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

Lakehal Abdelhak; Bouaza Amar; Benrabah Bedhiaf; Dalache Cherifa; Benhebal Hadj

doi:10.1515/htmp-2017-0185

Article Open Access

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

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

Published/Copyright: September 21, 2018

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From the journal High Temperature Materials and Processes Volume 38 Issue 2019

Abstract

In this article, manganese-doped cobalt oxide (Mn-doped Co₃O₄) thin films have been prepared on glass substrates using sol gel-based dip-coating technique in order to investigate their optical, structural and electrical properties. The Mn concentration was changed from 0 % to 9 %. The synthesized samples were characterized by ultraviolet-visible spectroscopy (UV-visible), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and complex impedance spectroscopy to elucidate the optical, structural, vibrational and electrical properties. Our optical results show that the transmittance of Mn-doped Co₃O₄ films decreases with increasing doping levels. The optical band gaps were found to be (Eg1=1.51 eV,Eg2=2.12eV) and (Eg1=1.23 eV,Eg2=1.72eV) for the case of undoped Co₃O₄ and 9 % Mn-doped Co₃O₄, respectively. This shift means that the impurities would create energy levels. The structural analysis provides evidence that obtained powders were crystallized in cubic spinel structure. The complementary phase information is provided by FTIR spectroscopy. The FTIR study depicted the presence of four distinct bands characterizing 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 found that the effective resistance Rp decreases, whereas the effective capacitance Cp increases with doping.

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

Introduction

Transparent conducting oxides (TCOs) are promising materials in various potential applications due to their dual property; they combine electrical conductivity and optical transparency in the visible range. These properties have attracted the attention for using them in optoelectronic devices such as photovoltaic solar cells and electrochromic windows [1, 2].

Among the TCOs, cobalt oxide (Co₃O₄) is considered as one of the broadly studied compounds due to its importance in various scientific fields such as solar selective absorber [1], energy storage [2], supercapacitors [3] and electrochromic sensor [4]. Co₃O₄ exhibits p-type semiconducting property and behaves like an antiferromagnet (AF) with the Néel temperature TN≈290 K [5], Optical absorption studies have shown that Co₃O₄ exhibits multiple band gap energies (Eg1=1.57eV,Eg2=2.28eV) [6]. Cobalt oxide exists in three different crystalline forms: the cobaltous oxide (CoO), the cobaltic oxide (Co₂O₃) and tricobalt tetraoxide (Co₃O₄). The most stable phase in the cobalt oxide system is a mixed valence compound [Co²⁺Co³⁺₂O₄] with a normal spinel structure [3].

In recent years, great efforts have been devoted to fabricating nanostructured systems of cobalt oxide with tunable physical–chemical properties for a broad range of applications, among which transition metal doping is a promising and efficient route to improve the optical absorption and the electrical behavior of Co₃O₄. In this regard, this study fabricates Mn-doped Co₃O₄ thin films in an effort to understand 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 optimized semiconductor thin films of Co₃O₄ with good optical and electrical properties, and we controlled the doping levels so that the Co₃O₄ phase did not change. We choose doping by manganese Mn²⁺ because its atomic radius (0.80 Å) is almost equal to that of cobalt Co²⁺ (0.72 Å). It can substitute the Co²⁺ ion. A small change in dopant concentration is significant for changing the band gaps, and the energy band gaps are expected to vary in a wide and very attractive energies intervals ranging from (Eg1=1.51eV,Eg2=2.12eV) in undoped Co₃O₄ and down to (Eg1=1.23eV,Eg2=1.72eV) in 9 % Mn-doped Co₃O₄.

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

Experimental procedure

Materials and methods

The principle of the dip-coating method is to dip the substrate in a viscous solution and to withdraw at constant speed under controlled and stable conditions to obtain a homogeneous film of regular thickness. The deposition of the thin films by the dip-coating technique is 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 thickness of the thin films such as sol viscosity, pressure, pulling force, gravitational force, surface tension and surface tension gradient. It is possible, in principle, to adjust some parameters to obtain the desired thickness.

In the present venture, a sol gel dip-coating setup operating at atmospheric pressure was used to deposit cobalt oxide thin films. In a typical experiment, cobalt nitrate hexahydrate (Co(NO₃)₂.6H₂O) was dissolved in an aqueous solution (100 ml) of sucrose (C₁₂H₂₂O₁₁) and stirred vigorously at 100 °C for about 2 h to form a burgundy homogeneous solution. To achieve Mn dopingMnCo, manganese nitrate hexahydrate (Mn(NO₃)2.6H₂O) was added to the precursor solution with different concentrations (3 wt%, 5 wt%, 7 wt% and 9 wt%) for all doped samples. The final solution (undoped Co₃O₄ and Mn-doped Co₃O₄) will be used either for film deposition or to obtain powder after calcination in muffle furnace at 400 °C for 3 h.

The procedure of cleaning glass substrates is very important to get well-adherent, smooth films. The substrates Pyrex pieces (75×25×1 mm³) were cleaned through dipping them in ultrasonic bath containing trichloroethylene, ethanol and aciton for 5 min, respectively, and finally rinsed by distilled water. The withdrawal speed of the substrates from the solution was 50 mm/min. The formed films pass through the drying step to evaporate the solvent. After that, the obtained films were calcined in muffle furnace at 400°C for 45 min. A schematic representation of the sol gel synthesis is given in Figure 1.

Figure 1:

Schematic diagram of sol gel process of Mn-doped Co₃O₄ preparation.

Characterizations

The as-prepared samples were characterized by using different physical techniques. The optical transmittance spectra and band gap energies of Mn-doped Co₃O₄ thin films were measured using Shimadzu-1650 spectrophotometer in the wavelength range from 300 to 900 nm. The phase and crystal structure of all samples were studied by XRD (Rigaku miniflex 600) with CuKα radiation (λ=1.5406 Å). The vibrational behavior of the samples was investigated by FTIR (Alpha Bruker spectrophotometer) in the wavenumber range from 400 to 4,000 cm⁻¹. Impedance measurements were carried out using Agilent4284A LCR-meter operating in the frequency range 75 KHz to 20 MHz with an oscillation amplitude of 1 V.

Results and discussion

Optical properties

In this part, the UV-Vis spectra of Mn-doped Co₃O₄ films were recorded in the wavelength range from 300 to 900 nm. Figure 2 shows the optical transmission spectra of Mn-doped Co₃O₄ thin films. All the obtained spectra manifest the presence of two sharp absorption edges in the visible region. The first one as reported in many researches is associated to the O^2– and Co²⁺ charge transfer process, while the other observed at 500–600 nm wavelength range is due to the O^2– and Co³⁺ charge transfer (ligand to metal charge transfer (LMCT) event). This indicates the presence of two band gap energies, in agreement with the literature [11, 12]. It is evident that the transmittance (Figure 2) gradually decreases while increasing Mn content. It can be seen from Figure 2 that the average transmission in the visible region has decreased with increasing doping levels. The absorption bands of Mn-doped Co₃O₄ vary with a change in the concentration of manganese. The band gap energies (allowed direct transition) of Mn-doped Co₃O₄ can be determined by the following equation [10]:

(1)αhυ=Ahυ−Eg12

Figure 2:

Transmittance spectra of Mn-doped Co₃O₄ films.

where A the edge parameter, Eg is the energy band gap of the material α is the absorption coefficient and hυ is the photon energy. Figure 3 shows the plots of αhυ2vs.hυ. The extrapolation of a straight portion to the energy axis at α=0 can give two values of band gap energies. Eg1 corresponds to (O⁻²→Co³⁺) and Eg2 corresponds to (O⁻² →Co²⁺). The extinction coefficient (k) of Mn-doped Co₃O₄ was calculated according to the following relation [13]:

(2)k=αλ4π

Figure 3:

Tauc plots of Mn-doped Co₃O₄ films.

where α is the absorption coefficient. The extinction coefficient spectra as a function of the incident wavelength (λ) are plotted in Figure 4. The increase of k in two wavelength regions [300–450 nm] and [600–770 nm] is assigned to the absorption by inter-band transitions (O⁻² →Co²⁺) and (O⁻²→Co³⁺), respectively. The refractive index is related to optical band gap (Eg) of a semiconductor by the following relationship [14]:

(3)n=4.084−0.68 Eg

Figure 4:

Extinction coefficient plots of Mn-doped Co₃O₄ thin films.

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

(4)d=λ1λ22nλ1−λ2

where n is the refractive index at two adjacent maxima or minima at wavelengths λ1 and λ2,Eg is the band gap energy. Results obtained here such as band gap energies, the average transmission, films’ thickness and the refractive index at different doping levels are reported in Table 1.

Table 1:

Band gap energies, average of transmission, refractive index and film thickness of Mn-doped Co₃O₄ films.

Samples	Eg1 (eV)	Eg2 (eV)	Refractive index	Film thickness (nm)
Undoped Co₃O₄	1.51	2.12	2.642	215.91
Co₃O₄: Mn (3 %)	1.45	2.07	2.676	230.77
Co₃O₄: Mn (5 %)	1.37	1.87	2.812	233.01
Co₃O₄: Mn (7 %)	1.23	1.75	2.894	216.05
Co₃O₄: Mn (9 %)	1.20	1.72	2.914	222.15

The band gap energies were found to decrease with increasing Mn content as shown in Figure 5. From these results, it can be seen that the shift in the band gap energies of our samples is generally due to individual or combined effects such as quantum confinement, crystallites size, charged impurities at grain boundaries, structural disorder, the network distortions caused by the introduction of impurities in the matrix and the creation of impurities energy levels. The optical properties of Mn-doped Co₃O₄ have attracted the attention for using it in solar cells.

Figure 5:

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

The refractive index value of undoped Co₃O₄ film is 2.642, in agreement with several authors [16, 17]. It can be observed that the refractive index gradually increases as the manganese concentration increases. This is explained by the substitution of Mn²⁺ ions in Co²⁺ ions which induces the densification of the material. Moreover, at the microscopic level, the increase of n refers to the modification of the polarizability of the ions and the local 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 solubility of the Mn ions into the Co₃O₄ cubic crystal. The main peak is 〈311〉 and the lattice parameter of undoped Co₃O₄ is a=8.069 n. These results have been previously confirmed by several authors [19, 20]. No other parasitic phase of manganese clusters are detected in all doped samples, which indicates a high purity of the samples. It is clear that the intensity of the diffraction peak decreases with increasing doping levels. Therefore, Mn doping decreases the crystallinity of Co₃O₄. The different structural parameters like grain size and lattice parameter of Mn-doped Co₃O₄ are calculated from the XRD analysis and listed in Table 2. The cubic network parameter of the samples has been computed from the Bragg’s cubic system formula [11]:

(5)dhkl=ah2+k2+l2

Figure 6:

XRD patterns of Mn-doped Co₃O₄ powders.

Table 2:

Structural parameters of Mn-doped Co₃O₄ powders.

Samples	(hkl)	2θ	d spacing (Ǻ)	Lattice parameter (Ǻ)	FWHM (deg)	Crystallite size D (Ǻ)
Undoped Co₃O₄	(311)	36.90	2.433	8.069	0.60	243
Co₃O₄: Mn (3 %)	(311)	36.87	2.438	8.089	0.45	325
Co₃O₄: Mn (5 %)	(311)	36.80	2.442	8.099	0.45	311
Co₃O₄: Mn (7 %)	(311)	36.79	2.443	8.102	0.45	324
Co₃O₄: Mn (9 %)	(311)	36.76	2.444	8.109	0.40	365

In Figure 7, we show the variations of the network parameter and grain size as a function of doping levels. The increase of the lattice parameter with increasing Mn content refers to the substitution of smaller Co²⁺ ions (0.72 Å) by larger Mn²⁺ ions (0.80 Å) in the system. The particle sizes of Mn-doped Co₃O₄ powders were calculated 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 size decreases:

(6)D=0.9 λβ cosθ

Figure 7:

Network parameter and grain size of Mn-doped Co₃O_4.

where D is the crystallite size, λ is the wavelength of CuKα radiation (1.5406 Å) and β is the FWHM of Bragg peak observed at Bragg angle θ. The obtained values fell in the [243–365 Å] range. The variation in the crystallite size may be due to the substitution of Mn²⁺ ions in Co²⁺ ions and the formation of nanostructured poly-crystalline Mn-doped Co₃O₄.

FTIR spectroscopy

In order to investigate the influence of Mn doping on vibrational properties, Alpha Bruker FTIR spectrometer experiments were carried out. Figure 8 shows FTIR transmission spectra of Mn-doped Co₃O₄ films. In the investigated region (4,000–400 cm−1), all obtained spectra have two absorption bands ʋ₁, ʋ₂ at 555 cm⁻¹ and 664 Cm⁻¹ assigned from stretching vibration of the metal–oxygen bond in Co₃O₄ spinel oxide [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. The band at approximately 555 cm⁻¹ can be attributed to the vibrations of the bonds formed by the Co³⁺ ion which are present in the octahedral holes of the crystal lattice. The other band at about 664 cm⁻¹ is associated to the vibrations of the bonds formed by the Co²⁺ ion which are present in the tetrahedral holes of the crystal lattice [22]. The absorption peak ʋ₄ at (2,350 cm⁻¹) is assigned to the bending vibration of O-H, and we suggest that the origin of this bond is due to the solvent involved in the synthesis of the films [10]. FTIR spectra show other low-intensity peaks which are assigned to the organic matter remaining after annealing. In the Mn-doped Co₃O₄ spectrum, the absorption band ʋ₃ in the region [600–630 cm⁻¹] confirms the vibrational frequency of Mn–O complex [23]. The obtained results are in good agreement with the XRD results (Figure 6).

Figure 8:

FTIR spectra of Mn-doped Co₃O₄ films.

Scanning electron microscopy studies

To understand the morphology of as-prepared powders, the scanning electron microphotographs of powders are recorded. Figure 9 shows the scanning electron microphotographs of manganese-doped Co₃O₄ powders at different doping levels (undoped, 3 wt%, 5 wt%, 7 wt% and 9 wt%) under the magnification of×2,000. All microphotographs are characterized by the presence of nanocrystalline grains with some overgrown clusters. Coalescence of smaller grains of different sizes and shapes can be noticed when the manganese concentration increases.

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.

Impedance spectroscopy

The measurement of the electrical properties of materials requires powerful tools to explore the electrical behavior, and that is through modeling them by an equivalent circuit [24]. In this method, we apply a sinusoidal disturbance of constant amplitude and a variable frequency to determine the conduction properties of a polycrystalline oxide and also, in theory, the different contributions to the conduction of a material (grains, grain boundaries and Pores, defects). It also characterizes the different electrically active regions in the material and demonstrates their existence by their individual electrical properties. The following complex expressions describe the electrical behavior of our films [15].

(7)complex impedance: Z*=Z′+jZ″=Rs−jωCs

(8)complex admittance: Y*=Y′−Y″=1RP+jωCp

(9)complex permittivity:ε*=ε′−jε″

(10)complex modulus:M*=M′+M″

(11)These expressions are interrelated as: M∗=1ε∗=jωC0Z∗=jωC01Y∗

where R_S and C_S are the series resistance and capacitance; R_P and C_P are the parallel resistance and capacitance, C₀ is the empty capacitance, the real and imaginary components are ω=2πʋ, ʋ is the applied frequency and j²=−1. The plot (Zˊˊ versus Zˊ) that describes the electrical behavior of our samples which have three components; each of these components corresponds to a particular relaxation frequency. The electrical response is due to the bulk properties (ν_b) at higher frequencies. At intermediate frequencies, the electrical response is due to the grain boundaries (ν_gb) and at low frequencies the electrical response corresponds to electrode process (ν_el), or processes occurring in the material/electrode interface (ν_el ≪ ν_gb≪ ν_b) [25]. Several factors influence the properties of grain boundaries as chemical composition, impurities, ageing and conditions of preparation. The electrical characteristic of a material is shown by the appearance of semicircular arcs in the Nyquist plots. Figure 10 is the Nyquist representation of undoped and Mn-doped Co₃O₄ thin films, whose f frequency varies from 75 KHz to 20 MHz at ambient temperature. The processes that occur in the electrode are modeled by an equivalent electrical circuit. The physical logic of the system indicates that the concurrent processes are connected in parallel. The capacity C_p of the layer was calculated using the following equation [10]:

(12) CP=12πfcRp

Figure 10:

Complex impedance spectrum (Nyquist plots) of Mn-doped Co₃O₄ thin films.

The variation of the resistance and capacitance as a function of Mn-doping levels are listed in Table 3 and shown in Figure 11. It is clear that the resistance of cobalt oxide decreases and the capacity increases with the increase of doping level; this shift is also due to the introduction of manganese ions in Co₃O₄ lattice which induces a variation in the particle size and consequently introduce more grain boundaries within the samples. Two conduction mechanisms are simultaneously present: conduction across the grain and the conduction through the grain boundaries. The effect of grain boundaries in samples becomes more dominant in the conduction mechanism compared to the grain contribution.

Figure 11:

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

Table 3:

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

Samples	fc (MHz)	Rp (Ω)	Cp (µF)
Undoped Co₃O₄	15.40	127.66	80.9
Co₃O₄: Mn (3 %)	15.29	119.02	87.5
Co₃O₄: Mn (5 %)	14.96	106.52	99.9
Co₃O₄: Mn (7 %)	14.74	102.37	105.5
Co₃O₄: Mn (9 %)	14.40	92.04	120.1

Conclusion

In conclusion, we have successfully synthesized Mn-doped cobalt oxide (Co₃O₄) thin films using a sol gel dip-coating technique. We used several experimental methods of investigation to study the effect of Mn doping on optical, structural and electrical properties of cobalt oxide as optical transmission spectroscopy (UV-visible and IR), XRD, FTIR and complex impedance measurement. The transmittance of all samples decreases with increasing doping levels. The band gap energies of our samples were determined by the Tauc plot. The values of the band gaps were found to decrease as the dopant concentration increases; it might be due to the formation of impurity energy level. XRD patterns revealed that Mn-doped Co₃O₄ samples were crystallized in cubic spinel structure, knowing that the crystallite size was found to be from 243 to 365 Å. The FTIR spectra of Mn-doped Co₃O₄ revealed four distinct bands due to the stretching vibrations of Co-O, Mn-O for all doped samples and O-H. The complex impedance spectroscopy indicates that the physical concurrent processes of Mn-doped Co₃O₄ are connected in parallel RC. The conduction mechanism of all samples is highly due to the grain boundaries.

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Received: 2017-12-15

Accepted: 2018-06-18

Published Online: 2018-09-21

Published in Print: 2019-02-25

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Articles in the same Issue

https://doi.org/10.1515/htmp-2017-0185

Keywords for this article

Cobalt oxide; Mn doping; sol gel dip-coating; thin films

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