396
J. Phys. Chem. 1987, 91, 396-401
(ii) The differences in surface chemistry of these two congeners, Pt and Pd, have been attributed to the greater effective bonding of the d electrons in platinum relative to palladium. The interaction of benzene, toluene, and pyridine is clearly stronger with the platinum surface. The driving force in the surface chemistry of these two aromatics on the platinum surface is the formation of covalent metal-carbon bonds, whereas this is not the case for palladium. The surface chemistry of hydrocarbons adsorbed on these two catalytically important metals, Pt and Pd, is demonstrably different. This study has focused on the clean metal surfaces. A comparison of the effects of adatoms on these two metals will be
an important step in our understanding of industrial catalyst.
Acknowledgment. This work was supported by the National Science Foundation Grant CHE-83-07159 and by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U S . Department of Energy under Contract DE-AC0376SF00098. We also thank G. C. Pimentel for many insightful discussions during the course of preparing this manuscript and L. Brewer for very helpful discussions on bonding in transition metals. Registry No. Pd, 7440-05-3; benzene, 71-43-2; toluene, 108-88-3; pyridine, 1 10-86-1.
Photoelectrochemistry in Particulate Systems. 6. Electron-Transfer Reactions of Small CdS Colloids In Acetonltrlle Prashant V. Kamat,* Nada M. DimitrijeviC, and Richard W. Fessenden Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 3, 1986; In Final Form: September 2, 1986)
Small colloidal CdS particles (particle diameter C 25-42 A) have been prepared in acetonitrile and their absorption and emission properties characterized. A transient bleaching was observed when CdS colloids were subjected to bandgap excitation with a 355-nm laser pulse. The recovery of the bleaching which consisted of at least two components has been attributed to the recombination of trapped charge carriers and to the process of anodic corrosion. The red emission of CdS colloid can be quenched with an electron scavenger such as methylene blue. The quantum yield for the reduction of oxazine and thiazine dyes varied from 0.04 to 0.08 and was dependent on the reduction potential of the dye. The electron-transfer reactions of CdS colloids have been investigated by using nanosecond laser flash photolysis and microwave absorption techniques.
Colloidal semiconductors are used as photocatalysts to carry out phototransformations of various organic and inorganic substrates. Recent studies have focused on the elucidation of the interfacial charge-transfer processes by using laser flash photolysis,'-2Raman spe~troscopy,~ and microwave absorption4,' techniques. Nanosecond and picosecond laser flash photolysis has been found useful in the investigation of the charge trapping and recombination processes in TiOz semiconductor colloids.6 In our earlier work,2 we investigated the dynamics and mechanism of electron-transfer reactions in the colloidal TiOz system and demonstrated that upon simultaneous capture of holes and electrons, one can enhance the yield of photoelectrochemical oxidation and reduction processes. Recently CdS colloids and dispersions have reeeived considerable attention because of their photocatalytic activity under visible light irradiation. A notable development in this area was to prepare extremely small colloidal semiconductor particles and to observe optical effects due to size q u a n t i ~ a t i o n . ~It~has ~ been ( 1 ) See for example: (a) Bahnemann, D.; Henglein, A.; Spankel, L. Faraday Discuss. Chem. SOC.1984, 78, 151. (b) Duonhong, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. SOC.1982, 104, 2977. (c) Brown, G. T.; Darwent, J. R.; Fletcher, P. D. I. J. Am. Chem. SOC.1985, 107, 6446. (2) (a) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986,90, 1389. (b) Kamat, P.V. Langmuir 1985, 1, 608. (c) Kamat, P. V. J. Chem. SOC., Faraday Trans. I 1985, 81, 509. (d) Kamat, P. V. J. Photochem. 1985, 28, 513. (3) Rossetti, R.; Brus, L. J. Phys. Chem. 1986, 90,558. (4) Fessenden, R. W.; Kamat, P.V. Chem. Phys. Lett. 1986, 123, 233. ( 5 ) (a) Warman, J. M.; De Hass, M. P.; Gratzel, M.; Infelta, P.P.Nature (London) 1984, 310, 306. (b) Kunst, M.; Beck, G.; Tributsch, H. J. Electrochem. SOC.1984, 131, 954. (6) (a) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. SOC.1985,107,8054. (b) Bahneman, D.; Henglein, A,; Lillie, J.; Spankel, L. J. Phys. Chem. 1984, 88, 709.
0022-3654/87/2091-0396$01.50/0
demonstratedg that changes in the medium or methods of preparation have marked effect on the particle size and their absorption and luminescence properties. The emission from CdS colloids serves as an excellent probe of the h+ and e- surface chemistry and physics.'O Albery et al." have reported a transient bleaching in colloidal CdS suspension under bandgap excitation, and they have attributed this behavior to trapped electrons. However, a recent pulse radiolysis study by Henglein and co-workersI2 suggests such a transient bleaching could also arise from trapped hole carriers. The study of trapped charge carriers is important as they can influence the photocatalytic activity and surface corrosion of the semiconductor. In view of this fact, we have undertaken a detailed investigation of the electron-transfer reactions of colloidal CdS in acetonitrile by using time-resolved microwave absorption and laser flash photolysis techniques, and the results are described here.
Experimental Section Materiuls. Methyiene blue chloride (Fluka puriss) was purified over a chromatography column of neutral a 1 ~ m i n a . l ~Oxazine (7) (a) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (b) Brus, L. E. J. Chem. Phys. 1984,80, 4403. ( c ) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984, 80, 4464.' (8) (a) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunrenges Phys. Chem. 1984,88, 969. (b) Weller, H.; Fojtik, A,; Henglein, A. Chem. Phys. Lett. 1985, 117, 485. (c) Nozik, A. J.; Williams, F.; NenadoviE, M. T.; Rajh, T.; MiEiE, 0. I. J . Phys. Chem. 1985, 89, 397. (9) Ramsden, J. J.; Webber, S. E.; Gratzel, M. J. Phys. Chem. 1985,89,
2740. (10) Rossetti, R.; Brus, L. J. Phys. Chem. 1982, 86, 4470. (11) Albery, W. J.; Brown, G. T.; Darwent, J. R.; Saievar-Iranizad,E. J Chem. SOC.,Faraday Trans. I 1985, 81, 1999. (12) Baral, S.; Fojtik, A.; Weller, H.; Henglein, A. J . Am. Chem. SOC. 1986, 108, 375.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 397
Spectra of CdS Colloids TABLE I: Characteristics of CIS Colloids in Acetonitrile-Water Mixtures
sample
counterion
% HzO
A B
clodc10,-5
C D E F
III- * I-
0.25 1.o 0.5 2.0 2.0 10.0
a
av particle diameter, 8,
aggregation no., no. of CdS molecules/particle
37 42 0
existence of a sulfur vacancy has been e ~ p l a i n e d ~ on J ~ the 9 ~ basis ~ of the scheme
t v) w
+
(1)
hd
(2)
Vs+ + hd’
(3)
CdS -!% CdS (hVB+ eCB-)
I z 0
hvB+
+ eCB-
hVB+ + Vs
E
z
+
5 WAVELENGTH
(nm)
Figure 3. Emission spectra of colloidal CdS suspension in acetonitrile (b) 2 mM Cd12, 2.0% H20; prepared from (a) 2 mM CdI,, 0.5% H20; (c) 0.25 mM Cd(ClO,),, 0.02% nafion, 0.25% H,O; and (d) 0.25 mM Cd(C10&, 0.02% nafion, 1% H 2 0 (excitation wavelength 410 nm; spectra have not been normalized to the scale).
-
2). Colloidal CdS prepared from Cd(C10,)2 which is larger in size (D, 40 A) exhibited considerably red-shifted absorption with a shoulder around 450 nm. The shift in the apparent absorption edge indicated an increase in the bandgap energy for decreasing particle diameter due to size quantization effects, which is in agreement with the concepts proposed by Brus’ and Henglein.’* An increase in the bandgap of -0.5 eV was estimated in the present case for the CdS particles in the range 25 A. Changes in the medium altered the emission properties considerably (Figure 3 and Table I). The emission spectrum was considerably blue shifted for small particle size CdS colloids. Colloidal suspensions prepared with 2 4 % (v/v) water gave a maximum emission yield of 1-2%. The emission of CdS colloids at 77 K exhibited a complex decay. The slower component of the emission at 650 nm had a lifetime of -750 ns at 77 K. This showed that changing the counterions or the mixed solvent system during the preparation of CdS colloid can influence its particle size and photophysical properties. Quenching of CdS Emission with an Electron Acceptor. The red emission of CdS colloids in acetonitrile which arises from the
+
+
-
Vs+ ecB- V, (4) where Vs is a sulfur vacancy which is frozen in surface kinetic processes and is predominant in mixed solvents to act as a luminescent center. (Sulfur is more soluble in organic solvents than in aqueous medium. When the CdS colloids are prepared in a medium such as acetonitrile, the equilibrium, CdS G CdS(Vs) + S,, shifts to the right, and this results in the creation of sulfur vacancy.) Other vacancies created by interstitial sulfur or Cd vacancies which yield green fluorescence (emission maximum 522 and 530 nm, r e s p e c t i ~ e l y were ’ ~ ~ ~not ~ predominant in the colloidal CdS prepared in the mixed solvent system. In a recent paper, Chestnoy et aL2’ have argued that photogenerated eCBwhich get trapped by Vs+ could also be responsible for the luminescence in small CdS clusters. The red emission of CdS can be quenched easily with electron acceptors such as methylene blue, MB (Figure 4). Reaction of methylene blue with the conduction band electrons is expected to affect the regeneration of the sulfur vacancy (reaction 4) which in turn would decrease the emission yield. The quenching data can be treated with the model based on Poisson statistic^'^,^^ which is described by the expression 4/40 = ex~(-Q) (5) or In ($Jo/$J) = Q (6) where and 4 are the emission yields of CdS colloid in the absence and presence of the quencher and Q is the mean concentration of the quencher (MB). If q is the number of CdS
-
-
(19) Ramsden, J. J.; Gratzel, M. J . Chem. Soc., Faraday Trans. I 1984, 80, 919.
(20) Lambe, J. J.; Klick, C. C. Phys. Rev. 1956, 103, 1715. (21) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393. ( 2 2 ) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978,100, 5951.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 399
Spectra of CdS Colloids u-r
I
I
Figure 5. Transient absorption spectra of 0.25 mM colloidal CdS (prepared from Cd(C104)2in acetonitrile (containing 1% H20)) recorded immediately after the flash ( 0 )and 3.5 ja after the flash (X).The upper insert is an absorption profile of the transient at 440 nm and the lower insert is the dependence of maximum bleaching vs. (dose)'I2.
molecules per colloidal particle, then the mean concentration of the quencher is given by the expression
Q = v[MBI/
[CdSI
(7)
It has been shown earlierI9 that one molecule of quencher can quench the emission of one colloidal particle. Under these circumstances, eq 6 can be written as In (40/4) = v[MBl/[CdSl
(8)
The straight line plot of In ($o/$) vs. [MB] (insert in Figure 2) confirmed this to be true. The value of 11 determined from this plot was 416 which is in close agreement with the aggregation number of 378 determined from the particle size measurements (Table I). Transient Bleaching of CdS Colloid. The transient absorption spectrum obtained upon excitation of a colloidal CdS suspension in the absence of any quenchers is shown in Figure 5 . Immediately after the flash, a transient bleaching was observed at wavelengths below 550 nm with maxima around 450 and 340 nm. The position of these bleaching maxima matched well with the shoulders observed in the absorption spectrum. This indicated that the transient bleaching originated from the depletion of CdS. The recovery of this bleaching was rather complex, exhibiting at least two components, and was similar to the one observed in microwave absorption experiments. The fast component had a lifetime of 10 ns, and the slow component had a lifetime of -2 p . As observed in an earlier study," the bleaching exhibited a square root dependence on the dose of excitation, suggesting this behavior as due to the trapped charge carriers. Bandgap excitation of CdS leads to charge separation followed by recombination (reaction 2) and trapping of charge carriers (reactions 9 and 10) at the semiconductor surface.
-
+ e(CdS), + h+ (CdS),
-
-
e;
(9)
h;
(10)
The possibility of the existence of a range of trap energies from deep traps to the lowest delocalized state in small CdS colloids has been proposed r e ~ e n t l y . l ~In+ the ~ ~ foregoing discussion we refer to the generalized term of trapped holes (h): or electrons (e;). The transient photobleaching observed in CdS colloids is attributed to trapped holes for the following reasons. (1) Hole scavengers such as triethanolamine reduced the amount of bleaching and enhanced the rate of .bleaching recovery. (2) Enhanced bleaching was observed when an electron scavenger such as zwitterionic viologen, ZV (N,N-dipropyl-4,4'-bipyridinium disulfonate), was present. (3) The formation of the ZV'- radical anion as monitored by its absorption at 600 nm was prompt, and no further growth in the absorption of ZV'- could be detected during the bleaching recovery.
Figure 6. Transient absorption spectra of 2 mM colloidal CdS (prepared from Cd12) recorded at (a) 0, (b) 0.2, (c) 0.9, and (d) 20 ps after the 355-nm laser pulse excitation. Inserts are the absorption profiles recorded at 420 nm at different time domains.
(4) The behavior of the transient bleaching observed in flash photolysis experiments was parallel to the recent findings of pulse radiolysis experiments in which trapped holes were generated by OH' attack on CdS colloids.12 As observed in the case of colloidal CdSe,23the fast recovery of photobleaching can be attributed to the recombination of trapped charge carriers (reaction 11). This process which is a e;
+ h,+
-
2(CdS),
(1 1) major component of the bleaching recovery (>80%) regenerated the semiconductor. The slower component (
