Direct Z-scheme P–TiO₂/g-C₃N₄ heterojunction for the photocatalytic degradation of sulfa antibiotics

Abstract

The construction of direct Z-scheme heterojunctions with high photocatalytic degradation ability is important for wastewater treatment, but there are still many unsolved challenges. In this article, we report the fabrication of a Z-scheme P–TiO₂/g-C₃N₄ (CNPT-X) heterostructure by the calcination method. Under simulated sunlight, CNPT-X composites are found to show excellent degradation performance against sulfonamide antibiotics sulfadiazine (SD), sulfamethazine (SM2), sulfamonomethoxine (SMM), and sulfamethoxazole (SMZ). CNPT-3 (400 mg L⁻¹) can be used to degrade four sulfa antibiotics within 90 min, with a degradation rate as high as 99%, which is higher than that for P–TiO₂ and g-C₃N₄ alone. The internal electron transfer paths and mechanisms for the composites are revealed by ESR radical detection experiments, XPS energy spectrum shifts, valence band positions and active material quenching experiments. Furthermore, the degradation products are analyzed by GC-MS, and four possible degradation pathways for sulfonamide pollutants are proposed. This photocatalyst provides new insights into the fundamental aspects of the photocatalytic degradation mechanism for composite pollutants, as well as new ideas for practical environmental applications.

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1 Introduction

Antibiotics are widely present in the environment due to their widespread use in modern medicine,^1,2 therefore, the antimicrobial and antibiotic resistance caused by the misuse or overuse of antibiotics represents a growing and significant threat to global public health.³ Since the discovery of penicillin in 1929, antibiotics have been widely used to treat bacterial infections in humans and animals for decades,⁴ and global antibiotic consumption has increased by 65% from 2000 to 2015.⁵ Based on their pharmacological properties, the main categories of antibiotics include aminoglycosides, β-lactams, glycopeptides, macrolides, quinolones, sulfonamides (SAs), and tetracyclines.⁶ SAs are broad-spectrum bacteriostatic antibiotics that have been shown to stimulate microbial resistance to sulfonamides even at ppt to ppb levels.⁷ The concentration of SAs ranks No. 2 only after tetracyclines in pharmaceutical wastewater, influents, and effluents of wastewater treatment plants (WWTPs), and natural water, with concentrations ranging from μg L⁻¹ to mg L⁻¹,^8,9 and more than 20 000 tons of SAs are introduced into the biosphere every year.¹⁰ In the Yangtze River Delta Reservoir in East China, SD and SMZ were detected at levels of 92.31% and 100%, with concentrations of 20.82 and 1.26–51.86 ng L⁻¹, respectively,¹¹ and the maximum concentration of SMM in the Beihe River was found to exceed 300 ng L⁻¹.¹² Therefore, there is an urgent need to develop a cost-effective method to degrade SAs without causing pollution to the environment.

Photocatalysis, as an environmentally friendly technology, not only can be used to degrade a variety of organic pollutants but also has the advantages of safety, nontoxicity, high stability, no secondary pollution, and recyclability of catalytic materials.¹³ In 1972, Honda and Fujishima proposed the electrochemical photolysis of water on a TiO₂ semiconductor electrode, paving the way for the utilization of artificial solar energy.¹⁴ Thus, photocatalysis has attracted widespread attention and shows great potential for various solar-driven reactions, such as CO₂ reduction, pollutant degradation, and water splitting.^15–18 However, the main shortcoming of TiO₂ is manifested by its large band gap, which can only utilize ultraviolet radiation,¹⁹ and the application of such photocatalytic materials in practical engineering remains insufficient, especially from the aspects of stability under practical conditions, utilization of solar energy, and the recycling and reuse of catalysts.^20–22 To improve the photoreduction performance, researchers have used various methods to modify TiO₂, such as heterojunctions.²³

g-C₃N₄ is a polymer inorganic nonmetallic photocatalyst that shows visible light responsiveness, with C and N atoms that are sp² hybridized to form a structure with a highly delocalized π-conjugated system.^24,25 g-C₃N₄ has been widely used for hydrogen conversion,²⁶ carbon dioxide reduction,²⁷ and removal of organic pollutants,²⁸ because it has high thermal and chemical stability. However, the photocatalytic performance of g-C₃N₄ is affected by its small specific surface area, fast electron–hole recombination rate and low visible light utilization rate.²⁹ In recent years, the photocatalytic performance of g-C₃N₄ has been improved by using various approaches, such as element doping,³⁰ morphology control and surface modification.²⁹ The construction of heterojunctions can enable one to enhance the photocatalytic degradation activity of g-C₃N₄,³¹ and the oxidation and reduction abilities can be enhanced by using composite materials with a more negative CB and more positive VB.³² Therefore, it is important to design and prepare photocatalysts with heterojunctions with synergistic effects for adsorption and degradation performance.³³

In this work, we designed and synthesized P–TiO₂/g-C₃N₄ composites based on thermal condensation reactions for the photocatalytic degradation of mixed sulfonamide antibiotics under simulated sunlight. Through characterization, the energy band structure of the composites is found to conform to the Z-scheme mechanism, and a photocatalytic degradation mechanism is proposed.

2 Materials and methods

2.1 Materials

SD (98%), SM2 (98%), SMM (98%), SMZ (98%), urea, n-butyl titanate, n-butanol, and ammonium dihydrogen phosphate were all purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Potassium dichromate (K₂Cr₂O₇, AR, 99.7%), sodium oxalate (Na₂C₂O₄, AR, 99.7%), ethanol (AR, 99.7%) and isopropyl alcohol (IPA, AR, 99.7%) were obtained from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). All reagents were not purified further before use, and deionized water was used throughout the experiments.

2.2 Preparation of P–TiO₂

P–TiO₂ was prepared according to a previously described method.³⁴ n-Butyl titanate was used as the precursor of TiO₂, and hydrated titanium hydroxide was prepared by a neutral amine sol–gel method. n-Butyl titanate was added drop by drop to a solution containing 300 mL of n-butanol (1–2 drops per min) until a milky white sol was formed. The pH of the solution was adjusted to 7, and then the solution was magnetically stirred for 12 h. Afterward, the solution was placed in a rotary evaporator at 80 °C to remove excess solvent, following which titanium hydroxide hydrate was obtained. Titanium dioxide phosphide was prepared by impregnating titanium hydroxide with an aqueous solution of ammonium dihydrogen phosphate. First, 0.085 g of ammonium dihydrogen phosphate was dissolved in water, and then 2 g of dried titanium hydroxide hydrate was added, followed by stirring for 4 h. Afterward, the solution was dried at 80 °C for 12 h and finally calcined at 600 °C for 4 h to obtain P–TiO₂. On the basis of no added dihydroammonium phosphate, the unphosphated TiO₂/g-C₃N₄ was obtained.

2.3 Preparation of g-C₃N₄

The g-C₃N₄ was prepared using a nitrogen-protected high-temperature calcination method.³⁵ First, 10 g of urea was placed into a crucible, and then the sample was placed into a programmed temperature-controlled atmosphere furnace and nitrogen was poured and heated at 550 °C for 2 h at a heating rate of 10 °C min⁻¹ to obtain g-C₃N₄.

2.4 Preparation of g-C₃N₄/P–TiO₂

The P–TiO₂/g-C₃N₄ composite photocatalytic material was prepared via a thermal condensation reaction. First, 50 mL of P–TiO₂ precursor solution with concentrations of 0.2, 0.6, 1.2, and 2.4 mg L⁻¹ was ultrasonically dispersed for 10 min, and then 10 g of urea was added. The solution pH was adjusted to 4–5 with 0.1 mol L⁻¹ HCI or NaOH solution, and then the solution was magnetically stirred at 40 °C for 4 h. Afterward, the solution was dried at 80 °C for 12 h. Finally, the obtained white solid was placed in a crucible and heated to 550 °C at a programmed heating rate of 10 °C min⁻¹ in a nitrogen atmosphere for 2 h. After cooling to room temperature, the obtained yellow solid was ground to a powder, sealed and placed into dry storage. The P–TiO₂/g-C₃N₄ heterostructures prepared with a varying weight ratio of P–TiO₂ to g-C₃N₄ of 1 : 1, 3 : 1, 6 : 1, and 12 : 1 were marked CNPT-1, CNPT-3, CNPT-6 and CNPT-12, respectively.

3 Results and discussion

3.1 Characterization

X-ray diffraction (XRD) was used to characterize the composition and crystal shape of P–TiO₂, g-C₃N₄ and P–TiO₂/g-C₃N₄ photocatalytic materials with different composite ratios roasted at 550 °C, and the results are shown in Fig. 1a. TiO₂ shows diffraction peaks at 25.12°, 36.94°, 48.04°, 54.28°, 62.80°, 68.98°, 70.36° and 75.16°, corresponding to the 101, 103, 200, 105, 204, 116, 220 and 215 crystal planes, respectively, of mineral phase TiO₂ (JCPDS No. 21-1272) with the square structure of anatase.³⁶ These results are also in good agreement with previous reports.^36,37 When forming the heterostructures, the characteristic diffraction peak pattern for the anatase phase of TiO₂ becomes more obvious with increasing P–TiO₂ content, especially the 101 crystal plane, indicating that the crystallinity of TiO₂ is improved. In previous studies, the loading of phosphorus can lead to the formation of a more stable anatase phase for TiO₂.^38,39 For the g-C₃N₄ sample, the characteristic peak is located on the 002 crystal plane (JCPDS No. 87-1526) at 2θ = 26.05°. The other characteristic peaks are not obvious in the spectrum due to the interlayer superposition of g-C₃N₄,⁴⁰ but the characteristic peaks due to P–TiO₂ and g-C₃N₄ at 25.12° and 26.05° can be found in CNPT-6 and CNPT-12. Thus, the P–TiO₂/g-C₃N₄ composite photocatalytic material was successfully synthesized. In addition, there is no obvious impurity peak in the P–TiO₂/g-C₃N₄ composite material, indicating that other impurities are not generated.

Fig. 1 (a) XRD patterns for g-C₃N₄, P–TiO₂ and P–TiO₂/g-C₃N₄ composite photocatalytic materials; (b) UV-Vis diffuse reflectance spectra for different materials.

UV-Vis DRS was used to analyze the light absorption characteristics of the P–TiO₂/g-C₃N₄ composite photocatalytic material. As shown in Fig. 1b, P–TiO₂ shows strong absorption in the ultraviolet region below 380 nm but weak absorption in the visible region above 400 nm. However, g-C₃N₄ shows obvious absorption above 400 nm, while P–TiO₂ combined with g-C₃N₄ still maintains visible light absorption performance. Compared with P–TiO₂, the light absorption edge of the P–TiO₂/g-C₃N₄ composite photocatalytic material extends to the visible light region.⁴⁵ Based on the above analysis, the g-C₃N₄ and TiO₂ composite has a high utilization rate for visible light (>400 nm), which endows the composite photocatalytic material with visible light catalytic activity. At the same time, due to the visible light response capability of g-C₃N₄ and the excellent light scattering performance of the formed heterostructure, the P–TiO₂/g-C₃N₄ composite photocatalytic material exhibits enhanced light absorption performance in both the ultraviolet and visible light regions.

The morphologies of the P–TiO₂, g-C₃N₄ and CNPT-3 catalysts were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Fig. 2. As shown in Fig. 2a, CNPT-3 is composed of clearly wrinkled nanosheets, which have a porous layered morphology and layered structure.⁴⁸ In Fig. 2b, the particle size of P–TiO₂ measured by nano measurement software (Nano Measure 1.2) is mainly 7.87–13.74 nm, and the TEM for g-C₃N₄ (Fig. 2c) shows that g-C₃N₄ contains obvious layered pores, and CNPT-3 also shows an obvious pore structure. Therefore, P–TiO₂/g-C₃N₄ composites retain the basic structure of g-C₃N₄.⁴¹ These pore structures can not only provide more photocatalytic active sites but can also show an increased contact area between the antibiotic and the P–TiO₂/g-C₃N₄ composite photocatalyst material, thereby improving the photocatalytic performance of the composite catalyst.⁴² Fig. 2d and e display TEM images of the TiO₂/g-C₃N₄ composites that were not phosphated. The picture clearly shows that there are more TiO₂ particle agglomerates on the g-C₃N₄ film. As marked in the picture, the lattice fringes of TiO₂(101) can be clearly observed when the picture is enlarged to 2 nm. Compared with Fig. 2d and f, the particle size of P–TiO₂ loaded onto g-C₃N₄ is smaller than that for TiO₂. As shown in Fig. 2g, it can be observed that the P–TiO₂ and g-C₃N₄ lattice size is 0.302 nm (101) and 0.25 nm (002), respectively, and that the edges are closely connected together, further indicating the formation of a heterojunction between P–TiO₂ and g-C₃N₄. Based on the above analysis, the P–TiO₂/g-C₃N₄ composite material was successfully prepared, and the P–TiO₂ nanoparticles are found to be uniformly deposited onto the g-C₃N₄ nanosheets and show a layered void structure. The elemental mapping images shown in Fig. 2h–m indicate that C, O, Ti, N and P elements are homogeneously distributed within P–TiO₂/g-C₃N₄.³⁷ These results indirectly demonstrate that the preparation of the P–TiO₂/g-C₃N₄ photocatalytic composite material was successful, which is consistent with the XRD scanning analysis results.

Fig. 2 (a) CNPT-3 composite photocatalytic material SEM; (b) P–TiO₂ and (c) g-C₃N₄ photocatalytic material TEM; (d) and (e) g-C₃N₄/TiO₂ (CNT-3) photocatalytic material TEM; (f) and (g) CNPT-3 photocatalytic material TEM; (h)–(m) CNPT-3 photocatalytic material element mapping.

X-ray photoelectron spectroscopy (XPS) was used to research the surface chemical composition and element valence of the P–TiO₂/g-C₃N₄ composite photocatalytic material. As shown in Fig. 3a that P–TiO₂/g-C₃N₄ mainly contains C, O, N, P and Ti, indicating the coexistence of P–TiO₂ and g-C₃N₄ in the nanocomposite. In addition, no characteristic peaks for other elements are observed in the XPS spectrum. As shown in Fig. 3c, the high-resolution P 2p spectrum shows a single peak at 137.26 eV, indicating that phosphorus exists in the P–TiO₂/g-C₃N₄ composite photocatalytic material in the pentavalent oxidation state (P⁵⁺).⁴³ According to previous studies,³⁸ the electron binding energy of P in P–TiO₂ is increased by 2.66 eV. Fig. 3d shows that the high-resolution O 1s spectrum of P–TiO₂/g-C₃N₄ can be divided into two peaks at 532.5 eV and 533.1 eV, which are attributed to O–Ti bonds and surface –OH groups in the TiO₂ lattice.⁴⁴ Phosphorus was found in the P–TiO₂/g-C₃N₄ composite photocatalytic material, indicating that the phosphorus element does not disappear during the preparation of the composite material. The C 1s spectral curve fitting result is shown in Fig. 3e. The characteristic C peak for g-C₃N₄ is located at 288.29 eV, corresponding to the carbon atom of the sp² bond of g-C₃N₄. At the same time, the deconvolution of the main peak reveals a subpeak at 287.62 eV. Fig. 3f shows that the high-resolution N 1s spectrum for P–TiO₂/g-C₃N₄ can be divided into three peaks at 401.31, 400.51 and 399.02 eV, which are attributed to amino C–NH_x, tertiary nitrogen N–(C)₃ and C=C–N, respectively.^45–47 The XPS results further confirm the successful preparation of the P–TiO₂/g-C₃N₄ composite photocatalytic material.

Fig. 3 XPS spectra for P–TiO₂/g-C₃N₄ composite photocatalytic materials: (a) total survey spectrum, (b) Ti 2p, (c) P 2p, (d) O 1s, (e) C 1s, (f) N 1s.

3.2 Photocatalytic performance for the degradation of antibiotics

Fig. 4a–d show the photocatalytic degradation for different composite ratios of P–TiO₂/g-C₃N₄ on four mixed antibiotics. The data show that P–TiO₂/g-C₃N₄ can achieve a high degradation effect on the four sulfa antibiotics after 120 min of light irradiation. Compared to other photocatalytic materials with different addition amounts, CNPT-3 can achieve high-efficiency degradation for four antibiotics in 30 min: SD: 99.3%, SM2: 99.6%, SMM: 99.6% and SMZ: 99.0%. Compared with g-C₃N₄ alone, the effect of CNPT-3 on the photocatalytic degradation of sulfonamide antibiotics within 30 min is increased by 1.96 times, 1.94 times, 2.32 times, and 1.02 times for SD, SM2, SMM, and SMZ, respectively, and compared with P–TiO₂ alone, it is increased by 3.06 times, 2.94 times, 2.32 times, and 0.88 times, respectively. However, as the proportion of P–TiO₂ increases, the photocatalytic degradation rate for sulfa antibiotics is decreased. This may be because excessive addition of P–TiO₂ can affect the direct composite effect of g-C₃N₄ and P–TiO₂, and a large amount of P–TiO₂ adheres to the surface of g-C₃N₄ to hinder the contact of active sites with antibiotics. In summary, the P–TiO₂/g-C₃N₄ composite photocatalytic material can be used to achieve a degradation efficiency of more than 97% for mixed sulfonamide antibiotics within 120 min, and CNPT-3 shows a better degradation effect under the condition of 30 min of light irradiation. As shown in the Fig. S1,† the pseudo first-order rate constants (k) of four sulfonamide pollutants are calculated. It is worth noting that CNPT-3 shows the fastest photocatalytic reaction rate, in which SM2, SMM, SD and SMZ are 0.093, 0.112, 0.104 and 0.031 min⁻¹ respectively, 2.88, 3.39, 3.05 and 2.82 times of pure g-C₃N₄, and 42.27, 45.90, 52.59 and 6.97 times of pure P–TiO₂. The photocatalytic degradation efficiency of four sulfonamides is SMM > SD > SM2 > SMZ.

Fig. 4 Degradation effect for four kinds of sulfonamide antibiotics in P–TiO₂/g-C₃N₄ composite photocatalytic materials: (a) SM2, (b) SMM, (c) SD, and (d) SMZ.

To evaluate the influence of different factors on the photodegradation efficiency of sulfonamides, the influence of the initial concentration of sulfonamides is shown in Fig. S2.† When the antibiotic concentration is 3 mg L⁻¹, the photocatalytic degradation effects are SD = 10.2%; SM2 = 10.22%; SMM = 19.95%; SMZ = 17.76%, indicating that the reactive group of the surface of the P–TiO₂/g-C₃N₄ composite photocatalytic material has a low probability of reacting with antibiotics. As the antibiotic concentration is increased from 5 to 10 mg L⁻¹, SD, SM2 and SMM reach a stable degradation effect of more than 92% after 90 min of light irradiation. However, for an antibiotic concentration of 15 mg L⁻¹, the four sulfonamide antibiotics all show poor degradation effects (SD = 90.2%; SM2 = 88.22%; SMM = 80.15%; SMZ = 40.46%). This phenomenon is attributed to the concentration of antibiotics reaching the saturation value of the photocatalytic reaction system, following which the degradation rate decreases with increasing concentration.

The influence of the amount of CNPT-3 catalyst on the degradation efficiency of antibiotics is shown in Fig. S3.† When the P–TiO₂/g-C₃N₄ composite photocatalytic material concentration is 100 mg L⁻¹, the number of active sites is relatively small, and the degradation rate for the four sulfonamide antibiotics after 120 min is small (SD = 13.43%; SM2 = 12.22%; SMM = 10.95%; SMZ = 13.76%). As the concentration of catalytic material is increased from 100 to 700 mg L⁻¹, the degradation rate for the four sulfonamide antibiotics reaches a value of more than 99% because of an increase in the number of catalytic active sites, which can lead to the absorption of more photons to produce active substances. However, for a higher catalyst concentration (1000 mg L⁻¹ and 2000 mg L⁻¹), the degradation rate for sulfonamides is reduced. This is due to the agglomeration phenomenon caused by the high catalyst concentration in the stirring process or the high concentration of the catalytic material affecting the permeability of the light beam in the solution preventing the active sites on the surface of the catalyst from absorbing the light intensity.

In addition, the effect of pH on the degradation of sulfonamides by P–TiO₂/g-C₃N₄ is shown in Fig. S4.† The degradation efficiency for sulfonamide antibiotics at pH = 3 and pH = 13 is better than that at pH = 7, indicating that acidic or alkaline conditions are conducive to the degradation of the four mixed sulfonamide antibiotics. Under the condition of pH = 7, the four sulfonamide antibiotics achieve higher degradation effects (SD = 97.61%; SM2 = 98.31%; SMM = 100%; SMZ = 100%). Although the degradation rate of sulfonamide antibiotics can reach a high degradation rate in a short time under the conditions of pH = 3 and pH = 13, considering the actual situation, it is necessary to consider the treatment cost, and secondary treatment under alkaline or acidic conditions under neutral conditions is efficient and environmentally friendly.

3.3 Reusability and stability of the P–TiO₂/g-C₃N₄ composites

The stability of the photocatalyst is one of the most important parameters in the water treatment process. In this study, the stability of P–TiO₂/g-C₃N₄ was determined by recyclability experiments for the photodegradation of four mixed sulfonamide antibiotics. After the end of each cycle, the photocatalyst was recovered through centrifugation, elution and drying before entering the next photocatalytic cycle. As shown in Fig. 5, after three cycles of experiments, the degradation rates for P–TiO₂/g-C₃N₄ for the four sulfonamide antibiotics are SD: 41.35%, SM2: 34.83%, SMM: 52.35% and SMZ: 5.60%.

Fig. 5 Stability analysis for the cyclic test for the photocatalytic degradation of antibiotics: (a) SD, (b) SM2, (c) SMM, and (d) SMZ.

3.4 Possible photocatalytic mechanism

To analyze the main active substances in the P–TiO₂/g-C₃N₄ composite material in the process of photocatalytic degradation of mixed sulfonamide antibiotics, isopropanol (IPA), benzoquinone (Ben) and sodium oxalate (Na₂C₂O₄) were used for quenching reactions to explore the active substances that are beneficial to photocatalytic degradation, and the results are displayed in Fig. S5.^49,50,† Compared with the experimental group without a quencher, the degradation rate for antibiotics after adding benzoquinone, sodium oxalate and isopropanol is decreased to 27.75%, 25.36% and 18.52%, respectively, indicating that ˙O₂⁻, h⁺ and ˙OH play a major role in the photocatalytic degradation of mixed sulfonamide antibiotics.

To further confirm the above conclusion, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as a spin trap for ESR analysis. As shown in Fig. S6a,† the ˙O₂⁻ signal was not detected in the absence of light irradiation. After the irradiation of the prepared sample with a Xe lamp, a 1 : 1 : 1 : 1 signal peak is observed. As the illumination time is increased, the signal intensity gradually stabilizes. As shown in Fig. S6b,† ˙OH is also observed in a similar phenomenon, and a 1 : 2 : 2 : 1 signal peak can be observed. Free radical capture experiments and ESR test results show that ˙O₂⁻ and ˙OH radicals are the main active components involved in the photocatalytic degradation process.^41,48,51 It is worth noting that if the charge transfer of the complex follows the traditional type II heterojunction mechanism, ˙O₂⁻ radicals cannot be generated. Obviously, the Z-scheme heterojunction mechanism is a better candidate for explaining the photocatalyst enhancement mechanism.

The VBXPS energy spectrum was used to measure the valence band positions of g-C₃N₄, P–TiO₂ and P–TiO₂/g-C₃N₄ composite materials, and the band gap energies (E_g) were evaluated using Kubelka–Munk plots:⁵²

αhν = A(hν − E_g)n^1/2

where α, h, ν, and A are the absorption coefficient, Planck constant, light frequency and proportionality constant, respectively.⁵³ The value for n was selected to be 1/2 for both g-C₃N₄ and P–TiO₂ with a direct bandgap, and E_g was determined from a plot of (αhν)² versus hν.⁵⁴ As shown in Fig. 6a, the band gaps (E_g) for g-C₃N₄, P–TiO₂, CNPT-3, CNPT-6 and CNPT-12 were calculated according to the Kubelka–Munk equation to be 2.72 eV, 3.10 eV, 2.79 eV, 2.88 eV and 3.05 eV, respectively. The results show that g-C₃N₄ reduces the band gap of P–TiO₂, but the band gap widths increase with increasing TiO₂ content and further enable the effective formation of heterojunctions between P–TiO₂/g-C₃N₄ composite photocatalytic materials. As shown in Fig. 6b, the valence band edges (VB) of g-C₃N₄, P–TiO₂ and P–TiO₂/g-C₃N₄ are estimated to be 1.60 eV, 2.81 eV, and 2.47 eV according to the XPS valence band spectrum, respectively, and the conduction band edges (CB) for g-C₃N₄ and P–TiO₂ are −0.29 eV and −1.21 eV, respectively.⁴⁷ The valence band potential of the P–TiO₂/g-C₃N₄ composite ranges between that of g-C₃N₄ and P–TiO₂ and is close to that of P–TiO₂. This valence band structure conforms to the Z-scheme system configuration, so it can be proven that the band structure of P–TiO₂/g-C₃N₄ has a Z-scheme architecture.⁵⁵ In summary, the P–TiO₂/g-C₃N₄ composite photocatalytic material has a Z-scheme energy band structure, and the heterojunction formed between g-C₃N₄ and P–TiO₂ can provide a directional transfer channel for charges, enhance visible light absorption, and reduce the recombination rate for current particles, which further promotes the photocatalytic degradation of antibiotics.⁵⁶

Fig. 6 (a) g-C₃N₄, P–TiO₂ and different ratios of composite band gap width; (b) XPS valence band spectra for the g-C₃N₄, P–TiO₂ and P–TiO₂/g-C₃N₄ composite photocatalytic material (CNPT-3); XPS spectrum migration for the CNPT-3 composites: (c) O 1s, (d) Ti 2p; (e) transient photocurrent response; (f) electrochemical impedance spectroscopy (EIS).

To directly observe the electron transfer direction inside the P–TiO₂/g-C₃N₄ composite material, in situ illumination XPS spectroscopy was used for characterization, and the experimental results are shown in Fig. 6c and d. Comparing the Ti 2p energy spectrum and O 1s energy spectrum for TiO₂, P–TiO₂ and P–TiO₂/g-C₃N₄, the P–TiO₂/g-C₃N₄ peak shifts to a high binding energy, and the two elements are mainly derived from P–TiO₂. The peak in the XPS spectrum due to the accumulated electron material shifts to a low binding energy. The peaks in the XPS spectrum shift to a high binding energy for materials that have lost electrons. According to the aforementioned results, the electrons and holes are concentrated in P–TiO₂ and g-C₃N₄ inside the P–TiO₂/g-C₃N₄ composite material, respectively. Such an electron transfer path conforms to the electron transport mechanism of the Z-scheme system, and the XPS spectrum proves that the P–TiO₂/g-C₃N₄ composite material conforms to the Z-scheme system configuration.⁵⁷

The transfer and separation properties for photogenerated carriers of the composite and individual components were investigated by transient photocurrent measurements and electrochemical impedance spectroscopy (EIS).⁵⁸ Fig. 6e shows the photocurrent response obtained for P–TiO₂, g-C₃N₄, and P–TiO₂/g-C₃N₄ (CNPT-3) under multiple switching cycles. The results show that CNPT-3 shows the highest photocurrent response intensity, which indicates that CNPT-3 has a better current separation efficiency and longer electron lifetime, much higher than that for a single component.⁵⁹ Moreover, this conclusion is also confirmed by the EIS results, as shown in Fig. 6f. The CNPT-3 shows the smallest arc radius, which indicates the lowest resistance performance, which further indicates that the combination of P–TiO₂ with g-C₃N₄ can significantly reduce the photoresistance and facilitate e⁻/h⁺ separation.⁶⁰ According to the above analysis, the P–TiO₂/g-C₃N₄ heterojunction composite catalyst can lead to a significantly improved visible light response, separation and transfer efficiency for photogenerated carriers, and photodegradation activity for sulfonamide antibiotics.⁶¹ Commonly, the separation rate of photogenerated electrons and holes decreases with an increase in PL emission intensity. Thus, PL spectra were investigated to determine the PL properties and separation ability. Fig. 6a compares the PL intensities of g-C₃N₄, P–TiO₂, and the CNPT-3 composite. CNPT-3 exhibited the lowest PL emission intensity (about 490 nm), which indicated that the introduction of P–TiO₂ played a key role in suppressing the recombination of the photogenerated carriers.

3.5 Analysis of the SA degradation pathway

GC-MS was applied to detect the photocatalytic degradation products of SAs, and the results are displayed in the ESI.†

3.5.1 Degradation pathway for sulfamethazine (SM2). Fig. 7a shows a possible pathway for the photocatalytic degradation of SM2. In path I, P2 and P3 are obtained due to the cleavage of the C–N bond and S–N bonds and the rearrangement of the molecular mechanism. P3 undergoes a ring-opening reaction, the C–N bond is broken, and C≡N is formed. In path II, the aniline is attacked by ˙OH, which finally leads to the para-position substitution of the hydroxyl group and the formation of C=N (P5). However, the C–N bond was broken to form P6 in P5. In path III, the molecular structure of pyrimidine is rearranged to form P7, and finally, the C–N bond is broken to form P8.^64–66

Fig. 7 Possible degradation pathway for (a) SM2; (b) SD; (c) SMM; (d) SMZ.

3.5.2 Degradation pathway for sulfadiazine (SD). Fig. 7b shows a possible pathway for the photocatalytic degradation of SD. Path I can involve attack of ˙OH on the aniline in the SD, leading to the para-substitution of the hydroxyl group to form P2, and finally, breakage of the C–S bond to form P5. Path II can involve the desulfurization reaction caused by the cleavage of N–S and S–C bonds, with the molecular structure rearranged to form a seven-membered heterocyclic compound (P6), followed by release of the sulfur atom on the sulfonamide group in the form of sulfate. Pathway III can involve the cleavage of the N–S bond and the rearrangement of the molecular structure to form two pairs of π–π conjugated structures (P4), and finally, cleavage of the S–C bond, which leads to the desulfurization reaction (P7).^62,63

3.5.3 Degradation pathway for sulfamonomethoxine (SMM). Fig. 7c shows a possible pathway for the photocatalytic degradation of SMM. In path I, due to the breaking of the C–N bond to form P2, the C–N bond in P2 breaks and forms C≡N(P3), and the C–N bond in P3 continues to break to form P4. In path II, the C–S bond in SMM is broken to form P5, and the C–O bond in P5 is broken to form P6. In path III, the C–C bond of SMM is broken to form P7, and in P7, the C–N bond is broken to form the smaller molecule P8.⁶⁷

3.5.4 Degradation pathway for sulfamethoxazole (SMZ). Fig. 7d shows a possible pathway for the photocatalytic degradation of SMZ. In path I, the C=C bond in SMZ is broken into C–C bonds (P2), the C–C bond in P2 is broken, a ring-opening reaction (P3) occurs, the C–O bond in P3 is broken, and a hydroxylation reaction occurs (P4). In path II, the S–N bond in SMZ is broken to form a smaller molecule P5, and the S=O bond in P5 is broken to form P6. In path III, P7 and P8 undergo the same reaction as in path I, and the S–N bond in P8 is broken to form P9.^68–70

3.6 Z-Scheme photocatalytic degradation mechanism analysis

Based on the above results and discussion, a photocatalytic mechanism for P–TiO₂/g-C₃N₄ can be proposed. For P–TiO₂, the photogenerated electron–hole pairs easily recombine, and only a part of the photocatalyst participates in the photocatalytic reaction, and the catalytic activity is relatively low. However, for the P–TiO₂/g-C₃N₄ composite material, the surface is partially covered by P–TiO₂ to form a heterojunction, which greatly reduces the recombination efficiency for electron–hole pairs.⁵¹ Fig. 8 presents the Z-scheme photocatalytic degradation mechanism for P–TiO₂/g-C₃N₄. The E_CB for P–TiO₂ is −0.29 eV. Because this level is lower than O₂/˙O₂⁻ (−0.33 eV), it cannot participate in the reaction with the dissolved oxygen in the water to form ˙O₂⁻. However, O₂/˙O₂⁻ (−0.33 eV) is higher than the E_CB (−1.12 eV) of g-C₃N₄, so the photogenerated electrons in the conduction band of g-C₃N₄ can reduce O to ˙O₂⁻.⁷¹ The E_VB for P–TiO₂ is 2.81 eV, which is higher than that for –OH/˙OH (2.69 eV), which enables reaction with adsorbed water molecules or hydroxides (–OH) to form hydroxyl groups (˙OH). Through the quenching experiment for active substances, ˙O₂⁻ and ˙OH are known to be the active substances in the system, which play a role in the catalytic process and undergo redox reactions with pollutants to degrade pollutants. The Z-scheme P–TiO₂/g-C₃N₄ composite photocatalytic material can be used to effectively improve the separation of photogenerated electron–hole pairs and greatly reduce the recombination efficiency for electron–hole pairs, thereby improving the photocatalytic activity of the photocatalyst material.⁷²

Fig. 8 Schematic diagram of the photocatalytic degradation mechanism.

Under light irradiation, the photogenerated electrons generated by the CB of P–TiO₂ can easily migrate to the VB of g-C₃N₄ for the recombination of photogenerated electrons and holes, which then leads to the accumulation of electrons in the CB of g-C₃N₄ and holes in the VB of P–TiO₂. Compared with the direct recombination of electrons and holes in a single material, the effective separation of electrons and holes is achieved after migration through different paths. The photogenerated holes of P–TiO₂ tend to remain in the VB, while electrons are transferred from the CB of P–TiO₂ to the VB of g-C₃N₄. The electron in the VB of g-C₃N₄ is further excited into the CB, which results in the effective separation of photogenerated carriers. Then, the electrons stored on the surface of g-C₃N₄ and the dissolved oxygen present in the water form superoxide radical ions (˙O₂⁻), and the holes generated by P–TiO₂ react with adsorbed water molecules or hydroxide radicals to form hydroxyl groups (˙ OH). Finally, organic pollutants are oxidized to carbon dioxide and water by these highly reactive free radicals. The free radical detection test, XPS energy spectrum shift, valence band position and active substance quenching experiments further confirm the Z-scheme electron transport mechanism. When P–TiO₂ and g-C₃N₄ are combined to form a Z-scheme electron transport, the effective separation for photogenerated carriers can be increased, thereby improving the photocatalytic degradation performance.⁷³

4 Conclusions

In this study, Z-scheme heterojunction P–TiO₂/g-C₃N₄ composite photocatalytic materials were prepared to degrade four sulfonamide antibiotics, showing a degradation efficiency of more than 99% after 120 min of illumination, indicating that the P–TiO₂/g-C₃N₄ composite photocatalytic material has an excellent degradation effect on sulfonamide antibiotics. To further understand the possible mechanism for the degradation of sulfonamides, the optical properties and valence band potential of P–TiO₂ were studied by UV-visible diffuse reflectance and valence XPS spectroscopy. The results show that the g-C₃N₄ and TiO₂ composite has a high utilization rate for visible light (>400 nm), which endows the composite photocatalytic material with visible light catalytic activity. The P–TiO₂/g-C₃N₄ composite photocatalytic material has a good energy band structure, and the heterojunction formed between g-C₃N₄ and P–TiO₂ can provide a directional transfer channel for charges, enhance visible light absorption, and reduce the carrier recombination rate, which further promotes the photocatalytic degradation of antibiotics. The quenching experiment proves that ˙O₂⁻, ˙OH and h⁺ play a major role in the photocatalytic degradation of mixed sulfonamide antibiotics by the P–TiO₂/g-C₃N₄ composite photocatalytic material. Free radical capture experiments and ESR test results show that the ˙O₂⁻ and ˙OH radicals produced by the P–TiO₂/g-C₃N₄ composite material under light irradiation are the main active components involved in the photocatalytic degradation process. A free radical detection test, XPS energy spectrum shift, valence band position and active substance quenching experiments further confirm the Z-scheme electron transport mechanism. When P–TiO₂ and g-C₃N₄ are combined to form a Z-scheme, the effective separation of photogenerated carriers is increased due to electron transport, thereby improving the photocatalytic degradation performance. Finally, four sulfonamide degradation products were analyzed by GC-MS, and four possible degradation pathways are proposed, among which the cleavage of S–N, S–C and C–N is concluded to be the main degradation pathway.

Author contributions

Dai Yongheng carried out experiments, formal analyzed data, and drafted the manuscript, Yuan Huayu and Su Qi put forward an original concept at beginning and modified the manuscript. Yi Qianwen and Zhang Yuntao assisted in the experiments. Li Jiang funded the whole study. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Science and Technology Program of Guizhou Provincial of China (No. [2020]1Z051) managed by Jiang Li.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra07289k

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