Electrochemical Reduction of Iron Oxide Nanoparticles on Mercury

Charlot, G. Les méthodes de la chimie analytique; Masson et Cie: Paris, 1966. ..... Lioz Etgar , Gregory Leitus , Leonid Fradkin , Yehuda G. Assaraf ...
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Electrochemical Reduction of Iron Oxide Nanoparticles on Mercury E. Dubois* and J. Chevalet† Laboratoire LI2C, UMR CNRS 7612, Universite´ Pierre et Marie Curie, case 51, 4 Place Jussieu, 75252 Paris Cedex 05, France Received July 17, 2003. In Final Form: September 19, 2003 The electrochemical reduction of well-defined γ-Fe2O3 nanoparticles dispersed and stabilized in aqueous media which are appropriate for electrochemical techniques (analysis and preparation) is studied. A medium, in which metallic iron can be formed and in which iron ions do not precipitate, is examined in detail ([citrate(NH4)3] ) 0.01 mol/L, NH4ClO4/NH3 pH ) 8.6). Our results clearly show that the differences between nanoparticles and ionic species as starting material do not consist in a simple scaling. At intermediate potentials (-0.4 to -1.2 V/ECS) we show that the nanoparticles can be reduced into FeII, with a fraction of it being free in solution. At more negative potentials (-1.5 to -1.8 V/ECS), nanoparticles are reduced into metallic iron nanoparticles that enter the Hg electrode due to the wetting properties of Hg on iron. There exist strong interactions between the oxide nanoparticles and the electrode, interactions which are shown to be partly of electrostatic nature, that is, depending on the concentration of the supporting electrolyte (NH4ClO4/NH3).

I. Introduction The final goal of the present investigation is the preparation of a dispersion of metallic iron in mercury, that is, a magnetic conductive liquid. We study here the electrochemical reduction of nanometric particles (around 10000 atoms) dispersed in water, that is, a colloidal dispersion. Such dispersions of metallic iron in mercury have been obtained from the reduction of ionic iron on a mercury cathod.1-3 Nevertheless, this method raises several difficulties during the synthesis: production of H2 and no control of the size of the iron aggregates formed. The final material shows a low stability that limits its applications. Consequently, the reduction of iron oxide nanoparticles instead of ionic iron appears as an advantage because nanoparticles could constitute nanoreactors, allowing a better control of the material prepared. Such a procedure corresponds to nonclassical electrochemistry, and few works deal with big objects. Several articles by Heyrovsky4-7 deal with nanoparticles, however, smaller than ours, studied with an analytical purpose and never totally reduced. Nevertheless, we shall compare our results to these works in the Discussion and show that there are indeed some similarities between both types of systems. The reoxidation of nanoparticles formed in situ by reduction has also been reported.8 Moreover, the interaction colloids/mercury interface has been studied in several cases, although in the absence of an electrochemical * To whom correspondence should be addressed. E-mail: [email protected]. † E-mail: [email protected]. (1) Luborsky, F. E. J. Electrochem. Soc. 1961, 108 (12), 1138. (2) Windle, P. L.; Popplewell, J.; Charles, S. W. IEEE Trans. Magn. 1975, Mag-11 (5), 1367. (3) Alekseev, V. A.; Veprik, I. Y.; Minukov, S. G.; Fedonenko, A. I. J. Magn. Magn. Mater. 1990, 85, 133. (4) Heyrovsky, M.; Jirkovsky, J. Langmuir 1995, 11, 4288. (5) Heyrovsky, M.; Jirkovsky, J.; Struplova-Bartackova, M. Langmuir 1995, 11, 4309. (6) Heyrovsky, M.; Jirkovsky, J.; Struplova-Bartackova, M. Langmuir 1995, 11, 4300. (7) Heyrovsky, M.; Jirkovsky, J. Langmuir 1995, 11, 4293. (8) Zutic, V.; Nicolas, E.; Ge´rard, P.; Gierst, L. Eletroanal. Chem. Interfacial Electrochem. 1973, 44, 107.

reaction: (i) with gold nanoparticles;9 (ii) with big objects such as living cells.10 The reduction of nanoparticles in view of preparative electrolysis raises several questions: (i) is it possible to reduce such nanoparticles; (ii) are they totally reduced; (iii) do they enter the mercury electrode? A first study gave a positive answer to these three questions.11 However, many difficulties appeared and were not taken care of: (i) the variation of intensity versus the time during electrolysis is erratic; (ii) the yield in current is bad; (iii) the stability of the material produced is not good enough. The resulting question is thus: do we really take advantage of the nanoreactor given the used procedure? We propose here to improve the understanding of the synthesis step and, in particular, the parameters and phenomena that govern the reduction of nanoparticles. II. Experiments 1. Colloidal Solution. The colloidal solutions, which constitute the electroactive species for the whole set of experiments, are composed of nanoparticles of γ-Fe2O3, that is, maghemite, which is a magnetic iron oxide. These nanoparticles were chemically synthesized.12-14 Due to the acidic properties of surface groups, the nanoparticle surface can be positively or negatively charged depending on the pH (see Figure 1). In acidic medium, the surface charge is positive whereas it is negative in alkaline medium, and the point of zero charge of the surface is around a pH value of 7.5. Consequently, there exist electrostatic repulsion forces between particles that allow stabilizing the suspensions in water if the conditions are appropriate. Nevertheless, such bare particles are not stable for 3.5 < pH < 10.5 because the density of the surface charge is too low. One solution for obtaining a stable dispersion is the coating of the surface with small citrate (9) Caselli, M.; Lippolis, G.; Gierst, L. J. Electroanal. Chem. 1972, 38, 451. (10) Svetlicic, V.; Ivosevic, N.; Kovac, S.; Zutic, V. Langmuir 2000, 16 (21), 8217. (11) Dubois, E.; Chevalet, J.; Massart, R. J. Mol. Liq. 1999, 83, 243. (12) Massart, R. IEEE Trans. Magn. 1981, Mag-17, 1247. (13) Massart, R.; Roger, J.; Cabuil, V. Braz. J. Phys. 1995, 25 (2), 135. (14) Magnetic Fluids and Applications Handbook; Berkovskii, U. B. M., Bashtovoi, V. G., Eds.; Begell House: 1996.

10.1021/la035294r CCC: $25.00 © 2003 American Chemical Society Published on Web 11/18/2003

Reduction of Iron Oxide Nanoparticles on Mercury

Figure 1. Characteristics of the different kinds of stable colloidal solutions: nature of the charge, choice of counterions, range of stability, nature of the supporting electrolyte, and maximum concentration of electrolyte compatible with the colloidal stability (a range is given because cmax depends on the size of the nanoparticles, with the smaller being stable for higher electrolyte concentrations). Note that, in the presence of a ligand L, the surface charge is only ensured by the ligand until pH 7 and is ensured both by the oxide sites and the ligand for higher pH, with the proportion of both sites depending on the pH. molecules in order to shift the point of zero charge.15 Under such conditions, the charge is negative for pH > 3.5. However, the value of the charge is not the only parameter controlling the colloidal stability: the nature of the counterions is crucial. The different kinds of surface/counterions associations are summarized in Figure 1: it is clear that stabilized dispersions can be obtained for any pH between 0.5 and 13.5. However, such conditions are not sufficient to determine a convenient composition of the supporting electrolyte used to perform electroreduction of the nanoparticles, and it will be shown that a specific study is necessary to determine the appropriate conditions (see section III.1 and the caption of Figure 1). These colloidal dispersions are characterized by several other parameters such as the size of the nanoparticles and the volume fraction Φ of solid. The size of the nanoparticles is determined from the magnetic properties of the solutions. Each particle is indeed a monocrystal and, due to the nanometric size, a magnetic monodomain, with a dipolar moment µ proportional to the volume of the particle.14,16 The size distribution is described by a lognormal distribution characterized by the mean diameter d0 and the polydispersity σ. The volume fraction Φ of the particles is determined from a chemical titration of iron:17 Φ (%) ) 1.577[Fe] (mol/L). The stock solution used in the present study is characterized by d0 ) 6.65 nm, σ ) 0.22, and Φ ) 3.4%. 2. Electrolytes and Ionic Species. All chemical used are pure analytical grade reactants except TMACl and TMAOH, which are 97% pure only (Aldrich). The solutions of ionic iron are prepared from FeSO4(NH4)2SO4 for divalent ionic iron and Fe2(SO4)3 for trivalent ionic iron. 3. Voltammetric Experiments. Depending on the purpose of the experiments, that is, preparative or analytic, two different geometries for the electrode are used. For preparative experiments, a mercury pool was used, the surface of which is around 1 cm2. It is associated with a calomel reference electrode and with a platinum counter electrode. The potentiostat is a Radiometer PGP201. For analytical purposes, the use of a static mercury drop electrode or of a dropping mercury electrode is required. A 303A system from PAR EGG is used in conjunction with an Ag/ AgCl reference electrode and a Pt counter electrode connected to a potentiostat, model EGG 273. The main differences between these two cells are the huge difference of surface between both mercury electrodes and the facility to renew the electrode. Indeed, the mercury drop electrode allows changing the surface very easily: more electrochemical (15) Dubois, E.; Boue´, F.; Cabuil, V.; Perzynski, R. J. Chem. Phys. 1999, 111 (15), 7147. (16) Bacri, J. C.; Boue´, F.; Cabuil, V.; Perzynski, R. J. Magn. Magn. Mater. 1986, 62, 36. (17) Charlot, G. Les me´ thodes de la chimie analytique; Masson et Cie: Paris, 1966.

Langmuir, Vol. 19, No. 26, 2003 10893 techniques can be applied as compared to the case of the mercury pool electrode; then the influence of the aging of the surface is easily studied. Conversely, the mercury pool cannot be changed easily, and essentially cyclic voltammetry or simple electrolysis is performed onto it. Figures 2-6 show results obtained with the mercury drop electrode. Potentials on the abscissa are referred to a Ag/AgCl/KClsat reference. 4. Magnetic Properties. The magnetization measurements are performed in order to detect the formation of metallic iron and to quantify this formation. An extraction method is used: a high magnetic field (10 000 Oe ) 180 kA‚m-1) is applied on the sample placed in a cylinder surrounded by the detection coil. The flux variation while extracting the tube from the field is then measured. This simple basic extraction method is needed in our case in order to avoid erroneous results due to inhomogeneous samples.

III. Determination of Appropriate Experimental Conditions The reduction of nanoparticles stabilized as colloidal dispersions raises several questions linked to their nonclassical electrochemical behavior as compared to that of pure ionic solutions. First of all, before performing an electrochemical reaction, the conditions have to be chosen in order to keep their colloidal stability. The second point is linked to the possibility of a reduction up to metallic iron formation under the previously fixed conditions, a question which raises the problem of the possible total or partial reduction of such big objects as nanoparticles. The third point is related to the specific behavior of these nanoparticles, which have to be compared with ionic iron. This comparison implies additional constraints on the composition of the solutions studied due to the complex chemistry of iron. To solve these different problems, the crucial points are the nature of the ions and their concentrations in the solutions. Both have to be determined in order to define appropriate conditions for studying the electroreduction of nanoparticles. 1. Stability of the Colloidal Suspension. The first point is the need for maintaining the stability of the colloidal solution, that is, of the electroactive species, despite both the nature and concentration of the supporting electrolyte and the nature of the new species produced during the electrolysis. Indeed, the salt concentration brought by the colloidal solution is too low to ensure a convenient electrical conduction. Moreover, the stability depends on several parameters, as pointed out in section II. (i) It depends on the value of the salinity: if it is too high, the electrostatic repulsion is screened and particles aggregate.18 (ii) It depends on the nature of the added ions: multivalent ions or small monovalent ions which have a charge opposite to that of the nanoparticles must be avoided, as well as ions that tend to strongly adsorb on the surface.19 (iii) As indicated previously, it depends on the pH: this parameter can shift during the electrolysis and can be strongly affected near the electrode. (iv) It also depends on the size of the nanoparticles: smaller particles are more stable (see the last column in and caption of Figure 1). Therefore, we worked with small γ-Fe2O3 particles of low polydispersity. The stability is checked at several scales. Macroscopically, a stable dispersion is black whereas a destabilized dispersion becomes brown and readily separates in two phases. At a microscopic scale, optical microscopy allows detecting aggregates of a few microns. Note that light scattering cannot be used for our (18) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (19) Jolivet, J. P. De la solution a` l′oxyde; Actuels, S., Ed.; CNRS Editions: Paris, 1994.

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Table 1. Summary of the Results of Preparative Experiments Performed To Select the Conditions for Metallic Iron Formation on the Mercury Cathodea surface charge + -

counterions -

ClO4 TMA+ Na+ Na+ TMA+ TMA+ TMA+ NH4+ TMA+ TMA+ TMA+

supporting electrolyte

Fe0

HClO4 pH ) 1.4 TMAOH 0.2 M Na3cit 0.15 M TMACl 0.2 M TMA3cit 0.15 M TMA2oxalate 0.2 M TMACl 0.2 M NH4cit/NH3 pH ) 8.6 TMAH2PO4/(TMA)2HPO4 pH ) 7.1 TMAHPO4/(TMA)3PO4 pH ) 11.1 TMAborate pH ) 9

no yes no yes no yes yes yes yes no no

E0(Fe)/EECS, V -1.85 -1.8 -1.8 -1.65 -1.65 -1.7

a The three columns on the left indicate the experimental conditions used to maintain stability while performing electrolysis. The two columns on the right give the result of nanoparticles’ reduction and the approximate reduction potential where metallic iron is obtained.

solutions due to the strong absorption of the particles in the visible range (except for very dilute dispersions). The presence of very small aggregates of several particles is checked by a technique taking advantage of the specific properties (magnetic and optical) of our magnetic oxide nanoparticles: the measurement of the relaxation of the birefringence induced by a low field perturbation.20,21 With the relaxation determined being proportional to the volume of the rotating nanobject, aggregates are very well detected. The pairs electrolyte/particles that fulfill the stability condition have thus been determined and are given in Table 1. 2. Conditions of Metallic Iron Formation. The second point in the setting of experimental conditions is to test whether the electrochemical reduction can be fully achieved, that is, oxide nanoparticles converted into metallic iron. This has been done using preparative experiments for a large variety of electrolytes over the whole range of pH where these different electrolytes were tested both with ionic iron (when possible) and nanoparticles. At the end of the preparative electrolysis, magnetization measurements are performed on the mercury electrode, to conclude if metallic iron was formed. The trials are summarized in Table 1. After these preliminary steps, four main conclusions arose: (i) nanoparticles can be reduced into metallic iron; (ii) the reduction into metallic iron always occurs for potentials more negative than -1.5 V/ECS, thus close to the border of the domain of electroactivity; (iii) metallic iron can be formed only if pH > 7; (iv) when alkali metal ions are present, they are simultaneously reduced with the iron. Metallic sodium, potassium, and so forth are dissolved into mercury, thus modifying the resulting material. Moreover, these alkali metals provoke a strong pH increase: this leads in all cases to the destabilization of the colloidal suspensions because these cations are not convenient counterions for negatively charged particles in alkaline medium. In a first series of experiments, we tried to avoid such a modification to simplify the problem. Consequently, to produce metallic iron from the reduction of dispersed oxide nanoparticles, these dispersions should be prepared in the absence of alkali metal counterions and are constituted of citrate surface coated particles. 3. Comparison between Ionic Iron and Nanoparticles as Starting Material. Such a comparison is important for the understanding of the nonconventional (20) Wilhelm, C.; Gazeau, F.; Roger, J.; Pons, J. N.; Salis, M. F.; Perzynski, R.; Bacri, J. C. Phys. Rev. E 2002, 65, 031404. (21) Bacri, J. C.; Perzynski, R.; Salin, D.; Servais, J. J. Phys. 1987, 48, 1385.

Figure 2. Influence of citrate concentration on the cyclic voltammograms of Fe3+. The electrolyte is 0.15 mol/L NH4ClO4, and [citrate(NH4)3] varies from 0 to 0.1 mol/L (see legend). [iron] ) 6 × 10-4 mol/L, twaiting ) 3 s (equilibration time at the initial potential, here always the most positive potential), and scan rate ) 50 mV/s.

electrochemical behavior of nanoparticles. Indeed, the electrochemical response of FeII or FeIII in the electrolyte tested must be at least characterized for identifying the different steps occurring over the entire potential range. Such a comparison of iron ions and oxide nanoparticles is, however, difficult due to the precipitation of iron species when their concentration is not low enough and/or when the pH is not sufficiently acidic. Nevertheless, at low enough iron ions concentrations, it is possible to study experimentally ionic iron for pH < 7 without precipitation during a short period of time (around 1 h), as observed during the set of experiments, for example in NH4ClO4. Under these conditions, FeIII is reduced into FeII at potentials more positive than the border of the domain of electroactivity, and the couple FeIII/FeII is thus not observable (see Figure 2). Only the reduction into metallic iron occurs. Furthermore, electrolytes such as NH4ClO4 do not stabilize the colloidal dispersions over a long period of time, since the citrate molecules adsorbed on the surface desorb with time. If the citrate concentration in the solution is zero, the surface charge, that is, the repulsion force between particles, becomes too low, and the particles then tend to precipitate. Solving this problem is possible by adding citrate in the supporting electrolyte: maghemite nanoparticles remain stable, and iron ions are complexed so that they do not precipitate. Moreover, the potential of FeIII/FeII is shifted inside the domain of electroactivity. Nevertheless, the reduction into metallic iron is also shifted toward the border of this domain while increasing citrate concentration (Figure 2). This implies choosing a

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Figure 3. Comparison of the behavior of ions and nanoparticles. Cyclic voltammograms in [NH4ClO4/NH3 pH ) 8.6] ) 0.015 mol/L, [citrate(NH4)3] ) 0.01 mol/L. [iron] ) 6 × 10-4 mol/L in all experiments (Fe2+, Fe3+, nanoparticles), twaiting ) 3 s, and scan rate ) 50 mV/s. Part b is the expansion of the nanoparticles’ signal plotted in part a.

citrate concentration as a compromise between its effect on the colloidal stability and its influence on the value of the potential of reduction into metallic iron. We have used [citrate] ) 0.01 mol/L in the experiments that follow. 4. Electroanalytical Methods: Preliminary Studies. Using the different electrolytes that fulfill the constraints described above, several electrochemical methods were applied at the mercury drop electrode in order to explore the behavior of ions and nanoparticles. The objective was to separate the specific contribution due to one type of electrolyte or to one ion from the general trends. Some general characteristics that are specific to the experiments with nanoparticles and that do not depend on the nature of the electrolyte can be extracted from our results. Experiments using cyclic voltammetry (CV) show that the intensities obtained depend (i) on the initial potential of the cycle, (ii) on the region of potential explored, (iii) on the waiting time (twaiting) at the initial potential, and (iv) on the values of potentials when the scan is blocked inside a cycle. These results mean that the intensities depend on the story of the mercury drop surface. Pulsed techniques that allow jumping above a potential region are thus interesting for the study of the present system. Normal pulse polarography (NPP) indeed shows that the intensity depends (i) on the initial potential, (ii) on the time spent at this potential, and (iii) on the story of the surface, because the intensities are totally modified if the drop is hanging instead of dropping (as in classical NPP). As no reoxidation of metallic iron is observed from CV experiments, reverse pulse polarography (RPP) was used in order to explore shorter times. No clear reoxidation of metallic iron was observed. Finally, chronoamperometric (CA) experiments show that the intensity after a potential step toward the potential of reduction into metallic iron deviates from the classical 1/t0.5 Cottrell’s law. As all these experiments have shown qualitatively similar behaviors in the various electrolyte solutions employed, further experiments were performed on the electrolyte that appears as the best compromise given all the constraints: NH4ClO4/NH3 pH ) 8.6, [citrate(NH4)3] ) 0.01 mol/L, with a ferrofluid coated with citrate and with NH4+ counterions. Indeed, the anion ClO4- is a low complexing agent, citrate ions complex iron and stabilize particles, and the solution is a buffer. Moreover, ions and nanoparticles can be directly compared in the same electrochemical environment.

Figure 4. Cyclic voltammograms of nanoparticles in 0.15 mol/L NH4citrate/NH3 pH ) 8.6. [iron] ) 4 × 10-3 mol/L, twaiting ) 3 s, and scan rate ) 50 mV/s. Dashed lines correspond to two cycles with nanoparticles without o-PNT. Plain lines correspond to two cycles in the same solution after addition of o-PNT.

IV. Studies of the Electrolyte NH4ClO4/NH3 pH ) 8.6, [citrate(NH4)3] ) 0.01 mol/L In the previous paragraph, the best conditions have been defined. It is now possible to compare iron ions’ and iron oxide nanoparticles’ reduction in order to determine the parameters that govern the electrochemical behaviors. The voltammograms obtained with FeII, FeIII, and nanoparticles in this electrolyte are plotted in Figure 3. The first obvious observation is that nanoparticles produce a much lower intensity than ions. In all cases, metallic iron is obtained around -1.5 V and there is no clear reoxidation peak. Nevertheless, for nanoparticles, there appears some current in the region -0.3 to -1.2 V, which may correspond to their partial reduction. This last point is the first striking difference between ions and nanoparticles. Such an original electrochemical behavior for nanoparticles can be expected given the complex chemistry of iron (complexation, formation of hydroxides and oxides, equilibrium between multiple species); nevertheless, it does not appear with ionic iron. 1. Intermediate Reduction States of the Nanoparticles. Figure 4 shows two cycles in the region [0 to -1 V]. The reoxidation peak noted A in Figure 4 (around -0.25 V) appears only if the species reduced for potentials in the region -0.4 to -0.8 V are formed. In the second cycle appears a new reduction peak noted B in Figure 4 (around -0.3 V). Given the results for ions shown in Figure

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3, one may suppose that these two peaks correspond to the couple FeIII/FeII. The potentials are not identical in Figures 3 and 4, however, as they vary with the intensity of the complexation (see Figure 2) and they depend on the ratio [ionic iron]/[citrate] that probably differs in the two experiments. This result means that the nanoparticles can be broken into FeII ions. This hypothesis is supported by other results. First, when adding a specific complexing agent of FeII ions, such as o-phenanthroline (o-PNT), the cyclic voltammogram is modified, as shown in Figure 4: the oxidation peak A no longer exists, and neither does the reduction peak B observed previously in the second cycle. This is consistent with the previous result: due to the complexation by o-PNT, FeII cannot be oxidized into FeIII; thus, the further reduction of FeIII (during the second cycle) is likewise not observed. Macroscopically, this complex FeII/o-PNT can be detected due to its very intense red color, allowing the direct detection of FeII formed close to the mercury drop. This is clearly observed with ionic FeIII. The same experiment with nanoparticles is more difficult to perform, due to the strong optical absorption of the particles around the same wavelengths as those for the complex. Nevertheless, this colored complex can be observed during nanoparticles’ reduction using a special cell of low thickness, an optimized nanoparticles’ concentration, and a simple optical device to magnify the mercury drop. The puzzling point is the lower quantity of FeII formed from nanoparticles than the amount formed from ferrous ions, as deduced from the difference of the color intensity around the mercury drop, although the global concentration of iron is identical: It is obvious that much less FeII is formed while nanoparticles are reduced. Several reasons can be suggested: (i) The quantity of reduced material is much lower than that for ions, as shown in Figure 3 (lower intensity for nanoparticles). (ii) Only a part of the ions are free ions, and some may precipitate: the local pH and concentration values near the drop may be different from the bulk conditions. (iii) The species produced may block the surface of mercury. (iv) The FeII ions may interact with negative particles and not with o-PNT. Another evidence of the partial reduction of nanoparticles can be deduced from another set of experiments: if a preparative electrolysis is made with the same system on a mercury pool, running a cyclic voltammetry after several hours of preparation shows the presence of FeII ions in the solution. Thus, from all this set of experiments, one can conclude that free FeII ions are obtained under the present conditions after a partial reduction of the nanoparticles in the region -0.4 to -1.2 V. Consequently, the particles are partly broken: such a situation has to be avoided during preparative experiments; otherwise, the benefit of the nanoreactor is lost. Indeed, several species are present in the solution: nanoparticles, ferrous iron, and/or precipitated oxides or hydroxides (depending on the pH and on ionic strength conditions), the characteristics of which are different from those of the initial nanoparticles and obviously not controlled. In such a case, a nanoparticle cannot be considered as a nanoreactor. 2. Interactions Particles/Mercury, celec ) 0.015 mol/ L. The preliminary experiments described in section III.4 have clearly shown from the influence of twaiting that accumulation of electroactive material takes place on the Hg electrode under specific conditions. It is important to precisely analyze these phenomena in order to understand their origin. All experiments with nanoparticles show an influence of the initial waiting time twaiting spent at the initial

Dubois and Chevalet

Figure 5. Influence of twaiting on chronoamperometry experiments on nanoparticles. The conditions are [NH4ClO4/NH3 pH ) 8.6] ) 0.5 mol/L, [citrate(NH4)3] ) 0.01 mol/L, and [iron] ) 6 × 10-4 mol/L. A potential step from Einitial ) 0 V toward Efinal ) -1.5 V is performed at t ) 0. The curve of the electrolyte is given as a reference. The inset plots the intensity versus t-0.5 for the data obtained with nanoparticles.

potential. For instance, in Figure 3, for nanoparticles, twaiting is 3 s (at 0 V). If this time is increased, the intensities over the whole cycle are higher. The same behavior is observed while performing NPP between 0 and -1.7 V: the intensity depends on the time spent at 0 V. This can also be clearly seen in Figure 5 using chronoamperometry: the intensity during the first second after the potential step is much higher if twaiting ) 120 s instead of 3 s, although it tends toward a limit independent of twaiting for long times. The inset of Figure 5 plots the intensity as a function of t-0.5: the decrease of the intensity clearly deviates from the classical t-0.5 law, showing that the process is not purely diffusional. These different experiments prove that there is accumulation of electroactive material on the electrode at least at 0 V (applied potential). After this first set of experiments, and in order to compare ions’ and nanoparticles’ behavior versus accumulation effects, another type of study has been performed. The idea was to check whether this type of interaction was weak or sufficiently strong to resist a complete removal of the electrolyte. With ionic iron (6 × 10-4 mol/L in the mentioned electrolyte), after twaiting ) 120 s under Einitial ) 0 V, if the solution is carefully replaced by pure deaerated electrolyte (the mercury drop always remaining under Einitial), a voltammogram shows the normal feature of a pure electrolyte response. Conversely, performing the same replacement of electrolyte with nanoparticles (6 × 10-4 mol/L in the indicated electrolyte), the first cycle of the set of voltammograms shows the same type of signal as does the solution of nanoparticles during a first cycle. The same situation occurs if chronoamperometry is applied in place of cyclic voltammetry. After such a treatment, the second cyclic voltammogram (or the second chronoamperogram) shows a peculiar shape. (i) There is no detectable singularity corresponding to the original cyclic voltammetry with nanoparticles. However, this shape is different from that of the cyclic voltammogram of the electrolyte alone; a higher intensity is observed and the limit of the domain for negative potentials is shifted, by approximately +100 mV. (ii) If nanoparticles are introduced again in the cell, the next voltammogram performed on the same drop produces a modified signal as compared to the reference signal of Figure 3b. This indicates a likely evolution of the surface. The influence of the initial accumulation potential has been then examined too, and the effects are as follows:

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Figure 6. Influence of the electrolyte concentration on the behavior of nanoparticles. Conditions: [NH4ClO4/NH3 pH ) 8.6] ) celec, [citrate(NH4)3] ) 0.01 mol/L, [iron] ) 6 × 10-4 mol/L, and twaiting ) 3 s. (a) Cyclic voltammetry: scan rate ) 50 mV/s. celec is given in the inset. (b) Chronoamperometry: Einitial ) 0 V and Efinal ) -1.5 V. celec is given in the inset.

(i) Under the same experimental protocol (exchange of electrolyte), and with nanoparticles, after twaiting ) 120 s at Einitial ) Ewaiting ) -1 V, no signal appears. (ii) When applying the NPP method on nanoparticles, some accumulation phenomena due to the time spent at the initial potential are observed (and are very efficient) only if the initial potential lies between 0 and -0.5 V. (iii) If cyclic voltammetry is performed between -1 and -1.7 V only, no reduction into metallic iron is observed except during the first cycle, whatever the initial waiting time at -1 V. Given these results, one can suppose that these accumulation phenomena are essentially related to electrostatic repulsion between the maghemite nanoparticles and the mercury electrode, since no real chemical affinity is expected between maghemite and mercury. However, high electric local fields could have an influence. For the nanoparticles, the structural charge is around 2 charges/ nm2, that is, 0.32 C/m2. The surface of mercury is positive in the range 0 to -0.5 V and negative in the range -0.5 to -1.7 V. Far from the point of zero charge (around -0.5 V), the charge of the mercury is of the same order of magnitude as that of the nanoparticles. With the concentration of 0.015 M in NH4ClO4/NH3, the electrolyte used here, the Debye length (which is the typical size of the electric double layer) is around 25 Å, so that the range of the electrostatic repulsion is high enough to avoid the particles approaching the electrode. 3. Influence of the Electrolyte Concentration celec. Given the very likely dependence on electrostatic phenomena deduced from the above experiments, it is reasonable to examine the influence of the ionic strength, that is, of the concentration of the supporting electrolyte. Indeed, contrary to fundamental behaviors of ions in electrolytes, which obey a diffusional regime except for very low concentrations (migration), an influence of the electrolyte concentration on electrochemical behavior exists for nanoparticles (see Figure 6). Increasing the electrolyte concentration changes the shape of the cyclic voltammograms (Figure 6a). The intensities corresponding to intermediate species increase and change in morphology. For the last reduction step (metallic iron), the intensity is much higher for high salinities. Chronoamperometry (Figure 6b) also shows that the intensity is much higher for short times and remains higher even at a very long time (15 s). Note that, given the time scale, no capacitive current influence is detectable. Concerning the accumulation phenomena described in the previous section, it is clear that they still exist at higher

salinities, as suggested from cyclic voltammetry, chronoamperometry, and experiments where the electrolyte is removed as described in section IV.2. The surface is also modified after experiments, as described previously. The interaction between oxide nanoparticles and mercury is thus not purely electrostatic. However, for high electrolyte concentrations, the particles approach the electrode for potentials in the range -1 to -1.7 V, contrary to what was observed for celec ) 0.015 mol/L (experiments in section IV.2). These results mean that, due to the high screening of the electrostatic repulsion if celec is high (the Debye length is around 4 Å at 0.5 mol/L), the oxide nanoparticles can approach the electrode by diffusion whatever the potential. The current measured for E ) -1.5 V is thus due both to adsorption and to diffusion. However, for low celec values, only adsorbed nanoparticles contribute to the cathodic current at -1.5 V. The change of behavior when celec increases is in agreement with an interaction partly of electrostatic nature between the maghemite nanoparticles and the mercury interface (repulsion effect at negative potential). V. Discussion The study of the electrochemical behavior of nanoparticles formed by about 10 000 atoms raises the question of the comparison to usual ionic solutions. In the present case, this comparison is difficult due to the nature of the element used, iron. The behavior of iron as an ionic species is complicated from the electrochemical point of view due to the number of different species that can exist, given the chemical properties of ferrous and ferric ions.22,23 Complexes are formed with many ions, several aquo/hydroxo complexes are formed depending on the pH, precipitation as a solid easily occurs, and intermediate species (polynuclear species or colloidal particles) can also be obtained. These species may have different electrochemical behaviors, and the local conditions near the electrode resulting from reactions inside the reaction layer can be very different from the bulk solution conditions, leading to locally different species and, therefore, increasing the complexity of the phenomena. Using nanoparticles of iron oxide such as the species studied here raises other difficulties. The first difficulty (22) Baes, C. F.; Mesmer, R. E. The hydrolysis of cations; WileyInterscience: 1976. (23) Ko¨nigsberger, L.-C.; Ko¨nigsberger, E.; May, P. M.; Hefter, G. T. J. Inorg. Biochem. 2000, 78, 175.

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is the precise characterization of the nanoparticle, which is the electroactive species. It is necessary to know its exact composition, the repartition of the components in the particles if several components are involved, the nature of the material (crystalline or not), the surface state (interface oxide/solution), as well as the size distribution of the particles. In the present case, the nanoparticles have been studied for many years using numerous techniques (see, for example, refs 14-16, 20, 21, 24, and 25). The particles are constituted of maghemite γ-Fe2O3, each particle is a monocrystal, and the size distribution can be measured by several techniques.14 The surface is covered with negatively charged citrate molecules that ensure a density of charge of 2 charges/nm2, and this charge is compensated with ammonium counterions.15 The second difficulty is the colloidal stability of such dispersions: they can be destabilized by the supporting electrolyte before any reaction. After electrochemical reactions occurred, the dispersions could be destabilized by the intermediate species formed, by the local conditions near the electrode surface, or as a result of the modification of the nanoparticles’ surface (partial dissolution, modification of the surface charges, and so forth). Such an evolution of the solution by formation of aggregates can be useful or not, depending on the aim of the experiment; however, it complicates the studies, modifying the nature of the electroactive species during the evolution of the experiments. In the present experiments, the colloidal suspensions are stable before electrochemical reaction and remain macroscopically stable after electrochemical reactions (checked as explained in section III.1). Moreover, no aggregation of the nanoparticles can occur on the electrode when no reaction occurs (due to interparticle repulsion); however, one cannot assert that there is no local aggregation on the electrode after reactions occurred. Note that if particles are partly dissolved into FeII, for example, we observe the behavior of ionic species, complicated by the presence of nanoparticles. Both behaviors are not independent: indeed, adsorption of iron ions can occur on the oxide nanoparticles. In a first very simple approximation, the difference between small species such as ions or molecules and big species such as nanoparticles might be seen as a simple change of scale. For ions, the intensity of the reduction current is supposed to be proportional to D[FeIII], with D being the diffusion coefficient. This can be applied to nanoparticles, considered as big electroactive species with a diffusion coefficient D. For FeIII, D ∼ 0.7 × 10-9 m2/s, although, for the nanoparticles, D is much smaller. A value of 2.7 × 10-11 m2/s has been measured for nanoparticles slightly bigger than the particles used in the present study.25 Here D is estimated as 6 × 10-11 m2/s. However, one nanoparticle of 6 nm contains around 4300 iron atoms. Comparing two solutions of the same concentration in iron, the current should be multiplied by 4300/12 ∼ 370. This is in contradiction with experiments that show a smaller current for nanoparticles as compared to ionic iron. The difference between true and colloidal solutions as well as the particularities of colloidal solutions have to be considered. 1. Comparison to Other Particles/Electrode Systems. Few electrochemistry experiments has been performed with colloidal particles; however, there exist very interesting works which are relevant to the present subject. Some studies deal with electrochemistry in the (24) Gazeau, F.; Dubois, E.; Bacri, J. C.; Boue´, F.; Cebers, A. R. P. Phys. Rev. E 2002, 65 (3), 031403. (25) Bacri, J. C.; Cebers, A.; Bourdon, A.; Demouchy, G.; Heegaard, B. M.; Kashevsky, B.; Perzynski, R. Phys. Rev. E 1995, 52, 3936.

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sense that there is an electrode and an applied potential but not always linked to an electronic transfer.9,26,27 Other studies deal with electron transfer on nanoparticles, however, not always through an electrode reaction.4-7,28-37 The results are sometimes difficult to interpret within a unified framework. Nevertheless, a survey of some of these studies will be made with comparisons with the present results wherever possible. First, several studies deal with reactions occurring at an electrode with particles, that is, with electron transfers. Pioneering works have been performed on the electrochemical reduction behavior of many insoluble substances (oxides, sulfurs, and so forth).28-32 The polarograms obtained usually show one or two reduction peaks. A transfer of electrons from the particles to the mercury electrode has been observed. The size of the particles seems to be an important parameter, as well as the interaction between the particles and the mercury electrode. However, one of the limitations of these studies is the lack of control and measurement of the size of the particles and of the colloidal stability of the solutions studied, parameters that appear to be crucial. Heyrovsky tried to clarify this field in a series of four papers introduced by an article in which he attempted to summarize the questions. The behaviors of nanoparticles of several semiconductive materials (TiO2, SnO2, TiO2/Fe2O3) have been studied on electrodes,4-7 in order to obtain information on their properties, especially on the reduction processes. The polarographic currents measured were diffusion-controlled in most cases. The current corresponds to the exchange of n electrons with the nanoparticle. This number n depends on the reaction involved between the electrode and the particle: it can take place in the bulk of the particle or on its surface. The number n can also depend on the value of the potential. Due to the size polydispersity, parameters such as n and D are also “dispersed”, leading to a spreading of the signals. For both the SnO2 and TiO2 nanoparticles studied, the electroreduction of the surface H+ ions on the particle surface is first observed. At more negative potentials, a direct reduction of metal surface ions may occur. For mixed TiO2/Fe2O3 colloids, the results obtained with pure TiO2 are superimposed with the reduction of FeIII to FeII, which is an electron transfer with the particle bulk. The present experiments are consistent with these results. Indeed, it is observed that an electronic transfer can occur between the electrode and the nanoparticle. With γ-Fe2O3, the reduction process reaches the bulk of the particle and not only the surface, in agreement with the results on TiO2/Fe2O3 colloids.5 (26) Andrade, E. M.; Molina, F. V.; Gordillo, G. J.; Posadas, D. J. Colloid Interface Sci. 1994, 165, 459. (27) Andrade, E. M.; Molina, F. V.; Posadas, D. J. Colloid Interface Sci. 1994, 165, 450. (28) Micka, K. Collect. Czech. Chem. Commun. 1956, 21, 647. (29) Micka, K. Collect. Czech. Chem. Commun. 1957, 22, 1400. (30) Micka, K. Depolarisation of the dropping mercury electrode by suspensions of insoluble substances. In Advances in Polarography; Second International Polarographic Congress, Cambridge 1959; Pergamon Press: 1960. (31) Micka, K. Collect. Czech. Chem. Commun. 1965, 30, 235. (32) Micka, K.; Kadlec, O. Collect. Czech. Chem. Commun. 1966, 31, 3837. (33) Heyrovsky, M.; Jirkovsky, J. NATO ASI Ser., Ser. 3: High Technology (Nanoparticles in Solids and Solutions) 1996, 18, 161. (34) Heyrovsky, M.; Jirkovsky, J. Scientific Papers of the University of Pardubice, Series A: Faculty of Chemical Technology 1998, 3, 227. (35) Mulvaney, P.; Cooper, R.; Grieser, F.; Meisel, D. Langmuir 1988, 4 (5), 1206. (36) Mulvaney, P.; Swayambunathan, V.; Grieser, F.; Meisel, D. J. Phys. Chem. 1988, 92, 6732. (37) Mulvaney, P.; Swayambunathan, V.; Grieser, F.; Meisel, D. Langmuir 1990, 6, 555.

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Second, several works deal with the behavior of particles toward electrodes in the absence of electronic transfers. It is shown that interactions between the particles and the surface exist and depend on the potential.9,26,27 With the aim of studying heterocoagulation, that is, the adsorption of colloidal particles on a planar surface, the behavior of gold nanoparticles (13-22 nm) on a mercury cathode has been studied in a large potential range.9 No reaction and no electron exchange occurs, but the nanoparticles are incorporated into the mercury, forming an amalgam and allowing us to study the rate of adsorption on the electrode. The rate of adsorption is shown to be dependent on the global interactions between the mercury surface and the gold nanoparticles. The latter are negatively charged whereas the charge of mercury depends on the potential. If the two metals bear charges of the same sign, no adsorption occurs. The adsorption of particles on a metallic surface was also studied with hematite (R-Fe2O3) on silver and mercury, however, with much bigger particles (1 µm) than the present ones.26,27 The amount of adsorption is shown to depend on the pH of the solution and on the electrode potential. The origin of this adsorption is not purely electrostatic because adsorption also occurs if the particles and the electrode have opposite charges, that is, if the interaction between the particles and the electrode is repulsive. This is due to the fact that all forces between both materials contribute to the interaction, and the details are not fully understood yet. These results on heterocoagulation are fully consistent with the present observations: nanoparticles remain on the surface of the mercury after a complete removal of the electrolyte (see section IV.2). This means that there is adsorption of the nanoparticles on the mercury surface, with the magnitude of the phenomenon depending on ionic strength and potential and, thus, on the surface charge and on the structure of the solid/electrolyte interface. The influence of the potential applied to the mercury electrode on adsorption indicates that an electrostatic interaction is probably involved in this adsorption. However, the existence of adsorption whatever the ionic strength indicates that the interactions are not purely electrostatic. In our experiments, the number of electrons exchanged can be calculated from the first chronoamperometric curve measured after this first electrolyte removal. If it is assumed that all these electrons are used to reduce FeIII into Fe° and that each particle is totally reduced, a maximum degree of coverage of the surface can be estimated. With the following typical conditions, [iron] ) 1.2 0.10-3 M, [NH4cit/NH3] ) 0.015 M, pH ) 8.6, and twaiting ∼120 s, a coverage around 30% is determined. If a simple calculation of coverage is assumed considering a random sequential adsorption, one expects for the fractional surface coverage θ: θ ) 2cparticulesS(D/π)1/2t0.5, where S is the particle section, t is the deposition time, and D is the diffusion coefficient.38 For particles of 7 nm, D ∼ 6, 10-11 m2/s, and [iron] ) 6 × 10-4 mol/L, this formula leads to a coverage of the surface of 14% for the experiments. This value is a good order of magnitude as compared to the experimental value of 30%. 2. Results of the Electron Transfer for Maghemite Nanoparticles on Mercury. When appropriate conditions are realized to observe an electron transfer (i.e. if the particles come sufficiently close or even in contact with the electrode), the main question is then what is the immediate future of the nanoparticle, that is, how are its

structure and its chemical composition modified by this overall reduction step? The present experiments indicate that metallic iron is formed at potentials negative enough. However, significant reduction currents are measured in the intermediate region (-0.3 V/ECS to -1.3 V/ECS). The injection of electrons in similar nanoparticles (hematite, R-Fe2O3, and goethite, R-FeOOH) has been studied, not by the means of an electrode but using radiolytically produced reducing free radicals.35-37 The metallic iron is never obtained because the reductive species is not a strong enough reductant. However, the authors show that three products are obtained: (i) Fe2+ ions dissolved in the aqueous phase; (ii) FeII hydroxide adsorbed at the particle surface; and (iii) FeII trapped within the lattice. The amount of each species is a function of the pH. These results are fully consistent with the present experiments. In the intermediate potential region, only FeII can be formed and one observes free FeII, visualized with o-PNT, but part of this FeII is not released in the solution (see section IV.1). Note that the free FeII is a quasi-reversible system although the reduction leading to FeII is not reversible. As the pH is high (8.6), the adsorption of FeII on the particles is favored but may be different from that of the experiments of ref 36 because of the nature of the complexing medium used (with sodium citrate). It is, however, not possible to differentiate in the present case surface adsorption from bulk migration. At far enough negative potentials, metallic iron is formed. This can be detected from the magnetic properties of iron (i.e. iron particles in mercury) after the preparative experiments. The voltammograms show that this reduction is totally irreversible: there is no reoxidation of iron (neither on the mercury drop nor on the mercury pool). After iron preparation, if the surface of the electrode is cleaned with concentrated acid (HNO3 10% or HCl 30%), the oxides adsorbed on the mercury surface are removed, and the electrode looks like pure mercury. Then, although no species adsorbed on the surface block the reactions, no iron is reoxidized in alkaline medium. It is interesting to note that, in the presence of HCl 30% (and without potential applied), no calomel is formed on the electrode if iron is present inside. The irreversibility of the reduction into metallic iron thus does not result from blocking species that could be adsorbed on the mercury surface. In acidic medium, that is, in HClO4 or 0.1 M H2SO4, a reoxidation current can be observed very close to the mercury oxidation wall. Such an electrolysis on a mercury pool, however, never allows extracting the whole iron formed. These rather unexpected results can be explained by the dependence on the potential of the wetting properties of iron by mercury. An iron thread in contact with a mercury pool is not wetted for E > -1 V/ECS but is wetted for E < -1.5 V/ECS. This has been shown and studied using iron plates,39 and has been observed with a thread in the present study. For very negative potentials, the traces of oxides on the metallic iron plate are removed, leading to a perfect wetting. Dewetting is then difficult. In the present case, once iron nanoparticles are formed, the wetting phenomena very likely favor their dispersion into the mercury electrode. The reoxidation would imply dewetting or mercury oxidation at the same time as iron oxidation; however, to date, no quantitative argument, to our knowledge, has been proposed to explain such a process. From the point of view of preparative experiments, several results can now explain the difficulties encoun-

(38) Johnson, C. A.; Lenhoff, A. M. J. Colloid Interface Sci. 1996, 179, 587.

(39) Winkler, K.; Krogulec, T.; Baranski, A.; Galus, Z. J. Electroanal. Chem. 1990, 291, 103.

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tered. Indeed, due to accumulation of nanoparticles under appropriate conditions, the electrode surface is modified. Moreover, the mercury surface can be easily polluted by adsorption of nonreduced particles or parts of particles if a reduction mechanism implies transient dissolution. The intermediate species could precipitate locally (the pH may be different from the pH in the solution, far from the electrode surface, and iron oxides seem to adsorb easily on mercury). Depending on the time scales of the different processes, if free FeII is formed, the benefit of the nanoreactor can be lost. VI. Conclusion In the present study, the electrochemical reduction of nanoparticles has been investigated using a well-defined colloidal system. Indeed, both the properties of the γ-Fe2O3 nanoparticles and the properties of the dispersions, especially their colloidal stability, are well-known. This allows improving the understanding of their electrochemical behavior, essentially through the comparison of nanoparticles (big species) and ions (small species). For this purpose, an appropriate medium is found in order to compare the behavior of ferric or ferrous ions with the behavior of oxide nanoparticles. This comparison shows that it does not correspond to a pure change of scale; that is, the nanoparticle cannot be considered as a “macroion”. Indeed, a nanoparticle is different from an ion because such an association of 10 000 atoms becomes a structured material (γ-Fe2O3 is a semiconductor); therefore, the parameters that control the phenomena are different. First, the interactions between the mercury electrode and the charged γ-Fe2O3 nanoparticles can control the approach of electroactive species toward the electrode, depending on the conditions (nature and concentration of ions); thus, the apparent electroactivity of the species can be modified. The result is either the adsorption of particles on the electrode if interactions are attractive, or the

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absence of reduction current if particles cannot approach the electrode due to repulsion. These interactions are shown to be strong and partly of electrostatic nature, so that they depend on the electrolyte concentration. Second, as for any material, wettability is an important parameter: the high wettability of iron by mercury at very negative potentials explains why here the nanoparticles of iron formed, whatever the process followed during the reduction step, enter the mercury electrode, while no solubility of iron in mercury is measurable. Finally, as a material, a nanoparticle can be broken into smaller crystallites down to the level of an atom. We show that this situation indeed occurs for potentials in the range -0.4 to -1.2 V/ECS, leading to ferrous ions. Such a reductive dissolution of the particles may also occur during the reduction into metallic iron, but this fact is not directly proven yet. However, depending on the media, the dissolution at intermediate stages of the reduction can generate problems. Ions can precipitate under the form of species adsorbed at the electrode interface and/or precipitate as nonreducible species. This is clearly linked to the complex chemistry of iron species in solution and explains the difficulties encountered in the preparative experiments. Therefore, the nanoparticle may not always play the expected role of a nanoreactor which would lead to a straightforward reduction into an iron object of similar nanometric size and shape. The next step in these investigations is now to improve the knowledge of the behavior of such nanobjects in order to control their electrochemical transformation. This implies, in particular, determining the characteristic times of the different phenomena, that is, of the adsorption on the electrode, of the electron transfer, and of the propagation of the reaction layer inside the material. LA035294R