Jiehe Sui

A high thermoelectric figure of merit ZT > 1 in Ba heavily doped BiCuSeO oxyselenides

A high ZT value of ∼1.1 at 923 K in the BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200–400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power generation applications.

Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides

We present a high ZT ∼ 1.4 in textured Bi0.875Ba0.125CuSeO obtained by a hot-forging process. The carrier mobility along the direction perpendicular to the pressing direction was significantly increased, resulting in increase in the electrical conductivity and maximization of the power factor at 923 K from 6.3 μW cm−1 K−2 for the sample before hot-forging to 8.1 μW cm−1 K−2 after the hot-forging process. Therefore, the maximum ZT was significantly increased from ∼1.1 to 1.4 through texturing for Bi0.875Ba0.125CuSeO, which is the highest ZT ever reported among oxygen containing materials.

Facile synthesis of MWCNT–ZnFe2O4 nanocomposites as anode materials for lithium ion batteries

Monodisperse ZnFe2O4 nanoparticles with sizes less than 10 nm have been successfully assembled on multi-walled carbon nanotubes (MWCNTs) by in situ high-temperature decomposition of the precursor iron(III) acetylacetonate, zinc acetate and MWCNTs in polyol solution. A possible formation mechanism was proposed, which suggests that the ZnFe2O4 nanoparticles are formed on the surface of MWCNTs through an aggregation thermochemical reaction process between ZnO and γ-Fe2O3 subparticles. It was found that the coverage density on the MWCNTs could be easily controlled by changing the concentration of the precursor. As anode materials for Li-ion batteries, the MWCNT–ZnFe2O4 nanocomposites showed high rate capability and superior cycling stability with a specific capacity of 1152 mA h g−1, which was much higher than that of ZnFe2O4 nanoparticles. The MWCNTs served as good electron conductors and volume buffers in improving the lithium performance of MWCNT–ZnFe2O4 nanocomposites during the discharge–charge process. Magnetic measurements showed that the MWCNT–ZnFe2O4 nanocomposites are superparamagnetic at room temperature and the magnetization of the samples can be controlled by the reaction conditions. The as-synthesized MWCNT–ZnFe2O4 nanocomposites are water dispersible and can be manipulated by an external magnetic field. Therefore, the nanocomposites have significant potential for application in the fields of energy storage, composites, wastewater treatment and biomaterials.

The roles of Na doping in BiCuSeO oxyselenides as a thermoelectric material

The thermoelectric properties of the Bi1−xNaxCuSeO (0.0 ≤ x ≤ 0.02) system have been investigated in the temperature range 300–923 K. Na doping significantly increased the carrier concentration to ∼0.92 × 1020 cm−3 at the doping amount of x = 0.02. Furthermore, a relatively high carrier mobility and a slight Seebeck coefficient enhancement was seen, thus resulting in a high power factor of 8.0 μW cm−1 K−2 at room temperature. Coupled with a low thermal conductivity reduced by point defects scattering, this leads to a ZT of 0.91 at 923 K for Bi0.985Na0.015CuSeO which is nearly twice the value observed in pristine BiCuSeO.

Lamellar MoSe2 nanosheets embedded with MoO2 nanoparticles: novel hybrid nanostructures promoted excellent performances for lithium ion batteries

A carbon-free nanocomposite consisting of MoO2 nanoparticles embedded between MoSe2 nanosheets, named MoO2@MoSe2, has been synthesized and demonstrated excellent electrochemical properties for lithium ion batteries. In such a composite, MoSe2 nanosheets provide a flexible substrate for MoO2 nanoparticles; while MoO2 nanoparticles act as spacers to retain the desired active surface to electrolyte and also introduce metallic conduction. In addition, the heterojunctions at the interface between MoSe2 and MoO2 introduce a self-built electric field to promote the lithiation/delithiation process. As a result, such lamellar composite has a long cycling stability with a reversible capacity of 520.4 mA h g-1 at a current density of 2000 mA g-1 after 400 cycles and excellent rate performance, which are attributed to the synergistic combination of the two components in nanoscale.

Design of coherent anode materials with 0D Ni3S2 nanoparticles self-assembled on 3D interconnected carbon networks for fast and reversible sodium storage

There has been tremendous progress in development of nanomaterials for energy conversion and storage, with sodium-ion batteries (SIBs) attracting attention because of the high abundance of raw materials and low cost. However, inferior cycling stability, sluggish reaction kinetics, and poor reversibility hinder their practical applications. In the present study, Ni3S2/carbon nanocomposites with coherent nanostructures were successfully used as anodes in half- and full-cells. Outstanding cycling and rate performances are attributed to a synergistic effect between the Ni3S2 nanoparticles and interconnected carbon networks. The coherent porous framework effectively alleviated volume changes of Ni3S2, shortened the Na+ diffusion path, and accelerated electron transport and ionic diffusion during the electrochemical reaction. More importantly, conversion reaction products can be confined by the entangled carbon networks, leading to reversible redox reactions as demonstrated in ex situ XRD studies. The coherent Ni3S2/C nanocomposites demonstrated a highly reversible charge capacity of 453 and 430 mA h g−1 at a current density of 0.1 and 0.4 A g−1 over 100 cycles, respectively. At a current density of 2.0 A g−1, high rate capacities of 408 mA h g−1 can be attained over 200 cycles. The high performance of Na3V2(PO4)3/Ni3S2 full-cells enrich prospects for future practical applications.

Oriented attachment growth of quantum-sized CdS nanorods by direct thermolysis of single-source precursor.

Quantum-sized CdS nanorods were synthesized by direct thermal decomposition of a single-source precursor in a monosurfactant system. The CdS nanorods were uniform, had high crystallinity, and exhibited strong quantum confinement effect. The nanorod growth was controlled by an oriented attachment mechanism, and the morphology was determined by the competition between dipole attraction and steric repulsion of nanodots. Increasing precursor concentration and prolonging reaction time were favorable for the formation of CdS nanorods.

Thermoelectric properties of Bi-based Zintl compounds Ca1−xYbxMg2Bi2

Bi-based Zintl compounds, Ca1−xYbxMg2Bi2 with the structure of CaAl2Si2, have been successfully prepared by mechanical alloying followed by hot pressing. We found that the electrical conductivity, Seebeck coefficient, carrier concentration, and thermal conductivity can be adjusted by changing the Yb concentration. All Ca1−xYbxMg2Bi2 samples have low carrier concentrations (∼2.4 to 7.2 × 1018 cm−3) and high Hall mobility (∼119 to 153 cm2 V−1 s−1) near room temperature. The partial substitution of Ca with Yb causes structural disorders, which lowers the thermal conductivity. The highest figure of merit of ∼1.0 is observed in Ca0.5Yb0.5Mg2Bi2, and ∼0.8 in the unsubstituted CaMg2Bi2 and YbMg2Bi2. A small amount of free Bi was found in all the samples except YbMg2Bi2. By reducing the initial Bi concentration, we succeeded in obtaining phase pure samples in all compositions, which resulted in a much better thermoelectric performance, especially much higher (ZT)eng and a conversion efficiency near 11%. Such a high efficiency makes this material competitive with half-Heuslers and skutterudites.

Large-scale preparation of CdS quantum dots by direct thermolysis of a single-source precursor

CdS quantum dots (QDs) have been synthesized on a large scale, based on the direct thermolysis of one single-source precursor, (Me(4)N)(4)[S(4)Cd(10)(SPh)(16)], in hexadecylamine (HDA). Transmission electron microscopy (TEM) observations show that the CdS QDs are well-defined, nearly spherical particles. The clear lattice fringes in high-resolution TEM (HRTEM) images confirm the crystalline nature of the QDs. The broad diffraction in the x-ray diffraction (XRD) pattern and diffuse diffraction rings of the selected-area electron diffraction (SAED) pattern are typical of nanomeric-size particles and indicative of the hexagonal phase of CdS QDs. The absorption spectra confirm quantum confinement of CdS QDs. The synthesis process for CdS QDs was investigated by ultraviolet-visible (UV-vis) absorption spectroscopy. The results demonstrate that the nucleation and growth stages were separated automatically in a homogeneous system.

Understanding and manipulating the intrinsic point defect in α-MgAgSb for higher thermoelectric performance

Nanostructured α-MgAgSb has been demonstrated as a good p-type thermoelectric material candidate for low temperature power generation. Nevertheless, the intrinsic defect physics that impedes further enhancement of its thermoelectric performance is still unknown. Here we first unveil that an Ag vacancy is a dominant intrinsic point defect in α-MgAgSb and has a low defect formation energy, shown by first-principles calculations. In addition, the formation of an Ag vacancy could increase the crystal stability. More importantly, intrinsic point defects, namely an Ag vacancy, can be rationally engineered via simply controlling the hot press temperature, due to the recovery effect. Collectively, a high peak ZT of ∼1.3 and average ZT of ∼1.1 are achieved when hot pressed at 533 K. These results elucidate the pivotal role of intrinsic point defects in α-MgAgSb and further highlight that point defect engineering is an effective approach to optimize the thermoelectric properties.