Rational Design of a Copper Cobalt Sulfide/Tungsten Disulfide Heterostructure for Excellent Overall Water Splitting.

Integrating different components into a heterostructure is a novel approach that increases the number of active centers to enhance the catalytic activities of a catalyst. This study uses an efficient, facile hydrothermal strategy to synthesize a unique heterostructure of copper cobalt sulfide and tungsten disulfide (CuCo2S4-WS2) nanowires on a Ni foam (NF) substrate. The nanowire arrays (CuCo2S4-WS2/NF) with multiple integrated active sites exhibit small overpotentials of 202 (299) and 240 (320) mV for HER and OER at 20 (50) mA cm-2 and 1.54 V (10 mA cm-2) for an electrolyzer in 1.0 M KOH, surpassing commercial and previously reported catalysts. A solar electrolyzer composed of CuCo2S4-WS2 bifunctional electrodes also produced significant amounts of hydrogen through a water splitting process. The remarkable performance is accredited to the extended electroactive surface area, reasonable density of states near the Fermi level, optimal adsorption free energies, and good charge transfer ability, further validating the excellent dual function of CuCo2S4-WS2/NF in electrochemical water splitting.

Heterostructure of polyoxometalate/zinc-iron-oxide nanoplates as an outstanding bifunctional electrocatalyst for the hydrogen and oxygen evolution reaction.

Electrocatalysts play an important role to increase the energy conversion efficiency of electrolysis processes. In this study, a heterostructure of zinc iron oxide (ZnFe2O4) and polyoxometalate (POM) nanoplates (POM-ZnFe2O4) was fabricated for the first time by a hydrothermal process. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) analysis of POM-ZnFe2O4 furnished low overpotentials of 268 and 356 mV, and 220 and 290 mV to achieve current densities of 20 and 50 mA cm-2, respectively. In addition, an electrolytic cell composed of a POM-ZnFe2O4 cathode and anode required an operating voltage of 1.53 V to deliver a current of 10 mA cm-2. The improved electrochemical performance of POM-ZnFe2O4 compared to commercial and recently reported catalysts is attributed to the high electrocatalytically active surface area, modulation in the electronic and chemical properties and the formation of heterojunction of ZnFe2O4 and POM, which are vital for accelerating HER and OER activity.

Metal-to-Semimetal Transition in Platinum Nanotubes: Dependence on Thickness.

The temperature dependence of the electrical conductivity of Pt nanotubes (NTs) with different thicknesses synthesized by a wetting method using an Al2O3 membrane was studied. Pt NTs exhibited circular pores with an average diameter of ∼200 nm. From XRD, the prepared Pt NTs displayed a cubic crystal structure. Pt metal was identified based on the binding energy peak at 71 eV via XPS analysis. Pt NTs with thicknesses of 5 and 12 nm behaved like a semimetal, whereas Pt NTs with thicknesses of 25 and 29 nm showed normal metallic electrical conduction characteristics. This metal-to-semimetal transition was induced as the thickness and grain sizes of the Pt NTs were decreased. The critical metal-to-semimetal transition temperature of Pt NTs with average tube wall thicknesses of ∼5 nm was measured at ∼37 °C. However, the critical temperature could not be measured for NTs with a thickness of 12 nm. It is assumed that the critical temperature would be far below 0 °C. This transition behavior resulted from both a discontinuity in the density of states due to the quantum confinement effect and the increased energy barrier for conduction of electrons accompanied by the increased density of grain boundaries. These results presented here signify a vital step in the direction of realizing high-performance nanoelectronic devices.