Enhanced Visible‑Light Photocatalytic Degradation of Rhodamine B Using Bi₄Ti₃O₁₂/Ag₃PO₄ Heterojunction Nanocomposites

Abstract

We report the synthesis of Bi₄Ti₃O₁₂/Ag₃PO₄ heterojunction nanocomposites via an ion‑exchange route. Comprehensive characterization (XRD, SEM, TEM, BET, XPS, UV‑vis DRS, PL, EIS, photocurrent) confirms intimate interfacial contact and efficient charge separation. When evaluated for Rhodamine B (RhB) photodegradation under simulated sunlight, the 10 % Bi₄Ti₃O₁₂/Ag₃PO₄ composite achieves 99.5 % removal in 30 min—2.6 times faster than pure Ag₃PO₄. The superior performance is attributed to the p–n heterojunction that facilitates electron transfer from Bi₄Ti₃O₁₂ to Ag₃PO₄, reducing recombination and enhancing reactive species generation.

Background

Industrial wastewater frequently contains non‑biodegradable dyes that pose severe environmental and health risks. Semiconductor photocatalysis, powered by solar energy, offers a green strategy for dye degradation. Visible‑light responsiveness is critical, as it constitutes ~45 % of solar radiation. Efficient photocatalysts must not only absorb visible light but also maintain long‑term stability and facilitate rapid separation of photogenerated electron–hole pairs.

Silver orthophosphate (Ag₃PO₄) is a promising visible‑light photocatalyst with a bandgap of ~2.4 eV and a high quantum efficiency (>90 % for λ > 420 nm). However, its positive conduction band makes it susceptible to self‑photocorrosion, and its slight solubility limits durability. Constructing heterojunctions with other semiconductors—such as TiO₂, SnO₂, g‑C₃N₄, CeO₂, and BiPO₄—has been shown to enhance charge separation and suppress photocorrosion.

Bi₄Ti₃O₁₂ is a layered n‑type semiconductor with a bandgap of ~3.3 eV and has demonstrated strong photocatalytic activity against organic pollutants. Its electronic structure—Ti 3d + Bi 6p in the conduction band and O 2p + Bi 6s in the valence band—promotes efficient charge transport. Integrating Bi₄Ti₃O₁₂ with the p‑type Ag₃PO₄ offers a promising route to create a robust p–n heterojunction with superior visible‑light performance.

Methods

Synthesis of Bi₄Ti₃O₁₂ and Ag₃PO₄ Nanoparticles

Bi₄Ti₃O₁₂ was prepared by a polyacrylamide gel method: bi(III) nitrate and titanium(IV) precursor were mixed in dilute nitric acid, followed by addition of citric acid, glucose, and acrylamide. After polymerization at 80 °C, the xerogel was calcined at 300 °C (3 h) and 500 °C (8 h) to yield crystalline nanoparticles.

Ag₃PO₄ was synthesized via ion‑exchange: AgNO₃ and Na₂HPO₄ were dissolved separately and then combined under stirring for 5 h. The resulting precipitate was washed, dried, and collected as powder.

Preparation of Bi₄Ti₃O₁₂/Ag₃PO₄ Nanocomposites

Bi₄Ti₃O₁₂ (5–15 wt %) was dispersed in water and sonicated for 1 h. Ag₃PO₄ was formed in situ by adding AgNO₃ and Na₂HPO₄ solutions dropwise to the dispersion, followed by 5 h stirring. The composites were washed, dried at 60 °C, and collected. The 10 % Bi₄Ti₃O₁₂ sample exhibited the best performance.

Characterization

XRD (Cu Kα) for phase identification.
SEM/TEM for morphology; EDS for elemental mapping.
BET for surface area.
XPS for oxidation states.
UV‑vis DRS for bandgap estimation.
PL (excitation 315 nm) for recombination assessment.
EIS and transient photocurrent for charge‑transfer analysis.

Photocatalytic Degradation of RhB

A 5 mg L⁻¹ RhB solution was mixed with 20 mg of photocatalyst in 100 mL, stirred for 20 min in the dark, then irradiated with a 200 W Xe lamp. Samples were cooled to ambient temperature. At set intervals, aliquots were centrifuged and the RhB concentration measured at 554 nm. Degradation % = (C₀−C_t)/C₀ × 100.

Detection of •OH Radicals

Terephthalic acid (TPA) was used as a probe: the reaction product 2‑hydroxyterephthalic acid emits at 429 nm. PL spectra of the TPA solution after 30 min irradiation were recorded.

Scavenger Tests

Ethyl alcohol (•OH scavenger), benzoquinone (O₂⁻ scavenger), and ammonium oxalate (h⁺ scavenger) were added separately to the reaction to identify dominant reactive species.

Results and Discussion

XRD

All samples displayed characteristic peaks of orthorhombic Bi₄Ti₃O₁₂ (JCPDS 035‑0795) and cubic Ag₃PO₄ (JCPDS 006‑0505). No secondary phases were detected, confirming phase purity.

SEM / TEM

Ag₃PO₄ particles are 300–600 nm spheres; Bi₄Ti₃O₁₂ particles are 60–90 nm spheres/ellipsoids. In the composite, Bi₄Ti₃O₁₂ nanoparticles decorate the surface of Ag₃PO₄ cores, evidencing intimate contact.

BET Surface Area

The 10 % composite has a BET surface area of 2.08 m² g⁻¹, typical of dense, non‑mesoporous materials.

XPS

Survey spectra confirm the presence of Bi, Ti, Ag, P, and O without contaminants. O 1s shows lattice and adsorbed oxygen peaks; shifts in binding energies in the composite suggest chemical interaction at the interface. Bi 4f and Ti 2p indicate Bi³⁺ and Ti⁴⁺ states; Ag 3d and P 2p confirm Ag⁺ and P⁵⁺.

UV‑vis DRS

Bi₄Ti₃O₁₂ absorbs up to 376.9 nm (E_g = 3.29 eV); Ag₃PO₄ up to 498.5 nm (E_g = 2.49 eV). The composite retains both absorption edges, enabling broad visible‑light harvesting.

Photoluminescence

The 10 % composite shows markedly lower PL intensity compared to bare Ag₃PO₄, indicating reduced electron–hole recombination due to effective charge transfer.

Electrochemical Impedance Spectroscopy & Photocurrent

EIS Nyquist plots reveal a smaller semicircle for the composite, reflecting lower charge‑transfer resistance. Transient photocurrent measurements show ~24 µA cm⁻² for the composite versus ~4 µA cm⁻² for Ag₃PO₄, confirming superior charge separation.

Photocatalytic Performance

Under simulated sunlight, the 10 % composite degrades 99.5 % of RhB in 30 min, compared with 66.7 % for Ag₃PO₄ and <10 % for Bi₄Ti₃O₁₂. The apparent first‑order rate constant k_app is 0.1789 min⁻¹ for the composite versus 0.0676 min⁻¹ for Ag₃PO₄—a 2.6‑fold increase.

Reactive Species Identification

Scavenger tests show negligible effect from ethanol (•OH), significant inhibition by benzoquinone (O₂⁻) and especially ammonium oxalate (h⁺). Thus, holes dominate the degradation mechanism, with superoxide as a secondary contributor. PL of TPA after irradiation confirms negligible •OH generation in the composite.

Proposed Mechanism

The CB of Bi₄Ti₃O₁₂ (≈ −0.6 V vs. NHE) lies more negative than that of Ag₃PO₄ (≈ +0.6 V), while its VB (≈ +2.7 V) is more positive. Upon contact, an internal electric field drives electrons from Bi₄Ti₃O₁₂ to Ag₃PO₄ and holes in the opposite direction, thereby suppressing recombination and enabling efficient oxidation of RhB by holes and reduction of O₂ to superoxide.

Conclusions

Bi₄Ti₃O₁₂/Ag₃PO₄ heterojunction nanocomposites, prepared by ion‑exchange, exhibit superior visible‑light photocatalytic activity toward RhB degradation. The optimal 10 % Bi₄Ti₃O₁₂ loading delivers 99.5 % removal in 30 min—2.6 times faster than bare Ag₃PO₄. Enhanced performance arises from efficient charge separation across the p–n junction, with holes as the primary oxidant and superoxide as a secondary species.