Thin Film Characterization of Co-Doped Titanium Dioxide

Abstract

The study investigates the synthesis and characterization of cobalt-doped titanium dioxide (TiO2: Co) thin films using the sol-gel method. Various analytical techniques including X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and optical measurements were employed to analyze the structural, morphological, and optical properties of the thin films. The results show that Co doping influences the crystalline structure, grain size, surface roughness, and optical properties of the TiO2 films. Additionally, electrical conductivity was examined, revealing its dependence on Co concentration and annealing temperature.

1. Introduction

Titanium dioxide (TiO2) is a prominent n-type semiconductor with a wide band gap of 3-3.2 eV. It exists in different crystalline phases, with rutile being the most common. TiO2 finds applications in photocatalysis, UV-induced water and air purification, self-cleaning surfaces, antibacterial activity, and transparent electronics, among others. Cobalt doping is essential for enhancing its photochemical and electromagnetic properties, resulting in visible light absorption and ferromagnetic behavior. The sol-gel method is a cost-effective approach to synthesize Co-doped TiO2 thin films, offering excellent control over the material's properties.

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2. Experimental Details

Cobalt-doped titanium dioxide (TiO2: Co) films were prepared on glass substrates using the sol-gel technique. The precursor solution was composed of titanium isopropoxide, isopropanol, acetic acid, cobalt (II) acetate tetrahydrate, and methanol. The samples underwent a series of steps, including immersion in the sol-gel solution, drying at 100°C, and annealing at temperatures of 300°C, 400°C, and 500°C. Various characterization techniques were employed to analyze the films.

2.1 X-ray Diffraction (XRD)

XRD patterns were recorded using a copper anti-cathode with step-scanning.

The analysis revealed crystallization starting at 350°C, with the presence of anatase and brookite phases. Grain size was estimated using Scherer's formula, showing an increase with annealing temperature.

2.2 Scanning Electron Microscopy (SEM)

SEM investigations demonstrated the uniform coating of TiO2 thin films on glass substrates, with no visible cracking even for multiple dipping layers.

2.3 Atomic Force Microscopy (AFM)

AFM studies indicated that Co-doped TiO2 films had lower average surface roughness compared to undoped films. The increase in surface roughness may affect the optical properties but is influenced by Co doping.

2.4 Optical Properties

Optical properties were characterized using UV-Vis spectroscopy. The optical band gap (Eg) was calculated using the Kubelka-Munk equation. The transmittance of TiO2 thin films decreased with increasing Co content and annealing temperature. Eg decreased with increasing temperature, suggesting a correlation with grain size.

2.5 Electrical Conductivity

The electrical conductivity of Co-doped TiO2 films decreased with higher Co doping levels and reached a minimum value at 450°C for 5% Co. Conductivity increased with temperature due to improved crystallographic quality.

3. Results and Discussion

3.1 Structural Properties

3.1.1 Differential Scanning Calorimetry (DSC)

The DSC analysis showed two singularities in the thermal curves, corresponding to water evaporation, decomposition of isopropanol, and the crystallization of titanium oxide. Higher temperatures and increased Co content shifted these peaks slightly, suggesting that annealing at or above 400°C was sufficient for complete titanium oxide formation.

3.1.2 SEM Investigations

SEM analysis confirmed the homogeneous coating of TiO2 films on glass substrates without cracking, even for multiple dipping layers.

3.1.3 X-ray Diffraction (XRD)

XRD patterns indicated crystallization from the amorphous phase at temperatures above 350°C. The presence of anatase and brookite phases and their evolution with Co concentration, annealing temperature, and number of dipping layers were observed. Grain size increased with temperature and dipping layers.

3.1.4 Atomic Force Microscopy (AFM)

AFM studies revealed lower average surface roughness for Co-doped TiO2 films compared to undoped films. The change in roughness could impact the optical properties and was influenced by Co doping.

3.2 Optical Properties

UV-Vis spectroscopy revealed that TiO2 thin films were transparent in the visible range (400-800 nm) and opaque in the UV range. Transmittance increased with Co content and temperature, while the threshold absorption shifted to longer wavelengths. Eg decreased with increasing temperature.

3.3 Electrical Conductivity

The electrical conductivity of Co-doped TiO2 films decreased with higher Co doping levels and reached a minimum value at 450°C for 5% Co. Conductivity increased with temperature due to improved crystallographic quality.

4. Conclusion

In summary, Co-doping of TiO2 thin films via the sol-gel method showed that crystallization began at 350°C. Anatase and brookite phases were observed, with grain size increasing with annealing temperature and dipping layers. The optical properties, including transmittance and optical band gap, were influenced by Co concentration and annealing temperature. Electrical conductivity decreased with higher Co doping levels but increased with temperature, indicating improved crystallographic quality. This study demonstrates the potential of Co-doped TiO2 thin films for various applications, including photocatalysis and electromagnetic devices.