TiO2 is an effective and extensively employed photocatalyst, but its practical use in visible-light-mediated organic synthesis is mainly hindered by its wide band gap energy. Herein, we have discovered that Rh-photodeposited TiO2 nanoparticles selectively dehydrogenate N-heterocyclic amines with the concomitant generation of molecular hydrogen gas in an inert atmosphere under visible light (λmax = 453 nm) illumination at room temperature. Initially, a visible-light-sensitive surface complex is formed between the N-heterocycle and TiO2. The acceptorless dehydrogenation of N-heterocycles is initiated by direct electron transfer from the HOMO energy level of the amine via the conduction band of TiO2 to the Rh nanoparticle. The reaction condition was optimized by examining different photodeposited noble metals on the surface of TiO2 and solvents, finding that Rh0 is the most efficient cocatalyst, and 2-propanol is the optimal solvent. Structurally diverse N-heterocycles such as tetrahydroquinolines, tetrahydroisoquinolines, indolines, and others bearing electron-deficient as well as electron-rich substituents underwent the dehydrogenation in good to excellent yields. The amount of released hydrogen gas evinces that only the N-heterocyclic amines are oxidized rather than the dispersant. This developed method demonstrates how UV-active TiO2 can be employed in visible-light-induced synthetic dehydrogenation of amines and simultaneous hydrogen storage applications.

One-dimensional (1D) wires are inherently unstable but can be stabilized by three-dimensional (3D) interaction with their environment, resulting in two-dimensional (2D) and 3D hybridization of 1D electronic states. The relevance of these interactions, which is still under debate, is exemplified by the prototypical Si(553)–Au system investigated here. This system forms double atomic 1D chains on each mini-terrace for the high-coverage phase, whereas in the low-coverage phase every second terrace is empty. The relevance of hybridization is demonstrated by the complete breakdown of the nearly free electron gas model, as revealed from plasmon dispersion. Nevertheless, the combined approach consisting of plasmon spectroscopy and first-principles calculations allows for a consistent and almost quantitative description. It further demonstrates that plasmon spectroscopy contains important information about the excitation spectrum of an electronic system. Because the coupling of the Au wires with higher dimensions through the substrate cannot be neglected, the wires are more appropriately described as an extremely anisotropic 2D object than as purely 1D.

The plasmon dispersion is inherently related to the continuum of electron-hole pair excitations. Therefore, the comparison of this continuum, as derived from band structure calculations, with experimental data of plasmon dispersion, can yield direct information about the form of the occupied as well as the unoccupied band structure in the vicinity of the Fermi level. The relevance of this statement is illustrated by a detailed analysis of plasmon dispersions in quasi-one-dimensional systems combining experimental electron energy loss spectroscopy with quantitative density-functional theory (DFT) calculations. Si(557)-Au and Si(335)-Au with single atomic chains per terrace are compared with the Si(775)-Au system, which has a double Au chain on each terrace. We demonstrate that both hybridization between Si surface states and the Au chains as well as electronic correlations lead to increasing deviations from the nearly free electron picture that is suggested by a too simple interpretation of data of angular resolved photoemission (ARPES) of these systems, particularly for the double chain system. These deviations are consistently predicted by the DFT calculations. Thus also dimensional crossover can be explained.

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We investigated initial steps of oxidation of the Si(557)-Au system by plasmon spectroscopy and first-principles calculations. The measurements, performed using an electron energy loss instrument with simultaneous high resolution in energy and momentum, reveal that metallicity is preserved under all oxidation conditions that are experimentally accessible in UHV. Corresponding simulations, performed within density functional theory, confirm this finding: Only the oxidation of the Si environment of the Au chains turned out to be strongly exothermic, with similar binding energy for adsorption on different structural elements. While large and site specific changes of the band structure were observed, the upper edge of the excitation spectrum of electron-hole pairs, to which plasmon dispersion is most sensitive, remains almost unchanged during the various steps of oxidation, due to the opposite and largely compensating contributions of different adsorption configurations. This investigation not only proves the robustness of metallicity of the gold chains upon oxidation of the surrounding environment of Si atoms, but also demonstrates the usefulness of plasmon spectroscopy in characterizing the electronic excitation spectrum of quasi-one-dimensional systems and unoccupied band structure.