Crystal defects have been introduced into inherently narrow-band-gap and non-toxic Ag₂Se quantum dots (QDs) via a facile and efficient thermal vibration approach during synthesis and studied by using electron holography and geometric phase analysis techniques. These crystal defects consequently demonstrated a solid possibility for tuning the optical band-gaps of Ag₂Se QDs, and thereby enhancing the visible-light-driven water splitting and hydrogen evolution performance of Ag₂Se QD-sensitized TiO₂ photocatalysts.