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Chapter 7 – Application of Graphene composites: Raman Strain Sensors electrochemically exfoliated graphene (FLG-5 and FLG-20) again shows few-layers in nature (~5 layers), similar to the observations by Abdelkader et al.123 Table 7-1: Average number of layers, thickness, Raman 2D band position, FWHM(2D), 𝐼2𝐷⁄𝐼𝐺 and 𝐼𝐷⁄𝐼𝐺 of all types of graphene used for epoxy composite coatings Graphene type FLG-0.5 FLG-5 MC monolayer 〈𝑵〉 or thickness (nm) < 6 layers ~ 5 nm - Raman 2D band position (cm-1) 2672.2 ± 0.4 2668.4 ± 3.5 2645 ± 1.5 FWHM(2D) 75 ± 0.7 80 ± 3.2 25 ± 0.9 𝑰𝟐𝑫 𝑰𝑮 0.4 ± 0.02 0.42 ± 0.03 > 2 𝑰𝑫 𝑰𝑮 0.42 ± 0.03 0.3 ± 0.25 - (𝑰𝑫) 𝑰𝑮 𝒄𝒐𝒎𝒑 0.45 ± 0.08 0.42 ± 0.12 - FLG-0.2 < 6 layers 2663.9 ± 1.6 76 ± 0.3 0.43 ± 1.18 ± 0.01 0.22 1.02 ± 0.35 FLG-1 layers 6–7 2663.2 ± 4.7 83 ± 4.3 0.05 0.11 0.47 ± 0.33 ± 0.38 ± 0.24 FLG-20 ~ 5 nm 2674.5 ± 3.7 74 ± 2.6 0.4 ± 0.04 0.33 ± 0.08 0.4 ± 0.11 As mentioned before, the 2D band of the graphene is the most strain sensitive of all the Raman bands. Hence the 2D band was fitted with one Lorentzian function for comparison. Table 7-1 show the 2D band position of all the FLGs used. The 2D band position varied between 2662 – 2675 cm-1 with FWHM(2D) ranging between 75-85 cm-1, confirming the few-layer nature of the samples. Generally, for the sonication-based exfoliation process, the 𝐼𝐷⁄𝐼𝐺 ratio is normally due to the edges of the flakes (see section 5.2.3.2) as the flakes are smaller than the laser spot (~2 μm).54 However for electrochemically exfoliated flakes, which have large sizes than laser spot the 𝐼𝐷⁄𝐼𝐺 ratio suggests that there is some in-plane defects present. Abdelkader et al. suggested these defects could be caused due to gas evolution between the layers or fatigue caused by repeated intercalation/deintercalation during the production process.123 The 2D band position, FWHM(2D), 𝐼2𝐷⁄𝐼𝐺 ratio and 𝐼𝐷⁄𝐼𝐺 ratio of a reference monolayer is also given for comparison. The low intensity of the 2D band for FLG when compared to a monolayer graphene (Table 7-1) and high fluorescence from the polymer at low graphene loadings meant reliable data could not be obtained for Raman strain sensing measurements. 201PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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