Rapid Synthesis Method of Ag₃PO₄ as Reusable Photocatalytically Active Semiconductor

Abstract

The widespread use of Ag₃PO₄ is not surprising when considering its higher photostability compared to other silver-based materials. The present work deals with the facile precipitation method of silver phosphate. The effects of four different phosphate sources (H₃PO₄, NaH₂PO₄, Na₂HPO₄, Na₃PO₄·12 H₂O) and two different initial concentrations (0.1 M and 0.2 M) were investigated. As the basicity of different phosphate sources influences the purity of Ag₃PO₄, different products were obtained. Using H₃PO₄ did not lead to the formation of Ag₃PO₄, while applying NaH₂PO₄ resulted in Ag₃PO₄ and a low amount of pyrophosphate. The morphological and structural properties of the obtained samples were studied by X-ray diffractometry, diffuse reflectance spectroscopy, scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of the materials and the corresponding reaction kinetics were evaluated by the degradation of methyl orange (MO) under visible light. Their stability was investigated by reusability tests, photoluminescence measurements, and the recharacterization after degradation. The effect of as-deposited Ag nanoparticles was also highlighted on the photostability and the reusability of Ag₃PO₄. Although the deposited Ag nanoparticles suppressed the formation of holes and reduced the degradation of methyl orange, they did not reduce the performance of the photocatalyst.

1. Introduction

Since the discovery and first application attempts of photocatalysis, numerous materials, such as TiO₂-based semiconductors and composites, other metal oxides, and salts have been investigated regarding their degradation of organic pollutants [1]. However, even the TiO₂-based ”flagships” (such as P25 [2] or other commercial titania) have severe limitations due to their relatively wide band gap, poor quantum efficiency, and/or fast recombination rate of photoexcited electron─hole pairs.

Silver-based semiconductors are promising candidates among the newer claimants to reach the throne of large-scale industrial applications due to their strong visible light response. One of their main benefits is that they can provide Ag nanoparticles that can be deposited in situ on the surface of catalysts. Depending on the light source applied during photocatalytic processes, deposited nanoparticles can act as charge separators (mostly under UV irradiation) or electron carriers (under visible light irradiation). Many photocatalysts (especially the Ag-based ones) are susceptible to photochemical corrosion, resulting in poor stability [3]. However, deposited nanoparticles can be effectively used to prevent the photocorrosion of semiconductors [4].

The application of Ag₃PO₄ as a photocatalytic material was first reported by Yi and coworkers [5]. Their investigation focused on water-splitting and degrading organic contaminants in wastewater by utilizing visible light. Ma et al. [6] focused on the structural peculiarities of Ag₃PO₄. They found that it can be characterized by a body-centered cubic structure (where all oxygen atoms are adjacent with three Ag and a P atom), a lattice parameter of 6 Å, and the space group of P4-3n [7,8]. Its band gap energy is relatively low (E_g ≈ 2.4 eV), which can be mainly modified by fine-tuning its morpho-structural characteristics. During its synthesis, pH and the appearance of pyro- and polyphosphates are key parameters. It is well known that two phosphoric acid molecules can easily condensate, resulting in these phosphates. Since pH can strongly influence the formation of oxo-acid-based Ag salts, it is not surprising that the type of phosphate precursor can be decisive (as it can also strongly influence pH). Accordingly, several phosphate-containing precursors have already been investigated, focusing on the morphology of the target semiconductor, including NH₄⁺ [9], Na⁺ [10], and K⁺ [11] based agents. Besides precursors, another decisive parameter is the applied synthesis pathway. Considering this aspect, the main methods applied to synthesize Ag₃PO₄ salts have been precipitation [9,10,12], ion-exchange [5,13], hydrothermal [14], ultrasonication [15], and other approaches [16]. It has been found that semiconductors prepared by ion-exchange reactions can yield electron─hole pairs with enhanced lifetime, resulting in higher photocatalytic efficiency [17]. Still, a deeper insight into the electronic structure of Ag₃PO₄ is necessary to understand the origin of the increased photocatalytic activity.

Thus, in this work, we focus on three main aspects related to the involvement of the Ag₃PO₄ semiconductor in photocatalytic approaches as follows:

• The effect of different phosphate sources on the synthesis and photocatalytic activity of Ag₃PO₄. For this purpose, H₃PO₄, NaH₂PO₄, Na₂HPO₄, and Na₃PO₄ · 12H₂O were used as phosphate sources. In this part, we propose a mechanism for the formation of Ag₃PO₄.
• Investigation of the stability of Ag₃PO₄ via recharacterization and reusability measurements.
• The effect of Ag nanoparticles on Ag₃PO₄, shown through the deposition mechanism of Ag nanoparticles. This aspect was demonstrated by photoluminescence measurements and their corresponding kinetic studies.

2. Materials and Methods

2.1. Materials

All chemicals were used as received: silver nitrate (AgNO₃, 99.8%, Penta industry; Prague, Czech Republic), methyl orange (MO, analytical reagent, Alfa Aesar; Tewksbury, MA, USA), Milli-Q (MQ; Budapest, Hungary) water, phosphoric acid (H₃PO₄, 85%, VWR Chemicals; Radnor, PA, USA), monosodium phosphate (NaH₂PO₄; >99.0%, Spektrum-3D; Debrecen, Hungary), disodium phosphate (Na₂HPO₄; >99.0%, Sigma-Aldrich; Schnelldorf, Germany), and trisodium phosphate dodecahydrate (Na₃PO₄·12H₂O; analytical reagent, Sigma-Aldrich, Schnelldorf, Germany).

2.2. Methods

2.2.1. Synthesis of Ag₃PO₄ Semiconductors

A precipitation method was used [13,18] to synthesize Ag₃PO₄ microcrystals. Four phosphate sources (MPO₄: H₃PO₄, NaH₂PO₄, Na₂HPO₄, Na₃PO₄·12 H₂O) and AgNO₃ were used. The weight ratio of the precursors was: MPO₄:AgNO₃ = 3:2.

In each case, two different initial concentrations of phosphate sources were used (0.2 M and 0.1 M). The aqueous solution of phosphate sources was stirred for 5 min then 1.247 g of AgNO₃ was added. A yellow suspension formed from the colorless and transparent solution. After another 5 min of stirring, the suspensions were washed with 3 × 45 mL of MQ water (for 10 min at 4400 RPM) and dried overnight at 40 °C.

The sample abbreviations used in the manuscript were conceived as follows: Ag₃PO₄_source_concentration, where concentration is the initial concentration of the used phosphate precursor (example: Ag₃PO₄_Na₃PO₄_0.1M denotes the sample that was prepared using 0.1 M Na₃PO₄·12 H₂O as the phosphate source). The word “after” was added to the names to designate samples investigated after photocatalytic tests (example: Ag₃PO₄_Na₃PO₄_0.1M_after).

2.2.2. Characterization and Instrumentation

The X-ray diffraction (XRD) measurements were performed on a Shimadzu 6000 X-ray diffractometer (Kyoto, Japan) at an accelerating voltage of 40 kV (30 mA), operated with CuKα radiation (λ_CuKα = 1.54 Å). The XRD patterns were recorded in 2θ range between 15 and 60°, with scan speed 1°·min⁻¹.

Fourier transform infrared spectroscopy (FT-IR) measurements were performed on a JASCO-6200 FT-IR spectrophotometer (Jasco, Tokyo, Japan) in the 4000─400 cm⁻¹ wavelength range, with 4 cm⁻¹ spectral resolution, using the well-known KBr pellet technique.

The morphology of the samples was identified with a Hitachi S-4700 Type II scanning electron microscope (SEM; Hitachi, Tokyo, Japan) equipped with an Everhart—Thornley detector using an electron beam with an acceleration voltage of 10 kV.

The band structure of the semiconductors was investigated by diffuse reflectance spectroscopy (DRS). The spectra were recorded in the 250–800 nm range with a JASCO-V650 spectrophotometer (equipped with an ILV-724 integration sphere; Jasco, Wien, Austria) using BaSO₄ as a reference. The band gap energy values were calculated based on the Kubelka-Munk theory [19].

X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Specs Phoibos 150 MCD system (SPECS Surface Nano Analysis GmbH, Berlin, Germany) equipped with Al-Kα source (1486.6 eV) at 14 kV and 20 mA, a hemispherical analyzer, and a charge neutralization device. Care was taken to completely cover the double-sided carbon tapes with the silver—phosphate samples.

Fluorescence measurements were carried out using a Jasco LP-6500 spectrofluorometer (Jasco, Japan; PL) equipped with a Xenon lamp (excitation source) coupled to an epifluorescence accessory (EFA 383 module). Fluorescence spectra were collected with a 1 nm spectral resolution in the 350─600 nm wavelength range using a fixed excitation wavelength of 325 nm. Bandwidths of 1 nm or 10 nm were employed during excitation and emission.

2.2.3. Photocatalytic Activity

The photocatalytic investigation of the samples was carried out in a double-walled photoreactor, where MO (C_MO = 125 μM) was the model pollutant. The reactor was surrounded by 6 × 15 W visible light-emitting lamps (λ > 400 nm). The system was kept at a constant temperature (25 °C), and the suspension (C_suspension = 1 g∙L⁻¹) was continuously stirred and purged by air at constant flow (40 L∙h⁻¹). The concentration change of MO was followed with a JASCO-V650 UV-Vis spectrophotometer (UV-Vis; Jasco, Wien, Austria) at λ_max = 513 nm (using a 1 mm optical path length quartz cell). The suspension was kept in the dark for 10 min to reach the adsorption–desorption equilibrium. The experiments were conducted for 2 h. Samples were taken every 10 min in the first hour and every 20 min in the second hour. Last, the samples were centrifugated and filtrated before quantitative analysis.

The conversion of MO was calculated by the following equation:

H = (100 − (C₁₂₀/C₀ × 100),

where C₁₂₀ is the concentration of MO after 120 min and C₀ is the initial concentration of MO.

The reaction order (n) and apparent rate constants (k₁, k₂) were calculated to investigate the kinetics of MO degradation. The apparent rate constants (k₁, k₂) were determined by plotting the MO concentration vs. the irradiation time, where the slope was considered the apparent rate constants. For k₁ values, we took the first hour (0–60 min) of the degradation process into account, whereas for k₂ values, we considered the second (60–120 min) hour.

The adsorption of MO on the samples was also investigated. The photocatalysts (50 mg) and MO (50 mL; C_MO = 125 μM) were added to a beaker. The beaker was stirred (500 RPR) and covered with aluminum foil to eliminate all light sources. Samples were taken every 5 min in the first 30 min, every 10 min between 30–60 min, and every 20 min between 1–2 h. Then, they were centrifuged and filtered, and the adsorption of MO was determined with a JASCO-V650 spectrophotometer.

To investigate reusability, we used the same setup for the photoactivity and adsorption measurements, but sampling times were changed to 30, 60, and 120 min. Samples collected between two cycles were washed with distilled water three times and dried at 40 °C for 12 h.

3. Results and Discussion

3.1. Characterization of Ag₃PO₄

Synthesizing Ag₃PO₄ by using different MPO₄ sources is a rather complicated process. MPO₄ (used as different phosphate sources; Figure 1) has different disproportional rates in aqueous media, which resulted in different pH values (

Table 1. pH, band gap energy, absorption band maxima, average particles sizes, and conversion values of the obtained Ag₃PO₄ samples (n.a.—no available).

	pH MPO4	Band Gap Energy (eV)	λmax (nm)	đSEM (μm)	Conversion −2 h-(%)
Ag3PO4_NaH2PO4_0.2M	4.22	2.3	507	n.a.	38.98
Ag3PO4_NaH2PO4_0.1M	4.27	2.3	504	n.a.	49.53
Ag3PO4_Na2HPO4_0.2M	9.14	2.33	506	0.92	89.72
Ag3PO4_Na2HPO4_0.1M	9.24	2.34	506	0.97	88.80
Ag3PO4_Na3PO4_0.2M	11.46	2.27	498	0.58	95.94
Ag3PO4_Na3PO4_0.1M	11.71	2.22	495	0.33	94.53
The colors of the samples are the same as those used in the entire article.

). Moreover, pH can also indirectly affect the samples’ morphological and structural properties. Using H₃PO₄ did not lead to the formation of Ag₃PO₄. The lack of Ag₃PO₄ can be explained by the acidic environment set by H₃PO₄, hindering the precipitation of Ag₃PO₄ (Figure 1). Moreover, the presence of H₃PO₄ can facilitate the formation of pyrophosphate. Thus, this sample was omitted from all the experimental work presented here.

XRD patterns were recorded to elucidate the effect of the other three phosphate sources on the formation of Ag₃PO₄ crystals. Cubic Ag₃PO₄ was identified (COD 00-101-0324) in all cases: the reflections of the Ag₃PO₄ were located at 2θ° ≈ 21.1°, ≈29.8°, ≈33.3°, ≈36.7°, ≈42.6°, ≈47.9°, ≈52.8°, ≈55.2°, and ≈57.3°, which were assigned to (110), (200), (210), (211), (220), (310), (222), (320), and (321) crystallographic planes (Figure 2a). Additional reflections were also observed in the Ag₃PO₄_NaH₂PO₄ sample series located at 2θ° ≈ 27.0°, ≈30.6° and ≈32.1° (Figure 2a), which could be associated with the typical reflections of Ag₄P₂O₇ [20]. These observations are consistent with the presumed mechanism (Figure 1). It must be emphasized that the pyrophosphate formation is much higher for NaH₂PO₄ than for the other two samples. In this case, crystallized pyrophosphate was formed since the materials have higher proton concentrations, and the possibility of condensation is much higher than in other cases. Interestingly, diffraction peaks of other Ag species, such as Ag nanoparticles or Ag_xO particles, were not found in the XRD patterns (Figure 2a).

FT-IR measurements (Figure 2b) were conducted to investigate the presence of pyrophosphate. The typical bands of asymmetrical vibrations of P─O─P bonds [20] were identified at ≈902 and ≈1116 cm⁻¹, confirming the presence of silver pyrophosphate (observed only in the Ag₃PO₄_NaH₂PO₄ sample series; Figure 2b). Additional bands were also observed [17]: at ≈554 cm⁻¹ (O=P─O); ≈1007 cm⁻¹ (_asO─P), ≈1389 cm⁻¹ (O=P), and H─O─H (≈1655 cm⁻¹). Thereby, it can be concluded that the formation of Ag₄P₂O₇ depends on the nature of the applied phosphate source (Figure 2b).

When NaH₂PO₄ was used as the phosphate source, the formation of Ag₃PO₄ was incomplete, while the formation of Ag₄P₂O₇ was detected (Figure 1). The reason for this could be that the formation of HNO₃ was not favored. This could be because, during the synthesis, Na⁺ exchange is more favored than that of H⁺. This makes the free formation of NaNO₃ favorable because the electropositivity of Na⁺ is higher than that of H⁺. Against the Ag₃PO₄_NaH₂PO₄ samples, the Ag₄P₂O₇ was not detected using Na₂HPO₄ or Na₃PO₄ as a phosphate source.

The morphological properties of the samples were analyzed using SEM. A correlation was found with the XRD measurements (Figure 2a). Two differently shaped and sized particles—spherical-like structure with 1.5 μm diameter and plates with 0.2 μm height—were obtained in the Ag₃PO₄_NaH₂PO₄ sample series (Figure 3). The particles could not be distinguished based on their elemental composition. This can be attributed to the co-presence of phosphate and pyrophosphate according to XRD patterns (Figure 2a). The samples prepared by using the other two phosphate sources had a much higher monodispersity (Figure S1) and a more defined shape (Figure 3). Their average particle size was ≈0.9 μm regardless of the used concentration. The particle size distribution was lower than for the Na₃PO₄ samples series; moreover, lower concentration resulted in smaller particles. The Ag₃PO₄_Na₂HPO₄ sample series contained more polyhedral particles compared to the Ag₃PO₄_Na₃PO₄ sample series.

Diffuse reflectance spectroscopy was used to analyze the optical properties of the materials and to understand the light absorption properties of Ag₃PO₄_NaH₂PO₄ (which contained Ag₄P₂O₇, a stand-alone photocatalyst [20]). As shown in Figure 4a, using different phosphate sources significantly influenced the visible light absorption properties of the samples, whose reflectance intensities decreased in the following order:

Ag₃PO₄_Na₂HPO₄ > Ag₃PO₄_NaH₂PO₄ > Ag₃PO₄_Na₃PO₄. These differences may result in different photocatalytic performances. On the other hand, no specific plasmon resonance band of Ag was detected (which could be found at ≈320 nm [21]). These observations agreement with the XRD results, where the reflection of Ag could not be detected.

The Kubelka-Munk theory was used to calculate the indirect band gap energies of the samples. No significant changes could be observed between them (E_g ≈ 2.22–2.34 eV; Table 1). Hence no clear conclusions could be drawn. To understand the relationship between the samples’ light absorption and photocatalytic activity, we analyzed their first derivative spectra (Figure 4b). Still, the λ_max values were almost identical (λ_max ≈ 495─507 nm; Figure 4). Since Ag₄P₂O₇ can be photoactive as well [20], the derivative DRS of sample Ag₃PO₄_NaH₂PO₄ should result in two specific electron transition peaks: (i) one corresponding to Ag₄P₂O₇ (observed at ≈300 nm [20]) and (ii) one corresponding to Ag₃PO₄, (observed at ≈548 nm (2.26 eV [11])).

The lack of the Ag nanoparticles was also demonstrated with high-resolution XPS (Figure 5). Symmetrical peaks were found in the Ag3d spectra (Ag 3d_5/2 and 3d_3/2 of Ag₃PO₄ corresponding to the peaks 373.67 and 367.67 eV, respectively [22]), which could be associated with Ag⁺ from Ag₃PO₄.

3.2. Photocatalytic Degradation

MO was employed as a model pollutant to investigate photoactivity. As shown in Figure 6, the photocatalytic activity of the samples can be correlated with the phosphate source used in the synthesis process. Regardless of the concentration of the phosphate source, the order was as follows: Ag₃PO₄_Na₃PO₄ > Ag₃PO₄_Na₂HPO₄ > Ag₃PO₄_NaH₂PO₄ (Figure 6). Because the samples had different absorption properties, similar photocatalytic activation was also expected to differ. According to our assumptions, samples containing Ag₄P₂O₇, a stand-alone photocatalyst, should have higher photocatalytic activity. Moreover, a photojunction between Ag₄P₂O₇ and Ag₃PO₄ could also form [23]. Ag₄P₂O₇ did not show any spectral features, that is, its characteristic secondary bands were not found in the DR and first-order derivate spectra (Figure 4). It could inhibit the photocatalyst and absorb electrons, which in turn could not be used in the photocatalytic process. As a result, a photojunction did not form. Similar to the DRS measurements, regardless of the used phosphate source, different photoactivity was observed (Figure 6). Using different phosphate sources resulted in different optical and structural properties, which influenced the degradation of MO. To clarify the degradation of MO, we evaluated the MO conversion values presented in Section 2.2.3. After two hours, 95.94 and 94.53% conversions were obtained for Ag₃PO₄_Na₃PO₄_0.2M and Ag₃PO₄_Na₃PO₄_0.1M, respectively (Table 1). It could also be noticed that the most photoactive materials (Ag₃PO₄_Na₃PO₄; Figure 6) had the highest degradation yield of MO degradation after one hour. On the other hand, the second-highest MO decolorization was achieved by using Na₂HPO₄ as a phosphate source. Based on these findings, we ascertained that the proposed mechanism of Ag₃PO₄ formation (Figure 1) is in good agreement with the photocatalytic performance. The more complete the transformation, the higher the photoactivity because when Na₃PO₄ was used during the synthesis, no Ag₄P₂O₇ or Ag nanoparticles could be observed.

As a parallel measurement, the adsorption capacity of the two most efficient photocatalysts was also investigated without using any light source (Figure S2). Adsorption did not occur throughout the process. Thus, it could be concluded that the photocatalytic degradation of MO could indeed be performed using these Ag-based materials.

Amornpitoksuk et al. [10] investigate the effect of different phosphates on the photocatalytic degradation of methylene blue. They found that using Na₂HPO₄ resulted in the highest photocatalytic activity. They also mentioned that the samples synthesized from Na₃PO₄ contained not only Ag₃PO₄ but Ag₂O as well, which inhibited photoactivity. Similarly, in our case, samples that did not contain Ag₂O had the highest photocatalytic activity. However, it cannot be conclusively declared which phosphate source is the most suitable for achieving high photoactivity. In our case, the samples containing pyrophosphate proved to be less effective. Thus, it can be concluded that synthesizing pure Ag₃PO₄ is the best approach since both Ag₂O and Ag₄P₂O₇ are stand-alone photocatalysts that do not improve the efficiency of Ag₃PO₄.

Regardless of the employed phosphate source, after one hour (Figure 6), a change in the photocatalytic reaction was observed during MO removal. A kinetic study of the MO degradation curves was carried out.

Regarding the kinetics of MO degradation, a two-step mechanism was proposed (

Table 2. Kinetics of methyl orange degradation of the Ag₃PO₄ samples.

	First Hour → 0 Order			Second Hour → 1 or 2 Order
	NaH2PO4//Na2HPO4//Na3PO4 →0th Order			NaH2PO4//Na2HPO4 →1st order Na3PO4 → 2nd Order
Sample	Reaction Order (n)	k1 (μM∙min−1)	R2	Reaction Order (n)	k2 (s−1)	R2
Ag3PO4_NaH2PO4_0.1M	0	0.3945	0.990	1	0.0063	0.988
Ag3PO4-NaH2PO4-0.2M	0	0.5626	0.991	1	0.0065	0.998
Ag3PO4_Na2HPO4_0.1M	0	1.0027	0.996	1	0.0244	0.996
Ag3PO4_Na2HPO4_0.2M	0	1.2889	0.997	1	0.0250	0.974
Ag3PO4_Na3PO4_0.1M	0	1.8342	0.996	2	0.0023	0.97
Ag3PO4_Na3PO4_0.2M	0	1.4366	0.993	2	0.0025	0.998
Sample	Reaction order (n)	k1 (μM∙min−1)	R2	Reaction order (n)	k2 (s−1/M−1)	R2

). The first step is the zero-order decolorization of azo dye bonds (R₁─N=N─R₂), probably by direct hole oxidation (0–60 min) since this is the thermodynamic and electrochemical facile pathway [24]. The second step (k₂) is the first-order mineralization (60–120 min) of the intermediates (benzenesulfonic acid; 4-hydrazinylaniline; phenyl diazene) by reactive radicals (•OH; O₂^•─) [25]. The k₁ values are in excellent agreement with the correlation coefficients (R²) regarding the MO conversion values after 1 h of visible light irradiation. The proposed mechanism can be applied to most samples; however, for Ag₃PO₄-Na₃PO₄-0.2M and Ag₃PO₄-Na₃PO₄-0.1M (Table 2), the degradation of intermediates follows second-order kinetics [26]. This could mean that the relatively fast mineralization of intermediates could also occur by direct hole oxidation and not by reactive radicals [27,28].

3.3. Stability Investigation of Ag₃PO₄

3.3.1. Recharacterization of the Investigated Ag₃PO₄

After the photocatalytic tests, we once again investigated the structural and optical parameters of the samples to examine the stability of the photocatalysts. After the photocatalytic activity, a slight modification was observed in the XRD patterns (Figure 7a) because not only the typical Ag₃PO₄ reflections (presented in Figure 2a), but a specific reflection of Ag (nano)particles was also observed at 38.1° (COD-00-110-0136). Based on our previous measurements [29,30], a silver-based material might lose photocatalytic activity due to the presence of Ag nanoparticles on the surface. Still, several publications in the literature state that Ag-based materials can be used quasi-unlimited times in (photo)catalytic processes [10]. Besides the deposited Ag nanoparticles, the typical reflection of Ag₄P₂O₇ was also observed after the photocatalytic degradation, which was noticed before the degradation. Moreover, the optical parameters (Figure 7b–c) were also evaluated, and the decreasing intensity of the absorption band of Ag₃PO₄ was observed. The typical plasmon resonance band could not be clearly identified. Thus, it can be concluded that the amount of Ag nanoparticles was lower than the detection limit of the device applied. The appearance of Ag_xO is also possible because their reflection is close to that of Ag, and they do not have plasmon resonance bands in the reflectance spectra.

3.3.2. Reusability of Ag₃PO₄ Samples: Recycling Tests and Photoluminescence Measurements

To investigate reusability, we have chosen the Ag₃PO₄_Na₂HPO₄_0.2M and Ag₃PO₄_Na₃PO₄_0.2M samples because these were the most efficient photocatalysts with a degradation of 89% and 95%, respectively (Table 1). The samples had photocatalytic activity after the second round as well (Figure 8a); however, a decrease in the degradation of MO was observed. The reason for this decrease was investigated by photoluminescence measurements (Figure 8b) to examine the recombination properties of Ag₃PO₄. Similar to Mohammadreza Batvandi et al. [31], two of the observed emission bands in our samples were in the violet and blue-cyan wavelength regions. The emission band observed in the violet region corresponds to charge transfer and self-trapping [32]. The blue-cyan wavelength can be assigned to surface oxygen vacancies and defects. After degradation, the intensity of these bands decreased, and in parallel, the band in the violet region increased slightly. The reason for the decreasing bands may be the disappearance of surface oxygen vacancies and defects, which may result from the deposition of Ag nanoparticles on the semiconductor surface. Based on our interpretation, the disappearance of oxygens vacancies and significant disappearances of defects resulted in a decrease in photocatalytic activity.

Considering all the results presented above, we proposed a possible charge transfer mechanism (Figure 9). Before elaborating on the mechanism, we summarized the main conclusions that are indispensable for understanding the mechanism as follows:

• Besides the Ag₃PO₄, only Ag₄P₂O₇ was observed after the synthesis. Typical bands for surface oxygen vacancies and defects were also observed in the PL spectra.
• Two different pathways were observed using MO as a model pollutant, and the kinetics parameters changed after the first hour.
• The deposition of Ag nanoparticles was observed after MO degradation, which resulted in the lack of surface oxygen vacancies and defects.

Defects and vacancies are essential for the degradation of MO. The degradation of MO occurs by reactive radicals/h⁺ formed in the valence bands of materials. After irradiation, structural changes were observed in our materials. We assume that Ag nanoparticles (Figure 9) overlap with or replace vacancies and defects (demonstrated by PL measurements; Figure 8b). These nanoparticles change the reaction mechanism, which was confirmed by the change in the kinetic parameters (from 0th→1st and 2nd order reaction; Table 2). This assumption was confirmed by the typical band observed in the PL spectra, attributed to charge transfer and self-trapping (Figure 8b). This also caused fewer holes to form. Thus, the charge transfer between Ag nanoparticles and Ag₃PO₄ occurs by utilizing their localized surface plasmon resonance effect (Figure 9). Thereby, it can be deduced that the formed Ag nanoparticles do not modify the photocatalysts; they only change the mechanism of MO degradation by promoting the transfer of electrons to the semiconductor conduction band. Consequently, the formation of holes is no longer favored. The lack of charge carrier holes also results in a lower photocatalytic efficiency, supported by the MO degradation rate slowing down during the 2-h-long experiment. This is also supported by the lower MO degradation in the second run of the reusability tests. In addition, the formation of Ag NPs could also degrade the Ag₃PO₄ and could damage the semiconductor via Ag⁺ ions being reduced into Ag NPs. These are observable by the change in the DR spectra of the samples before (Figure 4a) and after (Figure 7b) the degradation.

Although Ag nanoparticles were formed on the surface, they participated in electron transfer (transferring electrons to the conduction band of the semiconductor) processes, which did not promote the degradation of MO.

4. Conclusions

This paper examined the effect of different phosphate sources on the synthesis and photocatalytic activity of Ag₃PO₄. We proved that the formation of Ag₄P₂O₇ depends on the nature of the phosphate source. The type of phosphate sources influenced the light absorption properties and photocatalytic activity of the samples. We concluded that Ag₄P₂O₇ inhibits the photocatalytic activity of Ag₃PO₄. In addition to other similar publications in the literature, we also investigated the stability and reusability of Ag₃PO₄. We concluded that Ag species were formed on the Ag₃PO₄, which resulted in a slightly lower methyl orange degradation during the reusability processes. The difference could be attributed to the localized surface plasmon resonance of Ag nanoparticles, promoting the transfer of electrons within the semiconductor and preventing hole formation. This fact was supported by PL measurements. Considering the characterization results obtained before and after the photocatalytic tests, we concluded that Ag₃PO₄-based materials could be reliably used for the degradation of MO as they mostly retain their photoactivity during second recycling test.