The structural analysis of both pure and nitrogen-doped TiO₂ films was examined using X-ray diffraction (XRD), and the corresponding XRD patterns are presented in Figure 1. These patterns reveal distinct peaks consistent with the crystalline structure of anatase TiO₂ (Anatase XRD JCPDS Card No. 21-1272) and rutile TiO₂ (Rutile JCPDS Card No. 21-1276) [12][13]. Peaks related to the anatase phase were identified within the tetragonal I4₁/amd space group (No. 141), while those associated with the rutile phase were indexed in the tetragonal P4₂/mnm space group (No. 136). Phase identifications were carried out using ‘X’Pert HighScore Plus’ software. Notably, the XRD data presented in Figure 1 confirmed that both undoped and N-doped TiO₂ samples exhibit analogous peaks for anatase and rutile phases, indicating no phase transition induced by nitrogen doping.

Figure 1. XRD patterns of pure and 10%, 20%, 30%, and 40% nitrogen-doped TiO₂ compared to those of the anatase and rutile TiO₂ phases.

Figure 2. XRD profiles of the most intense peaks (1 0 1) of pure and 10%, 20%, 30%, and 40% N-doped TiO₂.

An enlargement of the most intense peak (1 0 1), as indicated in Figure 2, reveals a slight shift in the peak positions for the N-doped films towards lower diffraction angles (2θ) when compared to the pure TiO₂ sample. This change indicates successful incorporation of that nitrogen into the TiO₂ structure. Nitrogen was incorporated into titanium dioxide using the mixing method. Varying proportions of nitrogen were added to the chemical mixture using a graduated dropper. The successful incorporation of nitrogen into the material was confirmed by X-ray Diffraction (XRD) analysis. The resulting pattern showed a shift in the angles and peaks, which is indicative of nitrogen successfully integrating into the crystal lattice of the titanium dioxide [14]. The crystallite sizes (D) of the prepared thin films were calculated using the following Scherrer equation [15]:

Here, λ = 1.5406 Å denotes the X-ray wavelength, θ represents the Bragg angle associated with the anatase (101) peak, β indicates the line broadening at half the maximum intensity (FWHM), and K = 0.9 is referred to as the Scherrer constant. Table 1 shows the different parameters used in the Scherrer formula as well as the evolution of the crystallite size of the different samples. The estimated crystallite sizes (D) of all samples are found to be approximately 25 nm, indicating a nanoscale morphology of the synthesized materials. The findings presented in Table 1 demonstrate that there were minimal variations in the crystallite size across varying levels of N-doping content. This suggests that the introduction of nitrogen into the TiO₂ structure did not significantly alter the crystallite size of the material. This is consistent with previous studies [16][17]. The lack of significant variation in crystallite size with different N-doping levels in TiO₂ suggests that nitrogen incorporation primarily modifies electronic properties, such as light absorption, without disrupting crystallite formation.

Table 1. Values of the different parameters allowing the calculation of crystallite size using the Scherrer formula.

Figures 3(a)-(e) show SEM images of the pure and N-doped TiO₂ fabricated films, illustrating a well-dispersed microstructure and a consistent distribution of grains. The images exhibit both uniform and clustered grains, suggesting strong cohesion among individual particles and enhancing structural integrity [18]. Some minor voids are noticeable, which is expected due to the material’s porous nature. A high calcination temperature of 500˚C was employed during sample synthesis to improve crystallinity and achieve a more uniform microstructure. This elevated temperature facilitated grain enlargement, leading to enhanced crystalline properties. Furthermore, the SEM images of the pure and N-doped TiO₂ films reveal that these nanoparticles exhibit spherical morphology.

Figure 3. SEM images describing the surface morphology of pure and N-doped TiO₂ films: (a) Pristine TiO₂, (b) 10% N-doped TiO₂, (c) 20% N-doped TiO₂, (d) 30% N-doped TiO₂, and (e) 40% N-doped TiO₂.

By employing the Image J software to analyze the SEM images (refer to Figure 4), we calculated the average grain size for each sample. Consistent with the XRD analysis, there is no significant variation in the average grain size. This result is consistent with previously reported findings for similar materials [19]. The estimated sizes of the grains for both undoped and N-doped TiO₂ nanoparticles fall within the range of 40 nm to 45 nm. In contrast, the XRD patterns reveal smaller average crystallite sizes compared to those observed in the SEM images due to agglomeration caused by higher surface area-to-volume ratios [20]. This process merges crystallites into larger grains. Higher calcination temperatures enhance grain growth and the aggregation of adjacent grains into larger particles, reducing the system’s Gibbs free energy by minimizing the extended surface area [21].

Figure 4. Histograms of grain size distribution: (a) Pristine TiO₂, (b) 10% N-doped TiO₂, (c) 20% N-doped TiO₂, (d) 30% N-doped TiO₂, and (e) 40% N-doped TiO₂.

3.3.1. UV-VIS Absorbance Spectra

Figure 5 illustrates the absorbance (A) spectra versus wavelength (λ) for the pure and N-doped TiO₂ fabricated films within the UV (ultraviolet) and VIS (visible) radiation ranges. As depicted in this figure, the N-doped TiO₂ nanoparticles exhibit a progressive red shift towards higher wavelengths (red shift) in the UV-VIS regions as the N dopant concentration increases compared to pure TiO₂. This finding aligns with previous studies [22][23]. Conversely, the spectra reveal two primary absorption bands in the UV-VIS regions. The presence of these spectral features indicates that the films are well-suited for absorbing both UV and visible light, making them versatile for various applications requiring such light absorption. These include UV-VIS light absorption, photocatalysis, and dye-sensitized solar cells (DSSCs) [24][25]. In the realm of DSSCs, these prepared films have the potential to capture energy from absorbed UV and visible light, transforming it into electricity. Their ability to capture a wide range of wavelengths enhances their effectiveness in converting light energy into electrical power. Furthermore, in the context of photocatalysis, these samples can act as catalysts to accelerate chemical reactions when exposed to either UV or visible light.

Figure 5. UV-Vis spectra for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

The optical absorption coefficient (α) of the fabricated films was calculated through Equation (2), and the Tauc law, as defined in Equation (3) [26], was employed to determine the band gap energy (E_g) values for both the pure TiO₂ and N-doped TiO₂ films. Additionally, Equation (4) is employed to verify the optical transitions occurring within the samples.

where hν is the photon energy, d is the thickness of each sample, and A is the absorbance. From the [(αhν)² vs. hν] curves shown in Figure 6, the direct (E_gd) band gap values were found to be 3.12, 3.04, 3.00, 2.96, and 2.93 eV for pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂, respectively.

The plot of [ln(αhν) vs. ln(hν − E_g)] in Figure 7 reveals that the n exponent values are close to 0.5 for the prepared films. This suggests that both the undoped and N-doped TiO₂ nanoparticles exhibit direct optical transitions. This indicates a direct band gap, allowing electrons to transition from the valence band to the conduction band without intermediate energy levels. This distinctive feature, together with their exceptional light absorption and emission qualities, renders these materials promising candidates for use in dye-sensitized solar cells and optoelectronic devices.

Figure 6. Plots of (αhυ)² versus hυ for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

Figure 7. Plots of ln(αhυ) versus ln(hυ − E_g) for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

Table 2. Comparison of band gap energies for the pure and N-doped TiO₂ films with literature values.

The energy band gap values (E_g) of both undoped and N-doped TiO₂ films are compared in Table 2 with those of various semiconductors [27]-[29] and other TiO₂ films with different dopants [30]-[32]. The data indicated that the E_g energy decreases with nitrogen doping, aligning well with findings from other studies [17][30]. This decrease in band gap energy resulting from nitrogen doping may stem from the formation of isolated narrow bands above TiO₂’s valence band. This can occur by combining the 2p states of nitrogen and oxygen in the dopant and TiO₂ (interstitial doping), or by replacing oxygen-deficient sites with nitrogen (substitutional doping) [33][34]. Consequently, the decrease in band gap energy may be attributed to the impurity levels generated above the valence band of TiO₂ through interstitial nitrogen doping. It has also been noted that interstitial nitrogen doping significantly impacts and diminishes the band gap of TiO₂ in comparison to substitutional N doping [35]. In addition, the E_g values of the fabricated films, as depicted in Table 2, are lower than those of other doped TiO₂ films, such as Ni-doped TiO₂ and Cu-doped TiO₂ . Furthermore, the synthesized thin films have lower E_g values compared to wide-band gap semiconductors like ZnO, TiO₂, and CuO that absorb UV light [27]-[29]. Recent studies have focused on enhancing visible light absorption by developing materials with narrow band gap energies [36]. This makes the produced samples well-suited for capturing crucial visible light, which is essential for applications like solar cells and photocatalysis.

The observed decreases in both the direct (E_g) and indirect (E_u) band gaps are directly linked to the specific electronic states introduced by either interstitial or substitutional nitrogen. Interstitial nitrogen atoms introduce new energy levels deep within the band gap, providing additional pathways for electronic transitions and thus reducing both E_g and E_u. In contrast, substitutional nitrogen atoms replace host atoms, which modifies the overall electronic band structure and can lead to a general narrowing of the band gap. A recent study by Wang et al. (2024, Journal of Materials Science) supports this distinction, demonstrating that deep-level defects, primarily responsible for the significant reduction in band gap values, are predominantly formed by interstitial nitrogen, while substitutional nitrogen contributes more to the overall modification of the material’s band structure [37].

3.3.2. Urbach Energy

Urbach energy (E_u) serves as a crucial parameter in optical spectroscopy for evaluating the degree of disorder and impurity levels in a material [38]. It provides insights into the localized states within the band gap and the broadening of electronic transitions, thereby reflecting the material’s internal characteristics and electronic behavior. This energy value is associated with the exponential tail in the density of states near the band gap’s edges. A lower E_u value indicates a material with higher orderliness and fewer defects, whereas a higher E_u value indicates a heightened presence of disorder and defect concentration. The determination of E_u energy from the photon energy (hν) can be carried out from the following equation [39]:

where α is the absorption coefficient and α₀ is a constant. By logarithmically transforming both sides of Equation (6), we can derive this relation:

This logarithmic representation is utilized for analyzing experimental data and extracting the E_u energy from the absorption spectra based on the [ln(α) vs. (hν)] curve. The Urbach energy is derived from the ln(α) vs. hν curve by examining the linear segment located at the lower energy side of the absorption spectrum, below the band gap energy values. The determined E_u values for pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂ nanoparticles were 1.75, 0.91, 1.04, 1.54, and 0.86 eV, respectively. The observed decrease in Urbach energy values upon nitrogen doping suggests a potential decrease in structural disorder and impurities within the TiO₂ sample. Additionally, lower Urbach energy values indicate a more organized material structure with fewer localized states within the band gap in the prepared samples. The variation of E_u with nitrogen doping is shown in Figure 8.

Figure 8. Plot of ln(α) versus (hν) for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

3.3.3. Steepness Parameter and Electron-Phonon Interaction Energy

The Urbach energy (E_u) and the temperature-dependent steepness parameter S(T) can be expressed through the following [40]:

In this equation, the symbol k_B denotes the Boltzmann constant, and T represents the standard room temperature. The steepness parameter, denoted as S(T), quantifies the broadening of the absorption edge, which is attributed to various interactions such as electron-phonon or exciton-phonon couplings . It measures how rapidly the absorption coefficient changes with photon energy in the vicinity of the band edge. A higher S(T) value corresponds to a sharper absorption edge, indicating a wider energy range over which the absorption coefficient varies significantly. The calculated S values for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂ nanoparticles were 0.014, 0.028, 0.024, 0.017, and 0.030, respectively. Significantly, the increased S value observed in the 10% N-doped TiO₂ sample indicates a greater broadening of the absorption edge compared to the remaining samples. Following Equation (8), the electron-phonon interaction energy (E_e−ph) can be estimated based on the S parameter, as follows :

The E_e−ph value denotes the energy associated with the interaction between electrons and phonons in the material, serving as a significant parameter for understanding the material’s electronic and thermal characteristics. A higher E_e−ph-value indicates a stronger electron-phonon interaction, indicating a greater propensity for energy dissipation through lattice vibrations. In our study, utilizing Equation (8), we have computed the E_e−ph values for our samples as 47.62, 23.81, 27.78, 39.22, and 22.22 eV for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂ films, respectively. The heightened E_e−ph value observed in the pure TiO₂ sample indicates a stronger electron-phonon interaction and a higher likelihood for energy dissipation through lattice vibrations compared to the N-doped TiO₂ films.

3.3.4. Threshold Wavelength

Figure 9. Plots of (α/λ)² versus 1/λ for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

In optoelectronic devices, the critical wavelength (λ_T), also known as the threshold wavelength, plays a significant role in evaluating a material’s suitability for such applications by representing the maximum wavelength of incident radiation. This λ_T parameter signifies the shortest wavelength of light essential to trigger particular optoelectronic operations, like absorption or emission, within a material. To calculate the λ_T value, we have applied the following equation [42]:

where α represents the absorption coefficient, λ stands for the wavelength of incoming radiation, and C is a constant, the threshold wavelength (λ_T) values were determined as 442, 380, 389, 422, and 370 nm for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂ films, respectively, as indicated in Figure 9. The variation of λ_T values closely aligns with the band gap energy values. Typically, a lower band gap energy corresponds to a reduced λ_T value. This suggests that the 40% N-doped TiO₂ sample necessitates higher-energy photons (shorter wavelengths) to initiate optoelectronic processes.

3.3.5. Penetration Depth

The penetration depth (δ) acts as a measure that defines the distance to which incoming light or radiation can permeate within a substance. According to Equation (10) [43], the δ parameter can be estimated as follows:

The data illustrated in Figure 10 reveal that the determined penetration depth values (δ(λ)) are significantly reduced in the fabricated films. This indicates that the samples may be promising for photovoltaic purposes due to improved light absorption and potentially increased energy conversion efficiency resulting from the decreased penetration depth. Furthermore, their decreased penetration depth makes them ideal candidates for advanced photodetectors, enabling the precise detection of light spanning a wide range of wavelengths.

Figure 10. Penetration depth (δ) versus λ for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

3.3.6. Extinction Coefficient

The extinction coefficient (k) provides valuable insights into how light is absorbed and scattered within a material, indicating the material’s effectiveness in diminishing the intensity of incident radiation. This coefficient reflects both the absorption and scattering processes that contribute to the overall attenuation of light. According to Equation (11) [44], the extinction coefficient (k) can be calculated using the following formula:

Figure 11 displays the variation of the coefficient k versus photon energy (hυ) across the different types of films: pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂. It should be noted that k-values are higher at lower hυ values but decrease significantly as hυ rises, eventually approaching zero. This trend suggests minimal light loss in the higher energy range. Similar patterns of decreasing k values with rising photon energy have been noted in diverse materials [45][46], implying that the prepared samples allow light to pass through with minimal loss. The variations in k values are around 10⁻⁵, indicating that losses from scattering and absorption during light transmission through the samples are negligible. This behavior highlights the high transparency of the films. Importantly, these findings align with prior research by Mott and Davis [47].

Figure 11. Extinction coefficient (k) versus hν for the pure TiO₂, 10% N-doped TiO₂, 20% N-doped TiO₂, 30% N-doped TiO₂, and 40% N-doped TiO₂.

While high concentrations of nitrogen doping are known to enhance the electrochemical properties of titanium dioxide-coated anodes, particularly their performance in lithium-ion batteries, excessive nitrogen levels can lead to several undesirable consequences. Over-doping with nitrogen can induce lattice distortions in the titanium dioxide crystal structure, which may reduce its long-term stability. High nitrogen concentrations can also create unwanted defects or new, unstable phases, hinder lithium-ion diffusion, and negatively impact the battery’s charge and discharge rates. Furthermore, high nitrogen levels might decrease the material’s effective surface area, limiting available reaction sites for lithium ions and ultimately affecting the overall battery capacity. Future Experiment to Address These Drawbacks. To mitigate these challenges, we propose a future experiment focused on co-doping techniques. This study would evaluate the combined effect of doping with nitrogen and a second element, such as carbon or fluorine, to stabilize the titanium dioxide lattice. The experiment would involve preparing several anode groups:

A control group with an undoped anode. Groups were doped with nitrogen only at varying concentrations (low, medium, and high). Groups co-doped with nitrogen and the second element at different ratios.

We would then perform comprehensive analyses, including X-ray diffraction (XRD) to study crystal structure stability, X-ray photoelectron spectroscopy (XPS) to examine surface chemical composition, and galvanostatic cycling to measure key electrochemical properties like capacity and cycle life. We anticipate that co-doping will improve the anode’s performance more effectively than nitrogen doping alone, helping to identify the optimal nitrogen concentration that enhances performance without compromising structural integrity [48].