Photodegradation of Rhodamine B and Phenol Using TiO₂/SiO₂ Composite Nanoparticles: A Comparative Study

Abstract

Organic pollutants found in industrial effluents contribute to significant environmental risks. Degradation of these pollutants, particularly through photocatalysis, is a promising strategy ensuring water purification and supporting wastewater treatment. Thus, photodegradation of rhodamine B and phenol under visible-light irradiation using TiO₂/SiO₂ composite nanoparticles was within the main scopes of this study. The nanocomposite was synthesized through a wet impregnation method using TiO₂ and SiO₂ nanopowders previously prepared via a facile sol–gel approach and was fully characterized. The obtained results indicated a pure anatase phase, coupled with increased crystallinity (85.22%) and a relative smaller crystallite size (1.82 nm) in relation to pure TiO₂ and SiO₂ and an enhanced specific surface area (50 m²/g) and a reduced energy band gap (3.18 eV). Photodegradation of rhodamine B upon visible-light irradiation was studied, showing that the TiO₂/SiO₂ composite reached total (100%) degradation within 210 min compared to pure TiO₂ and SiO₂ analogues, which achieved a ≈45% and ≈43% degradation rate, respectively. Similarly, the composite catalyst presented enhanced photocatalytic performance under the same irradiation conditions towards the degradation of phenol, leading to 43.19% degradation within 210 min and verifying the composite catalyst’s selectivity towards degradation of rhodamine B dye as well as its enhanced photocatalytic efficiency towards both organic compounds compared to pure TiO₂ and SiO₂. Additionally, based on the acquired experimental results, ●O₂⁻, h⁺ and e⁻ were found to be the major reactive oxygen species involved in rhodamine B’s photocatalytic degradation, while ●OH radicals were pivotal in the photodegradation of phenol under visible irradiation. Finally, after the TiO₂/SiO₂ composite catalyst was reused five times, it indicated negligible photodegradation efficiency decrease towards both organic compounds.

1. Introduction

According to the World Water Development Report (2020) [1], the preservation of water resources linked to worldwide climate change has emerged as a crucial environmental concern within the realm of significant contemporary global issues. Presently, natural water ecosystems face susceptibility to an array of perilous chemical and biological compounds originating from various sources, such as civil society, the public sector, and notably, the industrial sector.

Numerous industries including textile, pharmaceutical, papermaking, printing, cosmetics, and food processing generate a vast quantity of wastewater containing diverse organic pollutants including dyes, thereby causing contamination. In the context of the textile industry, it is estimated that approximately 200,000 tons of dyes are discharged into water reservoirs annually throughout dyeing and finishing operations, primarily given the inefficiencies in the dyeing procedure [2].

Rhodamine B (RhB) and phenol, which are widely employed, represent a significant peril to freshwater ecosystems and human vitality. The aforementioned dyes are known for their mutagenic, toxic, chemically stable, and non-biodegradable properties, making them particularly hazardous even at concentrations below 1 mg/L [3], and they carry a high risk of carcinogenicity [4,5]. The presence of organic dyes, even in trace amounts, can lead to various ailments including abdominal disorders, irritations, anemia, etc. [6]. Consequently, pollutant dyes’ elimination from wastewater is imperative.

Currently, several treatment methods are employed, including chemical precipitation, separation, adsorption, and coagulation techniques [7]. However, these approaches often result in partial dye degradation and merely shift the contaminants from one phase to another, leading to the generation of secondary pollutants that necessitate additional treatment [8,9]. At present, there is growing attention towards advanced oxidation processes (AOPs) that utilize semiconducting materials as a viable alternative to traditional methods [10]. These processes offer several advantages, including simplified equipment requirements, non-selective oxidation, facile operational control, cost effectiveness, and thorough decomposition of organic dyes into benign byproducts [11]. A distinctive characteristic of AOPs is the generation of reactive species like hydroxyl radicals, which enable rapid and non-selective oxidation of organic contaminants. Heterogeneous photocatalysis, which employs nanomaterials based on oxides, is particularly intriguing, as it can effectively remove water-soluble organic pollutants from both water and wastewater when exposed to light irradiation [12].

In general, the process of photocatalytic degradation involves the use of suspended particles in a water solution, which serve as photocatalysts and are exposed to light. The photocatalyst plays a crucial role in this process. These materials constitute semiconductors that possess an electronic band structure, with a clear separation between the highest occupied energy band, known as the valence band (VB), and the lowest unoccupied band, the conduction band (CB), which is determined by an energy band gap (E_g). When photons with energies equal to or larger than the semiconductor’s bandgap are absorbed, electron–hole (e⁻−h⁺) pairs are generated within the semiconductor’s particles. As these photoproduced carriers migrate, charge separation occurs, resulting in the formation of various compounds (e.g., H₂O, ●OH, and ¹O₂). It is important to note that electrons and holes can also recombine with each other unless undergoing any chemical reactions. The oxidizing agents initiate the degradation of the organic pollutants that exist on or near the catalytic surface, ultimately leading to their complete mineralization into harmless substances [13].

Metal oxide semiconductors like TiO₂ have exceptional adsorption capabilities and act as efficient catalysts due to their high reactivity, enhanced photosensitivity, large specific surface area, cost effectiveness, non-toxicity, and improved catalytic efficiency in dye photodegradation [14].

Titanium dioxide (TiO₂) is considered as within the most commonly utilized photocatalysts. TiO₂ has been thoroughly studied and extensively applied in photocatalytic processes due to its favorable properties, including enhanced photocatalytic activity, chemical robustness, cost efficiency, and abundance [15,16]. TiO₂ exists in different crystal structures, with the most studied being anatase and rutile. These crystal structures have different bandgaps and surface properties, influencing their photocatalytic activity and selectivity. Anatase TiO₂ is generally considered the most active form of TiO₂ [16,17]. Due to its wide application and extensive research, TiO₂-based forms have been optimized and chemically modified in various ways to improve their efficiency and expand their range of applications. For instance, efforts have been conducted at enhancing the visible-light absorption of TiO₂ via doping it with transition metal ions or coupling it with other semiconductor materials [18,19].

Silicon dioxide (SiO₂), commonly known as silica, is not typically used as a photocatalyst for efficient photocatalysis; however, SiO₂ can still contribute to photocatalysis as a support material for other photocatalysts. SiO₂ nanoparticles or films are often used as substrates or matrices to immobilize and stabilize other photocatalytic semiconductors such as TiO₂ or zinc oxide (ZnO) [20]. In such composite systems, SiO₂ provides an enhanced specific surface area (SSA) and can increase the active photocatalyst’s dispersion, leading to an improved photocatalytic performance [20,21].

Within the framework of the present research, TiO₂ and SiO₂ nanopowders were initially prepared utilizing a facile sol–gel approach and were further utilized to prepare a TiO₂/SiO₂ composite material through a wet impregnation approach [22]. The physical aspects of the as-mentioned TiO₂/SiO₂ composite material were thoroughly investigated using various techniques like FESEM, XRD, micro-Raman, FTIR, BET, DLS, as well as DRS. Subsequently, the composite material’s photocatalytic efficiency towards the degradation of rhodamine B and phenol was evaluated under visible-light irradiation and compared, aiming to highlight the potential selectivity of the as-produced TiO₂/SiO₂ towards degradation of a specific organic dye.

Finally, the present research aimed also to elucidate, to some extent, the phenol’s photocatalytic degradation mechanism under visible-light irradiation via the quantitative study of certain phenol intermediates (p-benzoquinone and hydroquinone) using the fabricated TiO₂/SiO₂ composite material, as, according to authors’ best knowledge, the photodegradation mechanism of phenol and its derivatives under visible-light irradiation has not been fully established until now.

2. Materials and Methods

2.1. Synthesis of SiO₂ Powder

The SiO₂ sol–gel synthesis was performed according to the approach reported by Moji et al., 2020 [23]. SiO₂ was synthesized through the utilization of tetraorthosilicate (TEOS, Si(OC₂H₅)₄, 98%, Sigma-Aldrich, Darmstadt, Germany), ethanol (C₂H₅OH, 99.8%, Sigma-Aldrich, Darmstadt, Germany), nitric acid (HNO₃, 65%, Penta, Prague, Czech Republic), and lab-distilled water. TEOS (10 mL) and ethanol (10 mL) were mixed and also stirred for approximately 30 min. Nitric acid (5 mL) and double-distilled water (14 mL) were subsequently poured into the Si(OC₂H₅)₄ alcoholic solution. The mixture was stirred at ambient temperature until the formation of a gel. The volume ratio of TEOS:ethanol:H₂O:HNO₃ remained consistent and equal with 1:1:1.4:0.5. Then, the produced gel was air-dried at room temperature for 24 h. Afterwards, the air-dried gel was crushed to create powder and heat-treated at 100 °C for 2 h in order to eliminate solvent and organic ligands and for acquiring full densification. The powder underwent a process of re-crushing and was subjected to annealing at 800 °C for 2 h in a nitrogen environment. Finally, the obtained powder was further crushed to produce fine powder.

2.2. Synthesis of TiO₂ Powder

For synthesizing TiO₂ powder, 100 mL of double-distilled water was then acidified using nitric acid (HNO₃, 65%, Penta, Prague, Czech Republic) to properly adjust the pH (pH < 2), ensuring the progression of hydrolysis using a pH meter possessing accuracy equal to ±0.01 (ISOLAB Laborgeräte GmbH, Eschau, Germany). Next, 15 mL of titanium (IV) butoxide (C₁₆H₃₆O₄Ti, 97%, Sigma-Aldrich, Darmstadt, Germany) was poured into the acidified aqueous solution under potent stirring, and then, 30 mL of 2-propanol (C₃H₈O, 99.8%, Sigma-Aldrich, Darmstadt, Germany) was poured into to yield a translucent TiO₂ sol-gel. Afterwards, the solution was heated until the solvent was entirely evaporated. Then, the yielded gel was calcinated at 450 °C for 4 h, while the arising white powder was triturated and purified through rinsing and centrifugation to omit possible contingent impurities [17].

2.3. Synthesis of TiO₂/SiO₂ Composite Powder

The TiO₂/SiO₂ nanocomposites, possessing a 7:1 (TiO₂:SiO₂) molar ratio, were produced base on the impregnation approach described by Eddy et al., 2021 [22] The chosen molar ratio was based on existing experimental results [22], indicating the enhanced photocatalytic activity of TiO₂/SiO₂ nanocomposites with molar ratio equal to 7:1, with respect to other examined molar ratios (1:1 and 3:1). More specifically, 2.3263 g of TiO₂ powder was mixed with 80 mL of ethanol (C₂H₅OH, 99.8%, Sigma-Aldrich, Darmstadt, Germany) and sonicated for approximately 30 min. Afterwards, 0.25 g of the as-synthesized SiO₂ powder was added, followed also again by sonication (30 min). The produced mixture was stirred under heating (70 °C) until a white-colored paste was then formed. Subsequently, the as-developed paste was dried at 100 °C for 2 h and then calcinated at 500 °C for 5 h. Finally, the acquired powder was crushed into fine powder.

2.4. Characterization of the TiO₂/SiO₂ Composite Powder

The morphology of the TiO₂/SiO₂ composite powder was evaluated through field emission SEM (FESEM, JSM-7401F, JEOL, Tokyo, Japan).

For the XRD analysis and study of the as-synthesized composite powder, a Brucker D8 Advance (Brucker, Germany) X-ray diffractometer was used, employing a CuΚα radiation (λ = 1.5406 Å) (40 kV, 40 mA). The measurement was performed at a 2 theta (2θ) angle characterized by a range 20°–90°, with a counting diffraction intensity step equal to 0.01° per 0.8 s.

Raman spectroscopy was performed utilizing a micro-Raman system (inVia, Renishaw, Wotton-under-Edge, Gloucester, UK) using a λ = 532 nm laser line as the excitation source. Raman spectra were collected at 25 °C. An internal Si reference was used for calibrating the frequency shift value. 3–10 spots were acquired in total for the studied powder. The following parameters were set in order to record the Raman spectra: laser power: 1%; exposure time: 30 s; accumulations: 3; spectral range 100–1000 cm⁻¹; spectral resolution: 2 cm⁻¹. In order to support the Raman data, FTIR spectra were also collected in the 400–4000 cm⁻¹ range, with a 4 cm⁻¹ resolution at ambient temperature using a FTIR JASCO4200 apparatus (Oklahoma City, OK, USA) with a Ge crystal.

N₂ adsorption of the prepared powder was performed in a ChemBET 3000 Instrument (Yumpu, Diepoldsau, Switzerland) in order to ascertain the SSA BET. Prior to measurements, TiO₂/SiO₂ powder underwent a degassing procedure at 80 °C for 24 h.

Dynamic light scattering (DLS) (Malvern Zetasizer Nano ZS, Malvern Panalytical Ltd., Malvern, UK) was used to evaluate the hydrodynamic diameter and distribution of the powder’s particles in the aquatic dispersion. A 633 nm laser and a scattering angle of 173° was determined for recording scattering intensity.

Energy band-gap (E_g) values were assessed utilizing an ultraviolet–visible (UV–vis) spectrometer (Jasco UV/Vis/NIR Model name V-770, Interlab, Athens, Greece) equipped with an integrating sphere, thus allowing the recording of diffuse reflectance measurements.

The mineralization of rhodamine B (RhB) and phenol was ascertained through total organic carbon (TOC) analysis (TOC-LCSH/CSN, Shimadzu Scientific Instruments, Columbia, MD, USA), taking measurements at the same timepoints that were selected for the photocatalytic study experiments.

2.5. Photocatalytic Activity Study of the TiO₂/SiO₂ Composite Powder

The effectiveness of the visible-light photocatalytic activity of the as-synthesized TiO₂/SiO₂ composite was initially examined towards RhB degradation using 0.005 g of the composite in 10 mg·L⁻¹ RhB (C₂₈H₃₁CIN₂O₃, Penta-Chemicals Unlimited, Prague, Czech Republic) aqueous solution (250 mL) at ambient temperature and pH equal to 6.63 ± 0.01. Before each of the photocatalytic experiments (cycles), saturation of RhB solution for 1h was performed through extra-pure O₂ (99.999%) flow.

The evaluation of the composite’s photocatalytic activity was also evaluated under visible-light irradiation towards phenol degradation. More specifically, 0.005 g of the composite in 10 mg·L⁻¹ phenol (C₆H₅OH, 99.5%, Sigma-Aldrich, Darmstadt, Germany) aqueous solution (250 mL) at 25 °C and pH = 7.32 ± 0.01 was used. Prior to the photocatalytic tests, the phenol solution was also saturated with extra-pure O₂ (99.999%) for 1 h.

The photoreactor used was endowed with a system of four parallel lamps placed at a distance of 10 cm over the powder’s surface [17]. Furthermore, 15 W visible-light lamps of 900 lumens (made by Osram, OSRAM GmbH, Munich, Germany) with a 400 nm cutoff filter were used, and all the experiments were carried out under conditions of ambient temperature.

The photocatalytic degradation of RhB was estimated under visible light. The derived absorbance of the as-prepared TiO₂/SiO₂ composite powder was estimated at 554 nm using a spectrometer (Thermo Fisher Scientific Evolution 200 Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The ratio C/C₀, where C stands for the RhB concentration after a certain time of photocatalysis, and C₀ constitutes the initial concentration of RhB, was obtained indirectly through the assessment of the measured A (absorption at each time) to the initial (A_initial) [17].

Finally, high-performance liquid chromatography (HPLC, LC2050, Shimadzu Scientific Instruments, Columbia, MD, USA) integrated with a UV–vis detector for the identification of the degradation products was employed to evaluate the photocatalytic degradation of phenol under visible light over TiO₂/SiO₂ nanocomposites. A C-18 column as the stationary phase and a mixture of distilled water and methanol (H₂O:methanol = 70:30) as the mobile phase were used. The mobile phase was delivered at a flow rate equal to 1 mL/min, and the detection wavelength was set at 254 nm. A volume of 0.5 mL was received every 30 min and filtered via a cellulosic filter (0.45 μm pore size) before each injection into the HPLC chromatography, and the concentration was then estimated.

3. Results

3.1. Characterization of the TiO₂/SiO₂ Composite Powder

3.1.1. XRD Analysis

XRD analysis was applied to study the crystallinity of the as-synthesized TiO₂/SiO₂ composite powder. Figure 1 shows the diffraction diagrams that eventuated for the studied powder along with that of pure TiO₂ as reference. The dominant crystal phase of pure TiO₂ is anatase, while in the case of TiO₂/SiO₂, only crystal phase is spotted. The highest intensity diffraction peak of anatase (I_A) in TiO₂/SiO₂ as well as reference TiO₂ is at 2θ = 25.26°, corresponding to (101) crystal plain, and all the other peaks of anatase indexed are in accordance with the PDF No. 03-065-5714 [24]. Rutile is also observed in the case of pure TiO₂ at 2θ = 27.36° (I_R), 36.02°, 41.16°, and 54.25°, corresponding to (110), (101), (111), and (211) crystal planes, respectively (PDF No. 03-065-5714) [25].

Table 1. Crystallographic parameters of the as-synthesized TiO₂/SiO₂ powder.

Sample	Crystalline Phase	Space Group	Phases Percentage
TiO2	Tetragonal (anatase) Tetragonal (rutile)	I41/a m d P42/m n m	73.22% 26.78%
TiO2/SiO2	Tetragonal (anatase)	I41/a m d	100%

presents the percentage of the rutile phase that was calculated according to Equation (1) [26].

$$\%\mathrm{Rutile}=\left(\frac{1}{1+0.884\frac{{\mathrm{I}}_{\mathrm{A}\left(101\right)}}{{\mathrm{I}}_{\mathrm{R}\left(110\right)}}}\right)\times 100$$

(1)

The average crystallite size of the as-produced TiO₂/SiO₂ powder was determined using the Debye–Scherrer equation and the interplanar of TiO₂ and TiO₂/SiO₂ according to Bragg’s Law Equation, as previous studies have already analytically presented [27].

The crystallinity index (CI%) was calculated according to Equation (2):

$$\mathrm{CI}\%=\frac{\mathrm{Area} \mathrm{of} \mathrm{all} \mathrm{the} \mathrm{crystalline} \mathrm{peaks}}{\mathrm{Area} \mathrm{of} \mathrm{all} \mathrm{the} \mathrm{crystalline} \mathrm{and} \mathrm{amorphous} \mathrm{peaks}}$$

(2)

The crystal lattice index was determined using Equation (3) (

Table 2. d-spacing calculations for TiO₂ powder.

Bragg’s Angle		dhkl (Å)	dhkl (nm)	hkl
2θ	θ
25.26	12.63	3.5339	0.3534	101 (A)
27.36	13.68	3.2571	0.3257	110 (R)
36.02	18.01	2.4914	0.2491	101 (R)
36.87	18.44	2.4359	0.2436	103 (A)
37.72	18.86	2.3829	0.2383	004 (A)
38.48	19.24	2.3376	0.2338	112 (A)
41.16	20.58	2.1914	0.2191	111 (R)
47.99	23.99	1.8942	0.1894	200 (A)
53.85	26.93	1.7011	0.1701	105 (A)
54.25	27.13	1.6895	0.1689	211 (R)
54.97	27.49	1.6691	0.1669	211 (A)
62.67	31.34	1.4812	0.1481	204 (A)
68.84	34.42	1.3627	0.1363	116 (A)
70.29	35.15	1.3381	0.1338	220 (A)
74.99	37.49	1.2655	0.1266	215 (A)
82.81	41.41	1.1647	0.1165	224 (A)

and

Table 3. d-spacing calculations for TiO₂/SiO₂ powder.

Bragg’s Angle		dhkl (Å)	dhkl (nm)	hkl
2θ	θ
25.26	12.63	3.5202	0.35202	101 (A)
36.85	18.43	2.4894	0.24894	103 (A)
37.80	18.90	2.3781	0.23781	004 (A)
38.55	19.28	2.3335	0.23335	112 (A)
48.05	24.03	1.8920	0.18920	200 (A)
53.93	26.97	1.6988	0.16988	105 (A)
55.07	27.54	1.6663	0.16663	211 (A)
62.72	31.36	1.4802	0.14802	204 (A)
68.94	34.47	1.3610	0.13610	116 (A)
70.32	35.16	1.3376	0.13376	220 (A)
75.09	37.55	1.2641	0.12641	215 (A)
82.65	41.33	1.1665	0.11665	224 (A)

) [28]:

$$\frac{1}{{\mathrm{d}}_{\mathrm{hkl}}^2}=\frac{{\mathrm{h}}^2}{{\mathrm{a}}^2}+\frac{{\mathrm{k}}^2}{{\mathrm{b}}^2}+\frac{{\mathrm{l}}^2}{{\mathrm{c}}^2}$$

(3)

There are no significant changes detected in the peak positions, as it is obvious in all the XRD diffractograms. Additionally, no peaks of SiO₂ are indexed in composite powder, indicating that SiO₂ possesses an amorphous phase. According to Eddy et al. [29], the highest (100%) anatase phase percentage in the case of TiO₂/SiO² powder could be potentially attributed to the fact that little amounts of SiO₂ can efficiently inhibit the anatase phase from rutile phase transformation.

Table 4. Crystal lattice indices, average crystallite size, FWHM (full width at half maximum), and crystallinity of the produced TiO₂/SiO₂ powder.

Sample	Crystal Lattice Index (a = b ≠ c)			Average Crystallite Size (nm)	FWHM	Crystallinity (%)
	a	b	c
TiO2	3.7833	3.7833	9.4929	3.17	0.3330	69.49
TiO2/SiO2	3.7885	3.7885	9.5317	1.82	0.4878	85.22

indicates that the presence of SiO₂ might reduce the crystallite size. The decreased crystallite size in the case of TiO₂/SiO₂ powder implies potential compressive strain [29]. The Williamson–Hall plot between βcosθ/λ and sinθ/λ (Figure S1) is provided for validating the presence of compressive strain. In general, the negative slope in the plot points out the presence of compressive strain [30], while the positive slope denotes the potential presence of tensile strain [31]. Through the Williamson–Hall plot, we also calculated the lattice micro-strain of the prepared samples utilizing the following equation (Equation (4)):

$$\frac{\mathsf{\beta }\cos \mathsf{\theta }}{\mathsf{\lambda }}=\frac{1}{\mathrm{D}}+\frac{\mathsf{\epsilon }\sin \mathsf{\theta }}{\mathsf{\lambda }}$$

(4)

where β constitutes the full width at half maximum (FWHM), λ is the X-rays’ wavelength, θ is the diffraction angle, D is the particle size, and ε is the micro-strain [31]. According to the plot, the TiO₂ powder indicates a positive slope, and as a result, tensile strain is present, while the composite TiO₂/SiO₂ powder presents a negative slope, thus affirming the existence of compressive strain.

The observed micro-strain decrease could be associated with the increasing crystallite size and the reducing FWHM value, as presented in

Table 5. The average crystal size (D), FWHM value, and average micro-strain (ε) of the studied TiO2/SiO₂ powder.

Sample	Average Crystallite Size (D, nm)	Full Width at Half Maximum (β, FWHM)	Average Micro-Strain (ε, ×103)
TiO2	3.17	0.3330	3.39
TiO2/SiO2	1.82	0.4878	5.67

. The observed decrease in line broadening (β) in the case of the pure TiO₂ powder may be due to the size, micro-strain, or both [32].

3.1.2. Micro-Raman Analysis

The Raman spectra derived from the composite TiO₂/SiO₂ powder are presented in Figure 2. In the spectra of both TiO₂ and TiO₂/SiO₂ composite powders, the dominant Raman active modes can be allocated to the anatase TiO₂ phase. More specifically, a peak with strong intensity appears at approximately 144 cm⁻¹ (E_g(1)), followed by peaks of lower intensity at 197 (E_g(2)), 398 (B_1g(1)), 515 (combination of A_1g and B_1g(2) that overlaps at ambient temperature), and 639 cm⁻¹ (E_g(3)) (see Figure 2). The position of E_g(1) Raman mode for the reference TiO₂ powder and the studied TiO₂/SiO₂ powder varies from 144 to 145 cm⁻¹, as seen in Figure S2. Several parameters such as strain, phonon confinement, non-homogeneity of the particle size distribution, defects, and anharmonic and non-stoichiometry effects might lead to the position’s shift of the E_g(1) Raman mode of anatase TiO₂. Despite this, the division among the aforementioned factors is not perspicuous [33]. The prevalence of one or more of these factors, which is observable in the Raman spectra of TiO₂ and TiO₂/SiO₂ powders, is ascertained via the structural properties of the studied sample, such as crystallite size and crystallite size distribution, presence of a combination of phases (anatase combined with rutile and/or brookite phases), value and type of the strain (compressive or tensile), type of defects, etc. The main factors that affect E_g(1) Raman mode in the studied composite powder in relation to the reference TiO₂ powder could be ascribed to crystallite size and also the presence of an adequate amount of rutile TiO₂ phase combined with anatase in the case of pure TiO₂. These are verified by the plot that is presented in Figure S2, as the E_g(1) peak of the TiO₂/SiO₂ powder appears at ≈145 cm⁻¹, which is contrary to TiO₂, where the peak’s position appears to 144.53 cm⁻¹ given the fact that the as-synthesized composite powder owns the smallest crystallite size, and the rutile phase is not detected.

Additionally, the E_g(1) peak is significantly sensitive to anatase content, even in cases with low percentages of anatase (<50%). As the anatase content increases, the peak’s intensity becomes extremely strong, while the intensities of the other observed peaks are relatively low. This is also certified by the obtained Raman spectra (see Figure 2 and Figure S2), as TiO₂/SiO₂ composite powder consists of 100% anatase phase, while pure TiO₂ powder consists of 73.22% anatase phase. Taking into account the strong intensity of the E_g(1) peak of the composite powder, a magnification in the 350–700 cm⁻¹ range is provided (see Figure S3). As for pure TiO₂ powder, apart from the Raman active modes ascribed to the anatase TiO₂ phase, the E_g(R) and A_1g(R) modes of rutile phase are spotted at 447 and 612 cm⁻¹, respectively.

According to Zanatta, 2017 [34] and Jassinski et al., 2022 [35], the data provided by the Raman spectra analysis can be exploited in order to estimate the percentage of the rutile TiO₂ phase. The intensity and integrated intensity values of rutile bands at 447 and 612 cm⁻¹ (R3 and R4) and anatase bands at 398, 515, and 638 cm⁻¹ (A3, A4, and A5) were used to calculate the rutile phase percentage according to Equation (5), facilitating a semi-quantitative analysis:

$$\mathrm{Rutile}\%=\frac{{\mathrm{Int}}_{\mathrm{R}3}+{\mathrm{Int}}_{\mathrm{R}4}}{{\mathrm{Int}}_{\mathrm{A}3}+{\mathrm{Int}}_{\mathrm{A}4}+{\mathrm{Int}}_{\mathrm{A}5}+{\mathrm{Int}}_{\mathrm{R}3}+{\mathrm{Int}}_{\mathrm{R}4}}$$

(5)

where Int(x) corresponds to the intensity (maximum or integral) of particular rutile (R) or anatase (A) bands.

The obtained results are presented in

Table 6. Phase composition calculations obtained through Raman analysis.

Sample	Calculated Phase Composition (%)
	According to Maximum Intensity		According to Band Integral Intensity
	Anatase	Rutile	Anatase	Rutile
TiO2	71.25	28.75	72.88	27.12
TiO2/SiO2	100	-	100	-

. It can be observed that the results determined with the two methods (from the maximum intensity and band integral intensity) possess slight differences and are very close to the results obtained from the XRD analysis (see Table 1).

3.1.3. FTIR Analysis

The FTIR study of the synthesized composite TiO₂/SiO₂ powder presents the characteristics of the formation of TiO₂ of high purity. According to the FIIR spectra, several peaks at 490.01, 1390.27, 1622.87, 2852.84, 2919.08, and 3409.86 cm⁻¹ are depicted (Figure 3).

The broad absorption band present in the 400–1000 cm⁻¹ region is ascribed to the Ti–O stretching of the Ti–O–Ti linkages within TiO₂’s particles [36]. The peak at 1390.27 can be attributed to the asymmetric stretching mode of Ti-carboxylate [37]. Additionally, the peak spotted at 1622.87 cm⁻¹ and the wide band between 3200 and 3600 cm⁻¹ represent the O–H bending and stretching modes of hydroxyl groups respectively, verifying the existence of moisture in the studied powder [38]. Two weak bands present at 2852.84 and 2919.08 cm⁻¹, in the case of pure TiO₂, are associated with the characteristic frequencies of residual organic species emanating from the utilized precursors during the synthesis that were not completely eliminated after rinsing the as-prepared TiO₂ powders with distilled water. Thus, the aforementioned bands can be attributed to C–H stretching vibrations of alkane groups [37]. As for the composite powder, the strong and dominant absorption peak spotted at wave number 1102–1105 cm⁻¹ can be attributed to the asymmetric stretching vibration of the Si–O–Si (siloxane) bond [39].

3.1.4. BET Analysis

The photocatalytic effectiveness of a powder can be strongly affected by their texture and structure properties. The received plots from the BET technique are depicted in Figure 4. Both composite and pure TiO₂ powders are characterized by type IV isotherms with H3-like hysteresis loops (observed in the P/P₀ range of 0.8–0.95), indicating that the powders have a mesoporous texture (pores in the range 2–50 nm) with slits-like pores [40]. The SSA calculated by the BET method was found to be 15 m²/g for pure TiO₂ and 50 m²/g for the composite TiO₂/SiO₂ powder, while BJH plots were utilized to evaluate the distribution of the samples pore size. The inset figures in Figure 4 represent the BJH desorption cumulative pore size curve. The fact that the SSA of the composite TiO₂/SiO₂ is larger than that of pure TiO₂ could be possibly attributed to the increased surface area of amorphous SiO₂.The SSA’s enhancement of the composite powder would promote an increased photocatalytic efficiency towards degradation of RhB and phenol [41].

Table 7. Results of the BET method. (a) Micropore surface area via t-plot analysis, according to the Harkins and Jura model. (b) Cumulative volume of pores between 1.7 and 300 nm from N2-sorption data and the BJH desorption method. (c) Average pore diameter calculated by the 4 V/σ method; V was set equal to the maximum volume of N2 adsorbed along the isotherm as P/P_o → 1.0 [42].

Sample	BET Surface Area (m2/g)	Micropore Surface Area (m2/g)	Cumulative Volume (1.7–300 nm) (cm3/g)	Average Pore Diameter (nm)
TiO2	15	2	0.1	30
TiO2/SiO2	50	2	0.3	23
SiO2	185 [22]	-	-	-

presents data obtained from BET method regarding SSA, micropore surface area, average pore diameter, and cumulative volume.

3.1.5. Dynamic Light Scattering (DLS) Analysis

The DLS measurements were performed at pH values = 6.65 ± 0.01. The hydrodynamic radius distribution versus the scattering light intensity is presented in Figure S4 for reference TiO₂ as well as the as-prepared composite powder sample. Based on the acquired results, the TiO₂/SiO₂ composite presented an optimal particle size distribution compared to pure TiO₂, characterized by near monodispersity (PdI ≈ 0.1) (see

Table 8. Size distribution data obtained from DLS measurements from dispersion aqueous solutions of the composite ΤiO₂/SiO₂ powder.

Sample	Hydrodynamic Diameter (Dh) (nm)	PdI *
TiO2	35.94 ± 1.17	0.263 ± 0.011
TiO2/SiO2	27.03 ± 1.02	0.114 ± 0.009

Note: * PdI, polydispersity index.

). The size distribution of the composite powder was detected within the range of 10 and 100 nm with a maximum value ≈ 27 nm. Pure TiO₂ is characterized also by a size distribution in the range of 10–100 nm, thus presenting a greater maximum value (≈36 nm) and a greater PdI value (PdI ≈ 0.26), almost reaching polydispersity (PdIs in the range 0.3 to 0.7). TiO₂ is also characterized by a small fragment of few micro-sized (μm) particles, probably due to the presence of large agglomerates (see Figure S4b). The results of the DLS measurements are summarized in Table 8. Based on the obtained data, the addition of SiO₂ to TiO₂ resulted in particles’ size reduction and even distribution [22]. In general, the smallest particle size could potentially favor the redox rate’s enhancement for electrons and holes during the photocatalytic procedure as well as the recombination’s reduction of photogenerated electrons and holes, thus leading to a significantly improved photocatalytic efficiency of the studied composite powder [43].

3.1.6. Diffuse Reflectance UV–Vis Spectroscopy Analysis (DRS)

The determination of the energy band gap (E_g) is an important factor to be taken into account when conducting studies on photocatalysis. Figure 5a illustrates the diffuse reflectance spectra (DRS) of the TiO₂/SiO₂ composite powder as well as of pure TiO₂.

To assess the reflectance of the composite powder, the Kubelka–Munk (K-M) method was employed, as shown in Figure 5a, using Equation (6) [43]:

$$\mathrm{F}\left(\mathrm{R}\right)=\frac{{\left(1-\mathrm{R}\right)}^2}{2\mathrm{R}}$$

(6)

where R is the reflectance.

As depicted in Figure 5a, it is evident that the absorption edges of TiO₂ and TiO₂/SiO₂ are situated at 392 and 399 nm, respectively. Compared to pure TiO₂, the composite powder exhibits a wider absorption range of visible light.

Figure 5b presents the direct E_g of the developed composite powder using the K-M model vs. energy by extrapolating the linear region of the spectra (F(R)hv)^1/2 vs. hv. The energy band gap (E_g) was evaluated via Tauc’s equation (Equation (7)):

$$\mathrm{ahv}=\mathrm{A}{\left(\mathrm{hv}-{\mathrm{E}}_{\mathrm{g}}\right)}^{\mathrm{n}}$$

(7)

where E_g stands for the energy band gap, h corresponds to Planck’s constant, v is the frequency, α represents the absorption coefficient, and n = ½ [44].

The band gaps of pure TiO₂ and TiO₂/SiO₂ were calculated equal to 3.24 eV and 3.18 eV, respectively. Based on the aforementioned observations, the composite TiO₂/SiO₂ powder demonstrated an extended range of light absorption combined with a significantly reduced band-gap energy. These features are considered advantageous for improving the overall efficiency of its photocatalytic properties [45].

3.1.7. FESEM Analysis

The morphology of the synthesized nanomatrials was observed by using FESEM. Figure 6 indicates that while TiO₂ has a rugged morphology and many agglomerates (Figure 6a), SiO₂ appears to have a smoother and flatter surface and a kind of prism-shape morphology (Figure 6b). The texture of the composite material (Figure 6c) seeems to have been refined compared to either TiO₂ or SiO₂ nanoparticles with smaller particles and aggregates. Babyszko et al. found results that are in accoradnce with our SEM analysis [46].

3.2. Photocatalytic Study of TiO₂/SiO₂ Composite Powder

3.2.1. Study of the Photocatalytic Efficiency of TiO₂/SiO₂ towards RhB Degradation

The photocatalytic efficiency of the composite TiO₂/SiO₂ powder sample was initially evaluated by measuring its ability to degrade RhB dye in an aqueous solution upon visible-light exposure. The photocatalytic tests were conducted at ambient temperature (25 °C) and pH value of 6.63 ± 0.01. In Figure 7, the photocatalytic performance of the tested composite powder is presented. To assess the enhancement of the composite’s photocatalytic efficiency under visible-light irradiation, pure TiO₂ and SiO₂ were included as references along with the composite sample. Control experiments were also performed, including photolysis (RhB photolysis) as well as adsorption–desorption equilibrium (RhB dark), without light irradiation but under constant stirring for the same duration as the photocatalytic process. The results indicated that less than 1% of RhB was degraded upon visible-light irradiation, showing that the degradation rate of RhB without the presence of the tested powders was extremely low. Additionally, consistent outcomes were obtained from the experiments conducted in the dark state, confirming the stability of the dye [16].

During the experiments, the TiO₂/SiO₂ composite powder demonstrated a high efficacy compared to pure TiO₂ and SiO₂ powders. It achieved complete degradation (100%) of RhB dye within 210 min upon visible-light exposure. The results obtained from Figure 7 indicate that the addition of SiO₂ into TiO₂ led to an enhanced photocatalytic degradation of RhB dye compared to pure TiO₂ and SiO₂ powders, which exhibited a degradation rate of 45.01 ± 1.17% and 43.49 ± 0.89%, respectively, within the same 210 min timeframe.

To validate the findings of the RhB degradation experiments, TOC analysis was performed to assess the degree of RhB’s mineralization throughout the photocatalytic trials. The mineralization percentage of RhB was determined using Equation (8):

$$\mathrm{Mineralization}=1-\frac{{\mathrm{TOC}}_{\mathrm{final}}}{{\mathrm{TOC}}_{\mathrm{initial}}}\times 100\%$$

(8)

The percentage of RhB mineralization was determined by calculating the ratio between the initial total organic carbon concentration (TOC_initial) in the medium before the photocatalytic study and the final total organic carbon concentration (TOC_final) after conducting the photocatalytic treatment [47]. The corresponding data are illustrated in Figure S5. The TOC analysis results indicate that the composite TiO₂/SiO₂ powder exhibits an enhanced rate of RhB mineralization upon visible-light irradiation, thereby confirming the findings from the degradation study of RhB.

The superior photocatalytic performance of the composite powder compared to the reference powders can be attributed to several factors: (a) the composite powder exhibits a relatively smaller average crystallite size, as determined by XRD analysis; (b) it possesses a higher specific surface area, as evidenced by BET measurements; (c) it demonstrates a smaller hydrodynamic diameter (D_h), as indicated by DLS analysis; and (d) it showcases a reduced band-gap value (E_g), as observed during DRS analysis. It is well established that an increased SSA leads to enhanced light-harvesting capacity and faster interfacial charge-transfer rates. Moreover, the larger contact area between the catalyst and dye molecules promotes a maximum adsorbent effect. These characteristics facilitate the formation of numerous active sites for effective interactions between the composite TiO₂/SiO₂ powder and RhB molecules [10].

Kinetic Model Study

The results of the kinetic model investigations upon visible-light irradiation are depicted in Figure 8, showing the relationship between the variation of −ln(C/C₀) towards time for the studied composite powder. The rate of photocatalytic RhB absorption on the surface of the powders is notably higher for the composite powder in relation to pure TiO₂ and SiO₂, as indicated by the pseudo-first-order kinetics (0.018 min⁻¹), compared to the reference samples, which can be described by the following equation (Equation (9)) [6]:

$$-\ln \left(\frac{\mathrm{C}}{{\mathrm{C}}_0}\right)=\mathrm{kt}$$

(9)

where C₀ and C comprise the initial and reaction-time RhB concentrations, respectively; k stands for the apparent rate constant of the photocatalytic oxidation; and t represents the irradiation time. The slope of the linear fitted plot refers to the apparent rate constants of the studied composite powder as well as reference powders (TiO₂ and SiO₂) (see

Table 9. Kinetic parameters of the examined powders upon visible-light photocatalytic trials.

Sample Name	Pseudo-First-Order Kinetic Model		Pseudo-Second-Order Kinetic Model
	k1 (min−1)	R2	k2 (g/mg·min)	R2
TiO2	0.006	0.933	0.375	0.517
SiO2	0.002	0.979	0.846	0.921
TiO2/SiO2	0.018	0.975	0.381	0.920

).

More specifically, the apparent rate constant of the TiO₂/SiO₂ composite powder is as much as three times that of pure TiO₂ and more than nine times that of SiO₂ (Figure 8) under visible-light irradiation.

Additionally, the kinetics of photocatalytic trials could be described by the pseudo-second-order equation that is outlined below through Equation (10) [6]:

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}\mathrm{t}$$

(10)

where q_t and q_e refer to the amount of the pollutant adsorbed at time t and equilibrium, respectively (mg/g), while k₂ constitutes the rate constant (g/mg·min). As opposed to pseudo-first-order kinetics (Figure 8), the R² values of the pseudo-second-order kinetic study (Figure 9) are lower (Table 9).

Hence, taking into account the R² values of the kinetic model studies, the photocatalytic degradation of RhB under visible-light irradiation for the as-prepared composite as well as reference powders emulates pseudo-first-order reaction kinetics.

Study of the Photocatalytic Mechanism

The photocatalytic procedure involving RhB dye can be outlined through the following Equations (11)–(16) [48]:

$$\mathrm{RhB}+\mathrm{hv}\to {\mathrm{RhB}}^{*}$$

(11)

$${\mathrm{RhB}}^{*}+{\mathrm{TiO}}_2\to {\mathrm{RhB}•}^{*}+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}\right)$$

(12)

$${\mathrm{TiO}}_2\left({\mathrm{e}}^{-}\right)+{\mathrm{O}}_2\to {•\mathrm{O}}_2^{-}$$

(13)

$${•\mathrm{O}}_2^{-}+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}\right)+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(14)

$${•\mathrm{O}}_2^{-}+2{\mathrm{H}}_2\mathrm{O}\to •\mathrm{OH}+{\mathrm{OH}}^{-}+{\mathrm{O}}_2$$

(15)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}\right)\to •\mathrm{OH}+{\mathrm{OH}}^{-}+{\mathrm{O}}_2$$

(16)

In the process of photocatalytic oxidation, significant contributions are made by various key oxidative species, namely superoxide radicals (●O₂⁻), electrons (e⁻), holes (h⁺), and hydroxyl radicals (●OH). To gain deeper insights into the fundamental photocatalytic mechanism, extensive investigations were performed to identify the active species involved. To accomplish this, a series of experiments were conducted to scavenge and capture the specific species. Specifically, p-benzonquinone (p-BQ) (C₆H₄(=O)₂, ≥98%, Sigma-Aldrich, Darmstadt, Germany), silver nitrate (AgNO₃, >99%, Sigma-Aldrich, Darmstadt, Germany), disodiumethylene diaminetetraacetate (EDTA-2Na, C₁₀H₁₄N₂Na₂O₈●2H₂O, ≥97%, Sigma-Aldrich, Darmstadt, Germany), and t-butanol (t-BuOH, (CH₃)₃COH, ≥99.5%, Sigma-Aldrich, Darmstadt, Germany) were introduced into the RhB dye solution. These substances were utilized to selectively trap and identify the superoxide radicals (●O₂⁻), electrons (e⁻), holes (h⁺), and hydroxyl radicals (●OH), respectively [49].

According to the findings presented in Figure 10, the degradation efficiency of RhB on TiO₂/SiO₂ composite powder underwent a significant decrease to 21.31 ± 1.01%, 38.06 ± 1.13%, and 35.17 ± 0.98% respectively, upon the addition of p-BQ, AgNO₃, or EDTA-2Na to the photocatalytic reaction solution. These data provided confirmation that the superoxide radicals (●O₂⁻) and photogenerated electrons (e⁻) as well as holes (h⁺) indeed exerted a key influence in RhB’s photocatalytic degradation. Additionally, the inclusion of t-BuOH in the reaction solution resulted in a decrease in the degradation efficiency of RhB from 100% to 94.69 ± 1.03%, indicating that the hydroxyl radicals (●OH) were not the primary reactive species involved within the process.

Based on the analysis of the band-gap structure of free radicals as well as the experiments involving scavenging of electrons and holes, a plausible mechanism is proposed, as illustrated in Figure 11. When exposed to visible-light irradiation, TiO₂ becomes excited, simultaneously generating electrons (e⁻) and holes (h⁺). The holes reside in the VB, while the electrons are located in the CB. The electrons in the CB readily engage in reactions with dissolved O₂ in the water, leading to the formation of ●O₂⁻, H₂O₂, or ●OH radicals. Meanwhile, the photogenerated holes in the VB can directly interact with OH⁻ ions or H₂O molecules bound to the surface, forming ●OH radicals. These highly reactive species, such as ●O₂⁻, e⁻, and h⁺ radicals, relentlessly attack the RhB’s molecules until complete degradation is achieved, promoting efficient separation of electron–hole pairs and thereby augmenting the photocatalytic activity.

Reusability Study

As depicted in Figure S6, the reusability of the TiO₂/SiO₂ nanocomposite is demonstrated by subjecting it to visible-light irradiation over five consecutive photocatalytic cycles (catalyst loading = 5 mg, pH = 6.63 ± 0.01, C_0(RhB) = 10 mg/L). Following degradation in each cycle, the composite photocatalyst was subjected to centrifugation and multiple washes with distilled water. Subsequently, it underwent drying in a vacuum oven at 70 °C for a duration of 24 h in preparation for the subsequent trial without any further treatment [50,51]. It is noteworthy that the as-mentioned photocatalyst exhibited commendable photostability under visible-light irradiation, with an approximate 7% (7.16 ± 0.83%) reduction in its photocatalytic efficiency observed after five repeated cycles. These findings serve as evidence of the TiO₂/SiO₂ photocatalyst’s robust stability throughout the process of cyclic photocatalysis.

3.2.2. Photocatalytic Efficiency towards Phenol Degradation

The effectiveness of the TiO₂/SiO₂ composite’s photocatalytic activity was also estimated towards degradation of phenol aqueous solution upon visible-light irradiation. In the case of phenol, the photocatalytic experiments were carried out at room temperature (25 °C) and pH = 7.32 ± 0.01. Figure 12 illustrates the photocatalytic performance of the as-synthesized composite powder. In order to evaluate the improvement of the photocatalytic performance of the composite powder upon visible-light irradiation, pure TiO₂ and SiO₂ were also included as references. Additionally, control experiments were conducted, including photolysis (phenol photolysis) and adsorption–desorption equilibrium (phenol dark), without light irradiation and under constant stirring for the same time period as the photocatalytic procedure. The obtained results indicated that <2% of phenol was degraded upon visible-light irradiation, thus confirming that the phenol’s degradation rate in the absence of the tested powders was negligible. Furthermore, consistent results were acquired from the trials conducted under dark conditions, verifying the stability of phenol [6].

During the photocatalytic experiments, the TiO₂/SiO₂ composite indicated an increased efficiency in comparison to pure TiO₂ and SiO₂ powders. More specifically, the composite powder achieved phenol degradation equal to 64.95 ± 1.94% during visible- light irradiation for 270 min. The observed results in Figure 12 demonstrate that the addition of SiO₂ into TiO₂ also led to an enhanced photocatalytic degradation of phenol compared to the pure TiO₂ and SiO₂ powders that presented degradation rates equal to 41.85 ± 1.29% and 40.38 ± 1.54%, respectively, within 270 min. According to the results received from the photocatalytic efficacy’s studies for both RhB and phenol, it can be observed that the photocatalytic effectiveness of the TiO₂/SiO₂ composite under visible-light irradiation reached 100% within 210 min in the case of RhB, while only 49.13 ± 0.79% of phenol was degraded within the same time period. Thus, it can be concluded that the composite photocatalyst presents selective activity towards the photocatalytic degradation of RhB (Figure 13).

TOC analysis was performed to estimate the degree of phenol’s mineralization during the photocatalytic procedure in order to ensure about the results of phenol degradation trials. The mineralization percentage of phenol was calculated using Equation (10). The data are depicted in Figure S7. Based on the results of TOC analysis, the composite TiO₂/SiO₂ powder exhibits a higher rate of phenol mineralization under visible-light irradiation compared to pure TiO₂ and SiO₂ reference powders, thus verifying the results of phenol photocatalytic degradation study.

Study of the Kinetic Model

The kinetic model studies upon visible-light irradiation revealed data that are depicted in Figure 14, showing the relationship between the variation of -ln(C/C₀) towards time for the studied composite powder. Phenol photocatalytic rate absorption on the surface of the powders is increased in the case of the composite powder compared to pure TiO₂ and SiO₂, as indicated by the pseudo-first-order kinetics (0.003 min⁻¹) that can be described through Equation (11) (see

Table 10. Kinetic parameters of the examined powders under visible-light photocatalytic trials.

Sample Description	Pseudo-First-Order Kinetic Model		Pseudo-Second-Order Kinetic Model
	k1 (min−1)	R2	k2 (g/mg·min)	R2
TiO2	0.002	0.987	0.546	0.952
SiO2	0.001	0.992	0.930	0.743
TiO2/SiO2	0.003	0.975	0.738	0.903

).

Kinetics were also studied according to the pseudo-second-order kinetic model (Equation (12)), and the results are presented in Figure 15.

Contrary to pseudo-first-order kinetics (Figure 14), the R² values of the pseudo-second-order kinetic study (Figure 15) are lower (Table 10).

As a result, it can be assumed that a pseudo-first-order reaction kinetic model can describe the phenomenon of the photocatalytic degradation of phenol in the presence of the nanocomposite under visible-light irradiation.

Study of the Photocatalytic Mechanism

The results depicted in Figure 16 display the outcomes of HPLC and TOC measurements conducted on the reaction products of the visible-light-irradiated TiO₂/SiO₂ composite catalyst during phenol degradation. Several deductions can be made based on the findings shown in the figure. Following a 270 min irradiation period, slightly over 25% of the initial phenol underwent complete mineralization, as evidenced by the TOC curve indicating the conversion to CO₂ and H₂O. However, the disappearance of phenol itself occurred at a faster rate, with more than 65% of the substrate undergoing photocatalytic oxidation on the irradiated semiconductor. Notably, hydroquinone (HQ) and p-benzoquinone (p-BQ) were identified and quantitatively analyzed as the primary reaction intermediates. Since the photocatalytic oxidation of organic compounds on irradiated TiO₂ samples involves •OH attacking the substrate, dihydroxybenzenes were identified as the primary products of this process. Within this study, the concentrations of 1,4-hydroxybenzene (hydroquinone) as well as p-benzoquinone were quantitatively monitored throughout the photocatalytic reaction. After a 270 min irradiation period, the hydroquinone concentration was 14.7%, while p-benzoquinone comprised 6.5% of the total. When combined, these two compounds constituted approximately 21% of the initial concentration of phenol. However, the disparity between the phenol and the TOC curve in Figure 16 indicates that ≈38% of the organic matter remained in the reaction solution after 270 min of irradiation with visible light, which was also ascertained through the TOC analysis (see Figure S7). Therefore, around 17% of the initial phenol transformed into other intermediates not mentioned earlier. According to the existing literature [52], the superoxide radicals (●O₂⁻) and hydroxyl radicals (●OH) produced during the irradiation of TiO₂ catalysts by visible light can promote the oxidization of phenol to HQ and p-BQ, leading eventually to its mineralization into H₂O and CO₂.

Thorough investigations were also realized to identify those active species actually involved by focusing on the verification of the proposed photocatalytic mechanism for phenol degradation. Similarly, to the case of RhB, a series of experiments took place for scavenging and capturing the specific species. However, given the fact that p-BQ constitutes an ●O₂⁻ radical scavenger, as mentioned above (see part 2 in Section 3.2.1), as well as an intermediate product of phenol degradation, ●O₂⁻ radical scavenging trials were excluded in order not to affect the HPLC measurements. As a result, AgNO₃, EDTA-2Na, and t-BuOH were introduced into the phenol solution and utilized to selectively trap and identify electrons (e⁻), holes (h⁺), and hydroxyl radicals (●OH), respectively.

Based on the results illustrated in Figure 17, the degradation efficiency of phenol on TiO₂/SiO₂ composite powder presented a significant decrease to 15.23 ± 0.89% upon the addition of t-BuOH into the photocatalytic reaction solution. These data confirmed that the hydroxyl radicals (●OH) comprised a key parameter in phenol’s photocatalytic degradation. Additionally, the adding of AgNO₃ and EDTA-2Na into the reaction solution resulted in a slight decrease in the phenol’s degradation efficacy from 64.95 ± 1.94% to 56.44 ± 1.23% and 58.07 ± 1.08%, respectively, indicating that the photo-generated electrons (e⁻) and holes (h⁺) were not the primary reactive species involved during the photocatalytic procedure.

Due to the obtained data, a potential mechanism for phenol oxidation in the presence of visible-light-irradiated TiO₂/SiO₂ composite catalyst is proposed and illustrated in Figure 18. However, it is important to note that the mechanism depicted in Figure 18 is incomplete due to the fact that the complete photocatalytic oxidation mechanism of phenol as well as its intermediates products has not been fully examined up to now [53]. According to an existing study [53], HQ and p-BQ exist in equilibrium in an aerated solution, even in the presence of irradiated TiO₂. Consequently, p-BQ can be formed through three different routes: (1) through •OH attack on HQ molecules, (2) as a product of the reaction of HQ with photogenerated h⁺ in TiO₂, and (3) via direct oxidation of HQ by dissolved O₂ in H₂O. Furthermore, HQ can undergo further hydration, and it remains uncertain whether the oxidative opening of an aromatic ring solely occurs through BQ oxidation. Hence, the further oxidation of p-BQ can directly result in oxygen-containing aliphatic compounds without additional hydroxylation of the compound, following the pathway depicted in Figure 18.

Reusability Study

In Figure S8, the reusability of the TiO₂/SiO₂ nanocomposite is demonstrated under visible-light irradiation over five consecutive photocatalytic cycles (catalyst loading = 5 mg, pH = 7.32 ± 0.01, C_0(phenol) = 10 mg/L). The utilized procedure was the same as described in part 3 in Section 3.2.1. The composite photocatalyst presented remarkable visible-light photostability in the case of phenol, too, indexing ≈9% (7.16 ± 0.83%) reduction in its photocatalytic effectiveness after five repeated cycles. These observations certify the composite photocatalyst’s stability during the process of cyclic phenol photocatalysis.

4. Conclusions

Photocatalysis, using semiconductor materials like titanium dioxide (TiO₂), constitutes a promising alternative effective method for the removal of organic pollutants such as dyes and phenols from water and wastewater treatment. It involves the activation of TiO₂ by UV or visible light to generate ROS that can oxidize and ultimately degrade the organic contaminants.

Within the framework of the present study, silicon dioxide (SiO₂) was utilized to modify TiO₂, and both were initially synthesized via a facile sol–gel approach in a 7:1 molar ratio (TiO₂:SiO₂) through a wet impregnation method, expanding its photoactivation range in the visible-light region and developing visible-light-active TiO₂/SiO₂ nanocomposites.

The prosperous alteration of TiO₂ using SiO₂ was affirmed via FTIR analysis and the presence of Si–O–Si bonds in the composite powder. The TiO₂/SiO₂ composite exclusively consisted of the anatase crystal phase and enhanced crystallinity (85.22%) and also showed a lessened E_g value (3.18 eV) as opposed to the pure TiO₂ catalyst. The average crystallite size of the as-developed composite catalyst was evaluated at 1.82 nm.

Initially, the developed TiO₂/SiO₂ composite catalyst was studied in respect to its photocatalytic effectiveness regarding the visible-light photodegradation of the RhB aqueous solution. The composite nanomaterials presented exceptional efficacy, leading to total (100%) RhB degradation within 210 min, while pure TiO₂ and SiO₂ achieved significantly lower degradation rates in the same duration (45.01 ± 1.17% and 43.49 ± 0.89%, respectively). These results were additionally supported by the data acquired through the TOC analysis. Kinetic study on the visible-light photodegradation of RhB fitted the first-order kinetic model, while the apparent rate constant (k₁) of TiO₂/SiO₂ was as much as three times that of pure TiO₂ and more than nine times that of SiO₂. For asserting the durability of the prepared composite, five consecutive reusability tests were performed, proving the stability of the TiO₂/SiO₂ composite, as its photocatalytic efficacy’s loss was equal to 7.16 ± 0.83%.

Subsequently, the photocatalytic efficiency of the TiO₂/SiO₂ nanocomposites was tested with regards to phenol degradation, also utilizing visible-light irradiation. In that case, the TiO₂/SiO₂ catalyst accomplished 49.13 ± 0.79% phenol degradation within the 210 min time period, in which it reached complete degradation when RhB was the studied pollutants, thus proving that the composite catalyst exhibited selective photocatalytic activity based on the degradation of RhB. However, given the acquired results, the SiO₂-modified TiO₂ catalyst indicated enhanced phenol photodegradation (64.95 ± 1.94%) compared to pure TiO₂ and SiO₂, which reached degradation rates equal to ≈42% and ≈40% within 270 min, respectively. A first-order kinetic model also derived the optimum from the phenol photocatalysis kinetic study. Reusability trials verified the composite’s photo-durability (≈9% effectiveness loss) when using phenol as the pollutant.

According to photocatalysis mechanism study, ●O₂⁻, h⁺, and e⁻ were suggested to constitute the major ROS species responsible for RhB’s photodegradation under visible-light irradiation, potentially indicating a hole/electron pair recombination suppression mechanism. In the case of phenol, corresponding study and experimental data proved that ●OH radicals comprised the active species associated with its photocatalytic degradation.

Inferentially, the goal of the present study was to synthesize and thoroughly characterize a composite powder combining sol–gel (TiO₂ and SiO₂) and wet impregnation (TiO₂/SiO₂) methods, presenting ideal properties for enhanced photocatalytic performance towards photodegradation of organic compounds (rhodamine B and phenol) under visible-light irradiation. These compounds are commonly used in industrial applications and are reportedly labelled as harmful for both humans and the aquatic ecosystem. A comparison was also made to emphasize the composite’s potential selectivity in degrading a specific organic compound among the tested ones.

Furthermore, the present study aimed to shed light on the photocatalytic degradation mechanism of phenol under visible-light irradiation, particularly by quantitatively analyzing certain phenol intermediates (p-benzoquinone and hydroquinone) using the fabricated TiO₂/SiO₂ composite catalyst. It is noteworthy that, to the best of the authors’ knowledge, the complete understanding of the photodegradation mechanism of phenol and its derivatives under visible-light irradiation has not been established until now.

Taking into account all the aforementioned experimental results, the as-synthesized TiO₂/SiO₂ composite catalyst could be potentially utilized as an alternative, cost-effective, and environmentally friendly approach for water and wastewater treatment applications in various industries, including textile, pharmaceutical, papermaking, printing, cosmetics, and food processing industries, mainly towards the elimination of dyes (rhodamine B) as well as organic compounds (phenols).