Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.
Langmuir. 2013 May 7;29(18):5487-99. doi: 10.1021/la400313a. Epub 2013 Apr 22.
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.
对 SnO2 表面上水吸附的分子理解对于这种金属氧化物的几种应用至关重要，包括催化和气体传感。我们使用动力学和静态计算的组合研究了 SnO2(110)表面上水的吸附，以深入了解室温下的反应机制。通过元动力学计算在低和高覆盖率下研究水在 SnO2 表面上的吸附和离解的反应动力学。通过 Mulliken 电荷分析和投影态密度分析研究相关孤立极小值中的电子结构。表面桥氧（Obr）被发现对水吸附起着决定性的作用，形成带根的羟基，带水的 H 原子。与 H 的键合显著改变 Obr 的电子构型，据推测导致 SnO2 表面的能带弯曲减少。自由能估计表明，在室温下清洁的 SnO2(110)表面上，同时发生缔合吸附和离解吸附，后者在热力学上是有利的。氧覆盖率强烈影响缔合和离解吸附的 H2O 之间的比例，在高氧覆盖率（氧化表面）下有利于缔合吸附，在低氧覆盖率（还原表面）下有利于离解吸附。孤立表面极小值的电子分析表明，两种不同的电子转移现象发生在表面上，这取决于水的吸附机制。讨论了这些发现对解释基于 SnO2 的气体传感器在水吸附时电导率变化的相关性。缔合吸附导致金属氧化物表面电子富集，离解吸附诱导表面电子耗尽。这两种机制都与 SnO2 与水相互作用时发生的电导率变化一致，导致对后者的交叉敏感性。理论结果为将现有的原子模型与实验数据相关联提供了基础，并为室温下表面反应事件提供了一致的描述。
