# Effect of Cationic Dopant on the Physicochemical Properties and Visible Light Photocatalytic Degradation Activity of (M,N) Codoped TiO 2 Nanoparticles (M = Nb, Ta, W)

Qingyang Xi, Marc Lenertz, Nolwenn Le Breton, Clément Maheu, Valérie Keller, Bertrand Vileno, Thomas Cottineau

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Qingyang Xi, Marc Lenertz, Nolwenn Le Breton, Clément Maheu, Valérie Keller, et al.. Effect of Cationic Dopant on the Physicochemical Properties and Visible Light Photocatalytic Degradation Activity of (M,N) Codoped TiO 2 Nanoparticles (M = Nb, Ta, W). ACS Applied Nano Materials, 2025, 8 (14), pp.7073-7084. 10.1021/acsanm.5c00206. hal-05036517

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# Effect of Cationic Dopant on the Physicochemical Properties and Visible Light Photocatalytic Degradation Activity of (M,N) Codoped TiO₂ Nanoparticles (M = Nb, Ta, W)

Qingyang Xi,* Marc Lenertz, Nolwenn Le Breton, Clément Maheu, Valérie Keller, Bertrand Vileno, and Thomas Cottineau*

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Cite This: ACS Appl. Nano Mater. 2025, 8, 7073–7084

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## ACCESS

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**ABSTRACT:** (M,N) codoped TiO₂ nanoparticles (with M = Nb, Ta, or W) were synthesized via a sol–gel process, followed by a thermal nitridation in ammonia. The doping cation role in determining the codoped materials' physicochemical properties and their photocatalytic activity was studied. In situ temperature-dependent XRD analysis indicated an inhibition of rutile formation due to cation doping for all codoped TiO₂ nanomaterials. Depending on the thermal nitridation conditions, two types of absorption behaviors have been identified for each (M,N) codoped sample family: mild nitridation induced an absorption increase between 400 and 550 nm, while harsh nitridation induced additional absorption at a longer wavelength (>550 nm).

Furthermore, the presence of W in the TiO₂ lattice has been revealed to be more favorable than Nb and Ta to promote nitrogen insertion. A similar charge compensation mechanism, involving defects such as oxygen vacancies, was revealed for (Nb,N) and (Ta,N) codoped materials: low defect concentrations were achieved by the mild nitridation, while the harsh nitridation created a large amount of Ti³⁺ acting as charge recombination centers. On the opposite end, the defect concentration for (W,N) codoped TiO₂ nanoparticles remains low under both mild and harsh nitridation conditions. Additionally, for (W,N) codoped TiO₂ samples, W⁵⁺ was created by the harsh nitridation condition instead of Ti³⁺, resulting in an improved visible light photocatalytic activity.

**KEYWORDS:** photocatalysis, nanomaterials, TiO₂ doping, materials chemistry, synthesis, in situ XRD, EPR

M:TiO₂

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W,N:TiO₂

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M,N:TiO₂

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Nb,N:TiO₂ & Ta,N:TiO₂

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## INTRODUCTION

As a low-toxicity, chemically stable semiconductor that is widely available both naturally and industrially, TiO₂ is one of the most anticipated photocatalysts for large-scale solar-to-chemical applications, such as solar fuel generation or environmental remediation. The position of its energy bands permits a wide range of photocatalytic reactions; however, its wide band gap (>3.0 eV) limits its photoresponse to the UV region, leading to a low overall photoconversion efficiency.¹,²

To extend the absorption of TiO₂ toward the visible region, a common approach is to introduce anionic heteroatoms, among which nitrogen is the one that has been most widely researched.³⁻⁵ Although the N-doped TiO₂ materials exhibit visible-light-activated performance, these monodoped catalysts suffer from inefficient charge transport,⁶,⁷ which is mainly attributed to the generation of defects acting as charge recombination centers during the N-doping process.

To overcome the disadvantages of monodoping, the codoping of TiO₂ with cation and anion has been first proposed by ab initio calculations.⁸ It involves deeply modifying the TiO₂ optoelectronic structure by substituting

both O²⁻ and Ti⁴⁺ in the lattice with stoichiometric quantities of anion (A) and transition metal cation (M), respectively, according to⁹

$$ \mathrm{Ti}^{4+}_{(1-x)}\mathrm{M}^{(4+n)}_{x}+\mathrm{O}^{2-}_{(2-n)}\mathrm{A}^{(2+p)}_{p} \quad (1) $$

Due to the charge compensation between cations and anions, codoped materials are expected to exhibit fewer defects and a higher concentration of heteroatoms (>1%). While an ideal charge-balanced codoping has not yet been demonstrated, most synthesized (M,N) codoped TiO₂ materials exhibit enhanced visible-light activity.¹⁰⁻¹³ Recently, our group investigated the role of the doping sites and defects configuration in the charge compensation mechanism and

Received: January 16, 2025

Revised: March 20, 2025

Accepted: March 24, 2025

Published: March 31, 2025

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<figure><img src="image_15.png" /><figcaption>Scheme 1. Schematic Representation of the Sol–Gel Synthesis Process for M-Doped TiO₂ Gels (M = Nb, Ta, or W) and Thermal Treatment Conditions Used to Obtain the M:TiO₂, M,N:TiO₂-m, and M,N:TiO₂-h Photocatalyst Powders (* the Temperature and Duration for Each Nitridation Treatment Are Reported in Table S1)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 1.** Temperature-controlled in situ XRD patterns of TiO₂ (a), Nb-doped TiO₂ (b), Ta-doped TiO₂ (c), and W-doped TiO₂ (d) gels. The 2D XRD image is shown in the middle of each figure. The XRD pattern of the first (30 °C) and the last scan (700 °C for TiO₂ and 900 °C for the M-doped samples) are shown at the bottom and top of each figure, respectively. The brightness represents the intensity of the characteristic diffraction peaks.</figcaption></figure>

the electronic band structure of (Nb,N) codoped TiO₂.¹⁴ The findings emphasized that precise control of the thermal treatment conditions under ammonia, used for N insertion, is crucial to achieving good photocatalytic activity. However, as in most previous experimental studies, only a single cation–anion couple was investigated. Consequently, it remains unclear whether defect generation and the resulting photo-

catalytic performances are influenced only by the nitridation condition or also by the nature of the transition metal cation. To the best of our knowledge, aside from density functional theory (DFT) studies,¹⁵⁻¹⁷ no experimental work has systematically compared the photocatalytic activity of codoped TiO₂ materials with different cation anion pairs synthesized under similar conditions. In the literature, (Nb,N) codoping of
TiO₂ is the most studied case,¹⁰,¹⁸⁻²⁰ while fewer reports exist on TiO₂ codoping with 5d cations (Ta, W) and N. However, all these studies have also confirmed that (M,N) codoping is more effective than single doping in reducing the band gap of TiO₂ and enhancing visible light activity for photocatalytic degradation of pollutants or water oxidation.²¹⁻²⁶ On one hand, DFT calculations and experimental investigations on thin film or highly crystalline materials have provided valuable insight into the doping mechanism of (M,N) codoped TiO₂.¹⁷,²⁷,²⁸ However, these idealized structures do not necessarily reflect the materials relevant to practical photocatalytic experiments. On the other hand, only a few studies have successfully synthesized (M,N) codoped TiO₂ in the form of nanoparticles with a high concentration of doping species.¹⁹,²¹,²⁹ While the nanoscale dimension is primordial for high photocatalytic degradation,³⁰ a high concentration (>1%) of doping atoms is also required for a significant band gap reduction. Due to variations in terms of material synthesis, doping species concentration, photocatalytic applications, and photocatalytic test conditions across studies, direct comparisons between all these works remain challenging. As a result, the specific role of cations in (M,N)-codoped TiO₂ materials is not yet well understood.

Therefore, this study aims to compare the physicochemical properties of (M,N)-codoped TiO₂ materials containing niobium, tantalum, or tungsten as M cations and to correlate these properties with their photocatalytic activities as a function of the doping cation. These d transition cations were selected for their ionic diameter close to that of Ti⁴⁺, allowing for high doping concentration (>10%at.). Additionally, their empty d orbitals have energy levels comparable to the 3d orbitals of Ti, which form the conduction band of TiO₂. DFT studies have shown that the energy levels of Nb 4d and W 5d are theoretically close to that of Ti 3d, while Ta 5d is located at a higher energy level.¹⁵ By comparing (W,N) codoped TiO₂ with (Nb,N) and (Ta,N) codoped TiO₂, we investigate the impact of the doping cation charge, as W⁶⁺ is expected to allow the incorporation of more nitrogen than Nb⁵⁺ or Ta⁵⁺ according to eq 1. In this study, all (Nb,N), (Ta,N), and (W,N) codoped TiO₂ samples were synthesized using the same sol-gel method, followed by a thermal nitridation in ammonia. This approach ensures the production of codoped TiO₂ nanoparticles with comparable crystalline phases, particle morphology, and surface areas, allowing us to isolate the effects of the cation type on the physicochemical and photocatalytic properties. The optoelectronic properties, doping sites, and defects for each sample were characterized by combining UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR), and their photocatalytic properties were evaluated for methylene blue degradation under UV light and visible light irradiation.

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## RESULTS AND DISCUSSION

**M-Doped TiO₂ Synthesis and Characterizations.** M-Doped TiO₂ samples (hereafter labeled M:TiO₂) were synthesized using a sol-gel method described in our previous work:¹⁴ Briefly, titanium butoxide was dissolved in ethanol (EtOH) along with NbCl₅, TaCl₅, or WCl₆ to achieve a M/Ti molar ratio of M/Ti = 0.25 at. A solution containing water, EtOH, and HNO₃ was added dropwise under continuous stirring for 2 h at 80 °C to ensure gel formation. The resulting gel was dried at 80 °C for 48 h and subsequently ground (Scheme 1; details available in the ESI). As a reference

material, undoped TiO₂ was synthesized by the same procedure but without the addition of Nb, Ta, or W precursors.

To remove the organic residues from the synthesis and enhance the crystallinity of the photocatalyst nano powders, an annealing of the gels is required. To ensure that undoped and M-doped TiO₂ samples exhibit the same crystalline phase after gel annealing, temperature-dependent in situ X-ray diffraction (XRD) measurements were conducted to determine the temperature range in which only the anatase phase is present. At room temperature (bottom diffractogram of each panel in Figure 1), all samples exhibited broad anatase peaks, with a minor brookite contribution observed in the undoped TiO₂ gel. The intensity of the anatase peaks increases with temperature, while their width decreases (also shown in the original XRD patterns in Figure S3), indicating an increase in the crystallite size of the anatase with the temperature. For pure TiO₂, the anatase to rutile phase transition, which is determined by the first appearance of the (110) peak of rutile, occurs at 425 °C (indicated in red on the right of the figure). For the M-doped TiO₂ samples, the temperature of phase transition was observed to be increased to 775 and 850 °C for Nb-doped (Figure 2b) and Ta-doped TiO₂ (Figure 2c),

<figure><img src="image_18.png" /><figcaption>**Figure 2.** SEM images of TiO₂ (a), Nb-doped TiO₂ (b), Ta-doped TiO₂ (c), and W-doped TiO₂ (d).</figcaption></figure>

respectively. In the case of W-doped TiO₂, the absence of rutile peaks in the in situ XRD patterns (Figure 2d) indicates that the phase transition temperature is higher than 900 °C. A previous study reported an increased temperature for the phase transition from anatase to rutile due to Nb doping,³¹ and our results suggest a similar behavior in the case of Ta or W doping. These results indicate that the effect of cation doping on the inhibition of rutile formation follows the order W > Ta > Nb. It can also be noted from these in situ XRD measurements that a portion of the doping cation segregates from the anatase lattice of the M-doped TiO₂ at high temperature. For Nb-doped TiO₂, Nb begins to segregate at 750 °C to form TiNb₂O₇. For Ta-doped TiO₂ and W-doped TiO₂, the formation of Ta₂O₅ and WO₃ appears at 775 and 650 °C, respectively. Based on these results, the annealing temperature for TiO₂ photocatalyst powder was set at 400 °C for 2 h to prevent rutile formation while ensuring the removal of organic residues from the synthesis (thermogravimetric analysis is available in Figure S2). In comparison, M-doped TiO₂ gels required annealing at a higher temperature (500 °C for 12 h) to achieve complete removal of organic residues, but the presence of a doping cation inhibits the rutile phase formation at this temperature.

Scanning electron microscopy (SEM) images for TiO₂ and M:TiO₂ samples after annealing reveal a uniform distribution
of spherical nanoparticles (Figure 2). The average particle size of TiO₂ is 4.7 ± 1.3 nm (measured on 30 particles), while for the M-doped TiO₂ nanoparticles, it increases to 10.0 ± 2.0, 7.9 ± 1.2, and 7.2 ± 2.8 nm for Nb:TiO₂, Ta:TiO₂, and W:TiO₂, respectively. The ratios of Nb/Ti, Ta/Ti, and W/Ti determined by SEM-EDX are 0.22<sub>at.</sub>, 0.20<sub>at.</sub>, and 0.22<sub>at.</sub>, respectively, which are in close agreement with the expected synthesis ratio (M/Ti = 0.25<sub>at.</sub>).

**(M,N) Codoped TiO₂ Characterizations.** (M,N) codoped TiO₂ (M,N:TiO₂) samples were produced by annealing the corresponding M:TiO₂ nanopowders under ammonia flow (100 mL.min⁻¹). In a previous work, a significant influence of the thermal conditions of nitridation on the resulting (Nb,N) codoped TiO₂ materials has been identified.¹⁴ Therefore, in this study, for each (M,N) codoped TiO₂ sample family, mild and harsh nitridation conditions were established to produce M,N:TiO₂-m and M,N:TiO₂-h samples, respectively. The thermal parameters for each sample family, detailed in the ESI, Table S1, were determined by a trial-and-error process through the UV-visible absorption results after the nitridation treatment. As observed on the images in Scheme 1, samples obtained under mild conditions exhibit a yellow color (350 °C/12 h, 450 °C/1 h, and 250 °C/12h for Nb,N:TiO₂-m, Ta,N:TiO₂-m, and W,N:TiO₂-m, respectively). Harsh conditions produced darker samples (450 °C/1 h, 450 °C/12 h, and 350 °C/12h for Nb,N:TiO₂-h, Ta,N:TiO₂-h, and W,N:TiO₂-h, respectively).

The XRD patterns of the prepared samples are presented in Figure 3. The TiO₂ sample crystallized mainly in the anatase

<figure><img src="image_19.png" /><figcaption>**Figure 3.** XRD patterns of TiO₂, M-doped, and (M,N) codoped TiO₂ samples obtained under mild and harsh nitridation conditions.</figcaption></figure>

phase with small contributions (ca. 5%) of rutile and brookite allotropes. On the opposite, as expected by in situ XRD results, only anatase was observed for the M:TiO₂ and (M,N) codoped TiO₂ samples, and no characteristic diffraction peaks corresponding to any crystallized oxide, oxynitride, or nitride phases of the doped cation were detected. The lattice parameter and crystallite size τ of anatase for each sample were determined using Rietveld full profile refinement (Table 1). Compared to TiO₂ (τ = 5.0 nm), the M-doped and (M,N) codoped TiO₂ samples have a larger crystallite size, likely due to the growth of anatase crystallites during the extended initial

calcination in air for M:TiO₂ samples. The particle sizes measured by SEM are in good agreement with the crystallite size calculated from XRD data, indicating that each elemental particle consists of a single crystallite. The refined *a* and *c* lattice parameters of anatase are larger in M:TiO₂ and M,N:TiO₂ samples compared to TiO₂ (Table 1). A notable exception is observed for W-containing samples, where the *a* parameters are larger than that of TiO₂, while the *c* parameter is smaller. This behavior, predicted by DFT calculations, is attributed to the coexistence of two doping pair configurations: W in substitution of Ti (W<sub>Ti</sub>) associated with N in substitution of O (N<sub>O</sub>), and Ti vacancies (V<sub>Ti</sub>) associated with interstitial N (N<sub>i</sub>). In the case of (Nb,N) and (Ta,N) codoped TiO₂, Nb<sub>Ti</sub> (or Ta<sub>Ti</sub>) associated with N<sub>O</sub> is predicted to be the predominant doping pair.<sup>15,27</sup> The average anatase cell volume of Nb:TiO₂, Ta:TiO₂, and W:TiO₂ samples is 1.69, 1.19, and 0.70% larger than that of pure TiO₂, respectively. These results, consistent with the ionic radii of the M doping cations, confirm the incorporation of the cation into the TiO₂ crystal structure. Furthermore, a slight increase in unit cell volume is also observed for all samples submitted to nitridation, with this effect being significantly more pronounced under harsh conditions, confirming the better insertion of N in the nanoparticles in this case.

The N₂ adsorption-desorption isotherms for TiO₂, the M-doped TiO₂, and the (M,N) codoped TiO₂ present a type IV isotherm,<sup>32</sup> with a hysteresis of type H2 appearing in the relative pressure range between 0.4 to 0.8 (Figures S4 and S5).<sup>33</sup> This suggests a common mesoporous structure for all samples, which all exhibit a monodispersed pore distribution with a pore size distribution centered between 3 and 5 nm (Figures S4 and S5). The BET-specific surface areas (SSA) of each sample are reported in Table 1. TiO₂ (191 m².g⁻¹) exhibits a larger BET surface area compared to M:TiO₂ and (M,N) codoped TiO₂ samples (between 110 and 150 m².g⁻¹). This is consistent with the smaller particle and crystalline sizes observed for TiO₂ and can be attributed to the lower temperature used during its initial calcination in air.

XRD, BET, and SEM characterizations indicate that the optimized synthetic parameters ensure a high degree of similarity in phase structure, particle morphology, and pore structure among the different M doped and (M,N) codoped TiO₂ samples, with the exception of the TiO₂ reference, which has a smaller particle size and larger specific surface area. This structural consistency facilitates the establishment of structure-activity relationships between the nature of the cation dopant and the physicochemical and photocatalytic properties of the (M,N) codoped TiO₂ samples.

**Optical Characterizations.** The UV-vis absorption spectra of all samples are shown in Figure 4, and the corresponding band gap *E*<sub>g</sub> has been determined by fitting the linear part of the Tauc plot using the following equation:<sup>34</sup>

$$ (F(R) \times h\nu)^n = a(h\nu - E_g) \qquad (2) $$

where *F*(*R*), *h* and, *ν* are the Kubelka-Munk function, the Planck constant, and the photon frequency, respectively; *n* is a coefficient that depends on the nature of the band gap transition (1/2 in the case of an indirect band gap such as TiO₂ anatase). As expected, the undoped anatase TiO₂ nanoparticles show a band gap of 3.03 ± 0.05 eV. Single cationic doping does not induce a significant increase in absorption as the band gaps of Nb:TiO₂, Ta:TiO₂, and W:TiO₂ are determined as 3.03, 3.09, and 2.98 ± 0.05 eV, respectively (additional data in
<table><caption>Table 1. *a* and *c* Parameters of Anatase Lattice, Volume of the Lattice (*V*) and Volume Variation Compared to the TiO₂ Sample<sup>*a*</sup></caption><thead><tr><th>sample</th><th>a (Å)</th><th>c (Å)</th><th>V (Å³)</th><th>ΔV (%)</th><th>τ (Å)</th><th>BET (m² g⁻¹) (±8.0)</th></tr></thead><tbody><tr><td>TiO₂</td><td>3.7895(5)</td><td>9.478(2)</td><td>136.11</td><td>0.00</td><td>50(3)</td><td>191</td></tr><tr><td>Nb:TiO₂</td><td>3.8096(2)</td><td>9.5366(7)</td><td>138.41</td><td>1.69</td><td>73(5)</td><td>128</td></tr><tr><td>Nb,N:TiO₂-m</td><td>3.8099(2)</td><td>9.5376(6)</td><td>138.44</td><td>1.72</td><td>75(7)</td><td>120</td></tr><tr><td>Nb,N:TiO₂-h</td><td>3.8119(2)</td><td>9.5438(6)</td><td>138.68</td><td>1.89</td><td>75(4)</td><td>114</td></tr><tr><td>Ta:TiO₂</td><td>3.7985(5)</td><td>9.545(1)</td><td>137.72</td><td>1.19</td><td>61(6)</td><td>143</td></tr><tr><td>Ta,N:TiO₂-m</td><td>3.8004(5)</td><td>9.544(1)</td><td>137.84</td><td>1.28</td><td>62(5)</td><td>127</td></tr><tr><td>Ta,N:TiO₂-h</td><td>3.8043(4)</td><td>9.558(1)</td><td>138.33</td><td>1.63</td><td>65(4)</td><td>115</td></tr><tr><td>W:TiO₂</td><td>3.8091(3)</td><td>9.446(1)</td><td>137.05</td><td>0.70</td><td>76(7)</td><td>144</td></tr><tr><td>W,N:TiO₂-m</td><td>3.8099(5)</td><td>9.445(2)</td><td>137.10</td><td>0.73</td><td>73(6)</td><td>137</td></tr><tr><td>W,N:TiO₂-h</td><td>3.8107(5)</td><td>9.452(2)</td><td>137.26</td><td>0.84</td><td>73(6)</td><td>133</td></tr></tbody></table>

<sup>*a*</sup>Crystallite size (τ) was determined by Rietveld full profile refinement and BET specific surface area for the different (M,N) codoped TiO₂ samples.

<figure><img src="image_20.png" /><figcaption>**Figure 4.** UV–visible absorption spectra and Tauc plots of the (Nb,N) codoped samples (a, b) (mild:350 °C, 12 h, harsh: 450 °C, 1 h), (Ta,N) codoped samples (c and d) (mild: 450 °C, 1h, harsh: 450 °C, 12h), and (W,N) codoped samples (e and f) (mild: 250 °C, 12 h, harsh: 350 °C, 12 h).</figcaption></figure>

Figure S6). The slightly smaller band gap of W:TiO₂ likely indicates a lower conduction band bottom (CBB), attributed to the hybridization of W 5d and Ti 3d orbitals, whereas an opposite effect is observed in the case of Ta. Additionally, a weak absorption between 400 and 500 nm is present for Nb:TiO₂ and W:TiO₂ but absent for Ta:TiO₂. This absorption may be associated with energy levels just below the CBB, induced by Nb or W doping. Theoretically, Ta 5d orbitals are located at a higher energy level than Nb 4d and W 5d orbitals, explaining the absence of this effect in Ta:TiO₂.<sup>15</sup>

The UV–vis spectra of the (Nb,N), (Ta,N), and (W,N) codoped TiO₂ obtained under mild and harsh nitridation conditions are presented in Figure 4a,c,e, respectively. Two distinct absorption behaviors are observed depending on the thermal nitridation condition: (i) mild nitridation extends the visible light absorption of the M,N:TiO₂-m samples to wavelength below 550 nm; (ii) harsh nitridation further enhances absorption below 550 nm and additionally induces absorption at longer wavelengths (>550 nm). Furthermore, as illustrated in Figure S7, each (M,N) codoped TiO₂ sample demonstrates higher visible light absorption compared to the N-doped TiO₂ sample treated under identical nitridation

conditions. This result suggests that N-doping is facilitated by the prior presence of the cation, as corroborated by other studies.

The Tauc plots used for the band gap determination of the (Nb,N), (Ta,N), and (W,N) codoped samples are presented in Figure 4b,d,f, respectively. In each sample family, a significant reduction of the band gap energy is observed for the (M,N) codoped samples compared with that of the M-doped sample before nitridation. This observation indicates that the electronic band structure of TiO₂ has been effectively modified by (M,N) codoping. The (Nb,N) and (Ta,N) codoped samples have a similar band gap (~2.5 eV), larger than that of the (W,N) codoped samples (~2.2 eV). Besides the CBB lowered by the W 5d orbitals, another possible explanation for this difference is that the N 2p states, lying above the valence band top (VBT) of TiO₂, are at higher in energy in the (W,N) codoped TiO₂ than in (Nb,N) and (Ta,N) codoped TiO₂ samples.<sup>35</sup> Moreover, within each family of samples, no significant difference in band gap is observed between M,N:TiO₂-m and M,N:TiO₂-h samples (ΔEg < 0.2 eV), but the harsh nitridation leads to increased absorption. One explanation could be that the mild nitridation leads to the
hybridization of N 2p orbitals with O 2p orbitals, elevating the VBT of the M,N:TiO₂-m samples, while harsh nitridation introduces a greater number of nitrogen and thus these N 2p levels.

To more specifically highlight the effect of each M cation on the light absorption of the resulting (M,N) codoped TiO₂, we conducted a nitridation treatment on Nb:TiO₂, Ta:TiO₂, and W:TiO₂ under identical thermal conditions (350 °C, 12h). The measured UV visible spectra (Figure S8) indicate that the effectiveness of nitridation for improving visible absorption of (M,N)-codoped TiO₂ follows the order: W,N:TiO₂ > Nb,N:TiO₂ > Ta,N:TiO₂ ≈ N:TiO₂.

**Identification of Doping Species.** To elucidate these changes in optical properties, XPS and EPR characterization were performed to identify potential doping sites and defects induced by the nitridation treatment in the chemical structure of the doped TiO₂ nanoparticle.

The surface chemical composition of the prepared samples was analyzed by XPS. The O 1s spectral region (Figure S9) presents two contributions, located at 530.4 and 531.6 ± 0.2 eV, which are respectively attributed to lattice oxygen (O 1s-A) in the TiO₂ and a broad composite peak (O 1s-B) assigned to surface oxygen species such as hydroxyl and carboxyl groups.³⁶ In the Ti 2p area (Figure S10), a doublet with the binding energy of 459.0 and 464.8 ± 0.1 eV is observed for all the samples, consistent with the Ti 2p₃/₂ and Ti 2p₁/₂ of Ti⁴⁺.²⁹ Two additional peaks with a binding energy of 457.1 and 462.9 ± 0.2 eV, characteristic of Ti³⁺,²⁹ can be observed on all (Nb,N) and (Ta,N) codoped samples (zoom in Figure. S10.a and b). As reported in the case of (Nb,N) codoped TiO₂,¹⁴ an increase of the atomic proportion of Ti³⁺ to the total Ti with the nitridation condition is also observed for the (Ta,N) codoped TiO₂ samples (1.3% for Ta,N:TiO₂-m, 3.3% for Ta,N:TiO₂-h). This observation suggests a facilitation of Ti³⁺ generation by the thermal nitridation treatment for the (Nb,N) and (Ta,N) codoped samples, but this was not observed for the (W,N) codoped samples. Figure S11a-c represents the Nb 3d, Ta 4d, and W 5d regions, respectively. Each region presents a single doublet characteristic of Nb⁵⁺,³⁷ Ta⁵⁺,³⁸ and W⁶⁺.³⁹ The results indicate that the oxidation state of the M cation remains unchanged throughout the sol-gel process and nitridation treatment, consistent with the oxidation states of the precursors NbCl₅, TaCl₅, or WCl₆. The M/Ti ratio determined by XPS analysis is approximately 0.3 for all samples, which is close to the 0.25 value expected from the synthesis. Since XPS is mainly sensitive to the surface, these results suggest no significant enrichment in the M cation at the surface of the nanoparticles.

Figure 5a-c presents the spectral regions of N 1s for the Nb-, Ta-, and W-containing samples, respectively. All the samples exhibit a peak at 400.5 ± 0.2 eV (N 1s A), characteristic of surface chemisorbed nitrogen or nitrogen in interstitial position (Nᵢ).³,⁵ The broad peaks that appear in Figure 5a (395.2 ± 0.1 eV) and Figure 5b (404.8 ± 0.1 eV) are attributed to the Nb 3p satellite peak⁴⁰ and Ta 4p₃/₂⁴¹ respectively. The (M,N) codoped samples under mild nitridation (Nb,N:TiO₂-m, Ta,N:TiO₂-m, and W,N:TiO₂-m) present an increase of peak N 1s A compared with their respective M:TiO₂, indicating a successful nitrogen introduction in the interstitial sites. However, an additional contribution located at 395.6 eV (N 1s B), is also observed on Ta,N:TiO₂-m, indicating the appearance of substitutional nitrogen (N₀) in this sample.⁴² This may be ascribed to the

<figure><img src="image_21.png" /><figcaption>**Figure 5.** XPS spectra of N 1s for the (Nb,N) codoped samples (a), (Ta,N) codoped samples (b), and (W,N) codoped samples (c) (red lines are the fitted signals; blue, green, and cyan lines are the different contributions).</figcaption></figure>

higher thermal nitridation temperature for Ta,N:TiO₂-m (450 °C) compared to Nb,N:TiO₂-m (350 °C) and W,N:TiO₂-m (250 °C).

Under harsh nitridation conditions, M,N:TiO₂-h samples present characteristic peaks of both Nᵢ and N₀. In comparison to their counterparts subjected to mild nitridation, no further increase of the contribution Nᵢ is observed for Nb,N:TiO₂-h and W,N:TiO₂-h. These results suggest that the formation of substitutional nitrogen requires higher thermal energy. The atomic ratio of nitrogen to the M cation (N/M) for Nb,N:TiO₂-h, Ta,N:TiO₂-h, and W,N:TiO₂-h is 0.18 (N/Nb), 0.58 (N/Ta), and 0.31 (N/W), respectively. These values indicate that ideal balanced codoping, as defined by eq 1, is not achieved even under harsh nitridation conditions. The higher N/M ratio observed in Ta,N:TiO₂-h is unexpected, as this sample exhibits lower absorption compared to (Nb,N) and (W,N) codoped TiO₂. However, it should be noted that the broad Ta 4p peak interferes with the data treatment, potentially leading to an overestimation of the N content. A comparison between Nb,N:TiO₂-m (N/Nb<sub>at.</sub> = 0.12) and W,N:TiO₂-h (N/W<sub>at.</sub> = 0.31), both subjected to similar nitridation treatment (350 °C, 12 h), confirms an enhanced insertion of N with the presence of 6+ doping cation (W⁶⁺) compared to a 5+ cation (Nb⁵⁺).
To complete these XPS analyses and identify paramagnetic sites within the bulk of the nanoparticles, EPR analysis was conducted (Figure 6). The undoped TiO₂ presents three types

<figure><img src="image_22.png" /><figcaption>**Figure 6.** Low temperature (T = 100 K) EPR fingerprints of TiO₂, M-doped TiO₂, and (M,N) codoped TiO₂: (a) large sweep spectra normalized by sample mass and (b) narrow spectra normalized with peak-to-peak amplitude of the EPR signal.</figcaption></figure>

of EPR active centers. The signature at g = 2.002 (332 mT; indicated by Δ) corresponds to an unpaired electron trapped on an oxygen vacancy V₀⁺, which is a common defect created in metal oxides.⁴³⁻⁴⁵ The signals at 336, 332.5, and 330.0 mT (*) are assigned to the NO species captured at the surface of the porous TiO₂.⁴⁶,⁴⁷ Another weak signal, indicated I, characteristic of paramagnetic nitrogen species in a [O−Ti⁴⁺-N²⁻-Ti⁴⁺] unit,⁴⁸ could be attributed to the nitrogen residual from nitric acid used for synthesis due to the lower calcination temperature for this sample. After doping with the M cation, changes in the EPR signal are observed, depending on the type of doping cation. The V₀⁺ signal disappears (Nb:TiO₂) or decreases (Ta:TiO₂ and W:TiO₂) suggesting that EPR silent defect such as Ti vacancies (Vₜᵢ) could be involved in the

charge compensation mechanism as suggested by DFT calculations.¹⁷,²⁷ For Nb:TiO₂, an additional contribution of Ti³⁺ (g = 1.985,⁴⁹,⁵⁰ indicated by ●) is observed, but the overall defect concentration of the sample remains as low as that of TiO₂, as indicated by the large range spectra without normalization (Figure 6a). For Ta:TiO₂ and W:TiO₂, an increase in the contribution of the NO species is noted, which may be due to a preference for the adsorption of the NO species on the surface of these samples. In the case of W:TiO₂, we also observe the signals assigned to W⁵⁺ (g = 1.80 and 1.64, indicated by + in Figure 6a),⁵¹,⁵² suggesting a partial reduction from W⁶⁺ to W⁵⁺.

(Nb,N) and (Ta,N) codoped TiO₂ samples exhibit a similar evolution with the nitridation condition. For the samples that have been subjected to mild nitridation (Nb,N:TiO₂-m and Ta,N:TiO₂-m), the spectra are dominated by the signal of V₀⁺, and a small contribution of Ti³⁺ (●) can also be observed. However, the overall defect concentration of these samples remains comparable to that of their M:TiO₂ counterparts. Under the harsh nitridation conditions (Nb,N:TiO₂-h and Ta,N:TiO₂-h), a significant increase in the intensity of V₀⁺ and Ti³⁺ (●) is noted, with the appearance of a new signature corresponding also to Ti³⁺ but in different local coordination of the ion (g = 1.96, indicated by ■), in agreement with XPS results for the Ti³⁺ evolution.

In contrast, a different behavior was observed for (W,N) codoping. W,N:TiO₂-m presents only the signal of V₀⁺, with a lower signal intensity compared with those of Nb,N:TiO₂-m and Ta,N:TiO₂-m, suggesting improved defect passivation due to (W,N) codoping under mild nitridation conditions. The charge compensation is again expected to involve Vₜᵢ sites, as proposed by DFT studies, and since our XPS results indicate that the theoretical N/W ratio of 2 is not reached.²⁷ Under harsh nitridation conditions, both V₀⁺ and W⁵⁺ signals are detected in W,N:TiO₂-h, with signal intensity comparable to those of W:TiO₂. Thus, in comparison with (Nb,N) and (Ta,N) codoping, the overall defect and doping site concentration in (W,N) codoped TiO₂ are less sensitive to the nitridation conditions.

This distinctive behavior of the (W,N) codoped TiO₂ can also be observed by UV visible IR spectroscopy (Figure 7a,b). TiO₂, Nb:TiO₂, and Ta:TiO₂, as well as the associated (M,N) codoped TiO₂ samples subjected to mild nitridation, exhibit negligible absorption in the infrared region. Nb,N:TiO₂-h and Ta,N:TiO₂-h exhibit similar spectral features, where the absorption increases up to 2000 nm. The precise position of

<figure><img src="image_23.png" /><figcaption>**Figure 7.** UV-visible absorption spectra recorded in the region of (a) 300 to 2000 nm and (b) 500 to 2000 nm. (c) XPS valence band spectra of TiO₂, Nb:TiO₂, Ta:TiO₂, and W:TiO₂ (the inset shows magnification on the spectral region of 3 to −2 eV).</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure 8.** (a–f) MB degradation profiles and kinetics for TiO₂, M-doped and (M,N) codoped TiO₂ samples under the irradiation using UV LED (365 nm): (a, b) for Nb samples, (c, d) for Ta samples, and (e, f) for W samples. (g) MB degradation profiles and (h) kinetics for (M,N) codoped samples under visible light (450 nm). (i) Comparison of the pseudo-first-order photodegradation kinetic constant for all samples under UV or visible light (air: samples annealed under air; m: mild nitridation conditions; h: harsh nitridation conditions).</figcaption></figure>

this broad absorption peak is challenging to ascertain due to the absorption of molecules in the air, which increases the noise levels. However, this absorption is likely related to the transition from the occupied d orbital of Ti³⁺ sites, located in the band gap, to the conduction band. The presence of Ti³⁺ in Nb,N:TiO₂-h and Ta,N:TiO₂-h was revealed by EPR and XPS characterization. According to previous studies, the energy levels of the occupied d orbital when Ti⁴⁺ is reduced to Ti³⁺ are located ~ 0.8 to 0.5 eV below the conduction band.⁵³,⁵⁴ On the current UV visible IR spectra, the center of this transition is at a wavelength higher than 2000 nm, corresponding to energy transition <0.62 eV, which is in good agreement with these values. On the opposite, all the W-containing samples present a broad peak centered on 1300 nm (corresponding to 0.95 eV), and a decrease in this peak intensity is observed for W,N:TiO₂-m. We speculate that this absorption may correspond to another type of defect lying deeper in the band gap of the W-containing TiO₂. Since the absorption associated with this defect is reduced by the "mild" nitridation treatment, in correlation with the EPR result of the W-doped TiO₂ samples, we hypothesize that this defect state may relate to the W⁵⁺. Additionally, the VB region of the M-doped TiO₂ samples was recorded in XPS (Figure 7c). For all samples, the characteristic energy difference of 456 eV between the Ti 2p main emission line and the VB edge was measured for TiO₂ and the doped samples. It is in agreement with previous works on TiO₂ anatase.⁵⁵,⁵⁶ The VB recorded by XPS shows no edge variation after the metal doping, as expected, since the VB top is mainly constituted of O 2p orbitals. Nevertheless, an additional state is observed in the band gap for the W:TiO₂ sample. This broad state is centered on the Fermi level of the machine, aligned

with the Fermi level of the sample, and extends to at least 1.0 eV around this value. This position in the band gap is coherent with the absorption observed in UV visible spectroscopy and EPR; we thus attributed this state to W⁵⁺.

**Photocatalytic Results.** The photocatalytic activity of all samples was evaluated by following the kinetics of methylene blue (MB) photodegradation under UV (365 nm) and visible (450 nm). MB, a cationic dye, was chosen to exclude the possibility of photodegradation via the photosensitization of the photocatalyst at these two wavelengths of illumination.⁵⁷ For all experiments, before photocatalytic degradation, the adsorption-desorption equilibrium between the photocatalyst nanoparticles and MB was established in the dark for at least 60 min (Figure S12). The photodegradation kinetics for the reaction was described as a pseudo-first-order model:

$$ \ln\left(\frac{C_0}{C_t}\right) = k \times t \qquad (3) $$

where Cₜ is the MB concentration at time t, and C₀ is the concentration of MB when the light is turned on. The degradation profiles and associated kinetics for the (Nb,N), (Ta,N), and (W,N) codoped samples under UV light are shown in Figure 8a–f. For each (M,N) codoped TiO₂ family, the respective M-doped TiO₂ and undoped TiO₂ were used as references, and the calculated k for each (M,N) codoped sample are reported in Table 2 and Figure 8i.

Undoped TiO₂ degraded 79% of the MB within 5 h, with a kinetic constant k = 0.2814 h⁻¹. Upon cationic doping, all M-doped samples show reduced photocatalytic activities, with k decreasing by ~30% (Nb:TiO₂ and W:TiO₂) or ~60%
<table><caption>**Table 2.** Pseudo First Order of Photo-Degradation Kinetic Constant for All Samples under the Irradiation of UV LED (365 nm) and Visible LED (450 nm)</caption><thead><tr><th>sample</th><th>k-UV (h<sup>-1</sup>)<br>± 0.015</th><th>variation vs<br>TiO<sub>2</sub> (%)</th><th>variation vs M:<br>TiO<sub>2</sub> (%)</th><th>k-vis (h<sup>-1</sup>)<br>± 0.015</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>0.281</td><td></td><td></td><td>0.002</td></tr><tr><td>Nb:TiO<sub>2</sub></td><td>0.187</td><td>-33.5</td><td></td><td>0.004</td></tr><tr><td>Nb,N:TiO<sub>2</sub>-m</td><td>0.338</td><td>20.3</td><td>80.8</td><td>0.067</td></tr><tr><td>Nb,N:TiO<sub>2</sub>-h</td><td>0.175</td><td>-38.0</td><td>-6.7</td><td>0.071</td></tr><tr><td>Ta:TiO<sub>2</sub></td><td>0.101</td><td>-64.2</td><td></td><td>0.002</td></tr><tr><td>Ta,N:TiO<sub>2</sub>-m</td><td>0.122</td><td>-56.7</td><td>20.8</td><td>0.045</td></tr><tr><td>Ta,N:TiO<sub>2</sub>-h</td><td>0.055</td><td>-80.4</td><td>-45.2</td><td>0.055</td></tr><tr><td>W:TiO<sub>2</sub></td><td>0.198</td><td>-29.6</td><td></td><td>0.005</td></tr><tr><td>W,N:TiO<sub>2</sub>-m</td><td>0.292</td><td>3.6</td><td>47.3</td><td>0.108</td></tr><tr><td>W,N:TiO<sub>2</sub>-h</td><td>0.258</td><td>-8.3</td><td>30.3</td><td>0.105</td></tr></tbody></table>

(Ta:TiO₂) compared to undoped TiO₂. It should be noted that TiO₂ nanoparticles have smaller particle size and higher specific surface area, two properties that are expected to enhance the photocatalytic activity. As indicated by the UV- visible spectroscopy, a photon at 365 nm can excite electron excitation from the VBT to CBB in all samples. Based on our characterization results, two factors may explain the observed changes in degradation kinetic. First, the adsorption behavior of dye molecules onto the photocatalyst surface: all M:TiO₂ samples have higher MB adsorption (Figure S12) due to a more negatively charged surface as indicated by zeta potential measurements (Figure S13). This higher adsorption could promote the reaction between photogenerated reactive radicals and pollutant molecules. Second, the larger crystalline sizes for M:TiO₂ nanoparticles when compared to TiO₂ ones lead to an extended photogenerated charge carrier diffusion distance before reaching the surface. This, in turn, increases the possibility of electron-hole pair recombination, thereby reducing the photocatalytic activity. Additionally, the likely presence of Ti vacancies in the M:TiO₂ samples, which compensate for the additional positive introduction by the M cation, may also act as recombination centers.

For M,N:TiO₂ samples under UV light illumination, the photocatalytic performance depends on the nitridation conditions. Under mild nitridation, the degradation kinetics of M,N:TiO₂-m samples increase compared to their M:TiO₂ counterparts. Notably, Nb,N:TiO₂-m and W,N:TiO₂-m present slightly superior activities compared to undoped TiO₂. In addition to a better dye adsorption, the charge compensation between Nb⁵⁺, Ta⁵⁺, or W⁶⁺ and N³⁻, identified by EPR, is also expected to limit defect formation, thereby facilitating charge separation and transfer. On the opposite, harsh nitridation conditions lead to a deterioration of photocatalytic activity under UV light for Nb,N:TiO₂-h and Ta,N:TiO₂-h. Interestingly, (W,N) codoped samples exhibit a distinct behavior, with photocatalytic performances largely independent of the nitridation conditions. Both W,N:TiO₂-m (k = 0.2916) and W,N:TiO₂-h (k = 0.258) achieve MB degradation rates comparable to the one of TiO₂, and superior to W:TiO₂. These photocatalytic results under UV light suggested that both mild and harsh nitridation treatments limit the creations of defects acting as charge recombination centers in the M,N:TiO₂ nanomaterials. Nevertheless, Ti³⁺ and V⁰⁺ sites, identified by EPR, for the Nb,N:TiO₂-h and Ta,N:TiO₂-h samples could act as charge recombination centers, reducing the photocatalyst activity. In contrast, W⁵⁺ species and the

lower amount of V⁰⁺ measured for W,N:TiO₂-h appear to have no significant detrimental effect on the photocatalytic performance.

In order to determine the additional photocatalytic activities brought by the extended visible light absorption in the (M,N)-codoped TiO₂ nanopowders, their photocatalytic performances under visible illumination at 450 nm were also evaluated. At this wavelength, as illustrated by the UV visible spectroscopy results, the photon absorption strongly depends on the nitridation conditions. The photodegradation profiles and kinetics are shown in Figures 8.g and h, respectively, and the calculated kinetic constants k are summarized in Table 2 and Figure 8.i. As expected, no significant MB degradation is observed at this wavelength for the TiO₂ or M:TiO₂ samples. All (M,N)-codoped TiO₂ samples demonstrate visible-light photocatalytic activity, with no significant difference in the kinetic constant between those subjected to mild and harsh nitridation. We propose that the increased photon absorption at 450 nm may compensate for the higher charge carrier recombination caused by the formation of V⁰⁺ and Ti³⁺ sites. In the case of (W,N) codoping, both W,N:TiO₂-m and W,N:TiO₂-h demonstrate high visible-light photocatalytic activity, with a similar kinetic constant k = ~0.1050 h⁻¹ which is almost 1.6 and 2.3 times higher than that of Nb,N:TiO₂-m and Ta,N:TiO₂-m, respectively. The superior activities of the (W,N) codoped TiO₂ samples could be attributed to the lower overall defect concentration and the absence of Ti³⁺, as indicated by the EPR results, which enhance charge carrier availability for the MB photodegradation reaction.

These photocatalytic results demonstrate that cationic doping with the selected d elements alone does not enhance the photocatalytic activity of TiO₂ nanopowders in the UV region. However, (M,N) codoping under mild conditions effectively restores the UV-light photocatalytic activity, particularly for Nb,N:TiO₂-m and W,N:TiO₂-m samples. In addition to this restored UV-light activity, (M,N)-codoped TiO₂ samples demonstrate significant visible-light activity at 450 nm, consistent with their light absorption properties. Among the codoped samples W,N:TiO₂ demonstrate the highest activity followed by Nb,N:TiO₂ and Ta,N:TiO₂. While harsh nitridation conditions tend to decrease the photocatalytic activity, an exception is observed in the W,N:TiO₂-h sample. The ability to maintain the activity of TiO₂ in the UV regions while extending photocatalytic performance into the visible range makes these (M,N) codoped TiO₂ nanopowders promising photocatalyst candidates for pollutant degradation under solar irradiation.

## ■ CONCLUSIONS

In this study, we systematically investigated three (M,N)-codoped TiO₂ nanomaterials synthesized under similar conditions. By maintaining consistent structural and morphological properties across all samples, we establish clear structure-activity relationships between the nature of the M cation, the resulting optical properties, and the overall photocatalytic activity. For all (M,N) codoped samples, two distinct absorption behaviors were observed depending on the nitridation conditions. Samples subjected to mild nitridation exhibited a yellow color, attributed to N-doping, while those exposed to harsh nitridation developed a darker color due to additional absorption at a longer wavelength (>550 nm), which is linked to reduced metal cation centers located at deep
energy levels within the band gap. Among the codoped materials, (W,N) codoped TiO₂ nanoparticles present the narrowest band gap, likely due to a higher concentration of nitrogen, a different distribution of N 2p states, and the lower energy of W 5d orbital. XPS and EPR results suggest that mild nitridation promotes defect passivation in (M,N) codoped samples by the charge compensation between the doping cation and nitrogen. Notably, W,N:TiO₂-m appears to have the lowest defect concentration, as only a minimal amount of oxygen vacancies was detected. For samples subjected to harsh nitridation, we identified the origin of the extended light absorption: in Nb,N:TiO₂-h and Ta,N:TiO₂-h, it is related to an increase of Ti³⁺ concentration, while in W,N:TiO₂-h, it is attributed to W⁵⁺ and O_v⁺.

The photocatalytic performances of the as-prepared photocatalyst nanopowders were evaluated by MB degradation. In the UV region, the photocatalytic activity of (Nb,N) and (Ta,N) codoped TiO₂ samples was significantly influenced by the nitridation conditions. Benefiting from the charge compensation between Nb⁵⁺/Ta⁵⁺ and N³⁻, as well as increased MB adsorption, Nb,N:TiO₂-m and Ta,N:TiO₂-m demonstrated enhanced activities compared to their M:TiO₂ counterparts. However, harsh nitridation led to reduced activities due to the high concentration of Ti³⁺, which acts as a charge recombination center. In contrast, the photocatalytic activity of the (W,N)-codoped TiO₂ samples is less sensitive to nitridation conditions. Both W,N:TiO₂-m and W,N:TiO₂-h exhibited similar activity to TiO₂ in the UV spectral region combined with a superior activity compared to (Nb,N) and (Ta,N) samples in the visible region. The characterization results suggest that the dark color center W⁵⁺ in (W,N) codoped TiO₂ hardly affects the photodegradation kinetic, contrary to Ti³⁺ centers.

The outcomes for (Nb,N) and (Ta,N) codoped TiO₂ align with findings from previous studies and reveal a distinct charge compensation mechanism in (W,N) codoped TiO₂ nanoparticles, leading to improved visible light photocatalytic activity. Additionally, (W,N) codoped TiO₂ requires a lower temperature for nitrogen insertion, offering a practical advantage by reducing the production costs for the upscaling of such photocatalytic nanopowders. This study emphasizes the importance of selecting the appropriate cation and precisely controlling nitridation conditions to minimize defect formation and optimize the photocatalytic performance in (M,N) codoped TiO₂ materials.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsanm.5c00206.

Detailed experimental conditions for material synthesis and characterizations; thermogravimetric Analysis; supplementary XRD results; N₂ adsorption–desorption isotherms and porous distribution; supplementary UV–visible spectroscopy results; complete set of XPS spectrum; additional photocatalytic experiments; zeta potential measurements (PDF)

## AUTHOR INFORMATION

### Corresponding Authors

Qingyang Xi - CNRS - Université de Strasbourg, Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, (ICPEES UMR7515), Strasbourg 67087, France;

qingyang.xi@etu.unistra.fr

Thomas Cottineau - CNRS - Université de Strasbourg, Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, (ICPEES UMR7515), Strasbourg 67087, France;

orcid.org/0000-0001-8058-4824;

Email: cottineau@unistra.fr

Email: qingyang.xi@etu.unistra.fr

## Authors

Marc Lenertz - CNRS - Université de Strasbourg, Institut de Physique, Chimie et Matériaux de Strasbourg, (IPCMS UMR 7504), Strasbourg 67034, France

Nolwenn Le Breton - CNRS - Université de Strasbourg, Institut de Chimie, (UMR 7177), Strasbourg 67081, France

Clément Maheu - CNRS - Université de Nantes, Institut des Matériaux de Nantes Jean Rouxel, (IMN UMR6502), Nantes 44322, France;

Valérie Keller - CNRS - Université de Strasbourg, Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, (ICPEES UMR7515), Strasbourg 67087, France;

orcid.org/0000-0002-3381-1446

Bertrand Vileno - CNRS - Université de Strasbourg, Institut de Chimie, (UMR 7177), Strasbourg 67081, France

Complete contact information is available at:

https://pubs.acs.org/10.1021/acsanm.5c00206

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

Q.X. would like to thank the China Scholarship Council (CSC grant number 201908410202) for supporting his study in France. The authors would like to thank Thierry Dintzer and the SEM-Cro platform for the SEM characterizations and CNRS research infrastructure INFRANALYTICS (FR2054) for EPR support. XPS VB measurements were performed using the IMN's characterization platform, PLASSMAT. The financial support for this study was provided by ANR (BAGETE ANR-16-CE05-0001-01). C.M. thanks the Marie Skłodowska-Curie action for the project OMATSOLFUEL (no. 101105640).

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