Porous g-C₃N₄ with defects for the efficient dye photodegradation under visible light

Industrial wastewater contains various pollutants that have been recognized to cause further harmful effects on human beings and other creatures (Sim et al. 2013; Zhang et al. 2019). Among these, synthesized dye components with aromatic or azo functional groups are toxic and carcinogenic (Fatima et al. 2017; Waghmode et al. 2019). So the treatment of dye polluted water is a necessary part for clean and safe water supplies. Photocatalytic degradation is one of the most efficient and simple methods for the treatment of dye wastewater because almost all the organic compounds in water will be totally broken down into harmless fragments through photocatalytic procedures under the irradiation of visible or ultraviolet light (Khaki et al. 2017; Srikanth et al. 2017; Loeb et al. 2018; Natarajan et al. 2018).

Graphitic carbon nitride (g-C₃N₄) is one of the promising metal-free photocatalysts due to its marked properties, such as visible light adsorption, chemical stability, non-toxicity, low cost and reducibility (Ong et al. 2016; Wen et al. 2017; Gao et al. 2018). However, g-C₃N₄ is somewhat less effective due to the high combination rate of photoinduced electron–hole pairs in the photocatalytic procedure, which leads to a decrease in degradation efficiency (Wen et al. 2017; Chen et al. 2019a). Therefore, many research groups have focused on the development of catalytic efficiency for g-C₃N₄, such as heteroatom doping, nanostructure design, heterojunctions fabrication and functional component combination (Lam et al. 2016; Babar et al. 2018; Volokh et al. 2019; Xue et al. 2019). However, some of the g-C₃N₄-based catalysts require complex synthetic methods and purification procedures (Yuan et al. 2016; Iqbal et al. 2019; Li et al. 2019). Some reported g-C₃N₄ composite catalysts contain metal components, which may cause secondary pollution in the application (Pham & Shin 2018; Tao et al. 2019).

Defective engineering has been considered to play a crucial role in modifying the electronic structure and physical or chemical properties of photocatalytic catalysts. Cyanamide defects can be produced by the aid of chemical reagents (Lin et al. 2016; Yu et al. 2017; Zhang et al. 2018), carbon vacancies can be formed in bulk g-C₃N₄ by calcination in an NH₃ atmosphere (Liang et al. 2015), the number of hydroxyl groups on the surface of g-C₃N₄ can be increased by treating with oxygen gas plasma (Iqbal et al. 2018). All of these ways were intended to enhance visible light absorption, promote the separation of charge or adjust the band position of g-C₃N₄. In addition, defects can be beneficial in exposing numerous active sites, which are also the critical factor to affect catalytic performance.

Rhodamine B (RhB) is a kind of xanthene dye with high solubility and toxicity, which is widely used as a colorant in textile dyeing (Ding et al. 2020). Methyl Orange (MO) is a kind of azo dye, which is toxic, carcinogenic and difficult to degrade in water (Azam et al. 2020). Surface water quality could be significantly influenced even with low concentrations of such dyes. In this work, we reported that porous graphitic carbon nitride with defects (p-C₃N₄) was synthesized by simply calcinating g-C₃N₄ at 620 °C under an Ar atmosphere (Figure 1). The periodic structure of g-C₃N₄ was partially broken down and defects could be introduced into the structure by breaking some N − C, N = C bonds after thermal treatment. Compared with the original g-C₃N₄, p-C₃N₄ had a narrow band gap and lower recombination rate of photogenerated electron–hole pairs. As a result, the degradation efficiency of p-C₃N₄ for two different types of dyes, RhB and MO, was enhanced. p-C₃N₄ also exhibited good stability after several rounds of recycling. The relevant photocatalytic mechanism and the contribution of defects are also discussed.

All of the reagents were commercially purchased and used without further purification. Argon gas (≥99.999%) was purchased from Henan Yuanzheng Tech. Co., Ltd.

The photocatalyst p-C₃N₄ was synthesized as two facile steps (Liang et al. 2019; Xue et al. 2019). Firstly, the original g-C₃N₄ was prepared as follows: 10 g melamine powder was put in a covered crucible in a tubular furnace and heated to 550 °C with the heating rate of 10 °C·min⁻¹ for 4 h under an Ar atmosphere (30 sccm). After cooling down to room temperature, a pale yellow product was obtained. The porous C₃N₄ was prepared by calcining g-C₃N₄ to 620 °C with the heating rate of 10 °C·min⁻¹ and kept for 6 h under a pure Ar flow (30 sccm). The resulting orange powder was denoted as p-C₃N₄.

The powder X-ray diffraction (PXRD) patterns were generated on a Rigaku Ultima IV with CuKα radiation (λ = 1.540 Å. Fourier transform infrared (FT-IR) spectra were recorded on a WQF-510A FT-IR spectrometer (Beijing beifen-ruili). The scanning electron microscopy (SEM) measurements were conducted on a Hitachi S-4800 microscope. The transmission electron microscopy (TEM) images and energy dispersive (EDS) spectra and the elemental mapping were taken with a TEM, HT7700 microscope. The diffuse-reflectance spectra (DRS) were generated on a Cary300 spectrometer (Agilent). The photoluminescence (PL) spectra were carried out on an F-4700 (Hitachi) fluorescence spectrometer. All electrochemical and photoelectrochemical experiments were performed using a CHI-660E electrochemical workstation (Chenhua, China). The Brunauer-Emmett-Teller (BET) surface area data were collected on an AutoChem II 2920 automated chemisorption analyzer (Micromeritics).

All of the photocatalytic performances were investigated under visible light. A Xe lamp was used with a 400 nm optical filter as the light source. Working solutions of RhB and MO were prepared from a stock solution of 100 mg·L⁻¹ and diluted to needed concentration (from 5 mg·L⁻¹ to 50 mg·L⁻¹) in each experiment. Typically, 20 mg photocatalyst was dispersed in 100 ml of prepared dye aqueous solution in a photocatalytic reactor. Before irradiation, the mixture was stirred in darkness for 30 min to approach the adsorption–desorption equilibrium. After the photocatalytic process was started, 5 ml of sample solution was withdrawn from the reactor every 15 min. All collected solutions were filtered by 0.22 μm cellose acetate membrane filters and then analyzed by the UV-2600i spectrophotometer (Shimadzu, China).

For the detection of active species in the photocatalytic process, disodium ethylenediaminetetraacetic acid (EDTA-Na₂), benzoquinone (BQ), and tertiary butanol (TBA) were added into the RBh solution to trap holes (h⁺), superoxide radicals (·O₂⁻) and hydroxyl radicals (·OH), respectively. The concentrations of these radical scavengers were all 0.3 mmol·L⁻¹.

The Mott− Schottky measurements and transient photocurrent experiments were all recorded in a three-electrode cell, in which a platinum wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The working electrode was prepared by dispersing 2 mg photocatalyst in the mixture of 0.5 mL ultrapure water and 1.0 mL alcohol. After ultrasonication for 2 h, 20 μL of the homogeneous solution was dropped onto a indium tin-oxide (ITO) glass substrate (0.5 × 1 cm) and naturally dried; 0.5 M Na₂SO₄ solution was used as the electrolyte. The electrochemical impedance spectra (EIS) were also analyzed in a standard three-electrode system via an electrochemical analyzer CHI 660E over a frequency range of 0.01−10⁵ Hz.

Here C is the depletion layer capacitance, ε_r is the dielectric constant of semiconductor, ε₀ is the vacuum permittivity, N_d is the carrier donor density, V_fb is the flat band potential and (k_BT/e) is the temperature dependent correction term (Wang et al. 2015; Das et al. 2018).

As shown in Figure 2(a) and 2(b), the color of the original g-C₃N₄ was pale yellow, while the p-C₃N₄ became orange after the g-C₃N₄ was calcined under the Ar atmosphere. Such color changing indicated that the structural integrity of g-C₃N₄ had been broken after calcination. The surface functional groups and band structures of the samples were determined by FT-IR spectra (shown in Figure 2(c)). The p-C₃N₄ showed a similar IR spectrum with the original g-C₃N₄, which demonstrated that there was no evident structural change after calcination. Noticeably, the peak intensities at 3,200 and 2,310 cm⁻¹ increased, which were ascribed to the stretching vibration of N − H and the asymmetric stretching vibration of cyan groups (−C ≡ N), respectively. It indicated that some of the N − C and N = C bonds of triazine had been broken into N − H and C ≡ N bands after thermal treatment, which led to the formation of defects in the bulk catalysts. Both the defect sites and cyano groups are strong electron acceptors, which could capture the photon-generated electrons (Yuan et al. 2016; Iqbal et al. 2019).

The crystalline structures of g-C₃N₄ and p-C₃N₄ were measured by PXRD determination (Figure 2(d)). All obtained samples exhibited two distinct diffraction peaks. The spiky peak at 27.4° corresponded to the (002) diffraction plane of the hexagonal phase of polymeric g-C₃N₄ (JCPDS NO. 87-1526), confirming an interlayer packing of the graphene-like sheets. The decrease of the corresponding peak in p-C₃N₄ indicated that the conjugated interlayer stacking was partially destroyed after thermal treatment. The peak at 13.1° corresponded to the (100) diffraction plane related to the in-plane holistic structure, which illustrated p-C₃N₄ mainly kept the graphitic structure under the calcination process (Liang et al. 2019).

The morphologies of p-C₃N₄ were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). p-C₃N₄ was composed of sub-micron-sized sheets with plenty of cracks (Figure 3(a)). As shown in Figure 3(b), it demonstrated abundant pores with a diameter of about ∼50 nm appeared on the surface of p-C₃N₄ nanosheets. In contrast, the original g-C₃N₄ sample had a smooth surface and well parallel-arranged crystal morphologies (Figure S1). According to energy-dispersive X-ray (EDX) elemental mapping analysis (Figure S2), the catalyst was revealed to have uniform dispersion of C, N in the whole nanosheet. p-C₃N₄ was further characterized with high-resolution transmission electron microscopy (HRTEM). The lattice fringe spacing of 0.337 nm was revealed in the HRTEM image (Figure 3(c)), matching the d-spacing for the (002) crystallographic plane of C₃N₄. It indicated that the p-C₃N₄ still maintained the similar structure to g-C₃N₄.

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface elemental compositions of p-C₃N₄. It was found that the N/C ratios of the original g-C₃N₄ and p-C₃N₄ were very similar (1.20 and 1.22, atomic ratio), indicating the formation of cyano groups that did not result in marked N loss in the bulk (Tay et al. 2015; Huang et al. 2020). To further confirm the surface components, high-resolution XPS spectra of C 1s and N 1s were performed (Figure 4(a) and 4(b) ). The C1s XPS spectra for p-C₃N₄ contained three components located at 288.1, 285.6 and 284.6 eV, corresponding to N-C = N coordination in the framework of g-C₃N₄, C-NH_x (x = 1, 2) on the edges of heptazine units and hydrocarbons, respectively. Compared with g-C₃N₄, the intensity of N–C = N peak showed a small decrease, while the intensity of C-NH_x slightly increased. In the N 1s XPS spectra, there were fitted to triplets, locating at 398.6, 399.9, 401.0 eV, which were assigned to bicoordinated (N_2C) and tricoordinated (N_3C) nitrogen atoms and NH_x groups in the heptazine framework, respectively. Compared with g-C₃N₄, the ratio of tricoordinated (N_3C) nitrogen atoms also exhibited a small increase. These results further demonstrated some of the N − C bonds and N = C bonds of triazine had been broken in the p-C₃N₄. Such results were consistent with the data from infrared (IR) spectroscopy.

The BET surface area is vital for the enhancement of the photocatalytic degradation activities of the catalysts. The N₂ adsorption–desorption isotherms were obtained to detect the specific surface area of the catalyst. The BET surface area of g-C₃N₄ and p-C₃N₄ were 24.4 m²·g⁻¹ and 50.2 m²·g⁻¹, respectively (Figure S3). This indicated that thermal treatment was an efficient way to introduce the defects and enlarge the specific surface area of the catalyst.

To better understand the separation and migration efficiency of photoinduced electron–holes of the catalysts, the photoluminescene (PL) spectra and transient photocurrent were measured. As shown in Figure 4(c), the g-C₃N₄ sample displayed a broad emission peak centered at approximately 460 nm. Compared with the g-C₃N₄, the PL peak of p-C₃N₄ presented a red shift from 460 nm to approximately 480 nm. The bathochromic shift suggested that the band gap of p-C₃N₄ might have been changed after calcination. In addition, the peak intensity of p-C₃N₄ was far weaker than that of g-C₃N₄, which meant that the electron was relocalized on surface terminal sites and the recombination rate of photogenerated electrons–holes pairs was significantly decreased (Liang et al. 2019). Furthermore, it found that the p-C₃N₄ exhibited a larger photocurrent density response (approximately 5.5 μA·cm⁻²) under illumination than that of g-C₃N₄ (approximately 3 μA/cm²) in Figure 4(d). It demonstrated that the electron and holes can be efficiently separated in p-C₃N₄ under visible light irradiation. Thus, these results indicated that p-C₃N₄ with defects was more favorable for visible light adsorption, efficiently conduced to the transportation of the charge carriers, and further prohibited their recombination.

The optical absorption property is also an important factor in reflecting the photocatalytic ability of the photocatalysts. The photo-physical properties of the synthesized photocatalysts were examined by diffuse reflectance UV-vis light absorption in the range of 200−800 nm (wavelengths). As shown in Figure 5(a), both g-C₃N₄ and p-C₃N₄ had a primary peak at 385 nm, due to n → π* transition from nitrogen nonbonding orbitals to the aromatic nonbonding orbitals (Kroke et al. 2002; Yuan et al. 2016; Xue et al. 2019). Compared with g-C₃N₄, the adsorption edge of p-C₃N₄ exhibited the better visible light adsorption with a gradual red shift from 460 to 514 nm, consistent with the color change of p-C₃N₄ from pale yellow to orange. Additional electron states were formed above the valence band caused by structural defects, which resulted in the red shift of absorption edge (Niu et al. 2012; Xue et al. 2019). This meant that p-C₃N₄ was beneficial for the visible light utilization. The band gap energy of the catalysts was calculated according to the Kubelka–Munk equation (Shen et al. 2016). The corresponding band gap energies of g-C₃N₄ and p-C₃N₄ were 2.75 and 2.59 eV (Figure 5(b)), suggests that the involvement of defects in the structure could narrow the band gap of p-C₃N₄.

The flat band potentials (V_fb) values of the samples could be calculated from the x-intercept of the Mott–Schottky plots (1/C² = 0) as a function of the applied potential (Figure 5(c)). The V_fb of g-C₃N₄ and p-C₃N₄ were −0.99 and −0.94 V (vs. SCE), corresponding to −0.75 and −0.70 V (vs. NHE), respectively. Normally, the V_fb was 0.2 V more positive than the conduction band potential (V_CB) for an n-type semiconductor (Cui et al. 2019), so the potential of conductive band (CB) positions for g-C₃N₄ and p-C₃N₄ were −0.95 and −0.90 V (vs. NHE), respectively. This positive shift suggested a decrease in the bending of band edge, which was attributed to the enhancement of catalyst–electrolyte interactions on the surface of p-C₃N₄ (Liang et al. 2019). To clarify the charge-transfer processes on the electrode surface, EIS Nyquist plots of p-C₃N₄ samples (Figure 5(d)) exhibited a smaller impedance arc radius, which suggested a lower charge-transfer resistance than that of g-C₃N₄ (Lin et al. 2015). This indicated that the electrons were more easily transferred from interior to the surface of p-C₃N₄.

The photocatalytic degradation ability of g-C₃N₄ and p-C₃N₄ under visible light (λ > 400 nm) was assessed by using RhB and MO as representative pollutants with different functional groups. The degradation curves of RhB (20 mg·L⁻¹) using various catalysts are shown in Figure 6(a), in which the RhB concentration decreased faster with the existence of p-C₃N₄ than that of the original g-C₃N₄. The RhB degradation efficiency of p-C₃N₄ approached almost 100% after 90 min illumination, which was clearly higher than that of g-C₃N₄ (49% after 90 min). Similarly, the MO degradation efficiency for p-C₃N₄ (90% after 90 min) was also much higher than that of g-C₃N₄ (only 19% after 90 min). According to the pseudo-first-order kinetic plots and constants, the photocatalytic kinetic constant of p-C₃N₄ for RhB (5.208 × 10⁻² min⁻¹) was 6.7 times than for g-C₃N₄ (7.78 × 10⁻³ min⁻¹), while the kinetic constant for MO of p-C₃N₄ (2.76 × 10⁻² min⁻¹) was 7.1 times than for g-C₃N₄ (3.90 × 10⁻³ min⁻¹) (Figure 6(b)), indicating the enhancement of the photocatalytic activities.

To investigate the influence of the calcination time to the catalytic performance, the p-C₃N₄ samples were also thermal treated at 620 °C for 2 h (named as p-C₃N₄-2 h) and used for the degradation of RhB under the same experiment conditions. The RhB degradation efficiency of p-C₃N₄-2 h decreased slightly, but was still higher than that of the original g-C₃N₄ (Figure S4). It meant that the calcination time was one of the key factors in the formation of the porous structure and the concentration of the defects, which determined the degradation efficiency of the catalyst. The degradation efficiencies of p-C₃N₄ for different RhB concentrations were also investigated. The degradation curves showed that p-C₃N₄ retained the high degradation efficiencies even for the treatment of high concentration RhB solution (up to 90% at 105 min for 50 mg·L⁻¹ RhB, to see Figure S5).

The stability of the catalyst is an important parameter for the expected practical application. Recycling degradation experiments of RhB and MO with p-C₃N₄ were investigated. After each experimental cycle, the catalyst was separated by using high-speed centrifugation, cleaned with H₂O and then dried at 60 °C in an oven for the next cycle. As shown in Figure 7(a) and 7(b), there were no significant losses for both RhB and MO degradation efficiencies of p-C₃N₄ after three-cycle photodegradation reactions, indicating a sufficient stability and efficiency of the catalyst for pollutant degradation. To discuss the potential applications of the as-synthesized photocatalyst in real water situations, deionized water, tap water and lake water were utilized as the RhB solvents in the same photodegradation processes. The RhB degradation efficiency of p-C₃N₄ was 94% in deionized water, while the efficiency was 80% in tap water and 74% in lake water (Figure 7(c)). This decrease could be attributed to the existence of various components in different water sources, which might competitively consume the active species generated, which absorbed or scattered the light irradiation (Selvaraj et al. 2021). The results of the p-C₃N₄ in different water sources presented a promising method for photocatalytic degradation of dyes in industrial wastewater.

To investigate the possible mechanism of the degradation and find the main active species generated during the photocatalytic process, various scavengers including disodium ethylenediaminetetraacetic acid (EDTA-Na₂), benzoquinone (BQ), and tertiary butanol (TBA) solutions were introduced into the RhB solution with the catalyst p-C₃N₄ to quench the relevant active species holes (h⁺), superoxide radicals (·O₂⁻) and hydroxyl radicals (·OH) during the photocatalytic processes (Liang et al. 2019). As shown in Figure 7(d), the degradation efficiency of RhB decreased greatly with the addition of BQ, from 94% to 37%, respectively. The degradation efficiency was decreased to 61% with the addition of EDTA-Na₂. By contrast, the photocatalytic degradation efficiency of RhB was not obviously decreased when TBA is added. The scavenger experiments results indicated that superoxide radicals (·O₂⁻) was the main active species, while h⁺ was the secondary active species in the photodegradation process.

On the basis of the characterizations and discussion of the results above, the corresponding mechanism for the enhancement of photocatalytic activity of the p-C₃N₄ was presented and is illustrated in Figure 8. The CB of p-C₃N₄ (−0.90 eV) was lower than that of g-C₃N₄ (−0.95 eV), which indicated that the electrons from p-C₃N₄ had a higher reduction potential for easier formation of superoxide radicals (·O₂⁻) (Liang et al. 2019). Once irradiated by visible light, p-C₃N₄ with a narrow band gap could be more beneficial for the production of photogenerated holes and electrons than that of g-C₃N₄. The existence of defects in p-C₃N₄ could capture these electrons and restrain their recombination with photogenerated holes, which promoted the carriers migrating to the surface of catalyst to participate in the degradation process. As a result, the improved photocatalytic degradation ability of p-C₃N₄ was partly attributed to the easier formation of superoxide radicals and lower recombination rate of photogenerated electron–hole pairs than that of g-C₃N₄.

In summary, we successfully synthesized porous graphitic carbon nitride with defects using a facile calcination method as a kind of metal-free photocatalyst. The high efficiency of the photodegradation capability can be attributed to the following factors: (1) defects in the structure favored for visible light adsorption and aided the transportation of the charge carriers; (2) lower CB potential was beneficial for the formation of superoxide radicals (·O₂⁻); (3) narrow band gap promoted the electron-hole generation under visible light irradiation; (4) porous structure can increase the specific surface area to maximize active sites’ exposure and to promote the mass transfer during the photodegradation.

The project was supported by: the Natural Science Foundation of Henan (18B150003), Henan Provincial Natural Science Outstanding Youth Fund Project (212300410034), National Training Program of Innovation and Entrepreneurship for Undergraduates (202010463022) and Cultivation Programme for Young Backbone Teachers in Henan University of Technology (21420073 and 21420108).

All relevant data are included in the paper or its Supplementary Information.