Ab initio thermodynamic study of the SnO₂(110) surface in an O₂ and NO environment: a fundamental understanding of the gas sensing mechanism for NO and NO₂

Abstract

For the purpose of elucidating the gas sensing mechanism of SnO 2 for NO and NO 2 gases, we calculate the phase diagram of SnO 2 (110) surface in contact with an O 2 and NO gas environment by means of ab initio thermodynamic method. Firstly we build a range of surface slab models of oxygen pre-adsorbed SnO 2 (110) surfaces using (1×1) and (2×1) surface unit cells and calculate their Gibbs free energies considering only oxygen chemical potential. The fully reduced surface containing the bridging and in-plane oxygen vacancies in the oxygen-poor condition, while the fully oxidized surface containing the bridging oxygen and oxygen dimer in the oxygen-rich condition, and the stoichiometric surface in between, were proved to be most stable. Using the selected plausible NO-adsorbed surfaces, we then determine the surface phase diagram of SnO 2 (110) surfaces in (∆µ O , ∆µ NO ) space. In the NO-rich condition, the most stable surfaces were those formed by NO adsorption on the most stable surfaces in contact with only oxygen gas. Through the analysis of electronic charge transferring and density of states during NO x adsorption on the surface, we provide a meaningful understanding about the gas sensing mechanism.

Atomistic Descriptions of Gas-Surface Interactions on Tin Dioxide

“…Although a different study reports no O-down geometries [18], both studies agree that the most stable adsorption configuration is N-down onto a V br with an adsorption energy of about 1.2 eV. Analysis of the charge density transfer reveals that, in terms of the influence on the electronic structure, NO can act not only like a reducing gas but also like an oxidising one [18]. While adsorption onto O br results in a donor-like NO + state, the same molecule adsorbing onto a V br results in electron density being withdrawn from the surface and the formation of an NO -.…”

“…However, when present at the surface, the V br is again the preferred adsorption site. Although a different study reports no O-down geometries [18], both studies agree that the most stable adsorption configuration is N-down onto a V br with an adsorption energy of about 1.2 eV. Analysis of the charge density transfer reveals that, in terms of the influence on the electronic structure, NO can act not only like a reducing gas but also like an oxidising one [18].…”

“…Large SnO 2 single crystals, which are notoriously challenging to grow, preferentially expose the (110), ( 101) and (100) surfaces [26]. Of these three, the (110) was shown to have the lowest energy and is the subject of most computational adsorption studies [17,18,[27][28][29][30][31], though some results are also available for the (101) surface [32,33]. Both surfaces feature bridging oxygen sites (O br ), which protrude outwards from the surface determined by the plane of Sn atoms.…”

“…The former approach may be used to, for example, estimate the equilibrium density of oxygen adsorbates on O 2 or predict sensor behaviour in varying oxygen pressure [7,15]. The latter is essential in discovering possible adsorbate species and investigating their effects on the surface [17,18]. However, analysis of sensors working under conditions close to ambient has become increasingly more available with the advent of operando spectroscopy.…”

Metallicity at interphase boundaries due to polar catastrophe induced by charge density discontinuity

“…35 That of SnO 2 is obtained by removing the outermost O atoms while maintaining the bridging O atoms, 36 and that of SnO is obtained by removing half of the surface atomic species. In principle, therefore, polar-polar, nonpolar-nonpolar and polar-nonpolar (110) interfaces can be formed between SnO and SnO 2 .…”