6320
J Phys. Chem. 1988, 92, 6320-6327
Dihexadecyl Phosphate, Vesicle-Stabilized and I n Situ Generated Mixed CdS and ZnS Semiconductor Particles. Preparation and Utilization for Photosensitized Charge Separation and Hydrogen Generation Hyeong-Chan Youn, Subhash Baral, and Janos H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-1 200 (Received: October 19, 1987: In Final Form: January 29, 1988)
Semiconductor particles, comprised of either homogeneous mixed crystals of Zn,Cdl,S or crystals of CdS coated on the surface with ZnS, were in situ generated in and stabilized by dihexadecyl phosphate (DHP) vesicles. Introduction of H2S into solutions which contained Cd2+-and Zn2+-coatedDHP vesicles, at pH values higher than 9, resulted in the formation of colloidal mixed crystals of Zn,Cd,..,S. Surfactant vesicles were found to regulate the composition and band gap of the ZnxCdl-$ particles produced. Addition of ZnZ+to a DHP-stabilized CdS solution followed by the introduction of H2S led to the formation of ZnS-coated CdS particles. Microstructures of DHP-vesicle-incorporated semiconductor particles were investigated by absorption and fluorescence spectroscopies and by powder X-ray diffraction. Transfer of conduction band electrons or holes to acceptors or donors have been observed by flash or continuous irradiation. DHP-vesicle-incorporated Zn,Cdl-2 and ZnS-coated CdS particles efficiently sensitized water photoreduction even in the absence of noble metal catalysts. The hydrogen generation rate in the DHP vesicle system was found to be 5 times higher on using ZnS-coated CdS (ratio of Zn:Cd = 1:l) than pure CdS semiconductor particles as sensitizers.
Introduction Photochemical reduction of water utilizing dispersed colloidal semiconductors has been intensively investigated.'-I3 Both CdS'e24 and ZnS2S-27have been used as sensitizers. The 2.42-eV
( I ) Fendler, J. H . J . Phys. Chem. 1985,89, 2730. (2) (a) Kalyanasundaram, K.; Gratzel, M.; Pelizzetti, E. Coord. Chem. Rev. 1986,69, 57. (b) Gratzel, M. Homogeneous and Heterogeneous Photocatalysis; NATO AS1 Series, Series C; Kluwer: Dordrecht, 1985; p 91. (3) Memming, R. Photoelectrochemistry, Photocatalysis, and Photoreactors; Kluwer: Dordrecht, Holland, 1985; pp 107-153. (4) Serpone, N.; Pelizzetti, E. Photoelectrochemistry, Photocatalysis, and Photoreactors; Kluwer: Dordrecht, Holland, 1985; pp 5 1-90. (5) Reber, J. F. Photoelectrochemistry, Photocatalysis, and Photoreactors; Kluwer: Dordrecht, Holland, 1985; pp 321-349. (6) Henglein, A. Pure Appl. Chem. 1984, 56, 1215. (7) Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (8) Bard, A. J . Phys. Chem. 1982,86, 172. (9) Heller, A. Acc. Chem. Res. 1982, 14, 154. ( I O ) Albery, W. J. Acc. Chem. Res. 1982, 15, 142. ( 1 1) Organic Phototransformations in Nonhomogeneous Media; Fox, M . A,, Ed.; American Chemical Society: Washington, D.C., 1985. (1 2) Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983. ( 1 3) Rajeshwar, K.; Singh, P.; DuBow, J. Electrochim. Acta 1978, 23, 117. (14) (a) Darwent, J. R.; Porter, G. J . Chem. SOC.Commun. 1981, 145. (b) Darwent, J. R. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1703. (15) Harbour, J. R.; Wolkow, R.; Hair, M. L. J . Phys. Chem. 1981, 85, 4026. (16) (a) Kalyanasundaram, K.; Borgarello, E.; Dunghong, D.; Gratzel, M. Angew. Chem., Int. Ed. Engl. 1981, 20, 897. (b) Borgarello, E.; Kalyanasundaram, K.; Gratzel, M.; Pelizetti, E. Helu. Chim. Acta 1982.65, 243. (e) Dunghond, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. Soc. 1982,104,2977. (17) (a) Matsumura, M.; Saho, Y.; Tsubomura, H. J . Phys. Chem. 1983, 87, 3807. (b) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H . J . Phys. Chem. 1984, 88, 248. (18) Biihler, N.; Meier, K.; Reber, J. F. J . Phys. Chem. 1984, 88, 3261. (19) (a) Tricot, Y.-M.; Fendler, J. H . J . Am. Chem. Soc. 1984,106, 2475. (b) Tricot, Y.-M.; Fendler, J. H. J . A m . Chem. SOC.1984, 106, 7359. (c) Rafaeloff, R.; Tricot, Y.-M.; Nome, F.; Fendler, J. H . J . Phys. Chem. 1985, 89, 533. (d) Tricot, Y.-M.; Emeren, A,; Fendler, J. H . J . Phys. Chem. 1985, 89, 4721. (20) Mau, A. W.-H.; Huang, C. B.; Kakuta, B.; Bard, A. J.; Campion, A. C.; Fox, M. A,; White, M.; Webber, S. E. J . Am. Chem. Soc. 1984,106,7359. (21) Aruga, T.; Dorren, K.; Natio, S.; Onishi, T.; Tamura, K. Chem. Lett. 1983, 1037. (22) Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983.87, 474. (23) Mills, A.; Porter, G. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 3659.
0022-3654/88/2092-6320$01.50/0
band gap of CdS" allows the visible light irradiation for water photored~ction.'~ Efficient hydrogen generation in CdS-sensitized systems requires, however, the presence of a noble metal catal y ~ t . ' ~Incorporation ,'~ of ZnS into CdS has recently been shown to produce hydrogen upon water even in the absence of noble metal catalysts. When two semiconductors are present in a colloidal dispersion, they can form either a solid solution (microscopically homogeneous phase) or a phase-separated, colloidal (microscopically heterogeneous phase) dispersion, depending on the structure of the matrix and formation conditions. Understanding the microstructure of such colloidal semiconductor dispersions is crucial for designing an efficient solar energy conversion and storage system. Not only intrinsic (those defined by composition and crystal structure, i.e., band gap energy, electrical conductivity, etc.) but also extrinsic (those defined by the microstructure, Le., surface morphology, etc.) properties affect the chemistry mediated by mixed colloidal particles. Conditions employed during the generation of mixed colloidal particles and the nature of the matrices used for stabilization determine the properties of mixed colloidal particles. H e ~ a m e t a p h o s p h a t e , ~ ~ (24) Furlong, D. N.; Grieser, F.; Hayes, D.; Hayes, R.; Sasse, W.; Wells, D. J . Phys. Chem. 1986, 90, 2388. (25) Reber, J . F.; Meier, K. J . Phys. Chem. 1984, 88, 5903. (26) Kisch, H. Homogeneous and Heterogeneous Photocatalysis; NATO AS1 Series, Series C; Kluwer: Dordrecht, 1986 p 385. (27) (a) Yanagida, S.; Azuma, T.; Sakurai, H. Chem. Lett. 1982, 1069. (b) Yanagida, B.; Azuma, T.; Kawakami, H.; Kizumoto, H.; Sakurai, H. J . Chem. SOC.,Chem. Commun. 1984, 21. (c) Yanagida, S.; Kawakami, H.; Hashimoto, K.; Sakata, T.; Pac, C.; Sakurai, H. Chem. Lett. 1984, 1449. (d) Yanagida, S.; Azuma, T.; Midori, Y.; Pac, C. J . Chem. SOC.,Perkin Trans. 2 1985, 1487. (28) CRC Handbook of Physics and Chemisrry, 71st ed.; CRC Press: Boca Raton, FI, 1987. (29) (a) Mau, A. W.-H.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A,; Fox, M. A,; White, J. M.; Webber, S. E. J . A m . Chem. SOC.1984, 106, 6537. (b) Kakuta, N.; White, J. M.; Campion, A,; Bard, A. J.; Fox, M. A.; Webber, S . E.; J . Phys. Chem. 1985, 89, 48. (c) Kakuta, N.; Park, K.-H.; Finlayson, M. F.; Ueno, A,; Bard, A. J.; Campion, A,; Fox, M. A,; Webber, S . E.; White, J. M. J . Phys. Chem. 1985, 89, 732. (d) Ueno, A.; Kakuta, N.; Park, K.-H.; Finlayson, M. F.; Bard, A. J.; Campion, A,; Fox, M. A,; Webber, S . E.; White, J . M. J . Phys. Chem. 1985, 89, 3828. (e) Kakuta, N.; Park, K.-H.; Finlayson, Mm. F.; Bard, A. J.; Campion, A,; Fox, M. A,; Webber, S . E.; White, J. M. J . Phys. Chem. 1985, 89, 5028. (f) Enea, 0.; Bard, A. J. J . Phys. Chem. 1986, 90, 301. (30) Reber, J. F.; Rusek, M. J . Phys. Chem. 1986, 90, 824. (31) Kobayashi, J.; Kitaguchi, K.; Tanaka, H.; Tsuiki, H.; Ueno, A. J . Chem. SOC.,Faraday Trans. 1 1987, 83, 1395. (32) Henglein, A,; Gutierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 852.
0 1988 American Chemical Society
Mixed CdS and ZnS Semiconductor Particles colloidal ~ i l i c a ,alumina,31 ~ ~ ~ , ~ Nafion29b'c-c and clay29fhave been employed, to date, for the in situ generation and stabilization of coprecipitated Zn,Cdl-,S or ZnS-coated CdS semiconductor particles. Surfactant vesicles have been utilized in our laboratory as hosts for colloidal semiconductor particle^.'^*^^-^^ They provide nucleation sites, control the growth of the particles, act as stabilizers against coagulation, and enable compartmentalization of reactants and products.' Dihexadecyl phosphate (DHP) vesicles have been utilized as matrices for tailoring the band gap energy and microstructure of mixed CdS and ZnS colloidal particles in the study reported here. These DHP-vesicle-incorporated, mixed colloidal particles efficiently mediate water photoreduction and hydrogen generation in the presence of a sacrificial electron donor when irradiated by visible light, even in the absence of an added catalyst.
Experimental Section Materials. Dihexadecyl phosphate (DHP; Sigma), CdCI2. 2.5H20 (Baker), ZnC12 (Baker), gaseous H2S(99.5%, Matheson), acetone (Fisher), 2-propanol (Fisher), benzyl alcohol (Baker), and methylviologen dichloride (MVC12.4H20;Aldrich Chemical) were of analytical grades and used without purification. Water was purified with a Millipore Milli Q system containing a 0.4-pm Millistack filter system at the outlet. Five (A-E) different preparations were used for this study. Cd2+ and Zn2+ were externally added to the preformed vesicle solution for preparations A-D. Cd2+ and Zn2+were cosonicated with the surfactant for preparation E. Preparation of DHP Vesicles. DHP vesicles were prepared as described p r e v i o ~ s l y , ' ~by+ ~sonication ~ ~ ~ ~ in water at 80 OC. Typically, 49.2 mg of DHP was heated up to 80 OC in ca. 30 mL of water before sonication. After an initial dispersion by 5-10 min of sonication of DHP, 1.8 mL of 0.1 N N a O H (200% the stoichiometric amount of DHP) was injected to obtain a final pH of ca. 11. After 30 min of sonication at 80 OC, the solution was cooled to room temperature and diluted with water to a final M D H P (final volume = 45 mL). concentration of 2 X In Situ Generation of Pure CdS (Preparation A ) and Pure ZnS (Preparation B ) in DHP Vesicles. One hundred microliters of either 0.01 M CdC12 (preparation A) or 0.01 M ZnC12 stock solution (preparation B) was slowly added to 5.0 mL of 2 X M D H P vesicle solution with constant stirring to achieve a homogeneous distribution of Cd2+ (2 X lo4 M) or ZnZ+(2 X lo4 M) on the outer vesicle surface. After degassing by bubbling argon through the solution, H2S (2 times the stoichiometric amount of Cd2+or Zn2+)was injected into the sample solution. Subsequent to 10 min of incubation time, the unreacted H2S was removed by bubbling argon through the solution for at least 1 h. DHPincorporated CdS or ZnS was freshly prepared for each experiment. Storing in the dark for 1 week did not affect, however, the properties of these preparations. Single-Step Coprecipitation of Zn,Cdl-J on the Outer Surfaces of DHP Vesicles (Preparation C). Solutions of 0.01 M CdC12 and 0.01 M ZnC12 in various volume ratios (at a constant M DHP total volume of 100 pL) were added to 5 mL of 2 X vesicle solution. After constant stirring for 10 min, 44.8 pL (2 times the stoichiometric amount of total metal ions) of HzS was injected. Unreacted H2S was removed, as in preparations A and B, by argon bubbling after an incubation period of 10 min. This procedure led to the formation of Zn,Cd,-$ ( x being determined by the volume ratio of added CdCI, and ZnC12 stock solutions) on the outer surfaces of D H P vesicles. Preparation of ZnS-Coated CdS Particles on the Outer Surfaces of DHP Vesicles (Preparation D). Dropwise addition of 100 or 150 pL of 0.01 M CdCl, to 5 mL of 2 X M DHP vesicle solution, degassing by argon bubbling followed by injection of 89.6 or 134.4 pL of H2S (4 times the stoichiometric amount (33) Watzke, H. J.; Fendler, J. H. J . Phys. Chem.1987, 91, 854. (34) Youn, H.-C.;Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1987, 91, 581. (35) Tricot, Y.-M.; Furlong, D. N.; Sasse, W. H. F.; Davis, P.; Snook, I.; Van Megen, W. J. Colloid Interface Sci. 1984, 97, 380.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6321 of Cd2+)during vigorous stirring, and further argon bubbling led to the formation of 2 X or 3 X lo4 M CdS particles on the outer surfaces of D H P vesicles. An appropriate amount of 0.01 M ZnClz was added to this solution, followed by a further addition of H2S (4 times the stoichiometric amount of ZnC12). Removal of excess HIS by argon bubbling led to the formation of ZnScoated particles on the outer surfaces of D H P vesicles. Preparation of Zn,Cdl-J on both the Inner and the Outer Surfaces of DHP Vesicles (Preparation E ) . Appropriate volumes of 0.01 M CdC12and 0.01 M ZnC12were cosonicated with D H P surfactant to give final concentrations of 2 X M DHP, 2 X lo4 M Cd2+,and 2 X lo4 M Zn2+. The pH of the solution was adjusted to the required value (8-1 1) during sonication by the addition of 0.1 N NaOH. H2Swas injected into an argon stream flowing through the vesicle solution containing Cd2+ and Zn2+ distributed on both surfaces of the vesicle. The amount of H2S was typically 20% of the stoichiometric amount of Cd2+and Zn2+ present in the solution. The vesicle solution was degassed by argon prior and subsequent to the introduction of H2S. Spectroscopic Determinations of CdS and ZnS. Amounts of CdS and ZnS in ZnS-coated CdS particles, formed in the D H P vesicle solutions, were determined spectrophotometrically. Differences in absorbance at 400 or 285 nm prior and subsequent to the addition of H2S was utilized for assessing the amount of CdS or ZnS particles formed by taking tmnm = 1000 f 100 M-I cm-' for CdS19band t285nm = 1600 f 200 M-' cm-I for ZnS.36 The amounts of cadmium and zinc in coprecipitated Zn,Cd,-,S were estimated from the amounts of Cd2+ and Zn2+ ions precipitated from the solution in the form of sulfides. Absorption and emission spectra were measured with an H P 8450 diode array spectrometer and a Tracor Northern 6500 rapid scan spectrofluorometer system. X-ray Powder Dijfractometry. X-ray diffraction measurements of semiconductor powders were performed with a General Electric powder diffractometer (XRD-6) equipped with a Harshaw electronics package, and a scintillation counter as detector. Copper K a radiation (average X = 1.542 A), filtered through a nickel foil and obtained with power settings of 30 kV and 10 mA, was used as the beam, w-28 scan speeds were 2 and/or 0.4O/rnin. A time constant of 1 s and counting sensitivity of 500 counts/s were typically used to record the diffraction pattern. Four hundred milliliters DHP-vesicle-incorporated semiconductor particles (obtained by procedures for preparations, A-D) was rotary evaporated at 50-55 "C and 30 mmHg pressure. The molar ratio Cd:Zn was 1:l for preparations C and D. Just prior to complete removal of water, 200 mL of acetone and 2-propanol mixture (5050 by volume) was added to the residue to dissolve DHP. The resulting solution was filtered and the residue was extracted, once again, by 100 mL of warm acetone and isopropanol mixture (50:50 by volume). The residue was transferred on a slide glass while it was still wet and was spread evenly over a 3 cm by 1 cm area with the help of a few drops of acetone. Samples were completely dried in the dark in a desiccator prior to X-ray diffraction measurement. Flash Photolysis and Continuous Irradiation Experiments. In the flash photolysis experiments, a Quanta Ray DCR Nd:YaG laser was used, delivering 20-11s pulses of 353.4-nm irradiation (third harmonic) with 12 f 1 mJ per pulse en erg^.^^,^' The analyzing light was a 150-W mercury-xenon lamp fitted with a 550-nm cutoff filter to avoid preexcitation of CdS. Samples were placed in a 0.4 X 1.O cm quartz cell with the 1 .O-cm side facing the 0.4-cm-diameter laser beam. MV" formation was monitored at 605 nm, taking t = 13 700 & 300 M-I cm-1.38 Continuous irradation experiments were carried out by using a 450-W xenon lamp with a 400-nm cutoff filter and a 7.0-cm-long water cell. The 0.4 X 1.O cm quartz cell was also used for continuous irradiation experiments. Flash photolysis or continuous irradiation (36) The c value for ZnS was calculated from the slope of the plot in the insert of Figure 2 (see Results). (37) Reed, W.; Guterman, L.; Tundo, P.; Fendler, J. H. J . Am. Chem.SOC. 1984, 106, 1897. ( 3 8 ) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617.
6322 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Youn et al.
1.0 P)
E
2 D
n 4
-
0.5 -
I-
0.0 -
200
300
400
600
500
250
Wavelength,nm
Figure 1. Absorption spectra of Zn,Cd,_,S produced in situ in D H P vesicles at pH 9.7. (a) x = 1 (ZnS); (b) x = 0.8; (c) x = 0.6; (d) x = 0.4; (e) x = 0.2; (f) x = 0 (CdS). Total amount of Zn2+ Cd2+ was constant (2 X M). Absorption spectra were corrected for vesicle scattering. [ D H P ] = 2 X IO-' M. Variation in band gap energy with increasing amounts of Zn in ZnxCdl-$ is shown in the insert. The dotted line shows the ideal behavior.
+
experiments were performed on argon-bubbled (1 h) samples. Spectra were taken after every 10-s irradiation period, interrupting the irradiation by not more than 20 s for each recording. MVCI2 (4 X or 1 X IO-' M) was added to the degassed sample 15 min prior to the flash photolysis or continuous irradiation experiments. Benzyl alcohol [O.Ol% (v/v)] was used as a sacrificial electron donor in the continuous irradiation experiments. H2 Generation in the Presence of Benzyl Alcohol. Preparations A-D (containing in situ generated colloidal semiconductors at the outer surfaces of DHP vesicles) were employed in the H2 generation experiments. Samples were degassed by argon bubbling (2 h) and were subjected to irradiation by a 450-W xenon lamp through a 400-nm cutoff and a 6-cm water filter. Two hundred fifty microliters of benzyl alcohol was added to 25 mL of the sample just prior to irradiation. Details of the procedure have been published elsewhere.34 The number of photons falling on the photolysis cell was determined by potassium ferrioxalate a c t i n ~ m e t r yat~ ~404 - ~ nm using an interference filter. Amounts of hydrogen in the gas phase were measured by gas chromatography. as previously described.19 Results Figure 1 shows the absorption spectra of pure CdS, pure ZnS, and coprecipitated Zn,Cd,_,S mixed crystals incorporated on the outer surfaces of D H P vesicles. Corrections were made for the light loss by vesicle scattering. CdS particles in DHP vesicles (preparation A) have an absorption edge at 475 nm (Figure If). ZnS in D F P vesicles (preparation B) shows an absorption edge at 325 nm (Figure la). These wavelengths correspond to band gaps of 2.6 eV for CdS and 3.8 eV for ZnS, respectively, and are somewhat larger in energy than those reported for CdS and ZnS as macroscopic solids (2.42 a n d 3.67 e V , r e s p e ~ t i v e l y ) . ' ~Blue .~~ shifts in the absorption edge n small-sized colloidal semiconductors have been interpreted in terms of quantum size e f f e ~ t . ~ ~ , (39) Shizuka, H.; de Mayo, P. In Creation and Detection of the Excited State; Ware, W. R., Ed.; Dekker: New York, 1976; Vol. 4, Chapter 4, p 139. (40) Rabek. J. F. Experimental Methods in Photochemistry and Photophysics; Wiley-Interscience: New York, 1982. (41) (a) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A,, Ber. BunsenGes. Phys. Chem. 1984, 88, 649. (b) Fojtik, A,; Weller, H.; Koch, U.; Henglein, A,. Koch, U.; Fojtik, A,; Baral, S.; Henglein, A,; Ber. Bunsen-Ges. Phys. Chem. 1984,88, 969. (c) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A.; Baral, S.; Henglein, A., Kunath, W.; Weiss, K. Chem. Phys. Lerf. 1986, 124, 5 5 7 . (d) Spanhel, I-.; Weller, H.; Fojtik, A,; Henglein, A. Ber. Bunsen-Gcr. Phys. Chem. 1987, 91, 441. ( e ) Henglein, A,; Fojtik, A,: Weller, H. Bey. Burisen-Ges. Phvs. Chem. 1987, 91. 441
300
350
400
450
500
Wavelength,nm
Figure 2. Absorption spectra of sequentially precipitated ZnS-coated CdS in D H P vesicles at p H 10.0. [CdS] = 3 X IO4 M all cases. (a) [ZnS] = 0; (b) [ZnS] = 0.75 X IO4 M; (c) [ZnS] = 1.50 X lo4 M; (d) M; (e) [ZnS] = 3.0 X lo4 M; (f) [ZnS] = 4.5 X [ZnS] = 2.25 X IO-" M. Absorption spectra were corrected for vesicle scattering. 0.1 N N a O H was added to adjust p H to 10 after CdS formation. [DHP] = 2 x 10-3 M . 0.6
0.4
0
1
0.4
0
B
1
285
......
300
350
.... ..
400
450
500
Wavelength, nm
Figure 3. (A) Spectra obtained by subtracting those due to DHP-vesicle-incorporated CdS from the spectra shown in Figure 2. (B) Absorption spectra of ZnS produced in situ in D H P vesicles a t p H 10.0. [Zn2+] = 2 X IO4. Stoichiometry of H2S reacted with Zn? (a) 25%; (b) 50%; (c) 75%; (d) 100%; (e) 200%. Absorption spectra were corrected for vesicle scattering. A b s o r p t i o n s p e c t r a of Zn,Cd,,S coprecipitated on t h e o u t e r surfaces of DHP vesicles (preparation C) were found to lie between ~those ~ - ~of ~CdS and ZnS (see spectra b-e in Figure 1). Absorption edges of Zn,Cd,-,S systematically blue shifted from 475 to 325 (42) (a) Brus, L.; J . Phys. Chem. 1986, 90, 2555. (b) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. J . Phys. Chem. 1986, 90, 3393. (43) (a) Duonghong, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. SOC. 1982,104,2977. (b) Ramsden, J.; Gratzel, M. J . Chem. SOC.,Faraday Trans. 1 1984,80, 919. (c) Serpone, N.; Sharma, D. K.; Jamieson, M. A.; Gratzel, M.; Ramsden, J. Chem. Phys. Lett. 1985, 1 1 5 ( 6 ) , 473. (44) Metcalfe, K.; Hester, R. E. J . Chem. Soc., Chem. Commun. 1983, 133. (45) (a) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. J. J . Phys. Chem. 1985, 89, 397. (b) Nedeljkovic, J. M.; Nenadovic. M. T.; Micic, 0. I . ; Nozik, A. J . J. Phys. Chem. 1986, 90, 12.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6323
Mixed CdS and ZnS Semiconductor Particles
CdS pH 6.2 pH 8.6 pH 9.1
A pH 10.5
ZnS pH 9 . 4
pH 9 . 2 pH 8.8
B
i
300
400
I
3.
500
\.
\.+pH
600
10.9
700
Wavelength, nm Figure 4. Emission spectra at various pH values of CdS (A), ZnS (B), and Zno,5Cdo,sS (C) generated in situ on both surfaces of DHP vesicles.
nm on increasing the amount of zinc in Zn,Cdl-$. The variation in the band gap energy, with increasing amounts of Zn in the DHP-vesicle-supported Zn,Cd,-,S semiconductors, is shown in the insert of Figure 1. Deviation from the ideal behavior (dotted-straight line in the insert of Figure 1) might be the consequence of preferential CdS precipitation due to the smaller solubility product of CdS than that of ZnS. Absorption spectra of sequentially precipitated ZnS-coated CdS colloidal particles (Figure 2) show two separate absorption edges at 325 and 475 nm corresponding to those of pure CdS and ZnS. Sequentially precipitated ZnS-coated CdS colloids, unlike their coprecipitated counterparts, do not show any shift of absorpton edge at 475 nm with composition change. Instead their absorbances at wavelengths shorter than 325 nm increased with increasing amounts of Zn2+in the solution. The linear increase of the absorption at 285 nm with increasing amounts of Zn2+and H2S added to preformed DHP-incorporated CdS (see insert in Figure 2) suggests that quantitative conversion of Zn2+ to ZnS and permits the assessment of the extinction coefficient for ZnS in the in situ generated, DHP-incorporated, ZnS-coated CdS particles to be t285nm = 1600 f 200 M-' cm-I. Subtracting the absorption spectra of DHP-vesicle-incorporated CdS from those of sequentially precipitated CdS and ZnS (given in Figure 2) led to spectra (Figure 3A) that resembled those of ZnS produced on D H P vesicles by using preparation procedure B (Figure 3B). In both cases (see Figure 3), absorbances around 285 nm are linearly proportional to the amount of ZnS present in the colloid. The blue-shifted spectra with decreasing concentrations of ZnS may be. attributed to the formation of smaller sized particles. Figure 4 shows the fluorescence spectra of pure CdS, pure ZnS, and coprecipitated Zn,Cd,-,S (Zn2+:Cd2+= 1:l before HIS introduction) particles formed at different pH values on both surfaces of DHP vesicles. Zno,,Cd,,,S particles, like pure CdS,'9,46
do not fluoresce when present only at the outer surfaces of DHP vesicles. Incorporating these semiconductors on both sides of DHP vesicles led to broad emission bands for CdS and ZnS with onsets and maxima at 420 nm, 530 nm; 370 nm, and 460 nm, respectively. Fluorescence originates only from those semiconductor particles that are present in the interiors of D H P vesicles. This was confirmed by the absence of fluorescence quenching upon the external addition of 1 X lo-" methylviologen to vesicle solutions which contained CdS, ZnS, or Zn,Cd,-,S only in their interior. No fluorescence could be observed when methylviologen was cosonicated with D H P and Cd2+ (and/or Zn2+) prior to semiconductor formation. The pH of the solutions, at the time of sulfide precipitation, affected the intensity and the position of fluorescence emission bands of CdS and ZnS (see Figure 4A,B). The observed emission intensity of CdS was stronger when CdS particles were formed in solutions near neutral (pH 6.2) than at alkaline (pH 10.5) pH values (Figure 4A). Conversely, the intensity of ZnS emission was found to be stronger on forming this semiconductor in alkaline rather than in neutral solutions (Figure 4B). Emission maxima of CdS and ZnS particles in different systems have been found to range between 440 and 800 nm and between 380 and 450 nm.22,41-48 The spectrum of pure ZnS particles generated at the inner surface of DHP vesicles (emission maxima at 455 nmj is similar to that of hexametaphosphatestabilized ZnS (emission maxima at 425 nm).22*41a Similarly, the fluorescence of CdS particles incorporated in the interiors of DHP vesicles (emission maxima at 530 nm) also resembles that of hexametaphosphate-stabilized CdS (emission maxima at 500 nm).41b Both the position and the intensity of the emission band of coprecipitated particles were affected by the pH of the solution during sulfide formation (Figure 4C). Depending upon pH (in the pH 7.9-10.9 range), Zn,Cd,_xS (Zn2+:Cd2+= 1:l before HIS introduction) particles had onset of emission between 320 and 420 nm and maxima between 460 and 530 nm. The variation in the observed fluorescence intensities of CdS and ZnS particles distributed over the inner and outer surfaces of D H P vesicles with the pH of the solution during sulfide precipitation can be understood in terms of two opposing phenomena. An increase in the precipitation pH results in an increase in the inherent fluorescence intensity of CdS or ZnS particles residing on the inner vesicle surface. A very similar pH effect has been recently reported for hexametaphosphate-stabilized CdS particles.41e Increasing the basicity of solutions promotes the preferential formation of CdS or ZnS particles on the outer surfaces of the vesicles. As the sulfide particles on the outer vesicle surfaces do not fluoresce, this effect tends to decrease the observed fluorescence. In the case of CdS, which has a very small solubility product (Ksp = 1.4 X for ZnS28),nucleation and particle growth for CdS and on the vesicle surface is kinetically facile and, therefore, the second effect predominates even at the lowest pH. On the other hand, with ZnS, the increase in pH in the range of 7.9-10.9 does not considerably alter the particle distribution between in inner and outer vesicle surfaces and, hence, the observed fluorescence intensity increases with pH. A very similar effect was also observed with the coprecipitated crystals Zn,Cd,,$ (Figure 4C). The effect of pH on the rate of permeability of H2S or SH- ion through the vesicle bilayer is reflected in the shift in the fluorescence maximum which is caused by a change in the composition of the mixed crystalline particles on the inner vesicle surface. With the increase of pH, the permeation rate across the membrane was found to decrease and the mixed crystalline phase became richer in cadmium due to the lower solubility of CdS. This resulted in a redshift of the observed fluorescence maximum in Zn,Cd,-,S. The X-ray diffraction patterns of pure or mixed colloidal metal sulfides, generated on DHP-vesicle surfaces, are given in Figure 5. Principal diffraction peaks and Miller indices (in parentheses) (46) Tricot, Y.-M.; Fendler, J. H . J . Phys. Chem. 1986, 90, 3369. (47) (a) Rossetti, R.; Hull, R.; Gibson, J. M.;Brus, L. E. J . Chem. Phys. 1985, 82, 559. (b) Brus, L. E. J . Phys. Chem. 1986, 90, 2 5 5 . (48) Dannhauser, T.; O'Neil, M.;Johansson, K.; Whitten, D.; McLendon, G.J . Phys. Chem. 1986, 90, 6074.
6324 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 I
I
as 311
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cds
220
220
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Youn et al. I
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ZnS CdS 111 111
I
I
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I
30
40
20
2 0 , degree Figure 5. X-ray diffraction pattern of various pure and mixed sulfides generated in situ in DHP vesicle surfaces: (a) pure CdS; (b) Zno,sCdo,5S; (c) ZnS-coated CdS (ZnS:CdS = 1:l); (d) pure ZnS.
of pure CdS particles, preparation A, are at 20 values of 26.9' (1 1 I ) , 43.9' (220), and 52.0' (311), while those for pure ZnS particles, preparation B, are at 20 values of 28.5' (1 1 l ) , 47.8' (220), and 56.6' (31 1). These 28 values of the principal peaks, and their observed intensity ratios agree well with the most intense peaks listed for cubic @-CdSand cubic D-ZnS in standard reference tables.49 Coprecipitated Zno.sCdo,sSparticles, preparation C, appear at 20 values of 27.7', 44.5', and 52.6' (Figure 5b). The peaks were located approximately at the midpoints of corresponding (1 11,220, and 3 1 1 reflections) peaks for CdS and ZnS. Sequentially precipitated ZnS-coated CdS, preparation D, showed peaks at 26.9', 44.5', and 52.5'. These values were not much different from the 20 values obtained from pure CdS. XRD peaks corresponding to pure ZnS phase could not be observed in these samples. The average size of these microcrystallite particles can be estimated from their diffraction peak width assuming a complete random orientation in the crystallites and an absence of internal stress. In this case, the real diffraction broadening @ can be computed from the observed half peak width B by correcting for instrument broadening b which arises from extraneous factors (divergence of the X-ray beam, specimen geometry, and the wavelength spread of the Cu Ka,-Cu K a 2 doublets). @ was obtained from p2 = BZ- b2. The incompletely removed sodium chloride acts as an internal standard in the sample (crystallite size > lo4 cm), and the instrument broadening, b, is given by the half peak width of the sharp diffraction peaks of NaCl at 20 = 26.9', 45.5', and 56.5'. Assuming a spherical shape for the particles, the volume-averaged diameter of the sulfide particles normal to the diffracting plane D is related to the pure diffraction broadening, p in radian, by the Scherrer equation" D = A/(@ cos 0) (1) where X is the X-ray wavelength (1.542 A for Cu K a doublet) and 0 is the Bragg's angle of the diffraction peak. D is related to the actual particle diameter by a factor of 4/s. For example, for the 220 line of Z Q , ~ C ~ &homogeneous mixed crystals (Figure 5b), 20 = 45' and B = 3.75' (65.5 X IOw3 rad). Using the 28 = 45.5' peak of NaCl with b = 0.5' (8.7 X rad), the true diffraction peak width, p, can be calculated to be 60.7 X loW3rad.51 (49) Powder Diffraction File, ASTM, 1969. Card no. 10-454 and 6-0314 for CdS, 5-0566 and 5-0492 for ZnS. (SO) Taylor, A. In X-ray Metallography; Wiley: New York, 1983; p 614.
2 n e4 I.
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Figure 6. Electron transfer from excited semiconductor particles to MV2+ as acceptor. Excitation was at 353.4 nm with 204s laser pulses. Absorption change was monitored at 605 nm. All samples contain the same amount of CdS (2 X 10" M), MVC12 (1 X 10" M), DHP (2 X lo-' M), and pH (9.0). (A) ZnS-coated CdS; (0)Zn,Cd,_,S mixed crystals.
Data for mixed crystals were corrected for different absorptions at 354 nm of different samples. From eq 1, the mean particle diameter turns out to be 46 A. Very similar values (38-70 A) were obtained from the other diffraction peaks in Figure 5. Although other factors may contribute to peak broadening, these values can be taken as a lower limit for the average particle size. This suggests that extensive aggregation did not occur during the removal of water and surfactant in the course of sample preparation. Electron-transfer efficiency from the conduction band of mixed semiconductor particles, produced in situ on the outer surfaces of DHP vesicles, was assessed by flash photolysis and steady-state irradiation using methylviologen dichloride, MV2+, as electron acceptor and benzyl alcohol as valence band hole quencher. In the flash photolysis experiments, the amount of MV" radical formed upon flash excitation (duration 20 ns) at 354 nm was monitored by measuring the absorbance change at 605 nrn. (51) Jones, F. W. Proc. R. Soc. London, A 1938,116, 16.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6325
Mixed CdS and ZnS Semiconductor Particles
d
0
20
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11
Irradiation time, S e c Figure 7. Formation of MV'+ upon irradiation with X > 400 nm in the presence of 0.01% benzyl alcohol. (A) [MV2+]= 1 X lo4 M, [CdS + ZnS] = 2 X lo4 M, Zn,Cd,,S in situ generated on DHP vesicles. (a) [CdS] = 2 X lo4 M; (b) [CdS] = 1.6 X lo4 M, [ZnS] = 0.4 X lod M; (c) [CdS] = 1.2 X lo4 M, [ZnS] = 0.8 X lod M; (d) [CdS] = 0.8 X lo4 M, [ZnS] = 1.2 X lo4 M; (e) [CdS] = 0.4 X lo4 M, [ZnS] = 1.6 X lo4 M. (9) [MV2+]= 1 X lo4 M, [CdS] = 2 X lo4 M all cases. ZnS-coated CdS in situ generated on DHP vesicles. (a) [ ZnS] = 0; (b) [ZnS] = 0.4 X lo4 M; (c) [ZnS] = 0.8 X lod M; (d) [ZnS] = 1.2 X lo4 M; (e) [ZnS] = 1.6 X lo4 M; (f) [ZnS] = 2.0 X lo4 M.
Excitation at wavelengths longer than 330 nm resulted in energy deposition only into CdS since the absorption edge of ZnS particles lies below this wavelength. Formation of MV" radical was found to be instantaneous in all cases (
