J. Phys. Chem. 1994,98, 3859-3864
3859
Femtosecond Studies of Photoinduced Electron Dynamics at the Liquid-Solid Interface of Aqueous CdS Colloids J. Z. Zhang,’ R. H.O’Neil, and T. W . Roberti Department of Chemistry and Biochemistry, University of California, Santa Cruz. California 95064 Received: November 22, 1993; In Final Form: January 17, 1994Q
We report the direct measurements of the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale. The observed transient absorption is attributed to electrons trapped at the liquid-solid interface. We show that electron trapping due to surface states or defect sites occurs in less than 100 fs. The trapped electrons then decay by a double exponential with time constants of 2-3 ps and 50 ps at high excitation intensities, while a single-exponential decay (50 ps) is observed at low intensities. The slow, 50-ps decay is attributed primarily to geminate electron-hole recombination, which dominates the decay dynamics at low excitation intensities. The fast 2-3-ps decay observed at high intensities is assigned to nongeminate recombination, suggesting that nongeminate recombination plays an important role at high pump powers. The decay dynamics are also found to be sensitive to the solvent environment. The decay is slower when hole scavengers such as I- are added to the liquid and faster when the pH of the solution is lowered (decrease in concentration of the hole scavenger, OH-), in support of the interpretation of trapped electron-hole recombination for the observed decay.
Introduction The behavior of the electron at the liquid-solid interface has been an important area of interest in chemistry and physics. Understandingthe interfacial electron dynamics is of significance both fundamentally and technologically in the fields of photosynthesis, photocatalysis, and photoelectrochemistry.14 The electron dynamics in heterogeneous systems, such as the liquidsolid interface, remain poorly understood compared to their homogeneous counterpart,s7 partly due to the ultrafast nature of the electron-transfer process and the presence of two distinct condensed phases. One of the most important liquid-solid interfacial systems is the semiconductor-liquid interface, which plays a key role in several areas including solar energy conversion.14.8-11 Recent research efforts in the study of semiconductor photocatalysis have been focused on the development of ultrafine particulate or colloidal semiconductors with diameters of 10100 A.a-11 These colloidal particles provide a unique system for studying the electron dynamics at the liquid-solid interface, since the small size of the particles makes them practically transparent tolight and allows theuse ofversatile techniques such as transient absorption spectroscopy. Furthermore, colloidalsemiconductors exhibit optical and electronic properties different from those of bulk semiconductors or macroparticles (>lo0 A) due to the quantum size-confinement effect,12-18making them potentially useful in the design of new photoelectrochemical systems and electrooptical devices: For instance, the small size and the quantum nature of the particles could shorten the “diffusion” time significantly and reduce the probability of electron-hole recombination compared to that for the bulk semiconductor, resulting in enhancement of the charge-transfer efficiency across the liquid-solid interface,l4 In order to understand the overall interfacial electron-transfer process in these semiconductor colloidal systems, it is essential to study the primarydynamical processes includingcharge carrier trapping and recombination. Colloidal CdS has long served as a model system for the study of charge carrier dynamics in quantum particles both in nanocry~tals12.~~~~5.L7 and in the colloidal form (CdS particles dispersed in liquids).1”22 Colloidal CdS with different particle sizes and narrow size distributions can be Abstract published in Advance ACS Abstructs, March 1, 1994.
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prepared using various synthetic techniques.19-23 The static absorption and emission propertiesof colloidalCdS particles have been well c h a r a c t e r i ~ e d . ~ ~In- ~addition, ~ nanosecond and picosecond flash photolysis19-21as well as picosecond resonance Raman22 experiments have been performed to examine the interfacial charge-transfer process in colloidalCdS. It was found that reduction and oxidation reactions due to charge carrier transfer occur on the subnanosecond time scale, and charge carrier trapping was indicated to be faster than laser pulse duration of 18 ps.21 Recently, several femtosecond studies have been reported on the investigationof charge camer dynamicsin (solid) nanocrystals such as CdSe;18.24J5however, to date only one femtosecond study has been reported to examine the electron dynamics in aqueous CdS colloids.26 The study reported a transient absorption spectrum observed 330 fs after excitation by a 250-fs laser pulse at 308 nm,26which was assigned to the hydrated electron ejected into the liquid on the basis of the similarity between the observed transient absorption spectrumand theknown absorption spectrum of the h~dratedelectron.2~ This study also found that the spectrum did not decay on the time scale up to 10 ps and provided no indication of trapped electrons. This assignment of hydrated electrons is in direct contradictions with other studies that have suggested the observation of quickly trapped electrons in CdS particles dispersed in water21aor in polymer films28as well as in other aqueous colloidal systems such as Ti02,29,30ZnO,31 and W03,32 using picosecond and nanosecond flash photolysis techniques. Therefore,thedynamicsof the photoinducedelectrons at the liquid-solid interface of the colloidal CdS need further investigation. In this paper, we report a systematic study of the electron dynamics in aqueous CdS colloids using femtosecond laser spectroscopy. We show new evidence that supports the interpretation of trapped electrons at the liquid-solid interface. The data in ref 26 can actually be reproduced at a low pump power and explained in a similar manner in terms of trapped electrons. The implication is that the transient absorption spectrum of the trapped electron at the liquid-solid interface is similar to that of the hydrated electron. This is possible due to the fact that the red absorption spectra of localized or trapped electrons observed in various condensed-phase environments such as liq~ids2~~33-3~ and solids38 are usually broad and featureless and that theobserved 0 1994 American Chemical Society
3860 The Journal of Physical Chemistry, Vol. 98, No. 14, I994 red transient absorptionspectra in several colloidal semiconductors including CdS are also featureless and broad.11.28-32 We have observed that the average transient time for the creation of trapped electrons is
