(761c) A Rational Design of Different Synthesis Routes to Increase the Efficiency of Nanocrystalline Titania-Based Photocatalysts

Photocatalysis is an advanced oxidation process which is widely studied for the total mineralization of organic pollutants and the degradation of microorganisms by using natural or artificial light. In many applications, an activity as high as possible under ambient light (solar or artificial), that is mainly composed of visible light, is required.

One of the most frequently used photocatalyst is titanium dioxide, TiO₂, in the anatase crystallographic form. Its photocatalytic activity results from the injection of electrons e_cb^- into the conduction band, which leaves holes h_vb⁺ in the valence band, when exposed to UV light (λ < 380 nm). Both e_cb^- and h_vb⁺ speciescan migrate to the catalyst’s surface and react with oxygen and water to produce radical species such as O₂^– •superoxides and OH• which can promote the degradation of pollutants and/or microorganisms in the medium.

To be active beyond UV light, that is to additionally absorb visible photons, TiO₂ anatase has to be modified and different strategies can be used, separately or jointly: doping the TiO₂ lattice with metal ions or species such as phosphorus or nitrogen, dye photosensitization on the TiO₂ surface, deposition of noble metals, structural modification by mixing TiO₂ with other metal oxides, and synthesis of nano-crystalline TiO₂xerogels with a lower band gap.

The goal of this study is to present and compare different routes developed to obtain highly active TiO₂under both UV and visible light.

On the one hand, a non-aqueous method has been developed by introducing a dopant through the concomitant condensation of a functionalized precursor of that dopant and hydrolyzed titanium tetraisopropoxide (TTIP, Ti(OC₃H₇)₄). In this way, the dopant precursor is anchored to the TiO₂ skeleton during its growth which leads to a very homogeneous dispersion. For example, phosphorus doped TiO₂ samples have been prepared from the hydrolysis and co-condensation of diethyl‑[2-N-(aminoethyl)aminoethyl]phosphonate (EDAP) with TTIP in both 2-methoxyethanol and isopropanol. The materials obtained after drying contain Ti-O-P linkages resulting from hetero-condensation between EDAP and TTIP according to Raman and NMR spectroscopies [1]. Because they are mainly amorphous, no photocatalytic activity is observed for the degradation of 4-nitrophenol under halogen lamp. After a calcination at 650°C, porous anatase is partially produced resulting in a strong increase of the photocatalytic activity. A comparison with non-doped samples indicates that the presence of P in the anatase structure favours the activity through additional activation with the visible range of the light source (UV-Vis spectroscopy results). Interestingly, that presence facilitates the retention of small anatase nanocrystallites (8 nm from XRD) despite the high temperature treatment, which again favours a high activity.

The same synthesis method has been applied to the sensitization to visible light of TiO₂ xerogels with covalently linked porphyrins. For example, nickel (II) tetra(4-carboxyphenyl)porphyrin (TCPPNi) has been anchored to the TiO₂skeleton during its growth through co-condensation of hydrolyzed TTIP with the carboxy groups of TCPPNi. Since calcination is prohibited in that case since it would destroy the porphyrin, a drying at maximum 150°C and under vacuum during 10 days has enabled the production of samples containing anatase nanocrystallites (< 10 nm), in addition to amorphous titania, doped with TCPPNi and showing an improved activity for the degradation of 4-nitrophenol under halogen lamp [2].

On the other hand, an environmentally-friendly aqueous sol-gel method is under investigation to produce TiO₂ doped with metal cations (Cu²⁺, Ni²⁺, Zn²⁺, Pb²⁺) to again extend its activity to the visible range. It involves the initial production of a titania precipitate via the rapid hydrolysis of titanium tetraisopropoxide (TTIP) in the presence of acetic acid (HAc). After washing, small quantities of nitric acid (HNO₃) are then added under stirring to the precipitate in suspension to induce a repulsive electrostatic charge on the surface of the titania crystallites producing a well-dispersed nanoparticle sol (peptisation). Dopants are introduced in the form of nitrates at the beginning of the peptisation step. The translucent sols obtained are finally dried.

The latter approach yields photocatalysts exhibiting a remarkably high photocatalytic activity without requiring any calcination step. It has been found that, while produced at ambient temperature, the catalysts are exclusively composed of doped nanocrystallites of anatase with a size of 6-7 nm and exhibit a specific surface area varying from 184  to 275 m² g^-1.

[1] C. J. Bodson, S. D. Lambert, C. Alié, X. Cattoën, J.-P. Pirard, C. Bied, M. Wong Chi Man, B. Heinrichs, Microporous and Mesoporous Materials, 134, 157-164 (2010).

[2] L. Tasseroul, S. L. Pirard, S. D. Lambert, C. A. Páez, D. Poelman, J.-P. Pirard, B. Heinrichs, Chemical Engineering Journal, 191, 441-450 (2012).