# On the identification of BiOF, Bi7O5F11 and β-BiO0.5F2 bismuth oxyfluorides

Thomas Erbland, Sara Ibrahim, Guillaume Monier, Pierre Bonnet, Angélique Bousquet

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Thomas Erbland, Sara Ibrahim, Guillaume Monier, Pierre Bonnet, Angélique Bousquet. On the identification of BiOF, Bi7O5F11 and β-BiO0.5F2 bismuth oxyfluorides. 2025. hal-05261692v2

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# On the identification of BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> and β-BiO<sub>0.5</sub>F<sub>2</sub> bismuth oxyfluorides

Thomas Erbland<fnref n="1" />, Sara Ibrahim<fnref n="1" />, Guillaume Monier<fnref n="2" />, Pierre Bonnet<fnref n="1" />*, Angélique Bousquet<fnref n="1" />*

## 1. Introduction

Heterogeneous photocatalysis has emerged as a promising technology for both environmental remediation and sustainable energy production. Within this field, bismuth-based photocatalysts have attracted growing attention due to their unique optical, electronic, and chemical properties. These materials have demonstrated potential in applications such as the degradation of organic pollutants, H₂ generation, and CO₂ conversion [1].

Among them, bismuth oxyfluorides (BiOxF<sub>3-2x</sub>) represent a particularly interesting class of compounds [2]. Over the past decade, they have been investigated for their photocatalytic performance. The literature describes the existence of a structural and compositional continuum between BiF<sub>3</sub> and Bi<sub>2</sub>O<sub>3</sub> [3]. Nevertheless, the precise identification of the various oxyfluoride phases remains challenging, and misassignments are frequently encountered.

The present document aims to provide reliable analytical approaches for the unambiguous identification of three representative bismuth oxyfluorides: BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub>, and β-BiO<sub>0.5</sub>F<sub>2</sub>. A comprehensive characterization is carried out using XRD, FTIR, Raman spectroscopy, XPS, and SEM-EDX.

## 2. Experimental

### 2.1. Chemicals

All products were purchased from Sigma-Aldrich.

### 2.2. Synthesis

#### 2.2.1. BiOF

A simple co-precipitation method was used to synthesize BiOF. 8mmol of Bi(NO<sub>3</sub>)<sub>3.5</sub>H<sub>2</sub>O and 40mmol of NH<sub>4</sub>F are crushed in an agate mortar and dissolved in 40mL of ethylene glycol. The solution is left stirring for 24h at room temperature. The white powder obtained is separated from the solvent by centrifugation, washed 3 times with ultra-pure water and 3 times with ethanol and dried at 60°C for around 24h. BiOF is obtained after 4 hours of a 400°C heat treatment [4].

#### 2.2.2. Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> and BiO<sub>0.5</sub>F<sub>2</sub>

The hydrothermal method to prepare BiO<sub>0.5</sub>F<sub>2</sub> and Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> was adapted from Ren et al. works on the synthesis of BiO<sub>x</sub>F<sub>3-2x</sub> [5]. 2mmol of Bi(NO<sub>3</sub>)<sub>3.5</sub>H<sub>2</sub>O and 4mmol of NH<sub>4</sub>F are dissolved in a 5/25mL solution of acetic acid/water for β-BiO<sub>0.5</sub>F<sub>2</sub> and 25/5 for Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub>. After complete solubilization, the solution is poured in a 30mL PTFE line and heat at 160°C for 12h. The obtained product is separated from the solvent by centrifugation, washed 3 times with ultra-pure water and 3 times with ethanol and dried at 60°C for around 24h.

### 2.3. Physical characterization

#### 2.3.1. X-Ray diffraction

X-ray diffraction measurements were performed on PANalytical XPert'PRO diffractometer model (Cu Kα1 1.5406 Å, Cu Kα2 1.5444 Å, 40 kV, 30 mA, and an angle step of 0.0167°).

#### 2.3.2. Raman spectroscopy

Raman spectroscopy measurements were performed at room temperature using a JOBIN YVON T64000 spectrometer with a charge-coupled device multichannel. The radiation source was a 532

<footnote n="1">¹ Institut de Chimie de Clermont-Ferrand (ICCF) ; Université Clermont Auvergne, CNRS, Clermont Auvergne INP ; F-63000 CLERMONT-FERRAND</footnote>

<footnote n="2">² Institut Pascal (IP) ; Université Clermont Auvergne, CNRS, Clermont Auvergne INP ; F-63000 CLERMONT-FERRAND</footnote>

<footnote>* Corresponding authors. pierre.m.bonnet@uca.fr, angelique.bousquet@uca.fr</footnote>
nm argon laser line. The laser power was tuned to low power (100 mW) to avoid the sample decomposition under the beam. 3 scans of 30 s were made for a single spectrum. The resolution was equal to 0.08 cm⁻¹.

### 2.3.3. Infra-red spectroscopy

Infrared spectroscopy analyses were performed on a Thermo Scientific Nicolet Summit X FTIR Spectrometer with 1 cm⁻¹ step and 16 scans per spectrum.

### 2.3.4. X-Ray photoelectron spectroscopy

X-Ray photoelectron spectroscopy (XPS) were conducted at Institut Pascal, Clermont Auvergne University, using the Omicron brand EA 125 model with a DAR 400 X source. The following utilizes the Mg Kα anode (E = 1253.6 eV) and a hemispheric analyzer working at constant energy of 20 eV for the acquisition of high-resolution spectra. A high-resolution measurement was made for Bi 4f. The

exploitation of the experimental data was carried out by the XPSPEAK41 software. A Shirley baseline was used to subtract the background level on the spectra. A charge correction is necessary to find the “true” energy position of the different XPS signals. For this purpose, the position of the binding energy of carbon sp3 at 285.4 eV is taken as a reference.

### 2.3.5. SEM-EDX

Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) quantifications were acquired on a Zeiss Supra 55VP SEM equipped with a Gemini column. X-ray analysis was performed using a Bruker Silicon Drift detector, which allowed the study of bismuth, oxygen, and fluorine levels. The samples were sputter-coated with gold before introduction in the vacuum chamber. The general formulas of the compounds were estimated by considering only the anionic ratio O/F and an equilibrium of anionic and cationic charges.

## 3. Results

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<table><caption>Figure 1. (a) XRD patterns, (b) Raman spectra and (c) FTIR spectra of BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> and β-BiO<sub>x</sub>F<sub>3-2x</sub> (d) XPS spectra of Bi 4f from Bi<sub>2</sub>O<sub>3</sub>, BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub>, β-BiO<sub>x</sub>F<sub>3-2x</sub> and BiF<sub>3</sub> compounds.</caption><thead><tr><th>Compounds</th><th>Wavenumber (cm-1)</th><th>Raman assignment</th></tr></thead><tbody><tr><td rowspan="3">BiOF</td><td>169</td><td>A<sub>1g</sub> stretching mode of Bi-F</td></tr><tr><td>241</td><td>E<sub>g</sub> stretching mode of Bi-F</td></tr><tr><td>~400</td><td>E<sub>g</sub>/B<sub>g</sub> modes of oxygen atoms</td></tr><tr><td rowspan="5">Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub></td><td>91</td><td rowspan="5">Bi-O-Bi and Bi-F-Bi bending and stretching</td></tr><tr><td>152</td></tr><tr><td>185</td></tr><tr><td>444</td></tr><tr><td>595</td></tr><tr><td rowspan="3">BiO<sub>0.5</sub>F<sub>2</sub></td><td>157</td><td>v(Bi-O) vibrations</td></tr><tr><td>313</td><td>v(Bi-F) vibrations</td></tr><tr><td>449</td><td>Bi-O and Bi-F stretching vibrations<br>Bi-O-Bi and Bi-F-Bi stretching and bending vibrations</td></tr></tbody></table>

<table><caption>Table 1. Assignments of BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> and β-BiO<sub>0.5</sub>F<sub>2</sub> Raman bands [6,7].</caption><thead><tr><th>Compounds</th><th>Wavenumber (cm-1)</th><th>FTIR bands</th></tr></thead><tbody><tr><td rowspan="2">BiOF</td><td>550</td><td>v(Bi-O) vibrations</td></tr><tr><td>420</td><td>v(Bi-F) vibrations</td></tr><tr><td rowspan="2">Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub></td><td>604</td><td>v(Bi-O) vibrations</td></tr><tr><td>526</td><td>v(Bi-F) vibrations</td></tr><tr><td rowspan="2">BiO<sub>0.5</sub>F<sub>2</sub></td><td>545</td><td>v(Bi-O) vibrations</td></tr><tr><td>485</td><td>v(Bi-F) vibrations</td></tr></tbody></table>

<table><caption>Table 2. Difference in wavenumber for FTIR v(Bi-O) and v(Bi-F) vibration bands of BiOF, Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> and β-BiO<sub>0.5</sub>F<sub>2</sub> [6,7].</caption><thead><tr><th>Compounds</th><th>Binding energy (eV)</th><th>assignment</th></tr></thead><tbody><tr><td>BiOF</td><td>159.4</td><td rowspan="3">Bi 4f</td></tr><tr><td>Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub></td><td>158.9</td></tr><tr><td>BiO<sub>0.5</sub>F<sub>2</sub></td><td>160</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 2. SEM images of BiOF (a) x7k (b) x15k (c) x30k (d) EDX spectra of BiOF with atomic quantification (a%) (estimated formula BiO<sub>0.97</sub>F<sub>1.06</sub>).</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>Figure 3. SEM images of Bi<sub>7</sub>O<sub>5</sub>F<sub>11</sub> (a) x7k (b) x15k (c) x30k (d) EDX spectra of BiOF with atomic quantification (a%) (estimated formula Bi<sub>7</sub>O<sub>5.21</sub>F<sub>10.64</sub>).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 4.** SEM images of BiO<sub>0.5</sub>F<sub>2</sub> (a) x7k (b) x15k (c) x30k (d) EDX spectra of BiOF with atomic quantification (a%) (estimated formula BiO<sub>0.56</sub>F<sub>1.88</sub>).</figcaption></figure>

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