The synergistic photocatalytic effects of surface-modified g-C₃N₄ in simple and complex pollution systems based on a macro-thermodynamic model

Abstract

In this article, three types of surface-modified g-C₃N₄ (H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄) with unique photocatalytic properties have been successfully fabricated by simple and low-cost strategies. The morphology, elemental composition, textural properties, structural information, and photoelectric properties of the as-prepared materials are systematically characterized. Subsequently, the as-prepared materials are applied to Cr(VI) reduction, 4-FP degradation, and simultaneous Cr(VI) reduction with 4-FP degradation photocatalytic systems under normal and anoxic conditions. Based on the photocatalytic test results, we successfully constructed a macro-thermodynamic model to reveal the essential laws of synergistic photocatalytic effects. The constructed macro-thermodynamic model is suitable for complex photocatalytic systems and is of great significance for the application of photocatalytic technology in real wastewater treatment with multiple pollutants.

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Environmental significance

In the field of photocatalytic wastewater treatment, traditional research models focus too much on the micro-kinetics of photocatalytic reactions, such as the degradation routes of target pollutants and hydroxyl radicals generated by photocatalysts, but ignore the macro-thermodynamics of photocatalytic reactions, which is not conducive to the practical application of photocatalytic technology for wastewater treatment. In this article, we establish a macro-thermodynamic model to discuss the synergistic photocatalytic effects of various components in photocatalysts or multi-component pollution systems. Compared with the complex micro-kinetics model, this simple macro-thermodynamic model is more conducive to the industrial development of photocatalytic wastewater treatment.

1. Introduction

Heavy metal ions and organic pollutants in wastewater have caused long-term pollution to the natural environment with the continuous development of the world's industry.^1,2 Photocatalytic technology is an effective strategy for removing heavy metal ions and organic pollutants from wastewater based on the concept of sustainable development.^3,4 On the one hand, heavy metal ions in wastewater, such as highly toxic hexavalent chromium [Cr(VI)], can be reduced to low toxic trivalent chromium [Cr(III)] by photocatalytic reduction.^5–7 On the other hand, organic pollutants in wastewater can be decomposed into H₂O, CO₂ and other inorganic small molecules through photocatalytic oxidation.^8–10 In the past 40 years, humans have made some breakthrough progress in the field of environmental photocatalysis.^11–16 However, until now, the practical application of photocatalytic technology for wastewater treatment has not seen hope. We believe that there are some problems with the current research strategies on photocatalytic wastewater treatment and therefore need to be improved. In the study of photocatalyst development, many researchers tend to synthesize new photocatalytic materials and evaluate the activity of as-prepared materials by photo-generated carrier (e⁻–h⁺) separation efficiency while ignoring the connection between photocatalysts and target pollutants.^17–20 In the study of photocatalytic performance, the traditional research model only considers the removal rate and conversion process of target pollutants in the ideal single-component pollution system and ignores the interactions between various pollutant components in the multi-component pollution system.^21–24 Therefore, the research thoughts on photocatalyst development and photocatalytic performance study should be reconsidered in order to realize the application of photocatalytic technology for wastewater treatment.

Synergy, a ubiquitous natural law, is a relationship that inhibits or promotes each other among components in a complex system and is divided into internal and external synergy. Internal synergy is a global effect due to the collaboration between components in the system, and individual components may have no effect. External synergy is an additive effect due to the collaboration between components in the system, and individual components also have a certain effect. We believe that synergy in a photocatalytic system, that is, synergistic photocatalytic effect, is universal in nature. According to the meaning of internal synergy, the photocatalytic reduction of aqueous Cr(VI) under acidic conditions takes advantage of the internal synergy between a semiconductor photocatalyst and hydrogen ions (H⁺) because an individual semiconductor photocatalyst (such as g-C₃N₄) or H⁺ has no effect on the photocatalytic reduction of aqueous Cr(VI).^25–28 According to the meaning of external synergy, the construction of a heterojunction photocatalyst takes advantage of external synergy between two semiconductor components because each semiconductor component in the heterojunction photocatalyst usually has its own photocatalytic properties.^29–32 However, no systematic research has been conducted on the synergistic photocatalytic effect so far. The systematic study of synergistic photocatalytic effect in the photocatalytic system is beneficial to the application of photocatalytic technology for wastewater treatment.

In our previous work, we found that there is an external synergy in the reaction solution between oxidizing and reducing pollutants and clarified its mechanism.^33,34 That is, the electron transport effect of a photocatalyst accelerates the redox reaction between oxidizing and reducing pollutants, so the removal efficiencies of various pollutant components in a multi-component pollution system are simultaneously improved compared with that of a single-component pollution system. Some researchers have also found that there is a synergistic photocatalytic effect in the combined pollution system containing both Cr(VI) and organic pollutants, but they have not given the essential laws about the synergistic photocatalytic effect.^35–38 Whether a perfect research model could be established to clarify the synergistic photocatalytic effects between photocatalyst components, between photocatalyst and pollutant, and between pollutant components in a photocatalytic system is a very interesting scientific question and has great significance for the application of photocatalytic technology in wastewater treatment.

Construction of g-C₃N₄-based composites by surface modification is a simple and effective method to improve the photocatalytic performance of g-C₃N₄, which has been widely reported by many researchers in recent years.^39–41 For example, Dong et al. reported that a perylene imide (PI)-modified g-C₃N₄ was used to remove nitric oxide under visible-light conditions,⁴² Zhang et al. fabricated a surface hydroxyl-modified g-C₃N₄ with enhanced photocatalytic oxidation activity,⁴³ and Wang et al. synthesized an oxygen-containing ultrathin porous carbon quantum dots/g-C₃N₄ composite by a template-free method with superior photocatalytic activity for PPCPs remediation.⁴⁴ In this article, we successfully fabricated three surface-modified g-C₃N₄ photocatalysts (H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄) by simple and low-cost strategies and investigated the performance of as-prepared materials in single- and multi-component pollution systems under normal and anoxic conditions. Based on the photocatalytic test results, we successfully constructed a macro-thermodynamic model to reveal the essential laws of synergistic photocatalytic effects. The basic idea of the macro-thermodynamic model is that the elementary reactions in the photocatalytic process can be ignored, and the photo-generated carriers are the key to start the redox reaction in the photocatalytic system. Finally, the electron transport effect of the photocatalyst accelerates the electronic transmission between the oxidizing and the reducing agent. This macro-thermodynamic model is generally applicable to complex photocatalytic systems and is of great significance for the application of photocatalytic technology in wastewater treatment.

2. Experimental

2.1. Chemicals and reagents

Urea (CH₄N₂O, AR grade) and hydrochloric acid (HCl, 36–38%, AR grade) were purchased from Shanghai Xilong Chemical Co., Ltd. Waste Camellia oleifera shells (abbreviated WCOSs) were collected from Jiangxi Green Sea Oil Co., Ltd, China. Phloroglucinol (C₆H₆O₃, ≥99.0%) was purchased from Shanghai Macklin Biochemical Co., Ltd. Potassium dichromate (K₂Cr₂O₇, AR grade) and 4-fluorophenol (C₆H₅OF, 99%, abbreviated 4-FP) were procured from Aladdin Chemistry Co., Ltd. Diphenylcarbazide (C₁₃H₁₄N₄O, AR grade) was bought from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. Ultra-pure water was used in the high-performance liquid chromatography analysis. Deionized water was used in photocatalyst preparation.

2.2. Preparation

Bio-based carbon quantum dot (BCQD) aqueous precursor was prepared directly from WCOSs based on our previous work.⁴⁵ In a typical synthesis, 1.6 g of WCOS powder and 0.4 g of phloroglucinol were uniformly dispersed in 60 mL of water by using a 500 W ultrasonic crasher for 1 h. The resulting suspension was stirred for 6 h at room temperature and then hydrothermally treated at 230 °C for 12 h at a heating rate of 1 °C min⁻¹. After hydrothermal treatment, the black suspension was filtered using an organic filter with a pore size of 0.15 μm, and the obtained brown filtrate was BCQD aqueous precursor.

Pure g-C₃N₄ was prepared by directly calcining urea in air. Typically, 30 g of urea powder was put into a 100 mL alumina crucible with a cover. The crucible was heated to 550 °C from room temperature in a muffle furnace at a heating rate of 10 °C min⁻¹. After keeping the temperature at 550 °C for 2 h, the yellow g-C₃N₄ was obtained by natural cooling.

For the preparation of H⁺ surface-modified g-C₃N₄ (H⁺/g-C₃N₄), in a typical synthesis, 200 mg of g-C₃N₄ was uniformly dispersed in 100 mL of HCl diluted with a double volume of water and then stirred at room temperature for 3 h. After centrifugation, the obtained solid sample was washed several times with water. The yellow H⁺/g-C₃N₄ was obtained after drying at 60 °C for 12 h.

For the preparation of BCQD surface-modified g-C₃N₄ (BCQDs/g-C₃N₄), in a typical synthesis, 500 mg of g-C₃N₄ was uniformly dispersed in 100 mL of BCQD aqueous precursor by ultrasonic crashing for 1 h. The obtained suspension was stirred at 80 °C for 4 h and then separated by filtration. The brown BCQDs/g-C₃N₄ was obtained after fixing at 160 °C for 3 h in a vacuum.

For the preparation of H⁺ and BCQD surface-modified g-C₃N₄ (H⁺/BCQDs/g-C₃N₄), the preparation steps of H⁺/g-C₃N₄ were repeated but g-C₃N₄ was replaced with BCQDs/g-C₃N₄ to obtain brown H⁺/BCQDs/g-C₃N₄.

2.3. Characterization

Transmission electron microscopy (TEM) and scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images were recorded on a FEI Talos F200X field emission transmission electron microscope. The elemental mappings of the samples were determined using an energy-dispersive X-ray (EDX) spectrometer equipped on the TEM. Nitrogen gas porosimetry measurements were performed on a Quantachrome NOVA 2000e surface area and porosity analyzer after the samples were outgassed under a vacuum at 70 °C for 20 min and 120 °C for 6 h. X-ray diffraction (XRD) patterns were obtained using a D8ADVANCE diffractometer via Cu-Kα radiation with the 2θ angle scanning range of 10–80°. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS5 FTIR apparatus. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Ultra DLD instrument with a monochromated Al-Kα source at a residual gas pressure of less than 10⁻⁸ Pa. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements were carried out on an Edinburgh FS5 spectrofluorometer. Electrochemical impendence spectra (EIS), transient photocurrents, and Mott–Schottky plots were tested on a BioLogic VSP electrochemical workstation (Claix, France). Zeta potential measurements were performed using a Malvern MPT-2 automatic titrator. UV-vis/diffuse reflectance spectroscopy (UV-vis/DRS) was conducted using a Lambda 750S UV/VIS/NIR spectrometer.

2.4. Photocatalytic tests

A PLS-SXE 300 Xe lamp (300 W, Beijing PerfectLight Co., Ltd., China) with an output wavelength (λ) >320 nm served as the simulated sunlight source. Visible-light irradiation was obtained by removing UV irradiation from the lamp using a 420 nm cutoff filter, which could control λ >420 nm. Then, 30 mg of photocatalyst and 30 mL of reaction solution were poured into a 100 ml beaker with a quartz cover. For the single-component pollution systems, the initial concentrations of Cr(VI) and 4-FP were 10 mg L⁻¹. For the multi-component pollution systems, 30 mL of Cr(VI)/4-FP reaction solution was prepared by mixing 15 mL Cr(VI) and 4-FP solution at a concentration of 20 mg L⁻¹. The reaction solution was stirred in the dark until the adsorption–desorption equilibrium was reached. Subsequently, the light source was switched on, and fixed amounts of reaction solution were taken out at predetermined time intervals during irradiation. Changes in the Cr(VI) concentrations were analyzed via a diphenylcarbazide method by using a Lambda 750S UV/VIS/NIR spectrometer at λ = 540 nm. Changes in the 4-FP concentrations were examined using an Agilent 1100 series high-performance liquid chromatography (HPLC) system with a C₁₈ column and a UV detector (λ = 280 nm). Acetonitrile/water (60/40 v/v) was used as a mobile phase at a flow rate of 1.0 mL min⁻¹. The anoxic photocatalytic tests were realized by continuously passing argon gas into the reaction solution, and other steps were the same as above.

3. Results and discussion

3.1. Characterization

3.1.1. Morphology and elemental composition. The morphology and elemental composition of the as-prepared H⁺/BCQDs/g-C₃N₄ were studied by the observation of TEM, STEM-HAADF, and EDX elemental mappings. As shown in Fig. 1a and b, g-C₃N₄ prepared by the high-temperature polycondensation of urea exhibits two morphologies, namely, a large-scale structure with aggregation state and a small-scale structure with high dispersion. It can also be observed from Fig. 1a and b that BCQDs with different sizes are evenly distributed on the surface of g-C₃N₄. This result indicates that BCQDs were successfully modified on the surface of g-C₃N₄ during preparation (Scheme 1). By comparing Fig. 1c and d, we found that the BCQDs are bright in the STEM-HAADF image, which can be attributed to the secondary electron reflection of BCQDs. The EDX elemental mappings shown in Fig. 1e–h indicate that C, N, O, and Cl elements are present in the H⁺/BCQDs/g-C₃N₄. For the g-C₃N₄ component, the C and N elements are derived from the urea precursor, the O element is derived from the air during calcination, and the Cl element is derived from HCl. For the BCQDs component, the C, N, and O elements are derived from the WCOS precursor and the Cl element is derived from HCl. The Cl EDX elemental mapping shown in Fig. 1h can indirectly show that H⁺ was successfully modified on the surface of g-C₃N₄ and BCQDs by HCl acidification at room temperature (Scheme 1).

Fig. 1 TEM images of as-prepared H⁺/BCQDs/g-C₃N₄ with large (a) or small (b) size. TEM image (c), STEM-HAADF image (d), and corresponding C, N, O, and Cl EDX elemental mappings (e–h) of a large size H⁺/BCQDs/g-C₃N₄.

Scheme 1 The fabrication routes of surface-modified g-C₃N₄ and their synergistic photocatalytic effects based on the macro-thermodynamic model.

3.1.2. Textural properties and structural information. The textural properties of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ were characterized by nitrogen gas porosity measurement. As shown in Fig. 2a, all the tested materials show type IV isotherms with H3 hysteresis loops, indicating the mesostructures of the materials constructed by the secondary accumulation of g-C₃N₄ nanosheet structural units. Compared with g-C₃N₄, the BET (Brunauer–Emmett–Teller) specific surface areas of H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ are slightly reduced because the surface modifier H⁺ or BCQDs slightly destroy or block the geometrical surfaces of g-C₃N₄. As shown in Fig. 2b, the BJH (Barrett–Joyner–Halenda) pore-size distribution curves reveal that all the tested materials exhibit a narrow peak centered at ca. 3.7 nm and a broad peak centered at ca. 25.1 nm. The narrow peak can be attributed to the released NH₃ that acts as soft templates during the thermal polycondensation of urea precursor. The broad peak originated from the void space between g-C₃N₄ nanosheet structural units.

Fig. 2 Nitrogen gas adsorption–desorption isotherms (a), pore-size distribution curves (b), XRD patterns (c), and FTIR spectra (d) of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄.

The phase structures of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ were characterized by XRD. As shown in Fig. 2c, all the tested materials exhibit a typical interlayer stacking (002) peak at 27.5° which corresponds to an interlayer distance of d = 0.33 nm, while the (100) peak at 12.6° represents an in-plane structural packing motif with a period of 0.675 nm. The similar XRD patterns of the tested materials indicate that the phase structure of g-C₃N₄ is not affected by the surface modification.

The chemical structures of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ were studied by FTIR spectroscopy. As shown in Fig. 2d, for the BCQDs, the broad peak in the range of 3683–3045 cm⁻¹ is assigned to the stretching vibration of surface –OH, the peak at 2950 cm⁻¹ is assigned to the CH₂ units, and the peaks in the range of 1826–962 cm⁻¹ are assigned to C=O, C=N, C=C, C–O, C–N, and C–C stretching vibration. For g-C₃N₄, the broad peak in the range of 3650–2850 cm⁻¹ originated from the stretching vibration of primary (–NH₂) and secondary (=NH) amines. A series of peaks found in the range of 1800–1000 cm⁻¹ can be attributed to the stretching vibration modes like C–N and C=N in the CN heterocycles. The sharp peak at 815 cm⁻¹ is the typical bending vibration of s-triazine units. The similar FTIR characteristic absorption peaks of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ indicate that the chemical structure of g-C₃N₄ remains intact after surface modification. The FTIR signal of BCQDs is covered by that of g-C₃N₄, so the FTIR peaks regarding BCQDs cannot be observed in the BCQDs/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄.

The interactions of surface-modified H⁺ or BCQDs with g-C₃N₄ can be studied by high-resolution XPS. As shown in Fig. 3a, the XPS peak of C 1s centered at 284.8 eV is typically assigned to the adventitious reference C element. For the g-C₃N₄, the XPS peak of C 1s centered at 288.1 eV can be attributed to the sp² C bonded to N in an aromatic ring (N=C–(N)₂), whereas the XPS peak of C 1s centered at 288.9 eV is assigned to sp² C in the aromatic ring attached to the primary (N=C(N)–NH₂) and secondary (N=C(N)–NH) amines. For H⁺/g-C₃N₄, the shift of sp² C XPS peaks relative to g-C₃N₄ indicates that the N=C(N)–NH₃⁺Cl⁻ species were formed by the acid–base interaction between the surface –NH₂ of g-C₃N₄ and HCl (Scheme 1), and the decreased binding energy of sp² C can be attributed to the electron-withdrawing effect of H⁺. For the BCQDs/g-C₃N₄, the shift of sp² C XPS peaks relative to g-C₃N₄ indicates that the N=C(N)–NH₂–HO–C species were formed by the hydrogen bond interaction between the surface –NH₂ of g-C₃N₄ and the surface –OH of BCQDs (Scheme 1), and the increased binding energy of sp² C can be attributed to the electron-donating effect of BCQDs. As shown in Fig. S1a,† the C 1s XPS peaks of BCQDs centered at 284.8 and 288.9 eV overlaps with that of g-C₃N₄. Therefore, the characteristic C 1s XPS peaks of BCQDs cannot be observed in BCQDs/g-C₃N₄. The formation of N=C(N)–NH₃⁺Cl⁻ and N=C(N)–NH₂–HO–C species in the H⁺/BCQDs/g-C₃N₄ eventually caused the XPS peaks of sp² C to move slightly toward the high binding energy direction relative to g-C₃N₄. The XPS results of N 1s are consistent with that of C 1s. For the N 1s XPS peaks of g-C₃N₄ shown in Fig. 3b, the peak centered at 398.5 eV is assigned to sp² hybridized aromatic N atoms bonded to C atoms (C=N–C), the peak centered at 399.4 eV is related to either tertiary nitrogen (C)₃–N groups linking the structural motif (C₆N₇) or amino groups carrying hydrogen ((C)₂–NH, C–NH₂) to structural defects and incomplete condensation, and a weak peak centered at 401.1 eV corresponds to N atoms bonded to three C atoms in the aromatic cycles. The N 1s XPS peaks of H⁺/g-C₃N₄ shift to the low binding energy direction relative to g-C₃N₄ due to the formation of C–NH₃⁺Cl⁻ species by the acid–base interaction between the surface –NH₂ of g-C₃N₄ and HCl. The N 1s XPS peaks of BCQDs/g-C₃N₄ shift to the high binding energy direction relative to g-C₃N₄ due to the formation of C–NH₂–HO–C species by the hydrogen bond interaction between the surface –NH₂ of g-C₃N₄ and the surface –OH of BCQDs. The characteristic N 1s XPS peaks of BCQDs centered at 400.0 eV cannot be observed in BCQDs/g-C₃N₄, because the atomic concentration of the N element in BCQDs is very low (Fig. S1b and Table S1†). Compared with g-C₃N₄, the formation of C–NH₃⁺Cl⁻ and C–NH₂–HO–C species in H⁺/BCQDs/g-C₃N₄ makes the XPS peaks of N 1s move toward the high binding energy direction slightly. For the O 1s XPS peaks of g-C₃N₄ and H⁺/g-C₃N₄ shown in Fig. 3c, the XPS peak of O 1s centered at 532.5 eV can be attributed to the adsorbed H₂O, and the XPS peak of O 1s centered at 533.2 eV is derived from a foreign O element during calcination in air. Compared with g-C₃N₄ and H⁺/g-C₃N₄, the additional XPS peak of O 1s centered at 532.8 eV for the BCQDs/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ can be attributed to the O element of the aromatic carbon framework in the BCQDs (Fig. 3c and S1c†). For the Cl 2p XPS peaks of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ shown in Fig. 3d, the XPS peaks of Cl 2p centered at 197.1 and 198.6 eV can be attributed to the Cl 2p_3/2 and Cl 2p_1/2 electron orbits. The XPS results of Cl 2p further confirmed the successful introduction of H⁺ on the surface of g-C₃N₄ and BCQDs/g-C₃N₄ by the acidification with HCl at room temperature (Scheme 1). As shown in Fig. S1d,† the BCQD precursor prepared by using WCOSs mainly contains C and O elements, and the atomic concentrations of the C and O elements are 74.36% and 24.67%, respectively. In addition, the atomic concentrations of C, N, and O in g-C₃N₄ did not change obviously after surface modification because surface-modified BCQDs and/or HCl accounted for a small proportion in the as-prepared composites (Table S1†).

Fig. 3 High-resolution XPS of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ in the C 1s (a), N 1s (b), O 1s (c), and Cl 2p (d) binding energy regions.

3.1.3. Photoelectric property. The separation and recombination of photo-generated carriers are studied by the PL and TR-PL spectra. As shown in Fig. 4a, the broad PL emission peak of g-C₃N₄ in the range of 400–600 nm suggests that photo-generated e⁻–h⁺ pairs are generated and recombined within the g-C₃N₄. Compared with g-C₃N₄, the obviously reduced PL emission peak of BCQDs/g-C₃N₄ can be attributed to the electron-donating effect of BCQDs, which hinders the fall of the photo-generated electrons from the conduction band (CB) to the valence band (VB), thereby slowing down the recombination rate of photo-generated e⁻–h⁺ pairs. Compared with g-C₃N₄ and BCQDs/g-C₃N₄, the slightly enhanced PL emission peaks of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ are derived from the electron-withdrawing effect of H⁺ that induces the generation of more photo-generated e⁻–h⁺ pairs. As shown in Fig. S2,† the PL spectrum and corresponding fluorescence image of BCQD aqueous precursor proves that the as-prepared BCQDs are formed by quantum dots. The results of TR-PL spectra are consistent with that of PL spectra. As shown in Fig. 4b and Table 1, the PL decay curves of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ are fitted by the biexponential kinetic function [R(t) = B₁ exp(−t/τ₁) + B₂ exp(−t/τ₂)], and two decay components, namely, τ₁ and τ₂, are derived. The short-lifetime component (τ₁) usually originates from nonradiative relaxation related to material defects, whereas the long-lifetime component (τ₂) can be attributed to radiation produced by the recombination of photo-generated e⁻–h⁺ pairs. τ₂ of BCQDs/g-C₃N₄ is shorter than that of g-C₃N₄, indicating that the recombination number of photo-generated e⁻–h⁺ pairs is less in BCQDs/g-C₃N₄ than in g-C₃N₄ in the same time zone, suggesting that the electron-donating effect of BCQDs slow down the recombination of photo-generated e⁻–h⁺ pairs. Compared with g-C₃N₄ and BCQDs/g-C₃N₄, the slightly longer lifetime τ₂ of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ can be attributed to the increased number of photo-generated e⁻–h⁺ pairs by the electron-withdrawing effect of H⁺, resulting in an increased recombination number of photo-generated e⁻–h⁺ pairs in the same time zone. As a representative of the overall decay behavior, the average PL lifetimes of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ are 4.504, 4.516, 3.224, and 3.230 ns, respectively (Table 1).

Fig. 4 PL spectra (a), TR-PL spectra (b), EIS (c), and transient photocurrent (d) of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄.

Table 1 The fitted fluorescence decay components of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄

Photocatalysts	τ 1 (ns)	τ 2 (ns)	Ave. τ (ns)
g-C3N4	1.495	6.622	4.504
H^+/g-C3N4	1.526	6.898	4.516
BCQDs/g-C3N4	0.965	5.105	3.224
H^+/BCQDs/g-C3N4	1.007	5.144	3.230

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The transmission of photo-generated charges in the as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ is investigated by EIS and transient photocurrent. As shown in Fig. 4c, the EIS of BCQDs/g-C₃N₄ displays a smaller semi-cycle radius than that of g-C₃N₄, indicating that the charge directional transfer is smoother in BCQDs/g-C₃N₄ than in g-C₃N₄ due to the electron-donating effect of BCQDs. Compared with g-C₃N₄ and BCQDs/g-C₃N₄, the EIS of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ display a larger semi-cycle radius, which can be attributed to the enhanced charge directional transfer resistance by the electron-withdrawing effect of surface H⁺. As shown in Fig. 4d, the prompt increase in transient photocurrent from the light-off to the light-on state is ascribed to the quick directional transfer of photo-generated charges on the surfaces of working electrodes. Compared with g-C₃N₄, the higher transient photocurrent of BCQDs/g-C₃N₄ can be attributed to the smoother charge directional transfer due to the electron-donating effect of BCQDs. Compared with g-C₃N₄ and BCQDs/g-C₃N₄, the lower transient photocurrent of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ can be attributed to the enhanced charge directional transfer resistance by the electron-withdrawing effect of surface H⁺.

The relative amount of surface charges of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ at different pH values was characterized via zeta potential measurements. As shown in Fig. 5a, the as-prepared g-C₃N₄ is positively charged at pH <5 due to the surface adsorption of H⁺ and negatively charged at pH >5 due to the surface adsorption of hydroxide ions (OH⁻). Compared with g-C₃N₄, the increased surface negative charges of BCQDs/g-C₃N₄ can be attributed to the electron-donating effect of surface-modified BCQDs. Compared with g-C₃N₄ and BCQDs/g-C₃N₄, the increased surface positive charges of H⁺/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ are derived from the surface-modified H⁺. Compared with H⁺/g-C₃N₄, the decreased surface positive charges of H⁺/BCQDs/g-C₃N₄ can be attributed to the cancellation of surface positive charges derived from H⁺ and surface negative charges derived from BCQDs.

Fig. 5 Zeta potential (a), UV-vis/DRS (b), Mott–Schottky plots (c), and XPS VB spectra (d) of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄.

The light absorption properties of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ were studied by UV-vis/DRS. As shown in Fig. 5b, g-C₃N₄ shows a typical semiconductor absorption within the region of 250–450 nm, which originates from the electron transition from the VB populated by the N 2p electron orbit to the CB formed by the C 2p electron orbit. For the BCQDs, the light absorption in both UV- and visible-light regions can be attributed to the electronic transitions within the BCQDs. Compared with g-C₃N₄, the increased light absorption for the H⁺/g-C₃N₄ and BCQDs/g-C₃N₄ in the region of 250–450 nm can be attributed to the improved electron transition efficiency by the electron-withdrawing effect of surface H⁺ and the electron-donating effect of surface BCQDs. The combined effect of surface H⁺ and BCQDs further improves the light absorption capacity of H⁺/BCQDs/g-C₃N₄ in the region of 250–450 nm. With reference to the light absorption of BCQDs in the visible-light region, we can judge that the visible-light absorption of BCQDs/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ in the region of 450–800 nm originates from BCQDs.

The CB potentials (E_CB) of as-prepared g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄ are roughly determined by their corresponding Mott–Schottky plots. As shown in Fig. 5c, the flat band potentials (V_fb) obtained by extrapolation of the Mott–Schottky plots are −0.60, −0.55, and −0.53 V for g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄versus SCE at pH 7, respectively. After correction by adding +0.24 V, the E_CB values of g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄ are about −0.36, −0.31, and −0.29 V, respectively. The VB potentials (E_VB) of as-prepared g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄ are roughly determined by the XPS VB spectra. As shown in Fig. 5d, the E_VB values of g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄ are 2.09, 2.00, and 2.23 eV, respectively. The above Mott–Schottky plots and XPS VB spectra results indicate that the surface-modified H⁺ and BCQDs act as co-catalysts to change the E_CB and E_VB of g-C₃N₄, while surface-modified H⁺ slightly reduces the bandgap energy of g-C₃N₄ and the bandgap energy of g-C₃N₄ is slightly increased by BCQD surface modification.

3.2. Photocatalytic tests

In the field of photocatalytic wastewater treatment, traditional research models focus too much on the micro-kinetics of photocatalytic reactions, such as the degradation routes of target pollutants and hydroxyl radicals generated by photocatalysts, but ignore the macro-thermodynamics of photocatalytic reactions, which is not conducive to the practical application of photocatalytic technology for wastewater treatment. In this article, we establish a macro-thermodynamic model to discuss the synergistic photocatalytic effects of various components in photocatalysts or multi-component pollution systems. Compared with the complex micro-kinetic model, this simple macro-thermodynamic model is more conducive to the industrial development of photocatalytic wastewater treatment. Specifically, in the macro-thermodynamic model, all the elementary reactions will be ignored, there is a redox tendency between the substrates, and the excited photocatalyst looks like an electronic transmitter that switches on the electron transport from the reducing agent (RA) to the oxidizing agent (OA) (Scheme 1). Based on the macro-thermodynamic model, it is not difficult to understand that if one substrate is oxidized or reduced, the other reduced or oxidized substrate must be found in both single- and multi-component pollution systems. In the field of environmental photocatalysis, the photocatalytic reduction of Cr(VI) is essentially that of the electron transport effect of photocatalyst switches on the electron transport from O²⁻ to Cr⁶⁺ in Cr₂O₇²⁻ under acidic conditions, and the photocatalytic degradation of organic pollutants is essentially that of the electron transport effect of photocatalyst switches on the electron transport from organic pollutants to dissolved O₂. Based on the macro-thermodynamic model, from the perspective of photocatalytic substrates, we have confirmed in our previous studies that the synergistic photocatalytic effect in the 4-FP/Cr(VI) multi-component pollution system can be attributed to the electron transport from 4-FP to Cr(VI) being switched on by the electron transport effect of the photocatalyst.^33,34 Therefore, compared with the 4-FP or Cr(VI) single-component pollution system, the degradation and reduction rates of 4-FP and Cr(VI) are simultaneously improved in the 4-FP/Cr(VI) multi-component pollution system. However, based on the macro-thermodynamic model, from the perspective of the photocatalyst, one of the key scientific issues needs to be clarified; that is, what is the difference of the synergistic photocatalytic effect for the photocatalysts with different photocatalytic performance? In order to answer the above question, we fabricated three types of surface-modified g-C₃N₄ and systematically investigated the photocatalytic performance of as-prepared materials under normal conditions in Cr(VI) reduction, 4-FP degradation, and simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic systems. The photocatalytic test results can be perfectly explained by the constructed macro-thermodynamic model. In addition, in order to clarify the role of dissolved O₂, all photocatalytic tests were repeated under anoxic conditions.

3.2.1. Photocatalytic reduction of aqueous Cr(VI). As shown in Fig. 6a, the blank test shows that aqueous Cr(VI) cannot be reduced under visible-light irradiation because of the lack of a photocatalyst. The adsorption–desorption equilibria between Cr(VI) and the photocatalysts are reached before visible-light irradiation is administered. Compared with g-C₃N₄, the percentage of Cr(VI) adsorbed by the as-prepared H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ gradually increases due to the electrostatic effect between Cr₂O₇²⁻ and surface modifiers. Without adjusting the pH value of the reaction solution, g-C₃N₄ cannot reduce aqueous Cr(VI) under visible-light irradiation because the electron transport from O²⁻ to Cr(VI) cannot be switched on by the electron transport effect of g-C₃N₄ due to the high electron cloud density of O²⁻ in Cr₂O₇²⁻. As the pH value of the reaction solution is adjusted to 3, aqueous Cr(VI) can be slowly reduced by g-C₃N₄ under visible-light irradiation because of the internal synergy between g-C₃N₄ and H⁺ in the photocatalyst. Specifically, the electron transport from O²⁻ to Cr(VI) is successfully switched on by the electron transport effect of g-C₃N₄ because the electron cloud density of O²⁻ in Cr₂O₇²⁻ was reduced by the electron-withdrawing effect of H⁺ (equation A in Scheme 1). Similar to the above principle, H⁺/g-C₃N₄ can slowly reduce aqueous Cr(VI) without adjusting the pH value of the reaction solution due to the surface modification of H⁺. BCQDs/g-C₃N₄ can also slowly reduce aqueous Cr(VI) due to the internal synergy between g-C₃N₄ and BCQDs in the photocatalyst. Specifically, the electron transport effect of g-C₃N₄ switches on the electron transport from BCQDs to Cr(VI) due to the electron-donating effect of BCQDs (equation B in Scheme 1). Interestingly, compared with H⁺/g-C₃N₄ and BCQDs/g-C₃N₄, the as-prepared H⁺/BCQDs/g-C₃N₄ can rapidly reduce aqueous Cr(VI) without adjusting the pH value of the reaction solution due to the internal synergy of g-C₃N₄, H⁺, and BCQDs in the photocatalyst. Specifically, the electron transport from BCQDs to Cr(VI) is fully switched on by the electron transport effect of g-C₃N₄ because the electron cloud density of O²⁻ in Cr₂O₇²⁻ was reduced by H⁺ (equation C in Scheme 1). As shown in Fig. S3,† we further treated H⁺/BCQDs/g-C₃N₄ using HNO₃ with the same molar concentration as HCl diluted with a double volume of water and found that the photocatalytic activity of HNO₃-treated H⁺/BCQDs/g-C₃N₄ toward aqueous Cr(VI) reduction was significantly reduced. This result indicates that the strong oxidizing property of HNO₃ leads to the decomposition of BCQDs; therefore the internal synergy of g-C₃N₄, H⁺ and BCQDs in H⁺/BCQDs/g-C₃N₄ is suppressed. The Cr(VI) reduction rate was significantly decreased by NaOH-treated H⁺/BCQDs/g-C₃N₄ due to the lack of H⁺. Aqueous Cr(VI) cannot be reduced by BCQDs/SiO₂, indicating that the electron transport from BCQDs to Cr(VI) cannot be switched on by SiO₂. In addition, we systematically studied the effect of BCQDs aqueous precursor concentrations on the photocatalytic activities of H⁺/BCQDs/g-C₃N₄ for reducing aqueous Cr(VI). As shown in Fig. S4,† the adsorption and photocatalytic reduction abilities of H⁺/BCQDs/g-C₃N₄ to the Cr(VI) gradually increase with increasing concentration of BCQDs aqueous precursor. H⁺/BCQDs/g-C₃N₄ prepared by using the undiluted BCQDs aqueous precursor showed the best adsorption and photocatalytic reduction effect on the target pollutant Cr(VI) among the tested materials. The above results indicate that the surface-modified BCQDs play an important role in the capture and photocatalytic reduction of Cr(VI). We first prepared H⁺/BCQD precursor by freeze-drying and then obtained (H⁺/BCQDs)/g-C₃N₄ by the same method as that used in preparing BCQDs/g-C₃N₄. As shown in Fig. S5,† the adsorption capacity and photocatalytic reduction effect of (H⁺/BCQDs)/g-C₃N₄ for Cr(VI) are between those of BCQDs/g-C₃N₄ and H⁺/BCQDs/g-C₃N₄ due to the lack of H⁺ modified on the surface of g-C₃N₄.

Fig. 6 Photocatalytic reduction (a) and kinetic (b) curves toward aqueous Cr(VI) by using as-prepared materials. Photocatalytic reduction of aqueous Cr(VI) by using as-prepared materials under anoxic (c) conditions. Recyclability of as-prepared H⁺/BCQDs/g-C₃N₄ for the photocatalytic reduction of aqueous Cr(VI) (d).

The kinetics of aqueous Cr(VI) reduction was studied quantitatively by applying the Langmuir–Hinshelwood first-order model as expressed by the following equation:

ln(C_t/C₀) = −Kt

where K is the first-order rate constant. This model is generally used for the photocatalytic reduction or degradation process if the initial concentration of pollutant is low.^46–49 The relationships between ln(C_t/C₀) and reaction time for the aqueous Cr(VI) reduction over as-prepared photocatalysts are shown in Fig. 6b and the kinetic parameters in the current reaction system are summarized in Table S2.† The reduction rate of H⁺/BCQDs/g-C₃N₄ was the fastest among all the tested photocatalysts for aqueous Cr(VI) reduction under visible-light irradiation. It is worth noting that the correlation coefficient R² of g-C₃N₄ is very low because g-C₃N₄ cannot reduce aqueous Cr(VI) under visible-light irradiation without adjusting the pH value of the reaction solution.

As shown in Fig. 6c, under anaerobic conditions, the photocatalytic activities of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ for the aqueous Cr(VI) reduction are basically the same as those under normal conditions, indicating that dissolved O₂ in water did not participate in the photocatalytic reduction of aqueous Cr(VI). Compared with normal conditions, the as-prepared H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ have enhanced adsorption capacity toward Cr(VI) under anaerobic conditions, indicating that the surface modifiers compete for adsorption of Cr(VI) and dissolved O₂.

We further clarified the role of surface-modified H⁺ and BCQDs during aqueous Cr(VI) reduction through the recycling experiments of as-prepared H⁺/BCQDs/g-C₃N₄. As shown in Fig. 6d, the photocatalytic activity of H⁺/BCQDs/g-C₃N₄ decreased significantly after the first cycle. We regenerated the first cycled H⁺/BCQDs/g-C₃N₄ through H⁺ surface modification, but the activity of the first cycled H⁺/BCQDs/g-C₃N₄ did not return to its initial state. Then, we regenerated the second cycled H⁺/BCQDs/g-C₃N₄ by H⁺ and BCQDs surface modification and found that the activity of the second cycled H⁺/BCQDs/g-C₃N₄ returned to its original state. We can get the same result by regenerating the third cycled H⁺/BCQDs/g-C₃N₄ by H⁺ and BCQDs surface modification. The above results show that H⁺ and BCQDs on the surface of g-C₃N₄ are consumed simultaneously during aqueous Cr(VI) reduction. Specifically, H⁺ reduces the electron cloud density of O²⁻ in Cr₂O₇²⁻ and makes Cr(VI) have a strong electron-withdrawing ability, BCQDs provide electrons like sacrificial agents, and the electron transport effect of g-C₃N₄ switch on the electron transport from BCQDs to Cr(VI) (equation C in Scheme 1). Therefore, the as-prepared H⁺/BCQDs/g-C₃N₄ has the highest photocatalytic activity for the aqueous Cr(VI) reduction in all the tested materials. The full-range XPS and atomic concentrations results of H⁺/BCQDs/g-C₃N₄ before and after photocatalytic reduction of aqueous Cr(VI) are shown in Fig. S6 and Table S3.† The reduced atomic concentrations of C, O, and Cl in the H⁺/BCQDs/g-C₃N₄ after the photocatalytic reduction of aqueous Cr(VI) further proves that the surface-modified H⁺ and BCQDs are involved in the photocatalytic reduction of aqueous Cr(VI). Compared with the fresh H⁺/BCQDs/g-C₃N₄, the adsorption capacity of the first cycled H⁺/BCQDs/g-C₃N₄ toward Cr(VI) is obviously weakened. After regenerating the first cycled H⁺/BCQDs/g-C₃N₄ by H⁺ surface modification, the adsorption capacity of the first cycled H⁺/BCQDs/g-C₃N₄ toward Cr(VI) is restored somewhat. After regenerating the second cycled H⁺/BCQDs/g-C₃N₄ by H⁺ and BCQDs surface modification, the adsorption capacity of the second cycled H⁺/BCQDs/g-C₃N₄ toward Cr(VI) is restored completely. The above results indicate that both surface-modified H⁺ and BCQDs have an attractive effect on Cr(VI).

3.2.2. Photocatalytic degradation of aqueous 4-FP. As shown in Fig. 7a, the blank test shows that aqueous 4-FP cannot be degraded under visible-light irradiation because of the lack of a photocatalyst, and the adsorption–desorption equilibria between 4-FP and photocatalysts are reached before visible-light irradiation is administered. 4-FP can be slowly photocatalytically degraded by using g-C₃N₄ because g-C₃N₄ switches on the electron transport from 4-FP to dissolved O₂ (equation D in Scheme 1). Compared with g-C₃N₄, the higher photocatalytic activity of H⁺/g-C₃N₄ toward aqueous 4-FP degradation can be attributed to the electron-withdrawing effect of H⁺ that induces the generation of more photo-generated e⁻–h⁺ pairs, thus accelerating the electron transport from 4-FP to dissolved O₂. As shown in Fig. S7,† when g-C₃N₄ is treated with the same molar concentration of HCl or HNO₃, the photocatalytic activity of HCl-treated g-C₃N₄ is higher than that of HNO₃-treated g-C₃N₄. This result shows that HCl easily combines with the surface –NH₂ of g-C₃N₄ through acid–base interaction (Scheme 1). The photocatalytic activity of H⁺/g-C₃N₄ was decreased after NaOH treatment due to the loss of surface-modified H⁺. The electron transport from BCQDs to dissolved O₂ was switched on by the electron transport effect of g-C₃N₄ due to the electron-donating effect of BCQDs (equation E in Scheme 1). Therefore, compared with g-C₃N₄, the photocatalytic activity of BCQDs/g-C₃N₄ toward aqueous 4-FP degradation was decreased. The competitive effect of BCQDs and 4-FP on dissolved O₂ in water eventually leads to a decreased photocatalytic activity of BCQDs/g-C₃N₄ toward aqueous 4-FP degradation. The coexistence of surface-modified H⁺ and BCQDs finally makes possible the photocatalytic activity of H⁺/BCQDs/g-C₃N₄ between H⁺/g-C₃N₄ and BCQDs/g-C₃N₄.

Fig. 7 Photocatalytic degradation (a) and kinetic (b) curves toward aqueous 4-FP by using as-prepared materials. Photocatalytic degradation of aqueous 4-FP by using as-prepared materials under anoxic (c) conditions. Recyclability of as-prepared H⁺/BCQDs/g-C₃N₄ for the photocatalytic degradation of aqueous 4-FP (d).

The kinetics of aqueous 4-FP degradation was also studied quantitatively by applying the Langmuir–Hinshelwood first-order model. The relationships between ln(C_t/C₀) and reaction time for the aqueous 4-FP degradation over as-prepared photocatalysts are shown in Fig. 7b and the kinetic parameters in the current reaction system are summarized in Table S4.† The reduction rate of H⁺/g-C₃N₄ was the fastest among all the tested photocatalysts for aqueous 4-FP degradation under visible-light irradiation.

As shown in Fig. 7c, the photocatalytic activities of as-prepared g-C₃N₄, H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ decreased significantly under anoxic conditions because the electron transport from 4-FP to dissolved O₂ is impeded.

As shown in Fig. 7d, the as-prepared H⁺/g-C₃N₄ basically maintained the original photocatalytic activity toward aqueous 4-FP degradation after five recycles, indicating that the surface-modified H⁺ did not participate in the photocatalytic degradation of aqueous 4-FP. The as-prepared H⁺/g-C₃N₄ shows the highest photocatalytic activity toward aqueous 4-FP degradation in all the tested materials due to the internal synergy between g-C₃N₄ and H⁺ in the photocatalyst.

In addition, we added the same number of moles of HCl to the photocatalytic reaction system based on the atomic concentration of Cl element in the H⁺/BCQDs/g-C₃N₄. As shown in Fig. S8,† compared with H⁺/BCQDs/g-C₃N₄, both adsorption and photocatalytic activity of BCQDs/g-C₃N₄ (aq HCl) are slightly reduced. This result shows that the modified H⁺ on the surface of photocatalyst is more conducive to the capture and removal of target pollutants compared with H⁺ in the reaction solution.

3.2.3. Simultaneous reduction of Cr(VI) with 4-FP degradation. As shown in Fig. 8a, for the g-C₃N₄, the electron transport from 4-FP to dissolved O₂ and from 4-FP to Cr(VI) are switched on by the electron transport effect of g-C₃N₄ in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system (equation D and F in Scheme 1). Therefore, relative to single-component pollution systems, Cr(VI) can be smoothly photocatalytically reduced and 4-FP can be photocatalytically degraded in an accelerated manner in the multi-component pollution system due to the external synergy between Cr(VI) and 4-FP in the reaction solution. As shown in Fig. 8b, in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system, the Cr(VI) reduction and 4-FP degradation rates under anoxic conditions are similar to those under normal conditions due to the external synergy between Cr(VI) and 4-FP in the reaction solution (equation F in Scheme 1).

Fig. 8 Simultaneous reduction of Cr(VI) with 4-FP degradation by using g-C₃N₄ (a and b), H⁺/g-C₃N₄ (c and d), BCQDs/g-C₃N₄ (e and f), or H⁺/BCQDs/g-C₃N₄ (g and h) as a photocatalyst under normal and anoxic conditions.

As shown in Fig. 8c, for the H⁺/g-C₃N₄, in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system, the electron transport from O²⁻ to Cr(VI) in Cr₂O₇²⁻ is switched on by the electron transport effect of g-C₃N₄ because the electron cloud density of O²⁻ in Cr₂O₇²⁻ is decreased by the surface-modified H⁺, the electron transport from 4-FP to dissolved O₂ is accelerated because surface-modified H⁺ ions induce g-C₃N₄ to produce more photo-generated e⁻–h⁺ pairs, and the electron transport from 4-FP to Cr(VI) is accelerated by the surface-modified H⁺ because the electron cloud density of O²⁻ in Cr₂O₇²⁻ is decreased (equations A, D, and G in Scheme 1). Therefore, relative to single-component pollution systems, the external synergy between Cr(VI) and 4-FP in the reaction solution and the internal synergy between g-C₃N₄ and H⁺ in the photocatalyst significantly increase the reduction and degradation rates of Cr(VI) and 4-FP in the multi-component pollution system. In addition, the degradation rate of 4-FP decreased slightly after 40 min of photocatalytic reaction due to consumption of Cr(VI) and H⁺. As shown in Fig. 8d, in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system, the Cr(VI) reduction and 4-FP degradation rates under anoxic condition are similar to those of normal condition due to the external synergy between Cr(VI) and 4-FP in the reaction solution and the internal synergy between g-C₃N₄ and H⁺ in the photocatalyst (equations A and G in Scheme 1).

As shown in Fig. 8e, for the BCQDs/g-C₃N₄, the electron transport from BCQDs to Cr(VI), from 4-FP to dissolved O₂, from BCQDs to dissolved O₂, and from 4-FP to Cr(VI) are switched on by the electron transport effect of g-C₃N₄ in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system (equations B, D, E, and F in Scheme 1). Therefore, relative to single-component pollution systems, the Cr(VI) reduction and 4-FP degradation are simultaneously accelerated in the multi-component pollution system due to the external synergy between Cr(VI) and 4-FP in the reaction solution. As shown in Fig. 8f, under anoxic conditions, the electron transport from BCQDs to Cr(VI) and from 4-FP to Cr(VI) are switched on by the electron transport effect of g-C₃N₄ in the simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic system (equations B and F in Scheme 1). Relative to single-component pollution systems, both Cr(VI) reduction and 4-FP degradation can also be accelerated in the multi-component pollution system due to the external synergy between Cr(VI) and 4-FP in the reaction solution.

As shown in Fig. 8g, for the H⁺/BCQDs/g-C₃N₄, the electron transport from O²⁻ to Cr(VI) in Cr₂O₇²⁻ is switched on by g-C₃N₄ and the electron transport from BCQDs to Cr(VI), from 4-FP to dissolved O₂, from BCQDs to dissolved O₂, and from 4-FP to Cr(VI) are accelerated by surface-modified H⁺ (equations A, C, D, E, and G in Scheme 1). As shown in Fig. 8h, under anoxic conditions, the electron transport from O²⁻ to Cr(VI) in Cr₂O₇²⁻ is switched on and the electron transport from BCQDs to Cr(VI) and from 4-FP to Cr(VI) are accelerated (equations A, C, and G in Scheme 1). The reduction rate of Cr(VI) is fast in the Cr(VI) reduction photocatalytic system due to the coexistence of electron transport from O²⁻ and BCQDs to Cr(VI), and the degradation rate of 4-FP is slow in the 4-FP degradation photocatalytic system due to the coexistence of electron transport from 4-FP and BCQDs to dissolved O₂. Relative to single-component pollution systems, the accelerated Cr(VI) reduction and 4-FP degradation efficiencies can no longer be observed in the multi-component pollution system both under normal and anoxic conditions due to the competition between too many types of electron transport toward the photo-generated carriers both in the photocatalyst and in the reaction solution, resulting in a severe shortage of total supplied photo-generated carriers.

4. Conclusions

H⁺/g-C₃N₄, BCQDs/g-C₃N₄, and H⁺/BCQDs/g-C₃N₄ were successfully fabricated by simple and low-cost strategies. The as-prepared materials showed unique photocatalytic performance in the Cr(VI) reduction, 4-FP degradation, and simultaneous reduction of Cr(VI) with 4-FP degradation photocatalytic systems under normal and anoxic conditions. Based on the macro-thermodynamic model, the highest photocatalytic activity of H⁺/BCQDs/g-C₃N₄ toward Cr(VI) reduction among all the tested materials can be attributed to the internal synergy between H⁺, BCQDs, and g-C₃N₄ in H⁺/BCQDs/g-C₃N₄. The highest photocatalytic activity of H⁺/g-C₃N₄ toward 4-FP degradation among all the tested materials is due to the internal synergy between H⁺ and g-C₃N₄ in H⁺/g-C₃N₄. Compared with the single-component pollution system, the Cr(VI) reduction and 4-FP degradation by g-C₃N₄, H⁺/g-C₃N₄, and BCQDs/g-C₃N₄ are simultaneously accelerated in the multi-component pollution system due to the existence of external synergy between Cr(VI) and 4-FP in the reaction solution and internal synergy between g-C₃N₄ and H⁺ in the photocatalyst. However, the synergistic photocatalytic effects in the multi-component pollution system can no longer be observed by using H⁺/BCQDs/g-C₃N₄ as a photocatalyst because too many types of redox reactions in the photocatalyst and the reaction solution compete for the photo-generated carriers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Major International (Regional) Joint Research Program of China (51720105001), the National Natural Science Foundation of China (51568049) and the Natural Science Foundation of Jiangxi Province, China (20171ACB21035).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0en00759e

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