Eco-Friendly Colloidal Aqueous Sol-Gel Process for TiO2 Synthesis: The Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature

This work reviews an eco-friendly process for producing TiO2 via colloidal aqueous sol– gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal aqueous TiO2 are reviewed: the as-synthesized type obtained directly after synthesis, without any specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, obtained after a specific autoclave treatment. This eco-friendly process is based on the hydrolysis of a Ti precursor in excess of water, followed by the peptization of the precipitated TiO2. Compared to classical TiO2 synthesis, this method results in crystalline TiO2 nanoparticles without any thermal treatment and uses only small amounts of organic chemicals. Depending on the synthesis parameters, the three crystalline phases of TiO2 (anatase, brookite, and rutile) can be obtained. The morphology of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter 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 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-treated with thermal treatments and the commercial Evonik Aeroxide P25, which is produced by a high-temperature process. Indeed, as-prepared, the TiO2 photocatalysts present a high specific 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. Only a few works have explored these new properties, giving a lot of potential avenues for studying this eco-friendly TiO2 synthesis method for innovative implementations.


Introduction
Photocatalysis is a well-established process for the effective and sustainable removal of a large range of organic pollutants, both in liquid and gaseous media [1]. This phenomenon consists of a set of oxidation-reduction (redox) reactions between the organic compounds (pollutants) and the active species formed at the surface of an illuminated photo-catalyst (usually a photoactivable semiconductor solid). Generally, when the solid photocatalyst 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 at the surface of the photocatalyst, to produce hydroxyl ( • OH) and superoxide (O2 −• ) radicals. These radicals can attack organic molecules and induce their degradation in CO2 and H2O, if the degradation is complete [2]. Various semi-conductors can be used as photocatalysts, such as NiO [3], ZnO [4], 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]. TiO2 exists in three different crystallographic structures: anatase (tetragonal structure with a band gap of 3.2 eV), brookite (orthorhombic structure with a band gap >3.2 eV), and rutile (tetragonal structure with a band gap of 3.0 eV) [7]. The best phase for photocatalytic applications is anatase [7]. However, the use of TiO2 as a photocatalyst has two main limitations [7]: (i) the fast charge recombination, and (ii) the high band gap value which calls for UV light for activation. Therefore, the amount of energy required to activate anatase TiO2 is high. Indeed, its band gap width (3.2 eV) corresponds to light with a wavelength inferior or equal to 388 nm [7] and so, in the case of illumination by natural light, only the most energetic light will be used for activation, which corresponds to 5-8% of the solar spectrum [8]. To prevent these limitations, several studies have been conducted [9][10][11][12] to increase the recombination time and extend the activity towards the visible range. Most works consisted in modifying TiO2 materials by doping or modification with a large range of different elements, such as Ag [9], P [13], N [14], Fe [11,12], porphyrin [15,16], etc. Therefore, the synthesis process of TiO2 must be easily adjustable to incorporate such dopants/additives when needed, depending on the targeted application.
Several processes exist to produce TiO2 photocatalysts, the main methods being chemical or physical vapor deposition [17,18], aerosol process [19], microwave [20], reverse micelle [21], hydrothermal [22], and laser pyrolysis [23]. These processes often use severe synthesis conditions, such as high pressure, high-temperature, or complex protocols. Another possible synthesis pathway is the sol-gel method [24], which has proven to be effective for the synthesis of TiO2 in the form of powders or films, with control of the nanostructure and surface properties [25][26][27][28][29]. The sol-gel process is classified among "soft chemistry" protocols because reactions occur at low temperature and low pressure. The titanium precursor, usually an alkoxide, undergoes two main reactions: hydrolysis and condensation ((1)-(3) from Figure 2) [24,30,31]. The condensation gives the Ti-O-Ti network formation. By controlling the rate of the hydrolysis and condensation reactions, a liquid sol or a solid gel is obtained. In order to produce TiO2 by sol-gel processes, an organic solvent is often used. This organic solvent, such as 2-methoxyethanol, is able to complex the titanium precursor (for example, titanium tetraisopropoxide, TTIP, Ti-(OC3H7)4) to control its reactivity. A stoichiometric amount of water is added to avoid fast precipitation [24,31]. The material then undergoes drying and calcination steps to remove residual organic molecules and to crystallize amorphous TiO2 in anatase, brookite, or rutile phases [32]. In the last decade, attempts at reducing the use of large amounts of organic solvent have been heavily investigated, in order to develop greener syntheses. The use of water as the main solvent was made possible by the use of a peptizing agent. By definition, a peptizing agent (PA) is a substance that, even in small amounts, prevents the agglomeration/flocculation of particles and a decrease in viscosity through enhancing the dispersion in aqueous media [33]. The PA allows crystallization at low temperature, even if the titanium precursor has precipitated. The synthesis of high crystalline TiO2 nanoparticles, through colloidal aqueous sol-gel in presence of PA, has been successfully reported in the literature [34] and is the main subject of this review.
This synthesis path was first referenced at the end of the 1980s [35][36][37]. Water is present in a large excess compared to the Ti precursor, and peptizing agents are used to form small TiO2-crystalline nanoparticles from various Ti precursors at low temperature (<100 °C) [8,38,39], resulting in the formation of a crystalline colloid. Although it is seldom used in the development of TiO2 synthesis processes, since organic solvents are preferred to better control the Ti precursor reactivity, this preparation method presents a lot of advantages and fulfills the principles of green chemistry that are currently being promoted: (i) the synthesis conditions are soft as it is a sol-gel process; (ii) easy protocol with no risky conditions; (iii) low use of organic reagents, as water is the main solvent; and (iv) crystalline materials are obtained without thermal treatment. Additionally, this synthesis has other advantages, such as: (i) very stable colloids are obtained, allowing the elaboration of coatings very easily by classical deposition techniques (spray-, dip-, spin-, or bar-coating); (ii) protocol easily modified to introduce dopants or additives; and (iii) production at larger scale, up to 20 L.
The goal of this review is to evaluate the state of the art of the research into this not very well-known eco-friendly process for producing TiO2 via colloidal aqueous sol-gel synthesis, resulting in crystalline materials without a calcination step. A literature review allowed us to find about 115 articles making use of this synthesis process to produce TiO2 materials, spanning from 1987 to 2020. Figure 3 represents the year distribution of these 115 articles. The number of articles over the past 30 years was quite low, due to several reasons: (i) the hydrolysis of the Ti precursor is much easier to control in alcohol solvent and (ii) very fast in water, (iii) the use of water to replace organic solvents for greener processes is a quite recent requirement in chemical processes. Nevertheless, the development of this process has become more and more important over the last ten years. An increase of interest in this topic in the past ten years is clearly observed. Throughout this review article, the synthesis protocol will be detailed with a focus on the most important parameters, in order to template the resulting TiO2 material. Indeed, by changing synthesis parameters, the three different phases of TiO2 can be obtained, without any thermal treatments. Moreover, specific morphologies can also be produced. In some of the selected articles, thermal post-treatments (calcination or hydrothermal treatment) are applied to the as-synthesized materials, therefore their impact on the crystallinity and morphology of the resulting TiO2 materials will also be reviewed in this paper.
Finally, the photocatalytic properties of these aqueous TiO2 materials will be also reviewed and linked to their physico-chemical characteristics. In the end, new emerging applications will be highlighted.

Synthesis of TiO2 with PA in Water
The synthesis uses three main components: the Ti precursor, the peptizing agent, and water. Two operations will take place during the synthesis: the precipitation and the peptization. Indeed, usually the Ti precursor is very reactive on contact with water, resulting in its rapid hydrolysis and condensation. It produces a precipitate of mainly amorphous TiO2. Then, the addition of the peptizing agent will induce the peptization, i.e., the slow dissolution of the TiO2 precipitate and its crystallization into small TiO2 crystallites (<10 nm). Indeed, the introduction of peptizing agent modifies the pH of the solution and increases the solubility of the amorphous titania [39]. The heating of the solution further increases the dissolution of this amorphous TiO2 and accelerates the crystallization [40].
The high concentration of hydroxylated titanium leads to a rapid crystallization, with high nucleation rate [40]. Due to this rapid nucleation rate, metastable polymorphs (i.e., anatase and brookite phases) are favored. When the crystallization is slower, the stable rutile phase is produced [39,40]. Figure 4 presents the general scheme of the synthesis. Usually, the reaction medium can be heated up to 95 °C during peptization. The resulting colloids are very stable (up to years [41]) due to the surface charges of the nanoparticles and can be composed of different crystalline phases and morphologies, depending on the synthesis parameters. The parameters that can be varied are: the type and amount of peptizing agent, the temperature and duration of peptization, and the type of Ti precursor.
Numerous variants of these synthetic parameters have been collected and summarized in Table 1. In addition to the above-mentioned components, possible dopants, applied post-treatments, and shapings are also listed. From this summary, it appears clear that the most used Ti precursor is titanium isopropoxide (TTIP), used in 75 out of the 115 considered studies, due to its relatively low cost; while the peptizing agent is mainly nitric acid (in 71 out of 115 works). The reaction mixture is often heated to reduce the reaction time. When doping is performed, mainly metallic or nitrogen species are used, as they are the main dopants that are known to enhance TiO2 photoactivity. Each author tries to keep the synthesis protocol easy and eco-friendly by reducing the amount of additive/dopant used during the synthesis process. Some organic solvents can be added to stabilize the Ti precursor during the synthesis, but only in very small quantities (less than 10% in volume). With the obtained colloids, it is easy to produce materials with different shapes, such as coatings on various substrates, powders by just drying the colloids, or as colloids directly. The study of Douven et al. [42] refers to the possibility of easily synthesizing colloidal aqueous TiO2 at larger scale, up to 10 L batches. This shows the potential for scaling-up towards industrial scale.

Synthesis Parameters
Corresponding Parameters Collected in the Literature (Variants)

Crystallinity
One of the main advantages of this colloidal aqueous TiO2 synthesis method is to produce crystalline materials without any thermal treatment. Nevertheless, some studies performed post-synthetic hydrothermal and/or calcination steps in order to obtain specific physico-chemical properties. The following sections detail the crystalline properties obtained depending on these three possibilities: as-synthesized, after calcination, or hydrothermal treatments.

As-Synthesized Aqueous TiO2
As mentioned, after the synthesis, a stable TiO2 colloid in water is obtained. This suspension can be dried under ambient air or precipitated by a pH change to recover the assynthesized powder. This powder can be easily redispersed in acidic water [41]. In the majority of the reviewed studies, the powders are characterized by XRD in order to evaluate their crystalline phases.
Concerning the crystallinity, the peptizing agent seems to play a very important role. Indeed, the three different TiO2 phases, namely anatase/brookite/rutile, can be obtained by merely changing the amount of peptizing agent, its acid-basic character, or the nature of the counter ion [82]. In all these studies, the crystallite size remains in the same range, between 3 and 10 nm [47,78].

Acid Peptizing Agent
With the most used peptizing agent, HNO3, when it is used without any other additive, a mixture of anatase/brookite is often produced, [39,76,87,97,142]; with a higher proportion of anatase, as presented in Figure 5a. Only the peak at 30.8° is observed for brookite. An increase in the amount of HNO3 during peptization (from pH of 2 up to pH of 0.5) induces the formation of rutile phase, as show in [39,118,143]. A mixture of crystalline phases is often reported. When additives that cause a shift in pH value are used, the distribution can be modified. In the works of Burda et al. [133] and Chen et al. [95], only anatase is produced when amine is added with HNO3 at the beginning of the synthesis.
With HCl, which is the second most common peptizing agent used, anatase or anatase/brookite is also mainly reported [39,69,121,135,138,139]. A mixture of anatase/rutile is also produced when the amount of HCl increases [104,107,139]. At very high concentration, such as a Ti/H + molar ratio of 0.08, rutile alone is even observed [108]. Moreover, when different types of acids are used in the same concentration, different phase distributions can be obtained. As examples, Vinogradov et al. [87] used a Ti/H + ratio of 0.5, and obtained anatase/brookite mixtures with HNO3 or HCl, while only anatase was produced with acetic acid, and an anatase/Ti sulfate mixture with H2SO4. This suggests that the counter ion (Bronsted conjugate base) also plays an important role in the preferential crystalline phase formed [87]. In Kanna et al. [107], with a similar acid amount (not specified), anatase is produced with H2SO4 and H3PO4, and an anatase/rutile mixture with HNO3, HCl, or acetic acid. With carboxylic acids such as acetic, lactic, malonic, or citric acid, anatase is the main phase reported [57,74,76,87,125,133,138], as shown in Figure 5b. Only Kanna et al. [107] report an anatase/rutile mixture.
Globally, when inorganic acid is used, anatase and/or brookite phases are produced, but when the amount of acid leads to a pH smaller than 1, rutile phase is also produced. With organic acids, only anatase phase is formed. The different distributions of phases will impact the resulting surface area. Indeed, anatase and brookite phases lead to a higher specific surface area than rutile [63].

Basic Peptizing Agent
The basic peptization is far less common (about 8 out of 115 references considered in this review), but some studies still reference it. In Mashid et al. [38], NH4OH is used to synthesize anatase/brookite mixture, as illustrated in Figure 6 for pH 8 and 9. Similarly, with NaOH anatase/brookite is reported in Mutuma et al. [70]. In Yu et al. [91], only anatase is observed with NH4OH peptizing agent at high pH. Zhang et al. [113] report an anatase/rutile mixture with NH4OH at neutral pH. To conclude, the nature (acidic or basic) of the peptizing agent and the amount used will impact the resulting phases, but the type of phase is difficult to predict. As shown in the above paragraphs, both acidic or basic PA lead to crystalline TiO2 materials. It is worth mentioning that the resulting TiO2 materials are not 100% crystalline, as is the case when thermal treatments such as calcination or hydrothermal treatment (next paragraphs) are applied. Nevertheless, it was shown [63,65] that the crystalline fraction can be quite high (up to 85-90%) and that this fraction can be optimized by playing with the synthesis parameters, such as the time of reaction or the amount of PA.

Aqueous TiO2 after a Calcination Treatment
Even if a crystalline material is already obtained right after the synthesis, often composed of two or three TiO2 crystalline phases, as shown in the previous section, a large range of studies perform a calcination step to further crystallize the TiO2 materials, also leading to an increase in the crystallite sizes. When the calcination temperature is high (>600 °C), rutile is often produced, as it is the most stable phase at high-temperature, as represented in Figure 7  For temperatures below T <200 °C and pressure lower than 2 GPa the preferential crystalline phase is anatase, for calcinations in the same range of temperatures but with pressures higher than 2 GPa, the preferred crystalline phase formed is srilankite. On the other hand, for calcination performed at a temperature higher than 600 °C a preferential rutile phase is normally observed, independent of the applied pressure, Figure 7. Additionally, a phase anatase-rutile transition is often observed around 500 °C.
The phase transition from brookite to anatase or rutile has been less studied and no phase diagram is found in the literature. Nevertheless, some authors claimed that brookite evolves to anatase then rutile when the calcination temperature increases, [145,146], while others claim than brookite evolves directly to rutile [147,148].
In the considered studies, the temperature of calcination varies between 200 and 1000 °C. In all of these cases, the crystallite size increases, from 3-10 nm in the as-synthesized TiO2 materials, to a range of 20-100 nm, depending on the calcination temperature [45,46,86,149]. Obviously, the higher the temperature, the higher the obtained size.

Calcination after Acidic Peptization
In Borlaf et al. [93], a HNO3 peptized TiO2 colloid is calcined between 200 and 1000 °C, and the crystalline phases are compared at various temperatures. As-synthesized, the TiO2 material is composed of an anatase/brookite mixture, whose crystallite size increases, while keeping the same crystalline mixture until 500 °C. From 600 °C to 800 °C, the mixture is composed of anatase/rutile, with the proportion of rutile increasing with the temperature. From 800 to 1000 °C, only rutile is present. This is illustrated in Figure 8.

Basic Peptization Followed by Calcination
The same trends are globally observed in the case of the basic peptizers, even if these are less studied: an increase in anatase or anatase/brookite content is observed until a calcination temperature around 500-700 °C [52,58,70,91], then rutile becomes the main phase, as illustrated in Figure 9 [58,70,91].

Aqueous TiO2 after Hydrothermal Treatment
This treatment consists in placing the precursor suspension in water in an autoclave under pressure, and heated at a controlled temperature. Similarly to calcination, a hydrothermal treatment allows the increase of the crystallinity of the as-synthesized samples thanks to the Ostwald ripening mechanism [50]. The temperature of such a treatment is usually between 170 and 240 °C. The crystallite size increases compared to the as-synthesized TiO2 crystallite, in the range of 5 to 70 nm. When the treatment is very long (i.e., several days), a phase change may occur towards rutile (thermodynamically the most stable). A calcination step can be also applied after the hydrothermal treatment, and this will further increase the crystallite size of the phase present after the hydrothermal treatment [55,90,103,109,140,150], until the temperature of anatase-to-rutile transition is reached, where only rutile crystallites continue to grow [55,150]. For both types of peptizers, acid or basic, similar evolutions are observed. In [50,54,59,90,103,116,140,151], the hydrothermal treatment allows the increase of the crystallite size of the crystalline phase present in the as-synthesized sample. An increase of the duration, or temperature, of the hydrothermal treatment leads to larger crystallite size [50,55]. An as-synthesized anatase phase can also be converted into the rutile phase if the temperature or duration is sufficient, as illustrated in Figure 10, while an assynthesized anatase/brookite mixture is converted to rutile phase after hydrothermal treatment at 200 °C or 240 °C for 2 h.

Morphology
Besides the crystallite formation at low temperature, colloidal aqueous TiO2 synthesis allows the production of specific morphologies, depending on the synthesis conditions and the post-treatments applied. The following sections detail the TiO2 morphologies obtained, depending on the same three synthetic steps: as-synthesized, and after calcination or hydrothermal treatments. The morphology is linked to the crystalline phase produced. The morphology depends on the crystalline phases produced during the synthesis. Indeed, anatase and brookite phases mainly lead to spherical nanoparticles, while rutile gives rod-like nanoparticles [104].
A particularity of this synthesis method using peptization is that the crystallite size and the nanoparticle size are the same. Indeed, it was shown in many studies [8,38,41,60,61,63,66,80] that one particle is made of one crystallite, thanks to comparisons made between XRD (crystallite size estimated by Scherrer formula) and TEM imaging.

Morphology of As-Synthesized Aqueous TiO2
As-synthesized TiO2 materials are stable colloids that are composed of nanoparticles in the range of 3-10 nm [96,100]. For the materials composed of anatase or an anatase/brookite mixture, all studies report similar spherical nanoparticles below 10 nm, as shown in Figure 11a as an illustrative example [61]. When rutile phase is present, the morphology of rutile crystallites corresponds to nanorods, as depicted in Figure 11b [104]. Therefore, two main morphologies are observed, depending on the crystalline phases. The effect of PA on the final morphology of TiO2 will depend on the crystalline phase that is formed during the synthesis. Indeed, when anatase and/or brookite phases are formed, spherical nanoparticles are produced. Basic or acidic PA can lead to anatase/brookite phases, and thus basic or acidic PA can lead to spherical nanoparticles. When organic acid PA is used, spherical nanoparticles are produced because only anatase phase is formed. When rutile is produced, a nanorod morphology is obtained and, globally, it is when a large amount of acidic PA is used that this is the case. Therefore, in conclusion, it is difficult to state that one type of PA (acidic or basic) will produce a specific type of morphology, but it is rather linked to the resulting crystalline phase.

Morphology of Aqueous TiO2 after Calcination Treatment
As explained above, calcination permits further crystallizing the as-synthesized TiO2 materials, yielding an increase in the crystallite size. Therefore, as for the as-synthesized materials, two morphologies (sphere [88] and nanorod [73]) are observed depending on the crystalline phases, but the size range of the nanoparticles is larger than the as-synthesized (10-70 nm vs. 2-10 nm). Figure 12 presents the spherical [43] and nanorod [73] morphologies obtained after calcination at 500 °C.

Morphology of Aqueous TiO2 after Hydrothermal Treatment
As for the calcination, the hydrothermal treatment allows the increase of the crystallite size (comprised between 10 and 80 nm), while keeping the morphology of the as-synthesized materials (sphere or nanorod) [83,152]. Figure 13 gives an example of spheres [50] and nanorods [141] obtained by hydrothermal treatment.

Doping and Additives
As mentioned in the introduction, the two intrinsic limitations of TiO2 as a photocatalyst are (i) the fast charge recombination, and (ii) the high band gap value, which calls for UV light for activation [7]. Therefore, the doping and/or modification of colloidal aqueous TiO2 are also described in the literature to prevent these limitations. Throughout the liter-ature, four main modification strategies of aqueous TiO2 were found: doping with (i) metallic or (ii) non-metallic species, (iii) a combination with other semiconductors, and (iv) sensitization with dye molecules.
The modification of TiO2 with metallic species introduces metallic ions or metallic nanoparticles into the material. Metallic ions can produce intermediate levels of energy between the valence and conduction bands of TiO2, leading to a reduction of the energy necessary for electron photoexcitation. As a consequence, near-visible light can activate the photocatalytic process. These metallic ions can also act as photoelectron-hole traps, increasing the recombination time and enhancing the electron-hole separation. Metallic nanoparticles dispersed in the TiO2 matrix also act as electron traps due to their conductive nature. The metallic species listed are Ag [84,99,149], Fe [42,61,73], Cu [8,61], Rh [93], Pd, Ca [43], Cr [61,66], Pt [51], Zn [8,61,124], Nd [110,111], Tb [132], Ce [44,109,120], Eu [117,126], and W [123].
The doping with non-metallic elements is usually conducted with N, P, or S, and can reduce the band gap by creating an intermediate band for the electrons between the conduction band and the valence band. This doping allows the use of less energetic light to activate TiO2. Here, we mainly found N-doping (around 5 mol%), due to the frequent use of HNO3 as a peptizing agent, even in the materials referenced as pure TiO2. Supplementary sources of N were also used: mainly amine as trimethylamine [63,95,127,133], urea [54,63], melamine [116], hydrazine [133], ethylene diamine [63,75], etc. Many studies reported photoactivity under near-visible range illumination (see Section 6). The combination with other semiconductors in heterojunction is also reported: with ZrO2 [65,67], g-C3N4 [135], SnO [131], and Bi2O3 [77]. This modification produces a heterojunction at the interface of the two materials, which enhances the electron-hole separation due to the difference in energy levels of the conduction and valence bands of the two photocatalysts.
The introduction of dyes is reported in Mahy et al. [16]. In this case, the grafting of the porphyrin molecule at the surface allows the TiO2 activation in the visible range, due to the transfer of electrons from the dye by its excitation under visible illumination [16]. One study reports the production of composites made of aqueous TiO2 with carbon nanotubes [56]. In this case, the role of the carbon materials is similar to the introduction of metallic nanoparticles. As a carbon nanotube is a conductive material, it can trap the photo-generated electrons and decrease the recombination process.

Photocatalytic Properties
It is shown in the above paragraphs that colloidal aqueous TiO2 synthesis can produce crystalline TiO2 materials with specific morphologies, even without any thermal treatment. These crystalline materials are mainly being used for pollutant degradation. This section will summarize the photocatalytic activity of these aqueous TiO2 materials identified in the literature. A fraction of the articles dealing with aqueous TiO2 do not explore its photocatalytic properties and are limited to the description of the physicochemical properties. This represents 47 out of 115 articles, but in 10 cases, another application is also explored (see Section 7). Table 2 lists the parameters of the photocatalytic experiments in the studies using assynthesized TiO2 materials. The most tested molecule as a model "pollutant" is methylene blue (MB) [95,126,127,133,143], but 16 other molecules, such as methyl orange [125], pnitrophenol [42], and rhodamine B [42,84,87], have also been tested, showing the versatility of this material. The majority of these "pollutants" are model molecules (dyes); photocatalytic degradations of real wastewater or mixed pollutant solutions are very rare. The pollutant concentration is kept low as the photocatalysis process is a finishing water treatment step to remove residual pollution if still present, for example, after a classical wastewater treatment plant. Concerning the illumination, the information is often not very complete. Indeed, sometimes the wavelength and/or the intensity are not given.

Photoactivity of As-Synthesized Aqueous TiO2
Globally, UV-A light or visible light (~350-500 nm range) is used in most of the cases, as it corresponds to the band gap of TiO2. The time of irradiation can vary from minutes [106,126] to hours [42,120], up to 24 h [42], and depends on the power of the lamp.
Various dopants or additives are added at the beginning of the reaction to increase the photodegradation and/or the adsorption spectrum. Classically, metallic dopants such as Ag [84,99] or Fe [42] are added to enhance the electron-hole separation. As explained above, N-doping allows the increase of the light absorption in the visible range, and thus increases the photoactivity in the visible range [63,127].
Different shapes of photocatalysts can be used: powder [106,126,138], film deposited on various substrates [97,119,135], or even fabric [74]. Numerous studies [42,65,95,133,138] compare their photocatalysts to the most famous commercial TiO2, Evonik Aeroxide P25, which is produced by high-temperature process. Usually, similar or better activities are obtained with the aqueous TiO2. A direct comparison between all studies is very complicated, as the experimental conditions are different from one paper to another. Indeed, the lamp, illumination duration, concentration of photocatalyst or pollutant, and type of pollutant are the major parameters which differ from study to study (Table 2). Nevertheless, the high specific surface area obtained with the aqueous sol-gel process is referred to in most studies as the main reason for the increased photocatalytic activity compared to Evonik P25 (250 m 2 g −1 for aqueous sol-gel samples vs. 50 m 2 g −1 for P25). Therefore, the specific structure made of small nanoparticles (<10 nm, see Figure 10 from [8]) highly dispersed in water medium seems to play the most important role in its photocatalytic properties for pollutant removal in water. Table 2 demonstrates that it is possible to obtain a very efficient TiO2 material with an eco-friendly and easy synthesis without any additional high-temperature treatment. Indeed, the anatase phase, which is known to be the most efficient photocatalytic phase of TiO2, due to its better charge separation efficiency, is easily produced. UV-4 h 100% Table 3 summarizes the parameters of the photocatalytic experiments for the studies using calcined aqueous TiO2 materials. The observations are similar to Section 6.1 above: numerous pollutants can be degraded (but mainly model pollutants are studied, such as methylene blue), several efficient dopants are used to increase photo-degradation, and the various experimental conditions do not allow a direct comparison of the results. Nevertheless, the photoactivity of the calcined materials does not seem to be better than the assynthesized materials. Indeed, similar degradation rates are obtained with similar illumination times (compare Table 3 vs. Table 2). Acetone (gas phase-400 ppm) 365 nm-60 min Only kinetic constants given Table 4 summarizes the parameters of the photocatalytic experiments for the studies using aqueous TiO2 materials after a hydrothermal treatment. As for the calcined TiO2 materials, the photoactivity does not seem to be improved compared to the as-synthesized materials (compare Table 4 vs. Table 2). In terms of photoactivity, it can be deduced that a thermal treatment (calcination or hydrothermal) is not necessary to obtain an efficient photocatalyst with this type of synthesis method. Indeed, before thermal treatment, crystalline materials are already present with a high specific surface area. The thermal treatment increases the crystallite size and allows a 100% crystalline material to be obtained, but reduces the specific surface area, hence it is not advantageous because photocatalysis occurs at the surface.

Photoactivity of Aqueous TiO2 after Hydrothermal Treatment
One study [151] tested the photo efficiency of their catalysts on real wastewater, where multiple pollutants were present as pharmaceutical products, pesticides, and various organic chemicals. This study showed the effectiveness of the TiO2 photocatalysts for the degradation of these molecules. X3B (100 mg/L) 400-800 nm-120 min 95%

Addition Features for Aqueous Sol-Gel TiO2
Some other studies used colloidal aqueous TiO2 materials in applications other than photocatalytic pollutant degradation. All these applications used the other properties of titania, such as its hydrophilicity, its high refractive index, or its semi-conducting property. In Alcober et al. [123], aqueous TiO2 material is utilized to produce photochromic coatings with tungsten doping. In Antonello et al. [139], high refractive index coatings are produced from aqueous TiO2 suspensions. In Bugakova et al. [94], TiO2 inks, based on aqueous TiO2 colloids, are used for applications derived from the inkjet printing of microstructures for electronic devices. In Haq et al. [48] and Lin et al. [153], aqueous TiO2 suspensions give adsorbent materials for heavy metals and dye adsorption. Indeed, as aqueous synthesis of TiO2 suspensions produces TiO2 nanoparticles, the specific surface area of these materials is high compared to titania obtained by high-temperature synthesis. In Hore et al. [50] and Kashyout et al. [55], aqueous TiO2 materials are used in solar cell fabrication. In Papiya et al. [72], a cathode catalyst for microbial fuel cells is produced with aqueous TiO2 materials. In Salahuddin et al. [79], aqueous TiO2 is mixed with PLA to design a nanocomposite system for Norfloxacin drug delivery. Hydrophilic surfaces are also produced with aqueous TiO2 [62,138]. The use of photocatalyst materials such as aqueous TiO2 can be also implemented in energy related fields, such as the production of H2 by photocatalyzed decomposition of water [154]. The possibility of integrating heterogeneous photocatalysis with electrochemical processes to exploit their synergistic actions can be also envisaged [155]. Numerous further studies can be imagined to explore fully the properties of this green TiO2 synthesis pathway.

Conclusions and Outlook
The aim of this review was to establish the state of the art of the research in the area of the little known eco-friendly process of producing TiO2 via colloidal aqueous sol-gel synthesis, resulting in a crystalline material without a calcination step. From 1987 to 2020, about 115 articles were found dealing with colloidal aqueous sol-gel TiO2 preparation, taking into account three types of aqueous TiO2: the as-synthesized type obtained directly after synthesis, without any specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, obtained after this specific autoclave treatment.
This eco-friendly process is based on the hydrolysis of a Ti precursor in excess of water, followed by the peptization of the precipitated TiO2. Compared to classical TiO2 synthesis, this colloidal aqueous sol-gel method results in crystalline TiO2 nanoparticles without a thermal treatment, and it is a green synthesis method because it uses small amounts of chemicals, water as a solvent, and a low temperature for crystallization. Moreover, some works have shown that this synthesis method can be easily upscaled to 20 L.
Depending on the synthesis parameters, the three crystalline phases of TiO2 (anatase, brookite, rutile) can be obtained. The morphology of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter is the peptizing agent. Indeed, depending on its acidic or basic character and also on its amount, it can modulate the crystallinity, and so, the morphology of the material. HNO3 seems to be the most versatile PA. Indeed, it allows obtaining the three different phases of TiO2 and the corresponding morphologies (nanosphere or nanorod) just by changing its quantity during the synthesis.
The exact mechanism of the TiO2 material formation and the exact influence of the PA on the resulting TiO2 materials needs deeper studies, to understand clearly the formation of the different crystalline phases and morphologies. For example, the use of insitu XRD or FTIR to probe the exact formation mechanism of PA-assisted sol-gel synthesis of TiO2 could be a path to explore. Moreover, machine learning and big data analysis will open a new avenue in this TiO2 material research. Indeed, they could help to find a correlation between the many different experimental parameters and their ability to produce highly crystalline TiO2.
Even if crystalline TiO2 materials are obtained after aqueous sol-gel synthesis, some studies apply a thermal post-treatment, calcination, or hydrothermal to further crystallize the materials. These treatments can also increase the crystallite size of the as-synthesized material and modify its morphology. Moreover, the surface area will decrease during the calcination due to particle growth with the phase change. Furthermore, the increase in the calcination temperature causes the particles to coalesce, creating tightly connected agglomerates, blocking the entry of N2 gas during the BET analysis.
The aqueous TiO2 photocatalysts are mainly used in various photocatalytic reactions for organic pollutant degradation. More than 20 different molecules have been reported to be degraded with these materials, but mainly model pollutants. Experiments on real wastewater are lacking in the literature for this type of material. The numerous experimental conditions make it difficult to compare the performance of catalysts. Nevertheless, the as-synthesized materials seem to have an equivalent photocatalytic efficiency to the photocatalysts post-treated with thermal treatments. Indeed, as-prepared, the TiO2 photocatalysts are crystalline and present a high specific surface area. Thermal treatments do not seem to be necessary from a photocatalytic point of view. Moreover, studies showed that aqueous TiO2 presents better photoactivity than commercial Evonik Aeroxide P25, which is produced by high-temperature process.
Emerging applications are also referenced, such as elaborating catalysts for fuel cells, nanocomposite drug delivery systems, or the inkjet printing of microstructures. As the development of alternative energy sources is very prominent in current research activities, the use of this kind of photocatalyst to produce H2 from the photocatalyzed decomposition of water also seems a promising path to explore. Moreover, the development of electrophotocatalytic devices for various applications, in water pollution treatment for example, will be realized in the next few years. However, only a few works have explored these other properties, giving a lot of potential avenues for studying this eco-friendly TiO2 synthesis method for innovative implementations. Data Availability Statement: All data were taken from the articles of the bibliography section.