Browsing by Author "Wang, Weichao"
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Item D-Type Anti-Ferromagnetic Ground State in Ca₂Mn₂O₅(IOP Publishing Ltd, 2017-02) Liu, Pan; Wang, Wei-Hua; Wang, Weichao; Cheng, Ya-Hui; Lu, Feng; Liu, Hui; Wang, WeichaoWe study the electronic and magnetic properties of an oxygen-deficient perovskite Ca₂Mn₂O₅ based on the first principle calculations. The calculations show that the ground state of Ca₂Mn₂O₅ is a D-type anti-ferromagnetic structure with the anti-ferromagnetic spin coupling along the c-direction. The corresponding electronic structure of the D-type state is investigated, and the results display that Ca₂Mn₂O₅ is an insulator with an indirect energy gap of ~2.08 eV. By the partial density-of-state analysis, the valence band maximum is mainly contributed to by the O-2p orbitals and the conduction band minimum is contributed to by the O-2p and Mn-3d orbitals. Due to the Coulomb repulsion interaction between electrons, the density of state of Mn-3d is pulled to -6--4.5 eV. .Item Dual Sensitization Effect and Conductive Structure of Fe₃O₄@mTiO₂/C Photocatalyst Towards Superior Photodegradation Activity for Bisphenol A Under Visible Light(Elsevier B.V., 2019-05-31) Zhao, X.; Wang, R.; Lu, Z.; Wang, Weichao; Yan, Y.; Wang, WeichaoIn this paper, we prepared Fe₃O₄@mTiO₂/C (the mesoporous TiO₂ denoted as mTiO₂) photocatalyst via a simple synthetic method, and it exhibits strong light response under visible light region because of the dual sensitization effect of Fe₃O₄ and carbon. In addition, the composite photocatalyst also has double conductivity, which is beneficial to the transfer and separation of charge carriers due to the synergetic effect of Fe₃O₄, carbon and mTiO₂. As a consequence, the double conductive structure Fe₃O₄@mTiO₂/C photocatalyst displays prominent photocatalytic activity in degradation of BPA under visible light. Furthermore, the photocatalyst not only was recycled easily to reduce the secondary pollution of the environment but also had a superior stability. This work may provide a new inspiration toward constructing dual sensitization and double conductive structure with other semiconductor photocatalyst to effectively improve the photocatalytic activity under visible light for practical application.Item Electronic Structure and Ferromagnetism Modulation in Cu/Cu₂O Interface: Impact of Interfacial Cu Vacancy and Its Diffusion(Nature Publishing Group) Li, Hao-Bo; Wang, Weichao; Xie, Xinjian; Cheng, Yahui; Zhang, Zhaofu; Dong, Hong; Zheng, Rongkun; Wang, Wei-Hua; Lu, Feng; Liu, Hui; Wang, WeichaoCu/Cu₂O composite structures have been discovered to show sizable ferromagnetism (FM) with the potential applications in spintronic devices. To date, there is no consensus on the FM origin in Cu/Cu₂O systems. Here, first principles calculations are performed on the interface structure to explore the microscopic mechanism of the FM. It is found that only the Cu vacancy (VCu) adjacent to the outermost Cu₂O layer induces a considerable magnetic moment, mostly contributed by 2p orbitals of the nearest-neighbor oxygen atom (O_(NN)) with two dangling bonds and 3d orbitals of the Cu atoms bonding with the (O_(NN)). Meanwhile, the charge transfer from Cu to Cu₂O creates higher density of states at the Fermi level and subsequently leads to the spontaneous FM. Furthermore, the FM could be modulated by the amount of interfacial VCu, governed by the interfacial Cu diffusion with a moderate energy barrier (~1.2 eV). These findings provide insights into the FM mechanism and tuning the FM via interfacial cation diffusion in the Cu/Cu₂O contact.Item Investigation of High Oxygen Reduction Reaction Catalytic Performance on Mn-Based Mullite SmMn₂O₅(Royal Soc Chemistry, 2017-10) Liu, Jieyu; Yu, Meng; Wang, Xuewei; Wu, Jie; Wang, Changhong; Zheng, Lijun; Yang, Dachi; Liu, Hui; Yao, Yan; Lu, Feng; Wang, Weichao; 0000-0001-5931-212X (Wang, W); Wang, WeichaoAn alternative material SmMn₂O₅ mullite with regard to Pt/C is proposed to catalyze the oxygen reduction reaction (ORR) by combining density functional theory (DFT) calculations and experimental validations. Theoretical calculations are performed to investigate the bulk phase diagram, as well as the stability and electrocatalytic activity of the ORR under alkaline conditions for SmMn₂O₅ (001) surfaces, which are passivated by nitrogen atoms to avoid any spurious interference. The adsorptions of relevant ORR species (O*, OH*, OOH* and OO*) tend to compensate the coordination of manganese atoms to form Mn-centered octahedral or pyramidal crystal fields, and the corresponding binding energies fulfill a linear relationship. An oxygen molecule prefers to be reduced to OH⁻ via a four-electron pathway and this prediction is verified by electrochemical measurements on the as-prepared SmMn₂O₅ catalyst with a nanorod structure. Volcano curves are obtained to describe the trends in theoretical ORR activity as a function of a single parameter, i.e. the oxygen binding energy. An overpotential of 0.43 V is obtained at the O* binding energy around 3.4 eV, which is close to the experimental observation (0.413 V) in this work. SmMn₂O₅ mullite exhibits favorable ORR activity and superior stability with only ~5% decay in activity over 20 000 s of chronoamperometric operation in contrast to ~15% decrease of Pt/C, making it a promising candidate for a cathode catalyst.Item Investigation of the Hydrothermal Aging of an Mn-Based Mullite SmMn₂O₅ Catalyst of NO Oxidation(Royal Society of Chemistry, 2017-10-20) Xue, L.; Xiong, K.; Chen, H.; Cho, Kyeongjae; Wang, Weichao; 0000-0003-2698-7774 (Cho, K); 0000-0001-5931-212X (Wang, W); 369148996084659752200 (Cho, K); Cho, Kyeongjae; Wang, WeichaoHydrothermal aging tests are important to carry out when evaluating the hydrothermal durability of heterogeneous catalysts in vehicle exhaust emission. Here, we explored the effect of aging on an efficient Mn-based mullite catalyst (SmMn₂O₅) of NO oxidation. The mullite catalyst was prepared via the hydrothermal method and was subsequently aged in air with a 10% H2O stream at 750 °C for 16 hours. The fresh and aged catalysts were structurally characterized using Powder X-ray diffraction(XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), high resolution-transmission electron microscope (HR-TEM), Brunauer-Emmett-Teller (BET) and temperature programmed desorption (TPD). For the performance evaluations, the samples were placed into a U-shape reactor furnace, and NO and NO2 concentrations were in situ recorded with an NOx analyzer. In contrast to fresh mullite, the aged sample showed a 25 °C higher light-off temperature and 11% conversion loss at its maximum conversion temperature of 300 °C. O2-TPD of the aged sample displayed a large decrease of the desorption area, consistent with an ∼3-fold loss of the BET specific surface area. Moreover, HRTEM, XPS and Raman spectroscopy results together indicated that a small portion of the mullite decomposed into perovskite SmMnO3 and Mn2O3, which further reduced the total quantity of Mn active sites. The reduction of the BET surface area and mullite decomposition together caused the decrease of the catalytic performance. We therefore expect maintaining the specific surface area to be important for preventing the loss of catalytic performance during the hydrothermal aging process. © 2017 The Royal Society of Chemistry.Item Morphology Controlled Synthesis of SmMn₂O₅ Nanocrystals via a Surfactant-Free Route for Zn-Air Batteries(Elsevier B.V.) Yu, M.; Wei, Q.; Wu, M.; Wu, J.; Liu, J.; Zhang, G.; Sun, S.; Wang, Weichao; Wang, WeichaoDeveloping earth abundant and highly active electrocatalysts to overcome the sluggish oxygen reduction kinetics is one of the key toward the practical applications of air batteries with economic efficiency and high energy density. Herein, a shape-controlled synthesis of mullite SmMn₂O₅ is achieved through a surfactant-free one-step hydrothermal method. SmMn₂O₅ nanoparticles (SmMn₂O₅-NPs) and nanorods (SmMn₂O₅-NRs) are successfully synthesized and in half-cell tests, SmMn₂O₅-NRs perform enhanced oxygen reduction activity relative to SmMn₂O₅-NPs in terms of a more positive half-wave potential and reduced peroxide yield. Importantly, rechargeable zinc-air batteries constructed with SmMn₂O₅-NRs exhibit not only higher peak power density (217 mW cm⁻²) over commercial Pt/C catalyst (190 mW cm⁻²) but also excellent cycling stability during long-term charging-discharging test over 170 h. The facile synthesis of SmMn₂O₅-NRs together with the remarkable electrocatalytic performance endows mullite SmMn₂O₅ with great promise in replacing the precious metal as future catalysts for air batteries.