Enhanced Photoelectrochemical Water-Splitting Property on TiO₂ Nanotubes by Surface Chemical Modification and Wettability Control

We report a simple method to control the photoelectrochemical (PEC) water-splitting performance of TiO2 nanotube arrays (NTs) by surface chemical modification. Four types of modifier molecules with different surface energy and functional groups, including amine (−NH2), n-alkane (−CnH2n+1), perfluoroalkyl (−F), and polymer molecule (−polymer), were self-assembled to the surface of TiO2 NTs, which could change the surface chemical composition and wettability from superhydrophilicity to hydrophobicity. Interestingly, different from expected results, photoelectrochemical measurement results show that the n-octadecyltrichlorosilane-modified TiO2 nanotube arrays with a contact angle of about 134° present the highest PEC property with doubled photocurrent density and more negative onset potential. And the total PEC performance order of the monolayers-modified TiO2 NTs is (−CnH2n+1) > (−F) > (−NH2) > (−OH, pristine TiO2) > (−polymer), which is due to the molecular monolayers modification being able to suppress the recombination of photogenerated electrons and holes and facilitate water oxidation by regulating the interface electric double layer, whereas a thick polymer layer on the photoelectrode surface would affect the light absorbance and decrease the PEC performance. Further investigation indicates that the surface energy and wettability of the TiO2 photoelectrode adjusted by surface modification also have an important influence on the interface reaction of water oxidation and the adsorption/desorption of newly formed oxygen, which also provides a new method for controlling the surface photocatalytic reactions.