Electrical properties of TiO2 films synthetized by sol-gel process

Titanium oxide (TiO2) is one of the most used metal oxides for its multiple applications due to its high sensitivity, fast response, low cost, no toxicity and used in hostile environments. In this work was deposited TiO2 film on 304 stainless steel substrates using the spin coating technique. The synthesis of the deposited solution was with the sol-gel method a temperature environment, obtained TiO2 films on different thermal treatments. The results of the experiment show film: with a thickness from 0.6 to 1μm, anatase phase at the plane (101). The Raman spectroscopy confirms the TiO2 anatase phase. The resistivity decreases, and conductivity increases with increasing temperature. The micrograph SEM observes a compact and smooth homogeneous surface with an average size of 18.40nm to 7nm.


Introduction
Titanium oxide (TiO2) is one of the most studied metal oxides for its multiple applications due to its high sensitivity, fast response, low cost, no toxicity, and can be used in hostile environments [1].It is an n-type semiconductor with a bandgap width of 3 to 3.6 eV, it has three distinct crystalline structures: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic).The anatase crystalline phase can be achieved at low temperatures compared to the rutile phase, it has greater reactivity and a simple structure, crystallizing from 400°C, which is why it is the most used crystalline phase as a gas-sensitive film [2].TiO2 films can be synthesized by chemical and physical methods such as: Atomic Layer Deposition (ALD) [2], hydrothermal [3], Pulse Laser Deposition (PLD) [4] and sol-gel [5].The sol-gel method has some benefits with respect to other techniques such as: low equipment cost, good homogeneity, deposits at ambient temperature, control of film thickness, porous films, among others.The precursors used for film formation are: metal alkoxide or inorganic metal salts.the inorganic metals salts in the solgel process, they create impurities and original precipitates, for the above the most used is a metal alkoxide dissolved in hydrolyzed alcohol, in an acidic, neutral, or basic environment [5]- [7] By having the TiO2 sol-gel, films can be deposited by spin coating where the centrifugal speed, time, viscosity, and evaporation of the solution influence the homogeneity of the deposited film on different substrates such as: ITO/glass, polymer and 304 stainless steel (SS304) [7], [8].SS304 is used as a substrate because it has an accessible price.In this work, TiO2 films deposited on SS304 substrates by spin coating method modifying the thermal treatment are presented.

Sol-gel synthesis of TiO2
To realize the TiO2 synthesis, a precursor of titanium tetraisopropoxide (TTIP) was used with a concentration of 0.0025 M in ethanol, hydrochloric acid (HCl) was added to this solution at a concentration of 0.0014 M.This mixture was kept under continuous stirring at a surrounding temperature for 10 minutes obtaining a transparent and homogeneous solution.This solution was deposited on 4 x 4 cm SS304 by spin coating at 1900 rpm for 20 seconds at ambient temperature.The deposited films were dried at 200°C for 8 minutes to eliminate the organic solvent.The deposit-drying process was repeated 8 times.Finally, the dried films were annealed at 500°C(T500), 550°C(T550), and 600°C(T600) for 2 hours.

Characterizations
The TiO2 phases were determined using a Bruker D8 Advance X-ray diffractometer with a CuKα radiation source.Optical characterizations were performed by Raman spectroscopy using a He-Ne laser of 632.8 nm and photoluminescence using a Horiva NanoLog FR3 equipment with a resolution of 0.06nm with slit, 1200 lines/mm grid, lifetime measurement from 100 ps to 10 µs.Profilometry measurements were carried out with a Veeco Dektak 150 profilometer, performing a 3 mm sweep over the film surface with a micro-tip.The surface morphology of the TiO2 films was analyzed in a JEOL JSM-7600F scanning electron microscope with a magnification of 50,000.Finally, the TiO2 films were tested for resistivity at room temperature by injecting a voltage of -5v to 5v and an impedance of -10 mA to 10mA.

Results and discussion
Figure 1 shows X-ray diffraction results of these films, showing the anatase phase, which agrees with PDF #01-072-7058.The diffractograms show higher intensity in the (101) plane.The crystal size was calculated theoretically with the Scherrer equation 1, using the (101) peak [6]:

Figure 1. X-ray diffractograms of TiO2 films.
Table 1 contains the thicknesses obtained by profilometry and theoretical crystal size of the TiO2 films where with increasing temperatures the crystallite size increases and the thickness decreases.Figure 3 shows the Raman spectra of the TiO2 films these spectrums present vibrational modes at Eg (144 cm -1 , 197 cm -1 , and 638 cm -1 ), B1g (399 cm -1 and 519 cm -1 ), A1g (519 cm -1 ) confirming the presence of TiO2 anatase phase [9], with the increase of temperature the vibrational modes increase their intensity.The vibrational mode with the highest intensity is Eg(144 cm -1 ) which is related to a particle size smaller than 40 nm and is confirmed by the calculated grain size of the preferential plane (101) [10].The B1g modes correspond to the vibration of titanium atoms and the A1g mode to the vibrations of oxygen atoms, the peak at 519 cm-1 is superimposed with the B1g(2) mode so the increase of A1g + B1g is due to the oxygen vacancies [11].
The conductivity was calculated theoretically with the following equation 2 [12] based on the resistivity obtained.Table 2 shows that resistivity decreases, and conductivity increases with increasing temperature.

Conclusion
The results obtained by X-ray diffraction and Raman spectroscopy confirm TiO2 films in anatase phase with preferential in-plane growth (101).The crystallinity of the films is modified as a function of temperature.The resistivity of the films decreases, and the conductivity increases with temperature.The micrographs SEM observe a compact and smooth homogeneous surface with an average size of 18.40nm to 7nm, the size decreases for the temperature.The films deposited in this work have a high potential for different applications.

Figure 2
Figure2shows the micrographs of the TiO2 films with different thermal treatments.a) the micrograph of the T500, observing a compact and smooth homogeneous surface, composed of agglomerates of hemispherical structures with an average size of 18.40 nm.b) the T550 shows a compact homogeneous surface composed of agglomerates of flakes with an average size of 6.15nm.c) T600 film shows a compact homogeneous surface composed of agglomerates of hemispherical structures with an average size of 7nm.

Table 1 .
Thicknesses and crystal size of TiO2 films. )