Structural, energetic and electronic properties of (100) surfaces for alkaline earth metal oxides as calculated with hybrid density functional theory

Andrew J Logsdail,David Mora-Fonz,David O Scanlon,C Richard A Catlow,Alexey A Sokol

doi:10.1016/j.susc.2015.06.012

Andrew J Logsdail, David Mora-Fonz + Show 3 more

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https://doi.org/10.1016/j.susc.2015.06.012

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Journal: Surface Science	Publication Date: Jun 23, 2015
Citations: 19	License type: cc-by

Affiliation: University College London

Abstract

We perform a systematic investigation of (100) surfaces for rocksalt-structured group 2 metal oxides, namely MgO, CaO, SrO and BaO, using GGA and Hybrid-DFT exchange-correlation functionals. We examine the structural, energetic and electronic properties of the surfaces, with a specific focus on the surface ionisation potential and band bending; the latter of which we quantify by examining the density of states as a function of depth from the system surface. We report structural and energetic results in-line with previous experimental work when we use the Hybrid-DFT method, and for the electronic structure we find inequivalent band bending for the valence and conduction bands, which results in reduced ionisation potentials and the closure of the band gap at the surface when compared to bulk systems. We also report downward bending of the conduction band for MgO that brings it below the vacuum potential, unlike previous theoretical investigations, and thus indicates an origin of the positive electron affinity found in the experiment.

Highlights

Metal oxides are of technological importance due to their applications in fields such as heterogeneous catalysis and microelectronics [1,2,3,4,5], where surface stability is a desirable material characteristic [5,6]
The structural, energetic and electronic properties of the bulk materials are taken from our recent work on the ionisation potential of bulk rocksalt-structured metal oxides [59], where these bulk observables are included in the Supporting information (SI)
ΔCB is considerably reduced on comparison to the other materials investigated, with values ranging through both negative (PBE0: −0.15 eV) and positive (PBESol: 0.24 eV); the former is the only example for BaO that we discover of downwards band bending in the CB, though it is worth noting that, for all materials, ΔCB is always highest when using the PBESol XC functional and so for BaO this exceptional positive value may be an artefact of the functional itself

Summary

Introduction

Metal oxides are of technological importance due to their applications in fields such as heterogeneous catalysis and microelectronics [1,2,3,4,5], where surface stability is a desirable material characteristic [5,6]. For alkaline metal oxides that form in a rocksalt (NaCl) crystal structure, the (100) surface is well studied owing to its stability and ease of experimental preparation [5,7,8]. CaO and SrO are known to be better catalysts for the Tishchenko reaction: the dimerisation of aldehydes to form esters [18,19]. This superiority may be related to the basicity of the lattice oxygens, as suggested from calculations for the etherification of glycerol where the reactivity is ordered BaO N SrO N CaO N MgO [20,21]

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