(a) Column chart of ultrasonic treatment to SiO2 beads, PDA@SiO2 beads, sintered SiO2 beads, and C@SiO2 beads. (b) Compression test curves of SiO2 beads, PDA@SiO2 beads, sintered SiO2 beads, and C@SiO2 beads. (c) Cross-section and zoomed detailed images of C@SiO2 PC beads. Insert bar is 1 μm for (i) and 300 nm for (ii). (Red arrows are the positions of C granules.) (d, e) High-resolution XPS narrow scans of the N1s region of PDA (d) and py-PDA (e). N1s peak reveals graphitic N at 400.9 eV, pyrrolic N at 399.0 eV, and pyridinic N at 398.4 eV.