Formation of hot ice caused by carbon nanobrushes. II. Dependency on the radius of nanotubes

Assisted Self-Assembly of Nanoporous Ices via Carbon Nanomaterial Templates

Self-assembly is a widely used synthetic method in nanoscience to assemble well-organized structures. Self-assembly processes usually occur in a water solvent environment. However, the self-assembly of water molecules is rarely studied. Herein, we show a strategy to fabricate porous ice via carbon nanomaterial-assisted self-assembly. Diverse frameworks of nanoporous ice are formed by using orthorhombic and tetragonal arrays of carbon nanotubes or carbon-atom chains as templates. In contrast to many bulk ices discovered in nature, nanoporous ices are shown to be stable only under negative pressure. Hence, nanoporous ices cannot be produced through the direct nucleation of water at negative pressure. The template-assisted self-assembly method is shown to be the most effective method to fabricate nanoporous ice in quantity. Several key factors for the self-assembly of nanoporous ices are identified, including proper gap spacings in the carbon nanomaterial template and suitable interactions between water and the carbon nanomaterials.

Nanoporous ices: an emerging class in the water/ice family

The history of scientific research on diverse ice structures dates back to more than a century. To date, 20 three-dimensional crystalline ice phases (ice I-ice XX) have been identified in the laboratory, among which ice XVI and ice XVII belong to a class of low-density nanoporous ices. Nanoporous ices can also be viewed as a special class of porous materials or water ice, as they possess a relatively high fraction of nano-cavities and/or nano-channels built into the hydrogen-bonded water framework. As such, like the prototypical class of porous materials (e.g., MOFs and COFs), nanoporous ices can be named as water oxygen-vertex frameworks (WOFs). Because of their large surface-to-volume ratio, WOFs may be potential media for gas storage, gas purification and separation. They may be applied to the biomedical field owing to their excellent biocompatibility. The field of porous ices is still emerging, as many porous ice structures that are predicted to be stable by computer simulations require future experimental confirmation. For future theoretical/computational studies, as the machine-learning method becomes an increasingly popular research tool in the material science and chemical science fields, more reliable porous ice structures and phase diagrams will be predicted with the development of more accurate machine-learning force fields.

Formation of porous ice frameworks at room temperature

Bulk crystalline ices with ultralow densities have been demonstrated to be thermodynamically metastable at negative pressures. However, the direct formation of these bulk porous ices from liquid water at negative pressures is extremely challenging. Inspired by approaches toward porous media based on host-guest chemistry, such as metal-organic frameworks and covalent organic frameworks, we herein demonstrate via molecular dynamics simulations that a class of ultralow-density porous ices with upright channels can be formed spontaneously from liquid water at 300 K with the assistance of carbon nanotube arrays. We refer to these porous ice polymorphs as water oxygen-vertex frameworks (WOFs). Notably, our simulations revealed that the liquid-WOF phase transition is first-order and occurs at room temperature. All the WOFs exhibited the unique structural feature that they can be regarded as assemblies of nanoribbons of hexagonal bilayer ice (2D ice I) at their armchair or zigzag edges. Based on density functional theory calculations, a comprehensive phase diagram of the WOFs was constructed considering both the thermodynamic and thermal stabilities of the porous ices at negative pressures. Like other types of porous media, these WOFs may be applicable to gas storage, purification, and separation. Moreover, these biocompatible porous ice networks may be exploited as medical-related carriers.