Efficient degradation of organic pollutants and hydrogen evolution by g-C₃N₄ using melamine as the precursor and urea as the modifier

Abstract

Graphitic carbon nitride (g-C₃N₄) is an important photocatalyst; however the photocatalytic ability of pure g-C₃N₄ is restricted by rapid charge recombination and low specific surface area. To overcome the issue, we developed a facile and eco-friendly method to prepare g-C₃N₄ using melamine as the precursor and urea as the modifier. The samples were investigated by XRD, FTIR, SEM, TEM, EIS, UV-vis DRS, valence band XPS (VB-XPS) and Mott–Schottky plots. The results show that the urea-treated g-C₃N₄ has greatly improved photocatalytic efficiency. Compared with bulk-C₃N₄, the degradation rate was enhanced 7.2 and 3.7 times for Rhodamine B and methyl orange under visible-light irradiation. The as-prepared photocatalyst also showed a high rate of hydrogen evolution (498.9 μmol h⁻¹ g⁻¹) under visible light. The enhanced photocatalytic performance is attributed to both the increased specific surface area and efficient charge separation and transfer across the heterojunction. Due to the treatment with urea, g-C₃N₄/g-C₃N₄ isotypeheterojunction is formed, and the transfer of the photogenerated electrons and holes is driven by the conduction band offset of 0.49 eV and the valence band offset of 0.57 eV, respectively, which is confirmed by both VB-XPS and Mott–Schottky plots. This work demonstrates that graphitic carbon nitride with isotypeheterojunction is an excellent metal-free photocatalyst due to its visible-light-driven bandgap and suitable band edges, and leads the way for development of other efficient visible-light photocatalysts.

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Introduction

Semiconductor photocatalysis techniques have attracted considerable attention in recent years, as they provide a safer and environmentally friendly route for various catalyst applications in clean energy production and environmental remediation.^1–3 For practical applications, a desirable photocatalyst should possess a visible-light-driven bandgap and appropriate band edges. In addition, the photocatalyst should be non-toxic, stable, reusable, easy to prepare, and made from readily available raw materials. Carbon nitride is one such material that satisfies all these requirements. Graphitic carbon nitride (g-C₃N₄) was first developed as a metal-free photocatalyst with a visible-light driven bandgap and suitable band edges in 2009.⁴ It has found extensive applications in various fields due to its stability in ambient atmosphere,⁵ high thermal stability,⁶ and hydrothermal stability.⁴ In general, g-C₃N₄ can be synthesized by the thermal polycondensation of low-cost nitrogen-rich precursors, such as cyanamide, dicyandiamide, melamine, ammonium dicyanamide, urea and thiourea.⁷ However, the photocatalytic performance of g-C₃N₄ has been limited by its low catalytic efficiency, mainly due to the fast charge recombination process. Creating a heterojunction with type II alignment is an efficient method of accelerating the dissociation of excitons and minimizing the electron–hole recombination,⁸ thereby enhancing the photocatalytic performance.

In recent years, some semiconductor/g-C₃N₄ heterojunctions, such as TiO₂/g-C₃N₄, CdS/g-C₃N₄, Ag₂O/g-C₃N₄, NiS/g-C₃N₄, Fe₂O₃/g-C₃N₄, Zn₂FeO₄/g-C₃N₄, V₂O₅/g-C₃N₄ and MoS₂/g-C₃N₄ have been widely reported.^9–18 The common preparation methods of g-C₃N₄ based heterojunctions, such as chemical precipitation, hydrothermal reaction and covalent bonding, are generally multistep processes, hard to control and requiring rigorous conditions. The bandgap structure of g-C₃N₄ can be easily tuned by using different precursors, varying the pyrolysis conditions and doping. The band gap energies of g-C₃N₄ range from 2.4 to 2.8 eV depending on the different preparation conditions.^7,19,20 Noting this fact, we speculated that constructing two components of g-C₃N₄ to form type II band alignment of a g g⁻¹ C₃N₄ isotypeheterojunction would provide an alternative path way to address the intrinsic drawbacks of g-C₃N₄ for enhanced photocatalysis without relying on extra semiconductors.⁷ When g g⁻¹ C₃N₄ isotypeheterojunction is formed, the separation efficiency of photogenerated charge in g g⁻¹ C₃N₄ isotypeheterojunction is more intensive than pristine g-C₃N₄. The potential difference between the two g-C₃N₄ components in the heterojunction is the main driving force for efficient charge separation and transfer. Therefore, remarkable enhancement of photocatalytic capability of g g⁻¹ C₃N₄ isotypeheterojunction under vis-light irradiation can be achieved.

In this work, we developed a facile and eco-friendly method to prepare g g⁻¹ C₃N₄ isotypeheterojunction using melamine as the precursors and urea as the modifier. While products prepared from the precursors such as urea, thiourea, and cyanamide typically release harmful gases (NH₃, H₂S, HCN etc.), the g-C₃N₄ prepared from melamine shows excellent characteristics, such as high yield and less ammonia production.^21,22 The as-prepared isotypeheterojunction exhibited efficient photodegradation of organic pollutants and photocatalytic hydrogen evolution, which was attributed to both the increased specific surface area and efficient charge separation and transfer across the heterojunction. Due to the treatment of urea, g g⁻¹ C₃N₄ isotypeheterojunction is formed, and the transfer of the photogenerated electrons and holes was driven by the conduction band offset of 0.49 eV and the valence band offset of 0.57 eV, respectively. In this heterojunction, two types of the g-C₃N₄ (MCN and UCN) could be used to simulate its architecture, which is consistent with VB-XPS and Mott–Schottky plots results. This work demonstrated that graphitic carbon nitride with isotype-heterojunction, a visible-light driven bandgap and suitable band edges is an excellent metal-free photocatalyst.

Experiments

Synthetic methods

All starting materials were of analytical grade and used without further purification. For the synthesis of bulk-C₃N₄, 20 g of melamine powder (ABCR GmbH & Co. KG) was placed into an alumina crucible with a cover and then heated to 550 °C at a rate of 2.3 °C min⁻¹ in a muffle furnace and maintained at this temperature for 4 h. Then, the alumina crucible was cooled to room temperature (RT). The product was collected and ground to powder, denoted as MCN.

In the following experiment, 0.3 g MCN and urea (varying amounts) were added into 30 mL deionized water and sonicated for 5 min. The solution was then transferred to a 50 mL autoclave with an inner Teflon lining and maintained at 170 °C for 24 h and then cooled to RT. The product was collected by centrifugation, washed with deionized water, and dried at 80 °C. The products were denoted as CN-X (the representative of all intermediate products), where X represents the amount of urea used in the procedure (X = 0, 0.05, 0.1, 0.5, 0.8, 1.0 g). The CN-X powders were heated to 550 °C at a rate of 10 °C min⁻¹ and maintained for 2 h. The powders were then cooled to RT and denoted as CN-X-550 (the representative of all final product). The schematic of our preparation method is shown in Scheme 1.

Scheme 1 Mechanism of the reaction paths of modified graphitic carbon nitride.

For comparison, MCN powders, urea and mechanically mixed MCN (0.3 g)–urea (0.8 g) powders were also prepared by the above procedure without hydrothermal treatment, and the products were denoted as MCN-550, UCN and UMCN.

Characterization

The crystal structures of the resultant products were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (D/max-2000, Rigaku) with λ = 0.154 nm radiation. The Fourier-transform infrared (FT-IR) spectra were recorded on a FT-IR spectrometer (Nicolet-6700, Thermofisher) using a standard KBr pellet technique. The morphology of the products was observed at 10 kV on a field-emission scanning electron microscopy (FESEM) (S-4800, Hitachi). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a transmission electron microscope (Tecnai G² F20 S-Twin, FEI, USA) with a field emission gun at 200 kV. The TEM was coupled with an energy dispersive spectroscopy (EDS) detector with the ability to acquire EDS elemental mapping. Optical diffuse reflectance spectra were collected at room temperature with a UV-vis diffuse reflectance spectrometer (DRS) (U-4100, Hitachi). Absorption spectra were calculated from reflectance data using the Kubelka–Munk function. X-ray photoelectron spectra (XPS) were acquired using an electron spectrometer (ESCALAB 250, Thermo Scientific). Nitrogen adsorption isotherms were measured with an adsorption analyzer (ASAP-2020, Micro-metrics) at liquid nitrogen temperature. Thermogravimetric and differential scanning calorimetry (TG-DSC) curves were collected by a simultaneous thermal analyzer (STA 449 F3, Netzsch).

Photocatalytic experiments

The photocatalytic degradation ability of the as-prepared samples were evaluated by using them in the photodegradation reaction of two commonly used organic pollutant dyes, Rhodamine B (Rh.B) and methyl orange (MO), under visible-light irradiation at ambient temperature using a 150 W Xe arc lamp (Zolix LSP-X150, Beijing) with a 420 nm cutoff filter as the light source. In a typical photocatalytic experiment, 50 mg of the photocatalyst was dispersed in 50 mL Rh.B solution (10 mg L⁻¹) or MO solution (5 mg L⁻¹). Before starting the experiment, the solution was stirred for 30 min in the dark to achieve the adsorption–desorption equilibrium. During the photodegradation reaction, the dye-photocatalyst solution was stirred with a magnetic stirrer, and aliquots were taken at 10 or 30 min intervals, followed by centrifugation and filtration to remove the photocatalyst. The concentration of Rh. B (or MO) was determined by monitoring changes in the absorption spectrum at 554 (or 463 for MO) nm with a Hitachi U-4100 UV-vis spectrometer.

For the hydrogen evolution, reactions were carried out in a Pyrex top–irradiation reaction vessel connected to a glass enclosed gas circulation system (CEL-SPH2N, CEAULICHT). Typically, 100 mg of photocatalyst sample was dispersed in 120 mL of 10 vol% triethanolamine (TEOA) (used as sacrificial electron donor) and then sonicated for 10 min in an ultrasonic bath. In the case of deposition of Pt, an appropriate amount of H₂PtCl₆ solutions (3 wt% Pt with respect to the catalyst used) was dissolved in the reactant solution.⁴ The reactant solutions were degassed several times to remove air prior to irradiation under a 300 W Xe arc lamp equipped with a 420 nm cutoff filter and a water infrared (IR) filter under an argon (Ar) atmosphere at 100 mTorr. The produced gas was analyzed with a gas chromatograph (GC-7900, CEAULICHT) equipped with a thermal conductivity detector, with Ar as the carrier gas.

Electrochemical measurements experiments

Electrochemical measurements were performed on an electrochemical analyzer (IVIUMSTAT, Technologies BV) in a standard three-electrode configuration with a Pt wire as the counter electrode and Ag/AgCl (in saturated KCl) as a reference electrode. Na₂SO₄ (0.25 M) aqueous solution was used as the electrolyte. The typical working electrode was prepared as follows: 50 mg ground samples were mixed with 5 mL H₂O to make slurry. The slurry was then dispersed onto FTO glass with 0.38 cm² g-C₃N₄ exposed and then the FTO glass was dried at 60 °C for 12 h to obtain the electrode.

Results and discussion

The XRD patterns of MCN, CN-0.8, CN-0.8-550, UMCN, and MCN-550 are shown in Fig. 1(a), and that of CN-X and CN-X-550 are shown in Fig. S1.† The XRD patterns confirm the formation of graphitic stacking carbon nitride layers. The curve for MCN shows two peaks at 27.7° and 13.1°, which could be attributed to the typical (002) and (100) diffraction planes of the g-C₃N₄. The peak of CN-0.8 at (002) plane shifts to 27.9° and two new peaks appear at 10.8° and 12.4°. Shift of the (002) plane indicates a decrease in the interplanar spacing distance.²³ The appearance of the new peaks possibly results from the modification of a group containing N and O elements²⁰ and the dissolution of g-C₃N₄.^24,25 This phenomenon indicates that the MCN is dissolved in the hydrothermal process and certain groups (shown in Scheme 1) are introduced into the tri-s-triazine units. This is the reason why the intensity of the CN-0.8's peak is enhanced, and indicates the higher crystallinity of the sample.²⁶ This phenomenon becomes more obvious with the increasing amount of urea as shown in Fig. S1.† When the CN-0.8 was calcined at 550 °C, the peaks corresponding to (002) and (100) planes are restored to 27.7° and 13.1°, which are similar to the peaks of MCN and MCN-550. This suggests the recovery of the interplanar spacing distance and the in-plane repeated units. The curve for UMCN, shown in Fig. 1(a) possesses two peaks at 27.5° and 13.3°, which are different from MCN mainly because the g-C₃N₄ formed by urea has different interplanar spacing distances and in-plane repeated units compared with MCN.

Fig. 1 (a) XRD patterns, (b) FTIR spectra and XPS spectra of the as-prepared samples: (c) survey, (d) C 1s, (e) N 1s, (f) O 1s.

The FTIR spectra of the samples are shown in Fig. 1(b) and S2.† It can be seen that all the spectra contain six major peaks between 1200 cm⁻¹ and 1650 cm⁻¹, which are the characteristic stretching modes of tri-s-triazine heterocycles.^1,26,27 The peak at 803 cm⁻¹ is ascribed to triazine units in the g-C₃N₄.^1,26,27 The broad band in the range 3000–3500 cm⁻¹ can be assigned to the stretching vibration modes of NH₂ and NH groups.^1,26,27 For CN-0.8, two new peaks at 1780 cm⁻¹ and 1728 cm⁻¹ could be observed, which are assigned to the stretching and bending vibrations of the carbonyl and carboxyl groups.^28,29 The appearance of the new peaks indicates that a certain group (–NOOCN– as shown in Scheme 1) is introduced into the tri-s-triazine units by hydrothermal process. After the CN-0.8 was calcined at 550 °C, in the changed peaks revert back to the original, indicating that the graphitic carbon nitride is regenerated (as shown in Scheme 1). These results are consistent with the XRD results.

The XPS spectra were obtained to determine the chemical state of the surface elements of samples, and the results are shown in Fig. 1. Fig. 1(c) shows the survey spectra of the MCN, CN-0.8 and CN-0.8-550 samples, and it can be seen that C, N, O elements were detected. Fig. 1(d) shows the C 1s spectra of the three samples. The MCN sample has three peaks at 287.8 eV, 285.6 eV and 287.8 eV. The peak located at 287.8 eV can be assigned to C–(N)₃ group, the peak at 285.6 eV can be assigned to C–N–C coordination, and the peak at 284.7 eV is due to adventitious carbon.³⁰ It is evident that all three samples have the same peak at 284.7 eV, while the peaks at 287.8 eV and 285.6 eV shift to 287.9 eV and 285.5 eV for CN-0.8, and shift to 287.9 eV and 285.9 eV for CN-0.8-550, these shifts indicate that the hydrothermal and recalculation processes have changed the chemical nature of C element in the g-C₃N₄, and certain unstable groups introduced into g-C₃N₄ after hydrothermal processing. Fig. 1(e) shows the N 1s spectra of the three samples. The MCN sample exhibits three peaks at 400.1 eV, 398.8 eV and 398.2 eV. The peak located at 400.1 eV can be assigned to C–N–H group, the peak at 398.8 eV can be assigned to N–(C)₃ group, and the peak at 398.2 eV can be assigned to C–N–C coordination.³¹ For CN-0.8 and CN-0.8-550, the three peaks of N 1s show a slight shift, which indicates that the chemical condition of the N element in the g-C₃N₄ has changed. Fig. 1(f) shows the O 1s spectra of the samples. The peak at 532.2 eV is ascribed to the surface adsorbed H₂O in the three samples. The peak at 531.3 eV which appears in CN-0.8 sample is assigned to C=O bond³² which arises after hydrothermal treatment, and the peak at 533.4 eV which appears in CN-0.8-550 is assigned to the C–O bond³³ which arises after calcination. As shown in Table 2, oxygen content increases from 5.26% to 11.23% after hydrothermal process, and reduces to 5.22% after calcination. The above results suggest that groups containing C=O bond are introduced into g-C₃N₄ by hydrothermal processing, causing significant changes in the oxygen content. After calcination at 550 °C, a C–O bond appears in CN-0.8-550, which likely improves the photocatalytic performance.⁹

The surface morphology and microstructures of the samples were characterized by SEM as shown in Fig. 2 and S3.† When MCN is treated with urea in the hydrothermal process, the microstructures of the samples change significantly. When the loading amount of urea is less than 0.5 g, nanorods are formed as shown in Fig. S3.† As the amount of urea increases to 0.8 g, microspheres with microbundles are formed (Fig. 2(b)). This indicates that MCN experiences a regrowth process during the hydrothermal procedure, and this result is consistent with the XRD, FTIR and XPS results. After calcination, nanosized structures with higher specific surface area (S_BET) were formed, as shown in Fig. 2(c). However, the MCN-550 sample does not change significantly due to the existence of bulk-C₃N₄ (Fig. 2(d)). In the UMCN sample, as shown in Fig. 2(e) and (f), both porous layers and bulk-shaped microstructures could be found as it is prepared by two different precursors. The variation of microstructures results in changes in the S_BET values of samples, and therefore affects the photocatalytic performance. TEM images of MCN, CN-0.8 and CN-0.8-550 are shown in Fig. 2. It can be observed in Fig. 2(g) that the MCN sample is characterized by a large bulk microstructure measuring several microns. When the MCN is modified by hydrothermal process, microbundles (Fig. 2(h)) are formed. The CN-0.8-550 sample's TEM image is shown in Fig. 2(i), and the sample consists of smooth and thin sheets folded like papers, similar to the morphology of graphene nanosheets after calcination.

Fig. 2 SEM images of (a) MCN, (b) CN-0.8, (c) CN-0.8-550, (d) MCN-550, (e) UMCN, (f) amplification of the white region in image and TEM images of (g) MCN, (h) CN-0.8, (i) CN-0.8-550.

The N₂ adsorption–desorption behaviors and corresponding pore-size distribution curves are shown in Fig. 3(a), and the results are summarized in Table 1. The S_BET of MCN is only 6.09 m³ g⁻¹, while the S_BET of the g-C₃N₄ treated with urea in the hydrothermal process increases by 2.92 times for minimum and 8.58 times for maximum. After calcination at 550 °C, the S_BET of all samples further increase, especially for the samples where the amount of urea added is more than 0.5 g. This is attributed to the thermal polycondensation of functional group which growth into MCN in hydrothermal process and the microstructure variations after calcination.

Fig. 3 (a) N₂ adsorption–desorption isothermal and images of 100 mg as-prepared samples (inset) and (b) corresponding BJH pore-size distribution curves of as-prepared samples. The pore-size distribution was determined from the desorption branch of the isothermal curve. Comparison of photocatalytic activities for (c) Rh.B and (e) MO degradation; reaction rate constant (h⁻¹) for (d) Rh.B and (f) MO degradation under visible-light.

Table 1 Physicochemical properties of as-prepared samples

Sample	SBET (m^3 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter^a (nm)	Reaction constant (h^−1) for degradation of Rh.B^b
a Pore diameter was obtained from BJH pore-size distribution curve.b Reaction constant (h^−1) for degradation of Rh.B was calculated by eqn (1).
MCN	6.09	0.046	23.3	0.3227
CN-0.0	43.94	0.156	10.0	0.5544
CN-0.05	52.25	0.170	10.6	0.5163
CN-0.1	43.38	0.154	11.5	0.4688
CN-0.5	20.04	0.074	12.2	0.6367
CN-0.8	18.07	0.053	8.9	0.9274
CN-1.0	17.77	0.046	8.7	0.8380
CN-0.0-550	35.55	0.146	10.5	2.2115
CN-0.05-550	77.90	0.253	9.4	1.1982
CN-0.1-550	47.73	0.156	9.4	1.5504
CN-0.5-550	45.33	0.140	8.3	1.8606
CN-0.8-550	41.24	0.139	9.2	2.3193
CN-1.0-550	32.62	0.125	10.9	2.1335
UMCN	22.75	0.135	20.4	1.7070
MCN-550	10.24	0.062	20.6	0.4670

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The photocatalytic degradation property of the as-prepared samples was evaluated by monitoring their efficiency in degrading Rh.B and MO dyes and the results are shown in Fig. 3. It can be seen from Fig. 3(c) that after 60 min of visible-light irradiation, MCN could degrade less than 34% of Rh.B, while CN-0.8-550 shows significantly better photocatalytic performance with a degradation ratio of 97%. For the degradation of MO (Fig. 3(e)), MCN was able to degrade only 36.7% of the organic pollutants after 120 min, while the degradation ratio with CN-0.8-550 was as high as 83.3%. Clearly, the CN-0.8-550 sample exhibits the highest photocatalytic efficiency compared to CN-0.8, UMCN and MCN-550 samples, thus indicating that both hydrothermal and recalcination processes are necessary for achieving efficient photocatalytic performance.

In order to further investigate the degradation rate, the pseudo-first-order kinetic model can be adopted according to the below equation:^9,21,22

ln(C/C₀) = kt (1)

where C is the concentration of pollutant (mg L⁻¹), C₀ is the adsorption equilibrium concentration of pollutant before irradiation (mg L⁻¹), t is the reaction time (h) and k is the reaction rate constant (h⁻¹). As shown in Fig. 3(d), the reaction rate constants for the degradation of Rh.B by MCN, CN-0.8, CN-0.8-550, UMCN and MCN-550 samples are calculated to be 0.3227, 0.9274, 2.3193, 1.7069 and 0.4670 h⁻¹, respectively. The photocatalytic activity of CN-0.8-550 is 7.2 times higher than that of MCN. For the degradation of MO shown in Fig. 3(f), the reaction rate constants for the MCN, CN-0.8, CN-0.8-550, UMCN and MCN-550 samples are 0.2244, 0.2779, 0.8368, 0.7423 and 0.2656 h⁻¹, respectively. Although the g-C₃N₄ has lower photocatalytic activity for MO degradation than that of Rh.B, the photocatalytic activity of CN-0.8-550 is still 3.7 times higher than that of MCN.

Furthermore, the CN-0.8-550 photocatalyst also shows good reusability in photodegradation applications. To investigate the stability of CN-0.8-550, four successive cycles of photodegradation experiments were performed for both Rh.B and MO. The experimental results are shown in Fig. 4, and it can be seen that there is no apparent decrease in the photodegradation process even after the four cycles. The used CN-0.8-550 sample was analyzed by XRD, and the results are shown in Fig. S4.† It can be seen that there are no significant changes in the CN-0.8-550 sample after the four cycles of photodegradation reactions, indicating that CN-0.8-550 can be regarded as a stable photocatalyst.

Fig. 4 Stability investigation of the CN-0.8-550 sample in (a) Rh.B and (b) MO degradation; (c) time course of hydrogen evolution from water under visible-light of MCN and CN-0.8-550 with 3 wt% Pt co-catalyst in 10 vol% TEOA/water solution; (d) four successive cycle runs for the CN-0.8-550.

The excellent photocatalytic activity of the CN-0.8-550 sample is further demonstrated by the photocatalytic H₂ evolution from a 10 vol% TEOA/water solution with the help of 3 wt% Pt co-catalyst. The average hydrogen evolution rate (HER) with CN-0.8-550 under visible-light is 498.9 μmol h⁻¹ g⁻¹ after four successive H₂ evolution experiment cycles, which is 15.5 times higher than that with the MCN sample. In addition, the H₂ evolution rate and the structure of CN-0.8-550 remain quite stable through the experiment. After four cycles, there is no obvious decrease in the HER rate and no significant variation in the photocatalyst microstructure, as shown in Fig. 4(d) and S5,† indicating the stability of CN-0.8-550 as photocatalyst.

From above results, it can be concluded that fabricating g-C₃N₄ using melamine as the precursors and urea as the modifier could greatly improve photocatalytic performance and the CN-0.8-550 sample shows the best photocatalytic efficiency. As shown in Table 1, all samples' specific surface areas and photocatalytic efficiencies were increased after treatment by urea. However, it should be noticed that surface areas of these samples shows a increasing followed by decreasing with further increasing addition amount of urea, while photocatalytic efficiencies exhibit monotonously increasing. These results indicates that the increased surface area by urea incorporation is not the only reason for improved photocatalytic behavior.^34,35 The optical properties and band structures would also influence the photocatalytic behavior, and a large S_BET does not definitely indicate high photocatalytic efficiency.

To further clarify the photocatalytic mechanism, we investigated the optical properties and band structures of the as-prepared samples by UV-vis DRS, VB-XPS and Mott–Schottky plots. As shown in Fig. 5(a), the optical absorption spectra reveal that all the samples have an intrinsic semiconductor-like absorption in the blue region of the visible spectra. The absorption edge of MCN is about 463 nm, while that of CN-0.8 shifts to about 450 nm. When CN-0.8 was calcined at 550 °C, the absorption edge of CN-0.8-550 shifted to 470 nm, indicating the broader absorption for visible light. Fig. 5(b) and S6† show the bandgap of the MCN, CN-X, CN-X-550, UMCN and MCN-550 samples. The band gap energy (E_g) is estimated from the intercept of the tangents to the plots of (Ahν)² vs. photon energy. The corresponding values are shown in Table 1 and ESI.†

Fig. 5 (a) UV-vis DRS results and (b) plots of (Ahν)² vs. photon energy of as-prepared samples; (c) VB-XPS results for MCN, UCN, and CN-0.8-550; (d) mechanism of action for CN-0.8-550 heterojunction.

The VB-XPS, UV-vis DRS and Mott–Schottky plots results are summarized in Table 2. As shown in Table 2, the VB position of the CN-0.8-550 sample is offset of 0.27 eV (compared with the MCN sample) obviously, thus indicating that the band structure of the CN-0.8-550 sample has adjusted. Therefore, two types of the g-C₃N₄ (MCN and UCN) could be used to simulate CN-0.8-550's architecture. From the VB-XPS results shown in Fig. 5(c), the VB maximum of MCN and UCN are found to be 1.28 and 1.85 eV_NHE, respectively (NHE = normal hydrogen electrode), in agreement with previous reports.^36,37 The VB potential of UCN is found to be more positive than that of MCN. From the band gap energy shown in Table 2, the CB positions of MCN and UCN are calculated to be −1.37 and −0.88 eV, respectively, indicating that the CB potential of MCN is more negative than that of UCN. Meanwhile, the CB positions of MCN, UCN and CN-0.8-550 were also confirmed by Mott–Schottky plots. As shown in Fig. 6(a), the CB positions of MCN and UCN are calculated to be −1.40 and −0.89 eV, which is similar with the position calculated by VB-XPS and UV-vis DRS results. In addition, Mott–Schottky plots suggest n-type characteristics of MCN, UCN and CN-0.8-550 due to the positive slope of the linear plots.^38,39 On the basis of VB and CB levels of MCN and UCN, a band structure diagram for MCN/UCN phase heterojunction can be drawn as shown in Fig. 5(d). Once MCN and UCN are electronically coupled together, the band alignment between the two kinds of g-C₃N₄ materials results in the formation of a heterojunction with well-matched band structure. Upon visible-light irradiation of the CN-0.8-550 heterojunction, the photoresponsive electrons tend to transfer from MCN to UCN actuated by CB offsets of 0.49 eV, whereas the photoresponsive holes transfer from UCN to MCN actuated by VB offsets of 0.57 eV. Thus, the photoexcited e⁻–h⁺ pairs could be effectively separated using this technique, and the formed junction could further hinder the recombination of electrons and holes in charge transfer processes.

Table 2 Measured VB and CB positions, band gap energy, and elemental analysis of as-prepared samples

Sample	VB position (eV)	CB position^a (eV)	Band gap energy Eg^b (eV)	Elemental analysis^c (PP At%)
				C	N	O
a VB and CB positions were determined by VB-XPS and UV-vis DRS.b Band gap energy Eg was determined by UV-vis DRS.c Elemental analysis was determined by XPS.d Mott–Schottky plots results.
MCN	1.28	−1.37(−1.4)^d	2.75	47.98	46.77	5.26
CN-0.8	1.28	−1.56	2.84	44.55	44.22	11.23
CN-0.8-550	1.55	−1.21(−1.22)^d	2.79	48.72	46.06	5.22
UCN	1.85	−0.88(−0.89)^d	2.73	49.86	40.18	9.96

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Fig. 6 (a) Mott–Schottky plots and (b) EIS Nyquist impedance plots for MCN, UCN, and CN-0.8-550.

Electrochemical impedance spectroscopy (EIS) measurements were used to investigate charge transfer and recombination processes at solid/electrolyte interfaces. The reaction rate occurring on the surface of the electrode is reflected by the radius of the arc on the Nyquist plots.^40,41 As shown in Fig. 6(b), it was found that the arc radius of CN-0.8-550 is smaller than that of MCN and UCN. The smaller arc radius in the Nyquist impedance plot of CN-0.8-550 indicates a more effective separation efficiency of photoinduced electron hole pairs and a faster interfacial charge transfer.^42,43 Thus the EIS study gives further support for the higher photocatalytic activity of CN-0.8-550.

Conclusion

In summary, we developed a facile and eco-friendly method to prepare g-C₃N₄ using melamine as the precursors and urea as the modifier. The results exhibit that the urea treated g-C₃N₄ shows greatly improved photocatalytic efficiency. The enhanced photocatalytic performance is attributed to both the increased specific surface area and efficient charge separation and transfer across the heterojunction. Due to the treatment of urea, g g⁻¹ C₃N₄ isotypeheterojunction is formed, and the transfer of the photogenerated electrons and holes was driven by the conduction band offset of 0.49 eV and the valence band offset of 0.57 eV, respectively. This work demonstrates that graphitic carbon nitride with high specific surface area and isotype-heterojunction is an efficient metal-free photocatalyst with a visible-light driven bandgap and suitable band edges, and points the way for further development of efficient visible-light photocatalysts.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant Nos.: 61376020, 21301167, 61574021), the Natural Science Foundation of Jilin Province (20130101009JC), China are acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26890g

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