# Identification of the photocatalytic degradation mechanisms of pollutants by highly fluorinated graphitic carbon nitride

Tara Buzdon, Nicolas Batisse, Mohamed Sarakha, Daniel Zambon, Guillaume Monier, Pierre Bonnet

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Tara Buzdon, Nicolas Batisse, Mohamed Sarakha, Daniel Zambon, Guillaume Monier, et al. Identification of the photocatalytic degradation mechanisms of pollutants by highly fluorinated graphitic carbon nitride. 2025. hal-05336387

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# Identification of the photocatalytic degradation mechanisms of pollutants by highly fluorinated graphitic carbon nitride

Tara Buzdon<fnref n="1" />, Nicolas Batisse<fnref n="1" />, Mohamed Sarakha<fnref n="1" />, Daniel Zambon<fnref n="1" />, Guillaume Monier<fnref n="2" />,
Pierre Bonnet<fnref n="1" />*

**Keywords:** graphitic carbon nitride, fluorination, photocatalysis, methyl orange, carbamazepine.

## ABSTRACT

Fluorinated graphitic carbon nitride has demonstrated promising and interesting photocatalytic properties and more particularly for degradation of organic pollutants. In this study, graphitic carbon nitride (g-C₃N₄) was fluorinated using a F₂/N₂ mixture under controlled conditions and high amount of fluorine atoms are obtained (as about ~ 19 at.%). Electron paramagnetic resonance (EPR) analysis reveals that fluorination significantly enhances charge generation upon light exposure, leading to the production of highly reactive species (e.g., OH·) that can help for the efficient degradation of organic pollutants. The photocatalytic performance and mechanism of fluorinated g-C₃N₄ (g-C₃N₄-F) was evaluated through the degradation of methyl orange (MO) and carbamazepine (CBZ) used as model organic pollutants. The role of the

<footnote n="1">¹Université Clermont Auvergne, Institut de chimie de Clermont-Ferrand, F-63000 Clermont-Ferrand, France</footnote>

<footnote n="2">²Université Clermont Auvergne, Clermont Auvergne INP, CNRS, Institut Pascal, F-63000 Clermont-Ferrand, France</footnote>
inserted fluorine atoms, the acidity, the structural modifications on the photocatalytic mechanisms of g-C₃N₄ were explored. The results highlight the fact that fluorinated g-C₃N₄ greatly increases the acidity, from pH=6.1 to 3.1 with pristine g-C₃N₄ and fluorinated g-C₃N₄, of the contaminated solution inducing changes in the adsorption. The degradation increases from 19.9 % to 92.8 % after 2 hours under light irradiation with pristine g-C₃N₄ and fluorinated g-C₃N₄ respectively. The use of scavengers as evidence that OH⁻ and holes are the main active species for MO degradation. Additionally, the degradation of CBZ leads to various by-products distribution depending on whether g-C₃N₄ is fluorinated or not induced by different degradation pathways. Finally, the reversible fluorination of g-C₃N₄ was successfully demonstrated, highlighting the potential for material recycling. The defluorinated evidence similar photocatalytic performances as pristine g-C₃N₄, highlighting the important effect of the fluorination to improve the photocatalytic activity of g-C₃N₄.

# 1. Introduction

One of the main issues of our century is environmental pollution, especially water pollution which poses significant risks to ecosystems and human health. These contaminants includes non-degradable plastics, organic pollutants, heavy metals, nitrates and phosphate...¹ Some of these substances such as dyes or pharmaceuticals, are photosensitive.²,³ Given these concerns, exploring effective methods for water treatment appears to be crucial. Among the most widely used techniques is the Advanced Oxidized Process (AOP's) which generate highly reactive species such as hydroxyl radicals OH⁻, superoxide anions O₂⁻ (known as reactive oxygen species (ROS)), electrons (e⁻) and holes (h⁺). These reactive species can efficiently degrade a wide range of organic pollutants, transforming them into less harmful or even non-toxic substances.⁴⁻⁶ Photocatalysis, a commonly employed AOP process for water decontamination, is particularly advantageous owing to its ease of implementation and low cost.⁷,⁸
Since the discovery of the production of H₂ via water splitting in a photoelectrochemical cell (PEC) by Fujishima and Honda in 1972 with titanium dioxide (TiO₂)⁹, the use of semiconductors as photocatalysts widespread. When the light hits the semi-conductor, it generates charges that will then separate and migrate to the surface (if they do not recombine), where they are going to interact with the environment. Holes are going to oxidize water to generate OH⁺ and electrons are going to reduce atmospheric oxygen O₂ to generate O₂⁻. Among all the semi-conductors, graphitic carbon nitride (g-C₃N₄), a metal-free semiconductor, stands out thanks to its interesting optoelectronic properties, low cost and the availability of its components. The photocatalytic properties of g-C₃N₄ have been tested for various applications such as photochemical water splitting,¹⁰ CO₂ reduction¹¹ or pollutant degradation¹². In particular, the general photocatalytic properties of graphitic carbon nitride are usually tested on model pollutants, mostly dyes, like methyl orange (MO)¹³, rhodamine B (RhB)¹³,¹⁴ or methylene blue (MB)¹². Beyond water treatment, g-C₃N₄'s photocatalytic properties have broader applications such as energy conversion¹⁰ and storage¹⁵ like hydrogen production via water splitting¹⁶ and carbon dioxide reduction¹¹, or gas detection¹⁷. It's a π-conjugated polymer with a graphene-like structure. The sheets are mostly composed of tri-s-triazine units (C-N/C=N bond) giving to g-C₃N₄ the stable form of carbon nitride and significant thermal stability up to 600°C and week Van der Walls bond between the sheets.¹⁸ It has a tunable band gap around 2.7 eV,¹⁹ corresponding to a wavelength of 460nm, giving it a yellow light color so it can absorb in the visible range of the solar spectrum.

Nonetheless, g-C₃N₄ possesses a rapid recombination of the photogenerated charges and limited use of the solar spectrum.²⁰ The photocatalytic efficiency of g-C₃N₄ can be further enhanced through various modifications like elemental doping (e.g. Fe²¹, B²² or Br²³), coupling with other semiconductors in heterojunctions²⁴,²⁵, or introducing defects in the g-C₃N₄ structure via structural modifications (quantum dots²⁶, nano-structuration¹²), or exfoliation (thermal²⁰,
liquid<sup>27</sup> or gas<sup>28</sup> ways) that can improve its light absorption, charge separation, and overall reactivity. Such modifications have been shown to extend the light absorption range of g-C<sub>3</sub>N<sub>4</sub> into the visible spectrum and increase the lifetime of the charge carriers, leading to more efficient degradation of pollutants. Several articles have depicted the degradation of this pollutant with g-C<sub>3</sub>N<sub>4</sub>. Luo *et al*, have demonstrated that phosphorus doping of g-C<sub>3</sub>N<sub>4</sub> coupled with CeO<sub>2</sub> induces 4.9 times more degradation of MO than pristine g-C<sub>3</sub>N<sub>4</sub> thanks to improve of the charge separation and better visible light absorption. In this photocatalytic mechanism it's the superoxide radical anions (O<sub>2</sub><sup>•−</sup>) and the holes (<i>h</i><sup>+</sup>) which are mainly active.<sup>29</sup> With boron-doped g-C<sub>3</sub>N<sub>4</sub>, the degradation of MO is initiated by the electrons rather than the holes.<sup>30</sup> So, it can be highlighted that the degradation process of MO is highly dependent on how g-C<sub>3</sub>N<sub>4</sub> is modified. Few studies also focus on the degradation of other pollutants like drugs<sup>31</sup> or bisphenols<sup>12</sup>.

The functionalization by fluorine atoms can be considered as it is expected to induce strong
bonds with carbon due to the high electronegativity of fluorine, change the position of the
valence and conduction bands and induce a polarization of the sheets.³² For this reason, it
appears to be an efficient way to improve the photocatalytic properties of g-C₃N₄ as it is an
efficient way to change the morphology, optic and electronic properties of the material
simultaneously.¹⁹,³³⁻³⁷ The majority of the fluorination processes occur via solvothermal
synthesis with ammonium fluoride (NH₄F)¹⁹,³³, fluorhydric acid (HF)³⁸ or tetrahydrofuran
(THF)³⁵. However, in the less than 30 papers on the subject, the amount of fluorine content is
usually low, generally less than 1 at.%. Another way to fluorinate is the solid-gas fluorination,
which allows higher fluorine content because of the important reactivity of F₂ gas. To the best
of our knowledge, there is only two papers in which the authors used F₂ gas as a fluorination
method.³⁹,⁴⁰ Sun *et al*. were able to achieve 7 at.% of fluorine atoms in their material and
improved photocatalytic activity for H₂ production. To do so, they used diluted F₂ with N₂ and
“high” temperature (150°C). In our previous work, we were able to achieve 26 at.% of fluorine
atoms in g-C₃N₄ by pure fluorination with F₂ at room temperature, inducing important etching
of the material, the presence of numerous pores and a smaller band gap leading to improved
photocatalytic performances on the degradation of organic pollutants.

The aim of this article is to understand how the degradation of pollutants is influenced by
fluorine ions and highly fluorinated graphitic carbon nitride. Here, we have successfully
fluorinated g-C3N4 and obtained evidence of structural changes as well as improved
optoelectronic properties of g-C3N4. The photocatalytic properties of the non-fluorinated and
fluorinated g-C3N4 were evaluated on two distinct pollutants, methyl orange and
carbamazepine41, highlighting more significant degradation rates of methyl orange and several
degradation products of carbamazepine were identified. The various analyses presented in the
article provide a deeper understanding of the degradation mechanisms, regardless of whether
g-C3N4 is fluorinated or not. g-C3N4-F is, afterward and for the first time so far, defluorinated
with the use of a strong reducer enabling a clearer understanding of the effect of the F atoms on
the degradation mechanisms, as well as its influence on the structure and optoelectronic
properties.

**2. Materials and Methods**

2.1 *Preparation of the samples*

Graphitic carbon nitride is obtained by the thermal polycondensation of melamine at 550°C for 4 hours at a heating rate of 20°C/min.⁴² The fluorinated graphitic carbon nitride is obtained under a dynamic fluorination atmosphere adapted from the protocol described in our previous work.⁴⁰ The powder is placed in a nickel container and placed into the reactor. The fluorination is preceded and followed by a purging under a N₂ atmosphere for 90 min. The F₂ gas is added
for 45 min at room temperature with a flow rate of 28,5 mL/min of F₂, diluted with N₂. In the
following, the fluorinated g-C₃N₄ will be called g-C₃N₄-F.

Defluorination of fluorinated graphitic carbon nitride is done by stirring 30 mL of isopropanol with a certain quantity of fluorinated g-C₃N₄ for 15 min. Then, 5 mL of hydrazine (N₂H₄·H₂O) is added and the solution is stirred for 1h. The solution is centrifuged for 15 min at 6000 t/min. The powder obtained is then dried for around 2h at 60°C in air. This sample will be labeled g-C₃N₄-DF.

## 2.2 Characterization techniques

X-ray diffraction (XRD) of the samples is performed from 10° to 90° for 20 min on a XPERT-PRO and a Cu-K α radiation (λ= 1.542512 Å).

Attenuated total reflectance infra-red (IR-ATR) spectra are obtained from 400 cm⁻¹ to 4000 cm⁻¹ with a Nicolet Summit X Spectrometer.

Raman spectroscopy is performed at 785nm for 60s at 100mW, at room temperature, in the air
on a RENISHAW Invia Kontor coupled to a Leica confocal microscope.

X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultrahigh vacuum photoelectron spectrometer equipped with an Omicron DAR 400 X-ray source and an Omicron EA 125 hemispherical analyzer. The Mg Kα source (1253.6 eV) running at 300 W is separated from the analyzer by an angle of 55°. A high magnification mode, constant pass energy of the analyzer equal to 20 eV and an energy step of 0.1 eV were used for analysis. Photoelectron were recorded at a take-off angle of 90° (normal detection). The electron binding energies of the different core levels were corrected according to the C-C contribution of the C1s at 284.8 eV. Binding energies, line widths and areas of the different XPS peaks were obtained by a weighted least-squares fitting of a Lorentzian-Gaussian model curve to the experimental data and the Shirley method for removing of the background. The g-C₃N₄ particles were maintained
on a carbon tab for the XPS measurements. The atomic percentages were obtained using the
empirical formula of XPS signal intensity<sup>43</sup> considering a homogeneous atomic concentration
and the accurate spectrometer calibration described in reference<sup>44</sup>.

Gas adsorption is performed on an ASAP 2020 and the surface specific is measured with the
Brunauer-Emmett-Teller (BET) method under N₂ atmosphere at 77 K.

UV-vis spectra are recorded from 800 nm to 370 nm and Diffuse Reflectance Spectroscopy (DRS) are recorded from 800 nm to 200 nm (LSH 521, LOT). The DRS measurements are converted using the Kubelka-Monk method to obtain the absorbance of the powder, with BaSO₄ pellet used as a reference.

Electron Paramagnetic Resonance (EPR) analysis is performed at room temperature with a
quartz cell on a Bruker EMX EPR spectrometer with a X-band (with a microwave frequency
of 9.85 GHz). The measures are performed in the dark followed by light irradiation with a
Xenovia Xenon lamp (1000 W) from 100 nm to 800 nm with a UV-filter (λ<350 nm) then in
the dark again for 9 cycles for each condition.

Transmission electron microscopy (TEM) images are obtained via a Jeol 2100 Plus apparatus
at 80 kV. The powders are deposited on a holey carbon film on 300 mesh copper grids from
Agar scientific. The powders are dispersed in absolute ethanol for few seconds in an ultrasonic
bath. Then few drops of the solution are deposited on the grid.

The photoluminescence (PL) emission spectra were recorded at room temperature with a
Horiba-Jobin-Yvon set-up consisting of a Xenon lamp operating at 400 W monochromatized
through Triax 180 then the emitted light was dispersed through Triax 550 and detected by
means of a cryogenically cold charge coupled device (CCD) camera (Symphony LN2 series).
Luminescence decays were recorded at room temperature. A laser diode emitting at 375 nm
with a repetition rate of 2 MHz and a pulse duration of 65 ps was used as the excitation source.
The emitting signal was focused on the slit of a 300 mm focal length monochromator coupled to a photomultiplier.

Zeta potential is measured on a Zetasizer Malvern instrument with water as the eluant for g-C₃N₄, g-C₃N₄-F and MO.

HPLC measure are realized on a Water Alliance in isocratic mode, methanol/water 50:50. Finally, the LC-MS analysis are performed on a ThermoScientific UHPLC Ultimate 3000 RSLC, with a 5 µL injection with a gradient mode of a mixture of water and acetonitrile. The UHPLC is connected to a Orbitrap Q-Exactive, the detection range from 50 m/z to 750 m/z.

## 2.3 Photodegradation measurements

The degradation of several pollutants, methyl orange (MO, a common dye used as a model for organic pollutant degradation) and carbamazepine (CBZ, a neuroleptic drug) is tested, their properties are given **Table 1**. The pollutant solutions are prepared by adding the pollutant powders to distilled water to obtain a concentration of 6.10⁻⁵ M, the solution is stirred for an hour. Then, 2.5 mg of photocatalyst is added to a quartz spectroscopy cell and around 3.5 mL of the pollutant solution is added. Prior to light irradiation, the solution is stirred for an hour in the dark with a UV-vis measure every 30 min. After the adsorption-desorption equilibrium is mostly obtained, light irradiation is maintained for 2h with a Xenon lamp (560 W) and a UV-filter of 400 nm and for MO monitoring a UV-vis measurement taken every 20 min, between 370 nm and 800 nm. The absorbance used to calculate the concentration of the solution according to the initial concentration (C₀) is the maximum of absorbance after the first 30 min of stirring in the dark, the same value is kept for the other measurements. For CBZ degradation monitoring, HPLC measurements are carried out.

<table><caption>**Table 1.** Properties of Methyl orange and Carbamazepine.</caption><thead><tr><td>Propriety</td><td>Methyl Orange</td><td>Carbamazepine</td></tr></thead></table>
<table><tr><td>Chemical structure</td><td><img src="chemical_structure.png" alt="Chemical structure of 4-(trifluoromethylphenyl)-2,4,6-triisopropylanilin-3-carboxylic acid anion"></td><td><img src="chemical_structure.png" alt="Chemical structure of 4-(trifluoromethylphenyl)-2,4,6-triisopropylanilin-3-carboxylic acid anion with counter ion (Na<sup>+</sup>)"></td></tr><tr><td>Molecular weight (g/mol)</td><td>327.34</td><td>236.27</td></tr><tr><td>Empirical formula</td><td>C<sub>14</sub>H<sub>14</sub>N<sub>3</sub>NaO<sub>3</sub>S</td><td>C<sub>15</sub>H<sub>12</sub>N<sub>2</sub>O</td></tr><tr><td>Abbreviation</td><td>MO</td><td>CBZ</td></tr></table>

## 2.4 *Photoelectrochemical measurements*

Photocurrent measurements are carried out in a home-made quartz cell with a three-electrode configuration. A Pt electrode serves as the counter electrodes, an Ag/AgCl (KCl saturated) electrode acts as the reference electrode and the working electrodes is made with the prepared non-fluorinated/fluorinated/defluorinated samples. All the measures are realized in a 0.5 M of Na₂SO₄ electrolyte. The electrodes are prepared as follows: a solution of approximately 1g of 5 w.% of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) is stirred for at least 5 hours. Then, 20 w.% of the catalyst is added and the mixture is stirred overnight, if the mixture is too viscous 2 mL of NMP is added. 50 µL of the solution is then cast onto the conductive side of a fluoride-doped tin oxide (FTO) substrate, covering an area of 1.5 cm² and heated at 150°C for one night. Electrochemical Impedance Spectroscopy (EIS) is measured in the frequency range from 5 MHz to 1 mHz with a perturbation of 50 mV. Chronoamperometry is performed with a 30 sec on-off light cycle at 1.3 V with a 315 W lamp.

All the reagents were purchased from Sigma Aldrich. PVDF average molar mass is 534.000 (GPC), d 1.740. Anhydrous NMP is pure at 99.5 %.
### 3. Results and discussion

Photocatalysis is strongly influenced by the morphology, size, pores (size and shape) of the semi-conductor particles, the generation of active species and so on. In this article it will be presented how the photocatalytic performances have evolved according to identified parameters such as acidity, presence of fluorine atoms or structural changes, and how these parameters have changed with fluorination. The fluorinated graphitic carbon nitride will be referred to as g-C₃N₄-F.

#### 3.1 Fluorination

Macroscopic and microscopic changes are visible after the fluorination of g-C₃N₄. At the macroscopic level, the powder originally composed of thin light-yellow grains become slightly pasty if milled and the powder consists of heterogeneous size deep yellow grains. It is observed for g-C₃N₄-F that the mass increases by 40% after the fluorination, probably indicating the incorporation of fluorine atoms into the structure.

The microscopic structural changes are shown on X-ray patterns in **Figure 1**.a. The X-ray diffractogram of g-C₃N₄ exhibits two characteristic peaks namely one at 12.84°, (100) plane, corresponding to the in-plane organization and the distance between the tri-s-triazine units and another at 27.54°, (002) plane, corresponding to the interlayer stacking of conjugated aromatic heterocycles C-N.⁴⁵ As shown in **Figure 1**.a, the intensity of the peak of the (002) plane decreases with fluorination indicating perturbations on the lamellar structure, which is similar to the Br-doping of g-C₃N₄ which also induces the decrease of the intensity of this peak.⁴⁶ The peak at 12.84° disappears indicating less organization in the plane.

The changes are also characterized with infra-red spectroscopy (Figure 1.b) and Raman spectroscopy (Figure 1.c). The main active vibration mode in IR of g-C₃N₄ is located at 801 cm⁻¹, corresponding to the breathing mode of the triazine units. Peaks between 900 cm⁻¹ and
1700 cm⁻¹ correspond to the C-N and C=N bonds of the cycles.⁴⁷ Here, the peak corresponding to the breathing mode of the triazine units is located at 808 cm⁻¹ for g-C₃N₄ and at 803 cm⁻¹ for g-C₃N₄-F, showing clear a red shift with fluorination indicating a slight decrease of the strength of bonds in the ring. The major change in the spectra is the appearance of a large band at 1200 cm⁻¹ characteristic of the C-F bond proving the efficiency of the fluorination process (Figure 1.b).⁴⁸ The Raman active modes of graphitic carbon nitride are located between 400 cm⁻¹ and 1300 cm⁻¹ related to the vibration modes of the C-N heterocycle of the structure (Figure 1.c). It is admitted that the intensity ratios of the peaks located at 543 cm⁻¹ and 479 cm⁻¹ (I<sub>543</sub>/I<sub>479</sub>), appearing with the layer-layer interaction, and at 751 cm⁻¹ and 705 cm⁻¹ (I<sub>751</sub>/I<sub>705</sub>), appearing with the deformation of the layers, could provide information about the number of layers. Both ratios are slightly increasing from 0.72 to 0.79 for I<sub>751</sub>/I<sub>705</sub> and from 0.79 to 0.85 for I<sub>543</sub>/I<sub>479</sub> respectively with fluorination, indicating a decrease of the number of layers, as seen for ultrathin carbon nitride obtained by sulfuric acid intercalation and liquid exfoliation, I<sub>543</sub>/I<sub>479</sub> = 0.8 correspond to 2 layers and I<sub>751</sub>/I<sub>705</sub> = 1 correspond to 1 layer.⁴⁹
<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) XRD patterns of pristine g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF presenting two peaks for the (100) and the (002) planes (b) IR spectra of g-C₃N₄ with the appearance of the C-F bond around 1000 cm⁻¹ for g-C₃N₄-F and disappearance of the C-F bond after defluorination (visible with the purple highlight) (c) Raman spectra of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF with different intensities of the characteristic peaks with fluorination (identified by the purple highlight) and (d) the specific surface obtained with the BET methods for g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF with the values of the S<sub>BET</sub> given in the inserted table.</figcaption></figure>

As evidenced by various characterization techniques, fluorination induces changes in the structure of graphitic carbon nitride. The formation of C-F bonds has been identified and the
solid-gas reaction with F₂ causes some structural disorganization and a decrease of the number of layers.

It can be highlighted from TEM images that pristine g-C₃N₄ is composed of thick well-shape rectangular sheets between 3 µm² and 45 µm² (Figure 2.a), while g-C₃N₄-F presents smaller sheets (some are less than 0.2 µm²), holes in the structure, damaged edges and a certain exfoliation with thinner sheets (Figure 2.b). The exfoliation observed previously in XRD and Raman analyses is also confirmed by TEM. However, it appears that the specific surface measured with the BET methods is smaller for g-C₃N₄-F than for g-C₃N₄. Indeed, the specific surface decreases from 63 m²/g to 7.2 m²/g with fluorination, which has been seen only once by Ming *et al* with the fluorination of g-C₃N₄ via NH₄F because of a morphology change.³³

<figure><img src="image_3.png" /><figcaption>**Figure 2.** TEM images of (a) non-fluorinated pristine g-C₃N₄ (b) damaged g-C₃N₄-F, the red circle representing where the zoom is taken of the image below.</figcaption></figure>

Finally, to quantify the fluorine content in the materials, XPS measurements are carried on. The survey spectra of the g-C₃N₄ and g-C₃N₄-F are given in **Figure 3.a**. It can clearly be highlighted the presence of F atoms on the surface of g-C₃N₄-F by the detection of the F1s peak at around
686 eV. The high resolution F1s spectrum presents a strong contribution at around 686.4 eV characteristic of the C-F covalent bond, which is in good accordance with the IR spectra presented in (Figure 1.b). The contribution at 685 eV corresponds to the semi-ionic bond of C-F (Figure 3.b). This information proves the efficient fluorination process of graphitic carbon nitride. For the C1s spectrum, the first three contributions are not considered for the calculation of fluorine content as they are assumed to be due to environmental pollution and to the carbon tape used for the analysis. Contributions at 286.4 eV, 287.2 eV and 288.6 eV, in the Figure 3.c, are linked to the g-C3N4 structure, corresponding to C-N-H (or C-O), C=O and N-C=N (sp2) bonds for pristine g-C3N4, respectively. For g-C3N4-F, the contributions presented before shift at 286.3 eV, 287.1 and 288.5 eV, indicating a very slight decrease in bonding strength of C-N-H (or C-O), C=O and N-C=N (sp2), respectively. Other contributions appear proving that fluorine atoms are preferentially bonded to carbon atoms in the structure, with a peak around 289.8 eV synonym of a N-C-F bond. Then, contributions are found at 291.7 eV and 293 eV for F*N-C-F, indicating that surrounding fluorine atoms influence the N-C-F bonding and the N-C-F2 bond, respectively (Figure 3.d). For the N 1s spectrum (Figure 3.e), three contributions were attributed to the g-C3N4 structure with the C-N=C (sp2), N-(C)3, C-N-H (from the uncondensed edges) at 398.5 eV, 399.6 eV and 400.9 eV for pristine g-C3N4, respectively. After fluorination, the peaks are shifted to 398.9 eV, 400 eV and 401.4 eV for C-N=C (sp2), N-(C)3 and C-N-H respectively. These shifts correspond to the decrease of the electronic density around nitrogen atom because of the high electronegativity of fluorine and to the distortion of the sheets. Another contribution also appears at 402.7 eV corresponding to the N-C-F bond (Figure 3.f). It is possible to identify that nitrogen vacancies appear with the fluorination of g-C3N4 because a decrease of the area of the peak assigned to the C-N=C is visible. Moreover, the ratio of the areas between the C-N=C and N-(C)3 peaks changes from 4.87 to 1.09, indicating the likely formation of nitrogen vacancies in the heptazine units. Due to their high
electronegativity, fluorine atoms, have an extended attraction effect, then the electronic density of the N-C bond is strongly shifted towards the C-F bond.

With the chosen fluorination parameters, 19% of fluorine atoms in the materials has been achieved, which represents one of the highest amounts of fluorine atoms in g-C₃N₄ obtained to date.³⁹ Atomic percentages of C, N, F and O obtained by XPS measurements for g-C₃N₄ and g-C₃N₄-F are detailed in **Table 2**. A certain oxidation of the fluorinated g-C₃N₄-F has occurred as the amount of oxygen atoms as increased sharply. This increase is probably due to the adsorption of atmospheric gas.

<figure><img src="image_4.png" /></figure>

**Figure 3.** XPS spectra of (a) the survey spectra of g-C₃N₄ and g-C₃N₄-F, the appearance of fluorine atom is evidence with the purple line (b) the F 1s spectrum of g-C₃N₄-F (c, d) C 1s and N 1s of g-C₃N₄ and (e and f) C 1s and N 1s of g-C₃N₄-F showing new contributions corresponding to the presence of fluorine atoms near the elements.

To summarize, fluorination induces defects in the structure of g-C₃N₄, including etching, exfoliation with the decrease of the number of layer, the presence of damaged edges and holes
in the structure, and a strong C-F bond. With this fluorination process a high amount of fluorine atom is also achieved in the structure.

### 3.2 Defluorination

In order to study the potential recyclability of graphitic carbon nitride, to understand if the chemical and structural changes observed in the previous part are conversed in the absence of fluorine, and if they have an impact on the photocatalytic processes, a strong reducing agent, hydrazine, is used to induce the defluorination of fluorinated $g-C_3N_4$. This sample will be labelled $g-C_3N_4-DF$. Upon adding hydrazine in an isopropanol solution, there is a notable change of its color from transparent to intense yellow, possibly indicating dispersed fluorine atoms in the solution from $g-C_3N_4-F$. Defluorination is confirmed by XPS measurements, as shown on **Table 2**, as no fluorine is detected at the surface of the defluorinated sample (0 at.%) (**Figure S1**). Though, the oxygen amount in the catalyst tends to increase with defluorination.

**Table 2.** Atomic percentages of C, N, F and O obtained by XPS measurements for the corresponding non-fluorinated, fluorinated and defluorinated materials.

<table><thead><tr><th>(at.%)</th><th>C</th><th>N</th><th>F</th><th>O</th><th>C/N</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub></td><td>39</td><td>55</td><td>0</td><td>6</td><td>0.71</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>-F</td><td>32</td><td>35</td><td>19</td><td>15</td><td>0.91</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>-DF</td><td>31</td><td>36</td><td>0</td><td>33</td><td>0.86</td></tr></tbody></table>

Then, the IR-ATR spectrum indicates the efficiency of this defluorination process as the band at 1 200 cm⁻¹ attributed to the C-F bond disappeared with the hydrazine treatment (**Figure 1.b**). On the X-ray diffractogram, there is no critical changes in the structure after defluorination as both of the characteristic peaks at 12.84° and 27.63° of $g-C_3N_4$ are present with similar intensity as the pristine material (**Figure 1.a**). However, TEM images, in **Figure S2**, revealed that after defluorination, the sheets appeared smaller and thinner than $g-C_3N_4$ and $g-C_3N_4-F$, indicating
structural changes accompanying the loss of fluorine atoms. This result is to correlate to BET analysis which show a slight increase of the surface aera with defluorination as it increases from 7.2 m²/g to 10.3 m²/g for g-C₃N₄-F and g-C₃N₄-DF, respectively (Figure 1.f). This increase is probably du to the slight exfoliation of g-C₃N₄-DF.

To summarize, the efficiency of the defluorination process is proven with the use of hydrazine, indicating a reversibility of the fluorination process. The loss of the C-F is evidence with IR and XPS analysis and no major structural changes are observed according to the fluorinated material. Though, the defluorinated material is different from the pristine one (exfoliated and some defects are present) and by comparing their properties and photoactivity the influence of these structural defects and modifications on the photocatalytic performances can be identified.

### 3.3 Opto-electronics properties

The aim of fluorination is also to induced changes on the opto-electronic properties of the semiconductor, leading to changes in the band gap and in the quantity of photogenerated charges. A decrease of the band gap would lead to more absorption of light in the visible range and more photogenerated charges would lead to more redox reactions at the surface of the photocatalyst. Here, these properties are going to be compared between the pristine, fluorinated and defluorinated g-C₃N₄.

EPR measurements are carried out under light irradiation at the air to assess the number of photogenerated charges. The results indicate an increase in intensity for all three samples when exposed to light (Figure 4.a, b and c). All the samples exhibit only one signal at g = 2.003 which is attributed to the unpaired electrons of the localized π-conjugated structure (i.e in one tri-s-triazine unit). However, when irradiated, the electrons from σ-type bond, which did not participate originally to the formation of the paramagnetic species, can be transferred to the π-conjugated structure, explaining the increase of photogenerated charges.⁵⁰ In these three cases,
more unpaired electrons are generated with light irradiation as the g-factor stays at the same value (

**Table 3**). Specifically, for g-C<sub>3</sub>N<sub>4</sub>-F the intensity increases by 125% (Figure 4.a) compared to 47% for pristine g-C<sub>3</sub>N<sub>4</sub> and 49% for g-C<sub>3</sub>N<sub>4</sub>-DF (Figure 4.b and c). This significant increase for g-C<sub>3</sub>N<sub>4</sub>-F suggests that the changes induced by the fluorination leads to more photogenerated charges, likely due to the presence of defects, like nitrogen vacancies, in the material.<sup>50,51</sup> Indeed, the C/N ratio obtained with XPS analysis (Table 2) has increased with fluorination indicating the presence of nitrogen vacancies in the material. However, the increase of intensity under light irradiation can't be attributed only to the presence of defect, since the defluorinated material, which contains the defect induced by fluorination, shows a similar intensity to the non-fluorinated one. Moreover, after light irradiation, the peaks intensity of each sample decreases at around 20 % for g-C<sub>3</sub>N<sub>4</sub> and g-C<sub>3</sub>N<sub>4</sub> and at 85 % for g-C<sub>3</sub>N<sub>4</sub>-F. Thus, indicating that residual charges remain after light irradiation and that they are more important for the fluorinated compound than for the non-fluorinated and the defluorinated ones.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Effect of light on the EPR signal obtained in the dark (black), during light irradiation (red) and after irradiation (blue) for (a) the g-C<sub>3</sub>N<sub>4</sub>-F, (b) g-C<sub>3</sub>N<sub>4</sub> and (c) g-C<sub>3</sub>N<sub>4</sub>-DF.</figcaption></figure>

<table><caption>**Table 3.** Evolution of the intensity of the EPR signal according to if the samples are irradiated by light or not for the non-fluorinated, fluorinated and defluorinated g-C<sub>3</sub>N<sub>4</sub>.</caption><thead><tr><th>Name</th><th>Under light irradiation</th><th>After light irradiation</th><th>Difference before and after irradiation</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub></td><td>+47%</td><td>-20%</td><td>+17%</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>-F</td><td>+125%</td><td>-17%</td><td>+85%</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>-DF</td><td>+49%</td><td>-18%</td><td>+23%</td></tr></tbody></table>
Additionally, this effect is further supported by photoluminescence measurements, which show a substantial decrease in PL intensity for g-C₃N₄-F and g-C₃N₄-DF compared to g-C₃N₄ (Figure 5.a) along with a red-shift for the g-C₃N₄-F and g-C₃N₄-DF signals. The band at 475 nm originate from the emissions transitions from anti-bonding σ* and π* orbitals to the π state of the nitrogen lone pair (LP) hybrid orbital.⁵²⁻⁵⁴ Thus, indicating a very important decrease of the recombination via radiative emission for g-C₃N₄-F and g-C₃N₄-DF. Indeed, the photogenerated electrons do not recombine anymore, as a radiative emission, which induce the decrease of photoluminescence intensity as no photons are emitted. So, more charges are produced by g-C₃N₄-F when irradiated by light. Then, they recombine more slowly, giving the charges more time to induce photocatalytic reactions, or they undergo another type of emission, that is not radiative and therefore not visible by photoluminescence. For the g-C₃N₄-DF, the same amount of charges are generated under light as pristine g-C₃N₄ but the defluorinated material has a small PL intensity. This suggests a higher rate of charge recombination via radiative emission in the defluorinated material compared to g-C₃N₄-F.

Currently, the impact of fluorination on the band gap is not fully understood, in literature both decreases³⁴ and increases³⁶ in the band gap after fluorination have been reported. Here, under the chosen fluorination conditions (using diluted F₂), we observed a small decrease of the band gap from 2.69 eV for pristine g-C₃N₄ to 2.62 eV for g-C₃N₄-F (Figure 5.b) unlike Sun *et al* who obtained an increase from 2.66 eV to 2.69 eV with fluorination³⁹. The DRS also demonstrate that the band gap is not so impacted by defluorination as it goes from 2.62 to 2.65 for g-C₃N₄-F and defluorinated g-C₃N₄ respectively (Figure 5.b). This suggests that fluorine atoms may induce changes in the band positions. Indeed, the band structure of the g-C₃N₄ and g-C₃N₄-DF are similar compared to the g-C₃N₄-F, as presented in Figure 5.c. The VB was obtained with XPS measurements and the CB was deduced with the energy of the band gap. A clear shift to more positive potential of the VB is observed for the g-C₃N₄-F, probably due to
changes on the electronic density impacting local environment, making g-C₃N₄ more suitable for the oxidation of organic molecules. Nevertheless, the CB position of g-C₃N₄-F appears not to be suitable for photocatalytic production of H₂. Moreover, several studies have shown that nitrogen vacancies induced narrowing of band gap.⁵⁵,⁵⁶

The photogenerated charges decay curves recorded under excitation at 375 nm monitoring the emission at its maximum are gathered in **Figure 5.d**. In order to estimate the relative PL decay times of the photogenerated charges, the photoluminescence decay curves were fitted assuming a tri-exponential law given by the following equation:

$$I(t) = A_1e^{-t/\tau_1} + A_2e^{-t/\tau_2} + A_3e^{-t/\tau_3} \quad (Eq. 1)$$

Where A_n are the amplitudes of each decay component and τ_n represents the lifetime of each decay component. The mean lifetime value of the photogenerated charges is more important for g-C₃N₄-F than for g-C₃N₄ and for g-C₃N₄-DF, with 10.7 ns against 6.5 ns and 3.7 ns for g-C₃N₄-F, g-C₃N₄ and g-C₃N₄-DF respectively, which is significative and contribute to an improvement of the charge separation that could lead to better catalytic properties (Eq. 2).

$$\tau_m = \frac{A_1\tau_1^2 + A_2\tau_2^2 + A_3\tau_3^2}{A_1\tau_1 + A_2\tau_2 + A_3\tau_3} \quad (Eq. 2)$$

This result is in good accordance with the EPR and PL presented. This, showing that under light irradiation, more charges are generated, they recombined less by radiative emission and have a longer lifetime for g-C₃N₄-F. This increase observed with fluorination is in good agreement with other work on doped g-C₃N₄, and it can be considered as significant.³⁷,³⁹,⁵⁷,⁵⁸
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Effect of fluorination on (a) the photoluminescence of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF registered at λ<sub>exc</sub>= 350 nm. Effect of fluorination on (b) the band gap of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF, (c) the band diagram in potential of all three samples and (d) PL decay curve recorded under excitation at 375 nm for g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF.</figcaption></figure>

Electrochemical impedance spectroscopy (EIS) is performed under light illumination and Nyquist plot shows that with fluorination a better charge transfer occurs in the fluorinated material than in the pristine one as the arc radius is smaller for g-C₃N₄-F than for g-C₃N₄, also inducing less resistance (Figure 6.a).⁵⁹ The electrons generated under an applied current have less resistivity to transfer to the interface. Moreover, under light irradiation the photocurrent response is more important for g-C₃N₄-F than for g-C₃N₄ and g-C₃N₄-DF (Figure 6.b). Thus,
indicating an increase of the generation of the charge carriers under light with less recombination. g-C₃N₄-DF has a smaller arc circus than g-C₃N₄-F but in the same time no charges are generated under light illumination.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Photoelectrochemical activity of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF studied by (a) electrochemical impedance spectroscopy under light illumination from 5 MHz to 1 mHz at 50 mV of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF and (b) photocurrent responses of g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF at 1.3 V (vs sat. Ag/AgCl) with a 30 sec on-off cycle.</figcaption></figure>

These characteristics collectively suggest that the changes induced by fluorination enhances the generation of photogenerated charges when the photocatalyst is exposed to light irradiation, potentially leading to improved photodegradation performance. On the other hand, defluorination did not impact the molecular structure of g-C₃N₄-F, these defects have been shown to have a minor influence on the photo-physic process, and the band structure and elemental composition are similar to the non-fluorinated g-C₃N₄. Thus, the impact of fluorine atoms is evidence and benefits opto-electronics properties of g-C₃N₄ and it can be evidence that g-C₃N₄ and g-C₃N₄-DF have a similar activity under light irradiation compared to g-C₃N₄-F. These results are in good accordance as the increase of the generation of the charge, evidenced
by EPR, show less recombination, as evidence by PL, and have a longer lifetime as given by decay analysis.

### 3.4 Photocatalysis performances

To evaluate the photocatalytic efficiency of g-C₃N₄-F, two different substrates, used as model pollutants, were tested. First, methyl orange (MO) is used since it is a common dye, representative of numerous colorants used mostly in the textile, food, and paper industries but is also a pH indicator with a transition range from pH 3.1 to pH 4.4, changing from pink to orange.⁶⁰ Then, carbamazepine (CBZ), which is an antiepileptic, neurotropic and psychotropic drug, is also tested. Such compound is found in quantities in water that exceed regulation limits.⁶¹ Moreover, several studies have clearly demonstrated that halogens species can form radicals (Br*, Cl*, Cl₂⁻, Br₂⁻ or ClBr⁻) through photochemical or oxidation reactions able to degrade pollutants.⁶² Within the present work, specific electrodes have shown that fluorine ions are found to be released into water that can, in some way, interfere in the pollutant degradation process. **Figure 7** shows the amount of fluoride ions released in water after 1h of stirring (needed for the homogenization of the solution with the TISAB) for g-C₃N₄-F and g-C₃N₄, clearly showing that around 2.5 mg/L/g of fluoride ions are released in water for g-C₃N₄-F. Moreover, ionic chromatography also proves that fluoride ions are released, as for g-C₃N₄-F, 203 mg/L are obtained. However, the role of fluoride ions in such processes has not been yet extensively explored.
<figure><img src="image_8.png" /></figure>

**Figure 7.** Amount of F⁻ ions leaching in water measured by fluoride ion-selective electrode after 1h of stirring in water fluorine solution (TISAB) for 5.3 mg of g-C₃N₄-F and 5 mg of pristine g-C₃N₄, respectively.

## Methyl Orange degradation

In order to evaluate the photocatalytic efficiency of g-C₃N₄ before and after fluorination, the irradiation was undertaken every 20 min within the range 370 nm and 800 nm using methyl orange (MO) at a concentration of 6.10⁻⁵ M and the various photocatalyst materials g-C₃N₄; g-C₃N₄-F and g-C₃N₄-DF (2.5·10⁻³ g·L⁻¹) in aerated conditions.

It is noteworthy, that an important adsorption of almost 50% is observed after 1h of stirring in the dark when g-C₃N₄-F is added to the MO solution. Indeed, Zeta potential was assessed to further understand the surface charge and stability of the photocatalysts. It is important to note that MO (at pH=6) is negatively charged, with a Zeta potential of -57 mV and we can presume, given the important concentration of fluorine atoms on its surface, that g-C₃N₄-F possesses numerous depopulated adjacent sites, mostly around nitrogen atoms due to electron delocalization, that has been evidenced with XPS analysis, and fluorine's high electronegativity, making them probably highly active for MO adsorption.⁶³ The Zeta potential of g-C₃N₄-F is also negative, -12.2 mV, though the one of pristine g-C₃N₄ is -15.8 mV. Both values are below
30 mV representing slightly unstable particles.⁶⁴ However, the larger difference in zeta potential for MO/g-C₃N₄-F relative to MO/g-C₃N₄ may account for the stronger adsorption observed of MO on g-C₃N₄-F.

The degradation conversion of MO is 19.9% for g-C₃N₄ after 2h of light irradiation. With g-C₃N₄-F, though, the degradation percentage of MO is 92.8%, which is significantly higher than the pristine material (Figure 8.a). On the contrary, for g-C₃N₄-DF, the degradation conversion is 18.4%. So, g-C₃N₄ and g-C₃N₄-DF exhibit nearly identical degradation rates (Figure 8.b), suggesting that the structural changes caused by fluorination and defluorination are not the main factor in the enhanced photodegradation observed in presence of g-C₃N₄-F. Indeed, the absence of fluorine atoms in the photocatalyst leads to lower MO degradation. While the degradation rate of g-C₃N₄-F is more than 7-fold more efficient than g-C₃N₄ and g-C₃N₄-DF.

<figure><img src="image_9.png" /><figcaption>**Figure 8.** (a) effect of the F atoms in the photocatalyst and structural changes on the degradation of MO under light irradiation with a 560 W Xenon lamp with g-C₃N₄, g-C₃N₄-F and g-C₃N₄-</figcaption></figure>
DF registered at 464 nm for g-C₃N₄ and g-C₃N₄-DF and at 504 nm for g-C₃N₄-F every 20 min in aerated conditions (b) the degradation rate of MO associated to (a).

The second key parameter that should be taken into account here is the pH of the solution. When MO solution was added to g-C₃N₄-F, we observed a significant change of the pH. Notably, when the fluorinated photocatalyst is introduced, the pH decreases significantly, causing the solution to shift from orange (pH=6.1) to red (pH=3.09). Such acidification does not occur with the non-fluorinated or defluorinated g-C₃N₄. Further investigation (Figure 9.a) showed that while the strong non-reactive acid, perchloric acid (HClO₄), lowered the pH to 1.35, it had no impact on MO self-degradation under light exposure. In contrast, pristine g-C₃N₄ combined with HClO₄ nearly completely degraded MO within 40 min. This suggests that the synergy between the photocatalyst and a low pH enhances MO degradation.

In order to investigate this hypothesis, MO degradation was evaluated in the presence of fluoride ions from NH₄F, which dissociates into NH₄⁺ and F⁻ in solution, causing only a slight pH decrease from 6.1 to 5.7 (Figure 9.b). Ionic chromatography measurements indicate that the fluoride ion concentration can reached 360.25 mg/L, exceeding the value of 203 mg/L obtained with g-C₃N₄-F. With NH₄F only, the conversion rate increased to 23.7% under the same conditions. When both NH₄F and g-C₃N₄ were added, the conversion percentage rose to 56.7%. Thus, showing the impact of combining F ions and g-C₃N₄ to increase the degradation of MO in a given time.

Another fluorinated salt, KHF₂, is used to introduce both fluoride ions and acidity into the solution (Figure 9.c). The fluoride ion concentration in the solution is similar to that of g-C₃N₄-F, reaching 262.3 mg/L. The results show that after 120 minutes of light irradiation, the degradation of MO is not influenced by the presence of fluoride ions, regardless of their quantity, even in combination with acidity. Specifically, with 3.3 mg or 11.3 mg of KHF₂, the pH decreases from 6.1 to 3.5, yet in both cases, MO photodegradation does not exceed 8%.
Thus, the addition of KHF₂ alone has no significant effect on the degradation process, as doubling the mass of the fluorinated salt does not alter the conversion rate. In contrast, when pristine g-C₃N₄ is added along with 1.1x10⁻² M KHF₂, MO degradation reaches 96% after 120 minutes. Thus, confirming the importance of the presence of F ions along with g-C₃N₄ to improve MO degradation.

All the kinetics constants of the different materials and parameters tested are given **Table 4** and taken after 20 min of light irradiation. After this time, a competition between adsorption, MO degradation and the degradation of the by-product generated occurs.

<table><caption>**Table 4.** Degradation rates of MO for all the parameters tested: F⁻ ions, the acidity and the photocatalysts.</caption><thead><tr><th>Names</th><th>g-C<sub>3</sub>N<sub>4</sub></th><th>g-C<sub>3</sub>N<sub>4</sub>-F</th><th>g-C<sub>3</sub>N<sub>4</sub>-DF</th><th>HClO<sub>4</sub></th><th>HClO<sub>4</sub>+<br>g-C<sub>3</sub>N<sub>4</sub></th><th>NH<sub>4</sub>F</th><th>NH<sub>4</sub>F +<br>g-C<sub>3</sub>N<sub>4</sub></th><th>KHF<sub>2</sub></th><th>KHF<sub>2</sub> +<br>g-C<sub>3</sub>N<sub>4</sub></th></tr></thead><tbody><tr><td>k (min<sup>-1</sup>)</td><td>-0.0037</td><td>-0.0272</td><td>-0.0027</td><td>0.0005</td><td>-0.0988</td><td>-0.0026</td><td>-0.009</td><td>-0.0023</td><td>-0.0176</td></tr></tbody></table>

To further elucidate the role of the inserted F atoms in the photocatalyst in the degradation process of MO, several scavengers were employed, including isopropanol (IPA) and potassium dichromate (K₂Cr₂O₇), to assess the involvement of OH⁻ and O₂⁻ radicals, respectively. As shown in **Figure 9.d**, no significant difference is observed in MO degradation when the degradation with g-C₃N₄-F alone is compared to that with g-C₃N₄-F + K₂Cr₂O₇, with only a slight decrease from 92.8% to 86.4% after 120 min irradiation. This suggests that O₂⁻ does not play a major role in this photodegradation process. The increase in the conversion with g-C₃N₄ + K₂Cr₂O₇ could be explained by the trapping of O₂⁻, which ensures that holes remain available to induce OH⁻ radicals because of the reduced recombination phenomenon. In contrast, the addition of isopropanol (IPA) resulted in a significant decrease in degradation efficiency. The conversion percentage dropped from 92.8% to 41.7% after 120 min irradiation. This decrease
is attributed to the trapping of OH⁻ radicals during irradiation, with the remaining 41.7% possibly due to the presence of fluoride ions in the solution and adsorption effects. A slight decrease in MO degradation efficiency is also observed with g-C₃N₄ + IPA, indicating that g-C₃N₄ also generates OH⁻ radicals. Finally, when triethanolamine (TEA), known as a hole scavenger is combined with g-C₃N₄-F, the conversion percentage decreased to 27.1%, confirming that OH⁻ radicals, induced by the presence of holes, are the main active species. Moreover, TEA combined with g-C₃N₄ shows a slight decrease of the degradation of MO, as the degradation of MO decrease from 19.9% to 18.9%. This result can be attributed to the fact that only a small number of holes are produced for g-C₃N₄. The use of scavengers has shown that OH⁻ radicals and holes are the main active species for degradation while O₂⁻ does not seems to play an important role in the process. The enhancement of the degradation could likely be due to the drop of the valence band, for g-C₃N₄-F, promoting the generation of OH⁻, which appears to be the main species for MO degradation.⁶⁵
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9.** Effect of (a) the acidity (b) the F⁻ ions through NH₄F (c) the F⁻ ions through KHF₂ combine with acidity and (d) the scavengers on the degradation of MO under light irradiation of a 560 W Xenon lamp, all taken every 20 min under aerated conditions.</figcaption></figure>

Furthermore, several degradation cycles were performed to assess the reusability of the g-C₃N₄-F photocatalyst. Three cycles are performed, as shown on **Figure 10**, with a slight decrease in the degradation of MO observed for the third cycle. A reduction in the acidification properties of g-C₃N₄-F is, indeed, observed for the third cycle, as the pH increases compared to previous cycles. This is to link to the ionic chromatography results which is performed on solution after each cycle with catalyst particles removed by centrifugation. The results show a decrease in the
concentration of F⁻ ions released into the MO solution as the number of cycles increases. It drops from 203 mg/L for the first cycle to 95 mg/L in the second cycle, and further to 39 mg/L in the third cycle. These results are consistent with XPS measurements showing a loss of fluorine after MO degradation. From a cycle to the next one, less fluoride ions are released in water, as the F content decreases in g-C₃N₄-F, so the pH of MO decreased less with the number of cycle when g-C₃N₄-F is present in the solution. The kinetics constants of the degradation are also smaller as the number of cycle increases. With the exception of the first cycle which shows a smaller kinetic as the second cycle due to a strong adsorption (90%) (Table 5). However, with these experiments, we can postulate that the ideal pH medium is equal or below 4 and that the quantity of F ions in the solution must be more at least of 90 mg/L to achieve more than 90% degradation of MO. Thus, confirming the correlation between the presence of F atoms on the photocatalyst and the enhanced acidity, both of which are essential for efficient MO degradation.

After one cycle of the photodegradation process, the photocatalyst is recovered, and interesting features emerge, notably, the C-F bond around 1 200 cm⁻¹ in infrared spectrum becomes less intense and a broad one centered at 3100 cm⁻¹ appears (Figure S3). This is characteristics of the reduction of the azo group onto an amine or the presence of hydroxyl or carboxyl groups linked to the oxidation of g-C₃N₄ after photodegradation.⁶⁶ The disappearance of the C-F bond is consistent with the XPS analysis which shows that the fluorine content at the surface has decreased from 19 at.% to 0 at.% for g-C₃N₄-F before and after one cycle. That confirms the release of the F ions in water during the experiment (as presented in Figure 7 and Figure 10). The XRD diffractogram also reveals that the lamellar structure of g-C₃N₄ is disrupted after 3h of vigorous stirring, as evidenced by the significant decrease in the intensity of the peak at 27° for the (002) plane after the photodegradation (Figure S4).
<figure><img src="image_14.png" /></figure>

**Figure 10.** Evolution of the degradation efficiency of the MO by g-C₃N₄-F related to the amount of fluoride ions released in water and to the pH of the solution after 2h of light irradiation at 560 W with a Xenon lamp in aerated conditions. The powder is washed 3 times with water between each cycle.

<table><caption>**Table 5.** Degradation rates of MO during three cycles with g-C₃N₄-F.</caption><thead><tr><th>Cycle</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>k (min<sup>-1</sup>)</td><td>-0.0297</td><td>-0.0368</td><td>-0.0086</td></tr></tbody></table>

Based on this, one can conclude that acidity, F⁻ ions in the solution, and the photocatalyst play a crucial role in the degradation process of MO with g-C₃N₄-F. It is the interplay of these three factors that significantly enhances the degradation efficiency. The decrease of the pH, induced by the presence of F⁻ ions in solution, increases OH⁻ formation and the adsorption of MO on the surface of g-C₃N₄. The fluorinated g-C₃N₄ also contributes to the degradation of the pollutant with the generation of numerous charges under light irradiation. The possible mechanism for the MO degradation could be as follows, with the acidity and the released of F⁻ ions, MO is adsorbed on g-C₃N₄ surface, so the important quantity of produced OH⁻ radicals are closed to MO so that they can rapidly break the -N=N- bond of the molecule, to form -NH-NH- colorless
amine, making it less pollutant.⁶⁶ Moreover, the structural defects induced by fluorination appear to have few influence on the photocatalytic process.

## Carbamazepine degradation

The second model pollutant tested to evaluate the catalytic performances of g-C₃N₄ is carbamazepine (CBZ). The irradiation of the CBZ solution at a concentration of 6.10⁻⁵ M was undertaken every 20 min within the range 370 nm and 800 nm using various photocatalyst materials g-C₃N₄; g-C₃N₄-F and g-C₃N₄-DF (2.5·10⁻³ g/L) in aerated conditions. The degradation rates of CBZ are slightly lower than for MO. It reached 25% for g-C₃N₄. The, with the fluorinated g-C₃N₄-F, CBZ degradation rates increased to an average of 35%. Furthermore, g-C₃N₄-DF exhibited no catalytic activity. In addition, CBZ showed only a 15% adsorption after 1 hour of stirring in the dark in the presence of g-C₃N₄-F, this adsorption is not observable for g-C₃N₄ and g-C₃N₄-DF (Figure 11.a). Indeed, the Zeta potential of CBZ strongly depends on the formulation, for example it ranges from -21 to -38 mV for CBZ solid lipid nanoparticles, which are close values to the Zeta potential of g-C₃N₄ and g-C₃N₄-F.⁶⁷ Thus inducing that no strong adsorption is observed.

Like with MO, the influence of F⁻ ions is studied with two fluorinated salts, NH₄F (3·10⁻² M) and KHF₂ (1·10⁻² M), the last one bringing acidity at the same time (Figure 11.b). The pH doesn't change after 30 min of stirring but increases from 6 to 8 after the 2h light irradiation with NH₄F. On the other hand, it decreases from 6 to 3 with KHF₂. Both salts alone in CBZ solution, do not show any evidence of degradation. However, when combining g-C₃N₄ and both of the fluorinated salts, CBZ degradation is enhanced. Indeed, the degradation increases from 25% to 63% and to 64% with g-C₃N₄ + NH₄F and g-C₃N₄ + KHF₂, respectively (Figure 11.b).
Thus, showing a real impact of the presence of the F⁻ ions in the solution along with the presence of g-C₃N₄ for the degradation of CBZ. The kinetics constants are higher for g-C₃N₄ + NH₄F
and g-C₃N₄ + KHF₂, -0.0084 and -0.0086 respectively, than for g-C₃N₄ alone or the salts alone (Table 6). Here, though, the degradation is even superior than the one of g-C₃N₄-F.

Then, the same scavengers used for MO degradation were applied to determine the active species in CBZ degradation and to identify which species are predominantly produced. Figure 11.c illustrates that for the three scavengers used the degradation efficiency of CBZ decreases slightly in the presence of g-C₃N₄-F dropping from 35% to 22%, 26% and 30% for IPA (6.10⁻² M), TEA (3.10⁻² M) and K₂Cr₂O₇ (8.10⁻⁴ M), respectively. Thus, indicating that OH⁻ radicals seem to be active for CBZ degradation as well as holes. O₂⁻, on the other side, play a minor role in CBZ degradation. For the degradation of CBZ with pristine g-C₃N₄ and the scavengers, an important decrease of the degradation conversion is shown with TEA (h⁺ trapper) from 25% of degradation to 5%. While in the presence of IPA, no changes are observed. For K₂Cr₂O₇, however, the degradation is enhanced, the same behavior was observed for MO degradation, so we will postulate the same action of O₂⁻ trapping, which ensures that holes remain available to induce OH⁻ radicals. This last result indicates that OH⁻ and h⁺ are the main active species in the degradation of CBZ for both g-C₃N₄-F and g-C₃N₄, which is coherent with previous reported data.<sup>68,69</sup>

Finally, three cycles of degradation are realized in order to test the reusability of the fluorinated powder. Results evidence that the degradation rate efficiency decreases with the number of cycles. Going from the first at 42% degradation of CBZ to 18% for the second one and to 13% for the third one. Moreover, an increase of the pH is observed with the cycle. The initial pH of 6 dropped at 4 for the first cycle then increases at 5 and 6 for cycle 2 and 3, respectively. With the pH increasing, a decrease of the quantity of fluoride ions released in water with the number of cycles is also obtained, which is in good agreement with the results obtained for MO (Figure 11.d). Table 7 clearly evidence a decrease of the degradation rate with the cycles. This cycling test as evidence an important link of the pH, the concentration of fluoride ions and the catalyst
for CBZ degradation. The most adapted parameters for this degradation are a low pH (≤ 4) and a quantity of F⁻ ions superior to 100 mg/L combine with g-C₃N₄.

<figure><img src="image_15.png" /></figure>

**Figure 11.** (a) Degradation of CBZ under light illumination induced by g-C₃N₄, g-C₃N₄-F and g-C₃N₄-DF taken every 20 min after 1h of stirring in the dark (b) effect of F⁻ ions and acidity on the degradation of CBZ taken every 20 min after 1h of stirring in the dark (c) effect of the degradation with scavengers under light irradiation of a 560 W Xenon lamp (d) evolution of the efficiency of the MO degradation related to the amount of fluoride ion released in water and to the pH of the solution after 120 min of light irradiation, the powder is washed 3 times with water between each cycle. All the experiments are realized under aerated conditions.

**Table 6.** Degradation rates of CBZ for all the parameters tested: F⁻ ions, the acidity and the photocatalysts.
<table><caption>**Table 7.** Degradation rates of CBZ during three cycles with g-C<sub>3</sub>N<sub>4</sub>-F.</caption><thead><tr><th>Names</th><th>g-C<sub>3</sub>N<sub>4</sub></th><th>g-C<sub>3</sub>N<sub>4</sub>-F</th><th>g-C<sub>3</sub>N<sub>4</sub>-DF</th><th>NH<sub>4</sub>F</th><th>NH<sub>4</sub>F + g-C<sub>3</sub>N<sub>4</sub></th><th>KHF<sub>2</sub></th><th>KHF<sub>2</sub> + g-C<sub>3</sub>N<sub>4</sub></th></tr></thead><tbody><tr><td>k (min<sup>-1</sup>)</td><td>-0.0032</td><td>-0.0043</td><td>0.001</td><td>0.001</td><td>-0.0084</td><td>0.0018</td><td>-0.0086</td></tr></tbody></table>

<table><thead><tr><th>Cycle</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>k (min<sup>-1</sup>)</td><td>-0.0036</td><td>-0.002</td><td>-0.0007</td></tr></tbody></table>

Finally, our objective is to identified the degradation products of CBZ after the irradiation, several works have already studied them. Here, we would like to identified if unknown product can be obtained with g-C<sub>3</sub>N<sub>4</sub>-F which as high amount of fluorine and important structural changes. The first think to note is the presence of two main degradation products, identified by LC-MS, in both cases, pristine and fluorinated g-C<sub>3</sub>N<sub>4</sub>, with a m/z [M+H<sup>+</sup>] of 251 and 253. These correspond to the epoxide and hydroxyl forms of CBZ respectively. The epoxide form corresponds to the active metabolite of urine and serum of carbamazepine. Even if few papers have worked on the toxicity of the transformation products, the epoxide form is considered less toxic than CBZ.<sup>70</sup> According to the safety data sheet, the 10,11-Dihydro-10-hydroxycarbamazepine (CAS n°29331-92-8), this molecule have toxic long term effect on aquatic life. Though, it is worth highlighting that different generated by-products are obtained after 20% conversion of CBZ depending on whether g-C<sub>3</sub>N<sub>4</sub> is fluorinated or not. The chemical structures of the elucidation by-products are provided in **Table 8**. Concerning acridone molecule, obtained by the degradation of CBZ by pristine g-C<sub>3</sub>N<sub>4</sub>, its toxicity in comparison with the CBZ one is not clear. Some studies show divergent results.<sup>71,72</sup> The 11-Keto Oxcarbazepine (CAS n°537693-29-1), generated by the CBZ degradation with g-C<sub>3</sub>N<sub>4</sub>-F, is only irritating or toxic if ingested. Our results could suggest different degradation mechanisms
between CBZ degraded by g-C₃N₄-F and by pristine g-C₃N₄. Moreover, the presence of fluorine atoms is not detected on the structure of the degradation products. That means that fluoride ions released in the solution do not bind with the compounds produced.

<table><caption>**Table 8.** The different degradation products obtained by LC-MS analysis for (a) g-C₃N₄ and for (b) g-C₃N₄-F.</caption><thead><tr><th>Compound (name or abbreviation in the study)</th><th>M<sub>w</sub></th><th>ESI(+) MS m/z</th><th>Formula</th><th>Structure</th></tr></thead><tbody><tr><td>Carbamazepine</td><td>236</td><td>237</td><td>C<sub>15</sub>H<sub>12</sub>N<sub>2</sub>O</td><td>[Image of structure]</td></tr><tr><td>I<br>10,11-epoxycarbamazepine<sup>73</sup></td><td>250</td><td>251</td><td>C<sub>15</sub>H<sub>10</sub>N<sub>2</sub>O<sub>2</sub></td><td>[Image of structure]</td></tr><tr><td>II<br>Hydroxycarbamazepine<sup>73</sup></td><td>252</td><td>253</td><td>C<sub>15</sub>H<sub>12</sub>N<sub>2</sub>O<sub>2</sub></td><td>[Image of structure]</td></tr><tr><td>(a) III<br>Acridone<sup>73</sup></td><td>195</td><td>196</td><td>C<sub>13</sub>H<sub>9</sub>NO</td><td>[Image of structure]</td></tr><tr><td>(a) IV<sup>72</sup></td><td>223</td><td>224</td><td>C<sub>14</sub>H<sub>9</sub>NO<sub>2</sub></td><td>[Image of structure]</td></tr></tbody></table>
<table><tr><td>(a) V</td><td>286</td><td>287</td><td>C<sub>15</sub>H<sub>14</sub>N<sub>2</sub>O<sub>4</sub></td><td></td></tr><tr><td>(b) VI</td><td>128</td><td>129</td><td>C<sub>7</sub>H<sub>14</sub>NO</td><td></td></tr><tr><td>(b) VII</td><td>145</td><td>146</td><td>C<sub>7</sub>H<sub>15</sub>NO<sub>2</sub></td><td></td></tr><tr><td>(b) VIII</td><td>272</td><td>273</td><td>C<sub>15</sub>H<sub>15</sub>NO<sub>4</sub></td><td></td></tr><tr><td>(b) IX<br>11-Keto Oxcarbazepine<sup>73</sup></td><td>266</td><td>267</td><td>C<sub>15</sub>H<sub>10</sub>N<sub>2</sub>O<sub>3</sub></td><td></td></tr></table>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

All these results indicate that CBZ degradation is influenced by several parameters like the presence of the F⁻ ions, the pH and the catalyst used. Indeed, in order to obtain high degradation rate, we have evidence that a low pH, an important quantity of fluoride ions in the solution and g-C<sub>3</sub>N<sub>4</sub> are needed. It can also be evidence that it is possible to select the degradation products needed or the avoid their formation by selecting the pristine or the fluorinated g-C<sub>3</sub>N<sub>4</sub> for the degradation pathways not seem to be the same.

## 4. Conclusion

To conclude, highly fluorinated graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>-F) has demonstrated enhanced photocatalytic activity in the degradation of MO and CBZ. The changes induced by fluorination significantly improves the photoactivity by increasing the generation of photogenerated charges upon light exposure. This enhancement is accompanied by a notable reduction in photoluminescence, indicating slower charge recombination rates, which allows for more
efficient charge transfer to the surface, as evidence with Nyquist plot, and subsequent
production of hydroxide radicals. Moreover, under light irradiation more charges are generated.
These improvements are not obtained for g-C₃N₄-DF, indicating that the structural changes
induced by fluorination do not influence strongly the photocatalytic efficiency. Important
increase of the degradation of MO is obtained with g-C₃N₄-F, as the MO degradation increases
from 26 % to 96 % for g-C₃N₄ and g-C₃N₄-F respectively, with a decrease of the pH from 6.1
to 3.1. The OH⁻ were identified as the main active species for MO degradation. Then, for CBZ,
the holes, that have a more reducing power (i.e., a lower BV) in the fluorinated g-C₃N₄
compared to the pristine and defluorinated g-C₃N₄, and OH⁻ also seem to be the active species
for CBZ degradation. The degradation is enhanced by 10 % from 25% to 35 % with g-C₃N₄
and g-C₃N₄-F respectively, while g-C₃N₄-DF do not degrade CBZ. The identification of several
CBZ transformation products was also evidence and results show that a selection of wanted or
unwanted molecule can be made by using or fluorinated or non-fluorinated g-C₃N₄. The
observed improvements in photocatalytic performance are attributed to the high fluorine
content in g-C₃N₄-F, which induces structural changes such as the formation of holes, etching,
deformation, the introduction of C-F bonds but also to a down shift of the valence band making
the holes more oxidizing and smaller band gap inducing more light harvesting. Ongoing studies
are investigating the efficiency of g-C₃N₄-F in degrading various other pollutant molecules,
aiming to comprehensively assess its performance across different environmental contexts.

ACKNOWLEDGEMENT

This work was financially supported by Clermont Auvergne Metropole and I-site CAP 2025,
especially by the International Research Center “Innovation Transportation and Production
Systems” of the I-SITE CAP 20-25 but also by the “RESEAUX” project of FR Eau. The authors
would like to thank Lawrence Frezet for his help on EPR, Jonathan Colombet for the TEM
experiments, Vanessa Prevot for her help on the Zeta sizer and Martin Leremboure for LC-MS analysis. Finally, we thank Conchi Ania for her advices for electrodes preparation.

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