(25h) Investigating Mechanisms of Bimetallic Nanocrystal Formation with Liquid Phase Transmission Electron Microscopy

Liquid phase transmission electron microscopy (LP-TEM) has been widely used to investigate the formation mechanisms of metal nanoparticles in liquid, providing guidance to synthesize nanocrystals with desired structures and functional properties.^{1, 2} During LP-TEM imaging, the electron beam acts both as the imaging tool and the reducing agent to reduce either metal precursors to form nanocrystals, which enables visualizing nanocrystal formation in real time with nanometer scale spatial resolution. Despite numerous insights gained from these experiments, it is unclear how electron beam induced nanocrystal formation compares to nanocrystal formation during flask-based synthesis. Indeed, a systematic study comparing the chemistry and kinetics of nanocrystal formation during LP-TEM and flask-based colloidal synthesis has not been undertaken.

Here we establish ranges of LP-TEM experimental conditions that are representative of flask-based nanocrystal synthesis. We investigate Au/Cu bimetallic nanoparticle synthesis as a model system due to its promising application as a CO₂ reduction electrocatalyst.³ Controllable synthesis of alloyed bimetallic nanocrystals is difficult due to differing kinetics and thermodynamics of precursor reduction of different metal species.⁴ We utilized poly(ethylene glycol) methyl ether thiol (PEG-thiol, Mw=800 g/mol) to complex Au/Cu metal salts into prenucleation complexes (PNC) prior to nanoparticle synthesis.⁵ Our results showed that flask-based synthesis using sodium borohydride to reduce PNCs formed ~2-3 nm Au/Cu alloy nanocrystals. LP-TEM synthesis, which utilizes radiolysis to produce aqueous electrons and hydrogen radicals as reducing agents,⁶ formed 3 - 7 nm nanocrystals at low electron dose rates. Imaging at high electron dose rates (high image magnification and beam current) formed irregular aggregated particles. High electron dose rates increase the nucleation rate and create more oxidizing radical species,⁷ which oxidizes the sulfur group on the PEG-thiol ligand, leading to formation of uncapped aggregated particles. Our study identified key differences between flask-based and LP-TEM synthesis methods, giving important guidance on how to set experimental conditions during LP-TEM to study nanocrystal formation mechanisms at conditions relevant to flask-based synthesis.

1. Zheng, H.; Smith, R. K.; Jun, Y.-w.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P., Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories. Science 2009, 324 (5932), 1309-1312.
2. Loh, N. D.; Sen, S.; Bosman, M.; Tan, S. F.; Zhong, J.; Nijhuis, C. A.; Král, P.; Matsudaira, P.; Mirsaidov, U., Multistep nucleation of nanocrystals in aqueous solution. Nature Chemistry 2016, 9, 77.
3. Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P., Synergistic geometric and electronic effects for electrochemical reduction of carbon dioxide using gold–copper bimetallic nanoparticles. Nature Communications 2014, 5, 4948.
4. Wang, D.; Li, Y., Bimetallic Nanocrystals: Liquid-Phase Synthesis and Catalytic Applications. Advanced Materials 2011, 23 (9), 1044-1060.
5. Marbella, L. E.; Chevrier, D. M.; Tancini, P. D.; Shobayo, O.; Smith, A. M.; Johnston, K. A.; Andolina, C. M.; Zhang, P.; Mpourmpakis, G.; Millstone, J. E., Description and Role of Bimetallic Prenucleation Species in the Formation of Small Nanoparticle Alloys. Journal of the American Chemical Society 2015, 137 (50), 15852-15858.
6. Schneider, N. M.; Norton, M. M.; Mendel, B. J.; Grogan, J. M.; Ross, F. M.; Bau, H. H., Electron–Water Interactions and Implications for Liquid Cell Electron Microscopy. The Journal of Physical Chemistry C 2014, 118 (38), 22373-22382.
7. Wang, M.; Park, C.; Woehl, T. J., Quantifying the Nucleation and Growth Kinetics of Electron Beam Nanochemistry with Liquid Cell Scanning Transmission Electron Microscopy. Chemistry of Materials 2018, 30 (21), 7727-7736.