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J. Phys. Chem. B 1999, 103, 9302-9305
Chemisorption Effects on Colloidal Lead Nanoparticles Arnim Henglein Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: April 14, 1999; In Final Form: June 10, 1999
A stable aqueous lead sol (10 nm particles) is formed upon the γ-irradiation of Pb(ClO4)2 in the presence of (poly)ethyleneimine. Lead nanoparticles have an absorption band at 218 nm with ) 3.2 × 104 M-1 cm-1; the band appears at the wavelength that is expected for a surface plasmon oscillation. The changes in the shape of the absorption band, which occur upon the interaction of the nanoparticles with various solutes, are described and interpreted. Oxygen, nitrous oxide, carbon tetrachloride, and chloroform oxidize colloidal lead particles to Pb2+. Carbon disulfide oxidizes only surface lead atoms to yield a layer of PbS precursor. The oxidation of lead particles by the ions of noble metals is investigated for Ag+ and Cu2+. Silver ions oxidize lead nanoparticles incompletely, which is explained by the formation of mixed Ag-Pb structures. Cu2+ ions also do not completely oxidize lead particles, although Cu particles with a low Pb content can be obtained.
Introduction Little is known about colloidal lead nanoparticles. Particles of sufficiently good quality, whose absorption spectrum could be compared to a calculated spectrum, have been made by radiolytic reduction of lead ions in aqueous solution.1,2 In the present paper, the radiolytic method is further described, and the interaction of the particles with various oxidizing solutes is investigated. These interactions are accompanied by changes in the absorption spectrum of the lead particles. The changes may be caused by the formation of a surface layer (chemisorption), by full oxidation of the metal (corrosion), or by deep penetration of a solute into the particle (such as alloy formation). Experimental Section Solutions of Pb(ClO4)2, which also contained 2-propanol and a polymer stabilizer, were irradiated with 60Co γ-rays in a closed glass vessel. They were bubbled with argon or nitrous oxide prior to irradiation. The vessel carried a sidearm with an optical cuvette (the optical path is given in the figures on the abscissa scale of the spectra). Another sidearm carried a septum. Optical spectra could therefore be measured and substances be added via a syringe without exposure of the solutions to air. Electron microscopy was carried out with a Hitachi instrument, Type H 600 (80 kV). The grids were prepared in a glovebox under pure nitrogen. The reduction of Pb2+ occurs by the free radicals which are generated by γ-irradiation. The elementary reactions are wellknown. The hydrated electrons from the radiolysis of the aqueous solvent react according to
eaq- + Pb2+ f Pb+
(1)
and the OH radicals and H atoms from water radiolysis are scavenged by the alcohol:
OH (H) + (CH3)2CHOH f H2O (H2) + (CH3)2COH
(2)
The organic radicals produced in these reactions also reduce lead ions:
(CH3)2COH + Pb2+ f Pb+ + (CH3)2CO + H+
(3)
Figure 1. Absorption spectrum of a 1.0 × 10-4 M Pb(ClO4)2 solution before and after irradiation for 55 min at a dose rate of 9.3 × 104 rad/h under argon. Longer irradiation did not change the spectrum of the colloid; it is concluded that the reduction was complete. [PEI]: 1.0 × 10-4 M.
The processes that lead to colloidal particles are known in lesser detail. Zerovalent lead is formed via dismutation of Pb+
2 Pb+ f Pb0 + Pb2+
(4)
and larger particles via coalescence reactions of lead atoms:3
nPb0 f Pbn
(5)
Results Absorption Spectrum of Colloidal Lead. Figure 1 shows the absorption spectrum of a solution of 1.0 × 10-4 M Pb(ClO4)2, 1.0 × 10-4 M (poly)ethyleneimine (PEI), and 0.3 M 2-propanol before and after irradiation. Pb2+ in the presence of PEI absorbs at 207 with ) 9.3 × 103 M-1 cm-1. The solution of the colloid is almost colorless. Its spectrum contains a relatively narrow band which peaks at 218 nm; the absorption coefficient is 3.2 × 104 M-1 cm-1. A similar spectrum has been observed previously.1 (Poly)ethyleneimine turned out to be the best stabilizer among various polymers, which have little absorption in the UV: (poly)vinyl sulfate, (poly)acrylamide (10% carboxylated), (poly)vinyl alcohol, and(poly)phosphate. The absorption band of the
10.1021/jp991218e CCC: $18.00 © 1999 American Chemical Society Published on Web 07/20/1999
Colloidal Lead Nanoparticles
Figure 2. Absorption spectrum of a 1.0 × 10-4 M lead solution under N2O at various times of aging. [PEI]: 1.0 × 10-4 M.
lead particles became broader in the above order. The rate of reduction was the same for all stabilizers studied. A plasmon excitation occurs in a metal nanoparticle at the wavelength, where the negative real part of the dielectric constant is equal to twice the square of the refractive index of the solvent.4,5 From the dielectric data of lead6 one finds that this condition is fulfilled at 217 nm, i.e., at a wavelength which is very close to the observed position of the maximum in Figure 1. It is concluded that a plasmon oscillation contributes to the absorption, although the oscillation is probably strongly dampened, since the imaginary part of the dielectric constant is rather large at 218 nm.6 Particles with diameters around 10 nm were seen in the electron microscope. However, they melted and evaporated within seconds under the microscope beam; it was therefore not possible to support the optical findings by reliable micrographs. When the colloidal solution was kept under argon, no noticeable changes in the absorption spectrum occurred for many days. Lead is a slightly electronegative metal, and therefore a certain amount of dissolution to yield H2 would be expected. It seems that the overpotential for H2 production on the colloidal particles is high enough to retard this process substantially. Reaction of the Lead Particles with Nitrous Oxide. Figure 2 shows the absorption spectrum of a colloid which had been formed under an atmosphere of nitrous oxide. N2O scavenges hydrated electrons: N2O + eaq- + H2O f N2 + OH + OH-. The OH radicals generated produce reducing organic radicals, eq 2; thus, the same amount of reducing equivalents is available for Pb2+ reduction as in the absence of N2O. However, the absorption maximum right after the irradiation is less intense by 20% than in the irradiation under argon. This is attributed to the fact that N2O reacts with the Pb particles already during irradiation. The spectrum slowly fades away upon aging of the solution, the maximum shifting to shorter wavelengths till only the 207 nm band of Pb2+ is present. Reaction with Carbon Tetrachloride and Chloroform. Addition of 10-3 M CCl4 to a 1.0 × 10-4 M Pb colloid leads to the disappearance of the 218 nm absorption band within 30 s. After this time, the solution exhibits only the 207 nm band of Pb2+. Chloroform exerts a similar effect, although the time for corrosion is about 50 times longer. Reaction with Carbon Disulfide. Figure 3 shows the absorption spectrum of a 2.5 × 10-4 M lead colloid before and after the addition of various amounts of CS2. It is seen that the absorption band of lead decreases in intensity below 260 nm
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9303
Figure 3. Absorption spectrum of a 2.5 × 10-4 M Pb sol before and after addition of various concentrations of carbon disulfide. The spectrum was always measured at 10 min after an addition. [PEI]: 2.5 × 10-4 M.
Figure 4. The 207 nm absorption of CS2 and decrease in 218 nm absorption of Pb as functions of the CS2 concentration. (The absorption band of CS2 slightly extends to 218 nm. Knowing the spectrum of CS2, the 218 nm Pb absorption could be corrected for a small CS2 contribution.) PB and PEI concentrations as in Figure 3.
and slightly increases at longer wavelengths. CS2 has an intense absorption band at 207 nm ( ) 7.2 × 104 M-1 cm-1).7 This band does not show up in the first additions in Figure 3. In Figure 4, the absorbance of CS2 is shown as a function of the CS2 concentration. The straight line obtained reveals that there is practically no free CS2 in the solution below 27 µM; i.e., all the added CS2 had reacted with the colloid. The figure also shows the decrease in the lead absorption at 218 nm. This decrease tends toward a limiting value, which is interpreted as a saturation of the Pb particle surface with chemisorbed CS2. In the experiments of Figures 3 and 4, CS2 concentrations much smaller than that of Pb were used. In the experiment of Figure 5, a substantially higher CS2 concentration of 2.0 × 10-3 M was applied (curve 1). This solution did not undergo any further changes upon standing for several days. The solution was finally bubbled with argon to remove excess CS2 and exposed to air, which resulted in a decoloration within 1 min until a weak yellow color remained; the spectrum (curve 2) contained now the 207 nm absorption band of Pb2+ and a background absorption extending into the visible. The species responsible for the background absorption is very sensitive to UV light: exposure to the light of a 150 W xenon lamp (30 cm distance) led to the bleaching of this absorption, and only Pb2+ absorption was still present.
9304 J. Phys. Chem. B, Vol. 103, No. 43, 1999
Henglein
Figure 5. Spectrum of a 2.5 × 10-4 M Pb sol: (0) before and (1) after addition of 2.0 × 10-3 M CS2 (the steep absorption increase below 220 nm is due to CS2); (2) after exposure to air and removal of CS2. The UV illumination of the solution (2) leads to rapid disappearance of the broad absorption. [PEI]: 2.5 × 10-4 M.
Figure 7. Spectrum of a 2.5 × 10-4 M lead solution before (0) and after addition of Cu2+. Dashed line: extrapolation of Cu absorption. [PEI]: 2.5 × 10-4 M.
Figure 7 shows the optical changes which occur upon the addition of Cu(ClO4)2 to a lead sol. In the beginning, the changes are dominated by the decrease in the strong lead absorption, and little is seen of the absorption of copper, which is formed in the reaction
Pbn + nCu2+ f nPb2+ + Cun
Figure 6. Spectrum of a 2.5 × 10-4 M lead solution before (0) and after addition of various amounts of Ag+: (1) 2.0 × 10-4 M; (2) 4.0 × 10-4 M; (3) 5.8 × 10-4 M; (4) 6.8 × 10-4 M). (5) The solution was finally exposed to air. [PEI]: 2.5 × 10-4 M.
Reaction with Silver and Copper Ions. As an electronegative metal, lead should be readily dissolved by the ion of the more electropositive silver:
Pbn + 2nAg+ f nPb2+ + Ag2n
(6)
Colloidal silver has an intense plasmon absorption band in the 380-400 nm region ( ) 1.5-2.8 × 104 M-1 cm-1) and a much weaker band in the 200-300 nm region. Figure 6 shows the optical changes that occur when different amounts of AgClO4 were added to a 2.5 × 10-4 M lead sol. The changes always took place within seconds after the addition, and no more changes were then seen within 1 day of aging. Instead of the immediate development of the strong plasmon band of pure silver at 380-400 nm, a weak absorption increase around 350 nm is seen. At the higher Ag+ concentrations, the band moves to 375 nm. The 218 nm absorption band of lead decreases, but it does not decrease to zero when an excess over the stoichiometric amount of added Ag+ is present. In fact, when the solution was finally exposed to air, a substantial decrease in absorption occurred, which indicates that a lot of nonoxidized lead was still present. The final silver band is very broad; it peaks at 375 nm, i.e., still at a shorter wavelength than expected for pure silver particles.
(7)
This is due to the low specific absorption of colloidal copper, but probably also to the formation of intermediate Pb-Cu structures. The changes always occurred within less than 1 min after the addition of Cu2+, and then remained for days. When an excess of copper ion was present, the spectrum contained the Pb2+ absorption above a background which is mainly due to copper, as the typical copper absorption around 560 nm was also present. From the intensity of the 207 nm band of Pb2+, one could calculate that at least 5-10% of lead was not oxidized (in the determination of the Pb2+ absorption, one has to correct for the underlying copper absorption: the dashed line in Figure 7 is an extrapolation of this background, which might be even too conservative, as copper is expectd to have a rising absorption toward shorter wavelengths5). Discussion Various reactants were added to colloidal lead that have oxidizing properties, but quite different reaction paths were observed: Neat Oxidation by O2, N2O, CCl4, and CHCl3. Oxygen oxidizes lead to yield Pb2+. Nitrous oxide has the same effect, although the reaction occurs slowly:
nN2O + Pbn + nH2O f nN2 + nPb2+ + 2nOH-
(8)
Dissolution by nitrous oxide has also been observed for colloidal cadmium.8 Oxidation to Pb2+ is also initiated by organic chlorides, CCl4 and CHCl3. The reaction rates are remarkably high: in both cases, the rate of corrosion is greater than with N2O. Reaction with CCl4 has previously been observed for cadmium, zinc, and tin particles.9 Even silver particles have been found to be attacked by CCl4, although the reaction was much slower than with the electronegative metals and incomplete.10 An electrochemical mechanism, i.e., two-electron transfer to the chlorides, is postulated, the overall reaction being
Pbn + nCCl4 + nH2O f nPb2+ + nCHCl3 + nOH- + nCl(9)
Colloidal Lead Nanoparticles
Figure 8. Illustration of interaction of CS2 with lead nanoparticles.
The Fermi level in the colloidal Pb particles must be positioned at sufficiently low potential to allow this reaction to occur. The polarographic two-electron-reduction wave of CCl4 (to yield CHCl3) occurs at -0.48 V vs NHE.11 This wave is kinetically hindered, as one calculates a more positive potential of +0.32 V from the well-known thermodynamic data of the species in eq 9.12 In the case of chloroform, the two-electron reduction in polarography (to yield CH2Cl2) is even more hindered, as the wave appears at -1.4 V (vs NHE), whereas the thermodynamic potential is calculated as +0.22 V. The stronger hindrance for reduction of CHCl3 explains why CHCl3 is more slowly reduced than CCl4 in the present experiments. Incomplete Oxidation by CS2. The oxidation of colloidal lead by carbon disulfide occurs in a complex manner. Figure 8 illustrates the proposed mechanism. In the beginning of CS2 addition to a lead sol (up to 27 µM in Figure 4), the absorption band of CS2 is not seen. It is conluded that all the CS2 is adsorbed and experiences a chemical transformation. As has already been outlined in the previous work on the adsorption of CS2 on colloidal silver,7 a C-S bond is broken. Thus, the lead nanoparticles are covered with a layer of sulfur, or, more specifically, with a precursor structure of lead sulfide. The PbSlike layer is practically monoatomic, the amount of chemisorbed sulfur being about 15% of the lead present, which roughly equals the percentage of surface atoms in 10 nm particles. At higher CS2 concentrations, there is no additional oxidation of lead. However, oxidation of the intact lead nucleus takes place, when the solution is exposed to air. The solution then contains Pb2+ ions and PbS particles. In fact, the unstructured background absorption (in curve 2 of Figure 5) can be attributed to PbS: a 3 × 10-5 M PbS sol, as prepared from Pb(ClO4)2 plus Na2S in the presence of 2.5 × 10-4 M PEI, had the same absorption. Colloidal PbS is known to undergo rapid photoanodic dissolution;13 this explains the last step in the experiment of Figure 5, i.e., the removal of the unstructured background absorption by UV illumination. The decrease in the lead absorption band upon the formation of a PbS-like layer (Figure 4) is possibly due to damping of the plasmon oscillation. Typical effects of this kind have been investigated for silver particles.7 Incomplete Oxidation by Ag+ and Cu2+. If the reaction of eq 6 occurred literally, one would deal with the conversion of particles of atom number n into particles of number 2n according to the stoichiometric relationship, or speaking in terms of macromolecular chemistry, one would have an example of a polymer-analogous process (in contrast to processes of the type Pbn + 2nAg+ f nPb2+ + Agx + Ag(2n-x)). Unfortunately, the reaction appears rather complex, as it does not lead to pure silver particles. The spectral changes are interpreted as the formation of mixed bimetallic particles. The particles present after Ag+ was added in excess still have a strongly damped silver plasmon band, and much more absorption in the UV than pure silver particles. Silver ions possibly penetrate deeply into the lead
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9305 particles to form kind of an alloy. In a previous study, the reaction of Ag+ with gold (core)-lead (shell) particles has been investigated.14 It was found that Ag reached the gold core already in early stages of reaction, i.e., when much less Ag+ than the stoichiometric amount had been added; this also indicates a fast migration of silver in lead. The difference in redox potentials of the lead and silver redox systems is 0.9 V; i.e., there is a strong driving force for reaction 6 from the point of view of thermodynamics. The observed incompleteness of reaction 6 may be understood in terms of a strong shift of the Fermi level to a more positive potential when silver is taken up by the lead particles. This decreases the driving force for further oxidation of lead. The oxidation of lead particles by the copper ion is also incomplete as the optical changes indicate that less than the stoichiometric amount of Pb2+ is formed. However, the remaining lead in the final copper particles amounts to only 5-10%. Apart from this deficiency, reaction 7 may be used for the preparation of copper particles. It was not yet possible to obtain more detailed information about the intermediate Pb-Ag or Pb-Cu structures by electron microscopy, which was available in the present studies, the reasons being the extreme sensitivity of the particles toward air and the heating by the electron beam in the microscope. Studies with more sophisticated equipment, such as higher detection sensitivity in the electron microscope and sample transfer from glovebox to microscope under exclusion of air, are desirable. As far as the author knows, no systematic attempts have yet been made to study metal-metal-ion interactions on the nanoscale as in the examples of eqs 6 and 7.15 It might be interesting to study other metal combinations and elucidate the structure of the intermediate stages of reaction. Acknowledgment. The author thanks Prof. Dan Meisel for valuable discussions. The work described herein was supported by the Office of the Basic Energy Sciences of the U.S. Department of Energy. This is Contribution No. NDRL 4122 from the Notre Dame Radiation Laboratory. References and Notes (1) Henglein, A.; Mulvaney, P.; Holzwarth, A.; Sosebee, T. E.; Fojtik, A. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 754. (2) Mulvaney, P. Langmuir 1996, 12, 788. (3) Henglein, A.; Janata, E.; Fojtik, A. J. Phys. Chem. 1992, 96, 4734. (4) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Series in Materials Science 25; 1995. (5) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (6) Lemmonnier, J. C.; Priol, M.; Robin, S. Phys. ReV. B 1973, 8, 5452. (7) Henglein, A.; Meisel, D. J. Phys. Chem. 1998, 102, 8364. (8) Henglein, A.; Gutie´rrez, M.; Janata, E.; Ershov, B. G. J. Phys. Chem. 1992, 96, 4598. (9) Boronina, T.; Klabunde, K. J.; Sergeev, G. EnViron. Sci. Technol. 1995, 29 (9), 1511. (10) Henglein, A. Chem. Mater. 1998, 10, 444. (11) Kolthoff, I. M.; Lee, T. S.; Stocesova, D.; Parry, E. P. Anal. Chem. 1950, 22, 521. (12) Landoldt-Bo¨rnstein Zahlenwerte und Funktionen, Teil 4. Kalorische Zustandsgro¨ssen; Springer: Berlin, 1961. (13) Gallardo, S.; Gutie´rrez, M.; Henglein, A.; Janata, E. Ber. BunsenGes. Phys. Chem. 1989, 93, 1080. (14) Henglein, F.; Henglein, A.; Mulvaney, P. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 180. (15) The titration of copper colloid with silver ions was reported: Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J. Chem. Phys. Lett. 1992, 191, 351.
