Improvement of Photocatalytic Performance for the g-C3N4/MoS2 Composite Used for Hypophosphite Oxidation

The synthesized g-C3N4/MoS2 composite was a high-efficiency photocatalytic for hypophosphite oxidation. In this work, a stable and cheap g-C3N4 worked as the chelating agent and combined with the MoS2 materials. The structures of the fabricated gC3N4/MoS2 photocatalyst were characterized by some methods including X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS). Moreover, the photocatalytic performances of various photocatalysts were measured by analyzing the oxidation efficiency of hypophosphite under visible light irradiation and the oxidation efficiency of hypophosphite using the g-C3N4/MoS2 photocatalyst which was 93.45%. According to the results, the g-C3N4/MoS2 composite showed a promising photocatalytic performance for hypophosphite oxidation. The improved photocatalytic performance for hypophosphite oxidation was due to the effective charge separation analyzed by the photoluminescence (PL) emission spectra. The transient photocurrent response measurement indicated that the g-C3N4/MoS2 composites (2.5μAcm ) were 10 times improved photocurrent intensity and 2 times improved photocurrent intensity comparing with the pure g-C3N4 (0.25 μAcm) and MoS2 (1.25μAcm ), respectively. The photocatalytic mechanism of hypophosphite oxidation was analyzed by adding some scavengers, and the recycle experiments indicated that the g-C3N4/MoS2 composite had a good stability.


Introduction
Sodium hypophosphite is the most common reducing agent during the electroless plating, generating high concentration of hypophosphite wastewater [1,2]. The main components of plating wastewater are hypophosphite and phosphite, which should be further treatment before discharging into the river or lake avoiding the problem of eutrophication. Due to the high solubility of hypophosphite and phosphite, it is difficult to remove the contaminants by adding Ca 2+ and Fe 3+ ions to generate sediment following by precipitation [3,4]. Therefore, hypophosphite and phosphite should be oxidized to phosphate, which is easy to recover with the precipitation method. At the same time, the structure of hypophosphite and phosphite is relatively stable, and it is difficult to oxidize them by ordinary oxidation technology [5], so the technology with strong oxidation ability is needed to solve the problem of hypophosphite oxidation.
Semiconductor photocatalytic technology is a new kind of environmental pollutant reduction technology [6]. The photocatalytic oxidation treatment has the characteristics of easy to handle, no secondary pollution, and a broad potential application [7]. In particular, TiO 2 , as a photocatalytic material, can effectively catalyze the degradation of pollutants in water. It has the advantages of chemical stability, high catalytic activity, good harmlessness to human body, low reaction conditions, and mild selectivity, which has been widely used in the treatment of pollutants that are difficult to be degraded [8]. However, a large band gap (3.2 eV) of TiO 2 indicates that it can only absorb ultraviolet light (only about 3-5% of total sunlight), which largely inhabits its wide application [9,10]. So, it is necessary to synthesize a novel photocatalyst that can be responded under visible light irradiation.
Recently, a two-dimensional (2D) conjugated polymer, metal-free graphitic carbon nitride (g-C 3 N 4 ) has caused much attention due to its suitable band gap of 2.7 eV, which can be responded under visible light irradiation [11,12]. In addition, the excellent structural stability of g-C 3 N 4 is widely used in photocatalytic conversion of solar energy and purification of environmental pollutants [13]. Nevertheless, there are some shortcomings for g-C 3 N 4 as a photocatalyst, such as low utilization rate of light and high recombination rate of photon-generated carrier [14,15]. Some methods such as structure modification [16], semiconductors coupling [17], elements, and molecular doping [18,19] were used to modify the g-C 3 N 4 materials to improve the photocatalytic performance of g-C 3 N 4 .
Molybdenum disulfide (MoS 2 ), a 2D metal sulfide material, has the properties of excellent stability and low band gap of 1.2-1.9 eV, which can be easily responded under visible light irradiation and worked as photocatalyst [20,21]. The valence band electrons can detour into the conduction band under visible light irradiation and leave holes, thus it will generate electronhole pairs [22]. Due to the small band gap of MoS 2 , it can be used as a catalyst and cocatalyst, especially doped on some materials, and its photocatalytic performance has been greatly improved [23], which has been widely used in photocatalytic hydrogen evolution and the degradation of organic pollutants [24][25][26]. The photogenerated electrons of semiconductors could transfer through these noble metals rapidly, and the lifetime of these electrons and holes was prolonged for the noble metal-semiconductor heterostructure materials. Therefore, the prepared g-C 3 N 4 /MoS 2 composite photocatalyst is beneficial to achieve relatively large specific surface area with abundant active sites for the oxidation reactions.
Herein, a g-C 3 N 4 /MoS 2 photocatalyst was prepared and shown an improved photocatalytic performance for hypophosphite oxidation under visible light irradiation. The structures of the g-C 3 N 4 /MoS 2 photocatalyst was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS). Moreover, the separation mechanism of generated electron-hole pairs was investigated by photoluminescence (PL) emission spectra, and the photocurrent intensity was analyzed by photoluminescence emission spectra. The reactive species generated during the oxidation process were proved by the quenching experiment. The g-C 3 N 4 /MoS 2 composite was stable after recycle experiments.

Preparation of g-C 3 N 4 .
The g-C 3 N 4 was prepared as follows: added 5 g melamine into a corundum crucible and then heated at 550°C in a muffle furnace for 2 h to generate a raw g-C 3 N 4 . Later, the raw g-C 3 N 4 was dispersed into 50 mL anhydrous methanol and then stirred return for 3 h under 68~70°C. Later, the mixture was dried at 60°C for 12 h under vacuum drying. Finally, cooled down the mixture and grinded with agate mortars, then the yellow g-C 3 N 4 photocatalyst was synthesized [27].
2.3. Synthesis of g-C 3 N 4 /MoS 2 Composite. The g-C 3 N 4 /MoS 2 heterojunction was synthesized through the hydrothermal method and shown the following: dissolved 70 mg Na 2 MoO 4 ·2H 2 O and 140 mg C 2 H 5 NS into 20 mL deionized water to form a clear solution. Then, added 20 mg g-C 3 N 4 into the above solution and ultrasound for 45 minutes to generate suspension solution. After that, added 50 ml suspension solution into the teflon reactor and continuously heated at 220°C for 24 h in the drying oven. When the reaction was finished, washed the product with deionized water and anhydrous ethanol for 3 times and centrifugal separation. Finally, the product was heated at 60°C in a vacuum drying oven for 24 h [28].
2.4. Analysis. The concentration of hypophosphite was analyzed by ion the chromatography method [29]. The surface morphology of the samples was analyzed by scanning electron microscopy (SEM, Quanta FEG 250). The phase structure of the sample was analyzed by X-ray diffraction (XRD, bruker-d8 Advanc type). The surface properties and chemical states of the sample were analyzed by the X-ray  Journal of Nanomaterials photoelectron spectra (XPS, ULVAC-PHI, INC). The specific surface area and pore size were calculated based on the N 2 physisorption isotherms. At the same time, the photoluminescence spectra were measured with a F-4500 fluorescence (PL) spectrometer, and the photocurrents were measured on a CHI 660B electrochemical system.

Analysis of Photocatalytic
Performance. The photocatalytic performance of the generated photocatalyst was evaluated by the oxidation efficiency of hypophosphite under visible light irradiation. During the oxidation process, the light source (35 W) with a 420 nm UV-cutoff filter was placed 12 cm away from the surface of reaction solution. For each photocatalytic experiment, 5 mg photocatalyst was added into the hypophosphite solution (50 ml, 50 mg L −1 ). Before irradiation reaction, the solution was continuously stirred in the dark for 2 h until the adsorption saturation was reached. A small amount of hypophosphite solution was measured every 15 min, and the change of the hypophosphite concentration solution was measured to evaluate the photocatalytic perfor-mance of the prepared photocatalyst. The oxidation efficiency of hypophosphite was measured as follows: where C 0 was the initial concentration of hypophosphite (mg L −1 ); C t was the concentration of hypophosphite after irradiation for a certain time t (min).

Analysis of Structure and Morphology for Different
Materials. XRD patterns of the synthesized photocatalyst were analyzed to characterize the crystalline phases and were shown in Figure 1. Two characteristics peaks were observed at 13.2°and 27.6°for pure g-C 3 N 4 photocatalysts. The characteristics peaks observed at 13.2°and 27.6°were indexed as the (110) plane corresponding to the in-plane structure, and the diffraction peak at 27.6°was corresponded to  Journal of Nanomaterials interlayer stacking of the conjugated aromatic systems [30]. In addition, two characteristics peaks at 32.7°and 56.7°were observed, which were contributed to the (100) and (110) crystal planes of MoS 2 (JCPDS No. 37-1492) [31]. According to the results, the formation of g-C 3 N 4 /MoS 2 photocatalysts was successfully synthesized.
The microstructure of the different materials was shown in Figure 2, layered g-C 3 N 4 showed many thin nanosheets with a porous structure (Figure 2(a)), and the pure MoS 2 particles showed a flower-like nanostructure with thin nanosheets avoiding the disordered stacking of MoS 2 layers (Figure 2(b)). Moreover, the g-C 3 N 4 /MoS 2 photocatalyst was mainly composed of rhabditiform crystals, and the shape of the synthesized g-C 3 N 4 /MoS 2 photocatalyst was relatively uniform (Figure 2(c)).
To identify the elements and interaction of as-prepared samples, the XPS spectra of g-C 3 N 4 /MoS 2 were investigated. As shown in Figure 3(a), the high-resolution XPS spectrum of C 1 s exhibited two peaks at 284.8 and 288.2 eV in the g-C 3 N 4 /MoS 2 photocatalyst assigned to C-C and N=C(-N) 2 of g-C 3 N 4 , respectively [32]. In the N 1s XPS spectrum (Figure 3(b)), peaks at 398.3, 398.9 and 400.4 eV in the g-C 3 N 4 /MoS 2 photocatalyst were assigned to C-N, tertiary nitrogen N-C 3 , and C-N-H groups, respectively [33]. The characteristic peaks of 2p 1/2 and 2p 3/2 orbitals for S 2− were observed at 162.9 and 161.9 eV in the g-C 3 N 4 /MoS 2 photocatalyst, respectively (Figure 3(c)) [34]. The Mo 3d XPS spectrum of the g-C 3 N 4 /MoS 2 photocatalyst showed that the two peaks centered at 229.4 and 232.6 eV (Figure 3(d)) assigned to Mo 3d 5/2 and Mo 3d 3/2 , respectively, demonstrating that Mo was in the +4 valence state [35].
3.1.1. Surface Area and Pore Size Distribution. The nitrogen adsorption-desorption isotherms and pore size distribution of different materials were shown in Figure 4. The specific surface area was calculated to be 114.5, 55.4,, and 147.3 m 2 g -1 for MoS 2 , g-C 3 N 4, and g-C 3 N 4 /MoS 2 photocatalysts, respectively (Figure 4(a)). Generally, a catalyst with larger surface area could provide many active sites for adsorption and photodegradation towards organic pollutants, resulting in improving the photodecomposition performance. As a result, g-C 3 N 4 /MoS 2 photocatalysts with higher specific surface area were benefit for improving the photocatalytic oxidation of hypophosphite. Moreover, the pore size distribution of g-C 3 N 4 /MoS 2 was between 5 and 15 nm, and that of pure MoS 2 was mainly in the range of 10-25 nm, while no obvious mesopore structure was observed for g-C 3 N 4 (Figure 4(b)).   Journal of Nanomaterials

Analysis of Photocatalytic Activity for Different
Photocatalysts. The oxidation efficiency of hypophosphite for different photocatalysts was shown in Figure 5. The oxidation efficiency of hypophosphite for pure g-C 3 N 4 indicated the lowest photocatalytic performance (20.87%) due to the fast recombination of photo-generated electrons and holes, and the oxidation efficiency of hypophosphite for g-C 3 N 4 /MoS 2 photocatalyst was the highest (93.45%). The photogenerated electrons were transferred from g-C 3 N 4 to MoS 2 , which could efficiently improve the separate rate of photogenerated electrons and holes. The existence of heterostructure for g-C3N4/MoS2compositelimited the recombination of photogenerated electrons and holes. Therefore, the photocatalytic performance of the g-C 3 N 4 /MoS 2 photocatalyst was improved compared with the pure g-C 3 N 4 and MoS 2 photocatalyst.

Analysis of Photoluminescence Emission Spectra and Electrochemical
Properties. Enhanced photoactivity performance was ascribed to the fast separation of photogenerated electrons and holes, as confirmed by the photoluminescence technique and transient photocurrent response measurement [36]. According to PL results, the PL spectrum could directly reflect the separation efficiency of photogenerated electron-hole pairs, i.e., the higher intensity of the PL spectrum and the higher recombination rate of photogenerated electron-hole pairs [37,38]. The PL spectrum of the photocatalyst was shown in Figure 6(a), and all the photocatalysts exhibited a broad emission peak centered at around 460 nm. The PL intensity exhibited the highest value for pure g-C 3 N 4 photocatalyst, while the intensity had become weaker for the g-C 3 N 4 /MoS 2 composite photocatalyst, which was attributed to the improvement of the electron transport induced by quantum confinement effect. Moreover, for the g-C 3 N 4 /MoS 2 photocatalyst, the interaction for g-C 3 N 4 and MoS 2 in the redox potential, it caused the photogenerated electrons transfer from g-C 3 N 4 to MoS 2 , thus reducing the probability of its recombination with holes. In addition, comparing the pure MoS 2 , a distinct red shift was shown, indicating that the g-C 3 N 4 /MoS 2 photocatalysts were more efficient in light harvesting under visible light irradiation.

Journal of Nanomaterials
In addition, the separation of photoinduced carriers of the composites was clarified by the transient photocurrent response measurement [39]. To confirm superior photoinduced carriers in the composites of the g-C 3 N 4 /MoS 2 photocatalyst, the photocurrent response was evaluated in Na 2 SO 4 electrolyte [40]. As shown in Figure 6(b), when the g-C 3 N 4 /MoS 2 electrodes were irradiated under visible light, it showed rapid responsive photocurrents. Moreover, comparing with the pure g-C 3 N 4 (0.25 μA cm -2 ) and MoS 2 (1.25 μA cm -2 ), the g-C 3 N 4 /MoS 2 composites showed 10 times improved photocurrent intensity and 2 times improved photocurrent intensity, respectively. The photocurrent responses were repeatable during on/off cycles under visible light irradiation.

Analysis of Stability
Performance for the Prepared g-C 3 N 4 /MoS 2 Photocatalysts. The stability of the prepared g-C 3 N 4 /MoS 2 photocatalysts was an important consideration to evaluate the practical applications through four recycling experiments [41]. After each cycle, all the used photocatalysts were collected together, centrifuged, and washed with distilled water, then dried at 60°C overnight. As shown in Figure 7, the oxidation efficiency of hypophosphite in the recycling experiments was 93.45%, 92.43%, 89.89%, and 90.32%, respectively, which indicated that the structure of photocatalyst was stable during the oxidation of hypophosphite. The results of recycling experiments indicated that the prepared g-C 3 N 4 /MoS 2 photocatalyst had a strong binding force, which effectively reduced the dissolution of the bulk g-C 3 N 4 material during the photocatalytic process. Therefore, the g-C 3 N 4 /MoS 2 photocatalyst had a good stability in the oxidation of hypophosphite.

Photocatalytic Mechanism of Hypophosphite Oxidation.
To analyze the mechanism of photocatalytic oxidation of hypophosphite, the active oxygen species produced under the visible light irradiation were analyzed through the quenching experiment. According to some literature, isopropanol (IPA) worked as the radical quencher for ·OH radical, while the N 2 purging was used to reduce the superoxide O 2 ·radicals [42,43]. By the ways of adding different scavengers into reaction solutions to remove the corresponding reactive species, the functions of the corresponding reactive species generated in the photocatalytic process was related to the change of the photocatalytic oxidation efficiency of hypophosphite. As shown in Figure 8, the photocatalytic oxidation efficiency of hypophosphite was 93.45% without scavengers. When the IPA was added into reaction solution, and the photocatalytic oxidation efficiency of hypophosphite was decreased to 63%. When N 2 was blowing into reaction   Journal of Nanomaterials solution, the photocatalytic oxidation efficiency of hypophosphite was decreased to 46%. According to the results, it was clear that both ·OH and O 2 ·radicals were the major reactive species in the photocatalytic reaction system for hypophosphite oxidation. Accordingly, the ·OH and O 2 ·radials had a strong oxidation ability during the photocatalytic oxidation of hypophosphite. The mechanism of the photocatalytic oxidation of hypophosphite was shown as follows: Firstly, the semiconductor materials (g-C 3 N 4 /MoS 2 ) absorbed the visible light, and the electron-hole pairs were generated on the surface of photocatalyst. Then, hydroxyl radical was generated through the oxidation of hydroxyl ions (OH − ) with holes. In addition, the superoxide anions were also generated by the molecular reduction of O 2 , which may attribute to the presence of electrons on the surface of the photocatalyst. The reactive species generated in the photocatalytic system were responsible for hypophosphite oxidation. Furthermore, the photogenerated holes were also responsible for the direct oxidation of the hypophosphite.

Conclusion
In the photocatalyst system, the composite g-C 3 N 4 /MoS 2 indicated higher photocatalytic performance comparing with the pure g-C 3 N 4 and MoS 2 photocatalyst. The oxidation efficiency of hypophosphite for the g-C 3 N 4 /MoS 2 photocatalyst was 93.45%, while the oxidation efficiency of hypophosphite for g-C 3 N 4 and MoS 2 were 20.78% and 37.87%, respectively. The mechanism of the improved photocatalytic performance of the g-C 3 N 4 /MoS 2 photocatalyst for hypophosphite oxidation was analyzed by the photoluminescence technique and transient photocurrent response measurement. The recombination rate of photogenerated electronhole pairs was reduced, which improved the photocatalytic activity. Moreover, the generated active species were responsible for hypophosphite oxidation analyzed by the quenching experiment. OH and O 2 ·radials were responsible for the oxidation of hypophosphite. The results of recycling experi-ments indicated that the g-C 3 N 4 /MoS 2 photocatalyst had a good stability for hypophosphite oxidation. Therefore, the g-C 3 N 4 /MoS 2 photocatalyst was an efficient and promising materials for the application of hypophosphite oxidation under visible light irradiation.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
There are no conflicts to declare.