Confined water layers in graphene oxide probed with spectroscopic ellipsometry

Abstract

The confinement of water in quasi two-dimensional layers is intriguing because its physical properties can be significantly different when compared to those of the bulk fluid. This work describes spectroscopic ellipsometry study of confined water layers trapped between sheets of graphene oxide at varied thermal annealing temperatures. The wavelength-dependent refractive index of graphene oxide changes abruptly with annealing temperatures for Tann ≈ 125–160 °C, and we demonstrate that these changes are primarily governed by the expulsion of trapped water. This expulsion is associated with the decrease of interlayer separation of graphene oxide sheets from 7.8 Å to 3.4 Å. Graphene oxide annealed at high temperatures lacks trapped water layers and robust estimates of refractive index can be obtained within a Lorentz oscillator model. The trends in oscillator parameters are extended to lower annealing temperatures, where trapped water is present, in order to estimate the refractive index of confined water, whose value is found to be enhanced as compared to that of bulk. Temperature-dependent ellipsometry data show anomalous changes in ellipsometric parameters over a wide temperature interval (−10 to 10 °C) about the ice-point and these may be attributed to possible phase transition(s) of confined water.

Effect of layered water structures on the anomalous transport through nanoscale graphene channels

“…The 2D pentagonal and square water structures shown between 2 and 6 layers have not been reported before, but they show some similarity to the structure of Ice III [47], which is a high temperature and high pressure phase of ice, however, our simulation temperature is fixed at 300 K. Additionally, pentagonal shape have also been found in 2D ice simulations by Ji Chen et al [48]. It is possible to further validate the discovery of the water density and structure in our system through experimental techniques, such as x-ray reflectivity [49] and spectroscopic ellipsometry [50]. Figure 4(a) shows the density distribution of oxygen atoms as a function of channel height (i.e.…”

Electric-field-induced structural changes in water confined between two graphene layers

“…Depending on the value of the electric field, the external pressure may cause melting or freezing of water 11 . Recently, it was proposed experimentally that confined water exists as a quasi-two-dimensional layer with different properties than those of bulk water 12,13 , though this observation was challenged later 14 . Graphene, the two-dimensional allotrope of carbon 15 , was used in a recent experiment to confine water 13 into monolayer, bilayer and trilayer.…”

AA-stacked bilayer square ice between graphene layers

“…Recently, it was found experimentally that confined water exists as a quasi two-dimensional layer with different properties than those of bulk water 7,8 . Graphene, the two-dimensional allotrope of carbon 9 , was used in a recent experiment to confine water 8 into monolayer, bilayer and three layers.…”