Hydrogen energy is very important as a new clean energy source to combat the growing environmental problems. In this regard, novel photocatalyst materials for water splitting have a wide range of applications. Using first principles calculations, we theoretically studied three orthorhombic group-IVB nitride halide monolayers, Hf2N2Br2, Janus HfZrN2Br2 and Janus Hf2N2ClBr. The energy, dynamic and thermal stabilities are demonstrated for all three monolayers. Using the HSE hybrid functional, the calculations reveal that they are direct band gap semiconductors with suitable band edge positions, good optical absorptions, and anisotropic carrier mobilities, which makes them promising for water splitting applications. Importantly, the photogenerated carriers provide enough driving force to trigger the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) within wide pH ranges, and then overall water splitting can be achieved spontaneously. We conclude that orthorhombic group-IVB nitride halide monolayers have potential applications in photocatalytic nanodevices.
Using transition metal compounds as sulfur hosts is regarded as a promising approach to suppress the polysulfide shuttle and accelerate redox kinetics for lithium-sulfur (Li-S) batteries. Herein, we report that a new kind of compound, electrides (exotic ionic crystalline materials in which electrons serve as anions), is efficient sulfur hosts for Li-S batteries for the first time. Based on the first-principles calculations, we found that two-dimensional (2D) electrides M2C (M = Sc, Y) exhibit unprecedentedly strong binding strength toward sulfur species and surface functionalization is necessary to passivate their activity. The 2D electrides modified with the F-functional group exhibit the best performance in terms of the adsorption energy and sulfur reduction process. A comparative study with a nonelectride reveals that the anionic electrons (AEs) of electrides aid in anchoring the soluble polysulfides. These results open an avenue for the application of electrides in Li-S batteries.
Negative ion mode paper spray mass spectrometry (PS-MS) suffers from intense background noise and unstable MS signal. For the first time, we reported fluorinated boron nitride nanosheet (h-FBN) assisted negative ion PS-MS for the detection of a series of molecules. We demonstrated that the introduction of h-FBN can greatly improve the detection sensitivity and signal stability in the negative ion mode.
Boron Compounds/chemistry , Halogenation , Limit of Detection , Mass Spectrometry/methods , Paper , Nanostructures/chemistry , Signal-To-Noise Ratio
The structural diversity and multifunctionality of carbon nitride materials distinct from pure carbon materials are drawing increasing interest. Using first-principles calculations, we proposed a stable spiral structure of carbon nitride, namely spiral-C3N, which is composed of sp2-hybridized carbon and pyridine nitrogen with a 60° helical symmetry along the z-direction. The stability was verified from the cohesive energy, phonon spectrum, and elastic constants. Despite the strong covalent bonds of the spiral framework, the spiral-C3N exhibits a hardness lower than 12.00 GPa, in sharp contrast to the superhardness of cubic carbon nitrides reported in previous literature, which can be attributed to the unique porous configuration. The softness of the spiral-C3N was also confirmed by the small ideal strengths, which are, respectively, 33.00 GPa at a tensile strain of 0.22 along the [1Ì 21Ì 0] direction and 18.00 GPa at a shear strain of 0.52 in the (0001)[1Ì 21Ì 0] direction. Electronic band structure of spiral-C3N exhibits metallic features. A metal-semiconductor transition can be triggered by hydrogenation of the pyridine nitrogen atoms of spiral-C3N. Such a new three-dimensional spiral framework of sp2-hyperdized carbon and nitrogen atoms not only enriches the family of carbon nitride materials but also finds application in energy conversion and storage.
Nanostructured anode materials have attracted significant attention for lithium-ion batteries (LIBs) due to their high specific capacity. However, their practical application is hindered by the rather low areal capacity in the ultrathin electrode (â¼1 mg cm-2). Herein, we propose a new strategy of an all-conductive electrode to fabricate a flexible and free-standing vanadium nitride@N-doped carbon/graphene (VN@C/G) thick electrode. Due to the free-standing structure and absence of any nonconductive components in the electrode, the obtained thick electrode displays excellent cycling performances. With the high mass loading of 5 mg cm-2, VN based electrodes achieve a reversible capacity of 2.6 mAh cm-2 after 200 cycles. Moreover, the all-conductive electrode allows an ultrahigh areal capacity of 7 mAh cm-2 with a high mass loading of 18.3 mg cm-2, which is comparable to state-of-the-art graphite anodes (4 mAh cm-2). Theoretical calculations prove the metallic conductivity of VN, which allows fast charge transport in the thick electrode. This strategy of fabricating all-conductive electrodes shows great potentials to achieve high areal capacity in practical lithium-ion batteries.
The high carrier mobility, porous configurations and tunable electronic structures of two-dimensional (2D) carbon materials hold great promise in energy conversion and storage. However, few of them are capable of photocatalytic overall water splitting. Here, by means of first-principles calculations within the quasi-particle approximation and the Bethe-Salpeter equation, we demonstrated a unique framework of triphenylenes (sp2) and acetylenic linkages (sp), namely triphenyldiyne (TDY) that has the electronic band structure suitable for photocatalytic overall water splitting along with pronounced optical absorbance in visible light. The redox ability of its photogenerated electrons is high enough to drive the hydrogen evolution reaction (HER). Through Ni doping with TDY, its overpotential for the oxygen evolution reaction (OER) can be reduced to match the redox ability of its photogenerated holes, enabling the photocatalytic overall water splitting in sunlight without the need of sacrificial reagents. This work offers not only a low-cost, earth-abundant and environmental-friendly photocatalyst, but also a promising strategy for designing highly efficient photocatalysts for overall water splitting.
Multifunctional catalysts that integrate high efficiency hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalytic activity in a single material are attractive for unitized regenerative fuel cells and overall water splitting technologies. As the best-known HER and ORR electrocatalysts, Pt and its alloys have only moderate OER activity. Ruthenium and iridium oxides exhibit the highest OER activities but not as active as Pt for HER and ORR. Here, we proposed a general principle for achieving trifunctional electrocatalysis for three reactions in a single material. Using the newly-synthesized pyrazine-modified graphdiyne (PR-GDY) as an example, we demonstrated that the synergistic effect of the pyridinic nitrogen and anchored transition-metal (TM) single atoms renders highly-efficient HER/OER/ORR trifunctional electrocatalytic activity. For the Ni-doped PR-GDY, the overpotentials for HER, OER and ORR can be respectively as low as -0.05, 0.29 and 0.38 V, which are comparable or even superior to the best-known single-functional and bi-functional precious electrocatalysts. These computational results offer not only a promising trifunctional electrocatalyst but also a strategy for the design of multifunctional electrocatalysts.
As a promising means of renewable energy storage, the production of molecular hydrogen and oxygen from photocatalytic water splitting has gained increasing interest. The optimal photocatalyst for water splitting should have high solar energy conversion efficiency and strong photocatalytic redox ability to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, few photocatalysts have been reported to fulfil these two contradictive requirements. Here, we demonstrated from first-principles calculations that the recently synthesized two-dimensional carbon nitride (C3N5) multilayers can serve as promising candidates to reach this goal. The intrinsic electric field which is more pronounced in the multilayers alters the band alignment of the photocatalysts, making the HER and OER be driven solely by the photogenerated carriers. The thickness-dependent electronic band gap (2.95-2.16 eV) along with the high carrier mobility broadens the energy range of light absorption and promotes carrier separation and transfer, leading to high solar energy conversion efficiency. Our computational results offer not only low-cost, Earth-abundant and environmentally friendly photocatalysts but also a promising strategy for the design of photocatalysts for highly efficient overall water splitting without using sacrificial reagents.
The coexistence of ferroelectricity and magnetism in two-dimensional (2D) multiferroic materials with the thickness of few atomic layers offers a tantalizing potential for high-density multistate data storage but has been rarely verified in experiments. Herein, we propose a realistic 2D multiferroic material, VOCl2 monolayer, which is mechanically strippable from the bulk material. It has a large intrinsic in-plane spontaneous electric polarization of 312 pC m-1 and stable antiferromagnetism with the Néel temperature of 177 K. The off-center displacement of V ions that contributes to the ferroelectricity can be ascribed to the pseudo Jahn-Teller distortion. The energy barrier (0.18 eV) between two ferroelectric states with opposite electronic polarization renders the thermodynamic stability of the ferroelectricity and the switchability of the electric polarizations. The interplay between electric polarization and magnetism would lead to tunable ferroelectricity. Our findings are expected to offer a realistic platform for the study of 2D multiferroic materials as well as their applications in miniaturized memory devices.
