# Porous nitrogen-doped TiO2/graphene oxide derived from H2N-MIL-125(Ti)/graphene oxide composites as highly efficient visible light active photocatalysts for the degradation of dyes and carbamazepine

Askar Bakhadur, Halima Alem, Vincent Carré, Thomas Gries, Lavinia Balan, Jean-Louis Cantin, Ghouti Medjahdi, Zhandos Orazov, Bolat Uralbekov, Raphaël Schneider

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Askar Bakhadur, Halima Alem, Vincent Carré, Thomas Gries, Lavinia Balan, et al. Porous nitrogen-doped TiO2/graphene oxide derived from H2N-MIL-125(Ti)/graphene oxide composites as highly efficient visible light active photocatalysts for the degradation of dyes and carbamazepine. Applied Surface Science, 2025, 714, pp.164466. 10.1016/j.apsusc.2025.164466. hal-05231793

HAL Id: hal-05231793

https://hal.univ-lorraine.fr/hal-05231793v1

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# Porous nitrogen-doped TiO₂/graphene oxide derived from H₂N-MIL-125 (Ti)/graphene oxide composites as highly efficient visible light active photocatalysts for the degradation of dyes and carbamazepine

Askar Bakhadur<sup>a,b D</sup>, Halima Alem<sup>c</sup>, Vincent Carré<sup>d D</sup>, Thomas Gries<sup>c D</sup>, Lavinia Balan<sup>e D</sup>, Jean-Louis Cantin<sup>f D</sup>, Ghouti Medjahdi<sup>c</sup>, Zhandos Orazov<sup>b D</sup>, Bolat Uralbekov<sup>b</sup>, Raphaël Schneider<sup>a,∗ D</sup>

ARTICLE INFO

Keywords:

H₂N-MIL-125(Ti)

N-doped TiO₂

Graphene oxide

Heterostructured photocatalysts

Photocatalytic degradation

ABSTRACT

Due to the increasing release of pollutants in the environment, the development of efficient photocatalysts has become a major issue for water decontamination. Herein, heterostructured photocatalysts associating N-doped TiO₂ (N-TiO₂) and graphene oxide (GO) were prepared by thermolysis of the H₂N-MIL-125(Ti) MOF combined with GO. The high porosity of H₂N-MIL-125(Ti) is partially transferred to N-TiO₂ by calcination at 450 °C allowing the synthesis of porous N-TiO₂/GO catalysts with a specific surface area up to 131 m²/g. TEM shows that N-TiO₂/GO catalysts are composed of ca. 13 nm-sized N-TiO₂ particles deposited on GO sheets. GO not only allows to improve the light absorption in the visible range but also to enhance the charge carrier separation as demonstrated by photoluminescence and (photo)electrochemical measurements. The highest photocatalytic activity was reached for N-TiO₂ combined with 30 wt% GO and the obtained catalyst exhibits high activity for the degradation of dyes (Orange II and Eosin Y) and of the Carbamazepine drug under visible light irradiation. Radical scavenging experiments show that superoxide radicals O₂⁻, holes h⁺ and singlet oxygen ¹O₂ play a major role in the photodegradation process. The N-TiO₂/GO(30) photocatalyst also exhibits high stability and reusability and therefore has a high potential for real environmental applications.

1. Introduction

The intensification of industrial and agricultural activities around the world has led to the release of significant amounts of various pollutants into water effluents and thus to the pollution of aquatic environments. Access to drinking water has become a major issue in the 21st century. Among advanced oxidation processes (AOPs), heterogeneous photocatalysis has gained high interest due to its low cost and high efficiency for the degradation of pollutants in aqueous or gaseous phase [1,2]. Among inorganic photocatalysts, titania (TiO₂) is one of the most widely used due to its excellent performance, low cost, weak toxicity, high stability and availability [3–5]. The energy bandgap of anatase

TiO₂ (3.20 eV, 387 nm) restricts its photoactivation to the UV range which represents only 4 % of the solar spectrum and limits its use as photocatalyst [6]. Moreover, electrons photogenerated in the conduction band and holes in the valence band easily recombine before reaching the surface of the photocatalyst, which leads to a decrease in catalyst performance. In recent years, different strategies have been developed in order to extend the TiO₂ photoresponse to the visible range and limit the recombination of charge carriers.

To improve the optical absorption in the visible region, TiO₂ can be doped with transition metal ions like Fe³⁺, Mn²⁺, Cu²⁺ or Co²⁺ [7–9], with nonmetal elements like S, N or P [9,10] or coupled with low bandgap semiconductors [11–13] to engineer heterostructured

<footnote><sup>a</sup> Université de Lorraine, CNRS, LRGP, F-54000 Nancy, France</footnote>

<footnote><sup>b</sup> Al-Farabi Kazakh National University, Center of Physical-Chemical Methods of Research and Analysis, Al-Farabi Av., 71, Almaty 050040, Kazakhstan</footnote>

<footnote><sup>c</sup> Université de Lorraine, CNRS, LIL, F-54000 Nancy, France</footnote>

<footnote><sup>d</sup> Université de Lorraine, LCP-A2MC, F-57000 Metz, France</footnote>

<footnote><sup>e</sup> CNRS, Université d'Orléans, CEMTHI, 1 Av. de la Recherche Scientifique, Orléans, France</footnote>

<footnote><sup>f</sup> Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, UMR7588, F-75005 Paris, France</footnote>

<footnote>* Corresponding author.
E-mail address: raphael.schneider@univ-lorraine.fr (R. Schneider).</footnote>
photocatalysts. Recently, N-doped TiO₂ (N-TiO₂) has been widely investigated because this photocatalyst responds to visible light [14]. Theoretical calculations indicate an overlap between the p orbitals of N and O 2p orbitals of TiO₂, which improve the generation of electron-hole pairs under visible light irradiation [15]. Many processes have been developed for the preparation of N-TiO₂, of which nitridation of TiO₂ via hydrothermal or solvothermal methods using urea, ammonium hydroxide, triethylamine or nitric acid as N source [16–18] and by a sol gel process in which the source of N is added to the sol [19–21] are the most commonly used. Other methods such as annealing TiO₂ under NH₃ flow [22], the combination of reactive magnetron sputtering and ion implantation [23] and the calcination of Ti₃C₂ MXene and NH₄Cl mixtures [24] have also been reported. All these methods allow the efficient preparation of N-TiO₂ but the porosity of photocatalysts is generally low while it strongly influences their performance by the increase of active surface sites per unit catalyst mass.

The thermolysis of the H₂N-MIL-125(Ti) metal organic framework (MOF) has been demonstrated to be of high potential for the production of porous N-doped TiO₂ with good photocatalytic activity [25–32]. The calcination process induces a partial collapse of the H₂N-MIL-125(Ti) skeleton which leads to the disappearance of a large number of micro-pores and simultaneously new mesopores are generated. Despite this increase in porosity compared to N-TiO₂ prepared by conventional methods, the photocatalytic activity of H₂N-MIL-125(Ti)-derived N-TiO₂ remains too low for numerous photocatalytic applications due to the fast recombination of charge carriers. This barrier could be overcome by the association of N-TiO₂ with graphene-derivatives due to their high electronic mobility, thermal conductivity, electrical conductivity and large surface area [33,34]. Moreover, a two-dimensional carbon nanomaterial like graphene oxide (GO) not only allows to decrease or even suppress electron-hole-pair recombinations but also improves the visible light absorption of N-TiO₂/GO composites [33,34]. Recently, the preparation of catalysts combining N-TiO₂ with GO or reduced GO (rGO) has been the subject of many studies and the interest of N-TiO₂/GO or N-TiO₂/rGO heterostructures for the degradation of pollutants, CO₂ reduction or hydrogen photoproduction has been demonstrated [16,35–42]. These photocatalysts are most often prepared by self-assembly of N-TiO₂ with GO under hydrothermal conditions, by sonication [16,36,38,39,42] or by adding GO during the preparation of N-TiO₂ via a sol–gel process [35,37,40].

Herein, a facile synthesis of N-TiO₂/GO photocatalysts by thermolysis of H₂N-MIL-125(Ti)/GO composites is described. This process allows the efficient hybridization of N-TiO₂ with GO at a mild temperature of 450 °C as well as the preparation of porous photocatalysts with a S<sub>BET</sub> specific surface area up to 131 m²/g that provides a high number of active sites to boost the mass transport. Photocatalytic experiments demonstrate that the catalyst associating N-TiO₂ with 30 wt% GO exhibits the highest activity for the degradation of dyes (Orange II and Eosin Y) and Carbamazepine. The synthesis method described in this work provides a new design of porous metal oxide/GO heterostructured photocatalysts for environmental remediation.

## 2. Experimental part

### 2.1. Materials

Titanium (IV) propoxide (Ti(OPr)₄, 98 %, Merck), 2-aminoterephthalic acid (NH₂-BDC, 99 %, Fisher), graphite (99.99 %, Merck), dimethylformamide (DMF, 99.8 %, Fisher), Orange II (Fisher), eosin Y sodium salt (Fisher), carbamazepine (98 %, Fisher), *tert*-butanol (*t*-BuOH, Merck, >99 %), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL, Merck, >98 %), ethylenediaminetetraacetic acid disodium salt (EDTA, Fisher, >99 %), dimethylsulfoxide DMSO (Merck, >99.9 %), sodium azide (Merck, >99 %), 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO, Merck, > 97 %), α-(4-pyridyl-1-oxide)-N-*tert*-butylnitrone (PMBO, > 98 %, TCI), 2,2,6,6-tetramethylpiperidine (TMP, 98+%, Fisher), methanol (VWR) and ethanol (VWR) were used as received.

### 2.2. Synthesis of H₂N-MIL-125 (Ti)

H₂N-MIL-125 was prepared by solvothermal method as previously described [26]. Briefly, Ti(OPr)₄ (3.6 mL, 13.08 mmol) was added dropwise to a solution of 2-aminoterephthalic acid (H₂N-BDC) (3.31 g, 18.28 mmol) in 54 mL of anhydrous DMF and 6 mL MeOH. After stirring for 30 min at room temperature, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 48 h. After cooling, H₂N-MIL-125(Ti) particles were recovered by centrifugation (5000 rpm, 10 min) and then washed with DMF (3 x 30 mL), MeOH (3 x 30 mL) and dried at 80 °C for 12 h. The obtained yellow powder was used as precursor of N-TiO₂ particles.

### 2.3. Synthesis of graphite oxide and graphene oxide (GO)

Graphite oxide was prepared by a modified Hummer's method as previously described [43]. Graphene oxide GO was produced by exfoliation of graphite oxide using ultrasonication.

### 2.4. Preparation of H₂N-MIL-125(Ti)/GO and N-TiO₂/GO nanocomposites

For preparation of N-TiO₂/GO catalysts, a specific amount of GO was first dispersed in 30 mL of ultrapure water by sonication (15 min) followed by ultrasonication (15 min). Then, 400 mg of H₂N-MIL-125(Ti) particles were added to the GO solution followed by sonication during 30 min. After centrifugation (5000 rpm, 10 min), the H₂N-MIL-125(Ti)/GO composite was washed with ethanol (30 mL) and dried at 80 °C for 12 h. Then, the powder was placed in ceramic crucible and calcinated under air at 450 °C for 2 h (heating rate of 10 °C/min). The obtained N-TiO₂/GO (10, 20, 30 wt%) composites were used without additional treatment.

### 2.5. Photocatalytic experiments

The efficiency of the prepared photocatalysts was evaluated using the photocatalytic degradation of Orange II, Eosin Y and Carbamazepine under visible light irradiation. A 300 W Osram lamp with UV cut-off filter (λ > 420 nm) was used as a light source. In a typical experiment, 30 mg of photocatalyst was added into 30 mL of pollutant solution and stirred in the dark for 60 min to reach the absorption-desorption equilibrium. After light irradiation, 2 mL of the reaction solution were withdrawn at regular time intervals, centrifugated for 4 min at 15.000 rpm and analyzed by UV-visible spectroscopy to determine the concentration of remaining pollutant.

The same experimental protocol was used for scavenging experiments except that dimethyl sulfoxide (DMSO), ammonium oxalate (AO), sodium azide (NaN₃), *tert*-butanol (*t*-BuOH) and 4-hydroxy-2,2,6,6-tetramethylpiperidinoxy (TEMPOL) were added to the solution before light irradiation.

### 2.6. Characterizations

X-ray diffraction (XRD) was used to investigate the crystallinity of the photocatalysts using a Panalytical X'Pert pro MPD diffractometer with Cu Kα radiation = 0.15406 nm. The morphology of the catalysts was investigated by scanning electron microscopy (SEM, JEOL JSM-6490 LV instrument). The morphology and structure of N-TiO₂/GO catalysts were determined by transmission electron microscopy (TEM) on a JEOL ARM 200F cold FEG TEM/STEM equipped with a GIF quantum ER model 965. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and two-dimensional elemental mapping using energy-dispersive X-ray spectroscopy (EDX) were used to determine the composition of the catalysts.
The morphology and the crystallinity were studied by scanning electron microscopy (SEM, JEOL JSM-6490 LV and JEOL JSM IT800 instruments) and high resolution transmission electron microscopy (HR-TEM) using a Philips CM200 equipment equipped with energy dispersive X-ray (EDX) spectrometer.

The textural properties of the photocatalysts were studied using a Micromeritics 3Flex equipment. X-ray photoelectron spectroscopy (XPS) was used to study the surface composition and the chemical state of elements.

XPS spectra were recorded on a Gammadata Scienta SES 200-2 spectrometer and binding energies were calibrated with the binding energy of adventitious C 1 s located at 284.8 eV. Fourier transform-infrared (FT-IR) spectra were recorded on a Brucker ALPHA II FT-IR spectrometer. Raman spectra were recorded on a Horiba Xplora spectrometer using a 532 nm Nd:YAG laser. A Malvern Zetasizer Nano ZS equipment was used to determine the Zeta potential of the photocatalysts.

UV-visible absorption spectra in liquid phase were recorded on a Thermo Scientific Evolution 220 spectrometer. Diffuse reflectance spectra were recorded using a Shimadzu 2600-2700 spectrometer. Photoluminescence (PL) were recorded on a Horiba Fluoromax spectrometer equipped with a Xenon lamp as the excitation source. The mineralization degree of the pollutant was investigated by measuring the total organic carbon (TOC) using a Shimadzu TOC-VCSH analyzer.

Liquid chromatography coupled with ultraviolet and mass spectrometry (LC-UV-MS) analyses, including tandem mass spectrometry (MS/MS), were conducted using a Dionex Ultimate 3000 system (Thermo Fisher Scientific, France) interfaced with a dual linear ion trap mass spectrometer (LTQ Velos Pro, Thermo Fisher Scientific, MA, USA). Chromatographic separation was performed on a reverse-phase C18 column (Accucore C18, 2.1 × 100 mm, 2.6 µm, Thermo Fisher Scientific, France) maintained at a constant temperature of 30 °C. For each sample, an injection volume of 20 µL was used. The LC system operated at a continuous flow rate of 0.250 mL·min⁻¹.

The column effluent was split into two streams: one directed to the UV detector, monitoring absorbance at 255 and 285 nm, and the other to the mass spectrometer. The mobile phase initially consisted of 5 % acetonitrile and 95 % water, maintained for 5 min, followed by a linear gradient over 20 min to 80 % acetonitrile. This composition was held for 10 min before returning to the initial conditions.

For MS analysis, an electrospray ionization (ESI) source was employed in positive ion mode with an applied voltage of 4.2 kV. The ionization source was maintained at 200 °C and nitrogen (N₂) as the nebulizing gas. Mass spectra were acquired over an *m/z* range of 90–1000. Tandem mass spectrometry (MS/MS) was systematically performed for chromatographic peak confirmation using collision-induced dissociation (CID) at 20 eV, targeting the *m/z* value of the protonated molecules [*M* + *H*]⁺.

Electrochemical impedance spectroscopy (EIS) measurements were performed using a BioLogic SP150 potentiostat in a standard three-electrode setup combining a platinum coil counter electrode, Ag/AgCl (in saturated KCl) reference electrode and a working electrode composed of FTO coated glass substrate covered with N-TiO₂ or N-TiO₂/GO (1 cm²). The impedance analyses were carried out at open circuit potential (OCP) in frequency ranging 500 kHz – 10 mHz with an overlaid sinusoidal potential of ± 10 mV under dark conditions in 0.1 M Na₂SO₄ aqueous solution. Time-resolved photocurrent response tests were evaluated from chronoamperometric measurements (i(t)) under periodic light irradiation cycles (on/off) at a constant potential of + 0.5 V vs Ag/AgCl in a 0.1 M Na₂SO₄ electrolyte solution. A 300 W Xe lamp with a UV filter (λ > 420 nm) was used to illuminate the samples.

ESR spectra were measured at X Band frequency at room temperature on a Bruker ELEXSYS 500 spectrometer.

# 3. Results and discussion

## 3.1. Synthesis and characterization of N-TiO₂/GO photocatalysts

H₂N-MIL-125(Ti) was prepared under solvothermal conditions from H₂N-BDC and Ti(OPr)₄ in DMF at 150 °C. The obtained crystals were associated to GO by ultrasonication followed by drying. H₂N-MIL-125 (Ti)/GO was converted into N-TiO₂/GO by thermolysis as shown in Scheme 1. The calcination temperature of the H₂N-MIL-125(Ti)/GO composite was selected after studying the stability of H₂N-MIL-125(Ti) by TGA conducted under air from room temperature to 800 °C. The TGA profile of H₂N-MIL-125(Ti) shows a first weight loss of ca. 15 % between 100 and 200 °C that can be assigned to the removal of DMF and methanol from the MOF (Fig. S1). A second weight loss corresponding to the collapse of the 3D-network (oxidation of the H₂N-BDC linker and removal of the hydroxyl groups from the Ti oxo-clusters) can be observed between 300 and 450 °C, which agrees with literature [44]. The total weight loss is about 35 % and remains stable from 500 to 800 °C to give N-TiO₂. Noteworthy is also that the weight loss is lower than excepted for H₂N-MIL-125(Ti) (theoretical weight loss of ca 60 %), suggesting a partial collapse of the MOF structure during the synthesis. In preliminary experiments not described herein, the calcination temperature was varied (400, 450, 500 and 550 °C for 2 h) and the photocatalytic activity of N-TiO₂ was evaluated for the degradation of the Orange II dye under visible light irradiation. The highest activity was observed for N-TiO₂ prepared at 450 °C and these operating conditions have been maintained for the rest of the study.

The FT-IR spectra of N-TiO₂/GO composites show a single stretching vibration at 402 cm⁻¹ corresponding to the Ti-O bond, which indicates that H₂N-MIL-125(Ti) was fully decomposed into N-TiO₂ after 2 h heating at 450 °C (Fig. 1a). Fig. 1b shows the XRD patterns of N-TiO₂/GO nanohybrids obtained after calcination. Peaks can be observed at 2θ values of 25.2, 37.6, 47.7, 53.8, 54.9, 62.5, 68.8, 70.0, 75.1 and 82.6°. These signals correspond to the (101), (004), (200), (105), (211), (204), (116), (220), (215) and (224) planes of the anatase phase, respectively (ICCD PDF+/-5, No 00-021-1272). The XRD pattern of N-TiO₂, refined using the Rietveld method, confirms the anatase structure (Fig. S2). No XRD peaks related to the native H₂N-MIL-125(Ti) can be observed (Fig. S3). To provide further investigation, Raman analyses were conducted. For N-TiO₂, the signals observed at 144, 385, 516 and 640 cm⁻¹ correspond to the Eg(1), B1g(1), A1g and Eg(3) lattice vibrational modes of anatase TiO₂ [45] (Fig. 1c). A shift from 157 to 142 cm⁻¹ for the Eg(1) mode with the increase of the GO content from 0 to 30 wt%, indicating a distortion of the crystal structure [46]. As the Eg(1) mode is linked to symmetric stretching of O-Ti-O bonds, the observed shift is indicative of the introduction of oxygen vacancies Ov. The presence of Ov will cause Ti atoms to move closer to neighboring O atoms, which will result in a strenghtening of O-Ti-O bonds and a shift of the Eg(1) mode to lower frequency. The presence of Ov should improve the visible absorption and thus the photocatalytic activity. The shift observed for the Eg(1) mode suggests also strong interactions between N-TiO₂ and GO, which should favor the separation of photogenerated charge carriers. The signals at 1337 and 1571 cm⁻¹ correspond to the D band (disordered carbon like edge defects) and G band (graphitic carbon) of GO, respectively. The relative intensity of D and G bands (I<sub>D</sub>/I<sub>G</sub>) provides useful structural informations on nanocarbons and is found to be of ca. 0.68 (Fig. 1d). This relatively low value compared to native GO (0.90) (Fig. S4) is indicative of a defect increase during the thermal treatment used to transform H₂N-MIL-125 into N-TiO₂.

The valence states of elements and the chemical composition of the N-TiO₂/GO(30) catalyst selected as representative were studied by XPS. As expected, the overview XPS spectrum shows only C, O, Ti and N elements (Fig. S5). The C 1 s spectrum shows five components at 285.0 eV (C–C and C–H bonds), at 286.18 eV (C-O bonds), 287.60 (C=O bonds), 289.50 eV (O-C=O bonds) and 286.10 eV (C-N bonds) (Fig. 2a). This last signal suggests a strong interaction between N-TiO₂ and GO, which will
<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Schematic illustration of N-TiO₂/GO photocatalysts synthesis from H₂N-MIL-125/GO composites.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (a) FT-IR spectra and (b) XRD patterns of N-TiO₂ and N-TiO₂/GO composites. (c) and (d) Raman spectra of GO, N-TiO₂ and N-TiO₂/GO photocatalysts.</figcaption></figure>

benefit the photocatalytic activity [47]. The O 1 s spectrum can be deconvoluted into three peaks located at 530.73, 531.20 and 532.62 eV corresponding to lattice O atoms (Ti(+4)-O bonds), surface O vacancies or Ti(+3)-O bonds, and C-O, C=O and OH bonds, respectively (Fig. 2b) [48]. The signals located at 465.25 and 459.55 eV correspond to Ti 2p₁/₂ and Ti 2p₃/₂ for Ti in the + 4 oxidation state, respectively, which agrees well with literature (Fig. 2c) [49]. For N 1 s, the main signal is observed at 400.87 eV and can be assigned to Ti-O-N bonds and correspond to N atoms doped into the interstitial sites (Fig. 2d) [50,51]. A much weaker

signal is observed for N-Ti-O bonds (substitutional N) at 398.35 eV. Oxidized N atoms (O-N-O bonds) could also be detected at 402.76 eV and are likely formed during the thermolysis of H₂N-MIL-125(Ti) conducted under air. Using the XPS-associated elemental analysis, the percentage of N atoms relative to Ti was found to be of ca. 8.2 %. These results confirm the successful preparation of N-TiO₂ and its association with GO.

Fig. S6 shows SEM images of the H₂N-MIL-125(Ti)/GO composites. H₂N-MIL-125(Ti) particles are found to be homogeneously distributed
<figure><img src="image_4.png" /><figcaption>Fig. 2. HR-XPS spectra of (a) C 1s, (b) O 1s, (c) Ti 2p and (d) N 1s.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) STEM image and the mapping of (b) Ti, (c) O and (d) C elements. (e-f) TEM images, (g) HR-TEM image and (h) the corresponding SAED pattern.</figcaption></figure>
on the surface of GO sheets, likely due to electrostatic interactions between positively-charged H₂N-MIL-125(Ti) particles and negatively-charged GO sheets at neutral pH. The morphology of the N-TiO₂/GO catalysts obtained after calcination at 450 °C was first examined by SEM (Fig. S7). SEM images show that small-sized N-TiO₂ particles are well dispersed on the surface of GO sheets. N-TiO₂ also adhere to GO sheets, which should favor the delocalisation of photogenerated electrons in N-TiO₂ to GO and thus improve the photocatalytic activity.

The morphology and the size of N-TiO₂ particles associated to GO were also examined by STEM and TEM. Fig. 3a,e and f confirm that the thermal decomposition of H₂N-MIL-125(Ti) in the presence of GO generates N-TiO₂ particles of ellipsoidal morphology with an average diameter of ca. 13 ± 3 nm. The STEM-associated elemental analysis and the EDS analysis confirm the presence of Ti, O and C elements (Fig. 3b-d and Fig. S8). The lattice spacings of 0.36 and 0.24 nm measured on the HR-TEM image correspond to the (101) and (004) crystal planes of anatase N-TiO₂, which agrees well with XRD results and the associated selected area electron diffraction (SAED) pattern (Fig. 3g-h).

To determine the specific surface area and the pore size of the samples, N₂-adsorption-desorption measurements at 77 K were conducted and the results are shown in Fig. 4a, S9 and Table 1. H₂N-MIL-125(Ti) and H₂N-MIL-125(Ti)/GO show reversible type-IV isotherms, indicating microporous and mesoporous structures. The S<sub>BET</sub> specific surface area of H₂N-MIL-125(Ti) is of 415.5 m²/g, value lower than that reported in the literature [52] probably due to a partial collapse of the H₂N-MIL-125(Ti) 3D structure during synthesis, which is in agreement with the TGA analyses. S<sub>BET</sub> values increase gradually to 488.5 m²/g with the increase of the GO loading, indicating that the surface pores of H₂N-MIL-125(Ti) are not blocked by GO. A decrease of S<sub>BET</sub> values is observed upon calcination and decomposition of the H₂N-MIL-125(Ti) framework (107 m²/g for N-TiO₂) and, as previously, S<sub>BET</sub> values increase with the loading in GO (131.1 m²/g for N-TiO₂ associated to 30 wt% GO). For all samples, a combination of type-II and type-IV hysteresis loops can be observed. A marked increase of the pore size from ca. 5 to 9 nm is also observed after calcination, which should favor the adsorption and diffusion of organic pollutants into N-TiO₂/GO catalysts (Fig. 4b).

The light absorption capability of the photocatalysts plays a key role for the production of photogenerated charge carriers. As shown in Fig. 4c, N-TiO₂ only weakly absorbs light in the visible region with an absorption onset located at ca. 440 nm. After association with GO, the light absorption in the visible range significantly increases. The bandgap energy (Eg) was determined using the formula $(ah\gamma)^{1/n} = B(h\gamma - Eg)$ where $\alpha$ is the absorption coefficient, h the Planck's constant and n = 2 for a direct semiconductor like N-TiO₂. The extrapolation of curves obtained to $h\nu = 0$ allows to determine the bandgap energy. The bandgap for N-TiO₂ is 3.12 eV and this value is stable after association with GO (Fig. 4d). This constant value of the gap energy indicates that no covalent bonds like Ti-O-C bonds are formed during the thermolysis of H₂N-MIL-125(Ti) in the presence of GO, which agrees well with

<caption>**Table 1**</caption>

Textural properties of H₂N-MIL-125(Ti), H₂N-MIL-125(Ti)/GO, N-TiO₂ and N-TiO₂/GO materials.

<table><thead><tr><th>Sample</th><th>BET (m²/g)</th><th>Pore Volume (cm³/g)</th><th>Pore Size (nm)</th></tr></thead><tbody><tr><td>H₂N-MIL-125(Ti)</td><td>415.5</td><td>0.17</td><td>5.56</td></tr><tr><td>H₂N-MIL-125(Ti)/GO (10)</td><td>457.6</td><td>0.29</td><td>5.09</td></tr><tr><td>H₂N-MIL-125(Ti)/GO (20)</td><td>475.9</td><td>0.29</td><td>5.03</td></tr><tr><td>H₂N-MIL-125(Ti)/GO (30)</td><td>488.5</td><td>0.32</td><td>5.06</td></tr><tr><td>N-TiO₂</td><td>107.8</td><td>0.26</td><td>9.03</td></tr><tr><td>N-TiO₂/GO(10)</td><td>115.9</td><td>0.29</td><td>9.49</td></tr><tr><td>N-TiO₂/GO(20)</td><td>124.0</td><td>0.30</td><td>8.88</td></tr><tr><td>N-TiO₂/GO(30)</td><td>131.1</td><td>0.30</td><td>8.42</td></tr></tbody></table>

literature [53].

The decrease of the bandgap compared to pure anatase TiO₂ (3.20 eV) originates from the N-doping of TiO₂ [6]. Noteworthy is that GO allows the photocatalyst to be active under visible light.

## 3.2. Photocatalytic activity and stability

The Zeta potential of a photocatalyst in aqueous solution plays a key role on pollutant adsorption and is dependent on numerous factors such as pH and ions concentrations in solution. The evolution of the surface Zeta potential of the N-TiO₂/GO(30) catalyst vs pH is shown in Fig. S10. The point of zero charge (pzc) is 6.9, indicating that the photocatalyst is negatively-charged below pH 6.9 and positively-charged above this value.

In a first set of experiments, the photocatalytic system was optimized by varying the Orange II concentration and the photocatalyst dosage, the pH of the Orange II solution is of ca. 4.6 and the temperature was maintained at 20 °C. The amount of the photocatalyst (0.25, 0.5, 1 and 2 g/L) was varied using a concentration of Orange II of 10 mg/L (Fig. S11a). When the dose of the N-TiO₂/GO(30) catalyst is enhanced from 0.25 to 1 g/L, the degradation rate increased due to the increase of active sites and Orange II can be fully decomposed in 240 min using 1 g/L of photocatalyst. No improvement is observed by further increasing the catalyst loading to 2 g/L. The linear relationship of ln(C/C₀) vs reaction time, where C₀ is the initial concentration in Orange II and C the concentration at time t, indicates that the photocatalytic degradation follows pseudo-first-order kinetics (Fig. S11b). The photodegradation rate constants determined for catalyst loadings of 0.25, 0.5, 1 and 2 g/L are 0.00389, 0.00888, 0.01875 and 0.01787 min⁻¹, respectively.

As shown in Fig. S11c, Orange II is almost fully decomposed in 240 min when used at a concentration of 5 or 10 mg/L. The photodegradation rate decreases when increasing the Orange II concentration to 20 mg/L due to the saturation of the photocatalyst active sites by the dye and/or to a filter effect decreasing the amount of incident light wasted for the dye excitation rather than reactice oxygen species (ROS) production. The photodegradation rate constants determined for Orange II concentrations of 5, 10 and 20 mg/L are 0.01525, 0.01875 and 0.00986 min⁻¹, respectively (Fig. S11d).

Next, the photocatalytic degradation of the diazo Orange II dye under visible light irradiation was investigated using N-TiO₂ and N-TiO₂/GO catalysts (Fig. 5a-c). The dye is exposed to the photocatalyst in the dark for 1 h to reach the adsorption-desorption equilibrium and then visible light is turned on (light irradiance of 30 mW/cm² at 440 nm). Orange II is stable under visible light illumination and its self-bleaching is negligible. As can be seen on Fig. 5b, at pH 7, the adsorption of the dye at the surface of the photocatalysts increases with the GO content, the highest adsorption (ca. 23 %) being achieved for the N-TiO₂/GO(30) catalyst. The increase in adsorption with the GO content is likely related to the formation of hydrogen bonds between the OH function of Orange II and the carboxylates or alcohols functions of GO. All photocatalysts are active under visible light irradiation and the highest degradation efficiencies are obtained for N-TiO₂/GO(20) and N-TiO₂/GO(30) catalysts. N-TiO₂ is the photocatalyst with the lowest activity due to its low absorbance in the visible range. Using N-TiO₂/GO(20) and N-TiO₂/GO (30) photocatalysts, the degradation is complete after 4 h visible light irradiation as shown by UV-visible absorption spectra (Fig. 5c). The photodegradation rate constants for the N-TiO₂, and N-TiO₂/GO(10), (20) and (30) are 0.00098, 0.01130, 0.1875 and 0.01604 min⁻¹, further confirming that catalysts associating N-TiO₂ to 20 or 30 wt% GO exhibit the highest activity. Further increasing the GO content to 40 wt% has a deleterious influence on the photocatalytic activity likely due to the increased light absorption and shading effect of dark brown GO, which is consistent with previous studies [43].

In a second series of experiments, the photocatalytic degradation of the Eosin Y dye, commonly used in the textile and paper industries, was investigated using N-TiO₂ and N-TiO₂/GO(30) catalysts (Fig. 5d-f).
<figure><img src="image_6.png" /><figcaption>Fig. 4. (a) N₂ adsorption/desorption isotherms acquired at –196 °C of N-TiO₂/GO catalysts. (b) Pore volume distributions, (c) UV-visible absorption spectra of N-TiO₂ and N-TiO₂/GO catalysts and (d) the corresponding plots of (αhy)¹/² versus photon energy (hy).</figcaption></figure>

Eosin Y is both toxic to humans and aquatic organisms [54]. In humans, Eosin Y is toxic by contact and may cause irritation as well as burns. By ingestion, this dye causes deleterious effects on various organs such as the liver or the lung. The degradation of this dye is therefore a major issue. Eosin Y is also a photosensitizer commonly used for the production of singlet oxygen ¹O₂ and has found applications in photodynamic therapy. As can be seen from Fig. 5e, Eosin Y is bleached by ca. 40 % after 90 min of visible light irradiation and this process is directly linked to the formation of ¹O₂ [55,56]. Using the N-TiO₂/GO(30) catalyst, Eosin Y is effectively and fully decomposed in 30 min while photo-degradation requires more than 90 min with N-TiO₂ (Fig. 5e).

Another pollutant that has caused a lot of problems in recent years is Carbamazepine (CBZ) (Fig. 6a). Due to its widespread use as antiepileptic drug for the treatment of epilepsy or arrhythmia and its psychotropic activity, CBZ is one of the most commonly detected pollutants in groundwater, in surface water and even in drinking water [57]. The CBZ molecule is neutral and adsorbs very little (less than 3 %) on the surface of N-TiO₂ and N-TiO₂/GO catalysts. The activity of the N-TiO₂/GO(30) catalyst is much higher than that of N-TiO₂, which only breaks down 75 % of CBZ under the same conditions (Fig. 6b). UV-visible absorption spectra recorded during photodegradation using the N-TiO₂/GO(30) catalyst confirm that CBZ is fully decomposed after 210 min of irradiation (Fig. 6c). The performance of the N-TiO₂/GO(30) photocatalyst for the photocatalytic degradation of carbamazepine was also compared to that of catalysts recently described in the literature (Table S1). Although it is difficult to compare these data because the concentrations of carbamazepine or photocatalyst and irradiation conditions are different from those used in this work, the results show that the N-TiO₂/GO catalyst(30) is among the most effective for photocatalytic degradation of carbamazepine under visible irradiation

The mineralization of CBZ by the N-TiO₂/GO(30) photocatalyst was studied via total organic carbon (TOC) content measurements. The TOC value measured after 210 min irradiation is of 3.027 mg/L whereas the initial value was 7.625 mg/L. These results indicate that the TOC removal efficiency is of ca. 60 % after this duration of irradiation. Small organic molecules derived from CBZ are still present in the aqueous phase after 210 min and longer irradiation times may be required to fully decompose CBZ into CO₂ and H₂O.

The first intermediates produced during the photocatalytic degradation of Carbamazepine using the N-TiO₂/GO(30) catalyst were also identified via MS measurements (Fig. 7) and degradation pathways are proposed. Mono-, bis-, and tris-hydroxylated derivatives of Carbamazepine were detected (pathways A, B and C) that were further oxidized into quinones 1 or diazadibenzo[cd,h]azulene-3,10-dione 2. 10-Oxo 3 and 10,11-dioxo 4 carbamazepine derivatives could also be observed and these compounds were further decomposed into 5 and 6 by cleavage of the urea bond. This cutoff of the urea bond may also occur at the beginning of the photocatalytic oxidation (pathway D) to produce dibenzo[b,f]azepine 7 that is further hydroxylated and oxidized. The oxirene 8 derived from Carbamazepine was also detected (pathway E). Further oxidation of these intermediates produces first lower molecular weight compounds that are next further decomposed and mineralized into CO₂, H₂O, NH₄⁺, NO₂⁻ and NO₃⁻. The intermediates detected by MS using the N-TiO₂/GO(30) are in agreement with those observed during the degradation of carbamazepine using TiO₂ or ZnIn₂S₄ photocatalysts [58,59].

The reusability of a photocatalyst is a key parameter for real environmental applications because it provides important information on the feasibility of using this catalyst on a large scale. The stability and the reusability of the N-TiO₂/GO(30) catalyst was investigated by
<figure><img src="image_7.png" /><figcaption>Fig. 5. (a,c) Chemical structures of Orange II sodium salt and Eosin Y disodium salt, (b) Photocatalytic activity of N-TiO₂ and Ni-TiO₂/GO catalysts for Orange II degradation, (c) UV-visible absorption spectra of Orange II in the course of the photodegradation using the N-TiO₂/GO(30) catalyst, (d) Photocatalytic activity of N-TiO₂ and Ni-TiO₂/GO(30) catalysts for Eosin Y degradation, (e) UV-visible absorption spectra of Eosin Y in the course of the photodegradation using the N-TiO₂/GO(30) catalyst. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. (a) Chemical structure of CBZ, (b) Photocatalytic activity of N-TiO₂ and Ni-TiO₂/GO(30) catalysts for CBZ degradation and (c) UV-visible absorption spectra of CBZ during the photocatalytic degradation using the N-TiO₂/GO(30) catalyst under visible light irradiation.</figcaption></figure>

performing dark-adsorption and photocatalytic degradation cycles of Orange II under visible light irradiation (Fig. 8a). After each cycle, the percentage of Orange II degradation was evaluated by UV-visible spectroscopy. After five reuses without washing and drying, the photocatalyst retained ca. 93 % of the original activity, indicating its high stability. This was confirmed by XRD, FT-IR and SEM analyses that were used to identify the structural changes and surface chemistry of the photocatalyst after repeated use for the photocatalytic degradation of Orange II. As can be seen on Fig. 8b, the anatase crystalline structure of the photocatalyst is not altered after five reuses. Weak signals are observable by FT-IR at ca. 2915, 1457 and 1220 cm⁻¹ after reuses indicating the presence of organic by-products on the surface of the

photocatalyst, which agrees with TOC measurements (Fig. 8c). A slight decrease in S<sub>BET</sub> specific surface area is observed after recycling (443 m²/g vs 488 m²/g for the freshly prepared catalyst), indicating that these organic molecules do not block the pores of the catalyst and therefore do not alter its activity. The SEM analysis of the recycled catalyst also shows that there is no significant agglomeration of N-TiO₂/GO(30) particles (Fig. 8d).

### 3.3. Photocatalytic mechanism

Photoluminescence, transient photocurrent response and electrochemical impedance spectroscopy were used to study the
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Degradation mechanism of Carbamazepine using the N-TiO₂/GO(30) catalyst. In blue, the value of the mass to charge ratio corresponds to the [M + H]⁺ experimentaly measured by LC-MS(/MS). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

recombination, separation efficiency and transport of charge carriers in N-TiO₂ and N-TiO₂/GO catalysts.

After excitation at 350 nm, N-TiO₂ shows a weak and broad PL with main peaks located at ca. 482, 533 and 562 nm (Fig. 9a). After association with GO, the PL intensity significantly decreases indicating an inhibition the electrons-holes recombination and thereby improving the charge separation at the interface. As can be seen on Fig. 9b, a fast response is observed and an anodic photocurrent is generated upon illumination. During the first light-on and off cycles, the photocurrent intensity (i-t) of N-TiO₂/GO(30) is significantly higher than that of N-TiO₂/GO(20), N-TiO₂/GO(10) and N-TiO₂, indicating that photo-generated charge carriers produced by the N-TiO₂/GO(30) catalyst exhibit the highest separation efficiency. The observed decrease in photocurrent intensity for the three N-TiO₂/GO catalysts over time is related to their detachment from the FTO electrode surface and not to a deterioration of the photocatalysts. Fig. 9c shows EIS data for N-TiO₂ and N-TiO₂/GO catalysts. The N-TiO₂/GO(30) catalyst shows the smallest semicircular arc radius, which is indicative of its lower charge transfer resistance. This is indicative of a fast transport of photo-generated charge carriers and of a reduced accumulation of these carriers at the interface, which should favor the photocatalytic activity. The obtained results can well be fitted using the equivalent circuit shown in the inset of Fig. 9c and the charge transfer resistance RCT was determined (Rs is the resistance of the electrolyte and CPE is the constant

phase element). As seen in Table S2, the weakest RCT value is obtained for the N-TiO₂/GO(30) photocatalyst, further confirming that GO sheets acting as an electron sink and hybridized N-TiO₂ particles allow an efficient charge separation and transfer.

To investigate the mechanism, scavenging experiments were also conducted to identify the active species responsible for the pollutants degradation. DMSO, ammonium oxalate, sodium azide NaN₃, tert-butanol (t-BuOH) and TEMPOL were used to scavenge electrons, holes, singlet oxygen ¹O₂, hydroxyl ·OH radicals and superoxide O₂⁻ radicals, respectively. As shown in Fig. 9d, all species mentioned previously are involved in various degrees in the catalytic photodegradation of Carbamazepine. Free electrons are poorly involved in the photocatalytic degradation which remains at a high level of 91 % after addition of DMSO. H⁺, ¹O₂, and ·OH play a much larger role because the addition of the scavengers causes a sharp drop in the photodegradation (28, 35 and 51 % after addition of AO, NaN₃ and t-BuOH, respectively). Upon addition of TEMPOL, the photodegradation efficiency is almost reduced to 0, indicating that O₂⁻ radicals play the major role in the photocatalytic process.

ESR spin trap technique was further used to detect the active species generated by the N-TiO₂/GO(30) photocatalyst upon illumination. 5,5-Dimethyl-1-pyrrolidine-N-oxide (DMPO) was first used to trap O₂⁻ radicals. No ESR signal could be detected regardless of the concentration of the photocatalyst or DMPO used and on the irradiation time. Due to the
<figure><img src="image_10.png" /><figcaption>Fig. 8. (a) Photocatalytic efficiency of the N-TiO₂/GO(30) catalyst after five sequential cycles (Orange II, 10 mg/L, visible light irradiation). (b) XRD patterns. (c) FT-IR spectra of the N-TiO₂/GO(30) before and after five reuses and (d) SEM image of the reused photocatalyst. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

low rate constant of DMPO with O₂⁻ radicals (10 M⁻¹s⁻¹ at pH 7.0) [60], we assume that the photodegradation rate of DMPO is much higher than that of its reaction with O₂⁻ radicals. 2,2,6,6-Tetramethylpiperidine (TMP) was further used to detect ¹O₂ [61]. The strong triplet ESR signal with aN = 17.22 G, characteristic of the 2,2,6,6-tetramethyl-1-piperidinylxylo (TEMPO) adduct, was observed from the irradiation of TMP in the presence of the photocatalyst (Fig. 10a). A decrease in the intensity of TEMP is observed if the irradiation is prolonged to 15 min, suggesting that TEMPO is oxidized by species with a redox potential higher than + 0.75 V vs NHE like holes or hydroxyl radicals. The production of 'OH radicals was demonstrated using α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (PMBO) as spin trap [60]. Due to the short lifetime of the POBN-OH adduct, EtOH was added to convert 'OH radicals into ethoxy radicals 'CH(OH)CH₃ that can readily react with POBN (rate constant = 3 10⁷ M⁻¹s⁻¹) to form POBN/'CH(OH)CH₃ radicals that appear as a triple double-line spectrum (aN = 15.69 G and aH = 2.69 G) (Fig. 10b). ESR experiments confirm the production of ¹O₂ and 'OH radicals that are involved in the photocatalytic degradation.

The potentials of the valence band ($E_{VB}$) and of the conduction band ($E_{CB}$) of N-TiO₂ were calculated using formula (1) and (2):

$$E_{VB} = \chi - E^{e} + 0.5E_{g} \quad (1)$$

$$E_{CB} = E_{VB} - E_{g} \quad (2)$$

where χ is the absolute electronegativity of the N-TiO₂ semiconductor (5.79 considering the formula TiN₀.₀₈O₁.₉₂ based on XPS results), Eg is the bandgap energy determined from UV-visible absorption spectra (3.12 eV for the N-TiO₂/GO(30) catalyst), and Ee is the energy of free electrons on the hydrogen scale (4.5 eV) [62]. The values determined for $E_{VB}$(N-TiO₂/GO(30)) and $E_{CB}$(N-TiO₂/GO(30)) are 2.85 and -0.27 eV, respectively.

On the basis of the previously described results, the photocatalytic mechanism can be explained as follows (Fig. 11). The energy level of the N 2p dopant is located ca. 2.48 eV lower than the CB of N-TiO₂ [63]. Under visible light irradiation, electrons located in the N 2p dopant energy level and, to a lesser extent, those located in the VB of N-TiO₂ can be promoted to the CB of N-TiO₂ (Fig. 11a). Next, these excited electrons may transfer to the Fermi level of GO acting as an electron-transporting material and as an electron-sink. Moreover, the Schottky barrier at the
<figure><img src="image_11.png" /><figcaption>Fig. 9. (a) PL emission spectra, (b) transient photocurrent responses in 0.1 M Na₂SO₄ aqueous solution and under visible light irradiation, (c) EIS Nyquist plots recorded in the dark of N-TiO₂ and N-TiO₂/GO catalysts (the inset is the equivalent circuit used to fit EIS curves). (d) Influence of scavengers on the photocatalytic degradation of Carbamazepine using the N-TiO₂/GO(30) catalyst.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. (a) ESR spectra of the TEMPO radical after 5 and 15 min irradiation and (b) ESR spectrum of the POBN/CH(OH)CH₃ adduct.</figcaption></figure>

interface of GO and N-TiO₂ hinders the charge carrier recombination in N-TiO₂, as demonstrated by photoluminescence, photocurrent measurements and EIS Nyquist plots (Fig. 9a-c). At the same time as the electrons transport, holes are formed in the VB of N-TiO₂ and in the N 2p energy level.

The theoretical potential of the CB of N-TiO₂ (−0.27 eV) and of GO (−0.08 eV) are not perfectly suited to reduce O₂ into O₂⁻ radicals as the potential of the O₂/O₂⁻ couple is of −0.33 eV. However, the electric field generated at the interface between N-TiO₂ and GO is likely to change the energy levels and give them a more reducing potential allowing the
<figure><img src="image_13.png" /><figcaption>Fig. 11. (a) Band structure of the N-TiO₂/GO(30) photocatalyst, (b) schematic illustration of the photocatalytic process of the N-TiO₂/GO(30) catalyst.</figcaption></figure>

reduction of O₂ into O₂⁻ radicals that were demonstrated to play the major role in the photocatalytic degradation (Fig. 11b). These radicals can also be reduced by the holes to generate *O*₂. In the meantime, holes in the VB of N-TiO₂ and in the N 2p energy level oxidize either the pollutant into [pollutant]⁺ or react with H₂O molecules to give *·OH* radicals. All these species are involved in the degradation of pollutants and allow their mineralization.

## 4. Conclusion

In summary, photocatalysts associating N-doped TiO₂ to GO were efficiently prepared by thermolysis of H₂N-MIL-125(Ti) in the presence of GO. The obtained N-TiO₂/GO catalysts exhibit a high porosity (up to ca. 131 m²/g for N-TiO₂/GO(30)) and are composed of N-TiO₂ particles with an average diameter of 13 ± 3 nm deposited on GO sheets. The association of N-TiO₂ with GO enables to increase the charge carriers separation as well as the light absorption range, which significantly enhances the photocatalytic activity in the visible region. The N-TiO₂/GO(30) catalyst exhibits the highest activity for the degradation under visible light irradiation of dyes (Orange II and Eosin Y) and of the Carbamazepine drug. Moreover, the N-TiO₂/GO(30) catalyst shows high stability and reusability, maintaining a high activity over six cycles. Our results demonstrate the high potential of the N-TiO₂/GO(30) catalyst for visible-light driven photocatalytic degradations.

### CRediT authorship contribution statement

**Askar Bakhadur:** Writing – original draft, Methodology, Investigation, Data curation. **Halima Alem:** Methodology, Investigation, Data curation. **Vincent Carré:** Writing – original draft, Investigation, Data curation.

**Thomas Gries:** Writing – original draft, Investigation, Data curation. **Lavinia Balan:** Methodology, Investigation, Data curation. **Jean-Louis Cantin:** Data curation, Investigation, Methodology. **Ghouti Medjahdi:** Methodology, Investigation, Data curation. **Zhandos Orazov:** Methodology, Investigation, Data curation. **Bolat Uralbekov:** Writing – original draft, Supervision, Funding acquisition. **Raphaël Schneider:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP19679298). The authors acknowledge Dr Khalid Ferji (LCPM, Université de Lorraine) for Zeta potential analysis and Mélanie Emo from the CC3M of the IJL for the Electronic Microscopy experiments

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2025.164466.
# Data availability

Data will be made available on request.

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