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XPS Investigation of Surface Chemistry of Magnesium Electrodes in Contact with Organic Solutions of Organochloroaluminate Complex Salts Yossi Gofer,*,† Rachel Turgeman,† H. Cohen,‡ and Doron Aurbach† Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel; and Chemistry Department, The Weizmann Institute, Rehovot 76100, Israel Received October 2, 2002. In Final Form: November 27, 2002 Metallic magnesium can be reversibly deposited from ethereal solutions of magnesium-aluminum complex salts of the general structure Mg(AlCl4-nRn)2, where R ) alkyl. In contrast to the case of lithium surfaces, already thoroughly investigated, the surface state of magnesium electrodes in contact with organic solutions is yet unclear. In this paper we report on a systematic surface analysis of magnesium electrodes in contact with various organic solutions, using XPS. We find in both clean tetrahydrofurane (THF) and THF solutions of dibutylmagnesium or butylmagnesium chloride that the metal surface consists of magnesium oxide and hydroxide (probably developed during manipulation and sample transfer); however, it does not develop thick passivation layers. In THF solutions containing Mg(AlEtBuCl2)2, surface residuals of carbon, aluminum, and chlorine are detected yet are restricted to the outermost part of the surface, as physically adsorbed species. From their concentration one deduces that both the complex salt and the ether are not reduced at the magnesium surface but precipitate as an insoluble film. Metallic magnesium deposited from THF/ Mg(AlEtBuCl2)2 solution on gold electrodes shows a very similar surface chemistry, providing an additional proof that, even in the most frail conditions available during electrochemical deposition, pure magnesium is deposited. The surface chemistry of magnesium in contact with propylene carbonate (PC) exhibits as well layered surface chemistry, most of it composed of magnesium oxide and hydroxide, but no evidence is found for reduction products of PC. It is concluded that the magnesium metal behaves like a surface film-free electrode in organo-haloaluminate/THF solutions. Our conclusions support several other studies on the properties of magnesium in such solutions.
Introduction Magnesium is one of the most interesting materials in the context of a new anode for batteries.1 In contrast to the case of lithium, the surface chemistry of magnesium has not been explored in detail. In fact, as much as we could come across, no literature information could be found regarding the surface chemistry of magnesium in contact with nonaqueous solutions. Several publications indeed regard the surface chemistry of magnesium, but these few papers deal with magnesium in contact with either atmosphere or aqueous solutions.2 In the last several years a renewed interest in magnesium electrochemistry in nonaqueous solutions3-6 has emerged due to its potential capabilities for rechargeable batteries. To understand the electrochemical behavior of magnesium, extensive efforts have been focused on comprehensive studies of its surface chemistry in various nonaqueous solutions. Surface sensitive techniques have been employed, such as FTIR (Fourier transform infrared spectroscopy),7 SEM (scanning electron microscopy), EDAX (energy dispersive analysis by X-rays), STM * To whom correspondence should be addressed. † Bar-Ilan University. ‡ The Weizmann Institute. (1) Novak, P.; Imhof, R.; Haas, O. Electrochim. Acta 1999, 45, 351367. (2) Chen, C.; Splinter, S. J.; Do, T.; McIntyre, N. S. Surf. Sci. 1997, 382, L652-L657. (3) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724-727. (4) Novak, P.; Desilvestro, J. J. Electrochem. Soc. 1993, 140, 140144. (5) Shlover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novak, P. J. Solid State Chem. 1996, 123, 317-323. (6) Aurbach, D.; Gizbar, H.; Schechter, A.; Chusid, O.; Gottlieb, H.; Goldberg, I. J. Electrochem. Soc., in press.
(scanning tunneling microscopy), and EIS (electrochemical impedance spectroscopy).8,9 The emerging view, based on our increasing experience with magnesium electrochemistry, is that magnesium behaves essentially as a filmfree metal electrode in contact with the complex magnesium ions in ethereal solutions.7-10 In this paper we describe a systematic study on the surface chemistry of magnesium in contact with THF solutions containing a salt which is represented by the formula Mg(AlEtBuCl2)2 (DCC). This salt, as a THF solution developed in our laboratory, enables reversible electrochemical deposition/dissolution of magnesium, as well as intercalation of magnesium in a variety of hosts. One of the major tasks of this work is to learn whether the magnesium metal reacts with polar organic solvents in the same manner as lithium does. It is expected, as is the case for other active metals,11,12 that the reaction products form adherent, passivating films. In that case, our goal is to estimate the thickness of these layers and their chemical composition. The surface sensitivity of X-ray photoelectron spectroscopy (XPS) originates from the short escape depth of electrons from solids.13-16 We use this property, applying (7) Aurbach, D.; Turgeman, R.; Chusid, O.; Gofer, Y. Electochem. Commun. 2001, 3, 252-261. (8) Aurbach, D.; Schechter, A.; Moskovich, M.; Cohen, Y. J. Electrochem. Soc. 2001, 48, A1004-A1014. (9) Aurbach, D.; Cohen, Y.; Moskovich, M. Eelctrochem. Solid State Lett. 2001, 4, A113-A116. (10) Aurbach, D.; Gofer, Y.; Schechter, A.; Chusid, O.; Gizbar, H.; Cohen, Y.; Moshkovich, M.; Turgeman, R. J. Power Sources 2001, 978, 269-273. (11) Aurbach, D.; Skaletzky, R.; Gofer, Y. J. Electrochem. Soc. 1991, 138, 3536-3545. (12) Moshkovich, M.; Gofer, Y.; Aurbach, D. J. Electrochem. Soc. 2001, 148, E155-E167.
10.1021/la026642c CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003
Mg Electrodes in Contact with Mg(AlCl4-nRn)2 Solutions
also angle resolved XPS (ARXPS), to estimate the average thickness of the passivation layers.
Langmuir, Vol. 19, No. 6, 2003 2345 Table 1. Quantitative Results Calculated from the XPS Analysis for Magnesium Foil Prepared under a Glovebox Atmosphere before and after 120 s of Ar+ Sputtering
Experimental Section General. All sample preparation, salt synthesis, and solutions handling have been carried out in a high-quality, argon filled glovebox (VAC HE-493). Water and oxygen levels were maintained at lower than 1 and 10 ppm, respectively. All measurements were performed at ambient temperature. Materials. THF (Merck, selectipure, less than 10 ppm of H2O by Karl-Fisher titration), propylene carbonate, PC (Merck, selectipure), dibutylmagnesium, Bu2Mg (Aldrich, 1 M solution in heptane), ethyl dichloroaluminum, EtAlCl2 (Aldrich,1 M solution in heptane), and butylmagnesium chloride, BuMgCl (Aldrich, 2 M solution in diethyl ether) have been used as received. AlCl3 (Aldrich, 99.99%) has been sublimed under vacuum. 0.25 M solutions of Mg(AlEtBuCl2)2 in THF were prepared as described elsewhere.10 Magnesium samples for XPS analyses have been prepared as follows: thin (ca. 0.3 mm) magnesium foil was rolled from a magnesium bar (DSM, 99.7%). Small pieces of the foil (ca. 12 mm × 8 mm) were scraped free of the top layer by using the sharp edge of microscope glass slides. After introduction to the argon filled glovebox, and prior to any experiment, the foil was once again scraped, in the same manner, to clean its surface from any remaining passivation layer. In those experiments of magnesium immersed in solutions, the Mg foil was scraped by a glass slide once again, during immersion in the actual solution, in a Petri dish. By using this method, we made sure that the newly exposed magnesium surface would be totally free of contaminants arising from the glovebox. Hence, the samples had long (ca. 1 cm) linear grooves along their surfaces. Profilometer measurements indicated grooves of ∼0.5-2 µm in depth and 1-10 µm in width. XPS Analysis. XPS analyses were performed in a Kratos AXIS-HS spectrometer, equipped with an Ar+ sputtering gun (minibeam I), using a monochromatized Al KR source. Survey scans ran at 45 or 75 W. In the angle resolved work, an elevated power of 150 W was needed to obtain an improved S/N ratio. Long experiments did not reveal any susceptibility of the sample toward the X-ray irradiation. All data acquisitions were performed in a hybrid mode (using electrostatic and magnetic lenses) and detection pass energies of 40-80 eV. Ar+ etching was used in the x-y scan mode at ion acceleration of either 3 or 4 KV and current densities of 0.5-5 µA/cm2. Sputter rates were calibrated by standard samples (electrochemically anodized 150, 225, and 300 Å Ta2O5 on polished tantalum foil as well as 130 and 100 Å of vacuum evaporated gold on glass or Si(111) wafers). The tantalum oxide sputtering rates calculated for 4 and 3 KV Ar+ were ∼1.8 and ∼1.3 Å min-1 µA-1 cm-2, respectively. The corresponding sputtering rates calculated from the gold films were 8.7 and 5.8 Å min-1 µA-1 cm-2, respectively. Due to the anisotropic (macroscopic) roughness of the scraped magnesium foils, these samples were positioned with their linear scratches parallel to the direction of the argon beam, thus minimizing artifacts associated with sputtering of rough surfaces. Angle resolved XPS (ARXPS) was carried out with a manual tilt manipulator, with the angle θ defined as the angle between the sample normal and the detector. Signal collection was performed with a fully open iris, namely with a collection angle of (15°. These settings are not optimal for ARXPS work; however, in view of the more severe errors in depth resolution, arising from the surface roughness, these conditions were reasonable, while offering better sensitivity. Similar to the Ar etching considerations, here as well roughness effects were minimized by tilting in parallel to the linear scratches. All samples were transferred from the glovebox to the XPS prechamber by a homemade transfer device, with no exposure to the ambient. The device consisted of a magnetic manipulator (13) Fadely, C. S.; Baird, R. J.; Siekhaus, W.; Novakov, T.; Bergstorm, S. A. J. Electron Spectrosc. Relat. Phenom. 1974, 4, 93-137. (14) Powell, C. J.; Jablonski, A.; Tilinin, I. S.; Tanuma, S.; Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1999, 98-99, 1-15. (15) Gutner, P. L. J.; Gijzeman, O. L. J.; Niemantsverdriet, J. W. Appl. Surf. Sci. 1997, 115, 341-346. (16) Zalm, P. C. Surf. Interface Anal. 1998, 26, 352-358.
O(1s) C(1s) Cl(2p) Mg(2p) Mg2+/Mg0 pristine after 120 s of etching
40% 39%
25% 6%
0.81% 0.52%
35% 55%
64/36 55/45
terminated with a UHV compatible gate-valve. Up to three samples could be transferred simultaneously in this fashion. The samples were loaded on the probe in a glovebox, and the whole system was then delivered to the XPS prechamber and then attached through a Conflat flange sealed with Viton O-rings. The transfer of the samples was carried out after pumping the prechamber down to 1 × 10-6 Torr. All XPS measurements were carried out at room temperature, under vacuum conditions of (1.0-3.0) × 10-9 Torr. Unless otherwise indicated, the spectra were acquired without an electron flood gun for charge neutralization. Preliminary experiments with various excitation power values showed no discernible charging effects. The spectrometer energy scale was routinely calibrated according to the ISO TC/201 SC7 international procedure for binding energy (BE) with Au 4f7/2 ) 83.98 and Cu 2p3/2 ) 932.67. Data processing was done with either VISION 2.1 software (“Kratos”) or XI SDP V2.3 (XPS International, Kawasaki, Japan, 2001). Vision 2.1 sensitivity factors were used for quantification. In most cases a Shirley background was used. Plasmon loss features near the Mg(2p), Mg(2s), and Mg(1s) lines were not included in the quantification. Curve fitting was performed using a 80/20 Gaussian/Lorentzian line shape. Regularly, 100-1000 iterations were used to reach the best fitting.
Results Magnesium Metal Foil (Prepared under Glovebox Atmosphere). As a reference for the entire study, we first analyzed a magnesium foil that was subjected to the cleaning process described above, under the glovebox argon atmosphere. The magnesium foil was scraped clean only. Figure 1 presents spectra of this sample, before and after sputtering. As can be seen in Figure 1a, the Mg(2p) peak is composed of the sharp Mg metal peak (full width at half maximum, fwhm 0.533 eV) centered at 49.9 eV and a broader Mg2+ peak (fwhm 1.63 eV) centered at 51.3 eV (the plasmon excitations are not included in this figure).17-19 Figure 1b and d shows O(1s) before and after sputtering, respectively. At least four Gaussian/Lorenzian peaks are needed to fit this curve. The peak at 529.6 eV can be attributed to MgO, the peak at 530.5 to Mg(OH)2, and the peaks at 531.5 and 532.9 to organic ether oxygen.17-19 Yet, we believe that more types of materials may be present on the surface of this sample. A trace amount of chlorine was also detected, with Cl(2p) emission at 200 eV. Our quantitative results are given in Table 1. To further learn the surface, we Ar sputtered the surface for 120 s at a current density of 4.0 µA/cm2, for an estimated removal of ∼15-40 Å of surface layer. In general, the spectra of the sputtered sample are similar to the pristine ones. However, several features are slightly different. The Mg Auger spectral region at 300-400 eV shows sharper peaks and a flatter background after the sputtering. The Mg(2s) and Mg(2p) peaks are also sharper, and their subsequent plasmon satellites are better resolved. The O(1s) peak shape has also changed considerably, as can (17) “XI SpecMaster System”; Vincent, C., Ed.; XPS International: Kawasaki, Japan, 1998. (18) Wagner, C. D. In Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons Ltd: New York, 1990; Vol. 1, p 595. (19) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr. NIST Standard Reference Database 20, Version 3.2 (Web Version) (http://srdata.nist.gov/xps/).
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Figure 2. Mg(2p), C(1s), and O(1s) spectra of magnesium foils that were in contact for 1 h in THF solutions of (a) pure solvent, (b) Bu2Mg, (c) DCC, and (d) pure PC. Figure 1. XPS spectra obtained from magnesium foil that was scraped in an Ar atmosphere glovebox: (a and b) Mg(2p) and O(1s) regions, respectively, before sputtering; (c and d) Mg(2p) and O(1s) spectra after 120 s of Ar+ sputtering.
be seen in Figure 1c and d. All these facts indicate that with this modest dose of Ar+ ions a large part of the covering surface layer has already been removed. Indeed, the carbon C(1s) peak is almost completely removed, suggesting that the carbonaceous material on the magnesium surface is mostly organic adsorbates. It is likely that the preparation glovebox is saturated with organic solvents, mainly ethers and organic esters and carbonates, that are in use there. From comparison of the height ratio of O(1s) to Mg(2s) in both cases it is also obvious that some of the oxygen containing surface layer had been removed during the ion beam treatment. More detailed information can be gained from the comparison of parts b-d of Figure 1. It is clear that the most affected feature is the reduction of the contribution of the higher BE peaks that are attributed to the ethereal oxygen atom. The lower BE peaks have hardly changed in intensity or position at this stage. These observations are consistent with the almost complete disappearance of the C(1s) peak. Further support for these conclusions may be gained from the increase of the metallic Mg(2p) peak at the expense of the Mg2+ one after sputtering, as can be observed in Figure 1a and c. Magnesium in Contact with THF and THF Solutions of Dibutylmagnesium, Butylmagnesium Chloride, DCC, and PC. As mentioned earlier, magnesium is a light metal and is considered as a member of the reactive metals, such as lithium, potassium, barium, and so forth. It is a well-established fact that metallic lithium is reactive toward any organic, polar solvent. Usually, the reaction products of lithium with the solvents result in precipitation of surface films, which leads to “passivation”. Such passivated layers are composed of a mixture of organic and inorganic reaction products, usually lithium salts, which are, in many cases, electronic insulators and lithium ion conductors. First, we chose to learn the behavior of magnesium in DCC/THF solutions, from which magnesium might be plated and dissolved electrochemically.3 We also studied the surface chemistry of magnesium in contact with individual components of this solution, namely, pure THF, and THF solutions of BuMgCl and Bu2Mg. As an additional reference, we studied also the surface chemistry of magnesium in contact with PC. In
Table 2. Quantitative Results Calculated from the XPS Analysis for Magnesium Foils Dipped in the Specified Solutions for 2 h, without Further Treatment solution
Mg tot
Mg2+/ Mg0
O tot
Oorg/ Oinorg
C
Cl
THF/pure THF/Bu2Mg THF/BuMgCl THF/DCC PC/pure
33.7 24 30 20 36
62/37 63/37 65/35 61/39 65/35
36 34 36 33 30
82/18 77/23 78/22 89/11 75/25
29 40.5 28 34 34
1.2 1 7 7.8 0.6
Al
5.2
contrast to the electrochemistry in DCC/ether solutions, magnesium is passivated in most other polar, organic (dry) solvents. Also, distinctive from the “passivation” term, used in conjunction with lithium nonaqueous electrochemistry, the passivation phenomenon in magnesium is a “real” passivation; namely, the passive films are both electron and ion insulators. Thus, it is imperative to compare the behavior of magnesium in the ether solutions to at least one case of the most common solvents used in lithium electrochemistry. PC is a natural choice, since it is one of the most investigated solvents in this respect. To study the chemical behavior of magnesium with these solvents and solutions, we submerged magnesium foils, as previously described, in the various solutions for periods of 1 h and 2 months. In Figure 2, one can see comparison of Mg(2p), C(1s), and O(1s) spectra acquired from magnesium strips that were treated for 1 h with pure THF, THF solutions of Bu2Mg and DCC, and PC. Quantitative analysis of the surface chemical composition for the corresponding samples, including the results for BuMgCl, is presented in Table 2. Comparing the spectra in Figure 2 and the quantitative data in Table 2, one finds no marked differences in surface composition of the various samples. Indeed, the samples which were in contact with THF solutions of BuMgCl and DCC contain significantly higher concentrations of chlorine, or aluminum and chlorine, respectively. However, the shapes of the C(1s), O(1s), and Mg(2p) lines are very similar, suggesting that similar compounds appear on the various surfaces. Furthermore, these findings are not very different from the results obtained with the reference magnesium sample that was peeled in the glovebox, as described in the previous section. Note that the O(1s) and Mg(2p) lines of the sample put in contact with DCC are broader than the others. Besides the fact that all samples exhibited similar surface composition, it should be stressed that the C(1s) lines contained either no or only a trace of
Mg Electrodes in Contact with Mg(AlCl4-nRn)2 Solutions
Langmuir, Vol. 19, No. 6, 2003 2347 Table 3. Quantitative Results Calculated from the ARXPS Spectra Obtained from Magnesium Foil that Was Scraped and Immersed for 2 Months in a 0.25 M Solution of Mg(BuEtAlCl2)2 in THF
Figure 3. ARXPS spectra obtained from magnesium foil that was scraped and immersed for 2 months in a 0.25 M solution of Mg(BuEtAlCl2)2 in THF. The spectra present the Mg(2p) region acquired at the following takeoff angles: (a) 90°; (b) 30°; (c) 20°.
carboxylate or organic carbonate anion features (at BE ∼289 and ∼290 eV, respectively). In general, samples that were prepared identically and had been in contact with given solutions for 2 months exhibited only minor differences. In some cases the Mg KLL multiplets revealed a less resolved picture, and the Mg2+/Mg0 ratio increased slightly, probably as a result of a slight increase in the oxide layer thickness. However, these effects did not constitute a solid trend and were observed only in some of the samples. Hence, we conclude that the contact time between the solution and the magnesium metal has only a minor effect on the surface composition. Hence, the corrosion of magnesium submerged in all these solutions is minor. Here as well we applied Ar+ sputtering at a range of doses. Two minutes of sputtering was enough for a drastic reduction in the C(1s) signal and a complete removal of Cl and Al, if at all detected initially. Also, the O(1s) and the Mg peaks showed reduction in the Ohigh/Olow and Mg2+/ Mg0 ratios, respectively, as shown for the case of magnesium that was prepared under glovebox atmosphere (previous section). Further sputtering resulted in a further increase of the same trend. Notably interesting is the observation that the chemistry developing on clean magnesium surfaces in PC is qualitatively and quantitatively similar to those studied above. This fact implies that, even with a more reactive solvent, it is not the reaction products of PC with magnesium that dominate the film composition. (As with the former cases, a moderate sputtering dose, of 120 s, eliminated most of the C(1s) peak and part of the oxygen signal.) This fact is interesting, since PC is expected to be reduced by magnesium in a similar manner as by lithium, and hence to precipitate stable passivation layers. The sputtering depth profile suggests that the composition of the above magnesium surfaces is ordered in a layered fashion. The metal is covered by MgO, on which a layer rich with hydroxides is situated, and the latter is covered by an organic adsorbed overlayer containing the constituents of the organic solutions. Angle resolved analysis of some of the samples provided an additional support for this interpretation. Spectra acquired at 90°, 30°, and 20° takeoff angles on magnesium that was in contact with THF/DCC solution for 2 months are shown in Figure 3. Their quantitative analysis is presented in Table 3. These spectra cover the BE region of Mg(2p) and
take-off angle
O(1s)
C(1s)
Cl(2p)
Mg(2p)
Al(2p)
90° 30° 20°
36 30 31
23 29 31
9 9 10
25 16 11
8 15 12
Al(2p) emission. Three trends are well distinguished here: first, the Mg0/Mg2+ ratio decreases as the sample is tilted to lower angles. Second, the lower the tilt angle, the weaker the plasmon peaks (at ca. 58.3 and 71.5 eV), and third, the lower the tilt angle, the higher the ratio between the peak areas of both Al3+/Mg0 and Al3+/Mg2+ (the peak at BE ) ∼74 eV is attributed to Al3+). As lower tilt angles relate to enhanced surface sensitivity, all three trends above prove that the overlayers are ordered in such a way that the outermost surface is rich with aluminum ions. Beneath, a layer rich with magnesium compounds is situated, while underneath one deduces the presence of the magnesium metal substrate. A thin overlayer of carbon containing molecules, probably organic adsorbates, of nonuniform thickness, covers this whole multilayer structure. From the Mg0/Mg2+ ratio, the oxide layer was calculated to be ∼40 Å. It is important, though, to emphasize that, due to the inhomogeneity of the surface morphology, the thickness values calculated cannot be accurate and only present an order of magnitude. Generally speaking, electrodeposition of metal constitutes extreme conditions for reaction of the deposited metal with the electrochemical bath. Thus, to learn the influence of such conditions on the freshly deposited metal, we have electrochemically deposited magnesium from DCC/THF solutions onto clean gold substrates and analyzed the deposits. The samples were prepared by galvanostatic deposition (100 µA/cm2) of magnesium onto clean Au foils, from 0.25 M DCC/THF solutions at room temperature. We prepared samples with deposition charges of 50 and 500 mC/cm2; however, the analysis resulted in very similar results. It is interesting to note that in both cases magnesium, chlorine, and aluminum were detected on the gold surface. Moreover, in both cases, even after 1 min of sputtering all these elements were still clearly identified. These results provide a further evidence for the nature of these complex salts as strong adsorbates, at least on gold. The spectra recorded from the magnesium deposited gold samples were all similar, qualitatively and quantitatively, irrespective of the deposition charge. From the Mg2+/Mg0 ratio and from other features in the survey spectrum we deduce that the surface layers are very similar in composition and thickness to the ones measured from magnesium in contact with DCC/THF. A further evidence for the low thickness of these overlayers could be gained from the Mg(1s) peak. The emission of Mg(1s) electrons, at binding energies above 1300 eV, results in electrons of low kinetic energy of ∼180 eV. These electrons have a considerably lower attenuation length, about three times shorter than that of the 2s and 2p signals, and hence an enhanced surface sensitivity. The curve fitting of Mg(1s), before and after sputtering of ∼30-40 Å, verifies that the overlaying films cannot exceed 40-50 Å. Moreover, even after only 60 s of sputtering (removal of only 1.8-4 Å) the metallic Mg(1s) peak was already discernible. Thus, this led us to the conclusion that the overlayer films are, like in the former cases, only several angstroms thick. A more accurate assessment of the thickness of these films is not possible in these experiments, since the deposits on
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gold surfaces are granular and not uniform, thus, not allowing coherent sputtering to be carried out. An additional interesting observation was gained in the control experiments. It was found that gold plates that were dipped in DCC/THF solution for several minutes, followed by rigorous washing with THF, exhibited spectra that contained Mg, Al, Cl, and C. This observation supports the assumption that DCC is adsorbed tenaciously upon contact with solids. Conclusions As can be learnt from the value of the standard potential of the Mg2+/ Mg couple in the electrochemical series, -2.37 V, magnesium is a highly reactive metal. Therefore, it is expected that any exposed metal surface will react promptly with the ambient. Oxygen and water are, naturally, the greatest concern. Therefore, even under the atmosphere of a high-purity glovebox, the metal is expected to react with oxygen and humidity traces and form surface films. The expected reaction products are MgO and Mg(OH)2, as well as some hydrated forms of these compounds. Indeed, all of the O(1s) spectra could be deconvoluted into two peaks, at ∼529.6 and 530.5 eV (calibrated vs C(1s) ) 285.0 eV), the first one attributed to MgO, while the other to Mg(OH)2. Furthermore, in all depth profiles, it was demonstrated that the Mg(OH)2rich layer lays on top of the MgO one. Moreover, and not less important, there is no indication for the formation of MgCO3 from the C(1s) spectra. The main question remaining here is whether these compounds are the products of reactions between the magnesium and the solutions, or due to unavoided contact with the glovebox and the transfer stage atmosphere. Noting the intrinsic reduction potential of the organometallic reagentss Bu2Mg, BuMgCl, and EtAlCl2sit is unlikely that oxygen or water traces exist in these solutions. Therefore, we conclude that at least the major part of the oxidized magnesium layers is a result of postpreparation stages. One of the most important issues that this study aims at is whether magnesium metal reacts with solvents, such as THF, and the related solutions, to form stable surface layers, in the same manner as lithium does. Such reaction products are expected to be some magnesium salts of the reduction products of the organic species. The main feature of such compounds is the carbon C(1s) peak. Although in all the samples analyzed in this study a large C(1s) peak has been found, it contains only minor features expected from stable, organic, saltlike compounds, if any. First, the C(1s) signal originates in all samples from a carbonaceous material. Second, the ARXPS and the Ar+ sputtering show that this signal relates to weakly adsorbed molecules rather than a strongly adherent ionic species.
Gofer et al.
Interestingly, even with the sample that was in contact with PC, one of the more reactive (reducible) organic solvents, no evidence was found for either organic or inorganic carbonates, the expected reduction products of this solvent. These observations do not provide an unambiguous proof that there is no reaction whatsoever between magnesium metal and the solutions, but they do indicate that if there is a surface film formation it must be confined to a monolayer scale. Similar conclusions can be drawn for other components, such as DCC, dibutylmagnesium, and BuMgCl. The corresponding elements in these compounds were found at the outer surface and were, again, removed easily by sputtering. A further proof for the adsorption scheme can be found in the analysis of the gold electrode that was immersed in DCC solution only. Even without any electrochemical manipulation, and after rigorous washing, chlorine, aluminum, magnesium, and carbon were clearly detected on the surface. Since gold is inert to these solutions, only tenacious DCC and ether adsorption could have led to the presence of these elements. Probably, the most important information regarding the electrochemistry of magnesium in DCC/THF solution arises from the analysis of the gold electrode after electrodeposition of magnesium. In these spectra we could observe that the quality of the magnesium deposits is identical to that of pure magnesium metal that was in contact with the DCC/THF solution. This fact indicates that even under the most sensitive conditions, that is fine electrochemical growth, the magnesium grows as pure metal, with an interface that is most probably free of surface films, under the solution. On the basis of the above conclusions, we think that a more plausible scenario is that, upon immersion, the magnesium metal becomes covered with adsorbents, composed of the various solution species. Upon disturbance, for example, during washing, only part of this metastable film is removed. Since all these materials are very reactive, particularly when dry, the major part of them decomposes during sample transfer, to a variety of more stable magnesium compounds (and more stable aluminum compounds), upon exposure to trace water and oxygen. Since their source is not the underlying metal, they are probably loosely packed and adhered to the surface. Acknowledgment. Partial support for this work was obtained by the Israeli Ministry of Science & Technology, by the Israel Science Foundation of the Israeli Academy of Science, and by the BMBF, the German ministry of science, in the frame of the DIP project. LA026642C
