Novel LaAO_x/g-C₃N₄ (A = V, Fe, Co) Heterojunctions with Enhanced Photocatalytic Degradation of Norfloxacin under Visible Light

Abstract

In this study, novel photocatalysts LaAO_x/g-C₃N₄ (A = V, Fe, Co) were prepared by the hydrothermal method, through which LaAO_x and g-C₃N₄ were mixed and ultrasonically oscillated to gain heterojunction catalysts. All the samples were characterized by XRD, SEM, FT-IR, XPS, DRS, and PL to ensure the successful integration of LaAO_x with g-C₃N₄. The obtained results showed that LaAO_x/g-C₃N₄ (A = V, Fe, Co) could effectually improve the separation efficiency of photogenerated carriers during the photodegradation process, thus improving the photodegradation efficiency, while among them, LaFeO₃/g-C₃N₄ showed the best photocatalytic performance and degradation of norfloxacin under visible light, reaching up to 95% in 180 min, which was 9.32 times higher than pristine g-C₃N₄. From the discussed results above, the possible mechanism of the photodegradation process was put forward. This study supplies a promising method to gain g-C₃N₄-based photocatalysts for antibiotics removal.

1. Introduction

In recent decades, with the intensive research and production quantity, a significant number of antibiotic drugs have been widely used in agriculture, animal husbandry, aquaculture, clinical therapy, etc. [1]. One serious consequence of this are the increasing numbers of antibiotic residues in the natural environment. Among them, norfloxacin is a representative antibiotic [2,3,4] which is clinically prescribed in humans and animals for a wide range of diseases. Meanwhile, during the process of its production, massive wastewater is discharged directly into natural waters without treatment, which often contains antibiotics and can also cause serious ecological pollution [5,6]. Therefore, the rapid and effective removal of antibiotic residues from water has become an important research topic [7].

In order to solve the energy crisis and water pollution problems that limit contemporary human development, photocatalysis is a popular and promising research topic today. In 1972, Honda and Fujishima discovered that TiO₂ could decompose water molecules into hydrogen when exposed to ultraviolet light [8], which could be noted as the beginning of heterogeneous photocatalysts. In addition, as research further developed, more photocatalytic materials with a high visible light activity were successfully prepared [9], making it possible to utilize the high-energy visible light as the light source in the photocatalytic system.

Graphitic carbon nitride (g-C₃N₄) is an organic semiconductor polymer material with a graphite-like structure, consisting of tri-s-triazine units formed from sp², hybridized by C and N atoms, and interconnected by amino radicals [10]. Layer-to-layer interaction by van der Waals forces have demonstrated the structure by density functional theory (DFT) calculations [11]. g-C₃N₄ is endowed with excellent thermal, physical, as well as chemical stability. In addition, a bandgap of (~2.7 eV) makes g-C₃N₄ makes it possible to exhibit catalytic activities easily under visible light [12]. Since Wang [13] reported its application in photocatalytic water splitting in 2009, g-C₃N₄ has been extensively researched in the field of photocatalytic removal of refractory organic contamination. Nevertheless, its development in photocatalysis is still being restricted by such aspects: its difficulty in absorbing visible light specifically refers to wavelengths above 460 nm, the short lifetime of photogenerated carriers, its low electrical conductivity, as well as its small, specific surface area. In response to this situation, different strategies have been devised to improve the photocatalytic efficiency of g-C₃N₄, such as metal doping [14], non-doping [15], the regulation of its nanostructure [16], and construction of heterojunctions [17], among others.

Compared to other semiconductor modification methods, compounding two or more semiconductor materials to form a heterojunction photocatalyst can not only enhance the separation efficiency of photogenerated electrons and holes, but also regulate the redox chemistry and photocatalytic stability, thereby effectively increasing the photocatalytic activity, which is regarded as a simple, stable, and efficient modification method [18]. As we know about a photocatalyst, the photocatalytic activity is mainly limited to the adsorption efficiency, the photoresponse range, and the separation efficiency of the photogenerated electron-hole pairs. Inspired by plant photosynthesis in nature, Z-scheme heterojunction photocatalysts are considered as one of the most efficient photocatalytic systems [19]. In particular, the synthesis of the Z-scheme photocatalyst has been reported as an effective way to enhance the photocatalytic efficiency. Due to the appropriate energy band matching between each component materials, the photogenerated electrons are transferred and separated from holes effectively, which accumulate at the respective valence band (VB) and conduction band (CB) to form the redox centers. In the photocatalytic degradation process, the organic contaminants are decomposed into harmless small inorganic small molecules. Furthermore, the morphology of the photocatalysts also play a crucial role in the photodegradation process. As reported [20], the heterojunction photocatalysts with multilevel structure exhibit better photocatalytic performance than those with a single structure, mainly due to the interconnectedness of the multilevel nanostructures, which provide more multi-dimensional channels for the transport and diffusion of reactants, as well as the fact that they can be reflected or refracted on the surface of the multilevel structured composites, which contributes to the light utilization efficiency. The multilevel structure provides larger surface areas and active sites, improves the adsorption capacity, and helps the morphology to be maintained.

Therefore, the selection of suitable materials for the synthesis of Z-scheme composite photocatalysts for the degradation of norfloxacin is a highly desirable topic. Nowadays, nanometer-sized transition metal oxide particles have achieved a wide range of applications in the fields such as environmental protection [5], energy production and storage [14], and electronic devices [21]. Especially, perovskites presenting the atomic structure of ABO_x are the focus of research. Among such perovskite compounds, LaAO_x (A = V, Fe, Co), is a promising material with excellent catalytic oxidation capability, thermal stability, light absorption, electron migration efficiency, low-cost and no environmental impact. Graphite phase carbon nitride (g-C₃N₄) possesses high photo-sensitive and photosensitivity, is often combined with other semiconductor materials to form heterojunction photocatalysts to enhance the optical absorption and electron migration efficiency of the photodegradation system [22].

In this work, a novel Z scheme photocatalyst was synthesized by a facile ultrasound-assisted hydrothermal method, combining LaAO_x (A = V, Fe, Co) with g-C₃N₄ to form a binary LaCoO₃/g-C₃N₄ hybrid. Further, the crystal structure, surface morphology, optical properties and chemical composition of the samples obtained were investigated by XRD, SEM, FTIR, XPS, DRS and PL. The photocatalytic activities were investigated by the degradation of norfloxacin aqueous solution under visible light. Benefited by the construction of Z-scheme heterojunction, the optical absorption, and photogenerated electron transfer efficiency were enhanced, which increased the photocatalytic degradation activities over the samples obviously. This work provides a feasibility for the application of solar energy to remove the antibiotic contaminants from water over the g-C₃N₄ based ternary photocatalyst.

2. Experiment

2.1. Materials

Melamine, La(NO₃)₃·6H₂O, NH₄VO₃, Fe(NO₃)₃·9H₂O, Co(No₃)₃·6H₂O, NaOH, and citric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shenyang, China). Norfloxacin was obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2. Synthesis of g-C₃N₄

Typically, 10 g of melamine was placed in a sealed crucible and heated to 550 °C for 4 h, at a rate of 2 °C/min, then cooled down naturally, and then it was heated at 520 °C for 4 h at a rate of 2 °C/min in air to obtain g-C₃N₄ nanosheets [23].

2.3. Synthesis of LaAO_x (A = V, Fe, Co)

In order to obtain pure pin-like LaVO₄, 0.52 g of sodium hydroxide and 1.52 g of NH₄VO₃ were added to 30 mL of deionized water and dissolved by stirring to obtain aqueous solution of sodium metavanadate (A solution for short). Then, for LaVO₄, 5.84 g of lanthanum nitrate hexahydrate was added to a certain amount of water and dissolved by stirring to obtain B solution. The yellow suspension was formed by adding B solution to A solution, slowly. After mixing and stirring for 30 min, we added the yellow suspension to the 200 mL Teflon-lined reaction autoclave and the temperature was kept at 200 °C for 48 h, then cooled naturally to room temperature. The milky white product was centrifuged and washed successively with water and anhydrous ethanol for three times. Finally, the obtained products were dried at 80 °C for 24 h to form pure white LaVO₄ powder.

As for pure LaCoO₃, La(NO₃)₃·6H₂O, Co(NO₃)₃·6H₂O, and citric acid were mixed in 100 mL deionized water at a molar ratio of 1: 1: 5 and stirred magnetically for 30 min with adding 20 mL ethylene glycol. The mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 180 °C for 16 h. The obtained precipitate was washed with deionized water and anhydrous ethanol several times, dried at 80 °C for 6 h, then calcined at 900 °C for 2 h to gain pure LaCoO₃ nanospheres. LaCoO₃ and g-C₃N₄ were mixed at a 30: 100 mass ratio, then poured into deionized water, and ultrasonically shaken for 2 h. The mixture was dried at 80 °C for 2 h and calcined at 500 °C for 2 h to prepare LaCoO₃/g-C₃N₄ hybrid.

For pure LaFeO₃, 1 mmol Fe(NO₃)₃·6H₂O and 5 mmol critic acids with 20 mL ethylene glycol were dissolved in deionized water in a beaker. The mixed solution was stirred for 1 h and transferred onto the Teflon-lined reaction autoclave; the temperature was adjusted to 180 °C and kept for 12 h. The obtained solution was washed by deionized water and alcohol for 3 times, respectively. Then, the precursor was dried at 160 °C for 6 h and calcined at 850 °C for 4 h to obtain the LaFeO₃.

2.4. Synthesis of LaAO_x/g-C₃N₄ (A = V, Fe, Co)

LaAO_x/g-C₃N₄ (A = V, Fe, Co) composites were synthesized by ultrasonic dispersion method. We took a certain amount of g-C₃N₄ pure into 20 mL methanol in three different beakers, and the ultrasonic dispersing lasted for 30 min at room temperature. After 30 min we added a certain amount of LaVO₄, LaFeO_3, and LaCoO₃ powder, respectively, and stirred for 2 h under the fume hood. Then the resulting mixture was dried at 80 °C for 10 h. Finally, we placed the product in the crucible in muffle furnace at 5 °C per minute to 250 °C and kept for 1 h, letting it cool naturally to room temperature. The LaAO_x/g-C₃N₄ (A = V, Fe, Co) composites were obtained.

2.5. Characterization of the Samples

For all the samples obtained, the microstructure and chemical composition were characterization by X-ray Diffraction Analysis (XRD, Rigaku, Dmax-2000, Tokyo, Japan), Fourier transformed infrared (FTIR, Bruker AXS, TENSOR-27, Karlsruhe, Germany), and (XPS, Thermo VG, ESCALAB-250, Waltham, MA, USA). The surface morphology was investigated by scanning electron microscopy (SEM, Hitachi, S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, Hitachi, H-600, Japan). The optical capacity was measured by UV-vis diffuse reflection spectroscopy (DRS, PerkinElmer, Lambda-35, Waltham, MA, USA) and Photoluminescence (PL, Shimadzu, RF-540, Kyoto, Japan) spectra.

2.6. Photocatalytic Activity Examination

The photocatalytic activities of all the samples prepared were measured by the degradation of norfloxacin in aqueous solution under visible light illumination. In order to prove the degradation ability of La under visible light, we added a filter on the xenon lamp. The light source was provided by a xenon lamp with a 420 nm cutoff filter. 20 mg of all the samples obtained was mixed with 100 mL of 20 mg/L norfloxacin solution, magnetically stirred for 1 h to reach the adsorption-desorption equilibrium on the sample surface. Then, the xenon lamp was turned on, 5 mL of the suspension liquid was withdrawn, centrifuged and the absorbance the supernatant was measured by a UV–Vis spectrophotometer at 275 nm.

3. Result and Discussion

3.1. Characterization Results of Catalysts

The crystal phase structure of the catalyst prepared was analyzed using XRD, and the results are shown in Figure 1. As can be seen, the pure g-C₃N₄ (JCPDS 87-1526) shows two diffraction peaks of (100) and (002) planes at 13.02° and 27.56° [12], the diffraction peaks of LaVO₄ (JCPDS 50-0367) are observed at 18.28°, 20.36°, 24.46°, 26.16°, 27.76°, 29.0°, 30.12°, 32.88°, 35.1°, 39.66°, 40.42°, 41.28°, 45.06°, 46.5°, 47.24°, 49.48°, 50.8°, 51.96°, 53.98°, 55.5°, 57.56°, 67.78°, 70.24° and 73.32°, respectively, which corresponded to (011), (11-1), (020), (200), (120), (210), (012), (20-2), (21-2), (031), (31-1), (211), (212), (13-2), (103), (32-2), (132), (140), (40-2), (41-2), (21-4), (51-1), (41-4), and (33-4) planes [24]. Furthermore, the peaks of LaFeO₃ was observed as 22.7°, 32.24°, 39.83°, 46.19°, 57.53°, 67.41°, and 76.75°, which were matched to the planes (101), (121), (220), (202), (240), (242) and (204) of LaFeO₃ (JCPDS no. 37-1493) [25]. Nevertheless, the LaCoO₃ (JCPDS 48-0123) has strong diffraction peaks at 22.64°, 33.26°, 39.78°, 46.22, 57.44°, 68.94°, and 69.96° were well matched with (012), (110), (202), (024), (214) and (208) planes [26]. In the XRD patterns of the LaAO_x/g-C₃N₄ composite catalysts, the characteristic diffraction peaks of LaVO₄, LaFeO₃ and LaCoO₃ and g-C₃N₄ can be observed clearly and simultaneously. No impurity diffraction peaks were observed which confirmed the successful synthesis of LaAO_x/g-C₃N₄ (A = V, Fe, Co).

The morphologies of obtained samples were tested by scanning electron microscopy (SEM). In Figure 2a, pure LaVO₄ presents a pin-like structure and has a length about 200–300 nm. It can be seen in Figure 2c,e, that pure LaFeO₃ and LaCoO₃ show spherical particulates, and the nanospheres having a diameter about 70–150 nm, which can provide a rich space for electrons transport. As shown in Figure 2g, pure g-C₃N₄ presents smooth sheet structure, which provides well conditions for loading LaAO_x (A = V, Fe, Co). For LaVO₄/g-C₃N₄, we can clearly observe that pin-like LaVO₄ was grown on g-C₃N₄ nanosheet in Figure 2b. From Figure 2d,f, we can conclude that LaFeO₃ and LaCoO₃ nanosphere was evenly distributed on the g-C₃N₄ nanosheet. Based on the above analysis, the SEM results proved LaAO_x/g-C₃N₄ (A = V, Fe, Co) were successfully prepared.

The functional group of LaAO_x/g-C₃N₄ (A = V, Fe, Co) was examined by FT-IR spectroscopy. The spectra illustrated by FT-IR analysis were shown in Figure 3, which shows that there is absorption at whole spectra. For bare g-C₃N₄, the distribution of main characteristic peaks at 1100 cm⁻¹ to 1700 cm⁻¹, which were corresponding to aromatic C-N stretching vibration modes [27]. The peak related to N-H stretching vibration modes was shown at 3000 cm⁻¹ to 3700 cm⁻¹, which was consistent with previous reports [28]. For LaVO₄, the two characteristic peaks at 810 and 440 cm⁻¹ could be attributed to vibration of VO₄³⁻, and La-O [29]. As for LaFeO₃, the Fe-O bond was observed as 501 cm⁻¹. Apart from above peaks, another sharp peak at 808 cm⁻¹ can be attributed to bending vibration modes of the triazine ring units and C = N stretching vibration modes [10]. No other intermediate phases are existed in the FTIR spectra, which corresponds with the XRD results [30].

The surface chemical composition and chemical state of the LaAO_x/g-C₃N₄ (A = V, Fe, Co) catalyst were characterized through XPS. Figure 4a shows the X-ray photoelectron spectroscopy of the LaAO_x/g-C₃N₄ (A = V, Fe, Co) catalyst. All the signals of La, V, Fe, Co, O, C, and N were observed in the survey spectra of LaAO_x/g-C₃N₄ catalyst, which indicates its composite structure and tallies with the results of the XRD. Figure 4b–h show the high resolution X-ray photoelectron spectroscopy of La 3d, V 2p, Fe 2p, Co 2p, O 1s, C 1s and N 1s respectively. In Figure 4b, the binding energy (BE) of La 3d shows two pairs of diffraction peaks at 832.7, 836.8 eV, and 849.4, and 853.6 eV represent La 3d_5/2 and La 3d_3/2 [31]. As Figure 4c shows, the BE peaks at 522.5 and 514.7 eV are ascribed to V 2p_1/2 and V 2p_3/2, and the V 2p peak is assigned to V⁵⁺ [32]. In Figure 4d, the BE peaks at 708.9 eV and 722.8 eV of Fe 2p_3/2 and Fe 2p_1/2 were assigned to Fe³⁺ [33]. Moreover, the Co 2p XPS spectra shows in Figure 4e which present three primary peaks at 778.3, 787.1 and 795.7 eV, which corresponds with Co 2p_3/2 and Co 2p_1/2, indicating that all Co ions are present as Co³⁺ [27]. As for the element O 1s, shown in Figure 4f, the peak at 527.9 eV was attributed to the lattice oxygen, while the peak at 529.8 eV was assigned to surface adsorbed oxygen. In the high-resolution C 1s XPS spectrum shown in Figure 4g, the peak at 281.8 eV is attributed to the sp ²-type C-C bonds from the environment, the peak at 285.2 eV is assigned to the sp ²-type C = N bonds [19]. We can observe in Figure 4h that the N 1s peak at 395.7 eV to the C-N bonds and the peak at 398.3 eV to the sp ²-type C=N bonds, the peak at 401.5 eV is identified as originating from nitrogen surrounded by three carbons in an amorphous C-N network (N-(C)₃) [34]. In a nutshell, the XPS analysis proves that the LaAO_x/g-C₃N₄ (A = V, Fe, Co) composite catalysts were successfully prepared.

Figure 5 is the UV–vis diffuse reflectance spectra of g-C₃N₄, LaAO_x, and LaAO_x/g-C₃N₄ (A = V, Fe, Co) composite photocatalyst, and it is apparent that in Figure 5a, at the wavelength range from 300 to 800 nm, pristine LaCoO₃ and LaFeO₃ revealed an intense absorption over the total visible light range. It could be seen that the absorption edges of g-C₃N₄ are at about 490 nm. It could also be seen that the LaAO_x/g-C₃N₄ (A = V, Fe, Co) shows stronger absorption than pristine g-C₃N₄ and LaFeO₃. Among them, it can be clearly observed that the LaFeO₃/g-C₃N₄ composite showed a wide absorption range, approximately at 615 nm.

The band gap energies of all samples, as shown in Figure 5b–h, are determined according to the Kubelka-Munk equation; herein, the band gap (Eg) values of bare g-C₃N₄, LaVO₄, LaFeO₃ and LaCoO₃ are 2.68, 3.46, 2.02 and 2.72 eV, respectively. Furthermore, the values of Eg for LaVO₄/g-C₃N₄, LaFeO₃/g-C₃N₄, and LaCoO₃/g-C₃N₄ composite were calculated to be 2.98, 3.06 and 2.85 eV, indicating that the co-existence of LaAO_x (A = V, Fe, Co) and g-C₃N₄ materials.

Photoluminescence (PL) emission spectra of as-prepared samples, shown in Figure 6, study the transfer behavior of the photogenerated electrons and holes and understand the separation and recombination of photogenerated charge carriers [35]. Generally speaking, PL emission is the result of recombination of electrons and holes. In other words, a lower PL intensity is the same as a lower recombination rate and higher photocatalytic activity. Therefore, the PL spectra demonstrate that LaFeO₃/g-C₃N₄ has the best photocatalytic performance.

3.2. Photocatalytic Performance

After the above analysis, it can be found that the visible-light absorption capacity of LaAO_x/g-C₃N₄ (A = V, Fe, Co) composites and the separation efficiency of photogenerated carriers are significantly improved compared with g-C₃N₄, which may be beneficial to the improvement of photocatalytic performance. In order to verify the photodegradation performance of LaAO_x/g-C₃N₄ (A = V, Fe, Co), we studied the photocatalytic performance of NOF as target pollutant. In Figure 7, among them, the C₀ is the initial concentration, and the C_t is the concentration at t min. As can be seen, all the samples can degrade NOF for less than 8% after 30 min, while the pure g-C₃N₄, LaVO₄, LaFeO₃, and LaCoO₃ can only degrade 25%, 34%, 50%, and 39% NOF after 180 min visible-light illumination. As for LaAO_x/g-C₃N₄ (A = V, Fe, Co) composites, under the same conditions, LaVO₄/g-C₃N₄, LaFeO₃/g-C₃N₄, and LaCoO₃/g-C₃N₄ show higher photocatalytic efficiency, reached 80%, 95%, and 57%, respectively. Specifically, LaFeO₃/g-C₃N₄ composites show the highest photocatalytic efficiency, which can degrade 95% NOF after 180 min of visible-light illumination.

In addition, we calculated the kinetic curve of photocatalytic degradation of NOF by the equation as follow:

−ln(C_t/C₀) = kt

(1)

where C₀ is the initial concentration, the C_t is the concentration for t min, k is the rate constant and t is the reaction time. The calculation results are shown in Figure 8 and

Table 1. Photocatalytic results of all samples.

Samples	Degradation	k	R2
g-C3N4	25%	0.00147	0.98475
LaVO4	34%	0.00210	0.97228
LaFeO3	50%	0.00322	0.96608
LaCoO3	39%	0.00250	0.98143
LaVO4/g-C3N4	80%	0.00789	0.94428
LaFeO3/g-C3N4	95%	0.01371	0.94997
LaCoO3/g-C3N4	57%	0.00417	0.97579

. As we can see clearly that the reaction time and concentration of NOF degradation conform to the first order kinetics. Combining Figure 8 and Table 1, it can be concluded that the kinetic constants of NOF photodegradation over LaFeO₃/g-C₃N₄ (0.013 71 min⁻¹) is 9.32 times that of pristine g-C₃N₄ (0.001 47 min⁻¹), indicating that the coupling g-C₃N₄ with LaFeO₃ extremely enhances photodegradation efficiency of NOF among these photocatalysts.

3.3. Photodegradation Possible Mechanism

In order to explore the electron hole transfer path between LaFeO₃ and g-C₃N₄ under visible-light illumination, we proposed a possible photodegradation mechanism, as shown in Figure 9. According to previous reports [10], using the formula E_VB = X − E_e + 0.5E_g, and E_VB = E_CB + E_g, the conduction band (CB) potential and valence band (VB) potential of g-C₃N₄ were determined to be −1.01 eV and 1.57 eV LaFeO₃ were calculated to −0.37 eV and 1.65 eV, respectively. Furthermore, the CB of both g-C₃N₄ and LaFeO₃ are more negative than the reduction potential (−0.33 eV vsNHE) of O₂/•O₂⁻, which could be explained that electrons reacted with O₂ to produced •O₂⁻ [33], at the same time, •O₂⁻ participated in the NOF photodegradation process under visible-light illumination.

In simple terms, under illumination, the electrons on g-C₃N₄ and LaFeO₄ are photoexcited into their respective CB, leaving holes on the VB, thereby generating photogenerated electrons-hole pair. The e⁻ on the conduction band of LaFeO₄ rapidly transported to the valence band of LaFeO₄, while the h⁺ on the VB of the g-C₃N₄ reacts with the OH⁻ to form the •OH, and the e⁻ stayed on the CB of the g-C₃N₄ can react with O₂ generates •O₂⁻, and the finally formed •OH, •O₂⁻ and h⁺ participate in the photocatalytic degradation reaction of NOF, so that NOF can be transformed into products such as CO₂ and H₂O, and the Z-scheme mechanism formed effectively avoids photo-generated electrons and improves the separation efficiency of electron-hole pairs, thus, the photocatalytic degradation efficiency was improved.

4. Conclusions

In conclusion, a novel LaAO_x/g-C₃N₄ (A = V, Fe, Co) nanocomposite material was prepared by a facile hydrothermal method. This novel photocatalyst has the activity of initiating the decomposition of NOF under visible light illuminate. As shown in the XRD and FT-IR results, there were no impurity diffraction peaks observed, which confirmed the successful synthesis of LaAO_x/g-C₃N₄ (A = V, Fe, Co). We also observed that, in the SEM results, both the pin-like LaVO₄, spherical LaFeO₃ and LaCoO₃ were successful in loading on the sheet g-C₃N₄, respectively. And the results of DRS and PL characterization showed that LaAO_x has more efficient visible light response ability. In addition, it also exhibits excellent photocatalytic performance under visible light illumination. The enhanced photocatalytic performance of LaFeO₃/g-C₃N₄ is not only related to the energy band potential of LaFeO₃ and g-C₃N₄, but also related to the interconnected nanocrystalline heterojunction of g-C₃N₄. This study may provide an important strategy for the designation and preparation of high-performance photocatalysts induced by visible light.