J. Phys. Chem. 1994, 98, 6931-6935
6931
Radiolytic Formation of Colloidal Tin and Tin-Gold Particles in Aqueous Solution Arnim Henglein' and Michael Giersig Hahn-Meitner-Institut, Abteilung Kleinteilchenforschung, 14109 Berlin, FRG Received: January 4, 1994; In Final Form: April 20, 1994"
Colloidal tin is formed in y-irradiated solutions of SnC12, which also contain 0.5 M propanol-2 and 5 X 10-4 M polyethyleneimine. The reduction occurs in the first stages of irradiation by hydrated electrons. In the later stages, when some colloidal particles have been formed, the (1-hydroxymethy1)ethyl radicals, (CH&COH, which as well as the hydrated electrons are primary radiolysis products, contribute to the reduction. The absorption spectrum of Sn particles, which are essentially smaller than the wavelengths of light, is reported. In the presence of colloidal gold particles, the reduction of Sn(I1) by organic radicals is strongly enhanced. The effect is ascribed to the cathodic polarization of the Au particles by electron transfer from the radicals and subsequent reduction of Sn(I1) directly on the surface of the Au particles. The optical changes accompanying Sn deposition are also described. For small deposits, the plasmon band of gold is strongly damped and only part of the deposited tin can be reoxidized by oxygen. When larger amounts of tin are deposited, alloying occurs to a great extent.
Introduction
Radiolytic reduction of metal ions has frequently been used to prepare aqueous colloidal solutions of silver,' gold,2 iridium,3 platinum? t h a l l i ~ m ,cadmium? ~ lead,' nickel? copper? and bimetallic particles.*b The hydrated electrons generated in the radiolysis of aqueous solutions are able to form abnormal valence states of the metal ions, and the colloidal particles are produced in various dismutation and coalescence reactions of these states.1° In such radiolytic reduction experiments, an alcohol is added in order to scavenge the oxidizing hydroxyl radicals which are generated together with the hydrated electrons. The organic radicals produced in the reactions of OH with the alcohol often contribute to the reduction of the metal ions. For example, when propanol-2 is present, the strongly reducing (1-hydroxymethy1)ethyl radicals are formed:
OH + (CH,),CHOH
-
H 2 0 + (CH,),COH
(1)
Colloidal tin has not yet been prepared by this method. In fact,very littleis known either about theradiolysis of Sn2+solutions or about the colloidal state of tin as prepared by other methods. In previous radiolytic investigations, the oxidation of Sn(I1) to Sn(1V) was studied," and the rates of reduction of Sn022- and Sn032- by the hydrated electron were measured;l2 colloid formation was not reported in these studies. Whereas the absorption spectra of colloids of the noble metals have been investigated for a long time, the spectra of non-noble metals are less known. The colloidal particles of the non-noble metals often have a strong tendency to form clusters. In order to compare the spectrum of a metal colloid with the spectrum expected from Mie calculations,it is desirable to prepare spherical particles with a diameter substantially smaller than the wavelengths of light. Such comparisons have recently been made for various non-noble metals.l0 The methods of preparation of colloidal tin reported in the past do not yield particles with a defined structure. Colloidal tin in aqueous solution is formed upon the action of ultrasound on small metal pieces in water13 and in the electrolysis of alkaline solutions on tin e1e~trodes.l~ Organic solutionsof tin in alcohol, ether, and benzene are obtained in the reduction of SnCl2 by a solution of phosphorus in CS2.I5 As the significant absorptions of tin occur in the UV close to 200 .Abstract published in Advunce ACS Abstracts. June 1, 1994.
0022-365419412098-6931$04.5010
nm, organic liquids are not useful solvents for the spectrophotometric measurements. In the present paper, colloid formation in the reduction of SnCl2 in water by 7-rays is reported. Experiments are also described in which the reduction was carried out in the presence of colloidal gold particles to obtain composite Sn-Au particles. ExperimentalSection
A stock solution of 1 X 10-2M SnCl2 was prepared by bubbling water with argon and adding solid SnC1y2H20 (Merck, p.a.). This solution was slightly turbid because of the partial hydrolysis of Sn2+,agglomerated particles such as (SnCIOH), being present. The vessel containing the stock solution carried a side arm with a septum through which aliquots could be taken without bringing the solution into contact with air. A 50 cm3portion of a solution containing propanol-2 and poly(ethy1eneimine) (Fluka Chemie) was deaerated in an irradiation vessel by bubbling with argon. The vessel had a side arm carrying a 0.5 cm optical cuvette for measuring the absorption spectrum without exposing the solution to air and a septum. A 1 cm3 portion of the stock solution was added through the septum to obtain a Sn(1I) concentration of 2 X 10-4 M in the solution. The 7-irradiation occurred in the field of a 6oCosource. The amount of reduced tin was determined by adding 10-2 M NaOH and 5 X 10-3 M methylviologen (MV2+, 2,2-dimethylpyridiniumdichloride) to the irradiated solution and measuring the 600 nm absorption of the half-reduced methyl viologen, MV+, which is formed in the reaction
Sn
+ 2MV2++ 20H--
Sn(OH),
+ 2MV+
(2)
With the known absorption coefficient of MV+ (1.2 X 104 M-1 cm-I), the concentrationof reduced tin could readily be calculated. The gold solution was prepared by the reduction of HAuC14 by sodium citrate as described by Turkevich et a1.I6 It contained gold in an overall concentration of 3.2 X 10-4 M. The mean diameter of the gold particles was 190 A. Most of the ionic compounds in the solution were removed by treatment with Amberlite (MB1, Sigma Chemie). A Phillips CM12 microscope operating at 120 kV, equipped with an EDAX 9800 analyzer, was used for transmission electron microscopy. Samples for electron microscopy were prepared on a carbon-coated copper grid in a nitrogen-filled glovebox; the dried grid was then transferred into the vacuum holder (Gatan Model 648) from the glovebox to the microscope. 0 1994 American Chemical Society
6932 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
Henglein and Giersig 2.0
2.0
0 ..
-E
2 0 u Q
u
g 1.0
c
em 1.0
z
0
In
v)
n m
n
0
0.
X Inml
1
Figure 1. y-Irradiation of a solution containing 2 X 1W M SnC12,0.5 M propanol-2, and 4 X l e M poly(ethy1eneimine). Spectrum before irradiation (0) and after 3 h. Dase rate: 1.9 X lo5rad/h. The spectrum M methylviologen is also M NaOH and 3 X after addition of
shown.
2.0
10
400 600 iInml
Figure 3. Absorption spectrum of a solution containing 2 X lW M SnC12,0.5 M propanol-2, 1.5 X lo" M colloidal gold, 5 X 1W M poly(ethyleneimine), and 2.5 X 10-2 M N20 before and after 15 min of y-irradiation. The figurealsocontainsthe MV+absorptionbandobtaincd after addition of methylviologen to the irradiated solution.
0.6
E z
f
I
-
-1.0 E
L"
ooL
I
4: 10 =
2
u
G - 3 0 3
0.2
c
2 0 0.6
0
t [minl
t
n
Figure 2. Concentration of reduced tin as a function of irradiation time. Curve 1: concentrationsas in Figure 1. Curve 2: 0.2 M acetone present additionally.
0.4 0.2
ReSults Figure 1 shows the absorption spectrum of a SnCl2 solution before and after 3 h of y-irradiation. At longer irradiation times, the spectrum showed no further change. Spectrum0 is attributed to the complexed Sn(I1) species present in the solution, and the spectrum after 3 h is attributed to the colloid formed. The electron microscope pictures revealed well-separated crystalline tin particles with a rather broad size distribution, 60-200 A. The spectrum of colloidal tin has a maximum at 200 nm. The figure alsocontains the spectrumof the MV+formedafter addition of methylviologen to theirradiatedsolution. From theabsorbance of MV+, and taking into account a 5% dilution of the solution due to the addition of the MV2+ solution, an absorption coefficient of 2.4 X 104 M-I cm-1 at 200 nm of the tin colloid is obtained. Figure 2 shows the tin concentration (as determined by the MV2+ method) as a function of the irradiation time. In the experiment of curve 1, the solution contained only propanol-2 as organic additive, and in the case of curve 2, acetone was present as the second additive. Acetone is known to be an efficient scavenger for hydrated electrons. Organic radicals are produced in the reaction
e,,+
(CH,),CO
+ H+-(CH,)2COH
(3)
Thus, the organic radicals are the only species that can reduce Sn(I1) in the presence of acetone. It can be seen that an induction period exists in both cases in Figure 2. The induction period is longer for curve 2, and the rate of Sn formation at longer times is very much smaller than for curve 1. A radiation chemical yield G(Sn) = 2.6 atoms per 100 eV absorbed radiation energy is calculated from the slope in the steepest part of curve 1, whereas the yield is only 0.1/100 eV for curve 2. It is concluded that a good yield of tin reduction is achieved only when the hydrated electrons react with the Sn(I1) species.
'3kO
4b0
SbO X Inml
6b0
7O :
Figure 4. Absorption spectrum of the solution of Figure 3 before and after 30 min of y-irradiation (upper part) and at various times of aging under air (lower part).
In the experiment of Figure 3, the solution contained 1.5 X 1 V M colloidal gold, and it was saturated with nitrous oxide. N2O is a scavenger for hydrated electrons: N,O
+ e,, + H,O
-
N2
+ OH + OH-
(4)
The OH radicals generated in this reaction react with the alcohol (eq 1) to form additional organic radicals. Thus, the radicals are the only reducing species under theseconditions. Figure 3 contains the absorption spectrum of the solution before and after 15 min of irradiation. The 520 nm gold band can be seen to be damped by the irradiation. This is attributed to the deposition.of tin metal on the colloidal Au particles. Methylviologen was added after the irradiation. The absorption band of MV+ formed is also shown in the figure. From the absorbance of MV+, it can be calculated that 4 X le5M tin had been formed after 15 min. This corresponds to a radiation chemical yield of tin formation of 2.3 atoms/100 eV. A comparison with the results in Figure 1, curve 2, shows that the reduction of Sn(I1) by organic radicals occurs much more efficiently in the presence of gold particles. The absorption spectrum of gold particles carrying a tin deposit changes with time after exposure of the solution to air. This can be seen from Figure 4. In the upper part of the figure, the spectrum of the solution before and after deposition of tin by 30 min of irradiation is shown; in the lower part, the spectrum at various times after the admission of air can be seen. The 520 nm gold absorption band slowly recovers within about 1 day; at longer
Radiolytic Formation of Sn and Sn-Au Particles 2.0
t!
m c n
;;1.0 v)
n rg
I\ \
\
\
