Xiaoyang Shi

Capture CO2 from Ambient Air Using Nanoconfined Ion Hydration

Water confined in nanoscopic pores is essential in determining the energetics of many physical and chemical systems. Herein, we report a recently discovered unconventional, reversible chemical reaction driven by water quantities in nanopores. The reduction of the number of water molecules present in the pore space promotes the hydrolysis of CO3(2-) to HCO3(-) and OH(-). This phenomenon led to a nano-structured CO2 sorbent that binds CO2 spontaneously in ambient air when the surrounding is dry, while releasing it when exposed to moisture. The underlying mechanism is elucidated theoretically by computational modeling and verified by experiments. The free energy of CO3 (2-) hydrolysis in nanopores reduces with a decrease of water availability. This promotes the formation of OH(-), which has a high affinity to CO2 . The effect is not limited to carbonate/bicarbonate, but is extendable to a series of ions. Humidity-driven sorption opens a new approach to gas separation technology.

The Effect of Moisture on the Hydrolysis of Basic Salts

A great deal of information exists concerning the hydration of ions in bulk water. Much less noticeable, but equally ubiquitous is the hydration of ions holding on to several water molecules in nanoscopic pores or in natural air at low relative humidity. Such hydration of ions with a high ratio of ions to water molecules (up to 1:1) are essential in determining the energetics of many physical and chemical systems. Herein, we present a quantitative analysis of the energetics of ion hydration in nanopores based on molecular modeling of a series of basic salts associated with different numbers of water molecules. The results show that the degree of hydrolysis of basic salts in the presence of a few water molecules is significantly different from that in bulk water. The reduced availability of water molecules promotes the hydrolysis of divalent and trivalent basic ions (S2- , CO32- , SO32- , HPO42- , SO42- , PO43- ), which produces lower valent ions (HS- , HCO3- , HSO3- , H2 PO4- , HSO4- , HPO42- ) and OH- ions. However, reducing the availability of water inhibits the hydrolysis of monovalent basic ions (CN- , HS- ). This finding sheds some light on a vast number of chemical processes in the atmosphere and on solid porous surfaces. The discovery has wide potential applications including designing efficient absorbents for acidic gases.

Understanding flocculation mechanism of graphene oxide for organic dyes from water: Experimental and molecular dynamics simulation

Flocculation treatment processes play an important role in water and wastewater pretreatment. Here we investigate experimentally and theoretically the possibility of using graphene oxide (GO) as a flocculant to remove methylene blue (MB) from water. Experimental results show that GO can remove almost all MB from aqueous solutions at its optimal dosages and molecular dynamics simulations indicate that MB cations quickly congregate around GO in water. Furthermore, PIXEL energy contribution analysis reveals that most of the strong interactions between GO and MB are of a van der Waals (London dispersion) character. These results offer new insights for shedding light on the molecular mechanism of interaction between GO and organic pollutants.

Kinetic analysis of an anion exchange absorbent for CO2 capture from ambient air

This study reports a preparation method of a new moisture swing sorbent for CO2 capture from air. The new sorbent components include ion exchange resin (IER) and polyvinyl chloride (PVC) as a binder. The IER can absorb CO2 when surrounding is dry and release CO2 when surrounding is wet. The manuscript presents the studies of membrane structure, kinetic model of absorption process, performance of desorption process and the diffusivity of water molecules in the CO2 absorbent. It has been proved that the kinetic performance of CO2 absorption/desorption can be improved by using thin binder and hot water treatment. The fast kinetics of P-100-90C absorbent is due to the thin PVC binder, and high diffusion rate of H2O molecules in the sample. The impressive is this new CO2 absorbent has the fastest CO2 absorption rate among all absorbents which have been reported by other up-to-date literatures.

Development of a Transferable Reactive Force Field of P/H Systems: Application to the Chemical and Mechanical Properties of Phosphorene

We developed ReaxFF parameters for phosphorus and hydrogen to give a good description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus- and hydrogen-containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters, and phosphorus hydride molecules. The potential parameters were obtained by conducting global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extended ReaxFF by adding a 60° correction term, which significantly improved the description of phosphorus clusters. Emphasis was placed on the mechanical response of black phosphorene with different types of defects. Compared to the nonreactive SW potential ( Jiang , J.-W. Nanotechnology 2015 , 26 , 315706 ), ReaxFF for P/H systems provides a significant improvement in describing the mechanical properties of the pristine and defected black phosphorene, as well as the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon is observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also showed that the mechanical response of black phosphorene is more sensitive to defects in the zigzag direction than that in the armchair direction. In addition, we developed a preliminary set of ReaxFF parameters for P/H/O/C to demonstrate that the ReaxFF parameters developed in this work could be generalized to oxidized phosphorene and P-containing 2D van der Waals heterostructures. That is, the proposed ReaxFF parameters for P/H systems establish a solid foundation for modeling of a wide range of P-containing materials.

Molecular characteristics of H2O in hydrate/ice/liquid water mixture

The interfacial properties of hydrate and its ambient play an important role in hydrate technique. In this paper, the molecular characteristics of H2O in hydrate/ice/liquid water mixture system are investigated based on molecular dynamics (MD) simulations. The structure I (sI) methane hydrate is partially heated to obtain the studied system. The properties including hydrogen bond, radial distribution function (RDF) and F3 order parameter (tetrahedral coordinated parameter of H2O) indicate that there is little difference of water structure in the hydrate region and ice/liquid water mixture region. The F4 order parameter (parameter based on H–O–O–H torsion angles of H2O) could be used to distinguish the different region. The value of F4 experiences the continuous change at interface between mixture region and hydrate region.

Closed-edged bilayer phosphorene nanoribbons producing from collapsing armchair phosphorene nanotubes

A new phosphorous allotrope, closed-edged bilayer phosphorene nanoribbon, is proposed via radially deforming armchair phosphorene nanotubes. Using molecular dynamics simulations, the transformation pathway from round PNTs falls into two types of collapsed structures: arc-like and sigmoidal bilayer nanoribbons, dependent on the number of phosphorene unit cells. The fabricated nanoribbions are energetically more stable than their parent nanotubes. It is also found via ab initio calculations that the band structure along tube axis substantially changes with the structural transformation. The direct-to-indirect transition of band gap is highlighted when collapsing into the arc-like nanoribbons but not the sigmoidal ones. Furthermore, the band gaps of these two types of nanoribbons show significant size-dependence of the nanoribbon width, indicative of wider tunability of their electrical properties.

Prediction of a Two-dimensional Sulfur Nitride (S3N2) Solid for Nanoscale Optoelectronic Applications

Two-dimensional materials have attracted tremendous attention for their fascinating electronic, optical, chemical, and mechanical properties. However, the band gaps of most reported two-dimensional (2D) materials are smaller than 2.0 eV, which has greatly restricted their optoelectronic applications in the blue and ultraviolet range of the spectrum. Here, we propose a stable trisulfur dinitride (

Coalescence of Cu contacted nanoparticles with different heating rates: A molecular dynamics study

{\mathrm{S}}_{3}{\mathrm{N}}_{2}

Characterization of kinetic limitations to atmospheric CO2 capture by solid sorbent

) 2D crystal that is a covalent network composed solely of S-N