J. Phys. Chem. 1987, 91, 2096-2099
2096
Transient Photobleaching of Small CdSe Colloids in Acetonitrile. Anodic Decomposition Nada M. Dimitrijevit* and Prashant V. Kamat Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: May 27, 1986; In Final Form: November 21, 1986)
Small CdSe colloids (particle diameter 30-50 A) that exhibit quantization effects are prepared in acetonitrile. They show intensive transient photobleachingin the 370-500-nm region and transient absorption around 300 and 500-600 nm immediately after bandgap excitation of the semiconductor. The nature and kinetic behavior of the transient photobleaching, which is attributed to the trapped charge carriers, are elucidated by investigating the influence of laser dose and various hole and electron scavengers.
Introduction Considerable attention has been drawn in recent years to design photoelectrochemical systems for the conversion of light energy.' Short bandgap semiconductors such as cadmium selenide ( E g= 1.7 eV)2should be good candidates for this purpose as they respond to the major part of the visible spectrum. However, this semiconductor is known to undergo anodic photocorrosion when used in a photoelectrochemical ~ y s t e m . ~Most of the studies of CdSe so far have focussed the aspects of redox reactions on the surface of single-crystal electrode^,^" thin films,' and large particles (particle radius 1000 A).* N o major effort has been made to study the photochemical behavior of small colloidal CdSe particles. As reported in a number of recent papers, semiconductor particles with small particle size (particle diameter < 50 A) exhibit quantization effects9 In addition to very large effects on optical properties, size quantization can enhance photoredox chemistry'&'* on the surface of these small semiconductor particles. However, a large increase in effective bandgap, i.e. several electronvolts, can also lead to inherent instability of the particles, since cathodic and anodic corrosion potentials have new positions against band edges. In order to investigate the photochemical properties and to probe the mechanism of anodic photocorrosion, we have carried out laser flash photolysis experiments of small CdSe colloidal particles ( D p < 50 A) in acetonitrile. The transparency of the colloidal suspension facilitates the direct detection and characterization of the transients generated upon laser pulse excitation. Optical properties of the trapped excess charge carriers in the CdSe particles and their role in the anodic photocorrosion process are described. Experimental Section Preparation of CdSe Colloids. Colloids of CdSe were prepared in acetonitrile by slowly injecting 250 ~ AofL H2Segas (Matheson Gas Products) into 50 mL of previously deaerated 2 X IO4 mol dm-3 Cd(C10J2 solution containing 0.04% Nafion (Aldrich) as stabilizer. The injection was carried out separately at two different temperatures, -35 and 0-4 OC. The average particle size was estimated by electron microscopy using a Hitachi H600 transmission electron microscope. Standard preparation techniques were applied to take electron micrographs of the colloidal particles. The average particle size of colloids prepared at -35 OC was -30 A, while colloids prepared at 0-4 OC were 50 %, in diameter. Colloidal suspensions were prepared just before they were studied. Unless otherwise specifically mentioned, all of the experiments were done with small CdSe particles (Dp 30 A). Apparatus. The laser flash photolysis experiments were carried out using 355-nm laser pulses (6-11s pulse width) generated from a Quanta-Ray Nd:YAG system. A typical experiment consisted of 10-20 replicate shots per sample and the average signal was processed with a LSI-11 microprocessor interfaced with a PDP11/ 5 5 computer. The data collection system is described elsewhere.13 The solutions were saturated with Ar. All experiments were done at room temperature.
-
*Author to whom correspondence should be addressed.
0022-3654/87/2091-2096$01.50/0
Actinometry was performed from the flash photolysis experiment with a 355-nm laser pulse as the excitation source and anthracene triplet as the reference,14 employing the same experimental conditions as in the case of the investigated samples. Absorption spectra were recorded with a Cary 219 spectrophotometer and the emission spectra were recorded with a SLM single-photon-counting fluorescence spectrometer at room temperature. Results and Discussion Optical Properties. The absorption and emission spectra of the CdSe colloids of different particle sizes are presented in Figure 1. The absorption spectra exhibit shoulders at 400 and 450 nm for CdSe particles with D, 30 and 50 A, respectively. With decreasing particle size, the absorption edge shifted to shorter wavelengths and indicated an increase in the bandgap energy due to size quantization effects. An increase in the bandgap of -0.7 eV was estimated in the present case for the CdSe particles in the range of 30-50-A particle diameter. A shift in the photoluminescence spectra to higher energies was also observed. Emission maxima at 580 and 605 nm (with excitation at 400 nm) were
-
(1) See for example: (a) Bulatov, A. V.; Khidekel, M. L. Izv. Akad. Nauk SSSR, Ser. Khim. 1976, 1902. (b) Nozik, A. J. Appl. Phys. Lett. 1977,30, 567. (c) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 2239. (d) Inone, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature (London) 1979,277, 637. (e) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (f') Gerischer, H. In Photovoltaic and Photoelectrochemical Solar Energy Conversion, Cardon, F., Gomes, W. P., Dekeyser, W., Ed.; Plenum: New York, 1981; p 199. (2) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J . Am. Chem. SOC.1977,99, 2839. (3) Cahen, D.; Hodes, G.; Manassen, L.; Tenne, R. In Photoeffects at Semiconductor Electrolyte Interfaces, Nozik, A. J., Ed.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 146, p 387. (4) Frese, K. W. J. Electrochem. SOC.1983, 130, 28. (5) Tenne, R.; Giriat, W. J. Electroanal. Chem. Interfacial Electrochem.
1985, 186, 127.
( 6 ) Tenne, R. Hodes, G. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 74. (7) Curran, J. C.; Philippe, R.; Stremsdoerfer, G. J. Elecfroanal. Chem. Interfacial Electrochem. 1985, 187, 121. (8) Darwent, J. R. J. Chem. SOC.,Faraday Trans. 1 1984,80, 183. (9) (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. (d) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 649. (e) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (f) Nozik. A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.: Micic, 0. I. J. Phys. Chem. 1985,89, 397. (g) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985, 82, 552. (h) Weller, H.; Fojtik, A.; Henglein, A. Chem. Phys. Lett. 1985, 117, 485. (i) Fojtik, A,; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 120, 552. 6 ) Koch, U.; Fojtik, A,; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122. 507. (k) Chestnoy, N.; Hull, R.; Brus, L. E. J. Chem. Phys. 1986, 85, 2237. (I) Fischer, Ch.-H.; Weller, H.; Fojtik, A,; Lume-Pereira, C.; Janata, E.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1986,90,46. (m) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986,125,299. (n) Sandroff, C. J.; Hwang; D. M.; Chung, W. M. Phys. Rev.B 1986, 33, 5953. (10) Rossetti, R.; Nakahara, S.;Brus, L. E. J. Chem. Phys. 1983,79, 1086. (1 1) Williams, F.; Nozik, A. J. Nature (London) 1984, 31 I , 21. (12) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1986, 90, 12. (13) Das, P. K.; Encianas, M. V.; Small, Jr., R. D.; Scaiano, J. C. J. Am. Chem. SOC.1979, 101, 6965. (14) Amand, B.; Bensasson, R. Chem. Phys. Lett. 1975, 34, 44.
0 1987 American Chemical Society
Photobleaching of Small CdSe Colloids
The Journal of Physical Chemistry, Vol. 91, No. 8, I987 2097
0.70 CdSe
'1
in acetonitrile
WAVELENGTH (nm) Figure 1. Absorption and emission spectra of CdSe colloids in acetonitrile. Average particle size: (a) -30 A, (b) 50 A. Dashed line: absorption spectrum of colloidal CdSe (Dp 30 A) after steady-state irradiation for 20 min in an argon atmosphere (cutoff filter 380 nm).
-
observed for the 30- and 50-8, particles, respectively. The quantum yield of emission for these colloids was -O.O2.I5 Large CdSe colloidal particles (0,> 200 8,)have shown a weak emission with a maximum at 680 nm. Upon steady-state irradiation of deaerated CdSe colloidal solution in acetonitrile with >380-nm light for 20 min, a change in the absorption of colloidal CdSe was observed (Figure 1). A decrease of the absorption indicated decomposition of CdSe due to photocorrosion. The behavior of transients which are responsible for such corrosion processes was investigated by using time-resolved laser flash photolysis. Transient Absorption Spectra. The transient absorption spectrum recorded immediately after the excitation (A,, = 355 nm) of the colloidal CdSe in the absence of any quenchers is shown in Figure 2a. An intense photobleaching was observed at wavelengths below 500 nm when these colloids were subjected to ultra bandgag excitation. (Care was taken to exclude any interference from CdSe emission or scattered light.) Photobleaching of ground-state absorption upon flash excitation has also been observed for CdS colloids in aqueous16 and acetonitrile" solutions and in Nafion film.I8 Albery et a1.16 noted that such a transient bleaching was long-lived (milliseconds) and attributed it to electrons trapped in the intraband states, which exist on the surface of the particles. The CdS in Nafion film exhibited short-lived photobleaching (C1 F S ) a t 77 K'* and the recovery was attributed to the direct recombination of conduction and valence band carriers. In the present experiments transient bleaching recovery exhibited mixed kinetics consisting of at least two components. At low excitation doses, recovery of bleaching was almost completed in the submicrosecond time regime, and this recovery can be attributed to the recombination of trapped charges. At higher doses a second, long-lived intermediate appeared (Figure 2b) which revealed anodic decomposition of the semiconductor particles. The position of the bleaching maximum due to the second intermediate matched the position of the shoulder observed in the absorption spectrum of CdSe colloids. For particles with D, 30 8,the bleaching maximum was shifted to shorter wavelengths,
-
(15) DimitrijeviE, N. M. J. Chem. SOC.,Faraday Trans. 1 , in press. (16) Albery, W. J.; Brown, G . T.; Darwent, T. R.; Saievar-Iranizad, E. J. Chem. SOC.,Faraday Trans. 1 1985, 81, 1999. (17) Kamat, P. V.;DimitrijeviE, N. M.; Fessenden, R. W. J . Phys. Chem. 1987, 91, 396. (18) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J . Phys. Chem. 1984, 88, 980.
300
I
I
I
400
500
600
I
WAVELENGTH (nm)
Figure 2. Transient absorption spectra observed (a) immediately and (b) 25 p s after laser excitation of 2 X lo4 mol dm-) CdSe colloids in acetonitrile. (1, D, 30 A; 2, D, = 50 A).
-
confirming the quantization effect observed in the absorption and emission spectra. Transient absorption at longer wavelengths (500-600 nm) corresponded to Se'- which, as a radical, absorbs at 450 nm in aqueous s01ution.l~ Since Sea- species in the present experiments existed as a part of CdSe particle, its optical properties are influenced by the surface environment. The absorption spectrum of Sea- is characterized by the transfer of electrons into the unoccupied electronic levels of CdSe particle. Hence the absorption maximum of adsorbed Se'- is expected to be shifted to longer wavelengths compared to Sea- ions in solution. With decreasing particle size, the absorption maximum of Se'- shifts to shorter wavelengths as the unoccupied electronic levels of CdSe colloid shift to higher energies. Henglein and co-workersZ0observed similar spectra in reaction of OH radicals with CdS particles, as well as for ZnS and Cd,Pz. Chemical decomposition of colloidal particles observed during pulse radiolysis experiments was initiated by trapping of positive charge on the surface of particles.*O The absorption maximum near 310 nm (Figure 2a) could correspond to trapped electrons and the formation of Cd+ sites. As characterized in radiation chemical study,I5 small CdSe colloidal particles undergo cathodic corrosion. The formation of cadmium metal in such a process was initiated by localization of excess electrons on cadmium ions and the subsequent disproportionation of Cd+ to yield CdO. Strong absorption of the monitoring light by CdSe colloids at wavelengths < 350 nm limited the kinetic study of the transients absorbing in the UV region. Effect of Excitation Intensity on the Transient Photobleaching. The absolute value of the transient absorption at 400 nm measured immediately after pulse excitation showed a square root dependence on the dose. This square root dependence on the excitation intensity indicated that the transient photobleaching was due to the trapped charge carriers and did not reflect the direct decay of free charge carriers. The concentration of any transient observed immediately after laser pulse excitation depends upon the steady-state concentration of photogenerated carriers during the pulse. When the free charge carrier recombination occurs during the laser pulse, the steady-state concentration is expected to follow a square root dependence on the light intensity. The magnitude of the second intermediate (represented as the bleaching at 440 nm observed 25 hs after the pulse) was found (19) Schoeneshoefer, M.; Karrman, W.; Henglein, A. Int. J . Radiat. Phys. Chem. 1969, I , 407. (20) Baral, S.; Fojtik, A,; Weller, H.; Henglein, A. J . Am. Chem. SOC. 1986. 108, 375.
DimitrijeviE and Kamat
2098 The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
It
‘
CIY “ .
.
1
1
I
AA
0.0001 0
lb’
- 0002
- 0 001
I
hA
1i
7 . I t’ ’
0.0001 0 -0.OoolO
I
0.5-
I .o
5.0
x IO2’ charge / cm3
Figure 3. Amplitude of the photobleaching at 440 nm recorded 25 MS after the laser pulse as a function of produced charge density. Insert: Profile of transient photobleaching recorded at different excitation doses: (a) 6.5 X 1Odeinsteins; (b) 1.7 X IO-’einsteins; (c) 2.34 X 10-5einsteins.
to vary linearly with the laser dose (Figure 3). The dose of excitation in Figure 3 is presented as the number of charges produced per cm3 of the colloidal semiconductor. The number of charges produced in these experiments were in the range of 1 X 102*-7X loz1per cm3, with the approximation that one quantum produced one electron-hole pair. At lower doses (less than loz1charge/cm3), the absorption due to second intermediate was very small and its measurement was limited by the large noise/signal ratio. Identity of the Transients Responsible for Photobleaching. Bandgap excitation of CdSe leads to charge separation followed by recombination and trapping of charge carriers in the bulk and at the surface of the semiconductor:
(1)
The formation of the transient during the laser pulse, as monitored with the depletion of ground-state absorption of CdSe, was attributed to these trapped charge carriers. The charge trapping process may be considered as a process of localization of charge at the vacancy sites and/or other trapping centers in the bulk and/or surface of the semiconductor colloids. Long-range trapping energies from deep traps to the lowest delocalized state within the bandgap have been proposed for cadmium chalcogenides.21v22 Because the conduction and valence band of the CdSe colloidal particles (Dp< 50 A) are quantized into discrete states,” these particles exhibit hybrid molecular/solid-state character of their electronic proper tie^.^^ The “depletion” of CdSe could arise from either chemical changes produced by localization of the charge or changes in the electronic configuration in analogy with excited molecules. Other possibilities were also considered before assigning the transient photobleaching to the trapped charge carriers. The recombination of free charge carriers is a fast process” and occurs (21) Chestnoy, N.; Harris, T. D.; Hull, R.;Brus, L. E. J . Phys. Chem. 1986, 90, 3393. (22) Ramsden, J. J.; Grltzel, M. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 919. (23) Brus, L. J . Phys. Chem. 1986, 90, 2555.
A
6
I k
I
600 WAVELENGTH (nm) Figure 4. Transient absorption spectra recorded 25 ps after the laser pulse excitation of Ar-saturated acetonitrile solution containing (a) 2 X lo4 mol dm-’ CdSe with 1%H20;(b) 2 X lo4 mol dm-) CdSe, 2 X lod mol dm-’ Se2-with 1% H 2 0 ;(c) 2 X lod mol dm-’ CdSe, 0.1 mol dm-’ triethanolamine.
400
500
within the pulse duration ( 6 ns). Hence its contribution to the photobleaching recovery, if any, was negligible. Exciton formation in small semiconductors could also alter the absorption characteristics of the semicondu~tor.~ But recent workz5on small CdSe and CdS particles has shown that the exciton lifetimes are in the range of picoseconds and decrease rapidly as the particle diameter decreases and approaches the excitonic diameter. The trapped charge carriers could either recombine or participate in the corrosion processes. Corrosion processes are expected to be more efficient for a quantizied particle than for a large particle, since cathodic and anodic corrosion potential have new positions against band edges. As the particle size decreases, the anodic corrosion potential becomes more negative and cathodic corrosion potential becomes more positive with respect to the energy levels of valence and conduction bands, respectively. The anodic corrosion is caused by localization of holes on selenium ions while the cathodic corrosion is caused by the localization of electrons on cadmium ions of CdSe semiconductor (eq 2 and 3). anodic corrosion: (ht+)CdSe
-+
(Cdz+Se-)m(CdSe)n-m
+
S&/2(CdSe)wm/2
(2)
cathodic corrosion: As described in the previous section, the intermediates of such corrosion processes (Se-, Cd’) could be characterized by their optical absorption. The identity of the second long-lived intermediate was further assessed by the addition of various scavengers. When an excess amount of Sez-was present during the preparation of the colloidal CdSe, a decrease in the photocorrosion, as monitored by the photobleaching, and an increase in the absorption at 580 nm were observed (Figure 4b). This is attributed to the efficient hole scavenging action of the Se2-.26 Other hole scavengers such as sodium citrate or triethanolamine (TEA) exhibited a similar behavior. For example, no photobleaching could be seen when 0.1 mol dm-3 of TEA was added to the colloidal CdSe suspension. (24) Serpone, N.; Sharma, D. K.; Jameison, M. A.; Gratzel, M.; Ramsden,
J. J. Chem. Phys. Lett. 1985, 115, 413.
(25) Warnock, J.; Awschalom, D. D. Appl. Phys. Lett. 1986, 48, 425. (26) Heller, A,; Schwartz, G. P.; Vadimsky, R. G.; Menezes, S.; Miller, B. J . Electrochem. SOC.1978, 125, 1156.
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2099
Photobleaching of Small CdSe Colloids
the dependence of the observed quantum yield of ZV'- production on the concentration of ZV.28 The value of K, determined from vs. 1/[ZV] was 4.3 X lo4 mol-' dm3. the linear plot of l/40bsd The quantum yield of ZV" radical anion at different ZV concentrations was determined by monitoring its absorption at both 400 and 600 nm and taking the values of e = 4.18 X lo4 and 1.39 X lo4 mol-' dm3 cm-' ,29 respectively. Addition of a hole scavenger such as Se2- was found to enhance the efficiency of the reduction process at the colloidal surface. For example, the addition of 2 X lo4 mol dm-3 of Se2-led to an increase in the quantum yield from 0.1 to 0.4 for the reduction of viologen. Viologen radical anion exhibited a relatively slow decay. The initial decay of ZV'-, which can be seen in several microseconds, can be attributed to its reaction with the trapped holes at the surface:
I
-omst
OoooO
0.005
OW25
ZSOnc
I
h,'
I
WAVELENGTH ( n m ) Figure 5. Transient absorption spectrum recorded 250 ns after the laser pulse in acetonitrile solution containing 2 X lo4 mol dm-' CdSe, 1 X 10-4 mol dm-' ZV,and 1% H20.Inserts are the time profiles of transient absorption recorded at 400 and 440 nm.
However, an increase of transient absorption due to cathodic corrosion was observed (Figure 4c). The absorption seen below 450 nm was similar to the absorption of Cdo detected in radiation chemical studies upon injection of electrons into aqueous CdSe colloids. Electron scavengers such as zwitterionic viologen did not affect the photobleaching of CdSe. This further supports our assignment of photobleaching in CdSe colloids as being due to trapped holes. Reaction with Zwitterionic Viologen. In order to examine the charge-transfer processes on the surface of the semiconductor colloids and the nature of the first component of photobleaching, the flash photolysis experiments in the presence of 1,l'-disulfopropyl-4,4'-bipyridine (zwitterionic viologen, ZV)27 were conducted. The transient absorption spectrum recorded immediately after the laser flash excitation showed prompt formation of viologen radical, ZV'-, as characterized by its absorption peaks at 400 and 600 nm (Figure 5):
+ ZV
eCB-
-
ZV'-
(4)
The photobleaching of CdSe colloids, monitored at wavelengths where the absorption of ZV'- was negligible, was unaffected in the presence of ZV. The formation of ZV- was completed within the pulse duration and its formation did not correspond to the recovery of photobleaching. The apparent association constant, K,,for the association between CdSe colloid and zwitterionic viologen was determined from (27) Ford, W. E. Ph.D. Thesis, University of California, Berkeley, 1982.
+ ZV'-
-
ZV
(5) Nearly 50% of the ZV' radical remained unchanged in solution at the end of the process (after -200 K S ) , indicating that this fraction of ZV'- diffused away from the CdSe particle and was not accessible for the hole scavenging process. As a negatively charged radical ion, ZV'- had less ability for adsorption and was repelled by the negatively charged CdSe particles, which were stabilized by N a f i ~ n .The ~ ~ remaining unreacted holes could then participate in the anodic corrosion process (eq 2). Changing the laser intensity or the ZV concentration (6 X 10-5-10-3 mol dm-7 did not affect the fraction of unreacted ZV'-. This in turn indicated that the fraction of the holes which induced anodic corrosion was independent of the incident light intensity. This observation is in support of the previously reported results of the corrosion of some other selenide electrodes, which exhibit photocorrosion independent of incident light intensity at constant bias potentials, suggesting thereby that the corrosion of these materials occurs preferentially at the selected surface sites.31 Further evaluation of the role of the trapped charge carriers in the photocorrosion processes is in progress. Conclusion Transient photobleaching observed in the 370-500-nm region for CdSe colloidal particles (Dp 30 A) was attributed to trapped holes because hole scavengers reduced the amount of photobleaching, while electron scavengers had no effect. The recovery of the bleaching showed mixed kinetics in the absence of hole scavengers, and a second long-lived intermediate appeared which corresponded to the anodic decomposition of CdSe. The observed maximum of bleaching in the transient absorption spectrum showed a dependence on the particle size which was in good agreement with the trend observed in the absorption and emission spectra.
-
Acknowledgment. We thank Dr. William E. Ford for providing us with the sample of viologen compound. N.M.D. thanks the Faculty of Science, Belgrade University, for a leave of absence. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2857 from the Notre Dame Radiation Laboratory. (28) Kamat, P. Langmuir 1985, 1 , 608. (29) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617. (30) MiEiE, 0.;Meisel, D. In Homogeneous and Heterogeneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel: Dordrecht, 1986; NATO AS1 Ser., Ser C 174, p 227. (31) Gerischer, H.; Liibke, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 123.
