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 was studied using IR microscopy, and was attributed to physical adsorption due to the negligible peak shift of K. Photoelectrochemical 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 ∼2% 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.