Faculty Advisor: Masaru Kuno

Chalcohalide Nanocrystals for Solar Energy Harvesting: Synthesis, Characterization, and Photocatalytic Applications

Despite the pressing needs imposed by climate change, we have not yet managed to harvest the full potential of the sun as an abundant and fully renewable energy source. Scientists globally have invested significant resources to better understand how to capture this vast energy capital: the most renowned is arguably photovoltaics, but it is not the only one. Another promising route is storing the energy of the sun by forming new chemical bonds, through a process called photocatalysis. This idea draws inspiration from natural photosynthesis and is implemented technologically by using semiconductors, thus fueling the quest for novel materials of this class.

Recently, a new group of semiconductors known as chalcohalides has come to the attention of the community as promising active materials. Initially studied in the 1960s and then somehow neglected, these materials are still rather underexplored. However, many compounds of this class can be prepared with cheap, vastly abundant, and nontoxic elements, which would make them ideal candidates for large-scale applications. In this project, we aim to synthesize both known and novel nanocrystals of chalcohalide materials with varying compositions, structure, and morphology. We will optimize the synthetic methods for each material and characterize their optoelectronic properties to identify the best candidates for photocatalytic applications. Additionally, catalytically active metal tips will be grown on top of chalcohalide nanocrystals to form functionalized heterostructures with improved energy conversion efficiency. Finally, the most promising candidates will be tested as active materials in a liquid-medium photocatalytic test cell, hopefully leading to new industry-viable materials for light-to-chemical energy conversion.

Reserach Objectives 

Objective 1: synthesize metal chalcohalide nanocrystals with varying compositions, crystal structure, and morphology. We will consider both phases that have been already obtained colloidally, but whose photocatalytic potential is still to be explored (e.g., BiSX, Bi13S18X2, and Pb4S3X2, where X=Cl, Br, I), as well as materials known in bulk but never explored before at the nanoscale (e.g., Pb7S2Br10, or the lead-free AgBiSCl2 and CuBiSCl2). To achieve this goal, I will adopt and improve a generalized synthetic strategy based on the use of common precursors (e.g., metal oleates, benzoyl halides, and tetramethylsililsulfide), which was recently demonstrated to be successful on some of these materials. Finally, I will adopt known literature procedure to functionalize all nanocrystals with surface domains of catalytically active materials (e.g., gold, platinum): a comparison with the pristine material will enable the characterization of any positive influence of such domains during the photocatalytic tests (see Objective 3).

Objective 2: the so-obtained materials will be studied to achieve a complete understanding of their structure and properties. To do so, I will take full advantage of the vast variety of analytical techniques available at Notre Dame:

• Morphological characterization by transmission electron microscopy (TEM) and high-resolution TEM, available at the Notre Dame Integrated Imaging Facility (NDIIF).
• Crystal structure determination and refinement by powder X-Ray diffraction, available at the Notre Dame Materials Characterization Facility (MCF).
• Elemental composition and possible determination of defect-induced doping by a combination of energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS), available at the Notre Dame Integrated Imaging Facility (NDIIF) and at the Materials Characterization Facility (MCF) respectively.
• Optoelectronic properties of nanocrystals, including the extraction of the optical band gap and the determination of energy positions for conduction and valence band edges. This will be accomplished by optical absorption measurements and ultraviolet photoelectron spectroscopy (UPS), both available at the Notre Dame Materials Characterization Facility (MCF).

Objective 3: proof-of-concept demonstration of photocatalytic activity for both pristine and functionalized nanocrystals. As a target reaction we will select well-characterized CO2 photoreduction reactions, which will be tested both in water (if the materials stability allows it) or in polar organic solvents like ethyl acetate. And additional step of ligand-exchange might be included to improve the solubility of nanocrystals in polar solvents. This crucial objective will be tackled in collaboration with Prof. Prashant Kamat's laboratory, whose wide experience on these and similar processes is well-proven by their several publications on nanocrystals-catalyzed photoredox reactions.