Pd nanoparticles for interconnects and catalyst mimics · 2012. 9. 13. · Cross sections represent...

Milano Summer School

Pd nanoparticles for interconnects and catalyst mimics

Geoff Thornton University College London


Circuit Board for Nanoscale Electronics


TiO2(110) circuit board


Memristor


Outline –  TiO2(110) structure

–  Pd nanoparticle growth on TiO2(110) with STM and XPEEM

–  CO on nanoparticles--orientation from NEXAFS

–  Orientation of CO on individual nanoparticles with XPEEM

–  Multiple domains of CO formed at 78 K that are fluxional and are not the same as structures formed on a single crystal surface


4.59 Å

2.96 Å

[001]

[010][100]

Rutile TiO2 unit cell


4.59 Å

2.96 Å

[001]

[010][100]

Rutile TiO2 unit cell

(110)


Surface x-ray diffraction

Measurements on ID32 / SCL at the ESRF

(200 nm)2

STM


Rutile TiO2(110)-1x1

•  Anisotropic surface unit cell: [001] = 0.296 nm [110] = 0.649 nm

•  [001] - alternating rows of bridging oxygen/5-fold titanium

•  “Nano-tracks” for adsorbate motion Obr Ti5f

Obr Ti5f

[001]

[110] 6.49 Å

2.96 Å

LEED: Lindsay, Wander, Ernst, Montanari, Thornton, Harrison, Phys. Rev. Lett. 94 (2005) 246102 SXRD: Cabailth, Lindsay, Torrelles, Bikondoa, Zegenhagen, Thornton, Phys. Rev. B, Rap. Comm. 75 (2007) 241403


STM of TiO2(110)

Bright rows are Ti rows

Bright defects are H (OHbr), lower defects are Ovac Bikondoa, Pang, Ithnin, Muryn, Onishi, Thornton, Nature Materials 5 (2006) 189

LEED: Lindsay, Wander, Ernst, Montanari, Thornton, Harrison, Phys. Rev. Lett. 94 (2005) 246102 SXRD: Cabailth, Lindsay, Torrelles, Bikondoa, Zegenhagen, Thornton, Phys. Rev. B, Rap. Comm. 75 (2007) 241403


Surface O vacancies formed by reduction (removing O)


5% monolayer (1 ML=no. surface unit cells) coverage of bridging oxygen vacancies

Formation of Surface vacancies by annealing in UHV


Reaction of vacancies with H2O gives two OH


• Vacancies involved in water dissociation

Brookes, Muryn, Thornton, Phys. Rev. Lett. 87 (2001) 266103 Shaub et al , Phys. Rev Lett. 87 (2001) 266104

Milano Summer School 16

O2 also reacts with vacancies

•  Purple crosses indicate vacancies filled after 300 K exposure to 0.6 L O2, to form new features on adjacent bright rows. These are highlighted with yellow crosses.

•  Bikondoa, Pang, Ithnin, Muryn, Onishi, Thornton,

•  Nature Materials 5 (2006) 189

•  Consistent with theoretical calculations of O2 interacting with vacancies:

•  Rasmussen, Molina, Hammer, J. Chem. Phys. 120 (2004) 988.


Vacancies are reactive centres for H2O and O2


Schematic representation of crystallographic shear (CS) planes intersecting the TiO2(110) surface. Blue: Ti atoms Rhombi: cross sections of Ti atoms in pseudo-octahedral coordination Cross sections represent a cut parallel to the (001) plane One of these CS planes is highlighted with a red dashed perimeter. CS rhombi in gray.

CS planes induce half-height steps at the TiO2(110) surface:

(a)  up-down half-height step arrangement

(b)  staircase half-height step arrangement.


STM images of the crystallographically sheared film:

(a) 170 x 210 nm2, 0.30 V, 0.90 nA.

(b) 150 x 180 nm2, 0.15 V, 0.80 nA.

Arrows indicate both the principal azimuths of the TiO2(110) surface and the directions of the CS plane intersections at the (110) surface.


LEED of CSP

  TiO2(110) spots in open circles

  Streaked spots with a short (reciprocal) separation correspond to the CSP periodicity (~3.5 nm) c.f. STM image and FFT later …


STM image of the crystallographically sheared film (36 x 18 nm2, 0.15 V, 0.85 nA).

FFT

A dashed guideline shows that the TiO2(110) 1x1 rows are in-phase across the half-steps.


CSP -induced Facetting

The facets F1-F4 are equivalent and correspond to {17 16 1} planes

360 nm × 200 nm


Anthoula C. Papageorgiou, Chi L. Pang, Qiao Chen, and Geoff Thornton, ACS Nano 1 (2007) 566

Differentiated STM image of the crystallographically sheared film (360 x 360 nm2, 0.50 V, 4.82 nA).


Nanoparticle preparation on TiO2(110)

•  Prepare clean TiO2(110)-1x1

•  Substrate held at 1000 K

•  Pd ⇒ metal vapour deposition

Characterisation

STM UCL

PEEM/LEEM Diamond Light Source


Scanning tunneling microscopy•  Substrate held at 1000 K, Pd ⇒ metal vapour depostion

•  Nanowires and quasi-hexagonal islands


Scanning tunneling microscopy•  Nanowires and quasi-hexagonal islands

•  Nanowires align in [001] direction

⇒ same as the surface O bridging rows.

D.S. Humphrey, G. Cabailh, C.L. Pang, C.A. Muryn, S.A. Cavill, H. Marchetto, A. Potenza, S.S.

Dhesi, G. Thornton, Nano Lett., 9 155 (2009).


Nanoparticle preparation •  Prepare clean TiO2(110)-1x1

•  Substrate held at 1000 K

•  Pd ⇒ metal vapour depostion

Characterisation

STM UCL

PEEM/LEEM Diamond Light Source


Diamond Light Source


Elmitec LEEM III SPELEEM •  UVPEEM •  XPEEM •  µ-XPS •  LEEM

Beamline I06: Nanoscience Diamond Light Source Ltd (DLS)

17º


Low energy electron microscopy •  Nanowires and islands •  Nanoparticles appear bright on the TiO2 substrate •  Nanowires up to 1 µm in length


X-ray photoemission electron microscopySecondary electron XPEEM

hν = 345 eV

•  Select hν around Pd M4,5 absorption edge at 349 eV –  Below edge, no contrast


X-ray photoemission electron microscopySecondary electron XPEEM

hν = 345 eV hν = 420 eV

•  Select hν around Pd M4,5 absorption edge at 349 eV –  Below edge, no contrast –  Above edge, reveals features containing Pd •  Below the Ti L2,3 edge


X-ray photoemission electron microscopyCore level imaging XPEEM

hν = 420 eV, secondary electrons hν = 420 eV, KE = 342.1 - 334.1 eV

•  scan the kinetic energy of collected electrons at fixed hν

•  spectral information obtained directly from features containing Pd

•  Peaks are observed at 340.1 eV and 334.7 eV.



hν = 420 eV, secondary electrons






hν = 420 eV, secondary electrons




Nano Letters 9 (2009) 155


Motivation—catalyst mimics •  Why study oxide-supported metal nanoparticles?

–  Heterogeneous catalysts used in industrial applications usually consist of nano-sized metal particles distributed on an oxide support.

–  Metal nanoparticles exhibit properties deviating from their corresponding single crystal surfaces: kinetic interplay between facets, spill-over of adsorbates between particles and supports, and the strong metal support interaction (SMSI).

–  Pd/TiO2 systems have been shown to be active for a wide range of reactions such as nitrite reduction in drinking water, acetylene hydrogenation, NOx reduction.


CO Adsorption on Nanoparticles

–  Pd nanoparticle growth at 800 K to form

quasi-hexagonal structures


Previous work •  Gas adsorption on well-defined Pd nanoparticles grown on Al2O3/NiAl(110) was also

studied: such as CO, O2, NO, 1,3-butadiene, interaction of H2 with CO and C2H4, decomposition of NO and methanol, and olefin hydrogenation.

•  Only one of the above was investigated at the atomic level by Hansen et al. (Surf Sci 505, 25 (2002))

NO ~ 5×10-5 Torr

(c) STM image (5 x 5 nm2) taken following NO adsorption on Pd(111). The red rectangle indicates the hcp/fcc hollow c(4x2)-2NO unit-cell. (d) The corresponding model of NO adsorbed on Pd(111)

(a) Pd islands on Al2O3/NiAl(110). (b) atomically resolved image showing the (111) top-facet of the Pd island


Max-lab – Results •  LEED pattern recorded following the 2nd Pd dose

on as-prepared TiO2(110), KE = 139 eV

•  Red – TiO2(110)-(1x1); Blue – Pd(111)-(1x1)

•  The interatomic spacing on the Pd islands was calculated to be 2.75 Å, very close to the interatomic spacing (2.752 Å) found on the Pd(111) single crystal

•  Pd 3d XPS taken after the 1st and 2nd Pd doses.

•  Binding energy of the Pd 3d5/2 peak shifts from 334.5 eV to 333.9 eV between the 1st and 2nd doses, indicating a stronger interaction between Pd and the TiO2 substrate when only a smaller amount of Pd is present


Max-lab – Results •  Ti 2p XPS were taken before and after Pd

deposition.

•  The relative amounts of Ti4+ and Ti3+ species were calculated by performing numerical fits to the region near the Ti 2p3/2 peaks (Table)

•  The effective thickness of Pd overlayer on the TiO2 substrate can be deduced from the decrease in the Ti 2p3/2 peak intensity

•  Following the 2nd dose, the effective thickness of Pd is ~1 nm (~ 4.6 MLE).

As-prepared

After 1st dose

After 2nd dose

%(Ti4+) 98.5 ± 1.6 97.1 ± 1.9 96.3 ± 7.5

%(Ti3+) 1.5 ± 0.1 2.9 ± 0.1 3.7 ± 0.4


Max-lab – Results •  Carbon K-edge (280-320 eV) NEXAFS were

performed to determine the bonding geometry of CO on the Pd islands grown on TiO2

•  NEXAFS recorded at incident angles between 90° and 30°

•  Normalisation of NEAXFS spectra involves subtraction of spectra before and after CO dose (2000 L)

•  π* peak decreases in size with decreasing angle of incidence

•  The experimental data (blue) were fitted with a combination of a Gaussian (π* peak) and a Step*Gaussian (the FL Step). By doing so, the area of the π* peak at different incident angle can be determined

E CO 2π*


Max-lab – Results

•  The polar bond angle of CO adsorb on the Pd islands is determined by incorporating the polar-dependent π* peak intensity into the following equations:

where P is polarization factor (=0.94), θ is incident angle, γ is the angle between CO axis and the surface normal.

•  From the numerical fit, γ is determined to be 8.0° ± 22.1°


Elmitec LEEM III SPELEEM •  UVPEEM •  XPEEM •  µ-XPS •  LEEM

Beamline I06: Nanoscience Diamond Light Source Ltd (DLS)

17º


Diamond Light Source •  Undulator “rotates” the polarization vector by moving its magnet arrays


XPEEM Results •  C K-edge measurements with

rotating polarisation vector –  The Pd/TiO2 surface was

exposed to ~100 L of CO at 300 K

–  NEXAFS measurements were performed with the E vector in horizontal and vertical plane

–  As observed in the NEXAFS plot, the π* peak lights up at the position of the Pd islands only when the horizontal beam is employed, indicating CO bonds vertically on the Pd islands

XPEEM images of the same area scanned above (a) the Pd-edge and (b) at the π* resonance the C-K edge in horizontal polarisation. 20 µm FOV

NEXAFS spectra taken from a Pd nanoparticle at (black) horizontal and (red) vectical polzarization vector. The π* peak lights up only when

horizontal beam is used, indicating CO bonds vertically on the Pd islands

Photon


STM


STM results of CO on Pd(111) single crystals •  By performing sequential CO dosing on

the Pd(111) surface at 120 K, different CO ordered overlayers are observed (MK Rose et al, Surf Sci 512, 48 (2002))

c(4x2)-2CO at ½ ML, (left) bridge-bridge and (right) hcp-fcc hollow configuration

2x2-3CO at ¾ ML. Calculations suggest that 1/3 of the CO occupy the atop sites (giving rise to image contrast), while the other 2/3 of the CO occupy the hcp and fcc hollow site (invisible) (Inset) atomic arrangement of CO molecules in the (2x2)-3CO unit.

/3x/3-CO at 1/3 ML. CO occupies 3-fold hollow sites Pd-Pd = 0.27 nm


STM •  CO-saturated Pd/TiO2(110) surface was scanned in STM at 300 K

(left) A STM image (size = 120x120 nm2) of the pseudo-hexagonal Pd(111) island, which is 45 nm wide and 5 nm tall. (right) A zoom-in image taken on top of the islands (15 x 15 nm2). In the image, CO molecules (bright dots) adsorb in a disordered fashion on the surface. Other adsorbates (possibly H2O), which appear as extra-bright features, are observed.


STM at 78 K

•  Using the same recipe to grow Pd islands on the TiO2 surface, shortly after which the Pd/TiO2 surface was saturated with 2000 L of CO at 300 K, then immediately moved to STM stage (78 K) for scanning

•  (a) STM at 78 K after Pd deposition and subsequent CO exposure, (250 nm)2 in size. Pd islands have pesudo-hexagonal shape, averaged diameter of ~30 nm and height of ~5 nm

•  (b) Pd island of interest •  (c) (10 nm)2 picture showing ordered domains of

CO •  (d) same image as (c) but band-filtered, to

highlight different CO domains


STM at 78 K


STM

•  Different CO domains

•  BLUE – top-bridge c(4×2)-2CO

•  GREEN – bridge-bridge c(4×2)-2CO

•  PINK – bridge-bridge c(4×2)-2CO, separated from GREEN in angle by 60°

570±10 pm 460 ±10 pm 530±10 pm

480 ±10 pm 570±10 pm

460 ±10 pm

Distances expected from bulk lattice constant—550 x 476 pm2


STM

Tip-induced domain-conversion

(a)-(b) bridge-bridge c(4×2)-2CO, stayed unchanged for >15 min

(b)-(c) tip-parameter induced change from bridge-bridge to top-bridge c(4×2)-2CO

(c)-(d) while keeping STM parameters untouched after (c), the region tends to go back to the original bridge-bridge type


STM


Summary

–  Pd nanoparticles—one dimensional and quasi-hexagonal islands on TiO2(110)

–  Pd nanowires formed up to 1 µm long of 5x5 nm cross-section along [001] for interconnects.

–  CO adsorbs in c(4x2)-2CO domains in two types of site, only one of which is found on the surface of the bulk (111) termination. CO bonds along the surface normal.

–  Domains walls are mobile after displacement at 78 K exposing new reaction sites for eg oxidation


Co-workers •  University College London

–  Chi Ming Yim

–  Coinneach Dover

–  Anthoula Papageourgiou

–  Qiao Chen

–  Chi Pang

–  Greg Cabailh

–  John Venables

•  University of Manchester –  Chris Muryn

•  MAX Lab –  Karina Schulte

–  Alexel Zakharov

–  Jesper Andersen

•  Diamond Light Source –  Francesco Maccherozzi

–  Stuart Cavill

–  Sarnjeet Dhesi

Pd nanoparticles for interconnects and catalyst mimics · 2012. 9. 13. · Cross sections represent...

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