Faculty Advisor: Emily Tsui

Quantum Dot Metal Complex Hybrids, a New Platform for Tunable Electro- and Photocatalysis

Photocatalysis is a topic of great interest because it enables the use of solar energy for performing desirable energy-related reactions such as hydrogen evolution, CO2 reduction, and N2 reduction. Recent studies have demonstrated that the electrochemical potential and reactivity of molecular electrocatalysts for CO2 reduction can be tuned by modification of ligand electrostatics or electronic inductive effects around the catalyticallyactive metal center [1-3]. However, accessing these ligands can be synthetically taxing and require many chemical steps for minor electronic perturbations. A platform that readily allows for post-synthetic tuning of electrostatics would permit faster optimization of catalytic conditions. Further coupling this system to a lightabsorbing material would allow us to expand these systems to develop tunable photocatalyst hybrid systems.

We hypothesize that colloidal semiconductor quantum dots (QDs) can act as a tunable electrostatic platform to modify the activity of covalently-attached metal complex catalytic active sites. Our group has demonstrated that QD surface electrostatics can be modified in a number aways, for example by 1) reduction or oxidation of surface sites, 2) changes to the surface stoichiometry, and 3) post-synthetic ligand exchange [4]. In this proposed project, redox- active metal complexes such as 6-coordinate Ru or Fe complexes with pendant donor groups will be attached to different QD materials (CdSe, InP, etc.) via ligand exchange. The alteration to the surface of the QDs will change the surface electrostatics and the attached metal complex may display a change in redox potential in response. The change in the redox potential of the complex will be determined by electrochemical methods . After establishing QD surface electrostatic effects on these complexes, target reactions such as electrocatalytic hydrogen evolution will be performed to observe how the QD surface environment influences the catalytic activity of the tethered complexes. Once the effect of the QD surface electrostatics on these complexes is established on electrocatalysis the reactions will be applied in photocatalysis. Photocatalysis will be performed by photoexcitation of the QD and measuring yields of H2 in the hydrogen evolution reaction. The expected outcome of this research is to provide a proof-of-concept of a new method of tuning molecular catalyst activity and to establish a platform for photocatalysis where the catalytic conditions can be precisely tuned and easily adapted for different reactions.

[1] Reath A.; Ziller J.; Tsay C.; Ryan A.; Yang J. Inorg. Chem. 2017, 56, 3713−3718
[2] Chantarojsiri T.; Reath A.; Yang J. Angew. Chem. Int. Ed. 2018, 57, 14037 –14042
[3] Kang K.; Fuller J.; Reath A.; Ziller J.; Alexandrova A.; Yang J. Chem. Sci., 2019, 10, 10135
[4] Prather K.; Stoffel J.; Tsui E. Y. Chem. Mater., 2022, 34, 3976-3984

Research Objectives

The primary objective for this project is to prepare quantum dot (QD)-metal complex hybrid conjugates as tunable photocatalytic systems in which QD surface modifications can change the catalyst potentials and activities.

Aim 1: Covalent Attachment of Metal Complexes to QDs. Redox-active metal complexes (currently, Fe(bpy)32+ and Ru(bpy)32+ derivatives, bpy = bipyridine) with pendant donor groups (e.g., thiolates, carboxylates, and phosphonates) provided by collaborators will be covalently attached to QDs via exchange reactions with the native QD ligands. Different QD materials, including metal oxides like ZnO or TiO2, metal chalcogenides like CdSe, CdS, PbS, and III-V materials like InP will be targeted. These samples will be characterized using a combination of 1D and 2D NMR spectroscopy, IR spectroscopy, and elemental analysis (ICP-OES, XPS).

Aim 2: QD Surface Electrostatic Effects on Metal Complex Reduction Potential. Based on previous work from our group, the QD surface electrostatics will be tuned by changes to surface stoichiometry or by exchange of the dipole moments of the supporting ligands. These electrostatic changes can be measured using metal carbonyl spectroscopic reporters previously described by our group. Following these surface modifications, the redox potential of the attached metal complexes will be measured electrochemically in order to establish a relationship between metal reduction potential and QD surface electrostatics.

Aim 3: Effects of QD Surface Electrostatics on Electrocatalysis. After establishing any electrostatic effects on redox potentials, known H2 evolution catalysts such as nickel (II) salen compounds will be used as the attached species to the QD samples. Under reducing conditions with different proton sources, the formation of H2 will be monitored both electrochemically and by gas chromatography (GC). Changes in the production of H2 as the surface of the QDs are modified will indicate a relationship between the QD electrostatics and catalytic reactivity.

Aim 4: Photocatalysis. After demonstrating that the hybrid conjugates are active for electrocatalysis, the same effects will be studied under QD photoexcitation. Charge transfer to the metal complex can be measured through photoluminescence quenching experiments or using time-resolved methods in collaboration with other research groups. For photocatalysis, sacrificial reductants like ascorbic acid or ethanol will be added to the QD sample and yields of H2 will be measured as in Aim 3. In the long term, these same experiments can be extended to other QD platforms and other catalyst systems for more complicated reactions like CO2 reduction.