Surface atomic structure can play a critical role in the reactivity of materials, because many physicochemical processes occur at surfaces. As an important semiconductor material, titanium dioxide (TiO2) crystals with tailored facets have opened up intensive research in surface chemistry, such as photocatalytic hydrogen generation. However, the convincing measurement and explanation of the crystal facets effect for molecular hydrogen formation in photocatalysis has remained elusive. One longstanding controversy is between the exposed {0 0 1} and {1 0 1} facets. Here we demonstrate, through a combination of state-of-the-art electron microscopy and quantum chemical calculations, that the crystal facets of TiO2 do not play the critical role in photoreactivity; the bare TiO2 semiconductors without co-catalysts are catalytically inactive, whereas the location and species of the co-catalysts essentially determine the capacity for the hydrogen evolution reaction. Our results provide a unified explanation for the disparate results in the previous literature and highlight a feasible route to fabricating efficient photocatalysts.