Under pressure

Abstract

The application of large pressures on a solid induces lattice compression. In the case of metal oxides, this process favours the shortening of metal–oxygen bonds and simultaneously increases the diffusion activation energy of atoms and ions. The team therefore developed a thermal annealing process for WO₃ that relies on the application of gigapascal-range pressure — 12,000 bar — in an electrically heated high-pressure chamber. A suite of spectroscopy techniques as well as electron microscopy analysis confirmed that, unlike the pristine material or WO₃ samples annealed under H₂ atmosphere — which feature OVs both at the surface and in the bulk— the high pressure-assisted thermal annealing method yields WO₃ samples with OVs located selectively at the surface. Remarkably, this arrangement of the OVs has a positive impact on different properties of WO₃ that are crucial for photocatalysis, such as its exciton binding energy, and under light irradiation it leads to an increase in the carrier density and photocurrent. As a result, the system shows an enhancement in performance during the photocatalytic activation of molecular oxygen, as demonstrated by a few test reactions such as the light-promoted production of H₂O₂ or the aerobic oxidative coupling of amines.

At this stage, the applicability of the approach to other conventional semiconductors was not yet demonstrated, although it appears plausible. Therefore, further investigation is required to reveal the actual scope of this method in the context of photocatalysis and in other areas where OVs are known to trigger interesting reactivity.