J . Phys. Chem. 1992, 96, 2868-2873
2868
when the inhomogeneous broadening due to the particle size distribution is reduced. The photocatalytic polymerization of styrene is the most probable candidate for a photocatalytic reaction. The existence of styrene monomer was indispensable to the observation of the photoirradiation effect. In addition, the photoirradiation effect was enhanced by increasing the styrene monomer concentration. Photocatalytic polymerization of styrene would create a polystyrene coating on the surface of the CdS particle. The polymer layer prevents the particles from growing further. In spite of our efforts to obtain evidence for the formation of polystyrene by using such methods as FT-IR analysis, no trace of polystyrene has yet been detected. This is probably due to the low concentration of polystyrene on the surface. This model describing the polymerization of styrene on the surface of the growing particles thus remains hypothetical at present. It was initially thought that o-dicyanobenzene was necessary as an electron acceptor or mediator for generating active radicals for subsequent reactions or by simply making holes more reactive by removing electrons. However, the experimental results on the contribution of each component in the solution revealed that o-dicyanobenzene was not indispensable as long as enough styrene monomer is in the solution. As the irradiating light intensity increased, the wavelength of the absorption onset shifted to shorter wavelength, approaching 440 nm, as shown in Figure 1. With 0.21 W/cm2 intensity light, the onset wavelength almost coincided with the shortest wavelength (440 nm) of the irradiating light. This indicates that photoirradiation is effective for preventing the CdS particles from growing over a size corresponding to the shortest wavelength of the irradiating light. The photoirradiation effect is expected to become more effective when the CdS formation rate is small enough so that the photocatalytic reaction can take place before the growing particles have become larger than the size corresponding to the irradiation light wavelength. In fact, decreasing the H2Sconcentration in the H2Sand He gas mixture from 0.25 to 0.02 vol % resulted in a shift of the absorption onset wavelength to shorter wavelengths.
We also investigated the possibility that previously-grown particles undergo photobleaching, giving rise to smaller particles. When a previously-prepared CdS colloidal solution was irradiated without the presence of H2S, almost no change in the optical absorbance was observed. Thus, photobleaching does not occur easily. It was found by us that counteranions play an important role in photobleaching. We found that different cadmium compounds gave different results for the photoirradiation effect. When Cd(N03)2.4H20was used instead of Cd(C104)2.6H20,no photoirradiation effect was observed. Furthermore, the colloidal solution of CdS derived from the nitrate suffered from photobleaching. That is, a decrease in the absorbance of the solution over the entire spectral range was observed when irradiated. It is known that different cadmium compounds give different size distributions of colloidal particles. It is also well-known that the perchlorate counteranion primarily used in the present experiments shows little or no complexing tendency toward metal cations in comparison to the nitrate ion.13 This might explain the different behavior observed for the photobleaching experiments of CdS colloidal solutions. The nitrate ion might have a stronger tendency to coordinate metal cations, thus leading to an acceleration of the photobleaching process. Several items still remain that need to be studied. These include exact characterization of the particle size distribution of irradiated and unirradiated samples, as well as confirming the mechanism controlling particle growth. The type of photocatalytic reactions occurring also needs to be studied further to clarify the origin and other details of the absorption onset shift induced by photoirradiation.
Acknowledgment. This work was supported by N E D 0 (New Energy and Industrial Technology Development Organization). Registry No. CdS, 1306-23-6; H,S, 7783-06-4; Cd(C104)2,13760styrene, 100-42-5; o-dicyanobenzene, 91-15-6.
37-7;
(13) Moeller, T. Inorganic Chemistry, Modern Asia ed.;John Wiley & Sons: New York, 1970; p 237.
Recombination Luminescence Quenching of Nonstoichiometric CdS Clusters by ZnTPP J. Chrysochoos Department of Chemistry, The University of Toledo, Toledo, Ohio 43606 (Received: September 9, 1991; In Final Form: November 26, 1991)
The recombination luminescence of CdS(e-/h+) clusters is quenched effectively by ZnTPP. The quenching procass is described by static interactions at low [ZnTPP], leading to a quenching constant (KQ),and by Langmuir isotherms, leading to ranging from 5 X lo4 to 2.5 X IO5 M-I, increase slightly adsorption-desorption equilibrium constants K . Values of (KQ), with decreasing cluster size (or increasing Eg value) at constant CdS cluster composition. On the other hand, values of (KQ) decrease slightly with increasing Cd2+nonstoichiometry (Le., Cd2+excess) at constant Eg value. Values of K range from 1.5 X lo5 to 9.5 X lo5 (Le., kads>> kdss), implying a strong binding of ZnTPP on the surface of CdS clusters. Treatment of recombination luminescence quenching by Poisson statistics indicates that an average CdS cluster consists of at least 100-150 CdS units (Le., 25-30-A diameters). Fluorescence quenching of ‘ZnTPP*(S,)by CdS is less efficient, and it is dependent upon the size of the cluster. Values of kQ70(’ZnTPP*)range from 0 to 2 X IO3 M-l for Eg values varying from 3.1 to 2.9 eV.
Introduction During the past decade scientists from different disciplines demonstrated a very keen interest in the physical and chemical properties of semiconductor clusters. Although investigations in semiconductor clusters encompass a wide range of topics, two aspects of particulate semiconductors have attracted the interest of many investigators: optical properties (linear and nonlinear)
and interfacial electron transfer. The former relates to the significance of such species in electronic materials and devices whereas the latter to the role of semiconductor clusters as photocatalysts. Differences in the behavior of semiconductor clusters and that of bulk semiconductors are linked to both the “space confinement” imposed by the former on charge carriers (e-/h+), leading to “size quantization” effects, and the effect of surface
0022-3654/92/2096-2868%03.00/0 0 1992 American Chemical Society
Recombination Luminescence Quenching of CdS by ZnTPP
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2869 h
states and traps on their properties. The literature on these topics has been quite impressive, particularly in recent years. It was reviewed in several excellent recent articles'-s summarizing the earlier literature quite thoroughly. Any attempt to cite even the most relevant articles published in the field will inevitably lead to serious omissions due to space limitations. Although the absorption spectra of semiconductor clusters provide a measure of their band gap energy (Eg) via the absorption threshold (Xthr@h) and the absorption coefficient cr(hv), the luminescence spectra do not match the absorption spectra and the nature of the luminescence center(s) is often quite ambiguous. The luminescence emission of CdS clusters, whose spectral distribution and intensity depend strongly upon the preparation conditions of the cluster, is quenched by molecules and ions adsorbed on the surface of the cluster.61s Since the recombination luminescence observed is linked to trapped e- and h+ and surface defects rather than e-/h+ annihilation immediately after their formation, it has been employed to probe the physics and chemistry of e-CBand h+vBand to monitor interfacial electron transfer.'"" This brief paper examines the effect of particle size distribution, in terms of Eg, and CdS nonstoichiometry upon the recombination luminescence quenching of CdS clusters by ZnTPP (zinc tetraphenylporphyrin).
Experimental Section Colloidal CdS clusters were prepared by the well-established method of *arrested precipitation"" in 2-propanol at -78 O C and in the absence of colloidal stabilizers. The preparation was camed out with Cd(C104)2(99.9%) dissolved in 2-propanol and freshly prepared Na2S from anhydrous NaHS (99.9%) and slight excess of NaOH (99.99%) in methanol, in the absence of O2and H20. A nominal concentration of 2.0 X lo4 M CdS was prepared in all cases with an excess of Cd2+ ranging from 1 X lo4 to 8 X M, leading to CdS clusters with Cd2+-rich surface. The different Cd2+excess is defined as the "nonstoichiometry" of the clusters. Colloidal dispersions produced in this way were colorless and transparent. Their stability increased with their nonstoichiometry. Nonstoichiometric CdS clusters remained fairly stable for several months a t 0 O C . Solvents used in this study were of semiconductor purity. Zinc tetraphenylporphyrin was prepared from H2TPP (chlorin free) and zinc acetate using methods from the literaturea2' Results Absorption and emission spectra of all CdS clusters prepared were taken immediately following their preparation and routinely (1) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (2) Henglein, A. Chem. Reu. 1989, 89, 1861. (3) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (4) Bawendi, M. C.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477. (5) Wang, Y . ;Herron, N . J . Phys. Chem. 1991, 95, 525. (6) Weller, H.; Koch, V.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 649. (7) Ramsden, J. J.; Gratzel, M. J . Chem. SOC.,Faruduy Trans. I 1984, 80, 919. (8) Ramsden, J. J.; Webber, S.E.; Gratzel, M. J . Phys. Chem. 1985,89, 2740. (9) Chemoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J . Phys. Chem. 1986, 90, 3393. (10) Nosaka, Y . ;Fox, M. A. J . Phys. Chem. 1988, 92, 1893. (1 1) Kumar, A.; Janata, E.; Henglein, A. J . Phys. Chem. 1988, 92,2587. (12) Lee, Y . F.; Olshavsky, M.; Chrysochoos, J. J . Less-Common Met. 1989, 88, 259. (13) Chrysochoos, J. The Spectrum 1989, 2 (3), 16; 1990, 3 (3), 16. (14) Chrysochoos, J. Mol. Crysr. Liq. Crysr. 1991, 194, 247. (15) Chrysochoos, J. J. Lumin. 1991, 48&49, 709. (16) Kuczynski, J.; Milosavjevic, B. H.; Thomas, J. K. J . Phys. Chem. 1984, 88, 980. (17) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985, 82, 552. (18) Hiramoto, M.; Hashimoto, W. M.; Sakata, T. Chem. Phys. Lett. 1987, 133, 440. (19) Thomas, J. K. J . Phys. Chem. 1987, 91, 267. (20) Kamat, P. V.; Dimitrijevic, N. M.; Fessenden, R. W. J . Phys. Chem. 1987. 396. -. . , 91. ., -.. (21) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J . Inorg. Nucl. Chem. 1970, 32, 2443.
Figure 1. Plots of (a(hv))*vs hu of 2 X 10-4 M CdS clusters in 2-propanol with variable excess of Cd2+ (1 X 104-4 X lo4 M) immediately after their preparation (open symbols) and 42 days after preparation (filled symbols).
at several time intervals, while kept in storage. The absorption spectrum of 2 X M CdS with 1 X IO4 M excess Cd2+ exhibited an absorption threshold at about 500 nm and an absorption maximum at about 390 nm. The corresponding emission spectrum was extremely weak and broad (Le., 500-700 nm with a maximum at about 600 nm). This preparation, which consists of nearly stoichiometric CdS clusters, was quite unstable and precipitated out within a couple days. Cluster preparations with larger nonstoichiometries, Le., 2 X 10-4-8 X lo4 M excess Cd2+, were fairly stable. Their absorption threshold was blue-shifted to 440-450 nm, their absorption maximum moved to 360-370 nm, and the corresponding emission band became much more intense with & (,), at 540-560 nm. Evidence of partial particle dissolution at 0 "C was obtained at periods of several months after their preparation, manifested by a blue-shifted absorption band and reduction of the appropriate optical density (Lh = 410-390 nm). The corresponding emission spectra were blue-shifted accordingly = 500-510 nm). However, this effect was less pronounced in the presence of a larger excess of Cd2+, i.e., 4 x 10-"8 x M. The band gap energy (Eg) of all CdS clusters prepared was determined via the absorption coefficient of their edge-to-edge absorption, which is associated to a direct transition in CdS (k =0 k = 0) defined by22
-
a(hv) = [e2(2m*,,m*,/(m*,,
+ m*e))3/2/n~h2m*c]X (hv -
= A(hv - Eg)'/' (1)
where m*, and m*h stand for the effective mass of the electron and hole, respectively. Plots of {a(hv))2vs hv (eV) led to Eg values 0. a t a(hv) Equation 1 has been applied extensively for direct transitions of solid-state semiconductors. The same equation can be applied to semiconductor clusters, although their behavior is different from
-
(22) Pankove, J. J. In Optical Properties in Semiconductors; Prcntice-Hall: Englewocd Cliffs, NJ, 1971; p 36.
Chrysochoos
2870 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
TABLE I: Recombination Luminescence Quenching Constants ( K Q )of CdS(e-/h') in 2-Propanol by ZnTPP and Adsorption-Desorption Equilibrium Constants ( K ) of ZnTPP on CdS Clusters system Eg,eV ( L J m a x , nm Eg - (hvcm)mnx, eV (KQ), M-' 2 X lo4 M CdS,3 X M Cd2' =2.90 560-570 0.69-0.73 =5 x 104 2x M CdS,4 x M Cd2+ =2.90" 565-575b 0.71-0.75 =i.5 x 105 =2.95" 550-560 0.72-0.74 2X M CdS,4 X M Cd2' 2.1 x 105 3.05 =545 0.78 2X M CdS, 4 X M Cd2+ 1.1 x 105 3.0 555-565 0.77-0.81 2X M CdS,5 X lo4 M Cd2+ 1.7 x 105 3.05 550-560 0.82-0.84 2X M CdS, 5 X M Cd2+ 0.80-0.83 2.5 x 105 2X M CdS, 5 X M Cd2+ 3.10 540-545' 6 X lo4 3.10 535-545 0.78-0.83 2X M CdS, 8 X low4M Cd2+
10-5( K ) 1.5 9.5 5 4 7 7
"Broad distribution. bTraces. Weak.
that of bulk semiconductors, provided the effective-mass approximation is still in effect. The latter holds for crystallites containing as few as 95 Typical plots are shown in Figure 1, leading to Eg values equal to 2.87, 3.04, 3.06, and 3.08 eV for 2 X M CdS clusters with an excess of Cd2+equal to 1X 2 X loT4,3 X and 4 X M, respectively (1 h after preparation). The band gap energies of these clusters increased to 3.45, 3.37, and 3.26 eV in the same order shown above after a period of 42 days at 0 OC (Figure 1). Since the value of Eg increases with decreasing particle size25-27 Eg(c1uster) = Eg(bu1k) ( h2n2/2R2)(1/m*,+ 1/m*,,) 1.8e2/tR + polarization terms (2)
3
4t
+
there appears to be a gradual dissolution of CdS clusters which is less pronounced at higher nonstoichiometries. Such partial dissolution of CdS clusters in aerated colloidal dispersions upon illumination was documented in the past.28i29 Due to this stability pattern, CdS clusters used in this work (2 X lo4 M nominal) were associated with an excess of Cd2+in the range 3 X 10-4-8 X M. The recombination luminescence of CdS(e-/h+) clusters in deaerated and air-saturated dispersions in 2-propanol is quenched effectively by ZnTPP. The luminescence emission was excited at 365 nm and was monitored at 540 nm although the emission maxima of CdS clusters used were in the range 545-560 nm. This was necessary in order to avoid distortions in the emission band due to the absorption of ZnTPP consisting of the Q-bands, in the spectral range 550-600 nm, and the Soret band at 419 nm. The concentrations of ZnTPP used were low to avoid additional distortions of the luminescence recombination band of CdS by the 0-0 emission of ZnTPP (at 603 nm). Stern-Volmer plots of (PF/ZFZnTPP)corr vs [ZnTPP] were not linear, but they exhibited an upward trend at lower [ZnTPP] and a leveling-off trend at higher [ZnTPP] (Figure 3A). Semilogarithmic plots of (PF/ ZFZnTPP)corr vs [ZnTPP] at low [ZnTPP] were fairly linear. Such plots, implying static interactions between CdS(e-/h+) clusters and ZnTPP ( P F / ~ F = exp(K~[znTPP]) ~ ~ ~ ~ ~ ) ~ ~ (3) ~ ~ are shown in Figure 2 for 2 X lo4 M CdS with 5 X 10" M excess Cd2+. The slopes of these linear plots, Le., Kq (M-I), increase with increasing Eg value or decreasing particle size. These values should be viewed as average values arising from CdS clusters of different sizes (KQ) = ZKQ,$(Ri) (4) i
where X ( R i ) is the fraction of CdS clusters with radius Ri and KQ,j the corresponding quenching constant. Values of KQ determined from CdS clusters of various particle sizes and several nonstoichiometries are summarized in Table I. It is apparent that at a given nonstoichiometry the value of KQ increases with (23)Bonyai, L.;Koch, S. W. Phys. Reu. Left. 1986, 57, 2722. (24)Fu, 2.W.;Dow,J. D.Bull. Am. Phys. SOC.1986, 31, 557. (25)Brus, L. E.J . Chem. Phys. 1983, 79,5566;1984, 80,4403. (26)Brus, L. E.J . Phys. Chem. 1986, 90, 2055. (27) Brus, L. E. I € € € J . Quantum Electron. 1986, QE-22, 1909. (28)Henglein, A. J . Phys. Chem. 1982, 86, 2291. (29) Meissner, D.; Memming, R.; Shuben, L.; Yesodharan, S.;Gratzel, M. Ber. Bunsen-Ges. Phys. Chem. 1985, 85, 121.
't
I
-
IO'xCZnTPPI, M Figure 2. Semilogarithmic plots of (F'F/IFZnTPP)con vs [ZnTPP]of 2 X lo4 M CdS clusters in 2-propanol with variable excess of Cd2+:2 X lo4 M CdS,5 X M Cd2+:(0) Eg = 3.0 eV,,,,),X(, = 555-565 nm; (0) Eg = 3.05eV, (A,), = 550-560 nm; (A) Eg = 3.1 eV,,,,),X,( = 540-545 nm. (m) 2 X M CdS,4 X lo4 M Cd2+, Eg = 2.95 eV, (&,) 550-560 nm; (0)2 X lo4 M CdS,3 X lo4 M Cd2+, Eg = 2.90 eV;,,,)X,(, = 560-570 nm (A,, = 362 f 2 nm; A,, = 540 f 2 nm).
Eg, although the variation of Eg is rather limited. Extensive increase in Eg is accompanied by a reduction in the optical density of the clusters implying partial cluster dissolution, partial cluster precipitation, or both. In such cases the nominal concentration of CdS clusters may be less than 2 X lo4 M. In addition, at constant Eg values KQ decreases slightly with increasing nonstoichiometry (Table I). Excitation of mixtures of ZnTPP and CdS in deaerated and air-saturated solutions of 2-propanol at 546 nm, at which CdS does not absorb light, led to luminescence quenching of ZnTPP by CdS clusters whose Eg value was less than 3.0 eV. On the other hand, the luminescence of ZnTPP ('ZnTPP*(S,)), monitored at 603 nm (0-0 band), was not quenched by CdS clusters with Eg > 3.0 eV. The quenching of luminescence of 'ZnTPP*(S,) by CdS clusters obeyed both the Stern-Volmer and exponential laws fairly well. Typical plots are shown in Figure 3, leading to values of kQrO('ZnTPP*)defined by (pF/IFCdS)corr = 1 + kQrO('ZnTPP)*[CdS] (5) where so('ZnTPP*) is the fluorescence lifetime of 'ZnTPP* in the absence of CdS. Values of kQf ('ZnTPP*) were obtained from the slopes in Figure 3. Again, it should be emphasized that those are average values, Le., ZikQ,$(Ri). Values of kQT0('ZnTPP*) determined with different CdS cluster preparations are summarized in Table 11. Prolonged irradiation of deaerated CdS clusters and ZnTPP at either 365 or 546 nm caused no changes in either the absorption or the emission spectra of ZnTPP although recombination luminescence quenching took place. The same situation was ob-
Recombination Luminescence Quenching of CdS by ZnTPP
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2871
2.8-
4 h -
0.9 l.4--0
-
, I
(e)
A-A-
64
3'2 /
-
/-+----
(C)
served following light excitation of aerated mixtures of CdS clusters and ZnTPP at 546 nm ('ZnTPP* only). However, prolonged excitation of such mixtures at 365 nm led to very drastic changes in both the absorption and the emission spectra of ZnTPP (Figure 4). The photoreactions taking place could be completed in very short times under intense light excitation. Typical absorption spectra of aerated solutions of ZnTPP in the presence of CdS, irradiated by a tungsten lamp (entire spectrum), are shown in Figure 5. It is apparent that the characteristic features of the absorption spectrum of ZnTPP, including the very intense Soret band, were wiped out following 15-min irradiation. Because of such photoreactions, induced by CdS(e-/h+) in the presence of 02,recombination luminescence quenching experiments were carried out at very low light intensities and low [ZnTPP]. It should be pointed out that neither CdS nor ZnTPP was affected by light in the presence of O2 under similar conditions. The nature of photoproducts of ZnTPP formed in the presence of CdS and the mechanisms involved in such photoreactions will be presented in a separate report.
Discussion Although the origin of the recombination luminescence of CdS clusters is not completely unambiguous, it is linked to trapped electrons/holes and surface defects (traps). Photogenerated holes can migrate to the surface of CdS, forming S'-(s) traps. Such traps were shown to be about 1.5 eV above the valence band in solid n-CdS.'O Recombination of photogenerated electrons from the ~~
(30) Hodes, G.; Albu-Yaron, A. In Photoelectrochemistry and Elertrosynthesis of Semiconductor Materials; Ginley, D.S., et al., Eds.; The Electrochemical Society: Pennington, NJ, 1988; Proc. Vol. 88-14, p 298.
Figure 5. Absorption spectra of 2 X M CdS, 5 X lo4 M Cd2+,and M ZnTPP in 2-propanol: (A) unirradiated; (B) 5-min irra2.5 X diation by a tungsten lamp (air-saturated); (C) 15-min irradiation by a tungsten lamp (air-saturated); (D) 30-min irradiation: (E) 45-min irradiation.
conduction band or from electron traps with holes at S*-(s)traps competes against a rather efficient h+ transfer leading to a very weak emission, red-shifted considerably with respect to Eg. Such S'-fl traps are not expected to be dominant in CdS clusters with Cd -rich surfaces. A more likely surface defect in this case is an anion vacancy. Such a vacancy may not be the same as the V2+,defect formed in solid CdS under equilibrium conditions, but
2872 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 it may be a trap formed by several Cd2+ adatoms and surfaceadsorbed Cd2+ions due to the lack of additional S2 ions. Photogenerated electrons trapped in such vacancies giving rise to a defect resembling a V+, defect can recombine with free or trapped photogenerated holes in competition against electron transfer. The efficiency of the latter may be dependent upon the depth of such electron traps, leading to higher luminescence efficiencies in CdS clusters associated with large CdZCnonstoichiometries. It is of interest that the V+, vacancy is located about 0.7 eV below the conduction band of solid CdS3' Therefore, if electron traps in CdS clusters resemble V+, defects, one would expect a fairly strong recombination luminescence red-shifted by 0.7-1 .O eV with respect to Eg. This appears to be the case in most of the CdS clusters used in this study (Table I). Recombination luminescence quenching of CdS(e-/h+) clusters by ZnTPP is attributed to electron transfer from CdS(e-/h+) to the ground state of ZnTPP via the mechanism CdS
-+ -
CdS(e-/h+)
CdS(e-,,/h+,,) -!% CdS
+ hvRL
CdS(e-,,/h+,,) ZnTPP CdS(h+).ZnTPP'-; k,, tr The driving force for the electron-transfer process is AGOeI. defined by AGO,, 1r = e(Eo(CdS( h+)/CdS(e-/ h+)) EO(ZnTPP/ZnTPP'-)) - e2/tR (6) The redox potential of the conduction band of CdS clusters is more negative than that of bulk CdS by h2/8m*,eRZ,where R is the radius of the cluster. For CdS clusters with a diameter of 45 A eq 6 gives rise to AGO,, tr = 0 eV, whereas for CdS clusters with 2R < 45 A values of ACocltr< 0 are obtained, rendering electron-transfer thermodynamically allowed. It is anticipated that electron transfer from CdS clusters (2R > 45 A) to ZnTPP is rather inefficient. However, the value of Eo(CdS(h+)/CdS(e-/h+)) is based upon the corresponding value of Eo in solid CdS. The analogy between solid CdS and CdS clusters with a Cd2+-rich surface (Le., positively charged) may not be completely valid. In addition, adsorption of the electron acceptor (ZnTPP) on the surface of the cluster may render its conduction band more negative, thus making AGOeItr negative even at 2R > 45 A. This was observed with CdS electrodes on which several anions were adsorbed.jO Values of K, shown in Table I are in general agreement with this trend. The electron-transfer process described above appears to be accompanied by rapid back electron transfer in the absence of O2 (absence of photoproducts): CdS(h+).ZnTPP'-
back el
I1
CdS
+ ZnTPP
This process is very efficient thermodynamically: AGO,, tT = e(EO(ZnTPP/ZnTPP'-) - EO(CdS(h+)/CdS)) e2/tR = -3.1 eV for CdS clusters with a 45-Adiameter. These values are based on EO(ZnTPP/ZnTPP'-) = -1.07 V vs N H E (in several solvents)j2 and EO(CdS(h+)/CdS) = 1.90 V vs NHE (40-A diameter).2s Such highly efficient back electron transfer in the absence of O2implies that the electron acceptor is adsorbed on the surface of the cluster, making charge separation rather difficult. This assumption is supported by the shapes of the plots of PF/IFZnTPP vs [ZnTPP]. If the luminescence quantum yield of CdS clusters in the absence and presence of adsorbed ZnTPP is defined by +OF = kem/(kem+ knr) and $FZnTPP = k e m / ( k e m + keitr + knr) one can easily show that (4OF - +FZnTPP) /4OF = (PF - IFZ~~")/ P F = (k.1tr/(kel ti + kem + kn,r))B where B is the fraction of CdS clusters carying at least one ZnTPP (31) Vuyesteke, A. A,; Sihvonen, Y. T. Phys. Rev. 1959, 113, 40. (32) Felton, R. H.; Linschitz, H. J . Am. Chem. SOC.1966, 88, 113.
Chrysochoos
4
I
0.
..
I_.-
~
OLL
Y?: p ' ; ' : ~
0
2 ' 4
6
8
I0
I o-'x [ 2 nTPPI-,' M-'
0
IO
4
0
IO'x[ZnTPP],M
~
Figure 6. Quenching data plotted in accordance with eq 7. 2 X lo4 M CdS, 4 X M Cd*+,and ZnTPP in 2-propanol. A,, = 362 f 2 nm, A,, = 540 nm. Insert: data plotted according to eq 8.
molecule and 1 - B the fraction of free CdS clusters. Since B is defined via a Langmuir isotherm, at least at low [ZnTPP] B = K[ZnTPP] /( 1 K[ZnTPP])
+
where K = kads/kdcs,one obtains the following (PF - ZFZnTPP)/F'F=
I ~ t r /I( k e ~ tr + kern + knr)llK[ZnTPPl /(I + K[ZnTPPI)I
(7)
or PF/(PF - IFZnTPP)= (1 + (kem
+ k n r ) / h t r ) ( l + l/(K[ZnTPPI))
(8)
Some data are plotted according to eq 7 in Figure 6 and according to eq 8 in the insert of Figure 6. Values of K (i.e,, kads/kdcs) extracted from such plots are summarized in Table I. The values of K obtained in this way are high, implying k,, >> kd,. The desorption process must be characterized by a relatively high energy barrier. It is assumed that ZnTPP is adsorbed on CdS clusters via coordination of surface S2-ions to the central atom (Zn) of the metalloporphyrin. The intercepts of the straight lines shown in Figure 6 lead to appropriate values of kelvranging from 1.5(ke, + knr)to 2.5(ke, km). Therefore, at room temperature k,,,,ranges from 8 X lo7 to 1.3 X lo8 s-l for CdS clusters with k,, = 5 X lo7 s - ' . ~ diameters 20-30 A characterized by k,, The preceding approach is based on the assumption that one ZnTPP molecule adsorbed on a CdS cluster is sufficient to bring about luminescence quenching with an efficiency equal to kel@/(kd.+ k,, + k,). Excited CdS clusters in this study contain only one e-/h+ pair, and therefore additional ZnTPP molecules adsorbed on a single CdS cluster are unlikely to enhance the quenching efficiency. Therefore, one can consider recombination luminescence in terms of clusters free of ZnTPP and clusters carrying one or more ZnTPP molecules. In terms of Poisson statistics the fraction of CdS clusters carrying n ZnTPP molecules is given by
+
+
fn(r) = e-'r"/n!
where r = [ZnTPP]/[CdS clusters] = [ZnTPP]m/[CdS] with m describing the number of CdS units per cluster and [CdS] = 2 X M. In this casefo(r) represents the fraction of CdS clusters with no ZnTPP (4OF = k,,/k,, + knr)and 1 -fo(r) the fraction of clusters carrying n molecules of ZnTPP (n> 0, 4FznTpp = kem/(kem kelt,+ knr)). One can easily derive the following
+
d ~ ~ ~ =~(kern ~ +~ knr)/(kem / d ~ + Fb [keItr/(kern
t r
+ knr) +
+ keltr + knr)Vo(r) (9)
wherefo(r) = (exp(-[ZnTPP]/[CdS])Jm. Thus,fo(r) changes with both [ZnTPP] and m at constant [CdS]. Plots of 4FZnTPP/$'~ or ZFznTPp/PF vsfo(r) become linear at m = 100-150. Computer
J. Phys. Chem. 1992, 96, 2873-2879
fits were not attempted due to the limited number of experimental points. If anything, such plots suggest that an average CdS cluster in this study consists approximately of 100-150 CdS units or 200-300 atoms. Under these conditions one can, in principle, determine the average size of a cluster by determining the value of m at which experimental values of @‘m/40F match the values predicted via eq 9, provided that values of k,, ,,and k,, + k,, are available for the clusters under consideration and for the electron acceptor. Considering the lattice constant of zinc blende CdS and assuming that CdS clusters have a similar geometry, one can assign an approximate volume of 91.0 AS to a CdS unit viewed as a sphere. Therefore, clusters consisting of 100-150 CdS units are assigned diameters equal to 25-30 A. These values are smaller than expected. However, considering that 25-30 CdS units would be on the surface of such clusters, and taking into account the excess of Cd2+employed, the actual size of the clusters used may be larger. Furthermore, CdS clusters may be less compact than solid CdS, increasing the molecular volume of a CdS unit and therefore increasing the average cluster size. Light excitation of ZnTPP in the presence of CdS, under conditions a t which only ZnTPP absorbs light (A,, = 546 nm), is accompanied by fluorescence quenching dependent upon the size of the cluster: lZnTPP*dd, CdS ZnTPP+.CdS’-; k, The driving force of this reaction is given by AGO,, tr = dEO(ZnTPP+/’ZnTPP*) - EO(CdS/CdS*-)} - e Z / e R
+
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This value is approximately zero or slightly negative for CdS clusters with (2R L 60 A) and positive for CdS with smaller diameters, based upon the values E0(ZnTPP+/’ZnTPP*(SI)) ir -0.97 to -1.05 V vs N H E (several solvents)33and Eo(CdS/CdS‘) = -1.03 V vs N H E (for 2R = 60 A)25 Therefore, one would expect a quenching effect upon ‘ZnTPP* by CdS clusters with larger diameters (lower Eg values) and no effect by smaller CdS clusters. Results given in Table I1 are in general agreement with this pattern. In summary, electron transfer from CdS(e-/h+) to ZnTPP appears to gain importance with decreasing cluster size. However, the effect of adsorbed ZnTPP on redox potentials of both the CB and VB of the cluster is not clear at this time. At constant Eg values the electron-transfer process appears to decrease slightly with increasing excess of Cd2+, Le., increasing Cd2+ nonstoichiometry. This may be attributed to the reduction in the concentration of surface S” ions on which ZnTPP is assumed to bind. Recombination luminescence quenching of CdS(e-/h+) by ZnTPP is described fairly well by static interactions and Langmuir isotherms. Treatment of the quenching data by Poisson statistics indicates that an average CdS cluster in this work consists at least 100-150 CdS units.
Acknowledgment. Partial support for this project by The University of Toledo (URAFP) is gratefully acknowledged. (33) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 5 , Chapter 3.
Effect of Size Restriction on the Static and Dynamic Emission Behavior of Silver Bromide Katy P. JohanssonJgS Alfred P. Marchetti,t.s and George L. McLendon*vt.i University of Rochester, Department of Chemistry, Rochester, New York 14627; Eastman Kodak Company, Photoscience Research Diu., Rochester, New York 14650; and NSF Center For Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627 (Received: September 16, 1991; In Final Form: January 2, 1992)
The effect of size restriction on the photophysical properties of several different sizes of silver bromide crystallites, prepared either in AOT reverse micelle solutions or as gelatin-stabilized dispersions, is examined. The static and dynamic emission behavior is investigated. The principle effect of size restriction is found to be enhancement of the indirect exciton emission. The iodide bound exciton emission is also found to be vanishingly small or absent in the smallest of the size-restricted crystallites. The lifetime of the indirect exciton emission in the small crystallites is measured and is found to be shorter than that reported for bulk crystals. The temperature dependence of the enhanced indirect exciton is examined, and exciton binding energy is calculated.
Introduction The effect of restricted size on the photophysical properties of semiconductors has been under study for several years. For such “nanocrystalline” clusters, it has been noted that the optical properties of semiconductors are sensitive to the size of the crystallite, with both absorption and emission shifting to higher energies as the size of the semiconductor crystal is decreased.14 Most of these studies have been on direct bandgap semiconductors. The effect of size restriction on indirect gap semiconductors is interesting in that the effect may be 2-fold. First, the familiar result of a shift in the band edge with decreasing size could be expected. Second, the shape of the bands and consequently the selection rules governing transitions between the bands might also ‘University of Rochester, Department of Chemistry. Eastman Kodak Co. 1 University of Rochester, NSF Center For Photoinduced Charge Transfer.
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be affected. That is, in very small clusters, crystal momentum (k) is no longer an appropriate descriptor, and the associated selection rules break down. Thus, the band edge transition which is formally (momentum) forbidden for an indirect material can become “less forbidden” in a small cluster. This second effect could also occur in symmetry-forbidden direct gap materials. It is reasonable to expect that the nature of these band edge states might be affected as well. To expore these questions, spectroscopic studies of one particular indirect gap material, silver bromide, are presented here. Qualitative size quantization effects in the silver halides are in fact known. As early as 1970, Berry had noted anomalous (1) Brus, L. E. J . Chem. Phys. 1983, 79, 5566. (2) Henglein, A. Eer. Bunsen-Ges. Phys. Chem. 1982, 86, 301. (3) Kuczynksi, J.; Thomas, J. K. Chem. Phys. Lett. 1983, 79,445. (4) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R.J. J . Chem. Phys. 1987,
87. 7315.
0 1992 American Chemical Society
