Structural and Surface Morphological Characterization of Pure and Sr-doped TiO₂ Thin Film Prepared by Spin Coating Technique

Abstract

Transparent thin films of pure TiO₂ and 3% Sr-doped TiO₂ (Ti_0.97 Sr_0.03O₂) were prepared by spin coating technique onto well-cleaned glass substrate. These films were annealed at different temperatures. The structural analysis by GIXRD and Raman Spectroscopy confirms the anatase phase of TiO2. The study also shows dependence of structural parameters and crystallinity on the annealing temperature of films. Surface morphology of the prepared films studied using Atomic Force Microscopy (AFM) exhibits a homogeneous globular structure.

Introduction

TiO₂ possesses three polymorphs: anatase, rutile and brookite, with distinct crystalline structures. The rutile is the most common and well known structure of the three. In rutile, the structure is based on octahedrons of TiO₂, which share two edges of the octahedron with other octahedrons and from chains Titanium dioxide (TiO₂) has attracted the attention of many research workers due to its outstanding physical and chemical properties. It is found that TiO₂ is also antibacterial, self cleaning and super hydrophilic. It has large number of applications as a catalyst support, gas sensor, thermoelectric and photovoltaic cells [1-13]. TiO₂ is also a promising material for next generation of ultra-thin capacitors, due to its dielectric property [14-15].

With increase in the frequency, the dielectric constant of the film is found to be decrease. This is because the frequency increases dipoles start to lag behind the field and dielectric properties is going to decrease and at higher frequency dipoles are not able to follow the field for longer. The AC conductivity is linearly increase with respect to frequency and Sr-doped TiO₂ show the low dielectric property and high AC conductivity compare with pure TiO₂ films. An electrical property of the films has been reported [16].

Experimental & Characterization Details

In the present work, six films (three for pure TiO₂; three for Sr-doped TiO₂) of approximately 70-80 nm thickness were prepared. Titanium (IV) oxyacetylacetonate (0.3M) was used for Ti source and Strontium Chloride (GR) for Sr. Precursor samples were taken in polyethylene glycol (6ml) and water (4 ml). The solution was stirred for one hour at room temperature to obtain a viscous and transparent solution. The rpm of the spin coater was set at 2000 and during each rotation, three drops of solution were made to drop on the glass substrate. This process was repeated three times with the films being annealed at 300⁰C between each process. One film each of TiO₂ and Sr-doped TiO2 were then annealed for 30 minutes at 500⁰C and 600⁰C, respectively.

The films structure was studied by Grazing Incidence X-ray Diffraction (GIXRD) using Bruker D8 discover diffractometer,(Cu kα radiation, λ=1.5406 Å) complementary information concerning film microstructure was derived from Raman Spectra which were acquired by means of a Jobin Yvon Horibra Labram- HR visible spectrometer using the blue line (488 nm) of an Ar laser as excitation source. The collection time for each spectrum was 5 minutes in the range of 100 to 200 cm_-1.Film surface morphology were evaluated from AFM measurements.

Result and Discussion

Structural Characterization
GIXRD Analysis

Figure 1(a), GIXRD data of the TiO₂ thin films prepared by spin coating.

Figure 1(b), GIXRD data of the Sr-doped TiO₂ thin films prepared by spin coating.

Separate peaks for Sr-doped TiO₂ are not observed due to low dopant concentration. It is also difficult to predict whether the Sr ion exists as Sr-O on the surface of TiO₂ as SrTiO₃ from the GIXRD pattern due to low dopant concentration.

From the figure 1(a) and 1(b) it is clear that, without annealed film shows the amorphous nature and as we increase the annealing temperature the crystallinity of the film also increase. The anatase phase of TiO₂ was found in both samples annealed at high temperature. The crystalline size estimate using the Debye-Scherer formula is found to be 32 nm for pure TiO₂ and 37 nm for Sr-doped TiO₂.

We have used short and long time scan because of very low thickness of film and we took scan in the range of 100 to 200 cm^-1. The main peak of anatase phase of TiO₂ is observed near about 144 cm^-1. The Raman spectroscopy of the samples confirms the anatase phase of TiO₂ as determined by GIXRD analysis.

Raman spectra show broadening of the spectra with increase in annealing temperature. Post annealed film also show the shifting. Same result were observed in Sr-doped TiO₂ films but in whole sample we get peak near about 149 cm^-1 [17-19] and they are also shows the same type of shifting compare with TiO₂ Raman spectra. This result supports that the structural parameter is dependent on the annealing temperature and anatase phase is more temperature sensitive and TiO₂ shows the anatase to rutile phase transformation at high temperature. The Raman shifting of Sr-doped TiO₂ spectra also indicates that Sr is properly combined/attached with Ti atom.

Figure 2(a), Raman spectra of TiO₂ thin film prepared by spin coating technique.

The formation of TiO₂ and thin films deposited by spin coating is considered accumulating like “tiny island”. Figure 3 shows the surface topography image of sample obtained by AFM. All the as-deposited thin films indicate surfaces which are not highly rough, and that the film is homogeneously distributed on the substrate. Without post annealed films show rough surface compare to annealed films. The 2D image of the films show the spherical shape of the particles and post annealed films also show same result but compare to without annealed film the particle size is small and of uniform spherical shape. The particle size calculated from the AFM data is near about ~115 nm in post annealed films in both samples.

Figure 2(b), Raman spectra of Sr-doped TiO₂ thin film prepared by spin coating technique.

Figure 3(a), AFM data of TiO₂ thin film annealed at 600⁰C

Figure 3(b), AFM data of TiO₂ thin film annealed at 500⁰C

Figure 3(c), AFM data of Sr doped TiO₂ thin film annealed at 600⁰C

From the AFM result, it is believed that the thin film grow process consist of three aspects, the atoms adsorption, migration and desorption, which are all connected with the annealed temperature and viscosity of the sample solutions. AFM Images obtained on different regions of the samples showed that the films exhibit a homogeneous globular structure. The entire film surface is formed by small grains of the deposited material.

Conclusion

Acknowledgement

Rajiv Gandhi UGC fellowship to one of the authors (M. H. Mangrola) is fully acknowledged. The authors also wish to thank UGC DAE CSR Indore, Dr. B. S. Chakrabarty (Applied Physics Department, M. S. University of Baroda), Dr. Vandana N Rao (Metallurgy & Material Science Department, M. S. University of Baroda), Dr. Utpal Joshi (School of Science, Department of Physics, Gujarat University) and Dr. M. Roy (M. L. Sukhadiya University, Udaipur) for their support and valuable suggestions.

REFERENCES :

1. A.Nakaruk and C. Y. Lin, D. S. Perera and C. C. Sorrell,J Sol-Gel Sci Technol , 55 (2010), 328–334.
2. M.Hemissi and H. Amardjia-Adnani, Digest J. of Nanomaterials and Biostructures, 2 (2007) 299-305.
3. D.Kaczmarek, D. Wojcieszak, et. al., Cent. Eur. J. Phys., 9 (2), (2011)349-353.
4. M.Stamate, I. Vascan, I. Lazar, I. Caraman and M. Caraman, J. of Optoelectronics and Adv. Mate., 2 (2005) 771-774.
5. M. Catherine Blount, D. Hyun Kim and John L. Falconer, Environ. Sci. Technol. 35(2001), 2988-2994.
6. A. P.Caricato, R. Buonsanti, et. al. Appl. Phys. A., 104 (2011) 963-968.
7. S.Akbar, P. Dutta and C.Lee, Int. J. Appl. Ceram. Technol., 3(4) (2006) 302-311.
8. R. U. Mene,M. P. Mahabole, R. C. Aiyer and R. S. Khairnar, The Open Applied Physics Journal, 2010, 3,10-16.
9. G.Telipan, M. Ignat, C. Tablet and V. Parvulescu, J. of Optoelectronics and Materials, Vol. 10, No. 8, August 2008, p. 2138 – 2141.
10. D. P.Dubal, D. S. Dhawale, A. M. More and C. D. Lokhande, J Mater Sci (2011)46:2288–2293.
11. Yang Shengyuan, Zhu Peining, A. Sreekumaran Nair and S. Ramakrishna, J. Mater.Chem. 21 (2011) 6541-6548.
12. L.Kumaresan, M. Mahalakshmi, M. Palanichamy and V. Murugesan, Ind. Eng. Chem.Res. 49 (2010) 1480-1485.
13. Y. Cui,J. He, G. Amow and H. Kleinke, Dalton Trans., 39 (2010) 1031-1035.
14. Te-Cheng Mo, Hong-Wen Wang, San-Yan Chen and Yun-Chieh Yeh, Ceramics International34 (2008) 1767–1771.
15. Valentin Bessergenev, Materials Research Bulletin 44 (2009) 1722–1728.
16. M. H.Mangrola, A. S. Pillai, B. H. Parmar, V. G. Joshi, Advanced Materials Research Vol. 665(2013) pp 263-266.
17. Ahti Niilisk, Mart Moppel, et. al., Cent. Eur. J. Phys. 4(1) (2006) 105-116.
18. W. G. Nilsen and J. G. Skinner, The J. of Chemical Physics, 48 (5) (1968)2240-2248.
19. Danilo Bersani, Giovanni Antonioli,Pier Paolo Lottici, Tessy Lopez, Journal of Non-Crystalline Solids 232±234(1998) 175-181.