- Email: [email protected]
Optical and structural properties of TiO2 thin films prepared by sol–gel spin coating
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Optical and structural properties of TiO2 thin films prepared by solegel spin coating A. Elfanaoui a,*, E. Elhamri a, L. Boulkaddat a, A. Ihlal a, K. Bouabid a, L. Laanab b, A. Taleb c, X. Portier d a
Laboratoire Mate´riaux et Energies Renouvelables (LMER), Universite´ Ibn Zohr De´p. Physique, Faculte´ des sciences B.P.8106, Hay Dakhla, 80000 Agadir, Morocco b Centre de Microscopie, Faculte´ des sciences, Universite´ Mohamed V, 10 000 Rabat, Morocco c LECIME, Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France d CIMAP, Ensicaen, Bd du Mare´chal Juin, 14050 Caen, France
article info
abstract
Article history:
Hydrogen is a renewable and non-polluting fuel. Its production from water using renew-
Received 16 April 2010
able energy is an attractive challenge. In this work we report some results on the prepa-
Received in revised form
ration of titanium oxide TiO2 thin films for environmental applications such as water
8 July 2010
photosplitting. TiO2 thin films have been prepared by spin coating technique of sol
Accepted 11 July 2010
precursor onto glass substrates. The deposited films were annealed at different tempera-
Available online 15 August 2010
tures in air. The X-ray diffraction (XRD) experiments show that the two well-known anatase and rutile phases were observed depending both on the conditions of deposition
Keywords:
and on the temperature of annealing. The best conditions of crystallization were found to
TiO2
be around 400 C in air. The influence of the number of deposited layer on the crystalline
Nano-particles
quality of the films was investigated. The surface morphology of the deposited film was
Anatase
characterized by atomic force microscopy (AFM) and scanning electronic microscopy
Rutile
(SEM). The UVeViseNIR spectroscopy shows that the film exhibits a high transmission around 90%. The best layers were obtained when concentrated (HCl) was added to the sol solutions. The direct band gap of the films was found to be around 3.7 eV, and their refractive index was found to vary from 2 to 2.4. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Titanium oxide (TiO2) is currently the focus of intensive research thanks to the wide range of its potential applications. Thanks to its durability, non-toxicity, stability and its high refractive index, such material is a preferred material for optical purposes, white pigment, gas sensor, and corrosive-protective (for a review about TiO2 see Diebold [1]). For environmental concerns, TiO2 is extensively used for photodegradation of organic and inorganic pollutants [2], photovoltaic energy
production [3], and hydrogen production by water photosplitting [4e6]. For this later concern, the water photosplitting over semiconductor oxides yields high pure hydrogen and is the topic of many investigations that have been culminated in the publication of many articles [7]. Hydrogen is a renewable and nonpolluting fuel and its production from cheap raw materials like water using renewable energy is an attractive challenge. Because its abundance and non-toxicity, TiO2 is one of the attractive materials for such applications. TiO2 thin films have
* Corresponding author. Tel.: þ212 528220957; fax: þ212 28220100. E-mail address: [email protected] (A. Elfanaoui). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.057
4131
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
Titanium Tetraisopropoxyde
Brushing with a detergent
isopropanol
Mixing for 10min
Rinsing with bidistilled water
Concentrated hydrochloric acid HCl (0.1ml) + 0.2ml distilled water
Ultrasonic cleaning in a bidistilled water during 15 min
Mixing for 24h
Rinsing with bidistilled water
Gel
Substrat Cleaning with ultrasound in an acetone bath
Spin coating Cleaning with ultrasound in a bidistilled
Drying (70°C) for 1h
water bath.
Thermal treatment For 1h (300,350 and 400°C)
Rinsing with bidistilled
water
Stoving at 200°C (20min)
Fig. 1 e Flow chart showing the procedure for processing the TiO2 thin films (left) and substrate preparation (right).
been prepared using a variety of processes including plasma enhanced chemical vapour deposition [8] radio-frequency magnetron sputtering [5], metal organic chemical vapour deposition [9], spray pyrolysis [10,11], chemical bath deposition [12], sputtering [13], electrochemical deposition [14e16], solegel [17,18] photo-deposition [6] and cold plasma method
[19]. Non-vacuum methods are very attractive since they are cost effective, simple and suitable for large area scaling. In this work, we show that by using very simple and low cost solegel synthesis technique we can prepare granular TiO2 film.
2. A(101)
25
20
1 coating layers 3 coating layers 5 coating layers
A(215)
R(112)
A(211)
A(200)
5
A(204)R(002)
10 A(004)
I (au)
15
0 10
20
30
40
50
60
70
80
2θ (°) Fig. 2 e XRD pattern of TiO2 thin films annealed at 400 C for different thickness: 1 (red curve), 3 (blue curve) and 5 (green curve) coatings layers (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Experiments
The solegel process was used to synthesize TiO2 thin films. The alkoxide used during the current work is titanium isopropoxide TTIP (C12H28O4Ti) with purity of 99.99% which was mixed with isopropanol and hydrochloric acid (11M) as a catalyst and distilled water. The procedure of preparation includes the dissolution of 25 ml of isopropanol as solvent and 0.1 ml of hydrochloric acid (HCl), 0.2 ml of distilled water is added as well as 3.5 ml of titanium isopropoxide (C12H28O4Ti) (Fig. 1); this solution is transparent, of yellowish colour and is ready for the deposit. Thin films of TiO2 were deposited by the method of spin coating in air at room temperature, on corning glass (25 25 2 mm3) as substrate. The process of cleaning the substrate is summarized in the flow chart (Fig. 1). The films were dried at 100 C for 1 h and annealed at 300, 350 and 400 C in air during 1 h. The crystalline structure was characterized by an X-ray diffractometer (Philips PW) in 2q range from 10 to 80 by 0,02 s1 steps operating at 40 kV accelerating voltage and 40 mA current using Cu Ka radiation source. The incident angle was kept constant at 0.5 throughout the
4132
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
conditions, we observe in addition to anatase (101), (004), (200), (211), (204) and (215) the formation of rutile which crystallizes in the (002) and (112) plane parallel to the surface. We also observe that the intensities corresponding to the lines characteristic of anatase and rutile increase as the number of coating layers decreases. From the peaks intensity we can determine the variation of TiO2 amount as a function of the number of coatings layers. The grain size (L) of the prepared TiO2 thin films was obtained from XRD line broadening using the Scherrer equation: L¼
Fig. 3 e SEM micrograph of the surface of TiO2 annealed at 400 C (1 coating).
experimentation. Atomic force microscopy (AFM) measurements were performed at room temperature using a Nanoscope III instrument in a standard tapping mode to investigate the surface morphology of TiO2 spin-coated films. Scanning electronic microscopy SEM (JOEL JSM 6400 Scanning microscope) was used to study the surface morphology and pore distribution of the produced films. The transmission spectra were measured by Lambda 9 UV/VIS/NIR PerkineElmer spectrophotometer.
3.
Results and discussion
TiO2 has three well-known phases namely: Anatase, Rutile and Brookite. Rutile and Anatase are tetragonal whereas Brookite is orthorhombic. Rutile is the only stable phase whereas Anatase and Brookite are metastable at all temperatures and can be converted to rutile after heat treatment at high temperature. Fig. 2 shows the evolution of XRD pattern of (TiO2) thin films prepared with 1, 3 and 5 coatings layers on glass substrate and annealed at temperature of 400 C. These spectra show a peak corresponding to the (101) plane, which is attributed to the presence of Anatase regardless of number of coatings layers. At annealing temperatures of 400 C and for 1 and 3 coatings layers
0; 9l bcosq
A). l is the wavelength of X-ray beam (Cu Ka ¼ 1.5406 b is the full width at half maximum (FWHM) of the (hkl) diffraction peak. q is the Bragg angle. We have calculated the grain sizes of the thin films annealed at temperature of about 400 C and different numbers of coating 1 and 3. We found that the crystallinity of the obtained Anatase particles increased from 2.9 to 8.3 nm as the number of coatings layers increase, whereas the size of Rutile crystallites increases with increasing the number of coatings layers from 3.2 nm to 5.9 nm. From the obtained results, we identify the optimum conditions for complete crystallization of the prepared TiO2 films as 1 coating layer and annealing temperature of 400 C. In Fig. 3, we show SEM micrograph of TiO2 annealed film at temperature of 400 C. We can observe that the substrate is inhomogeneously covered by large aggregates with typically 10 mm wide and smooth surface with well-defined edges. The aggregates are separated by larges cracks probably formed during the drying process due to surface tension between the film and the air. Fig. 4 shows typical two- and three-dimensional AFM surface micrographs of TiO2 films prepared by 1 coating layer and annealed at temperature of 400 C. The deposited film exhibits smooth and compact granular morphology. From the AFM pattern we can observe that the film is formed by the nanoparticles of 20 nm in diameter and the corresponding average roughness is about 8.8 nm. Fig. 5 shows the typical evolution of (ahy)2 as a function of the photon energy in our films. The optical band gap value
Fig. 4 e 3D and 2D AFM images of TiO2 thin granular film recorded on the aggregates shown in Fig. 3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
[2] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33e177.
2 12 -1 2 (αhυ) 10 (eV Cm )
6
4
2
3,7
0 1
2
3
4
5
hυ (eV) Fig. 5 e Evolution of (ahy)2 vs the photon energy for the film of Fig. 4.
deduced from this figure is 3.7 eV. This value is in good agreement with those published by other authors [20,21]. However, it is still higher than the optimum range for solar energy conversion. It should be reduced by adding some metallic elements such as aluminuim and silver. Such work is currently undertaken in our laboratory.
4.
4133
Conclusions
This study deals with the preparation and characterization of TiO2 thin films using a simple and cost effective method: solegel spin coating. The deposition was performed on glass substrates at room temperature. The effect of the number of coatings and the annealing temperature were investigated. It has been shown that the complete crystallization of the films occurs after annealing at 400 C in air. The best XRD spectra were observed on the layers prepared with one coating. SEM analyses show that the films consist of agglomeration of large particles. These particles consist of clusters of Anatase and Rutile nanoparticles as revealed by XRD characterizations. The band gap value deduced from the optical spectra using UVeViseNIR spectrometry indicates a value around 3.7 eV. This value is still higher for an efficient utilization under sun light illumination. It should be reduced by addition of some metallic elements such as aluminium or incorporation of Nitrogen in the films. Intensive works are actually undertaken in our laboratory to achieve this concern.
Acknowledgements This work was partially supported by the CNRST/CNRS cooperation program (Chimie 05/06, 05/07, 05/08 and SPM05/09).
references
[1] Diebold U. The surface science of titanium dioxide. Surf Sci Rep 2003;48:53e229.
[3] O’Regan B, Gratzel M. Photoelectrochemical properties of Nbdoped TiO2 semiconductor ceramic membrane. Nature 1991; 353:737e9. [4] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [5] Dholam R, Patel N, Adami M. Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting. Int J Hydrogen Energy 2008;33(23):6896e903. [6] Xu S, Ng J, Zhang X, Bai H, Sun DD. Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water. Int J Hydrogen Energy 2010;35:52e4. [7] Akihiko K. Recent progress in the development of visible light-driven powdered photocatalysts for water splitting. Int J Hydrogen Energy 2007;32(14):2673e8. [8] Martinet C, Paillard V, Gagnaire A, Joseph J. Deposition of SiO2 and TiO2 thin films by plasma enhanced chemical vapor deposition for antireflection coating. J Non-Cryst Solids 1997; 216:77e82. [9] Sun SC, Chen TF. Effects of electrode materials and annealing ambients on the electrical properties of TiO2 thin films by metalorganic chemical vapor deposition. Jpn J Appl Phys 1997;36:1346e50. [10] Mulla IS, Soni HS, Rao VJ, Sinha APB. Deposition of improved optically selective conductive tin oxide films by spray pyrolysis. Mater J Sci 1986;21:1280e8. [11] Ennaoui A, Sankapal BR, Skryshelevsky V, Lux-Steiner MC. TiO2 and TiO2eSiO2 thin films and powders by one-step softsolution method: synthesis and characterization. Sol Energy Mater Sol Cel 2006;90:1533e41. [12] Suzuki S, Ohsaki H, Ando E. Materialistic difference in macroscopic friction coefficients of sputtered metal oxide thin films deposited on glass. Jpn J Appl Phys 1996;35:1862e7. [13] Guillard C, Beaugiraud B, Dutriez C, Herrmann JM, Jaffrezic H, Nicole JR, et al. Physicochemical properties and photocatalytic activities of TiO2-films prepared by sol-gel methods. Appl Cat B Environnemental 2002;39(12):331e42. [14] Sankapal BR, Sartale SD, Lux-Steiner MC, Ennaoui A. Chemical and electrochemical synthesis of nanosized TiO2 anatase for large-area photon conversion. Comptes Rendus Chim 2006;9(6):702e7. [15] Hepel M, Kumarihamy Indeewari D. Nanocrystalline structure and nanopore formation in modified thermal TiO 2 films. Int J Hydrogen Energy 2007;32:2693e702. [16] Palmas S, Polcaro AM, Rodriguez Ruiz J, Da Pozzo A, Mascia M, Vacca A. TiO2 photoanodes for electrically enhancedwater splitting. Int J Hydrogen Energy 2010;35(13): 6561e70. [17] Bouabid K, Ihlal A, Amira Y, Sdaq A, Assabane A, Ait-Ichou Y, et al. Optical study of TiO2 thin films prepared by solegel. Ferroelectrics 2008;372:69e75. [18] Mishra PR, Shukla PK, Srivastava ON. Study of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured TiO2 photoelectrodes. Int J Hydrogen Energy 2007;32:1680e5. [19] Zou JJ, Hea H, Cuib L, Dub HY. highly efficient Pt/TiO2 photocatalyst for hydrogen generation prepared by a cold plasma method. Int J Hydrogen Energy 2007;32:1762e70. [20] Teresa M, Viseu R, Isabel M, Ferreira C. Morphological characterization of TiO2 thin films. Vacuum 1999;52(1(2)): 115e20. [21] Park YR, Kim KJ. Structural and optical properties of rutile and anatase TiO2 thin films: effects of co doping. Thin Solid Films 2005;484:34e8.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Optical and structural properties of TiO2 thin films prepared by solegel spin coating A. Elfanaoui a,*, E. Elhamri a, L. Boulkaddat a, A. Ihlal a, K. Bouabid a, L. Laanab b, A. Taleb c, X. Portier d a
Laboratoire Mate´riaux et Energies Renouvelables (LMER), Universite´ Ibn Zohr De´p. Physique, Faculte´ des sciences B.P.8106, Hay Dakhla, 80000 Agadir, Morocco b Centre de Microscopie, Faculte´ des sciences, Universite´ Mohamed V, 10 000 Rabat, Morocco c LECIME, Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France d CIMAP, Ensicaen, Bd du Mare´chal Juin, 14050 Caen, France
article info
abstract
Article history:
Hydrogen is a renewable and non-polluting fuel. Its production from water using renew-
Received 16 April 2010
able energy is an attractive challenge. In this work we report some results on the prepa-
Received in revised form
ration of titanium oxide TiO2 thin films for environmental applications such as water
8 July 2010
photosplitting. TiO2 thin films have been prepared by spin coating technique of sol
Accepted 11 July 2010
precursor onto glass substrates. The deposited films were annealed at different tempera-
Available online 15 August 2010
tures in air. The X-ray diffraction (XRD) experiments show that the two well-known anatase and rutile phases were observed depending both on the conditions of deposition
Keywords:
and on the temperature of annealing. The best conditions of crystallization were found to
TiO2
be around 400 C in air. The influence of the number of deposited layer on the crystalline
Nano-particles
quality of the films was investigated. The surface morphology of the deposited film was
Anatase
characterized by atomic force microscopy (AFM) and scanning electronic microscopy
Rutile
(SEM). The UVeViseNIR spectroscopy shows that the film exhibits a high transmission around 90%. The best layers were obtained when concentrated (HCl) was added to the sol solutions. The direct band gap of the films was found to be around 3.7 eV, and their refractive index was found to vary from 2 to 2.4. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Titanium oxide (TiO2) is currently the focus of intensive research thanks to the wide range of its potential applications. Thanks to its durability, non-toxicity, stability and its high refractive index, such material is a preferred material for optical purposes, white pigment, gas sensor, and corrosive-protective (for a review about TiO2 see Diebold [1]). For environmental concerns, TiO2 is extensively used for photodegradation of organic and inorganic pollutants [2], photovoltaic energy
production [3], and hydrogen production by water photosplitting [4e6]. For this later concern, the water photosplitting over semiconductor oxides yields high pure hydrogen and is the topic of many investigations that have been culminated in the publication of many articles [7]. Hydrogen is a renewable and nonpolluting fuel and its production from cheap raw materials like water using renewable energy is an attractive challenge. Because its abundance and non-toxicity, TiO2 is one of the attractive materials for such applications. TiO2 thin films have
* Corresponding author. Tel.: þ212 528220957; fax: þ212 28220100. E-mail address: [email protected] (A. Elfanaoui). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.057
4131
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
Titanium Tetraisopropoxyde
Brushing with a detergent
isopropanol
Mixing for 10min
Rinsing with bidistilled water
Concentrated hydrochloric acid HCl (0.1ml) + 0.2ml distilled water
Ultrasonic cleaning in a bidistilled water during 15 min
Mixing for 24h
Rinsing with bidistilled water
Gel
Substrat Cleaning with ultrasound in an acetone bath
Spin coating Cleaning with ultrasound in a bidistilled
Drying (70°C) for 1h
water bath.
Thermal treatment For 1h (300,350 and 400°C)
Rinsing with bidistilled
water
Stoving at 200°C (20min)
Fig. 1 e Flow chart showing the procedure for processing the TiO2 thin films (left) and substrate preparation (right).
been prepared using a variety of processes including plasma enhanced chemical vapour deposition [8] radio-frequency magnetron sputtering [5], metal organic chemical vapour deposition [9], spray pyrolysis [10,11], chemical bath deposition [12], sputtering [13], electrochemical deposition [14e16], solegel [17,18] photo-deposition [6] and cold plasma method
[19]. Non-vacuum methods are very attractive since they are cost effective, simple and suitable for large area scaling. In this work, we show that by using very simple and low cost solegel synthesis technique we can prepare granular TiO2 film.
2. A(101)
25
20
1 coating layers 3 coating layers 5 coating layers
A(215)
R(112)
A(211)
A(200)
5
A(204)R(002)
10 A(004)
I (au)
15
0 10
20
30
40
50
60
70
80
2θ (°) Fig. 2 e XRD pattern of TiO2 thin films annealed at 400 C for different thickness: 1 (red curve), 3 (blue curve) and 5 (green curve) coatings layers (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Experiments
The solegel process was used to synthesize TiO2 thin films. The alkoxide used during the current work is titanium isopropoxide TTIP (C12H28O4Ti) with purity of 99.99% which was mixed with isopropanol and hydrochloric acid (11M) as a catalyst and distilled water. The procedure of preparation includes the dissolution of 25 ml of isopropanol as solvent and 0.1 ml of hydrochloric acid (HCl), 0.2 ml of distilled water is added as well as 3.5 ml of titanium isopropoxide (C12H28O4Ti) (Fig. 1); this solution is transparent, of yellowish colour and is ready for the deposit. Thin films of TiO2 were deposited by the method of spin coating in air at room temperature, on corning glass (25 25 2 mm3) as substrate. The process of cleaning the substrate is summarized in the flow chart (Fig. 1). The films were dried at 100 C for 1 h and annealed at 300, 350 and 400 C in air during 1 h. The crystalline structure was characterized by an X-ray diffractometer (Philips PW) in 2q range from 10 to 80 by 0,02 s1 steps operating at 40 kV accelerating voltage and 40 mA current using Cu Ka radiation source. The incident angle was kept constant at 0.5 throughout the
4132
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
conditions, we observe in addition to anatase (101), (004), (200), (211), (204) and (215) the formation of rutile which crystallizes in the (002) and (112) plane parallel to the surface. We also observe that the intensities corresponding to the lines characteristic of anatase and rutile increase as the number of coating layers decreases. From the peaks intensity we can determine the variation of TiO2 amount as a function of the number of coatings layers. The grain size (L) of the prepared TiO2 thin films was obtained from XRD line broadening using the Scherrer equation: L¼
Fig. 3 e SEM micrograph of the surface of TiO2 annealed at 400 C (1 coating).
experimentation. Atomic force microscopy (AFM) measurements were performed at room temperature using a Nanoscope III instrument in a standard tapping mode to investigate the surface morphology of TiO2 spin-coated films. Scanning electronic microscopy SEM (JOEL JSM 6400 Scanning microscope) was used to study the surface morphology and pore distribution of the produced films. The transmission spectra were measured by Lambda 9 UV/VIS/NIR PerkineElmer spectrophotometer.
3.
Results and discussion
TiO2 has three well-known phases namely: Anatase, Rutile and Brookite. Rutile and Anatase are tetragonal whereas Brookite is orthorhombic. Rutile is the only stable phase whereas Anatase and Brookite are metastable at all temperatures and can be converted to rutile after heat treatment at high temperature. Fig. 2 shows the evolution of XRD pattern of (TiO2) thin films prepared with 1, 3 and 5 coatings layers on glass substrate and annealed at temperature of 400 C. These spectra show a peak corresponding to the (101) plane, which is attributed to the presence of Anatase regardless of number of coatings layers. At annealing temperatures of 400 C and for 1 and 3 coatings layers
0; 9l bcosq
A). l is the wavelength of X-ray beam (Cu Ka ¼ 1.5406 b is the full width at half maximum (FWHM) of the (hkl) diffraction peak. q is the Bragg angle. We have calculated the grain sizes of the thin films annealed at temperature of about 400 C and different numbers of coating 1 and 3. We found that the crystallinity of the obtained Anatase particles increased from 2.9 to 8.3 nm as the number of coatings layers increase, whereas the size of Rutile crystallites increases with increasing the number of coatings layers from 3.2 nm to 5.9 nm. From the obtained results, we identify the optimum conditions for complete crystallization of the prepared TiO2 films as 1 coating layer and annealing temperature of 400 C. In Fig. 3, we show SEM micrograph of TiO2 annealed film at temperature of 400 C. We can observe that the substrate is inhomogeneously covered by large aggregates with typically 10 mm wide and smooth surface with well-defined edges. The aggregates are separated by larges cracks probably formed during the drying process due to surface tension between the film and the air. Fig. 4 shows typical two- and three-dimensional AFM surface micrographs of TiO2 films prepared by 1 coating layer and annealed at temperature of 400 C. The deposited film exhibits smooth and compact granular morphology. From the AFM pattern we can observe that the film is formed by the nanoparticles of 20 nm in diameter and the corresponding average roughness is about 8.8 nm. Fig. 5 shows the typical evolution of (ahy)2 as a function of the photon energy in our films. The optical band gap value
Fig. 4 e 3D and 2D AFM images of TiO2 thin granular film recorded on the aggregates shown in Fig. 3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 3 0 e4 1 3 3
[2] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33e177.
2 12 -1 2 (αhυ) 10 (eV Cm )
6
4
2
3,7
0 1
2
3
4
5
hυ (eV) Fig. 5 e Evolution of (ahy)2 vs the photon energy for the film of Fig. 4.
deduced from this figure is 3.7 eV. This value is in good agreement with those published by other authors [20,21]. However, it is still higher than the optimum range for solar energy conversion. It should be reduced by adding some metallic elements such as aluminuim and silver. Such work is currently undertaken in our laboratory.
4.
4133
Conclusions
This study deals with the preparation and characterization of TiO2 thin films using a simple and cost effective method: solegel spin coating. The deposition was performed on glass substrates at room temperature. The effect of the number of coatings and the annealing temperature were investigated. It has been shown that the complete crystallization of the films occurs after annealing at 400 C in air. The best XRD spectra were observed on the layers prepared with one coating. SEM analyses show that the films consist of agglomeration of large particles. These particles consist of clusters of Anatase and Rutile nanoparticles as revealed by XRD characterizations. The band gap value deduced from the optical spectra using UVeViseNIR spectrometry indicates a value around 3.7 eV. This value is still higher for an efficient utilization under sun light illumination. It should be reduced by addition of some metallic elements such as aluminium or incorporation of Nitrogen in the films. Intensive works are actually undertaken in our laboratory to achieve this concern.
Acknowledgements This work was partially supported by the CNRST/CNRS cooperation program (Chimie 05/06, 05/07, 05/08 and SPM05/09).
references
[1] Diebold U. The surface science of titanium dioxide. Surf Sci Rep 2003;48:53e229.
[3] O’Regan B, Gratzel M. Photoelectrochemical properties of Nbdoped TiO2 semiconductor ceramic membrane. Nature 1991; 353:737e9. [4] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [5] Dholam R, Patel N, Adami M. Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting. Int J Hydrogen Energy 2008;33(23):6896e903. [6] Xu S, Ng J, Zhang X, Bai H, Sun DD. Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water. Int J Hydrogen Energy 2010;35:52e4. [7] Akihiko K. Recent progress in the development of visible light-driven powdered photocatalysts for water splitting. Int J Hydrogen Energy 2007;32(14):2673e8. [8] Martinet C, Paillard V, Gagnaire A, Joseph J. Deposition of SiO2 and TiO2 thin films by plasma enhanced chemical vapor deposition for antireflection coating. J Non-Cryst Solids 1997; 216:77e82. [9] Sun SC, Chen TF. Effects of electrode materials and annealing ambients on the electrical properties of TiO2 thin films by metalorganic chemical vapor deposition. Jpn J Appl Phys 1997;36:1346e50. [10] Mulla IS, Soni HS, Rao VJ, Sinha APB. Deposition of improved optically selective conductive tin oxide films by spray pyrolysis. Mater J Sci 1986;21:1280e8. [11] Ennaoui A, Sankapal BR, Skryshelevsky V, Lux-Steiner MC. TiO2 and TiO2eSiO2 thin films and powders by one-step softsolution method: synthesis and characterization. Sol Energy Mater Sol Cel 2006;90:1533e41. [12] Suzuki S, Ohsaki H, Ando E. Materialistic difference in macroscopic friction coefficients of sputtered metal oxide thin films deposited on glass. Jpn J Appl Phys 1996;35:1862e7. [13] Guillard C, Beaugiraud B, Dutriez C, Herrmann JM, Jaffrezic H, Nicole JR, et al. Physicochemical properties and photocatalytic activities of TiO2-films prepared by sol-gel methods. Appl Cat B Environnemental 2002;39(12):331e42. [14] Sankapal BR, Sartale SD, Lux-Steiner MC, Ennaoui A. Chemical and electrochemical synthesis of nanosized TiO2 anatase for large-area photon conversion. Comptes Rendus Chim 2006;9(6):702e7. [15] Hepel M, Kumarihamy Indeewari D. Nanocrystalline structure and nanopore formation in modified thermal TiO 2 films. Int J Hydrogen Energy 2007;32:2693e702. [16] Palmas S, Polcaro AM, Rodriguez Ruiz J, Da Pozzo A, Mascia M, Vacca A. TiO2 photoanodes for electrically enhancedwater splitting. Int J Hydrogen Energy 2010;35(13): 6561e70. [17] Bouabid K, Ihlal A, Amira Y, Sdaq A, Assabane A, Ait-Ichou Y, et al. Optical study of TiO2 thin films prepared by solegel. Ferroelectrics 2008;372:69e75. [18] Mishra PR, Shukla PK, Srivastava ON. Study of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured TiO2 photoelectrodes. Int J Hydrogen Energy 2007;32:1680e5. [19] Zou JJ, Hea H, Cuib L, Dub HY. highly efficient Pt/TiO2 photocatalyst for hydrogen generation prepared by a cold plasma method. Int J Hydrogen Energy 2007;32:1762e70. [20] Teresa M, Viseu R, Isabel M, Ferreira C. Morphological characterization of TiO2 thin films. Vacuum 1999;52(1(2)): 115e20. [21] Park YR, Kim KJ. Structural and optical properties of rutile and anatase TiO2 thin films: effects of co doping. Thin Solid Films 2005;484:34e8.