The morphologies and pore sizes of the nanoporous graphene samples were characterized by SEM (JIB-4600F, JEOL Ltd.) at an operation voltage of 15 kV. The structure of the nanoporous graphene was observed by a HRTEM (JEM-2100F, JEOL Ltd.) equipped with an image aberration corrector. Raman spectroscopy (Renishaw) with an excitation wavelength of 514 nm and a laser power of 5 mW was used to characterize the structure and crystallinity of the nanoporous graphene. The ID/IG ratios were used to present the defects in graphene. The shape of the 2D peak, I2D/IG ratios, and FWHM were used to estimate the layer number of graphene walls in the nanoporous structure. The tube sizes were measured by an ImageJ software package from SEM images. At least two SEM images for each sample were used to evaluate tube sizes. All measured tube sizes were transferred to a histogram chart, and the peak values of the size distributions were used as the average tube sizes (fig. S10). In addition to Raman measurements and TEM characterization, the wall thickness (i.e., number of layer) of graphene, particularly multilayered ones, was also estimated by the specific surface area and density of nanoporous graphene@Ni after CVD growth under different conditions. The specific surface area was determined by the BET method using a high-precision surface area analyzer (BELSORP-mini II, BEL Japan Inc.) at 77.0 K. The wall thickness of graphene was then determined by the density of nanoporous graphene and the specific surface area of the nanoporous structure according to the Supplementary Materials.

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