Anatase nanoparticles surface modified with fused ring salicylate-type ligands (1-hydroxy-2-naphthoic acids): A combined DFT and experimental study

• Formation of the charge-transfer complexes results in a red shift of the TiO2 absorption.
• Extended aromatic ring systems reduce the effective bang gap.
• For the CT complexes formed stability constants in the order 103 M−1 were determined.
• Binding was found to be through bidentate binuclear-bridging complexes.
• Ligands interact with different active sites on the TiO2 surface that express energetic heterogeneity.

Sensitization of TiO2 crystals and nanoparticles with appropriately chosen organic molecules can lead to a significant shift of their absorption threshold from the UV to the visible, thus improving the absorption of the solar spectrum as well as the efficiency of photocatalytic and photovoltaic devices. Herein, the surface modification of nanocrystalline TiO2 particles (45 Å) with salicylate-type ligands consisting of an extended aromatic ring system, specifically 1-hydroxy-2-naphthoic acid and 1,4-dihydroxy-2-naphthoic acid, was found to alter the optical properties of nanoparticles in a similar way to salicylic acid. From both absorption measurements and steady-state quenching measurements of modifier fluorescence upon binding to TiO2 in methanol/water = 90/10 solutions, stability constants in the order of 103 M−1 have been determined at pH 2. Fluorescence lifetime measurements, in the presence and absence of colloidal TiO2 nanoparticles, indicated that the fluorescence quenching process is primarily static quenching, thus proving the formation of a nonfluorescent charge-transfer (CT) complex. The binding structures were investigated by using FTIR spectroscopy. Thermal stability of CT-complexes was investigated by using TPD analysis (TG/DTA/MS). 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 compared with the experimental data. 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. The formation of the inner-sphere 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.

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