Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H2

Abstract

The production of hydrogen from water with semiconductor photocatalysts can be promoted by adding small amounts of metals to their surfaces. The resulting enhancement in photocatalytic activity is commonly attributed to a fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, a step that prevents deexcitation back to the ground electronic state. Here we provide experimental evidence that suggests an alternative pathway that does not involve electron transfer to the metal but requires it to act as a catalyst for the recombination of the hydrogen atoms made via the reduction of protons on the surface of the semiconductor instead.

Keywords: core-shell nanostructures; gold–titania; hydrogen production; photocatalysis; time-resolved fluorescence.

Fig. 1.

Schematic representation of the mechanisms (Upper) and electronic transitions (Lower) proposed to explain the role of metals (Au) in the photocatalytic splitting of water with semiconductors (TiO₂). (A) In the conventional model, the metal acts as an electron trap that physically separates the excited electron used for proton reduction from the oxidation step that occurs on the surface of the semiconductor. (B) Our alternative model proposes that H⁺ reduction occurs at semiconductor sites but that the resulting hydrogen atoms need to migrate to the metal to recombine and produce the final H₂ product.

Fig. 2.

Time-resolved fluorescence data, integrated over wavenumbers in the range of 530 ± 90 nm, from different TiO₂-P25–based photocatalysts: (A–C: three different time regimes) TiO₂-P25 (blue) and 1 wt% Au/TiO₂-P25 (red), (D) × wt% Au/TiO₂-P25, x = 0–10 (various colors), and (E) TiO₂-P25 (blue) and 1 wt% Pt/TiO₂-P25 (red). All samples display the same time-resolved fluorescence behavior, within experimental error, whether any metal is present on the surface of the titania or not.

Fig. 3.

(A) Schematic representation of the mechanism by which the addition of reducible ions to water promotes photocatalysis with pure titania according to our model. (B) Initial rates of O₂ photoproduction from Ce⁴⁺, Ag⁺, and IO₃⁻ solutions using pure TiO₂-P25 as the photocatalyst. The value obtained with 1 wt% Pt/TiO₂-P25 for the iodate solution is also provided for reference. Error bars indicate the range of values expected based on the estimation of experimental errors. (C) Rates for H₂ photoproduction from 10 vol% methanol solutions and (D) HD production via room temperature scrambling of H₂ + D₂ gas mixtures measured by using pure titania (green) and titania with added carbon (blue), NiO (purple), gold (brown), and platinum (red) nanoparticles. Schematic representations of the mechanisms and electronic transitions for the reaction with Carbon/TiO₂ (E) and NiO/TiO₂ (F) catalysts.

Fig. 4.

TEM images of Au@SiO₂ core-shell structures by themselves (A and B) and after dispersion on TiO₂-P25 (C and D). Images for two types of samples are shown, with different silica shell thicknesses: ∼7 (A and C) and ∼15 (B and D) nm. The thin-shell samples were also aged in a water:ethanol mixture for 1 d to induce additional gelling. (E) Visible-light absorption spectra for three Au@SiO₂/TiO₂-P25 samples [with thin (∼7 nm, purple and red); thick (∼15 nm, orange and brown); and thin/aged (green and blue) silica shells] in 10 vol% methanol:water solutions before (purple, orange, green) and after (red, brown, blue) radiation with 365-nm light. No significant shifts in plasmon resonance frequency are seen with any of these samples upon photon excitation, indicating that the gold nanoparticles are electrically insulated from the titania. (F) Rates for H₂ photoproduction from 10 vol% methanol solutions and (G) Rates for HD production by scrambling of H₂ + D₂ gas mixtures measured with pure titania (green) and the three Au@SiO₂/TiO₂-P25 samples, with thick (blue), thin (purple), and thin/aged (red) silica shells. The two rates follow similar trends, suggesting that H₂ production via photocatalysis depends on the accessibility of the gold surfaces to the H atoms generated on the titania surface.