Preperation of Cobalt Oxide Thin Film using Chemical Spray Pyrolysis ...

International Journal of Scientific Research and Innovative Technology ISSN: 2313-3759

Vol. 2 No. 5; May 2015

Preperation of Cobalt Oxide Thin Film using Chemical Spray Pyrolysis Method and the Effect of Annealing Temperature in its Optical properties A.Elsakhi S. M. Hamed1, Mohamed A. Siddig1, Abdelrahman A. Elbadawi1, Asmma I. Mohamed1, M.Elhadi1,2* 1

Department of Physics, Faculty of Science and Technology, El-Neelian University P.O. Box 12702-KH-11121, Sudan 2 Department of Physics, Faculty of Science Ad Dawadimi, Shaqra University P.O. Box11911, KSA

Abstract In this work, Cobalt oxide thin films were prepared using chemical spray paralysis. The thin filmswere precipitated on glass substrates and were heated for various temperatures (100, 150, 200, 250 and 300οC). The annealed Cobalt oxide thin films were characterized using UV-visible spectroscopy. The optical properties of the prepared films were determined including Energy gap, Absorption coefficient, Extinction coefficient and refractive index. The annealing temperatures effect on the thickness of the films was profound. Whereas, the energy band gap were slightly affected by the selected annealing temperatures and the energy band gap were found to be 2.70, 2.60, 2.60, 2.60 and 2.57, respectively. The absorption coefficient, extinction coefficient and refractive index were increased with annealing temperature. Keywords: Absorption coefficient, chemical spray pyrolysis, optical properties, refractive index

catalytic behaviour of Co-containing MCM-41 and SBA[7] materials in Fischer–Tropsch synthesis[8]. In this regard, mesoporous materials obtained by a super molecular template mechanism have found wide applications in recent times due to their high surface area, narrow pore size distribution, large pore volume and hydrothermal stability[9]. Several mechanisms such as liquid crystal template,[10,11] cooperative template and legend-assisted templating[11] have been used to design mesophasic structures as part of the templating mechanisms. A number of papers have appeared in the literature on the synthesis and structures of metal oxides including laser ablation of cobalt and cobalt oxides (CoO and Co3O4) to prepare their nanoparticles. Therefore many procedures can be used for synthesis the nanocrystalline cobalt oxide. Mainly three routes namely, precipitation method (method I), sol/gel method (method II) and templating method (method III) and the efficacy of the catalyst obtained by each of the three methods was tested on a specific reaction. The final pore size distribution, surface area and

Introduction The supported catalysts containing cobalt in the metallic, oxidic, or sulphided state are very important in heterogeneous catalysis.[1,2] Both cobalt and nickel oxides are important materials that find applications in different fields such as catalysis, various types of sensors, electrochromic, electrical and other optoelectronic devices. The Co3O4 humidity-sensitive active in the visible wavelength region at room temperature has been reported[3]. Recently, there was an excellent review on the adsorption and applications of cobalt species on the interface, between aqueous solutions and metal oxides. These oxides pose various oxidation states; which motivate the researchers. They found special application as electrochromic materials exhibiting anodic electrochromism, changing colour from grey to pale yellow [4]. Nickel and cobalt oxides also have applications as electrochemical capacitors and in colouredglasses [5]. Cobalt-based catalysts are very interesting in the heterogeneous catalysis field, and have been used in hydrosulphurization and in hydrogenation also[6].Khodakovet al reported on the preparation and localization of cobalt and 112


local ordering if any, was related to the catalytic activity. In this study, the chemical spray paralysis was chosen for its simplicity. This followed by the preparation of Cobalt oxide thin films. The spray pyrolysis method is basically a chemical deposition method in which fine droplets of the desired material are sprayed onto a heated substrate. Continuous films are formed on the hot substrate by thermal decomposition of the material droplets [12].The thin films were precipitated on glass substrates and were heated for various temperatures (100, 150, 200, 250 and 300οC). The annealed Cobalt oxide thin films were characterized using UV-visible spectroscopy.

Vol. 2 No. 5; May 2015

The visible spectra obtained in shimadzo mini 1240 spectrophotometer scanning between 200 -1200 nm. The spectrophotometer measures how much of the light is absorbed by the sample.The intensity of light before going into a certain sample is symbolized by I0. The intensity of light remaining after it had gone through the sample is symbolized by I. the fraction of light transmitted is (I0/I) which is usually expressed as percent transmittance (%T)from this information,the absorbance of the sample is determined for that wavelength or as function for ranger of wavelength.Sophisticated UV/visible spectrophotometers often do this automatically.Although the sample could be solid (or even gaseous, they are usually liquid)[13]. A transparent cell, often called cavetti. It used to hold a liquid sample in spectrophotometer. The path length L through the sample is then the width of the cell through which the light passes through. Simple (economic) spectrophotometer may use cavetti shape like cylindrical test tubes, but more sophisticated one use rectangular cavity common 1cm in width for just visible spectroscopy, ordinary glass cavity may be used, but ultraviolet spectroscopy requires special cavities made of UV transparent materials such as quartz. An ultraviolet visible spectrum is essentially a graph of light absorbance versus wavelength in arrange of ultraviolet or visible regions[14].

Material and Method The Cobalt oxide thin films were prepared using the chemical spray paralysis and precipitated on glass substrates heated to (100, 150, 200, 250 and 300oC). Cobalt nitrate was coprecipitated with 4N Na2CO3 in a magnetic stirrer for 5 hrs at room temperature. The gel formed was washed repeatedly with deionizer distilled water till the filtrate was devoid of any impurities. The remaining solid was dried in air at 353 K and calcined at 673, 773 and 873 K for an hour each to get the required oxide. The reaction proceeds as follows,and the CoO thin was shown in figure (1)

Results and Discussion

Co (NO3)2 + Na2CO3 → CoCO3 + 2NaNO3,CoCO3 ¾D¾→D CoO + CO2, CoO → (in air and heating) Co3O4 (1)

Because of the crystallinity and higher transparency, the cobalt oxide films are suitable for optical analysis from which the absorption coefficient and energy band gap may be determined. The conversion of CoO into Co3O4 can also be shown by the determination of the optical band gap. For this, in the fundamental absorption region the optical absorption coefficient (α) was evaluated using α=(lnT-1)/t where t is the film thickness and T is the transmittance [15]. The best linear relationship is obtained by plotting α2 against hν, based on Eq. (2) below.

Figure (1):The CoO thin film before and after annealing; the slight from right to left show the heating effect gradually.

αhν= A(hν– Eg) n/2 (2)

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where α is absorption coefficient, A a constant (independent from ν) and n the exponent that depends upon the quantum selection rules for the particular material. The photon energy (hν) for yaxis can be calculated using Eq. (3). E = hv = hc/λ

Vol. 2 No. 5; May 2015

The absorption coefficient (α) of the prepared CoO thin films also of the thermal annealing were found from the following relation α = 2.303 A/t. where (A) is the absorbance and (t) is the film thickness. It is observed from Figure3 that absorption co-efficient (α) increases with photon wavelength in the UV region of spectrum. Absorption coefficient of the films decreases with the decrease of annealing temperature. It is obtained that absorption coefficient variation is the highest in case of substrate temperature 300 οC. This may be due to the presence of thermal lattice vibrations and imperfections. When the substrate temperature increases, it produces a significant shift in the energy absorption edge to the longer wavelength, which must be attributed to the growth of crystal grains.[17].The decrease of absorption coefficient with the increase of film thickness indicates that the thicker films contain more atoms and reasonably more states available for the photons to be absorbed [18].

(3)

where h is Plank’s constant (6.626x10-34Js), c is speed of light (3x108 m/s) and λ is the wavelength. A straight line shown in Fig.2 is obtained when α2 is plotted against photon energy (hν), which indicates that the absorption edge is due to a direct allowed transition (n = 1 for direct allowed transition). The intercept of the straight line on hν axis corresponds to the optical band gap (Eg) shown in Table 1. The determined optical band gap values for cobalt oxides are shown in Table 1. The band gaps of films that were obtained after annealing at different temperatures and thus, had the same microstructures do not differ significantly. However, the band gap values are in the expected range for cobalt oxide thin films [16]. The value of (Eg) obtained was 2.57 eV, which is approach the value of 2.7 eV .The value of (Eg) was decreased from 2.7 eV to 2.57 eV. The decreasing of (Eg) may be related to decrease in grain boundaries and their density due to the heating effect of the polycrystalline thin films. It was observed that the different structures of the films confirmed the reason for the band gap shifts.

Absorption coefficient (α) Figure (3) shows the plot of (α) with wavelength (λ), which obtained that the value of α >103 cm-1 for all films in the visible region, this means that the transition must corresponding to a direct electronic transition, and the properties of this state are important since they are responsible for electrical conduction. Also Figure (3) shows that the value of (α) for the annealed. The increase in absorbance was noticed after the thin films were annealing at 300οC for 2 hours.

Figure (2): Variation of (αhν) 2 with photon energy (hν) for cobalt oxide thin films with varying annealing temperatures.

Figure (3): Variation of absorption coefficient (α) with (λ) for cobalt oxides thin film with varying annealing temperatures.

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Extinction coefficient (K) Extinction coefficient (K) was calculated using the related K = λ α /4 π. Extinction coefficient exhibits very high value near UV region indicating high absorption in that wavelength region and begins to decrease as the wavelength increases and seem to be saturated in the visible and infrared region [19],which is nearly similar the behavior of absorption coefficient for ZnSe thin film. It is also observed that the highest value of the extinction coefficient decreases with the increase of annealing temperature which may be due to the variation of absorption of light at the grain boundaries which is almost consistent in the higher wavelength region. The increase of extinction coefficient with the photon energy indicates the probability of raising the electron transfers across the mobility gap with photon energy and greater attenuation of light in a thin film [20,21].The decrease of maximum value of extinction coefficient with the increase of film thickness agrees with the behavior of absorption coefficient [18].

Vol. 2 No. 5; May 2015

The refractive index (n) The refractive index (n) is the relative between speeds of light in vacuum to its speed in material which does not absorb this light. The value of n was calculated from the equation 1

(4)

where (R) is the reflectivity and (K) extinction coefficient. Figure 5 is the plot of refractive index versus wavelength for all different samples of CoOthin films. The samples with low temperature annealed show high refractive index 300, 250 and 200οC is 8.2, 7.1and 5.1 respectively, the high refractive index value makes this film a good material for photovoltaic application, and the other two samples 150 and 100οC is 2.0 and 1.4 respectively in the normal range of cobalt oxide, it is observed from Figure 5 that the refractive index in all samples pose a similar behavior in the entire spectral region. It can be seen that, the refractive index increase with annealing temperature increase.

The variations at the (K) values as a function of (λ) are shown in figure 4. It is observed that the spectrum shape of (K) as the same shape of (α). Figure (4) obtained the value of (K) at the visible region was depend on the film treatment method , where the value of (K) at 327 nm for untreated film is 4x108 while for annealed film at the same wavelength equal 1.5x108 at the same wavelength,this difference in (K) value become smaller at NIR region.

Figure (5): Variation of refractive index (n) with wavelength (λ) for cobalt oxides thin film with varying annealing temperatures.

Table (1): Energy gap, annealing temperature and the thickness of Cobalt Oxide thin film Temperature No of Energy gap Thickness sample (Eg)(eV) (nm) annealing (οC)

1 2 3 4 5

Figure (4): Variation of extinction coefficient (K) with wavelength (λ) for cobalt oxides thin film withvarying annealing temperatures.

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2.70 2.60 2.60 2.60 2.57

100 150 200 250 300

250 215 175 153 125


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[11]Antonelli D M and Ying J Y 1995 Angew. Chem., Intl. Ed. Engl. 34 2014 [12] J.H. Richter: Electronic Properties of Metal Oxide Films Studied by Core Level Spectroscopy, DigitalComprehensive Summaries of Uppsala Faculty of Sci-ence and Technology, 228, 69. [13]P. A. Ilenikhena, African Physical Review (2008) 2:0008. [14] C.M. Lampart and C.G. Granquist: SPIE, Optical Engineering Press, Bellingham WA 1990. [15] A.V. Adedeji, G.O. Egharevba, C. Jeynes and E.O.B.Ajayi: Thin Solid Films, 2002, 402, 49. [16] S.G. Bott, B.D. Fahlman, M.L. Pierson and R. Barron:J. Chem. Soc., Dalton Trans., 2001, 2148. [17]R. S. Rusu, and G. I. Rusu, Jour. of Optoelectronics and Advanced Materials 7(3), 1511 (2005). [18]H.T. Zhu, J. Luo, J.K. Liang, G.H. Rao, J.B. Li, J.Y. Zhang, Z.M. Du, Physica B 403 (2008) 3141– 3145 [19] H. Yoo and J.H. Lee: J. Phys. Chem. Solids, 1996, 57, 65. [20] S.N. Mott: Metal Insulator Transition, 2nd edition,Taylor and Francis, London 1990. [21] J. Wollenstein, M. Burgmair, G. Plescher, T. Sulima,J. Hildenbrand, H. Bottner and I. Eisele: Sensors andActuators, 2003, B93, 442.

Conclusion Cobalt oxide thin film was prepared using chemical spray pyrolysis method. The Cobalt Oxide thin films were annealed at (100, 150, 200, 250 and 300οC). The optical properties namely; absorption coefficient, extinction coefficient and refractive index were found to be increase with annealing temperature.The energy band gap was found to be decreased slightly as the annealing temperature is increased. It was possible to deduce that, the Cobalt oxide thin film thickness were found to decreasedwith as annealing temperature increased. Whereas absorption coefficient value andextinction coefficient (K) were found to increasewith annealing temperature increase.The samplesshowed high refractive index (1.4-8.2) at the same wavelength (327 nm) and this value was found to be in the range of cobalt oxide. References [1]Liotta L F, Pantaleo G, Macaluso A, Di Carlo G and Deganello G 2003 Appl. Catal. A245 167 [2]Lycourghiotis A, KordulisCh and Bourikas K 2002 In Encyclopedia of surface and colloid science (ed.) A Hubbard (New York: Marcel Dekker) p. 1366 [3] Fischer Rivera E, Atakan B and KohseHoinghaus K 2001 J. Phys. IV 11 3 [4] Bourikas K, Kordulis C, Vakros J and Lycourghiotis A 2004 Adv. Colloid Interface Sci. 110 97 [5]Fujii E 1996 Eur. Patent No. EP0733 721A1 [6] The encyclopedia of catalysis 2004 (New York: John Wiley& Sons) [7]Tsujia T, Hamagamia T, Kawamurab T, Yamakia J and Masaharu T 2005 ppl. Surf. Sci. 243 214 [8]Khodakov A Y, Zholobenko V L, Bechara R. and Durand D 2005 MicroporousMesoporous Mater. 79 2 [9] Beck J S, Vartuli J C, Roth W J, Lenowicz M E, Kresge C T Schmitt K D, Chu C T W, Olson D H, Shepard E W, McCullen S B, Higgins J B and Schlenker J L 1992 J. Am. Chem. Soc. 114 10834 [10]Huo Q, Margolese D I and Stucky G D 1995 Science 267 865 116