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CHAPTER 3

PREPARATION AND CHARACTERIZATION OF TiO2

THIN FILMS

3.1 INTRODUCTION

Nanocrystalline TiO2 thin films have been prepared by different

researchers using various techniques, such as evaporation (Toshihiro Miyata

et al 2006), spray pyrolysis (Okuya et al 2004), chemical vapour deposition

(Giovanni et al 1994), sputtering (Hossain et al 2008), laser ablation (Nicolas

Martin et al 1997) and sol-gel method (Verma et al 2005, Sabataityte

et al 2006, Guo et al 2005, Harizanov et al 2000, Yu et al 2000). In the

present study nanocrystalline TiO2 thin films have been prepared by sol-gel

dip coating technique. Dip coating technique has many advantages over other

techniques. This technique is simple and the deposition can be carried out at

room temperature. This method can be easily used for large area deposition at

low cost and the thickness of the deposited layer can be readily controlled by

varying the length of the deposition time.

In the present study TiO2 thin films have been prepared using

titanium tetra isopropoxide and isopropanol by the simple sol–gel dip coating

method at room temperature. The oxide network of TiO2 is formed by

hydrolysis and condensation of alkoxides followed by polymerization at

elevated temperatures. The ease of decreasing or increasing the chain length

of the alkyl group is being done easily by hydrolyzation of titanium

alkoxides. These reactions give polymeric species with –OH– or –O– bridges.



36

The coordination of water to the metal is the first hydrolytic step, a proton on

H2O could then interact with the oxygen of an OR group through hydrogen

bonding leading to hydrolysis. The high reactivity of alkoxide water causes

instantaneous precipitation. The stability of the sol can be increased byreplacing water with acetic acid. Here acetic acid acts as a catalyst and also as

a chelating agent. Acetic acid induces the initiating process of hydrolysis

mechanism. Acetic acid is also used to modify the precursor’s molecular

structure (Sanchez et al 1992). Many factors such as crystallinity, particle

size, surface area, amount of solvent used and preparation method strongly

affects the properties of the prepared TiO2 thin films (Wang et al 1999,

Adachi et al 2003).

3.2 PREPARATION OF TiO2 THIN FILMS

TiO2 thin films with anatase phase have been prepared using

titanium tetra isopropoxide and isopropanol by the simple sol–gel dip coating

method at room temperature. An attempt has been made to control the

hydrolysis/condensation reaction in the sol–gel of TiO2 by using acetic acid.

The properties of TiO2 films are not only influenced by the deposition

technique but also by the post-annealing treatment. The solution was prepared

by mixing 1.5ml of titanium tetra isopropoxide (Alfa Aaser 99.9%) with 15ml

of isoproponal (Aldrich 99.9%) at room temperature and stirred for half an

hour. To this solution 0.365ml of glacial acetic acid was added drop wise and

stirred vigorously for 2 hours to obtain a homogeneous mixture of TiO2 sol.

The pH of the solution is 5. Using the prepared solution thin films of TiO2 were

obtained on ITO glass plates by dip coating method. The ITO glass plates were

ultrasonically cleaned using acetone, ethanol and de-ionized water. It should be

noted that cleaning of the substrate is important for proper adhesion of the films.

TiO2 thin films have been coated on ITO glass plates and heated at 80°C for 10

minutes and then allowed to cool to room temperature.



37

Titanium isopropoxide Isopropanol

Stirring for30 minutes

Adding acetic acid

Stirring for 2 hours

TiO2 sol

Dip coating

Annealing at differenttemperature

TiO2 thinfilms

The dip coating, heating and cooling process was repeated five

times in order to get thicker films. The thickness of the films has been

determined using surface profilometer and was in the range of 0.8 – 0.83 m.

The heating of the film after each deposition in air is carried out to enhance

the inorganic polymerization and stabilize the formed mesophases. The films

were then dried at room temperature for 2 hrs and then annealed in air. The

films have been annealed at 400°C, 450°C and 500°C for 1 hr using a heating

rate of 2°C/min. TiO2 thin films were uniform and homogeneous. The flow

chart showing the procedure used for preparing TiO2 thin film is shown in

Figure 3.1.

Figure 3.1 Flow chart depicting the preparation of the TiO2 thin films



38

3.3 STRUCTURAL PROPERTIES

The structural properties of the prepared TiO2 films have been

studied using X-ray diffraction method, scanning electron microscope, atomic

force microscopy and high resolution transmission electron microscope. The

composition of the films have been analyzed using energy dispersive X-ray

analysis. During the formation process of TiO2, mono Ti–OH is formed

during the hydrolysis reaction and Ti–OH aggregation results in the formation

of crystalline nuclei and the primary particle size depends on the aggregation

degree. Finally the particles in the film are oxidized and turned into oxide

form during calcination at higher temperature and this promotes the formationof Ti–O–Ti anatase bonds.

The X-ray diffraction studies revealed that the as deposited films

are amorphous in nature. The prepared TiO2 films have been annealed at

400°C, 450°C and 500°C. The X-ray diffraction pattern of the TiO2 films

annealed at different temperatures is shown in figure 3.2. The diffraction

pattern shows a peak at 25.31 degrees which corresponds to the (101) plane ofthe anatase phase of TiO2. The presence of very small peaks in the 400°C

annealed film shows that nucleation of grain has started. The presence of

sharp peaks in the 450°C and 500°C annealed samples shows that the grains

have started to grow on annealing. The observed peaks corresponds to (1 0 1),

(0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) planes. No peaks corresponding to

the rutile or brookite phase has been observed in the X-ray diffraction pattern.

The diffraction pattern displays the coexistence of both amorphous and

crystalline TiO2 regions and this is observed by simultaneous presence of the

broad hump in the low 2 region demonstrating short range order and

amorphicity. It should be noted that the heat treatment temperature above

400°C makes the intensity of the peak to increase, with reduction in full width

at half maximum (FWHM). This is due to the transformation of TiO 2 from

amorphous to crystalline nature on heat treatment.



39

10 20 30 40 50 60 70 80

450 C

500 C

400 C

( 2 0 4 )

( 2 1 1 )

( 1 0 5 )

( 2 0 0 )

( 0 0 4 )

( 1 0 1 )

I n t e n s i t y ( a . u . )

2 (degrees)

Figure 3.2 X-ray diffraction pattern of TiO2 thin films annealed at

different temperatures

The lattice parameter values a and c have been calculated using the

relation,

2

2

2

22

2

1

c

l

a

k h

d (3.1)

The lattice parameters have been calculated and were found to be

a = 3.754 Å and c = 9.526 Å and the values are in good agreement with the

standard values (JCPDS No. 21-1272, a = 3.728 Å and c = 9.513 Å). The

crystallite size has been calculated using scherrer’s formula

cos

k D (3.2)



40

where, D is the grain size, K is a constant taken to be 0.94, is the

wavelength of the x-ray radiation, is the full width at half maximum and isthe angle of diffraction.

Table 3.1 shows the variation in grain size with annealing

temperature. Grain size is found to increase with annealing temperature.

Table 3.1 Grain size of TiO2 thin films annealed at different temperatures

Annealing

temperature (°C)

Grain

Size(nm)

400 11

450 19

500 28

Figure 3.3 shows the scanning electron microscope images of TiO2

thin films annealed at 400°C 450°C and 500°C. The films annealed at 400°C

contain pores and grains of smaller size. When the annealing temperature is

raised to 450°C and 500°C, the grain size is found to increase. The SEM

image of the TiO2 film shows the formation of nanosized smooth grains all

over the surface with meso-sized pores in between the grains. Such type of porous structure resulted due to the nucleation and coalescence of small

particles. The films are observed to have smooth surface morphology without

any cracks.



41

Figure 3.3 SEM images of TiO2 thin films annealed at a) 400°C,b) 450°C and c) 500°C

Energy dispersive x-ray analysis (EDAX) pattern of TiO2 thin films

annealed at 400°C, 450°C and 500°C are shown in Figures 3.4(a, b and c).

The origin of copper is from the copper coating on the film. The pattern

shows that the 400°C annealed TiO2 film has a composition of Ti - 30.35 at %

and O - 69.65 at %. The 450°C annealed film has a composition of Ti- 33.33

at % and O - 66.67 at %, and 500°C annealed film has a composition of Ti-

35.23 at % and O - 64.77 at %. As the annealing temperature increases there

is a decrease in oxygen content and this may be due to the evaporation of

oxygen on annealing.

(b)

(c)

(a)



42

Figure 3.4 EDAX pattern of TiO2 thin films annealed at (a) 400°C

(b) 450°C (c) 500°C

Figures 3.5(a, b and c) show the atomic force microscope images of

the dip-coated TiO2 thin films prepared and annealed at 400°C, 450°C and

I n t e n s i t y

( a .

u )

Energy (KeV)

Ti=33.33%

O=66.67%

(b)

I n t e n

s i t y

( a .

u )

Energy (KeV)

Ti=30.35%

O =69.65%

(a)

Ti = 35.23% O

= 64.77%

(c)

I n t e n s i t y

( a . u

)

Energy (KeV)



43

(a) (b)

(c)

500°C respectively. The image shows well-defined particle-like features with

granular topography and indicates the presence of small crystalline grains.

Because of the heat treatment, the nano crystalline phase has been formed and

this has led to the appearance of grains making the films to have higher surfaceroughness. Evaluation of surface pattern was conducted by estimating the

roughness of the film (root mean square). The atomic force microscope image

shows the formation of mesoporous TiO2 films. It can be seen that the particles

are distributed homogeneously with porosity consistent with a high surface area

structure. The root mean square surface roughness of the film was found to be 19

nm, 27nm and 32nm for the TiO2 films annealed at 400°C, 450°C and 500°C.

Figure 3.5 AFM images of TiO2 thin films annealed at a) 400°C,b) 450°C and c) 500°C



44

High-resolution transmission electron microscopy was used to

investigate the microstructure of TiO2 nanocrystalline films. For high-

resolution transmission electron microscopy analysis, the TiO2 film was

peeled off from the substrate and dispersed in acetone. The solution was

sonicated for 30 minutes. The high-resolution transmission electron

microscope images of the dip coated TiO2 films annealed at 400°C, 450°C

and 500°C are shown in Figures 3.6, 3.7 and 3.8. Figures 3.6 (a, b and c)

shows the high resolution transmission electron microscopy images of TiO2

film annealed at 400°C. Figure 3.6 (a) shows the closely-packed

agglomeration of the nanoparticles in the mesoporous structure. This

accumulation of nanoparticles creates narrow channels that may serve as

electronic injection membranes. It can be seen from figure 3.6(b) that the size

of the nano particles is extremely uniform. Figure 3.6(c and d) shows lattice

fringes and the interplanar distance is measured to be 0.35 nm. Figures 3.7and 3.8 shows the high-resolution transmission electron microscope images of

the films annealed at 450°C and 500°C. For films annealed at 450°C and

500°C the inter planar distance is measured using lattice fringes of

Figures 3.7(c) and 3.8(c) and the value is found to be 0.35 nm and 0.36 nm.

The obtained interplanar spacing values correspond to the (101) plane of

anatase phase TiO2. HRTEM images shows that there is an increase in grainsize with increase in annealing temperature.



45

(d)

2nm

(a)

5 nm

10 nm20 nm

(b)

(c)

0.35nm

(101)

Figure 3.6 HRTEM image of 400°C annealed TiO2 thin film



46

(d)(c)

20 nm

5nm 2nm

10nm

(a) (b)

0.35nm(101)




47

20nm 10nm

2nm

(a) (b)

(c) (d)

0.36nm

5nm

(101)


Figures 3.9 (a, b and c) shows the selected area electron diffractionimages of TiO2 nanocrysalline thin films annealed at 400°C, 450°C and

500°C. Selected area electron diffraction pattern is used to study about the

crystal properties of a particular region. The presences of rings with discrete

spots suggest that the TiO2 nanocrystalline film is made of small particles of

uniform size. Ring patterns corresponding to planes: (1 0 1), (0 0 4), (2 0 0),

(1 0 5), (2 1 1) and (2 0 4) are consistent with the peaks observed in XRD



48

(101)

(105)

(004)

(211)

(a)

(200)

(204) (204)

(200)

(004)

(101)

(105) (211)

(c)(204)

(200)

(004) (101)

(105) (211)

(b)(204)

patterns. The distance from centre to each ring (moving from smallest to

larger ones) is measured to be 2.810, 4.208, 5.155, 6.024 and 6.756 nm

respectively for film annealed at 400°C, 2.840, 4.232, 5.154, 5.847, 6.015 and

6.752 nm respectively for film annealed at 450°C and 2.717, 4.202, 4.950,5.617, 5.992, and 6.743 nm for film annealed at 500°C respectively. It is clear

from Table 3.2 that d values calculated from selected area electron diffraction

patterns of nanocrystalline TiO2 thin films are matching well with standard d

values of JCPDS data. The analysis of SAED patterns reveals the existence of

anatase phase without any secondary (brookite and rutile) or impurity phase.

Figure 3.9 Selected area electron diffraction images of TiO2 thin filmsannealed at a) 400°C, b) 450°C and c) 500°C

Table 3.2 Calculated ‘ d ’ spacing value of different planes of TiO2 thin films

Calculated ‘d ’ values from TEM

ÅReported ‘d ’ value

(JCPDS) Planes(hkl)

400°C 450°C 500°C

3.55 3.52 3.68 3.52 (101)

2.37 2.36 2.38 2.37 (004)

1.94 1.94 2.02 1.89 (200)

1.70 1.71 1.78 1.69 (105)

1.66 1.66 1.66 1.66 (211)

1.48 1.48 1.48 1.48 (204)



49

3.4 FOURIER TRANSFORM INFRA RED SPECTROSCOPY

Fourier Transform Infrared (FTIR) spectroscopy is a promising

method for observing molecular vibrations. Figure 3.10 shows the FTIR

spectra of TiO2 thin films annealed at 400°C, 450°C and 500°C. The thin

films annealed at 400,450 and 500°C have peaks above 3500 cm-1 which

represents the O–H stretching of hydrogen bonds present on the anatase TiO2

surface. A reaction with a hydroxyl anion is thought of as the initial step of

the photo oxidation of water. Since the TiO2 sol layer has relatively small

hardness and soft surface, it seems to be easy to dissociate the water or

oxygen molecules from the sol surface. The hydrate water vibration peak at

1550 cm-1 is still observed for all the three temperatures. This may be related

to water bound to TiO2 or may be due to the presence of reduced oxidant salt

species or TiO2 in a hydrate form (Lu et al 2006). The films annealed at

400°C exhibits a band at 3718 cm-1 which is not observed for the films

annealed at 450°C and 500°C respectively, and this is due to the removal of

hydroxide group after annealing at higher temperatures. The intense band

below 1550 cm-1 is due to Ti-O-Ti vibrations. The slight shift of the band to

the lower wave numbers and sharpening of the Ti-O-Ti band on annealing

may be due to the increase in size of the nanoparticles. In addition, the surface

hydroxyl groups in TiO2 increases with the increase of annealing temperature.

There is no band centered at 1389 cm-1 which clearly shows the absence of

the C-H bond in the TiO2 films (Wang et al 2001). Also, there are no

additional bands present in the spectra corresponding to the alkoxy groups.

This reveals that the addition of acetic acid has not introduced any residual

impurities on the TiO2 surface.



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4000 3500 3000 2500 2000 1500 1000

38423718 3610 2715

23371550

400ºC

948

450ºC

3842 29241735 1527 1373

956

500ºC

956

1550173525463842

T r a n s m i t t a n c e

( a . u . )

Wavenumber (cm-1)

Figure 3.10 FTIR spectra of TiO2 thin films annealed at 400°C, 450°Cand 500°C

3.5 RAMAN SPECTRA

Raman spectroscopy technique is a non-destructive method capable

of elucidating the titania structural complexity as peaks from each crystalline

phase is clearly separated in frequency, and therefore the anatase and rutile

phases are easily distinguishable. Figure 3.11 shows the Raman spectra of the

nano crystalline TiO2 samples annealed at 400°C, 450°C and 500°C. Raman

peaks originate from the vibration of molecular bonds, that is, vibrational

mode Eg, B1g and A1g peaks, which are related to different crystal planes.

According to factor group analysis, anatase has six Raman active modes (A1g

+ 2B1g + 3Eg). Ohsaka et al (1980) have reported the Raman spectra of

anatase TiO2 and have stated that six allowed modes appear at 144 cm 1 (Eg),

197 cm 1 (Eg), 399 cm 1 (B1g), 513 cm 1 (A1g), 519 cm 1 (B1g), and 639 cm 1

(Eg). The Raman spectra of the prepared nano crystalline TiO2 samples has

vibrational peaks at 143 cm 1, 197 cm 1, 396 cm 1, 519 cm 1 and 638 cm 1.

The absence of overlapping broad peaks show that the material is well

crystallized, with low number of imperfect sites. These peaks are



51

unambiguously attributed to the anatase modification. A special attention

must be given to the Raman peaks observed at 143 cm 1, 197 cm 1, 396 cm 1,

519 cm 1, 638 cm 1which are all slightly shifted, probably due to a smaller

size of TiO2 nano crystalline particles. The peaks corresponding to the rutile phase was not detected in the Raman spectra. This result agrees with Jorge

Medina - Valtierra et al 2006 who have observed that only the typical bands

of anatase are present in the spectra of TiO2 films annealed at temperatures

below 700°C.

TiO2 nanoparticles have been frequently investigated using Raman

spectroscopy because of the unusual band broadening and shifts of Raman

bands with decreasing particle size. However, there is no general agreement

about the origin of the broadening and shifts of the Raman bands. Xu et al

(2001) have tried to explain the variation in the Raman bands with a phonon

confinement model. On the basis of the Heisenberg uncertainty principle, the

relationship between particle size and phonon position can be expressed as

follows:

x. p 2/4 (3.3)

where x is the particle size, p is the phonon momentum distribution, and 2

is the reduced Planck’s constant. As the particle size decreases, the phonon is

increasingly confined within the particle and the phonon momentum

distribution increases. This broadening of the phonon momentum leads to a broadening of the scattered phonon momentum according to the law of

conservation of momentum. This phonon dispersion causes asymmetric

broadening and may lead to a shift of the Raman bands. From the Raman

spectra it is understood that the main spectral features of TiO2 films annealed

at different temperatures are closely similar which mean that the prepared

samples posses a certain degree of long range order of the anatase phase. The

spectra vary symmetrically with grain size. There is spectroscopic line



52

200 400 600 800 1000

500 C450 C

A1g

B1g

Eg

Eg

Eg

I n t e n s i t y ( a . u

)

Raman shift cm-1

400 C

broadening and merging, to line intensity decreases and there is position

shifting with decrease in annealing temperature. The full-widths at half-

maximum (FWHM) of the bands are given in Table 3.3. Comparing the three

Raman spectra, it is clear that the Raman bands shift towards higher wavenumber and their intensities relatively increase as the particle size increases.

However, the band broadening is very small and is insignificant.

Figure 3.11 Raman spectra of TiO2 film annealed at 400°C, 450°C and500°C

Table 3.3 FWHM of the peaks present in Raman spectra

Temp °C Eg Eg B1g A1g Eg

400 8 4 20 17 23

450 12 6 21 23 25

500 26 8 24 26 27



53

3.6 OPTICAL STUDIES

The optical properties of TiO2 films have been studied using the

transmittance spectra recorded using a UV-Vis spectrophotometer.

Figure 3.12 shows the optical transmittance spectra of the TiO2 films

annealed at different temperatures. A significant decrease in the transmittance

below 500nm wavelength can be assigned to absorption of light caused by the

excitation of electrons from the valence band to the conduction band of

titania. It should also be noted that the transmittance of the films decreases

with increase of the heat treatment temperature. This may be due to the

formation of larger particles on the surface of TiO2 thin film, which causes

the scattering of light. The UV–Vis transmittance spectra shows that the

absorption edge shifts to longer wavelength with increase in heat treatment

temperature and the red-shift of absorption edge can be attributed to the

increase in grain size of TiO2 films on annealing.

Using the optical transmittance spectra, the absorption coefficient

and the band gap of TiO2 films have been determined. The absorption

coefficient was calculated using the relation, =ln (1/T)/t, where T is the

transmittance and ‘t’ the thickness of the film. The absorption coefficient ( )

of the TiO2 films is of the order of 105cm-1. The band gap energy has been

calculated by plotting ( ) 2 versus the photon energy (h ), and extrapolating

the linear portion of the curve to intercept the h axis. Plot of ( ) 2 versus

photon energy (h ) of TiO2 films annealed at different temperatures is shown

in Figure 3.13.



54

0 500 1000 1500 2000 2500

20

40

60

80

100

500 C

450 C

400 C

T r a n s m i t t a n c e ( %

)

Wavelength (nm)

2.5 3.0 3.5 4.00.00E+000

4.00E+013

8.00E+013

1.20E+014

1.60E+014

2.00E+014

450 C

500 C

450 C

( a h ) 2 ( e V / m ) 2

h (eV)

Figure 3.12 Transmittance spectra of the TiO2 thin films annealed at400°C, 450°C and 500°C

Figure 3.13 Plot of ( ) 2

versus (h ) of TiO2 thin films annealed at400°C, 450°C and 500°C



55

Table 3.4 Band gap value of the TiO2 films annealed at differenttemperatures

Annealing temperature

(°C)

Band gap

(eV)400 3.73

450 3.69

500 3.61

The obtained band gap values of the TiO2 film annealed at different

temperatures are given in Table 3.4. It is found that the band gap energy of

the film decreases with increase in annealing temperature. This is due to the

increase in grain size on annealing. The estimated values of band gap are in

agreement with the reported values (Boschloo et al 1997).

3.7 PHOTOLUMINESCENCE STUDIES

Figure 3.14 shows the photoluminescence spectra of the

nanocrystalline TiO2 thin films annealed at 400°C, 450°C and 500°C. The

excitation wavelength used was 320 nm. The photoluminescence spectra of

the TiO2 thin films contain various emission features, with intensity and

position depending on the preparation conditions. The peak present at 387 nm

corresponds to the band-to-band transition due to excitation of electrons fromvalence band to conduction band. The band gap calculated using this peak is

3.60eV, which corresponds to the band gap of anatase phase of TiO2 thin

films. The presence of broader photoluminescence band in the visible green

region is attributed to the recombination of photo-generated holes with the

singly ionized oxygen vacancies. There is a weak broad band in the visible

region (bluish-green and yellow-green emission). The bluish-green and



56

yellow-green emissions are attributed to deep level defects in TiO2 films, such

as doubly-ionized and single-ionized interstitial Ti vacancies and oxygen

vacancies (Senthil et al 2011). The broadening of a visible band is attributed

to the porous structure of the films. The main peaks are located at about 387

and 485 nm. In the nanocrystalline regime, since the number of molecules at

the surface is more, surface defects play a vital role in determining the

luminescence characteristics. Lei et al (2001), Green et al (1997) have

reported that nano structured titania exhibit broad emission band centered at

360 – 550 nm at room temperature.

In general, the PL spectra of anatase TiO2 materials are attributed

to three kinds of physical origins: self-trapped excitons (Tang et al 1993,

Saraf et al 1998), oxygen vacancies (Saraf et al 1998, Serpone 1995) and surface

states (Fross et al 1993). Most of the surface states are oxygen vacancies or the

Ti4+ ions adjacent to oxygen vacancies (Redmond et al 1993, Lu 1994). The

PL of anatase TiO2 can be interpreted as due to the recombination via self-

trapped excitons located or trapped on TiO6 octahedra and this originates

from the defect states present. The oxygen impurities are a kind of intrinsic

defects in TiO2 lattice and form intermediate energy levels within TiO2 band

gap introducing many recombination centres for photo induced electrons and

holes (Gratzel 1989). The Ti-OH on TiO2 surface is a kind of surface state

different from bulk defects because they can act as surface recombination

centres (Kozuka et al 2000, Li 2002). The defect states in TiO2 can be easily

understood from the following relations (David et al 1995).

TiO2 TiO2 (e- / h+) (e-CB + h+

VB)

VOO+ e-

CB VO (e- trapping in shallow traps)

VO+ h+VB VO

O + h (radiative recombination)



57

350 400 450 500300

400

500

600

700

800

900

1000

1100

500°C

450°C

400°C

P L I n t e n s i t y ( a r b . u n

i t s )

Wavelength (nm)

where VOO in the Kroger notation is an ionized oxygen vacancy level which

rapidly traps a photo-generated conduction band electron (e-CB). This

subsequently interacts with a valence band hole (h+VB ) either radiatively or

non radiatively (David et al 1995). They have reported a narrow PL peak at

3.37eV for anatase TiO2 thin films and a broad band spread over the entire

visible region for the anatase phase. The strong PL band for the anatase TiO2

is mainly due to enhanced localization of impurity and defect trapped

excitons which induce impurity levels in the energy gap of TiO2.

Mathew et al (2012) have explained the photoluminescence spectraof the TiO2 nano particles and have reported that the luminescence is due to

the presence of defect levels caused by the oxygen vacancies.

Figure 3.14 Photoluminescence spectra of the TiO2 thin films annealedat 400,450 and 500°C



58

The structural and optical properties of TiO2 films reveal that the

prepared films are uniform, homogeneous, mesoporous and are of anatase

phase. When the annealing temperature was increased the grains started

growing and at 500°C the grain size was about 28nm. Since anatase TiO2

nanocrystalline films are more suitable than rutile and brookite structure for

solar cell applications the prepared TiO2 films are used for solar cell

applications.

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