Self-Assembled Nitrogen Gas Hydrates Layers on Graphite Surfaces under Ambient Conditions

Ing-Shouh Hwang^1*, Chung-Kai Fang¹, Chih-Wen Yang¹

¹物理所, 中研院, Taipei, Taiwan

* Presenter:Ing-Shouh Hwang, email:ishwang@phys.sinica.edu.tw

With high-sensitivity atomic force microscopy (AFM), we identified a special type of stripe structures at the interface between water and highly oriented pyrolytic graphite (HOPG) at room temperature. These structures are difficult to be detected with conventional AFM techniques, such as the tapping mode or contact mode. It usually takes many minutes for nucleation of the stripe structures and the domains grow slowly over tens of minutes at the interface. Our early AFM study suggested that formation of these structures might be related to nitrogen molecules dissolved in water [1,2]. However, it is very unlikely that the weak interaction among nitrogen molecules can form any ordered structure at room temperature. Our further study of nucleation of nanobubbles at the water-HOPG interface suggests that the stripe structures might be interfacial gas hydrates [3]. These ordered structures also play a crucial role in the formation of nanobubbles as well as in pinning of nanobubbles from movement in their lateral position [3,4]. Our study suggests that the stripe structures form through complicated bonding arrangement of dissolved N2 and water molecules at the interface. We have been looking for methods to provide direct evidence to support this concept of interfacial gas hydrates. Recently, we find that the row-like structures survive after removal of water, and they even survive under ultra-high vacuum. We have conducted x-ray photoemission spectroscopy several times and consistently obtained strong intensity of oxygen and weak intensity of nitrogen. We have also performed thermal desorption experiments and obtain strong intensity of water molecules (mass 18) and weak intensity of nitrogen molecules (mass 28). These experiments support the concept of interfacial nitrogen gas hydrates for the stripe structures. The interfacial gas hydrate layers may also form on many solid surfaces exposed to ambient air or immersed in water.

[1] Y.-H. Lu, C.-W. Yang, and I.-S. Hwang, Langmuir 28, 12691–12695 (2012)
[2] Y.-H. Lu, C.-W. Yang, and I.-S. Hwang, Appl. Surf. Sci. 304, 56–64 (2014)
[3] C.-K. Fang, H.-C. Ko, C.-W. Yang , Y.-H. Lu, I.-S. Hwang, Sci. Rep. 6, 24651 (2016)
[4] Y.-H. Lu, C.-W. Yang , C.-K. Fang, H.-C. Ko, I.-S. Hwang, Sci. Rep. 4, 7189 (2014)

Keywords: gas hydrate, graphite, atomic force microscopy, water, x-ray photoemission spectroscopy