Polymorphic control in titanium dioxide particles

The hydrolysis–condensation reaction of TiO2 was adapted to the phase inversion temperature (PIT)-nano-emulsion method as a low energy approach to gain control over the size and phase purity of the resulting metal oxide particles. Three different PIT-nano-emulsion syntheses were designed, each one intended to isolate high purity rutile, anatase, and brookite phase particles. Three different emulsion systems were prepared, with a pH of either strongly acidic (H2O : HNO3, pH ∼0.5), moderately acidic (H2O : isopropanol, pH ∼4.5), or alkaline (H2O : NaOH, pH ∼12). PIT-nano-emulsion syntheses of the amorphous TiO2 particles were conducted under these conditions, resulting in average particle diameter distributions of ∼140 d nm (strongly acidic), ∼60 d nm (moderately acidic), and ∼460 d nm (alkaline). Different thermal treatments were performed on the amorphous particles obtained from the PIT-nano-emulsion syntheses. Raman spectroscopy and powder X-ray diffraction (PXRD) were employed to corroborate that the thermally treated particles under H2O : HNO3 (at 850 °C), H2O : NaOH (at 400 °C), and H2O : isopropanol (at 200 °C) yielded highly-pure rutile, anatase, and brookite phases, respectively. Herein, an experimental approach based on the PIT-nano-emulsion method is demonstrated to synthesize phase-controlled TiO2 particles with high purity employing fewer toxic compounds, reducing the quantity of starting materials, and with a minimum energy input, particularly for the almost elusive brookite phase.


Bulk Synthesis of Rutile Phase TiO 2
A mixture of H 2 O:HNO 3 was prepared separately by diluting 6 mL of nitric acid in 100 mL of nanopure water. In a glass container, 50mL of the H 2 O:HNO 3 solution were added and heated in a hot plate at 35°C for 15 min. In another container, the titanium (IV) precursor solution was prepared by mixing 15 mL of ethanol, 5 mL of glacial acetic acid and 5 mL of titanium (IV) isopropoxide. The titanium (IV) precursor solution was added to the H 2 O:HNO 3 mixture and stirred at 350 rpm. Here, the glass container was sealed with aluminum foil to prevent evaporation. The resulting mixture was heated at 70°C for 75 min.

Bulk Synthesis of Anatase Phase TiO 2
A mixture of H 2 O:NaOH was prepared separately by diluting 6 mL of NaOH 1.0 M in 100 mL of nanopure water. In a glass container, 50mL of the H 2 O:NaOH solution were added and heated in a hot plate at 35°C for 15 min. In a separate container, the titanium (IV) precursor solution was prepared by mixing 15 mL of ethanol, 5 mL of glacial acetic acid and 5 mL of titanium (IV) isopropoxide. The titanium (IV) precursor solution was added to the H 2 O:NaOH mixture and stirred at 350 rpm. Here, the glass container was sealed with aluminum foil to prevent evaporation. The resulting mixture was heated at 70°C for 75 min.

Bulk Synthesis of Brookite Phase TiO 2
For the brookite phase TiO 2 bulk synthesis as control, the experimental procedure reported by Mamakhel et al. was followed. 1

Particle Size Determination of the Bulk Material.
Samples were prepared by taking 50 µL aliquots of the resultant suspension presumed to contain the bulk amorphous TiO 2 . They were transferred in disposable polystyrene cuvettes (REF: 67.754, 10 x 10 x 45 mm, Sarsted, Germany) and diluted with nanopure water in a 1:40 ratio. The cuvettes containing the samples remained undisturbed near the Zetasizer for 30 min prior to the measurements. Afterwards, size measurements were performed after 2 min of sample equilibration inside the instrument at room temperature (25°C).
Tables S1.2.1-S1.2.3 summarize the DLS parameters and values for the three different bulk syntheses products in nanopure water. Figures S1.2.1-S1.2.3 depict the DLS spectra showing the particle size distribution of amorphous TiO 2 particles from the three respective bulk syntheses.

Micro-powder X-ray Diffraction of Bulk Material.
Figures S1.3.1 depict an overlay of the experimental powder pattern of the TiO 2 particles obtained through the bulk syntheses without thermal treatment, with the spectra of the three standards rutile (ICSD 165920) 2 , anatase (ICSD 154601) 3 , and brookite (ICSD 154605) 3 . Figure  S1.3.2 depict an overlay of the experimental powder pattern of the TiO 2 particles obtained through the bulk syntheses and thermally treated with the respective standards.

Phase Inversion Temperature (PIT) Determination
For the H 2 O:HNO 3 /Heptane and H 2 O:NaOH/Heptane emulsion systems, the PIT was determined as follows. The respective mixture (respective aqueous phase, heptane and BrijL4®) was homogenized using an IKA T10 Basic Ultra Turrax (IKA Works Inc., Wilmington, NC), for 30 sec at a speed of "4" (14,450 rpm equivalent). The vial was situated in a jacketed beaker, with a 20.3 cm (8") stainless steel RTD temperature probe (VWR®, VWR International). The conductivity of the emulsion was measured with a Fisherbrand Accumet BasicAB30 conductivity meter (Fisher Scientific UK, Loughborough, UK). The bath temperature was controlled with a Julabo F32-ME Refrigerated/Heating Circulator (JULABO GmbH, Seelbach, Germany). Both the vial and the bath contained magnetic stir bars stirring at 300 rpm using a VWR® Professional Hot Plate Stirrer (97042-714, VWR®, VWR International). The temperature of the emulsion was allowed to reach 2°C in the bath before starting the measurements. The temperature profile started at 2°C and ended at 37°C at a heating rate of 1°C/min. The conductivity of the respective mixture was recorded in 1-degree intervals. Figure  S2

Particle Size Distribution of Amorphous TiO 2 Particles
Samples were prepared by taking 50 µL aliquots of the supernatant from the respective PIT-nano-emulsion synthesis aqueous phase. They were transferred in disposable polystyrene cuvettes (REF: 67.754, 10 x 10 x 45 mm, Sarsted, Germany) and diluted with nanopure water in a 1:40 ratio. The cuvettes containing the samples remained undisturbed near the Zetasizer for 30 min prior to the measurements. Afterwards, size measurements were performed after 2 min of sample equilibration inside the instrument at room temperature (25°C).
Tables S3.1-S3.3 summarize the DLS parameters and values for the three different PITnano-emulsion syntheses products in nanopure water. Figures S3.1-S3.3 depict the DLS spectra showing the particle size distribution of amorphous TiO 2 nanoparticles from the three respective PIT-nano-emulsion syntheses.

Raman Vibration Spectroscopy
Figures S4.1-S4.3 depict an overlay of the experimental Raman spectra of the TiO 2 particles obtained through the PIT-nano-emulsion synthesis and thermally treated, with the spectra of the three standards (rutile, anatase and brookite). Figure S4.4 depict an overlay of the experimental Raman spectra of the TiO 2 particles obtained through the PIT-nano-emulsion method without being thermally treated.

Micro-powder X-ray Diffraction (PXRD)
Figures S5.1-S5.3 depict an overlay of the experimental powder pattern of the TiO 2 particles obtained through the PIT-nano-emulsion synthesis and thermally treated, with the spectra of the three standards rutile (ICSD 165920) 2 , anatase (ICSD 154601) 3 , and brookite (ICSD 154605) 3 . Figure S5.4. depict an overlay of the experimental powder pattern of the TiO 2 particles obtained through the PIT-nano-emulsion method without being thermally treated. Figure

Size Comparison of the Synthesized TiO 2 Particles by Different Methods
Table S7.1 summarizes the particle size of the experimental rutile, anatase and brookite phase TiO 2 particles obtained from the PIT-nano-emulsion syntheses, determined by scanning electron microscopy (SEM), DLS, as well as the resulting crystallite size determined the Debye-Scherrer equation. Abbreviations: 2θ (Bragg reflection, °), τ (crystallite size, nm), τ Average (average crystallite size, nm).