Highly efficient photoelectrochemical (PEC) water splitting is achieved by Ag doped hierarchical porous TiO2 nanoparticles in compositing with GQDs (TiO2:Ag-GQDs). The unique optical properties of VO2 (m), monoclinic phase of VO2, is a motivation to use it in PEC processes. To further improve the efficiency of TiO2:Ag-GQDs photoanode, it is coupled with undoped and doped VO2 (m) with earth alkali metals. This coupling strategy is used to engineer the interfacial and optical properties and charge separation/transport on the photoanodes for PEC water splitting. Hence, the efficiency of the photoanodes with structures of TiO2:Ag-GQDs/VO2 or (Mg, Ca, Sr, or Ba)-doped VO2 in PEC water splitting process is investigated. A remarkable photocurrent density of 1.5 mA/cm2 at 1.23 V vs. RHE is achieved for FLGs/TiO2:Ag-GQDs/VO2:Ba photoanode. This value is 20 times higher than that of reported previously for pristine TiO2 photoanode (0.07 mA/cm2 at 1.23 V vs RHE).

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