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J. Phys. Chem. B 2000, 104, 6138-6142
Formation, Absorption Spectrum, and Chemical Reactions of Nanosized Colloidal Cobalt in Aqueous Solution B. G. Ershov,†,‡ N. L. Sukhov,‡ and E. Janata*,† Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany, and Institute of Physical Chemistry, Russian Academy of Sciences, 31 Leninsky Prospect, 117915 Moscow, Russian Federation ReceiVed: February 17, 2000; In Final Form: April 20, 2000
The radiation-chemical reduction of Co2+ ions in aqueous solution of Co(ClO4)2 and sodium formate is described. Stable metal sols containing spherical particles with a diameter of 2-4 nm are formed under γ-irradiation in the presence of polyacrylate as a stabilizing additive. Their optical absorption increases smoothly in the UV region without a maximum up to 200 nm (200 ) 1.3 × 104 M-1 cm-1). Pulse radiolysis is used to study the reduction of Co2+; the subsequent formation of colloidal Co occurs via an autocatalytic mechanism. The electrochemical potentials of Co2+/Co0 and Co+/Co0 are determined to be -2.3 and -2.9 V, respectively.
Introduction Theoretical calculations by Creighton and Eadon1 have shown that ultrafine cobalt particles in aqueous solution should have an optical absorption that increases smoothly in the UV region without a maximum up to 200 nm. The preparation of cobalt nanoparticles using the chemical reduction of Co2+ ions by NaBH4 in inverse micelles (water microdroplets dispersed in isooctane and stabilized by sodium diethylhexylsulfonil succinate) has recently been reported.2 In this investigation, optical measurements were only carried out at wavelengths above 250 nm. The absorption of these metal particles increased monotonically as the wavelength decreased. An increase in water content resulted in a decrease in particle size and an increase in the content of cobalt oxide, which exhibited an additional band at 350 nm. The magnetic properties of cobalt nanoparticles both in the self-assembled state and isolated in solution were also compared.3 In this method of preparation, the presence of some boron in the cobalt particles cannot be ruled out. The present paper describes the radiolytic preparation of small cobalt particles under conditions where no disturbing byproducts are formed. The hydrated electrons and the carboxyl radicals generated in the radiolysis of aqueous solution of formate are used for their strong reducing power. In the absence of a stabilizer, colloidal cobalt is not stable; it has the tendency to agglomerate with time. In the presence of sodium polyacrylate as a stabilizer, well-separated spherical particles with a diameter of 2-4 nm are formed. The pulse radiolysis method was used to investigate the reactivity of Co2+ toward the hydrated electron and CO2- radical. Experimental Section The aqueous solutions were exposed to the γ-rays of a 60Co source. The irradiation vessel had a sidearm carrying a 1.0 or 0.5 cm cuvette for optical measurements and a septum for injecting reactants into the irradiated solution without bringing the solution into contact with air. Co(ClO4)2 from Alfa was used while sodium polyacrylate was obtained from Aldrich. The †
Hahn-Meitner-Institut. Russian Academy of Sciences. * To whom correspondence should be addressed. E-mail:
[email protected].
‡
average molecular weight of the sodium polyacrylate is 2100. Its concentration in solution is expressed in moles of monomeric units. The solutions were prepared with triply distilled water and were deaerated by evacuation prior to irradiation. The volume of the irradiated sample was 10 mL. The irradiation of aqueous solution containing sodium formate results in the formation of hydrated electrons and CO2- radical ions, the latter due to the reaction of OH radicals and H atoms with HCOO-:
OH (H) + HCOO- f H2O (H2) + CO2-
(1)
The yield of the hydrated electrons is 2.6 per 100 eV of absorbed energy. The yield of H atoms is 0.6, and that of hydroxyl radicals is 2.7; if all H atoms and OH radicals react with HCOO-, the yield of CO2- is 3.3. The dose rate in all experiments was 1 kGy h-1. Thus, 4.2 × 10-6 M of hydrated electrons and 5.3 × 10-6 M of CO2- radical ions were generated during 1 min of irradiation. Samples for electron microscopic studies were prepared by placing a droplet of the sample solution onto a copper support, followed by drying in argon atmosphere. A Philips EM-301 transmission electron microscope was used. The amount of reduced cobalt metal was determined by adding methyl viologen to the solution under investigation, as described recently.4 The concentration of the reduced methyl viologen (MV+) was determined spectrophotometrically by recording the absorption of the MV+ radical cation (600 ) 1.2 × 104 M-1 cm-1). The hardware5 and the software6 of the pulse radiolysis facility ELBENA were described previously. Pulses of 3.8 MeV electrons and 0.1 µs duration from a van de Graaf accelerator were used. The optical signals are expressed as changes in OD divided by the concentration of hydrated electrons generated per pulse and by the optical path (1.5 cm), i.e., in units of (M-1 cm-1). The dose per pulse is given in terms of the concentration of hydrated electrons. Results Formation of Cobalt Sols. The γ-irradiation of deaerated solutions containing 2 × 10-4-5 × 10-2 M Co(ClO4)2, 10-310-1 M HCOONa, and 10-4-10-3 M polyacrylate results in
10.1021/jp000608u CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000
Colloidal Cobalt in Aqueous Solution
J. Phys. Chem. B, Vol. 104, No. 26, 2000 6139
Figure 1. Absorption spectrum before and at various times during γ-irradiation. Solution: 2 × 10-4 M Co(ClO4)2, 2 × 10-3 M HCOONa, and 2 × 10-4 M sodium polyacrylate. About 4.2 × 10-6 M of hydrated electrons and 5.3 × 10-6 M of CO2- radical ions are produced during 1 min of irradiation. The thickness of the optical cell is 1 cm.
the reduction of Co2+ ions. The solutions remain almost transparent and colorless during irradiation. Only when irradiated with an extremely high dose does the sample exhibit a faint yellow color. Figure 1 shows the absorption spectra before and after irradiation with various doses. The absorption increases monotonically toward shorter wavelengths; a small shoulder around 270 nm can also be observed. The optical density increases with an increasing dose while the shape of the absorption curve remains almost unchanged. Electron micrographs were taken from freshly prepared solutions and from solutions aged for several days. In both cases, the solution contained almost spherical particles with a diameter of 2-4 nm, which were well-separated from each other. These solutions were stable for a long time in the absence of oxygen. Colloidal cobalt is formed also in the absence of a stabilizer. Here, the solution becomes yellow-brown, and the intensity of the color increases with increasing irradiation time. The optical spectrum is characterized by a more intensive absorption in the visible region. The electron microscopic study shows the existence of cobalt particles with a wide size distribution. The average diameter of the particles ranges from 10 to 30 nm. However, much smaller particles (4-6 nm) as well as much larger ones and also agglomerates of these particles are present. At low concentration of Co(ClO4)2 (
