Terahertz investigations on photoisomerisable compounds

<p>We performed computational terahertz studies on organometallic photoisomerisable compounds, employing both gas phase and solid phase calculations. The calculations demonstrate the potential of employing terahertz techniques on photoisomerisable compounds. In particular, the <i>trans</i>-ligand, counterion and crystal effects are evaluated via the density functional theory calculations. In order to fully understand the terahertz responses of the photoisomerisable compounds, their experimental terahertz spectra were obtained and compared to the calculations. The calculated spectra generally predict the experimentally observed absorption peaks, while combined gas phase and solid phase calculations offer better agreement with the experiments. The first principles calculations also reveal the sensitivity of terahertz signal on the photoisomerisation processes, suggesting a photo-terahertz set-up that could be built in the future to fast screen and fully understand the photoisomerisable compounds, for related applications such as photo-transducer and photo-switch that require photoinduced geometrical changes.</p

First-Principles Study of Molecular Adsorption on Lead Iodide Perovskite Surface: A Case Study of Halogen Bond Passivation for Solar Cell Application

Organic molecules have recently been used to modify the surface/interface structures of lead halide perovskite solar cells to enhance device performance. Yet, the detailed interfacial structures and adsorption mechanism of the molecular modified perovskite surface remain elusive. This study presents a nanoscopic structural view on how organic molecules interact with the perovskite surface. We focus on the halogen bond passivated lead iodide perovskite surface, based on first-principles calculations. Our calculations show that organic molecules can interact with the perovskite surface via halogen bonds, which modifies the interfacial structures of the perovskite surface. We also constructed a detailed potential energy surface of the perovskite surface by moving the adsorbed molecule along different axes of the unit cell in order to comprehensively understand perovskite surface structures. This study demonstrates the effectiveness of modifying the perovskite surface structure via a molecular adsorption approach, and anticipates that the properties of perovskite materials can be further improved by a molecular engineering method

N–Co–O Triply Doped Highly Crystalline Porous Carbon: An Acid-Proof Nonprecious Metal Oxygen Evolution Catalyst

In comparison with nonaqueous Li–air batteries, aqueous Li–air batteries are kinetically more facile and there is more variety of non-noble metal catalysts available for oxygen electrocatalysis, especially in alkaline solution. The alkaline battery environment is however vulnerable to electrolyte carbonation by atmospheric CO₂ resulting in capacity loss over time. The acid aqueous solution is immune to carbonation but is limited by the lack of effective non-noble metal catalysts for the oxygen evolution reaction (OER). This is contrary to the oxygen reduction reaction (ORR) in acid solution where a few good candidates exist. We report here the development of a N–Co–O triply doped carbon catalyst with substantial OER activity in acid solution by the thermal codecomposition of polyaniline, cobalt salt and cyanamide in nitrogen. Cyanamide and the type of cobalt precursor salt were found to determine the structure, crystallinity, surface area, extent of Co doping and consequently the OER activity of the final carbon catalyst in acid solution. We have also put forward some hypotheses about the active sites that may be useful for guiding further work

High Electrochemical Performance of Monodisperse NiCo₂O₄ Mesoporous Microspheres as an Anode Material for Li-Ion Batteries

Binary metal oxides have been regarded as ideal and potential anode materials, which can ameliorate and offset the electrochemical performance of the single metal oxides, such as reversible capacity, structural stability and electronic conductivity. In this work, monodisperse NiCo₂O₄ mesoporous microspheres are fabricated by a facile solvothermal method followed by pyrolysis of the Ni_0.33Co_0.67CO₃ precursor. The Brunauer–Emmett–Teller (BET) surface area of NiCo₂O₄ mesoporous microspheres is determined to be about 40.58 m² g^–1 with dominant pore diameter of 14.5 nm and narrow size distribution of 10–20 nm. Our as-prepared NiCo₂O₄ products were evaluated as the anode material for the lithium-ion-battery (LIB) application. It is demonstrated that the special structural features of the NiCo₂O₄ microspheres including uniformity of the surface texture, the integrity and porosity exert significant effect on the electrochemical performances. The discharge capacity of NiCo₂O₄ microspheres could reach 1198 mA h g^–1 after 30 discharge–charge cycles at a current density of 200 mA g^–1. More importantly, when the current density increased to 800 mA·g^–1, it can render reversible capacity of 705 mA h g^–1 even after 500 cycles, indicating its potential applications for next-generation high power lithium ion batteries (LIBs). The superior battery performance is mainly attributed to the unique micro/nanostructure composed of interconnected NiCo₂O₄ nanocrystals, which provides good electrolyte diffusion and large electrode–electrolyte contact area, and meanwhile reduces volume change during charge/discharge process. The strategy is simple but very effective, and because of its versatility, it could be extended to other high-capacity metal oxide anode materials for LIBs

Molecular Engineering of the Lead Iodide Perovskite Surface: Case Study on Molecules with Pyridyl Groups

We computationally investigate the molecular engineering approach of the lead iodide perovskite surface employing a pyridyl anchor-based molecular adsorbate as an example. The molecular adsorption approach on lead halide perovskite surfaces has been employed for passivation purposes in perovskite solar cells and was demonstrated to successfully enhance the solar cell performance in previous experimental studies. It is an open question whether the structures and properties of the lead halide perovskite can be further modified via the molecular engineering approach, and this study serves to probe the molecular engineering approach in the lead halide perovskite surface. First-principles calculations are employed to determine the nanoscopic structure of the lead halide perovskite surface with pyridyl anchor-based molecular adsorbates and prove that the pyridyl anchor-based molecule resides stably on the perovskite surface and modifies the perovskite surface structure. In addition, the calculations demonstrate that the electronic and optical properties of the lead halide perovskites can be controlled by the molecular engineering method. Noteworthily, we find that the molecular engineering approach is effective to modify the optical properties of the lead halide perovskite layer investigated in this study. Such molecular engineering approach on the perovskite surface could be potentially applicable to further enhance the performance of perovskite solar cells and perovskite-based optoelectronic devices

Hollow MnCo₂O₄ Submicrospheres with Multilevel Interiors: From Mesoporous Spheres to Yolk-in-Double-Shell Structures

We present a general strategy to synthesize uniform MnCo₂O₄ submicrospheres with various hollow structures. By using MnCo-glycolate submicrospheres as the precursor with proper manipulation of ramping rates during the heating process, we have fabricated hollow MnCo₂O₄ submicrospheres with multilevel interiors, including mesoporous spheres, hollow spheres, yolk–shell spheres, shell-in-shell spheres, and yolk-in-double-shell spheres. Interestingly, when tested as anode materials in lithium ion batteries, the MnCo₂O₄ submicrospheres with a yolk–shell structure showed the best performance among these multilevel interior structures because these structures can not only supply a high contact area but also maintain a stable structure

Photoassisted High-Performance Lithium Anode Enabled by Oriented Crystal Planes

Lithium (Li) metal anodes are candidates for the next-generation high-performance lithium-ion batteries (LIBs). However, uncontrolable Li dendrite growth leads to safety issues and a low Coulombic efficiency (CE), which hinders the commercialization of Li metal batteries. Stable Li anodes based on the tailored plane deposition and photoassisted synergistic current collectors are currently the subject of research; however, there are few related studies. To suppress the growth of Li dendrites and achieve dense Li deposition, we design a low-cost customized-facet/photoassisted synergistic dendrite-free anode. The tailored (002) plane endows it with a nanorod array/microsphere composite structure and exhibits a strong affinity for Li, which effectively reduces the Li+ nucleation overpotential and promotes uniform Li deposition. Notably, during the photoassisted Li deposition/stripping process, due to electron–hole separation, a weakly charged layer is formed on the (002) surface and local charge carrier changes are induced, reducing the overpotential by 8.3 mV, enhancing the reaction kinetics, and resulting in a high CE of ∼99.3% for the 300th cycle at 2 mA cm–2. This work is of great significance for the field of next-generation photoassisted Li metal anodes