Abstract:

In situ liquid cell transmission electron microscopy (LCTEM) was used to observe the dynamic self-assembly behavior of gold nanorod (AuNR)/graphene (G) composites in real-time. Many important reactions in chemistry, physics, and biology occur in solution and real-time imaging of the reaction objects in a liquid medium can further our understanding of the reaction at the nanoscale. Observations of liquid samples using transmission electron microscopy (TEM) have historically been challenging due to issues with evaporation and difficulty in forming thin liquid layers that are suitable for election beam transmission. In situ LCTEM, as an emerging technology, provides novel opportunities for the real-time and high-resolution observation of dynamic processes in solution. In this communication, we report the use of in situ LCTEM to study the assembly behavior of graphene and AuNRs. By tracking and recording the changes in the positions and shapes of the AuNRs and graphene over time, novel composite formation mechanisms between AuNRs and graphene were observed. The AuNRs tended to approach the graphene edges tip-first due to charge attraction. After the assembled structures were formed, the AuNRs could rotate with the graphene edges, among which the edge-to-edge structure was more stable, without angle changes between the AuNR and graphene edge. Drifting motions of the self-assembled structures were observed. And compared with smaller self-assembled structures, the larger structures seem more effectively resisted pushing by liquid flow. In addition, the motions of the larger structure were more easily slowed due to the drag from the liquid cell window substrate. Graphene folding structures were also observed with LCTEM, suggesting that the folding structure can open and close in the liquid, causing apparent relative position changes between Au and graphene for a fixed AuNR on the graphene layer. Overall, the self-assembled structures are very stable and did not show any disassembly behavior in the liquid. Moreover, the AuNR/graphene composites were used as catalysts and showed improved catalytic performance compared to that of bare AuNRs in 4-nitrophenol reduction experiments. The self-assembled catalyst with a mass composite 1 : 5 AuNRs/G ratio exhibited the best performance with a k_app value of 0.5570 min⁻¹, 8 times that of the bare AuNRs. This significant improvement is closely related to the optimized and stable structure of the AuNR/graphene composites. In situ LCTEM provided a powerful characterization method for analyzing the complex self-assembly behavior of the composites in a liquid and will be useful for the development of high performance composite catalyst materials.

Key words: In situ liquid cell TEM, Self-assembly, Graphene, Gold nanorod, Catalytic performance