A simplistic and low-cost method that dramatically improves the performance of solution-grown hematite photoanodes for solar-driven water splitting through incorporation of nanohybrid metal oxide overlayers was developed. By heating the α-FeO/SnO-TiO electrode in an inert atmosphere, such as argon or nitrogen, the photocurrent increased to over 2 mA/cm at 1.23 V versus a reversible hydrogen electrode, which is 10 times higher than that of pure hematite under 1 sun (100 mW/cm, AM 1.5G) light illumination. For the first time, we found a significant morphological difference between argon and nitrogen gas heat-treated hematite films and discussed the consequences for photoresponse. The origin for the enhancement, probed via theoretical modeling, stems from the facile incorporation of low formation energy dopants into the FeO layer at the interface of the metal oxide nanohybrid overlayer, which decreases recombination by increasing the electrical conductivity of FeO. These dopants diffuse from the overlayer into the α-FeO layer readily under inert gas heat treatment. This simple yet effective strategy could be applied to other dopants to increase hematite performance for solar energy conversion applications.