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Materials Technology
Advanced Performance Materials
ISSN: 1066-7857 (Print) 1753-5557 (Online) Journal homepage: https://www.tandfonline.com/loi/ymte20
Influence of annealing temperature on structural,
morphological and optical properties of
nanostructured TiO
2
thin films
S Sönmezoğlu, G Çankaya & N Serin
To cite this article: S Sönmezoğlu, G Çankaya & N Serin (2012) Influence of annealing
temperature on structural, morphological and optical properties of nanostructured TiO2 thin films,
Materials Technology, 27:3, 251-256, DOI: 10.1179/1753555712Y.0000000008
To link to this article: https://doi.org/10.1179/1753555712Y.0000000008
Published online: 12 Nov 2013.
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structural, morphological and optical
properties of nanostructured TiO
2
thin films
S. So
¨nmezog˘lu*
1, G. C¸ankaya
2and N. Serin
3Thermal annealing is widely used to improve crystal quality, which affects electrical and structural
properties by reducing study defects in materials. Therefore, enormous research efforts were
focused on the control of material surface nanostructure through annealing processes, which is of
interest for various technologies. However, no work providing a detailed explanation for the
structural, morphological and optical parameters of nanostructured TiO
2thin films deposited on
glass at temperature above 500
uC by the sol–gel dip coating method has been presented to date.
In this work, we have grown nanostructured TiO
2thin films by sol–gel dip coating method on glass
substrates at room temperature and studied the effects of annealing temperature from 200 to
700
uC on optical performance, microstructural changes and surface morphology evolution. The
results of this work may be summarised as follows: the X-ray diffraction results show that annealed
TiO
2thin films have anatase crystal structure, and the intensities of the peaks of the crystalline
phase increased with the increase in annealing temperature; from atomic force microscopy
images, distinct variations in morphology of the thin films were also observed; and optical results
show that TiO
2films exhibit high visible transmittance, and it has a maximum transmittance of
,93?61% at 500
uC annealing temperature. The optical band gap of the as grown thin films
decreases from 3?68 to 3?31 eV with the increase in annealing temperatures. The TiO
2thin film
annealed at 500uC has the best optical property. The change in structural, morphological and
optical properties with annealing temperature demonstrates that this material has a potential to be
used as a novel technology such as nanoelectronics and possibly nano-optoelectronic devices
based on nanomaterial for insulating, semiconducting and electron and/or hole blocking layer,
etc.
Keywords: X-ray diffraction, Thin film structure and morphology, Optical properties of thin films, Nanoparticles
Introduction
Transparent semiconducting materials, which can be grown efficiently as thin films with low cost, are used extensively for a variety of applications, including architectural windows, solar cells, heat reflectors, light transparent electrodes and thin film photovoltaic and many other opto-electronic devices. Among the trans-parent conducting oxides, TiO2has been widely
investi-gated for its interesting optical, photocatalytic and electronic properties. For its high refractive index, wide
band gap and good chemical stability in adverse environment, TiO2 films are employed for a variety of
applications, such as in optics industry,1 single or multilayer optical coatings,2–4dye sensitised solar cells,5 dielectric applications,6 self-cleaning purposes2 and photocatalytic layers.7For example, as a photocatalytic, TiO2is considered a preferred and potential
semiconduc-tor material for applications requiring antimicrobial and sterilising characteristics.8–14The highly transparent TiO2
films have also been widely used as antireflection coatings for increasing the visible transmittance in heat mirrors.15 As a dielectric, TiO2is one of the most popular materials
for the purpose of antireflection coatings.16–18
TiO2can exist as an amorphous layer and also in three
crystalline phases: anatase (tetragonal), brookite (orthor-ombic) and rutile (tetragonal). Only the rutile phase is thermodynamically stable at higher temperature. The occurrence of crystalline phase depends upon the de-position method, comde-position, density and annealing
1Department of Materials Science and Engineering, Faculty of Engineering, Karamanog˘lu Mehmetbey University, Karaman 70100, Turkey
2Department of Materials Engineering, Faculty of Engineering and Natural Sciences, Yıldırım Beyazıt University, Ankara, Turkey
3Department of Physics Engineering, Faculty of Engineering, Ankara University, Ankara 06100, Turkey
temperature. Thermal annealing is widely used to improve crystal quality, which affects the electrical and structural properties by reducing study defects in
materials. During the annealing process, dislocations and other structural defects may smear out in the material, and adsorption/decomposition, especially at the surface regions, may change the structural, stoichio-metric and electrical properties of the material. Therefore, understanding the effects of annealing processes on TiO2
surfaces and films is of interest for various technologies employing this material.19
There are many deposition methods that can be us-ed to prepare TiO2 thin films, such as electron beam
evaporation,20 chemical vapour deposition,21 direct cur-rent–radio frequency reactive magnetron sputtering,22,23 pulsed laser deposition24 and sol–gel method.25 The generally used vacuum techniques are suitable for small area substrates. The thin films obtained by these methods are non-stoichiometric and non-uniform, and also costly equipment is needed. In comparison with other techniques, the sol–gel method has certain advantages: low process cost, low temperature of heat treatment, high evenness of the films, controllability, reliability, reproducibility and wide possibility to vary film properties by changing the composi-tion of the solucomposi-tion, etc.26–29
Many efforts have been directed to the control of TiO2 surface nanostructure; however, several
prob-lems still remain to be resolved. Annealing significantly changes, at the same time, the microstructure, the crystalline phase and the nanoparticle dimensions of nanostructured titania. This means that these para-meters cannot be controlled independently. Here, we report the structural, morphological and optical char-acterisation of anatase nanostructured TiO2 thin films
produced by sol–gel dip coating method. We have studied the effects of annealing temperature on optical performance, microstructural changes and surface mor-phology evolution.
Experimental
In order to prepare a TiO2solution, first, 2?4 mL titanium
tetraispropoxide [Ti(OC3H7)4, e.g. Ti >98%, Merck] was
added in 25 mL ethanol (C2H6O, 99?9%, Merck), and the
solution was kept in a magnetic stirrer for 1 h. Then, 5 mL glacial acetic acid (C2H4O2, 99?9%, Merck) and
25 mL ethanol were added in the solution, and after each additive component was added, it was mixed in the magnetic stirrer for 1 h. As a final step, 1?5 mL trietilamine [(C2H5)3N, 99%, Merck] was added in the
solution, and the final solution was subjected to the magnetic stirrer for 2 h. The solution was aged at room temperature for 1 day before deposition. Microscope glass slides were used as substrates for thin films. Before deposition, the glass slides were sequentially cleaned in an ultrasonic bath with acetone and ethanol. Finally, they were rinsed with distilled water and dried. The dipping process was performed using a homemade motorised unit, and each sample was dipped into the solution five times. After each dipping process, samples were subjected to repeated annealing processes at the temperature of 200, 300, 400, 500, 600 and 700uC for 1 h period in air using an electric oven (Vecstar VCTF-4).
The crystalline properties of the TiO2 thin films were
analysed by an X-ray diffractometer (Model-D8 Ad-vanced, Bruker) using Cu Karadiations (l51?5405 A˚ ) over
the range of 2h510–60u at room temperature. For morphological investigations, atomic force microscopy (AFM) images were recorded using an SPM Solver-PRO
1 X-ray diffraction patterns of TiO2 thin films annealed at
a 200uC, b 300uC, c 400uC, d 500uC, e 600uC and f 700uC
So¨nmezog˘lu et al. Influence of annealing temperature on TiO2thin films
(NT-MDT) AFM controller in tapping mode. The transmission spectra of TiO2thin films were measured at
room temperature by a Shimadzu ultraviolet–visible–near infrared (UV-VIS-NIR) 3600 spectrophotometer in the range of 300–1500 nm.
Results and discussion
Structural studies
Figure 1 illustrates the X-ray diffraction patterns of nanostructured TiO2 thin films deposited on glass
substrate and annealed at different temperatures. From the figure, it was found that all of them were poly-crystalline with a tetragonal structure, having anatase phase only. From the diffraction patterns, it was observ-ed that the nanostructurobserv-ed TiO2thin films, annealed at
200, 300, 400, 500 and 600uC, exhibited characteristic peaks of anatase crystal plane (101). Few weak peaks representing anatase (112) and (200) planes are also observed in the thin films annealed at 700uC. The appearance of those new peaks suggests a slight im-provement of crystallinity. No diffraction peaks of other impurity phases are found in the data. The relative peak intensity of these diffraction peaks also increases as the annealing temperature increases and resulted in better crystallinity. The crystallite sizes were calculated from the Scherrer relation30
D~ 0:9l
bcos h (1)
where D is the crystallite size (nm), l is the wavelength of Cu Karadiation (nm), h is the Bragg angle (u) and b is
the full width at half maximum of the diffraction peak. As shown in Table 1, the calculation results indicate that the crystallite size gradually increases with increasing annealing temperature in terms of crystal plane (101) of the anatase structure. These observations could be explained in terms of higher adatom mobility with increasing annealing temperature, which results in larger crystallite size and enhances the crystallinity of the thin films.
Morphological characterisation
Figure 2 shows the two- and three-dimensional AFM images of the thin films annealed at different tempera-tures. All the TiO2 thin films exhibit a smooth surface
with uniform grains. Although having the same granu-larity when deposited, the original nanograins transform into triangular pyramid particles with different dimen-sions, accounting for the crystallisation of nanostruc-tured TiO2into a polycrystalline material. As shown in
Fig. 2, the surface morphology reveals the nanostruc-tured TiO2grains, which combine to make denser films
significantly with increased annealing temperatures. As
shown in Table 1, the average grain size of thin films increases from about 382 to 598 nm with increasing annealing temperature. This may be due to the bigger clusters formed by the coalescence of two or more grains and increasing of roughness. In AFM analysis, the root mean square (rms) is the most widely used parameter to characterise surface roughness. The rms roughness of the thin films increases from ,0?488 to 0?888 nm with the increase in annealing temperature (Table 1).
Optical characterisation
The optical transmittance spectra of nanostructured TiO2thin films deposited by dip coating and annealed at
different temperatures are shown in Fig. 3. The trans-mission spectrum can be roughly divided into two regions: a transparent one with the interference pattern in the visible and near-infrared region and a zone of strong absorption in the near ultraviolet region, where the transmittance T decreases drastically owing to the effect of the absorption coefficient. TiO2 thin films
exhibit high transparency of 93?61% at 790 nm. This is the wavelength at which the optical thickness corre-sponds to half wave and shows maximum transmittance. All thin films showed good transmittance in the visible and near-infrared wavelength regions. As shown Fig. 3, the transmittance maxima increase with increasing annealing temperature up to 500uC and afterwards decrease slightly. The decrease/increase in transmittance mainly results from the absorption posed by vacancies of oxygen, and the transmittance edge shift to longer wavelength is attributed to the scattering of coating surface and absorption and increase in density of the film.
As we all know, a typical transmission spectrum at normal incidence has two spectral regions: the region of weak and medium absorption and the strong absorption region. In the weak and medium absorption regions, the refractive index n of the film can be calculated by the following expression31 n~ N z Nh 2{ ns21=2i 1=2 (2) where N ~1 2 1 z n 2 s z2 nsðTM{ TmÞ TMTm (3) where nsis the refractive index of the glass substrate, and
TMand Tmare points of the maxima and minima of the
transmission spectrum respectively. The basic equation for interference fringes is
2nd~ml (4)
where the order number m is the integer for maxima and half integer for minima. If n(l1) and n(l2) are the
Table 1 Values of thickness, energy band gap, refractive index, crystallite size, roughness and average grain size of TiO2thin films at different annealing temperatures
200uC 300uC 400uC 500uC 600uC 700uC
Thickness d/nm 249.14 208.63 176.49 103.01 82.74 54.25
Energy band gap/eV 3.68 3.65 3.45 3.40 3.38 3.31
Refractive index n 1.76 1.77 1.88 1.95 2.04 2.11
Crystallite size/nm 85 126.8 289.2 361.3 392.3 465.7
Roughness/nm 0.501 0.503 0.508 0.769 0.835 0.888
2 Two- and three-dimensional AFM images of TiO2 thin films annealed at a 200uC, b 300uC, c 400uC, d 500uC, e 600uC
and f 700uC respectively
So¨nmezog˘lu et al. Influence of annealing temperature on TiO2thin films
refractive indices at two adjacent maxima (or minima) at l1and l2, the film thickness d can be expressed by
d ~ l1l2
2 n l½ ð 1Þ l2{ n lð 2Þ l1
(5) Following the procedure described above, the physical parameters n and d were determined for all the annealed films. The curves and values of refractive index for as grown nanostructured TiO2thin are shown in Fig. 4 and
Table 1 respectively. As seen in Fig. 4, the refractive index values increase with increasing annealing tem-perature. The increase may be attributed to the higher packing density within the film and a slight increase in crystallinity. The thicknesses of the nanostructured TiO2
thin films were also determined from transmittance measurements and given in Table 1. As seen from
Table 1, the film thickness decreases with increasing annealing temperature.
Near the absorption edge or in the strong absorption zone of the transmittance spectra, the absorption coefficient a is related to the optical energy gap Eg
following the power law behaviour of Tauc’s relation32 ah v
ð Þ ~A hv{E g r
(6) where a is the absorption coefficient, A is an energy independent constant between 107and 108m21, Egis the
optical band gap and r is a constant, which determines the type of optical transition r5K, 2, 3=2 or 3 for allowed
direct, allowed indirect, forbidden direct and forbidd-en indirect electronic transitions respectively. Figure 5 shows the plot of (ahv)2versus energy (hv) according to equation (6). The optical energy gaps Egof the thin films
were determined by extrapolating the linear portion of this plot at (ahv)250 for r5K, which indicates that direct transition dominates in the nanostructured TiO2 thin
films. The direct band gaps Eg of thin films were
calculated from Fig. 5 and given in Table 1. As clearly shown from Fig. 5 and Table 1, it is evident that the increase in annealing temperature leads to a decrease in optical band gaps. The decrease in optical band gap is attributed to the lowering of interatomic spacing, which may be associated with a decrease in the amplitude of atomic oscillations around their equilibrium positions. Moreover, the increase in annealing temperature sig-nificantly enhances the photocatalytic and antimicrobial performance, which are related to a decrease in the band gap energy of titania.8–14,33–38
Conclusions
The results of this study on nanostructured TiO2 thin
films treated at different annealing temperatures may be summarised as follows.
1. The nanostructured titanium dioxide thin films have been prepared by sol–gel dip coating technique at ambient room temperature.
3 Transmission spectra for TiO2 thin films at different
annealing temperatures
4 Refractive index dispersion spectrum of TiO2 thin films
at different annealing temperatures
5 (ahv)2 versus (hv) plots of TiO2 thin films at different
2. The structure of all the films was tetragonal with a strong (1 0 1) preferred orientation.
3. The thin films annealed at different temperatures consisted of triangular pyramids having submicrometre diameters and had important change upon annealing. The rms roughness of the thin films increases from 0?488 to 0?888 nm, and at the same time, the average grain size as well grows up from ,382 to 598 nm with the increase in annealing temperature.
4. The thin films showed a high transmittance and a band gap decrease from 3?68 to 3?31 eV with the increase in annealing temperatures.
5. The increase in annealing temperature leads to an increase in refractive index.
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