Achieving Carbon Nanotube Encapsulated β-Sn—Direct Observation of the Encapsulation Mechanism by In Situ TEM

The fabrication of continuous one-dimensional Sn nanocrystals (nanowires) confined within chemically and mechanically robust multi-walled carbon nanotubes (MWCNTs) is highly compelling for high-density energy storage electrodes, electromagnetic absorption materials, and advanced nanoelectronics. Sn is an attractive element to encapsulate for these applications; however, its high vaporization temperature (above 1500 °C <i>in vacuo</i>) and non-wettability towards carbon have made it challenging to encapsulate directly within a highly crystalline carbon nanotube. Here, we present a novel synthesis method to overcome these challenges and achieve β-Sn encapsulation within highly crystalline MWCNTs through a vapor phase transport route.<br/><br/>In comparison to the encapsulation of catalytically active metals for CNT growth (such as Fe, Ni, and Co), where filling is achieved directly <i>in situ</i> with an inherently continuously filled core, the encapsulation of non-intrinsic metals such as Sn occurs post-synthesis of the CNT growth through either a vapor or solution synthesis route. Typically, <i>ex situ </i>encapsulation strategies often yield discrete, non-continuous particles embedded within the nanotube core. The success of encapsulation and the degree of filling are governed by three main factors: the capillarity of the nanotube core, wettability towards the inner carbon surface, and surface tension of the encapsulant, which all control the underlying mechanism of encapsulation. Understanding the interplay between these parameters and how they influence the overall mechanism is key to achieving continuous Sn-filled nanotubes.<br/><br/>To explore these parameters, we present the first-of-its-kind direct observation of the vapor encapsulation mechanism, achieved <i>in situ</i> using transmission electron microscopy (TEM). These findings pave the way for achieving filled nanotubes with non-intrinsic elements introduced through the vapor phase and demonstrate the importance of <i>in situ</i> imaging for providing a holistic understanding of the encapsulation mechanism at the fundamental level.<br/><br/>Beyond encapsulation, we will also exploit the robust and electron-transparent properties of hexagonal carbon as a 'nano-reactor', an emerging frontier for observing complex and dynamic reaction behaviour <i>in situ</i> using TEM at atomic resolution.^1,2<br/><br/>References<br/><br/>1. C. S. Allen, Ya. Ito, Al. W. Robertson, H. Shinohara, and J. H. Warner, ACS Nano, 2011, 12, 10084–10089<br/>2. N. Clark, D. J. Kelly, M. Zhou, Y.-C. Zou, C. W. Myung, D. G. Hopkinson, C. Schran, A. Michaelides, R. Gorbachev and S. J. Haigh, Nature, 2022, 609, 942–947.<br/><br/>Acknowledgements<br/><br/>The authors thank the Engineering and Physical Sciences Research Council (EP/T5171811/1), the Faraday Institution, and the Royal Society for the Royal Society Industry Fellowship for their financial support. The authors extend their gratitude to the Oxford Materials Characterisation Service and the David Cockayne Centre for Electron Microscopy, Department of Materials, University of Oxford, and the Electron Physical Sciences Imaging Centre (EM16967), Diamond Light Source, for access and support. We thank Gareth Hughes for his help with Ga FIB.