X-Ray Diffraction studies
The XRD analysis was used to certify the crystalline structure and phase composition of the as synthesized nanoparticles. The diffraction patterns are highly sharp this indicates that prepared sample shows high crystalline property and all the peaks were matched with the standard JCPDS card No.14–688. X-ray diffraction data recorded in the 2θ angle range of 15°–65°. Figure-3 shows the P-XRD patterns of BiVO4 samples. All diffraction peaks were indexed to monoclinic scheelite crystal structure with lattice parameter a = 5.1950Ǻ, b = 11.7010Ǻ, c = 5.0920Ǻ and α = 90°, β = 90.38° and γ = 90° belongs to 2/m point group. The diffraction peaks observed in the below figure is (110), (121), (040), (200), (002), (211), (150), (060), (240), (042), (202), (161) and (123) planes corresponding to the diffraction angle values at 19.06°, 28.86°, 30.56°, 34.58°, 35.4°, 40.1°, 42.64°, 46.84°, 47.4°, 50.36°, 53.3°, 58.5°, 59.7° respectively. No other peaks were identified this revealed that prepared bismuth vanadate sample was highly pure and strongly correlated with that of BiVO4 crystalline phase. Hence all the XRD patterns confirm the existence of pure BiVO4 peaks. The average crystallite size of the synthesized BiVO4 photo catalyst was calculated by using FWHM and 2 theta values corresponding to the most intense reflection plane (040) using the below Debye Scherrer’s equation.
\(\:\varvec{D}=\:\frac{k\lambda\:}{\beta\:\text{cos}\theta\:}\)------------ (2)
Where, D: Crystallite size of the samples, λ: X-ray wavelength, β: FWHM (Full width half maxima) and θ: Diffraction angle. Therefore, the average particle size of the synthesized BiVO4 nanoparticles is found to be 34 nm.
Morphological Studies
Surface morphology of synthesized BiVO4 nanoparticles was studied using a scanning electron microscope (SEM). The obtained image is given in the Fig. 4. The particles are in an range from 80 to 150 nm.
An Energy Dispersive X-ray Spectroscopy (EDS) was used to confirm the elemental composition. The results obtained from the EDX spectrometer indicated the presence of elemental Bismuth, Vanadium, Oxygen, in synthesized nano particles. The vertical axis shows the X-ray counts and the horizontal axis have energy displaced in keV (Fig. 5). The EDX spectrum of BiVO4 exhibits peak at main emission energies of Bismuth, Vanadium and Oxygen these findings confirm that the reactant employed in the nanoparticle synthesis was fully utilized and also indicates the purity of the material.
Photocatalytic performance of BiVO
The photocatalytic activity of the synthesized bismuth vanadate nanoparticles is evaluated by using methylene blue aqueous solution at room temperature using 300W/230V AC tungsten lamp. Generally, the photocatalytic activity involves four stems, firstly electrons and holes generated by the irradiation of light, secondly charge carriers separation and migration to the surface of the semiconductor, thirdly generated electrons and holes involves in the oxidation/reduction reactions to generate active species and intermediate products. Finally, recombination of charge carriers on the photocatalyst surface. All these steps are complementary and efficiency of the photocatalyst mainly depends on the thermodynamics and also kinetics. Further, the photocatalytic activity of the nanoparticles on dye depends on many factors such as size distribution, particle size, crystallinity, phase composition, morphology, the surface area of the catalyst and bandgap. In the typical procedure, the two mL of sample was withdrawn from the solution at a regular interval of 20 minutes, then the sample was centrifuged for the removal of the photocatalyst. The absorbance was recorded using UV-visible spectrophotometer at 665 nm (λmax).
Bismuth vanadate is oxygen deficient in nature and the oxygen present in the aqueous solution readily generate oxygen species like\(\:{\:\text{O}}^{-}\), \(\:{\text{O}}^{2-}\), \(\:{\text{O}}_{2}^{2-}\) by utilizing electrons present on the surface of the catalyst. These electrons are produced from the irradiation of visible light. These reactive oxygen species generates \(\:{\text{H}\text{O}}^{.}\:\) the holes present on the valance band react with water or hydroxyl group also generates \(\:{\text{H}\text{O}}^{.}\) react with methylene blue dye and degradation takes place. The catalytic activity of bismuth vanadate nanoparticles were investigated by taking 10.0 to 5..0 mg of nanoparticles with 5.0 ppm constant dye. The highest catalytic activity 98% was shown by 30 mg catalyst within 120 minutes and represented in the Figure-6. The 40.0 mg and 50.0 mg shown less activity due to scattering of light by increase in catalyst load, in turn decrease in generation of active species (charge carriers). All the three materials (BV4-4 ml fuel, BV6-6 ml fuel, BV8-8 ml fuel) are subjected for photocatalytic activity. After 120 minutes of visible light irradiation the removal efficiency of BV6, BV4 and BV8 were 98.8%, 95.1% and 81.6% respectively. The value corresponds to BV6 was significantly better than BV8 and BV4. These results confirms that the BV6 nanoparticles degrade methylene blue dye more efficiently than BV4 and BV8. Based on the above results proposed a possible photocatalytic mechanism to explain the photocatalytic activity of BiVO4 nanoparticles.
BiVO4 + visible light (hv) → BiVO4 [\(\:{h}_{VB}^{+}\)+\(\:{e}_{CB}^{-}\)]
BiVO4 (\(\:{e}_{CB}^{-}\)) + O2 → \(\:{\text{O}}_{2}^{.-}\) + BiVO4
BiVO4 (\(\:{h}_{VB}^{+}\)) + H2O → BiVO4 + \(\:{\text{H}\text{O}}^{.}\) +\(\:{\text{H}}^{+}\)
Methylene blue + \(\:{\text{H}\text{O}}^{.}\) → Oxidized product
Methylene blue + \(\:{\text{O}}_{2}^{.-}\) → Reduced product
Hence, the net reaction is:
Methylene blue + + → Degraded products
The generation and transfer of visible light driven charge carriers (electron-hole) are shown in the figure. Conduction band (ECB) and valence band (EVB) potential of the BiVO4 samples were predicted theoretically with the help of following formula.
ECB = χ (Aa Bb Cc)(1/a+b+c) - E0 − 0.5Eg
$$\:{\varvec{E}}_{\varvec{V}\varvec{B}}={E}_{CB}+\:{E}_{g}$$
Where χ (Aa Bb Cc) stands the geometric mean of the absolute electronegativity of the constituent atoms and a, b and c are the number of atoms present in the compound. Here the absolute electronegativity is the arithmetic mean of the atomic electron affinity and the first ionization energy (If the values are in kJ divided by 96.48 kJ/mol in order to get χ in terms of eV). E0 is the energy of free electrons on the hydrogen scale (E0 = 4.5eV) and Eg is the band gap energy of the BiVO4 sample. The band gap, determined using Tauc plot as shown in the figure-11, is found to be 2.4 eV.
Effect of MB dye concentration
Figure 7 shows the effect of methylene blue dye concentration on the catalytic activity of BVMW2 nanoparticles. Different concentrations, such as 5, 10, 15, and 20 ppm of MB dye in 100 mL of solution, were used along with 40 mg of catalytic load at pH 7. As illumination with visible light increases, the degradation efficiency also increases. By observing this experiment, it was noticed that the photo degradation efficiency of MB dye is inversely proportional to its concentration [12]. Hence, the lower the dye concentration, the higher will be the degradation efficiency. If the dye concentration increases, the production of reactive species becomes lesser. Therefore, the attack of holes and peroxide radicals decreases, leading to a decrease in degradation efficiency.
Variation of photocatalysts load
The degradation percentage versus irradiation time for different catalytic loads under visible light is shown in Figure-8. The optimal catalyst load was determined by conducting various experiments. This experiment was carried out by changing the photocatalyst load from 10 mg to 40 mg/100 mL of MB dye solution (5 ppm) while maintaining a neutral pH. From the figure, it can be concluded that the degradation of methylene blue dye proceeds to completion at around 180 min for all nanoparticle concentrations. Gradually, the degradation of methylene blue increases correspondingly with an increase in BiVO4 nanoparticles under the illumination of visible light [13]. It was observed that 60 mg is the optimal catalyst load because, as the catalyst load increases, the surface area of the prepared photocatalyst increases. Hence, there will be a larger surface area exposed to radiation, and therefore the degradation efficiency also increases. After the optimal catalyst load, the NPs aggregated, and finally, the degradation percentage decreases [14].
Effect of pH
The variation of pH is a crucial parameter in this work as the degradation efficiency was found to be different for different pH values. The experiments were conducted using solutions with different pH values covering all the pH states, i.e., acidic, basic, and neutral while keeping other parameters constant. From the data obtained in Figure-9, it was observed that the maximum degradation of MB dye occurred at a neutral pH medium (pH = 7–8) since the change in pH altered the surface adsorbent efficacy [15]. In acidic conditions (pH = 1–6), an electrostatic force of attraction was formed between the protonated dye and the surface of BVMW2 nanoparticles. On the other hand, in basic mediums (pH = 9–14), electrostatic repulsion started between OH− and the BiVO4 nanoparticles surface. Therefore, in both acidic and basic mediums, the degradation percentage was low. However, in neutral pH conditions (pH = 7–8), no attraction or repulsion forces existed, and maximum removal was observed due to Vander Waal’s forces.
Recyclability of BiVO 4 NPs.
By maintaining 40mg NBs and 5 ppm dye at pH-7, the recyclable potential of BiVO4 nanoparticles was investigated. The subjected NPs were centrifuged once the first cycle was completed. The particles were meticulously cleaned and dried 2–3 times. These retrieved NPs were then subjected to identical degrading experimental conditions again, and the degradation effectiveness was measured. A graph was created to show the number of cycles versus the degrading efficacy of BiVO4 nanoparticles. Figure 10 depicts the photo stability of the synthesized BiVO4 nanoparticles. This figure demonstrates that the decolorization of MB dye remained nearly constant even after the fourth cycle of degradation. As a result, it was determined that BiVO4 NPs have good catalytic recycling stability.
Cyclic voltametric studies
Initially, cyclic voltammetry (CV) studies were conducted to identify the redox properties and reaction mechanisms of paracetamol. CVs were recorded at the bare glassy carbon electron (GCE) in the absence of paracetamol, within the potential window of − 0.2 to 1 V vs. Ag/AgCl, with a scan rate of 0.05 V/s. The resulting CV of the bare GCE is shown in curve (a) of Fig. 11A. No oxidation peak or reduction peaks were observed at the bare GCE in the absence of paracetamol, and the material 6 mL-BiVO4 also exhibited no redox processes within this potential window. Under the same conditions, CV was recorded at the surface of the GCE in the presence of 5 mM paracetamol. The bare GCE was activated in the presence of paracetamol, showing a sensitive detection at paracetamol, showing a sensitive detection at this 5 mM concentration, with an anodic peak corresponding to the oxidation of paracetamol at 0.36 V and a cathodic peak corresponding to the reduction of paracetamol peak at 0.037 V.
The anodic current response for the detection of paracetamol at the bare GCE is 1.07 × 10–5 A. The modified GCE with 6 mL-BiVO4 NPs deposited, was then examined for paracetamol detection. The CV for this modified electrode is represented by curve (c), as shown in Fig. 11A. The highest current response, compared to the bare GCE, was observed at 0.55 V with a current of 5.23 × 10–5 A. Reversible electrochemical behaviour was observed at both the bare GCE and 6 mL-BiVO4/GCE. The cathodic scan showed a peak at − 0.18 V with a negative current of 2.394 × 10–5 A, indicating improved catalytic activity and sensitivity of the modified 6 mL-BiVO4. This enhancement is attributed to the active material and the high surface area of the modified 6 mL- BiVO4/GCE. Mamatha et al. reported oxidation and reduction potentials of + 0.68 V and − 0.34 V, respectively, for paracetamol in CV studies at the surface of a prepared CuO-graphite electrode [16]. Our results are consistent with the electrochemical investigation of paracetamol conducted by Vinay et al., using Fe₂O₃ nanoparticles modified carbon paste electrodes [17].
A comparison of the anodic peak currents obtained at the bare GCE and the modified GCE is shown in Fig. 11B. It is evident that the bare GCE shows a current 8.17 times higher than that of the CV obtained in the absence of paracetamol. In contrast, the modified 6 mL-BiVO4 displays a current 4.74 times higher than the current response of the bare GCE in the presence of paracetamol. This significant increase in current response indicates that the 6 mL-BiVO4 electrodes greatly enhance the electrocatalytic activity towards the oxidation of paracetamol, leading to improved sensitivity and detection limits. The 6 mL-BiVO4 electrode effectively facilitates electron transfer and lowers overpotential, making it a superior choice for sensitive electrochemical sensing of paracetamol compared to the bare GCE [18].
Linear sweep voltametric studies
LSV studies conducted with 6 mL-BiVO4, 4 mL-BiVO4, and 8 mL-BiVO4 modified GCEs demonstrated that the 6 mL-BiVO4 electrode achieved the highest current response at lower negative potentials compared to the 4 mL and 8 mL BiVO4 electrodes, as shown in Fig. 2a. At a constant scan rate of 50 mV/s and a fixed concentration of 5 mM paracetamol, the 6 mL-BiVO4 electrode showed superior electrochemical performance, with high current values and lower peak potential of 0.47 V. In contrast, the 4 mL and 8 mL BiVO4 electrodes exhibited lower currents and higher oxidation potentials, with peaks occurring at 0.5 V and 0.54 V, respectively (Fig. 2b). This indicates that the 6 mL-BiVO4 electrode not only enhances sensitivity but also improves the efficiency of paracetamol detection. The lower peak potential of the 6 mL-BiVO4 electrode suggests better electrochemical kinetics, making it particularly advantageous for sensitive and precise detection. Further studies using LSV at various concentrations can build on these findings to explore the detailed electrochemical behavior and optimize the sensor's performance.
Scan rate and linearity studies for the detection of Paracetamol
Electrochemical activity at the surface of 6 mL-BiVO4/GCE
Further electrochemical CV studies were conducted using 6 mL-BiVO4/GCE, with different scan rates ranging from 0.025-0.4 V/s performed to detect paracetamol within the potential window of -0.2 to 1.0 V. The corresponding CVs are shown in Fig. 3a. The data show an increase in the oxidation peaks, indicated by higher current responses as the scan rates increase, due to enhanced electrochemical activity for the detection of paracetamol and attributed to the greater rate of electron transfer at the electrode surface and more effective accumulation of the analyte, which results in a higher peak current. The relationship between the log of the current response (I) and the log of the scan rates (ν) is represented in Fig. 3b. A relationship between the log (I) and the log (ν) was expressed by the linear equation: log(I) = 0.271 log (ν) − 3.909, Pearson correlation coefficient (r) = 0.996.
Figure 3a) CVs at the surface of 6 mL-BiVO4/GCE at different scan rates (0.025–0.4 V/s) b) Calibration plot of the log of the current response for the oxidation of paracetamol at different scan rates vs. the log of scan rates (ν)
Additionally, the oxidation peak potential shifted slightly to the rightwards with increasing scan rates, due to the increased kinetic energy required to overcome the activation barrier for oxidation as the scan rate increased.The slope of 0.27, which is certainly closer to 0.5, suggests that the process is more consistent with a diffusion-controlled mechanism than an adsorption-controlled one [19, 20]. Factors such as experimental conditions, electrode surface properties, and the nature of the redox reactions could influence this deviation.
In the linearity studies conducted using 6 mL-BiVO4/GCE, paracetamol concentrations were varied from 4.05 mM to 5 mM, and linear sweep voltammetric (LSV) measurements were performed within the potential window of -0.2 to 1.0 V, and the scan rate is fixed to 0.05 V/s (Fig. 4a). As the concentration of paracetamol increased, the anodic peak currents corresponding to its oxidation were observed to increase (Fig. 4b). This pattern of behavior could be attributed to the effects of paracetamol adsorption, which intensify as the paracetamol concentration rises. Additionally, the constant slope suggests that uniform adsorbate coverage occurs even at higher paracetamol concentrations [21]. This shift to a higher potential is likely due to the enhanced interaction between paracetamol molecules and the electrode surface, which alters the electrochemical environment and requires a higher potential for oxidation [22].
The relationship between the oxidation current and concentration of paracetamol was quantified using the linear equation: I (A) = 0.0708 C (M) + 5.19 × 10− 6, r = 0.9929.
The limit of detection (LOD) of the 6 mL-BiVO4 sensor for paracetamol was determined according to the 3Sb/m formula [22] and is found to be 5.77 × 10− 4 M, indicating good linearity and sensitivity in the tested range and the lowest concentration that can be reliably detected by this method.
The concentration variation studies conducted with 6 mL-BiVO4, 4 mL-BiVO4, and 8 mL-BiVO4 modified GCEs reveal that the 6 mL-BiVO4 electrode exhibits the lowest detection limit of 5.77 × 10− 4 M for paracetamol, outperforming the 4 mL-BiVO4 (6.14 × 10− 4 M) and 8 mL-BiVO4 (6.42 × 10− 4 M) electrodes. This superior performance is attributed to the enhanced electrochemical properties and higher current intensity observed with the 6 mL-BiVO4 electrodes. The 6 mL-BiVO4 electrodes also demonstrate better linearity and sensitivity, as indicated by its improved slope in the calibration plot. This high sensitivity and lower detection limit make the 6 mL-BiVO4 electrodes particularly useful for applications requiring precise and reliable detection of low concentrations of paracetamol. These favorable potential characteristics of the 6 mL-BiVO4 electrodes make it a promising candidate for advanced analytical applications and further development in paracetamol sensing.