Study Of Interfacial Nanobubbles By Atomic Force Microscopy And Scanning Transmission Soft X-ray Microscopy

The study of interface is always an important field in frontier science. Lou et al. and Ishida et al. reported the first atomic force microscope(AFM) imaging of interfacial nanobubbles on different substrates in 2000, respectively. Since then, gas nanobubbles are gathering more and more attention from scientists. The astonishing long lifetime and large liquid-side contact angle are the most two mysterious properties of the nanobubbles.Various experimental and theoretical papers studying of nanobubbles have been published, but the explanation about the stability of nanobubble is still in debate. Many properties of nanobubbles including physical properties, the absorption to different substrates and inside structures need to be explored more deeply.Firstly, for the physical properties, we used PeakForce quantitative nanomechanics(PF-QNM) model of AFM to measure the stiffness of nanobubbles, and it was found that the stiffness of most nanobubbles is about 50-120 pN/nm. Furthermore, we measured the stiffness of bubble-like PDMS, and the preliminary data showed that the stiffness of PDMS is about 200-1200 pN/nm and larger than the nanobubble. We tried to distinguish nanobubbles from PDMS by using PF-QNM.Secondly, various previously studies demonstrated that the wettability of substrate has much influence on the interfacial property and stability of the nanobubbles. In the present study, we covered reduced graphene layers onto atomically flat Si3N4, mica, and HOPG substrates with different hydrophobicities, and we could simultaneously obtain information about the nanobubbles on two surfaces with different wettability ranges under the same conditions. We used PF-QNM to observe the nanobubbles produced by ethanol-water exchange on the substrate. Furthermore, the contact angles of nanobubbles were compared with those at macroscopic scale. It was found that the liquid-side contact angles were higher at more hydrophobic substrates. Moreover, we produced periodically nanopatterned substrates by electric beam lithography to explore the influence of substrates on the nanobubbles, and we observed the nanobubbles formed on them. It was found that the nanobubbles would be limited by the substrate if the periodically nanopattern is small enough.Finally, because of AFM lacking the ability of providing chemical information, we observed the adsorption of electrochemically generated oxygen nanobubbles in the solid/water interface by using scanning transmission soft X-ray microscopy(STXM). STXM has a high spatial resolution(30nm). The X-ray beam is focused to a spot on the sample as a probe, and it can give the images of the sample. More importantly, the distribution of different elements or elemental structures of the sample can be obtained by X-ray absorption near edge structure spectrum analysis. The experimental results showed that electrochemically method is a good way to generate nanobubbles on the silicon nitride window, and the nanobubbles are stable after a long time of soft X-ray irradiation.