Type-II AsP/Sc2CO2 van der Waals heterostructure: an excellent photocatalyst for overall water splitting

In this work, we explore the application potential of AsP/M₂CO₂ (M = Sc, Zr) van der Waals heterostructures in photocatalytic water splitting through the first-principles calculations. The calculated results show that AsP/Zr₂CO₂ heterostructure possesses an unfavorable type-Ⅰ band alignment, whereas AsP/Sc₂CO₂ exhibits a desirable type-Ⅱ band alignment, which is beneficial for separating the photogenerated electron-hole pairs. Also, the band edge positions of AsP/Sc₂CO₂ heterostructure stride the redox potential of water, ensuring favorable reaction kinetics. Besides, the strong optical absorption of AsP/Sc₂CO₂ heterostructure in both visible and ultraviolet regions (especially up to 10⁻⁶ cm⁻¹ at about 250 nm) makes it possible to utilize solar energy effectively. Meanwhile, AsP/Sc₂CO₂ heterostructure has an exciton binding energy as low as 0.09 eV, which quantitatively illustrates the high separation efficiency of photogenerated charge carrier. Thus, the type-Ⅱ band alignment, suitable band edge position, strong light absorption, and low exciton binding energy together indicate that AsP/Sc₂CO₂ heterostructure is a potential photocatalytic material. In addition, the obvious redshift phenomenon in the optical spectrum of AsP/Sc₂CO₂ heterostructure shows that biaxial strain can improve its light capture capability. Also, the interconversion between type-Ⅱ and type-Ⅰ can be achieved by applying different strains. All these findings suggest that the novel AsP/Sc₂CO₂ heterostructure has significant application prospects in next-generation photovoltaic and photocatalytic devices.     

钨中空位及其团簇的能量学和动力学性质参数

在总结前人钨中空位及其团簇的能量学和动力学行为的研究成果基础上，采用第一性原理方法系统计算了钨中空位及其团簇的结合能和扩散能垒。研究发现，交换关联泛函PW91和PBE较PBEsol、AM05和LDA更适合用于计算钨空位的能量学性质。基于第一性原理计算结果对文献中单空位形成能、双空位作用性质等争议性问题进行了讨论，并对钨经验势进行了评估。研究结果表明，钨中孤立单空位间总是相互排斥，而空位团簇（V_n>3）对单空位具有很强的吸引作用，其结合能随着所含空位个数增多呈现波动性增大的趋势。空位团簇稳定结构可通过最小化Wigner-Seitz表面积来确定，其结合能与V_n与V_n-1之间的Wigner-Seitz面积之差呈正比。     

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.     

Strategy to design high performance TiB2-based materials: Strengthen grain boundaries by solid solute segregation

TiB₂ exhibits a unique combination of excellent properties that make it promising candidate for applications in extreme environments, where retention of strength at high temperatures is essential. Tailoring grain boundary properties by segregation is believed a prominent way to design high-temperature performance of ceramics. In this work, segregation tendencies of solute elements, including Sc, Y, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, in TiB₂ grain boundaries and the strengthening/weakening effects induced by segregations are investigated by first-principles calculations. The results reveal that small atoms tend to segregate to grain boundary sites with local compression strains, while large atoms prefer grain boundary sites with local expansion strains. Deteriorated grain boundary strength is usually caused by additional expansion strain induced by segregation, while improved grain boundary strength results from either enhanced local bonding induced by segregation of small atoms or increased fracture strain due to segregation of large atoms. Cr and V, especially Cr, exhibit strong segregation tendency and improvement on grain boundary strength, which provides useful guidelines for the design of high performance TiB₂-based materials.     

B掺杂C3N对SO2催化氧化的第一性原理研究

通过第一性原理计算，采用Dmol3模块考察了B原子分别掺杂C3N二维材料的C位(B/C-C3N)和N位(B/N-C3N)所形成的两种单原子催化剂的结构、电子性质以及催化性能。首先，分析了B掺杂C3N的两种结构的稳定性与电子性质，发现B的掺杂使两种结构的稳定性均显著增强，B/C-C3N的电子性质变化更大，由半导体变成了导体。然后，分析了C3N、B/C-C3N和B/N-C3N这3种催化剂对SO2氧化反应的催化性能，计算比较了对O2与SO2的吸附能、电荷转移量、稳定构型、态密度以及O2活化后的键长。结果显示3种催化剂对O2的吸附能均大于SO2，均服从E-R机理，其中B/C-C3N对O2的吸附能最大、O2与SO2之间吸附能之差最大，对O2的活化程度最高，表现出最利于SO2催化氧化反应的性能。态密度分析也表明，O2与B/C-C3N在费米能级附近的杂化作用最强，这更利于反应的发生。因此，B/C-C3N具有成为SO2氧化反应优良催化剂的潜力。     

NiCrAlFe精密电阻合金中的L12有序结构研究

有序结构的形成是决定NiCrAlFe精密电阻合金电学性能的关键因素。本文利用第一性原理赝势平面波方法，计算了合金处于无序固溶体结构和形成L12型有序结构时的结合能、态密度、晶格常数等参数，并利用高分辨透射电子显微镜（HRTEM）对合金进行了结构表征，还测试了固溶态和时效态的电阻率。从结合能来看，该合金形成L12有序结构时比无序固溶体更加稳定；态密度以及部分态密度计算结果则表明，在L12有序结构中，Ni、Cr、Al、Fe会强烈成键而使得整个合金体系变得稳定；HRTEM分析结果证明固溶态合金经过和时效处理后出现了L12有序结构，而且该有序结构的晶格常数与计算值基本一致。对比无序固溶体与L12型有序结构费米能级处的态密度值发现，当形成L12有序结构时合金的导电能力较无序固溶体下降，电阻率升高，与实际测试结果吻合。     

Thermophysical and mechanical properties of novel high-entropy metal nitride-carbides

In this work, a novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)(N_0.5C_0.5) high-entropy nitride-carbide (HENC-1) with multi-cationic and -anionic sublattice structure was reported and their thermophysical and mechanical properties were studied for the first time. The results of the first-principles calculations showed that HENC-1 had the highest mixing entropy of 1.151R, which resulted in the lowest Gibbs free energy above 600 K among HENC-1, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N high-entropy nitrides (HEN-1), and (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy carbides (HEC-1). In this case, HENC-1 samples were successfully fabricated by hot-pressing sintering technique at the lowest temperature (1773 K) among HENC-1, HEN-1 and HEC-1 samples. The as-fabricated HENC-1 samples showed a single rock-salt structure of metal nitride-carbides and high compositional uniformity. Meanwhile, they exhibited high microhardness of 19.5 ± 0.3 GPa at an applied load of 9.8 N and nanohardness of 33.4 ± 0.5 GPa and simultaneously possessed a high bulk modulus of 258 GPa, Young's modulus of 429 GPa, shear modulus of 176 GPa, and elastic modulus of 572 ± 7 GPa. Their hardness and modulus are the highest among HENC-1, HEN-1 and HEC-1 samples, which could be attributed to the presence of mass disorder and lattice distortion from the multi-anionic sublattice structure and small grain in HENC-1 samples. In addition, the thermal conductivity of HENC-1 samples was significantly lower than the average value from the “rule of mixture” between HEC-1 and HEN-1 samples in the range of 300-800 K, which was due to the presence of lattice distortion from the multi-anionic sublattice structure in HENC-1 samples.     

Predictions of structural,electronic, mechanical,and thermodynamic properties of TMBCs (TM = Ti,Zr, and Hf) ceramics

In the present work, we have investigated the structural, electronic, elastic, and thermodynamic properties of transition-metal boron-carbon compounds (TMBCs) (TM = Ti, Zr, Hf) using the first-principles calculations. The results showed that TMBCs are energetically and thermodynamically stable, and the sequence of phase stability is HfBC > ZrBC > TiBC. B-C bonds can be formed in TMBCs ceramics due to the strong hybridization between B-2p and C-2p states. The elastic anisotropies of TMBCs were illustrated by elastic anisotropy indexes, 3D surface constructions, and 2D projections, and the results indicated that the sequence of elastic anisotropy is ZrBC > TiBC > HfBC. Finally, the calculated minimum thermal conductivities, based on the Clarke's and Cahill's models, of all TMBCs are anisotropic with the sequence of ZrBC > TiBC > HfBC.     

第一性原理计算Mo含量及压力对CrZrNbTiMox难熔高熵合金性能的影响

利用基于密度泛函理论的第一性原理方法研究Mo含量和压力对CrZrNbTiMo_x难熔高熵合金相结构、弹性常数和弹性模量的影响。计算了CrZrNbTiMo_x难熔高熵合金的混合焓(ΔH_mix)、混合熵(ΔS_mix)、原子半径差(δ)、价电子浓度(VEC)以及参数Ω。结果表明,CrZrNbTiMo_x难熔高熵合金主要以单一的BCC无序固溶体结构为主。随着Mo含量的增加,合金的晶格畸变不断减小,但力学性能稳定且始终以金属键结合。Mo的添加对合金的抗体积变形能力提升较大、对抗切应变能力及刚度影响较小,合金表现为韧性材料。在0～25 GPa压力作用下,CrZrNbTiMo_0.6合金表现出优异的相稳定性和力学稳定性,体弹性模量(B)增幅较小,剪切弹性模量(G)、杨氏模量(E)变化较小;泊松比μ及G/B计算结果表明合金为韧性材料。