Elucidating the photochemistry of inorganic nanostructures

纳米结构是具有一个或多个尺寸（颗粒尺寸、棒直径、膜厚度、孔径）的系统，范围从1E-10米（原子键的尺度）到1E-7（典型的生物病毒的尺寸）。大多数时候，这种纳米结构处于它们的低能量基态，但是当它们吸收光时，来自基态的一些电子可以被激发以形成所谓的激发态，其能量更高。然而，激发态是不稳定的，通常在1E-15到1E-6秒内，激发态的电子会回到基态，填满激发态留下的空穴。激发态的弛豫可以遵循不同的路径：首先，纳米结构可以在称为光致发光（PL）的过程中重新发光。其次，纳米结构可以经历化学反应，导致其原子的永久重排。第三，激发的电子和/或空穴可以转移到吸附在纳米结构上的分子，并且纳米结构可以加热。这些不同的弛豫路径具有重大的实际意义。无机纳米结构中的PL已成功应用于激光和节能固态照明等领域。在异质结太阳能电池和染料敏化太阳能电池中，激发的电子或空穴向吸附分子的转移是关键步骤。最后，由光引起的结构变化限制了太阳能电池和其他经常暴露在直射强烈阳光下的设备的使用寿命。现在，尽管上面讨论的应用程序的巨大的实际重要性，不同的激发态弛豫路径的基本知识是有限的。例如，激发态的最终结构往往是未知的。这种知识差距的出现是因为纳米结构固有的无序性和激发态的短寿命使得难以在实验中模拟相关过程。因此，这些应用的光活性材料开发的进展主要是通过试验和错误。我的奖学金的目的是通过使用理论方法来产生对无机纳米结构的光物理学和光化学的微观洞察来缩小知识差距。这将使我能够回答重要的实际问题，例如激发的电子可能被困在纳米结构的哪个部分，哪些材料特性决定了什么弛豫路径是主导的，以及如何通过实验成功地调整这些，在实践中，这意味着我将采用一种名为含时密度泛函理论（TD-DFT）的理论方法。以探测不同纳米结构中弛豫激发态的几何形状和化学性质。这种方法，当适当验证，给出准确的结果，而在同一时间被计算足够便宜，有效地研究感兴趣的系统。此外，在可能的情况下，我将比较所获得的结果，例如预测的PL光谱，与我的实验合作者获得的结果。在第一步中，我将研究一系列尺寸，形状和成分的化学计量纳米结构。在这项工作的基础上，我将把注意力转向纳米结构中激发态的命运，这些纳米结构错过了一些原子，掺杂了外来原子或吸附了分子。对后者系统的研究对于理解TD-DFT特别重要，迄今为止还没有用TD-DFT等理论方法研究过这一应用。最后，作为所提出的工作的后一部分，我将把所开发的理论方法应用于我的合作者在实验室中研究的现实光催化系统。汇集我们的实验和理论结果将使我们能够找到，例如，新的和改进的水分解催化剂，用于可再生氢气的生产。

Nanostructures are systems with one or more dimensions (particle-size, rod diameter, film thickness, pore-size) ranging from 1E-10 metres, the scale of atomic bonds, to 1E-7, the size of a typical biological virus. Most of the time such nanostructures are in their low energy ground state, but when they absorb light some electrons from the ground state can be excited to form a so-called excited state, which lies higher in energy. Excited states, however, are not stable and typically in 1E-15 to 1E-6 seconds the excited electrons will fall back to the ground state, filling the holes that they left upon excitation.The relaxation of an excited state can follow different paths: Firstly, the nanostructure can reemit light in a process called photoluminescence (PL). Secondly, the nanostructure can undergo a chemical reaction which results in a permanent rearrangement of its atoms. Thirdly, the excited electrons and/or holes can be transferred to a molecule adsorbed on the nanostructure and fourthly the nanostructure can heat up. These different relaxation paths have major practical implications. PL in inorganic nanostructures is successfully exploited in applications such as lasers and energy efficient solid state lighting. The transfer of excited electrons or holes to an adsorbed molecule is a critical step in both heterogeneous photocatalysis and in dye-sensitised solar cells. Finally, the structural changes induced by light impose a limit to the service life of solar cells and other devices that are routinely exposed to direct intense sunlight. Now, in spite of the enormous practical importance of the applications discussed above, fundamental knowledge of the different excited state relaxation paths is limited. For example the final structure of the excited state is often unknown. This knowledge gap arises because the inherent disorder of nanostructures and the short lifetimes of excited states make it difficult to characterise the relevant processes in experiment. As a result progress in photoactive materials development for these applications has been mostly through trial and error. The aim of my fellowship is to close the knowledge gap by using theoretical methods to generate microscopic insight into the photophysics and photochemistry of inorganic nanostructures. This will allow me to answer important practical questions such as on which part of the nanostructures the excited electrons are likely to get trapped, which material properties determine what relaxation path is dominant and how these could be successfully tuned experimentally, thus replacing serendipity by insight.In practice this means I will employ a theoretical method named time-dependent density functional theory (TD-DFT) to probe the geometry and chemical nature of the relaxed excited state in different nanostructures. This method, when properly validated, gives accurate results whilst at the same time being computationally cheap enough to efficiently study the systems of interest. Furthermore, where possible I will compare the obtained results, for example predicted PL spectra, to those obtained by my experimental collaborators. In a first step, I will study stoichiometric nanostructures for a range of sizes, shapes and compositions. Building on this work, I will then switch my attention to the fate of excited states in nanostructures that miss some atoms, are doped with foreign atoms or have molecules adsorbed on their surface. Study of these latter systems is especially important for understanding photocatalysis, an application that has to date not been studied with theoretical methods such as TD-DFT. Finally, as a latter part of the proposed work, I will apply the developed theoretical approach to realistic photocatalytic systems studied by my collaborators in the laboratory. Pooling our experimental and theoretical results will allow us to find, for example, new and improved water splitting catalysts for renewable hydrogen production.