Extended Data Fig. 4: Formation of nanoparticles around a hydrogen bubble evolved in GLC and in a batch reactor outside TEM.

a, Sequence of TEM images acquired by in situ analysis, showing the nucleation and subsequent growth and dissolution of nanoparticles on the surface of a bubble, recorded under high EDR. These particles are thought to be generated via the condensation of Pd ions around a bubble as gas molecules push other chemicals away. b, In situ HR-TEM images and corresponding FFTs of the nanoparticles formed on the surface of the bubble in a. Both fcc and hcp nanoparticles were generated. Notably, the interplanar distances of the fcc particles are 3–4% larger on average than those of pure fcc Pd, indicating the formation of fcc PdH_x particles. The production of fcc particles around hydrogen bubbles (which form when the radiolysis products exceed their critical concentration⁹) is another piece of evidence for the key role of H in the formation of the hcp structure. These bubbles remove supersaturated H, resulting in a lower H/Pd condition favoring the formation of fcc PdH_x around the bubble. c, Ex situ HR-TEM images of Pd nanoparticles grown on the surface of carbon black in a batch reactor that utilizes an electron beam with a dose rate of 1.9 × 10⁻³ e⁻ Å⁻² s⁻¹, showing only fcc particles. d, Le Bail fitting results for the XRD pattern of the Pd/C synthesized in the batch reactor. The asterisks and the “v” mark on the peaks correspond to internal standards of Si powder (NIST SRM 640e) and graphite (PDF# 04-006-5764), respectively. e, Schematic illustration of electron-beam-assisted Pd nanoparticle synthesis using a large-scale batch reactor. Scale bars, 20 nm (a), 5 nm (b), and 2 nm (c). ZA, zone axis.