A. Nakaruk, D. S. Perera and C. C. Sorrell
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1833-122X) Volume 6 November 2010
Degree of Crystallinity
Summary And Conclusions
Titania (TiO2) films were coated on microscope glass slides using
ultrasonic spray pyrolysis for 1 h in air at the deposition temperatures of
300°, 325°, 350°, 375°, and 400°C. The films were characterised using
glancing-angle X-ray diffraction (mineralogy), focussed ion beam milling
(thickness), and UV-VIS spectrophotometry (transmittance and optical band gap).
At 300°C, the films were amorphous; higher deposition temperatures yielded films
of the anatase polymorph of titanium dioxide. The thickness of the films
increased, the optical transmission decreased, and the indirect optical band gap
decreased as a function of increasing deposition temperature, with the latter
decreasing from 3.47 eV to 3.23 eV between 300° and 400°C, respectively. The
trend of decreasing band gap as a function of increasing deposition temperature
was considered in terms of the (a) mineralogy, (b) film thickness, (c)
interfacial reflections, (d) degree of crystallinity, and (e) grain size. In
explanation of this trend: (a) mineralogy may be responsible for the trend but
there are no data to support this conclusion, (b) thickness is irrelevant, (c)
interfacial reflections support the trend, (d) the degree of crystallinity
offers opposing perspectives in terms of the optical density and the refractive
index, and (e) a grain size effect is contrary to the present data.
Titanium dioxide (titania, TiO2) is one of the most widely used
materials in thin films because it has numerous applications, such as solar
cells, photocatalysts, gas sensors , optical coatings , and self-cleaning
materials . In general, titania is very inert, surpassing glass in its
resistance to attack by common solvents and acids. It acts as a catalyst for
various organic reactions and, as a thin film, it is used as a dielectric for
thin film capacitors and as an antireflection coating on silicon. In recent
years, it has been investigated as an electrode for photoelectrochemical cells
[4-6], and for detectors of H2O , oxygen , and hydrogen
There are many techniques to prepare titania films, including sputtering
, pulsed laser deposition (PLD) , sol-gel , gel oxidation ,
anodic oxidation , electrophoretic deposition , and spray pyrolysis
. Spray pyrolysis is an attractive, versatile, and practical method to
prepare titania thin films due to its advantages, such as low cost, simple
operation, simple experimental setup, no need for vacuum, capacity for mass
production, ease of doping, reproducibility, and rapid growth rates.
Briefly, spray pyrolysis is a simple technique, requiring only a liquid
source, atomiser, and heated substrate. The droplet size depends on the method
of atomisation, e.g., aerosol and ultrasonic spraying produce larger and smaller
initial droplets, respectively. Greater control over the transit of the initial
droplets to the substrate (heated typically with a hot plate) can be achieved
through air entrainment. These entrained droplets subsequently form thin or
thick films upon immediate approach to or impingement on the heated
From the preceding, it can be seen that the deposition temperature is one of
the most important parameters in the preparation of titania films. Earlier
reviews recognised the effects of deposition temperature and droplet size on
film qualities, including mineralogy, thickness, morphology, transmission, and
optical band gap [17-20]. However, these literature reviews were generic and so
did not concentrate on the preparation and characteristics of titania films;
this has been done recently by the authors . Other researchers have examined
the effects of the deposition temperature on titania thin films produced by
spray pyrolysis but these studies were done at conventionally higher
temperatures [22,23] while the present work was done at lower temperatures.
The precursor consisted of titanium butoxide
Reagent Grade, 97 wt%, Sigma-Aldrich) dissolved in methanol (Reagent Plus =99
wt%, Sigma-Aldrich) at a titanium concentration of 0.5 M. The aerosol was
produced using a commercial ultrasonic generator of 1.7 MHz frequency, placed
directly in the solution. Further details of the experimental conditions have
been described elsewhere . The aerosol was carried to the microscope glass
slides (25 mm x 25 mm x 1 mm) by using a conventional air entrainment
arrangement. The deposition temperature was varied at 300°, 325°, 350°, 375°,
and 400°C, with a 1 h deposition time and consistent solution feed rate.
The films were characterised by the following techniques: (1) The mineralogy
of the films was examined using glancing angle X-ray diffraction (GAXRD, angle
of incidence 1°, Phillips X’pert Materials Research Diffraction). (2) The film
thickness was determined using single-beam focussed ion beam (FIB) milling (FEI
XP200) following application of a ~20 nm thickness chromium (Cr) coating applied
by sputtering. In this method, gallium ions (Ga3+) are used to erode a square
hole in the film and an image of the cross-section of the layers is viewed at an
angle of 45°. (3) The transmission spectrum in the visible region (300-800nm)
was obtained using a dual beam UV-VIS spectrometer (Perkin Elmer Lambda 35).
Results And Discussion
Figure 1 shows the GAXRD patterns of titania films. It can be seen that the
as-deposited films at 300°C were amorphous. A small (101) peak of anatase
appeared for the as-deposited films processed at 325°C and the intensity of this
peak increased with increasing deposition temperatures. For the films
as-deposited at 350°, 375°, and 400°C, the GAXRD patterns showed that the films
were polycrystalline anatase, with the increase in peak intensities indicating
that increasing deposition temperature increased the amount of crystalline
Figure 1. Representative glancing angle X-ray diffraction
(GAXRD) patterns (measured at room temperature) for TiO2 processed at
The increase in film thickness as a function in increasing deposition
temperature is shown in Figure 2. These data, which are summarised numerically
in Figure 3, show that the thicknesses of the titania films increased over the
range 370, 460, 610, 720, and 800 nm for the deposition temperatures 300°, 325°,
350°, 375°, and 400°C, respectively (the bars represent the instrumental
variability). Since the solution feed rate was consistent for all of the
solutions, then it is implicit that the film thickness increased with increasing
deposition temperature owing to the corresponding increase in the reaction rate.
The increase is not due to increasing crystallinity since the specific volume of
a crystalline material is less than that of the analogue amorphous material
Figure 2. Representative focussed ion beam (FIB)
cross-section images of TiO2 films processed at different
Figure 3. Representative thin-film thicknesses as a
function of deposition temperature.
Figure 4 shows the transmission spectra of the films as a function of the
deposition temperature. It can be seen that all five of the spectra show
interference fringes in the form of periodic humps. This indicates that the
smoothness of these films is relatively high . These data are supported by
the FIB images in Figure 2, which show the consistency of the thicknesses and
smoothnesses of the films. The general decrease in transmission intensity in the
visible range with increasing deposition temperature, as exemplified over the
range 450-525 nm, is likely to be due to interfacial reflections. That is,
examination of Figure 2 suggests that the amounts of interfacial areas
associated with grain boundaries and subgrain boundaries increase as the
thicknesses (through-volumes) of the films increase .
Figure 4. Representative UV-VIS light transmission
spectra for TiO2 films processed at different temperatures.
The indirect optical band gap (Eg) can be evaluated by the optical
transmittance method . To achieve this, the absorption coefficient (a) is
determined according to the formula:
||= Absorption coefficient (obtained from light transmission and film
||= Film thickness (cm)|
||= Transmission (%)|
||= Constant that does not depend on hν|
||= Planck’s constant (4.135 x 10-15 eV.s)|
||= Frequency (s-1)|
||= Indirect band gap (eV)|
As shown in Figure 5, when (a)1/2 is plotted on the ordinate against h?
(photon energy) on the abscissa, then the intercept of the tangent to the
absorption edge with the abscissa gives an estimate of the band gap energy,
which is indirect in the case of anatase and rutile. The values obtained from
these data and the other data obtained are given in Table 1.
Figure 5. Representative optical band gaps calculated
from UV-VIS spectroscopy transmission data for films as a function of deposition
Table 1 shows that the band gap decreases with increasing deposition
temperature. This trend is the reverse of previous work on films annealed at
higher temperatures (600°-1000°C) to produce anatase/rutile mixtures by the
present  and other authors [27,28]. If the present data are a correct
reflection of the optical properties of these films, then the following
explanatory comments are relevant:
- Film Thickness
- Interfacial Reflections
- Degree of Crystallinity
- Grain Size
Table 1.Summary of Analytical Data.
||Deposition Temperature |
|Optical Band Bap
Since, it is known that the optical band gaps of anatase and rutile are in
the ranges 3.20-3.56 eV and 3.00-3.34 eV, respectively [24,27,28], and the
anatase-to-rutile phase transformation is known to occur at temperatures as low
as 390°C  and 465°C  in nanotitania, it is possible that incipient
anatase-to-rutile phase transformation occurs but is at a level below the limit
of detection of the GAXRD unit.
The reverse trend cannot be attributed to increasing film thickness because
this variable is included in the band gap calculation .
As discussed in relation to the transmission data, the amount of reflection
was assumed to increase with increasing thickness (assuming a relatively
constant grain size). For films in general, increasing reflection results in
decreasing transmission and band gap. Table 1 confirms this trend. Hence, the
reverse trend may result from the increasing reflection associated with
Degree of Crystallinity
The few data considering the effect of degree of crystallinity are
contradictory, with the present authors’ observing a direct relation between
degree of crystallinity and band gap  and others’ observing a converse
relation . This contradication is likely to result from the two concurrent
optical variables that are affected by the degree of crystallinity:
- Optical Density: As the crystallinity increases, the optical density
increases, the transmission decreases, and the band gap decreases. This is in
agreement with Aarik et al. .
- Refractive Index: As the crystallinity increases, the number of
interfaces decreases, the amount of interfacial reflection decreases, the
transmission increases, and the band gap increases. This is in agreement with
Nakaruk et al. .
If the grain sizes suggested by the asperities in the surface topography in
Figure 2 are not constant but increase with increasing deposition temperature,
which is a general observation for materials, then the amount of reflection as a
function of thickness would be expected to decrease. This is in agreement with
the data at higher temperatures [24,27,28], where grain growth occurs more
readily, but it is in disagreement with the present data at lower temperatures,
where less grain growth appears to occur.
Summary And Conclusions
In the present work, titania thin films were prepared by ultrasonic spray
pyrolysis using glass substrates at the deposition temperatures of 300°, 325°,
350, 375°, and 400°C. GAXRD patterns show that the films deposited at 300°C were
amorphous while those processed at higher temperatures consisted of anatase,
with the proportion of crystalline phase increasing with increasing temperature.
FIB images indicated that increasing deposition temperature resulted in
increasing film thickness, decreasing transmission, and decreasing band gap.
The decreasing band gap as a function of increasing deposition temperature
was considered in terms of the mineralogy, film thickness, interfacial
reflections, degree of crystallinity, and grain size. Mineralogy may be
responsible for the trend but there are no data to support this conclusion, the
thickness is irrelevant, interfacial reflection supports the trend, the degree
of crystallinity offers opposing perspectives in terms of the optical density
and the refractive index, and a grain size effect is contrary to the present
The authors are grateful for the financial support of Austral Brick Co. Pty.
Ltd., the National Hydrogen Materials Alliance, and the Australian Research
Council, which have allowed this and other developmental work to be
References1. C. Garzella, E. Comini, E.
Tempesti, C. Frigeri, and G. Sberveglieri, “TiO2 thin films by a
novel sol-gel processing for gas sensor applications”, Sens. Act. B: Chem., 68
2. S.R. Kurtz and R.G. Gordon, “Chemical vapour deposition of
doped TiO2 films”, Thin Solid Films, 147 (1987) 167-176.
Parkin and R.G. Palgrave, “Self-cleaning coatings”, J. Mater. Chem., 15 (2005)
4. T. Bak, J. Nowotny, M. Rekas, and C.C. Sorrell,
“Photo-electrochemical hydrogen generation from water using solar energy.
Materials-related aspects”, Int. J. Hydrogen Energy, 27 (2002) 991-1022.
T. Bak, J. Nowotny, M. Rekas, and C.C. Sorrell, “Photo-electrochemical
properties of the TiO2-Pt system in aqueous solutions”, Int. J.
Hydrogen Energy, 27 (2002) 19-26.
6. J. Nowotny, C.C. Sorrell, L.R. Sheppard,
and T. Bak, “Solar-hydrogen: Environmentally safe fuel for the future”, Int. J.
Hydrogen Energy, 30 (2005) 521-544.
7. L.L.W. Chow, M.M.F. Yuen, P.C.H. Chan,
and A.T. Cheung, “Reactive sputtered TiO2 thin film humidity sensor
with negative substrate bias”, Sens. Act. B: Chem., 76 (2001) 310-315.
Francioso, D.S. Presicce, P. Siciliano, and A. Ficarella, “Combustion conditions
discrimination properties of Pt-doped TiO2 thin film oxygen sensor”,
Sens. Act. B: Chem., 123 (2007) 516-521.
9. Y. Shimizu, N. Kuwano, T. Hyodo,
and M. Egashira, “High H2 sensing performance of anodically oxidized
TiO2 film contacted with Pd”, Sens. Act. B: Chem., 83 (2002)
10. H.H. Huang, C.C. Huang, P.C. Huang, C.F. Yang, and C.Y. Hsu,
“Preparation of rutile and anatase phases titanium oxide film by RF sputtering”,
J. Nanosci. Nanotechnol., 8 (2008) 2659-2664.
11. Y. Suda, H. Kawasaki, T.
Ueda, and T. Ohshima, “Preparation of high quality nitrogen doped
TiO2 thin film as a photocatalyst using a pulsed laser deposition
method”, Thin Solid Films, 453-454 (2004) 162-166.
12. M.M. Sasani Ghamsari
and A.R. Bahramian, “High transparent sol-gel derived nanostructured
TiO2 thin film”, Mater. Lett., 62 (2008) 361-364.
Abdullah and C.C. Sorrell, “Preparation and characterisation of TiO2
thick films by gel oxidation”, Mater. Sci. Forum, 561-565 (2007)
14. H.Z. Abdullah and C.C. Sorrell, “Preparation and
characterisation of TiO2 thick films fabricated by anodic oxidation”,
Mater. Sci. Forum, 561-565 (2007) 2159-2162.
15. H.Z. Abdullah and C.C.
Sorrell, “Preparation and characterisation of TiO2 thick films
fabricated by electrophoretic deposition”, Mater. Sci. Forum, 561-565 (2007)
16. A. Nakaruk, P.J. Reece, D. Ragazzon, and C.C. Sorrell,
“TiO2 films prepared by ultrasonic spray pyrolysis”, Mater. Sci.
Technol., in press. doi: 10.1179/026708309X12468927349299
17. J.C. Viguié and
J. Spitz, “Chemical vapor deposition at low temperatures”, J. Electrochem. Soc.,
122 (1975) 585-588.
18. J.B. Mooney and S.B. Radding, “Spray pyrolysis
processing”, Ann. Rev. Mater. Sci., 12 (1982) 81-101.
19. W. Siefert,
“Properties of thin In2O3 and SnO2 films,
prepared by corona spray pyrolysis, and discussion of the spray pyrolysis
Process”, Thin Solid Films, 121 (1984) 275-282.
20. D. Perednis and L.J.
Gauckler, “Thin film deposition using spray pyrolysis”, J. Electroceram., 14
21. A. Nakaruk and C.C. Sorrell, “Conceptual model for spray
pyrolysis mechanism: Fabrication and annealing of titania thin films”, J. Coat.
Technol. Res., in press, doi: 10.1007/s11998-010-9245-6
22. M. Okuya, K.
Nakade, and S. Kaneko, “Porous TiO2 thin films synthesized by a spray
pyrolysis deposition (SPD) technique and their application to dye-sensitized
solar cells”, Solar Energy Mater. Solar Cells, 70 (2002) 425-435.
Conde-Gallardo, M. Guerrero, N. Castillo, A.B. Soto, R. Fragoso, and J.G.
Cabañas-Moreno, “TiO2 anatase thin films deposited by spray pyrolysis
of an aerosol of titanium diisopropoxide” Thin Solid Films, 473 (2005)
24. A. Nakaruk, D. Ragazzon, and C.C. Sorrell, “Anatase-rutile
transformation through high-temperature annealing of titania films produced by
ultrasonic spray pyrolysis”, Thin Solid Films, in press.
25. W.D. Kingery, H.K. Bowen, and D.R. Uhlmann,
p. 92 in Introduction to Ceramics, 2nd Edition. John Wiley & Sons, New York,
26. R. Swanepoel, “Determination of surface roughness and optical
constants of inhomogeneous amorphous silicon films”, J. Phys. E: Sci. Instrum.,
17 (1984) 896-903.
27. D.J. Kim, S.H. Hahn, S.H. Oh, and E.J. Kim, “Influence
of calcination temperature on structural and optical properties of
TiO2 thin films prepared by sol-gel dip coating”, Mater. Lett., 57
28. D. Mardare, M. Tasca, M. Delibas, and G.I. Rusu, “On the
structural properties and optical transmittance of TiO2 r.f.
sputtered thin films”, Appl. Surf. Sci., 156 (2000) 200-206.
Kumar, “Growth of rutile crystallites during the initial stage of
anatase-to-rutile transformation in pure titania and in titania-alumina
nanocomposites”, Scripta Metall. Mater., 32 (1995) 873-877.
30. A.A. Gribb
and J.F. Banfield, “Particle size effects on transformation kinetics and phase
stability in nanocrystalline TiO2”, Amer. Miner., 82 (1997)
31. J. Aarik, A. Aidla, A.-A. Kiisler, T. Uustare, and V.
Sammelselg, “Effect of crystal structure on optical properties of
TiO2 films grown by atomic layer deposition”, Thin Solid Films, 305
C. C. Sorrell
School of Materials Science and Engineering
University of New South Wales, Sydney, NSW 2052, Australia
E-mail: [email protected]
This paper was also published in print form in "Advances in
Technology of Materials and Materials Processing", 12 (2010) 1-8.