Liu Baoshun, Zhao Xiujian
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei Province 430070, China.
Phys Chem Chem Phys. 2014 Oct 28;16(40):22343-51. doi: 10.1039/c4cp02243b.
The continuous time random walk (CTRW) simulation was used to study the photocatalytic kinetics of nanocrystalline (nc)-TiO2 assemblies in this research. nc-TiO2 assemblies, such as nc-TiO2 porous films and nc-TiO2 hierarchical structures, are now widely used in photocatalysis. The nc-TiO2 assemblies have quasi-disordered networks consisting of many tiny nanoparticles, so the charge transport within them can be studied by CTRW simulation. We considered the experimental facts that the holes can be quickly trapped and transferred to organic species just after photogeneration, and the electrons transfer to O2 slowly and accumulate in the conduction band of TiO2, which is believed to be the rate-limiting process of the photocatalysis under low light intensity and low organic concentration. Due to the existence of numerous traps, the electron transport within the nc-TiO2 assemblies follows a multi-trapping (MT) mechanism, which significantly limits the electron diffusion speed. The electrons need to undergo several steps of MT transport before transferring to oxygen, so it is highly important that the electron transport in nc-TiO2 networks is determined for standard photocatalytic reactions. Based on the MT transport model, the transient decays of photocurrents during the photocatalytic oxidation of formic acid were studied by CTRW simulation, and are in good accordance with experiments. The steady state photocatalysis was also simulated. The effects of organic concentration, light intensity, temperature, and nc-TiO2 crystallinity on the photocatalytic kinetics were investigated, and were also consistent with the experimental results. Due to the agreement between the simulation and the experiments for both the transient and the steady state photocatalysis, the MT charge transport should be an important mechanism that controls the kinetics of recombination and photocatalysis in nc-TiO2 assemblies. Also, our research provides a new methodology to study the photocatalytic dynamics from the random event viewpoint, which can be revised to investigate the kinetic processes of other kinds of material.
本研究采用连续时间随机游走（CTRW）模拟方法研究了纳米晶（nc）-TiO₂ 组装体的光催化动力学。nc-TiO₂ 组装体，如 nc-TiO₂ 多孔膜和 nc-TiO₂ 分级结构，目前在光催化领域得到了广泛应用。nc-TiO₂ 组装体具有由许多微小纳米颗粒组成的准无序网络，因此可以通过CTRW模拟研究其中的电荷传输。我们考虑了以下实验事实：光生空穴在产生后能迅速被捕获并转移到有机物种上，而电子向O₂ 的转移缓慢并在TiO₂ 的导带中积累，这被认为是低光强和低有机浓度下光催化的速率限制过程。由于存在大量陷阱，nc-TiO₂ 组装体内的电子传输遵循多陷阱（MT）机制，这显著限制了电子扩散速度。电子在转移到氧气之前需要经历几个MT传输步骤，因此确定nc-TiO₂ 网络中的电子传输对于标准光催化反应非常重要。基于MT传输模型，通过CTRW模拟研究了甲酸光催化氧化过程中光电流的瞬态衰减，结果与实验吻合良好。还模拟了稳态光催化过程。研究了有机浓度、光强、温度和nc-TiO₂ 结晶度对光催化动力学的影响，结果也与实验结果一致。由于瞬态和稳态光催化的模拟与实验结果相符，MT电荷传输应该是控制nc-TiO₂ 组装体中复合和光催化动力学的重要机制。此外，我们的研究提供了一种从随机事件角度研究光催化动力学的新方法，该方法可经修改用于研究其他材料的动力学过程。
