Fe3O4-Loaded g-C3N4/C-Layered Composite as a Ternary Photocatalyst for Tetracycline Degradation.

A ternary photocatalyst, Fe3O4-loaded g-C3N4/C-layered composite (g-C3N4/C/Fe3O4) was fabricated by a facile sonication and in situ precipitation technique. Carbon nanosheets were prepared using the remaining non-metallic components of waste printed circuit boards as carbon sources. In this hybrid structure, g-C3N4 was immobilized on the surfaces of carbon nanosheets to form a layered composite, and 10-15 nm Fe3O4 nanoparticles are uniformly deposited on the surface of the composite material. The photocatalytic performance of the catalyst was studied by degrading tetracycline (TC) under simulated sunlight. The results showed that the photoactivity of the g-C3N4/C/Fe3O4 composite to TC was significantly enhanced, and the degradation rate was 10.07 times higher than that of pure g-C3N4, which was attributed to Fe3O4 nanoparticles and carbon nanosheets. Carbon sheets with good conductivity are an excellent electron transporter, which promotes the separation of photogenerated carriers and the Fe3O4 nanoparticles can utilize electrons effectively as a center of oxidation-reduction. Moreover, a possible photocatalytic mechanism for the excellent photocatalytic performance was proposed.

Introduction

In recent years, the existence of pharmaceutical residues in wastewater and its harm to the living ecosystem have attracted wide attention all over the world.[1] Tetracycline (TC), as a broad-spectrum antibiotic, has been widely used to prevent human and animal infections because of its antibacterial, bactericidal effect and low price.[2,3] However, 70–90% of the administered dose of TC is excreted via urine and feces, which has been detected in several water sources.[4] In order to effectively remove and degrade TC, various technologies have been developed, such as physical absorption, electrolysis, photocatalysis, microbial decomposition, electrochemical oxidation, and membrane separation.[5−8] Moreover, semiconductor photocatalysis technology, as a green and efficient technology, has become the research hotspot of TC residue treatment in recent years. Graphitized carbon nitride (g-C3N4), as a typical metal-free polymer semiconductor material, has a unique two-dimensional structure, a suitable band gap to absorb visible light radiation and excellent chemical stability.[9,10] Although g-C3N4 has already shown great potential in the photocatalysis field for water splitting,[11] degradation, and CO2 reduction,[12] inevitable shortcomings such as low utilization efficiency of visible light, fast photo-induced carrier recombination, low BET surface area, and difficulty in restoring suspension/dispersion limit its photocatalytic activity.[13] Therefore, several strategies have been developed to enhance the photocatalytic performance of g-C3N4, such as porous structure design, metal or nonmetal element doping, surface modification, coupling with semiconductors, and so forth.[14−16]

Of special notice, carbon materials are an excellent conductor of electrons and can transfer photoexcited electrons quickly to avoid carrier recombination.[17] For instance, carbon dots, graphene, nanotubes, and others have been proven as effective components in g-C3N4-based hybrid systems due to their superior electrical, mechanical, thermal, and optical properties. Up to now, numerous significant research studies have been conducted via directly coupling carbon materials with g-C3N4-based materials, which include GO/g-C3N4/CDs,[18] MWNTs/g-C3N4,[19] g-C3N4/graphene/NiFe2O4,[20] and carbon@g-C3N4 core–shell nanostructures,[21] etc. Noticeably, since g-C3N4 is also a π-conjugated semiconducting material, the electronic integration of g-C3N4 with several carbonaceous materials including CNTs, carbon black, graphene, and carbon nanodots can significantly improve the delocalization, thus promoting the rapid migration of photoinduced electrons, hindering the recombination of carriers and improving the quantum efficiency of photocatalytic reactions.[22] However, the present g-C3N4-based photocatalysts are difficult to recover and separate after photocatalytic reactions, which cannot meet the requirements of practical application. An effective strategy to solve this defect is to deposit magnetic materials on g-C3N4 sheets. As a typical magnetic material, Fe3O4 has been widely used in the synthesis of magnetic photocatalysts due to its good stability, low cost, good magnetism, and environmental friendliness. In addition, it can also be used as a good redox medium to store electrons and further improve photocatalytic performance.[23−27]

Herein, as illustrated in Figure S1, we report a ternary heterojunction consisting of g-C3N4 sheets and Fe3O4 nanoparticles, with carbon layers attaching on the surfaces of the two components as an electron transfer mediator. The carbon material was prepared using the remaining nonmetallic fractions of waste printed circuit board (WPCB) as a carbon source according to our previous study.[28] The magnetically separable g-C3N4/C/Fe3O4 nanocomposite was synthesized via a simple sonication technique followed by an in situ precipitation method. Under simulated sunlight, the photocatalytic degradation activity of the prepared g-C3N4/C/Fe3O4 ternary composite for antibiotic TC was significantly improved compared to pure g-C3N4 and the g-C3N4/C composite. Moreover, the stability of the ternary g-C3N4/C/Fe3O4 photocatalyst and the mechanism of improving photocatalytic efficiency under solar light were studied.

Results and Discussion

The XRD patterns of all as-prepared samples are shown in Figure . The carbon sample shows two peaks at around 26.1 and 41.8°, which can be indexed to the (002) and (100) diffraction planes of the amorphous carbon, respectively. The peaks located at 12.9 and 27.2° correspond to the (100) and (002) planes of g-C3N4, which are attributed to the in-plane structural packing motif and interlayer stacking of the aromatic system, respectively. No carbon diffraction peak is observed in the g-C3N4/C complex, which should be attributed to the low carbon content and weak diffraction intensity in the g-C3N4/C complex. As for g-C3N4/C/Fe3O4, the characteristic peaks of Fe3O4 are found in the XRD pattern apart from that of g-C3N4, in which the peaks at 35.5, 57.1, and 62.6° can be indexed to the (311), (511), and (440) crystal planes of Fe3O4 (JCPDS 75-0449), respectively.[29] The XRD results indicate that the g-C3N4/C/Fe3O4 photocatalyst was successfully prepared.

Figure 1

XRD patterns of Fe3O4, carbon, g-C3N4, g-C3N4/C, and g-C3N4/C/Fe3O4.

The morphology and microstructure of bare g-C3N4, pure carbon, g-C3N4/C, and g-C3N4/C/Fe3O4 were investigated by TEM, and the results are shown in Figure . The sample of g-C3N4 (Figure a) displays an irregular sheet structure, and the carbon (Figure b) shows a typical two-dimensional nanosheet morphology with thin thickness. Figure c,d shows TEM images of the g-C3N4/C nanocomposite. It can be seen that the carbon nanosheets and the g-C3N4 nanosheets are successfully combined to form a 2D–2D interface. g-C3N4/C/Fe3O4 exhibits a similar layer structure to g-C3N4/C. In addition, many nanoparticles with a diameter of about 10 to 15 nm are evenly distributed on the surface of carbon and g-C3N4. In addition, the peaks of Fe, C, N, and O are clearly observed in the EDS result (Figure f) of g-C3N4/C/Fe3O4. The SAED pattern in Figure g consists of three diffraction rings, indicating the presence of Fe3O4. The three ring patterns observed in SAED are indexed to (311), (511), and (440) planes of Fe3O4, which are in good agreement with the XRD spectra.[30] In the HRTEM images of Figure h,i, the lattice fringe of 0.25 nm corresponds to the (311) plane of Fe3O4. All of the above observations suggested that the ternary hybrid photocatalyst g-C3N4/C/Fe3O4 was indeed formed.

Figure 2

TEM images of (a) g-C3N4, (b) carbon, (c, d) g-C3N4/C, and (e) g-C3N4/C/Fe3O4. (f) EDS of C3N4/C/Fe3O4, (g) SAED patterns, (h) HRTEM image of g-C3N4/C/Fe3O4, and (i) HRTEM image of Fe3O4.

The elemental compositions and the surface chemical states of g-C3N4/C/Fe3O4 were obtained by XPS spectroscopy. As depicted in Figure a, the XPS survey spectra of g-C3N4/C/Fe3O4 illustrated that the prepared sample was composed of C, N, O, and Fe elements. Figure b shows the C 1s core spectra of the g-C3N4/C/Fe3O4 sample. The peak at 284.6 eV is assigned to graphitic or hydrogenated C–C bonding. Also, the other two peaks located at 281.1 and 282.7 eV originate from the adventitious carbon.[31] The adventitious carbon on the sample surface may originate from the atmosphere, sample handling, and the contamination in the XPS chamber. In Figure c, a main peak with the strongest intensity located at 395.3 eV is attributed to the C=N—C groups in the triazine rings, and the weaker peak located at 397.1 eV is assigned to the amino groups located at the edges of the polymeric g-C3N4 sheets.[32] The binding energies of the Fe 2p1/2 and Fe 2p3/2 were observed at 724.9 and 710.2 eV,[33] which are associated with the spin–orbit peaks of Fe3O4 (Figure d). The presence of Fe3O4 can be further confirmed by the O 1s XPS peak at 530.1 eV (Figure e), which corresponds to the oxygen species in the Fe3O4 phase; the small O 1s peak at 531.8 eV in Figure e indicates the presence of oxygen-containing groups.[34]

Figure 3

(a) XPS survey spectra and high-resolution XPS spectra of (b) C 1s, (c) N 1s, (d) Fe 2p, and (e) O 1s core-level electrons of the g-C3N4/C/Fe3O4 sample.

Figure 4

N2 adsorption–desorption isotherms of as-prepared g-C3N4, carbon, g-C3N4/C and g-C3N4/C/Fe3O4.

The specific surface area and porous nature of the as-prepared photocatalysts were analyzed by the N2 adsorption–desorption technique. As depicted in Figure , the g-C3N4 sample exhibits a type IV with a H3 hysteresis loop, indicating the presence of a mesoporous structure within the sample.[35] The N2 adsorption isotherm of carbon shows a sharp increase in adsorption at a low relative pressure (P/P0) range of 0.01–0.1, revealing their microporous characteristics,[36] and the Brunauer–Emmett–Teller (BET) surface area was 2419.1 m2·g–1. The g-C3N4/C and g-C3N4/C/Fe3O4 nanocomposites exhibit a transitional isotherm from type I to type IV (Figure inset), which indicates the coexistence of micropores and mesopores. Notably, an H3-type hysteresis loop was observed in the range of P/P0 = 0.5–1.0, indicating the presence of mesopores. The amount of nitrogen adsorbed can be found in the low-pressure zone, further proving that there are abundant micropores, which are derived from the voids of carbon nanosheets. Also, the pore size distributions (Figure S2) of the as-prepared carbon, g-C3N4/C, and g-C3N4/C/Fe3O4 samples showed a narrow pore size distribution and were mainly microporous (<2 nm). This is consistent with the analysis of the N2 adsorption–desorption isotherm. As shown in Table S1, the surface areas calculated using the BET method of g-C3N4/C and g-C3N4/C/Fe3O4 were 146.6 and 125.1 m2·g–1, respectively, much higher than that of pure g-C3N4 (17.4 m2·g–1). It is clear that the surface area of g-C3N4/C/Fe3O4 decreased after loading of Fe3O4 nanoparticles. This may be due to the covering and blocking of some parts of the g-C3N4/C surface by Fe3O4 nanoparticles. The increased specific surface area was conducive to the adsorption and transfer of pollutant molecules and provided a large number of reaction sites for enhanced photocatalytic activity.

The optical properties of g-C3N4, carbon, g-C3N4/C, and g-C3N4/C/Fe3O4 were investigated by UV–vis DRS, and the results are shown in Figure a. For pristine g-C3N4, the response cutoff wavelength is 460 nm, and the high selectivity to the visible spectral range corresponds to its band gap; thus, the photoconversion efficiency in the visible range is rather low. It is known that the narrow gap of sp2carbon clusters embedded in the carbon layer has excellent optical absorption capacity in almost the whole wavelength, so the introduction of carbon materials can improve the optical absorption efficiency of the g-C3N4/C sample.[37] As expected, compared with pristine g-C3N4, the g-C3N4/C and g-C3N4/C/Fe3O4 samples show wider light absorption capacity and improved adsorption strength in the whole UV–visible region.

Figure 5

(a) UV–vis absorption spectra and (b) PL emission spectra of the as-prepared samples.

The photoluminescence spectroscopy (PL) was used to study the separation and recombination of photoelectrons and holes in semiconductor catalysts.[38] As shown in Figure b, compared with g-C3N4, the PL peak intensity of g-C3N4/C is significantly reduced, which is related to the inhibition of light-induced carrier recombination. The PL intensity of g-C3N4/C/Fe3O4 is much weaker than those of g-C3N4 and g-C3N4/C, indicating that the carbon layer can effectively transfer photoelectrons and extend the life of photoelectron–hole pairs. The results demonstrated that the introduction of a carbon layer and Fe3O4 can effectively inhibit the recombination rate of photocarriers, thus producing more active groups and improving the photocatalytic performance.

In order to determine the separation efficiency of the carrier, photochemical measurements were performed.[39]Figure a displays the transient photocurrent responses of g-C3N4, g-C3N4/C, and g-C3N4/C/Fe3O4 samples in several light on–off cycles. Compared with g-C3N4 and g-C3N4/C, g-C3N4/C/Fe3O4 significantly improves the photocurrent performance. The results show that it has the lowest electron and hole recombination rate, indicating that Fe3O4 and carbon nanosheets play an important role in the ternary photocatalyst. In Figure b, the charge migration rate was evaluated according to the arc radius in the EIS. Obviously, g-C3N4/C/Fe3O4 shows the minimum radius of curvature, indicating its highest electron–hole pair separation and electron transfer efficiency, which agreed well with the results of PL and photocurrent response.

Figure 6

(a) Photocurrent response curves and (b) Nyquist plots of g-C3N4, g-C3N4/C, and g-C3N4/C/Fe3O4.

The performance of degradation of TC by all synthesized samples under simulated sunlight is displayed in Figure a. As depicted in Figure S3, in the dark adsorption step, adsorption–desorption equilibrium was reached within 60 min and 30.2 and 28.6% of CIP can be adsorbed by g-C3N4/C and g-C3N4/C/Fe3O4, respectively. It is noted that the g-C3N4/C and g-C3N4/C/Fe3O4 photocatalysts exhibited a higher adsorption ability than pure g-C3N4, which can be ascribed to the increased specific surface area and the interactions between the graphitic carbon layer (sp2 bonding) and the aromatic rings of the TC molecules. In the control experiment, TC basically did not degrade without the photocatalyst, indicating that the self-decomposition of TC can be ignored. Under the same conditions, the carbon and pure g-C3N4 samples only degraded 3.6 and 28.8% of the TC in 120 min. Compared to pure g-C3N4, the coupling of carbon and g-C3N4 obviously improved the photodegradation efficiency. As expected, g-C3N4/C/Fe3O4 had the best degradation efficiency, with the photodegradation rate of TC, approaching 96.4% under the same irradiation time. According to the first-order kinetics model, the apparent rate constants (kapp/min–1) of g-C3N4, carbon, g-C3N4/C, and g-C3N4/C/Fe3O4 are calculated to be 0.0029, 0.0003, 0.0063, and 0.0292 min–1, respectively (Figure b). It is worth noting that the rate constant of the g-C3N4/C/Fe3O4 sample is the highest, which is 10.07 times that of the original g-C3N4. The enhanced activity of g-C3N4/C/Fe3O4 can be attributed to the formation of heterojunctions that can effectively separate photocarriers, and the introduction of carbon in the composite material also helps to expand the optical response range and realize more effective electron transfer.

Figure 7

Photocatalytic (a) activities and (b) kinetics for TC degradation over the as-prepared bare g-C3N4, carbon, g-C3N4/C, and g-C3N4/C/Fe3O4 samples under simulated solar light illumination.

Photocatalytic stability and recyclability are the main parameters of its practical application. Figure a shows the photocatalytic TC degradation performance of the g-C3N4/C/Fe3O4 sample under simulated sunlight for five successive runs. It can be seen that after five cycles, the removal rate of TC remains at 88.1%, indicating that the g-C3N4/C/Fe3O4 photocatalyst possesses high stability and can be used for repeated treatment of TC. Additionally, XRD patterns presented in Figure S4 shows that the position of the diffraction peaks for g-C3N4/C/Fe3O4 before and after the fifth use remained unchanged, with a slight decrease in the peak intensities. In this study, EDTA, p-benzoquinone, and t-BuOH were used to scavenge h, ·O2–, and ·OH, respectively, in order to investigate in depth the photocatalytic mechanism. In Figure b, it can be seen that the degradation rate of TC is significantly reduced after adding p-benzoquinone (1 mM) and EDTA (1 mM), indicating that •O2–and h play significant roles in the degradation process. However, little efficiency reduction with the addition of t-BuOH (1 mM) demonstrated that ·OH might not the predominant active species. Based on the analysis of tapping experiments, it can be concluded that •O2– and h primarily contributed to the photocatalytic removal of TC over the g-C3N4/C/Fe3O4 photocatalyst.

Figure 8

(a) Cycling photocatalytic degradation tests and (b) TC degradation rates in the presence of different radical scavengers over the g-C3N4/C/Fe3O4 catalyst.

In order to clarify the photodegradation pathway of TC under the action of the g-C3N4/C/Fe3O4 photocatalyst, the main intermediate products converted by TC in the photodegradation process were accurately identified by HPLC-MS, and the result is shown in Figure S5 and Table S2. Obviously, TC is completely transformed into seven main photoproducts, which are designated as P2–P7 in the order of retention time. Combining with these detection results and references,[40−42] degradation and removal processes can be divided into three main pathways (Figure ). Briefly, the m/z of 445.1 (P1) is the molecular ion of TC and also appeared in the mass spectrum at the early stage of degradation. The first pathway is that the TC was transformed into P2 with m/z 427.1, which is due to dehydration. Meanwhile, deprotonated product P3 with m/z 397 was generated via loss of the N-dimethyl group due to the relative low bond energy of C–N and loss of the hydroxyl group. When further increasing the reaction time, the formation of P4 with m/z 318.3 was proposed to form via dihydroxylation. Subsequently, the loss of methyl group occurred from intermediate P4 and then intermediate P5 was produced. The third possible degradation route is that the tetracycline molecule was attacked by ·OH to form its hydroxylated product P6 with m/z 453.3. The product P7 with m/z 362.3 was formed through the further oxidation of product P6 and ring opening. These ring-opening products were finally oxidized into CO2 and H2O.

Figure 9

Removal pathway of TC over g-C3N4/C/Fe3O4.

Figure shows the schematic representation of the proposed mechanism for target pollutant degradation over the g-C3N4/C/Fe3O4 ternary photocatalyst. First, the introduction of carbon nanosheets resulted in a larger specific surface area of the g-C3N4/C/Fe3O4 sample, thus providing more active reaction sites. According to previous studies, the CB and VB edge potentials of g-C3N4 were at −1.12 and 1.57 eV, respectively.[43] The g-C3N4 yields photoinduced electrons and holes after exciting by simulated solar light. Because the carbon nanosheets in the ternary complex have good electron transport capability, the electrons generated in the VB of g-C3N4 are quickly transferred to the Fe3O4 nanoparticles resulting in the decrease of the electron–hole recombination rate and the prolongation of positive hole half-life, along these lines, high photocatalytic activity was expected in the test of g-C3N4/C/Fe3O4. In addition, g-C3N4 and carbon nanosheets are layered systems with a tri-triazine structure and have electron delocalization, leading to high charge separation.[44] On the other hand, these captured electrons by Fe3O4 nanoparticles could react with oxygen to form active species ·O2–, and ·OH radicals also can be produced via multistep reduction of O2.[45] Moreover, the Fe3+ existing in Fe3O4 captures the electrons to generate Fe2+ and combines with O2 to generate more ·O2–. Meanwhile, the photogenerated holes left in the VB of g-C3N4 reacted with the target pollutants directly instead of oxidized H2O to produce active species ·OH to participate in the reaction, which can explain why the EVB value of g-C3N4 (+1.57 eV) is lower than the redox potential of ·OH/H2O (+2.68 eV).[46] These active substances produced in the photocatalytic process would further react with organic pollutants to achieve degradation.

Figure 10

Proposed photocatalytic mechanism of the as-prepared g-C3N4/C/Fe3O4 ternary nanocomposite.

Conclusions

In summary, the g-C3N4/C/Fe3O4 ternary nanocomposite was synthesized by a facile sonication and in situ precipitation technology. The as-prepared g-C3N4/C/Fe3O4 has significantly enhanced photocatalytic activity for the degradation of antibiotic TC, and the degradation rate is nearly ten times higher than that of g-C3N4 under simulated solar light. The significantly improved photocatalytic activity should be attributed to the enhancement of optical absorption, enlarged specific surface area, and effective separation efficiency of photogenerated carriers. According to the determination of free radical capture experiments, the main active substances responsible for photocatalytic degradation are photoinduced holes and ·O2– free radicals. Finally, the ternary photocatalyst showed reasonable stability during five successive runs.

Experimental Section

Preparation of Samples

Preparation of g-C3N4: 5 g of melamine was heated at 550 °C (2 °C·min–1) for 4 h. Then, the obtained yellow product was collected and ground into powder.

Synthesis of carbon nanosheets: the remaining WPCB nonmetallic fraction was carbonized in a microwave oven for 20 min at 600 W. The cooled product was mixed and ground with KOH at a mass ratio of 1:2 and then activated at 850 °C for 90 min under nitrogen protection at a heating rate of 1 °C·min–1. Finally, the samples were washed with DI water until the pH value was neutral and dried at 120 °C for 12 h.

Synthesis of g-C3N4/C/Fe3O4 photocatalyst: first, g-C3N4 (2.5 g) and carbon powder (0.25 g) were dispersed in 200 mL of ethanol/water (v/v, 1/3) and ultrasonicated for 2 h at ambient temperature. After that, FeCl3·6H2O (0.1081 g) and FeCl2·4H2O (0.0398 g) were dissolved separately in 5 mL of deionized water and added to the above suspension. The resulting mixture was stirred at 80 °C for 120 min, and then 8 mL of ammonia solution was quickly injected into the above reaction mixture and then stirred for 60 min. The products were collected, washed several times with deionized water and alcohol, and dried overnight at 65 °C vacuum.

Characterization of Photocatalysts

X-ray diffraction (XRD) data were obtained on an X-ray diffractometer (SmartLab, Rigaku) with Cu Kα radiation in the range of 10–70° (2θ). The morphology and microstructure of the samples were studied by transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) and high-resolution TEM (HRTEM). The elements of the sample prepared were analyzed by energy-dispersive spectroscopy (EDS). UV–vis diffuse reflectance spectra (DRS) of the samples were measured using a UV–vis spectrophotometer (UV-3600, Shimadzu). Photoluminescence (PL) with an excitation wavelength of 325 nm was obtained using a fluorescence spectrophotometer (Shimadzu RF-5301). The specific Brunauer–Emmett–Teller (BET) surface areas were determined by nitrogen adsorption using Micromeritics ASAP 2020 nitrogen adsorption apparatus. The analysis of intermediates was performed using an HPLC-MS system (Agilent 1290/6460, Triple Quad MS) equipped with a Zorbax XDB-C18 column (150 × 2.1 mm, 3.5 μm). The electrochemical measurement was performed with an electrochemical workstation (CHI660B, Chen Hua Instruments, Shanghai, China).

Photocatalytic Experiments

The photodegradation of tetracycline (TC) was performed in a photochemical reactor at room temperature. Also, a 500 W xenon lamp was used as the simulated solar light source. Typically, 10 mg of the as-prepared photocatalyst was suspended in 40 mL of TC solution (10 mg·L–1) and stirred magnetically for 60 min in the dark to ensure the establishment of the adsorption/desorption equilibrium between the catalyst and the simulated pollutant. In the course of the experiment, 1 mL of the sample was taken out every 5 min and a PTFE syringe filter (0.45 μm) was used to remove the particles. The concentration of TC was monitored by a high-performance liquid chromatograph (HPLC, Shimadzu LC-20A) equipped with an Inertsil ODS-SP column. The mobile phase consisted of acetonitrile and ultrapure water (with the addition of 0.2% formic acid) with a volume ratio of 25:75 at a flow rate of 0.8 mL min–1.