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

非技术性描述改进由廉价、耐用材料制成的太阳能收集装置的功能是使太阳能技术成为清洁、可再生能源战略的一个组成部分所必需的。光伏或光催化器件的效率通常由光吸收后最初几皮秒至纳秒内发生的事件决定，因此，对这种超快时间尺度上的电子和化学动力学的透彻理解对于指导下一代材料的设计至关重要。瞬态吸收光谱学长期以来一直被用于这一目的，其中超短激光泵浦脉冲同步地启动样品中的过程，并且探测脉冲在稍后的时间报告系统的状态。但是，虽然这种方法在提供溶液中反应的分子电影方面取得了巨大的成功，但激光诱导加热不可避免地会在固态材料中产生虚假信号。该项目旨在通过开创一种全新的超快X射线技术来解决这一困境，该技术探测原子核而不是电子。由于这项技术需要由大型储存环光源提供异常明亮的X射线脉冲，这项工作将在阿贡国家实验室的高级光子源进行。在这个世界一流的设施中的驻留将为罗得岛大学的研究团队提供独特的机会，首次展示TRSRM，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。技术描述射线瞬态吸收光谱学突出了这些传统方法用于研究固体中的光化学动力学的缺陷，由于激光诱导加热产生的大量热伪影的存在，该项目的目的是开发和演示时间分辨同步辐射M？ssbauer光谱（TRSRM），一种新颖而困难的X射线技术，将提供一个明确的探测激发态动力学的各种固态光催化和光伏材料，包括赤铁矿，钛酸铁，和有机铅卤化物钙钛矿。M？ssbauer光谱学保留了X射线技术的元素特异性和氧化/自旋状态灵敏度，同时拥有非常窄的线宽，简单的分配，最重要的是对于该技术的超快实现，对大的热变化相对不敏感。TRSRM也可以在电化学工作条件下进行，这是研究光电化学材料（如赤铁矿）功能的绝对要求。通过对这些材料如何发挥作用提供清晰的机理见解，这项工作将为化学和物理性质的关键评估（和重新评估）铺平道路，这些化学和物理性质增强或降低了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和执行初始实验。在皮秒分辨TRSRM成功演示之后，该团队将寻求在硬X射线自由电子激光设施中通过飞秒分辨TRSRM识别固态材料中的瞬态物质和过渡态。该奖项反映了NSF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.