Red-Shifted Absorptions of Cation-Defective and Surface-Functionalized Anatase with Enhanced Photoelectrochemical Properties

[Image: see text] Manipulating the atomic structure of semiconductors is a fine way to tune their properties. The rationalization of their modified properties is, however, particularly challenging as defects locally disrupt the long-range structural ordering, and a deeper effort is required to fully describe their structure. In this work, we investigated the photoelectrochemical properties of an anatase-type structure featuring a high content of titanium vacancies stabilized by dual-oxide substitution by fluoride and hydroxide anions. Such atomic modification induces a slight red-shift band gap energy of 0.08 eV as compared to pure TiO(2), which was assigned to changes in titanium–anion ionocovalent bonding. Under illumination, electron paramagnetic resonance spectroscopy revealed the formation of Ti(III) and O(2)(–) radicals which were not detected in defect-free TiO(2). Consequently, the modified anatase shows higher ability to oxidize water with lower electron–hole recombination rate. To further increase the photoelectrochemical properties, we subsequently modified the compound by a surface functionalization with N-methyl-2-pyrrolidone (NMP). This treatment further modifies the chemical composition, which results in a red shift of the band gap energy to 3.03 eV. Moreover, the interaction of the NMP electron-donating molecules with the surface induces an absorption band in the visible region with an estimated band gap energy of 2.25–2.50 eV. Under illumination, the resulting core–shell structure produces a high concentration of reduced Ti(III) and O(2)(–), suggesting an effective charge carrier separation which is confirmed by high photoelectrochemical properties. This work provides new opportunities to better understand the structural features that affect the photogenerated charge carriers.