First-principles Molecular Dynamics Simulations Research Articles

Enhanced Photosensitizer Wettability via Anchoring Competition of Violet Phosphorus Quantum Dots for Breakthroughs in Photodynamic Film Sterilization.

Wettability is important for photodynamic film sterilization since higher wettability enhances the capture of bacteria in contact with photosensitizers. Herein, a small number of violet phosphorus quantum dots (VPQDs) are anchored into hypericin bacterial cellulose films (VP/Hy-BC films) to improve wettability, reducing the water contact angle from 56.8° to 33.0°. This modification facilitated more effective interactions between the bacteria and photosensitizers, rapidly inactivating 7 log10 CFU/mL of Staphylococcus aureus within 60min. First-principles calculations and molecular dynamics simulations reveal that VPQDs, with their low spatial site resistance, reduced the intermolecular Hy self-aggregation force. This increased the solvent-accessible surface area of VP/Hy by ≈25.7%, thereby decreasing hydrophobic photosensitizer aggregation. Consequently, more active sites are exposed, remarkably improving the photoelectron transfer efficiency. VP/Hy-BC demonstrated exceptional efficacy in inhibiting bacterial proliferation; for instance, it extended beef shelf life by up to 10 days. The findings of this study will aid the development of health-conscious, eco-friendly, and efficient antimicrobial packaging films.

Molecular insights into the performance of promoters for carbon dioxide hydrate

Hydrate-based CO2 storage is a cost-effective and environmentally friendly approach to reduce carbon emission, and the addition of hydrate promoters has shown a promising avenue for enhancing CO2 hydrate formation. In this work, the promotion mechanism and promotion performance of five different hydrate promoters (denoted as DIOX, CP, THF, THP, and CH) were investigated and compared by first-principles calculations and molecular dynamics simulations. The results show that the hydrate promoters prefer to singly occupy 51264 cages of the sII hydrate, and CO2 molecules can singly occupy 512 cage or multiply occupy 51264 cages. The cohesive energy density indicates that the optimum CO2 storage capacity can reach up to ∼28 wt%. The stabilization effects of hydrate promoters on the hydrate stability should follow the order of CP > CH > DIOX > THF ≈ THP. The hydrate promoters can increase the water-water interactions, and the molecular diffusivity shows that the dynamic stability of the hydrates is THP ≈ CH > CP > DIOX > THF. Further, the hydrate promoters can accelerate the hydrate formation kinetics, which reduce the induction time and increase the nucleation and growth process.

Design and Optimization of Iron-Based Superionic-Like Conductor Anode for High-Performance Lithium/Sodium-Ion Batteries.

Metal selenides have received extensive research attention as anode materials for batteries due to their high theoretical capacity. However, their significant volume expansion and slow ion migration rate result in poor cycling stability and suboptimal rate performance. To address these issues, the present work utilized multivalent iron ions to construct fast pathways similar to superionic conductors (Fe-SSC) and introduced corresponding selenium vacancies to enhance its performance. Based on first-principles calculations and molecular dynamics simulations, it is demonstrated that the addition of iron ions and the presence of selenium vacancies reduced the material's work function and adsorption energy, lowered migration barriers, and enhances the migration rate of Li+ and Na+. In Li-ion half batteries, this composite material exhibites reversible capacity of 1048.3 mAh g-1 at 0.1 A g-1 after 100 cycles and 483.6 mAh g-1 at 5.0 A g-1 after 1000 cycles. In Na-ion half batteries, it is 687.7 mAh g-1 at 0.1 A g-1 after 200 cycles and 325.9 mAh g-1 at 5.0 A g-1 after 1000 cycles. It is proven that materials based on Fe-SSC and selenium vacancies have great applications in both Li-ion batteries and Na-ion batteries.

Corrosion inhibition of Ga-based thermal interface materials with Ni coating on Cu substrate

The gallium (Ga) base liquid metal (LM) with high thermal conductivity and deformability is applied widely in thermal management in electrical systems. However, Ga readily reacts with Cu to form intermetallic compounds (IMCs), which can cause failure in electronic devices. Herein, the thermal interface material (TIM) with Ga and diamond is prepared to analyze the reaction behavior of TIM and Cu/Ni-plated Cu under an aging test at 125 °C. The results show that Ga reacts with Cu substrate and forms CuGa2 bulk. Moreover, Ni coating can inhibit the corrosion of Ga and Cu. However, the as-volcano IMC structures were generated, also. The thermal resistance increased from 2.24 mm2·K·W−1 to 14.47 mm2·K·W−1 from initial time to 200 h. The interfacial corrosion behavior between TIM and Cu substrate is studied by first-principles molecular dynamics simulation. The diffusion coefficient in the z direction of Cu is 0.04 × 10−5 cm2/s. In a word, the work could provide the guidelines for the further design of high-performance corrosion resistance in the region where Ga was used for thermal management on electronic products.

Superionic iron hydride shapes ultralow-velocity zones at Earth’s core–mantle boundary

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary.

Reactant Discovery with an Ab Initio Nanoreactor: Exploration of Astrophysical N-Heterocycle Precursors and Formation Pathways.

The incorporation of nitrogen atoms into cyclic compounds is essential for terrestrial life; nitrogen-containing (N-)heterocycles make up DNA and RNA nucleobases, several amino acids, B vitamins, porphyrins, and other components of biomolecules. The discovery of these molecules on meteorites with non-terrestrial isotopic abundances supports the hypothesis of exogenous delivery of prebiotic material to early Earth; however, there has been no detection of these species in interstellar environments, indicating that there is a need for greater knowledge of their astrochemical formation and destruction pathways. Here, we present results of simulations of gas-phase pyrrole and pyridine formation from an ab initio nanoreactor, a first-principles molecular dynamics simulation method that accelerates reaction discovery by applying non-equilibrium forces that are agnostic to individual reaction coordinates. Using the nanoreactor in a retrosynthetic mode, starting with the N-heterocycle of interest and a radical leaving group, then considering the discovered reaction pathways in reverse, a rich landscape of N-heterocycle-forming reactivity can be found. Several of these reaction pathways, when mapped to their corresponding minimum energy paths, correspond to novel barrierless formation pathways for pyridine and pyrrole, starting from both detected and hypothesized astrochemical precursors. This study demonstrates how first-principles reaction discovery can build mechanistic knowledge in astrochemical environments as well as in early Earth models such as Titan's atmosphere where N-heterocycles have been tentatively detected.

First-principles study of non-linear thermal expansion in cadmium titanate by molecular dynamics incorporating nuclear quantum effects

First-principles molecular dynamics (FPMD) simulations were applied for analyzing structural evolutions around the paraelectric-ferroelectric phase transition temperature in the perovskite-type cadmium titanate, CdTiO3. Since the phase transition is reported to occur at the low temperature around 80 K, the quantum thermal bath (QTB) method was utilized in this study, which incorporates the nuclear quantum effects (NQEs). The structural evolutions in the QTB-FPMD simulations are in reasonable agreement with the experimental results, by contrast in the conventional FPMD simulations using the classical thermal bath (CTB-FPMD). Especially, the non-linear thermal expansion of lattice constants around the phase transition temperature was well reproduced in the QTB-FPMD with the NQEs. Thus, the NQEs are of importance in phase transitions at low temperatures, particularly below the room temperature, and the QTB is useful in that it incorporates the NQEs in MD simulations with low computational costs comparable to the conventional CTB.

Estimation of antigorite wave velocities in subduction conditions based on first-principles thermoelasticity

The most abundant serpentine mineral in subduction settings, antigorite has one of the highest water storage capacities and is involved in seismicity. Seismic wave velocities of antigorite are important for detecting and quantifying serpentinization within the mantle wedge and the subducting oceanic plate. At present, the elastic properties of antigorite at high pressures and temperatures are unclear. In this study, we have investigated pressure-volume-temperature (P-V-T) data and thermodynamic properties of antigorite using first-principles molecular dynamics (FPMD) simulations. Using these simulations results, we computed the relevant thermoelastic parameters and estimated compressional and shear wave velocities (vP and vS) of antigorite in subduction conditions. A simplified velocity model of antigorite with its coexisting mantle anhydrous phases was introduced to help us understand the potential effect of serpentinization on the seismic velocity of mantle rocks. Combined with seismic observations, we re-evaluated some velocity anomalies within forearc mantle wedges and established reliable serpentinization budgets. These results can provide preliminary evaluations and reliable constraints on serpentinization and water content in mantle rocks, which has important implications for understanding global plate dynamics and the deep water cycle.

The Hugoniot curve and sound velocity of forsterite to 1200 GPa

The comprehension of the composition and physical state of the deep interiors of large planets, as well as the impact events pertinent to planetary formation and evolution, necessitates an understanding of the properties of planetary materials under extreme conditions. Forsterite (Mg2SiO4), a significant geological mineral, has not been fully characterized in terms of its behavior under shock compression due to a lack of consensus among previous experiments and simulations aimed at determining its Hugoniot, as well as the absence of knowledge of sound velocity at high pressures, a critical parameter indicative of phase transformation and melting.In this study, we delineated the Hugoniot curve of the mineral forsterite up to immense pressures of 1200 GPa. For the first time, we successfully constrained its sound velocity along the Hugoniot curve up to 760 GPa by combining laser-driven shock experiments with first-principles molecular dynamics simulations. The measured Hugoniot data for forsterite corroborated previous findings and suggested the occurrence of incongruent melting during shock compression. Remarkably, along their respective Hugoniot curves, the sound velocity of forsterite was observed to fall between that of the minerals bridgmanite and periclase. The remarkable agreement between the experimental results and simulation data provides reliable sound velocity measurements on the forsterite Hugoniot, which is critical for comprehensively understanding the phase transition and melting behavior of forsterite under ultra-high pressures. This knowledge sheds invaluable light on the behavior of this significant geological mineral under extreme conditions akin to those found in the interiors of planets.

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications.

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air-water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first-principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface-rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed.

High-pressure phases of liquid silane studied using first-principles molecular dynamics simulations

High-pressure phases of liquid silane studied using first-principles molecular dynamics simulations

A first-principles study of the phase transitions in ultrahigh temperature shape memory alloy RuNb

Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P21/m symmetry instead of experimentally identified P2/m. The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost.

Machine Learning Force Field beyond the Limits of Classical and First-Principles Molecular Dynamics Simulations: The Case of Kaolinite Hydration

Machine Learning Force Field beyond the Limits of Classical and First-Principles Molecular Dynamics Simulations: The Case of Kaolinite Hydration

Effects of the Substitution of B and C for P on Magnetic Properties of FePCB Amorphous Alloys

In the present study, first-principles molecular dynamics simulations were employed to study the effects of small amounts of B and C substituted for P on the structure and magnetic properties of Fe80P13C7, Fe80P10C7B3, and Fe80P8C9B3 amorphous alloys. A small amount of B and C replacing P atoms increases the icosahedral structure of the amorphous alloys, especially the increase in the regular icosahedral structure. The saturation magnetization of the three kinds of amorphous alloys gradually increases with the addition of B and C atoms, and the results of experimental and simulated calculations show consistent trends. The substitution of P atoms by B and C atoms leads to the aggregation of Fe atoms, which increases the magnetic moment of the iron atoms. In addition, the improvement of local structural symmetry may be one of the reasons for the increase in saturation magnetization of amorphous alloys. The substitution of a small number of B and C atoms plays an important role in improving the saturation magnetization of the amorphous alloy, which has a certain guiding significance for the development of amorphous alloys with excellent soft magnetic properties.

Coherent Transfer of Lattice Entropy via Extreme Nonlinear Phononics in Metal Halide Perovskites

Entropy transfer in metal halide perovskites, characterized by significant lattice anharmonicity and low stiffness, underlies the remarkable properties observed in their optoelectronic applications, ranging from solar cells to lasers. The conventional view of this transfer involves stochastic processes occurring within a thermal bath of phonons, where the lattice arrangement and energy flow from higher- to lower-frequency modes. Here, we unveil a comprehensive chronological sequence detailing a conceptually distinct coherent transfer of entropy in a prototypical perovskite CH3NH3Pbl3. The terahertz periodic modulation imposes vibrational coherence into electronic states, leading to the emergence of mixed (vibronic) quantum beat between approximately 3 and 0.3 THz. We highlight a well-structured bidirectional time-frequency transfer of these diverse phonon modes, each developing at different times and transitioning from high to low frequencies from 3 to 0.3 THz, before reversing direction and ascending to around 0.8 THz. First-principles molecular dynamics simulations disentangle a complex web of coherent-phononic coupling pathways and identify the salient roles of the initial modes in shaping entropy evolution at later stages. Capitalizing on coherent entropy transfer and dynamic anharmonicity presents a compelling opportunity to exceed the fundamental thermodynamic (Shockley-Queisser) limit of photoconversion efficiency and to pioneer novel optoelectronic functionalities. Published by the American Physical Society 2024

Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface.

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

First-Principles Investigation of the Effects of UF4 and ThF4 Fuels on the Structural, Dynamic, and Thermodynamic Properties of LiF-NaF.

An in-depth understanding and characterization of molten salt properties are necessary for the optimized design, efficient operation, and safety assurance of molten salt reactors (MSRs). Investigating molten salt properties in experimental settings can be challenging and time-consuming due to the high temperatures of interest, the salt's corrosiveness, purity and composition control, and health and safety concerns. Therefore, it is beneficial to perform computational screening to assist in the ultimate experimental measurements. Herein, we used first-principles molecular dynamics simulations to calculate several thermophysical, structural, and dynamic properties of eutectic LiF-NaF with fuel additives UF4 and ThF4. We found that with the incorporation of uranium or thorium, a prepeak appears in the structure factor, indicative of a medium-range structural ordering. Furthermore, we explore the mechanism through which these structural changes enhance shear stress correlations, thereby increasing the salt's viscosity. This work highlights the importance of studying the atomic-scale structure of molten salts and how the addition of fuel elements can substantially affect it.

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

Hydrogen and silicon are the preferred light elements in Earth’s core

Hydrogen is an important light element in the Earth’s core for its high cosmochemical abundance and strong affinity to iron under core-formation conditions. Thus, constraining the core composition requires knowledge on the distribution of hydrogen between the liquid outer core and solid inner core. Here we investigate the chemical equilibrium of hydrogen at the inner-core boundary by calculating the chemical potential of hydrogen in solid and liquid iron-hydrogen alloys, respectively, using first-principles molecular dynamic simulations and neural network methods. We find that hydrogen partitions preferentially into the outer core and provides a major contribution to the density jump across the inner-core boundary. Combining geophysical constraints, mineral physics data, and chemical equilibrium, we evaluated light element abundances in the outer and inner cores simultaneously. Our results suggest hydrogen and silicon are the preferred light elements in the core, implying a relatively reduced environment during the Earth’s accretion and core-formation processes.

First-principle molecular dynamics and in situ DRIFTS study the stability of the intermediates of CO2 hydrogenation to methanol on ZnZrOx solid solution catalyst

Among the many catalysts used for CO2 hydrogenation to methanol, ZnZrOx solid solution is a highly efficient catalyst. Although the reaction mechanism has been widely explored at 0 K using DFT, critical aspects such as the atomic structure of the solid solution and the stability of reaction intermediates during CO2 reduction at experimental temperatures remain unclear. This study employs first-principle molecular dynamics (MD) simulations, FTIR and DRIFTS characterization to address these important questions. During the MD simulation, oxygen vacancies are formed in the system and same as the doped Zn atoms. The formation of oxygen vacancies was further confirmed by the Transmission FTIR and in situ DRIFTS. Due to the creating of oxygen vacancies, the coordination of Zn and Zr in the bulk is reduced to five and seven, respectively. And on the surface, their coordination is reduced to three、four and six、seven, respectively. Subsequently, the hydrogenation of CO2 to methanol using the formate and CO* pathways was studied. In the formate pathway, all intermediates remained stable at 593 K. However, in the CO* pathway, we observe instability of key intermediates which also confirmed by transmission FTIR and in situ DRIFTS characterization.