Self-hydrogenated shell promoting photocatalytic H2 evolution on anatase TiO2

Abstract

As one of the most important photocatalysts, TiO₂ has triggered broad interest and intensive studies for decades. Observation of the interfacial reactions between water and TiO₂ at microscopic scale can provide key insight into the mechanisms of photocatalytic processes. Currently, experimental methodologies for characterizing photocatalytic reactions of anatase TiO₂ are mostly confined to water vapor or single molecule chemistry. Here, we investigate the photocatalytic reaction of anatase TiO₂ nanoparticles in water using liquid environmental transmission electron microscopy. A self-hydrogenated shell is observed on the TiO₂ surface before the generation of hydrogen bubbles. First-principles calculations suggest that this shell is formed through subsurface diffusion of photo-reduced water protons generated at the aqueous TiO₂ interface, which promotes photocatalytic hydrogen evolution by reducing the activation barrier for H₂ (H-H bond) formation. Experiments confirm that the self-hydrogenated shell contains reduced titanium ions, and its thickness can increase to several nanometers with increasing UV illuminance.

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 equation (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

Fig. 2

Formation mechanism and reactivity of the hydrogenated TiO₂ shell. a Atomic structures of four key intermediates—initial state, sur3-1 state, sur3-2 state, and sub state—in the surface subsurface diffusion of a H atom. In the sur3-1 (sur3-2) state, atomic hydrogen is adsorbed on a surface three-fold O with the hydrogen pointing toward the vacuum (toward the bulk). In the sub (or final) state the hydrogen is adsorbed at a subsurface O atom. The O atom is red, H is gray blue, and the pink-red sticks represent the TiO₂ system. Red arrows indicate the three steps (Steps 1–3) for the diffusion of H (marked by the green dot circle) into TiO₂. b Potential energy profile for the surface subsurface diffusion of atomic hydrogen. Energies are relative to the initial state, and ‘TS’ indicates transition states. Blue and green bars represent energies in the absence and presence of adsorbed water, respectively. c Schematic representation of the photocatalytic process: under UV light, the photoexcited electron–hole pairs can split water to form hydrogen and oxygen. d Modified photocatalytic process described in this work: the hydrogen atoms diffuse first into TiO₂ (blue arrow) to form the hydrogenated TiO₂ shell on the NP^, before they are desorbed into water to form the hydrogen bubble. e, f Energy barriers and the desorption energies for the process of H₂ formation on the TiO₂(101) surface or subsurface as a function of the H/O ratio σ in the inner TiO₂, respectively

Fig. 3

Ex situ observation of the recovery of the hydrogenated shell on TiO₂ NPs. a–d Low magnification (top row) and high-resolution (second row) TEM images of dried TiO₂ NPs after photocatalytic reaction with UV illumination in water for 0, 12, 16, and 24 h, respectively. Fourier transform (FT) patterns from the surface (third row) and internal bulk areas (fourth row) of the corresponding high-resolution TEM images in a–d, showing the TiO₂, Ti₂O₃, and TiO structures, respectively. e, f HAADF image for the dried TiO₂ NPs after photocatlytic reactions in water (for 16 h). The EELS were acquired from the surface (red dot in e and red spectrum in f), and internal bulk (green dot in e, and the green curve in f)

Fig. 4

In situ LETEM observation of the photocatalytic water splitting process on TiO₂/Pt and ex situ observation of the surface shell on TiO₂/Pt NPs. a Schematic diagrams showing the deposition process of Pt on the TiO₂ NP under UV light illumination for 4 h. b In situ LETEM observation of bubble evolution near the TiO₂/Pt NPs in water under UV illumination for different period of times. The recording time of each TEM image was controlled within 1 s with an electron dose rate of about 3 e⁻ · Å⁻² s⁻¹. c A surface layer was observed on the TiO₂/Pt NPs during the photocatalytic water splitting process. d Evolution of surface layer thickness on Pt- loaded TiO₂ with increasing UV illumination time. e EELS of the bubble (red curve) at ~13 eV shows that the bubble contains hydrogen. Comparison between the EELS of pristine TiO₂ (black curve) and surface layer on TiO₂/Pt (blue curve) shows that the valence state of titanium in the surface layer has been reduced, same as the ones in Fig. 1f. f HAADF image and g EDS map of a TiO₂/Pt NP, after UV photocatalytic reaction in water for 6 h. h HRTEM image of surface area far from Pt (A1 in g) and the corresponding FT pattern (inset) from the surface shell, which is identified as cubic TiO. i HRTEM image of surface area near a Pt nanodot (A2 in g) and the FT pattern for Pt nanodot (inset). j HAADF image and k EDS map of a TiO₂/Pt NP after UV light illumination in water for 12 h. l HRTEM image of the TiO₂/Pt NP in surface area far from the Pt co-catalyst (A3 in k) and the corresponding FT pattern identified as TiO (inset), showing that the structure of the surface shell on TiO₂ is cubic TiO with thickness up to 4.5 nm. m HRTEM image of the TiO₂/Pt NP in surface area near the Pt co-catalyst (A4 in k) and the FT pattern for Pt nanodot (inset)