J. Phys. Chem. 1982, 86, 4470-4472
4470
Electron-Hole Recombination Emission as a Probe of Surface Chemistry in Aqueous CdS Colloids R. Rossetti and L. Brus+ Bell Laboratorles. Murray Hi//, New Jersey 07974 (Received Ju/y 8, 1982; I n Final Form: Sepfember 14, 1982)
Luminescence has been observed from dilute aqueous CdS colloidal solutions following irradiation above the semiconductor band gap. The emission is attributed to radiative recombination of short-lived (7 < 5 ns) h+ with e- in small particles of hydrodynamic radius -200 A. The luminescence quantum yield is sensitive to surface-adsorbedspecies that are able to undergo reduction. PbS and p-benzoquinone cause 50% luminescence M concentration, correspondingto far less than a monolayer surface coverage. These results quenching at N demonstrate that recombination emission in small CdS particles can be used as a probe of the h+ and e- surface chemistry and physics.
Introduction Aqueous semiconductor colloids and suspensions show interesting photochemical and catalytic pr~perties.l-~ Absorption of light by a small particle creates mobile electrons e- and holes h+ which may migrate to the surface and undergo redox processes with adsorbed chemical species. Such surface reactions have been characterized by analysis of the final chemical products, and by transient electronic absorption studies of radicals in solution after desorption from the surface. In some cases catalysts for specific reactions have been loaded onto the particle surface. It is clearly useful to have a more direct spectroscopic probe of these surface processes. The quantum yields for reaction are typically low, and the large fraction of carriers decay by recombination. We now report that, in the case of CdS colloids, the radiative recombination luminescence is detectable. We find that this luminescence is controlled by surface processes, and thus, can be used to monitor surface kinetics, in much the same way that molecular luminescence is used to follow excited-state molecular reactions. After completion of this work, a paper by Henglein was published reporting colloidal CdS emission and quenching.'O Experimental Section We prepared transparent yellow-green aqueous CdS colloids (1.5 X M in CdS) via controlled reaction of (1)For recent reviews of photoelectrochemistry see (a) A. Heller, Acc. Chem.Res., 14,154(1981);(b) A. J. Bard, J. Phys. Chem., 86,172(1982). (2)T.Inone, A. Fujishima, S. Konishi, and K. Honda, Nature (London), 277, 637 (1979). (3)B. Kraentler and A. J. Bard, J. Am. Chem. Soc., 100,5985(1978). (4)E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, and M. Gratzel, Nature (London),289,159 (1981). (5)K. Kalyanasundaram, E. Borgarello, D. Duonghong, and M. Gratzel, Angew. Chem., Znt. Ed. Engl., 20,987(1981). (6)M. Fujihira, Y. Satoh, and T. Osa, Nature (London), 293, 206 (1981). (7)R.Humphry-Baker, J. Lilie, and M. Gratzel, J. Am. Chem. Soc., 104,422 (1982). (8)A. Henglein, Ber. Bunsenges, Phys. Chem., 86, 241 (1982). (9)E. Borearello. K. Kalvanasundaram. M. Gritzel. and E. Pelizzetti. H e l a Chim. h a , 65,243 (1982). (10)A. Henglein, Ber. Bunsenges. Phys. Chem., 86,301 (1982). We have also received a preprint from Pressor M. Grltzel ["Dynamic of Interfacial Electron-Transfer Processes in Colloidal Semiconductor Systems", J . Am. Chem. Soc., to be published] reporting observation of CdS colloidal emission. We thank Professor Griitzel for this preprint. Note also that use of luminescence as a probe of the CdS surface on macroscopic electrodes has been reported by A. B. Ellis and B. R. Karas, J . Am. Chem. Soc., 101,236 (1979).
Cd2+ (99.999% pure on a metals basis) with S2- in copolymer stabilized solution, as described by Kalyanasundaram et al.5 The pH was adjusted to 3 with HC1. Bulk CdS has a band gap of E, N 2.4 eV, corresponding to an optical absorption edge near 5150 A. Figure 1 shows the collodial solution absorption origin is also in this region. The collodial particles are close to monodisperse and have a hydrodynamic radius near 200 A, as calculated by standard procedures from the photon correlation of scattered Rayleigh light.'?" The observed solution color reflects above band gap absorption in these small particles. Emission spectra were obtained with 10-Hz pulsed dye laser excitation at 365 nm and monitored with a GaAs photomultiplier at 1-nm resolution. The pulse widths are 5 ns (fwhm), and the peak pulse power is 3 X lo4 W/cm2. In this power range the luminescence intensity, emission spectra, and the percentage quenching with added benzoquinone in solution are linear with laser power. The sample was stirred, exchanging the solution between laser pulses. Luminescence quantum yields CP were obtained by comparison of emission yield with a fluorinated coumarin laser dye at a concentration which matched the optical densities at 365 nm. We assume the laser dye has a fluorescence quantum yield of N 1.
-
Results and Discussion Figure 1 shows that the visible emission from a freshly prepared equimolar (Cd2+and S2-) colloid is relatively weak (CP 2X and consists of a peak near 470 nm on top of a broad emission extending across the visible. In some preparations the relative strength of the broad emission was stronger than appears in Figure 1. Macroscopic CdS electrodes undergo a corrosion process (lattice dissolution) under illumination:
-
2h+ + CdS
-
Cd2++ S
(1)
This process can be overcome by preferential h+ oxidation of adsorbed S2- ions (SH- at acid pH) in a S2-rich solution.12 We observe that SH-rich CdS colloids undergo a type of photochemical aging process if left overnight in room light. The luminescence quantum yield increases by Figure 1shows that a factor of about ten to CP N 3 X the emission peak shifts to -505 nm; the absorption spectrum shows only a very minor increase on the red edge. (11)(a) G. D. Patterson, J. P. Jarry, and C. P. Lindsey, Macromolecules, 13,668 (1980); (b) B. J. Berne and R. Pecora, "Dynamic Light Scattering", Wiley-Interscience, New York, 1976. (12)B. Miller and A. Heller, Nature (London),262,680 (1976).
QQ22-3654f82f2086-447Q$OI .25/Q 0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86,No. 23, 1982 4471
Letters n.
1.4
""I I
LUMINESCENCE QUENCHING
0.9
t(5Xl01.
5 0.8 w
SOLUTION
I
- 1.2
e I ,
CdS COLLOIOAL
ii
I
I
\
', \
41.0 I
0.7b-
2E 0.6 E
E 0.6 a
A 0.6
cz 0.4
-0.4
i c
-2 0.5 -z 0.4 N
0 P
0.2c
0.3 -
i0.2
pb\
v)
1.y IO
O i
* 0.2-
10-6
CONCENTRATION(M) 0.1 -
' 300
0.0-
Flgure 2. Relative quenching yields as a function of concentration for the indicated species. 400
500
600
WAVELENGTH (NMI
Flgure 1. Emission and absorption spectra of CdS aqueous colloids. The weak emission spectrum corresponds to an fresh equimolar collold. The more intense spectrum corresponds to an aged SH- rich colloid.
Rayleigh scattering showed a N 15% decrease in hydrodynamic radius during the aging process. The hydrodynamic radius of aged CdS particles was N 200 A, with some variation from batch to batch. If the luminescence provides a measure of the same h+ and e- population that is able to undergo suface photochemistry, then 9 should vary as surface processes affect the carrier kinetics. In both aged and fresh colloids, the luminescence lifetime is
