Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer Boron–Nitride Sheets

Born–Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron–nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net positive charges on the contacting surface of the graphene and monolayer BN sheets, and such charge induction may affect electronic structure of the contacting graphene in view that graphene is a semimetal. Contact angles of nanodroplets of water in a supercooled state on the graphene are also computed. It is found that under the supercooled condition, water nanodroplets exhibit an appreciably larger contact angle than under the ambient condition

Direct Simulation Evidence of Generation of Oxygen Vacancies at the Golden Cage Au₁₆ and TiO₂ (110) Interface for CO Oxidation

We show Born–Oppenheimer molecular dynamics (BOMD) simulation evidence of the generation of oxygen vacancies at the golden cage Au₁₆ and TiO₂ (110) interface for CO oxidation. Unlike the conventional Langmuir–Hinshelwood (L-H) mechanism, the CO molecule adsorbed at the perimeter Au sites of Au₁₆ tends to attack a nearby lattice oxygen atom on the TiO₂ (110) surface rather than the neighboring co-adsorbed molecular O₂. Our large-scale BOMD simulation provides, to our knowledge, the first real-time demonstration of feasibility of the Mars–van Krevelen (M-vK) mechanism as evidenced by the generation of oxygen vacancies on the TiO₂ surface in the course of the CO oxidation. Furthermore, a comparative study of the CO oxidation at the golden cage Au₁₈ and TiO₂ interface suggests that the L-H mechanism is more favorable than the M-vK mechanism due to higher structural robustness of the Au₁₈ cage. It appears that the selection of either M-vK or L-H mechanism for the CO oxidation is dependent on the structural fluxionality of the Au cage clusters on the TiO₂ support

Catalytic Directional Cutting of Hexagonal Boron Nitride: The Roles of Interface and Etching Agents

Transition-metal (TM) nanoparticle catalyzed cutting has been proven to be an efficient approach to carve out straight channels in graphene to produce high-quality nanoribbons. However, the applicability of such a catalytic approach to hexagonal boron nitride (<i>h</i>-BN) is still an open question due to binary element compositions. Here, our ab initio study indicates that long and straight channels along either the zigzag or the armchair direction of the BN sheet can be carved out, driven by the energetically favored TM–zigzag or TM–armchair BN interface, regardless of roughness of the TM particle surface. Optimal experimental conditions for the catalytic cutting of either BN or BN/graphene hybrid sheet across the domain boundary are proposed via the analysis of the competition between TM–BN (or TM–graphene) interface and H-terminated BN (or graphene) edge. The computation results can serve to guide the experimental design for the production of highly uniform BN (or hybrid BN/graphene) nanoribbons with atomically smooth edges

Bilayer Phosphorene: Effect of Stacking Order on Bandgap and Its Potential Applications in Thin-Film Solar Cells

Phosphorene, a monolayer of black phosphorus, is promising for nanoelectronic applications not only because it is a natural p-type semiconductor but also because it possesses a layer-number-dependent direct bandgap (in the range of 0.3 to 1.5 eV). On basis of the density functional theory calculations, we investigate electronic properties of the bilayer phosphorene with different stacking orders. We find that the direct bandgap of the bilayers can vary from 0.78 to 1.04 eV with three different stacking orders. In addition, a vertical electric field can further reduce the bandgap to 0.56 eV (at the field strength 0.5 V/Å). More importantly, we find that when a monolayer of MoS₂ is superimposed with the p-type AA- or AB-stacked bilayer phosphorene, the combined trilayer can be an effective solar-cell material with type-II heterojunction alignment. The power conversion efficiency is predicted to be ∼18 or 16% with AA- or AB-stacked bilayer phosphorene, higher than reported efficiencies of the state-of-the-art trilayer graphene/transition metal dichalcogenide solar cells

Two Dimensional Epitaxial Water Adlayer on Mica with Graphene Coating: An <i>ab Initio</i> Molecular Dynamics Study

Motivated by a recent atomic-force-microscopy (AFM) study of water adlayers on mica by Heath and co-workers (Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. <i>Science</i> <b>2010</b>, <i>329</i>, 1188), we performed an <i>ab initio</i> molecular dynamics study of structural and dynamic properties of monolayer, bilayer, and trilayer water adlayers on the muscovite mica (001) surface with and without a graphene coating. We find that in the first epitaxial water adlayer, water molecules that form strong hydrogen bonds with the oxygen on the mica surface show little motions, thereby solid-like, while those “bridging” water molecules on top of the first water adlayer exhibit “itinerant” behavior, thereby liquid-like. Overall, the Born–Oppenheim molecular dynamics (BOMD) simulations (based on the BLYP-D functional) show that the first water adlayer on mica exhibits a unique hybrid solid–liquid-like behavior with a very low diffusion coefficient at ambient conditions. In particular, no dangling hydrogen bonds are found in the first water adlayer on mica. Moreover, the bilayer and trilayer water adlayers show slightly higher structural stability than the first water adlayer. A graphene coating on the water adlayer further enhances stability of the water adlayers. Most importantly, the bilayer water adlayer on mica with the graphene coating becomes fully solid-like, the structure of which is the same as the bilayer slice of ice-<i>I</i>_<i>h</i> with a thickness of 7.4 Å, consistent with the AFM measurement

Water-Promoted O₂ Dissociation on Small-Sized Anionic Gold Clusters

Although thermodynamically O₂ favors dissociative adsorption over molecular adsorption on small-sized anionic gold clusters (except Au₂^–), O₂ dissociation is unlikely to proceed under ambient conditions because of the high activation energy barrier (>2.0 eV). Here, we present a systematic theoretical study of reaction pathways for the O₂ dissociation on small-sized anionic gold nanoclusters Au_<i>n</i>^– (<i>n</i> = 1–6) with and without involvement of a water molecule. The density functional theory calculations indicate that the activation barriers from the molecular adsorption state of O₂ to dissociative adsorption can be significantly lowered with the involvement of a H₂O molecule. Once the O₂ dissociates on small-size gold clusters, atomic oxygen is readily available for other reactions, such as the CO oxidation, on the surface of gold clusters. This theoretical study supports previous experimental evidence that H₂O can be used to activate O₂, which suggests an alternative way to exploit catalytic capability of gold clusters for oxidation applications

Interaction between Iron and Graphene Nanocavity: Formation of Iron Membranes, Iron Clusters, or Iron Carbides

Motivated from a recent experimental study on filling of a graphene nanocavity by iron membrane at room temperature (<i>Science</i> <b>2014</b>, 343, 1228), we perform a comprehensive study of morphology changes of two-dimensional Fe membranes and iron carbides embedded in graphene nanocavities with specific sizes and shapes using the first-principles calculations and ab initio molecular dynamics simulations. Our simulations show that Fe atoms tend to gradually seal the graphene nanocavity via growing a metastable Fe membrane until the nanocavity is completely covered. Notably, a densely packed Fe membrane in the graphene nanocavity shows higher structural stability than a loosely packed one as long as more triangular lattices can form to release high tensile strain. The Fe membrane under high tensile strain tends to collapse and turns into a three-dimensional Fe cluster upon detaching from the edge. The structural transformation of Fe nanostructures follows the melting recrystallization mechanism at ambient temperatures in high vacuum. Moreover, the iron carbide can also exist in the graphene nanocavity and once formed can be highly stable even at 1200 K

Two Dimensional Epitaxial Water Adlayer on Mica with Graphene Coating: An <i>ab Initio</i> Molecular Dynamics Study

Motivated by a recent atomic-force-microscopy (AFM) study of water adlayers on mica by Heath and co-workers (Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. <i>Science</i> <b>2010</b>, <i>329</i>, 1188), we performed an <i>ab initio</i> molecular dynamics study of structural and dynamic properties of monolayer, bilayer, and trilayer water adlayers on the muscovite mica (001) surface with and without a graphene coating. We find that in the first epitaxial water adlayer, water molecules that form strong hydrogen bonds with the oxygen on the mica surface show little motions, thereby solid-like, while those “bridging” water molecules on top of the first water adlayer exhibit “itinerant” behavior, thereby liquid-like. Overall, the Born–Oppenheim molecular dynamics (BOMD) simulations (based on the BLYP-D functional) show that the first water adlayer on mica exhibits a unique hybrid solid–liquid-like behavior with a very low diffusion coefficient at ambient conditions. In particular, no dangling hydrogen bonds are found in the first water adlayer on mica. Moreover, the bilayer and trilayer water adlayers show slightly higher structural stability than the first water adlayer. A graphene coating on the water adlayer further enhances stability of the water adlayers. Most importantly, the bilayer water adlayer on mica with the graphene coating becomes fully solid-like, the structure of which is the same as the bilayer slice of ice-<i>I</i>_<i>h</i> with a thickness of 7.4 Å, consistent with the AFM measurement

Two Dimensional Epitaxial Water Adlayer on Mica with Graphene Coating: An <i>ab Initio</i> Molecular Dynamics Study

Motivated by a recent atomic-force-microscopy (AFM) study of water adlayers on mica by Heath and co-workers (Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. <i>Science</i> <b>2010</b>, <i>329</i>, 1188), we performed an <i>ab initio</i> molecular dynamics study of structural and dynamic properties of monolayer, bilayer, and trilayer water adlayers on the muscovite mica (001) surface with and without a graphene coating. We find that in the first epitaxial water adlayer, water molecules that form strong hydrogen bonds with the oxygen on the mica surface show little motions, thereby solid-like, while those “bridging” water molecules on top of the first water adlayer exhibit “itinerant” behavior, thereby liquid-like. Overall, the Born–Oppenheim molecular dynamics (BOMD) simulations (based on the BLYP-D functional) show that the first water adlayer on mica exhibits a unique hybrid solid–liquid-like behavior with a very low diffusion coefficient at ambient conditions. In particular, no dangling hydrogen bonds are found in the first water adlayer on mica. Moreover, the bilayer and trilayer water adlayers show slightly higher structural stability than the first water adlayer. A graphene coating on the water adlayer further enhances stability of the water adlayers. Most importantly, the bilayer water adlayer on mica with the graphene coating becomes fully solid-like, the structure of which is the same as the bilayer slice of ice-<i>I</i>_<i>h</i> with a thickness of 7.4 Å, consistent with the AFM measurement

Design of Ferroelectric Organic Molecular Crystals with Ultrahigh Polarization

Inspired by recent successful synthesis of room-temperature ferroelectric supramolecular charge-transfer complexes, i.e., tetrathiafulvalene (TTF)- and pyromellitic diimide (PMDI)-based crystals (Tayi et al.<i> Nature</i> <b>2012</b>, <i>488</i>, 485–489), three new ferroelectric two-component organic molecular crystals are designed based on the TTF and PMDI motifs and an extensive polymorph search. To achieve energetically favorable packing structures for the crystals, a newly developed computational approach that combines polymorph predictor with density functional theory (DFT) geometry optimization is employed. Tens of thousands of packing structures for the TTF- and PMDI-based crystals are first generated based on the limited number of asymmetric units in a unit cell as well as limited common symmetry groups for organocarbon crystals. Subsequent filtering of these packing structures by comparing with the reference structures yields dozens of promising crystal structures. Further DFT optimizations allow us to identify several highly stable packing structures that possess the space group of <i>P</i>2₁ as well as high to ultrahigh <i>spontaneous polarizations</i> (23–127 μC/cm²) along the crystallographic <i>b</i> axis. These values are either comparable to or much higher than the computed value (25 μC/cm²) or measured value (55 μC/cm²) for the state-of-the-art organic supramolecular systems. The high polarization arises from the ionic displacement. We further construct surface models to derive the electric-field-switched low-symmetry structures of new TTF- and PMDI-based crystals. By comparing the high-symmetry and low-symmetry crystal structures, we find that the ferroelectric polarization of the crystals is very sensitive to atomic positions, and a small molecular displacement may result in relatively high polarizations along the <i>a</i> and <i>c</i> axes, polarity reversal, and/or electronic contribution to polarization. If these newly designed TTF- and PMDI-based crystals with high polarizations are confirmed by experiments, the computer-aided ferroelectric material design on the basis of hydrogen-bonded charge-transfer complexes with flexible electron-donor and acceptor molecules would be proven valuable for expediting the search of room-temperature “displasive-type” ferroelectric organic crystals