44353-AC10
Oxidative Etching and Its Role in the Synthesis of Metal Nanostructures
Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal allows one to control its properties and enhancement of its usefulness for a given application. The aim of this project is to achieve a comprehensive understanding on the shape-controlled synthesis of metal nanocrystals. Specifically, we have focused on the use of oxidative etching to selectively remove twinned seeds formed in the initial nucleation stage of a synthesis. We found that the distribution of single-crystal versus twinned seeds can be manipulated through the use of oxidative etching, in which zero-valent metal atoms are oxidized back to ions. Since most syntheses are conducted in air, O2 is present in the reaction solution throughout the entire process. If a ligand for the metal ion is also present in the same solution, a combination of the ligand and O2 can result in a powerful etchant for both the nuclei and seeds. The defect zones in twinned seeds are much higher in energy relative to the single-crystal regions and thus are most susceptible to an oxidative environment, with their atoms being attacked by the enchant, oxidized, and dissolved into the solution. In contrast, single-crystal seeds are more resistant to oxidative etching as there are no twin boundary defects on the surface. By taking advantage of this selectivity, the population of different seed types in the reaction solution can be manipulated controllably. For example, in the polyol synthesis of Ag nanocrystals, all twinned seeds can be removed from the solution by adding a trace amount of Cl- to the reaction. As a result, single-crystal seeds and nanocrystals will prevail. By replacing Cl- with a less corrosive anion, Br-, it is possible to selectively eliminate only the multiply twinned seeds, leaving behind a mixture of single-crystal and singly twinned seeds in the solution. These seeds can grow into nanocrystals with drastically different shapes.
We have validated oxidative etching for a number of noble metals, including Ag, Pd, and Rh. In these examples, both O2 and a ligand are required in order to observe oxidative etching. For example, when a polyol synthesis for Ag nanocrystals is performed under argon, the multiply twinned seeds formed in the early stage of the reaction will grow quickly to form pentagonal nanowires. Likewise, if no Cl- is added, multiply twinned seeds will be formed which quickly evolve into quasi-spherical particles within 1 hour. Only when both O2 and Cl- (or another ligand) are present, will single-crystal seeds be obtained in high yields. Based upon the same mechanism, multiply twinned seeds can be saved by i) removing O2 from the reaction system by bubbling an inert gas through, ii) blocking oxygen adsorption to the seeds through the selection of suitable capping agents (e.g., citrate), or iii) diminishing the role of oxidative etching by scavenging oxygen in the solution with a redox pair (e.g., Fe(III/II) or Cu(II/I) salts).
It is worth pointing out that in many cases the counter ions of metal precursors or the miniscule amounts of ionic impurities present in the chemical reagents can facilitate oxidative etching and have a profound impact on the population of different types of seeds. For example, Na2PdCl4, a commonly used precursor for synthesizing Pd nanocrystals, contains the Cl- needed for oxidative etching. Also, in polyol syntheses based on ethylene glycol, Cl- may be present at sufficiently high concentrations (typically on the ppm level) to facilitate oxidative etching. Additionally, due to its synthesis and storage in steel vessels, ethylene glycol can be contaminated with trace Fe-containing species. Both FeII and FeIII ions have been shown to influence oxidative etching by coupling to O2 and the reductant. Knowledge of such impurities and their effects is essential to the reproducibility and scale-up of shape-controlled syntheses of metal nanocrystals.
