Impact of metal doping on the physical characteristics of anatase titanium dioxide (TiO2) films

This study focuses on the synthesis of anatase titanium dioxide (TiO2) films doped with metals (Mg, Ni, and Sn) using the sol–gel dip coating technique. The main objective was to investigate the chemical, crystal, and morphological structure of the Mg-, Ni-, and Sn-doped TiO2 films. The doping mechanism between the metals and the TiO2 films was confirmed through Fourier-transform infrared (FTIR) spectroscopy, which revealed characteristic absorption bands associated with the doping process. Additionally, X-ray diffraction (XRD) patterns confirmed that all films maintained an anatase phase, indicating the preservation of the desired crystal structure. Furthermore, the introduction of Mg and Sn metals into the TiO2 film resulted in a decrease in the crystallite size, reducing it from 53 to 16 nm and 24 nm, respectively. Conversely, the incorporation of Ni into the TiO2 film increased the crystallite size to 72 nm. Moreover, the presence of these metals in the TiO2 film contributed to a smoother film surface, thereby enhancing the hydrophilicity of the films. The optical bandgap of the TiO2 films decreased with the introduction of Mg, Ni, and Sn, exhibiting values of 3.24 eV, 3.11 eV, and 3.15 eV, respectively, compared to the original value of 3.33 eV. Additionally, the electrical conductivity (σ-value) increased upon the introduction of Mg, Ni, and Sn, reaching values of 0.25 mS.cm−1, 0.37 mS.cm−1, and 2.5 mS.cm−1, respectively. Overall, this work provides insights into the chemical, crystal, and morphological characteristics of Mg-, Ni-, and Sn-doped TiO2 films.


Introduction
Titanium dioxide (TiO 2 ) has recently garnered significant attention in the fields of solar-driven catalysis and solar cell applications due to its exceptional properties. These properties include its remarkable optical and electrical characteristics, high dielectric constant, elevated refractive index, wide bandgap (ranging from 3 to 3.4 eV) in the anatase phase, and excellent visible region transmission [1][2][3][4]. Additionally, TiO 2 crystals exist in three primary phases: anatase, rutile, and brookite, each exhibiting distinct structural, optical, and electronic properties [5,6]. Among these phases, the anatase phase of TiO 2 possesses unique attributes that make it highly suitable for optoelectronic applications, solar cells, and catalysis. These attributes encompass a larger surface area, a smaller bandgap, enhanced charge carrier mobility, and superior photocatalytic activity compared to other phases [7,8].
Numerous deposition techniques have been employed for the fabrication of titanium dioxide (TiO 2 ) films, including chemical vapor deposition [9], sol-gel technique [10,11], spray pyrolysis [12], hydrothermal method [13], pulsed laser deposition [14], and electro-beam evaporation [15]. Among these techniques, the sol-gel method was specifically selected for this research due to its inherent advantages, which encompass exceptional film uniformity, facile compositional control, the ability to coat large areas, low processing temperatures, cost-effectiveness, and superior photocatalytic performance [16]. The doping of TiO 2 films with metal ions has emerged as a captivating area of research owing to their crucial applications in solar-driven catalysis, solar cells, antimicrobial agents, sensors, self-cleaning surfaces, and environmental remediation [4]. Consequently, in this study, we incorporated three distinct metal ions as dopants into TiO 2 films: magnesium (Mg) as an alkaline earth metal, nickel (Ni) as a transition metal, and tin (Sn) as a post-transition metal. Alkaline earth metals, such as Mg, and posttransition metals, like Sn, were selected as dopants in TiO 2 to reduce the bandgap and diminish the electron-hole recombination rate [17,18]. Furthermore, transition metals, exemplified by Ni, were employed as dopants in TiO 2 to control the film morphology, decrease the bandgap, enhance conductivity, increase hydrophilicity, and improve magnetic properties [5,19]. The study aimed to investigate the influence of metal dopants (Mg, Ni, and Sn) on the structural, optical, electrical, and surface wettability characteristics of TiO 2 films. The primary objective was to assess the potential applications of these films, including their utility as ultraviolet filters, antireflection coatings for photovoltaic cells [20], photocatalysts for water and air purification and treatment [21], and thin ceramic films or membranes with controlled porosity [22,23]. To accomplish this, the research examined the structural features of metaldoped TiO 2 films, encompassing their chemical composition, crystal structure, and morphology, in order to determine the specific impact of the metals (Mg, Ni, and Sn) on the film structure. Additionally, the investigation encompassed the analysis of optical constants (extinction coefficient and refractive index), optical bandgap, band structure, and electrical conductivity to enhance our understanding of the optical, optoelectronic, and electrical properties of these films.
Stock solution 1 was prepared by vigorously stirring 2.95 mL of TTIP with 19.1 mL of 2-propanol at room temperature for 1 h. This solution served as the source of titanium. Concurrently, stock solution 2 was prepared by mixing 97.35 mL of DEA with 0.5 mL of distilled water under continuous stirring for 1 h at room temperature. This solution served as the source of oxygen. Subsequently, stock solution 1 was slowly added to stock solution 2 while stirring continuously for 4 h at room temperature to obtain the TiO 2 solution. The final solution was then filtered using a 0.45-lm filter paper.
Mg-, Ni-, and Sn-doped TiO 2 films were synthesized using the sol-gel dip coating technique. Stock solution 1 was prepared by mixing 2.95 mL of TTIP, 19.1 mL of 2-propanol, and 0.06 g of magnesium acetate (for Mg/TiO 2 ), nickel chloride (for Ni/TiO 2 ), or tin chloride (for Sn/TiO 2 ) under continuous stirring at room temperature for 1 h. Simultaneously, stock solution 2 was prepared by mixing 97.35 mL of DEA with 0.5 mL of distilled water under continuous stirring for 1 h at room temperature. The Mg-, Ni-, and Sn-doped TiO 2 solutions were then prepared by adding stock solution 1 slowly into stock solution 2 while stirring continuously for 4 h at room temperature. The final solutions were filtered using a 0.45lm filter paper. Afterward, the glass substrates were cleaned by rinsing with distilled water and ethanol, followed by air drying. Subsequently, pure TiO 2 as well as Mg-, Ni-, and Sn-doped TiO 2 films were deposited on the glass substrates using the dip coating technique. The glass substrates were immersed in the respective solutions for 2 h to achieve uniform films with a thickness of approximately 500 nm. Finally, the films were annealed at 550°C for 2 h (Fig. 1).

Sample characterization
The crystal structure analysis was performed using a powder X-ray diffraction (XRD) technique, employing a Malvern Panalytical Ltd. instrument (Malvern, UK) operating at 220-230 VAC, 50/60 Hz, and 40 A. Cu Ka1 radiation with a wavelength of 0.1540598 nm was used, and the measurements were conducted at room temperature. The incident photon angles ranged from 10°to 70°on the sample surface, with a step size of 0.02°and an energy resolution of 20%. For the determination of chemical and elemental structures, Fourier-transform infrared (FTIR) spectroscopy was employed using a Bruker Tensor 27 spectrometer. The measurements were performed at room temperature in the spectral range of 400-2000 cm -1 . Additionally, X-ray fluorescence (XRF) analysis was conducted using a NEX QC ? QuantEZ instrument (Rigaku) to further examine the chemical composition. The morphology of the films was investigated through scanning electron microscopy (SEM) using a Quanta FEG 450 microscope at room temperature. Surface wettability was evaluated by measuring the contact angle of a water droplet (pH = 7) with a volume of 10lL. Three separate measurements were taken to ensure accuracy and reproducibility. Optical transmittance and reflectance spectra were obtained using a UV-Vis spectrophotometer (U-3900H) at room temperature, covering the spectral range of 250-700 nm. The two-dimensional electrical conductivity was measured using a 4-point probe system (Microworld Inc.) equipped with a high-resolution multimeter (Keithley 2450 Sourcemeter).

Structural properties
The present study investigated the structural properties of pure TiO 2 and Mg-, Ni-, and Sn-doped TiO 2 films using various analytical techniques such as Fourier transform infrared (FTIR) spectroscopy, Xray diffraction (XRD) patterns, scanning electron microscopy (SEM) micrographs, and water contact angle measurements. The FTIR spectrum of pure TiO 2 film showed three distinct absorption bands at 430, 745, and 1640 cm -1 corresponding to stretching Ti À O, anatase titania, and stretching Ti À OH, respectively (Fig. 2). The presence of the Ti À OH band indicated the presence of hydrogen (H) impurities in the TiO 2 film, which were bound to oxygen vacancies through covalent bonding, leading to ntype conductivity in the TiO 2 film [25]. In contrast, the introduction of Mg, Ni, and Sn metals into the TiO 2 film resulted in the appearance of new absorption bands at 572 cm -1 (Mg À O), 530 cm -1 (Ni-O), and 613 cm -1 (O À Sn À O), respectively, which confirmed the doping mechanism between these metals and TiO 2 film. Figure 3 presents representative XRF spectra of pure TiO 2 films and TiO 2 films doped with Mg, Ni, and Sn. The analysis shows that the pure TiO 2 films exhibit a high Ti element percentage of 98.6%, with 1.4% impurities, indicating a relatively higher purity. It should be noted that the XRF technique is unable to directly detect oxygen atoms. Furthermore, the XRF analysis of Mg-, Ni-, and Sn-doped TiO 2 films demonstrates successful doping with all three metals, achieving a doping concentration of approximately 10%. However, the impurity content in Ni-and Sndoped TiO 2 films is relatively higher compared to the Mg-doped TiO 2 film. This discrepancy can be attributed to the different starting materials used for doping, where Ni and Sn were introduced as chlorides while Mg was added in the form of acetate.
XRD analysis was employed to assess the crystallinity and phase composition of pure TiO 2 films and TiO 2 films doped with different metals. The XRD patterns of pure TiO 2 and metal-doped TiO 2 films were subjected to qualitative phase analysis using software (Qualx2) (Fig. 4) [27]. Furthermore, the lattice constants of the metal-doped TiO 2 films were found to be consistent with those of the pure TiO 2 film ( Table 1). The microstructural properties of pure TiO 2 films and TiO 2 films doped with metals were determined using the Williamson-Hall (WH) method, which utilizes the diffraction peak characteristics and linewidths [28,29]. The total linewidth (b total ) of the XRD peaks was analyzed, and it was found to be the combination of the particle size linewidth (b size ) and the microstrain linewidth (b strain ), following the relationship provided by Eq. (1) [27]: The microstructural properties of the samples, including crystallite size (D) and microstrain (e), were determined using the modified Williamson-Hall equation based on the uniform deformation model (UDM). The particle size linewidth (b size ) was calculated as b size ¼ kk=Dcosh, where k represents the Scherrer constant (k = 0.94) and k is the X-ray wavelength (k = 1.54184 Å ) [30]. Additionally, the microstrain linewidth (b strain ) was obtained as b strain ¼ 4heitanh. Thus, the UDM-modified Williamson-Hall equation is given by Eq. (2) [31]: The b hkl cosh plotted against 4sinh allows us to determine the hei value based on the slope of the linear fit, while the D value is obtained from the yintercept. Table displays the average D and hei values estimated from the UDM. The inclusion of Mg and Sn in TiO 2 films leads to a reduction in crystallite size and microstrain, whereas the introduction of Ni increases the crystallite size and slightly raises the microstrain. This can be attributed to differences in the ionic radius of each doping metal compared to that of titanium [32]. Furthermore, the dislocation density (d) is calculated using the Williamson-Smallman equation as d ¼ 1=D 2 [33]. The d values increase with the addition of Mg and Sn to the TiO 2 film and decrease when Ni is introduced ( Table 1).
The modified Williamsons-Hall equation within the context of the uniform stress deformation model (USDM) was employed to calculate the stress-induced deformation for different crystallographic plane directions [34]. The equation used to modify the Williamsons-Hall equation in the USDM is expressed as [35,36] decrease upon the incorporation of Mg and Sn into TiO 2 film, whereas they increase with the introduction of Ni into TiO 2 film (Table 1). Furthermore, the energy density (E d ) was determined using the modified Williamsons-Hall equation within the framework of the uniform deformation energy density model (UDEDM), as described by the equation [37], According to Eq. (4), the r value is obtained from the slope of the linear fit function, while the y-intercept corresponds to the average D value. Additionally, the energy density (E d ) is calculated using the formula E d ¼ he 2 iY=2. The E d values decrease with the introduction of Mg and Sn into the TiO 2 film and increase when Ni is incorporated ( Table 1).
The morphological properties of pure TiO 2 and metal-doped TiO 2 films were examined using scanning electron microscopy (SEM) images, along with measurements of water contact angles. The results of this investigation are presented in Fig. 5, as documented in reference [38].
The analysis of the SEM micrograph for the pure TiO 2 film (Fig. 5a) reveals an irregular spheritic shape, with crystal sizes ranging from 0.3 to 1 lm. These crystals tend to agglomerate, leading to localized regions of increased crystal density within the film. Introducing Mg into the TiO 2 films results in a significant decrease in crystal agglomeration (Fig. 5b). This indicates that the incorporation of Mg in the TiO2 film effectively mitigates crystal clustering, resulting in a more dispersed distribution of crystals throughout the film. Comparatively, the Ni-doped TiO 2 film exhibits lower agglomeration compared to both the pure TiO2 film and the Mg-doped TiO 2 films (Fig. 5c). Additionally, the Ni-doped film shows a reduced surface density of spheritic particles, with sizes ranging from 0.5 to 3 lm. The larger particle sizes observed in this case may be attributed to the migration of MgO from the bottom layer to the top layer of TiO 2 during film formation. This further confirms that the introduction of Ni into the TiO 2 film promotes a more uniform distribution of crystals across the film. In contrast, the Sn-doped TiO 2 film exhibits a distinct morphology characterized by a smooth film surface (Fig. 5d). Moreover, the film exhibits a higher surface area, likely due to the presence of rod-like structures composed of SnO 2 . These rod-like structures have sizes falling within the range of 0.3 to 0.8 lm. The surface wettability of the pure TiO 2 and metaldoped TiO 2 films was assessed using static water contact angle (WCA) measurements, which involve observing the contact angle formed between the film surface and water droplets. The average WCA of the pure TiO 2 film was measured to be 30°, indicating its hydrophilic nature. When introducing metals (Mg, Ni, and Sn) into the TiO 2 film, the WCA decreased, indicating an increased hydrophilicity of the doped films (Fig. 6). This change can be attributed to the improved smoothness of the film surface as a result of metal doping (Fig. 5). The enhanced hydrophilicity of the films enhances the available surface area for photon-induced reactions or other chemical processes.

Optical properties
The optical properties of pure TiO 2 films and TiO 2 films doped with metals were investigated by analyzing the transmittance and reflectance spectra obtained through UV-Vis spectroscopy (Fig. 7). Both pure TiO 2 and metal-doped TiO 2 films exhibited high transmittance values (T [ 60%) in the visible region. However, the introduction of metals into the TiO 2 film further increased the transmittance values in the visible region (Fig. 7a). Notably, a sharp decline in transmittance values occurred at wavelengths prior to approximately 350 nm, indicating a region of high absorption. Additionally, the introduction of metals in the TiO 2 films led to a shift in the absorption edge towards the red region, resulting in a reduction of the bandgap energy upon metal doping (Mg, Ni, and Sn). This observation suggests that both pure TiO 2 and metal-doped TiO 2 films hold potential for UVshielding applications. Furthermore, the reflectance spectra of both pure TiO 2 and metal-doped TiO 2 films exhibited similar behavior, with a decrease in reflectance values upon metal doping (Fig. 7b). The refractive index (n) and extinction coefficient (k) were calculated using methods described in previous literature [39][40][41]. The k-spectrum displayed a significant drop for wavelengths exceeding 300 nm, indicating minimal interaction of photons with the film material in the visible region (Fig. 7c). Additionally, the n-spectrum of the TiO 2 film exhibited a gradual decrease from 3.15 to 2.39 as the incident wavelength increased from approximately 400 to 700 nm. Moreover, the introduction of metals into the TiO 2 film resulted in a decrease in the refractive index values (Fig. 7d). The notable drop in n-values between 250 and 400 nm can be attributed to the high absorption of photons and resonance between photons and electronic film polarization.
Tauc plots were employed to determine the bandgap energy (E g ) of pure TiO 2 and metal-doped TiO 2 films. The plots were generated by plotting ahv ð Þ 2 on the y-axis against hv on the x-axis, following the equation ahv ð Þ 2 ¼ bðhv À E g Þ for direct bandgap semiconductors [32]. The E g value of the pure TiO 2 film was found to be 3.33 eV. Upon introducing Mg, Ni, and Sn into the TiO 2 film, the E g values decreased to 3.24 eV, 3.11 eV, and 3.15 eV, respectively (Fig. 7e). This reduction can be attributed to the strong interaction between the s-and d-electrons of the TiO 2 film and the s-electron of Mg, the d-and selectrons of Ni, or the d-, s-, and p-electrons of Sn. The interplay of these electrons in the metal-doped TiO 2 film results in the creation of new energy levels between the valence band and conduction band, leading to increased localized state density within the mobility bandgap [42]. To investigate the band structure, including E VB , E CB bandgap energy, and substates, the ionization energy and electron affinity energy were employed, following the methods described in the literature [43,44]. The schematic diagram of the band structure for both pure TiO 2 and metal-doped TiO 2 films displayed a decrease in E CB and E VB , supporting the observed decrease in E g

Electrical conductivity
The electrical conductivity (r) of oxide films is influenced by various factors, including dopant materials, dopant concentrations, and the crystal and morphological properties of the films [45]. Figure 8 presents the electrical conductivity values of both pure TiO 2 and metal-doped TiO 2 films. The r-value of TiO 2 films is approximately 0.10 mS cm -1 , which can be attributed to the presence of hydrogen-related shallow donor defects, resulting in carrier density of O-and Ti-polar regions [46]. When Mg, Ni, and Sn are introduced into the TiO 2 film, the electrical conductivity (r) values increase to 0. 25  Additionally, the conductivity maps (1cm Â 1cm) of both pure TiO2 and metal-doped TiO 2 films exhibit variations in conductivity across the films, which can be attributed to the quality of the growth process and the transfer process (Fig. 9).

Conclusions
Anatase titanium dioxide (TiO 2 ) films doped with metals (Mg, Ni, and Sn) were synthesized using the sol-gel dip coating technique. The objective was to investigate the impact of Mg, Ni, and Sn as dopants in TiO 2 films on their structural, optical, electrical, and surface wettability properties to explore potential applications for these films. The Fourier-transform infrared (FTIR) spectrum of the TiO 2 film exhibited three absorption bands associated with stretching modes of Ti À O bonds, anatase titania, and stretching modes of Ti À OH bonds. Upon introducing Mg, Ni, and Sn metals to the TiO 2 film, new absorption bands emerged at 572 cm -1 (Mg À O), 530 cm -1 (Ni À O), and 613 cm -1 (O À Sn À O), confirming the successful doping mechanism between these metals and the TiO 2 film. Furthermore, X-ray diffraction (XRD) patterns revealed that all films exhibited an anatase phase, as evidenced by peaks at 25.36°(101), 37.86°(004), 48.06°(200), 53.94°(105), 55.14°(211), and 62.84°(204). However, the introduction of Mg and Sn metals into the TiO2 film led to a decrease in crystallite size from 53 to 16 nm and 24 nm, respectively. In contrast, introducing Ni into the TiO 2 film increased the crystallite size to 72 nm. Moreover, the introduction of these metals into the TiO 2 film resulted in smoother film surfaces and enhanced hydrophilicity. The refractive index (n) spectra of the TiO 2 film exhibited a decreasing trend from 3.15 to 2.39 as the incident wavelength increased from approximately 400 to 700 nm. Furthermore, the introduction of metals into the TiO 2 film led to a reduction in the n-values. The optical bandgap decreased from 3.33 eV to 3.24 eV, 3.11 eV, and 3.15 eV with the introduction of Mg, Ni, and Sn, respectively. Additionally, the electrical conductivity (r-value) increased from 0.10 mS cm -1 to 0.25 lS cm -1 , 0.37 lS cm -1 , and 0.25 lS cm -1 when  Fig. 9 Two-dimensional conductivity maps (1cm Â 1cm) of a pure TiO 2 , b Mg/TiO 2 , c Ni/TiO 2 , and d Sn/TiO 2 films measured at room temperature using a 4-point probe technique, with measurements taken at 12 points across the film introducing Mg, Ni, and Sn, respectively. However, introducing Ni into the TiO 2 film significantly increased the r-value to 18.38 lS.cm -1 .