Nano Pinstripes: TiO2 Nanostripe Formation by Nanoparticle-Mediated Pinning of Step Edges

Imaging the surface potential at the steps on the rutile TiO2(110) surface by Kelvin probe force microscopy

Although step structures have generally been considered to be active sites, their role on a TiO₂ surface in catalytic reactions is poorly understood. In this study, we measured the contact potential difference around the steps on a rutile TiO₂(110)-(1 × 1) surface with O₂ exposure using Kelvin probe force microscopy. A drop in contact potential difference was observed at the steps, indicating that the work function locally decreased. Moreover, for the first time, we found that the drop in contact potential difference at a <1-11> step was larger than that at a <001> step. We propose a model for interpreting the surface potential at the steps by combining the upward dipole moment, in analogy to the Smoluchowski effect, and the local dipole moment of surface atoms. This local change in surface potential provides insight into the important role of the steps in the catalytic reaction.

Tailoring the Catalytic Properties of Metal Nanoparticles via Support Interactions

The development of new catalysts for energy technology and environmental remediation requires a thorough knowledge of how the physical and chemical properties of a catalyst affect its reactivity. For supported metal nanoparticles (NPs), such properties can include the particle size, shape, composition, and chemical state, but a critical parameter which must not be overlooked is the role of the NP support. Here, we highlight the key mechanisms behind support-induced enhancement in the catalytic properties of metal NPs. These include support-induced changes in the NP morphology, stability, electronic structure, and chemical state, as well as changes in the support due to the NPs. Utilizing the support-dependent phenomena described in this Perspective may allow significant breakthroughs in the design and tailoring of the catalytic activity and selectivity of metal nanoparticles.

Nano-gold diggers: Au-assisted SiO(2)-decomposition and desorption in supported nanocatalysts

An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs.

Metal nanoparticle catalysts beginning to shape-up

The field of heterogeneous catalysis has received a remarkable amount of interest from scientific and industrial perspectives because of its enormous impact on the world's economy: more than 90% of chemical manufacturing processes use catalysts. Catalysts are also essential in converting hazardous waste into less harmful products (car exhaust) and in generating power (fuel cells). Yet in all applications, it remains a challenge to design long lasting, highly active, selective, and environmentally friendly catalytic materials and processes, ideally based on Earth-abundant elements. In addition, the field needs more satisfactory experimental and theoretical approaches to minimize trial and error experiments in catalyst development. Nanocatalysis is one area that is developing rapidly. Researchers have reported striking novel catalytic properties, including greatly enhanced reactivities and selectivities, for nanocatalysts compared to their bulk counterparts. Fully harnessing the power of nanocatalysts requires detailed understanding of the origin of their enhanced performance at the atomic level, which in turn requires fundamental knowledge of the geometric and electronic structures of these complex systems. Numerous studies report on the properties that affect the catalytic performance of metal naoparticles (NPs) such as their size, interaction with their support, and their oxidation state. Much less research elucidates the role played by the NP shape. Complicating the analysis is that the preceding parameters are not independent, since NP size and support will affect which NP shapes are most stable. In addition, we must consider the dynamic nature of NP catalysts and their response to the environment, since the working state of a NP catalyst might not be the state in which the catalyst was prepared, but rather a structural and/or chemical isomer that responded to the particular reaction conditions. In order to address the complexity of real-world catalysts, researchers must undertake a synergistic approach, taking advantage of a variety of in situ and operando experimental methods. With the continuous shrinking of the scale of material systems, researchers require more sensitive experimental probes and computational approaches that work across a wide range of temperatures and chemical environments. This Account provides examples of recent advances in the preparation and characterization of NP catalysts with well-defined shapes. It discusses how to resolve the shape of nanometer-sized catalysts via a combination of microscopy and spectroscopic approaches, and how to follow their evolution in the course of a chemical reaction. Finally, it highlights that, for structure-sensitive reactions, controlled synthesis can tune catalytic properties such as the reaction rates, onset reaction temperature, activity, and selectivity.