Modulation of the Microstructure and Enhancement of the Photocatalytic Performance of g-C 3 N 4 by Thermal Exfoliation

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Introduction
As industrialization progresses, environmental pollution issues, particularly those involving heavy metal ion contamination, have become an increasingly severe global challenge.Hexavalent chromium (Cr(VI)) is a typical heavy metal pollutant that mainly originates from the wastewater discharge of industries such as leather processing, electroplating, printing, and pigments [1][2].Due to its high toxicity, high extent, they are often accompanied by high costs, high energy consumption, and the potential for secondary pollution.Therefore, it is particularly important to develop a treatment technology that is both economical and environmentally friendly.Photocatalytic technology, which can utilize the energy of natural sunlight to drive the reaction of Cr(VI), is characterized by low cost and environmental friendliness, showing its potential in treating Cr(VI) contamination.Therefore, through photocatalytic technology, Cr(VI) can be converted into less toxic Cr(III), making it an effective and environmentally friendly pollution control strategy [1][2][3][4][5][6].
However, photocatalytic technology still needs to improve in practical applications, mainly due to the need for high-performance, costeffective, and environmentally friendly photocatalysts.Therefore, the development of high-performance photocatalysts is an important research direction for the development of environmental science.Graphitic carbon nitride (g-C3N4) is a layered photocatalyst with remarkable photocatalytic potential, which possesses non-toxicity, good chemical stability, and a suitable band gap structure [7][8][9][10][11][12].Nevertheless, due to disordered growth and interlayer intermolecular interactions, the conventional thermal polymerization method for preparing g-C3N4 makes the synthesized g-C3N4 usually present a bulk structure (bulk-g-C3N4).Its structural defects manifest in a small specific surface area, fewer surface active sites, and a high complexation rate of the photogenerated electronhole pairs [7,8].These factors limit its activity in visible light photocatalytic reactions [7][8][9][10][11][12].Its structural defects manifest in a small specific surface area, fewer surface active sites, and a high complexation rate of the photogenerated electronhole pairs [7,8].These factors limit its activity in visible light photocatalytic reactions [7][8][9][10][11][12].
Recent investigations have employed various strategies to enhance the performance of g-C3N4 to address these limitations, including morphological control [9], doping modification [8,10], heterojunction construction [11][12][13][14], chemical and thermal exfoliation [15][16][17], and dye sensitization [18].For instance, Yang et al. [8] successfully synthesized g-C3N4 nanosheets with excellent photocatalytic degradation performance for Rhodamine B through the synergistic effect of dual-element doping and secondary calcination.Nguyen et al. [11] prepared Ag/ZnO/g-C3N4 through a physical mixing calcination method, enhancing its visible light photocatalytic degradation activity for methylene blue (MB).Wang et al. [11] constructed a TiO2@C/g-C3N4 heterojunction for efficient removal of NO.Zhang et al. [15] used an aqueous sodium hydroxide solution to treat g-C3N4 to improve its photocatalytic activity for reducing Cr(VI) under visible light.On the other hand, Medeiros et al. [16] examined the effects of chemical and thermal exfoliation on the physicochemical and optical properties of carbon nitride and the underlying reasons.
Based on the various modification methods for g-C3N4, thermal exfoliation treatment has been widely studied for its simplicity, effectiveness, and minimal alteration of the material structure [8,17,19,20].However, in the existing works, the heat treatment temperature is higher than 590 °C [19], and gas protection is required [20], increasing energy consumption and costs.Additionally, further research is necessary to apply photocatalytic reduction of heavy metals.In this work, we modulated the microstructure of g-C3N4 by low-temperature thermal exfoliation (500-540 °C) in air, obtaining the effects of thermal exfoliation temperature on grain size and bandgap structure.By comparing the visible-light photocatalytic reduction activity of bulk-g-C3N4 and CN for Cr(VI), combining electrochemical tests and band structure, we propose the mechanism of CN photocatalytic reduction of Cr(VI) and discuss the possible reasons for the enhanced photocatalytic activity.

Synthesis
Weigh 3 g of dicyandiamide, place it into a capped crucible, and then put it into a muffle furnace.Heat it at a rate of 10 °C/min to a reaction temperature of 540 °C for 2 h.The resulting product is denoted as bulk-g-C3N4.The homemade bulk-g-C3N4 was put into a muffle furnace again and heated at 500 °C, 520 °C, and 540 °C with the same heating rate for 2 h.The products were labeled as CN-500, CN-520, and CN-540.It should be noted that as the temperature increases, the yield of CN obtained by thermal oxidation decreases.Considering the yield factor, select a temperature of up to 540 °C.

Characterizations
The composition of the synthesized materials was analyzed using an X-ray powder diffractometer (XRD, Ultima IV X, Rigaku Corporation, Japan), a Fourier-transform infrared spectrometer (FT-IR, ALPHA, Bruker, Germany), and an X-ray photoelectron spectrometer (XPS, Thermo escalab 250 XI, Thermo Fisher Scientific, USA).The morphological characteristics of the materials were obtained using a scanning electron microscope (SEM, SU8600, Hitachi, Japan).The ultraviolet-visible-near-infrared diffuse reflectance spectrum of the synthesized photocatalyst was obtained using a UV-Vis DRS spectrometer (Lambda750, PerkinElmer, USA).The optical properties of the samples were measured using a photoluminescence spectrometer (PL, F-2700, Hitachi, Japan).Transient photocurrent response curves (i-t), electrochemical impedance spectroscopy (EIS), and Mott-Schottky (M-S) curves were obtained using an electrochemical workstation (CHI 660E, Chenhua Instruments Co., Ltd., Shanghai, China, using a three-electrode system with Ag/AgCl as the reference electrode).The mineralization rate of organic pollutants was measured by total organic carbon/total nitrogen tester (TOC, Model TNM-L, Shimadzu, Japan).

Cr(VI) Reduction Experiments
The experiments for the visible-light photocatalytic reduction of aqueous Cr(VI) by g-C3N4 were conducted using a GHX-Z photochemical reaction apparatus.The experimental conditions were as follows: a 250 W Xe lamp (filtered to remove UV light with wavelengths less than 420 nm), a reaction temperature of 25 °C, with 1 mL of 0.5 mol/L citric acid as the hole scavenger, and 300 mg of photocatalyst added to 300 mL of 10 mg/L K2Cr2O7 solution.
First, an adsorption-desorption experiment was carried out for 40 min in the dark.During the photocatalytic reaction after turning on the light source, the reaction solution was pipetted at fixed intervals, and the post-reaction clarified Cr(VI) solution was obtained by filtering through a fiber filter membrane with a pore size of 0.22 µm.The concentration of the aqueous Cr(VI) was determined using a spectrophotometer.The removal rate of Cr(VI) was obtained using Equation (1).In the equation, c0 and ct represent the concentration of aqueous Cr(VI) at 0 and t min, respectively.

TC-HCl and RhB Degradation Experiments
The selective experiments of CN-540 were conducted through adsorption and photocatalytic experiments of 10 mg/L TC-HCl and RhB, with experimental parameters consistent with those of Cr(VI) reduction experiments.Rhodamine B (RhB) is an artificially synthesized rose-red, cationic dye commonly found in industrial wastewater from printing, textile, and food industries and is a common pollutant in such wastewater [21,22].Tetracycline hydrochloride is a water-soluble polar compound and a widely used antimicrobial drug in clinical settings.Due to the low effective utilization rate of tetracycline hydrochloride (TC-HCl), 75% of it is excreted as metabolites [23], posing a threat to human health and the ecological environment.Given the high chemical stability of TC-HCl and RhB [21], these organic pollutants are typically not directly oxidized by O2 in the air.Utilizing photocatalytic technology to purify organic wastewater is a potentially feasible strategy [21,24].

Structural and Compositional Characterization
Figure 1 shows the XRD patterns of bulk-g-C3N4, and CN obtained through thermal exfoliation.Compared with the standard card (JCPDS 87-1526) [8], all samples exhibit the two characteristic peaks of graphitic carbon nitride.The strong peak at 27.4° is attributed to the (002) plane of graphitic carbon nitride, formed by stacking aromatic rings [17,25].The weaker peak at 13.1° belongs to the 3-s-triazine units within the planar structure, corresponding to the (100) plane of g-C3N4 [17,25].Notably, the diffraction angle of bulk-g-C3N4 on the (002) plane is 27.47°, while the diffraction angle of the exfoliated CN photocatalyst on the same plane is 27.64°.According to the change of diffraction angle and Equation ( 2) [26], it can be inferred that the interlayer spacing decreases.
Figure 2(a) provides the FT-IR spectra of bulk-g-C3N4, CN-500, CN-520, and CN-540, which are similar to each other, indicating that the thermal exfoliation in the air atmosphere did not destroy the basic structure of g-C3N4.However, there are changes in the intensity of the characteristic peaks at the typical breathing vibration mode of the triazine ring at 810 cm −1 , as well as the vibrational modes of C−N hybridization within the range of 1200 cm -1 to 1400 cm −1 (Figure S1 Supporting Information).These variations are attributed to the adjustment of the interlayer spacing.The absorption peak observed at 810 cm −1 is characteristic of the bending vibration of the triazine ring [5,13,27].The absorption bands in the range of 1200 cm −1 to 1600 cm −1 are typical of the stretching vibrations of the aromatic CN heterocycles, with absorption peaks at 1230 cm −1 , 1315 cm −1 , and 1400 cm −1 attributed to the stretching vibrations of the aromatic C−N single bonds [5,13,27].Additionally, the absorption peaks at 1560 cm −1 and 1629 cm −1 are attributed to the stretching vibrations of −C=N and the stretching vibrations of C=O [5,13,27].The broad absorption peak near 3200 cm −1 is attributed to the stretching vibrations of O−H or N−H bonds [17].
Using X-ray photoelectron spectroscopy (XPS) technology, we conducted a detailed analysis of the chemical states of elements on the surface of the photocatalyst.As shown in Figure 2(b), the survey spectrum indicates that both bulk-g-C3N4 and CN-540 consist of carbon (C), nitrogen (N), and oxygen (O) elements, with the presence of oxygen mainly due to the adsorption of CO2 and H2O on the surface of the photocatalyst.Further high-resolution XPS analysis shows that in Figure 2(c), the C1s peak is fitted to two peaks located at 288.2 eV and 284.9 eV, corresponding to the C−C bonds of adventitious carbon and the carbon atoms in the N=C−N2 structure of the g-C3N4 molecular structure [10,13].The N1s peak in Figure 2(d) is fitted to two peaks, with the binding energies at 398.5 eV and 399.9 eV for CN-540 corresponding to the nitrogen atoms in the sp 2 hybridized C=N−C bonds [20] and the nitrogen atoms in N−(C)3, respectively.It is particularly noteworthy that compared to bulk-g-C3N4, the C1s binding energy of CN-540 is higher, while the N1s binding energy is lower.XPS peak separation software calculated the peak area of N1s spectra of bulk-g-C3N4 and CN-540.The N content of N−(C)3 in CN-540 decreased from 29.4% to 24.3%.This experimental result suggests that during the thermal exfoliation process in the air atmosphere, CN-540 may have undergone the removal of nitrogen atoms, thereby introducing nitrogen   vacancies [28,29], which could significantly affect the photocatalytic performance of the photocatalyst.
Figure 3 presents the SEM images of the prepared photocatalysts.Specifically, Figure 3(a) provides the morphology of bulk-g-C3N4, which exhibits a smooth bulk structure on its surface.Figures 3(b) to (d) display images of CN photocatalysts, indicating that as the temperature of thermal exfoliation treatment in the air atmosphere increases, the degree of surface porosity and sponginess of CN increases.Figures 3(e) and 3(f) present the EDX elemental distribution maps for bulk-g-C3N4 and CN-540, respectively, showing the uniform distribution of C and N elements.The nitrogen content in CN-540 has decreased from 45.55% to 37.35%, indicating the removal of nitrogen atoms from the structure of these materials.
Ultraviolet-visible diffuse reflectance absorption spectroscopy is an excellent means of evaluating the light-harvesting ability of photocatalysts.Figure 4(a) shows the UV-Vis spectra of bulk-g-C3N4, CN-500, CN-520, and CN-540, where CN-500, CN-520, and CN-540 all exhibit enhanced visible light absorption compared to the bulk-g-C3N4.g-C3N4 is an indirect band gap semiconductor [13], by plotting (αhν) 1/2 versus (hν) [13], Figure 4(b) can be obtained.Extrapolating the linear part of the figure to y = 0 obtains the band gap energies for bulk-g-C3N4, CN-500, CN-520, and CN-540, which are 2.70 eV, 2.69 eV, 2.67 eV, and 2.66 eV, respectively.It can be seen that the bandgap energies of CN-500, CN-520, and CN-540 are all smaller than that of bulkg-C3N4, and the bandgap energy decreases with the increase in thermal exfoliation temperature.reduction rate by bulk-g-C3N4 without citric acid was only 9.5% under the same conditions.This indicates that the photocatalytic performance of g-C3N4 can be improved by thermal exfoliation in the air atmosphere.

Photocatalytic Performance
A pseudo-first-order kinetic equation ( Eq. 3) was employed to further analyze the photocatalytic reduction process.Based on the relationship between ln(ci0/cit) and t shown in Figure 5(b), the reaction rate constants for the photocatalytic reduction of Cr(VI) by different catalysts were determined.The results show that the photocatalytic reaction rate constant for CN-540 on Cr(VI) is about 0.0298 min -1 , 6.21 times that of bulk-g-C3N4, indicating a faster reaction rate.
where, cit and ci0 denote the concentration of Cr(VI) solution when the light irradiation time is t and 0 min, respectively.
Figures 5(c-e nm remained unchanged.These results indicate that TC-HCl removal by CN-540 under light is a photocatalytic process.The inset of Figure 5(e) shows the adsorption and removal of RhB (40.2%) by CN-540 under dark conditions within 140 min.It was observed that the change in absorbance of RhB was prolonged, and the position of the absorption peak near 550 nm hardly moved.This suggests that the removal of RhB by CN-540 under dark conditions is mainly through adsorption.However, as the time of visible light illumination increases, the efficiency of CN-540 photocatalytic degradation of RhB gradually increases, and the absorption peak near 550 nm has shifted towards a smaller wavelength.This shift may be due to changes in the molecular structure of RhB. Figure 5(f) illustrates the mineralization rates of CN-540 during the adsorption and photocatalytic removal processes of TC-HCl, which were calculated based on the measured TOC and are 17.7% and 51.5%, respectively.Similarly, the adsorption and photocatalytic mineralization rates for RhB by CN-540 are 22.3% and 46.6%, respectively.The experimental results indicate that CN-540 exhibits varying catalytic capabilities towards different pollutants.It can be observed from the figures that as the illumination time increases, the absorption peak intensities of the functional groups decrease, indicating that the characteristic functional groups have been cleaved.

Photocatalytic Mechanism
To reveal the active species in the photocatalytic reduction of Cr(VI) by CN-540 photocatalyst, ammonium oxalate (AO), isopropanol (IPA), and benzoquinone (BQ) were introduced to capture holes (h + ), hydroxyl radicals (•OH), and superoxide radicals (•O2 -), respectively.Figure 6(a) shows that after the addition of AO, IPA, and BQ, the reduction rates of CN-540 for Cr(VI) decreased to 12.6%, 7.82%, and 26.5%, respectively, indicating that h + , •OH, and •O2 -all play essential roles in the reduction process of Cr(VI).Therefore, it can be inferred that h + , •OH, and •O2 -act as active radicals in the photocatalytic reaction.
It has been shown that a relatively weak PL peak typically indicates a lower recombination rate of photogenerated holes and electron pairs in semiconductor photocatalysts [30,31].The PL emission spectral intensities shown in Figure 6b decrease in bulk-g-C3N4 > CN-500 > CN-520 > CN-540, indicating that thermal etching in the air atmosphere can improve the separation efficiency of photogenerated electrons and holes.Transient photocurrent (TPC) and electrochemical impedance spectroscopy (EIS) were used to evaluate the generation and separation of interfacial charges and to investigate further the separation and migration of photogenerated electrons and holes in the photocatalytic process.According to the Nyquist plots, the smaller the semicircle diameter, the smaller the carrier transfer resistance of the sample [30].The Nyquist plots shown in Figure 7(a) indicate that the charge transfer resistances of the CN-500, CN-520, and CN-540 samples are smaller than that of bulk-g-C3N4, with CN-540 having the most minor charge transfer resistance.The transient photocurrent results in Figure 7(b) show that the current density of the CN-540 material is the highest, confirming its fastest separation efficiency of photogenerated carriers.
Figure 8 presents the Mott-Schottky (M-S) plots for bulk-g-C3N4 and CN-540.The positive slopes in the plots indicate that both bulk-g-C3N4 and CN-540 are n-type semiconductors [32] and that the thermal exfoliation in the air atmosphere has not changed the semiconductor type of g-C3N4.Using the extrapolation method where (1/C) 2 = 0, the flat band potentials (EFB) of bulk-g-C3N4 and CN-540 (vs.Ag/AgCl) are determined to be -0.75 and -0.76 eV, respectively.According to EFB(vs.NHE) = EFB (vs.SCE) + 0.222 + 0.0592 × pH [33], where the potential of the reference electrode AgCl/Ag is 0.222 V, and the conduction band edge (ECB) is typically 0.10 to 0.3 eV lower than the flat band potential (EFB) of n-type semiconductors (this article takes 0.3), the ECB (vs.NHE) of bulkg-C3N4 and CN-540 are calculated to be -0.414eV and -0.424 eV, respectively.Furthermore, based on the equation EVB = Eg + ECB, the valence band potentials (EVB) of bulk-g-C3N4 and CN-540 (vs.NHE) are calculated to be +2.286 and +2.236 eV, respectively.

Conclusions
This study has regulated the microstructure of g-C3N4 through thermal exfoliation in an air atmosphere, significantly enhancing its photocatalytic performance.The experimental results indicate that the CN obtained through thermal exfoliation treatment has reduced crystal grain size, decreased bandgap width, formed certain nitrogen-vacancy defects, and increased surface oxygen content, enhancing visible light absorption capability.In particular, the CN-540 sample exhibited a 96.9% reduction rate of Cr(VI), and its reaction rate constant was constant at 6.25 times that of the original g-C3N4.The photocatalytic degradation and mineralization rates for TC-HCl by CN-540 are 66.7% and 51.5%, Similarly, RhB was 60.6% and 46.6%, respectively.This indicates that CN-540 possesses good photocatalytic oxidation and reduction performance.Through transient photocurrent response and electrochemical impedance spectroscopy tests, it was confirmed that CN-540 has the best separation and transport efficiency of  photogenerated carriers.This study provides experimental evidence for optimizing the photocatalytic performance of g-C3N4 through thermal exfoliation strategies and offers new ideas for developing efficient photocatalysts.

Xinshan
Zhao and Yuanyuan Luo: Investigation, original draft, and Writing.Junwei Yu and Tingyu Meng: Formal analysis and Investigation.Zhao Li and Lin Tian: Revision and Funding.Yanzhen Fu and Limei Sun: Review and editing.Jing Li: Supervision, Design, Revision & Funding.

Figure 5 .
Figure 5. (a) Comparative activity chart of visible-light photocatalytic reduction of Cr(VI) by bulk-g-C3N4, CN-500, CN-520, and CN-540, (b) The pseudo-first-order reaction kinetics of samples, (c) Activity comparison of CN-540 for different pollutants, The UV-vise absorption spectra of CN-540 for the degradation and adsorption of organic pollutants (d) TC-HCl and (e) RhB, (f) The mineralization removal rate of TC-HCl and RhB by CN-540.

Figure 9 .
Figure 9. Schematic diagram of the photocatalytic Cr(VI) reduction by CN-540 under visible light.