Influence of interface defects and Zr doping on the photocatalytic activity of TiO2 nanofibers in water splitting

Mehrdad Mehdikhanloo, Zahra Hagheh Kavousi, Massomeh Ghorbanloo, Amr A Nada, Heba Hassan El-Maghrabi, Rasha Hosny, Fouad Zahran, Roman Viter, Djamel Bezzerga, Jisang Hong, et al.

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Mehrdad Mehdikhanloo, Zahra Hagheh Kavousi, Massomeh Ghorbanloo, Amr A Nada, Heba Hassan El-Maghrabi, et al. Influence of interface defects and Zr doping on the photocatalytic activity of TiO2 nanofibers in water splitting. Journal of Alloys and Compounds, 2025, 1020, pp.179439. 10.1016/j.jallcom.2025.179439. hal-05133323

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# Influence of interface defects and Zr doping on the photocatalytic activity of TiO₂ nanofibers in water splitting

Mehrdad Mehdikhanloo<sup>a</sup>, Zahra Hagheh Kavousi<sup>a,b</sup>, Massomeh Ghorbanloo<sup>a,*</sup>, Amr A. Nada<sup>b,c,**</sup>, Heba H. El-Maghrabi<sup>b,d</sup>, Rasha Hosny<sup>e</sup>, Fouad Zahran<sup>f</sup>, Roman Viter<sup>g</sup>, Djamel Bezzerga<sup>h</sup>, Jisang Hong<sup>h</sup>, Bernard Fraisse<sup>b</sup>, Fathi S. Soliman<sup>d</sup>, Igor Iatsunskyi<sup>i</sup>, Emerson Coy<sup>i</sup>, Yaovi Holade<sup>b</sup>, Mitra Saffari<sup>a</sup>, Mikhael Bechelany<sup>b,j,**</sup> ID

## ARTICLE INFO

Keywords:

Zirconium

Electrospinning

Photocatalysis

Hydrogen evolution

Water splitting

## ABSTRACT

Efficient hydrogen production by water photocatalysis requires effective charge separation and optimal reactant-catalyst interactions. However, producing catalysts with these features is still challenging. Here, electrospinning and calcination were used to fabricate zirconium (Zr)-doped TiO₂ nanofibers with controlled crystalline phase. In all tested nanofiber samples (pure TiO₂ and after doping with different Zr amounts), TiO₂ was detected mainly in the anatase form. All nanofibers displayed high purity and crystallinity. Moreover, X-ray photoemission spectroscopy showed that Zr was present as Zr⁴⁺ ions on the TiO₂ surface. The Zr-doped TiO₂ nanofiber band gaps (determined with the Kubelka-Munk function) ranged from 3.00 eV to 3.2 eV (vs 3.09 for TiO₂). The absorption of visible light was improved in Zr-doped TiO₂ nanofibers because Zr addition led to the creation of intermediate electronic levels between the conduction and valence bands, and to the decrease of the electron recombination rate. Then, Zr doping effect on TiO₂ electronic structure and photocatalytic properties was evaluated by density functional theory modeling. The nanofibers with 4 mol% of ZrOCl₂·8 H₂O exhibited the highest photocatalytic activity under visible light (hydrogen evolution rate of 2500 µmol·g⁻¹·h⁻¹, 23 times higher compared with pure TiO₂ nanofibers). These one-dimensional nanostructures also showed excellent recyclability and sustained high photocatalytic efficiency, promising features for environmental and energy applications.

## 1. Introduction

Energy is crucial for human survival and progress [1]. Rapid economic growth results in greater consumption of fossil fuels, leading to energy shortages and environmental decline [2,3]. Hydrogen, often termed the "fuel of the future", is considered a clean and renewable resource [4,5]. Hydrogen demand is progressively increasing worldwide

due to its extensive use in industrial processes and refining [6], as direct fuel and as a material to synthesize or upgrade fuels in order to meet stringent environmental regulations [7]. However, many scientific and technical issues still limit the wide implementation of a hydrogen-based economy. Hence, novel advanced materials and technologies are required to produce and extract high-purity hydrogen gas [7].

The semiconductor photocatalytic technology, which harnesses solar

<footnote><sup>a</sup> Chemistry Department, Faculty of Science, University of Zanjan, P. O. Box: 4537138791, Zanjan, Iran</footnote>

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

<footnote><sup>c</sup> Department of Analysis and Evaluation, Egyptian Petroleum Research Institute, Cairo 11727, Egypt</footnote>

<footnote><sup>d</sup> Department of Refining, Egyptian Petroleum Research Institute, Nasr city, Cairo PB 11727, Egypt</footnote>

<footnote><sup>e</sup> Department of Production, Egyptian Petroleum Research Institute (EPRI), Cairo 11727, Egypt</footnote>

<footnote><sup>f</sup> Faculty of Science, Helwan University, Cairo 11795, Egypt</footnote>

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

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

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

<footnote><sup>j</sup> Functional Materials Group, Gulf University for Science and Technology, Mubarak Al-Abdullah 32093, Kuwait</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding authors at: Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier, France.</footnote>

<footnote>E-mail addresses: m_ghorbanloo@znu.ac.ir (M. Ghorbanloo), amr.nada@umontpellier.fr (A.A. Nada), Mikhael.bechelany@umontpellier.fr (M. Bechelany).</footnote>
energy to split water into hydrogen, is a practical method for addressing some of the issues associated with the storage of solar energy and the generation of green hydrogen fuel [3,8]. Titanium dioxide (TiO₂) is a widely used semiconductor photocatalyst; however, its large-scale application is hindered by its wide band gap and inefficient use of solar light [9,10]. To improve TiO₂ photocatalytic efficiency, researchers have tested various methods that are generally classified in two main groups: i) chemical modifications whereby additional components are incorporated to modulate TiO₂ chemical characteristics [11–14] and ii) physical modifications to change TiO₂ morphology [15, 16]. These modifications can modulate key factors implicated in TiO₂ photocatalytic activity, such as surface area, absorption edge, crystallite size and charge transfer processes. Moreover, they can be tailored to meet specific applications [17]. Introducing dopants into a material can extend its responsiveness to visible light and suppress the recombination of electron-hole pairs, thereby enhancing photocatalytic efficiency. In the case of TiO₂, doping with transition metals can induce lattice distortions and alter surface characteristics, which influence its photocatalytic behavior. Studies have demonstrated that substituting an appropriate amount of Ti⁴⁺ with Zr⁴⁺ within the TiO₂ lattice can hinder electron-hole recombination through charge trapping, leading to enhanced photocatalytic properties [18]. Additionally, studies have shown that incorporating a small amount of Zr can enhance the thermal stability of the anatase phase and modify the surface characteristics of TiO₂, thereby boosting its photocatalytic activity [19].

Recently, electrospinning has gained prominence as an efficient technique for producing semiconductor photocatalysts with enhanced surface area, well-defined one-dimensional micromorphology, practical applicability [20], as well as high porosity and interconnected structures [21].

Here, Zr-doped TiO₂ NFs were produced using electrospinning. The interfacial contacts between Zr and TiO₂ favored the formation of Zr–O–Ti bonds and induced the generation of more surface defects and thus surface states in the TiO₂ band. After synthesis, the morphology, structure and optical properties of Zr-doped TiO₂ and TiO₂ NFs were precisely analyzed. Then, hydrogen generation upon exposure to visible light (wavelength > 400 nm and methanol as scavenger) was monitored in the presence of Zr-doped TiO₂ and TiO₂ NFs. Lastly, the density of states (DOS) of the different samples was computed using the density functional theory (DFT) method within periodic boundary conditions. This allowed constructing the complete band alignment scheme for the Zr-doped TiO₂ system.

## 2. Experimental

### 2.1. Materials

ZrOCl₂·8H₂O (Mw = 322.28 g/mol), titanium tetraisopropoxide (TTIP) (Ti(OCH(CH₃)₂)₄, 97 %), polyvinylpyrrolidone (PVP; Mw=1300000 g/mol), acetic acid (98 %), absolute ethanol (99 %), absolute methanol (99 %), N,N-dimethylformamide (DMF; 99.8 %), and Nafion™ perfluorinated resin solution, were from Sigma Aldrich. All chemicals were used without further purification.

### 2.2. Preparation of Zr-doped TiO₂ composite nanofibers

Zr-doped TiO₂ NFs with different mole percentages (mol%) of ZrOCl₂·8H₂O relative to TiO₂ [0.5 mol% (TZr0.5), 1 mol% (TZr1), 2 mol % (TZr2), 3 mol% (TZr3) and 4 mol% (TZr4)] were fabricated by electrospinning.

After stirring 0.3 g PVP in a solution with TTIP, 3 mL ethanol and 2 mL acetic acid at room temperature for 30 minutes and dissolving ZrOCl₂·8H₂O at different concentrations in a mixture of 2 mL ethanol and 0.5 mL DMF for 30 minutes, the two solutions were mixed together vigorously for 1 hour to homogeneity. Then, the mixture was loaded into a syringe with a stainless-steel needle (0.7 mm in diameter) and

electrospinning was carried out at a constant flow rate of 1 mL/h, chamber temperature of 38 ± 5 °C, and high voltage power (1.25 kV/cm). The resulting NFs were gathered on a rotating coil (400 rpm) covered with aluminum foil and calcinated in a furnace at 400 °C (heating rate of 5 °C/min in air) for 4 h.

### 2.3. Physicochemical characterization

The crystal structure and crystallite size of the different NFs were assessed by X-ray Diffraction (XRD) with a Bruker D8 Discover diffractometer that included a Johansson monochromator optimized for CuKα1 radiation and a LynxEye XE-T detector. This diffractometer, set at 40 kV and 40 mA, produced diffraction patterns ranging from 10° to 90°. Their morphology and structure were assessed using scanning (Hitachi S4800, Japan) and transmission (JEOL ARM 200 F) electron microscopy (SEM and TEM, respectively) at an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDX) and a Zeiss EVO HD15 microscope coupled to an Oxford X-MaxN EDX detector brought insights into their composition and elemental maps, respectively. Dispersive Raman spectroscopy (Model Sentera, Bruker, Germany), at 532 nm with a neodymium-doped yttrium aluminum garnet (60 mW), and a Bruker Fourier transform infrared (FT-IR) spectrophotometer with KBr disks allowed recording the Raman and FT-IR spectra, respectively. The band gaps and photoluminescence spectra were determined using an ultraviolet-visible light (UV–Vis) spectrophotometer (CARY 5E) and an optical fiber spectrometer (Ocean Optics usb2000), respectively. Elemental analysis and elemental composition were done by Varian Spectra 220 AA spectroscopy and X-ray photoelectron spectroscopy (XPS) with an Escalab 250 spectrophotometer (Thermo Fisher Scientific, USA) using monochromatic Al Kα (1486.6 eV) at 2 kV and 1 μA (surface diameter = 400 μm and acquisition time = 1203.5 s), respectively. Inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer Optima 8000 i) was used to investigate the elemental composition. Electron/hole generation, migration, and recombination were monitored by photocurrent response and electrochemical impedance spectroscopy (EIS) with a ModuLap XM potentiostat (Solartron Analytical, USA), a three-electrode cell, 0.5 M Na₂SO₄ aqueous solution as an electrolyte, Pt wire as counter electrode, Ag/AgCl as reference electrode, and TZrₓ coated on indium–tin–oxide (ITO) as working electrode. The light source was a 500 W linear halogen lamp (λ range = 420–600 nm and frequency range = 10–2–105 Hz). The working electrode was prepared by depositing 40 μL of nanocomposite suspension onto the ITO surface (10 × 10 mm) followed by drying at room temperature. The suspension was prepared by ultrasonic dispersion of 5 mg of photocatalyst in 80 μL deionized water, 20 μL ethanol, and 40 μL Nafion followed by ultrasonication for 60 min. The EIS data were analyzed and fitted to the equivalent circuit with the EC-Lab software. The photocurrent vs. irradiation time curve was recorded at 0.0 V/SCE to 1.1 V/SCE in the dark (20 s) and in the presence of light (20 s) at a scan rate of 10 mV·s⁻¹.

### 2.4. Photocatalytic H₂ generation

H₂ was produced by photocatalysis in an air-tight quartz reactor where the reaction temperature was maintained at 25 °C by a cold-water circulation system. The irradiation source was a 500 W linear halogen lamp that emitted light with a spectrum between 420 and 600 nm [22]. The lamp-aqueous solution distance was 10 cm. NFs (50 mg) were dispersed in 20 mL of 20 % methanol aqueous solution (i.e. the scavenger) and then transferred into the reactor. Before starting the reaction, oxygen was eliminated from the medium by pre-purging with nitrogen for 30 min. H₂ production was monitored every hour by gas chromatography (GC-1690, Jiedao, TCD, Ar carrier) with a 5 Å molecular sieve column (3 m × 2 mm). To understand the reactive species involved in the TZrₓ photocatalytic water decomposition process, scavenger studies were carried out under optimized conditions. Three different chemical
scavengers, i.e., 20 % methanol, 20 % isopropyl alcohol (IPA), and 20 % 1,4-benzoquinone (BQ)] were tested as H₂ scavengers in comparison with water alone in the air-tight quartz reactor.

### 3. Results and discussion

After characterization of the structure and morphology of pure and doped TiO₂ NFs, the effects of these factors on TiO₂ photocatalytic behavior were evaluated. Methanol was included as sacrificial reagent.

#### 3.1. Structure and morphology of the fabricated nanofibers

Analysis of the crystallinity and phase composition revealed that all as-prepared NFs exhibited the XRD pattern corresponding to the anatase tetragonal structures of TiO₂ (JCPDS No. 01-084-1285) (Fig. 1). The peaks at 2θ of 25.27°, 37.86°, 48.05°, 53.92°, 55.1° and 62.7° were due to the (101), (004), (200), (105), (211) and (204) lattice planes of TiO₂ anatase, respectively (Fig. 1) [23,24]. Notably, no characteristic peak of ZrO₂ was detected despite the addition of Zr reactants (0.5, 1, 2, 3 and 4 %) during NF preparation and the presence of Zr peaks in the EDX and field-emission SEM mapping results. This is mainly due to the fact that Zr (+IV) ions serve only as dopants in the TiO₂ matrix. Indeed, upon doping with low Zr⁴⁺ amounts, they can take the place of Ti⁴⁺ ions in an uniform manner or be incorporated in the interstitial sites of the TiO₂ lattice [25]. In samples with increasing Zr⁴⁺ amounts, the XRD peaks moved slightly to lower 2θ angles, possibly due to lattice distortions or changes within the crystal structure [26]. In agreement, the Rietveld refinement findings highlighted an increase of the unit cell volume and crystallite size in samples with increasing Zr⁴⁺ amounts: from 136.52 Å³ and 28 nm (pristine TiO₂) to 137.56 Å³ and 34 nm (TZr4 composites), based on the XRD patterns. This could be due to the replacement of Ti⁴⁺ (with smaller ionic radius) by Zr⁴⁺ with larger ionic radius [(r(Ti⁴⁺) = 60.5 pm vs r (Zr⁴⁺) = 72 pm)]. Similar findings have been reported by Luka et al., and Mohamed et al. [27,28]. Comparison of the lattice parameters in the Zr-doped TiO₂ NFs and pristine TiO₂ NFs did not highlight any significant change in the lattice parameters a and b. Conversely, the vertical parameter c was larger in TZr4 than in TiO₂ (9.53748 Å vs 9.50448 Å). Like for the cell volume, the increase of parameter c may be due to the ionic radius difference between Ti⁴⁺ and Zr⁴⁺. Altogether, these data

show that the crystallinity of the TiO₂ anastase NFs was increased by Zr addition. Moreover, Zr⁴⁺ presence in the octahedral interstitial sites of TiO₂ and its high oxygen affinities facilitate the creation of oxygen vacancies [29]. The TZr4 sample also exhibited higher micro-strain (57 %) than pure TiO₂ (43 %), possibly due to the lattice distortion and nano-sized crystallites [30]. This result can be considered another proof of the successful Zr (+IV) doping of the TiO₂ matrix.

XPS provided additional insights into how the presence of some elements affected the chemical composition of Zr-doped TiO₂ NFs (Fig. 2). After deconvolution, four peaks were detected in the O 1 s spectrum (Fig. 2a): at 529.97 eV (due to the lattice oxygen), 530.90 eV (due to the incorporation of hydroxyl groups in the lattice), 531.95 eV (due to C-O bonding from residual PVP), and 533.28 eV (indicative of chemisorbed water molecules) [31]. The Oads/Olatt ratio (a marker of surface oxygen vacancies) was higher in the TZr4 than in the undoped TiO₂ sample (0.175 vs 0.105), suggesting a greater amount of oxygen vacancies (Table 1). Indeed, upon Ti⁴⁺ replacement by Zr⁴⁺ in the TiO₂ lattice, the slightly larger ionic radius of Zr⁴⁺ causes strain. To alleviate this strain, nearby oxygen ions can escape to the surface, leaving oxygen vacancy donors that will increase the oxygen vacancies in the NFs. In the Ti 2p XPS spectra (Fig. 2b), two main peaks were observed at 458.67 eV and 464.38 eV, linked to the Ti 2 P₃/₂ and Ti 2 P₁/₂ states of Ti⁴⁺ in the TiO₂ lattice [32]. The Ti 2 P spin-orbit splitting value (5.7 eV) confirmed that Ti⁴⁺ was the main oxidation state in the TiO₂ sample [33]. In the TZr4 sample, the binding energies of the Ti 2 P peaks were decreased by ~0.05 eV, confirming Zr⁴⁺ ion addition in the TiO₂ lattice. As Ti has a Pauling electronegativity value higher than Zr (1.5 vs 1.3), charges are transferred from Zr⁴⁺ to Ti⁴⁺ via Ti-O-Zr bonds, and thus, the electron density of Ti ions increases. This explains the lower binding energies of the XPS peaks corresponding to Ti ions. In the Zr 3d spectrum of ZrO₂ (Fig. 2c), two peaks at 181.91 and 184.30 eV (due to Zr 3d₅/₂ and 3d₃/₂) were detected [34]. Their binding energy was increased by ~0.15 eV in TZr4 [35], highlighting the presence of Zr⁴⁺ ions in the TiO₂ lattice. Like for the lower binding energy of the Ti ion peaks, this shift is due to the charge transfers from Zr⁴⁺ to Ti⁴⁺ ions via Ti-O-Zr bonds and consequently to the change in electron density (increase for Ti ions and decrease for Zr ions) that leads to a change in the peak binding energies (decrease for Ti ions and increase for Zr ions) compared with pure TiO₂ and ZrO₂. The same Zr/Ti mole ratios (~0.04 %) for the TZr4 sample surface were obtained by XPS and EDX (Table 1). Additionally, the Olatt/Ti and Olatt/(Ti+Zr) mole ratios were lower and the Zr/Ti mole ratio was higher in TZr4 than TiO₂ NFs (Table 1). This validated the creation of oxygen vacancies at the TZr4 surface upon Ti⁴⁺ replacement by Zr⁴⁺ in the TiO₂ lattice. However, as Zr⁴⁺ and Ti⁴⁺ have the same oxidation state, doping with Zr ions should not directly increase the oxygen vacancies in Zr-doped TiO₂ NFs. The following hypothesis could explain their increase. In the TiO₂ anatase lattice, Ti⁴⁺ are at the center of the TiO₆ octahedral network, thus the coordination number for Ti⁴⁺ ions is 6. For such combination, the larger ionic radius of Zr⁴⁺ (72 pm vs 60.5 pm for Ti⁴⁺) generates a strain in the lattice. To alleviate this strain, nearby oxygen ions can escape to the surface, leaving oxygen vacancy donors that will increase the density of free electrons in the NFs [36]. Lastly, the peaks at 284.8, 286.19 and 288.73 eV (Fig. 2d), associated with different carbon atoms in the PVP molecule [37], indicated carbon contamination.

Next, the crystal structure was analyzed by Raman spectroscopy (Fig. 3a). In all samples, the Raman spectra showed the presence of Raman-active modes that corresponded to the TiO₂ anatase phase at 147.2 cm⁻¹ (Eg), 198.2 cm⁻¹ (Eg), 394.3 cm⁻¹ (B₁g), 511.3 cm⁻¹ (A₁g+B₁g), and 636.2 cm⁻¹ (Eg) [38,39], in agreement with the XRD results. In samples with increasing Zr⁴⁺ amounts, the Raman bands became larger and their wavenumbers decreased (by ~1.4 cm⁻¹), particularly the Eg mode near 145.8 cm⁻¹. This could be due to the TiO₂ lattice extension as a consequence of the increase in oxygen vacancies [40]. The vibrational properties are influenced by the NF size. When NFs expand, the force constants decrease due to the larger distances between

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** XRD profiles of TiO₂ (T) and of Zr-doped TiO₂ NFs with different mole percentages (mol%) of ZrOCl₂·8H₂O: 0.5 mol% (TZr0.5), 1 mol% (TZr1), 2 mol% (TZr2), 3 mol% (TZr3) and 4 mol% (TZr4).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. High-resolution XPS spectra of a) O 1 s b) Ti 2 p, c) Zr 3 d, and d) C 1 s for pure TiO₂ and Zr-doped TiO₂ NFs with 4 mol% ZrOCl₂·8H₂O (TZr4).</figcaption></figure>

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

The O<sub>latt</sub>/Ti, O<sub>latt</sub>/(Ti+Zr), Zr/Ti, and O<sub>ads</sub>/O<sub>latt</sub> ratios on the surface of TiO<sub>2</sub> and Zr-doped TiO<sub>2</sub> NFs with 4 mol% ZrOCl<sub>2</sub>·8H<sub>2</sub>O (TZr4).

<table><thead><tr><th>Ratio</th><th>TiO<sub>2</sub></th><th>TZr4</th></tr></thead><tbody><tr><td>O<sub>latt</sub>/Ti</td><td>2.12</td><td>2.04</td></tr><tr><td>O<sub>latt</sub>/Ti+Zr</td><td>2.12</td><td>1.96</td></tr><tr><td>Zr/Ti</td><td>0</td><td>0.044</td></tr><tr><td>O<sub>ads</sub>/O<sub>latt</sub></td><td>0.105</td><td>0.175</td></tr></tbody></table>

atoms and the wavenumber decreases during the vibrational transitions. This explains the lower wavenumbers of the Raman bands [41]. The decrease in scattering intensity, especially of the Eg mode, could also be attributed to the disruption of the long-range translational crystal symmetry caused by defects [42].

The structure of the different NF samples was assessed by FT-IR (Fig. 3b). The absorption bands in the range of 450–550 cm<sup>-1</sup> and 900–950, observed in all samples, were attributed to the TiO<sub>2</sub> anatase and Ti–O vibrations [43,44]. With increasing Zr<sup>4+</sup> amounts, these peaks were modified. The peak at 900 cm<sup>-1</sup> disappeared due to Zr-doped TiO<sub>2</sub> formation on the surface. Moreover, the band at 1632 cm<sup>-1</sup> could be explained by H<sub>2</sub>O bending frequency, and the bands at 2345 cm<sup>-1</sup> and 2800–2900 cm<sup>-1</sup> by Zr-OH and C-H stretching vibrations, respectively. Broad absorption bands at 690 cm<sup>-1</sup> were observed in all Zr-doped TiO<sub>2</sub> NFs and were associated with Zr-O vibrations [45]. In conclusion, the Zr-doped TiO<sub>2</sub> NFs contained both Zr and TiO<sub>2</sub>.

Then, SEM (Fig. 4a-f) revealed that the samples doped with 0.5, 1, 2, 3 or 4 % of Zr had a highly interconnected network morphology with continuous, but randomly oriented NFs. The NF diameter increased from

188 ± 3 nm in pristine TiO<sub>2</sub> to 208 ± 3 nm in TZr4 (Fig. S1). The larger diameters of Zr-doped TiO<sub>2</sub> NFs, compared with bare TiO<sub>2</sub> NFs, suggest the successful addition of Zr ions into the NFs, in line with the XRD results.

High-resolution TEM was used to elucidate the impact of Zr-doping in the TiO<sub>2</sub> lattice on the structural properties of TiO<sub>2</sub> phases (Fig. 4g-h). TiO<sub>2</sub> tetragonal arrangement in the TiO<sub>2</sub> and TZr4 samples was verified by measuring the interplanar distances. The interplanar spacing values for the (101) plane were (0.35 ± 0.01 and 0.34 ± 0.01) nm in TiO<sub>2</sub> and (0.37 ± 0.01) nm in TZr4 (insets in Fig. 4g and h). The higher value in TZr4 suggests an increase of the unit cell volume due to Zr<sup>4+</sup> ion presence in the TiO<sub>2</sub> lattice, in line with the XRD data and a previous study [46]. The interplanar distances of 0.35 ± 0.01 nm (TiO<sub>2</sub>) and of 0.36 ± 0.01 and 0.34 ± 0.01 nm (TZr4) corresponded to the (101) plane, and the spacing of 0.18 ± 0.01 nm (TiO<sub>2</sub> and TZr4) to the (211) crystallographic plane of the anatase structure (insets in Fig. 4g-h). Additionally, the interplanar distances of 0.24 ± 0.01 nm in TiO<sub>2</sub> and 0.12 ± 0.01 nm in TZr4 corresponded to the (004) and the (202) crystallographic plane of TiO<sub>2</sub> (anatase) phase structure [47], respectively. The additional planes in TZr4 could also indicate enhanced crystallinity due to Zr addition that promotes better crystallite formation and stability.

The results of the EDX analysis (Table S1) confirmed Zr presence on TiO<sub>2</sub> NFs. The Zr<sup>4+</sup> to Ti<sup>4+</sup> ratio became higher with higher ZrOCl<sub>2</sub>·4H<sub>2</sub>O amounts (i.e. 4 > 3 > 2 > 1 > 0.5 mol%), in agreement with the experimental values. The elemental maps showed that Zr was evenly distributed on the TiO<sub>2</sub> surface (Fig. S2).

The ICP-OES analysis indicated that Zr is 0.05 ± 0.01 and Ti is 1 ± 0.02. The Zr:Ti atomic ratio in the prepared TZr4 was approximately
<figure><img src="image_5.png" /><figcaption>Fig. 3. a) Raman spectra of the TiO₂, TZr0.5, TZr1, TZr2, TZr3, and TZr4 samples with the characteristic vibrational bands of the anatase phase. b) FT-IR spectra of the TiO₂, TZr0.5, TZr1, TZr2, TZr3, and TZr4 samples.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. SEM micrographs of a) TiO₂, b) TZr0.5, c) TZr1, d) TZr2, e) TZr3, f) TZr4, and high-resolution TEM micrographs of g) TiO₂ and h) TZr4 (insets: selected area diffraction patterns and fast Fourier transform patterns).</figcaption></figure>

0.05 ± 0.01–1.00 ± 0.02.

The Brunauer-Emmett-Teller surface area values indicated that the crystallite size and NF diameter decreased upon Zr doping. At the temperature of 400 °C, the specific surface area was higher in the TZr4

(65 m² g⁻¹) than TiO₂ (45 m² g⁻¹) sample. A larger specific surface area facilitates the diffusion of reactants, thus enhancing the photocatalytic reaction.

Photoluminescence was used to assess the optical properties of the
different samples. The wide peak centered at 520 nm in the TiO₂ photoluminescence spectrum is indicative of its photoluminescence emission (Fig. 5a). The presence of oxygen vacancies and self-trapped excitons contributes to explain this emission behavior [43]. In Zr-doped TiO₂ NFs, photoluminescence intensity was reduced. This was explained by the inhibition of donor level formation (e.g. charge transfer to excitons and vacancy states) within the TiO₂ band gap upon doping [48] that leads to photoluminescence quenching (Fig. 5a). The peaks in the wavelength range of visible light corresponded to oxygen vacancies [49]. The results showed that the lifetime of free electrons was longer in pristine TiO₂ than in Zr-doped TiO₂ NFs [50]. This also contributes to the photoluminescence intensity decrease in the doped samples.

Electrochemical impedance spectroscopy (EIS) was performed using a frequency range from 100 mHz to 1 MHz and a three-electrode system to investigate the electron transfer mechanism within the samples [51, 52]. The resulting Nyquist plots were interpreted using an equivalent circuit model, as illustrated in Fig. 5b. The spectra displayed two distinct curve types: a semicircle and a linear tangent, which appeared upon Zr doping. The semicircle diameter indicated the charge transfer resistance (Rct). Specifically, the Rct values decreased sequentially in the Zr-doped TiO₂ samples (TZr4 < TZr3 < TZr2 < TZr1 < TZr0.5), and they were lower in all ZrO₂/TiO₂ NF samples than in pure TiO₂ NFs. The measured resistances in TZr4, TZr3, TZr1, TZr2, TZr0.5 and TiO₂ were 20.5 ohms,

26.4 ohms, 68.5 ohms, 93.8 ohms, and 435 ohms, respectively. This significant reduction in resistance with increasing Zr doping greatly enhanced the conductivity and the overall efficiency of the materials [53].

Fig. 5c shows the transient on/off photocurrent response of TiO₂ and Zr-doped TiO₂ samples during chronoamperometry tests at a constant potential of 1.3 V vs RHE (reversible hydrogen electrode) in the presence of light. TZr4 NFs exhibited a much higher photocurrent density compared with pristine TiO₂ and the other Zr-doped TiO₂ NF samples (TZr4 > TZr3 > TZr2 > TZr1 > TZr0.5 > TiO₂). This indicates that in TZr4 NFs, charge separation and transfer are improved and the lifetime of photogenerated charge carriers is increased.

UV-Vis spectrometry data (Fig. 5d) and the Kubelka-Munk formula [54] were employed to calculate the band gap energy (Eg) of the fabricated composites.

$$F = ((1-R)^2)/2 R \quad (1)$$

$$(Fhv)^{1/2} = A(hv-Eg) \quad (2)$$

where F is the Kubelka-Munk function, R the reflectance, hv the photon energy, and Eg the band gap.

Comparison of the Eg values (Table 2) showed its increase from 3.09 eV (TiO₂) to 3.2, 3.18 and 3.1 eV in samples with low Zr amounts

<figure><img src="image_7.png" /><figcaption>Fig. 5. a) Photoluminescence spectra. b) EIS results for TiO₂, TZr0.5, TZr1, TZr2, TZr3, and TZr4 NFs. c) Current density of TiO₂, TZr0.5, TZr1, TZr2, TZr3, and TZr4 NFs during chronoamperometry tests at a constant potential of 1.3 V vs RHE in the presence of light d) Plots to determine the gap energy (with the transformed Kubelka-Munk function vs light energy) of the TiO₂, TZr0.5, TZr1, TZr2, TZr3, and TZr4 samples.</figcaption></figure>
<table><caption>**Table 2**
Band gap values of pure TiO₂ nanofibers and after doping with Zr.</caption><thead><tr><th>Sample</th><th>Band gap (eV)/ Eu (Urbach) (eV)</th><th>Sample</th><th>Band gap (eV)/ Eu (Urbach) (eV)</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>3.09/0.19</td><td>2 %</td><td>3.1/0.18</td></tr><tr><td>0.5 %</td><td>3.2/0.14</td><td>3 %</td><td>3.05/0.28</td></tr><tr><td>1 %</td><td>3.18/0.12</td><td>4 %</td><td>3.00/0.32</td></tr></tbody></table>

(0.5 %, 1 %, and 2 %, respectively). Conversely, with higher Zr amounts (3 % and 4 %), the Eg values decreased to 3.05 eV and 3.00 eV, respectively, due to the formation of defects in the structure. The Urbach Energy values showed that in the presence of higher Zr⁺⁴ amounts, more surface defects has been formed.

The band gap increase in the TZr0.5, TZr1 and TZr2 samples could be attributed to the higher conduction band levels, in agreement with other studies on Zr-doped TiO₂ [55,56]. In these samples, Zr addition might promote visible light absorption through the formation of intermediate electronic levels between the conduction and valence bands and through the concomitant decrease of the electron charge recombination rate [57]. The creation of trapping sites and lattice defects leads to a band gap increase and recombination rate reduction. This seems to be a promising approach to enhance the performance of TiO₂ NFs for various applications [58].

### 3.1.1. H₂ generation

The photocatalytic performance of the fabricated NFs was investigated by comparing their ability to produce H₂ in an aqueous solution with methanol as scavenger in the presence of visible light (Fig. 6). Methanol scavenges holes and reduces electron-hole recombination, thus promoting the reaction of electrons with protons and H₂ production [59]. In the presence of pure TiO₂, H₂ production was limited (110 µmolg⁻¹·h⁻¹) due to the rapid backward reactions, fast electron-hole recombination, and large energy band gap (3.19 eV) [60]. Doping with increasing Zr amounts (from 0.5 % to 4 %) improved the H₂ generation rate.

The best catalytic activity was obtained with the TZr4 sample (H₂ evolution rate of 2500 µmol·g⁻¹·h⁻¹) (Fig. 6). With lower Zr mol% (3 %, 2 %, 1 % and 0.5 %), the H₂ evolution rates decreased (1918, 1366, 1116, and 395 µmol·g⁻¹·h⁻¹, respectively), likely due to the absence of enough active sites to produce higher H₂ amounts.

These findings confirmed that Zr is a good co-catalyst for increasing H₂ production, likely due to the hole and electron stability and long lifetime, as revealed by the EIS data. Moreover, the electron transfer between the impurities levels close to the conduction band and valence band could enhance the NF photocatalytic activity under visible light.

Then, the TZr4 photocatalyst was tested over four consecutive cycles (six hours of visible light exposure/each) to assess its reusability and stability. After four cycles, the H₂ generation rate was only slightly reduced (Fig. 6b). This indicates that TZr4 is very stable and can be recycled. Specifically, the initial weight percentage of Zr/Ti elements in the TZr4 sample was 7.75/92.25 wt% and slightly decreased to 7.6/ 92.4 wt% following the photocatalytic reaction. Nevertheless, the ratio remained relatively stable, suggesting the NF durability.

During the photocatalytic decomposition of water, the photoinduced electron-holes combine with hydrogen ions as reduction component [61]. Therefore, sacrificial aqueous agents [i.e. 20 % methanol, 20 % isopropyl alcohol (IPA) and 20 % 1,4-benzoquinon (BQ)] were tested as H₂ scavengers in comparison with water alone in the air-tight quartz reactor. The TZr4 composite was selected to evaluate the best scavenger following the same procedure described in Section 2.4. Comparison of the H₂ yields (Fig. 6c) showed that the highest H₂ amount (2500 µmol g⁻¹·h⁻¹) was obtained in the presence of methanol, followed by IPA and BQ, compared with pure water. This confirmed that water alone is not an ideal medium for the photocatalytic generation of H₂. Methanol accelerates H₂ generation because of its ability to donate electrons to the

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** a) Photocatalytic performance (H₂ evolution) of the indicated nanofibers. b) Recycling test to monitor the stability of TZr4 photocatalytic activity (H₂ evolution) over time. c) H₂ yield in function of the sacrificial agent.</figcaption></figure>
conduction band [62].

Compared with the few studies on Zr-containing TiO₂ for H₂ generation (Table 3), the Zr-doped TiO₂ compound with a Zr content of 4.0 mol% (TZr4) showed a very good H₂ generation activity.

## 3.2. Electronic properties based on the DFT results

The band gap plays a crucial role in modulating the photocatalyst activity. Using optimized lattice parameters derived from experimental data, the DOS values for pure TiO₂ anatase and TZr4 were calculated using spin-polarized DFT with the PBEsol functional in the Generalized Gradient Approximation (GGA) specifically tailored for solids [63,64] through the Quantum ESPRESSO plane-wave pseudopotential code [65]. PBEsol is considered one of the most precise and accurate functionals for crystal structure optimization. Ultrasoft pseudopotentials [67] were used and the cut-off values for the kinetic energy and the charge density were set at 50 Ry and 400 Ry, respectively. A k-point grid was generated using a 10 × 10 × 10 Monkhorst-Pack scheme. The Hubbard U correction was implemented for the energy band gap corrections and to avoid the DFT underestimation error [68,69]. The Hubbard U parameter was set at 8 eV for the Ti 3d and Zr 4d states and at 4 eV for the 2p states of oxygen [70]. The Hubbard correction was applied to oxygen to avoid stress over the Zr-O and Ti-O covalent bonds that occurs if the correction is applied only to Zr 4d states [69]. The calculated Eg values for pure TiO₂ anatase and TZr4 (3.05 eV and 2.95 eV, respectively) are in good agreement with the experimental values (Table 2) and with the previously reported value for TiO₂ anatase (3.18 eV) [71]. In the TZr4 sample, the unit cell volume increased by 0.76 % and the energy band gap decreased by 3.28 % compared with pure TiO₂. The crystal distortion and defect creation can be attributed to alterations in the electronic structure, as shown in Fig. 7. Nevertheless, the small difference between the calculated Eg values of pure TiO₂ anatase and TZr4 suggests that the optical properties of the TZr samples are mainly influenced by TiO₂, the material present in the highest proportion. This is a characteristic of mixed oxides where this process depends on the amount of doping [72].

## 3.3. Mechanism of interaction upon exposure to visible light

For a good photocatalytic activity, a catalyst should have a big specific surface area and small particles and band gap (Eg) energy. Compared with pure TiO₂, TZr4 (the best photocatalyst in the present study) had a larger specific surface area and smaller energy band gap. It has been suggested that doping with metal ions might cause the

<figure><img src="image_9.png" /><figcaption>Fig. 7. Calculated total density of states of the TiO₂ anatase and TZr4 samples; the red dashed line at 0 eV indicates the Fermi level.</figcaption></figure>

formation of new phases dispersed into TiO₂, temporarily trapping the photogenerated charge carriers and inhibiting the recombination of photoinduced electron-hole pairs that migrate from inside to the surface of the photocatalyst [58].

The presence of impurity levels arising from oxygen vacancies and the doping of Zr in TiO₂ significantly enhances electron transfer from the valence band to the conduction band. These impurity levels act as intermediate electronic states, facilitating charge carrier migration and reducing recombination rates. As a result, the modified TiO₂ structure exhibits improved charge separation efficiency, which contributes to enhanced photocatalytic performance [30]. In the TZr4 composite, the energy states generated at the interface of the two oxides, due to the presence of defects, induced an upward band bending of the contacting oxides [73]. They could also trap photogenerated electrons that will not be available for recombination with photogenerated holes [74]. These changes promoted the photocatalytic activity of TZr4 (Fig. 8). The defects at the Zr-doped TiO₂ interface were explained by the introduction

<table><caption>**Table 3**
Comparison of studies on hydrogen evolution by water splitting using different Zr-containing TiO₂ catalysts.</caption><thead><tr><th>Catalyst</th><th>Preparation technique</th><th>Output: H<sub>2</sub> production</th><th>Sacrificial agent</th><th>Reference</th><th>Light source</th></tr></thead><tbody><tr><td>Zr-doped TiO<sub>2</sub></td><td>Electrospinning</td><td>2500<br>µmol g<sup>-1</sup> h<sup>-1</sup></td><td>MeOH</td><td>This work</td><td>500 W linear halogen lamp</td></tr><tr><td>Pd/ZTC</td><td>Polyol method</td><td>2080 µmolg<sup>-1</sup> h<sup>-1</sup></td><td>Na<sub>2</sub>S and Na<sub>2</sub>SO<sub>3</sub></td><td>[63]</td><td>Day light fluorescent lamp</td></tr><tr><td>Zr, 1 wt% CuO/ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>Wetness impregnation</td><td>100 µmolg<sup>-1</sup> h<sup>-1</sup></td><td>MeOH</td><td>[64]</td><td>UV Pen-Ray lamp</td></tr><tr><td>ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>Solvothermal</td><td>1877 µmol g<sup>-1</sup> h<sup>-1</sup></td><td>MeOH</td><td>[65]</td><td>Xenon lamp (PLS-SXE300/300UV, 15 A) with an UV filter</td></tr><tr><td>ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>UIO-66-NH<sub>2</sub> as precursor</td><td>698 µmol g<sup>-1</sup> h<sup>-1</sup></td><td>MeOH</td><td>[65]</td><td>Xenon lamp (PLS-SXE300/300UV, 15 A) with an UV filter</td></tr><tr><td>ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>Sol gel</td><td>39700<br>µmol g<sup>-1</sup> h<sup>-1</sup></td><td>MeOH</td><td>[65]</td><td>Xenon lamp (PLS-SXE300/300UV, 15 A) with an UV filter</td></tr><tr><td>ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>Deposition-precipitation with urea</td><td>950<br>3820</td><td>MeOH</td><td>[65]</td><td>Xenon lamp (PLS-SXE300/300UV, 15 A) with an UV filter</td></tr><tr><td>Sr-doped ZrO<sub>2</sub>-TiO<sub>2</sub></td><td>Hydrolysis</td><td>100<br>µmol g<sup>-1</sup></td><td>Na<sub>2</sub>S-9 H<sub>2</sub>O and Na<sub>2</sub>SO<sub>3</sub></td><td>[9]</td><td>300 W xenon lamp</td></tr><tr><td>ZrO<sub>2</sub>/TiO<sub>2</sub></td><td>Sol-gel and vacuum impregnation</td><td>40<br>µmol g<sup>-1</sup> at 8 h</td><td>Na<sub>2</sub>S-9 H<sub>2</sub>O and Na<sub>2</sub>SO<sub>3</sub></td><td>[9]</td><td>300 W xenon lamp</td></tr><tr><td>CdS QDs@ZrO<sub>2</sub>-TiO<sub>2</sub></td><td>Sol-gel and vacuum impregnation</td><td>664.3 mmol g<sup>-1</sup>, 8 h</td><td>Na<sub>2</sub>S-9 H<sub>2</sub>O and Na<sub>2</sub>SO<sub>3</sub></td><td>[66]</td><td>300 W xenon lamp</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Proposed mechanism for the separation and transport of photo-generated electrons and holes at the Zr-doped TiO₂ interface following exposure to visible light.</figcaption></figure>

of increasing Zr amounts (from 1 % to 4 % mol%).

The absorption of light in the visible range induces defect transition states and hole transfer to the conductance band of TiO₂. Electrons originating from the defects migrate to the surface where they generate reactive species. The scavenger methanol allows hole trapping from TiO₂ and the efficient use of the photogenerated charges, explaining the improved catalyst performance.

## 4. Conclusion

Zr doping of TiO₂ improves its photocatalyst activity during water splitting in the presence of visible light. This could pave the way to more efficient and sustainable photocatalytic processes for green energy production. TiO₂ photocatalysts (with and without Zr) could be easily produced by electrospinning. The analysis revealed that the composite with 4.0 % Zr-doped TiO₂ (TZr4) displayed the highest photocatalytic activity upon exposure to visible light (H₂ generation = 2500 µmol g⁻¹ h⁻¹: 23-fold higher than with pure TiO₂). The band gap was narrower and the photogenerated e⁻/h⁺ pair recombination was reduced in Zr-doped TiO₂ compared with pristine TiO₂ NFs. The presence of both Zr and TiO₂ NFs in the co-catalyst resulted in a beneficial interaction that effectively reduced charge recombination and increased charge transfer as well as the density of active adsorption sites and photocatalytic reaction centers. DFT modeling confirmed that the improved photocatalytic activity of the fabricated Zr-doped TiO₂ NFs was due to the Zr doping that facilitated effective charge transfer and separation. The proposed approach can be considered a promising technique to fabricate heterogeneous structures to be used in a wide variety of applications.

### CRediT authorship contribution statement

**Hagheh-Kavousi Zahra:** Writing – original draft, Validation, Formal analysis, Data curation.
**Iatsunskyi Igor:** Writing – review & editing, Visualization, Validation, Methodology, Formal analysis, Data curation.

**Ghorbanloo Massomeh:** Writing – review & editing, Validation, Supervision, Funding acquisition, Formal analysis, Conceptualization.
**Coy Emerson:** Writing – review & editing, Visualization, Validation, Resources, Formal analysis, Data curation.
**Fraisse Bernard:** Writing – review & editing, Validation, Software, Formal analysis, Data curation.

**Mehdikhanloo Mehrdad:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Soliman Fathi S.:** Writing – review & editing, Validation, Supervision, Resources, Formal analysis.

**Hosny Rasha:** Writing – review & editing, Visualization, Validation, Investigation, Formal analysis, Data curation.

**Bechelany Mikhael:** Writing – review & editing, Validation, Supervision, Resources, Methodology, Funding acquisition, Conceptualization.

**Zahran Fouad:** Writing – review & editing, Visualization, Validation, Formal analysis, Data curation.
**Nada Amr A.:** Writing – review & editing, Visualization, Validation, Methodology, Formal analysis, Conceptualization, Supervision.
**Holade Yaovi:** Writing – review & editing, Validation, Supervision, Investigation, Formal analysis.
**El-Maghrabi Heba H.:** Writing – review & editing, Visualization, Validation, Formal analysis, Data curation.
**Saffari Mitra:** Writing – review & editing, Visualization, Validation, Supervision, Investigation, Formal analysis.
**Viter Roman:** Writing – review & editing, Visualization, Validation, Methodology, Formal analysis, Data curation.
**Bezzerga Djamel:** Writing – review & editing, Validation, Software, Formal analysis, Data curation.
**Hong Jisang:** Writing – review & editing, Visualization, Validation, Supervision, Formal analysis.

### 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

The authors thank the University of Zanjan for financial support of this study. This work was partially supported by a National Research Foundation of Korea (NRF) grant from the Korean government (MSIT) (2022R1A2C1004440). The authors also thank the Deanship of Scientific Research at Shaqra University for funding this work.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jallcom.2025.179439.

### Data Availability

Data will be made available on request.

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