Influence of system configuration on the properties of Sol-Gel spin coating TiO 2 -ZnO films: A comparison of single-layer,composite,and bilayer

Abstract

This study investigates the influence of system configuration single-layer, composite, and bilayer on the structural, morphological, and optical properties of TiO₂-ZnO thin films. Four different structural designs were fabricated using the sol–gel spin-coating method: TiO₂, ZnO, TiO₂:ZnO composite and ZnO/TiO₂ bilayer. Structural analysis revealed that the ZnO/TiO₂ bilayer exhibited characteristic diffraction peaks corresponding to both crystalline oxides, confirming the formation of a dual-phase heterostructure. However, the TiO₂: ZnO composite film does not exhibit discernible Bragg reflections within the scanned 2θ range. The single layer TiO₂ and ZnO films displayed the expected anatase (101) and hexagonal (100) and (002) crystallographic orientations, respectively. Surface morphology analysis indicated apredominantly compact and relatively smoothfor TiO₂. The ZnO Several elongated surface features with lateral dimensions of ∼ 954 nm, 981 nm, and 1009 nm are observed. These features may correspond to coalesced grains or preferentially oriented growth structures formed during thermal treatment. All thin film configurations exhibited high optical transparency exceeding 60% in the 350–900 nm wavelength range. The optical band gap values varied with film configuration, ranging from 3.52 to 4.13 eV. The ZnO/TiO₂ bilayer showed the lowest band gap value (3.52 eV), which can be attributed to interfacial interactions at the heterojunction.

Keywords

Single layer bilayer composite thin film sol–gel spin-coating

Introduction

In recent years, thin film technology has attracted significant attention due to its extensive applications in optical, electronic, and optoelectronic devices. Although single-layer thin films have become increasingly prevalent, many modern applications necessitate multilayer thin films that integrate the properties of various materials. Such multilayer structures are frequently employed in the fabrication of optical reflectors, computer storage media, anti-reflective coatings, and solar cells.¹ Studies indicate that layering two semiconductors can enhance optical and electrical properties due to their synergistic interactions.^2,3

Metal oxide thin films, particularly titanium dioxide (TiO₂) and zinc oxide (ZnO), are extensively studied due to their unique optical, electronic, and structural characteristics, making them suitable for photocatalysis, photovoltaics, gas sensors, and transparent electronics. However, the intrinsic properties of these semiconductors differ, with ZnO having a direct energy bandgap and TiO₂ possessing an indirect energy bandgap.⁴ Both TiO₂ and ZnO thin films exhibit high transparency in the visible spectrum, chemical stability, and adjustable band gaps, allowing performance optimization according to application-specific requirements.

TiO₂, a well-known n-type semiconductor with a band gap of approximately 3.2 eV,⁵ is highly valuable for UV-responsive applications such as photocatalysis and gas sensing. Although undopedTiO₂ is transparent and stable, its limited visible-light absorption restricts its broader applicability. Strategies to improve TiO₂ performance include doping or forming composites with other metal oxides to extend its optical activity into the visible range and enhance photocatalytic efficiency. ZnO, another metal oxide semiconductor with a comparable band gap (∼ 3.37 eV), is distinguished by its high electron mobility,^6,7 strong exciton binding energy, and piezoelectric properties, making it ideal for optoelectronic devices, sensors, and energy-related applications. Similar to TiO₂, undoped ZnO predominantly absorbs in the UV range, yet its transparency, stability, and electronic properties make it suitable for transparent conductive oxides and robust device performance.

Numerous techniques have been utilized to deposit ZnO and TiO₂ thin films, including spray pyrolysis,⁸ sol–gel methods,⁹ and chemical vapor deposition.¹⁰ Among these, the sol–gel method stands out as an affordable technique that facilitates low-temperature synthesis while providing excellent control over film purity and uniformity.⁵ Combining ZnO and TiO₂ in a 50% mixed-metal oxide structure leverages the advantages of both materials. This composition enhances optical properties, extends light absorption across the UV-visible spectrum, and potentially improves photocatalytic efficiency. Mixed metal oxides, such as the 50% (TiO₂:ZnO) composition, are anticipated to improve charge separation, increase catalytic surface area, and enhance photocatalytic and optoelectronic performance, beneficial for environmental and energy applications.¹¹

The ZnO/TiO₂ bilayer thin film structure offers an alternative approach, sequentially depositing the two oxides to form an interface that improves charge separation, extends carrier lifetimes, and reduces recombination losses. This bilayer arrangement is particularly advantageous for photovoltaic and photocatalytic applications, where efficient charge transfer between ZnO and TiO₂ significantly enhances performance compared to individual or mixed-metal films. ZnO/TiO₂ bilayers thus demonstrate considerable promise for applications demanding superior optoelectronic characteristics.⁵

To the best of our knowledge, no comparative studies have been conducted on TiO₂, ZnO, (TiO₂:ZnO) mixed thin films, and ZnO/TiO₂ bilayer thin films deposited on glass substrates. Therefore, this study investigates ZnO/TiO₂ bilayers synthesized via the sol–gel spin-coating method, comparing their crystalline structure, morphology, and optical properties.

Experimental Details

Materials and Methods

Synthesis of TiO₂ Films

The process for synthesizing TiO₂ films begins with the addition 2.8 ml of TTIP (Titanium tetraisopropoxide) (Ti(OCH (CH₃)₂)₄) (+97% purity) to 2 ml of isopropanol (CH₃CHOH CH₃) (99.99% purity) which were added drop by drop, under vigorous magnetic stirring for 10 min at room temperature, 3.2 ml of acetic acid (CH₃COOH) (99.99% purity) is then introduced, and the mixture is stirred for another 15 min. After that, 10 ml of methanol was added, and the stirring continues for an additional hour. The sol obtained is subsequently deposited onto cleaned glass substrates using a spin coating method at 2000 r/min for 30 s per layer.

Each layer undergoes preheating at 120 °C for 10 min to evaporate the solvent. This spin coating process is repeated for a total of five times. Finally, the films are annealed at 500 °C for 90 min in a Nabertherm muffle furnace.

Synthesis of ZnO Films

ZnO films were synthesized using the sol–gel method. Zinc acetate dihydrate (C₄H₁₂O₆Zn-2H₂O) (99.99% purity) was used as the zinc precursor and dissolved in methanol under magnetic stirring, 2 ml of Diethanolamine (DEA) (C₄H₁₁NO₂, 98% purity) was used as a catalyst. The solution was stirred until a clear and homogeneous sol was obtained. The films were deposited using the same spin-coating parameters described above, followed by drying at 120 °C for 10 min after each layer, and final annealing at 500°C for 90 min in a Nabertherm muffle furnace.

Synthesis of TiO₂:ZnO Composite

The TiO₂: ZnO composite sol was prepared by mixing the previously prepared undoped TiO₂ and ZnO in a 1:1 volume ratio under magnetic stirring for 1 h to ensure complete homogenization. The composite films were deposited by spin coating at 2000 r/min for 30 s per layer. After each deposition, the films were dried at 120 °C for 10 min. The coating process was repeated five times to obtain the desired thickness. Finally, the films were annealed at 500 °C for 90 min to improve their structure and optical properties.

We mixed the prepared undoped TiO₂ solution with the undopedZnO solution in a 1:1 volume ratio.

Synthesis of ZnO/TiO₂ bilayer

Undoped TiO₂ and ZnO thin films were first prepared as described above. TiO₂ layer was initially deposited onto the cleaned substrate by spin coating at 3000 rpm for 30 s (was selected to better thickness control and improved interfacial uniformity), followed by drying at 200 °C for 10 min to ensure complete solvent removal. Subsequently, a ZnO layer was deposited onto the TiO₂ coated substrate under the same conditions. The spin-coating and drying processes were repeated five times for each layer to achieve the required thickness. Finally, the bilayer films were annealed at 500 °C for 90 min in a Nabertherm muffle furnace.

Characterization of sol-gel ZnO-TiO₂Thin films

The crystal structure of the films was analyzed using X-ray diffraction (XRD) with a D8 Advanced Bruker AXS θ–2θ diffractometer, where CuK_α of wavelength λ = 0.15406 nm was employed for the radiation, and the diffraction patterns were measured within the 2θ range of 20° to 80°.The samples’ surface morphology was analyzed by Scanning Electron Microscopy (SEM) using a (Jeol JSM-6360LV);the system was equipped with secondary and backscattered electron detectors, along with digital image capture capabilities and atomic force microscope (AFM). The elemental composition of the films was analyzed using SEM/EDS, incorporating a Bruker quantax energy-dispersive X-ray spectrometer. The optical transmittance of the thin films was measured at ambient temperature using a Shimadzu 3101 PC UV-visible spectrophotometer, and the FTIR spectra of the films were recorded using a Fourier transform infrared spectrometer (Nicolet Avatar 360).

Results and discussion

Structural Analysis

The crystal structures of TiO₂, ZnO, TiO₂:ZnO thin films, and ZnO/TiO₂bilayer thin films were analyzed using X-ray diffraction (XRD) to determine their phases and crystallinity. The diffraction pattern of TiO₂ reveals the presence of the anatase phase in Figure 1, with the most intense peak observed at 2θ = 25.3° corresponding to the (101) plane with JCPDS files N^o: 83–2243. Confirming the anatase phase, the most photocatalytically active phase of TiO₂, is the primary crystallization of the film, which is backed up by this dominant peak.^9,10 In addition to its chemical stability under visible light, the anatase phase is highly preferred in photovoltaic applications because of its enhanced photoactivity, extended lifespan of photogenerated holes and electrons, higher refractive index, and greater electron diffusion coefficient compared to other crystalline phases.¹² Some weak diffraction peaks can also be observed in Figure 1(a), which relate to the anatase and brookite (JCPDS file No: 76–1936) phases, each has two peaks at 2θ= 36° A(103), 48° A(200), 54.54° A(105), 30.3° B(121), 45.81° B(032), 62.7° B(204) planes, respectively.¹³ All the values corresponding to the XRD peaks confirm the polycrystalline nature and the presence of anatase and brookite phases of crystalline TiO₂, which is in good agreement with the reported.^14–16 The ZnO thin film diffraction pattern exhibits characteristic peaks of the hexagonal wurtzite structure (JCPDS card no. 36–1451) at 2θ= 31.40° and 33.30°, corresponding to the (100) and (002) planes, respectively.Although a standard powder diffraction pattern of wurtzite ZnO typically shows additional strong reflections such as (101), (102), and (110), the dominance of the (002) peak in our thin film indicates preferential c-axis orientation. The reduced intensity or absence of other reflections can be attributed to preferred orientation and limited film thickness rather than incomplete phase formation. For the ZnO/TiO₂ bilayer thin film, the diffraction peak observed at 2θ ≈ 25.6° can be indexed to the (101) plane of anatase TiO₂ (JCPDS Card No. 76-1936). Although this reflection is the most intense anatase peak, careful examination of the diffraction pattern shows no peak at 2θ ≈ 27.4°, which corresponds to the (110) plane of rutile TiO2 (JCPDS Card No. 21-1276), indicating the absence of rutile within the detection limits of XRD.¹⁴ Additionally, the diffraction peaks observed at 31.5°, 34.5°, and 47.6° are indexed to the (100), (002), and (102) planes of hexagonal wurtziteZnO (JCPDS Card No. 36-1451).^14–16 No extra diffraction peaks corresponding to other crystalline impurity phases were detected. Therefore, the XRD results suggest that the bilayer thin film predominantly contains anatase TiO₂ and hexagonal wurtzite ZnO phases within the sensitivity limits of the measurement.In contrast, the TiO₂:ZnO composite film does not exhibit discernible Bragg reflections within the scanned 2θ range. However, it should be noted that very thin composite films may appear XRD amorphous even ifthey arepartially crystalline, as diffraction intensity can be significantly reduced due to limited film thickness, reduced scattering volume, extremely small crystallite size, peak broadening, or instrumental sensitivity limitations. Therefore, the composite film is more appropriately described as amorphous or poorly crystalline within the detection limits of the XRD measurement. The suppression of crystallization upon Zn incorporation has been reported previously, where Zn addition inhibits anatase domain growth and promotes disordered structures.^2,17 However, other studies have reported different results, indicating that the synthesis of TiO₂:ZnO composite films produced by thermal evaporation can lead to the formation of mixed crystalline phases.¹⁸ The crystallites size of the nanostructure on thin films is able to be predicted by using the Scherer Formula (1)^19,20:

D = \frac{k λ}{β \cos θ}

(1)

Figure 1.

XRDspectra of tdeposited thin films.

Where λ represents the X-ray wavelength (1.54056 Å), θ denotes the Bragg angle of diffraction,the constant k is the shape factor (usually taken as 0.9), and β corresponds to the full width at half maximum (FWHM) of the peak at that specific angle θ.

δ Describes the dislocation density,which determines the length of dislocation lines within the crystal per unit volume.^7,21

δ = \frac{1}{D^{2}}

(2)

The FWHM values for the peaks at 25.15°, 31.35°, and 31.50° were 0.024, 0.0119, and 0.0165 for TiO₂, ZnO, thin films, and TiO₂/ZnO bilayer thin films, respectively. The corresponding crystallite sizes were 5.77, 11.65, and 8.4031 nm. As shown in Table 1.

Table 1.

Extracted XRD parameters corresponding to Figure 1.

Sample	FWHM/(rad)	θ (°)	Average crystallite size (D) (nm)	Dislocation line density (× $10^{16}$ )(m⁻²)
TiO₂	0.024	12.575	5.77	3
ZnO	0.0119	15.679	11.65	0.7367
TiO₂:ZnO	-	-	-	-
ZnO / TiO₂	0.3491	15.75	8.4031	1.44

Raman analysis

Raman spectroscopy is a highly sensitive technique for analyzing the crystal structure and phase of TiO₂ and ZnO without causing any damage to the material.⁵ The chemical bonding in ZnO is intermediate between ionic and covalent. The bond ionicity between Zn and O atoms is relatively high (about 0.62 on the Phillips scale), which is why they are commonly represented as Zn ⁺ ² and O⁻² ions. Under ambient conditions, ZnO typically adopts a thermodynamically stable hexagonal wurtzite structure. When ZnO is deposited on cubic substrate, it can also exhibit zinc-blende structure, respectively.²² Using excitation at 532 nm,as shown in Figure 2, prominent Raman shifts are evident from the distinct peaks that appear 396, 559, 639, and 797 cm⁻¹ confirmed E_g,B_1g,A_1g, and E_g phonon vibrational modes of the anatase TiO₂ phase in lattice respectively.²² ZnO Thin films characterized by a hexagonal wurtzite structure and excellent crystallinity exhibit a peak at approximately 1101 cm⁻¹ and exhibit higher intensity and narrower line-width, corresponding to a 2LO phonon mode process. Similar to Zhuo, R, et al..²³ Findings, excitation by laser lines at 325 nm indicates dominance of the 1 LO and multi-LO modes.The intensity of the bands increases notably when comparing the ZnO/TiO₂ bilayer thin films (554 cm⁻¹ and 790 cm⁻¹) to those of ZnO monolayer thin films. Additionally, in this case, the intensity of the main band is significantly elevated at 2820.35 cm⁻¹. Below 1000 cm⁻¹ was attributed to the Ti–O–Ti bond stretching vibrations in TiO₂. Therefore, the Raman spectra of the TiO₂:ZnO composite and ZnO/TiO₂ bilayer films exhibit vibrational features associated with phonon vibrational modes and Ti–O and Zn–O bonding environments. However, since Raman spectroscopy probes short-range structural order, these features indicate local structural organization or partial crystallinity rather than conclusively demonstrating fully developed long-range crystalline phases.

Figure 2.

Raman spectraof thin films: (a) TiO₂. (b)ZnO. (c) TiO₂:ZnOcomposite and (d)ZnO /TiO₂ bilayer.

FTIR analysis

FTIR is an effective method to analyze the composition of compounds or products. The FTIR spectra of the films were obtained using a FTIR (Nicolet Avatar 360) in the range of 400-4000 cm⁻¹. Metal oxides often show characteristic absorption bands in the fingerprint region (below 1000 cm⁻¹), which are ascribed to vibrations between atoms.²⁴ Figure 3 shows the FTIR spectrum of TiO₂, ZnO,TiO₂:ZnO composite, and ZnO/TiO₂ bilayers thin films. For the TiO₂ thin film, the strong absorption band observed in the 450–600 cm⁻¹ region is attributed to Ti–O–Ti lattice stretching vibrations, characteristic of the TiO₂ network structure.²⁵ A band around ∼1630 cm⁻¹ is assigned to the H–O–H bending mode of molecularly adsorbed water and Ti–OH surface groups,¹⁸ which commonly appear in sol–gel derived oxide films due to surface hydration.²⁶ These results well agree with earlier.^27,28 A weak absorption feature was observed around ∼2105 cm⁻¹, Vibrational modes in the 2100–2200 cm⁻¹ region are typically associated with C≡C, C≡N stretching vibrations, or adsorbed CO species on oxide surfaces.^29,30 However no precursors containing triple bonds(C≡C or C≡N) were used in the present synthesis, and no complementary evidence supports the presence of such functional groups, therefore, this weak band is cautiously attributed to (CO related adsorption). For ZnO thin films, Zn–O stretching vibrations are expected below ∼600 cm⁻¹. The broad absorption features observed in the 400–600 cm⁻¹ region are therefore assigned to Zn–O lattice vibrations. Bands appearing in the 1400–1600 cm⁻¹ region are attributed to surface carbonate species or residual organic fragments resulting from atmospheric CO₂ adsorption or incomplete removal of organic residues during annealing.²⁶ The TiO₂:ZnO composite and ZnO/TiO₂ bilayer thin films exhibit similar spectral characteristics, which is expected due to the common presence of metal oxygen frameworks and surface hydroxylation effects. FTIR spectroscopy confirms the formation of metal oxygen bonds in all films, however, it is not highly phase-selective for distinguishing crystalline phases. Therefore, phase identification and crystallinity are primarily supported by XRD and Raman analyses, while FTIR provides complementary information regarding chemical bonding and surface chemistry.^31,32

Figure 3.

FTIR spectra ofdeposited thin films.

AFM analysis

Atomic Force Microscopy (AFM) operated in tapping mode offers comprehensive insights into the surface morphology and nanoscale topography of thin films. This method allows for precise quantitative evaluation of parameters such as grain size, shape anisotropy, height distribution, and Root Mean Square (RMS) roughness, all of which are essential for analyzing film growth dynamics and defect formation.^30,31

Figure 4. shows the 2D and 3D topographic of thin films. As observed, the morphology of the TiO₂ films exhibits an RMS roughness of approximately 2.27 nm. Zulkiflee et al. Determined a similar RMS roughness of approximately 2.87 nm for TiO₂ thin films synthesizedusing the sol–gel method by dip-coating, followed by calcination at 500 °C.³³ This relativelysmoothmorphologyindicatesuniform grain growth and a compact structure, whichisbeneficial in optoelectronic applications where minimal surface scattering is desired. Such surfaces enhance electron mobility and improve interfacial contact with other layers in device architectures. ZnO thin films show a higher RMS roughness of about 6.07 nm, suggesting a more granular and porous surface structure. This increased roughness is advantageous for applications such as photocatalysis, UV sensors, and gas sensing, where a higher surface area promotes improved interaction with target molecules or enhanced light absorption.TiO₂:ZnO thin films, revealing rounded particles on the surface. However, these particles are not distinctly visible and not uniformly distributed and exhibit the lowest RMS roughness among the samples at 0.91 nm. This indicates excellent surface uniformity, likely due to the mutual modulation of TiO₂ and ZnO growth dynamics. This smooth surface is especially beneficial for optical applications, transparent conductive oxides (TCOs), and buffer layers in solar cells, where uniformity is critical for performance.The bilayer thin film ZnO/TiO₂ structure consists of compact grains, closely resembling those of the single TiO₂ layer. The thin films are clearly visible on the surface layers, formed through the incorporation of both TiO₂ and ZnO. Crystallization in the bilayer structure is enhanced by the TiO₂ coating on the ZnO layer. This configuration exhibits an intermediate RMS roughness of 2.78 nm. The stacking of layers tends to induce structural variations at the interface, thereby influencing the overall surface morphology. Such bilayer structures can harness the individual advantages of both oxides, enabling improved electron-hole separation,particularly beneficial in photovoltaic and photocatalytic devices.³⁴

Figure 4.

Morphology and surface roughness of: TiO₂, ZnO, (TiO₂: ZnO), and ZnO/TiO₂ thin films.

Surface morphology

Figure 5. shows SEM micrographs of the spin coating TiO₂, ZnO, TiO₂:ZnO compsite, and ZnO/TiO₂ bilayer thin films. Magnifications ranging from 50,000× to 80,000× were employed to provide an in-depth view of the thin films. The TiO₂ thin film Figure 5(a) exhibits a predominantly compact and relatively smooth surface. However, localized micro-cracks and small particle agglomerates are also visible. Such micro crack networks are commonly attributed to shrinkage induced tensile stress during solvent evaporation and densification in sol gel derived films, or annealing at 500 °C process.^5,35 Despite these localized defects, the overall surface morphology remains continuous and uniform. The ZnO thin film Figure 5(b) appears largely continuous and compact, but contrast variations are observed across the surface. These variations may arise from local differences in surface topography, grain orientation, density fluctuations, which are frequently encountered in SEM analysis of oxide films deposited on insulating substrates.³⁶ Several elongated surface features with lateral dimensions of ∼ 954 nm, 981 nm, and 1009 nm are observed. These features may correspond to coalesced grains or preferentially oriented growth structures formed during thermal treatment. However, their exact crystallographic nature cannot be conclusively determined from SEM imaging alone, Khan et al. Investigated ZnO films grown on glass substrates using the physical vapor condensation technique.³⁷ In Figure 5(c), the image shows particles distributed across the surface. These particles are relatively uniform in size, indicating good control over the deposition process;the average grain size is 246 nm, in addition elongated surface features with lateral dimensions of approximately 655 nm are observed. In Figure 5(d), the image shows clusters or agglomerations of particles on the surface. These might be due to the crystallization or particle aggregation during the deposition and annealing processes. The rough and granular structure could influence the optical and electrical properties of the thin films. The distribution of particles appears non-uniform, which is typical in thin films prepared via spin coatingthe average grain size is 211 nm.

Figure 5.

SEM images of thin films.

EDS analysis

The chemical compositions of the TiO₂, ZnO, TiO₂: ZnO composite, and ZnO/TiO₂ bilayer thin films are determined through EDS spectra, as illustrated in Figure 6. As anticipated, the spectra reveal the presence of Ti, Zn, and O elements. Their atomic and weight contents in the thin films have been calculated based on library standards and are listed in the table attached to each figure. It is observed that the oxygen content in the film slightly decreases as with the formation of TiO₂:ZnO composite and ZnO/TiO₂ bilayer compared to TiO₂ and ZnO. However, the measured oxygen content exceeds theoretical values, which is expected due to the nature of EDS analysis conducted via SEM, which cannot enable reliable quantification of light elements such as oxygen and carbon, etc.

Figure 6.

Typical EDS spectra of TiO₂, ZnO, TiO₂: ZnO composite, and ZnO/TiO₂ bilayer thin films.

Optical transmittance

Figure 7. Shows the optical transmittance spectra of the TiO₂, ZnO, TiO₂:ZnO composite, and ZnO/TiO₂ films deposited within the wavelength range of 300–900 nm. All the films exhibit high optical transmittance, exceeding 60% in the wavelength ranges from (400-800 nm).

Figure 7.

(a) Transmittance spectra of the films of TiO₂, ZnO, TiO₂:ZnO composite, and ZnO/TiO₂,(b) Enlarged optical transmission spectra in a short wavelength range of thin films deposited.

The transmittance spectra of the thin films reveal notable differences based on composition. In contrast, to provide a quantitative comparison, both the maximum transmittance (T_max) and the average visible transmittance (T_avg) were calculated and are summarized in Table 2. The TiO₂ film exhibits T_max = 71.7% and T_avg = 65.2%, whereas the ZnO film shows improved transparency with T_max = 75.9% and T_avg = 70.7% the higher visible transparency of ZnO is consistent with previous reports attributing this behavior to its wide direct band gap and relatively low absorption coefficient in the visible region.³⁸ The TiO₂:ZnO composite film presents intermediate optical behavior (T_max = 70.2%, T_avg = 64.7%), with a noticeable reduction near the absorption edge (∼395 nm) compared to the pure TiO₂ and ZnO films behavior indicates the presence of defect-related localized states and possible band tailing effects, which are commonly associated with structural disorder in oxide thin films. This optical response suggests that the composite film possesses a disordered or weakly crystalline structure. This interpretation is supported by the XRD analysis, where no distinct Bragg reflections were observed within the scanned 2θ range. The absence of sharp diffraction peaks indicates that the TiO₂:ZnO composite film is amorphous or poorly crystalline within the detection limits of the measurement. Therefore, the UV–Vis and XRD results are consistent, confirming that the structural disorder in the composite film significantly influences its optical behavior. Notably, the ZnO/TiO₂ bilayer demonstrates the highest transparency, with T_max = 79.4% and T_avg = 71.8%, indicating enhanced optical performance compared to the single-layer films.³⁹ Considering both the film morphology. From the AFM analysis, the bilayer surface consists of compact grains with a relatively intermediate RMS roughness (∼2.78 nm), which indicates a dense and fairly smooth surface. Such morphology reduces surface-induced light scattering and minimizes optical losses, thereby supporting higher measured transmittance.⁴⁰ The short wavelength portion of Figure 7(a) is expanded and shown in Figure 7(b).

Table.2.

Optical transmittance parameters and film thickness of TiO₂, ZnO, TiO₂:ZnO composite, and ZnO/TiO₂ bilayer thin films.

Thin films	TiO₂	ZnO	TiO₂:ZnO composite	ZnO/TiO₂ bilayer
T_max (%)	71.7	75.9	70.2	79.4
T_avg (%)	65.2	70.7	64.7	71.8
Thickness (nm)	308.07	662	400	851

The thickness (d) of the transparent thin films was estimated using the Swanepoel envelope method. The maximum (T_max) and minimum (T_min) transmittance values were extracted from the optical transmittance spectra Figure 7(a). Based on these parameters, the film thickness was calculated using the corresponding Swanepoel relation.⁴¹

d = \frac{λ_{1} λ_{2}}{2 (n_{1} λ_{2} - n_{2} λ_{1})}

(3)

Where, n₁ and n₂ are the refractive indices at two adjacent maxima (or minima) corresponding to the wavelengths λ₁ and λ₂.

n_{1, 2} = \sqrt{N + \sqrt{N^{2} - n_{s}^{2}}}

(4)

Where

N_{1, 2} = 2 n_{s} \frac{T_{M} - T_{m}}{T_{M} . T_{m}} + \frac{n_{s}^{2} + 1}{2}

(5)

T_M and T_m are maximum and minimum transmission respectively, on the envelope at a certain λ, and refractive index n_s = 1.44, of the substrate. The envelopes connecting the interference maxima and minima are considered to be continuous functions of the wavelength λ.

The optical energy gap E_gcan be estimated from the absorption spectrum (αhν) ⁿ, for both direct and indirect band gap semiconductors.For an indirect band gap, the optical energy gap can be estimated using the following relation^2,40:

(α h ν)^{1 / 2} = A (h ν - E_{g})

(6)

For a direct band gap, the optical energy gap can be estimated using the following relation:

(α h ν)^{2} = A (h ν - E_{g})

(7)

Here, hν is the photon energy. The absorption coefficient α can be calculated from the transmittance T, A is a constantby the equation:

α = \frac{1}{d} \ln (\frac{100}{T (%)})

(8)

The optical band gap values for thin film were measured using the linear extrapolating method, as shown in Figure 8. From these plots, for anatase TiO₂ thin films, which are typically characterized as indirect band gap semiconductors, the band gap was determined from the (αhν)^1/2 versus hν plot. The indirect band gap was determined to be 3.70 eV. A similar result was reported by Shakoor et al.,¹⁸ who investigated TiO₂ films fabricated via thermal evaporation and found an energy band gap of 3.8 eV, and TiO₂ films produced by spin coating and Meriem et al .⁵

Figure 8.

Tauc plot of thin films.

For the ZnO thin films, which are known to exhibit a direct band gap nature, the band gap was extracted from the linear region of the(αhν)² versus hν plot. The direct optical band gap was found to be 4.13 eV, which aligns with previously published values for ZnO films in the literature, typically ranging between 3.3 and 4.2 eV depending on structural quality and synthesis method, Khan et al. Reported a direct optical band gap of 3.54 eV for ZnO films prepared by the physical vapor condensation method.³⁷ Kumar et al. Determined a band gap of 3.75 eV for ZnO films synthesized using the chemical bath deposition (CBD) method.⁴²

For the TiO₂:ZnO composite and ZnO/TiO₂ bilayer films, both direct and indirect band gap were analyzed due of TiO₂ and ZnO phases and possible interfacial electronic coupling effects. The TiO₂:ZnO composite exhibited a direct band gap of 4.01 eV and an indirect band gap of 3.61 eV. The reduction in both band gap values compared to the individual oxides may be attributed to the formation of defect states, oxygen vacancies, and a possible charge transfer between Ti ⁺ ⁴ and Zn ⁺ ², which introduces sub-band gap states that facilitate visible light absorption.⁴³ The incorporation of Zn into the TiO₂ lattice also leads to lattice distortion and electronic interactions that modify the band structure, Arunachalam et al . Also demonstrated that the formation of Zn–Ti–O bonds inhibits crystal growth [26;47]. Similarly, the ZnO/TiO₂bilayer showed direct and indirect band gaps of 3.95 eV and 3.52 eV, respectively. These are the lowest band gaps observed among the samples. These estimated values align with the typical band gaps reported for TiO₂ and ZnO, which are relatively similar. This similarity makes the two materials well-suited to be combined, thereby enhancing their overall performance and suggesting a strong synergistic interaction at the heterojunction interface.⁴⁴

Conclusion

TiO₂, ZnO, (TiO₂: ZnO composite) single layer and bilayer of ZnO/TiO₂ thin films have been deposited by using a low-cost sol–gel spin coating method. After deposition, all the films were annealed at 500 °C. XRD and Raman analysis confirmed both the TiO₂ and ZnO particles present in the films. In terms of morphological properties, SEM analysis revealed notable surface morphology differences among the TiO₂, ZnO, TiO₂:ZnO composite, and ZnO/TiO₂ thin films. TiO₂ showed a relatively smooth surface with minor imperfections, while ZnO elongated surface features correspond to coalesced grains or preferentially oriented growth structures. The TiO₂:ZnO composite and ZnO/TiO₂ bilayer thin films displayed more defined particle distributions and surface roughness, reflecting the influence of composition and processing conditions on the films’ microstructure. AFM showed the nanoparticles with average sizes of TiO₂, ZnO, TiO₂: ZnO composite single layer and bilayer of ZnO/TiO₂ thin films are 2.7, 6.07, 0.91, and 2.78 nm respectively.FTIR analysis confirmed the presence of characteristic Ti–O–Ti and Zn–O bonds in the single and bilayer thin films. Absorption bands related to metal-oxygen vibrations, and organic residues were identified, indicating the successful formation of metal oxide structures and residual organics from the sol-gel process. The optical transmittance and band gap analyses revealed that all films exhibited high transparency. The TiO₂:ZnO composite thin films and ZnO/TiO₂bilayer thin films demonstrated improved optical properties and reduced band gaps, highlighting synergistic interactions and enhanced performance at the heterojunction interface.

Footnotes

Acknowledgments

The authors acknowledge the support of the Directorate-General for Scientific Research and Technological Development (DGRSDT), Ministry of Higher Education and Scientific Research of Algeria.

ORCID iDs

Meriem Boudiar

Faouzi Hanini

Loumafak Hafaifa

Mostefa Maache

Nilgun Baydogan

Ethical considerations

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Author contributions

Meriem Boudiar: Investigation, Writing - original draft.

Faouzi Hanini: Supervision, Writing - review & editing.

Loumafak Hafaifa: Writing - original draft.

Mostefa Maache: Writing - review & editing.

Nilgun Baydogan: Investigation, Formal analysis.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

The data presented in this study are available from the corresponding authors upon reasonable request.

References

Hafaifa

Maache

Arteev

, et al. Optimization of CZTS Absorber Layer for Enhanced Efficiency in CZTS/CIGS Tandem Solar Cells: A Comprehensive Loss Analysis. Phys Scr 2025; 100(9): 095530. http://iopscience.iop.org/article/10.1088/1402-4896/ae00fd .

Roberts

Chirumamilla

Wang

, et al. Ultra-thin titanium nitride films for refractory spectral selectivity [invited]. Opt Mater Express 2018; 8: 3717.

Hammad

Adabala

Ghosh

. Choice of layering and band alignment in 2D heterostructures. J Phys Chem C 2024; 128: 20635–20640.

Firdaus

Rizam

MSBS

Rusop

, et al. Characterization of ZnO and ZnO: tiO2 thin films prepared by Sol-gel spray-spin coating technique. Procedia Eng 2012; 41: 1367–1373.

Boudiar

Hanini

Bouabellou

, et al. Sol–gel derived Zn doped TiO2 thin films and their waveguides. J Sol-Gel Sci Technol 2023; 107: 430–440.

Hamani

Bourhefir

Hafaifa

, et al. Influence of Al and Mg co-doping on structural and optical properties of ZnO thin films deposited by spray pyrolysis. J Opt 2024; 53. https://doi.org/10.1007/s12596-024-02356-1.

Hafaifa

Maache

Djalab

. Performance Enhancement of CGS/CIGS Thin Flm Tandem Solar Cell Using Different Buffer Layers. J Opt 2024; 53. https://doi.org/10.1007/s12596-024-02035-1.

Vigil

Cruz

Santana

, et al. Influence of post-thermal annealing on the properties of sprayed cadmium-zinc oxide thin films. Appl Surf Sci 2000; 161: 27–34.

Khan

Bhatti

Qindeel

, et al. Results in physics investigations of the structural, morphological and electrical properties of multilayer ZnO / TiO 2 thin films, deposited by sol – gel technique. RESULTS Phys 2016; 6: 156–160.

10.

Haga

Kamidaira

Kashiwaba

, et al. Zno thin “ lms prepared by remote plasma-enhanced CVD method. J Cryst Growth 2000; 215: 77–80.

11.

Dai

Yao

, et al. ZnO/TiO 2 photocatalysts for degradation of methyl orange by low-power irradiation. Sci Prog 2025; 108. https://doi.org/10.1177/00368504251322606

12.

Fazli

FIM

Ahmad

Soon

, et al. Dye-sensitized solar cell using pure anatase TiO2 annealed at different temperatures. Optik (Stuttg) 2017; 140: 1063–1068.

13.

Qiao

Xie

, et al. Characterization and photoelectrochemical performance of Zn-doped TiO2 films by sol–gel method. Trans Nonferrous Met Soc China 2016; 26: 2109–2116.

14.

Ahmad

Singh

Mishrwan

, et al. Titanium dioxide nanomaterials: synthesis and applications. Asian J Chem 2023; 35: 1770–1774.

15.

Meena

Surela

Chhachhia

, et al. Investigation of the photocatalytic potential of C/N-co-doped ZnO nanorods produced via a mechano-thermal process. Nanoscale Adv 2025; 7: 1335–1352.

16.

Abbas

Bensaha

Taroré

. The influence of Zn+2 doping and annealing temperature on grown-up of nanostructures TiO2 thin films prepared by sol-gel dip-coating method and their photocatalytic application. Optik (Stuttg) 2019; 180: 361–369.

17.

Pereira Lessa

Lima

Jr Margalho

, et al. Application of nano-TiO2 and micro-ZnO on cementitious surfaces for self-cleaning façades by spray coating and dip coating: a comparative study. J Build Pathol Rehabil 2025; 10: 16.

18.

Shakoor

Nowsherwan

Alam

, et al. Fabrication and characterization of TiO2: znO thin films as electron transport material in perovskite solar cell (PSC). Phys B Condens Matter 2023; 654: 414690.

19.

Patterson

. The scherrer formula for X-ray particle size determination. Phys Rev 1939; 56: 978–982.

20.

Kitazawa

Choi

Yamamoto

, et al. Rutile and anatase mixed crystal TiO2 thin films prepared by pulsed laser deposition. Thin Solid Films 2006; 515: 1901–1904.

21.

Boudiar

. Synthesis and characterization of thin films of metallic oxides and pair oxides based on doped TiO2., n.d. https://doi.org/http://oldspace.univ-tebessa.dz:8080/xmlui/handle/123456789/http//localhost:8080/jspui/handle/123456789/12219.

22.

O’Regan

Grätzel

. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991; 353: 737–740.

23.

Zhuo

Feng

Liang

, et al. Morphology-controlled synthesis, growth mechanism, optical and microwave absorption properties of ZnO nanocombs. J Phys D Appl Phys 2008; 41: 185405.

24.

Khan

Naqvi

Ahmad

. Comparative study of the cytotoxic and genotoxic potentials of zinc oxide and titanium dioxide nanoparticles. Toxicol Reports 2015; 2: 765–774.

25.

Diebold

. The surface science of titanium dioxide. Surf Sci Rep 2003; 48: 53–229.

26.

Preface

. Infrared Spectroscopy Fundamentals and Applications, n.d.

27.

Eleutério

Sério

Teodoro

OMND

, et al. XPS And FTIR studies of DC reactive magnetron sputtered TiO2 thin films on natural based-cellulose fibers. Coatings 2020; 10: 87.

28.

León

Reuquen

Garín

, et al. FTIR And Raman characterization of TiO2 nanoparticles coated with polyethylene glycol as carrier for 2-methoxyestradiol. Appl Sci 2017; 7: 49.

29.

Coates

. Interpretation of Infrared Spectra, A Practical Approach. 2000: 10815–10837.

30.

Nakamoto

Griffiths

. Infrared and raman spectra of inorganic and coordination compounds. Wiley Interscience, 1978.

31.

Hallam

. Infrared and Raman spectra of inorganic compounds. R Inst Chem Rev 1968; 1: 39–61.

32.

Derri

Guezzoul

Mokadem

, et al. Insight into the photoluminescence and morphological characteristics of transition metals (TM = Mn, Ni, Co, Cu)-doped ZnO semiconductor: a comparative study. Opt Mater (Amst) 2023; 145: 114467.

33.

Zulkiflee

Hussin

Halim

, et al. Characterization of TiO2, ZnO, and TiO2/ZnO thin films prepared by sol-gel method. ARPN J Eng Appl Sci 2016; 11: 7633–7637.

34.

Hussin

Choy

Hou

. Fabrication of multilayer ZnO/TiO2/ZnO thin films with enhancement of optical properties by atomic layer deposition (ALD). Appl Mech Mater 2013; 465–466: 916–921.

35.

Brinker

Scherer

. Sol-gel science: the physics and chemistry of sol-gel processing. Gulf Professional Publishing, 1990.

36.

Southworth

. Scanning Electron Microscopy and Microanalysis springer, 1975. https://doi.org/10.1007/978-1-4684-2046-3_11

37.

Khan

Islamuddin Kumar

, et al. Optical and electrical characterization of ZnO thin film. Int J Nanosci 2010; 09: 423–429.

38.

Déssoudji

Baneto

Ako

. Structural, morphological and optical properties of ZnO thin films grown by time-dependent chemical bath deposition. Int J Renew Energy Dev 2026; 15: 66–75.

39.

Arifin

Mohamad

Hussin

, et al. Annealing treatment on homogenous n-TiO2/ZnO bilayer thin film deposition as window layer for p-Cu2O-based heterostructure thin film. Coatings 2023; 13: 06.

40.

Fawey

El Moula

AAA

El Hossary

, et al. Optical and structural properties of ZnO / TiO 2 bilayer thin films deposited by pulsed DC magnetron sputtering. J Mater Sci Mater Electron 2025; 36: 1–12.

41.

Ghimire

Dahal

Rai

, et al. Determination of optical constants and thickness of nanostructured ZnO film by spin coating technique. J Nepal Phys Soc 2021; 7: 119–125.

42.

Kumar

Husain

Khan

. Optical studies on amorphous ZnO film. Dig J Nanomater Biostructures 2011; 6: 1317–1323.

43.

Chakrabarti

Dutta

. Photocatalytic degradation of model textile dyes in wastewater using ZnO as semiconductor catalyst. J Hazard Mater 2004; 112: 269–278.

44.

Arifin

Mhd Noor

Mohamad

, et al. The impact of spinning speed on n-TiO2/ZnO bilayer thin film fabricated through Sol–gel spin-coating method. Coatings 2024; 14: 73.

45.

Hanaor

DAH

Sorrell

. Review of the anatase to rutile phase transformation. J Mater Sci 2011; 46: 855–874.

46.

Zhang

Banfield

. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2. J Phys Chem B 2000; 104: 3481–3487.

47.

Khan

Imran

Shahnawaz , et al. Annealing effect on the structural, morphological and electrical properties of TiO2/ZnO bilayer thin films. Results Phys 2018; 8: 249–252.

48.

Boughrara

Zaoui

Guezzoul

, et al. New alginic acid derivatives ester for methylene blue dye adsorption: kinetic, isotherm, thermodynamic, and mechanism study. Int J Biol Macromol 2022; 205: 651–663.

49.

Deepak Raj

Venkatesan

Dhivya

, et al. Influence of film thickness on structural and optical properties of nanocrystalline tellurium films. J Optoelectron Adv Mater 2014; 16: 782–787.

50.

Arunachalam

Dhanapandian

Manoharan

, et al. Influence of sprayed nanocrystalline Zn-doped TiO2 photoelectrode with the dye extracted from Hibiscus Surattensis as sensitizer in dye-sensitized solar cell. Ceram Int 2016; 42: 11136–11149.

Influence of system configuration on the properties of Sol-Gel spin coating TiO 2 -ZnO films: A comparison of single-layer,composite,and bilayer

Abstract

Keywords

Introduction

Experimental Details

Materials and Methods

Synthesis of TiO2 Films

Synthesis of ZnO Films

Synthesis of TiO2:ZnO Composite

Synthesis of ZnO/TiO2 bilayer

Characterization of sol-gel ZnO-TiO2Thin films

Results and discussion

Structural Analysis

Raman analysis

FTIR analysis

AFM analysis

Surface morphology

EDS analysis

Optical transmittance

Conclusion

Footnotes

Acknowledgments

ORCID iDs

Ethical considerations

Consent to participate

Consent for publication

Author contributions

Funding

Declaration of conflicting interests

Data availability

References

Synthesis of TiO₂ Films

Synthesis of TiO₂:ZnO Composite

Synthesis of ZnO/TiO₂ bilayer

Characterization of sol-gel ZnO-TiO₂Thin films