The catalytic mechanism offers an efficient tool to produce crystalline semiconductor nanowires, in which the choice, state, and structure of catalysts are active research issues of much interest. Here we report a novel solution-solid-solid (SSS) mechanism for nanowire growth catalyzed by solid-phase superionic conductor nanocrystals in low-temperature solution. The preparation of Ag2Se-catalyzed ZnSe nanowires at 100-210 °C is exampled to elucidate the SSS model, which can be extendable to grow other II-VI semiconductor (e.g., CdSe, ZnS, and CdS) nanowires by the catalysis of nanoscale superionic-phase silver or copper(I) chalcogenides (Ag2Se, Ag2S, and Cu2S). The exceptional catalytic ability of these superionic conductors originates from their structure characteristics, known for high-density vacancies and fast mobility of silver or copper(I) cations in the rigid sublattice of Se(2-) or S(2-) ions. Insights into the SSS mechanism are provided based on the formation of solid solution and the solid-state ion diffusion/transport at solid-solid interface between catalyst and nanowire.

First-order solid–solid phase transition of crystalline solids at the nanoscale has attracted an increasing interest in solid-state physics and chemistry, which can be used to alter the properties of materials without changing chemical compositions. Herein, we report the results of our comparative studies on phase transitions between tetragonal (t), orthorhombic (β), and cubic (α) polymorphs in Ag2Se nanocrystals. A significant discrepancy in stability and phase transition behavior is determined for t-Ag2Se nanocrystals, which were prepared separately by two different methods. Differential scanning calorimetry (DSC) and variable-temperature XRD studies reveal that the t-Ag2Se nanocrystals prepared by the oleylamine (OLA)-mediated method show a highly temperature- and time-sensitive metastability and undergo a t → β → α → β phase transition during the thermal cycling, in which the t → β transition is exothermic and irreversible, whereas the β → α transition is reversible. Similarly, the reversible β → α structure transition is detected in the β-Ag2Se nanocrystals, which were also prepared using the OLA-mediated method with different post-treatment manners and stabilized conditions. In contrast, the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method are more stable and exhibit a direct, reversible t → α phase transition without undergoing the β phase; however, when heated to a high temperature, for example, ≥250 °C, the stability of the t phase and the reversibility of the t → α transition will be destroyed due to the sintering and size increase of the sample, which is confirmed by the determination of the t → α → β phase transition in the DSC cycle. The formation of the t phase is attributed to the α → t structure transformation with the temperature cooled from synthetic temperatures (160–220 °C) to room temperature. Moreover, the reasons for the difference in the stabilities and phase transitions of t-Ag2Se nanocrystals prepared in our two methods are discussed based on the influences of size, surface (or shape), and defects on the thermodynamics and kinetics of a solid–solid structure transformation.

Transition-metal phosphide nanowires were facilely synthesized by Ullmann-type reactions between transition metals and triphenylphosphine in vacuum-sealed tubes at 350-400 degrees C. The phase (stoichiometry) of the phosphide products is controllable by tuning the metal/PPh(3) molar ratio and concentration, reaction temperature and time, and heating rate. Six classes of iron, cobalt, and nickel phosphide (Fe(2)P, FeP, Co(2)P, CoP, Ni(2)P, and NiP(2)) nanostructures were prepared to demonstrate the general applicability of this new method. The resulting phosphide nanostructures exhibit interesting phase- and composition-dependent magnetic properties, and magnetic measurements suggested that the Co(2)P nanowires with anti-PbCl(2) structure show a ferromagnetic-paramagnetic transition at 6 K, while the MnP-structured CoP nanowires are paramagnetic with Curie-Weiss behavior. Moreover, GC-MS analyses of organic byproducts of the reaction revealed that thermally generated phenyl radicals promoted the formation of transition-metal phosphides under synthetic conditions. Our work offers a general method for preparing one-dimensional nanoscale transition-metal phosphides that are promising for magnetic and electronic applications.

Seed-catalyzed, heteroepitaxial growth of high aspect-ratio, uniform ZnSe nanowires is reported, in which Ag 2 Se nanoparticles serve as heterogeneous seeds and as epitaxial substrates. Due to the difference in solubility product constants (K sp : Ag 2 Se ( ZnSe), Ag 2 Se is precipitated before ZnSe and in situ seeds and catalyzes the nanowire growth in a one-pot solution synthesis. The lattice match between metastable tetragonal Ag 2 Se{212} and cubic ZnSe{220} planes facilitates the heteroepitaxial growth of ZnSe nanowires. Conversion of small, uniform Ag particles, which are produced from the reduction of Ag + ions by reductants/surfactants such as polyvinyl pyrrolidone (PVP) and oleylamine (OA), into Ag 2 Se with small size and high uniformity is shown to be an effective strategy to control the diameter size and distribution of ZnSe nanowires. The third-order nonlinear optical behavior of the asprepared nanowires has been evaluated by Z-scan measurements. Experimental section Nanowire synthesisIn the experiments, a AgNO 3 ethanol solution (4 mmol L À1 ) was adopted as the silver source for the production of Ag 2 Se seeds.

A facile catalytic growth route was developed for the low-temperature solution synthesis of Ag2S-CdS matchstick-like heteronanostructures in oleylamine, which are composed of a Ag2S spherical head and a CdS rod-like stem. Ag2S nanoseeds acted as an effective catalyst for the growth of CdS nanorods and remained at the tip of the resultant nanorods, leading to the formation of Ag2S-CdS heterostructures with a matchstick shape. The diameter of the Ag2S heads and the length of the CdS stems could be easily controlled by varying the molar ratios of the Ag/Cd precursors. The differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction (XRD) studies confirmed that Ag2S catalytic seeds underwent a phase change, that is, they were in a high-temperature superionic conducting cubic structure during the CdS nanorod growth and then converted to a low-temperature monoclinic crystal structure as the reaction was cooled to room temperature. The influence of synthetic temperature on the product morphology was investigated and the morphological evolution at different growth stages was monitored using transmission electron microscopy (TEM). Furthermore, the growth kinetics of the Ag2S-CdS matchstick-like heteronanostructures, including the dissolution, nucleation and growth of CdS within the Ag2S catalyst, was reasonably discussed on the basis of the structural characteristics of the superionic cubic Ag2S catalyst and the low solubility of CdS in Ag2S derived from the Ag2S-CdS binary phase diagram.

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Composite core-shell nanostructures have greatly extended the properties of single-component counterparts, and display new bi-or multifunctionality in optical, electric, magnetic, catalytic, and biological performances. In this letter, we described the selective synthesis of uniform magnetic Fe 2 P/C and FeP/C core/shell nanocables via an organometallic route by thermolysis reactions of ferrocene (Fe(C 5 H 5 ) 2 ) with triphenylphosphine (PPh 3 ) in sealed vacuum tubes at ∼400 °C. The selective synthesis was readily accessed by controlling the heating rate and the molar ratio of reactants, and the Fe 2 P/C and FeP/C nanocables exhibited interesting ferromagnetic-paramagnetic transition with high blocking temperatures. Our method enriches core species and carbon sources for carbon shells, which can be widely extended to prepare new types of inorganic/carbon core-shell nanostructures, and the resulting magnetic nanocables may be used as components in nanomagnetic devices.

Diabetes has been reported to affect the microvasculature and lead to cerebral small vessel disease (SVD). Past studies using arterial spin labeling (ASL) at single post-labeling delay reported reduced cerebral blood flow (CBF) in patients with type 2 diabetes. The purpose of this study was to characterize cerebral hemodynamic changes of type 2 diabetes using a multi-inversion-time 3D GRASE pulsed ASL (PASL) sequence to simultaneously measure CBF and bolus arrival time (BAT). Thirty-six patients with type 2 diabetes (43–71 years, 17 male) and 36 gender- and age-matched control subjects underwent MRI scans at 3 T. Mean CBF/BAT values were computed for gray and white matter (GM and WM) of each subject, while a voxel-wise analysis was performed for comparison of regional CBF and BAT between the two groups. In addition, white matter hyperintensities (WMHs) were detected by a double inversion recovery (DIR) sequence with relatively high sensitivity and spatial resolution. Mean CBF of the WM, but not GM, of the diabetes group was significantly lower than that of the control group (p < 0.0001). Regional CBF decreases were detected in the left middle occipital gyrus (p = 0.0075), but failed to reach significance after correction of partial volume effects. BAT increases were observed in the right calcarine fissure (p < 0.0001), left middle occipital gyrus (p < 0.0001), and right middle occipital gyrus (p = 0.0011). Within the group of diabetic patients, BAT in the right middle occipital gyrus was positively correlated with the disease duration (r = 0.501, p = 0.002), BAT in the left middle occipital gyrus was negatively correlated with the binocular visual acuity (r = −0.408, p = 0.014). Diabetic patients also had more WMHs than the control group (p = 0.0039). Significant differences in CBF, BAT, and more WMHs were observed in patients with diabetes, which may be related to impaired vision and risk of SVD of type 2 diabetes.