Lagrow A P, Alyami N M, Lloyd D C, Bakr O M, Boyes E D, Gai P L
The York Nanocentre, University of York, York, U.K.
Department of Physics, University of York, York, U.K.
J Microsc. 2018 Feb;269(2):161-167. doi: 10.1111/jmi.12621. Epub 2017 Aug 29.
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.
了解过渡金属（如镍（Ni））的氧化和还原机制对于其在催化工业应用中的使用至关重要。一种用于研究氧化还原活性物种的强大技术是原位环境透射电子显微镜（ETEM），在该技术中可以实时跟踪氧化和还原过程。理解潜在反应的一个特别困难之处在于理解反应起始结构的潜在形态，特别是材料中包含的缺陷以及暴露的表面晶面。在此，我们在连续流动反应器中使用胶体纳米颗粒合成法来形成涂有油胺作为封端剂的镍纳米板。我们在真空中于300°C采用原位加热程序以去除油胺配体，然后在25°C下用2 Pa氧气氧化镍纳米颗粒，并跟踪纳米颗粒的初始氧化过程。之后，纳米颗粒在200°C和300°C下被氧化，使氧化壳的尺寸增加到约4 nm。氧化壳可以在500°C下于2 Pa氢气中还原至其初始尺寸约1 nm。高温氧化促使纳米颗粒形成纯NiO纳米颗粒，这是通过柯肯达尔效应导致中空和空洞形成而发生的。
