PES-Ag3PO4/g-C3N4 Mixed Matrix Film Photocatalyst for Degradation of Methyl Orange Dye

In the present study, we explored the effectiveness of PES-Ag3PO4/g-C3N4 film photocatalyst in degrading methyl orange dye under visible light irradiation. The PES-Ag3PO4/g-C3N4 film photocatalyst was prepared via a non-solvent-induced phase inversion process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser scanning microscopy (LSM), X-ray photoelectron spectra (XPS), UV-diffuse reflectance (DRS), and water contact angle. The incorporation of the Ag3PO4/g-C3N4 composite into the PES matrix improved the pristine PES film’s hydrophilicity, as evidenced by the reduction of water contact angle from 79.03° to 54.33° for a film containing 15 wt % of Ag3PO4/g-C3N4 composite. The film’s photoactivity showed that 13 wt % was the best loading of Ag3PO4/g-C3N4 composite, and the degradation performance was maintained up to three cycles. The •O2− and h+ were the predominant species responsible for the methyl orange degradation.


Introduction
Textile industries wastewater contains many types of dyes and chemical substances that pose an environmental challenge in wastewater disposal issues. Dye substances are toxic toward microorganisms, and their intense color blocks sunlight from penetrating the water, which causes severe problems for the aquatic ecosystem. Azo dyes, the largest group of synthetic colorants, are usually used in the textile industry. These dyes possess high stability against light, temperature, chemicals, and microbial attack, making conventional wastewater treatment less efficient. Moreover, the use of traditional physical and chemical methods produces secondary pollutants [1].
Advanced oxidation processes (AOPs) are the technologies that generally use hydroxyl radicals, the ultimate oxidant for remediating organic contaminants in wastewater by converting recalcitrant pollutants into biodegradable compounds. The efficacy of AOPs depends on the generation of reactive free radicals, mainly the hydroxyl radicals. The most favorable AOPs in wastewater treatment include Fenton oxidation [2], catalytic ozonation [3], and semiconductor-based heterogeneous photocatalysis.
In the past decades, heterogenous photocatalysis using transition metal oxides as catalysts stood out as a promising candidate in treatment methods because of its low cost and environmentally benign properties. Photocatalysis could mineralize organic compounds directly under solar irradiation and under mild conditions of temperature and pressure. However, the photocatalysts available to date are generally limited by either low photocatalytic efficiency in the visible light range or insufficient charge separation ability [4].
Recently, much effort is focused on the exploration of visible light active semiconductor photocatalysts. Silver phosphate, Ag 3 PO 4 , has been reported to possess superior photooxidative capabilities for the degradation of organic dyes under visible light irradiation. However, self-photo corrosion is the main limitation of Ag 3 PO 4 , which restricts its application for only one cycle. Coupling Ag 3 PO 4 with graphitic carbon nitride (g-C 3 N 4 ) is one of the promising techniques to suppress photo-corrosion and effectively enhance photocatalytic performance [5][6][7]. g-C 3 N 4 is known as a metal-free semiconductor (bandgap energy is 2.7 eV) with a π-conjugated structure and possesses high standard reduction potential (−1.15 eV vs. normal hydrogen electrode (NHE)) to produce more oxygen oxidation species (e.g., hydroxyl radical) [5]. The excellent photocatalytic activity of Ag 3 PO 4 /g-C 3 N 4 composite is ascribed to the efficient electron-hole separation [8].
Lately, PES gains more attention due to its resistance toward UV irradiation and hydroxyl radicals produced in the photocatalytic reaction, thus making it a suitable support material for photocatalysts [21][22][23]. However, most of the studies focused on UV active photocatalyst incorporated into PES matrix as film photocatalyst. To the best of our knowledge, incorporating visible light photocatalyst into PES has not been studied, mainly in the photocatalysis process. Herein, this study focuses on using immobilized Ag 3 PO 4 /g-C 3 N 4 -PES photocatalyst film in the degradation of methyl orange dye. The effects of operational parameters include composite loadings, initial pollutant concentration, initial pH, and film thickness, were investigated.

Materials
Polyethersulfone (PES Ultrason E6020P with M w = 75,000 g/mol) was purchased from Solvay Specialty Polymers, (Jurong Island, Singapore). Thiourea (CH 4 N 2 S) was purchased from HmbG (Kuala Lumpur, Malaysia). N-methyl-2-pyrolidone (NMP) and silver nitrate (AgNO 3 ) were purchased from Merck, di-sodium hydrogen phosphate (Na 2 HPO 4 ) was purchased from Sigma Aldrich (Petaling Jaya, Malaysia), and methyl orange (MO) was purchased from Bendosen Laboratory Chemicals (Kuala Lumpur, Malaysia). Distilled water was used throughout the study. All chemicals are of analytical grade and were used without any further purification.

Ag 3 PO 4 /g-C 3 N 4 Photocatalyst Preparation
In a typical synthesis of graphitic carbon nitride (g-C 3 N 4 ), 5 g of thiourea was heated in a muffle furnace at 500 • C for 2 h. The resulting yellow powder was washed via sonication in distilled water for 2 h and oven-dried at 60 • C overnight. The Ag 3 PO 4 /g-C 3 N 4 composite photocatalysts were fabricated by the facile co-precipitation method. Generally, 100 mL of 0.6 M of AgNO 3 was added into 100 mL of distilled water, containing 0.1 g of dispersed g-C 3 N 4 powder, and stirred for 1 h. Then, 100 mL of 0.25 M Na 2 HPO 4 was added dropwise into the suspended solution and stirred for 2 h. The obtained product was collected and washed with distilled water and ethanol before being dried overnight at 60 • C.

PES/Ag 3 PO 4 -g-C 3 N 4 (P-AgC) Composite Film Preparation
P-AgC was prepared by a previously reported methodology [14]. A certain amount of Ag 3 PO 4 /g-C 3 N 4 powder was added into the clear PES casting solution, which was prepared by dissolving a fixed amount of PES flakes in the organic solvent (NMP) and stirred at 60 • C for 24 h to uniformly dispersed the Ag 3 PO 4 /g-C 3 N 4 in the PES solution. The compositions of the prepared film are tabulated in Table 1. The solution mixture was cast on a glass plate (10 cm × 8 cm) using a casting knife with a gap setting of 100 µm. The glass plate was immersed in a nonsolvent media (distilled water) and stored in the dark for a day for the solvent-nonsolvent demixing process and separated the film from the glass. Lastly, the resulting films were air-dried at room temperature for 24 h and ready to be used in photocatalytic studies.

PES/Ag 3 PO 4 -g-C 3 N 4 (P-AgC) Composite Film Characterization
The crystallinity and phase formation of the PES and PES-Ag 3 PO 4 /g-C 3 N 4 film photocatalysts were analyzed using an X-ray diffractometer (PHILIPS PW 3040/60, Almelo, Netherlands) with CuKα radiation (λ = 0.154 nm, 30 kV, 30 mA) in the scan range of 2θ = 10 • to 80 • with a 2 • /min of scanning speed. The surface film's morphology was observed using a scanning electron microscope equipped with an electron dispersive X-ray analyzer (Fei Nova Nanosem 230, Eindhoven, Holland). Meanwhile, the cross-section film morphology was visualized by a scanning electron microscope equipped with an electron dispersive X-ray analyzer (SEM-EDX, HITACHI TM3000, Tokyo, Japan. A 3D measuring laser microscope (OLS5000 OLYMPUS, Hamburg, Germany) was used to capture the film's surface roughness. The X-ray photoelectron spectra (XPS) of the prepared samples were recorded using a Kratos Analytical Axis Ultra DLD photoelectron spectrometer (Manchester, UK) using an Al Kα radiation monochromatic source. The water contact angle of the film was analyzed by a water surface analysis system (VCA 3000S, Billerica, MA, USA).

Photocatalytic Studies
The photocatalytic activities of the photocatalyst were evaluated by degradation of methyl orange (MO), a model pollutant, under visible light irradiation The photodegradation was conducted in a 0.5 L glass photoreactor, as shown in Figure 1. Two pieces of P-AgC film photocatalyst (10 cm × 8 cm) were placed around the photoreactor. A known concentration of MO solution (0.3 L) was introduced into the photoreactor. Air was bubbled into the solution to ensure continuous oxygen supply. The film was put under dark condition for 20 min to achieve adsorption-desorption equilibrium before being irradiated by 23 W (fluorescent lamp) for 180 min. Then, 5 mL of sample was withdrawn from the bulk solution at 30 min time intervals, and the residual concentration of the MO was measured using a UV-vis spectrophotometer (Perkin Elmer Lambda 35, Waltham, USA) at λ max = 465.4 nm. The degradation efficiency and the amount of MO degraded were determined using Equations (1) and (2), respectively.
where C o is the concentration of methyl orange (MO) before irradiation and C t is the concentration of MO at 't' time. Chemical oxygen demand (COD) analysis was performed by digesting 2 mL of MO in a fixed amount of the oxidant reagent (Lovibond) at 150 • C for 1 h by using Thermoreactor RD125 (Lovibond). A colorimeter (DR/890) was used to read the COD concentration. The COD percentage was calculated according to the equation as follows: where COD o and COD t are the COD concentration initially and at a time "t", respectively.

Characterization of P-AgC Composite Films
The X-ray diffraction patterns of net PES, powder Ag 3 PO 4 , g-C 3 N 4 , and selected P-AgC film photocatalysts are depicted in Figure 2. No peak was observed for bare PES, indicating the amorphous nature of the polymer matrix. The addition of Ag 3 PO 4 /g-C 3 N 4 into the PES matrix contributed to the appearance of several Ag 3 PO 4 peaks that was indexed as body-centred cubic phase (JCPDS No. 060505), and no other impurities were observed, indicating that the samples were composed of pure phase Ag 3 PO 4 . No diffraction pattern for g-C 3 N 4 was observed due to the small amount of usage. It can be concluded that there were no Ag particles present in the composite film. The morphology of the PES-Ag 3 PO 4 /g-C 3 N 4 composite film photocatalyst was observed and depicted in Figure 3. The cross-sectional SEM images well revealed that the PES is made up of a top dense skin layer and finger-like void structures at the bottom layer, which represents asymmetric membranes characteristics. As a result of Ag 3 PO 4 /g-C 3 N 4 addition, the uniform finger-like microstructures of PES became elongated and non-uniform, with bottle-neck microvoids, appeared at the bottom layer. The presence of the Ag 3 PO 4 /g-C 3 N 4 composite might have enhanced the casting solution instability and accelerated the demixing process, which facilitated the development of non-uniform microvoids in the composite film [24,25]. The surface morphology and surface roughness of the composite films are evaluated based on FESEM and LSM images. The smooth surface of the neat PES film becomes rougher with the increased Ag 3 PO 4 /g-C 3 N 4 content. The increase in the average surface roughness may be due to the Ag 3 PO 4 /g-C 3 N 4 composite aggregation, as shown by the LSM image ( Figure 4). The surface hydrophilicity of the films was evaluated by contact angle analysis ( Figure 5). By increasing the Ag 3 PO 4 /g-C 3 N 4 composite content in PES membrane, the water contact angle reduced compared to neat PES, indicating that the hydrophilicity of the composite film had improved. The enhanced hydrophilicity can be due to Ag high affinity toward the water and the existence of hydrophilic groups such as N-H in g-C 3 N 4 [24,26]. Figure 6 shows the XPS spectra of the as-prepared Ag 3 PO 4 /g-C 3 N 4 composite, which reveals the chemical compositions and chemical states of the samples. From the XPS survey spectra in Figure 6a, the elements of Ag, P, C, N, and O were detected. Figure 6b shows the high-resolution Ag 3d spectrum and two deconvoluted peaks that ascribe to Ag + in Ag 3 PO 4 were seen at 373.3 eV and 367.8 eV corresponding to Ag 3d 3/2 and Ag 3d 5/2 , respectively [5]. The result confirmed that no Ag was formed during the composite preparation. In Figure 6c, the P 2p peak appeared at 132.6 eV, suggesting that the phosphorus in the sample existed in the pentavalent oxidation state (P 5+ ) [27]. It can be seen from the C 1s spectrum (Figure 6d) that the curve could be fitted into two peaks located at 284.6 eV, corresponding to the C-C bond with sp 2 orbital, and the peak located at 288.1 eV could be assigned to CN 3 bonds in the heterocycle ring of g-C 3 N 4 [6,28,29]. The N 1 S spectrum (Figure 6e    The optical properties of the Ag 3 PO 4 /g-C 3 N 4 composite photocatalyst were measured using UV-vis diffuse reflectant spectroscopy (DRS). As shown in Figure 7a, the absorption edge of pure Ag 3 PO 4 and g-C 3 N 4 were detected at approximately 440 nm and 550 nm, indicating that both the Ag 3 PO 4 and g-C 3 N 4 can absorb visible light energy. Meanwhile, the light absorption of the Ag 3 PO 4 /g-C 3 N 4 composite showed two edges and broad background in the visible light region, which means that the visible light absorption ability was improved. Based on the Kubelka-Munk function, the plots of (Ahv) 1/2 vs. hv for Ag 3 PO 4 , g-C 3 N 4, and Ag 3 PO 4 /g-C 3 N 4 composite are shown in Figure 7b. The indirect bandgap energies of the Ag 3 PO 4 and g-C 3 N 4 were 2.1 eV and 2.5 eV, respectively. For the Ag 3 PO 4 /g-C 3 N 4 composite, the bandgap for Ag 3 PO 4 maintained, while the bandgap for g-C 3 N 4 reduced to 2.3 eV significantly.

The Photocatalytic Activity of the P-AgC Film Photocatalysts
The photocatalytic activity of P-AgC film photocatalysts was evaluated by degrading methyl orange (MO) under visible light irradiation. Before the photocatalytic reaction, the sample was immersed in the MO solution for 30 min in dark to reach the adsorption-desorption equilibrium. Direct photolysis showed no degradation of MO in aqueous solution, which confirmed the stability of MO under visible light irradiation. The degradation efficiency and degradation rate are shown in Figure 8. Net PES did not show any photocatalytic properties. The degradation efficiency of P-AgC (13%) was 97%, which resulted in highest degradation efficiency and the rate of reaction among the samples tested (Table 2). It showed that the presence of carbon nitride enhanced the photocatalytic properties of Ag 3 PO 4 . The enhanced photodegradation efficiency could also be attributed to the increasing hydrophilicity of the photocatalyst film, as evidenced in Figure 5. The improved surface wettability of the film allows more contact between the MO and the Ag 3 PO 4 /g-C 3 N 4 photocatalyst, leading to a higher MO adsorption on the surface of the photocatalyst film ( Figure S1), hence higher photocatalytic efficiency. However, as the Ag 3 PO 4 /g-C 3 N 4 loadings increased up to 15 wt %, the degradation efficiency decreased. This phenomenon might be due to the agglomeration of Ag 3 PO 4 /g-C 3 N 4 particles in the PES matrix. The agglomeration resulted in lower active surface area of Ag 3 PO 4 /g-C 3 N 4 for photon absorption to promote photocatalyst activation. Thus, 13 wt % is the maximum amount of Ag 3 PO 4 /g-C 3 N 4 that could be immobilized onto the PES film to successfully achieve optimum photocatalytic activity.  The effect of MO initial concentration toward degradation efficiency is shown in Figure 9, and details are outlined in Table 3. It was found that the degradation efficiency decreased as the MO concentration increased due to the screening effect of MO dye that reduced the path length of the photon that arrived on the surface of the photocatalyst. At low MO concentration, the number of reactive radicals (•OH and •O 2 − ) is more than the number of MO molecules, increasing the chance of collision between the pollutant and the active site of catalyst. Hence, it increased the tendency of adsorption and oxidation of MO on the surface of catalyst [30]. As the MO concentration increased, the adsorption of substrate molecules on the photocatalyst surface suppressed the generation of reactive radicals. Practically, the reaction cannot be enhanced as the amount of active sites and the concentrations of the hydroxyl radicals are constant [31].  The effect of pH in the degradation efficiency of MO was studied by varying the pH of MO solution from 2 to 11 using 1 M of hydrochloric acid (HCl) and sodium hydroxide (NaOH), and the results are depicted in Figure 10a. It can be seen that the photodegradation favored acidic conditions as the degradation efficiency achieved 100% within 30 min under irradiation at pH 2. In acidic conditions, Ag 3 PO 4 particles dissolved in the solution to form Ag + ions. The presence of Cl − anion leads to the formation of AgCl. AgCl is known to possess excellent photocatalytic properties; hence, this AgCl formation hastens the rate of MO degradation [32][33][34]. However, as the initial pH increased up to 11, the degradation efficiency was depleted. The competitive adsorption between OH − and anionic MO onto the catalyst surface led to the reduction in degradation efficiency of MO in alkaline condition. The pH pzc of Ag 3 PO 4 is 6.8, which made its surface charge negative under basic condition [35]. Thus, strong repulsion existed between the anionic MO and negatively charged Ag 3 PO 4 surface, resulting in poor degradation activity. The effect of film thickness on the MO degradation is illustrated in Figure 10b. As the film thickness increased, the light penetration of the top layer of the film decreases, leading to a reduction in the activation of the Ag 3 PO 4 /g-C 3 N 4 catalyst in the membrane matrix; hence, it reduces the degradation efficiency of the composite film photocatalyst.
To ensure that the MO removal was via the photodegradation process, COD analysis was conducted. As depicted in Figure 11, the COD concentration decreased with increasing reaction time, confirming the degradation of the MO.

Reusability of P-AgC (13%)
For the reusability study, the photodegradation of MO (10 mg/L) by P-AgC (13%) film was tested at pH 6.2 for 3 h of irradiation. The catalyst was washed with deionized water and dried before being used in new MO solution for every cycle. After recycling four times, as shown in Figure 12, the degradation efficiency decreased sharply from 96.6% to 53.5%, which indicates that the P-AgC (13%) film possesses excellent photocatalytic stability up to three cycles. The high-resolution XPS spectrum of Ag 3d ( Figure S2) of the film after six experimental cycles showed dominant peaks of Ag 0 at binding energy 368.2 eV and 374.2 eV in P-AgC (13) in addition to the peaks attributed to the Ag + at 367.8 eV and 373.6 eV. Hence, the decreased in the activity may be due to the formation of Ag particles in the film.

Mechanism of Photodegradation of MO and Effect of Scavengers
The mechanism for photodegradation of MO was postulated based on the activation of Ag 3 PO 4 /g-C 3 N 4 composite photocatalyst by low energy visible light photons to generate electron-hole pairs (Figure 13a). Through a series of reactions between electrons and adsorbed oxygen and between holes and adsorbed water, OH radicals formed to eventually degrade MO molecules. The superior photocatalytic activity of P-AgC is mainly due to the performance of both Ag 3 PO 4 and g-C 3 N 4 to form electron-hole pairs for reactive species production in the solution. The conduction band (E CB ) and valence band (E VB ) potentials of the photocatalyst were calculated using Equations (4) and (5), respectively.
where E e is the energy of free electrons on the hydrogen scale (4.5 eV); E g is the bandgap energy, and χ is the electronegativity of semiconductor, which was calculated using the following equation: where a, b, and c are the number of atoms in the compounds. Based on the E g and χ values of 2.1 eV and 5.93 eV, the calculated E CB and E VB for Ag 3 PO 4 versus normal hydrogen electrode (NHE) are +0.38 eV and +2.48 eV, respectively [36,37]. Meanwhile, for the g-C 3 N 4 , the E g value is 2.65 eV and the χ values is 4.73 eV; hence, the estimated E CB and E VB were −1.09 eV and +1.55 eV, respectively, which agreed with previous research [38][39][40].
When the visible light irradiated the Ag 3 PO 4 /g-C 3 N 4 composites, the photons would be absorbed by Ag 3 PO 4 and g-C 3 N 4 , generating electron-hole pairs. The photogenerated electrons on the conduction band (CB) of g-C 3 N 4 react with adsorbed O 2 to form •O 2 − radicals, because CB of g-C 3 N 4 is more negative than the potential of O 2 /•O 2 − (E o (O 2 /•O 2 − ) = −0.33 eV/NHE) [41]. Since the CB edge potential of Ag 3 PO 4 (+0.38 eV) is more positive than the standard redox potential of O 2 /•O 2 − , Ag 3 PO 4 cannot reduce O 2 to •O 2 − . However, the electrons in the CB of Ag 3 PO 4 can be transferred to adsorbed oxygen molecules to produce H 2 O 2 , because the CB of Ag 3 PO 4 is more negative than E o (O 2 /H 2 O 2 = +0.682 eV vs. NHE). The produced H 2 O 2 molecules react with electrons to produce •OH radicals [37]. Through the z-scheme mechanism, the electrons can also be transferred to the valence band (VB) of the g-C 3 N 4 [42]. On the other hand, the holes in the valence band of Ag 3 PO 4 can oxidize the pollutant directly, since the VB potential of Ag 3 PO 4 is more positive than the E o (OH − /•OH = +2.4 eV).
To study the main reactive species involved in the photodegradation of MO, tertbutanol, p-benzoquinone, and sodium EDTA were used as a trapper for hydroxyl radicals (•OH), superoxide anion radicals (•O 2 − ), and holes (h + ), respectively, in the reaction system, and the results are shown in Figure 13b. When p-benzoquinone and sodium EDTA were used, the degradation efficiency declined to 23% and 10%, respectively. It can be concluded that •O 2 − and h + were the dominant radical species involved in the degradation of MO.

Conclusions
A novel PES-Ag 3 PO 4 /g-C 3 N 4 was successfully fabricated by the phase inversion technique. The incorporation of Ag 3 PO 4 /g-C 3 N 4 into the PES matrix widened the finger-like microvoid structure and formed a bottle-neck structure. The film hydrophilicity, surface roughness, and porosity were also enhanced significantly. The dye degradation experiment showed that 13 (wt %) is the optimum amount of the Ag 3 PO 4 /g-C 3 N 4 composite to be loaded into the PES matrix, which resulted in the highest photocatalytic activity. The PES-Ag 3 PO 4 /g-C 3 N 4 could be used up to three cycles before the performance was reduced to half. The •O 2 − and h + were the dominant species in the degradation of MO.