Proton-Transfer Mechanisms at the Water–ZnO Interface: The Role of Presolvation

The
dissociation of water is an important step in many chemical
processes at solid surfaces. In particular, water often spontaneously
dissociates near metal oxide surfaces, resulting in a mixture of H2O, H+, and OH– at the interface.
Ubiquitous proton-transfer (PT) reactions cause these species to dynamically
interconvert, but the underlying mechanisms are poorly understood.
Here, we develop and use a reactive high-dimensional neural-network
potential based on density functional theory data to elucidate the
structural and dynamical properties of the interfacial species at
the liquid-water–metal-oxide interface, using the nonpolar
ZnO(101̅0) surface as a prototypical case. Molecular dynamics
simulations reveal that water dissociation and recombination proceed
via two types of PT reactions: (i) to and from surface oxide and hydroxide
anions (“surface-PT”) and (ii) to and from neighboring
adsorbed hydroxide ions and water molecules (“adlayer-PT”).
We find that the adlayer-PT rate is significantly higher than the
surface-PT rate. Water dissociation is, for both types of PT, governed
by a predominant presolvation mechanism, i.e., thermal fluctuations
that cause the adsorbed water molecules to occasionally accept a hydrogen
bond, resulting in a decreased PT barrier and an increased dissociation
rate as compared to when no hydrogen bond is present. Consequently,
we are able to show that hydrogen bond fluctuations govern PT events
at the water–metal-oxide interface in a way similar to that
in acidic and basic aqueous bulk solutions.

水的解离是固体表面诸多化学反应中的关键步骤。具体而言，水常在金属氧化物表面附近自发解离，使得界面处形成H₂O、H⁺与OH⁻的混合体系。无处不在的质子转移（Proton-Transfer, PT）反应使得这些物种动态互变，但其背后的微观机制尚不明晰。本研究基于密度泛函理论数据，开发并应用了一种反应型高维神经网络势，以非极性ZnO(10̅10)表面作为典型模型体系，阐明液态水-金属氧化物界面处界面物种的结构与动态特性。分子动力学模拟结果表明，水的解离与复合过程通过两类质子转移反应实现：(i) 与表面氧化物及氢氧根阴离子之间的质子转移（“表面-PT”），(ii) 与邻近吸附态氢氧根离子和水分子之间的质子转移（“吸附层-PT”）。研究发现，吸附层-PT的反应速率显著高于表面-PT。对于两类质子转移反应，水的解离均由主导性预溶剂化机制调控：即热涨落会使吸附态水分子偶尔接受一个氢键，相较于无氢键存在的情况，此举可降低质子转移能垒并提升解离速率。综上，本研究证实，水-金属氧化物界面处的质子转移事件受氢键涨落调控，其调控机制与酸性、碱性本体水溶液中的情况高度相似。