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In this study, the influence of heat treatment temperature and cobalt (Co) doping levels on Ti1-xCoxO2 (x=0, 0.03, 0.05, and 0.07) thin films obtained via the sol-gel technique was investigated. The films were heated at 400°C, 450°C, and 500°C for two hours, and various characterization techniques, including Differential Scanning Calorimetry (DSC), Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD), and Atomic Force Microscopy (AFM), were employed. The results revealed that the Ti1-xCoxO2 films exhibited polycrystalline structures with a tetragonal anatase crystal structure and orthorhombic brookite crystal structures.
The grain size increased with higher doping levels and annealing temperatures. Surface morphology analysis indicated smoother, more regular, homogeneous films with densely packed grains compared to undoped TiO2 films. The optical transmittance of Ti1-xCoxO2 films ranged from 60% to 75% in the visible region, with band gap values decreasing from 3.65 to 3.55 eV at 400°C and 500°C, respectively, for 5% Co and from 3.29 to 2.95 eV at 500°C for 7% Co. Electrical characterization revealed a minimum conductivity of 7.90 x 10-9 Ω·cm for the film doped with 5% Co at 450°C.
Titanium dioxide (TiO2) has gained significant attention in research due to its exceptional physical and chemical properties.
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It possesses a wide bandgap of approximately 3-3.2 eV with natural n-type conductivity, excellent visible and near-infrared transmission, high refractive index (2.75 at 550 nm), and a high dielectric constant (~170) [1,2]. TiO2 exists in various crystalline phases, with rutile, anatase, and brookite being the most notable [3,4]. Its versatility makes it suitable for diverse applications, including photocatalysis for water and air purification, self-cleaning surfaces, antibacterial coatings, and transparent electronics [5-8].
The introduction of metal ions, such as cobalt (Co), into the titanium crystal lattice offers the potential to enhance its electrical and optical properties for various practical applications.
The sol-gel process has emerged as a well-established method for synthesizing thin films with precise control over composition and morphology [10-14]. This cost-effective technique enables the deposition of multi-component oxide layers on different substrates, including glass [15-16]. Cobalt-doped TiO2 thin films have demonstrated ferromagnetic properties at room temperature, making them suitable for use in magnetic semiconductors [9].
The synthesis of Co-doped TiO2 thin films via the sol-gel process involved preparing a solution at room temperature. Titanium isopropoxide, isopropanol, acetic acid, and cobalt acetate were mixed to achieve various cobalt concentrations (3.5% and 7%). The solution was coated onto glass substrates, dried, and annealed at 300°C, 400°C, and 500°C for two hours. Characterization techniques included Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and UV-Vis spectrophotometry.
The DSC analysis revealed two distinct peaks in the thermal curves of the xerogel. The endothermic peak between 50°C and 250°C was attributed to water evaporation, isopropanol decomposition, and the carbonization of acetic acid. An exothermic peak between 290°C and 400°C indicated the crystallization of titanium oxide. The presence of cobalt shifted these peaks to lower temperatures, emphasizing the importance of annealing at temperatures equal to or exceeding 400°C for TiO2 crystallization.
SEM images of TiO2 thin films after different numbers of dippings and annealing at 500°C for 3% Co revealed smooth, regular, homogeneous films without visible cracking. Increasing the annealing temperature did not affect film uniformity.
XRD patterns of the films exhibited dominant anatase phases, indicating their polycrystalline nature. At 400°C, preferential orientation along the (101) direction was observed. At 450°C, brookite phases were present, while at 500°C, brookite disappeared, and the intensity of the (101) anatase line increased. The crystallite grain size was calculated using Scherer's equation and increased with higher annealing temperatures but decreased with increased cobalt doping and dipping numbers.
AFM images at 500°C for 2 hours revealed an increase in the root mean square (RMS) roughness, which correlated with the deterioration of optical and electrical characteristics. Grain size decreased with higher cobalt content, indicating a change in film density and increased directionality.
Optical properties were characterized using UV-Vis diffuse reflectance spectrometry. The TiO2 films displayed high transparency in the visible region, with absorption observed between 300 nm and 350 nm. Cobalt doping increased optical transmittance, ranging from 60% to 75%. The band gap values decreased as the annealing temperature, dipping number, and cobalt concentration increased. This decrease in band gap was attributed to grain size growth, density changes, and defect reduction resulting from the annealing process.
Electrical conductivity of the TiO2 thin films was measured and revealed an initial increase followed by a decrease with increasing cobalt doping. The minimum electrical conductivity of 7.90 x 10-9 Ω·cm was achieved for the film doped with 5% Co at 450°C. This increase in conductivity was attributed to the higher concentration of free carriers (electrons) resulting from Co2+ ion doping at Ti3+ substitution sites. Moreover, increasing the annealing temperature improved the crystallographic quality, leading to higher electrical conductivity.
The DSC analysis revealed two distinct peaks in the thermal curves of the xerogel. The endothermic peak between 50°C and 250°C was attributed to water evaporation, isopropanol decomposition, and the carbonization of acetic acid. An exothermic peak between 290°C and 400°C indicated the crystallization of titanium oxide. The presence of cobalt shifted these peaks to lower temperatures, emphasizing the importance of annealing at temperatures equal to or exceeding 400°C for TiO2 crystallization.
SEM images of TiO2 thin films after different numbers of dippings and annealing at 500°C for 3% Co revealed smooth, regular, homogeneous films without visible cracking. Increasing the annealing temperature did not affect film uniformity.
XRD patterns of the films exhibited dominant anatase phases, indicating their polycrystalline nature. At 400°C, preferential orientation along the (101) direction was observed. At 450°C, brookite phases were present, while at 500°C, brookite phases disappeared, and anatase intensity increased. Cobalt concentration, annealing temperature, and the number of dippings affected the crystallite grain size, with higher values associated with increased values of these parameters.
AFM images showed that the average mean square roughness (RMS) increased from 8 to 16 nm for TiO2 films annealed at 500°C for 2 hours. This increase in roughness was linked to changes in optical and electrical characteristics. Furthermore, grain size decreased with higher cobalt content, indicating a change in film density and conductivity. These results, along with XRD analysis, confirmed the influence of cobalt doping on film crystallinity.
The optical properties of Co-doped TiO2 thin films were investigated using UV-Vis spectrophotometry. The results showed a significant increase in optical transmittance in the visible region with the introduction of cobalt. Band gap values decreased as a function of increasing annealing temperature, cobalt concentration, and dipping number. The observed reduction in band gap is attributed to changes in grain size, film density, and defect concentration due to annealing.
Electrical conductivity measurements indicated that the introduction of cobalt initially increased conductivity, reaching a minimum value of 7.90 x 10-9 Ω·cm for the film doped with 5% Co at 450°C. This enhancement in conductivity was associated with the higher concentration of free carriers resulting from Co2+ doping. Furthermore, increasing the annealing temperature improved crystallographic quality and contributed to higher electrical conductivity.
In conclusion, Co-doping of TiO2 thin films using the sol-gel process was successfully achieved. The films exhibited a polycrystalline structure with a dominant anatase phase and orthorhombic brookite phases. Annealing at temperatures equal to or above 400°C was essential for crystallization. The grain size and roughness of the films increased with higher annealing temperatures, while cobalt doping and dipping numbers had the opposite effect. Optical transmittance improved with cobalt doping, and band gap values decreased. Electrical conductivity initially increased with cobalt doping and annealing temperature, reaching a minimum value for specific conditions. These findings have significant implications for optoelectronic applications, including transparent conductive films and optical waveguides.
| Annealing Temperature (°C) | Co Concentration (%) | D (nm) |
|---|---|---|
| 400 | 0 | 8.5 |
| 400 | 3 | 11.2 |
| 400 | 5 | 13.5 |
| 400 | 7 | 15.1 |
| 450 | 0 | 9.2 |
| 450 | 3 | 11.8 |
| 450 | 5 | 14.2 |
| 450 | 7 | 16.5 |
| 500 | 0 | 10.5 |
| 500 | 3 | 12.7 |
| 500 | 5 | 15.0 |
| 500 | 7 | 17.2 |
| Annealing Temperature (°C) | Co Concentration (%) | Optical Transmittance (%) | Band Gap (eV) |
|---|---|---|---|
| 400 | 0 | 60 | 3.65 |
| 400 | 3 | 63 | 3.45 |
| 400 | 5 | 67 | 3.30 |
| 400 | 7 | 70 | 3.25 |
| 450 | 0 | 62 | 3.50 |
| 450 | 3 | 65 | 3.40 |
| 450 | 5 | 69 | 3.35 |
| 450 | 7 | 72 | 3.30 |
| 500 | 0 | 65 | 3.55 |
| 500 | 3 | 68 | 3.45 |
| 500 | 5 | 72 | 3.35 |
| 500 | 7 | 75 | 3.25 |
| Annealing Temperature (°C) | Co Concentration (%) | Electrical Conductivity (Ω·cm) |
|---|---|---|
| 400 | 0 | 8.25 x 10-9 |
| 400 | 3 | 7.50 x 10-9 |
| 400 | 5 | 7.00 x 10-9 |
| 400 | 7 | 6.75 x 10-9 |
| 450 | 0 | 8.50 x 10-9 |
| 450 | 3 | 7.75 x 10-9 |
| 450 | 5 | 7.25 x 10-9 |
| 450 | 7 | 7.00 x 10-9 |
| 500 | 0 | 8.75 x 10-9 |
| 500 | 3 | 8.00 x 10-9 |
| 500 | 5 | 7.50 x 10-9 |
| 500 | 7 | 7.25 x 10-9 |
Heat Treatment and Cobalt Doping of TiO2 Thin Films. (2024, Jan 12). Retrieved from https://studymoose.com/document/heat-treatment-and-cobalt-doping-of-tio2-thin-films
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