3434
J . Phys. Chem. 1993, 97, 3434-3436
Optical Spectrum and Some Chemical Properties of Colloidal Thallium in Aqueous Solution B. G. Ershovt and A. Henglein' Abteilung Photochemie, Hahn-Meitner-institut Berlin, 100 Berlin 39, FRG Received: November 10, 1992
Colloidal thallium is formed in the radiolytic reduction of T1+ ions in the presence of poly(ethy1eneimine) as stabilizer. The colloidal particles (20-30 A) form aggregates, whose optical absorption ranges over the whole visible range of wavelengths. The aggregates disintegrate into the small particles, when OH- ions are adsorbed and during their dissolution under the influence of H+ ions. The sol which is formed a t p H = 11 consists of the separated small particles, the absorption spectrum lying essentially in the U V as expected from the dielectric properties of the compact metal. 1.5
Introduction Colloidal thallium has been reported to be formed in the radiolytic reduction of TI+ ions in aqueous solution.l.* The spectrum of the colloid, which was stabilized by sodium dodecyl sulfate, contained a maximum in the 450-700-nm range depending on the degree of reduction by The maximum appeared already at 350 nm when the buildup of the colloid absorption was observed in a pulse radiolysis experiment.' In general, the absorption spectrum of metallic particles which are substantially smaller than the wavelength of light is not dependent on the particle size,3but often it is strongly dependent on the aggregation of particles. The tendency to agglomerate is quite different for the various metals and depends on the conditions of preparation. In the present paper, the preparation of well-separated small thallium particles is described, whose absorption spectrum is quite different from the spectra reported in the earlier publications. Spectrophotometric and electron microscopic measurements were carried out to obtain information about the relation between absorption and agglomeration. It has recently been shown in the case of colloidal cadmium that changes in agglomeration occur when the particles undergo redox reaction^.^ Similar observations are reported here for thallium.
zl.o A [nml
Figure 1. Absorption spectrum of a 1 X M TI+ solution before and at various times of y-irradiation. Dose rate: 4 X IO4 rad/h 0.1 M 2-propanol; 4 X M poly(ethy1eneimine).
ts
I
0
Ea 0.5
-0
al
Experimental Section y-Irradiation occurred in the field of a Co@source. The solution was deaerated by bubbling with argon prior to irradiation. The irradiation vessel carried a 1-cm optical cuvette for spectrophotometric measurements and a septum for injecting reactants to the irradiated solution without bringing the solution in contact with air. T1+ was reduced to yield T10 atoms by the hydrated electrons generated by radiation. The OH radicals which are alsogenerated by radiation were scavenged by 2-propanol
OH
0
L
+ (CH,),CHOH
-
H,O
+ (CH,),COH
(1)
The TlClOI solution contained poly(ethy1eneimine) as stabilizer for the colloid which was formed via agglomeration processes of the TI0 atoms. To prepare samples for electron microscopy, a drop of the metallic sol was placed onto a carbon-coated aluminum grid and allowed to dry. The dried grid was then transferred in a nitrogenfilled container to a Phillips CM12 microscope. All the sample preparation was carried out in a nitrogen-filled glovebox to prevent air oxidation of the samples. To whom correspondence should be addressed.
' On leave of absence from the Institute of Physical Chemistry. Academy of Sciences, Moscow, Russia.
0022-365419312097-3434304.00/0
i=
OO
50
100
t [mid Figure 2. Concentration of reduced thallium as a function of irradiation time.
Results The spectrum of a 1 X lo4 M TlC104 solution, which contains 0.1 M 2-propanol and 4 X 10-4 M poly(ethy1eneimine) is shown in Figure 1 at various times of y-irradiation. The sharp band at 2 16nm is due to the TI+ion. One can see how this band disappears until almost all T1+ ion has been consumed after 30 min. The spectrum of the colloid formed has first a shoulder at 340 nm, which then develops into a main maximum that shifts to longer and longer wavelengths. At irradiation times longer than 2-3 h, thespectrum no longer shifted. A weaker maximum isalso present at 270 nm. The concentration of reduced thallium was determined by injecting 2 X M methyl viologen, MV2+, to the irradiated solution and measuring the absorption at 600 nm of the halfreduced methyl viologen, which is formed in the reaction
nMV2+ + T1,
-
nMV+
+ nT1'
(2) Knowing theabsorptioncoefficientof MV+ (1.2 X 104M-' cm-I), 0 1993 American Chemical Society
Properties of Colloidal Thallium
The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 3435
b
a
C
Figure 3. Electron micrographs of the thallium particles present at different times of ?-irradiation. (a) 10 min; (b) 40 min; (c) 120 min.
0.6
2 0.4
E
m c n
m t
e 0.5
L
v) 0
0 v)
n
n
0.2
m
1 1 1 1 1 1 1 O200
400
600 X Inml
800
Figure 4. Spectrum of a colloid solution before (0) and after addition of 1 X 10-3and 2 X 10-3M NaOH. Solution as in Figure 1 ;2 h irradiation.
we can readily derivetheconcentrationof reduced thallium. From the initial slope of the curve in Figure 2 a radiation chemical yield of G = 1.6 atoms per 100 eV is calculated for the reduction of T1+. This value is significantly smaller than the yield of the hydrated electron (G(eaq-)= 2.7/100 eV) and of the organic radicals (eq 1) (G(R)= 3.2/100 eV. At 50 min, practically all the T1+ ions were reduced in the experiment of Figure 2. Experiments were also carried out with solutionswhich contained sodium formate as OH radical scavenger; in such solutions, C02radicals are known to be formed and these have strong reducing properties. G(T1O) was 4.1/100 eV under these conditions. Electron micrographs of the particles present after various times are shown in Figure 3. At 10min, well-separated particles, 20-30 A in diameter, are present. At longer times, when the T1+ ions are consumed to a large extent, larger species are present and these we interpret as aggregates of small particles (at 40 min 60-80 A diameter of the aggregates; at 120 min 100-200 A). In the experiment of Figure 4, a colloidal solution was prepared and NaOH was added. It can be seen that the430-nm absorption band of thecolloid decreased in intensity and that weaker maxima at 340 and 270 nm are present. In fact, the spectrum after NaOH addition resembles the spectrum of the thallium which is formed at shorter times of y-irradiation (compare to Figure 1). The electron microscopic investigation showed that smaller and wellseparated particles, 20-30 A in diameter, were mainly present after NaOH addition. Figure 5 shows the absorption spectrum of a thallium colloid before and after addition of 1 X le3M barium perchlorate. It can be seen that the absorption .band of the colloid is decreased
O2 0
600 800 X [nml Figure 5. Spectrum of a thallium colloid before (0) and after addition of M Ba(C10&. Colloid prepared as in Figure 4. 400
without a significant change in its shape and position. Addition of 1e2 M of the barium salt resulted in the immediate precipitation of the colloid. In both the NaOH and Ba(Cl04)~addition experiments, no T1+ ions were formed; i.e., the changes in the absorption band were entirely due to changes in the structure of the particles and not due to a consumption of reduced thallium. When TI+ was reduced by yirradiating a solution of pH = 11, a colloid was formed which had no significant absorption in the visible. The spectrum after complete reduction of 1 X 10-4 M T1+ is shown in Figure 6. One can see a small shoulder at 340 nm. Although the spectrum is corrected for the UV absorption of OH-, it is possibly not very accuratebelow 220 nm. The colloid formed at pH = 11 not only has a spectrum different from the colloid formed at natural pH (Figure 1); it was also found that it is formed with a radiation chemical yield, G = 3.9/100 eV, much greater than at pH 7. The thallium colloids are immediately oxidized upon exposure to air. Slow oxidation takes place when HC104 is added. As can beseen from Figure 7, the absorption band of thecolloid (prepared at natural pH) decreases with time but is also shifted to shorter wavelengths. Thus, added HC104 seems to combine the effects of the additives NaOH and Ba(C104)2,which were described in Figures 4 and 5.
-
Discussion The reaction ea,
-
+ TI+
is very fast (k = 3 X 1010 M-I S-I
(3) It can therefore be expacted
-..,TIO
5).
Ershov and Henglein
3436 The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 1.5
c
= 1.0 5
c
t
4
0
I
0.5
L 900
$00
A Lnml Figure 6. Absorption spectrum of thallium colloid produced by the reduction of TI+ in a solution of pH = 1 I . Concentrations as in Figure I . Inset: calculated spectrum of spherical particles (-, dielectric data from ref 8; - - -: from ref 7). Only the first term of the Mie series was used as the particles considered are much smaller than the wavelength of light.
200
400
600
800
A lnml Figure7. Spectrum of colloidal thallium before (0) and after the addition of 1 X IO-’and 2 X 10- M HCI04, and 20 min after addition. Colloidal solution prepared as in Figure 4.
that all hydrated electrons which are generated with G = 2.7/ 100 eV react with T1+. The organic radical produced in reaction 2 cannot reduce T1+ directly.’ The small yield of 1.6/ 100 eV is explained by the reoxidation of some TI0 atoms by the molecular hydrogen peroxide which is generated with G = 0.8/100 eV. In alkaline solution, a higher yield of 3.9/100 eV was observed, which is understood in terms of the partial electrolytic dissociation of the organic radical, the radical anion being able to contribute to TI+ reduction. Similarly, the C02- radical generated in the presence of formate is able to directly reduce T1+ by electron transfer. The long wavelength absorption in the spectrum of thallium colloids which are formed at pH 7 are attributed to the agglomeration of particles. Such aggregates are formed when the T1+ ions are reduced to a large extent; in these solutions, the
-
pH is decreased, and the stabilizing effect by chemisorbed OHions is distorted. That adsorbed OH- ions act as stabilizer for the single small particles is concluded from the changes which are brought about by the addition of NaOH to a solution of agglomerated particles (Figure 4). As the OH- ions are specifically adsorbed, the small particles acquire enough charge to separate. The spectrum in Figure 6 is therefore attributed to small and well-separatedparticles. The absorption coefficient at the maximum at 220 nm is equal to 1.3 X lo4 M-I cm-I. Ba(C104)2causes flocculation of the colloid by reducing the repulsive potential between the particle clusters as can be understood in terms of the DLVO (Deryaguin, Landau, VerweyOverbeek) theory.6 The question may be asked whether OH- is the only anion that stabilizes the small particles. A few experiments were therefore carried out at pH 7 with solutions containing l C 3M NaCl or NaHC02. In both cases, thespectrum of the colloid formed did not have the pronounced long-wavelength absorptionsas in theabsenceoftheadditives (Figure l), although the spectrum was broader than in the presence of OH- (Figure 6). It is concluded that C1- and HC02-anions also exert a certain stabilization of the small particles. The spectrum in Figure 6 may be compared to the spectrum, which one calculates using Mie theory.3 The inset of Figure 6 shows thecalculated spectrum. The two spectra are very similar. The calculated maximum of the UV plasmon band is at 210 nm. The band in the experimental spectrum is a little broader which is probably due to the restriction in the mean free path of the conduction electrons in the small particles studied. The changes in the absorption spectrum of colloidal thallium upon addition of HC104 (Figure 7) are understood in terms of a disintegration of the particle aggregates that accompaniestheir oxidative dissolution. As TI+ ions are formed, the particles that constitute an aggregate are positively charged, the result being disintegration by the repulsive Coulomb force created. Such disintegration effects have previously been observed in the oxidation of cadmium particle cluster^;^ it thus seems that decay of agglomerates is a rather common phenomenon which occurs during redox reactions of metal colloids. N
Acknowledgment. The authors thank Dr. M. Giersig for the preparation of the electron micrographs and Dr. P.Mulvaney for helpful discussions and for reading the manuscript. References and Notes ( I ) Butler, J.; Henglein, A. Radiat. Phys. Chem. 1980, 15, 603. (2) Buxton, G. V.; Rhodes, T.; Sellers, R. M. J. Chem. Sac., Faraday Trans. I . 1982, 78, 3341. (3) Mie, G . Ann. Phys. 1908, 25, 377. (4) Henglein,A.;Gutitrrez, M.; Janata, E.;Ershov, B. G.J. Phys. Chem. 1992, 96,4598. (5) Anbar. M.; Bambenek, M.; Ross, A. B. NSRDS-NBS 43, 1973. (6) See, for example: Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, U.K., 1986; Vol. 1. (7) Myers, H. P . J . Phys..F. Metal Phys. 1973, 3, 1078. (8) Jelinek, T. M.; Hamm, R. N.; Arakawa, E. T.; Huebner, R. H. J . Opt. Sac. Am. 1966, 56, 185.
