Beyond Single Dopants: The Chemistry of Multi-Atom Doping in Hematite Photoanodes for Water Splitting

Despite its great potential for solar-driven hydrogen production, including its noncorrosive nature, suitable bandgap (1.9–2.2 eV), abundance, high theoretical efficiency (15.4%), and photochemical stability, hematite photoanodes face limitations such as poor conductivity, low charge separation efficiency, short hole diffusion length (2–4 nm), and high onset potential. To address these challenges, several strategies have been investigated, including nanostructuring, morphological tuning, compositing, and doping. Among these, doping has proven to be the most effective, owing to its relative simplicity and significant impact on key properties. Each dopant plays a distinct role: Ti, Sn, Zr, and Ta enhance conductivity and band structure; Al and Si improve stability and reduce recombination; while Mn and Co boost catalytic activity. Despite extensive studies on single-element doping, multielement (co)doping remains limited, particularly in understanding synergistic effects. In this perspective, we discuss how the limitations introduced by one dopant can be mitigated through the incorporation of another. For example, Ge- or Sn-doped hematite exhibits high formation energies, while Al doping induces significant lattice shrinkage. However, introducing Ti into such systems can simultaneously reduce the formation energy and minimize strain, making hematite a more stable and efficient photoanode. We strategically explore the chemistry of combining dopants, particularly metal–metal and metal–nonmetal pairs, to tackle multiple bottlenecks concurrently. This perspective highlights these emerging concepts as a scalable and rational approach to unlocking the full potential of hematite-based photoanodes for efficient solar water splitting.