Experimental insights on the synthesis and characteristics of Fe_1−xBi_xVO₄ photocatalysts for efficient environmental and electrical applications

1 Introduction

In the modern world, one of the main causes of water pollution is pollutants from industrial organic dyes. It is known that 1 to 20 % of all dyes produced worldwide are washed out in the dyeing process. These dyes are very dangerous mainly due to their carcinogenic and non-biodegradable properties even at concentrations below 1 ppm (Velusamy et al., 2021). These residual dyes are also a major cause of hepatitis, stomach ulcers, cholera, diarrhea, high blood pressure, and other water-borne illnesses (Dahiya, 2022). In addition to the ideal solutions, effective green methods to remove hazardous dyes are required due to the depleting water supplies and deteriorating water quality (Saravanan et al., 2021). If stability, effectiveness, and adaptability are increased, semiconductor photocatalysis can be applied practically (Song et al., 2020). The development of affordable, environmentally benign, dependable, and visible-light-driven photocatalysts has received a lot of attention (Pirhashemi et al., 2018). Numerous metal composites, nanoclusters, transition metal oxides, sulphides, chalcogenides, metal and non-metal composites, and doped nanostructures were evaluated for this purpose (Al-Naggar et al., 2023). However, its practical uses are limited by the faster recombination and shorter separation of photogenerated electron-hole pairs (Lin et al., 2019; Su et al., 2020).

Photocatalysts should be active in the visible light region in order to benefit from the whole solar spectrum (Abbas et al., 2007). In this era, iron vanadate (FeVO₄) has emerged as one of the most effective metal oxides in photocatalytic water remediation (Sajid et al., 2018, 2020a). There are three structural sites for Fe³⁺ ions in FeVO₄: two at octahedral deformed FeO₆ and one at a rhombohedral bent bipyramid corresponding to FeO₅. To build three-dimensional models, six columns of doubly stable chains of Fe—O are connected with VO₄ tetrahedral. FeVO₄ is composed of four polymorphs: FeVO₄-I, FeVO₄-II, FeVO₄-III, and FeVO₄-IV, with FeVO₄-I being a stable form at normal temperatures and pressures and the others being intermediate metastable forms only appearing at high temperatures and pressures (Chen et al., 2017). However, the high rate of recombination, inefficient quantum yield, poor responsiveness to visible light, and bandgap restrictions hamper its performance compared to expectations. In order to make a metal vanadate combination, the bandgap of FeVO₄ can be tuned by introducing another metal cation (Akhoondi et al., 2021). If the metal cation is chosen correctly, it is possible to generate a favorable heterojunction with prolonged light absorption and carrier lifetime. It has been hypothesized that tuning ability in the bandgap is caused by the existence of two metal cations.

According to the literature, the B site in scheelite ABO₄ may contain some replaced material (Adhikari and Lachgar, 2016). By applying the same concept to the FeVO₄, the 8-coordinated Fe³⁺ with an ionic radius of 0.78 Å is substituted for the same coordinated Bi³⁺ that has an ionic radius of 1.17 Å. According to previously reported research, bismuth (Bi) in its d¹⁰ configuration forms a desired hybridized valence band (VB) when its 6 s orbital combines with the oxygen atoms 2p orbital. It benefits the enrichment of the photocatalytic activity of the bismuth-based semiconductor oxides and aids in the mobility of photogenerated holes in the VB. It is revealed that the Fe_1−xBi_xVO₄ heterostructure might be effective in producing electron-hole pairs efficiently, which may increase the photocatalytic activity of the catalyst (Thakur et al., 2020; Dozzi et al., 2016) In photocatalysis, a semiconductor is used as a catalyst, which is activated when the light of suitable energy (hν ≥ E_g) strikes it. The photochemical reaction mostly occurs on a solid surface, and during the photochemical reaction, electron-hole pairs (ehp) are generated, which enhance the redox reaction by which different organic pollutant compounds are decomposed into carbon dioxide and mineral water (Thakur et al., 2020; Das and Chowdhury, 2021; Singh et al., 2019). The lifetime of ehp, surface area, and surface adsorbent also affect the oxidation–reduction reactions that take place on the semiconductor material's surface (Isaac et al., 2022).

There are several ways to synthesize vanadate, such as the sonochemical approach (Koventhan et al., 2022), solid-state reactions (Moriomoto et al., 2022), sol–gel (Karami et al., 2022); coprecipitation (Ghazkoob et al., 2022), and photo-deposition processes (Chen et al., 2019). However, they have a number of disadvantages, including huge crystal sizes, atypical forms, a reduction in surface area, and crystallinity flaws. Due to its high yield, purity, and regulated particle sizes, hydrothermal synthesis is best suited for overcoming the aforementioned disadvantages. The hydrothermal process can produce more active sites and induce high-surface plasmon resonance (Zhang et al., 2021). Hybrid metal composites efficiently involve charge separation during photocatalytic reactions because of their amazing intrinsic properties (Chen et al., 2022). To explore the photocatalytic capabilities for the destruction of harmful dyes, however, advanced research is essential.

About a decade ago, research on mixed metal vanadates for photocatalytic applications was reported. Bera et al. (1953) reported the successful synthesis of a polycrystalline Fe_1−xBi_xVO₄ photocatalyst by a solid-state reaction method. The degradation of MB dye by photocatalysis on Fe_1−xBi_xVO₄ in the presence of visible light showed that heterostructures had greater and more efficient photocatalytic activity than single-phase photocatalysts. In comparison to other materials, the composite with x = 0.25 of the solid solutions exhibited the highest photocatalytic performance (Bera et al., 1953). However, it is unclear how exactly combined vanadates work to provide better photocatalytic performance or increased catalytic activity. Furthermore, there is no information in the literature on a high-performance mixed vanadate-based photocatalyst.

The present study adopts the hydrothermal method to synthesize mixed vanadate for improving crystallinity, photocatalytic, antimicrobial, and electrical potential applications of Fe_1−xBi_xVO₄. Visible light absorption and catalytic effectiveness are both improved by the presence of Fe and Bi in vanadate: specifically, the Fe_0.25Bi_0.75VO₄ composition exhibits good photocatalytic performance. Microscopic examination reveals that the Fe_0.25Bi_0.75VO₄ is made up of adjoining FeVO₄ and BiVO₄ nanoparticles, producing an advantageous heterojunction for charge separation. This study illustrates the synergistic impact of a (Bi)FeVO₄ vanadate heterojunction and offers suggestions on how to enhance the efficiency of vanadate-based photocatalysis.

2 Experimental

Without further purification, the following materials were used for the synthesis: deionized water (99.99%, PAEC PK), iron nitrate (Fe(NO₃)₃·9H₂O, Sigma Aldrich 99%), ammonium metavanadate (NH₄VO₃, Sigma Aldrich 99%), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, Sigma Aldrich 99%), nitric acid (HNO₃, Sigma Aldrich 99%), and sodium hydroxide (NaOH, Sigma Aldrich 99%). For photodegradation, crystal violet dye (C₂₅H₃₀ClN₃, Sigma Aldrich 99%) was utilized. A hydrothermal process was used to synthesize the Fe_1−xBi_xVO₄ photocatalyst. In order to prepare FeVO₄, a typical quantity of 5 mM of (Fe(NO₃)₃·9H₂O was added in a 250 mL beaker containing 40 mL of deionized water and 5 mL of nitric acid (2.5 M). In a different beaker, 40 mL of distilled water was used to dissolve 5 mM of NH₄VO₃, and a few drops of ammonia solution were then added. Both precursor solutions were combined and added to a flask for 30 min of stirring. After 20 min of sonication, the suspensions were left to agitate for 2 h. Then, this 100 mL Teflon-lined stainless autoclave reactor received the 80 mL suspension. The autoclave reaction temperature was set at 180 °C for around 24 h in order to produce FeVO₄ microstructures. Once the reaction period was optimized, brownish precipitates were filtered out and thoroughly washed with distilled water. The product was then dried in the open air overnight. Then the as-prepared FeVO₄ was employed as a precursor support to construct Fe_1−xBi_xVO₄ photocatalyst, and 50 mL of deionized water along with 200 mg of Bi(NO₃)₃·5H₂O powder were added to a flask that contained FeVO₄ solution. The mixture was kept on vagarious stirring for 3 h to get the absorption–desorption equilibrium, then the mixture solution was shifted to an autoclave of 200 mL capacity. The temperature for the autoclave reaction was maintained at 150 °C for 3 h. The finished product underwent filtering, an absolute washing in distilled water/absolute ethanol, and 100 °C drying. The finished mixture was ground into a fine powder using a mortar and pestle after drying. The finished product undergoes a 3-hour calcination process at 300 °C to increase purity and crystallinity. A series of hybrid catalysts Fe_1−xBi_xVO₄ containing different amounts of Bi (x = 0, 0.25, 0.50, 0.75, and 1.00) were produced by the hydrothermal method. The best conditions for constructing Fe_1−xBi_xVO₄ are revealed by the dosage variation impact of Bi³⁺ on the photocatalytic activity of the photocatalysts. Among these, the Fe_1−xBi_xVO₄ with 0.75% Bi³⁺ demonstrated an exceptional photocatalytic response when compared to other concentration ratios, might be ascribed to greater oxygen vacancies, deep energy level fractures, substantial charge separation, and a decreased rate of electron-hole recombination inside the catalyst. As a result, we proceeded with Fe_0.25Bi_0.75VO₄ for further characterization to save money and time to maintain the optimum manuscript length. Fig. 1 depicts the proposed hydrothermal process scheme for the preparation of Fe_0.25Bi_0.75VO₄.

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Fig. 1

Growth process for the synthesis Fe_1−xBi_xVO₄ photocatalysts.

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3 Results and discussion

The crystalline structure of the samples was evaluated through XRD (see Fig. 2). The XRD pattern of FeVO₄ (see Fig. 2a) exhibits peaks ascribed to the anorthic phase FeVO₄ (JCPD card 71–1592). In an anorthic phase, none of the sides and angles is equal i.e., a ≠ b ≠ c and α ≠ β ≠ γ. Further, by adding Bi³⁺ (see Fig. 2b–e) into the FeVO₄ sample, other diffraction peaks appeared at 23.87°, 33.83°, 35.65°, 39.88°, 44.76°, 52.32°, 55.18°, and 58.37° which correspond to (1 1 1), (2 0 0), (0 2 4), (2 1 1), (2 0 4), (2 2 0), (1 1 6), and (3 1 2) planes respectively, and these appearances matched very well with the BiVO₄ (JCPD card 85-1730). From the XRD results, it is revealed that the as-synthesized photocatalysts have a high degree of crystallinity. The maximum peak intensity in the spectrum relates to (−2 0 1), which is at 2θ = 34.04°. The peak shift is observed, which may be attributed to the presence of Bi³⁺ ions in the lattice. For crystallite sizes, Scherrer's relation was applied, and the obtained values for FeVO₄, BiVO₄ and Fe_1−xBi_xVO₄ are provided in Table 1 (Sajid et al., 2020a). Also, all materials containing (Bi³⁺)FeVO₄ (see Fig. 2) show diffractograms attributed to the pure FeVO₄ (JCPD card 71-1592) and BiVO₄ (JCPD card 85-1730). It is observed that with increasing bismuth concentration in Fe_1−xBi_xVO₄, the lattice strain is reduced; this might be due to the fact that the lattice constant increases as the Bi³⁺ has a larger ionic radius (1.17 Å) as compared to that of the Fe³⁺ (0.78 Å) ions. The lattice strain of Fe_1−xBi_xVO₄ composite was calculated using the William–Hall (WH) method. A gradual color change in the visual appearance of FeVO₄ from brownish-red to yellowish is observed as the Bi³⁺ concentration is increased.

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Fig. 2

PXRD patterns of synthesized FeVO₄, BiVO₄ and Fe_1−xBi_xVO₄ samples.

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Scanning electron microscopy (SEM) images of synthesized Fe_1−xBi_xVO₄ photocatalysts are shown in Fig. 3(a, c & e). SEM images present wire-like FeVO₄ (see Fig. 3a) and flake-like BiVO₄ (see Fig. 3c) morphologies, whereas the heterostructure Fe_0.25Bi_0.75VO₄ consists of mixed morphologies of rod and flake as shown in Fig. 3e. The results of energy dispersive X-ray spectroscopy (EDX) are shown in Fig. 3(b, d & f). The elemental composition of as-synthesized FeVO₄, BiVO₄ and Fe_0.25Bi_0.75VO₄ photocatalysts confirms the purity and presence of iron, bismuth, vanadium, and oxygen within the composites.

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Fig. 3

(a) SEM image of FeVO₄, (b) EDX analysis of FeVO₄, (c) SEM image of BiVO₄, (d) EDX analysis of BiVO₄, (e) SEM image of Fe_0.25Bi_0.75VO₄, and (f) EDX analysis of Fe_0.25Bi_0.75VO₄.

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The spectra produced by FTIR of the heterogeneous composite of FeVO₄, BiVO₄ and Fe_0.25Bi_0.75VO₄ is shown in Fig. 4. FTIR is commonly used in semiconductors to analyze the attached functional group in the composite. The V—O—V deformation band at 512 cm⁻¹ is brought on by the mechanism of Fe—O stretching. The peaks at 741, 837, 915, and 1052 cm⁻¹ reflect the short vanadyl bond stretching V⚌O and V—O—V coupled vibrations, while 474 and 1362 cm⁻¹ reflect Bi—O bending and asymmetric vibration, respectively. The bending vibration of the water molecule that was absorbed is what triggered the absorption at 1631 cm⁻¹. The stretching vibration of the OH associated with the absorbed water causes absorption between 3000 and 3600 cm⁻¹ (Sajid et al., 2018; Bera et al., 1953; Yang et al., 2020; Zhao et al., 2022).

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Fig. 4

FTIR spectra of. (a) FeVO₄, (b) BiVO₄, and (c) Fe_0.25Bi_0.75VO₄.

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To confirm the elemental, as well as chemical composition and surface bond analysis, an X-ray photoelectron spectroscopy (XPS) examination was performed. To validate the existence of necessary components in the sample, an XPS survey scan was carried out. The binding energy peaks of the main elements (Fe, V, O, and Bi elements) are seen in the survey spectrum of the Fe_0.25Bi_0.75VO₄ sample, as shown in Fig. 5. The high-resolution spectra for Bi, O, V, and Fe are shown in Fig. 5(b–f) in order to further examine the Fe_0.25Bi_0.75VO₄ composites. As shown in Fig. 5b, the XPS peaks of the Bi doublets (4f7/2 and 4f5/2) were observed at 159.73 eV and 165.07 eV (Kubendhiran et al., 2023). The predicted arrival of V 2p doublets (V 2p3/2 and V 2p1/2) at 517.10 and 524.63 eV can be seen in Fig. 5c. In the instance of V, the oxidation state of the V cation affects the binding energy of the V 2p3/2 core level. We additionally fitted the V 2p3/2 core peak in our data in light of other publications that showed curve fitting of this peak. Two peaks that appeared with binding energies of 517.10 and 518.10 eV were fitted to de-convolute the V 2p3/2 core peak. With the existence of V⁺⁴ ions in the Fe_0.25Bi_0.75VO₄ heterojunction composite, the peak fitting indicates that the valence state of V⁺⁵ is in bulk (see Fig. 5c) (Khawaja et al., 1989). From Fig. 5d, it is evident that O 1s exhibits asymmetrical behavior and can be fitted with two Gaussian peaks around 530.87 and 529.90 eV, respectively. As V—O and Bi—O have almost the same binding energies, the peak at 529.90 is caused by the lattice constant of Fe—O, V—O, or Bi—O bonding in Fe_1−xBi_xVO₄, while the peak at 530.87 eV is caused by the presence of the OH-group (Zhang et al., 2022). Fig. 5e displays the XPS spectra of the Fe 2P core peaks. To offer more accurate details on the valence state and presence of the Fe ions, Fe 2P peaks were further deconvoluted. For the valence states of Fe³⁺ and Fe²⁺, the de-convolution of Fe 2p3/2 reveals two peaks at 710.24 and 713.83 eV, while Fe 2p1/2 reveals two peaks at 723.87 and 728.16 eV (Karamat et al., 2009). The production of Fe-O-Bi bound, which causes a minor peak shift in these peaks towards greater binding energy and raises oxygen vacancies, was demonstrated to greatly boost photocatalytic activity. This peak shift was identified during the Bi³⁺ doping into FeVO₄ (Sajid et al., 2018; Li et al., 2015, 2019; Zhang et al., 2019). These findings support the elemental composition of the Fe_0.25Bi_0.75VO₄ photocatalyst.

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Fig. 5

High-resolution XPS spectra of Fe_0.25Bi_0.75VO₄ heterojunction composite material. (a) Total survey, (b) Bi 4f, (c) V 2p, (d) O 1s, and (e) Fe 2P.

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The optical characteristics of synthetic photocatalysts were examined using UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS). The optical absorption (red-shift) was extended by the Bi³⁺ concentration to the visible spectrum. The intensity of charges transferred between 450 and 730 nm is increased by the plasmonic action of Bi³⁺. The results of absorbance, photon energy (hʋ), and direct bandgap values are shown in Fig. 6, and the calculated values are summarized in Table 2. Photogenerated electrons between the VB and the conduction band (CB) cause the optical bandgap. The UV–Vis results demonstrate that the bi-metal enhances the charge transfer mechanism through surface plasmon resonance (SPR), which is required to produce reliable visible-light-propelled photocatalysts. The attachment of Bi³⁺ is considered to be superior on the FeVO₄ surface because new energy levels have been developed that serve as effective charge carrier traps between the VB and CB energy states.

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Fig. 6

DRS-Vis spectra (main graphic) and bandgap (inset) of Fe_0.25Bi_0.75VO₄ photocatalyst.

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Because the photocatalytic process involves an adsorption–desorption-liberating mechanism, organic contaminants that are adsorbed on the surface of photocatalysts play a key role in the photo-stimulated reaction. By plotting the nitrogen adsorption–desorption curves linearly, the specific surface areas of Fe_0.25Bi_0.75VO₄ were calculated as nitrogen adsorption–desorption isotherm plots, as shown in Fig. 7. The BET surface area was determined to be 56 m²/g. From the desorption isotherms, the pore size was determined using the Barret-Joyner-Halender (BJH) method (Ajmal et al., 2018). According to the BJH technique, the average pore volume and diameter were 0.071 cm³g⁻¹ and 1.68 nm, respectively, as shown in the inset of Fig. 7. The Fe_0.25Bi_0.75VO₄ seems to have a multichannel structure since it has a large enough surface area and volume. The surface area, volume, and pore diameters of all other samples are given in Table 3. Although the sample with a doping ratio of x = 0 has the highest surface area and pore volume due to microporosity that prevents the reactive from diffusing to the active sites, due to the high area's promotion of large dispersion and the tiny particle size, the metal support interactions increased while the activity decreased. Thus, the structure of Fe_0.25Bi_0.75VO₄ may stimulate gas dispersion, absorption, scattering, and mass transport, which in turn boosts the interaction of light with the photocatalyst's pores and expedites the degradation rate (Sajid et al., 2018). The mesoporous nature of the Fe_0.25Bi_0.75VO₄ was tested by the adsorption isotherm, which showed an H₃-type loop with a type IV (current–voltage) hysteresis curve. Nanopores enable the catalyst to capture more dye molecules on the material's surface, improving the procedure and boosting the photocatalytic reaction (Irfan et al., 2017a,b,c).

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Fig. 7

N₂ adsorption and desorption isotherm on Fe_0.25Bi_0.75VO₄ at 77 K. The inset is the BJH pore volume distribution.

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The fundamental qualities of photocatalysts, such as their light absorption range and charge separation effectiveness, have a significant impact on how quickly they degrade. The performance of photocatalysts as a whole is affected by the recombination of photoinduced liabilities; thus, increasing charge separation and the diffusion of charges to the active centers increase photocatalytic activity (Asahi et al., 2014). Thus, it is important to use the PL technique to study the charge excitation, transfer, and trapping processes. Since the separation of the charges has a substantial impact on the photocatalysis process, a PL investigation of pure FeVO₄, BiVO₄, and Fe_0.25Bi_0.75VO₄ photocatalysts was carried out to investigate the optical features. The PL spectrum exhibits the charge separation recombination rate within the photocatalyst (Bera et al., 1953). When the recombination rate is rapid, the degradation process is slow and the PL intensity level is high; while the low intensity suggests remittent and coordinated recombination, improved pollutant degradation, and higher photocatalyst efficiency are the end effects as a result of efficient charge transfer over the photocatalyst surface (Kumar et al., 2022). The difference between the PL intensities of the pure FeVO₄, BiVO₄, and Fe_0.25Bi_0.75VO₄ can be seen in Fig. 8.

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Fig. 8

Photoluminescence spectra of. (a) FeVO₄, (b) Fe_0.25Bi_0.75VO₄, and (c) BiVO₄.

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4 Crystal violet dye's photocatalytic degradation

The degradation of the CV dye was measured using the photocatalytic response of the powder sample. Fig. 9a displays the dye degradation findings as measured by a spectrophotometer. The plot shows the relationship between absorbance and concentration concerning how long it takes for the dye to degrade. The progressive degradation of the absorbance peak over time was caused by the breakdown of aromatic rings in CV dye. The produced Fe_0.25Bi_0.75VO₄ composite showed good photocatalytic activity for the breakdown of pollutant CV dye in visible light, in contrast to other photocatalysts. Fig. 9b compares the photodegradation efficiency of Fe_0.25Bi_0.75VO₄ with that of other prepared samples. According to observations, in FeVO₄, the increase of Bi³⁺ metal increases degrading efficiency from 45% to 99% due to improved light harvesting. The Schottky barrier at the Fe_1−xBi_xVO₄ interface facilitated charge separation due to the wide visible light response, SPR effect, and elevated electron populations over the semiconductor support such properties in the surface of the material.

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Fig. 9

(a) UV–Vis spectra of CV with Fe_0.25Bi_0.75VO₄ composite, and (b) Photocatalytic efficiencies of Fe_0.25Bi_0.75VO₄ composite with different concentrations of Bi³⁺.

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We describe the kinetics related to the degradation of the CV under visible light for the photocatalyst, where surface-controlled phenomena were observed and the dye concentration was low. The Langmuir Hinshelwood (LH) model is commonly used to fully convey the catalytic process. The mathematical expression concerning this model for the rate of reaction, r, can be written as:

The pH of the solution has also had a great impact on how quickly dye degrades (Boudghene Stambouli et al., 2021). It is obvious that when the pH concentration fluctuates, so do the chemical properties of the catalysts used and the dye that will decay. To change the pH and evaluate the pH impact, the reagents HCl and NaOH were introduced to the reaction mixture solution. The pH range for the photoreaction was chosen from 2 to 13 for 15 mg/L of CV dye and 10 mg of Fe_1−xBi_xVO₄ catalyst. Because of its strong affinity for the surface of the catalyst, CV dye faded slowly in an acidic pH medium. On the other hand, under the alkaline medium (base/high pH), the speed of dye degradation was increased because there were many hydroxide ions present and they were more capable of being transformed into ^•OH radicals. In another sense, stronger cationic dyes increased surface adsorption, enabling Fe_1−xBi_xVO₄ photocatalysts to function effectively in alkaline media. Fig. 10a shows the results of the photodegradation rate of CV on Fe_1−xBi_xVO₄ with an increase in pH values. The CV degradation percentage was observed at pH values of 2, 4, 8, 10, 11, and 13 and measured as 35.43%, 40.78%, 58.92%, 70.34%, 99.91%, and 90.83%, respectively. Degradation at pH 11 was observed to give an optimum result, which could be that at a lower value than pH 11, there are no sufficient OH radicals, while at a higher value than pH 11, charges start recombination within the catalyst and there are not sufficient charges on the surface of the catalyst to sustain the reaction, so the degradation reaction slows down.

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Fig. 10

(a) The effect of dye solution pH values on photocatalytic degradation of CV, (b) the effect of dye concentration on photocatalytic degradation of CV, (c) The effect of catalyst amount on photocatalytic degradation of CV dye, and (d) Cyclicity of the Fe_0.25Bi_0.75VO₄ sample under irradiation of visible light.

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On the effectiveness of degradation, the impact of CV dye's starting concentration was evaluated. After 60 min of photoreaction with 10 mg/L of Fe_0.25Bi_0.75VO₄ photocatalyst at pH 7, the degradation was checked. The results of evaluating various initial CV dye concentrations (0.05–0.25 g/L) are shown in Fig. 10b. Results show that more dye particles were adsorbed onto the catalyst surfaces when the dye concentration was raised. The extra quantities reduce the development of active sites during photoreaction by obstructing light penetration. The efficacy of the photocatalyst decreased due to fewer active sites. Less light harvesting results in fewer active sites at catalysts. Similarly, at low dye concentrations, more photons of incident light are absorbed in catalyst surfaces, which increases the number of photogenerated charges that are responsible for dye degradation. Additionally, because there is the same quantity of catalyst present, the photocatalytically produced ^•OH and ^•O₂^¯ on the photocatalyst surface remain constant. If the concentration of the catalyst has been maintained constant, increasing dye concentrations typically need larger quantities of ^•OH and ^•O₂¯ reactive species to maintain equilibrium.

The impact of photocatalyst dose on CV degradation was studied using 5–40 mg of Fe_1−xBi_xVO₄ photocatalyst for 500 mL of dye solution (15 mg/L) at an ideal pH of 7. Each photoreaction was carried out for 60 min. Because more catalytically active sites are created as the catalyst dosage is raised, the rate of ^•OH and ^•O₂^¯ production is enhanced. Fig. 10c illustrates the results. After 20 mg, the rate of degradation decreased as a result of the excess catalyst obstructing light penetration. Therefore, the best catalyst dose for the reactor was decided to be 20 mg/50 mL.

For practical applications of efficient photocatalysts, chemical stability and recycling potential are essential parameters (Ilyas et al., 2023). The Fe_0.25Bi_0.75VO₄ photocatalyst was centrifuged at 2000 rpm for 10 min in preparation for the subsequent run. The degradation efficiency of Fe_0.25Bi_0.75VO₄ declined from 99% on the first run to 94% on the fifth run. It has been noted that some of the catalyst particles were washed away during the recovery procedures, somewhat decreasing the photocatalytic effectiveness. The outcomes show that the photocatalyst maintains its catalytic efficacy even after five cycles, as shown in Fig. 10d. As a result, the high stability and remarkable reusability reveal that Fe_0.25Bi_0.75VO₄ is an effective photocatalyst for real-world uses.

4.1

4.1 Mechanism of the enhanced photocatalytic degradation

The probable photocatalytic system's scheme according to band structure and scavenger action for separating electrons and holes is depicted in Fig. 11. Because of the contact of the semiconductor with the Bi-metal during the photoreaction, a Schottky barrier is created, preventing the passage of rapidly recombining electrons in the opposite direction. Therefore, when the Fe_1−xBi_xVO₄ is exposed to visible light, the electron-hole pairs are produced (Lakshmi et al., 2020). At the same time, the electrons move to the CB and the holes to the VB of the FeVO₄ surfaces. Bi³⁺ functions as an SPR electrons promoter center for semiconductor support, activating the production of superoxide radical anion that breaks down the CV dye molecules (Passi and Pal, 2021; Kossar et al., 2021). Additionally, the presence of Bi³⁺ enhances the visible light absorption range (Allured et al., 2014). These characteristics lead to a large improvement in the photocatalytic activity when Bi³⁺ metal is present. The overall photocatalytic mechanism over Fe_1−x Bi _xVO₄ can be represented by the following Eqs. (4)–(10).

(4)

Fe_1−xBi_xVO₄ + hν → Fe_1−xBi_xVO₄* (e⁻ + h⁺)

(5)

H₂O + h⁺ → OH^• + H⁺

(6)

h⁺ + OH → OH^•

(7)

O₂ + e⁻ → ^•O₂^¯

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Fig. 11

Schematic illustration of the band structure diagram for photocatalytic degradation mechanism of CV dye over Fe_1−xBi_xVO₄ photocatalyst and the possible charge separation processes.

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These radicals engage with the harmful molecules of the CV dye and break them down into little, harmless pieces based on the following chemical processes:

(8)

^•OH + CV → CO₂ + H₂O + Intermediates

(9)

^•O₂^¯ + CV → CO₂ + H₂O + Intermediates

Hence, the overall degradation can be represented as:

(10)

C₂₅H₃₀ClN₃ + (^•OH, ^•O₂–) → CO₂ + H₂O + NO₃ + Other-Intermediate

To investigate the agents responsible for the photodegradation of CV dye, the effect of adding radical scavengers were analyzed. Fig. 12 displays the outcomes of the hydroxyl radical (^•HO), singlet oxygen (¹O₂), superoxide radical (^•O₂^¯), and photogenerated hole (h⁺) produced during photocatalytic degradation, according to the analyses. Ascorbic acid (^•O₂ radical) was added to the system, and the results revealed that this significantly reduced the rate of CV solution deterioration over Fe_0.25Bi_0.75VO₄. It is evident that di-ammonium oxalate monohydrate (h⁺ scavenger) and t-butanol (a source of the ^•OH radical), these two substances played a little role as the principal reactive species during photocatalytic degradation. Thus, as a reactive species, ^•O₂^¯ plays a significant role in the breakdown of CV dye solution; however, ^•OH, which is a high oxidant agent, also takes part and completes the photocatalytic activity.

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Fig. 12

The effects of adding a kind of scavenger on the CV dye concentration during photodegradation in Fe_0.25Bi_0.75VO₄, where the graphics are displayed as a function of an irradiation interval. The scavengers used in the experiment are AO, TBA, and AA.

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5 Antimicrobial activity of Fe_1−xBi_xVO₄

The antimicrobial activity of Fe_0.25Bi_0.75VO₄ was checked against Escherichia coli ATCC O157, Bacillus subtilis 168 ATCC 6051, and Bacillus licheniformis DW2 CCTCC M2011344, as shown in Fig. 13. These results proved that 1000 µL of the sample gave less OD₆₀₀ value of 0.66, 0.86, and 0.96 against B. subtilis, E. coli, and B. licheniformis, respectively. It is also suggested that Fe_0.25Bi_0.75VO₄ is a potential bacteriostatic nanoparticle against pathogenic microbes. Oxidative stress in bacterial cells is known to be caused by reactive oxygen species (ROS), which ultimately results in bacterial death. In order to increase the photocatalytic activity, customized BiVO₄ can yield more ROS with a high oxidation capacity. This study proved that modified Fe_0.25Bi_0.75VO₄ produces the hydroxyl radical (^•OH), singlet oxygen (¹O₂), and superoxide radical (^•O₂^¯) which attack the intramembrane transporter proteins as well as the respiratory chain in bacterial cells. Sharma and his co-workers demonstrated that introducing more m-BiVO₄ nanoparticles causes a gradual decrease in the cell biomass of E. coli Sharma et al., 2016. According to the findings of this study, m-BiVO₄ completely killed the E. coli after 8 h. To disrupt cellular metabolism and cause bacterial cell death, m-BiVO₄ nanoparticles alter the permeability of bacterial membranes (Boudghene Stambouli et al., 2021). Saleem et al., explained that BiVO₄ has strong antibacterial activity due to the production of ROS (^•HO, ¹O₂, ^•O₂^¯) which helps cause cell disruption, deformation and disorganization Saleem et al., 2020.

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Fig. 13

Antimicrobial activity of composite Fe_0.25Bi_0.75VO₄ nanoparticles against. (a) Bacillus licheniformis, E.coli, Bacillus subitlis, and (b–d) Fe_0.25Bi_0.75VO₄ against Bacillus licheniformis, E.coli, Bacillus subitlis bacterial strains at the various time intervals.

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The possible mechanism for antibacterial activity by Fe_1−xBi_xVO₄ was proposed (see Fig. 14). When Fe_1−xBi_xVO₄ was exposed to light, electrons and protons were produced. The produced electrons combine with oxygen to form ROS, which attacks the bacterial membrane proteins, DNA, and cellular metabolism, as shown in Fig. 14.

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Fig. 14

Schematic diagram of antibacterial mechanism of Fe_1−xBi_xVO₄.

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6 Electrical study

In this section, we discussed the electrical properties, such as dielectric constant, dielectric loss, and AC conductivity, to further comprehend the behavior of the motion of electrons (as electron-hole pairs produce active species that can oxidize the dye molecules) for the Fe_1−xBi_xVO₄ materials. They can be used to monitor photodegradation and efficacy in environmental applications.

6.1

6.1 Dielectric constant

The dielectric constant (εʹ) can be calculated from the capacitance C measured using the relation (Asahi et al., 2014).

(11)

εʹ = Cd/Aε₀

The fluctuation of the dielectric constant versus frequency (20 Hz–1 MHz) for FeVO₄, BiVO₄, and Fe_0.25Bi_0.75VO₄ at room temperature is shown in Fig. 15. According to the graph, as the frequency rises, the value of the observed samples' dielectric constant appears to decrease. Higher values of the dielectric constant are caused by a variety of causes, such as interfacial dislocation pileup, oxygen vacancies, and grain boundary defects (Devi et al., 2013). The dielectric constant was observed to drop at high frequencies before becoming almost constant between 1 KHz and 1 MHz. Because of the depolarization of randomly oriented dipoles, the values of the dielectric constant are larger at low frequencies. The inertia of the dipoles prevents the dipoles in the high-frequency domain from aligning, which causes a lag between the applied AC field and dipole orientation.

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Fig. 15

Dielectric constant vs frequency graphs of FeVO₄, BiVO₄ and Fe_0.25Bi_0.75VO₄.

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6.2

6.2 Dielectric loss

The dielectric loss (ε″) for FeVO₄, BiVO₄ and Fe_0.25Bi_0.75VO₄ at room temperature as a function of frequency (20 Hz–1 MHz) is shown in Fig. 16. Inextricably linked to the tangent loss is the dielectric loss. The amount of energy lost in the dielectric with an applied AC field is shown by the dielectric loss, which decreases as the frequency is raised. Due to the dominance of grain boundaries in the low-frequency domain, a higher value of the dielectric loss was found. However, in the high-frequency domain, the electrons start to shift their direction of motion near the grain boundaries, which reduces the probability that they will be reached and minimizes the dielectric loss. It is observed that the interfacial polarization at the grain boundaries is responsible for the significant increase in the dielectric loss at low frequencies. A high value of resistance was detected due to the grain boundaries acting as an insulating character, resulting in an increased amount of energy required for the exchange of electrons between Bi and Fe ions. As a result, a large dielectric loss value was recorded in the low-frequency domain (Mugutkar et al., 2020).

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Fig. 16

Dielectric loss vs frequency graph of FeVO₄, BiVO₄ and Fe_0.25Bi_0.75VO₄.

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6.3

6.3 AC conductivity

For all samples, AC conductivity (σ_AC) can be determined from (Patil et al., 2012):

(12)

σ_AC = 2πfε₀ε″

Fig. 17 depicts the change in AC conductivity at room temperature as a function of frequency (20 Hz–1 MHz). FeVO₄, BiVO₄, and Fe_0.25Bi_0.75VO₄ all have high-frequency domain conductivity due to the hopping of electrons between Bi and Fe ions toward octahedral sites. The hopping of electrons between Bi and Fe ions increased along with the frequency and polarization of the applied AC field, which in turn improved the AC conductivity. The conducting grains in the Fe_0.25Bi_0.75VO₄ material are separated from one another by thin layers of considerable resistive grain boundaries, according to the Maxwell-Wagner model (Ajmal et al., 2018; Gandhi et al., 2011). The AC conductivity in the low-frequency domain decreases as a result of the grain boundary effect. But the power law clarifies that AC conductivity and frequency are related to one another by following the relation (Kubendhiran et al., 2023; Mangalaraja et al., 2004).

(13)

σ_tot(ω) = σ_DC + Aωⁿ where A is the pre-exponential factor expressed in terms of electrical conductivity units, n is the fractional exponent of a dimensionless factor, and σ_DC denotes DC conductivity. As n gets closer to 0, the electrical conductivity equals DC conductivity and becomes frequency independent. When n ≤ 1, the electrical conductivity depends on frequency (Kumar et al., 2022). Since DC and AC conductivity can be thought by sum of both conductivity across the insulator matrix.

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Fig. 17

Conductivity vs frequency graph of. (a) FeVO₄, (b) BiVO₄, and (c) Fe_0.25Bi_0.75VO₄.

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7 Conclusions

In this study, the hydrothermal method was used to successfully manufacture FeVO_4, Fe_1−xBi_xVO₄ and BiVO₄ photocatalysts. Results from XRD, SEM, EDX, XPS, UV–Vis-DRS, FTIR, BET and PL were used to corroborate the synthesis of polycrystalline Fe_1−xBi_xVO₄ for structural morphologies and interfacial properties. The findings demonstrate better crystallinity and confirm the morphologies. It was observed that the lattice parameters, density, and grain size of Fe_1−xBi_xVO₄ is enhanced with the addition of Bi³⁺ concentration (x). The XPS study validated the existence of necessary components (Fe, V, O, and Bi elements) in the sample. Results from UV–Vis-DRS supported the prolonged absorbance in the visible light spectrum, demonstrating that Fe_1−xBi_xVO₄ are more efficient than FeVO₄ and BiVO₄. According to the findings, Bi³⁺ metal increases Fe_1−xBi_xVO₄′s to degrade from 45% to 99% efficiently in just 60 min. These enhanced photocatalytic activities explained the improvement of visible-light harvesting, SPR effect, and charge separation as well as the change in the Schottky barrier at the Fe_1−xBi_xVO₄ interface. Experiments on free radical entrapment are carried out, and the function of active chemical species is examined. According to this research, the major active species in the photoreaction responsible for dye degradation are the energetic ^•O₂^¯ radicals. Additionally, the effectiveness of dye degradation has been assessed and improved using a number of variables. At pH 10, 20 mg/500 mL of catalyst, higher CV-degradation activities have been observed. When this composite Fe_0.25Bi_0.75VO₄ was tested for antibacterial activity in LB against pathogenic microorganisms (E. coli, B. licheniformis, and B. subtilis), it showed improved antibacterial activity compared to that of pure FeVO₄ and BiVO₄.

CRediT authorship contribution statement

Muhammad Munir Sajid: Methodology, Material Experiments, Writing-Original draft preparation, Material Characterization, photocatalysis data Analysis, Investigation, Validation. Haifa Zhai: Material Characterization, Reviewing, Editing and Sources. Muhammad Aamir Iqbal: Reviewing, Editing, Proofreading, and Funding. Naveed Akhtar Shad: Reviewing, and Editing. Yasir Javed: Reviewing, and Editing. Ali Raza Ishaq: Experimental part of antimicrobial activity, antimicrobial data Analysis and Writing-Original draft, Validation. Baraa Abd Alreda: Reviewing, and Proofreading. Kareem Morsy: Funding, Validation, and Proofreading. Jeong Ryeol Choi: Reviewing, Editing, Proofreading, and Funding.