CQD‑Decorated BiFeO3 Nanoparticles Deliver Superior Visible and Near‑Infrared Photocatalysis for AO7 Degradation and Cr(VI) Reduction

Background

Industrial effluents containing heavy metals and organic dyes impose severe environmental and health risks. Hexavalent chromium, released from electroplating and tanning processes, is highly toxic, while many dyes are recalcitrant and carcinogenic. Photocatalytic and photo‑Fenton strategies are attractive for their low cost, broad substrate scope and operation simplicity. However, conventional photocatalysts with wide bandgaps respond only to UV light (~5 % of solar energy) and suffer from rapid electron‑hole recombination. Visible and NIR light comprise ~45 % and ~46 % of the solar spectrum, respectively, offering a larger energy pool for sustainable remediation.

BiFeO₃ (BFO), a perovskite oxide, is a promising visible‑light photocatalyst, yet its performance is limited by fast charge recombination and negligible NIR activity. Carbon quantum dots (CQDs) have emerged as versatile nanocarbon additives that provide large surface area, excellent conductivity, biocompatibility, and unique up‑conversion luminescence, enabling them to act as both electron acceptors/donors and NIR‑responsive sensitizers. Prior work showed CQD/BFO composites with enhanced dye‑degradation under visible light, but no systematic investigation of photo‑Fenton activity or NIR‑driven catalysis has been reported.

Here, we synthesize CQD/BFO hybrids and systematically examine their visible‑ and NIR‑induced photocatalytic degradation of AO7 and photo‑Fenton reduction of Cr(VI). The study elucidates the role of CQDs in charge separation, up‑conversion, and redox activity, providing a comprehensive framework for designing broadband photocatalysts.

Methods

Preparation of CQDs

CQDs were synthesized by hydrothermal carbonization of glucose (1 g in 80 ml water) at 180 °C for 4 h. The resulting reddish‑brown dispersion was filtered twice to remove large aggregates.

Fabrication of CQD/BFO Composites

BFO nanoparticles were synthesized by a polyacrylamide gel route. CQDs were then deposited onto BFO (0.1 g) by dropwise addition to a 70 ml water suspension under ultrasonication, followed by hydrothermal treatment at 130 °C for 4 h. By varying CQD volume (3, 6, 12, 24 ml) we prepared 3C/BFO, 6C/BFO, 12C/BFO and 24C/BFO composites.

The schematic illustration of CQD/BFO composite synthesis.

Photo‑Fenton and Photocatalytic Degradation of AO7

Photocatalytic and photo‑Fenton degradation were conducted in 200 ml AO7 (5 mg L⁻¹) under visible (420 nm cut‑off) or NIR (800 nm cut‑off) irradiation. After 30 min adsorption equilibrium, H₂O₂ was added for photo‑Fenton tests. Reaction progress was monitored by UV‑vis at 484 nm. Recyclability was assessed by repeated cycles with washing and drying of the catalyst.

Photocatalytic Reduction of Cr(VI)

Cr(VI) reduction experiments used 200 ml solutions (10 mg L⁻¹) at pH 2–3, with 0.2 g catalyst (1 g L⁻¹). Cr(VI) was quantified via the diphenylcarbazide (DPC) method.

Hydroxyl Radical Detection

Fluorescence of 2‑hydroxyterephthalic acid (TAOH) formed from terephthalic acid (TA) and ·OH was recorded (excitation 315 nm) to quantify radical production during irradiation.

Characterization

XRD, FTIR, TEM/HRTEM, DF‑STEM, XPS, UV‑vis DRS, PL, photocurrent, and EIS were employed to probe phase purity, morphology, composition, optical absorption, charge dynamics, and catalytic activity.

Results and Discussion

XRD Analysis

All samples crystallized in the rhombohedral BFO phase (JCPDS 74‑2016) with no secondary phases. CQD peaks were absent due to low loading.

XRD patterns of BFO, CQD and 24C/BFO.

FTIR Analysis

FTIR spectra confirmed characteristic Bi–O, Fe–O, and C–O vibrations, verifying successful CQD integration.

FTIR spectra of BFO, CQD and 12C/BFO.

Optical Absorption Property

UV‑vis DRS showed that CQD loading markedly enhanced absorption across the UV‑vis range while preserving the BFO band‑edge (~588 nm). The first‑derivative analysis confirmed unchanged bandgap, indicating that CQDs act as light‑harvesting and charge‑transfer agents rather than band‑gap modifiers.

a DRS spectra; b first‑derivative curves.

XPS Analysis

XPS confirmed the presence of Bi³⁺, Fe³⁺/Fe²⁺, lattice and chemisorbed oxygen, and C–C/C–O bonds from CQDs, confirming successful composite formation.

High‑resolution XPS of 12C/BFO.

Morphology Observation

TEM/HRTEM revealed ~120 nm BFO spheres decorated with ~15 nm CQDs. DF‑STEM elemental mapping confirmed uniform C distribution on BFO surfaces.

TEM/HRTEM and DF‑STEM images of BFO and CQD/BFO composites.

Photocatalytic and Photo‑Fenton Performance

Under visible light, 12C/BFO achieved 73 % AO7 degradation in 3 h, surpassing bare BFO (33 %) and outperforming other CQD loadings. The same trend held for Cr(VI) reduction (up to 80 % in 3 h). Photo‑Fenton tests with H₂O₂ further increased AO7 removal to 96 % (12C/BFO). Catalyst reusability remained >90 % after three cycles.

NIR irradiation yielded negligible activity for bare BFO but enabled 35 % AO7 degradation, 63 % Cr(VI) reduction and 49 % AO7 photo‑Fenton removal by 12C/BFO, underscoring CQDs’ role as NIR sensitizers.

a Visible‑light AO7 degradation; b Cr(VI) reduction; c photo‑Fenton AO7 degradation; d recyclability of 12C/BFO.

a NIR AO7 degradation; b NIR Cr(VI) reduction; c NIR photo‑Fenton AO7; d NIR recyclability.

Active Species Trapping

Scavenger studies revealed ·O₂⁻ as the dominant species in both photocatalytic and photo‑Fenton pathways, with h⁺ and ·OH contributing significantly. NIR‑driven processes also relied on these radicals, confirming that CQDs generate and transfer electrons to BFO under NIR excitation.

Photogenerated Charge Dynamics

Photocurrent measurements showed that 12C/BFO produced ~3.5× higher current density than BFO under visible light, while EIS Nyquist plots displayed a smaller semicircle for 12C/BFO, indicating reduced charge‑transfer resistance.

a Photocurrent response; b EIS spectra.

Catalytic Mechanism

Under visible light, BFO generates electron‑hole pairs that are efficiently separated by CQDs, which act as both electron donors and acceptors. CQD up‑conversion emits 400–580 nm photons under NIR excitation, directly exciting BFO and generating additional charge carriers. In photo‑Fenton cycles, H₂O₂ reacts with surface Fe²⁺ to produce ·OH, while photogenerated electrons reduce Fe³⁺ back to Fe²⁺, sustaining a catalytic loop. These synergistic processes account for the markedly enhanced AO7 degradation and Cr(VI) reduction.

Conclusions

We have demonstrated that CQD decoration on BiFeO₃ nanoparticles significantly boosts visible‑ and NIR‑driven photocatalysis and photo‑Fenton activity toward AO7 and Cr(VI). The improvements arise from enhanced charge separation, efficient electron transfer, and CQD up‑conversion. The composites maintain high stability over multiple cycles, positioning them as promising candidates for sustainable wastewater treatment.