Controllable and facile one-pot synthesis of high surface area amorphous, crystalline, and triphasic TiO2: catalytic and photocatalytic applications

Abstract

The present study reports a simple, template free and controllable one-step synthesis of a series of amorphous, crystalline, and triphasic TiO₂ nanoparticles utilizing titanium(IV) ethoxide as a molecular precursor. By reacting the titanium ethoxide in a mixture of ethanol, acetic acid, and water at reflux temperatures while varying the ratios of the reaction components, it was possible to obtain TiO₂ nanoparticles possessing high surface areas and pore volumes as well as diverse crystalline structures. The synthesized nanoparticles were characterized by FT-IR, XRD, SEM, TEM, TG, DTA, DTG, DRS, and argon sorption. X-ray diffraction and sorption analyses conclusively show that using an excess of ethanol resulted in the formation of an amorphous TiO₂ with a high surface area. Furthermore, an increase in the water content while slightly reducing the amount of acid but keeping ethanol still at excess resulted in the formation of a crystalline anatase phase, whereas the reaction in the absence of ethanol led to the formation of a triphasic material consisting of anatase (40.5%), rutile (25.5%), and brookite (34%). Under the optimized conditions, it was possible to produce amorphous and crystalline porous titanium dioxide with high surface areas of 350 and 310 m² g⁻¹ respectively at low temperatures without subsequent heat treatment, which is reflected in their catalytic properties. Investigation of the photocatalytic and catalytic abilities of the synthesized nanoparticles to degrade and to reduce methylene blue (MB), respectively, as well as reduction of nitroaromatic compounds in aqueous media revealed that the amorphous materials showed the best performance with degradation of up to 94–99% of MB within 10 minutes. The kinetic studies also revealed that all nanoparticles exhibited high catalytic activity for the reduction of MB within a short time (3–4 min) and showed good agreement with a pseudo-first-order reaction model. The derived rate constants for the catalytic reduction of 2-NP, 3-NP, 4-NP, 2-NA, and 3-NA (NP = nitrophenol and NA = nitroaniline) with amorphous TiO₂ nanoparticles (Ti-9), as the best catalyst, are 0.67 min⁻¹, 0.24 min⁻¹, 1.46 min⁻¹, 0.56 min⁻¹, and 0.21 min⁻¹, respectively. From the data, we can deduce that a high specific surface area, the occurrence of defects as well as the crystallinity of the materials play an essential role in the photocatalytic and catalytic performance.