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Gaetano Granozzi Francesco Sedona (PhD thesis)

Università degli Studi di Padova Dipartimento di Scienze Chimiche. TiOx NANOSTRUCTURES ON A MONOCRYSTALLINE Pt SUBSTRATE. Gaetano Granozzi Francesco Sedona (PhD thesis). Outline. ¨ Moti vations ¨ A brief summary of the results on TiO x nanophases on Pt(111)

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Gaetano Granozzi Francesco Sedona (PhD thesis)

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

  2. Outline ¨ Motivations ¨ A brief summary of the results on TiOx nanophases on Pt(111) ¨Use of the TiOx nanostructures for growing ordered arrays of Au nanoclusters with low dispersion ¨ Conclusions andPerspectives -Finetti et al. Core and Valence Band Photoemissionn Spectroscopy of Well-Ordered Ultrathin TiOx Films on Pt(111) J. Phys. Chem. C 2007 111, 869 - Barcaro et al. The structure of a TiOx zigzag-like monolayer on Pt(111) J. Phys. Chem. C 2007 ,111, 6095 -Sedona et al. Ordered arrays of Au nanoclusters by TiOx ultrathin templates on Pt(111) J. Phys. Chem. C 2007, 111, 8024 - Sedona et al. Ultrathin TiOx films on Pt(111): a LEED, XPS and STM investigation J. Phys. Chem. B 2005, 109, 24411 - Sedona et al. Ultrathin wagon-wheel-like TiOx phases on Pt(111): a combined LEED and STM investigation J. Phys. Chem. B 2006, 110, 15359

  3. Motivations for studying ultrathin oxide films (up to ca. 10 monolayers) Metal particles Metallic substrate 2D oxide ¨oxide functionality integrated in epitaxial devices (e.g. High-K dielectrics) ¨ 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

  4. Reactive deposition Oxidant agent metal (molecular oxygen, atomic oxygen, water,NO2) crystalline substrate oxide film Ultrathin Oxide films on metals: Methodology of preparation Preparations in vacuo (UHV) to drive the growth toward specific chemical composition, structure and morphology 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

  5. TiOx nanostructures on Pt(111) preparative procedure: Deposition reactive deposition @ RT in 1x10-7 mbar O2 Structural ordering of the film with a post-annealing Changing the Ti dose and post annealing condition (temperature and partial pressure of O2) 7 different surface stabilized phases

  6. TiOx films on Pt(111) Summary of Data Phase diagram: STM data rect-TiO2 rect’-TiO2 z-TiOx k-TiOx w-TiOx z’-TiOx w’-TiOx

  7. TiOx films on Pt(111) Results: XPS Annealing O2 pressure (Pa) 5x10-4 rect-TiO2 rect’-TiO2 10-5 z-TiOx k-TiOx w-TiOx 10-8 (UHV) z’-TiOx w’-TiOx 0.4 0.8 1.2 ≥2 equivalent monolayer (MLE) Chemical characterization

  8. TiOx films on Pt(111) Results: XPS Annealing O2 pressure (Pa) 5x10-4 rect-TiO2 rect’-TiO2 10-5 z-TiOx k-TiOx w-TiOx 10-8 (UHV) z’-TiOx w’-TiOx 0.4 0.8 1.2 ≥2 equivalent monolayer (MLE) Chemical characterization Strongly oxidized phases higher BE peak Ti2p 458.6 eV two chemically shifted Ti2p component

  9. TiOx films on Pt(111) Results: XPS Annealing O2 pressure (Pa) 5x10-4 rect-TiO2 rect’-TiO2 10-5 z-TiOx k-TiOx w-TiOx 10-8 (UHV) z’-TiOx w’-TiOx 0.4 0.8 1.2 ≥2 equivalent monolayer (MLE) Chemical characterization More reduced phases lower BE peak Ti2p 458.6 eV 456.4 eV two chemically shifted Ti2p component

  10. TiOx films on Pt(111) Results: XPS O Ti O Pt O Ti Pt stacking assignment 458.6 eV higher BE peak Ti sourrounded by oxygen O/Pt interface lower BE peak Ti at the interface with the substrate 456.4 eV

  11. TiOx films on Pt(111) analysis: structures and models Reduced phases obtained @ 10-5 Pa rect-TiO2 rect’-TiO2 z-TiOx k-TiOx w-TiOx z’-TiOx w’-TiOx DFT calculations carried out by A. Fortunelli (Pisa)

  12. TiOx films on Pt(111) analysis: structures and models @ positive bias a honeycomb habitus (not observed experimentally) @ negative bias a kagomé habitus (observed experimentally) k-TiOx -1V Ti2O3stoichiometry +1V O Ti Pt

  13. TiOx films on Pt(111) analysis: structures and models z-TiOx @positive bias O Ti6O8 stoichiometry Ti 4-fold oxygen coordinated : brighter Ti Pt Ti 3-fold oxygen coordinated : darker

  14. TiOx films on Pt(111) analysis: structures and models w-TiOx @positive bias Ti 4-fold oxygen coordinated : brighter TiO1.2 stoichiometry

  15. TiOx films on Pt(111) analysis: structures and models Ti vacancy w-TiOx @positive bias TiO1.2 stoichiometry Ti 3-fold oxygen coordinated : darker

  16. TiOx films on Pt(111) general trends rect-TiO2 rect’-TiO2 w-TiOx k-Ti2O3 z-Ti6O8 epitaxially oriented nanoparticles x=1.5 x=1.33 x=1.2 z’-Ti25O30 x=1.2 Evolution of the stoichiometry of the reduced phases with the Ti dose and oxygen pressure

  17. Gold Nanoparticles and catalysis Au nanoparticles over titania seem to be particularly active O2 activation

  18. 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)

  19. Au clusters on the TiOx phases templating effect three different phases have been tested rect-TiO2 rect’-TiO2 z-TiOx k-TiOx w-TiOx z’-TiOx w’-TiOx

  20. 35 x 35 nm2 95x80 nm2 templating effect: long range order w-TiOx w’-TiOx z’-TiOx 31 x 31 nm2 35 x 35 nm2 200x200nm2 100x100 nm2

  21. 35 x 35 nm2 30 x 14 nm2 95x80 nm2 templating effect: long range order w-TiOx w’-TiOx z’-TiOx 31 x 31 nm2 35 x 35 nm2 200x200nm2 100x100 nm2

  22. 35 x 35 nm2 templating effect: size distribution w-TiOx w’-TiOx z’-TiOx 31 x 31 nm2 35 x 35 nm2 200x200nm2 100x100 nm2

  23. Transformation from z’ to w TiOx phase ! 110x80 nm2 FT Au clusters on z’-TiOx.-phase Annealing in UHV @ 600 K for 20’ Au clusters form an hexagonal pattern with an average lattice of 1.8 nm

  24. w-TiOx w’-TiOx transformation 150x120 nm2 Au clusters on z’-TiOx.-phase: increasing the temperature Further annealing 30’ @ 700 K UHV

  25. No CO2 desorptions No Catalytic activity of w’-TiOx phase w’-TiOx phase without Au particles reproducible CO2 desorptions @ -140°C Catalytic activity of Au particles? w’-TiOx phase with 5’ of Au (~0.16 ML) Preliminary tests by using Thermal Programmed Desorption (TPD) on the catalytic conversion of CO  CO2 @ low T exposition: first 12L of O2, second 20L of CO @ -155°C w’-TiOx phase with 5’ of Au (~0.16 ML) after annealing @ 800K for 15’ No CO2 desorptions No Catalytic activity of Au 2D islands Experiments are in progress with 18O2

  26. Conclusions and perspectives -A rich panorama of TiOx 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. -The understoichiometric TiOx/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 annealingwhich determines the change of the template itself -Catalytic tests are in progress to evaluate the dependence of the chemical properties of the Au clusters as a function of their dimensions

  27. Collaboration and acknowledgments 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) Funding: PRIN 2005, EU VI PQ- NMP-Priority

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