Single-particle sepctroelectrochemistry on electrodeposited Palladium nanoparticles

Abstract

Palladium nanoparticles drive chemical innovation due to their high surface-to-volume ratio and tunable electronic properties. Their ability to readily activate hydrogen and form stable lattices makes them a cornerstone of modern chemistry. Therefore, to optimize chemical performance, controlling size and shape during synthetic processes is critical because these two factors dictate the catalyst's reactivity, stability, and cost-efficiency. While wet synthesis is an excellent approach to facilitate particle size and shape, surface ligands/surfactants removal together with particle immobilization on substrates remain problematic. Electrodeposition offers precise, template-free, and ligand-free control by tuning electrochemical parameters. In this dissertation, electrodeposition of Pd nanoparticles with controlled size and density on indiumdoped tin oxide substrate was firstly carried out using a combination of fast potential pulse and chronoamperometry techniques. As confirmed by electrochemical data and analysis of scanning electron microscopy images, a lower concentration of active species in the electrolyte solution results in a lower density of Pd nuclei during the nucleation step, and a larger particle size during the growth step. Meanwhile, at the same nuclei density, a higher concentration of active species gives a larger particle size during the growth phase. This approach provides a convenient means of fabricating nanostructures on substrates with desired sizes and densities and sheds light on the mechanism of growth. Secondly, in another approach, the growth of single Pd nanoparticles can be monitored with the power of dark-field microscope during electrochemical experiments. Through analyzing light scattering intensity of Pd nuclei exposed to a constant flow rate of electrolyte solution and chronoamperometric potential in a flow cell, a corresponding mechanism for the electrodeposition was uniquely obtained. This technique reveals a possibility to utilize optical data as a source of information in order to further understand the electrochemical processes at single particle level as opposed to average data retrieved from ensemble current. Lastly, we demonstrate a single-particle optical cyclic voltammetry technique that tracks hydrogen sorption dynamics in individual electrodeposited Pd nanoparticles by converting electrochemical processes into optical signals. This high-throughput method captures absorption and desorption across thousands of particles at once, bridging single-particle mechanisms with bulk statistical analysis. The technique exposes severe interparticle and intraparticle heterogeneity, along with cycling-induced deactivation caused by surface reorganization and deep trapping sites. Interestingly, the individual potential profiles show symmetric kinetic behavior via consistent shifts and broadening. These findings confirm that electrochemical activity is inherently heterogeneous and evolves dynamically under operando conditions. Ultimately, single-particle optical cyclic voltammetry offers a universal framework for mapping structure-activity relationships in energy materials, highlighting nanoscale behaviors that traditional ensemble measurements completely miss.