Site levels of VBM and CBM for OH:O=1 and OH:O=2 graphene oxide with different coverage rate: (a), (b), (c), (d), and (e) represented C36O8H4, C24O8H4, and C24O12H6 with OH:O=1, C20O6H4 and C16O6H4 with OH:O=2, respectively. The dot lines are standard water redox potentials. The reference potential is the vacuum level. Our results not only explain the recent experimental observations that graphene oxide is a promising two-dimensional material for visible-light photocatalysis but can be very helpful in designing the optimal composition for higher performance. Display omitted ► Show GO is a promising material for visible-light-driven photocatalyst. ► Explain recent observations. ► Helpful in designing GO. To elucidate the usage of graphene oxide (GO) as a photocatalysis material, we have studied the effect of epoxy and hydroxyl functionalization on the electronic structure, work function, CBM/VBM position, and optical absorption spectra of GO using density functional theory calculations. By varying the coverage and relative ratio of the surface epoxy (O) and hydroxyl (OH) groups, both band gap and work function of the GO materials can be tuned to meet the requirement of photocatalyst. Interestingly, the electronic structures of GO materials with 40–50% (33–67%) coverage and OH:O ratio of 2:1 (1:1) are suitable for both reduction and oxidation reactions for water splitting. Among of these systems, the GO composition with 50% coverage and OH:O (1:1) ratio can be very promising materials for visible-light-driven photocatalyst. Our results not only explain the recent experimental observations about 2-D graphene oxide as promising visible-light-driven photocatalyst materials but can also be very helpful in designing the optimal composition for higher performance.