ABSTRACT
In recent years, the potential applications of two-dimensional (2D) metal-organic framework (MOF) materials in fields like spintronics have drawn increasing attention. Inspired by the successful synthesis of a perthiolated coronene (PTC)-Fe MOF structure, this study explores the fine-tuning of its electronic and magnetic structure by substituting Fe elements with various transition metals. Our calculations demonstrate a substantial increase in the Curie temperature (Tc) by a factor of 5 for Co and 10 for Mn when replacing Fe. This enhancement is attributed to the elevated electron density near the Fermi level, facilitating the generation of additional itinerant electrons crucial for the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism. However, substituting Fe with V, Cr, Ni, and Cu leads to a loss of ferromagnetic ground state. Our work enhances the understanding of the electronic and magnetic behavior of the 2D PTC-TM (transition metal) MOF family, and provides a promising avenue for engineering 2D magnetic MOF systems.
ABSTRACT
Rapid bubble release at high current densities results in the detachment of catalysts and performance degradation, posing a persistent challenge in actual alkaline water electrolysis (AWE). Here, hierarchical nanosheet structures (CoNC@P-MoS2) are constructed, with P-doped MoS2 on the surface of Co,N-codoped carbon. It exhibits low hydrogen evolution reaction overpotentials of 30 and 354 mV at 10 and 1000 mA cm-2 in 1 M KOH, respectively, with a small Tafel slope of 36 mV dec-1. The constructed CoNC@P-MoS2||NiFe-DLH cell requires only 1.44 and 1.92 V to achieve overall water splitting at 10 and 1000 mA cm-2, which outperforms the traditional catalysts like Pt/C||IrO2. The introduction of P stabilizes surface hydroxyl (OH*) and increases the proton penetration depth, thereby greatly enhancing its intrinsic activity. It also makes the surface aerophobic by introducing more microfeatures, which greatly improves the geometric activity by increasing the bubble release rate (â¼5.8 times). Low energy consumption of 3.92 kW h Nm-3 was achieved with an energy efficiency close to 80%. Bubble growth kinetics analysis reveals that the time and growth factors for CoNC@P-MoS2 are increased to 0.54 and 11.79 from 0.45 and 6.09 for CoNC, respectively, which highlights its fast bubble reaction dynamics. The results suggest the feasibility of CoNC@P-MoS2 as a potential high-performance catalyst in commercial AWE.
ABSTRACT
Herein, using density functional theory, novel two-dimensional (2D) CrInX2 (X = S, Se, Te) structures are predicted to be practical ferromagnetic (FM) semiconductors. Phonon vibrations and molecular dynamics simulations verified their structural and thermodynamic stability. Sizable fully spin-polarized band gaps of 1.03 and 0.69 eV are found for CrInS2 and CrInSe2, while CrInTe2 exhibits half-metallic band nature (at 0 K with a perfect lattice). The high magnetic anisotropy energies are responsible for their long-range spin polarization. The Curie temperatures (Tc) are estimated to be 347, 397 and 447 K for CrInS2, CrInSe2 and CrInTe2, respectively, all well above the room-temperature. The high Tc originates from unusual FM direct exchange, the efficient super-exchange coupling between neighboring Cr eg-orbitals with zero virtual exchange gaps and the presence of dual Cr-X-Cr super-exchange channels. Our systematic study of the CrInX2 monolayer suggests that it could be a promising material for spintronics applications.
ABSTRACT
The multiple strategy design is crucial for enhancing the efficiency of nonprecious electrocatalysts in hydrogen evolution reaction (HER). In this work, we successfully synthesized N, P-codoped MoS2 nanosheets as highly efficient catalysts by integrating doping effects and phase engineering using a porous metal-organic framework (MOF) template. The electrocatalysts exhibit excellent bifunctional activity and stability in alkaline media. The N, P codoping induces electron redistribution to enhance conductivity and promote the intrinsic activity of the electrocatalysts. It optimizes the H* adsorption free energy and the dissociative adsorption energy, resulting in significant enhancement of HER activity. Moreover, the porous MOF structure exposes a large number of electrochemically active sites and facilitates the diffusion of ions and gases, which improve charge transfer efficiency and structural stability. Specifically, at a current density of 10 mA cm-2, the overpotential of the HER is only 32 mV, with a Tafel slope of 47 mV dec-1 and Faradaic efficiency as high as 98.51% (at 100 mA cm-2). Only a 338 mV overpotential is required to achieve a current density of 50 mA cm-2 for oxygen evolution reaction (OER), and a potential of 1.49 V (at 10 mA cm-2) is sufficient to drive overall water splitting. Further experimental measurements and first-principles calculations evidence that the exceptional performance is primarily attributed to the dual functionality of N and P dopants, which not only activate additional S sites but also initialize the phase transition of 2H to 1T-MoS2 to facilitate the rapid charge transfer. Through in-depth exploration of the combined design of multiple strategies for efficient catalysts, our work paves a new way for the development of future efficient nonprecious metal catalysts.
ABSTRACT
Electrochemical activation is an effective method for synthesizing economically feasible heterogeneous hydrogen evolution reaction (HER) electrocatalysts. Herein, we first synthesized MoO2-Co2Mo3O8precatalyst, which was electrochemically activated to produce K2Mo3O10within the original phase to form the heterogeneous structure. The electrochemically activated samples demonstrate exceptional HER activity in alkaline medium, which exhibit a low overpotential of 31 mV at current density of 10 mA cm-2(135 mV at 100 mA cm-2), as well as a small Tafel slope of 34 mV dec-1. This is due to the creation of multiphase heterostructures that prompt interfacial interactions and accelerate charge transfer. Simultaneously, the creation of additional active sites increases their intrinsic activities. The combined effects collectively enhance the HER performance. The application of this method in the preparation of HER catalysts is still relatively unexplored, thus rendering our work a pioneering contribution to the field.
ABSTRACT
Interfacial electronic characteristics are crucial for the hydrogen evolution reaction (HER), especially in nanoscale heterogeneous catalysts. In this work, we found that the synergistic activation of CoS2 and MoS2 (2H-MoS2 and 1T-MoS2) greatly enhances the HER activity in a wide pH range compared to those of each component. The Gibbs free energies for hydrogen adsorption at interfacial Co sites are as low as -0.08 (-0.25) eV and -0.20 (0.01) eV for 2H-MoS2/CoS2 and 1T-MoS2/CoS2 heterostructures in acidic (alkaline) media, respectively, which are even superior to that of Pt(111) (-0.09 eV). Moreover, the theoretical exchange current density of MoS2/CoS2 can reach â¼1.98 × 10-18 A site-1 (â¼8.43 A mg-1). Experimentally, MoS2/CoS2 exhibits a greatly reduced overpotential of 54 (46) mV and a Tafel slope of 42 (50) mV dec-1 under acidic (alkaline) conditions. The improved performance mainly originates from the synergistically activated interfacial Co atoms with better electron localization and local bonding. The interfacial effect enhances the electron conductivity and improves the H adsorption characteristics, making MoS2/CoS2 highly valuable as efficient HER electrocatalysts.
ABSTRACT
The supercapacitors possessing high energy storage and long serving period have strategic significance to solve the energy crisis issues. Herein, fluffy nano-dendrite structured cobalt phosphide (CoP) is grown on carbon cloth through simple hydrothermal and electrodeposition treatments (CoP/C-HE). Benefit from its excellent electrical conductivity and special structure, CoP/C-HE manifests a high specific capacity of 461.4 C g-1at 1 A g-1. Meanwhile, the capacity retention remains 92.8% over 10 000 cycles at 5 A g-1, proving the superior cycling stability. The phase conversion of Co2P during the activation process also contributes to the improved performance. The assembled two-electrode asymmetric supercapacitor demonstrates excellent performance in terms of energy density (42.4 W h kg-1at a power density of 800.0 W kg-1) and cycling stability (86.3% retention over 5000 cycles at 5 A g-1), which is superior to many reported cobalt-based supercapacitors. Our work promotes the potential of transition metal phosphides for the applications in supercapacitors.
ABSTRACT
Strong magnetic interfacial coupling in van der Waals heterostructures is important for designing novel electronic devices. Besides the most studied transition metal dichalcogenides (TMDCs) materials, we demonstrate that the valley splitting can be activated in two-dimensional tetragonald0metal oxide, SnO, via the magnetic proximity effect by EuBrO. In SnO/EuBrO, the valley splitting of SnO can reach â¼46 meV, which is comparable to many TMDCs and equivalent to an external magnetic field of 800 T. In addition, the valley splitting can be further enhanced by adjusting interlayer distance and applying uniaxial strains. A design principle of new spintronic device based on this unique electronic structure of SnO/EuBrO has been proposed. Our findings indicate that SnO is a promising material for future valleytronics applications.
ABSTRACT
The manipulation of valley splitting has potential applications in valleytronics, which lacks in pristine two-dimensional (2D) InSe. Here, we demonstrate that valley physics in InSe can be activated via the magnetic proximity effect exerted by ferromagnetic FeI2 substrate with spin-orbit coupling. The valley splitting energy can reach 48 meV, corresponding to a magnetic exchange field of ~800 T. The system also presents magnetic anisotropy behavior with its easy magnetization axis tunable from in-plane to out-of-plane by the stacking configurations and biaxial tensile strain. The d-orbital-resolved magnetic anisotropic energy contributions indicate that the tensile strain effect arises from the increase of hybridization between minority Fe dxy and dx2-y2 states. Our results reveal that the magnetic proximity effect is an effective approach to stimulate the valley properties in InSe to extend its spintronic applications, which is expected to be feasible in other group-III monochalcogenides.
ABSTRACT
For realizing two-dimensional (2D) spintronic devices, controllable manipulation of different spin channels is needed. In order to achieve this goal, the materials should have well-separated spin energies induced by spin-orbit coupling (SOC). However, a negligible SOC effect in 2D SnO limits its application in this prospect. Herein, we demonstrate that the stable Janus structure of SnPbO2 can exhibit large spin splitting (67 meV) at the valence band maximum (VBM) due to the breakdown of inverse symmetry. This spin splitting is larger than the thermal fluctuation at room temperature as well as some other 2D systems. In addition, the splitting can be further enhanced by strains (82 meV) or hole doping (147 meV). More importantly, there are no other electronic states except for the Rashba states at the VBM, which make it ideal for practical applications. Our study provides alternative ways to regulate the electronic structure of SnO and the feasibility for spin manipulation in this interesting 2D system.
ABSTRACT
Recently, SnO has attracted more and more attention, because it is a bipolar electronic material holding great potential in the design of p-n junction. In this paper, we examine the effect of extrinsic point defects on modifying the electronic and magnetic properties of SnO using density functionals theory (DFT). The surface adatoms considered are B, C, N, O and F with a [He] core electronic configuration. All adatoms are found energetically stable. B, C, N and F adatoms will modify the band gap and introduce band gap states. In addition, our calculations show that N, B and F can introduce stable local magnetic moment to the lattice. Our results, therefore, offer a possible route to tailor the electronic and magnetic properties of SnO by surface functionalization, which will be helpful to experimentalists in improving the performance of SnO-based electronic devices and opening new avenue for its spintronics applications.
ABSTRACT
Although MoS2 based heterostructures have drawn increased attention, the van der Waals forces within MoS2 layers make it difficult for the layers to form strong chemical coupled interfaces with other materials. In this paper, we demonstrate the successful strong chemical attachment of MoS2 on TiO2 nanobelts after appropriate surface modifications. The etch-created dangling bonds on TiO2 surfaces facilitate the formation of a steady chemically bonded MoS2/TiO2 interface. With the aid of high resolution transmission electron microscope measurements, the in-plane structure registry of MoS2/TiO2 is unveiled at the atomic scale, which shows that MoS2[1-10] grows along the direction of TiO2[001] and MoS2[110] parallel to TiO2[100] with every six units of MoS2 superimposed on five units of TiO2. Electronically, type II band alignments are realized for all surface treatments. Moreover, the band offsets are delicately correlated to the surface states, which plays a significant role in their photocatalytic performance.
ABSTRACT
Due to its high carrier mobility and tunable bandgap, phosphorene has been the subject of immense interest recently. Herein, we show using density functional theory based calculations that black phosphorus (BP) nanotubes are achievable. Moreover, the electronic properties of BP nanotubes are explored. In contrast to their monolayer and bulk counterparts, most BP nanotubes possess indirect band gaps. In addition, strong anisotropic electronic behaviors are observed between zigzag and armchair nanotubes. Semiconducting to semi-metallic transition occurs only for zigzag tubes when its diameter shrinks to â¼1.5 nm. This difference is strongly related to the bond bending after the formation of the nanotubes which governs the s-p hybridization, as well as electron distribution in different p orbitals and this eventually determines the electronic structure of BP nanotubes.
ABSTRACT
The two-dimensional layer of molybdenum disulfide (MoS2) exhibits promising prospects in the applications of optoelectronics and valleytronics. Herein, we report a successful new process for synthesizing wafer-scale MoS2 atomic layers on diverse substrates via magnetron sputtering. Spectroscopic and microscopic results reveal that these synthesized MoS2 layers are highly homogeneous and crystallized; moreover, uniform monolayers at wafer scale can be achieved. Raman and photoluminescence spectroscopy indicate comparable optical qualities of these as-grown MoS2 with other methods. The transistors composed of the MoS2 film exhibit p-type performance with an on/off current ratio of â¼10(3) and hole mobility of up to â¼12.2 cm(2) V(-1) s(-1). The strategy reported herein paves new ways towards the large scale growth of various two-dimensional semiconductors with the feasibility of controllable doping to realize desired p- or n-type devices.
ABSTRACT
The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high -quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5â nm thick, while rutile films reach their maximum activity for ~2.5â nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.
ABSTRACT
Metal oxide surfaces have been thought to be fairly rigid. On the example of rutile TiO2(011) we show that this is not necessarily the case. This surface restructures by interacting with molecules. The synergic effect of adsorbates causes a strictly directional reorganization of the substrate, which results in one-dimensional adsorbate cluster formation. The increase in the surface energy of the restructured surface is compensated for by the larger molecular adsorption energy. The reversible change of the surface structure suggests a dynamic surface that may change its properties in response to adsorbed molecules.
ABSTRACT
Titanium dioxide is the prototypical transition metal oxide photocatalyst. However, the larger than 3 eV bandgap of common bulk phases of TiO2 limits its light absorption to UV light, making it inefficient for solar energy conversion. Attempts at increasing visible light activity by narrowing the bandgap of TiO2 through doping have proven difficult, because of defect-induced charge trapping and recombination sites of photo-excited charge carriers. Here, we report the existence of a dopant-free, pure TiO2 phase with a narrow bandgap. This new pure TiO2 phase forms on the surface of rutile TiO2(011) by oxidation of bulk titanium interstitials. We measure a bandgap of only ~2.1 eV for this new phase, matching it closely with the energy of visible light.
Subject(s)
Phase Transition , Titanium/chemistry , Chemical Phenomena , Crystallization , Electrons , Microscopy, Scanning Tunneling , Oxidation-Reduction , Photochemical Processes , Photoelectron Spectroscopy , Spectrum Analysis/methods , Surface Properties , ThermodynamicsABSTRACT
We describe a simple and robust approach to fabricating an alpha-Fe2O3 switchable surface. The hydrophobicity of alpha-Fe2O3 nanostructures was observed for the first time. A remarkable surface wettability transition can be easily achieved by ultraviolet (UV) illumination. The distinctive properties of surface defects are disclosed by X-ray photoelectron spectroscopy (XPS) analysis. The nanoscale adsorption and photocatalytic properties of Fe2+ defects account for the highly amphiphilic character of the surfaces. We believe that the experiment will further the molecular-scale understanding and manipulation of the wetting behavior on smart devices.