Shaping the Morphology of Gold Nanoparticles by CO Adsorption Keith McKenna * and Alex Shluger London Centre for Nanotechnology 17-19 Gordon Street London WC1E 6BT UK Department of Physics and Astronomy University College London Gower Street London WC1 0AH UK modelling the structure of nanoparticles at finite temperature and pressure * e-mail: [email protected]
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Shaping the Morphology of Gold Nanoparticles by CO Adsorption Keith McKenna* and Alex Shluger London Centre for Nanotechnology 17-19 Gordon Street London.
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Shaping the Morphology of Gold Nanoparticles by CO Adsorption
Keith McKenna* and Alex Shluger
London Centre for Nanotechnology17-19 Gordon StreetLondon WC1E 6BTUK
Department of Physics and AstronomyUniversity College LondonGower StreetLondon WC1 0AHUK
modelling the structure of nanoparticles at finite temperature and pressure
N. Lopez et al., Journal of Catalysis, 223 232 (2004)
S. V. Ryabtsev et al., Semiconductors 35 869 (2001)
Pd/SnO2
Experimental probes
• Scanning probes– STM, AFM...
– topographic and spectroscopic
– in situ rare
• Temperature programmed desorption– adsorbed molecules - coverage and
energy
– ex situ, non-equilibrium
• Photoelectron spectroscopy– direct probe of electronic structure
– indirect probe of morphology
STM - G. Yang et al - Surface Science 589 129 (2005)
Au
TPD - C. Lemire et al, Surface Science 552 27 (2004)
• Transmission electron microscopy– atomic resolution possible
– e.g. Pd NPs on MgO(100) exposed to oxygen and annealed
– interpreted in terms of O modified surface energies (Wulff construction – large particles)
– possible role of electrons (metallic clusters have positive electron affinity)
H. Graoui, S. Giorgio and C.R. Henry, Surface Science 417, 350–360 (1998)B. Pauwels et al., PRB 62(15) (2000)
Pd/MgO
O2 and annealAu/MgO
• X-ray Absorption Fine Structure (XAFS)
– probe local structure (coordination)
– timescale ~ 1-10Hz
– e.g. Pd and cycled CO/NO
– also used in situ IR spectroscopy
– not just oxidation → structural change
M. A. Newton et al, Nature Materials 6 528 (2007)
• IR spectroscopy
– e.g. Au/TiO2 in CO pressures
– appearance of additional IR band on increasing pressure
– persists to low pressure (hysteresis)
– flattening of particle shapeT. Diemant et al, Topics in Catalysis 44, 83 (2007)
It can be very difficult to uniquely interpret what is happening using a single technique
Theoretical models
• What is the favoured nanoparticle structure in vacuum?
– thermodynamic equilibrium
– empirical potentials (Sutton-Chen, TB-SMA...)
– can also include NP-support interactions
– DFT for small clusters (static)
– global minimisation (genetic, simulated annealing...)
– dynamics (MD - small timescales) http://www-wales.ch.cam.ac.uk/CCD.html
many-body attraction
short-range repulsion
• Nanoparticles of different symmetry preferred as a function of size (e.g fcc):
100
111
111
111
111
Icosahedron (strained in bulk)
OctahedronTruncated octahedron
Wulffconstruction
• Compare the energy of clusters with different symmetry
– average excess energy per surface atom
– e.g. Baletto et al, J. Chem. Phys. 116 3856 (2002)
– icosohedral - small N
– truncated octahedral - large N
bulk strain∝ N
Atomic scale dynamics
• Finite temperature– surface diffusion
– kinetic barriers (lower at surface)
– metals (E=0.1-0.6eV) (ms at RT)
– transient configurations
– low probability configurations may be important (t> 103 s)
• Monte Carlo approach– probability to find a given structure
(equilibrium)
– statistical distributions of properties
– average properties of set of configurations from NPT ensemble (P=0)
– surface atom trial move
– embedded atom model potentials
C. L. Cleveland et al, PRL 81 (1998)K. P. McKenna et al, J. Phys. Chem. C Lett. 111, 2823-2826 (2007)K. P. McKenna et al, J. Chem. Phys. 126, 154704 (2007)
Free Au nanoparticle
• Effect of temperature structure (P=0)– Au NP with 1152 atoms (size ~3nm)– magic number truncated octahedron– full exploration of configurational space
• Results
K. P. McKenna et al, J. Chem. Phys. 126, 154704 (2007)
Typical room temperature morphologyIncreasing concentration of low coordinated atoms with temperature (3C)
Roughening transition associated with (111) facets
surface melting may occur at lower T than roughening
phase transition
Size of (111) facets
Energy
Au NP supported on MgO(100) surface
• The system:– 1-2nm diameter
– Au binds to O preferentially
– 3% lattice mismatch
– Epitaxial structure
– N=181 - 191
– T=250K - 800K
B. Pauwels et al, PRB 62 (2000)K. P. McKenna et al, J. Phys. Chem. C Lett. 111, 2823-2826 (2007)
Expectation energyDiscontinuity in configurational contribution to specific heatSmall compared to vibrational and electronic contributionsSecond order phase transition
9C sitesCorrespond to ideal Au(111) facetsAlmost independent of temperature below 500KRapid decrease in size of ideal facets after 500KPhase transition is associated with roughening of the (111) facets
Au-MgO Interface layer8C sites in the interface layer are fully coordinatedProbability distribution indicate magic numbers nmAbove 500K also get appreciable non-magic numbersDisordered interface layer
7C interface layer sites correspond to the perimeter sitesThe number of these decreases sharply after 500K
Therefore the roughening transition is a complex one involving Au(111) facets and the perimeter of the Au-MgO interface layer
4C
3C
Increasing concentration of low coordinated atoms with temperature (3C)
Effects of pressure: CO and Au NPs
• Molecules– adsorb and desorb from the NP
– may also react - catalysis
– can change NP morphology
• Thermodynamic equilibrium– equilibrium of molecular
adsorption/desorption
– equilibrium of NP configuration
– very large configuration space to investigate
• Constrained equilibrium– consider various possible
structures
– for each look at equilibrium with ambient
– configuration with lowest Gibbs' free energy of adsorption is favoured
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statistical mechanics: equate chemical potentials of gas and adsorbed phase
K. P. McKenna et al, J. Phys. Chem. C Lett. 111 18848 (2007)
• CO on an Au nanoparticle– NP active for CO → CO2
– CO adsorbs in the top position
– increased adsorption for low coordinated sites (cluster study)
N. Lopez et al., Journal of Catalysis, 223 232 (2004)
L. M. Molina and B. Hammer 69 155424 (2004)
CO→CO2 (Au/MgO(100))
• 79 atom Au neutral cluster– agreement on truncated octahedron structure for
many different empirical models (SC, EAM, etc)– optimise using DFT