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Article

Solvothermal Synthesis of g-C3N4/TiO2 Hybrid Photocatalyst with a Broaden Activation Spectrum

by
Amit Imbar
,
Vinod Kumar Vadivel
and
Hadas Mamane
*
Environmental Engineering Program, School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(1), 46; https://doi.org/10.3390/catal13010046
Submission received: 13 November 2022 / Revised: 19 December 2022 / Accepted: 20 December 2022 / Published: 26 December 2022
(This article belongs to the Special Issue Advances in Heterojunction Photocatalysts)
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Abstract

:
A solvothermal self-made composite of graphitic carbon nitride (g-C3N4) and commercially available titanium dioxide (TiO2) demonstrated the removal of commercial acid green-25 (AG-25) textile dye in a saline water matrix when activated by ultraviolet (UV) and visible light. The g-C3N4-TiO2 composite was characterized by X-ray diffraction (XRD), Nitrogen sorption–desorption recording and modeling by the Brunauer–Emmett–Teller (BET) theory, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and electron spin resonance (ESR). The solvothermal process did not modify the crystalline structure of the g-C3N4 and TiO2 but enhanced the surface area by interlayer delamination of g-C3N4. Under a simulated solar spectrum (including UVA/B and vis wavelengths), the degradation rate of AG-25 by the composite was two and four times higher than that of TiO2 and pure g-C3N4, respectively (0.04, 0.02, and 0.01 min−1). Unlike TiO2, the g-C3N4-TiO2 composite was activated with visible light (the UV portion of the solar spectrum was filtered out). This work provides insight into the contribution of various reactive oxidative species (ROS) to the degradation of AG-25 by the composite.

Graphical Abstract

1. Introduction

The textile industry is one of the world’s largest industries, providing jobs in low-income countries and playing a vital role in their economy. Over 100,000 synthetic dyes produced annually generate 7 × 105 tons of dyes [1,2]. The textile industry consumes large amounts of water and mainly contributes to wastewater production [3]. Due to process inefficiency, 10–15% of the dyes are washed out into sewer or water reservoir [4]. Synthetic dyes in water reservoirs pose a toxic threat to living organisms and vegetation [5] and hinder photosynthesis by blocking light penetration and increasing biological oxygen demand (BOD) [6,7]. Physical separation methods used in wastewater treatment (such as filtration and sorption) transfer the dyes from the water to a solid phase [8], biodegradation is generally inefficient in dye removal, and the use of oxidants (such as chlorine and ozone) is expensive [4,8]. Advanced oxidation processes (AOPs) are based on reactive oxygen treatment technologies aimed at degrading recalcitrant organic compounds in water through reaction with highly reactive species formed from O2 as hydroxyl radical (OH·) and superoxide (O2−) that breakdown the organic compounds [9]. Specifically, photocatalysis is of high potential due to the complete removal of organic compounds [10] and the capability to avoid by-products and can be executed without consuming chemicals. The main drawback of photocatalysis is the requirement for intensive energy and light for activation which plays a crucial role in the photocatalytic efficiency and applicability.
The use of TiO2 in water treatment has been extensively reported in the literature; it is inexpensive, non-toxic, and chemically stable [11,12,13,14,15]. Its main drawbacks are the high recombination rate of electron-hole (e−/h+) pairs and wide band gap (BG), ≥3.0 eV. This BG allows electron excitation only by UV wavelengths, making solar light inefficient as only 4% of the solar spectrum lies in the UV spectrum [16,17], making the process energy intensive [18,19].
Coupling photocatalysts to form heterojunctions is a known method for improving photocatalytic activity (PCA) and energy efficiency [20]. Graphitic carbon nitride (g-C3N4) gained interest due to its photocatalytic (PC) properties (BG of 2.7 eV), biocompatibility, low cost and easy production. g-C3N4 combined with TiO2 showed improved PCA [21,22,23,24,25,26,27] while allowing a broader range of solar spectrum use.
In the past decades, many reports of photocatalysts have been published. However, they mainly focused on pure contaminants in de-ionized (DI) water matrices and pure dyes. These conditions are far from the purity of the dyes used in textile dyeing and the water quality of the effluent discharged. At the same time, the existence of other ions is proved to be harmful in photocatalysis [28]. This study demonstrates the solvothermal coupling method of TiO2 and g-C3N4 for PC degradation of a commercial AG-25, a textile dye used for fabric coloring in a saline water matrix, imitating the salinity and electrical conductivity of dye house effluents.

2. Results and Discussion

2.1. X-ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) analysis is a technique used in materials science to determine the crystallographic structure of a material. The prepared PC composite (g-C3N4/TiO2) was analyzed by an XR diffractometer. The clear, sharp peaks of the diffraction pattern and low background noise indicate that the obtained product is of high purity. The resulting sharp peaks in Figure 1 reveal major intensive diffraction peaks for TiO2 at 25.6°, corresponding to the (111) planes, confirming that the crystalline structure matches the anatase phase of TiO2, as analyzed using JCPDS [29]. The graphitic carbon nitride has major intensive peaks at 12.3° and 26.7°, corresponding to the (100) and (200) planes as reported by Miranda et al. [24]. These are attributed to the planar, which repeats the cell structure and sandwich stack reflection of g-C3N4. After compositing with TiO2, the significant peaks of g-C3N4 still had decreased peak intensity. Thus, the generation of g-C3N4/TiO2 nanostructures can confirm by these structural characterizations.

2.2. Brunauer-Emmett-Teller (BET) Analysis

Adsorption and desorption experiments were carried out at 77 K, and N2 isotherms were applied to calculate the specific surface area. The surface area, pore size, and volume distribution of the g-C3N4/TiO2 composite were measured by the multi-point BET and BJH (Barrett, Joyner, and Halenda) method. As shown in Figure S3, mesoporous g-C3N4/TiO2 nanostructures are evident with an estimated pore volume of 0.039 cc/g, pore radius of ~2.038 nm, and pore size of 4.08 nm. As shown in Figure S4, the g-C3N4/TiO2 composite shows a hysteresis loop with a type H3 shape and shifts at high relative pressures (P/P0) between 0.8 and 1.0, which suggests a mesoporous structure. Mesoporosity is typical of wrinkled, sheetlike particles, and this result is consistent with SEM images [30]. The surface area was 15.43 m2 g−1, higher than obtained in our previous study (Kumar et al. [31]) for sole g-C3N4 after solvothermal treatment. The different results are possibly rooted in the large amount of TiO2 in the composite, which is smaller than g-C3N4, as shown in Figure 2.

2.3. Scanning Electron Microscopy (SEM)

EDX analysis was performed to obtain the composite composition. Figures S5–S7 show that the sheet-shaped particles are primarily composed of carbon and nitrogen, while the sphere-shaped particles are composed mainly of titanium and oxygen. The composition and the shape of the sheet-shaped particles both confirm that these particles are of g-C3N4. In contrast, the composition of the spherical particles is that of TiO2.
The SEM images of as-synthesized g-C3N4/TiO2 structures are presented in Figure 2. After compositing TiO2 with g-C3N4, TiO2 nanoparticles uniformly adhered to g-C3N4. However, g-C3N4 has a wrinkled sheetlike morphology, as reported in former publications [31,32,33], whereas TiO2 has smaller and sphere-shaped particles. g-C3N4 has a crystalline structure similar to the solvothermal-treated g-C3N4 and contrary to the untreated g-C3N4 [31]. This combination could promote the charge transfer between g-C3N4 and TiO2 by the Z-scheme route [30], as demonstrated in Figure 3.

2.4. Absorption Spectrum and DRS Analysis

UV–vis diffuse reflectance spectra were recorded to probe the prepared photocatalysts’ optical properties. As shown in Figure S8b, the absorption edges of g-C3N4, TiO2 and the composite are located at approximately 389, 334 and 340 nm, respectively. Compared to TiO2, the absorption edge positions of the composite exhibited a red shift toward the visible range alongside an extension of the absorbance spectrum toward ~430 nm wavelengths due to the g-C3N4 introduction with TiO2 [34]. Furthermore, the band gaps of photocatalysts could be determined by the following equation:
αhν = A(hν − Eg)n/2
where α, A, h, ν, and Eg represent the absorption coefficient, proportionality constant, Planck constant, light frequency, and band gap energy, respectively [35]. A plot based on the Kubelka–Munk function (Equation (1)) versus the energy of light is shown in Figure S8a. The estimated band gap values of the photocatalysts are about 2.80, 2.86, and 3.19 eV for g-C3N4, composite, and TiO2, respectively. The composite shows a narrower BG than TiO2 and is slightly wider than g-C3N4. As the band gap increases, the recombination of e−/h+ pairs decreases, enhancing the ROSs production, and the narrowing of the BG (compared with TiO2) enables activation by the visible-light region. Therefore, the UV–vis diffuse reflectance spectroscopy (DRS) results indicated more photogenerated charges when activating the composite are excited under UV–vis spectrum irradiation, which enhances their photocatalytic performance [36].

2.5. X-ray Photoelectron Spectroscopy (XPS)

Figure 4 shows the XPS spectra of g-C3N4, TiO2 and the composite. The full XPS spectrum presented all the peaks (C 1s, N 1s, Ti 2p, O 1s) of g-C3N4, TiO2 plus composite (TiO2/g-C3N4) nanomaterials. Thus, the generation of g-C3N4/TiO2 nanostructures can be confirmed by these structural characterizations and well supports the XRD analysis in Figure 1. However, the in the valence band (VB) of TiO2 and g-C3N4 is directly excited to its conduction band (CB) by absorbing photons with energy higher than or equal to its energy gap (Eg) and meanwhile resulting in the generation of a positive hole in the VB. Compared to typical photocatalysts such as TiO2, g-C3N4 has the most negative CB value of −1.69 eV versus the normal hydrogen electrode (NHE) and a neutral band gap (~3.19 eV), summarized in Table S2. Furthermore, a detailed analysis of the XPS valence band spectra confronted with band structure calculations was achieved for the three photocatalysts TiO2, g-C3N4 and composite (TiO2/g-C3N4). Furthermore, through valence band spectra analysis, conduction band offset (−0.63 eV) and valence band offset (2.23 eV) of g-C3N4–TiO2 heterojunction were estimated (Figure S9). The band gap (ECB) and valence band (EVB) positions of the photocatalyst materials can be calculated by the following equations:
ECB = X − EC − ½Eg
EVB = ECB + Eg
where EVB and ECB represent the valence band and conduction band position of the semiconductor material, EC is the hydrogen electron free energy (4.5 eV), and X represents the electronegativity of the semiconductor material.

2.6. Photo Luminescence (PL)

Figure 5 shows PL of g-C3N4, TiO2 and the composite. The spectrum shows emission at 350 nm corresponding to the recombination of e−/h+ pairs, with the intensity directly proportional to the photogenerated e−/h+ recombination in the catalyst. The composite’s intensity is between the two substances of origin, a decrease in intensity was obtained with the composite compared with g-C3N4 and increased intensity compared with TiO2.

2.7. Electron Spin Resonance Spectroscopy (ESR)

The production of ROSs by the g-C3N4, TiO2 and the composite under solar light was studied by ESR spin trap experiments. Aqueous suspensions of each catalyst were irradiated separately under solar light in the presence of 5-methyl-1-pyrroline N-oxide (BMPO), which traps both hydroxyl and superoxide radicals. To distinguish the signal corresponding to hydroxyl radical, dimethyl sulfoxide (DMSO), a hydroxyl radical scavenger, was added to the catalysts-BMPO suspensions.
The resulting signals are shown in Figure 6. When only BMPO is present, the signals show the formation of ROSs (either OH⸱ or O2−) by the irradiation of all three substances. The composite’s signals almost utterly correspond to those of BMPO-OOH, implying that it produces superoxide radicals. The consistent intensity of the signal suggests that the composite does not produce hydroxyl radicals.

2.8. Photocatalytic Degradation of AG-25 under Solar Irradiation in Saline Water

10-ppm AG-25 in saline water matrix was used as a target pollutant to study the degradation by the catalysts. The catalyst was stirred in the solution for 30 min (time −30 to 0) under dark conditions for sorption control, followed by simulated solar irradiation for 90 min (time 0 to 90). Under dark conditions, 16, 12, and 11% removal of AG-25 were observed by the composite, g-C3N4, and TiO2, respectively. The composite has an increased surface area, as mentioned in Section 3.2, promoting higher sorption over the catalyst.
Figure 7 shows the degradation rate of AG-25 by the three catalysts. AG-25 total removal by g-C3N4, TiO2, and the composite were 59, 86, and 98%, respectively, implying that all three catalysts can degrade AG-25. The degradation rate of AG-25 using the composite was twice as high as commercially available TiO2 and four times higher than that of pure g-C3N4. The higher results obtained by TiO2 and the composite can be explained by the type of radicals formed by the catalyst. TiO2 primarily produces hydroxyl radicals [37,38], which have higher oxidation potential than superoxide radicals, the main ROS produced by g-C3N4. Moreover, the PL results show significant recombination in g-C3N4 compared with TiO2. However, coupling the two catalysts in the composite can form a heterojunction that suppresses the recombination rate and improves photon energy usage, as shown in PL spectra in Figure 5 and the broadened absorption spectrum demonstrated in Figure S8, resulting in enhanced PCA [39].
Table 1 summarizes the degradation rate and total degradation of each catalyst. Adly et al. [40] degraded AG-25 with graphene oxide/titanium dioxide composites (GO-TiO2) under UV–vis irradiation, reporting 40% removal (vs. 98% in our work) of AG-25 after 90 min by a similar concentration of catalysts but a higher concentration of AG-25.

2.9. Photocatalytic Degradation of AG-25 under Solar Irradiation in Saline Water with a UV Cutoff

The role of UV and visible light in PCA was examined by degrading 10-ppm AG-25 in saline water under simulated solar irradiation, similar to Section 2.8, with a UV filter for UV cutoff. Figure 8 presents the degradation rate of AG-25 by the composite, g-C3N4, and TiO2 under the visible spectrum. AG-25 degradation by g-C3N4, TiO2, and the composite after 90 min was 37%, 0%, and 31%, respectively, demonstrating that TiO2 is a UV-driven catalyst. Moreover, a higher g-C3N4 ratio in the catalysts resulted in higher visible-based PCA in the absence of UV wavelengths. These results show the composite’s versatility as both a UV and a visible light-activated catalyst. In both cases (with and without the LP400 filter), the composite is superior to the commercial TiO2, while the cutoff results show increased degradation (37% compared with 31% removal) by g-C3N4. However, comparing the full spectrum results (without the LP400 filter) shows a clear advantage of the composite over pure g-C3N4. Generally, the composite’s versatile activation spectrum and improved results under total spectrum irradiation are advantageous.

3. Materials and Methods

3.1. Materials

Melamine and Titanium dioxide (99% purity) were purchased from Sigma-Aldrich, Darmstadt, Germany.

Acid Green-25 Textile Dye

AG-25, a light green textile acid dye, was provided by Colourtex Industries Ltd., Hamburg, Germany, and has a molecular formula of C28H20N2Na2O8S2. AG-25 stock solution of 300-ppm AG-25 was prepared and diluted with deionized (DI) water to reach a concentration of 10 ppm used for the experiments. The conductivity was adjusted to 36 mS cm-1 using NaCl to simulate a real textile dye effluent. Due to AG-25 physical properties, the concentration of AG-25 could be determined by spectroscopic measurement of the visible light spectrum. The maximum absorbance wavelength value of the dye solutions was 614 nm (Evolution 220 UV–Visible spectrophotometer, Thermo Scientific, Waltham, MA, USA) to determine the AG-25 concentration.
The removal of AG-25 was calculated using Equation (4).
AG-25 Removal = (Ci − Ct)/Ci
Here, Ci stands for the initial concentration, and Ct stands for concentrating the samples.

3.2. Characterization

The TiO2/g-C3N4 powder crystalline structure was characterized by a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with a secondary graphite monochromator, 2° Soller slits, and a 0.2 mm receiving slit. Low-background quartz sample holders were carefully filled with the powdered samples. The nanoparticle morphology and average particle size were further investigated by a Quanta 200 FEG environmental scanning electron microscope equipped with a field-emission electron gun (FEG). The samples were coated with a conductive carbon grid and imaged in a Hitachi S3200N SEM-EDS system at 20 kV accelerating voltage. The surface area, pore size and volume distribution of the modified and unmodified g-C3N4 were measured by N2 adsorption–desorption and the BJH (Barrett, Joyner, and Halenda) method, respectively. To probe the optical properties of the prepared photocatalysts, UV–visible diffuse reflectance spectra were recorded using a UV–Vis–NIR spectrophotometer (Cary 5000 + UMA, Agilent, Santa Clara, CA, USA).

3.3. Preparation of g-C3N4

g-C3N4 was synthesized via the thermal-condensation method. In short, 4 g melamine (Sigma-Aldrich, 99%) was heated to 540 °C in a packed aluminum crucible under ambient pressure in the air for 2 h with a heating rate of 15 °C min−1 in a muffle furnace. The resultant powder reached room temperature and was washed with acetone.

3.4. Solvothermal Method

TiO2 and g-C3N4 (named composite) were coupled via the solvothermal method. The TiO2-g-C3N4 mixture (2:1 ratio, 1 g) was stirred for 3 h in ethanol (70%) in a 50 mL PTFE pressure-tight liner at ~65 °C. Subsequently, the PTFE liner was placed in a stainless-steel jacket and heated to 180 °C for 10 h. The resulting catalyst was filtered and dried before usage.

3.5. Photocatalytic Degradation of AG-25 in Saline Water

The PC experiments were conducted under a 300-W solar simulator, ozone-free xenon arc lamp (Newport full-spectrum, 50.8 mm × 50.8 mm, Irvine, CA USA) equipped with a 1.5 Global Air Mass filter to remove infrared light, as shown in Figure S1. The xenon lamp’s incident spectral irradiance and photon fluence are recorded by a spectroradiometer (International Light, ILT 900R, Peabody, MA, USA), as in Figure S2. The integrated incident irradiance was 2.865 and 62.737 mW cm−2 for the UV (280–400 nm) and visible (400–1251 nm) ranges, respectively.
The PC experiments were conducted using AG-25 (100 mL, 10 ppm) in a crystalline cylinder (70 × 50 mm). The powdered catalyst (500 ppm) was sonicated and added to the AG-25 solution under stirring. The solution was constantly mixed under dark conditions for 30 min, followed by 90 min under solar-like irradiation. Samples of the slurry solution were taken in 30 min intervals and centrifuged (14600 RPM, 10 min) to separate the catalyst from the solution. The samples were analyzed in a spectroradiometer (Evolution 220 UV–Visible spectrophotometer, Thermo Scientific) to determine the AG-25 concentration.

3.6. Photocatalytic Degradation of AG-25 with UV Cutoff—LP400

Distinguishing between PCA driven by visible light and UV irradiation was performed similarly to as in Section 2.5, using a UV filter (400 HLP/130 mm, OMEGA OPTICAL, Brattleboro, VT, USA) to block UV wavelengths (i.e., λ < 400 nm) from reaching the slurry.

4. Conclusions

The composite showed improved photocatalytic activity compared with TiO2 and g-C3N4 under solar irradiation. As demonstrated by the BET results, the solvothermal method increased the surface area resulting in increased photocatalytic activity relative to the origin catalysts, reflected in the enhanced degradation rate of AG-25. The UV cutoff experiments showed a versatile spectrum ranging from UV to the visible spectrum, allowing composite activation at various wavelengths. This study provides a simple synthesis method of a composite that can efficiently remove acid-dye effluents in a versatile spectrum of wavelengths.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13010046/s1, Figure S1: Newport solar simulator, Figure S2: Spectral incident irradiance, Figure S3: Pore size distribution and volume curves, Figure S4: BET sorption\desorption curves, Figures S5–S7: EDX elemental analysis, Figure S8: UV−visible DRS spectra, Figure S9: XPS, S10:ESR, Table S1: XRD, Table S2: XPS.

Author Contributions

Supervision, reviewing and project administration, H.M.; methodology, V.K.V.; investigation, V.K.V. and A.I.; writing—original draft, A.I.; writing—review and editing, H.M., V.K.V. and A.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of the composite, TiO2 and g-C3N4.
Figure 1. XRD patterns of the composite, TiO2 and g-C3N4.
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Figure 2. SEM images of the composite. (a) magnification of 104 and (b) magnification of 2500.
Figure 2. SEM images of the composite. (a) magnification of 104 and (b) magnification of 2500.
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Figure 3. Z−scheme of TiO2 and g-C3N4.
Figure 3. Z−scheme of TiO2 and g-C3N4.
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Figure 4. Full XPS spectrum presented all the peaks (C 1s, N 1s, Ti 2p, O 1s) of g-C3N4, TiO2 and the composite (TiO2/g-C3N4) nanomaterials.
Figure 4. Full XPS spectrum presented all the peaks (C 1s, N 1s, Ti 2p, O 1s) of g-C3N4, TiO2 and the composite (TiO2/g-C3N4) nanomaterials.
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Figure 5. PL spectra of g-C3N4, TiO2 and the composite.
Figure 5. PL spectra of g-C3N4, TiO2 and the composite.
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Figure 6. BMPO-OOH with (red line) and without (black line) DMSO signal from the suspension of the composite.
Figure 6. BMPO-OOH with (red line) and without (black line) DMSO signal from the suspension of the composite.
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Figure 7. Photocatalytic degradation rate of 100 mL AG−25 10 ppm in brine for 90 min under solar irradiation with (filled shapes) and without (hollow shapes) UV spectrum cutoff by the composite (▲, ∆), g-C3N4 (●, ◌), and TiO2 (■, □).
Figure 7. Photocatalytic degradation rate of 100 mL AG−25 10 ppm in brine for 90 min under solar irradiation with (filled shapes) and without (hollow shapes) UV spectrum cutoff by the composite (▲, ∆), g-C3N4 (●, ◌), and TiO2 (■, □).
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Figure 8. Segmentation of sorption (blue background), visible light (orange background), and UV (grey background) contribution to the total degradation of 100 mL AG-25 10 ppm by TiO2, g-C3N4 and the composite after 90 min irradiation.
Figure 8. Segmentation of sorption (blue background), visible light (orange background), and UV (grey background) contribution to the total degradation of 100 mL AG-25 10 ppm by TiO2, g-C3N4 and the composite after 90 min irradiation.
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Table
Table 1. Total degradation (after 90 min) and degradation rate of AG-25 by coupled and uncoupled g-C3N4 -TiO2 under solar irradiation, with and without LP400 filter.
Table 1. Total degradation (after 90 min) and degradation rate of AG-25 by coupled and uncoupled g-C3N4 -TiO2 under solar irradiation, with and without LP400 filter.
CatalystPercent Degradation of AG-25 (%)Rate of AG-25 Degradation (min−1) × 10−3
g-C3N459%10.6
TiO286%21.5
Composite98%40.1
g-C3N4—LP40037%5.8
TiO2—LP4000%0
Composite—LP40031%4.7
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Imbar, A.; Vadivel, V.K.; Mamane, H. Solvothermal Synthesis of g-C3N4/TiO2 Hybrid Photocatalyst with a Broaden Activation Spectrum. Catalysts 2023, 13, 46. https://doi.org/10.3390/catal13010046

AMA Style

Imbar A, Vadivel VK, Mamane H. Solvothermal Synthesis of g-C3N4/TiO2 Hybrid Photocatalyst with a Broaden Activation Spectrum. Catalysts. 2023; 13(1):46. https://doi.org/10.3390/catal13010046

Chicago/Turabian Style

Imbar, Amit, Vinod Kumar Vadivel, and Hadas Mamane. 2023. "Solvothermal Synthesis of g-C3N4/TiO2 Hybrid Photocatalyst with a Broaden Activation Spectrum" Catalysts 13, no. 1: 46. https://doi.org/10.3390/catal13010046

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