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ISSN 1061-933X, Colloid Journal, 2009, Vol. 71, No. 4, pp. 487–492. © Pleiades Publishing, Ltd., 2009. Original Russian Text © B.G. Ershov, E.V. Abkhalimov, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 4, pp. 486–491.

Colloidal Copper and Peculiarities of Its Reaction with Silver Ions in Aqueous Solution B. G. Ershov and E. V. Abkhalimov Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia Received May 30, 2008

Abstract—Interactions between colloidal copper and silver ions lead to the formation of silver nanoparticles. The reaction proceeds through the intermediate stage of the formation of a copper–silver contact pair. The formation of bimetallic AgcoreCushell nanoparticles is observed in the presence of the “seeding” silver nanoparticles and upon the simultaneous radiochemical reduction of Ag+ and Cu2+ ions. DOI: 10.1134/S1061933X09040097

INTRODUCTION

EXPERIMENTAL

Recently, attention has been focused on the formation of mixed (heterometallic) nanoparticles characterized by a set of new (not inherent to individual metals) properties or their inherent properties are changed nonadditively in the presence of small amounts of other metal (see monograph [1] and relevant references). Therefore, it is urgent to develop synthesis procedures for the preparation of nanosized heterometallic particles by the simultaneous reduction of the ions of several metals, upon the interaction of nanoparticles of one metal with the ions of other (more precious) metal, by the deposition of metal shell on nanoparticle, or by other procedures.

We used Aldrich reagents. Average molecular mass of polyethylene imine (PEI) was 50 000. Its concentration in solution was expressed in moles of monomer units.

Systematic studies of the possible applications of these different methods can be performed using a pair of metals with substantially different electrochemical potentials as an example. We believe that copper and silver are convenient objects for these studies. The substantial difference in oxidation potentials (the E0 values for Cu2+/Cusolid and Ag+/Agsolid are equal to 0.337 and 0.799 V, respectively [2]) is responsible for the reaction between colloidal copper and silver ions, whereas distinct optical absorption spectra of nanoparticles and metal ions in the UV and visible ranges make it possible to study in detail reactions in aqueous solutions. The use of silver is significantly preferable because its optical band arises due to the absorption of surface plasmons and is very sensitive to changes in electron density in a metal nanoparticle [3]. This is very important for the analysis of electronic state of mixed nanoaggregates. The aim of this work is to synthesize mixed nanoparticles of copper and silver and to study specific features of the interaction between colloidal copper and silver ions in aqueous solutions.

Solutions were prepared with triply distilled water. Prior to irradiation, solutions were deaerated by deep evacuation. The irradiation was performed with 60Co source in a special glass vessels equipped with quartz cell for optical measurements. The volume of irradiated solution was usually equal to 10 ml. Optical measurements were conducted with an Specord M-40 spectrophotometer. As a result of the reactions [3] H 2O

e aq (2.7), •H (0.5), •OH (2.9), H2 (0.5), H2O2 (0.7), H+ (2.7), –

(CH3)2CHOH + •OH (•ç)

(CH3)2C•OH + H2O (ç2)

γ-radiation leads to the formation of hydrated electrons – e aq with a powerful reducing ability (reduction potential is –2.9 V) and (CH3)2C•OH radicals (–1.4 V) [4]. Values of the radiochemical yield of radiolysis products are indicated in brackets of the first reaction. When an aqueous solution was subjected to an experimental dose of γ-radiation of 60Co (3.5 kGy/h)for 1 min, approxi– mately 1.6 × 10–5 å e aq and 2.1 × 10–5 å (CH3)2C•OH were generated. The formation of silver nanoparticles was controlled with transmission electron microscopy (TEM). We used special software to process the obtained images. Nanoparticle sizes were determined by averaging over a minimum of 30 particles.

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oxidation of colloidal copper particles. In this case, the initial absorption of Cu2+ ions appears in the optical spectrum. According to [5, 6], in which the same radiochemical method of the synthesis of colloidal copper was used, the size of the formed spherical particles measured by the TEM technique in vacuum was equal to 6 ± 2 nm. Using known extinction coefficient ε of Cu2+–PEI complex (4.2 × 103 l mol–1 cm–1), from the data of Fig. 1, we find the ε values to be equal to 4.0 × 103 and 1.4 × 103 l mol−1 cm–1 at wavelengths of 300 and 565 nm, respectively.

Absorption 0.8 1 0.6

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Fig. 1. Absorption spectra of solution containing 1.5 × 10−4 å Cu(ClO4)2, 1 × 10–3 M PEI, and 0.1 M isopropanol: (1) prior to irradiation, (2), (3), and (4) after γ-irradiation for 3, 6, and 12 min, respectively.

RESULTS AND DISCUSSION 1. Colloidal Copper and Its Interaction with Silver Ions Upon the radiochemical reduction of Cu2+ ions in aqueous solution, the Cu2+ ion is formed at the intermediate stage as a result of the one-electron reduction of Cu+ ion. The first ion is hydrolyzed and forms nearly the insoluble compound Cu2O (–log [SP] = 14 [2], SP is solubility product). If precautions are not taken upon the reduction of Cu2+ ions in aqueous solution, Cu2O precipitate is formed at almost any pH value. Oxide, as well as copper, is yellow-red and their optical characteristics are very similar. In order to avoid the formation of Cu2O, upon the radiochemical reduction of Cu2+ ions, PEI was used to obtain stable colloidal copper solutions [5, 6]. In addition to the fact that this compound served as a stabilizer for forming copper nanoparticles, it was also a complexing agent for both Cu2+ ions and intermediate products of their reduction (Cu+ ions). As a result, the presence of PEI prevented the hydrolysis of Cu+ ions. Figure 1 illustrates consecutive stages of the reduction of Cu2+ ions upon γ-irradiation. The band with a maximum at 272 nm, which is typical for the solution prior to irradiation, is due to the presence of Cu2+–PEI complex. The irradiation of the solution leads to a decrease in the intensity of this band and the appearance of hidden band with maximum at 214 nm that belongs already to Cu2+–PEI complex [5]. Upon further irradiation, colloidal copper is formed. After the completion of the process, a broad absorption band of colloidal metal with unclear maxima at 300 and 565 nm is fixed in the optical spectrum (Fig. 1, curve 4). The absorption instantaneously disappears when the system is in contact with air oxygen, which is caused by the

According to [5], e aq (reduction potential is –2.9 V) and (CH3)2C•OH radical (–1.4 V) reduce Cu2+ ions. However, the disproportionation of Cu+ ions turned out to be thermodynamically disadvantageous. The reduction of Cu+ ions to copper atoms (for the Cu+/Cu0 pair, – E0 = –2.7 V [3]) only occurs in the reaction with e aq . The aggregation of atoms and clusters leads to the emergence of nucleation sites at which subsequent precipitation of metal occurs. The reaction of Ag+ ions with copper nanoparticles was studied by the consecutive addition of portions of silver ions (5 × 10–5 M) to copper colloidal solution (2.1 × 10–4 M for Cu atoms). It is also possible that, in this case, either the formation of a silver coat (shell) on copper particles occurs under the conditions when the concentration of the latter is much higher than the concentration of added silver or mixed copper–silver contact pairs are formed. According to data shown in Fig. 2, the consecutive addition of portions of Ag+ ions leads to the emergence of an absorption band at 400 nm attributed to silver nanoparticles and, simultaneously, to a decrease in the intensity of the absorption band of colloidal copper at 565 nm and an increase in the intensity of the absorption band of Cu2+ at 272 nm. Spectra barely changed with time after the first measurement, which is evidence of the fast reaction between silver ions and colloidal copper. An increase in the concentration of Ag+ ions was accompanied by strictly proportional changes in the intensities of aforementioned bands. From the known values of ε(Cu2+), ε(Cucoll), and ε(Agcoll) = 1.5 × 104 l mol–1 cm–1, strict correspondence of changes in reagent concentrations to the stoichiometry of the following reaction: Cun + 2nAg+

nCu2+ + Ag2n

was established (Fig. 3). If the silver shell had been formed on the surface of copper nanoparticles with diameters of about 6 nm, the addition of 5 × 10–5 M Ag+ ions would lead to the formation of approximately two atomic layers of metal. In this case, the disappearance of an absorption band of copper nanoparticles at 565 nm could be expected. However, this is not observed, even upon the addition of further portions of silver ions. The band at 565 nm only disappears upon the complete oxidation of copper COLLOID JOURNAL

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[Me] × 104, mol/l

5

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3 1.0 3

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Fig. 2. Absorption spectra of solutions: (1) 2 × 10–4 å Cu(ClO4)2 and 1 × 10–3 M PEI prior to irradiation, (2) colloidal copper formed after irradiation for 15 min, (3) after the addition of 0.5 × 10–4 å AgClO4; and (4), (5) after consecutive addition of portions of 5 × 10–5 å AgClO4.

0

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1.0

1.5 2.0 4 [Ag] × 10 , mol/l

Fig. 3. Dependences of variations in the concentration of (1) colloidal silver, (2) colloidal copper, and (3) Cu2+ ions on the concentration of Ag+ ions added to solution.

50 nm Fig. 4. TEM image of silver nanoparticles formed after the completion of interaction between silver ions (4 × 10–4 M AgClO4) and colloidal copper (2 × 10–4 å Cu0).

by silver. After the oxidation of copper (2 × 10–4 å Cu0) by silver ions (4 × 10–4 å AgClO4) is completed, the average size of formed silver nanoparticles is equal to approximately 10 nm (Fig. 4). The reduction of Ag+ ions occurs on the surface of copper particles, since this is most advantageous thermodynamically and leads to the formation of a copper– silver contact pair. The electron density of aggregateconstituting metals is shifted from copper to silver due to the noticeable difference in their potentials. Therefore, the aggregate represents the microelectrode sysCOLLOID JOURNAL

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tem where the cathode is silver and the anode is copper. The dissolution of copper and the precipitation of silver are caused by the electrochemical reactions of ionization on anode and discharge on cathode, respectively. Unfortunately, colloidal copper is quickly oxidized upon its contact with the air and this circumstance does not allow detecting copper–silver mixed aggregates by the TEM technique. However, similar mixed aggregates have been detected previously in the reaction of colloidal nickel particles with silver ions [7]. In contrast to the reaction with the participation of colloidal copper,

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Absorption 2.0

1.5 2

1.0

5 4 3

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1 0 200

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800 λ, nm

Fig. 5. Absorption spectra of solutions: (1) 0.7 × 10–4 M AgClO4, 3 × 10–3 M PEI, and 5 × 10–3 M acetone prior to irradiation; (2) after 30-min irradiation; (3) after the addition of 6 × 10–4 M Cu(ClO4)2 and 0.1 M isopropanol; (4) after 10-min irradiation; and (5) after 30-min irradiation.

this reaction developed rather slowly. In the course of the reaction between Ag+ ions and nickel particles, the intensity of the absorption band assigned to colloidal silver increased and narrowed, and its maximum shifted from 420 to 390 nm over tens of minutes. Apparently, the slow rate of this reaction was favored by the formation of a protective film of nickel oxide. Electron microscopy studies demonstrated that, upon the addition of silver ions to nickel colloidal solution, tiny nickel particles disappear first, while relatively large particles form mixed aggregates, which represent the contact pair composed of silver and nickel. The percentage of silver–nickel aggregates decreases with time and forming particles composed only of silver are characterized by the absorption band with the maximum at 390 nm. The potentials of electrochemical copper–silver contact pair differ greatly. The presence of PEI in the solution prevents the formation of the protective Cu2O film on the aggregate. Hence, the oxidation of copper by Ag+ ions proceeds very quickly and leads to the appearance of a band at 400 nm attributed to silver nanoparticles. In principle, this aggregate is thermodynamically unstable and the presence of ions of both metals in the solution favors exchange reactions and the formation of nanoparticles of individual metals. Thus, the reaction of nanoparticles of active metal with ions of more precious metal can lead to the formation of mixed contact aggregates of both metals. Their stability in aqueous solution increases with a decrease in the difference between electrochemical potentials and the formation of protective film on the surface of less precious metal.

2. Formation of Bimetallic AgcoreCushell Nanoparticles AgcoreCushell nanoparticles were synthesized by the radiochemical reduction of Cu2+ ions on the surface of silver particles. As in [5, 6], in order to avoid the reduction of Cu2+ ions in the solution bulk, acetone, which accepts hydrated electrons with extremely high reduction potential, was added to the solution. As a result, only (CH3)2C•OH radicals, which reduced copper ions adsorbed on the surface of silver nanoparticles, were generated upon the γ-irradiation of solutions containing isopropanol and acetone. We used 6-nm silver nanoparticles on which copper layers were deposited. It was shown that the intensity of a silver absorption band decreases and shifts to the UV region due to the drawing off the electron gas from copper to silver. Complete disappearance of the band was observed upon the formation of approximately five atomic layers of copper on the surface. We synthesized AgcoreCushell particles with a shell containing more than 13 copper layers. In this case, the absorption of colloidal copper was detected in the optical spectrum (Fig. 5). 3. Simultaneous Reduction of Ag+ and Cu2+ Ions The γ-irradiation of aqueous solution containing isopropanol leads, as was mentioned above, to the forma– tion of hydrated electrons e aq with a powerful reducing ability (reduction potential is –2.9 V) and (CH3)2C•OH radicals (–1.4 V) . Hydrated electrons are capable of reducing both ions (Ag+ and Cu2+) to the atomic state. Therefore, it could be expected that the radiochemical reduction of Ag+ and Cu2+ ions, which are simultaneously present in solution, resulted in the formation of mixed metal nanoparticles. However, according to the performed experiments (see Fig. 6), only silver nanoparticles are formed at the initial stage and Cu2+ ions cease to be reduced to form Cu+ ions. Curves 1–3 illustrate the disappearance of Cu2+ ions (band at 272 nm) with the formation of Cu+ ions (band at 214 nm) and the appearance of silver nanoparticles (band at 400 nm). Upon subtracting spectrum 1 from spectrum 3, changes in the solution composition become particularly notable (see inset in Fig. 6). As can be seen, the absorption of Cu2+ ions ceased to take place, the band of Cu+ ions appears, and the formation of silver nanoparticles completed. The latter conclusion stems from the fact that Cu+ ions, which are capable of reducing silver ions[5, 6], are present in the solution. Calculated extinction coefficient ε of silver atoms in nanoparticles turned out to be equal to 1.5 × 104 l mol–1 cm–1. Further γ-irradiation of solution (Fig. 6, curves 4, 5, and 6) leads to the reduction of Cu+ ions to a metal. In this case, the absorption band of silver nanoparticles shifts from 400 to nearly 370 nm and the absorption appears in the 250–350 nm range and at wavelengths longer than 450 nm. The blue shift of the silver band indicates that Ag nanoparticles are in contact with COLLOID JOURNAL

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formed copper particles and conduction electrons are transferred from copper to silver. This is the situation that is observed for bimetallic AgcoreCushell nanoparticles [5, 6]. The pumping of electrons into silver nanoparticles was done previously by the discharge of hydrated electrons and organic radicals on these particles [8, 9]. A similar process was more clearly put into effect upon the direct charging of silver on the cathode [10]. In all of the aforementioned cases, the electron polarization of silver particles was accompanied by the shift of the absorption band assigned to surface plasmons to the UV region (blue shift). Changes in the spectrum observed at wavelengths longer than 450 nm are associated with the formation of metal copper. The fact that there is no distinct absorption band with the maximum at 565 nm (inherent in copper nanoparticles) is worth mentioning. We believe that this also counts in favor of the fact that, in the process of reduction, copper is preferably deposited on silver nanoparticles. However, the thickness of the formed shell does not exceed about two to four atomic layers of metal. This observation was made when studying changes in the absorption spectra of AgcoreCushell nanoparticles with an increase in the thickness of the copper shell [5, 6]. Theoretical calculations confirm that the band at 565 nm appears at a copper-shell thickness of about 1 nm, which also corresponds to approximately four copper monolayers [6]. Thus, it can be concluded that, upon the simultaneous reduction of Ag+ and Cu2+ ions, silver nanoparticles are formed first, followed by the appearance of metallic copper, which covers the surface of silver. That is, the mechanism of the formation of mixed (copper and silver) nanoparticles with the core/shell structure is preferably implemented. Apparently, this mechanism is also common for other pairs of metals with substantially differing potentials; upon reduction, more-precious metal is precipitated first, followed by the precipitation of less precious metal. In this case, the first metal is partially covered with the second metal. CONCLUSIONS Thus, the interaction between colloidal copper and silver ions leads to the reduction of ions and the formation of silver nanoparticles. Most likely, the copper–silver contact pair is formed at the intermediate stage. The reduction of Cu2+ ions by (CH3)2C•OH radicals, which cannot reduce copper to Cu0 atoms, leads to the formation of bimetal AgcoreCushell nanoparticles in the presence of seeding silver nanoparticles. Upon the simultaneous reduction of Ag+ and Cu2+ ions with the participation of hydrated electrons, which can reduce copper to Cu0 atoms, nanoparticles with the core/shell structure are also formed preferably. Note that, upon the simultaneous reduction of Au–Pd [11] and Au–Pt [12–15] ion pairs, nanoparticles with the core/shell structure are COLLOID JOURNAL

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Absorption 1.6

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Fig. 6. Absorption spectra of the solution containing 1 × 10−4 M AgClO4, 2 × 10–4 M Cu(ClO4)2, 1 × 10–3 M PEI, and 0.1 M isopropanol: (1) prior to irradiation and after irradiation for (2) 3, (3) 9, (4) 21, (5) 27, and (6) 42 min, respectively. The inset is the spectrum obtained by the subtraction of spectrum 1 from spectrum 3.

also formed, the cores of which are formed by moreprecious metal. Alloys of precious metals can also be formed (see monograph [1] and relevant references). The structure of mixed nanoparticles is determined by the type of reductant used, the ratio between potentials, the concentrations of relevant metals, and the rates of the reduction and formation of nucleation centers, as well as by their nature. The studies of the copper–silver system are mainly associated with the preparation of a mixture of nanoparticles or alloys on a carrier [16–20]. Single copper and silver phases were detected in various preparation procedures. Note that, by analogy with absorption bands of silver and gold in the visible spectral region, the absorption band of colloidal copper at 565 nm is very often associated with plasmon absorption. Actually, this is not the case. The positions of the absorption maxima for silver and gold actually correspond to wavelengths for which the relationship ε1 = –2n0 (ε1 is the real part of the permittivity of metal and n0 is the refractive index of solvent) is fulfilled. This fact agrees rather well with the requirement for the implementation of the resonance absorption of surface plasmons. According to [21, 22], for copper at wavelength of 565 nm, ε1 = –5.94n0. This circumstance definitely indicates that the absorption of copper is caused to a considerable extent by the interband transfer of electrons. ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research, project no. 07-0300383.

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