Self-assembled layers of Bi2S3 nanoparticles have been used to sensitize nanocrystalline titanium dioxide, replacing the ruthenium dye used in conventional dye-sensitized solar cells. We report band-edge tuning of the bismuth sulfide quantum dot particles by adsorption of sulfide ions. In the absence of sulfide ions, only smaller Bi2S3 particles with a large band gap inject electrons into the TiO2. Progressive addition of sulfide ions shifts the conduction band of the Bi2S3 upward, so that larger nanoparticles begin to inject and the long wavelength component of the photocurrent response increases. The observation suggests that band-edge tuning may form a useful part of strategies designed to exploit semiconductor quantum dots as sensitizers.