Fining Crystal Facet and Oxygen Vacancy Engineering of Bifunctional BiOBr Facilitates Excellent Photocatalytic Hydrogen Evolution and Phenol Hydroxylation

The most cost-effective and environmentally benign method of producing hydrogen fuel from water is solar-driven hydrogen production, using efficient photocatalysts with low-energy photons. Engineering such a photocatalyst remains highly challenging. In this work, BiOBr(001) and BiOBr(010) nanosheets were synthesized using the hydrothermal method. Oxygen vacancies (OVs) were engineered in the BiOBr(001) and BiOBr(010) nanosheets by using a UV light-driven strategy. BiOBr(001) with OVs outperforms BiOBr(010) with OVs in photocatalytic activity for hydrogen evolution, producing 219.33 μmol·g^-1 within 4 h. So far, this is the highest photocatalytic hydrogen evolution performance of bismuth-based photocatalysts without cocatalysts. The electrochemical study results indicate that OVs considerably lower the overpotential of the hydrogen evolution reaction (HER). Experimental results show that OVs can enhance the reduction ability of photogenerated electrons and extend the lifetime of carriers. Their presence significantly enhances charge carrier separation by breaking the local structural symmetry, as demonstrated by density functional theory. Additionally, phenol hydroxylation is greatly enhanced due to the strong H₂O decomposition ability of BiOBr(001) with OVs. OVs increase the carrier concentration of the (010) crystal facet, while theoretical calculations reveal that OVs greatly suppress the carrier recombination probability on the (001) crystal facet. Here, we propose that the "hydroxyl blocking effect" on the (001) crystal facet of BiOBr is responsible for the low HER and phenol hydroxylation activities. By optimizing hydroxyl adsorption free energy via OVs, the conversion rate of phenol hydroxylation over the (001) crystal facet with OVs can be achieved at 99.2%. This work offers the fundamental understanding required to improve the photocatalytic efficiency of BiOBr and related photocatalysts by introducing oxygen vacancies (OVs) and altering the crystal facets.