Visible light sensitive mesoporous nanohybrids of lepidocrocite-like ferrititanate coupled to a charge transfer complex: Synthesis, characterization and photocatalytic degradation of NO

Abstract

The efficiency of photo-oxidation of pollutants catalysed by semiconductors is still limited for real-world applications due to several drawbacks, such as a) insufficient absorption of visible radiation, which predominates in solar spectrum, b) rapid free electron to hole recombination, c) small surface area, built from equilibrium crystallographic facets with low adsorption capacities and d) photo-corrosion. The present study disclosures new mesoporous heterostructures, built from exfoliated lepidocrocite-like ferrititanates and TiO 2 (anatase)-acetylacetone charge transfer complex, capable of reducing free electron-to-hole recombination rate through a robust charge separation and sensitive to the visible light spectrum. The synthesis route is based on soft-chemistry and low temperature calcination at 300°C. Two different partially pillarized heterostructures, denoted as HM-1 and HM-2, have been synthesized. It was observed that the heterostructure HM-1 was four times more active toward photocatalytic degradation of NO gas in comparison to the benchmark photocatalytic material P25. The lower activity of the heterostructure HM-2, comparable to that of P-25, was attributed to the high value of Urbach energy that indicates high number of defect sites within energy band-gap of the constituent semiconductor components. [Ti] anatase/[Ti] ferrititanate mol ratio might also play a role in photocatalytic efficiency.

The Influence of Calcination Temperature on Photocatalytic Activity of TiO2-Acetylacetone Charge Transfer Complex towards Degradation of NOx under Visible Light

“…X-ray powder diffraction (XRPD) patterns of TiO2-ACAC, TiO2-ACAC-300, TiO2-ACAC-400 and TiO2-ACAC-550 are presented in Figure 1 and demonstrate that all samples are crystalline. As previously reported by Habran et al [15] anatase xerogel (mean crystallite size of ~2.5 nm, as calculated by Le Bail method) has been formed through the sol-gel route followed by drying at 100 °C, without the need for additional calcination to crystallize xerogel [21]. On the other hand, Sannino et al [8,14] synthesized TiO2-ACAC xerogel in amorphous state.…”

“…On the other hand, ligand to metal charge transfer (LMCT) is based on the interaction between a semiconductor and a smaller organic molecule, for example, an oxygen-based bidentate, such as acetylacetone (ACAC) [14,15], dopamine, catechol or salicylic acid [11,16]. The mechanism is based on the direct injection of electrons from the HOMO of a bidentate molecule into the CB of a semiconductor [17].…”

“…The mechanism is based on the direct injection of electrons from the HOMO of a bidentate molecule into the CB of a semiconductor [17]. The LMCT mechanism is mostly reported for TiO 2 -based materials [6,15,18], although also documented for some other semiconductors, such as Mg 2 TiO 4 and BiOCl [7,19]. The LMCT complexes are capable to absorb visible light and to promote the formation of the superoxide anion radical •O 2 − under visible light irradiation [6].…”

“…It is worth noting that ACAC is originally proposed in sol-gel synthesis as a complexing agent to limit the rates of hydrolysis and condensation reactions and, therefore, to limit the size of the newly formed crystallites [20]. However, when used in excess it serves for CTC formation and promotes LMCT [8,14,15].…”

“…Previously, our research group [15] synthesized directly through the sol-gel method, followed by drying at 100 • C, nanocrystalline anatase (mean crystallite size of~2.5 nm) covered by ACAC. It was demonstrated that the as-prepared TiO 2 -ACAC nanocrystalline served as spacer for visible light sensitive mesoporous nanohybrids of lepidocrocite-like ferrititanate which was calcined only at 300 • C. TiO 2 -ACAC xerogel showed, under the visible light irradiation, photocatalytic activity for oxidative degradation of NO x higher than the one measured for Evonik P-25 TiO 2 powder.…”