© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 8, FEBRUARY 19, 1998
ARTICLES Ultrafast Charge Carrier Dynamics of SnO2 Nanoclusters: A Refined Interpretation of the Electron-Hole Kinetics in Metal Oxides Joseph J. Cavaleri, D. Philip Colombo, Jr., and Robert M. Bowman* Department of Chemistry, UniVersity of Kansas, Lawrence, Kansas 66045 ReceiVed: February 13, 1997; In Final Form: May 14, 1997X
Femtosecond experiments on 15 nm diameter SnO2 nanoclusters measure the elementary charge carrier reactions of electron trapping and electron-hole recombination. From the early time transient absorption data, an electron-trapping time of 200 ( 20 fs is determined. In addition, an experimental scheme to determine the effect of electron thermalization on the relaxation of photoexcited electrons is presented. Excess excitation energy above the conduction band increases the decay time to 500 ( 50 fs indicating that thermalization plays an important role in the electron-trapping kinetics. The dynamics of charge carrier recombination are investigated by an ultraviolet pump intensity study. A second-order rate constant of (1.0 ( 0.3) × 10-10 cm3/s is found to fit all of the decays. The early time decay kinetics in metal oxide nanoclusters do not agree with a recently proposed fractal kinetic study but are consistent with trapped electron/free hole recombination. The assignment of the early time transient absorption at 620 nm to trapped electrons is supported by comparing the transient absorption kinetics to ground state recovery results in both SnO2 and TiO2 nanoclusters.
Introduction The elementary charge carrier dynamics of nanometer-sized semiconductor particles have attracted great interest recently.1-9 The types of semiconductor materials of interest include colloidal solutions, thin films, and dye-sensitized thin films on optically transparent electrodes. The interest in these systems is due to their potential use in technological applications such as photocatalysis, chemical remediation, and solar energy conversion.7-9 The utility of nanocluster materials depends on the lifetime of the charge carriers, which has led to efforts aimed at determining the rate and extent of recombination of electronhole pairs. The ultrafast charge carrier kinetics in metal oxide colloidal solutions, including TiO2 and ZnO, have been the subjects of recent investigations.10-14 The results reveal that photogenerated electrons are rapidly trapped, in less than 200 * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 1, 1997.
fs,12 and undergo rapid electron-hole recombination, essentially complete within the first 50 ps.11 The recombination for TiO211 and ZnO13 is observed to proceed via second-order kinetics, i.e., the rate of recombination was proportional to the product of the concentration of photogenerated electrons and holes. These studies demonstrate that information on the charge carrier lifetimes and the potential efficiencies of these systems can be obtained directly with ultrafast spectroscopic techniques. SnO2 is similar to TiO2 and ZnO in that it is a large band gap semiconductor and has been studied for potential use in technological applications.15-19 Previous time-resolved experiments on SnO2 colloidal semiconductor solutions have been performed by P. V. Kamat and co-workers.15 These experiments characterize the electron-trapping process by laser flash photolysis and were done with microsecond time resolution. The results of these experiments show that electron-hole pairs were generated within the particle following the absorption of a 308 nm laser pulse and that the trapped electrons have a broad ab-
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1342 J. Phys. Chem. B, Vol. 102, No. 8, 1998 sorption centered around 620 nm. Transient absorption spectra obtained with a probe wavelength of 620 nm show electronhole recombination occurring even on a microsecond time scale. The kinetics of electron trapping and electron-hole recombination have not been resolved on an ultrafast time scale for colloidal SnO2. In addition to work on semiconductor colloids, the surfaces of nanocrystalline SnO2 thin films on optically transparent electrodes (OTE/SnO2) have been modified with chlorophyll-b in an effort to produce photosensitive electrodes (OTE/SnO2/ Chl-b), which can photoelectrochemically convert light energy into electricity.16 Time-resolved microwave absorption experiments with visible 532 nm light excitation on these systems exhibit a fast rise in absorption due to charge injection from the chlorophyll b to the SnO2. This process occurs within the laser pulse (10 ns) suggesting that the charge transfer takes place on an ultrafast time scale. SnO2 thin films that were not sensitized with the chlorophyll-b do not have a microwave absorption signal with the 532 nm excitation wavelength. Surface modification with thiazine and oxazine dyes17 has also been used to sensitize nanocrystalline SnO2 thin films. SnO2 films are highly porous and negatively charged, which favors strong adsorption and formation of dye aggregates on these surfaces. Charge injection from the dye aggregates into the semiconductor films is shown to occur in these systems as well. Kamat and co-workers18,19 have used picosecond transient absorption techniques to investigate the dynamics of cresyl violet H-aggregates adsorbed on SnO2 nanoclusters. SnO2 colloids themselves do not have significant absorption in the visible, but with the addition of adsorbed dye molecules there is a strong absorption in the visible region centered at ∼520 nm. The longtime transient absorption of the (CV+)2/SnO2 system has an absorption at 470 nm arising from the formation of the transient (CV)23•+ cation radical. The absorption in the red region > 600 nm results from trapped electrons in the SnO2 nanoclusters. The electron trapping and charge injection from the dye aggregate to the SnO2 nanocluster were measured. All of the previous work on nanoscale SnO2 systems points to the need to perform ultrafast experiments to measure the primary charge carrier reactions. The work presented here resolves the electron-trapping and electron-hole recombination kinetics using femtosecond transient absorption spectroscopy at 620 nm. Additional experiments are performed which clarify the role of electron thermalization in the trapping process and which support that the transient absorption at early times is due to trapped electrons. Comparisons with a recently published fractal kinetic model are made in an effort to understand the early time (
