Fig. 4

Energy-dispersive X-ray spectroscopy profiles between twhole particlesofiles large particleso. a, b Scanning electron microscopic images (left panels) showing the “whole particle” and “large particle,” indicated by the spectrum 4 and 12 (spectrum numbers refer to the characteristic location of the analyzed point), respectively, and their respective atomic compositions analyzed by energy-dispersive X-ray spectroscopy (right panels). For the energy-dispersive X-ray spectroscopy (EDX) profile of “whole particles” and “large particles” (right panels), each peak represents a different chemical element and specially sulfur element (red arrow) was detected in the “whole particles.” Note that these representative results were taken from the data of five independent EDX analysis with three independent preparations of UFPs. Scale bars: a 5 μm, b 5 μm