Fig. 1.

TEM/EELS analyses of surface shell growth on anatase TiO₂ and hydrogen nanobubble formation after UV light illumination. a Experimental setup of a fluidic TEM holder with function of in situ UV illumination (see details in Methods). Water-immersed TiO₂ NPs are injected into the liquid chamber of the fluidic holder for TEM imaging. The UV illumination area covers the whole liquid chamber, which is about 3 mm. b Photocatalysis experiments with different times of UV exposure. Each TEM image is recorded independently with fresh aqueous suspensions of TiO₂ NPs controlled by the fluidic holder. Red arrow indicates the gas bubble around the TiO₂ NPs. c Magnified views of the TEM images in b. After 12 h of UV illumination, a surface shell of 3.2 nm (indicated by a green arrow) covering the anatase TiO₂ NPs is observed. d Thickness of the surface shell on anatase TiO₂ vs. UV illumination time: experimental data (black squares) and fitted curve based on the KJMA equation²¹ (blue line). Growth of the surface shell abruptly decreases when the bubbles are visible at ~18 h (reddish area). e EELS spectra from the bubble (black line) and from the pure water region (dashed blue line). f EELS for the initial crystalline surface of TiO₂ (red curve), the surrounding water (blue curve), and the surface shell (black curve). Purple vertical lines highlight the pre-edge feature at ~527 eV, before the O-K edge in the spectrum of surface shell