II. Structure of Surfaces EEW508 II. Structure of Surfaces Reconstruction – (2x1) Reconstruction of Si(100) The outmost layer of the surface goes through more dramatic reconstruction. For semiconductor surfaces (Si, GaAs, Ge etc), which are covalently bonded, dangling bonds are created by the loss of nearest neighbors. This leads to more drastic rearrangement of surface atoms. Therefore, most semiconductor surfaces reconstruct, and major rebonding between surface atoms takes place. The number of broken bonds can be reduced. This process is called rehybridization. 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

II. Structure of Surfaces EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) DAS structure: dimer, adatom, and stacking fault 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.

II. Structure of Surfaces EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) 19 dangling bonds of (7x7) reconstructed surface (12 adatom, 6 rest atom, 1 corner hole)

II. Structure of Surfaces EEW508 II. Structure of Surfaces Reconstruction on metallic surface – Ir(100) (5x1) reconstruction Bulk structure:the square lattice Surface structure: hexagonally close packed layer

II. Structure of Surfaces EEW508 II. Structure of Surfaces Reconstruction on metallic surface –Ir (110) missing dimer row Missing row structure: every other row of surface atoms is removed. Au, Pt, Ir (110) surfaces have the (1x2) reconstruction -- But Ag, Cu, and Ni do not. (2x1) reconstruction structure

II. Structure of Surfaces EEW508 II. Structure of Surfaces Reconstruction – Ionic crystal Major deviation from the bulk structure is the movement of the surface Na+ towards the bulk, which causes a 0.12 A corrugation of the surface layer. The shift of surface cations results in a large surface dipole. Ionic crystal consists of charged spheres stacked in a lattice.

II. Structure of Surfaces EEW508 II. Structure of Surfaces 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

II. Structure of Surfaces EEW508 II. Structure of Surfaces Reconstruction of high-Miller-index surfaces 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. The ordering of steps at surface is due to an excess of electron charge. This charge excess may be viewed as a dipole. The repulsive interaction between dipoles imposes ordering. The reconstruction of stepped surface is often dominated by the dipole-dipole interaction. Roughening transition: If the surface is heated near the melting temperature, the steps become curved and break up into small islands

III. Molecular and Atomic Process on Surfaces

Structure of ordered monolayer EEW508 III. Molecular and Atomic Process on Surfaces 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 are Adsorbate-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.

Coverage of adsorbate molecules EEW508 III. Molecular and Atomic Process on Surfaces Coverage of adsorbate molecules “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li 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.

Up to one quarter of the coverage: Ni(100)-(2x2)-O EEW508 III. Molecular and Atomic Process on Surfaces Ordering of adsorbate molecules “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li Repulsive force between adsorbates makes atoms or molecules separated as far from each other as possible. When the coverage increased, the mean inter-adsorbate distance decreases to about 0.5-1.0 nm. Atomic oxygen on Ni (100) Up to one quarter of the coverage: Ni(100)-(2x2)-O Between one quarter and one half Ni(100)-c(2x2)-O

EEW508 III. Molecular and Atomic Process on Surfaces 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.

Adsorbate-induced reconstructuring EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate-induced reconstructuring Adsorbate-induced reconstructuring: Strong adsorbate-substrate bond remove the relaxation or the reconstruction observed for clean surface. The substrate surface atoms usually return to their bulk-like equilibrium position. Furthermore, adsorbate can also induce a new surface restructuring. This is called adsorbate-induced restructuring.

Adsorbate induced restructuring – Ni (100) – c(2x2) - C EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring – Ni (100) – c(2x2) - C “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li Adsorbate-induced reconstructuring: Strong adsorbate-substrate bond remove the relaxation or the reconstruction observed for clean surface. The substrate surface atoms usually return to their bulk-like equilibrium position. Furthermore, adsorbate can also induce a new surface restructuring. This is called adsorbate-induced restructuring. Carbon chemisorption induced restructuring of the Ni (100) surface. Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate.

Adsorbate induced restructuring – Fe (110) – (2x2)-S EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring – Fe (110) – (2x2)-S “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface.

EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring of steps to multiple-height step – terrace configuration On Pt (110) surface, hydrogen induces “nested” missing-row reconstruction, oxygen (1atm) microfacet reconstruction, and CO (1atm) unreconstructed (111) terraces. The repulsive dipole-dipole interaction that keep the steps apart can become attractive due to adsorption leading to step cluster or aggregation.

Sulfur-chemisorption-induced restructuring of the Ir (110) surface EEW508 III. Molecular and Atomic Process on Surfaces Sulfur-chemisorption-induced restructuring of the Ir (110) surface “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li Open and rough surfaces reconstruct more readily upon chemisorption. fcc(111) surface restructure more frequently upon chemisorption than do the closer-packed crystal faces.

Penetration of atoms through or below the first layer EEW508 III. Molecular and Atomic Process on Surfaces Penetration of atoms through or below the first layer “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li Adsorption of atoms (Na, S, Cl) involves the occupancy of high-coordination sites on metal surfaces which allows bonding to many substrate atoms. Smaller atomic adsorbates (H, N, C, O) prefer the high coordination sites. Small size of these atoms allow penetration within even below the first substrate layer.

Growth modes of metal surfaces EEW508 III. Molecular and Atomic Process on Surfaces Growth modes of metal surfaces Franck-van der Merwe growth: The deposited metal forms a thin film in a layer-by-layer fashion. Stranski-Krastanov growth mechanism: Subsequent thin film growth is replaced with the formation of three-dimensional islands. Volmer-Weber growth mode: Three-dimensional islands form from the very beginning of metal deposition.

Growth modes of metal surfaces EEW508 III. Molecular and Atomic Process on Surfaces Growth modes of metal surfaces Auger signal of substrate Franck-van der Merwe growth: The deposited metal forms a thin film in a layer-by-layer fashion. Stranski-Krastanov growth mechanism: Subsequent thin film growth is replaced with the formation of three-dimensional islands. Volmer-Weber growth mode: Three-dimensional islands form from the very beginning of metal deposition. Auger signal of adsorbate

Adsorption of CO on transition metal EEW508 III. Molecular and Atomic Process on Surfaces Adsorption of CO on transition metal For the early transition metal (the left of the periodic table), metal surfaces increasingly tend to dissociate CO. Then the separate C and O atoms bonds directly to individual hollow sites, as in atomic adsorption. On other metal substrate, CO remains intact and bonds through its carbon end to the surface, with the C-O axis perpendicular to the surface. CO most commonly adsorbs in onefold coordinated top sites or in twofold bridge sites. Occasionally, threefold hollow adsorption sites can be found. CO is found to adsorb dissociatively on the early transition metals (to the left of the periodic table) and molecularly on the late transition.

The preferred adsorption site of CO depends on three factors: EEW508 III. Molecular and Atomic Process on Surfaces 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 first Rh(111), Pt(111) the top sites are preferred at low coverages. The threefold site is occupied first on Pd(111).

Adsorption of Ethylene on metal EEW508 III. Molecular and Atomic Process on Surfaces 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.

Desorption of Ethylene on metal EEW508 III. Molecular and Atomic Process on Surfaces 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.

Case study – graphene on Pt(111) surface EEW508 III. Molecular and Atomic Process on Surfaces Case study – graphene on Pt(111) surface The clean surface was then exposed to ethylene at room temperature by 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.

Adsorption of ethylene on Rh(111) and Pt(111) EEW508 III. Molecular and Atomic Process on Surfaces Adsorption of ethylene on Rh(111) and Pt(111) Vibrational spectra from chemisorbed ethylene on Rh(111) at different temperature obtained by HREELS. SFG (Sum frequency generation) spectroscopy revealing di- bonded ethylene at 202 K on Pt(111), ethylidyne at 300K on Pt(111). HREELS spectrum at 77K has been attributed to pi-bonded ethylene adsorbed molecularly intact on Rh(111) surface. The vibration frequency is different from those for gas-phase ethylene due to a strong interaction between ethylene and Rh surface. When the temperature is increased to above 220K, ethylidyne (CCH3) is formed. One hydrogen atom is eliminated to produce this segment. SFG study on Pt(111) show the ethylene molecularly adsorbed on surface through di sigma bonds at 202K, and at higher temperature, ethylidyne is formed.

Formation of ethylidyne (CCH3 at high temperature (> 220K) EEW508 III. Molecular and Atomic Process on Surfaces Formation of ethylidyne (CCH3 at high temperature (> 220K) Ethylidyne has the adsorption site on three-fold hollow site. One hydrogen is eliminated to produce this surface fragment. Bonding geometry of ethylidyne on the Rh(111) and Pt(111) crystal surface.

Ethylidyne-chemisorption-induced restructuring of the Rh(111) surface EEW508 III. Molecular and Atomic Process on Surfaces 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.

SFG (Sum-frequency generation) vibrational spectroscopy EEW508 III. Molecular and Atomic Process on Surfaces SFG (Sum-frequency generation) vibrational spectroscopy In SFG, the two-photon transition can only occur in a medium that has no inversion symmetry. The interface satisfies this condition, but the bulk and the isotropic gas or liquid phases do not. Therefore, SFG detects only those molecules adsorbed on the surface. Physics Today, Somorjai and Park, Oct (2007)

EEW508 III. Molecular and Atomic Process on Surfaces Schematic of SFG (Sum-frequency generation) vibrational spectroscopy system

Detection of reaction intermediates on Pt(111) with SFG EEW508 III. Molecular and Atomic Process on Surfaces Detection of reaction intermediates on Pt(111) with SFG 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 Reaction intermediates detected by sum frequency generation (SFG) vibrational spectroscopy during ethylene Hydrogenation on Pt(111) surface. Out of the three, p-bonded ethylene is the most weakly adsorbed, and that is the one that turns over, the other two species are stagnant spectators during the reaction turnover.

High pressure STM and surface mobility – ethylene on Pt(111) EEW508 III. Molecular and Atomic Process on Surfaces High pressure STM and surface mobility – ethylene on Pt(111) (100 x 100) Å2 STM images of the Pt(111) surface under different pressures: (a) 20 mtorr H2, (b) 20 mtorr H2 and 20 mtorr ethylene, 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) R23.4° structure on the surface. (d) (200 200) Å2 STM image showing two rotational domains of ( 19  19)R23.4°. Scanning tunneling microscopy (STM) is a unique surface analysis probe that permits atomically resolved imaging of adsorbed species over a broad pressure range of 10-10-103 torr. mtorr pressure range of both hydrogen and ethylene, the latter of which converts to mostly ethylidyne (C2H3) on the surfaces, the species adsorbed on the metal surfaces are very mobile. However, when CO is introduced into the system the catalytic reaction ceases abruptly and static ordered structures of the adsorbates are formed that can be readily imaged by STM

High pressure STM and surface mobility – ethylene on Rh(111) EEW508 III. Molecular and Atomic Process on Surfaces High pressure STM and surface mobility – ethylene on Rh(111) (100 x 100) Å2 STM images of the Rh(111) surface under pressures 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 formed at 20 mtorr H2, 20 mtorr ethylene, and 5.6 mtorr CO, and (d) a schematic showing the proposed

Turnover rate during ethylene hydrogenation on Pt(111) EEW508 III. Molecular and Atomic Process on Surfaces Reactivity of ethylene hydrogenation – with and without CO Turnover frequency (TOF) is defined as the number of ethane molecules generated per Pt surface atom per second. When CO is adsorbed, the TOF is reduced by 2–3 orders of magnitude, depending on the temperature. Carbon monoxide is known to form an incommensurate hexagonal overlayer on the surface, with a coverage of 0.60 ML, when the species is present in the mTorr range and above. Turnover rate during ethylene hydrogenation on Pt(111)

Detection of reaction intermediates on Pt nanoparticles with SFG EEW508 III. Molecular and Atomic Process on Surfaces Detection of reaction intermediates on Pt nanoparticles with SFG In situ monitoring of nanoparticles by high-pressure SFG spectroscopy. NP: nanoparticle

UV/Ozone cleaning removes the organic capping layers of nanoparticles EEW508 III. Molecular and Atomic Process on Surfaces UV/Ozone cleaning removes the organic capping layers of nanoparticles TEM images of a Langmuir-Blodgett film of 10 nm platinum cubes (a) before and (b) after 2 h of UV-ozone treatment Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), 6150-6155

UV/Ozone cleaning removes the organic capping layers of nanoparticles EEW508 III. Molecular and Atomic Process on Surfaces UV/Ozone cleaning removes the organic capping layers of nanoparticles SFGVS spectra of a Langmuir-Blodgett film of 10 nm TTAB-capped platinum cubes.

Detection of reaction intermediates on Pt NP with SFG EEW508 III. Molecular and Atomic Process on Surfaces Detection of reaction intermediates on Pt NP with SFG Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), 6150-6155 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 species is also visible at 760 Torr and 298 K.

Molecular and Atomic Process on Surfaces EEW508 Molecular and Atomic Process on Surfaces

Cu84Al16 alloy (111) structure exhibiting 3 x  3 R30o EEW508 III. Molecular and Atomic Process on Surfaces Surface structure of alloy, AlCu “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li Cu84Al16 alloy (111) structure exhibiting 3 x  3 R30o The surface composition is 50%