Study On Two-dimensional Water Confined In Graphite Oxide

Water is the most common substance on earth.In the 150K-235K temperature range,the supercooled water can not be prepared,so this temperature range was named "no man's land".The nature of water in "no man's land" attracted people's attention.It is well known that confined space can significantly reduce the solid-liquid melting point.The study of confined water is an important way to study supercooled water.Generally believed that if the size of space is less than 2 nm,and the water will not freeze at low temperature.In this paper,we use water confined in graphite oxide.Because of a variety of oxygen-containing functional groups in graphite oxide,the water can be easily confined in layers.The interlayer spacing of the graphite oxide increases with increasing water contents.Therefore,comparing to other rigid confined systems,we can examine the properties of confined water at different sizes.The details are as follows:(1)In this work,the modified Hummers method was used to synthesize the graphite oxide,and the dried graphite oxide was separately stored in different humidity environment to absorb different water contents.The microstructures of graphite oxide and aqueous samples were characterized by NMR,FTIR,BET and XRD.Through the XRD and BET experiments,it was found that there were two kinds of confined spaces in the graphite oxide.One is the two-dimensional confined space between the graphite oxide layers,and the other is the slit-like channel(inter-grain voids)In the dielectric experiment,the influence of the conductance is eliminated by adding an extremely thin layer of PTFE between the electrode and the sample,so that the relaxation in the extreme conductive background can be revealed.The addition fo PTFE did not change the position of the relaxation peak,which proved the reliability of this method.The properties of confined water in graphite oxide were studied by using the dielectric impedance spectroscopy.We found that there were two different relaxation peaks in the aqueous graphite oxide.The high temperature P2 peaks is attributed to the water confined in graphite oxide layers,the P1 peak is attributed to the water confined in inter-grain voids.We believe that the water molecules in the graphite oxide layers form four hydrogen bonds and that they must break three hydrogen bonds during their rotation around a hydrogen bond.The activation of the P2 process is close to the three hydrogen bonds energy.This work found that the two-dimensional water confined in the layers remained supercooled state,and this result provides more experimental results about the physical problems in "no man's land".(2)We prepared graphite oxide with different water content.The functional groups content and the layered structure were characterized by XRD,13C NMR and XPS.In the dielectric experiment,the method of adding an insulating electrode between the upper and lower electrodes,which can eliminate the influence of the conductivity of the sample in the experiment.The effect of adding the insulating electrode of the experimental results is compared.It is determined that the three relaxation processes are derived from water confined in graphite oxide.In the previous work,it has been found the two relaxation processes:the relaxations of water confined in interlayers and water confined in inter-grain voids.We first discovered an anomalous relaxation process.Comparing to the normal relaxation process,the relaxation time increased with increasing temperature.The XRD and thermogravimetric experiments show that this anomalous relaxation process occurs with desorption of the interlayer water in the sample.Thus,this anomalous relaxation process is also attributed to water confined between layers.When the number of the interlayer water is reduced,the water molecules in the interlayer are difficult to rotate,and the water molecules move slowly.The results of dielectric and 1H NMR results show that the movement of water between layers become faster with increasing water content.The water molecules interact with graphite oxide with forming hydrogen bonds directly.This work deepens our understanding of the nature of interface water.(3)We prepared graphite oxide with different degree of oxidation.We used 13C NMR and XPS experimental methods to characterize the content of the functional groups.In the dielectric experiment,the aqueous sample has a large proton conductivity.We found that the proton conductivity is related to epoxy.In the13C cross-ploarization NMR experiment with 13C-labeled graphite oxide,we found that the epoxy in the graphite oxide has a strong interaction with the water molecules confined in the layers,and then we believe that the proton conductivity of graphite oxide is related to the Grotthuss mechanism.By comparing the activation energy of conductive process and relaxation process,we believe that the two processes are relavant.At low temperatures,the hopping of proton depends on the rotation of the water molecules.This work reveals the microscopic mechanism of proton transfer in graphite oxide.