State-selective dissociation of a single water molecule on an ultrathin MgO film

Shin, Hyung‐Joon; Jung, Jaehoon; Motobayashi, Kenta; Yanagisawa, Susumu; Morikawa, Yoshitada; Kim, Yousoo; Kawai, Maki

doi:10.1038/nmat2740

Cited by 178 publications

(195 citation statements)

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“…In this regard, the relative stability of molecularly and dissociatively bound species can be of critical importance with the preferred configuration being controlled by many factors including surface structure, acid/base properties, defects, impurities, water coverage, and temperature (8)(9)(10)(11)(12)(13). For oxides in particular, the relative stability of molecularly and dissociatively bound water has been widely debated even on the simplest, low-index surfaces.…”

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confidence: 99%

Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)

Wang

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

107

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Understanding adsorbed water and its dissociation to surface hydroxyls on oxide surfaces is key to unraveling many physical and chemical processes, yet the barrier for its deprotonation has never been measured. In this study, we present direct evidence for water dissociation equilibrium on rutile-TiO 2 (110) by combining supersonic molecular beam, scanning tunneling microscopy (STM), and ab initio molecular dynamics. We measure the deprotonation/ protonation barriers of 0.36 eV and find that molecularly bound water is preferred over the surface-bound hydroxyls by only 0.035 eV. We demonstrate that long-range electrostatic fields emanating from the oxide lead to steering and reorientation of the molecules approaching the surface, activating the O-H bonds and inducing deprotonation. The developed methodology for studying metastable reaction intermediates prepared with a high-energy molecular beam in the STM can be readily extended to other systems to clarify a wide range of important bond activation processes.W ater is ubiquitous in the environment and, as such, the nature of its interactions with interfaces can determine the outcome of a broad range of processes that include wetting, dissolution, precipitation, phase transformation, corrosion, and catalytic and environmental reactions (1-7). In this regard, the relative stability of molecularly and dissociatively bound species can be of critical importance with the preferred configuration being controlled by many factors including surface structure, acid/base properties, defects, impurities, water coverage, and temperature (8-13). For oxides in particular, the relative stability of molecularly and dissociatively bound water has been widely debated even on the simplest, low-index surfaces.Here we focus on resolving the fundamental question of water binding on rutile TiO 2 (110), one of the most studied oxide surfaces, which is often used as a prototype for reducible oxide surfaces and a model for understanding photocatalytic water splitting (3,(14)(15)(16)(17). Interestingly, despite the overwhelming wealth of literature, the nature of water adsorption and dissociation on nondefect titanium sites has been disputed for decades and to date remains unsettled (3, 14-17). The underpinning difficulty in resolving this debate is that it is practically impossible to prepare stoichiometric TiO 2 (110) surfaces. As such, bridging hydroxyl groups formed by water dissociation in oxygen vacancy defects interfere with determining the extent of dissociation on regular Ti sites (3,(14)(15)(16)(17)(18)(19). A number of recent studies by a variety of techniques including X-ray photoelectron spectroscopy (XPS) (20), infrared reflection absorption (21), photoelectron diffraction (PhD) (22), and scanning tunneling microscopy (STM) (23-25) arrived at conflicting conclusions. Whereas the XPS and PhD studies concluded partial dissociation of water in the hydrogenbonded chains on Ti sites at higher coverages (20,22), others are in favor of molecular bonding (21, 25).To address the adsorpt...

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confidence: 99%

Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)

Wang

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

107

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“…Water structuring on the insulating surfaces is more intriguing but more challenging than that on metal surfaces because of the complicated nature of waterinsulator interaction 10,[14][15][16][17] . Salt is one of the most widespread insulating materials in our daily life and also one of the most abundant components of atmospheric particles.…”

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confidence: 99%

An unconventional bilayer ice structure on a NaCl(001) film

et al. 2014

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Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.

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“…These results clearly show that catalytic activity on ultrathin MgO films can be controlled by film thickness as indicated in the previous experimental results. [7][8][9][10]14 Furthermore, it should be noted that dissociation of an isolated water molecule on the 1-ML MgO film surface becomes thermodynamically exothermic independent of the presence of neighboring water molecules or the defects known to be prerequisites for water dissociation on the MgO͑100͒ bulk surface. 3,4 The exothermic dissociation energy and the relatively low barrier height for water dissociation on the 1-ML MgO film surface strongly support recent experimental findings that defect-free terrace sites on an MgO film can enhance overall chemical reactivity.…”

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

et al. 2010

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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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State-selective dissociation of a single water molecule on an ultrathin MgO film

Cited by 178 publications

References 47 publications

Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)

Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)

An unconventional bilayer ice structure on a NaCl(001) film

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

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State-selective dissociation of a single water molecule on an ultrathin MgO film

Cited by 178 publications

References 47 publications

Probing equilibrium of molecular and deprotonated water on TiO 2 (110)

Probing equilibrium of molecular and deprotonated water on TiO 2 (110)

An unconventional bilayer ice structure on a NaCl(001) film

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

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Product

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Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)

Probing equilibrium of molecular and deprotonated water on TiO ₂ (110)