1 Introduction

For the survival of all kinds of animals and living organisms, clean water is an important factor. Our earth consists of 70% water but a minimum amount of 2.5% water is used for agriculture, drinking, industrial and domestic utilization. From the twenty-first century, water pollution, environmental pollution and global warming spread diseases, change our climate and create many problems for the survival of all kinds of life. Nowadays, environmental safety and remediation become major challenges for all mankind. Many industrial sectors like dyeing, printing, plastic, paper, cosmetics, ink, textiles ceramics, leather sectors and food processing release huge amounts of sewage and toxicity materials (Lotfi, 2022; You et al., 2019). According to survey, these industries release one thousand tons of waste material which consists of non-biodegradable dye stuff. Hence, it is very crucial to clean water for the protection of the environment and life. To overcome this problem, semiconductor photocatalysis has been observed eco-friendly because it has many properties such as being cost-effective, sustainable and non-toxic with greater photostability (Cheng et al., 2020; Kaur et al., 2020; Malathi et al., 2018).

These days, researchers have focused their research on two ways. The first way is by preparing a photocatalyst that will be able to utilize visible light which covers more than 50% of the solar spectrum. The second way is by improving the productivity of the photocatalytic reaction through adjusting synthesis conditions, using structure directing agent or producing new synthesizing methods (Deebasree et al., 2020). Nowadays, more than 150 semiconductor materials are developed for the cleanliness of water and environmental pollution (Kaur et al., 2020). Beyond these prepared semiconductors titanium dioxide, zinc oxide, bismuth, and bismuth vanadate based semiconductors (Bhowmick, et al., 2019; Kumar et al., 2018; Luo et al., 2022a) are considered as the best photocatalysts due to their different properties and multifunctional applications such as ionic conductivity, ferroelasticity, acousto optical photocatalytic activity, less toxicity, strong oxidizing power, greater stability, low cost, narrow band gap, gas sensor devices, lead-free paints, solid state electrolytes and lithium ion batteries, etc. (Aghakhaninejad et al., 2018; Ghotekar et al., 2020; Helal, et al., 2020; Jia et al., 2012; Kása et al., 2020; Luo et al., 2022b; Wu et al., 2018; Zhang et al., 2021; Zhong et al., 2023). TiO2 is considered as a useful material for many photocatalytic applications. The main drawback of TiO2 is its large bandgap, i.e. 3.2 eV, which needs ultraviolet light for photocatalytic activation (Chang & Liu, 2011; Zhang et al., 2014). The excellent properties of BiVO4 are resistance to corrosion, best dispersibility, non-toxicity, and long-term photostability (Choi et al., 2013; Zhang & Zhang, 2009).

The n-type semiconductor bismuth vanadate has good crystallinity and morphology. There are three types of polymorphs such as monoclinic scheelite, tetragonal scheelite and tetragonal zircon. Tetragonal scheelite is a natural polymorph also called pucherite and it has orthorhombic crystal structure. The dreyerite polymorph has tetragonal scheelite structure and the clinobisvanite has monoclinic scheelite structure. The tetragonal scheelite was not prepared by simple laboratory method but the monoclinic scheelite and tetragonal zircon can be prepared. At higher temperatures, tetragonal zircon can be changed into monoclinic scheelite. The reversible transition in BiVO4 tetragonal scheelite to BiVO4 monoclinic scheelite can be found at temperature 528 K (Trinh et al., 2019). The structure of monoclinic scheelite is considered a strong reactive phase due to its bandgap 2.4–2.5 eV (Chen et al., 2020; Helal, et al., 2020; Hemavibool et al., 2022; Hunge et al., 2021). In monoclinic scheelite, the transition in valance band is made by Bi6 or due to the hybrid orbital of O2p and Bi6 to a conduction band of V3d (Jiang et al., 2012, 2014; Li et al., 2016). Thus, in the monoclinic phase, smaller band gaps have appropriate band positions of conduction and valance band. These bands are almost near to H2O reduction and oxidation potential (Rani et al., 2019; Tayebi & Lee, 2019; Wu et al., 2021). Hence, the band gap and band position help BiVO4 crystal to absorb maximum visible light. Due to these characteristic, BiVO4 shows a stronger visible light–driven photocatalytic properties in case of water splitting, removal of organic pollutants, and photocatalytic O2 evolution (Kshetri et al., 2020; Senasu et al., 2021). The main disadvantage found in BiVO4 photocatalysts is fast recombination of photogenerated charge carriers in conduction band and valance band. To overcome this problem, scientists prepare different heterojunction photocatalysts (Nguyen, 2020).

Researchers use a variety of structure directing agents also called surfactants for preparing different kinds of semiconductors (Monfort & Plesch, 2018). The main advantage of adding surfactants in precursor solution is that, it will produce many changes in materials and played an important role for improving band gap, crystal structure, morphology, electron hole pair recombination, electron mobility, photocatalytic activity and develop a stable environment for growth kinetics of nanomaterials in different synthesis processes (Rani et al., 2018). Hence, the benefits of surfactants in synthesis process of nanostructure materials show the greater surface area, change in morphology, thin pore size, porosity, crystal phase formation and high crystallinity etc. (Afonso et al., 2014).

Furthermore, Yidan Luo et al. synthesized NiAl-LDH/biochar nanocomposites using the coprecipitation method. It was observed that, S-scheme heterojunction was found between NiAl-LDH and biochar. This junction enhanced the separation of photogenerated electrons and holes. The photogenerated electrons and holes increased photocatalytic activity. Due to greater photocatalytic activity, NiAl-LDH/biochar was considered the best photocatalyst and removes antibiotics from water (Luo et al., 2022b).

Monoclinic bismuth vanadate powder was synthesized through the hydrothermal method by Rong Ran et al. He observed that a bismuth vanadate catalyst shows better photocatalytic activity for the degradation of RhB dye i.e. 88% in 120 mint. This dye shows greater removal capacity (Ran et al., 2015). Yidan Luo et al. prepared bismuth oxychloride/biochar (BiOCl/BC) nanocomposites with abundant oxygen vacancies through the facile ball milling technique. These composites have superior adsorption for the removal of reactive red-120 dye in aqueous solution. It was observed that a strong interaction is found between BiOCl and BC after ball milling (Luo et al., 2022a).

Novel composites of the BiVO4/Bi2O3 heterostructure were fabricated through the hydrothermal technique. These composites show superior photocatalytic activity for RhB dye and TC under visible light irradiation. It was observed that 5% BiVO4/Bi2O3 catalyst have excellent photocatalytic activity as compared to other materials. The RhB dye degradation rate was almost 79%, while TC degrades 74%. This enhanced photocatalytic activity was due to p-n heterojunction (Liaqat et al., 2022).

Ch. Venkata Reddy et al. designed novel g-C3N4/BiVO4 heterostructured nanohybrids through the solvothermal technique. The photocatalytic activity of g-C3N4/BiVO4 was examined through MB and TC i.e. 88% and 89%. The enhanced catalytic performance was due to heterojunction between g-C3N4 and BiVO4. It was observed that these prepared composites of g-C3N4/BiVO4 are also suitable for environmental remediation applications (Reddy, et al., 2023).

Here, in this research work we have explained photocatalytic activity and morphology of BiVO4 nanostructures using surfactants through the hydrothermal route. To the best of our knowledge, this is the first report with respect to synthesis of monoclinic BiVO4/NH4F. It was also observed that ammonium fluoride (NH4F) is a good surfactant as a morphology controlling agent, enhancing optical properties of the prepared catalyst and photocatalytic activity. The prepared photocatalysts are most suitable for degradation of model organic dyes (MB and RhB) and tetracycline hydrochloride.

2 Experimental Procedures

2.1 Materials for Preparation

Bismuth nitrate (Bi(NO3)3·5H2O), ammonium metavanadate (NH4VO3), ethanol, ammonium fluoride, oxalic acid, CTAB, sodium hydroxide, nitric acid, acetone, methylene blue, Rhodamine B, tetracycline hydrochloride, and deionized water were utilized to fabricate desired materials. All chemicals and reagents were pure and directly used without performing any modifications.

2.2 Synthesis of Pure and Surfactant-Based BiVO4 Material

First of all, we dissolve 5 mmol of NH4VO3 into ethanol and stirred at 50 °C for 30 min. Then 5 mmol of Bi(NO3)3·5H2O was dissolved into ethanol and stirred at 50 °C for 30 min. Then by using burette add NH4VO3 solution dropwise into Bi(NO3)3·5H2O solution, and both mixtures were stirred continuously for 2 h during stirring add a little quantity of surfactants individually (ammonium fluoride, oxalic acid, and CTAB). The pH ~ 8 of this solution was maintained by using 50 vol. % NaOH and HNO3. After that, the whole mixture was poured into a Teflon-lined autoclave for hydrothermal synthesis at 180 °C for 12 h. Then, the obtained precursor was washed through deionized water, acetone and two times with ethanol. Hence, the washed material was put into a drying oven at 80 °C for 12 h. After drying, the whole material was calcined at 550 °C. At the end, the prepared material was collected to characterize and do photocatalytic tests. The pure BiVO4 was also prepared using the same procedure without adding surfactants. The schematic flowchart for preparation of BiVO4 using surfactants is displayed in Fig. 1.

Fig. 1
figure 1

Flowchart for synthesis of BiVO4

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2.3 Characterization of Photocatalysts

The phase structure of as-synthesized nanomaterials were explored by the powder X-ray diffraction machine through D/max 2500 having CuKα source (λ = 0.1541 nm). A scanning electron microscope (SEM; LEO 1530) was used to distinguish structural properties such as morphology and elemental composition of as-synthesized photocatalysts. For examining the UV–Visible absorption spectra of the photocatalysts, the UV-1800 spectrometer by Shimadzu was used. JASCO fluorescence spectrometer with FP-8200 was used to determine the photoluminescence spectra (PL) at a fixed excitation wavelength i.e. 400 nm for all prepared photocatalysts. Moreover, the FTIR spectroscopy measurements were used to investigate the prepared composites.

2.4 Photocatalytic Degradation Study

MB dye, RhB dye and antibiotic TC were used to examine the photocatalytic activity of prepared BiVO4 in a liquid medium through visible light irradiation. A photocatalytic reactor was used in photocatalytic experiments, consisting of tungsten halogen lamps (50 W per lamp) as a source of visible light. For checking photocatalytic activity, 20 mg of as-synthesized material was dissolved in 100 mL of MB solution i.e. 10 mg L−1 in a Pyrex glass vessel having pH of MB solution as almost 10. After that for attaining adsorption or desorption equilibrium on photocatalyst surfaces, the whole mixture was stirred about half an hour. After stirring, the whole solution took place at 15 cm away from the lamp and irradiation was started with light of lamp having wavelength range larger than 420 nm using a filter. After each half hour, 5 mL solution was taken from the reactor vessel. The prepared solution was centrifuged and concentration of dye degradation was measured by the UV–Visible spectrometer UV-1800 by Shimadzu. Hence, the whole procedure was repeated for RhB dye and TC.

3 Results and Discussion

3.1 Structure and Phase Identification

The crystalline phase structures of BiVO4 using surfactants were determined through X-ray diffractometer (D/max 2500) using Cu-kα radiation having wavelength 1.5406 Å. Figure 2a display XRD patterns of as-synthesized pure BiVO4 using the hydrothermal method. The diffraction peaks were observed at 2θ angles 19°, 27.48°, 28.26°, 31.64°, 34.75°, 35.64°, 45.0°, 46.78°, 47.55°, 55.31° and 57.34° corresponding to hkl values of (011), (− 121), (121), (113), (200), (002), (123), (240), (042), (206), (125), (321) and (134) respectively according to JCPDS Nos. 14–0688 and 14–0133. The largest peak at 28.26° shows maximum crystallinity and monoclinic structure with respect to other peaks. Figure 2b shows XRD pattern of BiVO4 using oxalic acid as a surfactant. Diffraction peaks observed in XRD patterns can be indexed as (101), (011), (013), (040), (002), (211), (051), (123), (240), (042), (202), (161), (321) and (134) at 2θ angles 18.84°, 19°, 29.02°, 30.0°, 35.64°, 39.87°, 42.56°, 45°, 46.78°, 47.55°, 50°, 53.0°, 58.54° and 59.88° respectively which show a pure monoclinic scheelite phase according to JCPDS No. 75–2480 and JCPDS No. 14–0688 (Kása et al., 2020; Wang et al., 2014). Figure 2c describe XRD pattern of BiVO4 using ammonium fluoride as a surfactant. The observed diffraction peaks at 2θ angles 11.04°, 19°, 27.48°, 28.26°, 29.02°, 31.64°, 35.64°, 37.12°, 40.94°, 42.56°, 45.0°, 46.78°, 49.28°, 53.0°, 55.31°, 58.54°, 59.88° and 61° corresponding to hkl values of (001), (011), (− 121), (121), (013), (113), (002), (141), (105), (051), (123), (240), (312), (161), (125), (321), (134) and (332) respectively. BiVO4 using surfactant ammonium fluoride shows narrow line widths and higher degree of crystallinity (Dabodiya et al., 2019; Sivakumar et al., 2015). This monoclinic scheelite phase is considered as a more active phase as compared to other phases of BiVO4. Figure 2d observed the XRD pattern of BiVO4 using CTAB as a surfactant. The detected diffraction peaks at 2θ angles 11.04°, 19°, 25.32°, 29.02°, 30.0°, 31.64°, 34.75°, 35.64°, 39.87°, 42.56°, 46.78°, 47.55°, 50°, 53.0°, 57.34°, 58.54° and 59.88° corresponding to hkl values of (001), (011), (111), (013), (040), (113), (200), (002), (211), (051), (240), (042), (202), (161), (206), (321) and (134) respectively. During calcination at high temperatures i.e. 550 °C peak at 35.20° splits into two other diffraction peaks (200) at (34.75°) and (002) at (35.64°). Two diffraction peaks show tetragonal phase of BiVO4 having hkl values (111) and (200) with respect to JCPDS 14–0133 (Brack, et al., 2015; Cheng et al., 2013; Khan, et al., 2020). These observed peaks prove that prepared material is a mixture of monoclinic and tetragonal BiVO4 (Cheng, 2012; Khan et al., 2020; Zhang et al., 2007). Hence by preparing BiVO4 using surfactants, it was concluded that all prepared materials are pure without finding any impurity of other materials’ diffraction peaks for example Bi2O3 and V2O5. Furthermore, it was concluded that XRD peak intensity of monoclinic BiVO4 was stronger as compared to tetragonal BiVO4 and FWHM of monoclinic BiVO4 diffraction peaks are little narrow or decreased and prove that higher crystallinity and maximum BiVO4 crystallites. Table 1 describes the detail of XRD such as crystal size, crystallinity index and nature of all prepared samples.

Fig. 2
figure 2

XRD results of pristine BiVO4 (a), BiVO4/oxalic acid (b), BiVO4/NH4F (c) and BiVO4/CTAB (d)

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Table 1 Detail of XRD
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3.2 SEM and EDX Characterization

The morphologies of BiVO4 samples synthesized through hydrothermal route were characterized by using high-resolution scanning electron microscope (SEM; LEO 1530). Figure 3a–d explain the morphologies of pristine BiVO4 and surfactant-based BiVO4 samples. Prepared samples show that there are major differences in the morphologies and particle shapes of BiVO4 using surfactants. As revealed in Fig. 3a and b the morphology of pure BiVO4 and BiVO4 using oxalic acid is agglomerated. Figure 3c describes the morphology of BiVO4 using CTAB as a surfactant. By using CTAB as a surfactant, the morphology of BiVO4 totally changed and nanoparticles were found. The average length of nanoparticles is 49.49 nm, while the mean diameter range of these nanoparticles is 36.05 nm. Interestedly, it can be seen that when surfactant ammonium fluoride was used nanorods will be as shown in Fig. 3d. The mean length and diameter of nanorods are 81.31 nm and 21.82 nm. These nanorods explain better results as compared to other surfactants (Helal, et al., 2020).

Fig. 3
figure 3

SEM images of pristine BiVO4 (a), BiVO4/oxalic acid (b), BiVO4/CTAB (c), BiVO4/NH4F (d) and EDX spectra of BiVO4 (e)

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For further confirmation of synthesis of BiVO4, the elemental composition of BiVO4 was examined via energy dispersive spectroscopy. Figure 3e illustrates the EDX analysis of BiVO4 and calculated atomic ratios of Bi, O, and V are 2:3:1. Hence, the presence of bismuth, vanadium and oxygen are clearly evidences of the formation of BiVO4 and the absence of any other impurity proves the highly pure nature of BiVO4.

3.3 UV–Visible Spectroscopy Analysis

In semiconductors, the optical absorption property has a greater role in photocatalytic performance. Optical properties of characterized samples were examined through a UV-1800 spectrometer by Shimadzu. The results of UV–Visible spectroscopy are displayed in Fig. 4a and b. The light absorption ranges of all characterized photocatalysts have 300–800 nm. This range is displayed in Fig. 4a and describes that all synthesized samples have good capability to absorb UV–Visible light and also have potential of photocatalysis in visible light. The absorption edge of all prepared samples are in green shift (Zhang et al., 2020). The maximum absorption in visible light region is produced because of the transfer of electrons from Bi 6 s and O 2p orbitals to V 3d orbitals (Ahmed et al., 2018).

Fig. 4
figure 4

UV–Visible absorption spectra of BiVO4 using surfactants (a) and Plot of (αhʋ)2 vs hʋ of BiVO4 using surfactants (b)

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In semiconductors, band gap values can be calculated with the help of Tauc relationship:

$$\alpha h\upsilon =A ({h\upsilon - {E}_{g}) }^\frac{n}{2}$$
(1)

By using Tauc formula, a plot of (αhʋ)2 versus (hʋ) is displayed in Fig. 4b.

The calculated band gaps of pure BiVO4, BiVO4/oxalic acid, BiVO4/CTAB and BiVO4/NH4F were estimated to be 2.41, 2.39, 2.37 and 2.44 at wavelengths 514 nm, 518 nm, 523 nm and 508 nm respectively. These band gap values are related with literature reports of the BiVO4 monoclinic structure (García-Pérez et al., 2012; Lin et al., 2019; Mousavi-Kamazani, 2019; Yu & Kudo, 2006; Zhou et al., 2006, 2007). The better optical properties of BiVO4/NH4F composites in visible light range are the best sign of their photocatalytic performance.

3.4 Photoluminescence Analysis

A photoluminescence spectroscopy is an appropriate method to observe the recombination rate, transfer capacity, trapping mechanism, separation of photogenerated charge carriers and oxygen defects. Lower PL intensity shows reduction in electron hole pair recombination rate and greater photocatalytic activity (Cheng et al., 2020). PL spectra illustrated in Fig. 5 were recorded by JASCO fluorescence spectrometer with FP-8200. The BiVO4 using surfactants was excited by 400 nm and three predominant emission peaks were obtained (Rani et al., 2019). These peaks situated in the blue region have wavelengths at 416 nm, 434 nm, and 453 nm. The decreasing order of PL intensity of prepared BiVO4 using surfactants is BiVO4/NH4F < BiVO4/CTAB < BiVO4/oxalic acid < pure BiVO4. The decrease in PL intensity of prepared samples show greater photocatalytic activity. Here, luminescence spectra produced through the emission of photons are produced due to the recombination of holes in O 2p orbital band and photo excitation of electrons in V 3d orbital band. The PL results relate with energy band diagram which proves low recombination and good photocatalytic activity. Ammonium fluoride shows the best photocatalytic activity because the recombination rate is too slow as compared to other surfactants. Hence, the PL results conclude that the surfactants produce a significant role for improving photoelectron hole pair separation in BiVO4 and enhanced its photocatalytic performance (Liu et al., 2018).

Fig. 5
figure 5

Photoluminescence spectra of BiVO4 using surfactant

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3.5 FTIR Analysis

To examine the chemical structure and bands of BiVO4 using surfactants, the FTIR spectra were used (Benisti et al., 2020). The spectra of BiVO4 using surfactants are illustrated in Fig. 6. In pure BiVO4 absorption band at 700 – 900 cm−1 is due to symmetric and asymmetric stretching vibrations of V – O bond. The peak at value of 698 cm−1 demonstrates the Bi – O bond while at 733 cm−1 stretching peaks relate with vanadium. The absorption band at 741 cm−1 shifted at 737 cm−1, while absorption band at 829 cm−1 also shows symmetric stretching of (VO4)3−. Hence, the peaks at 829 cm−1 and 831 cm−1 are due to variations in particle sizes and metal ion oxygen bond distance during calcination temperature.

Fig. 6
figure 6

FTIR spectra of BiVO4 using surfactant

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3.6 Photocatalytic Activity

The photocatalytic activity of BiVO4 using surfactants was calculated by using methylene blue and RhB model organic dyes in sunlight. The degradation efficiency was calculated with the help of formula i.e.

$$Degradation\;efficiency \left(\%\right)=\frac{{C}_{o}-C}{{C}_{o}} \times 100$$
(2)

Here, Co called initial concentration and C called final concentration of MB and RhB dye. Figure 7a and b demonstrate that no methylene blue and RhB degradation could be found in blank treatment (without catalyst) through visible light illumination. But Fig. 7c and d explain the percentage degradations of MB for prepared pristine BiVO4, BiVO4/oxalic acid, BiVO4/CTAB and BiVO4/NH4F are 42%, 47%, 59% and 77% while for RhB the degradation percentages of pristine BiVO4, BiVO4/oxalic acid, BiVO4/CTAB and BiVO4/NH4F are 48%, 54%, 61% and 69% within 2 h of irradiation time. Table 2 describes the comparison of photocatalytic activity of synthesized BiVO4 photocatalyst with reported literature.

Fig. 7
figure 7

Photocatalytic activity comparison of different photocatalysts for MB dye degradation (a), RhB dye degradation (b), MB dye degradation percentage (c) and RhB dye degradation percentage (d)

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Table 2 Comparison of photocatalytic activity of synthesized BiVO4 photocatalysts with reported literature
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3.7 Kinetic Study of BiVO4 Using Surfactants

The photocatalytic activity against MB and RhB dye can also be determined by using pseudo first-order reaction kinetics. For BiVO4-based photocatalyst, the degradation rate constant, i.e. k, was determined by using formula, i.e. ln (Co/C) = kt. The pseudo first-order reaction kinetics and k values for pristine BiVO4, BiVO4/CTAB, BiVO4/oxalic acid and BiVO4/NH4F in case of MB and RhB are displayed in Fig. 8a–d. These k values were estimated with the help of slopes calculated between ln (Co/C) vs. time. The calculated apparent constant values by using MB dye for prepared pristine BiVO4, BiVO4/oxalic acid, BiVO4/CTAB and BiVO4/NH4F are 0.0038 min−1, 0.0051 min−1, 0.0070 min−1 and 0.0103 min−1 but by using RhB dye values are 0.0036 min−1, 0.0043 min−1, 0.0059 min−1 and 0.0067 min−1. By comparing methylene blue and rhodamine B, it was observed that the kinetics value of BiVO4/NH4F is greater in case of methylene blue as compared to RhB. This may be due to many reasons such as the chemical structure, adsorption characteristics and susceptibility to degradation of both dyes is different.

Fig. 8
figure 8

Plot of ln (CO/C) versus irradiation time for MB dye degradation (a), RhB dye degradation (b), kinetic curves of MB degradation (c) and kinetic curves of RhB degradation (d)

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Moreover, tetracycline antibiotic pollutant was also used to overcome the effect of dye self-sensitization. Figure 9a exhibits photocatalytic ability of BiVO4-based photocatalysts for TC degradation through visible light exposure. There is 50% of TC that could be removed through pristine BiVO4 photocatalyst after irradiation time of 2 h. It was also observed that the BiVO4 using surfactants showed better photocatalytic efficiency and BiVO4/NH4F exhibited outstanding efficiency, i.e. 73% as compared to other synthesized materials. Figure 9b describes photocatalytic degradation percentage of TC. Additionally, the pseudo first-order reaction kinetics and kinetic constant values of pristine BiVO4, BiVO4/oxalic acid, BiVO4/CTAB and BiVO4/NH4F were 0.0046 min−1, 0.0058 min−1, 0.0072 min−1 and 0.0098 min−1 respectively which are displayed in Fig. 9c and d.

Fig. 9
figure 9

Photocatalytic degrading TC solution curve of BiVO4 using surfactants (a), TC degradation percentage (b), pseudo first-order reaction kinetics of BiVO4 using surfactants (c) and kinetic curves of TC degradation (d)

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3.8 Stability Test of BiVO4 Using Surfactants

To calculate the performance at the commercial level of as-prepared BiVO4/NH4F photocatalysts, the stability and reusability tests were done through the same parameters and conditions as for model pollutant MB degradation (described in Sect. 2.4). The obtained results are illustrated in Fig. 10a. These results examine that there was a minor reduction in BiVO4/NH4F after consecutive five cycles. These cycles prove the stability of BiVO4/NH4F as a novel photocatalyst. Furthermore, for more clarification of the stability and reusability of BiVO4/NH4F photocatalysts, XRD characterization was performed before and after the degradation process. The results of XRD characterization are displayed in Fig. 10b. It could be found that no new peaks came out before and after the reaction indicating that BiVO4/NH4F was not changed. These XRD results also explain that the BiVO4/NH4F photocatalysts have good stability, more efficient, economic, and have a large potential in industrial applications.

Fig. 10
figure 10

Reusability test of BiVO4/NH4F photocatalyst for five consecutive cycles (a), XRD pattern of BiVO4/NH4F before and after cycling stability test (b) and active species trapping experiment of BiVO4 using MB dye (c)

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3.9 Effect of Radical Scavenger

A radical trapping experiment was achieved to elucidate the as-involved active species which are responsible for degradation of model organic MB dye. In the radical trapping experiment, the radical scavengers like ethylenediaminetetraacetate, tert-butyl alcohol and 1,4-benzoquinone were used. This experimental method was like the degradation experiment discussed in Sect. 2.4 with addition of 1 mmol of quenchers EDTA, t-BuOH and BQ in the presence of aqueous solution MB. The obtained results are displayed in Fig. 10c. It was observed that due to addition of EDTA, t-BuOH and BQ quenchers the degradation percentage of MB reduces from 77 to 24% (h+), 35% (OH•) and 47% (\({\mathrm{O}}_{2}^{-\bullet }\)) respectively. This decomposition demonstrates that h+, OH• and \({\mathrm{O}}_{2}^{-\bullet }\) are primary reactive species involved for better photocatalytic performance of BiVO4/NH4F photocatalysts.

3.10 Photocatalytic Mechanism of BiVO 4

The transportation of photo-generated charge carriers can be assumed through relative band potentials; these are valence and conduction band of semiconductors. In semiconductors, the conduction and valance band potential can be calculated by using formula:

$${E}_{VB}=X-{E}_{e}+0.5 {E}_{g}$$
(3)
$${E}_{CB}= {E}_{VB}-{E}_{g}$$
(4)

Here, EVB and ECB describe valance band and conduction band potentials of semiconductor. While Eg called “band gap energy of semiconductor.” “Ee” is energy of free electron on hydrogen scale. X describes electronegativity of semiconductor and its value in the case of BiVO4 is 6.04 eV (Chen et al., 2020). The determined values of valance and conduction band potentials for pristine BiVO4, BiVO4/oxalic acid, BiVO4/CTAB, and BiVO4/NH4F are mentioned in Table 3. Figure 11 exposes the mechanism of charge transfer through photocatalytic degradation of MB dye. When light falls on BiVO4, the photogenerated electrons from the valence band of BiVO4 can be easily transferred towards conduction band leaving behind the holes in the valance band. The holes produced in valance band react with water molecules to create hydroxyl radicals that are capable of degrading the methylene blue dye molecules. Simultaneously, photogenerated electrons from the conduction band participated in oxidation reaction to generate superoxide radicals (\({\mathrm{O}}_{2}^{-\bullet }\)). Hence, hydroxyl radicals and superoxide radicals were responsible for degradation of methylene blue dye into non-toxic organic compounds. The detailed mechanism is summarized as:

Table 3 Values of band gap energy, valance band and conduction band of prepared materials
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Fig. 11
figure 11

Photocatalytic reaction mechanism of BiVO4 nanocomposite for enhanced photocatalytic activity

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$${BiVO}_{4}+hv \to {BiVO}_{4} \left( {e}^{-}+ {h}^{+}\right)$$
(5)
$${BiVO}_{4}+ {O}_{2} \to {BiVO}_{4}+ {O}_{2}^{. -}$$
(6)
$${BiVO}_{4}+ {H}_{2}O \to {BiVO}_{4}+ {}{}^{.}OH+ {H}^{+}$$
(7)
$${}^{.}OH+organic\;dyes \to Degradation\;products$$
(8)

Hence, the prepared BiVO4/NH4F materials exhibit greater photocatalytic performance in case of MB due to its monoclinic phase, greater crystallinity, superior morphological and optical properties. Furthermore, it has suppressed recombination rate of photogenerated charge carriers during photocatalytic reactions. Moreover, the use of FTIR spectroscopy in photocatalysis is of greater importance because it describes the qualitative and quantitative molecular insights into the interfacial process occurring in adsorption/desorption and chemical reaction (Atitar, et al., 2015).

4 Conclusion

In summary, we have synthesized bismuth vanadate (BiVO4) nanoparticles using a simple hydrothermal route by adding surfactants. Developed structural, morphological, and optical properties of prepared BiVO4 nanomaterials were examined. These prepared BiVO4 nanostructures had stronger intensity, greater crystallinity and multi-morphological characteristics with superior visible light to induce photocatalytic activity for degradation of MB and RhB dyes. It was also concluded that the BiVO4/NH4F material as compared to other materials show greater structural, morphological, optical properties and suppressed recombination rate which consequently resulted in enhanced photocatalytic activity. In addition, the hydrothermal method enables the production of excess interfacial \({}^{.}OH\) radicals on BiVO4 particles promoting reactions between electrons and adsorbed species on BiVO4 particles. The adsorbed species are O2 and H2O stop the photoexcited electrons and holes from recombining facilitating photocatalytic decomposition. Finally, the modified BiVO4 will be a potential material for photocatalytic activity and elimination of pollutants.