Characterization of Mn-Doped Co3O4 Thin Films Prepared by ...

Lakehal Abdelhak*, Bouaza Amar, Benrabah Bedhiaf, Dalache Cherifa and Benhebal Hadj

Characterization of Mn-Doped Co3O4 Thin FilmsPrepared by Sol Gel-Based Dip-Coating Process

https://doi.org/10.1515/htmp-2017-0185Received December 15, 2017; accepted June 18, 2018

Abstract: In this article, manganese-doped cobalt oxide(Mn-doped Co3O4) thin films have been prepared on glasssubstrates using sol gel-based dip-coating technique inorder to investigate their optical, structural and electricalproperties. The Mn concentration was changed from 0% to9%. The synthesized samples were characterized by ultra-violet-visible spectroscopy (UV-visible), X-ray diffraction(XRD), Fourier transform infrared spectroscopy (FTIR) andcomplex impedance spectroscopy to elucidate the optical,structural, vibrational and electrical properties. Our opticalresults show that the transmittance of Mn-doped Co3O4

films decreases with increasing doping levels. The opticalband gaps were found to be ðEg1 = 1.51 eV, Eg2 = 2.12eVÞand ðEg1 = 1.23 eV,Eg2 = 1.72eVÞ for the case of undopedCo3O4 and 9% Mn-doped Co3O4, respectively. This shiftmeans that the impurities would create energy levels. Thestructural analysis provides evidence that obtained powderswere crystallized in cubic spinel structure. The complemen-tary phase information is provided by FTIR spectroscopy.The FTIR study depicted the presence of four distinct bandscharacterizing Mn-doped Co3O4 cubic spinel-type structure.The Nyquist plots suggest that the equivalent circuit of Mn-doped Co3O4 films is an RpCp parallel circuit. It was foundthat the effective resistance Rp decreases, whereas theeffective capacitance Cp increases with doping.

Keywords: Cobalt oxide, Mn doping, sol gel dip-coating,thin films

Introduction

Transparent conducting oxides (TCOs) are promisingmaterials in various potential applications due to theirdual property; they combine electrical conductivity andoptical transparency in the visible range. These proper-ties have attracted the attention for using them in optoe-lectronic devices such as photovoltaic solar cells andelectrochromic windows [1, 2].

Among the TCOs, cobalt oxide (Co3O4) is considered asone of the broadly studied compounds due to its impor-tance in various scientific fields such as solar selectiveabsorber [1], energy storage [2], supercapacitors [3] andelectrochromic sensor [4]. Co3O4 exhibits p-type semicon-ducting property and behaves like an antiferromagnet (AF)with the Néel temperature TN � 290 K [5], Optical absorp-tion studies have shown that Co3O4 exhibits multiple bandgap energies ðEg1 = 1.57eV, Eg2 = 2.28eVÞ [6]. Cobalt oxideexists in three different crystalline forms: the cobaltousoxide (CoO), the cobaltic oxide (Co2O3) and tricobalt tetra-oxide (Co3O4). The most stable phase in the cobalt oxidesystem is a mixed valence compound [Co2+Co3+ 2O4] with anormal spinel structure [3].

In recent years, great efforts have been devoted tofabricating nanostructured systems of cobalt oxide withtunable physical–chemical properties for a broad range ofapplications, among which transition metal doping is apromising and efficient route to improve the optical absorp-tion and the electrical behavior of Co3O4. In this regard, thisstudy fabricates Mn-doped Co3O4 thin films in an effort tounderstand their enhanced optical and electrical properties.We will introduce a Mn metal impurity into the Co3O4

matrix by the sol gel technique in order to produce opti-mized semiconductor thin films of Co3O4 with good opticaland electrical properties, and we controlled the dopinglevels so that the Co3O4 phase did not change. We choosedoping by manganese Mn2+ because its atomic radius(0.80Å) is almost equal to that of cobalt Co2+ (0.72Å). Itcan substitute the Co2+ ion. A small change in dopantconcentration is significant for changing the band gaps,and the energy band gaps are expected to vary in a wideand very attractive energies intervals ranging fromðEg1 = 1.51eV,Eg2 = 2.12eVÞ in undoped Co3O4 and down toðEg1 = 1.23eV,Eg2 = 1.72eVÞ in 9% Mn-doped Co3O4.

*Corresponding author: Lakehal Abdelhak, Département de chimie,Université Ibn-Khaldoun, Zaaroura, Tiaret 14000, Algeria; Laboratoirede Génie Physique, Université Ibn-Khaldoun, Zaaroura, Tiaret 14000,Algeria, E-mail: [email protected] Amar, Benrabah Bedhiaf, Laboratoire de Génie Physique,Université Ibn-Khaldoun, Zaaroura, Tiaret 14000, AlgeriaDalache Cherifa, Laboratoire de synthèse et catalyse, Université Ibn-Khaldoun, Zaaroura, Tiaret 14000, AlgeriaBenhebal Hadj

High Temp. Mater. Proc. 2019; 38: 237–247

Open Access. © 2019 Walter de Gruyter GmbH, Berlin/Boston.This work is licensed under the Creative Commons Attribution 4.0 Public License.

Various methods have been reported for the synthesis ofCo3O4 thin films such as chemical vapor deposition (CVD) [7],spraypyrolysis [8] andprecipitationmethod [6]. In this paper,we have adopted a simple sol gel dip-coating method toobtain Mn-doped Co3O4 thin films. This method of preparingthin films has several advantages: its low cost, flexibility inthe deposition process and convenience for a large area.It has been used successfully in our laboratory to fab-ricate a variety of porous materials such as SnO2 [9]and TiO2 [10]. The synthesized samples were subjectedto characterizations such as ultraviolet-visible spectro-scopy (UV-visible), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning elec-tron microscopy (SEM) and impedance measurements.

Experimental procedure

Materials and methods

The principle of the dip-coating method is to dip thesubstrate in a viscous solution and to withdraw at con-stant speed under controlled and stable conditions toobtain a homogeneous film of regular thickness. Thedeposition of the thin films by the dip-coating techniqueis carried out in three steps:– preparation of substrates (cleaning).– the deposit of the film by the dip-coating technique– drying and heat treatment

There are several parameters that influence the thick-ness of the thin films such as sol viscosity, pressure,pulling force, gravitational force, surface tension andsurface tension gradient. It is possible, in principle, toadjust some parameters to obtain the desired thickness.

In the present venture, a sol gel dip-coating setup oper-ating at atmospheric pressure was used to deposit cobaltoxide thin films. In a typical experiment, cobalt nitrate hex-ahydrate (Co(NO3)2.6H2O) was dissolved in an aqueous solu-tion (100ml) of sucrose (C12H22O11) and stirred vigorously at100 °C for about 2h to form a burgundy homogeneous solu-

tion. To achieve Mn doping Mn½ �Co½ �

n o, manganese nitrate hexa-

hydrate (Mn(NO3)2.6H2O) was added to the precursorsolution with different concentrations (3wt%, 5wt%, 7wt%and 9wt%) for all doped samples. The final solution(undoped Co3O4 and Mn-doped Co3O4) will be used eitherfor film deposition or to obtain powder after calcination inmuffle furnace at 400 °C for 3h.

The procedure of cleaning glass substrates is veryimportant to get well-adherent, smooth films. The

substrates Pyrex pieces (75 × 25 × 1mm3) were cleanedthrough dipping them in ultrasonic bath containing tri-chloroethylene, ethanol and aciton for 5min, respec-tively, and finally rinsed by distilled water. Thewithdrawal speed of the substrates from the solutionwas 50mm/min. The formed films pass through the dry-ing step to evaporate the solvent. After that, the obtainedfilms were calcined in muffle furnace at 400°C for 45min.A schematic representation of the sol gel synthesis isgiven in Figure 1.

Characterizations

The as-prepared samples were characterized by usingdifferent physical techniques. The optical transmittancespectra and band gap energies of Mn-doped Co3O4 thinfilms were measured using Shimadzu-1650 spectrophot-ometer in the wavelength range from 300 to 900 nm. Thephase and crystal structure of all samples were studiedby XRD (Rigaku miniflex 600) with CuKα radiation(λ = 1.5406 Å). The vibrational behavior of the sampleswas investigated by FTIR (Alpha Bruker

Cobalt Nitrate

(Co(NO3)2.6H2O)

Sucrose (C12H22O11)+

Distilled water

Constant Stirring for 10 min

Manganese Nitrate

Mn(NO3)2 · 6H2O

Heat-treatment (T=100 ºC) for 2 h

Co3O4 :Mn Sol

Dip-Coating on glass substrates

Drying (T=100 ºC)for 15 Min

Heat-treatment (T=400 ºC) for 45 min

Co3O4 :Mn Thin films

Figure 1: Schematic diagram of sol gel process of Mn-doped Co3O4

preparation.

238 L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films

spectrophotometer) in the wavenumber range from 400to 4,000 cm−1. Impedance measurements were carriedout using Agilent4284A LCR-meter operating in the fre-quency range 75 KHz to 20MHz with an oscillationamplitude of 1 V.

Results and discussion

Optical properties

In this part, the UV-Vis spectra of Mn-doped Co3O4 filmswere recorded in the wavelength range from 300 to900 nm. Figure 2 shows the optical transmission spectraof Mn-doped Co3O4 thin films. All the obtained spectramanifest the presence of two sharp absorption edges inthe visible region. The first one as reported in manyresearches is associated to the O2– and Co2+ chargetransfer process, while the other observed at 500–600 nm wavelength range is due to the O2– and Co3+

charge transfer (ligand to metal charge transfer (LMCT)event). This indicates the presence of two band gapenergies, in agreement with the literature [11, 12]. It isevident that the transmittance (Figure 2) graduallydecreases while increasing Mn content. It can be seenfrom Figure 2 that the average transmission in the visi-ble region has decreased with increasing doping levels.The absorption bands of Mn-doped Co3O4 vary with achange in the concentration of manganese. The bandgap energies (allowed direct transition) of Mn-dopedCo3O4 can be determined by the following equation [10]:

αhυ=A hυ−Eg� �1

2 (1)

where A the edge parameter, Eg is the energy band gap ofthe material α is the absorption coefficient and hυ is thephoton energy. Figure 3 shows the plots of αhυð Þ2vs. hυð Þ.The extrapolation of a straight portion to the energy axis atα=0 can give two values of band gap energies. Eg1 corre-sponds to (O−2→Co3+) and Eg2 corresponds to (O−2 →Co2+).The extinction coefficient (k) of Mn-doped Co3O4 was cal-culated according to the following relation [13]:

k =αλ4π

(2)

where α is the absorption coefficient. The extinction coef-ficient spectra as a function of the incident wavelength(λ) are plotted in Figure 4. The increase of k in twowavelength regions [300–450nm] and [600–770nm] isassigned to the absorption by inter-band transitions (O−2

→Co2+) and (O−2→Co3+), respectively. The refractive indexis related to optical band gap (Eg) of a semiconductor bythe following relationship [14]:

n= 4.084−0.68 Eg (3)

The film thickness d has been calculated from UV-visibledata using the following equation [15]:

d=λ1λ2

2n λ1 − λ2ð Þ (4)

where n is the refractive index at two adjacent maximaor minima at wavelengths λ1 and λ2,Eg is the band gapenergy. Results obtained here such as band gap ener-gies, the average transmission, films’ thickness and therefractive index at different doping levels are reportedin Table 1.

The band gap energies were found to decrease withincreasing Mn content as shown in Figure 5. From theseresults, it can be seen that the shift in the band gapenergies of our samples is generally due to individual orcombined effects such as quantum confinement, crystal-lites size, charged impurities at grain boundaries, struc-tural disorder, the network distortions caused by theintroduction of impurities in the matrix and the creationof impurities energy levels. The optical properties of Mn-doped Co3O4 have attracted the attention for using it insolar cells.

The refractive index value of undoped Co3O4 film is2.642, in agreement with several authors [16, 17]. It can beobserved that the refractive index gradually increases asthe manganese concentration increases. This is explainedby the substitution of Mn2+ ions in Co2+ ions whichinduces the densification of the material. Moreover, at

300 400 500 600 700 800 9000

10

20

30

40

50

60

70

80

90

100

Tra

smitt

ance

(%

)

Wavelength (nm)

Undoped Co3O4

Co3O4: Mn (3%)

Co3O4: Mn (5%)

Co3O4: Mn (7%)

Co3O4: Mn (9%)

Figure 2: Transmittance spectra of Mn-doped Co3O4 films.

L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films 239

the microscopic level, the increase of n refers to themodification of the polarizability of the ions and thelocal field in the material [18].

Structural analysis

Figure 6 shows the XRD pattern of Mn-doped Co3O4

powders at different concentrations of manganese(3%, 5%, 7% and 9%). All obtained powders of Mn-doped Co3O4 show that multiple diffraction peaks

coincided well with the cubic spinel type structure(Fd3m space group), implying the complete solubilityof the Mn ions into the Co3O4 cubic crystal. The mainpeak is h311i and the lattice parameter of undopedCo3O4 is a= 8.069 n. These results have been pre-viously confirmed by several authors [19, 20]. Noother parasitic phase of manganese clusters aredetected in all doped samples, which indicates ahigh purity of the samples. It is clear that the intensityof the diffraction peak decreases with increasing dop-ing levels. Therefore, Mn doping decreases the

Figure 3: Tauc plots of Mn-doped Co3O4 films.

240 L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films

crystallinity of Co3O4. The different structural para-meters like grain size and lattice parameter of Mn-doped Co3O4 are calculated from the XRD analysis

and listed in Table 2. The cubic network parameterof the samples has been computed from the Bragg’scubic system formula [11]:

d hklð Þ =affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

h2 + k2 + l2ð Þp (5)

In Figure 7, we show the variations of the networkparameter and grain size as a function of doping levels.The increase of the lattice parameter with increasing Mncontent refers to the substitution of smaller Co2+ ions(0.72 Å) by larger Mn2+ ions (0.80 Å) in the system. Theparticle sizes of Mn-doped Co3O4 powders were calcu-lated using the full width at half maximum (FWHM)of (311) peak from the Debye–Scherrer formula [9],knowing that the width increases as the particle sizedecreases:

D=0.9 λβ cosθ

(6)

where D is the crystallite size, λ is the wavelength ofCuKα radiation (1.5406Å) and β is the FWHM of Braggpeak observed at Bragg angle θ. The obtained values fellin the [243–365Å] range. The variation in the crystallitesize may be due to the substitution of Mn2+ ions in Co2+

ions and the formation of nanostructured poly-crystallineMn-doped Co3O4.

FTIR spectroscopy

In order to investigate the influence of Mn doping onvibrational properties, Alpha Bruker FTIR spectrometerexperiments were carried out. Figure 8 shows FTIR trans-mission spectra of Mn-doped Co3O4 films. In the investi-gated region (4,000–400 cm−1), all obtained spectra havetwo absorption bands ʋ1, ʋ2 at 555 cm−1 and 664 Cm−1

assigned from stretching vibration of the metal–oxygenbond in Co3O4 spinel oxide [3–21]. The band at approxi-mately 555 cm−1 can be attributed to the vibrations of thebonds formed by the Co3+ ion which are present in theoctahedral holes of the crystal lattice. The other band atabout 664 cm−1 is associated to the vibrations of thebonds formed by the Co2+ ion which are present in thetetrahedral holes of the crystal lattice [22]. The absorptionpeak ʋ4 at (2,350 cm−1) is assigned to the bending vibra-tion of O-H, and we suggest that the origin of this bond isdue to the solvent involved in the synthesis of the films[10]. FTIR spectra show other low-intensity peaks whichare assigned to the organic matter remaining afterannealing. In the Mn-doped Co3O4 spectrum, the

Table 1: Band gap energies, average of transmission, refractiveindex and film thickness of Mn-doped Co3O4 films.

Samples Eg(eV)

Eg(eV)

Refractiveindex

Filmthickness (nm)

Undoped CoO . . . .CoO: Mn (%) . . . .CoO: Mn (%) . . . .CoO: Mn (%) . . . .CoO: Mn (%) . . . .

-1 0 1 2 3 4 5 6 7 8 9 101,0

1,2

1,4

1,6

1,8

2,0

2,2

Ene

rgy

band

gap

(eV

)

Mn content (%)

Eg1(eV)

Eg2(eV)

Figure 5: Energy band gap vs. Mn content of Mn-doped Co3O4 films.

300 400 500 600 700 800 900

x108

4,0

3,0

3,5

2,0

2,5

1,0

1,5

0,5

Ext

inct

ion

Coe

ffic

ient

Wavelength(nm)

Undoped Co3O4

Co3O4:Mn (3%)

Co3O4:Mn (5%)

Co3O4:Mn (7%)

Co3O4:Mn (9%)

0,0

Figure 4: Extinction coefficient plots of Mn-doped Co3O4 thin films.

L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films 241

absorption band ʋ3 in the region [600–630 cm−1] confirmsthe vibrational frequency of Mn–O complex [23]. Theobtained results are in good agreement with the XRDresults (Figure 6).

Scanning electron microscopy studies

To understand the morphology of as-prepared powders,the scanning electron microphotographs of powders arerecorded. Figure 9 shows the scanning electron micro-photographs of manganese-doped Co3O4 powders at dif-ferent doping levels (undoped, 3wt%, 5wt%, 7wt% and9wt%) under the magnification of × 2,000. All micropho-tographs are characterized by the presence of

nanocrystalline grains with some overgrown clusters.Coalescence of smaller grains of different sizes andshapes can be noticed when the manganese concentra-tion increases.

Impedance spectroscopy

The measurement of the electrical properties of materialsrequires powerful tools to explore the electrical behavior,and that is through modeling them by an equivalentcircuit [24]. In this method, we apply a sinusoidal distur-bance of constant amplitude and a variable frequency todetermine the conduction properties of a polycrystallineoxide and also, in theory, the different contributions to

20 30 40 50 60 70 80

Undoped Co3O

4

Co3O

4 :Mn (3%)

Co3O

4 :Mn (5%)ytisnetnI

(a.u

)

2θ (°)

Co3O

4 :Mn (7%)

Co3O

4 :Mn (9%)

(220

)

(311

)

(222

)

(400

)

(422

)

(511

)

(440

)

Figure 6: XRD patterns of Mn-doped Co3O4 powders.

Table 2: Structural parameters of Mn-doped Co3O4 powders.

Samples (hkl) θ d spacing (Ǻ) Lattice parameter (Ǻ) FWHM (deg) Crystallite size D (Ǻ)

Undoped CoO () . . . .

CoO: Mn (%) () . . . .

CoO: Mn (%) () . . . .

CoO: Mn (%) () . . . .

CoO: Mn (%) () . . . .

242 L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films

the conduction of a material (grains, grain boundariesand Pores, defects). It also characterizes the differentelectrically active regions in the material and demon-strates their existence by their individual electrical prop-erties. The following complex expressions describe theelectrical behavior of our films [15].

complex impedance : Z* = Z′+ jZ′′=Rs −j

ωCs(7)

complex admittance : Y* =Y ′−Y ′′ =1RP

+ jωCp (8)

complex permittivity : ε* = ε′− jε′′ (9)

complex modulus :M* =M′+M′′ (10)

These expressions are interrelated as : M* =1ε*

= jωC0Z*

= jωC01Y*

� �

(11)

where RS and CS are the series resistance and capaci-tance; RP and CP are the parallel resistance and capaci-tance, C0 is the empty capacitance, the real andimaginary components are ω = 2πʋ, ʋ is the applied fre-quency and j2 =−1. The plot (Zˊˊ versus Zˊ) thatdescribes the electrical behavior of our samples whichhave three components; each of these components

corresponds to a particular relaxation frequency. Theelectrical response is due to the bulk properties (νb) athigher frequencies. At intermediate frequencies, theelectrical response is due to the grain boundaries (νgb)and at low frequencies the electrical response corre-sponds to electrode process (νel), or processes occurringin the material/electrode interface (νel ≪ νgb≪ νb) [25].Several factors influence the properties of grain bound-aries as chemical composition, impurities, ageing andconditions of preparation. The electrical characteristic ofa material is shown by the appearance of semicirculararcs in the Nyquist plots. Figure 10 is the Nyquist repre-sentation of undoped and Mn-doped Co3O4 thin films,whose f frequency varies from 75 KHz to 20MHz at ambi-ent temperature. The processes that occur in the elec-trode are modeled by an equivalent electrical circuit.The physical logic of the system indicates that the con-current processes are connected in parallel. The capacityCp of the layer was calculated using the following equa-tion [10]:

CP =1

2πfcRp(12)

The variation of the resistance and capacitance as afunction of Mn-doping levels are listed in Table 3 andshown in Figure 11. It is clear that the resistance ofcobalt oxide decreases and the capacity increaseswith the increase of doping level; this shift is also dueto the introduction of manganese ions in Co3O4 lattice

-1 0 1 2 3 4 5 6 7 8 9 108,06

8,07

8,08

8,09

8,10

8,11

8,12

Lattice parameter ( )Grain size ( )

Mn content (%)

retemarap

ecittaL

100

120

140

160

180

200

220

240

Grain size

Figure 7: Network parameter and grain size of Mn-doped Co3O4.

L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films 243

Figure 8: FTIR spectra of Mn-doped Co3O4 films.

244 L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films

which induces a variation in the particle size and con-sequently introduce more grain boundaries within thesamples. Two conduction mechanisms are simulta-neously present: conduction across the grain and theconduction through the grain boundaries. The effect ofgrain boundaries in samples becomes more dominantin the conduction mechanism compared to the graincontribution.

Conclusion

In conclusion, we have successfully synthesized Mn-doped cobalt oxide (Co3O4) thin films using a sol geldip-coating technique. We used several experimentalmethods of investigation to study the effect of Mn dopingon optical, structural and electrical properties of cobaltoxide as optical transmission spectroscopy (UV-visible

a

bc

d e

a

Figure 9: SEM of Co3O4 powder at different doping levels of manganese (a) undoped, (b) 3%Mn, (c) 5%Mn, (d) 7%Mn and (e) 9% Mn.

L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films 245

and IR), XRD, FTIR and complex impedance measure-ment. The transmittance of all samples decreases withincreasing doping levels. The band gap energies of oursamples were determined by the Tauc plot. The values ofthe band gaps were found to decrease as the dopantconcentration increases; it might be due to the formationof impurity energy level. XRD patterns revealed that Mn-doped Co3O4 samples were crystallized in cubic spinel

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70-Z

'' (Ω

)

Z'(Ω)

Co3O4:Mn (7%)

Co3O4:Mn (3%)

Co3O4:Mn (5%)

Undoped Co3O4

Co3O4:Mn (9%)

CP

RP

Figure 10: Complex impedance spectrum(Nyquist plots) of Mn-doped Co3O4 thinfilms.

Table 3: Values of fc, Rp and Cp of Mn-doped Co3O4 films.

Samples fc (MHz) Rp (Ω) Cp (µF)

Undoped CoO . . .CoO: Mn (%) . . .CoO: Mn (%) . . .CoO: Mn (%) . . .CoO: Mn (%) . . .

-1 0 1 2 3 4 5 6 7 8 9 1090

95

100

105

110

115

120

125

130

Resistance (Ω)

Capacity(μF)

Mn Content(%)

RP(Ω

)

75

80

85

90

95

100

105

110

115

120

125

CP (μF)

Figure 11: Variation of capacity and resistance of Mn-doped Co3O4 thin films.

246 L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films

structure, knowing that the crystallite size was found tobe from 243 to 365Å. The FTIR spectra of Mn-doped Co3O4

revealed four distinct bands due to the stretching vibra-tions of Co-O, Mn-O for all doped samples and O-H. Thecomplex impedance spectroscopy indicates that the phy-sical concurrent processes of Mn-doped Co3O4 are con-nected in parallel RC. The conduction mechanism of allsamples is highly due to the grain boundaries.

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L. Abdelhak et al.: Characterization of Mn-Doped Co3O4 Thin Films 247