Ultrafast Coherent Spectroscopy of TiO2 Photocatalytic Processes

在化学系化学结构动力学和机制-A（CSDM-A）项目的支持下，匹兹堡大学的Hrvoje Petek教授正在飞秒（10^-15 s）时间尺度上对二氧化钛（TiO 2）半导体中的电子动力学进行时间分辨光电子能谱研究。TiO 2是一种具有众所周知的光催化性能的材料，可以将太阳能转化为电能或化学能，并通过光促进电子在分子/TiO 2界面上的转移来诱导有害化学物质的分解。这是有据可查的，这样的光诱导过程发生在二氧化钛，但如何使用光能进行有用的化学转化，在竞争松弛和降解成热的动力学，是知之甚少。这种理解存在于电子半导体和金属中，但对于大量可归类为金属氧化物的化学材料（如TiO 2）的理解却很少。TiO 2中电子和化学过程的表征可以促进金属氧化物在光活化应用中的使用。 该项目支持研究生和博士后助理的正规培训。Petek教授在COVID-19疫情封锁期间发起的虚拟国际超快知识咖啡馆（“IUNCH”）正在进行非正式教育和外展。单晶TiO 2表面的光致电子动力学正在通过超快时间分辨光电子能谱进行研究。在化学计量的TiO 2的光激发动力学的详细解释是困难的，因为测量探针的电子是不均匀分布在空间和动量。二氧化钛具有许多原子缺陷态，这些缺陷态产生尖锐的光电子谱，使得能够研究电子相和能量弛豫。超快飞秒（10-15秒）时间尺度的测量将在这样的缺陷功能，建立内在的能量和动量弛豫过程，广泛地说，在金属氧化物材料。二氧化钛还具有充分记载的等离子体增强的光催化活性。研究将进行如何在等离子体激元金属光激发激活超快电子过程在二氧化钛表面。参与该项目的学生和博士后将获得超快激光技术和超快相干动力学计算建模方面的经验。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。

With support from the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) Program of the Chemistry Division, Professor Hrvoje Petek of the University of Pittsburgh is performing time-resolved photoelectron spectroscopy studies of electron dynamics in titanium dioxide (TiO2) semiconductor on the femtosecond (10^-15 s) time scale. TiO2 is a material with well-known photocatalytic properties that can convert the solar to electrical or chemical energy, as well as induce decomposition of harmful chemicals, by light promoted transfer of electrons across the molecule/TiO2 interface. It is well documented that such light-induced processes occur in TiO2, but the dynamics of how light energy is used to perform useful chemical transformations, in competition with relaxation and degradation into heat, is poorly known. Such understanding exists for electronic semiconductors and metals, but is poorly understood for a large body of chemical materials that can be classified as metal oxides, such as TiO2. The characterization of electronic and chemical processes in TiO2 can advance the use of metal oxides in light-activated applications. The project supports the formal training of graduate students and post-doctoral associates. Informal education and outreach is being done via the virtual International Ultrafast Knowledge Coffee House ("IUNCH") that Professor Petek initiated during the COVID-19 pandemic lockdown.Photoinduced electron dynamics at single crystal TiO2 surfaces are being examined by ultrafast time-resolved photoelectron spectroscopy. Detailed interpretation of photoexcitation dynamics in the stoichiometric TiO2 is difficult, because the measurements probe electrons that are inhomogeneously distributed in space and momentum. Titanium dioxide has many atomic defect states, which give sharp photoelectron spectra, that enable the study of electron phase and energy relaxation. Ultrafast femtosecond (10-15 s) time scale measurements will be performed on such defect features to establish intrinsic energy and momentum relaxation processes, broadly, in metal oxide materials. Titanium dioxide also has well-documented plasmonically enhanced photocatalytic activity. Studies will be performed on how light excitation in a plasmonic metal activates ultrafast electronic processes at TiO2 surfaces. The students and post-doctoral associates involved in this project will gain experience in ultrafast laser techniques and computational modeling of ultrafast coherent dynamics.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.