Generally, the band gap (equals to the optical gap reduced by the exciton binding energy (EBE)) of a semiconductor is a direct reflector of its light absorption capability, which should be smaller than 3 eV so that the visible part of the solar spectrum can be captured. As exhibited in Figure , the solar energy conversion efficiency of overall water splitting increases with increasing the wavelength of irradiation at different quantum efficiencies. Meanwhile, the valence band maximum (VBM) and the conduction band minimum (CBM) should be more negative and positive than the oxygen evolution reaction (OER) potential (E(O 2 /H 2 O), 5.67 eV vs. vacuum) and hydrogen evolution reaction (HER) potential (E(H + /H 2 ), 4.44 eV vs. vacuum) (Figure ), respectively, so that photon-induced OER and HER can occur. As a result, a potential 2D polymer photocatalyst should have a band gap in range of 1.5 -3.0 eV to meet the requirements of having sufficient light absorption ability and appropriate band edge alignments. Definitely, a narrowed band gap in this range results in higher absorption (Figure ), as this energy range covers the most intense part of the solar spectrum (at the expense of energy dissipation of high-energy photons). Besides, the positions of CBM and VBM at the same k-point or at different k-points leading to the formation of direct or indirect band gaps, respectively, can also affect light absorption and the generated excitons of 2D polymers. Typically, for semiconductors with indirect band gaps, optical absorption relies on phonon-assisted processes. However, the photogenerated electrons and holes have different momentum and the change of electron wavevector is required for the transition from CBM to VBM, which will suppress exciton annihilation and show longer exciton lifetime than that with direct band gaps , thereby promoting water splitting. The electronic structures of 2D polymers can be slightly adjusted by controlling the thickness, while applying out-of-plane electric field and external strain are also possible strategies . However, many 2D polymers exhibit large band gaps due to weak electronic interaction between building units, while the requirement for appropriate band edge alignments further excludes a number of 2D polymers from possible photocatalysts that can be used for overall water splitting.