Statistically meaningful grain size analysis of CVD graphene based on the photocatalytic oxidation of copper

CVD graphene is a polycrystalline material, with grains typically smaller than a few microns. Although it is known that graphene’s grain size affects its properties, not much attention has been paid to developing methods similar to those existing for metals to perform grain size analysis. Reported strategies thus far are either limited to extremely small areas or incapable of providing real grain size statistics. Here we report a novel, straightforward laboratory-bench method that uses TiO2 suspended in water and UV light to selectively and precisely oxidize copper through graphene defects, resulting in well-contrasted darker copper oxide lines on a light copper background. The graphene grains are then visible via optical microscopy, and the quality is unprecedented. This allows for the statistical measurement of graphene grain size distributions across large areas, in accordance with ASTM standards and automated image analysis. Raman spectroscopy shows further that graphene is only moderately damaged after the photocatalytic process. Measurements of three monolayer CVD graphene samples with grain sizes ranging from 1.4 to 3.9 microns are presented, showing Gaussian distributions. For the first time, an experiment can be used to show how graphene grain size distributions become wider for larger average grain sizes, as it is common for polycrystalline materials.