Chem120A+Notes+-+Charge+Separation+at+Interfaces

Chem120A+Notes+-+Charge+Separation+at+Interfaces - Charge...

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Charge Distribution at Interfaces
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The Surface Space Charge (Surface Dipole) • Free electrons spill out into vacuum and leave partial positive charge in the bulk to form a surface dipole layer. • For metals, the spatial extent is limited to the topmost layer of atoms at the interface. • For semiconductors or insulators, this dipole layer may be tens or hundreds of atomic layers into the bulk. • The electron density exhibits fluctuations on the bulk side (Friedel Oscillation).
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Spatial Extent of Surface Dipole Layer The Debye length increases when the bulk electron density decreases. This is why the surface dipole layer extends much deeper into semiconductors or insulators than in metals. - - - - - - - + + + + + + + + + + Surface Charge (-) Conduction band edge Fermi level Ionized donor x x=0 x=d Space charge region Region of space charge neutrality 0 2 the height of the effective potential energy at the interface relative to the bulk Fermi level the electron density in the bulk s bulk e s bulk e V d en V n εε = Debye length
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Surface Space Charge at Solid-Liquid Interface • The formation of Helmholtz layer affects the charge transfer at the solid-liquid interface. • The charged solid-liquid interfacial layer helps stabilize colloid systems. Electrode Electrolyte - - - - - - - - + + + + + + + + + + + + + + - + - - - - - - + + + + + + + Helmholtz Layer Diffuse Layer Potential Distance to Electrode
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Work Function
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The Work Function Work function () x ce l e c t r F v bulk v E φ =+ Δ x Potential energy v eff (x) v elect (x) v xc (bulk) E F Δ v elect bulk vacuum The correlation between atomic ionization potential and the work function of solid Work Function Ionization Potential Element (eV) (10 -19 J/mole) (eV) (10 -19 J/mole) Lithium 2.9 4.6 5.392 8.639 Sodium 2.75 4.41 5.139 8.234 Potassium 2.30 3.69 4.341 6.955 Rubidium 2.16 3.46 4.177 6.692 Cesium 1.81 3.03 3.894 6.239
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Adsorption-Induced Change of Work Function Inert gas on metal Δ φ < 0 Alkali-metal on transition-metal Δ φ < 0 Molecule chemisorbed on metal ------- + - + + + + + + + + + + + + + + + - + + + + + + + + - Xe Na Rh Pd Rh C O
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Adsorption-site Dependence of Work Function Change The enthalpy change (a), the entropy (b), and the work function change (c) for xenon adsorption as a function of xenon coverage on the Pd(810) stepped crystal surface. The change of work function is greater when xenon is adsorbed on the step site.
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CO Adsorbed on Rh(111) 28 3 When 0.33, CO molecules occupy the top sites with 0.2 Debye, and 0.3 10 m . When 0.33, CO molecules begin to occupy the bridge sites with a larger surface-dipole moment than that on the CO CO CO CO θ μα < =− = × > top site.
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Na Adsorbed on Rh(111) 28 3 When 0.2, the 5.4 eV work function of Rh(111) deceases rapidly to 2.5 eV.
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This note was uploaded on 09/28/2009 for the course CHEM 120A taught by Professor Whaley during the Spring '07 term at University of California, Berkeley.

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Chem120A+Notes+-+Charge+Separation+at+Interfaces - Charge...

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