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TiO2 thin films prepared by sol - gel method

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					Processes in Isotopes and Molecules                                                         IOP Publishing
Journal of Physics: Conference Series 182 (2009) 012080                 doi:10.1088/1742-6596/182/1/012080




TiO2 thin films prepared by sol - gel method

                R C Suciu1, E Indrea1, T D Silipas1, S Dreve1, M C Rosu1, V Popescu2, G Popescu2
                and H I Nascu2
                1
                 National Institute for Research and Development of Isotopic and Molecular
                Technologies, 65-103 Donath, 400293 Cluj-Napoca, Romania
                2
                 Technical University of Cluj-Napoca, Physics Department, 15 C Daicoviciu,
                400020 Cluj-Napoca, Romania

                E-mail: ramona.suciu@itim-cj.ro

                Abstract. There is a growing awareness that titania (TiO2) and TiO2-based oxide systems are
                the most promising candidates for the development of photoelectrodes for
                photoelectrochemical cell (PEC) for solar-hydrogen production [1]. The PEC is equipped with
                a single photoelectrode (photoanode) and cathode, both of which are immersed in an aqueous
                electrolyte. In this work we present a sol-gel method to prepare TiO2 thin films on ITO using
                tetraisopropoxides of titanium, acetylacetone, 1-butanol and Tween 80 as surfactant. The films
                were deposited on ITO coated glass slides by spray pyrolysis. UV-VIS spectra and
                fluorescence measurements were made for the solutions and films. X-ray diffraction was used
                for structural investigations and the morphology of the film was studied by Scanning Electron
                Microscopy.



1. Introduction
TiO2 has found wide and important applications in many fields of chemical engineering and materials
engineering including either traditional catalysis, or photocatalysis, dye-sensitized solar cells, lithium-
insertion-based devices, integrated circuits, gas sensors and in the paint industry [2]. Most of these
applications are a consequence of its n-type semiconducting property and realized with micro or nano
structured TiO2 powders or thin films.
    A number of methods have been employed to prepare TiO2 films, including e-beam evaporation,
sputtering, chemical vapor deposition and sol-gel process. The sol-gel conventional method uses the
hydrolytic route, which involves the initial hydrolysis of the alkoxide precursor followed by continual
condensations between the hydrolysed particles forming the gel.
    This process is carried out at room temperature, and the desired morphological properties of the
particles are obtained by controlling the conditions under which the synthesis is carried out. Moreover,
sol-gel processing route is particularly attractive for the scaling-up of oxide thin films fabrication,
since the liquid precursor can easily be applied on a substrate by dipping, spinning or spraying [3] and
heat treated at lower temperatures.
    The spray-coating technique potentially offers the advantage of conformal film deposition on non-
planar structures (e.g. steps, stacks or trenches) on semiconductor chips [4].
    In this paper, we report the preparation of TiO2 films using the hydrolytic sol-gel process to obtain
the oxide and the spray-coating technique to deposit the films on ITO glass substrates.


c 2009 IOP Publishing Ltd                            1
Processes in Isotopes and Molecules                                                       IOP Publishing
Journal of Physics: Conference Series 182 (2009) 012080               doi:10.1088/1742-6596/182/1/012080

2. Experimental

2.1. Preparation of coating solution
Firstly, 1-butanol sol was made, which include 0.496M commercial ultrapure titanium isopropoxide
(TTIP, Fluka), 0.28M acetylacetone (99%, Alfa Aesar), 0.92M H2O and 1-butanol (absolute).
Subsequently, Tween 80 and 1-butanol solvent were added to the 1-butanol sol. During the sol
preparation, the alkoxide solution was vigorously stirred at room temperature, so as to keep a
homogeneous mixture of the chemical compositions.

2.2. Deposition and thermal treatment of precursors films
The film consists on five layers of TiO2 was obtained by spray pyrolisis. The as obtained films were
heat-treated at 600ºC, 1 h.

2.3. Analysis
UV–VIS absorption spectra of the TiO2 coating solutions and thin films deposited on ITO glass were
taken on a JASCO V-550 spectrometer.
    X-ray diffraction (XRD) measurements were performed using a BRUKER D8 Advance X-ray
diffractometer, working at 45 kV and 45 mA. The Cu Kα radiation, Ni filtered, was collimated with
Soller slits. A germanium monochromator was used. The data of the X-ray diffraction patterns were
collected in a step-scanning mode with steps of ∆2θ = 0.01°. Pure silicon powder (standard sample)
was used to correct the data for instrumental broadening. The microstructural informations obtained by
single X-ray profile Fourier analysis of the TiO2 anatase nanoparticles were the effective crystallite
mean size (Deff) and the root mean square (rms) of the microstrains, averaged along the [hkl] direction,
<ε2>1/2hkl [5]. The Warren-Averbach X-ray profile Fourier analysis of the (101) and (200) anatase
peak profiles were processed by the XRLINE [6] computer program. The unit cell parameters were
calculated by Rietveld refinement using the PowderCell software [7]. PowderCell program enables a
quantitative phase (volume fractions) analysis method by comparison of the different scattering
powers of the component materials.
    The morphology of the films was investigated by Scanning Electron Microscopy using a JSM 5600
LV field emission – high resolution scanning electron microscope equipment with an Oxford INCA
Crystal electron backscattering diffraction (EBSD) systems.

3. Results and Discussion

3.1. Precursor characterization
The X-ray diffraction pattern of the dried (100ºC) hydrolyzed precursors, shown in figure 1, evidences
its amorphous nature with a slight crystallization tendency. The powder prepared by sol-gel method
was quasi-amorphous to X-ray as long as it was calcined below 500°C. On heating to 500°C, for 1h
the reflections corresponding to titania anatase are detected (figure 1).
    Figure 2 shows the effective crystallite size distribution for the powder obtain by the heat treatment
of the precursor solution at 500°C, respectively 600ºC.

3.2. Films characterization
The UV absorption property of TiO2 films is a important factor for the photocatalyst. The UV spectra
of TiO2 films before and after calcinations were shown in figure 3. It has been reported that the band-
gap electronic transition of anatase TiO2 is indirect [8]. After heat treatment the intensities of
absorption peaks of TiO2 film increased and the peak position slightly shifted to a higher wavenumber,
because formation of TiO2.




                                                     2
Processes in Isotopes and Molecules                                                                                                                                                                                                                                                                                                                                                                    IOP Publishing
Journal of Physics: Conference Series 182 (2009) 012080                                                                                                                                                                                                                                                                                                                            doi:10.1088/1742-6596/182/1/012080



                                                                                                                                                                                                                                                                                                                                                                     0.02

                                   1600
                                                     TiO anatase (101)




                                                                                                                                                            TiO anatase (105)

                                                                                                                                                                                TiO anatase (211)
                                                                                                                                        TiO anatase (200)
                                                                                              TiO anatase (004)
                                                                          TiO anatase (103)


                                                                                                                   TiO anatase (112)




                                                                                                                                                                                                                                                                TiO anatase (215)
                                   1400




                                                                                                                                                                                                                                            TiO anatase (220)
                                                                                                                                                                                                                        TiO anatase (116)




                                                                                                                                                                                                                                                                                    TiO anatase (301)
                                                                                                                                                                                                    TiO anatase (204)
                                                                                                                                                                                                                                                                                                                                                                    0.015




                                                                                                                                                                                                                                                                                                                                     Distribution probability
                                   1200
                                                                  2
Intensity (a. u.)




                                                                                                                                                                         2



                                                                                                                                                                                             2
                                                                                                                                                     2
                                                                                                           2
                                                                                       2




                                                                                                                                2




                                   1000




                                                                                                                                                                                                                                                                             2
                                                                                                                                                                                                                                                         2
                                                                                                                                                                                                                                     2




                                                                                                                                                                                                                                                                                                 2
                                                                                                                                                                                                                 2
                                                                                                                                                                                                                                                                                                                                                                     0.01
                                    800
                                                                                                                                                                                                                                                                                                               O
                                                                                                                                                                                                                                                                                                            600 C
                                    600
                                                                                                                                                                                                                                                                                                               O                                                    0.005
                                    400
                                                                                                                                                                                                                                                                                                            500 C

                                                                                                                                                                                                                                                                                                               O
                                    200                                                                                                                                                                                                                                                                     300 C                                                                              500 C
                                                                                                                                                                                                                                                                                                                                                                                                    O
                                                                                                                                                                                                                                                                                                                                                                                                                                              O
                                                                                                                                                                                                                                                                                                                                                                                                                                          600 C
                                                                                                                                                                                                                                                                                                               O
                                                                                                                                                                                                                                                                                                            100 C                                                         0
                                      0
                                                                                                                                                                                                                                                                                                                                                                              50       100   150        200       250   300   350       400       450
                                          20                         30                                           40                                 50                                        60                                           70                                                          80
                                                                                                                                                            O                                                                                                                                                                                                                                  Real space distance R ( A )
                                                                                                                                       2θ (                            )


Figure 1. The X-ray diffraction pattern of the                                                                                                                                                                                                                                                                                 Figure 2. Effective crystallite size distribution
titania precursor dried at 100ºC, and calcined at                                                                                                                                                                                                                                                                              along the [101] crystallographic direction for
300ºC, 500ºC and 600ºC.                                                                                                                                                                                                                                                                                                        the TiO2 anatase structure.

    With regard to the relationship between the absorption coefficient α and the incident photon energy
hν near the band edge, one can write out a good approximation [8] as (α × hν)1/2= A (hν - Eg )1/2,
where the photon energy is hν, h being the Planck constant, and Eg is the indirect optical band-gap.
From the function curve (α × hν)1/2 vs hν, shown in figure 4, the band gap energy of indirect transition
is calculated to be about 3.45 eV, which is larger than of 3.2 eV reported for the bulk TiO2 anatase,
indicating a quantum size effect [8].

                                     1,8                                                                                                                                                                                                                                                                                                                        6

                                                                                                                                                                                                                                                                                                                                                                                                   E = 3,45eV
                                     1,6                                                                                                                                                                                                                                                                                                                                                            g
                                                                                                                                                                                                                                                                                                                                                                5

                                     1,4
             Absorbance ( a. u.)




                                                                                                                                                                                                                                                                                                                                                                4
                                     1,2
                                                                                                                                                                                                                                                                                                                         1/2
                                                                                                                                                                                                                                                                                                                          (α x hν)




                                                                                                                                                                                                                                                                                                                                                                3
                                          1


                                     0,8                                                                                                                                                                                                                                                                                                                        2

                                                                                                                                                                                                                                                                                                        a
                                     0,6
                                                                                                                                                                                                                                                                                                                                                                1

                                     0,4                                                                                                                                                                                                                                                                b
                                                                                                                                                                                                                                                                                                                                                                0
                                               300                        400                                                    500                          600                                   700                                                   800                                                900                                                    1,5            2         2,5              3         3,5         4         4,5

                                                                                                                            Wavenumber (nm)                                                                                                                                                                                                                                                              hν (eV)


  Figure 3. UV-VIS measurement of TiO2 thin                                                                                                                                                                                                                                                                             Figure 4. (αhν)1/2 as a function of hν for the
  films before (a) and after (b) the heat treatment.                                                                                                                                                                                                                                                                    TiO2 film after the heat treatment.

   The diffraction pattern of the TiO2/ITO thin film (figure 5) exhibit the diffraction peaks owning to
anatase phase however, thiny amount of the rutile phase (7.4% volume fraction) at 2θ of 27.43° was
also detected. The value of the anatase particles size are Deff = 5.4 nm, indicating low crystallinity of
the anatase phase.




                                                                                                                                                                                                                                                                                                                    3
Processes in Isotopes and Molecules                                                                                                                                         IOP Publishing
Journal of Physics: Conference Series 182 (2009) 012080                                                                                                 doi:10.1088/1742-6596/182/1/012080


                               4
                          3 10
                                        TiO anatase (101)

                                               ITO (222)
                               4
                         2.5 10
                                   TiO rutile (101)




                          2 104
   Intensity ( a. u.)




                                             2




                                                                        TiO anatase (004)
                                                            ITO (400)
                                      2




                         1.5 104
                                                                                             ITO (440)



                               4                                                                                 ITO (622)
                          1 10
                                                                                      2




                           5000



                              0
                                                 30                     40                  50                   60          70   80   Figure 5. The XRD patterns of TiO2/ITO thin
                                                                                            2θ (
                                                                                                         O
                                                                                                             )                         film.

   The films were free from the pinholes and cracks as generally observed in sprayed films due to its
high deposition temperature (figure 6). Formation of was study SEM using quantitative analyses such
as EDX (figure 7). This analysis confirms the TiO2 thin films formation also.




                        Figure 6. SEM images of anatase TiO2.                                                                          Figure 7. EDX measurements of TiO2 thin film.

4. Conclusions
TiO2 thin films were prepared by a sol-gel spray coating process using titanium alkoxide. XRD
analysis of our titanium precursor powder shows that starting from 500°C annealing temperature the
TiO2 anatase is the main crystalline phase. The band gap energy of indirect transition of the TiO2 thin
films is calculated to be about 3.45 eV, indicating a quantum size effect. Our nano structured TiO2 thin
films which has a little red-shifted compared with the band-gap energy of the TiO2 indirect electronic
transition may be a more efficient candidates in the development of photoelectrodes for
photoelectrochemical cell (PEC) used in solar-hydrogen production.

References
[1] Nowotny J, Sorrell C C, Sheppard L R and Bak T 2005 Int. J. Hydrogen Energy 30 521-44
[2] Abou-Helal M O and Seeber W T 2002 Appl. Surf. Sci. 195 53-62
[3] Knoth K, Schlobach B, Hunne R, Schultz L and Holzapfel B 2005 Physica C 426-431
[4] Schwartz R W, Schneller T and Waser R 2004 C. R. Chimie 7 433-461
[5] van Bercum J G M, Vermeulen A C, Delhez R, T H de Keijser and Mittemeijer E M 1994 J.
        Appl. Phys. 27 345-353
[6] Aldea N and Indrea E 1990 Comput. Phys. Commun. 601 55-159
[7] Kraus W and Nolze G 1996 J. Appl. Crystallogr. 29 301-303
[8] Linsebigler A L, Lu G Q and Yates J T 1995 Chem. Rev. 95 735-758.




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Description: Abstract. There is a growing awareness that titania (TiO2) and TiO2-based oxide systems are the most promising candidates for the development of photoelectrodes for photoelectrochemical cell (PEC) for solar-hydrogen production [1]. The PEC is equipped with a single photoelectrode (photoanode) and cathode, both of which are immersed in an aqueous electrolyte. In this work we present a sol-gel method to prepare TiO2 thin films on ITO using tetraisopropoxides of titanium, acetylacetone, 1-butanol and Tween 80 as surfactant.