Photoinduced Ultrafast Heterogeneous Electron Transfer at Molecule-Semiconductor Interfaces

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO₂ reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO₂ conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO₂. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs^•-) with TiO₂ compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs^•- formed at adjacent TiO₂ without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

A possible channel effect of the organics adsorbed to the electrode surface on interfacial electron transfer in the alkaline Pb electrodeposition process

A possible electron transfer channel in solid–liquid interface.

Ultrafast core-excited electron dynamics in model crystalline organic semiconductors

Resonant photoemission measurements show that ultrafast electron dynamics in core-excited states of large organic semiconductors depends on both the nature of the core-hole and the proximal chemical environment.

Attaching titania clusters of various size to reduced graphene oxide and its impact on the conceivable photocatalytic behavior of the junctions-a DFT/D + U and TD DFTB modeling

DFT/D  +  U and density functional based tight binding (DFTB) molecular modeling was used to investigate the role of the structural, electronic and optical properties of reduced graphene oxide surface (r-GO), hybridized with hydrated TiO₂ moieties of various size, ranging from small molecular Ti₂O₄ clusters into extended Ti₄₃O₈₆ rutile type nanocrystals of ~5 nm diameter. The calculated adhesion energies, varying from  -5.048 eV (r-GO|Ti₂O₄), -12.159 eV (r-GO|Ti₅O₁₀), -18.499 eV (r-GO|Ti₁₅O₃₀) to  -42.484 eV (r-GO|Ti₄₃O₈₆), indicate high stability of these composites. It was shown that electronic interactions at the r-GO|(1 1 0)TiO₂ interface give rise to net charge flow from the r-GO substrate towards the TiO₂ moieties, analyzed in terms of the partial charge density 3D plots and an interfacial dipole moment formation. The DOS structure of the composites was calculated by means of the time dependent DFTB approach, and the position and composition of the VB and CB edges, along with the presence of weak mid-gap 2p  C states originating from the intact graphene-like patches in the r-GO substrate were discussed in detail in the context of conceivable photocatalytic activity of the composites. The constructed band alignment diagram implies formation of the staggered type II scheme, with the electric field offset that is sensitive to the titania cluster size. In the case of the nano-reticular TiO₂, where only a fraction of the Ti atoms is engaged in the Ti-O-C linkers formation, recombination of the photogenerated charges is inhibited owing to favorable spatial separation effect. For small molecular TiO₂ clusters with all Ti cations anchored to the r-GO layer fast cross-relaxation quenches the beneficial interfacial charge separation effect, since the strong hybridization of the oxygen and carbon states provides a convenient pathway for the efficient electronic coupling between the CB edge states of r-GO and the VB edge states of the TiO₂ moieties. A phenomenological model of the molecular r-GO|Ti₂O₄ and the reticular r-GO|Ti₄₃O₈₆ composites was constructed in account for different photocatalytic behavior of both junctions.

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO₂, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO₂. In free PCA, the pyrene's forbidden S₀→S₁ transition has increased intensity, exhibits a long progression in 1400cm^-1 A_g mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO₂ causes a broadening and red shift of the S₀→S₁ absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Morphology-Preserving Sensitization of ZnO Nanorod Surfaces via Click-Chemistry

Films of ZnO nanorods grown by chemical vapor deposition were functionalized with a chromophore in a stepwise process that preserves the surface morphology. In the first step, the ZnO nanorods were functionalized by exposure to prop-2-ynoic acid (propiolic acid) in vacuum, which did bind through the COOH group leading to a ZnO surface functionalized with ethyne moieties (ethyne/ZnO). In the second step, 9-(4-azidophenyl)-2,5-di-tert-butylperylene (DTBPe-Ph-N₃) was reacted with the ethyne/ZnO surface via copper-catalyzed azide-alkyne click reaction (CuAAC) in solution to form the DTBPe-functionalized surface (DTBPe/ZnO). The ZnO morphology was preserved after each step, as demonstrated by scanning electron microscopy (SEM). Each step was probed by X-ray photoelectron spectroscopy (XPS), and transient absorption spectroscopy (TA) of the resulting DTBPe/ZnO surface shows interfacial electron transfer following visible light excitation. As expected, attempts to bind the reference compound 1-(4-(8,11-ditert-butylperylen-3-yl)-phenyl)-1H-1,2,3-triazole-4-carboxylic acid (DTBPe-Ph-Tz-COOH) directly from solution lead to etched surfaces (confirmed by SEM) and undefined binding modes (confirmed by TA).

Titania nanotube stabilized BiOCl nanoparticles in visible-light photocatalysis

Photocatalysis is a green approach in environmental organic pollutant decomposition.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Excited Electron Dynamics at Semiconductor-Molecule Type-II Heterojunction Interface: First-Principles Dynamics Simulation

Excited electron dynamics at semiconductor-molecule interfaces is ubiquitous in various energy conversion technologies. However, a quantitative understanding of how molecular details influence the quantum dynamics of excited electrons remains a great scientific challenge because of the complex interplay of different processes with various time scales. Here, we employ first-principles electron dynamics simulations to investigate how molecular features govern the dynamics in a representative interface between the hydrogen-terminated Si(111) surface and a cyanidin molecule. Hot electron transfer to the chemisorbed molecule was observed but was short-lived on the molecule. Interfacial electron transfer to the chemisorbed molecule was found to be largely decoupled from hot electron relaxation within the semiconductor surface. While the hot electron relaxation was found to take place on a time scale of several hundred femtoseconds, the subsequent interfacial electron transfer was slower by an order of magnitude. At the same time, this secondary process of picosecond electron transfer is comparable in time scale to typical electron trapping into defect states in the energy gap.

Vibronic and Coherent Effects on Interfacial Electron Transfer Dynamics

This Letter examines fundamental issues for electron transfer (ET) dynamics, such as adiabatic versus nonadiabatic effects during interfacial ET, the influence of vibrational degrees of freedom on the electronic dynamics, the occurrence of electronic coherences and the ensuing dephasing effects. The interplay of these mechanisms during the ultrafast ET is discussed. A theoretical method for the quantum dynamics of electrons in flexible molecular systems is used to study such issues on the interfacial ET from the perylene chromophore to the TiO2 semiconductor surface. By analyzing the Fourier transform of the survival probability curves, it is possible to discern the oscillating features that are caused by electronic coherences and vibronic effects. The vibronic degrees of freedom are treated within the atomistic level of description and their effects identified on the charge transfer dynamics. The insights revealed are general and thus can be useful for the analysis of other ET phenomena.

Crucial Role of Nuclear Dynamics for Electron Injection in a Dye-Semiconductor Complex

We investigate the electron injection from a terrylene-based chromophore to the TiO2 semiconductor bridged by a recently proposed phenyl-amide-phenyl molecular rectifier. The mechanism of electron transfer is studied by means of quantum dynamics simulations using an extended Hückel Hamiltonian. It is found that the inclusion of the nuclear motion is necessary to observe the photoinduced electron transfer. In particular, the fluctuations of the dihedral angle between the terrylene and the phenyl ring modulate the localization and thus the electronic coupling between the donor and acceptor states involved in the injection process. The electron propagation shows characteristic oscillatory features that correlate with interatomic distance fluctuations in the bridge, which are associated with the vibrational modes driving the process. The understanding of such effects is important for the design of functional dyes with optimal injection and rectification properties.

Disrupted Attosecond Charge Carrier Delocalization at a Hybrid Organic/Inorganic Semiconductor Interface

Despite significant interest in hybrid organic/inorganic semiconductor interfaces, little is known regarding the fate of charge carriers at metal oxide interfaces, particularly on ultrafast time scales. Using core-hole clock spectroscopy, we investigate the ultrafast charge carrier dynamics of conductive ZnO films at a hybrid interface with an organic semiconductor. The adsorption of C60 on the ZnO surface strongly suppresses the ultrafast carrier delocalization and increases the charge carrier residence time from 400 attoseconds to nearly 30 fs. Here, we show that a new hybridized interfacial density of states with substantial molecular character is formed, fundamentally altering the observed carrier dynamics. The remarkable change in the dynamics sheds light on the fate of carriers at hybrid organic/inorganic semiconductor interfaces relevant to organic optoelectronics and provides for the first time an atomistic picture of the electronically perturbed near-interface region of a metal oxide.