J. Phys. Chem. C 2007, 111, 5701-5707
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Spectroscopic Evidence of Platinum Negative Oxidation States at Electrochemically Reduced Surfaces J. Ghilane,† C. Lagrost,† M. Guilloux-Viry,† J. Simonet,† M. Delamar,‡ C. Mangeney,*,‡ and P. Hapiot*,† Sciences Chimiques de Rennes, UniVersite´ de Rennes 1, UMR CNRS n°6226, Campus de Beaulieu, 35042 Rennes, France, and Itodys, UniVersite´ de Paris 7-Denis Diderot, UMR CNRS n°7086, 1 rue Guy de la Brosse, 75005 Paris, France ReceiVed: December 22, 2006; In Final Form: January 21, 2007
Electrochemical “reduction” of Pt was investigated using electrochemical and X-ray photoelectron spectroscopy (XPS) techniques. This transformation of a platinum metal, which is possible with a large variety of organic and inorganic cations, was investigated in DMF containing CsI salts chosen as a test system. Experiments show that considerable modifications of the chemical nature of the starting material accompany the morphological changes that were previously reported. With rather mild reduction conditions, cesium-platinideslike structures were detected. XPS investigations bring a direct spectroscopic evidence for the formation of a reduced platinum state from a metallic electrode held at a negative potential. XPS spectra clearly showed a significant and continuous shift of the Pt formal oxidation degree with the injected charge of -1.6. This behavior is analogous to a charging process. Additionally, proofs of the full reversibility of the phenomenon was provided by the XPS analyses showing that the “reduced Pt” returns to its metallic state after reaction with a diazonium salt. It is noticeable that the formation of negative oxidation state of Pt exists even when a large competitive electrochemical process occurs, as in our case with the reduction of protons from residual water.
Introduction Formation of intermetallic compounds from the uptake (or exchange) of guest ions by host lattices is a well-known phenomenon in the field of solid-state chemistry.1,2 A particular class of intermetallic materials are the Zintl phases, where alkali or alkaline metals cations are blended into a post-transitionmetal matrix.3 Such compounds can be produced by chemical or electrochemical methods. Among the metals that can be reduced electrochemically to form the intermetallic phases are those belonging to the IV and V main group of elements such as antimony, bismuth, tin, or lead, which yield the so-called Zintl phases.3 The Zintl (Klemm) concept has been developed for intermetallic phases involving post transition metals and assumes a complete transfer of the valence electrons from the more electropositive component to the more electronegative one. A natural extension of the Zintl-Klemm concept is to consider transition metals as the electronegative component. Among the transition elements, platinum exhibits very high electron affinity (2.13 eV) and of a magnitude comparable to those of typical nonmetals.4 However, only few works report on the chemistry of platinides. Red transparent Cs2Pt crystals showing full charge separation, Ba-Pt systems (BaPt, Ba3Pt2, Ba2Pt, BaPt2, BaPt5), Ca3Pt2 structures or the more complex Pt2In14Ga3O8F15 crystal can thus be quoted.5-8 It is worth outlining that the synthesis and isolation of these platinides structures require drastic experimental conditions. In this context, it has been recently shown that platinum metal electrodes can form new reducing phases upon reduction in dry dimethylformamide (DMF) * To whom correspondence should be addressed. † Sciences Chimiques de Rennes - University of Rennes 1. ‡ Itodys -University of Paris 7.
containing tetraalkylammonium or alkali metal iodide salts.9,10 These electrogenerated phases resembling to the Zintl phases are produced from an elemental electrochemical process, which should formally correspond to the reduction of the platinum metal.11 It occurs when the platinum electrode is held at negative potential depending on the considered electrolyte cation (typically -1.5 to -1.9 V vs SCE with an alkali metal cation), that corresponds to relatively mild reductive conditions. The electrochemical process involves an electron transfer coupled to the insertion (or deposition) of the electrolyte cation. The resulting metal-organic phase was anticipated to be of the general formula [Ptn-, M+, MX], where M+ is the cation electrolyte and X- is the anion electrolyte.9 The cathodic modifications of the platinum electrode have been investigated by different electrochemical methods and scanning electron microscopy (SEM). These investigations evidenced impressive morphological changes of the metal surfaces.9 More recently, in situ electrochemical atomic force microscopy (EC-AFM) experiments performed with microstructured platinum metal samples allowed us to monitor the morphological surfaces changes as a function of the nature of the salts and of the injected charge.10a,c Although the same dramatic modifications of the morphology were observed upon the cathodic treatment, the process was found to be fully reversible as the sample retained its starting structure after reoxidation.9i,10c The so-generated phases have been shown to exhibit strong reducting properties.9c,e In exploitation of this specific reactivity, the reduced metallic phases were then used for inducing the grafting of aryl groups onto platinum surfaces from the electroless reduction of diazonium salts.10b All these results tend to demonstrate that the metallic nature of the cathodically activated platinum electrodes corresponds to a reduced platinum state. Nevertheless,
10.1021/jp068879d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007
5702 J. Phys. Chem. C, Vol. 111, No. 15, 2007 any direct experimental proof has never been provided. Moreover, the quantification and the evolutions of the negative oxidation state of the platinum inside these phases during the reduction remain an open and puzzling question for the chemistry of this metal. The main goal of the present work is thus to provide new insights about the chemical nature that accompanies the deep morphological modifications of the platinum surfaces upon the cathodic activation based on X-ray photoelectron microscopy (XPS) measurements. Recent XPS investigations performed on the barium platinide compounds have shown that the Pt(4f7/2) levels shift toward lower binding energies by ca. 1.2 eV per oxidation state unit as compared to metallic platinum, which corresponds to negative oxidation states of platinum.6d However, we should insist on the fact that if the formation of such negative oxidation platinum states are somewhat reasonable when platinum is in presence of a very strong reducing agent, the energetic conditions considered here are completely different and remain relatively mild. We should also remind that platinum has been used for decades as a supposed inert electrode material in numerous cathodic processes. Thus, it is likely that the concomitant transformation of the Pt with the reduction of organic molecules in an aprotic solvent may require some reexaminations of published data. The second and consecutive question addressed in this work concerns the evolutions of the surface modifications (composition and XPS chemical shifts) when the reduced platinum in used for inducing a chemical reaction triggered by an electron transfer. This is for example the case for the electrogeneration of aryl radical leading to the formation of aromatic layers by the reduction of diazonium salt. Besides the possible interest of this reaction for preparing structured organo-metallic structures, the reaction provides a convenient way for investigating the regeneration of the metallic Pt.
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Figure 1. Voltammogram of Pt surface in DMF with 0.1 mol L-1 CsI on a 1 mm diameter disk Pt electrode. Scan rate 0.1 V/s.
Experimental Section Chemicals. Cesium iodide, CsI, from Fluka (electrochemical grade) was used as supporting electrolyte without further purification. Dry DMF (Puriss) was purchased from Fluka and stored over molecular sieves. The diazonium salt (4-bromophenyl diazonium tetrafluoroborate) was purchased from Acros and used as received. Electrochemical Procedures and Pt Samples Preparations. Electrochemical modifications and XPS investigations were performed on microstructured platinum substrates with a low surface roughness that were used as a working electrode. These samples allow EC-AFM examinations and thus to control the occurrence of the reduction through the appearance of the morphology changes as we have previously reported.10 Pt samples were prepared by direct current sputtering of Pt onto a (100)MgO single-crystal substrate (5 mm × 5 mm). This leads to a distribution of (100)Pt platelets (around 100-200 nm size and an average thickness around 50 nm), and the samples behave like a very flat native electrode.10c,12 The direct grafting of aryl compounds after cathodic modification of Pt was performed with platinum foils (5 mm × 5 mm) purchased from Goodfellow. Platinum samples were cleaned in an ultrasonic bath in DMF before any experiments to remove any dust impurities. A conventional three-electrode cell was used for the classical cyclic voltammetry experiments and for the electrochemical modification of the platinum foils prior to the reduction of the diazonium salts. The potentiostat was a PAR potentiostat/galvanostat model M 273 (EG&G Princeton Applied Research). An Ag/AgNO3 electrode system and a 50-µm-twisted platinum wire were used as quasireference and counterelectrodes. The potentials were
Figure 2. XPS survey scans of platinum substrate (a) without any polarization and (b) after a 500 s cathodic polarization at -1.8 V/SCE in a DMF solution of 0.1 mol L-1 CsI.
checked vs the ferrocene/ferrocenium couple (taken as E° ) 0.400 V/SCE) and potentials were scaled vs the SCE electrode. All solutions were deoxygenated by bubbling argon gas for 15 min before any experiment. For further XPS analysis, the samples were rapidly transferred under air and store under vacuum in the spectrometer chamber. We have shown that during a short time (some 10 s), action of oxygen is negligible when Cs cation is used as supported electrolyte. For longer times and with other cations, we have simply observed the reoxidation of the cathodically modified surface with the concomitant regeneration of the Pt metal. (See ref 10). XPS Analyses. XPS measurements were performed using a Thermo VG ESCALAB 250 instrument equipped with a monochromatic Al KR (hν ) 1486.6 eV) 200 W X-ray source. The X-ray spot size was 650 µm. The pass energy was set at 150 and 40 eV for the survey and the narrow scans, respectively. Additional high-resolution C1s regions were recorded using a pass energy of 10 eV. Data acquisition and processing were achieved with the Avantage software, version 2.20. Spectral calibration was determined by setting the main C1s component at 285 eV.13 Atomic percentages have been determined using this sofware. They take into account photoemission cross sections, analyzer transmission, and variation of electron mean free paths with kinetic energy. However, since we analyze a series of samples that are not homogeneous in the analyzed
Platinum Negative Oxidation States
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Figure 3. Relative signal intensities (normalized against the sum of C(1s) and I(3d5/2) intensities) as a function of the polarization time for Pt(4d5/2) (b), Cs(3d5/2) (1), and O(1s) (2). For a sake of clarity, the Pt(4d5/2)/[C(1s) and I(3d5/2)] and O(1s)/[C(1s) and I(3d5/2)] ratios were multiplied by 2 and 9, respectively.
SCHEME 1: Modifications of the Pt Metal during the Reducing Process in a Solution of DMF + CsI
depth, these values only indicate trends in the composition changes occurring at the surface and near surface region. Results and Discussion Electrochemical Behavior of Pt Samples in DMF Containing CsI. Let us first recall representative experiments that were already described, using cyclic voltammetry and EC-AFM techniques.9,10 Figure 1 displays a typical cyclic voltammogram recorded in dry (typically containing less than 100 ppm of water) and deoxygenated DMF containing 0.1 mol L-1 CsI. Hereafter, CsI was chosen as supporting electrolyte because cesium and iodide are particularly suitable elements for XPS tracking experiments since they exhibit high sensitivity coefficients.14 Moreover, the potential required for the formation of the Pt reduced phase is not too negative for the reduced Pt phase being sufficiently stable to easily handle the samples (for example during a short time under air).15 As previously reported, the voltammogram shows a quasireversible redox system with a cathodic wave located at -1.8 V/SCE for a scan rate of 0.1 V s-1.9,10 On the reverse scan, a smaller anodic current corresponding to the anodic counterpart is observed. On the basis of detailed electrochemical investigations, the reduction peak was ascribed to the “quasireversible” formation of the iono-metallic phase.9,10 Examination of the transformation by AFM imaging shows the formation of large spheroids (diameters around 150 nm) on the Pt surfaces when the cathodic polarization was applied as already described. (See for example, the EC-AFM pictures in refs 10a-c). XPS Analyses of Cathodically Modified Pt Surfaces. XPS is a versatile tool for accurately analyzing the surface chemical composition of a given material. Basically, the binding energy of a selected atomic level is correlated to the chemical environment of the targeted atom. Since the binding energies
of elements are related to the atomic partial charges in a given substance, XPS analyses can also provide clean determination of the oxidation states of an atom in the substance.6d,16 For instance, a linear relationship was found between the binding energies of the Au (4f7/2) level and the oxidation states of gold for a series of gold compounds.16 XPS investigations were conducted on the Pt samples as a function of injected charge. A series of platinum samples were polarized at -1,8 V/SCE in DMF containing 0.1 mol L-1 CsI for increasing polarization time, namely, 10, 20, 100, 200, and 500 s. These polarization times correspond to total injections of 13, 40, 80, and 250 mC, respectively.17 The samples were subsequently rinsed with large quantities of DMF, then immediately transferred under air to the spectrometer vacuum chamber to avoid further evolution of the reduced surface (vide infra).18 A blank experiment was carried out with a platinum sample immersed in the electrolyte without any polarization being applied. The Figure 2 compares the XPS survey spectra of Pt substrate without any polarization and after the longest cathodic polarization time (500 s). The spectrum of the nonpolarized Pt substrate mainly displays the platinum signals, which originate from Pt(4f) core electrons. The Pt(4f7/2) and Pt(4f5/2) doublet are observed at 71.1 and 74.1 eV, respectively, in agreement with literature reports.19 Minor peaks are also identified such as cesium Cs(3d) (at 730 eV) and iodine I(3d) (at 630 eV) that are assigned to some loosely adsorbed ions, together with carbon C(1s) (at 285 eV) and oxygen O(1s) (at 531 eV), which are both ascribed to classical surface contamination. After 500 s of cathodic polarization, clear modifications of the platinum surface chemical composition are observed. The platinum signal is strongly attenuated while the Cs(3d) (at 730 eV) and O(1s) (at 532 eV) relative intensities considerably increase. As compared with the nonpolarized sample, iodine and carbon signals remain approximately constant. For both samples, iodine concentration remains very low, suggesting that the corresponding signal arises from low amount of loosely adsorbed species. Similar features to those described for the 500 s-polarized sample were observed from the 10, 20, 100, and 200 s-polarized samples. In particular, C(1s) and I(3d5/2) signal intensities are almost insensitive to the increase of the polarization time. In fact, the sum of C(1s) and I(3d5/2) intensities are almost constant in the whole series of experiments including the blank experiment confirming that the signal of I element is due to surface adsorbed species. Figure 3 shows a plot of the Cs(3d5/2), O(1s), and Pt(4d5/2) relative signal intensities (normalized against the sum of C(1s)
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Figure 5. Pt(4f7/2) binding energy variation as a function of the polarization time. Figure 4. XPS Pt(4f) region of platinum substrate for 0, 10, 20, 100, and 500 s of polarization times, at -1.8 V/SCE in DMF containing 0.1 mol L-1 CsI.
and I(3d5/2) intensities) as a function of the polarization time. A quasi-plateau value is reached for the longest polarization times ca. 100 s with almost no further modification of the survey spectra. This analysis shows that the chemical composition is strongly modified upon the cathodic treatment and that these modifications are progressive with the charge injected into the Pt substrate. A closer examination of the XPS spectra provides other information about the chemical nature of the cathodically modified phase. The background increase in the XPS spectra on the high binding energy side (i.e., lower kinetic energy side) of the Cs(3d) peaks is to be remarked. Indeed, such a background shape means that a higher proportion of Cs(3d) photoelectrons are inelastically scattered. This observation shows that the cesium is located more deeply inside the substrate (Figure 2). The I(3d) does not show any similar increase of the background after the peak, confirming that iodide remains located on the outside surface. Similarly, the oxygen O(1s) signal shows no background increase after the peak, which also suggests that oxygen atoms would rather be located at the outside surface (Figure 3). Moreover, the O(1s) peak for the polarized samples is observed at 532 eV, in agreement with an O element coming from hydroxide ions. The production of hydroxide ions is likely a competitive process arising from the reduction of residual water on Pt during the cathodic polarization of the samples. All these observations can be rationalized as follow. First, some cesium ions are inserted inside the platinum sample. Second, during the cathodic polarization, hydroxide ions are produced in the immediate vicinity of the electrode surface resulting in the formation of a CsOH crust on the sample surface, leading to an overall attenuation of the platinum XPS signals (see Scheme 1). It should be noticed that the charge passed during our experiments is higher than the theoretical values required for reducing all the Pt, around 5-10 times, indicating that the electrogeneration of platinide occurs even in the occurrence of a large electrochemical competitive process. To further analyze the chemical nature of the cathodically modified platinum phases, the platinum signal was acquired at high resolution. Figure 4 displays the high-resolution spectra at the Pt(4f) level of the platinum electrode for 0, 10, 20, 100, and 500 s of polarization time.
Initially, the Pt(4f7/2) level is observed at 71.1 eV, corresponding to the binding energy of Pt(4f7/2) level for elemental platinum or Pt(0) compounds.6d,14b When the cathodic polarization is applied to the electrode, two major features are observed. First, two new peaks appear on the high binding energy side of the Pt(4f) doublet. Their intensities increase as the polarization time is increasing. These two peaks correspond to the cesium Cs(3d) energy levels: Cs(3d5/2) at 74.9 eV and Cs(3d3/2) at 77.2 eV. This progressive appearance of the Cs(3d) peaks near the Pt(4f) signal is indicative of the insertion of cesium ions within the platinum sample as discussed above. Second, the binding energy of the Pt(4f7/2) level shifts progressively to lower energies, from 71.1 eV for nonpolarized Pt to 69.4 eV for the longest cathodic polarization time. It is remarkable that what is observed is not the transformation of one chemical state of Pt into another one but a global shift of the Pt photoelectron line without any deformation nor broadening. The Pt(4f) peak shift toward the lower binding energies, indicates an elevated electronic density on the platinum atoms as compared to elemental platinum. This is a direct experimental proof of the formation of a negative oxidation state at platinum. It is worth outlining that the binding energies for all the Pt levels shift in a similar fashion. The binding energy monotonically varies with increasing polarization time, i.e., with increasing the injected charge as shown in Figure 5. As observed for the intensity, the amplitude of shift seems to reach a plateau for the longest polarization times indicating a decrease of the modification rate. This observation evidences a progressive modification of the electrode with the injected charge, the binding energy decreasing down to a minimum value (ca. 69.4 eV) probably corresponding to a “fully reduced” Pt phase within the XPS sampling depth. Such binding energy shifts are of the same order of magnitude than those recently reported on a series of solid-state barium platinide compounds.6d Interestingly, the XPS measurements performed on these compounds have revealed a virtually linear relationship between the binding energies of the Pt(4f7/2) levels and the negative oxidation states of platinum, with a maximum shift of -1.9 eV observed for Ba2Pt as compared to elemental platinum. To quantitatively assign a valence state to the electrogenerated platinum phases, we used a calibration curve as described by Jansen et al.6d The reported values for the shifts in binding energies measured for potassium tetrachloroplatinate, potassium hexachloroplatinate, and the barium platinides compounds vs the elemental platinum are plotted against the corresponding formal oxidation states for each compounds, yielding a linear calibration line (Figure 6).6d From this relation,
Platinum Negative Oxidation States
Figure 6. Correlation between formal oxidation states of platinum and the Pt(4f7/2) binding energy shifts. The circles correspond to the previously published values in ref 6d on barium platinide and chloroplatinate samples, while the triangles correspond to cathodically modified platinum samples with various polarization times.
Figure 7. XPS survey scans of platinum substrate (a) after a 100-s cathodic polarization, (b) after a 100 s cathodic polarization and reaction with 4-bromophenyl diazonium, and (c) unpolarized electrode dipped into the 4-bromophenyl diazonium solution.
we can derive a partial oxidation state from the Pt(4f7/2) binding energies shifts that were measured for the different polarization times: -0.7 for a polarization time of 10 s, -0.9 for 20 s, -1.3 for 100 s, and -1.6 for 500 s. The platinum partial oxidation state thus appears to vary progressively with increasing the injected charge until a value corresponding to a partial oxidation state of -1.6 is reached, showing a continuous and homogeneous evolution of the oxidation state of platinum during the electrochemical charge injection. It is worth emphasizing that the increase of the polarization time results in both a larger shift of the Pt(4f7/2) binding energy and a higher intensity of the cesium signal. In others words, increasing the quantity of injected charge in the platinum sample results both in a larger insertion of cesium ions and in a higher absolute value of the partial oxidation state of the platinum. Offering more electrons by increasing the polarization time makes the platinum to be less capable to forming Pt-Pt bonds. In agreement with this expectation, more cesium ions are allowed to be inserted and more “negative” appears the platinum. This behavior has a lot of similarities with
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Figure 8. Reaction of reduced platinum substrates with 4-bromophenyl diazonium. XPS Pt(4f) region: (a) polarization time 100 s, unreacted with diazonium; (b) polarization time 10 s + reaction; (c) polarization time 100 s + reaction; (d) polarization time 200 s + reaction.
the properties reported with the barium platinides compounds, where the degree of isolation of platinide ions increases with increasing the Ba/Pt ratio.6 The structures of the “negative platinum” in theses Ba compounds range from isolated atoms (Ba2Pt), through dumbbells (Ba3Pt2), to chains (BaPt).6 Accordingly, we expect that a further increase of the charge injection would tend to produce platinum phases with partial oxidation state closer to -2, corresponding to the quasi-inert electronic configuration 5d106s2.20 Analogy with a charging process can be made from the smooth evolution of the negative oxidation states of the platinum samples. Reactivity of Cathodically Modified Pt Surface with 4-Bromophenyldiazonium. As explained previously, the reduced Pt can be used for triggering electron-transfer reactions. In that connection, we have previously reported on the activation of diazonium salts by the cathodically modified Pt in the absence of any externally applied potential. This procedure allows the transformation of the reduced Pt to a stable and functional organometallic interface.10b XPS investigations were performed with modified platinum samples being, or not, immersed in DMF containing a diazonium salt. Indeed, the reduction of diazonium salts leads to the formation of aryl radicals in the vicinity of the surface which can react with a large variety of substrates like the metallic Pt.21 As a test system, we choose the 4-bromophenyldiazonium salt. The experimental procedure first involved the electrogeneration of the reduced platinum as described in the previous part. Similar polarization times (10, 100, and 200 s) were used and the sample was quickly transferred into a deoxygenated DMF solution containing 0.1 mol L-1 of 4-bromophenyldiazonium salt immediately after the Pt reduction. The system was left to react for 10 min under argon to allow a sufficient time for the reaction to be completed without applying any external potential. Then, the electrode was thoroughly rinsed several times with DMF and sonicated in DMF for 30 min to remove the organic material that is only weakly bound on the platinum surface. Figure 7 displays the survey spectra of reduced platinum electrodes, before and after reaction with 4-bromophenyldiazonium. Comparison of the spectra leads to the following comments: (i) The platinum signal intensity increases considerably. Relative intensity (vs the sum of the C(1s) and I(3d5/2) intensities) varies from 0.89 to 8.1 before and after dipping into the diazonium
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TABLE 1: Apparent Surface Chemical Composition as Determined by XPS for Unreacted, Reduced Electrode and Blank and Reduced Electrodes Treated with Diazonium Salt (Given in Atomic Percentages)a sample/atomic %
100 s reduced Pt unreacted
blank + diazonium
10 s reduced Pt + diazonium
100 s reduced Pt + diazonium
200 s reduced Pt + diazonium
C O Pt Cs I Br N
22 42 2 33 0.2 0 0.6
49 19 26
50 13 24 4 0.1 4 5
53 8 30
52 8 29
6
0.3 5 4
0.2 5 5
a It should be noticed that these atomic percentages allow comparisons but do not totally reflect the surface composition since this is not an in-depth homogeneous phase. (See Experimental section.)
salt solution. (ii) The cesium and oxygen contents decrease. Before reaction, the ratios I(Cs3d5/2)/[I(I3d5/2) + I(C1s)] and I(O1s)/ [I(I3d5/2) + I(C1s)] were 5.3 and 0.66, respectively. After the electroless reaction was completed, these ratios drop to 0 and 0.38, respectively. (iii) The carbon content increases, while new signals assigned to bromide appear (Br(3p3/2)at 183.6 eV and Br(3p1/2) at 190.8 eV). This last point confirms the presence of the grafted organic layer on the platinum surface. For comparison, blank experiments were done on the native platinum sample without any electrochemical modification prior the transfer into the solution of 4-bromophenyldiazonium salt. No signal corresponding to Br could be evidenced (Figure 8c), indicating that bromophenyl groups require prior cathodic activation of the platinum surface to be grafted. The first two points show that when the cathodically modified samples are allowed to react with the diazonium salts, Cs and O signals vanish as a simultaneous increase of the platinum signal is clearly visible. The atomic percentages derived from the relative ratio of the peaks are gathered in Table 1. These results show that the reduced Pt phase produced from the electrochemical treatment of Pt is regenerated to its initial state by the reaction with the diazonium salt. As shown in Figure 8, the Pt(4f) high-resolution spectra strengthens this idea. Upon reaction with the 4-bromophenyldiazonium salts, the Pt(4f) photopeaks shift back to the binding energy of the native Pt(0) platinum electrode, i.e., 70.9 (for 4f7/2) and 74.2 eV (for 4f5/2). Platinum is thus clearly restored to its initial metallic oxidation state. Conclusion The cathodic polarization of a platinum electrode in DMF containing CsI salts deeply modifies the chemical nature of the starting material. With rather mild reactions conditions, cesiumplatinides-like structures were produced. Our XPS investigations bring the first direct spectroscopic evidence for the formation of a reduced platinum phase from a metallic electrode held at a negative potential. XPS spectra clearly show a significant shift of Pt photopeaks to lower energies, depending on the injected charge, up to 1.7 eV, corresponding to a Pt formal oxidation degree of -1.6. The platinum partial oxidation state varies progressively with increasing the injected charge until a value corresponding to a partial oxidation state of -1.6 is reached. This means that there is no crossover between the elemental platinum and reduced platinum signals, showing a continuous and homogeneous evolution of the oxidation state of platinum during the electrochemical charge injection. This behavior is analogous to a charging process. It is noticeable that the formation of negative oxidation state of Pt exists even when a large competitive electrochemical process occurs, as in our case with the reduction of protons from residual water.
Additionally, proofs of the full reversibility of the phenomenon are provided by the XPS analyses showing that Pt returns to its metallic state after electroless reduction of 4-bromophenyl diazonium by the reduced phase. References and Notes (1) Scho¨llhorn, R. Angew. Chem., Int. Ed. 1980, 19, 983 and references therein. (2) Chlistunoff, J. B.; Lagowski, J. J. J. Phys. Chem. B 1998, 102, 5800 and references therein. (3) (a) Zintl, E.; Harder, A. Z. Phys. Chem. A 1931, 154, 47. (b) Zintl, E.; Dullenkopf, W. Z. Phys. Chem. B 1932, 16, 183. (c) Corbett, J. D. Chem. ReV. 1985, 85, 383. (d) Ryan, C. M.; Svetlicic, V.; Kariv-Miller, E. J. Electroanal. Chem. 1987, 219, 247. (4) Andersen, T.; Haugen, H. K.; Hotop, H. J. Phys. Chem. Ref. Data 1999, 28, 1511. (5) Karpov, A.; Nuss, J.; Wedig, U.; Jansen, M. Angew. Chem., Int. Ed. 2003, 42, 4818. (6) (a) Schulz, H.; Ritapal, W.; Bronger, W.; Klemm, W. Z. Anorg. Allg. Chem. 1968, 357, 299. (b) Karpov, A.; Nuss, J.; Wedig, U.; Jansen, M. J. Am. Chem. Soc. 2004, 126, 14123. (c) Karpov, A.; Wedig, U.; Dinnebier, R. E.; Jansen, M. Angew. Chem., Int. Ed. 2005, 44, 770. (d) Karpov, A.; Konuma, M.; Jansen, M; Chem. Commun. 2006, 838. (7) Palenzona, A. J. Less-Common Met. 1981, 78, P49. (8) Ko¨hler, J.; Chang, J.-H.; Whangbo, M.-H. J. Am. Chem. Soc. 2005, 127, 2277. (9) (a) Simonet, J.; Labaume, E.; Rault-Berthelot, J. Electrochem. Commun. 1999, 1, 252. (b) Simonet, J.; Peters, D. G. Electrochem. Commun. 2000, 2, 325. (c) Cougnon, C.; Simonet, J. Platinum Met. ReV. 2002, 46, 94. (d) Cougnon, C.; Simonet, J. Electrochem. Commun. 2002, 4, 266. (e) Cougnon, C.; Simonet, J. J. Electroanal. Chem. 2002, 531, 179. (f) Simonet, J. Electrochem. Commun. 2003, 5, 439. (g) Simonet, J. J. Electroanal. Chem. 2005, 578, 79. (h) Simonet, J. J. Electroanal. Chem. 2006, 593, 3. (i) Simonet, J. Platinum Met. ReV. 2006, 50, 180. (10) (a) Bergamini, J.-F.; Ghilane, J.; Guilloux-Viry, M.; Hapiot, P. Electrochem. Commun. 2004, 6, 188. (b) Ghilane, J.; Delamar, M.; GuillouxViry, M.; Lagrost, C.; Mangeney, C.; Hapiot, P. Langmuir 2005, 21, 6422. (c) Ghilane, J.; Guilloux-Viry, M.; Lagrost, C.; Hapiot, P.; Simonet, J. J. Phys. Chem. B 2005, 109, 14925. (11) (a) In the 80s and early 90s, Kariv-Miller et al.11b-f demonstrated that cathodically produced phases could be formed with organic cations such as N,N-dimethylpyrrolium and metals such as Hg, Sn, and Sb. Even if these materials are different in detail from those studied in the present work, the similarities with this materials should be noticed. (b) Kariv-Miller, E.; Lawin, P. B.; Vajtner, Z. J. Electroanal. Chem. 1985, 195, 435. (c) Kariv-Miller, E.; Andruzzi, J. Electroanal. Chem. 1985, 187, 175. (d) Svetlicic, V.; Lawin, P. B.; Kariv-Miller, E. J. Electroanal. Chem. 1990, 284, 185. (e) Fidler, M. M.; Svetlicic, V.; Kariv-Miller, E. J. Electroanal. Chem. 1993, 360, 221. (f) Kariv-Miller, E.; Christian, P. D.; Svetlicic, V. Langmuir 1995, 11, 1817. (12) Ducle`re, J. R.; Guilloux-Viry, M.; Perrin, A.; Cattan, E.; Soyer, C.; Re`miens, D. Appl. Phys. Lett. 2002, 81, 2067. (13) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers, Scienta ESCA300 Database; Chichester: John Wiley & Sons, 1992. (14) (a) 19.9 and 23.8 are the sensibility coefficients for I(3d5/2) and Cs(3d5/2), respectively.14b (b) NIST XPS Database (http://srdata.nist.gov/ xps/index.htm) and references therein. (15) It has been previously shown that the potential required for the cathodic modifications depends on the nature of the supporting electrolyte.9,10 (16) Knecht, J.; Fischer, R.; Overhof, H.; Hensel, F. J. Chem. Soc., Chem. Commun. 1978, 905. (17) These values correspond to the global charges that include the quantity required both for the Pt modification and the existence of the competitive processes. According to the geometrical parameters of the Pt
Platinum Negative Oxidation States layer, the theoretical charge to reduce Pt to Pt2- is equal to approximately 26 mC. (See Discussion.) (18) Exposure of the cathodically modified surface to air leads to its re-oxidation to the platinum metal. For a thin layer, the process is fully reversible.10 We found that, with a short time (some 10 seconds) and when using the Cs cation, the re-oxidation of the surface is negligible. (19) The Pt(4f7/2) binding energy value is in good agreement with the values reported in the literature for elemental platinum or Pt(0) compounds (71.2, 71.6, 72.0 eV).13b The separation between Pt(4f5/2) and Pt(4f7/2) is due to the spin-orbit coupling. This doublet splitting is known to be quasi-
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5707 independent of the oxidation state. (See for example ref 6d.) (20) Unfortunately, it was not possible to increase the amount of charge unless destroying the Pt deposit onto MgO substrates.10 (21) The electroless reduction of diazonium salts has been previously reported.21b,c For Pt, we did not observed this spontaneous reaction certainly because Pt is too difficult to be oxidized.10b (b) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370. (c) Adenier, A.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.; Pinson, J.; VautrinUl, C. Chem. Mater. 2005, 17, 491.
