Tunable Ordered Nanostructured Phases by Co-assembly of Amphiphilic Polyoxometalates and Pluronic Block Copolymers.

The assembly of polyoxometalate (POM) metal-oxygen clusters into ordered nanostructures is attracting a growing interest for catalytic and sensing applications. However, assembly of ordered nanostructured POMs from solution can be impaired by aggregation, and the structural diversity is poorly understood. Here, we present a time-resolved small-angle X-ray scattering (SAXS) study of the co-assembly in aqueous solutions of amphiphilic organo-functionalized Wells-Dawson-type POMs with a Pluronic block copolymer over a wide concentration range in levitating droplets. SAXS analysis revealed the formation and subsequent transformation with increasing concentration of large vesicles, a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually a hexagonal phase formed at concentrations above 110 mM. The structural versatility of co-assembled amphiphilic POMs and Pluronic block copolymers was supported by dissipative particle dynamics simulations and cryo-TEM.

Controlled Synthesis of Tellurium Nanowires.

One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour-solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 µm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 µm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.

Vacancy-Rich MXene-Immobilized Ni Single Atoms as a High-Performance Electrocatalyst for the Hydrazine Oxidation Reaction.

Single-atom catalysts (SACs), on account of their outstanding catalytic potential, are currently emerging as high-performance materials in the field of heterogeneous catalysis. Constructing a strong interaction between the single atom and its supporting matrix plays a pivotal role. Herein, Ti3 C2 Tx -MXene-supported Ni SACs are reported by using a self-reduction strategy via the assistance of rich Ti vacancies on the Ti3 C2 Tx MXene surface, which act as the trap and anchor sites for individual Ni atoms. The constructed Ni SACs supported by the Ti3 C2 Tx MXene (Ni SACs/Ti3 C2 Tx ) show an ultralow onset potential of -0.03 V (vs reversible hydrogen electrode (RHE)) and an exceptional operational stability toward the hydrazine oxidation reaction (HzOR). Density functional theory calculations suggest a strong coupling of the Ni single atoms and their surrounding C atoms, which optimizes the electronic density of states, increasing the adsorption energy and decreasing the reaction activation energy, thus boosting the electrochemical activity. The results presented here will encourage a wider pursuit of 2D-materials-supported SACs designed by a vacancy-trapping strategy.

Solid-state synthesis of few-layer cobalt-doped MoS2 with CoMoS phase on nitrogen-doped graphene driven by microwave irradiation for hydrogen electrocatalysis.

The high catalytic activity of cobalt-doped MoS2 (Co-MoS2) observed in several chemical reactions such as hydrogen evolution and hydrodesulfurization, among others, is mainly attributed to the formation of the CoMoS phase, in which Co occupies the edge-sites of MoS2. Unfortunately, its production represents a challenge due to limited cobalt incorporation and considerable segregation into sulfides and sulfates. We, therefore, developed a fast and efficient solid-state microwave irradiation synthesis process suitable for producing thin Co-MoS2 flakes (∼3-8 layers) attached on nitrogen-doped reduced graphene oxide. The CoMoS phase is predominant in samples with up to 15 at% of cobalt, and only a slight segregation into cobalt sulfides/sulfates is noticed at larger Co content. The Co-MoS2 flakes exhibit a large number of defects resulting in wavy sheets with significant variations in interlayer distance. The catalytic performance was investigated by evaluating the activity towards the hydrogen evolution reaction (HER), and a gradual improvement with increased amount of Co was observed, reaching a maximum at 15 at% with an overpotential of 197 mV at -10 mA cm-2, and a Tafel slope of 61 mV dec-1. The Co doping had little effect on the HER mechanism, but a reduced onset potential and charge transfer resistance contributed to the improved activity. Our results demonstrate the feasibility of using a rapid microwave irradiation process to produce highly doped Co-MoS2 with predominant CoMoS phase, excellent HER activity, and operational stability.