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Abstract— Iron (Fe) doped titanium dioxide (TiO2) thin films

have been successfully prepared by using spin coating method. X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) characterization were conducted to know the microstructure and crystallite morphology of the films. It was indicated that the ankerite crystal orientation appears due to introduction of doping Fe on the thin films. Furthermore, increasing the dopant concentration of the TiO2 thin films yielded an increasing of porosity value which is related to the application on gas sensor films.

Index Terms—Fe doped TiO2, Spin coating, Porosity value.

I. INTRODUCTION

itanium dioxide (TiO2, titania) appears as a most interesting candidates for gas detection. The sensing capability of TiO2

sensors depend on the concentrations of two factors, i.e. intrinsic defects and extrinsic impurities. Extrinsic impurities which recognized as dopant have been put into TiO2 to improve its electrical and mechanical properties. Few authors report about iron (Fe) doped TiO2, though Fe is always present in TiO2 as an intrinsic impurity. It has been reported that Fe doping in TiO2 induces a structural transformation from anatase at low Fe concentration to rutile for Fe concentrations larger than 0.32 at. % [3] and also increases the dielectric constant of the films.

Fe doped TiO2 (TiO2:Fe) thin films are used commonly to investigate the application of TiO2:Fe due to the advantages of ease and low cost in preparation, and also flexibility of use in comparison to bulk materials. One of the effective methods for the synthesis of titania films is spin coating. It is believed to be the best alternative method for producing titania thin films.

In the literature there are almost no clear reports about the effect of Fe on mechanical properties of TiO2:Fe, esspecially about its porosity. Therefore the aim of this present work was to investigate the effect of Fe on the crystalline structure and porosity value of TiO2:Fe thin films. In particular, this work focused on the X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) data of TiO2 thin films doped with up to 6 g/L Fe.

This work was supported in part by the Indoensian Ministry of National

Education under Grant No. 2676/H.23.4.FST/PN.01/2010. Mukhtar Effendi and Bilalodin are with the Physics Study Program, Faculty

of Science and Engineering, Jendral Soedirman University, Purwokerto, Indonesia (corresponding author, tel/fax: +62 281 638793; e-mail: mukhtar.effendi@gmail.com).

II. EXPERIMENTAL

TiO2 and TiO2:Fe thin films were deposited on glass substrates using spin coating method. Titania (99%, Merck) and iron oxide (99%, Merck) were used as supplied. Precursors were prepared using TiO2 solution of 0.5 M (390 mg of titania powder dilutes to 10 mL volume with distilled water) and Fe solutions (of 2 g/L, 4 g/L and 6 g/L) which were stirred continuously by magnetic stirrer without heating at 700 rpm for 3 hours. TiO2 solution was mixed with Fe solution to yield TiO2:Fe solution.

Spin coating process was done by dropping ~ 0.2 mL of solution onto glass substrates (13 mm × 13 mm × 1 mm, CAT. No. 7101) spun in air for 15 s at 1370 rpm. Instantaneous heating at 150 °C for 30 min was following this spin coating process. Subsequently annealing was carried out using a furnace at a heating rate of about 20 °C/min and soak time of 8 hours at temperatures of 600 °C.

A PAN analytical X’Pert PRO, Philips XRD with Cu Kα radiation (λ=1.540562 Å) and a JSM-6380LA, JEOL SEM were employed to determine the structure and the surface morphology of the TiO2:Fe thin films, respectively. Data from XRD were examined by using Cohen model to find out the value of lattice parameter. Meanwhile the SEM photographs were analyzed to investigate the grain size, homogenity and porosity of the films. The porosity calculation procedure of M. Rosi et al was used to calculate the porosity of the thin films.

III. RESULTS AND DISCUSSION

The introduction of donor type atoms, such as Fe, was expected to decrease its resistance and lower power consumption. In order to examine the structure of the obtained thin films, XRD characterization has been performed. All of XRD patterns of the Fe-doped TiO2 thin films (Fig. 1) show that these thin films are consistent with the anatase structure. In the Fe-doped TiO2 thin films, structure of Calcium Iron Magnesium Carbonate or usually called as Ankerite appears due to doping Fe. The Ankerite structure seems to be increase by increasing of Fe dopant. We supposed that Fe diffused in the TiO2 by mechanism of interstitial and led to increasing Ankerite structure.

Effect of Doping Fe on TiO2 Thin Films Prepared by Spin Coating Method

Mukhtar Effendi and Bilalodin Physics Study Program, Faculty of Science and Engineering, Jenderal Soedirman University, Jl. dr. Soeparno 61 Purwokerto 53123, Indonesia

T

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Fig. 1. shows that rutile phase crystal structures were not observed. It can be explained assuming that annealing temperature in the thin films fabrication process, i.e. 600 °C, was not sufficient to transform the crystal phase from anatase to rutile. For annealing temperature of 800 °C, we found that this annealing temperature being able to transform anatase to be

rutile crystal phase [7]. It is different with niobium dopant which transform anatase crystal phase of TiO2 thin films to be rutile at stabilization temperature of 600 °C [8]. Whereas brokite crystal phase which rarely discuss in the crystal phase transformation known as unstable structure due to it will transform to be another phase at low or high temperature [8].

10 20 30 40 50 60 70 80 90

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T = AnataseK = Ankerite

0.4 g/L

0.6 g/L

0.2 g/L

Inte

nsi

ty (

a.u

)

2θθθθ (degree)

undoped

Fig. 1. X-ray diffraction patterns of 0 to 0.6 g/L Fe-doped TiO2 thin films.

Furthermore, the lattice parameter of Fe-doped TiO2 thin films which has tetragonal crystal structure could be calculated by following the lattice parameter calculation method which introduced by Cohen [6]. Calculated values of the lattices parameters of the Fe-doped TiO2 thin films are summarized in Table I.

TABLE I

LATTICE PARAMETER VALUE OF TiO2:FE THIN FILMS Iron concentration (g/L) a=b ( Ǻ ) c ( Ǻ )

0 (undoped) 3.926 9.143 0.2 7.662 19.005 0.4 6.526 22.015 0.6 10.782 99.471

Table I shows that the crystal structure significantly depends on the concentration of iron dopant. Lattice parameter value of the tetragonal crystal structure of undoped TiO2 thin films which has anatase crystal structure are a=b= 3.926 Å and c= 9.143 Å. It seems that increasing consentartion of iron dopant enlarges the lattice parameter value of Fe-doped TiO2 thin films. We guest that all of ferum dopant diffused well in the TiO2 thereby enlarges the crystal constant of TiO2 thin films which apparent on rising of peaks in the XRD spectra. Moreover, it

will be related to the surface morphologies of TiO2:Fe thin films which has been analyzed from SEM micrographs (Fig. 2).

These SEM show that the grain shape of the TiO2:Fe crystal are uniformly round and increasing the consentration of iron dopant will decreases the crystal grain size. Crystal grain size of undoped and iron-doped TiO2 are 100 nm and 83 nm, respectively. It is assumed that crystal growth of the TiO2 was impeded by iron as dopant. In addition, a porosity analysis procedure, proposed by M. Rosi, et. al. were employed to calculate the porosity value of undoped and Fe-doped TiO2 thin films. These porosity value are presented in Table II.

TABLE II POROSITY VALUE TiO2:FE THIN FILMS

Iron concentration (g/L) Porosity value (%) 0 (undoped) 37.1

0.2 37.3 0.4 37.7 0.6 40.1

It is clearly seemed that increasing consentration of Fe dopant

will increase porosity value of TiO2:Fe thin films. Porosity of TiO2:Fe thin films will be very useful for gas sensor application due to its capability for trapping the detected gas.

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It is supposed that crystal growth mode of film formation [11] of undoped TiO2 and iron doped TiO2 thin films, which is initially follow the layer or Frank-van der Merwe mechanism, converts to be Stranski-Krastanov (combination of island and

layer growth mode) due to increasing the consentration of iron dopant. The crystal growth mode convertion on the film formation process causes the alteration of crystal orientation and porosity value (or vacancy) of the thin films.

Fig. 2. SEM images for the undoped, 0.2, 0.4 and 0.6 g/L Fe-doped TiO2 thin films.

IV. CONCLUSION

Increasing the concentration of iron dopant on the titanium dioxide thin films prepared by spin coating method causes the alteration in crystal structure and morphology of the films. Our experiment results indicated that the Ankerite crystal structure appear due to iron doping on the TiO2 thin films. However, the concentration of iron doping up to 0.6 g/L did not transform the anatase crystal phase of TiO2 thin films to be rutile yet.

Porosity value could be calculated well by M. Rosi’s procedure. Increasing the concentration of iron doping will increase the porosity value of TiO2 thin films. Regarding to the application on gas sensor films iron doping on TiO2 thin films provide a superior capability due to the increasing of porosity value.

ACKNOWLEDGMENT

The authors wish to acknowledge Rakhmat Randhy Prathama for his contributions in this research.

REFERENCES

[1] A. Ruiz, G. Dezanneau, J. Arbiol, A. Cornet, J. R. Morante, “Study of the influence of Nb content and sintering temperature on TiO2 sensing films”, Thin Solid Films, 436, 2003, pp. 90-94.

[2] E. Sotter, X. Vilanova, E. Llobet, M. Stankova, X. Correig, “NIOBIUM-DOPED TITANIA NANOPOWDERS FOR GAS SENSOR APPLICATIONS”, J. Optoelectron. Adv. Mater., 7 (3), 2005, pp. 1395-1398.

[3] A. R. Bally, E. N. Korobeinikova, P. E. Schmid, F. Levy and F Bussy, “Structural and electrical properties of Fe-doped TiO2 thin films”, J. Phys. D: Appl. Phys. 31, 1998, pp. 1149-1154.

[4] D. Mardare, G. I. Rusu, “COMPARISON OF THE DIELECTRIC PROPERTIES FOR DOPED AND UNDOPED TiO2 THIN FILMS”, J. Optoelectron. Adv. Mater., 6 (1), 2004, pp. 333-336.

Undoped (0 g/L Fe)

0.2 g/L Fe

0.6 g/L Fe

0.4 g/L Fe

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[5] A. Nakaruk, C. Y. Lin, D. S. Perera, C. C. Sorrel, “Effect of annealing temperature on titania thin films prepared by spin coating”, J. Sol-Gel Sci. Technol., 55, 2010, pp. 328-334.

[6] M. Rosi, F. D. Eljabbar, U. Fauzi, M. Abdullah dan Khairurrijal, “Pengolahan Citra SEM dengan Matlab untuk Analisis Pori pada Material Nanopori”, Journal Nanosains dan Nanoteknologi, Vol. Edisi Khusus (Agustus), 2009, start page: 29.

[7] M. Effendi and Bilalodin, “EFFECT OF ANEALING TEMPERATURE ON FERRUM DOPED TITANIUM DIOXIDE THIN FILMS PREPARED BY SPIN COATING TECHNIQUE”, Indonesian Journal of Applied Physics, 2012, accepted (in press).

[8] A. Ruiz, A. Calleja, F. Espiell, A. Cornet, and J. R. Morante, “Nanosized Nb-TiO2 Gas Sensors Derived From Alkoxides Hydrolization”, IEEE Sensors Journal, 3(2), 2003, pp. 189-194.

[9] M. Hemissi and H. A. Adnani, “Optical and Structural Properties of Titanium Oxide Thin Films Prepared by Sol-Gel Method”, J. Nanomat. and Biostruct., 2 (4), 2007, pp. 299-305.

[10] B. D. Culity, Element of X-Ray Diffraction, Addison Wesley Publishing Company Inc., Massachusets, 1978.

[11] M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego, 1991, ch.5..

Mukhtar Effendi received the S. Si. (equivalent with B. Sc) degree in the Physics Department from Faculty of Mathematics and Natural Sciences, Gadjah Mada University, Yogyakarta, Indonesia in 2002. He had continuing his educational background at Department of Materials Science and Technology, Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan, and received the M. Eng, and Dr. Eng. degrees in 2006 and 2009, respectively. Currently, his job carrier as a lecturer in Physics Study Program, Faculty of Science and Engineering, Jenderal Soedirman University, Purwokerto, Indonesia. Bilalodin received the S. Si. and M. Si. (equivalent with B. Sc., and M. Sc., respectively) degrees in Physics Department from Faculty of Mathematics and Natural Sciences, Univeristas Diponegoro, Semarang, Indonesia in 1994 and Bandung Institute of Technology, Bandung, Indonesia in 2000, respectively. Since 1995, he has been with the Physics Study Program, Faculty of Science and Engineering of the Jenderal Soedirman University, Purwokerto, Indonesia.