Sunlight-driven fast photo-degradation of Eriochrome Black T dye using highly efficient La-doped Ag₃PO₄ decorated with ZnS QDs

Abstract

The untreated discharge of dye-contaminated effluents into aquatic environments poses serious risks to both environmental integrity and public health. Therefore, the development of efficient dye removal strategies is essential for pollution control and ecosystem protection. This study aims to investigate and optimize the photocatalytic degradation of Eriochrome Black T (EBT) in aqueous solutions using novel Ag₃PO₄-based composites, including lanthanum-doped Ag₃PO₄, and ZnS quantum dots under sunlight irradiation. Pure Ag₃PO₄, 2% and 6% La-doped Ag₃PO₄, and ZnS quantum dots were synthesized via a co-precipitation method, and their composite photocatalysts were fabricated using a hybrid mixing approach. The characterizations of the materials were carried out using X-ray diffraction, BET surface area analysis, UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) to evaluate their structural, optical, and morphological properties. The synthesized photocatalysts, including pure Ag₃PO₄, 2% and 6% La-doped Ag₃PO₄, ZnS quantum dots, and their composite systems, exhibited well-defined crystalline structures, as confirmed by X-ray diffraction (XRD) analysis. UV-vis DRS analysis showed that pure Ag₃PO₄ had a band gap of 2.41 eV, which decreased to 2.39 eV for 2% La-doped Ag₃PO₄ and 2.36 eV for 6% La-doped Ag₃PO₄, while ZnS quantum dots exhibited a band gap of 3.6 eV, and the 6% La-doped Ag₃PO₄/ZnS QD composite showed a significantly reduced band gap of 1.5 eV. Upon the incorporation of ZnS QDs into the 6% La-Ag₃PO₄ particles, the surface area of the 6% La-Ag₃PO₄ heterojunction composite increased from 133.446 to 141.120 m² g⁻¹. The photocatalytic activity of the synthesized Ag₃PO₄-based materials was evaluated through the degradation of Eriochrome Black T (EBT) under light irradiation. The influence of key operational parameters, including the solution pH (3–10), dye concentration (10–20 ppm), photocatalyst dosage (0.05–0.15 g), and irradiation time (5–120 min), was systematically investigated to assess their effect on degradation efficiency. Among the examined photocatalysts, 6% La-doped Ag₃PO₄ exhibited the highest photocatalytic performance, demonstrating the beneficial role of lanthanum incorporation. The degradation efficiencies of 97.84% and 84.88% were achieved using 6% La-doped Ag₃PO₄ and the 6% La-doped Ag₃PO₄/ZnS QD composite, respectively, under the optimized conditions of pH 6 and an irradiation time of 120 min. Overall, these results indicate that La-modified Ag₃PO₄-based photocatalysts are promising and sustainable materials for the effective treatment of dye-contaminated wastewater, offering significant potential for environmental remediation applications.

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1 Introduction

Rapid population growth and ongoing industrial development are major contributors to water pollution, creating serious environmental, economic, and public health challenges by contaminating vital water resources.^1–4 Organic substances like dyes and hazardous pollutants like heavy metals are the main sources of water contamination.^5,6 Azo dyes, commonly applied in textile industries, are a significant source of water pollution. These dyes are extremely resistant to degradation, even under sunlight, with half-lives exceeding two thousand hours.⁷ More than 50% of the dye produced worldwide is Eriochrome Black T (EBT).^8–10 Many industries, including pharmaceutical, paper printing, leather, paint, and cosmetics, utilize it, which contaminates the water.¹¹ Its intricate chemical structure, resistance to photodegradation, and light stability make it a significant target for removal.^12–14

Indiscriminate effluent discharge from industries is largely responsible for the rise in water pollution, causing rapid deterioration in water quality and posing serious environmental threats. Pollutants like EBT accumulate in different parts of plants or plant products, which are then consumed by humans and animals, entering the food chain and having negative effects. Additionally, groundwater may become contaminated by EBT-containing industrial effluents, rendering it unsafe for human consumption.^15–18 Numerous health problems, such as pulmonary toxicity, genetic lesions, genetic mutation, congenital malformations, and oncogenesis, can result from long-term exposure to high concentrations of EBT dye.^19,20 For this reason, wastewater must be properly treated before being released into the environment. It is also vital to develop affordable, efficient, and eco-friendly technology for removing EBT and azo dyes from both water and wastewater sources.

All dyes, especially reactive dyes, cannot always be completely removed using conventional dye removal techniques, which include physical and biological processes. Photocatalysis is one of the most promising processes (AOPs) which helps in the oxidative elimination of organic contaminants. The breakdown of organic pollutants is accelerated by this process, which uses catalysts and light sources. Studies reveal that the pigments in industrial effluents can be removed by photocatalysis at a rate of 70–80%. In an effort to achieve efficient dye degradation, numerous scientists are studying heterogeneous photocatalysts, including WO₃, ZnO, and TiO₂.²¹

Ag₃PO₄ has been synthesized in various shapes as reported in literature such as tetrapod, tetrahedral, cubic, spherical, and trisoctahedral.^22–25 When exposed to visible light, the produced Ag₃PO₄ showed 100% photodegradation of the MB dye. Concave trioctahedral Ag₃PO₄ particles were created. using the same process but with a different phosphate precursor. Furthermore, when employed as a photocatalyst for the degradation of RhB dye, they similarly achieved 100% degradation under visible-light illumination.²³ Many studies have shown that silver phosphate displays excellent photocatalytic performance. However, it also has a high photogenerated charge-carrier recombination rate. Therefore, doping has been incorporated into the host Ag₃PO₄'s lattice site to increase its photocatalytic potential. Dopants such as nickel, bismuth, lanthanum, fluorine, sulfate, carbonate, and rare-earth elements (dysprosium, erbium, and gadolinium) have been identified in several studies. These elements have the potential to enhance the chemical characteristics and photocatalytic efficiency of Ag₃PO₄.^26–32 It has also been reported that lanthanum, as a dopant, alters the surface structure of the host lattice.^33,34 Amirulsyafiee et al. used the coprecipitation method to synthesize La-doped Ag₃PO₄ and studied its photocatalytic performance under visible light. The 1% La-doped Ag₃PO₄ demonstrated superior photocatalytic activity, degrading 81% of methylene blue in 60 minutes and 94% of methyl orange in 30 minutes. The enhanced photocatalytic efficiency was attributed to reduced electron–hole recombination and morphological changes resulting from lanthanum incorporation.^32,35 Ag₃PO₄ suffers from photocorrosion due to Ag⁺ reduction under visible light. Therefore, La³⁺ doping confers Ag₃PO₄ antiphotocorrosion properties by altering internal electron migration pathways, introducing new electronic states, and improving electron migration, thereby enhancing charge separation, facilitating internal electron transfer, and increasing photocatalytic stability.³⁶

Zinc sulfide (ZnS) is one of the most attractive metal chalcogenides due to its intrinsic properties, including high electronic mobility, water insolubility, low cost, and non-toxicity.^37,38 ZnS QDs have been modified for increased photocatalytic effectiveness using techniques such as heterojunction creation and dopant inclusion.³⁹ Recent studies have revealed that tuning the electronic structure and coordination environment of catalysts is an effective strategy to enhance charge separation and improve photocatalytic efficiency.⁴⁰ Furthermore, the construction of heterojunction systems has been widely explored to promote interfacial charge transfer and extend light absorption. In addition, the local structural distortion of catalytic sites can regulate electron density and facilitate charge transfer, which plays a crucial role in determining catalytic performance.⁴¹ The photocatalytic activity of a material is also strongly influenced by its band gap. An effective photocatalyst typically has a band gap in the range of 2–3 eV, allowing efficient visible-light absorption while providing sufficient redox potential for photocatalytic reactions, whereas materials with too low a band gap (<1.23 eV) are generally not suitable, as they cannot facilitate the necessary redox processes effectively.⁴²

In the current work, Ag₃PO₄, La-doped Ag₃PO_4, and ZnS QDs were synthesized via a co-precipitation method. The composites were formed by mixing. The photocatalytic performance of individual components and the composites was studied for the degradation of Eriochrome Black T (EBT) under sunlight irradiation. Comprehensive characterizations of the synthesized materials were carried out using X-ray diffraction, BET surface area analysis, UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) to evaluate their structural, optical, and morphological properties. The influence of key factors, including the dye concentration, catalyst dose, pH, and temperature, was thoroughly investigated. Interestingly, the individual components exhibited higher degradation efficiency for EBT dye than the composites under the conditions studied. No studies are available on undoped Ag₃PO₄ and doped Ag₃PO₄ for EBT degradation. Furthermore, the ZnS quantum dots synthesized via the presented co-precipitation approach have not been explored for EBT degradation. However, direct comparison with previously reported studies is limited, as, to the best of our knowledge, Ag₃PO₄-based photocatalysts have not been explored for EBT degradation. Therefore, this study not only introduces a new photocatalytic system but also extends the application of Ag₃PO₄-based materials to a previously unexplored EBT azo dye, providing new insights into their photocatalytic behaviour.

2 Materials and methods

The chemicals used for synthesis in this research were silver nitrate (AgNO₃; Sigma-Aldrich), potassium dihydrogen phosphate (KH₂PO₄; Scharlau), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O; Sigma-Aldrich), zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O; Scharlau), sodium sulfide (Na₂S; Sigma-Aldrich), sodium dodecyl sulfate (Sigma-Aldrich), deionized water, distilled water, and Eriochrome Black T (Sigma-Aldrich).

2.1 Synthesis of Ag₃PO₄ and La-doped Ag₃PO₄

Silver phosphate (Ag₃PO₄) and lanthanum-doped silver phosphate (La-Ag₃PO₄) were synthesized via a modified co-precipitation method.³⁵ Briefly, 0.1 M AgNO₃ and 0.3 M KH₂PO₄ solutions were prepared separately in 20 mL of deionized water. The KH₂PO₄ solution was added dropwise to the AgNO₃ solution under continuous stirring, resulting in the formation of yellow precipitates (confirming the formation of Ag₃PO₄). The suspension was left undisturbed for 2 hours to allow the precipitates to settle, followed by vacuum filtration and repeated washing with deionized water. The obtained product was dried in an oven for 2 hours and stored.

For La-doped Ag₃PO₄ synthesis, the same procedure was followed with the addition of an appropriate amount of La(NO₃)₃·6H₂O to the AgNO₃ solution to achieve 2% and 6% La doping. The schematic representation of the synthesis route of Ag₃PO₄ and La-doped Ag₃PO₄ is shown in Fig. 1(a).

Fig. 1 Schematic of the synthesis routes to (a) doped and undoped Ag₃PO₄, (b) ZnS QDs, and (c) Ag₃PO₄/ZnS QD and La-doped Ag₃PO₄/ZnS QD composites.

2.2 Synthesis of ZnS quantum dots

The co-precipitation method was used to synthesize ZnS quantum dots with slight modifications.⁴³ In the standard synthesis procedure, a 0.5 M zinc acetate dihydrate solution and a 0.5 M sodium sulfide solution were prepared in 25 mL of distilled water. Then, the zinc acetate solution was added to the sodium sulfide solution dropwise under constant stirring at neutral pH. Additionally, 25 mL of a 0.5 M SDS solution was added dropwise to the solution while being constantly stirred. The above solution was stirred for 2 hours until white precipitates were formed. The solution was filtered and washed with a mixture of water and ethanol several times. The precipitates were dried in an oven at 60 °C for 5 hours. In Fig. 1(b), a schematic of the synthesis of ZnS quantum dots is presented.

2.3 Synthesis of composites

The Ag₃PO₄/ZnS composite was prepared by combining Ag₃PO₄ and ZnS quantum dots in a 1 : 1 weight ratio. In the typical process, 0.5 g of the synthesized Ag₃PO₄ was dispersed in 25 mL of deionized water and ultrasonicated at 30 °C for 15 minutes. Subsequently, 0.5 g of ZnS quantum dots was added to the mixture, followed by ultrasonication for 15 min to achieve uniform dispersion. The mixture was then stirred for 2 hours, during which dark-green precipitates formed, indicating the formation of the Ag₃PO₄/ZnS composite. The product was allowed to settle, filtered, washed several times with deionized water, and dried in an oven for 2 hours at 50–60 °C. Fig. 1(c) presents the schematic illustration of the synthesis method of Ag₃PO₄/ZnS QDs and La-doped Ag₃PO₄/ZnS QDs. The synthesis of La-Ag₃PO₄/ZnS composites followed the same procedure, using La-doped Ag₃PO₄ (2% and 6%) instead of pure Ag₃PO₄.

The crystal structure and crystalline size of the synthesized materials were analysed using XRD. FTIR spectroscopy was utilized to examine the functional groups of the synthesized materials. An FTIR spectrophotometer (SHIMADZU-8400) was used to record FTIR spectra in the range from 400 to 4000 cm⁻¹. The surface morphology of the synthesized materials was analysed by scanning electron microscopy (SEM). The elemental composition of the synthesized materials was determined using energy-dispersive X-ray spectroscopy (EDS). The thermal behaviour of the synthesized materials was analysed by thermogravimetric analysis (TGA) in an inert gas atmosphere. Raman spectroscopy was used to investigate the structural and vibrational properties of the synthesized materials.

2.4 Adsorption experiments

The adsorption studies were conducted using a method already reported in the literature, with slight modifications.⁴⁴ Briefly, an adsorption experiment was conducted using a 15 ppm dye solution at 25 °C. An appropriate quantity of the catalyst (0.1 g) and 50 mL of the dye solution were combined, and the mixture was stirred for 1 hour in the dark. After 1 hour, an aliquot of the solution was taken out, followed by the separation of the catalyst by centrifugation. The absorbance was measured at 550 nm using a UV-visible spectrophotometer. Fig. S1(a) presents the schematic representation of the adsorption experiment.

2.5 Photocatalytic degradation experiments

The photocatalytic degradation experiment was conducted in direct sunlight using a previously reported method.^45,46 Briefly, a known quantity of the catalyst (0.1 g) was added to 50 mL of a dye solution. The dye solution was exposed to direct sunlight for 2 hours under continuous stirring. An adequate volume of the solution was removed after 5, 15, 30, 45, 60, and 120 min. The catalyst was removed from the sample via centrifugation, and the absorbance was measured at 550 nm. The schematic representation of the photocatalytic experiment is shown in Fig. S1(b). The equation used to determine the percentage of photocatalytic degradation is given below.⁴⁷where A_o is the dye's initial absorbance, and A_t is its absorbance at time t.

3 Results and discussion

3.1 Characterization of the synthesized photocatalysts

Fig. 2(a) displays the XRD patterns of pure and doped silver phosphate (Ag₃PO₄).^26,32 In the XRD patterns of Ag₃PO₄ and La-doped Ag₃PO₄, the characteristic peaks observed at different 2θ values correspond to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), and (411) planes, respectively.^26,32 All these peaks are well-matched with the JCPDS card no. 06-0505.^26,32 Compared to pure Ag₃PO₄, no change in the crystal structures and no impurity peaks are observed in the XRD patterns of 2% and 6% La-doped Ag₃PO₄, which indicates that the La(III) ions are successfully integrated and uniformly distributed in the host lattice. The XRD pattern of ZnS quantum dots (ZnS QDs) is also shown in Fig. 2(a).^48,49 In the XRD pattern of ZnS QDs, the major peaks observed at 2θ values of 29.20°, 48.53°, and 57.52° correspond to the (111), (220), and (311) planes, respectively. All these peaks are in accordance with JCPDS card no. 05-0566. The observed XRD pattern closely resembles the typical pattern of the cubic phase of the zinc blende structure.⁴⁹ The XRD patterns of the composites are displayed in Fig. 2(a). In the XRD patterns (Fig. 2(a)) of the composites, the diffraction peaks corresponding to Ag₃PO₄ and ZnS QDs are observed, confirming the coexistence of both phases in the composites. The crystal structure of Ag₃PO₄ is shown in Fig. 2(b).

Fig. 2 (a) XRD patterns of the synthesized nanomaterials. (b) Crystal structure of silver phosphate.

The crystalline size of the synthesized materials was calculated using the Debye–Scherrer equation, as shown in eqn (2).⁵⁰

The dislocation density (δ) was calculated using the following formula:⁵¹

The crystal microstain (ε) was calculated using the following formula:⁵²

All calculated parameters are given in Table S1 in the SI. The average crystalline size of pure Ag₃PO₄ is found to be 158.13 nm. A clear reduction in the crystallite size is observed after La incorporation. The average crystallite sizes of 2% La-doped Ag₃PO₄ and 6% La-doped Ag₃PO₄ samples are 73.07 nm and 118 nm, respectively. This decrease can be attributed to the introduction of lattice strain and structural defects due to the addition of La, which restricts crystal growth, which is consistent with a previous report.⁵³ The calculated average crystallite size of the ZnS quantum dots is 2.26 nm, closely matching the previously reported value of 2.09 nm for ZnS quantum dots.^43,54 The Ag₃PO₄/ZnS composite further exhibits a reduced crystallite size of 10.33 nm. After the incorporation of La into this composite, the crystallite sizes are 37.80 and 21.54 nm for the 2% and 6% La-doped Ag₃PO₄, respectively.

FT-IR analysis was utilized to examine the functional groups of the synthesized materials. The FT-IR spectra of pure and La-doped Ag₃PO₄ are presented in Fig. 3(a). The main characteristic peaks are observed at 507, 550, 558, 670, 1007, and 2350 cm⁻¹, respectively. The peak observed at 507 cm⁻¹ is due to the stretching vibrations of the Ag–O band, while the peak at 550 cm⁻¹ corresponds to the O–P–O bending vibrations.³⁵ The stretching and bending vibrations of the P–O bond in the PO₄³⁻ group appear at 1007 and 558 cm⁻¹, respectively.^55–57 The peak at 2350 cm⁻¹ is due to atmospheric carbon dioxide.⁴⁸ In the case of La-doped Ag₃PO₄, the main peak is observed at 610 cm⁻¹, corresponding to the La–O bond vibrations.³⁵

Fig. 3 (a) FTIR spectra of the synthesized nanomaterials. (b) Raman spectra of the synthesized nanomaterials.

In the FTIR spectrum of ZnS quantum dots (Fig. 3(a)), the main characteristic peaks appear at 651 cm⁻¹ and 484 cm⁻¹, corresponding to the stretching vibrations of the Zn–S bond.^49,58 The absorption peaks at 1384,^59,60 1419,⁶¹ 1563⁶² and 1620 cm⁻¹ (ref. 61) correspond to the symmetric stretching vibrations of the –COO⁻ bond. The broad peak observed at 3400 cm⁻¹ is assigned to the stretching vibrations of the O–H bond, which arises from the adsorption of water molecules on the surface of ZnS.⁶³ The absorption peaks observed at 2852 cm⁻¹ and 2921 cm⁻¹ are attributed to the C–H stretching,⁶⁴ while the peaks at 1458 cm⁻¹ and 1466 cm⁻¹ correspond to the CH₃ bending vibrations.⁶⁵ The peak observed at 928 cm⁻¹ is assigned to the S–C bond stretching vibrations.⁶⁶

The Raman spectra of pure and doped Ag₃PO₄ are shown in Fig. 3(b). The Raman spectrum of pure Ag₃PO₄ displays distinct vibrational bands characteristic of the phosphate tetrahedron, which are closely related to those reported in a previous study.⁶⁷ The high-intensity band at 912 cm⁻¹ is assigned to the stretching vibration of the terminal oxygen of the PO₄³⁻ groups. A weak band is observed at 555 cm⁻¹, which corresponds to the asymmetrical bending vibration of the P–O–P bond. In the case of La-doped Ag₃PO₄, no new band is identified in the range of 200–1800 cm⁻¹. However, the band at 912 cm⁻¹ becomes progressively weaker and narrower with an increase in the concentration of La. Thus, the incorporation of La³⁺ alters only the intrinsic phosphate vibrations.

In the Raman spectrum of ZnS quantum dots (Fig. 3(b)), the characteristic bands appear at 270 cm⁻¹ and 347 cm⁻¹, which correspond to the transverse optical (TO) and first-order longitudinal optical (1LO) phonon modes of cubic ZnS, respectively.^68–70 Additional bands appearing between ∼997 and 1562 cm⁻¹ are linked to multiphonon processes and SDS vibrational modes.⁷¹ These spectral characteristics not only confirm the structural crystallinity of ZnS QDs but also reveal the phonon confinement effects in nanocrystals. The Raman spectra of the composites display the vibrational modes of both components with slight band broadening, confirming that the two crystalline phases coexist in the composites.

The thermal stability of the synthesized photocatalysts was evaluated using thermogravimetric analysis (TGA), and the results are shown in Fig. 4(a). A slight weight loss is observed in the case of pure and doped Ag₃PO₄ up to 750 °C, demonstrating high thermal stability. The minor weight loss in these TGA curves is correlated with water loss. Similarly, the TGA curve of ZnS quantum dots, shown in Fig. 4(a), displays approximately 21% weight loss. The 7% weight loss at 150 °C is due to the removal of adsorbed water from the quantum dots' surface. However, the loss of sulfuric acid groups and degradation occur up to 400 °C. At 600 °C, the weight loss is due to the oxidation of ZnS to ZnO. Furthermore, in the case of Ag₃PO₄/ZnS quantum dot composites, three weight loss regions are observed up to 600 °C. The first weight loss at 150 °C corresponds to the loss of adsorbed water molecules, while the second one at 400 °C involves the evaporation of organic compounds. The final weight loss at 600 °C relates to the breakdown of the residual inorganic content, the oxidation of metal species, or internal structural modification.

Fig. 4 (a) TGA analysis of the synthesized nanomaterials. (b) Relactance vs. wavelength spectra. (c) Tauc plots showing vs. hv for band gap estimation. (d) PL spectra of selected samples.

The composite exhibits a 12% weight loss, indicating superior thermal stability compared to pure ZnS. In the doped composites, overall, 14% and 7.15% weight loss are observed for 6% La-doped Ag₃PO₄/ZnS and 2% La-doped Ag₃PO₄, respectively.

The optical characteristics of the synthesized catalysts were analysed by UV-visible diffuse reflectance spectroscopy (UV-DRS). The data were collected in terms of the reflectance (R) versus wavelength (λ), and the spectra are presented in Fig. 4(b). The band gap energies (E_g) were estimated using the Kubelka–Munk function,,⁷² and Tauc plots were created by plotting (F(R)·hv)^1/2 against the photon energy (hν) (Fig. 4(c)). The band gap values were determined by extrapolating the linear portion of each Tauc plot to the energy axis. The calculated band gap energies are shown in Table 1.

Table 1 Calculated band gap energies of different nanocatalysts

S. no.	Catalyst	Band gap (eV)
1	Pure Ag3PO4	2.41
2	2% La-doped Ag3PO4	2.39
3	6% La-doped Ag3PO4	2.36
4	ZnS quantum dots (QDs)	3.60
5	6% La-doped Ag3PO4/ZnS QD composite	1.50

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Pure Ag₃PO₄ exhibits a band gap of 2.41 eV. Upon the addition of 2% and 6% La to the lattice, the band gap decreases slightly to 2.39 eV and 2.36 eV, respectively. The decrease in the band gap is possibly due to the creation of oxygen vacancies within the lattice upon La doping.³⁵ In contrast, ZnS QDs have a wider band gap of 3.60 eV and mainly absorb in the UV region. The 6% La-doped Ag₃PO₄/ZnS QD composite shows a markedly reduced band gap of 1.50 eV.

Photoluminescence (PL) spectroscopy was employed to identify defect sites and oxygen vacancies within the synthesized nanoparticles. As shown in Fig. 4(d), the results clearly indicate that all samples exhibit two distinct emission peaks. Pure Ag₃PO₄ exhibits two peaks at 436 and 463 nm.⁷³ 6% La-doped Ag₃PO₄ shows three peaks at 439, 468, and 507 nm. ZnS quantum dots (QDs) show two peaks at 439 and 467 nm. The 6% La-doped Ag₃PO₄/ZnS QD composite shows two peaks at 436 and 464 nm. It is observed that 6% La-doped Ag₃PO₄ shows an additional peak. The emission band observed within the 490–550 nm range is attributed to the radiative recombination of photogenerated charge carriers at surface-active oxygen vacancies.^74,75

The surface area of the synthesized samples was investigated using BET analysis (Fig. 5(a)). The BET specific surface areas are determined to be 130.109, 89.934, 133.446, 116.537, and 141.120 m² g⁻¹ for pure Ag₃PO₄, 2% La-Ag₃PO₄, 6% La-Ag₃PO₄, ZnS QDs, and 6% La-Ag₃PO₄/ZnS QDs, respectively. Upon the incorporation of ZnS QDs into the 6% La-Ag₃PO₄ particles, the surface area of the 6% La-Ag₃PO₄ heterojunction composite increases from 133.446 to 141.120 m² g⁻¹.

Fig. 5 (a) BET adsorption isotherms of the synthesized materials. (b) BJH adsorption-derived pore size distributions of the synthesized materials. (c) Comparison of the pore size, pore volume, and crystallite size of different catalysts.

The pore size distribution of the samples was analysed by the BJH adsorption method (Fig. 5(b)). The pore radii of Ag₃PO₄, 2% La-Ag₃PO₄, 6% La-Ag₃PO₄, ZnS QDs, and 6% La-Ag₃PO₄/ZnS nanoparticles are 15.578, 15.564, 15.581, 15.610, and 14.761 Å, respectively. The pore sizes of Ag₃PO₄, 2% La-Ag₃PO₄, 6% La-Ag₃PO₄, ZnS QDs, and 6% La-Ag₃PO₄/ZnS nanoparticles are 3.116 nm, 3.113 nm, 3.116 nm, 3.122 nm, and 2.952 nm, respectively. This indicates a mesoporous structure according to the IUPAC classification (pores between 2 and 50 nm).⁷⁶

Additionally, the pore volumes of Ag₃PO₄, 2% La-Ag₃PO₄, 6% La-Ag₃PO₄, ZnS QDs, and 6% La-Ag₃PO₄/ZnS are 0.036, 0.031, 0.038, 0.040, and 0.053 cm³ g⁻¹, respectively. This suggests that the BET surface area and pore size of the material are not the primary factors influencing its photocatalytic activity.⁷⁷ The comparison between different catalysts is presented in Fig. 5(c).

The surface morphology of the catalysts was studied using SEM images at various magnifications, as shown in Fig. 6(a–l). The pure Ag₃PO₄ particles have irregular spherical shapes with a porous, rough surface, and some are aggregated⁷⁸ (Fig. 6(a–d)). The SEM images of La-doped Ag₃PO₄ reveal that the morphology of silver phosphate changes from spherical to tetrapod, with a rough surface consisting of some pores (Fig. 6(e–h)). In the case of ZnS QDs, a regular morphology with slight agglomeration is observed, as shown in Fig. 6(i–l).⁷⁹

Fig. 6 SEM images of (a–d) Ag₃PO₄, (e–h) 6% La-doped Ag₃PO₄, and (i–l) ZnS quantum dots at different magnifications.

The elemental composition of the materials was determined using energy-dispersive X-ray spectroscopy (EDS). The presence of Ag, P, and O elements in the EDS spectrum (Fig. 7(a)) of Ag₃PO₄ confirms the formation of Ag₃PO₄. In the case of 6% La-doped Ag₃PO₄, the EDS spectrum (Fig. 7(b)) displays the peaks of Ag, P, La, and O. The existence of the peaks of La in the EDS spectrum of 6% La-doped Ag₃PO₄ confirms that lanthanum (La) is successfully introduced into the lattice. The existence of carbon in the EDS spectra of pure and doped Ag₃PO₄ is probably due to the latex material of the SEM sample holder.⁸⁰ The presence of Zn and S peaks in the EDS spectrum (Fig. 7(c)) confirms the synthesis of the ZnS QDs. The presence of the small peaks of copper (Cu) and silicon (Si) in the EDS spectrum is due to the sample holder and EDS detector, respectively.

Fig. 7 EDS spectra of (a) Ag₃PO₄, (b) 6% La-doped Ag₃PO₄, and (c) ZnS QDs.

Fig. 8(a–f) presents the TEM micrographs of pure and 6% La-doped Ag₃O₄ at varying magnifications, revealing the nanostructured morphology of the synthesized photocatalysts. The TEM micrograph of the nanoparticles shows that the synthesized nanoparticles are nearly spherical, tend to form soft agglomerates due to magnetic interactions, which range in size from 0.5 µm to less than 100 nm, and exhibit a smooth surface texture due to proper dispersion. The TEM images of ZnS are presented in Fig. 8(g–h).

Fig. 8 TEM images of (a–c) pure Ag₃PO₄, (d–f) 6% La-Ag₃PO₄, and (g–h) ZnS QDs at different magnifications.

The XPS survey spectra of La-Ag₃PO₄/ZnS nanocomposites reveal the presence of peaks corresponding to silver (Ag), oxygen (O), zinc (Zn), sulfur (S), phosphorus (P), lanthanum (La), and carbon (C), as illustrated in Fig. 9(a–f) and S2. The two characteristic peaks observed at 366.4 eV and 370.2 eV correspond to Ag 3d_5/2 and Ag 3d_3/2, respectively, indicating the presence of Ag⁺ in Ag₃PO₄.⁸¹ The O 1s peak at 530.2 eV corresponds to lattice oxygen in Ag₃PO₄.⁸² As shown in Fig. 9(d), the Zn 2p spectrum displays two characteristic peaks at 1020.2 eV and 1044.5 eV, assigned to Zn 2p_3/2 and Zn 2p_1/2, respectively, confirming the presence of ZnS.⁸³ In the S 2p spectrum shown in Fig. 9(e), three distinct peaks are observed. The two peaks at binding energies of 162 and 163.1 eV are attributed to the S 2p_3/2 and S 2p_1/2 orbitals, respectively, confirming the presence of metal–sulfur bonds, such as those found in ZnS. The additional peak appearing at 169.1 eV is assigned to sulfur species in a higher oxidation state, which likely results from surface oxidation effects.⁸³

Fig. 9 XPS spectra of the La-doped Ag₃PO₄ decorated with ZnS quantum dots: (a) Ag 3d, (b) P 2p, (c) O 1s, (d) Zn 2p, (e) S 2p, and (f) La 3d.

The P 2p XPS spectrum shown in Fig. 9(b) exhibits a characteristic peak at a binding energy of 133.72 eV, which is attributed to phosphorus in the phosphate species.⁸⁴ The lanthanum 3d XPS spectrum is illustrated in Fig. 9(f). For lanthanum, the closed-shell La³⁺ ion shows La 3d_5/2 and La 3d_3/2 peaks. As shown in the spectrum, the La 3d peaks are deconvoluted into distinct subpeaks, with binding energies of 832.5 eV and 837.1 eV, corresponding to the La 3d_5/2 and La 3d_3/2 levels, respectively, confirming the presence of La in the trivalent oxidation state.⁸⁵ The deconvoluted C 1s XPS spectrum of Ag₃PO₄ nanoparticles, shown in Fig. S2(a), displays two distinct peaks at 284.2 eV and 286.9 eV. The C1 peak at 284.2 eV is assigned to sp²-bonded carbon C–C, while the higher-binding-energy peak, C2, at 286.9 eV corresponds to C=O species. These results are consistent with the literature.⁸⁶ The complete XPS survey is shown in Fig. S2(b).

3.2 Application of the synthesized photocatalysts

3.2.1 Adsorption capacity of catalysts. The adsorption efficiency of all seven catalysts is illustrated in Fig. S3 of the SI. Results indicate that 6% La-doped Ag₃PO₄ exhibits the highest adsorption efficiency, at 64.93%, compared to other catalysts. Variations in adsorption efficiency can be linked to changes in the textural properties and surface functional groups of the catalysts.⁸⁷

3.2.2 Photodegradation of azo dye (EBT). The photocatalytic activity of the prepared catalyst was tested for the photodegradation of Eriochrome Black T (EBT) under direct sunlight irradiation. The change in the color intensity of changes in 15 ppm standard EBT dye solutions under exposure to sunlight in the presence of different photocatalysts is shown in Fig. 10(a–f). Initially, the colour of the standard dye solutions is deep blue. As shown in Fig. 10(a–f), the colour of standard dye solutions changes from deep blue to colourless, indicating the successful breakdown of Eriochrome Black T in the presence of photocatalysts.

Fig. 10 Photographs showing colour intensity changes in 15 ppm standard dye solutions over time under exposure to sunlight using different photocatalysts: (a) pure Ag₃PO₄, (b) 2% La-doped Ag₃PO₄, (c) 6% La-doped Ag₃PO₄, (d) ZnS QDs, (e) Ag₃PO₄/ZnS quantum dots, and (f) 2% La-doped Ag₃PO₄/ZnS quantum dots.

The results are presented in Fig. 11(a–f) and S4. The extent of dye degradation was examined by monitoring the change in the absorbance of the dye at regular intervals. The continuous decrease in absorbance with increasing irradiation time clearly indicates the effective photodegradation of EBT molecules. The photocatalytic activities of pure Ag₃PO₄, 2% La-doped Ag₃PO₄, 6% La-doped Ag₃PO₄, and ZnS quantum dots are 94.71%, 95.63%, 97.84% and 97.59%, respectively. Furthermore, the photocatalytic degradation efficiencies of the composite materials, i.e., Ag₃PO₄/ZnS QDs, 2% La-doped Ag₃PO₄/ZnS QDs, and 6% La-doped Ag₃PO₄/ZnS QDs, is 83.33%, 79.54%, and 84.88%, respectively.

Fig. 11 Changes in the absorbance of a standard dye solution (15 ppm) with time for (a) pure Ag₃PO₄, (b) 2% La-doped Ag₃PO₄, (c) 6% La-doped Ag₃PO₄, (d) ZnS QDs, (e) Ag₃PO₄/ZnS quantum dots, and (f) 2% La-doped Ag₃PO₄/ZnS quantum dots. In each plot, the curves represent absorbance taken at different time intervals. The black, red, azure-blue, green, purple, amber orange and tiffany-blue lines correspond to 0, 5, 15, 30, 45, 60 and 120 min, respectively.

La³⁺ doping improves photocatalytic activity primarily through lattice distortion and defect formation. As shown in Table S2, the crystallite size decreases with increasing La content, while the microstrain and dislocation density increase, indicating the introduction of structural defects. These defects increase the number of active sites and slightly reduce the band gap, enhancing light absorption and photocatalytic performance.

Although heterojunctions generally enhance photocatalytic activity, the lower degradation efficiency of the 6% La-Ag₃PO₄/ZnS composite than that of 6% La-Ag₃PO₄ can be explained by surface coverage effects. A similar behaviour has been reported for Ag₃PO₄/g-C₃N₄ composites, where the excessive loading of one component reduces photocatalytic activity due to the shielding of active sites.⁸⁸ In this work, the presence of ZnS may reduce the accessibility of Ag₃PO₄ active sites, leading to lower efficiency.

Although heterojunctions usually improve photocatalytic activity by reducing electron–hole recombination,⁸⁹ the 6% La-Ag₃PO₄/ZnS composite showed lower degradation efficiency than 6% La-Ag₃PO₄, likely because its low band gap (1.5 eV) can lead to faster charge recombination.

The slightly lower photocatalytic activity of the composite materials can be attributed to several factors. First, the combination of different components may have led to poor interfacial contact or inefficient charge transfer between the constituents, which hinders the separation and migration of photogenerated charge carriers. Second, there may have been an unfavourable band alignment between materials, preventing efficient electron–hole pair separation. Additionally, synthesis conditions or improper doping ratios might have resulted in surface defects, recombination centres, or agglomeration, all of which can negatively affect photocatalytic efficiency. These limitations collectively reduced the overall degradation efficiency of composites compared to individual photocatalysts.

3.3 Factors affecting the photocatalytic degradation of EBT

3.3.1 Effect of the amount of dye on photocatalytic efficiency. The concentration of dye plays an important role in the photocatalytic activity of a material. Three different concentrations (10, 15, and 20 ppm) of EBT were used to study their effect on the photocatalytic efficiency of seven different catalysts under direct sunlight. A fixed amount of the catalyst (0.1 g) was added to all samples in this experiment. The absorbance of all samples was measured at regular time intervals, and the results are presented in Fig. 12(a–f) and S5. The results indicated that the photodegradation efficiencies decreased with increasing concentration of EBT. The reason for this reduction in photodegradation is the decrease in the number of available surface active sites of photocatalysts to break down the dye molecules at a given time. Furthermore, the quantity of photons reaching the catalyst's surface decreases as the concentration of dye rises. This results in a low percentage of degradation because few OH radicals are produced.^44,90

Fig. 12 (a–f) Effect of the initial dye concentration on the photocatalytic degradation of Eriochrome Black T using seven different catalysts. The black, red and blue curves correspond to 10, 15, and 20 ppm, respectively.

3.3.2 Effect of the amount of catalyst on photocatalytic efficiency. The effect of the amount of photocatalysts on the rate of dye degradation was studied using different concentrations of the catalyst, i.e., 0.05 g, 0.1 g, and 0.15 g, in a fixed amount of dye (15 ppm) at room temperature. The results are illustrated in Fig. 13(a–f) and S6. According to the results, all catalysts showed increased photodegradation efficiencies with increasing amount of the catalyst from 0.05 to 0.15 g. An increase in the catalyst quantity results in more active sites on the photocatalyst surface, which raises the quantity of ^˙OH radicals that can contribute to the dye solution's decolorization.⁹¹

Fig. 13 (a–f) Effect of catalyst loading on the time-dependent photodegradation efficiency of EBT using seven different catalysts, each tested at three dosage levels. The black, red and blue curves correspond to 0.05, 0.1, and 0.15 g, respectively.

3.3.3 Effect of pH on photocatalytic efficiency. pH is an important parameter in the photocatalytic degradation process because it influences the photocatalyst's surface charge, which influences dye degradation.^92,93 To study the effect of pH on the rate of EBT degradation, the experiment was conducted at three different pH levels, i.e., 4, 6 (the natural pH of an EBT solution), and 10, using 6% La-doped Ag₃PO₄ and the 6% La-doped Ag₃PO₄/ZnS QD composite. Fig. S7(a and b) presents the influence of pH on the photodegradation efficiency of EBT using two different catalysts. At pH 6 (the original pH of the solution), the highest photodegradation efficiency of 97.84% was achieved using 6% La-doped Ag₃PO₄. The results indicated that the degradation efficiency increased as the pH changed from acidic (pH = 4) to near-neutral (pH = 6). Furthermore, the degradation efficiency decreased as the pH changed from neutral to basic conditions (pH = 10).

The impact of pH on the photocatalytic degradation performance of the catalyst is fundamentally explained by variations in the electric double layer at the solid-electrolyte interface. This variation in the electric double layer influences the adsorption–desorption process, generating electron–hole pairs on the photocatalyst surface. At low pH values (e.g., pH = 4), the stronger adsorption of dye molecules reduces the number of active sites on the photocatalyst surface, thereby decreasing light absorption and photodegradation. Moreover, strong adsorption leads to the accumulation of multiple layers of dye molecules on the catalyst, hindering direct contact between dye molecules and the catalyst surface. Therefore, they do not participate in photodegradation.^94–96

In case of 6% La-doped Ag₃PO₄/ZnS QD composite, the highest photodegradation efficiency of 88% was observed at pH 4. The higher photocatalytic activity is due to the effective electron–hole pair separation, higher dye adsorption capacity, and suitable band alignment between the 6% La-doped Ag₃PO₄ and ZnS QDs.

3.3.4 Effect of temperature on photocatalytic efficiency. Temperature plays a crucial role in the degradation of EBT. The effect of temperature on the degradation rate was studied by conducting experiments at 25 °C, 40 °C, and 50 °C using 6% La-doped Ag₃PO₄ and 6% La-doped Ag₃PO₄/ZnS QDs as photocatalysts. Fig. S7(c and d) illustrates the effect of temperature on the photodegradation efficiency of EBT. The rate of photodegradation gradually increased with increasing temperature. This increase in photocatalytic activity can be attributed to several factors. Firstly, raising the temperature enhances bubbling in the solution, which may increase the generation of free radicals. Furthermore, higher temperatures increase the oxidation of dye molecules at the catalyst-solution interface, further leading to improved photodegradation.⁹⁷

Although degradation curves showed slight overlap for various factors (dye concentration, catalyst dose, pH, and temperature), it can be attributed to minor experimental variations in the sunlight intensity,⁹⁸ catalyst dispersion,⁹⁹ and light attenuation by dye molecules and other components in the solution.⁹⁸ Overall, the trends still clearly demonstrated how each factor affects EBT degradation.

3.4 Kinetic study of the photodegradation process

The kinetic study of photodegradation was conducted using three kinetic models: zero-order, pseudo-first-order, and pseudo-second-order models.

In the case of pure Ag₃PO₄, 2–6% La-doped Ag₃PO₄, and the Ag₃PO₄/ZnS quantum dot composite, regression coefficient (R²) values from the pseudo-second-order kinetic model were higher than the regression coefficient values from the zero-order kinetic model and pseudo-first-order kinetic model. The result showed that the degradation of EBT followed second-order kinetics. By contrast, in the case of ZnS quantum dots, the regression coefficient value (R² = 0.94308) from the pseudo-first-order kinetic model indicated that the degradation of EBT followed the first-order kinetics. In Fig. S8(a–c), the kinetic study of the photodegradation process of EBT for all catalysts is displayed.

3.5 Catalyst reusability

For practical applications, photocatalysts must exhibit not only high catalytic performance but also good stability and reusability. The reusability of the 6% La-doped Ag₃PO₄ photocatalyst was examined through repeated photocatalytic cycles. After each cycle, the catalyst was recovered, washed, and reused under the same reaction conditions. The results are shown in Fig. 14(a and b). The photocatalytic degradation efficiency of 6% La-silver phosphate is 97%, 91%, 85%, 77%, and 65% in cycles 1, 2, 3, 4, and 5, respectively. The decrease in degradation efficiency may be due to the loss of the photocatalyst during washing between cycles. The SEM images after 5 cycles are shown in Fig. 14(c and d).

Fig. 14 (a and b) Catalyst reusability for up to 5 cycles using 6% La-Ag₃PO₄ in a 15 ppm dye solution. (c and d) SEM images after 5 cycles.

Table S3 presents the recently reported efficiencies of various photocatalysts for the degradation of Eriochrome Black T (EBT). 6% La-doped Ag₃PO₄ and ZnS QDs exhibit superior performance under natural sunlight, achieving 97.84% and 97.59% degradation of EBT, respectively, within 120 minutes. Compared modified efficiency and operational simplicity.

3.6 Mechanism of photocatalytic degradation

Based on the UV-vis diffuse reflectance spectroscopy (DRS) results and photocatalytic degradation performance, a plausible photocatalytic mechanism is proposed. Under sunlight, Ag₃PO₄ (2.41 eV), 2% La-doped Ag₃PO₄ (2.39 eV), 6% La-doped Ag₃PO₄ (2.36 eV), ZnS QDs (3.6 eV), and 6% La-doped Ag₃PO₄/ZnS QDs (1.5 eV) generate electron–hole (e⁻/h⁺) pairs. The reduced band gap of the composite enhances sunlight absorption; however, an excessively low band gap may decrease the redox potential required for efficient photocatalytic reactions.⁴² The photogenerated electrons are expected to react with dissolved oxygen to produce superoxide radicals (^˙O₂⁻), while holes oxidize water to generate hydroxyl radicals (^˙OH), which are responsible for the degradation of the dye. These reactive species attack the dye adsorbed on the catalyst surface, breaking it down into simpler, less-toxic molecules.^37,100 Nevertheless, interfacial interactions in the composites may promote charge recombination, resulting in lower photocatalytic efficiency compared to the individual components. The chemical reactions involved in the degradation of EBT are presented in Fig. 15(a). The schematic diagram for EBT degradation via photocatalysts is shown in Fig. 15(b).

Fig. 15 (a) Photocatalytic degradation equations. (b) Schematic of EBT degradation using the proposed photocatalysts.

4 Conclusion

In summary, pure Ag₃PO₄, La-doped Ag₃PO₄ (2% and 6%), and ZnS quantum dots were successfully synthesized via a co-precipitation method, while the composites were synthesized through a hybrid mixing method. Different characterization techniques were used to analyse the structural, morphological, and optical properties of the synthesized materials. The XRD results confirmed the successful formation of crystalline Ag₃PO₄, La-doped Ag₃PO₄, ZnS quantum dots, and composites. The morphological study of the synthesized materials was performed using SEM micrographs. The elemental composition of the synthesized materials was confirmed by EDS analysis. TGA analysis indicated that all synthesized materials exhibited good thermal stability. UV-DRS results further verified that La incorporation into the Ag₃PO₄ lattice altered its band gap energy.

The photocatalytic capability of the synthesized materials was examined by observing the breakdown of Eriochrome Black T (EBT), an azo dye, under solar illumination. Notably, all synthesized individual materials showed higher degradation efficiency than the composites. This may be due to limitations such as inefficient charge separation and the limited availability of surface active sites on the composites. Additionally, particle clustering or weak interactions between the two components may have reduced the photocatalytic efficiency. Under the optimized reaction conditions, EBT photodegradation efficiencies of 97.84% and 84.88% were achieved using 6% La-doped Ag₃PO₄ and the 6% La-doped Ag₃PO₄/ZnS QD composite, respectively, at pH 6, with an initial dye concentration of 15 ppm, a photocatalyst dosage of 0.1 g, and an irradiation time of 120 min. Among the tested materials, 6% La-doped Ag₃PO₄ showed the highest photocatalytic performance, which can be attributed to enhanced charge carrier utilization and favourable electronic interactions resulting from lanthanum doping. These results indicate that La-modified Ag₃PO₄-based photocatalysts are promising and sustainable materials for the effective treatment of dye-contaminated wastewater, offering significant potential for environmental remediation applications.

Author contributions

Shabana Bibi: experimental work, writing original draft; Dr Amna Bashir, Dr Noshabah Tabassum: supervision, data analysis, writing, proofreading. The rest of the authors helped in the characterization of materials.

Conflicts of interest

There is no conflict of interest.

Data availability

Data would be made available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra00544f.

Acknowledgements

This research work was supported by the Ministry of Higher Education (MOHE) under the 2023 Translational Research Program for the Energy Sustainability Focus Area (Project ID: MMUE/240001), the 2024 ASEAN IVO (Project ID: 2024-02), and Multimedia University, Malaysia.

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