Fe₃O₄ nanoparticles are good catalysts for, e.g., ammonia synthesis and the water-gas shift reaction. Catalytic efficiency varies depending on the surface orientation and molecular activity at adsorption sites, and thus clarifying surface structures of Fe₃O₄ nanoparticles at the atomic scale is crucial for a better understanding of catalytic reaction mechanisms. Although numerous studies reported the observation of nanoparticles using transmission electron microscopy [1], they only revealed shapes and sizes of nanoparticles, but not the surface structure. Because Fe₃O₄ nanoparticles are often functionalized by organic ligands such as oleic acid, direct observation of surface structures is challenging. In this study, we investigate, using scanning tunneling microscopy (STM), the effect of ultraviolet (UV) ozone treatment and annealing in ultra-high vacuum (UHV) on the removal of oleic acid from the surface of Fe₃O₄ nanoparticles. Because flat samples are essential for STM observations, we employed a Langmuir-trough to fabricate monolayers of Fe₃O₄ nanoparticles. The solution of Fe₃O₄ nanoparticles (Ocean NanoTech, LLC) was 100 times diluted with chloroform. The diluted solution was gently placed on the water subphase with a microsyringe and transferred to the Au (111) thin film on mica. The prepared sample was first treated by the UV ozone cleaner for 20 min. We then checked the overall sample morphology using amplitude-modulation atomic force microscopy (AFM) in ambient. As shown in Fig. 1(a), we confirmed that the nanoparticles existed as monolayers. The monolayer was composed of tiny islands (approximately 50-60 nm wide), which did not appear before the UV ozone treatment. It may indicate the removal of oleic acid. Next, we introduced the sample to an UHV-STM and conducted STM observations. We found Fe₃O₄ nanoparticles, but the probe tip was easily contaminated during the scan. This implies that oleic acid still remains, or air-gas molecules are adsorbed on the nanoparticle surfaces. We thus annealed the sample up to 510 ℃ in UHV until the base pressure was recovered. After the annealing procedure, we were able to scan more stably, and Fig. 1(b) shows an example of STM images. Since the diameters of nanoparticles are approximately 15 nm according to the company specification, the island shown in Fig. 1(b) is likely an aggregate of three nanoparticles. Although the Fe₃O₄ nanoparticles are supposed to exist as polyhedral structures [2], the shapes of nanoparticles varied. On the surface of nanoparticles, we found many protrusions separated by1-2 nm. The distance between protrusions is larger than the expected atomic arrangement of Fe₃O₄, but the density is smaller than that of oleic acid on the Fe₃O₄ (001) surface [1]. These differences may imply the removal of oleic acid, but there is a possibility that residual organic compounds, decomposed oleic acids, cause surface protrusions. Further verification is needed.

References

[1] Dreyer, A., et al. Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties. Nat. Mater. 15, 522-528 (2016).

[2] Meng, Y. et al. Prediction on morphologies and phase equilibrium diagram of iron oxides nanoparticles. Appl. Surf. Sci. 480, 478-486 (2019).