Gaetano Granozzi Francesco Sedona (PhD thesis) TiOx NANOSTRUCTURES ON A MONOCRYSTALLINE Pt SUBSTRATE Università degli Studi di Padova Dipartimento di Scienze Chimiche

Outline  Motivations  A brief summary of the results on TiO x nanophases on Pt(111)  Use of the TiO x nanostructures for growing ordered arrays of Au nanoclusters with low dispersion  Conclusions and Perspectives - Sedona et al. Ultrathin TiO x films on Pt(111): a LEED, XPS and STM investigation J. Phys. Chem. B 2005, 109, Sedona et al. Ultrathin wagon-wheel-like TiO x phases on Pt(111): a combined LEED and STM investigation J. Phys. Chem. B 2006, 110, Finetti et al. Core and Valence Band Photoemissionn Spectroscopy of Well-Ordered Ultrathin TiO x Films on Pt(111) J. Phys. Chem. C , Barcaro et al. The structure of a TiO x zigzag-like monolayer on Pt(111) J. Phys. Chem. C 2007,111, Sedona et al. Ordered arrays of Au nanoclusters by TiO x ultrathin templates on Pt(111) J. Phys. Chem. C 2007, 111, 8024

Motivations for studying ultrathin oxide films (up to ca. 10 monolayers)  oxide functionality integrated in epitaxial devices (e.g. High-K dielectrics) Metal particles 2D oxideMetallic substrate  a way to control defectivity and to study its role  model systems for oxide supported catalysts  if the ultrathin film is nanostructured, it can act as a template for growing ordered metallic nanocluster arrays

Ultrathin Oxide films on metals: Methodology of preparation Preparations in vacuo (UHV) to drive the growth toward specific chemical composition, structure and morphology Reactive deposition metal Oxidant agent (molecular oxygen, atomic oxygen, water,NO 2 ) oxide film crystalline substrate Deposition parameters to be optimised in order to obtain a nanostructured film:  Choice of the substrate  Deposition rate  Substrate temperature  Nature and concentration (partial pressure) of the oxidising agent  Temperature and time of heat treatments

TiO x nanostructures on Pt(111) preparative procedure: Deposition reactive RT in 1x10 -7 mbar O 2 Structural ordering of the film with a post-annealing Changing the Ti dose and post annealing condition (temperature and partial pressure of O 2 ) 7 different surface stabilized phases

Annealing O 2 pressure (Pa) 5x (UHV) ≥2≥2≥2≥2 equivalent monolayer (MLE) k-TiO x z-TiO x z’-TiO x w-TiO x w’-TiO x rect-TiO 2 rect’-TiO 2 TiO x films on Pt(111) Summary of Data Phase diagram: STM data

≥2≥2≥2≥ Annealing O 2 pressure (Pa) equivalent monolayer (MLE) (UHV) x10 -4 k-TiO x z-TiO x z’-TiO x w-TiO x w’-TiO x rect-TiO 2 rect’-TiO 2 Chemical characterization TiO x films on Pt(111) Results: XPS

≥2≥2≥2≥ Annealing O 2 pressure (Pa) equivalent monolayer (MLE) (UHV) x10 -4 k-TiO x z-TiO x z’-TiO x w-TiO x w’-TiO x rect-TiO 2 rect’-TiO eV Chemical characterization Ti 2p Strongly oxidized phases higher BE peak two chemically shifted Ti 2p component TiO x films on Pt(111) Results: XPS

≥2≥2≥2≥ Annealing O 2 pressure (Pa) equivalent monolayer (MLE) (UHV) x10 -4 k-TiO x z-TiO x z’-TiO x w-TiO x w’-TiO x rect-TiO 2 rect’-TiO eV Chemical characterization Ti 2p two chemically shifted Ti 2p component More reduced phases lower BE peak eV TiO x films on Pt(111) Results: XPS

higher BE peak Ti sourrounded by oxygen O/Pt interface Pt Ti O Pt Ti O O lower BE peak Ti at the interface with the substrate eV eV TiO x films on Pt(111) Results: XPS stacking assignment

Annealing O 2 pressure (Pa) 5x (UHV) ≥2≥2≥2≥2 equivalent monolayer (MLE) k-TiO x z-TiO x z’-TiO x w-TiO x w’-TiO x rect-TiO 2 rect’-TiO 2 Reduced phases Pa TiO x films on Pt(111) analysis: structures and models DFT calculations carried out by A. Fortunelli (Pisa)

k-TiO x TiO x films on Pt(111) analysis: structures and models +1V -1V Pt Ti positive bias a honeycomb habitus (not observed negative bias a kagomé habitus (observed experimentally) Ti 2 O 3 stoichiometry

z-TiO x Pt Ti O Ti 4-fold oxygen coordinated : brighter Ti 3-fold oxygen coordinated : darker TiO x films on Pt(111) analysis: structures and bias Ti 6 O 8 stoichiometry

w-TiO x Ti 4-fold oxygen coordinated : brighter TiO x films on Pt(111) analysis: structures and bias TiO 1.2 stoichiometry

w-TiO x TiO x films on Pt(111) analysis: structures and bias TiO 1.2 stoichiometry Ti 3-fold oxygen coordinated : darker Ti vacancy

Annealing O 2 pressure (Pa) 5x (UHV) ≥2≥2≥2≥2 equivalent monolayer (MLE) epitaxially oriented nanoparticles rect-TiO 2 rect’-TiO 2 k-Ti 2 O 3 w-TiO x z-Ti 6 O 8 x=1.5x=1.33 x=1.2 Evolution of the stoichiometry of the reduced phases with the Ti dose and oxygen pressure TiO x films on Pt(111) general trends z’-Ti 25 O 30 x=1.2

Gold Nanoparticles and catalysis O 2 activation Au nanoparticles over titania seem to be particularly active

experiments are in progress with the deposition of Au clusters on the TiOx/Pt nanophases The ultrathin TiOx films are exploited as possible templates (preferential nucleation at the defects)

k-TiO x z-TiO x rect-TiO 2 rect’-TiO 2 three different phases have been tested Annealing O 2 pressure (Pa) 5x (UHV) ≥2≥2≥2≥2 equivalent monolayer (MLE) z’-TiO x w-TiO x w’-TiO x Au clusters on the TiO x phases templating effect

35 x 35 nm 2 31 x 31 nm 2 200x200nm 2 100x100 nm 2 95x80 nm 2 w’-TiO x z’-TiO x w-TiO x templating effect: long range order

35 x 35 nm 2 31 x 31 nm 2 30 x 14 nm 2 200x200nm 2 100x100 nm 2 95x80 nm 2 w’-TiO x z’-TiO x w-TiO x templating effect: long range order

35 x 35 nm 2 31 x 31 nm 2 200x200nm 2 100x100 nm 2 w’-TiO x z’-TiO x w-TiO x templating effect: size distribution

110x80 nm 2 Au clusters on z’-TiO x.-phase Au clusters form an hexagonal pattern with an average lattice of 1.8 nm FT Annealing in 600 K for 20’ Transformation from z’ to w TiO x phase !

150x120 nm 2 Au clusters on z’-TiO x.-phase: increasing the temperature Further annealing 700 K UHV w-TiO x  w’-TiO x transformation

exposition: first 12L of O 2, second 20L of -155°C No CO 2 desorptions No Catalytic activity of w’-TiOx phase w’-TiOx phase without Au particles reproducible CO °C Catalytic activity of Au particles? w’-TiOx phase with 5’ of Au (~0.16 ML) No CO 2 desorptions No Catalytic activity of Au 2D islands w’-TiOx phase with 5’ of Au (~0.16 ML) after 800K for 15’ Experiments are in progress with 18 O 2 Preliminary tests by using Thermal Programmed Desorption (TPD) on the catalytic conversion of CO  CO low T

Conclusions and perspectives -The understoichiometric TiO x /Pt(111) nanophases are effective templates for growing Au nanoclusters of a very low dimensionality and size dispersion, whose chemistry is still unexplored (studies underway) -The cluster arrays present a large degree of long range order and is stable at relatively high temperatures -We can manipulate an entire array of nanoclusters inducing a cooperative change of their mutual positions by a thermal annealing which determines the change of the template itself -A rich panorama of TiO x nanophases can be obtained by a careful choice of the preparative conditions. -They present a different structural arrangement where a different stoichiometry is connected to a different coordination environment around Ti atoms. -Catalytic tests are in progress to evaluate the dependence of the chemical properties of the Au clusters as a function of their dimensions

Collaboration and acknowledgments Funding: PRIN 2005, EU VI PQ- NMP-Priority Internal Collaborators: Permanent: M. Sambi, A. Vittadini (theory), G. Andrea Rizzi Non permanent: S. Agnoli, P. Finetti, L. Artiglia Present external collaborations: CNR of Pisa, Italy (A. Fortunelli) Univ. of Brescia(L. Gavioli)