Nonhydrolytic Synthesis of High-Quality Anisotropically Shaped Brookite TiO₂ Nanocrystals

Abstract

A surfactant-assisted nonaqueous strategy, relying on high-temperature aminolysis of titanium carboxylate complexes, has been developed to access anisotropically shaped TiO2 nanocrystals selectively trapped in the metastable brookite phase. Judicious temporal manipulation of precursor supply to the reaction mixture enables systematic tuning of the nanostructure geometric features over an exceptionally wide dimensional range (30-200 nm). Such degree of control is rationalized within the frame of a self-regulated phase-changing seed-catalyzed mechanism, in which homogeneous nucleation, on one side, and heterogeneous nucleation/growth processes, on the other side, are properly balanced while switching from the anatase to the brookite structures, respectively, in a continuous unidirectional crystal development regime. The time variation of the chemical potential for the monomer species in the solution, the size dependence of thermodynamic structural stability of the involved titania polymorphs, and the reduced activation barrier for brookite nucleation onto initially formed anatase seeds play decisive roles in the crystal-phase- and shape-tailored growth of titania nanostructures by the present approach.

Brookite, the Least Known TiO2 Photocatalyst

“…Inorganic and organic precursors such as TiCl 4 [21,24,[49][50][51][52][53][54][55][56][57][58], Ti(SO 4 ) 2 [18,[59][60][61][62], TiOSO 4 [63,64], TiCl 3 [3,19,[65][66][67][68], TiOCl 2 [16,69,70], titanate nanotubes [69,[71][72][73], titanium tetraisopropoxide [25,[74][75][76], titanium butoxide [77][78][79][80][81] and titanium tetraethoxide [82] have been employed.…”

“…Buonsanti et al [57] developed a surfactant-assisted non aqueous strategy to prepare anisotropic brookite nanocrystals with tunable geometric features. The syntheses relayed on the high temperature aminolysis of titanium carboxylate complexes obtained by heating mixtures of TiCl 4 , oleic acid and oleyl amine in 1-octadecene at 290 °C for 30 min under air-free conditions.…”

“…Recently, nanostructured brookite films were deposited by the matrix-assisted pulsed laser evaporation technique [115,116]. The target was a frozen toluene suspension of brookite nanorods covered with an oleate/oleyl ammine capping layer synthesized by surfactant-assisted aminolysis of titanium oleate complexes at 280 °C under air-free conditions [57].…”

Strategies for designing metal oxide nanostructures

“…This wet chemical synthetic method relies on organic ligands to passivate particle surfaces during growth to provide size control and colloid stabilization. Ultrafine TiO2 nanoparticles with sizes below 10 nm were also synthesized via this arrested precipitation method in high boiling point solvents such as TOPO, benzophenone, and even aqueous-contained solutions [65][66][67][68]. One typical example is the synthesis of brookite TiO2 nanocrystals via a surfactant-assisted nonaqueous strategy [67].…”

“…Ultrafine TiO2 nanoparticles with sizes below 10 nm were also synthesized via this arrested precipitation method in high boiling point solvents such as TOPO, benzophenone, and even aqueous-contained solutions [65][66][67][68]. One typical example is the synthesis of brookite TiO2 nanocrystals via a surfactant-assisted nonaqueous strategy [67]. In a general procedure, 3 g of 1-octadecene (ODE), 13-52 mmol of oleyl amine (OLAM), and 0.5-2 mmol of oleic acid (OLAC) were loaded into a three-neck flask and degassed at 120°C for 30 min, after which the mixture was cooled down to 50°C under flowing N2 gas.…”