The evolution of the Raman spectra of hydrogenated graphene was measured at different temperatures (
Fig. 2b). Prior studies have noted that the sp
2 graphene lattice (G peak, 1580 cm
−1) as well as related defects in the sp
2 lattice (D peak, 1350 cm
−1) can be identified by Raman spectroscopy [
25]. The intensity ratio of the D peak and the G peak (
ID/
IG) is an index for measuring the density of defects in graphene. As mentioned in the literature review, a benchmark of
ID/
IG = 2 is equivalent to ~10% atomic coverage of hydrogen [
33]. From 100 ℃ to 160 ℃, the
ID/
IG value generally increased with temperature (
Fig. 2c). However, the
ID/
IG value of HG-180 is smaller than HG-160. This tendency is consistent with that of the XRD results and confirms our previous explanation. HG-160 showed the highest
ID/
IG value at 1.61. Although this value does not reach the benchmark of 2, it still confirmed that HG-160 has a high density of defects due to the hydrogenation process. In
Fig. 2d, we present the Gaussian fitting 2D Raman band of the samples. As can be seen in HG-140, HG-160 and HG-180, the 2D band can be fitted to 4 peaks, located at 2658 cm
−1, 2688 cm
−1, 2706 cm
−1 and 2721 cm
−1 respectively, which were consistent with the reported results from L.M. Malard about Raman spectra for few-layer graphene [
37]. In his study, he demonstrated that the 2D band of the sample that has higher intensity of the low-frequency Gaussian fitting peak tends to have fewer layers. According to the observation of Gaussian fitting peak intensity in the samples, the number of layers of HG-160 was the fewest. When the number of layers is more than 6, the Raman peak of graphene coincides with the Raman peak of graphite, fitting two peaks at 2688 cm
−1 and 2721 cm
−1, and therefore the HG-100 and HG-120 is more than 6 layers [
37]. ICP is an invaluable instrument to carefully check the content of the ferromagnetic impurities in all our samples. In
Fig. 2e, the results of ICP showed that only less than 0.05 ppm of the ferromagnetic impurities (Fe, Ni, Co and Mn) could be detected. This result indicated that the content of ferromagnetic impurities is negligible, and the contribution of magnetic impurities can be ignored.