Review of the sol–gel method in preparing nano TiO2 for advanced oxidation process

Cheng Chang; Saeed Rad; Lei Gan; Zitao Li; Junfeng Dai; Asfandyar Shahab

doi:10.1515/ntrev-2023-0150

Article Open Access

Review of the sol–gel method in preparing nano TiO₂ for advanced oxidation process

Cheng Chang , Saeed Rad , Lei Gan , Zitao Li , Junfeng Dai and Asfandyar Shahab

Published/Copyright: November 9, 2023

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From the journal Nanotechnology Reviews Volume 12 Issue 1

Abstract

Application of nano titanium dioxide (TiO₂) in various fields such as advanced oxidation process (AOP) has led to the development of its preparation technologies. The sol–gel process is a widely used chemical wet method for preparing nanoscale TiO₂ gels. This technique offers numerous advantages, such as the potential to produce large quantities of homogeneous materials with high purity, surface area, porosity, and reactivity, as well as being cost-effective, simple to implement, and capable of controlling the size and shape of the resulting particles. This review provides a comprehensive overview of the chemicals, reaction conditions, and procedures required for preparing nano TiO₂ using the sol–gel method. It covers the selection of necessary compounds, such as TiO₂ precursors, solvents, hydrolysis agents, and additives, along with their composition and sequences of adding, reaction order, and impact on the final product. Additionally, it provides detailed information on the routes of gel formation and ambient conditions, including temperature, humidity, stirring speed, injection rates of compounds, aging process, and storage conditions. This information serves as a basic reference for understanding the sol–gel process and the relative contribution rates of the influencing factors, which is essential for controlling the size, morphology, crystallinity, and other physicochemical properties of the resulting TiO₂ gel/powder for targeted applications.

Graphical abstract

Keywords: nano titanium dioxide; sol–gel method; TiO2 preparation

Abbreviations

AcAc: acetylacetone
DEA: diethanolamine
EDTA: ethylenediaminetetraacetic acid
HAD: hexadecylamine
HPC: hydroxypropyl cellulose
PEG: polyethylene glycol
PVA: polyvinyl alcohol
PVP: polyvinylpyrrolidone
TiCl₃: titanium trichloride
TiCl₄: titanium tetrachloride
TTB: titanium tetrabutoxide
TNBT: titanium(IV) n-butoxide
TTE: titanium tetraethoxide
TTIP: titanium isopropoxide

1 Introduction

Nano titanium dioxide (TiO₂) has been extensively applied in various sectors such as in film coating industry [1,2], construction materials [3,4], electronic components [5,6], photocatalytic fuel generations [7], and degradation of pollutants [8,9], just to mention a few. Advanced oxidation process (AOP), on the other hand, is formulated to use various chemical compounds for the removal of organic/inorganic contaminants through the oxidation process [10,11]. Employing semiconductors for AOPs, especially TiO₂, has been among the most dynamic research areas [12,13]. This is due to its remarkable properties, including high photo-responsiveness, eco-friendliness, chemical stability, strong oxidizing property, and low cost [14–16]. It has emerged as a highly promising candidate for photocatalytic degradation of water and air pollutants [8,9,17,18], and a significant potential in environmental remediation [19,20]. Based on ISI’s annual data analysis of papers related to AOP and TiO₂ (Figure 1), it is evident that research interest in AOP and nano-TiO₂ has experienced a notable surge since 2001. Furthermore, the research prominence of TiO₂ and AOP aligns closely with the overall upward trajectory in research interest. This compelling correlation underscores the intricate relationship between TiO₂ and the field of AOP research. Consequently, the development of nano TiO₂ preparation methods and material characterization technologies has received considerable attention, as it greatly contributes to the advancement of these applications. TiO₂-based products with different properties and applicability in specific fields have been the focus of many scholars’ research efforts.

Figure 1

Number of publications per year on the topic of TiO₂ and AOP based on ISI web of science.

Numerous studies have reviewed various preparation technologies, synthesis principles, characterization techniques, modification methods, and applications of diverse types of TiO₂ in a large number of papers. Scholars have described the preparation methods and properties of TiO₂ nanoparticles with distinctive dimensions including zero-dimensional near spherical TiO₂ nanoparticles in organic chemistry, preparation methods of one-dimensional nanowires, nanorods, and nanobelts [21,22], two-dimensional and porous nanosheets as barrier films, building blocks, etc. [23], and three-dimensional nanocomposite for fast lithium storage [24]. Previous studies [25,26,27] have discussed the growth mechanism of nano-TiO₂ from the perspective of synthesis principles and the factors influencing the resulting material. In some work [28,29], the physicochemical properties of TiO₂ are comprehensively summarized across multiple dimensions, encompassing mechanical, optical, and catalytic aspects. This synthesis of information is aimed at fostering a profound understanding of TiO₂ properties, thereby enabling the optimization of their efficacy in chemical applications and facilitating the rational design of practical solid materials. Certain studies [30–33], driven by the ultimate objective of enhancing its overall properties and broadening its application scope, have undertaken extensive reviews on the doping of TiO₂ and other elements, with the explicit purpose of elucidating the resultant alterations in its properties. In addition, the applications of TiO₂ [6,29,33,34,35] in many fields such as building, information, and environmental science, have been widely reviewed.

Various synthesis techniques have been employed for the preparation of TiO₂ in different forms, such as powder, thin film, and doped complexes. These methods encompass a wide array of approaches, including hydrothermal synthesis to produce TiO₂ (B) nanowires with an exceptionally high surface area [36], the utilization of a low-temperature solvothermal process for the creation of fluorine–sulfur co-doped TiO₂ materials, particularly useful in photocatalytic dye degradation [37], the development of anatase TiO₂ supported on porous substrates through chemical vapor deposition [38], sol–gel synthesis for the fabrication of mixed TiO₂ composite photocatalysts tailored for environmental applications [39,40], the creation of nanostructured TiO₂-based nanotubes via template-assisted synthesis for environmental purposes [41], and the production of TiO₂- and ZnO-based photocatalysts using flame spray pyrolysis for water treatment [42]. Furthermore, innovative methods such as solvothermal microwave synthesis have been explored to generate doped-TiO₂ nanoparticles for the sustainable production of green hydrogen fuel, aimed at mitigating carbon dioxide (CO₂) emissions [43]. These represent just a subset of the diverse procedures employed in TiO₂ synthesis and modification. Owing to its simple synthesis, homogeneity, low cost, and ease of control, the sol–gel method is extensively used [35,39,44,45]. Earlier studies have synthesized TiO₂ via modified sol–gel technique optimized by Plackett–Burman system design [2], Zn²⁺ doped TiO₂ catalyst with aqueous sol–gel process [46], nanocomposites of TiO₂-P25 (Degussa) using a peroxotitanic acid modified sol–gel [47], centrifugation and storage precipitation [48], and low-temperature sol–gel [49]. While researchers have conducted analyses on the synthesis methods of nano TiO₂ using the sol–gel approach and its subsequent applications in photocatalysis [50], equal attention has been devoted to the process of creating various forms of TiO₂ and its composites for environmental engineering purposes via the sol–gel method [40]. Additionally, the sol–gel processes applied in the production of TiO₂ thin films through dip coating have been explored with a particular emphasis on discerning the interplay between their microstructure and the optical properties of nano-crystalline materials [51]. Overall, the sol–gel method has the potential for large-scale synthesis of TiO₂ for AOP, especially in fields such as water treatment and environmental remediation.

However, even within a single method like sol–gel, the final TiO₂ product features can vary significantly depending on the experimental conditions and techniques applied. Therefore, generalizing the principles for preparing nanomaterials through this method is not feasible. Instead, it requires precise experimental protocols tailored to specific material requirements to achieve desired functions and properties. Especially, the morphology of TiO₂-based materials prepared by the sol–gel method is inseparable from the properties of the sols. Thus, the key to preparing the most suitable TiO₂ sol–gels lies in the experimental design. Nevertheless, relatively few scholars have focused on the sol–gel formulation mechanisms in detail. It is important to note that the choice of chemicals and preparation routes required for sol–gel, can greatly affect the properties of the targeted TiO₂ nanoparticles, which makes it difficult in the absence of a solid reference. Therefore, in this work, we have reviewed the TiO₂ sol–gel preparation mechanisms in-depth to show how the selection of TiO₂ precursors, solvents, hydrolysis agents, and additives during the process affects the gel properties. Moreover, how other influencing factors such as reaction temperature, humidity, stirring speed, aging time, and the sequences of adding and injection rate of chemicals, can determine the crystal types and morphology of the prepared TiO₂. Based on a total of 218 literature, this work offers a useful experimental guideline about the sol–gel process, which can also be a practical reference for researchers to prepare high-quality targeted nano TiO₂ materials via this method.

1.1 TiO₂ preparation via sol–gel in brief

Sol preparation is the first step of the sol–gel process to obtain any phase (such as anatase or rutile as presented in Figure 2) or type (such as dense film, powder form, etc.) of nano TiO₂ materials. To formulate the nano TiO₂ solution or the “sol,” usually, TiO₂ precursor (titanium alkoxides such as titanium isopropoxide; TTIP, titanium(iv) n-butoxide; TNBT, etc.) is added into an alcohol-based solvent (such as ethanol or methanol) under continuous stirring, to be thoroughly mixed. Then, an appropriate amount of acid-mixed H₂O or H₂O-containing solution (e.g., HCl solution or acetic acid solution) will be added to the mixture, dropwise. After a certain time of hydrolysis-condensation reactions, TiO₂ sol is obtained. Finally, the sol will undergo an aging process to form the gel which can be later used for film coating or will be further dried to xerogel to obtain fiber, dense ceramic, or powder form of nano TiO₂. In this process, the addition of a pH-adjusted aqueous solution catalyzes the hydrolysis reaction. The hydrolysis-condensation rate in the solvating system can be regulated via different types and concentrations of these solutions. Considering the designed application of TiO₂ materials, various additives such as complexing agents (for adapting the hydrolysis process) and metal salts (to modify the TiO₂ sol) can be utilized to improve the properties of TiO₂ nanoparticles.

Figure 2

Anatase (a) and rutile (b) molecular 3D structures.

The general synthesis routes for the sol formation are schematically presented in Figure 3 [52]. As can be seen, titanium alkoxide, such as TTIP (Ti(OR)₄), reacts with water (H₂O) to form titanium hydroxide (Ti(OH)₄) and alcohol (ROH). This reaction is typically initiated by adding a small amount of acid or base as a catalyst. The resulting gel retains the shape of the container and contains a high concentration of the metal oxide. The gelation involves the formation of a three-dimensional network structure through the interconnected Ti–O–Ti bonds. As the condensation reaction progresses, the gelation of the metal oxide network occurs. The gel is then subjected to aging or maturation, which allows further condensation reactions to occur and promotes the development of desired material characteristics. The aging process can range from hours to several days, depending on the specific requirements. Finally, the wet gel is dried to remove the remaining solvent or alcohol. Various methods such as evaporation, supercritical drying, or freeze-drying can be used to obtain a solid material, typically a xerogel or aerogel, with controlled porosity and high surface area.

Figure 3

Hydrolysis-condensation reaction process of titanium alkoxide in sol–gel.

In Sections 2 and 3, the effects and properties of the common chemicals required for the sol–gel as well as additives, the dosage used, and process sequences with the essential time in each step are introduced and exemplified. Also, the ambient condition for the preparation and storage of nano TiO₂ gel by the sol–gel method is illustrated. The related works are presented in Table 1 for reference.

Table 1

Common chemicals used in the sol–gel method to obtain nano TiO₂

#	Precursor	Solvent	Acidity modifier	Additive
1	Titanium tetrachloride (TiCl₄) [126]	Ethylene glycol (C₂H₆O₂) [126]		PVP ((C₆H₉NO)_n) [126]
2	TTIP (C₁₂H₂₈O₄Ti) [133]	Isopropanol (C₃H₈O) [133]		PEG (HO(C₂H₂O)_nOH) [133]
3	TTIP (C₁₂H₂₈O₄Ti) [175]	Ethanol (C₂H₆O) [175]	Hydrochloric acid (HCl) [175]	Methylcellulose (C₂₀H₃₈O₁₁) [175]
4	TTIP (C₁₂H₂₈O₄Ti) [141]	Ethanol (C₂H₆O) [141]		Oleic acid (C₁₈H₃₄O₂) [141]
5	TTIP (C₁₂H₂₈O₄Ti) [98]	Nitric acid (HNO₃) [98]	Ammonia solution (NH₅O) [98]	EDTA (C₁₀H₁₆N₂O₈) [98]
6	Tetrabutyl titanate (C₁₆H₃₆O₄Ti) [176]	Distilled water (H₂O) [176]	Ammonia solution (NH₅O) [176]	Citric acid (C₆H₈O₇) [176]
7	TTE (C₈H₂₀O₄Ti) [125]	Ethanol (C₂H₆O) [125]	Distilled water (H₂O) [125]	PVA ([C₂H₄O]_n) [125]
8	TTIP (C₁₂H₂₈O₄Ti) [177]	Ethanol (C₂H₆O) [177]	Acetic acid (C₂H₄O₂) [177]	Cetyltrimethyl ammonium bromide (C₁₉H₄₂BrN) [178]
9	TTIP (C₁₂H₂₈O₄Ti) [179]	Isopropyl alcohol (C₃H₈O) [179]	Nitric acid (HNO₃) [179]	HPC (C₃H₇O) [179]
10	Tetra-η-butyl titanat (C₁₆H₄₀O₄Ti) [180]	Ethanol (C₂H₆O) [180]	Distilled water (H₂O) [180]	Ethyl aceto acetate (C₆H₁₀O₃) [180]
11	TTIP (C₁₂H₂₈O₄Ti) [39]	Ethanol (C₂H₆O) [39]	Ammonia solution (NH₅O) [39]	Pluronic P123 (C₁₈H₃₆O₅) [39]
12	TTIP (C₁₂H₂₈O₄Ti) [82]	Propanol (C₃H₈O) [82]	Hydrochloric acid (HCl) [82]	AcAc (C₅H₈O₂) [82]
13	Titanium(iv) butoxide (C₁₆H₃₆O₄Ti) [181]	Ethanol (C₂H₆O) [181]	Nitric acid (HNO₃) [181]	Water-soluble chitosan (C₆H₁₁NO₄X₂) [181]
14	TiO₂ P25 [140]	Distilled water (H₂O) [140]	Ammonia solution (NH₅O) [140]	([C₁₂mim]Br)_n (C₁₆H₃₁BrN₂) [140]
15	TTIP (C₁₂H₂₈O₄Ti) [136]	Ethanol (C₂H₆O) [136]	Ammonia solution (NH₅O) [136]	HDA (C₁₆H₃₅N) [136]
16	TTIP (C₁₂H₂₈O₄Ti) [127]	T-butanol (C₄H₁₀O) [127]	Distilled water (H₂O) [127]	DEA (C₄H₁₁O₂N) [127]
17	Titanium butoxide (C₁₆H₃₆O₄Ti) [182]	Distilled water (H₂O) [182]		Ethanolamine (C₂H₇ON) [182]
18	Tetrabutyl titanat (C₁₆H₄₀O₄Ti) [183]	Ethanol (C₂H₆O) [183]	Hydrochloric acid (HCl) [183]	Fluoroalkylsilane (C₁₆H₁₉F₁₇O₃Si) [183]
19	TTIP (C₁₂H₂₈O₄Ti) [61]		Nitric acid (HNO₃) [61]	Poly (diallyldimethylammonium) chloride (C₂₄H₅₄Cl₃N₃X₂) [61]
20	Titanium(iv) tert-butoxide (C₁₆H₃₆O₄Ti) [184]	Butanol (C₄H₁₀O) [184]	Formic acid (CH₂O₂) [184]	Polyethylenimine(H(C₂H₅N)_nNH₂) [184]
21	TTIP (C₁₂H₂₈O₄Ti) [185]	Ethanol (C₂H₆O) [185]	Nitric acid (HNO₃) [185]	Triton X-100 (t-Oct-C₆H₄-(OCH₂CH)xOH, x = 9–10) [185]

2 Required chemicals

2.1 TiO₂ precursor

Precursor is the direct source of titanium applied in the sol–gel method, among which the most widely used type is titanium alkoxide. Alkyl groups in these alkoxides can either enhance or hinder the hydrolysis reaction of the alkoxide, with electron-withdrawing groups increasing reactivity and electron-donating groups decreasing reactivity [53–55]. Other inorganic titanium salts also have been applied as precursors to prepare TiO₂ materials by sol–gel methods, such as titanium tetrachloride (TiCl₄) [56,57], and titanium trichloride (TiCl₃) [58,59]. The most frequently used titanium alkoxides include ethyl titanate [60], isopropyl titanate [61,62,63], butyl titanate [64,65], and their isomeric chemicals [2,66,67]. The precursor type, concentration, and dosage can impact the crystalline structure, surface area, and pore size of the targeted TiO₂ produced through hydrolysis and condensation reactions of the alkoxides.

Several studies have explored the choice of precursor impacts on the properties of TiO₂ gels. For instance, Fröschl et al. [69] as well as Simonsen and Søgaard [68] have observed that the choice of precursor can significantly influence various characteristics, including particle size, morphology, crystallinity, and surface area. This influence arises from differences in the reactivity of the precursor’s alkoxy group, with titanium tetraethoxide (TTE) being more reactive than titanium tetrabutoxide (TTB) and titanium tetraisoproxide (TTIP). In the context of their study, TiO₂ treated with TTIP exhibited the most noteworthy BET result (6 m²/g) and the smallest particle size (3 nm), making it particularly conducive for AOP. Muthee and Dejene [70] delved into the impact of titanium precursor concentration on TiO₂ sol–gels and established that lower precursor concentrations (specifically, a volume ratio of 1:63 of TTIP to ethanol) resulted in a reduced anatase percentage (42.07%) compared to more optimal concentrations (1:9 or 1:7 of TTIP to ethanol volume ratios). In a similar study, Kwon et al. [60] prepared TiO₂ films using four different precursors, including TTIP titanium propoxide, titanium ethoxide, and titanium butoxide. They found that the crystal size of the films made with the former three precursors was larger than that of the film made with titanium butoxide. This was due to the exchange rate of the alkoxy group in the precursors, which dominated the ratio of TiO₂ crystalline phases during the heat treatment and resulted in different grain sizes [70].

Another two types of frequently utilized precursor compounds for the synthesis of TiO₂ through hydrolysis are TiCl₄ and TiCl₃ in which TiCl₃ can also be synthesized from TiCl₄. However, the reduced purity of the produced titanium salt poses a significant drawback [71], which despite their lower cost, limits their potential and scalability in TiO₂ production. In particular, for large-scale manufacturing, the ease of control and higher purity achieved through the hydrolysis reaction of the titanium alkoxide make it a prominent choice based on literature [29,33,72]. While TiO(OH)₂, Ti(SO₄)₂, and titanium(4) bis-(ammonium lactate) dihydroxide (C₆H₁₈N₂O₈Ti) have also been employed as precursors in previous studies, however, due to their limited applicability they will not be further discussed here.

Furthermore, different precursors have various hydrolysis rates owing to their molecular structures, as pointed out by Kinoshita et al. [73]. A slower rate of hydrolysis-condensation promotes the formation of small colloidal clusters, which contributes to the formation of colloidal homogeneous sol. Nolan et al. [74] successfully accomplished the phase transformation of anatase to rutile (consisted of 77% rutile at 600℃ and 95% rutile at 700°C) at relatively low temperatures for rutile (which is normally obtained about 13% at 700°C [74]) by employing TTIP as the precursor. This achievement serves as a valuable reference point for guiding the selection of the preferred crystalline phase orientation in TiO₂ materials. Ge et al. [75] used TiOSO₄ as the Ti precursor and ammonium hydroxide as a precipitating agent to generate Ti(OH)₄, an intermediate of TiO₂. Muthee and Dejene [70] used precursor solutions with different concentrations to organize TiO₂ nanoparticles and found that the percentage of anatase phase particles increased with the increase in precursor concentration, while the grain size showed a non-regular variation. The optimal precursor concentration for crystallinity was found to exist, beyond which the crystal size tended to decrease with the increase in the precursor concentration due to a lower number of reactive ions producing TiO₂. Finally, Bahloul et al. [72] investigated the mechanism by which TiO₂ production from precursors (TNBT) with different mass fractions was affected and found that the rate of hydrolysis-condensation reaction amplified with higher precursor mass fractions, resulting in a shorter time to generate TiO₂, decreased the precursor utilization efficiency. They also concluded that the higher mass fraction of the precursor is more sensitive to water, leading to a more violent reaction that can make the reaction less complete. Similarly, in our previous study (unpublished), we found that the use of a higher concentration of TiO₂ precursor solution can reduce the aging time for gel formation and result in a more viscous gel [76].

Overall, the sol–gel method for producing TiO₂ requires careful consideration of the precursor used in terms of type and dosage as it determines several properties of the final product. These properties include particle size, morphology, crystallinity, surface area, and pore size. The formation of colloidal homogeneous sols is facilitated by a slower rate of hydrolysis-condensation, which develops smaller colloidal clusters. Different types of precursors exhibit varying structures, sensitivity to water, as well as distinctive alkoxy exchange behavior. The hydrolysis rates of dissimilar precursors grow with their sensitivity to water during the sol–gel process, resulting in crystals of various sizes and types. Higher precursor concentrations lead to faster hydrolysis rates and more viscose gels, which can cause particle agglomeration and limit the effective sites on materials. Selecting appropriate precursors for TiO₂ material preparation allows for better control of the sol–gel reaction rate, leading to homogeneous sols, and enabling the subsequent production of diverse morphologies and material types, accordingly. Thus, the choice of the precursor is a critical factor in the sol–gel method for producing TiO₂ with specific desired properties.

2.2 Solvent

The primary function of a solvent in the sol–gel process is to dissolve the titanium alkoxide effectively and disperse the active components in the solution [77]. This will help to achieve more uniform hydrolysis and prevent the final product from agglomerating. Moreover, the selective use of solvents can introduce a spatial barrier effect that hinders the efficient hydrolysis of titanium alkoxides [78]. This approach also serves to stabilize the oligomers formed during the hydrolysis-condensation process of titanium alkoxides. Commonly used solvents include methanol [79,80], ethanol [70,81], propanol [82,83], butanol [83], and their isomers, such as ethylene glycol [84], 2-propanol [37], and glycerol [85]. The concentration and dosage of solvent used directly affect the hydrolysis-condensation reaction rate and the degree of reaction in the system, ultimately determining the properties of the resulting TiO₂ sol [86,87]. During the formation of TiO₂ gel, the evaporation of the solvent, especially if heat treatment is applied, can have a significant impact on the crystallization and morphology of the material. For instance, in Jia and Jimmy’s work [88], samples were subjected to a controlled evaporation process at 100°C within an oven for approximately 24 h in an air environment. This step effectively removed residual water and alcohol. Consequently, this solvent evaporation-induced crystallization method yielded nano-sized TiO₂ particles characterized by a bi-phase composition comprising anatase and brookite phases. Notably, these particles exhibited an impressive specific surface area of 265 m²/g and a diminutive grain size of approximately 5 nm, all achieved at relatively low temperatures. The choice of the solvent type can also considerably impact the properties of the TiO₂ gel produced via the sol–gel process. This impact can be attributed to several factors, such as solvent polarity, acidity, basicity, and hydrogen bonding interactions between the solvent and the TiO₂ precursor [88]. However, it is also possible to produce a gel without the conventional use of solvents by directly adding the Ti precursor into acidic water [61,89]. It is crucial to highlight that the non-aqueous sol–gel method has emerged as a prominent technique for synthesizing TiO₂. This method entails dispersing Ti precursors in non-aqueous solvents, including organic solvents [90] and supercritical CO₂ [91] to produce gels that exhibit exceptional stability [92]. However, this technique is not considered practical for TiO₂ production due to its demanding safety requirements and high production costs [93]. Additionally, the use of organic and supercritical fluid solvents further contributes to its impracticality.

In a primary research work [81], three TiO₂ gels were synthesized by combining different solvents of ethanol, isopropyl alcohol, and 2-ethoxyethanol. It was found that the addition of isopropyl alcohol to the ethanol solvent increases the consistency of the gels. This is because the supplement of isopropyl alcohol gives a more monomeric structure to the complex formed by the solvent and TiO₂ precursor (TTIP) than with ethanol only, which helps to speed up the hydrolysis of the precursor. Polar solvents, like alcohol, are commonly applied in the sol–gel method, facilitating faster gel formation due to their polar functional groups. Nevertheless, low solvent amounts may prolong gelation time and result in a highly viscous gel, whereas high amounts may reduce the precursor solubility and cause incomplete hydrolysis. The viscosity of sol–gel is also influenced by the molecular structure of the solvent. Longer carbon chain solvents like octanol and hexanol intensify the viscosity, while smaller ones like methanol or ethanol decrease the viscosity due to their respective molecular weights and polarities [94]. Applying TiO₂ gels to silica glass plates using dip coating revealed that the thicker the gels, the greater the film thickness. This is explained by the fact that gels with higher viscosities have stronger adhesion due to the advanced toughness of the adhesive gel [95]. Nonetheless, the application of a thicker TiO₂ sol–gel coating may, at times, induce issues such as film detachment from the supporting substrate (Figure 4a vs Figure 4d) and the agglomeration of TiO₂ particles (Figure 4b and c compared to Figure 4e and f). This is primarily attributed to an excessive thickness of the colloidal coating, which can lead to the uppermost layer drying more rapidly than the underlying ones, thereby causing stress disparities. Consequently, this drying incongruity can ultimately culminate in the formation of undesirable peelings [96].

Figure 4

Morphology of TiO₂ films and particles prepared with a thicker gel (a)–(c) and a thin gel (d)–(f).

Golobostanfard and Abdizadeh [79] prepared TiO₂ sols using methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, and mixtures of them and analyzed their properties in terms of stability, gel transparency, gelation time, and other features. Among the gels produced, the 1-propanol-treated and 1-butanol-dominated-treated sols showed a longer gelation time (aging) and the best stability later, which was attributed to their higher order complexes, higher boiling points, and lower dipole moments. These were possibly through reducing the likelihood of TiO₂ particles aggregating, lowering the rate of solvent evaporation, and limiting the availability of reactive sites for gelation. The same justification was argued earlier in another work done by Hu et al. [81]. As an isomer of 1-butanol, the 2-butanol-treated sol has a very short gelation time and is less stable, which is supposed to be due to its smaller dipole moment and dielectric coefficient [97]. It is worth mentioning that the gel made with the mixed solvents neutralized its properties, compared with the gels from each one of the two solvents alone, which can be used as a reference for the choice of solvent type [79].

Mahyar et al. [80] found that the solvent type has an important contribution rate to the TiO₂ crystalline phase composition. The crystalline phase of TiO₂ nanoparticles synthesized with multiple solvents differed significantly (isopropanol-treated of 100 wt% anatase, methanol-treated of 55 wt% anatase and 45 wt%, ethanol-treated of 85 wt% anatase and 15 wt% rutile). This is because the TiO₂ particles, synthesized using isopropanol, have better thermal stability which can avoid the conversion of the anatase to the rutile phase during the annealing process at high temperatures (500℃ and above [98]). The use of methanol and ethanol in this work resulted in larger TiO₂ grain sizes because their higher polarity and lower viscosity allowed for faster hydrolysis and diffusion of precursor. Han et al. [99] prepared TiO₂ films using four alcohol chemicals as solvents for TTIP, and their morphology and photocatalytic properties were investigated in response to methanol, ethanol, isopropanol, and 1-butanol. According to this literature, the TiO₂ films prepared using methanol (with the highest dielectric constant) as the solvent showed pronouncedly higher grain size and surface roughness with a lower specific surface area than the other solvent-dominated combinations. The dielectric constant of solvent on TiO₂ material morphology in this study was consistent with that reflected in the study by Lucky et al. [97]. This should be because solvents with high dielectric constant facilitate faster hydrolysis reactions of TiO₂ precursors, and the rapid growth of nanoparticles results in larger grain sizes, as does the surface roughness. A TiO₂ thin film with high roughness has more active sites, which helps to enhance its photocatalytic reaction efficiency.

In summary, selecting appropriate solvents is a highly influencing factor in producing TiO₂ materials with targeted shapes and crystalline phases. The differences in dielectric coefficient, viscosity, boiling point, and dipole moment among various solvents have a significant impact on the crystallinity, phase, and morphology of TiO₂ materials produced by the sol–gel process. These effects are not solely due to individual solvent properties, but rather a combination of them. The amount of solvent used is relative to the TiO₂ precursor. A lower precursor-to-solvent ratio not only leads to a more homogeneous solute, smaller crystal particles, and a larger specific surface area but also results in a lesser viscosity of gel and a reduced crystallinity of the final material, in general.

2.3 Hydrolysis

Hydrolysis reaction normally takes place via adding an adjusted pH water, dropwise, into the mixture of precursor and solvent. Water is necessary for hydrolysis reactions, while acidity modifiers act as catalysts in the sol–gel process, influencing both hydrolysis and polymerization reactions [100]. The commonly used acidity modifiers are hydrochloric [62,101], orthophosphoric [102], nitric [61], carboxylic [45], sulfuric [103], and perchloric acid [63]. Sodium chloride [58] and sodium hydroxide [104] also can mutually regulate the hydrolysis-condensation rate. TiO₂ particles of different sizes were prepared using several ratios of precursor to water by Li et al. [45]. They found that the TiO₂ particles with a TTIP to water volume ratio of 1:2.5 were the smallest and had the best effect in degrading the methyl orange dye. When either the water or precursor content was too high, it promoted the hydrolysis reaction rate resulting in larger particle size and agglomeration. The specific surface area of the material was also reduced and as a result, the photocatalytic efficiency of the final product was lower. Powders with low hydrolysis completion need higher temperatures for crystallization than the larger ones, due to smaller particle sizes which require more thermal energy for crystallization [105]. The powders with the most photocatalytic efficiency were produced with moderate water consumption (the molar ratio of water-to-TTIP was between 2 and 4) in this research. It means that there is an optimum value for the complete degree of the reaction and that the amount of water is an effective control factor. Hafizah and Sopyan [86] investigated the effect of the hydrolysis reaction completion rate (adjusting the amount of water) on the properties of the TiO₂ powders formed. The powders with high hydrolysis completion were rod-shaped and had a low specific surface area, which was supposed to be due to the fast hydrolysis reaction leading to the agglomeration of nanoparticles.

The rate and extent of the hydrolysis reaction can be effectively regulated by selecting the type of acidity modifier [106], determining the appropriate amount of it [2,107], and controlling the quantity of water employed [45,49,86]. The selection of the acid type is crucial as it determines whether the introduced ions in the solution will or will not undergo unfavorable reactions with other ions during the hydrolysis-condensation process [108]. The introduction of acid serves to regulate pH and exerts a notable impact on both the reaction process and the resulting gel. In this context, it was observed that HNO₃ played a role in promoting the growth of brookite, whereas NH₄OH not only slowed down the phase transformation of TiO₂ powders from amorphous to anatase and anatase to rutile but also inhibited the growth of brookite. [109]. Furthermore, the characteristics of the TiO₂ gel generated through the sol–gel method are notably influenced by the quantity of water and its level of acidity. The findings suggest that achieving a sufficiently low pH, preferably around pH = 2, and employing a moderate excess of water, specifically with a water dosage ratio of at least 120 (the molar ratio of water to vanadium oxyacetyl acetonate-doped precursor of tetrabutyl titanate), are essential prerequisites for obtaining crystallized vanadium-doped TiO₂ at room temperature [110].

Lower pH values (1–5) lead to well-crystallized TiO₂ gel with a high degree of order [111,112], and the use of different acid types in the sol–gel process can impact TiO₂ gel properties [113]. In a word, the purpose of regulating the amount of water, pH, and acid types is to influence the properties of TiO₂ gels by controlling the degree of hydrolysis reaction [86]. Cruz et al. [62] prepared TiO₂ sols using HCl solution as an acidity modifier to adjust the solution pH to 1.5–1. According to the study, the density of TiO₂ sols increased (thick gel with lower viscosity) when decreasing the pH of the reaction solution (or increasing the dosage of acidic water) under a strongly acidic environment. They indicated that it may be due to this reason that the more acidic reaction conditions allowed the hydrolysis reaction to proceed faster and more completely, resulting in the production of denser TiO₂ gels. As shown in Figure 5a and b, using the same types and ratios of chemicals (3 mL TTIP, 30 mL ethanol, and HNO₃ solution at pH 1.2), two different viscosities of TiO₂ gel were obtained when various amounts of the acidic water (1–1.5 mL) were injected for hydrolysis during the same time of 38 min [76].

Figure 5

TiO₂ gels prepared via different amounts of acidic water: (a) 1 mL and (b) 1.5 mL.

In the research by Bai et al. [114], TiO₂ sols were obtained by adjusting the pH of the solution to 3, 4, 5, 7, 10, and 13 using NaOH solution to produce a powder form of TiO₂. It was found that as the pH increased, the particle size and agglomeration of TiO₂ powder gradually decreased and there was a shift in the shape, from dandelion-like (pH = 3–5) to nanorods (pH = 7 and 10), and to nanoparticles (pH = 13). This should be attributed to the fact that the increase in pH of the solution lowers the completion of the hydrolysis reaction, resulting in a gradual decrease in the amount of TiO₂ produced, and a decrease in powder agglomeration [115]. Analysis of the crystalline phase composition revealed that at low pH (pH = 3–5), only the rutile phase was presented in the TiO₂ grains, with the anatase phase appearing at pH = 7, and the highly crystalline anatase phase at pH = 10. Other studies have shown that acidic reaction conditions contribute to the formation of the rutile phase [114], but not essentially [116]. In contrast, Bano et al. [117] used HCl and NaOH to adjust the pH of the solution from 1 to 10 to obtain various types of Ag–TiO₂ nanocomposites and tested their surface morphology. They reported that the specific surface area of the composites shows an increase and then a decrease with the increase in the pH of the solution (the maximum value occurred at pH = 4). The smaller specific surface area implies agglomeration of nanoparticles, which should be related to the nucleation growth and photoreduction of AgCl (pH < 4) and Ag(OH) (pH > 9) precipitates, rather than the acidity affecting the hydrolysis reaction. In addition, pH can also influence the TiO₂ quantum confinement by regulating the rate and completion of the hydrolysis reaction. The crystallinity of TiO₂ can be controlled through hydrolysis, thereby influencing the optical band gap energy of the resulting material. Sridevi et al. [115] observed that elevating the pH of the solution, indirectly reduces the TiO₂ band gap by affecting the crystallinity, leading to enhanced light utilization efficiency and improved photocatalytic activity.

In summary, the type of the acidity modifier applied, its dosage, the amount of water used, and the injection rate will impact the rate and completion of the hydrolysis-condensation reaction, resulting in diverse properties of the TiO₂ gels [49,118]. Therefore, the TiO₂ powders or thin films prepared via the same process may differ in their grain sizes, morphology, and optical properties. In general, a low pH (around 1–2 [62]), moderate amounts of water to precursor (molar ratio varies from 2 to 4 [86]), and slow injection rates (dropwise [119,120]), through slowing down of hydrolysis-condensation, are in favor of producing a desirable gel with acceptable transparency and optical properties, nano TiO₂ particle size, and photocatalytic efficiency. However, the selection of a suitable experimental protocol cannot be generalized and needs to be analyzed in light of practical needs.

2.4 Additives

In this review, TiO₂ precursor, solvent, and acidic water were defined as basic chemicals applied in the sol–gel method; while other compounds are introduced as additives that can modify the properties of TiO₂ when being added to the solution. Three main categories of additives including stabilizers, surfactants, and metal/nonmetal dopants are being extensively used in different research works for various purposes. Stabilizers are normally added to the solution to provide a more stable gel as the gel is an unstable structure by nature [121]. Surfactants are mainly used for surface modification and enhancing the adherence of the TiO₂ nanoparticles on the support surface via reducing surface tension and increasing the solubility of reactants [122]. And finally, nanoparticles, such as metal or metal oxides, can be applied as dopant additives to the sol which creates hybrid nanoparticles with TiO₂ that have superior properties (especially optical) [123]. These three categories are explained and exemplified further in the following subsections.

2.4.1 Stabilizers

The addition of stabilizers such as hydroxypropyl cellulose (HPC) [124], polyvinyl alcohol (PVA) [125], polyvinylpyrrolidone (PVP) [126], acetylacetone (AcAc) [118], and diethanolamine (or DEA which is also one of the most frequently used additives) [127] can boost the dispersion of TiO₂ nanoparticles in the sol [128]. This happens by providing steric and electrostatic repulsion to prevent particle agglomeration which provides a stable structure during the sol–gel process [129] as illustrated in Figure 6 [76]. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA) [130] and citric acid [131] also can be added to control the stability and reactivity of the TiO₂ sol–gel solution [132]. Thompson et al. [2] used polypropylene glycol as a shape control agent to prepare TiO₂ thin films by sol–gel. The crystalline phase and grain size were investigated and it was found that increasing the dosage of polypropylene glycol resulted in larger grains due to more space for grain growth after heat treatment and solvent evaporation. Chen and Dionysiou [133] synthesized macroporous films using polyethylene glycol (PEG) as a porogenic agent in the sol–gel process; where PEG escapes during the heat treatment and leaves the pores.

Figure 6

TiO₂ gels resulted with (a) and without (b) using DEA as stabilizer.

In general, in the sol–gel process for TiO₂ synthesis, stabilizers serve the functions of regulating the properties of TiO₂ sol–gels, thus affecting particle size, preventing particle aggregation, and maintaining sol stability. They play a crucial role in achieving a controlled and consistent production of TiO₂ nanoparticles by overseeing aspects such as size, shape, and distribution. Stabilizers exercise control over the growth and clustering of particles, ensuring that the resulting material possesses the intended properties. Furthermore, they uphold the uniformity of the sol, avoiding premature gel formation, and promoting the even dispersion of nanoparticles within the gel precursor.

2.4.2 Surfactants

For the coating purpose, surfactants [134] act to reduce surface tension, increase the solubility of reactants, and serve as excellent media for surface directing agents, nucleation, and crystal growth [122], thus playing a crucial role in achieving the desirable size, shape, dispersion, and stability of TiO₂ nanoparticles formed via sol–gel [135]. Compounds, such as hexadecylamine (HDA) [136], PEG [133], and Triton X-100 [137] can be used to control the particle size and distribution of the TiO₂ nanoparticles. A proposed mechanism has been suggested for the formation of monodisperse precursor beads to illustrate the significant role of HDA in determining the morphology and mono-dispersity of TiO₂ during the sol–gel synthesis process in which HDA was used as a structure-directing agent to control the monodispersity of the resultant titania beads, along with the spherical shape, varying the amount of structure-directing agent involved in the sol–gel process [136]. Modifying the ammonia concentration (the molar ratio of HDA/TTIP = 0.33–1) in a mild solvothermal process gives rise to mesoporous anatase titania beads having controllable crystallite size (320–830 nm), specific surface area (from 89 to 120 m²/g), and pore size distribution (from 14 to 23 nm). The resultant mesoporous titania materials are expected to have potential applications in areas of energy conversion and environmental cleanup. Nonionic surfactants like PEG [138], on the other hand, can enhance the dispersion and stability of TiO₂ nanoparticles within the gel. When adsorbed onto the surface of the particles, surfactants can create mesoporous structures, which increase the specific surface area of the particles and improve accessibility to active sites [139]. Wei et al. [59] utilized different mass concentrations of methylcellulose to induce the synthesis of micro-spherical TiO₂ powders. The concentration of methylcellulose affected the shape and phase of the synthesized particles and formed a three-dimensional network of high-density hydroxyl group-exposed methylcellulose molecules. Curcio et al. [44] added Triton X as a surfactant to improve the wettability of TiO₂ gels for film coating. Xin et al. [140] used [C₁₂mim]Br as a surfactant to synthesize ionic liquid/TiO₂ composite films with good self-cleaning properties, where [C₁₂mim]Br acted as lower surface tension material to enhance the hydrophobicity of the final product and improved its self-cleaning properties. Jin et al. [141] synthesized TiO₂ nanorods using oleic acid as a surfactant and 1-hexadecylamine as co-surfactant, which was found to be an effective substance to control the diameter of TiO₂ nanorods. Additionally, surfactants can promote the formation of highly crystalline mesoporous TiO₂, high surface area (245–300 m²/g), and tunable mesopore diameter (2.2–3.8 nm) by means of anionic surfactant templating effects [142,143], which can enhance the photocatalytic activity of TiO₂ nanoparticles.

Surfactants function at the interfaces between liquids and either gases or other liquids to reduce surface tension and promote the dispersion of TiO₂ nanoparticles. Their main advantage lies in their ability to thwart particle aggregation and improve the uniform distribution of particles. This enhanced dispersion holds significant importance, particularly in applications such as coatings, where achieving uniform coverage is of utmost importance. Surfactants assume a central role in upholding the integrity of TiO₂ nanoparticles within the sol, diminishing the probability of particle clustering, and guaranteeing consistent properties of the end product.

2.4.3 Dopants

To modify the photocatalytic properties of TiO₂ nanoparticles, dopants like nitrogen [30,144], carbon [31,145], and transition metal ions [146] such as Fe can be used. Metal/nonmetal solutions have been investigated as additives in the sol–gel process to prepare TiO₂ composites for improved light utilization efficiency and wider applications in water treatment [147,148]. Transition metal dopants such as Fe, Co, and Ni can narrow the bandgap and increase visible light absorption [149], while nonmetal dopants such as N and C can introduce defect states and modify the electronic structure of TiO₂ [150,151]. The dopant concentration [152] and species [149] can affect the size, shape, crystallinity, and surface area of TiO₂ nanoparticles, thereby influencing their photocatalytic activity. For instance, Chen et al. [153] found that nitrogen-doped TiO₂ nanoparticles had higher photocatalytic activity than pure TiO₂. The incorporation of other elements such as Ag, Au, and ZnO can enhance the photocatalytic activity of TiO₂ by promoting charge separation and improving the light-harvesting efficiency [154].

Additionally, the integration of other nanoparticles with TiO₂ can modify the surface area, surface energy, and surface defects of TiO₂, which can enhance its photocatalytic performance [155]. For example, Fu and Zhang [156] reported that the addition of Au to the TiO₂ gel improved its photocatalytic activity for the degradation of pollutants. Catalysts supplements such as noble and non-noble metals and metal oxides can promote the separation of electron-hole pairs and increase the surface area of TiO₂ nanoparticles, which supports their photocatalytic efficiency [157]. For instance, applying Pt, Pd, and Au as noble metal catalysts has reported to greatly enhance the photocatalytic activity of TiO₂ [158,159]. Moreover, metal oxides such as Fe₂O₃ and WO₃ have been utilized to boost the photodegradation ability of TiO₂ by acting as electron sinks [160,161].

Dopants are deliberately introduced into the TiO₂ matrix to bring about specific property modifications. These additives have the capacity to influence the electronic structure, optical characteristics, and reactivity of TiO₂, tailoring it for particular applications. For example, incorporating metals like Fe, N, or Pt as dopants can enhance the photocatalytic prowess of TiO₂ [32]. Dopants wield significant influence over TiO₂ properties; nitrogen (N) doping, for instance, can shift TiO₂’s bandgap, rendering it responsive to visible light and thereby improving its photocatalytic performance [144]. Achieving the desired property adjustments necessitates careful control over the type and concentration of dopants.

In summary, incorporating suitable additives can enhance the properties or structural characteristics of TiO₂ nanomaterials when they fail to meet practical requirements, efficiently. The selection of additives depends on the intended application and desired properties of the final TiO₂ sol–gel product. Some additives can indirectly influence the sol–gel process by inducing and controlling chemical processes to prepare TiO₂ materials with specific structural features or properties. Chelating agents, for instance, affect the properties of TiO₂ sols and gel materials by influencing the rate and completion of hydrolysis, while shape control agents induce the corresponding shape of the generated TiO₂, thereby retrofitting the properties of nano TiO₂. Meanwhile, the addition of nanoparticles and metal/nonmetallic solutions will incorporate some properties of the additives, such as electrical conductivity and photosensitivity, directly into the TiO₂ gels. In either approach, the choice of additives aims to minimize any negative effects and maximize the efficiency of the targeted product.

2.5 Summary of chemical selection

The sol–gel method for the preparation of TiO₂ relies on the hydrolysis reaction, which requires Ti precursor, solvent, and water. However, some researchers have been able to achieve the desired results without the use of solvents [46,61], acidity modifiers [70,83], and additives [45,162]. The hydrolysis-condensation reaction in the sol–gel process is influenced by the selection of precursor type and its concentration, the amount of solvent used, acidity regulators, H₂O, and additives. However, due to the complexity of the chemical process, it is difficult to obtain the exact target material by solely controlling the use of chemicals in a specific manner, or even to replicate the identical product in the first attempt using the same process. Achieving control over the required molar ratio and the properties of the target product using a single chemical is also challenging and might require trial and error at the initial stages. In some cases, a combination of two or more chemicals (such as a mixture of solvents) and physical methods may be required to produce the desired convergent modification. Table 1 presents the different types of precursors, solvents, acids, and modifiers used in numerous studies done for producing nano TiO₂ via the sol–gel method.

3 Reaction conditions for the sol–gel process

Although not very complex, the preparation of TiO₂ sols is a process that requires accuracy in every step for successful chemical reactions. Each one of the steps and factors in this process plays a crucial role in formation of the final form of the gel. The properties of TiO₂ sol–gel can be influenced by factors like reaction temperature, humidity, stirring speed, the order of adding chemicals, the aging process, and injection rates of chemicals which are explained as follows.

3.1 Reaction temperature

The hydrolysis and condensation reaction rates in the sol–gel process are sensitive to temperature and time, which in turn affect the properties of the resulting TiO₂ gel. Specifically, the size, morphology, and crystallinity of TiO₂ particles can be impacted [163]. Higher temperatures during the gel-making process promote faster gelation, while longer times (aging) can result in increased crystallinity and larger particle sizes [164]. At lower temperatures, the hydrolysis and condensation reactions that form TiO₂ gel proceed more slowly, resulting in the formation of more amorphous TiO₂ due to a lower reaction rate and longer time for precursor molecules to form non-crystalline structures [121]. As temperature increases, the reaction rate accelerates, leading to the formation of crystalline structures due to the higher energy state of the system [165]. The higher temperature also promotes the growth of TiO₂ crystals, which develops crystallinity [166]. Particle size is influenced by the rate of reactions, the concentration of precursor molecules, and the degree of agglomeration or sintering of particles. At higher temperatures, precursor molecules may hydrolyze and condense more rapidly, leading to the formation of larger particles [73]. Porosity is related to the degree of crosslinking between precursor molecules, with lower temperatures developing a highly porous structure due to the slower reaction rates and more uncross-linked chains [167]. Higher temperatures can lead to increased density and decreased porosity due to increased crosslinking and removal of solvents [168]. Surface area is related to both particle size and porosity, with lower temperatures leading to a greater surface area due to the formation of smaller particles and more porous structures [169]. However, higher temperatures can cause a denser structure with larger particles, resulting in a smaller surface area [86].

3.2 Humidity

Humidity affects both TiO₂ sol [170] and gel processes [171], causing changes in its surface area, pore structure [162], as well as crystallization [172]. Ti precursors are water-sensitive substances; thus, when the humidity of the experimental environment is high, the precursors will precipitate, which will destroy the hydrolysis reaction process. Therefore, the preparation of TiO₂ gel via the sol–gel method is recommended to be done under the air conditioner [173], if it is not in the glovebox. Slunecko et al. [174] found that samples prepared under different humidity conditions differed in their number of unhydrolyzed alkyl and rutile particles, indicating that increasing atmospheric relative humidity causes fewer water-soluble alkyl particles and larger rutile elements.

According to Barlier et al. [170], air humidity inhibits Ti(OeCarb)₄ hydrolysis activity when it comes to its environmental reactivity. Moreover, higher humidity levels result in a slower drying rate of TiO₂ gel [186], thereby affecting the crystallinity, formation of agglomerates, size, and shape of nanoparticles, and mechanical properties of TiO₂ gel. The rapid drying speed of TiO₂ gel can lead to uneven internal and external stresses, which can impact the morphology of the gel. If the drying process is too intense, the gel may even crack [187]. Matsuda et al. [172] found that under high temperatures and humidity, the structure of porous TiO₂–SiO₂ film is easy to be eroded by water vapor, resulting in the generation of more Ti–O–Ti bonds and anatase crystal phase. Generally, the TiO₂ films prepared at high room humidity (>75% RH) were rough and inhomogeneous, with significant differences compared to the uniform and more conductive films prepared at low RH (<15%) [188].

3.3 Stirring speed

The stirring rate is an important criterion in the preparation of TiO₂ via sol–gel, as it affects the subsequent product in multiple ways. Stirring at an appropriate rate helps to uniformly distribute the reactants, producing a more homogeneous and consistent sol–gel [189]. This, in turn, affects the size and morphology of the particles formed during the process [190], ultimately influencing the properties of the final TiO₂. Besides, the stirring rate can influence the gelation kinetics, accelerating the process, and developing a highly stable and mechanically solid gel [191]. Moderately increasing the stirring speed of a TiO₂ solution can decrease the particle size of the gel by applying a greater shear force to the solution, according to a study by Sun et al. [192]. Furthermore, increasing the stirring speed can lead to an increase in the specific surface area and crystallinity, a more homogeneous distribution of particles, and higher elastic modulus and hardness due to the densification of the gel network [193]. A stirring speed of 500 rpm is recommended for mixing the solution in the sol–gel method [194].

3.4 Sequences of adding chemicals

The order of adding chemicals during the preparation of TiO₂ sol–gel can significantly impact the properties and performance of the resulting material. It can affect the kinetics and thermodynamics of the hydrolysis and condensation reactions, the stability and colloidal behavior of the sol–gel precursor, the degree of agglomeration, and the surface area of the resulting TiO₂ material [195,196]. Hence, consideration of the chemicals’ adding order is necessary to achieve the desired properties and performance of the TiO₂ material. During the preparation of TiO₂ via sol–gel, this order can impact the size and uniformity of TiO₂ particles. Adding water before acid can result in larger particles while adding acid before water can produce smaller particles. The reason for this is that the acid serves as a catalyst, preventing the hydrolysis of the precursor and ensuring that excessive hydrolysis does not occur, which can lead to the formation of smaller gel particles [180]. Incorporating a chelating agent before the titanium alkoxide precursor can lead to more uniform particles with smaller sizes [197]. A general order of adding chemicals is illustrated in Section 4 of this article.

3.5 Injection rate of chemicals in the solution

The injection rate or dropping speed of the chemicals during the sol–gel process can affect the properties of TiO₂ gel in various aspects. Tian et al. [120] found that a slower dropping speed of chemicals results in smaller particle size, higher crystallinity, porosity, surface area, photocatalytic activity, hardness, and elastic modulus, as well as a lighter color. When the chemicals are dropped slowly, the reaction between the precursors is allowed to proceed more gradually and uniformly, leading to a relatively controlled formation of the material [198,199]. This can cause smaller particle size, as the slower reaction kinetics allow for future homogeneous nucleation and growth of particles [200,201].

Additionally, lower injection rates can result in a higher degree of crystallinity, as the slower reaction kinetics allow for a complete conversion of the precursors to the crystalline form [202]. This can also lead to higher porosity and surface area, as the slower reaction kinetics provide a more thorough formation of voids and defects within the material [203]. The prolonged reaction kinetics can also result in higher photocatalytic activity, as the gentler formation process can allow for more effective integration of photoactive species [204]. Higher hardness and elastic modulus can also be derived from the more controlled formation process, as the gradual reaction kinetics can offer higher uniform packing and bonding of the material [205]. Hydrolysis is one of the most injection rate-sensitive steps in the sol–gel process which is normally recommended to be performed dropwise [206], and the dropping speed can be adjusted accordingly, considering the size of the experiment.

3.6 Aging process

The aging process, also known as maturation, is the last step in the preparation of TiO₂ sol–gel, and it can impact the resulting sol–gel from various points of view. First, aging promotes the hydrolysis and condensation of precursor molecules, which increases the cross-linking between TiO₂ particles, providing a more stable and mechanically robust sol–gel [207]. Second, aging can affect the size and morphology of TiO₂ particles through Ostwald ripening [208], which causes the smaller particles to dissolve while the larger particles continue to grow, thus increasing the average particle size. Third, aging can alter the surface chemistry of the TiO₂ particles, making them more hydrophobic and improving adhesion to hydrophobic substrates [209], and can impact photocatalytic properties. Moreover, aging can decrease the porosity of the gel, increase the density and mechanical strength, and decrease the surface area and reactivity [210]. Controlling the aging time and temperature can optimize the properties of the resulting sol–gel for its intended application.

Finally, aging time can affect the thermal stability of the TiO₂ gel too [211]. Panić et al. [212] found that aging led to improved thermal stability in TiO₂–SiO₂ composites. On the other hand, as aging time increases, particle aggregation can occur, causing a decrease in surface area [213]. Fajriati et al. [214] reported that increased crystallinity due to aging results in improved photocatalytic activity, while prolonged aging times can lead to decreased porosity and specific surface area due to larger TiO₂ particle formation [215]. Similarly, Venkatachalam et al. [164] reported that prolonged aging times decrease the TiO₂ nanoparticle size but increase the surface hydroxyl groups, improving the photocatalytic activity. Liu et al. [216] found that increasing the aging time can lead to a better TiO₂ gel cross-linking, resulting in improved mechanical strength and stability. Aging time can vary depending on the kind and dosage of chemicals used for the sol–gel. A thermal treatment process at low temperatures can shorten the required aging time. Some experiments have applied a 7 day aging (25℃) while using heat treatment (160–180℃) it can be reduced to 1 h [217]; however, this might impact the properties of the final product as mentioned above. Indeed, sometimes the gels might quickly become hard, even within 30 min at room temperature (22°C) [121], unless they are covered and stored in a refrigerator before hardening. Therefore, low-temperature storage (4℃ in the refrigerator) is an effective way to avoid excessive aging of TiO₂ sol–gel and maintain its availability [2,218].

3.7 Summary of reaction conditions

Only through employing carefully controlled parameters such as temperature, humidity, stirring rate, sequences of chemical addition, injection rate, and aging time, the properties of TiO₂ sol–gel can be optimized. These are significant factors that influence the sol–gel process in TiO₂ preparation to achieve specific characteristics. Higher temperatures accelerate gelation, resulting in larger particles and increased crystallinity, while, lower temperatures yield slower reactions, causing a smaller, more amorphous particles. Humidity, on the other hand, affects the resulting gel’s surface area, pore structure, and crystallization through the reaction completion of the sol–gel process. High humidity can deteriorate water-sensitive drugs, impacting sol–gel reaction completion, and subsequent processes such as crystallization and molding. The lower stirring speed triggers the deposition of colloidal particles with larger sizes. Stirring speed is important in achieving uniform distribution of chemicals and influencing particle size, surface area, crystallinity, and mechanical properties of the resulting gel by affecting gelation kinetics. The sequence of adding chemicals during the sol–gel process affects hydrolysis, stability, and particle size due to variations in kinetics, thermodynamics, colloidal behavior, and agglomeration. The injection rate controls the severity of hydrolysis and the overall sol–gel process, impacting particle size, crystallinity, porosity, and mechanical properties. Aging the gel after its formation promotes stability and alters the surface chemistry. It affects the properties of TiO₂ sol–gel through ongoing hydrolysis, condensation, Ostwald ripening, and crystallization processes, over time. These processes influence particle size, morphology, crystallinity, and surface properties, ultimately impacting the overall characteristics of the sol–gel material. Therefore, adequate storage conditions must be maintained to prevent deterioration or alteration of the gel over time.

4 A practical example of TiO₂ gel preparation

The following example provides a better understanding of the routes’ selection and ambient conditions for gel formation. To prepare 275 mL of gel [62,165], use 25 mL ethanol as a solvent in a beaker. While the solvent is stirred at 500 rpm, drop 1.25 mL of TTIP for 10 min and continue stirring for an hour. If additives such as stabilizers are required, they can be added at this stage (DEA:TTIP molar ratio R = 1 to 2 [127]). In the next step, the hydrolysis takes place by adding 250 mL of acid-mixed water (at pH 1.5) using a dropping funnel, dropwise during 240 min while stirring. Continue stirring the mixture overnight until a viscose transparent (or slightly translucent) gel is formed. In case the gel has not formed yet, an aging process can take place by leaving the sol undisturbed at room temperature. This step can be shorter if a low-temperature heat (at 30–60°C) is used (it will cause evaporation of some components). After the gelation, the final product can be kept at 4℃ for around a month. The general order of adding chemicals is roughly depicted in Figure 7.

Figure 7

Schematic sol–gel process to prepare 275 mL of TiO₂ gel.

The justifications behind the choice of chemicals in the abovementioned example are interpreted as follows. TTIP is a commonly used Ti precursor with ethanol chosen as the solvent since its relatively high polarity and low viscosity help the rapid diffusion of TTIP and avoid causing colloidal agglomeration of colloidal particles. Moreover, ethanol itself is a chemical with more daily exposure, increasing the convenience of its storage and use. HNO₃, as an inorganic acid, has solution ions that do not conflict significantly with the other chemicals used and do not generate additional reactions (e.g., precipitation reactions) that might make the process less efficient [75].

5 Concluding remarks and future perspectives

The preparation of TiO₂ gels via the sol–gel is a process that requires careful selection of chemicals, precise reaction conditions, and appropriate procedures. This method offers numerous advantages over others, including the ability to produce nanoscale TiO₂ gels with controlled properties. The choice of precursors, solvents, acid, and additive types and concentrations, as well as the reaction conditions, all play crucial roles in the final product properties including the size, morphology, crystalline phase of the nanoparticles, and various physical and chemical characteristics of the resulting TiO₂ gels. Proper control of these parameters is essential for an optimized preparation of TiO₂ gels for various applications, including catalysis, energy storage, environmental remediation, etc.

In the future, the preparation of TiO₂ via the sol–gel process is expected to continue as an active area of research. One future trend is the use of novel TiO₂ precursors and additives to tailor the properties of the resulting gels for specific applications. Another trend is the development of new reaction conditions that can enhance the efficiency and reproducibility of the sol–gel process, such as the use of microwave heating or continuous flow reactors. Additionally, the integration of TiO₂ with other materials, such as graphene or metal nanoparticles, is expected to accelerate the development of new composite materials with enhanced properties. The application of TiO₂ gels in fields such as AOP, photocatalysis, and biomedical engineering is also likely to expand in the future. Overall, the continued exploration and optimization of the sol–gel process for TiO₂ preparation is predicted to result in the development of new materials with significant applicability in numerous fields.

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Funding information: The financial support for this work was received from the National Natural Science Foundation of China (Grant number 52150410408) and the Research Initiation Funds of Guilin University of Technology (2012027, 2017025).
Author contributions: In this review, C. Chang was responsible for conceptualizing and designing the study, conducting the literature search, and drafting the manuscript. S. Rad reviewed and selected studies for inclusion, developed the data extraction form, and analyzed the studies’ results. L. Gan provided expert guidance on analyzing the data and interpreting the findings. Z. Li conducted a comprehensive quality assessment of the included studies. J. Dai assisted in the screening of articles and in resolving disagreements between review authors. Finally, A. Shahab provided overall support and feedback throughout the process. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2023-07-11

Revised: 2023-09-28

Accepted: 2023-10-18

Published Online: 2023-11-09

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/ntrev-2023-0150

Keywords for this article

nano titanium dioxide; sol–gel method; TiO2 preparation

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BY 4.0

Review of the sol–gel method in preparing nano TiO2 for advanced oxidation process

Article

Abstract

Graphical abstract

Abbreviations

1 Introduction

1.1 TiO2 preparation via sol–gel in brief

2 Required chemicals

2.1 TiO2 precursor

2.2 Solvent

2.3 Hydrolysis

2.4 Additives

2.4.1 Stabilizers

2.4.2 Surfactants

2.4.3 Dopants

2.5 Summary of chemical selection

3 Reaction conditions for the sol–gel process

3.1 Reaction temperature

3.2 Humidity

3.3 Stirring speed

3.4 Sequences of adding chemicals

3.5 Injection rate of chemicals in the solution

3.6 Aging process

3.7 Summary of reaction conditions

4 A practical example of TiO2 gel preparation

5 Concluding remarks and future perspectives

References

Articles in the same Issue

Articles in the same Issue

Articles in the same Issue

Review of the sol–gel method in preparing nano TiO₂ for advanced oxidation process

1.1 TiO₂ preparation via sol–gel in brief

2.1 TiO₂ precursor

4 A practical example of TiO₂ gel preparation