Effect of sensitization by quantum-sized CdS on photoacoustic and photoelectrochemical current spectra of porous TiO2 electrodes

We report the optical absorption characteristics of highly porous, polycrystalline ${\mathrm{TiO}}_2$ electrodes, on which CdS nanocrystal particles are adsorbed, measured by the photoacoustic (PA) method together with the photoelectrochemical (PEC) current characteristics. The PA spectrum shifts to the lower photon energy region and the intensity below the band-gap energy of ${\mathrm{TiO}}_2$ increases rapidly with an increase in the number of CdS coating layers to 20 and becomes almost constant at more than 20 layers, indicating the possibility of the growth of CdS coated layers. The increase in PEC current intensity and a clear shift of spectra to the lower photon energy region are observed with the increased number of CdS coated layers, indicating the possibility of the transfer of photoexcited electrons of the CdS nanocrystals to the ${\mathrm{TiO}}_2$ conduction band. The PEC current spectrum shifts to the lower photon energy region and the intensity below the band-gap energy of ${\mathrm{TiO}}_2$ increases rapidly with an increase of in the number of CdS coating layers to 20 and decreases with a further increase in the number of layers coated to 40. Thus, nanocrystals of low-band-gap semiconductors are suitable for the sensitization of ${\mathrm{TiO}}_2$ PEC solar cells as well as dye sensitization.