Hierarchically Structured CuCo₂S₄ Nanowire Arrays as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Hydrogen produced from water splitting offers a green alternative to conventional energy such as fossil fuels. Herein, CuCo₂S₄ nanowire arrays were synthesized on a nickel foam substrate by a two-step hydrothermal approach and utilized as highly efficient bifunctional electrocatalyst for overall water splitting. The CuCo₂S₄ nanowire arrays were identified as an exceptionally active catalyst for the hydrogen evolution reaction (HER) in a basic solution with an extremely low overpotential of 65 mV to reach a current density of 10 mA/cm². The hierarchically structured CuCo₂S₄ electrode was also highly active toward the oxygen evolution reaction (OER), achieving a high current density of 100 mA/cm² at an overpotential of only 310 mV. Consequently, an alkaline electrolyzer constructed using CuCo₂S₄ nanowire arrays as both anode and cathode can realize overall water splitting with a current density of 100 mA/cm² at a cell voltage of 1.65 V, suggesting a promising bifunctional electrocatalyst for efficient overall water splitting

Hierarchically Structured Ni Nanotube Array-Based Integrated Electrodes for Water Splitting

The development of high-performance nonprecious electrocatalysts for overall water splitting has attracted increasing attention but remains a vital challenge. Herein, we report a ZnO-based template method to fabricate Ni nanotube arrays (NTAs) anchored on nickel foil for applications in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). On the basis of this precursor electrode, the three-dimensional NiSe₂ NTAs of unique sandwich-like coaxial structure have been fabricated by electrodeposition of NiSe₂ on Ni NTAs, which exhibits high performance toward the HER in both acidic and alkaline media. The method based on Ni NTAs can be readily extended to fabricate Ni₂P NTAs by gas–solid phosphorization for the HER, and NiFeO_<i>x</i> NTAs by anodic codeposition of Ni and Fe for the OER. Consequently, an alkaline electrolyzer has been constructed using NiFeO_<i>x</i> NTAs and NiSe₂ NTAs as anode and cathode, respectively, which can realize overall water splitting with a current density of 100 mA cm^–2 at an overpotential of 510 mV

Heterostructured Arrays of Ni_<i>x</i>P/S/Se Nanosheets on Co_<i>x</i>P/S/Se Nanowires for Efficient Hydrogen Evolution

The development of efficient electrocatalysts for hydrogen evolution reaction (HER) is of increasing importance in energy conversion schemes. The earth-abundant transition-metal phosphides, especially CoP and Ni₂P, have emerged as promising catalysts for HER. We describe here the preparation and characterization of a hybrid catalyst of Ni₂P nanosheets@CoP nanowires on a carbon cloth for the reaction. The heterostructure and synergistic effects of the Ni₂P and CoP components result in an extremely low overpotential of 55 mV for achieving a catalytic current density of 10 mA cm^–2, which is remarkable for transition-metal phosphide electrocatalysts. The synthetic procedure could be readily extended to related, heterostructured bimetallic sulfides or selenides for HER

N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen Production

Molybdenum (Mo) carbide-based electrocatalysts are considered promising candidates to replace Pt-based materials toward the hydrogen evolution reaction (HER). Among different crystal phases of Mo carbides, although Mo₂C exhibits the highest catalytic performance, the activity is still restricted by the strong Mo–H bonding. To weaken the strong Mo–H bonding, creating abundant Mo₂C/MoC interfaces and/or doping a proper amount of electron-rich (such as N and P) dopants into the Mo₂C crystal lattice are effective because of the electron transfer from Mo to surrounding C in carbides and/or N/P dopants. In addition, Mo carbides with well-defined nanostructures, such as one-dimensional nanostructure, are desirable to achieve abundant catalytic active sites. Herein, well-defined N,P-codoped Mo₂C/MoC nanofibers (N,P-Mo_<i>x</i>C NF) were prepared by pyrolysis of phosphomolybdic ([PMo₁₂O₄₀]^3–, PMo₁₂) acid-doped polyaniline nanofibers at 900 °C under an Ar atmosphere, in which the hybrid polymeric precursor was synthesized via a facile interfacial polymerization method. The experimental results indicate that the judicious choice of pyrolysis temperature is essential for creating abundant Mo₂C/MoC interfaces and regulating the N,P-doping level in both Mo carbides and carbon matrixes, which leads to optimized electronic properties for accelerating HER kinetics. As a result, N,P-Mo_<i>x</i>C NF exhibits excellent HER catalytic activity in both acidic and alkaline media. It requires an overpotential of only 107 and 135 mV to reach a current density of 10 mA cm^–2 in 0.5 M H₂SO₄ and 1 M KOH, respectively, which is comparable and even superior to the best of Mo carbide-based electrocatalysts and other noble metal-free electrocatalysts

Au Nanorod Decoration on NaYF₄:Yb/Tm Nanoparticles for Enhanced Emission and Wavelength-Dependent Biomolecular Sensing

We introduce gold nanorods (GNRs) decoration on NaYF₄:Yb/Tm upconversion nanocrystals (UCNCs) by functionalizing the UCNCs with polyamidoamine generation 1 (PAMAM G1) dendrimer, followed by a single-step seed-mediated growth of long-range GNRs to enhance “biological window” upconversion emission. The up-conversion emission of GNR-decorated UCNCs can be enhanced beyond the level typically obtainable using shell-like structures up to 27-fold enhancement. Also, the enhancement can be tuned at different wavelength regions by varying the GNR aspect ratio. The GNR-decorated UCNC is further modified with 2-thiouracil for nonenzymatic detection of uric acid, revealing a detection limit as 1 pM

Lanthanide-Doped Na_<i>x</i>ScF_3+<i>x</i> Nanocrystals: Crystal Structure Evolution and Multicolor Tuning

Rare-earth-based nanomaterials have recently drawn considerable attention because of their unique energy upconversion (UC) capabilities. However, studies of Sc³⁺-based nanomaterials are still absent. Herein we report the synthesis and fine control of Na_<i>x</i>ScF_3+<i>x</i> nanocrystals by tuning of the ratio of oleic acid (OA, polar surfactant) to 1-octadecene (OD, nonpolar solvent). When the OA:OD ratio was increased from low (3:17) to high (3:7), the nanocrystals changed from pure monoclinic phase (Na₃ScF₆) to pure hexagonal phase (NaScF₄) via a transition stage at an intermediate OA:OD ratio (3:9) where a mixture of nanocrystals in monoclinic and hexagonal phases was obtained and the coexistence of the two phases inside individual nanocrystals was also observed. More significantly, because of the small radius of Sc³⁺, Na_<i>x</i>ScF_3+<i>x</i>:Yb/Er nanocrystals show different UC emission from that of NaYF₄:Yb/Er nanocrystals, which broadens the applications of rare-earth-based nanomaterials ranging from optical communications to disease diagnosis