Li2Ti0.5Mn0.5O3 samples sintered at different temperatures were prepared via a solid-phase method,and their photocatalytic performance in degrading 2,4-dichlorophenol (2,4-DCP) under UV light was investigated.Photocatalytic experiments revealed that Li2Ti0.5Mn0.5O3 sintered at 900℃(LTMO-900) exhibited superior photocatalytic activity,achieving a 94.46% degradation rate of 2,4-DCP within 30 min under UV irradiation.UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) confirmed that Mn doping significantly enhanced the light absorption capacity of the material.Mott-Schottky and electrochemical impedance spectroscopy (EIS) analyses indicated that LTMO-900 possessed a more positive valence band potential and higher efficiency in photogenerated charge carrier separation.Radical scavenging experiments revealed that holes (h+) and superoxide radicals ($\mathrm{O}_{2}^{·-}$) are the dominant active species in the degradation process.First-principles calculations based on density functional theory (DFT) indicated that Li2Ti0.5Mn0.5O3 is a semiconductor with an optical absorption bandgap of approximately 1.79 eV.The Mn-3d and O-2p orbitals predominantly constitute the valence band maximum,whereas the conduction band minimum is mainly formed by the hybridized states of Mn-3d,Ti-3d,and O-2p orbitals.This electronic structure elucidates the generation and migration mechanisms of the photogenerated charge carriers.
本研究基于密度泛函理论(DFT),采用Vienna ab initio Simulation Package(VASP)中的投影缀加平面波(PAW)方法进行第一性原理计算[20]。Kohn-Sham波函数通过平面波基组展开,截断能设为 600 eV。几何结构优化使用广义梯度近似(GGA)下的Perdew-Burke-Ernzerhof(PBE)交换关联泛函[21]。所有原子均充分弛豫,直至原子间最大作用力低于0.01 eV/Å。采用Monkhorst-Pack方法对布里渊区进行采样,k点网格密度设置为11×6×10[22]。Li2Ti0.5Mn0.5O3中Mn掺杂浓度为50%,即在Li2TiO3结构中有一半的Ti原子被Mn原子替代。采用反铁磁构型进行结构优化并计算其电子结构。
为探究LTMO-900在光催化降解2,4-DCP过程中的反应机理,进行了自由基捕获实验。从图11结果可以看出,当体系分别加入0.1 mL TBA、0.1 mmol P-BQ及1 mmol EDTA-2Na后,2,4-DCP的光催化降解效率依次下降至89.98%、55.96%和27.01%。与未加捕获剂的对照组相比,降解率分别降低了约4.48%、38.50%和67.45%,说明空穴(h+)、超氧自由基($\mathrm{O}_{2}^{·-}$)和羟基自由基(·OH)均为光催化过程中的活性物种,其作用重要性依次增强。
基于上述分析表征,提出了LTMO-900的光催化降解2,4-DCP的机理。通常情况下,涉及半导体光催化氧化降解污染物机制的活性物种有·OH,$\mathrm{O}_{2}^{·-}$和h+。根据图12所示,光生电子从导带迁移至材料表面,与吸附的氧气发生还原反应,生成超氧自由基(O2/$\mathrm{O}_{2}^{·-}$:-0.33 eV vs.NHE),半导体LTMO-900的EVB和ECB分别是1.73 eV和-0.59 eV。当光照时,导带上电子将O2还原为$\mathrm{O}_{2}^{·-}$,而h+可以直接氧化2,4-DCP分子和将H2O氧化为·OH,这与自由基捕获实验保持一致。
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