ADVERTISEMENT

Vanishing Air-Bubble Layer And Ion Adsorption On Graphene In Aqueous Media

Potential applications of graphene in bioanalytic devices and functional composites have recently attracted wide attention. For these applications, graphene often needs to be exposed to, or in intimate contact with, aqueous systems containing ions and bio(macro)molecules.

Understanding the surface structure of graphene in aqueous media is, therefore, important. Several studies on the interactions between biomolecules and graphene and its derivatives in aqueous media have been reported. However, the methods employed in these studies provided limited structural information on the adsorbed molecular layer at the buried graphene-water interface.

ADVERTISEMENT

Synchrotron X-ray reflectivity (XRR) is a quantitative technique for unraveling the structural information at such buried interfaces.1,2 Our recent paper3 reported unprecedented results on the structure of the same batch of graphene samples in air, using XRR along with several complementary methods, further demonstrating the feasibility of XRR for probing the graphene interfacial structure. In this study,4 synchrotron XRR (European Synchrotron Radiation Facility beamline BM28; energy 14 keV) has been applied to investigate the CVD graphene on SiO2 (widely used by the research community for graphene properties and applications), when graphene was immersed in water and phosphate buffered saline (PBS) at 25-60 °C. Contact angle measurements have also been performed to assess the wettability of the graphene, and atomic force microscopy (AFM) imaging was attempted for complementary topographic information.

Fig. 1. a) Physical model used to fit the XRR curves of graphene in water includes a diffuse air-bubble layer atop a contaminant layer on graphene. b) The water contact angle (CA) of graphene before and after its submergence in Milli-Q water for 24 h. Figure republished with permission from Elsevier from https://doi.org/10.1016/j.carbon.2018.10.093

Fitting the scattering length density (SLD) to the XRR curves on graphene indicated the presence of a diffuse air-bubble layer (SLD ρd = 6.72×10-6 Å-2, thickness td = 84.9 Å, and roughness Ra,d = 31.9 Å) on top of graphene upon initial immersion in water (Fig. 1a). In contrast, AFM imaging provided inconclusive topographic information due to the intrinsic experimental difficulties, but the results were consistent with an inhomogeneous interface.

Interestingly, the diffuse air-bubble layer diminished after the graphene was submerged in water for 24 h at 25 °C. This is also evident from its enhanced wettability, with the water contact angle on graphene decreasing from 84.9±0.4° to 55.6±0.4° after submergence (Fig. 1b). Furthermore, an additional 10.1 Å nanolayer appeared atop graphene after soaking at room temperature. The thickness of this nanolayer increased to 11.8 Å as graphene was heated 60 °C in PBS buffer, and was retained upon cooling back to the room temperature. We attribute this nanolayer to ion adsorption on graphene from PBS (Fig. 1b), a process that was enhanced by heating. Such ion adsorption would manifest as a result of a series of environmental stimulations, such as temperature-enhanced ion mobility, possible formation of a silanol layer on the SiO2 substrate, and water structure disruption at higher temperatures at the graphene/water interface.

These unprecedented results point to the complex interfacial structure of graphene in contact with aqueous media, and possible structural evolution in response to the surroundings. Such information is relevant to bioanalytic and nanotechnological applications of graphene in which its structure at the interface between water and electrolyte solutions is an important consideration to the efficacy and functionality of the devices or the processes.

ADVERTISEMENT

These findings are described in the article entitled Graphene surface structure in aqueous media: Evidence for an air-bubble layer and ion adsorption, recently published in the journal Carbon.Acknowledgment: The XRR measurements were performed at XMaS beamline (BM28), the UK-CRG, European Synchrotron Radiation Facility (ESRF), 6 Rue Jules Horowitz, BP 220, 38043 Grenoble CEDEX 9, France. The project was partially supported by Household Care Analytical, Procter & Gamble Newcastle Innovation Centre Whitley Road, Longbenton, Newcastle NE12 9TS, UK.

References:

  1. Briscoe, W. H., Speranza, F., Li, P. X., Konovalov, O., Bouchenoire, L., van Stam, J., Klein, J., Jacobs, R. M. J. & Thomas, R. K. Synchrotron XRR study of soft nanofilms at the mica-water interface. Soft Matter 8, 5055-5068 (2012)
  2. Speranza, F., Pilkington, G. A., Dane, T. G., Cresswell, P. T., Li, P. X., Jacobs, R. M. J., Arnold, T., Bouchenoire, L., Thomas, R. K. & Briscoe, W. H. Quiescent bilayers at the mica-water interface. Soft Matter 9, 7028-7041 (2013)
  3. Zhou, L., Fox, L., Włodek, M., Islas, L., Slastanova, A., Robles, E., Bikondoa, O., Harniman, R., Fox N., Cattelan, M., Briscoe, W.H. “Surface structure of few layer graphene”, Carbon 136, 255-261 (2018)
  4. Zhou, L., Islas, L., Taylors, N., Robles, E., Bikondoa, O., Briscoe, W.H. “Graphene surface structure in aqueous media: Evidence for an air-bubble layer and ion adsorption”, Carbon 143, 97-105 (2019)

Comments

READ THIS NEXT

How Do Precipitation Error Characteristics Affect The Simulation Of Streamflow?

The hydrologic application of precipitation estimation from remote sensing is a promising way to assess streamflow variability and its downstream […]

Appreciating The Importance Of Upper Motor Neurons

When I first started working with upper motor neurons, I did not realize that the path would have such high […]

Superhydrophobic Nanotip And Nanopore Arrays: A Practical Substrate For Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a kind of powerful spectroscopic analytical technique which can detect the molecular arrangement, intermolecular interaction, […]

In The Future, The Best Solution To Cavities Might Be A Vaccine

The least favorite thing for most people is a visit to the dentist. Whether it is a simple checkup or […]

The Driving Forces Of Grain Growth And Densification During Sintering Evolution

Manufactured ceramics are one of the first technological achievements of humankind, and after thousands of years, although their spectrum of […]

Newborn Turtles Carry The Pollution History Of Their Mothers

Scientists detected a large number of chemical pollutants in freshly-laid eggs of hawksbill turtles along the Yucatán coast of Mexico. […]

Functions Of The Medulla Oblongata

The medulla oblongata is a cone-shaped neuronal mass in the brain located in the brain stem, directly below the pons and anterior […]

Science Trends is a popular source of science news and education around the world. We cover everything from solar power cell technology to climate change to cancer research. We help hundreds of thousands of people every month learn about the world we live in and the latest scientific breakthroughs. Want to know more?