Transition metal (Ti, Cu, Zn, Pt) single-atom modified graphene/AS2 (A = Mo, W) van der Waals heterostructures for removing airborne pollutants.

Air pollution is a worldwide issue that affects human health and the environment. The scientific community tries to control it through different approaches, from experimental to theoretical assessments. Here, we perform DFT calculations to describe CO2, NO2, and SO2 detection on a single-atom (Ti, Cu, Zn, Pt) graphene supported on 2D molybdenum disulfide (MoS2) and tungsten disulfide (WS2). Transition metal single atoms on graphene improve the monolayer reactivity by generating an effective way to remove airborne pollutants. Results indicate that SO2 and NO2 chemically adsorb on all tested transition metals, whereas CO2 stands on top of the incorporated atoms through van der Waals interactions. Since strong Ti-O interactions appear, the Ti single-atom graphene/MoS2(WS2) systems efficiently remove CO2 from the environment. Compared to pristine graphene, our proposed heterostructures improve the SO2, NO2, and CO2 adsorption energies. The heterostructures' electronic properties change once the molecules interact with the transition metals, generating sensible and selective pollutant molecule detection and removal.

Ab initio study of the adsorption of SO2 on single-atom Cu-decorated ZnO(0001) surface.

CONTEXT: The World Health Organization has cataloged sulfur dioxide (SO2) as harmful for the human health and the environment. It also contributes to generate acid rain, which affects the ecosystems. To reduce its negative effects, new strategies to control the emissions are required. New and engineered materials are investigated to detect, capture, and eradicate toxic gases from the environment. Zinc oxide is considered a promising candidate. Here, we investigate the Cu-decorated ZnO(0001) surfaces as a single-atom catalyst (SAC) to reduce SO2 by first-principles calculations. We propose a two-step reduction mechanism. First, one of the S-O bonds is broken on the pristine surface, with a calculated activation energy of 14.76 kcal/mol, 1.84 kcal/mol larger than the one obtained in the Cu SAC. In the second step, the SO reduction is viable only for Cu SAC, with calculated activation energy of 29.28 kcal/mol. Our results point that Cu SAC improves the SO2 reduction, pointing it as a potentially efficient device to eradicate such harmful pollutant from the environment. METHODS: The calculations were performed using the density functional theory, as implemented in quantum ESPRESSO package. The exchange-correlation energy was calculated within the generalized gradient approximation with the Perdew-Burke-Ernzerhof parameterization. Van der Waals dispersion-corrected interactions were considered. Spin-polarization was considered for studying dangling bonds in transition states. The minimum energy pathways were calculated by using the climbing image nudged elastic band.

Theoretical investigation of the MXene precursors MoxV4-xAlC3 (0 ≤ x ≤ 4).

By first-principles total-energy calculations, we investigated the thermodynamic stability of the MAX solid solution MoxV4-xAlC3 in the 0 ≤ x ≤ 4 range. Results evidence that lattice parameter a increases as a function of Mo content, while the c parameter reaches its maximum expansion at x = 2.5. After that, a contraction is noticed. Mo occupies VI sites randomly until the out-of-plane ordered Mo2V2AlC3 alloy is formed. We employed the Defect Formation Energy (DFE) formalism to evaluate the thermodynamic stability of the alloys. Calculations show five stable compounds. At V-rich conditions and from Mo-rich to Mo-moderated conditions, the pristine V4AlC3 MAX is stable. In the region of V-poor conditions, from Mo-rich to Mo-moderated growth conditions, the solid solutions with x = 0.5, 1, and 1.5 and the o-MAX Mo2V2AlC3 are thermodynamically stable. The line profiles of the Electron Localization Function and Bader charge analysis show that the V-C interaction is mainly ionic, while the Mo-C is covalent. Also, the exfoliation energy to obtain a MXene layer is ~ 0.4 eV/Å2. DFE also shows that MXenes exfoliated from the MAX phase with the same Mo content and atomic arrangement are thermodynamically stable. Our results get a deeper atomic scale understanding of the previously reported experimental evidence.

Electronic and optical properties of the buckled and puckered phases of phosphorene and arsenene.

Using first-principles calculations, we have investigated the structural, electronic, and optical properties of phosphorene and arsenene, group V two-dimensional materials. They have attracted the scientific community's interest due to their possible applications in electronics and optoelectronics. Since phosphorene and arsenene are not planar monolayers, two types of structures were considered for each system: puckered and buckled arrangements. Computations of band gap were performed within the GW approach to overcome the underestimation given by standard DFT and predict trustable band gap values in good agreement with experimental measurements. Our calculated electronic band gaps lie in the range from near-infrared to visible light, suggesting potential applications in optoelectronics devices. The computed electronic band gaps are 2.95 eV and 1.83 eV for blue and black phosphorene systems. On the other hand, the values for buckled and puckered arsenene are 2.56 eV and 1.51 eV, respectively. Moreover, the study of the optical properties has been dealt by computing the dielectric function imaginary part, which was obtained using the Bethe-Salpeter approach. The use of this technique allows the consideration of excitonic effects. Results indicate strong exciton binding energies of 830 meV for blue phosphorene, 540 meV for black phosphorene, 690 meV for buckled arsenene, and 484 meV for puckered arsenene. The results of our study suggest the possibility of using these materials in electronic and optoelectronic devices.

The effect of shape and size in the stability of triangular Janus MoSSe quantum dots.

Asymmetric Janus transition metal dichalcogenide MoSSe is a promising catalytic material due to the intrinsic in-plane dipole of its opposite faces. The atomic description of the structures observed by experimental techniques is relevant to tuning and optimizing its surface reaction processes. Furthermore, the experimentally observed triangular morphologies in MoSSe suggest that an analysis of the chemical environment of its edges is vital to understand its reactivity. Here we analyze the size-shape stability among different triangular structures-quantum- dots proposed from the ideal S(-1010) and Mo(10-10) terminations. Our stability analysis evidenced that the S-Se termination is more stable than Mo; moreover, as the size of the quantum dot increases, its stability increases as well. Besides, a trend is observed, with the appearance of elongated Mo-S/Se bonds at symmetric positions of the edges. Tersoff-Hamann scanning tunneling microscopy images for both faces of the stablest models are presented. Electrostatic potential isosurfaces denote that the basal plane on the S face of both configurations remains the region with more electron density concentration. These results point toward the differentiated activity over both faces. Finally, our study denotes the exact atomic arrangement on the edges of MoSSe quantum dots corresponding with the formation of S/Se dimers who decorates the edges and their role along with the faces as catalytic sites.

A first-principles study of the atomic layer deposition of ZnO on carboxyl functionalized carbon nanotubes: the role of water molecules.

The formation of heterostructures that combine a large surface area with high surface activity has attracted the attention of the scientific community due to the unique properties and applications of these heterostructures. In this work, we describe - at the atomic level - the full reaction mechanisms involved in the atomic layer deposition of a hybrid ZnO/CNT inorganic structure. First, the pristine CNTs are chemically activated with a carboxylic acid, a process unique to carbon materials. Diethylzinc (DEZ) and water are used as gas-phase precursors to form ZnO. Our findings show that DEZ is physically adsorbed on the CNTs during the exposure of the first precursor. The ligand-exchange to generate chemisorbed ethyl zinc on the O side of the COOH group needs to overcome an energy barrier of 0.06 eV. This is a very small energy if compared to the values (0.5-0.6 eV) obtained in previous studies for OH functionalized surfaces. The height of the barrier is associated with the C[double bond, length as m-dash]O side, which mediates the H proton's exchange from the OH group to the C2H5 ligand. Furthermore, upon exposure to the oxidizing agent (H2O), ethyl zinc exchanges its last ligand as ethane, and it accepts a hydroxyl group through a self-limiting reaction with an energy barrier of 0.88 eV. Notice that the energy barrier of the second ligand-exchange is larger than of the first. We have also analyzed the effect in the saturation of the second precursor: as the quantity of water molecules increases, the long-range interactions tend to repel them. However, the energy barrier of the second ligand-exchange decreases from 1.53 eV to 0.88 eV for one and two water molecules, showing a clear dependence on the oxidizing agent. Non-covalent interactions are used as a tool to visualize the driving forces that take place during each partial reaction in real space. Our study points out the importance of using the right functionalization agent to achieve a controlled and conformal ALD growth at the initial steps of the formation of hybrid ZnO/CNT structures, as well as the role played by the oxidizing agent to lower the energy barrier on the second ALD step.

Structural properties and thermal stability of multi-walled black phosphorene nanotubes and their operation as temperature driven nanorotors.

In this paper, we explore the influence of structural properties, thermal stability, and temperature on the rotational frequency of (0,n) armchair multi-walled black phosphorene nanotubes (MWßPNTs). Using Density Functional Theory (DFT) calculations, we first determine the influence of the outer wall rotation on the structural stability of (0,12)@(0,19) DWßPNTs, and (0,12)@(0,19)@(0,26) TWßPNTs. The results indicate that the relative energies of the DW- and the TWßPNTs do not change with the rotation angle. Therefore, the outer wall rotation is not important for the structural formation of the MWßPNTs. Then, using molecular dynamics (MD) simulations, we study the structural properties, thermal stability, and rotational frequency of (0,12)@(0,19) DWßPNTs, (0,12)@(0,19)@(0,26) TWßPNTs, and (0,12)@(0,19)@(0,26)@(0,33) QWßPNTs from 1 K to 400 K. The calorific curve, the mean-squared displacement, and the radial distribution function are analyzed to characterize the temperature behavior of the MWßPNTs. In all cases, the nanotubes are rotating: they act as thermal-driven rotors as the temperature increases, with a maximum rotational frequency of 16.7 GHz (clockwise direction) at 5 K for the DWßPNTs. Our results suggest that MWßPNTs could be used to construct room temperature nanomotors.

Structural transition induced by compression and stretching of puckered arsenene nanotubes.

The stretching and compression effects on puckered arsenene nanotubes (AsNTs) are investigated by using density functional calculations. The atomic arrangement determines the nanotube properties and relative stability; therefore, zigzag, chiral, and armchair present different properties. Since the AsNT properties depend on the diameter, three cases are considered: (a) (0, 9) and (9, 0), (b) (0, 14) and (14, 0), and (c) (0, 19) and (19, 0) NTs. For all calculated parameters of the smallest NTs, it is found that the armchair (0, 9) nanotube is always more stable than the zigzag (9, 0) nanotube. On the other hand, for the two largest NTs, a structural transition from armchair to zigzag is found upon stretching. Phase transitions are of great interest, in part because they result in changes of the properties of the material under study, changes that can be used in many technologies. To our knowledge, this is the first time that a structural transition in a puckered nanotube has been predicted. Our results show that the electronic band gap of the AsNTs can be modulated by increasing or decreasing the axial lattice parameter. It is also found that semiconductor NTs are more stable than metallic NTs.

Doped penta-graphene and hydrogenation of its related structures: a structural and electronic DFT-D study.

The structure of penta-graphene (penta-C), an irregular pentagonal two-dimensional (2D) structure, has been predicted recently. In this communication we carried out a dispersion-corrected density functional theory (DFT-D) study of the penta-C doped with Si, Ge and Sn atoms and its related hydrogenated penta-C structures (H-penta-C-X). We predict various new structures as thermally stable based on Born-Oppenheimer molecular dynamics (BOMD) calculations. Moreover, their dynamical stability is attested by phonon dispersions spectra. In general, we found that the bandgap value of doped structures reduces, while H-penta-C-X show large bandgap values. This feature can be exploited for potential uses of hydrogenated doped-penta-C structures as dielectric layers in electronic devices.

Chiral phosphorus nanotubes: structure, bonding, and electronic properties.

The study of black phosphorus nanotubes (PNTs) had been devoted to zigzag and armchair structures, with no consideration of chiral structures to date. In this communication, we studied the structural and electronic (band structure) properties of chiral nanotubes using a periodic plane wave-pseudopotential approach. We found that some chiral nanotubes display similar bandgaps and binding energies per atom (BEA) as armchair PNTs and Born-Oppenheimer molecular dynamics (BOMD) calculations attest their thermal stability. Interestingly, we determined that the bandgap is tuned by varying the PNTs chirality and it is not related to their diameters. This feature can be exploited in optical and electronic applications wherein a direct and sizable bandgap is required.