In situ synthesis of silver nanocluster in AOT reverse micelles

May 17, 1993 - properties.1 Many reports on quantum size effect on photo- ... micelles.8 The ternary systems alkane/AOT/water present ... effect.15-16...
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J. Phys. Chem. 1993,97, 12974-12983

12974

In Situ Synthesis of Silver Nanocluster in AOT Reverse Micelles Christophe Petit,t**Patricia Lixonf and Marie-Paule Pileni’*+v* Laboratoire SRSI, URA CNRS 1662, Universitb P. and M. Curie, 75005 Paris, France, and CEA- DRECAM, Service de Chimie Molbcuiaire, CE Saclay 91 191 Gif Sur Yvette Cedex, France Received: May 17, 1993; In Final Form: September 13, 1993’

Silver nanosized crystallites have been synthesized in AOT reverse micelles. By using a functionalized surfactant, silver sulfosuccinate (Ag(AOT)), as reactant, a high concentration of monodisperse and stable nanosize particles can be obtained even in the presence of air. This study focused on the various parameters controlling the nanosize of silver particles such as micellar structure and nature of the reducing agent or of the solvent. At low water content, a precursor such as Agd2+ can be stabilized. The changes in the size of the crystallites with the number of initiators per micelle and with the micellar exchange processes can be explained from a kinetic model.

Introduction Synthesis of nanoparticles is a new emerging field in solidstate chemistry. Due to their small size, these crystallites exhibit novel material properties which largely differ from the bulk properties.’ Many reports on quantum size effect on photochemistry,2 nonlinear optical properties of semiconductor), or the emergence of metallic properties with the size of the particles4 have appeared during the past few years. The chemical way is often employed to synthesize such nanosize particles. Silver nanocrystallites have been largely studied because of their ability in having surface-enhanced Raman spectroscopyS and for their application in the photographic process.6 To be able to make these nanostructures, reverse micelles are good candidates.7 Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) is the most commonly surfactant used to form reverse micelles.* The ternary systems alkane/AOT/water present enormous advantages: they are spheroidal and monodisperse aggregates (the size distribution is around 20%) where water is readily solubilized in the polar core, forming a “water-pool” characterized by the ratio of water to surfactant concentration ( W = [H20]/[AOT]). Inisooctaneas the bulksolvent,thewaterpool radius, R,, is found to be linearly dependent on the water content (R, (A) = 1.5 W).9 Another property of reverse micelles is their dynamic character; the water pools can exchange their contents by a collision process.8 This makes possible chemical reaction or coprecipitation between compounds solubilized in two different reverse micelles. Hence semiconductor,’O magnetic,’ I or metallic particles12 were synthesized in situ in AOT reverse micelles. Some groups have used microemulsions to synthesize silver particles. They operate in degassed solution and with cationicI3 or nonionic microemulsion.l4 In the first case (water/CTAB/ hexanol system), silver particles are coated by AgBr.13 However, in this system, a large part of the microemulsion phase is a bicontinuous medium and not water in oil droplets. This fact does not allow us to study the micellar size effect on the silver particles. In the case of nonionic surfactant, the polar headgroup can reduce, by thermal or photochemical action, the silver ions,I4 which perturbs the study. In all these cases, silver ions are always present as a salt like AgN03 which is poorly soluble in AOT reverse micelles. We report here for the first time, the use of functionalized surfactant Ag(A0T) to obtain calibrated nanosize silver particles

* To whom correspondence should be addressed.

Laboratoire SRSI. CEA-DRECAM. 0 Abstract published in Advance ACS Abstracts, October 15, 1993.

f f

0022-365419312097-12974$04.00/0

in AOT reverse micelles. We present the different parameters which control the size of the silver particles synthesized in situ and we focus on the kinetic parameters which regulate the size of the particles.

Optical Properties of Silver Colloidal Particles As for other metals, optical properties, related to the excitation of plasma resonance or interband transition, have been intensively investigated during these last years particularly on the size effect.I5J6 To sum up, the UV-vis absorption spectra of a fairly dilute dispersion of colloidal silver can be calculated from the “Mie” theory from the wavelength dependence of optical constant of the particles relative to the surrounding medium.I7 For small spherical particles (diameter < 20 nm), the absorption spectrum depends only on the contribution of the dipole term in the Mie summation.18 Thus, for a dispersion of Nparticles per unit volume, the absorbance is given by A = CN1/2.303 (1) where C and 1 are the absorption cross section and the optical path length, respectively. The absorption cross section is given by C = l 8 ~ V ~ ~ ( o ) / X [ ( q+( o2eJ2 )

+c~(w)~]

(2) where V and X are the volume of the spherical particle and the incident wavelength (which corresponds to a frequency o), respectively. The complex relative permittivity of the metal is B(W) = q(o) + iq(u). A maximum in the absorption occurs at el(w) = -2e,. The wavelength of this resonance is given by the wavelength dependence of q ( w ) (this depends also on the surrounding medium). The width and the intensity of the resonance are determined by the value of cz(o) at the resonance. In the case of silver, q(o) is a linear function of the frequency and, in consequence, the absorption band is Lorentzian in shape, with the bandwidth of the absorption peak and the reciprocal of the height proportional to Q ( w ) . For silver, as for free electrons metals, Q(W) is fairly constant in UV-vis range when the diameter of the particles is larger than the electron mean free path (520 8, in the case of silver). When the size decreases, it is necessary to correct ~ ( w )for electron scattering at the particles boundaries. In such a case, 4 o ) dependson the size of the particles.15b This involves a broadening and a decrease of the height of the plasmon absorption b a n d ~ . l ~ ~ ’ ~ This is well-known and has been evaluated and discussed in the literature.lSJ6 Thus from relations 1 and 2 it is possible to simulate the absorption of a solution of silver particles with variable size. The size correction is introduced in the form suggested in the 0 1993 American Chemical Society

Synthesis of Ag Nanocluster in AOT

" 1

250

'I

" 1

250

360

*

350 4bO &O wavelength (nm)

The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12975

500

550

W = 12.5

360

350 460 4 0 wavelength (nm)

560

5 b

Figure 1. (A) Simulated absorption spectra of metallic silver particles. Values of particles diameter, D (in angstroms), are indicated. (B) Absorption spectra of silver particles formed in (30% AgAOT-70% NaA0T)-water-isooctane reverse micelles at various water content. M, [NaAOT] = 3.5 X 10-2 [NaBHd] = 1 P M; [AgAOT] = 1.5 X M; optical path length = 1 mm. 1 i t e r a t ~ r e . IParameters ~~ required for this simulation, plasma frequency (up= 1.38 X 10l6s-l), the Fermi velocity (vf = 1.4 X 106 ms-'), and optical constant n(w) and k(w) of the bulk silver were taken in the l i t e r a t ~ r e . ~ ~ b l ~Figure * ~ ~ ~ J1A ~ J ~shows a very broad absorption band centered a t 400 nm, characteristic of very small nanosize particles. By increasing the size, the absorption spectrum is narrower with an increase in the intensity of the plasmon band and a decrease in the bandwidth. In fact, there is a linear variation of the half-width with the reciprocal of the particle diameter.1692O The position and the number of peaks in the absorption spectra are also an indication of the shape of the particles: for spherical silver particles there is only one band centered at 400 nm whereas for an ellipsoidal particle there are two peaks.'* The absorption in the UV range results from interbands transitions which are fairly constant with the size;'* hence the optical density at 250 nm can be taken as a measure of the silver concentration. Thespectroscopicbehavior seems to benow well established.16-21 For small particles (130 A), the maximum and the bandwidth of the plasmon peak strongly depend on the surrounding medium. When crystallites strongly interact with the matrices, a blue shift and a large broadening of the absorption band are observed whereas with low interactions a red shift and a lower broadening take place.16 Hence a lot of information in the size, the shape, and the interactions of the silver crystallites with the surrounding medium can be easily deduced from UV-vis spectroscopy study.

Experimental Section Products. All the materials were used without further purification Ag(N03) Gold label and AOT were purchased from

Sigma. Isooctane was obtained from Fluka. Sodium tetrahydroboride is from Alpha Products and hydrazine from Prolabo. Synthesis of the Silver Sulfosuccinate (Ag(A0T)). The AOT salt is changed to its acid form by ion exchange on a previously protonated strong cationicresin (Lewatit SP 1080). Subsequently, the acid form is passed through a weakly acid macroporous cation exchange resin (Bio-Rex 70) and put into the silver form. The preparation of the Ag(A0T) is carried out in a 50-50 mixture of water and ethanol as AOT is very slightly soluble in pure water. The product is finally dried under vacuum. Theefficiency of the exchange of the counterion is determined by silver ions concentration titration using VoPhard's method: precipitation of thiocyanate in presence of ammonium ferric sulfateas indicator. More than 99%of sodiumions was exchanged by silver. Residual sodium is measured by atomic absorption (0.004 wt a). In isooctane solution, this compound is surprisingly stable; there is no evolution either by irradiation with an intense light (Oriel Hg Lamp at 500 W) or by heating. It is not necessary to store it in the absence of air or in the dark. This behavior is characteristic for Ag(A0T) surfactant; the same exchange of sodium counterions from SDS by silver ions yields quickly to a black compound due to the photoreduction of the silver ions. Then all the experiments were done a t room temperature without removing the oxygen. The metallic silver particles synthesized in situ, in reverse micelles, are very stable for months, in the presence of oxygen. Apparatus. Absorption spectra have been recorded on a conventional Varian Cary 1 spectrophotometer (in 1- or 2-mm cuvettes). The fast absorption kinetics were obtained with a HP 8452A diode array spectrophotometer (Figure 1SA,C,E,G). For electron microscopy, a drop of the solution was placed on a carboncoated copper grid. After evaporation of the solvent, micrographs were recorded on a transmission electron microscope (JEOL 100CX2). Histograms are obtained by measuring the diameter Di of all the particles from five micrographs from different parts of the grid (X160 000). Depending on the experimental parameters, the number of particles, n, contained is between 200 and 500. The standard deviation, u, is calculated from the experimentally determined distributions: u

= { [ C ( D i- D ) 2 ] / [ n- 1]]''2

where D is the average diameter. Synthesis of Silver Particles in Reverse Micelles. The concentrations given in the text are the overall concentrations. The preparation of colloidal silver particles is achieved by mixing AOT reverse micellar solutions at the same water content containing 30%Ag(AOT), 70%Na(AOT),andNaB& (0rN2&) respectively. The average number of silver ions per micelles, nAg+, is between 2 (at W =2) and 67 (at W = 15). The maximum solubility of NaBH4 in 0.1 M AOT reverse micelles is very poor and equal to 3 X 10-4 M at W = 7.5. Under our experimental conditions, the maximum average number of NaBH, per micelle, n d , is 0.32. Thus, with NaBH4, there is a large excess of silver ions, The maximum solubility of N2H4 is 10-1 M, and the maximum average number of hydrazine per micelle can reach 111 (at W = 15). Because of such large amounts of hydrazine per micelle, it is possible to totally reduce silver ions solubilized in the water pool. Thus, the extinction coefficient per atom of silver particle can be deduced (see below). SAXSExperiments. The scattered X-ray intensity a t low angles was measured from 0.025 to 0.5 A-1 (which corresponds to a resolution in real space from 10 to 250 A), using a GDPA 30 goniometer and Cu Ka radiation (1.54 A). The experimental arrangement has been described elsewhere.22 The scattering patterns analysis has been described in a recent papere23A good agreement between the various water-pool radius, deduced from different representations of the scattered intensity (Le., Porod's or Guinier's plots), supports spherical droplets h y p o t h e s i ~ . ~ ~ , ~ ~ This can be further confirmed by a simulation of the scattering

Petit et al.

The Journal of Physical Chemistry, Vol. 97, No. 49, 1993

I

mfl

0.05

0

20

40

60

80

100

__

! + % of

AOT links

to Ag+

Figure2. Variation of the limit of water solubilityin mixed micelleswith the percentage of silver sulfosuccinate concentration,Ag(A0T). The total AOT concentration is kept equal to 0.1 M.

patterns carried out using the known composition of the sample and the estimated micellar radius. In this simulation, the polydispersitycan be estimated as a Gaussian distribution of the micellar size centered on the estimated radius, Results and Discussion

I. Structural Study of Mixed Ag(AOT)/Na(AOT) Reverse Micelles. Silver bis( 2-ethylhexyl) sulfosuccinate,Ag(AOT), can easily dissolve water in alkane solution. In water-Ag(A0T)isooctane solution, the maximum amount of water is much lower than that obtained in water-Na(A0T)-isooctane. As a matter of fact, the maximum water content is found to be equal to 8 in water-Ag(A0T)-isooctane solution and to 55 with Na(A0T). In (Ag(AOT)/Na(AOT)) mixed micelles, progressive exchange of Ag(A0T) by Na(A0T) (keeping the total surfactant concentration constant) induces an increase of the maximum water solubility (Figure 2). Above 20% of Ag(AOT), a linear relationship between the water content limit and the relative percentageof Ag(A0T) is observed. This means a random mixing of the two surfactants in the micelles without Ag(A0T) micro domain^.^' However, thediscontinuityin the linear variation below 20% could indicate a change in the aggregation mode of mixed micelles at low Ag(A0T) concentration. In our experimental conditions, the average number of Ag(A0T) per micelle is 30% of the micellar aggregation number (27 at W = 7.5). Small-Angle X-ray Scattering. Figure 3 shows the Porod's plot (I(q)q' vs q, where q is the wave vectorZ4) of the scattered intensityof pure Na(A0T) and mixed (30%Ag(A0T)-70% Na(AOT)) reversemicellesat W = 7.5. Thegoodagreement between the three different determinations of the radius (Table I) and between the simulated and experimental curves (Figure 3) indicates spherical droplets. In 30% Ag(AOT)-7O% Na(A0T)-water-isooctane micellar solution, the water-pool radius increases with the water content as has been observed in Na(A0T)-water-isooctane solution.8 No significantchange in the polydispersity is observed. Replacing isooctane by cyclohexane as bulk solvent does not change the shape and the size of the mixed micelles (Table I). 11. Synthesis in Situ of Silver Particles in Reverse Micelles. The reduction of silver ions has been mainly performed by using sodium borohydride, NaBH4, as a reducing agent. However, in some specified cases hydrazine, N2H4, was used. Silver nitrate, [AgNO3] l e 3 M,dissolved in 0.1 M AOT-water-isooctane micelle, at W 10 (nAg+= 0.48), is reduced in the presence of air. Figure 4 shows an absorption spectrum different to that previously calculated (Figure 1A) or published13J4J8 for pure

0.1

0.15

0.2 q

0.25

--

AOT70%-ApAOT30%

X

Na(A0T)

0.3

0.k

0.4

1

Figure 3. Porod's plot (normalized by the invariant: Z(q)#/Q) vs q ) of NaAOT-water-iosoctane (*), AgAOT-NaAOT-water-isooctane (+) reverse micelles at W = 7.5, [AOT]-l= 0.1 M. The solid line on each plot is the best fit for polydisperse (Gaussiandistribution) non interacting spheres.26

metallic silver particles. Such a strong absorption in the visible region is characteristicof the formation of large silver aggrega" As matter of fact, after 1 h all particles flocculate. On replacing nitrate by AOT silver derivative (keeping same average number of silver per micelle, nAG+ = 0.48), Figure 4 shows a well-defined plasmon peak characterized by a maximum centered at 400 nm. According to the calculated absorption spectra (Figure 1A), this can be attributed to small metallic silver particles. Because of thesedifferences in the behavior between AgNO3 and Ag(AOT), it has been chosen to use mixed micelles containing 30% of Ag(AOT) and 70% of AOT. The overall AOT concentration is maintained toO.lM (exceptwhenitispreciseinthetext). Because of the high stability of Ag(A0T) (see above), all the syntheses have been performed in the presence of air. II.1. Micellar Effect on the Syntbesis of Silver Particles. ( A )At Fixed AOT Concentration and Variable Water Content, W. Figure 1B shows the absorption spectra of colloidal silver particles obtained in micelles at various water content, W, These spectra are characteristicof metallic silver particles with a plasmon absorption band centered at 400 nm. At low water content, the absorption spectrum is broad with a low optical density. On increasing the water content, the plasmon peak is narrower, more intense, and slightly blue-shifted. On the other hand, the absorbance at 250 nm increases slightly with the water content. The comparison between Figure 1A and 1B shows an increase in the size of silver particles with the water content, W. This is confirmed from electron microscopy pictures. Figure 5 and Table I1 show an increase in the particles size with the water content (i.e., the size of the micelles). It can be noticed that there is a decrease in the size distribution at low water content compared to that obtained at higher Wvalues. Hence, a decrease in the micellar size induces formation of smaller and more monodisperse silver particles. Similar data have been obtained for other inorganic particleslO and have been attributed to a protecting effect of the surfactant. Electron diffraction pattern (Figure 6) and high-resolution electron microscopy (Figure 7A) reveal a crystalline order. On the Figure 6 spacings are observed at 2.36, 2.04,1.44, 1.22, 1.18, and 0.94 A which are all within 2% of the values reported for the 111,200,220,3 11,222, and 33 1 reflection, respectively, for metallic ~ilver.2~ The deduced spacing distance of the crystalline lattice is equal to 4.08 A. This value is similar to that reported for fccmetallicsilver(4.0862 A). High-resolution electron microscopy (Figure 7A) reveals the lattices fringes of the silver nanoparticles (which again gives a spacing distance of 4.09 A for the crystalline lattice). The invariance of the spacing

The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12977

Synthesis of Ag Nanocluster in AOT

TABLE I: Micellar Size Obtained bv SAXS Exoeriments W = 7.5 isooctane Na(A0T) 0.1 M

19.5 i 1

17f 1

1912

I5

0.28

21.5 11

21 f 1

21 1 2

17

0.28

22f 2

22f2

23 f 2

18

0.30

35f2

36f2

33i2

30

0.28

W = 7.5 isooctane Ag(A0T) 0.03 M,Na(A0T) 0.07 M

W = 7.5 cyclohexane Ag(A0T) 0.03 M, Na(A0T) 0.07 M

W = 15 isooctane Ag(A0T) 0.03 M,Na(A0T) 0.07 M 1.0

56

W =5

0.9

0.8

0.7

b

'g

0.6

tl

3a

0.5

30 25

0.4

20 IS

0.3

IO

5

0.2

0

.

0.1 '

250

350

450

550

-

.

I 6 32 4 6 62 78 9 4 I 1 0 1 2 5 1 4 1 I 5 6 I)IAMETER I A I

35

30

650

vavelength (nm)

Figure 4. Absorption spectra of silver particles synthesized in situ in NaAOT reverse micelles by using (- - -) silver nitrate and (-) silver sulfosuccinate [AOTJt,l = 0.1 M; [AgNO3] = [AgAOT] = It3M; [NaBH4] = l(r M. W = 10. Optical path length = 1 cm (nb+ = 0.48 and n d = 0.048).

distancewith the particle sizeconfirms metallic properties appear for small silver particle size.4b The blue shift of the absorption spectrum maximum with the water content (Figure 1B and Table 11) can be correlated to the sizeof the crystallitesdeterminedfrom electronmicroscopy (Table 11). A linear variation of the half-width of the plasmon band with the reciprocal of the diameter, as predicted from the theory, is observed (Figure 8). The change in the slope and absorption intensity compared to theoretical behavior16*20 with the size of the particles (Table 11) can be attributed to matrix interactions with the metallic particles.16 Additionof a largeexcessofwater to the microemulsioninduces a phase transition. After centrifugation at 9000 rpm during 15 min, silver nanoparticlesare totally transferred into the aqueous phase. The supernatant contains both the oil and the major part of surfactant. Figure 9A shows no differences in the absorption spectra, indicating no change in the size of the crystallites. This is confirmed in Figure 9, B and C, where no changes are observed in size and distribution of individual particles before and after extraction, respectively. However, after extraction, the particles form large clusters groups without coalescence(Figure 9C). This indicates the crystallites are coated by surfactant molecules protecting them from coalescence. From the molar extinction coefficient per atomof silver particles (determined below), the yield of reduction of silver by NaBH4 given in Table I1 is very low. This could be due to the low NaBH4

' 1I

16

32

6 2 78 9 4 1 1 0 1 2 5 1 4 1 I 5 6 DIAMETER l i b

46

Figure 5. Electron microscopy and size distributionof silver crystallites synthesized in AgAOT-NaAOT-water-isooctane reverse micelles at various water content. [AgAOT] = 3 X 1 t 2 M, [NaAOT] = 7 X M, [NaBH4] = 2.5 X 10"' M. The marks represent 208 A.

TABLE 11: Characteristics of Silver Particles Obtained in Various Experimental Conditions in 30%Ag(A0T)-70% Na(A0T) in Isooctane W 2 5 7.5 7.5 7.5 15 15 7.5 7.5 7.5 7.5

reducing agent NaBH4 NaBH4 NaBH4 NaBH4 NaBH4 NaBH4 NaBH4 N2H4 N2H4 N2H4 N2H4

AX112 diameter

[Red](M) 2.5 X 10-4 2.5X 10-4 lt5

10-4 2.5 X 10-4 2.5X 10-4 6.8 X 6.8 X 6.8 X

(nm)

421 419 430 414 10-4 406 409 10-4 400 422 10-4 420 l t 3 413 I t 2 406

@&a

(nm)

0.024 0.038 0.012 0.059 0.066 0.089 0.11 0.017 0.045 0.49 1

170 138 200 108 82

90 75 160 120 87 80

(A)

-

(i) -

27

10

45 55 60 70 45 50 65 75

15 21 20 30 20 21 25 30

-

-

a @& is the ratio of the silver ions reduced (taking 7200 M-km-' for the molar extinction coefficient per atom of silver particles (see text)) on the initial Ag(A0T) concentration.

solubility in AOT reverse micelles and to reduction of water by BH4-, catalyzed by metallic ions.3' (B) Influence of the Polar VolumeFraction on the Size of the Particles. At constant water content, W = 7.5, the micellar

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12978 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993

(331) d = 0.94 A

r

(222) d = l.18A (311) d=1.22A (220) d = 1 . U A (200) d=2.04A (111) d=2.36A FwHM-35+501OrO -

0.01

1

0.02

-

1

-

1

-

0.03 0.04 1/D (A-')

1

0.05

-

N2H4 0 1

0.06

-

1

0.07

Figure 8. Linear variation of the bandwidth with the reciprocal of the particles diameters determined by electron microscopy (see Table 11). The dashed line represents the theoretical variation as obtained from the simulation of Figure 1A.

Figure 6. Electron diffraction pattern of the particles [AgAOT] = 3 X 10-2 M, [NaAOT] = 7 X 10-* M, [NaBHd] = 2.5 X lo-" M. W = 15. .

-

**

.F

ci!

0

= -

*.I---

wavelength (nm)

B

Figure 9, (A) Absorption spectra of silver metallic particles before (-) and after (- - -) extraction. (B) Electron microscopy of silver metallic particles before extraction. ( C ) Electron microscopy of silver metallic particles after extraction. [AgAOT] = 3 X 1P2M; [NaAOT] = 7 X M; W = 7.5, [NaBHd] = 2.5 X l0-C M. Optical path length = 1 mm. The marks represent 208 A.

Figure 7. High-resolution transmission electron microscopy on silver particles (X4 700 000). The marks corresponds to 21 A. (A) W = 15, [AgAOT] = 3 X M,[NaAOT] = 7 X 1 t 2 M,[NaBH4] = 2.5 X 1 P M. (B) W = 7.5, [AgAOT] = 3 X le2M,[NaAOT] = 7 X 1P2 M, [N~HI]= 6.8 X 1C2M.

concentration, [micelle], changes with the increase of AOT concentration. ( 5 X l e 2 M 5 [AOT] I0.4 M). The relative percentage (30%/70%) and the ratio - of An(AOT)-Na(AOT) -. , I

.

I

[Ag+]/[NaBHd] = 300 remain constant. Hence, the water-pool radius and the average number of silver ions and reducing agent per micelle remain constant and are equal to 2.1 nm, 9, and 0.03, respectively. The micellar concentration increases from 1.7 X 10-3 to 1.3 x WM. The absorption spectra of colloidal silver particles are quasihomothetics (Figure 10A). No change is observed in the halfwidth and in the maximum absorption. In the same way, the absorption at 250 nm changes linearly with the micellar concentration. Hence, the increase in the micellar concentration

The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12979

Synthesis of Ag Nanocluster in AOT

1 34

1

0.4 M

m

n

Diameter (A) Figure 12. Size distribution of silver metallic particles made by NaBH4 or N2H4. W = 7.5, nrd = 3 X 1kz, n b + = 9, [AOT] = 0.1 M.

so w

450 m (nm)

wo

I

Figure 10. Absorption spectra of silver metallic particles at various micellar concentration. W = 7.5; optical path length = 1 mm. 5 X M < [AOTImlI 0.4 M;1.7 X 10-3 M I [micelle] S 1.3 X 10-2 M. (A) PM.7.5 [Ag+]/[NaBH,] = 300; n d = 3 X lo-*. (B) [NaBHd] = 1 X 10-3 In

d I 6 X 10-2,

[NaBH41 = 2.5 l W 4 M

1.54

I \

wavelength (nm)

Figure 11. Absorption spectra of silver particles synthesized in mixed reverse micelles at various NaBH4 concentration. [AgAOT] = 3 X M. W = 7.5. Optical path length = 2 mm. M; [NaAOT] = 7 X induces an increase in the number of silver particles formed without any changes in the average size. (C) Influence of the Reducing Agent Concentration on rhe Size of Metallic Crystallites. At fixed W value, keeping 30% Ag(AOT)-7O% Na(AOT), the average number of NaBH4 per micelle, nrd, decreases either by decreasing the reducing agent concentration or by increasing the micellar concentration: (i) On increasing NaBH4 concentration, that is to say on increasing the average number of NaBH4 per micelle from 3 X 10-3 to 7.5 X 10-2, Figure 11 and Table I1 show a red shift in the maximum of plasmon peak with a decrease of its intensity. This indicates a decrease in the size of the metallic particles with the average number of reducing agent per micelle. This is due to the

increase in the protecting effect of surfactant molecules on the crystallite growth. (ii) If the reducing agent concentration is kept constant ([NaBHd] = 1WM),the increase in the micellar concentration (that is to say by decreasing the average number of NaBH4 per micelle from 6 X 10-2 to 7.5 X 10-3) induces a red shift in the plasmon peak maximum and an increasein the bandwidth (Figure 10B). This indicates a decrease in the average size of the silver particles due to the decrease in the average number of reducing agent per micelle. This can be associated with the change in the particle sizes by increasing the water content. As the matter of fact, the increase of the water content, W,at constant AOT concentration (Figure 1A) induces a decrease in the micellar concentration with an increase in the average number of reducing agent per micelle from 0.01 (W= 2) to 0.146 (W = 12.5). In both cases, the 250-nm absorption is almost invariant, indicating an unchanged value in the concentrationof reduced silver (Figures 1B and 10 B). From these data, it can be concluded that the size of the particles decreases with the average number of NaBH4 per micelle and stays unchanged with the increase in the polar volume fraction that is to say with the number of droplets. 11.2. Effect of the Reducing Agent. On replacing sodium tetrahydroboride with hydrazine, N2H4, changes in size, polydispersity, kinetics, and number of particles formed are observed. For a given average number of reducing agent per micelle (% = 3 X 10-2) by replacing NaBH4 by N2H4, Figure 12 shows an unchanged value of the average size of the particles with an increasein the polydispersity. The formationof metallicparticles is slower with N2H4 (4 h) than with NaBH4 (30 min). Because the intermicellar exchange process is shorter (on a millisecond time scale) than thegrowth of theparticles, thechemical reduction of silver ions takes place in a pseudocontinuous phase and the surfactant plays the role of a protecting agent to prevent particles flocculation. As previously observed by using NaBH4, Figure 13 and Table I1 show an increase in the particles size and in the polydispersity with the average number of N2H4 per micelle (2.1 X 10-2 C < 20.4). At fixed Wvalue and reducing agent concentration, the average size of the crystallite remains the same. However, the polydispersity is higher by using NzH4 instead of NaBH4. As shown above, a linear relationship is obtained by plotting the width of the band of the plasmon peak with the reciprocal of the particle sizes (Figure 8). However, the slope is different from that obtained by using NaBH4. This could be attributed either to the increase in the polydispersity1s*~20 or to change in the crystallites interactions with the medium.16 High-resolution electron microscopy of nanoparticles (Figure 7B) shows defects for particles obtained from hydrazine instead of sodium tetrahydroboride (Figure 7A). This could be due to the slower kinetics of nanosize particles formation. This could

Petit et al.

12980 The Journal of Physical Chemistry, Vol. 97, No. 49,1993

w

= 7.5 [N2H41 = 7.10-4 M

-1 5 a b

'm

I

6 12 19 25 11 38 44 50 56 63 69 75 81 88 94 100 IO6 112 1 I9 I25 131

Diameter I

(A)

.-

w

= 7.5 [NzHq]= 7.1Os2 M

h

5 s

10

I

6

12 19 25 31 38 44 50 56 63 69 75 81 88 94 100 106 112 I19 125

11:

Diameter (A) Figure 13. Influence of N2H4 concentration on the size distributionof silver particles. [AgAOT] = 3 X 1W2M;[NaAOT] = 7 X 1W2M;W = 7.5.

The marks represent 208 A.

have a significant impact on the particle interactions with the medium and, as mentioned above, could partly explain the differencein theoptical behavior of silver metalliccolloidsobtained with different reducing agents. M,the At high hydrazine concentration, [N2H4] = 6.8 X reduction of silver ion is total ( n d = 20.4; nAg= 9). The optical density (0.1-mm cuvette) is 2.15 at 400 nm and the molar extinction coefficient per atom of silver particles is found equal to 7200 M-1 cm-l. This value is in agreement to those reported by Henglein et al.31aand Mostafavi M.3* for silver particles in water. II.3. Solvent Effect.The intermicellar exchange process is governed by the attractive interactions between droplets. It is known that the bulk solvent plays an important role in the intermicellar potential. As a matter of fact, theuseofcyclohexane compared to isooctane as bulk solvent induces a decrease in the interaction potential which decreases the intermicellar exchange rate constant by a factor of By using cyclohexane as the bulk solvent, electron micrographs reveal a decrease in the size, polydispersity, and number of silver particles compared to that obtained with isooctane (Figure 14 and Table 111). It should be noticed that no change in the micellar size has been observed by SAXS (see above). Hence, at a given water content, the decrease of the intermicellar exchange rate constant induces a decrease in particles size. The number of silver metallic particles obtained is lower than that observed by using isooctaneas the bulk solvent, indicating a decrease in the reduction yield. This is confirmed by a decrease in the 250-nm absorbance when using cyclohexane instead of isooctane (see below). 111. Kinetics Analysis. According to Wokaun et al.I4 the reduction of silver ions by NaBH4 in aqueous solution takes place in two fast steps involving eight electrons to form eight silver

atoms.3' This is followed by an aggregation process yielding metallic silver colloids where some intermediate cluster can be stabilized depending on the experimental ~ e t u p . ~ IBy - ~ pulse ~ radiolysis studies Belloni et al.31b*c report an aggregation rate constant (Ag, + Ag, Ag,,+,) equal to 2 X 10-8 M - I d and show a drastic decrease of this aggregation process by membrane system.3lb In 30% AgAOT-70% NaAOT-water-isooctane solution ([AOT],l = 0.1 M,[NaBHd] = 10-4 M), the mechanism of reduction differs with water content: ( i ) At Very Low Wuter Content (W 5 1.5). After NaBH4 addition, an absorption band characterized by a sharp peak centered at 275 nm appears (Figure 15, A and B). It is attributed to Agq2+.31JhJ3 The percursors Ag2+and Ago cannot be observed. This could be explained as follows. (i) The positively charged clusters can be stabilized by strong electrostatic interactions with the negatively charged head polar group of the surfactant., when the reducing agent BH4-is repelled due to its negative charge. Such interactions are stronger with clusters having two positive charges than one. (ii) According to Henglein et al.,34the clusters are long-lived in the presence of an excess of silver ions (under our experimental condition [AgAOT]/[NaBH4] = 120). (ii)At 2.5C W S 5. Duringthefirst30safterNaBH4additio11, an absorption spectrum with a shoulder centered at 275 nm appears. At longer time, two peaks centered at 275 and 400 nm are observed (Figure 15C,D). After 25 min, the 275-nm peak decreases to disappear after 24 h. This indicates the formation of Agq2+clusters soon after mixing the reactants. Then two processes take place with formation of A a 2 +clusters and larger particles characterized by the plasmon peak. The latter can be due to the aggregation of unstable c1usters.u

-

The Journal of Physical Chemistry, Vol. 97, No.49, 1993 12981

Synthesis of Ag Nanocluster in AOT

60 50

401 30

" .

.

16

.

.

46

32

62

78

94

110

DIAMETER

.

' e

;

..

w = 7.5

II

Isooctane

32

46

L 78

62

DIAMETER

94

110

(A)

Figure 14. Electron microscopy and size distribution of silver particles synthesized in isooctane and cyclohexane. [ A O T ] a = 10-' M, [AgAOT] = 3 X 1k2M; [NaAOT] = 7 X 1W2 M; [NaBH,] = 2.5 X 10-4 M;W = 7.5. The marks represent 208 A.

TABLE IIk Change in the Size of Silver Particles with the

Kinetics Models

Bulk Solvent

L x W

AX112 diameter

solvent [Rad](M) (nm) i p d (nm) 2.5 X l W 406 0.066 82 7.5 isooctane 7.5 cyclohexane 2.5X 10-4 415 0.032 120

(A)

(i)

55 30

21 11

(iii) Above W = 5. Immediately after the reducing agent addition, large aggregates are formed characterized by a plasmon band centered at 400 nm. However, a shoulder can be noticed at 275 nm due to A&2+clusters (Figure 15E,F). As has been previously said, on replacing isooctane by cyclohexane as the bulk solvent, the intermicellar exchange processes are slower. Figure 15G shows that immediately after NaBH4 addition, the formation of A a 2 +clusters characterized by its absorption at 275 nm. Compared to the results obtained with isooctane (Figure 15C), no absorption at 400 nm is yet observed, indicating no formation of large particles. At longer time (Figure 15H), the plasmon peak appears. After 2 min, the 275-nm peak decreases with an increase in the plasmon peak. There is no difference in the formation kinetic of Agg2+clusters in isooctane and cyclohexane. Hence, as in homogeneous solution we can observed the two major steps of the silver formation, the nucleation, and then the aggregation. However, these processes, obtained after chemical reaction,are drastically slowed down in regard to what is observed in aqueous solution in steady state328 or pulse radiolysis.3*J4

A kinetic model, first developed by Schmoluchowski,3~has been largely used to explain cluster formation. This model can be described as follows: After nucleation, the growth phase starts with a suspension of monomers of concentration, eo, which agglomerate. Thus the total concentration of particles is given by Ccn(t)

= cO/(l+

t / ~ )

(3)

where c,, is themcentration of n-mer (n is the aggregationnumber of monomer in the particles), t is the time, and T is the half-time of the reaction (time at which the total concentration of particles is divide by a factor 2). In the case agglomeration is due to Brownian motion, eq 3 can be resolved analytically and r is given by (4)

where kd is the diffusion-controlled rate constant. The concentration of particular n-mer at any particular time is given by cn = c0(t/r)"l (1

+ f/r)-("+l)

Petit et al.

12982 The Journal of Physical Chemistry, Vol. 97,No. 49,1993

I "1

W

.

8 IN CYCLOHEXANE

0 (tarUUm150a)

Figure 15. Variation of the formation of silver particles in AgAOT-NaAOT-water-solvent with time. Optical path length = 1 mm. [AgAOT] = 3 x 10-2 M;[NaAOT] = 7 x 10-2 M;[NaBH,] = l e M. (Aand B) W = 1.5;bulk solvent = iosoctane. (Cand D) W = 5; bulk solvent = isooctane. (E and F) W = 7.5; bulk solvent = isooctane. (G and H) W = 5; bulk solvent = cyclohexane.

which is maximum at t,,,

(6) t,,, = ( n - 1 ) T / 2 To take into account the fact that the aggregation process is due to the intermicellar exchange, it was recently proposed to slightly modify this scheme by replacing kd with k,,, the intermicellar exchange rate constant.30 Under our experimental conditions, tmx is constant and found equal to 30 min. By using the kinetic model described above, our experimental results can be explained as follows: (i) Assuming that the monomer concentration, CO, is directly related to the average number of reducing agent per micelles, the decrease in co (eq 4) induces an increase of T and then a decrease of the total concentration of particles (eq 1). Because t,,, is constant and T increases, this induces a decrease in the aggregation number, n (eq 6), that is to say a decrease in the size of the

particles. This is consistent with the data presented above (Table 11) whereadecreasein thesizeisobservedeitherwith thedecrease in the water content (number of droplets increases and then the average number of reducing agent per micelle decreases) (Figure 5 ) or with the increase in the micellar concentration (at fixed reducing agent concentration) (Figure 10A) or a decrease in the overall reducing agent concentration (Figure lOB), a t a constant micellar concentration. (ii) Replacing isooctane by cyclohexane induced a decrease in the intermicellar exchange rate constant from k, = lo7M-* s-I to k,, = 106 M-1 s-I .30 Hence, the increase of 7 induces a decrease in the aggregation number of particles. As is observed above, the size of the particles is smaller by using cyclohexane instead of isooctane (Figure 4). From the average size of silver particles and the silver density,36 the aggregation number of the final particles can be estimated and the T value calculated (eq 6): at

Synthesis of Ag Nanocluster in AOT

W = 1.5, the r value is found equal to 0.5 and 4.5 s in isooctane and cyclohexane, respectively. The ratio riM1/rcyc,o is equal to 9 which is similar to the ratio of the exchange rate constant. From eq 4, the concentration of initial monomers is found close to 2 X le7 M in both cases. This confirms the fact, mentioned above, that only a few part of the NaBH4 reduced the silver ions (see Table 11). Conclusion The use of functionalizedsurfactant as silver bis(2-ethylhexyl) sulfosuccinate instead of sodium AOT favors the synthesis of various sizes of silver metallic particles with a narrow size distribution whereas using silver nitrate in sodium AOT reverse micelles floculate is observed. The stability of these metallic particles is very long in presence of air compared to what observed in former study with cationic or non ionic reverse micelles. Both electronic microscopy and UV-vis absorption spectra show the synthesis of pure silver nanocrystallites. By varying the water content, the reducing agent concentration, and the solvent, the size of the particles can be changed. The scheme of formation of these clusters is described and found very close to that obtain in aqueous water in the presence of a protecting agent. It is possible to stabilize the primary cluster A g P , showing the two steps of the precipitation process: nucleation and aggregation. The average size of final silver metallic colloids is found to be dependent on the micellar size, but also on the nature of the solvent and on the reducing agent concentration. This is explained considering a modified Schmoluchowsky’s scheme taking into account the intermicellarrate constant.30 On changing the solvent, the intermicellar rate constant changes without changing the micellar size which yields a change in the intermicellar growth process and then in the particles size. This kinetics study shows also that the reverse micelles were used essentially to stabilize and stop the aggregation process; the micellar size has only an indirect effect on the particles size. The nature of water seems to play also an important role on the reaction: at very low water content the nature of water is drastically different from bulk water which can explain the exceptional stability of the silver cluster. The drastic increase of the reaction times allows us to obtain the kinetics of formation without requiring fast kinetics methods.

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