VS₂/Graphene Heterostructures as Promising Anode Material for Li-Ion Batteries

Two-layer freestanding heterostructure consisting of VS₂ monolayer and graphene was investigated by means of density functional theory computations as a promising anode material for lithium-ion batteries (LIB). We have investigated lithium atoms’ sorption and diffusion on the surface and in the interface layer of VS₂/graphene heterostructure with both H and T configurations of VS₂ monolayer. The theoretically predicted capacity of VS₂/graphene heterostructures is high (569 mAh/g), and the diffusion barriers are considerably lower for the heterostructures than for bulk VS₂, so that they are comparable to barriers in graphitic LIB anodes (∼0.2 eV). Our results suggest that VS₂/graphene heterostructures can be used as a promising anode material for lithium-ion batteries with high power density and fast charge/discharge rates

Toward Analysis of Structural Changes Common for Alkaline Carbonates and Binary Compounds: Prediction of High-Pressure Structures of Li₂CO₃, Na₂CO₃, and K₂CO₃

The behavior of alkaline carbonates at high pressure is poorly understood. Indeed, theoretical and experimental investigations of the pressure induced structural changes have appeared in the literature only sporadically. In this article we use evolutionary crystal structure prediction algorithms based on density functional theory to determine crystal structures of high-pressure phases of Li₂CO₃, Na₂CO₃, and K₂CO₃. Our calculations reveal several new structures for each compound in the pressure range of 0–100 GPa. Cation arrays of all high-pressure structures are of the AlB₂ topological type. The comparison of cation arrays of ambient and high-pressure structures with that of binary A₂B compounds indicates an analogy between high-pressure behavior of alkaline carbonates and alkaline sulfides (oxides, selenides, tellurides), which under compression go through the following series of phase transitions: anti-CaF₂ → anti-PbCl₂ → Ni₂In → AlB₂. All structures presented in this trend are realized in the high-pressure trend of alkaline carbonates, although some intermediary structures are omitted for particular compounds

Construction of Polarized Carbon–Nickel Catalytic Surfaces for Potent, Durable, and Economic Hydrogen Evolution Reactions

Electrocatalytic hydrogen evolution reaction (HER) in alkaline solution is hindered by its sluggish kinetics toward water dissociation. Nickel-based catalysts, as low-cost and effective candidates, show great potentials to replace platinum (Pt)-based materials in the alkaline media. The main challenge regarding this type of catalysts is their relatively poor durability. In this work, we conceive and construct a charge-polarized carbon layer derived from carbon quantum dots (CQDs) on Ni₃N nanostructure (Ni₃N@CQDs) surfaces, which simultaneously exhibit durable and enhanced catalytic activity. The Ni₃N@CQDs shows an overpotential of 69 mV at a current density of 10 mA cm^–2 in a 1 M KOH aqueous solution, lower than that of Pt electrode (116 mV) at the same conditions. Density functional theory (DFT) simulations reveal that Ni₃N and interfacial oxygen polarize charge distributions between originally equal C–C bonds in CQDs. The partially negatively charged C sites become effective catalytic centers for the key water dissociation step <i>via</i> the formation of new C–H bond (Volmer step) and thus boost the HER activity. Furthermore, the coated carbon is also found to protect interior Ni₃N from oxidization/hydroxylation and therefore guarantees its durability. This work provides a practical design of robust and durable HER electrocatalysts based on nonprecious metals

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite

The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphene-based spintronic devices. A combined approach of topmost-surface-sensitive spectroscopy utilizing spin-polarized metastable He atoms and <i>ab initio</i> calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La_0.7Sr_0.3MnO₃ (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure

Photocatalysis with Pt–Au–ZnO and Au–ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles

Metal–semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au–ZnO particles (Pt–Au–ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au–ZnO particles (Au@Au–ZnO). An epitaxial connection at the Au–ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on–off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation properties of Pt–Au–ZnO hybrids compared to Au–ZnO. In contrast, Au–ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au–ZnO (Au, ∼5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi-level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au–ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au–ZnO. Pt–Au–ZnO hybrids showed better photodegradation and mineralization properties compared to both Au–ZnO and Au@Au–ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt–Au–ZnO, Au@Au–ZnO, and Au–ZnO were 84, 66, and 39%, respectively. In the interest of effective metal–semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired

Aragonite-II and CaCO₃‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO₃

The importance for the global carbon cycle, the <i>P</i>–<i>T</i> phase diagram of CaCO₃ has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO₃. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO₃-VII. CaCO₃-VII is a structural analogue of CaCO₃-<i>P</i>2₁/<i>c</i>-l, predicted theoretically earlier. The <i>P</i>–<i>T</i> phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO₃-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO₃-VI was found to be metastable in the whole pressure–temperature range