Confined water layers in graphene oxide probed with spectroscopic ellipsometry

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 T_ann ≈ 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.