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Oct 7, 2018 - photocatalytic composite has great application prospects in organic pollutant ... Nevertheless, these methods face many drawbacks like high-cost, ... carbon nitride structure, using a cyanuric acid-melamine (CM) ... Beijing Reagent Co. ..... indicated that the cyanuric acid modified g-C3N4/kaolinite will make ...
minerals Article

Fabrication of Novel Cyanuric Acid Modified g-C3N4/Kaolinite Composite with Enhanced Visible Light-Driven Photocatalytic Activity Zhiming Sun 1, * , Fang Yuan 1 , Xue Li 1 , Chunquan Li 1 , Jie Xu 1 and Bin Wang 2, * 1

2

*

School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; [email protected] (F.Y.); [email protected] (X.L.); [email protected] (C.L.); [email protected] (J.X.) Beijing Key Laboratory of Clothing Materials R & D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing 100029, China Correspondence: [email protected] (Z.S.); [email protected] (B.W.); Tel.: +86-10-62339920 (Z.S.)

Received: 2 September 2018; Accepted: 1 October 2018; Published: 7 October 2018

 

Abstract: A novel kind of cyanuric-acid-modified graphitic carbon nitride (g-C3 N4 )/kaolinite (m-CN/KA) composite with enhanced visible light-driven photocatalytic performance was fabricated through a facile two-step process. Rhodamine B (RhB) was taken as the target pollutant to study the photocatalytic performance of the synthesized catalysts. It is indicated that the cyanuric acid modification significantly enhanced photocatalytic activity under visible light illumination in comparison with the other reference samples. The apparent rate constant of m-CN/KA is almost 1.9 times and 4.0 times those of g-C3 N4 /kaolinite and bare g-C3 N4 , respectively. The superior photocatalytic performance of m-CN/KA could be ascribed, not only to the generation of abundant pore structure and reactive sites, but also to the efficient separation of the photogenerated electron-hole pairs. Furthermore, the possible photocatalytic degradation mechanism of m-CN/KA was also presented in this paper. It could be anticipated that this novel and efficient, metal-free, mineral-based photocatalytic composite has great application prospects in organic pollutant degradation. Keywords: kaolinite; photocatalysis; cyanuric acid modification; g-C3 N4

1. Introduction In the past few decades, the problem of industrial and domestic wastewater was becoming more and more serious. A great amount of wastewater was discharged directly due to its complicated and expensive treatment process, resulting in severe damage to soil and water resources. Accordingly, highly efficient removal and degradation of wastewater has attracted a great deal of attention from environmental scientific research in the past few years [1–3]. Several approaches have been proposed to resolve this problem, such as ultrafiltration [4], absorption [5], and biological treatment [6]. Nevertheless, these methods face many drawbacks like high-cost, low-efficiency, non-recyclable, etc. Hence, semiconductor-based photocatalysis, a highly efficient, non-toxic, and low-cost method, which can degrade a variety of pollutants in wastewater via a series of simple redox reactions under illumination should be one kind of promising technology in treating wastewater [7,8]. However, some conventional semiconductor photocatalysts like ZnO and TiO2 in a pure form have less visible responses, which severely restrict their practical application [9]. Therefore, the fabrication of high-activity, visible light-driven photocatalysts have become a hot topic.

Minerals 2018, 8, 437; doi:10.3390/min8100437

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In recent years, graphitic carbon nitride (g-C3 N4 ) has been widely researched in photocatalysis, owing to its superior visible-light response, stable structure, metal-free and environmentally friendly superiority, after being reported by Wang et al. [10,11]. Since Niu et al. [12] successfully developed covalent solid carbon nitride using laser sputtering technology, extensive research has investigated preparation methods of g-C3 N4 , such as HTHP (high temperature and high pressure) process [13], solid-phase synthesis [14], and liquid-phase electrodeposition [15]. Compared with the other methods, thermal polymerization is considered to be the most common method for synthesizing g-C3 N4 because of it is easy-to-operate, low cost, there is an availability of raw materials, and has a better crystal form [16]. However, a photocatalyst prepared by the thermal polymerization method has the disadvantages of low specific surface area and low utilization of visible light. According to previous reports, if g-C3 N4 could be completely exfoliated to monolayers, the specific surface area would increase several times [17]. In order to improve the photocatalytic properties of visible-light irradiation, many researchers have put great effort into modification using various methods, such as metallic elements doping, construction of semiconductor heterojunction, precious metal deposition, and organic acid modification. For instance, Wang et al. [18] successfully designed a K-modified g-C3 N4 by facile thermal polymerization with KBr as the K source, where the rate of H2 evolution was 5.6 times compared to pure g-C3 N4 . Sridharan et al. [19] synthesized g-C3 N4 /TiO2 using thermal transformation methodology, resulting in better pollutant degradation due to a synergistic heterojunction between g-C3 N4 and TiO2 . Shalom et al. [20] reported a new and facile approach to form an ordered hollow carbon nitride structure, using a cyanuric acid-melamine (CM) complex in ethanol as the starting precursor. These methods are mainly used to restrain the recombination and prolong the life of photo-carriers in g-C3 N4 by increasing the specific surface area of the catalyst, replacing the in-situ lattice, producing a Schottky barrier, generating an impurity level, and then improving photoactivity. Although these modification methods can partially improve the defects of g-C3 N4 , it is still hard to avoid the disadvantages of easy aggregation, difficult separation, high production cost, and low adsorption capacity [21]. Currently, a number of studies have paid close attention to composites based on natural minerals supporting catalyst nanoparticles due to their low-cost, non-toxic, high adsorption, and chemical stability [22,23]. It has been demonstrated that photocatalytic efficiency can be improved after the introduction of natural minerals, which can, not only retain the photocatalytic activity of the catalyst, but also promote rapid pollutant adsorption on the surface of the composite photocatalyst [24,25]. Among the various natural minerals carriers, kaolinite appears to be a superior candidate as a g-C3 N4 carrier, because kaolinite is a 2D layered silicate composed of one tetrahedrally-coordinated sheet of silicon and one octahedrally-coordinated sheet of aluminum, which is similar to the 2D structure of g-C3 N4 [26]. Therefore, a 2D/2D composite structure could be established by the combination of the kaolinite and g-C3 N4 photocatalyst, which would significantly improve the photocatalytic efficiency of catalysts. For the past few years, our group has focused on the photocatalytic performance of g-C3 N4 /kaolinite composite [26–28]. However, the photocatalytic performance, especially in visible light, is still limited. Hence, how to further improve the visible photocatalytic activity of g-C3 N4 /kaolinite composite has become the key to the large-scale application of this material. In our present study, we synthesize a novel kind of g-C3 N4 /kaolinite composite through a two-step process using cyanuric acid as a modifier. The physicochemical properties of as-prepared composites were systematically analyzed. Furthermore, the photocatalytic performance of cyanuric-acid-modified g-C3 N4 /kaolinite composites were evaluated by the photo-degradation of rhodamine B under visible-light irradiation. The influence of the cyanuric acid modifier amounts on visible-light photocatalytic efficiency of the as-prepared photocatalysts was probed. The possible mechanism of photocatalytic performance was also proposed and discussed. Our work is beneficial for better understanding the enhancement mechanism of mineral-based composite photocatalysts and designing more efficient metal-free, mineral-based photocatalytic composites for wastewater treatment.

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2. Materials and Methods 2.1. Materials The purified kaolinite (denoted as KA) was acquired from Suzhou City, China. Melamine (C3 H6 N6 ) was bought from Jiaxing Sicheng Chemical Co., Ltd. (Zhejiang, China). Cyanuric acid (C3 H3 N3 O3 ) and p-Benzoquinone (BQ) were purchased from Sinopharm Chemical Reagent Co. (Beijing, China). Beijing Reagent Co. (Beijing, China) provided rhodamine B (C28 H31 C1 N2 O3 , RhB), tert-butanol (TBA), edetate disodium (EDTA-2Na), and other chemicals used in our studies. The commercially available Degussa P25 was employed as reference. All of the chemicals were analytical reagent grade without further treatment and deionized water was used throughout this study. 2.2. Catalysts Preparation The g-C3 N4 (denoted as CN) sample was synthesized by thermal polymerization of melamine in air, which was based on our previous reports [29]. Twenty grams of melamine was added into an alumina crucible with a cover and heated to 550 ◦ C at a heating rate of 2.3 ◦ C·min−1 and kept at this temperature for 4 h. After cooling down to room temperature, the sample was further heated to 500 ◦ C for 2 h in an open-air-atmosphere tube furnace with a heating rate of 5 ◦ C·min−1 . The final resultant yellow product was ground into powder for further use. For synthesizing cyanuric-acid-modified g-C3 N4 /kaolinite (denoted as m-CN/KA) composites, we followed a facile two-step procedure; wet impregnation and thermal polymerization processes. In detail, first, 4 g of melamine and various amounts of cyanuric acid were added into 50 mL of deionized water and stirred until the chemicals dissolved completely. Then, 2 g of purified kaolinite (KA) were dispersed into the above solution with uniform stirring for 12 h. After, the mixture was evaporated at 60 ◦ C using a water bath with continuous stirring. The resulting powder was calcined under the same conditions as those of the CN catalyst. Thermogravimetric analysis (TG) results showed that the content of graphitic carbon nitride in the CN/KA composite could be calculated as 38.82%, which was in accordance with the theoretical loading amount (40%). For comparison, g-C3 N4 /kaolinite (denote as CN/KA) composites were also synthesized according to the above steps in the absence of the cyanuric acid modifier. The m-CN/KA samples with different cyanuric acid modifier amounts with respect to m-CN/KA were labeled as m-CN/KA-0.5, m-CN/KA-1, m-CN/KA-2, m-CN/KA-3, and m-CN/KA-4, respectively. The number at the end of each label corresponds to the additive mass of cyanuric acid. 2.3. Characterization The phase properties of the as-prepared composites were detected using a D8 advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu-Kα radiation (λ = 0.154056 nm), which were scanned in the range of 2θ from 10◦ to 80◦ with a 0.02◦ step at a scanning speed of 4◦ ·min−1 . The Brunauer-Emmett-Teller (BET) specific surface areas and pore size distributions of the as-prepared composites were determined by nitrogen adsorption instrument equipped with a JW-BK nitrogen adsorption apparatus (JWGB Sci. & Tech, Beijing, China) at 77 K. The morphology of the as-received composites was determined on an S-4800 scanning electron microscope (SEM) (Hitachi, Tokyo, Japan). Fourier transform infrared (FT-IR) spectroscopy was investigated using a ThermoFisher Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), which was prepared using potassium bromide (KBr) pellets. A METTLER SF/1382 thermal analyzer (Columbus, OH, USA) was employed to probe the thermogravimetric analysis (TG) under an O2 atmosphere at a heating rate of 10 ◦ C·min−1 , from ambient temperature to 1000 ◦ C. UV-vis diffuse reflectance spectrometry (UV-vis DRS) was also applied in this study, and the UV-vis absorption spectra were obtained using a UV-vis spectrophotometer (U-3100, Hitachi, Tokyo, Japan), where BaSO4 was used as the reference.

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2.4. Photoactivity Measurements To evaluate the photocatalytic activities of the as-received catalysts, the photo-degradation performance of rhodamine B (RhB) aqueous solution was determined using a 500-W Xenon lamp with a 420-nm cut-off filter, which could guarantee the same wavelength as visible-light irradiation (BL-GHX-V, Shanghai Bilang plant, China). In a typical experiment, 0.2 g of the as-received composites were dispersed in 100 mL of RhB (10 mg·L−1 ) aqueous solution by sonicating for 5 min in a quartz tube. To achieve the establishment of an adsorption/desorption equilibrium, 1 h of dark reaction was conducted before visible-light irradiation. At specific time intervals, 4 mL of suspension was collected and centrifuged, then the absorbance of the supernatant was measured at 554 nm using a UV-vis spectrophotometer, which reflected the photodegradation performance of the composites. Throughout the reaction, the catalyst was uniformly dispersed in the solution using a magnetic stirrer. The Degussa P25, CN and CN/KA composites served as references under the same conditions to carry out comparative experiments. 2.5. Computational Method In this work, theoretical calculations were conducted in the CASTEP code using the Density Function Theory (DFT) approach. During the calculation, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function and spin-polarized generalized gradient approximation (GGA) exchange-correlation functions were used. The interaction effects between the layered mineral (i.e., kaolinite) and the g-C3 N4 were described by the ultrasoft pseudopotential. The modification effect of cyanuric acid towards the g-C3 N4 /kaolinite system was also researched. During model construction, the structure of g-C3 N4 was introduced into the primitive cell of kaolinite to construct the composites. The kinetic cut-off energy was set at 340 eV for the plane wave expansion, the threshold values were 0.001 Å for maximum displacement based on the convergence criteria, the maximum stress was 0.05 GPa, maximum force was 0.03 eV/Å, energy tolerance was 1.0 × 10−5 eV/atom, and self-consistent field tolerance was 1.0 × 10−6 eV/atom. 3. Results and Discussion 3.1. XRD Analyses XRD patterns of CN, KA, CN/KA, and m-CN/KA composites with various cyanuric acid modifier amounts are shown in Figure 1. For kaolinite, the characteristic diffraction peaks at 12.43◦ , 20.00◦ , 25.01◦ , and 38.75◦ can be indexed as (001), (020), (002), and (200) diffraction planes, which are in line with the patterns of kaolinite (JCPDS No. 29-1490). It was found that the kaolinite characteristic diffraction peaks were not detected in the XRD patterns of CN/KA or m-CN/KA composites, which indicated that the kaolinite in composites were presumably becoming amorphous after calcination [26–28]. Additionally, the XRD pattern of CN exhibited a weak peak at 13.12◦ and an intense peak at 27.67◦ , consistent with the interlayer distance of 0.671 nm and 0.324 nm, which could be indexed as the (100) and (002) lattice planes of g-C3 N4 (JCPDS No. 87-1526), respectively. The former peak was due to the S-triazine structural arrangement of inter-layer packing and the latter peak could be ascribed to the long-range interplanar stacking of conjugated aromatic systems of the g-C3 N4 catalyst. From the XRD patterns of CN/KA and m-CN/KA composites, it was obvious that they displayed only one peak at 27.67◦ (002), which signified that g-C3 N4 was successfully loaded onto the surface of kaolinite. As for the characteristic peak of g-C3 N4 at 13.12◦ (100), it disappeared, as exhibited in the composites, owing to the low catalyst content and low intensity. Furthermore, there was no obvious difference among the XRD patterns of CN/KA and m-CN/KA composites. Hence, it can be inferred that the introduction of cyanuric acid has little significant influence on the crystallographic structure of g-C3 N4 in composites.

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Figure 1. composites with various 1. XRD XRD patterns patterns of of KA, KA, CN, CN, CN/KA CN/KA composite composite and and m-CN/KA m-CN/KA composites cyanuric acid modifier amounts.

3.2. Morphology Analysis Analysis 3.2. Morphology The and m-CN/KA-4 m-CN/KA-4was wasidentified identified using using aa scanning scanning electron electron The morphology morphology of of CN, CN, KA, KA, CN/KA, CN/KA, and microscope (SEM), as observed in Figure 2. From Figure 2a, KA possesses a typical two-dimensional microscope (SEM), as observed in Figure 2. From Figure 2a, KA possesses a typical two-dimensional layered which might, might, not not only only provide layered structure structure composed composed of of many many parallel parallel nanosheets, nanosheets, which provide large large surface area for for enhancing enhancing adsorption adsorptionability, ability,but butalso alsoclosely closelyintegrate integratewith withg-C g-C 34Nnanosheets 4 nanosheets surface area 3N in in composites. Furthermore, smooth regular surface, without impurities, would thethe composites. Furthermore, the the smooth andand regular surface, without impurities, would alsoalso be be significantly beneficial in loading the loading of3N g-C to Figure 2b, a two-dimensional 3 N4 . According significantly beneficial in the of g-C 4. According to Figure 2b, a two-dimensional layered layered structure with bulk morphology was formed, owing to the aggregation effect, forming in structure with bulk morphology was formed, owing to the aggregation effect, forming in the process the process of thermal polymerization. This would be the disadvantage to the contact between of thermal polymerization. This would be the disadvantage to the contact between catalysts and catalysts pollutioninmolecules in photocatalytic and further decrease the migration of pollutionand molecules photocatalytic processes,processes, and further decrease the migration of the the photo-generated carrier, which results in a poor adsorption capacity and low quantum efficiency photo-generated carrier, which results in a poor adsorption capacity and low quantum efficiency of of pure N4 . illustrated As illustrated in Figure as-prepared CN/KA composite maintained pure g-Cg-C 3N43. As in Figure 2c, 2c, the the as-prepared CN/KA composite still still maintained the the original lamellar structure kaolinite, lamellarsurface surfacebecome becomerougher rougherand and the the edges original lamellar structure of of kaolinite, butbut thethelamellar edges became was successfully became smoother. smoother. The The morphology morphology varieties varieties showed showed that that the the g-C g-C33N N44 was successfully loaded loaded onto onto the kaolinite surface and had an effective interfacial bond. Compared with CN, the introduction the kaolinite surface and had an effective interfacial bond. Compared with CN, the introduction of of promoted dispersion of g-C N in CN/KA composite, which would be favorable for KAKA promoted thethe dispersion of g-C 3N43 in4 CN/KA composite, which would be favorable for the the improvement of photoactivity. As shown in Figure compared with CN/KA composite, improvement of photoactivity. As shown in Figure 2d, 2d, compared with the the CN/KA composite, the the surface of g-C N /kaolinite became more porous and uniform after cyanuric acid modification, 3 4 surface of g-C3N4/kaolinite became more porous and uniform after cyanuric acid modification, which in precursor the which should should be be responsible responsible for for the the complete complete volatilization volatilization of of cyanuric cyanuric acid acid in precursor during during the calcination process. In summary, the introduction of cyanuric acid effectively improved the pore calcination process. In summary, the introduction of cyanuric acid effectively improved the pore N44 in in aa binary binary system, system, providing providing more more active active sites sites for for pollutant pollutant photodegradation. photodegradation. structure structure of of g-C g-C33N

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Figure 2. SEM images of (a) KA, (b) CN, (c) CN/KA composite, (d) m-CN/KA-4 composite. Figure2.2.SEM SEMimages imagesof of(a) (a)KA, KA,(b) (b)CN, CN,(c) (c)CN/KA CN/KA composite, (d) m-CN/KA-4 m-CN/KA-4 composite. Figure composite.

3.3.BET BETAnalysis Analysis 3.3. 3.3. BET Analysis To further investigate the pore structure in CN, KA, CN/KA, and m-CN/KA-4 composites, the To further investigate the the pore porestructure structure inCN, CN,KA, KA,CN/KA, CN/KA, and m-CN/KA-4 composites, furtherarea, investigate and m-CN/KA-4 composites, the BETTosurface pore volume and average in pore diameter (calculated by Barrett-Joyner-Halenda the BET surface area, pore volume and average pore diameter (calculated by Barrett-Joyner-Halenda BET surface area, pore volume and average pore diameter (calculated by Barrett-Joyner-Halenda method) were examined by the N2 adsorption-desorption isotherms, and the obtained results are method) were examined by N adsorption-desorption isotherms, and and the the obtained obtained results results are method) examined by the theto N22Table adsorption-desorption isotherms, are listed inwere Table 1. According 1, among the as-prepared samples, it is apparent that listed in Table 1. According to Table 1, among the as-prepared samples, it is apparent that m-CN/KA-4 listed in Table 1. According to Table 1, among the pore as-prepared samples, is apparent that m-CN/KA-4 possesses the largest BET surface area and volume, which are it almost twice those possesses the possesses largest BET surface area and pore volume, which are almostwhich twice are those of CN.twice The larger m-CN/KA-4 the largest BET surface area and pore volume, almost those of CN. The larger surface area and total pore volume of the m-CN/KA composites could effectively surface arealarger and total pore area volume the pore m-CN/KA composites could effectively enhance ofenhance CN. The surface andof total of the m-CN/KA composites could adsorption effectively adsorption and photocatalytic ability,volume which account for the uniform distribution of g-C3N4 and photocatalytic ability,photocatalytic which account for thewhich uniform distribution of g-C3 Ndistribution and 4 on kaolinite enhance adsorption ability, account uniform of g-C3the N4 on kaolinite and theand volatilization of cyanuric acid according tofor thethe previous morphological analysis volatilization of cyanuric acid according to the previous morphological analysis results. As shown onresults. kaolinite the in volatilization of N cyanuric acid according toisotherms the previous morphological analysis As and shown Figure 3, the 2 adsorption-desorption of CN/KA and m-CN/KA in FigureAs 3, ashown the N2 adsorption-desorption isotherms CN/KAisotherms andon m-CN/KA displayed a typical results. inIVFigure 3, theisotherm N2 adsorption-desorption CN/KA m-CN/KA displayed typical adsorption with a H3 of hysteresis loop theofbasis of theand International IV adsorption isotherm with a H3 hysteresis loop on the basis of the International Union of Pure and Union ofaPure and Chemistry which is theloop standard displayed typical IVApplied adsorption isotherm(IUPAC), with a H3 hysteresis on thestate basisofofthe the mesoporous International Applied Chemistry (IUPAC), which is the standard state of the mesoporous structure [30]. As for CN structure [30]. and As for CN and KA, both showedwhich a typical II standard adsorption isotherm, was Union of Pure Applied Chemistry (IUPAC), is the state of the which mesoporous and KA, both typical IIKA, adsorption isotherm, which was attributed to the nonporous and attributed to showed the and microporous structure. Furthermore, as shown in Figure 3b, after structure [30]. Asnonporous for aCN and both showed a typical II adsorption isotherm, which was microporous structure. Furthermore, as shown in Figure 3b, after modification, the number of pores modification, thenonporous number of and pores within 2 nm structure. to 4 nm increased significantly, and in a wider attributed to the microporous Furthermore, as shown Figurepore 3b, size after within 2 nm tothe 4 nm andtoa4wider pore sizesignificantly, distribution, from 1 nm to 20and nm, distribution, from 1increased nm of to pores 20significantly, nm, was obtained, which would also favor of and the modification, number within 2 nm nm increased aadsorption wider pore size was obtained,from which favor of the adsorption and photodegradation a target pollutant. photodegradation a target distribution, 1ofwould nm to also 20pollutant. nm, was obtained, which would also favor ofofthe adsorption and photodegradation of a target pollutant.

Figure3.3.(a) (a)NN2 2 adsorption adsorption isotherm isotherm and (b) pore-size pore-size distribution distribution curves Figure curves of of CN, CN, KA, KA, CN/KA CN/KA composite,and andm-CN/KA-4 m-CN/KA-4 composite. composite. composite, Figure 3. (a) N2 adsorption isotherm and (b) pore-size distribution curves of CN, KA, CN/KA composite, and m-CN/KA-4 composite.

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Table 1. BET surface area, pore volume, and average pore size of m-CN/KA composites with various Table 1. BET surface area, pore volume, and average pore size of m-CN/KA composites with various modifier amounts. modifier amounts.

Samples Samples KA KA CN CN CN/KA CN/KA m-CN/KA-0.5 m-CN/KA-0.5 m-CN/KA-1 m-CN/KA-1 m-CN/KA-2 m-CN/KA-2 m-CN/KA-3 m-CN/KA-3 m-CN/KA-4 m-CN/KA-4

Surface Area (m2·g−1) Pore Volume (cm3·g−1) Average Pore Size (nm) 3 −1 Average Pore Size (nm) Surface Area (m2 ·g−1 ) Pore Volume 22.957 0.059 (cm ·g ) 6.979 22.957 0.059 6.979 29.672 0.066 7.100 29.672 0.066 7.100 27.378 0.062 6.610 27.378 0.062 6.610 28.562 0.063 6.591 28.562 0.063 6.591 33.835 0.066 6.445 33.835 0.066 6.445 40.632 0.076 6.434 40.632 0.076 6.434 44.519 0.082 6.309 44.519 0.082 6.309 49.522 0.088 6.251 49.522 0.088 6.251

3.4. Optical Optical Properties Properties 3.4. A UV-vis A UV-vis diffuse diffuse reflection reflection spectrum spectrum (DRS) (DRS) was was applied applied to to inspect inspect the the optical optical absorption absorption properties of KA, CN, CN/KA, and m-CN/KA composites. As illustrated in Figure 4a, it is implied properties of KA, CN, CN/KA, and m-CN/KA composites. As illustrated in Figure 4a, it is implied that all these as-prepared catalysts exhibited superior absorption capacity from the UV that all these as-prepared catalysts exhibited superior absorption capacity from the UV to to visible visible light region. region. KA KA absorbed absorbed light light from from the the UV UV to to the the visible visible region, region, while while the the bare bare CN light CN started started from from around 480 nm. Moreover, it is obvious that the optical absorption of the as-prepared CN/KA around 480 nm. Moreover, it is obvious that the optical absorption of the as-prepared CN/KA and and m-CN/KAcomposites compositeswas wasgreatly greatly enhanced enhanced within within the the scope scope of of both both UV UV and and visible visible light light compared compared m-CN/KA with and KA, KA, which which can can be be attributed attributed to to the the defects defects and and vibration vibration produced produced by by intimate intimate with those those of of CN CN and interfacial combination between g-C N and kaolinite. Therefore, due to the stronger light absorption 3 43N4 and kaolinite. Therefore, due to the stronger light interfacial combination between g-C ability and the larger range of wavelengths the photon absorption of as-prepared catalysts, more absorption ability and the larger range of and wavelengths and the photon absorption of as-prepared photogenerated electron-hole pairs would be created under irradiation, which might be a key factor catalysts, more photogenerated electron-hole pairs would be created under irradiation, which might to increase photodegradation efficiency. In addition, theInband gaps of as-prepared catalysts were be a key factor to increase photodegradation efficiency. addition, thethe band gaps of the as-prepared calculatedwere and calculated are shown and in Figure 4c,d. The calculated gap of CNband was 2.72 andwas all the catalysts are shown in Figure 4c,d. band The calculated gap eV, of CN 2.72band eV, gaps of the m-CN/KA composites were around 2.72 ± 0.01 eV, indicating that the introduction of and all the band gaps of the m-CN/KA composites were around 2.72 ± 0.01 eV, indicating that the cyanuric acidof had little oracid no effect on the band gaps g-C 3 N4 .gaps of g-C3N4. introduction cyanuric had little or no effect onofthe band

(a,b) UV-vis DRS and Figure 4. (a,b) and (c,d) (c,d) band band gaps gaps ofofCN, CN,KA, KA,CN/KA CN/KA composite, composite, and and m-CN/KA m-CN/KA composites with different cyanuric acid modifier modifier amounts. amounts.

3.5. FT-IR Analysis The functional groups of KA, CN, CN/KA, and m-CN/KA-4 were analyzed using FT-IR spectroscopy, and the results are shown in Figure 5. Regarding KA, the strong peak at 3689 cm−1 and

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3.5. FT-IR Analysis The2018, functional groups of KA, CN, CN/KA, and m-CN/KA-4 were analyzed using FT-IR Minerals 8, x FOR PEER REVIEW 8 of 15 spectroscopy, and the results are shown in Figure 5. Regarding KA, the strong peak at 3689 cm−1 and −1 denoted the OH stretching of inner surface hydroxyl 3621 3621cm cm−1 denoted the OH stretching of inner surface hydroxyl and OH stretching of inner hydroxyl, −1 −1 respectively. respectively. The The band band in inthe therange rangeofofaround around1600 1600cm cm−1 to to1700 1700cm cm−1 represented the the presence presence of of −1−1to 1000 cm−−1 1 were co-intercalated water molecules. The peaks located in the range of 1100 cm co-intercalated water molecules. The peaks located in the range of 1100 cm to 1000 cm were −1 assigned assigned to tothe theSi–O Si–Ostretching. stretching.The Theband bandatat917 917cm cm−1 displayed the Al–OH Al–OH bending bending vibrations, vibrations, and and − 1 − 1 −1 −1 the bands at 787 cm and 695 cm were attributed to O–Al–OH absorption vibrations. The bands the bands at 787 cm and 695 cm were attributed to O–Al–OH absorption vibrations. The bands at −1 and 540 −1 were related to the Al–O–Si deformations, and the bands−1 at 465 cm−1−1 −1 and at 757757 cmcm 540 cm−1 cm were related to the Al–O–Si deformations, and the bands at 465 cm and 434 cm − 1 and 434 cm the Si–O–Si The deformations. The characteristic infrared bands referred to the referred Si–O–Si to deformations. characteristic infrared absorption bandsabsorption between CN/KA between CN/KA and m-CN/KA-4 were similar, but the intensity of infrared absorption bands of and m-CN/KA-4 were similar, but the intensity of infrared absorption bands of m-CN/KA-4 was − 1 −1 m-CN/KA-4 was obviously thanThe thatstrong of CN/KA. The strong peak between 3000corresponded and 3300 cm to obviously weaker than thatweaker of CN/KA. peak between 3000 and 3300 cm corresponded to stretching NH2 caused or NH groups caused by incomplete polycondensation of stretching of terminal NH2oforterminal NH groups by incomplete polycondensation of g-C3N4 at the g-C N at the defect sites of the aromatic ring. Compared with CN/KA, the intensity of absorption defect 3 4 sites of the aromatic ring. Compared with CN/KA, the intensity of absorption peaks peaks corresponding to stretching of terminal NH groups obviously decreased, indicating corresponding to stretching of terminal NH2 NH or NH obviously decreased, indicating thatthat the 2 or groups the addition of cyanuric acid promoted the polycondensation of g-C N in the as-prepared composites, addition of cyanuric acid promoted the polycondensation of g-C33N4 4in the as-prepared which NH22 or orNH NHgroups. groups.The Theabsorption absorptionbands bands range which significantly significantly reduced the terminal NH inin thethe range of − 1 −1 corresponded of 1100–800 corresponded Si–Ostretching stretchingvibration vibrationand and Al-OH Al-OH bending bending vibration in 1100–800 cmcm to to thetheSi–O in kaolinite, kaolinite, where where m-CN-KA-4 m-CN-KA-4 is is significantly significantly smoother. smoother. Hence, Hence, itit shows shows that that the the reactive reactive sites sites on on the the kaolinite significantly, and the were combined with g-C3 with N4 more kaolinitesurface surfacedecreased decreased significantly, andothers the others were combined g-Ceffectively. 3N4 more Iteffectively. could be demonstrated that the g-Cthat nanosheets had been attached the layered It could be demonstrated g-C3N4 nanosheets had beentoattached to thekaolinite layered 3 N4 the successfully and the distribution of g-C N on the surface carrier became more uniform due to due the kaolinite successfully and the distribution 3 of 4 g-C3N4 on the surface carrier became more uniform introduction of cyanuric acid. In addition, there werethere no newborn in thegroups m-CN-KA-4, to the introduction of cyanuric acid. In addition, were nofunctional newborngroups functional in the indicating thatindicating the introduction ofintroduction cyanuric acidofdid not change groups of g-C3 N4groups in the m-CN-KA-4, that the cyanuric acid the didfunctional not change the functional as-prepared of g-C3N4 incomposites. the as-prepared composites.

Figure Figure5.5.FT-IR FT-IRspectra spectraofofCN, CN,KA, KA,CN/KA CN/KA composite, composite,and andm-CN/KA-4 m-CN/KA-4 composite.

3.6. 3.6. DFT DFT Analysis Analysis CASTEP, Cambridge Sequential Total Energy Package, is a frequently-used modulemodule based CASTEP,i.e., i.e.,the the Cambridge Sequential Total Energy Package, is a frequently-used on density functional theory (DFT). Through accessing the CASTEP module embedded within the based on density functional theory (DFT). Through accessing the CASTEP module embedded within material studio software (Materials Studio 8.0, Accelrys, San Diego, CA, USA), we can realize electronic the material studio software (Materials Studio 8.0, Accelrys, San Diego, CA, USA), we can realize structure in many fields, including molecules, liquids, and liquids, amorphous electroniccalculations structure calculations in many fields,surfaces, including surfaces,solids, molecules, solids, and amorphous materials according to the first-principles calculations. In order to identify the interaction properties between g-C3N4 and kaolinite, as well as the function of cyanuric acid towards g-C3N4/kaolinite composite, we constructed a crystal model and calculated the band structure and density of states based on DFT calculations. Figure 6 presents the crystal structures of CN, KA, CN/KA, and m-CN/KA. The unit cell of kaolinite (α = 91.93°, β = 105.04°, γ = 89.79°, a = 5.149 Å , b =

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materials according to the first-principles calculations. In order to identify the interaction properties between g-C3 N4 and kaolinite, as well as the function of cyanuric acid towards g-C3 N4 /kaolinite composite, we constructed a crystal model and calculated the band structure and density of states based DFT Figure 6 presents the crystal structures of CN, KA, CN/KA, and m-CN/KA. Mineralson 2018, 8, xcalculations. FOR PEER REVIEW 9 of 15 The unit cell of kaolinite (α = 91.93◦ , β = 105.04◦ , γ = 89.79◦ , a = 5.149 Å, b = 8.934 Å, c = 7.384 Å) 8.934 Å , c = 7.384 Å )research investigated inin this research is also in line with results previously results [31]. investigated in this is also line with previously reported [31].reported All the structures of All samples the structures of the samples constructed anda exhibited based on aon (010) surface. report, Based on the were constructed andwere exhibited based on (010) surface. Based a previous it isa previous that report, is indicated that theg-C cyanuric acid modified g-C 3Nsubstitution 4/kaolinite will the indicated the it cyanuric acid modified will make the frommake nitrogen 3 N4 /kaolinite substitution from nitrogen atomintoFigure carbon6datom, atom to carbon atom, as shown [32]. as shown in Figure 6d [32].

Figure 6. Side view of the basal (010) surfaces of (a) CN (C6 N21 ), (b) KA (H8 Si4 Al4 O18 ), (c) CN/KA Figure 6. Side view of the basal (010) surfaces of (a) CN (C6N21), (b) KA (H8Si4Al4O18), (c) CN/KA (2C 6 N15 -H8 Si4 Al4 O18 ), and (d) m-CN/KA (2C7 N14 -H8 Si4 Al4 O18 ). Color scheme (used for all figures): (2C6N15-H8white; Si4Al4Ooxygen, 18), and (d) m-CN/KA (2C7N14-H8Si4Al4O18). Color scheme (used for all figures): hydrogen, red; aluminum, pink; silicon, yellow; carbon, gray; nitrogen, blue. hydrogen, white; oxygen, red; aluminum, pink; silicon, yellow; carbon, gray; nitrogen, blue.

To further elucidate the enhanced photocatalytic performance of cyanuric-acid-modified To4 /kaolinite, further elucidate the enhanced photocatalytic performance of cyanuric-acid-modified g-C3 N the band structure and density of states of CN, KA, CN/KA, and m-CN/KA were g-C3N4/kaolinite, band structure and density of states of CN, KA, CN/KA, and m-CN/KA were calculated using the Generalized Gradient Approximation/Perdew-Burke-Ernzerhof (GGA/PBE) calculated As using Generalized method. seenthe from Figure 7, Gradient bare g-C3Approximation/Perdew-Burke-Ernzerhof N4 has a narrow band gap (0.896 eV) (GGA/PBE) while bare method. As from Figure bareeV), g-C3demonstrating N4 has a narrow gap (0.896 eV) whileand bareinsulation kaolinite kaolinite hasseen a wide band gap 7, (4.502 theband semiconductor property has a widerespectively. band gap (4.502 eV),combination, demonstrating semiconductor property and insulation property, property, After thethe calculated band gaps of CN/KA and m-CN/KA respectively. After combination, the calculated band gaps of CN/KA and m-CN/KA were obviously were obviously decreased, which might contribute to the photogenerated carriers’ excitation and decreased, Nevertheless, which might the contribute toband the gaps photogenerated carriers’ excitation and migration. migration. calculated were much lower than the experimental results, Nevertheless, calculated bandwell-known gaps were limitations much lower experimental results, which which should the be ascribed to the of than DFT. the As for CN/KA and m-CN/KA, should be ascribed to the well-known limitations of DFT. As for CN/KA and m-CN/KA, the electron the electron orbital density was increased greatly in conduction band minimum (CBM), especially orbital density was increased greatly in conduction band minimum especially m-CN/KA, for m-CN/KA, indicating an improvement in the transfer efficiency(CBM), of carriers and thefor reaction rate. indicating an improvement in the transferthe efficiency carriersacid and rate. Density of Density of states were also taken to research effect of of cyanuric onthe g-Creaction N /kaolinite composite. 3 4 states were also taken to research the effect of cyanuric acid on g-C 3 N 4 /kaolinite composite. The The results showed that the orbit was primarily located on the valance band and mainly composed of thatthe thesingle orbit kaolinite. was primarily located on 3the band composed of s sresults and p showed orbitals for For the bare g-C N4 ,valance the CB of g-Cand was mostly composed 3 N4 mainly and p orbitals for the single kaolinite. For the bare g-C3N4, the CB of g-C3N4 was mostly composed by s and p orbitals. As for CN/KA and m-CN/KA, consecutive orbitals were mainly distributed in the range of −25 to 2.5 eV. It is indicated that the introduction of cyanuric acid caused the change in orbital constitutions. The increase of s and p orbital intensity in valence band maximum (VBM) could promote the augmentation of band density and be conducive to the improvement of quantum

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by s and p orbitals. As for CN/KA and m-CN/KA, consecutive orbitals were mainly distributed in the range of −25 to 2.5 eV. It is indicated that the introduction of cyanuric acid caused the change in orbital constitutions. The increase of s and p orbital intensity in valence band maximum (VBM) could promote the augmentation of band density and be conducive to the improvement of quantum efficiency [31]. Minerals 2018, 8, x FOR PEER REVIEW 10 of 15

Figure 7.7.Band Band structures partial statesfor (PDOS) for6 N(a) (C(H 6N21Si (b)4 OKA structures and and partial densitydensity of statesof(PDOS) (a) CN (C (b) KA 21 ), CN 8 ), 4 Al 18 ), (H8CN/KA Si4Al4O18(2C ), (c) CN/KA (2C 6N 15-H 8and Si 4Al 4Om-CN/KA 18), and (d) m-CN/KA (2C 7Al N 14O -H 8Si 4Al4O18). (c) N -H Si Al O ), (d) (2C N -H Si ). 6 15 8 4 4 18 7 14 8 4 4 18

3.7. Photocatalytic Photocatalytic Performance Performance 3.7. To specify effect of cyanuric acid concentration on adsorption and photocatalytic performance, To specifythethe effect of cyanuric acid concentration on adsorption and photocatalytic the photodegradation of RhB (10 mg/L) under visible light illumination was carried out for m-CN/KA performance, the photodegradation of RhB (10 mg/L) under visible light illumination was carried composites using P25, CN, KA, and CN/KA as reference samples under the same conditions. All out for m-CN/KA composites using P25, CN, KA, and CN/KA as reference samples under the same of the results average values ± standard deviation of three replicates. Asreplicates. shown in conditions. Allare ofpresented the resultsin are presented in average values ± standard deviation of three Figure 8a, the RhB degradation ratio curves were calculated by C/C of the as-prepared catalyst, where 0 As shown in Figure 8a, the RhB degradation ratio curves were calculated by C/C0 of the as-prepared C is the initial concentration of RhB and C is the concentration of RhB at time t. Clearly, all samples 0 catalyst, where C0 is the initial concentration of RhB and C is the concentration of RhB at time t. achieved equilibrium adsorption and desorption between solution and catalysts in 60 min under dark Clearly, all samples of achieved equilibrium of adsorption and desorption between solution and condition prior to irradiation. Among all the as-prepared catalysts, the adsorption ability of CN was the catalysts in 60 min under dark condition prior to irradiation. Among all the as-prepared catalysts, lowest. On the other the adsorption ability of m-CN/KA were ability gradually enhanced the adsorption abilityhand, of CN was the lowest. On the other hand,composites the adsorption of m-CN/KA with increasing cyanuric acid amounts. All the as-prepared m-CN/KA composites had higher composites were gradually enhanced with increasing cyanuric acid amounts. All the as-prepared adsorption abilities compared with other samples, except KA. Under visible light illumination, KA m-CN/KA composites had higher adsorption abilities compared with other samples, except KA. showedvisible no photoactivity towards RhB, while single CN had certain visible RhB, light photocatalytic activity. Under light illumination, KA showed no photoactivity towards while single CN had The CN/KA composite showed a higher photocatalytic performance than CN, because the graphite certain visible light photocatalytic activity. The CN/KA composite showed a higher photocatalytic carbon nitridethan in the composite distributed on the surface of the carrier kaolinite, performance CN, because was the uniformly graphite carbon nitride in the composite was uniformly solving the on problem of lowofquantum efficiency caused by the the thermal process [31]. distributed the surface the carrier kaolinite, solving problem agglomeration of low quantum efficiency Furthermore, the introduction of kaolinite as the supporter of g-C N provided more surface defects 3 4introduction of kaolinite as the caused by the thermal agglomeration process [31]. Furthermore, the for the composites improved separation of photogenerated electron-hole pairs. supporter of g-C3N4and provided morethe surface defectsefficiency for the composites and improved the separation efficiency of photogenerated electron-hole pairs. The addition of cyanuric acid could also effectively improve the adsorption ability of the composites. The improvement should be ascribed to the fact that cyanuric acid is decomposed at around 330 °C during the thermal polymerization process. As a result, a large number of pore structures and reactive sites were generated in the composites, greatly increasing the specific surface area of the composites, improving the adsorption ability and further

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The addition of cyanuric acid could also effectively improve the adsorption ability of the composites. The improvement should be ascribed to the fact that cyanuric acid is decomposed at around 330 ◦ C during the thermal polymerization process. As a result, a large number of pore structures and reactive sites were generated in the composites, greatly increasing the specific surface area of the composites, improving the adsorption ability and further enhancing the photocatalytic performance. The removal rate of 2018, RhB8,after min of visible light illumination for m-CN/KA-4 was up to 93%, which Minerals x FOR180 PEER REVIEW 11 was of 15 around two and nine times higher than bare g-C3 N4 and standard P25, respectively. Table 2. Parameters of the pseudo-first order for RhB degradation underof visible light over P25, CN, To further quantitatively evaluate the photocatalytic performance the m-CN/KA composites, KA, CN/KA, and m-CN/KA composites with various amounts. the relative concentrations of RhB were fitted using modifier the apparent pseudo-first-order rate equation, ln(C0 /C) = kt, where C and C0 were explained above and k is the apparent reaction rate constant. Sample k (min−1) Correlation Coefficient (R2) The results are listed in Table 2 and the curves are displayed in Figure 8b. From Table 2, it is obvious P25 0.0012 0.99 that the photo-degradation kinetics of RhB in the as-received catalysts showed significant linear CN 0.0021 0.99 relationships. In addition, with increasing cyanuric acid modifier amounts, the reaction constant KA 2.502 × 10−4 0.87 rate of m-CN/KA composites exhibited an obvious increasing trend. The reaction constant rate of CN/KA 0.0043 0.99 m-CN/KA-4 was over four times and seven times as much as that of bare g-C3 N4 and standard m-CN/KA-0.5 0.0048 0.99 P25, respectively. m-CN/KA-1 0.0059 0.99 m-CN/KA-2 0.0064 0.97 Table 2. Parameters of the pseudo-first order for RhB degradation under visible light over P25, CN, m-CN/KA-3 0.0074 0.98 KA, CN/KA, and m-CN/KA composites with various modifier amounts. m-CN/KA-4 0.0086 0.98 Sample

k (min−1 )

Correlation Coefficient (R2 )

To further quantitatively evaluate the photocatalytic performance of the m-CN/KA composites, P25 0.0012 0.99 CN of RhB were fitted 0.0021 0.99 the relative concentrations using the apparent pseudo-first-order rate equation, −4 KA 0.87 2.502 × 10 ln(C0/C) = kt, where C and C0 were explained above and k is the apparent reaction rate constant. The CN/KA 0.0043 0.99 results are listed in Table 2 and the curves are displayed in Figure 8b. From Table 2, it is obvious that m-CN/KA-0.5 0.0048 0.99 the photo-degradation kinetics of RhB in0.0059 the as-received catalysts0.99 showed significant linear m-CN/KA-1 relationships. In addition, with increasing cyanuric m-CN/KA-2 0.0064 acid modifier amounts, 0.97 the reaction constant rate m-CN/KA-3 0.0074 increasing trend. The 0.98reaction constant rate of of m-CN/KA composites exhibited an obvious m-CN/KA-4 0.98g-C3N4 and standard P25, m-CN/KA-4 was over four times and seven0.0086 times as much as that of bare respectively.

of RhB by P25, KA,CN, CN/KA and m-CN/KA Figure 8.8.(a)(a)Photodegradation Photodegradation of RhB byCN, P25, KA, composite, CN/KA composite, and composites m-CN/KA with the average values ± thevalues standard deviation three replicates visible lightvisible and (b)light the composites with the average ± the standardofdeviation of threeunder replicates under corresponding first-order kinetics plots. and (b) the corresponding first-order kinetics plots.

3.8. Possible Mechanism 3.8. Possible Mechanism As is known, the three main active species in the photocatalytic process are photo-generated As is known, the three main active species in the photocatalytic process are photo-generated holes, ·OH radicals, and ·O2−− radicals. Hence, in order to determine the crucial active species in the holes, ·OH radicals, and ·O2 radicals. Hence, in order to determine the crucial active species in the photodegradation process of as-prepared m-CN/KA-4 catalyst under visible light illumination, we photodegradation process of as-prepared m-CN/KA-4 catalyst under visible light illumination, we carried out a battery of radicals trapping experiments by employing edentate disodium (EDTA-2Na), 1,4-benzoquinone (BQ), and tert-Butanol (TBA) as scavengers for photo-generated holes (h+), hydroxyl radicals (·OH), and superoxide radicals (·O2−), respectively. The dosage of all scavengers was 5 mM and the results are displayed in Figure 9. In absence of adding scavengers, it is

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carried out a battery of radicals trapping experiments by employing edentate disodium (EDTA-2Na), 1,4-benzoquinone (BQ), and tert-Butanol (TBA) as scavengers for photo-generated holes (h+ ), hydroxyl radicals (·OH), and superoxide radicals (·O2 − ), respectively. The dosage of all scavengers was 5 mM and the results are displayed in Figure 9. In absence of adding scavengers, it is obvious that the photocatalytic rate in degrading of RhB aqueous solution was up to almost 80% after 180 min of visible-light illumination. The addition of TBA exhibited a negligible influence on the RhB degradation of m-CN/KA-4 composite, revealing that the ·OH radicals are not main radical Minerals 2018, 8, x FOR PEER REVIEW 12 of 15 species in the photocatalytic reaction system. On the other hand, the addition of EDTA-2Na had a significant on influence on the photodegradation visible which indicatesthat that the the influence the photodegradation processprocess under under visible light,light, which indicates + ) should be the most vital active species in the photodegradation process of photo-generated holes (h photo-generated holes (h+) should be the most vital active species in the photodegradation process of the as-received as-received m-CN/KA m-CN/KAcomposites. composites. the

Figure 9. Trapping Trapping experimental experimentalresults resultsofofactive activespecies speciesforfor the photocatalytic degradation of RhB Figure 9. the photocatalytic degradation of RhB by by m-CN/KA-4 composite under 180 min visible light illumination. m-CN/KA-4 composite under 180 min visible light illumination.

According to the above experimental experimental results and previous literature literature reports [26,28], the main According reactions could be inferred as follows: m-CN/KA + hv → e− + h+ m-CN/KA + hv → e− + h+ − − O2 → →· e−e ++O ·O O22 − 2

− − 2·2· OO O→ ·OH +O22 → 22· OH++ 2OH 2OH− +O 2 2− + 2H22O

The possible RhB degradation mechanism mechanism and and potential potential electrons electrons transfer transfer pathway pathway over over m-CN/KAcomposites compositesare aredescribed describedininFigure Figure10.10. Under visible-light irradiation, electrons m-CN/KA Under visible-light irradiation, the the electrons (e−) − (e the ) of valence the valence band (VB) over m-CN/KA composites excited theconduction conductionband band (CB), (CB), of band (VB) over thethe m-CN/KA composites excited totothe + + forming holes holes(h(h) )ininthethe which could directly oxidize RhB. we allelectrostatic know, electrostatic VB,VB, which could directly oxidize RhB. As we As all know, repulsion exists between electrons, and electrostatic attraction exists between repulsion existsnegatively-charged between negatively-charged electrons, and electrostatic attraction existsnegatively between negatively chargedand electrons and charged positively charged electrons. For its kaolinite, layered is charged electrons positively electrons. For kaolinite, layereditssurface is surface generally − − generally charged [26–28,30]; therefore, e onbeVB should be to negativelynegatively charged [26–28,30]; therefore, the excited etheonexcited VB should accelerated to accelerated migrate to CB − CB migrate andCB thecould e− onbe the CB couldwith be combined the oxygen surface of and the eto on the combined the oxygenwith absorbed on the absorbed surface ofon thethe composite to − −). − would the composite to produce (·O 2 Then, the · O 2 react with water molecules to produce superoxide (·O2 − ).superoxide Then, the ·O would react with water molecules to produce hydroxyl 2 − and ·OH produce OH), O2, and ·OH could also reactelectrostatic with RhB. radicals (·hydroxyl OH), OH−radicals and O2 ,(· and theOH product couldthe alsoproduct react with RhB. Furthermore, Furthermore, electrostatic attraction between the negatively-charged layers and RhB in the attraction between the negatively-charged kaolinite layers and RhB inkaolinite the solution can also make an solution can also make an enhancement on the photocatalytic enhancement on the photocatalytic performance of m-CN/KAperformance composite. of m-CN/KA composite.

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Figure 10. illustration of the separation and photocatalytic mechanismmechanism of m-CN/KA 10.Schematic Schematic illustration of charge the charge separation and photocatalytic of composites under visible light irradiation. m-CN/KA composites under visible light irradiation.

4. Conclusions 4. Conclusions In summary, a novel kind of cyanuric-acid-modified g-C3 N4 /kaolinite (m-CN/KA) composite In summary, a novel kind of cyanuric-acid-modified g-C3N4/kaolinite (m-CN/KA) composite with enhanced visible light photocatalytic activity was prepared through a facile two-step method in with enhanced visible light photocatalytic activity was prepared through a facile two-step method in this study. The as-received cyanuric-acid-modified g-C3 N4 /kaolinite composite possesses enhanced this study. The as-received cyanuric-acid-modified g-C3N4/kaolinite composite possesses enhanced photocatalytic performance in RhB degradation under visible-light irradiation, the apparent rate photocatalytic performance in RhB degradation under visible-light irradiation, the apparent rate constant of which was almost two times that of the g-C3 N4 /kaolinite and over four times that of bare constant of which was almost two times that of the g-C3N4/kaolinite and over four times that of bare g-C N44.. Based thethe characterization resultsresults and DFT analyses, the improvement of the photocatalytic g-C33N Basedonon characterization and DFT analyses, the improvement of the performance should be ascribed, not only to the generation of abundant pore structures and reactive photocatalytic performance should be ascribed, not only to the generation of abundant pore sites, but also the efficient separation of efficient the photogenerated pairs because of the structures and to reactive sites, but also to the separation ofelectron-hole the photogenerated electron-hole intimate interfacial combination of g-C3 N4 and kaolinite. Combined with the characterization results pairs because of the intimate interfacial combination of g-C3N4 and kaolinite. Combined with the and the radical scavenger experiments, it isscavenger indicated that the formeditintimate interfacial characterization results and the radical experiments, is indicated that combination the formed between g-C N and kaolinite could reduce the combination of the photogenerated electron-hole 3 4 intimate interfacial combination between g-C3N4 and kaolinite could reduce the combination ofpairs the efficiently, which electron-hole improves the utilization efficiency of a photo-generated carrier, thusefficiency enhancingofthea photogenerated pairs efficiently, which improves the utilization photocatalytic activity of thus composites. Ourthe present study canactivity provideofnew insight into the preparation photo-generated carrier, enhancing photocatalytic composites. Our present study of visible light-driven composite photocatalysts based on natural minerals, which exhibit potential can provide new insight into the preparation of visible light-driven composite photocatalysts based industrial in the field of organic dyeindustrial degradation. on naturalapplications minerals, which exhibit potential applications in the field of organic dye degradation. Author Contributions: Conceptualization, Z.S. and B.W.; data curation, Z.S. and F.Y.; investigation, F.Y., X.L. and J.X.; project administration, Z.S.; software, C.L.; supervision, Z.S.; writing—original draft, Z.S. and F.Y.; writing—review and editing, Z.S. and B.W. Z.S. and B.W.; data curation, Z.S. and F.Y.; investigation, F.Y., X.L. Author Contributions: Conceptualization, and J.X.; project administration, Z.S.; C.L.; supervision, Z.S.;ofwriting—original draft, Z.S.the and F.Y.; Funding: This research was funded by software, the National Key R&D Program China (2017YFB0310803-4), Young writing—review and editing,Program Z.S. andby B.W. Elite Scientists Sponsorship CAST (2017QNRC001), the Yueqi Funding Scheme for Young Scholars (China university of Mining & Technology, Beijing), the Beijing Excellent Talent Training Subsidy Program—Youth Funding: (2017000020124G089), This research was funded by Youth the National KeyTeacher R&D Program of ChinaPlan (2017YFB0310803-4), Backbone and the Backbone Growth Support of Beijing Institutethe of Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Yueqi Funding Scheme for Young Fashion Technology (BIFTQG201807). Scholars (China university of Mining & Technology, Beijing), the Beijing Excellent Talent Training Subsidy Conflicts of Interest: The authors declare no conflicts of interest. Program—Youth Backbone (2017000020124G089), and the Youth Backbone Teacher Growth Support Plan of Beijing Institute of Fashion Technology (BIFTQG201807).

References

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