Hydrothermal Versus Physical Mixing: Superior Photocatalytic Activity of TiO₂/WO₃ Nanocomposites for Water Treatment Applications

Abstract

The photocatalytic efficiency of TiO₂ was significantly enhanced by coupling with WO₃ to form a TiO₂/WO₃ heterostructure, designed to operate effectively under UV-LED irradiation. The nanocomposites were synthesized via a hydrothermal route, and their activity was evaluated through the degradation of the pharmaceutical pollutant venlafaxine. Contaminants are rarely addressed in photocatalytic studies. Unlike a simple physical mixture of commercial TiO₂ and WO₃ powders, the hydrothermally synthesized TiO₂/WO₃ photocatalyst exhibited superior efficiency, attributable to its nanoscale dimensions achieved via the hydrothermal route, which promoted improved charge carrier separation, enhanced surface homogeneity, and the formation of an effective heterojunction interface. An optimization study varying the WO₃ content (5, 10, 20, and 30 wt.%) within the TiO₂ revealed that the 10 wt.% WO₃ composition achieved the highest performance, with ~52% venlafaxine degradation within 60 min. SEM, TEM, FTIR, Raman spectroscopy, XRD, and UV-Vis DRS revealed the successful incorporation of WO3 into the TiO₂ matrix, confirming phase purity and composition-dependent structural evolution of the nanocomposite, and evidencing extended light absorption and superior charge-transfer properties. Importantly, the optimized photocatalyst thin film retained excellent stability and reusability, maintaining high degradation efficiency over three consecutive cycles with negligible activity loss, which avoids slurry separation. These findings establish hydrothermally synthesized TiO₂/10%WO₃ thin film heterostructures as effective and sustainable photocatalytic platforms for the removal of pharmaceutical pollutants in wastewater under UV-LED irradiation.

1. Introduction

Water pollution has emerged as one of the most serious environmental challenges of the 21st century, largely driven by rapid urbanization, industrialization, and the overutilization of synthetic organic chemicals. Among the different classes of pollutants, pharmaceutical residues are of increasing concern owing to their continuous release into the environment via domestic, hospital, and industrial effluents. Classified as emerging pollutants, they are capable of exerting long-term ecological and health effects, even at low concentrations, and are not effectively eliminated by conventional groundwater purification methods [1,2,3]. Pharmaceuticals, such as anti-inflammatories, antibiotics, and antidepressants, are designed to be biologically active and stable, which makes them resistant to the natural degradation processes. Their presence in aquatic environments has been widely documented. One such compound is venlafaxine (Venla), a serotonin-norepinephrine reuptake inhibitor (SNRI) used extensively for the treatment of depression and anxiety disorders. Due to its widespread use, high water solubility, and poor biodegradability, Venla is frequently detected in surface waters, groundwater, and even drinking water supplies [3,4,5]. Researchers have reported its presence at different concentrations ranging from ng/L to several µg/L, raising serious concerns about its long-term ecotoxicological impacts, including behavioral and reproductive disorders in aquatic organisms [6].

The inefficiency of conventional groundwater purification in removing Venla and other micropollutants necessitates the use of more effective and advanced treatment methods [7,8]. Among these, heterogeneous photocatalysis has generated significant attention as a green and sustainable process capable of degrading a wide spectrum of organic pollutants into non-toxic end products (CO₂ and H₂O) [9,10,11]. Upon light irradiation, the semiconductor photocatalyst is photoexcited, generating electron–hole pairs. The photogenerated electrons can react with dissolved oxygen to produce reactive species such as superoxide radicals (O₂·⁻), while holes in the valence band can directly oxidize organic molecules or react with surface-adsorbed water to form hydroxyl radicals (HO·). These reactive species, together with direct hole oxidation, act synergistically to attack and degrade the organic pollutants. Titanium dioxide (TiO₂) is widely recognized as a highly active photocatalyst and is frequently employed as a benchmark for evaluating the performance of emerging photocatalysts due to its chemical stability, reduced cost, non-toxicity, and strong oxidative power. TiO₂ has two major disadvantages: its wide band gap (3.2 eV), which restricts activation to UV light, and the high electron–hole recombination rate, thus limiting its quantum efficiency [12,13,14]. To overcome these limitations, heterojunction engineering has been proposed as an improved method for boosting photocatalytic activity. Associating TiO₂ with a suitable narrow band gap semiconductor, such as tungsten trioxide (WO₃), has shown promising results. WO₃, with a band gap of 2.6–2.8 eV, is photoactive under UV and near visible light (440–480 nm), and can act as an effective electron acceptor co-coupled with TiO₂. The formation of TiO₂/WO₃ heterojunction facilitates spatial separation of charge carriers, prolongs their lifetime, and enhances the generation of reactive species [15,16]. Furthermore, WO₃ possesses high chemical stability and good compatibility with TiO₂, making the nanocomposite a promising candidate for photocatalytic applications.

In addition to material optimization, the choice of the light source plays a crucial role in the efficiency and sustainability of the photocatalytic process. Traditional UV mercury lamps are energy-intensive and environmentally hazardous due to mercury content. In contrast, UV-A light-emitting diodes (LEDs) offer several advantages, including low energy consumption, long operational life, low heat generation, and wavelength specificity, with a narrow spectral band near 365 nm. These features make LEDs more suitable for practical, scalable photocatalytic systems for water purification applications [17,18].

Given this context, the present study focuses on the photocatalytic degradation of Venla using TiO₂/WO₃ thin film under LED³⁶⁵ irradiation, with a particular emphasis on ecological applications. It explores how the preparation method of the photocatalyst, specifically hydrothermal synthesis versus physical mixing, significantly influences its photocatalytic efficiency. Furthermore, the systematic variation of WO₃ loading provides new insights into the optimal composition required for enhanced activity. To strengthen this understanding, a comprehensive set of characterization techniques (Raman spectroscopy, FTIR, SEM, and UV-Vis spectroscopy) was employed to establish a clear correlation between the structural features, morphological properties, and photocatalytic performance.

2. Materials and Methods

2.1. Materials

The precursors employed in this study were titanium dioxide (TiO₂ Aeroxide^® P25, Evonik industries AG, Essen, Germany), tungsten (VI) oxide (WO₃, Sigma Aldrich, St. Louis, MO, USA), sodium tungstate dihydrate (Na₂WO₄·2H₂O, Loba Chemie, Mumbai, India, 99, hydrochloric acid (HCl, Carl Roth, Karlsruhe, Germany), deionized water, isopropanol (C₃H₈O, Chem solute, Renningen, Germany), acetylacetone (C₅H₈O₂, Carl Roth, Karlsruhe, Germany), nitric acid (HNO₃, Carl Roth, Karlsruhe, Germany), titanium isopropoxide (Ti(O-iPr)₄, Sigma Aldrich, St. Louis, MO, USA), and polyethylene glycol (PEG, Sigma Aldrich, Overijse, Belgium).

2.2. Synthesis of Nanocomposites

In this study, two sorts of nanocomposites will be treated as photocatalysts and will be compared with an unmodified sample of nano-TiO₂. Nano-TiO₂ P25 was used as the reference photocatalyst. P25 consists of 70–80% anatase and 20–30% rutile, and its mixed-phase composition provides higher photocatalytic activity compared to pure anatase, owing to the synergistic interaction between the two crystalline phases [19]. The first type of nanocomposite was achieved by mixing two commercial oxides (C.O), TiO₂ and WO₃, through simple physical mixture. The oxides were ground using a clean mortar and a pestle for 30 min. The second sort of nanocomposite preparation will be based on synthesized oxides (S.O) by the hydrothermal method. The TiO₂/WO₃ nanocomposites with different WO₃ loading (5, 10, 20, and 30%) values were synthesized using 0.3 g of TiO₂ as the base and Na₂WO₄·2H₂O as the tungsten source. In a typical preparation, the required stoichiometric amount of Na₂WO₄·2H₂O corresponding to the desired WO₃ percentages was dissolved in 20 mL of deionized water under continuous magnetic stirring (300 rpm) for 30 min. The pH of the solution was carefully adjusted by the dropwise addition of a concentrated HCl (1 M) until the pH reached 2, thereby promoting the formation of WO₃ species. Subsequently, the predetermined amount of P25 TiO₂ powder was dispersed into the mixture under vigorous stirring (500 rpm) for one hour to ensure homogeneous mixing and intimate contact between TiO₂ and WO₃ precursors. The resulting suspension was transferred into a 45 mL Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 150 °C and autogenous pressure for 12 h. After cooling to room temperature, the solid products were washed with deionized water and ethanol to remove residual ions, and then dried at 100 °C for 3 h. The as-prepared nanocomposites, denoted as TiO₂/x%WO₃ (x = 5, 10, 20, and 30%), were subsequently stored for further physicochemical and photocatalytic characterization.

2.3. Preparation of Thin Film

All the thin films were deposited onto square fluorine-doped tin oxide (FTO) glass substrates with the same thickness (5 cm × 5 cm, 3 mm thick, Sigma Aldrich). As a first step, a base solution was prepared via the sol–gel route. Initially, the deionized water was placed in a beaker under magnetic stirring (300 rpm) and mixed with isopropanol. Subsequently, acetylacetone was introduced and stirred for 5 min, followed by acidification with nitric acid. The mixture was stirred further before increasing the speed to 500 rpm, at which point titanium isopropoxide was added dropwise. The resulting solution was stirred for 2 h at ambient temperature until it had a transparent yellow appearance. The precise sequence of addition was essential to ensure controlled hydrolysis and to avoid premature precipitation.

As a second step, for each sample, 1 mL of the base sol–gel solution was mixed with 0.1 g of the prepared nanocomposites in a reactor under magnetic stirring for 1 h/300 rpm. Afterwards, the mixture was placed in an ultrasonic bath. Then, 0.25 mL of polyethylene glycol, 0.5 mL of 2-propanol, and 0.2 mL of acetylacetone were added sequentially to the reactor, ensuring that the components were added in the correct order. The coagulated mixture was coated on the surface of FTO glass to enable heating in an oven at 300 °C for 2 h.

2.4. Experimental Setup and Procedure

The experimental reactor was specifically designed to enable measurements of pollutant degradation. The unit consists of a 145 mL reactor vessel, in which the photocatalyst coated onto FTO glass was mounted on one side, oriented towards the interior. A magnetic stirrer was integrated at the base of the vessel to ensure effective turbulence. Illumination is provided by an LED array comprising nine UV-A diodes (365 nm, 1 W, Seoul, Republic of Korea, CUN66A1B). The array is positioned parallel to the catalyst at a distance of 5.8 cm, allowing the radiation to pass through the FTO substrate and reach the photocatalyst surface.

For the investigation of the photocatalytic performance of the nanocomposites, Venla was chosen as a pharmaceutical pollutant. The UV-Vis measurements were performed with Mettler Toledo UV5 Bio (Gießen, Germany) at 365 nm. The photocatalytic degradation generates reactive species that break down organic pollutants into less harmful substances like CO₂ and H₂O.

Photocatalyst (HO·/O₂·⁻) + pollutant → CO₂ + H₂O

This reaction involves the excitation of electrons, formation of electron–hole pairs, and subsequent redox reactions with the pollutant. The degradation efficiency is calculated using the equation:

$$\mathrm{Degradation}(\%)=({\displaystyle \frac{C_0-C_t}{C_0}})\times 100$$

(1)

where $C_0$ is the initial concentration, $C_t$is the concentration at time t.

Assuming that the photocatalytic degradation of Venla follows a first-order kinetic model, the reaction can be described by the following equation:

$$r=-{\displaystyle \frac{dC}{dt}}\mathrm{and}\ln \left({\displaystyle \frac{C}{c_0}}\right)=-Kt$$

(2)

In the expression, r represents the reaction rate, C is the concentration of contaminant at a given irradiation time t (min), C₀ is the initial concentration (mg/L), K is the rate constant (min⁻¹), and t is the irradiation duration.

The structural, morphological, and optical properties of the synthesized samples were characterized using Fourier-transform infrared spectroscopy (FTIR) (PerkinElmer FT-IR spectrometer, Castries, France, L 160000A Spectrum Two, 2.5 µm to 25 µm, LiTaO₃ detector), Raman spectroscopy (Thunder Optics Gurzil Raman Microscope TO-RM-S-785 nm, Montpellier, France), Energy-Dispersive X-Ray Spectroscopy (EDX) (QUANTAX Brucker, Berlin, Germany), Scanning Electron Microscopy (SEM) (Coxem Tabletop EM-30N, Daejeon, Republic of Korea, Tungsten filament (W), detector SE and BSE), Transmission Electron Microscopy (TEM/HRTEM) and Selected Area Electron Diffraction (SAED) were performed using the same instrument (Tecnai G² 20, FEI Company, Hillsboro, OR, USA, LaB₆ filament, 200 kV), The crystal structure was examined by the powder X-ray diffraction (XRD)(Bruker AXS GmbH, Karlsruhe, Germany, CuK radiation (k_1/4 1.54056Å) and a graphitic monochromator), and Diffuse reflectance spectroscopy (DRS) (PerkinElmer UV-Vis spectrometer (OPDI.MA, ODM98) Castries, France, 250–1100 nm, BaSO₄).

2.5. Analytical Method

The calibration curve of Venla is presented in Figure 1. It exhibits a characteristic absorption band in the UV region, with maximum absorbance around 225–230 nm. Venla was established over a concentration range of 1–30 mg/L, showing a strong linear relationship between absorbance and concentration. The correlation coefficient (R²) confirmed excellent linearity, ensuring the accuracy and reliability of quantitative analysis.

3. Results and Discussion

3.1. Characterization of Nanocomposites

3.1.1. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectroscopy (Figure 2a) was employed to investigate the structural and vibrational features of the synthesized TiO₂/WO₃ nanocomposites with varying WO₃ loadings (5, 10, 20, and 30 wt.%). Spectra of commercial TiO₂ and WO₃ were used as reference materials. The TiO₂ spectrum displays a prominent absorption band between 450 and 700 cm⁻¹, attributed to Ti-O stretching vibrations_. This feature was also present in the nanocomposites, though progressively shifted and reduced in intensity with increasing WO₃ content, suggesting modifications to the Ti-O bonding environment. Pure WO₃ showed characteristic bands at 507–610 cm⁻¹, corresponding to W-O bending vibrations, and at 804 cm⁻¹ assigned to W-O-W bridging vibrations within WO₆ octahedra. These peaks also appeared in nanocomposites, albeit slightly displaced, which may indicate partial structural interaction or distortion of WO₃ within the TiO₂ matrix. Additional weak bands observed in the 1800–2200 cm⁻¹ region were assigned to C=O stretching, most likely arising from adventitious carbonates or atmospheric CO₂ incorporated during synthesis or measurement. A broad absorption band between 3000 and 3600 cm⁻¹, characteristic of O-H stretching, was detected in all samples and is commonly associated with surface hydroxyl groups or absorbed water, particularly in nanocomposites prepared by hydrothermal routes [20].

To assess the influence of the synthesis route, the FTIR spectrum of the TiO₂/10%WO₃ (S.O) nanocomposite was compared with TiO₂/10%WO₃ (C.O) in Figure 2b. The hydrothermal sample exhibited subtle shifts in the Ti-O (450–700 cm⁻¹) and W-O (507–610 cm⁻¹) vibrational bands, together with sharper and more intense absorption features. The spectral changes suggest a stronger interaction between TiO₂ and WO₃, most likely through the formation of Ti-O-W linkages. Such structural modifications are indicative of improved incorporation of WO₃ within the TiO₂ and WO₃ framework and may account for the enhanced photocatalytic performance observed in hydrothermally prepared samples.

3.1.2. X-Ray Diffraction of Synthesized TiO₂/WO₃ Nanocomposites

XRD analyses were conducted to examine the crystalline phases present in the prepared nanocomposites with varying WO₃ loadings. The diffraction pattern of pure TiO₂ confirms predominant crystallization in the anatase phase, with characteristic reflections at 2θ = 26.4°, 38.4°, and 48°, corresponding to the (101), (004), and (200) planes, respectively, in agreement with JCPDS card No. 88–1175. Additional peaks attributable to the rutile phase of TiO₂ (space group P4₂/mmm) were also detected, assigned to the (110), (101), (111), (210), and (220) planes (Figure 3). For WO₃, diffraction peaks characteristic for the monoclinic crystalline phase were identified, corresponding to the (001), (020), and (200) planes. The XRD diffractograms of the TiO₂/WO₃ nanocomposites reveal the coexistence of TiO₂ and WO₃ phases, with no evidence of secondary phases or impurities, confirming the high purity of the synthesized nanomaterials. Moreover, the progressive increase in WO₃ content from 5% to 30% leads to a corresponding intensification of the WO₃ diffraction peaks, demonstrating the successful incorporation of WO₃ and its compositional influence on the crystallographic features of the nanocomposites.

3.1.3. Raman Spectroscopy

Raman spectroscopy was used in this study to probe the structural properties and interfacial interactions within TiO₂/10%WO₃ thin film. The Raman-active modes characteristic of anatase are well-defined in the spectra of Figure 4, including a symmetric bending vibration at 399 cm⁻¹ (B₁g), and an antisymmetric bending mode around 516 cm⁻¹ (A₁g). In addition, a distinct peak at 635 cm⁻¹ (Eg), corresponding to the antisymmetric stretching vibration of the rutile phase, was also observed. An intense band confirms the presence of WO₃ at 807 cm⁻¹, attributed to W-O stretching vibrations, together with additional features at 717, 272 cm⁻¹, as well as a lower-frequency band at 325 cm⁻¹, all of which can be assigned to W-O-W bending and stretching modes [21,22]. Notably, a slight shift in the vibrational modes of TiO₂/10%WO₃ film was detected when compared with the reference spectra of TiO₂ and WO₃. This shift is indicative of strong interfacial interactions between the oxides, suggesting possible Ti-O-W bond formation or local structural distortions at the heterojunction.

3.1.4. Energy Dispersive X-Ray Spectroscopy (EDX)

The EDX spectrum presented in Figure 5 for the pure TiO₂ thin film deposited on a standard glass slide as a substrate indicates titanium (Ti) and oxygen (O) as the principal elements. Distinct Ti peaks were observed at Kα (4.52 keV) and Kβ (4.93 keV). Although the characteristic O Kα peak at 0.52 keV was not detected in the spectrum, oxygen was nevertheless quantified in the elemental composition table, with a normalized mass concentration at 58.99%. This value is consistent with the theoretical stoichiometry of TiO₂, and the apparent absence of the oxygen peak is attributed to the well-recognized limitation of EDX in detecting light elements, which emit low-energy X-rays that are often absorbed before reaching the detector. In contrast, the EDX spectrum of the TiO₂/10%WO₃ nanocomposite thin film revealed, in addition to Ti, distinct peaks associated with tungsten: Mα at 1.75 keV, Lα at 8.4 keV, and a weaker peak of Lβ at 9.65 keV. The corresponding normalized mass concentration for W is 27.24%, confirming the presence of WO₃. In this case, the O Kα peak remained discernible, reflecting contributions from both oxides. Minor signals corresponding to Na, C, and Cl were also detected, most likely arising from trace impurities introduced during the precursor chemistry or film preparation process. Collectively, the EDX results substantiate the successful synthesis of the TiO₂/10%WO₃ composite while preserving the compositional integrity of the thin film.

3.1.5. Scanning Electron Microscopy (SEM) of TiO₂/WO₃ Composites

The surface morphology of TiO₂/WO₃ thin films with varying WO₃ loadings (0, 5, 10, 20, 30 wt.%) was investigated by SEM imaging to evaluate the homogeneity, particle distribution, and structural integrity of the deposits. As shown in Figure 6, the pure TiO₂ thin film exhibited a relatively uniform coverage but was characterized by the presence of cracks, discernible grain boundaries, and occasional agglomerates. With the incorporation of 5% WO₃, the thin film displayed enhanced particle dispersion and elimination of surface cracks, indicating a positive effect of WO₃ even at a low ratio. The TiO₂/10%WO₃ (S.O) film demonstrated the most compact and homogeneous morphology, consisting of a continuous and smooth layer with uniformly dispersed particles of the smallest dimension, suggesting a strong interaction between the TiO₂ and WO₃ phases and improved interfacial contact [23]. In contrast, the nanocomposite TiO₂/10%WO₃ (C.O) exhibited large agglomerates, poor uniformity, and less homogeneous structures, highlighting the advantage of using synthesized oxides to achieve finer particle distribution and enhanced interfacial interactions. Increasing the WO₃ content to 20% led to some surface irregularities and moderate agglomeration. Whereas at 30% thin film exhibited markedly larger agglomerates and rougher textures. These observations indicate that, although higher WO₃ loading (30%) induces progressively irregular surfaces, larger agglomerates, and rougher textures, the 10 wt.% WO₃ nanocomposite maintains a more uniform morphology with reduced agglomeration, thereby possessing the most favorable structural characteristics and emerging as the most promising candidate for photocatalytic applications.

3.1.6. Transmittance Electronic Microscopy (TEM) of TiO₂/WO₃ Composite

The TEM images of TiO₂/WO₃ nanocomposite are presented in Figure 7a,b. They reveal two distinct morphologies: nanoplatelets and a hexagonal structure. TiO₂ crystallizes predominantly as nanoplatelets, whereas the secondary WO₃ phase adopts a hexagonal form. The average size of the nanoplatelets ranges from 12 to 30 nm, while the hexagonal structures exhibit a characteristic thickness of approximately 7 nm.

High-resolution TEM (HRTEM) analysis of the TiO₂/WO₃ nanocomposites (Figure 7c) displays alternating bright and dark fringes. The interplanar spacing of d = 2.38 Å is attributed to the (103) plane of TiO₂, whereas the spacing of d = 2.60 Å corresponds to the (220) plane of WO₃. The selected area electron diffraction (SAED) pattern (Figure 7d) further shows concentric diffraction rings, which can be indexed to the (101), (110), and (103) planes of TiO₂, along with the (011) plane of WO₃, thereby confirming the successful formation of TiO₂/WO₃ nanocomposites.

3.1.7. Optical Band Gap (Eg) Estimation via Kubelka-Munk Analysis

The Kubelka-Munk function, F(R) = (1 − R)²/(2R), was applied to the reflectance data of TiO₂, WO₃, and TiO₂/WO₃ (S.O) thin films (Figure 8a), and the optical band gap (Eg) was estimated from the Tauc plots (F(R). hν)ⁿ versus hν. Although TiO₂ and WO₃ are generally reported as indirect band gap semiconductors, the indirect transition model (F(R). hν)^1/2 versus hν was adopted in this work. Pure TiO₂ presented a band gap of 3.12 eV, corresponding to an absorption edge at ~400 nm, which confines its photoresponse mainly to the ultraviolet region. While pure WO₃ exhibited a narrower band gap of 2.60 eV, extending absorption well into the visible range and enabling stronger utilization of solar light. The incorporation of WO₃ into TiO₂ progressively narrowed the band gap and red-shifted the absorption edge: 2.99 eV (~415 nm) at 5 wt.% WO₃, 2.91 eV (~426 nm) at 10 wt.% WO₃, 2.83 eV (~438 nm) at 20 wt.% WO₃, 2.75 eV (~451 nm) at 30% wt.% as it was showed in Figure 8b. These shifts, although modest and within the experimental uncertainty (0.01–0.03 eV), clearly demonstrate a systematic trend. These results indicate that low WO₃ contents retain predominantly UV absorption similar to TiO₂, whereas higher WO₃ loading significantly extends absorption further into the visible range. Notably, composites containing 20 and 30 wt.% WO₃ exhibit band gaps of 2.83 eV and 2.75 eV, with absorption edges near 438 nm and 451 nm, respectively, indicating their potential to harvest a larger fraction of sunlight compared to other samples. The observed gradual band gap narrowing is attributed to interfacial electronic interactions between TiO₂ and WO₃. While excessive WO₃ loading may lead to particle agglomeration, as confirmed by SEM analysis, it potentially compromises heterojunction quality. Overall, moderate WO₃ contents (5–10 wt.%) appear optimal for balancing UV and visible absorption, whereas higher loading (20–30 wt.%) may further enhance solar light utilization at the expense of charge separation efficiency [24,25].

3.2. Photocatalytic Activity

3.2.1. Evaluating Photocatalytic Activity Against Controls

A comparative analysis was carried out to assess photolysis, adsorption, and photocatalysis of the nanocomposite TiO₂/10%WO₃ (S.O) over 30 min. The Venla was selected as a pharmaceutical pollutant in this study, with a concentration of 30 mg/L, and the source light was UV-LEDs. It should be noted that the Venla concentration used in our experiments (30 mg/L) is substantially higher than the levels typically detected in the environment, which generally range from ng/L to low µg/L. This elevated concentration was chosen to ensure accurate and reproducible spectrophotometric measurements, facilitate reliable kinetic analysis, and allow meaningful comparison of photocatalyst performance under controlled laboratory conditions. While the absolute degradation efficiencies reported here may not directly reflect those achievable in natural waters, the observed trends in photocatalytic activity, interfacial charge separation, and material stability are expected to remain relevant at environmentally realistic concentrations. Moreover, the mechanistic insights obtained at higher pollutant loads can inform the design of scaled-up systems, where factors such as longer residence times, enhanced light penetration, or pre-concentration strategies may be employed to achieve effective Venla removal in real-world purification applications. Under identical conditions, as shown in Figure 9a, photolysis (irradiation in the absence of a photocatalyst) and adsorption (dark experiment with a photocatalyst) produced negligible substrate removal, confirming that neither direct irradiation nor physical adsorption plays a significant role. By contrast, the photocatalytic experiment (irradiation in the presence of nanocomposite) led to a pronounced reduction in substrate concentration within 30 min, thereby demonstrating that the degradation originates predominantly from photocatalytic activity rather than from adsorption or photolysis

3.2.2. Comparison of the Photocatalytic Performance Between the Synthesized Nanocomposite and That Prepared from Commercial Oxides

An assessment of photocatalytic activity was conducted to evaluate the efficiency of TiO₂/10%WO₃ thin film nanocomposite synthesized Via a controlled method versus a nanocomposite prepared from commercial TiO₂ and WO₃ powders during 120 min. Under identical experimental conditions as the previous test. The thin film TiO₂/10%WO₃ (S.O) achieved a photocatalytic degradation of 31%, as shown in Figure 9b, while the thin film TiO₂/10%WO₃ (C.O) exhibited a photocatalytic efficiency of 25%. The superior performance of the synthesized binary system was attributed to its nanoscale dimensions, markedly smaller than those of commercial oxides, together with its enhanced structural homogeneity, confirmed by SEM and TEM imaging, improved interfacial contact between TiO₂ and WO₃, and more efficient charge separation, which collectively promote higher photocatalytic activity [26,27]. The result highlights the significance of the synthesis method and composite quality in ameliorating the performance of photocatalytic materials.

3.2.3. The Effect of WO₃ Amount on the Photocatalytic Behavior of TiO₂

The photocatalytic degradation of Venla (30 mg/L) under UV-LEDs irradiation was evaluated using pure TiO₂ and TiO₂/WO₃ synthesized nanocomposites with varying WO₃ content (5, 10, 20, 30 wt.%). The results showed in Figure 10a a clear dependence of the photocatalytic efficiency on the WO₃ loading. Pure TiO₂ reached 18% of degradation during 120 min. A slight enhancement to 20% was observed with 5% WO₃, suggesting improved charge separation due to heterojunction formation between TiO₂ and WO₃. The highest photocatalytic activity, 31% was achieved with TiO₂/10%WO₃, indicating that this photocatalyst provides an optimal balance between charge separation and UV-light absorption. However, further increases in WO₃ amount led to a decline in efficiency, 24% for 20 wt.% WO₃ and 15% for 30% wt.% WO₃. This decrease is likely due to the excessive presence of WO₃, which can act as a recombination center and diminish the effective surface area available for light penetration, thereby limiting the potential for enhanced visible-light absorption, as discussed in the band gap analysis. These findings highlight that while the addition of WO₃ can enhance TiO₂ performance under UV light, optimal loading is crucial to avoid detrimental effects on photocatalytic activity [28].

3.2.4. Effect of Initial Concentration of the Pollutant

The effect of the initial concentration of Venla on its photocatalytic degradation was investigated using TiO₂/10%WO₃, identified as the most efficient photocatalyst in this study, under UV LEDs irradiation for 60 min. The results in Figure 10b showed a clear inverse relationship between the initial concentration of Venla and the degradation efficiency. At lower concentrations, 5 and 10 mg/L, the photocatalyst achieved a high degradation efficiency of 52%, indicating that a greater proportion of the pollutant molecules could interact with the available active sites on the catalyst surface, and the generation of reactive oxygen species was sufficient for effective degradation. However, as the initial concentration increased to 20 mg/L, the degradation efficiency decreased to 39%, and further dropped to 21% at 30 mg/L. This decline can be attributed to several factors. The higher concentration of the pollutant increases the number of molecules competing for the same active sites, potentially leading to their saturation. Higher concentrations can limit light penetration due to increased solution turbidity, reducing photon absorption by the photocatalyst. Moreover, the fixed amount of generated reactive species may become insufficient to degrade a larger number of pollutant molecules [29,30]. These observations confirm that lower concentrations of pollutants favor more efficient photocatalytic degradation and prove the importance of optimizing operational parameters for real-world wastewater treatment applications.

3.2.5. Kinetic Study

The kinetics of Venla degradation by TiO₂/10%WO₃ under UV-LEDs irradiation were examined at different concentrations (5, 10, 20 mg/L) over one hour. The applicability of pseudo-first-order kinetics was validated by the linear relationship obtained from ln(C₀/C) versus time plots, with good correlation coefficients. The apparent rate constant (k_app) and half-life time (t_1/2) were calculated for each concentration as presented in Figure 11a. At 5 mg/L, the reaction presented a k_app of 0.0114 min⁻¹ with a correlation coefficient R² of 0.9675, and a t_1/2 of 57.76 min. For 10 mg/L, k_app increased slightly to 0.012 min⁻¹, with a higher R² of 0.9818, and t_1/2 was 60.8 min, indicating a good fit to the pseudo-first-order model and efficient degradation at lower concentrations. However, when the initial concentration was increased to 20 mg/L, k_app decreased to 0.0085 min⁻¹, despite a higher R² of 0.9955, and the t_1/2 extended to 81.54 min. This decline in reaction rate at higher concentrations may be attributed to the saturation of active sites on the catalyst surface, reduced availability of hydroxyl radicals per pollutant molecule, and increased light attenuation. These results confirm that the degradation rate is strongly influenced by the initial concentration of Venla, with lower concentrations favoring faster and more efficient photocatalytic reactions.

3.2.6. Stability of Photocatalyst

The stability and reusability of the TiO₂/10%WO₃ thin film photocatalyst were assessed over 3 consecutive cycles of Venla degradation (5 mg/L) under UV LEDs irradiation (Figure 11b). The results demonstrated good photocatalytic durability. In the first cycle, the degradation efficiency reached 52%, confirming the high initial activity of the photocatalyst. In the second cycle, a slight decrease was observed, with 46% degradation, likely due to partial surface fouling, minor loss of active sites, or accumulation of intermediate byproducts. However, in the third cycle, the efficiency slightly improved to 49%, suggesting that the photocatalyst retained most of its activity and that any deactivation observed was not progressive or irreversible. This recovery may also indicate the partial desorption or degradation of surface-bound intermediates during the cycle. Overall, the TiO₂/10%WO₃ thin film exhibits good stability and reusability, maintaining consistent performance over multiple uses, which is essential for practical and sustainable wastewater treatment applications.

3.2.7. Photocatalytic Charge Transfer Pathways in TiO₂/10%WO₃ Nanocomposite

The band alignment of the TiO₂/10%WO₃ heterostructure plays a crucial role in its enhanced photocatalytic performance. For a semiconductor, the CB and VB potentials vs. normal hydrogen electrode (NHE) can be estimated using these equations:

$${\mathrm{E}}_{\mathrm{C}\mathrm{B}}=\mathsf{\chi }-{\mathrm{E}}_{\mathrm{e}}-0.5{\mathrm{E}}_{\mathrm{g}}$$

(3)

$${\mathrm{E}}_{\mathrm{V}\mathrm{B}}={\mathrm{E}}_{\mathrm{C}\mathrm{B}}+{\mathrm{E}}_{\mathrm{g}}$$

(4)

where E_CB is the conduction band edge potential (V vs. NHE), E_VB is the valence band edge potential (V vs. NHE), χ is the absolute electronegativity of the semiconductor (eV), E_g is the band gap energy (eV), and E_e is the energy of free electrons on the hydrogen scale (4.5 eV).

TiO₂ exhibits a conduction band (CB) potential of −0.25 V and a valence band (VB) potential of +2.87 V (vs. NHE). In comparison, WO₃ shows a CB at +0.79 V and a VB at +3.39 V. Upon illumination, as presented in Figure 12, electrons excited into TiO₂ CB readily transfer to the lower-energy CB of WO₃, leading to electron accumulation in WO₃. In contrast, the corresponding holes remain in the VB of TiO₂. This spatial separation effectively suppresses electron–hole recombination. Owing to its positive potential, the TiO₂ VB (+2.87 V) possesses sufficient power to generate hydroxyl radicals (HO·) from water or hydroxide ions, and can also directly oxidize adsorbed Venla molecules. In contrast, electrons accumulated in the WO₃ CB (+0.79 V) are not thermodynamically capable of reducing O₂ to superoxide radicals (O₂·⁻), rendering this pathway negligible. Consequently, the degradation of Venla is primarily driven by direct hole oxidation and HO· attack, with the TiO₂/WO₃ heterojunction ensuring more efficient charge separation and sustained photocatalytic activity.

4. Conclusions

This study demonstrated that the addition of WO₃ to TiO₂ markedly enhances the photocatalytic degradation of Venlafaxine under UV-LEDs irradiation as a safe, energy-efficient, and compact alternative for water purification. Among the synthesized materials, the hydrothermally prepared TiO₂/10%WO₃ nanocomposite achieved the highest degradation efficiency, outperforming both the pure TiO₂ and the nanocomposite obtained from commercial oxides. The superior activity is attributed to the formation of a heterojunction, which promotes efficient charge separation and suppresses electron–hole recombination. In contrast, higher WO₃ loadings (20-30 wt.%) led to reduced performance, likely due to surface coverage effects and the introduction of recombination sites. Characterization analyses reinforced these findings: Raman and FTIR spectra confirmed the successful formation of TiO₂/WO₃ heterostructure, XRD analysis confirmed the coexistence of TiO₂ phases together with monoclinic WO₃, with no evidence of secondary phases, thereby verifying the structural purity of the synthesized nanocomposites. SEM and TEM micrographs revealed homogeneous thin films with favorable morphology at the nanoscale, as confirmed by TEM, thereby indicating enhanced UV-light absorption. Photocatalytic efficiency decreased with increasing initial pollutant concentration, particularly at 30 mg/L Venla, due to limited active sites and reduced light penetration. Importantly, the TiO₂/10%WO₃ thin film also demonstrated excellent reusability over three consecutive cycles with the same efficiency (~50%/1 h), highlighting practical advantages such as immobilized thin-film configuration, reusability, and avoidance of slurry separation. which underlines its potential as a promising photocatalyst for groundwater purification.