Enhanced photosensitization of nanocrystalline TiO2 film with a sulfonic sensitizer by surface protonation

The interaction between an azo sensitizer containing an -SO3- anchoring group, 4-amino-5-hydroxy-6-azobenzene- 1,7-naphthyl disulfonic sodium (K), and nanocrystalline TiO2 film as studied using IR microscopy, and was attributed to physical adsorption due to the negligible peak shift of K. Photoelectro-chemical experiments were carried out to characterize the interface charge transfer. Maximum incident photon-to-current conversion efficiency (IPCE) was greatly increased from 7.7 to 43.7% by HCl pretreatment of TiO2 film. This strong photosensitization enhancement was assigned to surface protonation of the TiO2 film, which enables K anions to be closely attracted to the TiO2 nanocrystalline surface by electrostatic forces, thus facilitating interface charge transfer. An overall energy conversion efficiency of similar to2% was achieved for the sandwich-type photoelectrochemical cell constructed with K sensitized protonated TiO2 film, while this value was 0.37% for the untreated TiO2 film. Surface protonation provides a general solution to improving the photosensitization of TiO2 film with dyes containing SO3- anchoring groups.