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Nucleation and stabilization of quantized AgI clusters in aqueous solution
Colloids and Surfaces A; Physicochemical and Engineering Aspects, 81 (1993) 23 l-238 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.
231
Nucleation and stabilization of quantized AgI clusters in aqueous solution Paul Mulvaney’ Hahn-Meitner
Institut, Abteilung Photochemie,
Glienicker
Strasse 100, W-1000, Berlin 39, Germany
(Received 16 March 1993; accepted 8 June 1993) Abstract Stable, quantized AgI clusters (Q-AgI) have been prepared in aqueous solution using poly(ethyleneimine) (PEI) to inhibit particle growth. The optical absorption spectrum shows an exciton band at 323 nm, the shape and intensity of which improve when the colloid contains excess iodide ion. The cluster can be generated by peptization of fresh AgI sols with PEI. Under illumination, colloidal silver metal is produced and I- is released from the lattice. PEI is photooxidized. The cluster formed is the smallest AgI particle which still displays an exciton absorption band. The nucleation is complete within 200 us. Solid state titrations of Q-AgBr and Q-PbI, with I- and Ag+ respectively, yield mixed Q-state colloids with excitonic bands lying between the bands of the pure colloids. Key words: Aqueous
solution;
Nucleation;
Quantized
AgI clusters; Stabilization
1. Introduction
The shift in the optical band edge of semiconductor crystals due to size quantization provides a simple method for monitoring the nucleation and growth of colloids in aqueous solution. Such spectroscopic measurements complement the earlier methods used to follow particle growth such as light-scattering, stop-flow spectrophotometry and electrophoresis [l]. In this regard the nucleation of silver halides is particularly attractive because they have long been the stalwart of colloid chemists. In two recent pulse radiolysis studies Meisel and co-workers [2] demonstrated that silver iodide can display dramatic quantum size effects. By using nanosecond electron beam pulses to generate iodide ions in situ in silver salt solutions, they were able to follow the fundamental nucleation processes ‘Present address: Department of Physical Chemistry, of Melbourne, Parkville, 3052 Vie., Australia.
University
involved in the formation of colloidal silver iodide. Most remarkably they found that the first discernible excitonic absorption bands were blue-shifted by over 0.9 eV from the bulk value of 2.94 eV (420 nm) to about 3.92 eV (315 nm). Their kinetic data suggested that the AgI crystallites were about 23 A in diameter at this time. Micic and co-workers [3] reported that silver iodide colloids with maxima at 303 nm (25 A diameter) and 334 nm (35 A diameter) could be stabilized in acetonitrile using polybrene. In this paper the preparation of stable silver iodide clusters (Q-AgI) in aqueous solution is described. The cluster is stabilized by poly(ethyleneimine) which can totally inhibit the formation of silver iodide colloids. (In order to distinguish the quantized clusters from normal colloidal AgI, the colloids are henceforth denoted N-AgI.) Based on the nucleation kinetics, it appears that clusters become polymer stabilized at a size where the exciton absorption first becomes fully developed. This paper focusses on the role of the polymer in
P. Muloaney/Colloids
232
stabilizing unusual
the nascent
demonstrated observed
crystallites.
semiconductor that
in colloid
A number
cluster
have
not
reactions previously
of
AgClO,
has an optical
Eng. Aspects 81 (1993) 231-238
edge at about
pronounced
exciton
been
is strongly
determined
absorption
420 nm, and a
band whose shape
by the particle
size and
shape, as first shown by Berry some 25 years ago [S]. It fluoresces weakly with maximum excitonic
systems.
and KI were analytical
grade reagents
emission observable at about 430 nm [6]. However when AgClO, (1 ml, 10 mM) is rapidly added to 50 ml of solution containing 0.4 mM KI in the presence
(PEI)
band
are
2. Experimental
from Aldrich. Poly(ethyleneimine)
Surfaces A: Physicochem.
was purchased
as a
50% aqueous solution from Sigma, and had a nominal molecular weight of 5000. All polymer concentrations are based on the monomer molecular weight of 43. Particle sizes were determined using a Philips CM 12 electron microscope. The samples were prepared by allowing 10 ul of colloidal solution to dry on carbon-coated copper grids. The average particle size of the Q-AgI clusters absorbing at 323 nm was determined to be 30 f 5 A, based on a histogram of 100 particles. The particles were very sensitive to electron irradiation. The clusters disappeared within 5-10 s when examined under high resolution in the electron microscope. Time-dependent diffraction patterns were also obtained. The microscope was equipped
of 1 mM
PEI,
a colourless
solution
is
obtained. The absorption spectrum is shown in Fig. 1 (curve C). A band is present with a sharp maximum at 323 nm. The extinction coefficient is 3750 M-i cm-’ based on the AgI monomer concentration. Sols containing 0.2 mM excess silver ions had much weaker maxima at 323 nm (curve B) whilst in the presence of 0.6 mM excess silver ions the band is broadened and red-shifted to about 335 nm. N-AgI was formed in sols containing 0.6 mM excess silver ions over the course of several hours. The material fluoresces very weakly (4
with a 9800 EDAX analyser which was used to determine the elemental composition of the colloids. In nanoprobe mode, the EDAX analyser can be used to determine the elemental composition of single particles down to about 50 A. EDAX analysis of individual colloid particles and groups of about ten AgI clusters consistently yielded an Ag : I ratio of 1.0 +_0.06. Pulse radiolysis was carried out with 0.5~us pulses from a 3.6 MeV Van de Graaff accelerator. The set-up has been described in detail elsewhere
c41. 3. Results 3.1 Preparation
and characterization
of Q-Agl
When solutions of AgC104 and KI are rapidly mixed, a pale yellow sol of AgI forms which ripens over several hours before settling out. Bulk AgI
A [nml Fig. 1. Absorption and fluorescence spectra of 0.2 mM Q-AgI. Solution contained 1mM PEI and 0.2 mM AgI plus: (curve A) 0.6 mM excess Ag+; (curve B) 0.2mM excess Ag+; (curve C) 0.2 mM excess KI. The fluorescence spectrum is due to solution (C) with excitation at 280 nm.
P. MulvaneylColloids Surfaces A: Physicochem. Eng. Aspects 81 11993) 231-238
caused
the fluorescence
of a stoichiometric
sol to
233
quantum
size limit
is a longer
tail
and
more
undergo a blue shift and the band became more intense and narrower, while excess silver ion caused
pronounced peak around the bulk exciton peak. An unprecedented effect was observed when the
strong
polymer
broadening
and red-shifting.
was added to AgI prepared
in the absence
The effect of the polymer concentration on the absorption spectrum of colloidal AgI is shown in
of a stabilizer.
Fig. 2. When smaller concentrations of polymer (10 PM) were used, the spectrum obtained resembled that of normal colloidal AgI (spectrum 1). As
was observed (Fig. 3). PEI was then added to the fresh sol after 1 min, and the spectrum measured
the concentration
was increased,
the fraction
of
N-AgI decreased, the exciton band became weaker and a small peak at 335 nm appeared (spectra 2 and 3). At PEI concentrations > 0.1 mM predominantly Q-AgI was formed, and eventually at concentrations above 0.25 mM, only the cluster material was generated (spectrum 4). Whether the polymer actually forms a well-defined stoichiometric complex with the silver iodide is difficult to ascertain since the polymer contains primary, secondary and tertiary amine groups in unknown ratios. The fact that the bulk exciton peak at 425 nm initially decreases in magnitude as the PEI:AgI ratio is increased (spectra 1 and 2) indicates that the average particle size of the bulk crystallites decreases before the formation of Q-AgI becomes dominant. Mie calculations show that the primary effect of increased particle size above the
and iodide
Immediately
salt solutions,
after mixing the spectrum
the silver of N-AgI
again. The intensity of the bulk exciton band was strongly reduced, and simultaneously the band of the cluster material appeared at 323 nm. After 40 min in contact with the PEI, the bulk AgI absorption was further reduced and the quantized AgI cluster absorption was dominant. However the absorption of the colloidal AgI could never be completely suppressed. The longer the time between the preparation of the AgI sol and the addition of the PEI, the less efficacious was the polymer in regenerating Q-AgI. The polymer apparently peptizes the colloid particles, which is only
1.0 \\\ ;\
0.8
\
‘\
0.2
O-
250
X Inml Fig. 2. Spectra of 0.2 mM AgI and 0.2 mM KI in the presence of (curve 1) 10 uM; (curve 2) 100 uM; (curve 3) 200 PM; (curve 4) 500 uM PEI.
300
350 X hnl
400
450
Fig. 3. Spectra of 0.2 mM AgI: obtained immediately after mixing 0.2 mM AgCIO, and 0.4 mM KI (-); 1 min -after addition of 1 mM PEI to the fresh AgI sol (--); after ageing the solution for 40 min (--). The pH of the sol prior to addition of PEI was 6.0 and after addition of PEI it rose to 7.3.
P. MulvaneylColloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
234
possible than
if they had formed
ripening.
by aggregation
The peptization
reaction
rather may
be
(1)
It seems astounding that although the exciton band had red-shifted to 420 nm, indicating electronic between
the clusters
AgI, peptization
of Q-AgI
peptization
observed,
which constituted
of the colloidal
(curve AgI was of
sols, the cluster peptiza-
tion was never complete and the longer the solution remained at pH 3, the poorer was the regeneration of the cluster solution.
the 3.2 Solid-state
was still possible.
The stabilization is apparently due to strong complexation of surface silver ions with the amine groups of the polymer. The pH of freshly prepared sols was 7.3, and at this pH PEI is only weakly protonated. The effect of the solution pH on the stabilization of the clusters is shown in Fig. 4. When the pH was progressively lowered to about 4.5, the absorption spectrum red-shifted to about 345 nm (curve 2). Further acidification to pH 3.2 caused the clusters to aggregate and the spectrum of colloidal AgI with the characteristic 420 nm excitonic band appeared (curve 3). Only the unprotonated amine groups stabilize the clusters. When the pH of the acidic sol was then raised to 8.9, the bulk exciton band again diminished in intensity
1.61
reappeared
but as in the case of addition
PEI to fresh, unstabilized
N-AgI + PEI + Q-AgI-PEI
contact
4), i.e. partial again
written
colloidal
and the 323 nm band
cluster reactions
In Fig. 5, results of a solid-state cluster titration are shown. 0.2 mM AgBr was precipitated in the presence of PEI to yield Q-AgBr crystallites. The sol contained excess Br- to ensure that no Agf ions remained in solution. The AgBr was only weakly stabilized by the polymer and the spectrum slowly red-shifted over the course of several hours. KI was then added stepwise to the fresh sol. The absorption band of Q-AgBr at 275 nm red-shifted and gained in intensity with each aliquot of KI added, as can be seen in spectra I-5 of Fig. 5, until when the stoichiometric amount of KI had been added, the spectrum obtained was that of pure Q-AgI. The quantized cluster materials appear to be fully miscible, although only about 40 mol% of iodide can be dissolved in bulk AgBr crystals [7].
I 1. 2. 3. 4.
ptl 7.3 OH 4.5 pH 3.2
I
pH 8.9
250 X km1 Fig. 4. Spectra of 0.2 mM Agl containing 0.2 mM excess KI and stabilized with 1 mM PEI at: initial pH 7.3 (curve I); after acidification to pH 4.5 (curve 2) or pH 3.2 (curve 3); after raising the pH again to 8.9 (curve 4).
300
350
X [nml Fig. 5. Spectra of Q-AgBr clusters prepared by adding 0.2 mM Ag+ to 0.4 mM KBr in the presence of 2 mM PEI (solid line) and after the addition of: (curve 1) 40 FM; (curve 2) 80 FM; (curve 3) 120 PM; (curve 4) 160 pM; (curve 5) 240 pM of KI.
23.5
P. Muluaney/Colloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
Similar effects could be obtained with 0.2 mM PbIz stabilized on PEI (0.2 mM excess of PbZf ions) when A&10, was used as the titrant. Again the absorption bands of the Pb12 were steadily redshifted from 270 to 323 nm as the silver ions replaced lead lattice ions. In both cases, the stoichiometry of the clusters could be altered without any significant particle growth.
3.3 Photolysis
of Q-Agl
The photoreactivity of the AgI clusters was very dependent on whether there was an excess of silver ions or iodide in the solution. This was also observed previously by Henglein et al. for PVA stabilized N-AgI sols [6]. In the presence of an excess of silver ions (0.2-0.8 mM), the 0.2 mM Q-AgI solutions became yellow, orange and then brown upon standing, even in the dark. In daylight, the sols turned orange within an hour due to the formation of colloidal silver. However, if an excess of I- was present then the solution was stable for a week in daylight, and for several months in the dark. Under illumination, the solution slowly became orange. The reaction was faster if the solution had been previously deoxygenated. In Fig. 6, the spectrum of a 0.2 mM Q-AgI sol is shown as a function of the illumination time at 308 nm. The incident light intensity was 3.45 kJ cm-’ s-l. Note that an isosbestic point exists at 350 nm. When the photolysis was carried out in the presence of excess silver ions, the absorption simply increased at all wavelengths, as also observed for N-AgI by Henglein et al. [6]. No hole scavenger was added to the solution. After the photolysis, iodide ion was present in the solution (c229= 11 100 M - ’ cm-‘). Since iodide ions were not oxidized, PEI had to have been photo-oxidized during the reaction. The formation of colloidal silver is much more rapid when excess silver ion is present. The pathway for photodecomposition is therefore written Q-A@ - hv e,+h,i,
(2a)
0; 250
300
350
400 X [nml
450
500
Fig. 6. Photolysis of 0.22 mM Q-AgI in the presence of 0.2 mM excess KI. The solution was evacuated on a vacuum line prior to photolysis. Spectra of the solution after various periods of illumination with monochromatic light at 308 nm are shown.
(2’4 h,?,,+ PEI,,, + PEI,+,,
(2c)
I&r --) I,,,
(2d)
The efficiency of the reaction is determined by step (2b), the trapping of the electron at a surface site, which allows hole scavenging to compete with recombination. The fluorescence yield is < 0.001, so the vast majority of carriers recombine without emission of radiation. The point of zero charge (PZC) of AgI is at pAg 4.0. Sols prepared in the presence of excess silver ion (0.2 mM) are positively charged, and the dramatic increase in the photoinstability is attributed to the large number of surface traps created by adsorbed silver ions. Oxidation of PEI is facilitated by the fact that the valence band of AgI contains some 30% Ag 4d character [7]. Adsorbed amine groups coordinated via surface silver ions may therefore be effectively coupled to the valence band. Oxidation of PEI transfers an electron from the solution to the particle. The final step in the decomposition is then release of iodide from the lattice so that the surface potential is held constant, as is required by double layer theory. When the photolysed solution was subsequently exposed to air, the plasmon absorption band
P. Muluaney/Colloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
236
decreased dramatically, same time, the cluster
as seen in Fig. 7. At the exciton band reappeared
and after 60 min the absorption
band
at 323 nm
was almost as intense as it was prior to the photolysis. These spectral changes can be explained as follows.
Colloidal
silver is only oxidized
AgI directly; trapped
as
it simply captures silver
atoms
them after they are (or
colloidal
silver
particles). 3.4 Q-Agl nucleation kinetics
by air
when a strong complexing agent for silver ions is present, e.g. CNor S2-. PEI has a similar
During the nucleation of AgI in the absence of a stabilizer, Hayes et al. [2(b)] found that immedi-
chelating power. silver colloid
ately after the generation of iodide ion, the silver ions reacted quantitatively to form molecular AgI. Then over several hundred microseconds, an absorption band appeared at 315 nm, which was attributed to sharply quantized exciton absorption. This band then slowly red-shifted until it reached 420 nm after several seconds. The observed position of the cluster band in the presence of PET is 323 nm. In order to determine whether the clusters formed in the presence of PEI are related to the species absorbing at 3 15 nm in the absence of PEI, pulse radiolysis was used to follow the nucleation of Q-AgI in the presence of PEI under conditions similar to those of Hayes et al. [2(b)]. Details of the radiation chemical processes occurring in solution which enable iodide to be generated are given in detail in their paper.
Ag-PEI
Following
the oxidation
+ (n/4)02 + nH + -+ nAg+-PEI
the silver ions recombine
nAg+-PEI
+ (n/2)H,O
(3a)
with the iodide
free by the initial photolysis iodide clusters
of the
to regenerate
ions set the silver
+ nI_ + nQ-AgI
(3b)
No N-AgI was produced by the oxidation of the colloidal silver. From these results it is clear that the reason why the photocorrosion is slower in aerated solution is that the colloidal silver is continually being reoxidized as it forms. The oxygen does not have to scavenge the photoelectrons from the
1.6I
1
Hydrated electrons and H atoms, formed by pulsed electron irradiation of the solution, reduce methylene iodide within the 0.5 us electron pulse to yield II: CH,I,
+ e-(aq) -+ CH,I
+ I-
At the doses used, 8.8 uM iodide was created
I
250
300
I
350
I
400
I
450
I 500
X [nml Fig. 7. Effect of air on the spectrum of colloidal silver produced by photolysis of 0.2 mM AgI clusters (see Fig. 6). Spectra before admission of air and 5, 15 and 60 min after admission of air.
(4) per
pulse assuming G(e-) + G(H) = G(II)= 3.2. Immediately following the pulse, an absorption band rising into the UV was seen (Fig. 8), which is attributed to molecular AgI [2]. Over the following 100 us the absorption band at 323 nm built up in a single step until the spectrum resembled that of the solutions prepared by direct mixing. A typical oscilloscope trace at 325 nm is shown in the inset. No further changes to the solution were observed up to 10 s after the pulse, using iodide ion concentrations of between 1 and 15 PM. The growth appeared to follow second-order kinetics, and the half-life for the build-up of the absorption band at
P. Mulvaney/Colloids
Surfaces
A: Physicochem.
Eng. Aspects 81 (1993
0.08. . -\
J 231-238
231
found that AgI particles
in acetonitrile
with diame-
ters of about 25 A had a maximum at 304 nm, and 35 A-sized particles a peak at 334 nm. Mie calcula-
0
tions
carried
dielectric
out
data
on
100-A
of Bedikyan
AgI sols using et al. [lo]
the different dielectric constants barely influence the position
the
show that
of the two solvents of the absorption
maxima, and that the position of the maxima should be comparable for the two solvents. 3.5 Kinetics of lattice ion substitution It was shown in section 2 that the addition of iodide leads to a strong red shift in the exciton absorption band of quantized AgBr particles. Quantized mixed crystals of silver iodobromide
X km1
are formed, Fig. 8. Spectra of an Ar-saturated solution containing 0.1 mM AgCIO,, 0.5 M tert-butanol, 2 mM CH,I, and 2 mM PEI immediately after a pulse generating 8.8 PM iodide ion, and 400 ps after the pulse; solution pH 7. Inset: kinetic trace showing the rate of formation of the absorption band at 325 nm.
323 nm was just 30 f 5 us. This is somewhat faster than the rate observed by Meisel and co-workers [2] for the formation of the 315 nm absorption band under similar conditions. Since the silver ions are strongly bound to the amine groups on the polymer, the nucleation and growth of the clusters must take place along the polymer chains. Under such conditions, the observed rate of reaction may be increased due to a reduction in dimensionality of the reaction space, as previously observed in micellar [S] and polymeric systems [9]. No redshift of the absorption
band
could
be seen either
during or after the build-up at 323 nm. It therefore seems that the silver iodide clusters are the same as those absorbing at 315 nm in the absence of PEI. The PEI stabilizer apparently shifts the exciton energy to slightly longer wavelengths. Hayes et al. [2(b)] predicted, using a particle-ina-box model, that the clusters absorbing at 3 15 nm should have a radius of about 12 A, in good agreement with the value found here by electron microscopy of 15 A. These values are also in accord with the data of Micic and co-workers [3] who
and Q-AgI.
Q-AgBr + I- + Q-AgI + Br-
(5)
The pulse technique can be used to monitor the rate of reaction (5). A 0.2 mM AgBr sol containing an excess of 0.2 mM bromide ion was prepared in the presence of 2 mM PEI (see Fig. 5) and then irradiated with a dose that produced about 9 uM iodide ion. The excess bromide ion ensured that silver iodide was not nucleated in solution. Nevertheless, the reaction was found to be very rapid (half-life 50 f 10 us), and the build-up of the long wavelength absorption band following ion exchange was complete within 200 us of the electron pulse. The difference spectrum obtained (Fig. 9) showed very little growth at 280-300 nm which confirms that the iodide ions displaced bromide ions, and did not lead to new AgI clusters directly in solution. (Compare the titration in Fig. 5.) If the reaction is diffusion controlled, then it is readily shown that the half-life for the disappearance of iodide after the electron pulse should be given by zijZ =693
r2/3VmD [AgBr]
(6)
where I is the colloid radius, D the iodide diffusion coefficient, V, the molar volume of silver bromide and [AgBr] the molecular concentration of colloid. Taking r= 15 x 10e8 cm, D= 10m5 cm* S-‘, V,=
g
P. Mulvaney/Colloids
238 0.80
Surfaces A: Physicochem.
I-
released
oxidized.
1
0.04
0.64
$ 2
0.48
Z : (D
0.01
0.16
f-I.00 250
I
0.02
0.32
g
300
350 Wavelength
400 [nm]
0.00 450
Fig. 9. Absorption spectrum of 0.2 mM Q-AgBr containing 0.2 mM excess Br- and 2 mM PEI and the difference spectrum obtained 400 ps after the generation of 10 pM I in the Q-AgBr sol. The solution contained 2 mM CH,I,, 0.5 M tert-butanol and was saturated with argon.
29 cm3 mol -I and [AgBr] = 0.2 mM, half-life of 89 ps in good agreement observed value.
we find a with the
in the presence
Stable, quantized AgI clusters have been prepared in aqueous solution. The optical absorption spectrum shows an exciton band at 323 nm, the shape and intensity of which improve when the colloid contains excess iodide ion. However the presence of the polymer, which enables the cluster to be prepared, also drastically changes the redox chemistry of the system. The cluster can be generated by peptization Under illumination
of fresh AgI sols with Ag is produced
colloidal
PEI. and
while PEI is photo-
silver is readily
oxidized
of PEI. From the kinetics
it was found
that the cluster
by air
of cluster formed
is
the smallest AgI cluster which still displays an exciton absorption band. The nucleation is complete within 200 ~1s. Solid state titrations of AgBr and PbI, tonic
yield mixed
bands
clusters.
Q-state
lying between
The substitution
in AgBr by Icontrolled.
was found
colloids
the bands of bromide
with exciof the pure lattice
ions
to be close to diffusion
References I
2
3
4. Conclusions
from the lattice,
Colloidal
nucleation
0.03
z 2
D
Eng. Aspects 81 (1993) 231-238
(a) E.J. Meehan and W.H. Beattie, J. Phys. Chem., 65 (1961) 1522. (b) M.J. Jaycock and G.D. Parfitt, Trans. Faraday Sot., 57 (1961) 791. (c) T. Tanaka, H. Saigo and T. Matasubara, J. Photogr. Sci. Eng., 26 (1982) 92. (a) K.H. Schmidt, R. Pate1 and D. Meisel, J. Am. Chem. Sot., 110 (1988) 4882. (b) D. Hayes, K.H. Schmidt and D. Meisel, J. Phys. Chem., 93 (1989) 6100. (a) M.I. Vucemilovic and 0. Micic, Radiat. Phys. Chem., 32 (1988) 79. (b) 0.1. Micic, M. Meglic, D. Lawless, D.K. Sharma and N. Serpone, Langmuir, 6 (1990) 487. A. Kumar, E. Janata and A. Henglein, J. Phys. Chem., 92 (1988) 2587. C. Berry, Phys. Rev., 161 (1967) 848; 153 (1967) 989. A. Henglein, M. Gutierrez, H. Weller, A. Fotjik and J. Jirovsky, Ber. Bunsenges. Phys. Chem., 93 (1989) 593. A. Marchetti and R.S. Eachus, Adv. Photochem, 17 (1992) 145. A.J. Frank, M. Grltzel and J.J. Kozak, J. Am. Chem. Sot., 98 (1976) 3317. P. Mulvaney and A. Henglein, J. Phys. Chem., 94 (1990) 4182. L.D. Bedikyan, V.K. Miloslavskii and L.A. Ageev, Opt. Spektrosk, 47 (1979) 225.
231
Nucleation and stabilization of quantized AgI clusters in aqueous solution Paul Mulvaney’ Hahn-Meitner
Institut, Abteilung Photochemie,
Glienicker
Strasse 100, W-1000, Berlin 39, Germany
(Received 16 March 1993; accepted 8 June 1993) Abstract Stable, quantized AgI clusters (Q-AgI) have been prepared in aqueous solution using poly(ethyleneimine) (PEI) to inhibit particle growth. The optical absorption spectrum shows an exciton band at 323 nm, the shape and intensity of which improve when the colloid contains excess iodide ion. The cluster can be generated by peptization of fresh AgI sols with PEI. Under illumination, colloidal silver metal is produced and I- is released from the lattice. PEI is photooxidized. The cluster formed is the smallest AgI particle which still displays an exciton absorption band. The nucleation is complete within 200 us. Solid state titrations of Q-AgBr and Q-PbI, with I- and Ag+ respectively, yield mixed Q-state colloids with excitonic bands lying between the bands of the pure colloids. Key words: Aqueous
solution;
Nucleation;
Quantized
AgI clusters; Stabilization
1. Introduction
The shift in the optical band edge of semiconductor crystals due to size quantization provides a simple method for monitoring the nucleation and growth of colloids in aqueous solution. Such spectroscopic measurements complement the earlier methods used to follow particle growth such as light-scattering, stop-flow spectrophotometry and electrophoresis [l]. In this regard the nucleation of silver halides is particularly attractive because they have long been the stalwart of colloid chemists. In two recent pulse radiolysis studies Meisel and co-workers [2] demonstrated that silver iodide can display dramatic quantum size effects. By using nanosecond electron beam pulses to generate iodide ions in situ in silver salt solutions, they were able to follow the fundamental nucleation processes ‘Present address: Department of Physical Chemistry, of Melbourne, Parkville, 3052 Vie., Australia.
University
involved in the formation of colloidal silver iodide. Most remarkably they found that the first discernible excitonic absorption bands were blue-shifted by over 0.9 eV from the bulk value of 2.94 eV (420 nm) to about 3.92 eV (315 nm). Their kinetic data suggested that the AgI crystallites were about 23 A in diameter at this time. Micic and co-workers [3] reported that silver iodide colloids with maxima at 303 nm (25 A diameter) and 334 nm (35 A diameter) could be stabilized in acetonitrile using polybrene. In this paper the preparation of stable silver iodide clusters (Q-AgI) in aqueous solution is described. The cluster is stabilized by poly(ethyleneimine) which can totally inhibit the formation of silver iodide colloids. (In order to distinguish the quantized clusters from normal colloidal AgI, the colloids are henceforth denoted N-AgI.) Based on the nucleation kinetics, it appears that clusters become polymer stabilized at a size where the exciton absorption first becomes fully developed. This paper focusses on the role of the polymer in
P. Muloaney/Colloids
232
stabilizing unusual
the nascent
demonstrated observed
crystallites.
semiconductor that
in colloid
A number
cluster
have
not
reactions previously
of
AgClO,
has an optical
Eng. Aspects 81 (1993) 231-238
edge at about
pronounced
exciton
been
is strongly
determined
absorption
420 nm, and a
band whose shape
by the particle
size and
shape, as first shown by Berry some 25 years ago [S]. It fluoresces weakly with maximum excitonic
systems.
and KI were analytical
grade reagents
emission observable at about 430 nm [6]. However when AgClO, (1 ml, 10 mM) is rapidly added to 50 ml of solution containing 0.4 mM KI in the presence
(PEI)
band
are
2. Experimental
from Aldrich. Poly(ethyleneimine)
Surfaces A: Physicochem.
was purchased
as a
50% aqueous solution from Sigma, and had a nominal molecular weight of 5000. All polymer concentrations are based on the monomer molecular weight of 43. Particle sizes were determined using a Philips CM 12 electron microscope. The samples were prepared by allowing 10 ul of colloidal solution to dry on carbon-coated copper grids. The average particle size of the Q-AgI clusters absorbing at 323 nm was determined to be 30 f 5 A, based on a histogram of 100 particles. The particles were very sensitive to electron irradiation. The clusters disappeared within 5-10 s when examined under high resolution in the electron microscope. Time-dependent diffraction patterns were also obtained. The microscope was equipped
of 1 mM
PEI,
a colourless
solution
is
obtained. The absorption spectrum is shown in Fig. 1 (curve C). A band is present with a sharp maximum at 323 nm. The extinction coefficient is 3750 M-i cm-’ based on the AgI monomer concentration. Sols containing 0.2 mM excess silver ions had much weaker maxima at 323 nm (curve B) whilst in the presence of 0.6 mM excess silver ions the band is broadened and red-shifted to about 335 nm. N-AgI was formed in sols containing 0.6 mM excess silver ions over the course of several hours. The material fluoresces very weakly (4
with a 9800 EDAX analyser which was used to determine the elemental composition of the colloids. In nanoprobe mode, the EDAX analyser can be used to determine the elemental composition of single particles down to about 50 A. EDAX analysis of individual colloid particles and groups of about ten AgI clusters consistently yielded an Ag : I ratio of 1.0 +_0.06. Pulse radiolysis was carried out with 0.5~us pulses from a 3.6 MeV Van de Graaff accelerator. The set-up has been described in detail elsewhere
c41. 3. Results 3.1 Preparation
and characterization
of Q-Agl
When solutions of AgC104 and KI are rapidly mixed, a pale yellow sol of AgI forms which ripens over several hours before settling out. Bulk AgI
A [nml Fig. 1. Absorption and fluorescence spectra of 0.2 mM Q-AgI. Solution contained 1mM PEI and 0.2 mM AgI plus: (curve A) 0.6 mM excess Ag+; (curve B) 0.2mM excess Ag+; (curve C) 0.2 mM excess KI. The fluorescence spectrum is due to solution (C) with excitation at 280 nm.
P. MulvaneylColloids Surfaces A: Physicochem. Eng. Aspects 81 11993) 231-238
caused
the fluorescence
of a stoichiometric
sol to
233
quantum
size limit
is a longer
tail
and
more
undergo a blue shift and the band became more intense and narrower, while excess silver ion caused
pronounced peak around the bulk exciton peak. An unprecedented effect was observed when the
strong
polymer
broadening
and red-shifting.
was added to AgI prepared
in the absence
The effect of the polymer concentration on the absorption spectrum of colloidal AgI is shown in
of a stabilizer.
Fig. 2. When smaller concentrations of polymer (10 PM) were used, the spectrum obtained resembled that of normal colloidal AgI (spectrum 1). As
was observed (Fig. 3). PEI was then added to the fresh sol after 1 min, and the spectrum measured
the concentration
was increased,
the fraction
of
N-AgI decreased, the exciton band became weaker and a small peak at 335 nm appeared (spectra 2 and 3). At PEI concentrations > 0.1 mM predominantly Q-AgI was formed, and eventually at concentrations above 0.25 mM, only the cluster material was generated (spectrum 4). Whether the polymer actually forms a well-defined stoichiometric complex with the silver iodide is difficult to ascertain since the polymer contains primary, secondary and tertiary amine groups in unknown ratios. The fact that the bulk exciton peak at 425 nm initially decreases in magnitude as the PEI:AgI ratio is increased (spectra 1 and 2) indicates that the average particle size of the bulk crystallites decreases before the formation of Q-AgI becomes dominant. Mie calculations show that the primary effect of increased particle size above the
and iodide
Immediately
salt solutions,
after mixing the spectrum
the silver of N-AgI
again. The intensity of the bulk exciton band was strongly reduced, and simultaneously the band of the cluster material appeared at 323 nm. After 40 min in contact with the PEI, the bulk AgI absorption was further reduced and the quantized AgI cluster absorption was dominant. However the absorption of the colloidal AgI could never be completely suppressed. The longer the time between the preparation of the AgI sol and the addition of the PEI, the less efficacious was the polymer in regenerating Q-AgI. The polymer apparently peptizes the colloid particles, which is only
1.0 \\\ ;\
0.8
\
‘\
0.2
O-
250
X Inml Fig. 2. Spectra of 0.2 mM AgI and 0.2 mM KI in the presence of (curve 1) 10 uM; (curve 2) 100 uM; (curve 3) 200 PM; (curve 4) 500 uM PEI.
300
350 X hnl
400
450
Fig. 3. Spectra of 0.2 mM AgI: obtained immediately after mixing 0.2 mM AgCIO, and 0.4 mM KI (-); 1 min -after addition of 1 mM PEI to the fresh AgI sol (--); after ageing the solution for 40 min (--). The pH of the sol prior to addition of PEI was 6.0 and after addition of PEI it rose to 7.3.
P. MulvaneylColloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
234
possible than
if they had formed
ripening.
by aggregation
The peptization
reaction
rather may
be
(1)
It seems astounding that although the exciton band had red-shifted to 420 nm, indicating electronic between
the clusters
AgI, peptization
of Q-AgI
peptization
observed,
which constituted
of the colloidal
(curve AgI was of
sols, the cluster peptiza-
tion was never complete and the longer the solution remained at pH 3, the poorer was the regeneration of the cluster solution.
the 3.2 Solid-state
was still possible.
The stabilization is apparently due to strong complexation of surface silver ions with the amine groups of the polymer. The pH of freshly prepared sols was 7.3, and at this pH PEI is only weakly protonated. The effect of the solution pH on the stabilization of the clusters is shown in Fig. 4. When the pH was progressively lowered to about 4.5, the absorption spectrum red-shifted to about 345 nm (curve 2). Further acidification to pH 3.2 caused the clusters to aggregate and the spectrum of colloidal AgI with the characteristic 420 nm excitonic band appeared (curve 3). Only the unprotonated amine groups stabilize the clusters. When the pH of the acidic sol was then raised to 8.9, the bulk exciton band again diminished in intensity
1.61
reappeared
but as in the case of addition
PEI to fresh, unstabilized
N-AgI + PEI + Q-AgI-PEI
contact
4), i.e. partial again
written
colloidal
and the 323 nm band
cluster reactions
In Fig. 5, results of a solid-state cluster titration are shown. 0.2 mM AgBr was precipitated in the presence of PEI to yield Q-AgBr crystallites. The sol contained excess Br- to ensure that no Agf ions remained in solution. The AgBr was only weakly stabilized by the polymer and the spectrum slowly red-shifted over the course of several hours. KI was then added stepwise to the fresh sol. The absorption band of Q-AgBr at 275 nm red-shifted and gained in intensity with each aliquot of KI added, as can be seen in spectra I-5 of Fig. 5, until when the stoichiometric amount of KI had been added, the spectrum obtained was that of pure Q-AgI. The quantized cluster materials appear to be fully miscible, although only about 40 mol% of iodide can be dissolved in bulk AgBr crystals [7].
I 1. 2. 3. 4.
ptl 7.3 OH 4.5 pH 3.2
I
pH 8.9
250 X km1 Fig. 4. Spectra of 0.2 mM Agl containing 0.2 mM excess KI and stabilized with 1 mM PEI at: initial pH 7.3 (curve I); after acidification to pH 4.5 (curve 2) or pH 3.2 (curve 3); after raising the pH again to 8.9 (curve 4).
300
350
X [nml Fig. 5. Spectra of Q-AgBr clusters prepared by adding 0.2 mM Ag+ to 0.4 mM KBr in the presence of 2 mM PEI (solid line) and after the addition of: (curve 1) 40 FM; (curve 2) 80 FM; (curve 3) 120 PM; (curve 4) 160 pM; (curve 5) 240 pM of KI.
23.5
P. Muluaney/Colloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
Similar effects could be obtained with 0.2 mM PbIz stabilized on PEI (0.2 mM excess of PbZf ions) when A&10, was used as the titrant. Again the absorption bands of the Pb12 were steadily redshifted from 270 to 323 nm as the silver ions replaced lead lattice ions. In both cases, the stoichiometry of the clusters could be altered without any significant particle growth.
3.3 Photolysis
of Q-Agl
The photoreactivity of the AgI clusters was very dependent on whether there was an excess of silver ions or iodide in the solution. This was also observed previously by Henglein et al. for PVA stabilized N-AgI sols [6]. In the presence of an excess of silver ions (0.2-0.8 mM), the 0.2 mM Q-AgI solutions became yellow, orange and then brown upon standing, even in the dark. In daylight, the sols turned orange within an hour due to the formation of colloidal silver. However, if an excess of I- was present then the solution was stable for a week in daylight, and for several months in the dark. Under illumination, the solution slowly became orange. The reaction was faster if the solution had been previously deoxygenated. In Fig. 6, the spectrum of a 0.2 mM Q-AgI sol is shown as a function of the illumination time at 308 nm. The incident light intensity was 3.45 kJ cm-’ s-l. Note that an isosbestic point exists at 350 nm. When the photolysis was carried out in the presence of excess silver ions, the absorption simply increased at all wavelengths, as also observed for N-AgI by Henglein et al. [6]. No hole scavenger was added to the solution. After the photolysis, iodide ion was present in the solution (c229= 11 100 M - ’ cm-‘). Since iodide ions were not oxidized, PEI had to have been photo-oxidized during the reaction. The formation of colloidal silver is much more rapid when excess silver ion is present. The pathway for photodecomposition is therefore written Q-A@ - hv e,+h,i,
(2a)
0; 250
300
350
400 X [nml
450
500
Fig. 6. Photolysis of 0.22 mM Q-AgI in the presence of 0.2 mM excess KI. The solution was evacuated on a vacuum line prior to photolysis. Spectra of the solution after various periods of illumination with monochromatic light at 308 nm are shown.
(2’4 h,?,,+ PEI,,, + PEI,+,,
(2c)
I&r --) I,,,
(2d)
The efficiency of the reaction is determined by step (2b), the trapping of the electron at a surface site, which allows hole scavenging to compete with recombination. The fluorescence yield is < 0.001, so the vast majority of carriers recombine without emission of radiation. The point of zero charge (PZC) of AgI is at pAg 4.0. Sols prepared in the presence of excess silver ion (0.2 mM) are positively charged, and the dramatic increase in the photoinstability is attributed to the large number of surface traps created by adsorbed silver ions. Oxidation of PEI is facilitated by the fact that the valence band of AgI contains some 30% Ag 4d character [7]. Adsorbed amine groups coordinated via surface silver ions may therefore be effectively coupled to the valence band. Oxidation of PEI transfers an electron from the solution to the particle. The final step in the decomposition is then release of iodide from the lattice so that the surface potential is held constant, as is required by double layer theory. When the photolysed solution was subsequently exposed to air, the plasmon absorption band
P. Muluaney/Colloids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 231-238
236
decreased dramatically, same time, the cluster
as seen in Fig. 7. At the exciton band reappeared
and after 60 min the absorption
band
at 323 nm
was almost as intense as it was prior to the photolysis. These spectral changes can be explained as follows.
Colloidal
silver is only oxidized
AgI directly; trapped
as
it simply captures silver
atoms
them after they are (or
colloidal
silver
particles). 3.4 Q-Agl nucleation kinetics
by air
when a strong complexing agent for silver ions is present, e.g. CNor S2-. PEI has a similar
During the nucleation of AgI in the absence of a stabilizer, Hayes et al. [2(b)] found that immedi-
chelating power. silver colloid
ately after the generation of iodide ion, the silver ions reacted quantitatively to form molecular AgI. Then over several hundred microseconds, an absorption band appeared at 315 nm, which was attributed to sharply quantized exciton absorption. This band then slowly red-shifted until it reached 420 nm after several seconds. The observed position of the cluster band in the presence of PET is 323 nm. In order to determine whether the clusters formed in the presence of PEI are related to the species absorbing at 3 15 nm in the absence of PEI, pulse radiolysis was used to follow the nucleation of Q-AgI in the presence of PEI under conditions similar to those of Hayes et al. [2(b)]. Details of the radiation chemical processes occurring in solution which enable iodide to be generated are given in detail in their paper.
Ag-PEI
Following
the oxidation
+ (n/4)02 + nH + -+ nAg+-PEI
the silver ions recombine
nAg+-PEI
+ (n/2)H,O
(3a)
with the iodide
free by the initial photolysis iodide clusters
of the
to regenerate
ions set the silver
+ nI_ + nQ-AgI
(3b)
No N-AgI was produced by the oxidation of the colloidal silver. From these results it is clear that the reason why the photocorrosion is slower in aerated solution is that the colloidal silver is continually being reoxidized as it forms. The oxygen does not have to scavenge the photoelectrons from the
1.6I
1
Hydrated electrons and H atoms, formed by pulsed electron irradiation of the solution, reduce methylene iodide within the 0.5 us electron pulse to yield II: CH,I,
+ e-(aq) -+ CH,I
+ I-
At the doses used, 8.8 uM iodide was created
I
250
300
I
350
I
400
I
450
I 500
X [nml Fig. 7. Effect of air on the spectrum of colloidal silver produced by photolysis of 0.2 mM AgI clusters (see Fig. 6). Spectra before admission of air and 5, 15 and 60 min after admission of air.
(4) per
pulse assuming G(e-) + G(H) = G(II)= 3.2. Immediately following the pulse, an absorption band rising into the UV was seen (Fig. 8), which is attributed to molecular AgI [2]. Over the following 100 us the absorption band at 323 nm built up in a single step until the spectrum resembled that of the solutions prepared by direct mixing. A typical oscilloscope trace at 325 nm is shown in the inset. No further changes to the solution were observed up to 10 s after the pulse, using iodide ion concentrations of between 1 and 15 PM. The growth appeared to follow second-order kinetics, and the half-life for the build-up of the absorption band at
P. Mulvaney/Colloids
Surfaces
A: Physicochem.
Eng. Aspects 81 (1993
0.08. . -\
J 231-238
231
found that AgI particles
in acetonitrile
with diame-
ters of about 25 A had a maximum at 304 nm, and 35 A-sized particles a peak at 334 nm. Mie calcula-
0
tions
carried
dielectric
out
data
on
100-A
of Bedikyan
AgI sols using et al. [lo]
the different dielectric constants barely influence the position
the
show that
of the two solvents of the absorption
maxima, and that the position of the maxima should be comparable for the two solvents. 3.5 Kinetics of lattice ion substitution It was shown in section 2 that the addition of iodide leads to a strong red shift in the exciton absorption band of quantized AgBr particles. Quantized mixed crystals of silver iodobromide
X km1
are formed, Fig. 8. Spectra of an Ar-saturated solution containing 0.1 mM AgCIO,, 0.5 M tert-butanol, 2 mM CH,I, and 2 mM PEI immediately after a pulse generating 8.8 PM iodide ion, and 400 ps after the pulse; solution pH 7. Inset: kinetic trace showing the rate of formation of the absorption band at 325 nm.
323 nm was just 30 f 5 us. This is somewhat faster than the rate observed by Meisel and co-workers [2] for the formation of the 315 nm absorption band under similar conditions. Since the silver ions are strongly bound to the amine groups on the polymer, the nucleation and growth of the clusters must take place along the polymer chains. Under such conditions, the observed rate of reaction may be increased due to a reduction in dimensionality of the reaction space, as previously observed in micellar [S] and polymeric systems [9]. No redshift of the absorption
band
could
be seen either
during or after the build-up at 323 nm. It therefore seems that the silver iodide clusters are the same as those absorbing at 315 nm in the absence of PEI. The PEI stabilizer apparently shifts the exciton energy to slightly longer wavelengths. Hayes et al. [2(b)] predicted, using a particle-ina-box model, that the clusters absorbing at 3 15 nm should have a radius of about 12 A, in good agreement with the value found here by electron microscopy of 15 A. These values are also in accord with the data of Micic and co-workers [3] who
and Q-AgI.
Q-AgBr + I- + Q-AgI + Br-
(5)
The pulse technique can be used to monitor the rate of reaction (5). A 0.2 mM AgBr sol containing an excess of 0.2 mM bromide ion was prepared in the presence of 2 mM PEI (see Fig. 5) and then irradiated with a dose that produced about 9 uM iodide ion. The excess bromide ion ensured that silver iodide was not nucleated in solution. Nevertheless, the reaction was found to be very rapid (half-life 50 f 10 us), and the build-up of the long wavelength absorption band following ion exchange was complete within 200 us of the electron pulse. The difference spectrum obtained (Fig. 9) showed very little growth at 280-300 nm which confirms that the iodide ions displaced bromide ions, and did not lead to new AgI clusters directly in solution. (Compare the titration in Fig. 5.) If the reaction is diffusion controlled, then it is readily shown that the half-life for the disappearance of iodide after the electron pulse should be given by zijZ =693
r2/3VmD [AgBr]
(6)
where I is the colloid radius, D the iodide diffusion coefficient, V, the molar volume of silver bromide and [AgBr] the molecular concentration of colloid. Taking r= 15 x 10e8 cm, D= 10m5 cm* S-‘, V,=
g
P. Mulvaney/Colloids
238 0.80
Surfaces A: Physicochem.
I-
released
oxidized.
1
0.04
0.64
$ 2
0.48
Z : (D
0.01
0.16
f-I.00 250
I
0.02
0.32
g
300
350 Wavelength
400 [nm]
0.00 450
Fig. 9. Absorption spectrum of 0.2 mM Q-AgBr containing 0.2 mM excess Br- and 2 mM PEI and the difference spectrum obtained 400 ps after the generation of 10 pM I in the Q-AgBr sol. The solution contained 2 mM CH,I,, 0.5 M tert-butanol and was saturated with argon.
29 cm3 mol -I and [AgBr] = 0.2 mM, half-life of 89 ps in good agreement observed value.
we find a with the
in the presence
Stable, quantized AgI clusters have been prepared in aqueous solution. The optical absorption spectrum shows an exciton band at 323 nm, the shape and intensity of which improve when the colloid contains excess iodide ion. However the presence of the polymer, which enables the cluster to be prepared, also drastically changes the redox chemistry of the system. The cluster can be generated by peptization Under illumination
of fresh AgI sols with Ag is produced
colloidal
PEI. and
while PEI is photo-
silver is readily
oxidized
of PEI. From the kinetics
it was found
that the cluster
by air
of cluster formed
is
the smallest AgI cluster which still displays an exciton absorption band. The nucleation is complete within 200 ~1s. Solid state titrations of AgBr and PbI, tonic
yield mixed
bands
clusters.
Q-state
lying between
The substitution
in AgBr by Icontrolled.
was found
colloids
the bands of bromide
with exciof the pure lattice
ions
to be close to diffusion
References I
2
3
4. Conclusions
from the lattice,
Colloidal
nucleation
0.03
z 2
D
Eng. Aspects 81 (1993) 231-238
(a) E.J. Meehan and W.H. Beattie, J. Phys. Chem., 65 (1961) 1522. (b) M.J. Jaycock and G.D. Parfitt, Trans. Faraday Sot., 57 (1961) 791. (c) T. Tanaka, H. Saigo and T. Matasubara, J. Photogr. Sci. Eng., 26 (1982) 92. (a) K.H. Schmidt, R. Pate1 and D. Meisel, J. Am. Chem. Sot., 110 (1988) 4882. (b) D. Hayes, K.H. Schmidt and D. Meisel, J. Phys. Chem., 93 (1989) 6100. (a) M.I. Vucemilovic and 0. Micic, Radiat. Phys. Chem., 32 (1988) 79. (b) 0.1. Micic, M. Meglic, D. Lawless, D.K. Sharma and N. Serpone, Langmuir, 6 (1990) 487. A. Kumar, E. Janata and A. Henglein, J. Phys. Chem., 92 (1988) 2587. C. Berry, Phys. Rev., 161 (1967) 848; 153 (1967) 989. A. Henglein, M. Gutierrez, H. Weller, A. Fotjik and J. Jirovsky, Ber. Bunsenges. Phys. Chem., 93 (1989) 593. A. Marchetti and R.S. Eachus, Adv. Photochem, 17 (1992) 145. A.J. Frank, M. Grltzel and J.J. Kozak, J. Am. Chem. Sot., 98 (1976) 3317. P. Mulvaney and A. Henglein, J. Phys. Chem., 94 (1990) 4182. L.D. Bedikyan, V.K. Miloslavskii and L.A. Ageev, Opt. Spektrosk, 47 (1979) 225.