Impact of the Precursor on the Physicochemical Properties and Photoactivity of TiO2 Nanoparticles Produced in Supercritical CO2

Abstract

The synthesis of TiO₂ nanoparticles (NPs) in supercritical media has been reported over the last two decades. However, very few studies have compared the physicochemical characteristics and photoactivity of the TiO₂ powders produced from different precursors, and even fewer have investigated the effect of using different ratios of hydrolytic agent/precursor (HA/P) on the properties of the semiconductor. To bridge this knowledge gap, this research focuses on the synthesis and characterization of TiO₂ NPs obtained in a supercritical CO₂ medium from four different TiO₂ precursors, namely diisopropoxytitanium bis (acetylacetonate) (TDB), titanium (IV) isopropoxide (TIP), titanium (IV) butoxide (TBO), and titanium (IV) 2-ethylhexyloxide (TEO). Further, the effect of various HA/P ratios (10, 20, 30, and 40 mol/mol) when using ethanol as a hydrolytic agent has also been analyzed. Results obtained have shown that the physicochemical properties of the catalysts are not significantly affected by these variables, although some differences do exist between the synthesized materials and their catalytic performances. Specifically, photocatalysts obtained from TIP and TEO at the higher HA/P ratios (HA/P = 30 and HA/P = 40) led to higher CO₂ photoconversions (6.3-7 µmol·g^-1·h^-1, Apparent Quantum Efficiency < 0.1%), about three times higher than those attained with commercial TiO₂ P-25. These results have been imputed to the fact that these catalysts exhibit appropriate values of crystal size, surface area, light absorption, and charge transfer properties.

Keywords: CO2 photoreduction; TiO2; precursor; semiconductor nanoparticles synthesis; supercritical CO2.

Figure 1

Representation of the SC synthesis set-up.

Figure 2

Production yield of TiO₂ catalysts, with solid color columns representing the HA/P ratio average and the clear areas behind representing the precursor maximum.

Figure 3

SEM images of selected catalysts: (a) TDB-30, (b) TIP-30, (c) TBO-30, and (d) TEO-30.

Figure 4

TEM images of the selected catalysts: (a) TDB-30, (b) TIP-30, (c) TBO-30, (d) TEO-30, and (e) TEO-30 zoomed image to observe anatase fringes.

Figure 5

XRD diffractograms of the catalysts, indicating the anatase crystal planes: (a) diffractograms of TDB-30, TIP-30, TBO-30, and TEO-30; (b) diffractograms of the different AH/P ratios for TIP catalysts.

Figure 6

Crystallite sizes of all the catalysts synthesized. Solid color columns represent the HA/P ratio average and the clear areas behind represent the precursor maximum.

Figure 7

BET area of all the catalysts synthetized. Solid color columns represent the HA/P ratio average and the clear areas behind represent the precursor maximum. A commercial P-25 catalyst is also presented as a black dotted line for reference.

Figure 8

FTIR spectra of TiO₂ catalysts: (a) FTIR spectra of the different TIP AH/P ratios, (b) FTIR spectra of TDB-30, TIP-30, TBO-30, and TEO-30. In clear transparent colors, functional groups are represented. P-25 spectrum is also presented.

Figure 9

UV-vis DRS spectra of the catalysts: (a) UV-vis DRS spectra of the different TIP catalysts at AH/P ratios; (b) UV-vis DRS spectra of the selected catalysts of TDB, TIP, TBO, and TE. In both graphics, the spectrum of P-25 is presented (dotted lined).

Figure 10

Absorption thresholds of all the catalysts and HA/P ratios, with solid color columns representing the HA/P ratio average and the clear areas behind representing the precursor maximum. A commercial P-25 catalyst is also presented as a black dotted line for reference.

Figure 11

Band gap energies of all the catalysts and HA/P ratios, with solid color columns representing the HA/P ratio average and the clear areas behind representing the precursor maximum. A commercial P-25 catalyst is also presented as a black dotted line for reference.

Figure 12

EIS Nyquist plots of selected catalysts.

Figure 13

Normalized photocatalytic conversion, with solid color columns representing the HA/P ratio average and the clear areas behind representing the precursor maximum. Normalized conversion = 1 for commercial P-25 catalyst is also presented as a black dotted line for reference.