# Innovative electrospinning approach to fabricate TiO2/NiO nanofibers for effective acetaminophen degradation

Mahmoud Abid, Tia Maria Howayek, Olga Mazur, Roman Viter, Maged Bekheet, Amr A Nada, Djamel Bezzerga, Jisang Hong, Philippe Miele, Igor Iatsunskyi, et al.

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## To cite this version:

Mahmoud Abid, Tia Maria Howayek, Olga Mazur, Roman Viter, Maged Bekheet, et al. Innovative electrospinning approach to fabricate TiO2/NiO nanofibers for effective acetaminophen degradation. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2025, 709, pp.136077. 10.1016/j.colSURFA.2024.136077. hal-05154418

HAL Id: hal-05154418

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Submitted on 9 Jul 2025

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Innovative electrospinning approach to fabricate TiO₂/NiO nanofibers for
effective acetaminophen degradation

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Mahmoud Abid<sup>a</sup>, Tia Maria Howayek<sup>a</sup>, Olga Mazur<sup>a</sup>, Roman Viter<sup>b</sup>, Maged F. Bekheet<sup>c</sup>,
Amr A. Nada<sup>a,d</sup>, Djamel Bezzerga<sup>e</sup>, Jisang Hong<sup>e</sup>, Philippe Miele<sup>a</sup>, Igor Iatsunskyi<sup>f</sup>,
Emerson Coy<sup>f</sup>, Geoffroy Lesage<sup>a</sup>, Roland Habchi<sup>g,h</sup>, David Cornu<sup>a</sup>, Mikhael Bechelany<sup>a,h,*</sup>

HIGHLIGHTS

* Nanofibers composed of titanium and nickel oxides were fabricated.

* More than 98 % of acetaminophen was degraded within 4 hours under visible light.

* After five cycles, a degradation efficiency decrease of 19 % was recorded.

* Acute toxicity and scavenging tests highlighted the key roles of O₂⁻, and h⁺ species in acetaminophen degradation.

* DFT calculations reveal charge transfer mechanisms, enhancing NiO-TiO₂ photocatalytic efficiency.

GRAPHICAL ABSTRACT

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ARTICLE INFO

Keywords:
Nanofibers
TiO₂
NiO
Electrospinning
Photocatalysis

A B S T R A C T

Photocatalysis offers a promising method for water treatment, utilizing light-activated photocatalysts to trigger chemical reactions that degrade organic pollutants. In this study, composite fibers of polyvinylpyrrolidone (PVP) and titanium dioxide (TiO₂) were synthesized using sol–gel process and electrospinning techniques, by integrating controlled concentrations of nickel oxide (NiO) ranging from 1 to 5 wt%, followed by calcination at 500 °C. The structural and morphological properties of these nanocomposites were meticulously characterized using a suite of advanced analytical techniques, including scanning and transmission electron microscopy,

<footnote><sup>a</sup> Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier 34095, France</footnote>

<footnote><sup>b</sup> Institute of Atomic Physics and Spectroscopy, University of Latvia, 19 Raina Blvd., Riga LV 1586, Latvia</footnote>

<footnote><sup>c</sup> Technische Universität Berlin, Faculty III Process Sciences, Institute of Materials Science and Technology, Chair of Advanced Ceramic Materials, Straße des 17. Juni 135, Berlin 10623, Germany</footnote>

<footnote><sup>d</sup> Material Research and Technology Department, Luxembourg Institute of Science and Technology, 28 Avenue des Hauts-Fourneaux, Esch-sur-Alzette 4362, Luxembourg</footnote>

<footnote><sup>e</sup> Department of Physics, Pukyong National University, Busan 48513, Korea</footnote>

<footnote><sup>f</sup> NanoBioMedical Centre, Adam Mickiewicz University, Wszechnicy Piastowskiej 3, Poznan 61-614, Poland</footnote>

<footnote><sup>g</sup> EC2M, Faculty of Sciences 2, Campus Pierre Gemayel - Fanar, Lebanese University, Fanar 90656, Lebanon</footnote>

<footnote><sup>h</sup> Gulf University for Science and Technology, GUST, Kuwait</footnote>

<footnote>* Corresponding author at: Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier 34095, France.
*E-mail address:* mikhael.bechelany@umontpellier.fr (M. Bechelany).</footnote>
Acetaminophen
DFT

Brunauer-Emmett-Teller (BET) analysis, Raman spectroscopy, X-ray diffraction and photoelectron spectroscopy. These analyses revealed that the introduction of NiO into the TiO₂ matrix not only modified the structural characteristics but also enhanced the optical properties, as demonstrated by the reflectance and photoluminescence spectra of the fibers. Photocatalytic performance was assessed through the degradation of acetaminophen under visible light, where NiO-TiO₂ nanocomposites containing 5 wt% NiO achieved a 98.8 % degradation after 4 hours, demonstrating a significant enhancement compared to the 6 hours needed for pure TiO₂ nanofibers. Thermal regeneration at 500 °C effectively restores the photocatalytic efficiency of 5 wt% NiO-TiO₂ after a gradual decline over five cycles. Furthermore, acute toxicity and scavenging tests indicated that hydroxyl radicals (OH⁻), superoxide anions (O₂⁻), and holes (h⁺) played significant roles in the degradation process. Density Functional Theory (DFT) calculations provided detailed insights into the electronic structure, band gaps, and charge transfer processes, revealing the mechanisms by which NiO-TiO₂ composites achieve enhanced photocatalytic efficiency through optimized electron-hole separation and photon energy retention. These results underscore the potential of NiO-TiO₂ composite nanofibers for photocatalytic applications and open pathways for the development of customizable photocatalysts tailored to specific environmental challenges.

# 1. Introduction

Human activities have significantly escalated the consumption of chemicals due to population growth, industrialization, and global development [1]. This has led to the introduction of various pollutants into aquatic environments, including pharmaceuticals, pesticides, dyes, and personal care products, which are now considered emerging contaminants [2]. These micropollutants, often discharged untreated from various sources, pose a severe threat to water quality, potentially impacting human health and ecosystems even at low concentrations. Addressing water pollution and scarcity has become a pressing global priority, necessitating the development of effective purification systems to remove these contaminants [3-5].

Recent research has focused on two primary strategies for removing chemical pollutants from water: adsorption on solid sorbents and mineralization [2,6-8]. Although adsorption is widely used due to its high purification performance, it has significant drawbacks, including the challenge of regenerating sorbents and the transfer of pollutants to the sorbent itself. Consequently, there is a growing need for energy-efficient and cost-effective treatment methods.

Advanced oxidation processes (AOPs) have emerged as a promising alternative, offering environmental friendliness, low energy consumption, and the ability to degrade and mineralize non-biodegradable molecules [9,10]. Within the realm of advanced oxidation processes (AOPs), heterogeneous photocatalysis is distinguished by its effectiveness, particularly through the utilization of titanium dioxide (TiO₂) as a semiconductor photocatalyst.

TiO₂, especially in its anatase form, is favored for its activity, stability, and low cost [11-14]. However, its limitations, such as a wide band gap restricting light absorption to the UV spectrum and rapid electron-hole recombination, necessitate enhancements [15]. Developing composites with other metal oxide semiconductors, such as SnO₂, WO₃, CuO, ZnO, Fe₂O₃, and NiO, has shown promise in overcoming these limitations [15-20].

Nickel oxide (NiO) has emerged as a particularly effective component for improving photocatalytic performance due to its enhanced charge separation capabilities, rapid hole mobility, stability, and cost-effectiveness. Incorporating NiO into TiO₂-based nanocomposites improves the separation of electron-hole pairs, thereby enhancing interfacial charge transfer and overall efficiency. Studies have demonstrated the efficacy of TiO₂-NiO nanocomposites under direct sunlight, showing significant improvements in the degradation of various pollutants. Padmavathy *et al.* examined the degradation efficiency of a TiO₂-NiO photocatalyst synthesized *via* the sol-gel method on various pollutants, including Crystal Violet, Acid Orange 7 and Alizarin Yellow. Their findings indicated that increasing the concentration of NiO within the composite notably enhanced its photocatalytic activity [21]. In a similar study, Zayed *et al.* created a TiO₂ nanotube and NiO nanoparticle composite for solar Methyl Blue degradation, achieving an impressive 99.5 % degradation after 45 minutes compared to 73 % with pure TiO₂

nanotubes [22]. Further research by Wahyuningsih *et al.* highlighted the significant improvements in the photodegradation of Rhodamine B (RhB) using a TiO₂-NiO composite synthesized *via* the sol-gel method. The incorporation of NiO resulted in a remarkable 98.25 % degradation after 360 minutes, compared to 43.15 % with TiO₂ alone [23]. Liu *et al.* investigated the photodegradation of Methyl Blue using black NiO-TiO₂ nanorods, revealing a substantial improvement, achieving 7.94 times higher degradation efficiency compared to TiO₂ nanorods alone [24]. Castaneda *et al.* examined NiO/F-TiO₂ for degrading 4-chlorophenol. They found that incorporating various percentages of NiO (0.5 wt%, 1.0 wt%, and 2.0 wt%) significantly affected the photocatalytic activity, with 1.0 wt% NiO being optimal. UV irradiation for 180 minutes increased the degradation rate by 1.4 times when using NiO/F-TiO₂ compared to F-TiO₂ alone [25]. Recent work by Pankaj and Pethaiyan on core-shell nanoparticles of TiO₂-NiO-0.5 demonstrated enhanced degradation of RhB under sunlight, achieving 77.6 % degradation, compared to 18.3 % with pure TiO₂ [26]. Wongburapachart *et al.* investigated NiO-TiO₂/TiO₂ bilayer films, which showed a significant improvement in degrading Acid Orange 7, achieving a 50 % degradation rate compared to 9 % for pure TiO₂ [27]. Mannaa *et al.* reported high photodegradation rates for Brilliant Green and Phenol using an optimal photocatalyst of 10.0 wt% NiO/TiO₂ nanoparticles, achieving 86.7 % and 74.3 % degradation, respectively [15]. Similarly, Ahmed *et al.* synthesized mesoporous NiO/TiO₂ nanoparticles with 5 mol% NiO, showing superior activity in methylene blue mineralization with a rate of 95 % compared to 65 % with pure TiO₂ [28]. These findings underscore the effectiveness of metal oxide semiconductor photocatalysts, which can be optimized through careful control of factors such as morphology, crystallinity, and surface characteristics during the synthesis process.

Our research introduces a novel approach by using electrospinning to directly incorporate nickel oxide (NiO) into titanium dioxide (TiO₂) nanocomposites, marking a first in this field. Unlike traditional methods that rely on precursors for coupling, we achieved a direct integration of NiO oxides with TiO₂ *via* sol-gel process and electrospinning, followed by calcination at 500 °C. This innovative method enhances the photocatalytic performance of the nanocomposites and sets a new standard in material fabrication. Our comprehensive analysis, including acetaminophen degradation, high-performance liquid chromatography (HPLC), and toxicity screening, highlighting the significant advancements achieved with this novel approach. Additionally, Density Functional Theory (DFT) was employed to investigate the electronic properties and charge transfer mechanisms of NiO-TiO₂ composites.

# 2. Materials and methods

## 2.1. Materials

Table 1 lists the chemicals utilized in this study without undergoing additional purification processes.
<table><caption>**Table 1**

Materials used in this study.</caption><thead><tr><th>Material</th><th>Supplier/ Source</th><th>Purity</th><th>CAS number</th></tr></thead><tbody><tr><td>Nickel (II) oxide</td><td>Sigma-Aldrich</td><td>99.8 %</td><td>1313-99-1</td></tr><tr><td>Titanium tetraisopropoxide (TTIP)</td><td>Sigma-Aldrich</td><td>97 %</td><td>546-68-9</td></tr><tr><td>Polyvinyl pyrrolidone (PVP)</td><td>Sigma-Aldrich</td><td>Mw = 1300000</td><td>9003-39-8</td></tr><tr><td>Acetaminophen (ACT)</td><td>Sigma-Aldrich</td><td>≥ 99 %</td><td>103-90-2</td></tr><tr><td>2-propanol (IPA)</td><td>Sigma-Aldrich</td><td>99.9 %</td><td>67-63-0</td></tr><tr><td>p-benzoquinone (BQ)</td><td>Sigma-Aldrich</td><td>≥ 99.5 %</td><td>106-51-4</td></tr><tr><td>Ethylenediaminetetraacetic acid (EDTA)</td><td>Sigma-Aldrich</td><td>99.995 %</td><td>60-00-4</td></tr><tr><td>Acetic acid</td><td>VWR chemicals</td><td>-</td><td>64-19-7</td></tr><tr><td>Ethanol</td><td>VWR chemicals</td><td>≥ 99.8 %</td><td>64-17-5</td></tr><tr><td>Deionized water</td><td>Milli-Q® Academic</td><td>> 18.2 MΩ</td><td>-</td></tr></tbody></table>

## 2.2. Fabrication of NiO-TiO₂ composite nanofibers

To prepare the solution to be spun, different NiO amounts (1, 3 and 5 wt% of TiO₂ mass) were well dispersed in 3 mL of absolute ethanol for one hour. Concurrently, titanium tetraisopropoxide was dissolved in 2 mL acetic acid and 3 mL ethanol, incorporating 0.3 g polyvinylpyrrolidone. This resulting solution was then loaded into a 22 mL syringe connected to a high-voltage power source. Electrospinning was performed under specified conditions: a needle-collector distance of 10 cm, applied voltage tension of 20 kV, and a solution flow rate of 1 mL per hour [29,30]. Subsequently, the collected nanofibers were calcined at 500 °C.

The nanofibers used in this research are detailed in Table 2.

## 2.3. Structural characterizations

The comprehensive characterization of the composite nanofiber samples involved the application of several advanced analytical techniques. Scanning electron microscopy (Hitachi S-4800) was used to examine the surface morphology, while transmission electron microscopy (JEOL ARM 200FS) provided insights into the internal structure. X-ray diffraction (PANalytical X'pert-PRO) was employed for crystalline phase identification, and Brunauer-Emmett-Teller (BET) analysis (ASAP 2020) measured the surface area and porosity. Raman spectroscopy (Horiba Xplora at 532 nm) offered molecular vibrational data, and X-ray photoelectron spectroscopy (ESCALAB 250) enabled elemental surface analysis. To evaluate optical properties, reflectance measurements were performed using a Shimadzu 3600 UV-VIS-NIR spectrophotometer (300–800 nm, with an integrating sphere, slit width 3 nm, and wavelength change step 0.5 nm). Photoluminescence analysis was conducted using an instrument equipped with a Nd:YAG laser (266 nm, 15 mW, 15 ns) for excitation and an Ocean Optics HR4000 spectrometer (350–750 nm, resolution 1.2 nm) for detection. Finally, high-performance liquid chromatography (RP18, Nucleoshell) coupled with mass spectrometry (Quattro-Micro instrument) facilitated the separation and identification of specific compounds within the nanofibers, providing a comprehensive understanding of their structural and functional properties.

<table><caption>**Table 2**

NiO-TiO₂ samples are used in this study.</caption><thead><tr><th>Sample name</th><th>Sample description</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>TiO<sub>2</sub></td></tr><tr><td>TN1</td><td>NiO (1 wt%)-TiO<sub>2</sub> (NF)</td></tr><tr><td>TN3</td><td>NiO (3 wt%)-TiO<sub>2</sub> (NF)</td></tr><tr><td>NiO-TiO<sub>2</sub> or TN5</td><td>NiO (5 wt%)-TiO<sub>2</sub> (NF)</td></tr></tbody></table>

## 2.4. Photocatalytic and kinetic measurements

The photocatalytic experiments were conducted using a visible light halogen lamp (R7S Quartz Linear, 400 W, 3000 K, 220–240 V, 118 mm with reference 2430800281468). The lamp emits a continuous spectrum of light, primarily in the visible range, with a color temperature of 3000 K. The spectral range includes wavelengths from approximately 400 to 700 nm, corresponding to its visible light classification, with some minimal infrared contributions beyond 700 nm due to the nature of halogen lamps. Importantly, this lamp has negligible emission in the UV region, ensuring that the observed photocatalytic activity is exclusively driven by visible light. Based on lamp specifications and experimental setup distance, the estimated power intensity of the light at the sample surface was estimated to be 318 mW/cm².

The photocatalytic efficiency of the prepared samples for ACT degradation under visible light was evaluated using Eq. 1 as described in reference [31]:

$$ \text{Degradation efficiency (\%)} = [(C_0-C)/C_0] \times 100 \quad (1) $$

Here, C₀ signifies the initial concentration of ACT, while C indicates the concentration after a specified exposure time to visible light.

The kinetic data for ACT photocatalytic degradation under visible light were analyzed using a pseudo-first-order kinetic model, as described by Eq. 2 [32]:

$$ \ln(C_0/C) = K_{\text{app}} t \quad (2) $$

In this equation, Kapp denotes the apparent rate constant, with the initial concentration C₀ and C the concentration at time t.

## 2.5. Toxicity tests

To assess the potential toxicity of byproducts formed during acetaminophen degradation, a bioluminescence toxicity analysis was conducted. This approach relies on measuring the luminescence response of marine bacteria. The strain Vibrio fischeri LCK 487 was employed for this purpose, with measurements performed using the Microtox® Model 500 Analyzer (Modern Water Inc., United Kingdom) integrated with the MicrotoxOmni® software.

As reported in previous studies [29,30,33], the bacterial reagent was reconstituted by adding 5 mL of diluent at 5 °C. Subsequently, 200 µL of the bacterial suspension was placed into cuvettes and stabilized at 15 °C for 15 minutes. To enhance the activity of the Vibrio fischeri bacteria, samples were diluted to 81.9 % of their original concentration by adding a 22 % NaCl solution. The luminescence intensity served as an indicator of toxicity; a reduction in luminescence signals the presence of toxic compounds inhibiting bacterial activity. Before luminescence measurements, all samples were filtered using 0.45 µm filters to eliminate any particulate matter or precipitates.

The toxicity levels were quantified based on the inhibition of bacterial activity, and calculated using the following equation:

$$ I_c(t) (\%) = \left(1 - \frac{L U(t)}{L U_0 x R(t)}\right) \times 100 \quad (3) $$

where LU(t) represents the luminescence intensity emitted by bacteria after t = 15 min of contact with the sample, LU₀ is the initial bacterial luminescence intensity before the addition of the sample, and R(t) is the corrected factor accounting for natural bacterial luminescence variability over time due to environmental conditions.

Indeed, in the absence of toxicity, the bacterial luminescence decreases over time under the influence of environmental conditions. Therefore, the natural luminescence variability must be taken into account to avoid errors. The corrected term is given by the following equation:
$$R(t) (\%) = \left( \frac{LU0(t)}{LU0} \right) \times 100 \qquad (4)$$

## 2.6. Computational methods

Utilizing the optimized lattice parameters obtained from the experimental data, the electronic density of states (DOS) was calculated using the density functional theory (DFT), employing a novel parameterization that specifically targeted solids PBEsol approximation as implemented in the plane-wave pseudopotential method Quantum ESPRESSO code [34–36]. The kinetic energy cut-off was optimized at 50 Ry, along with a charge density cut-off of 500 Ry. A Monkhorst-Pack scheme of 10 × 10 × 10 was used to sample a k-point grid. An ultrasoft (US) pseudopotentials [37] describes the interaction between the core and valence states. Spin-polarized calculations using (GGA+U) were used to accurately determine the energy band gap [38,39], and the effective Hubbard U parameter was determined to be 7.25 for Ti 3d, and 7.6 eV for Ni 3d states.

# 3. Results and discussion

## 3.1. Morphology and structure of the NiO-TiO₂ nanocomposites

Nanocomposites were synthesized using electrospinning by gradually increasing NiO amounts within TiO₂ nanofibers. After annealing at 500 °C in air for 4 hours, the samples exhibited a subdued grayish tone, and SEM analysis confirmed that their nanofibrous structure was preserved. This specific architecture not only maintains the nanofiber morphology but also enhances internal light reflection, potentially improving efficiency across various applications. In-depth SEM investigations of TN1, TN3 and TN5 samples containing 1 wt%, 3 wt%, and 5 wt% of NiO, respectively, revealed uniformly distributed nanofibers oriented randomly (illustrated in Fig. 1a-c). Despite using electrospinning parameters consistent with previous research [29,30], a slight increase in the mean nanofiber diameter was observed. The diameter increased from 254 ± 10 nm for TN1 to 278 ± 10 nm for TN3, and further to 286 ± 10 nm for TN5. This increase in diameter is likely attributable to the higher NiO content, suggesting a relationship between NiO content and nanofiber diameter.

The XRD patterns for both unmodified TiO₂ nanofibers and NiO-modified TiO₂ using 5 wt% NiO (TN5 sample) are displayed in Fig. 2a. The XRD pattern of pure TiO₂ reveals distinct peaks at 2θ angles of 25.3° (101), 37.7° (004), 48.0° (200), 53.8° (105), 55.0° (211), 62.6° (204), 68.7° (116), 70.2° (220), and 75.0° (215), confirming its tetragonal anatase structure (space group I4₁/amd) according to JCPDF CARD (21–1272) [29].

The incorporation of NiO resulted in the formation of the rutile TiO₂ phase (space group P4₂/mnm) as indicated by its characteristic XRD reflections at 27.4° (110), 36.0° (101), 39.1° (200), 41.2° (111), 44.0° (210), 56.5° (220), and 63.9° (310) according to JCPDS no. 21–1276 [40,41]. This result suggests that NiO promotes the transition from the anatase phase to the rutile phase. No XRD reflections corresponding to

the crystalline NiO phase are observed in the XRD pattern, which can be explained by its amorphous/nanocrystalline nature or its incorporation in the TiO₂ lattice. As shown in Fig. 2a and Fig S1 the anatase (101) peak positions shift to lower 2θ angles when NiO is introduced during the electrospinning process. The predominant shift to lower angles indicates significant incorporation of Ni²⁺ into the TiO₂ lattice, primarily through substitutional modes [42–45]. Rietveld refinement of the XRD pattern reveals that the TN5 sample consists of 56.9 wt% anatase and 43.1 wt% rutile. Furthermore, the unit cell volumes of anatase (136.57 Å³) and rutile (62.72 Å³) are slightly larger than the values reported in the literature (136.31 Å³ and 62.43 Å³, respectively) [29,40,41]. These findings confirm the partial substitution of Ti⁴⁺ by larger Ni²⁺ cations in both anatase and rutile phases (r(Ti⁴⁺) = 60.5 pm, r (Ni²⁺) = 69 pm, all cations are 6-fold coordinated) [46]. These results suggest that the interaction between NiO and TiO₂ leads to the formation of the rutile phase through the incorporation of Ni²⁺ cations in the TiO₂ lattice. This substitution lowers the activation energy for the anatase-to-rutile phase transition, facilitating the conversion. Additionally, the difference in ionic radius and charge between Ni²⁺ and Ti⁴⁺, along with the presence of oxygen vacancies in the TiO₂ lattice, further accelerates this phase conversion [42–45].

Raman spectroscopy typically reveals six active vibrational modes (A₁g, 2 B₁g, 3 Eg) for anatase TiO₂ (black curve) [29]. Some of these modes are particularly sensitive to structural defects and the presence of dopants within the anatase lattice. The Eg mode is associated with the symmetrical stretching vibrations of O–Ti–O groups, whereas the A₁g and B₁g modes correspond to the antisymmetric and symmetric bending vibrations of the O–Ti–O groups [47,48].

For NiO-TiO₂ (TN5) samples (red curve), the total intensity of the Raman signal increases. Additionally, all modes exhibit a slight redshift, particularly the Eg mode at 139 cm⁻¹. This redshift is attributed to phonon confinement and non-stoichiometry effects in the modified nanoparticles, likely due to the variable presence of oxygen vacancies [47–49].

The absence of the Eg mode at 195 cm⁻¹ for TiO₂ anatase can be attributed to structural and compositional changes caused by the incorporation of NiO nanoparticles. Specifically, the inclusion of NiO may lead to a phase transition from anatase to rutile, along with the substitution of Ti⁴⁺ by Ni²⁺, disrupting the vibrational modes associated with the anatase phase [42–45].

High-resolution transmission electron microscopy (HRTEM) images (Fig. 3a,b) demonstrates that the NiO-TiO₂ (TN5 sample) composite exhibits rough and extensive regions, displaying lattice spacings of 0.350 nm and 0.205 nm corresponding to the (101) crystalline plane of TiO₂ anatase and the (200) plane of the cubic structure of NiO (space group Fm-3m), respectively. This result indicates that not all NiO is incorporated in the TiO₂ lattice, and a small amount of nanocrystalline NiO phase is present in the TN5 sample, which can not be detected in XRD patterns, corroborating findings from prior studies [50,51].

In addition, elemental mapping (Fig. 3c-f) demonstrated a consistent and uniform distribution of titanium (Ti) and nickel (Ni) throughout the nanofiber matrix. This uniform distribution is likely due to the

<figure><img src="image_6.png" /><figcaption>Fig. 1. Scanning electron microscopy photographs of TN1 (a), TN3 (b) and TN5 (c) samples (described in Table 2).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 2. X-ray diffraction patterns (a) and Raman spectra (b) of nanofibers composites of the pure TiO2 and NiO-TiO2 (TN5) samples.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 3. (a,b) TEM images and (c-f) STEM-EDX chemical mapping of TN5 sample (NiO-TiO2).</figcaption></figure>

synergistic interaction between NiO and TiO₂, which facilitates effective charge separation and transfer. This interaction enhances the photo-catalytic performance of the composite by promoting more efficient electron-hole pair separation, reducing recombination rates, and thereby increasing the overall photocatalytic activity.

Moreover, the specific surface areas of TiO₂ and TN5 sample (NiO-TiO₂), calculated using the BET method, were determined to be 59.9 and 67.3 m²/g, respectively. The incorporation of NiO resulted in an increased surface area available for photocatalytic reactions, thereby improving the efficiency of pollutant degradation.

To gain deeper insights into the surface morphology and chemical composition of TN5 sample (NiO-TiO₂), an analysis using X-ray photo-electron spectroscopy (XPS) was conducted. Significant signal intensities were observed in the XPS survey (Fig. 4a), with prominent peaks to Ti 2p and O 1s, whereas signals for C 1s and Ni 2p were comparatively weaker.

In the O 1s spectrum, three prominent peaks were identified:

530.0 eV, corresponding to lattice oxygen; 531.4 eV, attributed to hydroxyl groups (O-H); and 532.9 eV, associated with adsorbed water (Fig. 4b). The Ti 2p spectrum exhibited peaks at 458.7 eV (Ti 2p₃/₂) and 464.5 eV (Ti 2p₁/₂), which indicates the presence of titanium in the Ti⁴⁺ oxidation state (Fig. 4c). In the Ni 2p spectrum (Fig. 4d), two peaks observed at 856.7 eV and 856.2 eV are attributed to Ni(OH)₂ and NiO, respectively. Satellite features characteristic of NiO are observed at 862.5 eV and 866.3 eV, associated with the Ni 2p₃/₂ region. Additional satellite peaks at 874.2 eV and 880.1 eV are detected in the Ni 2p₁/₂ region, further corroborating the presence of the nickel oxide phase. The observed satellite features and their positions provide strong evidence for the existence of NiO within the sample.

This thorough analysis highlights the uniform distribution of NiO within NiO-TiO₂ composite, providing valuable insights into its surface chemistry and reactivity. Such detailed characterization is crucial for understanding the material's potential applications in various fields, including catalysis and electronic devices.
<figure><img src="image_9.png" /><figcaption>**Fig. 4.** Chemical State Analysis of TN5 sample (NiO-TiO₂) *via* X-ray Photoelectron Spectroscopy: survey (a), O 1s (b), Ti 2p (c) and Ni 2p spectra (d).</figcaption></figure>

Fig. 5a illustrates the reflectance spectra of the samples. The TiO₂ nanofibers exhibited a distinct absorption band within the 320–400 nm range, aligning with their known bandgap properties. The incorporation of NiO into the TiO₂ matrix *via* electrospinning (TN5) significantly reduced reflectance in the visible range compared to bare TiO₂, indicating enhanced light absorption. This enhancement makes the TN5

nanofibers highly suitable for photocatalytic applications. Bandgap calculations revealed that the inclusion of NiO slightly increased the effective bandgap from 2.94 eV (bare TiO₂) to 3.1 eV (TN5). Additionally, the electrospun nanofibers demonstrated improved visible light absorption, emphasizing their potential for visible-light-driven photocatalysis. Fig. 5b displays the photoluminescence (PL) spectra of the

<figure><img src="image_10.png" /><figcaption>**Fig. 5.** (a) Reflectance and (b) photoluminescence spectra of TN5 (NiO-TiO₂) and TiO₂.</figcaption></figure>
samples. Bare TiO₂ nanofibers showed a prominent peak at 385 nm, corresponding to band-to-band transitions of photogenerated electrons. However, upon incorporating NiO into the TiO₂ matrix, the PL spectra underwent substantial changes. The 385 nm peak disappeared, and a new peak emerged at 500 nm in the visible range, attributed to oxygen vacancies. These vacancies are instrumental in enhancing visible-light photocatalytic activity by introducing localized states within the bandgap. Furthermore, the overall PL intensity of the TN5 sample was significantly reduced compared to bare TiO₂, indicating efficient photo-generated charge separation between NiO and TiO₂. This suppression of charge recombination strongly supports the enhanced photocatalytic performance of the TN5 nanocomposite, highlighting the synergistic interaction between NiO and TiO₂ in promoting effective charge transfer and visible-light absorption.

## 3.2. Photocatalytic investigations

The photocatalytic performance of TiO₂ and NiO-TiO₂ composites (containing 1 wt%, 3 wt%, and 5 wt% NiO) was evaluated by tracking the degradation of acetaminophen (ACT) under visible light for 4 hours. ACT was selected due to its widespread use, frequent detection in natural water and wastewater, and resistance to photodegradation without a catalyst. While TiO₂ and NiO are semiconductors that primarily absorb UV light, visible light was selected as the irradiation source to better replicate real-world conditions, specifically the solar spectrum, which is predominantly composed of visible wavelengths. This choice is vital for reflecting natural processes, where visible light plays a key role in driving photocatalytic reactions. Additionally, the exceptional photocatalytic performance of electrospun TiO₂ in its anatase form is attributed to its high surface area, which provides abundant active sites, and oxygen vacancies that enhance visible-light absorption and charge carrier separation. The nanofiber morphology further improves charge dynamics and surface interactions, making it highly efficient for applications such as environmental remediation [11–14]. Moreover, visible light is more energy-efficient than UV light, which requires more energy to generate and leads to higher energy consumption. By using visible light, the experimental setup more closely simulates the energy distribution of natural sunlight, making it a more practical and environmentally representative irradiation source for photocatalytic applications [52–55]. After 4 hours, degradation efficiencies were recorded at 52.1 %, 77.3 % for NiO, TiO₂, 75.9 % for TN1, 92.3 % for TN3, and 98.8 % for TN5, as illustrated in Fig. 6a.

These findings demonstrate that the addition of NiO greatly enhances the photocatalytic efficiency of the composites, with TN5

demonstrating the highest degradation efficiency.

The observed enhancement can be attributed primarily to the formation of p-n heterojunctions at the interface between NiO and TiO₂ [56–59]. As a p-type semiconductor, NiO creates a junction with n-type TiO₂, which promotes the efficient separation of photogenerated electron-hole pairs. This effective separation diminishes the recombination rate of the charge carriers, thereby increasing the availability of both electrons and holes for photocatalytic reactions [56–59]. Furthermore, the synergistic interactions between NiO and TiO₂ are recognized to enhance the production of reactive oxygen species (ROS), thereby further elevating the photocatalytic activity [60].

The photocatalytic performance was optimal at 5 wt% NiO, where an efficient p-n heterojunction formed, enhancing charge separation and visible light absorption, significantly improving acetaminophen degradation. However, higher NiO concentrations (>5 wt%) led to reduced performance due to blocked active sites, increased recombination centers, and practical challenges in electrospinning, such as poor solubility, inhomogeneous mixtures, and disrupted fiber formation.

Overall, these findings underscore the significant role of NiO, particularly at optimal concentrations, in enhancing the photocatalytic efficiency of TiO₂ nanocomposites under visible light irradiation.

ACT degradation kinetics modeled as pseudo-first order, confirmed by the linearity of the degradation curve and an R² close to 1 (Fig. 6b). ACT degradation rate was higher with TN5 than with the TiO₂, TN1 and TN3 samples due to synergistic effects between NiO and TiO₂. The maximum rate constant of 0.0133 min⁻¹ was observed for TN5, while the rates for TN1 and TN3, TiO₂ and NiO were 0.0091 min⁻¹ and 0.0055 min⁻¹, 0.0052 min⁻¹ and 0.0032 min⁻¹ respectively.

In this study, the degradation efficiency of ACT (10 mg/L, pH 7) was monitored over five cycles using a TN5 catalyst. After each cycle, the photocatalyst was recovered by filtration, thoroughly washed with deionized water, and dried at 100 °C for 12 hours in accordance with established protocols [29]. The degradation efficiency exhibited a progressive decline, decreasing from 98.8 % in the first cycle to 93.8 % in the second, 85.5 % in the third, 76.4 % in the fourth, and ultimately reaching 70.3 % in the fifth cycle (Fig. 7a). The observed decline can be attributed to surface fouling by reaction intermediates and minor structural or compositional changes, such as the passivation of oxygen vacancies or the reduction of active surface species. These factors impaired light absorption, charge separation, and surface reactions, leading to reduced efficiency. Subsequent BET analysis revealed specific surface areas of 67.3 m²/g for TN5 and 54.6 m²/g for the TN5 obtained after 5 cycles. XRD and SEM analyses confirmed that the morphology and crystallinity were preserved for both samples (Fig S2 and Fig S3).

<figure><img src="image_11.png" /><figcaption>Fig. 6. (a) Degradation of ACT with varying NiO concentrations on TiO₂ under visible light, and (b) corresponding photocatalytic kinetics.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 7.** (a) Reusability and regeneration of TN5, (b) ACT degradation in the presence of NiO (5 wt%)-TiO₂ and of EDTA, isopropanol (IPA), or p-benzoquinone (BQ) and (c) V. fischeri fluorescence inhibition during ACT degradation by NiO (5 wt%)-TiO₂ in the presence of visible light.</figcaption></figure>

However, thermal regeneration at 500 °C restored the photocatalytic activity to its initial level (99 %). This recovery is likely due to the removal of surface fouling, the reactivation of oxygen vacancies, and the reoxidation of surface species, thereby reinstating the material's original structural and functional properties.

In conclusion, while the decrease in catalytic activity over five wash cycles highlights some challenges, the overall stability exhibited by the NiO-TiO₂ composite underscores its potential for industrial applications. Further optimization is needed to mitigate the observed loss in activity, but the results are promising for the development of durable and efficient photocatalysts.

Following this, degradation experiments for ACT were performed under identical conditions using the NiO-TiO₂ nanocomposite and a variety of scavengers, each at a concentration of 0.06 M [29,50,61] (refer to Fig. 7b). The degradation efficiency of ACT was reduced to

94.1 %, 12.8 %, and 19.5 % with the addition of isopropanol (acting as *OH scavenger), EDTA served as h⁺ scavenger, and benzoquinone acted as an O₂⁻ scavenger, respectively. These results clearly indicate that h⁺ and O₂⁻ radicals are crucial in the degradation process of ACT when exposed to visible light.

To examine the formation of toxic by-products during ACT degradation, V. fischeri was exposed to TN5 and ACT solution, with fluorescence intensity measured at various intervals under visible light (Fig. 7c) by subjecting V. fischeri to TN5 and ACT solution, and monitoring V. fischeri fluorescence intensity at various time intervals during visible light exposure. The fluorescence inhibition of V. fischeri was 40.6 % after 5 minutes, decreasing progressively to 20.4 % at 1 hour, 17.4 % at 2 hours, 16.3 % at 4 hours, 16.0 % at 6 hours, 8.16 % at 12 hours, 6.5 % at 16 hours, 5.8 % at 20 hours, and 4.11 % at 24 hours. These findings suggest that both ACT and its by-products were progressively

<table><caption>**Table 3**

NiO-TiO₂ and other photocatalysts (previous studies) used to degrade water pollutants.</caption><thead><tr><th>Photocatalyst (g.L<sup>-1</sup>)</th><th>Pollutant (mg.L<sup>-1</sup>)</th><th>Synthesis method</th><th>Irradiation type</th><th>Removal efficiency</th><th>Degradation time (min)</th><th>Ref</th></tr></thead><tbody><tr><td>NiO-TiO<sub>2</sub><br>(0.5 g/L)</td><td>ACT<br>(10 mg/L)</td><td>Sol-gel +<br>Electrospinning</td><td>Halogen linear lamp</td><td>98.8 %</td><td>240</td><td>Present<br>Work</td></tr><tr><td>TiO<sub>2</sub> (0.5 g/L)</td><td></td><td>Sol-gel +<br>electrospinning</td><td></td><td>77.3 %</td><td></td><td></td></tr><tr><td>TiO<sub>2</sub> (NT) +<br>NiO (Np)<br>(0.05 g)</td><td>MB<br>(5 mg/L)</td><td>Hydrothermal</td><td>Solar light</td><td>99.5 %</td><td>25</td><td>[22]</td></tr><tr><td>TiO<sub>2</sub>-NiO<br>Thin films<br>(2,5 × 2,5 cm<sup>2</sup>)</td><td>MB<br>(2µmol/L)</td><td>DC magnetron sputtering</td><td>UV</td><td>25 %</td><td>60</td><td>[69]</td></tr><tr><td>NiO(1.0)/F-TiO<sub>2</sub><br>(0.5 g/L)</td><td>4-chlorophenol (40 mg/L)</td><td>Sol-gel+ impregnation</td><td>UV</td><td>84.9 %</td><td>180</td><td>[25]</td></tr><tr><td>TiO<sub>2</sub>/NiO-0.5 core shell (Np)<br>(1 g/L)</td><td>RhB<br>(No details)</td><td>Wet chemical method</td><td>Sunlight</td><td>98.3 %</td><td>60</td><td>[26]</td></tr><tr><td>NiO-TiO<sub>2</sub>/TiO<sub>2</sub> bilayer<br>(No details)</td><td>Acid orange 7<br>(20 mg/L)</td><td>Spin coating</td><td>UV-A illumination</td><td>50 %</td><td>240</td><td>[27]</td></tr><tr><td>TiO<sub>2</sub>-NiO (10 %)<br>(Np)<br>(1 g/L)</td><td>Brillant green<br>(10 mg/L)<br>Phenol<br>(10 mg/L)</td><td>Surfactant assisted sol-gel</td><td>Simulated sunlight</td><td>86.7 %</td><td>180</td><td>[15]</td></tr><tr><td>TiO<sub>2</sub>-NiO<br>(Np)<br>(50 mg)</td><td>Crystal violet<br>(10 mg/L)<br>Acid orange<br>7<br>(10 mg/L)<br>Alizarin yellow<br>(10 mg/L)</td><td>Sol-gel</td><td>Direct sunlight</td><td>93</td><td>270</td><td>[21]</td></tr><tr><td>TiO<sub>2</sub>-NiO (6/1) nanorods<br>(0.2 g/L)</td><td>MB<br>(20 mg/L)</td><td>Sol-gel</td><td>Direct sunlight</td><td>77</td><td></td><td>[21]</td></tr><tr><td>TiO<sub>2</sub>-NiO (Np)<br>(0.4 g/L)</td><td>RhB<br>(5 mg/L)</td><td>Sol-gel</td><td>Direct sunlight</td><td>97</td><td></td><td>[21]</td></tr><tr><td>TiO<sub>2</sub>-NiO (Np)<br>(1 g/L)</td><td>MB<br>(10<sup>-5</sup> mol/L)</td><td>Hydrothermal method</td><td>Solar light<br>300 W Xe-lamp</td><td>80</td><td>360</td><td>[24]</td></tr><tr><td></td><td></td><td>Sol-gel</td><td>Visible light source</td><td>98.25 %</td><td>360 min</td><td>[23]</td></tr><tr><td></td><td></td><td>Surfactant + sol-gel</td><td>UV</td><td>95 %</td><td>120 min</td><td>[28]</td></tr></tbody></table>
transformed into non-toxic compounds throughout the duration of the experiment.

The superior performance of these nanofibers, as demonstrated in Table 3, underscores their potential as a highly efficient solution for environmental remediation and offers a significant advancement over existing photocatalysts.

Fig. 8 illustrates a simplified model of the photocatalytic degradation mechanism of acetaminophen (ACT) facilitated by the NiO-TiO₂ photocatalyst under visible light. Upon light irradiation, the NiO-TiO₂ catalyst absorbs visible light, generating photogenerated electron-hole pairs [62]. Electrons in the conduction band of TiO₂ reduce molecular oxygen to form superoxide anions (O₂⁻), while holes (h⁺) in the valence band participate directly in oxidative reactions. Scavenger tests have confirmed that O₂⁻ and h⁺ are the primary reactive species responsible for driving the degradation process. The suppression of photocatalytic activity upon their inhibition underscores their critical roles in the reaction mechanism [63,64]. The NiO-TiO₂ heterojunction enhances photocatalytic efficiency by promoting charge separation and minimizing electron-hole recombination. The resulting charge carriers migrate to the catalyst surface, where O₂⁻ and h⁺ initiate oxidative reactions with adsorbed acetaminophen molecules [64–66]. The degradation pathway begins with an oxidative attack on the aromatic ring of acetaminophen, leading to the formation of key intermediates such as p-aminophenol and p-nitrophenol. These intermediates undergo further oxidation, producing secondary compounds such as hydroquinone, catechol, and benzoquinone. Subsequent cleavage of aromatic structures yields short-chain organic acids, including maleic acid, oxalic acid, and formic acid. Simultaneously, nitrogenous functional groups are transformed into ammonium (NH₄), nitrite (NO₂), and nitrate (NO₃). Complete mineralization results in environmentally benign end products, carbon dioxide (CO₂) and water (H₂O) [67,68]. The structural advantages of the nanofibrous composite, including its high specific surface area and abundant active sites, further amplify the catalytic efficiency. Together with the synergy of the NiO-TiO₂ heterojunction and the dominant roles of O₂⁻ and h⁺, this system achieves effective visible-light-driven degradation of acetaminophen, making it a promising approach for environmental remediation.

## 4. Electronic properties

It was crucial to comprehend the electronic properties and density of states (DOS) of the individual oxides that comprised the composites. The band gap is crucial for the photocatalytic process. For this reason, we performed (DOS) calculations of TN5 nanocomposite, as shown in Fig. 9. The calculated band gap closely aligns with experimental values and is comparable to previously reported findings [70–73].

Moreover, we found that the DOS profile of TN5 is dominated by anatase TiO₂ and was characterized by O 2p-dominated valence bands and Ti 3d-dominated conduction bands, while the valence bands comprised slightly hybridized Ni 3d and Ti 3d states of TiO₂. DOS calculations indicate that the photocatalytic activity of NiO-TiO₂

<figure><img src="image_13.png" /><figcaption>Fig. 9. Calculated total and projected density of states (T/PDOS) calculations of TN5 nanocomposite.</figcaption></figure>

composites (TN5 sample) is enhanced by the large dispersion of Ni 3d and Ni 2p/s states and the optical indirect transition, which promotes photon energy storage and enhances electron-hole separation. The first principal calculations indicate a net charge transfer of 1.248 electrons from Ni to O ions [73]. The charge transfer properties of NiO result in longer carrier lifetimes [74], which facilitate the separation of e⁻/h⁺ for higher efficiencies. We expected that e⁻ will be transferred from the conduction band maxima (CBM) of NiO to CBM of TiO₂, and h⁺ will migrate from the valence band maxima (VBM) of TiO₂ to VBM of NiO, fully driven by the band energetics in for TN5.

## 5. Conclusion

NiO-TiO₂ composite nanofibers were synthesized using sol-gel process and electrospinning, incorporating controlled amounts of nickel oxide (1–5 wt%) followed by calcination at 500 °C. Detailed characterization techniques, including SEM, TEM, Raman spectroscopy, and XRD, confirmed that the nanofibrous structure was preserved and NiO was successfully integrated. BET analysis revealed an increase in surface area, which is advantageous for photocatalytic reactions and enhances pollutant degradation efficiency. XPS analysis identified Ni²⁺ through the presence of Ni 2p peaks. The photocatalytic activity under visible light showed that NiO-TiO₂ nanocomposites (5 wt% NiO) exhibited superior acetaminophen degradation compared to TiO₂ nanofibers alone, as evidenced by changes in reflectance and photoluminescence. Thermal regeneration at 500 °C effectively restores the photocatalytic efficiency of TN5 after a gradual decline over five cycles, highlighting its stability and potential for reuse. Additionally, acute toxicity and scavenging tests highlighted the significant roles of O₂⁻, and h⁺ species in

<figure><img src="image_14.png" /><figcaption>Fig. 8. Simplified model of NiO-TiO₂ catalyst activity for ACT degradation under visible light.</figcaption></figure>
acetaminophen degradation. The DOS results suggest that the photo-catalytic activity of NiO-TiO₂ composites is enhanced by the dispersion of Ni 3d and 2p states, promoting efficient photon energy storage and electron-hole separation. Overall, the NiO-TiO₂ nanocomposite with 5 wt% NiO exhibited notable photocatalytic efficiency and long-term stability, presenting itself as a cost-effective option for large-scale photocatalyst production for water pollutant removal. However, further optimization of the NiO content and exploration of alternative dopants are essential to improve the material's performance. Additionally, testing the composites under real-world conditions and for the degradation of a broader range of pollutants will be critical to assessing their scalability and practical applications. These future directions will pave the way for enhanced photocatalytic materials with greater efficiency and versatility.

### CRediT authorship contribution statement

**Roland Habchi:** Writing - review & editing, Visualization, Validation, Supervision. **Mahmoud Abid:** Writing - original draft, Validation, Methodology, Formal analysis, Data curation, Conceptualization. **Emerson Coy:** Writing - review & editing, Validation, Resources, Funding acquisition, Formal analysis, Data curation. **Geoffroy Lesage:** Writing - review & editing, Visualization, Validation, Formal analysis. **Philippe Miele:** Writing - review & editing, Validation, Supervision, Formal analysis. **Igor Iatsunskyi:** Writing - review & editing, Visualization, Validation, Resources, Data curation. **Djamel Bezzerga:** Writing - original draft, Validation, Software, Resources, Formal analysis, Data curation. **Jisang Hong:** Writing - review & editing, Visualization, Validation, Supervision, Resources, Formal analysis. **Roman Viter:** Data curation, Formal analysis, Investigation, Validation, Writing - review & editing. **Olga Mazur:** Writing - review & editing, Validation, Methodology, Investigation, Formal analysis, Data curation. **Maged F. Bekheet:** Data curation, Formal analysis, Validation, Visualization, Writing - review & editing. **Amr A. Nada:** Writing - review & editing, Visualization, Validation, Formal analysis. **David Cornu:** Writing - review & editing, Visualization, Validation, Supervision, Formal analysis. **Mikhael Bechelany:** Writing - review & editing, Visualization, Validation, Supervision, Project administration, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Tia Maria Howayek:** Writing - original draft, Validation, Investigation, Formal analysis, Data curation.

### Declaration of Competing Interest

The authors declare no conflict of interest.

### Acknowledgements

This research was funded by the National Research Foundation of Korea (NRF) (2022R1A2C1004440/2024-CRE-0093). T.H. acknowledges support from the National Council of Lebanon (CNRS-L) and the University of Montpellier (UM) for the doctoral fellowship.

### Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.colsurfa.2024.136077.

### Data availability

Data will be made available on request.

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