Synthesis of Spherical and Layered Ag-SiO₂-TiO₂/TiO₂-Ag Structures

Abstract

Due to the unique properties of titanium dioxide (TiO₂), titanium oxide catalysts hold significant potential for photo-oxidative processes involving organic substances in liquid media. The current research has focused on developing new preparation methods that enable the manipulation of the properties, structure, and geometric shape of catalysts to enhance their efficiency in targeted reactions. This study developed a method for preparing Ag-SiO₂-TiO₂/TiO₂-Ag layered structures with a spherical shape, featuring particle diameters ranging from 232 to 653 μm and mesopores of 2–20 nm. This was achieved by combining sol–gel and template synthesis methods. A comprehensive analysis of the obtained materials was conducted using methods such as X-ray phase analysis, micro-X-ray spectral analysis, X-ray microanalysis, and scanning electron spectroscopy. The photocatalytic properties were assessed by measuring the degree of decomposition of methyl orange in a model oxidation reaction under light radiation. The obtained spherical Ag-SiO₂-TiO₂/TiO₂-Ag layered structures demonstrated high efficiency in the photooxidation of methyl orange in the model reaction.

1. Introduction

In today’s world, characterized by rapid technological advancement, humanity is increasingly confronted with environmental pollution issues, primarily resulting from hazardous emissions produced by industry, energy generation, and transportation [1,2,3]. Therefore, the development of environmental technologies aimed at protecting the ecosystem has become highly relevant. For air purification, approaches based on adsorption and photocatalysis are mainly used [4]. In the case of adsorption, pollutants accumulate on the surface of the adsorbent, often without changing their chemical structure [5]. As a result, the efficiency of adsorption-based purification decreases, adsorbent regeneration is required, and the further utilization of accumulated pollutants during purification remains a problem [6]. In this regard, photocatalytic technologies that utilize titanium dioxide (TiO₂) as a photocatalyst are promising purification methods, usually realizing the complete degradation of organic pollutants to carbon dioxide (CO₂), water, and mineral acids [7,8,9].

Materials based on titanium dioxide are practically significant because of the stability of their physicochemical properties [10,11]. However, due to the characteristics of the TiO₂ band structure, these materials exhibit photocatalytic properties only when exposed to UV radiation, which utilizes only about 4% of sunlight’s radiation intensity [12,13]. For this reason, the development of titanium-dioxide-based materials that are more efficient in visible light spectrum is an urgent task.

There are several methods of enhancing the photocatalytic activity of titanium dioxide, including the directed synthesis of nanoscale TiO₂ structures with specific phase compositions and morphologies, as well as the development of composite materials incorporating metal nanoparticles and semiconductor nanocrystals [14,15]. This approach increases the lifetime of nonequilibrium charge carriers in the semiconductor, partly due to the redistribution of photogenerated charge carriers between the contacting particles. Also, the absorption edge of the composite shifts into the visible spectrum [16]. In this case, the absorption of visible light occurs through the nanoparticles of the modifying semiconductor or through metal nanoparticles via surface plasmon resonance [17]. It is important to note that the morphology of the nanocomposites can have a significant impact on their properties.

To modify TiO₂ with metal nanoparticles, the use of silver is advantageous due to its stability against oxidation in air in the nanocrystalline state [18,19]. However, silver atoms cannot be incorporated into the crystal lattice of TiO₂ because the radius of the Ag atom (0.126 nm) is significantly larger than that of the titanium (Ti) atom (0.067 nm). This challenge can be addressed by employing a bonding agent to facilitate contact between the metal and oxide particles [20]. When Ag and TiO₂ particles come into contact, surface plasmon resonance enables the transfer of non-equilibrium electrons from Ag to TiO₂, while the photogenerated holes migrate in the opposite direction. The well-studied TiO₂–SiO₂ system, where amorphous SiO₂ acts as the bonding agent, is suitable for modification with silver particles [21,22,23].

The global practice of using photocatalysts shows that an increase in their dispersity correlates with enhanced photocatalytic activity primarily due to the quantum-size effect [24]. Therefore, the majority of synthesis methods developed to date have focused on producing titanium-containing photocatalysts only in highly dispersed powder form. However, the use of powdered materials in photocatalytic processes, conducted in liquid phases, causes serious difficulties related to the removal of the spent photocatalysts by filtration [25]. This issue inevitably results in the contamination of target products with photocatalyst particles and, therefore, increased material costs during the use of powdered photocatalysts in industrial liquid-phase processes. This limits the practical application of promising photocatalytic systems [26]. The larger forms of catalytic materials obtained from nanoscale particles by creating layered structures can offset the influence of the quantum-size effect. Also, they can offset operational difficulties while maintaining high efficiency in the target reactions. The homogeneity of the composition and the uniformity of the physicochemical properties of the formed layers are important for layered materials’ functional applications.

Thus, research aimed at the development of methods for obtaining materials based on nanosized titanium dioxide modifications with enhanced photocatalytic activity for use in liquid-phase oxidation processes is relevant and interesting from both the theoretical and practical points of view.

• In the present research work, for obtaining spherical, layered Ag-SiO₂-TiO₂/TiO₂-Ag structures, we developed an approach that combines the sol–gel and template synthesis methods using thermal treatment. This process involves the following steps (Figure 1):
• Spherical template preparation;
• Preparation of stable Ti-Si-Ag-containing film-forming solution and coating the prepared template;
• Thermal treatment.

The template preparation stage enables the production of spherical particles with an average size ranging from 800 to 1000 microns. During the thermal treatment phase, an inner TiO₂-Ag layer forms. To create the outer layer of Ag-SiO₂-TiO₂ in the Ag-SiO₂-TiO₂/TiO₂-Ag layered structure, the preparation of a Ti-Si-Ag-containing film-forming solution and the coating of the prepared template are necessary. Modifying the outer layer of the structure with amorphous SiO₂, acting as a bonding agent, can enhance the adhesion between the particles but can also give plasticity to the layered structure and resistance to changes in the spherical shape with changes in the linear parameters due to temperature fluctuations. The application of thermal treatment destroys the organic matrix of the template and forms the photocatalytically active phases of the Ag-SiO₂-TiO₂/TiO₂-Ag layered structure.

While constructing this approach for developing a method of producing photocatalytic materials based on titanium dioxide, the authors established the goal of this study: the synthesis of spherical, layered Ag-SiO₂-TiO₂/TiO₂-Ag structures and the investigation of temperature’s influence on the phase formation and the linear parameters of the layered, spherical structure. Special attention was given to the rheological properties of Ti-Si-Ag-containing film-forming solutions, the morphological and physicochemical properties of the initial systems and the obtained materials, and the evaluation of their photocatalytic activity.

2. Materials and Methods

2.1. Template Preparation

Ion-exchange resins (ionites) with a divinylbenzene matrix of different structures and functional groups were used as templates of aspherical shape: macroporous carboxylic ionite, gel sulfocationite, and macroporous sulfoionite.

Table 1. Labels of ion exchange resins used and their characteristics.

Labeling	Organic Matrix	Functional Group	Structure	Average Grain Size, µm
GS	styrene-divinylbenzene	sulfogroup	gel	830
MK	acrylic-divinylbenzene	carboxylic group	macroporous	950
MS	styrene-divinylbenzene	sulfogroup	macroporous	830

presents the labels and characteristics of the ion-exchange resins used.

The preparation of the template was performed via the ion exchange method. Ti⁴⁺ and Ag⁺ ions were introduced into the pores of ionites labeled as GS, MK, and MS from solutions of Ti(SO₄)₂ and AgNO₃ to obtain the inner TiO₂-Ag layer via the heat treatment of the template. This process was performed according to the following scheme: ion-exchange resins GS, MK, and MS with masses of 20 g each were immersed in a solution of titanium sulfate (45% wt) for 28 h at 20 °C; then they were dried for 2 h at 80 °C. Afterwards, they were immersed in an aqueous solution of silver nitrate (40% wt) for 9 h and were dried for 2 h at 80 °C. As a result of these operations, three types of prepared templates with a mass of 21 g each with different organic matrix structures were obtained and labeled as GS (Ti⁴⁺, Ag⁺), MK (Ti⁴⁺, Ag⁺), and MS (Ti⁴⁺, Ag⁺). Also, at this stage, the data on the total exchange and sorption capacities concerning Ti⁴⁺ and Ag⁺ for all used ionites, the decomposition temperatures of their organic matrices, and the phase formation temperatures of TiO₂-Ag were established.

2.2. Preparation of Ti-Si-Ag-Containing Film-Forming Solution and Its Application on the Prepared Template

Ti-Si-Ag-containing film-forming solutions were prepared via the sol–gel method at 20 °C by the sequential mixing of components in the following ratio (% wt): distilled water (0.2–2.0), nitric acid (0.01–0.06), silver nitrate (0.01–0.06), tetraethoxysilane (0.2–0.5), tetrabutoxytitanium (0.3), and butanol (remaining). The influence of the quantitative composition on the rheological properties of the solutions was investigated within the given concentration ranges of the components. The obtained stable Ti-Si-Ag-containing film-forming solutions were obtained with the following components composition (% wt): distilled water (0.2), nitric acid (0.06), silver nitrate (0.05), tetraethoxysilane (0.3), tetrabutoxytitanium (0.3), and butanol (remaining) were applied to the prepared templates through an immersion. The total volume of the solutions was 50 mL. Afterwards, drying was performed at 60 °C to accelerate the evaporation of the solvent (butanol) and to promote the process of gelation, resulting in the formation of a film on the surface of the prepared templates.

The reagents used for the preparation of film-forming solutions, along with their purity grades and physicochemical characteristics, are presented in

Table 2. List of substances and their characteristics.

Product Name	Formula	Mr, g/mol	ρ, g/mL
Tetrabutoxytitanium (ACROS)	Ti(OC4H9)4	340.35	0.99
Tetraethoxysilane (extra-pure grade)	Si(OC2H5)4	208.34	0.94
Butyl alcohol (pure for analysis)	C4H9OH	74.12	0.81
Silver nitrate (chemically pure)	AgNO3	169.87	–
Nitric acid (extra-pure grade)	HNO3	63.01	1.40

.

2.3. Heat Treatment

The next stage involved heat treatment to remove the organic matrix of the template, forming the outer Ag-SiO₂-TiO₂ layer from the film-forming solution and the inner TiO₂-Ag layer from the prepared templates GS (Ti⁴⁺, Ag⁺), MK (Ti⁴⁺, Ag⁺), and MS (Ti⁴⁺, Ag⁺). The thermal treatment was performed with a heating rate of 10 deg/min in three stages: 220 °C—30 min; 360 °C—30 min; 500 °C—30 min. As a result, from each prepared GS (Ti⁴⁺, Ag⁺), MK (Ti⁴⁺, Ag⁺), and MS (Ti⁴⁺, Ag⁺) template coated with a film-forming solution, after annealing, 6 g of each material was obtained. The structural parameters, as well as the morphological and functional properties of the synthesized materials, were analyzed using modern physicochemical methods.

2.4. Physicochemical Methods of Analysis

The total exchange and sorption capacities of the ion-exchange resins MK, MS, and GS were studied using sorption methods. The Ag⁺ content was determined through complexometric titration following Folgard’s method [27], while the content of Ti⁴⁺ was measured using the spectrophotometric method with 3% hydrogen peroxide on the “EKROS PE5400 UV” (LLC "Ekros-analitica" Saint-Petersburg, Russia) device.

The study of acid–base properties was performed through the potentiometric titration of a series of individual suspensions, each weighing 0.1 g, of cation exchange resins in their sodium form (Na-form) with a 0.1 M NaOH solution. The experiments were performed at a constant ionic strength (I) of 0.1 and maintained using NaNO₃, and the total volume of the solution was 25 mL. After equilibrium was established (approximately 5 h of shaking the solution with the ion exchange resin), the pH of the solutions was measured using a pH meter “ITAN” (LLC “NPP Tomanalit”, Tomsk, Russia). The effective dissociation constants of the functional groups (pK_a) of the ion exchange resins were calculated using the Henderson–Hasselbalch equation:

$$p{K}_a=pH-n\cdot \lg ({\displaystyle \frac{\alpha }{1-\alpha }})$$

a—neutralization rate;

n—parameter related to the change in electrostatic free energy of a macromolecule during its neutralization.

The absorption of Ag⁺ and Ti⁴⁺ from the dilute solutions of GS, MK, and MS was investigated through isotherm plotting. Sorption experiments were conducted using solutions with an ionic strength of 0.1 (NaNO₃) in the concentrations range of (0.8 ÷ 4)·10⁻³ mmol/mL at a pH of ~ 4.5 for Ag⁺ and (0.7 ÷ 3.5)·10⁻⁴ mmol/mL at a pH of approximately 2.0 for Ti. The concentrations of Ag⁺ in the equilibrium solutions were determined complexometrically after two days, while concentrations of Ti⁴⁺ were determined spectrophotometrically (“EKROS PE5400 UV” (LLC "Ekros-analitica" Saint-Petersburg, Russia)).

The viscosity of the prepared film-forming solutions was measured using a glass capillary viscometer “VPF-2” (LLC "Ekros-analitica" Saint-Petersburg, Russia). The determination of viscosity is based on measuring the flow time of the film-forming solutions through the viscometer’s reservoir, which has a specific volume [28]. The calculation of kinematic viscosity was performed using the average flow time:

$$\nu ={\displaystyle \frac{g}{9807}}\cdot T\cdot K$$

where K denotes the viscometer constant, mm²/s²;

T denotes flow time, s;

G denotes free-fall acceleration 9.81 m/s²;

Ν denotes kinematic viscosity, m²/s.

The viscometer constant was estimated based on the time it took for water to flow through the viscometer reservoir, which has a diameter of 0.99 mm. For this viscometer, the viscometer constant (K) is equal to 0.0716 mm²/s².

The values of the hydrogen index were obtained using an “ITAN” pH meter. A chlorosilver electrode was used as a reference electrode for measurements.

The study of the thermal decomposition of templates with varying structures of organic matrices and sorbed Ag⁺ and Ti⁴⁺ ions was performed using “NETZSCH STA 499C” (NETZSCH-Gerätebau GmbH, Selb, Germany). The analysis was performed over a temperature range of 25–1000 °C, with calcined Al₂O₃ serving as the standard. The experiments were performed in an air atmosphere and heating rate of 10 deg/min, utilizing Al₂O₃ crucibles [29].

A “NETZSCH DIL 402 E/7/G-Py” (NETZSCH-Gerätebau GmbH, Selb, Germany) high-temperature vacuum dilatometer was utilized to investigate the sintering processes of the obtained samples and to determine the coefficient of linear thermal expansion of the sintered samples. Heating was performed in an argon gas atmosphere. The sintering program involved heating the samples to a temperature of 1300 °C (900 °C) at a rate of 2 K/min and cooling to 50 °C at a rate of 5 K/min. Then, under the same conditions, a standard of the same size was taken for correction. Standard samples made of aluminum were taken as references. Before and after the sintering survey, the length of the samples was measured. The data were processed via “Proteus Analysis 8.15” software.

The surface morphology of the samples was examined on a “TM-3000” scanning electron microscope (SEM) (Hitachi, Tokyo, Japan) with an accelerating voltage of 15 kV (electron gun 5 × 10⁻² Pa and camera for the sample 30–50 Pa). X-ray microanalysis was performed on the “Quanax-70” console (Hitachi, Tokyo, Japan) using energy-dispersive X-ray.

The phase composition of the initial components and the obtained composite materials were determined using a diffractometer “XRD-7000” (Shimadzu, Kyoto, Japan) via CuKα radiation. Survey imaging was performed in the range of reflection angles 2θ = 3–100 with a step size of 0.05°. The accumulation time for each point was 3 s. The phases were decoded and identified using the ICDD diffraction database (PDF-2/Release 2012 RDB).

The porous structure and specific surface area of the samples were studied using nitrogen adsorption at 77 K on an automatic gas adsorption analyzer, “TriStar II” (30–20), manufactured by Micromeritics (Norcross, MN, USA). The specific surface area and pore size distribution of the studied samples were determined based on data obtained from the “3Flex” automated sorption unit, also manufactured by Micromeritics (Norcross, MN, USA), utilizing the Brunauer–Emmett–Teller (BET) method. Before the analysis, the samples were degassed in a vacuum for 2 h at a temperature of 200 °C.

The photocatalytic activity of the synthesized materials was evaluated using the photometric method, which measures the change in the concentration of a methyl orange indicator solution (4-dimethylaminoazobenzene-4’-sulfonic acid) due to its oxidation when exposed to light. The photocatalytic material was placed in the indicator solution and irradiated with light at an incident power of 15 mW/cm² and a wavelength of 440 nm. The amount of methyl orange to oxidize was 3 mg per 50 mL of aqueous solution, and the mass of the catalyst was 0.25 g. Before dye photodegradation, dark sorption was carried out for 1 h. The concentration of the methyl orange solution before and after irradiation was determined by measuring the optical density with a photoelectrocolorimeter at a wavelength of λ = 463 nm. By utilizing a calibration curve, the mass concentration of the methyl orange solution was calculated, and the change in concentration was used to assess photocatalytic activity. The kinetics of the dye photodegradation reaction was studied in a closed vessel under stirring. During irradiation, aliquots of the dye solution were taken at regular intervals (20 min). The progress of the photodegradation reaction was monitored via a decrease in the intensity of the dye absorption band at λ_max = 463 nm. The experiment was performed until reaching the complete decolorization of the solution within 2 h. The obtained spectra were used for plotting the kinetic curves and for calculating the photodegradation reaction rate.

3. Results and Discussion

3.1. Template Preparation

To create a spherical shape, it is essential to substitute and distribute the maximum amount of Ag⁺ and Ti⁴⁺ ions within the organic matrix of the template, as these ions serve as the nucleation sites for the growth of Ag and nanosized TiO₂ particles. For this purpose, it is important to obtain data on the total exchange and sorption capacities of the ion-exchange materials chosen as templates.

The measured sorption capacities indicate a high affinity of ionites relative to the studied particles, with values ranging from 2.85 to 3.37 mmol-eq/g for Ag⁺ ions and 1.61 to 1.62 mmol-eq/g for Ti⁴⁺ ions (

Table 3. The total exchange capacity (TEC) and sorption capacity (SC) values of the ion exchange resins for the ions under study.

Name	SC(Ag+), mmol-eq/g	SC(Ti4+), mmol-eq/g	TEC, mmol/g
MK	3.37 ± 0.20	1.62 ± 0.13	8.09 ± 0.34
MS	3.18 ± 0.26	1.22 ± 0.15	5.60 ± 0.04
GS	2.85 ± 0.15	1.01 ± 0.15	4.87 ± 0.54

).

The capacity indexes for the ion-exchange resin MK are higher compared to those of MS and GS, and they (the values) increase in the order of Ag⁺ > Ti⁴⁺. A similar trend is observed for both MK and GS. In general, the sorption capacity for silver particles ranges from 42% to 57% of the total exchange capacity, while for titanium particles, it ranges from 20% to 33%. This indicates that only some of the functional groups in the ion-exchange resins are available for ion exchange.

For more comprehensive characterization of Ag⁺ and Ti⁴⁺ absorption on the Na form of MK, MS, and GS ionites, the isotherms of sorption from metal solutions were plotted (Figure 2). The isotherms for Ti⁴⁺ sorption by both sulfonic and carboxylic ionites within the studied concentration range are nearly linear. This linearity can be explained by the energy homogeneity of the sorption centers and the lack of sorption equilibrium reaching. The isotherms for Ag⁺ sorption on the MK ionite have similar shapes, while the Ag⁺ isotherms on GS and MK ionites have less monotonic characteristics.

A distinctive characteristic of the sorption isotherms for all ions on MS is their linear progression, accompanied by a sharp increase at the initial site. This observation suggests a high selectivity for the absorption of Ti⁴⁺ and Ag⁺ within the studied concentration range. This finding is significant for the application of ion exchange materials in the development of spherical structures that remain stable during thermal treatment.

The quantitative estimation of particle absorption was conducted using the distribution coefficient D. The order of distribution coefficients, approximately 10³, indicates a high level of absorption for Ti⁴⁺ and Ag⁺ ions (

Table 4. Values of distribution coefficients (D, mg/mL) of Ti⁴⁺ and Ag⁺.

Name	MK D,·103 mg/mL	MS D,·103 mg/mL	GS D,·103 mg/mL
Ti4+	0.75–1.10	0.13–0.42	0.13–0.25
Ag+	1.29–2.59	1.10–1.75	0.62–0.68

).

The carboxylic ionite MK has the highest selectivity for Ti⁴⁺ and Ag⁺, along with increased sorption capacity. As a result of the ion exchange process, templates with various organic matrices were obtained: MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺).

The formation of a photosensitive TiO₂-Ag layer from sorbed Ti⁴⁺ and Ag⁺ ions using ion-exchange resins, along with the removal of the organic matrix of resins, requires high-temperature treatment. This process requires an investigation of the effects of increased temperatures on the composition and structure of the prepared templates.

Thermogravimetric analysis (TGA) was used to investigate the temperature regime of the organic matrix in the prepared templates MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺) and thermal degradation (Figure 3). Within the temperature range of 20–220 °C, a slight mass loss of approximately 7% was observed. This fact can be attributed to the loss of adsorbed water. The physical nature of this process is confirmed by the calculated energy values using the Metzger–Horowitz method, taking values 65 to 80 kJ. In the temperature range from 300 to 500 °C, three exothermic effects are observed on the DTG curves for all investigated samples. The high values of activation energy, ranging from 354 kJ to 415 kJ, indicate the chemical nature of the processes associated with the destruction of the carbon matrices MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺). The mass loss is ~63%.

The control of phase formation was performed at temperatures of 400 °C, 500 °C, and 600 °C (Figure 4). The results of the X-ray phase study of obtained samples by annealing at 400 °C (Figure 4a) showed that all samples are amorphous and have broad and low-intensity peaks.

The diffractograms of TiO₂ samples obtained at 600 °C (Figure 4c) are characterized by narrow peaks, which indicate a high degree of crystallization of the studied samples. However, as the annealing temperature is increased to 600 °C, the anatase phase of titanium dioxide undergoes the process of rutilization. The TiO₂ sample annealed at 500 °C (Figure 4b) also has an identical crystalline structure as the sample obtained at 600 °C, but it is presented by the anatase phase and does not contain the rutile phase, which is crucial for functional applications.

The destruction of the spherical shape of the samples is observed at a temperature of 400 °C (Figure 5). Heat treatment temperatures increasing to 600 °C further contribute to the destruction of the spherical geometry. The microphotographs of these samples are similar to the images presented in Figure 5.

The destruction of the spherical shape of the samples is probably associated with the effects of linear expansion and contraction processes, which lead to changes in the linear parameters of the spherical structures. The change dependency in the relative linear parameters of the studied samples on temperature is characteristic for all samples examined, each with different organic ion exchange matrices, as illustrated in Figure 6. Dilatometric measurements conducted in the temperature range from 20 °C to 600 °C indicated that the samples undergo significant compression with respect to the linear parameters related to spherical shapes.

This is confirmed by the results of calculating the coefficient of linear thermal expansion (CLTE) (

Table 5. Average values of coefficient of linear thermal expansion (CLTE) when the samples are exposed to temperatures.

CLTE α ∙ 107, K−1	T, °C
−76	25–220
−120	220–360
−362	360–400
−298	400–500
−247	500–600

). The values of the temperature coefficient of linear expansion do not depend on the structure of the ionites used as templates and have similar values for all ionites. When the samples are heated from 20 °C to 600 °C, the coefficient of linear thermal expansion assumes negative values, indicating a contraction in the linear parameters of the studied samples. Significant compression of the material is observed in the temperature range from 25 °C to 400 °C, which probably occurs due to the loss of moisture and the degradation of the organic matrix of the ionite occurring between 220 °C and 400 °C.

As the temperature rises, the value of the coefficient of linear thermal expansion increases due to the expansion of the linear parameters of the studied samples. Such deformations in the linear parameters of the structure can ultimately lead to its destruction.

3.2. Preparation of Film-Forming Solution

To enhance the thermal stability of the spherical-shaped structure, the next step is to apply the Ag-SiO₂-TiO₂ layer on the surface of the prepared template before heat treatment. The film obtained via the sol–gel method can serve as this layer. The synthesis of films from Ti-Si-Ag-containing film-forming solutions using the sol–gel method is well studied and does not require complex technical equipment. A notable advantage of this method is the ability to control the composition of the components.

To obtain films, precursor solutions must comply with several conditions determined by their states. During the dissolution of the components, a period known as the solution “maturation” or sol formation process is required. The duration of this period can range from a few minutes to several days depending on the type of substances involved. At this stage, a transition occurs from a true solution to a colloidal solution due to the processes of solvation, hydrolysis, and condensation. Polycondensation is the primary chemical process throughout all stages of sol–gel technology in the production of titanium-dioxide-based materials. Absolute butanol is more readily available and less hygroscopic than ethanol, making it easier to control the water content, which significantly influences the “lifetime” of the solution. This is particularly advantageous in butanol medium, despite its relatively higher viscosity (3.6 mm²/s at 20 °C) compared to ethanol-based solutions [30].

In solutions, alkoxide undergoes active hydrolysis to form hydroxo-derivatives, which then condense into hydroxo-oligomers and hydroxopolymers of varying molecular weights. These compounds exhibit three-dimensional and tetrameric structural units of cyclical configurations [31]. A key indicator of these processes is the change in the viscosity of the solutions (Figure 7). As illustrated using the presented data, an increase in the concentration of water in the solution results in a reduction in the time interval of solution stability, leading to rapid gelation that renders the solution unsuitable for film production.

Water initiates the hydrolysis process of Ti(OC₄H₉)₄. The formed hydroxo-derivatives of tetrabutoxytitanium undergo a condensation reaction catalyzed by C₄H₉OH₂⁺ ions in butanol medium (Equations (1)–(5)).

2H₂O → H₃O⁺ + OH,

(1)

2C₄H₉OH → C₄H₉OH₂⁺ + C₄H₉O,

(2)

Ti(OC₄H₉)₄ + H₂O → Ti(OC₄H₉)₃OH + C₄H₉OH,

(3)

$$2\mathrm{Ti}({\mathrm{OC}}_4{\mathrm{H}}_9{)}_3\mathrm{OH}\to ({\mathrm{H}}_9{\mathrm{C}}_4\mathrm{O}{)}_3\mathrm{Ti}<\begin{array}{c}\mathrm{OH}\\ \mathrm{OH}\end{array}>\mathrm{Ti}({\mathrm{OC}}_4{\mathrm{H}}_9{)}_3,$$

(4)

$$2\mathrm{Ti}({\mathrm{OC}}_4{\mathrm{H}}_9{)}_3\mathrm{OH}\to ({\mathrm{H}}_9{\mathrm{C}}_4\mathrm{O}{)}_3\mathrm{Ti}<\begin{array}{c}\mathrm{OH}\\ \mathrm{OH}\end{array}>\mathrm{Ti}({\mathrm{OC}}_4{\mathrm{H}}_9{)}_3,$$

(5)

As a result, the molecular weight of the formed polymers increases, leading to a rapid increase in the viscosity of the solution. The dissociation processes of butanol result in a higher concentration of C₄H₉OH₂⁺ ions in the solution, as indicated by the gradual decrease in the pH of the freshly prepared solution (

Table 6. Variation in the pH values of Ti-Si-Ag-containing film-forming solutions with different H₂O concentrations depending on the time elapsed after mixing the components.

C(H2O) in Solution	pH Value 1 min	pH Value 5 min	pH Value 10 min	pH Value 20 min
0.2 M	7.71	6.42	5.22	4.65
0.4 M	7.67	6.39	5.31	4.58
0.6 M	7.61	6.31	5.18	4.52
0.8 M	7.56	6.26	5.15	4.54
1.0 M	7.49	6.20	5.07	4.59
1.2 M	7.52	6.36	5.16	4.62
1.4 M	7.50	6.24	5.28	4.51
1.6 M	7.47	6.18	5.25	4.55
1.8 M	7.55	6.29	5.12	4.58
2.0 M	7.59	6.31	5.23	4.57

).

In an acidic medium, the hydrolysis of tetrabutoxytitanium occurs at an accelerated rate, resulting in reduced time for the solution to reach a stable state. Considering this fact, it was decided to introduce nitric acid into the composition of film-forming solutions.

The study of the influence of the acidity of the medium on the stability of Ti-Si-Ag-containing film-forming solutions in time showed that increasing the concentration of HNO₃ from 0.01 to 0.06 M results in a corresponding increase in viscosity (Figure 8).

The addition of nitric acid in concentrations ranging from 0.01 to 0.02 M leads to rapid gelation, as a result of which the solution becomes unsuitable for film production. It was observed that at nitric acid concentrations exceeding 0.03 M, the viscosity of the studied colloidal systems stabilizes. For a solution with a nitric acid concentration of 0.06 M, viscosity stabilization occurs after 6 h, reaching values between 5.52 and 5.64 mm/s². This stabilization is accompanied by an increase in the solution’s stability interval, extending beyond 40 days.

Further measurements of the viscosity of Ti-Si-Ag-containing film-forming solutions with nitric acid concentrations ranging from 0.01 to 0.06 M enabled the determination of their stability time for film production (

Table 7. Effect of HNO₃ concentration on time of stabilization and period of stability of Ti-Si-Ag-containing film-forming solutions.

C(HNO3) in Solution	Time of Solution Stabilization, Hour	Period of Stability, Days
0.01 M	more than 24	-
0.02 M	more than 24	-
0.03 M	6	28
0.04 M	6	27
0.05 M	6	34
0.06 M	6	42

). The stability time of the solutions was assessed using the time interval preceding the formation of a white precipitate of Ti(OH)₄. Solutions exhibiting this precipitate are unsuitable for film formation.

The addition of tetraethoxysilane to film-forming solutions is essential for producing films with the Ag-SiO₂-TiO₂ composition. An increase in the concentration of tetraethoxysilane in Ti-Si-Ag-containing film-forming solutions results in a gradual increase in viscosity (Figure 9).

The slow increase in viscosity contributes to an extended stabilization time for solutions, reaching up to 8 h. Furthermore, the stability interval of these solutions increased to 62 days (

Table 8. Effect of Si(OC₂H₅)₄ concentrations on the time of stabilization and the period of stability of Ti-Si-Ag-containing film-forming solutions.

C(Si(OC2H5)4) in Solution	Time of Solution Stabilization, Hour	Period of Stability, Days
0.2 M	6	43
0.3 M	6	47
0.4 M	8	56
0.5 M	8	62

).

The addition of AgNO₃ to film-forming solutions within the selected concentration range from 0.01 to 0.06 M has minimal impact on the viscosity changes and stability of these solutions.

The results obtained allowed for the establishment of the concentration composition of stable Ti-Si-Ag-containing film-forming solution: Ti(OC₄H₉)₄ at 0.1 M, Si(OC₂H₅)₄ at 0.5 M, AgNO₃ at 0.06 M, HNO₃ at 0.06 M, and H₂O at 0.2 M. This composition is suitable for the production of Ag-SiO₂-TiO₂ films.

The powder was obtained from Ti-Si-Ag-containing film-forming solutions through heat treatment at 500 °C, and X-ray phase analysis was performed (Figure 10). The reflections in the X-ray diffraction pattern indicate the presence of the phase of titanium dioxide and crystalline silver. The presence of noise on the X-ray diagram indicates the presence of silicon dioxide, which has higher crystallization temperatures than the temperature used for heat treatment.

The next stage of the study involved the preparation of spherical Ag-SiO₂-TiO₂/TiO₂-Ag layered structures. To achieve this, the stable Ti-Si-Ag-containing film-forming solutions were applied to the prepared templates GS (Ti⁴⁺, Ag⁺), MK (Ti⁴⁺, Ag⁺), and MS (Ti⁴⁺, Ag⁺).

3.3. Destruction of the Template

Taking into account the results of thermogravimetric, X-ray diffraction, and dilatometric analyses of the samples MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺), heat treatment was conducted in a stepwise manner over three stages, with a heating rate of 10 °C/min. The calcination time was determined based on the physical and chemical characteristics of the processes occurring under the influence of temperature:

1. Heating to 220 °C and calcining for 30 min: Adsorbed water is removed during this stage.

2. Heating to 220 °C and calcining for 30 min: The calcination process at a given temperature results in the combustion of the organic matrix, which is accompanied by significant alterations in the linear parameters of the structure.

3. Heating to 500 °C and calcining for 30 min: At the final stage, the crystalline anatase phase of titanium dioxide and the silver phase are formed.

The results of the study on the structural and photocatalytic properties are similar and characteristic for all samples obtained after the thermal treatment of MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺), and they do not depend on the organic matrices used in the synthesis.

After a stepwise heat treatment of MK (Ti⁴⁺, Ag⁺), MS (Ti⁴⁺, Ag⁺), and GS (Ti⁴⁺, Ag⁺), all studied samples have a spherical shape (Figure 11). The particle diameters range from 232 to 653 μm. The surfaces of the spheres display a relief characterized by distributed agglomerations of particles throughout the entire area. As a result of micro-X-ray spectral analysis, it was possible to establish that these agglomerations consist of clusters of silver particles, with interlayers of silicon, titanium, and oxygen atoms present in the spaces between them.

Figure 12a illustrates a unit of spherical material that has been obtained following the mechanical removal of a portion of the outer layer. The spherical particle is represented by two layers with a recurrent geometry. The inner layer exhibits a longitudinal crack, which was probably obtained during the mechanical process used to remove part of the outer layer. Both layers, as shown in Figure 12b, possess a porous structure.

The results of the porosity study indicate a monomodal pore size distribution within the structure. The spherical particles exhibit both through and closed pores, characterized by drop-shaped and cone-shaped geometries with varying entrance hole radii. The analyzed samples are predominantly mesoporous, containing a significant fraction of fine pores ranging from 2 to 20 nm in size. The specific surface area measurement is recorded at 36 m²/g.

The phase composition of spherical layered structures is characterized by the anatase phase of titanium dioxide and silver particles (Figure 13). The results of qualitative studies employing methods such as X-ray phase analysis and micro-X-ray spectral analysis indicate that the obtained material is highly crystalline titanium dioxide in the anatase modification, with embedded metallic silver particles.

The photocatalytic activity of the samples under investigation was assessed by measuring the change in the concentration of oxidized methyl orange in the solution (Figure 14). Prior to the initiation of the photochemical degradation of the dye, a dark sorption of dye molecules on the surface of Ag-SiO₂-TiO₂/TiO₂-Ag was conducted for one hour. A control solution of methyl orange without any photocatalytic material was maintained under identical conditions. Subsequently, all dye solutions were subjected to irradiation with light at a wavelength of 440 nm.

Upon completion of dark sorption, a 5–10% change in the dye concentration is observed in the solution of methyl orange with Ag-SiO₂-TiO₂/TiO₂-Ag photocatalyst particles. With irradiation beginning, the concentration loss occurs linearly and proportionally in the measured time intervals in the presence of Ag-SiO₂-TiO₂/TiO₂-Ag. The average reaction rate is equal to 1.3 × 10⁻⁶ mol/(L·min).

After dark sorption on the IR spectrum of the Ag-SiO₂-TiO₂/TiO₂-Ag sample (Figure 15a), the bands characteristic of organic molecule vibrations were observed. In the region of 3300–2900 cm⁻¹, the band is characteristic of the C-H bonds of benzene rings, while bands in the region of 2000–1400 cm⁻¹ are associated with C-N and N=N bonds. This observation confirms the adsorption of methyl orange molecules on the surface of the Ag-SiO₂-TiO₂/TiO₂-Ag.

The IR spectrum of the Ag-SiO₂-TiO₂/TiO₂-Ag sample after light irradiation (Figure 15b) does not exhibit any bond vibrations characteristic of organic molecules. This observation can be attributed to the photodegradation of organic molecules on the surface of the granules, resulting in the formation of carbon dioxide, as indicated by the band in the region of 2400 cm⁻¹.

The complete degradation of methyl orange (Equation (6)) on the surface of Ag-SiO₂-TiO₂/TiO₂-Ag is achieved within two hours, as evidenced by the discoloration of the solution and by the results of IR spectroscopy.

2C₁₄H₁₄N₃SO₃⁻ + 35.5O₂ → 28CO₂ + 13H₂O + N₂ + 2HSO₄⁻

(6)

The efficiency of photocatalytic properties of the obtained material is explained by its physicochemical properties such as phase composition, crystallinity, particle size, structure, and porosity. For this reason, the catalyst-obtaining process in this work was focused on the optimization of these properties. Despite the fact that the obtained layered spherical material consists of particles with a size of 232–653 μm, which is much larger than the particle size of powder photocatalysts (average particle size in the range of 0.2–0.5 μm), the limitations of the quantum-size effect for Ag-SiO₂-TiO₂/TiO₂-Ag particles were overcome due to the formation of layered structures with developed porosity based on the anatase phase of titanium dioxide modified with silver particles. Upon the contact of Ag and TiO₂ particles, due to the effect of surface plasmon resonance, there is a possibility of the transfer of nonequilibrium electrons from Ag to TiO₂ and photogenerated holes in the opposite direction, which shifts the absorption edge of Ag-SiO₂-TiO₂/TiO₂-Ag to the visible region [21,22,23]. The porous structure with a developed system of mesopores causes an increase in the specific surface area of the material due to the inner surface and provides accessibility for the diffusion of methyl orange molecules. Such a structure provides the efficiency of the obtained Ag-SiO₂-TiO₂/TiO₂-Ag material in photocatalytic processes of organic dye destruction at the level comparable to the efficiency of known powder photocatalysts (

Table 9. Averaged values of the characteristics of the obtained photocatalytic materials Ag-SiO₂-TiO₂/TiO₂-Ag and powder photocatalysts TiO₂, TiO₂-Ag, and Ag-SiO₂-TiO₂.

Composition	Particle Size, μm	Specific Surface Area, m2/g	Pore Volume, cm3/g	Absorption Edge, nm	Degree Decomposition of Organic Dyes, %	Decomposition Time, min	Ref
Ag-SiO2-TiO2/TiO2-Ag	442	36.0	220	440	100	120	-
TiO2	0.34	54.0	0.17	365	100	140	[32]
TiO2-Ag	0.24	30.1	0.42	449	100	120	[33]
Ag-SiO2-TiO2	0.52	56.5	0.29	440	100	120	[34]

). The obtained Ag-SiO₂-TiO₂/TiO₂-Ag material particles, due to their larger size, can be more suitable for applications in industrial liquid-phase processes in comparison with powder ones. The choice between these types of different-sized structures depends on the specific application, the required process time, and the reaction conditions.

4. Conclusions

The method proposed in this study enables the synthesis of spherical layered Ag-SiO₂-TiO₂/TiO₂-Ag structures. These structures consist of a mixture of metallic Ag with anatase TiO₂ in the inner TiO₂-Ag layer, along with an Ag-SiO₂-TiO₂ layer on the surface of the spherical agglomerate. The synthesized spherical materials exhibit diameters ranging from 232 to 653 μm, a specific surface area of 36 m²/g, and mesoporous structure. The oxidation reaction of methyl orange on the surface of the Ag-SiO₂-TiO₂/TiO₂-Ag structures demonstrated photocatalytic properties when irradiated with light at a wavelength of 440 nm. This finding is promising for the further study of synthesized materials in addressing practical photocatalytic applications.

Some results obtained during research on the rheological properties of multicomponent colloidal systems for the development of Ti-Si-Ag-containing film-forming solutions, as well as the characteristics of phase formation, structures, and physicochemical properties of the intermediate materials Ag-SiO₂-TiO₂ and TiO₂-Ag, may be of interest to specialists engaged in inorganic chemistry and materials science research.