Synthesis and Characterization of High Efficiency and Stable Spherical Ag 3 PO 4 Visible Light Photocatalyst for the Degradation of Methylene Blue Solutions

1Guangxi Key Laboratory of Farm Products Processing (Cultivation Base), Colleges and Universities Key Laboratory for Efficient Use of Agricultural Resources in the Southeast of Guangxi, College of Chemistry and Material, Yulin Normal University, Yulin, Guangxi 537000, China 2School of Chemistry and Chemical Engineering and Institute of Physical Chemistry, Lingnan Normal University, Zhanjiang 524048, China 3School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, China


Introduction
Semiconductor based photocatalysts have received great attention due to their applications in environmental pollution mediation and solar energy conversion [1][2][3][4][5][6].However, the major challenge in the practical applications is their poor efficiency in visible light range or insufficient charge separation ability.To resolve this problem, significant efforts have been devoted to the exploration and fabrication of novel photocatalytic materials for improving photocatalytic properties [7,8].To address this issue, diverse techniques in recent years have been focused on the exploration and fabrication of novel semiconductor catalysts to improve photocatalytic properties [9][10][11].Recently, Bi and coworkers have reported the new use of a Ag 3 PO 4 semiconductor in photocatalytic applications, which exhibits extremely high photooxidative capabilities for O 2 evolution from water as well as organic dyes decomposition under visible light irradiation [12,13].Otherwise, this novel photocatalyst can achieve quantum efficiency up to 90% at wavelengths longer than 420 nm, significantly higher than previously reported values.Interestingly, they have further studied the facet effect on photocatalytic performances based on single-crystalline Ag 3 PO 4 rhombic dodecahedrons exposed with {110} planes and cubes bound by {100} planes [14].Unfortunately, Ag 3 PO 4 nanoparticles suffer from stability issues in practical applications because they can photochemically decompose if no sacrificial reagent is involved.Furthermore, the consumption of a large amount of noble metal, silver, strongly limits its practical environmental applications.So it remains a great challenge to develop feasible methods for the synthesis of well-defined hierarchical Ag 3 PO 4 nanoparticles.
Herein, we successfully fabricated spherical Ag 3 PO 4 submicron particles by a simple hydrothermal method.The asprepared samples were characterized and used for photocatalytic degradation of MB under visible light irradiation.

Experimental
In a typical experiment, using CH 3 COOAg and K 2 HPO 4 as starting materials, (A) 0.50 g of CH 3 COOAg (0.003 mol) is dissolved in 20 mL of deionized water and stirred for 10 minutes, and the solution pH adjusts to 9 with ammonia.(B) 0.1742 g of K 2 HPO 4 (0.001 mol) is dissolved in 15 mL of deionized water and stirred for 10 minutes.Then solution (B) is added dropwise to solution (A) with stirring.After the resulting solution is magnetically stirred for 30 minutes, the suspension is transferred into a 50 mL Teflon-lined stainlesssteel autoclave and heated at 120 ∘ C for 24 h.After being cooled naturally to room temperature, the products are collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried under vacuum at 60 ∘ C for 24 h.The as-synthesized samples are denoted as Ag 3 PO 4 -120.
As a comparison, a precipitation method is used to prepare to Ag 3 PO 4 .0.50 g of CH 3 COOAg (0.003 mol) is dissolved in the beaker with 20 mL of deionized water and magnetically stirred for 10 minutes, then 0.1742 g of K 2 HPO 4 (0.001 mol) and 0.5182 g C 6 H 5 Na 3 O 7 ⋅2H 2 O (0.002 mol) are added to the solution.The solution pH adjusts to 5 with HNO 3 (0.01 M).The suspension is stirred for 6 h at room temperature.The products are collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried under vacuum at 60 ∘ C for 24 h.The assynthesized samples are denoted as Ag 3 PO 4 -P.
X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Bruker D8-Advance, Germany) using Cu Ka radiation.The surface morphology was examined by a scanning electron microscopy (SEM, Quanta 250, FEI Company).UV-Vis diffuse reflectance spectra (DRS) were recorded with a UV-Vis spectrophotometer by using BaSO 4 as the reference (UV-2500, Shimadzu).The Fourier transform infrared (FTIR) spectrum was recorded with a GX spectrophotometer (Spectrum 100, Perkin-Elmer) with the KBr wafer technique.The photoluminescence (PL) spectrum was conducted on a Hitachi F-2500 fluorescence spectroscope.
The photocatalytic reaction was conducted in a 200 mL cylindrical glass vessel fixed in the XPA-II photochemical reactor.A 500 W Xe lamp was used as the simulated solar light source (UV-visible light), and a house-made filter was mounted on the lamp to eliminate infrared irradiation.The visible-light was obtained by using the cut-off filter.The cut-off filter was made up of 1 M sodium nitrite solution which can absorb the light with wavelength under 400 nm [15,16].This solution was injected into the interlayer of the quartz cool trap and was placed behind the water filter to completely remove the UV portion of the radiation and to supply a visible-light source.MB with the concentration of 20 mg/L was used as contamination.In order to obtain an optimally dispersed system and reach complete adsorption/desorption equilibration, 20 mg photocatalyst powder dispersed in 200 mL reaction solutions by supersonic for 15 min and then the suspension was magnetically stirred in dark for 1 h.During the photocatalytic reaction, air was blown into the reaction medium at a flow rate of 200 mL/min.At regular intervals, 8 mL of the suspension was filtered and then centrifuged.The concentration of the remaining MB was measured by its absorbance (A) at 664 nm with a Hitachi UV-2550 spectrophotometer.The degradation ratio of MB can be calculated by  = ( 0 − )/ 0 × 100%.

SEM and XRD Analyses of the As-Synthesized Samples.
The XRD patterns of the as-prepared samples synthesized by different methods are shown in Figure 1.It is clear to see that all of the diffraction peaks of Ag 3 PO 4 -120 corresponded to the cubic phase of Ag 3 PO 4 (JCPDS number 06-0505) [14], as well as the characteristic diffraction peaks (110), (200), (210), (211), (220), (310), ( 222), (320), (321), (400), and (420) (Figure 1(a)).The crystal type of Ag 3 PO 4 -P is the same as that of Ag 3 PO 4 -120.It is noted that the as-prepared samples are all of the cubic phase of Ag 3 PO 4 .
Figure 2 shows the typical SEM images of Ag 3 PO 4 products synthesized by different reacting temperatures of hydrothermal method.It can be truly seen that the single-crystalline Ag 3 PO 4 submicrocubes with spherical and smooth surfaces have been fabricated in large quantities, and their main diameters range from 500 to 800 nm (Figure 2(a)).With the increase of the reaction temperature, the morphology of crystalline Ag 3 PO 4 was unchanged obviously (Figures 2(a)-2(c)).However, the diameters of singlecrystalline Ag 3 PO 4 are more and more large.Ag 3 PO 4 -P is irregular oval ball and the main diameters range from 5 m (Figure 2(d)).

DRS and IR Analyses of the As-Prepared Samples.
Figure 3 shows the UV-Vis absorption spectra of Ag 3 PO 4 -120, Ag 3 PO 4 -P, and N-TiO 2 .The UV-Vis spectrum of the Ag 3 PO 4 -120 indicates that it absorbs sunlight with a wavelength less than 548 nm, corresponding to 2.26 eV of band gap energy.This result does not agree with the light-absorption property of Ag 3 PO 4 powders reported by another group [17].This maybe was attributed to the surface plasmon resonance effect of the coexistence of silver [18,19].However, the absorption band edge of Ag 3 PO 4 -P is the same as Ag 3 PO 4 -120; N-TiO 2 appears at 424 nm based on its UV-Vis spectrum, respectively.

Photocatalytic Activities of the As-Synthesized Samples.
Figure 5 shows the effect of reacting temperature of hydrothermal on the rate of MB degradation under visible light irradiation.The photodegradation efficiency of Ag  catalysts have also been studied and compared in Figure 6.
As shown in Figure 6, it can be clearly seen that except for N-doped TiO 2 catalysts, all these Ag 3 PO 4 samples exhibit excellent photocatalytic activities for the MB degradation.Ag 3 PO 4 -120 especially exhibits the highest photocatalytic activity, which can nearly completely degrade MB dye in only 4 min.The absorption variation of MB dye over Ag 3 PO 4 -120 at different irradiation times is shown in Figure 7.The Ag 3 PO 4 -120 sample needs about 6 min that the MB dye is nearly colorless.At present, we consider that Ag 3 PO 4 -120 can effectively improve light harvesting as a result of its high surface area.On the other hand, such novel spherical structures could promote the separation of photoexcited carriers and minimize the probability of electron-hole recombination [22].However, the photocatalytic mechanism over Ag 3 PO 4 -120 structure cannot be completely understood and a more detailed study is still needed.As reusability of photocatalyst is a key issue for practical application, Figure 8 shows that the reusability of Ag 3 PO 4 -120 for degradation of MB under visible light irradiation.As no obvious decrease of degradation is observed after three runs, the Ag 3 PO 4 nanoparticles can be concluded to be stable during the photocatalytic reaction.
Notably, the photocatalytic activity of Ag 3 PO 4 -120 is higher than that of Ag 3 PO 4 -P and N-TiO 2 , which can be explained from the PL analysis.The PL spectra are widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron-hole pairs in a semiconductor, since PL emission arises from the recombination of free carriers.Figure 9 shows the PL spectra of Ag 3 PO 4 -120, Ag 3 PO 4 -P, and N-TiO 2 excited by 254 nm.As shown in Figure 9, it is clear to see that there is a significant decrease in the PL intensity of Ag 3 PO 4 -120 compared to that of Ag 3 PO 4 -P and N-TiO 2 .A weaker intensity of the peak represents a lower recombination probability of photogenerated charge carriers.The lower of the observed PL intensity can be associated with the electron transfer process from excited CB to VB in Ag 3 PO 4 lattice, through which the process of electron transfer takes place.Hence, the samples prepared by this method prevent the charge carrier recombination probabilities and improve the separation of charge carriers.The decrease in intensity can also be attributed to the generation of new defect or vacancy sites like oxide ion vacancy [22].Therefore, Ag 3 PO 4 -120 could effectively inhibit the recombination of photogenerated charge carriers, which helps the separation of photogenerated electron-hole pairs.It can be established that Ag 3 PO 4 -120 is very promising for the photocatalysis with satisfying efficiency.

Conclusion
In summary, we have demonstrated a facile and efficient hydrothermal method for fabricating Ag 3 PO 4 submicron particles at low temperature.Moreover, its photocatalytic performance studies indicated that this novel Ag 3 PO 4 structures exhibited much higher activities than that of Ag 3 PO 4 -P and N-doped TiO 2 for the organic contaminant degradation under visible light irradiation.This study clearly reveals that the rational fabrication of semiconductor nanostructure may be an effective technique for the development of highly efficient visible light sensitive photocatalysts.
This feature of UV-Vis light absorption properties of the asprepared samples suggests that the Ag 3 PO 4 -120 photocatalyst could be used for visible light photocatalytic reactions.The IR patterns of the Ag 3 PO 4 -120, Ag 3 PO 4 -P are shown in Figure4.The IR peaks of the samples prepared by hydrothermal and precipitation methods have all appeared in 551 cm −1 and 1015 cm −1 , which is attributable to the P-O antisymmetric stretching mode of PO 4 3− .The IR peak at 846 cm −1 may be C-O vibration, which is the impurities of precursor.The peaks appearing at 1399 cm −1 and 1655 cm −1 are due to torsional vibration H 3 O + .The weak signal of C-H bond is observed at 3230 cm −1[20,21].

Figure 7 :
Figure 7: UV-Vis absorption spectra of MB solution as a function of irritation time in the presence of Ag 3 PO 4 -120 (inset shows the corresponding photographs of MB solution).