An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films

Materials Chemistry and Physics xxx (2015) 1e8

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An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films S. Grace Victoria a, *, A. Moses Ezhil Raj b, C. Ravidhas c a

Department of Physics, Women's Christian College, Nagercoil 629 001, India Department of Physics & Research Centre, Scott Christian College (Autonomous), Nagercoil 629 003, India c Department of Physics, Bishop Heber College (Autonomous), Tiruchirappalli 620 017, India b


Position of the obtained diffraction peaks coincided well with the standard cubic spinel structure of polycrystalline Co3O4.
The Co3O4 phase is stable because after film formation, no structural change was visible.
The crystallinity of the films was found to decrease with increase in temperature.
The preferential orientation of the films was the (311) plane irrespective of temperature.

g r a p h i c a l a b s t r a c t 778.78

20000 Co2p

Relative Intensity (a. u.)

h i g h l i g h t s

15000 794.12

10000

788.33

Co2p 802.8

5000

0

-5000 770

775

780

785

790

795

800

805

Binding Energy (eV)

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2014 Received in revised form 18 June 2015 Accepted 5 July 2015 Available online xxx

The spray pyrolysis deposition of stable and single phase cobalt oxide (Co3O4) thin films at three different deposition temperatures viz. 350  C, 420  C and 470  C was undertaken in the present work. It was found that substrate temperature played a pivotal role in obtaining pure thin layers. X-ray diffraction analysis confirmed the cubic spinel type structure (a ¼ 8.094 Å) of polycrystalline Co3O4 with predominant orientation along (311) plane. The average crystallite size was found to be around 49 nm. Fourier transform IR spectra revealed the presence of an OB3 vibration at 540 cm1 and an ABO3 vibration at 640 cm1 where, B denotes Co3þ ions in an octahedral hole and A denotes Co2þ ions in a tetrahedral hole. The Raman spectra recorded in the wave number range 200e800 cm1 indicated five Raman peaks and the strong characteristic peak at 694 cm1 was assigned to the A1g mode which corresponds to CoeO breathing vibration of Co2þ ions in tetrahedral coordination. Surface analysis was carried out with the aid of SEM and AFM. The formation of Co3O4 was confirmed by analyzing the binding energies and intensities of Co 3s, Co 2p and O 1s obtained from XPS. Temperature dependent resistivity was realized from the two probe electrical measurements validating the semiconducting behaviour of the prepared films. Co3O4 films had low transmittance values in the order of 23%e43% in the UVeVis-NIR region. Estimation through Tauc plots revealed direct optical band gap energies around 2e2.24 eV due to charge transfer transition between p states of O2 and t2 states of Co2þ. Related optical parameters like extinction coefficient (k) and refractive index (m) were calculated using Swanepoel method. © 2015 Elsevier B.V. All rights reserved.

Keywords: Thin films Fourier Transform Infrared spectroscopy (FTIR) X-ray photo-emission spectroscopy (XPS) Optical properties Electrical properties

* Corresponding author. E-mail address: grace_fl[email protected] (S.G. Victoria). http://dx.doi.org/10.1016/j.matchemphys.2015.07.015 0254-0584/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction Cobalt oxide thin films have been widely studied for their immense applications as heterogeneous catalysts, solid state gas sensors and electrode materials for lithium ion batteries [1]. Most of the rechargeable batteries for portable applications are cobalt based on account of its high energy density. One of the main criteria in designing rechargeable batteries is to maintain the electrode's withstanding capacity during numerous dischargeecharge cycles [2]. Cobalt oxides have successfully endured in this challenge. They usually occur in 2 stable covalent states CoO and Co3O4 [3]. Co3O4 is 2þ a normal cubic spinel Co2þ[Co3þ]2O2 ions in 4 with high spin Co d7 configuration occupying tetrahedral sites and low spin Co3þ ions with d6 configuration occupying octahedral sites [4]. Co3O4 is a suitable alternative for the highly expensive and traditional Rubidium oxide, Iridium oxide electrodes used in super capacitors owing to its large surface area, electrochemical stability and high corrosion resistance [5,6]. Being an antiferromagnet with Neel temperature around 40 K, Co3O4 creates an impact in the field of spintronics [7,8]. Co3O4 has a great potential as a catalyst in the reduction of molecular oxygen to O-2 in alkaline solution and in oxidation of hydrocarbons and carbon monoxide [9] and is being credited to have the highest capacity to exchange its lattice oxygen with atmosphere [10] among nine other tested oxides. These films exhibit anodic colouration and cathodic bleaching and thus are prominent in electrochromic applications such as smart windows, glare free mirrors, high contrast displays etc [11,12]. Co3O4 films exhibited excellent chemiresistive gas sensing characteristics for a host of test gases (CH4, CO, NO2, Cl2, NH3 and H2S) [13]. A variety of physical and chemical processes are available for the tailoring of microstructure and properties of thin films. Various routes of synthesis of Co3O4 films have been undertaken such as electrodeposition [14], chemical vapour deposition [15], sol gel method [16], hydrothermal oxidation [17], metal organic chemical vapour deposition (MOCVD) [18]. But these methods are complex and expensive. Chemical solution deposition (CSD) methods are more reliable, less expensive and can produce films with new and improved properties than the physical and chemical methods. Spray pyrolysis is a sophisticated and promising CSD method to obtain well textured and low defect species. It utilizes aqueous or organic solutions for film growth. The method of spray pyrolysis has high degree of compositional control and direct patterning of thin films [19]. The present work deals with synthesizing the versatile Co3O4 films and characterizing it to get a clear insight of its structural, morphological, electrical and optical properties. The experiment, characterizations and interpretation of results are furnished in detail. 2. Experimental procedure In the present venture, a compact spray pyrolysis setup operating at atmospheric pressure was used to deposit cobalt oxide thin films. Co3O4 films were prepared with 0.1 M of cobalt acetate [Co(CH3COO)2$4H2O)] in 100% ethyl alcohol. Considering the fact that the cleaning procedure of the glass substrates is very vital for obtaining well adherent, smooth and pinhole free films, utmost care was taken for the cleaning process. Initially, the 7.5  2.5 cm glass substrates were washed in soap solution and rinsed off with deionised water. Then they were soaked in chromium trioxide and heated at 80  C for half an hour following which they were immersed in deionized water. Next the glass plates were rubbed with cotton swab dipped in isopropanol and scrubbed with cotton swab and finally dipped in acetone, dried and packed in air tight packets. The substrate was inserted into a tubular furnace controlled with an SANSEL (Model STC 002) temperature controller.

The tubular furnace was 30 cm in height and 12 cm in diameter capable of heating upto 800  C with a current limit of 25 A and a power of 5750 W. The compressed air from an ELGI compressor, along with the precursor was allowed to enter into the spray nozzle and a fine aerosol was created. Pyrolysis is a form of incineration that chemically decomposes organic materials by heat in the absence of air. Oxides of cobalt were formed by the pyrolytic action on the substrate surface. To research the effect of substrate temperature on the quality and properties of samples, Co3O4 films were deposited at 3 different temperatures viz. 350  C, 420  C and 470  C. The optimized deposition conditions are listed in Table 1. Structural investigations were carried out by employing X-ray diffraction using PANalytical 3040 X'pert pro diffractometer using a copper target (l ¼ 1.5405 Å) in step size of 0.03 . The phase formation of the films was confirmed by utilizing Fourier Transform Infrared and Laser Raman spectroscopy using a Nexus 670 spectrometer and a Renishaw Laser Raman microscope respectively. Surface morphology of the films was examined by a Scanning Electron Microscope (JSM-6390). Energy Dispersive X-ray Spectroscopy (EDS) was utilized to determine the composition of Co3O4 films. The chemical composition and binding states of the cobalt oxide film was characterized by X-ray photoelectron spectroscopy (XPS) using a PHI 5600ci ESCA spectrometer employing monochromated Al Ka (1486.6 eV) radiation. The conductivity of the films was measured using standard two probe method. A Cary eclipse fluorescence spectrophotometer in the emission mode of scanning recorded the photoluminescence spectra of the films. A Shimadzu 2401 spectrophotometer recorded the transmittance, absorbance and reflectance of the films in the ultraviolet, visible and near infrared regions to gain deep insight of the optical properties of the films. 3. Results and discussions 3.1. Structural and morphological characterization 3.1.1. X-ray diffraction Fig. 1 shows that the position of the obtained diffraction peaks coincided well with the cubic spinel type structure of polycrystalline Co3O4. No other crystallite phase (Co hydroxide, Co monoxide) was detected, thus proving that the only formed phase was the spinel cobalt oxide. The Co3O4 phase is stable because no structural change was visible after film formation. The lattice parameter of the cubic Co3O4 films was calculated to be 8.094 Å. The density and volume of the films were found to be 6.029 Å and 530.359 Å3 respectively. These values were in good agreement with standard values of Co3O4 powder diffraction (JCPDS 80-1533, a ¼ b ¼ c ¼ 8.098 Å, density ¼ 6.026 Å, volume ¼ 530.81 Å3). The preferential orientation of the films was the (311) plane irrespective of temperature. This dominance is the same as in already reported results [20]. The crystallinity of the films was found to decrease with increase in temperature. The rough topography of the films revealed by the Table 1 Optimized deposition parameters. Spray parameters

Optimized values

Concentration of precursor Volume of precursor Solvent Spray rate Substrate temperature Carrier gas pressure Nozzle substrate distance

0.1 M 50 ml 100% Ethanol 5 ml/min 350  C, 420  C, 470  C 0.4 kg/cm2 30 cm

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S.G. Victoria et al. / Materials Chemistry and Physics xxx (2015) 1e8

cubic spinel structures with space group Fd-3m and serves as a clear evidence for the presence of crystalline Co3O4. Co3O4 crystallises in the normal spinel structure Co2þ(Co3þ)2O2 4 with Co2þ and Co3þ ions placed at tetrahedral and octahedral sites respectively. The primitive unit cell contains 14 atoms [23]. The reduction of the 42-dimensional representation G of the vibrational modes into irreducible representations is given as,

470 C

200

420 C (311)

0 400

(220)

Intensity (a.u.)

0 200

350 C

G ¼ A1g þ Eg þ 3F2g þ 5F1u þ 2A2u þ 2Eu þ 2F2u

200 100 0 JCPDS 80-1533

50 0 20

30

40

50

60

3

70

2θ (degrees) Fig. 1. XRD spectra of Co3O4 films.

AFM analysis together with the possibility of precipitants' formation as pointed by the XPS study may be the reason for this crystallinity loss. The Scherrer equation was utilized with the prominent peak (311) to estimate the grain size of the films and it was found to be 49 nm. 3.1.2. FTIR and RAMAN Fourier transform IR spectra were recorded between 400e4000 cm1 for the cobalt oxide thin films prepared for three different deposition temperatures viz. 350  C, 420  C, 470  C. Fig. 2 reveals weak absorption bands appearing around 3610 cm1 and 1640 cm1 that are due to weakly hydrogen bonded hydroxide ions present in the entrapped water and this absorption is found to decrease with increasing deposition temperature. The conspicuous presence of some carbonaceous materials (CO2) is evident from the strong bands at around 2357 cm1 and 1272 cm1. The band at 2357 cm1 is attributed to the asymmetric stretching mode of CO2. The IR spectra reveal two distinct bands that arise due to the stretching vibrations of the metal (Co)eoxygen bonds in the finger print region. The first band around 540 cm1 is associated with OB3 vibration in the spinel lattice of Co3O4, where B denotes Co3þ ions in an octahedral hole. The second band at 640 cm1 is the ABO3 vibration, where A denotes Co2þ ions in a tetrahedral hole [21,22]. The band around 640 cm1 corresponds to the bending mode vibration of cobalt oxide. This IR spectrum is typical of all

(1)

Fig. 3 shows a typical Raman spectrum of the Co3O4 film samples with five Raman peaks centered at 197,484,522,623,694 cm1 in the range of 200e800 cm1[1]. The A1g, Eg, and the three F2g modes are Raman active. From the five Flu modes given in Table 2, four are infrared active and one is an acoustic mode. The remaining F1g, 2A2u, 2Eu, and 2F2u modes are inactive. The strong peak at 694 cm1 in the Raman spectrum is typical characteristic of all spinel structures. It is assigned to the A1g mode, which corresponds to CoeO breathing vibration of Co2þ ions in tetrahedral coordination. Thus the crystalline structure of cobalt oxide was identified by Raman spectroscopy.

3.1.3. AFM, SEM, EDS Fig. 4(a,b) shows the typical morphology of the cobalt oxide film captured with the help of atomic force microscope. It shows a rough topography consisting of compact and spherical particles. The layer is also found to be dense and well covered without any cracks and pinholes. SEM images in Fig. 5 reveal that the substrate is uniformly covered by the oxide and the coating appears smooth and compact without cracks. The presence of nano crystalline grains with some overgrown clusters is also evident. Over growth can be explained on the basis of nucleation and coalescence process. Initially grown nano grains may have increased their size by further deposition and come closer to each other resulting in coalescence. Such morphology with nano sized grains may offer increased surface area feasible for super capacitor and gas sensing applications [20]. Energy-dispersive X-ray spectroscopy (EDS) is an analytical technique used for the chemical composition of a sample. The EDS spectrum of Co3O4 film in Fig. 6 reveals characteristic peaks corresponding to the constituents cobalt and oxygen. The additional peaks may be due to the constituents of the glass substrate.

1 0 0

(4 7 0

C )

9 0

Intensity(a.u.)

470 C

Intensity(a.u.)

8 0

(4 2 0

C )

420 C F

1 0 0

(1 9 7 )

(3 5 0

E

9 0

350 8 0

4 0 0 0

3 0 0 0

2 0 0 0

W a v e n u m b e r(c m

Fig. 2. FTIR spectra of Co3O4 films.

A (6 9 4 )

(4 8 4 )

C )

1 0 0 0

)

2 0 0

C

4 0 0

F

F

(5 2 2 )

(6 2 3 )

6 0 0

W a v e n u m b e r(c m

8 0 0

)

Fig. 3. Raman spectra of Co3O4 films.

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Table 2 Line positions and widths (FWHM) of the Raman-active phonon modes of Co3O4. Phonon mode

Wave number (cm1)

FWHM (cm1)

F2g Eg F2g F2g A1g

197 484 522 623 694

2 5.1 4.2 4.3 5.2

3.2. Elemental analysis XPS X-ray photoelectron spectroscopy is a surface sensitive probe that can provide valuable information on the surface composition and the valence and coordination state of the constituent elements. The C 1s level of unintentionally absorbed carbon species (285 eV) is used to calibrate the binding energies of the elements in the deposited Co3O4 film. The survey scan information of this spectroscopic analysis is useful, particularly in the identification of the elements present at the surface of the film. For the present work, the XPS survey scan analysis of Co3O4 films was carried out in the binding energy (Eb) range of 0e1000 eV. The survey spectrum in Fig. 7 clearly shows the peaks corresponding to Co 2p, O 1s and C 1s electron ejection from the inner-shell orbitals. Presence of only Co and O confirmed the product as Co3O4 and there is no significant existence of impurities except for the contaminant carbon. The C 1s peaks may be due to amorphous carbon in 285 eV and some organic species containing carbon [24]. In order to further confirm the formation of Co3O4, the Co 2p

spectrum is deconvoluted as shown in Fig. 8. In cobalt oxide, the cobalt ions are present in different valence and coordination states. Hence there occur peaks due to ionization of both low spin Co3þ ions in the octahedral and high spin Co2þ ions in the tetrahedral oxygen environment. Hence it exhibits two major peaks with binding energy values of 778.78 and 794.12 eV, corresponding to Co 2p3/2 and Co 2p1/2 doublet core level peaks, respectively of the Co3O4 phase. The Co 2p3/2eCo 2p1/2 peak separation is approximately 15.34 eV. Earlier reports suggested that pure Co3O4 spectra is characterized by a Co 2p3/2 peak at 780.2 eV, a shake-up satellite peak at 789.3 eV, a Co 2p1/2 peak at 795.6 eV and the satellite at 802.8 eV [25e27]. The close proximity of separation among the spineorbit component, the satellites shape and its positions with already reported results confirms the formation of Co3O4 films. The formation of Co3O4 is further confirmed by the presence of weak satellite peak between the main peaks Co 2p3/2 and Co 2p1/2 [28,29]. The typical satellite peak is observed at a binding energy value of 788.43 eV, about 10.14 eV higher than 778.29 eV, where the peak of Co 2p3/2 appears. The Co2p core-level spectra of the high spin (HS) ions contain characteristic multiplet splitting satellite peaks which is due to the presence of unpaired 3d electrons in the valence shells of Co3þ ions [30]. Moreover, Co 2p1/2 satellite to main peak intensity ratios of 0.9 is characteristic of the cubic cobalt monoxide (CoO); small values below 0.3 have been measured for Co3O4 [31]. In the present investigation, the intensity ratio of satellite to main peak is well below 0.3, which again confirms the formation of single phase Co3O4. The O 1s level of the Co3O4 nanostructure is deconvoluted into two peaks at 528.66 and 530.91 eV [32e34] as shown in Fig. 9.

Fig. 4. (a) Two dimensional AFM image of Co3O4 film. (b) Three dimensional AFM image of Co3O4 film.

Fig. 5. SEM micrographs of Co3O4 film at 10,000 and 20,000 magnification.

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5

Fig. 6. EDS spectrum of Co3O4 film.

The lower binding energy peak at 528.66 eV corresponds to oxygen species in the spinel Co3O4 phase [35]. The peak at 530.91 eV indicates the presence of eOH (hydroxyl) species adsorbed on the surface of the sample. Previous studies have shown that the lattice O 1s binding energy for transition metal monoxides is relatively insensitive to changes in near-surface stoichiometry. However, present investigation shows sensitiveness to changes in adsorption of hydroxyl species. This is an additional evidence for the formation of single phase Co3O4 films. The calculated molar ratio of Co:O is very close to 3:4, confirming the stoichiometry of the prepared Co3O4 nano films.

Relative intensity (counts/sec)

300000

Co2p

O1s 200000

100000

C 1s

3.3. Electrical analysis 0

0

200

400

600

800

1000

Co3O4 is a p-type semiconductor. The electrical response of the Co3O4 films was studied by using standard two probe setup employing reliable electrometers. Important electrical parameters like resistivity, conductivity, activation energy (Ea) and temperature coefficient of resistance (TCR) were determined by subjecting the

Binding energy (eV) Fig. 7. XPS survey spectrum of Co3O4 film.

778.78

20000

50000

15000

Relative Intensity (a. u.)

Relative Intensity (a. u.)

Co2p3/2

794.12

10000

788.33

Co2p1/2

802.8

5000

0

530.91

O1s

40000 30000

528.66

20000 10000 0

-5000 770

775

780

785

790

795

Binding Energy (eV) Fig. 8. Cobalt core levels of Co3O4 film.

800

805

525

528

531

534

B inding Energy (eV) Fig. 9. O 1s level of Co3O4 film.

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2 0 0

Intensity(a.u.)

1 0 0

4 7 0

C

42 0

C

0

2 0 0

1 0 0

0 1 0 0

5 0

3 5 0 0

Fig. 10. Arrhenius plot of Co3O4 films.

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

W a v e le n g th (n m )

films to temperature in the range of 300e470 K. The DC electrical resistivity of the films at room temperature was in the order of 104e105 Ucm which is low enough to be a good super capacitor electrode material. This is in good agreement with already reported results [5]. Activation energy is the minimum amount of energy required to activate atoms. It also represents the location of trap levels below the conduction band. For the regions in which the electrical conductivity increases with the increase in temperature, the well known dependence of semiconductor electrical conductivity on temperature is utilized.

s ¼ so exp ð  Ea =2KTÞ

3 5 0

C

(2)

Fig. 10 shows the Arrhenius plots drawn for the films deposited at 3 deposition temperatures and from the slope the activation energies are calculated. Temperature coefficient of resistance symbolizes the resistance change factor per degree of temperature range. The value of activation energies and temperature coefficient of resistance obtained for the Co3O4 films are tabulated in Table 3. The increase of activation energy with increasing temperature may be due to the fact that due to disorder in the lattice, charge carriers are scattered and requires more energy to hop into conduction band. This is termed as extrinsic or hopping conduction. The negative value of TCR again indicates the semiconducting nature of Co3O4 films. From the thickness point of view, the increase of activation energy with decreasing film thickness may be understood from island structure theory based on tunnelling of charged carriers between islands separated by a short distance. 3.4. Optical characterization 3.4.1. Photoluminiscence studies The PL spectra of the Co3O4 films were recorded with a Cary eclipse photometer. The as-deposited Co3O4 films could emit visible light at room temperature. Fig. 11 shows the room temperature PL spectrum for the cobalt oxide thin films. The PL spectrum shows two strong PL peaks in the visible range

Fig. 11. PL emission spectra of Co3O4 films.

centered at 480 nm and 520 nm. The peak around 520 nm corresponds to energy of 2.38 eV. This value is very close to the reported direct optical band gap of 2.10 eV for Co3O4 thin films [36]. Hence these Co3O4 films might be applied as visible light emitting materials. Another emission peak is evident in the UV region around 360 nm. Since Co3O4 is a transition metal oxide, there are cobalt interstitials, oxygen vacancies, cobalt vacancies and oxygen interstitials in it [37]. Emission peaks could occur as a result of two processes. One is a radiative transition from donors (cobalt interstitial, oxygen vacancy) to acceptors (cobalt vacancy and oxygen interstitial) and the other may be due to recombination of excitons. Only the p and d electrons have the chance to become excitons and these inturn can recombine by UV light and can give out the excited emission in the UV region.

3.4.2. UVeVis-NIR spectroscopy Optical transmittance was measured in the wavelength range of 300e1200 nm and the obtained transmittance spectra of Co3O4 films deposited at various temperatures is shown in Fig. 12. Optical transmittance of the films increases with increase in deposition temperature while absorbance decreases with temperature. This may be due to the thickness effect. Availability of more Co species leads to thick surfaces thereby reducing optical transmittance at low deposition temperature. Co3O4 is a cubic spinel known as cobalt cobaltite Co2þ[Co3þ]2O4 with Co2þ ions in high spin tetrahedral sites and Co3þ ions in low spin octahedral sites. Due to the electronic configuration of Co3O4, a number of optical transitions are possible. These include charge transfer between Co2þ / Co3þ d orbitals, ligand field transition between split d orbitals of Co2þ, and ligand to metal charge transition from O2 to Co2þ ions. Since the p states of O2 ions are located closely to the d states of Co3þ ions, p electrons can easily undergo a transition. At low temperatures this peak splits and results in a doublet corresponding to p(O2) / eg (Co3þ) and p(O2) / t2(Co2þ). These are

Table 3 Electrical parameters of Co3O4 films. Temperature,  C

Thickness, mm

Activation energy, Ea (eV)

Temperature coefficient of resistance (TCR)

350 420 470

1.31 1.19 1.06

1.3 1.6 2.05

0.022 0.1 0.024

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Transmittance (%)

50

o

470 C

40 o

420 C 30 20

o

350 C

10 0

400

600

800

1000

Wavelength (nm)

7

to the stretching vibrations of the metal CoeO bonds in the finger print region. The IR spectra were typical of a cubic spinel structure with space group Fd-3m and served as a clear evidence for the presence of crystalline Co3O4. The Raman spectrum showed five Raman peaks in the range of 200e800 cm1. The strong peak at 694 cm1 in the Raman spectrum is typical characteristic of all spinel structures. It was assigned to the A1g mode which corresponds to CoeO breathing vibration of Co2þ ions in tetrahedral coordination. XPS investigations showcased that the O 1s/Co 2p intensities were consistent with the formation of Co3O4. The DC electrical resistivity of the films at room temperature was in the order of 104 Ucm which is low enough to be a good super capacitor electrode material. Optical studies concluded that Co3O4 films had band gap energies of 2e2.24 eV and is assigned to the charge transfer transition between p states of O2 and t2 states of Co2þ. All these results supported the formation of single phase Co3O4 films.

Fig. 12. Transmittance spectra of Co3O4 films.

Acknowledgements called band gap energy transitions. From the obtained optical transmittance data, Tauc plot has been constructed to determine the optical band gap energy as shown in Fig. 13. The band gap energies were obtained by extrapolating the linear portion of (ahy)2 versus hy plots to the energy axis at x ¼ 0 in the higher energy domain. The direct band gap energies of Co3O4 films is found to be 2e2.24 eV and is assigned to the charge transfer transition between p states of O2 and t2 states of Co2þ [38]. Other optical parameters like extinction coefficient (k) and refractive index (n) are calculated using Swanepoel method in the transmittance region. For the optimized films, the refractive index and extinction coefficient values were respectively 1.77 and 0.09 at 600 nm. The close agreement with the reported values of these optical parameters further serves as a confirmatory tool for the formation of cubic spinel Co3O4 films. 4. Conclusions Single phase nano sized Co3O4 thin films have been synthesized successfully by spraying cobalt acetate dissolved in 100% ethanol. The effect of substrate temperature on crystallinity and structural parameters was evident from the recorded XRD spectra and the lattice structure of the Co3O4 was found to be cubic. The IR spectra of Co3O4 films revealed two distinct bands that arise due

1.0x10 o

350 C αhν (eV/cm)

2

2.24eV 8.0x10 o

420 C 2.15eV 6.0x10

o

470 C 2.08eV

4.0x10

2.0x10

0.0

2.1

2.4

2.7

hν (eV) Fig. 13. Tauc plot of Co3O4 films.

3.0

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Please cite this article in press as: S.G. Victoria, et al., An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.015