Preconcentration mechanism of trivalent lanthanum on eQCM electrodes in the presence of α-hydroxy isobutyric acid

Journal of Electroanalytical Chemistry 857 (2020) 113731

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Preconcentration mechanism of trivalent lanthanum on eQCM electrodes in the presence of α-hydroxy isobutyric acid Adan Schafer Medina a, Nathalie A. Wall b, Cornelius F. Ivory a, Sue B. Clark b,c, Haluk Beyenal a,⁎ a b c

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, USA Department of Chemistry, Washington State University, Pullman, WA, USA Pacific Northwest National Laboratory, Energy & Environment Directorate, Richland, WA, USA

a r t i c l e

i n f o

Article history: Received 23 August 2019 Received in revised form 3 December 2019 Accepted 5 December 2019 Available online 07 December 2019 Keywords: Electroprecipitation Lanthanum Complexing ligand HIBA Electrochemical quartz crystal microbalance

a b s t r a c t Electroprecipitation can be used to preconcentrate lanthanum on carbon electrode surfaces. The use of complexing ligands is expected to improve the electroprecipitation of lanthanum by protecting La ions in solution from the alkaline region near the electrode surface. However, the electroprecipitation mechanism of La in the presence of a complexing ligand is not known. The goal of this work is to 1) determine the effect of the complexing ligand, α-hydroxy isobutyric acid (HIBA), on the electroprecipitation of La onto the gold electrodes, and 2) identify the changes in the mechanism of accumulation when preconcentrating in the presence of HIBA. We used an electrochemical quartz crystal microbalance (eQCM) and needle type pH microelectrodes to determine pH near the electrode surface in combination with cyclic voltammetry to understand the electroprecipitation mechanism. We used the bi-dentate ligand HIBA as a ligand and found that lanthanum electroprecipitation is hindered in the presence of HIBA. The presence of HIBA also delayed the onset of film formation during a cyclic voltammetric experiment by ~100 mV compared to experiments performed without HIBA. The shift in onset potential is attributed to the buffering action of HIBA (pKa = 3.7) since the shift is not present in subsequent scans. The precipitated film was characterized by scanning electron microscopy, X-ray photoelectron spectrometry, and Auger nanoprobe spectrometry. While we found that La(OH)3 was the predominant chemical state of the film on electrodes in the absence of HIBA, La2O3 was found for films created in the presence of HIBA. Our finding demonstrates that La(OH)3 can be electrodeposited at room temperature. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The lanthanides are of great interest to researchers in a variety fields with implications in the manufacturing of catalysts and electronics, corrosion resistant alloys and coatings, and molten salt electrochemistry and nuclear waste management [1–7]. These metals have implications to environmental health and national security due to their chemical similarity to the trivalent actinides [1,2,8]. Lanthanum, the first and most abundant lanthanide, is often used as a chemical analog to the trivalent actinides. Direct electrochemical reduction of La is not possible in aqueous solutions as its standard reduction potential, −2.57 VAg/AgCl, is outside the stability limits of water [9]. Early attempts to separate lanthanides electrochemically involved the use of mercury pool electrodes and mercury drop electrodes, and some leveraged the use of complexing ligands [10–15]. Preconcentration of lanthanum has been investigated recently by various techniques, including electrochemical preconcentration on both bare and mercury-filmed electrodes and ligand-modified carbon paste electrodes [16–21]. Mercury-filmed ⁎ Corresponding author. E-mail address: [email protected] (H. Beyenal).

https://doi.org/10.1016/j.jelechem.2019.113731 1572-6657/© 2019 Elsevier B.V. All rights reserved.

electrodes were described to undergo surface amalgamation during analyte preconcentration, but the mechanism has not been confirmed. Electrochemical preconcentration via electroprecipitation (also known as electrochemical generation of base) has been used to form mixed metal oxide films and corrosion resistant coatings [7,22–24]. However, it is not a selective preconcentration process as it relies on the precipitation of metal ions at elevated pH. Amalgamation is more selective due to its dependence on applied potential but is problematic due to environmental concerns with the use of mercury. On the other hand, carbon paste electrodes modified with the complexing ligand αhydroxyisobutyric acid (HIBA) were proposed to work by an absorption mechanism that was dependent on the presence of the ligand in the carbon paste [18]. In summary, if the target application is for La detection, electroprecipitation can be used for sample preconcentration. If, however, electrodeposition of La coatings is desirable, electroprecipitation can be used at room temperature to deposit La on surfaces as a preliminary step. On the other hand, complexing ligands can be used to remove lanthanides from the solution. For example, Choppin and Silva [25] demonstrated that separation of the lanthanides was achievable on a cation exchange resin through the use of HIBA as a complexing ligand. The

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authors noted separation factors varying from 34.1 to 0.055 for different lanthanides – with larger numbers referring to the species eluting at subsequently higher elution volumes based on the elution of gadolinium, and lower numbers referring to the opposite [25,26]. The ability of HIBA to chelate lanthanide ions stems from its carboxylate and hydroxyl moieties. The use of complexing agents to raise the reduction potential of lanthanide species in solution is seen in the works of Podlaha's group, among others [27,28], but rely on the phenomenon of induced co-deposition. This may be unfavorable for separations processes considering the necessary addition of co-reductants. The complexation of HIBA with La (and the other lanthanides) has been described as tridentate and monoanionic by Chen et al. [29]. One HIBA molecule is a bidentate ligand to a unique La while also being a monodentate ligand to a second unique La [29]. Furthermore, the data suggests that the complexation scheme described above was the only one that exists in the conditions tested, but this may change at lower concentrations of Ln/HIBA. Notably, the existence of anionic (but not polyvalent) [La-HIBA4]− complexes has been evidenced for large HIBA concentrations [30]. The complexing ligand HIBA has been used in conjunction with a carbon paste electrode to directly preconcentrate lanthanides via chronoamperometry [20]. While there was a general trend that the heavier lanthanides preconcentrated more compared to the lighter lanthanides, the mechanism was not determined. Previously, we demonstrated that lanthanum electroprecipitates as La(OH)3 on cathodically polarized gold or carbon electrode surfaces [16,17]. During cathodic polarization, the rapid increase in surface pH could result in some lanthanum hydrolyzing before reaching the surface of the electrode, so a method to protect lanthanum as it approaches the electrode may improve the performance of this technique. These literature studies indicated that it would be possible to electroprecipitate La in the presence of HIBA which could slow down hydrolyzation during cathodic electroprecipitation. This could also change the chemistry of the deposits on the surface. Based on our previous work, we hypothesized that the addition of HIBA would allow complexed La to reach the surface of the electrode and result in thicker films compared to La alone, and that it could also act as a buffer. The goals of this work are to 1) determine the effect of complexing ligand HIBA on the electroprecipitation of La on gold electrodes, and 2) identify the changes in mechanism of accumulation when preconcentrating with HIBA. The effect of HIBA on the electroprecipitation of Ln's to their hydroxides has not been studied. HIBA is expected to improve delivery of La to a cathodically polarized electrode surface by protecting the La from hydrolysis before diffusing to the vicinity of the electrode but may also delay the onset potential for electroprecipitation by acting as a buffer. Simultaneous surface pH and mass-shift measurements are used to show how film formation changes in the presence of HIBA. Electrochemical quartz crystal microbalance (eQCM) and needle type pH microelectrodes were used to determine pH near the electrode surface in combination with cyclic voltammetry. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are used to determine the chemical state of the La film when produced in the presence of HIBA. 2. Materials and methods 2.1. Reagents The following chemicals were used without modification: nitric acid (225711-475ML, Sigma Aldrich), lanthanum chloride (203521-25G, Sigma Aldrich), lithium chloride (746460-500G, Sigma Aldrich), αhydroxybutyric acid (323594, Sigma Aldrich). 2.2. Electrochemical quartz crystal microbalance measurements Fig. 1A illustrates the gold-coated eQCM electrodes used in this work. A complete (electrochemical quartz crystal microbalance)

Fig. 1. A) Illustration of a gold-coated eQCM electrode. The eQCM undergoes an oscillating shear deformation that is sensitive to changes in adhered mass. A 10-MHz crystal can detect changes in mass as small as ~1 ng in the form of a shift in its resonant frequency. B) Schematic of an all-in-one pH microelectrode. The outer diameter of the pH microelectrode tip is smaller than 20 μm.

eQCM hardware/software laboratory kit was purchased from Gamry Instruments (992-00083, Gamry® Instruments). All experiments were run in a 4 mL Teflon cell (971-00003, Gamry® Instruments). Aucoated electrodes (971- 00006, Gamry® Instruments) were used for electrochemical experiments as well as the substrate for XPS experiments. The gold surface allowed for convenient charge referencing. The cell was soaked in 30% w/w nitric acid for 30 min to remove adsorbed metal ions. Prior to use, the cell was washed three times with 18 MΩ-cm deionized water and subsequently dried. The eQCM electrode was then mounted and background solution (100 mM NaCl solution) was added to the assembled cell. A Pt wire counter electrode (MW-1032 Bioanalytical Systems, Inc.) and a 3 M NaCl silver/silver chloride reference electrode (MF-2052 Bioanalytical Systems, Inc.) were inserted into the filled cell. The resonator that oscillates the eQCM electrode was then turned on. A stable background resonant frequency was obtained prior to all experiments. As mass is added to the electrode surface, the resonant frequency decreases, resulting in a frequency shift. The frequency shift is converted to a mass shift (apparent mass) using the calibration factor (226 Hz μg−1 cm−2) of the crystal, the electroactive area (0.209 cm2 for Au-coated electrodes) and the Sauerbrey relation [31]. 2.3. Electrochemical setup A Gamry Interface 1000™ potentiostat (Gamry® Instruments) was used to run all voltammetry experiments. Unless specified, cyclic voltammograms were performed between 0 VAg/AgCl and −2.65 VAg/AgCl at 50 mV/s. The eQCM driver was connected to a Gamry Interface 1000™ potentiostat to control the potential of the working electrode. If an eQCM electrode exhibited a constantly shifting baseline frequency or a poorly defined frequency spectrum, the electrode was discarded. 2.4. Microelectrode construction The fabrication of all-in-one needle-type pH-sensing microelectrodes is also described in detail elsewhere [17,32–34]. Briefly, pH microelectrodes are constructed by pulling a glass capillary to an outer diameter of 20 μm, followed by filling the tip with a liquid ion exchange (LIX) membrane. The capillary was then epoxied to a glass outer case

A.S. Medina et al. / Journal of Electroanalytical Chemistry 857 (2020) 113731

with its sensing-tip protruding 1–1.5 mm from the outer case. A liquid junction made by filling the outer case orifice with agar solution (18.2 g/L) containing 100 mM Na2SO4. The outer case is then filled with a solution of saturated KCl/AgCl and an Ag/AgCl wire and used as the built-in reference electrode for the all-in-one pH microelectrode. The proximity of the reference electrode “frit” to the sensing tip of the pH microelectrode and it's fixed position relative to the tip allows for accurate pH measurement near polarized electrodes [33]. The all-inone pH microelectrodes were then calibrated in standard buffer solutions (pH 4, 7, and 10). The potential between the reference and the pH sensing tip was measured by a Gamry Interface 1000™ potentiostat (Gamry® Instruments). 2.5. Measurements of pH near the electrode surface using pH microelectrodes All-in-one pH microelectrodes were recently developed and successfully used on polarized surfaces [33] (Fig. 1). The microelectrode tip position was controlled using a Mercury Step motor controller PI M-230.10S Part No. M23010SX (Physik Instrumente, Auburn, MA). A Leica M80 stereomicroscope (Leica Microsystems Inc.) was used to locate the microelectrode tip above the Au-coated eQCM electrode. Using the motor and the stereomicroscope, the microelectrode tip was placed at ca. 100 μm above the electrode surface. The measurement solution was then added to the cell without disturbing the microelectrode tip and the pH measurement was initiated. 2.6. Sample preparation and XPS analysis X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique with the capability to determine the elemental composition of an analyte's surface. Removal of surface layers through sputtering allows for limited sub-surface analysis while also serving to remove surface contaminants from handling of the sample. Survey spectra reveal the elements present on the surface while highresolution spectra are used to analyze the chemical state of the exposed material. Sample preparation procedures were slightly altered between pH-adjusted samples and electroprecipitated samples as described in our previous publication [17]. Electroprecipitated samples were prepared by applying a linear potential sweep from 0 V to −2.65 V vs. Ag/AgCl reference electrode (3 M NaCl) at 20 mV/s. The sample preparation solution contained 10 mM LaCl3, 10 mM HIBA, 100 mM NaCl, and was adjusted to a pH of 3. Upon completion of the potential sweep the electrode was promptly removed from the solution, rinsed in a stream of 18 MΩ water and dried in a stream of Argon. The dried sample was packaged and sealed in an argon atmosphere and shipped overnight for analysis. Prior to mounting of the sample, the gold electrode was isolated from the surrounding quartz using a diamond cutter. Alternatively, pH-adjusted samples were prepared by pipetting 100 μL of the electroprecipitation solution described above directly onto the electrode surface. An equal measure of 1 M NaOH was mixed into the solution to raise the pH and dried under a stream of Argon. The rest of the procedure is identical to the electroprecipitation samples. Au-coated eQCM electrodes were used for both procedures. Samples were analyzed using a PHI 5600 spectrometer located at the Imaging and Chemical Analysis Laboratory (ICAL) at Montana State University. Au-coated QCM electrodes were used in placed of carboncoated electrodes to facilitate the acquisition of XPS spectra. The XPS data was collected and analyzed by AugerScan software using Gaussian/Lorentzian line shape and Shirley background correction. 2.7. XPS peak deconvolution Peak fitting was accomplished after a Shirley type background subtraction using Voigt functions. Sunding, Hadidi, Diplas, Løvvik, Norby and Gunnæs [35] describes the process to deconvolute the acquired

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signal to determine whether the analyzed sample is of the form La (OH)3 or La2O3 [35]. The La 3d5/2 and 3d3/2 spin-orbit spectra have four different components, even when only one chemical state is present, and the multiplet splitting and intensity ratios can be used to distinguish between different chemical states [35,36]. The 3d5/2 spin orbit spectrum region was chosen because it exhibits two peaks which can be deconvoluted into three multiplet contributions. 2.8. SEM imaging and auger nanoprobe analysis Scanning electron microscopy (SEM) and AES (Auger electron spectroscopy) analyses were carried out using an integrated Physical Electronics (PHI) 710 Scanning Auger NanoProbe (NanoAuger). NanoAuger instrumentation provides fast and quantitative elemental and chemical state mapping, and line scans with highest resolution and surface sensitivity at several analysis areas from spot to large areas simultaneously through AES. It also provides surface morphology information and assistance for area specific Auger analysis with high resolution SEM imaging capability. In this study, Auger and SEM analyses were carried out at a primary electron beam energy of 10 keV. The SEM images and energy dispersive X-ray spectroscopy (EDX) scans were collected at variable field of views. The SEM images and AES data were collected using PHI SmartSoft software and analyzed using PHI MultiPak software. 3. Results and discussion We determined the effect of HIBA on the electroprecipitation of La on electrode surfaces. We have previously demonstrated the simultaneous measurement of current, mass shift, and surface pH response of eQCM electrodes during La electroprecipitation without HIBA [17]. Based on our previous work, our hypothesis was that the addition of HIBA would allow complexed La to reach the surface of the electrode and result in larger/thicker films compared to La alone and that HIBA could also act as a buffer. The eQCM and needle type pH microelectrodes were used to determine pH near the electrode surface in combination with cyclic voltammetry. XPS and AES provided a detailed analysis of the electrode surface chemistry after electroprecipitation in the presence and absence of HIBA. 4. Effect of HIBA on La deposition Fig. 2 shows the current, mass change, and pH response to a cyclic voltammetry experiment performed on an Au-coated eQCM electrode in the absence (top) and presence (bottom) of HIBA. Two differences were observed in the QCM response compared to the case of the analyte alone. First, the film onset potential occurred at −1.1 VAg/AgCl with HIBA vs. −1 V Ag/AgCl without HIBA. Second, a broad peak in the current response appeared at −0.8 VAg/AgCl, which was consistent with the HIBA voltammetric response observed in the absence of lanthanum. In addition to delaying film onset potential, the presence of HIBA decreased the total amount of mass deposited by ~5%. The subsequent scan (not shown) lacks the broad peak observed at −0.8 VAg/AgCl. Over the course of the second scan, the experiment in the presence of HIBA only adds ca. 0.25 μg while the experiment without HIBA added 1.1 μg of material. 4.1. Comparison of surface pH response in the presence and absence of HIBA The surface pH was measured simultaneously using an all-in-one pH microelectrode and is shown in Figs. 2C1 and C2. The pH sharply increases in both cases as the potential is scanned cathodically, but the addition of HIBA delays the onset of pH change by about 300 mV. Additionally, the surface pH falls below 10 faster in the presence of HIBA by about the same margin. This data shows that HIBA has a buffering action on the surface pH during electroprecipitation of La.

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Fig. 2. pH response on an Au-coated eQCM electrode during a cyclic voltammogram measured using an all-in-one pH microelectrode positioned 100 μm above the electrode surface. TOP: Response from 10 mM La. Bottom: Response from 10 mM La + 10 mM HIBA. The supporting electrolyte for both experiments was 100 mM NaCl at pH 3.0. The potential was swept from 0 VAg/AgCl → −2.65 VAg/AgCl → 0 VAg/AgCl at 20 mV/s.

La-HIBA complexes are expected to have a smaller positive charge than free lanthanum ions. The decreased attraction of the complex to the electrode surface may reduce the amount of lanthanum that electroprecipitates on the electrode surface. This effect is more apparent as the ratio of La:HIBA changes. The presence of HIBA at low La:HIBA ratios (≤0.1) substantially reduces the deposition of La and the film immediately disassociates when the cell is turned off (data not shown). Likewise, HIBA does not influence the deposition of La at a high La: HIBA ratios (≥10) (data not shown) as there is not enough HIBA present in solution to complex the La ions.

additional samples were prepared with the solution pH adjusted in the absence of a polarized electrode. These pH-adjusted samples showed a layer of La(OH)3 on the surface of their precipitates, but

4.2. Elemental composition and chemical state of the La precipitates with HIBA The elemental composition and chemical state of the film formed in the presence of HIBA was determined by XPS analysis. The film contains lanthanum at a lanthanum to oxygen ratio of 1:4. The peaks for La 3d5/2 are deconvoluted according to Sunding et al. [35] and shown in Fig. 3. The multiplet splitting yielded a binding energy separation of ~4.6 eV and the asymmetry in the peak envelope suggests that the chemical state of lanthanum precipitate is lanthanum oxide. The main peak corresponds to the contribution of the final state without charge transfer, and the two satellite peaks correspond to the final state with charge transfer from the ligand bonding and antibonding components. This is, to the best of the author's knowledge, the first report of lanthanum oxide being generated without thermal dehydration. Direct electrochemical production of La2O3 has not been reported, as the electroprecipitation of La resulting in La(OH)3 requires thermal treatment to drive off water [23,24]. Considering the large reduction potential of La, it was surprising to observe La2O3 directly from an electroprecipitated film in the presence of HIBA only. Therefore,

Fig. 3. High-resolution XPS spectrum of the La 3d5/2 region acquired from an electroprecipitated La film on a gold-coated QCM substrate. The acquired data (blue markers) were background subtracted and fitted to construct the multipeak 3d5/2 envelope (solid yellow line). The envelope was deconvoluted according to Sunding et al. [35]. The peak without charge transfer from the ligand is shown at 840.45 eV (red squares), and the satellite peaks with charge transfer from the ligand are shown at 835.9 eV and 836.9 eV for the bonding and antibonding components, respectively. The chemical state of La was confirmed to be La2O3.

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Fig. 4. SEM image of Au surface (lighter region) with A) La2O3 precipitate (darker region) encroaching onto bare surface and B) La(OH)3 precipitate (darker region) in segregated precipitated on a separate Au surface. EDX spectra showing elemental analysis of the square regions in A) is shown in Fig. S2. The darker regions are La-enriched and the lighter portions are La-poor.

showed evidence of La2O3 after the top 1 nm of film was sputtered away (Fig. S1). Only La(OH)3 was observed with sample prepared without HIBA, as previously reported [17]. 5. SEM, EDX, and auger spectra of La precipitates with HIBA Fig. 4 shows a scanning electron micrograph (SEM) of a representative section of the sample surface that was analyzed by XPS with (Fig. 4A) and without (Fig. 4B) HIBA. Our previous work [17] shows that the precipitates on the electrode surface were situated in discrete clusters, with La, O, and Cl collocating on the precipitates. The precipitates observed in Fig. 4A exist as a film that coats the surface much more evenly compared to the case without HIBA (Fig. 4B).

O1

20 µm 20 µm

0.0

0.0

0.0

K1

331939

20 µm 20 µm

20 µm

0.0

C1

287769

20 µm

Au3

231371

115579

20 µm

20 µm

Cl1

632230

20 µm

440028

20 µm

La2

Fig. S2 shows the electron diffraction X-ray spectra of the dark squares shown in Fig. 4A, indicating that there are La-rich regions (around the dendrites) and La-poor regions (the exposed gold surface was devoid of La). Auger elemental maps for La, O, Cl, Au, C, and K are overlaid onto the SEM image from Fig. 4. The collocation of La, O, and Cl is similar to the Auger elemental maps of La precipitates in the absence of HIBA [17]. The lack of Au signal on areas where the precipitate exists indicates that the precipitates are at least several layers thick considering the limitation of Auger spectroscopy to penetrate through materials [36,37]. Interestingly, Fig. 5 shows that C is not collocated with La, indicating that HIBA is not likely to be a large component of the La film – or at least not on the surface.

20 µm

0.0

20 µm

0.0

Fig. 5. Auger elemental maps of the region shown in Fig. 4. The elemental maps show a section of exposed Au surface surrounded by La/O/Cl rich regions. The carbon shown may be a result of HIBA precipitating alongside La.

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A.S. Medina et al. / Journal of Electroanalytical Chemistry 857 (2020) 113731

Declaration of competing interest The authors have no competing interests to declare. Acknowledgements

Fig. 6. The proposed mechanism for the electroprecipitation of lanthanum in the presence of HIBA on a polarized electrode. Electroprecipitated films had La(OH)3 on the surface (from contact with water from the solution) and La2O3 underneath the surface. The numbers in the circles refer to the description in the text.

5.1. HIBA-facilitated electroprecipitation of trivalent lanthanum Fig. 6 describes a proposed mechanism for the involvement of HIBA in the electroprecipitation of lanthanum hydroxide and lanthanum oxide. The first process is the generation of a high pH on the surface of the working electrode, accomplished by the electrogeneration of hydroxyl ions activated by cathodic polarization. In aqueous solution, La is sequentially hydrolyzed to La(OH)3(aq) as the pH increased above 8, as shown in step 2. However, HIBA is a complexing ligand, and likely protects some La from hydrolysis near the surface while simultaneously buffering the solution as the pH increases past its pKa of ca. 3.7, as shown in step 3. Based on the results from the XPS measurements, we propose that HIBA can protect non-hydrolyzed La as it approaches the surface in order to form lanthanum oxide on the surface of the electrode. The lanthanum oxide directly in contact with water in solution will immediately rehydrate to form lanthanum hydroxide, which protects the precipitates underneath it.

6. Conclusions In this study, we investigated the effect of introducing the complexing ligand HIBA to La when electroprecipitating onto bare electrodes. Instead of acting as a source for soluble La near the electrode surface when the surface pH was high, we found that HIBA acts as a buffer, delaying the increase of surface pH at the cathodically polarized electrode and thereby decreasing the time that La could precipitate directly onto the electrode. X-ray photoelectron spectroscopy was used to determine the chemical state of the film when produced in the presence of HIBA, and Auger electron spectroscopy was used to gain insight into the uniformity of the film. The presence of HIBA delayed the onset potential of film formation and resulted in ca. 0.25 μg less film being formed. The shift in onset potential did not occur in subsequent scans with HIBA. La2O3 was observed in the precipitates formed in the presence of HIBA below a surface layer of La(OH)3 – no thermal treatment required. La precipitate morphologies were more evenly distributed compared to the absence of La. Lastly, La and O are collocated as a coating on the electrode surface in a ratio of 1:4.

Credit author statement Adan Schafer Medina: Data Curation, Investigation, Writing - Original Draft, Validation, Nathalie A. Wall: Writing - Review & Editing, Methodology, Cornelius F. Ivory: Funding acquisition, Writing - Review & Editing, Methodology, Sue B. Clark: Funding acquisition, Writing - Review & Editing, Methodology, Haluk Beyenal: Funding acquisition, Supervision, Writing – Original Draft, Review & Editing, Methodology.

This work was funded by HDTRA-1-14-10069. The authors would like to thank Gretchen Tibbits for providing the pH microelectrodes. A.S.M. acknowledges NIGMS training grant T32 GM008336 and the ARCS Foundation of Seattle. Surface analysis part of this work was performed at the Montana Nanotechnology Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant# ECCS1542210). The authors acknowledge Professor Recep Avci for his help with surface analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113731. References [1] P. Janoš, Analytical separations of lanthanides and actinides by capillary electrophoresis, Electrophoresis 24 (12−13) (2003) 1982–1992. [2] S. Cotton, The lanthanides - principles and energetics, Lanthanide and Actinide Chemistry, John Wiley & Sons, Ltd 2006, pp. 9–22. [3] B. Zhang, Y. Yao, Z. Shi, J. Xu, Y. Liu, Z. Wang, Communication—direct roomtemperature electrodeposition of La from LaCl3 in an organic solvent supported by LiNO3, J. Electrochem. Soc. 166 (6) (2019) D218–D220. [4] X. Song, J. Zhang, M. Yue, E. Li, H. Zeng, N. Lu, M. Zhou, T. Zuo, Technique for preparing ultrafine nanocrystalline bulk material of pure rare-earth metals, Adv. Mater. 18 (9) (2006) 1210–1215. [5] A. Amadeh, B. Pahlevani, S. Heshmati-Manesh, Effects of rare earth metal addition on surface morphology and corrosion resistance of hot-dipped zinc coatings, Corros. Sci. 44 (10) (2002) 2321–2331. [6] Q.B. Zhang, C. Yang, Y.X. Hua, Y. Li, P. Dong, Electrochemical preparation of nanostructured lanthanum using lanthanum chloride as a precursor in 1-butyl-3methylimidazolium dicyanamide ionic liquid, Phys. Chem. Chem. Phys. 17 (6) (2015) 4701–4707. [7] X. Huang, N. Li, L. Cao, J. Zheng, Electrodeposited lanthanum film as chromate replacement for tinplate, Mater. Lett. 62 (3) (2008) 466–469. [8] P. Yan, M. He, B. Chen, B. Hu, Fast preconcentration of trace rare earth elements from environmental samples by di(2-ethylhexyl)phosphoric acid grafted magnetic nanoparticles followed by inductively coupled plasma mass spectrometry detection, Spectrochim. Acta B At. Spectrosc. 136 (2017) 73–80. [9] A.J. Bard, R. Parsons, J. Jordan, Standard potentials in aqueous solution, Monographs in Electroanalytical Chemistry and Electrochemistry, Marcel Dekker, Inc., New York, NY 1985, p. 834. [10] L.F. Audrieth, E.E. Jukkola, R.E. Meints, B.S. Hopkins, Observations on the rare earths. XXXVII. electrolytic preparation of rare earth amalgams. 1. Preparation of amalgams of lanthanum and neodymium, J. Am. Chem. Soc. 53 (5) (1931) 1805–1809. [11] E.I. Onstott, Separation of the lanthanons at amalgam cathodes. II. The separation of samarium from gadolinium and purification of europium at a lithium amalgam cathode1, J. Am. Chem. Soc. 78 (10) (1956) 2070–2076. [12] E.I. Onstott, Separation of the lanthanons at amalgam cathodes. III. Electrochemical fractionation of the lanthanons at a lithium amalgam cathode1, J. Am. Chem. Soc. 81 (17) (1959) 4451–4458. [13] R.F. Large, A. Timnick, Polarographic behavior of dysprosium(III) in aqueous solutions, Anal. Chem. 36 (7) (1964) 1258–1264. [14] B.S. Hopkins, Electrochemistry of the rare earth group, Trans. Electrochem. Soc. 89 (1) (1946) 295–300. [15] L.M. Dennis, B.J. Lemon, The electrolysis of solutions of the rare earths, J. Am. Chem. Soc. 37 (1915) 131–137. [16] J.T. Babauta, A. Medina, H. Beyenal, EQCM and surface pH studies on lanthanum accumulation on electrodes in aqueous solution, J. Electrochem. Soc. 163 (9) (2016) H866–H870. [17] A.S. Medina, C.F. Ivory, N.A. Wall, S.B. Clark, H. Beyenal, Electrochemical preconcentration mechanism of trivalent lanthanum, J. Electrochem. Soc. 165 (13) (2018) D654–D661. [18] P.D. Schumacher, K.A. Fitzgerald, J.O. Schenk, S.B. Clark, Preconcentration of felements from aqueous solution utilizing a modified carbon paste electrode, Anal. Chem. 83 (4) (2011) 1388–1393. [19] P.D. Schumacher, S.M. Miley, J.O. Schenk, S.B. Clark, Optimization of Nd(III) preconcentration on a rotating disk mercury film electrode in aqueous solution, Proceedings in Radiochemistry A Supplement to Radiochimica Acta 2011, p. 21. [20] P.D. Schumacher, N.A. Woods, J.L. Doyle, J.O. Schenk, S.B. Clark, Cathodic preconcentration of f-elements on a mercury film carbon fiber disk microelectrode, Anal. Chem. 83 (12) (2011) 4788–4793.

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