Enhanced Photocatalytic Degradation of Rhodamine B Using SrTiO3/Bi5O7I Nanocomposites: Fabrication, Characterization, and Mechanistic Insights

Abstract

We report a thermal‑decomposition route to synthesize SrTiO₃/Bi₅O₇I nanocomposites that combine perovskite SrTiO₃ nanoparticles with tetragonal Bi₅O₇I nanorods. XRD, XPS, SEM, EDS, FTIR, DRS, and PL analyses confirm phase purity, intimate heterojunction formation, and effective charge separation. Under simulated solar irradiation, the 30 wt % SrTiO₃/Bi₅O₇I composite achieves 89.6 % Rhodamine B (RhB) removal in 150 min—3.97‑fold higher than pure Bi₅O₇I and 12.5‑fold higher than SrTiO₃. •O₂⁻ and holes dominate the oxidative process, while PL data reveal suppressed recombination. The study elucidates a Z‑type heterojunction mechanism that drives superior photocatalysis, offering a blueprint for high‑performance, visible‑light photocatalysts.

Background

Dye contaminants from textile effluents pose severe ecological and health risks, prompting the search for cost‑effective, green remediation methods. Conventional adsorption, coagulation, and advanced oxidation processes often suffer from high operational costs and secondary waste. Semiconductor‑based photocatalysis, especially under solar light, emerges as a promising alternative due to its renewable energy source and minimal by‑products. TiO₂ remains the benchmark photocatalyst, yet its wide band gap limits visible‑light activity and promotes rapid electron‑hole recombination. Recent strategies involve surface modification or semiconductor coupling to enhance light absorption and charge separation.

Bi₅O₇I, a p‑type oxyiodide, exhibits a relatively high valence band that generates strong oxidizing holes, but suffers from poor charge transfer efficiency. Coupling with n‑type SrTiO₃, a thermally stable perovskite with a suitable band alignment, can form an effective heterojunction that facilitates electron migration and reduces recombination. This work pioneers the synthesis of SrTiO₃/Bi₅O₇I nanocomposites and evaluates their photocatalytic performance and underlying mechanism.

Methods

Preparation of SrTiO₃/Bi₅O₇I Composites

SrTiO₃ nanoparticles and SrTiO₃/BiOI precursors were first prepared via a sol‑gel route. The composites were then subjected to thermal decomposition (ramp 5 °C min⁻¹ to 500 °C, hold 3 h, cool naturally) to yield SrTiO₃/Bi₅O₇I nanocomposites with varying SrTiO₃ loadings (10–40 wt %).

Characterization

Phase analysis employed XRD (Cu Kα). Morphology and elemental mapping were captured by FE‑SEM with EDS. Surface chemistry was probed by XPS. UV‑vis diffuse reflectance (DRS) assessed optical absorption, while PL spectroscopy evaluated charge recombination. FTIR identified functional groups.

Photocatalytic Evaluation

Photocatalytic activity was measured by RhB degradation under a 500 W Xe lamp (simulated solar light). 100 mg of catalyst was dispersed in 100 mL of 20 mg L⁻¹ RhB, stirred for 30 min in the dark (adsorption equilibrium), then irradiated. Every 30 min, 3 mL aliquots were centrifuged, and RhB concentration was quantified by UV‑vis spectroscopy.

Results and Discussion

XRD Analysis

Pure SrTiO₃ exhibited the expected perovskite peaks (JCPDS 35‑0734). Bi₅O₇I showed characteristic tetragonal reflections (JCPDS 10‑0548). The 30 wt % composite displayed a superposition of both patterns with no secondary phases, confirming successful heterojunction formation.

XPS Analysis

Survey scans revealed Ti, Sr, Bi, I, and O. Core‑level spectra confirmed Ti⁴⁺, Sr²⁺, Bi³⁺, and I⁻ states, and the O 1s peak split into lattice and vacancy‑related components, indicating oxygen vacancies that aid charge transport.

SEM & EDS

SrTiO₃ particles were spherical (50–300 nm). Bi₅O₇I nanorods (100–300 nm × 80 nm) wrapped SrTiO₃ particles, creating intimate contact. EDS confirmed the 30 wt % SrTiO₃ content.

Optical Properties

DRS showed a UV edge at 380 nm for SrTiO₃ and a visible‑light edge at 520 nm for Bi₅O₇I. The composite absorbed strongly between 480–520 nm, improving visible‑light harvesting. Band gaps were calculated as 3.2 eV (SrTiO₃), 2.31 eV (Bi₅O₇I), and 2.38 eV (30 wt % composite).

Photocatalytic Activity

Under simulated solar light, the 30 wt % composite achieved 89.6 % RhB removal in 150 min, outperforming both pristine materials. The apparent rate constant (k_app) was 1.45 × 10⁻² min⁻¹, 12.5‑fold higher than SrTiO₃ and 2.97‑fold higher than Bi₅O₇I. Reusability tests over five cycles showed negligible activity loss, confirming structural stability.

Mechanistic Insights

Scavenger studies revealed that •O₂⁻ and holes are the dominant oxidative species; hydroxyl radicals played a negligible role. PL spectra displayed a markedly reduced emission for the composite, indicating suppressed electron‑hole recombination. Calculated band positions (E_VB = 2.92 eV for Bi₅O₇I, 2.03 eV for SrTiO₃) support a Z‑type heterojunction: electrons flow from SrTiO₃ to Bi₅O₇I, while holes migrate oppositely, driving efficient charge separation and •O₂⁻ generation.

Conclusions

We have engineered SrTiO₃/Bi₅O₇I nanocomposites via a simple thermal decomposition method that yields a robust Z‑type heterojunction. The 30 wt % composite delivers superior photocatalytic degradation of RhB under simulated solar light, attributed to enhanced visible‑light absorption and efficient charge separation. These findings provide a scalable route for high‑performance photocatalysts applicable to wastewater treatment and beyond.