Construction of nitrogen-doped graphene quantum dots-BiVO₄/g-C₃N₄ Z-scheme photocatalyst and enhanced photocatalytic degradation of antibiotics under visible light

Abstract

Development of high-efficiency photocatalysts for photocatalysis research has been a hotspot in recent years. In this study, an effective nitrogen-doped graphene quantum dots (NGQDs)-BiVO₄/g-C₃N₄ Z-scheme heterojunction has been successfully prepared for environmental remediation. Antibiotics (tetracycline (TC), oxytetracycline (OTC) and ciprofloxacin (CIP)) were chosen as target pollutants to explore the photocatalytic performance. Results showed that both the BiVO₄/g-C₃N₄ ratio (Bi/CN ratio) and NGQDs amount have an important influence on the antibiotics degradation. Under our experimental conditions, the optimum Bi/CN ratio was 1 : 2 and optimum NGQDs addition amount was 5 wt%, with the corresponding 5% NGQDs-Bi/2CN sample showing the highest photocatalytic efficiency (91.5% in 30 min with TC). By further study based on electron spin resonance (ESR) and active species trapping experiments, the enhanced photocatalytic property could be ascribed to the crucial role of NGQDs and Z-scheme photocatalytic system, which not only accelerated the separation of the charge carriers but also exhibited a strong oxidation and reduction ability for efficient degradation of organic pollutants.

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Introduction

Antibiotics, accounting for the largest production and usage of drugs, have been widely used to treat bacterial infections worldwide. However, they are now producing serious threats to human health and ecosystems, as evidenced by their continuous input and accumulation in the environment due to the proliferation of bacterial drug resistance. Consequently, it is a serious problem to eliminate antibiotics from aqueous resources. To date, various advanced technologies, such as ozonation and UV, have limited applicability because of their high maintenance, energy cost, and construction. Therefore, more efficient treatment techniques are needed to efficiently eliminate emissions of antibiotics to the environment.

In recent times, photocatalytic oxidation has been regarded as an environmentally friendly and predominant pathway to remove antibiotics in natural water^1–6 and significant efforts have been made in the design and synthesis of highly efficient photocatalysts for water treatment to transform organic pollutants into harmless compounds.^7–9 To improve the photocatalytic activity, several synthetic strategies have been proposed by researchers from the viewpoint of the separation of photocatalyst charge-carriers. The construction of heterojunction systems is seen to be an effective way to limit the recombination of photogenerated charge-carriers.^10–12 Nevertheless, it is difficult for high charge-separation efficiency and strong redox ability to exist simultaneously.¹³ By contrast, a Z-scheme construction, in which the charge transfer directly quenches the weaker oxidative holes and reductive electrons, is preferable. Very often, Z-scheme systems contain two different photocatalysts with an appropriate intermediate or mediator (e.g. redox pair (Fe³⁺/Fe²⁺, IO₃⁻/I⁻),^14,15 or noble metals Ag and Au),^16–19 which will bring some disadvantages, such as low stability, high cost and difficulty in practical application. Thus, developing mediator-free direct Z-scheme photocatalytic systems as an ideal and effective method, which has become a hotspot for application in environmental purification and hydrogen generation from water.

On the one hand, as a robust organic semiconductor, graphitic carbon nitride (g-C₃N₄) has recently aroused much interest due to its thermal, electrical and optical characteristics.^20,21 It possesses many superior properties such as absence of metals, as well as being inexpensive, non-toxic and easily prepared. In particular, as a novel visible-light-driven photocatalyst, g-C₃N₄ (E_g = 2.7 eV) exhibits extremely efficient H₂ evolution from water and organic pollutant degradation. However, g-C₃N₄ also has some shortcomings such as limited visible-light absorption below 460 nm and rapid recombination of photogenerated electron–hole pairs, both of which lead to low photocatalytic activity. Therefore, further study is required for enhancing the photocatalytic activity of g-C₃N₄. In this regard, various effective methods have been developed to improve the catalytic performance for single semiconductors, such as constructing semiconductor heterojunctions,^22,23 doping with metal/non-metal,^24,25 and dye sensitization.^26–30 Meanwhile, as a ternary oxide semiconductor, BiVO₄ has recently become a promising photocatalyst due to its narrow band gap (2.4 eV), low toxicity and high stability. In addition, as a visible light response photocatalyst, BiVO₄ has been reported for both oxygen evolution from water splitting³¹ and for pollutant degradation.^32–34 In recent years, many researchers have been interested in coupling g-C₃N₄ with BiVO₄ to construct heterostructures.^35–40 For example, Kong et al. reported an efficient sulfur-doped g-C₃N₄/BiVO₄ composite photocatalyst for water oxidation under visible light.⁴¹ Ji and co-workers have investigated the catalytic performance of a g-C₃N₄/BiVO₄ heterostructure, finding that the coupled semiconductor exhibited higher photocatalytic activity for RhB dye degradation than that of the single semiconductor under visible light illumination.⁴² However, compared to the heterostructure, a Z-scheme system will possess high charge-separation efficiency and strong redox ability at the same time. It is well known that the VB position of g-C₃N₄ is at about 1.58 eV, and the CB position of BiVO₄ is at about 0.34 eV, which can match well to form a Z-scheme system. In this way, the photoinduced electrons in the CB of BiVO₄ may transfer to the VB of g-C₃N₄ rapidly, which leaves rich electrons in the CB of g-C₃N₄ and holes in the VB of BiVO₄ to participate in the redox process, exhibiting strong redox ability, respectively.

On the other hand, graphene quantum dots (GQDs), zero-dimensional materials with a typical lateral size of 3–20 nm, have been envisioned to fuel the development of research in electrochemical biosensors, drug delivery, bioimaging and energy conversion^43–52 due to their excellent electric conductivity, high surface area, and superior mechanical and thermal properties.^43–52 Additionally, GQDs also exhibit a strong effect of quantum confinement and a quantum size effect.⁵³ Moreover, when nitrogen atoms are introduced into the carbon lattice of quantum dots (NGQDs), they can modulate the electronic properties of the quantum dots and induce “activated regions” on the GQDs surface. This kind of activated region can participate in catalytic reactions directly, such as the oxygen reduction reaction.⁵⁴ To the best of our knowledge, heterointerfaces of composites based on N-graphene (or NGQDs) can exhibit much higher photocatalytic performance compared to the individual components. Hence, we constructed a ternary NGQDs-BiVO₄/g-C₃N₄ catalyst to investigate the photocatalytic activities. To the best of our knowledge, no reports exist at present on the preparation of ternary NGQDs-BiVO₄/g-C₃N₄ catalysts.

Herein, for the first time, we report an effective, facile and scalable route to prepare the novel Z-scheme system of NGQDs-BiVO₄/g-C₃N₄ photocatalysts. The photocatalytic performance was monitored by the degradation of antibiotics under visible light irradiation (λ ≥ 420 nm). The as-prepared NGQDs-BiVO₄/g-C₃N₄ samples could significantly enhance photocatalytic activity in comparison with pure g-C₃N₄ and BiVO₄. Furthermore, a mechanism for the enhanced photocatalytic activities is proposed in detail, based on active species trapping experiments and electron spin resonance (ESR) analysis.

Experimental section

Materials

Bismuth trichloride (BiCl₃), ammonium metavanadate (NH₄VO₃), ethanolamine (H₂NCH₂CH₂OH), urea, ammonium citrate (C₆H₁₇N₃O₇), tetracycline (TC), oxytetracycline (OTC), ciprofloxacin (CIP) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Sinopharm Chemical Reagent. All chemicals were of analytical reagent grade and used without further purification; deionized water was used in all experiments.

Synthesis

The g-C₃N₄ powders were obtained by thermal treatment. Briefly, 10 g of urea were placed in an alumina crucible with a lid and heated in a muffle furnace at 550 °C for 4 h with a heating rate of 2.3 °C min⁻¹.

BiVO₄ was prepared by a hydrothermal process. Typically, 1 mmol BiCl₃ was added into 35 mL of distilled water to obtain a hydrolyzed white flocculent suspension. When 1 mmol NH₄VO₃ was added into the above suspension, the color of the solution changed to orange. Then, 0.5 mL (1 M) aqueous ethanolamine was added, the suspension turned light yellow, and became a colloidal solution. After being stirred for 30 min, the obtained precursor was transferred into a 50 mL Teflon-lined stainless autoclave, and heated at 160 °C for 12 h. Finally, the obtained precipitates were collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried at 60 °C for 12 h.

The NGQDs were obtained by directly pyrolyzing ammonium citrate. 1 g of ammonium citrate and 20 mL of water were placed in a beaker and the mixture heated to 200 °C with an oil-bath pan. Within 30 min, the color of the solution slowly became orange, implying the formation of NGQDs. Subsequently, a quantity of NaOH (10 mg mL⁻¹) solution was added to adjust the pH value of 7.

The NGQDs-BiVO₄/g-C₃N₄ samples were prepared as follows. First, different quantities of BiVO₄ and g-C₃N₄ were added into an agate mortar and mixed together. Then, the mixture was calcined at 400 °C for 4 h to obtain the BiVO₄/g-C₃N₄ photocatalysts (abbreviated as Bi/CN, Bi/2CN, Bi/3CN, Bi/5CN and Bi/7CN). The NGQDs-Bi/2CN samples were synthesized by a facile low-temperature process. Different amounts (0.001, 0.003, 0.005, 0.007 or 0.010 g) of the NGQDs were dissolved in 25 mL of water, and then 0.1 g Bi/2CN was dispersed in the above solution for 24 h with continued stirring. Finally, the obtained products were dried at 80 °C overnight, to give the samples 1% NGQDs-Bi/2CN, 3% NGQDs-Bi/2CN, 5% NGQDs-Bi/2CN, 7% NGQDs-Bi/2CN and 10% NGQDs-Bi/2CN.

Characterization

X-Ray diffraction (XRD) pattern measurements were undertaken using a D/MAX-2500 diffractometer (Rigaku, Japan) with a nickel-filtered Cu-Kα radiation source (λ = 1.54056 Å). Scanning electron microscopy (SEM) was obtained using a Hitachi S-4800 field emission SEM (FESEM, Hitachi, Japan) to observe the morphology of the as-prepared samples. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were gathered on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.), equipped with a 200 kV accelerating voltage. Diffuse reflectance UV-Vis absorption spectra were collected using a Shimadzu UV-Vis 2550 spectrophotometer. Reflectance measurements were performed on powdered samples, using standard BaSO₄ as a reference. Total organic carbon (TOC) analyses were conducted on a multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer. The nitrogen adsorption and desorption isotherms of the samples were measured by a NOVA 3000e (America) nitrogen adsorption apparatus. The photocurrent and electrochemical impedance spectroscopy (EIS) measurements were conducted by use of a CHI-852C (CH Instrument) electrochemical workstation and a CHI 760D workstation, respectively. The electron spin resonance (ESR) signals of radicals were examined on a Bruker EPR A 300-10/12 spectrometer to detect the generation of activated species.

Photocatalytic analysis

The photocatalytic activity of as-prepared samples were evaluated by the degradation of antibiotics (TC, CIP and OTC) under visible light in a photochemical apparatus. A 250 W xenon lamp with a cut-off filter was used to remove wavelengths less than 420 nm. In detail, 50 mg of sample powders were dispersed in 100 mL solution of TC (10 mg L⁻¹), and in order to ensure adsorption equilibrium, the suspensions were kept stirring for 30 min in the dark before irradiation. At given irradiation intervals, 6 mL aqueous solutions were sampled and separated from the suspended catalyst particles for analysis. The photocatalytic degradation ratio was tested via the intensity changes of the absorption peaks at 357, 277 and 275 nm to determine the concentration of TC, CIP and OTC, respectively, at different times by the same UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).

Active species trapping and ESR experiments

In the active species trapping experiments, 1 mM TEOA, 1 mM BQ and 1 mM IPA were employed as the scavengers for h⁺, ˙O₂⁻ and ˙OH, respectively. The method used was similar to the above photocatalytic activity test. Furthermore, ˙OH and ˙O₂⁻ radicals were measured by the ESR technique by DMPO. Before determining the hydroxyl radicals (DMPO-˙OH) and superoxide radicals (DMPO-˙O₂⁻), 10.0 mg samples were dissolved in 0.5 mL deionized water (DMPO-˙OH) or 0.5 mL methanol (DMPO-˙O₂⁻), and then 45 μL DMPO was added with ultrasonic dispersion for 5 min, respectively.²⁰

Results and discussion

Characterization of NGQDs

The surface chemical compositions and states of the as-prepared N-GQDs were initially examined by XPS (Fig. S1†). Fig. S1a† displays the survey spectrum of NGQDs which shows three predominant peaks at 530.5, 400.2 and 284.1 eV which can be assigned to O 1s, N 1s and C 1s, respectively. The N 1s signal is fitted to four peaks centered at 398.6, 399.3, 400.4 and 401.9 eV in Fig. S1b,† which can be attributed to the pyridinic-like N, pyrrolic-like N, graphitic-like N and N–H, respectively.^54,55 Fig. S1c† shows the C 1s XPS expanded spectrum. The peaks at 284.6, 285.2, 287.3 and 289.0 eV correspond to C–C, C–N, C=O and O–C=O, respectively.^56,57 Finally, the above XPS results clearly confirmed the successful incorporation of N atoms into the GQDs.

Characterization of NGQDs-BiVO₄/g-C₃N₄

Morphology. The morphologies of the as-prepared samples were detected by SEM. It can be observed that the m-BiVO₄ products are nanoplates with an average thickness of 20–30 nm (Fig. 1a). In Fig. 1b, the pure g-C₃N₄ displays a layered structure with smooth surface. The microstructure of 5% NGQDs-Bi/2CN was determined by TEM and HRTEM. The TEM image in Fig. 1c shows that the g-C₃N₄ and BiVO₄ are close enough to form an intimate interface and are mixed with each other. While NGQDs are not observed by TEM due to their small size they are clearly seen in the HRTEM (Fig. 1d) and the distinct lattice fringe of the NGQDs is at 0.200 nm, which is consistent with a graphite carbon source. At the same time, distinct lattice fringes with spacings of d = 0.259 and 0.325 nm can be observed, which coincide with the (200) plane of BiVO₄ and the (002) plane of g-C₃N₄, respectively. The results clearly demonstrated that composites of BiVO₄, g-C₃N₄ and NGQDs have been formed.

Fig. 1 SEM images of prepared samples: BiVO₄ (a), g-C₃N₄ (b); TEM image of 5% NGQDs-Bi/2CN (c), HRTEM image of 5% NGQDs-Bi/2CN (d).

Compositional and structural information

The crystalline phases of the composites were investigated via XRD. As can been seen from Fig. 2a, the diffraction peaks at 13.1 and 27.4° can be indexed to pure g-C₃N₄ (JCPDS 87-1526), while BiVO₄ shows good consistency with the structure of monoclinic scheelite (JCPDS no. 14-0688). For the BiVO₄/g-C₃N₄ composites, the peak intensities of g-C₃N₄ become stronger with increasing of g-C₃N₄ content. However, when the NGQDs were introduced on the surface of BiVO₄/g-C₃N₄ (Fig. 2b), there are no obvious diffraction peaks, which may be due to the small amounts and low crystallinity. Similar observations have been noted in previous reports.^58,59

Fig. 2 XRD patterns of the as-prepared samples.

The Raman absorption spectra of 5% NGQDs-Bi/2CN and Bi/2CN are shown in Fig. 3. Raman bands at ca. 120, 210, 324, 366 and 826 cm⁻¹ are typical vibrational bands of BiVO₄.^60–62 However, the peaks located at 475, 705 and 1234 cm⁻¹ arise from the vibration modes in g-C₃N₄.⁶³ Compared with Bi/2CN, the 5% NGQDs-Bi/2CN sample displayed two relatively obvious extra peaks at 1350 and 1585 cm⁻¹, corresponding to the D and G band of NGQDs, respectively. The results suggested that the composites of BiVO₄, g-C₃N₄ and NGQDs have been coupled together successfully.

Fig. 3 Raman spectra of Bi/2CN and 5% NGQDs-Bi/2CN.

UV-vis absorption spectra

The light absorption ability of pure BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples were investigated by UV-vis absorption. As shown in Fig. 4, pure BiVO₄ shows an absorption edge at around 520 nm, which corresponds to a band gap energy of 2.4 eV, while bare g-C₃N₄ has a sharp absorption edge at about 460 nm, which corresponds to a band gap energy of 2.7 eV. The Bi/2CN sample is a mixture of BiVO₄ and g-C₃N₄ samples in the mole ratio of 1 : 2, which has an absorption edge between that of BiVO₄ and g-C₃N₄ samples. When 5 wt% NGQDs were decorated with Bi/2CN, the light absorption spectrum shows a strong absorption above 520 nm, which illustrates that the NGQDs can strengthen the visible-light spectral responsive range. A similar phenomenon was also observed by other researchers, which can be ascribed to the absorption of NGQDs.⁶⁴

Fig. 4 UV-vis adsorption spectra for as-prepared samples (a) and energy spectra to calculate band gaps (b).

Photocatalytic behaviors

In recent years, TC as an organic pollutant has received much attention.^23,65,66 For example, Zhou and co-workers had synthesized CdSe/rGO photocatalyst, which exhibited an excellent photocatalytic performance of 85.6% TC degradation under visible light irradiation within 60 min.⁶⁷ In addition, Z-scheme V₂O₅/g-C₃N₄ heterojunctions were prepared by the group of Shi, showing a degradation rate of TC of 75.7% under visible light irradiation for 2 h.²⁰ In our study, we also selected TC as the target pollutant to explore the photocatalytic performance (Fig. 5). As shown in Fig. 5a, pure g-C₃N₄ shows weak activity, on which about only 10.2% of TC was degraded after 30 min irradiation. Pure BiVO₄ had a higher activity than g-C₃N₄, on which 31.3% of TC could be removed over the same time. When the two semiconductors were coupled together this resulted in enhanced ability for the degradation of TC. The optimum sample of Bi/2CN showed the highest photocatalytic efficiency of 72.3%. After further modification with the NGQDs on the surface of the Bi/2CN, the photocatalytic activities can be significantly further enhanced, which can be ascribed to the excellent electron transfer properties of NGQDs. After 30 min irradiation, the 5% NGQDs-Bi/2CN sample exhibited superior photocatalytic performance, with TC removal efficiency of 91.5%. However, further increasing the NGQDs amount from 5 to 10%, the TC degradation was decreased because a larger NGQDs amount results in more limited light absorption. Meanwhile, the total organic carbon (TOC) removal rate of 5% NGQDs-Bi/2CN sample for degradation of TC is shown in Fig. S2.† It can be observed that the removal rate of TOC reached 68.9% after photoreaction, indicating that the photocatalyst can efficiently mineralize the TC molecules. Moreover, in order to make a comparison, commercial P25 TiO₂ powder was also selected to degrade TC under visible-light irradiation in Fig. S3.† Within 30 min, only 9% TC solution was found to be removed.

Fig. 5 Photocatalytic degradation of TC with as-prepared samples (a and b); the pseudo-first-order reaction kinetics for TC degradation with as-prepared samples (c and d); photocatalytic degradation of OTC and CIP with the NGQDs-Bi/2CN sample (e); four cycling experiments of 5% NGQDs-Bi/2CN sample for TC degradation (f).

The degradation kinetics of TC using as-prepared samples were investigated and the results are shown in Fig. 5c and d. It can be observed that the changes of the TC concentration vs. the irradiation time over the as-prepared samples conform to pseudo-first-order kinetics. The reaction rate constant (k) values were enhanced through the introduction of NGQDs. The 5% NGQDs-Bi/2CN sample (0.0804 min⁻¹) exhibited the highest k value among all the samples, which is about 23-fold and 6.6-fold higher than that of pure g-C₃N₄ (0.0035 min⁻¹) and BiVO₄ (0.0121 min⁻¹).

As broad-spectrum antibiotic agents, oxytetracycline (OTC) and ciprofloxacin (CIP) were also selected as target pollutants for photodegradation to acquire better insight about the catalytic activities over 5% NGQDs-Bi/2CN. As shown in Fig. 5e, under visible light irradiation for 120 min, 72.4% CIP and 66.7% OTC could be degraded, suggesting that the 5% NGQDs-Bi/2CN sample is an efficient photocatalyst for antibiotic degradation.

The photocatalytic reusability and repeated performance of the 5% NGQDs-Bi/2CN sample were also studied by the repeating experiments of photocatalytic TC degradation. After each cycle, the photocatalyst was filtered off and dried thoroughly, and then the fresh TC solution was added. It can be seen from Fig. 5f, after four successive cycles, the 5% NGQDs-Bi/2CN sample still maintained good photocatalytic stability under visible light.

The nitrogen adsorption–desorption isotherms were conducted to determine the surface area of the as-prepared samples.⁶⁸ As shown in Fig. S4,† the specific surface areas of the pure BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples are calculated as 5.96, 25.15, 42.69 and 54.51 m² g⁻¹, respectively. Moreover, it is known that the photocatalytic activity highly depends on the specific surface areas, and the larger surface area of the 5% NGQDs-Bi/2CN sample could absorb more active species and reactants on the surface, which thus favors the improvement of photocatalytic performance.

The intermediates of TC degradation

As shown in Fig. 6, the intermediates of TC degradation using 5% NGQDs-Bi/2CN were also investigated by LC-MS. It is clearly observed in Fig. 6a that there is an intense prominent ion with m/z = 445, corresponding to TC. During the decomposition process, TC molecules were attacked by the major reactive oxidising species and the relative sequence of successive ions fragmentation during collision-induced dissociation is in the following order: m/z = 445 → m/z = 406 (by loss of OH, H and CH₃) → m/z = 362 (by loss of CONH₂) → m/z = 318 (by loss of N, CH₂ and CH₃) → m/z = 274 (by loss of CH, C, H and OH). Based on the conclusions of the experimental and the reported studies,⁶⁶ the possible processes of the degradation are shown in Fig. 7. Finally, the intermediate products would be degraded to inorganic molecular material.

Fig. 6 Typical LC-MS chromatogram and m/z of degraded tetracycline: (a) tetracycline, (b) degradation of tetracycline after 30 min, and (c) degradation of tetracycline after 60 min.

Fig. 7 Proposed degradation pathways for photocatalytic degradation of TC with 5% NGQDs-Bi/2CN sample.

Electrochemistry analysis

In order to evaluate the kinetics of charge transfer with the BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples, photocurrent measurements and electrochemical impedance spectroscopy (EIS) were carried out. Fig. 8a shows that the photocurrent increases rapidly when the light irradiation is activated. Meanwhile, after several cycles of intermittent on–off irradiation, the photocurrent values still keep steady and are reproducible. The Bi/2CN composite exhibits a higher photocurrent response compared to pure BiVO₄ and g-C₃N₄, indicating that after BiVO₄ is coupled with g-C₃N₄ to form a Z-scheme system, the separation ration of the photogenerated electrons and holes could be increased. However, with NGQDs modification of Bi/2CN, the photocurrent is much higher than that of Bi/2CN, which could be attributed to the fact that the photogenerated electrons can be rapidly transferred from the CB to NGQDs, restricting the direct recombination of electrons and holes.

Fig. 8 EIS (a) and transient photocurrent response (b) for pure BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples.

EIS was carried out to explore the process of charge transfer resistance. The radius of the arc about the EIS Nyquist plot represents the charge transfer rate occurring at the contact interface between the working electrode and electrolyte solution. A smaller radius of the Nyquist circle indicates a lower charge-transfer resistance.⁶⁹ As shown in Fig. 8b, the 5% NGQDs-Bi/2CN sample displays the smallest arc radius among all the samples, which indicated a faster interfacial charge transfer and a more effective separation of electron–hole pairs in the Z-scheme system by the introduction of NGQDs. The conclusion was consistent with the photocurrent analysis, which clearly suggested that Z-scheme system of 5% NGQDs-Bi/2CN is effective to enhance photocatalytic activity.

Photocatalytic mechanism

A series of active species trapping experiments were carried out to examine the photocatalytic oxidation mechanism with pure g-C₃N₄, BiVO₄, Bi/2CN and 5% NGQDs-Bi/2CN (Fig. 9). For bare BiVO₄ (31.3%), as TEOA (13.2%) and IPA (22.4%) were injected into the solution to trap holes (h⁺) and ˙OH, the photodegradation activity was inhibited.^70–73 On the contrary, the addition of BQ (30.2%) to trap ˙O₂⁻,⁷⁴ did not obviously change the photocatalytic efficiency. The results indicated that h⁺ and ˙OH take part in the TC degradation. After TEOA (9.5%) was added into reaction solution for g-C₃N₄ (10.4%), the photocatalyst degradation rate scarcely changed. However, the addition of BQ (5.1%) and IPA (6.2%), slightly decreased the photodegradation activity, suggesting that the ˙O₂⁻ and ˙OH radicals were the major reactive species in the reaction system. On the other hand, for the Bi/2CN sample (72.3%), as TEOA (65.4%), BQ (28.5%) and IPA (34.4%) were added into the reaction system, the results showed that the photodegradation activity was changed by different degrees, indicating all the active species have an influence on the photocatalytic reaction (˙O₂⁻ > ˙OH > h⁺).^75–77 The 5% NGQDs-Bi/2CN (91.5%) sample showed similar behavior (TEOA (89.2%), BQ (24.8%) and IPA (29.4%)) as the Bi/2CN sample, suggesting that ˙O₂⁻ and ˙OH are the major reactive species for organic TC degradation.

Fig. 9 The species trapping experiments for degradation of TC over BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs- Bi/2CN photocatalysts under visible light irradiation.

The reactive species evolved in the process of photocatalytic reaction were investigated with the electron spin resonance (ESR) technique by DMPO. It can be observed from Fig. 10a, that the four characteristic peaks of DMPO-˙OH with intensity 1 : 2 : 2 : 1 exist in g-C₃N₄, BiVO₄, Bi/2CN and 5% NGQDs-Bi/2CN, indicating that these samples generated ˙OH in the process of photocatalytic reaction. However, the intensity of the characteristic peaks of DMPO-˙OH was highest for 5% NGQDs-Bi/2CN among all the samples, suggesting that the amount of ˙OH radicals generated on the 5% NGQDs-Bi/2CN surface was much higher than that of the other samples. Meanwhile, Fig. 10b shows that there are no obviously characteristic peaks of ˙O₂⁻ in the BiVO₄, indicating that no ˙O₂⁻ radicals are generated in the BiVO₄ reaction system. By contrast, six characteristic peaks of DMPO-˙O₂⁻ adducts are found in the g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples. According to the result of the characteristic peaks intensity (5% NGQDs-Bi/2CN > Bi/2CN > g-C₃N₄), it can be inferred that the 5% NGQDs-Bi/2CN sample generated the highest amount of ˙O₂⁻ radicals. Based on the above results of the active species trapping experiments and the ESR analysis, we can conclude that ˙O₂⁻ and ˙OH played major roles in the photocatalytic reactions over 5% NGQDs-Bi/2CN. At the same time, singlet oxygen generation was also investigated and detailed information is described in the ESI† (Fig. 11).

Fig. 10 DMPO spin-trapping ESR spectra of BiVO₄, g-C₃N₄, Bi/2CN and 5% NGQDs-Bi/2CN samples for DMPO-˙OH (a) and DMPO-˙O₂⁻ (b) irradiated for 60 s.

Fig. 11 Schematic of the separation and transfer of photogenerated charges in the 5% NGQDs-Bi/2CN sample combined with the possible reaction mechanism of photocatalysis.

In order to illustrate the photocatalytic process, the band positions can be evaluated according to a previous paper²¹ as follows:

E_CB = X − E^e − 0.5E_g

E_VB = E_CB + E_g

where X is the electronegativity of the semiconductor (X for BiVO₄ is 6.04 and for g-C₃N₄ is 4.73) and E^e is the energy of free electrons on the hydrogen scale (4.5 eV). According to the UV-vis diffuse reflectance absorption spectra, the band gaps of BiVO₄ and g-C₃N₄ are 2.4 and 2.7 eV, meanwhile, the conduction bands (CB) of BiVO₄ and g-C₃N₄ could be calculated to be 0.34 and −1.12 eV, and the valence bands (VB) of BiVO₄ and g-C₃N₄ are 2.74 and 1.58 eV, respectively.

As is well known, many reactive species such as h⁺, ˙O₂⁻ and ˙OH play major roles in pollutant decomposition. For pure BiVO₄, ˙O₂⁻ could not be produced because the CB of BiVO₄ is at 0.34 eV, which is more positive than the reduction potential of −0.33 eV/NHE (O₂/˙O₂⁻) by electrons.⁷⁸ However, the photoexcited holes in the VB of BiVO₄ can oxidize H₂O to give ˙OH, because the VB potential of BiVO₄ is more positive than the standard redox potential of ˙OH, h⁺/H₂O (2.72 eV vs. NHE). For the bare g-C₃N₄, ˙O₂⁻ could be produced due to a more negative value than the potential of O₂/˙O₂⁻ (−0.33 eV vs. NHE).⁷⁴ Whereas, the VB potential of g-C₃N₄ is lower than the standard redox potential of ˙OH, h⁺/H₂O (2.72 eV vs. NHE), so ˙OH could not be produced by the reaction of h⁺ and H₂O. However, the conclusion of the active species trapping experiments and ESR identify that ˙OH can be produced in the g-C₃N₄ photocatalytic process. Therefore, the ˙OH may be generated by further reduction of ˙O₂⁻, which is an indirect way to form ˙OH:²⁰

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + e⁻ + 2H⁺ → H₂O₂

˙O₂⁻ + H₂O → ˙OH + OH⁻ + O₂

H₂O₂ → 2˙OH

Based on the ESR and active species trapping experiments, we know that the ˙O₂⁻ and ˙OH play major roles in the 5% NGQDs-Bi/2CN reaction system. If the charge transfer path of photogenerated electrons and holes is like the typical heterojunction system, which is the common separation of the electron–hole process for a great number of composite photocatalysts, the electrons in the CB of g-C₃N₄ will migrate to the CB of BiVO₄, and the holes in the VB of BiVO₄ will migrate to the VB of g-C₃N₄. As a result, these accumulated electrons in the CB of BiVO₄ can not reduce O₂ to yield ˙O₂⁻, and the holes in the VB of g-C₃N₄ can not oxidize H₂O to give ˙OH. Therefore, when the charge carriers of the photocatalyst transfer in accordance with the traditional model, it is not favorable for the formation of active species, and leads to lower photocatalytic activity of the reaction system. From the above analysis, we explained the enhanced photocatalyst activity to the Z-scheme system. When the BiVO₄ coupled with the g-C₃N₄, the e⁻ in the CB of BiVO₄ could be rapidly combined with the holes in the VB of g-C₃N₄. Meanwhile, the e⁻ in the CB of g-C₃N₄ could react with O₂ to yield ˙O₂⁻; the h⁺ in the VB of BiVO₄ would react with H₂O to generate ˙OH, which improved the efficiency of the redox reaction to destroy pollutants into small molecules. When the NGQDs were introduced, e⁻ transfer was accelerated, which would further improve the charge separation efficiency and therefore enhance the photocatalytic degradation activity. In addition, in a separate report, we have characterized a further heterostructured ternary photocatalyst as shown in Fig. S6.†⁷⁹

Conclusion

In summary, a novel NGQDs-BiVO₄/g-C₃N₄ Z-scheme photocatalyst has been successfully fabricated using NGQDs to accelerate the transfer of electrons. Antibiotics (TC, OTC and CIP) were chosen as target pollutants to explore the photocatalytic performance, and it was found that the synthetic sample of 5% NGQDs-Bi/2CN has the highest photodegradation rate among all the photocatalysts studied. Under our experimental conditions, the enhancement of activity could be ascribed to the crucial role of NGQDs and Z-scheme photocatalytic system, which could accelerate effective charge separation and exhibit a strong oxidation and reduction ability to degrade organic pollutants. In general, Z-scheme photocatalytic systems could address the problem of low photocatalytic activity for environmental purification.

Acknowledgements

The authors would like to acknowledge the National Natural Science Foundation of China (21276116, 21477050, 21301076, 21303074, 21522603 and 21576121), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), Natural Science Foundation of Jiangsu Province (BK20141304), Special Financial Grant from the China Postdoctoral (2015T80500), China Postdoctoral Science Foundation funded project (2014M551508), Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068), Six Talents Peak Project in Jiangsu Province (XCL-025) and Graduate Research and Innovation Program of Jiangsu University (KYXX_0022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07589d

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