The development of an industrialized society has brought convenience to our daily lives. However, more problems are emerging with regard to environmental pollution. In particular, there is a large amount of antibiotics that show adverse effects in terms of the environment and human health [1-3]. The abuse of antibiotics can lead to the spread of various chronic diseases and microorganisms with a certain resistance to the antibiotics [4]. Therefore, it is highly necessary to find an efficient and environmentally friendly method to solve the vestigial problems associated with the use of antibiotics. In this context, photocatalysis has the outstanding advantages of making full use of the sunlight spectrum, reduced energy consumption, and completely mineralizing the organics, and has been widely used in advanced treatment of wastewater [5]. Semiconductor photocatalysts can degrade organic pollutants that are a widespread environmental concern [6, 7]. Natarajan's group synthesized an efficient semiconductor photocatalyst that had a high photocatalytic activity under visible light for the degradation of pollutants [8]. Dong et al. [9] prepared g-C3N4 that displayed excellent performance for the photodegradation of tylosin. Therefore, it is crucial to find a semiconductor photocatalyst that has good photocatalytic properties. This photocatalyst should enable the rapid separation of electron-hole pairs and improve resource utilization [10, 11].
Recently, AgCl, a silver salt semiconductor, has been widely used in the photocatalytic field in order to take advantage of the incident visible light and thereby enhance the photocatalytic performance in this region of the spectrum [12-14]. Furthermore, in the non-poisonous and facile preparation of several materials, AgCl has been considered as an ideal material that promotes the photocatalytic activity during the process of photocatalytic degradation. For example, Shen's group obtained a highly efficient catalyst based on Ag/AgCl-modified SrTiO3 nanotubes that was considered as a major breakthrough in the photocatalysis research field [15]. Pang et al. [16] revealed that Ag@AgCl-modified Cu(Ⅱ) catalyst has an excellent photocatalytic ability. Reddy's group fabricated AgX/Ag3PO4 (X = Cl, Br, I) heterostructures that better promoted the degradation of phenol than pure Ag3PO4 [17, 18]. However, the prepared AgCl nanoparticles are susceptible to agglomeration and photocorrosion during the photocatalysis process.
On one hand, the nanoparticles are susceptible to agglomeration. It is an efficient way to load certain support materials. Graphitic carbon nitride (g-C3N4) as a support material is a promising n-type semiconductor having an appropriate band gap that has attracted wide consideration in the area of photocatalytic degradation because of its non-toxicity, rich resources, and recyclability [19-21]. However, bulk g-C3N4 has a low efficiency of detachment of the photogenerated electrons and holes, and therefore exhibits inefficient photocatalytic performance [22]. Therefore, it is necessary to exfoliate bulk g-C3N4 in order to overcome its shortcomings. Two-dimensional (2D) layered structure nanosheets have shown great potential in photocatalysis owing to their unique structural properties [23-25]. This is because g-C3N4 nanosheets (CN) have higher specific surface areas compared to bulk g-C3N4. Moreover, CN can increase the carrier path length, thus increasing the carrier lifetime [26]. For example, Tan's group prepared a g-C3N4/TiO2 composite in which g-C3N4 played an influential role in the dispersion of TiO2 nanoparticles [27]. Bi et al. [28] synthesized NiCoP/g-C3N4, which is an excellent photocatalyst for hydrogen production, through an easy one-step method. Highly efficient visible-light-driven g-C3N4/Ag2O heterostructured photocatalysts were prepared by Xu et al. [29]. However, the AgCl/g-C3N4 composite photocatalyst has caused widespread concern. Murugesan's group synthesized photocatalysts displaying excellent photocatalytic performances by improving the properties of g-C3N4 by adding AgCl and provided new insights into solving the environmental problems associated with this catalyst [14]. Kang et al. [30] studied the influence of a g-C3N4 self-assembled coating on the photocatalytic activity and stability of Ag/AgCl microspheres. The deposition of AgCl on g-C3N4 leads to an excellent photocatalytic activity that has been confirmed by a number of studies to be attributed to surface plasmon resonance effect [31, 32]. Consequently, it is possible to load AgCl onto the CN photocatalyst. The AgCl/g-C3N4 binary complex can not only form a heterojunction that decreases the rate of recombination of photogenerated electrons and holes and increases the electron transfer rate, but also enhance the light absorption range [33, 34].
On the other hand, the stability of the photocatalyst is a significant factor determining its photocatalytic performance. The AgCl particles in the AgCl/CN composite photocatalysts are vulnerable to photocorrosion. In this context, electron transfer cannot be ignored. There are many reports on conductive polymers that are widely utilized in the process of photocatalysis by combining with semiconductors. These polymers can enhance the electrical conductivity, cycle utilization, photocorrosion resistance, and electronic transmission rate of semiconducting materials [35]. For example, Yu's group synthesized polyaniline (PANI)-g-C3N4 composite photocatalyst that displayed a remarkably improved photocatalytic ability for the degradation of the contaminants in wastewater [36]. Sui et al. [37] fabricated conductive polymer modified g-C3N4 with improved solar-driven H2 production rates that were higher than that obtained with pure g-C3N4. Vellaichamy's group reported that g-C3N4-PANI composites boost catalysis through synergistic interconstituent interactions [38]. Obviously, coupling conducting polymers with other semiconductors is a remarkable way to increase the photocatalytic activity. Poly-o-phenylenediamine (PoPD) has many advantages, such as variable conductivity, excellent optical properties, and strong electroactivity [39]. Notably, finding a low-cost, high-stability, and environment-friendly photocatalyst is very important for practical applications [40]. Inspired by the above analysis, we speculate that a combination of AgCl/CN with PoPD may be an ideal material for improving the photocatalytic activity of the composite and preventing the photocorrosion phenomenon.
Therefore, we prepared CN by the simple ultrasonic stripping method. The AgCl/CN composite photocatalyst was prepared by a simple precipitation method. To prevent the photosensitization of AgCl/CN, the PoPD-modified AgCl/CN photocatalyst was prepared by photoinitiated polymerization method. The prepared composite significantly facilitated the photocatalytic degradation of tetracycline (TC), which is attributed to the introduction of PoPD to AgCl/CN and the enhanced efficiency of separation of the photogenerated carriers, as revealed by the photoluminescence (PL) results. Finally, the possible photocatalytic reaction mechanism for the PoPD/AgCl/CN system was proposed.
A photocatalyst composed of bulk g-C3N4 was synthesized by heating melamine (15 g) to 550 ℃ for 4 h. The heating rate of the calcination process carried out in a muffle furnace was 2.5 ℃/min. Then, bulk g-C3N4 was ground into fine powder and 3.0 g of the powder was calcined twice by heating to 500 ℃ for 2 h at the rate of 5 ℃/min. In the next step, 640 mg of the above obtained secondary calcined product was dispersed in 100 mL nitric acid (HNO3) at room temperature and exfoliated by ultrasonic treatment for 3 h. The treated mixed solution was centrifuged to obtain pure g-C3N4 nanosheets that were repeatedly washed with deionized water to remove superfluous HNO3 until a neutral solution was obtained. The final product was obtained by drying overnight at 80 ℃ in an oven. After natural cooling, the resulting yellow powders were g-C3N4 nanosheets, which are called CN.
AgCl/CN composite photocatalysts were prepared by precipitation reaction method in the dark condition at room temperature. The previously synthesized CN (1 g) was dispersed in deionized water (80 mL) by magnetically stirring for 30 min. The suspension was stirred vigorously for a further 1 h, during which 0.29 mg of NaCl was added to 10 mL of the suspension. Then, 0.1 mol/L AgNO3 solutions of volume 24.4 mL were added dropwise and the solution vigorously stirred for 3 h. In the obtained sample, the mass percentage of CN was 35; such a sample has been defined here as AgCl-35/CN. The sample was washed and dried with deionized water and ethanol. Samples AgCl-10/CN, AgCl-20/CN, AgCl-30/CN, AgCl-40/CN, and AgCl-50/CN were also prepared likewise by modifying the initial amounts of AgNO3 and NaCl. Finally, AgCl/CN composite photocatalysts with different mass ratios were obtained.
o-Phenylenediamine (oPD, 0.2 g) was dispersed in 40 mL of deionized water to form a homogeneous solution after magnetic stirring for 40 min. The binary photocatalyst exhibiting the best performance was selected from the samples listed above. Then, 1 mol/L hydrochloric acid (HCl) was added and the solution stirred well to adjust the pH to 3.0. The solution was stirred for 24 h in order to achieve self-polymerization equilibrium, before it was irradiated by using an ultraviolet lamp for 40 min. Afterwards, the purified precipitates were rinsed several times with deionized water and ethanol until the solutions were pH neutral, followed by centrifugation. The obtained powders were dried in an oven for 12 h at 80 ℃. The proposed mechanism of formation of the PoPD/AgCl/CN photocatalysts is shown in Scheme 1.
The crystalline properties of the as-prepared photocatalysts were characterized by X-ray diffraction (XRD, MAC Science, Japan) with Cu-Kα radiation. The surface electronic states and the elements present on the surface of the photocatalysts were determined through X-ray photoelectron spectroscopy (XPS) by using a Thermo ESCALAB 250X (America) electron spectrometer. The functional groups of the as-synthesized photocatalysts were revealed by Fourier transform infrared spectroscopy (FT-IR). The energy dispersive spectroscopy (EDS) images and transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the as-prepared samples were obtained by using a JSM-7001F electron microscope and a JEM-2010 electron microscope with an accelerating voltage of 200 kV, respectively. The mode of the Brunauer-Emmett-Teller (BET) technique used was Nova3000e. This can be used to determine the specific surface areas of the catalysts. The optical properties of the obtained catalysts were measured by UV-vis diffuse reflectance spectroscopy (DRS) in the wavelength range 200 to 800 nm. The PL spectra were recorded by using a F4500 (Hitachi, Japan) PL detector. The electrochemical properties were evaluated with an electrochemical system (VersaSTAT3). To obtain the transient photocurrent response, Na2SO4 solution (0.5 mol/L) was used as the supporting electrolyte at 0.5 V. Electrochemical impedance spectroscopy (EIS) analysis was performed by using a CHI 760E electrochemical workstation. The total organic carbons (TOC) were measured by using a multi N/C 2100 (AnalytikJena AG, Germany) TOC analyzer.
The photocatalytic activities of the various samples were determined based on the level of photodegradation of TC, with an initial concentration of 20 mg/L, under visible-light irradiation. Briefly, 0.05 g of the photocatalyst was placed in 100 mL of a TC solution containing recycled water (25 ℃) to maintain a constant reactor temperature. The solution was stirred for 0.5 h in the dark to achieve dynamic adsorption equilibrium. The suspension was illuminated for 1.5 h through a filter by using a 250-W Xe arc lamp to simulate the visible solar light and sampled at intervals of fifteen minutes during the reaction. The concentration of TC was measured with a UV-vis spectrophotometer at the wavelength of 357 nm.
The XRD patterns of CN and the as-prepared AgCl/CN photocatalyst samples of different mass ratios are presented in Fig. 1(A). CN and the as-prepared photocatalysts exhibit the typical interplanar stacking peak corresponding to conjugated aromatic systems. It is easy to observe that the CN peaks are located at around 27.4° and 13.0°. These peaks are attributed to the (002) and (100) diffraction planes, respectively [41]. It is apparent that diffraction peaks are also detected at 27.8°, 32.3°, 46.3°, 54.9°, 57.6°, 67.5°, 74.6°, and 76.8°. As the content of AgCl increases, the intensities of the corresponding peaks become stronger, and the observed peaks correspond to the (111), (200), (220), (311), (222), (400), (331), and (420) planes of the AgCl crystal (Entry # 96-901-1667), which matches with the results of a preceding report [42]. The intensity of the characteristic peak at 27.96° increases. It could be attributed to a convergence of the (111) plane of AgCl (2θ = 27.8°) and the (002) plane of g-C3N4 (2θ = 27.4°) [43, 44]. As discussed above, AgCl/CN binary composites have already been successfully fabricated. The XRD patterns of AgCl-35/CN and PoPD/AgCl-35/CN are presented in Fig. 1(B). The peak positions and shapes of the PoPD/AgCl-35/CN photocatalyst do not change compared to those of AgCl-35/CN, indicating that the presence of PoPD does not affect the construction of the AgCl-35/CN lattice. Thus, the PoPD/AgCl-35/CN composite may be successfully fabricated [45, 46].
The elemental compositions and chemical states of the PoPD/AgCl-35/CN photocatalyst were characterized based on the XPS pattern, and the results are shown in Fig. 2. The elements C, N, O, Ag, and Cl are observed in the full survey spectrum of Fig. 2(A), suggesting that the PoPD/AgCl-35/CN photocatalyst is composed of these elements. Fig. 2(B) shows a high-resolution C 1s spectrum that reveals three obvious peaks located at 283.9, 285.2, and 287.2 eV that are ascribed to the C=C, C–N, and N=C–N bonds, respectively [58, 59]. In Fig. 2(C), the peaks at 397.8, 399.3, 400.5, and 403.6 eV could be ascribed to C=N–C, N–(C)3, N–H, and π excitation, respectively [47]. The emerging peak at ∼404.2 eV is attributed to protonated g-C3N4 nanosheets, and corresponds to the localization of the charging effects in heterocycles or in the cyano functional groups [31, 48]. In the high-resolution Ag 3d spectrum shown in Fig. 2(D), the two peaks observed could be deconvoluted into four peaks located at 366.7, 368.0, 372.7, and 373.7 eV, which are attributed to the Ag 3d5/2 and Ag 3d3/2 binding energies. While the peaks at 366.7 and 372.7 eV could be attributed to the Ag+ of the AgCl, the peaks at 368.0 and 373.7 eV could be ascribed to Ag0 [49, 50]. In order to explore the existence of Ag0, the XPS patterns of the AgCl, AgCl-35/CN, and PoPD/AgCl-35/CN samples are presented in Fig. S2. The survey spectra of all the as-prepared samples reveal the existence of the Ag component. As shown in Fig. S2(D), Ag0 does exist during the initial synthesis of AgCl because the prepared AgCl nanoparticles are susceptible to photocorrosion through photoreduction of the ambient light. The presence of two peaks indicates that a fraction of silver is produced by the oxidation of Ag+. Besides, the synthesized PoPD was irradiated by using an ultraviolet lamp for 40 min. which may also result in the formation of Ag0 [32, 42]. As displayed in the high-resolution O 1s spectrum of Fig. 2(E), only one type of elemental O is detected at 531.6 eV, corresponding to the C–O binding energy. This is due to the fact that O2 molecules from the air attack the coordinated N atoms of the aromatic C–N heterocycle, which results in the fracture of the C–N heterocycle; the O atoms substitute the coordinated N atoms to form a new C–O heterocycle [51]. We can observe two peaks at 196.9 and 198.5 eV in the XPS pattern of Cl 2p (Fig. 2(F)), which could be attributed to Cl 2p3/2 and Cl 2p1/2, respectively [49].
The bonding structures of CN, PoPD, AgCl-35/CN, and PoPD/AgCl-35/CN photocatalysts were further confirmed by FT-IR spectroscopy. In Fig. 3, the characteristic peaks located at 810 cm−1 could be attributed to the characteristic breathing mode of the tri-s-triazine units. The chemical band at 1200-1640 cm-1 could be attributed to the skeletal vibrations of aromatic CN heterocycles [33, 51]. The peaks of the AgCl stretching vibration are weak and explain why AgCl-35/CN and CN have similar characteristic peaks. PoPD is a conductive polymer containing only C–N and N–H functional groups, therefore, it has the same characteristic peak as CN, further indicating that PoPD/AgCl-35/CN photocatalyst retains the core heterocycle structure of CN [52, 53].
The morphologies of the as-synthesized photocatalysts were studied by SEM. The SEM image of CN (Fig. 4(A)) shows nanosheets, and CN increases the contact area compared to bulk g-C3N4 (Fig. S1(A)), which is beneficial for the enhancement of the photocatalytic performance [54, 55]. As presented in the inset of Fig. S1(C), the EDS image of the CN sample reveals that CN has been successfully synthesized. From Figs. S1(B) and 4(B), it is obvious that the smooth surfaces of the AgCl particles have been successfully dispersed onto the surface of CN, which indicates that a heterojunction may have formed between CN and AgCl [56, 57]. Fig. 4(C) shows the PoPD-modified AgCl particles deposited on the surface of CN. The magnified SEM image (Fig. 4(D)) shows that the surfaces of the AgCl particles change roughly, indicating that PoPD has successfully wrapped around AgCl-35/CN. In Fig. 4(E), the EDS pattern of the PoPD/AgCl-35/CN composite shows the presence of Ag, Cl, C, and N elements, indicating that this composite has been successfully prepared.
In order to further understand the nanostructure, the as-prepared samples were characterized by TEM and HRTEM, and the corresponding images are shown in Fig. 5. Dark spherical AgCl particles are observed in Figs. 5(A) and (B) that are loaded on the surface of CN. As can be seen in the TEM image of PoPD/AgCl-35/CN (Fig. 5(C)), the interface of three components is indicated by a yellow dotted line. Further, the composite has a lattice spacing of 0.32 nm, which corresponds to the (111) plane of the AgCl particles (JCPDS 31-1238) (Fig. 5(D)). These results reveal that CN, AgCl-35, and PoPD are in close contact with each other, which is beneficial for the separation of photogenerated electrons and holes for enhancing the photocatalytic activity [58, 59]. The elemental mappings of the PoPD/AgCl-35/CN photocatalyst with different color contrasts are presented in Fig. 5(E). It indicates that Ag, Cl, C, and N are uniformly distributed on the surface of PoPD/AgCl-35/CN. The maps correspond to the XPS and EDS results, indicating that AgCl-35/CN and PoPD/AgCl-35/CN have been successfully prepared.
To determine the specific surface area, the experimental data collected are presented in Fig. 6 and Table S1. As shown in Fig. 6(A), the BET surface areas of the AgCl-10/CN (20.4 m2/g), AgCl-20/CN (40.3 m2/g), AgCl-30/CN (41.0 m2/g), AgCl-35/CN (53.3 m2/g), AgCl-40/CN (49.0 m2/g), and AgCl-50/CN (27.6 m2/g) samples show differences, suggesting that the areas affect the photocatalytic performance. From Fig. 6(B), the BET surface area of CN was measured to be 90.0 m2/g, which is four times that of bulk g-C3N4 (22.3 m2/g). It indicates that CN has larger specific surface areas and contact areas. Moreover, more reaction sites are available for photocatalytic degradation [60]. The BET surface area of AgCl-35/CN (53.3 m2/g) photocatalyst gradually decreases after the addition of the AgCl particles, which is attributed to the dense growth of the AgCl particles loaded on the g-C3N4 [61, 62]. It is clearly observed that the introduction of PoPD enhances the specific surface area of AgCl-35/CN (Table S1). All the results suggest that PoPD has coupled well with AgCl-35/CN binary complex and that the increased specific surface areas are beneficial in providing more reactive sites, which in turn enhances the photocatalytic efficiency.
The photoabsorption properties of CN, AgCl-35/CN, and PoPD/AgCl-35/CN were studied by UV-vis DRS. The spectra of AgCl and PoPD and the proportionate sketch of (αhv)2 vs. hν are shown in Fig. S2. CN displays a wide range of intrinsic absorption edge at about 480 nm, and AgCl-35/CN and PoPD/AgCl-35/CN reveal slight redshifts in Fig. 7(A) in proportion to the bandgap (Eg) of CN, which is around 2.49 eV (Fig. 7(B)). Moreover, the bandgaps of AgCl-35/CN and PoPD/AgCl-35/CN are narrowed to 2.43 and 2.0 eV, respectively (Figs. 7(C) and (D)). Therefore, the bandgap of the ternary photocatalyst becomes smaller, which indicates that the electrons require lower energy to be excited. That PoPD/AgCl-35/CN reveals a significantly widened absorption range may be ascribed to the electronic transfer rate and the effective separation of charge carriers, which is conducive for improving the photocatalytic ability [63].
To investigate the recombination behavior of the charge carriers, the PL spectra were recorded and are presented in Fig. 8. The PL intensity of AgCl-35/CN composite photocatalyst has slightly delayed than that of the PL emanation peak of CN. This may be attributed to the fact that AgCl and its heterojunction together with CN can produce a synergistic effect when the holes are separated and captured, thereby suppressing the direct luminescent charge recombination [49, 64]. It is observed that PoPD/AgCl-35/CN exhibits a much constrained PL peak compared to AgCl-35/CN. Recombination of the charge carriers of PoPD/AgCl-35/CN is obviously suppressed, demonstrating that PoPD can increase the electronic transmission rate to improve the carrier separation capability. Therefore, the photogenerated electron-hole pairs can be efficiently transferred across the heterojunction interface and PoPD/AgCl-35/CN composites exhibit a higher photocatalytic performance than CN, AgCl, PoPD, and AgCl-35/CN [65-67]. This demonstrates that the recombination of the photogenerated carriers is greatly suppressed following the modification of AgCl-35/CN by using PoPD.
The efficiency of separation of the photogenerated electron-hole pairs has a great influence on the photocatalytic reaction when semiconductors are excited. In order to determine the photoelectrochemical properties of the sample, photocurrent response experiments were performed. It is understood that the intensity of the EIS pattern corresponds to the process of charge separation [68]. In the schematic diagram (Fig. 9(A)), PoPD/AgCl-35/CN exhibits the highest photocurrent density, which is three times that of CN. It indicates that the lifetime of the photogenerated carriers and the effective transfer of the photogenerated electron-hole pairs are apparently increased. The carrier separation efficiency at the interface was assessed by EIS and the result is presented in Fig. 9(B). The principle of the impedance measurement is an electrode and an electrolyte solution are in contact with each other during the test, and a charge transfer resistance is generated at the contact interface [63]. The magnitude of the charge transfer resistance can be directly obtained from the radius of the semicircle observed in the impedance spectrum. It can be observed that the radius of the arc in the impedance curve of PoPD/AgCl-35/CN is minimal. This radius becomes smaller in the EIS pattern, which indicates a lower resistance to charge transfer during the reaction [69]. These observations indicate that PoPD/AgCl-35/CN has a lower charge transfer resistance compared to AgCl-35/CN. It is shown that PoPD increases the electronic transmission rate and reduces the carrier recombination rate [70].
TC was selected to evaluate the performances of the as-synthesized photocatalysts under visible-light irradiation, which are presented in Fig. 10. Specifically, 50 mg of the AgCl/CN composite photocatalysts (with different mass ratios of CN) was added into the TC solution (20 mg/L) of volume 100 mL and stirred for 0.5 h in dark conditions to achieve dynamic adsorption equilibrium. The rate of degradation of TC suggested that no significant changes occurred in the absence of irradiation. The performance tests conducted on the different catalysts (AgCl-10/CN, AgCl-20/CN, AgCl-30/CN, AgCl-35/CN, AgCl-40/CN, and AgCl-50/CN) show that the degradation rates are as follows: 63.79%, 66.19%, 72.47%, 74.7%, 73.79%, and 65.84%, respectively (Fig. 10(A)). As shown in the results of Fig. 10(A), the best catalytic performance was obtained with AgCl-35/CN. In Fig. 10(B), the PoPD/AgCl-35/CN sample exhibits a much higher photocatalytic performance than AgCl-35/CN. In the process of photocatalytic degradation, about 83.06% of TC is photodegraded and PoPD/AgCl-35/CN displays a higher degradation rate than AgCl-35/CN for the same irradiation time. A photocatalytic activity for the degradation of TC was also detected in the case of the ternary material PoPD/AgCl-35/bulk g-C3N4; about 66.87% of TC was photodegraded, which is far less than that achieved by using PoPD/AgCl-35/CN. This can be attributed to the fact that CN prevents the agglomeration of the AgCl particles and increases the carrier transmission path lengths, which in turn increase the carrier lifetime [51]. Therefore, PoPD/AgCl-35/CN composite photocatalyst displays a high degradation rate. To quantitatively understand the reaction kinetics, a pseudo-first-order model (Eq. (1)) was used to fit the TC photodegradation reaction:
where C (mg/L) is the concentration of TC remaining at the irradiation time of t (min), C0 (mg/L) is the initial concentration of TC, and the slope k (min−1) is the apparent rate constant. Fig. 10(C) and (D) show the kinetic curves of all the samples. Obviously, the PoPD/AgCl-35/CN complex has the highest k value, which is about 0.0375 min−1 and is 7.98, 4.31, 1.40, and 1.57 times higher than those of CN (0.0047 min−1), AgCl (0.0087 min−1), AgCl-35/CN (0.0267 min−1), and PoPD/AgCl-35/bulk g-C3N4 (0.0239 min−1), respectively. It indicates that PoPD/AgCl-35/CN exhibits the highest photocatalytic activity, which is consistent with the result of the photocatalytic degradation of TC.
To evaluate the photocatalytic degradation abilities of the as-prepared photocatalysts, the mineralization activities must also be measured. In Fig. 11, it can be seen that only 42.34% and 58.18% of the TOC are removed by using CN and AgCl-35/CN, respectively. However, the removal rate of TOC reaches up to 69.04% within 120 min in the presence of PoPD/AgCl-35/CN. This indicates that PoPD/AgCl-35/CN has a much higher mineralization efficiency for TC photodegradation than CN or AgCl-35/CN [71].
It is known that the stability and recyclability of photocatalysts are crucial in solving the environmental problems. Therefore, the synthesized photocatalysts were subjected to cyclic experimental testing, and the results are presented in Fig. 12. We can obviously find that the photocatalytic efficiency of AgCl-35/CN complex decreases during four recycle tests for the degradation of TC under visible light irradiation. However, there is a slight change in the degradation performance of PoPD/AgCl-35/CN. It is remarkable that there is a huge enhancement in the photoactivity of PoPD/AgCl-35/CN compared with that of AgCl-35/CN, indicating that PoPD-modified AgCl/CN effectively inhibits the photocorrosion of AgCl and still maintains its stability [72, 73]. We further investigated the stability of the photocatalysts in this experiment, and the XRD patterns of AgCl-35/CN and PoPD/AgCl-35/CN after the four cycles and before the degradation are presented in Fig. 13(A). The location of the peak at 44.8° suggests it is a new peak. Correspondingly, after the cycle test, Ag0 is produced [74, 75]. The main feature of the ternary diffraction peak observed in Fig. 13(B) is almost the same as that before the experiment, indicating that the crystal structure does not change after the cycling test. Therefore, the prepared PoPD/AgCl-35/CN can be considered as a stable photocatalyst, in which PoPD is added as a charge carrier and acts as a photosensitive protective layer on the surface of the AgCl.
In order to determine the main active species during the photocatalytic process involving PoPD/AgCl-35/CN, an active species trapping experiment was performed. The presence of superoxide radicals (•O2−), vacancies (h+), and hydroxyl radicals (•OH) mainly influence the photocatalytic process [60, 61]. The influences were directly investigated by adding different trapping scavengers, namely ascorbic acid, ethylene diamine tetra acetic acid (EDTA), and isopropyl alcohol (IPA), respectively. Fig. 14 shows the photocatalytic degradation results obtained from the free radical capture experiments. The degradation of the solution is 66%, which indicates that •OH has little effect on the reaction system. However, when ascorbic acid (1 mmol) and EDTA (1 mmol) were added to the photocatalytic process, the degradation of TC is greatly suppressed. Therefore, •O2− and h+ are the main reactive species in the photodegradation of TC [76, 77].
Additionally, the ESR technique was used to analyze the main active species during the photocatalysis process. 5, 5-dimethyl-1-pyrroline (DMPO) was used in this test to confirm the presence of •OH and •O2− in the photocatalytic process of PoPD/AgCl-35/CN. As can be seen in Fig. 15(A), obvious DMPO-•O2− signals appear upon illumination compared to the dark. It indicates that •O2− as the active species plays an extremely important role over the course of photocatalytic degradation. Surprisingly, compared to CN and AgCl-35/CN, PoPD/AgCl-35/CN photocatalyst exhibits the highest peak intensities, suggesting that PoPD/AgCl-35/CN has a higher ability to generate the active •O2− species during the photocatalytic process than CN and AgCl-35/CN. The enhanced ability of the composite photocatalysts for the photocatalytic degradation reaction is also noted. In Fig. 15(B), •OH radicals are not detected in the characteristic signals corresponding to the dark conditions and weak peaks are detected compared to the •OH radicals under visible light illumination. It is easy to determine that the •OH radicals do not play an important role in the chemical reaction [78, 79]. From the above analysis, it can be concluded that the results of the ESR characterization correspond well with those of the active species trapping experiments.
The bandgap energies estimated from the DRS of CN (Fig. 7(B)), AgCl, and PoPD are 2.49, 3.21, and 1.78 eV, respectively (Fig. S3). To determine the positions of the band edges of CN, AgCl, and PoPD, their XPS valence band spectra were analyzed (Fig. 16), which correspond to the valence band positions associated with the mechanism [80, 81]. The peak locations are 1.67, 3.11, and 1.64 eV, respectively. Based on the previous discussion and the conclusions, a probable mechanism for the efficient degradation of TC over PoPD/AgCl-35/CN was proposed (Fig. 17). Under visible light irradiation, both CN and PoPD semiconductors can be easily excited to produce electron-hole pairs in the conduction band (CB) and valence band (VB), whereas the AgCl catalyst has a wide bandgap, which suggests that the charge carriers could not be excited during the degradation process [82, 83]. The CB energy levels of CN and PoPD are more negative than the CB potential of AgCl, therefore, the photogenerated electrons of CN and PoPD could be transferred to the CB of AgCl. The CB potential of AgCl (−0.09 eV vs NHE) is more negative than that of O2/•O2− (−0.046 eV vs NHE). Therefore, the CB of AgCl can photogenerate electrons that react with O2 to produce •O2−. Then, •O2− react with TC to produce carbon dioxide (CO2) and water (H2O) [84]. The VB of CN and PoPD accumulate h+, which can directly react on TC because h+ have a prominent affinity to directly capture electrons from organic pollutants. The h+ can also oxidize Cl−, which results in •Cl; the •Cl can oxidize TC. The Cl− produced during the process of oxidation can be recycled for the next catalytic reaction [84]. The above results indicate that h+ and •O2− are the major active species for the photocatalytic degradation, as revealed by the trapping and ESR experiments.
g-C3N4 nanosheets were successfully synthesized by using a facile ultrasonic stripping approach that increased the specific surface area compared to that of bulk g-C3N4, which results in more exposed active edges and enhanced light harvesting. Moreover, PoPD/AgCl-35/CN was also successfully fabricated, and exhibits the highest photocatalytic performance during the degradation of TC under visible-light radiation. The cycling experiment proves that PoPD/AgCl-35/CN composite photocatalyst is more stable than AgCl-35/CN. This study synthesized a g-C3N4-based photocatalyst that has a high stability and exhibits fast transport of electron-hole pairs. Besides, the photosensitized catalysts were modified with a conductive polymer to improve their catalytic properties. It provides a new perspective for the subsequent improvement of the photocatalytic degradation process of pollutants.
2019,
Vol. 40 Issue (1): 80-94 
PDF
2019,
Vol. 40 Issue (1): 80-94 PDF