Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy = 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of <100 fs, and a slower component that is sensitive to sample condition. Similar ultrafast electron injection times were measured in TiO2 films sensitized by Ru(dcbpy)2(X)2 (X2 = 2CN- and dcbpy). Electron injection in these systems was found to compete with the vibrational energy relaxation process within the excited state of the molecules, leading to an injection yield that depends on the excited-state redox potential of the adsorbate. The injection rate from Ru(dcbpy)2(SCN)2 to different semiconductors was found to obey the trend TiO2 > SnO2 > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO2 sensitized by three adsorbates, Re(L n )(CO)3Cl [L n is a modified dcbpy ligand with n (=0, 1, 3) CH2 units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH2 units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)2X2 (X2 = 2SCN-, 2CN-, and dcbpy) sensitized SnO2 thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.

The photophysics and electron injection dynamics of Ru(dcbpy)2(NCS)2 [dcbpy = (4,4‘-dicarboxy-2,2‘-bipyridine)] (or Ru N3) in solution and adsorbed on nanocrystalline Al2O3 and TiO2 thin films were studied by femtosecond mid-IR spectroscopy. For Ru N3 in ethanol after 400 nm excitation, the long-lived metal-to-ligand charge transfer (3MLCT) excited state with CN stretching bands at 2040 cm-1 was formed in less than 100 fs. No further decay of the excited-state absorption was observed within 1 ns consistent with the previously known 59 ns lifetime. For Ru N3 absorbed on Al2O3, an insulating substrate, the 3MLCT state was also formed in less than 100 fs. In contrast to Ru N3 in ethanol, this excited state decayed by 50% within 1 ns via multiple exponential decay while no ground-state recovery was observed. This decay is attributed to electron transfer to surface states in the band gap of Al2O3 nanoparticles. For Ru N3 adsorbed onto the surface of TiO2, the transient mid-IR signal was dominated by the IR absorption of injected electrons in TiO2 in the 1700−2400 cm-1 region. The rise time of the IR signal can be fitted by biexponential rise functions:  50 ± 25 fs (>84%) and 1.7 ± 0.5 ps (<16%) after deconvolution of instrument response function determined in a thin silicon wafer. Because of the scattering of the pump photon in the porous TiO2 thin film, the instrument response may be slightly lengthened, which may require a faster rise time for the first component to fit the data. The first component is assigned to the electron injection from the Ru N3 excited state to TiO2. The amplitude of the slower component appears to vary with samples ranging from ca. 16% in new samples to <5% in aged samples. The subsequent dynamics of the injected electrons have also been monitored by the decay of the IR signal. The observed 20% decay in amplitude within 1 ns was attributed to electron trapping dynamics in the thin films.

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO 2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4′-dicarboxy-2,2′-bipyridine) 2 (NCS) 2 ] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO 2 at 1.52 µm and in the range of 4.1-7.0 µm, we have directly observed the arrival of electrons injected into the TiO 2 film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al 2 O 3 and ZrO 2 and found no indication of electron injection at probe wavelengths in the mid-IR (4.1-7.0 µm).

Transient infrared (IR) absorption of injected electrons in colloidal TiO2 nanoparticles in the 1900−2000 cm-1 region are measured by femtosecond IR spectroscopy. The direct detection of electrons in the nanoparticles with subpicosecond time resolution provides a new approach to study ultrafast interfacial electron transfer between semiconductor nanoparticles and molecular adsorbates. The dynamics of electron injection from sensitizers to nanoparticles and the subsequent back-transfer and relaxation dynamics of the injected electrons correspond to the rise and decay of the transient IR signal of injected electrons. Using this technique, the injection time for coumarin 343 sensitized TiO2 nanoparticles in D2O is determined to be 125 ± 25 fs. The subsequent decay dynamics of the injected electrons in nanoparticles are found to be different from conduction band electrons in a bulk TiO2 crystal.

Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN) 6 4sensitized TiO 2 nanoparticles in D 2 O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN) 6 4to TiO 2 occurs in <50 fs, indicating a direct photoinduced charge-transfer process. The back ET from TiO 2 to Fe(III)(CN) 6 3in the <1 ns time scale is found to be a non-single-exponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al.

Electron injection and back electron transfer dynamics of xanthene dyes adsorbed on TiO2 nanoparticles have been studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. When the xanthene dyes are adsorbed on the TiO2 surface, a good fraction of the dye molecules forms charge transfer (CT) complex with the TiO2 nanoparticle. On excitation of the above system, electron transfer from dye molecule to nanoparticle takes place. Electron injection has been observed by direct detection of electron in the conduction band of nanoparticle and bleach of the dye as detected by picosecond transient absorption spectroscopy. The corresponding dynamics have been determined by monitoring the recovery kinetics of the bleach of the dye in the visible region. Electron injection in the above systems can take place in two different ways:  (1) through the excited state of the dye and (2) through direct injection to the conduction band on excitation of the charge transfer complex. For the charge transfer complex, when the recombination reaction takes place, charge transfer (CT) emission has been observed. Monitoring the CT emission, we have determined the back ET rate. We have also found that the back ET rate for the xanthene dye-sensitized TiO2 CT complex decreases as the relative driving force increases. Assuming a negligible change in electronic coupling, our results provide the evidence for the Marcus inverted region kinetic behavior for an interfacial ET process.

Electron injection and back electron transfer dynamics in coumarin 343 (C-343) adsorbed on TiO2 nanoparticles are studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. The direct detection of electrons in the nanoparticles and the parent cation are monitored using picosecond transient absorption spectroscopy, and the corresponding dynamics of the adsorbate are monitored by time-resolved absorption spectra of the cation radical of C-343 in the visible region. When the electron returns from the nanoparticles to the parent cation, a low quantum yield red-shifted charge transfer emission is observed. Measuring the charge transfer emission lifetimes by a picosecond time-resolved fluorimeter, we get an exact rate of back electron transfer reaction from the nanoparticle to the parent cation.

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