To overcome the increasing demand of energy worldwide and global warming due to CO2 emissions from the use of traditional fuel sources, renewable and clean energy sources are in high demand. Solar energy is one of the important renewable energy sources since it can be converted into hydrogen fuel via water splitting. To produce highly efficient and low-cost H2 from the reduction of water and CO2, photocatalytic materials have been developed. Presently, the efficiency of H2 production using transition metal-oxide and non-metal oxide photocatalysts via water splitting is quite low. The main issues reported are low light absorption and poor charge separation. The reasons for these two issues are the large band gaps and band edge positions of the present photocatalysts used for H2 production. To produce H2 to a larger extent from the decomposition of water, the present photocatalysts have been modified by co-catalysts or dopants using different techniques, including the reduction of the band gap and adjustment of the morphology, band edge positions, crystallinity, surface structure, etc., such that these photocatalysts can absorb sufficient light in the visible-light region. This type of modified nanostructured photocatalysts (both oxide and non-oxide) can enhance the efficiency of H2 production via absorbing sufficient light in the visible-light region of the solar spectrum and improving the charge separation by suppressing charge recombination. In this regard, we reviewed both UV- and visible-light active nanostructured photocatalysts and modified photocatalysts reported in theoretical studies.
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