Core-Hole Excitation Spectra of the Oxides and Hydrates of Fullerene C60 and Azafullerene C59N.

The interaction of fullerenes and their derivatives with environmental molecules such as oxygen or water was crucial for the rational design of low-dimensional materials and devices. In this paper, the near-edge X-ray absorption fine structure (NEXAFS), X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) shake-up satellites were employed to distinguish the oxides and hydrates of the fullerene C60 and azafullerene C59N families. The study includes various isomers, such as the open [5,6] and closed [6,6] isomers of C60O, C60H(OH), C60-O-C60, C60H-O-C60H, C59N(OH) and C59N-O-C59N, based on density functional theory. These soft X-ray spectra offered comprehensive insights into the molecular orbitals of these azafullerene molecular groups. The oxygen K-edge NEXAFS, carbon and oxygen K-edge XPS shake-up satellite spectra provided valuable tools for distinguishing oxides or hydrates of fullerene C60 and azafullerene C59N. Our findings could significantly benefit the development of fullerene functional molecular materials and expand the application scope of soft X-ray spectroscopy as a molecular fingerprinting tool for the fullerene family.

Rational Design of Photocontrolled Rectifier Switches in Single-Molecule Junctions Based on Diarylethene.

The construction of multifunctional, single-molecule nanocircuits to achieve the miniaturization of active electronic devices is a challenging goal in molecular electronics. In this paper, we present an effective strategy for enhancing the multifunctionality and switching performance of diarylethene-based molecular devices, which exhibit photoswitchable rectification properties. Through a molecular engineering design, we systematically investigate a series of electron donor/acceptor-substituted diarylethene molecules to modulate the electronic properties and investigate the transport behaviors of the molecular junctions using the non-equilibrium Green's function combined with the density functional theory. Our results demonstrate that the asymmetric configuration, substituted by both the donor and acceptor on the diarylethene molecule, exhibits the highest switching ratio and rectification ratio. Importantly, this rectification function can be switched on/off through the photoisomerization of the diarylethene unit. These modulations in the transport properties of these molecular junctions with different substituents were obtained with molecule-projected self-consistent Hamiltonian and bias-dependent transmission spectra. Furthermore, the current-voltage characteristics of these molecular junctions can be explained by the molecular energy level structure, showing the significance of energy level regulation. These findings have practical implications for constructing high-performance, multifunctional molecular-integrated circuits.

A non-heat-source process for preparing graphene oxide with low energy consumption.

As the most widely used method for preparing graphene oxide (GO), Hummers' method always involves a key step, that is adding water to concentrated sulfuric acid. We found that if this process is cancelled, the oxidation degree of GO will be significantly reduced. This means that the heat released during concentrated sulfuric acid dilution will promote further oxidation of GO. In this paper, we fully utilize the heat released during concentrated sulfuric acid dilution to develop a new non-heat-source process without any low-/high-temperature auxiliar, exponentially reducing the energy consumption and largely avoiding the frequent temperature control. The result shows that GO prepared by Hummers' method and that prepared by the proposed process show a similar structure, composition, morphology, and defect degree. Meanwhile, the corresponding reduced GO (rGO) obtained after reduction shows similar capacitive behavior. Their specific capacitances are 243.6 F g-1 and 240.3 F g-1 at 1 A g-1, respectively, and they both have a long-term cycling performance (with a 100% capacitance retention after 10 000 cycles at 30 A g-1). This study provides a new strategy for the preparation of GO with low energy consumption.

Controlling the electrochemical activity of dahlia-like ß-NiS@rGO by interface polarization.

Transition metal sulfides have become more and more important in the field of energy storage due to their superior chemical and physical properties. Herein, dahlia ß-NiS with a rough surface and ß-NiS@reduced graphene oxide (rGO) have been green synthesized by a one-step hydrothermal method. The interface characteristics of ß-NiS@ rGO composites have been systematically studied by XPS, Raman, and first-principles calculations. It is found that the residual O atoms in the interface and the polarization charge generated by them play an important role in performance enhancement. The NiS@rGO composite material has the best electrochemical performance when the C/O ratio is 6.48. Furthermore, we designed a NiS@rGO//rGO asymmetric supercapacitor with a potential window of 1.7 V. Its excellent energy density and power density demonstrate the advantages of the optimized NiS@rGO electrode. When the power density is 850 W kg-1, the energy density can reach 40.4 W h kg-1. Even at a power density of up to 6800 W kg-1, the energy density can be maintained at 17.6 W h kg-1. These encouraging results provide a possible pathway for designing asymmetric supercapacitors with ultra-high performance and a feasible strategy for the precise control of electrochemical performance.

An intramolecular-locked strategy for designing nonlinear optical materials with remarkable first hyperpolarizability.

To meet the expanding demands of high performance nonlinear optical (NLO) materials, an unprecedented intramolecular-locked strategy is proposed to design NLO materials with remarkable static first hyperpolarizability (ß0). This strategy means that importing a large steric hindrance group diphenylmethane (DPM) decreases the torsion angles (θ) between the donor {triphenylamine (TPA)} and acceptor {9-H-thioxanthen-9-one-10,10-dioxide (TXO)} units, as well as between the donor (TPA) and π-bridge (benzene) fragments. The decrease of θ can accelerate the intramolecular charge transfer and enhance the contributions of the TPA, TXO and quinoxaline-6,7-dicarbo-nitrile (QCN) fragments to the axial component of the ß0 value, and then the ß0 values of TPA-TXO (ß0 = 10 762 au) and TPA-QCN (ß0 = 22 495 au) are increased by 14.9% and 34.4%, respectively. Overall, the intramolecular-locked strategy is very effective for designing high performance NLO materials.

Effects of Fluorination and Molybdenum Codoping on Monoclinic BiVO4 Photocatalyst by HSE Calculations.

Monoclinic phase bismuth vanadate (BiVO4) is one of the most promising photoelectrochemical materials used in water-splitting photoelectrochemical cells. It could be even better if its band gap and charge transport characteristics were optimized. Although codoping of BiVO4 has proven to be an effective strategy, its effects are remarkably poorly understood. Using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, we estimate the formation energy, electronic properties, and photocatalytic activities of F and Mo codoped BiVO4. We find that Mo atoms prefer to replace V atoms, whereas F atoms prefer to replace O atoms (FOMoV-doped BiVO4) under oxygen-poor conditions according to calculated formation energies. BiVO4 doped with FOMoV is found to be shallow-level doped, occurring with some continuum states above the conduction band edge, which is advantageous for photochemical catalysis. Moreover, FOMoV-doped BiVO4 shows absorption stronger than that of pure BiVO4 in the visible spectrum. Based on the band-edge calculation, BiVO4 doped with FOMoV still retains a high oxidizing capacity. It has been shown that FOMoV-doped BiVO4 exhibits a very high photocatalytic activity under visible light.

Corrosion-controlled surface engineering improves the adhesion of materials for stable free-standing electrodes.

Directly anchoring active materials on porous conductive substrates is considered an effective strategy to obtain a high-activity electrode since the direct contact between active materials and substrates benefits charge transfer, and the presence of porous structures provides more active sites. However, due to the presence of strong stress and weak adhesion, active materials loaded on the substrate are very easy to peel off during assembly and use, which can greatly shorten the lifetime of use. Herein, an ultrasonic corrosion strategy is proposed to regulate the surface of a metal substrate. We find that ultrasonic oxygen corrosion and interfacial water control play key roles in fabricating the complex electrode, which can help the surface of Cu foam to form special lamellar cross-linked CuO nanoarchitectures with strong adhesion and then overcome the defect of the deposited NiCo layered double hydroxides (NC LDH) on the stress and adhesion. The expected electrode shows more than 70% improvement in cycling stability at an ultra-high current density of 20 A g-1, relative to the active material layer of the electrode with strong stress and weak adhesion. Meanwhile, benefiting from its lamellar cross-linked nanoarchitectures having large specific surface area and many nano-pores, it presents a high specific capacitance of 3010.8F g-1 at 1 A g-1 and a good rate capability of 59.3% at 50 A g-1. It is foreseen that this finding provides a novel, universal strategy for managing the surface and interface of the metal substrate, thereby obtaining a reliable, stable electrode.

Second-Order Nonlinear Optics Response of the Boron-Dipyrromethenes-Based Mislinked Expanded Porphyrins: Revealing the Role of the -BF2 Group.

Here, the mislinked expanded porphyrins singly (labeled A) and doubly (labeled B) neo-confused [22]smaragdyrin, the boron-dipyrromethenes-based mislinked expanded porphyrins singly (labeled C) and doubly (labeled D) neo-confused [22]smaragdyrin, where both C and D include a -BF2 group, are chosen to serve as the study objects, and theoretical calculations are carried out to study the role of the -BF2 group in the second-order nonlinear optics (NLO) behaviors. Results highlighted that the -BF2 group plays an important role for the second-order behaviors in mislinked expanded porphyrins; namely, embedding the -BF2 group well enhanced the hyper-Rayleigh scattering (HRS) value {ßHRS(0;0,0)}, C{ßHRS(0;0,0)}A{ßHRS(0;0,0)} = 2.0 and D{ßHRS(0;0,0)}B{ßHRS(0;0,0)} = 2.9, main owning to the fact that installing -BF2 increases the electron delocalization degree and decreases the excited energy of the crucial excited state.

Two-dimensional ZrC2 as a novel anode material with high capacity for sodium ion battery.

Rational design of high-performance anode materials is of paramount importance for developing rechargeable lithium ion batteries (LIBs) and sodium ion batteries (SIBs). In this work, a ZrC2 monolayer is predicted by performing the particle swarm optimization (PSO) algorithm. The high energetic, dynamic, and thermal stabilities of the ZrC2 monolayer are confirmed by cohesive energy, phonon dispersion, and molecular dynamics simulations, respectively. Unexpectedly, we find that the theoretical specific capacity for Na on the ZrC2 monolayer reaches as high as 932 mA h g-1, which is even higher than that of Li. Meanwhile, the diffusion energy barrier of Na on the ZrC2 monolayer is only 0.02 eV, ensuring the ultrafast charge/discharge rate. Additionally, the calculated open-circuit voltage (OCV) suggests that the change of Na intercalation voltage is steady. Therefore, our results consistently demonstrate that the ZrC2 monolayer can be an ideal anode material for SIBs.

Adsorption of Organic Molecules and Surfactants on Graphene: A Coarse-Grained Study.

The research studies on the adsorption of surfactants on graphene help us to know how to use surfactants to exfoliate graphene from graphite or functionalize the graphene surface. Among them, molecular dynamics (MD) simulation has been widely used to investigate the adsorption of organic molecules and surfactants on graphene. In particular, coarse-grained (CG) MD simulation greatly improves the computational efficiency by simplifying the complexity of the studied systems, allowing us to explore the structure and dynamics of complex systems on larger spatial scales and longer time scales. However, an accurate prediction of the adsorption of surfactants on graphene is required by optimizing the interaction between surfactants and graphene, which is often overlooked by some CG models. In this work, we found that an accurate prediction of the adsorption enthalpies of organic molecules on graphene can be achieved by optimizing the interactions between organic molecules and benzene. Meanwhile, we simulated the adsorption of a surfactant on single-layer and double-layer graphene nanosheets, respectively. Our results revealed that increasing the temperature would favor the interactions between hydrophilic groups of surfactants. In addition, we discovered that the surfactant prefers to be adsorbed on the inner surfaces of double-layer graphene compared with the outer surfaces, and this is owing to the dehydration in the middle of double-layer graphene, which is beneficial to the hydrophilic interactions between surfactant molecules inside the double-layer graphene.

Two-dimensional ScN with high carrier mobility and unexpected mechanical properties.

Two-dimensional (2D) semiconductors with desirable bandgaps and high carrier mobility have great potential in electronic and optoelectronic applications. In the present work, 2D M-ScN, H-ScN, and O-ScN are predicted by the swarm-intelligent global structure search method. The low formation energies and high dynamical and thermal stabilities indicate the high feasibility of experimental synthesis of these ScN monolayers. The electronic structure calculations reveal that M-ScN and O-ScN are both direct bandgap semiconductors with the bandgaps of 1.39 and 2.14 eV, respectively, while H-ScN has a large indirect bandgap of 3.21 eV. In addition, both M-ScN and H-ScN exhibit ultra-high electron mobilities (3.09 × 104 cm2 V-1 s-1 and 1.22 × 104 cm2 V-1 s-1, respectively). More notably, O-ScN is found to be a promising 2D auxetic and ferroelastic material. The values of negative Possion's ratios and reversible strain of this monolayer are predicted to be -0.27% and 15%, respectively.

Sc/C codoping effect on the electronic and optical properties of the TiO2 (101) surface: a first-principles study.

Extension of the light absorption range and a reduction of the possibility of the photo-generated electron-hole pair recombination are the main tasks to break the bottleneck of the photocatalytic application of TiO2. In this paper, we systematically investigate the electronic and optical properties of Sc-doped, C-doped, and Sc/C-codoped TiO2 (101) surfaces using spin-polarized DFT+U calculations. The absorption coefficient of the Sc/C-codoped TiO2 (101) surfaces were enhanced the most compared with the other two doped systems in the high energy region of visible light, which can be attributed to the shallow impurity states. Furthermore, we studied the optical absorption properties with the change of the impurity concentration. The Sc/C-codoped TiO2 (101) surface with 5.56% impurity concentration exhibited optimal photocatalytic performance in the visible region. These results may be helpful for designing the high-performance of the photocatalysts by doping.

Hybrid Density Functional Theory Study of Native Defects and Nonmetal (C, N, S, and P) Doping in a Bi2WO6 Photocatalyst.

Native defects and nonmetal doping have been shown to be an effective way to optimize the photocatalytic properties of Bi2WO6. However, a detailed understanding of defect physics in Bi2WO6 has been lacking. Here, using the Heyd-Scuseria-Ernzerhof hybrid functional defect calculations, we study the formation energies, electronic structures, and optical properties of native defects and nonmetal element (C, N, S, and P) doping into Bi2WO6. We find that the Bi vacancy (Bivac), O vacancy (Ovac), S doping on the O site (SO), and N doping on the O site (NO) defects in the Bi2WO6 can be stable depending on the Fermi level and chemical potentials. By contrast, the substitution of an O atom by a C or P atom (CO, PO) has high formation energy and is unlikely to form. The calculated electronic structures of the Bivac, Ovac, SO, and NO defects indicate that the band-gap reduction of Ovac 2+, Bivac 3-, and SO defects is mainly due to forming shallow impurity levels within the band gap. The calculated absorption coefficients of Ovac 2+, Bivac 3-, and SO show strong absorption in the visible light region, which is in good agreement with the experimental results. Hence, Ovac 2+, Bivac 3-, and SO defects can improve the adsorption capacity of Bi2WO6, which helps enhance its photocatalytic performance. Our results provide insights into how to enhance the photocatalytic activity of Bi2WO6 for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

Effects of native defects and cerium impurity on the monoclinic BiVO4 photocatalyst obtained via PBE+U calculations.

In this article, we report a periodic density functional theory (DFT) investigation on the formation of the native defects and cerium doping in monoclinic BiVO4 (m-BiVO4) and their effect on the electronic structures, using the Perdew-Burke-Ernzerhof functionals corrected for on-site Coulombic interactions (PBE+U). From the point defect formation energies and transition levels, the Bivac (Bi vacancy), Vvac (V vacancy), Oint (O interstitial) and CeV (Ce doping on V site) defects in m-BiVO4 are identified as shallow acceptors. For Ce doping in m-BiVO4, the substitution of Bi by Ce is energetically favorable in the single positively charged state (Ce) under Bi/V-poor conditions, while the substitution of V by Ce is in the single negatively charged state (Ce) under O-rich conditions. The calculated electronic structures suggest that Ce degrades the activity by an unoccupied deep level in the gap region, mainly composed of Ce 4f orbitals, which makes this defect as the photogenerated electron-hole recombination center, in good agreement with the experimental results. For Ce, no localized state exists within the calculated band gap. Its formation energy is sensitive to the chemical potentials and Fermi energy, suggesting that the Bi/V-poor and O-rich conditions are desirable to eliminate the deep-level states and improve photocatalysis. Our results provide insights into enhancing the photocatalytic activity of m-BiVO4 for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

Atom-Pair Catalysts Supported by N-Doped Graphene for the Nitrogen Reduction Reaction: d-Band Center-Based Descriptor.

Achieving an effective nitrogen reduction reaction (NRR) under mild conditions is a great challenge for industrial ammonia synthesis. NRR is often accompanied by a competing hydrogen evolution reaction (HER), which causes an extremely low Faraday efficiency. We systematically investigated the NRR reactivity of atom-pair catalysts (APCs) formed by 20 transition metal (TM) elements supported by N-doped graphene via three reaction pathways. By analyzing the correlation among the limiting potential, Gibbs free energy, and d-band center, we evaluated the activity trends of the TM APCs. Our computations revealed that the enzymatic pathway is the most suitable reaction pathway for the TM APCs, and the intrinsic activity trend of these APCs can be determined by the d-band center-based descriptor, which has a simple linear correlation with the bonding/antibonding orbital population. In addition, the NRR APCs with excellent performance have been screened out through selective analysis of the competing HER in the electrocatalytic NRR process.

Electron-Phonon Scattering Is Much Weaker in Carbon Nanotubes than in Graphene Nanoribbons.

Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) are lower-dimensional derivatives of graphene. Similar to graphene, they exhibit high charge mobilities; however, in contrast to graphene, they are semiconducting and thus are suitable for electronics, optics, solar energy devices, and other applications. Charge carrier mobilities, energies, and lifetimes are governed by scattering with phonons, and we demonstrate, using ab initio nonadiabatic molecular dynamics, that charge-phonon scattering is much stronger in GNRs. Focusing on a GNR and a CNT of similar size and electronic properties, we show that the difference arises because of the significantly higher stiffness of the CNT. The GNR undergoes large-scale undulating motions at ambient conditions. Such thermal geometry distortions localize wave functions, accelerate both elastic and inelastic charge-phonon scattering, and increase the rates of energy and carrier losses. Even though, formally, both CNTs and GNRs are quantum confined derivatives of graphene, charge-phonon scattering differs significantly between them. Showing good agreement with time-resolved photoconductivity and photoluminescence measurements, the study demonstrates that GNRs are quite similar to molecules, such as conjugated polymers, while CNTs exhibit extended features attributed to bulk materials. The state-of-the-art simulations alter the traditional view of graphene nanostructures and demonstrate that the performance can be tuned not only by size and composition but also by stiffness and response to thermal excitation.

Effect of Cholesterol and 6-Ketocholestanol on Membrane Dipole Potential and Sterol Flip-Flop Motion in Bilayer Membranes.

A variety of experimental and theoretical approaches have been employed to investigate the sterol flip-flop motion in lipid bilayer membranes. However, the sterol effect on the dipole potential of lipid bilayer membranes is less well studied and the influence of dipole potential on sterol flip-flop motion in lipid bilayer membranes is less well understood. In our previous works, we have demonstrated the performance of our coarse-grained (CG) model in the computation of the dipole potential. In this work, five 30 µs CG simulations of dimyristoylphosphatidylcholine (DMPC) bilayers were carried out at different sterol concentrations (in a range from 10 to 50% mole fraction). Then, a comparison was made between the effects of cholesterol (CHOL) and 6-ketocholestanol (6-KC) on the dipole potential of DMPC lipid bilayers as well as the sterol flip-flop motion. Our CG simulations show that the membrane dipole potential is impacted more significantly by 6-KC than by CHOL. This finding is consistent with recent experimental studies. Meanwhile, our work suggests that the sterol-sterol interactions (in particular, electrostatic interactions) should be critical to the formation of sterol-sterol clusters, which would hinder the sterol flip-flop motion inside the lipid bilayers. This is in support of the recent experimental study on the sterol transportation in lipid bilayer membranes.

Single Pt atoms stabilized on Mo2TiC2O2 for hydrogen evolution: A first-principles investigation.

Single-atom catalysis offers an effective way to reduce the amount of used noble metals and maximizes their catalytic activity. We systematically explore electrocatalytic performances of Pt doped Mo2TiC2O2 monolayer by the first principles calculations. Our results show that the presence of donor defects in Mo2TiC2O2 can always increase the reaction free energy of hydrogen adsorption and further promotes the performance in hydrogen evolution reaction (HER). More interestingly, the substitution of Pt for O in the Mo2TiC2 can modify the free energy to an ideal value and is responsible for the significantly enhanced catalytic activity. Furthermore, the large value of diffusion barrier indicates that single Pt atoms can be stabilized onto the O vacancy sites, which can effectively prevent them to aggregate into nanoparticles. Our works are useful for understanding the recent experimental observations and pave the way for further experimental improvements of catalytic activity for the HER.

Interface Schottky barrier in Hf2NT2/MSSe (T = F, O, OH; M = Mo, W) heterostructures.

The Schottky barrier height (SBH) is a critical parameter that determines the carrier transfer at metal/semiconductor interfaces. In this work, the interfacial properties of Hf2NT2/MSSe (T = F, O, OH; M = Mo, W) heterostructures are systematically investigated using first-principles calculations. It is found that, for MoSSe and WSSe, the use of S or Se atomic layers in contact with Hf2NT2 can give significantly different SBHs. In addition, SB-free contact for electron injection can be realized for F-S interfaces in Hf2NF2/MoSSe and Hf2NF2/WSSe heterostructures. Furthermore, the SBHs of the heterostructures can be tuned by applying compressive strain and p-type ohmic contact can be obtained for O-Se interfaces in Hf2NO2/MoSSe and Hf2NO2/WSSe heterostructures. This work proposes a feasible strategy to regulate the SBHs of interfaces.

Molecular Dynamics Simulations of Ether- and Ester-Linked Phospholipid Bilayers: A Comparative Study of Water Models.

Membrane dipole potential influences a variety of important biological processes involving cell membranes. Because it is quite challenging to directly measure the membrane dipole potential in experiments, molecular dynamics (MD) simulation has emerged as a powerful tool for a reasonable prediction of the dipole potential. Although MD predictions agree well with experiments about the sign of the dipole potential, the magnitude of the dipole potential varies significantly with the force field parameters. It has been shown that the positive dipole potential of phosphatidylcholine (PC) bilayer membranes would be overestimated by a nonpolarizable model owing to the treatment of many-body polarization effects in a mean-field fashion. In this work, we carried out atomistic MD simulations of the diphytanyl PC (ether-DPhPC) and diphytanoyl PC (ester-DPhPC) bilayers and made a comparative study of three different nonpolarizable water models (TIP3P, TIP4P, and TIP5P). Interestingly, we discover that the calculated dipole potential by the TIP5P model shows good agreement with the result obtained using the cryoelectron microscopy experiment, suggesting that a better description of electrostatic interactions in a nonpolarizable water model can effectively ameliorate the overestimation in the calculation of the dipole potential. In addition, our MD results show that the substitution of the ether linkage for the ester linkage of phospholipid bilayers would bring about a change in the orientation of the linkage group with respect to the bilayer normal, leading to the difference in the membrane dipole potential. Surprisingly, although water molecules provide a major contribution to the positive dipole potential, they have a limited impact on the difference of the dipole potential between the ether-DPhPC and ester-DPhPC bilayer membranes.