Precursor and Surface Reactivities Influence the Early Growth of Indium Oxide Nanocrystals in a Reagent-Driven, Continuous Addition Synthesis

The structural attributes and structure-dependent properties of metal oxide nanocrystals are often defined during the earliest stages of nanocrystal growth. The species formed early in the growth process are notoriously difficult to study due to their small size and rapidly changing structures. Thus, despite many studies on the formation and initial growth of nanocrystals, little is known about how to control these steps during synthesis. Here, we investigate how the choice of reagent─oleyl alcohol or oleylamine─influences the earliest stages of indium oxide nanocrystal growth in a reagent-driven, continuous addition synthesis. The metal oxide precursor is activated through reaction with the reagent (through amidation or esterification) as opposed to thermal decomposition. Amidation proceeds faster than esterification, and this difference has a significant influence on the early steps of nanocrystal formation and growth. Fewer, larger nanocrystals are formed in the presence of oleylamine whereas more, but smaller, nanocrystals are formed in oleyl alcohol. Our studies suggest that differences in surface reactivity in the presence of the two reagents influence the transition from nanocrystal formation to growth. In the case of oleylamine, the reagent activates the surface of the nanocrystal through amidation reactions that expose more reactive metal hydroxyl species. Due to the higher concentration of these species, the rate of attachment of activated precursor to existing nanocrystals outpaces the rate of condensation of precursors to form new nanocrystals. As a result, the formation of new particles ends earlier in oleylamine than it does in oleyl alcohol, resulting in fewer particles being formed in the presence of amine. When mixtures of the amine and alcohol are used, the reactions also proceed through reagent-driven, rather than thermally driven, growth, and the ratio of reagents can be used to control the number of nanocrystals formed.