Understanding the Structural Dynamics of Polarons in Transition Metal Oxide Semiconductors by Vibrational Spectroscopy and Charge-Carrier Mobilities

利用太阳能电池或光电化学产生太阳能燃料无疑是当今最重要和最有前途的技术方法之一。特别是光催化水分解，其中水被分离成气态氧和氢气，最近引起了广泛的兴趣，因为它被设想为能量存储的关键技术。值得注意的是，尽管过去几十年在研究方面付出了巨大的努力，但仍然没有可以经济地分解水的光催化剂。可用于驱动光催化水分解的一类材料是过渡金属氧化物。这些氧化物是半导体，其中当光子被吸收时，电子从价带被激发到导带。由此发生电荷分离，并且所产生的自由电子和电子空穴可以分别单独地用作还原剂或氧化剂。这些电荷载流子通常被捕获在过渡金属氧化物中，因为它们形成极化子，其中电荷周围的原子发生位移，从而产生电势最小值。当涉及到在原子水平上表征极化子态时，理论方法是不可避免的。然而，这样的电子结构计算，这通常是基于混合DFT，可以是模糊的，使得极化子的确切几何形状显着（甚至定性）取决于所选择的功能。动力学相关的性质，如迁移率或电子转移速率常数，显然取决于极化子的几何形状。因此，详细了解确切的极化子结构对于以下研究项目至关重要。在这个项目中，我提出了一个先进的理论模拟研究的极化态BiVO 4，这是一个领先的候选人，有效的光催化分解水。在混合密度泛函理论水平上对BiVO 4中的空穴极化子进行了从头算分子动力学模拟。根据这些模拟，计算振动光谱（IR和拉曼）和电荷载流子迁移率。所有模拟，包括振动光谱的计算，都将通过机器学习来加速，这对于获得统计收敛的数据是绝对必要的。 与著名的实验学家合作安排，谁将测量相应的振动光谱和迁移率。因此，该项目的目的是为电子结构理论提供基准;在这种情况下，明确确定BiVO 4中正确的电子空穴极化子几何形状，但所应用的方法也可以转移到任何其他金属氧化物材料。从联合的理论/实验数据，我还将获得详细的了解BiVO 4中的电子空穴极化子的结构动力学。这对于寻求优化太阳能燃料转换技术作为低碳能源系统一部分的社区可能具有重要价值。

The generation of solar fuels by photocatalysis or photoelectrochemistry is certainly one of the most important and promising technological processes nowadays. Especially photocatalytic water splitting, where water is separated into gaseous oxygen and hydrogen, attracted a wide interest recently because it is envisaged as a key technology for energy storage. Remarkably, there is yet no photocatalyst available which can split water economically, despite the vast efforts in research over the last decades. One class of materials which can be used to drive photocatalytic water splitting are transition metal oxides. These oxides are semiconductors, where an electron is excited from the valence to the conduction band when a photon is absorbed. Thereby charge separation occurs and the resulting free electron and the electron hole can be used individually as reducing or oxidizing agent, respectively. These charge carriers are often trapped in transition metal oxides since they form polarons, where the atoms in the surrounding of the charge displace and thus create a potential minimum. When it comes to characterizing polaronic states on an atomistic level, theoretical methods are inevitable. However, such electronic structure calculations, which are usually based on hybrid DFT, can be ambiguous such that the exact geometry of a polaron significantly (and even qualitatively) depends on the chosen functional. Dynamically relevant properties, such as mobilities or electron transfer rate constants, obviously depend on the geometry of the polaron. A detailed knowledge of the exact polaronic structure is therefore crucial for following research projects. In this project, I propose an advanced theoretical simulation study of polaronic states in BiVO4 which is a leading candidate for efficient photocatalytic water splitting. Ab initio molecular dynamics simulations on the hybrid DFT level of theory are to be conducted of the hole polaron in BiVO4. From these simulations, vibrational spectra (IR and Raman) and charge-carrier mobilities are to be calculated. All simulations, including the calculation of the vibrational spectra, will be accelerated by machine learning which is absolutely necessary to obtain statistically converged data. Collaborations with renowned experimentalists are arranged, who will measure the corresponding vibrational spectra and mobilities. The aim of this project therefore is, to provide a benchmark for the electronic structure theory; in this case to definitively identify the correct electron hole polaron geometry in BiVO4, but the applied methods are transferable to any other metal oxide material as well. From the joint theoretical/experimental data, I will moreover gain a detailed understanding of the structural dynamics of the electron hole polaron in BiVO4. This could be of significant value to the community seeking to optimise solar-to-fuel conversion technologies as part of a low-carbon energy system.