Ag3PO4/BiFeO3 Heterojunctions: Superior Visible‑Light Photocatalytic Degradation of Acid Orange 7

Abstract

We present a facile, scalable synthesis of Ag₃PO₄ microparticles deposited on BiFeO₃ microcuboids, forming p‑Ag₃PO₄/n‑BiFeO₃ heterojunctions. Under visible‑light irradiation, these composites achieve markedly higher photocatalytic degradation of Acid Orange 7 (AO7) compared with bare BiFeO₃. Their intrinsic photocatalytic capability is also confirmed via phenol degradation, and the composites exhibit photo‑Fenton‑like activity. Photocurrent, impedance, and PL analyses reveal that the p‑n junction effectively suppresses electron–hole recombination, leading to enhanced charge separation and interfacial charge migration.

Background

Photocatalysis has emerged as a key technology for renewable energy and environmental remediation. Conventional TiO₂ catalysts are limited by UV activation, which constitutes only ~5% of the solar spectrum. Consequently, visible‑light‑active semiconductors such as BiFeO₃ (bandgap ≈2.18 eV) have attracted significant interest for dye degradation and water splitting. However, BiFeO₃ suffers from rapid charge recombination, limiting its practical performance.

Coupling BiFeO₃ with a narrow‑bandgap semiconductor to form a heterojunction is a proven strategy to enhance charge separation. Prior studies have reported improved activity in BiFeO₃–Bi₂WO₆, BiFeO₃–AgCl/Ag, and BiFeO₃–g‑C₃N₄ composites. Silver orthophosphate (Ag₃PO₄) is a notable visible‑light photocatalyst with a narrow bandgap (~2.35 eV) and strong oxidative power. It has been successfully combined with various oxides to form high‑performance composites. Importantly, BiFeO₃ is n‑type while Ag₃PO₄ is p‑type, making them ideal partners for a p‑n heterojunction.

Despite the potential, limited work has explored Ag₃PO₄/BiFeO₃ composites. Here we report their facile synthesis, comprehensive characterization, and superior photocatalytic performance toward AO7 and phenol degradation, as well as photo‑Fenton‑like activity.

Methods

Preparation of Ag₃PO₄/BiFeO₃ Composites

BiFeO₃ microcuboids were synthesized hydrothermally: 0.005 mol each of Bi(NO₃)₃•5H₂O and Fe(NO₃)₃•9H₂O were dissolved in 20 mL of dilute HNO₃ (5 mL) + 15 mL deionized water. KOH (4.5 M, 60 mL) was added dropwise under stirring, followed by 8 min sonication and 30 min vigorous stirring. The mixture was sealed in a 100 mL Teflon‑lined autoclave and heated at 200 °C for 6 h. After cooling, the precipitate was washed and dried at 80 °C to yield BiFeO₃. Ag₃PO₄ microparticles were prepared by precipitating 3 mmol AgNO₃ in 30 mL deionized water and 1 mmol Na₃PO₄·12H₂O in 30 mL deionized water. The latter solution was added dropwise to the former under vigorous stirring for 7 h, producing a yellow suspension that was centrifuged, washed, and dried at 60 °C. For composite synthesis, 0.1 g BiFeO₃ was dispersed in 30 mL deionized water and sonicated for 2 h. AgNO₃ was dissolved in the suspension, then Na₃PO₄ solution (30 mL) was added dropwise under vigorous stirring for 7 h. The resulting composites were centrifuged, washed, and dried at 60 °C. Samples with Ag₃PO₄ loadings of 5, 10, 20, and 40 wt % were prepared (designated 5wt%–40wt%Ag₃PO₄/BiFeO₃). A mechanically mixed 20wt% sample (20wt%Ag₃PO₄/BiFeO₃-M) served as a control.

Photoelectrochemical Measurements

Photocurrent and impedance studies employed a three‑electrode setup (platinum counter, Ag/AgCl reference, working electrode: 15 mg catalyst + 0.75 mg carbon black + 0.75 mg PVDF in NMP, coated on FTO). Visible light (300 W Xe lamp, 420‑nm cutoff) illuminated the cell at 0.2 V bias. EIS was recorded (5 mV amplitude, 10⁻²–10⁵ Hz).

Photocatalytic Activity Test

AO7 (5 mg/L, pH ≈ 6.8) and phenol (5 mg/L, pH ≈ 6.2) were degraded under visible light (50 mW cm⁻²) with 0.5 g/L catalyst. Samples were stirred in the dark for 30 min to reach adsorption equilibrium, then irradiated. Aliquots were taken periodically, centrifuged, and analyzed by UV‑Vis (AO7 λ_max = 484 nm, phenol λ_max = 270 nm). Reusability tests involved three consecutive cycles with catalyst recovery by centrifugation and drying. Photo‑Fenton experiments added 5 mmol/L H₂O₂ to the reaction mixture.

Characterization

Phase purity: XRD (Cu Kα). Morphology: SEM (JEOL JSM‑6701F) and TEM (JEOL JEM‑2010). Elemental analysis: EDX. Chemical states: XPS (PHI‑5702). Optical absorption: UV‑Vis DRS (PERSEE TU‑1901). Photoluminescence (SHIMADZU RF‑6000, 350 nm excitation).

Results and Discussion

XRD Analysis

Patterns confirm rhombohedral BiFeO₃ (PDF 74‑2016) and cubic Ag₃PO₄ (PDF 06‑0505). Composite peaks match both constituents with no impurities, and Ag₃PO₄ peak intensity rises with loading, indicating successful deposition.

Morphology Observation

SEM shows BiFeO₃ cuboids (200–500 nm, smooth surfaces). TEM reveals Ag₃PO₄ as irregular spheres (110–180 nm). In composites, Ag₃PO₄ microspheres decorate BiFeO₃ cuboids, forming clear p‑n interfaces. HRTEM confirms lattice spacings of 0.288 nm (BiFeO₃ (110)) and 0.267 nm (Ag₃PO₄ (210)). EDX confirms all expected elements.

XPS Analysis

Ag 3d peaks at 373.8/367.7 eV confirm Ag⁺. P 2p peak at 133.2 eV indicates P⁵⁺. Bi 4f (164.1/158.8 eV) and Fe 2p (723.7 eV Fe³⁺, 709.9 eV Fe²⁺) confirm mixed Fe oxidation states. Slight shifts in Bi and Fe binding energies for composites reflect interfacial interaction. O 1s spectra show lattice and surface‑adsorbed oxygen, with a composite shift indicating charge transfer.

Optical Absorption Property

All samples absorb strongly below 600 nm. Ag₃PO₄ bandgap: 2.35 eV (527 nm); BiFeO₃: 2.18 eV (567 nm). Composite absorption edges remain unchanged, implying bandgap preservation.

Photocatalytic Activity Measurement

AO7 degradation: Bare BiFeO₃ achieves 27% after 120 min; composites show progressive improvement with loading: 40 wt % (91%), 20 wt % (87%), 10 wt % (69%), 5 wt % (46%). The 20 wt % composite nearly matches the 40 wt % result, indicating an optimal loading around 20 %. The mechanically mixed 20 wt % sample underperforms, confirming the necessity of heterojunction formation.

Phenol degradation: Bare BiFeO₃ degrades ~9% after 120 min, while 20 wt % composite achieves significantly higher conversion, demonstrating intrinsic photocatalytic activity beyond dye sensitization.

Reusability: The 20 wt % composite retains high activity over three cycles; XRD and TEM after cycling show intact heterostructures, confirming robustness. Ag₃PO₄ alone shows marked activity loss, underscoring the composite’s stability advantage.

Photo‑Fenton‑like Catalytic Activity

In the presence of H₂O₂, AO7 degradation improves markedly for both BiFeO₃ and the composite. The composite’s higher activity is attributed to efficient electron transfer to Fe³⁺, generating more •OH radicals via the photo‑Fenton cycle. Trapping experiments confirm •OH and h⁺ as primary active species, with superoxide playing a minor role.

Photogenerated Charge Performance

Transient photocurrent: Composite exhibits ~4× higher photocurrent than bare BiFeO₃, indicating reduced recombination. EIS shows smaller semicircle for the composite, implying lower charge‑transfer resistance. PL intensity of the composite is significantly lower, further confirming enhanced charge separation.

Proposed Photocatalytic Mechanism

Energy‑band calculations yield CB/VB potentials: BiFeO₃ (0.34/2.52 V), Ag₃PO₄ (0.31/2.66 V vs. NHE). Upon contact, a built‑in electric field drives electrons from Ag₃PO₄ CB to BiFeO₃ CB and holes from BiFeO₃ VB to Ag₃PO₄ VB, suppressing recombination. This efficient charge migration fuels the oxidation of pollutants (via •OH, h⁺) and the photo‑Fenton cycle.

Conclusions

Ag₃PO₄/BiFeO₃ p‑n heterojunctions, fabricated by simple precipitation, exhibit superior visible‑light photocatalytic degradation of AO7 and phenol, and act as robust photo‑Fenton catalysts. The performance gains stem from effective electron–hole separation driven by the heterojunction, validating this strategy for advanced photocatalyst design.