Publications

2025

1. SLICES-PLUS: A crystal representation leveraging spatial symmetry

   Baoning Wang, Zhiyuan Xu, Zhiyu Han, Qiwen Nie , Xi Chen,  Hang Xiao , and Gang Yan

   Materials & Design, May 2025

   DOI
2. Improving SLICES crystal representation through CHGNet integration and parameter tuning

   Bizhu Zhang, Kedeng Wu, Chang Zhang,  Hang Xiao , and Liangliang Zhu

   AIP Advances, May 2025

   Abs DOI

   In the rapidly evolving field of materials science, the inverse design of crystals has become increasingly significant with the widespread adoption of generative models. However, current crystal representation methods face persistent challenges in maintaining both invertibility and invariance. To bridge this gap, we present an enhanced approach to the simplified line-input crystal-encoding system (SLICES) representation by incorporating the Crystal Hamiltonian Graph Neural Network (CHGNet) machine learning force field model. Comprehensive evaluations across multiple datasets (MP-20, MP21-40, and MOF) using different neighbor recognition algorithms (EconNN and CrystalNN) show that our approach outperforms the original in reconstructing structures with fewer than 20 atoms per unit cell, achieving up to a 1.34% improvement in reconstruction rate (from 92.55% to 93.89%). Furthermore, through systematic optimization of key parameters, including bond scaling, Δx, and lower bounds of lattice scaling, we achieved enhanced reconstruction performance. Our research represents a step forward in developing more efficient solutions for crystal inverse design utilizing the SLICES representation framework.

2024

1. K_\textrm2 CO_\textrm3 on porous supports for moisture‐swing CO_\textrm2 capture from ambient air

   Shiqiang Zheng, Xinyue Cheng , Wenjia Zhou, Tong Wang, Liangliang Zhu,  Hang Xiao , and Xi Chen

   Asia-Pacific Journal of Chemical Engineering, May 2024

   Abs DOI

   Direct air capture (DAC) of CO2 is an important technology to mitigate mobile carbon emissions, reduce atmospheric CO2 concentration, and cope with climate change. Moisture-swing adsorption is regarded as one of the most promising technologies in DAC due to its low energy consumption and ease of operation. In this work, a cheap and easily available moisture-swing adsorbent of potassium carbonate loaded on porous supports (i.e., activated carbon, magnesium oxide, and zeolite) was prepared for CO2 capture from ambient air. The composite adsorbent of potassium carbonate on activated carbon showed the best performance with a DAC capacity of 0.562 mmol/g at 25 C and 5% relative humidity. The effects of temperature, relative humidity, and CO2 concentration on the adsorption performance were investigated systematically, as well as the cyclic DAC performance. In 50 adsorption–desorption cycles, the adsorption capacity of the composite adsorbent decreased by 40% due to potassium carbonate leaching loss during water evaporation but can be fully recovered simply by re-impregnating with potassium carbonate again.

2. MatterGPT: A Generative Transformer for Multi-Property Inverse Design of Solid-State Materials

   Yan Chen, Xueru Wang, Xiaobin Deng, Yilun Liu , Xi Chen, Yunwei Zhang, Lei Wang, and  Hang Xiao

   Aug 2024

   arXiv:2408.07608 [cond-mat]

   Abs DOI

   Inverse design of solid-state materials with desired properties represents a formidable challenge in materials science. Although recent generative models have demonstrated potential, their adoption has been hindered by limitations such as inefficiency, architectural constraints and restricted open-source availability. The representation of crystal structures using the SLICES (Simplified Line-Input Crystal-Encoding System) notation as a string of characters enables the use of state-of-the-art natural language processing models, such as Transformers, for crystal design. Drawing inspiration from the success of GPT models in generating coherent text, we trained a generative Transformer on the next-token prediction task to generate solid-state materials with targeted properties. We demonstrate MatterGPT’s capability to generate de novo crystal structures with targeted single properties, including both lattice-insensitive (formation energy) and lattice-sensitive (band gap) properties. Furthermore, we extend MatterGPT to simultaneously target multiple properties, addressing the complex challenge of multi-objective inverse design of crystals. Our approach showcases high validity, uniqueness, and novelty in generated structures, as well as the ability to generate materials with properties beyond the training data distribution. This work represents a significant step forward in computational materials discovery, offering a powerful and open tool for designing materials with tailored properties for various applications in energy, electronics, and beyond.

2023

1. Confronting the Carbon-Footprint Challenge of Blockchain

   Xiaoyang Shi,  Hang Xiao , Weifeng Liu, Klaus. S. Lackner, Vitalik Buterin, and Thomas F. Stocker

   Environmental Science & Technology, Jan 2023

   Publisher: American Chemical Society

   Abs DOI

   The distributed consensus mechanism is the backbone of the rapidly developing blockchain network. Blockchain platforms consume vast amounts of electricity based on the current consensus mechanism of Proof-of-Work (PoW). Here, we point out a different consensus mechanism named Proof-of-Stake (PoS) that can eliminate the extensive energy consumption of the current PoW-based blockchain. We comprehensively elucidate the current and projected energy consumption and carbon footprint of the PoW- and PoS-based Bitcoin and Ethereum blockchain platforms. The model of energy consumption of PoS-based Ethereum blockchain can lead the way toward the prediction of other PoS-based blockchain technologies in the future. With the widespread adoption of blockchain technology, if the current PoW mechanism continues to be employed, the carbon footprint of Bitcoin and Ethereum will push the global temperature above 1.5 °C in this century. However, a PoS-based blockchain can reduce the carbon footprint by 99% compared to the PoW mechanism. The small amount of carbon footprint from PoS-based blockchain could make blockchain an attractive technology in a carbon-constrained future. The study sheds light on the urgency of developing the PoS mechanism to solve the current sustainability problem of blockchain.

2. An invertible, invariant crystal representation for inverse design of solid-state materials using generative deep learning

   Hang Xiao , Rong Li, Xiaoyang Shi , Yan Chen, Liangliang Zhu , Xi Chen, and Lei Wang

   Nature Communications, Nov 2023

   Abs DOI

   Abstract The past decade has witnessed rapid progress in deep learning for molecular design, owing to the availability of invertible and invariant representations for molecules such as simplified molecular-input line-entry system (SMILES), which has powered cheminformatics since the late 1980s. However, the design of elemental components and their structural arrangement in solid-state materials to achieve certain desired properties is still a long-standing challenge in physics, chemistry and biology. This is primarily due to, unlike molecular inverse design, the lack of an invertible crystal representation that satisfies translational, rotational, and permutational invariances. To address this issue, we have developed a simplified line-input crystal-encoding system (SLICES), which is a string-based crystal representation that satisfies both invertibility and invariances. The reconstruction routine of SLICES successfully reconstructed 94.95% of over 40,000 structurally and chemically diverse crystal structures, showcasing an unprecedented invertibility. Furthermore, by only encoding compositional and topological data, SLICES guarantees invariances. We demonstrate the application of SLICES in the inverse design of direct narrow-gap semiconductors for optoelectronic applications. As a string-based, invertible, and invariant crystal representation, SLICES shows promise as a useful tool for in silico materials discovery.

3. Rippled blue phosphorene with tunable energy band structures and negative Poisson’s ratio

   Rong Li,  Hang Xiao , and Yan Chen

   AIP Advances, Nov 2023

   Publisher: AIP Publishing

2022

1. Controllable porous perovskite with three-dimensional ordered structure as an efficient oxygen reduction reaction electrocatalyst for flexible aluminum-air battery

   Ziyi Shui, Wei Zhao,  Hang Xiao , Liangliang Zhu, Yilun Liu, Xiaobin Deng , and Xi Chen

   Journal of Power Sources, Mar 2022

   Abs DOI

   Perovskite materials have recently attracted extensive attention since tailoring their chemical compositions has led to remarkable activity toward oxygen reduction reaction. However, the desired electrocatalytic activity is limited by the morphological effect, and lack of methods to achieve large surface area. Herein we report an effective strategy to synthesize three-dimensional ordered macroporous (3DOM) perovskite oxides, where La0.75Sr0.25MnO3 (3DOM LSMO) displays excellent ORR activity and durability with considerable specific surface area (43.1 m2 g-1). The electrochemical results exhibit that the electron transferred numbers (n) is close to 4 and the H2O2 yield (% H2O2) is as low as 10% for 3DOM LSMO, which mainly attributes to comprehensive effect of the reduced Mn valence state, the increased specific surface, and the exposed high activity crystal planes. Firstprinciples study confirms that the lowest overpotential obtained by LSMO is in good agreement with the experimental results. Our work demonstrates perovskite oxides with larger surface area could be advanced oxygen catalysts with wide applications.

2. Enhanced CO2 electroreduction on Co active site of cobalt phthalocyanine by electronic effect

   Huiying Tian, Kai Wang, Ziyi Shui, Muhammad Ali Raza,  Hang Xiao , Meidan Que, Liangliang Zhu , and Xi Chen

   Materials Letters, Mar 2022

   Abs DOI

   Metal phthalocyanines molecular catalysts exhibit the unique ability of CO2 electrochemical reduction reaction (CO2ERR) thanks to their well-defined macrocycle structure. In this work, we introduced different substituting functional groups at the phthalocyanine ring of cobalt phthalocyanine to study the relationship of molecular structure optimization and CO2ERR activity. An optimal nitro-substituted cobalt phthalocyanine catalyst can mediate CO2 to CO in a H-cell with maximum selectivity of ~94% together with a current density of 12.6 mA cm−2 at −0.877 V vs. RHE. The insights of this work on designing and optimizing of the molecular catalysts contribute to lower energy as well as cost-effective CO2ERR.

2021

1. Nanomaterials for adsorption and conversion of CO2 under gentle conditions

   Chao Lu, Xiaoyang Shi, Yilun Liu,  Hang Xiao , Junjie Li , and Xi Chen

   Materials Today, Nov 2021

   Abs DOI

   Global warming effect caused by excessive emission of greenhouse gases, such as CO2 from modern industries, are emerging as serious environmental problem nowadays. Many strategies have been developed to address this issue, including solar energy utilization, green plants cultivation and coal desulfurization. These existing strategies always need sophisticated equipment and harsh reaction conditions, and are usually with limited efficiency. With the development of nanoscience and nanotechnology, it is becoming an effective strategy to directly capture and convert CO2 with nanomaterials into valuable chemicals under gentle conditions. Herein, we summarize recent progress on nanomaterials for adsorption and conversion of CO2 under gentle conditions, including various physical and chemical processes, and artificial photosynthesis. Future perspective and research direction of nanomaterials for CO2 adsorption and conversion have been put forward on the basis of existing research works and our experimental experience.

2. Fractal-inspired soft deployable structure: a theoretical study

   Zechen Xiong,  Hang Xiao , and Xi Chen

   Soft Matter, May 2021

   Publisher: The Royal Society of Chemistry

   Abs DOI

   The study of soft deployable structures is an emergent field that is highly correlated with metamaterial design, soft robotics, medical devices, etc. This paper studies a novel two-dimensional (2D) soft deployable structure that has a fractal layout with hierarchically coupled thin walls, which buckles upon actuation and deforms into a “peacock tail” pattern that is over 10 fold its original dimension. Large deflection theory and finite-element (FE) modeling are used to characterize its mechanical performance and to investigate its potential application in multiple fields. Further, 2D FE homogenization is implemented to extend the novel design into an active plane lattice metamaterial, on which parametric studies are carried out to explore its effective stiffness and large strain properties. The results show that, besides excellent deformability, the “peacock tail” soft deployable structure and its lattice metamaterial derivates exhibit intriguing properties such as multi-stiffening, strong anisotropy, zero/negative Poisson’s ratio, a unique post-buckling collapse mechanism, etc. Three-dimensional generalization of the fractal compliant system is modeled to elaborate on the practical use of the structures. This paper aims to enrich the spectrum of soft deployable structures, shedding light on the research of novel soft robots, hierarchical structures, and metamaterials.

3. Post-wrinkling behaviors of a bilayer on a soft substrate

   Youlong Chen, Xiangbiao Liao, Wei Zhao, Pengfei Yang,  Hang Xiao , Yilun Liu , and Xi Chen

   International Journal of Solids and Structures, Apr 2021

   Abs DOI

   The instability behavior of a compressive bilayer on a soft substrate is of significance for epidermal electronics adhered to human skin. Conventionally, two different wrinkling modes dependent of the modulus of intermediate layer were proposed to evolve from flat feature. Here, we uncover a new periodic pattern of secondary wrinkling feature with further compression in the case of a moderate-modulus intermediate layer. FEM simulation results illustrate a linear relationship between this wavelength of secondary wrinkling and the modulus of intermediate layer. An analytical model based on Timoshenko beam theory is utilized to predict the wavelength and critical strain of this secondary wrinkling, which agrees well with that in FEM simulations. Further, a phase diagram of instability feature in the system of bilayer/soft substrate is proposed. The secondary wrinkling feature with such large wavelength has broad applications in tunable surface morphologies for smart glasses, color controlling and manipulation of water droplets.

4. Gas diffusion in polymer nanocomposites: Role of defects and caves in fillers

   Jianfeng Wan, Wenyan Bi, Xiangbiao Liao,  Hang Xiao , Xi Chen , and Junjie Chen

   Journal of Polymer Research, Apr 2021

   Abs DOI

   Gas barrier in polymers is able to be effectively enhanced by adding air-impervious nano-fillers. It is inevitable that there are some defects or caves on the surface of nano-fillers, which can have a certain effect on the gas barrier. The effect of defects and caves in nano-fillers was investigated using the finite element model in order to better understand the diffusion process in a polymer nanocomposite film with staggered array nano-fillers by varying the microstructural features such as volume fraction, aspect ratio, gaps between fillers, and so on. The results showed that the effect of defects and caves on relative diffusivity arises from the following two aspects: the first is the effect of cave volume which dominates, and the second is the induced effect from defects and caves. A theoretical model for predicting relative diffusivity was developed based on the volume effect, and then was generalized into three dimensions. In order to estimate the value range of the model, the upper and lower boundaries of the model have been derived. Comparisons were then made between the estimated and simulated values, and the results showed that the upper and lower boundaries are basically correct, and especially the upper boundary is on the same order of magnitude as the real value.

5. Strong bases behave as weak bases in nanoscale chemical environments: implication in humidity-swing CO2 air capture

   Yingying Han, Liangliang Zhu, Yutong Yao, Xiaoyang Shi, Yayun Zhang,  Hang Xiao , and Xi Chen

   Physical Chemistry Chemical Physics, Jul 2021

   Publisher: The Royal Society of Chemistry

   Abs DOI

   Hydration of ions/molecules in nanometer-sized clusters or nanoscopic pores is ubiquitous and plays a key role in many chemical and physical systems. In this work, guanidine–H2O reactions with n = 1–8 water molecules were systematically studied by ab initio methods. The result suggests that the reduced availability of water molecules greatly inhibits the strong base guanidine from producing OH−. That is, guanidine exhibits the behavior of a weak bases in low-humidity nanoscale environments. Intriguingly, this effect is not limited to guanidine but could be applied to other strong bases. Furthermore, we demonstrate that the direction of guanidine–CO2 reactions can be controlled by changing the number of water molecules present, which in turn responds to the humidity change in air. These findings not only shed some light on unconventional chemical reactions of strong bases in atmospheric clusters and on solid porous surfaces, but also provide insights into the development of guanidine-based CO2 air-capture sorbents.

2020

1. Moisture-Driven CO2 Sorbents

   Xiaoyang Shi,  Hang Xiao , Kohei Kanamori, Akio Yonezu, Klaus S. Lackner , and Xi Chen

   Joule, Aug 2020

   00010

   Abs DOI

   An energy-saving system containing ion-exchange or nanoporous materials and carbonate ions is proposed, which is capable of capturing CO2 from ambient air simply by controlling the amount of water (moisture) in contact with the sorbent. The system binds CO2 from the air when the surrounding is dry, whereas it desorbs CO2 when it is wet. A design of such CO2 sorption and desorption systems is investigated using quantum mechanics simulations and is verified by experiments. Its working mechanism is revealed as the free energy change of the chemical reaction of the carbonate ions and water molecules; the free energy change decreases when the number of water molecules in the materials decreases. The influence of pore size, spacing of cations, and surface hydrophobicity of the sorbents on CO2 capture efficiency are elucidated. The study sheds light on ways to optimize an efficient direct air capture system and therefore contributes to the development of “negative emission technologies.”

2. Highly efficient reduction of O2-containing CO2 via chemical looping based on perovskite nanocomposites

   Yan’e Zheng, Xiangbiao Liao,  Hang Xiao , Vasudev Haribal, Xiaoyang Shi, Zhen Huang, Liangliang Zhu, Kongzhai Li, Fanxing Li, Hua Wang , and Xi Chen

   Nano Energy, Dec 2020

   Abs DOI

   Purification/separation of CO2 stream from carbon capture or other carbon source is highly energy consuming process. However, oxidative impurity of O2 either deactivates catalysts in most carbon reduction systems, and thus reduces CO2 conversion efficiency. Here we report an effective method for splitting O2-containing CO2 into CO, through a chemical looping scheme with Cu (5 at%) doped LaFeO3 perovskites as efficient oxygen carriers. Up to 2.28 mol/kg CO yield was achieved with high stability in the presence of O2, five times higher than that with the state-of-the-art oxygen carrier, while pristine LaFeO3 perovskite only showed efficient capability of reducing pure CO2. Furthermore, the syngas productivity was doubled with Cu modification. Through experimental characterizations and ab initio calculations, we uncovered that the exsolution of metallic Cu on the surface of reduced perovskite was able to mitigate the competition between CO2 and O2 in the re-oxidation step. We envision that the efficient CO2 splitter with well-designed oxygen carriers have the potential to facilitate economical combination of impure carbon feedstock and carbon utilization system.

3. Investigation of Water-Stable Perovskite DMASnIxBr3- x for Photoenzyme Catalysis in Aqueous Solution

   Dianxing Ju, Gang Lin,  Hang Xiao , Yuanyuan Zhang, Shigang Su, and Jian Liu

   Solar RRL, Oct 2020

   Publisher: Wiley Online Library
4. Broadband CrOCl Saturable Absorber with a Spectral Region Extension to 10.6 μm

   Mengxia Wang, Jian Zhang, Zhengping Wang, Ci Wang, Sander Smaalen,  Hang Xiao , Xi Chen, Chenlin Du, Xinguang Xu, and Xutang Tao

   Advanced Optical Materials, Oct 2020

   Publisher: Wiley Online Library
5. Sorbents for the direct capture of CO2 from ambient air

   Xiaoyang Shi,  Hang Xiao , Habib Azarabadi, Juzheng Song, Xiaolong Wu , Xi Chen, and Klaus S Lackner

   Angewandte Chemie International Edition, Oct 2020

   Publisher: Wiley Online Library
6. Comment on “Accelerated Discovery of New 8-Electron Half-Heusler Compounds as Promising Energy and Topological Quantum Materials”

   Hang Xiao , Yong Dan , Bingbing Suo , and Xi Chen

   The Journal of Physical Chemistry C, Oct 2020

   Publisher: ACS Publications
7. Interaction between mechanosensitive channels embedded in lipid membrane

   Liangliang Zhu, Wei Zhao, Yuan Yan, Xiangbiao Liao, Athanasios Bourtsalas, Yong Dan,  Hang Xiao , and Xi Chen

   Journal of the Mechanical Behavior of Biomedical Materials, Mar 2020

   Abs DOI

   The study of the gating mechanism of mechanosensitive channels opens a window to the exploration of how different mechanical stimuli induce adaptive cellular behaviors of both the protein and the lipid, across different time and length scales. In this work, through a molecular dynamics-decorated finite element method (MDeFEM), the gating behavior of mechanosensitive channels of small conductance (MscS) in Escherichia coli (E. coli) is studied upon membrane stretch or global bending. The local membrane curvature around MscS is incorporated, as well as multiple MscL (mechanosensitive channels of large conductance) molecules in proximity to MscS. The local membrane curvature is found to delay MscS opening and diminishes moderately upon membrane stretching. Mimicking the insertion of lysophosphatidylcholine (LPC) molecules into the lipid, both downward and upward bending can active MscS, as long as the global membrane curvature radius reaches 34 nm. Based on the different MscS pore evolutions observed with the presence of one or more MscLs nearby, we propose that when coreconstituted, multiple MscL molecules tend to be located at the local membrane curvature zone around MscS. In another word, as MscL “swims around” in the lipid bilayer, it can be trapped by the membrane’s local curvature. Collectively, the current study provides valuable insights into the interplay between mechanosensitive channels and lipid membrane at structural and physical levels, and specific predictions are proposed for further experimental investigations.

2019

1. On the surface hydrophilization of a blended polysulfone membrane: atomic force microscopy measurement and molecular dynamics simulation

   Kazunori Miyamoto, Daiki Ikeshima, Takumi Furutani,  Hang Xiao , Akio Yonezu , and Xi Chen

   Surface Topography: Metrology and Properties, Jul 2019

   Abs DOI

   This study elucidates a mechanism to improve fouling resistance for a polysulfone (PSf) membrane by blending polyvinylpyrrolidone (PVP). PSf is generally hydrophobic in nature and tends to be fouled by natural organic matter (NOM) due to a hydrophobic bonding during membrane filtration. Thus, to reduce hydrophobic bonding, a hydrophilic treatment is conducted using non-solvent induced phase separation, in which PVP is added to the PSf membrane. To investigate the chemical properties of the membrane surface, a surface elemental analysis using x-ray photoelectron spectroscopy and contact angle measurement of water droplets is conducted. It is found that, with an increase in PVP additive, macroscopic hydrophilicity increases. Next, to measure the surface adsorption of hydrophobic bonding, atomic force microscopy (AFM) is used. In this study, the AFM probe tip is chemically modified with hydrophobic matter that mimics hydrophobic NOM. With this AFM tip, contact force measurements are conducted in order to measure the absorption characteristics between the membrane surface and the NOM. Furthermore, a cross-flow filtration test is carried out to measure adsorption amounts of tannic acid (as a model hydrophobic substance) into the PSf membrane. Finally, these experimental results were phenomenologically investigated using a molecular dynamics simulation to clarify the hydrophilicity mechanism and improvement of anti-fouling performance of the membrane.

2. Moisture Swing Ion-Exchange Resin-PO4 Sorbent for Reversible CO2 Capture from Ambient Air

   Juzheng Song, Liangliang Zhu, Xiaoyang Shi, Yilun Liu,  Hang Xiao , and Xi Chen

   Energy & Fuels, Jul 2019

   Publisher: American Chemical Society

   Abs DOI

   Reversible CO2 capture from ambient air by a humidity swing has shown great potential in mitigating the greenhouse effect. In this work, we developed a new humidity-swing absorbent based on PO43–/HPO42–/H2PO4– ions exhibiting superior CO2 absorption capacity and kinetics compared to that of CO32–/HCO3–-based absorbent. After ion exchange with PO43– ions, the ion-exchange resin (IER-PO4) containing positive quaternary ammonium groups and movable PO43– ions is able to reversibly capture CO2 from the ambient air by a humidity swing, which triggers the transformation between the PO43– ions and HPO42–/H2PO4– ions in the resin. In a dry environment, PO43– ions in IER-PO4 are hydrolyzed into OH– ions and HPO42– ions, which are further hydrolyzed into H2PO4– ions and OH– ions. Both hydrolysis reactions produce OH– ions for CO2 absorption, while the adsorbed CO2 can be released in a humid environment. The results of quantum chemical calculation show that the hydrolysis of the ions is promoted by the reduction of water molecules in the nanoscale hydrated cluster. The adsorption capacity of IER-PO4 during the moisture swing is 80% larger than that of IER-CO3, and the adsorption rate of the IER-PO4 resin at a temperature range of 15–35 °C is much higher than that of the IER-CO3 absorbent. A modified pseudo-first-order (MPFO) kinetic is developed, which can describe the experimental results well. The present study sheds light on the design of high performance moisture-swing absorbents with PO43– ions.

3. Three-dimensional auxetic properties in group V–VI binary monolayer crystals X 3 M 2 (X = S, Se; M = N, P, As)

   Yan Chen, Xiangbiao Liao, Xiaoyang Shi,  Hang Xiao , Yilun Liu , and Xi Chen

   Physical Chemistry Chemical Physics, Jul 2019

   Publisher: Royal Society of Chemistry

   DOI
4. Mechanical modeling of pimple growth

   Xiangbiao Liao, Xiaobin Deng, LiangLiang Zhu, Xiaoyang Shi,  Hang Xiao , and Xi Chen

   Journal of the Mechanical Behavior of Biomedical Materials, Jul 2019

   Abs DOI

   Pimple is one of the most common skin diseases for humans, whose growth cause pain yet the corresponding mechanical analysis is lacking. A finite element model is developed to quantify the deformation field with the expansion of follicle, and then the mechanical stimulus is related to the sensation of pain during the development of pimple. Parametric studies show the dependence of mechanical stimulus and pain level on the pimple-surrounded structures, follicle depth and mechanical properties of the epidermis. The findings in this paper may provide useful insights on prevention or pain mitigation of pimples, as well as those related to other tissue growth and respective cosmetic concerns.

5. Elementary Slender Soft Robots Inspired by Skeleton Joint System of Animals

   Pengfei Yang, Xueru Wang, Fei Dang, Zhe Yang, Zeming Liu, Yingbo Yan, Liangliang Zhu, Yilun Liu,  Hang Xiao , and Xi Chen

   Soft Robotics, Mar 2019

   Publisher: Mary Ann Liebert, Inc., publishers

   Abs DOI

   In this article, we demonstrate elementary slender soft robots mimicking the skeleton joint system of animals with excellent locomotion performance and scalability. Inspired from bending characteristics of mammal joints, the soft joint, consisting of two Pneu-nets (PNs), one with three chambers and the other with two chambers, was developed. By using simple constraints and connections, the soft joint can be separately controlled by 180° bending and unbending, and exhibits large force output with limited volume and deformation. Then, a skeleton joint-like slender robot was proposed by including a different number of the soft joint and stiff skeleton for different functionalities. This robot exhibits excellent locomotion performance due to the skeleton joint design to achieve large deformation level and proper selection of gait to fully utilize the skeleton joint-like motion, that is, moving 0.5 body length per loading cycle and navigating both continuous terrains, such as a slope, and unprecedented terrains with “discrete” obstacles, such as a wall and stairs. Besides, the slender soft robot is scalable in joint and skeleton, and additional tuning joints can be introduced to achieve turning and spinning locomotion. The slender robot presented herein has not only the advantages of simple fabrication, light weight, good adaptability and scalability, and human-friendly interface but also provides insights for future development of mobile soft robots.

6. Gating and inactivation of mechanosensitive channels of small conductance: A continuum mechanics study

   Liangliang Zhu, Qiang Cui,  Hang Xiao , Xiangbiao Liao , and Xi Chen

   Journal of the Mechanical Behavior of Biomedical Materials, Feb 2019

   Abs DOI

   Mechanosensitive channels of small conductance (MscS) in Escherichia coli (E. coli) serve as a paradigm for understanding the gating behaviors of the MscS family of ion channels. In this work, we develop a continuum mechanics framework to explore the conformational states of MscS during the gating transition. A complete gating transition trajectory from the closed to the open state along with partially open intermediates is obtained, and the open structure is close to the available structural model from crystallographic studies. The computational efficiency of the modeling framework makes it possible to explore the roles of various structural elements (e.g., loops that connect transmembrane helices) and specific interactions in the gating transition. It is observed that removing either the Asp62-Arg131 salt bridge or the Phe68-Leu111 non-polar interaction leads to essentially non-conducting structures even with a membrane tension close to the lysis limit. The loop connecting TM2 (the second transmembrane helix) and TM3 is found to be essential for force transmission during gating, while the loop connecting TM1 and TM2 does not make any major contribution. Based on the different structural evolutions observed when the TM3 kink is treated as a loop or a helical segment, we propose that the helical propensity of the kink plays a central role in inactivation; i.e., under prolonged sub-threshold membrane tension, transition of the initially flexible loop to a helical segment in TM3 may lead to MscS inactivation. Finally, the gating transition of MscS under different transmembrane voltages is explored and found to be essentially voltage independent. Collectively, results from the current continuum mechanics analysis provide further insights into the gating transition of MscS at structural and physical levels, and specific predictions are proposed for further experimental investigations.

7. Strain-Guided Oxidative Nanoperforation on Graphene

   Xiangbiao Liao, Baidu Zhang, Takumi Furutani , Youlong Chen,  Hang Xiao , Yong Ni, Akio Yonezu , and Xi Chen

   Small, Feb 2019

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/smll.201903213

   Abs DOI

   Increased applications of nanoporous graphene in nanoelectronics and membrane separations require ordered and precise perforation of graphene, whose scalablility and time/cost effectiveness represent a significant challenge in existing nanoperforation methods, such as catalytical etching and lithography. A strain-guided perforation of graphene through oxidative etching is reported, where nanopores nucleate selectively at the bulges induced by the prepatterned nanoprotrusions underneath. Using reactive molecular dynamics and theoretical models, the perforation mechanisms are uncovered through the relationship between bulge-induced strain and enhanced etching reactivity. Parallel experiments of chemical vapor deposition (CVD) of graphene on SiO2 NPs/SiO2 substrates verify the feasibility of such strain-guided perforation and evolution of pore size by exposure of varied durations to oxygen plasma. This scalable method can be feasibly applied to a broad variety of 2D materials (e.g., graphene and h-boron nitride) and nanoprotrusions (e.g., SiO2 and C60 nanoparticles), allowing rational fabrication of 2D material-based devices.

8. Effect of Local Terrace on Structure and Mechanics of Graphene Grain Boundary

   Yan Chen, Xueru Wang, Yilun Liu,  Hang Xiao , and Xi Chen

   The Journal of Physical Chemistry C, Nov 2019

   Publisher: American Chemical Society

   Abs DOI

   In this study, the effects of local terrace of the substrate on the structure and mechanical properties of graphene grain boundaries (GBs) during chemical vapor deposition (CVD) growth have been explored by phase-field crystal modeling and molecular dynamics simulations. It is found the GBs are significantly disturbed for a weak surface disturbance with a bulge height of only 3.4 Å. The distance between GBs and the bulge plays an important role in determining the morphologies of GBs, and the aperiodic and curved GBs can be observed, which is attributed to several representative structures, like 5–6\textbar6–7 and 5–6–7 defects and GB deflection. In general, there are four fracture modes for GBs with weak surface disturbance depending on the existence of 5–6–7 and 5–6\textbar6–7 defects. While for a strong surface disturbance with a bulge height of 10 Å, the interaction between topological defects at bulged graphene and GBs will locally offset nearby 5–7 dislocation pairs, and there are two fracture modes depending on the structural integrality of GBs. Besides, it is also found that the aperiodic and curved morphologies of GBs widely exist for other orientation angles with surface disturbance. The results presented herein explore the mechanism for the interaction of GBs with surface disturbance during CVD growth and may provide some useful insights for designing and regulating the morphologies of GBs.

2018

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

   Xiangbiao Liao,  Hang Xiao , Xiaobo Lu , Youlong Chen, Xiaoyang Shi , and Xi Chen

   Nanotechnology, Nov 2018

   00002

   Abs DOI

   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.

2. Prediction of a two-dimensional S3N2 solid for optoelectronic applications

   Hang Xiao , Xiaoyang Shi, Xiangbiao Liao, Yayun Zhang , and Xi Chen

   Physical Review Materials, Feb 2018

   Abs DOI

   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 (S3N2) 2D crystal that is a covalent network composed solely of S-N σ bonds. The S3N2 crystal is dynamically, thermally, and chemically stable, as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. GW calculations show that the S3N2 crystal is a wide, direct band-gap (3.92 eV) semiconductor with a small-hole effective mass. In addition, the band gap of S3N2 structures can be tuned by forming multilayer S3N2 crystals, S3N2 nanoribbons, and S3N2 nanotubes, expanding its potential applications. The anisotropic optical response of the 2D S3N2 crystal is revealed by GW–Bethe-Salpeter-equation calculations. The optical band gap of S3N2 is 2.73 eV and the exciton binding energy of S3N2 is 1.19 eV, showing a strong excitonic effect. Our result not only marks the prediction of a 2D crystal composed of nitrogen and sulfur, but also underpins potential innovations in 2D electronics and optoelectronics.

3. Molecular Dynamics-Decorated Finite Element Method (MDeFEM): Application to the Gating Mechanism of Mechanosensitive Channels

   Liangliang Zhu, Qiang Cui, Yilun Liu, Yuan Yan,  Hang Xiao , and Xi Chen

   In Handbook of Nonlocal Continuum Mechanics for Materials and Structures, Feb 2018

   Abs DOI

   Many fundamentally important biological processes rely on the mechanical responses of membrane proteins and their assemblies in the membrane environment, which are multiscale in nature and represent a significant challenge in modeling and simulation. For example, in mechanotransduction, mechanical stimuli can be introduced through macroscopic-scale contacts, which are transduced to mesoscopic-scale (micron) distances and can eventually lead to microscopic-scale (nanometer) conformational changes in membrane-bound protein or protein complexes. This is a fascinating process that spans a large range of length scales and time scales. The involvement of membrane environment and critical issues such as cooperativity calls for the need for an efficient multi-scale computational approach. The goal of the present research is to develop a hierarchical approach to study the mechanical behaviors of membrane proteins with a special emphasis on the gating mechanisms of mechanosensitive (MS) channels. This requires the formulation of modeling and numerical methods that can effectively bridge the disparate length and time scales. A top-down approach is proposed to achieve this by effectively treating biomolecules and their assemblies as integrated structures, in which the most important components of the biomolecule (e.g., MS channel) are modeled as continuum objects, yet their mechanical/physical properties, as well as their interactions, are derived from atomistic simulations. Molecular dynamics (MD) simulations at the nanoscale are used to obtain information on the physical properties and interactions among protein, lipid membrane, and solvent molecules, as well as relevant energetic and temporal characteristics. Effective continuum models are developed to incorporate these atomistic features, and the conformational response of macromolecule(s) to external mechanical perturbations is simulated using finite element (FEM) analyses with in situ mechanochemical coupling. Results from the continuum mechanics analysis provide further insights into the gating transition of MS channels at structural and physical levels, and specific predictions are proposed for further experimental investigations. It is anticipated that the hierarchical framework is uniquely suited for the analysis of many biomolecules and their assemblies under external mechanical stimuli.

4. Theoretical investigation on the oxidation mechanism of dibutyl phthalate by hydroxyl and sulfate radicals in the gas and aqueous phase

   Huanxuan Li, Yayun Zhang, Jinquan Wan,  Hang Xiao , and Xi Chen

   Chemical Engineering Journal, May 2018

   Abs DOI

   A thorough thermodynamic analysis of different DBP conversion paths induced by hydroxyl (OH) and sulfate radicals (SO4−) is performed using Density Functional Theory (DFT) calculations in the gas and aqueous phases. We propose the thermodynamics favorable reaction chains for the major degradation products formation, and the involved reaction mechanisms include radical adduct formation (RAF), formal hydrogen atom transfer (FHAT), OH oxidative cleavage of CCdouble bond, and acid-catalyzed decomposition of C–C single bond. Theoretical results indicate that phthalic anhydride (PA) and acetophenone (ACP) are respectively the dominant and minor products in water, which is consistent with the experimental data. Computational results reveal that the reactivity of OH for the H-abstraction reaction is higher than that of SO4−. Interestingly, the unsaturated CC bonds cleavage reaction can directly occur with OH attack in aqueous solutions. However, it needs to overcome an extra radical adduct formation step when it is attacked by SO4−. The present work opens a new window to illustrate the reaction mechanism and reactivity of OH and SO4− toward different chemical structures of organics from the view of molecular level.

5. Quaternized Chitosan/PVA Aerogels for Reversible CO2 Capture from Ambient Air

   Juzheng Song, Jie Liu, Wei Zhao , Yan Chen,  Hang Xiao , Xiaoyang Shi, Yilun Liu , and Xi Chen

   Industrial & Engineering Chemistry Research, Apr 2018

   Abs DOI

   Developing inexpensive and highly efficient CO2 air capture technologies is an important solution for solving the greenhouse problem. In this work, we used the low-cost quaternized chitosan (QCS)/poly(vinyl alcohol) (PVA) hybrid aerogels with quaternary ammonium groups and hydroxide ions to reversibly capture CO2 from ambient air by humidity swing. The CO2 capture capacity and adsorption rate of the aerogels were investigated over the temperature range 10–30 °C. The CO2 capture capacity of the aerogels was measured to be about 0.18 mmol/g, which is 38% higher than the state-of-the-art commercial membrane. In addition, we proposed a modified pseudo-first-order kinetic model considering both the CO2 adsorption and the H2O desorption, which describes the experimental results very well. For the first time, the moisture-swing CO2 adsorbent is built by low-cost biomass material, which opens up a new approach for the design of the moisture-swing CO2 adsorbent.

6. Effects of Temperature and Strain Rate on Mechanical Behaviors of Stone–Wales Defective Monolayer Black Phosphorene

   Yan Chen,  Hang Xiao , Yilun Liu , and Xi Chen

   The Journal of Physical Chemistry C, Mar 2018

   00002

   Abs DOI

   The mechanical behaviors of monolayer black phosphorene (MBP) are explored by molecular dynamics (MD) simulations using a reactive force field. It is revealed that the temperature and strain rate have a significant influence on the mechanical behavior of MBP, and they are further weakened by SW (Stone–Wales) defects. In general, the tensile strength for both the pristine and SW defective MBP decreases with the increase of temperature or decrease of strain rate. Surprisingly, for relatively high temperature (\textgreater300 K) and low strain rate (\textless5.0 × 10–8 fs–1), a phase transition from the black phosphorene to a mixture of β-phase (β-P) and γ-phase (γ-P) is observed for the SW-2 defective MBP under armchair tension, while self-healing of the SW-2 defect is observed under zigzag tension. A deformation map of SW-2 defective MBP under armchair tension at different temperature and strain rate is established, which is useful for the design of phosphorene allotropes by strain. The results presented herein yield useful insights for designing and tuning the structure, and the mechanical and physical properties of phosphorene.

7. Investigation of inner mechanism of anisotropic mechanical property of antler bone

   Zhongqi Fang , Bin Chen, Shiyun Lin, Wei Ye,  Hang Xiao , and Xi Chen

   Journal of the Mechanical Behavior of Biomedical Materials, Dec 2018

   Abs DOI

   Bones have different functions and various applications depending on the roles they play in different mammal bodies. The internal relationships between the functions and microstructures of bones need further expounding to understand their specific mechanical properties. In this study, the relationships between the mechanical properties and microstructures of the compact bone of antler (called as antler bone for short) along its three different orientations are investigated. First, the bending mechanical properties of the specimens of the antler bone along its three different orientations are tested with material-testing machine, followed by the observations of the crack-extending routes and the fracture surfaces of the three different orientations with a scanning electron microscope (SEM). The results of the tests reveal that the antler bone possesses anisotropic mechanical property. Namely, the mechanical properties of the antler bone are closely related to its orientations. Concretely, the fracture strength, elastic modulus and work-of-fracture along the transversal orientation of the bone are remarkably larger than those of the longitudinal and radial orientations. The results of the observation of the SEM show that there are different crack-extending routes and fracture-surface characteristics along the three different fracture orientations of the bone. Specifically, there are crack deflections and crack twists along the transversal fracture orientation, crack bridging along the longitudinal fracture orientation and crack rounding of osteons along the radial fracture orientation. Based on the tested and observed results, the fractal models of the crack-extending routes along the three different fracture orientations are presented. The fractal dimensions and critical crack extension forces along the three different fracture directions are calculated based on the fractal models. Further, the box-counting method is adopted to verify the correctness of the models. It is indicated that the fractal dimension and fracture energy of the transversal orientation are obviously larger than those of the longitudinal and radial orientations, which are in accordance with the experimental results.

8. Predicting a two-dimensional P2S3 monolayer: A global minimum structure

   Hang Xiao , Xiaoyang Shi, Yayun Zhang, Mingjia Li, Xiangbiao Liao , and Xi Chen

   Computational Materials Science, Sep 2018

   00000

   Abs DOI

   A new 2D crystal, P2S3, is found based on extensive evolutionary algorithm driven structural search. Furthermore, P2S3 is confirmed to be stable by the computed phonon spectrum and ab initio molecular dynamics simulations. This 2D crystalline phase of P2S3 corresponds to the global minimum in the Born-Oppenheimer surface of the phosphorus sulfide monolayers with 2:3 stoichiometry. It is a wide band gap (4.55 eV) semiconductor with PsbndS σ bonds. The electronic properties of P2S3 structure can be fine-tuned by stacking into multilayer P2S3 structures, forming P2S3 nanoribbons or P2S3 nanotubes, expanding its potential applications in the emerging field of 2D electronics.

9. Humidity effect on ion behaviors of moisture-driven CO2 sorbents

   Xiaoyang Shi,  Hang Xiao , Xiangbiao Liao, Mitchell Armstrong , Xi Chen, and Klaus S. Lackner

   The Journal of Chemical Physics, Oct 2018

   00000

   Abs DOI

   Ion hydration is a fundamental process in many natural phenomena. This paper presents a quantitative analysis, based on atomistic modeling, of the behavior of ions and the impact of hydration in a novel CO2 sorbent. We explore moisture-driven CO2 sorbents focusing on diffusion of ions and the structure of ion hydration complexes forming inside water-laden resin structures. We show that the stability of the carbonate ion is reduced as the water content of the resin is lowered. As the hydration cloud of the carbonate ion shrinks, it becomes energetically favorable to split a remaining water molecule and form a bicarbonate ion plus a hydroxide ion. These two ions bind less water than a single, doubly charged carbonate ion. As a result, under relatively dry conditions, more OH− ions are available to capture CO2 than in the presence of high humidity. Local concentrations of dissolved inorganic carbon and water determine chemical equilibria. Reaction kinetics is then driven to a large extent by diffusion rates that allow water and anions to move through the resin structure. Understanding the basic mechanics of chemical equilibria and transport may help us to rationally design next-generation efficient moisture-driven CO2 sorbents.

10. Reversible Band Gap Narrowing of Sn-Based Hybrid Perovskite Single Crystal with Excellent Phase Stability

    Dianxing Ju, Xiaopeng Zheng, Jialiang Liu , Yan Chen, Jian Zhang, Bingqiang Cao,  Hang Xiao , Omar F Mohammed, Osman M Bakr, and Xutang Tao

    Angewandte Chemie International Edition, Nov 2018

    Publisher: Wiley Online Library
11. Narrow band gap and high mobility of lead-free perovskite single crystal Sn-doped MA 3 Sb 2 I 9

    Dianxing Ju, Xiaomei Jiang,  Hang Xiao , Xi Chen, Xiaobo Hu, and Xutang Tao

    Journal of Materials Chemistry A, Oct 2018

    Publisher: Royal Society of Chemistry
12. Oxidation-induced negative Poisson’s ratio of phosphorene

    Feng Hao, Xiangbiao Liao, Mingjia Li,  Hang Xiao , and Xi Chen

    Journal of Physics: Condensed Matter, Jul 2018

    Publisher: IOP Publishing

    Abs DOI

    Mechanical properties of phosphorene oxides are investigated by using density functional theory calculations. Intriguingly, as the oxygen coverage approaches 50%, negative Poisson’s ratios are found in the in-plane principal directions, originating from the asymmetric atomic structure upon oxygen adsorption. It is demonstrated that the structure of phosphorene oxide is largely weakened compared with that of pristine phosphorene. Our work indicates that certain unconventional mechanical properties, such as the negative Poisson’s ratio, can be achieved by tuning the atomic structure through simple processes, which offers a new avenue to design specific properties at the nanoscale.

13. Strain and defect engineering on phase transition of monolayer black phosphorene

    Yan Chen, Xiaoyang Shi, Mingjia Li, Yilun Liu,  Hang Xiao , and Xi Chen

    Physical Chemistry Chemical Physics, Aug 2018

    Publisher: The Royal Society of Chemistry

    Abs DOI

    The phase transition of monolayer black phosphorene (MBP, α-P) to β-P and γ-P is explored by density functional theory (DFT) calculations and molecular dynamics (MD) simulations using reactive force fields. It is found that MBP can convert to a mixed phase of β-P and γ-P under biaxial strain, while the Stone–Wales defect (SW-2) in MBP can serve as an excellent ‘phase transition catalyzer’, significantly decreasing the critical strain for phase transition and increasing the homogeneity of the phase transition. The biaxial strain state (i.e. the strain components in the armchair and zigzag direction) and loading mode (i.e. the proportional and staged loading) have significant effects on the phase transition of MBP. In general, the phase transition of MBP is driven by the tension strain in the armchair direction, but large tension or compression strain in the zigzag direction can also promote the phase transition. Besides, MBP has a larger fracture strain under staged loading, generating a more uniform phase transition structure. The effects of curvature and SW-2 defect concentration on the phase transition of MBP are also studied, which shows an easier phase transition for a larger curvature and higher SW-2 defect concentration. The systematic results presented herein provide useful insights for designing and tuning the structure of MBP through phase transition facilitated by strain and defect engineering.

14. Unconventional localization prior to wrinkles and controllable surface patterns of film/substrate bilayers through patterned cavities

    Xiangbiao Liao , Youlong Chen, Takumi Nagakura, Liangliang Zhu, Mingjia Li, Xiaoyang Shi, Akio Yonezu,  Hang Xiao , and Xi Chen

    Extreme Mechanics Letters, Nov 2018

    Abs DOI

    Wrinkle formation followed by sharp strain localization is commonly observed in compressed stiff film/soft substrate systems. However, cavities or defects beneath the film may directly trigger the formation of local ridges and then folding configurations at a relatively small compressive strain, and a mixture of wrinkles and folds upon further compression. The morphological transition is different than those of defect-free substrates. Numerical simulations of continuously compressed bilayer with pre-patterned cavities are carried out to elucidate the transition mechanism of surface patterns. Parallel experiments of cavities-patterned bilayer prototypes by 3D-printing are also performed to validate the findings in simulations. A rich diversity of periodic surface topologies, including overall spreading waves, localizations, saw-like and co-existing features of folds and wrinkles can be obtained by varying the diameter, depth and spacing of cavities, which provides a potential approach to engineer various surface patterns for applications.

15. Tunable surface morphology via patterned cavities in soft materials

    Xiangbiao Liao, Takumi Nagakura , Youlong Chen, Liangliang Zhu, Xiaoyang Shi, Akio Yonezu , Xi Chen, and  Hang Xiao

    Physical Review E, Nov 2018

    Publisher: APS

2017

1. CO2 adsorption and separation from natural gas on phosphorene surface: Combining DFT and GCMC calculations

   Yayun Zhang, Chao Liu, Feng Hao,  Hang Xiao , Shiwei Zhang , and Xi Chen

   Applied Surface Science, Mar 2017

   00000

   Abs DOI

   We have examined the performance of phosphorene-based material, phosphorene slit pores (PSP), in CO2 adsorption and separation from natural gas by using Density Function Theory (DFT) calculation and Grand Canonical Monte Carlo (GCMC) simulations. First, the adsorption of CH4 and CO2molecules on phosphorene sheet were conducted by DFT study. Then, adsorption performances of natural gas components as well as their binary CO2/CH4 gas mixture were investigated at 300 K with the pressure up to 3.0 MPa. The effects of slit pore width, H, and mole ratio of CO2/CH4in the gas phase on the separation of CO2 from mixtures of CO2/CH4 were also investigated. Our DFT calculation results show that the CO2 moleculehas higher adsorption energy than that of CH4, which implies that it can be easily adsorbed to the phosphorene surface than CH4. Detailed GCMC simulations reveal that the phosphorene slit pore has a high performance in separating CO2fromnature gas and achieves the highest gas selectivity at H = 1.0 nm at pressures lower than 0.1 MPa. Moreover, the selectivity of CO2 overCO2/CH4gas mixture increases with increasing the mole ratio of CO2/CH4due to the enhanced adsorbate-adsorbent interactions for the favorable component. Therefore, it is suggested that the phosphorene is a promising candidate for natural gas purification and possessing practical potential applications in gas adsorption.

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

   Hang Xiao , Xiaoyang Shi, Feng Hao, Xiangbiao Liao, Yayun Zhang , and Xi Chen

   The Journal of Physical Chemistry A, Aug 2017

   00007

   Abs DOI

   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.

3. The catalytic effect of H 2 O on the hydrolysis of CO 3 2- in hydrated clusters and its implication in the humidity driven CO 2 air capture

   Hang Xiao , Xiaoyang Shi, Yayun Zhang, Xiangbiao Liao, Feng Hao, Klaus S Lackner , and Xi Chen

   Physical Chemistry Chemical Physics, Aug 2017

   Publisher: Royal Society of Chemistry
4. Self-assembled nanocapsules in water: a molecular mechanistic study

   Hang Xiao , Xiaoyang Shi , and Xi Chen

   Physical Chemistry Chemical Physics, Aug 2017

   Publisher: The Royal Society of Chemistry

   Abs DOI

   The self-assembly mechanism of one-end-open carbon nanotubes (CNTs) suspended in an aqueous solution was studied by molecular dynamics simulations. It was shown that two one-end-open CNTs with different diameters can coaxially self-assemble into a nanocapsule. The nanocapsules formed were stable in aqueous solution under ambient conditions, and the pressure inside the nanocapsule was much higher than the ambient pressure due to the van der Waals interactions between the two parts of the nanocapsule. The effects of the normalized radius difference, normalized inter-tube distance and aspect ratio of the CNT pairs were systematically explored. The electric field response of the nanocapsules was studied using ab initio molecular dynamics simulations, which shows that the nanocapsules can be opened by applying an external electric field, due to the polarization of carbon atoms. This discovery not only sheds light on a simple yet robust nanocapsule self-assembly mechanism, but also underpins potential innovations in drug delivery, nano-reactors, etc.

2016

1. Thermal conductivity of armchair black phosphorus nanotubes: a molecular dynamics study

   Feng Hao, Xiangbiao Liao,  Hang Xiao , and Xi Chen

   Nanotechnology, Aug 2016

   Abs DOI

   The effects of size, strain, and vacancies on the thermal properties of armchair black phosphorus nanotubes are investigated based on qualitative analysis from molecular dynamics simulations. It is found that thermal conductivity has a remarkable size effect, because of the restricted paths for phonon transport, which is strongly dependent on the diameter and length of the nanotube. Owing to the intensified low-frequency phonons, axial tensile strain can facilitate thermal transport. In contrast, compressive strain weakens thermal transport due to the enhanced phonon scattering around the buckling of the nanotube. In addition, the thermal conductivity is dramatically reduced by single vacancies, particularly those with high defect concentrations.

2. Capture CO2 from Ambient Air Using Nanoconfined Ion Hydration

   Xiaoyang Shi,  Hang Xiao , Klaus S. Lackner , and Xi Chen

   Angewandte Chemie, Mar 2016

   00035

   Abs DOI

   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 CO32− 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 CO32− 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.

3. The Effect of Moisture on the Hydrolysis of Basic Salts

   Xiaoyang Shi,  Hang Xiao , Xi Chen, and Klaus S. Lackner

   Chemistry - A European Journal, Dec 2016

   DOI
4. Effects of intrinsic strain on the structural stability and mechanical properties of phosphorene nanotubes

   Xiangbiao Liao, Feng Hao,  Hang Xiao , and Xi Chen

   Nanotechnology, May 2016

   00025

   DOI
5. Hydrogen separation by porous phosphorene: A periodical DFT study

   Yayun Zhang, Feng Hao,  Hang Xiao , Chao Liu, Xiaoyang Shi , and Xi Chen

   International Journal of Hydrogen Energy, Dec 2016

   Abs DOI

   We have examined theoretically the stability of porous phosphorene and its application in hydrogen separation from gas mixture by employing first-principles calculations. The self-passivated pore of phosphorene was designed by removing six phosphorous atoms, reaching to the formation of covalent bonds among marginal atoms spontaneously. The gas permeability and selectivity were obtained for the porous phosphorene membrane. The results indicated that the self-passivated defect in phosphorene is inert to the gas mixture containing N2, CO, CO2, H2O, and CH4 molecules, and the porous phosphorene performed high selectivity for hydrogen over other gas molecules compared with previous graphene and silicene-based membranes. Our results unveiled the great potential of porous phosphorene as a promising membrane in hydrogen purification.

2015

1. A mechanical model of overnight hair curling

   Hang Xiao , and Xi Chen^*

   The European Physical Journal E, Sep 2015

   00000

   Abs DOI

   Based on the observation of overnight hair curling procedure, we establish a mechanical model to describe the temporary wave formation of straight hair (initial curvature is zero), which incorporates the contact between hair and hair roller. Systematic studies are carried out to explore the effects of radius ratio between hair and hair roller, hair’s average axial strain, creep time, Poisson’s ratio and gravity on the curl retention. The variation of curl retention with respect to time obtained from our numerical model is validated by a simple theoretical model and by overnight curling experiments on hair samples. The results of simulation show that overnight hair curling is suitable to create a wavy hairstyle within about 7 hours, while the combined usage with hair fixatives enables a wavy hairstyle with desired curvature that lasts for a day or more. Graphical abstract

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

   Jun Liu, Peng Li,  Hang Xiao , Yayun Zhang, Xiaoyang Shi, Xiaomeng Lü , and Xi Chen

   AIP Advances, Nov 2015

   Abs DOI

   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.

2011

1. Modeling and simulation of curled dry leaves

   Hang Xiao , and Xi Chen

   Soft Matter, Nov 2011

   Abs DOI

   Based on the observation of the natural senescence and experiment of the drying process of leaves, we establish phenomenological buckling models to explain the curled configuration of dried leaves, where the driving force is the differential contraction strain field. In the minimalist model, through a systematic study, the averaged buckling curvature is correlated with the aspect ratio and normalized size of the leaf, as well as the magnitude of the differential strain. In the refined model, the role of the vascular system is emphasized. Several main characteristics discovered through theoretical/numerical studies are validated by proof-of-concept experiments using various types of leaves. The findings in this paper not only shed some light on the intriguing natural phenomena, but also may enable potential applications in three-dimensional fabrications using mechanical self-assembly.