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

Northwestern University, Evanston IL

Investigators

Abstract

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.