The Starting Material Concentration Dependence of Ag3PO4 Synthesis for Rhodamine B Photodegradation under Visible Light Irradiation

Synthesis of Ag3PO4 photocatalyst under the varied concentrations of AgNO3 and Na2HPO4·12H2O as starting material has been successfully synthesized using the co-precipitation method. The concentration of AgNO3 is 0.1; 0.5; 1.0; and 2.0 M, whereas Na2HPO4·12H2O is 0.03; 0.17; 0.33; and 0.67 M, respectively. The coprecipitations were carried out under aqueous solution. As-synthesized photocatalysts were examined to degrade Rhodamine B (RhB) under blue light irradiation. The results showed that varying concentrations of starting materials affect the photocatalytic activities, the intensity ratio of [110]/[200] facet plane, and their bandgap energies of Ag3PO4 photocatalyst. The highest photocatalytic activity of the sample was obtained by synthesized using the 1.0 M of AgNO3 and 0.33 M of Na2HPO4·12H2O (AP-1.0). This is due to the high [110] facet plane and increased absorption along the visible region of AP-1.0 photocatalyst. Therefore, this result could be a consideration for the improvement of Ag3PO4 photocatalyst.

Recently, the different morphology, particle size, crystallinity, and absorption profiles could be resulted in the different concentrations of KH 2 PO 4 as starting material in the Ag 3 PO 4 synthesis. This might be due to the different concentration influenced the nucleation and crystal growth during photocatalyst synthesis. However, the high photocatalytic activity of Ag 3 PO 4 photocatalyst can be found in the sample with smaller particle size, higher crystallinity, and UV-Vis absorption ability of Ag 3 PO 4 (Afifah et al., 2019). Besides KH 2 PO 4 , the reactant of Na 2 HPO 4 ·12H 2 O as a source of phosphate has been applied for Ag 3 PO 4 design synthesis (Chen, Dai, Guo, Bu, & Wang, 2016;Wang et al., 2015;Wu et al., 2013). Nevertheless, the investigation using the variation of Na 2 HPO 4 ·12H 2 O concentration has not been reported. Based on these reports, it is a good idea to obtain high photocatalytic activity by using Na 2 HPO 4 ·12H 2 O. Therefore, we have tried to observe the effect of the varied concentration of AgNO 3 and Na 2 HPO 4 ·12H 2 O as starting material for Ag 3 PO 4 coprecipitation. The activities of the products were investigated using the Rhodamine B (RhB) photodecomposition under blue light irradiation.

Photocatalyst Characterization
Determination of crystallinity and purity of samples was done using X-ray diffraction (XRD) Bruker AXS D2 Phaser. UV and visible light absorption profiles were conducted using Diffuse Reflectance Spectroscopy (DRS) JASCO V-670. Photocatalyst samples were examined under blue light irradiation of LED Skyled (3W/220V) for RhB photodegradation. Lampsuspension surface distance was adjusted at 10 cm. The absorbance of filtrate after the photodegradation process was analyzed using UV-Visible spectroscopy Shimadzu 1800.

Photocatalytic Activity Analysis
An amount of 0.2 g of Ag 3 PO 4 photocatalyst was poured into 100 mL of 10 mg/L RhB under stirring condition. Initially, the reaction was carried out under the dark condition for 20 minutes to ensure the adsorption-desorption of dye and Ag 3 PO 4 photocatalyst. After that, the suspension was irradiated under blue light, and the sample was drawn ±5 mL at a certain time. The suspension was centrifuged at 1.500 rpm, and the absorbance of the filtrate was measured (Febiyanto et al., 2016).

XRD Analysis of Ag 3 PO 4 Photocatalyst
The XRD profile of as-synthesized Ag 3 PO 4 was shown in Fig. 2. The diffraction of AP-0.1; AP-0.5; AP-1.0; and AP-2.0 are similar. As shown in Fig. 1, the photocatalysts synthesized under the varied concentration have a body-centered cubic of Ag 3 PO 4 . It is suitable with the JCPD #06-0505 (Huang et al., 2019;Yan et al., 2014). High intensities of peaks and no others peak except Ag 3 PO 4 , suggesting that the samples are highly crystalline and high purity (Ge, 2014).

Band gap energy analysis of Ag 3 PO 4 photocatalyst
Analysis of UV-Vis absorption profiles of as-synthesized Ag 3 PO 4 photocatalyst was analyzed using DRS and depicted in Fig. 2 (A-B). Based on Fig. 2 (A), samples have a good-absorption ability on the UV up to visible light (<600 nm). In addition, the difference in absorption ability could also be found in the visible wavelength at around 400-500 nm. The sample of AP-1.0 showed the highest absorption in the visible region (540-800 nm). ( 3) where α, h, v, A, E g , and n are absorption coefficient, Planck's constant, light frequency, a constant number, band gap energy, and value depend on the direct (n=1) and indirect (n=4) characteristics of nature semiconductor transition. According to Eq. 3, the (αhv) 2 plot as a function of hv where the band gap energy of each sample was resulted using extrapolation of a straight line towards the axis (x)-direction. The direct band gap energy of samples was summarized in Table. 2. According to Table 2, the varied molar concentrations slightly affect the light absorption as same as the band gap energy of photocatalyst samples.   Fig. 3. Based on Fig. 3 (A), the control or RhB photolysis process without the addition of photocatalyst material showed a small decrease in RhB concentration under blue light irradiation. The percent RhB degradation using photolysis reaction over 80 minutes of light exposure was less than 6% (yellow color). The slight decrease in RhB photodegradation without catalyst addition could be explained by Wilhelm and Stephan (Wilhelm and Stephan, 2007) as follows: the excitation of RhB compounds (Eq. 4) is followed by O 2 reduction to be O 2-• and photogenerated RhB + • (Eq. 5) under the light irradiation. The O 2-• radicals react with H + from H 2 O autoprotolysis, and then OOH• was resulted (Eq. 6). Subsequently, RhB cationic was degraded, resulting in CO 2 , H 2 O, and acid minerals (Eq. 7). Qu and Zhao reported that these reaction mechanisms are a very slow reaction, and the photocatalysis reaction using a catalyst is the best reason to explain a significant decrease in RhB photodecomposition (Qu and Zhao, 1998).
In addition, the same trends were also observed in the adsorption ability of photocatalysts under the dark condition, with a percent decrease of less than 10%, as shown in Fig. 3 (A) (green color). It concluded that RhB photodecomposition under blue light irradiation or photolysis and adsorption process at least could be neglected since there is no apparent concentration changing compared to the RhB photodecomposition with the addition of Ag 3 PO 4 photocatalyst. However, the dye photodegradation using radical species that is resulted from lightactivated photocatalyst was a plausible mechanism to decrease the absorbance or dye concentration than photolysis reaction (Febiyanto et al., 2019). A significant decrease in RhB concentration was found when the RhB sample was mixed with Ag 3 PO 4 photocatalyst, as shown in Fig. 3 (A). Fig. 3 (A) (red color) showed that all samples could effectively degrade the RhB dyes. The high photocatalytic activity can be found in the AP-1.0 sample, with the percent degradation of 82.5%. The high photocatalytic activity might be induced by the highest intensity of [110] facet plane and different properties obtained by coprecipitation under the starting material of AP-1.0 photocatalyst. This preparation generates high absorption along the visible region, as shown in Fig. 2 (A). The AP-1.0 is the high absorption in the visible region. This phenomenon increases the photogenerated electron and holes that lead to improving the photocatalytic activity. The percent degradation of 69.2, 57.4, and 54.3% could be observed in AP-0.1, AP-0.5, and AP-2.0, respectively. These photocatalytic activities were relatively similar and highly different from the AP-1.0 sample.
Photocatalytic kinetic analysis of Ag 3 PO 4 photocatalyst with varied molar concentrations was performed in Fig. 3 (B). Rate constant can be determined from a slope that is resulted from linear regression. These can be determined using a formula as follows (Eq. 8): where C o , C, t, and k are initial concentration, the concentration at a certain time, irradiation time, and a rate constant of pseudo-first-order kinetic (Yan et al., 2014). Based on Fig. 3 ( B) and Table 2, the rate constant of samples shows a good-linearity and following the pseudo-first-order rate kinetic (Pradhan et al., 2014). However, the sample with AP-1.0 showed a high photocatalytic rate kinetic of 0.0218 min -1 , whereas samples of AP-0.1, AP-0.5, and AP-2.0 were 0.0147, 0.0107, and 0.0098 min -1 , respectively. The results showed that varying molar concentrations affect the rate constant, as same as photocatalytic activities of Ag 3 PO 4 photocatalyst.
The RhB oxidation and reduction mechanism chain of photocatalyst generally might be happened on the photocatalyst surfaces of Ag 3 PO 4 under visible light irradiation.
This following the photodegradation mechanisms below (Guo et al., 2015): Photocatalyst under blue light excites electron (e -) from valence band (VB) to the conduction band (CB) resulted in electron (e -CB ) and hole (h + VB ) (Eq. 9). The electron-hole pairs transferred to the Ag 3 PO 4 surfaces and oxidation/reduction of water molecules (H 2 O) (Eq. 10) and OHions or dissolved oxygen (O 2 ) (Eq. 11-12) can take place. Subsequently, this can produce a radical species of hydroxyl (•OH) and superoxide (•O 2 -). The benefits of this chemical radical decompose the RhB dye structures to be small molecules such as H 2 O, CO 2 , and acid minerals (Eq. 13).

CONCLUSION
The photocatalyst of Ag 3 PO 4 under the varied concentration of starting materials have been successfully synthesized using the coprecipitation method. The results showed that the varied concentrations of starting materials affect the photocatalytic activities, the intensity ratio of