Enhanced Visible‑Light Photocatalysis via In‑Situ Z‑Scheme BiOBr0.3I0.7/Ag/AgI Nanocomposites

Abstract

A series of all‑solid‑state Z‑scheme photocatalysts comprising BiOBr_0.3I_0.7, Ag and AgI were synthesized through an in‑situ precipitation and photo‑reduction route. Under visible‑light irradiation, the BiOBr_0.3I_0.7/Ag/AgI composites displayed markedly higher photocatalytic performance than either BiOBr_0.3I_0.7 or AgI alone in the degradation of methyl orange (MO). The optimum catalyst, containing 15 % Ag by weight, achieved 89 % MO removal within 20 min. The superior activity originates from efficient charge‑carrier separation via a Z‑scheme pathway, where Ag nanoparticles mediate electron transfer. •O₂⁻ and h⁺ are the dominant reactive species, with a minor contribution from ·OH.

Background

Photocatalysis offers a sustainable route to convert solar energy into chemical fuels and to mineralise organic pollutants under ambient conditions. BiOBr_1−xI_x solid solutions are particularly attractive because their layered crystal structure allows band‑gap tuning; however, their positive conduction band limits redox power. Z‑scheme systems, in which two semiconductors are coupled through an electron mediator, preserve the most negative conduction band and the most positive valence band, thereby enhancing redox capabilities. Noble‑metal mediated Z‑schemes have proven more robust and efficient than shuttle‑redox systems. Silver iodide, with its negative conduction band and strong photolysis, is a suitable partner for constructing Z‑schemes. Here we report an in‑situ strategy to assemble BiOBr_0.3I_0.7 with Ag and AgI, forming an all‑solid‑state Z‑scheme that markedly improves visible‑light photocatalysis.

Methods / Experimental

Materials

Bismuth nitrate pentahydrate, silver nitrate, potassium bromide, potassium iodide, methyl orange and tert‑butanol were obtained from commercial suppliers and used as‑is.

Synthesis of BiOBr_0.3I_0.7 and BiOBr_0.3I_0.7/Ag/AgI

BiOBr_0.3I_0.7 was prepared by ultrasound‑assisted hydrolysis. For the Z‑scheme composites, 0.5 g of BiOBr_0.3I_0.7 was dispersed in 50 mL of AgNO₃ solution (Ag:I = 15 % by weight). The suspension was stirred for 1 h to precipitate AgI, then irradiated with a 300 W Xe lamp (200 mW cm⁻²) for 10 min to photo‑reduce Ag⁰ nanoparticles. The solids were collected, washed, and dried at 60 °C. By varying the AgNO₃ concentration, samples BAA‑1 to BAA‑4 (5 %, 10 %, 15 %, 20 % Ag) were obtained.

Characterization

XRD (Bruker D8), SEM (Zeiss Ultra 55), TEM/HRTEM (JEM‑2100), EDS, XPS (Thermo ESCALAB 250Xi), EPR (Bruker ER 200‑SRC) and UV–Vis DRS (HITACHI U‑41000) were employed to probe crystal structure, morphology, composition, electronic states and optical properties.

Photocatalytic Activity Tests

Photocatalytic degradation of 10 mg L⁻¹ MO was evaluated under visible light (400 nm cut‑off) using a 300 W Xe lamp. 100 mg of catalyst was dispersed in 150 mL of MO solution, stirred for 30 min in the dark to reach adsorption equilibrium, then irradiated. Samples were withdrawn every 5 min, centrifuged, and the MO concentration measured at 465 nm.

Results and Discussion

Structural and Morphological Analysis

XRD patterns confirmed that the BiOBr_0.3I_0.7 phase remained unchanged after AgI deposition and Ag⁰ formation. The 23.7° peak (AgI (111)) intensified with higher Ag content. SEM/TEM images revealed nanoplates (200–600 nm) of BiOBr_0.3I_0.7 decorated with ~10 nm Ag/AgI particles. HRTEM resolved lattice fringes of Ag (0.233 nm), AgI (≈0.350 nm) and BiOBr_0.3I_0.7 (≈0.285 nm), confirming intimate contact required for Z‑scheme transfer.

Compositional and Optical Properties

XPS showed Bi, Br, I, O and Ag, with distinct Ag⁺ and Ag⁰ peaks, confirming the presence of both ionic and metallic silver. SEM‑EDS confirmed ~2.8 % AgI in BiOBr_0.3I_0.7/AgI, and higher Ag content in BAA samples after photo‑reduction. UV–Vis DRS revealed that BAA composites possess strong absorption between 400–575 nm, slightly reduced at high Ag loading. Band‑gap calculations gave 1.61 eV for BiOBr_0.3I_0.7 and 2.83 eV for AgI, with corresponding VB and CB potentials of 2.71 eV / 1.10 eV and 2.52 eV / −0.31 eV versus NHE.

Photocatalytic Performance

Under visible light, BAA‑3 (15 % Ag) achieved 89 % MO removal in 20 min, outperforming BiOBr_0.3I_0.7 and BiOBr_0.3I_0.7/AgI. Lower Ag loadings (5 %, 10 %) were less effective due to insufficient Z‑scheme formation, while 20 % loading led to excess Ag⁰ that promoted recombination. The enhanced activity is attributed to efficient charge separation: photo‑excited electrons in BiOBr_0.3I_0.7 migrate to Ag, then to the VB of AgI, leaving highly reducing electrons in AgI’s CB and highly oxidising holes in BiOBr_0.3I_0.7’s VB.

Mechanistic Insight

Radical trapping experiments with t‑BuOH (·OH scavenger), KI (hole scavenger) and N₂ (superoxide scavenger) revealed that h⁺ and ·O₂⁻ are the primary active species; ·OH plays a minor role. EPR spectra under illumination confirmed the generation of ·O₂⁻ and ·OH, and the decrease in the TEMP signal (hole indicator) corroborated hole involvement. The proposed Z‑scheme mechanism (illustrated in Figure 6) aligns with band‑edge calculations and experimental observations.

Conclusion

The in‑situ synthesized BiOBr_0.3I_0.7/Ag/AgI Z‑scheme composites exhibit markedly improved visible‑light photocatalytic degradation of MO, achieving 89 % removal in 20 min with 15 % Ag loading. The enhanced performance stems from an efficient Z‑scheme charge‑transfer pathway that preserves strong oxidation and reduction potentials. These materials hold promise for practical applications in the remediation of organic pollutants.