**3. Band structure and surface properties for photocatalytic water splitting**

Photocatalytic activity of water splitting reactions has been affected by various factors such as surface structure chemical functionality deactivating destructive surface state, band edge position, and extract of carriers to improve catalytic activity [32, 33]. In spite of surface area properties responsible for the photocatalytic activity, there is an adjustment between charge diffusion length and light absorption. An increase in surface area may lead to a decrease in photovoltage and an increase in the surface recombination process. Therefore, before surface modification, it is necessary to study the loss mechanism. It can be observed that sheet-like structures

exhibited higher light absorption capacity in comparison with spherical morphologies. The band gap shift towards lower energies with the reduction in crystallite size [34]. The most important example of this is BiVO4 with a narrow band gap of 2.4 eV with control and desirable structure and morphology which is mandatory for the photocatalytic activity [35, 36]. Photocatalytic activity if BiVO4 depends upon the facets which are exposed to irradiations [37]. Improved photocatalytic activity due to charge separation is reported for CdS/ZnO and CDs/TiO2 heterostructure [38–41]. The incorporation of cations and anions into UV-active materials turned into visible light-active materials.
