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Figure 4: Morphology observations of CNT growth in the channels of CL-wood after 1 min Joule heating. (a, b) Low-magnification SEM images of CL-wood after Joule heating showing numerous stacked cup-structured CNTs grew in the vessels. (c) High-magnification SEM image of CNTs, indicating the “stacked cup” structure. (d) HRTEM and TEM (inset) images of CNTs, confirming the hollow core, highly crystalline structure of the inner wall and amorphous structure of the outer wall.