Structural and Electronic Impact on Various Substrates of TiO 2 Thin Film Using Sol-Gel Spin Coating Method

Titanium dioxide (TiO 2 ) thin films have been deposited on Corning 7059 glass and Fused quartz silicate substrates using Sol-Gel spinning coating technique. The effect of annealing temperature on the structure, surface morphology, optical and electrical properties of these films are characterized by Raman, XRD, FT/IR, UVvis and four-point-probes measurements. On glass substrates, there are four Raman active bands are observed: 3Anatase [A<149 cm -1 >, A<523 cm -1 > and A<646 cm -1 >] and 1 Rutile B<401 cm -1 >. On silica substrates, additional two more bands which are R<859 cm -1 > and B<1068 cm -1 > detected. The deposited films show polycrystalline nature with high XRD intensity peaks in (110), (200) and (211) orientation corresponding to anatase and rutile phases respectively with tetragonal BCC structure. The other orientations (101), (111), (210), (211), (220), (201), (002), (204) and (116) are also observed for all films with low intensities. XRD crystal sizes are found to increase with increasing annealing temperature on both substrates. Maximum crystal sizes are found to be ~31 nm on silica substrates and ~23 nm on glass substrates at 500 o C. On glass substrate, TiO 2 thin film shows the agglomeration of various non-uniform flaky-type of structures. On silica substrate, the FESEM micrographs shows the following observations: (i) particles are spherical in shape with forming different islands (ii) particles are soft agglomerates/spongy in nature with uniform surface, (iii) each spherical agglomerate contains many particles in the nanometric range and (iv) the agglomerate size is in between 40 and 110 nm. FE-SEM TiO 2 particles size distribution at 500 o C showed that the average particle size is 89.55 and 110.35 nm on glass and silica substrates respectively.


Introduction
Titanium dioxide (TiO 2 ) is the most fascinated materials in nano-technology because it has a lot of attention-grabbing properties (Castillo, et al., 2004).TiO 2 thin film research has been widely expanded due to the growing demands for optical devices, sensors, and corrosion-resistant coating and high efficiency solar cell.TiO 2 is a useful transition-metal oxide.It has three different crystallographic phases (brookite, anatase and rutile).Generally, TiO 2 is found as bulk form and thin film form as well.As it is known that bulk form of TiO 2 is a polymorphous material and there are three main crystalline structures existed: 1) Two tetragonal structures, 2) the anatase & the rutile phases, and 3) an orthorhombic structure with brookite phases as well.However, in thin film form, only anatase and rutile structures are more effortlessly synthesized (Agnarsson et al., 2013;Wiggins, Nelson, & Aita, 1996).The suitability of this material in the thin film form is mostly governed by its crystalline structure.Anatase and rutile phases are comprised of interconnected TiO 6 octahedral chains which are interrelated in different configurations, causing in diverse physical and chemical properties (Diebold, 2003;Lee et al., 2013), which are potentially eye-catching in many areas.For instance, the high temperature equilibrium, rutile phase depending on its crystal orientations, is preferred for optical and microelectronic applications due to high refractive index and dielectric constant.Anatase phase shows high photocatalytic activity which can be used in a variety of potential applications that include dye-sensitized solar cells (DSSC), photo-chemical degradation of toxic chemicals, electrode material in lithium batteries etc. (Lee & Park, 2013).
In order to accomplish the above properties, TiO 2 thin films can be prepared by a variety of methods such as pulsed laser deposition (Kim, Lee, & Im, 1999), chemical vapor deposition (Hu & Gordon, 1992), spray pyrolysis (Islam et al., 1996) and Sputtering techniques (Nezar et al., 2017).
In the present work, we demonstrate the preparation of TiO 2 thin films on corning 7059 and silicate substrates by sol-gel spin coating method.We also discuss the effects of annealing temperatures on the various properties of the TiO 2 thin films.The structural, optical, morphological and electrical properties have been investigated by XRD, Raman, FT/IR, SEM and AFM measurements.

Experimental
In the preparation of TiO 2 sol-gel 1 ml of hydrochloric acid (Hcl) mix with DI water with 1:1 ratio (i.e.Hcl: DI = 0.5 ml:0.5 ml) was prepared as Acid-Water.Titanium isopropoxide (TIP) mix with absolute ethyl alcohol (AEA) having the ratio of (TIP:AEA) 1:8 was prepared and stirring as Solvent-Solution as shown in Figure 1.Few min later, adding the pre-prepared acid water with the solvent-solution and the mixture is vigorously stirred using a magnetic stirrer for 2 hrs at room temperature to get homogeneous of TiO 2 .Then the TiO 2 sol-gel is ready for thin film deposition.The 927 diaphragm vacuum pump is connected with the spin coater in order to mount the substrate tightly on the vacuum-chuck.Due to precise dispensing of precursor-liquid uniformly onto substrate, a liquid dispenser is used.Nitrogen (N 2 ) gas cylinder as a back pressure is connected to the liquid dispenser.Sol-gel liquid holder is also connected to it with a stand just above the spin coater chuck as shown in Figure 2.With adjusting the back pressure, the amount of precursor liquid (~0.2 ml) can be dispersed on the substrate.Corning 7059 glass (Glass) and Fused quartz silicate (Silicate) substrates were cutting with the dimension of 10 mm x 5 mm and 5mm x 5 mm for different measurements.The substrates were cleaned for 15 min using acetone, 15 min using ethanol and 15 min using di-ionized (DI) water in an ultrasonic cleaner and dry-out with N 2 gun.The substrates were fixed on spin coater vacuum chuck and its constant rpm speed was adjusted at 2000 rpm (stage 1: 1000 rpm and stage 2: 1000 rpm).0.2 ml of TiO 2 solvent-solution was released from the sol-gel holder located just above the spin coater on the substrate and TiO 2 thin film deposition was performed by spinning the coater for 10 s (both stages).The experiment was repeated to grow six samples on both glass and silica substrates.All TiO 2 thin films then were dried at 25 o C for 15 min.Subsequently, all the samples on both substrates were annealed in air at different temperatures 100 o C, 200 o C, 300 o C, 400 o C and 500 o C for 1 hr each.The resultant films were characterized by using XRD, Raman, FT/IR, FESEM, AFM, UVvis, Resistivity and XPS.

Characterizations of the Films
The structural properties were investigated using an XRD instrument Bruker D8 Advance diffractometer (Bruker, Germany) with Cu Kα (λ ∼ 1.54 Å) radiation in 2θ ∼ 20 ○ − 70 o , scan rate 0.2 and 0.0484 step size.The average crystallite size, [δ], was estimated from the Full width half maximum (β) of the XRD spectra using Scherrer's formula as follows: [δ] = (1) Where κ, λ, β and θ are a constant, shape factor value as 0.9, the wavelength of X-ray (1.54 Å), the full width at half maximum (FWHM) and Bragg angle of the diffraction peak respectively.
The Raman spectra of the films were recorded using a portable iRaman (B&W TeK) with the argon ion laser having an excitation wavelength of 514 nm was used and the power was less than 5 mW.
The surface morphologies of TiO 2 films were examined by AFM (Agilent 5500 non-contact mode).The topological images were taken by Field emission scanning electron microscopy (FESEM, 1450VP, Phenom Pure).
The FTIR absorption was measured by a Thermo Scientific, Nicolet iS10 FTIR spectroscopy with the wavenumber range of 400 to 4,000 cm -1 and resolution of 2 cm -1 .
Thicknesses of the films were obtained by using a Film Sense Multi-Wavelength Ellipsometer (FS-1EX Gen. 3 Multi-Wavelength Ellipsometer System integrated on a motorized r theta 300mm sample stage for automated mapping measurements) includes 6-wavelength of 405-950 nm.The resistivity ρ of the films were measured using four point probes by the following equation: where v and I are voltage and current respectively.
The optical band gap energy of TiO 2 thin films were calculated by Tauc's equation; where h ν is photon energy and n is equal to ½ and 2 respectively.The linear variation of (α h ν) 2 vs hν at absorption edge.The Extrapolating the straight line portion of the plot (αhν) 2 versus hν for zero absorption coefficient values gives the optical band gap (Eg).
Surface roughnesses (Ra) of the films were measured using Surftest SJ-310 Mitutoyo series 178 portable surface roughness tester.

Raman Spectrum Analysis
Figure 3 shows the Raman spectra of TiO 2 thin films annealed at different temperatures (Td) in air on glass and silica substrates.It corresponds unambiguously to the rutile phase on both substrates.
On glass substrates, there are four characteristics of active peaks pointing at R<149 cm -1 >, R<401cm -1 >, R<523 cm -1 ) and R<646 cm -1 > observed.Raman intensity of all the bands was found to be increasing with the increment of annealing temperature as shown in Figure 3(b).However, on the silica substrates, additional two more bands which are R<859 cm -1 > and B<1068 cm -1 > detected.On the other hand, rutile band R<149 cm -1 > shifted to R<211 cm -1 > at higher annealing temperature of 500 o C observed.Moreover, all the Raman intensity at lower annealing temperature was found to be very low, which were also increasing sharply with the increment of annealing temperature, indicating that the crystallinity has improved evidently.
Figure 4 shows the peak position of the rutile band of Raman spectra for TiO 2 films deposited on glass and silica substrates annealed in the air for different temperatures.However, the rutile peak positions were almost unchanged for the films deposited on glass substrates as shown in Figure 4(b).However, the peak-shifts for the films deposited on silica substrates were found to be moved towards higher wavenumber at the higher annealing temperatures as shown in Figure 4.

XRD Analysis
Figure 5 shows the x-ray diffraction pattern of the TiO 2 samples deposited on glass and silica substrates as a function of annealing temperatures.Figure 5a shows XRD spectra of TiO 2 on silica substrate as a function of annealing temperature.The deposited films show polycrystalline nature with high intensity peaks in ( 110), ( 200) and ( 211) rutile phases respectively with tetragonal BCC structure.The other orientations ( 101), ( 111), ( 210), ( 211), ( 220), ( 201), ( 002), ( 204) and ( 116) are also observed for all films with low intensities.From the XRD pattern, it is observed that the overall width of the peaks decrease and intensity of the peaks increase with increasing the annealing temperature as shown in Figure 6.It is evident that XRD intensities are found to be higher values for glass substrates than silica substrates (Figure 6a).On the other hand, show polycrystalline nature with high intensity peaks in ( 110), ( 200) and ( 211) orientation corresponding to anatase and opposite relationships are observed for FWHM (Figure 6b) Figure 7 shows the XRD grain sizes as a function of annealing temperature on glass and silica substrates.Crystal sizes are found to increase with increasing annealing temperature on both substrates.Maximum crystal sizes are found to be ~31 nm on glass substrates and ~23 nm on silica substrates at 500 o C.This result is consistent with the result above.
Figure 7. shows the variation of Grain sizes of TiO2 thin film on two different substrates (Glass and Silica) as a function of annealing temperature

FT/IR Analysis
Figure 8 shows the bonding properties of thin film are analyzed by FTIR in the range of 400 to 4000 cm −1 for films deposited on glass and silica substrate as a function of different annealing temperatures.There are significant amounts of water and carbonaceous materials are present at 3000-3700 cm −1 and 1300-1800 cm −1 in all films.Spectrum of the TiO 2 film exhibits a strong, broad absorption band at 400-800 cm −1 .The presence of a broad band in this region corresponds to the formation of TiO and TiO 2 bonds which is also developing the titanium dioxide network in the films (Calzada & Del Olmo, 1990;Kamada, Kitagawa, & Shibuya, 1991).
The basis of peak broadening associated to the Ti-O bond might arise from the amorphous nature of TiO 2 thin film which is due to the amalgamation of carbon and/or hydroxyl groups into the Ti O bond system.With increasing the annealing temperatures, the broad band at 400-800 cm −1 are found to be sharpened with increasing intensity, corresponding to an increase in the degree of direct Ti O bonding.The content of water and its related materials in TiO 2 films were diminished after high annealing temperature.
Figure 8. FTIR spectra for TiO 2 films on glass and silica substrates annealed at different temperatures in Air Figure 9 shows the Ti-O peak shift for the films deposited on glass and silica substrates as a function of annealing temperature.No significant peak shift is observed for the films on silica substrates however, films on glass substrates has a drastic peak shift towards the higher wave number at the annealing temperature of 300 and 400 o C are observed.
Figure 9. Ti-O peak shift for the films deposited on glass and silica substrates as a function of annealing temperature

UV-vis Analysis
Figure 10 shows the band gap energy (Eg) for the films on glass and silica substrates with various annealing temperatures.It is found that the TiO 2 films on both glass and silica, Eg decreases with increasing annealing temperature as shown in Figure 10 below.Band gap energy (Eg) is decreasing from 4.88 eV to 3.68 eV for the films on glass substrates having the lowest value of ~3.68 eV at 500 o C. On the other hand, on silica substrates band gap energy decreases from 5.88 eV to 3.88 eV with increasing annealing temperature.At the highest annealing temperature, Eg is found to be smaller on glass than silica substrates.The main reason for the larger band gap values for the film on silica substrates might be due to an axial strain effect from lattice distortion produced by a mismatch between film and substrate lattice constant (Ong, Zhu, & Du, 2002).This effective decrease in the optical band gap is suggested to be excellent efficiency of the photocatalytic performance of TiO 2 .This decrease in the band gap energy is also due to an increase in the grain size of TiO 2 thin films as shown in Figure 7. Also, it is speculated that several sub-bands may be created due to defect levels in the forbidden band and the high temperatures accelerate the evaporation of oxygen during annealing process of TiO 2 thus reducing the band-gap energy.The variation of Eg is recommended that these films will be proper for window material for the fabrication of high efficiency solar cells (Chitra & Easwaramoorthy, 2015).

Resistivity Analysis
Figure 11 shows the variation of average resistivity of TiO 2 thin films as a function of annealing temperature.The resistivity, ρ, of the films was calculated from the Equation 2. From this figure, it is seen that the resistivity decreases with increasing the annealing temperature on both substrates having the overall higher value on the silica substrates.Therefore, the conductivity of films increased with the annealing temperature.These results are in good agreement with the XRD results.The resistivity is measured for an electric field of 1MV/cm.It is observed that the resistivity of films at 500 o C on glass and silica substrates is 1.3x107Ω-m and 1.7x107 Ω-m respectively (Angadi & Nallamshetty, 1988).
As shown in Figure 7, XRD grain size is increased as a function of annealing temperature on both substrates.It is speculated that the dislocation density is reduced as a result the electrons are easily moving from grain to grain and improved the electrical properties (Fuyuki, T., & Matsunami, 1986;Giannakopoulou et al., 2014;Matějová et al., 2016).
Figure 11.Average resistivity of TiO2 thin films as a function of annealing temperature on glass and silica substrates

SEM Analysis
Figure 12(a) and 12(b) show the surface morphologies of TiO 2 on silica and glass substrates at 500 o C using FESEM, respectively.The micrographs revealed that surface morphology depends strongly on substrate selectivity.
On glass substrate (Figure 12b), TiO 2 thin film shows the agglomeration of various non-uniform flaky type of structures.On silica substrate, the FESEM micrographs shows the following observations: (i) particles are spherical in shape with forming different islands (ii) particles are soft agglomerates/spongy in nature with uniform surface, (iii) each spherical agglomerate contains many particles in the nanometric range and (iv) the agglomerate size is in between 40 and 110 nm. Figure 13 shows the FESEM TiO2 particles size distribution on glass and silica substrates at 500 o C. Statistical analysis of FESEM data showed that the average particle size is 89.55 and 110.35 nm on glass and silica substrates respectively.As shown in these diagrams on glass substrate, the roundish-like roughness having uniform heights were observed.On the other hand, on silica substrate more homogeneous grains distribution were observed.In general, the proper surface roughness is advantageous to the nucleation formation and grain growth, resulting in larger grain size in the films was obtained at higher annealing temperature.These results are well consistent with the results of Raman and XRD measurements.The C 1s peak is located at the binding energy value of 284.6 eV for the TiO 2 film on glass substrate at 500 o C.However, it shifts to 285.7 eV for the film deposited on silica substrate observed.
Figure 15.Ti 2p and O1s core level spectra of the surface of theTiO 2 film deposited on Silica and glass substrates at 500 o C Figure 15 also shows the Ti 2p doublet of the pristine of TiO 2 films on glass substrate (binding energies at 458.5 ± 0.2 eV) which is disappeared on silica substrate.These changes could be caused by the dropping of Ti 4+ ions to Ti3+ defect states which are usually accompanied by a loss of oxygen from the surface of TiO 2 (Kitui et al., 2015;Byun et al., 2000) XPS spectrum shows O 1s peaks for the films deposited on both substrates.Lower binding energy at 529.7 ± 0.2 eV and the other 532.8 ± 0.2 eV are attributed for the TiO 2 films on glass and silica substrates respectively.

Growth Mechanism
As discussed before, several mechanisms are responsible for determining the crystalline properties of TiO 2 thin film on glass and silica substrates should be considered.However, one or more of these mechanisms may be jointly responsible for the crystalline properties.As shown in Figures.3-7 & 13-14, a high-quality TiO 2 thin film can be deposited on a substrate with a smooth surface.
However, the surfaces of the resultant TiO 2 thin films with the highest crystalline quality are likely to be the most lowly roughened (Figure 14a), due, most likely to the higher annealing temperature values.Similar results have also been observed for TiO 2 thin films on silica substrates.Further-more, a similar improvement in the crystalline quality, corresponding to the smooth surface of the TiO 2 thin film, has been observed.The optimum conditions for obtaining improved TiO 2 thin films have been found to occur, as the annealing temperature is increased as 500 o C on silica substrates.
Accordingly, considering substrates-dependent, better nucleation due to smooth surface on silica substrates if annealing temperature rises above 300 o C, the roughness of the substrate may decrease further, improving the crystalline quality of the TiO 2 on silica substrate.Furthermore, as stated, the crystalline quality of the resultant TiO 2 films has been reported to depend strongly on the shape of the roughened surface of the substrate as well as the degree of roughness.These results, due to mechanism reported before will be one reason why opposite relationships between the substrate roughness and the crystalline qualities have been reported.In addition, note that each peak, as shown in Figure 5, may be composed of a number of crystal grains.Based on the results shown in Figs. 3 and 5, the number of O atoms incorporated decreases with increasing annealing temperature, corresponding to the improved crystalline qualities of the TiO 2 films.
Furthermore, as shown in Figure 8, for the films with low annealing temperature, enhanced incorporation of O atoms is obtained after air exposure.Therefore, these O atoms will be imported from air after film deposition.We believe the change in the O atom density with increasing annealing temperature to be independent of the effects of the chemical cleaning for O contamination during film growth.In addition, the results obtained by increasing the annealing temperature on silica substrate and those obtained on glass substrate as stated above, may minimize the effectiveness of the chemical cleaning effect.Although we believe that surface roughness contributes predominantly to the control of the crystalline properties.

Conclusion
1).Titanium dioxide (TiO 2 ) thin films have been deposited on glass and silica substrates using Sol-Gel technique.
2).XRD intensities are found to be higher values for glass substrates than silica substrates and opposite relationships are observed for FWHM .
3).XRD crystal sizes are found to increase with increasing annealing temperature on both substrates.Maximum crystal sizes are found to be ~31 nm on silica and ~23 nm on glass substrates at 500oC.This result is consistent with the Raman result.
4).FESEM micrographs, on glass, TiO 2 thin film shows the agglomeration of various non-uniform flaky types of structures.On silica substrates, shows the particles with spherical shape forming different islands, particles of soft agglomerates/spongy in nature with uniform surface and the agglomerate size is in between 40 and 110 nm.
5).Statistical analysis of FESEM TiO 2 particles size distribution data showed that the average particle size is 89.55 and 110.35 nm on glass and silica substrate respectively.

Figure 1 .
Figure 1.Overall steps to prepare TiO 2 sol-gel for this experiment

Figure 2 .
Figure 2. Two-stage spin-coater used in this experiment

Figure 3 .
Figure 3. Raman spectra for TiO 2 films deposited on a) Silica and b) glass substrates as annealed in air for different temperatures

Figure 5 .
Figure 5. XRD spectra for TiO2 films annealed at different temperatures in presence of air

Figure 10 .
Figure 10.Band gap energy of TiO 2 thin Films on glass and silica substrates with different annealing temperatures

Figure 12 .
Figure 12.FESEM images of TiO2 thin films deposited on (a) Silica and (b) Glass substrates at 500 o C

Figure 13 .
Figure 13.FESEM particles size distribution of TiO 2 thin film on (a) Silica and (b) Glass substrates at 500 o C

Figure 14 .
Figure 14.AFM micrographs of TiO2 thin films at 500oC on (a) Silica and (b) Glass substrates

Table 1 .
Wet cleaning Conditions used to disinfect substrates and remove any fragments of material of oil