KR102723122B1 - method of preparing highly efficient graphitic carbon nitride using hard-template method for degradation of organic pollutants - Google Patents

Abstract

The present application relates to a method for producing nanostructured porous graphite carbon nitride having nitrogen defects to simultaneously improve the active site, light absorption efficiency and charge separation ability of the catalyst.

Description

Method of preparing highly efficient graphitic carbon nitride using hard-template method for degradation of organic pollutants

The present application relates to a method for producing graphite carbon nitride using a hard template method for decomposing organic pollutants.

Various types of organic pollutants such as dyes, antibiotics, and phenols are released into the environment every year from various industries including pharmaceutical industry, food industry, fish and shrimp farming. The release of organic pollutants generated during the cleaning process of industrial sites into natural waters is accompanied by serious environmental problems and has serious adverse effects on humans, animals, and the entire ecosystem.

Therefore, an efficient method to treat wastewater and remove hazardous pollutants is urgently needed. To solve this problem, several traditional methods have been performed, including biological treatment, adsorption, electrochemical methods, ion exchange, and chemical precipitation. However, these methods have limitations such as low removal efficiency, incomplete decomposition into non-toxic substances, long removal times, and the need for post-treatment.

In recent years, advanced oxidation processes (AOPs) have been used as an alternative to non-destructive water treatment methods to remove a wide range of industrial pollutants from aqueous solutions. AOPs, including sonochemical degradation, ozonation, electrochemical remediation, Fenton reaction, and photocatalysis, are based on the in situ production of highly active radical species aimed at the degradation of organic compounds.

Among these AOPs, photocatalytic decomposition using semiconductor catalysts has emerged as a method for removing hazardous pollutants because it is simple to design, regenerable, low-cost, non-toxic, and does not cause serious secondary pollution.

Recently, photocatalytic degradation of RhB and TC has been reported using various semiconductor photocatalysts such as TiO ₂ , ZnO, and SrTiO ₃ . However, in most cases, photocatalysts react to ultraviolet light, which accounts for less than 5% of the total solar irradiance. To solve this problem, research on highly efficient photocatalysts active under visible light irradiation is required.

Among the numerous reported photocatalysts, graphitic carbon nitride (gC ₃ N ₄ ), a metal-free polymer semiconductor with a layered structure, has attracted much attention in many fields such as water splitting, photodegradation of environmental organic pollutants, organic synthesis, and CO ₂ reduction. This is because gC ₃ N ₄ has excellent properties such as visible-light responsiveness, extreme stability, non-toxicity, easy preparation, and low-cost production. Nevertheless, the applications of conventional bulk gC ₃ N ₄ are generally limited by its unfavorable photocatalytic efficiency due to high recombination rate of photoinduced charges, small specific surface area, and insufficient sunlight absorption. Various modification approaches, including nanostructure design, elemental and molecular doping, supramolecular preorganization, interfacial engineering, and dye sensitization, have been studied to complement the above shortcomings and enhance the photocatalytic performance.

However, constructing a high-performance visible-light-responsive photocatalyst for the purification of organic pollutants from wastewater remains a great challenge in recent years. An important strategy to enhance the visible-light photocatalytic effect of g-C3N4 is to secure porosity in its structure. Introducing porosity increases the surface area, accessible channels, and active sites, which enhances photogenerated charge separation, molecular mass transfer, and surface reactions.

In addition, recent studies have reported that introducing nitrogen defects into the lattice structure of gC ₃ N ₄ can significantly enhance the photocatalytic activity in various applications such as bisphenol A degradation, hydrogen evolution, and CO ₂ reduction. This is mainly because the defect sites tend to capture photogenerated electrons and promote charge carrier separation.

The present application relates to a method for producing nanostructured porous graphite carbon nitride having nitrogen defects to simultaneously improve the active site, light absorption efficiency and charge separation ability of the catalyst.

One aspect of the present application relates to a method for producing graphite carbon nitride.

In one example, the manufacturing method may include the steps of forming a gC ₃ N ₄ and SiO ₂ complex; and removing a SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, after the step of removing the SiO ₂ template, the step of centrifuging the residue from which the SiO ₂ template has been removed; the step of washing with distilled water until the pH becomes 7; and the step of drying in an oven at 60 to 80° C. may be additionally included.

In one example, the manufacturing method may include the steps of drying and then pulverizing a colloidal SiO ₂ dispersion to prepare a SiO ₂ cluster template; mixing the SiO ₂ template and melamine to prepare a mixture; heating the mixture to form a gC ₃ N ₄ and SiO ₂ complex; and removing the SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, the colloidal SiO ₂ dispersion can be dried in an oven at 80 to 120 °C.

In one example, the colloidal SiO ₂ dispersion can be dried for 10 to 14 hours.

In one example, the SiO ₂ template and melamine can be mixed in a weight ratio of 1:1 to 1.5.

In one example, the mixture can be heated to 550 °C and held for 2 to 4 hours to form a gC ₃ N ₄ and SiO ₂ complex.

In one example, the SiO ₂ template can be removed from a gC ₃ N ₄ and SiO ₂ complex by adding a hydrogen fluoride (HF) solution.

In one example, the manufacturing method may include the steps of: preparing SiO ₂ microspheres; mixing molten cyanamide into the SiO ₂ microspheres to prepare a mixture; heating the mixture to form a gC ₃ N ₄ and SiO ₂ complex; and removing a SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, the weight ratio of cyanamide and silica microspheres can be from 1:0.1 to 2.5.

In one example, the average diameter of the silica microspheres can be 200 to 400 nm.

In one example, the mixture can be mixed at 70 to 90 °C for 20 to 40 minutes, cooled, and then heated again at 500 to 600 °C to form a gC ₃ N ₄ and SiO ₂ complex.

In one example, the SiO ₂ template can be removed from the gC ₃ N ₄ and SiO ₂ complex by adding an ammonium hydrogen difluoride solution.

Another aspect of the present application relates to graphite carbon nitride produced by such a production method.

In one example, the average pore volume may be 0.2 to 0.3 cm ³ g ^-1 .

In one example, the average pore diameter may be 10 to 20 nm.

In one example, the BET surface area can be 30 to 70 m ² g ^-1 .

Another aspect of the present application relates to a visible light photocatalyst comprising such graphitic carbon nitride.

According to one embodiment of the present application, a technology for manufacturing a high-performance graphite carbon nitride photocatalyst for removing organic pollutants can be provided.

According to one embodiment of the present application, a technology for controlling the morphology/porosity/surface area and improving the performance of a photocatalyst using a hard template technique (silica nanocluster) can be provided.

According to one embodiment of the present application, an organic nano-photocatalyst design with a desired constant shape can be provided by creating an Inverse Opal structure in a hard template technique (silica microsphere).

According to one embodiment of the present application, an efficient treatment method for various difficult-to-decompose organic pollutants (RhB, MB, TC, etc.) can be provided.

According to one embodiment of the present application, it is possible to establish a photodecomposition mechanism and provide a toxicity reduction effect.

Figure 1 is a schematic diagram illustrating a method for synthesizing graphite carbon nitride using a hard template method.
Fig. 2a is a SEM image of a SiO2 cluster, Fig. 2b is a SEM image of NC MCN, Fig. 2c is a SEM image of bulk CN, and Figs. 2d to 2h are TEM images of NC MCN.
Fig. 3a is a SEM image of SiO2 microspheres, Fig. 3b is a SEM image of IO CN 1:0.5, Fig. 2c is a SEM image of IO CN 1:1, Figs. 2d and 2e are SEM images of IO CN 1:2, and Fig. 2f is a TEM image of IO CN 1:2.
Figure 4a shows the N2 adsorption/desorption isotherms of NC MCN and bulk CN, and Figure 4b shows the corresponding pore size distribution curves.
Figure 5a shows the N2 adsorption/desorption isotherms of IO CN and bulk CN, and Figure 5b shows the corresponding pore size distribution curves.
Figure 6a is a photocatalytic degradation curve for rhodamine B (RhB) degradation, Figure 6b is a photocatalytic degradation curve for tetracycline (TC) degradation, Figure 6c is a pseudo-first-order kinetic curve for the photocatalytic degradation of rhodamine, and Figure 6d is a pseudo-first-order kinetic curve for the photocatalytic degradation of tetracycline.
Figure 7 is a graph of the MB decomposition UV-vis spectrum of NC MCN.
Figure 8 is a graph of the decomposition UV-vis spectra (RhB and MB peaks) of the RhB and MB mixture of NC MCN.
Figure 9 is a graph of the decomposition UV-vis spectrum (total peak) of RhB and MB mixture of NC MCN.
Figure 10a is the UV-Vis absorption spectrum of MB in the presence of IO CN 1:2, and Figure 10b is the photocatalytic degradation curve for MB decomposition in the presence of the prepared photocatalyst.
Figure 11a is the UV-Vis absorption spectrum of RhB in the presence of IO CN 1:2, and Figure 11b is the photocatalytic degradation curve for RhB decomposition in the presence of the prepared photocatalyst.
Figure 12 is a graph showing the photocatalytic stability of NC MCN in TC decomposition within 8 cycles.
Figure 13 shows SEM images of unused and used NC MCNs.
Figure 14 is a schematic diagram of the TC and RhB photocatalytic degradation of NC MCN.
Figure 15 shows the proposed intermediate production pathway during the photodecomposition process of TC.
Figure 16 is a graph showing the results of toxicity evaluation of TC intermediates generated by photocatalytic decomposition using NC MCN, including (a) acute toxicity LD50; (b) bioaccumulation factor; (c) developmental toxicity (d) mutagenicity.

The terminology used in this application is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, the terms "comprises" or "has" and the like are intended to specify that the features, components, etc. described in the specification are present, but do not mean that one or more other features or components are not present or cannot be added.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms defined in commonly used dictionaries, such as those defined in common usage, should be interpreted as having a meaning consistent with the meaning they have in the context of the relevant art, and will not be interpreted in an idealized or overly formal sense unless expressly defined in this application.

The term "nano" in this application may mean a size in nanometers (nm), for example, but not limited to, a size of 1 to 1,000 nm. In addition, the term "nanoparticle" in this specification may mean a particle having an average particle diameter in nanometers (nm), for example, but not limited to, a particle having an average particle diameter of 1 to 1,000 nm.

One aspect of the present application relates to a method for producing graphite carbon nitride.

In one example, the manufacturing method may include the steps of forming a gC ₃ N ₄ and SiO ₂ complex; and removing a SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, after the step of removing the SiO ₂ template, the step of centrifuging the residue from which the SiO ₂ template has been removed; the step of washing with distilled water until the pH becomes 7; and the step of drying in an oven at 60 to 80° C. may be additionally included.

In one example, the manufacturing method may include the steps of drying and then pulverizing a colloidal SiO ₂ dispersion to prepare a SiO ₂ cluster template; mixing the SiO ₂ template and melamine to prepare a mixture; heating the mixture to form a gC ₃ N ₄ and SiO ₂ complex; and removing the SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, the colloidal SiO ₂ dispersion can be dried in an oven at 80 to 120 °C. In one example, the colloidal SiO ₂ dispersion can be dried for 10 to 14 hours. In one example, the SiO ₂ template and melamine can be mixed in a weight ratio of 1:1 to 1.5. In one example, the mixture can be heated to 550 °C and maintained for 2 to 4 hours to form a gC ₃ N ₄ and SiO ₂ complex. In one example, the SiO ₂ template can be removed from the gC ₃ N ₄ and SiO ₂ complex by adding a hydrogen fluoride (HF) solution.

Specifically, to prepare mesoporous gC ₃ N ₄ , a colloidal silica dispersion (12 nm, Ludox HS-40, 40% wt.% suspension in water) is dried in an oven at 100 ° C. for 12 h and then ground in a mortar. The obtained SiO ₂ cluster template (1.5 g) is thoroughly mixed with melamine (1.5 g) and transferred to a crucible. Then, the mixture is heated from room temperature to 550 ° C. in air for 4 h and maintained at this temperature for 3 h. The resulting yellow gC ₃ N ₄ /SiO ₂ hybrid is added to 40 mL of 10% HF solution for 24 h to remove the silica template. The suspension is then centrifuged, washed several times with distilled water until the pH becomes 7, and finally dried in an oven at 70 ° C. The resulting catalyst is referred to as NC MCN in the present application. As a reference sample, 1.5 g of melamine was calcined with the same heat treatment as before to synthesize bulk gC ₃ N ₄ , which is referred to as bulk CN.

In addition, the manufacturing method may include a step of preparing SiO ₂ microspheres; a step of mixing molten cyanamide into the SiO ₂ microspheres to prepare a mixture; a step of heating the mixture to form a gC ₃ N ₄ and SiO ₂ complex; and a step of removing a SiO ₂ template from the gC ₃ N ₄ and SiO ₂ complex.

In one example, the weight ratio of cyanamide and silica microspheres can be 1:0.5 to 2.5. In one example, the average diameter of the silica microspheres can be 200 to 400 nm. In one example, the mixture can be mixed at 70 to 90 °C for 20 to 40 minutes, cooled, and then heated again at 500 to 600 °C to form the gC ₃ N ₄ and SiO ₂ complex. In one example, the SiO ₂ template can be removed from the gC ₃ N ₄ and SiO ₂ complex by adding an ammonium hydrogen difluoride solution.

Specifically, uniformly dispersed silica microspheres having a diameter of about 300 nm are synthesized based on the well-known Stober method. Specifically, solution A is prepared by mixing 154 ml of ammonia water, 304 ml of ethanol, and 370 ml of deionized water in a stirrer for 10 minutes. Solution B containing 683 ml of ethanol and 67 ml of tetraethoxysilane (TEOS) is added to solution A within about 1 minute, during which the solution is vigorously stirred (800 r/min). The solution is then maintained under magnetic stirring (400 r/min) for 16 hours to allow the silica particles to grow. The white precipitate is then collected by centrifugation (11000 rpm), washed three times with water and ethanol, mixed with 100 ml of ethanol, sonicated for 15 minutes, and then dried in an oven at 100°C. Finally, the produced white solid is ground in a mortar and pestle, heated in air from room temperature to 550°C for 4 hours, and maintained at this temperature for 6 hours to obtain monodispersed SiO ₂ microspheres.

Figure 1 is a schematic diagram illustrating a method for synthesizing graphite carbon nitride using a hard template method.

Cyanamide is used as a precursor for synthesizing the inverse opal structure of gC ₃ N ₄ . First, 1.5 g of cyanamide is placed on a ceramic plate and melted by stirring at a temperature of 80 ° C. Then, a certain amount of SiO ₂ microspheres is gradually added to the molten cyanamide. Then, the mixture is continuously mixed at the same temperature (80 ° C) for about 30 min. After that, the mixture is cooled, ground, and transferred to a calcination crucible. The crucible is heated in a muffle furnace in air at 550 ° C for 2 h at a ramping rate of 2.2 ° C/min. The resulting yellow sample of gC ₃ N ₄ /SiO ₂ composite is mixed with 40 ml of 4 M ammonium bifluoride for 48 h to completely remove the SiO ₂ hard template. Then, the suspension is washed several times with distilled water until the pH becomes 7 and then dried in an oven at 80 ° C. for 24 hours. The obtained catalyst is referred to as IO CN x:y, where x and y are the mass ratios of cyanamide:silica microspheres, which are IO CN 1:0.5 (IO CN1), IO CN 1:1 (IO CN2), and IO CN 1:2 (IO CN3). As the control catalyst of the present application, bulk gC ₃ N ₄ is synthesized by calcining cyanamide without silica particles by the same heat treatment method represented by bulk CN.

Another aspect of the present application relates to graphite carbon nitride manufactured by the above manufacturing method. In one example, the average volume of the pores can be 0.2 to 0.3 cm ³ g ^-1 . In one example, the average diameter of the pores can be 10 to 20 nm. In one example, the BET specific surface area can be 30 to 70 m ² g ^-1 .

Another aspect of the present application relates to a visible light photocatalyst comprising such graphitic carbon nitride.

Below, the present application is described in more detail through experimental examples.

[SEM and TEM measurements]

Mesoporous graphitic carbon nitride nanoclusters

SEM and TEM were used to explore the morphology and microstructure of the silica clusters and synthetic materials. Fig. 2a is a SEM image of SiO ₂ clusters, Fig. 2b is a SEM image of NC MCN, Fig. 2c is a SEM image of bulk CN, and Figs. 2d to 2h are TEM images of NC MCN.

Fig. 2a is a SEM image of SiO ₂ clusters after drying. As shown in Fig. 2a, the formation of silica clusters can be confirmed through the adhesion and aggregation of small silica nanoparticles. After mixing SiO ₂ clusters and melamine, the growth of the gC ₃ N ₄ plane during the calcination process is restricted and controlled by the shape of the silica clusters.

Figure 2b shows the obtained morphology of NC MCN. In contrast, gC ₃ N ₄ synthesized without using silica clusters shows a highly layered morphology in Figure 2c.

Referring to FIGS. 2d to 2f, the internal structure and morphology of the NC MCN can be further confirmed, which confirms the composition of small clusters and mesoporous architecture.

Figures 2g and 2h show HRTEM fringes of NC MCN with an interplanar spacing of 0.33 nm.

Inverse Opal Graphite Carbon Nitride

Additionally, SEM and TEM were used to explore the morphology and microstructure of the silica clusters and synthetic materials.

In the present application, the content of cyanamide and silica is 1:0.1 to 2.5, preferably 1:0.5, 1:1, and 1:2, and more preferably 1:2.

Fig. 3a is a SEM image of SiO ₂ microspheres, Fig. 3b is a SEM image of IO CN 1:0.5, Fig. 2c is a SEM image of IO CN 1:1, Figs. 2d and 2e are SEM images of IO CN 1:2, and Fig. 2f is a TEM image of IO CN 1:2.

Referring to Fig. 3a, the SEM image of SiO ₂ microspheres at an average size of 300 nm confirms a uniform spherical structure. Fig. 3b is a SEM image of inverse opal gC ₃ N ₄ synthesized using a cyanamide/SiO ₂ mass ratio of 1/0.5. Fig. 3c is SEM images for a cyanamide/SiO ₂ mass ratio of 1/1, and Figs. 3d and 3e are SEM images for a cyanamide/SiO ₂ mass ratio of 1/2, which show a much higher uniform structure. Also, Fig. 3f is a TEM image of IO CN3, which is considered a desirable sample with uniform porosity.

[Specific surface area (BET)]

Mesoporous graphitic carbon nitride nanoclusters

Nitrogen adsorption/desorption measurements were performed to further investigate the surface properties and porosity of the catalyst.

Figure 4a shows the N ₂ adsorption/desorption isotherms of NC MCN and bulk CN, and Figure 4b shows the corresponding pore size distribution curves.

The plots of the two obtained catalysts showed typical type IV isotherms with H3 hysteresis loops in the range of 0.4–0.95P/P0 based on the International Union of Pure and Applied Chemistry (IUPAC) classification, confirming the presence of mesoporous structure.

The BJH pore size distribution confirmed that NC MCN has very abundant mesopores in the range of 2 to 20 nm compared to bulk CN. In addition, the sample also contains pores larger than 20 nm up to 80 nm, which is due to the retroradiation of silica clusters of various sizes.

The pore volume, pore diameter and BET surface area of the sample are shown in Table 1.

Pore volume (cm ³ g ^-1 ) Pore diameter (nm) BET surface area (m ² g ^-1 ) NC MCN 0.226 17.66 51.34 Bulk CN 0.126 23.63 21.49

As shown in Table 1, the NC MCN consistently exhibited a surface area of 51.34 m ² g ^-1 and a pore volume of 0.226 cm ³ g ^-1 , which were higher than those of bulk CN (21.49 m ² g ^-1 , 0.126 cm ³ g ^-1 ), supporting that using silica clusters as a solid template can significantly generate mesoporous structures with increased surface area and enlarged pore volume.

Inverse Opal Graphite Carbon Nitride

Figure 5a shows the N ₂ adsorption/desorption isotherms of IO CN and bulk CN, and Figure 5b shows the corresponding pore size distribution curves.

The plots of both catalysts showed typical type IV isotherms with H3 hysteresis loops in the range of 0.4–0.95P/P0 based on the International Union of Pure and Applied Chemistry (IUPAC) classification, confirming the presence of mesoporous structure. The BJH pore size distribution also confirmed that IO CN 1:2 possessed highly abundant mesopores in the range of 2–80 nm compared to the other samples.

The pore volume, pore diameter and BET surface area of the sample are shown in Table 1.

Pore volume (cm ³ g ^-1 ) Pore diameter (nm) BET surface area (m ² g ^-1 ) IO CN3 0.25 15 66.77 IO CN2 0.12 13.56 37.5 IO CN1 0.12 14.8 33.34 Bulk CN 0.035 21.43 6.5

As shown in Table 2, IO CN3 exhibited a surface area of 66.77 m ² g ^-1 and a pore volume of 0.25 cm ³ g ^-1 , which were higher than those of other samples. This confirms that using silica microspheres as a solid template can significantly increase the surface area and generate a hierarchical mesoporous structure with an enlarged pore volume.

[Photocatalytic performance evaluation]

To determine the photocatalytic performance of the synthesized gC ₃ N ₄ sample for TC and RhB decomposition, the following experiments were performed.

Before the photocatalytic test, 15 mg of catalyst was mixed with 15 mL of aqueous solution of organic pollutants (TC or RhB) at a concentration of 15 mg L ^-1 . The mixture was stirred for 15 min in the dark before irradiation to allow the solution and the catalyst to reach adsorption-desorption equilibrium. After that, a 300 W xenon lamp (with a UV blocking filter and an IR filter (400 < λ < 800 nm)) was used as the light source, and visible light was irradiated to the mixture. The solution was recovered every 15 min during the photocatalytic reaction, centrifuged at a speed of 4000 rpm for 10 min, and the residual concentration of pollutants was measured using a UV-Vis spectrometer. The following equation 1 represents the photocatalytic degradation efficiency (PDE) of TC and RhB.

[Equation 1]

PDE= (C ₀ -C _n )/C ₀ Х 100%

Here, C _O (mg L ^-1 ) is the starting concentration of organic pollutant and C _n is the pollutant concentration after investigation time t.

Mesoporous graphitic carbon nitride nanoclusters

To evaluate the photocatalytic performance of g-C3N4 samples, RhB, TC, and MB were selected as target pollutants. Photocatalytic degradation of the pollutants was performed under visible light irradiation.

Figure 6a is a photocatalytic degradation curve for rhodamine B (RhB) degradation, Figure 6b is a photocatalytic degradation curve for tetracycline (TC) degradation, Figure 6c is a pseudo-first-order kinetic curve for the photocatalytic degradation of rhodamine, and Figure 6d is a pseudo-first-order kinetic curve for the photocatalytic degradation of tetracycline.

As shown in Fig. 6, the blank test (photolysis) induced by visible light showed negligible degradation of the pollutants since the concentration did not change, confirming that the presence of the tested catalyst mainly contributed to the removal of TC and RhB.

The functions of adsorption and photocatalysis were analyzed separately. In terms of adsorption capacity, NC MCN showed a significant increase in adsorption compared to bulk CN under the same conditions after 30 min of reaction under dark conditions, and the adsorption-desorption equilibrium of the suspension was completely achieved. It was observed that the decomposition efficiency of bulk CN for RhB was very low, and it took too much time to completely remove RhB from the solution (see Fig. 6a). On the other hand, NC MCN significantly shortened this time, and RhB was completely decomposed within 30 min under visible light irradiation.

TC degradation is shown in Fig. 6b. The TC degradation ability of bulk CN was very poor due to its low adsorption amount and slow removal rate. In contrast, NC MCN showed very high photocatalytic activity with a degradation rate of more than 70% within 15 min and complete degradation within 30 min, which is verified using LC-MS analysis (Section 3.10). In addition, the photocatalytic degradation data of both pollutants obtained within 25 min were suitable for a pseudo-first-order kinetic reaction, which is calculated using Equation 2 below.

[Equation 2]

ln(C ₀ /C _n ) = kt

Here, k is the first-order kinetic rate constant (min ^-1 ), and C ₀ and C _n are the concentrations of pollutants at reaction times of 0 and n min, respectively.

As shown in Fig. 6c, the kinetic plot of RhB decomposition shows that the reaction rate constant of NC MCN (0.099 min ^-1 ) is about 3.9 times higher than that of bulk CN (0.025 min ^-1 ).

The same results were obtained for TC decomposition, where the reaction rate constant of NC MCN was increased by about 3.4 times compared to bulk CN. As a result, NC MCN showed much higher efficiency and ability in removing RhB and TC from solution. The significantly improved photocatalytic performance of NC MCN is attributed to its larger specific surface area, porous structure, and the introduction of nitrogen defects into the structure.

In addition, the photocatalytic performance of NC MCN was evaluated using MB as a pollutant. The decomposition process was carried out under visible light irradiation, and the UV-vis spectrum is shown in Fig. 7. The adsorption capacity of the sample during the reaction showed that NC MCN showed a high adsorption capacity for MB. After about 30 min of reaction (in the dark) without light irradiation, adsorption-desorption equilibrium was achieved. Therefore, the additional removal of pollutants after the 30 min adsorption experiment was mainly due to the photocatalytic decomposition. The NC MCN catalyst showed efficient performance to completely decompose MB in a 140 min photocatalytic reaction. However, bulk CN showed very poor performance degradation, taking more than 300 min to degrade 50% MB.

Since it is important to analyze the ability of NC MCN for actual wastewater decomposition, additional experiments were conducted by mixing 7.5 ml MB 15 ppm and 7.5 ml RhB 15 ppm into the solution to simulate actual wastewater.

Fig. 8 is a graph of the decomposition UV-vis spectrum (RhB and MB peaks) of the RhB and MB mixture of NC MCN. Fig. 9 is a graph of the decomposition UV-vis spectrum (total peaks) of the RhB and MB mixture of NC MCN.

The same amount of NC MCN photocatalyst as in the previous experiment was added to the solution. MB has two absorption peaks between 600 and 700 nm, and RhB has one absorption peak around 550 nm. The absorption peaks of MB and RhB disappear in a shorter time compared with the time with pure MB, and the complete degradation of the two pollutants was achieved in 100 min under visible light illumination. This indicates that RhB and MB can be completely degraded in a short time. Figure 9 shows the full spectrum from 200 nm to 800 nm. It can be observed that another peak around 200 to 300 nm is generated and becomes higher as the reaction proceeds. This is because some by-products are generated as the reaction proceeds due to the decomposition of original RhB and MB or the combination of other molecules.

Inverse Opal Graphite Carbon Nitride

The photocatalytic performance of inverse opal graphite carbon nitride was evaluated using RhB and MB as target pollutants. The degradation process was performed individually under visible light irradiation and in the presence of each photocatalyst.

Figure 10a is the UV-Vis absorption spectrum of MB in the presence of IO CN 1:2, and Figure 10b is the photocatalytic degradation curve for MB decomposition in the presence of the prepared photocatalyst.

Blank experiments without photocatalyst under visible light irradiation showed very little degradation of MB and RhB, confirming that the removal of pollutants from the solution in the presence of the fabricated photocatalyst was mainly due to the photocatalytic degradation process. In terms of the adsorption capacity of the samples during the reaction, IO CN3 showed superior adsorption of pollutants compared to the other samples.

For MB, adsorption-desorption equilibrium was achieved after approximately 60 min of reaction (under dark conditions) without light irradiation. Therefore, the additional removal of MB after the 60-min adsorption experiment was mainly due to photocatalytic degradation. As shown in Fig. 10b, bulk CN showed the lowest performance in terms of MB decomposition, with only 45% of the pollutants removed after 200 min of visible light irradiation. IO CN1 and IO CN2 showed improved efficacy compared to bulk CN. On the other hand, IO CN3 showed the performance of completely decomposing MB in 110 min of photocatalytic reaction, confirming that this sample had superior efficacy compared to the other samples.

Subsequent experiments on the decomposition of RhB were performed. Fig. 11a is the UV-Vis absorption spectrum of RhB in the presence of IO CN 1:2, and Fig. 11b is the photocatalytic degradation curve for the decomposition of RhB in the presence of the prepared photocatalyst.

The RhB degradation ability of bulk CN was very poor with small adsorption and slow removal rate. In contrast, IO CN 1:2 showed very high photocatalytic activity with a degradation rate of more than 80% within 15 min and complete degradation of RhB within 25 min.

[Stability and reusability of photocatalyst]

In the process of analyzing the ability of photocatalysts for practical applications, the stability and reusability of the catalyst are very important. Therefore, in order to investigate the reusability of NC MCN under visible light irradiation, eight consecutive recycling experiments were performed under the same conditions.

Figure 12 is a graph showing the photocatalytic stability of NC MCN in TC decomposition within 8 cycles.

As shown in Fig. 12, no deactivation of NC MCN was detected and the TC removal rate did not change significantly after 8 cycles of degradation.

In addition, SEM images of the catalysts used for comparison were taken and are shown in Fig. 13. Fig. 13 shows SEM images of unused NC MCN and used NC MCN.

It was found that the morphology of NC MCN did not change after use. These results indicate that NC MCN has high reusability and stability. Through this, NC MCN can be used as a stable and efficient photocatalyst for practical use of pollutant photodegradation by visible light irradiation.

[Enhanced photocatalytic mechanism of NC MCN]

Based on these experimental results, the mechanism for the enhanced photocatalytic activity of NC MCN for the decomposition of TC and RhB is shown in Fig. 14.

As illustrated in Fig. 14, the process of photocatalytic reaction consists of (i) light harvesting; (ii) charge excitation and electron-hole pair generation; (iii) separation and surface transport of electron-hole pairs; and (iv) surface adsorption and oxidation-reduction reaction. Therefore, all of these four steps were optimized in designing NC MCN with excellent photocatalytic ability.

First, the higher surface area together with the porous structure of NC MCN provides abundant exposed active sites for surface reactions and enhanced mass transfer processes, which leads to efficient photodegradation of RhB and TC. In addition, the mesoporous structure of NC MCN enables more efficient light harvesting and utilization because of the higher accessible surface area compared to bulk samples.

Second, the presence of abundant cyano groups and nitrogen vacancies in the structure of NC MCN greatly enhances the light harvesting and charge separation. In the presence of the mid-gap state, in addition to being excited into the conduction band, electrons can also absorb longer-wavelength photons to be excited into the mid-gap state. As a result, the visible light harvesting capability is greatly enhanced. The mid-gap energy state below the conduction band of NC MCN can confine the electrons excited into the conduction band and prevent them from recombination with holes in the valence band, thereby reducing the recombination rate of the photogenerated charges. The trapped electrons react with oxygen dissolved in water to generate superoxide radicals, which play a leading role in the decomposition of RhB and TC.

Finally, since the VB edge potential of NC MCN is more positive than that of bulk CN, it exhibits much better oxidation ability, resulting in superior photodegradation efficiency.

Therefore, it can be confirmed that the NC MCN synthesized in this application exhibits remarkable activity in all four steps, thereby exhibiting remarkable photocatalytic activity in the decomposition of TC and RhB.

[Possible Mechanism of Photocatalytic Decomposition and Toxicity Assessment]

Liquid chromatography-mass spectrometry (LC-MS) technique is used to identify possible degradation pathways and intermediates formed during the photocatalytic process at various study times.

Figure 15 shows the proposed intermediate production pathways in the photodegradation process of TC. The product TC with m/z 445 corresponds to the molecular weight of tetracycline, which can be degraded in three major pathways. Consequently, it is important to investigate the toxicity of the intermediates generated during the water purification process. Therefore, the acute toxicity, bioaccumulation factor, developmental toxicity, and mutagenicity of these organic compounds were estimated to analyze the toxicity of TC and its generated photodegradation intermediates. This estimation was performed based on the quantitative structure-activity relationship (QSAR) method using the toxicity estimation software tool (T.E.S.T.).

Figure 16 is a graph showing the results of toxicity evaluation of TC intermediates generated by photocatalytic decomposition using NC MCN, including (a) acute toxicity LD50; (b) bioaccumulation factor; (c) developmental toxicity (d) mutagenicity.

As shown in Fig. 16a, the LD50 value of TC for rats is 806.96 mg.kg ^-1 , which is evaluated as "highly toxic". Except for P12 and P13, all other intermediates showed higher LD50 values than TC, confirming that the acute toxicity of the intermediates was reduced. Although P12 and P13 are highly toxic, the major final intermediates of TC are P16, P17, and P18, which have high LD50 values and lower toxicity than TC. In addition, although the bioconcentration factors of most intermediates are higher than that of TC, the final product of P18 has a low bioaccumulation factor, which is advantageous (see Fig. 16b). It can be seen from Fig. 16c that the developmental toxicity of all intermediates generated, except for P3, is lower than that of TC. However, the developmental toxicities of P4, P9, P10, P14, and P16 were very close to the developmental non-toxic region. Also, TC is "mutagenic" and some intermediates have higher mutation values, but the final intermediates from P11 to P18 should be in the negative mutagenic range and much lower than TC (see Figure 16d). This confirms that the toxicity of most of the generated intermediates was reduced during visible light irradiation. However, since a completely non-toxic solution should be obtained, it is desirable to extend the photocatalytic reaction time to purify polluted water.

According to the present invention, various shapes and structures of graphite carbon nitride photocatalysts can be synthesized by using silica particles of various shapes. Both mesoporous gC ₃ N ₄ nanoclusters and inverse opal gC ₃ N ₄ showed excellent activity for the decomposition of various organic pollutants.

The manufacturing method of this present application is a simple manufacturing method, which can provide easy availability of raw materials, shape and porous engineering of photocatalyst, enhanced photocatalytic decomposition ability of photocatalyst, high stability and reusability. These advantages make it suitable for practical applications. The present application on the composition of defective shape-controlled gC ₃ N ₄ and decomposition, mineralization and toxicity reduction of TC can provide a method for producing highly efficient photocatalyst in applications related to wastewater and environmental pollution improvement.

Although the above has been described with reference to preferred embodiments of the present application, it will be understood by those skilled in the art that various modifications and changes can be made to the present application without departing from the spirit and scope of the present invention as set forth in the claims below.

Claims (18)

delete delete A step of drying a colloidal SiO ₂ dispersion in an oven at 80 to 120° C. for 10 to 14 hours and then pulverizing it to prepare a SiO ₂ cluster template;
A step of preparing a mixture by mixing SiO ₂ template and melamine in a weight ratio of 1:1 to 1.5;
A step of heating the mixture to 550 ℃ and maintaining it for 2 to 4 hours to form a gC ₃ N ₄ and SiO ₂ complex; and
A method for producing graphitic carbon nitride, comprising the step of removing a SiO ₂ template from a gC ₃ N ₄ and SiO ₂ complex by adding a hydrogen fluoride (HF) solution. delete delete delete delete delete A step of preparing SiO ₂ microspheres having an average diameter of 200 to 400 nm;
A step of preparing a mixture by mixing molten cyanamide into SiO ₂ microspheres at a weight ratio of cyanamide and silica microspheres of 1:0.1 to 2.5;
A step of mixing the mixture at 70 to 90°C for 20 to 40 minutes, cooling, and then heating again at 500 to 600°C to form a gC ₃ N ₄ and SiO ₂ complex; and
A method for producing graphite carbon nitride, comprising the step of removing a SiO ₂ template from a gC ₃ N ₄ and SiO ₂ complex by adding an ammonium hydrogen difluoride solution. delete delete delete delete Graphite carbon nitride manufactured by the manufacturing method of claim 3 or 9. In Article 14,
Graphitic carbon nitride having an average pore volume of 0.2 to 0.3 cm ³ g ^-1 . In Article 14,
Graphitic carbon nitride having an average pore diameter of 10 to 20 nm. In Article 14,
Graphitic carbon nitride with a BET surface area of 30 to 70 m ² g ^-1 . A visible light photocatalyst comprising graphitic carbon nitride according to claim 14.