TiO2 Thin Film’s Fabrication and Characterization in Response to Heat Treatment

Introduction

Environmental improvement with TiO₂ photocatalyst is receiving huge attention today because of the rise in environmental issues ¹. TiO₂ is a suitable material Because of many prime physical properties that are required for thin film applications. TiO₂ also has prime chemical, optical, and electrical properties, making it useful in a variety of optical applications, including- gas-sensors ², multiple-layer optical filters ³, photocatalytic water purification ⁴, planer waveguides ⁵. TiO₂-based solar cells (especially- DSSCs) are a popular area of exploration due to their economical, easy manufacturing, environment friendly, and comparatively good energy conversion efficiency ⁶. TiO2 shows good transmittance, high bandgap, high dielectric constant, chemical stability, low cost, non-toxicity, and the capability to be readily doped with ions ^7-9.

Because of its importance in applied physics, we can use various techniques in the TiO2 thin film preparation like chemical vapor deposition (CVD) [10], sputtering ¹¹, and sol-gel method ¹². The condition of synthesis and chemistry of the surface will affect the TiO2 thin film stability. Herein, the sol-gel procedure was adopted because it is simple and reproducible ¹³.

The Sol-gel procedure has several advantages in comparison to other approaches. The Sol-gel procedure shows excellent control of the chemistry, purity, homogeneity, and crystalline phase because of its simple use and economical machinery. The sol-gel method can retrieve new morphology and new physical properties ¹⁴.

The properties of TiO₂ thin film can be modified by different heat temperature treatments. When we increase the temperature, the particle size of TiO₂ increases, resulting in precipitate formation ¹⁵. Usually, the initial crystalline phase (anatase) of TiO₂ is secure up to 400 °C, it transforms into the rutile phase at a higher temperature, which has less photoactivity for solar cells. This conversion takes place at about 500 °C temperature ¹⁶. By doping metallic ions in varying ratios, we can influence the phase conversion from anatase to rutile and the band-gap energy of TiO₂. We can control band-gap energy and the anatase to the rutile phase transition of TiO₂ by doping metallic ions in various ratios. The factors impacting the stability of the doped TiO₂ are oxidation state, bonding of dopant, and ionic radius, it also improves the photocatalytic activity ¹⁷.

To enhance the qualities of solar applications, the current study aims to synthesize and characterize multiple TiO₂ thin film properties, such as its makeup, morphology, and optics, at 3 different annealing temperatures: 300, 400, and 500 °C. We prepared TiO₂ sol using the sol-gel method, and we applied TiO₂ to plain substrates made of glass using the spin coating technique. The optical, morphological, and structural outcomes have been analyzed for the photovoltaic device’s stability, performance, and capacity.

Experimental

Cleaning process of the glass substrates

We cleaned simple glass substrates sequentially in ethanol, isopropanol, acetone, and then in Deionized water for 10 minutes each. Substrates were kept under Deionized water before use.

Chemicals

TTIP [Ti(O-i-C₃H₇)₄], ethanol (CH₃CH₂OH), HCl (“Hydrochloric Acid”), and DI water all materials obtained from CDH Pvt. LTD, Daryaganj, Delhi. All the materials are AR grade for the preparation of TiO2 sol.

Solution Preparation Methodology

Firstly, we take 25 ml Ethanol (CH₃CH₂OH) and 0.1 Mol TTIP [Ti(O-i-C₃H₇)₄] in a beaker Stirred for 2.30 hr at 50 ⁰C. A solution of 25 ml Ethanol+0.1mol HCl+0.1 mol DI WATER was added by dropper and Stirred for 2.30 hr at 50 ⁰C. then we received a Transparent TiO₂ solution. After that, the prepared solution was spin-coated onto a non-conducting glass substrate at 2200 RPM for 40 sec. The deposited TiO₂ thin films have been baked on a hot plate at 110ᵒC for 3 minutes to remove the solvents and other impurities. After that heat the baked film at 300ᵒC, 400℃, and 500 ⁰C temperature for 30 min.

Characterization Methods

The X-rd machine (Panalytical X Pert Pro) was employed to the microstructures of the TiO₂ nanocomposites at a temperature of 22℃. The crystal size is evaluated using Scherrer’s equation. The FESEM (Nova Nano FE-SEM 450) investigated the surface morphology and chemical configuration. Atomic force microscopy (AFM) having a Multimode Scanning Probe Microscope (Bruker) was employed to investigate the topographical feature.

The absorbance and transmittance measurements of films were analyzed as a sample and non-conducting glass as a standard using UV Spectrophotometer-LAMBDA-750-UV-Vis-NIR. Raman spectroscopy (STR 500 CONFOCAL MICRO) was employed to determine the Raman shift.

Chemical Composition

Spin-coated TiO₂ thin films have undergone an energy dispersive spectroscopy (EDS) analysis to ascertain the presence and reveal the chemical makeup of components. The EDS spectrum shows four principal peaks, three for Ti and one for O. The existence of Ti and O materials, as well as their atomic percentages, were verified by EDS analysis. According to EDS analysis, the highlighted atomic concentration in thin films of TiO₂ matches the predicted proportion of atoms at 500°C annealing temperature. The EDS plot is revealed below in Figure (1). Table 1 comprises TiO₂ thin film EDS data.

Figure 1: Chemical composition of TiO2 thin film annealed at 500℃ Click here to View Figure

 

Table 1: Composition of TiO₂ in Atomic % of composition

Elements	Atomic %	Weight %
Ti	38.57	64.67
O	62.78	36.06

Structural Study

X-Rd patterns have been applied to examine the crystalline phase and size of the crystallite of the deposited nanostructured thin film of TiO₂ at various annealing temperatures of 300, 400, and 500 ⁰C. At the post-annealing temperature of 300 ℃, we did not get any peak, and it shows an amorphous graph. After that, when the annealing temperature goes to 400 ⁰C, the X-rd pattern shows a single peak at 2Ѳ =25⁰.3635^’. It is identified as the anatase phase at the peak (1 0 1).

The anatase phase of the peak (101) becomes sharper with the increase in the heat treatment. that shows that crystal size is increasing concerning increasing in annealing temperature. The anatase phase starts to transform into the rutile and brookite phase gradually when we increase the annealing temperature ¹⁸.

At 500 ⁰C annealing temperature, X-rd spectra show three peaks at 2Ѳ of 25.2692⁰, 37.7928^0, and 48.0008⁰, which correspond to the lattice planes (1 0 1), (0 0 4), and (2 0 0). The blend of both anatase and rutile phase structure in the final TiO₂ film has been analyzed. In this mixture, the anatase phase has more influence because of the high activity of photocatalysis ¹⁹.

The crystalline size has been increased with the increase in heat treatment as can be seen in table (2).

Table 2

Annealing temp.	Crystallite Size(nm)
400 0C	2.10
500 0C	3.16

 

Here we evaluate crystal size(D) by Scherrer’s equation-

D indicates the average of the crystalline, k represents the shape factor, λ signifies the Cu-Kα x denotes ray wavelength, β indicates the FWHM (“Full Width Half Maxima”), θ presents the Bragg-angle.

The average crystallite size of TiO2 thin film made using the sol-gel methodology and accumulated by spin coating process at 2000 rpm for 30 sec came out to be 2.10 nm at 400 ⁰C and 3.1601 nm at 500 ⁰C.

Dislocation density (ẟ)- Williamson and Smallman’s formula (e) was utilized to determine the ẟ present in the TiO2 thin film.

The determined ẟ value for TiO2 thin film is 0.2267 nm^-2 and 0.07672 nm^-2.

The Raman spectra display distinct peaks linked to the attendance of TiO2 in anatase and brookite phases at annealing temperatures of 400⁰C and 500⁰C. Bands about 153 and 193 cm^-1 are ascribed to the TiO2 anatase phase in these spectra ²⁰.

Figure 2: XRD pattern of TiO2 films annealed at various temperatures Click here to View Figure

Figure 3: Raman spectra of TiO2 thin film at different temperature Click here to View Figure

Investigation of optical characteristics

The optical features of thin films of TiO₂ can be analyzed with Transmittance spectra (Fig), absorbance spectra (fig-5), and reflectance spectra (fig-6) from 200 to 800 nm which are displayed. The transparency on average is 80% in the visible range. The film transmission aggrandized moderately in the influence of annealing temperature in the near-infrared region. An incisive decrease is observed in each absorbance spectra at 300–345 nm probably because of imperfection in the crystallinity of the material. The Tauc expression is applied to compute the energy band gap (Eg) ²¹:

Where  α, h, E_g, A, and n have their usual meaning

A straight line with an intercept to the X-axis is expected to offer the optical band gap in its direct form on a graph between (αhν)² and hν. The curve between (αhν)² and hν is displayed in fig. (6). The essence of acquired data shows that TiO₂ thin films have a direct band-gap- gap and when the annealing temperature was increased the direct band-gap was decreased. This acquired band gap was higher than the proclaimed optical band gap for single-crystal 3.2 ev. The large optical band gap value obtained for spin-coated TiO₂ thin film might be because of the quantum size impact and size of finite crystallite ²².

Figure 4: UV transmittance spectrum of the TiO2 Click here to View Figure

Figure 5: UV absorption spectrum of the TiO2 Click here to View Figure

Figure 6: Band – Gap Graph of the TiO2 Click here to View Figure

Surface morphology study

SEM was employed to investigate the shape of the surface (surface morphology) and the dispersion of pores of the developed films. Its images show particles are divergent, spherical, and extended, with large particle size distribution. SEM investigates morphological changes of annealed TiO₂ thin films from 300 ⁰C, 400 ⁰C, and 500 ⁰C confirming that the surface morphology of the film has been modified with the annealing temperature.

Fig. (7) displays the SEM images in the top view, and it is examined that the larger particles are made of smaller fragments, the distance between nanoparticles reduces with the rising temperature, and thin films will be more closely packed. The SEM analysis verifies that annealing temperature treatment plays a vital role in the morphology of TiO₂ thin film, thus the crystallization of layers. Above, X-rd data and AFM also support the SEM observations that the size of crystallite increases from 300 ⁰C to 500 ⁰C as a result of densification.²³.

Figure 7: SEM micrographs of the TiO2 at post-heat treatment (a.) 300 ℃ (𝑏) 400 ℃ & (c.) 500 ℃ Click here to View Figure

AFM is utilized to check the surface morphology mitigation and evaluation of TiO₂ thin films at 500 ⁰C annealing temperature. AFM images at 500 ⁰C (Fig. 8) show that the grain agglomeration is in a satisfactory state.

Figure 8: AFM images of the TiO2 thin film annealed at 500 ℃ Click here to View Figure

Conclusion

This study describes the formation and characteristics of the TiO₂ thin films employing a simple and economical method: sol-gel spin coating methodology. As it generates samples with high homogeneity and repeatability, the sol-gel method is acknowledged as one of the most straightforward procedures for preparing TiO₂ thin films and the most effective in terms of layer quality. The deposition was carried out at a temperature of 22 ℃ and post-annealed at distinct temperatures of 300 ℃-400 ℃-500 ℃. The influence of temperature on the features of TiO₂ is discussed in this study.

The X-rd pattern reveals an amorphous graph at an annealing temperature of 300 ℃, it shows an anatase phase around 400 ℃. Furthermore, at a higher annealing temperature (500 ℃), it shows the presence of both initial phases (rutile-anatase) of TiO₂. The Raman spectra also help this fact.

The SEM microscopy shows no breaks in the film, which is made up of microscopic flaky clusters at 300 ℃ heat treatment. Further, grain size increases with an increase in heat treatment. The study of UV-Vis spectra reveals that absorbance decreases abruptly around wavelength 340-345 nm and the band gap increases gradually with increasing temperature.

Acknowledgment

We are thankful to MNIT Jaipur and Manipal University, Jaipur for providing facilities for this work and also grateful to USIC, Rajasthan University, Jaipur.

Conflict of Interest

There is no disagreement of interest, as stated by the corresponding author with the permission of all authors.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Data availability

Yes, Original data is available

Ethical approval

The experiment and study do not involve the use of humans or animals.

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