J. Phys. Chem. 1986, 90, 3369-3374
3369
I n Situ Generated Colloidal Semiconductor CdS Partkles in Dihexadecyl Phosphate Vesicles: Quantum Size and Asymmetry Effects Yves-M. Tricot'. and Janos H. Fendler*Ib Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676 (Received: August 26, 1985; In Final Form: January 7, 1986)
Colloidal semiconductor CdS particles were in situ generated by exposure to gaseous H2S of Cd2+ions in the presence of anionic dihexadecyl phosphate (DHP) vesicles. The Cd2+ions were adsorbed either at the inner or outer surface or at both surfaces of the vesicles. The vesicle dispersionswere characterized by atomic and optical absorption, fluorescence spectroscopy, and static and dynamic light scattering. CdS fluorescence was observed only from inner-surface-generated CdS and with a DHPCdS ratio smaller than 40,although CdS absorbance could be measured up to a DHPCdS ratio of 1OOO. Fluorescence excitation spectra were blue-shifted by about 60 nm from the absorbance spectra when Cd2+was completely precipitated by HIS. Partial formation of CdS produced two types of particles, one with a band gap or absorption edge near 430 nm, corresponding to the excitation threshold, and another with a band gap near 490 nm. The 430 nm band gap, fluorescing CdS could be stabilized only at the inner surface of the vesicles. This selectivity is understood in terms of protection against aggregation with CdS from other vesicles and of the reduced effect of aging by dissolution-precipitation equilibrium. The variations in band gap are due to quantum size effects appearing for CdS colloids smaller than 50 A in diameter.
Introduction
The potential of colloidal semiconductor particles for solar energy conversion devices and, more generally, photocatalysis has been recognized by a rapidly growing number of laboratories ~ o r l d w i d e . ~ -Because ~~ of its optimum characteristics among
(1) (a) Present address: Department of Materials Research, The Weizmann Institute of Science. Rehovot 76100, Israel. (b) Present address: Department of Chemistry, Syracuse University, Syracuse, NY 13210. (2) Fox, M. A., Ed. Organic Phototransformations in Nonhomogeneous Media; American Chemical Society: Washington, DC,1985; ACS Symp. Ser. No. 278. (3) Ramsden, J. J.; GrBtzel, M. J. Chem. SOC.,Faraday Trans. I 1984, 80, 919. (4) Serpone, N.; Sharma, D. K.; Jamieson, M. A.; GrBtzel, M.; Ramsden, J. J. Chem. Phys. Lett. 1985, 115, 473. ( 5 ) Moser, J.; Grltzel, M. J . Am. Chem. Soc. 1984, 106, 6557. (6) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 302. (7) Henglein, A.; Gutibrrez, M. Ber. Bunsenges. Phys. Chem. 1983.87, 852. (8) Weller, H.; Koch, U.; Gutibrrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1984,88,649. (9) Fojtik, A.; Weller, M.; Koch, U.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1984,88,969. (10) Rossetti, R.; Nakahara, S.;BNS, L.E. J. Chem. Phys. 1983, 79, 1086. (11) Brus, L. E. J . Chem. Phys. 1984,80,4403. (12) Rossetti, R.; Ellison, J. L.;Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984, 80, 4464. (13) Ramsden, J. J.; Webber, S.E.;GrBtzel, M. J. Phys. Chem. 1985,89, 2740. (14) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1984,88, 4278. (15) Williams, F.; Nozik, A. J. Nature (London) 1984, 312, 21. (16) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rahj, T.; Micic, 0. I. J. Phys. Chem. 1985,89, 397. (17) Mau, A. W.-H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S.E. J. Am. Chem. Soc. 1984, 106, 6537. (18) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (19) Kuczynski, J. P.; Milosajevic, B. H.; Thomas, J. K. J . Phys. Chem. 1984, 88, 890. (20) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Phys. Chem. 1985,89, 626, 1922. (21) Meyer, M.; Wallberg, C.; Kurihara. K.;Fendler. J. H. J . Chem. Soc., Chem. Commun. 1984.90. (22) Tricot, Y.-M.; Fendler, J. H. J . Am. Chem. Soc. 1984, 106, 7359.
many available semiconductor materials, CdS has been the most popular and the best described. In several of the chemical systems used to generate and stabilize CdS colloids, the diameter of the particles was smaller than 50 A. The minute size of these crystallites produced new effects due to the quantum confinement of charge carriers as, for instance, increase of band gap energy and resolution of an absorbance peak due to exciton formation.*l2>l6 In addition to the fundamental interest in their unique properties, CdS particles may also alter kinetics of photochemical reactions and improve efficienciesof energy conversion processes.*5 In previous works, ultrasmall CdS particles have been prepared in nonaqueous media, in mixtures of organic solvents with water, or in aqueous media in the presence of stabilizing polymers or inorganic oxide particles. We report here observation of marked size effects in CdS colloids prepared in aqueous dihexadecyl phosphate (DHP) vesicles. D H P and other surfactant vesicle dispersions have recently been shown to be convenient media for in situ generation and catalyst coating for CdS colloids, thereby producing efficient solar energy conversion model systems.22-20 This work demonstrates that D H P vesicles provide a simple and efficient way to vary the size of CdS colloids, by controlling either the number of Cd2+ions (CdS precursors) on each vesicle or the amount of H2S introduced to precipitate CdS. Moreover, surfactant vesicles are shown to generate different populations of CdS particles whether their formation is done at the inner or at the outer vesicle surface. The large observed variations in photophysical properties of CdS, induced by minor changes in preparation conditions, demand a very precise description of experimental procedures. Experimental Section Dihexadecyl phosphate (DHP, Sigma), CdC12-21/2H20(Baker), methylviologen dichloride (MVCl,, Aldrich), and NaOH (Fischer) were of analytical grade and used without further purification. Hydrogen sulfide (Matheson, 99.5%) was used as received. Deionized water was distilled in a quartz apparatus. DHP vesicles were prepared, as described p r e v i o ~ s l yby , ~ ~sonication in water a t 80-85 "C. Typically, 25 mL of distilled water was added to 49.2 mg of D H P and heated until D H P was liquid (mp 76-77 (23) Rafaeloff, R.; Tricot, Y.-M.; Nome, F.; Fendler, J. H. J . Phys. Chem. 1985, 89, 533. (24) Tricot, Y.-M.; Emeren, A.; Fendler, J. H. J . Phys. Chem. 1985,89, 4721. (25) Tricot, Y.-M.; Furlong, D. N.; Sasse, W. H. F.; Daivis, P.; Snook, I.; Van Megen, W. J. Colloid Interface Sci. 1984, 97, 380.
0022-3654/86/2090-3369$01 .50/0 0 1986 American Chemical Society
3370 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 "C). After initial dispersion by a Braunsonic cell disruptor operated at 70 W, appropriate amounts of 0.1 M aqueous NaOH and 0.1 M aqueous CdCl, were added by using microsyringes. Sonication was then continued for 15 min. After cooling down in air at room temperature, the samples were centrifuged for 1 h at 3000 rpm to separate traces of titanium released by the sonicator microtip. Dilution to 45 mL with distilled water was done to adjust D H P concentration to 2 X lo-) M, and the pH was measured with a glass electrode.26 The amount of N a O H injected at the beginning of sonication was adjusted so that the final pH was in the range 7.5-8.5. This corresponded to a stoichiometric amount of NaOH with respect to DHP, or 900 pL of N a O H (0.1 M), plus up to 150 pL of N a O H (0.1 M) to compensate for the effect of CdCI2 on the dispersion pH (with M CdC12 or 405 pL of 0.1 M CdCl,). up to 9 X Of adsorbed Or nonadsorbed Cd2+ions was achieved by passing the dispersions through a Bio-Rad AG5ow-x2cation exchange resin (100-200 mesh, hydrogen form). Removal of excess HCl left by the cation exchange resin and reconcentration to the initial D H P concentration were done in an ultrafiltration stirred cell (Amicon, 65-mL capacity) equipped with a Diaflo membrane of 100000 nominal molecular weight cutoff (type XMlOOA). Exact Cd2+concentrations were determined by atomic absorption spectroscopy with a Perkin-Elmer instrument (Model 5000). Hydrodynamic radii of the vesicles and qualitative estimates of size distributions were obtained on a Coulter N4 submicron particle analyzer. CdS formation was achieved by exposure to gaseous H2S. As the exact procedure was found to play a critical role, particularly with low amounts of HIS, further details are given below. Samples (10 mL) were degassed by argon bubbling at a flow rate of approximately 10 mL/min, for 45 min prior to H2S exposure, under gentle magnetic stirring. Argon bubbling was maintained during H2Sexposure. Two different techniques were used to bring H2S into the samples: (a) A Pasteur pipet connected to the H2Sbottle was placed with its orifice approximately 5 mm above the surface of the sample, and H2S was admitted during 5-60 s. The flow of H2S was approximately 30 mL/min. CdS formation was usually completed in less than 20 s. For samples with variable amounts of Cd2+where complete precipitation of CdS was desired, this technique was used with an exposure time to H2Sof 30 s. This technique did not allow reproducible formation of partially precipitated CdS, due to the too high rate of addition of H2S and the lack precise control of volume of H2S used. Bubbling H2S directly through the dispersions produced, of course, even faster CdS formation and was also unsuitable for adequate control of partial CdS formation. (b) Precise volumes of gaseous H2S were injected, with microsyringes, 30-40 cm upstream in the argon flow (Tygon tubing, 1/4-in.id.) so that H2S,diluted in argon, took about 1 min to reach the vesicle samples. The volume of each sample was 5 mL, with M DHP. Using 100% stoichioM Cd2+and 2 X 8X metric H2S vs. Cd2+ corresponded to 90 p L of gaseous H2S. Injection of H2S was done slowly (10-20 s for volumes in the 50-350-pL range). The samples were protected from light during CdS formation. This technique allowed partial and slow formation of CdS, in a very controlled and reproducible fashion. Absorbance spectra were recorded on a Hewlett-Packard 8450A diode array spectrophotometer. Fluorescence spectra were recorded on a Spex Fluorolog and/or on a Perkin-Elmer LS-5 spectrofluorometers, without correction for the response of the photomultiplier tube. Results and Discussion CdS Size Effects and Asymmetry from Variation of Cd2+ Surface Density. These effects were studied by using method a of H2Sexposure (see Experimental Section). The dispersion pH (26) Due to a drift in the measured pH, induced by interaction of the vesicles with the glass electrode,*' the pH was always measured after stirring for a few seconds and leaving the electrode response stabilize without stirring for 1-2 rnin.
Tricot and Fendler
'
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Figure 1. Absorbance, fluorescence emission, and fluorescence excitation spectra of 3 X 10-4 M co]]oidal CdS generated on both sides of 2 X 10-3 M DHp vesicles, at pH 8. The absorbance spectrum does not contain the contribution due to vesicle light scattering and is scaled to have the same slope as the fluorescence excitation spectrum. The excitation was at 370 nm for the emission spectrum, and the observed emission was at 500 nm for the excitation spectrum. The spectral characteristics monitored in Figures 4 and 7 are graphically defined.
was maintained in the range 7.5-8.5 since we have reported22that CdS fluorescence was maximized at mild alkaline pH in D H P vesicle systems. The photophysical properties of colloidal CdS were studied as a function of its concentration either at the inner or outer surface or at both surfaces of DHP vesicles. They were monitored by absorbance, fluorescence emission, and fluorescence excitation spectroscopies. Figure 1 represents a typical example of these spectra, in this case for CdS 3 X lo4 M on both surfaces M and at pH 8.0. The excitation of D H P vesicles at 2 X threshold and absorption edge are defined graphically in Figure 1. This was found to be the most convenient way of comparing different samples, particularly their absorption edge. The wavelength of this absorption edge may not, however, correspond strictly to the band gap energy. For a direct band gap semiconductor such as CdS, the band gap energy is usually defined by the x axis intercept of (cyhu)'
VS.
hu
(1)
where CY is the absorption c o e f f i ~ i e n t . ~Applied ~ to the near-edge absorption in Figure 1, this relation is linear only from 460 nm toward higher energy. Extrapolation of this linear part to the x axis yields a band gap energy equal to 2.65 eV (468 nm), significantly different from the absorption edge at 2.56 eV (484 nm) in Figure 1. Absorption at wavelengths longer than 468 nm could be attributed to the exciton state, the lowest excited state where the charge carriers do not move independently of each other, situated slightly below the conduction edge.9-'2 Its absorption becomes relatively stronger as the density of states in the conduction band is decreased by the small size of the particles. However, due to the presence of size, and hence band gap energy, distributions, the determination of one band gap energy by use of relation 1 is somewhat arbitrary. Because of this complexity, we defined the absorption edge of our samples as shown in Figure 1 and took it as an apparent band gap which is, as well as the one defined by relation 1, of higher energy than the macrocrystalline CdS band gap energy of 2.42 eV, as found in a physics table. It seems, however, that Brus12 did not agree with this value since he used a bulk CdS band gap energy of 2.53 eV in his calculations. The near-edge absorption could be better fitted to a relation of the type proposed by Dutton28 and applied by G r a t ~ e l : ~ , ' ~ In
CY
= @hv/kT
(2)
We found a parameter @ = 0.36, similar to @ = 0.33 found by Gratze13 for hexametaphosphate-stabilized aqueous CdS. For macrccrystalline CdS, Dutton determined fl = 2.13, corresponding (27) Buttler, M. A. J . Appl. Phys. 1977, 48, 1914 (28) Dutton, D. Phys. Rev. 1958, 112, 785.
The Journal of Physical Chemistry, Vol, 90, NO. 15, 1986 3371
CdS in Dihexadecyl Phosphate Vesicles I
I
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2
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lo4 initial CdC12 concentration, M F w 2. Effect of the initial CdC12concentration on its retained fraction after cation exchange resin treatment, with 2 X M DHP and at pH 1
7.5-8.5.
to a steeper increase of absorption toward higher energies. As discussed above, the lower absorption of very small colloidal particles is due to the lower density of states near the limit of the conduction band. The maxima observed in the excitation spectrum (380 nm in Figure 1) and in the emission spectrum (500 nm in Figure 1) were also monitored. As no correction was applied, these maxima were used only on a comparative basis. Figure 1 reveals a shift of about 50 nm between the absorbance and fluorescence excitation spectra. This is quite unusual but was observed systematically in many different samples of various ages (up to several weeks) and with two different spectrofluorometers. We reported previously22that only CdS generated at the inner surface of D H P vesicles showed a detectable fluorescence. No fluorescence was observed from CdS generated at the outer surface of D H P vesicles, and methylviologen (MV2+),a very efficient quencher of CdS fluorescence when inside of the vesicles, could not quench CdS fluorescence when added externally to D H P vesicles having CdS colloids a t both surfaces.22 The shift between fluorescence excitation and absorbance spectra was also found for CdS colloids located selectively a t the inner surface of D H P vesicles. This peculiar behavior of CdS colloids in D H P vesicles will be discussed further in view of the other results reported here. In order to produce CdS colloids selectively at the inner surface of D H P vesicles, samples sonicated in the presence of Cd2+ ions were treated with a cation exchange resin (see Experimental Section). To investigate size effects in the resulting CdS particles, the initial Cd2+ concentration was varied through the whole accessible range. The absorbance of CdS could be measured with less than 5% uncertainty a t 400 nm down to 2 X 10" M, as determined by atomic absorption of the precursor CdZ+. CdS absorbances were measured by difference against the same sample prior to CdS formation, to eliminate the D H P turbidity contribution. The higher limit was determined by the vesicle stability against Cd2+-inducedflocculation. D H P concentration was always 2X M, and the maximum tolerable amount of Cd2+,symmetrically distributed, was about 8 X 10-4 M. The cation exchange resin treatment was monitored via the retained or nonexchanged fraction of Cd2+, either by atomic absorption or by absorbance of subsequently formed CdS. Atomic absorption was found more reliable, due to variations in extinction coefficients (see below). Figure 2 shows the effect of initial Cd2+ concentration on its retained fraction. As this fraction should be essentially surfa~e-controlled:~due to the strong adsorption of Cd2+, one would expect to find 4045% of Cd2+not removed by the cation exchange resin (assuming spheres of 500-A external radius and 50-8, bilayer thickness). We pointed out earlieS2that the experimental retained fraction of Cd2+ was in this range for initial DHPCd2+ratios of 40:l to 1O:l ((0.5-2.0)X lo4 M Cd2+ with 2 X M DHP) (29) As described strong adsorption of Cd2+ions on negatively charged DHP vesicles took place, up to a density of one Cd2+per three DHP molecules. Therefore, at 2 X M DHP concentration, up to about 6.5 X lo4 mol of Cd2+ was adsorbed. Any excess remained in the outer bulk aqueous solution.
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,
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2
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4
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Figure 3. Effect of colloidal CdS location and concentration on its 400-nm extinction coefficient, with 2 X 10" M DHP vesicles and at pH 7.5-8.5 after sonication: (0)CdS at the outer surface; (0) CdS at the inner surface; ( 0 )CdS on both surfaces of the vesicles.
at neutral pH but increased above 50% a t higher CdZ+concentrations. This was taken as an indication of a change in adsorption strength with Cdz+concentration. The results in Figure 2 confirm the same trend at high Cd2+concentrations and reveal a reverse effect a t very low Cd2+ concentrations (less than 0.5 X lo4 M Cd2+). One possible explanation is that Cd2+adsorbs preferentially a t the outer surface of the vesicles (at least during sonication at 80 "C), at low Cd2+concentrations. The turbidity of DHP vesicles increased strongly in the presence of adsorbed Cd2+. The absorbance (due to light scattering) was about 7 times stronger with 8 X lo4 M Cd2+ (with 2 X lob3M DHP) compared with naked D H P vesicles, for wavelengths in the range 400-600 nm. This indicated clearly the presence of larger aggregates. However, dynamic light-scattering (DLS) experiments detected only a slight increase in hydrodynamic diameter (&) from 800 f 200 to lo00 f 2000 A over the whole range of Cd" concentrations, as reported which is hardly significant. Estimates of size distributions from the DLS data seemed to indicate the presence of bigger aggregates of diameter up to several thousand angstroms, but the statistical error was always too large to allow meaningful conclusions. DLS may not pick up these aggregates very well if most of the diffusion occurs via the single vesicles, by fast dispersionaggregation equilibrium, in a way analogous to which surfactant molecules diffuse in micellar solutions.30 The higher retained fraction of Cd2+after cation exchange resin treatment could then be a consequence of the reversible formation of larger aggregates, induced by high CdZ+concentrations. The low pH of the resin might freeze these aggregates, and a part of the Cd2+ ions, adsorbed at the outer surface of single vesicles, were protected from ionic exchange. The conclusion from Figure 2 is that CdS could be formed exclusively at the inner surface of D H P vesicles only M, with 2 when Cd2+ concentrations were lower than 3 X X M D H P and at pH 7.5-8.5. The extinction coefficients of CdS were determined by combining atomic absorption of CdZ+with absorbance of resulting CdS, after complete precipitation with H2S. The effect of CdS location and concentration on its extinction coefficient (at 400 nm) is shown in Figure 3. It appears that when formed at the outside surface, a constant extinction coefficient of e = 1050 f 50 M-' cm-l was found. Conversely, inner-surface-generated colloidal CdS revealed a clear trend, e(400 nm) varying from 650 M-' cm-I, at the lowest detectable concentration, to the outer surface value of 1050 M-' cm-I at or above 3 X loT4M. In agreement with these results, symmetrically distributed CdS resulted in an intermediate trend. A decrease in extinction coefficient at a constant wavelength can be explained, in the monotonic CdS absorption spectrum, by a blue shift of absorption edge, suggesting formation of smaller particles at the inner surface of ~
(30)(a) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1977. (b) Israelachvili, J. N.; Marcelja, S.;Horn, R. G . Q.Reu. Biophys. 1980,13, 121.
3372 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 I
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at pH 7.5-8.5 after sonication: (0)CdS at the outer surface; (0) CdS at the inner surface; ( 0 )CdS at both surfaces of the vesicles. r
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Figure 5. Effect of colloidal CdS location and concentration on its fluorescence at 500 nm under 370-nm excitation in 2 X lo-) M DHP vesicles and at pH 7.5-8.5 after sonication: (A) relative fluorescence quantum yield (intensity divided by CdS absorbance at 370 nm); (B)
relative fluorescence intensity. (a) CdS at the inner surface; (Of CdS at both surfaces of the vesicles. No fluorescence was observed from CdS at the outer surface of the vesicles.
the vesicles. The variations in absorption edge are in fact quite complex, as shown in Figure 4. Unlike in Figure 3, the average behavior of CdS on both surfaces of the vesicles was not a simple combination of the behaviors on each surface. It may be important to stress that the concentrations of CdS in Figures 3-5 were the final concentrations, after resin treatment and reconcentration M D H P when applicable, as determined by atomic to 2 X absorption of Cd2+. The samples with inner-surface CdS had, therefore, a higher initial concentration of Cd2+ (see Figure 2). Although quite surprising, the opposite variations of absorption edge for CdS at low concentrations may be explained by the same effect as in Figure 2. If Cd2+adsorbed preferentially at the outer surface with less than 0.5 X M Cd2+,the majority of CdS formed will have had its absorption edge at long wavelength near 490 nm. As CdS concentration increased, a growing fraction will have been at the inner surface resulting in a blue shift of the apparent absorption edge. The faster red shift of absorption edge for inner-surface CdS, compared to that for CdS on both surfaces, was due partly to the higher CdS surface density (see above). For example, inner-surface CdS at 2 X IO4 M corresponded to CdS on both surfaces at about 5 X IO4 M. Outer-surface CdS showed a moderate trend, similar to the behavior of CdS at both surfaces at low concentration. There is, however, the possibility that outer-surface-generated CdS was different than the outer part of CdS generated at both surfaces. In the former case, Cd2+was added to already sonicated vesicles, a technique which may have
Tricot and Fendler resulted in nonstatistical distribution of CdZ+on the vesicles. Adsorption of Cd2+was very fast (an excess of it could flocculate vesicles instantly), probably diffusion-controlled, while desorption or equilibration with naked vesicles may have been very slow because of the very low equilibrium concentration.22 Although care was taken to add Cd2+very slowly to DHP vesicle dispersions, mixing by sonication was still likely to result in a better, homogeneous distribution of Cd2+on vesicles. All curves in Figure 4 tend to reach high absorption edges (490-500 nm) at high Cd2+concentrations, suggesting formation of larger CdS colloids. One very interesting observation was the minimum of 474 nm obtained at 0.6 X lo4 M CdS concentration when it was symmetrically distributed. This concentration corresponded to the sudden rise of the fluorescence emission intensity, pictured in Figure 5 . Detectable fluorescence rose very sharply, M at 0.7 X lo4 M for CdS at both surfaces and at 0.4 X for CdS at the inner surface. This difference in concentration threshold and the fact that inner-surface CdS fluorescence reached about twice the quantum yield of that of CdS at both surfaces confirm again that only inner-surface CdS fluoresced. This conclusion could be made from the fluorescence behavior up to 1X M CdS. At higher concentrations, CdS fluorescence still originated from inner-surface Cds only, as shown by fluorescence quenching experiments.22 However, the quantum yield ratio between inner surface CdS and CdS at and both surfaces did not maintain its initial value. With (2-3) X lo4 M CdS, nearly the same quantum yield was observed in both cases (Figure 5). This suggested that the inner part of CdS from vesicles having CdS at both surfaces fluoresced more strongly at higher concentrations. In fact, the fluorescence quantum yield kept increasing as CdS concentrations increased which means that the fluorescence intensity increased more than linearly with CdS concentration. It was found that the higher the Cd2+concentration, the more critical the exposure time to H2S became in determining CdS fluorescence intensity. Other workers have reported3s8that excess of Cd2+or sulfur vacancies could be responsible for the observed fluorescence. Incomplete reaction with H2S would obviously produce sulfur vacancies and, therefore, enhance fluorescence. M with 2 At relatively low Cd2+concentrations (up to 2 X X 10" M DHP), it was found that incomplete formation of CdS only produced proportionally reduced fluorescence, although a large excess of H2S would also decrease this fluorescence. But more concentrated Cd2+on DHP vesicles seemed to require more careful exposure to H2S,and maximum fluorescence was obtained with apparently incomplete CdS formation. More precise information on CdS photophysical properties at high concentrations could be obtained by exposure to H2Sin a more controlled fashion. CdS Size Effects by Volume-Controlled Exposure to H2S. A concentration of Cd2+of 8 X lo4 M with 2 X lo-' M DHP was chosen to illustrate the effects of H2Samounts on the properties of CdS. At this concentration, the highest fluorescence intensity could be obtained. It was about an order of magnitude more M Cd2+ under equally optimum intense that at, say, 2 X conditions. As shown in Figure 2, it was not possible with 8 X M Cd2+to entirely remove outer-surface Cd2+. Therefore, only samples with a symmetrical distribution of Cd2+were used. Figure 6 shows the large variations in absorbance and fluorescence spectra induced by changing the amount of H2S injected in the argon gas flow to identical samples. The most interesting observation was the appearance of colloidal CdS having an absorption edge around 430 nm, simultaneously with the usual one at about 490 nm. This is most visible in curve b in Figure 6, where an equimolar amount of gaseous H2Swas injected in the Cd2+vesicle dispersions. As it was likely that a fraction of H2S, even diluted in argon, may have escaped the samples before reacting with Cd2+, the amounts of H,S (in Figures 6-8) give only the upper limits of CdS formation. Curve b in Figure 6 also corresponded to the highest fluorescence intensity. More samples than shown in Figure 6 were prepared, with variable amounts of H2S and with excellent reproducibility. Samples prepared with less than 90-100% stoichiometric H2S increased their fluorescence over several days
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3373
CdS in Dihexadecyl Phosphate Vesicles
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J. Phys. Chem. 1986,90, 3314-3311
3374
vesicles to form interstitial sulfur, as reported by Gratze13 in polymer-stabilized CdS colloids. In fact, no anion effect was found upon changing C1- by NO< in the D H P vesicle system,22where NO,- was reported to produce red fluorescence in polymer-stabilized system^.^ However, we cannot exclude an interaction between the phosphate head groups of the vesicles and the CdS colloids. This could create surface states and radiative recombination centers. The present data do not tell whether the crystal structure of DHP-stabilized CdS colloids is cubic (zinc blende), hexagonal (wurtzite), or even amorphous. A controversy exists about which form is more active for photochemical hydrogen In vesicle systems, neither the location of CdS generation.l’~~~ nor the type of vesicle (polymerized or not) was found critical for hydrogen production rate, but rather the nature of the electron donor and the charge of the vesicles (positive or negative) were found determinant.24 A cubic structure was found for CdS colloids in both water and water/acetonitrile mixture^.^^'^*'^ Further work is under current investigation to determine the crystal structure and the fluorescence mechanism of vesicle-stabilized colloidal CdS particles. Summary and Conclusions In situ generation of colloidal CdS in D H P vesicles has been demonstrated to be a very flexible technique allowing wide var(31) Matsumura, M.; Furukawa, S.; Saho, y.; Tsubomura, H.J . Phys. Chem. 1985, 89, 1327.
iations of CdS photophysical properties in aqueous medium. Vesicles possess both an inner and an outer interface, and these two sites have different properties. This asymmetry makes it possible to generate two different populations of CdS colloids, either separately or on the same vesicle. Variation of the surface density of Cd2+is an easily controllable parameter which alters the size and properties of CdS particles. Unlike in other preparation media,9 the fluorescence excitation threshold was blueshifted from the absorption edge due to the presence of different particle sizes, even when the particles were all at the inner surface. Partial growth of colloidal CdS produced only the fraction having a very small size (ca. 25-A diameter). This fraction was responsible for the fluorescence, and its absorption edge (ca. 430 nm) corresponded to the excitation threshold. The growth of colloidal CdS was shown to occur by distinct steps with increasing amounts of H2S and could be stoppped at any of these steps. The capability of such control over CdS properties may be very useful to optimize the coupling of CdS with other semiconductors such as ZnS32or to achieve interparticle electron transfers.33 Acknowledgment. Support of this work by the U S . Department of Energy is gratefully acknowledged. Registry No. CdS, 1306-23-6; DHP,2197-63-9. (32) Emeren, A.; Tricot, Y.-M.; Fendler, J. H., unpublished results. (33) Serpone, N.; Borgarello, E.; Grltzel, M. J. Chem. SOC.,Chem. Commum 1984, 342.
A Fourier Transform Infrared Study of Bllayer Membranes of Double-Chain Ammonium Amphiphlles Naotoshi Nakashima, Norihiro Yamada, Toyoki Kunitake,* Department of Organic Synthesis,t Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
Junzo Umemura, and Tohru Takenaka Institute f o r Chemical Research, Kyoto University, Kyoto 61 1, Japan (Received: September 12, 1985; In Final Form: March 14, 1986)
Fourier transform infrared spectroscopy was applied to an examination of the phase transition behavior of aqueous bilayer membranes of double-chain ammonium amphiphiles. Large spectral changes were observed at the respective gel-to-liquid crystal phase transitions (T,) of the bilayers. The frequency change in the antisymmetric CH2 stretching band indicated formation of the gauche conformation at Tc The ester groups became either hydrated or disordered in the liquid crystalline state, but the amide units remained strongly associated. It is concluded that intermolecular hydrogen bonding is not necessarily weakened at T,, although the molecular packing is loosened.
Introduction It has been established that a large number of synthetic amphiphiles produce stable bilayer membranes.’ These synthetic bilayers possess self-assembling properties which are fundamentally the same as those of biolipid bilayers. The structures of some of the synthetic bilayers have been analyzed by X-ray diffraction of their single crystals and ordered f i l m ~ . ~ 9It~ is desirable, however, that the structure and dynamics of synthetic bilayers be studied directly in the aqueous phase. Fourier-transform infrared spectroscopy has been increasingly used for examining the nature of aqueous molecular aggregates such as micelle formation of aqueous surfactant^?^ and phase transitions of phospholipids.@ Contribution No. 807.
Fine review articles on the latter subject have been published re~entIy.~J~ (1) Kunitake, T.; Okahata, Y. J. Am. Chem. SOC.1977, 97, 3860, and subsequent publications from these and other laboratories. (2) Okuyama, K.; Soboi, Y.; Hirabayashi, K.; Harada, A.; Kumano, A,; Kajiyama, T.; Takayanagi, M.; Kunitake, T. Chem. Lett. 1984, 21 17. (3) Shimomura, M.;Kunitake, T.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M.Thin Solid Films 1984, 121, L89. (4) Umemura, J.; Cameron, D. G.; Mantsch, H. H . J . Phys. Chem. 1980, 84, 2272. ( 5 ) Kawai, T.; Umemura, J.; Takenaka, T. Colloid Polym. Sci. 1984,262, 61. (6) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665. (7) Mantsch, H. H.; Martin, A.; Cameron, D. G. Biochemistry 1981, 20, 3138.
0022-365418612090-3374%01.5010 0 1986 American Chemical Society
