Calcium Adsorption on MgO(100):  Energetics, Structure, and Role of Defects

Abstract

Abstract:The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (∼4 nm thick) were grown epitaxially on a 1 µm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of ∼1 × 10 12 islands/cm 2 . Ca adsorbs initially at defect sites with a very high heat of adsorption (∼410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at ∼0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at ∼1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. 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.

Experimental measurements of the energetics of surface reactions

“…The calorimetry results in Table 4 are probably the best understood for the case of Ca on MgO(1 0 0), since this calorimetry study [77] included complementary DFT calculations by our collaborators (Graeme Henkelman and Lijun Xu) [77,81]. These DFT calculations showed that terrace sites bind Ca very weakly but that steps and kinks bind Ca very strongly.…”

“…Here we will review in detail our studies of metal adsorption on ordered, 4-nm-thick MgO(1 0 0) thin films grown epitaxially on Mo(1 0 0) [24,68,70,71,[74][75][76][77][78]. These calorimetric measurements have allowed us for the first time to directly assess the energetic stability of metal atoms on a well-defined oxide surface and of metal atoms within metal nanoparticles supported on this oxide surface.…”

Tailoring band gaps of insulators by adsorption at surface defects: Benzoic acids on NaCl surfaces

“…Even on pristine NaCl͑100͒ surface, monatomic steps and kinks are also present inevitably. Indeed defects ͑vacancies, steps, etc.͒ are often decisive in chemical reactivity on insulator surfaces, [13][14][15] and they play an important role in both the adsorption geometry and the electronic structure. For example, a few experiments and theoretical studies suggest dissociative adsorption of water on NaCl͑100͒ in the presence of F s center.…”

Structural and electronic characterization of the MgO/Mo(001) interface using STM

“…37 19 If φ varies across the surface, large modulations in the image contrast occur due to the different contributions of the FERs to the electron transport. , 21 The fact that the oxide square pattern appears in the STM around the reported work-function values of the MgO/Mo system (3.3-3.9 eV) 25,38 This assumption is verified by measuring the bias position of FERs at different regions of the coincidence cell via dz/dV spectroscopy (Fig. 5b).…”