Facile Preparation, Mechanism Insight, and Application as Efficient

Research Article pubs.acs.org/journal/ascecg

Phosphorus- and Sulfur-Codoped g‑C3N4: Facile Preparation, Mechanism Insight, and Application as Efficient Photocatalyst for Tetracycline and Methyl Orange Degradation under Visible Light Irradiation Longbo Jiang,†,‡ Xingzhong Yuan,*,†,‡ Guangming Zeng,†,‡ Xiaohong Chen,§ Zhibin Wu,†,‡ Jie Liang,†,‡ Jin Zhang,†,‡ Hui Wang,†,‡ and Hou Wang*,∥ †

College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China Key Laboratory of Environmental Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, P. R. China § Mobile E-business 2011 Collaborative Innovation Center of Hunan Province, Hunan University of Commerce, Changsha 410205, China ∥ School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore ‡

ABSTRACT: Phosphorus- and sulfur-codoped graphitic carbon nitride has been successfully synthesized by in situ thermal copolymerization of hexachlorocyclotriphosphazene and thiourea. The phosphorus doping, together with the sulfur doping, would enhance light trapping, surface area, and charge separation, making it serve as a more efficient photocatalyst than its pure g-C3N4 and single-doped g-C3N4 counterpart for the removal of tetracycline (TC) and methyl orange (MO). The optimum photocatalytic activities of a P-, S-codoped g-C3N4 sample for the degradation of TC and MO were about 5.9 times and 7.1 times higher than that of individual g-C3N4, respectively. Furthermore, the optimum TOC removal reached 70.33% and 55.37% for TC and MO within 120 min, respectively. The introduction of a P atom and S atom could significantly change the electronic property of g-C3N4 and suppress the recombination of photogenerated charges. Moreover, the defects in the framework of samples caused by the doping of P and S could serve as centers to trap the photoinduced electrons, thus inhibiting the charge recombination and improving its photocatalytic performance. KEYWORDS: g-C3N4, Codoping, Visible light photocatalysis, Tetracycline, Methyl orange

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The conduction band (CB) position of g-C3N4 is about −1.23 eV, which is able to promote H2 evolution and •O2 − production.3,17 The valence band (CB) position of pure gC3N4 is 1.47 eV, which can trigger water oxidation.18 Due to the appropriate band position, along with its excellent thermal and chemical stability, g-C3N4 is widely applied in photocatalytic organic pollutant degradation, water splitting, CO2 reduction, and organic synthesis under visible light.19−21 However, pristine g-C3N4 is always restricted by unsatisfactory photocatalytic efficiency caused by insufficient visible absorption, fast recombination of photogenerated charges, and low surface area.3,22,23 Thus, various modification strategies, such as elemental doping,24,25 preparation of mesoporous g-C3N4,26 design of g-C3N4nanosheets,27 combination with conductive materials,28,29 construction of heterojunctions with other

INTRODUCTION Semiconductor photocatalysis technology has become a hot research field, serving as a promising and green technology for its potential applications in the removal of organic contaminants and solar energy conversion.1−3 TiO2 has become the most popular and widely used photocatalyst since Fujishima and Honda applied TiO2 as the photoanode for water splitting in 1972.4 However, the use of TiO2 is limited by its wide bandgaps (∼3.2 eV) and negligible activity under visible light irradiation.5,6 In order to overcome the drawbacks of a wide bandgap TiO2, a large amount of semiconductors with a narrow bandgap, such as Ag3PO4, BiVO4, CdS, In2S3, ZnIn2S4, and Bi2WO6, have been designed to efficiently utilize the visible light in solar light.1,7−13 Recently, graphitic C3N4 (g-C3N4), a metal-free polymer semiconductor with tri-s-triazine units, has attracted much attention for its promising applications in photochemistry and photocatalysis.14,15 In detail, g-C3N4 is a promising semiconductor with good visible light response (up to 455 nm).16 © 2017 American Chemical Society

Received: February 22, 2017 Revised: May 24, 2017 Published: June 6, 2017 5831

DOI: 10.1021/acssuschemeng.7b00559 ACS Sustainable Chem. Eng. 2017, 5, 5831−5841

Research Article

ACS Sustainable Chemistry & Engineering semiconductors,9 and dye sensitization30 are adopted to improve photocatalytic activity. Among the methods mentioned above, doping with nonmetal (e.g., P, B, S, O, and Br)16,18,31−35 or metal (e.g., K, Fe, and Cu)36−38 atoms has been considered as an efficient strategy. To date, some researchers have reported that P-doped g-C3N4 or S-doped g-C3N4 exhibited enhanced visible light photocatalytic performance.18,23,39,40 Zhang et al. prepared Pdoped g-C3N4 using dicyandiamide and the ionic liquid [Bmim]PF6 as the precursors.41 The electrical conductivity and the separation efficiency of photogenerated charges were obviously enhanced after P doping. However, the ionic liquid [Bmim]PF6 is expensive and complicated to synthesis, making it not an ideal phosphorus source. Hexachlorotriphosphazene (HCCP) has an s-triazine-like P−N heterocyclic ring which can match the tri-s-triazine structure of g-C3N4 better.24 It suggests that HCCP will easily incorporate the P atom into the tri-striazine structure of g-C3N4. On the other hand, Liu et al. synthesized S-doped g-C3N4 by heat treating pure g-C3N4 in gaseous H2S at high temperature.40 However, it would discharge much poisonous and foul odor gas to the surrounding environment. Recently, an in situ sulfur doping strategy is identified as a facile and environmental friendly approach to obtain S-doped g-C3N4 with enhanced photocatalytic activities.18,42−44 For example, Hong et al. designed in situ S-doped mesoporous g-C3N4 using thiourea as the precursor with promoted photocatalytic hydrogen evolution under visible light.42 This heteroatom doping strategy is beneficial for tuning the band gap structure, increasing the surface area, enhancing light trapping, accelerating charges separation, and creating more active sites.2,3,18 However, it is worth noting that single element doping always could not achieve all the enhancements above and obtain the satisfactory photocatalytic properties. Codoping could combine the merits of both dopants, resulting in enhanced photocatalytic activity. Codoping of TiO2 has been extensively developed.45−47 However, few studies focused on the design and properties of codoped gC3N4. Recently, Fe-, P-codoped g-C3N4,48 P-, O-codoped gC3N4,49 B-, S-codoped g-C3N4,50 and S−Co−O-tridoped gC3N451 have been developed and exhibit much higher photocatalytic activities than that of single-doped g-C3N4. In this study, the P-doping and in situ S-doping strategies were combined for the synthesis of metal-free P-, S-codoped g-C3N4. HCCP was used as the phosphorus source, and thiourea served as both the sulfur source and raw materials of g-C3N4. P-, Scodoped g-C3N4 was fabricated by a facile copyrolysis of HCCP and thiourea mixtures in the muffle furnace. The obtained P-, Scodoped g-C3N4 showed much enhanced visible light photocatalytic activities toward the degradation of TC and MO solution than that of bare g-C3N4 and single-doped g-C3N4. Furthermore, the influences of element doping are systematically discussed.

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Pure g-C3N4 was obtained by using melamine as the single raw material and is referred to as CN. P-doped g-C3N4 was prepared by mixing melamine (2.0 g) and HCCP (200 mg) as the starting materials. The obtained catalyst was denoted as PCN. Characterization. XRD analyses were conducted on a Bruker AXS D8 advance diffractometer with a Cu Ka source. Field emission scanning electron microscopy (SEM) (JSM-6700F, Japan) was used to examine the morphologies of resulting samples. Transmission electron microscopy (TEM) (Tecnai G2 F20, USA) was applied to analyze the structural details of the catalyst. X-ray photoelectron spectroscopy (XPS) was obtained from a Thermo Fisher Scientific spectrometer. The N2 adsorption−desorption isotherm was measured by a surface analyzer (Micromeritics ASAP 2020). The absorbance spectrum was recorded with a Varian Cary 300 spectrometer. The photoluminescence (PL) spectra were measured by using a PerkinElmer LS-55 spectrofluorimeter with an excitation wavelength of 350 nm. Total organic carbon (TOC) was analyzed using a Shimadzu TOCVCPH analyzer. The photoelectrochemical experiments were carried on a CHI 660C electrochemical analyzer (CHI 660C, China) in a three-electrode cell. A platinum wire and Ag/AgCl and FTO electrodes deposited with samples were used as the counter, reference, and working electrodes, respectively. The electron spin resonce (ESR) experiments were carried out on a Bruker ER200-SRC spectrometer. Photocatalytic Experiments. TC and MO were selected as target pollutants to test the photocatalytic activities. Visible light (λ ≥ 420 nm) was provided by a 300 W xenon lamp (Beijing China Education Au-light, Co., Ltd.). The average visible light intensity was ca.100 mW cm−2 measured by a light meter (HS1010). In a typical procedure, 1.0 g/L and 0.5 g/L photocatalysts were used for TC (10 mg L−1) and MO (10 mg L−1) degradation, respectively. The suspension was stirred for 60 min in the dark before irradiation. At a certain time intervals, 4 mL of suspension was withdrawn and then centrifuged for analysis. The concentrations of TC and MO were examined by a UV−vis spectrophotometer (UV-2250, SHIMADZU Corporation, Japan). Five successive cyclic tests for TC and MO degradation were conducted.

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RESULTS AND DISCUSSION XRD Analysis. Figure 1 displays the XRD patterns of P-, Scodoped CN in comparison with those of P-doped CN, S-

MATERIALS AND METHODS

Figure 1. XRD patterns of the prepared CN, PCN, SCN, and PSCN samples.

Preparation of Photocatalysts. In a typical experiment, a certain amount (2.0 g) of thiourea and different amount of HCCP were grounded together in an agate mortar. The resultant powders were calcined at 550 °C for 4 h in a covered crucible with a heating rate of 10 °C min−1.52 After being cooled to room temperature, the obtained products were collected and milled into powder. For clarity, the P-, Scodoped C3N4 with expected HCCP contents of 25, 50, and 75 mg are referred to as PSCN-25, PSCN-50, and PSCN-75, respectively. For comparison, the S-doped g-C3N4 sample was obtained according to the same method above without HCCP and is referred to as SCN.

doped CN, and pure CN. Two characteristic peaks of g-C3N4 were observed in the XRD patterns, located at 13.0° and 27.7 o, respectively.3,22,24 The characteristic peak of pure g-C3N4 around 27.7°, resulting from the interplanar stacking peak of the aromatic systems, could be indexed as the (002) diffraction plane.24 The minor diffraction peak at 13.0°, representing the 5832


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Figure 2. SEM images of as-prepared samples (a) SCN and (b) PSCN-50. EDS elemental mapping of PSCN-50 (c, d, e, and f).

Figure 3. TEM micrographs of PSCN-50 (a, b). 5833


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Figure 4. XPS spectra of PSCN-50 sample: (a) survey scan, (b) C 1s, (c) N 1s, (d) O 1s, (e) P 2p, and (f) S 2p.

exhibited aggregated, lamellar, and different nanosized crystal stacking layers.52,54 After phosphorus doping, lamellar SCN was observed in irregular stacked decorated sheets, probably due to random doping (Figure 2b). The size of the layered structure of PSCN-50 was obviously shrunk compared with SCN. This means that phosphorus doping could restrain crystal growth of g-C3N4, which is helpful to the faster photogenerated electron− hole pairs separation.55 Meanwhile, the energy dispersive spectroscopy (EDS) elemental mapping of PSCN-50 was performed to display the distribution patterns of the component elements. As shown from Figure 2c−f, all four major elements (C, N, P, and S) are uniformly distributed in the layer structure of the PSCN-50 sample. The TEM image of PSCN-50 also confirmed the sheet-like structure (Figure 3). This morphology of PSCN-50 was due to aggregation and random gathering of the g-C3N4 nanosheets. XPS Analysis. The surface elemental composition and chemical state of PSCN-50 were further investigated by XPS. As shown in Figure 4, four elements of C, N, O, and P can be easily found. However, a characteristic peak for sulfur is less prominent due to its low doping level as suggested by the

in-plane structural packing motif of the tri-s-triazine units, could be perfectly indexed to the (100) diffraction plane for graphitic materials.22 As for phosphorus-doped g-C3N4, the featured peaks are still retained, indicating that the textural structure of g-C3N4 is not evidently changed during the phosphorus-doping procedure. The stronger intensity of the peak indicated that CN and PCN have better crystallinity. The peaks of PSCN show positions similar to those of g-C3N4 but slightly broadened, implying that more defects in the structure appear along with phosphorus and sulfur doping.53 Additionally, the peak location of the (002) plane is gradually shifted from 27.7° to 27.5°, which is probably due to the phosphorus and sulfur heteroatoms being doped into the lattices of g-C3N4, leading to greater lattice distortions and interplanar distance.34 Moreover, XRD patterns indicated that the P , S codoping has no significant influence on the structure of g-C3N4, and no other characteristic peaks can be detected, indicating that P-, Scodoped g-C3N4 retains the original crystal structure of g-C3N4. Morphological Analysis. The morphology and microstructure of the samples were characterized via SEM and TEM. As depicted in Figure 2a, it can be seen that the SCN sample 5834


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regions. Moreover, the absorption edge was red shifted from 460 nm of pure CN to 464, 466, 468, 473, and 477 nm of PCN, SCN, PSCN-25, PSCN-50, and PSCN-75, respectively. The red shift indicated that the doping of phosphorus and sulfur resulted in the trapping of more light energy to generate more photoinduced electron−hole charges.58 The enhanced light absorbance and red shift of PCN, SCN, and PSCN could be because the content of phosphorus atoms and sulfur atoms incorporation into the g-C3N4 matrix has a significant effect on the band structure and electronic properties which are closely related to the optical absorption of a semiconductor.34,42,54 As a result, the optical properties of g-C3N4 would be influenced by phosphorus or sulfur doping. In general, the optical absorption band edge (Eg) of the asprepared samples can be estimated according to the following formula:62

elemental analysis result. In order to confirm the presence of sulfur in the PSCN-50 sample, the atomic composition of S was determined as 0.06% by CHNOS Elemental Analyzer Vario EL III. The chemical states of the elements were further investigated by their corresponding high-resolution spectra. High-resolution spectra of C 1s (Figure 4b) can be fitted into two main peaks at 284.8 and 288.1 eV, respectively. The former is ascribed to the adventitious carbon species, whereas the latter one is assigned to a sp2-hybridized carbon in an N-containing aromatic ring (N−CN) in the g-C3N4 lattice.17,44,56 As shown in Figure 4c, the N 1s spectra can be fitted with four peaks at 398.8, 399.6, 401.4, and 404.2 eV, respectively. The main N 1s peak located at 398.8 eV could be attributed to an sp2-hybridized nitrogen atom bonded to a carbon atom (C N−C), whereas the component at 399.6 eV usually corresponds to either N−(C)3 groups. The weak peak located at 401.4 eV corresponds to the N−(C)3 group in the aromatic cycles.22,57 Additionally, the negligible peak at 404.2 eV is caused by charging effects.24 In the O 1s region (Figure 4d), the peak at 532.3 eV was attributed to adsorbed H2O or CO2.18,54 Another peak located at 530.4 eV was attributed to the HO− CO bond.58 The peak of P 2p is located at 133.9 eV, which corresponds to P−N coordination (P−C bonding would be centered at about 131.5 eV), suggesting that P most probably substitutes for C in triazinerings to form P−N bonds (Figure 4e).22,34,59 In Figure 4f, the S 2p peak was deconvoluted into four peaks at 161.5, 163.7, 165.4, and 167.6 eV. The weak peaks at 163.7 and 165.4 eV were ascribed to the C−S bond and N−S bond which were formed by substituting sulfur with lattice nitrogen and carbon, respectively.18,60 The peaks at 161.5 and 167.6 eV belong to S2− and oxidized S (SOx), indicating that the S elements are partially doped into the g-C3N4 sheets.1,61 The P content is determined to be 3.02 atom % by XPS analysis of PSCN-50, and the atomic composition of S in the PSCN-50 was 0.06%. Moreover, the corresponding C/N atomic ratio (0.737) is close to the theoretical value of g-C3N4 (0.75). Therefore, the experimental results revealed the successful preparation of P-, S-codoped g-C3N4. UV−Vis DRS Analysis. The optical properties of the CN, PCN, SCN, and PSCN samples were characterized using UV− vis diffuse reflectance spectroscopy. As depicted in Figure 5, phosphorus doping, sulfur doping, and P, S codoping could enhance the light trapping in both ultraviolet and visible light

Eg = 1240/λ

(1)

where λ is the wavelength (nm) of the absorption edge. Thus, the band gaps were estimated to be about 2.70, 2.67, and 2.66 eV for CN, PCN, and SCN, respectively, which were similar to the previous literatures.52,58,63 Additionally, as for the other P-, S-codoped samples (from PSCN-25 to PSCN-75), the energy band gaps were found to be 2.65, 2.62, and 2.60 eV, respectively. Nitrogen Adsorption Analysis. Nitrogen adsorption− desorption isotherms of the photocatalyst samples were measured to characterize the specific surface area and pore distribution. As shown in Figure 6, the isotherms of all samples exhibit a classical type IV, indicating the presence of mesopores. A type H3 hysteresis loop in the relative pressure range of 0.9− 1.0 is related to the larger pores formed between secondary particles.48 Owing to the emission of ammonia gas during the thermal condensation process, some pores would be generated on the surface of g-C3N4-based photocatalysts. The BET surface areas, pore size, and pore volume of the as-prepared samples are collected in Table 1. As shown in Table 1, the BET specific surface areas (SBET) of CN, PCN, SCN, and PSCN-50 were calculated to be 6.42, 10.21, 20.03, and 25.87 m2 g−1, respectively. The much enhanced surface area of PSCN-50 was due to the P atoms and S atoms being doped into the g-C3N4 lattices inhibiting crystal growth.58 Furthermore, the introduction of P and S atoms (larger than C and N atoms) as doping and leaving motifs could disturb the structure of g-C3N4, akin to the heteroatom-assisted “bottom-up” synthesis of g-C3N4 nanosheets.39,64 Generally, larger SBET can absorb more active species and reactants on its surface, which is favorable to the photocatalytic performance. Photogenerated Electron−Holes Separation and Transport. PL emission is helpful to investigate the separation efficiency of electron−hole charges. In this study, the PL emission spectra were excited under a wavelength of 350 nm, and the peaks were centered at around 453 nm. As depicted in Figure 7, CN shows the highest PL peak intensity, which means a high recombination efficiency between the photogenerated electrons and holes. Obviously, the PL intensities of PCN and SCN were much lower than CN, indicating that the separation rates of charge pairs in the PCN and SCN samples were faster, which was caused due to the generated electrons being trapped in the defects, impeding a direct recombination of photogenerated charges.3,18 Furthermore, PSCN-50 showed a much lower PL peak intensity, indicating that the existence of P and S elements does not act as a recombination center for

Figure 5. UV−vis spectrum of as-prepared samples. 5835


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Figure 6. N2 adsorption−desorption isotherms of as-prepared CN, PCN, SCN, and PSCN-50 samples.

Figure 8a depicts the transient photocurrent responses of pure PCN, SCN, and PSCN-50. The photocurrent for PSCN50 is much higher than that of PCN and SCN, revealing that sample PSCN-50 has higher separation efficiency of photogenerated changes. The results indicated that the defects in the structure of samples caused by the doping of P and S could serve as the trapping centers of photoinduced electrons, thus promoting the electron−hole separation, inhibiting the photogenerated charges recombination, and prolonging the lifetime of charge carriers.3,18 The same result was obtained using an electrochemical impedance spectroscopy experiment. Figure 8b depicts the EIS changes of PCN, SCN, and PSCN-50 electrodes. The data in Figure 8b displays that the relative arc sizes for the three electrodes are PSCN-50 < SCN < PCN. This confirms that the PSCN-50 sample has the highest efficiency in charge separation and electron transfer. The PL, PC, and EIS results indicated that the doping with P and S could greatly restrain the recombination of electron−hole charges and efficiently facilitate the separation of photogenerated charges as the defects trapping of electrons. Photocatalytic Performance and Mechanism. To assess the photocatalytic activity of P-, S-codoped g-C3N4 samples, the photocatalytic degradation of TC and MO were performed under visible light irradiation (Figure 9). As depicted in Figure 9a, the concentration of TC decreased gradually with illumination time increasing for all samples. Pure CN exhibited the lowest TC removal efficiency of only ca. 32.95% within 60 min. PCN and SCN showed slightly increased activity for 43.48% and 73.50%, respectively. It is worth noting that P, S codoping could significantly enhance the photocatalytic performance, and PSCN-50 displayed the highest degradation

Table 1. BET Surface Areas, Pore Size, and Pore Volumes of As-Prepared Samples samples

SBET (m2 g−1)

pore size (nm)

pore volume (cm3 g−1)

CN PCN SCN PSCN-50

6.42 10.21 20.03 25.87

28.61 12.52 34.57 34.27

0.04 0.07 0.14 0.16

Figure 7. Photoluminescence (PL) spectra of as-prepared samples.

photogenerated electrons and holes. Generally, the inhibited recombination rate is always beneficial to increasing photoactivities and quantum yield.39

Figure 8. Transient photocurrent responses (a) and electrochemical impedance spectroscopy (EIS) changes (b) of PCN, SCN, and PSCN-50 samples. 5836


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Figure 9. Photocatalytic activities and apparent rate constants of as-prepared samples on degradation of TC (a, b) and MO (c, d).

Figure 10. Species-trapping experiments for degradation of TC and MO over PSCN-50 photocatalysts: (a) TC and (b) MO.

be found that the photocatalytic activity of P-, S-codoped gC3N4 for TC and MO degradation is much higher than that of CN, PCN, and SCN. PSCN-50 exhibited the highest rate constant, which was ascribed to the larger surface area, improved light absorbance, and improved separation efficiency of photoinduced charges. The k value for TC degradation over PSCN-50 (0.03823 min−1) is about 5.9 times higher than that of CN (0.00648 min−1). Similarly, the k value for MO degradation over PSCN-50 (0.02141 min−1) is about 7.1 times higher than that of CN (0.003 min−1). During the photocatalytic degradation of organic pollutants, holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O2−) are expected to be generated in the photocatalytic process. The predominant active species generated in the reaction system was first detected by using p-benzoquinone (i.e., •O2− scavenger), triethanolamine (i.e., h+ scavenger), and isopropanol (i.e., •OH scavenger). As shown in Figure 10, it can be seen that the addition of triethanolamine restrains the photocatalytic activity of the PSCN-50 material significantly

efficiency (ca. 85.85%) under the identical condition. This is probably due to the synergistic effect of P and S codoping which improved the SBET, enhanced light absorbance, and inhibited recombination of photogenerated electron−hole pairs. Similar to TC degradation, the photocatalytic performance is significantly enhanced with 73.25% of MO photodegraded in 60 min for the PSCN-50 sample, while the photocatalytic efficiency of pure g-C3N4, PCN, and SCN for MO are 15.26%, 22.67%, and 24.86%, respectively (Figure 9c). The kinetic behaviors of the obtained samples for removal of TC and MO were studied. All of them fit well with the pseudofirst-order kinetics model:

−ln(C /C0) = kt

(2)

where C0 (mg L−1) is the initial concentration, C (mg L−1) is the remaining concentration at irradiation time of t, and k (min−1) is the apparent first-order rate constant. The k values of different samples are shown in Figure 9b and d, and it could 5837


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Figure 11. DMPO spin-trapping ESR spectra with PSCN-50 sample: (a) DMPO-•O2− and (b) DMPO-•OH.

from 85.85% to 25.55%, indicating that h+ is the predominant active species for TC degradation. Similarly, •O2− also possessed a crucial role in the photocatalytic process as the inhibited effect of p-benzoquinone. However, ignorable inhibition is shown by isopropanol, indicating that •OH does not contribute to the degradation of TC. A similar result could be found in the photodegradation of the MO solution. Therefore, it was indicated that h+ and •O2− were the dominant active species for TC and MO degradation. In order to confirm the presence of •O2− and •OH radicals in the PSCN-50 photocatalytic reaction systems, the ESR spintrap technique was implemented. DMPO (5,5-dimethyl-1pyrroline N-oxide), a nitrone spin-trapping reagent, is utilized to capture the superoxide (•O2−) and hydroxyl radicals (•OH).65 As depicted in Figure 11a, four characteristic peaks of DMPO-•O2− were observed when the light is turned on, which imply the •O2− radical species were produced in the reaction system.66 It was reasonable that the •O2− production as the photogenerated electrons of PSCN-50 has the ability to trap molecular oxygen to generate •O2−. This could be explained by the CB potentials (−1.02 eV vs NHE) of g-C3N4 being more negative than the reduction potential (−0.33 eV vs NHE) of O2/•O2−. No DMPO-•OH signals were observed, which indicated that •OH is not produced during the photocatalysis process (Figure 11b). Additionally, the potentials of VB and CB for P-, S-doped gC3N4 can be calculated according to the following equations: EVB = X − Ee + 0.5Eg, ECB = EVB − Eg. On the basis of the above formulas, the VB potential and CB potential of the as-prepared samples are summarized in Table 2. As shown in Table 2, both P doping and S doping could decrease the bandgap energy of pure g-C3N4, implying the electronic integration of P and S atoms in the lattice of g-C3N4 and thus a bandgap narrowing, which was consistent with previous studies.3,18,34,43,59 This kind of electronic band structure is meaningful because it not only endows the thermodynamical enhancement in the photo-

catalytic reaction with more efficient visible light utilization but also inhibits electron−hole recombination.59 On the basis of the above experimental results, a probable mechanism for the photocatalytic oxidation of TC and MO over PSCN-50 was proposed. As illustrated in Scheme 1 and Scheme 1. Photocatalytic Mechanism Scheme of PSCN-50 Sample under Visible Light Irradiation

eqs 5−7, PSCN-50 could generate electrons and holes under visible light irradiation. Subsequently, the photoinduced electrons in CB of PSCN-50 could be trapped by the defects in the structure of samples and then react with the dissolved oxygen to produce •O2−, which can react with the MO molecule to form its degraded products. Meanwhile, the photogenerated holes left behind in the valence band of PSCN50 could directly oxidize the TC and MO molecules. As discussed above, PSCN-50 possesses higher specific surface area and numerous active sites for the photoreaction process. The doping of P and S can broaden the visible light response region of the photocatalyst, which can promote solar energy utilization efficiency and decrease band gap energies. Moreover, P and S doping into the crystal lattice of g-C3N4 would introduce more defects into the samples, which could serve as the trapping centers of photoinduced electrons and thus enhance the electron−hole separation, inhibiting the photogenerated charges recombination and prolonging the lifetime of charge carriers. In this work, P- and S-codoped g-C3N4 simultaneously improves the surface area, light absorption

Table 2. Bandgap (Eg), Relative Conduction Band (ECB), and Valence Band (EVB) of As-Prepared Samples samples

Eg (eV)

ECB

EVB

CN PCN SCN PSCN-25 PSCN-50 PSCN-75

2.70 2.67 2.66 2.65 2.62 2.60

−1.13 −1.11 −1.11 −1.10 −1.09 −1.08

1.57 1.56 1.55 1.55 1.53 1.52 5838


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Figure 12. Recycling runs (a) of PSCN-50 for degradation of TC and MO. XRD spectra (b) before and after photocatalytic reactions.

capability, and separation efficiency of photogenerated charges, thus leading to higher activity. PSCN + hv → PSCN (e− + h+)

(3)

e− + Defects → Defects (e−)

(4)

O2 + Defects (e−) → •O−2

(5)

• − O2

(6)

+ TC/MO → degraded or mineralized products

h+ + TC/MO → degraded or mineralized products

(7)

Recyclability and Mineralization Ability. Considering the actual application of the photocatalyst, the repeatability and stability of the photocatalyst are very important. To test the stability of P-, S-codoped CN, the PSCN-50 sample was collected after the photodegradation experiment of TC for the recycle experiments. As shown in Figure 12a, the photocatalytic performance of the PSCN-50 sample has no obvious deactivation even after five cycling runs for the removal of TC (from 85.85% to 80.52%) and MO (from 73.25% to 68.78%), which indicates its high stability. To further demonstrate the stability of PSCN-50, XRD spectra of the fresh and used photocatalysts have been provided for comparison. It could be found that the phase and structure of the recycled PSCN-50 sample had almost no obvious discrepancy after the photocatalytic reaction process. The results revealed the sample presents excellent recycling in the photocatalytic degradation processes. The mineralization ability is also an important parameter to evaluate the photocatalytic properties of the as-prapared samples. Figure 13 depicted the TC and MO degradation in terms of TOC removal by pure CN and PSCN-50. Within the irradiation time of 60 min, the TC decomposition efficiencies reached about 11.53% and 42.57% for pure CN and PSCN-50, respectively, while they were about 5.63% and 33.58% for MO degradation. When the illumination time was lengthened to 120 min, the TOC removal over the PSCN-50 sample increased to 70.33% and 55.37% for TC and MO, respectively. Only 18.24% and 7.22% removal efficiencies were obtained for pure CN, suggesting that much higher mineralization ability was achieved by P-, S-codoped CN. The results indicated that P-, S-codoped g-C3N4 could effectively mineralize TC and MO into small intermediates or directly CO2 and H2O, validating a promising application potential for wastewater treatment.

Figure 13. TOC removal in the presence of pure CN and PSCN-50 under visible light irradiation.

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CONCLUSIONS P-, S-codoped g-C3N4 was fabricated by a facile copyrolysis of HCCP and thiourea mixtures in the muffle furnace. The obtained P-, S-codoped g-C3N4 showed much enhanced visible light photocatalytic activities toward degradation of TC and MO solution than those of bare g-C3N4 and single-doped gC3N4. Active species trapping experiments and ESR analysis indicated that h+ and •O2− were the dominant active species. Photoluminescence, photocurrent response, and EIS results demonstrated that doping with P and S could greatly inhibit the recombination of electron−hole pairs and efficiently facilitate the separation of photogenerated charges. Moreover, the P atoms and S atoms being doped into the g-C3N4 lattices could inhibite crystal growth to enhance the surface area of PSCN-50. Lastly, the doping of P and S can broaden the visible light response region of the photocatalyst, which can promote the solar energy utilization efficiency and decrease the bandgap energies.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 731 88821413. Fax: +86 731 88821413. E-mail: [email protected] (X.Z. Yuan). *Tel.: +86 731 88821413. Fax: +86 731 88821413. E-mail: [email protected] (H. Wang). ORCID

Xingzhong Yuan: 0000-0001-6226-4085 Jie Liang: 0000-0003-4559-4378 5839


Research Article

ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51521006), Key Project of National Nature Science Foundation of China (No. 71431006), National Natural Science Foundation of China (No. 51479072, No. 51679082), and Key Research and Development Project of Hunan Province, China (No. 2016SK2015, No. 2016SK2045).

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