High-purity chemicals were used throughout the study to ensure the reliability and reproducibility of the synthesis and photocatalytic experiments. Bismuth nitrate pentahydrate (Bi(NO₃)₃$\cdot$5H₂O, $\ge$98%), potassium bromide (KBr, $\ge$99.0%), iron (III) chloride (FeCl₃, 97%), and silver nitrate (AgNO₃, $\ge$99.8%) were purchased from Sigma Aldrich (Louis, MO, USA) and employed as precursor materials for the synthesis of Fe-doped BiOBr photocatalysts. Rhodamine B (RhB) was used as the model organic pollutant for photocatalytic degradation studies. Reactive species trapping experiments were conducted using p-benzoquinone (BZQ) as a superoxide radical (•O₂^–) scavenger, isopropanol (IPA, $\ge$99.5%) as a hydroxyl radical (•OH) scavenger, and silver nitrate (AgNO₃) as an electron scavenger. All scavenger chemicals were also obtained from Sigma Aldrich (Louis, MO, USA). Sodium hydroxide pellets (NaOH, 99%), supplied by Qrec (Auckland, New Zealand), were used to adjust the pH during the synthesis process. Sodium chloride (NaCl, 99%), purchased from RCI Lab scan (Bangkok, Thailand), was employed under controlled reaction conditions. Deionized water (DI) and ethanol (EtOH, 99.8%) were used as solvents for precursor dissolution, reaction media, and washing of the synthesized materials to remove residual impurities. All chemicals were used as received without further purification.

Pure and Fe-doped BiOBr photocatalysts were synthesized via a microwave-assisted method. In a typical procedure, 1 mmol of bismuth nitrate pentahydrate (Bi(NO₃)₃$\cdot$5H₂O) was dissolved in 80 mL of DI water under continuous magnetic stirring to obtain a clear Bi³⁺ solution. Subsequently, 1 mmol of potassium bromide (KBr) was added as the bromide source and stirred until completely dissolved. Iron(III) chloride (FeCl₃) was then introduced as the dopant at varying molar concentrations (0.00, 0.25, 0.50, and 0.75 mol % relative to Bi³⁺), followed by continuous stirring at 65 °C for 20 min to ensure homogeneous mixing of the precursor solution. The pH of the resulting suspension was adjusted to 8.0 by dropwise addition of 3 M NaOH to facilitate BiOBr nucleation. The mixture was then transferred to a microwave reactor (Samsung ME81KS-1, Samsung Electronics Co., Ltd. Suwon-si, Gyeonggi-do, Republic of Korea) and irradiated at 450 W for 10 min to induce rapid nucleation and crystal growth of pure and Fe-doped BiOBr. After microwave treatment, the reaction mixture was allowed to cool naturally to room temperature. The resulting white precipitates were collected by filtration, washed several times with DI water to remove residual ions, and further rinsed with 95% ethanol to eliminate remaining impurities. Finally, the obtained BiOBr and Fe-doped BiOBr powders were dried at 80 °C for 12 h before characterization and photocatalytic evaluation. A schematic diagram summarizing the microwave-assisted synthesis and photocatalytic evaluation of pure and Fe-doped BiOBr is presented in Fig. 1.

Fig. 1.

Microwave-assisted synthesis and photocatalytic evaluation of pure and Fe-doped BiOBr. BiOBr, bismuth oxybromide; DI water, Deionized water.

The structural and physicochemical properties of the synthesized samples were systematically characterized using various analytical techniques. Phase identification and crystallographic analysis were carried out by X-ray diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany) using Cu K$\alpha$ radiation ($\lambda$ = 1.5406 Å) over a 2$\theta$ range of 10–80° with a step size of 0.0001°. The morphology and microstructure of the samples were examined by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7800F, Akishima, Tokyo, Japan) operated at accelerating voltages ranging from 100 V to 30 kV, and by transmission electron microscopy (TEM, JEOL JEM-2010, Akishima, Tokyo, Japan) operated at an accelerating voltage of 200 kV. Elemental composition and surface chemical states were analyzed using X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, Manchester, UK) with monochromated Al K$\alpha$ radiation. The binding energies were calibrated using the C 1s peak at 285.1 eV as a reference. Optical properties, including photoluminescence excitation and emission spectra, were measured using a spectrofluorometer (Shimadzu RF-6000, Shimadzu Corporation, Kyoto, Japan). Ultraviolet-visible (UV-vis) absorption spectra were recorded using a UV-Vis spectrophotometer (Agilent Cary 60, Agilent Technologies, Santa Clara, CA, USA). Functional group identification and chemical bonding analysis were performed by Fourier-transform infrared spectroscopy (FT-IR, Bruker INVENIO-S, Bruker Optics GmbH & Co. KG, Ettlingen, Baden-Württemberg, Germany).

For RhB degradation, 0.05 g of catalyst was added to 200 mL of 1.5 $\times$ 10^-5 M RhB. After 30 min of dark equilibration, a 35 W Xe lamp provided visible light. Samples (5 mL) were taken every 10 min, centrifuged, and analyzed at $\lambda$_max = 554 nm. Degradation was calculated using Eqn. 1, where A₀ and A_t represent the initial and absorbance at time t, respectively.

(1)$$\text{Decolorization efficiency (\%)}=\left|\frac{A_0-A_t}{A_0}\right|\times 100\%$$

The photocatalytic performance of the Fe-doped BiOBr system was evaluated through the degradation of RhB dye under visible light irradiation. The corresponding UV-Vis absorption spectra of the 0.50 mol % Fe-doped BiOBr sample recorded before and after irradiation are presented in Fig. 10. The RhB solution exhibits a distinct absorption maximum at approximately 554 nm, which is a characteristic of its chromophoric structure.

Fig. 10.

Time-dependent UV-Vis spectra of RhB during adsorption and photodegradation over 0.50% Fe-BiOBr. (A) RhB spectra in the dark. (B) Under visible light with 0.5% Fe-BiOBr for 90 min. RhB, Rhodamine B.

Prior to light irradiation, the suspension was kept in the dark for 30 min to achieve adsorption-desorption equilibrium between RhB molecules and the photocatalyst surface. During this period, only a slight decrease in absorbance was observed (Fig. 10A), indicating limited adsorption of dye molecules on the catalyst surface. This behavior suggests weak dark-phase interactions and slow molecular diffusion of RhB toward the BiOBr nanosheets in the absence of photoactivation [28, 29].

Upon visible light irradiation, the absorption intensity at 554 nm decreased progressively with increasing exposure time (Fig. 10B), indicating continuous photodegradation of RhB. A marked decline in intensity during the first 20 min reflects the rapid photoexcitation of charge carriers and efficient generation of reactive oxygen species on the Fe-doped BiOBr photocatalyst surface. Correspondingly, the color of the RhB solution gradually changed from deep pink to nearly colorless, confirming substantial dye decomposition. After approximately 70 min, the main absorption peak at 554 nm nearly disappeared and shifted toward 498 nm, which can be attributed to the stepwise de-ethylation of RhB into rhodamine intermediates [29, 30, 31]. The complete disappearance of absorption bands within the 400–700 nm range after 90 min indicates near-total mineralization of RhB into CO₂, H₂O, and other inorganic products [32, 33, 34]. These results clearly demonstrate that Fe doping notably enhances the visible-light photocatalytic efficiency of BiOBr, primarily by improving charge separation and accelerating the redox reactions involved in dye degradation. The 0.50 mol % Fe-doped BiOBr sample, in particular, exhibited rapid degradation kinetics, establishing it as an effective photocatalyst for organic pollutant removal under visible-light irradiation.

The decolorization efficiencies of RhB degradation over pure BiOBr and Fe-doped BiOBr under visible light illumination are presented in Fig. 11A. The incorporation of Fe effectively narrows the optical band gap and introduces shallow impurity states, thereby enhancing visible-light absorption and promoting charge-carrier separation. However, excessive Fe doping can introduce recombination centers that deteriorate photocatalytic performance. Pure BiOBr exhibits relatively low photocatalytic activity, with the reaction rate during the initial 50 min remaining slow and reaching a steady state decolorization efficiency of approximately 70%. When 0.25 mol % Fe is introduced, the degradation rate becomes slightly faster than that of pure BiOBr, resulting in a decolorization efficiency of about 75%. This limited improvement indicates that a small amount of dopant provides only a marginal enhancement in photocatalytic performance. In contrast, the 0.50 mol % Fe-doped BiOBr sample shows a pronounced enhancement in photocatalytic activity. A rapid decrease in RhB concentration occurs within the first 20 min of visible light irradiation, demonstrating significantly improved charge carrier separation and transport. This sample maintains high activity throughout the reaction period and achieves more than 95% decolorization. For the 0.75 mol % Fe-doped BiOBr sample, the photocatalytic efficiency decreases compared with the 0.50 mol % sample. Although the degradation performance remains higher than that of pure BiOBr, the decolorization efficiency reaches only about 80%. This decline is attributed to excessive Fe content, which introduces recombination sites and suppresses the beneficial effects of doping. Overall, these results indicate that moderate Fe incorporation provides an optimal balance between enhanced visible light absorption and reduced charge carrier recombination. The kinetic behavior of RhB photodegradation over pure and Fe-doped BiOBr under visible light irradiation was analyzed using a pseudo-first-order reaction model, as described by Eqn. 4.

(4)$$\ln \left(\frac{C_t}{C_0}\right)=Kt$$

Fig. 11.

Photocatalytic performance and active-species identification for RhB degradation over Fe–BiOBr under visible-light irradiation. (A) Decolorization efficiency. (B) Pseudo-first-order kinetics. (C) Cycling stability. (D) Scavenger effects. BZQ, benzoquinone; IPA, isopropanol.

Where C₀ and C_t represent the initial and time-dependent RhB concentrations, respectively, anddenotes the apparent rate constant. Fig. 11B presents the pseudo-first-order kinetic plots for RhB degradation over pure BiOBr and Fe-doped BiOBr photocatalysts under visible-light irradiation. The calculated apparent rate constants clearly demonstrate a strong dependence on Fe doping concentration. The 0.50 mol % Fe-doped BiOBr exhibits the highest rate constant (0.0660 min^-1), which is significantly higher than those of 0.75 mol % Fe-doped BiOBr (0.0460 min^-1), 0.25 mol % Fe-doped BiOBr (0.0249 min^-1), and pure BiOBr (0.0137 min^-1). This trend is consistent with the degradation efficiency results, confirming that optimized Fe doping significantly accelerates photocatalytic kinetics rather than merely increasing the final degradation percentage. The superior kinetic performance of the 0.50 mol % Fe-doped BiOBr is attributed to its optimized charge-carrier dynamics. As evidenced by PL analysis, this sample exhibits the lowest PL intensity, indicating the most effective suppression of electron-hole recombination and the highest charge separation efficiency. Enhanced charge separation allows a greater number of photogenerated carriers to participate in surface redox reactions, resulting in an increased reaction rate constant. In contrast, lower Fe content provides insufficient charge trapping sites, while excessive Fe doping (0.75 mol %) introduces defect states that act as recombination centers, thereby reducing kinetic efficiency despite relatively strong light absorption. The structural stability of the 0.50 mol % Fe-doped BiOBr photocatalyst was evaluated through XRD analysis.

As shown in Fig. 11C, the long-term stability and reusability of the 0.50 mol % Fe-doped BiOBr photocatalyst were evaluated through five successive photocatalytic cycles. After each cycle, the catalyst was separated, rinsed, and dried prior to reuse. A slight decrease in degradation efficiency was observed after the fifth cycle, with RhB removal remaining at approximately 90%–92% after about 450 min of irradiation. XRD analysis of the recycled photocatalyst confirmed the preservation of the tetragonal BiOBr crystal structure without the appearance of impurity phases, indicating that no phase transformation or structural degradation occurred during repeated photocatalytic cycles. Radical trapping experiments were conducted to identify the dominant reactive species involved in RhB degradation (Fig. 11D). The addition of p-benzoquinone (BQ) and AgNO₃ produced only minor changes in photocatalytic efficiency, suggesting that superoxide radicals (•O₂^–) and electrons play a limited role in the degradation process. In contrast, the presence of NaCl and isopropanol (IPA) resulted in a significant reduction in degradation efficiency, indicating that photogenerated holes (h⁺) and hydroxyl radicals (•OH) are the dominant active species in the process. These results confirm that the 0.50 mol % Fe-doped BiOBr photocatalyst has strong reusability and operates primarily through h⁺ and •OH radicals, highlighting its potential for efficient and sustainable photocatalytic remediation.

The photocatalytic mechanism of Fe-doped BiOBr under visible light irradiation is presented in Fig. 12. The CBE and VBE positions of 0.50 mol % Fe-doped BiOBr were determined to be E_CB = –1.63 eV and E_VB = 1.22 eV, respectively, with a band gap energy of 2.85 eV. Under visible light irradiation, Fe-doped BiOBr undergoes photoexcitation, generating electron-hole pairs (e^–/h⁺) within its band structure [35]. The photoexcited electrons (e^–) in the conduction band (CB) react with dissolved O₂ molecules to generate superoxide radicals (•O₂^–), because the standard redox potential of O₂/•O₂^– at –0.33 V is more positive than the CBE potential of 0.50 mol % Fe-doped BiOBr photocatalyst [29, 36]. These radicals subsequently transform into hydrogen peroxide (H₂O₂), which is further reduced to produce highly reactive •OH radicals. Simultaneously, the photogenerated holes (h⁺) in the valence band directly oxidize the RhB dye molecules to form RhB*. The oxidation of OH^– and H₂O to generate h⁺ does not occur because the standard redox potentials of OH^–/•OH at +1.23 and H₂O/•OH at +2.38 eV are higher than the valence band edge position of the 0.50 mol % Fe-doped BiOBr [26]. The incorporation of Fe³⁺/Fe²⁺ in the BiOBr lattice plays a major role in suppressing electron-hole recombination, enhancing charge transfer efficiency [37]. The Fe dopants act as electron mediators and facilitate the interfacial electron transfer through reversible Fe³⁺/Fe²⁺ redox cycling, thereby prolonging the lifetime of reactive species [38]. Ultimately, the generated h⁺ and •OH radicals break down RhB dye molecules into less harmful intermediates, which are further mineralized into CO₂ and H₂O, as summarized in the following equations [39, 40].

Fig. 12.

Proposed mechanism under visible light.

(5)$$\mathrm{BiOBr}+\mathrm{h}\nu \mathit{\hspace{1em}}\to \mathit{\hspace{1em}}\mathrm{BiOBr}\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)$$

(6)$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to \mathit{\hspace{1em}}\bullet {\mathrm{O}}_2^{-}$$

(7)$$\bullet {\mathrm{O}}_2^{-}+{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\mathit{\hspace{1em}}\to \mathit{\hspace{1em}}{\mathrm{H}}_2{\mathrm{O}}_2$$

(8)$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\mathit{\hspace{1em}}\to \mathit{\hspace{1em}}\bullet \mathrm{OH}+{\mathrm{OH}}^{-}$$

(9)$${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\mathit{\hspace{1em}}\to \mathit{\hspace{1em}}{\mathrm{Fe}}^{2+}$$

(10)$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\mathit{\hspace{1em}}\to \mathit{\hspace{1em}}{\mathrm{Fe}}^{3+}+\bullet \mathrm{OH}+{\mathrm{OH}}^{-}$$

(11)$$\begin{array}{c}\hfill \mathrm{RhB}+\bullet \mathrm{OH}+{\mathrm{h}}^{+}+{\mathrm{O}}_2^{-}\to \\ \hfill {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\text{degraded products}\end{array}$$

The structural stability of the 0.50 mol % Fe-doped BiOBr photocatalyst was evaluated through XRD analysis before and after five consecutive photocatalytic cycles, as presented in Fig. 13. The diffraction patterns of both fresh and reused samples exhibit identical characteristic peaks corresponding to the tetragonal BiOBr phase (JCPDS No. 90-0393), with no detectable secondary phases or impurity reflections. After repeated cycles, only a slight decrease in peak intensity is observed, while the peak positions remain unchanged, indicating that the crystal structure of Fe-doped BiOBr is well preserved. These results confirm that the photocatalyst maintains high phase stability and excellent structural integrity under visible-light irradiation. Overall, the negligible variation in XRD patterns after five cycles demonstrates the robustness and recyclability of Fe-doped BiOBr, highlighting its suitability as a durable photocatalyst for long-term visible-light-driven wastewater treatment applications.

Fig. 13.

XRD of 0.5% Fe-BiOBr before/after five cycles.

Although Fe-doped BiOBr nanostructures exhibited enhanced visible-light-driven photocatalytic performance, several limitations should be acknowledged. First, the photocatalytic activity was evaluated using a single model organic dye (Rhodamine B) under controlled laboratory conditions, which may not fully represent the complexity of real wastewater systems containing mixed organic and inorganic pollutants. Second, although the proposed photocatalytic mechanism is supported by band structure analysis, photoluminescence results, and radical scavenging experiments, direct identification of intermediate species during the degradation process was not performed. In addition, long-term stability and large-scale applicability of the photocatalyst were not investigated within the scope of this study. Future work will focus on evaluating the photocatalytic performance under real wastewater conditions, assessing long-term durability, and exploring scalable synthesis routes for practical environmental remediation applications.