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. 2024 Jun 12;9(25):26983–27001. doi: 10.1021/acsomega.3c10281

Highly Efficient Photocatalytic Degradation of Imidacloprid Based on Iron Metal–Organic Frameworks of Mesoporous NH2-MIL-88b/Graphite Carbon Nitride Nanocomposites by Visible Light Driven in Aqueous Media

Elham Chiani †, Shahram Ghasemi ‡,*, Seyed Naser Azizi †
PMCID: PMC11209690  PMID: 38947846

Abstract

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Pesticides that protect crops from insects and other pests are some of the main causes of water pollution. Imidacloprid (IMC) is the most widely used insecticide in the world and should be removed from the environment. This work aims to prepare mesoporous nanocomposites to increase the photodegradation efficiency of IMC. To improve the surface properties and enhance the photocatalytic activity, mesoporous nanocomposites with different weight ratios of graphite carbon nitride (CN = 125, 250, and 500 mg) were prepared by the solvothermal method. Mesoporous NH2-MIL-88b(Fe)/graphite carbon nitride (CN = 250 mg, NH2-MCN-2) nanocomposites showed the best photocatalytic performance. To save the time and cost of the experiments, central composite design (CCD) and response surface methodology (RSM) were used and the results were obtained as the initial concentration of IMC (20 mg L–1), amount of photocatalyst (0.76 g L–1), pH = 5, and degradation time ∼46 min. The maximum photocatalytic degradation efficiency estimated by the model was obtained at 96.31%, which is very close to the actual value of 95.47%. The mesoporous NH2-MCN-2 nanocomposite showed excellent stability and suitable reusability with a maximum degradation of 84.5% after five cycles. Results obtained from kinetic studies indicated a rate constant value of 0.08 min–1, and isotherm models showed that equilibrium data are more consistent with the Langmuir model in photocatalytic degradation. Electrochemical experiments showed significant improvement in the electron transfer rate and photocatalytic activity of the mesoporous NH2-MCN-2 nanocomposite. Different trapping agents were used to investigate the effective active species in IMC photodegradation, and it was determined that the hole (h+) and OH radical (•OH) play the main role. The possible mechanism for IMC photocatalytic degradation was suggested by Mott–Schottky (M-S) electrochemical impedance.

1. Introduction

Pesticides are chemicals used to control weeds and harmful insects.1 Although the use of insecticides and herbicides in the production of agricultural products increases the yield and improves the quality of farm products, they have destructive effects that cannot be ignored.2

Based on the results of research, imidacloprid (IMC) is considered one of the most dangerous insecticides for the environment,3 and among the top 10 global agrochemicals, it is ranked second.4 IMC is extensively utilized for pest control in agriculture, gardens, and lawns, foundations to prevent termite damage, flea control in domestic dogs,4 protecting trees from boring insects,3 etc. IMC decomposes very slowly in the dark at pH between 5 and 7, and its half-life at pH = 9 is about 1 year; therefore, it remains in underground water for a long time. The stability of IMC in soil under aerobic conditions is 1 to 3 years with a half-life of 39 days on the soil surface. Due to the high use of IMC, it usually enters the water flow as a spray drift or runoff after use and its high solubility in water (0.61 g L–1)5 remains stable approximately between 30 and 50 days, making it a severe pollutant for water. This insecticide is a neonicotinoid that disrupts the transmission of stimuli in the nervous system of insects. IMC poses severe threat to human life in addition to its killing effect on insects.6 It causes severe damage to human skin and eyes.4 Also, the effects of poisoning with IMC can be seen in the form of fatigue, irritability, diarrhea, and muscle weakness, especially in the respiratory muscles.7

Various methods, such as microbial decomposition,8 adsorption,9 electrochemistry,10 ozonation,11 and advanced oxidation processes (AOPs),12 have been used to remove poisons from the aquatic environment. AOPs are one of the most effective methods in the decomposition of resistant pollutants, which have received much attention in recent years. Active species of hydroxyl and sulfate radicals have high reactivity due to having free electrons. AOP is widely used in wastewater treatment for the degradation of various pollutants with complex and rigid structures that are not quickly and completely decomposed by conventional methods. This process has very high power in material degradation such as aromatic compounds, petroleum, organic and inorganic materials with strong bonds, etc. After complete degradation by AOP, these pollutants turn into harmless compounds such as H2O and CO2.

Recently, the use of efficient photocatalysts based on visible light has received particular attention for water purification. Graphite carbon nitride (g-C3N4), which is a metal-free semiconductor, has a structure similar to that of graphene. This structure is used for various applications such as reducing carbon dioxide,13 water splitting,14 and degrading pollutants.15 g-C3N4 is a photocatalyst with a suitable energy gap (ca. 2.7 eV)14 that responds to visible light. Its advantages include nontoxicity, chemical stability, simple fabrication method, easy preparation from available raw materials, and high solar absorption. However, g-C3N4, due to its low specific surface area, exhibits low performance in practical applications. To decrease the rate of recombination of the produced charge carriers (electron–hole pair), improve sensitivity to light, increase stability, and as a result, increase the activity of its photocatalyst, the use of approaches such as connecting with other semiconductors, doping, loading with a co-catalyst, utilization of ionic and non-ionic surfactants, and preparation of various composites with compounds like metal–organic frameworks (MOFs), has been studied.16

On the other hand, the research in recent decades shows the widespread use of MOFs as photocatalysts for the degradation of pollutants in aqueous environments.15 MOFs are crystal structures that are formed by the connection of metal ions and organic ligands. These frameworks have a unique design capability, and it is easy to synthesize and achieve different physicochemical properties and pore size by changing the ligand or the central metal. Although active carbon and zeolites have a high surface area, a tiny dead volume in MOFs has made them more porous than active carbon and zeolites.17

Also, the use of prefunctionalized MOFs with additional functional groups such as amine and hydroxyl has received much attention. Among the vast family of MOFs, MIL-88 (Lavazier Institute materials) has a high absorption capacity due to its large pore volume. Its high chemical and thermal stability has made it widely used in various processes.18,19 In particular, amine-doped MOFs have a high affinity for acid gas molecules and active sites for catalysis.20 NH2-MIL-88b (Fe) is a subset of MIL compounds introduced to heterogeneous photocatalytic systems due to its high surface area, unsaturated metal sites, specific pore size, and amine groups to improve light absorption.

It is expected that MOF/g-C3N4 composites improve photocatalytic activity by increasing the surface area and reducing the recombination rate of photogenerated charges. For example, in NH2-MIL-101(Fe)@C3N4, an efficient transfer of photogenerated charge carriers between the constituent components of the composite can be observed; because of these properties, the photocatalyst can be used in photodegradation of many kinds of organic and inorganic pollutants. Zhao et al. proposed the photocatalytic mechanisms for the reduction of Cr(VI) and oxidation of bisphenol A by NH2-MIL-101(Fe)/C3N4.21 The introduction of g-C3N4 into NH2-MILbase on Fe increased both light absorption and interfacial charge carrier separation.22−24

Experimental design includes mathematical techniques used for statistical modeling and systematic analysis.25 The response surface methodology (RSM) in experimental design is superior to the customary time-consuming approach of one variable at a time due to the reduction in the number of experiments, the increase in speed, and the systematicity of processing variables.26,27 Box–Wilson’s second-order central compound design (CCD) is an efficient and RSM standard design that has attracted many researchers’ attention; for example, Kumar et al. used the CCD approach to optimizing the separation of leukemic acid by reactive components.28 Sarai et al. evaluated and optimized the effect of various factors on total organic carbon (TOC) degradation using the RSM based on CCD.29

In recent decades, MOFs as a new category of porous materials have found a good perspective in the preparation of composites for photocatalytic applications due to their high structural diversity and suitable energy gap in the visible region. Among the large number of reported structures, MOFs based on carboxylate ligands and high-valent metal ions such as Fe (ΙΙΙ) have attracted the attention of many researchers due to their unique physical and chemical properties.

One of the important factors in reducing the rate of recombination of charge carriers produced in the photocatalytic process is the surface area of the photocatalyst. As the surface area increases, the recombination rate decreases. Since g-C3N4 has a high recombination rate, forming a composite with NH2-MIL88b (Fe) with its appropriate specific surface area leads to a higher surface area compared to g-C3N4 alone and improves the photocatalytic performance. As the specific surface area of mesoporous nanocomposites increases, the recombination rate decreases compared to g-C3N4 alone.

For this purpose, in this work, for the first time, mesoporous nanocomposites based on iron-MOF and amine-functionalized carboxylate ligand, which is an effective structural block, were studied with g-C3N4 for efficient photocatalytic degradation of IMC from aqueous solutions. Mesoporous NH2-MCN nanocomposites were synthesized by the solvothermal method by introducing various weight ratios of g-C3N4 (CN = 125, 250, and 500 mg). Among these different composites, NH2-MIL-88b(Fe)/graphite carbon nitride (CN = 250 mg, NH2-MCN-2) showed the best photocatalytic performance due to its higher specific surface area and lower energy gap. In the following, the statistical method of CCD was used to measure the effects of different factors. With RSM, the impact of the independent variables on the response was examined with suitable optimum conditions. Typically, a 20 mg L–1 (ppm) IMC solution was degraded up to 95.47% by the mesoporous NH2-MCN-2 nanocomposite. NH2-MCN-2 showed excellent stability and reusability for five recycling cycles under the same experiment conditions. In the following, the kinetics of the reaction was investigated by pseudo-first-order and pseudo-second-order models. In addition, the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich adsorption isotherm models were also studied. Electrochemical impedance spectroscopy (EIS) and photocurrent responses of mesoporous NH2-MCN-2 nanocomposites, NH2-MIL-88b(Fe), and g-C3N4 were also reviewed. A significant improvement in the electron transfer rate and photocatalytic activity of NH2-MCN-2 compared to g-C3N4 and NH2-MIL-88b(Fe) was observed. To investigate the effective reactive species in the photodegradation process of IMC, different trapping agents were used, and the efficient role of h+ and •OH in the photodegradation process was determined. The proposed mechanism was investigated based on the results achieved from M-S electrochemical impedance.

2. Experimental Section

2.1. Chemicals and Reagents

Urea (CH4N2O, >99%), iron trichloride hexahydrate (FeCl3·6H2O, ≥97%), sodium sulfate (Na2SO4, >99.5%), potassium hexacyanoferrate (III) (K3Fe(CN)6, >99%), potassium hexacyanoferrate (II)(K4Fe(CN)6, >99%), potassium chloride (KCl, >99.5%), methanol (CH3OH, 98%) and ethanol (C2H5OH, 98%) were supplied from Merck. 2-Aminoterephthalic acid (NH2-BDC, 99%,), N,N′-dimethylformamide (DMF, ≥99%), imidacloprid (IMC, ≥98%), ammonium oxalate (AO, >99%), isopropyl alcohol (IPA, ≥99.5%), 1,4-benzoquinone (BQ, ≥97%), and silver nitrate (AgNO3, ≥99%) were purchased from Sigma-Aldrich.

2.2. Preparation of g-C3N4, NH2-MIL-88b(Fe), and Mesoporous NH2-MCN Nanocomposites

g-C3N4 was synthesized through thermal polycondensation of urea. Ten grams of urea was heated in an alumina crucible with a cover at a rate of 10 °C min–1 to 550 °C and then calcined at this temperature for 3 h in an ambient air atmosphere. After natural cooling to room temperature, a light yellow powder product was obtained and used for the synthesis of composites.

Using a thermal solvent process, iron salt (FeCl3·6H2O) and NH2-BDC were used as a ligand to bind to iron sites for the synthesis of NH2-MIL-88b (Fe) with a hexahedral structure. For this purpose, 0.74 g of iron salt was dissolved with 0.25 g of H2BDC in 30 mL of DMF, subjected to ultrasonic waves for 30 min, and then heated in a stainless steel autoclave with Teflon coating for 24 h at a temperature of 110 °C. The obtained product was naturally cooled to room temperature, then washed with methanol, and dried in vacuum at 60 °C for 12 h. After grinding, reddish-brown dry powder NH2-MIL-88b(Fe) was obtained.

Mesoporous NH2-MCN nanocomposites were prepared by a solvothermal method. Different weight ratios of g-C3N4 (CN = 125, 250, and 500 mg) were added to 30 mL of DMF and ultrasonicated for 30 min. Then, 0.25 g of NH2-BDC and 0.74 g of FeCl3·6H2O were added to the above solution, and the mixture was ultrasonicated for another 30 min. The solution was then added to a 100 mL Teflon-lined stainless steel autoclave and heated in an oven at 110 °C for 24 h. After the reaction, the product was separated by a centrifuge, washed several times with methanol, and finally dried in a vacuum oven at 60 °C for 12 h. Final products were labeled according to the weight ratios of g-C3N4 in mesoporous nanocomposites as NH2-MCN-1, NH2-MCN-2, and NH2-MCN-3, respectively.

2.3. Characterization

An advanced X-ray diffractometer D8 (XRD, Philips PW1730) with a scanning speed of 0.05°/s and Cu Kα radiation (λ = 1.5405 Å) was utilized to determine the crystal structure of the samples. Fourier transform infrared (FT-IR, Bruker-Vector 22) spectroscopy was obtained using a spectrometer. Weight and atomic percentage, distribution of catalyst elements, and morphology were measured using energy-dispersive X-ray spectroscopy (EDX/MAP, ZEISS Sigma 300) and field emission scanning electron microscopy (FE-SEM, ZEISS Sigma 300). The structural studies of the materials were carried out with transmission electron microscopy (TEM, Philips EM 208S, 100 kV). N2 physisorption measurements (mini PROSLEB 100) were done at 77 K. The specific surface area was determined from the Brunauer–Emmett–Teller (BET) method, and the Barrett–Joyner–Halenda (BJH, mini PROSLEB 100) method was performed to determine the pore size distribution and volume. Thermogravimetric analysis (TGA, STA6000) was carried out under N2 gas with a heating rate of 10 °C min–1. Zeta potential analysis (Zeta SZ-100z) was utilized to determine the electric charge of particles. UV–vis diffuse reflectance spectroscopy (DRS, SCINCO, S-4100) and dual-beam optical spectroscopy (UV–vis, T90+ PG) were performed to record data. For recording steady-state photoluminescence (PL), a fluorescence spectrometer (PL, FP-8300 JASCO) was used. Nuclear magnetic resonance (NMR, Bruker ULTRASHIELD 400 AVANCE III) was registered. X-ray photoelectron spectroscopy (XPS, SPECS UHV) analysis was utilized to determine the chemical composition and type of bond on the surface of the sample as well as the possibility of identifying and determining the number of elements.

High-performance liquid chromatography (HPLC, Waters 1525) equipped with a UV detector (Waters 2487) and a column C18 (250 × 4.6 mm, 5 μm) was used with mobile phase acetonitrile/water (70:30, V/V)) at a flow rate of 1.0 mL min–1. Electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M-S) electrochemical impedance experiments were carried out using a potentiostat/galvanostat (IRASOL) with a standard three-electrode cell containing platinum wire as a counter electrode, Ag|AgCl|KCl (3M) as the reference electrode, and glassy carbon electrode (GCE) as the working electrode (internal diameter, 2 mm). For this purpose, the open circuit potential (OCP) was used by applying a disturbance amplitude of 5 mV, and the frequency was swept from 100 kHz to 1 Hz.

2.4. Investigation of Photocatalytic Activity

Photocatalytic experiments for the degradation of IMC were carried out in a box with dimensions of 40 × 40 × 40 cm, containing a source of visible light radiation, and in a two-shell Pyrex glass cylindrical reactor with a capacity of 100 mL (inner diameter, 7 cm; height, 20 cm). The reaction medium was cooled to ambient temperature by circulating water using a circulator to maintain the temperature. Artificial radiation (λ > 400 nm) was supplied by a 300 W halogen lamp. The UV light-locking filter was placed on top of the reactor. In each step, 15 mg of mesoporous NH2-MCN nanocomposites with various weight ratios was added to 50 mL of the IMC solution with a concentration of 30 mg L–1. At first, the mixture was stirred in the dark for 15 min to equilibrate the absorption of IMC before irradiation, and then irradiation was carried out for 60 min. The distance between the source of radiation and the reactor was retained at 10 cm in all measurements. At certain time intervals of irradiation, a 3 mL IMC solution was coming out from the reactor and was centrifuged, and finally, the absorption of IMC solution was measured after passing through a syringe filter with dimensions of 0.45 μm using a UV–vis spectrophotometer at λmax ∼ 270 nm. The photocatalytic degradation efficiency (%R) and IMC adsorption capacity (qe) on mesoporous NH2-MCN nanocomposites were calculated according to eqs 1 and 2, respectively:

2.4. 1
2.4. 2

where Ci and Ce are the initial concentration and equilibrium concentration of IMC in the solution, respectively, V is the volume of the solution (L), and M is the mass of the photocatalyst (g).

2.5. Photoelectrochemical and Electrochemical Experiments

For photoelectrochemical studies, 3 mg of the mesoporous NH2-MCN-2 nanocomposite, NH2-MIL-88b(Fe), and g-C3N4 was ultrasonically placed in 1 mL of DMF for 30 min to obtain slurry. Then, 100 μL of a slurry was spread on a fluorine–tin oxide surface (FTO, 1 × 1 cm) and dried at 60 °C for 30 min. The photocurrent responses of the photocatalysts were measured at OCP under visible light (λ > 400 nm) using a potentiostat/galvanostat (Auto Lab 302 N) and in a solution (0.5 M Na2SO4, pH= 6.8) as an electrolyte. EIS was carried out in 5 mM potassium hexacyanoferrate (II and III) and 0.1 M KCl. M-S electrochemical impedance experiments were performed with modified GCE as a working electrode and Pt counter electrode at 6000 Hz in a Na2SO4 solution (0.5 M, pH = 6.8). Equation 3 was used to convert the obtained potential against Ag|AgCl|KCl (3M) to a reversible hydrogen electrode (RHE) (normal hydrogen electrode (NHE) at pH = 0).

2.5. 3

2.6. Experimental Design

Nowadays, it has become inevitable to design experiments with suitable software to reduce costs, save time, and attain optimal experiment conditions. For this purpose, experiments were designed with RSM. Design-Expert software (version 12) was used to examine the response of various independent factors. The experimental conditions for the maximum photocatalytic degradation efficiency of IMC by the CCD method were investigated to study influential factors: pH, amount of mesoporous photocatalyst, IMC concentration, and irradiation time, as well as interactions between parameters. The design consists of five different levels with α = ±2 (the alpha (α) value can be defined as the calculated distance of each axial point (star point) from the center in the CCD). The pH ranged from 3 to 11, the photocatalyst amount ranged from 5 to 50 mg, the IMC concentration ranged from 10 to 50 mg L–1, and the irradiation time was variable from 5 to 60 min. In this study, three replications were performed to distinguish the significance of error during the experimental run. It should be noted that the experiments were conducted to identify the optimal points during the photodegradation process.

2.7. Adsorption Isotherm Studies

Experimental data in photocatalytic degradation of IMC by the mesoporous NH2-MCN-2 nanocomposite were investigated with Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. To 50 mL IMC aqueous solutions prepared with concentrations of 10, 20, 30, 40, and 50 mg L–1 adjusted at pH = 5, the photocatalyst (0. 76 g L–1) was added and stirred at 300 rpm. Next, the concentration of the remaining IMC in the solution was calculated after centrifuging and passing it through a syringe filter with a UV–vis spectrometer using the calibration curve. The equations for Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms are provided in the Supporting Information (Sections 2.7.1–2.7.4), respectively.

2.8. Investigation of Photocatalytic Degradation Kinetics of IMC

To determine the photocatalytic degradation kinetics of IMC, pseudo-first-order and pseudo-second-order kinetic models, as the two most important models, were investigated. For this purpose, 0.76 g L–1 mesoporous NH2-MCN-2 nanocomposite was added to 50 mL IMC solutions in concentrations of 10, 20, 30, 40, and 50 mg L–1 at pH = 5. The solutions were stirred at 300 rpm at 25 °C for 10, 20, 46, and 60 min. The samples were centrifuged, and after passing through the syringe filter, the absorption of the solution was recorded with the UV–vis spectrometer. The results showed that the photocatalytic degradation of IMC in all concentrations reached equilibrium after ∼46 min. The equations for the pseudo-first-order and pseudo-second-order models are provided in Section 2.8.1 of the Supporting Information.

3. Results and Discussion

3.1. Characterization of Photocatalysts

X-ray diffraction (XRD) analysis is used to identify the crystalline phase present in the material and shows chemical composition information. Figure 1a shows the XRD patterns of g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN nanocomposites with different weight ratios. For g-C3N4, the strong peak at 2θ ∼ 27.4°, which corresponds to the (002) crystal plane, indicates the accumulation of the aromatic conjugated system.30 The small peak at 2θ ∼ 13°, corresponding to the (100) crystal plane, is attributed to the stacking of g-C3N4 layers.30

Figure 1.

Figure 1

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(a) XRD patterns, (b) FT-IR spectra, (c) N2 adsorption–desorption isotherm, and (d) BJH of g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN nanocomposites with different weight ratios.

The XRD pattern of NH2-MIL-88b(Fe) shows the main diffraction peaks of its structures at 2θ ∼ 9.25, 10.5, 16.7, 18.9, and 21.05, which is in complete agreement with the reported works.31,32 As can be seen from the patterns of NH2-MCN with different weight ratios, the prominent distinct peaks of NH2-MIL-88b(Fe) are preserved in the mesoporous nanocomposites. Also, the peak of 2θ ∼ 27.4° belonging to g-C3N4 has been observed in the XRD patterns. In addition, the specific peak of g-C3N4 becomes more evident with an increasing amount of g-C3N4 in mesoporous NH2-MCN nanocomposites.

FT-IR studies were performed to peruse the functional groups and the structures of the mesoporous compounds (Figure 1b). In the spectrum of g-C3N4, out-of-plane bending modes of C–N heterocycles appear at 810 cm–1.33 The absorption bands from 1200 to 1600 cm–1 (1244, 1322, 1414, 1462, and 1573 cm–1) and 1641 cm–1 can be related to C–N aromatic stretching and C=N stretching, respectively.33,34 For the NH2-MIL-88b(Fe) sample, the symmetrical and asymmetrical vibrations of the NH2 groups are indicated by absorption bands at 3463 and 3340 cm–1.35 Also, the peaks at 1606, 1383, and 1581 cm–1 are attributed to the C=O stretching mode and the carboxyl vibration.36,37 The absorption bands at 1255 and 769 cm–1 indicate the C–N stretching and C–H bending vibration of the benzene ring, respectively.38,39 In NH2-MCN, absorption bands at 800 cm–1 corresponding to g-C3N4 appear in the mesoporous nanocomposite and become stronger with increasing percentage in the sample, indicating that g-C3N4 is successfully incorporated into the structure.

N2 adsorption–desorption is a valuable method for determining the physical properties of porous material. Data related to the porous texture of the samples are presented in Figure 1c,d, as well as Table 1. The results showed that the adsorption isotherm for all photocatalysts is type IV with the hysteresis loop of type H3.35,40 According to the IUPAC classification, the type IV isotherm is related to mesoporous materials. Combining g-C3N4 with different weight ratios with MOF causes a decrease in the specific surface area of nanocomposites compared to NH2-MIL-88b(Fe), indicating that portions of the pores are blocked due to the presence of structural plates of g-C3N4.41,42 The structural properties of mesoporous g-C3N4 (with a size average of approximately 7.30 nm) provide active sites for the improvement of a photocatalytic process.43

Table 1. Physical Parameters of g-C3N4, NH2-MIL-88b(Fe), and NH2-MCN by N2 Adsorption–Desorption.

sample SBET (m2/g)a Vp (cm3/g)b D (nm)c
g-C3N4 51.18 0.14 7.30
NH2-MIL-88b (Fe) 142.13 0.45 13.78
NH2-MCN-1 67.04 0.22 14.74
NH2-MCN-2 86.61 0.45 22.85
NH2-MCN-3 76.58 0.56 29.96
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a

Specific surface area.

b

Pore volume.

c

Pore diameter

Photoluminescence (PL) analysis was used to investigate the rate of recombination of electron–hole pairs produced by radiance. The rate of recombination is linked to the emission peak intensity of the PL spectrum. If emission peaks are generated with high intensity, the material has a speedy recombination rate of electron–hole pair.44,45 The PL spectra of C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN nanocomposites at excitation wavelengths of 333 and 338 nm are presented in Figure 2a. g-C3N4 and mesoporous NH2-MCN-2 nanocomposites have the highest and lowest PL emission intensities, respectively, at a wavelength of ∼450 nm. The low separation efficiency of the electron–hole pair in g-C3N4 is due to faster recombination.

Figure 2.

Figure 2

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(a) Photoluminescence analysis at the excitation wavelengths of 333 and 338 nm, (b) DRS spectrum, and (c) Kubelka–Munk plots of mesoporous NH2-MCN nanocomposites with different weight ratios.

The gap energy is one of the critical parameters for photocatalysts. A wide absorption band of mesoporous NH2-MCN nanocomposites in the range of 220–1000 nm is dominated by an intense absorption band at ∼283 nm. In addition to this intense band, there are two weak absorption bands at ∼380 and ∼588 nm. The peaks observed are related to electron transitions from the highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbitals (LUMOs), which could be assigned to the existence of the amino group of ligand and Fe3-μ 3-oxo clusters. It can be said that HOMOs are more of the nature of 2p orbitals of oxygen (organic linkers) and LUMOs are more formed from the participation of metal orbitals.46,47 The π–π* transition in the organic linkers can be seen in the absorption in the UV region, while another absorption in the visible region is related to the Fe–O clusters, −NH2 groups, and g-C3N4.47

DRS analysis was performed to investigate the amount of energy required for the electron to jump from the valence band to the conduction band (Figure 2b). For mesoporous NH2-MCN-1, NH2-MCN-2, and NH2-MCN-3 nanocomposites, the energy gap was calculated to be ∼2.49, 2.29, and 2.69 eV, respectively, using Kubelka–Munk plots (Figure 2c) and eq 4.

3.1. 4

where α, h, ν, A, and Eg are the absorption coefficient, Plank constant, light frequency, proportionality constant, and energy gap, respectively.

Figure 3 shows the FE-SEM images of g-C3N4, NH2-MIL-88b(Fe), and the mesoporous NH2-MCN-2 nanocomposite. g-C3N4 has a layered structure formed by placing its plates on top of each other (Figure 3a), which is in agreement with previous work.48 NH2-MIL-88b(Fe) has a hexahedral shape with a smooth surface (Figure 3b).48 The FE-SEM images of the mesoporous NH2-MCN-2 nanocomposite showed the presence of both NH2-MIL-88b(Fe) and g-C3N4 in the structure of the nanocomposite (Figure 3c), which indicates that MOF preserves its structure during the synthesis process and has high stability.49

Figure 3.

Figure 3

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FE-SEM images of (a) g-C3N4, (b) NH2-MIL-88b(Fe), and (c) mesoporous NH2-MCN-2 nanocomposite.

The chemical composition and distribution of the elements present in the mesoporous NH2-MCN-2 nanocomposite were determined by EDX elemental analysis (Figure 4a) and MAP (Figure 4b), so the presence of Fe, O, N, C, and Cl elements was confirmed in its structure.

Figure 4.

Figure 4

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(a) EDX spectrum, (b) MAP, and (c) TEM image of the mesoporous NH2-MCN-2 nanocomposite.

TEM images of NH2-MCN-2 (Figure 4c) indicated the close connection of NH2-MIL-88b(Fe) and g-C3N4, which confirmed the formation of the mesoporous nanocomposite. Fe atoms with higher atomic mass cause the formation of a dark area in the images. g-C3N4 with C and N elements are also observed in the images as nanosheets.47

The TGA curve of the mesoporous NH2-MCN-2 nanocomposite recorded in a N2 atmosphere and with a heating rate of 10 °C min–1 showed several stages of weight loss (Figure 5a). The first and second stages happen in the range of 25–400 °C, which is related to the loss of free water molecules, water molecules and solvent (DMF) inside the pores, and the amino group in the MOF.50 In the TGA curve, another weight loss can be seen at a temperature above 400 °C related to the decomposition of ligands in NH2-MIL-88b(Fe). Finally, the fourth weight loss in the temperature range of 650–850 °C displays the destruction of g-C3N4 species.51

Figure 5.

Figure 5

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(a) TGA curves and XPS survey spectra of mesoporous NH2-MCN-2 nanocomposite and (b) full scans, (c) Fe 2p, (d) C 1s, (e) N 1s, and (f) O 1s.

In AOP, the electric charge on the catalyst surface is considered an influential factor. When pH > pHpzc, the catalyst’s surface has a negative charge, while at pH < pHpzc, the surface has a positive charge. In this work, the solid addition method and zeta potential were used to measure pHpzc. Determination of pHpzc of g-C3N4, NH2-MIL-88b(Fe), and NH2-MCN-2 was performed using a sodium nitrate solution. For this purpose, 0.5 g L–1 mesoporous composites were added to 40 mL of sodium nitrate solution (0.1 N) and the pH was adjusted using HCl and NaOH solutions (0.1 M) at pH 2, 4, 6, 9, 12, and 14. The suspension was placed on an incubator shaker (400 rpm) for 24 h at room temperature. Eventually, the pH of each solution was measured and recorded as the final pH.

The results were plotted as ΔpH (final pH – initial pH) vs initial pH in Figure S1a. The pHpzc values for g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN-2 nanocomposites were obtained as ∼6.9, 7.8, and 6.5, respectively. In addition, the variation in zeta potential (−33.3 to 44.7 mV) at different pH values confirmed the results of pHpzc. The plot of zeta potential vs pHs is shown in Figure S1b.

In the XPS spectrum of NH2-MCN-2, signals of Fe, C, N, and O were observed, which offers the successful synthesis of the mesoporous nanocomposite (Figure 5b). The peaks of Fe 2p spectra with high resolution are presented in Figure 5c. Three peaks at 712.0, 725.7, and 716.7 eV are attributed to Fe 2p3/2, Fe 2p1/2, and their satellite, respectively, which display the presence of Fe(III).47,52 The C 1s spectrum indicates three peaks centered at 284.4, 286.0, and 288.5 eV (Figure 5d). The 284.8 eV peak was considered as the calibration reference. The peak in the 284.4 eV region corresponds to the alkyl carbon components in the benzene ring, C–H, and C–C bonds. The peaks at 286 and 288.5 eV are related to the area C–N–C group in g-C3N4 and the carboxylate functional group (O–C=O) of H2BDC-NH2 or the sp2 carbon in the thiazine ring (N–C=N), respectively.47,53 The N 1s spectrum of the mesoporous NH2-MCN nanocomposite provides three peaks at 399.1, 400.9, and 404.0 eV, which are matched to the CN=C of sp2-bonded aromatic, N–(C)3 of tertiary N, and C–N–H groups from g-C3N4, respectively (Figure 5e).47,54 Eventually, the O 1s spectrum demonstrates three peaks at around 528.6, 530.4, and 531.6 eV, which are ascribed to the oxygen components of terephthalate linkers, Fe–O bonds in the ligand, and possibly H2O absorption on the surface of the catalyst, respectively (Figure 5f).21,47,55

3.2. Degradation of IMC

In this part, the photocatalytic degradation rate of IMC was investigated with mesoporous NH2-MCN nanocomposites with various weight ratios of g-C3N4. Specific amounts of each photocatalyst (0.3 g L–1) were added separately to 50 mL IMC solutions with a specific concentration (30 mg L–1) under the same experimental conditions (pH = 3 and ambient temperature). The solutions were stirred at 300 rpm for 15 min without light, and sampling was done at specified time intervals. After centrifuging and passing the solutions through a syringe filter (0.45 μm), the IMC residue in the solution was investigated by a two-beam UV–vis spectrophotometer. Next, the solutions were exposed to the visible light of a halogen lamp (λ > 400 nm with a power of 300 W) for 60 min, sampling was done at specified time intervals, and the IMC concentration was determined. The photodegradation rates for NH2-MCN-1, NH2-MCN-2, and NH2-MCN-3 were obtained at 75.00, 92.05, and 55.04%, respectively (Figure S2). It was observed that for each photocatalyst, degradation increases with increasing time. The mesoporous NH2-MCN-2 nanocomposite has higher photocatalyst efficiency in the IMC degradation process than other mesoporous nanocomposites because it has a lower energy gap and higher surface area. In the mesoporous NH2-MCN-2 nanocomposite, a low rate of hole–electron recombination caused the degradation of IMC with higher efficiency. Figure S3a–c shows the morphology, EDX elemental analysis, and MAP analysis of mesoporous NH2-MCN-2 nanocomposites, respectively, which defined the high structural stability of photocatalysts during the process (1st).

3.3. Designing Experiment in Photocatalytic Degradation of IMC

3.3.1. Investigating the Photocatalytic Process Based on CCD and RSM

The RSM method was used to optimize the photocatalytic degradation of IMC using four factors. The total number of experiments was 30 runs using five CCD levels with three repetitions. A, B, C, and D are the coded values of IMC initial concentration, pH, amount of photocatalyst, and irradiation time, respectively, presented in Table 2. The experimental results of IMC photocatalytic degradation (%) were predicted based on the designed initial conditions, and they are shown in Table 3 and eq 5.

3.3.1. 5
Table 2. Independent Factors and Their Coded Levels for CCD.
factor unit low (−1) middle (0) high (+1) α– α+
(A) initial concentration of IMC mg L–1 20 30 40 10 50
(B) pH   5 7 9 3 11
(C) amount of photocatalyst mg 16.25 27.5 38.75 5 50
(D) irradiation time min 18.75 32.5 46.75 5 60
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Table 3. Experiments Performed Based on the Experimental Design and Answers Obtained.
run factor 1 (A) factor 2 (B) factor 3 (C) factor 4 (D) response (R)
1 0 0 0 2 62.84
2 –1 1 1 1 41.82
3 –1 1 1 –1 29.81
4 0 0 0 0 55.24
5 0 –2 0 0 90
6 2 0 0 0 38.87
7 0 0 0 0 54.26
8 1 1 1 1 34.39
9 1 1 –1 1 30.15
10 1 –1 –1 –1 46.94
11 0 0 –2 0 28.96
12 1 –1 1 1 75.97
13 –1 –1 –1 1 85.28
14 1 –1 1 –1 50.24
15 0 0 2 0 63.11
16 1 –1 –1 1 69.93
17 –1 –1 –1 –1 55.89
18 1 1 –1 –1 20.48
19 0 2 0 0 25.36
20 –1 1 –1 1 36.34
21 0 0 0 0 50.26
22 0 0 0 0 56.27
23 0 0 0 0 58.01
24 0 0 0 0 52.22
25 –2 0 0 0 64.01
26 1 1 1 –1 27.36
27 0 0 0 –2 18.02
28 –1 1 –1 –1 23.36
29 –1 –1 1 –1 67.95
30 –1 –1 1 1 94.85
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Based on the values of the coefficients in the regression model, the order in which the variables affect the photocatalytic degradation of IMC is pH (B) > irradiation time (D) > initial concentration of IMC (A) > amount of photocatalyst (C). The significance and adequacy of the proposed model were perused by ANOVA. The results presented excellent agreement between the predicted and experimental values of photocatalytic degradation of IMC. The model is statistically significant with linear terms, interactions, and quadratic terms. The effectiveness and significance of the proposed model were gained with the F-value, p-value, related R2, predicted R2, and adjusted R2, which are given in Table 4 and Table S1.

Table 4. ANOVA for Optimization of Photocatalytic Degradation of IMC.
source sum of squares df mean square F-value p-value  
model 12,299.46 14 878.53 49.08 <0.0001 significant
A-A 705.47 1 705.47 39.41 <0.0001  
B-B 7798.34 1 7798.34 435.67 <0.0001  
C-C 623.42 1 623.42 34.83 <0.0001  
D-D 2327.36 1 2327.36 130.02 <0.0001  
AB 109.94 1 109.94 6.14 0.0256  
AC 10.73 1 10.73 0.5992 0.4509  
AD 15.72 1 15.72 0.8783 0.3635  
BC 3.92 1 3.92 0.2190 0.6465  
BD 250.59 1 250.59 14.00 0.0020  
CD 0.7056 1 0.7056 0.0394 0.8453  
A2 11.46 1 11.46 0.6404 0.4361  
B2 22.89 1 22.89 1.28 0.2759  
C2 109.46 1 109.46 6.12 0.0258  
D2 316.88 1 316.88 17.70 0.0008  
residual 268.49 15 17.90      
lack of fit 229.35 10 22.94 2.93 0.1235 not significant
pure error 39.14 5 7.83      
cor total 12567.96 29        
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The p-value and F-value of the model for the photocatalytic degradation of IMC were obtained as <0.0001 and 49.08, respectively, which means that the model is significant. All independent variables such as A, B, C, D, BD, AB, C2, and D2, which have p < 0.05, indicate the significance of the term in the model. The R2-predicted value shows good agreement with the R2 and R2-adjusted values. The F-value of the lack of fit (LOF) is obtained as 2.93, which is insignificant because the p-value is 0.1235. Figure S4 shows good agreement between the predicted and actual values, indicating the adequacy and significance of the mode.

3.3.2. Effect of Factors in the Photocatalytic Process

Three-dimensional (3D) response surface and two-dimensional contour (2D) plots were utilized to peruse the effects of the experiment variables and their interaction on the photocatalytic degradation of IMC. Since the time factor has a direct relationship with process efficiency, the impact of each factor has been studied along with the time factor (Figure 6a–f).

Figure 6.

Figure 6

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3D surface plots and 2D contour for the interactions of (a, b) initial concentration and degradation time, (c, d) pH and degradation time, and (e, f) amount of photocatalyst and degradation time.

3.3.2.1. Effect of Initial Concentration of IMC

Figure 6a,b shows that the photocatalytic degradation efficiency of IMC decreases with increasing concentration. It can be attributed to the following reasons: (a) The increase in IMC concentration leads to the intensification of dispersion in the solution and turbidity, which reduces the penetration of radiance into the solution, as well as photon absorption by the photocatalyst. (b) At high IMC concentrations, the formation of oxidizing species is reduced through the coverage of active sites by IMC molecules or the absorption of photons by them. Consequently, a finite number of oxidizing species can degrade IMC molecules.

3.3.2.2. Effect of pH

One of the influential factors in the photocatalytic degradation of pollutants is solution pH, which affects the surface charge of the catalyst and its structure.56 In this study, the kinetics of photocatalytic degradation of IMC was studied in the range of pH from 3 to 11, and the results show high degradation efficiency in acidic media (Figure 6c,d). At lower pH values of solution, the mesoporous NH2-MCN-2 nanocomposite with pHpzc ∼ 6.5 has a positive charge. In an acidic environment, IMC with pKa = 11 cannot release H+ ions to the reaction solution. Due to the presence of electron-rich aromatic rings in the IMC molecule, it probably adsorbed on the positively charged surface of the photocatalyst. In addition to electrostatic/acid–base/π–π interactions, hydrogen bonding may affect the photocatalytic degradation efficiency in aqueous solutions.13

3.3.2.3. Effect of Photocatalyst Amount

The relevance between the photocatalytic degradation efficiency of IMC and the amount of photocatalyst is shown in Figure 6e,f. Increasing the amount of mesoporous NH2-MCN-2 nanocomposite leads to an increment in active sites and, therefore, the adsorption of IMC molecules on its surface. In addition, by increasing the amount of photocatalyst, the number of photons absorbed increases, and as a result, by increasing the production of electron–hole pairs, conditions are provided for more photodegradation.

3.4.3. Determining the Optimum Condition of Factors

After investigating the effect of the main factors and their interaction in photocatalytic degradation of IMC, to save time, cost, and high efficiency, certain levels of effective factors were selected to determine the maximum degradation capacity of the mesoporous NH2-MCN-2 nanocomposite (Figure S5). According to the results, the optimal points were obtained in the IMC initial concentration of 20 mg L–1, the amount of photocatalyst of 0.76 g L–1, pH 5, and the irradiation time of 46′ 25″. Also, the predicted value (96.31%) agrees with the actual values of the maximum degradation of IMC (95.47%).

3.4. Stability and Reusability

One of the characteristics of a functional photocatalyst is its stability and reusability. For this purpose, the mesoporous NH2-MCN-2 nanocomposite was used in the photocatalytic degradation of IMC. After the end of each cycle, the photocatalyst was separated using a centrifuge, washed with ethanol, and then dried at 60 °C for 12 h in a vacuum oven. The results in Figure 7a show that the mesoporous NH2-MCN-2 nanocomposite has strong reusability performance after five cycles. Figure 7b indicates that the XRD patterns of the mesoporous NH2-MCN-2 nanocomposite before and after the reaction are similar. In addition, the FT-IR spectra of the materials confirm that there is no significant change in the structural properties of the photocatalyst (Figure 7c). FE-SEM of the mesoporous NH2-MCN-2 nanocomposite (Figure 7d) shows that the morphology of the mesoporous NH2-MCN-2 nanocomposite remains unchanged after the photocatalytic reaction. For more investigation, EDX and MAP analyses of the sample were performed, and the results confirm that significant components of the catalyst including NH2-MIL-88b(Fe) and g-C3N4 can be found after recycling experiment (Figure S6a,b). After five cycles, the efficiency of photocatalytic degradation of IMC decreases from 95.47 to 84.5%. This reduction may be the absorption of resistant intermediate products on the surface of the photocatalyst and those occupying the active sites, resulting in the absorption of less light and the production of charge carriers on the surface of the photocatalyst, followed by less degradation of IMC.

Figure 7.

Figure 7

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(a) Reusability of the mesoporous NH2-MCN-2 nanocomposite in optimal experiment conditions (initial concentration of 20 mg L–1 IMC, amount of photocatalyst of 0.76 g L–1, pH = 5, and degradation time ∼46 min), (b) XRD patterns, (c) FT-IR and (d) FE-SEM image of photocatalytic degradation of IMC after five cycles, and (e) photocatalytic degradation in the presence of trapping agents (2 mmol).

3.5. Effects of Trapping Agents

To peruse the main active species in photocatalytic degradation of IMC by the mesoporous NH2-MCN-2 nanocomposite, BQ (O·2–), IPA (•OH), AO (h+), and AgNO3 (e–) species were added as trapping agents with a concentration of 2 mmol to the aqueous solution containing IMC. The results show that when AgNO3 and BQ species are added to the IMC solution, they have a negligible effect on the degradation (Figure 7e). The photocatalytic degradation of IMC by the mesoporous NH2-MCN-2 nanocomposite is clearly decreased by the addition of AO. In addition, the results exhibited that the degradation decreases with the addition of IPA. Therefore, it is proved that h+ is the main active species in the reaction, and •OH is the auxiliary active species.

3.6. Mechanism of Photocatalytic Degradation

To determine the flat band potential (VFB) of mesoporous composites, an M-S plot is recorded at a frequency of 6000 Hz (Figure 8a). g-C3N4 has higher negative VFB of ∼−1.4 V vs Ag|AgCl|KCl (3M) compared to NH2-MIL-88b(Fe), and mesoporous NH2-MCN-2 nanocomposites have EFB of ∼−0.3 and ∼−0.21 V, respectively. The CB band positions of g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN-2 nanocomposites have been obtained as −1.20, −0.10, and −0.01 V vs NHE, respectively.47 Moreover, the values of the energy gap were calculated by using DRS analysis. Next, using the following equation, EVB was obtained.

3.6. 6

Figure 8.

Figure 8

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(a) Mott–Schottky plots at a frequency of 6000 Hz, (b) photocurrent response and (c) Nyquist plots of g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN-2 nanocomposite, and (d) kinetic plot of pseudo-first-order model in photocatalytic degradation of IMC by NH2-MCN-2.

The EVB values of g-C3N4, NH2-MIL-88b(Fe), and the mesoporous NH2-MCN-2 nanocomposite were estimated as 1.56, 1.58, and 2.27 V vs NHE, respectively. A positive slope at the M-S plot displays an n-type semiconductor. Irradiation of visible light to the mesoporous NH2-MCN-2 nanocomposite causes the production of charge carriers (electron–hole pair) by NH2-MIL-88b(Fe) and g-C3N4; thus, the photogenerated holes in the VB of g-C3N4 and the photogenerated electrons in the CB of NH2-MIL-88b(Fe) increase the separation efficiency of photocarriers and finally cause the improvement of the photocatalytic performance (eqs 7–13).

3.6. 7
3.6. 8
3.6. 9
3.6. 10
3.6. 11
3.6. 12
3.6. 13

3.7. Electrochemical and Photoelectrochemical Investigations

The photocurrent response and EIS of g-C3N4, NH2-MIL-88b(Fe), and mesoporous NH2-MCN-2 nanocomposites were recorded to investigate the effect of charge transfer and separation of generated charge carriers (electron–hole pair). Figure 8b shows the photocurrent–time response of different photocatalysts with four on–off cycles. Compared to g-C3N4 and NH2-MIL-88b(Fe), the photocurrent response of the mesoporous NH2-MCN-2 nanocomposite is significantly increased, which proves that g-C3N4 and NH2-MIL-88b(Fe) help to separate the charges produced in the photodegradation process. EIS studies express the difficulty of charge transfer on the catalyst surface. In the Nyquist plot, the mesoporous NH2-MCN-2 nanocomposite has the most minor diameter of the semicircle, which indicates less charge transfer resistance and a fast electron transfer rate in the electrode–electrolyte interface (Figure 8c).

3.8. Kinetic Studies

One of the critical factors in the process of adsorption is to examine and study its kinetics. The most critical isotherm models including first- and second-order pseudo models were investigated. The photocatalytic degradation of IMC as a change of Ln C/C0 vs irradiation time (t) follows a straight line with a high correlation coefficient (R2) (Figure 8d). The results illustrated that the photocatalytic degradation of IMC follows first-order kinetics. The rate constant (k) obtained in the conditions of catalyst amount = 0.76 g L–1, different initial concentrations of IMC (10–50 mg L–1), pH = 5, and irradiation time ∼46 min is equal to 0.08 min –1 (Table S2).

3.9. Adsorption Isotherm

The adsorption behavior of IMC on the surface of the mesoporous NH2-MCN-2 nanocomposite was investigated by Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich models (Figure S7a–d). The obtained coefficients (R2) show that the adsorption process agrees better with the Langmuir model. The parameters of the isotherm models for adsorption are presented in Table 5. The photocatalytic performance for the decomposition of IMC was compared with other photocatalysts in Table 6.

Table 5. Isotherm Parameters for Photocatalytic Degradation of IMC by the Mesoporous NH2-MCN-2 Nanocomposite.

isotherm parameter result
Langmuir qm(mg g–1) 35.7142
KL (L mg–1) 0.1351
RL (20 mg L–1) 0.2702
R2 0.9913
Freundlich KF (L mg–1) 28.2106
1/n 1.9296
R2 0.9360
Temkin B 8.3180
KT (L g–1) 1.1297
bT (J mol–1) 297.8566
R2 0.9681
Dubinin–Radushkevich qm (mg g–1) 25.2796
KDR (mol2 kJ–2) 2 × 10–6
E (kJ mol–1) 2
R2 0.9405
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Table 6. Comparison of Different Photocatalytic Performances for IMC Degradation.

photocatalyst photocatalytic degradation efficiency of IMC (%) reference
Au@PPy-C/SnO2 93.18 (57)
Bi12.7Co0.3O19.35 96 (58)
g-C3N4/TiO2 93 (59)
g-C3N4 /ZnO (20:80) 56 (60)
TiO2 90 (61)
Bi2Co2C42H36N6O33 81 (62)
PANI/ZnO-CoMoO4 97 (63)
SO4/Ag3PO4 75 (64)
PANI/WO3-CdS 94 (65)
Ag/AgBr 90 (66)
GO@TiO2 93 (67)
GO/Fe3O4/TiO2-NiO 97.47 (68)
ZnO/CoFe2O4 98.1 (69)
TiO2 90.24 (70)
U-g-C3N4 (from urea) 90 (71)
M-g-C3N4 (from melamine) 42.9 (71)
NH2-MCN-2 95.47 this study
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3.10. Investigation of Photocatalytic Degradation of IMC

Comparing the results of XPS analysis on the surface of the mesoporous NH2-MCN-2 nanocomposite before and after the photodegradation process shows good agreement (Figure 9a–e). All peaks have almost the same bonding energy, and no significant change has occurred in their position. Therefore, it can be said that there are no physicochemical interactions between IMC and NH2-MCN-2.

Figure 9.

Figure 9

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XPS survey spectra of the mesoporous NH2-MCN-2 nanocomposite: (a) full scan, (b) Fe 2p, (c) C 1s, (d) N 1s, and (e) O 1s after photocatalytic degradation of IMC.

The HPLC method was also used to investigate the photodegradation process. Two solutions of IMC (50 mL) were prepared under the optimal experiment conditions. One of the solutions was kept as a stock solution, and the mesoporous NH2-MCN-2 nanocomposite was added to the other solution in the dark. Next, the solution was exposed to visible light. After the necessary time (∼46 min) in the photodegradation process of IMC, the catalyst was filtered, and the solution was injected into the HPLC instrument with a detector adjusted at the wavelength of 270 nm. The results shown are consistent with the UV–vis results reported in this work and confirm the photodegradation of IMC (Figure S8).

4. Conclusions

In this work, mesoporous NH2-MCN nanocomposites with different weight ratios of g-C3N4 (CN = 125, 250, and 500 mg) were synthesized by the solvothermal method and used for photocatalytic purposes. Among them, the mesoporous NH2-MCN-2 nanocomposite was proposed as a promising candidate for photodegradation of IMC due to its lower energy gap (∼2.29 eV) and higher specific surface area (86.61 m2 g–1). In the following, the CCD method was used to investigate four influential factors in the photocatalytic degradation of IMC from aqueous solutions. RSM was used to optimize the photodegradation process. The optimal points were obtained as follows: initial concentration of 20 mg L–1 IMC, amount of photocatalyst of 0.76 g L–1, pH 5, and degradation time of ∼46 min. The photocatalytic degradation of 95.47% for IMC was achieved after irradiation by NH2-MCN-2. The agreement between the parameters and the response of the model based on the results of ANOVA shows the validity of the model. NH2-MCN-2 showed high stability and reusability. According to the coefficient of determination (R2) obtained from the kinetic and isotherm models, the photodegradation process is controlled using the pseudo-first-order model (k = 0.08 min–1), and the isotherm data were more compatible with the Langmuir model. The EIS results as well as the photocurrent response of NH2-MCN-2 showed a significant improvement compared to g-C3N4 and NH2-MIL-88b(Fe) according to electron transfer rate and photocatalytic activity. The reason is due to the synergistic effect between g-C3N4 and NH2-MIL-88b(Fe). The ECB band positions of the g-C3N4 and NH2-MIL-88b(Fe) photocatalysts as well as the EVB band vs NHE were obtained using M-S electrochemical impedance at −1.20, −0.10, 1.56, and 1.58 V, respectively. The results achieved from trapping experiments identified the role of the effective active species of h+ and •OH for the photocatalytic degradation of IMC.

Acknowledgments

The authors acknowledge the scientific and technical supports of University of Mazandaran, Iran for this research work.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c10281.

  • Determination of pHpzc and zeta potentials (Figure S1); photocatalytic degradation of IMC by mesoporous NH2-MCN nanocomposites (Figure S2); FE-SEM, EDX elemental, and MAP analyses of the mesoporous NH2-MCN-2 nanocomposite after photocatalytic degradation of IMC (1st) (Figure S3); statistical parameters related to the proposed model of photocatalytic degradation of IMC (Table S1); normal probability plot of residuals, standard plots of residuals vs predicted and residuals vs run number, and plot of predicted vs actual values (Figure S4); suggested points (optimum points) of model in photocatalytic degradation of IMC (Figure S5); EDX and elemental mapping analyses of the NH2-MCN-2 photocatalyst after five cycles of IMC degradation (Figure S6); pseudo-first-order kinetic parameters in photocatalytic degradation of IMC (Table S2); isotherm plots of IMC adsorption on the surface of the mesoporous NH2-MCN-2 nanocomposite (Figure S7); chromatograms of IMC solution before and after photocatalytic degradation (Figure S8) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c10281_si_001.pdf (1MB, pdf)

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