Vacuum Ultraviolet Spectroscopic Analysis of Structural Phases in TiO₂ Sol–Gel Thin Films

Abstract

This study investigates the structural and electronic transitions of sol–gel derived titanium dioxide (TiO₂) thin films using vacuum ultraviolet (VUV) spectroscopy, to elucidate the impact of annealing-induced phase evolution. As the annealing temperature increased from 400 °C to 800 °C, the films transitioned from amorphous to anatase, mixed anatase–rutile, and finally rutile phases. VUV spectroscopy revealed distinct absorption features: a high-energy σ → π* transition below 150 nm, associated with bonding to antibonding orbital excitations, and lower-energy absorption bands in the range 175–180 nm and near 280 nm, attributed to π → π* and t_2g(π) → t*_2g(π*) transitions, respectively. These spectral features highlight the material’s intrinsic electronic states and defect-related transitions. A slight redshift of the absorption band from 176 nm to 177 nm with annealing reflects bandgap narrowing, attributed to increased rutile content, crystallite growth, and defect-induced effects. Broadening and additional absorption features around 280 nm were attributed to oxygen vacancies and reduced titanium oxidation states (Ti³⁺), as corroborated by X-ray photoelectron spectroscopy (XPS). XPS further confirmed the presence of Ti³⁺ species and oxygen vacancies, providing complementary evidence of defect-mediated transitions observed in the VUV spectra. While complementary techniques such as X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed phase transitions and the reduction of hydroxyl groups, respectively, VUV spectroscopy uniquely captured the dynamic interplay between structural defects, phase evolution, and optical properties. This study underscores the utility of VUV spectroscopy as a powerful tool for probing the electronic structure of TiO₂ films, offering insights critical for tailoring their functional properties in advanced applications.

1. Introduction

Titanium dioxide (TiO₂) is a versatile material that has been extensively studied for applications in photocatalysis, solar energy conversion, gas sensing, and other optoelectronic technologies [1,2]. Its widespread use stems from its unique electronic structure, chemical stability, and nontoxicity [3]. TiO₂ exists in three main polymorphs—anatase, rutile, and brookite—of which anatase and rutile are the most significant due to their distinct optical and electronic properties [4,5]. Anatase, with a larger bandgap of ~3.2 eV, is favored for photocatalytic activity due to efficient charge separation and high surface energy [6]. Rutile, with a narrower bandgap (~3.0 eV), is thermodynamically more stable and exhibits enhanced light absorption in the visible region, making it suitable for high-temperature or optical applications [7].

The optical behavior of TiO₂ is governed by its electronic transitions, which involve charge transfer between oxygen 2p orbitals in the valence band and titanium 3d orbitals in the conduction band [8]. These transitions are highly sensitive to the material’s crystal structure, phase composition, and defect density. For instance, in anatase, the distorted TiO₆ octahedral geometry induces crystal field splitting, separating the Ti(3d) orbitals into lower-energy t_2g (d_xy, d_yz, d_xz) orbitals and higher-energy e_g (d_x²_−y², d_z²) orbitals [3]. This splitting, denoted as Δ, defines the energy separation between bonding (π) and antibonding (π*) states, directly impacting the bandgap energy. In rutile, the TiO₆ structure is less distorted, resulting in smaller crystal field splitting and stronger orbital overlap, thereby reducing the bandgap. These differences in bandgap and electronic structure significantly influence TiO₂’s optical absorption and application-specific properties.

The vacuum ultraviolet (VUV) range, corresponding to wavelengths below 250 nm, captures electronic transitions involving high-energy charge transfer between molecular orbitals formed by TiO₂’s Ti(3d) and O(2p) states [9], where O(2p) is valence band and Ti(3d) conduction one [10,11]. The primary transitions include σ → π* and π → π*, where electrons are excited from bonding (σ or π) orbitals to antibonding states (π*). These transitions reflect the material’s intrinsic electronic structure, including the effects of crystal field splitting, orbital hybridization, and defect-induced states. For anatase, the distorted lattice enhances hybridization between O(2p) and Ti(3d) orbitals, resulting in sharper and more intense transitions. In rutile, the more symmetric structure leads to increased orbital overlap, broadening the transitions and reducing their energy. The absence of long-range order in amorphous TiO₂ diminishes the definition of these transitions, leading to broader, featureless absorption profiles in the VUV spectrum.

The optical properties of TiO₂ are also highly sensitive to its structural phase, crystallinity, and defect density. Amorphous TiO₂, for instance, typically exhibits a slightly lower bandgap than crystalline anatase due to its lack of long-range order and the presence of defect-induced states near the band edges. Previous studies have reported a photonic bandgap of 3.18 eV (390 nm) for amorphous TiO₂, slightly smaller than the bandgap of anatase (~3.2 eV) [5]. Doping with nitrogen can further narrow the bandgap to 2.99 eV (415 nm) by introducing localized electronic states within the bandgap [5]. These findings highlight the significant role of structural disorder and defect density in modulating TiO₂’s bandgap, making it crucial to investigate high-energy electronic transitions in detail [12].

While much attention has been given to the UV-visible absorption of TiO₂, its behavior in the vacuum ultraviolet (VUV) range—where high-energy electronic transitions occur—remains less explored [13]. Vacuum ultraviolet (VUV) spectroscopy provides a powerful tool to analyze electronic transitions, such as σ → π* and π → π*, that are sensitive to phase evolution and defect states [13].

Unlike UV-visible (UV-Vis) spectroscopy, which is limited to lower-energy transitions in the 200–800 nm range and primarily focuses on bandgap estimation and visible-light absorption, VUV spectroscopy operates below 250 nm and captures high-energy features that are inaccessible through UV-Vis. These transitions are influenced by structural defects, crystal field splitting, and changes in chemical bonding.

In sol–gel derived films, the porous structure and dynamic defect chemistry play an important role in determining their electronic and optical properties. Both sol–gel and DC sputtered TiO₂ films exhibit water absorption features, highlighting the presence of water and hydroxyl groups [2,9], which are commonly detectable by FTIR spectroscopy. Notably, in DC films, water absorption is also detected by VUV spectroscopy, revealing sharp absorption peaks related to water and hydroxyl groups [9].

Sol–gel films exhibit distinct behavior due to their unique structural and chemical characteristics, which likely affect defect states and phase evolution, which could potentially be observed through VUV spectroscopy. Despite this, VUV spectroscopy has been underused in the study of sol–gel TiO₂ films, leaving a gap in understanding how their structural and electronic properties evolve in this context.

To the best of our knowledge, no prior studies have employed VUV spectroscopy to systematically investigate the optical properties of sol–gel derived TiO₂ thin films. Building on this gap, we analyze the VUV optical properties of these films subjected to annealing at temperatures of 400 °C, 600 °C, and 800 °C. These temperatures induce a phase transition from amorphous to anatase and eventually to rutile, providing a platform to study the effects of crystal structure, defect density, and grain size on electronic transitions. The evolution of absorption features in the VUV spectrum is analyzed to elucidate key mechanisms, including bandgap narrowing, defect-induced broadening, and quantum confinement effects. These insights offer a deeper understanding of the interplay between structural and electronic properties in TiO₂, with implications for its design in advanced energy and environmental applications.

2. Materials and Methods

2.1. Preparation and Characterization of TiO₂ Thin Films

TiO₂ thin films were synthesized using a sol–gel method, a versatile and effective approach for controlling the properties of the resulting films. The synthesis process was carried out under conditions where the water-to-alkoxide ratio was intentionally kept low to control the reaction. The process involved the following steps:

• Sol Preparation: A 0.2 M solution of titanium (IV) isopropoxide (TTIP, Ti(i-OPr)₄, 97%, Aldrich, Darmstadt, Germany) was prepared by dissolving it in ethanol (EtOH, 99.9%, Fisher Scientific, Porto Salvo, Portugal). Acetylacetone (AcAc, 99%, Fisher Scientific) was added as a chelating agent in a 1:1 molar ratio with TTIP to stabilize the titanium alkoxide. Deionized water and glacial acetic acid (CH₃CO₂H, 1 M) were added dropwise under continuous stirring. The solution was then stirred under reflux at 75 °C for approximately 12 h to ensure proper reaction and stabilization. After cooling to room temperature, a few drops of distilled water were added dropwise under vigorous stirring to facilitate the hydrolysis of the titanium precursor. The solution was stirred for 2 h at room temperature until a clear, light-yellow homogeneous sol was obtained.
• Film Deposition: The TiO₂ films (thickness~200 nm) were deposited on Si wafers using a single cycle of spin-coating at 500 rpm for 15 s with a Laurell WS-400-6NPP-LITE spin coater (Laurell Technologies Corporation, USA). Si wafers were used for FTIR and XPS analyses. The sol was also deposited onto calcium fluoride (CaF₂) substrates (1 mm thick) using the same spin-coating technique. CaF₂ substrates were chosen for VUV analysis. The substrates were spun with the gel solution at a controlled rate to achieve a uniform film thickness of approximately 500 nm. The spinning parameters, including acceleration and duration, were optimized to ensure the desired thickness and homogeneity of the films.
• Drying and Annealing: The coated substrates were dried at 120 °C for 1 h to remove excess solvent. The films were then annealed at various temperatures (400 °C, 600 °C, and 800 °C) for 6 h each to promote different levels of crystallization.
• Characterization: The structural and morphological properties of the TiO₂ films were characterized using X-ray diffraction (XRD) with a Siemens D-5000 diffractometer and CuKα radiation. The diffracted intensity was measured over a 2θ range from 22° to 35°, which includes the main diffraction peaks for both the anatase and rutile phases, specifically at 25.3° (101) for anatase and 27.4° (110) for rutile. The step size used was 0.02°. FTIR spectra were recorded in transmission mode with a resolution of 2 cm⁻¹, using a Nicolet iS5 FTIR spectrometer (Thermo Scientific, Waltham, MA, USA), specifically to analyze the OH vibrations in the range of 4000–2000 cm⁻¹. The spectra were averaged from 100 scans at room temperature. XPS measurements were conducted on an ESCALAB 250 spectrometer with dual aluminum–magnesium anodes and monochromatic Al Kα radiation (hν = 1486.6 eV). The procedure followed was as described in [5], with an energy resolution of 0.1 eV for the Ti 2p spectra, including the spin-orbit split components (2p₃/₂ and 2p₁/₂), and for the O 1s spectrum.

2.2. VUV Photoabsorption Measurements

VUV photoabsorption spectroscopy was conducted across a wavelength range of 120–350 nm using the UV1 beamline at the ASTRID synchrotron radiation facility, located at Aarhus University’s ISA. This experimental procedure was performed as previously published in [9] and so we highlight here only the essential aspects of the configuration, which included a vacuum monochromator and a sample chamber capable of accommodating up to three sample discs alongside a reference disc. As the VUV beam passed through the discs, the transmitted intensity was recorded at 1.0 nm intervals with a photomultiplier tube detector. Continuous monitoring of both the transmitted light intensity and the synchrotron beam ring current was performed at each wavelength, achieving a typical resolution of better than 0.1 nm.

The intensity of the light transmitted through the reference disc was also recorded, serving as the I₀ value for absorbance calculations based on the equation A = log(I₀/I), where A is absorbance, I₀ is the incident light intensity, and I is the transmitted light intensity. In this setup, the electric field vector of the VUV photons was oriented horizontally, consistent with the design of the apparatus. During the measurements, the sample chamber was evacuated using a turbo pump to achieve a pressure of 0.01 Pa and was separated from the monochromator’s vacuum by a LiF window, enabling radiation down to 120 nm to enter the sample chamber.

Photon intensity was detected by a UV-enhanced photomultiplier tube (Electron Tubes Ltd., Ruislip, Middlesex, UK), positioned after the exit window of the sample chamber. To prevent absorption by molecular oxygen in the air, the small volume between the sample chamber’s exit window and the photomultiplier was purged with helium for measurements conducted below 180 nm. For measurements at wavelengths above 220 nm, this purging was ceased, allowing molecular oxygen to effectively filter out any contamination from higher-order flux emitted by the monochromator, up to the maximum measured wavelength of 350 nm.

3. Results and Discussion

3.1. TiO₂ Film Evolution

In our sol–gel process, titanium alkoxide reacts with water to undergo hydrolysis, producing titanium hydroxide (Ti–OH) and ethanol (C₂H₅OH). The hydrolysis reaction can be represented as follows: Ti(i-OPr)₄ (l) + H₂O (l) → Ti–OH (s) + C₂H₅OH (l). As the reaction progresses, the Ti-OH groups condense, forming a titanium–oxygen–titanium (Ti–O–Ti) network and releasing water as a byproduct. This condensation reaction is represented as follows: Ti–OH (s) + Ti–OH (s) → Ti–O–Ti (s) + H₂O (l). Additionally, the titanium alkoxide can react with Ti–OH to further promote the extension of the Ti–O–Ti network, with ethanol being released in the process: Ti(i-OPr)₄ (l) + Ti–OH (s) → Ti–O–Ti (s) + C₂H₅OH (l).

Previous studies showed that the limited amount of water led to an incomplete reaction of the alkoxide, resulting in a slower reaction rate and a gradual rearrangement of the Ti–O–Ti network [14]. Based on these findings and considering that our samples were prepared with low water content, we hypothesize that our films may also exhibit the formation of surface defects, such as Ti³⁺. These defects could be attributed to the removal of unreacted alkoxide during the subsequent annealing process [14].

After sol–gel synthesis, the resulting TiO₂ films are generally amorphous [9]. As anatase formation occurs from the Ti–O–Ti network during the annealing process, accompanied by the removal of inorganic and organic residues [15], XRD analysis is necessary to monitor the crystallization process.

3.2. XRD Analysis

The X-ray diffraction (XRD) patterns shown in Figure 1 illustrate the structural evolution of TiO₂ thin films under four distinct conditions: as-deposited (a), annealed at 400 °C (b), annealed at 600 °C (c), and annealed at 800 °C (d), each for 6 h. The analysis of these patterns highlights the temperature-dependent phase transformations in the material, transitioning from an amorphous state to anatase and finally to rutile as the dominant crystalline phase at higher temperatures. In (a), the as-deposited TiO₂ film exhibits a broad and diffuse XRD profile, with no discernible diffraction peaks. This pattern is indicative of an amorphous structure, where the lack of long-range crystalline order prevents the formation of sharp reflections. Amorphous TiO₂ is often observed immediately after deposition, and its disordered nature results in no clear phase being present in this initial state. Upon annealing at 400 °C, as shown in (b), the XRD pattern reveals a sharp diffraction peak at approximately 25.3°, corresponding to the (101) plane of anatase TiO₂ (card no: # 00-021-1272). The emergence of this peak signifies that the amorphous structure has undergone crystallization into the anatase phase, which is the thermodynamically stable phase of TiO₂ at moderate temperatures [16]. Importantly, no peaks associated with the rutile phase are observed at this stage, confirming that the film consists entirely of anatase TiO₂. This finding highlights that annealing at 400 °C is sufficient to induce crystallization but not high enough to trigger the anatase-to-rutile transition.

The XRD pattern for the film annealed at 600 °C, (c), indicates the coexistence of anatase and rutile phases. The characteristic anatase (101) reflection at 25.3° remains visible, but a new peak emerges at ~27.5°, corresponding to the (110) plane of rutile TiO₂ (card no: # 00-021-1276). This coexistence reflects the partial transformation of anatase into rutile, a phase transition that typically begins at this temperature range. The presence of both phases at 600 °C suggests a transitional state, where the anatase phase remains dominant, but rutile begins to form. This mixed-phase composition is particularly significant for certain applications, as the interplay between anatase and rutile can enhance photocatalytic activity [1,17].

Finally, the XRD pattern for the film annealed at 800 °C, (d), demonstrates a clear dominance of the rutile phase. The peak associated with rutile, at ~27.5° (110), becomes significantly more intense, while the anatase (101) peak at 25.3° is substantially weakened. This indicates that the anatase-to-rutile transformation is nearly complete, with rutile becoming the primary phase in the film. Residual traces of anatase suggest that the transformation has not reached full completion, though rutile dominates the structure at this temperature. The transition from anatase to rutile aligns with the expected thermal behavior of TiO₂, where rutile becomes the stable phase at high temperatures [6,7]. The crystallite sizes of the TiO₂ films were estimated by applying Scherrer’s formula [18]. The calculated average crystallite size values for anatase (D_A) and rutile (D_R) are presented in Figure 1. The correction for line broadening caused by the instrument was applied using silicon as a reference. The relationship $B^2=B_S^2-B_R^2$, where B_S and B_R represent the measured full widths at half maximum (FWHM) of the diffraction peaks from the sample and the silicon reference, respectively, was applied to account for the instrumental broadening. After this correction, the crystallite size of the sample was determined using the Scherrer equation, with the calculation performed via the online crystallite size calculator provided in [18].

3.3. VUV Analysis

The vacuum ultraviolet (VUV) spectrum of sol–gel derived TiO₂ thin films undergoes notable transformations with increasing annealing temperature, highlighting the electronic transitions between oxygen 2p (O2p) and titanium 3d (Ti3d) orbitals. These transitions are crucial for understanding the electronic structure of TiO₂, which is influenced by phase composition and crystallinity. The VUV absorption spectra, shown in Figure 2a,b for the 120–230 nm and 220–330 nm regions, respectively, display prominent charge-transfer processes from O(2p) to Ti(3d) orbitals. A key absorption peak in the range 175–180 nm is particularly sensitive to annealing conditions, with its position, width, and intensity revealing structural changes in the films as the annealing temperature increases.

3.3.1. Observed Electronic Transitions

The VUV spectra presented in Figure 2 reveal a decline in absorbance beyond 125 nm, which can be attributed to high-energy charge-transfer transitions between the oxygen 2p (O(2p)) orbitals and titanium 3d (Ti(3d)) orbitals. These transitions can be categorized into two primary types [9]:

• High-energy charge-transfer transitions (~125–150 nm): These involve electron excitation from the bonding (σ) orbitals of the O(2p) states to the antibonding (π*) orbitals in the Ti(3d) conduction band (σ → π*). These transitions require relatively higher energy due to the excitation of electrons to a higher unoccupied state, resulting in broader absorption features in the spectra.
• Lower-energy charge-transfer transitions (~160–200 nm): These transitions involve electron excitation from the π-bonding orbitals (primarily in the O(2p) states) to the antibonding π* orbitals in the Ti(3d) conduction band (π → π*). These transitions are typically characterized by sharper peaks, indicative of lower-energy electronic transitions compared to the high-energy σ → π* transitions.

These electronic transitions in TiO₂ are highly sensitive to its phase composition, defect density, and the interaction between Ti(3d) and O(2p) orbitals, which govern the material’s electronic structure. These interactions form the conduction and valence bands, which are essential for understanding TiO₂’s optical properties. The degree of crystal field splitting, denoted as Δ_π, is influenced by the lattice symmetry of the TiO₂ phase. In anatase, the distorted TiO₆ octahedral structure leads to enhanced hybridization between Ti(3d) and O(2p) orbitals, resulting in significant crystal field splitting. Conversely, rutile’s more symmetric lattice structure exhibits weaker hybridization and smaller splitting. The crystal field splitting can be quantified as the energy difference between the π*-antibonding and π-bonding states, expressed as follows:

$${\mathsf{∆}}_{\pi }=E_{{\pi }^{\ast }}-E_{\mathsf{\pi }}$$

(1)

This degree of splitting differs between the phases, with ${\mathsf{∆}}_{rutile}<{\mathsf{∆}}_{anatase}$. Beyond lattice symmetry effects, this splitting is further modulated by defect states, such as oxygen vacancies or titanium in lower oxidation states (Ti³⁺), which introduce localized electronic states within the bandgap. These defect contributions, represented by Δ_def, describe the interaction between Ti(3d) orbitals and defect-related states. While Δ_π pertains to intrinsic lattice effects, Δ_def accounts for extrinsic influences that further shape the material’s electronic properties. In anatase TiO₂, the distorted TiO₆ octahedral structure leads to the splitting of the Ti(3d) orbitals into lower-energy t_2g orbitals (d_xy, d_xz, d_yz) and the higher-energy e_g orbitals (d_x²_−y², d_z²). These orbitals interact with oxygen 2p orbitals to form molecular orbitals, with the valence band dominated by O(2p) states and the conduction band by Ti(3d) states. The distortion in anatase weakens the orbital overlap, especially in the t_2g configuration, increasing the energy gap and resulting in a wider bandgap (~3.2 eV) compared to rutile [12]. In contrast, rutile’s symmetric TiO₆ octahedral structure exhibits smaller crystal field splitting and stronger orbital overlap, producing a narrower bandgap (~3.0 eV) [12]. The reduced splitting in rutile leads to decreased separation between the valence and conduction bands, enhancing electronic delocalization.

Based on the molecular orbital energy level diagram of TiO₂ [19], the feature at approximately 175–180 nm (Figure 2a) involves a combination of e_g(σ) → t*_2g(π*) transitions and charge transfer from O(2p) → Ti3d(e_g). In the amorphous phase, the σ → π* transitions are poorly defined due to the absence of orbital hybridization and structural symmetry, resulting in a broad and featureless absorption.

The anatase phase exhibits moderate e_g(σ) → t*_2g(π*) transitions, with minor contributions from O(2p) → Ti(3d) charge transfer. The mixed phase displays stronger transitions, combining anatase-like e_g(σ) → t*_2g(π*) transitions with rutile-like O(2p) → Ti(3d) charge transfer, leading to broader absorption and enhanced intensity. In the rutile phase, the e_g(σ) → t*_2g(π*) transitions dominate due to higher orbital overlap and reduced crystal field splitting. Additionally, charge transfer from O(2p) → Ti(3d) contributes to the overall intensity, making this transition the sharpest and most intense among the phases.

The transition at ~280 nm (Figure 2b), dominated by charge transfer from O(2p) orbitals to Ti3d(t_2g) orbitals, is assigned to t_2g(π) → t*_2g(π*) transitions, as indicated by the molecular orbital energy level diagram of TiO₂ [19]. In the amorphous phase (red), this transition exhibits a broad and diffuse absorption due to the lack of defined orbital overlap. The structural disorder introduces localized electronic states that smear the O(2p) → Ti(3d) transitions, resulting in less distinct features. In the anatase phase (blue), a strong O(2p) → Ti(3d) charge transfer occurs, driven by a well-defined crystal field splitting (Δ) and the distorted TiO₆ octahedral structure, which enhances the sharpness and intensity of the transition compared to the amorphous phase. The mixed phase (green) combines anatase-like O(2p) → Ti3d(t_2g) transitions with rutile-like contributions, producing intermediate intensity and absorption features. In the rutile phase (purple), the reduced crystal field splitting (Δ) increases electronic delocalization and orbital overlap, leading to the most intense O(2p) → Ti3d(t_2g) transitions, characterized by a broader and more defined absorption spectrum than in anatase or mixed phases.

Finally, the shoulder at approximately 145 nm is tentatively assigned from π → π* transitions, specifically t_2g(π) → t*_2g(π*), with minor contributions from e_g(σ) → t*_2g(π*) in rutile and mixed phases. In the amorphous phase, no significant π → π* transitions are observed due to the lack of defined t_2g and t*_2g states, resulting in a featureless spectrum. The anatase phase exhibits moderate π → π* transitions, driven by crystal field splitting and the distortion of the TiO₆ octahedral structure. The mixed phase shows stronger π → π* transitions, combining anatase-like and rutile-like contributions, leading to broader and more intense absorption features. The rutile phase, with its reduced crystal field splitting (Δ), higher symmetry, and strong orbital overlap, shows the most intense π → π* transitions, resulting in sharper and more defined absorption peaks.

Table 1. Optical transitions and absorption features in amorphous, anatase, mixed (A + R), and rutile (dominant) phases of TiO_2.

Aspect	Am (Red)	A (Blue)	A + R (Green)	R (Purple)
Absorption characteristics	Minimal orbital hybridization, broad disordered states dominate.	Ti t2g(π) → t*2g (π*); moderate eg(σ) contributions.	Anatase t2g → t*2g and rutile eg(σ) → t*2g, enhanced overlap of π → π*.	Strong eg(σ) → t*2g (π*) and t2g(π) → t*2g (π*) due to symmetry and smaller Δ.
Transition at ~280 nm	Broad charge-transfer transition O(2p) → Ti3d(t2g).	Strong O(2p) → Ti(3d) charge-transfer due to high Δ.	Enhanced O(2p) → Ti3d(t2g); rutile overlaps increase intensity.	O(2p) → Ti3d(t2g) dominates; smaller Δ enhances electronic delocalization, increasing the transition intensity.
Transition at ~175–180 nm	Diffuse σ → π* or charge transfer O(2p) → Ti3d(eg)	Moderate eg(σ) → t*2g(π*); overlaps with charge-transfer transitions.	Mixed contributions: anatase-like eg(σ) → t*2g, and rutile-like charge-transfer O(2p) → Ti(3d) states enhance intensity.	Dominated by eg(σ) → t*2g; intense due to higher symmetry and smaller bandgap (~3.0 eV).
Shoulder at ~145 nm	Weak or absent; no sharp π → π* transitions due to amorphous structure.	Moderate π → π*: t2g(π) → t*2g(π*).	Strong shoulder: dual contributions from anatase π → π* and rutile’s σ → π*.	Most intense π → π*: t2g(π) → t*2g(π*), combined with σ → π* transitions from eg.
Bandgap-related effects	Broad, featureless absorption, defect and localized states dominate.	Crystal field splitting Δ (~3.2 eV) shapes sharper absorption features.	Intermediate Δ due to mixed phases: slightly broader features compared to anatase, sharper than amorphous.	Smallest Δ (~3.0 eV), strongest and sharpest features from well-ordered orbital overlap.

Am—amorphous; A—anatase; R—rutile.

summarizes the key optical transitions observed in the different TiO₂ phases, including the charge transfer and orbital interactions that define their absorption features. The transitions at approximately 280 nm, 175–180 nm, and 145 nm are analyzed for each phase, highlighting the variations in orbital overlap, crystal field splitting (Δ), and the intensity of the transitions.

3.3.2. Temperature-Induced Variations in Absorbance

The variations in absorbance observed in the VUV spectra correspond to temperature-induced changes in crystallinity and phase composition. Amorphous TiO₂ and smaller nanocrystals have defect states that introduce localized energy levels within the bandgap, enabling absorption at shorter wavelengths. Quantum confinement effects in nanocrystalline TiO₂ further shift the absorption edge to higher energies, broadening absorption peaks due to structural disorder or small crystal sizes. This shift and broadening are more pronounced in anatase, where the grain sizes are smaller, and quantum confinement effects are stronger.

The presence of σ → π* transitions (prominent around 125–150 nm) is attributed to strong bonding between O(2p) and Ti(3d) orbitals. These transitions involve the excitation of electrons from σ bonds to antibonding π* states in the conduction band. In contrast, π → π* transitions (160–200 nm) occur between O(2p) bonding orbitals and antibonding π* orbitals, requiring less energy and typically observed at longer wavelengths.

3.3.3. Temperature-Induced Phase Evolution

A prominent feature of the VUV TiO₂ behavior is the band near 175–180 nm, whose position, width, and intensity vary across samples annealed at 400 °C, 600 °C, and 800 °C, as shown in Figure 3. This band is linked to π → π* transitions, where electrons from the O(2p) orbitals are excited to higher Ti(3d) states, characteristic of the anatase phase.

At 400 °C, the spectrum is dominated by the anatase phase, with a sharp peak near 176 nm. This peak corresponds to the O(2p) → Ti(3d) (t₂g) charge-transfer transition, a signature feature of anatase’s unique crystal structure. Anatase has a distorted octahedral TiO₆ lattice with significantly strained Ti–O–Ti bond angles, resulting in greater lattice disorder compared to rutile. This peak corresponds to the O(2p) → Ti(3d) (t₂) charge-transfer transition, characteristic of anatase’s distorted lattice. Anatase’s higher thermodynamic instability promotes defect formation, such as oxygen vacancies, which introduce localized electronic states and broaden the absorption peaks. This broader peak at 400 °C is attributed to defects, which cause a less ordered crystal structure, leading to a range of transitions within the bandgap.

Following baseline correction, the position of the peaks of Figure 3 was determined using the wavelength at which the first derivative of the smoothed absorbance curve changes sign. The peak wavelengths were calculated as 176.02 nm for both 400 °C and 600 °C and 177.02 nm for 800 °C. These results confirm the observed redshift trend with increasing temperature. As the temperature increases to 600 °C, the spectral profile reflects a mixed anatase–rutile phase. The absorption peak at ~176 nm shifts slightly to the right, approaching ~177 nm, indicating a narrowing of the bandgap as rutile begins to form. This redshift results from the interaction between anatase and rutile phases, where localized strain introduces defect states within the bandgap, lowering its energy. The shift can be attributed to the increasing fraction of rutile, which has a smaller bandgap (~3.0 eV) compared to anatase. Rutile’s less distorted structure leads to smaller crystal field splitting (Δ) and stronger Ti-Ti orbital overlap, reducing the energy separation between the valence and conduction bands, thereby narrowing the overall bandgap. This relationship can be expressed as follows:

$$E_{\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{g}\mathrm{a}\mathrm{p},\mathrm{m}\mathrm{i}\mathrm{x}\mathrm{e}\mathrm{d}}=x\cdot E_{\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{s}\mathrm{e}}+(1-x)\cdot E_{\mathrm{r}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{e}}$$

(2)

where x represents the fraction of anatase. The interfaces between anatase and rutile create additional effects: Strain, localized electronic states, and structural distortions at these boundaries modify the electronic structure. These interface effects can either narrow or broaden the effective bandgap, depending on the nature and density of the defects introduced by mismatched Ti-O bond lengths. Defects such as oxygen vacancies or surface states play a critical role in modifying the bandgap. These defects introduce localized states within the bandgap, effectively reducing the material’s bandgap. This relationship is described as follows:

$$E_{gap}=E_{\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{c}}-E_{\mathrm{d}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{c}\mathrm{t} \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}}$$

(3)

Such defects are more prevalent in the anatase phase due to its greater thermodynamic instability and distorted lattice, leading to broader absorption peaks compared to rutile. These defects also affect phase coexistence, as seen in the mixed anatase–rutile systems where strain at the interfaces amplifies the defect contribution.

At 600 °C, the absorption peak broadens less than at 400 °C, suggesting that the mixed-phase structure introduces fewer defects at the anatase–rutile interface. The more ordered rutile phase is less prone to defect formation than anatase, reducing the overall broadening. However, strain at the phase boundaries contributes slightly to the peak broadening. Additionally, the slight increase in grain size at this temperature reduces quantum confinement effects, further influencing the shift and peak broadening.

At 800 °C, the spectrum predominantly reflects the rutile phase, as anatase almost completely transforms into rutile. The absorption peak shifts slightly further to around 177 nm, indicative of rutile’s narrower bandgap (~3.0 eV) and the enhanced overlap between Ti(3d) and O(2p) orbitals. This redshift is due to the change in crystal structure and the denser packing of rutile compared to anatase. Rutile’s more symmetric structure results in less distortion in the TiO₂ lattice, strengthening the orbital overlap and lowering the energy gap between the valence and conduction bands.

While the peak width at 800 °C is similar to that at 600 °C, its height is slightly lower. This suggests that although the more ordered rutile structure reduces defect density, leading to a more defined absorption feature, the overall intensity is lower than in the mixed-phase structure at 600 °C. The presence of residual defects in rutile still contributes to localized states, but their impact on broadening the peak is less significant than in the anatase phase.

3.3.4. Defect Creation and Phase Transition in TiO₂

Previously, in Section 3.1, we discussed how employing a low water-to-alkoxide ratio limits the hydrolysis of alkoxide groups, leaving some of these groups unreacted or partially hydrolyzed in the gel matrix before annealing. This controlled deficiency of water gives rise to the introduction of defects, as opposed to stoichiometric or excess water conditions, which typically yield more stoichiometric TiO₂ with fewer intrinsic defects [14]. During annealing at 400 °C, these residual alkoxide groups decompose, leading to the formation of oxygen vacancies (VO) and Ti³⁺ centers. This process aligns with the amorphous-to-anatase phase transformation observed at this temperature, where the introduction of defects stabilizes the anatase phase and enhances its surface reactivity.

Specifically, during the crystallization of amorphous TiO₂ into anatase, terminal isopropoxide groups (i-OPr) bound to Ti⁴⁺ sites are decomposed during the annealing of amorphous TiO₂ at temperatures ranging from ~200 °C to 450 °C [14]. As the material undergoes the amorphous-to-anatase phase transition, the i-OPr groups break away, leading to the formation of an oxygen vacancy (VO), represented by ${\mathrm{V}\mathrm{O}}^{••}$, and the release of acetone (CH₃COCH₃) into the gas phase [14]. The reaction can be represented as follows:

$$\mathrm{T}\mathrm{i}–\mathrm{O}–\mathrm{i}–\mathrm{O}\mathrm{P}\mathrm{r}(s)\to {\displaystyle \frac{1}{2}}{\mathrm{V}\mathrm{O}}^{••}+{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}}_3(g)$$

(4)

This process facilitates the creation of Ti³⁺ surface defects through the removal of oxygen atoms from Ti⁴⁺ sites. These defects significantly impact the material’s optical and electronic properties by introducing mid-gap states, which modify the electronic structure of the material. These changes are directly reflected in the VUV spectra, where defect-induced broadening and shifting of the absorption features are observed.

The Ti⁴⁺–i-OPr–Ti⁴⁺ structure can evolve into a more ordered TiO₂ network with defects (like Ti³⁺ and oxygen vacancies) after the alkoxide is removed, typically during calcination at temperatures above 400 °C. The removal of these alkoxide groups contributes to the reduction of Ti⁴⁺ to Ti³⁺.

3.3.5. Linking Defects and Quantum Confinement Effects

The observed redshift from 176 nm at 400 °C to 177 nm at 800 °C aligns with the theoretical relationship between bandgap energy ${(E}_g)$ and absorption edge wavelength (λ):

$$\lambda ={\displaystyle \raisebox{1ex}{$hc$}\!\left/ \!\raisebox{-1ex}{$E_g$}\right.}$$

(5)

As the bandgap narrows from anatase to rutile, the absorption edge moves to longer wavelengths. This behavior reflects a reduction in crystal field splitting (Δ) and diminished quantum confinement effects at higher annealing temperatures, where larger grain sizes dominate. Changes in peak shape and width correlate with disorder introduced by strain and defects, particularly at the anatase–rutile interfaces at 600 °C. These defects contribute to spectral broadening near the band edges, leading to a redshift that is more pronounced in the anatase phase and decreases as the material becomes more ordered at higher temperatures.

The shift of the 176–177 nm peak across different annealing temperatures can be understood in terms of crystal field splitting and quantum confinement. The larger crystal field splitting in anatase results in a wider bandgap, while the smaller splitting in rutile leads to a narrower bandgap. The redshift from 400 °C to 800 °C reflects these structural and electronic changes, as the material transitions from a more ordered anatase phase, through a mixed anatase–rutile phase, to a rutile-dominated phase.

In addition to structural effects, defects such as oxygen vacancies and surface states significantly modify the bandgap and absorption features. Defects act as localized states within the bandgap, with their concentration being higher in the mixed phase and rutile films due to strain at the anatase–rutile interface and transition-induced disorder. These defects contribute to spectral broadening near the band edges, leading to a redshift. This broadening is most pronounced at 400 °C, where disorder is significant, and decreases at 800 °C as the material becomes more ordered. The degree of broadening is influenced by temperature and the anatase-to-rutile phase ratio, with higher temperatures generally resulting in fewer defects and slightly narrower absorption peaks.

Quantum confinement effects are also significant, particularly in nanocrystalline films. In smaller crystallites, the bandgap is larger due to the confinement of charge carriers in finite-size particles. At 400 °C, the anatase phase exhibits pronounced quantum confinement effects, leading to larger bandgap and sharper absorption peaks. At higher temperatures, where grain growth occurs and the rutile phase dominates, quantum confinement effects diminish, contributing to the observed redshift and reduced broadening of the peak. Quantum confinement effects further influence the bandgap in nanocrystalline films. In smaller crystallites, the bandgap increases as charge carriers (typically electrons) are confined within finite-size particles. This effect is particularly noticeable in anatase, where grain sizes are typically smaller, and it is described by the equation:

$$E_{\mathrm{g}\mathrm{a}\mathrm{p},\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{u}\mathrm{m}}=E_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}+{\displaystyle \frac{{\hslash }^2{\pi }^2}{2m^{\ast }{R}^2}}$$

(6)

Here, E_bulk is the bandgap of the film (which reflects the bulk material’s properties, typically either rutile or anatase without quantum confinement), m∗ is the effective mass of the charge carrier (usually electrons in the conduction band), and R is the particle radius, representing the size of the nanocrystal. As annealing temperature increases, grain sizes grow, which diminishes the quantum confinement effects. This reduction in confinement leads to a narrower bandgap, reflected in the redshift observed in the VUV spectra as the TiO₂ transitions from a nanocrystalline to a more bulk-like form.

3.3.6. Hybridization of Conduction Band States

The hybridization of conduction band states in TiO₂ films, particularly in mixed-phase systems, plays a critical role in modulating the material’s optical properties. In anatase and rutile phases, this hybridization involves the interaction between the Ti(3d) orbitals and the O(2p) orbitals, with the relative contributions of these orbitals to the conduction band being phase-dependent. The phase boundary energy alignment between anatase and rutile is crucial, as it governs the interaction strength and the overall electronic structure, affecting both the bandgap and the electronic transitions observed in the UV and VUV regions.

At higher annealing temperatures, the anatase and rutile phases are better ordered, leading to stronger orbital overlap between the Ti(3d) and O(2p) states or hybridization and a modified electronic structure [20,21]. The alignment of the conduction band states across the two phases allows for the formation of hybridized states, which results in a shift in the absorption spectrum. The energy of these hybridized states can be approximated by the average energy of the individual anatase and rutile conduction bands, expressed as follows:

$$E_{\mathrm{h}\mathrm{y}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}}={\displaystyle \frac{E_{\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{s}\mathrm{e}}+E_{\mathrm{r}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{e}}}{2}}$$

(7)

This equation explains the redshift and narrowing of the bandgap as the system transitions from anatase to rutile at higher annealing temperatures [20]. As rutile becomes more prevalent, the hybridization increases, leading to the merging of the electronic structure, which sharpens the absorption peaks and facilitates electronic transitions, particularly in the 175–180 nm and 280 nm regions

The hybridized states are responsible for the enhanced transition probabilities observed at higher annealing temperatures. As rutile phase content increases, the electronic structure shifts towards a stronger overlap of Ti(3d) and O(2p) orbitals, increasing the density of states (DOS) in the conduction band. This enhanced DOS and hybridized states contribute to enhanced transition probabilities, described by Fermi’s Golden Rule:

$$W_{i \to f}={\displaystyle \frac{2\pi }{\hslash }}{\left|⟨f\left|H^{\prime }\right|i⟩\right|}^2\rho {(E}_f)$$

(8)

Here, $W_{i \to f}$ represents the transition rate (probability per unit time) from an initial state i to a final state f, H^′ is the perturbation Hamiltonian (which encapsulates the interaction driving the transition), and $\rho \left(E_f\right)$ is the density of final states at the energy $E_f$ of the final state. As rutile exhibits a higher degree of electronic delocalization and orbital overlap, the transition probability between electronic states is significantly enhanced, which leads to sharper and more intense absorption features, particularly in the 175–180 nm and 280 nm regions.

The reduced crystal field splitting (Δ) in rutile compared to anatase contributes to a smaller bandgap and facilitates greater electronic delocalization across the conduction band. This delocalization enables a more efficient transfer of electronic excitations, resulting in sharper and more intense absorption peaks. In mixed-phase systems, the interaction between anatase and rutile phases further modulates these effects, leading to the broadening of certain spectral features due to phase boundary effects.

The spectroscopic behavior observed in the 175–180 nm peak is not only indicative of the improved crystallinity but also reflects the structural evolution of TiO₂ thin films. With higher annealing temperatures, the reduction in defect density and quantum confinement effects leads to narrower absorption peaks and a redshift in the absorption onset, as the films transition from an amorphous or anatase-rich state to a more crystalline rutile phase. This shift is directly linked to the increased orbital overlap and hybridization of the conduction band states, which are critical factors in determining the material’s optical properties at the nanoscale.

To better understand the transitions across different phases of TiO₂,

Table 2. Orbital interactions and energy transitions in TiO₂ phases.

Phase	Transition Type	Orbital Interaction	Energy (eV)	Wavelength (nm)	Key Notes
Amorphous	σ → σ*	Ti(3d) (σ-bonding) → O(2p) (σ*-antibonding	>9.9	<125	Broad absorption due to high disorder; localized defect states.
Anatase	σ → π*	O(2p) (σ-bonding) → Ti(3d) (π*-antibonding)	~9.0–9.4	~125–138	Stronger in anatase due to distortion in TiO6 octahedra.
Anatase	π → π*	O(2p) (π-bonding) → Ti(3d) (π*-antibonding)	~6.2–7.0	~176–200	Sharp peaks at 176 nm; indicative of crystallinity and an increase in defect density.
Rutile	σ → σ*	Ti(3d) (σ-bonding) → O(2p) (σ*-antibonding)	~8.5	~146	Less pronounced due to symmetric lattice structure.
Rutile	π → π*	O(2p) (π-bonding) → Ti(3d) (π*-antibonding)	~6.6–6.8	~182–190	Broader peaks due to less distortion compared to anatase.
Mixed phases	Interfacial states	Defect-mediated transitions	~5.8–6.5	~190–215	Resulting from oxygen vacancies or anatase–rutile grain boundary effects.
Quantum confinement	π → π*	O(2p) (π-bonding) → Ti(3d) (π*-antibonding)	>7.5	<165	Observed in nanoscale TiO2 grains; absorption edge blue shifted.

summarizes the key electronic transitions, associated orbital interactions, energy values (in eV), and corresponding wavelengths (in nm) for different TiO₂ phases, including amorphous, anatase, rutile, mixed phases, and nanoscale quantum confined TiO₂. It highlights the spectroscopic behavior of each phase, from broad absorption in disordered states to sharper transitions in crystalline forms, and the impact of phase and defects on optical properties.

The hybridization of conduction band states in mixed-phase TiO₂ films not only governs the electronic structure but also explains the transition from broad, featureless absorption in amorphous films to sharp, intense absorption peaks in crystalline rutile. The interplay between crystal field splitting, phase interactions, and defect density provides a comprehensive understanding of the electronic transitions in our samples.

3.4. FTIR and VUV Spectra Analysis of TiO₂ Films

TiO₂ films synthesized via the sol–gel method typically retain residual water and hydroxyl (OH⁻) groups as a result of the hydrolysis and condensation processes of titanium alkoxides during film formation. These water and hydroxyl species, introduced during the sol–gel synthesis, are commonly detected in Fourier Transform Infrared (FTIR) spectra, where they manifest as O–H stretching and bending modes. The broad band around 3300 cm⁻¹ corresponds to the stretching vibrations of hydroxyl groups, which are associated both with molecular water adsorbed on the oxide surface and with structural hydroxyl groups (Ti-OH) embedded within the TiO₂ film matrix. Additionally, a bending vibration at 1563 cm⁻¹ is attributed to adsorbed water molecules (H–O–H), further supporting the presence of water within the film. Another feature, observed around 2366 cm⁻¹, corresponds to stretching vibrations of hydroxyl groups, possibly associated with surface hydroxyl groups or Ti–OH groups within the film’s structure [22].

As shown in Figure 4, the FTIR spectrum of TiO₂ samples reveals characteristic bands corresponding to water and hydroxyl groups. In the as-deposited amorphous TiO₂ films, the broad O–H stretching band around 3300 cm⁻¹ confirms the presence of both adsorbed water and structural hydroxyl groups. The bending vibration at approximately 1563 cm⁻¹ further supports the presence of water-related species, which are critical to the amorphous TiO₂ structure. As the temperature of the samples increases during thermal annealing, these water-related bands (specifically the O–H stretching and bending bands) gradually diminish, which is consistent with the desorption of molecular water from the film’s surface and the condensation of hydroxyl groups [23]. This process is accompanied by densification and a rise in crystallinity, which is evident in the sharpness of the Ti–O bands. This behavior corresponds to the transition of the amorphous phase to the anatase and rutile crystalline phases [24].

It is important to note that the vibrations due to hydroxyl groups from molecular water disappear at temperatures above 800 °C, which suggests the removal of water from the TiO₂ structure as the material undergoes crystallization.

It is important to note that the vibrations due to molecular water and hydroxyl groups vanish at temperatures above 800 °C, suggesting the removal of residual water from the TiO₂ structure as the material undergoes crystallization. This thermal evolution of the film is central to the observed changes in the FTIR spectrum and indicates the increasing degree of crystallinity, as well as the transformation from amorphous to crystalline phases.

Apparent Contradiction Between FTIR and VUV

The apparent discrepancy between FTIR and VUV analyses of sol–gel TiO₂ films arises from the different aspects of the material each technique probes. FTIR spectroscopy detects the vibrational modes of water and hydroxyl groups within the films, providing insights into molecular water and structural hydroxyl groups (Ti-OH). On the other hand, VUV spectroscopy primarily captures the electronic transitions that are influenced by the material’s structural properties, such as crystallinity and defect states.

In FTIR analysis, sol–gel derived TiO₂ films, particularly those in the as-deposited or low-temperature states, exhibit clear water-related features. The broad O–H stretching vibration around 3300 cm⁻¹ and the H–O–H bending vibration at approximately 1563 cm⁻¹ are indicative of molecular water adsorbed on the surface and structural hydroxyl groups (Ti–OH) within the amorphous matrix [25]. These features are progressively diminished upon annealing, particularly at temperatures exceeding 800 °C, consistent with the desorption of molecular water and the condensation of hydroxyl groups. This process facilitates densification and the phase transformation of TiO₂ into the more crystalline anatase and rutile phases [23]. However, VUV spectra lack a distinct water-related absorption band at ~145 nm, observed in dense sputtered films [9], where molecular water retention is more pronounced. Instead, the VUV spectrum of sol–gel films consistently shows a shoulder at ~145 nm across annealed samples.

The persistence of the shoulder at ~145 nm, even after annealing at 800 °C, underscores the role of defect states such as oxygen vacancies and Ti³⁺ centers in influencing the electronic structure of sol–gel TiO₂ films. These defects are minimized in dense sputtered films, resulting in sharp water-related absorption peaks, but are prominent in sol–gel films due to their porous nature and surface chemistry [25]. This highlights the critical role of annealing in tailoring defect density and crystallinity. By optimizing these parameters, sol–gel TiO₂ films can be engineered to enhance optical absorption in the VUV range, a feature crucial for UV-light-driven photocatalysis and other optoelectronic applications. The difference in behavior between sol–gel and DC-sputtered TiO₂ films can be attributed to the structural differences between the two. DC-sputtered films, due to their dense and compact nature, retain more molecular water, which leads to a sharp absorption peak at 145 nm in the VUV spectrum, as reported in previous studies [9]. In these films, the water absorption peak is prominent because the oxygen partial pressure leads to a large amount of adsorbed H₂O molecules [9]. On the other hand, sol–gel films, which are more porous, lose much of their molecular water during the drying and annealing processes. As a result, these films exhibit only a weak shoulder around 145 nm in the VUV spectrum, a feature that is likely linked to intrinsic electronic transitions within the anatase conduction band, as well as contributions from defect states such as oxygen vacancies [10] and Ti³⁺ centers [26].

This shoulder at 145 nm is a hallmark of defect-related absorption in TiO₂ and is strongly influenced by the evolution of crystallinity during annealing. In the as-deposited amorphous state, the presence of hydroxyl groups and adsorbed water suppresses this feature. However, as the material undergoes crystallization into anatase and eventually rutile phases, the formation of defect sites such as oxygen vacancies and Ti³⁺ centers enhances the shoulder. In contrast, as rutile becomes the dominant phase at higher annealing temperatures, this feature begins to diminish due to the distinct electronic structure of rutile, which shows less defect-related absorption compared to anatase.

Despite the presence of OH groups, as revealed by FTIR, VUV spectroscopy focuses on electronic transitions and is more influenced by the material’s electronic structure, including defect states and phase transitions from amorphous to crystalline TiO₂. Therefore, the shoulder at 145 nm observed in the VUV spectra of sol–gel TiO₂ films is more appropriately attributed to defect-related absorption rather than molecular water absorption, as noted in [9].

3.5. XPS and VUV Spectra Analysis of TiO₂ Films

3.5.1. XPS Analysis

To further explore the origin of defect-related features, XPS measurements were performed to examine the presence and chemical states of titanium in both as-deposited (red) and anatase (blue) films as differences in their chemical states were observed. The results help clarify the defect evolution during the transition from amorphous to crystalline phases, which is also seen in the VUV spectra. Specifically, the 145 nm shoulder and 178 nm peak are present in the crystalline films but absent in the as-deposited sample.

The high-resolution XPS spectrum of the Ti 2p region of the 400 °C sample (Figure 5a) shows two peaks at the binding energies 458.1 eV and 463.8 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively. These peaks are well de-convoluted into four components: Ti³⁺ 2p_3/2 (457.4 eV), Ti⁴⁺ 2p_3/2 (458.1 eV), Ti³⁺ 2p_1/2 (463.2 eV), and Ti⁴⁺ 2p_1/2 (463.8 eV), confirming the presence of both Ti³⁺ and Ti⁴⁺ ions [27]. The position of the Ti 2p_3/2 peak at 458.1 eV is close to the value reported for Ti⁴⁺ states in the anatase phase (458.7 eV) [28]. The Ti⁴⁺ ions are reduced by lattice oxygen in TiO₂, and the binding energy at 459.5 eV corresponds to the Ti³⁺ defect state in the anatase phase, consistent with XRD analysis. The presence of Ti³⁺ observed in the Ti 2p spectra indicates the formation of oxygen vacancies due to the reduction of Ti⁴⁺.

In contrast, the Ti 2p spectrum of the as-deposited film (Figure 5b) corresponds entirely to the Ti⁴⁺ oxidation state. As-deposited films typically remain amorphous due to the conditions under which they are prepared (e.g., low-temperature sol–gel processes). The structural unreacted alkoxide (i-OPr–Ti⁴⁺–O–Ti⁴⁺–i-OPr) results from the partial reaction between the alkoxide and water or Ti-OH groups. As shown in Figure 5a, both the unreacted alkoxide and terminal isopropoxide (i-OPr), which bond to Ti⁴⁺–O–Ti⁴⁺, are removed during the annealing treatment. Specifically, as the amorphous Ti–O–Ti network crystallizes upon heating to 400 °C, the removal of the terminal i-OPr groups leads to the formation of Ti³⁺ surface defects, coinciding with an increase in the crystalline structure of the anatase phase.

Figure 6 presents the high-resolution O 1s XPS spectra for as-deposited and annealed TiO₂ films at 400 °C, highlighting differences in oxygen species during the amorphous-to-crystalline transition. In the as-deposited film (Figure 6a), two prominent oxygen species are identified. The lower binding energy (BE) peak at approximately 529.2 eV is attributed to lattice oxygen in the TiO₂ structure, representing bulk stoichiometric oxygen. The higher BE peak at 531.7 eV corresponds to surface-bound oxygen species, such as hydroxyl groups (OH) and adsorbed molecular oxygen (O₂). These surface species are typically formed during the deposition process or through exposure to ambient moisture, especially in amorphous or poorly crystallized TiO₂ films, which tend to exhibit higher surface reactivity [23]. The presence of surface hydroxylation is corroborated by the FTIR spectrum, where a band at 3300 cm⁻¹ is observed. The alignment of these features in both XPS (531.7 eV) and FTIR highlights the substantial surface hydroxylation characteristic of the amorphous state. The annealed film at 400 °C (Figure 6b) exhibits significant changes in its O 1s XPS spectrum. The main peak shifts slightly to 531.3 eV, reflecting the reduction of surface hydroxyl groups and the stabilization of lattice oxygen. A shoulder emerges at a higher BE of 533.2 eV, suggesting the presence of adsorbed oxygen species, including molecular O₂, interacting with oxygen vacancies formed during the annealing process. These vacancies are a hallmark of the crystallization process, as the amorphous TiO₂ transitions into the anatase phase [29]. The corresponding FTIR spectrum shows a significant decrease in the intensity of the O-H stretching band at 3300 cm⁻¹, indicating a reduced concentration of hydroxyl groups. The annealing process not only lowers the hydroxyl content but also promotes the adsorption of oxygen species, which corresponds to the formation of a more ordered anatase structure [9].

3.5.2. Cause–Effect Analysis: Defects in XPS and VUV Spectra

Our findings reveal a direct relationship between the structural and electronic properties of TiO₂ films, observed through both VUV spectroscopy and XPS analysis.

XPS results indicate that anatase typically shows an increase in Ti³⁺ concentration and oxygen vacancies (VO) as the material transitions from the amorphous (as-deposited) state to 400 °C (anatase phase). These defects are clearly reflected in the VUV spectra.

At 400 °C, the spectra exhibit broad absorption peaks with high intensity around 176 nm, which corresponds to a higher concentration of defects, including Ti³⁺ and oxygen vacancies. This broadness is attributed to localized electronic states that arise from mid-gap states, which are defect-induced states between the conduction and valence bands. The broad absorption bands observed at 280 nm and 175–180 nm reflect these defect states, which arise due to oxygen vacancies and Ti³⁺ centers. The localized nature of these states weakens the symmetry of the band structure, leading to the broadening of the absorption peaks.

At 600 °C, as the material begins transitioning towards a mixed anatase–rutile phase, the VUV spectra show a slight narrowing of the absorption peaks, particularly around 176 nm, with a slight reduction in intensity at 176 nm. These changes suggest a reduction in defect density and the onset of rutile stabilization. The narrowing of the peaks can be explained by the transition to a more ordered crystalline structure. As the rutile phase starts to emerge, the symmetry of the lattice improves, leading to a reduction in defect-related states, resulting in sharper absorption peaks. The red shift of the absorption edge further supports this idea, as the absorption peak shifts slightly from 176 nm to 177 nm, consistent with the onset of rutile stabilization.

At 800 °C, as the material predominantly consists of rutile, the VUV spectra show sharper, more defined absorption peaks, particularly around 177 nm, indicating a reduced concentration of defects, especially Ti³⁺ and oxygen vacancies. The intensity at 177 nm is further reduced, confirming the reduction in defect states and stabilization of the rutile phase. The sharper peaks can be attributed to the improved lattice ordering of the rutile phase, where the more symmetrical crystal structure reduces defect formation. In rutile, the TiO₂ lattice has a more regular arrangement of Ti⁴⁺ ions, which leads to fewer Ti³⁺ defects and less oxygen vacancy formation, thus reducing mid-gap states and resulting in narrower, more defined absorption peaks. The slight narrowing of the peaks and red shift of the absorption edge further indicate that the material is becoming more ordered and transitioning to rutile, which is known to have a lower density of defect-induced electronic states due to its symmetrical crystal lattice.

Although XPS data for the 600 °C and 800 °C samples are not available, the observable changes in the VUV spectra—such as peak narrowing, sharpening, and reduced intensity—support the hypothesis that Ti³⁺ concentration decreases as the material becomes more ordered in the rutile phase. This aligns with the general understanding that rutile, with its more symmetrical lattice structure, has a lower concentration of defects compared to anatase, resulting in sharper absorption features and reduced defect-related transitions. As the material transitions to a predominantly rutile phase, these defects become stabilized, reducing their density [30]. On the other hand, rutile, with its smaller crystal field splitting and more symmetric lattice, is less prone to defect-induced mid-gap states. In mixed phases of anatase and rutile, defects are often concentrated at the grain boundaries between the two phases. These boundaries can act as pathways for defect-mediated electronic transitions. Therefore, even as the material transitions predominantly to rutile, defects at the grain boundaries can still play a role in the observed electronic properties, especially in the VUV spectra.

4. Conclusions

This study investigated the structural and electronic evolution of sol–gel derived TiO₂ thin films under thermal annealing using vacuum ultraviolet (VUV) spectroscopy. The films transitioned from an amorphous state to anatase at 400 °C, to a mixed anatase–rutile phase at 600 °C, and predominantly to rutile at 800 °C. These phase transitions significantly influenced the films’ optical and electronic properties, evidenced by changes in VUV absorption features, including a redshift of the absorption edge and spectral broadening.

At 400 °C, the anatase phase was characterized by prominent electronic transitions, with oxygen vacancies and Ti³⁺ states contributing to defect-mediated absorption and a broader bandgap. The mixed anatase–rutile phase at 600 °C exhibited broadened absorption features, consistent with the coexistence of anatase-like and rutile-like electronic states. The interaction between these phases likely led to localized strain and defect states at the phase boundaries, as inferred from the VUV spectral broadening and redshift. By 800 °C, the rutile phase dominated, characterized by improved crystallinity, reduced defect density, and stronger orbital overlap, resulting in sharper electronic transitions and a narrower bandgap.

The analysis identified critical high energy σ → π* and lower energy π → π* transitions, whose intensities and shifts were closely tied to phase composition, crystallinity, and defect density. The observed redshift of the VUV absorption edge corresponded to bandgap narrowing, influenced by phase evolution and the progressive reduction in structural distortions and quantum confinement effects as grain sizes increased. X-ray photoelectron spectroscopy (XPS) confirmed the presence of oxygen vacancies and Ti³⁺ states, particularly in films annealed at intermediate temperatures, while X-ray diffraction (XRD) confirmed the phase transitions and crystallite growth. FTIR analysis further demonstrated the removal of water and hydroxyl groups during annealing, correlating with densification and phase evolution.

This study demonstrates the unique capabilities of VUV spectroscopy in probing the electronic structure of TiO₂ thin films. By capturing high-energy transitions such as σ → π* and π → π*, which are inaccessible to UV-Vis spectroscopy, this work provides a deeper understanding of the interplay between structural defects, phase evolution, and electronic properties. These findings extend the characterization of TiO₂ thin films beyond conventional UV-Vis analyses, offering critical information for tailoring their functional properties in advanced applications.

These results underscore the pivotal role of annealing conditions in tailoring TiO₂’s optical and electronic properties, providing valuable understanding for optimizing its performance in photocatalytic and optoelectronic applications.