# Polycrystalline BiVO4 visible-light photocatalytic activity: Rietveld refinement, microstructural, vibrational, and photocatalytic properties

Safia Lotfi, Mohamed El Ouardi, Madjid Arab, Abderrazzak Assani, Amal Baqais, Henrik Haspel, Zoltán Kónya, Hassan Ait Ahsaine

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Safia Lotfi, Mohamed El Ouardi, Madjid Arab, Abderrazzak Assani, Amal Baqais, et al., Polycrystalline BiVO4 visible-light photocatalytic activity: Rietveld refinement, microstructural, vibrational, and photocatalytic properties. OXFORD OPEN MATERIALS SCIENCE, 2025, 5 (1), 10.1093/oxf-mat/itaf005. hal-05141648

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Polycrystalline BiVO₄ visible-light photocatalytic activity:
Rietveld refinement, microstructural, vibrational, and
photocatalytic properties

<table><tr><td>Journal:</td><td>Oxford Open Materials Science</td></tr><tr><td>Manuscript ID</td><td>OXFMAT-2024-027</td></tr><tr><td>Manuscript Type:</td><td>Research article</td></tr><tr><td>Date Submitted by the Author:</td><td>20-Oct-2024</td></tr><tr><td>Complete List of Authors:</td><td>Lotfi, Safia; Mohammed V University of Rabat, chemistry<br>El Ouardi, Mohamed; Mohammed V Agdal University, chemistry<br>Arab, Madjid; Universite de Toulon, chemistry<br>Assani, Abderrazzak; Mohammed V Agdal University, chemistry<br>BaQais, Amal; Princess Nora bint Abdul Rahman University, chemistry<br>Haspel, Henrik; HUN-REN Hungarian Research Network, SZTE Reaction Kinetics and Surface Chemistry Research Group; University of Szeged, Applied and Environmental Chemistry<br>Ait Ahsaine, Hassan; Mohammed V University of Rabat, chemistry</td></tr><tr><td>Keywords:</td><td>Photocatalysis, Materials, Inorganic chemistry, Bismuth vanadate</td></tr></table>
# Polycrystalline BiVO₄ visible-light photocatalytic activity: Rietveld refinement, microstructural, vibrational, and photocatalytic properties

Safia Lotfi<fnref n="1" />, Mohamed El ouardi<fnref n="1" />,<fnref n="2" />, Madjid Arab<fnref n="2" />, Abderrazzak Assani<fnref n="1" />, Amal BaQais<fnref n="3" />, Henrik Haspel<fnref n="4" />,<fnref n="5" />, Hassan Ait Ahsaine<fnref n="1" />,*

Corresponding author: prof. Hassan Ait Ahsaine, h.aitahsaine@um5r.ac.ma

## Abstract

Herein, we have successfully synthesized BiVO₄ photocatalysts via a simple chemical precipitation process followed by heat treatment at 400, 500, and 600 °C. The resulting samples were characterized by using X-ray diffraction (XRD) combined with Rietveld refinement, Raman spectroscopy, scanning electron microscopy (SEM), and UV-vis diffuse reflectance spectroscopy (DRS). The Rietveld refinement from powder diffraction data confirmed the monoclinic structure of the BiVO₄ with I112/b space group in accordance with the literature, whereas morphology changed from well-rounded to faceted and bulky particles. The presence of the (VO₄)<fnref n="3" />⁻ vibrational mode in the vibrational spectra indicated high-purity BiVO₄ samples. The photocatalytic performance was assessed by photodegradation studies of organic contaminants, namely methylene blue (MB) and methyl orange (MO) under UV-vis irradiation. The 400-BiVO₄ exhibited the highest MB (95 %) and MO (80 %) photodegradation efficiencies during 150 min visible light illumination compared to 500 and 600-BiVO₄. The photocatalytic mechanism was investigated based on UV-visible diffuse reflectance spectra, photocurrent and Mott-Schottky analysis, and radical trapping measurements.

**Keywords:** Bismuth vanadate; photocatalysis; visible-light; dye degradation, active species

<footnote n="1">¹Laboratoire de Chimie Appliquée des Matériaux, Centre des Sciences des Matériaux, Faculty of Sciences, Mohammed V University in Rabat, Morocco.</footnote>

<footnote n="2">²Université de Toulon, AMU, CNRS, IM2NP, CS 60584, Toulon Cedex 9, France.</footnote>

<footnote n="3">³Department of Chemistry, College of Science, Princess Nourah Bint Abdulrahman University, P. O. Box 84428, Riyadh 11671, Saudi Arabia.</footnote>

<footnote n="4">⁴HUN-REN-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary</footnote>

<footnote n="5">⁵Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1, H-6720 Szeged, Hungary</footnote>
# 1. Introduction

Recent decades have seen rapid growth of urbanization of the global population and industrial development that led to water pollution and scarcity on every continent [1,2]. This phenomenon is a consequence of the direct discharge of domestic and industrial wastes, including heavy metals, residual non-biodegradable dyes, pesticides, and bacteria [3-8]. These dangerous substances have harmful impacts on the environment and the human health, while it was also reported that 80% of diseases and 50% of deaths can be linked to wastewater [9]. Therefore, various treatment technologies were utilized [10,11], such as coagulation [12], flocculation [13], ion exchange [14], chemical precipitation [15], sedimentation [16], and chlorination [17]. Despite having been used for decades, the above methods proved to be expensive and partly ineffective as they cannot completely remove pollutants [5,18,19]. As an alternative approach, semiconductor photocatalysis has attracted the attention of researchers in the past decades [20]. This process is a promising environmental-friendly technology, which utilizes the sunlight as a renewable source for the generation of electron-hole pairs in semiconducting catalyst particles. The excited charge carriers lead to the production of reactive oxygen species (•O₂⁻ and •OH), which, in turn, participate in the chemical degradation of pollutants until their mineralization to CO₂ and H₂O [3,5,21-25]. In the past decades, titanium dioxide (TiO₂) was commonly used as photocatalyst for the degradation of organic contaminants due to its high photocatalytic activity, robust chemical stability, wide commercial availability, affordability, and non-toxic nature [5,26-28]. However, the use of TiO₂ was limited because of its wide bandgap of 3.2 eV, which makes it suitable only for the UV region [2,29]. To overcome this, non-titania-based narrow bandgap semiconductors were developed with an effective visible light response [30], like FeVO₄ [31], Bi₂WO₆ [32,33], BiVO₄ [26], LaVO₄ [19], CeVO₄ [34], and YVO₄ [35]. Among these photocatalysts, bismuth vanadate (BiVO₄) has been extensively studied in organic pollutant photodegradation under visible light illumination, as its narrow band gap, non-toxicity, excellent dispersibility, chemical stability, and high-performance make it an ideal candidate [5,36-38]. Although BiVO₄ has three crystal phases: 1) tetragonal scheelite, 2) tetragonal zircon, and 3) monoclinic scheelite [39], only the monoclinic scheelite BiVO₄ (m-s) possess excellent photocatalytic activity due to the combination of its narrow band gap (2.4 eV) and high crystal distortion of BiO₈ hybridization between the lone pair states of Bi-6s and the O-2p [39-41]. Various BiVO₄ synthesis methods have been reported in the literature, e.g., hydrothermal [42], sol-gel [43], co-precipitation [26,44], sonochemical [45] etc., most of them
use templates [46] or surfactants [47]. To avoid the latter, the researchers developed a green and sustainable synthesis without the assistance of any additives [48].

Herein, we synthesized monoclinic BiVO₄ by a simple precipitation process without using any organic solvents. The as-prepared powder was then calcined at 400, 500, and 600 °C, respectively. The obtained samples were characterized by X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV-vis diffuse reflectance spectroscopy (DRS). Finally, the photodegradation activities were characterized by using the model organic pollutants of methylene blue (MB) and methyl orange (MO) under visible light illumination. The re-usability of the catalysts was determined in recycling tests, and the photocatalytic mechanism was unveiled by investigating the behavior of the photogenerated charge carriers.

## 2. Experimental

### 2.1. Materials

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, 98%, Alfa Aesar), ammonium metavanadate (NH₄VO₃, 98%, ACROS Organics), nitric acid (HNO₃ 65%, Merck), and sodium hydroxide (NaOH, 99%, Fisher Scientific) were used as received without any further purifications. All solutions were prepared by using de-ionized water as solvent.

### 2.2. Synthesis of BiVO₄

BiVO₄ was synthesized via a simple chemical precipitation method. First, solution A was obtained after the dissolution of Bi(NO₃)₃·5H₂O in diluted HNO₃, and then solution B of NH₄VO₃ dissolved in NaOH was added dropwise to the former under constant stirring. After 1 hour stirring the pH of the resulting yellow solution was adjusted to 5 by using NaOH, and a yellow precipitate is formed under vigorous stirring for 30min. The final product was filtered and washed with the de-ionized water for several times, and dried at 80 °C overnight [49]. The as-prepared BiVO₄ samples were calcined at 400, 500, and 600 °C for 3 hours under air [26,50].

### 2.3. Characterization of BiVO₄

X-ray diffraction (XRD) was used to identify the crystal structure of the samples by employing Cu Kα radiation (λ = 1.5406 Å) in a Rigaku Miniflex II powder diffractometer at a scan speed of 4° min⁻¹ in the angular range from 2θ = 10° to 60°. The crystalline structure determination was done by Rietveld refinement using the FullProf Suite software package.
Raman spectra were recorded using 532 nm laser excitation at 0.25 mW power in a SENTERRA II Raman spectrometer. The morphology of the BiVO₄ powders was characterized by scanning electron microscopy (SEM, Thermo-Fisher Scientific Apreo C) with an accelerating voltage of 10 kV. UV-vis measurements were carried out with a miniature fiber optic-based spectrometer (high resolution HR4000 USB spectrometer HR4000CGUV-vis-NIR, Ocean Optics) in the wavelength region of 200 to 800 nm using a DH-2000-BAL (Ocean Optics) Deuterium Tungsten Halogen light source [51].

## 2.4. Photocatalytic degradation experiments

The photocatalytic activity of BiVO₄ was examined by the removal of 5 ppm methylene blue (MB) and 5 ppm methyl orange (MO) in aqueous solution of pH = 5.4 and 5.8, respectively. First, the catalysts were stirred in the dye solutions in dark to reach the adsorption-desorption equilibrium, then the dispersions were irradiated in the visible light range by a 300 W Xenon lamp. At specified time periods, aliquots of 3 mL were taken and centrifuged at 13400 rpm for UV-vis analysis in the 400-800 nm wavelength range using a Shimadzu UV 2600 spectrophotometer. Radical trapping tests were done by using ethylenediamine disodium tetra-acetate (EDTA), isopropanol (IPA), and L-ascorbic acid as scavengers for the active species h⁺, OH•, and O₂•, respectively.

## 2.5. Determination of the point of zero charge (pzc)

The pH<sub>pzc</sub> of the BiVO₄ surface was determined via the method reported by Al-Harahsheh [52]. A sample mass of 50 mg was weighted into six beakers each containing 50 ml of 0.1 M potassium nitrate (KNO₃) solution. The zero-point charge measurements curve was obtained after setting the pH<sub>initial</sub> of the beakers to 2.1, 4.0, 6.5, 8.7, 10.1, and 12.5 by adding either 0.1 M NaOH or 0.1 M HNO₃ solution dropwise.

## 2.6. Photoelectrochemical measurements

Photoelectrochemical studies were conducted using an Origalys ElectroChem electrochemical workstation in a three-electrode system, where working electrodes (WE) were obtained by drop-casting the samples onto indium tin oxide (ITO)/glass substrates. In a typical experiment, 10 mg photocatalyst powder was dispersed into 2 mL of absolute ethanol by ultrasonication for 15 min. After dropping 100 µL of the suspension onto 1 × 1 cm² ITO/glass, the electrode was dried at 100 °C for 5 hours. Measurements were carried out in a 0.1 M sodium sulfate (Na₂SO₄) aqueous solution, using Ag/AgCl and platinum foil as
reference and counter electrode, respectively. Visible light irradiation was done by a 300 W Xe lamp equipped with a 420 nm cut-off filter (λ > 420 nm). Transient photocurrent curves were recorded at a bias of 0.4 V, while Mott-Schottky measurements were carried out at a frequency of 200 Hz.

## 3. Results and discussion

### 3.1. Structural analysis of BiVO₄

**Figure 1.** shows the powder X-ray diffractograms of the BiVO₄ materials obtained through co-precipitation, then calcined at 400-600 °C for 3 h. The diffraction profiles were first compared to the well referenced BiVO₄ in the ICSD database. All characteristic peaks at 2θ = 18.7, 28.9, 30.6, 34.5, and 46.7° were indexed with a single-phase monoclinic scheelite BiVO₄ structure (ICSD datasheet File Card N°96-901-3437). The sharp diffraction peaks in the XRD patterns indicate the formation of a well crystallized BiVO₄. The average D crystallite size was calculated by the Scherrer formula:

$$D = \frac{k\lambda}{\beta \cos(\theta)} \qquad (1)$$

Where k is the Scherrer-constant set to k = 0.9, λ is the wavelength of the X-ray radiation (0.15406 nm), β refers to the full width at half maximum (FWHM) of each peak in radian determined through Gaussian peak fitting, and θ is the Bragg position.

<figure><img src="image_2.png" /><figcaption>Figure 1. XRD pattern of BiVO₄ samples calcined at 400, 500, and 600 °C.</figcaption></figure>
The crystallite size was found to be 20, 24, and 30 nm for samples prepared at temperatures 400, 500, and 600 °C, respectively, showing only a small variation with calcination temperature. This latter influences the crystal growth by welding primary particles together resulting in the formation of bigger particles.

The formation of the monoclinic scheelite structure with the I2/b space group was confirmed in all samples by the Rietveld refinement of the XRD patterns. The observed and the calculated XRD patterns of BiVO₄ calcined at 400, 500, and 600 °C, along with the corresponding difference curves are shown in **Figure S1**. The figures of merit characterizing the quality of the structural refinement are listed in **Table S1**, and the refined atomic positions are shown in **Table S2-4**.

## 3.2. Vibrational analysis of BiVO₄

**Figure 2** shows the Raman spectra of the samples prepared at 400, 500, and 600°C. The Raman shift at around 211.5, 325.5, 369, 711, and 830.1 cm⁻¹ correspond to the monoclinic BiVO₄ according to the literature [53,54]. The most intense bands centered at 827.7, 828.9, and 830.1 cm⁻¹ referred to the symmetrical stretching mode of the V–O vibrations, while its anti-symmetric counterpart is located at 711 cm⁻¹. The bands at 369 and 325.5 cm⁻¹ are the symmetric and asymmetric deformation modes of the VO₄³⁻ tetrahedron. Besides, the bands located at 211.5 and 129 cm⁻¹ are the external modes in BiVO₄ involving rotation and translation. Therefore, results from Raman spectroscopy agree well with those from the previous XRD analysis [55,56]. These findings are summarized in **Table 1**, where we can observe a red shift in the Raman bands positions with increasing calcination temperature. This behavior is due to the thermal expansion, bond population, less agitation of particles [57,58].
<figure><img src="image_3.png" /><figcaption>Figure 2. Raman spectra of BiVO₄ calcined at 400, 500, and 600 °C.</figcaption></figure>

<table><caption>Table 1. Band identification in the Raman spectra of BiVO₄ calcined at 400, 500, and 600°C.</caption><thead><tr><th>Calcination temperature (°C)</th><th>Raman stretching frequency (cm<sup>-1</sup>)</th><th>Vibrational modes assignment</th><th>Refs</th></tr></thead><tbody><tr><td>400</td><td>827.7</td><td></td><td></td></tr><tr><td>500</td><td>828.9</td><td>υ<sub>s</sub>(V-O)</td><td></td></tr><tr><td>600</td><td>830.1</td><td>Symmetric stretching V-O</td><td></td></tr><tr><td></td><td>711</td><td>υ<sub>as</sub>(V-O)<br>Antisymmetric stretching V-O</td><td>[59–61]</td></tr><tr><td></td><td>369</td><td>δ<sub>s</sub>(VO<sub>4</sub><sup>3-</sup>)<br>Symmetric deformation V-O in VO<sub>4</sub><sup>3-</sup></td><td></td></tr><tr><td></td><td>325.5</td><td>δ<sub>as</sub>(VO<sub>4</sub><sup>3-</sup>)<br>Antisymmetric deformation V-O in VO<sub>4</sub><sup>3-</sup></td><td></td></tr><tr><td>400</td><td></td><td></td><td></td></tr><tr><td>500</td><td></td><td></td><td></td></tr><tr><td>600</td><td></td><td></td><td></td></tr><tr><td></td><td>211.5</td><td></td><td></td></tr><tr><td></td><td>129</td><td>External modes (Rotation/Translation)</td><td>[62]</td></tr></tbody></table>
### 3.3. Morphological analysis of BiVO₄

SEM images of BiVO₄ synthesized via the co-precipitation process after heat treatment at 400–600 °C for 3h were shown in **Figure 3**. The sample calcined at 400 °C consists of uniform spherically shaped particles (**Figure 3a**), whereas agglomerated smaller particles are seen in the 500 °C sample by assembling pseudospherical grains with distinct edges and facets (**Figure 3b and d**). The 600 °C heat treatment gave rise to more massive and dense assemblies in **Figure 3c**. Thus, increasing calcination temperature in BiVO₄ led to an increase in particle size along with increasing degree of particle agglomeration. Moreover, pores are also observed in each sample, at lower temperatures, probably microporosity due to surfaces defects and grain boundaries, followed by the formation of macroporosity by coalescence at 600 °C.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** SEM micrographs of the BiVO₄ calcined at (a) 400, (b) 500, and (c) 600 °C. The 500 °C sample at higher magnification where edges and facets are recognizable (d).</figcaption></figure>

### 3.4. UV-visible optical absorption of BiVO₄

Diffuse reflectance UV-vis spectroscopy (DRS) was utilized at room temperature to determine the optical properties of the BiVO₄ powders, and the optical spectra for BiVO₄ calcined at 400, 500, and 600 °C are presented in **Figure 4a**. The absorption onset wavelength of 480 nm corresponds to a band gap of 2.58eV, which makes these catalysts suitable for visible light range application.
For a more precise band gap determination, the spectra were converted to the pseudo
absorption function F(R) via the Kubelka-Munk relation (R) = (1 - R)²/2R, where R is the
absolute reflectance. The Tauc plots, i.e., (F(R) hv)¹/n versus the photon energy hv, were
plotted (Figure 4b), and band gaps E_g were calculated from the equation (F(R) hv)¹/n = A (hv
-Eg), where A is the proportionality constant, and n = ½ or 2 is a coefficient for direct and
indirect band gap, respectively. The band gap energies E_g were extracted from the linear fit of
the Tauc plots of each sample [54], and E_g = 2.24, 2.27, and 2.31 eV were obtained for the
400, 500, and 600 °C samples, respectively. These results are in good agreement with
previous literature results [50].

<figure><img src="image_5.png" /><figcaption>Figure 4. (a) UV-vis DRS spectra and (b) the corresponding Tauc plots of the BiVO₄ samples calcined at 400, 500 and 600 °C.</figcaption></figure>

**3.6. Photocurrent analyses**

The efficient charge carrier separation in the semiconductor is a prerequisite for a high photocatalytic performance, thus charge separation efficiency was investigated in transient photocurrent experiments (Figure 5). The photocurrent response of BiVO₄ decreases with increasing calcination temperature, where the highest photocurrent density in BiVO₄ 400 implies a facilitated charge carrier separation. Moreover, the well-preserved photocurrent density in the subsequent excitations indicates high photo-electrochemical stability. Calcination at 400 °C resulted in the smallest crystallite size among the studied samples. This, on the one hand, increases the specific surface area, which provides more active site for pollutant adsorption, while on the other hand, give rise to a more pronounced crystal defect formation. The latter, in turn, deteriorates charge carrier mobility through unwanted photoinduced electron-hole recombination.
<figure><img src="image_6.png" /><figcaption>Figure 5. The transient photocurrent curves of BiVO₄ calcined at 400, 500, and 600 °C.</figcaption></figure>

### 3.5. Photocatalytic activity of BiVO₄

The photocatalytic activity of BiVO₄ calcined at 400, 500, and 600 °C was characterized by the decomposition of the model pollutants MB and MO dye molecules at room temperature under visible light illumination. The photocatalytic dye degradation results are shown in **Figures 6-7**. In highly diluted solutions (ppm) the Beer-Lambert law applies, which means direct proportionality between the analyte concentration and the measured absorbance. The photocatalytic decomposition was followed by the change in the Cₜ/C₀ ratio with time, where Cₜ, C₀, and t are the dye concentration at the beginning of the experiment and at t reaction time (**Figure 6b and 7b**). Kinetic plots of the photodegradation reaction are constructed by plotting the negative logarithm of the former against time, and the dataset is fitted by a linear equation by using a pseudo-first-order model with the rate constant k [63]:

$$- \ln \frac{c_t}{c_0} = kt \qquad (2)$$

#### 3.5.1. Photocatalytic degradation of MB dye

**Figure 6a** shows the decreasing intensity of the absorption peak of MB at around 664 nm with increasing irradiation time in the presence of BiVO₄ (400 °C). In the absence of irradiation, only 9% of the initial MB was decomposed after 1 h, which implies that dye removal is mainly the result of photocatalytic degradation rather than adsorption. Kinetic plots of the photodegradation reaction are depicted in **Figure 6b-c**, where degradation kinetics was described by equation (2). BiVO₄ calcined at 400 °C shows the highest photocatalytic performance of 95% MB removal in 150 min, compared to those of BiVO₄ calcined at 500 °C
(86%), and 600 °C (76%). Pseudo-first order kinetic rate constants of k = 0.0198, 0.0131, and 0.0097 min⁻¹ for BiVO₄ 400, 500, and 600 °C are found, respectively (Figure 6c). The poor photocatalytic performance of the samples calcined at higher temperatures corroborates with the transient photocurrent results above, indicating rapid recombination of the photoinduced charge carriers (Figure 5). The catalytic activity of BiVO₄ 400 °C is supported by its optical characteristics, particularly its narrow band gap along with higher visible light absorption and the small particle size with higher accessible surface area [5,19,26,50,64]. Furthermore, it also depends on the point of zero charge (pHₚzc) of the catalyst, (Figure 6d) influenced by the electrical charge and the state of the surface [65]. Results show, that BiVO₄ surfaces are negatively charged during the photocatalytic test reaction, as the pH of the dye solution (pH = 5.1) is higher than that of the pHₚzc = 4.1. The electrostatic attraction between the negative charged catalyst surface and the positively charged cationic dye MB enhances dye adsorption, and consequently, improves photocatalytic activity.

<figure><img src="image_7.png" /><figcaption>Figure 6. (a) UV-vis absorption spectra of MB during photocatalytic degradation on BiVO₄ calcined at 400 °C, (b) variation of the Cₜ/C₀ MB ratio over 150 min visible light irradiation, (c) the pseudo-first-order photodecomposition kinetics, and (d) the determination of the zero-point charge of the BiVO₄ 400 °C particles.</figcaption></figure>
### 3.5.2. Photocatalytic degradation of organic MO dye

**Figure.7a** shows the time dependent UV-vis absorption spectrum of MO with intensity maximum at 465 nm upon visible light irradiation using the BiVO₄ 400°C catalyst. The MO loss due adsorption was found to be only 4% in the dark after 1 h, which verifies that MO removal also takes places due photocatalytic degradation just like in the case of MB in 3.5.1. The corresponding kinetic curves are plotted in **Figure 7b** and in their linearized form in **Figure 7c**.

<figure><img src="image_8.png" /><figcaption>**Figure 7.** (a) UV-vis absorption spectra of MO during photocatalytic degradation on BiVO₄ calcined at 400 °C, (b) variation of the Ct/C₀ MO ratio over 150 min visible light irradiation on the BiVO₄ catalysts calcined at 400, 500, and 600 °C, (c) alongside with the pseudo-first-order photodecomposition kinetics.</figcaption></figure>

The photodegradation efficiency of 80% on BiVO₄ calcined at 400 °C was the highest compared to those if 500 °C (75 %) and 600 °C (69 %) samples after 150 min visible light exposure. The kinetic constants were extracted from Figure 7c using the pseudo-first order model of equation (2), and the corresponding k values were 0.0114, 0.0096, and 0.0086 min⁻¹ for the BiVO₄ samples calcined at 400, 500, and 600 °C, respectively. Since the point of zero charge of BiVO₄ is pHₚzc = 4.1 (Figure 6d), the surface of the samples in the MO degradation measurements (pH = 3.1) is positively charged. The electrostatic attraction between the
positively charged catalyst and the negative charge of the anionic dye, in turn, further
promotes MO photodegradation.

**3.6. Trapping test and recyclability**

Trapping tests were conducted for the determination of the major oxidizing species
responsible for dye degradation. Ethylenediaminetetraacetic acid (EDTA), L-ascorbic acid (L-
asc) and isopropanol alcohol (IPA) were selected as specific radical trap moieties for
identifying any short-lived species potentially involved in MB and MO decomposition, i.e. h⁺,
O²⁻, and OH·. **Figure 8.a** depicts the impacts of trapping agents on the photodegradation
efficiency of MB and MO on the BiVO₄ 400 °C catalyst. In the absence of trapping agents,
the photocatalytic decomposition rates were 95% for MB and 80% for MO (**Figure 6-7**).
Upon addition of L-ascorbic acid, EDTA, and IPA, the activities dropped to 39%, 51%, and
77% for MB and 42%, 59%, and 69% for MO, respectively. This indicates that holes h⁺ and
superoxide O²⁻ appear to be the major species in the photocatalytic decomposition of MB and
MO, whereas hydroxyl radicals OH· have a secondary role in the photocatalytic
decomposition. The stability of the BiVO₄ 400 °C sample was assessed by reusing the catalyst
at the end of every photocatalytic reaction. The collected photocatalyst has been washed and
then dried for subsequent reuse. The results in **Figure 8.b** show only a minor drop in the
decomposition efficiency after 4 successive tests.

<figure><img src="image_9.png" /><figcaption>**Figure 8.** (a) Photocatalytic decomposition efficiency of MB and MO using BiVO₄ 400 in the presence of trapping agents, and (b) photocatalyst recycling in four successive tests.</figcaption></figure>
### 3.7. Proposed mechanism

The band gap of BiVO₄ 400 °C, i.e. the energy difference of the band edges of the conduction (CB) and valence bands (VB), was determined from its optical properties in **Figure 4**, whereas the band edge of the CB (the so-called flat-band potential) can be independently calculated from electrochemical measurements using the Mott-Schottky equation:

$$C^{-2} = \frac{2}{N_D e \epsilon \epsilon_0} \left( V - V_{fb} - \frac{kT}{e} \right) \quad (3)$$

where C is the interfacial capacitance, N_D is the carrier density, e is the electronic charge, ε and ε₀ is the dielectric constant of the semiconductor and the free space, V and Vfb is the applied and the flat band potential, k is the Boltzmann constant, and T is the absolute temperature.

<figure><img src="image_10.png" /><figcaption>**Figure 9. (a) Mott-Schottky graph of the BiVO₄ 400 photocatalyst, and (b) the scheme of the proposed degradation mechanism.**</figcaption></figure>

Plotting C⁻² against the applied potential, a linear trend is seen in a limited potential region in **Figure 9a**. A positive slope of the latter indicates that the fabricated BiVO₄ 400 °C is a n-type semiconductor, while fitting equation (3) to these data provides its flat band potential (Vfb) from the x-intercept. The resulted Vfb = -0.61 V (vs. Ag/AgCl) can then be translated onto the normal hydrogen electrode scale (NHE) by EAg/AgCl = ENHE - 0.197 [66,67], however, it is known from the literature that conduction band potentials are more negative by 0.2 V than the measured Vfb flat band potential in n-type semiconductors [68–70]. Thus, the valence and conduction band edge positions in BiVO₄ 400 was calculated by the following equations:
$$E_{CB} = V_{fb} - 0.2 \qquad (4)$$

$$E_g = E_{VB} - E_{CB} \qquad (5)$$

From Equation (4) the E<sub>CB</sub> value of -0.81 V (vs. Ag/AgCl) is given, which equals to -0.613 V (vs. NHE). The corresponding E<sub>VB</sub> = 1.627 (V vs. NHE) is derived from equation (5) by adding E<sub>CB</sub> and E<sub>g</sub>, while the active species of the BiVO<sub>4</sub> 400 °C photocatalyst are characterized by the potentials of the E(H<sub>2</sub>O/OH•) = (1.99 (V vs. NHE) and the E(O<sub>2</sub>/O<sub>2</sub><sup>-</sup>) = -0.33 (V vs. NHE) redox couples [71]. As can be seen in **Figure 9b**, the CB edge potential is more negative compared to that of O<sub>2</sub>/O<sub>2</sub><sup>-</sup>, which in turn implies that the BiVO<sub>4</sub> 400 °C catalyst is able to produce O<sub>2</sub><sup>-</sup>. The VB edge potential, on the other hand, is more negative than that of H<sub>2</sub>O/OH• making it difficult to generate OH• radicals. As a consequence, OH• radicals play a minor function in the degradation process [72,73]. This aligns well with the active moieties trapping results in **Figure 8**, which showed that superoxide radicals are the major species responsible for degradation. The photocatalytic performance of various BiVO<sub>4</sub>-based systems were collected from the literature, and their performance metrics are presented in **Table 2** along with the experimental parameters of the tested model contaminants and the irradiation sources. The material developed in this study shows a high photocatalytic performance compared to the other reported BiVO<sub>4</sub>-based photocatalysts.

<table><caption>**Table 2.** A comparative analysis of the photocatalytic efficacy of the BiVO<sub>4</sub> photocatalyst with other reported photocatalysts.</caption><thead><tr><th>Photocatalyst</th><th>Pollutant</th><th>Synthesis method</th><th>Concentration (mg/L)</th><th>Light sources (W)</th><th>Efficiency</th><th>Ref</th></tr></thead><tbody><tr><td rowspan="2">1 wt% Pd/BiVO<sub>4</sub></td><td rowspan="2">MO</td><td rowspan="2">Impregnation</td><td rowspan="2">10</td><td>Lamp</td><td>100%</td><td rowspan="2">74</td></tr><tr><td>(12 W)</td><td>15 h</td></tr><tr><td rowspan="2">BiVO<sub>4</sub>-1</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td></td><td>44%</td><td rowspan="2"></td></tr><tr><td></td><td>4 h</td></tr><tr><td rowspan="2">BiVO<sub>4</sub>-2</td><td rowspan="2">MO</td><td rowspan="2">hydrothermal</td><td rowspan="2">10</td><td>Xe lamp</td><td>60%</td><td rowspan="2">75</td></tr><tr><td>(300 W)</td><td>4 h</td></tr><tr><td rowspan="2">BiVO<sub>4</sub>-3</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td></td><td>68%</td><td rowspan="2"></td></tr><tr><td></td><td>4 h</td></tr><tr><td>BiVO<sub>4</sub>-4</td><td></td><td></td><td></td><td></td><td>87%</td><td></td></tr></tbody></table>
<table><tbody><tr><td>0.04 mol%<br>B/BiVO<sub>4</sub></td><td>MO</td><td>Sol gel</td><td>10</td><td>Halogen<br>lamp<br>(250 W)</td><td>98%<br>50 min</td><td>76</td></tr><tr><td>6.5 wt% Ag/BiVO<sub>4</sub></td><td>MB</td><td>Hydrothermal</td><td>5</td><td>Xe lamp<br>(350 W)</td><td>98%<br>6 h</td><td>77</td></tr><tr><td>BiVO<sub>4</sub><br>+</td><td>MB</td><td>Hydrothermal</td><td>5</td><td>LED<br>light</td><td>99%<br>90 min</td><td>78</td></tr><tr><td>Peroxymonosulfate<br>(PMS) + Visible<br>0 wt%-BiVO<sub>4</sub>-<br>paint</td><td></td><td></td><td></td><td></td><td>27%</td><td></td></tr><tr><td>20 wt%-BiVO<sub>4</sub>-<br>paint</td><td>MB</td><td>Coating</td><td>5</td><td>2<br>Havells<br>bulbs<br>(15 w)</td><td>240min<br>67%<br>240 min</td><td>79</td></tr><tr><td>40wt%-BiVO<sub>4</sub>-<br>paint</td><td></td><td></td><td></td><td></td><td>72%<br>240 min<br>95%</td><td></td></tr><tr><td>BiVO<sub>4</sub> 400 °C</td><td>MB<br>MO</td><td>Co-<br>precipitation</td><td>5</td><td>Philips<br>lamps<br>(300 W)</td><td>150 min<br>80%<br>150 min</td><td>This<br>study</td></tr></tbody></table>

Conclusions

In summary, we have successfully prepared BiVO₄ by a facile precipitation method without using any organic additives. The as-prepared materials were subjected to a subsequent calcination at 400, 500, and 600 °C. It was found that the heat treatment had a significant impact on the structural, morphological, optical, and photocatalytic properties of the resulting BiVO₄. All synthesized samples possessed a single phase monoclinic scheelite structure with a uniform spherical particle morphology, where increasing calcination temperature resulted in an increase in the crystallite size. The bandgap of the semiconductor catalysts was determined by optical spectroscopy (Eg = 2.24 eV), which shows considerably visible light absorption and makes the catalyst suitable for sunlight activation. The BiVO₄ calcined at 400 °C showed the highest photocatalytic activity among the studied materials, decomposing 95% and 80% of the initial methylene blue and methyl orange after 150 minutes of visible light illumination,
respectively. Moreover, the same sample provided the highest photocurrent density implying an effective charge carrier separation. Finally, the trapping experiment indicated that the primary reactive species utilized in the photocatalytic degradation of MB and MO are holes ($h^+$) and superoxide ($O_2^-$), which corroborated well with the band structure determination.

## Acknowledgements

Safia Lotfi thanks the support of CNRST (Centre National pour la Recherche Scientifique et Technique) in the Excellence Research Scholarships Program. The authors are thankful to the faculty of Science, Mohammed V University in Rabat, Morocco. Profs. Ait Ahsaine and Madjid thank the IM2NP for the characterization of the prepared materials. Dr. BaQais thanks Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R230), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. H. Haspel thanks the financial support of the János Bolyai Research Grant of the Hungarian Academy of Sciences (BO/682/22). Project no. RRF-2.3.1-21-2022-00009 (National Laboratory for Renewable Energy) was undertaken with the support provided by the Recovery and Resilience Facility of the European Union within the framework of Programme Széchenyi Plan Plus. This project has received funding from the HUN-REN Hungarian Research Network.

## Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

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SUPPLEMINARY INFORMATION

Polycrystalline BiVO₄ visible-light photocatalytic activity:
Rietveld refinements, microstructural, vibrational and
photocatalytic properties

Safia Lotfi¹, Mohamed El ouardi¹,², Madjid Arab², Abderrazzak Assani¹, Amal BaQais³, Henrik Haspel⁴,⁵, Hassan Ait Ahsaine¹,*

¹Laboratoire de Chimie Appliquée des Matériaux, Centre des Sciences des Matériaux, Faculty of Sciences, Mohammed V University in Rabat, Morocco.

²Université de Toulon, AMU, CNRS, IM2NP, CS 60584, Toulon Cedex 9, France.

³Department of Chemistry, College of Science, Princess Nourah Bint Abdulrahman University, P. O. Box 84428, Riyadh 11671, Saudi Arabia.

⁴HUN-REN-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, H-6720 Szeged, Hungary

⁵Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1, H-6720 Szeged, Hungary

Corresponding author: prof. Hassan Ait Ahsaine, h.aitahsaine@um5r.ac.ma

<table><caption>Table S1. Crystal structure data of BiVO₄ calcined at 400, 500 and 600°C.</caption><thead><tr><th>Sample</th><th>BiVO<sub>4</sub> 400 °C</th><th>BiVO<sub>4</sub> 500 °C</th><th>BiVO<sub>4</sub> 600 °C</th></tr></thead><tbody><tr><td>Structure</td><td></td><td>Monoclinic</td><td></td></tr><tr><td>Space group</td><td></td><td>I112/b</td><td></td></tr><tr><td>Lattice parameters<br>(Å)</td><td>a= 5.1866 (3)<br>b= 5.0879 (4)<br>c= 11.6873 (3)</td><td>a = 5.1912 (3)<br>b = 5.0881 (4)<br>c = 11.692 (3)</td><td>a = 5.1908 (6)<br>b = 5.0868 (5)<br>c = 11.691 (2)</td></tr><tr><td>γ (°)</td><td>90.3865 (4)</td><td>90.369 (3)</td><td>90.377 (5)</td></tr><tr><td>Volume (Å<sup>3</sup>)</td><td>308.41</td><td>308.83</td><td>308.72</td></tr><tr><td>R<sub>p</sub></td><td>6.459</td><td>6.550</td><td>6.956</td></tr><tr><td>R<sub>WP</sub></td><td>8.141</td><td>8.701</td><td>9.285</td></tr><tr><td>R<sub>exp</sub></td><td>7.033</td><td>7.003</td><td>7.204</td></tr><tr><td>R<sub>Bragg</sub></td><td>2.482</td><td>3.323</td><td>4.810</td></tr><tr><td>χ<sup>2</sup></td><td>1.34</td><td>1.54</td><td>1.66</td></tr></tbody></table>

<table><caption>Table S2. Refined atomic positions of BiVO₄ calcined at 400 °C.</caption><thead><tr><th>Atom</th><th>x</th><th>y</th><th>z</th><th>U<sub>iso</sub>*/U<sub>eq</sub></th></tr></thead><tbody><tr><td>Bi</td><td>0.00000</td><td>0.25000</td><td>0.63289</td><td>-0.0042*</td></tr><tr><td>V</td><td>0.00000</td><td>0.25000</td><td>0.12929</td><td>-0.0056*</td></tr><tr><td>O1</td><td>0.13938</td><td>0.48744</td><td>0.20075</td><td>-0.0106*</td></tr><tr><td>O2</td><td>0.26745</td><td>0.37957</td><td>0.44581</td><td>-0.0316*</td></tr></tbody></table>
<table><caption>Table S3. Refined atomic positions of BiVO₄ calcined at 500 °C.</caption><thead><tr><th>Atom</th><th>x</th><th>y</th><th>z</th><th>U<sub>iso</sub>*/U<sub>eq</sub></th></tr></thead><tbody><tr><td>Bi</td><td>0.00000</td><td>0.25000</td><td>0.63318</td><td>0.00431*</td></tr><tr><td>V</td><td>0.00000</td><td>0.25000</td><td>0.12904</td><td>0.00646*</td></tr><tr><td>O1</td><td>0.14672</td><td>0.49747</td><td>0.20429</td><td>-0.0040*</td></tr><tr><td>O2</td><td>0.26642</td><td>0.37696</td><td>0.45042</td><td>-0.0089*</td></tr></tbody></table>

<table><caption>Table S4. Refined atomic positions of BiVO₄ calcined at 600 °C.</caption><thead><tr><th>Atom</th><th>x</th><th>y</th><th>z</th><th>U<sub>iso</sub>*/U<sub>eq</sub></th></tr></thead><tbody><tr><td>Bi</td><td>0.00000</td><td>0.25000</td><td>0.63308</td><td>0.00569*</td></tr><tr><td>V</td><td>0.00000</td><td>0.25000</td><td>0.12628</td><td>0.00310*</td></tr><tr><td>O1</td><td>0.15228</td><td>0.48886</td><td>0.19337</td><td>-0.0091*</td></tr><tr><td>O2</td><td>0.28248</td><td>0.36558</td><td>0.43979</td><td>-0.0127*</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>Figure S1. Rietveld refinement of BiVO₄ calcined at (a) 400, (b) 500 and (c) 600 °C.</figcaption></figure>