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Eco-Friendly Colloidal Aqueous Sol-Gel Process for Tio2 Synthesis: the Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature

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Eco-Friendly Colloidal Aqueous Sol-Gel Process for Tio2 Synthesis: the Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature catalysts Review Eco-Friendly Colloidal Aqueous Sol-Gel Process for TiO2 Synthesis: The Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature Julien G. Mahy 1,* , Louise Lejeune 1, Tommy Haynes 1 , Stéphanie D. Lambert 2 , Raphael Henrique Marques Marcilli 3, Charles-André Fustin 3 and Sophie Hermans 1,* 1 Molecular Chemistry, Materials and Catalysis (MOST), Institute of Condensed Matter and Nanosciences (IMCN), Université Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium; [email protected] (L.L.); [email protected] (T.H.) 2 Department of Chemical Engineering—Nanomaterials, Catalysis, Electrochemistry, B6a, University of Liège, B-4000 Liège, Belgium; [email protected] 3 Bio and Soft Matter Division (BSMA), Institute of Condensed Matter and Nanosciences (IMCN), Université Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium; [email protected] (R.H.M.M.); [email protected] (C.-A.F.) * Correspondence: [email protected] (J.G.M.); [email protected] (S.H.); Tel.: +32-10-47-28-10 (S.H.) Abstract: This work reviews an eco-friendly process for producing TiO via colloidal aqueous sol– 2 gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal Citation: Mahy, J.G.; Lejeune, L.; aqueous TiO2 are reviewed: the as-synthesized type obtained directly after synthesis, without any Haynes, T.; Lambert, S.D.; specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, Marcilli, R.H.M.; Fustin, C.-A.; obtained after a specific autoclave treatment. This eco-friendly process is based on the hydrolysis of a Hermans, S. Eco-Friendly Colloidal Ti precursor in excess of water, followed by the peptization of the precipitated TiO2. Compared to Aqueous Sol-Gel Process for TiO2 classical TiO2 synthesis, this method results in crystalline TiO2 nanoparticles without any thermal Synthesis: The Peptization Method to treatment and uses only small amounts of organic chemicals. Depending on the synthesis parameters, Obtain Crystalline and Photoactive Materials at Low Temperature. the three crystalline phases of TiO2 (anatase, brookite, and rutile) can be obtained. The morphology Catalysts 2021, 11, 768. https:// of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter doi.org/10.3390/catal11070768 is the peptizing agent. Indeed, depending on its acidic or basic character and also on its amount, it can modulate the crystallinity and morphology of TiO2. Colloidal aqueous TiO2 photocatalysts Academic Editor: Ewa Kowalska are mainly being used in various photocatalytic reactions for organic pollutant degradation. The as- synthesized materials seem to have equivalent photocatalytic efficiency to the photocatalysts post- Received: 4 June 2021 treated with thermal treatments and the commercial Evonik Aeroxide P25, which is produced by Accepted: 21 June 2021 a high-temperature process. Indeed, as-prepared, the TiO2 photocatalysts present a high specific Published: 24 June 2021 surface area and crystalline phases. Emerging applications are also referenced, such as elaborating catalysts for fuel cells, nanocomposite drug delivery systems, or the inkjet printing of microstructures. Publisher’s Note: MDPI stays neutral Only a few works have explored these new properties, giving a lot of potential avenues for studying with regard to jurisdictional claims in this eco-friendly TiO2 synthesis method for innovative implementations. published maps and institutional affil- iations. Keywords: TiO2; photocatalysis; sol–gel synthesis; peptization; doping; pollutant degradation; mild temperature Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 1. Introduction This article is an open access article distributed under the terms and Photocatalysis is a well-established process for the effective and sustainable removal of conditions of the Creative Commons a large range of organic pollutants, both in liquid and gaseous media [1]. This phenomenon Attribution (CC BY) license (https:// consists of a set of oxidation-reduction (redox) reactions between the organic compounds creativecommons.org/licenses/by/ (pollutants) and the active species formed at the surface of an illuminated photocatalyst 4.0/). (usually a photoactivable semiconductor solid). Generally, when the solid photocatalyst is Catalysts 2021, 11, 768. https://doi.org/10.3390/catal11070768 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 29 Catalysts 2021, 11, 768 2 of 29 catalyst (usually a photoactivable semiconductor solid). Generally, when the solid photo- catalyst is illuminated (Figure 1), electrons from the valence band are promoted to the conduction band. This results in electron–hole pairs, which can react with O2 and H2O, adsorbed atilluminated the surface (Figure of the 1photocatalyst,), electrons from to produce the valence hydroxyl band are(●OH) promoted and superoxide to the conduction −● band. This results in electron–hole pairs, which can react with O and H O, adsorbed at the (O2 ) radicals. These radicals can attack organic molecules and induce their2 degradation2 surface of the photocatalyst, to produce hydroxyl (•OH) and superoxide (O −•) radicals. in CO2 and H2O, if the degradation is complete [2]. 2 These radicals can attack organic molecules and induce their degradation in CO2 and H2O, if the degradation is complete [2]. Figure 1. Schematic representation of photocatalytic TiO2 NP: photogenerated charges (electron and hole) upon absorption of radiation. Figure 1. SchematicVarious representation semi-conductors of photocatalytic can be TiO used2 NP: as photogenerated photocatalysts, charges such (electron as NiO and [3], ZnO [4], hole) upon absorption of radiation. CeO2 [5], MnO2 [6], or TiO2 [7]. The most widely used solid photocatalyst is TiO2 [7,8], which is a non-toxic and cheap semiconductor sensitive to UV radiation [8]. TiO exists in Various semi-conductors can be used as photocatalysts, such as NiO [3], ZnO [4], 2 three different crystallographic structures: anatase (tetragonal structure with a band gap of CeO2 [5], MnO2 [6], or TiO2 [7]. The most widely used solid photocatalyst is TiO2 [7,8], 3.2 eV), brookite (orthorhombic structure with a band gap >3.2 eV), and rutile (tetragonal which is a non-toxic and cheap semiconductor sensitive to UV radiation [8]. TiO2 exists in structure with a band gap of 3.0 eV) [7]. The best phase for photocatalytic applications three different crystallographic structures: anatase (tetragonal structure with a band gap is anatase [7]. However, the use of TiO2 as a photocatalyst has two main limitations [7]: of 3.2 eV), (i)brookite the fast (orthorhombic charge recombination, structure and with (ii) a band the high gap band >3.2 gapeV), valueand rutile which (tetrago- calls for UV light nal structure with a band gap of 3.0 eV) [7]. The best phase for photocatalytic applications for activation. Therefore, the amount of energy required to activate anatase TiO2 is high. is anatase [7].Indeed, However, its band the gap use width of TiO (3.22 as eV) a photocatalyst corresponds tohas light two with main a wavelengthlimitations [7]: inferior (i) or equal the fast chargeto 388 recombination, nm [7] and so, and in the (ii) case the ofhigh illumination band gap value by natural which light, calls only for UV the light most energetic for activation.light Therefore, will be used the foramount activation, of energy which required corresponds to activate to 5–8% anatase of theTiO2 solar is high. spectrum [8]. Indeed, itsTo band prevent gap thesewidthlimitations, (3.2 eV) corresponds several studies to light have with been a wavelength conducted [inferior9–12] to or increase the equal to 388recombination nm [7] and so, time in andthe extendcase of theillumination activity towards by natural the visible light, range.only the Most most works en- consisted ergetic light will be used for activation, which corresponds to 5–8% of the solar spectrum in modifying TiO2 materials by doping or modification with a large range of different [8]. To preventelements, these such limitations, as Ag [ 9several], P [13 ],studies N [14 ],have Fe [ 11been,12 ],conducted porphyrin [9–12] [15,16 to], etc.increase Therefore, the the recombinationsynthesis time process and of extend TiO2 must the activity be easily towards adjustable the to visible incorporate range. such Most dopants/additives works consisted inwhen modifying needed, TiO depending2 materials on theby doping targeted or application. modification with a large range of different elements,Several such processes as Ag [9], exist P [13], to produce N [14], TiOFe [11,12],2 photocatalysts, porphyrin the [15,16], main methodsetc. There- being chem- fore, the synthesisical or physical process vapor of TiO deposition2 must be [ 17easily,18], aerosoladjustable process to incorporate [19], microwave such do- [20], reverse pants/additivesmicelle when [21], needed, hydrothermal depending [22], on and the laser targeted pyrolysis application. [23]. These processes often use se- Severalvere processes synthesis exist conditions, to produce such TiO as high2 photocatalysts, pressure, high-temperature, the main methods or complex being protocols. chemical orAnother physical possible vapor synthesisdeposition pathway [17,18], aerosol is the sol–gel process method [19], microwave [24], which [20], has provenre- to be verse micelleeffective
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