非受限条件下二维冰的形成及生长机制

Formation and growth of two-dimensional ices without confinement

受限和界面水/冰在自然界中无处不在,在岩石断裂、摩擦和纳米流体等领域发挥着重要作用.纳米级高度受限的环境可以破坏水中氢键网络,从而增加二维水/冰的可能多晶型物的数量.然而近期许多研究揭示了在不需要纳米级限制的情况下也可以形成二维冰的可能性,比如在石墨烯、Pt(111)表面、Ru(0001)衬底和Au(111)表面上都观察到二维双层六角冰(bilayer hexagonal ice,BHI)的生长.本文综述了非受限条件下,二维冰形成和生长的理论研究进展,着重阐述表面浸润性以及表面结构对二维冰形成所起的关键作用.在非受限条件下,BHI冰岛锯齿形边缘通过集体桥接机制生长.相比之下,扶椅形边缘的生长涉及局部播种和边缘重构.在应用方面,这种BHI的存在可以决定冰晶在亲水表面的取向,进而控制冰晶在表面的生长模式.

Confined and interfacial water/ice is ubiquitous in nature and plays an important role in a broad range of physicochemical phenomena and technological processes such as rock fracture,friction,and nanofluids.The highly confined environment at the nanoscale can disrupt the hydrogen bonding network in water,thereby increasing the number of possible polymorphs of two-dimensional(2 D)water/ice.It is also known that water molecules next to a solid surface can undergo stratification that extends two to three molecular diameters.A number of recent studies have revealed the possibility of forming 2 D ice without the need of nanoscale confinement.For example,the presence of an ice-like water layer has been detected in scanning polarization force microscopy on the hydrated muscovite surface at room temperature.A sum-frequency generation experiment revealed no free OH bonds in the adsorbed water layer on mica.Scanning tunneling microscopy imaging evidence of a helical ice monolayer in which every six water molecules are helically arranged along the normal to the basal plane has reported.In addition,the growth of 2 D bilayer hexagonal ice(BHI)structures has been widely observed on different surfaces,such as graphene,Pt(111)substrate,Ru(0001)substrate and Au(111)substrate,without the need of confinement in the laboratory.In this paper,we review the recent theoretical progress on the formation and growth of 2 D ices without confinement.In particular,we focus on the important role of surface wettability and surface structure in the formation of 2 D ices.We find that BHI formation on various surfaces and the dependence of the 2 D crystalline structure on the hydrophobicity and morphology of the underlying surface.The tendency toward the formation of BHI without confinement reflects a proper water-surface interaction that can compensate for the entropy loss during the freezing transition.Moreover,both experiments and simulations show that armchair-type edges coexist with the zigzag edges in BHI.In the case of zigzag edge of BHI whose growth processes follow a collective bridging mechanism,a periodic but unconnected array of pentagons is initially formed at the zigzag edge,and subsequent incoming water molecules collectively connect these pentagons,resulting in a 565-chain structure.The addition of further water molecules leads to the formation of a fully connected hexagon array.By contrast,armchair edge growth involves edge reconstruction into a periodic structure of 5756-type membrane rings.Addition of two water pairs converts the 575-type member rings to 656-type member rings.The 656-type member rings then grow laterally to form a 5656-type edge.Further,inserting one water pair into the hexagon of the5656-type edge leads again to the formation of a 5756-type edge.At low temperature,BHI forms only on the hydrophilic surface and not on the hydrophobic surface.Because of the close lattice match between the BHI and the ice Ihbasal face,the orientation of the ice crystal on the hydrophilic surface is dictated by the preexistence of the BHI.Ice growth is anisotropic.Among the three prism crystal faces of ice Ih,namely,basal face(BF),prism face(PF)and secondary prism face(SPF),the growth of the BF is the slowest.For the hydrophilic surface,because the SPF and PF are perpendicular to the solid surface,the ice crystal grows along the surfaces.By contrast,for the hydrophobic surface,an angle is observed between the solid surface and SPF,indicating that the ice crystal grows off the solid surface.