Enhanced Photocatalytic Degradation of Rhodamine B Using an ATP/TiO₂/Ag₃PO₄ Ternary Nanocomposite Under Simulated Solar Light

Abstract

A highly efficient ternary photocatalyst was engineered by integrating silver orthophosphate (Ag₃PO₄), attapulgite (ATP), and TiO₂. The resulting heterojunction promotes effective separation of photogenerated electrons and holes, markedly boosting photocatalytic performance. Structural analysis via SEM, XRD, and UV‑vis diffuse reflectance confirmed a lath‑particle morphology and the presence of all three components. Compared to binary composites, ATP/TiO₂/Ag₃PO₄ achieved superior Rhodamine B (RhB) degradation under simulated solar irradiation, while simultaneously enhancing the stability of Ag₃PO₄ and reducing its required loading. This work offers a practical blueprint for designing Ag‑based photocatalysts with lower cost and higher durability.

Background

Organic pollutants pose a persistent threat to environmental health, and photocatalytic degradation has emerged as a promising, environmentally benign remediation strategy. Since Fujishima’s pioneering discovery of TiO₂-driven water splitting in 1972, TiO₂ has been extensively studied for its chemical stability, non‑toxicity, and low cost. However, its wide band gap (3.2 eV) limits visible‑light absorption, curbing its practical efficiency.

In contrast, semiconductors such as Ag₃PO₄, Bi₂MoO₆, WO₃, and g‑C₃N₄ exhibit strong visible‑light activity. Ag₃PO₄, in particular, delivers high photo‑oxidation performance but suffers from poor photostability due to Ag⁺ reduction to metallic Ag and high material cost. Numerous composite strategies (e.g., TiO₂/Ag₃PO₄, Ag₃PO₄/graphene) have been pursued to mitigate these drawbacks.

Attapulgite (ATP) is a rod‑shaped, hydrated magnesium–aluminum silicate featuring a high specific surface area and excellent adsorption capability. Its rod morphology makes it an attractive scaffold for anchoring photocatalytic nanoparticles, yet ternary ATP‑based composites remain underexplored.

In this study, we synthesized an ATP/TiO₂/Ag₃PO₄ ternary nanocomposite via a simple two‑step route. Comprehensive characterization revealed a robust heterojunction that not only improves charge separation but also stabilizes Ag₃PO₄ and reduces silver usage, while achieving rapid RhB degradation under simulated solar light.

Experimental section

Materials

ATP nanofibers (average diameter <100 nm, length <1 µm) were supplied by Jiangsu Qingtao Energy Science and Technology Co., Ltd. Rhodamine B (RhB, A.R.), EDTA disodium salt, tert‑butanol, stearyl trimethyl ammonium chloride, silver nitrate, and disodium dihydrogen phosphate dihydrate were obtained from Macklin. TiO₂ anatase nanoparticles (5–10 nm, 99.8 % purity) were sourced from Aladdin.

Enhanced Photocatalytic Degradation of Rhodamine B Using an ATP/TiO₂/Ag₃PO₄ Ternary Nanocomposite Under Simulated Solar Light

Synthesis of samples

The ternary composite was fabricated in two steps. First, ATP nanorods and TiO₂ nanoparticles (mass ratio 5:2) were dispersed in deionized water and stirred for 4 h, allowing TiO₂ to adsorb onto ATP surfaces. After centrifugation, washing, and drying at 60 °C for 6 h, ATP/TiO₂ was obtained. Next, Ag₃PO₄ nanoparticles were precipitated onto ATP/TiO₂ via a simple co‑precipitation: 20 mL 0.1 M AgNO₃ was mixed with 0.7 g ATP/TiO₂ in 50 mL water under sonication for 30 min; then 20 mL 0.1 M Na₂HPO₄ was added dropwise in the dark for 40 min. The resulting light‑yellow precipitate was centrifuged, washed with absolute ethanol, and dried at 60 °C for 12 h, yielding ATP/TiO₂/Ag₃PO₄. Analogous procedures produced Ag₃PO₄, Ag₃PO₄/ATP, Ag₃PO₄/TiO₂, and ATP/TiO₂ samples.

Characterization

XRD data were collected on a Rigaku D/max‑RB diffractometer (40 kV, 30 mA). SEM imaging was performed on an INSPECTF FEI instrument. UV‑vis diffuse reflectance spectra were recorded on a Hitachi U‑3010 spectrophotometer with BaSO₄ as reference.

Photocatalytic experiment

Photocatalytic degradation of RhB (5 mg L⁻¹) was carried out under simulated solar irradiation using a 300 W Xe lamp (150 mW cm⁻²). 50 mg of ATP/TiO₂/Ag₃PO₄ was added to 100 mL RhB solution and stirred in the dark for 40 min to reach adsorption–desorption equilibrium. Upon lamp illumination, aliquots (4 mL) were withdrawn at set intervals, centrifuged (10,000 rpm, 10 min), and analyzed by UV‑vis spectroscopy (λ_max=554 nm). Degradation efficiency was calculated as %D = (1–A/A₀) × 100 %.

Results and discussion

Characterization of the ATP‑Ag₃PO₄‑TiO₂ composites

Figure 1 confirms the crystalline integrity of each component: ATP matches JCPDS #21–0958 (monoclinic), TiO₂ shows anatase peaks, and Ag₃PO₄ aligns with JCPDS #06‑0505. No impurity phases appear in the composites, indicating successful fabrication. In the ternary pattern (Fig. 1g), the ATP peaks are weak, suggesting TiO₂ and Ag₃PO₄ nanoparticles coat the ATP rods.

SEM images (Fig. 2) reveal ATP as sub‑micron rods (<1 µm) with <100 nm diameter. TiO₂ nanoparticles (~40 nm) attach uniformly to ATP surfaces, forming ATP/TiO₂. In the ternary composite, the rods are completely covered by both TiO₂ and ~50 nm Ag₃PO₄ spheres, producing a lath‑particle architecture that maximizes interfacial contact.

Absorption spectra

UV‑vis spectra (Fig. 3a) show Ag₃PO₄ absorbs up to ~500 nm, TiO₂ absorbs only in the UV, and ATP has weak absorption. The ternary composite combines these features, yielding strong UV absorption from TiO₂ and ATP and extended visible‑light response from Ag₃PO₄. Band‑gap energies were extracted from Tauc plots: TiO₂ 3.20 eV (indirect), Ag₃PO₄ 2.49 eV (direct), ATP 3.75 eV (direct). The composite exhibits two absorption edges—385 nm (E_g 3.64 eV) and 510 nm (E_g 2.49 eV)—demonstrating retention of both TiO₂ and Ag₃PO₄ characteristics.

Photocatalytic activities

Figure 4a displays the time‑dependent UV‑vis spectra of RhB during irradiation with ATP/TiO₂/Ag₃PO₄. The 554 nm peak nearly vanishes after 20 min, indicating >90 % degradation. In comparison (Fig. 4b), pure TiO₂ and ATP achieve <50 % removal after 60 min, whereas Ag₃PO₄ performs better but suffers from stability loss. The ternary composite reaches 81.1 % degradation after only 3 min and >99 % after 20 min—outperforming all binary counterparts while requiring less Ag content.

Stability tests (Fig. 5) show that Ag₃PO₄ loses activity after successive cycles due to Ag accumulation on its surface. In contrast, ATP/TiO₂/Ag₃PO₄ retains >95 % of its initial performance over five cycles, confirming the protective effect of TiO₂ and ATP in the heterojunction.

Possible mechanism in photocatalytic process

Scavenger experiments (Fig. 6) identified the dominant reactive species: holes (h⁺) and superoxide radicals (O₂^•–) are essential, while hydroxyl radicals (•OH) play a minor role. The proposed mechanism (Fig. 7) involves electron transfer from TiO₂ CB (−0.5 eV) to Ag₃PO₄ CB (+0.45 eV), and hole migration from Ag₃PO₄ VB (+2.97 eV) to TiO₂ VB (+2.70 eV). This charge separation reduces recombination, enhances O₂ reduction to O₂^•–, and protects Ag⁺ from photoreduction, thereby improving both activity and stability.

Conclusions

We successfully fabricated an ATP/TiO₂/Ag₃PO₄ ternary nanocomposite via a straightforward two‑step process. The resulting heterojunction markedly improves charge separation, leading to rapid and stable RhB degradation under simulated solar light. Compared with pure Ag₃PO₄, the composite requires less silver, reduces cost, and offers superior durability. These findings provide a scalable strategy for designing Ag‑based photocatalysts with enhanced performance and longevity.