Controlling the Synthesis of Palladium and Platinum Nanocrystals for Electrocatalytic Applications

Noble-metal nanocrystals with well-controlled attributes have found use in a variety of applications, including those related to electrocatalysis and energy conversion. Since the electrocatalytic performance of noble-metal nanocrystals critically depends on their sizes, shapes, and internal structures, gaining deep insights into their nucleation and growth is essential to achieving an ultimate control over their colloidal synthesis. In this dissertation, I present a number of methods for controlling the synthesis of Pd and Pt nanocrystals to improve their purity and uniformity in terms of size, shape, and structure, together with evaluation of their electrocatalytic performance toward oxygen reduction. First, the synthesis of Pt nanobars with aspect ratios tunable up to 2.1 was demonstrated by simply heating a Pt(IV) precursor in N,N-dimethylformamide in the presence of poly(vinyl pyrrolidone). A mechanistic study revealed that both particle coalescence and localized oxidative etching followed by preferential growth contributed to the anisotropic growth vital to the formation of nanobars. Recognizing the challenge in mass production of metal nanocrystals without compromising the product quality, a continuous and scalable route to Pt multipods was then developed by switching from a batch to a continuous flow reactor. Investigation on the morphological evolution of Pt multipods indicated that the anisotropic growth arose from a combination of fast autocatalytic surface reduction and attachment of smaller particles. When supported on carbon, the Pt multipods exhibited enhanced activity toward oxygen reduction relative to the commercial Pt/C catalyst. In addition to scaling up the production volume, the tubular flow reactor was further demonstrated as a powerful tool to tightly control the nucleation step of a synthesis. With Pd nanocrystals as an example, the nucleation and growth could be decoupled from each other using a flow reactor to trigger the nucleation, generating nanocubes highly uniform in both size and shape. Both the temperature and duration for nucleation were found to significantly impact the seed diversity and thus the quality of resultant nanocrystals. This methodology was also successfully extended to the preparation of uniform, sub-5 nm Pt nanocubes. Utilizing this strategy, Pt right bipyramids with a single twin plane and covered by {100} facets were prepared in high quality. The Br- ions involved in the synthesis, as well as the pair of temperatures used for the nucleation and growth steps, played critical roles in mediating the formation of singly-twinned seeds and directing their evolution into right bipyramids. The ability to control the nucleation and growth, coupled with in-depth mechanistic understanding of these processes, will contribute to the rational synthesis of noble-metal nanocrystals with desired features for electrocatalytic and other applications.Ph.D