Chlorophenols are toxic organic compounds and are listed by the Environmental Protection Agency as priority water pollutants ^[1]. They have been used extensively in many industrial products such as petrochemicals, pharmaceuticals, dyes, pulps, pesticides, and paints ^{[2, 3, 4, 5, 6]}; therefore, chlorophenols are common chloroaromatic pollutants ^[7]. Removing these contaminants from water is a significant challenge because of ever-increasing pollution and the shortage of high-quality fresh water ^[8]. The removal of these hazardous organic pollutants has become necessary and important for environmental safety. These organic compounds can be oxidized using chemical, photochemical, and microbiological processes.

Heterogeneous photocatalytic oxidation has aroused considerable interest as a potential efficient method for degrading recalcitrant environmental contaminants. Semiconductors, which have a filled valence band and an empty conduction band, are important materials because of the electronic structures of the metal atoms in chemical combinations ^[9]. A wide range of chloroaromatics have been eliminated by this process, mainly using TiO₂ as a semiconductor photocatalyst. TiO₂ is the most widely used photocatalyst because of its low cost, abundance, high chemical stability, and low toxicity ^[10]. However, it has two main defects. First, it is a relatively high energy band gap material (E_g ≈ 3.2 eV), which can be excited under ultraviolet (UV) irradiation (λ < 387 nm). This practically rules out the use of sunlight to induce photocatalytic reactions efficiently. Secondly, the low rate of electron transfer to reducible species (e.g., oxygen in oxidation reactions, protons in hydrogen production from water) and high recombination rate of the photoproduced electron-hole pairs limit the rates of photocatalytic reactions ^[11]. Research to find alternative high-performance, low-cost photocatalysts is therefore important ^{[12, 13, 14]}. Among the strategies adopted to minimize the effects of these drawbacks of TiO₂ photocatalysts, coupling TiO₂ with another metal oxide is a promising route.

Improvement of the photocatalytic activity of TiO₂ by doping with transition metals such as Pt, Fe, Mo, Ru, and V or nonmetals, and mixing with other metal oxides to form composite semiconductors has been investigated. The combination of a semiconductor substrate and metal cluster improves the photocatalytic activity by trapping the photoinduced charge carriers, thereby improving the charge-transfer processes, as a result of the recombination of electrons and holes prior to the superoxide activation process ^[15]. Additionally, surfactants have often been used to increase the photodegradation efficiencies of UV-TiO₂ systems ^{[16, 17]}. Several attempts have been made to enhance the efficiencies of TiO₂ catalysts using visible light ^{[18, 19, 20]}. For example, V-doped TiO₂, which has a band gap of 3.0-3.2, compared with 2.5-2.7 eV for TiO₂, is a promising visible-light-driven photocatalyst ^{[21, 22]}. Catalytic systems based on vanadium oxides are highly active and selective for a number of industrially relevant chemical transformations. It has already been established that the catalyst support plays an important role in both the activity and selectivity ^{[23, 24, 25, 26]}. ZrO₂-TiO₂ and Fe₃O₄-TiO₂ composite photocatalysts are examples of mixed metal oxides used in the degradation of chlorophenols ^{[27, 28]}. This type of catalyst has been thoroughly investigated in the treatment of organic wastewater. Catalysts are usually prepared using sol-gel and hydrothermal methods. There are no reports of photocatalysts for 2, 4-dichlorophenol removal prepared by solid-state dispersion. The synthesis of inexpensive, active, and nanosized photocatalysts using simple methods therefore needs to be investigated.

In the present work, binary metal oxide catalysts (V₂O₅-TiO₂) were prepared by solid-state dispersion. 2, 4-Dichlorophenol was used as a model pollutant to evaluate the photocatalytic activities of the catalysts under UV irradiation. The effects of the metal oxide ratio and surfactant additives on the photocatalytic efficiency were examined. The catalysts were analyzed using X-ray diffraction (XRD), diffuse-reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, and N₂ adsorption-desorption, and the relationship between the catalyst structure and photocatalytic activity was examined.

The starting materials for catalyst preparation were V₂O₅ (Merck), TiO₂ P25 (Degussa), ammonium vanadium oxide (Alfa Aesar, 99%), ethanol (absolute), titanium tetrachloride (≥ 99%), TiCl₄ (carbon tetrachloride; Merck), cetyltrimethylammonium bromide (CTAB; Merck), hexadecyltrimethylammonium bromide (HTAB; Merck), and poly(vinyl alcohol) (PVA; Merck). The organic compounds used in the photocatalytic experiments were 2, 4-dichlorophenol, phenol, 4-chlorophenol, 2-chlorophenol, hydroquinone, catechol, and methanol (for high-performance liquid chromatography (HPLC), ≥ 99%)); they were purchased from the Fluka Company and used without further purification. Deionized water was used for the preparation of all the catalysts and to dilute the 2, 4-dichlorophenol solution.

Pure TiO₂ was prepared using a sol-gel method ^[29]. TiCl₄ (1.5 mL) was slowly added dropwise to ethanol (15 mL) at room temperature. A large amount of HCl gas was exhausted during the mixing process. The obtained light-yellow solution was gelatinized for several days to form a sol-gel. The gel was dried in an oven at 105 ℃ for 1 d, ground to a fine powder, and calcined at 600 ℃ for 4 h.

V₂O₅-doped TiO₂ catalysts were prepared using a solid-state dispersion method. V₂O₅ and TiO₂ samples were mixed in specific weight percentage proportions, namely 10:90, 50:50, 70:30, 90:10; for example, 10V₂O₅-TiO₂ denotes the catalyst nominally containing 10 wt% V₂O₅. The binary oxide catalysts were dried at 110 ℃ for 1.5 h, calcined in air at 450 ℃ for 6 h, and ball milled.

V₂O₅-TiO₂ binary oxides and surfactants (CTAB, HTAB, or PVA) were mixed in weight ratios of 1:1 using a solid-state dispersion method. The resultant catalyst was dried at 110 ℃ for 1.5 h, heated at 150 ℃ in air for 5 h at a heating rate of 10 ℃/min, and ball milled.

The BET surface areas of the samples were determined from N₂ adsorption-desorption isotherm measurements at -196 ℃. The samples were degassed at 200 ℃ prior to the actual measurements.

Powder XRD patterns of the samples were obtained using a Rigaku D/Max-2200 diffractometer with Cu K_α (λ = 1.540 Å) radiation. Samples were scanned from 10° to 80° (2θ) at a rate of 2°/min. The sizes of the crystalline domains were calculated using the Scherrer equation, t = Cλ/Bcosθ, where λ is the X-ray wavelength (Å), B is the full width at half maximum, θ is the Bragg angle, C is a factor that depends on the crystallite shape (taken to be 1), and t is the crystallite size (Å).

The morphologies and size distributions of the photocatalysts were determined using SEM (JEOL/JSM-6335F).

Samples for FT-IR spectroscopy were prepared as KBr pellets. All spectra were recorded at a 4 cm^-1 resolution and 100 scans were performed. The surface OH groups of the photocatalysts were detected by FT-IR spectroscopy (Perkin Elmer Precisely Spectrum One).

UV-visible (UV-vis) DRS was performed using a UV-vis spectrophotometer (Shimadzu UV-3600), with BaSO₄ as the reference.

Photoluminescence (PL) spectra were recorded at room temperature using a fluorescence spectrophotometer (Agilent Technologies-Cary Eclipse), with a xenon lamp as the excitation source. The sample was dispersed in ethanol using an ultrasonic bath and the excitation wavelength used in the PL measurements was 530 nm.

Photoactivity studies were performed at atmospheric pressure and room temperature (25 ℃). In a typical experiment, the catalyst (100 mg) was dispersed in 2, 4-dichlorophenol solution (50 mL) of initial concentration 25 mg/L and neutral pH (pH 5) under magnetic stirring. Phenol photooxidation runs were performed using a cylindrical quartz batch photoreactor. A Luzchem LZC-5 photoreactor system was used in all experiments. The light source was a 64-W UV-B lamp (Luzchem LZC-UVB), with a maximum light intensity at 312 nm and illumination distance 18 cm from the target. The light intensity of the UV lamp used for degradation experiments was recorded using a UV/vis power meter (Smart Sensor-AR823). The photoreactor system had a magnetic stirrer, which was used to achieve uniform conditions in the reaction mixture. Before the UV light was turned on, the solution was stirred for 1 h to ensure good adsorption equilibrium between the catalyst and the solution. After irradiation for 5 h, the 2, 4-dichlorophenol solution was filtered through a membrane filter (pore size 0.45 mm) and the filtrate was used for total organic content (TOC) measurements (TOC-V, Shimadzu).

The concentrations of 2, 4-dichlorophenol and products were determined using an HPLC (Thermo Finnigan) system equipped with a C-18 column. The mobile phase was a methanol/water (70/30, v/v) mixture at a flow rate of 1 mL/min.

The BET surface areas of the catalysts are shown in Table 1. The surface areas of the prepared V₂O₅ and TiO₂ are 25 and 40 m²/g, respectively. For the V₂O₅-TiO₂ series, the surface areas decrease with increasing V₂O₅ content. This is because surface coverage by the V₂O₅ particles increases, and this prevents the entry of nitrogen probe molecules. In addition, when CTAB is added to 50V₂O₅-TiO₂, the surface area decreases 17 to 10 m²/g. However, the photocatalytic activity does not only depend on the BET surface area. This point was clarified by a series of detailed experiments.

Table 1 The crystallite sizes, specific surface areas, band gap, morphology of materials and 2, 4-dichlorophenol degradation efficiencies over 30 min. Catalyst V2O5 crystallite size (nm) SBET (m2/g) Band gap (eV) 2, 4-Dichlorophenol degradation (%) kr (mg/(L·min)) 10V2O5-TiO2 27 22 2.31 60 0.87 50V2O5-TiO2 30 17 2.26 85 1.49 70V2O5-TiO2 33 12 2.24 79 1.19 90V2O5-TiO2 37 8 2.21 75 1.13 (50V2O5-TiO2)-CTAB 35 10 2.21 100 2.22 (50V2O5-TiO2)-HTAB 38 6 2.24 93 1.84 (50V2O5-TiO2)-PVA 42 4 2.26 18 0.12 V2O5 40 25 2.35 72 1.09 TiO2 (synthesis) 43 40 3.12 66 0.95 TiO2 (P25) 32 50 3.12 59 0.77

Table 1 The crystallite sizes, specific surface areas, band gap, morphology of materials and 2, 4-dichlorophenol degradation efficiencies over 30 min.

Fig. 1 shows the XRD patterns of the V₂O₅-TiO₂ catalysts, and pure V₂O₅ and TiO₂. The diffraction peaks can be indexed to orthorhombic V₂O₅ (JCPDS 41-1426) and anatase TiO₂ (JCPDS 89-4921). No peaks for other phases were detected. The patterns show that the pure V₂O₅ catalyst consists of an orthorhombic phase and pure TiO₂ has significant peaks from the anatase crystal phase. Orthorhombic V₂O₅ is the dominant phase in the catalysts prepared using solid-state dispersion, whereas anatase TiO₂ prevails in the catalysts prepared by coprecipitation. XRD confirms the presence of V₂O₅ and TiO₂ in the composites, confirming the successful introduction of V₂O₅ onto TiO₂. Many studies have confirmed that the photocatalytic activity of anatase-phase TiO₂ is higher than those of the brookite or rutile phases ^[30]. For 50V₂O₅-TiO₂, the dominant V₂O₅ phase is orthorhombic, with some TiO₂ embedded on the V₂O₅ particles. The phase structure, crystallite size, and crystallinity of V₂O₅ play important roles in the photocatalytic activity.

Fig. 1. XRD patterns of V2O5, TiO2, 50V2O5-TiO2, CTAB-assisted V2O5-TiO2, HTAB-assisted V2O5-TiO2 and PVA-assisted V2O5-TiO2 catalysts.

Figure 1 shows that the XRD patterns of all the surfactant-assisted 50V₂O₅-TiO₂ nanoparticles have similar profiles and all the diffraction peaks can be indexed to orthorhombic V₂O₅ and anatase TiO₂. Although all the samples show the same morphologies, the crystallite sizes and relative peak intensities change on addition of a surfactant. The crystallite sizes are 42, 38, and 35 nm for (50V₂O₅-TiO₂)-PVA, (50V₂O₅-TiO₂)-HTAB, and (50V₂O₅-TiO₂)-CTAB, respectively. The size differences may lead to morphological and structural differences ^[31].

The nanostructural morphology has an important effect on the photocatalytic activity. The SEM images of 50V₂O₅-TiO₂ and surfactant-assisted V₂O₅-TiO₂ catalysts are shown in Fig. 2. Fig. 2(a) shows a SEM-energy dispersive X-ray spectroscopy (EDS) image of the 50V₂O₅-TiO₂ catalyst. The figure shows that the sample has a mixed morphology consisting of micro- and macro-clusters. EDS analysis indicates the presence of Ti, V, and O on the 50V₂O₅-TiO₂ surface. The SEM-EDS results clearly show that the synthesized 50V₂O₅-TiO₂ consists of particles. Fig. 2(b) shows that on CTAB addition the particle morphology changes to rod-like and significant particle interactions occur. However, agglomeration is seen in the SEM photograph of the (50V₂O₅-TiO₂)-PVA and (50V₂O₅-TiO₂)-HTAB catalysts. There are irregular and coarse grains in addition to agglomeration in the morphology of (50V₂O₅-TiO₂)-HTAB. These nanoparticles gradually self-assemble into aggregates with a specific morphology as a result of van der Waals interactions with surfactant molecules adsorbed on the nanocrystal surfaces and the tendency to minimize the interfacial energy ^{[32, 33]}.

Fig. 2. SEM images of (a) 50V2O5-TiO2, (b) (50V2O5-TiO2)-CTAB, (c) (50V2O5-TiO2)-HTAB, and (d) (50V2O5-TiO2)-PVA catalysts.

The surfactants act as structure-directing agents and can reduce the particle size; therefore, they influence the particle morphology ^[34]. Surfactants greatly affect the morphology of V₂O₅-TiO₂, and the crystal structure, as shown by the XRD results.

The optical absorption properties of a semiconductor, which are related to the electronic structure, are key factors in determining its photocatalytic activity ^[35]. The UV-vis DRS spectra of various samples are shown in Fig. 3. The band gaps of the samples were calculated from the onset of the absorption edges to be 2.26, 2.21, 2.24, and 2.26 eV for 50V₂O₅-TiO₂, (50V₂O₅- TiO₂)-CTAB, (50V₂O₅-TiO₂)-HTAB, and (50V₂O₅-TiO₂)-PVA, respectively (Table 1). The band gaps are close to each other.

Fig. 3. UV-vis DRS spectra of 50V2O5-TiO2 and surfactant loaded 50V2O5-TiO2 catalysts.

The absorption of visible light by V₂O₅-TiO₂ catalysts is better than that by TiO₂. V₂O₅ loading therefore significantly improves visible-light absorption by TiO₂ photocatalysts. Surfactant addition has a minor impact on the catalyst band gap energy. However, doping of the pure catalyst with a surfactant such as CTAB decreases the absorption wavelength into the visible region. In solution, CTAB, which is a cationic surfactant, ionizes and is selectively adsorbed on the crystal surfaces. This changes the growth rates of different crystal surfaces, and different morphologies are formed ^[36]. This change in the band gap energy positively affects the photocatalytic activity. (50V₂O₅-TiO₂)-CTAB therefore shows strong absorption in the UV region, indicating that this material would show photocatalytic activity under UV irradiation.

Fig. 4 shows the FT-IR spectra in the range 450-4000 cm^-1 for various samples. The samples all have similar FT-IR spectra, indicating that the structures of V₂O₅ and TiO₂ did not change after surfactant loading. The Ti-O-Ti bending vibration at 420-650 cm^-1 confirms the formation of anatase TiO₂. This is in agreement with the XRD results. The band at 1016 cm^-1 from the V-O (vanadyl oxygen) stretching vibration and the bands at 844 and 580 cm^-1 from O-V-O stretching modes are observed for pure V₂O₅. The bands at 1000-1030 cm^-1 represent the O-V-O stretching vibration and the bands at 800-830 cm^-1 due to Ti-O-V linkage ^[37] was observed for V₂O₅-TiO₂ and surfactant doped V₂O₅-TiO₂ nanocomposites. Furthermore, the sharp bands at about 2850, 2918 and 1474 cm^-1 are assigned to the C-H and C=C stretches. The absorption bands at 2918 and 2850cm^-1are due to the C-H asymmetric and symmetric stretching vibrations of CTAB and HTAB, respectively ^[38]. The IR spectra show two intense bands, assigned to asymmetric (2918 cm^-1) and symmetric (2850 cm^-1) stretching vibrations of C-CH₂ in methylene chains. The sharp bands in the region 1450-1500 cm^-1 are attributed to deformation of -CH₂- and -CH₃ ^[39] of the incorporated surfactants. These results indicate that the higher catalytic activities of CTAB- and HTAB-doped V₂O₅-TiO₂ can be attributed to higher concentrations of methylene chains compared with that in pure V₂O₅-TiO₂.

Fig. 4. FT-IR spectra of 50V2O5-TiO2 and surfactant loaded 50V2O5-TiO2 catalysts.

Fig. 5 shows the PL spectra of V₂O₅-TiO₂, (50V₂O₅-TiO₂)- CTAB, (50V₂O₅-TiO₂)-HTAB, and (50V₂O₅-TiO₂)-PVA. The PL emission is the result of separation and recombination of excited electrons and holes ^[40]. A lower PL intensity implies decreased recombination, and possibly higher photocatalytic activity ^[41]. This is reasonable because CTAB and HTAB addition can improve the separation of photogenerated electron-hole pairs and provide faster transport of charge carriers, thereby enhancing the photocatalytic performance ^{[42, 43]}. As shown in Fig. 5, all the samples exhibited a strong emission peak at around 530 nm. The intensities varied, and those for (50V₂O₅-TiO₂)-CTAB and (50V₂O₅-TiO₂)-HTAB were much lower than those for the other catalysts. This suggests that the addition of CTAB or HTAB inhibits electron-hole recombination and improves the photocatalytic activity. These results are in good agreement with the activity results.

Fig. 5. PL spectra of 50V2O5-TiO2 and surfactant loaded 50V2O5-TiO2 catalysts.

The photocatalytic activities of the samples were investigated based on degradation of 2, 4-dichlorophenol in aqueous solution in the presence of a small amount of H₂O₂ under UV-B irradiation. The reactions follow pseudo-first-order kinetics. The rate constants are listed in Table 1.

In general, the diffusion rate of adsorbed reactive species on the surface is faster than the photocatalytic reaction rate. The photocatalytic reaction is therefore the rate-controlling step. Photocatalytic degradation can be described using the Langmuir-Hinshelwood equation ^[44]:

Heterogeneous photocatalytic processes involve many steps such as diffusion, adsorption, and reaction. An appropriate pore distribution promotes diffusion of reactants and products, and this favors the photocatalytic reaction ^[45]. Before the photocatalytic reaction, dark reactions were performed, i.e., in the presence of a catalyst without irradiation, to ensure adsorption-desorption equilibrium. The effects of strongly oxidizing additives such as H₂O₂ are controversial and depend on the substrate type and various experimental parameters ^[46]; their usefulness should be carefully checked under the operating conditions. In our case, the highest degradation rate was achieved with addition of 0.15 mL of H₂O₂. The improved catalytic activity of V₂O₅-TiO₂ in the presence of H₂O₂ is attributed to the reactive radical intermediates (•OH) formed from the oxidants by reaction with photogenerated electrons; these intermediates can themselves act as strong oxidants and as electron scavengers, inhibiting electron-hole recombination at the semiconductor surface. The activities were calculated and are given in Table 1.

The photocatalytic degradation performances of V₂O₅, TiO₂, and V₂O₅-TiO₂ binary oxide catalysts in the oxidative degradation of 2, 4-dichlorophenol were monitored; the results are shown in Fig. 6. The data in Table 1 show that 72% degradation of 2, 4-dichlorophenol was achieved with the pure V₂O₅ catalyst under UV-B irradiation for 30 min. Under the same conditions, the photodegradation rate was only 66% for the synthesized TiO₂ catalyst. The 50V₂O₅-TiO₂ catalyst gave the highest percentage of 2, 4-dichlorophenol degradation (85%); however, higher weight percentages (70-90 wt%) did not noticeably change the reactivity and the value decreased slightly with further loadings of V₂O₅. 2, 4-Dichlorophenol degradation decreased to 60% for 10 wt% V₂O₅ mixed with TiO₂.

Fig. 6. Degradation activities of V2O5, TiO2 (synthesis), TiO2 (P25) and V2O5-TiO2 catalysts.

These results confirm that the optimum V₂O₅-TiO₂ weight ratio is 1:1. This is because few V₂O₅-TiO₂ heterostructures can be formed on the surface when the V₂O₅ content is too low; when it is too high, excess V₂O₅ can cover active sites on the TiO₂ surface and few V₂O₅-TiO₂ heterostructures are formed. V₂O₅-TiO₂ nano-heterostructures with a V₂O₅-TiO₂ weight ratio of 1:1 show the highest photocatalytic activity.

The activity of the photocatalyst is influenced by its crystallinity and other factors such as surface area, crystal size, synthetic method, band gap, crystal phase, and surface OH groups.

The activity results show that the surface area, particle size, and surface OH groups are not the only factors that contribute to high reactivity in 2, 4-dichlorophenol degradation. The interactions in binary oxide catalysts are more important than the surface area, particle size, and surface OH groups. The optimum photodegradation activity was achieved with 50V₂O₅-TiO₂. Our XRD observations clearly show that V interacts with Ti.

The specific surface area decreases with increasing V₂O₅ content (70% to 90%) in the binary oxide catalysts because of TiO₂ poreblockage. In addition, a high V₂O₅ content (70% or 90%) prevents TiO₂ crystallization. The results show that the photocatalytic activity of V₂O₅-TiO₂ was significantly decreased by high loading with V₂O₅ clusters (70% and 90%). These results are in agreementwith those in the Refs. ^{[47, 48]}.

Neppolian et al. ^[49] reported that electron injection was the major factor in the activities of binary oxide catalysts, along with other physicochemical characteristics. The transfer of electrons from ZrO₂ to TiO₂ through chemical interactions between ZrO₂ and TiO₂ to form Ti-O-Zr- bonds was the main process observed in binary oxide catalysts. Wu et al. ^[50] described mutual chemical interactions between pure oxides when they were coprecipitated (-Ti-O-Zr-), and this profoundly affected the photocatalytic properties.

In this study, the photocatalytic activity of 50V₂O₅-TiO₂ was higher than those of nano-TiO₂ (synthesized) and Aeroxide P25. Although the surface area of the 50V₂O₅-TiO₂ catalyst was comparable to or even lower than those of TiO₂ and P25, it showed high activity. However, the changes in surface areas were not as great as these in the catalytic activities; therefore, differences among the activities were associated with the active species on the surfaces. 2, 4-Dichlorophenol removal of 59% was achieved with Degussa P25. Various synthesized novel catalysts with higher photocatalytic activities than that of Degussa P25 have been reported. Xu et al. ^{[51, 52, 53]} investigated the photocatalytic activity of TiO₂ supported on ZSM-5, zeolite A, silica, and alumina in the photooxidations of 4-chlorophenol and acetophenone. The photocatalytic activity of TiO₂/ZSM5 was higher than those of prepared TiO₂ powder and Degussa P25.

The performance data of modified V₂O₅-TiO₂ catalysts with various HTAB, CTAB, and PVA loadings are shown in Fig. 7(a). The data in Table 1 show that CTAB and HTAB loading improved the degradation activity of 50V₂O₅-TiO₂. The photocatalytic performance in 30 min over the prepared catalysts decreased in the order (50V₂O₅-TiO₂)-CTAB (ca. 100%) > (50V₂O₅-TiO₂)- HTAB (ca. 93%) > (50V₂O₅-TiO₂) (ca. 85%) > (50V₂O₅-TiO₂)- PVA (ca. 18%). The (50V₂O₅-TiO₂)-CTAB sample achieved complete degradation in 30 min (Table 1) and gave the highest reaction rate (2.22 mg/(L·min)). One explanation of the results is strong interactions between V₂O₅-TiO₂ and HTAB. Only 18% degradation of 2, 4-dichlorophenol was achieved in 30 min with the (50V₂O₅-TiO₂)-PVA catalyst. The decrease in the activity can probably be explained by partial blocking of the active species of the V₂O₅-TiO₂ binary oxide by large PVA particles. The SEM photograph of V₂O₅-TiO₂ loaded with PVA shows particle agglomeration, which would cause a drop in the photocatalytic activity. These results show that surfactant loading changed the surface areas and particle sizes of the samples.

Fig. 7. (a) Degradation activities of 50V2O5-TiO2 and surfactant-loaded 50V2O5-TiO2 catalysts. (b) TOC removal for photocatalytic degradation of 2, 4-dichlorophenol.

These observations indicate that the local structure, crystallinity, surface morphology, and particle shape of the V₂O₅-TiO₂ powder affect the photocatalytic performance. The crystal structure, crystallinity, morphology, and surface area of a material are important factors in its photocatalytic performance ^{[54, 55, 56]}. The photocatalytic activity results suggest that the enhanced photocatalytic activity of (50V₂O₅-TiO₂)-CTAB can be attributed to an appropriate pore distribution and high separation rate of photoinduced charge carriers. The addition of CTAB to the synthesis system not only alters the surface parameters, especially the pore distribution, but also enhances the photoinduced charge separation rate of V₂O₅-TiO₂. These results show that the pore distribution and photoinduced charge separation rate both play important roles in promotion of the photocatalytic activity of (50V₂O₅-TiO₂)-CTAB.

Fig. 7(b) shows the TOC removal results for photocatalytic degradation of 2, 4-dichlorophenol with 50V₂O₅-TiO₂, (50V₂O₅- TiO₂)-CTAB, (50V₂O₅-TiO₂)-HTAB, and (50V₂O₅-TiO₂)-PVA. The (50V₂O₅-TiO₂)-CTAB sample achieved complete degradation of 25 mg/L 2, 4-dichlorophenol and 97% TOC removal in 30 min. These results show that total mineralization was achieved.

The concentration profiles of 2, 4-dichlorophenol, 2-chlorophenol, phenol, catechol, and ring-opening products (others) during photocatalytic oxidation using 50V₂O₅-TiO₂ and (50V₂O₅-TiO₂)-CTAB are shown in Fig. 8. The intermediate distributions were also compared.

Fig. 8. Concentration profiles for intermediates obtained during the photocatalytic oxidation reaction using (a) 50V2O5-TiO2 (b) (50V2O5-TiO2)-CTAB.

HPLC results showed that 2-chlorophenol, phenol, and catechol were the main intermediate when 50V₂O₅-TiO₂ was used. Ring-opening products (others) were also detected in low concentrations. When CTAB was added to the pure catalyst, the detected concentrations of 2-chlorophenol, phenol, and catechol were low. It was observed that the concentrations of ring-opening products (others) were considerably higher than those of intermediate products when CTAB was used. These intermediates undergo further photocatalytic oxidation to give ring cleavage and yield carboxylic acids and aldehydes, which give CO₂ and H₂O by decarboxylation.

It should be noted that some structural features of catalysts play a significant role during photocatalysis. These results confirm that (50V₂O₅-TiO₂)-CTAB is more active than some other catalysts. These results also indicate that the photochemical reaction is important in the degradation of 2, 4-dichlorophenol under these conditions.

V₂O₅-TiO₂ binary oxide catalysts prepared using a solid-state dispersion method were used in the photocatalytic degradation of 2, 4-dichlorophenol under UV-B irradiation. The photocatalysts showed high photocatalytic activities in 2, 4-dichlorophenol degradation. The chemical interactions between V₂O₅ and TiO₂ to form V-O-Ti- bonds affect the photoefficiencies of binary oxide catalysts. The effects of various surfactants (CTAB, HTAB, and PVA) on the photocatalytic degradation of 2, 4-dichlorophenol were also investigated. (50V₂O₅-TiO₂)-CTAB gave the highest percentage of 2, 4-dichlorophenol degradation (100%) and the highest reaction rate (2.22 mg/(L·min)) in 30 min. The catalytic activity of (50V₂O₅-TiO₂)-CTAB was higher than those of nano TiO₂ and P25 photocatalysts. The structural, morphological, optical, and photocatalytic properties of the catalyst play a significant role during photocatalysis. In summary, the effectiveness of using a surfactant such as CTAB to construct an efficient visible-light-driven 50V₂O₅-TiO₂ photocatalyst was shown. The local structure and crystallinity significantly affected the photocatalytic performance. The results indicate that the photocatalytic degradation of wastewater containing chlorophenols can be achieved using (50V₂O₅-TiO₂)-CTAB. Such heterostructured nanocomposites may help in the design of highly efficient photocatalysts for organic compounds, and have potential technological applications.