Daojiang Gao

Microstructure and electrical properties of La-modified K0.5Na0.5NbO3 lead-free piezoelectric ceramics

Lead-free ceramics (K0.5Na0.5)1−3xLaxNbO3 (0 ≤ x ≤ 0.0175) have been fabricated by a conventional sintering technique. The results of XRD show that the ceramics possess a perovskite structure with orthorhombic symmetry. Moreover, doping inhibits the grain growth, decreases the ferroelectric–paraelectric phase transition temperature and induces a diffuse phase transition. At low doping levels (x ≤ 0.0075), the observed remanent polarization (Pr) and coercive field (Ec) remain almost unchanged. As x increases from 0.0075 to 0.0175, Pr starts to decrease while Ec increases. Nevertheless, due to the increase in relative permittivity, the ceramic with x = 0.0125 exhibits the optimum piezoelectric properties, giving a large piezoelectric coefficient (d33 = 135 pC N−1) and a high planar electromechanical coupling coefficient (kP = 0.40).

Piezoelectric and ferroelectric properties of (Bi0.94−xLaxNa0.94)0.5Ba0.06TiO3 lead-free ceramics

Lead-free piezoelectric ceramics (Bi0.94−x LaxNa0.94)0.5Ba0.06TiO3 have been fabricated by an ordinary sintering technique, and their piezoelectric and ferroelectric properties have been studied. The results of x-ray diffraction reveal that La3+ and Ba2+ diffuse into the Bi0.5Na0.5TiO3 lattices to form a new solid solution with a pure perovskite structure. After the partial substitution of La3+ for Bi3+ in the (Bi0.94−x Lax Na0.94)0.5Ba0.06TiO3 ceramics (x = 0–0.04), the ceramics exhibit a lower coercive field Ec and a larger remanent polarization Pr. Because of the large Pr and low Ec, the ceramics with x = 0.02–0.04 exhibit optimum piezoelectric properties: d33 = 181–196 pC N−1 and kP = 33.2–36.3%. The depolarization temperature Td decreases with increasing x (x = 0 − 0.04). At the high La3+ level (x = 0.06–0.12), the ceramics exhibit weak ferroelectricity and thus possess very poor piezoelectricity, and the low dielectric anomaly at Td disappears. In addition, the ceramics exhibit a relaxor characteristic, which probably results from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics with x = 0–0.04 may contain both polar and non-polar regions at temperatures above Td, while for the ceramics with x = 0.06–12, the polar and non-polar regions coexist at room temperature.

Encased Copper Boosts the Electrocatalytic Activity of N-Doped Carbon Nanotubes for Hydrogen Evolution

Nitrogen (N)-doped carbons combined with transition-metal nanoparticles are attractive as alternatives to the state-of-the-art precious metal catalysts for hydrogen evolution reaction (HER). Herein, we demonstrate a strategy for fabricating three-dimensional (3D) Cu-encased N-doped carbon nanotube arrays which are directly grown on Cu foam (Cu@NC NT/CF) as a new efficient HER electrocatalyst. Cu nanoparticles are encased here instead of common transition metals (Fe, Co, or Ni) for pursuing a well-controllable morphology and an excellent activity by taking advantage of its more stable nature at high temperature and in acidic or alkaline electrolyte. It is discovered that metallic Cu exhibits strong electronic modulation on N-doped carbon to boost its electrocatalytic activity for HER. Such a nanostructure not only offers plenty of accessible highly active sites but also provides a 3D conductive open network for fast electron/mass transfer and facilitates gas escape for prompt mass exchange. As a result, the Cu@NC NT/CF electrode exhibits superior HER performance and durability, outperforming most of the reported M@NC materials. Furthermore, the etching experiments together with X-ray photoelectron spectroscopy (XPS) analysis reveal that the electronic modulation from encased Cu significantly enhances the HER activity of N-doped carbon. These findings open up opportunities for exploring other Cu-based nanomaterials as efficient electrocatalysts and understanding their catalytic processes.

Magnetic, recyclable PtyCo1−y/Ti3C2X2 (X = O, F) catalyst: a facile synthesis and enhanced catalytic activity for hydrogen generation from the hydrolysis of ammonia borane

Exploring the applications of two dimensional layered Ti3C2X2 (X = OH, F) is of great importance because of its excellent physical and chemical properties. Herein, we report the synthesis of the bimetallic catalyst PtyCo1−y/Ti3C2X2via a facile one-pot approach using Ti3C2X2 as the support. The in situ synthesized PtyCo1−y/Ti3C2X2 is subsequently applied as a catalyst for hydrogen evolution from the hydrolysis of ammonia borane at 25 °C. Experimental results show that Pt0.08Co0.92/Ti3C2X2 exhibits excellent catalytic activity for the hydrolysis of ammonia borane with a high hydrogen generation rate of 100.7 L H2 (min gPt)−1 and turnover frequency of 727 mol H2 (min molPt)−1 because of the synergistic effect between Pt and Co. Moreover, Pt0.08Co0.92/Ti3C2X2 could be recovered from the reaction mixture by a magnet and recycled at least seven times, thus showing its high recycling efficiency.

Controlled Synthesis of CaWO4 Microcrystalline via Surfactant-Assisted Precipitation Method

CaWO4 microcrystallines have been synthesized via a surfactant-assisted precipitation method using cetyltrimethylammonium bromide (CTAB) and /or Polyvinylalcohol (PVA) as surfactants, respectively. The as-prepared CaWO4 microcrystallines were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectra and photoluminescence spectra (FL). The growth process of these CaWO4 microcrystallines obtained in the presence of CTAB and PVA has been discussed. The structural characterization results indicate that the obtained CaWO4 are ellipsolid-like microcrystallines with the average size of ∼350 nm in short axis and ∼800 nm in long axis, and the CaWO4 microcrystallines possess a pure scheelite structure with a tetragonal symmetry. The FTIR spectra provide the evidence of W–O stretching vibration in [WO4]2− tetrahedrons at 796 cm−1 and W-O bending bands at 438 cm−1. FL spectra reveal that the CaWO4 microcrystallines exhibit the only green peak at 430 nm excited by ultraviolet ray (UV) with the wavelength at 220 nm, 240 nm and 260 nm, respectively. The SEM results indicate that the morphology of the CaWO4 microcrystallines can be affected by the concentration of surfactants.

Hydrothermal Synthesis of Ca(1-3x/2)Tb x WO4 Microcrystallines and Their Luminescent Properties

Ca(1–3x/2)Tb x WO4 solid solution microcrystallines have been synthesized by hydrothermal method; the crystal structure, surface morphology and room temperature photoluminescence properties of the as-synthesized microcrystallines were investigated by through X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and fluorescence analysis (FA). Our results reveal that the obtained Ca(1–3x/2)Tb x WO4 microcrystallines are single-phase scheelite structure with tetragonal symmetry. The doping of Tb3+ inhibits the grain growth, decreases the intrinsic emission of WO4 2− complex ions and enhances the green emission of the microcrystallines. Moreover, the green emission intensity (λ = 545 nm) of the as-synthesized Ca(1–3x/2)Tb x WO4 microcrystallines phosphors increases with the increasing doping level of Tb3+ ions, giving the maximum value at x = 0.07.

Tunable magnetic pole inversion in multiferroic BiFeO3–DyFeO3 solid solution

In ferromagnets, the magnetic moment can generally be reversed by applying a sufficiently high external magnetic field of opposite polarity. Temperature, on the other hand, is usually known to affect only the magnitude of a magnetic moment, rather than its sign or polarity (most magnets exhibit a monotonic increase in magnetization upon cooling below their magnetic phase transition temperature). As a result, temperature-induced magnetization reversal (i.e. magnetic pole inversion) remains a very rare phenomenon which lacks proper understanding and explanation because of the extreme difficulties encountered in controlling the thermodynamics of magnetization of classical metal or metal oxide magnets. Herein, we report an unusual magnetic pole inversion behaviour in multiferroic (1 − x)BiFeO3–xDyFeO3 solid solution (alloy), which can be tuned by varying the concentration of the magnetic ion Dy3+ in the solid solution. It is found that the temperature-induced magnetic pole inversion occurs in a wide composition range (x = 0.14–0.90). Moreover, for the first time in any ferrites, multiple magnetic pole inversions are observed in the solid solution compounds of high Dy3+-concentrations. Our results may provide a better understanding of the temperature- and composition-induced magnetic pole inversion and related phenomena and point to new potential applications for magnetic and multiferroic materials.

Facile synthesis of effective Ru nanoparticles on carbon by adsorption-low temperature pyrolysis strategy for hydrogen evolution

The facile synthesis of efficient catalysts for hydrogen evolution from electrochemical water splitting and ammonia borane (AB) hydrolysis is highly important. Here, we develop an adsorption-low temperature pyrolysis method for facilely preparing uniformly dispersed Ru nanoparticles on carbon (Ru/C) with an outstanding catalytic property toward hydrogen generation from both electrochemical water splitting and AB hydrolysis. The experimental results indicate that the Ru/C synthesized by calcination at 300 °C (Ru/C-300) exhibits the highest catalytic activity for the hydrogen evolution reaction (HER) in basic solution, which requires an overpotential of only 14 mV at 10 mA cm−2. Additionally, this catalyst also displays high activity and reusability toward hydrogen evolution through AB hydrolysis, leading to a high turnover frequency of 643 mol H2 (min molRu)−1. Moreover, the Ru/C-300 shows excellent stability and reusability for both reactions. The adsorption-low temperature calcination strategy ensures that the small Ru nanoparticles are confined onto the carbon matrix, which can provide abundant highly reactive surface sites. It is discovered that the excellent catalytic activity of Ru/C depends largely on the size and dispersion of Ru nanoparticles as well as on their chemical states. This work may provide a facile and environmentally friendly strategy for preparing uniformly distributed metal nanocatalysts with high catalytic efficiencies for HER and AB hydrolysis.

Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen Functionalized Carbon toward Advanced Electrocatalysts for Alkaline Water Splitting

Developing efficient catalytic materials for electrochemical water splitting is important. Herein, uniformly dispersed and size-controllable iridium (Ir) nanoparticles (NPs) were prepared using a nitrogen-functionalized carbon as the support (Ir/CN). We found that nitrogen functionalization can simultaneously modulate the size of Ir NPs to substantially enhance the catalytically active sites and adjust the electronic structure of Ir, thereby promoting electrocatalytic activity for water splitting. Consequently, the as-synthesized Ir/CN shows excellent electrocatalytic performance with overpotentials of 12 and 265 mV for hydrogen and oxygen evolution reactions in basic medium, respectively. These findings may pave the way for designing and synthesizing other similar materials as efficient catalysts for electrochemical water splitting.

Palladium Supported on Titanium Carbide: A Highly Efficient, Durable, and Recyclable Bifunctional Catalyst for the Transformation of 4-Chlorophenol and 4-Nitrophenol

Developing highly efficient and recyclable catalysts for the transformation of toxic organic contaminates still remains a challenge. Herein, Titanium Carbide (Ti3C2) MXene modified by alkali treatment process was selected as a support (designated as alk-Ti3C2X2, where X represents the surface terminations) for the synthesis of Pd/alk-Ti3C2X2. Results show that the alkali treatment leads to the increase of surface area and surface oxygen-containing groups of Ti3C2X2, thereby facilitating the dispersion and stabilization of Pd species on the surface of alk-Ti3C2X2. The Pd/alk-Ti3C2X2 catalyst shows excellent catalytic activity for the hydrodechlorination of 4-chlorophenol and the hydrogenation of 4-nitrophenol in aqueous solution at 25 °C and hydrogen balloon pressure. High initial reaction rates of 216.6 and 126.3 min−1·gpd−1 are observed for the hydrodechlorination of 4-chlorophenol and hydrogenation of 4-nitrophenol, respectively. Most importantly, Pd/alk-Ti3C2X2 exhibits excellent stability and recyclability in both reactions without any promoters. The superior property of Pd/alk-Ti3C2X2 makes it as a potential material for practical applications.