Barium-Impregnated Ag₃PO₄ for Enhanced Visible Light Photocatalytic Degradation of Methyl Orange

Abstract

In this study, we highlight the use of the alkaline earth metal barium (Ba) for the impregnation of Ag₃PO₄ (AgP). AgP was synthesized via co-precipitation and subsequently impregnated with a Ba²⁺-containing solution, followed by hydrothermal treatment to obtain Ba-AgP. The addition of barium significantly influenced both the crystallinity and crystallite size. Ba impregnation enhanced the crystallinity of AgP and promoted the growth of its crystallites. It was confirmed that Ba²⁺ was homogeneously distributed on the surface of AgP, with only a slight effect on particle shape. Ba-impregnated Ag₃PO₄ (Ba-AgP) exhibited improved photocatalytic activity for the degradation of methyl orange (MO) under visible light compared to bare AgP. The optimal impregnation concentration of Ba²⁺ was determined to be 6%. This enhancement is attributed to the role of Ba²⁺ in facilitating the separation of photogenerated electron–hole pairs, which also contributed to the improved stability of AgP. The active species h⁺, ·OH, and O₂·⁻ were all identified as essential for the MO degradation process, with h⁺ being the most significant contributor.

1. Introduction

Semiconductor photocatalysis has recently gained significant attention for environmental pollution remediation, including the degradation of organic molecules in aqueous solutions [1] and air purification [2]. This process relies on the excitation of a semiconductor by light, generating electron–hole pairs (e⁻/h⁺). These charge carriers can participate in redox reactions with pollutants either directly or indirectly by producing reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and superoxide radicals (O₂·⁻), leading to pollutant degradation [3,4]. However, significant challenges face photocatalysis, among the need to identify semiconductors with high quantum efficiency and the preference for renewable solar light as the energy source [5,6,7].

Among visible-light-driven photocatalysts, silver orthophosphate (Ag₃PO₄) has demonstrated remarkable potential due to its narrow band gap (~2.30 eV), which enables strong absorption in the visible light range, as well as its high efficiency in the photocatalytic degradation of a wide variety of organic pollutants—an effect attributed to its highly positive valence band potential [8,9]. However, its practical application is limited by the rapid recombination of photogenerated charge carriers and the photoreduction of Ag⁺ to metallic silver (Ag⁰), both of which compromise the long-term stability of Ag₃PO₄ during extended photocatalytic operation [10,11].

To address these limitations, various strategies have been explored, including the incorporation of foreign metal and non-metal elements through doping or impregnation, which can modify the electronic structure and enhance charge separation [12,13,14,15,16,17]. Another effective approach involves the construction of heterojunction photocatalysts by coupling Ag₃PO₄ with other semiconductors, thereby facilitating more efficient charge separation and suppressing recombination. Notable examples include Ag₃PO₄@ZnIn₂S₄ [18], Ag₃PO₄/C₃N₅ [19], Ag₃PO₄/Ag₄P₂O₇ [20], MoO₃/Ag₃PO₄ [21], Bi₂MoO₆/Ag₃PO₄ [22], and rGO/Ag₃PO₄/CeO₂ [23].

In contrast, the modification of Ag₃PO₄ using alkaline earth metals has received relatively limited attention. When introduced through doping or impregnation, alkaline earth metals have been reported to enhance photocatalytic activity; however, the underlying mechanisms remain a subject of debate [24]. Some studies argue that these elements, due to their tendency to lose two electrons and achieve a stable electron configuration, do not introduce mid-gap states within the host material [25]. Conversely, other reports suggest that alkaline earth metals can behave similarly to transition metals by introducing electronic sublevels into the band gap, thereby narrowing the band gap and acting as electron acceptors—ultimately promoting charge separation [26,27,28,29]. Despite these theoretical discrepancies, numerous studies have confirmed the effectiveness of alkaline earth metals in improving photocatalytic performance [30,31,32,33].

In the present study, a series of Ba²⁺-impregnated Ag₃PO₄ (Ba-AgP) photocatalysts were synthesized with varying Ba²⁺ contents (2%, 4%, and 6%). To the best of our knowledge, the impregnation of AgP with alkaline earth metals has not been previously reported. AgP was first synthesized via a co-precipitation method, followed by Ba²⁺ incorporation through a wet impregnation technique and subsequent hydrothermal treatment. Comprehensive structural characterization confirmed the successful incorporation and homogeneous distribution of Ba²⁺ on the catalyst surface, as evidenced by HRTEM/EDX analysis. XRD results further revealed that Ba impregnation enhanced the crystallinity of AgP, with crystallinity increasing proportionally with Ba²⁺ content. Although the optical properties remained largely unchanged, a slight decrease in light absorption intensity was observed for the Ba-impregnated samples compared to the pristine phase. Photocatalytic degradation tests using MO, a persistent azo dye, demonstrated that Ba incorporation improved the photocatalytic efficiency of AgP, with the highest performance achieved with 6% Ba-AgP. Additionally, durability assessments showed that 6% Ba-AgP exhibited significantly enhanced stability compared to pure AgP, highlighting the beneficial effect of Ba²⁺ impregnation on both photocatalytic activity and structural integrity.

2. Materials and Methods

2.1. Materials

The commercial raw materials used in this study include AgNO₃ (100.5%), Na₂HPO₄ (98–100.5%), Ba(NO₃)₂ (≥99%), HNO₃ (≥65%), NaOH (≥98%), methyl orange (MO, 100%), isopropyl alcohol (IPA, ≥98%), ethylenediaminetetraacetic acid (EDTA, 99.4–100.6%), and L-ascorbic acid (LAA, ≥99%). All reagents were supplied by Sigma-Aldrich, Darmstadt, Germany.

2.2. Synthesis

Stoichiometric Ag₃PO₄ (AgP) was synthesized via a co-precipitation method using AgNO₃ and Na₂HPO₄ as precursors. Specific quantities of each precursor, as described in our previous work [34], were individually dissolved in deionized water to form aqueous solutions. The Na₂HPO₄ solution was then added dropwise to the AgNO₃ solution under vigorous stirring, resulting in the immediate formation of a golden-yellow AgP precipitate. The precipitate was subsequently collected and thoroughly washed with distilled water, followed by drying at 80 °C. The resulting light-green AgP was finely ground and sieved to ensure a uniform particle size distribution. Ba-modified Ag₃PO₄ (Ba-AgP) was prepared via a wet impregnation method using the previously synthesized AgP. The AgP was dispersed in an aqueous solution of Ba(NO₃)₂ at concentrations corresponding to the desired loadings (2%, 4%, and 6%). The mixtures were stirred for 1 h, followed by hydrothermal treatment in an oven at 90 °C for 2 h. The resulting materials were then collected, thoroughly washed with distilled water, and dried at 90 °C.

2.3. Characterization of Materials

The structural properties of the materials were analyzed using powder X-ray diffraction (XRD) on a D2 PHASER diffractometer from Bruker AXS GmbH, Karlsruhe, Germany (CuKα radiation, λ = 1.54 Å, 30 kV, 10 mA) over a 2θ range of 10° to 80°. The textural and morphological characteristics were investigated using scanning electron microscopy (SEM, Quanta 600, 30 kV) from FEI Company, Hillsboro, OR, USA; transmission electron microscopy (TEM, JEOL 1011, 80 kV) from JEOL Ltd., Tokyo, Japan; and field emission scanning electron microscopy (FESEM, SCIOS 2) from FEI Company, Hillsboro, OR, USA. The elemental composition and distribution were examined via energy-dispersive X-ray spectroscopy (EDX) coupled with high-resolution transmission electron microscopy (HRTEM/EDX, JEOL JEM-2010) from JEOL Ltd., Tokyo, Japan. The Fourier transform infrared (FTIR) spectra were recorded in the range of 400–4000 cm⁻¹ using a SP-FTIR-1 spectrophotometer from SCO-TECH, Germany. The optical properties were assessed by UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS) using an Agilent Cary 5000 spectrophotometer from Agilent technologies Santa Clara, CA, USA.

2.4. Photocatalytic Activity Measurements

The photocatalytic activity of the synthesized materials was evaluated through the degradation of MO. All experiments were carried out in a home-made photoreactor equipped with a 250 W metal halide lamp fitted with a cutoff filter (λ > 400 nm) and a cooling fan to maintain a stable reaction temperature. Typically, 20 mg of catalyst was dispersed in 100 mL of MO solution with a concentration of 10 mg/L. Prior to light irradiation, the suspension was stirred in the dark for 60 min to achieve adsorption–desorption equilibrium. During the experiment, 5 mL aliquots were withdrawn at regular intervals and analyzed using a UV–Vis photometer to monitor the residual concentration of MO.

To investigate the influence of operating parameters, the effects of initial dye concentration and solution pH were also examined. MO concentrations of 10, 15, and 20 mg/L were tested, along with pH values of 3, 5, 7, and 9.

The photocatalytic degradation kinetics were analyzed using a pseudo-first-order model, expressed by the following equation:

ln(C/C₀) = −k.t

(1)

where C (mg/L) is the MO concentration at time t, C₀ (mg/L) is the initial concentration at t = 0, k (min⁻¹) is the apparent rate constant, and t is the reaction time in minutes.

3. Results

3.1. Characterization

3.1.1. XRD Analysis

The differences in crystal phase and crystallite size between pristine AgP and Ba-AgP were investigated using X-ray diffraction (XRD), and the resulting patterns are presented in Figure 1. The diffraction peaks corresponding to the Ag₃PO₄ phase exhibit fifteen characteristic Bragg reflections at 2θ values of 20.549°, 29.169°, 32.690°, 35.900°, 41.694°, 46.880°, 51.690°, 53.950°, 56.171°, 60.449°, 64.720°, 68.480°, 70.654°, 72.380°, and 76.460°. These peaks are indexed to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (411), (420), (421), (332), and (422) crystallographic planes, consistent with a body-centered cubic structure and a space group $P\overline{43}n$, in accordance with JCPDS No. 06-0505.

The Ba-AgP samples exhibit the same set of peaks, although slight shifts in peak positions and notable changes in intensities were observed, indicating structural modifications due to Ba incorporation. For comparison, we focused on the peaks corresponding to the (200), (210), and (211) planes. In pristine AgP, these peaks appear at 29.169°, 32.690°, and 35.900°, respectively. Upon impregnation with 2% Ba, the positions shifted to 29.190°, 32.700°, and 35.920°, followed by 29.189°, 32.710°, and 35.921° for 4% Ba-AgP, and finally to 29.140°, 32.669°, and 35.880° for 6% Ba-AgP. These shifts in 2θ values reflect variations in the interplanar spacing, suggesting changes in the unit cell dimensions due to Ba²⁺ incorporation. The intensity enhancement observed with increasing Ba content further supports this structural modification, consistent with previous findings [35].

The calculated unit cell volumes were 229.55 ų for AgP, 228.89 ų for 2% Ba-AgP, 229.60 ų for 4% Ba-AgP, and 229.14 ų for 6% Ba-AgP. These changes in lattice parameters are attributed to the partial substitution of Ag⁺ by Ba²⁺ in the outer lattice layers of AgP, where Ag⁺ deficiency is likely. The incorporation of larger Ba²⁺ ions causes slight lattice distortions, leading to the observed structural differences.

The influence of Ba content on crystallite growth was also examined using the well-known Scherrer equation to estimate the average crystallite size:

$$D={\displaystyle \frac{k\lambda }{\beta cos\theta }}$$

(2)

where D is the average crystallite size (nm), λ is the X-ray wavelength (0.15406 nm), k is the shape factor (typically 0.9), β is the full width at half maximum (FWHM) of the selected diffraction peak (in radians), and θ is the Bragg angle (in radians).

Based on this equation, the calculated average crystallite sizes were 14.33 nm for pristine AgP, 15.19 nm for 2% Ba-AgP, 17.40 nm for 4% Ba-AgP, and 224.48 nm for 6% Ba-AgP. A clear trend of increasing crystallite size with increasing Ba content was observed. This is particularly evident in the XRD patterns, where the 6% Ba-AgP sample displays the sharpest and most intense diffraction peaks, indicative of enhanced crystallinity and larger crystallite domains. The observed growth in crystallite size can be attributed to the hydrothermal treatment, which promotes the partial dissolution and infiltration of Ba²⁺ ions into the AgP lattice. This incorporation likely induces local lattice distortions and facilitates strain relaxation. These effects reduce the density of nucleation sites during crystallization, thus favoring the growth of larger crystallites.

3.1.2. FTIR Analysis

The chemical composition of the samples was confirmed by analyzing the vibrational modes of functional groups through Fourier transform infrared (FTIR) spectroscopy, with the spectra recorded in the range of 4000–400 cm⁻¹, as shown in Figure 2. The characteristic bands of Ag₃PO₄ observed in the pristine sample are also present in the Ba-impregnated AgP photocatalysts, with only minor variations in frequency and intensity. The fingerprint region of Ag₃PO₄ is defined by five bands between 1400 and 400 cm⁻¹. The band at 553 cm⁻¹ corresponds to the symmetric and asymmetric stretching vibrations of the O–P=O bond. The bands at 678 and 867 cm⁻¹ are attributed to the symmetric stretching modes of the P–O–P linkage. The corresponding asymmetric and antisymmetric stretching vibrations are observed at 1020 cm⁻¹ (narrow and intense) and 1383 cm⁻¹, respectively. The large band between 3688 and 2508 cm⁻¹ is attributed to the stretching vibration of bounded O-H; its bending vibration is represented by the narrow band at 1663 cm⁻¹.

In the Ba-impregnated Ag₃PO₄ samples, the characteristic FTIR bands appear slightly more intense than in the pristine material, particularly in the 2% Ba-AgP sample. Additionally, noticeable shifts in wavenumber are observed. These variations can be attributed to the incorporation of Ba²⁺ ions into the Ag₃PO₄ lattice, as previously confirmed by XRD analysis. The integration of Ba²⁺ alters the local chemical environment surrounding the PO₄³⁻ groups. Since vibrational frequencies are highly sensitive to changes in bond strength and interatomic distances, any substitution involving ions of different electronegativities or ionic radii—such as replacing Ag⁺ with Ba²⁺—can significantly affect the electronic distribution around the phosphorus center. This, in turn, influences the P–O and P=O bond characteristics, leading to shifts in the corresponding vibrational modes. Therefore, the observed changes in both position and intensity of the FTIR bands further support the successful incorporation of Ba²⁺ into the Ag₃PO₄ structure, consistent with the structural modifications inferred from the XRD data.

3.1.3. SEM, TEM, FESEM, and HRTEM/EDX Analysis

The morphology and particle size of AgP, as well as the changes induced by Ba impregnation, were investigated using various electron microscopy techniques. SEM and TEM images of AgP, 2% Ba-AgP, and 4% Ba-AgP are presented in Figure 3, while detailed SEM, FESEM, and HRTEM/EDX images of 6% Ba-AgP are shown in Figure 4. The SEM and TEM/HRTEM analyses reveal that both pristine and Ba-impregnated AgP samples consist predominantly of spherical particles, indicative of isotropic growth during synthesis. These particles tend to form agglomerates, likely due to high surface energy. Pristine AgP contains particles with a diameter of 466 nm, which increases to 557 nm for 2% Ba-AgP, then slightly decreases to 553 nm for 4% Ba-AgP, and further reduces to 484 nm for 6% Ba-AgP. This trend suggests that while initial Ba addition promotes crystal growth, higher Ba concentrations may introduce structural distortions or nucleation barriers that limit particle expansion. Moreover, the TEM images reveal uniformly contrasted particles following Ba impregnation, suggesting the formation of a single phase with no evidence of secondary phases.

In Figure 4c, a distinct morphological feature can be observed in which one particle appears enclosed within another, resembling the pattern of dots on a die. This configuration suggests the formation of a core–shell-like structure composed of the same material (AgP). Such a feature may result from the sequential nucleation and growth of smaller particles during synthesis, likely influenced by the stepwise addition of precursors. The presence and distribution of Ba on the surface of AgP were further confirmed by HRTEM/EDX elemental mapping and quantification. As shown in Figure 4d, EDX analysis confirms the presence of Ba, while the corresponding elemental mapping in Figure 4e reveals that Ba is uniformly distributed across the surface of the AgP particles, indicating successful surface impregnation and homogeneous dispersion of Ba²⁺.

3.1.4. UV–Vis DRS Analysis

The optical properties of AgP and Ba-AgP photocatalysts were studied using UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), with the resulting spectra presented in Figure 5. The absorption edges of the materials were analyzed to estimate their optical band-gap energies, which were determined using the Tauc–Mott method.

(αhυ)^1∕2 = A(hυ − E_g)

(3)

where α is the absorption coefficient, h is Planck’s constant, ν is the frequency of light, A is the proportionality constant, and E_g is the band-gap energy determined by extrapolating the linear portion of the plot to the x-axis.

The Tauc–Mott plots reveal that both pristine AgP and Ba-AgP samples exhibit the same optical band-gap energy of approximately 2.31 eV. This value corresponds to an absorption edge near 518 nm, as determined from the absorbance versus wavelength spectra. Despite having identical band-gap energies, differences in absorption behavior were observed. Pristine AgP displays a higher overall absorption intensity compared to Ba-AgP, indicating superior light-harvesting capability in the visible range. This suggests that while Ba incorporation does not significantly alter the band gap, it may influence the optical absorption efficiency due to changes in surface properties or electronic structure.

The edge potentials of the valence band E_vb and conduction band E_cb were calculated using the following empirical equations:

E_vb = χ − E_e + 1/2·E_g

(4)

E_cb = E_vb − E_g

(5)

where χ is the mean electronegativity, and E_e represents the energy of free electrons (̴ 4.5 eV). The calculated values for AgP are E_vb = 2.59 eV and E_cb = 0.28 eV.

3.2. Photocatalytic Activity Evaluation

3.2.1. Effect of Ba on the Surface of AgP

Figure 6 presents the kinetics of MO degradation for AgP and Ba-AgP photocatalysts at varying Ba concentrations (2%, 4%, and 6%). The incorporation of Ba onto the surface of AgP significantly enhances its photocatalytic efficiency. After 75 min of reaction, the degradation efficiencies were 65.63%, 87.11%, 79.18%, and 85.45% for AgP, 2% Ba-AgP, 4% Ba-AgP, and 6% Ba-AgP, respectively. Among these, the 6% Ba-AgP catalyst exhibited the highest degradation efficiency, with a kinetic rate constant of 0.0268 min⁻¹, which is 1.8 times greater than that of pristine AgP.

Table 1. MO degradation rate constants (min⁻¹) for AgP and Ba-AgP under different conditions.

	Concentration Effect			pH Effect
	10 mg/L	15 mg/L	20 mg/L	pH = 3	pH = 5	pH = 7	pH = 9
AgP	0.0146	0.0036	0.0078	0.0021	0.019	0.0146	0.0111
2%Ba-AgP	0.0243	0.0126	0.0119	0.0023	0.0218	0.0243	0.0275
4%Ba-AgP	0.0223	0.0119	0.009	0.0012	0.0184	0.0223	0.0311
6%Ba-AgP	0.0268	0.0141	0.0112	0.0024	0.0186	0.0268	0.0336

summarizes the rate constants for each sample. The enhanced degradation performance of Ba-AgP can be attributed to several factors. Ba²⁺ ions on the surface likely act as electron acceptors, capturing photoexcited electrons in their vacant orbitals. This helps to inhibit electron–hole recombination, a major factor contributing to the lower activity of photocatalysts.

3.2.2. Effect of MO Concentration

The photocatalytic efficiency is significantly influenced by the initial concentration of the dye solution, particularly when the photocatalyst dosage is kept constant. In this study, a fixed catalyst loading of 6% Ba-AgP was maintained to ensure consistent reaction conditions. The performance of the photocatalysts was evaluated at an initial concentration of MO of 10 mg/L, 15 mg/L, and 20 mg/L. Figure 7 graphically illustrates the influence of the initial MO concentration on the photocatalytic performance of 6% Ba-AgP catalyst, while Table 1 summarizes the corresponding kinetic parameters.

As shown in the figure, an increase in MO concentration results in a noticeable decline in degradation efficiency. This trend can be attributed to several interrelated factors. At higher dye concentrations, a larger number of MO molecules compete for active sites on the catalyst surface, limiting the probability of effective interaction under fixed photocatalyst loading. Furthermore, the increased dye concentration leads to a shielding effect, wherein the excess MO molecules hinder light penetration by absorbing and scattering the incident photons. This reduces the intensity of light reaching the catalyst surface, thereby diminishing the generation of photoinduced charge carriers essential for the degradation process. In addition, MO itself absorbs a significant portion of the incoming light, further reducing the available photon flux. Collectively, these effects contribute to the reduced photocatalytic efficiency observed at elevated dye concentrations.

3.2.3. Effect of pH

The effect of pH on the photocatalytic performance of 6% Ba-AgP was investigated under four different pH conditions: 3, 5, 7, and 9. For consistency, all other experimental parameters were maintained constant. The results of the photocatalytic experiments are presented in Figure 8A. The photocatalytic performance of 6% Ba-AgP varied significantly with changes in the pH of the solution. At pH 3, corresponding to a strongly acidic environment, the catalyst showed markedly reduced activity, achieving only 30% degradation efficiency and a low rate constant of 0.0024 min⁻¹. This inhibition is likely due to the increased concentration of H⁺ ions, which may compete with the dye molecules for adsorption sites or neutralize reactive species such as hydroxyl radicals. As the pH increased to 5, the degradation efficiency improved substantially to 76%, accompanied by a higher rate constant of 0.0186 min⁻¹. Enhanced performance was observed near neutral pH, with the rate constant peaking at 0.0268 min⁻¹ at pH 7. Notably, the best photocatalytic activity occurred in alkaline conditions (pH 9), where a degradation efficiency of 90% was achieved, along with the highest rate constant of 0.0336 min⁻¹. This enhancement can be attributed to the greater availability of OH⁻ ions, which promote the formation of highly reactive hydroxyl radicals under light irradiation, thus accelerating the degradation process.

To better interpret these results, the surface charge properties of the photocatalysts were examined by determining the point of zero charge (pH_Pzc), as presented in Figure 8B The pH_Pzc values were found to be 9.19 for 6% Ba-AgP and 9.51 for pristine AgP. These values indicate that the surface of the photocatalyst is positively charged at pH values below the pHₚzc and becomes negatively charged above it.

At pH 3, MO primarily exists in its zwitterionic form. Under these conditions, the photocatalyst surface is also positively charged (pH < pH_Pzc), leading to electrostatic repulsion between the positively charged sites on MO and the catalyst surface. This repulsion likely hinders MO adsorption, contributing to the observed low photocatalytic activity. Furthermore, the strongly acidic environment can suppress the availability of hydroxide ions (OH⁻), which are essential for generating reactive oxygen species involved in the degradation process. Additionally, such acidic conditions may induce partial dissolution or surface etching of the photocatalyst, further reducing its effectiveness.

At pH 5, the photocatalytic degradation efficiency improved significantly. This enhancement can be attributed to the fact that pH 5 is above the pKa of MO (pKa = 3.5), where the dye predominantly exists in its deprotonated (anionic) form. This anionic MO form experiences stronger electrostatic attraction toward the positively charged surface of the photocatalyst (since pH 5 < pH_Pzc), promoting better adsorption and subsequent degradation. Additionally, this mildly acidic environment is less likely to affect the structural stability of the catalyst, ensuring sustained photocatalytic performance.

The best photocatalytic activity was observed at pH values above 7. In this alkaline range, the surface of the photocatalyst remains positively charged (as pH < pH_Pzc), promoting strong electrostatic attraction toward the negatively charged sulfonate groups (–SO₃⁻) of MO. Additionally, the increased concentration of OH⁻ plays a crucial role in enhancing photocatalytic degradation. These OH⁻ ions can be readily oxidized by photogenerated holes to produce hydroxyl radicals (·OH), which are highly reactive species responsible for breaking down organic pollutants. The synergistic effect of enhanced dye adsorption and greater ·OH generation explains the superior photocatalytic efficiency under basic conditions.

3.2.4. Photocatalytic Mechanism

In photocatalysis, the primary reactive species responsible for the degradation of pollutants are photogenerated holes (h⁺), hydroxyl radicals (·OH), and superoxide radicals (O₂·⁻). When a photocatalyst absorbs electromagnetic radiation with energy equal to or greater than its band gap, electrons (e⁻) in the valence band (VB) are excited to the conduction band (CB), leaving behind positively charged holes in the VB. These photogenerated charge carriers can then participate in redox reactions at the catalyst surface. The nature of the active species involved depends strongly on the positions of the valence and conduction band edges relative to the redox potentials of the species in the solution. For instance, holes can directly oxidize organic pollutants or react with surface-adsorbed water or hydroxide ions to form ·OH radicals. Simultaneously, excited electrons in the CB can reduce dissolved oxygen to generate O₂·⁻ radicals. The interplay between these reactive species determines the overall efficiency and pathway of photocatalytic degradation.

To determine the predominant reactive species involved in the photocatalytic degradation of MO by Ba-AgP, scavenger-based quenching experiments were conducted. Isopropyl alcohol (IPA) was used as a scavenger for hydroxyl radicals (·OH), L-ascorbic acid (LAA) was used to quench superoxide radicals (O₂·⁻), and ethylenediaminetetraacetic acid (EDTA) was used to capture photogenerated holes (h⁺). The obtained results are compared with the test in the absence of any scavenger and are illustrated in Figure 9.

Based on the obtained results, it can be concluded that the photocatalytic degradation of MO by Ba-AgP involves a synergistic action of multiple reactive species, namely holes (h⁺), superoxide radicals (O₂·⁻), and hydroxyl radicals (·OH). Among these, h⁺ plays the most dominant role in the degradation process, as evidenced by the complete suppression of photocatalytic activity upon the addition of EDTA. O₂·⁻ also contributes significantly as its presence resulted in a marked reduction in the degradation efficiency. ·OH had a comparatively lesser impact, with only a slight decrease in activity observed after the introduction of IPA. Based on these findings, a possible mechanism for the photocatalytic degradation of MO is proposed. The Ba²⁺ ions play a key role in accepting the excited electrons from the conduction band, thereby significantly reducing the recombination of photogenerated electron–hole pairs. These electrons then interact with O₂ to generate O₂·⁻. It is noteworthy that this reaction does not occur directly between O₂ and the conduction band, as the energy level of the conduction band (0.28 eV) is insufficient. The generated O₂·⁻ radicals participate in the degradation of MO either directly, as shown in the scavenger experiment, or indirectly by reacting with water (H₂O) to produce ·OH. Meanwhile, the photogenerated holes (h⁺) in the valence band contribute to the photocatalytic process by either directly transferring charge to the MO molecules or reacting with OH⁻ in the solution to form more ·OH. Therefore, the photocatalytic degradation of MO is a collaborative process involving the active species h⁺, O₂·⁻, and ·OH, which participate either directly or indirectly in the degradation pathway.

3.2.5. Recyclability

Evaluating the durability of the newly developed Ba-modified AgP catalyst is essential, as the primary goal was to enhance the stability of AgP while extending its visible light absorption capability. After confirming its improved photocatalytic efficiency in degrading MO, we assessed the recyclability of Ba-AgP over three consecutive cycles and compared it to the unmodified AgP catalyst (Figure 10). The Ba-AgP catalyst retained over 90% of its initial activity after three cycles, whereas the performance of pristine AgP declined significantly to 74%. This decline is consistent with literature reports attributing AgP’s reduced stability to partial dissolution and self-corrosion during repeated photocatalytic cycles [36,37]. The improved stability of Ba-AgP is likely due to the enhanced separation of photogenerated electron–hole pairs, promoted by the presence of Ba²⁺ ions on the catalyst surface. These ions serve as electron acceptors, minimizing charge recombination. Furthermore, the incorporation of Ba appears to mitigate the dissolution of AgP in aqueous environments by reducing the leaching of Ag⁺ ions. This protective effect contributes to the sustained photocatalytic performance of Ba-AgP over multiple uses, highlighting its potential for long-term application.

4. Conclusions

In this study, we developed a novel approach for modifying Ag₃PO₄ by impregnating it with varying amounts of barium. Ag₃PO₄ was synthesized via a co-precipitation method, followed by Ba incorporation through wet impregnation and subsequent hydrothermal treatment. The comprehensive characterization revealed that Ba addition enhanced the crystallinity and promoted crystallite growth. Notably, the FESEM analysis of 6% Ba-Ag₃PO₄ displayed unique dice-like morphologies indicative of a core–shell structure, while HRTEM/EDX mapping confirmed uniform Ba distribution across the AgP surface. Although Ba²⁺ incorporation did not significantly alter the band gap of AgP, a slight reduction in absorbance was observed. The photocatalytic activity, assessed via methyl orange degradation, showed marked improvement with Ba modification, with 6% Ba-AgP demonstrating the highest efficiency. pH-dependent studies revealed enhanced degradation under basic conditions, attributed to favorable surface charge interactions and increased hydroxyl radical formation. Scavenger experiments identified holes (h⁺) as the dominant reactive species, followed by superoxide radicals (O₂⁻) and hydroxyl radicals (·OH), all playing a role in MO degradation. Recyclability tests further confirmed the enhanced stability of Ba-AgP compared to pristine AgP, likely due to suppressed Ag⁺ leaching and improved charge separation. Overall, this work underscores the potential of alkaline earth metals like Ba²⁺ to enhance the photocatalytic performance and durability of AgP, offering a viable alternative to more commonly used transition metals.