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Article

Synthesis of Ag3PO4/G-C3N4 Composite with Enhanced Photocatalytic Performance for the Photodegradation of Diclofenac under Visible Light Irradiation

by
Wei Zhang
,
Li Zhou
,
Jun Shi
* and
Huiping Deng
*
State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
*
Authors to whom correspondence should be addressed.
Catalysts 2018, 8(2), 45; https://doi.org/10.3390/catal8020045
Submission received: 3 December 2017 / Revised: 17 January 2018 / Accepted: 23 January 2018 / Published: 25 January 2018
(This article belongs to the Special Issue Organic Photoredox Catalysis)
Browse Figures
Versions Notes

Abstract

:
A new visible-light-driven heterojunction Ag3PO4/g-C3N4 was prepared by a simple deposition-precipitation method for the degradation analysis of diclofenac (DCF), a model drug component, under visible-light irradiation. The heterojunction photocatalysts were characterized by a suite of tools. The results revealed that the introduction of Ag3PO4 on the surface of g-C3N4 greatly promoted its stability and light absorption performance. In addition, the effects of the heterojunction mixing ratios were studied, when the molar ratio of Ag3PO4 to g-C3N4 in the composite was 30%, the as-prepared photocatalyst Ag3PO4/g-C3N4 (30%) possessed the best photocatalytic activity toward the photodegradation of DCF, and the optimal photocatalyst showed a DCF degradation rate of 0.453 min−1, which was almost 34.8 and 6.4 times higher than those of pure g-C3N4 (0.013 min−1) and Ag3PO4 (0.071 min−1) under visible light irradiation (λ ≥ 400 nm). The trapping experimental results showed that h+, ·OH, and ·O2− were the main reactive oxygen species during the photocatalytic reaction. The improved performance of the composites was induced by the high charge separation efficiency of the photogeneration electron-hole pairs as well as the surface plasmon resonance (SPR) endowed in the Ag0 nanoparticles, and ultimately enhanced the DCF photodegradation.

1. Introduction

Recently, semiconductor-based photocatalysts have attracted intense attention as they hold promising potential for the use of solar light for pollution degradation and environmental remediation [1,2]. The conventional TiO2 photocatalyst has been extensively investigated due to its great photoactivity, stability, and innocuousness [3]. However, since the band gap of TiO2 is 3.2 eV, it is only active under UV-light irradiation [4]. Therefore, the performance of TiO2 is limited due to the limited visible-light utilization.
Currently, some visible-light-driven photocatalysts have been reported, such as g-C3N4, Ag-based photocatalysts, as well as Bi2WO6 [5,6,7,8]. However, it is very difficult to obtain a single-component photocatalyst with a wide light-absorption range and high charge-separation efficiency. Multi-component photocatalysts, which have more potential to be perfect photocatalysts, have received widespread attention [9,10]. As the energy levels of coupling semiconductors should be well matched, one of the basic conditions for constructing a heterojunction is to find two semiconductors with suitable band structure.
After Ye et al. first reported Ag3PO4 with photooxidation properties under visible light irradiation in 2000 [11], more and more research demonstrated its promising photocatalytic performance in water oxidation and organic-contaminant photodegradation. It has been reported that Ag3PO4 can achieve a quantum efficiency of up to 90% at wavelengths greater than 420 nm, which are significantly higher than that of previously reported semiconductors [12,13]. However, due to the conduction band potential (CB) in Ag3PO4 (+0.45 eV, vs. NHE) being more positive than the reduction potential of O2/·O2− (−0.33 eV, vs. NHE), the generated electrons cannot combine with the O2 to form active species for photo-oxidization during the photocatalytic process [11]. In addition, Ag3PO4 can be easily reduced to Ag0 by the electrons generated during the photocatalytic reactions, leading to the photocorrosion of Ag3PO4. Furthermore, Ag3PO4 is slightly soluble in aqueous solution, which seriously reduces its structural stability and separating efficiency in an aqueous solution [13,14]. Therefore, some strategies should be taken to enhance the photocatalytic performance and stability of Ag3PO4. Up until now, Ag3PO4-based composites synthesized by various strategies have been widely reported, such as Ag3PO4/BiVO4 [15], WO3/Ag3PO4 [16], and Ag3PO4/TiO2 [17]. The prepared photocatalysts have exhibited good stability and superior photocatalytic performance. Graphite carbon nitride (g-C3N4), a two-dimensional polymeric metal-free semiconductor, has been recently investigated in regards to water oxidation and pollution decomposition under visible light irradiation [18]. The g-C3N4 photocatalyst exhibits high thermal and chemical stability as well as novel electronic properties [19]. Furthermore, g-C3N4 presents a similar structure with graphene, which makes it a valuable candidate for combinations with other semiconductors to synthesize heterojunction composites [20,21].
As the CB and valence band (VB) positions of g-C3N4 are −1.30 and +1.40 eV (vs. NHE), and the CB and VB positions of Ag3PO4 are +0.45 and +2.90 eV (vs. NHE) [13,22], g-C3N4 and Ag3PO4 are two appropriate candidates for the construction of a heterostructured photocatalyst. Furthermore, the deposition of slight-soluble Ag3PO4 on the surface of the insoluble g-C3N4 sheet can effectively protect Ag3PO4 from dissolution due to the chemical adsorption between the g-C3N4 and Ag3PO4, which could enhance the structural stability of the Ag3PO4/g-C3N4 composites [14,23]. Ag3PO4/g-C3N4 composites have shown a positive photocatalytic performance for the degradation of dyes such as methylene blue [24], rhodamine B [25], and orange methyl [26]. In addition, the composites also show an effective ability in the CO2 photoreduction process [27]. Nevertheless, Ag3PO4/g-C3N4 composites are scarcely employed in the degradation of pollution materials, which are more difficult to be oxidized.
Diclofenac (DCF), a model non-steroidal anti-inflammatory drug (NSAID), is one of the most frequently detected pharmaceutical compounds in the aquatic environment [28], and its adverse impact on the current environment and ecosystems is evident. For example, DCF is toxic to fish and terrestrial vertebrates even at low concentrations [29,30]. The presence of DCF residuals in aqueous environment has also been attributed to the decline in the vulture population and the death of rainbow trout [31,32]. Traditional water treatment processes (e.g., activated carbon adsorption and coagulation) are usually insufficient for the complete removal of DCF [29,30,31,33,34], therefore, novel enhanced water-treatment technologies and processes are urgently needed to tackle the DCF challenges, as well as other emerging pharmaceutical pollutants in water treatment industries.
In this work, a series of Ag3PO4/g-C3N4 composite photocatalysts were synthesized via a deposition-precipitation method. The DCF photodegradation under visible-light irradiation over the heterojunctions was evaluated to better understand the photocatalytic properties and activity of this novel composite catalyst. The effects of different mass ratios on photocatalytic activity were systematically compared. In addition, the stability of Ag3PO4/g-C3N4 was assessed through five successive cycles. The structure, composition, morphology, and optical properties of the Ag3PO4/g-C3N4 photocatalysts were also characterized and discussed. Finally, a possible mechanism for the enhanced activity of Ag3PO4/g-C3N4 was discussed based on the synergetic effects of the Ag3PO4/g-C3N4 interface and the trapping experimental results.

2. Results and Discussion

2.1. Structure and Composition of Ag3PO4/G-C3N4 Photocatalysts

Figure 1 shows the XRD patterns of the g-C3N4, Ag3PO4, and as-prepared Ag3PO4/g-C3N4 samples. There are two characteristic peaks at 13.1° and 27.5° in the XRD pattern of g-C3N4, which correspond to the (100) and (002) planes, respectively, (JCPDS files: 87-1526) [35]. All diffraction peaks of the as-prepared Ag3PO4 coincide with the cubic Ag3PO4 (JCPDS No. 06-0505) [24]. The Ag3PO4/g-C3N4 composites exhibited diffraction peaks corresponding to g-C3N4 and Ag3PO4. By increasing the content of the Ag3PO4, its diffraction peaks gradually intensified at the expense of the g-C3N4 peaks, thus reflecting their contents in the Ag3PO4/g-C3N4 hybrids. The XRD analysis also confirmed the high purity of the Ag3PO4 and g-C3N4.
The FTIR spectra of the as-prepared sample are shown in Figure 2. For the pure g-C3N4, two main absorption regions are clearly observed, the strong absorption bands in the 1200~1650 cm−1 region were ascribed to the typical skeletal stretching vibration modes of the s-triazine or tri-s-triazine, which corresponded to the CN heterocycles [22]. The sharp peak centered at 808 cm−1 indicated a characteristic breathing mode of the triazine units, and verified that the prepared photocatalyst was a g-C3N4 or g-C3N4-based catalyst [36,37,38]. For Ag3PO4, the observed strong peak at 546 cm−1 was related to the O=P-O bending vibration, while the peak at 860 cm−1 was assigned to the symmetric and asymmetric vibration modes of P-O-P [39]. All of the characteristic peaks of g-C3N4 and Ag3PO4 could be observed in the spectra of Ag3PO4/g-C3N4 composites, and the intensity of the peaks at 546 cm−1 and 860 cm−1 rose with the increase of Ag3PO4 content.
The morphology and microstructure of Ag3PO4/g-C3N4 were investigated by SEM and TEM. As shown in Figure 3a,b, the C, N, O, P, and Ag elements were all detected in the Ag3PO4/g-C3N4 by the element mapping results, which indicated Ag3PO4 nanoparticles were deposited on the surface of g-C3N4 and the heterojunction was formed. In order to further confirm the combination of Ag3PO4 and g-C3N4, EDS analysis was further used, as shown in Figure S1. Figure 3c,d are the TEM images of g-C3N4 and Ag3PO4/g-C3N4, respectively. According to previous research [40], the light part is g-C3N4, and the dark part is Ag3PO4, which further demonstrates that Ag3PO4 was well-deposited on the surface of g-C3N4. The SEM and TEM images of Ag3PO4, Ag3PO4/g-C3N4 (20%), Ag3PO4/g-C3N4 (40%) are shown in Figure S2 in the Supplementary Materials. With the increasing ratio of Ag3PO4 in the composites, the size of Ag3PO4 nanoparticles increased slightly following an aggregation.
Surface chemical states of the Ag3PO4/g-C3N4 composite were studied by XPS. Figure 4a shows the XPS spectrum of Ag3PO4/g-C3N4 (30%) and Ag3PO4. All signals of C, N, Ag, P, and O were detected in the Ag3PO4/g-C3N4 composite, which is consistent with the XRD and FTIR results. The typical high-resolution XPS spectra of C 1s, Ag 3d, and P 2d in the composite are shown in Figure 4b–d. C 1s has two peaks at 284.6 and 287.4 eV that are separately attributed to the adventitious carbon and the sp2-hybridized C of N-C=N in g-C3N4 [41,42]. The Ag 3d5/2 and 3d3/2 of Ag3PO4 were located at 367.8 and 373.8 eV, which are consistent with the value of Ag+ in the Ag3PO4 [43]. The P 2p has a peak at 132.4 eV, corresponding to P5+ in the Ag3PO4 [15].
The optical properties of the prepared photocatalysts were measured via UV-vis DRS. As shown in Figure 5a, the absorption edge of pure g-C3N4 and Ag3PO4 is approximately 460 and 505 nm, respectively, indicating that both the g-C3N4 and Ag3PO4 could absorb solar energy. Based on the Kubelka–Munk function, the plots of (Ahν)1/2 vs. hν for g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (30%) are shown in Figure 5b [5]. The indirect band gap energies of the g-C3N4 and Ag3PO4 are 2.70 and 2.45 eV, respectively, which is consistent with previous results [13,44]. Compared to g-C3N4, the Ag3PO4/g-C3N4 (X) composites show an intense and broad background absorption in the visible-light region from 300 to 500 nm, which means that the visible-light absorption ability of the Ag3PO4/g-C3N4 (X) was improved.
Figure 6 displays the PL spectra of Ag3PO4/g-C3N4 (X) composites and g-C3N4 at an excitation wavelength of 368 nm. Compared with the g-C3N4, the Ag3PO4/g-C3N4 (X) composites showed significant PL quenching as a result of the efficient charge transfer at the heterostructure interface. In particular, when the molar ratio of Ag3PO4 to g-C3N4 was 30%, the PL peak in the Ag3PO4/g-C3N4 (30%) reached the lowest intensity, indicating the most effective inhibition for the recombination of photogenerated electron-hole pairs.

2.2. Photocatalytic Activity of the Ag3PO4/G-C3N4 Composites

The photocatalytic activities of the as-prepared Ag3PO4/g-C3N4 (X) composites were evaluated by the photodegradation of DCF under visible light. For comparison, the activities of pure g-C3N4 and Ag3PO4 were also tested under the same conditions. As shown in Figure 7a, all of the Ag3PO4/g-C3N4 (X) composites show a higher photocatalytic degradation rate than the g-C3N4 and Ag3PO4, and the photocatalytic activities are closely related to the molar ratio of Ag3PO4 to g-C3N4. With the increase of Ag3PO4 doped proportion from 10% to 30% in the composites, the photocatalytic property of the Ag3PO4/g-C3N4 composite increased gradually, while further increasing the mass ratio to 40%, which led to a decrease in the photocatalytic activity. The reason may be due to excessive deposits of Ag3PO4 shielding the active site on g-C3N4 surfaces, which will subsequently cause a decrease in the photocatalytic activity. To investigate the DCF degradation kinetics, a pseudo-first-order reaction model was used to describe the experimental data. It was found that the photocatalytic DCF degradation followed the first-order kinetics, and the rate constant for the different photocatalysts is shown in Figure 7b. The Ag3PO4/g-C3N4 (30%) composite exhibited the highest DCF photodegradation rate (0.453 min−1), which was 34.8 and 6.4 times faster than those of the g-C3N4 (0.013 min−1) and Ag3PO4 (0.071 min−1), respectively. Several studies have investigated DCF removal in the advanced oxidation processes. Michael observed an 80% DCF (the initial concentration was 10 mg L−1) removal efficiency in 120 min with TiO2 as the catalyst under a sonocatalytic process [45]. Shan found that 80% DCF (the initial concentration was 20 mg L−1) was removed in 40 min with FeCeOx under an ultrasonic system [46]. Cheng found that the photodegradation rate of DCF (the initial concentration was 5 mg L−1) was 0.08945 h−1 with Pd/TNTs photoelectrode under xenon lamp irradiation [47]. Shan has studied the degradation of DCF in a Fenton like system, FeCeOx-H2O2 in detail, and the reaction rate constant of DCF degradation was 0.073 min−1 [48]. Liu has found that the rate constant of DCF (initial concentration was 10 mg L−1) degradation was 1.0498 h−1 in a Bi2MoO6/Cu/PEC/PS system [49]. Horiya has reported the degradation of DCF (the initial concentration was 10 mg L−1) with mont-La(6%)-Cu0.6Cd0.4S under NUV-Vis irradiation, and the removal of DCF was ca. 92% in 240 min [50]. Martinez has studied the degradation of DCF (the initial concentration was 8 mg L−1) under UV irradiation, and the optimal conditions for a complete removal were obtained using synthesized anatase (0.5 g L−1) and 50% O2 (v/v) under UV irradiation, with rate constants ca. 0.9 min−1 (half-life time ca. 0.8 min−1) [51]. Thus, the obtained results in this study indicated that DCF photodegradation by Ag3PO4/g-C3N4 (30%) under visible-light irradiation illustrated an excellent performance for DCF removal.
Since it was quite possible that the intermediate products during the photocatalytic process were more poisonous, the removal of TOC was used to indicate the degree of mineralization of DCF degradation. As shown in Figure 8, DCF was completely decomposed in the first 12 min, while the removal of TOC was only 43.2%, indicated the DCF was completely degraded but partly mineralized.
The stability and reusability of the Ag3PO4/g-C3N4 composites were also examined to provide insight into the practical applications. A five-run cycling test of DCF degradation was performed to evaluate the changes of the DCF photodegradation kinetics. Figure 9 shows no apparent decrease in the DCF degradation rate over the five consecutive cycles (totaling 75 min), indicating good photocatalyst stability. Furthermore, the leaching of Ag+ after each re-cycle was analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES), and the result was shown in Table S1.
Furthermore, the XRD patterns of the Ag3PO4/g-C3N4 (30%) before and after the photocatalytic reaction were studied to evaluate structural stability. As shown in Figure 10, several small diffraction peaks of Ag that formed from the reduction of Ag3PO4 appear in the XRD pattern without evident crystalline structure changes during the reaction. From the SEM and TEM images of the cycled Ag3PO4/g-C3N4 (30%), shown in Figure S3, there was no significant change in the morphology and microstructure before and after the reaction.

2.3. Possible Photocatalytic Mechanisms

2.3.1. The Roles of ROS

Figure 11 shows the effects of different scavengers for the DCF photodegradation with Ag3PO4/g-C3N4 (30%) under visible light irradiation. Adding sodium azide did not appreciably inhibit the degradation rate of DCF, while the addition of isopropanol, benzoquinone, and disodium EDTA decreased the DCF degradation rate from 0.453 to 0.196, 0.173, and 0.011 min−1, respectively, indicating that ·OH, ·O2− and h+ were all responsible for ROS during the photocatalytic processes.
In order to further verify the active species in the photocatalytic reaction, the existence of ·O2− and ·OH was detected by ESR [24]. In Figure 12a,b, it can be seen that no characteristic peaks of DMPO- ·O2− and DMPO- ·OH adducts were generated without light irradiation, while the characteristic peaks of the DMPO-·O2− and DMPO-·OH adducts were observed over the Ag3PO4/g-C3N4 under visible light irradiation, which proves that the ·O2− and ·OH radicals were indeed generated and played an important role during the reaction. Moreover, compared with Ag3PO4/g-C3N4, the intensities of the DMPO-·O2− peaks were lower and no DMPO-·OH peaks could be found, indicating that less O2− and no ·OH were generated in the reaction process over g-C3N4.

2.3.2. Proposed Mechanisms

From the result of the trapping experiment, OH, ·O2− and h+ were all responsible for ROS during the photocatalytic processes, from the result of ESR, ·O2− and ·OH radicals were indeed generated and played an important role during the reaction. Generally speaking, there are two types of electron separation processes for the photogeneration electron-hole pairs during the photocatalytic reaction, one is a double-transfer mechanism and the other is a Z-scheme mechanism. The double-transfer mechanism is a common separation process for a large amount of composite photocatalysts, such as Ag2O/g-C3N4 composites [22]. If the Ag3PO4/g-C3N4 heterojunctions follow this mechanism, the schematic diagram of Ag3PO4/g-C3N4 can been seen in Figure 13a, as the potentials of the CB and VB edges of the g-C3N4 are −1.30 eV and +1.40 eV (vs. NHE), respectively, and the CB and VB edge potentials of the Ag3PO4 are +0.45 eV and +2.90 eV (vs. NHE), respectively [52,53]. Electrons in the CB of g-C3N4 would transfer to that of the Ag3PO4, and the holes accumulate on the VB of g-C3N4. However, as the CB of the Ag3PO4 (+0.45 eV, vs. NHE) is more positive than the potentials of the O2/·O2− (−0.33 eV, vs. NHE) [54], the photogenerated electrons on the CB of the Ag3PO4 cannot reduce the O2 to yield ·O2−. In addition, due to the potentials of the ·OH/OH− (+2.38 eV, vs. NHE) and ·OH/H2O (+2.72 eV, vs. NHE) [55], which are more positive than the VB of g-C3N4 (+1.4 eV, vs. NHE), the holes on the VB of g-C3N4 cannot react with the OH− and H2O to form an ·OH radical. As this result contradicts the results in the reactive species-trapping experiments, the double-transfer mechanism played a minor or negligible role. In contrast, the electron transfer might follow a direct Z-scheme on the Ag3PO4/g-C3N4. During the photocatalytic reactions, Ag0 could be formed through the reduction of Ag3PO4 by the photoexcited electron that is deposited on the Ag3PO4 surface. In this scheme, the Ag0 nanoparticles could act as a charge transmission bridge to form the Ag3PO4/Ag/g-C3N4 Z-scheme system, which is shown in Figure 13b. The Ag had a relatively low Fermi level (EF = 0.4 V vs. NHE), which could serve as the electron acceptor for the photoexcited electrons. The photogenerated electrons in the Ag3PO4 could be rapidly transferred to the g-C3N4 through Ag0 nanoparticles that promote the effective separation of photogenerated electron-hole pairs and suppress the Ag3PO4 reduction. Simultaneously, the holes in the VB of the g-C3N4 could move to the Ag0 nanoparticles and combine with the electrons. The type of charge transmission could efficiently enhance the electron-hole pairs separation and enable the electrons and holes to remain on the CB of the g-C3N4 and VB of Ag3PO4, respectively. On the other hand, the surface plasmon resonance (SPR) endowed in the Ag nanoparticles could effectively promote the visible-light absorption ability of the Ag3PO4/g-C3N4 composite. Furthermore, under this scheme, as the CB of g-C3N4 (−1.30 eV, vs. NHE) was more negative than the potentials of O2/·O2− (−0.33 eV, vs. NHE), and the VB of Ag3PO4 (2.90 eV, vs. NHE) was more positive than the potentials of the ·OH/OH− (2.38 eV, vs. NHE) and the ·OH/H2O (2.72 eV, vs. NHE), the photogenerated electrons on the CB of g-C3N4 could reduce the O2 to yield ·O2−. The enriched holes on the VB of Ag3PO4 could react with the OH− or H2O to form ·OH radicals, which is consistent with the trapping experimental results.

3. Experimental

3.1. Materials

All chemicals were used as received without any further purification. Melamine (C3H6N6), silver nitrate (AgNO3), and sodium dihydrogen phosphate (NaH2PO4) were purchased from Sinopharm Chemical Reagent Corp. (Beijing, China). Diclofenac sodium was purchased from Sigma Chemical Reagent Co. (St. Louis, MO, USA).

3.2. Preparation of the Photocatalysts

3.2.1. Synthesis of g-C3N4

Graphitic carbon nitride (g-C3N4) was synthesized according to previous reports in the literature [56]. Typically, 10 g of acid-treated melamine was put in a muffle furnace and heated at a ramping rate of 4 °C min−1 from room temperature to 550 °C, and then kept at 550 °C for 4 h.

3.2.2. Synthesis of Ag3PO4/g-C3N4 Composite

g-C3N4 nanosheets (0.1 g) were added to 100 mL of deionized water and ultrasonicated for 20 min. Different amounts of [Ag(NH3)2]+ solution were then added to the g-C3N4 mixture and magnetically stirred for 30 min. Finally, NaH2PO4 was added to the mixture solution under vigorous stirring. After 3 h, the product was collected by filtration, washed with deionized water, and dried in an oven at 70 °C for 10 h. The obtained samples were denoted as Ag3PO4/g-C3N4 (X), where X (10%, 20%, 30%, or 40%) represents the molar ratio of Ag3PO4 to g-C3N4 in Ag3PO4/g-C3N4. The Ag3PO4 sample was prepared following the same procedures, excluding the addition of g-C3N4.

3.3. Characterization of the As-Prepared Photocatalysts

The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray Diffractometer (Billerica, MA, USA) equipped with Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range of 10° to 80°. The photocatalyst chemical structures were confirmed by a Nicolet 5700 Fourier transform-infrared spectrometer (Thermo Fisher Scientific, WI, USA) with a scanning range from 4000 to 400 cm−1 at room temperature. An observation by transmission electron microscope (TEM) was taken with a JEOL-JEM-2100 (JEOL, Akishima, Tokyo, Japan) operated at up to 200 kV. Observations from a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS) were taken using a Hitachi S4800 (Hitachi, Tokyo, Japan). Surface composition and chemical bonds were examined with a PHI 5000C ESCA X-ray photoelectron spectroscopy (XPS). UV-Vis diffuse reflectance spectra (UV-Vis DRS) were obtained on an UV-Vis spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan). BaSO4 was used as a reflectance standard in the UV-Vis diffuse reflectance experiments. Photoluminescence spectroscopy (PL) of the samples was obtained on a Horiba FL-3000 spectrofluorometer with a 368 nm excitation wavelength. The scanning speed was 1200 nm min−1, and the photomultipliers’ voltage was 700 V. Electron spin resonance (ESR) of the spin-trapped paramagnetic species with DMPO were tested by the Bruker Elexsys E500 spectrometer (Billerica, MA, USA).

3.4. Experimental Procedures

The photocatalyst performance was evaluated through DCF photocatalytic degradation under visible light. The source of radiation was a 300 W Xenon lamp (BL-GHX-Xe-300, Shanghai, China) with a suitable optical cut-off filter (λ ≥ 400 nm), and the temperature of the solution was maintained at 25 °C with a jacketed cooler. In a typical experiment, 10 mg of the photocatalyst was suspended in 100 mL of DCF aqueous solution (1 mg L−1) in a quartz reactor. The photocatalytic tests were performed with magnetic stirring. During the reaction, the aqueous samples were collected at given time intervals and filtrated through 0.22 μm polyether sulfone membranes. The DCF concentration was measured by a high-performance liquid chromatography (HPCL) system (Agilent 1200, Santa Clara, CA, USA). The analytical column was a C18 column (Gemini 5 μm, 150 mm × 4.6 mm, from Phenomenex, Torrance, CA, USA). The mobile phase consisted of a mixture of 80% methanol and 20% water at a constant flow rate of 1.0 mL min−1, and the detection wavelength was set at 276 nm.
To test the stability and reusability of the photocatalyst, cyclic experiments of DCF photodegradation were conducted. After each cycle, the photocatalyst was filtered through 0.22 μm polyether sulfone membranes and washed several times with deionized water for the subsequent cycle.

4. Conclusions

In summary, a series of novel visible-light-driven Ag3PO4/g-C3N4 (X) composites were prepared by a deposition-precipitation method. The as-prepared photocatalyst exhibited excellent photocatalysis for DCF degradation, which was much higher than that of pure g-C3N4 and Ag3PO4. The DCF photodegradation under visible light irradiation followed first-order kinetics; when the molar ratio of Ag3PO4 and g-C3N4 was 30%, the composites showed optimal photocatalytic performance, and the reaction rate constant on Ag3PO4/g-C3N4 (30%) was 0.453 min−1, which was almost 17.9 and 6.4-time as much as that of pure g-C3N4 and Ag3PO4, respectively. The enhanced photocatalytic activity of the Ag3PO4/g-C3N4 could be beneficial for the heterojunction formed between g-C3N4 and Ag3PO4, as well as to the SPR effects of formed Ag0 nanoparticles during the reaction. Based on the energy band position and the trapping experimental results, it was found that the electron transfer style of the Ag3PO4/g-C3N4 followed the Z-scheme mechanism. This study provides a new idea in the design of heterojunction photocatalysts for the photodegradation of NSAIDs in aquatic environments.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/8/2/45/s1, Figure S1: EDS analysis of Ag3PO4/g-C3N4 (30%), Figure S2: SEM images of (a) Ag3PO4, (c) Ag3PO4/g-C3N4 (20%) and (e) Ag3PO4/g-C3N4 (40%). and TEM images (b) Ag3PO4, (d) Ag3PO4/g-C3N4 (20%) and (f) Ag3PO4/g-C3N4 (40%), Figure S3: SEM and TEM images of Ag3PO4/g-C3N4 (30%) after the reaction, Table S1: The concentration of Ag+ after each re-cycle test.

Acknowledgments

This study was supported by State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRY16001), and Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07501001), China. We are grateful to the Key Laboratory of Yangtze River Water Environment, Ministry of Education, for providing facilities for the experiments and the analysis.

Author Contributions

For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Wei Zhang. Jun Shi and Huiping Deng conceived and designed the experiments; Wei Zhang performed the experiments; Wei Zhang and Jun Shi analyzed the data; Li Zhou contributed reagents/materials/analysis tools; Wei Zhang wrote the paper.” Authorship must be limited to those who have contributed substantially to the work reported.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) composites.
Figure 1. X-ray diffraction patterns of g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) composites.
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Figure 2. FTIR spectra of the as-prepared g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) composites.
Figure 2. FTIR spectra of the as-prepared g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) composites.
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Figure 3. (a) SEM image of Ag3PO4/g-C3N4 (30%); (b) Element mapping of the Ag3PO4/g-C3N4 (30%); (c) TEM image of g-C3N4; and (d) TEM of Ag3PO4/g-C3N4 (30%).
Figure 3. (a) SEM image of Ag3PO4/g-C3N4 (30%); (b) Element mapping of the Ag3PO4/g-C3N4 (30%); (c) TEM image of g-C3N4; and (d) TEM of Ag3PO4/g-C3N4 (30%).
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Figure 4. XPS spectrum of the Ag3PO4 and Ag3PO4/g-C3N4: (a) survey spectrum; (b) C 1s; (c) Ag 3d; and (d) P 2p.
Figure 4. XPS spectrum of the Ag3PO4 and Ag3PO4/g-C3N4: (a) survey spectrum; (b) C 1s; (c) Ag 3d; and (d) P 2p.
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Figure 5. (a) UV-Vis diffused reflectance spectra of the as-prepared photocatalyst; (b) Plots of (Ahν)1/2 versus energy (hν) for the g-C3N4 and Ag3PO4/g-C3N4 (30%).
Figure 5. (a) UV-Vis diffused reflectance spectra of the as-prepared photocatalyst; (b) Plots of (Ahν)1/2 versus energy (hν) for the g-C3N4 and Ag3PO4/g-C3N4 (30%).
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Figure 6. PL spectra of g-C3N4 and Ag3PO4/g-C3N4 (X).
Figure 6. PL spectra of g-C3N4 and Ag3PO4/g-C3N4 (X).
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Figure 7. (a) DCF photodegradation with different photocatalysts, [DCF] = 1 mg L−1, [photocatalyst] = 0.1 g L−1; (b) The kinetic constants of g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) for the DCF photodegradation.
Figure 7. (a) DCF photodegradation with different photocatalysts, [DCF] = 1 mg L−1, [photocatalyst] = 0.1 g L−1; (b) The kinetic constants of g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 (X) for the DCF photodegradation.
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Figure 8. The photodegradation of DCF and the TOC removal during the reaction. [DCF] = 1 mg L−1, [Ag3PO4/g-C3N4 (30%)] = 0.1 g L−1.
Figure 8. The photodegradation of DCF and the TOC removal during the reaction. [DCF] = 1 mg L−1, [Ag3PO4/g-C3N4 (30%)] = 0.1 g L−1.
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Figure 9. Circling runs for the DCF photodegradation in the presence of Ag3PO4/g-C3N4 (30%) under visible-light irradiation. [DCF] = 1 mg L−1, [Ag3PO4/g-C3N4 (30%)] = 0.1 g L−1.
Figure 9. Circling runs for the DCF photodegradation in the presence of Ag3PO4/g-C3N4 (30%) under visible-light irradiation. [DCF] = 1 mg L−1, [Ag3PO4/g-C3N4 (30%)] = 0.1 g L−1.
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Figure 10. XRD patterns of Ag3PO4/g-C3N4 (30%) before and after the photocatalytic reaction.
Figure 10. XRD patterns of Ag3PO4/g-C3N4 (30%) before and after the photocatalytic reaction.
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Figure 11. The effects of different scavengers for the DCF photodegradation with Ag3PO4/g-C3N4 (30%) under visible-light irradiation.
Figure 11. The effects of different scavengers for the DCF photodegradation with Ag3PO4/g-C3N4 (30%) under visible-light irradiation.
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Figure 12. ESR signals of (a) DMPO-·O2− in methanol dispersion; and (b) DMPO-·OH in aqueous dispersion with visible-light irradiation.
Figure 12. ESR signals of (a) DMPO-·O2− in methanol dispersion; and (b) DMPO-·OH in aqueous dispersion with visible-light irradiation.
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Figure 13. Schematic diagram of the photoexcited electron-hole transfer process (a) Double-transfer mechanism; and (b) Z-scheme mechanism.
Figure 13. Schematic diagram of the photoexcited electron-hole transfer process (a) Double-transfer mechanism; and (b) Z-scheme mechanism.
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MDPI and ACS Style

Zhang, W.; Zhou, L.; Shi, J.; Deng, H. Synthesis of Ag3PO4/G-C3N4 Composite with Enhanced Photocatalytic Performance for the Photodegradation of Diclofenac under Visible Light Irradiation. Catalysts 2018, 8, 45. https://doi.org/10.3390/catal8020045

AMA Style

Zhang W, Zhou L, Shi J, Deng H. Synthesis of Ag3PO4/G-C3N4 Composite with Enhanced Photocatalytic Performance for the Photodegradation of Diclofenac under Visible Light Irradiation. Catalysts. 2018; 8(2):45. https://doi.org/10.3390/catal8020045

Chicago/Turabian Style

Zhang, Wei, Li Zhou, Jun Shi, and Huiping Deng. 2018. "Synthesis of Ag3PO4/G-C3N4 Composite with Enhanced Photocatalytic Performance for the Photodegradation of Diclofenac under Visible Light Irradiation" Catalysts 8, no. 2: 45. https://doi.org/10.3390/catal8020045

APA Style

Zhang, W., Zhou, L., Shi, J., & Deng, H. (2018). Synthesis of Ag3PO4/G-C3N4 Composite with Enhanced Photocatalytic Performance for the Photodegradation of Diclofenac under Visible Light Irradiation. Catalysts, 8(2), 45. https://doi.org/10.3390/catal8020045

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