Vicinal Rutile TiO₂ Surfaces and Their Interactions with O₂

Slightly miscut TiO₂(110) surfaces with high densities of step edges were studied by scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and ultraviolet photoemission spectroscopy (UPS). STM measurements provided information on the surface morphology and the density of defects and adstructures, whereas UPS measurements revealed information on the electronic structure and the surface reduction state before and after the conduction of O₂ TPD experiments. It was found that the presence of step edges and adstructures has a strong influence on the O₂–TiO₂ interaction. The growth of TiO_<i>x</i> islands occurred in the same way on stepped surfaces as on flat TiO₂(110) surfaces, but the island densities were smaller. TPD measurements revealed that significantly less O₂ desorbed between 300 and 410 K from stepped surfaces than from surfaces with large terraces. Importantly, the stepped TiO₂ surfaces were characterized by clearly lower surface reduction states than flat TiO₂(110) surfaces

Long-Range Order Induced by Intrinsic Repulsion on an Insulating Substrate

An ordered arrangement of molecular stripes with equidistant appearance is formed upon the adsorption of 3-hydroxybenzoic acid onto calcite (10.4) held at room temperature. In a detailed analysis of the next-neighbor stripe distances measured in noncontact atomic force microscopy images at various molecular coverages, we compare the observed stripe arrangement with a random arrangement of noninteracting stripes. The experimentally obtained distance distribution deviates substantially from what is expected for a random distribution of noninteracting stripes, providing direct evidence for the existence of a repulsive interaction between the stripes. At low molecular coverage, where the average stripe distance is as large as 16 nm, the stripes are significantly ordered, demonstrating the long-range nature of the involved repulsive interaction. The experimental results can be modeled with a potential having a 1/<i>d</i>² distance dependence, indicating that the observed long-range repulsion mechanism originates from electrostatic repulsion of adsorption-induced dipoles solely. This effect is particularly pronounced when local charges remain unscreened on the surface, which is characteristic of nonmetallic substrates. Consequently, the observed generic repulsion mechanism is expected to play a dominant role in molecular self-assembly on electrically insulating substrates

Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface

Prototypical molecular switches such as azobenzenes exhibit two states, <i>i.e.</i>, <i>trans</i> and <i>cis</i>, with different characteristic physical properties. In recent years various derivatives were investigated on metallic surfaces. However, bulk insulators as supporting substrate reveal important advantages since they allow electronic decoupling from the environment, which is key to control the switching properties. Here, we report on the light-induced isomerization of an azobenzene derivative on a bulk insulator surface, in this case calcite (101̅4), studied by atomic force microscopy with submolecular resolution. Surprisingly, <i>cis</i> isomers appear on the surface already directly after preparation, indicating kinetic trapping. The photoisomerization process is reversible, as the use of different light sources results in specific molecular assemblies of each isomer. The process turns out to be very efficient and even comparable to molecules in solution, which we assign to the rather weak molecular interaction with the insulator surface, in contrast to metals

Chemical Identification at the Solid–Liquid Interface

Solid–liquid interfaces are decisive for a wide range of natural and technological processes, including fields as diverse as geochemistry and environmental science as well as catalysis and corrosion protection. Dynamic atomic force microscopy nowadays provides unparalleled structural insights into solid–liquid interfaces, including the solvation structure above the surface. In contrast, chemical identification of individual interfacial atoms still remains a considerable challenge. So far, an identification of chemically alike atoms in a surface alloy has only been demonstrated under well-controlled ultrahigh vacuum conditions. In liquids, the recent advent of three-dimensional force mapping has opened the potential to discriminate between anionic and cationic surface species. However, a full chemical identification will also include the far more challenging situation of alike interfacial atoms (i.e., with the same net charge). Here we demonstrate the chemical identification capabilities of dynamic atomic force microscopy at solid–liquid interfaces by identifying Ca and Mg cations at the dolomite–water interface. Analyzing site-specific vertical positions of hydration layers and comparing them with molecular dynamics simulations unambiguously unravels the minute but decisive difference in ion hydration and provides a clear means for telling calcium and magnesium ions apart. Our work, thus, demonstrates the chemical identification capabilities of dynamic AFM at the solid–liquid interface

Designer Titania-Supported Au–Pd Nanoparticles for Efficient Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au–Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd_shell–Au_core nanoparticles immobilized on TiO₂ exhibit extremely high quantum efficiencies for H₂ production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO₂ is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis