Cellulose and Graphene: A Raman Study on their Interface and Design of a Triboelectric Nanogenerator

Abstract

Cellulose nanocrystals and graphene have attracted huge interest from the research community in the past 5-10 years. Both materials have a variety of desirable and unique material properties. This makes them attractive material choices for composites and device design. Despite the vast quantities of literature using both of these materials, so far there has been no experimental study on the graphene/cellulose interface. The first study of this thesis uses a Raman monitored four-point bending test to study a model cellulose nanocrystal/graphene bilayer. Changes in peak positions of characteristic Raman bands for both graphene and cellulose have been used to quantify stress transfer at the interface. Using a novel approach, stress transfer at the interface has been deconvoluted into stress transfer efficiencies. A stress transfer efficiency of 66\% was determined at this interface. The cellulose nanocrystals are rod shaped. In this model cellulose/graphene composite, the thin film of cellulose nanocrystals are oriented using a magnetic field. The alignment of the cellulose nanocrystals in this film is shown to be concentric, centred on the middle of the film. This meant that there are some regions of the cellulose film which are oriented parallel and some perpendicular to the loading axis. It is shown that the cellulose film is stiffer when the cellulose is oriented parallel compared to when it is oriented perpendicular. By performing the Raman monitored deformation test in both parallel and perpendicular regions of the model composite, the effect of film stiffness on stress transfer efficiency can be isolated and compared. Unfortunately, conclusions from this test were limited as there was no statistical significance between the data sets. This is likely due to the very low sample size and large variability in the data sets. The data from these tests took an exceedingly long time to collect. A variation on the testing method using Raman mapping is suggested. This would allow a much larger sample size to be collected and hopefully more meaningful conclusions to be drawn. Finally, a cellulose/graphene triboelectric nanogenerator (TENG) is created. TENGs create electrical power from small, otherwise wasted mechanical energy and are used to drive various small sensors and devices. The development of TENGs is thought to be a vital step in powering future electronics. Here, the cellulose/graphene TENG is created using an all water based manufacturing technique which is suitable for roll to roll mass production. To create the TENG, a laminated graphene and cellulose structure is created on top of a flexible substrate, forming a `single-electrode mode TENG'. Several variations of this device are created. Some devices used unmodified cellulose nanocrystals whilst others used cellulose nanocrystals modified with amine (--NH$_2$) functional groups. The electrical response of these devices was recorded whilst they contacted with polytetrafluoroethylene. The devices with the modified cellulose showed an improvement in electrical response compared to their unmodified counterparts.