Sol-Gel Synthesis of TiO2 from TiOSO4 (Part 2): Kinetics and Photocatalytic Efficiency of Methylene Blue Degradation Under UV Irradiation

Abstract

The sol-gel process was used to create titanium dioxide (TiO₂) nanoparticles, a nanocrystalline semiconductor. How several synthesis factors, such as titanium precursor concentration, annealing temperature, and peptization temperature, affected the structural and morphological properties of TiO₂ nanoparticles were thoroughly explored. X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), measurements of the specific surface area and pore size using the BET method, and UV-visible diffuse reflectance spectroscopy were all used in this investigation. The specific surface area determined by BET analysis decreased with increasing calcination temperature. The XRD analysis showed that a composite sample consisting mainly of anatase with minor brookite phases was obtained when the titanium precursor concentration ranged between 0.2 and 0.4 M, whereas a concentration of 0.5 M resulted in the formation of pure anatase. The photocatalytic activity of the synthesized TiO₂ powders under different operational parameters was evaluated for the common commercial textile dye, i.e., methylene blue (MB). It was experimented that the model pollutant decoloration follows the Langmuir–Hinshelwood (L-H) model. In view of this detailed research work, it was observed that the TiO₂ produced with a titanium precursor concentration of 0.3 M, a pH value of 5 during the peptization step, and an annealing temperature of 600 °C were found to be the best conditions for this catalytic degradation process. When used in conjunction with a TiO₂ concentration of 0.04 g/L and a reactor suspension pH value of 6.0, the TiO₂ catalyst produced a stunning 98% degradation of methylene blue under these circumstances.

1. Introduction

Semiconductor photocatalysis is a key instrument in resolving a variety of environmental issues thanks to its broad range of applications in environmental engineering and technology. It specifically plays a significant function in the fields of air and water purification, providing many ways to raise the caliber of our natural resources. Many semiconductors possess band gap energies that are adequate for catalyzing a variety of chemical processes of environmental interest. Among the various semiconductors, titanium dioxide (TiO₂) has emerged as the most effective photocatalyst for wastewater remediation because of its stability against chemical and photochemical corrosion, biological and chemical inertness, non-toxic nature, practically insoluble in water, and cost-effectiveness [1]. By absorbing light with a wavelength shorter than 384 nanometers, TiO₂ activates its noteworthy photocatalytic characteristics. The creation of electron-hole pairs (e⁻/h⁺) is initiated by this absorption process. In particular, the energy band gap is energized to excite electrons from the valence band into the conduction band. These generated electron-hole pairs operate as strong catalysts for the oxidation and reduction of TiO₂ at its surface. Hydroxyl ions (OH⁻) or water molecules effectively scavenge the positively charged holes (h⁺), resulting in the production of extremely reactive hydroxyl radicals (OH^•). Parallel to this, the photoinduced electrons (e⁻) interact with oxygen from an external source to produce radical species such as superoxide radicals (O₂^•−) and hydroperoxide radicals HO₂^•−. These generated radicals initially set out to oxidize adsorbed organic or chemical substrates that were present on the TiO₂ surface, turning them into intermediary compounds. These intermediates then undergo a thorough mineralization process, which ultimately causes the chemicals to completely decompose into safe byproducts, including water, carbon dioxide, and mineral acids. The extraordinary potential of TiO₂ in the remediation and purification of numerous environmental pollutants is highlighted by this complex series of photocatalytic processes [2,3]. Various methodologies have been used in the past for the synthesis of nanoparticles of TiO₂, encompassing approaches such as chemical precipitation [3], microemulsion with heat treatment [4], hydrothermal or solvothermal methods [5], flame spray pyrolysis [6], sol-gel processes [7], and microwave or sonochemical techniques [8]. Within this array of methods, the sol-gel process stands out as a particularly effective and straightforward approach for producing nanoscale TiO₂ materials with exceptional photocatalytic capabilities [9]. The sol-gel process offers advantages such as (i) sol preparation and gel processing occur at ambient temperature, (ii) product purity and homogeneity, (iii) drying of gel below 100 °C, (iv) ease of developing multicomponent materials, and (v) good control over particle shape and size as well as size distribution [10]; therefore, chemically pure nanocrystalline products can be produced by carefully managing the processing variables and tailoring the chemical structure of the primary alkoxide precursors (titanium butoxide, titanium isopropoxide, titanium tetrachloride etc.), the later complex methods can be compensated using inorganic precursors (titanium sulfate, titanium oxysulfate). The physicochemical properties (particle size, surface area, crystalline structure, degree of surface hydroxyl groups, etc.) of TiO₂ have a significant effect on its photocatalytic properties. These physicochemical properties are dependent on precursor concentration, peptization pH value, annealing temperature, etc., which are managed in the processing steps (hydrolysis and polycondensation, peptization, aging followed by drying and thermal annealing) of the sol-gel synthesis process. By optimizing and controlling these effecting variables one can tune these properties of TiO₂ towards enhanced photocatalytic activity. The influence of various photocatalytic process parameters (addition of H₂O₂ as oxidizing agent, TiO₂ concentration, dye content, pH) on the removal of aromatic compounds has been reported [11,12,13]. Chelbi et al. [14] undertook a thorough investigation to probe the impact of two crucial variables on the structural and morphological traits of titanium dioxide (TiO₂). They specifically examined the effects of the temperature and alkoxide titanium precursor concentration used during the calcination process. This was accomplished using a complex and sophisticated sol-gel method that involved significant acid alteration. Titanium tetra-isopropoxide was used as the precursor in this procedure. It is also important to note that the synthesis method required the use of supercritical drying conditions using ethanol as the solvent, which increased the complexity of the experimental process by a level. K.H. Rehman and A.K. Kar [15] stated the effect of alkoxide titanium precursor (titanium butoxide) concentration on TiO₂ thin film properties using the hydrothermal synthesis method. Vishwanathan et al. [16] studied the effect of experimental parameters on the photocatalytic decomposition efficiency of TiO₂ prepared by an electrochemical method using titanium sheets as electrode material. Shahrezaei et al. [3] studied the effect of synthesis parameters on the preparation of modified TiO₂ using NaOH and commercial TiO₂ (Degussa P25) sing hydrothermal method. In summary, the effect of photocatalyst synthesis parameters and photocatalytic process parameters are most reported with alkoxide titanium precursors and or with commercial TiO₂.

In this study, we used titanium oxysulfate (TiOSO₄), an inorganic titanium precursor, as the starting material for the synthesis of titanium dioxide (TiO₂) nanoparticles. Utilizing the sol-gel technique, we systematically changed several operational factors during the synthesis process. These parameters included the titanium precursor concentration, the pH level during the critical peptization step, and the temperature during the calcination process. We used the commonly used commercial textile dye methylene blue (MB) degradation as a probe reaction to assess the photocatalytic efficacy of the produced TiO₂ nanoparticles. This decision gave us the opportunity to quantitatively assess the photocatalytic activity of the different TiO₂ nanoparticle preparations, giving us important information about their potential for use in environmental remediation applications. MB is a common synthetic dye used in the clothing and textile industries, dyeing papers and leather industries, and also for food, cosmetics, and pharmaceutical industries [17]. Moreover, on the surface of many materials, MB has substantial adsorption. In addition, it is highly water-soluble, thus forming a very stable solution in water at room temperature. In addition, MB is toxic, carcinogenic, non-biodegradable, and unlikely to be degraded by light illumination; therefore, it is an appropriate dye for experimental research on photocatalytic degradation. The influence of some operational parameters (such as photocatalyst dosage and pH of photocatalytic suspension) on the photocatalytic removal of MB was also studied. Finally, the relative importance of each operational parameter was determined for the optimal condition that guarantees the highest photocatalytic activity of the final material for the model pollutant. The literature lacks such kind of comprehensive, detailed study toward finding the optimal operational parameters that guarantee the highest activity of the prepared photocatalyst against the model pollutant.

2. Results and Discussions

In photocatalysis research, the ultimate objective is to synthesize TiO₂ with physicochemical properties that ensure its enhanced activity against the degradation of the model pollutants; therefore, under the effect of varying the titanium precursor concentration i.e., titanium oxysulfate (TiOSO₄) in the range of 0.2–0.5 M, keeping the other processes variables (precipitating pH valve with the addition of NH₄OH = 7.0, peptization pH value with the addition of HNO₃ = 4.0, calcination temperature = 600 °C) as constant. XRD patterns showed (Figure S1, Supplementary Information) that the calcined powders consist of a composite mixture of anatase and brookite. We used formulae put forward by Zhang and Banfield [18] to ascertain the weight composition of TiO₂ polymorphs. These equations consider the strength of the XRD peaks connected to various crystalline phases. In our situation, the {101} peak of anatase, {121} peak of brookite, and {110} peak of rutile were the main XRD peaks that we concentrated on. Additionally, each crystalline phase’s unique coefficients (k_a = 0.884, k_b = 2.721, k_r = 1) are included in the equations. Composition of anatase phase $X_a={\displaystyle \frac{k_a{I}_a}{k_a{I}_{a+k_b{I}_b+I_r}}};$ composition of brookite phase $X_b={\displaystyle \frac{k_b{I}_b}{k_a{I}_{a+k_b{I}_b+I_r}}};$ composition of rutile phase $X_r={\displaystyle \frac{I_r}{k_a{I}_{a+k_b{I}_b+I_r}}}$. It was observed that the composition of brookite increases with the increase in TiOSO₄ concentration, the results are shows in Table 1.

In calculating the crystallite size of the TiO₂ phases in the prepared powder, the Debye–Scherrer equation is used by considering the main peak reflection of anatase {101} and brookite{121}. As depicted in Table 1, the crystallite size increases with an increase in precursor concentration. In the sol-gel synthesis process, the precursor (TiOSO₄) solution is completely neutralized with the addition of NH₄OH until a pH of 7.0 is attained in the precipitation step; this results in the formation of titanium hydroxide, which grows to colloidal size and then from the solution precipitate of (TiOH)_n take place. At low precursor concentrations, a high nucleation rate in comparison to crystal growth resulted in a high proportion of titanium hydroxide from the diluted solution. As a consequence, TiO₂ with a smaller crystallite size is formed [19]. On the other hand, an increase in precursor concentration results in a large crystallite size of the final material because of the oversaturation and agglomeration phenomena of titanium hydroxide. In other words, nucleation may initially increase due to supersaturation; however, beyond a certain point, crystal growth dominates, resulting in larger crystallite sizes due to the abundance of precursors. This result shows that higher TiOSO₄ concentration yields more monomer and oligomer units to condense; due to the dominant crystal growth process, bigger particles of dried titania particles are formed. The nucleation rate may also be affected because of different phase formation. In addition, due to the secondary phase of brookite formation, the nucleation rate may be affected in addition to crystal growth suppression of the anatase phase. According to Zhang and Banfield [20], brookite is more stable than anatase for crystallite sizes greater than 11 nm. When the concentration of TiOSO₄ is increased, there is a noticeable rise in the primary peak reflection of anatase and brookite. The extension of the inter-planar distance in the direction under study, as observed, is a sign of a significant structural change. This expansion essentially tells us that the atomic planes in the material are moving farther apart. As a result, the O-O distances along the crystal’s c-axis vary noticeably as a result of this change in the structure, becoming significantly shorter while the O-O distances perpendicular to the “c” axis grow [21]. The size of the crystallites also increases in parallel with the increase in titanium precursor concentration. This phenomenon hints strongly towards the bonding and accumulation of titanium atoms on the surface of the TiO₂ crystallites. The consequence is that these titanium atoms are located outside of the crystallites rather than taking part in the precipitation process, which results in the creation of titanium oxide [14]. This specific circumstance affects the formation and evolution of the crystalline structure by significantly delaying the nucleation process.

The results mentioned earlier were supported by the BET-specific surface area analysis in Table 1. It was observed that low TiOSO₄ concentration produced particles with high surface area, whereas high TiOSO₄ concentration resulted in particles with low surface area. The presence of the brookite phase in the precursor concentration range of 0.1–0.4 M could be responsible for inhibiting the growth of crystallites compared with the nucleation rate. When the TiOSO₄ concentration was further increased to 0.5 M, only the anatase phase was produced with a larger crystallite size. This suggests that at high TiOSO₄ concentrations, the precursor cations interact rapidly, which may impede the rearrangement of TiO₆ octahedrons required for brookite structure formation [22].

The effects of varying calcination temperature on the structural and morphological properties of the prepared TiO₂ were investigated by subjecting the TiO₂ powder (prepared with a TiOSO₄ concentration of 0.3 M, precipitation pH value of 7, and peptization pH value of 6) to different annealing temperatures: 300, 400, 500, 600, 700, and 800 °C for a duration of 2 h. The intention was also to determine the temperature at which the crystalline phase (brookite–anatase–rutile) transformation occurs. Figure 1a shows the XRD patterns of the TiO₂ samples that were annealed at these varied temperatures. It is important to note that when the annealing temperature rises, the intensity of the XRD peaks also becomes stronger. This finding suggests that the annealing procedure aids in improving the crystalline quality of the TiO₂ samples that were initially synthesized. In plainer terms, the TiO₂ material’s crystalline structure becomes more clearly defined and organized as it is heated, producing stronger and more pronounced XRD peaks in the diffraction pattern. Understanding how temperature affects the qualities of the TiO₂ material starts with this improvement in crystalline purity. In other words, the amorphous content of the dried powders decreases which will be fruitful to boost the photocatalytic activity. The powders, which have been annealed between 300 and 600 °C, comprise a composite blend of primarily anatase and a small amount of brookite phase. If the temperature is increased to 700 °C, pure anatase is produced, and at 800 °C, there is a partial phase transformation from anatase to rutile; therefore, in the original powder, the phase transformation sequence appears to be brookite to anatase to rutile. The transition from anatase to rutile is a process of nucleation and growth. Research has indicated that in pure anatase, the nucleation of rutile occurs at the interface of anatase because these locations possess a similar structure to rutile [23]. According to the findings, it has been disclosed that rutile demonstrates a lack of oxygen, and it is expected that the transition to rutile will increase in anatase because of the easier rearrangement of ions caused by oxygen vacancies [24]. The present study indicates that heating anatase in the air can enhance the number of oxygen vacancies within its lattice. This is achieved by encouraging the transition from anatase to rutile, which typically occurs at a temperature range of 700 to 800 °C.

The different crystalline phases that are present in the TiO₂ powder, which underwent calcination at the prescribed temperatures, are all listed in detail in Table 2, along with their weight percentage compositions. As shown graphically in Figure 1b,c, the estimated crystallite size agrees well with our electron microscopy data. These micrographs make it evident that the TiO₂ particles have a spherical morphology, and we can also see that as the calcination temperature is raised, so does their size in a linear fashion. Several significant conclusions may be drawn from our investigation of the FTIR spectra of the TiO₂ samples that were annealed at various temperatures, as shown in Figure 1d. For instance, the FTIR spectra of the sample that was calcined at 300 °C have a broad band that is roughly centered at 3200 cm⁻¹ and spans the 3000–3700 cm⁻¹ region. This noticeable characteristic denotes the presence of hydroxyl (OH⁻) groups situated on the titania’s surface [25].

Furthermore, either adsorbed water or Ti-OH coordination on the TiO₂ surface is responsible for the signal seen at 1635 cm⁻¹. It is important to note the N-H bonding, which is probably responsible for the short, sharp peak at 1435 cm⁻¹ [26]. It is possible to trace the creation of this peak to the HNO₃ addition during the peptization process. Along with peaks at 1070, 1139, and 1210 cm⁻¹, additional spectral characteristics are seen, including a minor shoulder peak at 1000 cm⁻¹. These characteristics imply that the Ti-O-C and C-O-C bond stretching vibrations are present. Numerous peaks were found to be absorption bands connected to the flexion vibrations of Ti-O and O-Ti-O in the spectral area below 1000 cm⁻¹ [27].

It is interesting to note that as the annealing temperature increased, we noticed peaks between 1000 and 1450 cm⁻¹ disappearing. This shows that during the annealing process, the assigned peaks underwent deterioration. Additionally, as the annealing temperature increased, the peak at 1665 cm⁻¹ that corresponded to adsorbed water decreased, indicating that water was evaporating from the TiO₂ surface. The broad band in the 3000–3700 cm⁻¹ range indicates the presence of OH⁻ groups; however, it continued to exist even at an annealing temperature as high as 800 °C, albeit with just a slight drop in peak strength.

In the context of photocatalysis processes, in particular, this observation has important ramifications. Even at high annealing temperatures, the persistence of adsorbed OH groups is favorable. When TiO₂ is exposed to UV radiation, these OH groups are critical in the production of hydroxyl radicals during the photocatalysis reaction. The subsequent efficient use of these hydroxyl radicals in the breakdown of pollutant molecules emphasizes the significance of these radicals for the photocatalytic activity of TiO₂.

The specific surface area (S_BET) decreases as the crystallite size and calcination temperature rise, according to the BET study. Two causes are principally responsible for this decrease in S_BET and the concomitant loss in BJH pore volume (as seen in Table 2). First, the amorphous titania undergoes a change into a crystalline state during high-temperature annealing, which causes the pores in the TiO₂ powder to close. Second, as the particles expand, the small, narrow pores tend to separate and join to form bigger pores. In essence, these mechanisms work together to cause the observed drop in specific surface area and change in the properties of pore size. The N₂ adsorption–desorption isotherms are mentioned in the Supplementary Information (Figure S2).

In-depth information on the band gap values of the synthesized powders is provided in Table 2, which also offers insightful information on the correlation between phase composition and band gap energy. Notably, the band gap range of the anatase-brookite composite sample ranges from 3.12 to 3.22 eV. This difference in band gap values is an indication of how the electrical properties of the material are impacted by various phase compositions. 3.0 eV was calculated as the band gap for pure anatase, which is in good agreement with previously published values in the literature [28]. Additionally, the anatase-rutile composite’s band gap was discovered to reach 2.9 eV (as seen in Figure 1e). These results highlight the close relationship between the TiO₂ material’s phase composition and band gap energy.

The correlation between an increase in absorption wavelength and higher calcination temperatures was also an intriguing pattern that was noticed. This behavior suggests that the band gap energy will also drop. This behavior can be linked to improvements in crystallinity brought on by a growth in crystallite size as well as the appearance of new energy levels within the TiO₂ band gap, which was previously hidden [29]. The existence of oxygen vacancies produced by the removal of OH⁻ ions from the surface [30] or the repositioning of Ti-O octahedral units at the interface during the calcination process [31,32] may be related to the presence of these newly developed energy levels. As a result, changes in the TiO₂ band gap take place, which causes the absorption edge in the visible light area to shift sharply to the red. This finding highlights the dynamic interaction between band gap energy, phase composition, and the optical characteristics of TiO₂.

To consider the influence of peptization pH value on the TiO₂ physicochemical properties, samples were prepared in the pH range of 2–6, while the other constant process variables are precursor concentration = 0.4 M, precipitation pH = 7, and calcination temperature = 600 °C. It was observed that precipitation pH value has a profound effect on crystallite size and phase composition. The function of peptization acid in the sol-gel process is to disintegrate large aggregates into smaller ones by the electrostatic repulsion of the charged particles [33]. Peptization acid addition causes the TiO₂ precipitate particles dissolution, stabilizing the sol solution to recrystallize them into different phases. At low peptization pH value of 2 to 3, rutile has been the dominant phase; at pH value of 3 to 5, anatase is the only phase formed, and in between pH values of 5 and 6 hybrid sample composed of anatase and brookite is formed. The addition of more HNO₃ amount for the pH value 2–3 in the peptization step results in strong protonation ([H⁺/Ti]) of the surface of the hydrated TiO₂ particle. This creates a net positive charge, which yields an electrostatically stabilized sol with enhanced particle agglomeration and rutile phase formation. This implies that in a solution, the behavior of protonated particles and their capability to form aggregates depends on the value of the surface charge [34]. When the peptization pH value went beyond 4, a decrease in crystallite size (as seen in Table 3) was seen, indicating a promotion of crystallization processes in which an increase in nucleation instead of crystal growth became the limiting factor. In conclusion, the evidence points to a structural arrangement that favors the production of anatase and brookite phases at lower H⁺ concentrations. Corner-sharing octahedral chains typical of the rutile phase emerge from structural changes brought on by a decrease in the amount of Ti-O-Ti bonds as the H⁺ concentration rises. These aforesaid XRD results under the effect of peptization pH value are well supported by the BET surface analysis. Particles prepared at a low peptization pH value have reduced S_BET area compared with the particles prepared at a high peptization pH value, as illustrated in Table 3.

3. Photocatalytic Activity

The photocatalysis concept is centered on the fact that the photocatalyst’s ability to absorb sufficient light to produce excitant (electron (e⁻) and hole (h⁺)) pairs for the ongoing process of redox reactions. The absorption of light and the generation of e^−/h⁺ pairs depend on the material physicochemical properties, which depend on the synthesis method process parameters. Figure 2a shows the TiO₂ photocatalytic activity prepared at different titanium precursor concentrations (0.1–0.5 M) against the model MB pollutant. It was observed that TiO₂ photocatalyst synthesized at 0.2 M precursor concentration showed the highest activity, resulting in 81% degradation of MB. This increase in activity may be ascribed to the particle’s increased surface area via decreased crystallite size. A high surface area results in enhanced pollutant absorption on the photocatalyst surface [35]. A lower rate of electron-hole (e⁻/h⁺) recombination is also a result of the photocatalyst’s composite character, which entails the coexistence of both anatase and brookite phases. The higher separation of e⁻/h⁺ pairs inside the TiO₂ crystalline phases is responsible for the increased activity of the hybrid sample. This distinction implies that the valence band of brookite has a higher positive redox potential than anatase [20]. Because of the effective interfacial hole transfer that takes place at the intersection of these two phases, holes are effectively utilized in both oxidation and reduction processes throughout the process. Increasing precursor concentration results in decreased pollutant decomposition because the TiO₂ crystallite size increases, the decrease as well in the proportion of the brookite phase occurs. Therefore, the observed activity trend (Figure 2a) was 0.3 M (13% brookite) > 0.2 M (10% brookite) > 0.1 (7% brookite > 0.4 M (6% brookite) > 0.5 (pure anatase).

These kinetic data related to the effect of an increase in precursor concentration versus pollutant degradation is consistent with pseudo-first-order kinetics of MB decomposition in the presence of UV irradiation. For the photocatalytic degradation of organic pollutants (in our case, MB) in solution, the Langmuir–Hinshelwood (L-H) kinetic model is frequently employed [36]. The L-H model expression is as follows for an ideal constant volume batch reactor containing the model pollutant:

(1)$-{\displaystyle \frac{\mathrm{d}\mathrm{C}}{\mathrm{d}\mathrm{t}}}={\displaystyle \frac{k{K}^o{C}_{MB}}{1+K^o{C}_{MB}}}$

where K^o is the equilibrium concentration (L/mg) of the adsorbed MB molecules on the catalyst surface at the reaction temperature, C is the decomposed MB molecules concentration (mg/L) in reactor suspension, and k is the reaction rate constant (mg/L min). The Equation (1) above becomes:

(2)$-{\displaystyle \frac{\mathrm{d}\mathrm{C}}{\mathrm{d}\mathrm{t}}}={\displaystyle \frac{k^{\prime }{C}_{MB}}{1+K^o{C}_{MB}}}$

k′ is the observed reaction rate constant (min⁻¹) equal to kK^o.

When K^oC_MB << 1 at low concentrations, Equation (2) becomes the following first-order equation.

(3)$-{\displaystyle \frac{\mathrm{d}\mathrm{C}}{\mathrm{d}\mathrm{t}}}=k^{\prime }{C}_{MB}$

whose integral gives:

(4)$-\mathrm{l}\mathrm{n}{\displaystyle \frac{C_{MB}}{C_{{CB}_o}}}=k^{\prime }\mathrm{t}$

where C_MBo is the MB organic contaminant initial concentration. The rate of degradation expressed in k′ is obtained from the slope of the linear regression of −ln(C_MB/C_MBo) versus the time plot (Figure S3, Supplementary Information). Linear graphs with R² values larger than 0.98 are observed, indicating that the reaction kinetics of the MB degradation process followed first-order kinetics. Under UV light, the sample with a precursor concentration of 0.3 M exhibits the highest rate constant (k′) of 0.03 min⁻¹ (Figure 2b).

Figure S4a illustrates the photocatalytic efficiency of the as-prepared TiO₂ powder annealed at different temperatures (300–800 °C). The observed activity trend follows TiO₂ 600 °C > 300 > 400 > 500 > 700 > 800 °C. XRD analysis showed that powder crystallinity increases by increasing the temperature. Improved powder crystallinity is also a factor for enhanced photocatalytic activity because poor crystallinity leads to defect structures [37] that are responsible for the photoinduced charge carrier recombination. As observed, increased crystallinity caused the well-ordering of the atoms in the crystal lattice; this may also be the reason that the proportion of the secondary brookite phase increases the most at 600 °C. In addition, on closed observation, it was also demonstrated that degradation activity versus crystalline phases follows the trend of brookite–anatase > anatase > rutile–anatase. The decreased activity of the hybrid sample (rutile–anatase) is due to the increased rutile content in the powder. It is reported elsewhere that rutile has a 10-fold enhanced rate of hole-trapping capability compared with anatase and, as a consequence, a higher rate of e⁻/h⁺ recombination; our experimental observation supports the result obtained by Akpan and Hameed [38]. Moreover, the decreased band gap at high annealing temperature is beneficial for enhanced photon absorption, but the removal of absorbed water and hydroxyl groups from the catalyst surface [39] is detrimental and contributes a lot to the decrease in activity of the photocatalyst. The sample annealed at 600 °C has the highest value of k′ (0.02 min⁻¹, Figure 2c); the value of k′ was obtained from the slope of the plots (−ln(C_MB/C_MBo) versus time plot (Figure S4b, Supplementary Information)) drawn under the effect of different annealing temperature.

Figure S5a shows the activity of the prepared TiO₂ powder at different peptization pH values against the model MB pollutant. The powder synthesized at pH 5 has the highest activity, resulting in 97% degradation, while the powder prepared at pH 2 has the lowest activity of 35%. The photocatalyst characterization analysis showed that low peptization pH value during the synthesis process favors rutile phase formation. The final material has a very low surface area that results in decreased adsorption of MB molecules on the photocatalyst surface. In addition, the electrons lying in the rutile conduction band have low reductive power compared with anatase conduction band electrons; thus, these may be the reason for the lower photocatalytic activity. Following the L-H kinetics, the highest value of k′ (0.06 min⁻¹) was observed for the powder prepared at a peptization pH value of 5.0 (Figure 2d). The values of k′ were obtained from the slope of the curves −ln(C_MB/C_MBo) versus time (Figure S5b, Supplementary Information) plotted under the effect of TiO₂ synthesis with different peptization pH values.

The effect of various TiO₂ concentration levels (from 0.01 to 0.06 g/L) on the pollutant degradation is shown in Figure S6a. According to Wei and Wan [40], the quantity of catalyst affects the rate of photodegradation in a variety of ways, having both positive and negative effects on the procedure. As per our observations, the first reaction rates show a direct proportionality to the catalyst concentration, which is a sign of a heterogeneous regime. This shows that as the catalyst concentration is increased up to a specific ideal value (0.04 g/L), the breakdown rate for the degradation of the MB pollutant increases. These results are very consistent with those of earlier investigations [41,42]. It is crucial to remember that the degradation rate decreases as TiO concentration above the ideal level of 0.04 g/L is exceeded. There are several contributing reasons to this phenomenon. First off, a higher photocatalyst loading raises the opacity of the solution, which reduces photon flux penetration (a shielding effect) inside the reactor [13,43]. The rate of photocatalytic breakdown is adversely affected by this decrease in photon penetration. Additionally, a high solid concentration may cause particle agglomeration, which reduces surface area as a result of particle–particle interactions [44].

The increased light scattering caused by the suspended particles also gradually slows down the reaction rate. Additionally, there may not be enough MB molecules left to effectively adsorb onto an overwhelming amount of TiO₂ particles [42]. The presence of active sites on the photocatalyst surface and the entry of photoactivating light into the reactor suspension can both be used to explain this characteristic. To achieve a successful photo-mineralization process, it is crucial to add the right quantity of photocatalyst. This not only stops the unnecessary addition of an extra photocatalyst, but it also ensures that all light photons are absorbed, permitting effective photo-mineralization. It is crucial to stress that the ideal photocatalyst loading depends on several variables, including the kind of pollutant, the initial pollutant concentration, and the photoreactor’s operating circumstances. This complex relationship emphasizes how important it is to customize photocatalyst loading to conditions to obtain the best outcomes in photocatalytic processes. Plotting the −ln(C/C_o) versus UV irradiation time (Figure S6b, Supplementary Information) for different loading of TiO₂ content in the photocatalytic reactor. The highest value of k′ was 0.07 min⁻¹ (Figure 2e), recorded for TiO₂ content of 0.04 g/L in the photoreactor.

The effect of the pH of photoreactor suspension, which ranges from 2 to 6, on the MB degradation is shown in Figure S7a. It is important to remember that during the testing procedure, the pH value is precisely controlled by the addition of HNO₃ and NaOH. It is a challenging task to assess how pH affects the effectiveness of the photodegradation process. This intricacy results from the fact that several potential reaction pathways, such as (a) hydroxyl radical (OH^•) attack, (b) direct oxidation by positive holes (h⁺), and (c) direct reduction by conducting band electrons, can all contribute to MB disintegration. The kind of substrate and the pH of the reactor suspension determine the relative importance of each mechanism [45]. The electrical double layer at the solid-electrolyte interface is greatly influenced by the pH of the suspension, which also has an impact on sorption-desorption processes and the separation of photoinduced electron-hole (e⁻/h⁺) pairs on semiconductor particle surfaces. TiO₂ has an amphoteric nature, which means that the charge on its surface can be either positive or negative. As a result, pH changes can have a big impact on how well MB dye adheres to TiO₂ photocatalyst surfaces [46].

Our findings support Bubacz et al. [47] observation that the rate of MB breakdown increases as the reactor suspension’s pH is raised. According to Ling et al. [48], at basic pH circumstances, robust adsorption is encouraged by strong electrostatic interactions between negative TiO⁻ and MB cations, which speeds up the rate of breakdown. Thus, MB adsorption is significantly enhanced by a rise in the pH level of reactor suspension. It is interesting that the amphoteric behavior of TiO₂ causes its surface charge characteristics to alter along with pH fluctuations. The large concentration of protons (H⁺) in highly acidic circumstances (reactor suspension pH < 3) prevents the photodecomposition of MB dye. Additionally, under acidic conditions, photocatalyst particles tend to aggregate, decreasing the surface area available for MB molecule adsorption and photon absorption. Reduced decomposition rates are the outcome, with pH values of 3 (49%), 2 (27%), and 1 (20%) indicating poorer breakdown efficiency. On the other hand, the presence of OH⁻ ions neutralizes any acidic byproducts that may be produced during the photodecomposition reaction in mildly acidic to basic conditions (reactor suspension pH > 4). Higher degradation rates are consequently seen, with efficiency being noticeably increased at pH values of 4 (67%), 5 (86%), and 6 (98%) in particular. In conclusion, it is crucial to thoroughly analyze the pollutants that are intended to be degraded and identify the ideal pH conditions necessary for successful photocatalytic degradation. When constructing photocatalytic processes, it is important to consider the nature of the pollutant as well as the current pH level. As said earlier, the degradation of MB follows the L-H kinetics, plotting these data (−lnC/C_o versus irradiation time, Figure S7b) under the influence of pH value of photocatalytic reactor suspension, it was observed that the highest value of k′ (0.064 min⁻¹, Figure 2f was for reactor suspension pH value of 6.0.

4. Experimental Methods

4.1. TiO₂ Synthesis

The key ingredient of our experiment, titanium oxysulfate, came from Sigma Aldrich (Saint Louis, MO, USA). This technical-grade titanium oxysulfate has a minimum titanium content of 29.0% in the form of titanium dioxide (TiO₂). We also purchased a number of other compounds that were necessary for our investigation. We received nitric acid, precisely a 10% v/v aqueous solution, from Ricca Chemical (Arlington, TX, USA) as one of these ingredients. We also bought 20% v/v aqueous ammonium hydroxide from Ricca Chemical. We also purchased anhydrous barium chloride (BaCl₂) from Fisher Scientific (Norristown, PA, USA). We used laboratory deionized (DI) water as the dispersing medium in our tests. It is significant to note that none of these substances underwent any additional purifying procedures; they were all used just as received.

We started the meticulously controlled sol-gel method [28] by making a solution with 0.4 M of titanium oxysulfate (TiOSO₄). We gradually added ammonium hydroxide (NH₄OH) drop by drop until the solution’s pH reached the desired level of 7.0. Titanium hydroxide, therefore, precipitated from the reaction mixture and was painstakingly separated from the mother liquor using centrifugation. We then thoroughly washed the precipitate using warm DI water that was kept at a temperature of 40 °C. This lengthy washing procedure continued until the elimination of sulfate ions was verified, which we accomplished by conducting the BaCl₂ test. After dispersing the sulfate-free precipitate in 40 mL of DI water, we peptized the mixture by adding drops of nitric acid (HNO₃) until the appropriate pH level was reached. After being aged at room temperature for 12 h, the acid-peptized sol produced eventually produced a stable gel.

The solid gel was then dried in an oven for the following step, which took place at an oven temperature of 80 °C for 12 h. The solid gel pellets were painstakingly crushed using a motor and pestle to enable further processing. Finally, we heated these processed pellets gradually at a rate of 2 °C per minute for 2 h at the designated temperature.

4.2. Characterization

Using a Phillips (New York, NY, USA) PW 1710 diffractometer and monochromatic high-intensity, Cu Ka radiation with a wavelength of 0.15418 nm, X-ray diffraction (XRD) patterns were recorded to identify crystalline structure and crystallite sizes. Nitrogen adsorption–desorption isotherms were measured on Micromeritics TriStar (Las Vegas, NV, USA) 3000 (V6.07 A) analyzer to elucidate the textural characteristics of the powder. Fourier-transform infrared (FTIR) spectroscopy analysis was performed on a Bruker (Billerica, MA, USA) Tensor 27 spectrometer to investigate the nano-TiO₂ surface functional groups in the range of 400–4000 cm⁻¹. In addition to these investigations, we also looked at the surface morphology of the synthesized powder with a scanning electron microscope (Hitachi S-4700 FE-SEM, Tokyo, Japan), operating at an acceleration voltage of 200 kV.

A thermoscientific (Waltham, MA, USA) UV-Vis Evolution 300 spectrophotometer equipped with a DRS accessory was used to obtain the diffuse reflectance spectra (DRS) of the annealed powders in the wavelength range of 190–600 nm. Using the Kubelka–Munk function method, the Tauc plots were plotted (αhυ)^1/2 vs. hυ) to calculate the indirect band gap value of the synthesized powders. The band gap energy, designated as E_g in the spectrum, is the location where the linear portion of the spectra intersects the hυ axis (x-axis).

4.3. Activity Tests

We used a cylindrical acrylic batch photoreactor with a capacity of 2 L in our laboratory to conduct photocatalytic studies at room temperature. Precision amounts of MB at a concentration of 0.085 g/L and the model pollutant, TiO₂ powder (0.04 g/L), were painstakingly manufactured for each study run. These compounds were individually suspended in DI water in separate flasks to provide a stable suspension and solution and to reduce particle agglomeration. In the following step, we sonicated these suspensions and solutions for 30 min.

After completing this stage of preparation, the batch reactor was filled with a mixture of the MB solution and TiO₂ suspension. The reactor was filled with additional DI water until it had a total volume of 160 mL. We maintained a constant flow of filtered air from an external source throughout the experiment, which was directed into the reactor suspension.

With the use of this experimental setup, we were able to thoroughly analyze the photocatalytic degradation process in a controlled environment, revealing the effectiveness of our created TiO₂ nanoparticles in the elimination of the model pollutant methylene blue. We used magnetic stirring, keeping the reactor suspension under such agitation for a length of 30 min before beginning UV irradiation, to achieve a thorough mixing of the TiO₂ suspension and MB solution and to create adsorption/desorption equilibrium. A one-hour UV exposure was then administered after that. To achieve this, we used a single UV light source, especially the GPH212T5L/4-10W type from Atlantic Ultraviolet Corp., Hauppauge, NY, USA. This UV lamp was safely enclosed inside a quartz sleeve and produced powerful radiation at a wavelength of 254 nm. It was positioned in the middle of the batch reactor, vertically. We took an exact 5 mL aliquot from the irradiation sample every 10 min. A Millipore syringe filter with a 0.22 µm porosity was used to filter this aliquot, which successfully eliminated the photocatalyst particles from the solution. Then, using a UV-visible spectrophotometer, we performed analyses by determining the highest absorbance of methylene blue (MB) at 664 nm. Throughout the experiment, these measurements were crucial in determining the photocatalytic activity.

5. Conclusions

The photocatalytic properties of TiO₂-based oxides are closely correlated to their physiochemical properties. It has been shown that improvement in the physicochemical properties (crystalline size, structure and morphology, crystallinity, specific surface area, pore size, band gap, crystalline phase transition, degree of surface hydroxyl groups, etc.) is possible through the selection of the optimum range of the photocatalyst synthesis process parameters. In this connection, we selected the sol-gel process, which is a very useful technique for the synthesis of nanocrystalline TiO₂ photocatalyst. It was observed that titanium precursor (TiOSO₄) concentration, calcination temperature, and peptization pH value have a profound effect on the physical properties of the final material. The solid findings from this current work were;

(i) TiO₂ crystallite size increases with an increase in TiOSO₄ precursor concentration.

(ii) TiO₂ is composed mainly of anatase with the minor brookite phase by using titanium precursor in the range of 0.2–0.4 M, while further increase in precursor concentration (0.5 M) results in anatase phase formation.

(iii) The Brookite phase transforms into anatase above 600 °C, while the transition of anatase into the rutile phase starts by increasing the annealing temperature beyond 700 °C.

(iv) It was observed that the band gap of the final material is dependent on the crystalline phases, which vary with an increase in calcination temperature.

(v) Rutile is the dominant phase at low peptization pH values (2.0 and 3.0), anatase is the only phase obtained at peptization pH value of 3 to 5, and a hybrid sample consisting of mainly anatase with minor brookite phase is formed at peptization pH value of 5 and 6.

(vi) TiO₂ photocatalytic activity against the model MB pollutant is found to depend on the usage of photocatalyst concentration and the pH value of the reactor suspension.

(vii) Finally, the influence of different sol-gel synthesis process parameters and the influence of photocatalyst concentration and reactor suspension pH value demonstrated that using the following set of parameters guarantees the highest photocatalytic activity (>98%) of the TiO₂ against MB degradation: titanium precursor concentration (0.3 M), calcination temperature (600 °C), peptization pH value (5.0), photocatalyst concentration (0.04 g/L) and reactor suspension pH value (6.0).

Data Availability Statement

Original data are included in the article and the Supplementary Materials. Further inquiries can be directly addressed to the author.

Acknowledgments

The author gratefully acknowledges the King Faisal University, Saudi Arabia (KSA), for supporting this study.

Conflicts of Interest

The author declares no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. (a) XRD patterns of TiO2 powder annealed at 300, 500, 600, and 800 °C, respectively, (b,c) SEM images of TiO2 powder calcined at 300 and 800 °C, (d) FTIR spectra of TiO2 powder annealed at 300 and 600 °C and (e) Tauc plot of TiO2 powder calcined at 300 and 800 °C (photocatalyst synthesis conditions: precursor concentration 0.4 M, precipitation pH value 7.0, peptization pH value 5.0, aging time 12 h and drying temperature and time is 80 °C and 12 h).

Figure 2. Photocatalytic activity of the as-prepared TiO2 powder (a) at different TiOSO4 concentrations against the decomposition of MB, (b) plot of k′ versus precursor concentration, (c) plot of k′ versus calcination temperature, (d) plot of k′ versus peptization pH value, (e) plot of k′ versus loading TiO2 content, (f) plot of k′ versus reactor suspension pH value.

Table 1

Influence of titanium precursor concentration of TiO₂ crystallite size, phase composition, and specific surface area.

Precursor Concentration (M)	Crystallite Size (nm), (% Fractional Composition)		SBET (m2 g−1)
	Anatase	Brookite
0.1	8.0 (0.93)	9 (0.07)	137.0
0.2	10.5 (0.9)	11 (0.1)	142.0
0.3	15.7 (0.87)	13 (0.13)	156.0
0.4	17 (0.94)	8 (0.06)	147.0
0.5	20.3 (1.0)		98.0

Table 2

Influence of annealing temperature on TiO₂ crystallite size, phase composition, specific surface area, pore size, and band gap.

Calcination Temperature (°C)	Crystallite Size (nm), (% Fractional Composition)			SBET (m2 g−1)	BJH PoreVolume (cm3 g−1)	Band Gap (eV)
	Anatase	Brookite	Rutile
300	9.0 (0.92)	6.0 (0.08)		175.0	0.28	3.12
400	10.0 (0.9)	7.0 (0.1)		170.0	0.27	3.19
500	13.6 (0.91)	9.5 (0.1)		162.0	0.23	3.22
600	16.0 (0.86)	6.0 (0.14)		154.0	0.2	3.1
700	27.0 (1.0)			78.0	0.13	3.0
800	39.0 (0.06)		55.5 (92.4)	25.0	0.07	2.9

Table 3

Influence of peptization pH value on the TiO₂ crystallite size, phase composition, and specific surface area.

Peptization pH Value	Crystallite Size (nm), (% Fractional Composition)			SBET (m2 g−1)
	Anatase	Brookite	Rutile
2	22.0 (0.54)		42.5 (0.45)	68.0
3	16.0 (0.85)		22.0 (0.15)	104.0
4	11.0 (1.0)			136.0
5	9.0 (0.93)	5.0 (0.07)		185.0
6	8.0 (0.92)	5.0 (0.08)		182.0

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15010064/s1, Figure S1: XRD patterns of calcined samples prepared with different precursor concentrations (0.1–0.5 M) at precipitation pH = 7, peptization pH = 5, and calcination temperature T = 600 °C. Figure S2: N₂ adsorption–desorption isotherm for samples calcined at different temperatures. Figure S3: Plot of −ln(C/C_o) versus irradiation time under the effect of precursor concentration. Figure S4: (a) Photocatalytic activity of the as-prepared TiO₂ powders calcined at different calcination temperatures and (b) Plot of −ln(C/C_o) versus irradiation time under the effect of calcination temperature. Figure S5: (a) Photocatalytic activity of the as-prepared TiO₂ powders prepared under the influence of different peptization pH values and (b) Plot of −ln(C/C_o) versus irradiation time under the effect of peptization pH value. Figure S6: (a) Effect of different TiO₂ loading on photocatalytic activity of MB pollutant and (b) Plot of −ln(C/C_o) versus irradiation time under the effect of different TiO₂ loading in a photocatalytic reactor. Figure S7: (a) Effect of reactor suspension pH value on the photodegradation of MB model pollutant and (b) Plot of −ln(C/C_o) versus irradiation time under the effect of different reactor suspension pH values.

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