Composition dependent reactivity of titanium oxide clusters

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

TD-DFT calculations were performed on neutral Ti_nO_2n, Ti_nO_2n-1, and Ti_nO_2n-2 clusters, where n ≤ 7. Calculations show the Ti_nO_2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the suboxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcitations of Ti_nO_2n clusters are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters are all characterized by d-d transitions that occur at a much smaller optical gap. Upon low energy photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized around the cluster. The properties of the clusters, including the oxygen binding energies and structures, were calculated to account for the variation in relative populations found in experimental cluster distributions. Several Ti_nO_2n-2 clusters contain higher symmetry which is reflected in their relative stability. In particular, the tetrahedral symmetry of Ti₄O₆ inhibits charge carrier localization and therefore exhibits higher stability.

Effect of oxidation on excited state dynamics of neutral TinO2n-x (n < 10, x < 4) clusters

Excited state lifetimes of neutral titanium oxide clusters (Ti_nO_2n-x, n < 10, x < 4) were measured using a sequence of 400 nm pump and 800 nm probe femtosecond laser pulses. Despite large differences in electronic properties between the closed shell stoichiometric Ti_nO_2n clusters and the suboxide Ti_nO_2n-x (x = 1-3) clusters, the transient responses for all clusters contain a fast response of 35 fs followed by a sub-picosecond (ps) excited state lifetime. In this non-scalable size regime, subtle changes in the sub-ps lifetimes are attributed to variations in the coordination of Ti atoms and localization of charge carriers following UV photoexcitation. In general, clusters exhibit longer lifetimes with increased size and also with the addition of O atoms. This suggests that the removal of O atoms develops stronger Ti-Ti interactions as the system transitions from a semiconducting character to a fast metallic electronic relaxation mechanism.

Surface-Enhanced Raman Spectroscopy on Amorphous Semiconducting Rhodium Sulfide Microbowl Substrates

Exploring highly surface-enhanced Raman scattering (SERS)-active semiconductors is urgently required for practical applications. Here, with the guidance of theoretical calculations, amorphous rhodium sulfide microbowls with high enhancement factor (1 × 105) and low limit of detection (10-7 M) for rhodamine 6G are successfully developed. This remarkable sensitivity is attributed to quasi-resonance Raman effect and multiple light scattering. The first-principles calculations show that the energy gap of 4-nitrobenzenethiol adsorbed on Rh3S6 is greatly decreased by shifting its lowest unoccupied molecular orbital (LUMO) energy level close to the LUMO of Rh3S6, enabling quasi-resonance Raman effect by visible light. The finite-difference time-domain simulations demonstrate the efficient photon trapping ability enabled by multiple light scattering. The optimum wavelength of ∼633 nm for SERS is predicted in simulations and confirmed in experiments. Our results provide both a deep insight of the photo-driven charge transfer process and an important guidance for designing SERS-active semiconductors.

A first-principles study of titanium oxide clusters formation and evolution in a steel matrix

The structures and evolution law of Ti_mO_n clusters were explored and clusters prefer a Ti : O ratio of 1 : 1.