  Photocatalytic water splitting can produce hydrogen in an environmentally
friendly way and provide alternative energy sources to reduce global carbon
emissions. Recently, monolayer fullerene networks have been successfully
synthesized [Hou $\textit{et al., Nature}$ $\textbf{2022}$, 606, 507], offering
new material candidates for photocatalysis because of their large surface area
with abundant active sites, feasibility to be combined with other 2D materials
to form heterojunctions, and the C$_{60}$ cages for potential hydrogen storage.
However, efficient photocatalysts need a combination of a suitable band gap and
appropriate positions of the band edges with sufficient driving force for water
splitting. In this study, I employ semilocal density functional theory and
hybrid functional calculations to investigate the electronic structures of
monolayer fullerene networks. I find that only the weakly screened hybrid
functional, in combine with time-dependent Hartree-Fock calculations to include
the exciton binding energy, can reproduce the experimentally obtained optical
band gap of monolayer C$_{60}$. All the phases of monolayer fullerene networks
have suitable band gaps with high carrier mobility and appropriate band edges
to thermodynamically drive overall water splitting. In addition, the optical
properties of monolayer C$_{60}$ are studied, and different phases of fullerene
networks exhibit distinct absorption and recombination behavior, providing
unique advantages either as an electron acceptor or as an electron donor in
photocatalysis.
