(141a) Using Raman Spectroscopy to Probe the Distribution of Bio-Graphene Layers Grown on Iron and Cobalt Foils in Vacuum and Hydrogen Free Environment

Graphene, the thinnest 2D material, was synthesized on iron and cobalt metal foils via a novel patented method using biochar as a renewable carbon precursor. Biochar was placed on the metal foil and heated to 1000 °C in an electric quartz tube furnace. At this temperature, carbon atoms from biochar dissolve into the metal catalyst, forming carbides. Upon cooling, for Co, these carbides become unstable and precipitate, resulting in graphene formation at the eutectic temperature. XRD characterization confirmed the presence of graphene on both the iron and cobalt foils. The presence of uncovered regions and metastable iron carbide (Fe₃C) was also detected with multi-layer graphene on the iron foil as iron carbide is stable below the eutectoid temperature of iron (727 ^oC). Unlike the traditional CVD process, this process resulted in simultaneous growth of graphene on both sides of the metal foils. Raman spectroscopy is used to identify the number of graphene layers formed. Initially, graphene synthesis with biochar (B) left some uncovered areas on the cobalt foil with a mixture of both monolayer (2D/G > 1) and multi-layer graphene (2D/G < 1), while oxides of iron (Fe₂O₃ and Fe₃O₄) were observed with the presence of multi-layer graphene on iron foil. A number of experimental iterations were performed to enhance coverage. To increase the solid-solid contact to facilitate carbon dissolution, a stainless-steel plate (W) was placed on the biochar (B+W), resulting in enhanced graphene coverage for both metal catalysts. To investigate the effect of concentration gradient, biochar was placed on both sides of the metal foil (2B). Enhanced coverage of graphene was observed for both metal catalysts. For Fe, the uncovered regions were reduced with formation of multilayer graphene (solubility of C in Fe is 1.4 wt% at 1000 ^oC) and for Co, more monolayer formation was observed on both sides due to low carbon solubility (0.3 wt% at 1000 °C). Additional weight on cobalt foil with biochar on both sides (2B+W) further enhanced contact which resulted in formation of monolayer graphene on both sides. Performing a second run (R2) of graphene synthesis, by removing the used biochar and replacing it with a fresh batch of biochar on the already graphene coated samples, was also attempted. The results indicate that a combination of adding weight to improve solid-solid contact, coupled with biochar on both sides to control the concentration gradient together with a second run gave the best coverage, especially for Fe. This method has resulted in one of the best graphene coverage on Fe when compared to literature. The synthesized graphene on metal foils was then thoroughly analyzed using Raman spectroscopy at various points on the same sample to comprehend the graphene layer distribution and explain the effect of each experimental iteration. Furthermore, from literature, it is understood that multi-layer graphene acts as a good anti-corrosive coating. Since iron foil exhibited complete multi-layer graphene coverage, corrosion resistance property of graphene coating on iron foil was explored by electrochemical corrosion testing the samples in 3.5 wt% NaCl solution.