Hsin-Yi Tiffany Chen

A DFT Study of the Reactivity of Anatase TiO2 and Tetragonal ZrO2 Stepped Surfaces Compared to the Regular (101) Terraces

It is generally assumed that low-coordinated sites at extended defects of oxide surfaces like steps or edges are more reactive than the regular, fully coordinated sites at the flat terraces. In this work we have considered the properties of stepped surfaces of anatase TiO2 and tetragonal ZrO2 by means of periodic DFT+U calculations. For both oxides, the stability of oxygen vacancies located near the step edges is compared to that of the same defects at the regular terraces. The capability of the steps to induce nucleation of metal nanoparticles on the surface has been evaluated by simulating the adsorption of a single ruthenium adatom. We conclude that, for anatase, step edges have no particular role in favouring the reduction of the oxide by reducing the cost for oxygen abstraction; in the same way, there is no special role of the stepped anatase surface in stabilizing adsorbed Ru atoms. On the contrary, step edges on zirconia display some capability to stabilise oxygen vacancies and ruthenium adatoms.

Role of Oxide Reducibility in the Deoxygenation of Phenol on Ruthenium Clusters Supported on the Anatase Titania (1 0 1) Surface

The deoxygenation of phenol on stoichiometric and reduced Ru10/TiO2 anatase (1 0 1) surfaces has been studied by using DFT with the Hubbard correction (DFT+U). If the molecule orients with the OH group towards the metal–oxide interface, the direct deoxygenation of phenol can occur. However, on the stoichiometric TiO2 surface, the reaction is thermodynamically unfavorable. Two kinds of reduced surfaces have been considered: one in which Ti3+ centers are generated by hydrogen addition, and a second one in which a water molecule is removed from a hydroxylated surface with the formation of O vacancies and Ti3+ centers. On the surface reduced by hydrogen addition (Ti3+ ions), the phenol molecular and dissociative adsorptions (C6H5+OH fragments) become isoenergetic; the barrier to dissociate the C−OH bond is 1.19 eV, which indicates a possible channel for the deoxygenation of phenol. On the surface reduced by O vacancies, the dissociative adsorption is 0.22 eV more stable than the molecular adsorption, which indicates a thermodynamically favorable process; however, the C−OH activation energy is higher, 1.50 eV. The results show that the C−O scission can be an important step towards the direct deoxygenation. The reduction of the surface facilitates the direct deoxygenation of phenol significantly.

Size-Dependent Penetration of Gold Nanoclusters through a Defect-Free, Nonporous NaCl Membrane

Membranes and their size-selective filtering properties are universal in nature and their behavior is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation. Exploring and understanding penetration and transmission mechanisms of nanoparticles in thin-film systems may provide new opportunities for size selective deposition or embedding of the nanoparticles. Here, we demonstrate an unexpected finding that the sieving of metal nanoparticles through atomically thin nonporous alkali halide films on a metal support is size dependent and that this sieving effect can be tuned via the film thickness. Specifically, relying on scanning tunneling microscopy and spectroscopy techniques, combined with density functional theory calculations, we find that defect-free NaCl films on a Au(111) support act as size-dependent membranes for deposited Au nanoclusters. The observed sieving ability is found to originate from a driving force toward the metal support and from the dynamics of both the nanoparticles and the alkali halide films.

Lateral Manipulation of Atomic Vacancies in Ultrathin Insulating Films

During the last 20 years, using scanning tunneling microscopy (STM) and atomic force microscopy, scientists have successfully achieved vertical and lateral repositioning of individual atoms on and in different types of surfaces. Such atom manipulation allows the bottom-up assembly of novel nanostructures that can otherwise not be fabricated. It is therefore surprising that controlled repositioning of virtual atoms, i.e., atomic vacancies, across atomic lattices has not yet been achieved experimentally. Here we use STM at liquid helium temperature (4.5 K) to create individual Cl vacancies and subsequently to laterally manipulate them across the surface of ultrathin sodium chloride films. This allows monitoring the interactions between two neighboring vacancies with different separations. Our findings are corroborated by density functional theory calculations and STM image simulations. The lateral manipulation of atomic vacancies opens up a new playground for the investigation of fundamental physical properties of vacancy nanostructures of any size and shape and their coupling with the supporting substrate, and of the interaction of various deposits with charged vacancies.

TiO2 and ZrO2 in biomass conversion: why catalyst reduction helps

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase TiO2, a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm. This article is part of a discussion meeting issue ‘Providing sustainable catalytic solutions for a rapidly changing world’.

Spontaneous doping of two-dimensional NaCl films with Cr atoms: aggregation and electronic structure

Scanning tunneling microscopy (STM) experiments combined with density functional theory (DFT) calculations reveal that deposited Cr atoms replace either Na or Cl ions, forming substituting dopants in ultrathin NaCl/Au(111) films. The Cr dopants exchange electrons with the support thus changing the electronic properties of the film and in particular the work function. The Cr atoms spontaneously aggregate near the edges of the bilayer (2L) NaCl islands, forming a new phase in the insulator with a remarkably dense population of Cr dopants. The spectra of differential conductance yield evidence that, compared to the undoped or Cr-poor 2L NaCl films on Au(111), the Cr-rich region shows different interface states, shifted image-potential states, and a reduced work function. This demonstrates the potential of doping ultrathin films to modify their adsorption properties in a desired manner.