Surface modification of anatase nanoparticles with fused ring salicylate-type ligands (3-hydroxy-2-naphthoic acids): a combined DFT and experimental study of optical propertiesElectronic supplementary information (ESI) available: Absorption spectra used for determining stability constants. Fluorescence emission intensity of modifiers vs. pH. Quenching of modifiers' emission with the addition of TiO2. Time-resolved fluorescence decay profiles in both presence and absence of TiO2. FTIR band assign

• Main Authors: Savi, Tatjana D, aponji, Zoran V, omor, Mirjana I, Nedeljkovi, Jovan M, Dramianin, Miroslav D, Nikoli, Marko G, Veljkovi, Duan, Zari, Sneana D, Jankovi, Ivana A
• Format: Article
• Language: English
• ISSN: 2040-3364; 2040-3372
• DOI: 10.1039/c3nr01277h

The surface modification of nanocrystalline TiO 2 particles (45 ) with salicylate-type ligands consisting of an extended aromatic ring system, specifically 3-hydroxy-2-naphthoic acid, 3,5-dihydroxy-2-naphthoic acid and 3,7-dihydroxy-2-naphthoic acid, was found to alter the optical properties of nanoparticles in a similar way to salicylic acid. The formation of the inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and a reduction in the band gap upon the increase in the electron delocalization when including an additional ring. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of a salicylate-type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From both absorption measurements in methanol/water = 90/10 solutions and steady-state quenching measurements of modifier fluorescence upon binding to TiO 2 in aqueous solutions, stability constants in the order of 10 3 M 1 have been determined at pH 2 and pH 3. Fluorescence lifetime measurements, in the presence and absence of colloidal TiO 2 nanoparticles, indicated that the fluorescence quenching process is primarily static quenching, thus proving the formation of a nonfluorescent CT complex. The binding structures were investigated by using FTIR spectroscopy. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain the vibrational frequencies of charge transfer complexes, and the calculated values were then compared with the experimental data. Modified TiO 2 nanoparticles have enhanced visible light absorption compared to bare TiO 2 .