In Situ Studies of Single-Nanoparticle-Level Nickel Thermal Oxidation: From Early Oxide Nucleation to Diffusion-Balanced Oxide Thickening

Abstract

High-temperature oxidation mechanisms of metallic nanoparticles have been extensively investigated; however, it is challenging to determine whether the kinetic modeling is applicable at the nanoscale and how the differences in nanoparticle size influence the oxidation mechanisms. In this work, we study thermal oxidation of pristine Ni nanoparticles ranging from 4 to 50 nm in 1 bar 1%O2/N2 at 600 °C using in situ gas-cell environmental transmission electron microscopy. Real-space in situ oxidation videos revealed an unexpected nanoparticle surface refacetting before oxidation and a strong Ni nanoparticle size dependence, leading to distinct structural development during the oxidation and different final NiO morphology. By quantifying the NiO thickness/volume change in real space, individual nanoparticle-level oxidation kinetics was established and directly correlated with nanoparticle microstructural evolution with specified fast and slow oxidation directions. Thus, for the size-dependent Ni nanoparticle oxidation, we propose a unified oxidation theory with a two-stage oxidation process: stage 1: dominated by the early NiO nucleation (Avrami–Erofeev model) and stage 2: the Wagner diffusion-balanced NiO shell thickening (Wanger model). In particular, to what extent the oxidation would proceed into stage 2 dictates the final NiO morphology, which depends on the Ni starting radius with respect to the critical thickness under given oxidation conditions. The overall oxidation duration is controlled by both the diffusivity of Ni2+ in NiO and the Ni in Ni self-diffusion. We also compare the single-particle kinetic curve with the collective one and discuss the effects of nanoparticle size differences on kinetic model analysis.

Partial Chemicalization of Nanoscale Metals: An Intra‐Material Transformative Approach for the Synthesis of Functional Colloidal Metal‐Semiconductor Nanoheterostructures

“…[21d] The authors showed that the oxidation of Ni started with the formation of an initial NiO thin shell (with 3 nm thickness). Once the Ni/NiO interface had been created, either a single or multiple voids nucleated [36] Copyright 2022, American Chemical Society.…”

“…Sainju et al explored the oxidation kinetics of Ni NCs with different sizes (4‐50 nm) by using in situ gas‐cell environmental TEM (ETEM). [ 36 ] While smaller Ni NCs were entirely oxidized into NiO, larger Ni NCs could be converted only partially, resulting in Ni@NiO core‐shell MSHs. With increases in size, the core‐shell structure changed from symmetric to asymmetric (Figure 7a).…”

“…Dashed green lines indicate the faceted internal Ni surface; thick green arrows point to the diffusion direction of the faceted internal Ni surface. Reproduced with permission [36]. Copyright 2022, American Chemical Society.…”

Direct Observation of Sample Dynamics in the Gaseous Environment: A Perspective on Current Trends and Future Directions of In Situ Gas Cell Transmission Electron Microscopy

“…Information on microstructural evolution and oxidation kinetics can be obtained by in situ TEM studies conducted in an oxidizing environment. 56 Direct observations using in situ gas cell STEM can reveal details such as the evolution of surfaces under oxidation, identifying those surface-specific sites most prone to oxidation, and interfacial and orientation details of the oxide surface. 57 An example of one such study is the analysis of carbon oxidation of a thermal protection system (TPS) material phenolic-impregnated carbon ablator (PICA)�which is composed of carbon fiber preform, FibreForm, and a phenolic resin�used in NASA deep space missions.…”

“…The oxidation of a material can be beneficial by forming a protective coating, or detrimental by corroding the material. Information on microstructural evolution and oxidation kinetics can be obtained by in situ TEM studies conducted in an oxidizing environment …”