The Ability of CO2 Capture on Transition Metal-modified 1T'-MoS2 Monolayers Controlled by an Electric Field.

Herein, we evaluate the CO2 capture ability on the transition metal-modified 1T'-MoS2 monolayers (TM@1T'-MoS2 , TM represents a transition metal atom from 3d to 4d except Y, Tc and Cd) under different external electric fields via first-principles calculations. As the screened results revealed that Mo@1T'-MoS2 , Cu@1T'-MoS2 and Sc@1T'-MoS2 monolayers possess higher sensitivity for electric field than pristine 1T'-MoS2 monolayer. Among the above candidates, Mo@1T'-MoS2 and Cu@1T'-MoS2 monolayers only require the electric field strength of 0.002 a.u. to reversibly capture CO2 and can absorb up to four CO2 molecules with the electric field of 0.004 a.u. Furthermore, Mo@1T'-MoS2 can selectively capture CO2 molecule from the mixture of CH4 and CO2 . Our findings not only provide useful insights that the synergistic effect of electric field and transition metal doping is beneficial for CO2 capture and separation, but also guide the application of 1T'-MoS2 in the field of gas capture.

Cu-Doped MoSi2 N4 Monolayer as a Potential NH3 Sensor.

Cu doped MoSi2 N4 monolayer (Cu-MoSi2 N4 ) was firstly proposed to analyze adsorption performances of common gas molecules including O2 , N2 , CO, NO, NO2 , CO2 , SO2 , H2 O, NH3 and CH4 via density functional theory (DFT) combining with non-equilibrium Green's function (NEGF). The electronic transport calculations indicate that Cu-MoSi2 N4 monolayer has high sensitivity for CO, NO, NO2 and NH3 molecules. However, only NH3 molecule adsorbs on the Cu-MoSi2 N4 monolayer with moderate strength (-0.55 eV) and desorbs at room temperature (2.36×10-3  s). Thus, Cu-MoSi2 N4 monolayer is demonstrated as a potential NH3 sensor.

Nitric oxide electrochemical reduction reaction on transition metal-doped MoSi2N4 monolayers.

Nitric oxide electrochemical reduction (NOER) reactions are usually catalyzed by noble metals. However, the commercial applications are limited by the low atomic utilization and high price, which prompt researchers to turn their attentions to single-atom catalysts (SACs). Recently, a novel two-dimensional semiconducting material MoSi2N4 (MSN) has been synthesized and is suitable for the substrate of SACs due to its high stability, carrier mobility and mechanical strength. Herein, we employed first principles calculations to investigate the catalytic properties of transition metal doped MoSi2N4 monolayers (labelled as TM-MSN, where TM is a transition metal atom from 3d to 5d except Y, Tc, Cd, La-Lu and Hg) in NO reduction. The calculated results demonstrate that the introduction of Zr, Pd, Pt, Mn, Au, or Mo atoms can greatly improve the catalytic NOER performance of a pristine MSN monolayer. Zr-MSN and Pt-MSN monolayers at low coverage exhibit the most superior catalytic activity and selectivity for NH3 production with a limiting potential of 0 and -0.10 V. This work may help guide the application of MSN monolayer in the area of energy conversion.

Binary self-assembly of ordered Bi4Se3/Bi2O2Se lamellar architecture embedded into CNTs@Graphene as a binder-free electrode for superb Na-Ion storage.

With the development of various flexible electronic devices, flexible energy storage devices have attracted more research attention. Binder-free flexible batteries, without a current collector, binder, and conductive agent, have higher energy density and lower manufacturing costs than traditional sodium-ion batteries (SIBs). However, preparing binder-free anodes with high electrochemical performance and flexibility remains a great challenge. In this study, a binary self-assembly composite of an ordered Bi4Se3/Bi2O2Se lamellar architecture wrapped by carbon nanotubes (CNTs) was embedded in graphene with strong interfacial interaction to form Bi2O2Se/Bi4Se3@CNTs@rGO (BCG), which was used as a binder-free anode for SIBs. A unique "one-changes-into-two" phenomenon was observed: the layered Bi2Se3 was transformed into a unique layered Bi4Se3/Bi2O2Se heterojunction structure, which not only provides more electrochemical channels but also reduces internal stress to improve the stability of the material structure. BCG-2 showed excellent sodium-ion storage, delivering a reversible capacity of 346 mA h/g at 100 mA/g and maintaining a capacity of 235 mA h/g over 50 cycles. Even at a high current density of 1 A/g, it retains a capacity of 105 mA h/g after 1000 cycles. This unique design concept can also be employed in synthesizing other binder-free electrodes to improve their properties.

A high-performance asymmetric supercapacitor-based (CuCo)Se2/GA cathode and FeSe2/GA anode with enhanced kinetics matching.

The performance of asymmetric supercapacitors (ASCs) is limited by the poorly matched electrochemical kinetics of available electrode materials, which generally results in reduced energy density and inadequate voltage utilization. Herein, a porous conductive graphene aerogel (GA) scaffold was decorated with copper cobalt selenide ((CuCo)Se2) or iron selenide (FeSe2) to construct positive and negative electrodes, respectively. The (CuCo)Se2/GA and FeSe2/GA electrodes exhibited high specific capacitances of 672 and 940 F g-1, respectively, at 1 A g-1. The capacitance contributions from the Co3+/Co2+ and Fe3+/Fe2+ redox couple for the positive and negative electrodes were determined to elucidate the energy storage mechanism. Furthermore, the kinetics study of the two electrodes was performed, revealing b values ranging between 0.7 and 1 at various scan rates and demonstrating that the surface-controlled processes played the dominant role, leading to fast charge storage capability for both electrodes. Fabrication of an ASC device with a configuration of (CuCo)Se2/GA//FeSe2/GA resulted in a voltage of 1.6 V, a high energy density of 39 W h kg-1, and a power density of 702 W kg-1. The excellent electrochemical performances of the (CuCo)Se2/GA and FeSe2/GA electrodes demonstrate their potential applications in energy storage devices.

CO oxidation over defective and nonmetal doped MoS2monolayers.

Defective (missing S atoms) and nonmetal (C- and N-) doped MoS2monolayers in the 2H and 1T' phases have been evaluated for catalyzing CO oxidation based on first-principles calculations. For the reaction 2CO + O2→ 2CO2, the oxidization of the first CO molecule is fairly easy and sometimes is even spontaneous, as the O2 molecule is highly activated or dissociates upon adsorption. However, for the defective (2H-), C-doped (1T'-), and N-doped (2H- and 1T'-) MoS2monolayers, the remaining O*adatom often refuses to react with other CO molecules and is hard to be removed (barrier > 1.20 eV). Only when over the C-doped 2H- and defective 1T'-MoS2monolayers, the removal of the second O*adatom requires to overcome moderate barriers (0.74 and 0.88 eV, respectively) by reacting with another CO molecule via the Eley-Rideal mechanism and the catalysts are recovered. The barriers can be further reduced by applying either tensile or compressive strain to the MoS2nanosheet. In contrast, the Langmuir-Hinshelwood mechanism is followed over the metal-containing MoS2nanosheets, as the bigger size of metal dopants allow the co-adsorption of CO and O2. Therefore, the C-doped 2H- and defective 1T'-MoS2monolayers are promising nonmetal-doped catalysts for CO oxidation.

NO disproportionation over defective 1T'-MoS2 monolayers.

The MoS2 monolayers are usually created with vacancies (most likely missing S atoms). At an S vacancy, the exposed and under-coordinated Mo atoms become reactive and can strongly bind to small molecules. Here, by using first-principles calculations, it is proved that 1T'-MoS2 monolayers are an efficient catalyst for NO disproportionation. The reaction starts with NO adsorptions at the exposed Mo atoms. Later the incoming NO molecules react with the ones already adsorbed to give NO2 molecules, which readily desorb. The remaining N-doped MoS2 sheets can then easily react with NO molecules to produce N2O, and can be heated to desorb them. Thus, the defective 1T'-MoS2 monolayers are recovered and the catalytic cycle is completed. The NO2 formation step has a relatively high activation barrier of 1.58 eV, but it can be lowered to 0.19 or 0.56 eV by applying biaxial -3% or 3% strain, respectively. The reaction mechanism is totally different from those catalyzed by metal-centered catalysts (complexes, clusters, or metal-organic frameworks), which feature the N2O formation as the rate-limiting step and the NO2 in the metal-nitrite complexes cannot be released. This work paves the way for strain engineering two-dimensional (2D) materials into efficient NO disproportionation catalysts.

Separation Properties of Porous MoS2 Membranes Decorated with Small Molecules.

Using first-principles simulations, we surveyed the interactions between porous MoS2 monolayers in the 2H phase and 15 small molecules (H2, O2, H2O, H2S, CO, CO2, SO2, N2, NO, NO2, NH3, HF, HCl, CH4, and CH3OH). Four types of molecules including H2, O2, H2S, and NO2 directly dissociate and saturate the corners of the most common S-rimmed triangular pores, while other molecules only molecularly adsorb. The trisublayered structure of a MoS2 monolayer allows a new in-pore stable adsorption configuration in addition to the most studied above-pore adsorption configuration. Furthermore, the gas penetration pathways through the MoS2 membranes are no longer the conventional single-peak curve with one transition state like in the case of porous graphenes but are the "M"-shaped curve featuring two transition states connected by a stable in-pore adsorption state. The irreversible pore passivation via dissociative adsorption and reversible pore decoration by molecular adsorption will lead to very different separation performances of the MoS2 membranes, largely by changing the effective pore size. For example, the S-rimmed pores in the pore-3Mo2S membrane allow free pass of CH4 and CO2 molecules. If passivated by H atoms, the membrane can be used to separate gas mixtures like H2/CH4 and H2/CO2 with selectivities of 109:1 and 108:1, respectively. The permeance value of H2 is estimated to be about 0.15 mol m-2 s-1 Pa-1 at room temperature and 0.1 bar pressure drop across the membrane. In contrast, the medium strong adsorption of a SO2 molecule in the center of the pore will completely block the passage of CO2 and CH4, whose removal only needs heating. Our work reveals the complex behaviors of porous transition metal dichalcogenides (TMDs) toward guest molecules.

Ligand induced structure and property changes of 1T-MoS2.

Stabilizing metastable 1T-MoS2 sheets is significant for their potential applications. In this work, we investigate the influence of surface adsorption of a series of functional groups (including -H, -O, -SH, -NH2, -CH3, -CF3, -SCH3 and -OCH3) on the structural and electronic properties of 1T-MoS2 by using first-principles calculations. Strong adsorptions of these functional groups eventually transform 1T-related MoS2 monolayers into the 1T' phase (featuring zigzag Mo-Mo chains). The adsorptions of functional groups on 1T'-MoS2 monolayers highly prefer half of the surface sites (the St sites) and try to form adsorbate pairs at the St sites of the second nearest neighbors, which means that the study of surface-decorated 1T'-MoS2 monolayers should not be based on randomly generated configurations. Factors like the type of functional group as well as its coverage and configurations make the relationship between the structure of the adsorbate-MoS2 complex and its electronic properties (e.g. band gap) unclear, which implies that the band gap engineering through surface adsorption is unpredictable.