Free Energy and Electronic Properties of Water Adsorption on the SnO₂(110) Surface

Abstract

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

Ambient Humidity Influence on CO Detection with SnO2 Gas Sensing Materials. A Combined DRIFTS/DFT Investigation

“…In the case of defective (101) and (110) surfaces, the molecular adsorption is enhanced above a vacancy thanks to shorter H-bonds that can be formed between the water molecule and the O O atoms placed in close vicinity 34,43 . Noteworthy, we observed a molecular adsorption (Fig.…”

“…) because of the formation of a H-bond between the dissociated H atom coming from the water molecule and the dissociated OH adsorbed on Sn atom (d(H bond)= 1.79 Å)34,35,43,44 . Here the two bonds between Sn-Oo are slightly enlarged, from 2.00 Å to 2.16 Å but not broken.…”

Sub-wavelength waveguide properties of 1D and surface-functionalized SnO2 nanostructures of various morphologies

“…OTS molecules are more reactive with –OH groups on the metal oxide surfaces [31–33]. Molecular dynamics simulations revealed that the O B site of SnO 2 was the most preferable site for the formation of –OH groups [34–35]. Thus, OTS binds dominantly at O B sites over the O P sites of SnO 2 NPs.…”

“…8). DFT calculations using the generalized gradient approximation (GGA) indicated that the luminescence transitions around 2 and 2.4 eV belonged to bridging and in-plane oxygen vacancies, respectively [34–35 37–38]. Similarly Liu et al [37] demonstrated that the in-plane oxygen vacancies were responsible for the creation of shallow donor states while the bridging oxygen vacancies were related to relatively deeper donor states.…”

Surface and grain boundary energies of tin dioxide at low and high temperatures and effects on densification behavior

“…On the other hand, other authors have reported that for the (110) surface, reduced conditions will benefit dissociative behavior. 28 This is because upon exposure to water molecules, the reduced state is first oxidized. This scenario suggests that reduced states would have higher heats of adsorption as the energy for oxidation is exothermic, and contrasts with the data we have acquired.…”