Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, United States.
Department of Earth Sciences, Utrecht University, Princetonlaan 8a, 3548 CB Utrecht, The Netherlands.
ACS Appl Mater Interfaces. 2023 Apr 12;15(14):18414-18426. doi: 10.1021/acsami.2c22595. Epub 2023 Mar 30.
Understanding the mechanism of charge dynamics in photocatalysts is the key to design and optimize more efficient materials for renewable energy applications. In this study, the charge dynamics of a CuO thin film are unraveled via transient absorption spectroscopy (TAS) on the picosecond to microsecond timescale for three different excitation energies, i.e., above, near, and below the band gap, to explore the role of incoherent broadband light sources. The shape of the ps-TAS spectra changes with the delay time, while that of the ns-TAS spectra is invariant for all the excitation energies. Regardless of the excitations, three time constants, τ ∼ 0.34-0.59 ps, τ ∼ 162-175 ns, and τ ∼ 2.5-3.3 μs, are resolved, indicating the dominating charge dynamics at very different timescales. Based on these observations, the UV-vis absorption spectrum, and previous findings in the literature, a compelling transition energy diagram is proposed. Two conduction bands and two defect (deep and shallow) states dominate the initial photo-induced electron transitions, and a sub-valence band energy state is involved in the subsequent transient absorption. By solving the rate equations for the pump-induced population dynamics and implementing the assumed Lorentzian absorption spectral shape between two energy states, the TAS spectra are modeled which capture the main spectral and time-dependent features for > 1 ps. By further considering the contributions from free-electron absorption during very early delay times, the modeled spectra reproduce the experimental spectra very well over the entire time range and under different excitation conditions.
理解光催化剂中电荷动力学的机制是设计和优化用于可再生能源应用的更高效材料的关键。在这项研究中，通过在皮秒到微秒时间尺度上的瞬态吸收光谱（TAS）来研究 CuO 薄膜的电荷动力学，对于三种不同的激发能，即带隙以上、带隙附近和带隙以下，来探索非相干宽带光源的作用。 ps-TAS 光谱的形状随延迟时间而变化，而 ns-TAS 光谱的形状在所有激发能下都是不变的。无论激发如何，都解析出三个时间常数 τ∼0.34-0.59 ps、τ∼162-175 ns 和 τ∼2.5-3.3 μs，表明在非常不同的时间尺度上存在主导的电荷动力学。基于这些观察结果、紫外可见吸收光谱和文献中的先前发现，提出了一个引人注目的跃迁能量图。两个导带和两个缺陷（深和浅）态主导初始光致电子跃迁，而亚价带能量态参与随后的瞬态吸收。通过求解泵浦诱导的群体动力学的速率方程，并在两个能量态之间采用假设的洛伦兹吸收光谱形状，对 TAS 光谱进行建模，该模型捕捉了 >1 ps 的主要光谱和时间相关特征。通过进一步考虑在非常早的延迟时间内自由电子吸收的贡献，所建模的光谱在整个时间范围内和在不同的激发条件下很好地重现了实验光谱。
