Epitaxial TiO2 nanoparticles on Pt(111): a structural study by photoelectron diffraction and scanning tunneling microscopy

Abstract

Angle-scanned X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM) are used to characterise the structure of TiO2 nanoparticles grown on a Pt(111) single crystal surface. The nanoparticles grow over a well-ordered oxide interfacial layer that displays a (square root 43 x square root 43) - R7.6 degrees superstructure with a unit cell (18.2 x 18.2 A), as demonstrated by STM and low-energy electron diffraction (LEED). Our XPS Ti 2p core level spectra suggest a significant contribution from reduced titanium ions within the interfacial layer. On the contrary, according to XPS binding energy data, the nanoparticles are mostly composed of Ti(IV) ions. During the initial stage of the growth, the nanoparticles are on the average 2 nm high and about some tens of nm wide, and show a flat on-top surface, while the interparticle region show the structure of the ordered interfacial layer. During later stages of the deposition, the particles become larger and they show a more irregular, globular-like shape as well as coalescence. But, even at this stage of the growth, large interparticle regions are present. Moreover, the nanoparticles produce a distinct XPD pattern which demonstrates that they grow with a preferential azimuthal orientation with respect to the substrate surface. A simple geometrical analysis of the XPD data in terms of forward scattering events suggests that the particles crystallize in the rutile TiO2 structure and expose the (100) surface. This hypothesis is supported by means of multiple scattering simulations of the XPD patterns.

Stability and chemisorption properties of ultrathin TiOx/Pt(111) films and Au/TiOx/Pt(111) model catalysts in reactive atmospheres

“…This may be compatible with the results of a previous investigation where, using angle-scanned photoelectron diffraction (PhD), the presence of rutile-like, flat TiO 2 nanostructures supported on the Pt substrate, with the (100) surface exposed, has been demonstrated. 39,40 In this case both the TiO 2 coverage and the thermal treatment of the system were almost identical to the one described above. Similar rutile-like titania nanostructures with the (100) surface being exposing and typical dimensions of 5-20 nm, have recently been reported on Au(111) after annealing to 900 K. 41 In conclusion, although both reduced systems seem to follow an oxidative dewetting when being exposed to HP CO-O 2 (1 : 1), the reduction process of the two differently prepared films upon annealing in UHV is rather different: after thermal treatment in UHV up to 950 K, the z 0 -TiO x film is brought back to its original oxidation state and structure, while the w 0 -TiO x one is not.…”

Cation site environment in ultrathin TiOx films grown on Pt(111) probed by X-ray absorption spectroscopy at the Ti 2p edge

“…These high-coverage films do not show well-defined LEED patterns. According to previous STM investigations [20], in such coverage region the deposit is formed by fully oxidised TiO 2 nanoparticles on a wetting monolayer (ML), whose dimensions and morphologies are strongly dependent on the actual coverage [20]. The highest coverage herein explored can be taken as a reference for a bulk-like titania system, as demonstrated by the close similarity with the pertinent literature data [18].…”

“…The notation used to label the phases is determined by to the shape of the surface unit cell or to their appearance in STM. This notation is consistent with the one used in previous experimental and theoretical studies [10][11][12][13][14][15][16]20]. The Ti coverage is gauged from Ti 2p XPS intensity (peak area) normalised to the average XPS intensity of the wagon-wheel w structure, which serves as a reference (see Fig.…”

Nanoparticle Characterization

“…3.9) [46]. Only elastically scattered electrons (changing direction without energy loss) contribute to the diffraction pattern; the lower energy (including secondary) electrons are removed by energy-filtering grids placed in front of the fluorescent screen that is employed to display the pattern.…”