In situ oxidation and reduction of triangular nickel nanoplates via environmental transmission electron microscopy

Summary Understanding the oxidation and reduction mechanisms of transition metals, such as nickel (Ni), is important for their use in industrial applications of catalysis. A powerful technique for investigating the redox reactive species is in situ environmental transmission electron microscopy (ETEM), where oxidation and reduction can be tracked in real time. One particular difficulty in understanding the underlying reactions is understanding the underlying morphology of the starting structure in a reaction, in particular the defects contained in the material, and the exposed surface facets. Here‐in, we use a colloidal nanoparticle synthesis in a continuous flow reactor to form nanoplates of nickel coated with oleylamine as a capping agent. We utilise an in situ heating procedure at 300 °C in vacuum to remove the oleylamine ligands, and then oxidise the Ni nanoparticles at 25 °C with 2 Pa oxygen, and follow the nanoparticles initial oxidation. After that, the nanoparticles are oxidised at 200 and 300 °C, making the size of the oxide shell increase to ∼4 nm. The oxide shell could be reduced under 2 Pa hydrogen at 500 °C to its initial size of ∼1 nm. High temperature oxidation encouraged the nanoparticles to form pure NiO nanoparticles, which occurred via the Kirkendall effect leading to hollowing and void formation. Lay description Understanding how a metal system reacts under gaseous environments is important for their application in catalysis where reactions are routinely carried out in oxidising or reducing conditions. A powerful technique for investigating metal systems in reactive environments is environmental transmission electron microscopy, where changes in the materials structure and phase with temperature and reactive gas environment can be tracked in real time. In this article we study nickel nanoplates that have been synthesised chemically in a flow reactor. Nickel is an important catalytic material which is very prone to oxidation. Using the nanoplate morphology as our starting point, we can precisely understand the initial morphology of the nickel nanoplates and readily track their changes during oxidation and subsequent reduction. We study the nickel nanoplates under oxygen gas at low temperature (25 °C), medium temperature (200–300 °C) and high temperature (500 °C) and show how the nickel nanoplates native oxide shell thickens with time and temperature, until finally at high temperatures the nickel is completely oxidised. We also study the