Rosemary leaves extract mediated green synthesis of silver functionalized BiFeO₃ nanoparticles with improved visible light photocatalytic performance

Abstract

Silver-functionalized bismuth ferrite (Ag-BFO) nanoparticles were successfully synthesized via a rosemary (Rosmarinus officinalis) leaf extract–assisted green route and demonstrated markedly enhanced visible-light photocatalytic performance. Structural analyses confirmed the formation of a rhombohedral perovskite BiFeO₃ phase with effective Ag incorporation and suppressed secondary phases. Ag functionalization reduced the optical bandgap from 2.11 to 2.07 eV and significantly increased the BET surface area from 1.86 to 9.35 m² g⁻¹, promoting improved light absorption and charge separation. Under visible-light LED irradiation, Ag-BFO achieved 95% degradation of methylene blue dye within 120 min, compared to 75% for pristine BFO under identical conditions. Kinetic studies revealed a pseudo-first-order behavior with a higher rate constant for Ag-BFO. Scavenger experiments identified hydroxyl radicals (•OH) as the major reactive species. The enhanced photocatalytic activity is attributed to Ag-induced bandgap narrowing, increased surface area, and suppressed charge-carrier recombination. These findings demonstrate that rosemary-mediated green synthesis of Ag functionalized BFO as an effective strategy for developing high-performance photocatalysts for sustainable wastewater treatment.

Introduction

Water contamination caused by synthetic dyes and industrial effluents is causing a severe threat to ecosystems as well as human health [1]. The development of efficient and sustainable remediation strategies are critical scientific challenge [2]. Industries like pharmaceutical, chemical, paper, plastic, food and textile industries discharge various types of organic pollutants into the environment. The release of these pollutants significantly harm aquatic and terrestrial ecosystems [3]. Among these pollutants, textile dyes such as methylene blue are extremely stable, toxic, and non-biodegradable, and thus the removal of these pollutants from wastewater has been a persistent challenge [4].

Photocatalysis is a process which makes use of light-responsive catalysts to break down organic contaminants. This process has gained attention as a sustainable strategy for water remediation [5]. Bismuth ferrite (BFO) nanoparticles are of interest because of their narrow bandgap, responsiveness to visible light, multiferroic properties, and capability to produce charge carriers upon light exposure [6]. These properties has made BFO as a fascinating material for the efficient degradation of complex organic pollutants [7]. However, challenges such as low stability, moisture sensitivity, and limited operational performance have hindered their broader application [8].

Several approaches have been employed to synthesize BFO such as sol–gel, co-precipitation, hydrothermal, reduction, and irradiation-assisted chemical reactions [9]. These methods typically carry issues of long reaction time, complicated procedures, high temperature, expensive instruments and environmentally unfriendly solvents [10]. In this regard, green chemistry approaches have emerged as a sustainable alternative for achieving a sustainable future by reducing the use of toxic chemicals and using plant or microbial extracts as reducing and stabilizing agents [11]. Plant-mediated green synthesis has attracted great attention owing to its milder reaction conditions, ease, environmentally benign, and the presence of bioactive phytochemicals such as flavonoids, tannins, and phenolic acids in the presence of reduction and stabilization of nanoparticles [12]. Recent studies have shown that the photocatalytic efficiency and chemical stability [13] as well as the visible light absorbance could be improved by metal doping especially the addition of silver in BFO nanoparticles [14]. Among other green sources, Rosmarinus officinalis (rosemary) is remarkably promising material being high in antioxidants, flavonoids, and polyphenolic compounds, that act as natural reducing and stabilizing agents [15].

However, only few studies have explored the synthesis of BFO via green methods and its exploration for the degradation of textile dyes. This study covers this gap and aims to synthesize Ag-BFO nanoparticles by Rosmarinus officinalis leaf extract and assess their structural, morphological, and photocatalytic properties for methylene blue degradation. The research demonstrates the potential of these green-synthesized nanoparticles for effective and sustainable water pollution remediation. Advanced characterization techniques such as XRD, SEM, EDX, N₂ surface area analysis, UV-DRS analysis and FTIR were used to determine their structural and functional properties whereas their photocatalytic performance under visible light was investigated using MB dye as model water pollutant.

To the best of our knowledge, there is no prior report on the green synthesis of this material and its application for dye degradation. Accordingly, the novelty of the present work is in the eco-friendly (green) synthesis approach, as well as the first-time investigation of the photocatalytic performance toward organic dye degradation under visible light irradiation. In addition, this research offers a systematic correlation among the structural, optical, and photocatalytic properties, which illustrates the importance of green-fabricated nanostructures to enhance charge separation and degradation efficiency.

Materials and methods

Materials

Rosemary (Rosmarinus officinalis) leaf extract, Silver Nitrate (AgNO₃, 99%), Bismuth (III) Nitrate Pentahydrate (Bi(NO₃)₃·5H₂O, 98%), Ferric Nitrate Nonahydrate (Fe(NO₃)₃·9H₂O, 98%), Hydrochloric Acid (HCl, 37%), Methylene Blue (99%,), and distilled water were used in the synthesis of Ag-BFO nanoparticles.

Green synthesis of bismuth ferrite (BFO) nanoparticles

Preparation of plant extract

Rosmarinus officinalis leaves were collected in 2023 by Ms. Nadia Nawaz from a private cultivated garden located in Sialkot, Punjab, Pakistan. All relevant national guidelines were fully followed during the collection of the plat material. No permission/ license was required as the plant was collected from a privately owned garden. The collected leaves were thoroughly washed with running tap water to remove dust impurities and were shade-dried in a clean and ventilated room. The dried leaves pulverized into fine powder. Five grams of leaves were added to 100 ml of distilled water. The mixture was allowed to mix under vigorous stirring at 70 °C for successive two hours using a hot plate/magnetic stirrer assembly. After cooling to room temperature, the reaction mixture was filtered immediately using Whatman filter paper No 9. To obtain a clear, brown-colored filtrate. The freshly prepared filtrate was used immediately for BFO and Ag-BFO synthesis and was not stored for extended periods.

For the nanoparticle synthesis, salt solutions of iron and bismuth were prepared by addition of 2.5 g of iron nitrate nonahydrate [Fe (NO3)3.9H2O] and 5.7 g of bismuth nitrate pentahydrate [Bi (NO3)3.5H2O] in 100 mL of distilled water with continuous stirring to yield a homogeneous solution (Solution A). Separately, 0.51 g of silver nitrate was dissolved in a minimum amount of distilled water to obtain Solution B. Solution B was added slowly to Solution A under continuous stirring and the mixture obtained was homogenized completely with the help of a magnetic stirrer to obtain Solution C.

Green synthesis of BFO and Ag-BFO nanoparticles

To synthesize BFO, the plant extract was added dropwise to solution A, and the reaction temperature was kept at 70 °C using a hot plate and magnetic stirrer. A change in colour of the solution from pale yellow to dark brown suggested the formation of BFO. A similar procedure was adopted to synthesize Ag-BFO, where the plant extract was slowly added to the solution C at 70 °C. A change in colour from pale brown to blackish brown indicated the formation of the composite material. Both the reaction mixtures were stirred for three hours to maximize the product formation at the prescribed temperature. The material was filtered and washed with plenty of distilled water. Both BFO and Ag-BFO were dried at 105 °C in an oven and were calcined at 420 °C for three hours. The ambient calcination temperature was selected to obtain the pure perovskite phase of BFO. Verma et al. reported that calcination temperatures below 400 °C favour the formation of secondary phases such as Bi₂Fe₄O₉ and Bi₂₅FeO₄₀. However, increasing the calcination temperature promotes the formation of the pure perovskite phase of bismuth ferrite [16]. Figure 1 depicts the general scheme for the synthesis of BFO or Ag-BFO nanocomposites using Rosmarinus officinalis leaves extract.

Fig. 1.

Flowchart to describe the synthesis of Ag-BFO nanoparticles using extracts of rosemary leaves

Characterization techniques

The synthesized photocatalyst was thoroughly characterized using various techniques.

X-ray diffraction analysis was carried out using a Bruker D8 Advance diffractometer operating in reflection mode and equipped with a Cu–Kα source (λ = 1.5406 Å), set at 40 kV and 30 mA. The functionalities of the synthesized catalysts were determined using a Bruker Alpha II FTIR spectrometer. The spectra were recorded from 4000 to 400 cm⁻¹. The surface morphology of the synthesized samples was examined using a MIRA3 TESCAN scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The SEM images were acquired under high-vacuum conditions at an accelerating voltage of 15 kV, with a working distance of approximately 9.5 mm. High-magnification micrographs (∼15,000 ×) were recorded to investigate particle agglomeration, surface texture, and morphological variations induced by Ag doping. The elemental composition was determined using the same instrument used for SEM analysis. Surface area analysis of the synthesized photocatalyst was carried out using a NOVA 1200 analyzer. The optical properties and bandgap determination were performed using a SPE 3000 UV–Vis diffuse reflectance spectrophotometer (UV–DRS).

Reaction setup

The photocatalytic activity (PCA) was conducted in a Pyrex glass 100 mL beaker. MB was used as a model pollutant. For each photocatalytic experiment, 0.04 g of the catalyst was dispersed in 50 mL of a 50 ppm dye solution and the mixture was kept in the dark for 30 min to establish adsorption–desorption equilibrium. The reaction mixture was then exposed to visible light using a standard white LED light of 12-W capacity, having a luminous flux of 800 lumens and a visible wavelength range (~ 400–700 nm). The distance between the LED source and the reaction solution was fixed at 5 inches (≈12.7 cm). The reaction temperature was maintained at 30 ± 0.5 °C using a water-bath setup to avoid thermal effects. Aliquots of the reaction mixture were taken at regular intervals, centrifuged and analysed by a UV–Visible spectrophotometer to determine any decrease in the concentration of the dye. The reaction temperature was controlled with the help of a water-bath setup placed on a hotplate–stirrer unit. The reactions were mostly conducted at room temperature, i.e. 30 °C ± 0.5.

The efficiency of the catalysts was determined using the following equation

1

here [MB]_o represents the initial concentration of the MB dye (mg L⁻¹), while [MB]_t denotes its concentration at a given time ‘t’.

Results and discussion

Characterizations of the synthesized BFO nanoparticles

XRD analysis

The X-ray diffraction (XRD) patterns of pure BFO and Ag-BFO, and metallic silver are presented in Fig. 2. The primary diffraction peaks of the BFO sample are well indexed to the rhombohedral perovskite structure with space group R3c, under the standard JCPDS card No. 01–071-2494, confirming successful phase formation of BFO. The prominent reflections observed at 2θ ≈ 22.5°, 31.7°, 32.1°, 39.5°, 45.6°, 51.0°, 56.2°, and 66.1° correspond to the (012), (104), (110), (202), (024), (116), (122), and (300) crystallographic planes of BFO, respectively, which is consistent with earlier reports [17].

Fig. 2.

XRD pattern of Ag (Green), BFO (Black) and Ag-BFO(Red)

In addition to the main phase, the BFO sample exhibits minor impurity peaks at around 30° and 33°, which can be attributed to Bi₂O₃ and Bi₂₅FeO₄₀ secondary phases, respectively. These impurity peaks are more pronounced in the undoped BFO sample, indicating partial decomposition or incomplete reaction during synthesis, which has also been reported in similar systems synthesized via solid-state and sol–gel routes [18].

Doping BFO with silver retained the crystal structure of BFO nanoparticles without disrupting its crystal lattice. However, additional low intensity peaks labeled with asterisks (*) are observed in the Ag-BFO pattern at 2th around 38.1⁰, 44.3⁰ and 64.4⁰ representing (111), (200) and (220) crystalline planes of metallic silver (Ag⁰), which indicates the presence of a small amount of metallic Ag phase corresponding to JCPDS 04–0783. These peaks indicate the partial reduction of Ag + ions to Ag0 nanoparticles on the BFO surface or at the grain boundaries [19].

The minor variation in the diffraction peaks in the Ag-BFO sample relative to the pure BFO sample implies a distortion in the crystal lattice due to the addition of Ag⁺ ions. This shift indicates the successful incorporation of Ag⁺ into the BFO lattice, most likely substituting at the Bi³⁺ or Fe³⁺ sites due to their comparable ionic radii. Substitutional doping results in lattice strain, which is reflected in a peak shift of the XRD pattern.

X-ray diffraction analysis of the undoped samples of BiFeO₃ shows the appearance of secondary impurity phases, which were identified as Bi₂WO₃ and Bi₂₅FeO₄₀.The creation of these impurities is a typical issue in BFO synthesis and it is usually linked to partial decomposition or incomplete reactions during the fabrication process [20].Bismuth ferrite is known to be a metastable material and the synthesis parameters such as the purity of the starting materials and the thermal treatment conditions critically affect the phase composition and can lead to the manifestation of secondary phases [21]. For example, precise control over calcination temperature is very important, with inappropriate temperatures potentially favoring the formation of such non-perovskite phases [22].In contrast, Ag doped BFO samples show a noticeable decrease of these impurity peaks. This suppression of secondary phases upon Ag doping can be explained by improved phase stability and modified crystallization behaviour generally induced by dopants during synthesis. The addition of dopants such as silver can have a significant impact on the properties of the material and its crystallization kinetics, resulting in a more stable perovskite structure and preventing formation of undesirable secondary phases [23]. Although the exact mechanism by which Ag suppresses the formation of Bi₂WO₃and Bi₂₅FeO₄₀ may vary, dopants typically interact at the atomic level, modifying lattice parameters, electronic structure, and thermodynamic stability [24]. These changes help stabilize the desired single-phase structure and inhibit the formation of secondary impurity phases.

The mean crystallite size of the green synthesized BFO nanoparticles was calculated using the Debye- Scherrer equation:

2

The average crystallite sizes of the green synthesized nanoparticles were calculated to be 20.6 nm for BFO and 13.7 nm for Ag-BFO, respectively [25].

FTIR analysis

The FTIR spectra of rosemary leaves extract, undoped BFO, fresh Ag-BFO and used Ag-BFO nanoparticles are shown in Fig. 3 to establish the structural changes caused by silver doping and further photocatalytic application. The FTIR spectra provides vital evidence of how the bioreduction, and stabilization mechanism was assisted by the rosemary leaf extract. In the extract spectrum (black), the characteristics peaks of 3415 cm⁻¹ (stretching, O–H), 1628 cm⁻¹ (stretching, C=O) and 1040 cm⁻¹ (stretching, C–O) confirm the presence of bioactive polyphenols and flavonoids (Rosmarinus and carnosic acid) [26]. These phytochemicals not only facilitate the reduction of metal precursors, they also serve as capping agents, thereby preventing the agglomeration of the nanoparticles [27].

Fig. 3.

FTIR of leaves extract, undoped BFO, Unused Ag-BFO, and Used Ag-BFO

For the undoped BFO (in red) the vibrational bands at 1669 cm⁻¹ and 1496 cm⁻¹ signal the successful attachment of organic functional groups to the metal surface, while the sharp peaks at 630 cm⁻¹ and 498 cm⁻¹ are associated to the characteristic Fe–O stretching and Bi–O bending vibration of the octahedrally coordinated FeO₆ units in the perovskite lattice. Upon doping with silver (blue), the Ag-doped BFO (unused) shows a clear shift in these peaks to 1645 cm⁻¹ and 1478 cm⁻¹, in addition to having a new stretching vibration at 2855 cm⁻¹ (C-H symmetric stretching) implying that this changes the surface chemistry and interaction with the extract’s stabilizing agents [28].

Most notably, the metal–oxygen stretching band shifts from 498 cm⁻¹ in the undoped sample to 480 cm⁻¹ after Ag doping. The red shift and band broadening verify Ag⁺ substitution in the BFO framework, which generates lattice strain and perturbs the vibrational energy of FeO₆ octahedral units [29]. An additional peaks at 3750 cm⁻¹ (undoped) and 3727 cm⁻¹ (Ag-doped) corresponds to surface-adsorbed hydroxyl group. The stability of Ag-BFO after use (green) demonstrates that the rosemary-derived organic shell effectively protects the perovskite BFO structure, while the silver remains retained even after the reaction [30, 31].

SEM and EDX analysis

The surface morphology of green-synthesized BFO and 2.5% Ag–BFO was examined using scanning electron microscopy (SEM), as shown in Fig. 4. The pure BFO sample (Fig. 4a), synthesized using Rosmarinus officinalis (rosemary) leaf extract, exhibits a highly agglomerated microstructure composed of fine, nearly spherical to irregular nanoparticles. These nanoparticles are densely packed, forming clusters with relatively small intergranular voids. The particle boundaries are visible but closely interconnected, indicating strong agglomeration, which is commonly observed in green-synthesized metal oxide nanoparticles due to the presence of phytochemicals that promote particle bridging during growth. Overall, the morphology suggests a compact granular structure with moderate surface roughness and limited porosity [31].

Fig. 4.

a SEM micrograph of BFO; b SEM micrograph of Ag–BFO; c EDX spectrum of BFO; d EDX spectrum of Ag–BFO

In contrast, the 2.5% Ag-doped BFO sample (Fig. 4b) shows a noticeable change in morphology. The microstructure appears more heterogeneous, consisting of irregularly shaped grains along with plate-like and flake-like features. Compared to pure BFO, the Ag–BFO sample exhibits looser agglomeration and the presence of larger inter-particle voids, indicating increased porosity. The incorporation of Ag ions during synthesis likely disturbed the uniform growth of BFO nanoparticles, leading to grain refinement as well as localized grain coalescence and defect formation. This morphological evolution results in a rougher surface texture and a more open structure, which is beneficial for photocatalytic applications. The increased surface irregularity and porosity can enhance surface area, facilitate light absorption, and provide additional active sites for pollutant adsorption, thereby potentially improving the photocatalytic performance of Ag–BFO. These results confirm that Ag doping via a green synthesis route using Rosmarinus officinalis extract induces favorable microstructural modifications in BFO [32].

EDX analysis

Energy Dispersive X-ray Spectroscopy (EDX) was used to determine the elemental composition of the produced nanocomposites. The EDX spectra of bismuth ferrite (Fig. 4c) and Ag-doped bismuth ferrite (Fig. 4d), synthesized using rosemary leaves extract, confirm their elemental composition. The undoped sample exhibited prominent peaks of Bi and Fe, verifying the formation of BFO, along with minor peaks of Ca, K, Na, Mg, Si, and Cl, which are attributed to trace phytochemical and mineral residues from the plant extract. The doped sample displayed the characteristic Bi and Fe peaks along with a distinct Ag peak, confirming successful silver incorporation into the BFO lattice. Similar low-intensity peaks from bio-derived elements are also observed, indicating their origin from the green synthesis medium [33]. Table 1 shows the energy dispersive X-ray spectroscopy of silver-doped bismuth ferrite nanoparticles.

Table 1.

Elemental composition of synthesized photocatalysts

	Ag-BFO		BFO
Elements	Mass%	Atom%	Elements	Mass%	Atom%
Oxygen	37.66	46.33	Carbon	53.31	65.35
Carbon	18.31	29.98	Oxygen	30.98	28.51
Sodium	21.21	18.17	Sodium	5.02	3.22
Chlorine	3.46	1.93	Silicon	2.48	1.30
Bismuth	12.22	1.14	Calcium	1.24	0.46
Calcium	1.38	0.67	Magnesium	0.64	0.39
Silicon	0.75	0.53	Chlorine	0.54	0.22
Silver	3.01	0.55	potassium	1.02	0.38
Iron	2.02	0.71	Iron	0.66	0.17
Total	100.0	100.0			100.0

Optical characterization

Figure 5a, b presents the UV–Vis absorption spectra of pristine BFO and Ag-doped BFO nanoparticles in the wavelength range of 350–650 nm, along with the corresponding Tauc plots shown as insets. Both samples exhibit strong absorption in the visible region, confirming their suitability for visible-light-driven photocatalytic applications. The absorption edge of pristine BFO is observed around ~ 560 nm, whereas Ag-BFO shows a noticeable red shift toward longer wavelengths, indicating enhanced visible-light absorption upon silver incorporation. The improved absorption behavior of Ag-BFO can be attributed to the introduction of Ag-related electronic states and improved charge carrier dynamics, which facilitate better light harvesting. The optical bandgap energies were estimated using the Tauc relation by plotting versus photon energy (hν), assuming a direct allowed transition. The extrapolation of the linear region to the energy axis yields bandgap values of approximately 2.11 eV for BFO and 2.05 eV for Ag-BFO. The reduction in bandgap energy after Ag doping suggests the formation of intermediate energy levels within the BFO lattice, which lowers the excitation energy required for electron–hole pair generation. This bandgap narrowing enhances visible-light absorption and suppresses charge carrier recombination, thereby contributing to the superior photocatalytic performance of Ag-BFO compared to pristine BFO. The improved electron-harvesting ability of Ag-BFO therefore leads to a more efficient degradation of MB dye under visible-light irradiation [34].

Fig. 5.

UV–Vis absorption spectra of a BFO and b Ag-BFO nanoparticles. Insets show the corresponding Tauc plots used for bandgap estimation

Surface area analysis

The nitrogen adsorption–desorption isotherms for both pristine BFO and Ag-doped BFO are shown in Fig. 6. Both isotherms exhibit Type IV behaviour according to IUPAC classification, characterized by pronounced H3-type hysteresis loops. These types of hysteresis loops are associated with mesoporous structures and the formation of slit-shaped pores. The undoped BFO shows a very low BET surface area of only 1.86 m² g⁻¹. A marked increase in surface area (32.4 m² g⁻¹) was observed upon incorporation of Ag into the BFO lattice. This increase in surface area enhanced the number of active sites for dye molecule adsorption, facilitating efficient interaction between the doped catalyst and the dye, which resulted in improved photocatalytic activity compared to pristine BFO [35].the textural properties of both the catalysts is shown in Table 2

Fig. 6.

BET surface area plot of BFO and Ag-BFO powders (Inset pore size distribution)

Table 2.

Surface textural properties of pristine BFO and Ag-doped BFO catalysts

Parameter	BFO	Ag-BFO
BET surface area (m2 g⁻1)	6.8	9.35
Total pore volume (cm3 g⁻1)	0.012	0.098
BJH average pore diameter (nm)	12.6	27.8

Photocatalytic activity of BFO and Ag-BFO

The undoped and Ag-doped BFO were prepared by the green synthesis technique using rosemary leaves extract and were investigated for the degradation of methylene blue under LED light. Various operational parameters, including contact time, catalyst dose, pH, recyclability, effect of temperature, and the initial dye concentration of MB, were studied to thoroughly understand the reaction behaviour under different conditions. A good catalyst is expected to exhibit high absorption capacity with fast reaction kinetics. As per expectations, the reaction rate was found to be fast at the start of the reaction due to the free availability of the active sites on the catalyst surface. The reaction rate, however, was found to decrease as the surface of the catalyst got saturated. A maximum of 95% MB was degraded by Ag-BFO, compared to BFO, which degraded 75% dye in 120 min under the same reaction conditions. The addition of silver to BFO resulted in a decrease in the particle size, improvement in the surface area, and band gap energy. This rendered a better PCA of Ag-BFO, in comparison to undoped BFO. The effect of contact time on methylene blue degradation using rosemary-assisted Ag-BFO and BFO photocatalysts was investigated, with the results depicted in Fig. 7a. The corresponding UV–Vis absorption spectra, showing the gradual decrease in methylene blue concentration over time in the presence of the Ag-BFO photocatalyst, are presented in Fig. 7b. A summary of the degradation of MB using BFO and Ag-BFO is presented in Table 3.

Fig. 7.

a Effect of contact time on the performance of BFO and Ag-BFO photo catalysts b UV–Vis spectra of MB at different time intervals over the Ag-BFO photocatalyst. [MB]o = 50 mL, 50 mg/L, catalyst 0.04 g, normal pH (6.5)

Table 3.

Summary of removal efficiency using BFO and Ag-BFO

Sample	Removal efficiency (%)
	90 min	120 min
BFO	67 ± 0.2	75 ± 0.17
2.5% Ag-BFO	86 ± 0.2	95 ± 0.2

Effect of initial dye concentration on MB degradation

The effect of initial dye concentration of MB, Fig. 8a, was investigated in a range of 25–100 mg/L at normal pH and optimal catalyst dose, i.e., 0.04 g of Ag-BFO. At lower concentration (25 mg/L), the reaction rate was faster, about 99% degradation within 120 min; However, increasing the dye concentration led to a reduction in the degradation efficiency, with values of 95% for 50 mg/L, 77% for 75 mg/L, and 47% for 100 mg/L. At low concentration, the availability of active sites to the MB was abundant; however, as the dye concentration increased, the number of active sites to the dye molecules decreased. Another reason could be the access to light through the reaction mixture. Being a photocatalytic process, the more accessible the light is to the catalyst, the better the photocatalytic activity. Hence, at low concentration, the light penetrates easily, producing an enhanced dye degradation, but a higher dye concentration hinders the light from reaching the catalyst surface, thereby reducing the photocatalytic activity [36].

Fig. 8.

Photocatalytic degradation of methylene blue using BFO and Ag-BFO under varying conditions: a dye concentration, b catalyst dosage, c stirring speed, d temperature, and e stirring speed

Effect of catalyst dose on degradation efficiency of MB dye

The effect of catalyst dosage on the photocatalytic degradation of (MB) dye was evaluated using both pure BFO and Ag-BFO. The results demonstrated that degradation efficiency increased with catalyst dosage up to an optimal value of 0.04 g, beyond which the photocatalytic activity declined. At this optimum dose (0.04 g), Ag-BFO achieved the highest degradation of 95%, while BFO showed 83% degradation. A further increase in catalyst amount led to reduced efficiencies: at 0.06 g, degradation dropped to 85.1% for Ag-BFO and 73% for BFO, and at 0.08 g, the degradation further declined to 80.3% for Ag-BFO and 61% for BFO. At a lower dose of 0.02 g, Ag-BFO showed 85.1% degradation, while BFO achieved 77%. The initial increase in efficiency can be attributed to the availability of more active sites and enhanced generation of reactive species. However, beyond the optimal dose, excessive catalyst loading leads to light scattering, reduced photon penetration, and particle agglomeration, which collectively reduce the effective surface area and limit dye adsorption. These observations highlight the critical role of optimizing catalyst dosage to balance photocatalytic activity and degradation efficiency. The results are shown in Fig. 8b.

Effect of pH on photocatalytic degradation of MB dye

The pH of the reaction medium plays a crucial role in influencing the photocatalytic degradation efficiency of dyes. MB dye, being a cationic dye, is also influenced by the reaction pH. The findings demonstrate that the degradation efficiency is markedly dependent on the solution pH, exhibiting a gradual increase from acidic to basic conditions (Fig. 6c). At pH 2 and 4, the degradation efficiencies were 68% and 80%, respectively. Due to the cationic nature of the dye, there is a reduced formation of hydroxyl radicals (•OH) due to the electrostatic repulsion between the positively charged catalyst surface and dye. At natural pH (~ 6.5), efficiency improved to 92%. At pH 8 and pH 10, the degradation improved further, reaching 96% and 99% respectively, within 90 min. The superior degradation efficiency in alkaline media is primarily due to the elevated generation of highly reactive •OH radicals. Another reason could be the better adsorption of the MB dye under basic conditions. Under alkaline conditions, the catalyst surface acquires a negative charge, which strengthens electrostatic interactions with the MB dye, leading to improved adsorption. Hence, an alkaline medium favours the degradation efficiency of Ag-BFO due to synergistic effects of enhanced dye-catalyst interaction and radical formation [37].

Effect of temperature

The influence of reaction temperature on the photocatalytic degradation of MB dye using Ag-BFO was investigated by varying the temperature from 30 °C to 60 °C, while keeping all other parameters constant. As shown in Fig. 6d, the degradation efficiency exhibited a temperature-dependent trend. At 30 °C, a degradation efficiency of 92.3% was achieved after 90 min of irradiation. Increasing the temperature to 40 °C led to a notable improvement in performance, with the degradation rising to 98.1%. The initial enhancement in degradation efficiency from 30 °C to 40 °C can be attributed to increased kinetic energy of dye molecules, which improves mass transfer, adsorption–desorption dynamics, and collision frequency between the dye and reactive species [38].Moreover, moderate thermal energy facilitates the separation and migration of photogenerated electron–hole pairs, leading to enhanced formation of reactive hydroxyl radicals (•OH), thereby accelerating the degradation kinetics. However, the degradation efficiency decreased beyond this point. The degradation efficiency was found to be decreased as follows: 91.2% at 50 °C, then to 83.4% at 60 °C after 90 min of irradiation. The photocatalytic efficiency decreases due to several concurrent factors. Elevated temperatures promote faster recombination of photogenerated charge carriers [39], which reduces the lifetime of reactive species and limits hydroxyl radical availability [40, 41].

In addition, excessive thermal agitation weakens dye adsorption on the catalyst surface by shifting the adsorption–desorption equilibrium toward desorption, resulting in fewer dye molecules available at active sites [42]. Furthermore, higher temperatures may induce partial surface dehydroxylation and destabilization of surface-adsorbed oxygen species, which are essential for •OH generation [43]. These combined effects outweigh the benefits of increased molecular kinetics, leading to a net reduction in photocatalytic activity at elevated temperatures.

A first-order kinetic model was used to assess the temperature data from the catalytic degradation of the dye. Rate constants for the degrading reaction at different temperatures were determined by the slope of the graphs: 0.02488 at 30 °C, 0.03783 at 40 °C, 0.02984 at 50 °C, and 0.0214 at 60 °C. This shows that the rate constant of the reaction increases from 0.02488 to 0.03783 as the temperature climbs from 30 to 40 °C, indicating an enhanced reaction rate. This impact is due to molecules' increased energy activation, which encourages more frequent and potent collisions, optimizing dye degradation. On the other hand, degradation activity decreased at higher temperatures. Increased electron–hole pair recombination on the catalyst surface is the cause of this decline.

Effect of stirring speed (RPM)

The influence of stirring speed on the photocatalytic degradation efficiency of methylene blue (MB) using Ag-BFO was examined within the range of 200–1000 RPM, as shown in Fig. 8e. The degradation percentage initially increased with stirring speed, indicating enhanced interaction between dye molecules and the catalyst surface. At 200 RPM, the degradation efficiency was 81.8%, which increased to 87% at 400 RPM due to improved mass transfer and dispersion of catalyst particles. Maximum degradation of 92.3% was achieved at 600 RPM, suggesting that this speed ensures optimal suspension of the catalyst, prevents particle sedimentation, and allows better light penetration and contact with MB molecules. However, a further increase to 800 and 1000 RPM resulted in a decline in efficiency to 89.1% and 77.2%, respectively. This decrease may be attributed to excessive turbulence at higher RPMs, which can lead to re-aggregation of catalyst particles and disruption of light absorption zones. Thus, 600 RPM was identified as the optimum stirring speed for maximum photocatalytic performance [44].

Scavenging experiments

Scavenger experiments were carried out to identify the key reactive species in MB degradation over Ag-BFO, using IPA to quench •OH radicals, BQ for O₂^•− radicals, and EDTA for photogenerated holes (h +). In these experiments, the photocatalytic performance was assessed with each scavenger under identical reaction conditions. The effect of each scavenger on degradation efficiency was found to be as follows: Without scavenger: 95% degradation of MB, With IPA (10 mM), the degradation was reduced to 58.6% with BQ (10 mM), the degradation remained relatively high at 89.2%, while with EDTA (10 mM), the degradation slightly decreased to 85.7%. The significant reduction in degradation efficiency upon the addition of isopropanol (IPA) confirms that hydroxyl radicals (OH) are the dominant reactive species responsible for the breakdown of MB dye. IPA is a well-known hydroxyl radical scavenger, and its presence effectively quenches the active species, thereby suppressing the degradation process. However, only a minimal impact of benzoquinone (BQ) and EDTA indicates that superoxide radicals (O₂⁻) and photogenerated holes (h⁺), respectively, do not play a major role in the photocatalytic reaction. The nesar-unchanged degradation efficiency with BQ suggests that the electron-induced formation of superoxide radicals is not a significant pathway under the applied conditions. Similarly, the slight reduction in activity with EDTA shows that direct oxidation by holes is not the primary mechanism. The results are summarized in Fig. 9a.

Fig. 9.

a The scavenger experiments and b Proposed mechanism for the photocatalytic degradation of MB over Ag-BFO

Proposed mechanism

The photocatalytic degradation of methylene blue (MB) using Ag-doped BFO (Ag-BFO) occurs predominantly through the generation and action of hydroxyl radicals (OH). Scavenging experiments confirmed that neither superoxide radicals (O2⁻) nor photogenerated holes (h +) significantly contribute to the degradation pathway. Therefore, the following mechanism is proposed:

Upon visible light irradiation, Ag-BFO absorbs photons and generates electron–hole pairs:

3

The conduction band electrons react with molecular oxygen to form superoxide free radical

4

The peroxide free radical may react with the proton to produce Hydroperoxyl radical

5

6

7

The photogenerated holes in the valence band react with water molecules and hydroxide ions adsorbed on the surface of the catalyst to produce hydroxyl radicals:

8

9

The highly reactive and non-selective hydroxyl radicals (•OH) interact with MB dye molecules, promoting their gradual oxidative decomposition into non-toxic products such as CO₂, H₂O, and other small intermediates (Fig. 9b). Although superoxide radicals may form during the reaction, their primary contribution is indirect, as they serve as precursors for hydroxyl radical generation, explaining why •OH appears as the dominant reactive species in the scavenger experiments.

10

Degradation kinetics

The photocatalytic degradation of methylene blue (MB) using Ag-doped BFO synthesized via rosemary extract follows the Langmuir–Hinshelwood (L–H) mechanism, typical for heterogeneous reactions. The mechanism proceeds through the following steps: (i) attachment of methylene blue molecules onto the surface of the catalyst, (ii) absorption of light leading to electron–hole pair generation, (iii) production of reactive radical species, and (iv) oxidative breakdown of the dye molecules.

According to the Langmuir–Hinshelwood kinetic model, the rate of dye degradation can be expressed as,

11

Since the photocatalytic reaction is carried out under continuous irradiation, the rate constant becomes independent of light intensity and can be simplified as

12

Rearranging

13

Integrating between the limits C_o (initial concentration) and C_t (concentration at time t), we get

14

15

Taking antilog, we get.

15

or

17

Equation (12) was employed to evaluate the kinetics of MB dye degradation at various initial dye concentrations; the slopes of the linear plots were obtained and used to calculate the rate constants using the corresponding formula. The results demonstrated that the equation provided an excellent fit to the concentration data for the studied range (25–100 mg/L), as illustrated in Fig. 10. Furthermore, it was observed that the calculated rate constants decreased progressively with increasing dye concentration, indicating that higher concentrations lead to slower degradation rates due to active site saturation and reduced photon penetration in the reaction medium. The kinetic rate constants and corresponding regression coefficient values are presented in Table 4.

Fig. 10.

Application of Eq. (12) to evaluate the kinetics of MB dye degradation at various initial dye concentrations (25–100 mg/L)

Table 4.

Rate constants (k) and regression coefficients (R²) for MB degradation at different initial dye concentrations

Initial concentration (mg/L)	Rate constant (k, min⁻1)	Regression coefficient (R2)
25 mg/L	0.0342	0.955
50 mg/L	0.0226	0.993
75 mg/L	0.0106	0.955
100 mg/L	0.0056	0.986

Comparison of photocatalytic performance of BFO-based catalysts for MB dye degradation

Table 5 compares the photocatalytic performance of various BFO-based catalysts for MB dye degradation. While doped BFO systems, such as Ag- and Ca-doped variants, exhibited high efficiencies under visible light, they required either longer reaction times or worked at lower dye concentrations. In contrast, our Ag–BFO photocatalyst demonstrated higher degradation efficiency within a shorter reaction time and at higher dye concentrations, highlighting its superior photocatalytic performance over previously reported BFO-based systems. Although experimental conditions vary among studies, the present Ag–BFO photocatalyst demonstrates high degradation efficiency at a relatively higher dye concentration and lower catalyst dose under visible light, highlighting its competitive performance.

Table 5.

Comparison of the photocatalytic performance of various catalysts for methylene blue degradation in aqueous solution

Catalyst	MB concentration (mg/L)	Catalyst dose	Light source	Reaction time (min)	Degradation efficiency (%)	References
BFO	100	Not specified	Visible light	120	79	[45]
BiFeO₃–25 wt% ZnFe₂O₄ composite	15	0.1 g	Visible light (2 × 100 W xenon lamps with λ > 420 nm)	30	97	[46]
Ni-doped BFO	20	50 mg	UV light	120	84.06	[47]
La-doped BFO	3.2	0.05 g	Visible light	50	96	[48]
Sr-doped BiFeO₃ (x = 0.05)	20	0.1 g	Visible light (420 nm, 450 W high-pressure mercury lamp, 45 cm distance)	280 min	91	[49]
Ca-doped BFO	10	0.1 g	Visible light	60	98	[50]
Ag-BFO	50	0.04 g	Visible light (LED)	120	95	This work

Conclusion

This work establishes an eco-friendly approach for the synthesis of silver-doped bismuth ferrite (Ag-BFO) nanoparticles using Rosmarinus officinalis leaf extract as a green reducing and stabilizing agent. The incorporation of silver markedly improved the photocatalytic activity of BFO nanoparticles, achieving up to 95% degradation of methylene blue under visible light within 120 min at room temperature (30 °C) and neutral pH (6.5), compared to 75% degradation by undoped BFO. XRD and surface area analysis confirmed the formation of crystalline cubic BFO with a porous, heterogeneous morphology upon Ag doping, which increased active surface sites and improved light absorption. Scavenger and pH studies confirmed that hydroxyl radicals (•OH) were the primary species responsible for the dye degradation. The photocatalytic degradation followed first-order kinetics, with increased reaction rates observed at optimal temperature and catalyst doses. Overall, this work validates the effectiveness of green synthesis routes using rosemary extract to produce Ag-BFO nanoparticles with superior catalytic efficiency and stability, positioning them as promising materials for sustainable water remediation applications. Future studies could focus on optimizing the synthesis and exploring the application of Ag-BFO nanoparticles for the degradation of a broader range of organic pollutants under real wastewater conditions.

Author contributions

NZ: Conceptualization, experiments, writing. MS: editing and Supervision. AA: analysis, editing, methodology, Data curation. NZ, AA, MS, editing and review, JB, NB, editing, review and funding HMA, MEAZ, ERS.

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

The author(s) used ChatGPT to improve the clarity, coherence, and correctness of the English grammar. After using this tool, the author (s) reviewed and edited the content as needed and take full responsibility for the content of the publication.

Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.