gaetano granozzi francesco sedona (phd thesis)

Gaetano GranozziFrancesco Sedona (PhD thesis)

TiOx NANOSTRUCTURES ON TiOx NANOSTRUCTURES ON A MONOCRYSTALLINE Pt SUBSTRATEA MONOCRYSTALLINE Pt SUBSTRATE

Università degli Studi di Padova Dipartimento di Scienze Chimiche

OutlineOutlineMotivations

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 and Perspectives

- 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 investigationJ. Phys. Chem. B 2006, 110, 15359

-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

Motivations for studying ultrathin oxide filmsMotivations for studying ultrathin oxide films(up to ca. 10 monolayers)(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:Ultrathin Oxide films on metals:Methodology of preparationMethodology of preparation

Preparations in vacuo (UHV) to drive the growth towardspecific chemical composition, structure and morphology

Reactive deposition

metal Oxidant agent

(molecular oxygen,atomic oxygen, water,NO2)

oxide film

crystalline substrate

Deposition parameters to be optimised in orderto 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

TiOTiOxx nanostructures on Pt(111) 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

An

nealin

g O

An

nealin

g O

22 pressu

re (Pa)

pressu

re (Pa)

5x105x10 -4-4

1010 -5-5

1010 -8 -8 (U

HV

)(U

HV

)

0.40.4 0.80.8 1.21.2 ≥≥22

equivalent monolayer (MLE) equivalent monolayer (MLE)

k-k-TiOTiOxx z-z-TiOTiOxx

z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx

rect-rect-TiOTiO22 rect’-rect’-TiOTiO22

TiOx films on Pt(111) Summary of Data

Phase diagram: STM data

≥≥221.21.20.80.80.40.4

An

nealin

g O

An

nealin

g O

22 pressu

re (Pa)

pressu

re (Pa)


1010 -8 -8 (U

HV

)(U

HV

)1010 -5-5

5x105x10 -4-4


z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx


Chemical characterizationTiOx films on Pt(111) Results: XPS

≥≥221.21.20.80.80.40.4

An

ne

alin

g O

An

ne

alin

g O

22 pre

ss

ure

(Pa

) p

res

su

re (P

a)


10

10 -8

-8

(UH

V)

(UH

V)

10

10 -5-5

5x

10

5x

10 -4-4


z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx


inte

nsi

ty (

arb

. un

its)

472

472

468

468

464

464

460

460

456

456

452

452

binding energy (eV)

w-Ti0x

w'-Ti0x

z'-Ti0x

z-Ti0x

rect-Ti02

k-Ti0x

rect'-Ti02

458.6 eV

Chemical characterization

Ti2p

Strongly oxidized phases

higher BE peak

two chemically shifted

Ti2p component

TiOx films on Pt(111) Results: XPS

≥≥221.21.20.80.80.40.4

An

ne

alin

g O

An

ne

alin

g O

22 pre

ss

ure

(Pa

) p

res

su

re (P

a)


10

10 -8

-8

(UH

V)

(UH

V)

10

10 -5-5

5x

10

5x

10 -4-4


z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx


inte

nsi

ty (

arb

. un

its)

472

472

468

468

464

464

460

460

456

456

452

452

binding energy (eV)

w-Ti0x

w'-Ti0x

z'-Ti0x

z-Ti0x

rect-Ti02

k-Ti0x

rect'-Ti02

456.4 eV

Chemical characterization

Ti2p

two chemically shifted

Ti2p component

More reduced phases

lower BE peak

458.6 eV


higher BE peakTi sourrounded by oxygen

O/Pt interface

inte

nsi

ty (

arb

. un

its)

472

472

468

468

464

464

460

460

456

456

452

452

binding energy (eV)

w-Ti0x

w'-Ti0x

z'-Ti0x

z-Ti0x

rect-Ti02

k-Ti0x

rect'-Ti02

PtTiO

Pt

TiO

O

lower BE peak

Ti at the interface with the substrate

458.6 eV

456.4 eV


stacking assignment

An

nealin

g O

An

nealin

g O

22 pressu

re (Pa)

pressu

re (Pa)

5x105x10 -4-4

1010 -5-5

1010 -8 -8 (U

HV

)(U

HV

)

0.40.4 0.80.8 1.21.2 ≥≥22



z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx


Reduced phases obtained @ 10-5 Pa

TiOx films on Pt(111) analysis: structures and models

DFT calculations carried out byA. Fortunelli

(Pisa)

k-TiOx


+1V -1V

Pt

TiO

@ positive bias a honeycomb

habitus(not observed

experimentally)@ negative bias a kagomé habitus

(observed experimentally)

Ti2O3 stoichiometry

z-TiOx 011

PtTiO

Ti 4-fold oxygen coordinated : brighter

Ti 3-fold oxygen coordinated : darker


@positive bias

Ti6O8 stoichiometry

w-TiOx

Ti 4-fold oxygen coordinated : brighter


@positive bias

TiO1.2 stoichiometry

w-TiOx


@positive bias

TiO1.2 stoichiometry Ti 3-fold oxygen coordinated : darkerTi vacancy

An

nealin

g O

An

nealin

g O

22 pressu

re (Pa)

pressu

re (Pa)

5x105x10 -4-4

1010 -5-5

1010 -8 -8 (U

HV

)(U

HV

)

0.40.4 0.80.8 1.21.2 ≥≥22


epitaxially epitaxially oriented oriented

nanoparticlesnanoparticles


k-Ti2O3w-TiOxz-Ti6O8

x=1.5 x=1.33 x=1.2

Evolution of the stoichiometry of the reduced phases with the Ti dose and oxygen pressure

TiOx films on Pt(111) general trends

z’-Ti25O30

x=1.2

Gold Nanoparticles and catalysisGold Nanoparticles and catalysis

O2 activation

Au nanoparticles

overtitania seem to

beparticularly

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)



three different phases have been tested

An

nealin

g O

An

nealin

g O

22 pressu

re (Pa)

pressu

re (Pa)

5x105x10 -4-4

1010 -5-5

1010 -8 -8 (U

HV

)(U

HV

)

0.40.4 0.80.8 1.21.2 ≥≥22


z’z’-TiO-TiOxx

ww-TiO-TiOxx

w’-w’-TiOTiOxx

Au clusters on the TiOx phases templating effect

35 x 35 nm235 x 35 nm231 x 31 nm2

200x200nm2 100x100 nm2

95x80 nm2

w’-w’-TiOTiOxx z’-z’-TiOTiOxxw-w-TiOTiOxx

templating effect: long range order

35 x 35 nm235 x 35 nm231 x 31 nm2 30 x 14 nm2

200x200nm2 100x100 nm2

95x80 nm2


templating effect: long range order

35 x 35 nm235 x 35 nm231 x 31 nm2

200x200nm2 100x100 nm2


templating effect: size distribution

110x80 nm2

Au clusters on z’-TiOx.-phase

Au clusters form an hexagonal pattern with an average lattice of 1.8 nm

FT

Annealing in UHV @ 600 K for 20’

Transformation from z’ to w TiOx phase !

150x120 nm2

Au clusters on z’-TiOx.-phase: increasing the temperature

Further annealing

30’ @ 700 K UHVw-TiOx w’-TiOx transformation

exposition: first 12L of O2, second 20L of CO @ -155°C

No CO2 desorptionsNo 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)

No CO2 desorptionsNo Catalytic activity

of Au 2D islands

w’-TiOx phase with 5’ of Au (~0.16 ML) after

annealing @ 800K for 15’

Experiments are in progress with 18O2

Preliminary testsby using Thermal Programmed

Desorption (TPD)on the catalytic conversion

of CO CO2 @ low T

Conclusions and perspectives

-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 annealing which determines the change of the template itself

-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.

-Catalytic tests are in progress to evaluate the dependence of the chemical propertiesof the Au clusters as a function of their dimensions

Collaboration and acknowledgments Collaboration and acknowledgments

Funding:Funding: PRIN 2005, EU PRIN 2005, EU VI PQ- NMP-PriorityVI PQ- NMP-Priority

Internal Collaborators:Permanent: M. Sambi, A. Vittadini (theory), G. Andrea RizziNon permanent: S. Agnoli, P. Finetti, L. Artiglia

Present external collaborations:CNR of Pisa, Italy (A. Fortunelli) Univ. of Brescia (L. Gavioli)

gaetano granozzi francesco sedona (phd thesis)

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