Chi Lun Pang

Chemical reactions on rutile TiO2(110)

Understanding the surface chemistry of TiO2 is key to the development and optimisation of many technologies, such as solar power, catalysis, gas sensing, medical implantation, and corrosion protection. In order to address this, considerable research effort has been directed at model single crystal surfaces of TiO2. Particular attention has been given to the rutile TiO2(110) surface because it is the most stable face of TiO2. In this critical review, we discuss the chemical reactivity of TiO2(110), focusing in detail on four molecules/classes of molecules. The selected molecules are water, oxygen, carboxylic acids, and alcohols-all of which have importance not only to industry but also in nature (173 references).

Electron traps and their effect on the surface chemistry of TiO2(110)

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO2(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Growth of copper and palladium on α-Al2O3(0001)

Non-contact atomic force microscopy (NC-AFM) has been used to image the room-temperature growth of copper and palladium on the (1 x 1) and (root 31 x root 31) R +/- 9 degrees terminations of alpha-Al2O3(0001). Three-dimensional (3D) clusters of palladium are observed on both the (1 x 1) and the (root 31 x root 31) R +/- 9 degrees terminations, with 3D clusters of copper observed on the reconstructed surface. There is evidence of step-edge-dominated growth of palladium on the (root 31 x root 31) R +/- 9 degrees termination

Low-Dimensional, Reduced Phases of Ultrathin TiO2

Reduced phases of ultrathin rutile TiO(2)(110) grown on Ni(110) have been characterized with scanning tunneling microscopy and low-energy electron diffraction. Areas of 1 x 2 reconstruction are observed as well as {132} and {121} families of crystallographic shear planes. These phases are assigned by comparison with analogous phases on native rutile TiO(2)(110).

Imaging reconstructed TiO2 surfaces with non-contact atomic force microscopy

Abstract We have used non-contact atomic force microscopy (NC-AFM) to study TiO2(110), identifying a row with twice the thickness of a TiO2(110)1×2 row. This can be explained by a [110] extension of the added row model of TiO2(110)1×2. In the [001] direction, this reconstruction narrows into a 1×2 row giving strong evidence that the two structures are very closely related. For the TiO2(100) surface, we present NC-AFM data which supports the intermediate 1×3-β model previously proposed on the basis of an STM experiment.

Non-contact atomic force microscopy imaging of TiO2(100) surfaces

Abstract Atomically resolved non-contact fm mode atomic force microscopy images have been obtained from TiO2(100) surfaces. The 1×1 surface is observed, as well as the 1×3 phase previously imaged with STM. The morphology of the latter reconstruction consists of (110) microfacets. An additional reconstruction with 1×3 symmetry is observed, which is assigned to a phase intermediate between the 1×1 and 1×3-microfacet terminations.

Self-Assembled Metallic Nanowires on a Dielectric Support: Pd on Rutile TiO2(110)

Palladium nanoparticles supported on rutile TiO(2)(110)-1 x 1 have been studied using the complementary techniques of scanning tunneling microscopy and X-ray photoemission electron microscopy. Two distinct types of palladium nanoparticles are observed, namely long nanowires up to 1000 nm long, and smaller dotlike features with diameters ranging from 80-160 nm. X-ray photoemission electron microscopy reveals that the nanoparticles are composed of metallic palladium, separated by the bare TiO(2)(110) surface.

Influence of support morphology on the bonding of molecules to nanoparticles

Significance Supported metal nanoparticles often exhibit properties differing from those of their single-crystal counterparts. There have been several suggested explanations for this, including quantum size effects, strong metal support interactions, and some interplay between facets. In this article, we show that the support morphology can also have a decisive role in the nanoparticle properties. Using scanning tunneling microscopy, we show that Pd nanocrystals formed across steps of a TiO2 support have a curved top facet, where unusual adsorption behavior of CO is found. Calculations suggest that the different adsorption behavior arises from strain originating in the curved top facet. Our observations open the way for the tailoring of nanoparticle functionality by tuning the morphology of the support. Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the CO–metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.

Scanning tunnelling microscopy study of ammonia adsorption on TiO2(110)

We have used scanning tunnelling microscopy to study the adsorption of ammonia on TiO2(110). Our results confirm that NH3 adsorbs on 5-fold coordinated Ti sites, in line with previous studies. At higher coverages, our images show that the NH3 has a preferred periodicity in the [001] direction of twice the primitive unit cell distance.