[en] Monodisperse Ni nanoparticles with sizes varying from 4.8 to 11.3 nm are prepared via a one-pot reaction that involves the reduction of nickel(II) acetylacetonate in oleylamine in the presence of trioctylphosphine and 1,2-hexadecanediol. Reaction parameters such as temperature and the concentration of capping agent and metal precursor are critical for the adjustment of particle size. The decrease of crystallinity is observed for the samples with smaller particle sizes, which significantly affects the magnetic properties. Three-dimensional (3D) superlattices that are composed of Ni nanoparticles with different sizes are obtained on different substrates by a facile self-assembly process, and are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and small-angle X-ray diffraction (SAXRD). The Ni nanoparticle superlattices formed on carbon-coated TEM copper grids exhibit a dominant hexagonal close-packed (hcp) symmetry, although local fcc packing is also occasionally observed. The formation of 3D nanoparticle superlattice structures on Si substrates is confirmed from the SAXRD measurements. The method revealed in this study for the preparation of 3D superlattices composed of Ni nanoparticles with tunable sizes offers the potential to explore their interesting collective properties for multiple applications. - Highlights: • A facile solution route to monodisperse Ni nanoparticles with tunable sizes. • 3D Ni nanoparticle superlattices are formed on different substrates. • A dominant large-scale hcp symmetry is observed on carbon film substrate. • Typical structural parameters are obtained on the 3D nanoparticle superlattices. • The demonstrated method gives a convenient access to study collective properties

[en] MoSe₂ represents an important layered transition metal dichalcogenides (LTMDs) that have high electrocatalytic activity in hydrogen evolution reaction (HER). A key issue to achieve excellent electrochemical properties of MoSe₂ is to synthesize nanostructures that are composed of few-layer MoSe₂ with maximized exposure of active edge sites. In this study, we report the synthesis of MoSe₂ nanostructures with network-like and flower-like morphologies via an organic solution approach that utilizes molybdenum hexacarbonyl as Mo precursor and Se powders as Se source. The prepared MoSe₂ nanostructures consist of few-layer ultrathin nanosheets which can have different orientations. Depending on the volume ratio of oleylamine to oleic acid, the arrangement of ultrathin MoSe₂ nanosheets is adjustable and can form porous network-like or discrete flower-like nanostructure which has large specific surface area and a wealth of exposed edge sites. Excellent HER activities with low overpotentials, small Tafel slopes and long-term durability are observed on the as-synthesized MoSe₂ nanostructures. The acetic acid-washing step that may effectively remove the organic molecules capped on MoSe₂ surfaces is also found to be efficient in improving the HER activity of the synthesized MoSe₂ nanostructures.

[en] Nickel nanoparticles were prepared from the thermal decomposition of nickel(II) acetylacetonate in alkylamines and characterized by powder x-ray diffraction, transmission electron microscopy and magnetic measurement. The reaction temperature, heating rate and solvent type play an important role in the control over the crystalline phase. Depending on the reaction conditions, face-centered cubic (fcc) or hexagonal close-packed (hcp) nickel nanoparticles can be obtained. Monodisperse nickel nanoparticles were also obtained by introducing surfactants. The results of magnetic characterization showed that the magnetic properties of the hcp nickel nanoparticles are quite different from those of the fcc nickel nanoparticles

[en] Magnetically recyclable Ag-Ni core-shell nanoparticles have been fabricated via a simple one-pot synthetic route using oleylamine both as solvent and reducing agent and triphenylphosphine as a surfactant. As characterized by transmission electron microscopy (TEM), the as-synthesized Ag-Ni core-shell nanoparticles exhibit a very narrow size distribution with a typical size of 14.9 ± 1.2 nm and a tunable shell thickness. UV-vis absorption spectroscopy study shows that the formation of a Ni shell on Ag core can damp the surface plasmon resonance (SPR) of the Ag core and lead to a red-shifted SPR absorption peak. Magnetic measurement indicates that all the as-synthesized Ag-Ni core-shell nanoparticles are superparamagnetic at room temperature, and their blocking temperatures can be controlled by modulating the shell thickness. The as-synthesized Ag-Ni core-shell nanoparticles exhibit excellent catalytic properties for the generation of H₂ from dehydrogenation of sodium borohydride in aqueous solutions. The hydrogen generation rate of Ag-Ni core-shell nanoparticles is found to be much higher than that of Ag and Ni nanoparticles of a similar size, and the calculated activation energy for hydrogen generation is lower than that of many bimetallic catalysts. The strategy employed here can also be extended to other noble-magnetic metal systems.

[en] The 1.5 at.% Er³⁺ doped gadolinium scandium gallium garnet (GSGG) laser crystal with high optical quality was successfully grown by the Czochralski method. The structural parameters were obtained by x-ray Rietveld refinement. A high crystalline quality of Er:GSGG was determined by x-ray rocking curve. The spectroscopic parameters of Er³⁺ ion were calculated and analyzed using the Judd–Ofelt theory. Furthermore, the stimulated-emission cross-section spectra were investigated for the ⁴I_13/2  →  ⁴I_15/2 transitions at 1.5–1.6 µ m, which indicates the great potential of Er:GSGG for multi-wavelength emission at 1.5–1.6 µ m. The larger emission cross-section and long fluorescence lifetimes around 0.55 and 0.67 µ m mean that the 1.5 at.% Er:GSGG crystal is beneficial for green and red laser generation. Meanwhile, the intensity of the emission spectra was compared between the 1.5 at.% and 2.0 at.% Er-doped GSGG. The roles of cross-upconversion, cross-relaxation and the excited-state absorption process were discussed in the 1.5 at.% Er:GSGG crystal for generating visible and 1.5–1.6 µ m lasers. (paper)

[en] In this paper, we report the anisotropic optical and catalytic properties of wurtzite-type hexagonal CoO (h-CoO) nanocrystals, an unusual nanosized indirect semiconductor material. h-CoO nanoplates and nanorods with a divided morphology have been synthesized via facile solution methods. The employment of flash-heating and surfactant tri-n-octylphosphine favors the formation of plate-like morphology, whereas the utilization of cobalt stearate as a precursor is critical for the synthesis of nanorods. Structural analyses indicate that the basal plane of the nanoplates is (001) face and the growth direction of the nanorods is along the c axis. Moreover, the UV–vis absorption spectra, the corresponding energy gap and the catalytic properties are found to vary with the crystal shape and the dimensions of the as-prepared h-CoO nanocrystals. Furthermore, remarkable catalytic activities for H₂ generation from the hydrolysis of alkaline NaBH₄ solutions have been observed for the as-prepared h-CoO nanocrystals. The calculated Arrhenius activation energies show a decreasing trend with increasing extension degree along the 〈001〉 direction, which is in agreement with the variation of the charge-transfer energy gap. Finally the maximum hydrogen generation rate of the h-CoO nanoplates exceeds most of the reported values of transition metal or noble metal containing catalysts performing in the same reaction system, which makes them a low-cost alternative to commonly used noble metal catalysts in H₂ generation from the hydrolysis of borohydrides, and might find potential applications in the field of green energy. (paper)

[en] Noble metal–semiconductor hybrid nanocrystals represent an important class of materials for many potential applications, especially for photocatalysis. The utilization of transition metals to form alloys with noble metals can not only reduce the preparation costs, but may also offer tunable optical and catalytic properties for a broader range of applications. In this study, we report on the solution synthesis of AuCu_3–ZnO hybrid nanocrystals with three interesting morphologies, including urchin-like, flower-like and multipod-like nanocrystals. In the synthetic strategy, Au–Cu bimetallic alloy seeds formed in situ are used to induce the heteroepitaxial growth of ZnO nanocrystals on the surface of bimetallic alloy cores; thus different types of morphologies can be achieved by controlling the reaction conditions. Through high-resolution transmission electron microscopy observations, well-defined interfaces between ZnO and AuCu_3 are observed, which indicate that ZnO has a (0001) orientation and prefers to grow on AuCu_3 {111} facets. The as-prepared hybrid nanocrystals demonstrate morphology- and composition-dependent surface plasmon resonance (SPR) absorption bands. In addition, much higher photocatalytic efficiency than pure ZnO nanocrystals is observed for the hybrid nanocrystals in the degradation of methylene blue. In particular, the multipod-like AuCu_3–ZnO hybrid nanocrystals show the highest catalytic performance, as well as more than three times higher photocurrent density than the pure ZnO sample. The reported synthetic strategy provides a facile route to the effective combination of a plasmonic alloy with semiconductor components at the nanoscale in a controlled manner. (paper)

[en] We report facile solution approaches for the phase-controlled synthesis of rock-salt cubic CoO (c-CoO) and wurtzite-type hexagonal CoO (h-CoO) nanocrystals. In the syntheses, the cobalt precursor cobalt (II) stearate is decomposed in 1-octadecene at 320 °C, and the crystalline phase of synthesized products depend critically on the amounts of H₂O. While the presence of small amounts of H₂O promotes the generation of c-CoO, h-CoO is obtained in the absence of H₂O. The as-prepared c-CoO nanocrystals exhibit a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together whereas the h-CoO nanocrystals show a multi-rod structure with several rods growing on the same base facet along the c -axis. The formation mechanisms are discussed on the basis of FTIR spectrometry data and color changes of the reaction mixture. Finally the magnetic properties of as-prepared CoO nanocrystals are measured and the results show that c-CoO nanocrystals are intrinsically antiferromagnetic with a Néel temperature of about 300 K but the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Weak ferromagnetic contributions are also observed for both c-CoO and h-CoO nanocrystals with obvious magnetic hysteresis at 5 and 300 K. The uncompensated spins that can be induced by crystalline defects such as cation-vacancy may account for the observed weak ferromagnetism. (paper)

[en] Research highlights: → In this paper, the Al-doped zinc oxide (AZO) films were prepared on quartz glass flakes and silicon wafers by radio frequency (RF) magnetron sputtering. Evolution of the structural, optical, electrical and mechanical properties of the AZO films as a function of substrate temperatures ranging from room temperature to 400 ^oC were analyzed. The results indicate that AZO films with a low resistivity value of 4.97 x 10^-4 Ω cm a relatively higher adhesion and a high transparency above 90% can be prepared at a substrate temperature of 400 ^oC. Especially, the adhesion and hardness were seldom studied before but were investigated in details in this paper. - Abstract: The Al-doped zinc oxide (ZnO:Al) films were prepared on quartz glass flakes and silicon wafers by radio frequency (RF) magnetron sputtering which uses an aluminum-doped zinc oxide ceramic target. Meanwhile, their properties were characterized by scanning electron microscopy, X-ray diffraction, infrared-UV spectrophotometry, resistance measurement, nano-scratch and indentation test. Evolutions of the structural, optical, electrical and mechanical properties of the ZnO:Al films as a function of substrate temperatures ranging from room temperature to 400 ^oC were analyzed. The results indicate that the ZnO:Al films with a low resistivity value of 4.97 x 10^-4 Ω cm, a relatively higher adhesion and a high transparency above 90%, can be prepared at a substrate temperature of 400 ^oC.

[en] Magnetic metal-semiconductor hybrid nanocrystals containing ferromagnetic Ni and semiconductor ZnO have been prepared via a hot-injection route. The Ni-ZnO hybrid nanocrystals have a flower-like morphology that consists of Ni inner cores and ZnO petal shells. In spite of their large lattice mismatch, ZnO nanocrystals can still grow on faceted Ni nanocrystals to form stable interfaces. The composition of Ni-ZnO hybrid nanocrystals is readily controlled, and the average size of Ni core is tunable from 25 to 50 nm. Room temperature ferromagnetic properties are observed in these hybrid nanocrystals, and tunable magnetic properties also can be achieved by varying the size of Ni core. The as-prepared Ni-ZnO hybrid nanocrystals exhibit enhanced photocatalytic performance under ultraviolet light illumination as compared to pure ZnO nanocrystals. Furthermore, the superior reusability of hybrid nanocrystals for photocatalytic application is achieved by virtue of their magnetic properties. The facile and efficient seed-mediate strategy is particularly attractive to construct hybrid magnetic-semiconducting heterostructures. The as-obtained Ni-ZnO hybrid nanocrystals offer great potential for various applications due to their combined magnetic and semiconducting properties and low-cost earth-abundant availability.