Atomically Thick Bismuth Selenide Freestanding Single Layers Achieving Enhanced Thermoelectric Energy Harvesting

Thermoelectric materials can realize significant energy savings by generating electricity from untapped waste heat. However, the coupling of the thermoelectric parameters unfortunately limits their efficiency and practical applications. Here, a single-layer-based (SLB) composite fabricated from atomically thick single layers was proposed to optimize the thermoelectric parameters fully. Freestanding five-atom-thick Bi₂Se₃ single layers were first synthesized via a scalable interaction/exfoliation strategy. As revealed by X-ray absorption fine structure spectroscopy and first-principles calculations, surface distortion gives them excellent structural stability and a much increased density of states, resulting in a 2-fold higher electrical conductivity relative to the bulk material. Also, the surface disorder and numerous interfaces in the Bi₂Se₃ SLB composite allow for effective phonon scattering and decreased thermal conductivity, while the 2D electron gas and energy filtering effect increase the Seebeck coefficient, resulting in an 8-fold higher figure of merit (<i><i>ZT</i></i>) relative to the bulk material. This work develops a facile strategy for synthesizing atomically thick single layers and demonstrates their superior ability to optimize the thermoelectric energy harvesting

Ammonia-Induced Size Convergence of Atomically Monodisperse Au₆ Nanoclusters

Developing effective synthetic protocols for atomically monodisperse Au nanoclusters is pivotal to their fundamental science and applications. Here, we present a novel synthetic protocol toward atomically monodisperse [Au₆(PPh₃)₆]²⁺ nanoclusters (abbreviated as Au₆) via ammonia-induced size convergence from polydisperse Au_<i>x</i> (<i>x</i> = 6–11) nanocluster mixture. The analogous ammonia-induced size conversion reactions starting from individually prepared Au₇ and Au₉ nanoclusters to Au₆ were traced by time-dependent ultraviolet–visible absorption and electrospray ionization mass spectra. It is observed that in both cases the size conversion is achieved through gradual release of the ion–molecule complex [NH₄AuPPh₃Cl]⁺ from the larger Au nanoclusters until the formation of thermodynamically stable Au₆ nanoclusters with the stability against the etching reaction. The role of ammonia ions in this size convergence synthesis is to accelerate the depletion of [Au­(PPh₃)]⁺ fragments from the PPh₃-protected Au nanoclusters, by the formation of the stable complex [NH₄AuPPh₃Cl]⁺

Strong Surface Hydrophilicity in Co-Based Electrocatalysts for Water Oxidation

Developing efficient and durable oxygen evolution electrocatalyst is of paramount importance for the large-scale supply of renewable energy sources. Herein, we report the design of significant surface hydrophilicity based on cobalt oxyhydroxide (CoOOH) nanosheets to greatly improve the surface hydroxyl species adsorption and reaction kinetics at the Helmholtz double layer for high-efficiency water oxidation activity. The as-designed CoOOH-graphene nanosheets achieve a small surface water contact angle of ∼23° and a large double-layer capacitance (<i>C</i>_dl) of 8.44 mF/cm² and thus could evidently strengthen surface species adsorption and trigger electrochemical oxygen evolution reaction (OER) under a quite low onset potential of 200 mV with an excellent Tafel slope of 32 mV/dec. X-ray absorption spectroscopy and first-principles calculations demonstrate that the strong interface electron coupling between CoOOH and graphene extracts partial electrons from the active sties and increases the electron state density around the Fermi level and effectively promotes the surface intermediates formation for efficient OER

Graphene Activating Room-Temperature Ferromagnetic Exchange in Cobalt-Doped ZnO Dilute Magnetic Semiconductor Quantum Dots

Control over the magnetic interactions in dilute magnetic semiconductor quantum dots (DMSQDs) is a key issue to future development of nanometer-sized integrated “spintronic” devices. However, manipulating the magnetic coupling between impurity ions in DMSQDs remains a great challenge because of the intrinsic quantum confinement effects and self-purification of the quantum dots. Here, we propose a hybrid structure to achieve room-temperature ferromagnetic interactions in DMSQDs, <i>via</i> engineering the density and nature of the energy states at the Fermi level. This idea has been applied to Co-doped ZnO DMSQDs where the growth of a reduced graphene oxide shell around the Zn_0.98Co_0.02O core turns the magnetic interactions from paramagnetic to ferromagnetic at room temperature, due to the hybridization of 2p_<i>z</i> orbitals of graphene and 3d obitals of Co²⁺–oxygen-vacancy complexes. This design may open up a kind of possibility for manipulating the magnetism of doped oxide nanostructures

Unidirectional Thermal Diffusion in Bimetallic Cu@Au Nanoparticles

Understanding the atomic diffusions at the nanoscale is important for controlling the synthesis and utilization of nanomaterials. Here, using <i>in situ</i> X-ray absorption spectroscopy coupled with theoretical calculations, we demonstrate a so far unexplored unidirectional diffusion from the Au shell to the Cu core in thermally alloying Cu@Au core@shell architecture of <i>ca.</i> 7.1 nm. The initial diffusion step at 423 K is found to be characterized by the formation of a diffusion layer composed of a Au-dilute substitutional CuAu-like intermetallic compound with short Cu–Au bond length (2.61 Å). The diffusion further happens by the migration of the Au atoms with large disorder into the interior Cu matrix at higher temperatures (453 and 553 K). These results suggest that the structural preference of a CuAu-like compound, along with the nanosized effect, plays a critical role in determining the atomic diffusion dynamics

Half-Unit-Cell α‑Fe₂O₃ Semiconductor Nanosheets with Intrinsic and Robust Ferromagnetism

The synthesis of atomically thin transition-metal oxide nanosheets as a conceptually new class of materials is significant for the development of next-generation electronic and magnetic nanodevices but remains a fundamental chemical and physical challenge. Here, based on a “template-assisted oriented growth” strategy, we successfully synthesized half-unit-cell nanosheets of a typical transition-metal oxide α-Fe₂O₃ that show robust intrinsic ferro­magnetism of 0.6 μ_B/atom at 100 K and remain ferromagnetic at room temperature. A unique surface structure distortion, as revealed by X-ray absorption spectroscopy, produces nonidentical Fe ion environments and induces distance fluctuation of Fe ion chains. First-principles calculations reveal that the efficient breaking of the quantum degeneracy of Fe 3d energy states activates ferro­magnetic exchange interaction in these Fe_5‑co–O–Fe_6‑co ion chains. These results provide a solid design principle for tailoring the spin-exchange interactions and offer promise for future semi­conductor spin­tronics

Vacancy-Induced Ferromagnetism of MoS₂ Nanosheets

Outstanding magnetic properties are highly desired for two-dimensional ultrathin semiconductor nanosheets. Here, we propose a phase incorporation strategy to induce robust room-temperature ferromagnetism in a nonmagnetic MoS₂ semiconductor. A two-step hydrothermal method was used to intentionally introduce sulfur vacancies in a 2H-MoS₂ ultrathin nanosheet host, which prompts the transformation of the surrounding 2H-MoS₂ local lattice into a trigonal (1T-MoS₂) phase. 25% 1T-MoS₂ phase incorporation in 2H-MoS₂ nanosheets can enhance the electron carrier concentration by an order, introduce a Mo⁴⁺ 4d energy state within the bandgap, and create a robust intrinsic ferromagnetic response of 0.25 μ_B/Mo by the exchange interactions between sulfur vacancy and the Mo⁴⁺ 4d bandgap state at room temperature. This design opens up new possibility for effective manipulation of exchange interactions in two-dimensional nanostructures

Understanding the Local and Electronic Structures toward Enhanced Thermal Stable Luminescence of CaAlSiN₃:Eu²⁺

It is a great challenge to maintain thermally stable luminescence of red phosphors in white light-emitting diodes (LEDs), because of the large Stokes shift. For the purpose of overcoming this challenge, this work elucidates the intrinsic mechanism of the thermal quenching luminescence of CaAlSiN₃:Eu²⁺. The empty 5d orbital of Eu²⁺ is partly filled with electrons upon Eu²⁺ increasing, as observed using XANES; and the exceptional expansion of the local Eu–N bond length, the ratio of which is far larger than the volume expansion of crystal lattice brought by doping Eu²⁺, is measured using EXAFS. The shift of Fermi level predicted with the first-principles calculations is confirmed by the valence band spectra. Therefore, the changeable distribution of electrons on the excited substates and then thermal delocalization to the conduction band are the intrinsic mechanisms of thermal quenching luminescence of CaAlSiN₃:Eu²⁺. The results provide a solid basis for exploring the methods to enhance the thermal stable luminescence of CaAlSiN₃:Eu²⁺

Fast Photoelectron Transfer in (C_ring)–C₃N₄ Plane Heterostructural Nanosheets for Overall Water Splitting

Direct and efficient photocatalytic water splitting is critical for sustainable conversion and storage of renewable solar energy. Here, we propose a conceptual design of two-dimensional C₃N₄-based in-plane heterostructure to achieve fast spatial transfer of photoexcited electrons for realizing highly efficient and spontaneous overall water splitting. This unique plane heterostructural carbon ring (C_ring)–C₃N₄ nanosheet can synchronously expedite electron–hole pair separation and promote photoelectron transport through the local in-plane π-conjugated electric field, synergistically elongating the photocarrier diffusion length and lifetime by 10 times relative to those achieved with pristine g-C₃N₄. As a result, the in-plane (C_ring)–C₃N₄ heterostructure could efficiently split pure water under light irradiation with prominent H₂ production rate up to 371 μmol g^–1 h^–1 and a notable quantum yield of 5% at 420 nm

Probing Nucleation Pathways for Morphological Manipulation of Platinum Nanocrystals

Understanding the formation process in the controlled synthesis of nanocrystals will lead to the effective manipulation of the morphologies and properties of nanomaterials. Here, <i>in-situ</i> UV–vis and X-ray absorption spectroscopies are combined to monitor the tracks of the nucleation pathways in the solution synthesis of platinum nanocrystals. We find experimentally that the control over nucleation pathways through changing the strength of reductants can be efficiently used to manipulate the resultant nanocrystal shapes. The <i>in-situ</i> measurements show that two different nucleation events involving the formation of one-dimensional “Pt_<i>n</i>Cl_<i>x</i>” complexes from the polymerization of linear “Cl₃Pt–PtCl₃” dimers and spherical “Pt_<i>n</i>⁰” clusters from the aggregation of Pt⁰ atoms occur for the cases of weak and strong reductants; and the resultant morphologies are nanowires and nanospheres, respectively. This study provides a crucial insight into the correlation between the particle shapes and nucleation pathways of nanomaterials