micropad2 polishing pads (PACE technologies). The stainless steel surfaces were then rinsed with ethanol and copious amounts of Millipore water before drying in air.
3.1. Characterization of Au@SnO2 SHINs Physical characterization of Au@SnO2 SHINs was done by observing the particle size and shell thickness using TEM. Fig. 1 shows representative TEM images of the particles. The thin SnO2 shell is visible as a lighter area around the dark Au core. Generally, the SHINs are found in clusters of between 3 and 8 particles and are interconnected either by their Au cores or by their SnO2 shells. The SnO2 shell appears to be continuous around the clusters of particles and have an average thickness of 3.7 nm with a standard deviation of 1.2 nm based on 55 measurements of the shell thicknesses. The gold cores are spheroids, having a long axis that is on average 40.2 nm with a standard deviation of 4.3 nm and a
Fig. 1. TEM images of Au@SnO2 SHINS. 2
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short axis of 30.2 nm with a standard deviation of 2.5 nm. The optical extinction of freely dispersed Au@SnO2 SHINs exhibits a maximum at 535 nm with a long wavelength tail that extends for several hundred nanometers. Although the localized surface plasmon polariton (LSPP) is not resonant with the selected Raman laser excitation wavelength, the deposition of the SHINs on solid supports clearly leads to partial particle aggregation which is known to shift the LSPP resonance to longer wavelengths. Alternatively, the diameter of the Au cores could potentially be tuned in an attempt to maximize the resonance between the Raman laser line and the deposited particles. However, this approach was not pursued in this work. 3.2. SHINERS and pinhole test of SHINs using 4-methoxypyridine Fig. 2 contains spectra collected in 102 M MOP solution for (a) SHINs on a Au substrate, (b) a Au substrate with no SHINs and (c) SHINs on a Si substrate. By comparison of Fig. 2 (a) and (b), it is evident that the Au@SnO2 SHINs enhance the MOP signal as peaks are only present in the spectrum of the Au substrate when SHINs are present on the surface. However, this experiment doesn’t differentiate between MOP molecules adsorbed on the underling gold surface or on the Au cores of the SHINS if defects in the SnO2 shells expose the Au core to the MOP containing solution. To rule out the latter, the SERS intensity was also measured using Au@SnO2 SHINs deposited on a flat Si wafer in place of the Au substrate. Comparison of Fig. 2 (a) and (c) gives evidence that the SHINs are free of pinholes. Since MOP does not adsorb on Si or SnO2, any MOP signals appearing in the spectrum for SHINs on the Si substrate must arise from MOP adsorbed on the Au cores of the SHINs through pinhole defects in the shells. SHINs are considered to be suitably pinhole free if the MOP signal originating from MOP adsorbed on the flat substrate is very large compared to the SERS signal originating from MOP adsorbed on Au nanoparticle cores. This condition is clearly satisfied and the MOP spectrum in Fig. 2a relative to that shown in Fig. 2c is indicative of pinhole free Au@SnO2 SHINs.
Fig. 3. Cyclic voltammograms (10 mV/s scan rate) for 304 stainless steel in 102 M KClO4 in the presence (a) and absence (b) of 102 M KCl. Trace c) is the same as trace b) but for a 304 s s working electrode decorated with Au@SnO2 SHINs. The crosshair on each voltammogram represents the origin.
b
scan is shown for each sample. In general, the same features are present in Fig. 3 (b) and (c) and their similarity is evidence of both the insulating nature of the particle shells and the fact that the Au@SnO2 are electrochemically inert. For electrolyte free of KCl (Fig. 3b and c), a feature in the positive going potential scan is observed at 0.46 V. This peak is often attributed to iron oxidation and the formation of either magnetite (Fe3O4) or FeOOH, through the intermediate Fe(OH)2 [22–24]. The current density drops as the potential is scanned further positive, reaching a small but stable value for potentials in the approximate range of 0.20 V–0.20 V. This corresponds to the 304 stainless steel passive state. A broad feature appears in the positive potential scan with a local current maximum at 0.55 V and an increasingly positive current is observed at potentials greater than 0.85 V. Both features are attributed to the formation of oxides of increasingly higher oxidation states of chromium [22,24,25]. In the negative going potential scan there are two main features, a small peak around 0.0 V which is attributed to the reduction of Cr(VI) to Cr(III) [22,24,25] and a broad reduction feature between 0.65 V and 1.0 V that we interpret to be the convolution of reduction peaks for molecular oxygen and iron oxides. The voltammetry in the presence of 102 M KCl was quite similar except for the larger current density of the peak observed in the positive potential scan at around 0.61 V. Depending on how quickly the local pH near the electrode drops after the most negative polarizations, the peak at 0.61 V corresponds to either active iron oxidation and the dissolution of Fe2þ species or oxidation and the precipitation of a layer of Fe(OH)2 [22]. In the case of the former, the increased current density of the voltammetric feature at 0.61 V in Fig. 3a may suggest that a thicker Fe(OH)2 layer is formed when chloride is present in the electrolyte. Alternatively, if the pH is low enough to prevent the formation of an insoluble layer, the presence of chloride likely accelerates the active dissolution of the iron.
c
3.4. Electrochemical SHINERS on stainless steel
3.3. Electrochemistry of stainless steel substrates
Raman intensity / a.u.
Fig. 3 contains cyclic voltammograms for grade 304 stainless steel (304 s s) in 102 M KClO4 with (a) 102 M KCl, (b) no KCl and (c) no KCl where the substrate is decorated with Au@SnO2 SHINs. It was found that the voltammetry changed during the first 2–3 potential cycles but became largely invariant with further cycling. Thus, the fourth potential
a
800
900
wavenumber / cm
1000
Prior to conducting the SHINERS experiments, the potential of the working electrode was cycled several times between 1.0 V and þ1.0 V vs. SCE until the current-potential traces became largely invariant with scan number. As described below, this also provided a mechanism to demonstrate the inertness of Au@SnO2 to alkaline conditions. As the unbuffered electrolyte was not de-oxygenated, the application of negative potentials results in significant pH increase near the steel electrode due to the reduction of oxygen. The increase in pH had a deleterious
1100
-1
Fig. 2. Results of pinhole testing used to determine the presence of defects in the SnO2 shells. Raman spectra of a 102 M 4-methoxypyridine (MOP) solution covering different substrates were measured. The substrates were a) Au@SnO2 SHINs on a smooth gold Au substrate b) a smooth Au substrate with no SHINs and c) Au@SnO2 SHINs on a Si substrate. 3
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remains relatively constant in the negative potential scan between 1.0 V and approximately 0.0 V. However, this band does become more intense between 0.0 V and 0.60 V. At further negative potentials the intensity of the 560 cm1 band decreases until it disappears at 1.0 V. The band position also shifts to lower wavenumbers during negative potential stepping and returns to 540 cm1 at 0.8 V. Fig. 4b contains analogous potential dependent spectra for 304 s s in 102 M KClO4 with the addition of 102 M KCl to the electrolyte. Fig. 5b contains the corresponding peak integrals and voltammetry. The presence of chloride gives rise to several changes in the spectra. During the positive potential sweep, the appearance of the main band at 540 cm1 requires a more positive potential before reaching its plateau intensity compared to the chloride-free experiment. The voltammetric peak at ca. 0.61 V is unique to the chloride containing electrolyte but is not correlated with any obvious new SHINERS signals. However, the main Raman band centered at 540 cm1 and appearing at E 0.60 V is a much broader feature in Fig. 4b compared to Fig. 4a. This may imply the presence of unresolved chloride species in the iron oxidation products that are formed prior to the onset of the 304 s s passive regime. A band at 265 cm1, which is not observed in the chloride free spectra, is evident at E þ0.40 V in the positive potential scan direction. This band is present in the negative potential scan direction for E 0.60 V (see Fig. 5b). A third difference in the spectra for the chloride containing electrolyte is the delayed appearance of the 795 cm1 peak. For the chloride free electrolyte this peak is evident around þ1.0 V in the positive potential scan while in the presence of chloride this band is not noticeable until ~0.60 V in the negative potential scan direction. The broad peak centered between 540 and 560 cm1 has been reported in SERS experiments with potentiostatically grown oxide films on Fe in solutions of borate[5], carbonate [4], nitrate [4], and sulfate [3]. In the work of Oblonsky and Devine [5], the passive film on Fe was grown by polarization at 0.1 V vs SCE for 1 h and then reduced by an initiating a negative potential scan to 0.90 V. At wavenumbers below 800 cm1, their spectra are similar in appearance to those presented in this work. Specifically, one broad band in the range of 540–560 cm1 was observed and found to disappear when the applied potential was more negative than 0.70 V. This band was attributed to the symmetric stretch (A1 mode) of amorphous or microcrystalline Fe(OH)2 although the authors noted that the spectra were also indicative of small amounts of either Fe3O4 or Fe2O3. Therefore, we interpret the broad peak centered between 540 and 560 cm1 in our electrochemical SHINERS data to be the symmetric stretch of amorphous Fe(OH)2[26]. The band appearing at 265 cm1 at 0.40 V in the presence of chloride likely arises from the formation of lepidocrocite also known as γ-FeOOH [4,27]. Chloride ions are known to stabilize this phase and a strong Raman band has been reported at ~255 cm1 [4,5]. The intermediate species in the conversion between Fe(OH)2 and γ-FeOOH is green rust, which can take several forms depending on the exact environmental conditions [2,28]. Previous in situ Raman studies of green rust showed an intense band at 665 cm1 which is notably absent in our SHINERS results [29]. In addition, the absence of strong bands in the 660-680 cm1 range also rules out any significant contribution from Fe3O4[5]. The band at 795 cm1 cannot be attributed with any degree of certainty to iron oxides as the closest reported bands are 745 cm1 for β-FeOOH and 710-740 cm1 for γ-Fe2O3 [6]. The possibility that this band is related to γ-Fe2O3 is inconsistent with the literature as additional intense bands associated with this phase should be found in the range of 645–670 cm1 [29–31]. Previous studies have attributed an 827 cm1 peak to Cr(III) oxides, specifically Cr2O3 [5, 32]. However, the appearance of the 795 cm1 band is also coincident with features in the voltammetry that are attributed to the oxidation of Cr(III) to Cr(VI). In situ thermal oxidation of Cr coupons has shown a band at 826 cm1 which was assigned to Cr(VI) incorporated into a mixed valence Cr2O3-like structure [33]. Ex situ work with Cr3O8 and Cr2O5 found that the Cr(VI)–O–Cr(VI) asymmetric stretch falls in the range of 719–848 cm1 and is likely the origin of the 795 cm1 band. This assignment is bolstered by the fact that the peak completely disappears at
effect on the stability of the shell of Au@SiO2 nanoparticles whereas Au@SnO2 nanoparticles provided stable responses during repeated potential cycling. Electrochemical SHINERS spectra of the 304 stainless steel in 102 M KClO4 are presented in Fig. 4a and are separated into positive (top) and negative (bottom) scan directions. Peak integrals for the major features are presented in Fig. 5a along with the corresponding voltammetry. The potential was first held at 1.0 V for 120s and a spectrum was collected. The potential was then stepped in the positive direction in 0.20 V intervals up to 1.0 V with a SHINERS spectrum collected at each potential step position. The working electrode potential was then stepped in the reverse direction and spectra were collected. At the initial bias of 1.0 V the spectra are essentially featureless indicating that any Raman active components of the oxide layer are completely reduced at this potential. As the potential is stepped in the positive direction a broad peak at 540 cm1 becomes evident in the spectrum acquired at 0.60 V. This peak grows in intensity, and shifts to higher wavenumbers over the next several hundred millivolts until the intensity plateaus at a potential of ~0.0 V. The intensity of this band remains more or less constant throughout the rest of the positive potential scan (see Fig. 5a). At 1.0 V, an additional weak band is observed around 795 cm1. This band becomes more prominent during the negative potential scan, reaching a maximum around 0.0 V, before decreasing in intensity and eventually disappearing around 0.60 V. The low frequency band
Fig. 4. SHINERS spectra of 304 stainless steel in 102 M KClO4 as a function of applied potential in the absence (a) and presence (b) of 102 M KCl. Both sets of spectra are divided into two panels top: spectra acquired during a series of positive-going potential steps and bottom: spectra acquired during a subsequent series of negative-going potential steps. 4
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Fig. 5. Integrated Raman intensities from Fig. 4 as a function of applied potential (top) and a linearized cyclic voltammogram (bottom) for 304 s s in 102 M KClO4 in the absence (a) and presence (b) of 102 M KCl.
potentials more negative than 0.6 V in the negative potential scan which is consistent with the fact that Cr(VI) is reduced to Cr(III) in this potential range [22,24,25].
evidenced by the SHINERS data for this system (vide infra). The reduction feature in curves (b) and (c) is likely the convolution of the reduction of surface oxides, double layer [Fe(H2O)6]3þ and molecular oxygen in the electrolyte. When 102 M BTA is present in the electrolyte (Fig. 6a), the voltammetry is quite different compared to the CV obtained in additive free electrolyte. In general, in the presence of BTA the current density is higher in the passive region and there is a small anodic feature at approximately 0.0 V in the positive potential scan. The main anodic feature observed at ~0.9 V when BTA is absent, although a significant anodic current is observed at the positive limits of the applied potential scan. The new feature is not associated with the formation of [Fe(H2O)6]3þ as will be shown in the SHINERS data and is likely related to oxidation of BTA.
3.5. Electrochemistry of carbon steel substrates Fig. 6 contains cyclic voltammetry traces for carbon steel in (a) 102 M KClO4 with 102 M BTA, (b) 102 M KClO4, and (c) Au@SnO2 SHIN decorated surface in 102 M KClO4. The similarity of curves (b) and (c) again provides evidence that the SHINs do no alter the electrochemistry of the substrate steel nor do they appear to be electroactive. There is a slight decrease of current density when the carbon steel surface is decorated with SHINs likely caused by an effective decrease in electrode surface area due to the presence of the SHINs. In the absence of BTA, the curves show only two features, an anodic peak is observed at ~0.9 V in the positive going potential scan and a cathodic peak appears at approximately 0.7 V in the negative potential scan. We interpret the anodic feature to arise from the formation of [Fe(H2O)6]þ3 as will be
3.6. Electrochemical SHINERS on carbon steel Electrochemical SHINERS spectra for carbon steel in 102 M KClO4 solution are presented in Fig. 7a, separated into positive and negative potential step directions (top and bottom respectively). A spectrum was first collected at OCP (0.20 V) followed by applied potentials stepped in the positive direction up to 1.2 V, then descending steps in the negative direction. The weak band at 931 cm1 is present in all spectra for carbon steel even in the absence of SHINs. This, combined with the fact that the intensity of the band is largely completely invariant with potential, indicates that this is not a SHINERS response. This peak can be assigned to the Cl–O stretching of the solution perchlorate ions [34]. The normal Raman perchlorate peak is also present in the 304 s s spectra. However it is not on scale with the much higher intensity SHINERS signals observed in those spectra. As the electrode potential is stepped positive from 0.20 V in 0.20 V intervals, the peak corresponding to the perchlorate stretch remains relatively unchanged while the peak observed at approximately 304 cm1 disappears above 0.40 V which is likely associated with the formation of a thin layer of Fe(II) corrosion products. At 0.80 V a new, relatively intense peak, is observed at 454 cm1 with a lower wavenumber shoulder around 400 cm1. The intensity of this peak continues to grow throughout the remaining positive potential steps and persists during the negative-going potential steps until a maximum intensity is reached at ~0.40 V on the return cycle. The peak intensity remains relatively constant as the potential is further decreased to 0.20 V before becoming completely absent at around 0.80 V. The
Fig. 6. Cyclic voltammograms (10 mV/s scan rate) for carbon steel in 102 M KClO4 in the presence (a) and absence (b) of 102 M BTA. Trace c) is the same as trace b) but for a carbon steel working electrode decorated with Au@SnO2 SHINs. The crosshair on each voltammogram represents the origin. 5
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Fig. 7. SHINERS spectra of carbon steel in 102 M KClO4 as a function of applied potential in the absence (a) and presence (b) of 102 M BTA. Both sets of spectra are divided into two panels top: spectra acquired during a series of positive-going potential steps and bottom: spectra acquired during a subsequent series of negativegoing potential steps.
454 cm1 peak demonstrates the sensitivity advantage of SHINERS as compared to normal Raman spectroscopy as the potential dependent spectra in the absence of SHINs did not show this band. The 454 cm1 band does not correspond to any known iron oxides or oxyhydroxides but has been attributed to Fe–OH2 stretching modes in Fe(III) water complexes such as [Fe(OH2)6]3þ [35,36]. The shoulder around 400 cm1 has also been documented and has been assigned to arise due to stretching modes associated with Fe–OH or Fe–OH2 bonding in solution species of [Fe(OH2)6]2þ [35,36]. The SHINERS spectra of carbon steel are quite different when the electrolyte contains 102 M BTA (Fig. 7b). At 0.20 V (OCP) the peak at 304 cm1 is absent, however, there are three additional weak peaks that can be assigned to BTA at 782 cm1, 1012 cm1 and 1387 cm1. The band at 1387 cm1 is assigned to the triazole ring stretching mode with some contributions from C–C stretching and C–H out of plane bending. The band at 1012 cm1 is assigned to the trigonal ring breathing mode while the peak at 782 cm1 can be assigned to a C–C in-plane stretch [37, 38]. It should be noted that in a separate experiment in the absence of SHINs, the bands at 782 cm1 and 1012 cm1 were clearly visible, indicating that these bands do not arise from SHINERS and correspond to BTA in solution. The band at 1387 cm1 on the other hand, was only present when SHINs were on the surface. Enhancement of this band suggests an end-on orientation of the BTA, meaning the BTA is coordinated via the nitrogen atoms in the ring [37,38]. It is also interesting to note that other bands which are expected to show enhancement based on surface selection rules for this end-on orientation are absent from the spectrum. These are the 1576 cm1 band associated with the C–C in-plane stretching mode as well as the 1290 cm1 band associated with CH in-plane bending and CC in-plane stretching [37,38]. The main feature at 454 cm1 is absent from the SHINERS spectra when the electrolyte contained BTA. In addition, despite the increased passive current density and features in the voltammetry, no peaks associated with Fe corrosion products are found in the spectra obtained at positive potentials. This supports the idea that BTA enhances the dissolution of Fe
through a very thin passive film, however it inhibits the formation of the Fe-water complexes at the surface of the electrode that form in its absence. 4. Conclusions There have been no previous demonstrations of the use of SnO2 protected gold nanoparticles for surface enhanced Raman studies despite previous reports in the literature of their synthesis. Herein, we have shown for the first time that SHINERS experiments are possible using Au@SnO2 SHINs. The synthesis procedure yielded clusters of ~35 nm diameter particles, encased in SnO2 with an average thickness of 3.7 nm. SHINERS data was gathered for a pyridine derivative on a gold substrate as a proof of concept followed by the first demonstration of electrochemical SHINERS on 304 stainless steel and carbon steel substrates. For 304 stainless steel, the SHINERS spectra were dominated by a broad feature centered around 550 cm1, which was taken to be mostly amorphous Fe(OH)2. An additional SHINERS peak at ~795 cm1 is thought to correspond to Cr(VI)–O bonds in a mixed valence (Cr(III)/ Cr(VI)) oxide. When the electrolyte contained 102 M KCl, the SHINERS spectra also had some contribution from γ-FeOOH which was formed at E 0.4 V during positive-going potential increments. This peak persisted when the potential was decreased until the applied potential was less than 0.4 V. We found no evidence for green rust, the intermediate species in the formation of γ-FeOOH from Fe(OH)2, in our SHINERS data. For carbon steel, the only feature in the SHINERS spectra was a broad feature centered at 454 cm1, which also contained a shoulder at approximately 400 cm1. These two features had been previously reported and are not due to iron oxides, hydroxides or oxyhydroxides, but rather iron water complexes present at the surface of the electrode. When the corrosion inhibitor BTA was added to the electrolyte, these features were not observed at any potential and there were no features in the Raman data attributable to Fe species, thus demonstrating the ability of BTA to inhibit the oxidation of steel. We did not observe a film of 6
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corrosion products forming on the electrode surfaces for either substrate using optical microscopy, nor did we observe any regular Raman signal from Fe species. This indicates that the Raman signals reported in this work arise from very thin films and demonstrates that enhancement from the Au@SnO2 particles is critical.
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