Surface Chemistry of Materials

1. Surface Chemistry of Materials • Introduction to Surfaces • Why are surfaces different from the bulk? • Why we need a vacuum (no Hoover jokes, please) • Methods for probing surfaces • Adsorption of gases • Adsorption isotherm • Surface Area • Adsorption from solution

2. L[m] 1 10-3 10-6 macroscopic regime 10-9 mesoscopic regime t[s] 1 10-12 10-9 10-6 10-3 microscopic regime Surfaces

3.  Computational surface science Construction of models Experimental surface science Surface Science methods AB+C=>AC+B

4. Atoms at a surfaceare low-coordinate relative to the bulk Surface atom, 5 bonds to nearest neighbors vacuum Surface Bulk Bulk atom, 6 bonds to nearest neighbors

5. Unused surface bonds can interact, causing change in surface structure Surface dimerization

6. Reconstruction of Si(100)

7. Redistribution of Charge near surface sets up the Surface Dipole - - - - - + + + + + Bulk

8. Charge distribution at Surfaces electrons spill out from the surface

9. Surface relaxations at metal surfaces Smoluchowski smoothing at metal surfaces, Finnis and Heine, J. Phys. Chem. B 105, L37 (1973) The charge density will be redistributed at the surface such the charge is moved from the regions directly above the atom cores to the regions between the atoms. The atoms in the surface layer experience a charge imbalance. This give rise to an inward electrostatic force which leads to a compression of the separation between the surface layers.

10. Bulk

11. Surfaces A I II The surface break the 3D-periodicity of the bulk crystal Total energy of the system: GI+II=GI+GII+DGsurface

12. A g= 1 (GI+II(T,p)-SiNimi]) g= 1 (Esurf -Ebulk) A A Surface energy I II Gibbs free energy: G(T,p) = E-TS + pV=SjNjmj where the chemical potential is defined Surface energy g = Energy cost to create a surfaces Solids (low T): G(T,p) ~ G(0,0) ~ Etot

13. Work function is the extra energy needed to promote an electron from the HOMO (Fermi level) into the vacuum different for different surfaces e.g~ 4.3 eV, W ~ 5.3 eV, Pt EVacuum Work Function E EFermi

14. Work function Work function F surface dipole d + d - Potential difference Df=f ()-f (-)=4pd

15. Work function Work function F Chemical potential of the electrons m=E(N+1)-E(N)=EF Work function F=f ()-m = Df-m Potential difference Df=f ()-f (-)=4pd Lang and Kohn, PRB 1,4555(1970)

16. quiz ???? • Describe Surface atoms, its bonding and surface charge? • Describe Surface Gipps energy? • What is surface work function? • Where is the vacuum level fits? • Where is the Frmi level fits? • How surface atom overcome low coordination? • Complete figure 1 ??? E EFermi

17. Surface AnalysisUHV, XPS, TEM

18. UHV Technology Experimental surface science only possible in UHV. Reason: The surface composition should remain unchanged (clean) during the experiment. From kinetic gas theory it follows: Impingement rate: Mean free path: Molecular density: Monolayer formation time: Some important numbers:

19. Why do we need a vacuum? H2O CO2 hydrocarbons O2 • Atoms at the surface directly interact with gases in the environment • Rxns occur at the surface that don’t occur in the bulk • We need to control this

20. Typical Atom Surface Density: ~ 1015 atoms/cm2 Flux of atoms of mass M to this surface from the gas phase (F) is given by (at gas temperature T) : F (atoms/cm2-sec) = 3.51 x 1022 P(Torr) x [M(g/mole) T]-1/2 (Somorjai) Note: At P = 3 x 10-5 Torr, M = 28 gr/mole; T = 300 K F ~ 1015 atoms/cm2-sec. Thus, assuming a “sticking coefficient” of 1, the surface is covered by a fresh monolayer every second under a mild vacuum

21. Sticking Coefficient = probability/collision that an atom coming from the vacuum and colliding with the surface will stick! • Sticking coefficients are often small (e.g., N2 on Au) but can approach 1 for , e.g., N2 on clean W. • We need to keep surface contaminant concentrations low over the course of an experiment (~ 1 hour, say). Therefore, pressures ~ 10-9 or lower are required. • This is known as ultra-high vacuum (UHV). • Important: in measuring surface concentrations of adsorbed atoms, it is NOT pressure, but Pressure x Time [Exposure] that is important. • 1 Langmuir = 10-6 Torr-sec is the standard unit of exposure

22. Ultra-High-Vacuum (UHV) Technology Material: Take only low outgassing and temperature stable materials! Stainless steel (304), copper, aluminum, refractory metals (Ta, W, Mo) μ-metal, glass, ceramics, teflon, viton, capton Do not take: plastics, rubber, zinc plated steel, brass, glue, Pumping systems: Rotary pumps Cryosorption pumps Ion pumps Turbomolecular pumps Pressure gauges: Ion gauge (Bayard-Alpert) Thermocouple and Pirani

23. X-Ray Photoelectron Spectroscopy (XPS) In XPS core levels are excited, the spectrum reflects the energy levels of the atom. Therefore elemental characterization is possible. In addition to the photoelectrons there is a number of additional features in the spectrum, like continuous background, Auger peaks, plasmon losses. Furthermore, the cross section for excitation may be different for individual levels. Valence band electrons are only weakly excited. Qualitative evaluation of XPS spectra involves the comparison of spectra in the XPS-atlas. Quantitative evaluation can be done similarly to that described for AES. In general this method is more accurate for XPS, because less electrons are involve. Ni

24. High resolution XPS can yield a number of additional information: In particular the fine structure of the core levels, i.e. spin-orbit coupling can easily be seen. This splitting increases with binding energy. Furthermore, slight changes in the binding energies due to different chemical environment can be measured (typically 1 – 10 eV): Chemical shift. Different oxidation states will have different chemical shifts. The ability to investigate chemical composition is the reason for the name: ESCA The atomic environment on the surface normally differs from that in the bulk. Therefore, bulk and surface features are observed simultaneously. The surface sensitivity can be enhanced by grazing incidence light, and/or increasing the detection angle.

25. Transmission Electron Microscopy (TEM) The principle is the same as for optical microscopy, but using electron lenses. Due to the small de Broglie wavelength of high energetic electrons (100 keV  Δ≈2Å) the resolution is much higher. Due to the limited penetration depth the samples should be very thin: about 100 - 1000Å. In classical TEM metals were deposited on alkali halides, covered by a thin film of carbon and then the alkali halide substrate was removed by dissolving in water. In this way nucleation, growth and coalescence of metal islands can be studied. Furthermore, the surface structure of alkali halides can be studied by this step decoration method. Another method to obtain thin samples is by mechanical cutting, electrochemical etching and ion milling. Cross section of hetero-structures with atomic resolution can be studied. NaCl cleavage surface decorated with Au Si/TbSi2/Si double heterostructure

26. Figure 1 quiz What is UHV, why it is used for surface? What is XPS, principle and application? How is XPS different from XRD? What is TEM, how it is helpful for surface? What is in the figure 1 related to? What is in the figure 2 related to? What is in the figure 3 related to? Figure 2 Figure 3 Turbomolecular pumps

27. What is on the surface?Adsorption of gas

28. DEFINITIONS Adsorption: The uptake of gaseous or liquid components of mixtures from external and/or internal surface of porous solids. Adsorbate:Substance in the adsorbed state. Adsorptive:Adsorbable substance in the fluid phase. Adsorbent:Solid material on which adsorption occurs. Absorption:When the species of the adsorbate travel between the atoms, ions or the molecules of the adsorbent. Desorption: The process of removal of an adsorbed substance from the surface on which it is absorbed

29. Adsorption PHASE I ‘PHASE’ 2 Absorption (“partitioning”) PHASE I Henry’s Law PHASE 2

30. Causes of Adsorption • Dislike of Water Phase – ‘Hydrophobicity’ • Attraction to the Sorbent Surface • van der Waals forces: physical attraction • electrostatic forces (surface charge interaction) • chemical forces (e.g., - and hydrogen bonding)

31. AdsorptionPhenomenon The surface of a solid shows a strong affinity for molecules that come into contact with it. Certain solid materials concentrate specific substances from a solution onto their surfaces. • Physical adsorption (physisorption): • Physical attractive forces • (van der Waals forces) • e.g. Carbon ads, Activated alumina Adsorption Phenomenon • Chemical adsorption (chemisorption): the adsorbed molecules are held to the surface by covalent forces. • (little application in ww treatment)

32. Adsorption of gas

33. Adsorption Energy Activation barrier Ediss z Physisorption well Eads Chemisorption well

34. Thermodynamics for adsorption a ma Host Definition of adsorbate energy: Eads=DG=G[host+ads]-{G[host]+Nama} where G(T,p)= E-TS + pV=F+pV Ftrans, Frot, pV negligible for solids, but not in the gas phase The adsorbates vibrate at the surface: Fvib(T,w)=Evib (T,w)-TSvib (T,w) This gives the adsorption energy Eads={E[host+defect]+Fvib(T,w)}-{E[host]+Nama}

35. Thermodynamics for adsorption Convert the energy values of the chemical potential into T and p-dependence of the gas phase reservoir mi(T,pi)=mDFT+DG(T,p0)+ kTln(pi /p0) InterpolateDG(T,p0) from tables. Reuter and Scheffler, PRB 65, 035406 (2002). Eads(T,p)={E[host+defect]+Fvib(T)}-{E[host]+ma(T,pa)} The adsorbate concentration can be estimated in the dilute limit C=Nexp(-Eads/kT) where N is the number of adsorbtion sites

36. metal r’ r z z + - - + Taylor expand in terms of 1/z: Physisorption The electrostatic energy:

37. van der Waals interaction Cohesive energy for graphite as function of a- and c-lattice parameters. Calculated with GGA XC-functional Rydberg et al., Surf. Sci. 532, 606 (2003).

38. Physisorption of O2 on graphite h=3.4 Å DFT-GGA: Eads=0.04 eV/O2 TPD-experiment: Eads=0.12 eV/O2 Ulbricht et al.,PRB 66, 075404 (2002)

39. Chemisorption A gas molecular bond to surface molecule or atom or ion including charge transfer

40. Adsorption sites Top site Bridge site Hollow FCC-site Hollow HCP-site T B B F H H F T Close packed (111)-surface

41. e sp-band Weak chemisorption limit If the interaction between the substrate and the adsorbate is weak, i.e. Vak is small compared to the bandwidth of the substrate band. Ex for a sp-band. D is then independent of energy which means that L =0. The projected density of states for the adsorbate atom is then a Lorentzian with a width D, centered around ea | a > D

42. Strong chemisorption limit When the adsorbate interacts with a narrow d-band, then the ek can be approximated by center value ec such that the denominator in the Green’s function becomes: Solving this equation gives two roots corresponding to bonding and anti bonding levels of the absorbate system. e | a > d-band

43. Charge transfer • Gurney suggested that the atomic levels of a adsorbate atom would broaden and that there would be a charge transfer between the substrate and the adsorbate atom. • Charge would be donated to the substrate if the atom has low ionization energy and • charge would be attracted from the substrate if the atom has a high ionization energy. Gurney, Phys Rev. 47, 479 (1933)

44. ADSORPTION Summary

45. Figure 1 quiz What is adsorption, desorption, adsorbate, absorption, adsorbent? What cause adsorption to occur? What forces makes physical adsorption? What is chemical adsorption? What difference between physical and chemical adsorption? What is weak adsorption? What is strong adsorption? What is charge transfer? Complete gaps in figure 1? Energy ????????? Ediss z ????? well Eads ?????? well

46. Adsorption Isotherm

47. Adsorptive Equilibration in a Porous Adsorbent Pore Early Later Laminar Boundary Layer GAC Particle Equilibrium Adsorbed Molecule Diffusing Molecule

48. ADSORPTION ISOTHERMS Adsorption isotherm is the equation that relates the amount of a substance attached to a surface to its concentration in the gas phase or in solution, at a fixed temperature. • The process of Adsorption is usually studied through graphs know as adsorption isotherm. It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature. Different adsorption isotherms have been Freundlich, Langmuir and BET theory. • In the process of adsorption, adsorbate gets adsorbed on adsorbent.

49. According to Le-Chatelier principle, the direction of equilibrium would shift in that direction where the stress can be relieved. In case of application of excess of pressure to the equilibrium system, the equilibrium will shift in the direction where the number of molecules decreases. Since number of molecules decreases in forward direction, with the increases in pressure, forward direction of equilibrium will be favored. From the graph, we can predict that after saturation pressure Ps, adsorption does not occur anymore. This can be explained by the fact that there are limited numbers of vacancies on the surface of the adsorbent. At high pressure a stage is reached when all the sites are occupied and further increase in pressure does not cause any difference in adsorption process. At high pressure, Adsorption is independent of pressure. Basic Adsorption Isotherm

50. Langmuir Isotherm In 1916 Langmuir proposed another Adsorption Isotherm known as Langmuir Adsorption isotherm. This isotherm was based on different assumptions one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. Assumptions of Langmuir Isotherm Langmuir proposed his theory by making following assumptions. 1. Fixed number of vacant or adsorption sites are available on the surface of solid. 2. All the vacant sites are of equal size and shape on the surface of adsorbent. 3. Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released during this process. 4. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.