Zhao Fengyi, Xu Zihao, Suo Sa, Lin Lu, Hill Craig L, Musaev Djamaladdin G, Lian Tianquan
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
Cherry L. Emerson Centre for Scientific Computation, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, USA.
J Chem Phys. 2024 Sep 7;161(9). doi: 10.1063/5.0226128.
In photoelectrochemical cells, promising devices for directly converting solar energy into storable chemical fuels, the spatial variation of the electrostatic potential across the semiconductor-electrolyte junction is the key parameter that determines the cell performance. In principle, electric field induced second harmonic generation (EFISH) provides a contactless in situ spectroscopic tool to measure the spatial variation of electrostatic potential. However, the total second harmonic generation (SHG) signal contains the contributions of the EFISH signals of semiconductor space charge layer and the electric double layer, in addition to the SHG signal of the electrode surface. The interference of these complex quantities hinders their analysis. In this work, to understand and deconvolute their contributions to the total SHG signals, bias-dependent SHG measurements are performed on the rutile TiO2(100)-electrolyte junction as a function of light polarization and crystal azimuthal angle (angle of the incident plane relative to the crystal [001] axis). A quadratic response between SHG intensity and the applied potential is observed in both the accumulation and depletion regions of TiO2. The relative phase difference and amplitude ratio are extracted at selected azimuthal angles and light polarizations. At 0° azimuthal angle and s-in-p-out polarization, the SHG intensity minimum has the best match with the TiO2 flatband potential due to the orthogonal relative phase difference between bias-dependent and bias-independent SHG terms. We further measure the pH-dependent flatband potential and probe the photovoltage under open circuit conditions using the EFISH technique, demonstrating the capability of this contactless method for measuring electrostatic potential at semiconductor-electrolyte junctions.
在光电化学电池（有望将太阳能直接转化为可储存化学燃料的装置）中，半导体 - 电解质结处静电势的空间变化是决定电池性能的关键参数。原则上，电场诱导二次谐波产生（EFISH）提供了一种非接触式原位光谱工具来测量静电势的空间变化。然而，总二次谐波产生（SHG）信号除了包含电极表面的SHG信号外，还包含半导体空间电荷层和双电层的EFISH信号贡献。这些复杂量的干扰阻碍了对它们的分析。在这项工作中，为了理解并解卷积它们对总SHG信号的贡献，我们在金红石TiO₂(100)-电解质结上进行了与偏压相关的SHG测量，测量结果是光偏振和晶体方位角（入射平面相对于晶体[001]轴的角度）的函数。在TiO₂的积累区和耗尽区都观察到SHG强度与施加电势之间的二次响应。在选定的方位角和光偏振下提取了相对相位差和振幅比。在方位角为0°且s偏振光入射、p偏振光出射时，由于与偏压相关和与偏压无关的SHG项之间的正交相对相位差，SHG强度最小值与TiO₂平带电势最匹配。我们进一步测量了pH依赖的平带电势，并使用EFISH技术在开路条件下探测光电压，证明了这种非接触方法测量半导体 - 电解质结处静电势的能力。
