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Document Type |
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Article In Conference |
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Document Title |
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The Structures and Adsorption Energies of Ca and Li on the MgO(100) Surface The Structures and Adsorption Energies of Ca and Li on the MgO(100) Surface |
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Subject |
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كيمياء |
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Document Language |
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English |
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Abstract |
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The adsorption of Ca and Li on the MgO(100) surface at 300 K has been studied
using microcalorimetry, in combination with LEED, AES, ISS, work function,
sticking probability measurements. The MgO(100) thin films (~4 nm thick) were
grown epitaxially on a 1 m thick Mo(100) single-crystal. The sticking probabilities
of Ca and Li on MgO(100) at 300 K are near unity. Initially, both metals adsorb
strongly at defect sites, estimated to cover ~7-9% of the surface, with a high heat of
adsorption (~425 kJ/mol for Ca, ~260 kJ/mol for Li). Based on comparison of this
energy for Ca to DFT calculations at different sites, these defects initially are kink
sites, and then possibly fill in along the steps. A sharp initial decease of the work
function by ~1 eV at low Ca coverages suggests that the adsorbed Ca is cationic,
donating electron density to the MgO substrate. For Li, work function measurements
yielded a change with coverage typical for alkali metal adsorption on late transition
metals, with a 2.2 eV initial decrease associated with cationic Li adatoms, followed by
an increase back to the value for bulk Li metal as the Li depolarizes at higher
coverages due to dipole-dipole repulsions in the Li adlayer.
Both AES and ISS measurements demonstrate that Ca grows mainly as 3D
particles on MgO(100) with a density of ~1x1012 islands/cm2, while Li initially wets
on MgO(100) up to 0.5 ML, after which 3D islands of Li grow. The heat of adsorption
for both metals decreases rapidly with coverage, reaching a minimum of 163 kJ/mol
for Ca and 149 kJ/mol for Li at ~0.2 ML. Afterwards, the heat increases to the metal’s
bulk heat of sublimation (179 kJ/mol for Ca, 159 kJ/mol for Li) by 1.2 ML, attributed
to the increase in stability with increasing metal particle size.
Sputtering the MgO(100) surface with Ar+ ions generates defects that have
different effects on Ca and Li adsorption. For Ca adsorption, light ion sputtering of
the MgO(100) surface generates point defects, but these do not change the heat of
adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen
vacancies are a likely candidate; DFT calculations show that F and F+ center
vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates
extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up
to ~ 400 kJ/mol, similar to that at the intrinsic defect sites. However, for Li adsorption,
the defects created by Ar+ ion beam damage adsorb Li more strongly than MgO(100)
terraces. The more defects created, the higher the initial heat of adsorption of Li on
MgO(100). |
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Conference Name |
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Tenth International Symposium on Heterogeneous Catalysis |
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Duration |
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From : 22/8/1429 AH - To : 26/8/1429 AH
From : 23/8/2008 AD - To : 27/8/2008 AD |
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Publishing Year |
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1429 AH
2008 AD |
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Article Type |
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Article |
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Added Date |
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Saturday, February 14, 2009 |
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