Low-temperature sol–gel synthesis of crystalline materials

Abstract

Sol–gel chemistry has opened a new era of modern materials science by enabling the production of ceramic materials at near-room temperature. Thousands of papers have been published since its inception, and new hybrid materials and composites widely used in our everyday life have been obtained. From a chemical point of view, these materials actually have compositions identical to their high-temperature ceramic analogs, but there is a drastic difference in structure and phase composition. In the majority of cases, oxide systems produced using the sol–gel method possess an amorphous structure and huge surface area with narrow micro/mesopore size distribution. At the same time, there are a great variety of oxides and mixed-oxide systems with quite a number of polymorphic modifications and, consequently, certain properties can only be produced by high-temperature treatment. Investigation of the mechanisms and methods of crystallization for such systems in the colloidal state at temperatures less than 100 °C would significantly contribute to the development of new materials obtained by low-temperature sol–gel synthesis. Taking into account the millions of different thermosensitive organic, inorganic, and bio-organic substances that could be used in producing hybrids and composites, the potential of low-temperature sol–gel technology is immense. In fact, it is a ‘second wind’ for developing classical sol–gel technology, with its more than a hundred-year history. The present review describes the fundamental principles of crystallization of oxide sol–gel systems in solution and gives examples of the applications of composites produced by low-temperature sol–gel synthesis.

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Alexandr V. Vinogradov

Alexandr V. Vinogradov obtained his Ph.D. degree from the Ivanovo State University of Chemistry and Technology (ISUCT) in 2011, and was appointed as an Assistant Professor in the Department of Ceramic Technology and Nanomaterials (ISUCT). He undertook postdoctoral research at Leipzig University, Germany in the research group of Prof. Evamarie Hey-Hawkins. In 2014, he joined ITMO University, where he is currently the Vice-head of the International Laboratory of Solution Chemistry of Advanced Materials & Technologies. His main research focus is in developing low-temperature crystal growth for the synthesis of photoactive nanomaterials.

Vladimir V. Vinogradov

Vladimir Vinogradov was born in Ivanovo, Russia, in 1985. He received a MS Degree in Materials Science from the Ivanovo State University of Chemistry and Technology in 2007. He was awarded a PhD in Inorganic Chemistry from the Institute of Solution Chemistry of RAS (with Professor Alexander V. Agafonov) in 2010. Thereafter, he did postdoctoral training in materials science in Prof. David Avnir's research group at the Hebrew University of Jerusalem. In 2014, he joined ITMO University, where he is currently the Head of the International Laboratory of Solution Chemistry of Advanced Materials & Technologies.

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1. Motivation

Metal oxides are one of the most intensively studied and promising classes of modern materials, numbering hundreds of various compounds. Counting the quantity of possible solid solutions and the mixed oxide systems produced on their basis is barely possible. At the same time, the application of these systems is significantly limited by the methods of their production. As a rule, these imply high-temperature treatment required for the formation of crystals in the course of solid-phase topochemical reactions. Taking into account the huge number – multimillions – of various organic substances, finding the conditions for synthesis of such crystalline systems in solution at temperatures less than 100 °C would, of course, result in the emergence of new materials with unique properties and practical applications. Despite the fact that fundamental principles are to be developed, the main motivation is practical:

✓ The number of simple oxides is limited. However, the variability of mixed oxides and solid solutions based on them is practically infinite. When considering these compounds as a potential source of new materials, a low-temperature conception may turn out to be fruitful.

✓ Turning any high-temperature process into a low-temperature one is always preferable due to both lower power consumption and more practical application.

✓ The scope of organic and thermosensitive compounds as well as that of oxide systems is immense; merging both dramatically enlarges the scope of hybrid materials.

✓ An organic dopant can either deliver a required property to the matrix-oxide, or provide a multiple increase in the property already exhibited.

Despite almost a hundred-year¹ history of sol–gel synthesis, developing methods of producing crystalline sols of metal oxides remains a rather difficult problem.

In our studies, we have realized many of these predictions: new materials for electronics and spintronics, photocatalysis, biomedicine, textile, and more have emerged.

2. The methodology

2.1. Terminology

The classical interpretation of the term “sol–gel synthesis” does not specify whether the state of the product is amorphous or crystalline. The term, “low-temperature sol–gel synthesis” introduced in the present review only concerns the production of crystalline materials and imposes the following requirements on the system:

✓ The presence of a pronounced crystalline phase with an amorphous content of no more than 10%;

✓ High sedimentation stability of a sol and its capability for polycondensation and gelation;

✓ Crystallization processes carried out at a temperature of no more than 100 °C and at atmospheric pressure.

2.2. Proof of concept

The development of sol–gel synthesis methods, as one of the soft chemistry sections, started with studying the phase transitions of silicon–organic species in aqueous and aqueous–alcoholic solutions.^2–5 Formation of colloidal nanoparticles as a sol was instantly accompanied by polycondensation of disperse phases, resulting in a stable gel state.⁶ This approach was very promising for many important and spectacular applications in such domains as, for example, the stabilization of organic dyes,⁷ entrapment of enzymes⁸ and proteins⁹ for prolonged and improved activity,¹⁰ and creation of “impossible” organic catalysts for complex, one-pot acid–base and redox transformations.¹¹ The possibility of entrapping various species into a highly porous silica matrix became a revolutionary stage of development for this method.¹² Results obtained by researchers in the 60–90 s of the 20th century^13–16 allowed silica-based sol–gel materials to gain significant popularity, including in real industrial applications. However, at present, the vigorous trend in studying silica-based materials is showing an obvious deceleration. It is due to the fact that the silica matrix itself is highly inert to various physical and chemical processes owing to its amorphous structure. As a consequence, it is not active by itself and cannot the increase the activity of an entrapped component.

Despite the fact that there are no data on low-temperature crystallization of a silica sol so far, a large number of oxides and mixed oxide systems (see Section 3) have been successfully synthesized in solution at a temperature of less than 100 °C. Below, we will try to prove the possibility of this phase transition theoretically and show the practicability of this approach visually. The development of methods aimed at obtaining new sol–gel materials displaying their own activity in various processes is determined by the chances to successfully adjust the crystalline structure formed in the course of synthesis. It is quite obvious that the unique properties of a material are not only provided by the presence of a developed, ordered structure, as is the case for silica, but also by the presence of the long-range order, that is, the crystal structure. Taking into account that the formation of these sol–gel materials from solutions at low temperatures is initiated, as a rule, by irreversible reactions of hydrolysis and alcoholysis, the particles formed at the initial stage appear to be rather amorphized (more than 90%).¹⁷ In this case, it is possible to represent the structure of the particles produced by a soft chemistry method as a nucleus and a cover, where in the center is a crystal germ shrouded in a “fur coat” of ligands and intermediate products; see Fig. 1.

Fig. 1 Visualization of the growth of a crystal nucleus covered with an amorphous coating.

Three main stages of the process of crystallizing particles from a solution can be distinguished:^18–20 turning the system into a metastable state, the emergence of crystals (formation of germs) and the growth of crystals. It means that in order to achieve the maximum degree of crystallinity of hydrosols, one needs to carefully control two processes: (1) nucleation and (2) growth of the formed crystal nuclei. Thus, the transition of the system to a metastable state is the key factor determining the possibility of beginning the crystallization of a new phase, formed from a theoretically homogeneous liquid medium. In the case of the molecular precursor hydrolysis reaction yielding ultradisperse phases, the size of heterophase fluctuation in the bulk of the formed product is responsible for the emergence of a metastable state. Despite the fact that the formation of a nanoparticle as a new phase in a homogeneous solution is related to the reduction of the chemical potential of a substance (formation of intramolecular and intermolecular bonds), the change in free energy (or the thermodynamic potential) for the system may be positive. This is due to the fact that at initial stages of phase formation, an important role is played by the interface possessing the excess positive surface energy. Fig. 2A shows the change in free energy vs. the size of the formed crystal nucleus as a new phase. A maximum is distinctly observed at a certain critical value of the radius r_crit. Thus, for r < r_crit, an increase in the size of a germ of a new phase results in an increase in free energy, thereby increasing the entropy factor and destabilizing the system. For r > r_crit, the growth of the crystallization center becomes a thermodynamically favorable process at the expense of reducing free surface energy, providing a spontaneous start of the two subsequent stages (nucleation and growth of crystals).¹⁸

Fig. 2 (A) The change in free energy vs. the size of the formed crystal nucleus. (B) Illustration of the LaMer¹⁸ theory depending on the change in the concentration of the molecular precursor in solution with time.

The mechanism of further system behavior is perfectly illustrated and proved in the theory of LaMer,¹⁸ Fig. 2B. When the concentration of the molecular precursor in solution approaches the critical level required for formation of nuclei, C_crit, the system enters the initial stage of nucleation. However, this process quickly reaches a saturation maximum due to the equilibrium between the level of delivery of “building” ions and the “norm” of their consumption in the construction of the crystal framework, Fig. 2.

Thus, the theoretical proof for performing low-temperature crystallization of oxide materials in aqueous solutions using the sol–gel chemistry method is quite persuasive. A key question in this case is the selection of optimal conditions that provide specified parameters. In practice, this question is very individual and realized empirically in most cases. In the present review, we only consider the most appropriate examples, including those performed by our scientific group.

2.3. Ways to increase crystallinity in a solution

A general approach to increasing the crystallite sizes and the degree of crystallinity is promoting the diffusion of ions to the crystallization center.^17,24 In the case of aqueous solutions, boiling methods are the most popular.¹⁷ They initiate the thermal dehydration of a highly hydrated cover with its gradual decomposition; see Fig. 1. The application of this approach was considered by several scientific groups dealing with the crystallization of sol–gel materials at low temperatures.^21–27 The use of biocatalytic processes can be another interesting method for the growth of crystallites under neutral pH conditions,²⁸ with the application of biological precursors.²³ It was shown that in this case,²⁸ biodestruction of the amorphous cover containing intermediate products of transition metal complexes is performed in the course of an enzymatic reaction, transforming stable amorphous areas into ionic configurations. As a result, this also promotes the formation of nanocrystalline phases in aqueous solutions. Removal of organic ligands and curing of the defects can be achieved even by increasing the polarity of the medium and facilitating protolytic mechanisms by creating extreme pH. Thus, fully crystalline particles of BaTiO₃ can easily be produced at 90 °C by introducing titanium alkoxides into an aqueous solution of the strong base Ba(OH)₂.²⁹ The same effect is achieved by the reported techniques for improving the crystallinity of TiO₂ by peptization. Indeed, “re-dissolving” in HNO₃ the XRD amorphous precipitate from the hydrolysis of titanium alkoxides provides a stable sol consisting of uniform anatase nanoparticles.^24,30–33 Among other ways of activating the crystallization (not the least important, and gaining popularity at present) are external physical factors (ultrasound, microwaves, UV) under atmospheric pressure.^26,35 The general mechanisms for the effects of a medium's pH and external physical factors on the growth of crystalline phase are considered below.

We would like to highlight the two approaches (chemical and physical activation) and to consider these in more detail, see Fig. 3. Attention is paid to the prospects of the present approaches, not only from the viewpoint of successful crystallization of oxides and their derivatives in aqueous solutions, but also to demonstrate successful stabilization of the crystalline sols, which are capable of participating in the further stages of polycondensation and sol–gel process.

Fig. 3 Effect of acidic peptizers on the growth of TiO₂ crystallites in aqueous solutions.

2.4. Effect of acidic peptization on the formation of crystalline sols

Sol formation consists of hydrolysis and condensation reactions, which are catalyzed in the presence of an acid. The hydrolysis reaction leads to the formation of original nuclei or basic units, while the condensation reaction leads to particle growth as predicted by the LaMer theory, with the strength of the acid and its concentration affecting the crystallization of the amorphous shell of the crystal-shell particles formed during condensation. In particular, the effect of the concentration ratio [Meⁿ⁺]/[H⁺], temperature and type of precursor is reported by Liu et al.,³⁵ as predicted by the LaMer theory,¹⁸ Fig. 2B. The addition of an acid to amorphous hydroxide MeO(OH)_n shell was found to result in breaking oxo- bonds and protonation of the crystalline particle surface. A decrease in particle size also results in the stimulation of crystallization processes,¹⁹ with an increase in nucleation, rather than growth of the formed crystals, being the rate-limiting step.

In Fig. 3 we compare the size of the anatase crystals vs. the concentration ratio (Ti]/[H⁺]) and protonating agent strength, log(K_d(acid)). To obtain the data, four acids with various dissociation degrees in aqueous solution were used. More oxo bonds can be broken among titanium atoms by using a stronger acid, which can affect the nucleation and crystal growth of TiO₂, leading to bigger crystallites as well as colloid particles of TiO₂. It is expected that this could beneficially influence the size of final colloidal particles, in particular, the size of the crystalline core. This method allows not only the formation of highly photoactive crystalline particles as sol colloid, but also nano-dispersed powders capable of re-suspension in aqueous solution, given their initial colloid state.

2.5. Physical activation of colloidal particles and growth of crystals

Physically- and mechanically-assisted syntheses are appealing because they can dramatically reduce reaction time, improve product yield, and enhance material properties when compared to conventional synthesis routes.^36–38 While conventional heating is limited by thermal conduction from the vessel walls, physical fields (ultrasonic, electric, microwave, ultraviolet, etc.) can quickly and uniformly heat a solution by directly coupling molecules within the solution through polarization or conduction. Polarization is the process of dipole formation from bound charges and aligning polar molecules along an oscillating electric field. Conduction is the motion of free charge carriers and ions in response to an electric field.

Thus, it is possible to provide significant activation of colloidal phases both by disaggregating agglomerates and increasing ion mobility to initiate the accelerated processes of crystallization, as illustrated in Fig. 4.

Fig. 4 Comparative visualization of the mechanism of growth of crystals formed under the influence of (A) physical and (B) acidic peptization.

For example, using the microwave-assisted sol–gel route, crystalline colloids of ZnO,³⁹ ZrO₂,⁴⁰ TiO₂,^26,41 SiO₂,⁴² WO₃,⁴³ AlF₃,⁴⁴ MgO⁴⁵ have been obtained, as well as some mixed oxides such as BaTiO₃,^46,47 etc.⁴⁸ Using ultrasonic treatment, the crystalline particles of TiO₂,^34,49 AlOOH,²⁷ ZrO₂ (ref. 50) and some mixed oxides Pb(Zr,Ti)O₃ (ref. 51) were synthesized at room temperature. However, although the existing approaches in most cases lead to an increase in crystallinity, they do not allow the formation of stable colloidal systems at neutral pH values. For this purpose, stabilizers are usually used, or solvents with different polarities are selected. At the same time, only a small number of papers report original methods for the formation of highly crystalline sol–gel systems in aqueous solutions at the neutral pH that are promising for biochemical engineering and electronics.^27,34 For instance, our group has succeeded in employing a low-temperature, ultrasonic-assisted method to prepare nanocrystalline TiO₂ (ref. 31) and AlOOH²⁷ sol–gel materials directly from the hydrolysis products of alkoxide precursors in an aqueous medium with no addition of modifying agents.

2.6. Peculiarities of sol–gel transition for nanocrystalline particles

As was mentioned before, the crystallization of colloid particles inevitably leads to the formation of core–shell structures. The further behaviour of the particle is defined by the composition and size of the amorphous shell. It is well known that the key parameters for inorganic polymer growth are the thermodynamic characteristics of the particle state (such as surface energy, the sign of the surface potential, crystallographic energy of interaction within the crystal, etc.) as well as the external parameters (temperature, PH, medium etc.)

Obviously, the main role in further particle growth, their aggregation and the formation of inorganic frame (gel) is given to residual ligands acting as interface to the surrounding solution.

The role of controlled hydrolysis has already been determined, whereas the reaction control by polycondensation between the amorphous shells of the crystalline particles in solution will define the possibility of low-temperature sol–gel transition.

The role of ligands in the formation of the primary particles was found to be the facilitation of hydrolysis–polycondensation and also the stabilization of emerging particles in solution against aggregation through enhanced interaction with the solvent molecules. The direct kinetic data show that the speeds of hydrolysis and polycondensation objectively increase when chelating heteroligands are introduced in the precursors.¹⁷ Simply put, the ligands act as surfactants in the stabilization of the particles, and quantitative evaluation of this role has been reported.⁴² These particles are both individual molecular species, as well as colloid particles stabilized in solution by interactions between the ligands and the solvent by the forces typical for colloid systems. This finding led to a proposal to denote them as Micelles Templated by Self-Assembly of Ligands (MTSALs).¹⁷

3. Scope of nanocrystalline objects for low-temperature sol–gel synthesis

A large number of papers are devoted to producing crystalline ceramic oxides from aqueous solutions at low temperatures.⁶⁰ Among them (in order of increasing atomic number of metal) are Al₂O₃,⁵² AlOOH,²⁷ TiO₂,^53–59 V₂O₅,⁶¹ MnO₂,⁶² Mn₃O₄,⁶³ β-FeOOH/α-Fe₂O₃,⁶⁴ Fe₃O₄,⁶⁵ CoO,⁶⁶ Co₃O₄,⁶⁷ NiO,^68–70 NiFe₂O₄,⁷¹ Cu₂O,^72,73 CuO,⁷⁴ ZnO,^75–81 ZrO₂,⁸² AgO,⁸³ Ag₂O,⁸³ CdO,^84,85 Cd₂SnO₄,⁸⁶ In₂O₃,⁸⁷ SnO₂,^88,89 LnMO₃ (Ln = La, Nd; M = Cr, Mn, Fe, Co),^90,91 LaNbO_x,⁹² CeO^2+x,⁹³ Tl₂O₃,⁹⁴ α-PbO₂ (ref. 95) and many others not presented here.

Using already-perfected methods of chemical and physical peptization, the production of stable sols is, as a rule, not a problem for practically all of the presented systems. In this review, we only dwell upon crystalline sols, which are the most intensively studied and used because they are stable in a wide range of temperatures and pH values.

3.1. Alumina

Aluminum oxides and hydroxides amount to dozens of various polymorphic modifications. In a single γ-Al₂O₃ → α-Al₂O₃ process, nine transition modifications of alumina have been found (δ, ε, η, θ, ι, κ, π, ρ, χ).⁹⁶ Annual production of alumina amounts to more than 100 million tons, and, of course, one of the main usage applications is catalysis. The presence of acid–base centers, its high specific surface area, economical availability and relative stability determine its use in the major processes of oil refining and organic synthesis as a support of catalytically active phases.^97–103 Apart from its immense role in catalysis, crystalline aluminum hydroxide also finds application as a fire retardant filler for polymer applications.¹⁰⁴ It decomposes at about 100–300 °C, absorbing a considerable amount of heat and giving off water vapor. In addition to behaving as a fire retardant, it is very effective as a smoke suppressant in a wide range of polymers, especially in polyesters, acrylics, ethylene vinyl acetate, epoxies, PVC and rubber.¹⁰⁴ Aluminum hydroxides are indispensable for pharmaceutics owing to their high activity as antacids and adjuvants.^27,105 Boehmite is the only aluminum hydroxide approved by FDA for parenteral injections.

Four polymorphs of aluminum hydroxide exist, all based on the common combination of one aluminum atom and three hydroxide molecules into different crystalline arrangements that determine the appearance and properties of the compound. The four combinations are: gibbsite¹⁰⁶ (γ-Al(OH)₃), bayerite¹⁰⁷ (β-Al(OH)₃), nordstrandite and doyleite (both designated as Al(OH)₃). Two oxyhydroxides are also available: boehmite (γ-AlOOH) and diaspore α-AlOOH. Each of the presented hydroxides and oxyhydroxides has its own unique structure and set of functional properties. The literature provides thousands of different examples of preparing the sols of both monohydroxides (gibbsite,¹⁰⁶ bayerite¹⁰⁷) and trihydroxide,¹⁰⁸ but only few papers on entrapping organic molecules into a matrix of crystalline aluminum hydroxide are available.^{27,105,109,110}

In Section 4.2, we consider various examples of entrapping organic molecules in detail and provide evidence for prospects of developing this direction.

Three classes of alumina precursors are typically used for the preparation of crystalline sols: (1) aluminum alkoxides, (2) inorganic alumina salts, and (3) aluminum oxide hydroxide (boehmite) or aluminum hydroxide (bayerite), either in the form of a gel or as a dispersed nanopowder. The hydrolysis of aluminum alkoxides was first described by Yoldas in 1973.¹¹¹ In this work, it was shown that depending on the reaction temperature, the hydrolysis and polycondensation can result in boehmite or amorphous aluminum monohydroxide. A preparation method for porous, transparent aluminum oxide films from aluminum alkoxide sols was also described.¹¹² This opened a new route in alumina sol–gel chemistry. Aluminum alkoxides are very reactive, so that the addition of chelating agents to control both the hydrolysis and polycondensation rate is often required.¹¹³ Aluminum isopropoxide and aluminum sec-butoxide were both used to obtain high-surface-area monolithic gels with crystalline structure.¹¹⁴

It is well known that aluminum salts hydrolyze and condense into aluminum oxide hydroxide gels under basic conditions.¹¹⁵ However, aqueous aluminum salt solutions are acidic due to partial aluminum hydroxo complex hydrolysis, as shown by the following equation:¹¹⁶

[Al(H₂O)₆]³⁺ + nH₂O = [Al(OH)_n(H₂O)_6−n]⁽³⁻ⁿ⁾⁺ + nH₃O⁺

Increasing pH shifts the equilibrium to the right. During basification, Al³⁺ ions undergo several intermediates,¹¹⁷ and the subsequent condensation (via olation and oxolation) results in the formation of polynuclear hydroxides or oxo-hydroxides, eventually leading to sol–gel transition. Focus has been put on controlling the hydrolysis by the gradual and homogenous increase of pH.

Several examples of both chemical and physical peptization of crystalline nanoparticles of aluminum hydroxides exist.^27,118–121 Fig. 5 shows images of boehmite xerogels produced using treatment by acetic acid (Fig. 5a and c) and ultrasound (Fig. 5b and d). Despite a substantial difference in preparation, both samples demonstrate similar structure.

Fig. 5 SEM and TEM images of boehmite xerogels prepared with acetic acid (a and b) and ultrasound treatment (c and d).

3.2. Iron oxides

Iron oxides play a crucial role in the development of modern technology and materials science. A large number of different phases of iron oxides, hydroxides or oxy-hydroxides are well known to date. Among those, the most studied are Fe(OH)₃, Fe(OH)₂, Fe₅HO₈·4H₂O, Fe₃O₄, FeO, five polymorphs of FeOOH and four of Fe₂O₃.^122,123 These oxides can be synthesized by all known wet chemical methods, but tailoring the particle size in nano range and morphology towards a particular application still remains a challenging task. Synthesis of iron oxides in the nano range for various applications has been an active area of research during the last two decades. In order to achieve the required results, one has to carefully select the pH of a solution, concentration of reagents, temperature, mixing procedure and oxidation rate.¹²⁴ Morphology of the formed particles also depends on several processes, such as nucleation, growth, aggregation and adsorption of impurities. At the same time, producing an iron oxide with selected particle size and morphology often fails, and for this purpose, approaches to achieve the transition of one polymorphic modification of iron oxide to another are employed.¹²⁵ The sensitivity of the preparative method complicates both the reproducibility and scale-up of the process. Recently, several colloidal chemical synthetic procedures have been developed to produce mono-disperse nanoparticles of various materials. This includes the classical LaMer mechanism, Fig. 2B, wherein a short burst of nucleation from a supersaturated solution is followed by the slow growth of particles without any significant additional nucleation, thereby achieving a complete separation of the nucleation stage.¹⁸ Here are some examples of sol–gel synthesis of iron oxides with different crystalline structure. Again, no calcination has been applied to achieve crystallinity.

The most popular method of preparing a sol of iron oxy-hydroxide employs the hydrolysis of the Fe³⁺ cations. However, aging of the sol prepared by pouring fresh ferric solutions into concentrated NaOH or KOH solutions has to take place at 60–80 °C for a period of time ranging from a few days to several weeks.¹²⁶ In ref. 127, the sol–gel synthesis of α-Fe₂O₃ was carried out from condensed ferric hydroxide gels obtained from FeCl₃ solutions in NaOH. After aging the gel at 100 °C for 8 days, mono-disperse, pseudo-cubic α-Fe₂O₃ particles were obtained. The reaction proceeded through a two-step phase transformation from precipitated Fe(OH)₃ gel, to a fibrous β-FeOOH, and finally to α-Fe₂O₃. Oxidation of the Fe²⁺ ions in a solution at neutral pH values usually results in the formation of finer crystallites than those produced in alkaline Fe³⁺ solutions. Moreover, oxidation of the ferrous salt solutions by air bubbling yields one or several of the following products: goethite (α-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe₃O₄) and hematite (α-Fe₂O₃).¹²⁸ Crystallization of iron oxides also strongly depends on anions and cations being adsorbed on the surface of the formed particles. Furthermore, various phases formed during the oxidation of aqueous ferrous systems suggest that the oxidation rate, which can be influenced by pH, temperature and other additives (anions or cations), is the dominant factor in determining the hydrolysis product. Using the same precursors, α-FeOOH, γ-FeOOH, and Fe₃O₄ with different morphology can be obtained by adjusting the synthesis conditions.¹²⁹ Acicular goethite particles were obtained after aerial oxidation of iron(II) solutions at 20–80 °C in acidic conditions. Depending on the temperature, the length of the particles varied between 0.1 and 0.5 μm, and the aspect ratio lay between 5 and 10. Poly-disperse microcrystals of magnetite were produced at neutral and basic pH at 90 °C by the addition of KNO₃ to FeCl₂ and KOH solution.¹²⁹

Despite the fact that not all types of iron oxides have been obtained using the sol–gel method, we actually believe that this is only a question of time and motivation, since very attractive mechanisms of chemical and physical peptization exist at present.

3.3. Titania

Since the discovery of the unique activity of Pt_cat/TiO_2(anode) photocatalysts in the oxidation of H₂O molecules to hydrogen and oxygen under the influence of sunlight, titania-based materials have become some of the most studied to date.^130,131 Numerous investigations have been performed to study and improve the photoactivity of TiO₂.^132–134 For this purpose, a large number of various methods of production have been analyzed to realize the possibility of adjusting its properties in the course of synthesis and to achieve the maximum effect.^{17,24–26,34,134–137} We would like to give prominence to the most important issue in this multitude, namely, the controllable growth of TiO₂ crystalline phase from aqueous solutions of molecular precursors as the main method for increasing the photoactivity of the materials that are produced using it as a basis. Moreover, synthesis of crystalline particles of TiO₂ phases at low temperatures (<100 °C) eliminates the need for using high temperatures, which is economically extremely important. Thus, one can achieve efficiency not only by a decrease in applied energy, but also by reducing the time between production of a material and its final application. This commercial interest was instantly followed by a huge number of papers aiming at avoiding high-temperature or harsh chemical conditions and devoted to low-temperature formation of TiO₂ nanocrystals, which results in a number of certain benefits: (1) it allows in situ functional and structural modification with thermally unstable species, such as biomaterials,¹³³ metal–organic frameworks (MOF),¹³⁸ dyes,¹³⁹ and organic LED by entrapping these directly into the inorganic polymer matrix, which was previously considered impossible in crystalline titania gel; (2) it enables adjusting the growth of crystallites, average particle size, as well as the degree of crystallinity; (3) it facilitates carrying out the processes of forming nanocomposites in a polymer matrix, using polymer solutions as reaction media and performing the polymerization of monomers in the presence of the formed sol system to achieve a homogeneous distribution of particles. The main drawback of low-temperature methods is the low crystallinity of obtained materials and, as a consequence, low photocatalytic activity of species. The key approach providing a solution to this problem is the sol–gel method.^{17,24–26,34,135–137,140,141} A low-temperature procedure for the synthesis of TiO₂ using a water-soluble titanium complex and enzymes was developed.²⁸ The formation of nanocrystalline and monodisperse TiO₂ from a water-soluble and stable precursor, ammonium oxo-lactato-titanate, (NH₄)₈Ti₄O₄(lactate)₈·4H₂O, was also described.²³ Previously, we have studied mechanisms of the growth of TiO₂ crystalline phase caused by chemical peptization in aqueous solutions.²⁴ Thermal dehydration of hydrolysis products of TiCl₄ to obtain a crystalline sol of anatase was investigated.²² Ref. 25 reports an original method of obtaining a suspension of nanocrystalline TiO₂ powder using titanium methoxide as a precursor, which possesses an anomalously high specific surface area.

However, all similar investigations imply either chemical modification of the surface of the formed particles, or chelation of the used precursors at the hydrolysis stage. Moreover, the use of aqueous solutions for producing a crystalline sol of TiO₂ is complicated by a high degree of hydrolysis for the used precursors. Most of those are inorganic salts, the use of which inevitably results in consequent purification stages for the materials obtained. The use of alkoxides in the hydrolysis process is complicated by residual terminal alkoxide groups in the structure of TiO_2−n(OR)_2n, which cannot be removed even with a large excess of water.³⁵ Above all, a complex effect of low-temperature conditions on the formation of crystalline titania, growth of crystalline phase and nucleation remains unclear.

A solution to these problems is the use of physical and chemical peptization to activate the colloidal phases in nano-sized, titania-based aqueous solutions; see Fig. 4. For example, Reeja-Jayan et al. were the first to perform microwave-assisted formation of TiO₂ nanocrystalline films on a flexible polymer substrate, which are suitable for use in electronics.²⁶ In our recent paper,¹⁴² we also report potential applications of low-temperature sol–gel methods for the formation of conducting coatings. Ref. 41 also reports a successful synthesis of TiO₂ particles by hydrolysis of tetraisopropyltitanate under ultrasonic irradiation. However, they used acetic acid as a dispersant, protonating the surface of the formed particles, which prevents the formation of stable sol–gel systems.

It is paradoxical, but according to previous results^{17,24–26,31–34} the maximum growth of TiO₂ crystalline phase is attained using aqueous solutions, despite a high rate of hydrolysis for alkoxides under these conditions. In this case, pH has a crucial effect on phase composition, and peptization promotes the preservation of crystallinity and formation of stable sol–gel systems. Fig. 6 suggests that the use of strong acids, HNO₃ and HCl, leads to anatase–brookite crystallites with almost equimolar ratio, see Fig. 6A and B. A decrease in acid strength illustrated by acetic acid results in crystallization of anatase TiO₂, Fig. 6C, which is analogous to the effect of ultrasonic and microwave treatment at neutral pH values.

Fig. 6 X-Ray diffraction patterns of prepared TiO₂ samples using different protonating agents: (A) TiO₂ (HNO₃), (B) TiO₂ (HCl), (C) TiO₂ (acet.), (D) TiO₂ (H₂SO₄).²⁴

Obtaining titania by the sulfate method forms a mixture of crystalline Ti(SO₄)₂ and anatase, Fig. 6D, sometimes called thetta-phase.¹⁴³ The behavior of protonated particles in a solution and their ability to form aggregates in an aqueous medium depend on the value of the surface charge. A change in zeta potential of particles results in the formation of titania-based particles with different morphology. In the case of a high protonating degree, particles with narrow size distribution, about 15 nm, are formed (see Fig. 7A); the use of acetic acid as a stabilizer promotes the formation of nanorods, Fig. 7B, and the formation of nanocrystalline titania sol under ultrasonic treatment leads to dense packing of particles with the size of about 15 nm. The absence of a strong surface charge in the latter case also determines the possibility of gelation, with the subsequent formation of a xerogel, which undoubtedly increases the practical significance of this approach.

Fig. 7 Morphology of the surface of dip-coating films produced from aqueous solutions of nanocrystalline TiO₂ sols prepared using (A) HNO₃, (B) acetic acid as a peptization agent, (C) ultrasonically activated TiO₂.

3.4. Mixed oxides

Numerous oxides have been produced as polycrystalline thin films from aqueous solutions at low temperatures. Although most of the examples reported to date are single oxides, multicomponent films have been deposited, including doped single oxides (ZnO:Ni, Cu, Cd, Al, Sn; SnO₂:Sb; In₂O₃:Sn), solid solutions (spinel, ferrites; ZrO₂–Y₂O₃), and stoichiometric compounds (CdSnO₄; perovskites). From a practical point of view, the relative metal concentrations in solutions must be adjusted empirically to obtain a desired ratio of metals in the mixed oxides; different precipitation kinetics can make it difficult to achieve simultaneous and uniform precipitation of both components in the ratios desired for stoichiometric mixed oxides. Usually, oxides of high-valence (III–V) metal oxides can be prepared from acidic solutions, whereas low-valence (II–III) metal oxides have been deposited at neutral to basic pH. This reflects a fundamental difference in the precipitation chemistry of these metals and affects the formation of multicomponent oxides. The recipes to solve these problems may be found in the extensive literature on the deposition of non-oxide films from aqueous solutions, or in the sol–gel literature where films of such mixed-oxide compounds as Pb(Zr, Ti)O₃, BaTiO₃ and LiNbO₃ have been successfully synthesized. These methods are based on direct interaction of components of a multiphase system, in which at least two types of solid ultradisperse particles, generated from molecular predecessors, are present. In this case, the formation of a nanocomposite takes place due to a deep intercomponental penetration of disperse phases at the nanolevel during the course of sol–gel reactions.^31,33 Such an approach, in combination with chemical or physical peptization, will obviously make it possible to realize a simple way to synthesize sols of crystalline mixed oxides at low temperatures directly.

For example, we have studied³³ the formation of crystalline CoTiO₃ depending on stoichiometric ratios of reactants and subsequent annealing temperature. Synthesis of reactants proceeded in a single stage by mixing two solutions. The first solution was obtained by peptization of pyrochroite in highly acidic aqueous medium upon stirring and heating to 70 °C, and the second one was prepared by mixing isopropyl alcohol and titanium isopropylate (C = 0.05 M). Stirring of the resultant solution was carried out for 4 hours at 80 °C. Then, the effect of stoichiometric ratios of components on the structure of the formed products was studied. Table 1 lists molar ratios of reagents and the results for each experiment.

Table 1 Stoichiometric ratios of components for each experiment

Experiment no.	1	2	3	4
Ti(OC3H7)4 mol	0.027	0.027	0.027	0.027
[Co(OH2)](OH)2 mol	0.002	0.008	0.011	0.013
HNO3 mol	0.012	0.012	0.012	0.012
H2O mol	2.778	2.778	2.778	2.778
Temperature of formation for CoTiO3	900 °C	900 °C	80 °C	530 °C
Image of the formed product

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Visual data of thermal analysis for systems 1, 3, and 4 (see Fig. 8) demonstrate that only under certain conditions, such as in Sample 3, is the formation of a single-phase system that does not undergo phase transitions during heating observed. This conclusion is confirmed by the absence of characteristic thermal effects in the region of 450 °C from the DTA and TG analysis. The most essential result is presented in Fig. 9c. It shows that annealing the hydrated cobalt–ilmenite-type crystallites does not lead to phase and polymorphic transformations in a material, except for removal of the adsorbed and chemically bonded water. This conclusion confirms the possibility of forming the CoTiO₃ prestructures as a result of performing a low-temperature sol–gel synthesis without subjecting the material to subsequent heat treatment.

Fig. 8 Thermal analysis of samples obtained according to experimental conditions of experiments: (a) 1; (b) 3; (c) 4.

Fig. 9 The SEM images of free (1) and entrapped AgNWs: AgNW@Al₂O₃ (3), AgNW@TiO₂ (4). Demonstration of AgNW@Al₂O₃ as a transparent and conductive film – the LED bulb is lit (2).

The number of articles on producing sol–gel mixed oxides possessing a high degree of crystallinity and, as a consequence, high functional characteristics is continuously increasing. In our papers,^31,33 we have proved the possibility of obtaining crystalline mixed oxides using sol–gel chemistry for several systems without an annealing stage.

4. Properties and application

The development of low-temperature methods of producing crystalline phases of metal oxides from solutions extends the areas of their practical application substantially. Modern materials science numbers about 0.5 million of inorganic substances, the most part of which are thermally stable, whereas there are more than 27 million possible organic substances. Selection of optimal conditions for sol–gel synthesis of crystalline phases of inorganic oxides opens new possibilities for producing organo–inorganic hybrids with unique properties, the majority of which are still expected to be produced and studied. In this section, we present some examples, which perfectly combine the boundless possibilities of low-temperature sol–gel synthesis of crystalline oxide systems.

4.1. Electronics

Among conductive ceramic materials, the most popular transparent films are ITO and FTO ceramics. High optical transparency and low film resistance have resulted in its wide application in many areas.^144–147 However, these materials are completely unsuitable for flexible devices because of their brittle nature. Moreover, a rapid increase in the price of indium and the absence of methods for obtaining similar systems at room temperature and atmospheric pressure with a minimum set of equipment motivated us to create a new class of transparent, conductive and flexible systems with good adhesion. This was achieved by entrapment of silver nanowires within sol–gel materials.¹⁴² We will dwell here only upon the samples which showed excellent combination of transparency/conductivity, and both of these samples were prepared with crystalline hydrosols – boehmite (AlOOH) and anatase (TiO₂). A very simple procedure was applied for preparation of the films: mixing AgNWs and hydrosols at different ratios with subsequent spraying on a substrate. In contrast to the other transparent conductive oxides, this method can be mastered by any living person. It is interesting to note that the greatest electrostability of films with an optimum set of transparency and conductivity was observed for crystalline matrices, which reveals a crucial role of nanocrystals in improving operational characteristics of the samples.

SEM images of such films are shown in Fig. 9. In that work, four key parameters of these films were determined: conductivity comparable to ITO analog, excellent electrostability, high flexibility and exceptional adhesion to the substrate. Results obtained in the this work¹⁴² are of great interest, not only from the point of view of producing promising materials for flexible electronics, but also because they are actually precursors for creating absolutely new devices. Entrapping an organic dopant into a crystalline sol–gel matrix allows the development of new sensors for bio-application (due to biocompatibility of boehmite and silver), as well as electrochromic coatings, solar cell elements, accumulators and supercondensers (due to crystalline anatase in the AgNW@TiO₂ composite).

4.2. Bioapplication

A major obstacle in the introduction of bioactively-doped sol–gel based materials for medical applications has been the fact that the most widely studied sol–gel material, silica, despite being a GRAS material widely used as an additive in food and drug formulations, is still not approved by regulatory agencies for parenteral injections. Some works^27,109 point to a potential solution of this problem by shifting the weight to alumina, which is approved by FDA and EMA for injections as the most common immunologic adjuvant. Again, we have to address here the one crystalline phase of alumina that can be used. Out of the six crystalline phases of alumina, the biologically active form of alumina used as an adjuvant in current vaccines is boehmite. Boehmite (AlOOH) adjuvants, used in commercial vaccination for more than 70 years, have gained widespread recognition and are considered to be safe for humans. Prospects of employing sol–gel boehmite as an alternative to existing alumina-based adjuvants consist not only in the possibility of entrapping bioactive molecules, but also in the possibility of widely adjusting the porous structure of the matrix carrier by the initial conditions of synthesis.

An original approach was developed to obtain the matrix (boehmite) itself using highly pure and biocompatible materials. Previously, this was considered impossible without the use of acids or additional chelate molecules. Adding therapeutic enzymes to the crystalline hydrosol substantially increases the stabilization of an enzyme. The thermal stability was studied in detail by kinetics follow-up, differential scanning calorimetry and circular dichroism. These techniques indicate that the entrapment shifts the temperatures of denaturation higher by 30–50 °C. The authors have even shown that the boehmite matrix, over a certain temperature range, quite significantly enhances the activity of the enzymes with temperature, whereas free enzymes die out at these conditions. The DCS signals of temperature denaturation for free and entrapped acid phosphatase, which is used for treatment of bone dysplasia, are shown in Fig. 10.

Fig. 10 DSC analysis of free and entrapped acid phosphatase. An increase by 51 °C in the denaturation temperature is observed for AcP@alumina (right curve) as compared to free AcP (left curve).

Similar observations were also found for horseradish peroxidase (HRP), used for the release of toxic drugs in vivo from their pro-drugs, and asparaginase (ASP), used for starving cancerous leukemia cells. Thus, when HRP or ASP are heated in solution to 75 °C, their activity drops by 65% and 72%, respectively, but when entrapped, it drops only by 1.2% and 1.9%, respectively.

The exceptional thermal stability in the case of entrapment within alumina leads us to suggest that the rotational mobility of the enzyme within alumina is restricted even to a higher degree than in silica, providing the needed conditions for thermodynamic stability. This observation can be related to the crystalline nature of the alumina matrix. Amorphous silica is relatively soft compared to alumina, and during heating, the structure can still rearrange, providing more freedom, eventually leading to earlier unfolding. The dense structure of crystalline boehmite nanorods of alumina keeps enzymes tighter, preventing easy unfolding.¹⁰⁹

Controlling the release rate for medications in crystalline sol–gel matrices provides large discretion due to the possibility of adjusting not only the size of particles, but also their shape. The results obtained, see Fig. 11, confirm the possibility of a slow release from crystal matrices. Cisplatin, an anti-neoplasmic medicine, was used as a model drug in this study. Cisplatin was entrapped within alumina by adding the drug during the sol–gel manufacturing process. Furthermore, the research aim is the determination of the dynamics of cisplatin release from sol–gel processed alumina xerogels. Taking into account the volume of global market of biomaterials with slow drug release and controlled delivery, the present work is of major applied importance.

Fig. 11 Short-time (a) and long-time (b) release profile of CSP from alumina and its fit to the Weibull model.

The kinetics of CSP release from the alumina into histidine buffer (pH = 7.4) was measured, and the results are presented in Fig. 11. It is seen that the profile of cisplatin release from alumina is represented by gradual release. This behavior was found to fit the Weibull model. 30% of the total amount of CSP was released in 12 hours (Fig. 11a). The long-release kinetics of CSP was studied as a function of time over two weeks (Fig. 11b). After 12 days of assay, the studied systems showed similar trends of release. During this period, alumina showed 88% release of CSP.

4.3. Magnetic properties

Diluted magnetic semiconductors (DMS) are one of the most promising materials in spintronics.^148,149 Among a number of materials belonging to DMS, the greatest interest is paid to the systems based on crystalline zinc and titanium oxides doped with 3d metals, since the Curie point for such nanomagnetics is close to room temperature.^150,151 It is worth noting that at present, the methods of point physical manipulation are, as a rule, employed for producing DMS structures, implying the use of high-vacuum equipment and high temperatures.¹⁵² Therefore, obtaining DMS nanocomposites with an organic matrix becomes impossible, so that using low-temperature sol–gel synthesis in this case also seems to be rather promising. Existing approaches to producing ZnO or TiO₂ DMS doped with the 3d metals represent various variants of the classical sol–gel method, employing various precursors and reductants of metal ions, the action of UV irradiation, various catalysts of precursor hydrolysis, additives of polymers, or surfactants to achieve homogeneity for a system of sols or gels.^153,154 Production of solid semiconductor oxide films doped with the 3d metals was the result of such syntheses. Such an approach implies thermal treatment of the product of sol–gel synthesis (temperatures up to 700 °C), which affects the structure and properties of products, often resulting in undesirable effects – the formation of chemical compounds, a change in the degree of oxidation, etc. This is confirmed by the results of numerous works on the physics of diluted magnetic semiconductors. The originality of our studies comes from the fact that a new approach to the synthesis of DMS has been developed for the first time, which allows the production of well-crystallized materials in liquid phase in the process of sol–gel synthesis without an annealing stage.

During the realization of the project, we have used a methodology based on the comparative analysis of magnetic and transport characteristics of materials possessing identical chemical composition, but different structures and degrees of oxidation of elements forming the basis of magnetic phases. Conditions leading to the formation of both magnetic and non-magnetic phases upon interaction of the system components have been simulated. It revealed the effect of the presence of non-magnetic phases, emerging in the magnetic process, on magnetic properties of the final products. Studies performed have shown that our suggested conditions of sol–gel synthesis allow the use of low-temperature methods for producing DMS possessing ferromagnetic properties at room temperature. This success is, first of all, determined by crystallization of products from aqueous solutions with the possibility of forming stable sol–gel systems. Fe₂TiO₅ (ref. 30) produced in the process of low-temperature sol–gel synthesis is a solid solution of the Ti⁴⁺ ion isomorphically replaced by Fe³⁺ in a xerogel of titania crystallized in the structure of anatase, with the Fe/Ti stoichiometric ratio corresponding to pseudo-brookite containing a number of the Fe²⁺ ions. Magnetic properties arise in this material due to an incomplete transition of Fe²⁺ to Fe³⁺ upon peptization of Fe₃O₄ nanoparticles in the medium of nitric acid. Apparently, the architecture of the produced material is composed of nanoparticles constructed according to the nucleus-cover type. These heterostructures include nuclei of magnetite nanoparticles whose surfaces are wrapped in a Fe₂O₃ cover, in which the Fe³⁺ ions are isomorphically replaced by the Ti⁴⁺ ions.

Apparently, the phase volume of the cover of Fe₂O₃ with the Fe³⁺ ions, isomorphically replaced by the Ti⁴⁺ ions, is considerably greater than that of the Fe₃O₄ nucleus. Introducing additional quantities of titania upon producing materials similar to diluted magnetic semiconductors results in the distribution of such particles over the material volume. The analysis of magnetization curves for some of the produced materials (Fig. 12A) shows that all of them possess weak coercivity in weak fields, i.e. PVP–Fe₂TiO₅ + TiO₂ nanocomposites possess ferrimagnetic properties.

Fig. 12 (A) Effective magnetization of nanocomposites with the inorganic phase content of 50% in polyvinylpyrrolidone, calculated for conditions of a uniform material vs. magnetic field intensity. PVP is used as a filler. (B) Curves of magnetization for Fe₂TiO₅ + TiO₂ composites in a PVP matrix were calculated with respect to the magnetic phase content.

Reducing magnetic characteristics to the relative magnetic phase content levels the characteristics of materials with high concentration of magnetic compounds, Fig. 13B. As Fig. 12B suggests, at 7% Fe₂TiO₅ content in the PVP matrix, the magnetization parameters of the nanocomposite decrease as compared to the materials containing 25 and 50% Fe₂TiO₅. Apparently, this is due to a decrease in the sizes of nanoparticles in the composite and a decrease in the volume of the magnetite nucleus in the structure of nanoparticles that comprise it. It may also be due to the peculiarities of synthesis: while producing a 5% composite, synthesis of the magnetite sol is performed in diluted solutions, which results in the formation of ultrasmall magnetite particles, smaller than those produced in the synthesis of more concentrated magnetite suspensions.

Fig. 13 Visualization of the formation of Fe(III)-doped TiO₂ nanocrystals by a soft chemistry method.³⁰

4.4. Photovoltaic and photocatalytic activity

Nowadays, it is a great challenge to synthesize crystalline TiO₂ possessing high photoactivity¹⁵⁴ by using low-temperature methods without an annealing stage. Its popularity as a photocatalyst is mainly due to its high chemical and thermal stability, as well as low toxicity and cost.¹⁵⁵ Application of titania strongly depends on its specific physico-chemical and semiconductor features.¹⁵⁶ The degree of crystallinity and specific surface area are considered to be the main characteristics affecting photoactive properties. An increase in crystallinity results in prolongation of the rate of recombination of photoinjected electrons and holes, which increases the reducing and oxidizing ability of the photocatalyst. An increase in the surface area leads to an enhanced adsorption ability for target molecules and, hence, results in acceleration of the catalytic process. However, both of these parameters are mutually exclusive, and the search for an optimum ratio is still ongoing.⁴⁴ High crystallinity is achieved upon annealing the samples at temperatures of 400 °C and above. The same approach is, as a rule, also used^158,159 for obtaining materials with high photocatalytic properties. However, an increase in the degree of crystallinity results in the collapse of the porous structure and a sharp decrease in specific surface area. Data listed below in Table 2 comparing the morphological and photocatalytic properties evidently show that using low-temperature techniques of TiO₂ crystallization provides photodestruction effects analogous to those observed with the commercial product Degussa P-25, therewith maintaining high specific surface area and saving megawatts of energy expended in thermal treatment (annealing temperature 350 °C).¹⁵⁷

Table 2 Structural and photocatalytic properties of synthesized samples^24a

Sample	Crystallite size, (nm)	Phase ratio, (%)	Aggregate size D, (DLS) (nm)	Zeta potential, (mV)	Surface area, (m^2 g^−1)	t1/2, (min)
a A – anatase, B – brookite, TS – titanium sulfate.
TiO2 (Acet.)	4.91	A – 100	56	16	88	90
TiO2 (H2SO4)	A – 3.7	TS – 42.8	38	20	156	45
	TS – 2.77	A – 57.2
TiO2 (HCl)	A – 3.97	B – 41.7	24	23	374	26
	B – 2.76	A – 58.3
TiO2 (HNO3)	A – 5.48	B – 42	32	21	252	33
	B – 3.97	A – 58

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Highly photoactive titania-doped nanocrystals prepared by an annealing-free approach. However, pure TiO₂ absorbs only a rather insignificant part of the solar spectrum due to its bandgap width. In this case, the most economically effective modifiers are the Fe³⁺ ions, which are characterized by the red absorption region. Besides, the Fe³⁺ ion is the only species having a 0.062 ± 0.003 nm ionic radius similar to that of the Ti⁴⁺ ion. The Fe(III)-doped TiO₂ photocatalysts are considered among the most promising of those working under the effect of solar light. In particular, Kisch et al.¹⁶⁰ reported the ability of Fe₂O₃·nTiO₂ (n = 1, 2) based compounds to non-enzymatically bind nitrogen molecules to form ammonia in the presence of air moisture and solar light. Such nitrogen fixation is the second most important chemical process in nature next to photosynthesis. However, doping crystal lattice under the low-temperature sol–gel synthesis conditions is a difficult technical problem compared to the production of simple oxides. Mechanisms of low-temperature doping of anatase by a soft chemistry method were first reported in ref. 30. Such doping led to a sharp increase in photoactivity under visible-light irradiation. Fig. 13 demonstrates the mechanism of the formation of Fe(III)-doped TiO₂ nanocrystals using nitric acid as an activator of ion mobility.

It was the first example of successfully performed synthesis of Fe(III)-doped titania by a low-temperature sol–gel route without annealing. Results of assessing photocatalytic and photovoltaic activity (Fig. 14) have revealed a drastic increase in the photoactivity of Fe(III)-doped TiO₂ nanocrystals as compared to pure titania. The most pronounced photocatalytic effect was attained upon doping the anatase crystal lattice with more than 7 at.% and less than 13 at.% of iron. It has been shown that excessive dopants can act as recombination centers,³² promoting the recombination of electron–hole pairs and increasing semiconductor properties. These results imply a high potential for application in the utilization of solar light, with the absence of an annealing stage for preserving functional characteristics of materials allowing the coating of a photocatalytically active layer of the Fe–TiO₂ nanoparticles on finished surfaces, including those possessing low thermal stability. According to a photovoltaic test, these films demonstrate an excellent superiority of photoresponse, even those that were annealing-free.

Fig. 14 Photocatalytic activities (A) and photocurrent responses (B) of Fe(III)-doped TiO₂ samples containing different amounts of iron: (2) 13 at.% Fe(III), (3) 7 at.% Fe(III), (4) 10 at.% Fe(III) and pure TiO₂, (1) under visible light.

Photoactivity of nanosized titania produced using low-temperature sol–gel method in the treatment of textile materials. The production of textile materials is an age-old branch of industry, which provides multibillion dollar profits for the enterprise owners. The basis for the modern development of this branch is determined by the adoption of new and economically super-accessible methods promoting substantial changes in functional properties of a product with minimum expenses for additional treatment prior to output. Below, we give two examples in which a low-temperature sol–gel method proved to be the most successful way to solve such problems.

A simple modification of the cotton fabric surface with crystalline titania nanosols. The main (if not the most important) subdivision of the innovative “smart textile” industry represents products that display unique self-cleaning properties under solar light irradiation.¹⁶¹ Achieving this effect is performed by chemically cross-linking a textile fiber to nanosized titania crystallites.¹⁶² For this purpose, the majority of classical methods employ suspensions of highly crystallized TiO₂¹⁶³ powders comprising solutions of intermediates, which provide tight covalent bonding and exhibit unique properties of the final product. It is hard to imagine what made the textile giant factories develop and inculcate these approaches, since multi-stageness, non-uniformity, and commercial inexpediency are obvious drawbacks of these methods. It is clear that the use of low-temperature sol–gel method, which allows the formation of crystalline TiO₂ sols in aqueous solutions with activity analogous to that of annealed samples, is the most proper and adequate solution to be considered for these problems. Moreover, the presence of gelation stage allows uniform coating of optically transparent, nanosized photoactive layers without using modifiers and additional cross-linking agents. In particular, we have carried out experiments on modifying cotton (Fig. 15) and polyester (Fig. 16) fibers with crystalline titania nanoparticles produced using the low-temperature sol–gel method.^24,34

Fig. 15 Morphology of cotton fibers for non-treated (A and B) and treated TiO₂ (acac. as a peptizer),²⁴ (C and D), and TiO₂ (physical activation).³⁴

Fig. 16 Left side: decomposition of Rhodamine B dye: (1) Rhodamine B dye solution with no fabric; (2) Rhodamine B dye solution containing control white fabric; (3) Rhodamine B dye solution containing cotton fabric treated with titania nanosols and kept in the dark; (4) Rhodamine B dye solution containing cotton fabric treated with titania nanosols prepared as in ref. 24, (5) Rhodamine B dye solution containing cotton fabric treated with titania nanosols, prepared as in ref. 34 and exposed to UV irradiation for different periods of time. Right side: images of Rhodamine B dye decomposition in aqueous solution containing (A) control white fabric cotton; (B) cotton fabric treated with titania nanosols prepared as in ref. 24, (C) cotton fabric treated with titania nanosols prepared as in ref. 34.

Fig. 17 SEM images of polyester fibers treated by TiO₂ hydrosol.¹³⁶

As seen in this case, the formation of uniform external coating on the fiber is observed, with characteristic composition provided by the EDX analysis data. Photocatalytic measurements have revealed that almost complete destruction of the model dye is achieved in 45 minutes of UV irradiation at a specific content of the active component with respect to cotton fiber of no more than 0.1% (see Fig. 16).

Coating TiO₂ hydrosol on the surface of a polyester fiber is the second example that successfully demonstrates the prospects of the low-temperature sol–gel process.¹³⁶ A problem with coating a crystalline hydrosol is poor wettability of polyester. In our work,¹³⁶ we have developed an original method simultaneously solving the problems of wettability and tight fixation of titania nanoparticles on the surface of a polyester fiber, which can be easily adopted to the analogous systems Fig. 17. The method consists of using a co-solvent. Hydrosol-containing TiO₂ nanocrystals were first mixed with isopropanol and then with chloroform. As a result, a homogeneous sol was formed. Chloroform dissolves the external surface of the polyester fiber, providing conditions for penetration of nanoparticles into the near-surface layer of the fiber. After drying, we obtained a composite with uniform distribution of titania nanoparticles over the entire fiber surface.

As a result of such modification, we have attained a unique self-cleaning property of the polyester fiber, which was exhibited even after 20 washing cycles (see Fig. 18).

Fig. 18 Self-cleaning test using Rhodamine B versus number of washing cycles (left side) and UV irradiation time (right side). The images shown are the exposed sides of only polyester substrates (a), titania²⁴-coated original polyester substrates (b), and titania³⁴-coated polyester substrates (c). The numbers stand for washing cycles before the test (A) and UV irradiation time (B).

5. Conclusions

The materials science paradox lies in its infinity. In producing new materials, we do not approach the end point when all possible materials are obtained and studied, but rather, to some extent, we move on to infinity. The diversity of produced materials yields an even greater diversity of all possible composite derivatives. The revolution in materials science arises from the discovery of new methods of synthesis that are usually simpler and less resource-consuming. The history of developing materials science in itself is a perfect example. Many great discoveries have been made by improvised methods.

In this review, we draw the reader's attention to the obvious facts. Despite an already-large number of papers devoted to the low-temperature preparation of crystalline sols of various oxides, the vast majority of studies still deal with the preparation of amorphous products. Shifting the trend of development towards the synthesis of crystalline hydrosols using low-temperature methods significantly broadens the outlook for practical applications and leads to the development of low-cost and energetically effective approaches that are easy to handle even by untrained personnel.

Acknowledgements

This work was supported by the Russian Government, Ministry of Education (Research was made possible due to financing provided to the Customer from the federal budget aimed at maximizing Customer's competitive advantage among world's leading educational centers), RFBR, Research Projects no. 12-03-97538, 14-03-31046. The authors are grateful to the Center for Nanoscience and Nanotechnology at Hebrew University for assistance in performing HRTEM and SEM experiments.

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