Transformation of CdS Colloids: Sols, Gels, and Precipitates - The

Highly concentrated CdS colloids (about 30 wt %) are prepared in acetone through ... The sol−gel transformation results from an aggregation mechanis...
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J. Phys. Chem. B 2001, 105, 10228-10235

Transformation of CdS Colloids: Sols, Gels, and Precipitates T. Gacoin,† K. Lahlil,‡ P. Larregaray, and J-P. Boilot* Groupe de Chimie du Solide, Laboratoire de Physique de la Matie` re Condense´ e, CNRS UMR 7643, EÄ cole Polytechnique, 91128 Palaiseau Cedex, France ReceiVed: May 8, 2001; In Final Form: August 5, 2001

Highly concentrated CdS colloids (about 30 wt %) are prepared in acetone through the grafting of 4-fluorophenylthiol at their surface. 19F NMR measurements show that the controlled oxidation by an aqueous solution of hydrogen peroxide leads to a mixture of dithiol and fluorosulfonate species. Aggregation of particles produces either transparent gels or opaque precipitates, depending on the nature and the concentration of the oxidant. The sol-gel transformation results from an aggregation mechanism, which requires the release of surface groups because of their interaction with water molecules. Considering the formation of mass-fractal clusters, small-angle X-ray scattering measurements show, at least in the first steps, that the growth kinetics is in agreement with the usual reaction limited cluster aggregation mechanism previously observed in the silica system. This can be followed by a saturation effect, enhanced at low water concentration, which is due to the poisoning of the surface by chemical groups produced by oxidation.

Introduction The colloidal processing of ceramics is an emerging field of scientific research.1 A controlled aggregation of highly concentrated colloids provides the opportunity to reliably produce ceramic films and bulk forms with optimized micro- or nanostructures. In this field, the preparation of optically transparent materials represents the ultimate challenge, which has led to the development of new chemical processes such as sol-gel chemistry.2 The synthesis of solid materials made from nanoparticles as building blocks involves the aggregation of the particles to form a solid network. A transparent material will be obtained rather than a diffusive powder only if the aggregation is well-controlled so that the physical structure of the aggregates preserves the homogeneity of the material in the visible-wavelength scale. The stability of colloidal suspensions is a major problem for the synthesis of solid particles with a controlled size, shape, and concentration. Particles suspended in a liquid medium undergo strong attractive van der Waals forces which usually lead to the formation of a precipitate of aggregated particles rather than a stable suspension. This natural tendency for aggregation can be, to some extent, limited by the proper choice of the chemical environment of the particles (solvent, pH, ionic strength, complexing molecules, etc.) so that electrostatic or steric hindrance ensures a sufficient physical repulsion. In a somewhat limited number of cases, colloidal suspensions with a high weight fraction (more than a few percent) of the solid phase can be obtained with particles in the nanometer-size range (SiO2,3 ZnO,4 TiO2,5 etc.). The theoretical aspects of colloid aggregation have been the subject of a large number of investigations based on experimental results gathered from model systems such as gold,6 silica,7 or latex8 colloids. Most of these works use the formalism * Corresponding author. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail: [email protected].

of fractal geometry to give a mathematical description of the morphology of the aggregates. Their main characteristics are then found in their fractal dimension, D, which determines the variation of their mass, m, as a function of their radial dimension, ξ, through the relation

m ∝ ξD

(1)

A value of D ) 3 corresponds to dense solids or solids with a regular porosity, whereas lower values (1 < D < 3) are found for lacunar aggregates corresponding to mass-fractal objects. The sol-gel processing of transparent materials is based on the controlled formation of such lacunar aggregates within the solution which is called a “sol”. Percolation of the aggregates, corresponding to the gelation, results either from rapid solvent evaporation (deposition of thin films by spin or dip coating) or simply from their continuous growth within the sol. A gel can then be described as a rigid skeleton of particles enclosing a continuous liquid phase. The careful removal of the solvent from the liquid phase yields the final solid material. This material can be either an aerogel with a very low density if the drying is achieved under supercritical conditions or a dense xerogel if the drying is made slowly at low temperatures (20-100 °C) to avoid fractures from capillary stresses.9 Until recently, sol-gel chemistry was restricted to a limited number of compounds, mostly oxides and especially silica. The main barrier for its extension to other compounds lies in the difficulty of the synthesis of highly concentrated colloids with a controllable state of dispersion. In a previous work,10,11 we have qualitatively shown that highly concentrated CdS colloids could be stabilized in acetone through the grafting of 4-fluorophenylthiol at their surface. Because thiols are well-known to be easily oxidized and acetone does not have a strong interaction with the particle surface, the controlled oxidation of the thiol leads to the aggregation of the particles, opening the way toward the development of sol-gel chemistry to chalcogenide materials.

10.1021/jp011738l CCC: $20.00 © 2001 American Chemical Society Published on Web 09/28/2001

Transformation of CdS Colloids This work is devoted to a better understanding of the aggregation mechanism, concerning both the chemical reactions which are involved and the growth kinetics of the fractal aggregates. Experimental Section Synthesis. All reagents were purchased from Aldrich and used as received without further purification. (1) CdS Colloids. A huge amount of work has been devoted to the controlled synthesis of CdS colloids as a consequence of the interesting physical properties related to quantum confinement.12-16 In our previous studies concerning the sol-gel transformation of CdS nanoparticles, two methods have been used to obtain concentrated colloids stabilized in acetone with 4-fluorophenylthiol. The first one consists of the precipitation of CdS within the water pools of a microemulsion. This allows for the synthesis of CdS particles in a rather small amount but with a controllable particle size in the 2-6 nm range. The second consists of the precipitation of the CdS particles directly in the presence of thiol, producing a large amount of particles with a size of about 1-2 nm. In this work, these two preparations are used to deduce a qualitative evolution of CdS colloids after the oxidation of capping thiolates. In contrast, a quantitative study of the aggregation mechanism is performed by using a single batch of 2 nm colloids prepared by the second method. Typical syntheses in microemulsion are conducted as follows: 17 di(2-ethylhexyl)sulfosuccinate sodium salt or AOT (111.14 g, 0.25 mol) is dissolved in 500 mL of heptane. Into this solution is added 22.5 mL of an aqueous solution of cadmium nitrate with a variable concentration (0.01-0.2 M). The resulting solution is vigorously stirred until it becomes optically clear. At this stage, the cadmium ions are dispersed within the water pools formed by the inverted micelles of the microemulsion. The formation of the CdS nanoparticles occurs with the injection of an excess of H2S. As deduced from absorption spectrometry and from the size/gap correlation,18 the average size of the particles varies between 2 and 6 nm depending on the initial cadmium concentration. Excess H2S is removed by bubbling with N2. The addition of 4-fluorophenylthiol and triethylamine with a concentration equal to 5 times the initial Cd concentration induces the flocculation of the particles which can be recovered as a powder by centrifugation. After washing with heptane, the particles can be dispersed in acetone. In the second process, particles are prepared by direct precipitation in the presence of 4-fluorophenylthiol. A total of 100 mL of acetone containing 4-fluorophenylthiol (128 mg, 1 mmol), hydrogen sulfide (12 mL, 0.5 mmol), and triethylamine (101 mg, 1 mmol) is added dropwise to the same volume of a vigorously shaken acetone solution containing cadmium nitrate Cd(NO3)2‚4H2O (124 mg, 0.4 mmol). After the addition is completed, the solution is left under vigorous stirring for 1 h. The solvent is removed by evaporation, and the resulting powder is washed with ethanol and finally dispersed in acetone. As deduced from the size/gap correlation, the size of the particles is about 2 nm in diameter. In all cases, particles can be dispersed in acetone at a concentration up to 30 wt %, corresponding to a Cd atomic concentration of about 5 M according to 113Cd NMR measurements. (2) Dance’s Cluster Synthesis. Well-defined phenylthiolatecapped CdS clusters have been extensively studied by Dance et al.19 and later by Herron et al.20 The process we used for the

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10229 fluorinated complex was adapted from the Dance process, taking into account the much higher solubility of the fluorinated complex in methanol compared to that of the nonfluorinated one. The synthesis of (N(CH3)4)4[S4Cd10(SC6H4F)16] is achieved under an inert atmosphere of argon. A solution of cadmium nitrate Cd(NO3)2‚4H2O (5.25 g, 17 mmol) in 15 mL of methanol is slowly added to a vigorously stirred solution of 15 mL of methanol containing 4-fluorophenylthiol (4.84 mL, 45.5 mmol) and triethylamine (6.34 mL, 45.5 mmol). During the addition, a precipitate forms which immediately dissolves, and the final solution is perfectly clear. Tetramethylammonium chloride (2.1 g, 19.25 mmol) dissolved in 15 mL of methanol is then added, and the resulting solution is stirred for a few minutes. The addition of water (4:1 by volume) results in the precipitation of a very viscous phase which is separated by centrifugation and dissolved in 10 mL of methanol. Tetramethylammonium chloride (2.1 g, 19.25 mmol) is added, followed by sulfur powder (0.136 g, 4.25 mmol) under vigorous stirring. After a few minutes, a white precipitate begins to form. It is recovered by filtration after 45 min, washed with ethanol, and dried in a vacuum. The obtained powder is crystallized by the slow evaporation of the acetone from an acetone-butanol mixture (1:4 by volume). The formula and the structure of the complex are confirmed by microanalysis and 19F and 113Cd NMR spectroscopy. Because the grafting of 4-fluorophenylthiol at the surface of the CdS nanoparticles is achieved without using the tetramethylammonium salt, the Dance complex with the triethylammonium ion as the counterion of the surface charges is expected to be more interesting to mimic the surface structure of the CdS particles. The (HN(C2H5)3)4[S4Cd10(SC6H4F)16] complex is then synthesized using exactly the same procedure as described above for the fluorinated Dance complex except that triethylammonium chloride is used instead of the tetramethylammonium salt. Triethylammonium chloride was made simply by bubbling HCl gas into a solution of triethylamine in diethyl ether. Gelation Experiments. Investigation of the gel formation is achieved on colloids with a CdS concentration between 0.1 and 5 M in acetone. In a typical experiment, 0.1 mL of a solution containing H2O2 is added to 0.9 mL of the starting colloid under vigorous agitation. The H2O2 solution is obtained by diluting a commercial 30% H2O2 solution in either water or acetone. The final concentration is chosen depending on the desired molar ratio (X) between H2O2 and the grafted thiolate in the colloid. This latter concentration was deduced from 19F NMR measurements. Investigation of the influence of water on the kinetics of aggregation is made after the addition of an appropriate amount, in addition to the water from the 30% H2O2 solution. NMR Spectroscopy. 19F NMR spectra are recorded on a Bruker WP200 spectrometer. FC6H4NO2 was chosen as an internal reference for the chemical shift, which is taken to be equal to -101.75 ppm compared to the standard CFCl3 (Freon 11). Synthesis of the references for the oxidation products of 4-fluorophenylthiol are adapted from previous studies. (1) The dithiol, FC6H4SSC6H4F, is made by the oxidation of the thiol using dimethyl sulfoxide.21 A 25 mL flask equipped with a magnetic stirring bar is charged with 5 g (39 mmol) of 4-fluorophenylthiol and 6 g (78 mmol) of dimethyl sulfoxide. The solution is stirred at room temperature for 24 h, and the produced dimethyl sulfide is removed under vacuum. The obtained oil crystallizes after cooling at 0 °C. The solid is then purified by recrystallization from hot ethanol to give white flakes (19F NMR chemical shift in acetone, -112.26 ppm).

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Figure 1. Schematic representation of the time evolution of CdS colloids after oxidation by aqueous hydrogen peroxide solutions depending on the X molar ratio between H2O2 and the grafted thiolate in the colloid.

(2) FPhSO3K is made by the oxidation of the thiol in the presence of KOH using an excess of H2O2.22 A solution is prepared containing 50 mL of ethanol, 20 mL of H2O, 1 g (7.9 mmol) of 4-fluorophenylthiol, and 2.2 g (40 mmol) of potassium hydroxide. A total of 1.6 mL of 30% H2O2 (15.8 mmol) is added, and the resulting solution is stirred for 30 min. The solvent is removed in a vacuum, and the resulting solid is crystallized from 2 mL of hot ethanol. An 19F NMR spectroscopy experiment performed in ethanol indicates a mixture of FPhSO3K (-108.7 ppm, 85%) and FPhSO2K (-110.76, 15%). (3) FPhSO2Na is made according to the method of Dumont and Rumpf.23 In the first step, the reaction of an excess of chlorosulfonic acid (5 equiv) with pure fluorobenzene (1 equiv) leads to sulfochloride, which is isolated by distillation under vacuum (75 °C, 0.5 mmHg). Then, sulfochloride is reduced at 70 °C for 3 h by anhydrous sodium sulfite under basic conditions (with the addition of a 40% NaOH solution). FPhSO2Na is purified by crystallization from water. Small-Angle X-ray Scattering (SAXS) Experiments. SAXS experiments are performed using synchrotron radiation at LURE (Orsay, France). The white beam from the storage ring is monochromatized using a double crystal monochromator of silicon tuned for 8.5 keV (λ ) 0.152 16 nm). A one-dimensional position-sensitive detector is used to record the scattered intensity from the CdS samples in a capillary tube. The parasitic scattering from air, slits, and the sample holder is subtracted from the experimental SAXS intensity. Absorption Spectroscopy. The evolution of the absorption spectra of a sample during its gelation is recorded on a Shimadzu UV-160A spectrophotometer in quartz cells with a 2 mm path length. Because the samples have too high of an absorbency, measurements are performed on small aliquots diluted about 100 times in acetone. Results Macroscopic, Visual Evolution of the Sol. The controlled aggregation of CdS colloids requires the progressive oxidation of the stabilizing thiolates. As previously shown,10,11 a CdS solgel transition can be observed by using H2O2 as the oxidant. Surprisingly, other usual oxidants, such as bromine or benzoyl peroxide, do not induce any gelation but instead only a rapid precipitation at high concentrations.

Gacoin et al.

Figure 2. Photographs of CdS colloids (average particle size of 2 nm and Cd concentration of 0.1 M) after oxidation by an aqueous hydrogen peroxide solution (10% H2O) for different values of the X molar ratio between H2O2 and the grafted thiolate in the colloid. Note the clear evolution from transparent gels to opaque precipitates when the X value increases.

Concerning the H2O2 oxidation, Figure 1 gives a qualitative description for the time evolution of CdS sols as a function of the X parameter (molar ratio of H2O2 and the grafted thiolate). (1) The addition of a small amount of H2O2 does not induce any visual evolution of the solution until a minimum value, Xmin, is reached for which gelation is observed. Note that the amount of water in the H2O2 solution has a strong influence on the Xmin value. As an example, the value of Xmin is about 0.2 for a colloid (2 nm in diameter, 0.1 M in CdS) in the presence of 10% H2O, whereas it increases up to 0.5 when the H2O amount is less than 1%. (2) Above Xmin, higher amounts of oxidized thiolate lead to the gelation of the sol after a period of time which decreases from a few days down to a few minutes with increasing X values. Concerning the reversibility of the sol-gel transition, the increase of the temperature or the dilution in a large excess of solvent does not show any effect. Vigorous shaking only breaks the gel into smaller gelatinous pieces. (3) Increasing X values lead to the syneresis of the gels as a consequence of the shrinkage of the solid network associated with the solvent expulsion out of the pores. (4) In the case of very high amounts of oxidant (typically 5Xmin), gelation is not observed, and the particles only precipitate. For higher concentrations of oxidant, the particles are partially dissolved as a consequence of the oxidation of the sulfide ions of the CdS particles themselves. Photographs (Figure 2) summarize this evolution of CdS colloids (2 nm in diameter, 0.1 M in CdS) in the presence of 10% H2O as a function of the X value. Transparent gels, opaque gels, and opaque precipitates are successively observed by increasing the X value from X ) 0.2 to X ) 5. Chemical Evolution of the Sol Followed by 19F NMR Spectroscopy. 19F NMR spectroscopy was used in order to characterize the chemical evolution of the sol after the addition of H2O2. Figure 3 shows the time evolution of the NMR spectra recorded on a CdS colloid with a particle size of 2 nm and a concentration of 0.1 M. The starting colloid exhibits two broad bands located around -119 and -122 ppm, which can be assigned in reference to the spectrum of the Dance complex. These two bands correspond to terminal (Cd-tSC6H4F) and bridging (Cd-µSC6H4F-Cd) thiolates, respectively. The observed broadening is attributed to a distribution of the chemical environment at the surface of the particles.

Transformation of CdS Colloids

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10231

Figure 4. 19F NMR spectra (domain around -112 ppm) showing the products of the oxidation of fluorophenylthiolate groups by an aqueous hydrogen peroxide solution (X ) 1). The more intense peak corresponds to dithiol. The weak peak corresponds to the sulfonate FC6H4SO3-. In contrast with the signal of dithiol (-112.26 ppm), note the shift of the latter when the water content increases from 1% to 10% by volume.

Figure 3. (a) Structure of the Dance complex. (b) Schematic description of the surface of the CdS particles. (c) Time evolution of the 19F NMR spectra for fluorophenylthiolate-capped CdS colloids (average particle size of 2 nm and Cd concentration of 0.1 M) after oxidation by an aqueous hydrogen peroxide solution (X ) 0.2-0.2% H2O). The two sharp peaks correspond to terminal (Cd-tSC6H4F) and bridging (Cd-µSC6H4F-Cd) thiolates existing in the Dance-type complex.

Just after the addition of hydrogen peroxide (X ) 0.2 and 0.2% of water in the solution), a peak located at -112.2 ppm appears with an increasing intensity. This signal is associated with the progressive release of the oxidized thiol species such as the dithiol (FC6H4S-SC6H4F). At the same time, the two initial thiolate bands turn into a very broad band shifted downfield. This shift is not well understood but might be related to the chemical or electrostatic changes of the surface of the nanoparticles. As the time increases, the thiolate signal continues to decrease without producing more dithiols. Aggregation of nanoparticles produces clusters of increasing size which tumble more and more slowly in the solution. This accounts for the continuous fall of the signal as a consequence of NMR dipolar broadening, as previously observed in the case of silica.24 This evolution clearly shows that the aggregation kinetics is not controlled by the oxidation reaction. This can be generalized for samples with different X values (up to X ) 1) and different water contents (up to 10% by volume) for which kinetic studies of the oxidation, as revealed by NMR spectroscopy, show that this reaction is nearly complete after about 15 min. Moreover, the evolution of the large thiolate signal remains mostly the same. Nevertheless, thiol molecules are well-known to undergo oxidation, providing different products depending on the reaction conditions.22 Thus, for X ) 1 and focusing our attention on the band around -112 ppm, we observe the presence of another band whose chemical shift is strongly dependent on the water contents (Figure 4). The addition of reference species directly into the NMR sample confirms the presence of the dithiol with a chemical shift (-112.26 ppm) which is almost independent of the water contents. The other signal corresponds to the sulfonate FC6H4SO3-, whose NMR signal shifts from -112.42 to -111.61 ppm when the water quantity increases from 1% to 10% by volume. Note that the chemical shift of the sulfonate

does not depend only on the water content, because the reference product has a chemical shift of -112.58 ppm in acetone containing 2% H2O, while its position is at -112.01 ppm when the CdS particles are present in this solution. Furthermore, the integrated intensity of both signals does not change with the water content, although the width of the sulfonate band clearly decreases when the water content is increased. This suggests some interactions between the surface of the particles and sulfonate species which are progressively weakened in the presence of water. In fact, at 10% water by volume, the relative concentration of the sulfonate increases from a few percent for X ) 0.2 to 40% for X ) 1. Thus, for X ) 1, about 50% of the thiol is oxidized after 15 min, among which 40% is transformed into sulfonate and 60% into dithiol. Because the formation of dithiol and sulfonate uses half and three H2O2 molecules, respectively, the yield of the oxidation is about 75%. Finally, the chemical reaction which occurs after the addition of hydrogen peroxide in the CdS colloids can be schematically written as

Structural Evolution of the Sol Followed by SAXS. SAXS experiments were performed in order to study the structure of the aggregates and their growth kinetics. Figure 5 presents the results obtained in the case of a representative sample similar to the one studied by NMR spectroscopy (CdS colloid with an average particle size of 2 nm and a concentration of 0.1 M). This shows, at different times, the evolution of the scattered intensity (I) as a function of the wave vector [Q ) (4π/λ) sin(θ)] on a log-log scale (λ is the X-ray wavelength and θ is the diffusion angle). At low Q, the curves show a plateau corresponding to the Guinier regime. Assuming the usual fractal analysis, the intermediate Q range corresponds to the mass-fractal regime. It spreads out between 2πξ-1 and 2πa-1, where ξ is the coherence length above which the material is no longer fractal and a is the average particle size below which the material is considered to be fully dense. Considering an assembly of spherical particles aggregated into clusters with a disordered fractal geometry, the correlation function is expressed by the

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Gacoin et al.

Figure 5. Time evolution of the scattered intensity for a CdS colloid (average particle size of 2 nm and Cd concentration of 0.1 M) after oxidation by an aqueous hydrogen peroxide solution (X ) 0.2-10% H2O). Solid lines represent a fit using eq 3 assuming the formation of mass-fractal aggregates. This leads to the fractal dimension D ) 1.9 and to the average size of the aggregates ξ. The inset shows a schematic representation of a mass-fractal aggregate of size ξ formed of CdS elementary units of size a.

Figure 6. Time dependence of the average mass of aggregates for CdS colloids (average particle size of 2 nm and Cd concentration of 0.1 M) oxidized by an aqueous hydrogen peroxide solution (10% H2O) for different values of the X molar ratio between H2O2 and the grafted thiolate in the colloid. The average mass values are deduced from fits of scattered curves with eqs 1 and 2. The inset shows the exponential time dependence of the average mass at short times which is in agreement with the RLCA aggregation mechanism (see eq 5).

relation25

g(r) ∝ (r/a)D-3 exp(-r/ξ)

(2)

where ξ may be associated to the average size of the aggregates and D to the fractal dimension. This leads to the following analytical variation of the scattered intensity as a function of Q:26,27

Γ(D + 1) sin[(D - 1) arctan(Qξ)] I(Q) ) AF2ξ2 2 2 (D-1)/2 (D - 1)Qξ (1 + Q ξ ) (3) A is a constant depending on the square of the average scattering length, Γ is the gamma function, and F is the bulk density. Fitting the experimental curves with the above formula provides an estimate of the fractal dimension and of the average correlation length. This fit, applied to the crossover between the fractal and the Guinier regime, gave a fractal dimension of 1.9 for the scattering curves at different aging times. Note that this fractal dimension seems to be independent of the size of starting colloids as shown by other experiments performed from particles of 3 and 5 nm in size. Using the characteristic relation for a mass-fractal aggregate (m ∝ ξD, where m is the average mass of a cluster of size ξ), one can deduce the time evolution of the average mass of the aggregates after the sol oxidation. These are shown both for the first experiments (Figure 6) in which the water amount is 10% and for the second ones (Figure 7) with a low water concentration (less than 1%). From these curves, it is clearly seen that both the X value and the amount of water have a strong impact on the cluster-growth kinetics and that the data reported here give more quantitative indications than just by considering the gelation time. In fact, many works have been devoted to the theoretical description of colloid aggregation. The simplest models rely on the mathematical descriptions provided by solving the Smoluchowski equations after assuming simple approximations. In all cases, the colloid aggregation is thought to occur through cluster-cluster aggregation, and two extreme cases can be examined considering the sticking probability, R, after each particle encounter. In the diffusion-limited cluster aggregation model (DLCA), the sticking probability is almost

Figure 7. Time dependence of the average mass of aggregates for CdS colloids (average particle size of 2 nm and Cd concentration of 0.1 M) oxidized by different aqueous hydrogen peroxide solutions. The numbers labeling the curves give both the X molar ratio between H2O2 and the grafted thiolate in the colloid and the volume percentage of water in the solution. Labels X ) 0.05 + 0.05 and X ) 0.05 + 0.1 indicate that an aqueous solution of hydrogen peroxide (X ) 0.05 or X ) 0.1) was added to a sample which had attained the saturation regime after a previous oxidation by a first solution (X ) 0.05).

equal to 1, so the cluster growth is limited by the diffusion of the aggregates. In this case, the mean cluster mass is expected to scale linearly with the time:

m(t) ) m0(1 + ct)

(4)

In this equation, m0 is the mass of the elementary particles while c is a constant which is proportional to the concentration of particles and also depends on the temperature and the viscosity of the solvent.28 In the reaction-limited cluster aggregation (RLCA), the sticking probability is now assumed to be significantly less than one, and the solution of the Smoluchowski equation predicts that the mean cluster mass should behave as

m(t) ) m0 exp(cRt)

(5)

The results obtained from our experiments do not show a perfect correlation with the above models. However, especially

Transformation of CdS Colloids for the highest X values, a very good correlation with RLCA is found at shorter times, t, for the sample with 10% H2O (Figure 6). In this case, for X ) 0.15, the linear dependence of the constant k (k ) cR) as a function of the starting concentration of particles is also well-verified in the range from 0.03 to 0.3 M. When t increases, this system progressively deviates from the simple RLCA model. This is probably related to the effect of saturation which interferes with the mechanisms involved in this model. Such a saturation can be seen very clearly for samples with a low water concentration (Figure 7) and for those at lowest X values with a 10% H2O concentration. In fact, at a given time t, the evolution of the system could then still be described by the characteristic relation of the RLCA mechanism dm/dt ) km, but the kinetic constant would be a decreasing function of M. The saturation effect can then be associated to a decrease of the cluster sticking probabilities as their size grows. This is confirmed by a simple experiment (Figure 7) in which an aqueous solution of H2O2 was added into the sample which had already reached its saturation cluster size after being treated with X ) 0.05 (0.05% H2O). In this case, the clusters start to grow again as a consequence of the increase of reactivity induced by the addition of the fresh aqueous solution of H2O2. Evolution of the Optical Properties. The II-VI semiconductor nanoparticles are well-known to exhibit a strong change of their optical absorption when their size is reduced down to a few nanometers. This is a consequence of the quantum confinement of electrons and holes, which is basically a blue shift of the absorption threshold and the appearance of discrete energy levels associated to the progressive transition from the bulk state down to a molecular behavior. In the case of CdS particles of a few nanometers in diameter, the effect is very sensitive to small changes in the size of the particles. Absorption spectroscopy is, then, a good way to study any change in the particle size which could occur during the aggregation of the particles. In fact, during the aggregation of CdS colloids, no significant change is observed in the absorption threshold, indicating that the aggregation process only proceeds from a very small change of the chemical nature of the surface of the particles. This is not the case when the colloid is treated with much more H2O2 (i.e., X > 0.8 in the case of this sample). A blue shift is then observed, which indicates a partial dissolution of the particles through the oxidation of the sulfide ions. In fact, the only change in the absorption spectra is the appearance of an additional signal which continuously increases with time. The intensity of this signal decreases as a function of the wavelength, which is characteristic of the light diffusion induced by the aggregates whose size becomes close to the wavelength of measurements (Rayleigh scattering). Luminescence spectroscopy is also well-known to be very sensitive toward small changes of the surface chemistry of the II-VI semiconductor nanoparticles. Nevertheless, in the case of our CdS nanoparticles, the surface complexation by thiols provides colloids with almost no detectable luminescence. This is attributed to the ability of thiols to act as hole traps, thus preventing any efficient radiative recombination of the excited carriers. As in absorption spectroscopy, no significant change (i.e., no increase of luminescence signal) could be detected while the aggregation of the particles proceeded. Here again, the only detectable changes in the luminescence behavior occur when the colloid is treated with an excess of H2O2. In this case, a broad luminescence signal appears, indicating some kind of passivation toward the nonradiative processes which might be related to the elimination of most of the surface thiolates.

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10233 Discussion The beginning of this work is concerned with the very high stabilization of the CdS nanoparticles through the grafting of 4-fluorophenylthiol at their surface. Concentrations of more than 30 wt % have thus been obtained in organic solvents such as acetone or tetrahydrofuran. This stabilization power in organic polar solvents is much more important than it is for other complexing molecules which have been previously proposed and, more particularly, as compared to the nonfluorinated phenylthiol. In a previous work on the synthesis of small CdSx(SC6H4)y particles, Wang et al. have shown that the reaction performed under specific experimental conditions (mixed acetonitrile/methanol solvent) leads to particles with a better solubility as a consequence of a higher (SC6H4)/S molar ratio in the final particles.12 The stabilization of such particles in organic solvents is then explained both by the interaction of the surface thiolate with solvent molecules and by the electrostatic repulsion caused by negative surface charges from thiolates in excess. Obviously, this stabilization behavior still remains when using 4-fluorophenylthiolate instead of phenylthiolate as the complexing agent. Nevertheless, the fluorinated Dance complex ((N(CH4)4)4[S4Cd10(SC6H4F)16]) has a very high solubility in acetone, while the solubility of the nonfluorinated one ((N(CH4)4)4[S4Cd10(SC6H4)16]) is very poor. Because these two clusters exhibit exactly the same structure, the improvement of the stabilization does not occur either from an increase of the surface electrostatic charge or from an increase of the thiolate surface coverage. We may then conclude that the stabilization is a consequence of the much higher dipole moment of the 4-fluorophenylthiol as compared to the phenylthiol. This leads to high dipolar interactions between the surface thiolate and the solvent molecules, and the resulting thick solvating shell improves the stabilization of the particles. Starting from very well-stabilized CdS particles in acetone, the addition of an oxidant induces the aggregation of the particles through the removal of their stabilizing surface thiolates. Experiments have shown that the evolution of the colloid after the partial oxidation of the thiolates strongly depends on the chemical nature of the oxidant, its concentration, and the amount of water present in the solution. As checked by NMR, oxidation of the thiol produces either the dithiol or the sulfonate. This transformation of the thiolates requires a charge compensation. Thus, the first step of the evolution of the particles is the partial replacement of the thiolates by the reduced form of the oxidant: either Br-, C6H5COO-, OH-, or FC6H4SO3-. As a consequence, the differences observed in the further evolution of the colloid may then be found in the differences in affinity between these anions and the surface of the particles or with the solvent. Concerning the oxidation with bromine or benzoyl peroxide, no evolution of the colloid is observed for increasing amounts of oxidant up to a point when the particles suddenly precipitate. This means that, in addition to the remaining thiolate, the Brand C6H5COO- groups have enough interactions both with the surface of the particles and with the solvent molecules to limit the distance of approach between the particles. Nevertheless, these interactions are not strong enough to ensure the stabilization of the particles when a large portion of their surface thiolate has been oxidized. At this critical point, van der Waals interactions prevail and aggregation occurs. This leads to a precipitate rather than a gel because a large portion of the surface is free of a solvating shell and the particles can rearrange themselves into dense aggregates.

10234 J. Phys. Chem. B, Vol. 105, No. 42, 2001 On the contrary, the observation of a sol-gel behavior when using an aqueous H2O2 solution as the oxidant means that, in this case, the activation of the surface is much more efficient for aggregation so that the particles can stick together even for a small fraction of oxidized thiolates. This explains why a gel is obtained rather than a precipitate because, as a small part of the surface is activated, no reconstruction can occur that would lead to the precipitation of dense aggregates. This much higher reactivity of the particles treated with H2O2 seems to occur through the concomitant effects of the chemical species involved on the surface (OH- and FC6H4SO3-) and the presence of water. Water is indeed always present when the colloid is treated with H2O2, and all of our experiments have shown a drastic increase of the aggregation kinetics when the amount of water is increased. The starting CdS particles are negatively charged with HN(CH3)3+ counterions. When treated with H2O2, the oxidation of the thiolates leaves the surface cadmium ions bonded to either OH- or FC6H4SO3- groups. The influence of water must then be found in its ability to solvate these highly hygroscopic ions. This is typically what is shown in the case of the sulfonate (FC6H4SO3-), for which 19F NMR experiments performed on samples with increasing amounts of water show an increase of the solvation of this ion which is then progressively released from the surface and may finally form a soluble salt with HN(CH3)3+. In the case of OH- surface groups, such a mechanism is enhanced because an acid-base reaction with the HN(CH3)3+ ion provides an irreversible pathway for their release from the surface of the particles. Here again, the mobility of the ionic species is greatly improved by their solvation with water molecules. Considering this mechanism, it can be stated that the waterassisted release of the OH-, FC6H4SO3-, and HN(CH3)3+ from the surface of the particles is the main reason for the high reactivity of the H2O2-treated colloids. Such a release does not seem to occur, or occurs with much less efficiency, when the Br- or the C6H5COO- is located on the surface as a result of oxidation by bromine or benzoyl peroxide. In these latter cases, the ions are less hygroscopic, and no acid-base reaction can pull the ions out of the surface as in the case of OH-. Moreover, the reported synthesis of the (N(CH4)4)2[Cd4(SC6H4)6Br4] cluster, as the result of the addition of Br2 to a solution of (N(CH4)4)2[Cd4(SC6H4F)10], gives an indication that Br- is a good ligand of cadmium atoms.29 Once activated by the elimination of surface ions (OH-, FC6H4SO3-, and HN(CH3)3+), the decrease of the charge leads to the sticking of particles in order to complete the free valence of the cadmium atoms. The progressive aggregation then leads to the formation of either new bridging thiolates or new Cd-S bonds to connect the particles themselves. This aggregation mode clearly implies directionality constraints in the structure of aggregates, leading to the formation of lacunar clusters which are required for gelation. This is in agreement with the observation of fractal clusters by SAXS experiments (fractal dimension of 1.9) which result from an RLCA mechanism at high water concentration, showing a clear analogy between cadmium sulfide and silica gelation.2 In this case, the aggregation mechanism is clearly verified at high water concentration both by the exponential dependence of the average mass of clusters as a function of time and by the linear dependence of the kinetic constant as a function of the concentration of the particles. As described before, the growth kinetics of the aggregates deviates from the usual RLCA mechanism because of the existence of a saturation effect which is not taken into account

Gacoin et al. in simple models. In our case, such a saturation is clearly seen for small X values but is much more pronounced when the amount of water is low. In a rough model at low water concentration, everything happens as if the growing of the aggregates occurred through an RLCA mechanism but with a sticking probability which decreases as the size of the aggregates increases. Considering the influence of water on the aggregation mechanism as discussed above, the saturation must be related to the consumption of the water molecules through the solvation of the ionic species formed by the oxidation of the thiolates. Saturation corresponds to the moment when there is no more water to ensure the solvation of the remaining OH- or FC6H4SO3- groups, thus leaving the surface inactive for the aggregation. This is strongly supported by the fact that, once the saturation is reached, the addition of water ensures the further aggregation of the particles until the sol-gel transition is observed. Concerning the optical properties, it is clear that the use of thiol complexing molecules to stabilize the starting particles is greatly unfavorable for their luminescence. It is indeed wellknown that semiconductor particles exhibit high luminescence quantum yields only if they have a perfect surface state with specific complexing agents such as trioctylphosphine oxide.15 The chemical process used in this work is obviously not compatible with an optimized luminescence of the particles. Future improvements might be found either by using a molecule which would act both as the fluorophenylthiol to stabilize the particles and as the phosphine oxide molecule to passivate the surface or, more probably, by using nanoparticles with a core/ shell structure30 in order to separate the interface optimized for luminescence and the surface state optimized for the sol-gel processing of the colloids. Conclusion Concerning thiolate-capped CdS particles, the aggregation mechanism requires the formation of active surface sites. This implies both the oxidation of surface thiolate complexing groups and the release from the surface of the new groups resulting from this oxidation. We have shown that oxidation by aqueous H2O2 solutions satisfies these two conditions because the release of the new groups in their ionic forms is assisted by the water molecules. This contrasts with the use of other oxidants such as bromine for which the aggregation is only observed after oxidation of a large part of thiolate surface groups leading to the sudden formation of opaque precipitates. Finally, after optimization of the experimental conditions, a clear analogy appears between the gelation mechanism of CdS colloids and the well-known one observed in silica systems. In both cases, the formation of lacunar (mass-fractal) aggregates results from an RLCA mechanism. This leads to transparent percolating gel structures which can be described as homogeneous assemblies of fractal clusters whose sizes are inferior to visible-range wavelengths. Acknowledgment. We are grateful for the important contribution of Laurent Malier. SAXS measurements were carried out using synchrotron radiation at LURE (Orsay, France). We thank Pierre Lesieur for technical assistance. We also thank Nicolas Mezailles and Patrick Rosa for their kind help in the use of their NMR equipment. References and Notes (1) Lewis, J. A. J. Am. Ceram. Soc. 2000, 83 (10), 2341.

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