RESUMEN
Searching for two-dimensional (2D) materials with a high phase-transition temperature and magnetic anisotropy is critical to the development of spintronics. Herein, we investigate the electronic and magnetic properties of 2D TiX3 (X = F, Cl, Br and I) monolayers based on density-functional theory (DFT). We show that the 2D TiX3 monolayers are stable dynamically and thermodynamically as evidenced by phonon and molecular dynamics calculations, respectively, and show their semiconducting nature. We find that the TiBr3 and TiI3 monolayers are ferromagnetic with magnetic anisotropy out of plane, which are intrinsic without the need for external intervention. The magnetic anisotropy energies of the TiBr3 and TiI3 monolayers are 0.8 and 2.5 meV per s.f., respectively. The Curie temperatures of TiBr3 and TiI3 are 75 K and 90 K, respectively. We further show that the interlayer magnetic coupling and magnetic anisotropy energies (MAE) of the bilayer TiI3 can be tuned by the interlayer distance. Additionally, a two-step transition in the magnetic state is observed in the bilayer TiI3 with AB' stacking under applied strain in a vertical direction. It is expected that our design may enrich two-dimensional functional materials, which may find versatile applications.
RESUMEN
Active catalysts for nitrogen fixation (N2-fixation) have been widely pursued through constant efforts for industrial applications. Here, we report a family of catalysts, MXenes (M2X: M = Mo, Ta, Ti, and W; X = C and N), for application in N2-fixation based on density functional theory calculations. We find that the catalytic performance of MXenes strongly depends on the reaction energy in each reaction step. More exothermic steps lead to higher catalytic performance in the course of N2-fixation. We show that the reaction energy in N2-fixation is strongly affected by the charge transfer: (1) if N atoms gain more electrons in a step, the reaction is exothermic with a larger reaction energy; (2) if N atoms lose electrons in a step, the reaction is endothermic in general. We further show that Mo2C and W2C are highly active for N2-fixation due to their exothermic reactions and strong charge transfer, which may be applicable in the chemical-engineering industry.
RESUMEN
Ambient air quality data, including atmospheric visibility, of Foshan city, a highly polluted city in the Pearl River Delta (PRD), and data obtained by the On-line Air Pollutant Exhaust Monitoring Network (OAPEMN), recently established by the National Emission Monitoring and Control Network for major industrial enterprises, were analyzed and are reported here for the first time, revealing the change in air pollution patterns and its impact on visibility degradation in the last decade. Reduced visibility of less than 8 km (after elimination of rainy and foggy periods) was found 22% of the time from 1998 to 2008, accompanied by elevated levels of pollutants, especially SO2 and PM10, in comparison with that of other developed cities. However, PM10 showed a steady decreasing trend (0.004 mg m⻳) year⻹) during 2001-2008, in contrast to the noticeable increase in ambient NO2 concentrations from ~0.020 mg m⻳ before 2005 to above 0.050 mg m⻳ afterward. Multiple regression analysis revealed that the percentage of reduced visibility strongly correlated with PM10 concentration, suggesting that visibility degradation was directly proportional to the loading of particles. Moreover, the fairly significant correlation between reduced visibility and NO2 concentration also implied that the impact of primary emissions of NO2 and enhanced secondary pollutants, formed via photochemical processes in the atmosphere, could not be ignored. The decreased PM10levels were obviously the predominant factor for the improvement in visibility (5.0% per 0.01 mg m⻳) and were likely due to the implementation of stricter air pollution control measures for industrial exhaust, which also resulted in reduced SO2 pollution levels in the recent 2 years. In particular, the OAPEMN records showed an overall enhanced SO2 removal by 64% in major industrial sectors. The continuous increase in road traffic and lack of efficient NO(x) control strategies in the PRD region, however, caused an increase in ambient NO2 concentrations.