Anatase, Brookite, and Rutile Nanocrystals via Redox Reactions under Mild Hydrothermal Conditions:  Phase-Selective Synthesis and Physicochemical Properties

Abstract

Anatase, rutile, and especially brookite nanocrystals have been selectively synthesized in this work via a redox route under mild hydrothermal conditions (180 °C, 3 h), employing trichloride as the titanium source and ammonium peroxodisulfate (APS), hydrogen peroxide, nitric acid, or perchloric acid as the oxidant. Characterizations of the three pure phases were achieved by XRD, Raman spectroscopy, FTIR, TG, HRTEM, UV−vis, and BET. The use of APS consistently yields anatase, but the particle morphology can be tuned from wormhole-structured agglomerates to more dispersed nanocrystallites. The use of other oxidants yields almost identical results, and phase selection can be attained in this case by controlling the reactant concentration and solution pH. The three phases show their distinctive crystal shapes: rounded nanocrystals for anatase, nanoplates for brookite, and nanorods for rutile. Both the optical band gap (3.11 eV) and the indirect band gap (2.85 eV) of brookite were found to lie in between those of anatase and rutile. Under the same surface area of loaded TiO2, the brookite nanoplates exhibit the highest efficiency in the beaching of methyl orange solution under UV irradiation. The mechanism of phase selection was discussed based upon a systematic investigation into the effects of synthetic parameters on phase constituents of the hydrothermal products.

Brookite, the Least Known TiO2 Photocatalyst

“…Anyway, for the interpretation of the diffractograms it is necessary to take into account that the main (101) diffraction peak of anatase at 2θ = 25.28° overlaps with the (120) and (111) peaks of brookite at 2θ= 25.34° and 25.69°, respectively, so that apparently pure brookite samples can be a mixture of anatase and brookite. lower than this value [3]. This does not imply necessarily the presence of anatase since samples synthesized at low temperatures are often scarcely crystalline and the corrected ratio could be easily reached by calcination at relatively high temperatures.…”

“…Na + ions were considered to stabilize the lattice of brookite but Schwarzmann and Ognibeni [40] obtained well crystallized brookite by hydrothermal treatment of TiO 2 ·xH 2 O between 100 and 300 °C in the absence of Na + ions. Basic environments are not essential for the formation of brookite and this phase was also prepared by oxidation of an aqueous acidic TiCl 3 solution containing CH 3 COONa under a flux of air at 95 °C [41,42]. Moreover, brookite was synthesized by hydrothermal oxidation of Ti metal in a NaF solution at 200-500 °C [43].…”

“…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.…”

“…Li et al [3] selectively synthesized brookite nanocrystals via a redox route under mild hydrothermal conditions (180 °C, 3 h), employing TiCl 3 as the titanium source and H 2 O 2 as the oxidant. Pure brookite was attained by carefully controlling the reactant concentration (0.0625-0.075 M) and the solution pH (1.1-1.32).…”

Low-Temperature Sol-Gel Synthesis of Nitrogen-Doped Anatase/Brookite Biphasic Nanoparticles with High Surface Area and Visible-Light Performance

“…Both NA-145 and HA-165 are phase-pure anatase. From the XRD peak intensities [29], the brookite phase contents of sample NA-185 and NA-165 were calculated to be~10% and~6%, respectively. The crystal size was calculated by Scherrer equation (Table 1).…”