CHAPTER 2. EFFECT OF COBALT DOPING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF TiO 2 FILMS PREPARED BY SOL-GEL PROCESS

Transcription

1 3 CHAPTER EFFECT OF COBALT DOPING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF TiO FILMS PREPARED BY SOL-GEL PROCESS.1 INTRODUCTION Titanium dioxide (TiO ) thin films possess high transmittance and high refractive index in the visible region with chemical stability and good durability in hostile environments (Bach and Krause 1997). Due to their promising structural, optical, electrical, and chemical properties, TiO thin films have been widely used for many applications such as photocatalytic purifier (Fujishima and Honda 197), optical thin film devices like antireflection coating for solar cell (Hara et al 001), multilayer optical coating (Dakka et al 000), and also as a sensor material (Tang et al 1995). TiO films are used in capacitors of microelectronic devices due to their high dielectric constant (Alexandrov et al 1996). The photocatalytic and photoelectrochemical activities of TiO thin films have been improved by doping with noble metals (Zhao et al 1996). The structural and optical properties have been enhanced by impurity doping and heat treatment (Mardare et al 000; Mardare and Rusu 000). Even though, several dopants have been studied, the Co doped TiO thin films have gained much importance as they exhibit ferromagnetism at room temperature conditions, which are suited for spintronic applications (Matsumoto et al 001; Daniel Bryan et al 004) and also for the realization of photocatalytic properties in the visible region (Mitsunobu Iwasaki et al 000).

2 4 Many deposition techniques have been used to prepare cobalt doped titanium dioxide (Co:TiO ) thin films, such as oxygen-plasma-assisted molecular beam epitaxy (Chambers et al 00), sputtering (Wan Kyu Park et al 00), laser ablation (Shinde et al 003), ion implantation (Lee et al 006) and sol-gel (Daniel Bryan et al 004; Suryanarayanan et al 005). Though all these samples did show unambiguous sign of ferromagnetic behavior at 300 K and above, it is difficult to exclude the possibility of formation of Co clusters and thus attribute the observed magnetism entirely to Co substituted TiO. Several arguments, for and against the formation of clusters were proposed by different groups (Matsumoto et al 001; Chambers et al 00). Daniel Brian et al (004) and Suryanarayanan et al (005), have demonstrated that it is possible to prepare colloidal cobalt doped TiO nanocrystals without the formation of metallic cobalt and cobalt oxide in Co:TiO. Mitsunobu Iwasaki et al (000) reported the visible light photocatalytic activity of Co + ion doped TiO. Earlier reports on Co doped TiO revealed that most of the studies focus on the spintronic properties of Co:TiO and there is no systematic study on the optical and structural properties of Co:TiO for different cobalt concentrations. Poniatowski et. al (1994) reported a red shift upon Co doping (up to %) in TiO powders. Simpson et al (004) reported a blue shift in the Co:TiO films prepared by pulsed laser deposition. The formation of Co doped TiO is reported to be very much depended on the preparation methods. The films have also been prepared by a non-vacuum approach like the sol-gel process, which is efficient to prepare large area, transparent, doped films with controlled porosity and multi-component oxide layers of many compositions on various substrates (Trapalis et al 1993; Yu et al 00). In the present investigation, Co:TiO films have been prepared by sol-gel process up to 15 wt% of cobalt concentration and their structural and optical properties have been studied by variety of characterization methods.

3 5. EXPERIMENTAL PROCEDURE..1 Sample Preparation Cobalt doped titanium dioxide (Co:TiO ) films have been prepared by sol-gel technique using titanium (IV) butoxide and cobalt acetate tetrahydrate as precursors. For the preparation of Co:TiO sol, cobalt acetate tetrahydrate was dissolved in a mixture of 0 ml absolute ethanol and 1 ml H O and refluxed at 80 C for 10 min. After cooling the solution to room temperature, titanium (IV) butoxide was added followed by the addition of 0.1 ml of concentrated nitric acid and a few drops of acetyl acetone. In order to obtain a homogeneous mixture of Co: TiO, the solution was stirred vigorously for h. The above process was carried out in an ice bath. Figure.1 shows the flow chart for the preparation of cobalt doped TiO thin films. Cobalt acetate + Ethanol + H O Refluxed Titanium (IV) butoxide + HNO 3 Stirring Adding Acetyl acetone Stirring Spin coating Dried at 100 C Repeated for several times Annealing at 600 C Co:TiO film Figure.1 Flow chart for the sol-gel synthesis of cobalt doped TiO thin films

4 6 Typical sol was prepared for various wt% of cobalt concentrations Ti 1-x Co x O (x = 0.01, 0.0, 0.03, 0.04, 0.05, 0.08, 0.15) and the films were deposited on microscopic slides and Si(100) substrates by spin coating technique. Before coating, the substrates were cleaned in an ultrasonic bath for 10 min using ethanol, acetone and deionised water. The sol was spin coated on these substrates at 500 rpm and then dried at 100 C for 10 min and the deposition process was repeated for several times. The coated films on glass and Si substrates were annealed at 350 C and 600 C for 1 h respectively using a resistive heated furnace controlled with an accuracy of ±0.1 C... Surface Morphology Effect of cobalt doping on the surface morphology of Co:TiO films was studied by scanning electron microscopy (SEM) employing a Hitachi S-4800 model high resolution SEM. Figures.a and.b show the SEM images of Ti 0.95 Co 0.05 O and Ti 0.85 Co 0.15 O films deposited on Si (100) substrates. From the SEM analysis, it has been observed that the grains are almost uniform with smooth surface. As shown in Figure.a, for less cobalt concentration the films possess large number of pores and density of the pores decreases upon increasing the cobalt concentration as evidenced from Figure.b. From the SEM studies, it is also observed that upon increasing the cobalt concentration the grains are more equiaxed with continuous grain boundary. This indicates that the density of the films increases upon increasing the cobalt concentration. The average grain size of the samples lies between 10 to 30 nm.

5 7 (a) 00 nm (b) 00 nm Figure. Surface morphology of (a) Ti0.95Co 0.05O and (b) Ti0.85Co 0.15O thin films studied by high resolution scanning electron microscope

6 8..3 Film Composition The composition and thickness of the deposited films were determined by Rutherford backscattering spectroscopy (RBS) method. A tandetron accelerator with 1.4 MeV 4He + particles and a current of 14 ma was used for RBS measurements. The backscattered particles were detected at an angle of 170º with respect to the incident beam direction by a semiconductor detector with an energy resolution of 10 kev. The thickness of cobalt doped titanium dioxide layer was calculated from the energy width of the Ti peak taking into account the energy loss of 4He + per unit depth in the cobalt doped titanium dioxide matrix. The stoichiometry of the films has been estimated from the height of the peaks. Figure.3 shows the typical RBS spectrum recorded for a 5 wt% cobalt doped titanium dioxide film. In order to derive the stoichiometry of the films, the experimental data has been simulated theoretically by using Rutherford universal manipulation program (RUMP). The simulated curve shows good agreement with the experimental observations, which indicates that the film properties are described preciously within the error bar limitations of our experimental conditions (Figure.3). Table.1 shows the precursor stoichiometry and the film stoichiometry measured by RBS. For low cobalt concentrations, up to wt% no cobalt incorporation was detected in the TiO films, as the values of both the dopant concentration and the error bar is equal (0.). Upon increasing the cobalt concentration, the cobalt signal is clearly observed in the RBS spectrum. From Table.1, it is confirmed that the film stoichiometry is almost equal to the precursor stoichiometry.

7 9 Energy MeV O O TiOx Experiment --- Simulation Normalized Yield Normalized Yield Ti Co Channel Figure.3 Rutherford backscattering spectrum for 5 % Co:TiO film Table.1 Comparison of RBS derived stoichiometry of the films to the starting precursor values Sl. No. Expected stoichiometry as per precursor preparation Stoichiometry measured by RBS 1 Ti 1 O Ti 1 O Ti 0.99 Co 0.01 O Ti 1 O 3 Ti 0.98 Co 0.0 O Ti 0.98 Co 0.01 O 4 Ti 0.95 Co 0.05 O Ti 0.96 Co 0.04 O 5 Ti 0.9 Co 0.08 O Ti 0.9 Co 0.07 O 6 Ti 0.85 Co 0.15 O Ti 0.85 Co 0.14 O

8 30..4 X-ray Diffraction Analysis Grazing angle X-ray diffraction (GXRD) analysis was performed to study the film structure. To record the X-ray diffraction (XRD) patterns of the films, the grazing angle geometry with CuK radiation was used. Measurements were performed at an incidence angle of 0.75º and the typical XRD patterns were recorded for θ varying from 15º to 75º using a Philips X pert X-ray diffractometer. For these measurements, an incidence angle of 0.75 was chosen and for this incidence angle the effective penetration depth of X-rays is a few hundred nanometer, which is slightly less than the thickness of the films under investigation. Co:TiO films of approximately 400 nm thickness deposited on silicon substrate was used. The GXRD patterns for the films annealed at 600 C with varying dopant concentration are depicted in Figure.4. The as-deposited films and the films annealed at 350 C showed amorphous behavior (not shown in figure). The films annealed at 600 C showed crystalline anatase structure without any secondary or mixed phase. The experimental peak positions were compared with the standard ICDD files (card No. 1-17) and the corresponding miller indices were indexed. In the X-ray patterns, no peaks corresponding to cobalt titanate and cobalt oxide phases were observed. Analysis of the full width at half maximum of (101) peak using the Scherrer equation (West 199) revealed the grain size of the Ti 1-x Co x O thin films and the calculated grain size is given in Table....5 Raman Spectroscopy Studies The Raman spectroscopy measurements were performed in the range cm -1 employing a dispersive Raman spectrometer (Thermoelectron Corporation) that employs a YAG laser with an excitation wavelength of 53 nm.

9 31 Ti 0.85 Co 0.15 O Intensity (arb. units) Ti 0.95 Co 0.05 O Ti 0.96 Co 0.04 O (101) (004) (00) Ti 0.97 Co 0.03 O (105) (11) (04) (degree) Figure.4 Grazing angle XRD patterns for different wt. % of Co:TiO thin films (a) 3% Co:TiO (b) 4% Co:TiO (c) 5% Co:TiO and (d) 15% Co:TiO Table. Gain size values of Co:TiO thin films Sample Grain size (nm) Ti. 99 Co.01 O 8 Ti. 98 Co.0 O 5 Ti. 97 Co.03 O 15 Ti. 96 Co.04 O 0 Ti. 95 Co.05 O 16 Ti. 85 Co.15 O 19

10 3 The X-ray diffraction studies revealed that the films annealed at 600 C possess anatase structure. This result is complimented by Raman spectroscopy studies, which show almost all the expected vibrational modes of anatase TiO. Figure.5 shows the Raman spectra of titanium dioxide films for different cobalt dopant concentrations after crystallization. The asdeposited and the films annealed at 350 C did not show any TiO vibration modes, which confirm the amorphous nature of these films. A strong peak at 50 cm -1 corresponding to the LO-phonon mode of Si(100) substrates was observed in all measurements. (e) 15% Co:TiO Intensity (log scale) (d) 7% Co:TiO (c) 5% Co:TiO (b) 3% Co:TiO E g Si B 1g Si E g Si-O-Ti (a) TiO Wave number (cm) -1 Figure.5 Raman spectra of pure and Co doped TiO thin films (a) TiO (b) 3% Co: TiO (c) 5% Co:TiO (d) 7% Co:TiO and (e) 15% Co: TiO

11 33 In Figure.5, except the substrate peak at 50 cm -1, all other peaks observed at 14.5, 198, 396 and 637 cm -1 were attributed to the anatase phase of TiO. A broad band centered at 950 cm -1 is attributed to the silicon-oxygentitanium bond formation upon annealing (Kamada et al 1991), indicating a strong interaction between the titania film and the native silicon dioxide layer on the silicon substrate. Within the allowed experimental shift of a few wave numbers, no discernable shift in Raman peaks was observed for higher Co concentrations. This confirms that the anatase structure is retained even for higher cobalt doping. This result is in agreement with the X-ray diffraction studies. The absence of characteristic vibrational modes of cobalt oxide in the, Raman spectrum suggests that there is no cobalt oxide segregation into TiO which indicates that cobalt dopants might occupy the substitutional sites in the host lattice...6 Optical Properties In order to study the influence of cobalt concentration on the optical properties of the films, both optical transmittance and spectroscopic ellipsometry measurements were carried out in the UV-visible region. The optical transmittance of the films was recorded by using a Perkin Elmer lambda-5 spectrometer. The transmittance data were recorded in the spectral range from 1. (10000 cm -1 ) to 6. ev (50,000 cm -1 ). The ellipsometric data were recorded in the energy range from 0.7 (5807 cm -1 ) to 5. ev (41940 cm -1 ) using an ellipsometer (JASCO Model M-10). In order to derive the film properties like thickness, refractive index and band gap (E g ), theoretical simulations have been performed using SCOUT software package (Theiss 1998). The data obtained from the ellipsometry have been used as the input for simulations, to reveal the film properties. A model of O Leary et al (1997) was used for computer simulation. Expressions for the joint density of states are given for optical transitions from the valance band to the conduction band.

12 34 By using this density of states, the imaginary part of the dielectric function is modeled and then the real part is calculated by Fast-Fourier-Transformation to satisfy causality (Kramers-Kronig relation) and the details on this modeling is described elsewhere (Weis et al 1999; Lange et al 000). From the dielectric function, the refractive index and extinction coefficient were calculated. The refractive indices of the films were also measured in time resolving mode for a fixed wavelength of 63 nm. Figure.6 shows typical transmittance spectra recorded for different Co:TiO films. From the figure, it is clear that the films are fully transparent in the visible region and starts absorbing in the range 350 to 400 nm. With increasing cobalt concentration the absorption edge shifts towards higher wavelength side indicating the decrease in bandgap of the films TiO Transmittance % Co:TiO 10% Co:TiO 15% Co:TiO Absorption edge decreases upon increasing Co doping Wavelength (nm) Figure.6 UV-VIS Transmittance spectra of Co:TiO

13 35 Figure.7 shows the typical spectroscopic ellipsometry measurement carried out for 1% Co:TiO film coated on a microscopic slide. The experimental data were analyzed by an adjustable oscillator model, to obtain a viable optical model compromising both microstructrual parameters and optical constants of the films. We have used OJL model (O Leary et al 1997) to describe the optical constants of the material. The simulated curves (solid line) agree well with the experimental data (lines with circles), and hence the material property can be described accurately. The variation of refractive index for 1% Co:TiO is shown in Figure.8. It is evident that the as-deposited films dried at 100 C possess very low refractive index value, which is due to the amorphous phase (confirmed by XRD) with less film 5 0 Experiment Simulation [ 0 ] [ 0 ] % Co:TiO Annealed at C Energy [ev] Figure.7 Spectroscopic ellipsometry measurement for 1% Co:TiO annealed at 600C

14 36 density. This is further confirmed by time resolved mode measurements in which a He-Ne laser (63.8 nm wavelength) was used to measure the refractive index of the films as a function of time. It is well known that, in solgel process organic products from the precursors would also influence the crystallinity as well as the optical properties of the films (Suresh et al 1998; Mahanty et al 004). Upon annealing at higher temperatures, due to the evaporation of organic products, the films become dense with an increased crystallinity, which significantly improves the refractive index of the films as shown in Figure.8. The refractive index (calculated at 650 nm) and bandgap values for different crystalline Co:TiO films are tabulated in Table.3. It is clear that the refractive index of the films increases with increasing cobalt dopant concentration and is presumably due to the increase in film density (Subramanian et al 007). The dependence of refractive index on the film density could be easily explained by the well known Clausius-Mossotti relation (Kittel 1971; Heitman 1970). The relation between the packing density (P), defined as the ratio of the film density ( ) to the bulk density ( b ) of the material, and the corresponding refractive index of the material is given as (Kittel 1971) f f P b n n f f 1 n n b b 1 (.1) where TiO. n f and nb are the refractive index of the film and the bulk material of f and b are the density of the film and bulk material, respectively. The packing density of the film was calculated from the refractive index values. Figure.9 depicts the variation of packing density of Co:TiO films as a function of cobalt concentration. It is seen that the packing density of the films gradually increase upon increasing cobalt concentration, indicative of the increase in film density with Co dopant concentration (Subramanian et al 007).

15 37 Refractive index n Amorphous film : Film dried at 100 o C Crystalline film : Annealed at 600 o C (b) (a) Energy [ev] Figure.8 Variation of the refractive index for 1% Co:TiO thin film (a) Dried at 100 C (b) Annealed at 600 C. The index of refraction increases upon increasing annealing temperature Table.3 Thickness, Refractive index and band gap of Co:TiO films determined by spectroscopic ellipsometry. The refractive index of the films increases with simultaneous decrease of optical band gap Wt% of Co doping Thickness [nm] Refractive index n at 650 nm Band gap E g [ev]

16 Packing density f nf P b nf 1 nb nb Cobalt concentration (wt. %) Figure.9 Variation of packing density of Co:TiO thin films with different cobalt concentrations For pure TiO, a band gap value of 3.19 ev was obtained within the error limit, which is almost equal to the band gap of bulk-like TiO. The calculated band gap values are given in Table.3. It is also clear that the band gap decreases with increase in Co dopant concentration. The reduction in the band gap of Co:TiO may be due to the introduction of electronic states by the impurity Co 3d electrons. Similar trend of decrease in optical band edge has also been reported for Nd: TiO (Li et al 003) and Fe: TiO (Natalie Smirnova et al 001). Optical studies of Co:TiO films suggest that it is possible to tune the band gap of TiO films for any desired applications, which makes this material to be more attractive for photocatalytic applications in addition to their potential applications for spintronics devices. It is well known that the photocatalytic activity of TiO depends on three factors (i) the electron-hole

17 39 pair production capacity, (ii) the separation efficiency of the photogenerated charge pair, and (iii) the charge transfer efficiency of the holes and electrons (Chao et al 00). Due to the reduction in band gap, the yield of both photogenerated electron-hole pair and the photogenerated charge-pair increases, which is more favourable for the enhanced photocatalytic activity of TiO. The photogenerated electron-hole pairs are produced in the near visible region because of the reduced band gap of TiO doped with cobalt..3 CONCLUSION Effect of cobalt doping (up to 15 weight percentage) on the structural and optical properties of titanium dioxide films prepared by sol-gel spin coating technique has been studied. The deposition conditions have been optimized to obtain nanocrystalline films of pure and doped titanium dioxide with grain size ranging from 10 to 30 nm. RBS measurements revealed the stoichiometry of the films and the film compositions are comparable to the precursor stoichiometry. X-ray diffraction and Raman spectroscopy studies reveal that the as-deposited films are amorphous in nature and the films annealed at 600 C become crystalline with anatase structure. Scanning electron microscopy measurements showed that the pore density depends on cobalt concentration and nearly porous free films can be prepared with 15 wt% cobalt doping. Optical transmittance studies confirmed that the films are fully transparent in the visible region and the absorption edge shifts towards the higher wavelength side upon increasing the cobalt concentration. The refractive index of the films was found to be increased with Co concentration with a simultaneous decrease of the band gap. The observed decrease in band gap upon increasing Co concentration is attributed to the formation of Co impurity band into the TiO energy bands. The reduction in band gap is favourable for the enhanced photocatalytic activity of TiO in the visible region.