Langmuir 1992,8, 1049-1053
1049
Articles Synthesis of Cadmium Sulfide in Situ in Cadmium Bis(ethy1-2-hexyl)Sulfosuccinate Reverse Micelle: Polydispersity and Photochemical Reaction L. Motte,t C. Petit,tJ L. Boulanger,§ P. Lixon,t and M. P. Pileni*itJ CEN Saclay, DRECAM-SCM, 91191 Gif sur Yvette, France, Universite P . et M . Curie, Laboratoire S.R.I. batiment de Chimie Physique, 11 rue P. et M . Curie, 75005 Paris, France, and CEN Saclay, CEREM-DTM-SRMP, 91191 Gif sur Yvette, France Received April 12, 1991. In Final Form: November 25, 1991 Functionalized reverse micelles are used to control the size of CdS semiconductor particles. We show that the use of mixed sodium-cadmium AOT reverse micelles favors the formation of monodispersed particles. By increase of the water content, the size of the particle increases, maintaining a high monodispersity. Photoelectron transfer reactions are more efficient at smaller particle size than at higher. At low water content, CdS semiconductors are protected against photocorrosion.
Introduction
Surfactantsdissolved in organic solventsform spheroidal aggregates called reverse micelles.’ Water is readily solubilized in the polar core, forming a so-called “water pool”, characterized by w, w = [H20]/[AOTl. For AOT as a surfactant, the maximum amount of bound water in the micelle corresponds to a water-surfactant molar ratio w = [HzO]/[AOT] of about 10. Above w = 15, the water pool radius is found to depend linearly on the water content (R, = 1 . 5 ~ Another ) ~ ~ property of reverse micelles is their dynamic character.’ They can exchange the content of their water pools by a collision process. The synthesis of CdS particles at very high micellar concentration ([AOT] = 0.5 MIw = 5) shows the formation of small crystallites.3-5 In dilute reverse micellar solution ([AOTI = 0.1 M), in the presence of a protecting polymer such as hexamethylphosphate (HMP), similar behavior is observed, namely, an increase in the size of particles with the water content? The polydispersity strongly decreases by using reverse micelles: even if it slowly increases with the water content. The use of a functionalized surfactant such as cadmium AOT favors the formation of more monodispersed particles with a strong increase in the photocorrosion process.’ In the present paper we present quantitative data using reverse micelles formed by a mixture of cadmium and sodium bis(ethy1-2-hexyl) sulfosuccinate surfactants. CdS particles have been extracted from the micellar solution and electron microscopy has been performed. Photocor-
* To whom correspondence should be addressed. t
CEN Saclay, DRECAM-SCM.
* Universite P. et M. Curie.
CEN Saclay, CEREM-DTM-SRMP.
(1) Structure and reactivity in reverse micelles; Pileni, M.
P., Ed.; Elsevier: Amsterdam, 1989. (2) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985,118,414. (3) Meyer, M.; Wallberg, C. C.; Kurihada, K.; Fendler,J. H. J. Chem. SOC.,Chem. Commun. 1984,90,90. (4) (a) Lianos, P.;Thomas,J. K. Chem. Phys. Lett. 1986,125,299. (b) Lianoe, P.; Thomas, J. K. J. Colloid Interface Sci. 1987, 117, 505. (5) Atkineon, P. J.; Crimson, M. J.; Heenan, R. K.; Howe, A. M.; Robinson, B. H.; J. Chem. SOC.,Chem. Commun. 1989, 1807. (6) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (7) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598.
rosion and photoelectron transfer reactions are studied in various experimental conditions.
Experimental Section Products. Sodium bis(ethy1-2-hexyl) sulfosuccinate is produced by Sigma and sodium sulfide (Na2S) by Janssen. The synthesis of functionalized surfactant has been previously described.7 The various dialkylviologens have been synthesized as described previously.8 Synthesis of CdSe6v7The synthesis is carried out by mixing two micellar solutions with the same ratio of water (w = [H20]/ [AOT]),one containing a solutionin which the sulfideions (NaZS) are solubilizedand the other mixed sodium (AOT)and cadmium (AOT)reverse micelles in isooctane. The concentrations of the two reactants (Cd2+and S2-)are equal to 3 X lo-‘ mol L-l. The mixing is produced by rapid injection (manuallyor witha Biologic SFM3 stopped flow apparatus) of a variable volume of solutions of cadmium ions and sulfide ions of the same concentration. A ratio x is defined as [Cd2+l/[S2-l. Preparation of Samples for Electron Microscopy Experiments. Water-acetone is added to the mixed micellar solutioncontaining CdS particles. A two-phase separation takes place and CdS particles migrate to the interface. A drop of a suspension of extracted CdS particles in acetone is spread on a copper plate with a carbon film. The solvent is removed by vacuum. Apparatus, The small-angleX-ray scattering was done on a GDPA3O goniometer using copper K a radiation (1.54 A). The experimental arrangement used has been described previo~sly.~ The absorption spectra were obtained with a Perkin-Elmer Lambda 5 and Hewlett-Packard spectrophotometer and the fluorescence spectra with a Perkin-Elmer LS5 spectrofluorometer. A Philips electron microscope (Model CM 20, 200 kV) was used for electron microscopy. Continuous irradiation was performed using a 1000-W Oriel lamp with a 20-cm water filter and 385-nm cutoff filter. Flash photolysis experiments were performed using an Applied Photophysics apparatus. Results and Discussion By use of mixed AOT reverse micelles ([(AOT)Nal = mol L-l), small angle 0.1 M and [(AOT)zCdl = 3 X (8) Chevalier, $3.; Lerebours, B.; Pileni, M. P. J. Photochem. 1984,27, 301. (9) Zemb,T.; Charpin, P. J. Phys. (Paris) 1986, 46, 249.
0743-746319212408-10~9$03.00/0 0 1992 American Chemical Society
Motte
1050 Langmuir, Vol. 8, No. 4, 1992
X-ray scattering observations are similar to those obtained using sodium AOT reverse micelles:2 at high water content (up to lo), the aggregates remain spherical and a linear relationship between the water pool radius and the water content ( R , = 1 . 5 ~is) observed. No changes in the size, in comparison to sodium AOT, are observed at low Cd2+ content. I. Size Determination. Cadmium sulfide suspensions are characterized by their absorption spectrum in the visible range. In the case of small particles, a quantum size effect1@17J8J9is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change.lg This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. These phenomena occur as the crystallite size is comparable or below the excitonic diameter of 50-60 A.19b In the first approximation a simple "electron-hole in a box" model can quantify this blue shift with the size variation.14J8JgbThus the absorption threshold is directly related to the average size of the particles in solution. In the presence of an excess of cadmium ( x = 2), Figure 1shows a red shift in the absorption spectra by increasing the water content. According to the data previously published using sodium bis(ethy1-2-hexyl) sulfosuccinate (AOT) as a ~ u r f a c t a n t ,this ~ , ~ can be attributed to an increase in the average size of particles with the water content. Below the absorption onset several shoulders are observed (Figure 1) and can be clearly recognized in the second derivative (insets in Figure 1). These weak absorption bands correspond to the excitonic transitions. This clearly shows a narrow size distribution.20 At low water content the first excitonic peaks is well-resolved and is followed by a bump. The second derivative shows a very high intensity of this bump (Figure 1A inset). With small crystallites, according to the data previously published,20several bumps due to several excitonic pics are expected. Figure 1A inset shows only one bump. This is due to the fact that the others are blue shifted and are not observable under our experimental conditions. By increasing the water content, that is to say by increasing the size of the particles, several bumps are observed (Figure 1B inset). The intensity of these bumps decreases with the water content, w (Figure 1 insets). This indicates a decrease in the number of excitonic transitions with the size of the particle. This is in agreement with the theoretical calculations previously published for the Q particles.20 In the presence of an excess of sulfide ions, a strong change in the absorption spectra at low water content is observed as shown Figure 2, compared to what it is obtained (10) Brus, L. E. J . Chem. Phys. 1983, 79, 5566. (11)Rossetti, R.; Ellison, J. L.; Bigson, J. M.. Brus, L. E. J . Chem. Phys. 1984, 80, 4464. (12) Nozik, A. J.; Williams, F.; Nenadocic, M. T.; Rajh, T.; Micic, 0. I. J. Phys. Chem. 1985,89, 397-399. (13) Bawendi, M. G.;Steigerwald, M. L.; Brus, L. E. Annu. Reo. Phys. Chem. 1990,41, 477. (14) Henglein, A. Chem. Reu. 1989, 89, 1861. (15) (a) Wang, Y.; Herron, N. Phys. Reo. B 1990, 41, 6079. (b) Wang, Y.; Herron, N. J . Phys. Chem. 1991, 95, 525. (16) Kayanuma, Y. Phys. Reu. B 1988, 38, 9797. (17) Lippens, P. E.; Lannoo, M. Phys. Reo. B 1989, 39, 10935. (18) Brus, L. E. J . Chem. Phys. 1983, 79, 5566. (19) (a) Wang, Y.; Herron, N. Phys. Reo. B 1990,41,6079. (b) Wang, Y.; Herron, N. J . Phys. Chem. 1991, 95, 525. (20) Katsikas, L.; Eychmuller, A.; Giersig, M.; Weller, H. Chem. Phjs. Lett. 1990, 172, 201.
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at x = 2. With an increase in the water content, the sharp peak disappears and similar behavior to that observed at x = 2 is observed, a red shift in the absorption spectrum. The sharp peak observed at low w value is more intense
Langmuir, Vol. 8,No.4, 1992 1051
Synthesis of CdS Water content W
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at x = l / 4 than at x = l/2. This peak could be attributed to polymers formed by sulfideassociation and called sulfide clusters21 on the CdS particles. Its disappearance by increasing the water content could be explained by the fact that sulfide ions which have two negative charges are repelled in the center of the droplet and freely diffuse inside the water pool. From the relation b e t ~ e e n l O J " ~the ~ , ~absorptiononset and the size of CdS particle, the average radius, T is deduced. Figure 3 shows a strong change in the size of the particle with the relative ratio of cadmium and sulfide ions ( x = [Cd2+]/[S2-]).The biggest sizes are obtained for x = 1and the smallest for x = 2. It can be noticed that the size of CdS is always smaller when one of the two reactants is in excess ( x = l/4,l/2,2). This confirms that the crystallizationprocess is faster when one of the species is in excess.22 The absorption spectrum depends on the rate of mixing the solutions and changes with time: when the solutions are mixed very fast (200 ms), the absorption spectrum is blue shifted in comparison to that obtained after 1s or by mixing by hand. Similarly the absorption spectrum is red shifted with time, whatever is the water content. At low water content, the shift is very small and corresponds to an increase in a size of 2 A. Above w = 10, the size of the particle increases with the water content and with decreasing x value; at x = 2 the increase in the size of the particles is about 10%. Such increase reaches a factor of 3 at high water content and x = l/4. In other words, a t high water content, the particles formed in the presence of an excess of sulfide ions are not stable with time. Electron microscopy has been performed using a sample synthesized at w = 10 and x = 2, in mixed reverse micelles, characterized by 430-nm absorption onset corresponding to a CdS diameter equal to 25 A. The microanalysis study shows the characteristic lines of sulfide and cadmium ions indicating that the observed particles are CdS semiconductor crystallites. Figure 4 shows spherical particles with average size in the range of 40 f 20 A. The electron ray diffraction pattern shows concentrical circles (Figure 5A). This is compared to a simulated diffractogram of bulk CdS (Figure 5B). Good agreement between the two spectra is obtained indicating the particles keep ZnS crystalline structure (fcc) with a lattice constant equal to 5.83 A.The (21) Barnes, D.; Kenyon, A. S.; Zaieer, E. M.; Lamer, V. K. J. Colloa Sci. 1947,2, 349. (22) Fisher, C. H.;Weller, H.; Lume-Periera, C.; Janata, E.; Heinglein,
A. Ber. Bunsen-Ges. Phys. Chem. 1986,90,46.
8
Figure 4. Electron microscopyof a sampleafter extraction from a micellar solution.
Figure 5. Electron ray diffraction spectrum of CdS particle: (A) sample; (B) simulated.
radius of the particle can be deduced from the line widening of the diffraction signals given by the following relationship: 23 2r = Z/ARwhere X is electron wavelength (A = 0.0251 A), L is camera length (L= 4900 mm), and AR is the widening line (AR = 3 mm). The size determined from such calculation is equal to 41 A. The differences in the size of the particles are probably due to the extraction of CdS from the droplets with appearanceof some aggregation processes. 11. Polydispersity. The fluorescence quantum yield depends on the x value and the water content: it decreases with increasing the water content and reaches the same value a t w = 40 for the various x values. The red shift of the maximum fluorescence with w can be related to the increase in particle size. The increase of the sulfide (23) Electron microscopy of thin crystallite. Hish, P. B.; Howie,A.; Nicholson, R. B.; Pashley, D. W.; Whelm, M. J. Butterworth Press: London, 1971.
Motte et al.
1052 Langmuir, Vol. 8, No. 4 , 1992
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Figure 6. Variation of the fluorescence spectra with water content at various excitation wavelengths (Aexc: 0,380 nm; m, 400 nm; +, 420 nmj. The intensities are normalized to the emmision maximum. [(AOTjNal = 0.1 M. X = 2, [(AOT)&d] = 2 x 10-4M: A, = 5 ; B,w = io; c, w = 20; D, w = 40. x = ' I 2 , [(AOT)*Cd]= 1 X lo-' M: E,w = 5 ; F,w = 10. vacancies at the surface ( x = 2), favors a fluorescence centered at 550nm24-26dueto the recombination of charges previously trapped a t the surface. Figure 6A-D gives the fluorescence spectra, at x = 2 and various water content. These spectra are normalized at the maximum of emission. The unchanged spectra with the excitation wavelength indicate stronger monodispersity of the particles formed in mixed reverse micelles than that obtained in the same experimental conditions using cadmium ions and HMP instead of cadmium AOT. In the first case the variation of the maxima by changing the excitation wavelength is found equal to AX = 40 nm, whereas by using functionalized surfactant this difference is in the range of the spectrofluorometer resolution (Ax = 1 nm). The shift of the maximum of the emission with the water content, w ,can be related to the average size of the particles. Most of CdS particles formed in the presence of an excess of sulfide ions are characterized by two fluorescence bands. The first is centered at 450 nm and attributed to a direct recombination of charge carriersZ4and a red fluorescence band centered in the 550-650 nm range and related to the size of the particle^.^' Similar behavior was observed in AOT reverse micelles, at w up to 10 in the presence of protecting polymer (HMP). PreviouslyG in sodium AOT reverse micelles obtained by mixing cadmium nitrate and sodium sulfide, in the presence of a protecting polymer, (24) Ramsden J. J. and Graetzel M.: J. Chem. Soc.. Faraday Trans. I 1984, 90, 919. (25)Chestnoy, N.; Harri, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393. (26) Thomas, J. K. J. Phys. Chem. 1987, 91, 267. (27) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987, 109, 5649.
Figure 7. Percentage of disappearance of CdS semiconductor w = with time at various water content: w = 5 (m); w = 10 (0); 20 (0);w = 40 (+). A, X = '/4; B,X = '/z; C, X = 1, D,X = 2. similar behavior was observed. By use of a functionalized surfactant (cadmium AOT), Figure 6E,F shows one fluorescence band centered at 550 nm (w = 5) which is red shifted by increasing the water content (centered at 620 nm at w = 10). The disappearance of the first fluorescence band centered at 450 nm could indicate that functionalized surfactant (cadmium AOT) prevents a direct recombination of charge carriers. The red shift observed by increasing the water content can be attributed, as previously, to an increase in the size of the particle. For a sample synthesized in the given experimental condition (fixed w and x values) the fluorescence excitation spectra obtained are unchanged by changing the emission wavelength. This confirm the monodispersity in the size of the CdS particles. 111. Photocorrosion. The photocorrosion is observed on the change in the absorption spectrum at 300 nm. Such a process is inherent with CdS in the presence of oxygen.28 A photodissolution of CdS is observed by the following process:
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CdS + 20, CdSO, Cd2++ SO-: Figure 7 shows the photocorrosion efficiency with the x value and w. The yield is negligible at low water content: at w = 5, no (or very few) photocorrosion is observed after 10 min of irradiation. This is probably due to a protecting effect of the micellar solution which results in an increase in the rigidity of the water pool making an "immobilized" particle. At x = (Figure 7A), and w = 5, the crystallites are stable for a relatively long period of time. The comparison of the photocorrosion yield with w and x shows the lowest yield is obtained at x = '14. In this case sulfide ions could trap the holes formed during the irradiation of the particles: the excess S2- is oxidized by h+, which then cannot react with CdS.29 IV. Photoelectron Transfer. The photoelectron transfer from CdS to various dialkylviologens, ((C,)zV2+}, has been studied by flash photolysis. The size of the particles remains unchanged by adding viologen either before or after CdS synthesis. (28) Henglein, A.; Fojtik, A.; Weller, H. Ber. Bunsen-Ges.Phys. Chem. 1987, 91, 441. (29) Henglein, A. Ber. Bunsen-Ces. Phys. Chem. 1982, 86, 301.
Langmuir, Vol. 8,No. 4, 1992 1053
Synthesis of C I S
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In the presence of an excess of sulfide ( x = l/d, Figure 8 shows a decrease in the yield of reduced dialkylviologen with the water content. This can be related to the fact that, a t low water content, Figure 2 shows a change in the absorption spectrum probably due to sulfide aggregates adsorbed a t the CdS interface. Such sulfide ions are able to give an electron to the hole formed by light excitation and then prevent the back electron transfer reaction. The yield of reduced viologen increases with chain length of viologen. I t can be attributed to an increase in the amount of viologen anchored to the CdS semiconductor surface. The photoelectron transfer is characterized by a firstorder kinetic rate constant in the microsecond time scale. This rate constant increases with the water content and
with the chain length of dialkylviologen. This can be explained in terms of size effect: Figure 3 shows that, at x = V 2 , the size of CdS particle is strongly changed between w = 5 ( R = 11A) and w = 10 (R= 13 A). With an increase in water content, no drastic change in the size is obtained (for 15 < w < 40, RCds # 16A). The photoelectron transfer rate constant is more efficient by using small particle and long chain dialkylviologen. In the presence of an excess of cadmium ions ( x = 21, the photoelectron transfer rate constant and the reduced viologen yield are constant and independent of the length of the alkyl chain and to the water content. This can be attributed to the fact that there is no internal electron donor to prevent the back electron transfer reaction as it takes place in the presence of an excess of sulfide in which the latter playsa role. The addition of an external electron donor such as cysteine, adenine, or benzylnicotinamide does not prevent the back reaction. This could be explained in terms of accessibility of the electron donor to the holes formed by CdS excitation. The back electron transfer takes place before the external electron donor can reach the CdS hole.
Conclusion We report here the synthesis, in situ, of CdS in reverse AOT micelles from functional surfactants. The location of one of the reactants a t the micelle interface makes it possible to favor the formation monodispersed particles. This small size is confirmed by electron microscopy and from electron ray diffraction. Semiconductor particles are protected against corrosion a t low water content. This process is stronger in the presence of sulfide aggregates a t the CdS interface. The photoelectron transfer reaction depends on the length of the alkyl chain of viologen and on the size of the particle. Registry No. [(AOT)Na], 577-11-7; [(AOT)zCd], 12446179-6; CdS, 1306-23-6.
