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Platinum Metals Rev., 2006, 50, (4), 180

doi:10.1595/147106706x152703

The Platinised Platinum Interface Under Cathodic Polarisation

MORPHOLOGY CHANGES UNDER THE INFLUENCE OF TETRALKYLAMMONIUM SALTS IN SUPER-DRY SOLUTION

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Article Synopsis

Under the use of super-dry dimethylformamide containing tetraalkylammonium salts (TAAX with X = Cl, Br, I, ClO, BF4, etc.) platinised platinum layers may exhibit a reversible charging process that occurs at quite negative potentials (more negative than −2.2 V vs. saturated calomel electrode (SCE)). The mode of reactivity of the electrolyte and the reversibility of the platinum charging of platinised layers (whatever the type of conducting substrate – gold and glassy carbon can also be used successfully) are discussed in terms of the nature of the salt. After cathodic charge and oxidation by air of samples removed from the cell, a huge change of morphology of the original platinised layer was observed. During repeated reduction/oxidation stages, the original amorphous platinised layer was progressively transformed, with a noticeable swelling of the original layer. This transformation, based essentially on cathodic swelling due to the peculiar reactivity of platinum in the presence of bulky tetraalkylammonium salts, is the precondition for a new kind of platinum interface.

Platinum is now considered one of the most practical materials for achieving electrochemical conversions (1, 2). In particular it can easily be cleaned to a polished surface, and employed in most electrocatalytic transformations, especially where adatoms deposited at the surface may induce a specific activity (3–13). Applications of the perfectly defined interfaces obtainable with platinum monocrystals are also noteworthy (14, 15). In the field of anodic processes, platinum is known (16) to be electrochemically stable even at very positive potentials. However, when platinum is employed as the cathode material, the situation is more complex (17): platinum indeed possesses a very low overpotential towards proton reduction, and this phenomenon seriously limits the accessible domain for achieving conversions of organic molecules when rather negative potentials are required. In general, beyond −1 V vs. SCE (saturated calomel electrode), hydrogen is frequently evolved (18–20). Thus even in organic solvents the level of moisture strongly limits the cathodic domain.

To circumvent the inconvenience of the low hydrogen evolution overpotential at platinum, the use of solvents of low proton availability has been developed. This may extend the cathodic domain by about 1 V. Given the desirability of reaching extremely negative potentials (e.g. −3 V vs. SCE, known to be easily obtained in organic solvents in the presence of tetraalkylammonium salts if a conventional mercury cathode is used), the application of “super-dry” media was developed (21, 22). Significantly dry media have been obtained by maintaining the solvent-electrolyte mixture over drying reagents such as strongly activated alumina. Extremely low moisture levels (often less than 50 ppm of water) have been attained. Unexpectedly, however, the use of platinum cathodes in contact with a super-dry solvent-electrolyte mixture brought to light (2327) a form of ion insertion of (or reactivity with) the electrolyte (often an alkali metal or tetraalkylammonium salt) at the platinum-solution interface. Thus, in the presence of an electrolyte MX under conditions which effectively prevent hydrogen evolution at the platinum surface, the formation of a thin “ionometallic” layer of a general structure [Ptn, M+, MX] could be demonstrated. Here n depends on the saturation level of the platinum layer and therefore on its maximum degree of reduction. In general, it was found that n = 2 for N,N-dimethylformamide (DMF) containing dissolved alkali metal iodides or bulky tetraalkylammonium halides (NR4+X), according to Equation (i):

  nPt + e + MX → [Ptn, M+, MX]    (i)

In some respects, such a cathodic reaction of platinum with electrolytes – and the subsequent formation of an ionometallic layer – strongly resembles the formation of Zintl phases (28–30) via the reaction of an electropositive alkali metal with most of metalloids (such as Pb, Si or Ge). The classical synthesis of Zintl phases generally involves heating a mixture of these reagents in a closed tantalum or niobium vessel. Another reported method is the reduction of a salt of a post-transition metal in the presence of sodium metal in liquid ammonia (28, 29). Electrochemical methods may also be used (30–35): for example, cathodically polarised lead (or any other post-transition metal) immersed in a solution of a non-electroactive cation (such as K+) may produce a reduced polyanionic form as shown in Equation (ii):

  4K+ + 4e + 9Pb → [Pb94–, 4K+]    (ii)

(Pb is the cathode material.)

Similar phases have already been described (34, 35) with polarised mercury in the presence of tetraalkylammonium ions. However, the implication of the choice of anion in the electrolyte was never considered.

Returning to platinum, the analogous electrochemical building of a complex layer owing to the involvement of ions concomitantly with the associated electron motion raises a number of issues (36, 37) about this new kind of interface: its stability, its conductivity, the reversibility of electron storage, the maximum attainable thickness as well as its reducing power. Additional factors to be taken into account include progressive distortion of the platinum lattice induced by the motion of electrolyte cations, and progressive swelling evidently associated with the stoichiometry of insertion into platinum.

The present paper aims to describe in terms of morphologic changes the cathodic behaviour of platinised platinum, which has been found to be much more reactive towards electrolytes than smooth platinum. This study focuses especially on the effect of tetraalkylammonium salts (TAAX) on the electrochemical reduction of platinum. An explanation of the effect of the TAA+ cation size on structural changes to the layer is proposed. In particular the behaviour of tetramethylammonium salts (TMAX) appeared anomalous. This behaviour provides information on the interfacial activity of the TMA+ ion, considered as the smallest non-acidic ammonium cation. Moreover, TMA+ cannot form an ylide through the Hoffmann degradation (38, 39). Also at non-reactive cathodes, the TMA+ ion has been reported as by far the most readily electroactive (39), meaning that the electrochemical reactivity of TMA+ could occur at a potential much less negative than those of other NR4+ ions, especially within a potential range where there is no appreciable reduction of residual water (even when present in significant amounts (> 500 ppm)).

Experimental Procedure

Salts and Solvent

In most of the experiments, the concentration of TAAX (X = I, Br, Cl, BF4, ClO4) was 0.1 M. All salts studied were obtained from Fluka; their purity was at least 99%, and all were employed without any additional purification after being thoroughly dried under vacuum at 60°C (except for TAAClO4 which, for reasons of safety, was used as received). Anhydrous DMF was obtained from SDS, France; its water content was claimed to be 0.005%. Nevertheless, DMF was maintained continuously over alumina activated at 340°C in vacuo for at least four hours before solutions were prepared. All experiments were performed under an argon atmosphere, and the absence of dioxygen in the solutions was carefully checked. The continuous use of alumina (acidic, Brockmann I, standard grade from Aldrich) in solutions and in situ in the electrochemical cell during the experiments kept the moisture levels no higher than 50 ppm.

Electrochemistry

All potentials are given with respect to the saturated calomel electrode (SCE), although measurements were obtained with a Ag|AgI|0.1 M TBAI system in DMF; this electrode, which is particularly suitable for scrupulously dry solutions, has a potential of −0.52 V vs. SCE at 25°C. For voltammetry, a three-electrode cell was used without a separator. The counter electrode was a glassy carbon or graphite rod. Platinum working electrodes had a diameter of 1 mm. The working electrodes were polished with Norton polishing papers (grades 02 and 03) or DP Paste M made from monocrystalline diamonds (with particle size appropriate to the finest polishing). After polishing, the working electrode was sonicated for five minutes, rinsed twice with alcohol and acetone, and then dried at about 60°C. Chronocoulometric and ECQM (electrochemical quartz microbalance measurement) procedures have been fully described in a recent paper (40). (Here the electrode acts specifically as a base provider, making possible at its interface chemical reactions catalysed by strongly basic media.)

Electrode Plating

Coulometric experiments were carried out with platinised platinum electrodes prepared by deposition of the metal from an aqueous solution of 10 g l−1 H2PtCl6 (Aldrich) in 0.1 M HCl onto a metal disk (effective area: 0.78 mm2). Gold was plated under the same conditions. Metal was deposited at a constant current density (30 mA cm−2). Morphology changes of the platinum layer during charging/discharging cycles were followed by SEM analysis after rinsing each sample with alcohol and acetone, and sonication to eliminate alumina particles.

Results

Voltammetry for Tetramethylammonium Salts

Figure 1 (curve (b)) summarises the voltammetric response of smooth platinum under standard aprotic conditions. In 0.1 M TMAClO4 (as with other TMA+ salts), a diffusion step (IC) with a half-peak potential of about −2.2 V vs. SCE is observed in the course of the first scan. At more negative potentials (beyond about −2.5 V vs. SCE), the cathodic limit is reached. This limit is attributable to electrolyte decomposition. Both the potential and the current for the main cathodic step (IC) vary with the mode of polishing of the platinum and the moisture level of the electrolyte solution. If the solution contains much more than 200 ppm water, step IC is progressively shifted towards less negative potentials and becomes totally irreversible. Higher water content leads to a “cathodic wall” attributable to the water reduction.

Fig. 1

Voltammetric responses of platinum electrodes in 0.1 M TMAClO4 in DMF. Super-dry conditions. Scan rate: 0.1 V s−1. Effective electrode area: 0.78 mm2. Potentials are referred to aqueous SCE: (a) Response of a platinised platinum electrode (two first scans) with a pause of 20 s at −2.8 V. Average thickness of the platinum deposit: 0.25 µm; (b) For comparison, the response of a smooth platinum electrode under the same conditions

Voltammetric responses of platinum electrodes in 0.1 M TMAClO4 in DMF. Super-dry conditions. Scan rate: 0.1 V s−1. Effective electrode area: 0.78 mm2. Potentials are referred to aqueous SCE: (a) Response of a platinised platinum electrode (two first scans) with a pause of 20 s at −2.8 V. Average thickness of the platinum deposit: 0.25 µm; (b) For comparison, the response of a smooth platinum electrode under the same conditions

However, the current at step IC depends on the mode of polishing of the platinum electrode. Given particularly careful polishing (always under super-dry conditions), the magnitude of step IC may decrease, and it may even disappear. The nature of the surface, grain boundaries, as well as the possible presence of fractals and activated sites would be expected to favour an interfacial charge-discharge process.

If the platinum interface described above is now platinised galvanostatically so as to produce a much larger active surface, the shape of the voltammetric curves, as shown in Figure 1(a), changes dramatically. For a relatively thick film of electrodeposited platinum (with, for example, an average thickness significantly greater than 0.1 µm), the use of a super-dry solvent-electrolyte enables a quasi-reversible step (IIC), at least in the cases of TMAI, TMAClO4 and TMABF4 (Figures 1 and 2). For example, platinised platinum in 0.1 M TMAClO4 exhibits a pair of broad main steps (IIC and IA) whose half-peak potentials are E0.5pc = −2.0 V and E0.5pa = −1.7 V vs. SCE, respectively. At scan rates greater than 50 mV s−1, the relative areas of these two steps are approximately equal. At potentials more negative than −2.3 V vs. SCE, another cathodic step may arise. Its presence apparently depends on the history of the electrode surface and on the amount of platinum galvanostatically deposited onto the substrate. Limiting currents for step IIC (iII) do not depend directly, at a given sweep rate, on the amount of electrodeposited platinum. The platinising procedure shifts the main reduction step towards much less negative potentials, and the rate of the charging process (although not measured here) appears significantly greater at a platinised platinum surface. The shapes of these steps and their current values would suggest that only the external part of the deposit – then in contact with the liquid phase – reacts with the components of the electrolyte. With longer electrolysis times, a sudden decay of the cathodic current is observed at very negative potentials (Figures 1 and 2). This may be induced by a self-inhibiting phenomenon. This inhibition is attributable to the weak electronic conductivity of the ionometallic layer formed during the reduction process. Conceivably, compacting associated with a structural change in the platinised layer further restricts the motion of species participating in this charging phenomenon.

Fig. 2

Electrochemical behaviour of platinised platinum in 0.1 M TMABF4 in DMF. Super-dry conditions. Effective electrode area: 0.78 mm2. Average thickness of the galvanostatic platinum deposit: 0.3 µm. Scan rate: 0.1 V s−1. Two first scans. Reference electrode: aqueous SCE

Electrochemical behaviour of platinised platinum in 0.1 M TMABF4 in DMF. Super-dry conditions. Effective electrode area: 0.78 mm2. Average thickness of the galvanostatic platinum deposit: 0.3 µm. Scan rate: 0.1 V s−1. Two first scans. Reference electrode: aqueous SCE

Pauses were introduced into forward voltammetric scans on both smooth platinum surfaces and substrates covered by very thin, strongly heterogeneous platings. In these cases, at very negative potentials (as shown in Figure 1(a)), the following scans towards negative potentials may be significantly altered. The total cathodic current is then strongly increased, and step IC completely disappears. Recurrent scanning and/or pauses in scanning would be expected to contribute to changes in the nature of the platinum surface. Thus the presence of only the pair of steps IIC and IA would suggest that smooth platinum zones at the electrode surface have disappeared under cathodic treatment. This could correlate with the observation that the presence of steps IC and IIC together depends on the conditions prevailing during the plating, and on the amount of electricity passed. Step IIC is observed alone in some cases; this could be attributable to changes in the plating homogeneity.

For timescales much longer than those currently used for voltammetric experiments, the charging processes remained reversible, but generally showed current efficiencies lower than 60%. Figure 3 shows the two distinct branches in the chronocoulometric response of a platinised platinum microelectrode in a 0.1 M solution of TMAClO4 in DMF. The very steep slope at the beginning of the discharge process is notable; it is associated with the thinness of the reduced film at the interface.

Fig. 3

Chronocoulometric curve for a platinised platinum electrode of area 0.78 mm2 in 0.1 M TMAClO4 in DMF. Superdry conditions. Plating thickness: 0.2 µm. Charge at −2.3 V vs. SCE for 10 s followed by discharge at −0.5 V until nil current

Chronocoulometric curve for a platinised platinum electrode of area 0.78 mm2 in 0.1 M TMAClO4 in DMF. Superdry conditions. Plating thickness: 0.2 µm. Charge at −2.3 V vs. SCE for 10 s followed by discharge at −0.5 V until nil current

Voltammetry for Other Tetraalkylammonium Salts

Voltammetric data obtained with bulky TAA+ salts (from tetra-n-butyl- to tetra-n-octylammonium salts) indicated much less reversibility than that already described for TMAX salts. The cathodic step IIIC (Figure 4) attributable to the reduction of platinum in the presence of the salt is shifted to more negative potentials. The corresponding anodic step IIIA is shifted in the anodic direction. Half-peak potentials are −2.4 V and −1.5 V vs. SCE respectively. The current of the anodic step is relatively small. At significantly more positive potentials, two reversible steps H1 and H2 may appear, even under super-dry conditions. These are readily assigned to the reversible anodic oxidation of hydrogen formed at very negative potentials at the polycrystalline platinum surface during the forward scan. The steps could not be induced merely by residual water reduction beyond −2.8 V. These steps were not observed with TMAX salts. Their presence tended to support the concept of a “probase” cathode (40) previously proposed for tetramethylammonium salts. On the other hand, it would be expected that large TAA+ ions under the conditions given here would produce hydrogen evolution, leading to reductive degradation of the salt. This may be regarded as a kind of cathodic Hoffmann reaction, in which the free electron plays the role of a base as shown in Equation (iii):

  R3N+CH2–CH2R' + e → R3N + CH2=CHR' + ½H2    (iii)

Fig. 4

Repeated voltammetric scans at a platinised platinum electrode of effective area 0.78 mm2 in 0.1 M TBAClO4 in DMF. Super-dry conditions. Average thickness of the platinum deposit: 0.4 µm. Scan rate: 0.5 V s−1. Potentials are with respect to aqueous SCE

Repeated voltammetric scans at a platinised platinum electrode of effective area 0.78 mm2 in 0.1 M TBAClO4 in DMF. Super-dry conditions. Average thickness of the platinum deposit: 0.4 µm. Scan rate: 0.5 V s−1. Potentials are with respect to aqueous SCE

In a parallel process, the expected two-electron cathodic cleavage of TAA+ (forming the anion R) may trigger the classical Hoffmann degradation (protonation of the anion R by the salt) – a partial explanation for the poor reversibility of the charging process. In overall terms, this type of degradation leads to a broadly similar product distribution, with the formation of a tertiary amine and an alkene. Only the hydrogen evolution (as proposed in Equation (iii)) appears specific to the behaviour of TAA+ at the platinum interface.

Microcoulometry

Coulometric measurements were carried out at platinised interfaces in super-dry DMF-TAAX solutions. The conditions were close to those shown in Figure 3 (i.e. reduction beyond −2.4 V and oxidation at about −0.8 V), having reached saturation of the deposited layer. Only the amount of electricity recovered during the oxidation process was taken into account. With tetra-n-butyl-, tetra-n-hexyl- and tetra-n-octylammonium halides, the stoichiometry was found to correspond to one electron per two atoms of platinum. This suggests a formula for the complex of the following form (Equation (iv)):

  [Pt2, TAA+, TAAX]     (iv)

A large series of TMAX salts was tested (with X = ClO4, BF4, I, Br and Cl), as well as the corresponding sulfate salt (Figure 5). It was confirmed that bare platinum as such does not rapidly accumulate a high charge within the timescale necessary for such processes (e.g. a few minutes). All coulometric experiments were conducted in very carefully dried DMF-electrolyte solutions. Complete dryness and neutrality of the platinised platinum electrode were ensured by rinsing with alcohol and/or DMF with the addition of Me4NOH, followed by double rinsing with acetone and drying. The gradient of the plot in Figure 5 reveals that four platinum atoms (abscissa) are correlated with the charge of one electron (ordinate), which supports the conclusion that the stoichiometry of the phase, after saturation within the timescale of charging, can be written as follows (Equation (v)):

  [Pt4, TMA+, (TMAX)m]    (v)

Fig. 5

Microcoulometry of platinised platinum (area: 0.8 mm2) for different plating thicknesses. Charging at −2.2 V vs. SCE (reduction for 100 s, allowing full saturation of the deposit) and then oxidation at 0 V until nil current. Qd is the quantity of electricity recovered during discharge. Qf/4 is one quarter of the quantity of electricity required to convert PtIV to Pt0. Five TMAX salts were tested at 0.1 M concentration in DMF: + (X = Cl), • (X = ClO4), Δ (X = Br), ○ (X = BF4), ▪ (X = I)

Microcoulometry of platinised platinum (area: 0.8 mm2) for different plating thicknesses. Charging at −2.2 V vs. SCE (reduction for 100 s, allowing full saturation of the deposit) and then oxidation at 0 V until nil current. Qd is the quantity of electricity recovered during discharge. Qf/4 is one quarter of the quantity of electricity required to convert PtIV to Pt0. Five TMAX salts were tested at 0.1 M concentration in DMF: + (X = Cl), • (X = ClO4), Δ (X = Br), ○ (X = BF4), ▪ (X = I)

Analysis of platinum microdeposits onto the gold substrate (via the EQCM technique) confirmed that m = 1.

Morphology Changes to Large Electrodes on Macroelectrolysis

A large number of potentiostatic electrolyses were performed in two-compartment cells on platinised sheets of commercial platinum in the presence of tetramethylammonium salts (TMAClO4, TMABF4 and TMAI), tetra-n-butyl ammonium iodide (TBAI), tetra-n-hexylammon-ium bromide (ThexABr) and tetra-n-octyl-ammonium bromide (ToctABr). Experimental conditions were as described for the voltammetric and coulometric studies. The main point of these experiments was to use super-dry solvent-electrolyte; the addition of activated alumina to the cell appeared crucial to this. In order to compare the surface modifications and draw conclusions, the following parameters were considered:

– the initial thickness of the platinum deposit;

– the applied potential for the reduction;

– the theoretical amount of electricity per unit area required to charge (partially or totally) the platinised layer.

After cathodic polarisation of the platinised samples, two alternative modes of oxidation were used and compared. The sample was either simply exposed to air after the electrolysis, or the sample was kept in the cell at a potential generally between −0.5 and +0.3 V vs. SCE until the current decayed completely. The present study describes only morphology changes caused by contact with air at the completion of the reduction process. Surface modifications were generally controlled so as to occur without any noticeable mass change. However, when thick deposits are over-reduced for a long time, charging of the platinum layer could possibly lead to a local collapse of the original plating. Figure 6 shows the original appearance of a platinised platinum sample before any cathodic treatment in the presence of salts.

Fig. 6

SEM image of a platinised layer galvanostatically deposited onto smooth platinum from H2PtCl6 solution in 0.1 M HCl . Current density: 5 mA cm−2. Total quantity of electricity: 1.6 C cm−2. Average thickness of plating: 0.4 µm

SEM image of a platinised layer galvanostatically deposited onto smooth platinum from H2PtCl6 solution in 0.1 M HCl . Current density: 5 mA cm−2. Total quantity of electricity: 1.6 C cm−2. Average thickness of plating: 0.4 µm

A few results have been selected as being the most representative from a large number of experiments demonstrating the huge modification of platinum surfaces by TAA+ salts under cathodic polarisation.

Modification of Platinum Films

In the absence of a visible degradation of the film (which was easily checked with a magnifying glass and confirmed by the absence of mass changes), the amount of electrodeposited platinum could be related to the amount of electricity necessary to achieve the deposit. For example, by using the initial platinised layer shown in Figure 6, the original deposit was made with a quantity of electricity of 1.6 C cm−2 (total reduction of PtIV), which corresponds approximately to an average thickness of the deposit of δ = 0.4 µm. The reduction of the deposit at −2 V vs. SCE in the presence of TMAClO4 was performed until an amount of electricity of 0.8 C cm−2 was passed. This theoretically corresponds to twice the amount of electricity required for total saturation of the layer, given that the charging yield would be about 50% platinum. The stoichiometry is given by Equation (v) with m = 1. Cathodic polarisation was followed by oxidation, simply by contact of the samples with air during rinsing. The samples were then thoroughly sonicated. This electrochemical treatment resulted in a complex coverage consisting of more or less touching platinum spheroids, with average diameter depending on the thickness of the initial layer. The average diameter is estimated at about 0.5 µm for the structure shown in Figure 7(a). Apart from the zones of spheroids, large flat zones were attributed to the original smooth platinum substrate.

Fig. 7

Reduction of thin platinised film in the presence of TAAX salts in DMF under super-dry conditions. After reduction, samples were rinsed and sonicated in contact with air: (a) Reduction in 0.1 M TMAClO4 of film of mean thickness 0.2 µm) at −2.5 V. Total amount of electricity: 0.8 C cm−2. Oxidation by air; (b) and (c) Reduction in 0.1 M TBAI of a platinum deposit of 0.22 mg cm−2 onto platinum substrate. Electrolysis at −2.5 V vs. SCE. Quantity of electricity: 1.7 C cm−2. No mass loss after oxidation by air, sonication and rinsing; (d) Reduction in 0.1 M tetra-n-octylammonium bromide. Deposit of platinum: 0.30 mg cm−2. Electrolysis at −2.5 V. Quantity of electricity passed: 1 C cm−2. No mass loss

Reduction of thin platinised film in the presence of TAAX salts in DMF under super-dry conditions. After reduction, samples were rinsed and sonicated in contact with air: (a) Reduction in 0.1 M TMAClO4 of film of mean thickness 0.2 µm) at −2.5 V. Total amount of electricity: 0.8 C cm−2. Oxidation by air; (b) and (c) Reduction in 0.1 M TBAI of a platinum deposit of 0.22 mg cm−2 onto platinum substrate. Electrolysis at −2.5 V vs. SCE. Quantity of electricity: 1.7 C cm−2. No mass loss after oxidation by air, sonication and rinsing; (d) Reduction in 0.1 M tetra-n-octylammonium bromide. Deposit of platinum: 0.30 mg cm−2. Electrolysis at −2.5 V. Quantity of electricity passed: 1 C cm−2. No mass loss

Similar morphologies were observed with platinised films, in particular with TBAI (See Figures 7(b) and 7(c)) and ToctABr (Figure 7(d)). However, with thicker platinised films, and with large amounts of electricity involved in the reduction process, the layer swells to different extents. (The layer swells by transformation into spheres during the cathodic reaction of platinum with bulky salts, as indicated by the stoichiometry of the ionometallic complexes.)

The case of some TMA+ salts is, however, more complex. These afford relatively limited swelling in terms of the stoichiometry given by Equation (v). Primary spheres are covered by smaller spheroids, giving the overall morphology the appearance of a layer of blackberry-like structures. The uniform volume of all the spheres is also striking. (See Figure 8 for the case of TMAI.)

Fig. 8

Reduction (E = −2.5 V vs. SCE) of platinised platinum in 0.1 M TMAI. Thickness of platinum deposit: 0.2 µm. Moderate quantity of charge corresponding to half that theoretically required to saturate the platinised film. No mass loss. Two images of the layer at different magnifications; note the appearance of “cauliflower-like” structures

Reduction (E = −2.5 V vs. SCE) of platinised platinum in 0.1 M TMAI. Thickness of platinum deposit: 0.2 µm. Moderate quantity of charge corresponding to half that theoretically required to saturate the platinised film. No mass loss. Two images of the layer at different magnifications; note the appearance of “cauliflower-like” structures

The swelling may sometimes become chaotic, particularly where large amounts of electricity are involved. Large, regularly swollen cauliflower-like structures then arise (Figure 9). These are reminiscent of the structures observed during the anodic polymerisation of pyrrole or thiophene (41, 42), in which the polymers are rendered electrically conductive by the progressive insertion of anions as “dopants” inside the polymer matrix. In the present work, cauliflower and macrospherical structures were found, principally with thick platinised layers (Figures 9(a), 9(b) and 9(c)). During the oxidation of the platinised layer, movement of the salt out of the platinum causes a shrinkage, as clearly evidenced by the formation of large cracks (see Figure 9(d)) and large zones of uncovered platinum substrate (Figure 7(c)). The shrinkage of cathodically generated spheres under oxidation by air, due to the expulsion of ions, is strongly expected to yield almost empty structures. By contrast, the anodic oxidation of spheroid layers leads to profound changes, until a quasi-flattening of the platinum surface occurs (43) through the bursting of the spheres.

Fig. 9

Formation of more chaotic structures of cauliflower-like morphology. (a) Reduction of a platinised layer of thickness 0.2 µm in 0.1 TMAClO4 at −2.5 V under weak current density, with a moderate total quantity of electricity; (b) Reduction of a platinised layer (0.30 mg cm−2) in 0.1 M tetra-n-octylammonium bromide with a large over-reduction of the layer. Part of the deposit then collapses; (c) Reduction of a platinised film of thickness 2 µm in 0.1 M TBABF4. A large quantity of electricity passed. Evidence for strongly swollen structures; (d) Same conditions as image (c). During oxidation by air, the film shrinks strongly and forms deep cracks

Formation of more chaotic structures of cauliflower-like morphology. (a) Reduction of a platinised layer of thickness 0.2 µm in 0.1 TMAClO4 at −2.5 V under weak current density, with a moderate total quantity of electricity; (b) Reduction of a platinised layer (0.30 mg cm−2) in 0.1 M tetra-n-octylammonium bromide with a large over-reduction of the layer. Part of the deposit then collapses; (c) Reduction of a platinised film of thickness 2 µm in 0.1 M TBABF4. A large quantity of electricity passed. Evidence for strongly swollen structures; (d) Same conditions as image (c). During oxidation by air, the film shrinks strongly and forms deep cracks

Discussion

The use of tetraalkylammonium salts as electrolytes in aprotic (super-dry) DMF can undoubtedly lead to the reversible charging of platinised layers electrochemically deposited onto substrates such as platinum, glassy carbon or gold. The finely divided structure of platinised deposits apparently enhances the cathodic reactivity of platinum. Under these conditions, tetraalkylammonium salts (R4NX with R bulkier than Me) react cathodically with the platinum surface, but at potentials significantly more negative than those observed with R = Me. At such potentials, significant reduction of residual water may occur, and the charging process observed in conjunction with hydrogen evolution is always poorly reversible in terms of charge recovery.

By contrast, cyclic voltammetric experiments carried out at platinised layers in the presence of cathodically more reactive TMA+ salts provide evidence for a quasi-reversible charge process. Thus, under super-dry conditions and for rather thick platinum deposits, the quantity of electricity specifically involved in the charging process was found to be an increasing function of the scan rate. Electrochemical charging is definitely first associated with the external part of the platinised layer. However, with sufficiently high currents and long electrolysis times, the platinum layer may become saturated with the electrolyte salt. The charge recovered during the re-oxidation process reaches a limit, and becomes proportional to the amount of deposited platinum. Thus a simple calculation performed for several TMA+ salts demonstrates that, at least with thin platinum films, the charging process involves four atoms of platinum for each transferred electron.

All the ionometallic complexes obtained under similar conditions with bulky NR4+ salts and alkali metal iodides exhibit reducing properties. They can therefore produce anion-radicals from π-acceptors such as fluorenone and 1,4-dinitrobenzene. Similarly, the reducing power of the layer was recently used to cleave diazonium cations, making possible the chemical functionalisation of the platinum surface (44).

With TMAX, as stated above, the structure of the platinum layer at the saturation stage is quite different from those previously found (37) with bulky NR4+ (such as tetra-n-butyl-, tetra-n-hexyl-, and tetra-n-octylammonium salts). This difference seems to be due to the smaller size of the TMA+ cation producing a change in the spatial structure of the ionometallic complex. After cathodic polarisation of platinum and air exposure of the interface obtained in the presence of NR4+ cations, SEM images most frequently revealed either cauliflower-like structures or layers of spheres, depending on such experimental parameters as the thickness of the platinised layer and the amount of electricity involved in the reduction process. The appearance of spheres of uniform size, as well as huge changes to the morphology of the original platinum layer are highly intriguing. Does the formation of spheres or swollen structures occur during the reduction process or later when the surface is oxidised by contact with air? Recent in situ electrochemical scanning tunnelling microscopy (EC-STM) experiments in this laboratory (45) revealed, particularly in the case of tetra-n-butylammonium iodide, that the reversible swelling of platinum monocrystals can be followed using cyclic voltammetry over relatively short durations. The swelling of platinum was found to be directly related to the cathodic reaction of surface platinum atoms with the electrolyte. The complete restoration of the surface at the end of the reverse scan was highly effective, at least for short reaction times and limited amounts of electricity. This finding strongly supports the conclusion that the platinum reactivity is fully reversible, at least in the case of tetra-n-butylammonium salts. Over-reduced ionometallic layers in the presence of TMA+ (up to one TMA+ ion per platinum atom) could be reached in certain cases. Over-reduction of platings could cause spectacular swelling and deformation of platinum surfaces.

Returning to the quasi-uniform size of the spheres (as shown in Figures 7 (a)–(d)), it may be proposed that these structures grow under thermodynamic conditions. (This assumption is supported by the quasi-reversibility of the electrochemical process). The total conformational energy per sphere, Esp, may be split into two terms, relating to the bulk (ρV) and surface (ρS) energies respectively as shown in Equation (vi):

  Esp = (4/3)πR3ρV – 4πR2ρS     (vi)

where R is the radius of the spheres.

At the equilibrium (Equation (vii)):

  (dE/dR)eq = 4πR2ρV – 8πRρS    (vii)

The equilibrium radius, Req, of the spheres is given by Equation (viii):

  Req = 2ρS/ρV            (viii)

Thus the radii of the spheres depend only on energy factors specific to the experimental conditions (such as the nature of the salt used, its concentration, its level of dissociation, as well as the number of available platinum atoms). The formation of contiguous spheres of very similar volume suggests that the ionometallic layer turns out to be very mobile on the conducting substrate. Therefore the model of growth of swollen structures from randomly distributed activated centres onto the substrate surface is probably incorrect.

A range of experiments were performed to visualise clearly the occurrence of the platinum swelling. The degree of swelling may be approximately estimated by adopting the following formula for the charged phase obtained with TMAX salts (Equation (ix)):

  [Pt4, TMA+, TMAX]         (ix)

The example of TMAI is then taken as representative. On the basis of literature values of ionic radii for the phase constituents (43, 46), i.e., Pt = 0.80 Å, I = 2.2 Å, and TMA+ = 3.01 Å, it is possible to assess the degree of swelling for closely-adjacent ions. This calculation predicts a volume increase of approximately 30 times – which is huge. Similar swelling factors can be estimated for X = ClO4 and BF4. By contrast, the swelling at platinum without a specific insertion of the salt would be limited to 14 times – again in the case of TMAI, on attaining complete reduction of the platinum layer. Thus the deposition of a few platinum atoms onto a conducting surface would lead to the progressive formation of a sphere-like volume, until attainment of the final stoichiometry based on the availability of strongly reactive platinum atoms. Although the mode of swelling is unclear so far, the rate of growth of such spheres would be determined both by the number of starting points (perhaps randomly distributed on the surface) and the diffusion of ions. The mobility of the spheres on the platinum substrate (under the condition that an electric contact promotes their growth) should probably also be taken into account. Since the volume of the spheres is not directly dependent on the amount of platinum deposited, one can foresee the existence of zones weakly covered by a small number of spheres (see, for example, Figures 7(c) and 7(d)).

It would be expected that the huge swelling described above starts from sites located at the interface between the platinised layer and the electrolyte solution, causing substantial volume changes and, in particular, the formation of spheres. A tentative approach is depicted in Scheme I. The charging process might involve a front of charge moving progressively from the external interface until it reaches the much less reactive platinum substrate. The formation of spheres or more complex and bulky structures by extrusion due to the swelling (cauliflower-like features for thicker platinum deposits, Figure 9) may explain some of the alterations to the electrode surfaces. If the ionometallic layer is oxidised simply by contact with air (presumably due to a slow electron transfer at the boundary of the spheres), then the interface would be expected to keep roughly its original structure. The oxidation first forms a platinum shell, and continues by the diffusion of dioxygen through this shell. The action of dioxygen would tend to preserve the general features of the layer, including cracks and smaller spheres (often not immediately adjacent) owing to shrinkage. Scheme II depicts swelling processes on a much larger scale, which would lead to cauliflower-like structures in the case of thicker platinum layers and/or over-reduction of deposits.

Scheme I

Schematic representation of the morphology change of a platinised platinum interface (shown in green) during the charging process by TMAX followed by air oxidation. The ionometallic layer (the fully reduced form of the platinised layer) is shown in red. The oxidation by dioxygen would lead to smaller spongy spheres, of which the shrinking factor illustrated here is arbitrary

Schematic representation of the morphology change of a platinised platinum interface (shown in green) during the charging process by TMAX followed by air oxidation. The ionometallic layer (the fully reduced form of the platinised layer) is shown in red. The oxidation by dioxygen would lead to smaller spongy spheres, of which the shrinking factor illustrated here is arbitrary

Scheme II

Schematic representation for the swelling of the platinised platinum surface. Note the possible transformation of (c) for a moderate swelling into (d), obtained with larger amounts of electricity injected into the platinum layer (termed “cauliflower-like” structures in the text). Arrows indicate the supposed direction of the swelling

Schematic representation for the swelling of the platinised platinum surface. Note the possible transformation of (c) for a moderate swelling into (d), obtained with larger amounts of electricity injected into the platinum layer (termed “cauliflower-like” structures in the text). Arrows indicate the supposed direction of the swelling

Conclusions

Under super-dry conditions, platinised platinum layers exhibit a high cathodic reactivity towards the electrolyte. In particular, with TMA+ salts, a fully reversible ion insertion has been demonstrated. A stoichiometry for this insertion process has been proposed for the condition that the platinised layer reaches is full reactivity. Coulometric results for a large number of TAAX salts strongly support the conclusion that two or four platinum atoms are associated with the transfer of one electron to produce thin modified (ionometallic) layers (up to 1 µm in thickness). Under appropriate conditions, the salt-infused ionometallic layer shows a huge swelling, which depends on the applied potential, the nature of the interface and the mode of the discharging process. With the aid of SEM techniques, it has been found that charging and discharging of platinum layers are correlated with their swelling and shrinkage. Dramatic structural changes of the platinised platinum layer have been analysed and explained in terms of both the existence of a reversible charging-discharging process and the specific nature of the electrolytes employed. Finally, analysis of reduction processes revealed that side reactions may occur, such as the slow decomposition of the ionometallic layer obtained at quite negative potentials. For example, methane may be evolved with TMAX salts as shown in Equations (x) and (xi):

  [Ptn, TMA+, TMAX] → nPt + (CH3)3N + CH3 + TMAX    (x)

In the presence of a source of electrons and protons:

  CH3 → CH4                      (xi)

Thus controlling the roughness of the platinum interface by the various treatments presented here might be used to impart particular properties to the resulting surface – for instance, by achieving a clean platinum surface in the absence of dioxygen, for catalytic and electrocatalytic applications.

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Acknowledgments

The author is grateful to the Centre de Microscopie Electronique à Balayage et MicroAnalyse (Dr Le Lannic, Université de Rennes) for its efficient help, and to Professor C. Amatore (ENS, Université de Paris VI) for fruitful technical discussions.

The Author

Emeritus Professor Jacques Simonet is Directeur de Recherche, Electrochemistry Group, Université de Rennes, France. His principal interests are organic electrochemistry, the activation of organic reactions by electron transfer, electro-polymerisation and the formation of redox polymers. He also researches on the reversible cathodic charging of precious metals (platinum and palladium) in super-dry conditions, in contact with polar organic solvents containing electrolytes, mimicking Zintl phases for transition metals.

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