Jaeil Bai

Multiwalled ice helixes and ice nanotubes

We report six phases of high-density nano-ice predicted to form within carbon nanotubes (CNTs) at high pressure. High-density nano-ice self-assembled within smaller-diameter CNT (17,0) exhibits a double-walled helical structure where the outer wall consists of four double-stranded helixes, which resemble a DNA double helix, and the inner wall is a quadruple-stranded helix. Four other double-walled nano-ices, self-assembled respectively in two larger-diameter CNTs (20,0 and 22,0), display tubular structure. Within CNT (24,0), the confined water can freeze spontaneously into a triple-walled helical nano-ice where the outer wall is an 18-stranded helix and the middle and inner walls are hextuple-stranded helixes.

Polymorphism and polyamorphism in bilayer water confined to slit nanopore under high pressure

A distinctive physical property of bulk water is its rich solid-state phase behavior, which includes 15 crystalline (ice I–ice XIV) and at least 3 glassy forms of water, namely, low-density amorphous, high-density amorphous, and very-high-density amorphous (VHDA). Nanoscale confinement adds a new physical variable that can result in a wealth of new quasi-2D phases of ice and amorphous ice. Previous computer simulations have revealed that when water is confined between two flat hydrophobic plates about 7–9 Å apart, numerous bilayer (BL) ices (or polymorphs) can arise [e.g., BL-hexagonal ice (BL-ice I)]. Indeed, growth of the BL-ice I through vapor deposition on graphene/Pt(111) substrate has been achieved experimentally. Herein, we report computer simulation evidence of pressure-induced amorphization from BL-ice I to BL-amorphous and then to BL-VHDA2 at 250 K and 3 GPa. In particular, BL-VHDA2 can transform into BL-VHDA1 via decompression from 3 to 1.5 GPa at 250 K. This phenomenon of 2D polyamorphic transition is akin to the pressure-induced amorphization in 3D ice (e.g., from hexagonal ice to HDA and then to VHDA via isobaric annealing). Moreover, when the BL-ice I is compressed instantly to 6 GPa, a new very-high-density BL ice is formed. This new phase of BL ice can be viewed as an array of square ice nanotubes. Insights obtained from pressure-induced amorphization and crystallization of confined water offer a guide with which to seek a thermodynamic path to grow a new form of methane clathrate whose BL ice framework exhibits the Archimedean 4⋅82 (square-octagon) pattern.

Guest-free monolayer clathrate and its coexistence with two-dimensional high-density ice

Three-dimensional (3D) gas clathrates are ice-like but distinguished from bulk ices by containing polyhedral nano-cages to accommodate small gas molecules. Without space filling by gas molecules, standalone 3D clathrates have not been observed to form in the laboratory, and they appear to be unstable except at negative pressure. Thus far, experimental evidence for guest‐free clathrates has only been found in germanium and silicon, although guest‐free hydrate clathrates have been found, in recent simulations, able to grow from cold stretched water, if first nucleated. Herein, we report simulation evidence of spontaneous formation of monolayer clathrate ice, with or without gas molecules, within hydrophobic nano-slit at low temperatures. The guest-free monolayer clathrate ice is a low-density ice (LDI) whose geometric pattern is identical to Archimedean 4·82-truncated square tiling, i.e. a mosaic of tetragons and octagons. At large positive pressure, a second phase of 2D monolayer ice, i.e. the puckered square high-density ice (HDI) can form. The triple point of the LDI/liquid/HDI three-phase coexistence resembles that of the ice-Ih/water/ice-III three-phase coexistence. More interestingly, when the LDI is under a strong compression at 200 K, it transforms into the HDI via a liquid intermediate state, the first direct evidence of Ostwald’s rule of stages at 2D. The tensile limit of the 2D LDI and water are close to that of bulk ice-Ih and laboratory water.

Thermal stability of nanostructurally stabilized zirconium oxide

Nanostructurally stabilized zirconium oxide (NSZ) hard transparent films were produced without chemical stabilizers by the ion beam assisted deposition technique (IBAD). A transmission electron microscopy study of the samples produced below 150 °C revealed that these films are composed of zirconium oxide (ZrO2) nanocrystallites of diameters 7.5 ± 2.3 nm. X-ray and selected-area electron diffraction studies suggested that the as-deposited films are consistent with cubic phase ZrO2. Rutherford backscattering spectroscopy (RBS) indicated the formation of stoichiometric ZrO2. The phase identity of these optically transparent NSZ films was in agreement with cubic ZrO2, as indicated by the matching elastic modulus values from the calculated results for pure cubic zirconium oxide and results of nanoindentation measurements. Upon annealing in air for 1 h, these NSZ films were found to retain most of their room temperature deposited cubic phase x-ray diffraction signature up to 850 °C. Size effect and vacancy stabilization mechanisms and the IBAD technique are discussed to explain the present results.

Melting temperature of ice Ih calculated from coexisting solid-liquid phases

We carried out molecular-dynamics simulations by using the two-phase coexistence method with the constant pressure, particle number, and enthalpy ensemble to compute the melting temperature of proton-disordered hexagonal ice I(h) at 1-bar pressure. Four models of water were considered, including the widely used TIP4P [W. L. Jorgensen, J. Chandrasekha, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys.79, 926 (1983)] and TIP5P [M. W. Mahoney and W. L. Jorgensen J. Chem. Phys.112, 8910 (2000)] models, as well as recently improved TIP4P and TIP5P models for use with Ewald techniques-the TIP4P-Ew [W. Horn, W. C. Swope, J. W. Pitera, J. C. Madura, T. J. Dick, G. L. Hura, and T. Head-Gordon, J. Chem. Phys.120, 9665 (2004)] and TIP5P-Ew [S. W. Rick, J. Chem. Phys.120, 6085 (2004)] models. The calculated melting temperature at 1 bar is T(m) = 229 +/- 1 K for the TIP4P and T(m) = 272.0 +/- 0.6 K for the TIP5P ice I(h), both are consistent with previous simulations based on free-energy methods. For the TIP4P-Ew and TIP5P-Ew models, the calculated melting temperature is T(m) = 257.0 +/- 1.1 K and T(m) = 253.9 +/- 1.1 K, respectively.

Ab initio studies of quasi-one-dimensional pentagon and hexagon ice nanotubes

Ab initio plane-wave total-energy calcuation is carried out to study the relative stability of the quasi-one-dimensional (Q1D) pentagon and hexagon ice nanotubes. Electronic structure calculations indicate the two Q1D ice nanotubes have nearly the same band structures and energy bandgap as those of proton-ordered bulk ice Ih. Ab initio molecular-orbital and density-functional theory calculations, as well as three classical potential models of water, are also employed to investigate the relative stability of the pentagon and hexagon water clusters (H2O)30, (H2O)60, and (H2O)120. Clusters of this kind can serve to bridge the gap between the small polygonal water rings and the infinitely long Q1D polygon ice nanotubes. It is found that the polygon water prisms with the size (H2O)120 begin to show the relative energetic behavior of the infinitely long polygon ice nanotubes.

Highly Confined Water: Two-Dimensional Ice, Amorphous Ice, and Clathrate Hydrates

Understanding phase behavior of highly confined water, ice, amorphous ice, and clathrate hydrates (or gas hydrates), not only enriches our view of phase transitions and structures of quasi-two-dimensional (Q2D) solids not seen in the bulk phases but also has important implications for diverse phenomena at the intersection between physical chemistry, cell biology, chemical engineering, and nanoscience. Relevant examples include, among others, boundary lubrication in nanofluidic and lab-on-a-chip devices, synthesis of antifreeze proteins for ice-growth inhibition, rapid cooling of biological suspensions or quenching emulsified water under high pressure, and storage of H2 and CO2 in gas hydrates. Classical molecular simulation (MD) is an indispensable tool to explore states and properties of highly confined water and ice. It also has the advantage of precisely monitoring the time and spatial domains in the sub-picosecond and sub-nanometer scales, which are difficult to control in laboratory experiments, and yet allows relatively long simulation at the 10(2) ns time scale that is impractical with ab initio molecular dynamics simulations. In this Account, we present an overview of our MD simulation studies of the structures and phase behaviors of highly confined water, ice, amorphous ice, and clathrate, in slit graphene nanopores. We survey six crystalline phases of monolayer (ML) ice revealed from MD simulations, including one low-density, one mid-density, and four high-density ML ices. We show additional supporting evidence on the structural stabilities of the four high-density ML ices in the vacuum (without the graphene confinement), for the first time, through quantum density-functional theory optimization of their free-standing structures at zero temperature. In addition, we summarize various low-density, high-density, and very-high-density Q2D bilayer (BL) ice and amorphous ice structures revealed from MD simulations. These simulations reinforce the notion that the nanoscale confinement not only can disrupt the hydrogen bonding network in bulk water but also can allow satisfaction of the ice rule for low-density and high-density Q2D crystalline structures. Highly confined water can serve as a generic model system for understanding a variety of Q2D materials science phenomena, for example, liquid-solid, solid-solid, solid-amorphous, and amorphous-amorphous transitions in real time, as well as the Ostwald staging during these transitions. Our simulations also bring new molecular insights into the formation of gas hydrate from a gas and water mixture at low temperature.

Ferroelectric hexagonal and rhombic monolayer ice phases

Two new phases of water, the mid-density hexagonal monolayer ice and the high-density flat rhombic monolayer ice, are observed in our molecular dynamics simulations of monolayer water confined between two smooth hydrophobic walls. These are in addition to the two monolayer ices reported previously, namely, the low-density 4·82 monolayer ice and the high-density puckered rhombic monolayer ice (HD-pRMI). Stabilities of the structures are confirmed by ab initio computation. Importantly, both new phases and the HD-pRMI are predicted to be ferroelectric. An in-plane external electric field can further stabilize these ferroelectric monolayer ices.

Compression Limit of Two-Dimensional Water Constrained in Graphene Nanocapillaries.

Evaluation of the tensile/compression limit of a solid under conditions of tension or compression is often performed to provide mechanical properties that are critical for structure design and assessment. Algara-Siller et al. recently demonstrated that when water is constrained between two sheets of graphene, it becomes a two-dimensional (2D) liquid and then is turned into an intriguing monolayer solid with a square pattern under high lateral pressure [ Nature , 2015 , 519 , 443 - 445 ]. From a mechanics point of view, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of the 2D monolayer water. Here, we perform a simulation study of the compression limit of 2D monolayer, bilayer, and trilayer water constrained in graphene nanocapillaries. At 300 K, a myriad of 2D ice polymorphs (both crystalline-like and amorphous) are formed from the liquid water at different widths of the nanocapillaries, ranging from 6.0 to11.6 Å. For monolayer water, the compression limit is typically a few hundred MPa, while for the bilayer and trilayer water, the compression limit is 1.5 GPa or higher, reflecting the ultrahigh van der Waals pressure within the graphene nanocapillaries. The compression-limit (phase) diagram is obtained at the nanocapillary width versus pressure (h-P) plane, based on the comprehensive molecular dynamics simulations at numerous thermodynamic states as well as on the Clapeyron equation. Interestingly, the compression-limit curves exhibit multiple local minima.

New Computational Approach to Determine Liquid–Solid Phase Equilibria of Water Confined to Slit Nanopores

We devise a new computational approach to compute solid-liquid phase equilibria of confined fluids. Specifically, we extend the multibaric-multithermal ensemble method with an anisotropic pressure control to achieve the solid-liquid phase equilibrium for confined water inside slit nanopores (with slit width h ranging from 5.4 Å to 7.2 Å). A unique feature of this multibaric-multithermal ensemble is that the freezing points of confined water can be determined from the heat-capacity peaks. The new approach has been applied to compute the freezing point of two monolayer ices, namely, a high-density flat rhombic monolayer ice (HD-fRMI) and a high-density puckered rhombic monolayer ice (HD-pRMI) observed in our simulation. We find that the liquid-to-solid transition temperature (or the freezing point) of HD-pRMI is dependent on the slit width h, whereas that of HD-fRMI is nearly independent of the h.