Xue-fang Yu

Monolayer Ti2CO2: A Promising Candidate for NH3 Sensor or Capturer with High Sensitivity and Selectivity

Ti2C is one of the thinnest layers in MXene family with high potential for applications. In the present study, the adsorption of NH3, H2, CH4, CO, CO2, N2, NO2, and O2 on monolayer Ti2CO2 was investigated by using first-principles simulations to exploit its potential applications as gas sensor or capturer. Among all the gas molecules, only NH3 could be chemisorbed on Ti2CO2 with apparent charge transfer of 0.174 e. We further calculated the current-voltage (I-V) relation using the nonequilibrium Greens function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V relation before and after NH3 adsorption on Ti2CO2. Thus, we predict that Ti2CO2 could be a promising candidate for the NH3 sensor with high selectivity and sensitivity. On the other hand, the adsorption of NH3 on Ti2CO2 could be further strengthened with the increase of applied strain on Ti2CO2, while the adsorption of other gases on Ti2CO2 is still weak under the same strain, indicating that the capture of NH3 on Ti2CO2 under the strain is highly preferred over other gas molecules. Moreover, the adsorbed NH3 on Ti2CO2 could be escapable by releasing the applied strain, which indicates the capture process is reversible. Our study widens the application of monolayer Ti2CO2 not only as the battery material, but also as the potential gas sensor or capturer of NH3 with high sensitivity and selectivity.

Penta-graphene: A Promising Anode Material as the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate.

Recently, a new two-dimensional (2D) carbon allotrope named penta-graphene was theoretically proposed ( Zhang , S. ; et al. Proc. Natl. Acad. Sci. U.S.A. 2015 , 112 , 2372 ) and has been predicted to be the promising candidate for broad applications due to its intriguing properties. In this work, by using first-principles simulation, we have further extended the potential application of penta-graphene as the anode material for a Li/Na-ion battery. Our results show that the theoretical capacity of Li/Na ions on penta-graphene reaches up to 1489 mAh·g-1, which is much higher than that of most of the previously reported 2D anode materials. Meanwhile, the calculated low open-circuit voltages (from 0.24 to 0.60 V), in combination with the low diffusion barriers (≤0.33 eV) and the high electronic conductivity during the whole Li/Na ions intercalation processes, further show the advantages of penta-graphene as the anode material. Particularly, molecular dynamics simulation (300 K) reveals that Li ion could freely diffuse on the surface of penta-graphene, and thus the ultrafast Li ion diffusivity is expected. Superior performance of penta-graphene is further confirmed by comparing with the other 2D anode materials. The light weight and unique atomic arrangement (with isotropic furrow paths on the surface) of penta-graphene are found to be mainly responsible for the high Li/Na ions storage capacity and fast diffusivity. In this regard, except penta-graphene, many other recently proposed 2D metal-free materials with pentagonal Cairo-tiled structures may be the potential candidates as the Li/Na-ion battery anodes.

Modulating the strength of tetrel bonding through beryllium bonding

AbstractQuantum chemical calculations were performed to investigate the stability of the ternary complexes BeH2···XMH3···NH3 (X = F, Cl, and Br; M = C, Si, and Ge) and the corresponding binary complexes at the atomic level. Our results reveal that the stability of the XMH3···BeH2 complexes is mainly due to both a strong beryllium bond and a weak tetrel–hydride interaction, while the XMH3···NH3 complexes are stabilized by a tetrel bond. The beryllium bond with a halogen atom as the electron donor has many features in common with a beryllium bond with an O or N atom as the electron donor, although they do exhibit some different characteristics. The stability of the XMH3···NH3 complex is dominated by the electrostatic interaction, while the orbital interaction also makes an important contribution. Interestingly, as the identities of the X and M atoms are varied, the strength of the tetrel bond fluctuates in an irregular manner, which can explained by changes in electrostatic potentials and orbital interactions. In the ternary systems, both the beryllium bond and the tetrel bond are enhanced, which is mainly ascribed to increased electrostatic potentials on the corresponding atoms and charge transfer. In particular, when compared to the strengths of the tetrel and beryllium bonds in the binary systems, in the ternary systems the tetrel bond is enhanced to a greater degree than the beryllium bond. Graphical AbstractA tetrel bond can be strengthened greatly by a beryllium bond

The band gap modulation of monolayer Ti2CO2 by strain

Ti2C is the thinnest member in the MXene family with great potential for applications. In the present study, we have investigated the band gap modulation of O-terminated Ti2C (Ti2CO2) monolayer under biaxial or uniaxial strain through first-principles simulation. Our results reveal that monolayer Ti2CO2 could undergo an indirect to direct band gap transition under the biaxial strain of ∼4%, and the uniaxial strain of ∼6%. Similarly, the band gap modulation also occurs in monolayer Zr2CO2 and Hf2CO2 when 10% and 14% biaxial strains are applied, respectively. The detailed reasons for the band gap modulation are also discussed. Our studies have crucial implications for the application of monolayer MXene in opto-electronics and optical devices.

Beryllium decorated armchair BC2N nanoribbons: coexistence of planar tetracoordinate carbon and nitrogen moieties

Planar tetracoordinate carbon and nitrogen (ptC and ptN) are the most important members in planar tetracoordinate chemistry. On the other hand, the isostructure of graphene, hexagonal monolayer boron–carbon–nitrogen (i.e., h-BC2N) has been viewed as one of the most potential materials in many fields. In this paper, we make the first attempt to design a structure with the coexistence of ptC and ptN moieties by using edge-decoration of armchair BC2N nanoribbons (aBC2NNR) using various atomic types. Among them, one new structure with alternate ptC and ptN moieties via Be-decorated aBC2NNR has been obtained, and its excellent thermal stability is confirmed by using the global minimization method and molecular dynamic simulations. The electronic conductivity of aBC2NNR undergoes a change from semiconductor to conductor before and after Be atom decoration. Especially, large spin-splitting behavior is induced in aBC2NNR via partially Be atom decoration. Our work will be helpful not only for enriching ptC and ptN chemistry, but also providing a promising candidate to act as a spintronic device.

Double-proton transfer mechanism in 1,8-dihydroxydibenzo[a,c]phenazine: a TDDFT and ab initio study

The double-proton transfer mechanism in the newly synthesized molecule 1,8-dihydroxydibenzo[a,c]phenazine containing two hydrogen bonds is investigated by means of the time-dependent density functional theory and resolution-of-identity second-order approximate coupled-cluster methods. Both reaction paths of the concerted and the stepwise mechanisms are examined, and the results show that the double-proton transfer is likely to occur in the excited state following the stepwise mechanism in the gas phase at the lowest excitation energy. Based on the calculated potential energy scheme, the single-proton transfer and the double-proton transfer are likely to coexist, with the former is expected to occupy in a significant number. The concerted reaction path of C2 symmetry is found to be less preferred compared with the stepwise reaction path, and also could be dynamically open due to the small activation energy.

Structure and magnetic properties of open-ended silicon carbide nanotubes

Nanotubes with open ends have been found to offer interesting opportunities in many applications. In this paper, the structures and magnetic properties of a series of open-ended SiCNTs have been studied through first-principles simulation. Our results reveal that the structures and magnetic properties of the open-ended SiCNT are strongly dependent on the tube diameter and chirality: (i) the open-ended armchair SiCNT is nonmagnetic due to the formation of spin-antiparallel pairs between the Si and C atoms at the tube mouth, and the self-closure behavior occurs at the tube mouth; (ii) the magnetic moment of the C-rich-ended zigzag or chiral SiCNT is nearly equal to the number of dangling bonds on the tube mouth; (iii) for the Si-rich-ended zigzag or chiral SiCNT, the spin density mainly locates on the isolated Si atoms at the tube mouth, the magnetic moment of each Si atom at the tube mouth is strongly dependent on the tube diameters. Our results might be helpful in deeply understanding the magnetic properties of SiCNTs, as well as provide guidance to design novel SiCNT-based nanodevices such as spintronic or field-emission display devices.

Protic vs Aprotic Solvent Effect on Proton Transfer in 3-Hydroxyisoquinoline: A Theoretical Study

In this work, the structures, energetics, and tautomerizations in 3-hydroxyisoquinoline (3HIQ) in both the ground state and the excited state have been theoretically investigated by the MP2, TDDFT, and CASPT2 methods, respectively. The solvent effect including the implicit solvent and explicit solvent on the structures, energetics, and tautomeizations are revealed. We found that the explicit solvent plays a more important role in the structures, energetics, and tautomerizations in 3HIQ than implicit solvent in both the ground state and the excited state. The proton transfer is more facilitated in explicit solvent (water or methanol) compared to that in the gas phase and in the implicit solvent in the excited state, and the reactive role of the molecular solvent is found to be related with the two linear hydrogen bonds.

Carbon Excess C3N: A Potential Candidate as Li-Ion Battery Material

Xu et al.s recent experimental work ( Adv. Mater. 2017, 29, 1702007) suggested that C3N is a potential candidate as Li-ion battery with unusual electrochemical characteristics. However, the obvious capacity loss (from 787.3 to 383.3 mA h·g-1) occurs after several cycles, which restricts its high performance. To understand and further solve this issue, in the present study, we have studied the intercalation processes of Li ions into C3N via first-principle simulations. The results reveal that the Li-ion theoretical capacity in pure C3N is only 133.94 mA h·g-1, the value is obviously lower than experimental one. After examining the experimental results in detail, it is found that the chemical component of the as-generated C xN structure is actually C2.67N with N excess. In this case, the calculated theoretical capacity is 837.06 mA h·g-1, while part of Li ions are irreversibly trapped in C2.67N, resulting in the capacity loss. This phenomenon is consistent with the experimental results. Accordingly, we suggest that N excess C3N, but not pure C3N, is the proposed Li-ion battery material in Xu et al.s experiment. To solve the capacity loss issue and maintain the excellent performance of C3N-based anode material, the C3N with slightly excess C (C3.33N), which has been successfully fabricated in the experiment, is considered in view of its relatively low chemical activity as compared with N excess C3N. Our results reveal that the C excess C3N is a potential Li-ion battery material, which exhibits the low open circle voltage (0.12 V), high reversible capacity (840.35 mA h·g-1), fast charging/discharging rate, and good electronic conductivity.