Towards the Enhancement in Photocatalytic Performance of Ag3PO4 Nanoparticles through Sulfate Doping and Anchoring on Electrospun Nanofibers

Present work reports the enhancement in photocatalytic performance of Ag3PO4 nanoparticles through sulfate doping and anchoring on Polyacrylonitrile (PAN)-electrospun nanofibers (SO42−-Ag3PO4/PAN-electrospun nanofibers) via electrospinning followed by ion-exchange reaction. Morphology, structure, chemical composition, and optical properties of the prepared sample were characterized using XRD, FESEM, FTIR, XPS, and DRS. The anchoring of SO42−-Ag3PO4 nanoparticles on the surface of PAN-electrospun nanofibers was evidenced by the change in color of the PAN nanofibers mat from white to yellow after ion-exchange reaction. FESEM analysis revealed the existence of numerous SO42−-Ag3PO4 nanoparticles on the surface of PAN nanofibers. Photocatalytic activity and stability of the prepared sample was tested for the degradation of Methylene blue (MB) and Rhodamine B (RhB) dyes under visible light irradiation for three continuous cycles. Experimental results showed enhanced photodegradation activity of SO42−-Ag3PO4/PAN-electrospun nanofibers compared to that of sulfate undoped sample (Ag3PO4/PAN-electrospun nanofibers). Doping of SO42− into Ag3PO4 crystal lattice could increase the photogenerated electron–hole separation capability, and PAN nanofibers served as support for nanoparticles to prevent from agglomeration.


Introduction
The breakthrough work carried out by Yi et al. [1] opened a new door to engineer and synthesize silver phosphate (Ag 3 PO 4 )-based photocatalysts with enhanced performance that can find potential applications in dye photodegradation, hydrogen evolution, and killing microbes [2][3][4][5]. Ag 3 PO 4 is a narrow band gap (2.36 eV) semiconductor material, which can generate reactive oxygen species (ROS) like OH • or O 2 • − from electron-hole pairs under visible light irradiation. Thus, generated ROS are responsible for its photocatalytic activities [6,7]. Application of Ag 3 PO 4 as a visible light-driven photocatalyst is limited due to poor chemical stability, when applied in absence of sacrificial agent [1,8]. Therefore, successive investigations have been carried out for designing and fabricating Ag 3 PO 4 -based photocatalysts to overcome this limitation and improve their performance. In this regard, various studies have been reported, such as fabricating composites [9][10][11][12][13], coupling with other semiconductor materials [14,15] and doping suitable ions [16][17][18]. Out of these, doping of suitable ions into the crystal lattice of semiconductor materials could be an alternate strategy to enhance their photocatalytic property. It is reported that semiconductor material doped with suitable ions could prevent the recombination of photogenerated electron-hole pairs and consequently increases its stability and photocatalytic performance [19]. On that note, various results regarding the enhanced photocatalytic performance of Ag 3 PO 4 via cations doping into its crystal lattice have been reported [20,21]. On the contrary, photocatalytic activities of Ag 3 PO 4 doped with suitable anions have not been reported frequently. Meanwhile, a recent report has demonstrated the enhanced photocatalytic activity of sulfur-doped Ag 3 PO 4 on the basis of hybrid density-functional calculation [22]; however, due to the strong P-O bond, doping of sulfur into Ag 3 PO 4 crystal lattice seems more difficult. Therefore, instead of sulfur, SO 4 2− might be a suitable anion as a dopant to replace PO 4 3− from Ag 3 PO 4 crystal lattice due to smaller radius of SO 4 2− (0.218 nm) compared to that of PO 4 3− (0.230 nm) [23,24]. On the other hand, use of photocatalyst nanoparticles in powder form creates a serious problem of agglomeration during photocatalysis, which leads to a reduction of surface area and ultimately, a decrease in photocatalytic performance [25].
In addition, the separation process of photocatalyst from solution becomes more difficult after use.
To deal with these difficulties and avoid the loss of photocatalyst, polymer-electrospun nanofibers are being widely used as supports for nanoparticles [26][27][28]. Due to its excellent characteristics, like environmental stability, easy processability, and low density, PAN polymer is being extensively used for the fabrication of nanofibers as flexible support for photocatalyst nanoparticles using simple and versatile electrospinning technique [29,30]. Hence to realize the synergistic effect of sulfate-doped Ag 3 PO 4 nanoparticles and PAN-electrospun nanofibers as support, our work is focused on the fabrication of SO 4 2− -Ag 3 PO 4 /PAN-electrospun nanofibers by combining electrospinning and ion-exchange reaction. Visible light photocatalytic activity of as fabricated sample was evaluated by observing photodegradation of MB and RhB dye solutions. Finally, we hope this visible-light-driven photocatalyst would be a promising candidate for the degradation of organic dyes from waste water to avoid negative effects to the dependent living ecology. To the best of our knowledge this type of work has not been reported so far.

Chemicals
The

Fabrication of Na 2 HPO 4 /PAN Nanofiber
First, fine powder of Na 2 HPO 4 (0.46 g) was dispersed in DMF (14 mL) and ultrasonicated for 1h. Then 1.5 g powder of PAN polymer was added to the above dispersion and magnetically stirred for 12h to prepare electrospinning solution. Electrospinning of the prepared solution was carried out by loading into a plastic syringe fitted with plastic micro-tip. The applied voltage and distance between needle tip to collector were set as 18 kV and 12 cm, respectively. The developing nanofibers were collected on rotating drum collector connected to DC motor. Thus obtained Na 2 HPO 4 /PAN-electrospun nanofibers were vacuum dried for 12 h at 70 • C.

Investigation of Photocatalytic Activity
To investigate the photocatalytic performance, as fabricated samples (pristine PAN, AP/PAN, and SAP/PAN) were utilized as visible-light-driven photocatalysts for the degradation of MB and RhB solutions at 10 ppm using a solar simulator having an internal xenon lamp (DYX300P, DYE TECH Co., Seoul, Korea) equipped with a UV cutoff filter. The experiments were carried out in a

Investigation of Photocatalytic Activity
To investigate the photocatalytic performance, as fabricated samples (pristine PAN, AP/PAN, and SAP/PAN) were utilized as visible-light-driven photocatalysts for the degradation of MB and RhB solutions at 10 ppm using a solar simulator having an internal xenon lamp (DYX300P, DYE TECH Co., Seoul, Korea) equipped with a UV cutoff filter. The experiments were carried out in a glass vial containing dye solution (50 mL) and photocatalyst (100 mg). Prior to irradiation, the suspension was magnetically stirred under dark conditions for 30 min to establish adsorption/desorption equilibrium. Afterward, visible light obtained from the 200-W xenon lamp was irradiated under continuous magnetic stirring. Aliquots were taken at regular time intervals (10 min) and the concentration of the dye solution was measured spectrophotometrically by recording the absorbance using a UV-vis spectrophotometer (HP 8453 UV-vis spectroscopy system, Hudson, MA, USA). The total organic carbon (TOC) content in residual solution was determined with a TOC analyzer (multi N/C 3100, Analytik Jena, Konrad-Zuse-Strasses 1 07745 Jena, Germany).

Results and Discussion
XRD analysis was applied to investigate the crystallinity and effect of sulfate doping into Ag 3 PO 4 crystal lattice. Figure  glass vial containing dye solution (50 mL) and photocatalyst (100 mg). Prior to irradiation, the suspension was magnetically stirred under dark conditions for 30 min to establish adsorption/desorption equilibrium. Afterward, visible light obtained from the 200-W xenon lamp was irradiated under continuous magnetic stirring. Aliquots were taken at regular time intervals (10 min) and the concentration of the dye solution was measured spectrophotometrically by recording the absorbance using a UV-vis spectrophotometer (HP 8453 UV-vis spectroscopy system, Hudson, MA, USA). The total organic carbon (TOC) content in residual solution was determined with a TOC analyzer (multi N/C 3100, Analytik Jena, Konrad-Zuse-Strasses 1 07745 Jena, Germany).

Results and Discussion
XRD analysis was applied to investigate the crystallinity and effect of sulfate doping into Ag3PO4 crystal lattice. Figure 1a displays XRD patterns of pristine PAN, AP/PAN, and SAP/PAN. A broad and noncrystalline peak at 2θ of 20-30° in all formulations was assigned to the (110) crystal plane of PAN polymer [29]. Besides, the diffraction peaks at 2θ of 20.          [32]. Moreover, the absorption bands located at about 1600 cm −1 and 3400-3500 cm −1 were assigned to stretching vibration of H-O-H and bending O-H to denote the presence of physically absorbed water molecules [33]. Furthermore, absorption bands located at about 550 cm −1 and 981 cm −1 in AP/PAN and SAP/PAN were due to the molecular vibration of PO 4 3− [30,34]. However, the absorption band that locates at about 983 cm −1 [35,36]    The coexistence of SO4 2− in Ag3PO4 and PAN in SAP/PAN was confirmed by performing XPS analysis. As shown in survey spectrum (Figure 5a), P, S, Ag, and O elements coming from SO4 2− -Ag3PO4 and C and N elements corresponding to PAN were clearly observed. Moreover, specific nature of S in SAP/PAN and Ag, P, and O in both samples was obtained from high-resolution XPS spectra. As depicted in Figure 5b, a peak located at around 168.38 eV in high-resolution spectra of S 2p in SAP/PAN was attributed to S 6+ [37]. This result also indicated the incorporation of SO4 2− into Ag3PO4 crystal lattice during synthesis process. In case of Ag 3d and P 2p peaks of SAP/PAN, slight shifting of these peaks to higher values of binding energies was observed compared to that of AP/PAN (Figure 5c,d). This shifting might happen due to doping of SO4 2− , which could decrease electron density around Ag and P due to higher electronegativity of S [38]. Similar behavior was observed for O 1s peak of SAP/PAN compared to AP/PAN (Figure 5e). Hence, all these XPS results further confirmed the existence of S in the form of SO4 2− in Ag3PO4 crystal lattice due to its strong electronic interactions with Ag, P, and O [19].   The coexistence of SO4 2− in Ag3PO4 and PAN in SAP/PAN was confirmed by performing XPS analysis. As shown in survey spectrum (Figure 5a), P, S, Ag, and O elements coming from SO4 2− -Ag3PO4 and C and N elements corresponding to PAN were clearly observed. Moreover, specific nature of S in SAP/PAN and Ag, P, and O in both samples was obtained from high-resolution XPS spectra. As depicted in Figure 5b, a peak located at around 168.38 eV in high-resolution spectra of S 2p in SAP/PAN was attributed to S 6+ [37]. This result also indicated the incorporation of SO4 2− into Ag3PO4 crystal lattice during synthesis process. In case of Ag 3d and P 2p peaks of SAP/PAN, slight shifting of these peaks to higher values of binding energies was observed compared to that of AP/PAN (Figure 5c,d). This shifting might happen due to doping of SO4 2− , which could decrease electron density around Ag and P due to higher electronegativity of S [38]. Similar behavior was observed for O 1s peak of SAP/PAN compared to AP/PAN (Figure 5e). Hence, all these XPS results further confirmed the existence of S in the form of SO4 2− in Ag3PO4 crystal lattice due to its strong electronic interactions with Ag, P, and O [19]. -Ag 3 PO 4 and C and N elements corresponding to PAN were clearly observed. Moreover, specific nature of S in SAP/PAN and Ag, P, and O in both samples was obtained from high-resolution XPS spectra. As depicted in Figure 5b, a peak located at around 168.38 eV in high-resolution spectra of S 2p in SAP/PAN was attributed to S 6+ [37]. This result also indicated the incorporation of SO 4 2− into Ag 3 PO 4 crystal lattice during synthesis process. In case of Ag 3d and P 2p peaks of SAP/PAN, slight shifting of these peaks to higher values of binding energies was observed compared to that of AP/PAN (Figure 5c,d). This shifting might happen due to doping of SO 4 2− , which could decrease electron density around Ag and P due to higher electronegativity of S [38]. Similar behavior was observed for O 1s peak of SAP/PAN compared to AP/PAN (Figure 5e). Hence, all these XPS results further confirmed UV-vis diffusive reflectance spectra (DRS) of pristine PAN, AP/PAN, and SAP/PAN were measured to determine their light absorption behavior and the results are plotted in Figure 6. As seen, two absorption bands presented in the range of 200-350 nm were assigned to the pristine PAN, which is in agreement with the result of a previously reported study [39]. After loading sulfate undoped/doped nanoparticles on PAN nanofibers, visible light absorption behavior could be observed. Both the samples (AP/PAN and SAP/PAN) displayed continuous absorption in visible range (520-700 nm), however the absorption intensity of SAP/PAN was found to be slightly increased. Therefore, these results signified the visible light harvesting capability of AP/PAN and SAP/PAN. UV-vis diffusive reflectance spectra (DRS) of pristine PAN, AP/PAN, and SAP/PAN were measured to determine their light absorption behavior and the results are plotted in Figure 6. As seen, two absorption bands presented in the range of 200-350 nm were assigned to the pristine PAN, which is in agreement with the result of a previously reported study [39]. After loading sulfate undoped/doped nanoparticles on PAN nanofibers, visible light absorption behavior could be observed. Both the samples (AP/PAN and SAP/PAN) displayed continuous absorption in visible range (520-700 nm), however the absorption intensity of SAP/PAN was found to be slightly increased. Therefore, these results signified the visible light harvesting capability of AP/PAN and SAP/PAN. Nanomaterials 2020, 10, x 8 of 14 Figure 6. UV-vis diffusive reflectance spectra of prepared photocatalysts.
Photodegradation performances of different samples under visible light irradiation were investigated using MB and RhB dye solutions and results are presented in Figure 7a,b. The photodegradation is represented as the variation of (Ct/Co) with irradiation time, where Co is the initial concentration and Ct is remaining concentration of dyes solution at time t. As presented in figure, pristine PAN could show negligible capability of photodegradation of MB and RhB dye solutions. In contrary, more than 95% of MB was degraded by SAP/PAN within 40 min, while only 88% of MB was degraded within this time period utilizing AP/PAN. Likewise, SAP/PAN could exhibit superior performance over AP/PAN towards the photodegradation of RhB. In this case, SAP/PAN could degrade 95% of RhB within 50 min, but within this time period, AP/PAN could degrade about 87% of RhB. On the basis of these results, SAP/PAN was found to be a more advantageous visible-light-driven photocatalyst over AP/PAN. Figure 7c,d show the time-dependent absorbance variations of MB and RhB dye solutions utilizing SAP/PAN under visible light irradiation. Corresponding absorbance peaks of MB at 665 nm and RhB at 554 nm are gradually diminished with the increase in irradiation time. Importantly, the maximum wavelength of MB and RhB were not found to be shifted, which indicated that benzene/heterocyclic rings were decomposed rather than decolorized due to adsorption of dye molecules on the surface of photocatalyst [40,41]. Insets (Figure 7c These results showed that TOCs were lower than that of original dye solutions (MB = 3.9 mg/L and RhB = 4.56 mg/L). Furthermore, the rate of TOC change for both MB and RhB dyes was lower compared to their photodegradation rate, which is assigned to the partial decomposition of dye molecules into intermediate products resulting in the disappearance of color and partial mineralization [42,43]. The photodegradation is represented as the variation of (C t /C o ) with irradiation time, where C o is the initial concentration and C t is remaining concentration of dyes solution at time t. As presented in figure, pristine PAN could show negligible capability of photodegradation of MB and RhB dye solutions. In contrary, more than 95% of MB was degraded by SAP/PAN within 40 min, while only 88% of MB was degraded within this time period utilizing AP/PAN. Likewise, SAP/PAN could exhibit superior performance over AP/PAN towards the photodegradation of RhB. In this case, SAP/PAN could degrade 95% of RhB within 50 min, but within this time period, AP/PAN could degrade about 87% of RhB. On the basis of these results, SAP/PAN was found to be a more advantageous visible-light-driven photocatalyst over AP/PAN. Figure 7c,d show the time-dependent absorbance variations of MB and RhB dye solutions utilizing SAP/PAN under visible light irradiation. Corresponding absorbance peaks of MB at 665 nm and RhB at 554 nm are gradually diminished with the increase in irradiation time. Importantly, the maximum wavelength of MB and RhB were not found to be shifted, which indicated that benzene/heterocyclic rings were decomposed rather than decolorized due to adsorption of dye molecules on the surface of photocatalyst [40,41]. Insets (Figure 7c These results showed that TOCs were lower than that of original dye solutions (MB = 3.9 mg/L and RhB = 4.56 mg/L). Furthermore, the rate of TOC change for both MB and RhB dyes was lower compared to their photodegradation rate, which is assigned to the partial decomposition of dye molecules into intermediate products resulting in the disappearance of color and partial mineralization [42,43].   (Figure 9a,b). For cycling experiments, the used sample was separated, washed, and dried at room temperature then reapplied for photodegradation under similar conditions. Experimental results showed good stability of SAP/PAN up to third cycle, however slight decrease was observed in the performance during cycling experiments, which could happen due to loss of photocatalyst during separation process. Furthermore, Langmuir-Hinshelwood model was applied to evaluate the photodegradation kinetics of MB and RhB solutions utilizing AP/PAN and SAP/PAN.   (Figure 9a,b). For cycling experiments, the used sample was separated, washed, and dried at room temperature then reapplied for photodegradation under similar conditions. Experimental results showed good stability of SAP/PAN up to third cycle, however slight decrease was observed in the performance during cycling experiments, which could happen due to loss of photocatalyst during separation process. Furthermore, Langmuir-Hinshelwood model was applied to evaluate the photodegradation kinetics of MB and RhB solutions utilizing AP/PAN and SAP/PAN.  (Figure 9a,b). For cycling experiments, the used sample was separated, washed, and dried at room temperature then reapplied for photodegradation under similar conditions. Experimental results showed good stability of SAP/PAN up to third cycle, however slight decrease was observed in the performance during cycling experiments, which could happen due to loss of photocatalyst during separation process. Furthermore, Langmuir-Hinshelwood model was applied to evaluate the photodegradation kinetics of MB and RhB solutions utilizing AP/PAN and SAP/PAN. (1) where Co and Ct represent the initial concentration and concentration at time (t), respectively. kapp is the apparent rate constant (min −1 ), which can be obtained by plotting ln (Co/Ct) vs. reaction time.
Hence, degradation kinetics of MB and RhB solutions were calculated by applying equation (2) and the results are shown in Figure 9c,d. The linear relationship between ln (Co/Ct) vs. reaction time suggested the pseudo first-order kinetics of photodegradation. From the results, apparent rate constants of MB degradation utilizing AP/PAN and SAP/PAN were determined to be 0.057 min −1 and 0.075 min −1 , respectively. Similarly, the apparent rate constants 0.043 min −1 and 0.064 min −1 were determined for RhB degradation utilizing AP/PAN and SAP/PAN, respectively. All these results indicated that sulfate doping could provide significant capability to Ag3PO4 to enhance its photodegradation performance. On the basis of above results, an overall mechanism is proposed for photodegradation of organic dyes. Ag3PO4, being a semiconductor material, generates electron-hole pairs under visible light irradiation. The photo-excited electrons travel to conduction band (CB) from valence band (VB) and react with dissolved oxygen molecules to produce ROS, i.e., oxygen peroxide radicals (O2 • − ), which are strong oxidizing agents and degrade dye molecules effectively. On the other hand, holes at VB directly react with dye molecules [45]. In this way, these ROS and holes can photocatalytically degrade organic dyes as Since the initial concentration of MB and RhB was very low (C o = 10 mg/L), equation (1) can be considered a pseudo first-order kinetics equation [44] as where C o and C t represent the initial concentration and concentration at time (t), respectively. k app is the apparent rate constant (min −1 ), which can be obtained by plotting ln (C o /C t ) vs. reaction time.
Hence, degradation kinetics of MB and RhB solutions were calculated by applying equation (2) and the results are shown in Figure 9c,d. The linear relationship between ln (C o /C t ) vs. reaction time suggested the pseudo first-order kinetics of photodegradation. From the results, apparent rate constants of MB degradation utilizing AP/PAN and SAP/PAN were determined to be 0.057 min −1 and 0.075 min −1 , respectively. Similarly, the apparent rate constants 0.043 min −1 and 0.064 min −1 were determined for RhB degradation utilizing AP/PAN and SAP/PAN, respectively. All these results indicated that sulfate doping could provide significant capability to Ag 3 PO 4 to enhance its photodegradation performance.
On the basis of above results, an overall mechanism is proposed for photodegradation of organic dyes. Ag 3 PO 4 , being a semiconductor material, generates electron-hole pairs under visible light irradiation. The photo-excited electrons travel to conduction band (CB) from valence band (VB) and react with dissolved oxygen molecules to produce ROS, i.e., oxygen peroxide radicals (O 2 • − ), which are strong oxidizing agents and degrade dye molecules effectively. On the other hand, holes at VB directly react with dye molecules [45]. In this way, these ROS and holes can photocatalytically degrade organic dyes as It is well known that semiconductor photocatalysts having improved separation capability and low recombination rate of photoinduced electron-hole pairs can exhibit enhanced performances. Hence, in this work the enhanced photocatalytic performances of SAP/PAN compared to that of AP/PAN can be explained with the role of SO 4 2− as dopant, which could play an important role to trap and transfer photoinduced electrons to CB, thereby providing improved separation capability of electron-hole pairs to SO 4 2− -Ag 3 PO 4 ( Figure 10). Moreover, SO 4 2− -Ag 3 PO 4 could receive additional electrons due to higher electronegativity of S than that of P. As a result, the Fermi level of SO 4 2− -Ag 3 PO 4 gets shifted towards CB and possesses n-type conductivity. In this way, doping of SO 4 2− into the Ag 3 PO 4 crystal lattice can improve its separation capability and lower recombination rate of photoinduced electron-hole pairs, which ultimately increases the production of ROS and enhances the photocatalytic performance of SO 4 2− -Ag 3 PO 4 [22,46,47].
It is well known that semiconductor photocatalysts having improved separation capability and low recombination rate of photoinduced electron-hole pairs can exhibit enhanced performances. Hence, in this work the enhanced photocatalytic performances of SAP/PAN compared to that of AP/PAN can be explained with the role of SO4 2− as dopant, which could play an important role to trap and transfer photoinduced electrons to CB, thereby providing improved separation capability of electron-hole pairs to SO4 2− -Ag3PO4 ( Figure 10). Moreover, SO4 2− -Ag3PO4 could receive additional electrons due to higher electronegativity of S than that of P. As a result, the Fermi level of SO4 2− -Ag3PO4 gets shifted towards CB and possesses n-type conductivity. In this way, doping of SO4 2− into the Ag3PO4 crystal lattice can improve its separation capability and lower recombination rate of photoinduced electron-hole pairs, which ultimately increases the production of ROS and enhances the photocatalytic performance of SO4 2− -Ag3PO4 [22,46,47].

Conclusions
In summary, SO4 2− -Ag3PO4/PAN-electrospun nanofibers were fabricated successfully by combining electrospinning and ion-exchange reaction. Different characterization techniques were used to study the morphology, structure, chemical composition, and optical properties of the samples. Photocatalytic activity of the fabricated samples was investigated by photodegradation of MB and RhB dye solutions under visible light irradiation. In both investigations, SO4 2− -Ag3PO4/PANelectrospun nanofibers could show enhanced performance compared to Ag3PO4/PAN-electrospun nanofibers. We believe that the enhanced performances of SO4 2− -Ag3PO4/PAN-electrospun nanofibers were attributed to the sufficient electron-hole separation capability of SO4 2− -Ag3PO4 nanoparticles to produce ROS due to doping effect of SO4 2− ions into the Ag3PO4 crystal lattice. Therefore, thus fabricated SO4 2− -Ag3PO4/PAN-electrospun nanofibers can find potential application as visible-lightdriven photocatalyst with good flexibility and reusability for wastewater treatment.
Author Contributions: Conceptualization, G.P.; methodology, G.P.; data analysis, G.P. and K.RG.; manuscript preparation, G.P. and K.RG.; writing-review and editing, G.P. and M.P.; resources and supervision, M.P.; funding acquisition, M.P. All authors have read and agreed to the published version of the manuscript.

Conclusions
In summary, SO 4 2− -Ag 3 PO 4 /PAN-electrospun nanofibers were fabricated successfully by combining electrospinning and ion-exchange reaction. Different characterization techniques were used to study the morphology, structure, chemical composition, and optical properties of the samples.