Visible Region Response of Cobalt Doped TiO 2 via Sol Gel Technique

TiO2 nanoparticles were prepared via sol gel technique, and then doped with cobalt ions to form TiO2 doped cobalt sol. Thin films were prepared from this sol using dip coating method and underwent calcination at a temperature of 550 C.The films were characterized by UV–Vis spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). The UV–Vis. Absorption spectra show a red shift, which indicates that TiO2 doped cobalt will responds to the visible region of spectrum rather than UVregion for pure TiO2.


1-Introduction
Titanium dioxide (TiO 2 ) is one of the most interesting n-type semiconductor materials due to its wide applications such as self-cleaning surfaces, antifogging function, antibacterial function, and decomposition of pollutants [1,2].TiO 2 has two main drawbacks it has a photocatalytic activity under UV-light irradiation only since its energy gap is wide (3.2 eV) for anatase phase, (λ= 387nm), and UV forms less than 5% of solar spectrum while the dominant spectral region of solar spectrum is visible.
Another drawback is the fast electron-hole recombination.To overcome these limitations, several efforts have been made by modifying the surface or bulk properties of TiO 2 through doping with metals and non-metals in order to reduce the band gap and delay electron-hole recombination [3] .
In this work, cobalt doped TiO 2 was studied since there are a few published papers on this transition metal.
Cobalt doped TiO 2 sol was achieved by sol gel technique, and thin films of this sol were prepared using dip coating method.

2-Experimental
Titanium tetra isopropoxide (TTIP), Ti[OCH(CH 3 ) 2 ] 4 , (purity 97%) from (Sigma-Aldrich) was used as a precursor in the preparation of TiO 2 nanoparticles sol via sol gel technique.Different weights of cobalt nitrate (0.05, 0.1, 0.15, 0.2, 0.25)gm were dissolved in (5 ml) of Ethanol.Each of these weights was added to TiO 2 sol was gotten with stirring for 24 hours and adjusting the pH to (1.5) by adding drops of HNO 3 .Hence, got cobalt doped TiO 2 sol.After that, a thin film of that sol was achieved by using dip coating method on glass substrates.The substrates with different weights of cobalt were calcined in a furnace at 550 C for three hours with rate temperature 10 C/ min.

UV-Visible Spectroscopy
Figure (1-a) shows the absorption spectra for pure and different weights of cobalt doped TiO 2 .From this figure, it is clear that with the increase of the weight content of cobalt, the absorption edge shows a red shifted (i.e.shifting towards longer wavelengths), which indicates that the samples with cobalt doped TiO 2 will be responsd to visible region, and the optimum sample is (0.2gm of cobalt doped TiO 2 ).The electrons of the six oxygen atoms in TiO 2 cell will repulse the electrons in the d-orbital of cobalt ion Co 2+ , and hence split d-orbital of cobalt ion [5].
.  The X-ray diffraction patterns of cobalt doped TiO 2 thin films show no much change as compared with that of pure TiO 2 thin film, which confirms that there is no additional phase formation.
The measured crystal sizes using Debye formula were (10.9, 12.6, 13.1, 13.6, 13.9) nm for pure TiO 2 , and (0.05, 0.1, 0.15, 0.2) gm cobalt doped TiO 2 respectively.According, we notice that the crystal size increases with the increasing the concentration of cobalt, and this due to that the radius of cobalt ion Co 2+ , (0.74 A ), which is greater than that of Ti 4+ ion (0.6 A ), so it will be on the surface of Ti 4+ ion [6]. Figure (3) shows the AFM image of the optimum sample, (0.2 gm cobalt doped TiO 2 )and thin film annealed at 550°C.As a result of the doping a clear particle structures with granular morphology were formed and this leds to the appearance of grains making the films to have higher surface roughness [7].

4-Conclusion
Pure TiO 2 and cobalt with different weights doped TiO 2 thin films were synthesized via sol gel technique and calcined at 550 .From XRD test, the samples become in anatase phase and with large crystal size after doping with cobalt since the radius of cobalt ion is greater than that of Ti ion.From UV-Vis spectroscopy, the sample doped with (0.2gm) of cobalt shows response to the visible region spectrum.AFM test shows higher levels of surface roughness.

Figure ( 1
Figure(1-b)  shows the transmittance for pure and different weights of cobalt doped TiO 2 , and it reduces with increase the weight content of cobalt.Cobalt ions (Co 2+ ) substitute (Ti 4+ ) site generates additional oxygen vacancies in the TiO 2 unit cell, which introduces extra energy levels, and hence, reduces the indirect energy band gap of TiO 2 nanoparticles[4]

Figure ( 1
Figure (1-a): The Absorption Spectra for Pure and Different Weights of Cobalt Doped TiO 2 .

Figure ( 2 )
Figure (2): XRD Patterns for Pure TiO 2 and Different Weights of Cobalt Doped TiO 2 Thin Films