Surface modification of anatase nanoparticles with fused ring salicylate-type ligands (3-hydroxy-2-naphthoic acids): a combined DFT and experimental study of optical properties

Abstract

The surface modification of nanocrystalline TiO2 particles (45 Å) 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 TiO2 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 TiO2 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.

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“…2B). The determined molar ratio is in agreement with literature data concerning composition of CT complexes between surface Ti atoms and catecholate-or salicylate-type of ligands [26][27][28][29][30][31].…”

“…By plotting 1/A versus 1/[L], the straight line was obtained, and from the ratio of the intercept and the slope, Kb value was found to be 3.76×10 3 M -1 . The determined stability constant is of the same order of magnitude as the reported values for surface-modified colloidal TiO2 NPs with catecholate-and salicylate-type of ligands[26][27][28][29][30][31].The stoichiometric ratio between Tisurf atoms and ligand (AA) in the CT complex was checked by Job's method of continuous variation[40] assuming that only one type of complex is present in solution. According to Job's method, the stoichiometric ratio (n) is determined from the plot of the absorbance as a function of the mole fraction (x) of metal or ligand.…”

“…Consequently, surface Ti atoms are very reactive and their binding to electrondonating ligands leads to the formation of charge transfer (CT) complexes accompanied with a red absorption shift. So far, the CT complex formation has been reported for colloidal TiO2 NPs surface-modified with either catecholate-or salicylate-type of ligands [26][27][28][29][30][31]. Also, the appearance of absorption in the visible spectral region has been observed upon surface modification of colloidal TiO2 NPs with ascorbic acid (AA) [25,32].…”

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

“…Several papers have investigated CT complex formation between catecholate [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] and salicylate [27,[36][37][38]40,41] type ligands and colloidal TiO 2 nanoparticles (d $ 45 Å), where the binding of modifiers induces the surface reconstruction of the nanoparticles. Recently, combined theoretical (DFT) and experimental studies were performed, mainly on catecholate-type molecules as surface modifiers of rutile [32] and anatase [33][34][35] TiO 2 nanoparticles, reporting different adsorption geometries.…”

“…Recently, combined theoretical (DFT) and experimental studies were performed, mainly on catecholate-type molecules as surface modifiers of rutile [32] and anatase [33][34][35] TiO 2 nanoparticles, reporting different adsorption geometries. However, only a few papers have investigated the binding of salicylic acid [36][37][38]41]. Interfacial charge-transfer emission measurements and fluorescence anisotropy measurements [36] in catecholate-type molecule-TiO 2 nanoparticle complexes showed that the excitation is localized, with the charge transferred from the ligand to the localized Ti atom and not to the entire nanoparticle.…”