The chemistry of Cu3N and Cu3PdN nanocrystals

Abstract

Cu3N and Cu3PdN nanocrystals are attractive materials with numerous applications ranging from optoelectronics to catalysis. However, their chemical formation mechanism and surface chemistry are unknown or contested. In this work, we first optimize the synthesis and purification to yield phase pure, colloidal stable Cu3N and Cu3PdN nanocubes. Second, we elucidate the precursor conversion mechanism that leads to the formation of Cu3N from copper(II) nitrate and oleylamine. We find that oleylamine is both the reductant and nitrogen source. Oleylamine is oxidized to a primary aldimine and the latter reacts further with oleylamine to a secondary aldimine, eliminating ammonia. Ammonia reacts with Cu(I) to form Cu3N. Third, we investigated the surface chemistry of the nanocrystals using solution NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). We find a mixed ligand shell of aliphatic amines and carboxylates. The carboxylate is produced in situ during the synthesis. While the carboxylates appear tightly bound, the amines are easily desorbed from the surface. Finally, we analyze the optoelectronic properties by UV-Vis and XPS. Doping with palladium decreases the bandgap but the material remains a semiconductor. These results bring insight into the chemistry of metal nitrides and will help the development of other metal nitride nanocrystals.

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…Ammonia reacts with Cu­(I) to copper nitride (again releasing three protons). The overall pathway is shown in Scheme . There is a high similarity with the pathway toward InN formation, where In­(III) is reduced to In(0), forming also the primary aldimine.…”

“…Copper nitride nanocrystals are typically synthesized by a simple heat-up method. − Copper nitrate is heated in oleylamine, and after 15 min at 260 °C, copper nitride nanocubes are formed with a cube edge length of around 10 nm. We found that Cu­(II) is first reduced to Cu­(I) by oleyamine around 190 °C . The by-products are a primary aldimine and two protons; see Scheme .…”

“…Therefore, the first step in colloidal synthesis is usually the conversion of the precursors into the intended product. Given its importance, precursor conversion has been studied in many nanocrystal systems: metals, ,− metal sulfides and selenides, ,− metal oxides, − lead halides, and metal pnictides. − In such efforts, nanochemists aim to write down the complete and balanced chemical equation, after determining the correct stoichiometry of the reaction and identifying reaction by-products. Ideally, this is complemented with a rigorous kinetics study to obtain further details about the mechanism.…”

“… a Reproduced from ref under the terms of the Creative Commons Attribution 4.0 International License (CC BY; ). …”

“…As a second part of our investigation, we focused on the source of nitride in copper nitride . The nitrate anion also oxidizes the primary amine to a primary aldimine, albeit at slightly higher temperatures (above 220 °C).…”