Controlling water dissociation on an ultrathin MgO film by tuning film thickness

Abstract

Periodic density-functional theory calculations at the single-molecule level were used to study dissociation of water on ultrathin MgO films with varying thickness deposited on the Ag͑100͒ surface. The enhanced chemical activity for water dissociation on MgO/Ag͑100͒ originates from the greater stability of dissociated products, which is due in turn to the strong hybridization of their electronic states at the oxide-metal interface. Our results provide insights into the superiority of the monolayer MgO film surface over the bulk surface and the use of the film thickness to control heterogeneous catalysis in water dissociation. DOI: 10.1103/PhysRevB.82.085413 PACS number͑s͒: 68.43.Bc, 68.47.Gh, 82.65.ϩr The interaction of water with oxide surfaces has long been of fundamental importance to various fields of science, including geochemistry, electrochemistry, and catalysis. 1,2Understanding how water molecules dissociate on oxide surfaces is crucial not only for revealing various solid-liquid interface phenomena but also for controlling chemical reactions involving water dissociation. Water dissociation on the MgO͑100͒ bulk surface has so far been observed to occur only if the dissociation products are stabilized by defects or neighboring water molecules. [3][4][5][6] Compared with the MgO bulk surface, the chemical activity of water dissociation is remarkably enhanced on a single monolayer ͑ML͒ MgO film deposited on a Ag͑100͒ surface. [7][8][9][10] In studies using highresolution electron energy loss spectroscopy combined with x-ray photoemission spectroscopy, Savio et al. reported that the dissociation probability of water at room temperature on the 1-ML MgO film surface increases by a factor of 6 compared with a multilayer film. 7,8 They suggest that this remarkable enhancement in chemical reactivity originates in the border sites of the 1-ML oxide film in contact with a metal substrate. However, a theoretical study by Ferrari et al. does not support the role of the metal substrate at the border of the 1-ML oxide film as an explanation for the high chemical activity of the 1-ML MgO film compared with bulk MgO.11 In photoemission and Auger electron spectroscopy experiments, Altieri et al. found that both border and terrace sites enhance water dissociation compared with corresponding sites of bulk MgO.9,10 They suggest that the metal substrate beneath the terrace sites of MgO film plays a pivotal role in water dissociation. The discrepancy between these experimental results might be due to the difficulty in precisely controlling the number of defects, the size, and the thickness of the MgO thin film at a monolayer limit. 12,13 Although experimental studies so far have definitely demonstrated the superiority of ultrathin MgO film over bulk MgO for water dissociation, such details as the phenomenon's dependence on film thickness and the contribution of defect-free terrace sites as venues for chemical activity have not been clarified and demand systematic study. [7][8][9][10] In particular, Shin et al. have demons...

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