SusChEM: Visible Light-Driven Reduction of Carbon Dioxide using Heavy Metal-Free Colloidal Quantum Dots as Sensitizers

利用阳光将二氧化碳（CO2）转化为各种可以用作燃料的小分子，是一种开发丰富的太阳能的可持续方式，可以在白天和晚上使用。这种转化所涉及的化学反应是复杂和多步骤的，通常涉及电子从一个分子转移到另一个分子。“光催化剂”是一种利用光能驱动化学反应，从而降低复杂反应的能垒，从而促进复杂反应的物质。在化学部化学催化项目的资助下，西北大学的Emily Weiss博士正在进行基础研究，以确定和优化催化剂设计中的二氧化碳转化光催化剂，这些催化剂包括由无毒、富含地球的材料制成的半导体纳米颗粒表面吸附的分子。她的研究涉及这些无机-有机杂化配合物的化学分析和激光光谱，以监测光能激发后它们在超快时间尺度上的行为。Weiss博士积极参与该项目本科生研究人员的指导工作，参与西北大学普通化学新课程的开发，以增加女性和少数族裔学生在STEM领域的代表性不足，以及Gateway科学研讨会等项目，该项目为STEM课程的学生提供结构化的学习课程，特别是那些高中准备较差的学生。在化学催化项目的资助下，西北大学的Emily Weiss博士正在确定和优化最重要的热力学和动力学参数，这些参数影响了三元无重金属胶体CuInX2 （X = S, Se）量子点（QDs）作为可溶性、多位点、胶体增敏剂的性能，用于光催化溶液中二氧化碳的还原。这项工作阐明了胶体量子点敏化剂的特定和独特性质如何增强具有已知二氧化碳还原特异性的分子催化剂的性能的机制，通过解决与该任务相关的几个基本挑战，即：(i)具有足够还原能力的量子点敏化剂的合成和表面功能化，以向助催化剂提供多个电子；（ii）控制局部氢离子浓度；（iii）从敏化剂中快速提取空穴，抑制复合和光氧化降解；（iv）敏化剂和助催化剂之间的最大电子耦合，为快速电子传递提供途径。一维和二维核磁共振（NMR）光谱、电子显微镜、电化学测量以及稳态和时间分辨光学测量可以定量表征QD-催化剂对的结合亲和力、QD表面的质子（H+）浓度、催化底物和中间体的吸附程度、基本电子和空穴转移步骤的速率常数以及空穴清除的效果。这些技术补充了气相色谱和液相色谱，核磁共振和红外光谱表征产物分布和反应速率。除了使用无毒的、富含地球的胶体催化剂从阳光和二氧化碳中寻找新的途径来生产碳基和氢基燃料的社会影响之外，这项工作还具有广泛的影响，因为本科生将参与Weiss博士提出的研究和正在进行的普通化学课程改革，以增加西北大学STEM领域女性和代表性不足的少数民族学生的保留。

The use of sunlight to convert carbon dioxide (CO2) to various small molecules that can be used as fuels is a sustainable way to exploit abundant solar energy for use both during day and night. The chemical reactions involved in this conversion are complicated and multi-step, and typically involve the transfer of electrons from one molecule to another. A "photocatalyst" is a species that lowers the energy barriers for, and thereby facilitates, such complex reactions by using light energy to drive chemical reactions. With funding from the Chemical Catalysis Program of the Chemistry Division, Dr. Emily Weiss of Northwestern University is conducting fundamental studies to identify and optimize photocatalyts for CO2 conversion in catalyst designs that comprise molecules adsorbed to the surfaces of semiconductor nanoparticles made of non-toxic, earth abundant materials. Her studies involve both chemical analysis of these hybrid inorganic-organic complexes and laser spectroscopy to monitor their behaviors on ultrafast timescales after excitation with light energy. Dr. Weiss is actively involved in the mentoring of undergraduate researchers on this project, in the development of a new curriculum for General Chemistry at Northwestern University to increase retention of women and minority students under-represented in STEM fields, and in programs like the Gateway Science Workshop, which offers structured study sessions for students in STEM courses, particularly those students with weaker high school preparation. With funding from the Chemical Catalysis Program of the Chemistry Division, Dr. Emily Weiss of Northwestern University is identifying and optimizing the most important thermodynamic and kinetic parameters in the performance of ternary heavy metal-free colloidal CuInX2 (X = S, Se) quantum dots (QDs) as soluble, multi-site, colloidal sensitizers for photocatalysis of the reduction of CO2 in solution. This work illuminates the mechanisms by which specific and unique properties of a colloidal QD sensitizer enhance the performance of a molecular catalyst with known specificity for CO2 reduction, by addressing several fundamental challenges associated with this task, namely: (i) synthesis and surface functionalization of a QD sensitizer with enough reducing power to donate multiple electrons to the co-catalyst; (ii) control of the local concentration of hydrogen ions; (iii) fast hole extraction from the sensitizer to inhibit recombination and photo-oxidative degradation; and (iv) maximal electronic coupling between the sensitizer and the co-catalyst to provide pathways for fast electron delivery. One and two dimensional nuclear magnetic resonance (NMR) spectroscopy, electron microscopy, electrochemical measurements and steady-state and time resolved optical measurements allow for quantitative characterization of the binding affinity for the QD-catalyst pair, the proton (H+) concentration on the QD surface, the degree of adsorption of the catalytic substrates and intermediates, the rate constants for elementary electron and hole transfer steps, and the efficacy of hole scavenging. These techniques complement the gas and liquid chromatography, NMR, and infrared spectroscopy characterization of product distributions and reaction rates. In addition to the societal impact of finding new pathways to produce carbon- and hydrogen-based fuels from sunlight and CO2 using a non-toxic, earth-abundant colloidal catalyst, this work has broad impact in that undergraduates will be involved both in the proposed research and in ongoing curriculum reform of General Chemistry by Dr. Weiss, in order to increase retention of women and under-represented minority students in STEM fields at Northwestern University.