RII Track-4: Time-Resolved Mossbauer Spectroscopy, a New Tool for Investigating Ultrafast Dynamics in Solid-State Photocatalytic and Photovoltaic Materials

改善太阳能收集装置的功能是必要的，这些装置由廉价、耐用的材料制成，要使太阳能技术成为清洁、可再生能源战略的组成部分。光伏或光催化装置的效率通常由光吸收后最初几皮秒到纳秒内发生的事件决定，因此，在这个超快时间尺度上彻底理解电子和化学动力学对于指导下一代材料的设计至关重要。瞬态吸收光谱学早已被用于此目的，其中超短激光泵浦脉冲同步启动样品中的过程，探针脉冲报告稍后时间系统的状态。但是，虽然这种方法在提供溶液中反应的分子膜方面已经被证明是非常成功的，但激光诱导的加热不可避免地会在固态材料中产生虚假信号。这个项目旨在通过开创一种全新的超快x射线技术来解决这个难题，这种技术探测原子核而不是电子。由于这项技术需要大规模存储环形光源提供的异常明亮的x射线脉冲，这项工作将在阿贡国家实验室的先进光子源进行。在这个世界级的设施的驻留将为罗德岛大学的研究团队提供第一次展示TRSRM的独特机会，并跟踪几种固态太阳能转换材料中短寿命化学物质的形成。最近在超快光学和x射线瞬态吸收光谱方面的工作突出了这些传统方法在研究固态光化学动力学方面的缺陷，这是由于激光诱导加热产生的大量热伪影的存在。该项目旨在开发和演示时间分辨同步辐射M？斯堡尔光谱（TRSRM）是一种新颖而困难的x射线技术，它将为各种固态光催化和光伏材料（包括赤铁矿、钛酸铁和有机卤化铅钙钛矿）的激发态动力学提供明确的探测。M ?斯堡尔光谱保留了x射线技术的元素特异性和氧化/自旋态灵敏度，同时具有非常窄的线宽，直接的赋值，最重要的是对技术的超快速实现相对不敏感大的热变化。TRSRM也可以在电化学工作条件下进行，这是研究赤铁矿等光电化学材料功能的绝对要求。通过提供对这些材料如何运作的清晰的机械洞察，这项工作将为关键评估（和重新评估）化学和物理特性铺平道路，这些特性会增强或降低许多光活性材料的性能，并指导材料工程师通过合理设计提高太阳能转换效率。由于TRSRM只能在时间分辨的硬x射线光谱光束线上进行，研究小组将前往先进光子源准备并执行初始实验。在皮秒分辨率TRSRM的成功演示之后，该团队将寻求通过硬x射线自由电子激光设备上的飞秒分辨率TRSRM，不仅可以识别瞬态物质，还可以识别固态材料中的过渡态。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical DescriptionImproving the function of solar harvesting devices made from cheap, durable materials is necessary to make solar technology an integral part of a clean, renewable energy strategy. The efficiency of a photovoltaic or photocatalytic device is generally determined by events that occur within the first few picoseconds to nanoseconds following absorption of light, and thus a thorough understanding of electronic and chemical dynamics on this ultrafast timescale is crucial for guiding the design of next-generation materials. Transient absorption spectroscopy has long been deployed to this end, wherein an ultrashort laser pump pulse synchronously launches processes in a sample and a probe pulse reports on the state of the system at a later time. But while this approach has proven enormously successful in providing molecular movies of reactions in solution, laser-induced heating inevitably gives rise to spurious signals in solid-state materials. This project aims to solve that dilemma by pioneering an entirely new ultrafast X-ray technique that probes atomic nuclei instead of electrons. Because this technique requires the exceptionally bright X-ray pulses provided by large-scale storage ring light sources, this work will be performed at the Advanced Photon Source at Argonne National Laboratory. A residency at this world-class facility will provide the University of Rhode Island research team the unique opportunity to demonstrate TRSRM for the first time and track the formation of short-lived chemical species in several solid-state solar energy conversion materials.Technical Description Recent work in ultrafast optical and X-ray transient absorption spectroscopy has highlighted the deficiencies of these traditional approaches for studying photochemical dynamics in the solid-state due to the presence of massive thermal artifacts that arise from laser-induced heating. This project aims to develop and demonstrate time-resolved synchrotron radiation M?ssbauer spectroscopy (TRSRM), a novel and difficult X-ray technique that will provide an unambiguous probe of excited state dynamics in a wide variety of solid-state photocatalytic and photovoltaic materials, including hematite, iron titanate, and organolead halide perovskites. M?ssbauer spectroscopy retains element specificity and oxidation/spin state sensitivity of X-ray techniques while boasting exceptionally narrow linewidths, straightforward assignments, and most importantly for ultrafast implementation of the technique a relative insensitivity to large thermal variations. TRSRM may also be performed under electrochemical working conditions, an absolute requirement for investigating the function of photoelectrochemical materials such as hematite. By providing clear mechanistic insight into how these materials function, this work will pave the way for critical assessments (and reassessments) of the chemical and physical properties that enhance or diminish performance in many photoactive materials and guide the efforts of materials engineers to improve solar energy conversion efficiency through rational design. Because TRSRM can only be performed at a time-resolved hard X-ray spectroscopy beamline, the research team will travel to the Advanced Photon Source to prepare and execute the initial experiments. Following successful demonstration of picosecond-resolved TRSRM, the team will seek to identify not only transient species but also transition states in solid-state materials through femtosecond-resolved TRSRM at hard X-ray free electron laser facilities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.