eew508 ii. structure of surfaces reconstruction – (2x1) reconstruction of si(100) the (2x1)...
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EEW508II. Structure of Surfaces
Reconstruction – (2x1) Reconstruction of Si(100)
The (2x1) reconstruction of Si (100) crystal structure as obtained by LEED crystallography. Note that the surface relaxation extends to three atomic layer into the bulk
EEW508II. Structure of Surfaces
(7x7) Reconstruction of Si (111)
LEED and STM image of (7x7) reconstructed structure of Si (111)The total number of dangling bonds is reduced from 49 to 19 through this reconstruction.
DAS structure: dimer, adatom, and stacking fault
EEW508II. Structure of Surfaces
(7x7) Reconstruction of Si (111)
19 dangling bonds of (7x7) reconstructed surface (12 adatom, 6 rest atom, 1 corner hole)
EEW508II. Structure of Surfaces
Reconstruction on metallic surface – Ir(100)
Bulk structure:the square latticeSurface structure: hexagonally close packed layer
(5x1) reconstruction
EEW508II. Structure of Surfaces
Reconstruction on metallic surface –Ir (110) missing dimer row
(2x1) reconstruction structure
EEW508II. Structure of Surfaces
Reconstruction – Ionic crystal
Ionic crystal consists of charged spheres stacked in a lattice.
Surfaces with strong chemical bonds exhibits more drastic rearrangement of surface atoms
Generally speaking, surfaces with weak chemical bonds (van der Waals, hydrogen, dipole-dipole and ion-dipole) exhibits less pronounced reconstructed structure -- for example, Graphite (0001) surface
EEW508II. Structure of Surfaces
EEW508II. Structure of Surfaces
Reconstruction of high-Miller-index surfaces
Roughening transition: If the surface is heated near the melting temperature, the steps become curved and break up into small islands
Reconstruction at Cu(410) stepped surface. Atoms in the first row at the each step become adatoms which are pointed out in the side view of the reconstructed surface.
III. Molecular and Atomic Process on Surfaces
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Structure of ordered monolayer
When atoms or molecules adsorb on ordered crystal surface, they usually form ordered surface structure over a wide range of temperature and surface coverages.
Two factors which decide the surface ordering of adsorbates areAdsorbate-adsorbate(AA) interaction and adsorbate-substrate(AS) interaction
Chemisorption – adsorbate-substrate interaction is stronger than adsorbate-adsorbate interaction, so the adsorbate locations are determined by the optimum adsorbate-substrate bonding, while adsorbate-adsorbate interaction decides the long-range ordering of the overlayer.
Physisorption or physical adsorption – AA interaction dominates the AS interaction –the surface could exhibit incommensurate structures.
III. Molecular and Atomic Process on Surfaces
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Coverage of adsorbate molecules
Definition of coverage: one monolayer corresponds to one adsorbate atom or molecules for each unit cell of the clean, unreconstructed substrate surface.For example, the surface coverage of atom on fcc(100) is one-half a monolayer.
III. Molecular and Atomic Process on Surfaces
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
Atomic oxygen on Ni (100)Up to one quarter of the coverage: Ni(100)-(2x2)-OBetween one quarter and one half Ni(100)-c(2x2)-O
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Ordering of adsorbate molecules
III. Molecular and Atomic Process on Surfaces
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
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Epitaxial Growth
With metallic adsorbates, very close packed overlayers can form because of attractive force among adsorbed metal atoms.When the atomic sizes of the overlayer and substrate metals are nearly the same, we can observe a one-monolayer (1x1) surface. This is called epitaxial growth.
III. Molecular and Atomic Process on Surfaces
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Adsorbate-induced reconstructuring
III. Molecular and Atomic Process on Surfaces
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Adsorbate induced restructuring – Ni (100) – c(2x2) - C
Carbon chemisorption induced restructuring of the Ni (100) surface. Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate.
III. Molecular and Atomic Process on Surfaces
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
EEW508
Adsorbate induced restructuring – Fe (110) – (2x2)-S
S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface.
III. Molecular and Atomic Process on Surfaces
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
Hydrogen: 1.7 atm.73 nm × 70 nm
Oxygen: 1 atm.90 nm × 78 nm
Carbon Monoxide: 1 atm.77 nm × 74 nm
“nested” missing-row reconstructions
fcc (111) microfacets
Unreconstructed (111) terraces separated by multiple height steps
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Adsorbate induced restructuring of steps to multiple-height step – terrace configuration
III. Molecular and Atomic Process on Surfaces
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Sulfur-chemisorption-induced restructuring of the Ir (110) surface
fcc(111) surface restructure more frequently upon chemisorption than do the closer-packed crystal faces.
III. Molecular and Atomic Process on Surfaces
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
EEW508
Penetration of atoms through or below the first layer
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
III. Molecular and Atomic Process on Surfaces
EEW508III. Molecular and Atomic Process on Surfaces
Growth modes of metal surfaces
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Growth modes of metal surfaces
Auger signal of adsorbate
Aug
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III. Molecular and Atomic Process on Surfaces
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Adsorption of CO on transition metal
CO is found to adsorb dissociatively on the early transition metals (to the left of the periodic table) and molecularly on the late transition.
III. Molecular and Atomic Process on Surfaces
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Adsorption of CO on transition metal
The preferred adsorption site of CO depends on three factors:The metal, the crystallographic face, and the CO coverage
Ni (111) face: CO occupies the bridge sites firstRh(111), Pt(111) the top sites are preferred at low coverages. The threefold site is occupied first on Pd(111).
III. Molecular and Atomic Process on Surfaces
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Adsorption of Ethylene on metal
Unsaturated hydrocarbon adsorption on clean transition metal is that it is mainly irreversible.
Once unsaturated hydrocarbon molecules are adsorbed on the surface, if the surface is heated, Then the adsorbed molecules will decompose to evolve hydrogen and leave the surface covered with the partially dehydrogenated fragments or carbon.
III. Molecular and Atomic Process on Surfaces
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Desorption of Ethylene on metal
Thermal desorption of hydrogen from chemisorbed ethylene on Rh(111) due to thermal dehydrogenation for several coverages.
To determine the structure and bonding of these various surface fragments, vibration spectroscopy or HREELS over a temperature range can be used.
III. Molecular and Atomic Process on Surfaces
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Case study – graphene on Pt(111) surface
The clean surface was then exposed to ethylene at room temperatureby backfilling the chamber with ethylene. Exposures were typically greater than 10 Langmuir to ensure saturationof the Pt(111) surface.
After exposure, the sample was heated to about 1250 K, resulting in the decomposition of ethylene and formation of a single monolayer of graphite on the Pt(111) surface.
M. Enachescu et al. Phys. Rev. B. 60 16913 (1999). AFM image of moiré superstructure.
Image size is 10 nm310 nm.
III. Molecular and Atomic Process on Surfaces
Adsorption of ethylene on Rh(111) and Pt(111)
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(a) Vibrational spectra from chemisorbed ethylene on Rh(111) at different temperature obtained by HREELS.
(b) SFG (Sum frequency generation) spectroscopy revealing di- bonded ethylene at 202 K on Pt(111),
(c) ethylidyne at 300K on Pt(111).
III. Molecular and Atomic Process on Surfaces
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Formation of ethylidyne (CCH3 at high temperature (> 220K)
Bonding geometry of ethylidyne on the Rh(111) and Pt(111) crystal surface.
III. Molecular and Atomic Process on Surfaces
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Ethylidyne-chemisorption-induced restructuring of the Rh(111) surface
Metal-metal distances expand for those Rh atoms that bind to the carbon of the ethylidyne molecule located in the three fold site.
Rh atoms in the second layer moves also upwards, closer to the organic molecules.
III. Molecular and Atomic Process on Surfaces
SFG (Sum-frequency generation) vibrational spectroscopy
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Physics Today, Somorjai and Park, Oct (2007)
III. Molecular and Atomic Process on Surfaces
Schematic of SFG (Sum-frequency generation) vibrational spectroscopy system
EEW508III. Molecular and Atomic Process on Surfaces
Detection of reaction intermediates on Pt(111) with SFG
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SFG spectrum of the Pt(111) surface during ethylene hydrogenation
The spectrum was measured with 100 Torr of H2, 35 Torr of C2H4, and 615 Torr of He at 295 K
III. Molecular and Atomic Process on Surfaces
(100 x 100) Å2 STM images of the Pt(111) surface underdifferent pressures: (a) 20 mtorr H2, (b) 20 mtorr H2 and 20 mtorrethylene, and (c) 20 mtorr H2, 20 mtorr ethylene, and 2.5 mtorr CO.The presence of CO induced the formation of a (19 x 19) R23.4structure on the surface. (d) (200 200) Å2 STM image showing tworotational domains of ( 19 19)R23.4.
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High pressure STM and surface mobility – ethylene on Pt(111)
III. Molecular and Atomic Process on Surfaces
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High pressure STM and surface mobility – ethylene on Rh(111)
(100 x 100) Å2 STM images of the Rh(111) surface underpressures of (a) 20 mtorr H2 and (b) 20 mtorr H2 and 20 mtorr ethylene.(c) (50 50) Å2 STM image of c(4 2)-CO + C2H3 structure formedat 20 mtorr H2, 20 mtorr ethylene, and 5.6 mtorr CO, and (d) aschematic showing the proposed
III. Molecular and Atomic Process on Surfaces
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Reactivity of ethylene hydrogenation – with and without CO
Turnover rate during ethylene hydrogenation on Pt(111)
III. Molecular and Atomic Process on Surfaces
Detection of reaction intermediates on Pt nanoparticles with SFG
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In situ monitoring of nanoparticles by high-pressure SFGspectroscopy. NP: nanoparticle
III. Molecular and Atomic Process on Surfaces
Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), 6150-6155
TEM images of a Langmuir-Blodgett film of 10 nmplatinum cubes (a) before and (b) after 2 h of UV-ozone treatment
UV/Ozone cleaning removes the organic capping layers of nanoparticles
EEW508III. Molecular and Atomic Process on Surfaces
SFGVS spectra of a Langmuir-Blodgett film of 10 nm
TTAB-capped platinum cubes.
UV/Ozone cleaning removes the organic capping layers of nanoparticles
EEW508III. Molecular and Atomic Process on Surfaces
SFG spectra of a drop-cast film of a 10 nm TTAB-capped platinum cube under ethylene hydrogenation conditions. The spectrum shows contributions from ethylidine and di-σ-bonded ethylene adsorbates. A very small contribution from the intermediate π-bonded speciesis also visible at 760 Torr and 298 K.
Detection of reaction intermediates on Pt NP with SFG
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Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), 6150-6155
III. Molecular and Atomic Process on Surfaces
EEW508Molecular and Atomic Process on Surfaces
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Surface structure of alloy, AlCu
Cu84Al16 alloy (111) structure exhibiting 3 x 3 R30o
The surface composition is 50%
“Introduction to Surface Chemistry and Catalysis”G. A. Somorjai and Y. Li
III. Molecular and Atomic Process on Surfaces