Inverse opal anatase/rutile TiO₂ multi-heterojunctions enable efficient photoelectrochemical water splitting

TiO₂ has garnered significant attention in the field of photoelectrochemical (PEC) water splitting due to its non-toxicity, cost-effectiveness, and exceptional photochemical stability. However, its practical efficiency in H₂ production is greatly hindered by inherent limitations such as low electron mobility, a short carrier diffusion length, and a wide optical band gap. Herein, we present a strategy of combining a crystal phase heterojunction and crystal facet heterojunction to enhance electron-hole separation efficiency in TiO₂. The crystal facet heterojunction of rutile TiO₂ extends the photogenerated electron lifetime by exploiting discontinuous band gaps and accelerates space charge separation. Moreover, the band alignment between rutile and anatase TiO₂ is favorable for electron transfer from rutile to anatase through a phase heterojunction. Consequently, the inverse opal anatase/rutile TiO₂ nanorod (IO-TiO₂/NRs-TiO₂) photoanode affords an excellent hydrogen production rate (682 μmol h^-1 g^-1), which is 1.6 times higher than that of an inverse opal anatase/rutile TiO₂ single heterojunction and 3 times higher than that of inverse opal anatase. This work provides valuable insights into the rational design of photoanodes with a 3D hierarchical structure.