Nano-confined Systems Research Articles

Modulating electronic structure of NiFe layered double hydroxide membrane by interlayer-confinement for enhanced water decontamination

Herein, the role of nanoconfinement on modulating the electronic structure of the catalyst that influences the efficiency of advanced oxidation processes (AOPs) was investigated. Peroxymonosulfate (PMS)-based AOP was conducted within the nanoconfined sub-nanometer interlayer channels of a 2D NiFe layered double hydroxide membrane (NiFeM). The interlayer-confined NiFeM exhibited complete removal of sulfamethoxazole, with a degradation rate constant of 211.67 s−1 that was 2.6–946.2 times higher than previously reported nanoconfined AOPs systems, and also demonstrated broad pH working range and anti-interference ability with excellent catalytic stability in 25 h. Theoretical calculations suggested such interlayer confinement upshifted the catalyst metal d-band center, accounting for strengthened PMS adsorption and electron transfer from catalyst to PMS molecules. This work embodies the interlayer confinement effect on modulating the electronic structure of catalyst and provides insights for designing catalysts with excellent capability for environmental remediation, energy conversion and chemical synthesis.

What keeps nanopores boiling.

The liquid-to-vapor transition can occur under unexpected conditions in nanopores, opening the door to fundamental questions and new technologies. The physics of boiling in confinement is progressively introduced, starting from classical nucleation theory, passing through nanoscale effects, and terminating with the material and external parameters that affect the boiling conditions. The relevance of boiling in specific nanoconfined systems is discussed, focusing on heterogeneous lyophobic systems, chromatographic columns, and ion channels. The current level of control of boiling in nanopores enabled by microporous materials such as metal organic frameworks and biological nanopores paves the way to thrilling theoretical challenges and to new technological opportunities in the fields of energy, neuromorphic computing, and sensing.

Bridging adsorption behavior of confined CH[formula omitted]-CO[formula omitted] binary mixtures across scales

An accurate understanding of the competitive adsorption of CH4-CO2 binary mixtures in nano-confined systems is critical for engineering CO2 storage in shale gas reservoirs. Due to difficulties in making reliable experimental observations in nano-scale, atomistic simulations (ASs), such as the Grand Canonical Monte Carlo (GCMC) method, provide a viable approach to studying the adsorption behavior of confined fluids. ASs are, however, limited in the size of the compositional domain due to the high computational cost. This work proposes a framework that combines AS and the lattice Boltzmann (LB) method to bridge the physics of confined fluids across scales. The Peng–Robinson equation of state (PR-EoS) produces fugacity coefficients, which serve as input for conducting multi-component GCMC simulations. These GCMC simulations explore the competitive adsorption behavior of CH4-CO2 in nano-slits at various composition, pressure, and channel-width conditions. Both components generate adsorption layers with high densities near the walls with CO2 preferentially adsorbing compared to CH4 on the organic walls of carbon sheets. At the mesoscale, a pseudopotential model represents the intermolecular forces in multi-component, multiple-relaxation-time LB simulations. The LB simulations are in good agreement with the GCMC results, allowing us to obtain values for tunable LB parameters. We then extend the use of LB to simulate adsorption behavior in complex networks with nano-sized channels. The phase behavior and fluid properties in the complex geometries of nano-channels differ from nano-slits and bulk systems. The bridging of physics from GCMC (microscale) to LB (mesoscale) via the macroscale PR-EoS connects the adsorption behavior of binary systems across scales.

The synergistic effect of space and surface charge on nanoconfined ion transport and nanofluidic energy harvesting

Diverse nanoconfined ion transport behaviors have attracted increasing attention in constructing ion-channel-inspired nanoconfined systems for high nanofluidic energy conversion performance. Conventional nanofluidic systems cannot simultaneously obtain high ion-selectivity and high ion-flux in a large region due to the decreasing effect of the electrical double layer (EDL) formed on the charged surface. Here, space charge is introduced in the nanoconfined system to effectively solve this problem by employing the synergistic effect of surface and space charges. The charge effect contribution on controlled-ion-transport is further decoupled into surface charge, space charge and synergistic effects for the first time. The surface charge induces charge separation within the EDL in a smaller diameter channel while the space charge dominates the ion-selectivity by the ion-exchange mechanism in a larger diameter channel. The synergistic effect, contributed by EDL and ion-exchange effects, is extremely important in large (around 100 nm), short nanochannel (around 100 nm), and high concentration fold (around 1000-fold) conditions to achieve high ion-selectivity, ion-flux, and nanofluidic energy conversion performances, which is systematically analyzed by theoretical simulation results. These results reveal the mechanism for enhanced ion transport behaviors and provide a solid foundation for designing high-performance nanofluidic devices in fields of desalination, sensor, energy conversion and storage.

Effect of Microstructure on Ionic Transport in Silica-Based Sodium Containing Nanoconfined Systems and Their Electrochemical Performance as Electrodes

Glasses having compositions xNa2O·10P2O5·(100–(10+x))SiO2, with x having values of 20 and 35, were synthesized by a facile sol–gel technique within a mesoporous silica (SBA-15) template. Nanocrysta...

Water flow inside various geometric nano-confinement channels

In nano-confined systems, the properties of a fluid are different from those of macroscopic systems, and the properties of a nanotube can significantly affect water transport. However, our knowledge of the effects of nanotube shape is far from adequate. In the present work, we study the properties of a fluid transporting in different nano-confined configurations by molecular dynamics simulations. This study is aimed at gaining insight into the transport of water molecules in carbon nanotubes with different configurations. We find that the closer of channel shape to the circular nanotube (more sides of the channel), the lower friction coefficient of the solid-liquid interface has and the friction coefficient of nanochannels increases with R when R < 1.0 nm. The friction coefficient converges to a stable value (close to the friction coefficient of graphene/water) when R > 1.0 nm. A variety of configurations leads to the variation of the fluid properties in nanotubes. Our results can be applied to the nanofluid properties of a complex channel structure and water nanochannel microscopic design.

Ice-Liquid Oscillations in Nanoconfined Water.

Nanoscale confinement has a strong effect on the phase behavior of water. Studies in the last two decades have revealed a wealth of novel crystalline and quasicrystalline structures for water confined in nanoslits. Less is known, however, about the nature of ice-liquid coexistence in extremely nanoconfined systems. Here, we use molecular simulations to investigate the ice-liquid equilibrium for water confined between two nanoscopic disks. We find that the nature of ice-liquid phase coexistence in nanoconfined water is different from coexistence in both bulk water and extended nanoslits. In highly nanoconfined systems, liquid water and ice do not coexist in space because the two-phase states are unstable. The confined ice and liquid phases coexist in time, through oscillations between all-liquid and all-crystalline states. The avoidance of spatial coexistence of ice and liquid originates on the non-negligible cost of the interface between confined ice and liquid in a small system. It is the result of the small number of water molecules between the plates and has no analogue in bulk water.

Nano confinement and catalytic conversion of energy molecules

Catalysis, as a key and enabling technology, plays an increasingly important role in fields ranging from energy, environment and agriculture to health care. Rational design and synthesis of highly efficient catalysts has become the ultimate goal of catalysis research. Thanks to the rapid development of nanoscience and nanotechnology, and in particular a theoretical understanding of the tuning of electronic structure in nanoscale systems, this element of design is becoming possible via precise control of nanoparticles’ composition, morphology, structure and electronic states. In the present paper, we will illustrate the results on the structural and electronic properties of catalytically active centers in the nano-confined systems, as well as their catalytic activities. Based upon the concept of “Nano-Confined Catalysis”, novel catalyst has been designed by embedding low-valent iron atoms within the lattice of silicon dioxide. The active single iron site could catalyze the direct and selective conversion of methane exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C−C coupling, further oligomerization and hence coke deposition. At 1090 °C, methane conversion reaches a maximum at 48.1% and ethylene selectivity 48.4%, while the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The surface-confined iron sites delivered stable performance, with no deactivation observed during a 60-h test. This technology would allow the removal of the energy-intensive process of syngas production from the conventional natural gas utilization technologies. In addition, we recently established the concept of nano-composite catalysts comprising of metal oxides and zeolites, which effectively separates the active sites for CO and H2 activation from those for C−C coupling. CO and H2 are activated over the partially reduced metal oxides forming intermediates, which are subsequently converted to products within the confined pores of zeolites. As such, surface polymerization, a long-standing problem in the conventional Fischer-Tropsch synthesis invented more than 90 years ago, could be circumvented. This novel process for syngas conversion could achieve a high selectivity of light olefins (80%), surpassing the limit of 58% for C2−C4 light hydrocarbons by the ASF distribution. Furthermore such a bifunctional composite catalyst delivers stable performance and no obvious deactivation was observed within a over 3000 h stability test. These achievements open up new avenues for the developments of clean technologies for the efficient utilization of carbon resources, such as natural gas, coal and biomass, and are recognized as the “milestone in the direct synthesis of light olefins”. Now, his team is collaborating with companies to explore industrial applications, starting from a pilot-scale study.

A non-equilibrium molecular dynamics study of methane transport in clay nano-pores

Accurate characterization of shale reservoir properties is vitally important for reliable gas production forecasting and reserve estimation. In this work, we use boundary-driven non-equilibrium molecular dynamics (BD-NEMD) to investigate methane transport in porous clays, which are one of the major mineral components of shale. One of the critical issues in BD-NEMD simulations is robust temperature control schemes to maintain isothermal conditions when the external force is applied to drive fluid flow. To this end, we scrutinize the performance of six temperature control schemes for BD-NEMD. We identify a number of effective temperature control schemes, and we show that careful algorithm selection can considerably reduce the number of simulations required to estimate transport diffusion coefficients. Using a robust temperature control scheme identified in our study, we examine the validity of the Knudsen model for predicting methane diffusivities in clay nano-pores. Although the Knudsen model is one of the most widely used theoretical approaches for predicting reservoir properties, we find that it fails to accurately predict methane transport diffusivities in small clay nano-pores. Our findings therefore suggest that reservoir models based on Knudsen theory may result in significant over- or under-prediction of gas production, depending on reservoir conditions. These findings are consistent with other studies, which posit that the Knudsen diffusion model breaks down in nano-confined systems because it neglects to account for the effects of adsorption on gas transport.

A review of the structure and dynamics of nanoconfined water and ionic liquids via molecular dynamics simulation.

During the past decade, the research on fluids in nanoconfined geometries has received considerable attention as a consequence of their wide applications in different fields. Several nanoconfined systems such as water and ionic liquids, together with an equally impressive array of nanoconfining media such as carbon nanotube, graphene and graphene oxide have received increasingly growing interest in the past years. Water is the first system that has been reviewed in this article, due to its important role in transport phenomena in environmental sciences. Water is often considered as a highly nanoconfined system, due to its reduction to a few layers of water molecules between the extended surface of large macromolecules. The second system discussed here is ionic liquids, which have been widely studied in the modern green chemistry movement. Considering the great importance of ionic liquids in industry, and also their oil/water counterpart, nanoconfined ionic liquid system has become an important area of research with many fascinating applications. Furthermore, the method of molecular dynamics simulation is one of the major tools in the theoretical study of water and ionic liquids in nanoconfinement, which increasingly has been joined with experimental procedures. In this way, the choice of water and ionic liquids in nanoconfinement is justified by applying molecular dynamics simulation approaches in this review article.

Ångström-scale chemically powered motors

Like their larger micron-scale counterparts, Ångström-scale chemically self-propelled motors use asymmetric catalytic activity to produce self-generated concentration gradients that lead to directed motion. Unlike their micron-scale counterparts, the sizes of Ångström-scale motors are comparable to the solvent molecules in which they move, they are dominated by fluctuations, and they operate on very different time scales. These new features are studied using molecular dynamics simulations of small sphere dimer motors. We show that the ballistic regime is dominated by the thermal speed but the diffusion coefficients of these motors are orders of magnitude larger than inactive dimers. Such small motors may find applications in nano-confined systems or perhaps eventually in the cell.

A new and effective method for thermostatting confined fluids

We present a simple thermostatting method suitable for nanoconfined fluid systems. Two conventional strategies involve thermostatting the fluid directly or employing a thermal wall that couples only the wall atoms with the thermostat. When only a thermal wall is implemented, the temperature control of the fluid is true to the actual experiment and the heat is transferred from the fluid to the walls. However, for large or complex systems it can often be computationally prohibitive to employ thermal walls. To overcome this limitation many researchers choose to freeze wall atoms and instead apply a synthetic thermostat to the fluid directly through the equations of motion. This, however, can have serious consequences for the mechanical, thermodynamic, and dynamical properties of the fluid by introducing unphysical behaviour into the system [Bernardi et al., J. Chem. Phys. 132, 244706 (2010)]. In this paper, we propose a simple scheme which enables working with both frozen walls and naturally thermostatted liquids. This is done by superimposing the walls with oscillating particles, which vibrate on the edge of the fluid control volume. These particles exchange energy with the fluid molecules, but do not interact with wall atoms or each other, thus behaving as virtual particles. Their displacements violate the Lindemann criterion for melting, in such a way that the net effect would not amount to an additional confining surface. One advantage over standard techniques is the reduced computational cost, particularly for large walls, since they can be kept rigid. Another advantage over accepted strategies is the opportunity to freeze complex charged walls such as β-cristobalite. The method furthermore overcomes the problem with polar fluids such as water, as thermalized charged surfaces require higher spring constants to preserve structural stability, due to the effects of strong Coulomb interactions, thus inevitably degrading the thermostatting efficiency.

Signatures of Nanoconfinement on the Linear and Nonlinear Vibrational Spectroscopy of a Model Hydrogen-Bonded Complex Dissolved in a Polar Solvent

The one-dimensional IR (1D-IR) absorption and IR pump-probe spectra of a hydrogen stretch in a model hydrogen-bonded complex dissolved in a polar solvent confined in spherical hydrophobic cavities of different sizes were simulated using ground-state mixed quantum-classical dynamics. Due to a thorough analysis of key properties of the complex and solvent from equilibrium trajectory data, we were able to gain insight into the microscopic details underlying the spectra. Both the 1D-IR and IR pump-probe spectra manifested the effects of confinement on the relative stabilities of the covalent and ionic forms of the complex through pronounced changes in their peak intensities and numbers. However, in contrast to the 1D-IR spectra, the time-resolved pump-probe spectra were found to be uniquely sensitive to the changes in the molecular dynamics as the cavity size is varied. In particular, it was found that the variations in the time evolutions of the peak intensities in the pump-probe spectra reflect the differences in the solvation dynamics associated with the various forms of the complex in different locations within the cavities. The ability to detect these differences underscores the advantage of using pump-probe spectroscopy for studying nanoconfined systems.

Influence of nanoconfinement on morphology and dehydrogenation of the Li11BD4–Mg(11BD4)2 system

The decomposition of a nanoconfined mixture of lithium–magnesium borohydride, Li11BD4–Mg(11BD4)2, has been investigated and compared to the corresponding mixture in the bulk form. The systems were investigated by thermal analysis, small-angle neutron scattering, 11B nuclear magnetic resonance and transmission electron microscopy. The dehydrogenation temperatures decreased by up to 60 °C in the nanoconfined system, with gas evolution following different steps, compared to the behaviour of the bulk material under the same conditions. Most importantly, desorption from the nanoconfined hydride proceeds without formation of diborane, B2D6, which evolves from the bulk mixture. From small-angle neutron scattering, differences in morphology between the bulk and the nanoconfined systems are also demonstrated. Evidence of a complete decomposition has been found in the nanoconfined system, after heating up to 460 °C. Furthermore, 11B NMR data show that nanoconfinement inhibits the formation of dodecaborane, [B12D12]2−, during decomposition, a result which is important for practical applications of borohydrides.

Quest for Order in Chaos: Hidden Repulsive Level Statistics in Disordered Quantum Nanoaggregates

The local distribution of exciton levels in disordered cyanine-dye-based molecular nano-aggregates has been elucidated using fluorescence line narrowing spectroscopy. The observation of a Wigner-Dyson-type level spacing distribution provides direct evidence of the existence of level repulsion of strongly overlapping states in the molecular wires, which is important for the understanding of the level statistics, and therefore the `functional properties, of a large variety of nano-confined systems.

Effect of Shape of Silver Nanoplates on the Enhancement of Surface Plasmon Resonance (SPR) Signals

The surface plasmon resonance (SPR)-based sensor technique is based on the utilization of a noble metal to generate a surface plasmon electromagnetic field, which is then used to probe changes in the optical properties. Using nanoplates, it is possible to considerably enhance the sensitivity of a conventional SPR device. In this work, we investigated the effect of shape of silver nanoplates (AgNPs) on the enhancement of SPR sensitivity. By the use of bovine serum albumin (BSA) which has reversible pH-driven conformational change property, signal amplification effect on various shapes of AgNPs was studied. The proposed technique allows an enhancement of SPR sensitivity by coupling with a localized surface plasmon resonance of AgNPs. This novel platform could be used for sensing applications, or to detect and investigate the unique behaviors of nano-confined molecular systems.

Simplified gauge-cell method and its application to the study of capillary phase transition of propane in carbon nanotubes

A modification of the gauge-cell method for Monte Carlo studies of phase equilibrium in nano-confined systems is presented and employed for studying the capillary phase transition of propane in single-walled carbon nanotubes as a function of tube diameter. It is shown that if an analytical equation of state for the vapor phase is known, the acceptance rule for the trial move of particle exchange can be modified to reference the bulk system through its chemical potential. Under these conditions, the simulation procedure is simplified and acquires many characteristics of a Monte Carlo simulation conducted in the grand canonical ensemble. It is also shown that the critical temperature can be estimated by interpolation of sub- and supercritical values of the slope of the inverse isotherm at the inflection point. To enhance the sampling of propane molecules in the hollow space of the nanotubes the configurational-bias scheme is employed. The simulation results show that the confinement of propane increases its critical density, reduces the critical temperature and narrows the binodal curve with decreasing tube diameter until the system approaches one-dimensional behavior.

Detection of nano-confined DNA using surface-plasmon enhanced fluorescence

Surface-plasmon resonance (SPR)-induced enhanced fluorescence was used to detect the presence of fluorescent TOTO-1 bis-intercalated lambda-DNA complexes introduced into a periodic nanopore array. As we show, the proposed technique allows a high detection sensitivity by providing a strong fluorescence-signal efficiency from nano-confined fluorescent complexes. This novel platform could be used for sensing applications, or to detect and investigate the unique behaviors of nano-confined molecular systems.