Engineering Electron Transport Pathways in Cobalt-Doped g-C₃N₄ Photocatalysts: Enhanced Tetracycline Degradation Through Interlayer Bridging

Abstract

The exploration of visible light-responsive, efficient, and durable photocatalysts is of great concern for removing organic dyes and antibiotics from wastewater. This work involved the preparation of a CoCN0.02 photocatalyst by simple thermal polymerization. The synthesized catalysts were mainly used for the photocatalytic degradation of tetracycline (TC) pollutants. The photocatalytic efficiency of one of the catalysts reached 97% in 30 min, which was much higher than that of pure g-C₃N₄ (CN). The consistency between the results of kinetic simulations and characterization supported the strong role of Co intercalation sites in photocatalysis. Additionally, using the active species capture experiments, the predominant active species were determined to be •OH, •O₂⁻, and h⁺, thereby allowing us to explore the electron transportation and redox reactions during the process of photocatalysis. This investigation establishes a basis for exploring the evolution of active species in the context of antibiotic pollutants.

1. Introduction

With the rapid advancement of urbanization and industrialization, many pollutants that were previously unknown have become common in our environment [1,2,3]. Specifically, the emergence of antifungal drugs and the irresponsible overuse of antibiotics have resulted in antibiotic substances being classified as a new type of pollutant [4,5,6]. Tetracycline (TC) is a commonly used antibiotic that has attracted considerable attention because of its widespread use in human medicine, animal husbandry, veterinary practices, agriculture, and aquaculture [7,8,9]. Based on extensive statistical data, the production of TC in China alone has reached 210,000 tons, with its regional consumption accounting for about half of global consumption [10,11,12]. Regrettably, due to the hydrophilicity of tetracycline, it cannot be fully absorbed in the body and its metabolic transformation is unbalanced, resulting in at least 75% of the intake of tetracycline being discharged into sewage through urine and feces in its active form [13,14,15]. Unfortunately, the biotoxic effect of TC on microorganisms reduces their effectiveness in biological wastewater treatment processes, hindering the reduction of TC levels before the water is discharged into the wider environment [16,17,18]. Moreover, tetracycline remains in the environment for long periods after being released, thanks to its stable structure and resistance to degradation. Alarmingly, it has been found in rivers, lakes, seawater, groundwater, and even drinking water [19,20,21]. The highest level of TC found in Brazilian rivers reached 100,000 mg/L [22,23,24]. Unfortunately, prolonged exposure to TC leads to chronic toxicity in non-target organisms, affecting both aquatic and terrestrial animals ecologically [25,26,27]. Additionally, the increase in TC fosters the development and spread of antibiotic-resistant bacteria, leading to various infectious diseases, and posing serious threats to human health [28,29,30]. In view of this, several technologies have been developed to remove TC from water, such as physical adsorption, electrochemical oxidation, and membrane filtration [31,32,33]. However, these technologies suffer from major drawbacks such as high capital intensity and high energy requirements [34,35,36].

Carbon nitride (CN), a metal-free material, has attracted great attention in the field of heterogeneous photocatalysis, owing to its good thermal and chemical stability, good band structure, and easy synthetic process [37,38,39]. However, CN exhibits shortcomings in terms of the photogenerated carrier separation efficiency and redox reactions to reactants [40]. Over the years, various strategies have been explored to overcome these challenges, such as morphology control, heteroatom doping, heterojunction creation, and defect engineering [20,41]. In recent years, the advantages of nitrogen defects in photocatalytic redox have been of interest to many researchers [19,30]. It has been found that surface nitrogen defects in the CN heptazine framework can be used as electron traps and O₂ adsorption sites, enhancing the interaction between molecular oxygen and semiconductors [42,43,44]. However, the mismatch between the photogenerated carrier lifetime and the oxygen redox kinetics of this one-component semiconductor material still leads to low electron mobility and a large amount of charge recombination in the process of photocatalytic molecular O₂ activation [45,46,47]. These shortcomings seriously affect its overall efficacy, despite its advantageous photocatalytic oxygen activation performance [48,49,50,51,52].

In this context, doping with Co is one of the effective ways to improve carrier separation efficiency and guide the charge transfer to achieve a specific migration route. Interlayer insertion sites can efficiently facilitate electron transfer. Specifically, these sites provide more electron transport channels to ensure the spatial separation of redox processes and optimize the high-energy redox capability of the carriers. Inspired by this, we speculate that choosing appropriate semiconductors and introducing specific elements to construct defective CN-based photocatalysts can catalyze an efficient molecular oxygen activation process.

In this study, we successfully demonstrated the synthesis of layered carbon nitride (CoCN0.02) with nitrogen defects and cobalt doping through in situ thermal polymerization. Based on experimental results and density functional theory (DFT) calculations, the factors influencing the CoCN0.02 photocatalyst are explained. Notably, the presence of nitrogen vacancies enhances the availability of active sites within CoCN0.02. Introducing cobalt impurities creates a new electron transfer pathway, improving the efficiency of electron transfer. Our results indicate that CoCN0.02 establishes a novel electron transport channel via the interlayer insertion of cobalt impurities. This innovative structural feature enables the involvement of oxygen and hydrogen peroxide in redox reactions, producing active species. As a result, the photocatalytic degradation efficiency of tetracycline is notably enhanced.

2. Results and Discussion

2.1. Synthesis and Characterization of Catalysts

Figure 1 shows the preparation, morphology, and elemental analysis results of CoCN002. Figure 1a illustrates the thermal polymerization process used to produce CoCN0.02. Briefly, the crystal phase structures of the CoCN0.02 and CN were characterized using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). As shown in Figure 1b–e, Figure S1 and S2, the CoCN0.02 and CN samples displayed an irregular shape with a block-like structure. Notably, numerous pores were observed to be randomly distributed across the surface of the CoCN0.02 sample. Additionally, the elemental composition of the CoCN0.02 sample, as analyzed by EDS (Figure S3), confirms the presence of C, N, and Co elements within the sample. Further investigation of the elemental content of CoCN0.02 was carried out using elemental mapping. Figure 1f,g shows that the elements were uniformly distributed throughout the sample, in agreement with the EDS results.

Figure S4 displays the XRD patterns of different samples that reflect their crystal structures. The characteristic peaks of the pure CN sample are located at 13.2° and 27.4°, corresponding to the (100) and (002) crystal planes of CN, respectively. Importantly, the peak positions hardly changed, which shows that the introduction of the Co element did not disrupt the triazine ring structure of pure CN. FT-IR analysis was carried out to reveal the chemical molecular structures of CN and CoCNx (x = 0.01, 0.02, 0.03, 0.04, and 0.05). As shown in Figure S5, clear peaks around 810 cm⁻¹, 1200 cm⁻¹, 1750 cm⁻¹, 3000 cm⁻¹, and 3400 cm⁻¹ were observed in the spectra of CN and CoCNx samples. The peak around 810 cm⁻¹ represents the out-of-plane bending mode of the triazine unit. The absorption peaks between 1200 cm⁻¹ and 1750 cm⁻¹, 3000 cm⁻¹, and 3400 cm⁻¹ correspond to the stretching vibration modes of the heterocycle and N-H in bridged or terminal amino groups, respectively. These findings are consistent with the results of XRD pattern.

The chemical compositions and valence states of CN and CoCN0.02 were further characterized by X-ray photoelectron spectroscopy (XPS). As depicted in Figure 2a, the XPS spectrum of CN displayed distinct peaks corresponding only to C, N, and O, mirroring the spectrum of pure CN. The C1s XPS spectrum in Figure 2b can be deconvoluted into two peaks: the peak at 284.8 eV attributable to a C-C bond, and the other peak at 288.3 eV, indicative of sp² carbon (C-N=C). Figure 2c shows high-resolution XPS spectra of the N1s peaks of CN and CoCN0.02. Specifically, the peaks due to CN and CoCN0.02 are situated at 401.6 eV, 400.5 eV, 398.8 eV, 401.4 eV, 400.3 eV, and 398.9 eV. These peaks arise due to diverse nitrogen configurations: amino groups (C-N-H) in sp2-bonded nitrogen, tertiary nitrogen N-C3 groups, and aromatic rings containing nitrogen (C=N-C). Interestingly, compared with the N1s spectrum of pure CN, the C-N2 peak in the CoCN0.02 spectrum shifts to a higher binding energy. These peak position shifts suggest that there is a chemical interaction between Co and CN. Simultaneously, as shown in Table S1, the C=N-C peak area of CoCN0.02 decreases upon fitting, indicating the predominant formation of N defects in CoCN00.2. It can be seen from Figure 2d that both CN and CoCN0.02 exhibited an O1s peak located at 532.4 eV, representative of oxygen adsorption. Finally, in the high-resolution Co3d XPS spectrum (Figure 2e), two peaks at 794.1 and 795.1 eV, belonging to Co⁴⁺ (3d_5/2) and Co⁴⁺ (3d_3/2), respectively, were observed. These results indicated that in CoCN0.02, Co⁴⁺ was the main existing form. In summary, we can preliminarily infer that the doping of Co ions at N sites results in the occurrence of nitrogen defects.

Both CN and CoCN0.02 (Figure 2f) exhibited characteristic peaks at g = 2.004 in the EPR spectra, which could be attributed to the unpaired electrons in the sp² carbon atoms within the pi-conjugated aromatic ring. Notably, the amplitude of the CoCN0.02 curve was much higher than that of the CN curve after doping with Co. This observation suggested the presence of defects in CoCN0.02. When considering the findings from the XPS analysis, it becomes apparent that the C/N ratio of CN was higher than that of C-N=C and N=C-N within CoCN0.02. This signifies that the peak area associated with pure CN is greater than that of CoCN0.02. This observation strongly suggests that N defects predominantly manifest at bridging nitrogen sites, aligning harmoniously with the conclusions presented in Figure 2c. Remarkably, this trend remains in line with the outcomes of the EPR analysis.

Furthermore, there are several ways Co ions can be integrated into CN, such as interlayer insertion, substitution of N or C atoms, and filling of CN vacancies. To determine the mode of inclusion of Co, density functional theory (DFT) was used to calculate the Gibbs free energy of Co under various doped modes. As shown in Figure 2g, the calculated results demonstrated that Co ions need the Gibbs free energies of 1.707, 1.710, 0.454, 0.308, and 2.069 eV to undertake the interlayer insertion of triazine, internal N and C atom substitutions, bridged N substitutions, and hole filling, respectively. It is worth noting that the free energy of interlayer insertion was the lowest compared to the other modes of Co. Thus, it seemed to be the easiest mode for Co incorporation.

2.2. Surface Area and Optical Properties

Through nitrogen adsorption and desorption equilibrium experiments, the specific surface areas of pure CN and CoCN0.02 were compared. It is widely understood that a larger specific surface area indicates stronger adsorption. Hence, to improve the adsorption of antibiotic pollutants on the semiconductor surface, it is beneficial to increase the number of active sites in the sample. The adsorption and desorption ability of the CoCN0.02 sample was found to be higher than that of the pure CN, indicating that the introduction of Co resulted in an increase in the specific surface area of the sample. Table S2 shows that the specific surface area of CoCN0.02 is 28.62 m² g⁻¹. This value is 2.56 times higher than that of pure CN (11.06 m² g⁻¹). Additionally, the pore diameter of CoCN0.02 was larger than that of CN, while the pore volume of CoCN0.02 was smaller than CN.

The bandgap and light absorption of the samples were investigated using diffuse reflectance spectroscopy (DRS). As shown in Figure 3a, the absorption bands and light absorption capabilities of the CoCN samples were better than those of pure CN. This shows that with the introduction of Co, the absorption edge of the photocatalyst is significantly enhanced. The Kubelka–Munk function for the catalyst was derived from the DRS information. According to Figure 3b, the narrower band gap in the case of CoCN0.02 (2.66 eV) than that of CN (2.80 eV) suggested the CoCN0.02 photocatalyst had a better photo response range. The VB-XPS method was used to measure the valence band potential (E_VB, XPS) through the VB-XPS plot (Figure 3c,d). According to the formula:

$${\mathrm{E}}_{\mathrm{V}\mathrm{B},\mathrm{N}\mathrm{H}\mathrm{E}}=\mathsf{\phi }+{\mathrm{E}}_{\mathrm{V}\mathrm{B},\mathrm{X}\mathrm{P}\mathrm{S}}+4.44$$

(1)

where φ is the work function of the instrument (4.63 eV). The corresponding E_VB (E_VB,NHE) of the standard hydrogen electrode can be calculated as 1.99 eV and 2.48 eV. Additionally, according to the equation:

The E_CB of CN and CoCN0.02 were −0.13 eV and −0.48 eV, respectively. The more negative ECB of CoCN0.02 than CN translates into a higher electron transfer capacity of CoCN0.02.

$${\mathrm{E}}_{\mathrm{C}\mathrm{B}}={\mathrm{E}}_{\mathrm{V}\mathrm{B}}-{\mathrm{E}}_{\mathrm{g}}$$

(2)

The E_CB of CN and CoCN0.02 were −0.13 and −0.48 eV, respectively. The more negative E_CB of CoCN0.02 than CN translates into a higher electron transfer capacity of CoCN0.02.

2.3. Photoelectric Properties of the Photocatalyst

The carrier separation and electron transfer capabilities of the prepared photocatalysts were investigated by PL analysis. An emission peak around 460 nm in the case of pure CN (Figure 4a), corresponding to its band gap, could be attributed to the recombination of photogenerated electron and hole pairs. With the introduction of Co, the luminescence intensity of CoCN0.02 was notably reduced compared to that of pure CN. This reduction implied that the doped CoCN0.02 photocatalyst could effectively govern the recombination rate of electron–hole pairs and greatly improve the separation efficiency of photogenerated carriers.

The transient fluorescence lifetime decay of photocatalysts also represents the migration time of carriers during the photocatalytic process. As seen from Figure 4b, the transient fluorescence lifetime of CoCN0.02 was longer than that of pure CN, indicating more efficient utilization of photogenerated electrons. This further proved the better photocatalytic performance and higher photocatalytic efficiency of the CoCN0.02 composite. At the same time, all these results suggested that an appropriate concentration of Co doping could promote the effective separation of photogenerated carriers, accelerate their transport to the surface of photocatalysts, and actually enhance electron efficiency, resulting in good photoactivity.

Electrochemical impedance spectroscopy (EIS) is a commonly used technique to analyze the separation capacity of photo-generated electron (e⁻) and hole (h⁺) pairs. The EIS curve depicted in Figure 4c provides insights into the conductivity of the samples. The slope of CoCN0.02 was lower than that of CN, indicating that the pure samples showed higher electrical impedance compared to CoCN0.02. Specifically, according to the data in Table S3, the resistance of CoCN0.02 is about 3.73 times that of pure carbon nitride (CN), which makes its conductivity significantly lower than that of pure CN. This result shows that doping of cobalt significantly affects the electrical properties of the material. Excellent conductivity enabled the sample to have strong electron transfer performance. Furthermore, the transient photocurrent response curve reflects the ability of different photocatalysts to excite e⁻ and h⁺. The transient photocurrent curves are displayed in Figure 4d, with light activation and deactivation at 50-s intervals. All the samples showed a clear photocurrent signal when the light was turned on. This photocurrent remained basically unchanged until subsequent light deactivation and then dropped to the background value. In contrast to pure CN, the CoCN0.02 exhibited increased photocurrent intensity, emphasizing the enhanced charge separation capability imparted by the introduced Co ions. In summary, these results demonstrate that introduction of Co significantly enhanced the excitation of photo-generated electrons, promoted the separation and transfer of charge carriers, and thus achieved more efficient photocatalytic performance. This is consistent with the results of steady-state fluorescence and fluorescence attenuation described above.

2.4. Photocatalytic TC Degradation Performance

The efficacy of the prepared photocatalysts was assessed by studying the degradation of the pollutant TC. Photocatalytic experiments were conducted under xenon lamp irradiation using pure CN and a series of different CoCN sample photocatalytic samples. The photocatalyst was subjected to a dark reaction initially for 0.5 h to evaluate the equilibrium adsorption capacity of the sample. Then, a sample was taken out every 5 min for a total of 30 min. The photocatalytic degradation rate of TC by pure CN was only 24%. Importantly, within the same time frame of 30 min, the degradation rate of TC by CoCN0.02 reached 97% (Figure 5a). In addition, the photocatalytic degradation rate of TC was different for all CoCN samples, potentially arising from different Co contents.

To determine the potential active species involved in the photocatalytic degradation of TC using CoCN0.02 and pure CN photocatalysts, photocatalytic active species capture experiments were performed for active species capture experiments, and 100 mg of CoCN0.02 photocatalyst along with 1 mL of K₂Cr₂O₇ (0.1 mg/L), EDTA (0.1 mg/L), PBQ (0.1 mg/L), and IPA (0.1 mg/L) solutions were used (Figure 5b,c). It is worth noting that after adding IPA, the capture of •OH and photocatalytic efficiency were significantly reduced, followed by EDTA, PBQ, and K₂Cr₂O₇ quenching h⁺, •O₂⁻, and e⁻, respectively [53]. Similar capture experiments were performed on CN and similar inhibitory effects were also observed. Therefore, in summary, it can be indicated that •OH is the main photocatalytic active species, followed by h⁺ and •O₂⁻. The direct impact of e⁻ can be ignored.

Next, investigations on the kinetics of degradation of TC were carried out based on the Langmuir–Hinshelwood model:

$$K=-ln{\displaystyle \frac{C_t}{C_0}}$$

(3)

where K is the apparent rate constant, C₀ is the initial pollutant concentration (mg·L⁻¹), and C is pollutant concentration at different time intervals (mg·L⁻¹). As shown in Figure 5d, the apparent rate constant of CoCN0.02 (K = 3.507) was the highest, which was about 14.1583 times higher than that of CN (K = 0.2477). The enhanced photocatalytic efficacy of CoCN0.02 can be attributed to the presence of doped Co and induced N vacancies. These factors enhanced TC adsorption and served as active sites to expedite TC degradation, ultimately enhancing its photocatalytic performance.

Finally, to verify the stability of the photocatalysts and to study the degradation under different water environments, cyclic experiments were performed using the CoCN0.02 sample. As shown in Figure 5e, the degradation rate of TC was still about 90% after a 5 h cycle, indicating the notable stability of CoCN0.02. Furthermore, as illustrated in Figure 5f, when subjected to photocatalysis in tap water, river water, and lake water conditions, the TC degradation efficiency of the CoCN0.02 photocatalyst was found to be significantly superior to that of pure CN. These outcomes collectively highlight the potential applications of the CoCN0.02 photocatalyst.

2.5. Mechanism of TC Degradation by Photocatalysis

As shown in Figure 6a, CoCN0.02 produced significantly higher quantity of hydrogen peroxide (H₂O₂) within 30 min than that produced by pure CN (Figure 6b). Since the decomposition of hydrogen peroxide may produce •OH, we boldly speculate that the CoCN0.02 photocatalyst may produce a large amount of •OH, while CN produces a small amount of •OH. As shown in Figure 6c, from the beginning of the dark treatment to the end of the 30 min reaction, the signal value of CoCN0.02 for hydroxyl radicals was consistently higher than that of CN (Figure 6d). Interestingly, the E_VB of CoCN0.02 was equal to the •OH/OH⁻ potential (1.99 V), thereby enabling direct generation of •OH. In addition, •O₂⁻ may also participate in the generation of H₂O₂ via alternative reaction pathways. In summary, CoCN0.02 may generate more hydroxyl radicals (•OH) through the decomposition of H₂O₂, thereby promoting photocatalytic degradation.

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {•\mathrm{O}}_2^{-}$$

(4)

$${\mathrm{O}}_2^{-}+{2\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to { \mathrm{H}}_2{\mathrm{O}}_2$$

(5)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{OH}}^{-}+•\mathrm{OH}$$

(6)

$${•\mathrm{O}}_2^{-}/•\mathrm{OH}/{\mathrm{h}}^{+}+\mathrm{TC} \to \mathrm{Products}$$

(7)

The mechanism of the photocatalytic degradation process of TC is shown in Figure 7. Under visible light irradiation, CoCN0.02 generates electrons and holes. Subsequently, these electrons are transferred through the intercalated Co, acting as a bridge. This transfer leads to the rapid conversion of the dissolved oxygen into •O₂⁻. A fraction of the resulting •O₂⁻ is used for oxidative degradation of TC. The remaining portion of •O₂⁻ is transformed into H₂O₂, which, in turn, can be converted to •OH. Finally, these active substances mineralize TC, transforming it into water and carbon dioxide.

3. Experiment

3.1. Materials

All chemicals used were analytical reagent grade and were of analytical purity and had no further purification.

3.2. Synthesis of Catalysts

The Co-doped g-C₃N₄ was prepared by annealing a mixture of g-C₃N₄ and cobalt nitrate powder. Cobalt nitrate pentahydrate powder (0.01 g, 0.02 g, 0.03 g, 0.04 g, and 0.05 g) was uniformly mixed with 4 g melamine and each resulting mixture was transferred to separate beakers containing 100 mL boiling water. The solution is pink. The mixtures were then heated under stirring for 60 min by placing the beakers in a water bath at 100 °C. The contents of the beakers were then transferred to evaporating dishes while still hot. The evaporating dishes were placed in an oven and dried at 100 °C for 5 h. Subsequently, the mixtures were placed in covered crucibles and heated in a muffle furnace at 600 °C for 5 h with a heating rate of 5 °C·min⁻¹. After heating, the crucibles were removed from the muffle furnace and cooled to room temperature. The products were ground into fine powders to yield CoCN0.01, CoCN0.02, CoCN0.03, CoCN0.04, and CoCN0.05 (where 0.01, 0.02, 0.03, 0.04, and 0.05 refer to the weight of Co). The mass of the sample was determined to be 2.8 g using a calibrated analytical balance.

3.3. Characterization of the Catalysts

The phase composition of each catalyst was determined by X-ray powder diffraction (XRD, Bruker-D8, Karlsruhe, Germany, KIT). Fourier transform infrared spectrometry (FTIR, Nicolet 6700, Thermos Fisher Scientific, Lenexa, KS, USA) was utilized to detect the functional groups in each sample. Electron paramagnetic resonance (EPR, JES-FA 200, Tokyo, Japan) at a frequency of 100 kHz and power of 0.99 mW was employed to detect the bond signals of the catalysts. The surface morphologies of the catalysts were studied using scanning electron microscopy (SEM, Hitachi-4800S, Tokyo, Japan) and transmission electron microscopy (TEM, Tecnai G20, Hillsboro, OR, USA) at accelerating voltages of 20 kV and 200 kV, respectively. Energy-dispersive X-ray spectroscopy (EDS) and elemental mapping were used to analyze the elemental composition of the samples. X-ray photoelectron spectroscopy (XPS, Multilab 2000, Surrey, UK) was carried out to determine the surface elemental compositions and chemical states of the samples with C 1s peak at 284.8 eV as a reference. Nitrogen adsorption–desorption isotherms were obtained at 77 K on a Micromeritics ASAP2020 system. Each sample was vacuum dried at 180 °C overnight prior to obtaining the isotherms. The pore characteristics and the adsorption properties of the samples were analyzed using O₂ temperature-programmed desorption (TPD). UV–visible solid diffuse reflectance spectroscopy (DRS, UV-2550, Tokyo, Japan) was employed to determine the light absorption of the catalysts using BaSO₄ as a reference. Steady-state photoluminescence spectroscopy (PL, F-7000, Hitachi, Tokyo, Japan) was used to determine the separation efficiency of the photoinduced carriers. The transient photocurrent responses (TPR) and electrochemical impedance spectroscopy (EIS) curves of the samples were recorded in a KCl solution (0.1 mol/L, pH = 7) using an electrochemical workstation (CHI 760E). A saturated calomel electrode (SCE) and a Pt plate were employed as the reference and counter electrodes, respectively.

3.4. Photoelectrochemical Experiment

The volume ratio of naphthol solution to anhydrous ethanol is 98:2 mL because the amount of naphthol used is very small and has less impact on light absorption. We just use this solution to dissolve it, making it easy to form a membrane. The photocurrent experiments were performed in a three-electrode quartz cell containing 100 mL of 0.1 M KCl as the electrolyte. A platinum sheet and saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. The working electrodes were prepared using different catalysts. Briefly, ITO glass substrates were first washed with distilled water and ethanol in an ultrasonic cleaner for 30 min. A catalyst slurry was obtained by dispersing the catalyst (10 mg) in 0.2 wt% α-naphthol solution followed by grinding for 10 min. This slurry was loaded onto pretreated ITO glass substrates (1.5 cm × 1.5 cm). All loaded ITO glass substrates were dried at 100 °C under air to form the photoelectrodes. A 300 W xenon lamp was used as the light source, and the lamp was kept 15 cm away from the photoelectrode.

3.5. Photocatalytic Degradation Efficiency for TC

The degradation efficiency of the catalysts for TC was used as an index to measure their photocatalytic performance. Visible light was provided by a 300 W xenon lamp (PLS-SXE300/300 UV) and a glass filter was used to remove ultraviolet light with wavelengths below 420 nm. Typically, 100 mg of the photocatalyst was added to a 100 mL solution containing 50 mg L⁻¹·TC. This mixture was stirred for 30 min under dark conditions to achieve adsorption–desorption equilibrium. Next, the mixture was irradiated by visible light, and 2 mL of the mixture was sampled at 3 min intervals. The TC solution and the photocatalyst in each 2 mL sample were separated by a 0.22 µm filter membrane. The concentration of the TC aqueous solution was measured by high-performance liquid chromatography (HPLC, Shimadzu, Kyoto, Japan) with an EC-C18 column (4.6 × 150 mm). The column temperature was set to 25 °C and the detection wavelength was 359 nm. The ratio of 0.3% aqueous phosphoric acid solution to methanol in the mobile phase was 6:4 and the flow rate was 1 mL·min⁻¹. The sample volume for each injection was 20 μL. Finally, the degradation rate was calculated according to Formula (S1) (Text S1 of Supplementary Materials).

3.6. Theoretical Calculations

For the bulk, a cutoff energy of 450 eV was used and a 2 × 2 × 1 K-point mesh sampling based on the Monkhorst–Pack scheme was chosen. The energy was converged to 10⁻⁶ eV/atom. Geometry optimization was carried out using a BFGS method, keeping a force tolerance criterion of 0.05 eV/Å and the maximum displacement was less than 0.002 Å (Text S2 of Supplementary Materials).

4. Conclusions

This study involved the preparation of Co-doped g-C₃N₄ photocatalysts via a simple thermal polymerization process. The mechanism of photocatalytic degradation of TC by CoCN0.02 under visible light irradiation was described. The photocatalytic degradation efficiency of CoCN0.02, as high as 97% within 30 min, is particularly noteworthy. According to the theoretical calculations, when the Co elements are doped at the intercalation site, a new electron transfer channel is successfully established, thereby improving electron transfer efficiency. Moreover, photochemistry observations demonstrate the enhanced transfer of photogenerated charges. The photogenerated carrier dynamics and the expanded light absorption range of CoCN0.02 accelerate the transformation of active species, especially the production of •OH. This advancement contributes to the improvement of the photocatalytic removal and mineralization efficiency for TC. This study thus provides a basis for improving photocatalytic degradation of pollutants by doping.