TiO2/g-C3N4 Visible-Light-Driven Photocatalyst for Methylene Blue Decomposition

Hoa, Dang Thi Ngoc; Tu, Nguyen Thi Thanh; Thinh, Huynh Quoc An; Van Thanh Son, Le; Son, Le Vu Truong; Quyen, Nguyen Duc Vu; Son, Le Lam; Tuyen, Tran Ngoc; Thong, Pham Le Minh; Diem, Ly Hoang; Khieu, Dinh Quang

doi:https://doi.org/10.1155/2023/9967890

Journal of Nanomaterials

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Research Article | Open Access

Volume 2023 | Article ID 9967890 | https://doi.org/10.1155/2023/9967890

TiO₂/g-C₃N₄ Visible-Light-Driven Photocatalyst for Methylene Blue Decomposition

Dang Thi Ngoc Hoa,^1,2Nguyen Thi Thanh Tu ,³Huynh Quoc An Thinh,²Le Van Thanh Son,⁴Le Vu Truong Son,⁴Nguyen Duc Vu Quyen,²Le Lam Son,²Tran Ngoc Tuyen,²Pham Le Minh Thong,⁵Ly Hoang Diem,⁶and Dinh Quang Khieu²

Academic Editor: Ganesan Sriram

Received07 Jun 2022

Revised05 Oct 2022

Accepted13 Oct 2022

Published01 Feb 2023

Abstract

In this work, graphitic carbon nitride (g-C₃N₄)/titanium dioxide (TiO₂) nanoparticles with heterostructures were synthesized in situ from a mixture of melamine and peroxo-titanium complexes in a calcination process. The TiO₂ nanoparticles are well-dispersed on the g-C₃N₄ nanosheets. The prepared TiO₂/g-C₃N₄ composites have a heterostructure and excellent photocatalytic activity for decomposing methylene blue (MB) under visible light irradiation. The as-obtained g-C₃N₄ embroiled with TiO₂ has a much larger surface area than its components (66.7 and 6.6 m²·g⁻¹ for TiO₂ and g-C₃N₄ against 95.5–143.8 m²·g⁻¹ for the composite, respectively). It enhances the separation of photo-generated charge carriers. The TiO₂/g-C₃N₄ photocatalytic degradation of MB was investigated in aqueous heterogeneous suspensions. The experimental kinetic data for the photocatalytic process follow the pseudo-first-order kinetic model. Furthermore, TiO₂/g-C₃N₄ retains high photocatalytic activity after four reaction cycles. In addition to prompt removal of the color, the TiO₂/g-C₃N₄ photocatalyst can oxidize MB almost completely to final oxidation products. The pathway of MB decomposition was also addressed. Additionally, the TiO₂/g-C₃N₄ photocatalytic system was employed to eliminate other typical organic pigments, such as malachite green, methyl blue, and methyl red. The TiO₂/g-C₃N₄ material, with remarkable dye degradability, is a promising catalyst in industrial textile treatment and can find applications in light-harvesting systems.

1. Introduction

Recently, graphitic carbon nitride (g-C₃N₄) has become prominent as a stable photocatalyst and gained significant attention [1] owing to its relevant bandgap (2.7 eV) [2] and, therefore, can be active under visible light. In addition, the negative potential (−1.12 eV vs. normal hydrogen electrode (NHE)) of the conduction band (CB) enables g-C₃N₄ to form heterojunctions with other semiconductors. Despite its disadvantages, such as small surface area and high recombination rate of photoinduced charge carriers, g-C₃N₄ is still a potential candidate for hybrid heterostructures with wide bandgap semiconductors [3].

Besides various semiconducting materials, such as SnO₂, ZnO, ZnS, WO₃, and V₂O₅ [4], TiO₂ has attracted tremendous attention in photocatalytic applications because of its low cost, high stability, nontoxic nature, and environmental friendliness. Although TiO₂ has been used in several photocatalytic processes of wastewater treatment and air purification [5], its wide bandgap (3.2 eV) [6] limits its application only to the ultraviolet region of visible light, which is around only 4% of the solar spectrum. To overcome this disadvantage, doping other nonmetallic or trace metallic elements, such as N, P, Fe, Ni, and V, to develop composite materials with narrowband energy has been studied intensively [4, 7, 8]. Numerous studies have presented the feasibility of synthesizing TiO₂/g-C₃N₄ photocatalysts [9]. Using g-C₃N₄ with TiO₂ to form a heterojunction is one of the efficient approaches for either narrowing bandgap or enhancing photoinduced electron–hole pair separation. For example, Liu et al. [10] synthesized TiO₂/g-C₃N₄ from TiO₂ and urea and utilized this material to promote the photocatalytic reduction of U(VI) in water. Zhang et al. [11] and Alcudia-Ramos et al. [12] fabricated TiO₂/g-C₃N₄ from tetrabutyl titanate or TiOSO₄ and urea to enhance photocatalytic hydrogen evolution under visible light. Titanium alkoxides, TiCl₃, and TiCl₄ in a hydrochloric acid aqueous solution or Ti(SO₄)₂ in a sulfuric acid aqueous solution are mainly used as a titanium source [11, 12]. However, titanium alkoxides, such as tetra-isopropoxide, are highly unstable, flammable, and readily hydrolyze in a moist atmosphere. Moreover, titanium chlorides in these acidic solutions are difficult to handle because of their aggressive and toxic nature. For solving these issues, insensitive approaches have been investigated in developing water-soluble titanium complexes, such as peroxo-titanium oxalate and peroxo-titanium complexes [13], for synthesizing titanium-dioxide-based materials [14]. To our best knowledge, reports on the TiO₂-based materials synthesized via water-soluble titanium species are limited. Therefore, we developed a strategy for preparing TiO₂/g-C₃N₄ from a water-soluble titanium complex and melamine solution to form a homogeneous mixture in this study. The peroxo–hydro titanium complexes were synthesized from titanium oxide in a hydrothermal process, followed by the formation of water-soluble titanium complexes with hydrogen peroxide. TiO₂/g-C₃N₄ was then prepared by evaporating the homogeneous complex solution, followed by calcination. The photocatalytic activity of TiO₂/g-C₃N₄ was studied via the kinetics, mineralization, photocatalytic mechanism, and decomposition pathway of methylene blue.

2. Experimental

2.1. Materials

Anatase (TiO₂, 99%) and melamine (C₃H₆N₆, ≥99.5%) were purchased from Merck. Potassium iodide (KI, ≥99%), sodium chloride (NaCl, ≥99.5%), sodium hydroxide (NaOH, ≥96%), hydrochloric acid (HCl, 38%), hydrogen peroxide (H₂O₂, ≥30%), and isopropanol (CH₃CH(OH)CH₃, ≥99.8%) were obtained from Xilong, China. Methylene blue (C₁₆H₁₈N₃SCl·3H₂O, MB), methyl red (C₁₅H₁₅N₃O₂, MR), methyl blue (C₃₇H₂₇N₃Na₂O₉S₃, MyB), malachite green (C₂₈H₃₀N₂O₃, ≥96.5%, MG), and p-benzoquinone (C₆H₄O₂, ≥99%) were provided by Sigma–Aldrich. All the chemicals were of analytical grade and used as received without any further purification.

2.2. Synthesis of g-C₃N₄

According to Yan et al. [1], the g-C₃N₄ material was synthesized from melamine. Briefly, 10 g of melamine powder was weighed, placed in a mortar, and finely ground. The powder was put into a porcelain cup and calcined at 500°C for 4 hr with a heating rate of 5°/min. The powder was cooled to ambient temperature and finely grounded to obtain a yellow product, denoted as g-C₃N₄.

2.3. Synthesis of Peroxo-Titanium Complexes

According to Chau et al. [15] and Le et al. [16], the peroxo–hydroxo titanium(IV) complex was synthesized. Briefly, 0.25 g of TiO₂ powder was dispersed into 12.5 mL of a 20 M NaOH solution under sonication for about 15 min. Then, the entire solution was transferred to a Teflon flask and heated at 130°C for 10 hr. After cooling to ambient temperature, the white solid was separated and rinsed with distilled water and a 0.1 M HCl solution to completely remove the alkali then dried at 80°C for 2 hr (around 0.189 g of dried TiO₂ obtained). The 4.62 mg of hydrothermal-treated TiO₂ was added 35 mL of 30% H₂O₂ at 90°C, stirring for 1 hr to obtain a clear yellow solution (the solution is stable at 10°C for several days). The obtained peroxo-titanium complexes solution has a composition of 4.62 mg TiO₂/35 mL determined with the gravity method.

2.4. Synthesis of TiO₂/g-C₃N₄

TiO₂/g-C₃N₄ composites were synthesized via an aqueous solution process: a mixture of titanium peroxo-titanium complex solution (35 mL; 4.62 mg TiO₂) and (0.00; 1.16; 4.62; 18.48 mg) with known TiO₂/melamine mass ratios was sonicated for 1 hr, followed by calcination at 500°C for 4 hr to obtain a TiO₂/g-C₃N₄ composite. The mixtures are denoted as (10/0)TiO₂/g-C₃N₄, (8/2)TiO₂/g-C₃N₄, (5/5)TiO₂/g-C₃N₄, (2/8)TiO₂/g-C₃N₄, and (0/10)TiO₂/g-C₃N₄, where the fraction in the parentheses is the TiO₂/melamine mass ratio. The synthetic diagram is shown in Scheme 1.

2.5. Material Characterization

X-ray diffraction (XRD) was performed on a D8 Advance device (Bruker, USA) with a Cu/Kα radiation source and λ = 0.154056 nm. Scanning electron microscopy (SEM) images were collected by using an SEM Hitachi-S-4800, Japan, scanning electron microscope. FT infrared (FTIR) spectra for the samples were obtained on an IRAffinity-1S-Shimadzu spectrometer (Japan). Specific surface areas were determined from N₂ adsorption–desorption isotherms in liquid nitrogen at 77 K by using a Micromeritics ASAP 2020 analyzer. UV–vis DR spectra were obtained on a UV2600-Shimadzu spectrometer (Japan).

2.6. Catalytic Activity

The catalytic activity of the obtained materials was studied via the decomposition of methylene blue (10 ppm) on the catalyst (0.2 g/L). The mixture was stirred in the dark for 40 min to reach the adsorption/desorption equilibrium, then illuminated with an Osram, 160 W filament lamp (filter cutoff λ < 420 nm). The MB concentration in the supernatant was determined by using UV–vis spectroscopy at a maximum wavelength of 664 nm. The pH value of the dye solutions was adjusted with 0.1 M HCl or 0.1 M NaOH solutions. In addition, the concentration of malachite green was determined at 617 nm and methyl red at 521 nm. The UV–vis spectra were recorded on a Spectro UV-2650 spectrometer (Labomed, Inc., USA). The chemical oxygen demand (COD) was determined by using a Thermoreactor Lovibond ET108 analyzer according to the dichromate method. The intermediates that appear during MB degradation were extracted and accumulated by chloroform, then determined by the ion trap method in combination with EIS ionized mass spectroscopy on a LC/MSD Trap-SL ion trap mass spectrometer. ICP analysis for titanium was performed using a 7800 ICP-MS instrument (Agilent Technologies, Santa Clara, CA, USA).

3. Results and Discussion

Figure 1 presents the XRD patterns of the products synthesized under different conditions. Anatase (TiO₂) reacts with NaOH to create several forms of sodium titanate at 130°C and 10 hr. The characteristic peaks of Na₂Ti₃O₇ indexed as (011), (–311), (–313), and (–105); Na₂Ti₆O₁₃ indexed as (201), (110), (401), (402), (313), (223), and (713) and Na₂Ti₉O₁₉ indexed as (003), (006), (007), (60–1), and (020) according to JCPDS 31-1329, JCPDS 73-1398, JCPDS 33-1293, respectively, are present in the XRD patterns (Figure 1(a)). The sodium titanates after being rinsed with HCl until neutral filtrates change to anatase TiO₂ (JCPDS 21-1272) (Figure 1(b)). The preparation of water-soluble peroxo-titanium complexes is associated with the formation of peroxo-titanium complexes, such as [Ti(O₂)(OH)]⁺, [Ti(O₂)(OH)₂], and [Ti(O₂)(OH)₃]⁻, which dimerize and result in various oxo–peroxo–hydroxo di-titanium complexes, such as Ti₂(O)(O₂)₂(OH)₂, [Ti₂(O)(O₂)₂(OH)₃]⁻, and [Ti₂(O)(O₂)₂(OH)₄]²⁻. These dimeric complexes can undergo further self-condensation to form oligomers. As a result, polynuclear oxo–peroxo–hydroxo titanium complexes precipitate as [Ti₂(O)(O₂)₂(OH)₂]_n [17] (Equations (1)–(4)).

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In the synthesis of the TiO₂/g-C₃N₄ composite, different peroxo-titanium complexes/melamine mass ratios were used. Figure 2(a) presents the XRD patterns of pristine g-C₃N₄, TiO₂, and TiO₂/g-C₃N₄ composites. For the pristine g-C₃N₄ and TiO₂, the characteristic peaks of the g-C₃N₄ phase are observed on the XRD pattern of (0/10)TiO₂/g-C₃N₄ at 13° and 27°, indexed as (100) and (002) (JCPDS 87-1526). These two peaks are likely assigned to the structure of the tri-s-triazine unit with interplanar spacing and the conjugated aromatic system [18]. This observation confirms the presence of g-C₃N₄ in the sample. The XRD pattern of (10/0)TiO₂/g-C₃N₄ exhibits characteristic peaks at 25.2°, 37.8°, 48.0°, 53,8°, 62.6°, 70.3°, and 75.0° corresponding to (101), (004), (200), (105), (204), (220), and (215) Miller index, respectively (JCPDS 21-1272). These peaks suggest the TiO₂ anatase form. The characteristic peaks of both TiO₂ and g-C₃N₄ are observed in the XRD patterns of TiO₂/g-C₃N₄ composites. The increasing amount of g-C₃N₄ augments the intensity of its characteristic peaks.

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The textural properties of the obtained composites were studied via their nitrogen adsorption/desorption isotherms (Figure 2(b)). All the isotherm curves of g-C₃N₄, TiO₂, and its composite belong to the IV type according to the IUPAC classification with the H3 hysteresis loop at relative pressures 0.6–1, suggesting the mesoporous structures formed from fine particles. The surface area of g-C₃N₄ and TiO₂ is similar to what was reported previously [19, 20]. It is worth noting that the composites have a significantly larger surface area than g-C₃N₄ and TiO₂ (95.5–143.8 m²·g⁻¹ compared with 66.7 and 6.6 m²·g⁻¹) (Table 1).

Table 1 shows that the surface area of the composite results mainly from the mesoporous area. Both g-C₃N₄ and TiO₂ enhance the microporous and mesoporous spaces. The reason could be that the initial precursor of both titania and melamine in the liquid form is favorable for creating a homogeneous mixture. During heating, the gas products are generated to form the porous structure of TiO₂/g-C₃N₄. The (2/8)TiO₂/g-C₃N₄ sample exhibits the largest surface area (143.8 m²·g⁻¹), higher than the TiO₂/g-C₃N₄ samples synthesized from titanium n-butoxide and urea (62.1 m²·g⁻¹) [21] and TiO₂ powder and melamine (97.26 m²·g⁻¹) [22].

Figure 3(a) presents the FTIR spectra of g-C₃N₄, TiO₂, and TiO₂/g-C₃N₄ composites. For bare g-C₃N₄ and TiO₂, the vibration at 810 cm⁻¹ is related to the C–N ring of the tri-s-triazine unit. The peaks at 1,238, 1,316, and 1,402 cm⁻¹ are attributed to the stretching vibrations of the C–NH bond in g-C₃N₄. The peaks at 1,637 and 3,000–3,500 cm⁻¹ are assigned to the stretching vibration of C = N [21, 23] and –N–H or = N–H bond [23, 24]. The stretching vibrations of the Ti–O–Ti bonds at 470 cm⁻¹ are typical for TiO₂ [21]. The vibrations of both g-C₃N₄ and TiO₂ in the FTIR spectra of TiO₂/g-C₃N₄ composites again confirm the composite formation between g-C₃N₄ and TiO₂.

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The (2/8)TiO₂/g-C₃N₄ composite was also characterized from Raman spectra (Figure 3(b)–3(d)). The characteristic peaks of g-C₃N₄ at 470, 543, 702, 746, 978, and 1,147 cm⁻¹ are observed, which are similar to those as reported by Bai et al. [25]. For TiO₂, the vibration modes at 144 cm⁻¹ (E_g), 197 cm⁻¹ (E_g), 399 cm⁻¹ (B_1g), 513 cm⁻¹ (A_1g), and 639 cm⁻¹ (E_g) belong to anatase [26, 27]. For the TiO₂/g-C₃N₄ composite, the vibrations of TiO₂ and g-C₃N₄ observed in the Raman spectra confirm the coexistence of TiO₂ and g-C₃N₄.

UV–vis diffuse reflectance spectra were employed to evaluate the bandgap of the material. Compared with pristine TiO₂, TiO₂/g-C₃N₄ displays a significant red shift because of the band alignment of coupled TiO₂ and g-C₃N₄ (Figure 4(a)). This shift suggests that introducing g-C₃N₄ into TiO₂ could enhance the visible light absorption available to TiO₂/g-C₃N₄. The bandgap of g-C₃N₄ and TiO₂/g-C₃N₄ was estimated from the Tauc equation [28]: α(hν) = A(hν – E_g)^n/2, where α is the absorption coefficient; hν is the energy of photon; E_g is the bandgap of the semiconductor; and A is the constant. The index n depends on the properties of the transition of semiconductors: n = 1 for direct transition and n = 4 for indirect transition semiconductors. TiO₂ and g-C₃N₄ are indirect bandgap semiconductors. Thus, the value of n for g-C₃N₄, TiO₂, and TiO₂/g-C₃N₄ is 4. The calculated bandgaps are 3.20, 2.80, 2.75, 2.70, and 2.65 eV for (10/0)TiO₂/g-C₃N₄, (8/2)TiO₂/g-C₃N₄, (5/5)TiO₂/g-C₃N₄, (2/8)TiO₂/g-C₃N₄, and (0/10)TiO₂/g-C₃N₄, respectively (Figure 4(b)). It is possible that g-C₃N₄, a nitrogen-rich molecule, also acts as a nitrogen source to form TiO_2−xN_x. The substitution of N in the TiO₂ lattice results in a visible light response owing to an additional N level above the TiO₂ VB that triggers the bandgap to narrow to 0.55 eV. These results confirm that the formation of heterojunction between g-C₃N₄ and TiO₂ could narrow the bandgap of pristine TiO₂ and enhance visible light availability.

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The elemental composition in the composite was studied by EDX-mapping. Figure 5 illustrates the EDX-mapping of the (2/8)TiO₂/g-C₃N₄ sample. The electron image shows that this sample has a porous structure that agglomerates in large particles of several micrometers (Figure 5(a)). EDX spectrum exhibits the existense of C, N, and Ti elements (Figure 5(b)). Figure 5(c)–5(f) shows that C, N, O, and Ti are evenly dispersed in the material.

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The morphology of TiO₂, g-C₃N₄, and (2/8)TiO₂/g-C₃N₄ is shown in Figure 6. Figure 6(a) shows the layered structure of g-C₃N₄ with a nanometer stacking that is typical for the C₃N₄ graphite layers. The morphology of TiO₂ consists of titania fibers with a 2 × 100 nm diameter and irregular sheets around 50–100 nm in diameters. Composite TiO₂/g-C₃N₄ (Figure 6(c)) is formed through embroidering the titania fiber and the g-C₃N₄ layers in a nanoscale, suggesting the formation of the heterojunction interface in the TiO₂/g-C₃N₄ composite.

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3.1. Visible-Light-Driven Photocatalyst for Methylene Blue (MB) Decomposition

3.1.1. Kinetics of MB Decolorization

Figure 7(a) displays the photocatalytic activity of the g-C₃N₄, TiO₂, and TiO₂/g-C₃N₄ composites. The experiment was conducted in two stages: adsorption in the dark and photocatalytic decomposition of MB. The adsorption lasted for 40 min to ensure the adsorption/desorption equilibrium, followed by visible-light irradiation for 40 min. It is obvious that the pristine g-C₃N₄ and TiO₂ possess either low adsorption capacity or poor decolorization, indicating that these materials do not catalyze MB decomposition. The color of the MB solution remains almost unchanged after 40 min of irradiation, suggesting MB is stable under these conditions. The TiO₂/g-C₃N₄ composites substantially enhance MB decolorization compared with the components. The MB decolorization rate increases with the increasing amount of g-C₃N₄ and peaks for the (2/8)TiO₂/g-C₃N₄ sample. Therefore, this sample was selected for further experiments.

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The leaching of active metal species to the liquid phase was studied by removing the catalyst after 50 min of illumination, and the MB decolorization was studied. It was found that, although illumination continued for up to 120 min (Figures 7(b) and 7(c)), the MB decolorization did not occur in the absence of the catalyst. Only a small amount of leaching titanium was also detected in the supernatant (0.336 mg·L⁻¹) by ICP analysis. This fact confirms that (2/8)TiO₂/g-C₃N₄ is a heterogeneous catalyst in MB photocatalytic degradation.

The kinetics of MB decomposition is shown in Figure 7(d). The process was conducted in two stages, as described above. The (2/8)TiO₂/g-C₃N₄ sample exhibits adsorption equilibrium after around 40 min with an equilibrium adsorption efficiency of ∼8%–10%, depending on the initial concentration. The higher the MB concentration, the larger the adsorption capacity is because of the increasing driving force. The kinetics of photocatalytic decomposition is widely described by using the Langmuir and Hinshelwood (L–H) mechanism [29–31]. In this case, MB adsorption on the catalyst surface follows the Langmuir isotherm, and the adsorption equilibrium maintains during the photocatalytic reaction, that is, the adsorption rate is larger than the reaction rate of the electrons or holes, which is the rate-determining step. The reactions and kinetics equation are expressed as follows:where k_f and k_b are the forward and backward adsorption rate coefficients; k_r is the rate constant of the photocatalytic reaction; C_0e (ppm) is the MB concentration at the equilibrium of dark adsorption; and k_app (min⁻¹) is the apparent rate constant.

The value of k_app can be obtained from the linear plot of vs. t. It was found that the fitted lines of against irradiation time exhibit high linearity (R² = 0.9679), and these values indicate that the MB photodegradation apparently fits well with pseudo-first-order kinetics [32]. The pseudo rate constant at different MB concentrations is listed in Table 2. A comparison of k_app with those of previous reports is also presented. It was found that the rate constant of MB decolorization in this paper is a bit higher than those of previous reports, confirming the advantage of synthesizing TiO₂-based materials from water-soluble titanium complexes.

3.1.2. Effect of pH and Some Scavengers on MB Decomposition

The pH effect of dark adsorption and photocatalytic decomposition of MB is shown in Figure 8(a). When pH > 3.8 (pKa of MB 3.8 [40]), MB is positively charged. The pH_PZC value of TiO₂/g-C₃N₄ determined with the pH shift method is 7.4. At pH less than pH_PZC, the material’s surface carries a positive charge because of protonation. The MB degradation efficiency increases gradually with pH from 4 to 10. When pH > 7.4, the electrostatic interaction between the material’s negatively charged surface and positively charged MB causes decolorization efficiency to increase.

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Figure 8

(a) Effect of pH on MB decolorization; the inset presents the estimation for point of zero charge (pH_PZC) of (2/8)TiO₂/g-C₃N₄; (b) yield of MB decolorization with present of the scarvengers using (2/8) TiO₂/g-C₃N₄ catalyst (conditions: V = 100 mL, C_0(MB) = 10 ppm, m_catalyst = 0.02 g, V_scarvenger = 20 mL, illumination time: 120 min, ambient temperature); (c) UV–vis spectra; (d) kinetics of disappearance of chemical oxygen demand (COD) on (2/8)TiO₂/g-C₃N₄ (conditions: c) V = 100 mL, C_0(MB) = 10 ppm, m_catalyst = 0.02 g, illumination time: 200 min; (d) V = 100 mL, C_0(MB) = 500 ppm, m_catalyst = 1.00 g, illumination time: 120 min, ambient temperature.

The results of free radical scavenging experiments are shown in Figure 8(b). The experiments were conducted to determine the free radicals formed during irradiation, causing MB decomposition. The three free radical scavengers used in the research are based on Tu et al. [41]: potassium iodide to capture h⁺, isopropanol to capture ^•OH, and benzoquinone to capture O₂^•−. The results indicate that benzoquinone significantly reduces MB decolorization, followed by isopropanol and potassium iodide, indicating that free radicals play an essential role in the MB degradation in the following order: O₂^•− > ^•OH > h⁺.

3.1.3. Photocatalytic Degradation Pathway of MB

The UV–vis absorption spectrum of MB solutions is shown in Figure 8(c). The absorption peaks at 245 and 292 nm correspond to the excitation of π-electrons from the π– interaction in the benzene ring, while those at 615 and 664 nm are assigned to the benzene ring and the heteropolyaromatic linkage [42]. The maximum absorption occurs at 664 nm. The intensity of the adsorption peaks decreases with increasing illumination time, and the color of the MB solution almost disappears after 100 min of illumination. This decolorization reveals that the MB structure was destroyed during illumination. The mineralization of carbons is also evidenced by the COD values, as shown in Figure 8(d). COD gradually decreases from an initial 271 to 11.1 mg·L⁻¹ after 100 min of irradiation and 8.3 mg·L⁻¹ after 120 min. This decolorization is associated with the aromatic ring opening with the transient formation of carboxylic acids, followed by the evolution of CO₂ according to the “photo-Kolbe” reaction: R–COO⁻ + h⁺ ⟶ R^• + CO₂. Methylene blue is broken down into harmless or less harmful substances, such as carbon dioxide (CO₂) and water (H₂O), via certain intermediates.

The degradation of MB was also studied quantitatively by using HPLC before and after degradation. The peak area reveals that an insignificant amount of MB remains after the 100 min illumination (only 3.7% remains (Figure 9)).

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The intermediate compounds were identified by using liquid chromatography–mass spectrometry (LC–MS), as shown in Figure 10. Based on the mass/charge ratio (m/z) in the mass spectra, we proposed the compounds and their structural formulas (Scheme 2). The signal at m/z of 284 presents before degradation is assigned to MB. Then, the cleavage of each methyl group on amine groups, in turn, leads to the formation of Azure B, Azure A, and thionin, with m/z of 268, 252, and 225, respectively (Figure 10(a)–10(e)). These products result from the reduction of the methyl group in the MB molecule via photocatalytic degradation [35, 43]. The peak at 1.8 min (Figure 10(f)) corresponding to m/z 192 is attributed to HO–C₆H₃–SO₃H(NH₃⁺). Accordingly, the decomposition is due to the cleavage of the bonds in the C–S⁺=C and C–N=C functional groups. The C–S⁺=C group is attacked by HO^• radicals and oxidized to the C–S(=O)–C sulfoxide group, causing the formation of a central imino group (–NH) at the para-position of the middle aromatic ring.

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The H atoms for the formation of C–H and N–H bonds may result from the deprotonation by photogenerated electrons to form H⁻ radicals [44]. The saturation of the two amino groups by H⁻ radicals produces substituted aniline. Then, the amino group can be replaced by an OH⁻ radical, forming the corresponding phenol and releasing an NH₂⁻ radical producing NH₃ and ammonium ions. The sulfoxide group can be attacked by HO⁻ radicals for the second time to produce sulfone. At the same time, the HO⁻ radicals cause ring cleavage; then, the sulfone group can be further attacked by HO⁻ radicals to produce sulfonic acid. The peak at 2.1 min (Figure 10(g)) corresponding to m/z of 164 is assigned to (HO)₂C₆H₃–N(CH₃)–CHO. Accordingly, the first step is due to the cleavage of the bonds in C–S⁺=C and C–N=C in Azure B, which are then attacked by HO^• radicals to form NH₂ and NH₂ groups. They are further attacked by HO radicals and form –OH groups in phenol, and the –CH₃ group in N(CH₃)₂ forms the –CHO group. Thus, the obtained structure is consistent with the structure of the fragment with m/z of 167, as proposed by Houas et al. [44]. The peak at 2.7 min (Figure 10(h)) corresponding to m/z of 137 has the proposed structure as (HO)₂C₆H₃–NH–CH₃. Accordingly, the –CHO group can be attacked by HO^• radicals to form the –COOH group. Then, the N–COOH group is attacked by h⁺ to form N^•, and it combines with H⁺ to form the –NH– group.

3.1.4. Mechanism of MB decomposition

The valence band (VB) and CB potential edges of g-C₃N₄ and TiO₂ were calculated according to the equations proposed by Xu and Schoonen [45] and Lin et al. [46] as follows:where E_CB is the CB edge energy, E_VB is the VB edge energy, χ is the electronegativity (χ = 4.73 eV for g-C₃N₄ and χ = 5.81 eV for TiO₂ [45]), E_e is the free energy of electrons with respect to the NHE (E_e = 4.5 eV [47]), and E_g is the bandgap energy of the semiconductor.

The values of E_CB and E_VB for g-C₃N₄, calculated from Equations (7) and (8), are –1.11 and 1.55 eV, and those for TiO₂ are –0.29 and 2.91 eV, respectively. The position of the edge energy of g-C₃N₄ and TiO₂ is shown in Scheme 3. First, MB is adsorbed by TiO₂/g-C₃N₄, and its aromatic rings interact with g-C₃N₄ or TiO₂ on the surface via π–π bonding [48]. Under visible-light illumination, the MB molecule can be excited, and the photoelectrons in HOMO of MB (–0.82 eV) migrate to LUMO of MB (–1.04 eV) (Equation (9)) [49]. These photoelectrons could transfer to the CB of TiO₂. The VB electrons of g-C₃N₄ would be excited, and the photoelectron–hole pairs are, thus, generated. These photoelectrons have a chance to transfer to the CB of TiO₂, which enhances the photoelectron–hole separation (Equation (10)). The electrons and holes could react with oxygen and water to form free radicals (Equations (11) and (12)). They, in turn, can decompose MB to different products with decreasing molecular mass under prolonged light illumination. MB can be completely mineralized to inorganic substances (CO₂, H₂O, Cl⁻, NO₃⁻, SO₄²⁻, and NH₄⁺), similar to what was reported by Houas et al. [44].

The following photocatalytic degradation mechanism for MB decomposition on the TiO₂/g-C₃N₄ catalyst is illustrated as follows:

3.2. Recyclability of TiO₂/g-C₃N₄

The catalyst’s reuse after a catalytic reaction is a critical requirement besides its high catalytic activity. The used catalyst was regenerated by calcining in the nitrogen atmosphere at 500°C for 3 hr. The SEM observation of the catalyst after the dark adsorption following the dye degradation is shown in Figures 11(a) and 11(b). As seen from the figure, the particles of the catalyst present the large agglomerates resulted in the combination of fine particles and dyes molecules. After the third recycle, fine particles are separated clearly indicating that the MB dyes are removed from the surface of (2/8)TiO₂/g-C₃N₄ due to the photocatalytical degradation of MB. The MB decomposition efficiency after three recycling is found as 98.5%, 93.0%, 87.5%, and 82.5%. It is obvious that the efficiency decreases gradually after each recycling. It drops only around 6% after three recyclings, compared with the first reaction. The XRD patterns of the catalyst remain unchanged, indicating that the catalyst is stable and promising for treating dyes wastewaters (Figure 11(c)).

(a)

(b)

(c)

(d)

3.3. Photocatalytic Activity TiO₂/g-C₃N₄ for Other Dyes

To confirm the photochemical degradation ability of TiO₂/g-C₃N₄ materials for other dyes under visible illumination, we also conducted other experiments with malachite green (MG), methyl blue (MyB), and methyl red (MR). The assessment of color degradation was based on the change in the main absorption peak’s intensity at λ_max of 617 nm for MG, 607 nm for MyB, and 521 nm for MR. Figure 11(d) shows that the TiO₂/g-C₃N₄ material exhibits high catalytic ability in the decomposition of these organic dyes, especially the cationic ones. The photochemical degradation efficiency of MG with the cationic configuration is 100% after 80 min of illumination. Methyl blue with the anionic configuration decomposes by 97.8% after 100 min, and MR with the neutral configuration decomposes by 65.9% after 120 min. This result shows that TiO₂/g-C₃N₄ is possibly a photocatalyst working in the visible light region to decompose organic dyes in aqueous media.

4. Conclusion

A novel and facile approach to the in situ synthesis of TiO₂/g-C₃N₄ nanoparticles with heterostructures and enhanced photocatalytic activity was demonstrated. The composites were synthesized from a homogeneous mixture of melamine and a water-soluble titanium (IV) complex, which initiates an intimate interfacial contact between TiO₂ nanoparticles and g-C₃N₄ nanosheets with a relatively large surface area. The obtained catalyst exhibits excellent catalytic activity toward methylene blue decomposition. The dye was degraded via a free radical mechanism in which the radical activity followed the order O₂^•− > ^•OH > h⁺. First, methylene blue was degraded by opening its central aromatic rings, then the oxidation of their subsequent metabolites, and finally, the evolution of CO₂. In addition, TiO₂/g-C₃N₄ also exhibited efficient photocatalytic degradation toward other dyes, such as malachite green, methyl red, and methyl blue. The TiO₂/g-C₃N₄ material is a promising catalyst for treating diluted wastewater in textile industries.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Funding

This work was financially supported by Hue University, Vietnam, under project code DHH 2021-04-151 and Van Lang University, Vietnam.

Acknowledgments

The authors (Nguyen Thi Thanh Tu and Dang Thi Ngoc Hoa) thank the financial support from Hue University and Van Lang University.

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Copyright © 2023 Dang Thi Ngoc Hoa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

Advances in Nanoporous-based Materials for Water Treatment

TiO2/g-C3N4 Visible-Light-Driven Photocatalyst for Methylene Blue Decomposition

Abstract

1. Introduction

2. Experimental

2.1. Materials

2.2. Synthesis of g-C3N4

2.3. Synthesis of Peroxo-Titanium Complexes

2.4. Synthesis of TiO2/g-C3N4

2.5. Material Characterization

2.6. Catalytic Activity

3. Results and Discussion

3.1. Visible-Light-Driven Photocatalyst for Methylene Blue (MB) Decomposition

3.1.1. Kinetics of MB Decolorization

3.1.2. Effect of pH and Some Scavengers on MB Decomposition

3.1.3. Photocatalytic Degradation Pathway of MB

3.1.4. Mechanism of MB decomposition

3.2. Recyclability of TiO2/g-C3N4

3.3. Photocatalytic Activity TiO2/g-C3N4 for Other Dyes

4. Conclusion

Data Availability

Conflicts of Interest

Funding

Acknowledgments

References

Copyright

TiO₂/g-C₃N₄ Visible-Light-Driven Photocatalyst for Methylene Blue Decomposition

2.2. Synthesis of g-C₃N₄

2.4. Synthesis of TiO₂/g-C₃N₄

3.2. Recyclability of TiO₂/g-C₃N₄

3.3. Photocatalytic Activity TiO₂/g-C₃N₄ for Other Dyes