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The study focuses on investigating the influence of heat-treatment temperature and cobalt (Co) doping on the structural, optical, and electrical properties of TiO2 thin films. Pure TiO2 and Co-doped TiO2 films (Co: TiO2, Co: 0-3-5, and 7 at. %) were deposited onto glass substrates using the sol-gel method at room temperature. Subsequently, the films were annealed at three different temperatures: 400°C, 450°C, and 500°C. Various characterization techniques, including Differential Scanning Calorimetry (DSC), Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD), and Atomic Force Microscopy (AFM), were employed to analyze the films.
The results confirmed that all Co: TiO2 films exhibited a polycrystalline structure with a tetragonal anatase phase having a preferential orientation along the (101) plane, along with traces of orthorhombic brookite.
Moreover, an increase in annealing temperature and doping level led to a reduction in grain size. Surface topography, as observed by AFM, indicated that Co-doped TiO2 films had smoother surfaces compared to undoped TiO2 films. Additionally, the refractive index of the films increased as the band gap decreased from 3.03 to 2.96 eV, accompanied by a decrease in porosity.
Keywords: TiO2, anatase, annealing temperature, optical band gap, refractive index, sol-gel, morphology, optical transmittance.
Cobalt-doped TiO2 thin films were prepared on glass substrates (15 mm × 15 mm × 2 mm) via the sol-gel process.
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The sol-gel solution [17] was prepared at ambient temperature as follows: 2 cm3 of titanium isopropoxide (Fluka, 99.9%) were mixed with 0.7 cm3 of isopropanol, added drop by drop, and stirred for 10 minutes. Subsequently, 2.2 cm3 of acetic acid were added and stirred for 15 minutes. Cobalt acetate tetrahydrate (98+% powder) was dissolved in methanol and added to create Co: TiO2 films with various cobalt concentrations (x = 0.00, 0.03, 0.05, 0.07).
Finally, 5.2 cm3 of methanol were added and stirred for 1 hour. The glass substrates were then immersed in the sol-gel solution, drawn at a speed of 12 cm/min, dried at 100°C for 15 minutes, and annealed at 400°C, 450°C, and 500°C for 2 hours.
The structural properties of the films were investigated using X-Ray Diffraction (XRD) with Cu-ka radiation (wavelength λ = 0.15418 nm). Microstructure analysis was performed using scanning electron microscopy (SEM). Optical transmittance measurements were conducted on a UV-VIS spectrophotometer. Atomic force microscopy (AFM) was employed to observe the surface morphology of Co: TiO2 films deposited on glass substrates within a 4.68 × 4.68 µm2 area. Electrical properties were assessed through current-voltage (I-V) characteristics using the two-probe technique with coplanar structures and two evaporated gold electrodes.
The thermal analysis of the xerogel revealed two significant characteristics. First, an endothermic peak was observed in the temperature range of 50 to 250°C. This peak corresponds to the evaporation of water, thermal decomposition of isopropanol, and the combustion of acetic acid and certain alkoxide elements within the sample. Second, an exothermic peak appeared in the range of 290 to 400°C, corresponding to the crystallization of titanium oxide. Interestingly, it was noted that both peaks were affected by low temperatures and increased cobalt content. This analysis suggests that annealing at or above 400°C is sufficient for complete titanium oxide formation.
Scanning electron microscopy (SEM) was employed to examine the morphology and pore size of the TiO2 thin films. The film's morphology was uniform without visible cracking, even when subjected to higher treatment temperatures. The uniformity of the film remained unaffected as the annealing temperature increased.
The X-ray diffraction (XRD) patterns of the TiO2 thin films exhibited a dominant anatase phase, indicating the polycrystalline nature of the films. At 400°C, the films showed a preferential orientation along the [101] direction. At higher temperatures (450°C), the brookite phase was present, accompanied by the disappearance of anatase. Finally, at 500°C, the brookite phase diminished, and the intensity of the (101) line of anatase increased. The presence of multiple phases, grain size variation, and shifting peak angles indicated an improvement in crystallinity.
Atomic force microscopy (AFM) was employed to study the thin layer structure and morphological evolution with varying cobalt concentration and heat treatments. The average root mean square roughness (RMS) increased as the cobalt content rose. Additionally, the grain size decreased with higher cobalt content, suggesting a directional change in film density. The AFM analysis further confirmed the influence of cobalt doping on crystallinity.
The optical properties of Co: TiO2 films were characterized using UV–Vis diffuse reflecting spectrometry. The band gaps were determined by the Kubelka–Munk equations. The transmittance spectra of TiO2 layers in the visible range revealed their transparency. They began to absorb light between 300 and 350 nm. As the annealing temperature increased, the absorption edge shifted toward lower energy, indicating a reduction in the band gap. This decrease in transmittance may be attributed to inadequate oxygen incorporation during deposition, particularly at higher substrate temperatures.
The electrical conductivity (σ) of undoped TiO2 and Co-doped films was investigated as a function of doping level and annealing temperature. The conductivity initially increased slightly and then decreased, reaching a minimum value at a specific annealing temperature. This behavior could be explained by an increased concentration of free carriers (electrons) from Co2+ ions in Ti3+ substitution sites.
In summary, the sol-gel method allowed for the successful incorporation of cobalt into TiO2 thin films. The films exhibited preferential orientation, multiple phases, and changes in crystallinity. Additionally, as annealing temperature, dipping number, and cobalt concentration increased, grain size, roughness, and electrical conductivity showed corresponding trends. The optical properties, including band gap and transmittance, were influenced by annealing temperature, dipping number, and cobalt concentration, illustrating the correlation between X-ray diffraction and optical properties of TiO2 thin films characterized using the sol-gel method.
In conclusion, the sol-gel method enables the straightforward incorporation of cobalt into TiO2 thin films. Initially, the as-deposited samples are amorphous, but they crystallize into the anatase phase, starting at 400°C, while maintaining their (101) preferential orientation. Additionally, our findings indicate that the anatase and brookite phases are highly dynamic, with the crystalline size of anatase and brookite increasing as the annealing temperature and dipping number increase.
The mean grain size and roughness of the samples also increase with higher annealing temperatures, increased dipping numbers, and higher cobalt concentrations. Analyzing the transmission spectra reveals that TiO2 thin films exhibit transparency in the visible spectrum but become opaque in the UV range. Furthermore, both transmission and calculated optical band gap decrease with rising temperature, increasing the number of dippings, and higher cobalt concentrations.
Conductivity values decrease with increasing Co doping levels and increase with higher annealing temperatures. The refractive index of the titanium oxide thin films increases regardless of Co concentration, temperature, or the number of dippings, while porosity decreases. Consequently, the observed changes in conductivity, absorption edge, and band gap of TiO2 enable a correlation between the results obtained from X-ray diffraction and the optical properties of TiO2 thin films characterized by the sol-gel method.
The Effects of Heat-Treatment Temperature and Co-Doping on TiO2 Thin Films. (2024, Jan 12). Retrieved from https://studymoose.com/document/the-effects-of-heat-treatment-temperature-and-co-doping-on-tio2-thin-films
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