微波消解-双道原子荧光光谱法同时测定大米中微量砷和汞

建立了微波消解—双道原子荧光光谱法同时测定大米中砷和汞含量的方法。通过试验优化了负高压、灯电流、载气流速、原子化器高度、载流酸度和硼氢化钾浓度等测试条件。砷、汞含量分别在1.0~100μg/L 和0.1~10μg/L范围内与荧光强度呈良好的线性关系,线性相关系数均大于0.9996,方法检出限分别为0.052,0.028μg/L,回收率分别为93.4~100.9,90.95~103.6,相对标准偏差均小于5%（n=6）。     

TiO2含量对镁铝尖晶石烧结性能的影响

以分析纯氧化镁、氧化铝、氧化钛为原料,按氧化镁与氧化铝质量比28.33∶71.67配料,在合成体系中分别引入质量分数为0、0.5％、1％、2％、4％、6％、8％和10％的氧化钛,在钼丝炉中1600℃保温2h,烧结法合成镁铝尖晶石.用X射线衍射、扫描电子显微镜和能谱对烧后试样进行分析.结果表明:引入适量的TiO2可显著提高镁铝尖晶石的烧结性能;当TiO2含量低于4％时,随着TiO2的含量增加,试样的线变化率减小,显气孔率下降,体积密度增大,常温耐压强度增大.TiO2的引入提高了镁铝尖晶石晶体空位浓度,活化了晶格,促进镁铝尖晶石的烧结.当TiO2含量高于4％时,试样的线变化率、显气孔率和体积密度没有显著变化,常温耐压强度有所下降;尖晶石晶粒尺寸没有明显变化,且生成了一定量的Mg0.6Al0.8Ti1.6O5,阻碍了镁铝尖晶石之间的接触,影响镁铝尖晶石烧结性能的提高.     

Enhanced photocatalytic activity of nonstoichiometric crystalline TaO2F and Ta2O5 with carbon coating

Pure and carbon-coated tantalum-based oxides photocatalysts were synthesized via the mesocrystalline precursor transformation method by annealing pure and polydopamine-coated (NH₄)₂Ta₂O₃F₆ mesocrystals in Ar. The oxygen-poor atmosphere thermal annealing process assisted the formation of nonstoichiometric TaO₂F mesocrystals with more F and Ta₂O₅ nanorods with oxygen vacancies and the associated lower valence state Ta ions (Ta⁴⁺). Furthermore, the carbon coating, decomposed from coated polydopamine, helped to control their particle size within 100 nm by isolating the connection of (NH₄)₂Ta₂O₃F₆ subunits. Hence, as-synthesized products, particularly carbon-coated Ta₂O₅ nanosheets, owning large surface area (67.6 m² g^?1), fine particle size (<100 nm), excellent electronic conductivity, decreased bandgap energy, enhanced and extended absorption in the visible range, exhibited preferable photocatalytic activity in the photodegradation of methylene blue, reaching a 76.54 % and 41.71 % removal under ultraviolet and visible light illumination, suggesting a promising candidate for wide-range responsive photocatalytic applications.     

Ultrafast high-temperature sintering of barium titanate ceramics with colossal dielectric constants

Application of Ultrafast High-temperature Sintering (UHS) technique to rapidly densify barium titanate ceramics has been explored for the first time. Bulk ceramic with ~94% density was obtained by UHS at ~1340 °C for 60 seconds. The densification process was accompanied with progressive sample discolouration from light to dark grey. Further analysis indicates that oxygen vacancy and its associated Ti-rich phase Ba₄Ti₁₂O₂₇ are present in the ceramics. Their roles in ultrafast densification and sample discoloration are discussed. Due to the presence of oxygen vacancies, the UHSed ceramics generally exhibit a colossal dielectric constant of ~ 15–30k at 1 kHz, with dielectric loss of ~0.07–0.10, while the ceramics without oxygen vacancy retain a dielectric constant of ~3000–6000 and dielectric loss of ~ 0.06 at 1 kHz which are comparable to that of the conventionally sintered ceramics. Furthermore, the challenges in applying UHS to sinter thick BT ceramics are discussed, aided by thermal simulations.     

Sintering of high-entropy nanoparticles obtained by polyol process: A case study of (La0.2Y0.2Nd0.2Sm0.2Gd0.2)2Ce2O7-δ

High-entropy (HE) ceramics nanoparticles have received much attention due to their interesting properties. However, very limited studies have been conducted on their sintering. Here, we report the sintering behavior of HE A₂B₂O₇ type rare earth oxide nanoparticles obtained by polyol process. HE cerate (HECe) (La_0.2Y_0.2Nd_0.2Sm_0.2Gd_0.2)₂Ce₂O_7-δ is chosen as an exemplary case, which is considered as a good candidate for thermal insulation. HECe nanoparticles with size of 2.6–7.1 nm can be synthesized through polyol process followed by annealing in air at 300–700 °C. HECe nanoparticle compact can be densified by directly sintering at 1500 °C. The sintering temperature could be further decreased using a two-step sintering process, i.e., 1500 °C 5 min-1300 °C 5 h. Our results show that fine particle and abundant oxygen vacancies probably dominate the densification process. By controlling the sintering regime, we can tune the microstructure of HECe ceramics and thermal conductive properties accordingly.     

Magnetic field-enhanced photocatalytic nitrogen fixation over defect-rich ferroelectric Bi2WO6

Photocatalytic N₂ fixation is a promising and sustainable manufacturing process of ammonia (NH₃); however, the NH₃ production rate by this method is very low, thus severely restricting further application of this sustainable technology. Therefore, developing an efficient photocatalyst for N₂ fixation under mild conditions is urgently required. Herein, ferroelectric Bi₂WO₆ materials with different surface oxygen defects were prepared, and the concentration of corresponding defects was controlled by adjusting the thermal reduction time. The abundant oxygen defects in Bi₂WO₆ can provide more reactive sites to promote the effective adsorption of N₂, and the photogenerated charge carrier can be efficiently separated benefiting from the internal electric field. These would weaken the N₂ triple bond and reduce the activation energy barrier for the conversion of N₂ to NH₃ under mild conditions. In the absence of sacrificial agents and cocatalysts, the optimized Bi₂WO₆ with oxygen defects shows an indigenous NH₃ yield of 132.175 μmol·g^-1·L^-1·h^-1, which is more than two times higher than that of the original Bi₂WO₆. Surprisingly, the Bi₂WO₆ with oxygen defects produced more than eight times NH₃ (471.13 μmol·g^-1·L^-1·h^-1) than that of the original Bi₂WO₆ when assisted by an external magnetic field, thus providing a new perspective for further enhancing the N₂ fixation performance.     

Simultaneously improving the electrical properties and long-term stability of ZnO varistor ceramics by reversely manipulating intrinsic point defects

Excellent electrical properties and the improved long-term stability of ZnO varistor ceramics were simultaneously achieved by doping NiO. The microstructural features were investigated using X-ray diffractometer, scanning electron microscopy, and energy dispersive spectroscopy, while the intrinsic point defects were characterized using frequency domain dielectric spectroscopy and verified by photoluminescence and Raman spectra. The results indicated that in the ZnO varistor ceramics, a reverse manipulation of donor point defects, i.e., suppressing mobile zinc interstitial but increasing stable oxygen vacancy, was achieved. The long-term stability of NiO-doped ZnO ceramics was improved via a decrease in zinc interstitial density, with a degradation rate of 0.064 μA cm^?2 h^?0.5. Meanwhile, due to an increase in oxygen vacancy density, the excellent nonlinear current–voltage performance, i.e., a high nonlinear coefficient (72.9), low leakage current density (0.08 μA cm^?2), and low grain resistivity (13.43 × 10^?3 Ω m), was maintained. The findings of this study provide a possible method for developing high-performance ZnO varistor ceramics by manipulating point defects.     

Porous TiO2/rGO nanocomposites prepared by cold sintering as efficient electrocatalyst for nitrogen reduction reaction under ambient conditions

Haber–Bosch process as the current dominant artificial NH₃ production process in industry, requires relatively high temperature (350–550 °C) and pressure (150–350 atm). Electrocatalytic nitrogen reduction reaction (NRR) as a green and sustainable strategy for ammonia production has raised intensive research interest in recent years but still remains a significant challenge because of the lack of high performance electrocatalysts. In this work, porous TiO₂-reduced graphene oxide (TiO₂/rGO) nanocomposite as self-supporting efficient electrocatalyst for NRR under ambient conditions were prepared by cold sintering associated with sacrificial template method. The porous TiO₂/rGO nanocomposite with grain size of ～40 nm were prepared by cold sintering process at 220 °C and 147 MPa. Given the 220 °C as cold sintering temperature, anatase TiO₂ were preserved as the final phase which exhibit much better NRR electrocatalytic performance than the rutile phase. The oxygen vacancy densities in the nanocomposites were also tuned by heat treatment at 450 °C under different atmosphere, while samples heat treated under H₂/Ar atmosphere gave the best electrocatalytic NRR performance with a FE of 8.88 % and an NH₃ yield of 7.75 μg h^?1 cm^?2 at ambient conditions. Experiments also shows that the addition of rGO significantly improved the electrocatalytic NRR performance especially the conductivity. This work not only designed a framework of ceramic nanocomposites based self-supporting and durable electrocatalysts system but also paves a feasible way towards preparing electrocatalysts that are sensitive to high temperature fabrication process.     

Surface energies in high-entropy carbides with variable carbon stoichiometry

The carbon vacancy in high-entropy carbides (HECs) has a significant impact on their physical and chemical properties, yet relevant studies have still been relatively few. In this study, we investigate the surface energies of HECs with variable carbon vacancies through first-principles calculations. The results show that the surface energy of the (1 0 0) surface of the stoichiometric HECs is significantly lower than that of (1 1 1) surface. With the decrease in carbon stoichiometry, the surface energies of both (1 0 0) and (1 1 1) surfaces increase gradually, which is mainly due to the weakening of covalent bonding and the decrease of metal Hirshfeld-I (HI) charges. However, the surface energy of (1 0 0) surface increases more quickly than that of (1 1 1) surface and will exceed that of (1 1 1) surface when the carbon stoichiometry decreases to a certain extent, which is primarily attributed to the greater decrease rate of metal HI charges of (1 0 0) surface.     

Insight into the selective oxidation mechanism of glycerol to 1,3-dihydroxyacetone over AuCu–ZnO interface

Directional regulation of polyol oxidation selectivity by constructing active sites with specific structure is a critical yet challenging problem. Herein, the specific Au-based catalyst with efficient Au–Cu–ZnO interfacial active sites was successfully designed to promote selective oxidation of glycerol to 1,3-dihydroxyacetone under mild conditions. X-ray absorption spectroscopy revealed that the increased electron transfer between Au and Cu increases the content of Au⁺, resulting in the higher catalytic activity (turnover frequency: 402.5 h⁻¹). Meanwhile, small AuCu alloy nanoparticles (ca., 2.7 nm) could be inserted into the ZnO lattice with the formation of Au(Cu)–O–Zn linkages, resulting in the enrichment of interfacial oxygen vacancies. These interfacial oxygen vacancies induce the activation and adsorption of the secondary hydroxyl group of glycerol on the interfacial active sites, improving the selectivity of 1,3-dihydroxyacetone (83.4%). Furthermore, in situ Fourier transform infrared, structure-dependent kinetics and density functional theory calculation demonstrated that Au–Cu–ZnO interfacial active sites could enhance the participation of OH* and oxygen vacancies in activating the O H and C H bonds, respectively, promoting the improvement of the catalytic performance. The outcome of this work offers new insights for the rational design of high effective catalyst for the selective oxidation of polyol.