Jun Ke

Ru Nanocrystals with Shape-Dependent Surface-Enhanced Raman Spectra and Catalytic Properties: Controlled Synthesis and DFT Calculations

Despite its multidisciplinary interests and technological importance, the shape control of Ru nanocrystals still remains a great challenge. In this article, we demonstrated a facile hydrothermal approach toward the controlled synthesis of Ru nanocrystals with the assistance of first-principles calculations. For the first time, Ru triangular and irregular nanoplates as well as capped columns with tunable sizes were prepared with high shape selectivity. In consistency with the experimental observations and density functional theory (DFT) calculations confirmed that both the intrinsic characteristics of Ru crystals and the adsorption of certain reaction species were responsible for the shape control of Ru nanocrystals. Ultrathin Ru nanoplates exposed a large portion of (0001) facets due to the lower surface energy of Ru(0001). The selective adsorption of oxalate species on Ru(10-10) would retard the growth of the side planes of the Ru nanocrystals, while the gradual thermolysis of the oxalate species would eliminate their adsorption effects, leading to the shape evolution of Ru nanocrystals from prisms to capped columns. The surface-enhanced Raman spectra (SERS) signals of these Ru nanocrystals with 4-mercaptopyridine as molecular probes showed an enhancement sequence of capped columns > triangle nanoplates > nanospheres, probably due to the sharp corners and edges in the capped columns and nanoplates as well as the shrunk interparticle distance in their assemblies. CO-selective methanation tests on these Ru nanocrystals indicated that the nanoplates and nanospheres had comparable activities, but the former has much better CO selectivity than the latter.

Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in NaxREF3+x Nanocrystals

Efficient tailoring of upconversion emissions in lanthanide-doped nanocrystals is of great significance for extended optical applications. Here, we present a facile and highly effective method to tailor the upconversion selectivity by engineering the local structure of lanthanides in Na(x)REF(3+x) nanocrystals. The local structure engineering was achieved through precisely tuning the composition of nanocrystals, with different [Na]/[RE] ([F]/[RE]) ratio. It was found that the lattice parameter as well as the coordination number and local symmetry of lanthanides changed with the composition. A significant difference in the red to green emission ratio, which varied from 1.9 to 71 and 1.6 to 116, was observed for Na(x)YF(3+x):Yb,Er and Na(x)GdF(3+x):Yb,Er nanocrystals, respectively. Moreover, the local structure-dependent upconversion selectivity has been verified for Na(x)YF(3+x):Yb,Tm nanocrystals. In addition, the local structure induced upconversion emission from Er(3+) enhanced 9 times, and the CaF2 shell grown epitaxially over the nanocrystals further promoted the red emission by 450 times, which makes it superior as biomarkers for in vivo bioimaging. These exciting findings in the local structure-dependent upconversion selectivity not only offer a general approach to tailoring lanthanide related upconversion emissions but also benefit multicolor displays and imaging.

Sodium Doping Controlled Synthesis of Monodisperse Lanthanide Oxysulfide Ultrathin Nanoplates Guided by Density Functional Calculations

Doping of nanocrystals is an intriguing field, since the intentional introduction of impurities has long been regarded as a major way of tailoring the properties of materials. Furthermore, it was recently found that the use of dopants in the synthesis of inorganic colloidal nanocrystals can not only introduce novel properties but also influence shape and size evolution during nanocrystal nucleation and growth. For example, Liu and co-workers showed that NaYF4-based nanocrystals can be precisely tuned in size, phase, and upconversion emission through Gd doping. Wang and co-workers reported that lanthanide doping of alkaline earth metal fluoride nanocrystals can lead to a significant increase in monodispersity. On the other hand, the special 4f electron configurations of lanthanides endow their compounds with promising functionalities, such as luminescence, catalytic activity, and permanent magnetism. This has therefore stimulated recent efforts to synthesize colloidal lanthanide-based nanocrystals (Ln-based NCs) with tunable morphologies and unique material properties. For example, both the small size and excellent luminescence properties make Ln-based NCs a potential new type of fluorescent probes. Specifically, lanthanide oxysulfides (Ln2O2S; Ln=La, Gd, Y) can serve as one of the most effective hosts for fluorescence applications, and research on Ln2O2S NCs is therefore highly intriguing. [4] However, the synthesis of monodisperse Ln2O2S NCs remains a challenge, since the theory of hard and soft acids and bases (HSAB) predicts a lack of affinity between the hard Lewis acid Ln and the soft Lewis base S . Previously, Gao and coworkers presented pioneering work in the synthesis of monodisperse lanthanide oxysulfide nanocrystals, yet due to the difficulty in preparing the corresponding single-source precursors, this method was limited to only a few lanthanides (i.e., Sm, Eu, Gd). On the basis of both experimental characterization and DFT calculations, we now demonstrate that introduction of monovalent Na ions as dopants in trivalent Ln host lattices can significantly facilitate the formation of Ln2O2S NCs in oleic acid (OA)/oleylamine (OM)/1-octadecene (ODE) mixed solvent by creating oxygen vacancies in the host lattice during sulfurization reactions. In a typical synthesis, monodisperse and single-crystalline Na-doped La2O2S nanoplates were formed with a diameter of (22.3 2.0) nm (Figure 1a and b). High-resolution transmission electron microscopy (HRTEM) revealed that the morphology of the as-synthesized nanoplates was mainly hexagons with six {100} facets as their side planes (Figure 1b). When the dispersion of nanoplates in cyclohexane was highly

Microwave-assisted rapid synthesis of mesoporous nanostructured ZnCo2O4 anode materials for high-performance lithium-ion batteries

Cobalt-based oxides have attracted much attention due to their extensive application in energy storage. In this work, a microwave-assisted second-level rapid synthesis method is developed to prepare ZnCo2O4 anode materials for lithium-ion batteries. Mesoporous rose-like nanostructured ZnCo2O4 is obtained by heat hydrolysis of a (Zn,Co)-organic hybrid precursor obtained by a rapid microwave-assisted solvothermal route. Systematic investigation on optimization of synthesis conditions is conducted to understand the synthesis-controlled process. Requisite characterization is carried out on the obtained ZnCo2O4 by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption. As an anode material, the mesoporous rose-like ZnCo2O4 exhibits high capacity and excellent cyclability, which can be ascribed to the easy penetration of electrolytes into the inner part of active materials through numerous pores, stable microstructure, and alleviated volume expansion induced by the porous structure during the Li+ insertion/extraction process.

Controllable Synthesis of Monodispersed Middle and Heavy Rare Earth Oxysulfide Nanoplates Based on the Principles of HSAB Theory

Based on the theory of hard and soft acids and bases, trivalent ions of middle and heavy rare earths belong to very hard acids, which possess weak affinity towards S ions but strong affinity to O ions. So it is difficult to synthesize middle and heavy rare earth oxysulfide nano-materials through the thermolysis method in high-boiling-point organic solvent. In this article, monodispersed oxysulfide nanoplates of Y, Eu, Gd, Er and Yb were synthesized through this thermolysis method we developed. Both sodium-doped and undoped rare earth oxysulfide nanoplates could be prepared, and the doping of sodium ions could promote the crystallization of the nanoplates. Rare earth acetylacetonates were used as metal precursors and H2S gas as the sulfurizing reagent. The reactions were conducted in oleylamine at 280 °C for 1 hour. The thermogravimetric analysis of the precursor showed that the initial decomposition temperature of the rare earth acetylacetonates is about 200 °C, which is much lower than that of rare earth oleates. The transmission electron microscopy observation and energy dispersive X-ray analyses of the intermediate products during the synthesis of the nanoplates showed that rare earth oxide nanoplates formed firstly at about 220 °C, and these nanoplates transformed to oxysulfide nanoplates gradually during the temperature ramping period. Density functional theory calculation was used to compare the total free energy of the oxide and oxysulfide of different rare earth elements. According to this thermodynamical comparison, we concluded that, from light rare earths to heavy rare earths, higher chemical potential of sulfur is needed to obtain the oxysulfide rather than oxide. On one hand, H2S gas has higher sulphurizing power than sulfur. On the other hand, a comparatively low reaction temperature favors the dissolving of H2S in oleylamine. As a result, the chemical potential of sulfur in synthetic system could be effectively increased by using rare earth acetylacetonates as the precursors instead of rare earth oleates, and using H2S gas as sulphurizing agent instead of sulfur, which made it possible to prepare middle and heavy rare earth oxysulfide nanoplates in oleylamine. Fluorescent measurements showed that as-synthesized Y2O2S:Eu nanoplates could emit red light under 251 nm ultraviolet light excitation. For Y2O2S:Eu nanoplates, the fluorescence life time was shorter and quantum yield was lower in comparison with the corresponding bulk counterpart, possibly due to its much higher portion of surface atoms as well as lower crystallinity.