[en] Graphical abstract: We reported a novel hetero-valence cation exchange route to synthesize Ln: NaGdF4 upconversion nanocrystals for the first time. -- Highlights: • The Ln3+: NaGdF4 nanocrystals were synthesized based on the Ln3+: CaF2 precursors. • The microstructures of nanocrystals were characterized. • The multi-color upconversion emissions were easily realized. -- Abstract: Lanthanide-doped upconversion nanomaterials have attracted great attention recently for their potential applications in the fields of bio-label, three-dimensional display, solar cell and so on. In this article, we report a new strategy to prepare hexagonal Ln³⁺:NaGdF₄ upconversion nanocrystals. Unlike the routine way of synthesizing NaGdF₄ nanocrystals through nucleation and growth, the formation of hexagonal NaGdF₄ nanocrystals herein is realized based on the Ln³⁺-doped cubic CaF₂ precursors, following a hetero-valence cation exchange process between Gd³⁺/Na⁺ and Ca²⁺. Evidently, Ln³⁺ dopants in the CaF₂ precursors are retained in the finally formed hexagonal NaGdF₄ nanocrystals and, subsequently, multi-color upconversion emissions are easily realized by simply adjusting the Ln³⁺ dopant species and contents in the CaF₂ precursors. This novel hetero-valence cation exchange route may open up a new pathway to synthesize nanomaterials that cannot be fabricated directly

[en] In this communication, a thermolysis route is developed to synthesize novel Cu_1.94S–ZnS–Cu_1.94S nanoheterostructures with interesting sandwich-like architectures, taking Cu_1.94S nanoplates as precursors. Evidently, the trimeric nanostructure is formed by a three-stage process, which includes the Zn-oleate induced assembling of Cu_1.94S nanoplate couples, the heteronucleation and growth of a ZnS layer between two Cu_1.94S plates dominated by interfacial diffusion, and the catalyst assisted axial growth of ZnS nanorod following the solution–liquid–solid mechanism. With epitaxial growth of ZnS nanocrystal between two Cu_1.94S nanoplates, the localized surface plasmon resonance frequency of Cu_1.94S shifts from 1875 to 1323 nm, indicating that this new material is potentially applicable as a light absorbing agent in laser photothermal therapy. The reported growth mechanism may provide new strategies for designing and fabricating various technologically important polymeric nanoheterostructures. (paper)

[en] A series of Er³⁺-doped (1 − x)CaF₂–xYbF₃ (0 ≤ x ≤ 0.6) disordered solid-solution nanocrystals with various mean sizes were successfully prepared by a facile solvothermal route. Interestingly, abnormal size-dependent upconversion emissions were demonstrated in these nanocrystals for the first time. With increasing grain size, an obvious enhancement of red to green emission ratio was observed in the Er³⁺ (2 mol%): 0.4CaF₂–0.6YbF₃ nanocrystals, which is the opposite of the routine size-dependent upconversion emission behavior reported previously. Taking Eu³⁺ ions as a structural probe, we investigated the influence of a disordered solid-solution structure on Ln³⁺ luminescence, and proposed that Ln³⁺ clusters formed in the host should play a key role to induce this unusual size-dependent upconversion emission phenomenon. As a consequence, multi-colors such as green, yellow, and red upconversion emissions can be easily realized by appropriately modifying the Yb³⁺ content in the Er³⁺-doped (1 − x)CaF₂–xYbF₃ nanocrystals. The reported results will deepen the understanding of size effects on the lanthanide upconversion in nanocrystals. (paper)

[en] Highlights: • The die upset magnets with Ce-Fe-B addition consist of Nd-rich and Ce-rich main phases. • The Nd-rich and Ce-rich domains present different magnetization behaviors. • The superior magnetic properties of magnet with x = 20 wt% Ce-Fe-B are obtained. The anisotropic hot deformed magnets consisting of Nd-rich and Ce-rich 2:14:1 main phases were prepared using dual alloy process. After heat treatment, crystallographic alignment was improved and compositional heterogeneity reduced. Coercivity and remanence descended with increasing Ce-Fe-B addition, and then were improved after diffusing heat treatment compared with magnets without heat treatment. Moreover, the magnetization behaviors of magnets were discussed and the coercivity mechanism was controlled by inhomogeneity of the domain wall pinning. The interaction domains were observed and the moment of Ce-rich grain was prior to reversal by comparison with Nd-rich grain in an applied field due to the lower magnetocrystalline anisotropy.

[en] Highlights: • Semicrystalline yellow TiO₂ nanoparticles are new synthesized. • Lattice OO bonds is firstly in-situ created within semicrystalline TiO₂. • Lattice OO bonds significantly enhance photocatalytic activity for H₂ production. • Reaction mechanism is proposed that describes the role of lattice OO bonds. -- Abstract: Atomic defects (e.g., Ti³⁺, oxygen vacancies) have been intenstively investigated for modifying TiO₂ in order to reach visible light active photocatalytic H₂ production. However, the atomic defects within TiO₂ could easily act as photo-generated charge-carrier recombination centers, resulting in relatively low H₂ conversion efficiency. In this paper, semicrystalline yellow TiO₂ nanoparticles rich of superoxide ions are new synthesized by a simple aqueous solution method. Instead of introducing atomic defects, we show for the first time that catalytic performance can also be significantly improved via in-situ creating lattice OO bonds within metastable semicrystalline TiO₂. The synthesized semicrystalline yellow TiO₂ exhibits significantly enhanced photocatalytic activity for H₂ production after cycle tests. The formaldehyde in aqueous solution is used as target pollutant to simulate industrial wastewater. In-situ created elastic lattice OO bonds are proposed to improve catalytic performance through facilitating the breakage of CH bonds of HCHO. A series of internally consistent reaction equations is proposed that describes the role of in-situ created lattice OO bonds for improving the catalytic performance. This is strongly supported by that the H₂ production rate at the end of the fourth cycle test is significantly more than that of the beginning of the first cycle test.

[en] The interfacial diffusion between the electrode and thermoelectric legs is a key factor to determine thermoelectric (TE) module performance. Metal elements M (Sn, Ni, Ag, Cu) are frequently used in lead-free solder SAC serials and barrier layers. M’s effects on thermoelectric properties are investigated in this work, through doping 0.1 wt% M in Bi_0.5Sb_1.5Te₃ (BST) prepared by melting and spark plasma sintering method. The results show that the additive of Ag and Cu could enhance the average ZT value 69.4% and 75.5% in the temperature from 323 K to 573 K, and transfer ZT summit to 423 K, while Sn and Ni doping have little influence on the BST’s thermoelectric performance. The ZT peak value of Cu doped BST reaches 1.01 at 423 K, and Ag 0.96 at 423 K. This work exhibits a prospect that the efficiency of thermoelectric device can be improved by finely choosing different solders at cold and hot side of leg-electrode joint at different applied temperature. (paper)

[en] Pressless heat treatment (PHT) at different temperatures was carried out on the samples cut off from the hot-deformed (HD) Nd-Fe-B magnets. It showed remarkably influence on both the remanence and coercivity of the HD samples. When the PHT temperature was higher than 850 °C, the HD samples exhibited a drastic remanence increase from 13.58 kGs to 14.45–14.69 kGs and dramatic coercivity decrease. This declined coercivity could be raised by 3 kOe when the HD sample experienced a second stage of PHT at 700 °C for 2 h. The texture of the HD samples with enhanced remanence was evaluated by X-ray diffractometry (XRD), suggesting the texture improvement after the PHT at 900 °C and 1080 °C. The microstructures of these HD samples were examined by scanning electron microscopy (SEM), which revealed the inherited crystalline orientation during the growth of nanosized Nd₂Fe_12.8Co_1.2B platelets into microsized grains after PHT. Backscatting electron (BSE) SEM and energy dispersive spectroscopy (EDS) revealed the aggregation of Nd-rich phase both at ribbon boundary and grain boundary at the PHT temperature of 900 °C, which plays an important role in the dramatic decrease of coercivity. The BSE-SEM further revealed that the increase of coercivity during the secondary stage of PHT at 700 °C was ascribed to the improved distribution of aggregated Nd-rich phase.

[en] We investigate the thermal conductivities of silicon nanowires (SiNWs) and their arrays based on molecular dynamics simulations. It is found that diminishing diameter, roughing surface and doping impurity of SiNWs can reduce their thermal conductivities by two or three orders of magnitude compared with that of bulk silicon crystals due to the strong phonon boundary and phonon impurity scattering. The simulated thermal conductivities of SiNW arrays demonstrate that arraying nanowires can further lower the thermal conductivity owing to the laterally-coupled effect, and the thermal conductivity of arrays decreases notably with the increased nanowire volume fraction, resulting in an ultralow thermal conductivity for the doped SiNW arrays with rough surfaces, which provides theoretical guidance of thermal management for semiconductor nanowire based microelectronic and thermoelectric devices. (paper)

[en] Bismuth nanowire arrays have been synthesized within the pores of ordered mesoporous silica templates using a supercritical fluid (SCF) inclusion technique. The formation of nanowires within the mesopores was confirmed by powder X-ray diffraction (PXRD), N₂ adsorption experiments and transmission electron microscopy (TEM). The formation of the bismuth nanowire arrays occurred through the initial binding of the bismuth precursor to the inner walls of the mesoporous channels, forming bismuth crystal seeds, which subsequently developed into wire-like structures. By varying the concentration of the bismuth precursor in the SCF phase, the loading of bismuth nanocrystals within the mesoporous channels can be controlled. The effect that temperature had on the formation of bismuth nanocrystals within the mesopores was also investigated. The highest loading of bismuth nanocrystals inside the mesopores was obtained at reaction temperatures near the critical point of toluene

[en] The thermoelectric (TE) performance of polycrystalline SnSe is limited by its low electrical properties, and normally doped with metals like Ag, Na etc by simply mixing with SnSe in melting. In this paper, a facile electroless plating is introduced to improve its TE performance for the first time. The results showed Ag was coated on the surface of SnSe powders homogeneously even after sintered with SPS. The influence of the Ag dopant on the electrical and thermal transport is analyzed. The silver coating with electroless plating could obviously improve average power factor (nearly fourfold) with minor effects (only 13% enhancement) on thermal conductivity. Therefore, a peak ZT value of 0.68 is achieved at 873 K in 1.0% Ag doped SnSe parallel to SPS pressing direction. In addition, the anisotropic properties are also found in this Ag doping SnSe by electroless plating. Compared with sample along parallel direction in 0.5% Ag coating, the power factor along the perpendicular direction improves over 50%, and thermal conductivity only increases 12.35%, resulting in a ZT peak value of 0.85 at 873 K. Compared with traditional mixing method, electroless plating is a facile and more homogenous doping method, and could be an improved way for doping to finer tune other thermoelectric materials performance. (paper)