The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Grätzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Grätzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.

A nonadiabatic molecular dynamics (MD) simulation of the photoinduced electron transfer (ET) from a molecular electron donor to the TiO 2 acceptor is reported. The system under study is typical of the dye sensitized semiconductor nanomaterials used in solar cell, photocatalysis, and photoelectrolysis applications. The electronic structure of the dye-semiconductor system and the adiabatic dynamics are simulated by ab initio MD, whereas the nonadiabatic effects are incorporated by the quantum-classical mean-field approach. A novel procedure separating the nonadiabatic and adiabatic ET pathways is proposed. The simulation provides a detailed picture of the ET process. For the specific system under study, ET occurs on a 30 fs time scale, in agreement with the ultrafast experimental data. Both adiabatic and nonadiabatic pathways for the ET are observed. The nonadiabatic transfer entirely dominates at short times and can occur due to strong localized avoided crossing as well as extended regions of weaker nonadiabatic coupling. Although the adiabatic ET contribution accumulates more slowly, it approaches that of the nonadiabatic ET pathway asymptotically. It follows from the simulation that the nonadiabatic ET rate expressions, such as the Fermi golden rule, can be rigorously applied only for the fastest 30% of the ET process. The electron acceptor states are formed by the d orbitals of Ti atoms of the semiconductor and are localized within the first three to four layers of the surface. About 20% of the acceptor state density is localized on a single Ti atom of the first surface layer. The simulation predicts a complex non-single-exponential time dependence of the ET process. † Part of the special issue "John C. Tully Festschrift".

lution of Al. Since the electric field strength across the channel bottom (i.e., the barrier layer) is much greater than that across the channel wall, the Al dissolution rate at the bottom is far greater than that at the wall, resulting in perpendicular growth of the channel with high aspect ratio.However, other structural features of the anodic tin oxide such as gaps between the oxide layers, irregular pore shape, and a wide range of pore sizes imply that the formation of porous tin oxides is more complicated than that of porous anodic alumite, which is characterized by continuous channels consisting of regular shaped pores with a narrow size distribution. Vigorous gas evolution possibly plays a significant role in creating the irregular porous structure. It is also noted that the growth rate of anodic tin oxide is much faster than that of anodic alumite. Further investigations are needed to characterize the physical and microscopic properties of the barrier layer, the defective nature of tin oxide, and the conductivities of ions (O 2± /OH ± and Sn ions) through the tin oxide film, which are critical to clarifying the growth mechanism.In conclusion, porous tin oxides with nanochannels have been successfully prepared using an anodic oxidation process. This represents a novel way of creating porous structures that allow fast transport of gas/liquid and rapid electrochemical reactions due to high surface area. The present tin oxides with additional pores between the layers are ideally suited for electrodes in electrochemical devices such as batteries and chemical sensors. In particular, the as-prepared and annealed porous tin oxides are being directly used as meso-to macroporous anodes for lithium secondary batteries and the annealed tin oxide with stannic form is being tested for solidstate gas sensors. The electrochemical and catalytic properties of these unique structures will be reported in subsequent communications. Furthermore, the porous tin oxide can be used as a template for preparation of other functional materials in nanostructured forms. ExperimentalHigh-purity tin (Alfa Aesar, 99.998 %) was used as the working electrode (the anode). Before anodizing, the tin surface was polished to a mirror finish with 1.0 and 0.3 lm alumina powder (in order to remove the native oxide film) and then rinsed in distilled water. A platinum wire was used as the counter electrode (the cathode) for anodization. The distance between anode and cathode was kept at 1 cm. A constant voltage (5 to 14 V) was applied to the substrate using a Solartron 1285 potentiostat in an electrolyte of 0.5 M oxalic acid (Aldrich) at room temperature. Anodization was performed in a stationary electrolyte solution without stirring or N 2 bubbling. After anodization, some samples were annealed at 500 C for 3 h. Elemental analysis of as-prepared and annealed samples with an energy dispersive X-ray spectrometer (EDS) showed that the only constituents of the samples are tin and oxygen. The as-prepared samples appear to be dark brown, indicating the pr...

A nonadiabatic molecular dynamics (MD) simulation of the photoinduced electron transfer (ET) from a molecular electron donor to the TiO2 semiconductor acceptor is carried out in a microcanonical ensemble with an average temperature of 350 K. The electronic structure of the dye–semiconductor system and the adiabatic dynamics are simulated by ab initio MD, while the nonadiabatic (NA) effects are incorporated by a quantum‐classical mean‐field approach. The ET dynamics are driven by thermal fluctuations that dominate ionic motions at the simulated temperature. The ground and excited state ion dynamics are similar; therefore, the change in the quantum force due to the electronic photoexcitation can be neglected, and the analysis is greatly simplified. The simulated ET occurs on a 5‐fs timescale, in agreement with recent ultrafast experimental data. Vibrational motions of the chromophore ring carbons induce an oscillation of the photoexcited state energy, resulting in a bimodal distribution of the initial conditions for ET. At low energies the photoexcited state is localized primarily on the chromophore, while at high energies the photoexcited state is substantially delocalized into the TiO2 surface. Thermally driven adiabatic transfer is the dominant ET mechanism. Compared to the earlier simulation at 50 K, the rate of NA transfer at 350 K remains almost unchanged, whereas the rate of adiabatic ET increases substantially.

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