Abstract

1. Introduction

In the last decade, transparent conductive oxides (TCO) have gained considerable interest in the research community due to their intriguing properties, 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, electrochromic sensor¹1 Yao L, Xi Y, Xi G, Feng Y. Synthesis of cobalt ferrite with enhanced magnetostriction properties by the sol-gel-hydrothermal route using spent Li-ion battery. Journal of Alloys and Compounds. 2016;680:73-79. DOI: 10.1016/j.jallcom.2016.04.092
https://doi.org/10.1016/j.jallcom.2016.0... ^,22 Razmi H, Habibi E. Amperometric detection of acetaminophen by an electrochemical sensor based on cobalt oxide nanoparticles in a flow injection system. Electrochimica Acta. 2010;55(28):8731-8737. DOI: 10.1016/j.electacta.2010.07.081
https://doi.org/10.1016/j.electacta.2010... .

Among the transparent conductive oxides (TCO), cobalt oxide (Co₃O₄) is one of the most studied oxides due to its importance for various scientific fields¹1 Yao L, Xi Y, Xi G, Feng Y. Synthesis of cobalt ferrite with enhanced magnetostriction properties by the sol-gel-hydrothermal route using spent Li-ion battery. Journal of Alloys and Compounds. 2016;680:73-79. DOI: 10.1016/j.jallcom.2016.04.092
https://doi.org/10.1016/j.jallcom.2016.0... such as supercapacitors³3 Shinde VR, Mahadik SB, Gujar TP, Lokhande CD. Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis. Applied Surface Science. 2006;252(20):7487-7492. DOI: 10.1016/j.apsusc.2005.09.004
https://doi.org/10.1016/j.apsusc.2005.09... , solar selective absorber⁴4 Smith GB, Ignatiev A, Zajac G. Solar selective black cobalt: preparation, structure, and thermal stability. Journal of Applied Physics. 1980;51(8):4186-4196. DOI: 10.1063/1.328276
https://doi.org/10.1063/1.328276... , and energy storage owing to its electrochemical stability¹1 Yao L, Xi Y, Xi G, Feng Y. Synthesis of cobalt ferrite with enhanced magnetostriction properties by the sol-gel-hydrothermal route using spent Li-ion battery. Journal of Alloys and Compounds. 2016;680:73-79. DOI: 10.1016/j.jallcom.2016.04.092
https://doi.org/10.1016/j.jallcom.2016.0... , large surface area⁴4 Smith GB, Ignatiev A, Zajac G. Solar selective black cobalt: preparation, structure, and thermal stability. Journal of Applied Physics. 1980;51(8):4186-4196. DOI: 10.1063/1.328276
https://doi.org/10.1063/1.328276... , and high conductivity. It is also characterized by good resistance to thermal shocks, oxidation, UV radiation, humidity and corrosion. Co₃O₄ exhibits p-type semiconducting property and behaves like an antiferromagnet (AF) with the Néel temperature T_N ≈ 290 K⁵5 Patil PS, Kadam LD, Lokhande CD. Preparation and characterization of spray pyrolysed cobalt oxide thin films. Thin Solid Films. 1996;272(1):29-32. DOI: 10.1016/0040-6090(95)06907-0
https://doi.org/10.1016/0040-6090(95)069... , Optical studies have shown that Co₃O₄ exhibits multiple direct band gap energies (E_g1 = 1.48 eV, E_g2 = 2.24 eV)⁶6 Gulino A, Dapporto P, Rossi P, Fragalà I. A Novel Self-Generating Liquid MOCVD Precursor for Co3O4 Thin Films. Chemistry of Materials. 2003;15(20):3748-3752. DOI: 10.1021/cm034305z
https://doi.org/10.1021/cm034305z... . It has three well-known valence states, the cobaltous oxide (CoO), the cobaltic oxide (Co₂O₃), and the cobalt cobaltite (Co₃O₄)⁷7 Young RS, ed. Cobalt: Its Chemistry, Metallurgy and Uses. New York: Reinhold Publishing Corp.; 1960.. The most stable phase in the cobalt oxide system is a mixed valence compound [Co²⁺Co³⁺ ₂O₄] with a normal spinel structure³3 Shinde VR, Mahadik SB, Gujar TP, Lokhande CD. Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis. Applied Surface Science. 2006;252(20):7487-7492. DOI: 10.1016/j.apsusc.2005.09.004
https://doi.org/10.1016/j.apsusc.2005.09... .

In recent years, several efforts have been devoted to fabricating nanostructured systems of cobalt oxide with tunable physical-chemical properties for 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 Ni-doped Co₃O₄ thin films in an effort to understand their enhanced optical and electrical properties. In our work, we choose doping by nickel because its atomic radius is almost equal to that of cobalt. A small change in doping concentration is significant for changing the band gaps, the energy band gaps are expected to vary in a wide and very attractive energies intervals.

Various routes of synthesis of Co₃O₄ films have been undertaken such as chemical vapor deposition⁸8 Armelao L, Barreca D, Gross S, Tondello E. Sol-Gel and CVD Co3O4 thin films characterized by XPS. Surface Science Spectra. 2001;8(1):14-23. DOI: 10.1116/11.20010601
https://doi.org/10.1116/11.20010601... , spray pyrolysis⁹9 Victoria SG, Raj AME, Ravidhas C. An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films. Materials Chemistry and Physics. 2015;162:852-859. DOI: 10.1016/j.matchemphys.2015.07.015
https://doi.org/10.1016/j.matchemphys.20... , sol-gel method⁸8 Armelao L, Barreca D, Gross S, Tondello E. Sol-Gel and CVD Co3O4 thin films characterized by XPS. Surface Science Spectra. 2001;8(1):14-23. DOI: 10.1116/11.20010601
https://doi.org/10.1116/11.20010601... ^,1010 Tang Q, Zhu H, Chen C, Wang Y, Zhu Z, Wu J, et al. Preparation and Characterization of Nanoscale Cobalt Blue Pigment for Ceramic Inkjet Printing by Sol-Gel Self-Propagating Combustion. Materials Research. 2017;20(5):1340-1344. DOI: 10.1590/1980-5373-MR-2017-0322
https://doi.org/10.1590/1980-5373-MR-201... ^,1111 Patil V, Joshi P, Chougule M, Sen S. Synthesis and Characterization of Co3O4 Thin Film. Soft Nanoscience Letters. 2011;2(1):1-7. DOI: 10.4236/snl.2012.21001
https://doi.org/10.4236/snl.2012.21001... , metal organic chemical vapor deposition (MOCVD)¹²12 Švegl F, Orel B, Grabec-Švegl I, Kaučič V. Characterization of spinel Co3O4 and Li-doped Co3O4 thin film electrocatalysts prepared by the sol-gel route. Electrochimica Acta. 2000;45(25-26):4359-4371. DOI: 10.1016/S0013-4686(00)00543-0
https://doi.org/DOI: 10.1016/S0013-4686(... . In this paper we have adopted a simple sol gel dip-coating method, this method of preparing thin films has several advantages: its low cost, flexibility in the deposition process, and it is convenient for a large area and it can produce homogeneous thin films with very regular crystallites sizes¹³13 Chatelon JP, Terrier C, Bernstein E, Berjoan R, Roger JA. Morphology of SnO2 thin films obtained by the sol-gel technique. Thin Solid Films. 1994;247(2):162-168. DOI: 10.1016/0040-6090(94)90794-3
https://doi.org/10.1016/0040-6090(94)907...

14 Zarbali M, Göktaş A, Mutlu IH, Kazan S, Şale AG, Mikailzade F. Structure and Magnetic Properties of La0.66Sr0.33MnO3 Thin Films Derived Using Sol-Gel Technique. Journal of Superconductivity and Novel Magnetism. 2012;25(8):2767-2770. DOI: 10.1007/s10948-011-1260-z
https://doi.org/10.1007/s10948-011-1260-... ^-1515 Göktas A. Sol-gel derived Zn1-xFexS diluted magnetic semiconductor thin films: Compositional dependent room or above room temperature ferromagnetism. Applied Surface Science. 2015;340:151-159. DOI: 10.1016/j.apsusc.2015.02.115
https://doi.org/10.1016/j.apsusc.2015.02... . It has been used successfully in our laboratory to fabricate a variety of porous materials such as SnO₂⁹9 Victoria SG, Raj AME, Ravidhas C. An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films. Materials Chemistry and Physics. 2015;162:852-859. DOI: 10.1016/j.matchemphys.2015.07.015
https://doi.org/10.1016/j.matchemphys.20... , TiO₂¹⁶16 Kharoubi A, Bouaza A, Benrabah B, Ammari A, Khiali A. Characterization of Ni-doped TiO2 thin films deposited by dip-coating technique. The European Physical Journal Applied Physics. 2015;72(3):30301. DOI: 10.1051/epjap/2015150282
https://doi.org/10.1051/epjap/2015150282... .

2. Experimental Details

2.1. Films and Powders preparation

In the present venture, a sol gel dip-coating setup operating at atmospheric pressure was used to deposit Ni-doped cobalt oxide thin films on glass substrates. To prepare 0.1 M solution of pure Co₃O₄, hexahydrate nitrate cobalt (Co(NO₃)₂.6H₂O) was dissolved in an aqueous solution of citric acid (C₆H₈O₇ total solutions volume was 30 ml) (citric acid can also act as an effective chelating agent to produce fine particles) and the resulting solution was stirred and heated at 80°C for 2 hours in order to obtain the burgundy homogeneous solution which is further converted into thick gel. The stoichiometric equilibrium reaction between the cobalt nitrate and citric acide used in this study,can be described by the following equation:

To achieve Ni-doping$\left\{\frac{\left[\mathit{Ni}\right]}{\left[\mathit{Co}\right]}\right\}$, nickel nitrate was added to the precursor solution with different concentrations (3 wt%, 5 wt%, 7 wt% and 9 wt%) for all doped samples. This mixture was equally stirred and heated at 80 °C for 2 hours to obtain viscous solution. The obtained viscous gel was calcined in muffle furnace at 400°C for 3 hours in a static air atmosphere to obtain Ni-doped Co₃O₄ powders. A schematic representation of the sol gel synthesis is given in Fig.1.

2.2. Film deposition

The procedure of cleaning glass substrates is very important to get well adherent, smooth films. The substrates Pyrex pieces (75 x 25 x1mm³) were cleaned through dipping them in ultrasonic bath containing trichloroethylene, ethanol, aciton for 5 minutes respectively, and finally rinsed by distilled water. The substrates were dipped in the solution, The withdrawal speed of the substrates from the solution was 5Cm/min. The optimized deposition conditions are listed in Table 1. After coating process was completed, the films were heated at 100°C for 2 hours in the ambient to evaporate the solvent and then annealed at 400°C during 45 minutes to remove organic residues and for densification.The obtained samples were subjected to microstructural, optical and electrical analysis.

2.3. Characterizations

The as-prepared samples were characterized by using different physical techniques. The optical transmittance spectra and band gap energies of Ni-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 X-ray diffraction (Rigaku miniflex 600) with CuKα radiation (λ= 1.5406 Å). The vibrational behavior of the samples was investigated by FT-IR (Alpha Bruker spectrophotometer) in the wavenumber range from 400 to 4000 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 1V.

3. Results and discussion

3.1. Optical analysis

In this part, the UV-Vis spectra of Ni-doped Co₃O₄ films were recorded in the wavelength range from 300 to 900 nm. Fig.2 shows the optical transmission spectra of Ni-doped Co₃O₄ thin films. All the obtained spectra manifest the presence of two sharp absorption edges in the visible region, which are attributed to the ligand to metal charge transfer (LMCT) event of (O^-2 →Co²⁺) and (O^-2→Co³⁺) in Co₃O₄. This indicates the presence of two energy band gaps, in agreement with the literature¹⁷17 Kandalkar SG, Gunjakar JL, Lokhande CD, Joo OS. Synthesis of cobalt oxide interconnected flacks and nano-worms structures using low temperature chemical bath deposition. Journal of Alloys and Compounds. 2009;478(1-2):594-598. DOI: 10.1016/j.jallcom.2008.11.095
https://doi.org/10.1016/j.jallcom.2008.1... .

Fig.2 shows a high transmittance in the range of visible light (T ~ 85% for pure Co₃O₄ and between 60% and 75% is observed for all doped samples), attributed to a better structural homogeneity. It is well known that Co₃O₄, (i.e. Co²⁺ [Co³⁺] ₂O₄) has a normal spinel crystal structure, knowing that the Co²⁺ ions occupy the tetrahedral sites, while Co³⁺ ions occupy the octahedral sites¹⁸18 Thota S, Kumar A, Kumar J. Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol-gel derived oxalates. Materials Science and Engineering: B. 2009;164(1):30-37. DOI: 10.1016/j.mseb.2009.06.002
https://doi.org/10.1016/j.mseb.2009.06.0... . Since the p states of O^2- ions are located closely to the d states of Co³⁺ ions, p electrons can easily undergo a transition. At low temperatures this peak splits and results in a doublet corresponding to p(O^2-)→e_g(Co³⁺) and p (O^2-)→t₂(Co²⁺). The higher band gap should be associated to the O^2-→Co²⁺charge transfer (valence to conduction band excitation) and the lower band gap associated to the O^2-→Co³⁺charge transfer (with the Co³⁺ level located below the conduction band)¹⁹19 Pal J, Chauhan P. Study of physical properties of cobalt oxide (Co3O4) nanocrystals. Materials Characterization. 2010;61(5):575-579. DOI: 10.1016/j.matchar.2010.02.017
https://doi.org/10.1016/j.matchar.2010.0...

20 Jacobs JP, Maltha A, Reintjes JGH, Drimal J, Ponec V, Brongersma HH. The Surface of Catalytically Active Spinels. Journal of Catalysis. 1994;147(1):294-300. DOI: 10.1006/jcat.1994.1140
https://doi.org/10.1006/jcat.1994.1140... ^-2121 Shelef M, Wheeler MAZ, Yao HC. Ion scattering spectra from spinel surfaces. Surface Science. 1975;47(2):697-703. DOI: 10.1016/0039-6028(75)90218-6
https://doi.org/10.1016/0039-6028(75)902... . Jacques Pankove. I²²22 Pankove JI. Optical Processes in Semiconductors. Englewood Cliffs: Prentice-Hall; 1971. suggest that the multiple band gap energy for the Co₃O₄ thin films may be due to the valence band degeneracy. Moreover, the electrical conduction of Co₃O₄ occurs by the hopping of small polarons between two different valency states of the cobalt ions²³23 Ali GAM, Fouad OA, Makhlouf SA. Structural, optical and electrical properties of sol-gel prepared mesoporous Co3O4/SiO2 nanocomposites. Journal of Alloys and Compounds. 2013;579:606-611. DOI: 10.1016/j.jallcom.2013.07.095
https://doi.org/10.1016/j.jallcom.2013.0... . A Schematic representation of the band structure of Pure Co₃O₄ is given in Fig.3. The variation of absorption coefficient against photon energy hν for direct band-to-band transition has the form of:

where α is the absorption coefficient $\left(\mathit{\alpha }=\frac{1}{\mathit{d}}\ln \left\{\frac{1}{\mathit{T}}\right\}\right)$, hν is the photon energy, E_g the band gap energy and A the edge parameter. The value of x is 2 for indirect allowed transitions and ½ for direct allowed transitions²⁴24 Louardi A, Rmili A, Ouachtari F, Bouaoud A, Elidrissi B, Erguig H. Characterization of cobalt oxide thin films prepared by a facile spray pyrolysis technique using perfume atomizer. Journal of Alloys and Compounds. 2011;509(37):9183-9189. DOI: 10.1016/j.jallcom.2011.06.106
https://doi.org/10.1016/j.jallcom.2011.0... . Fig.4 shows the plots of (αhυ)² vs (hυ). The extrapolation of a straight portion to the energy axis at α = 0 can give two values of band gap E_g1 = 1.50eV corresponds (O^-2→Co³⁺) and E_g2 = 2.20eV corresponds (O^-2 →Co²⁺). Louardi et al.²⁴24 Louardi A, Rmili A, Ouachtari F, Bouaoud A, Elidrissi B, Erguig H. Characterization of cobalt oxide thin films prepared by a facile spray pyrolysis technique using perfume atomizer. Journal of Alloys and Compounds. 2011;509(37):9183-9189. DOI: 10.1016/j.jallcom.2011.06.106
https://doi.org/10.1016/j.jallcom.2011.0... have obtained the same results. The refractive index is calculated by Ravindra relation ship²⁵25 Ravindra NM. Energy gap-refractive index relation - some observations. Infrared Physics. 1981;21(5):283-285. DOI: 10.1016/0020-0891(81)90033-6
https://doi.org/10.1016/0020-0891(81)900... :

The film thickness d has been calculated from UV-visible data using the following equation²⁶26 Tareen JAK, Malecki A, Doumerc JP, Launay JC, Dordor P, Pouchard M, et al. Growth and electrical properties of pure and Ni-doped Co3O4 single crystals. Materials Research Bulletin. 1984;19(8):989-997. DOI: 10.1016/0025-5408(84)90212-5
https://doi.org/10.1016/0025-5408(84)902... :

Where, n is the refractive index at two adjacent maxima or minima at wavelengths λ₁ and λ₂. Results obtained for the direct band gap energy, films' thickness and refractive index of our films at different doping levels are reported in Table 2. In Fig.5, we show the variations of the band gap energies as a function of doping level.

Ni-doped Co₃O₄ at different doping levels show slight decrease in the band gap compared to pure Co₃O₄ with the increase of doping levels (the lower band gap energy shifted from1.50 for pure Co₃O₄ and to 1.42 eV for the 9wt% Ni-doped Co₃O₄, the higher band gap energy shifted from 2.20 for pure Co₃O₄ and to 2.07 eV for the 9wt% Ni-doped Co₃O₄), This behavior may be due to the network distortions caused by the introduction of nickel ions in the Co₃O₄ matrix and the formation of impurity energy levels (acceptor level) within the band gap. On the other hand nickel contributes to the creation of holes and increases its role by the number of charge carriers (holes) which contribute to the conductivity knowing that Co₃O₄ is a P type semiconductor.

Refraction index increases from 2.588 (pure) to 2.676 (9%Ni). This can be explained by the crystallization and the densification of the material. J.A.K. Tareen et al.²⁶26 Tareen JAK, Malecki A, Doumerc JP, Launay JC, Dordor P, Pouchard M, et al. Growth and electrical properties of pure and Ni-doped Co3O4 single crystals. Materials Research Bulletin. 1984;19(8):989-997. DOI: 10.1016/0025-5408(84)90212-5
https://doi.org/10.1016/0025-5408(84)902... suggest that the Ni atoms are located in the octahedral sites of the spinel lattice. 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 material²⁷27 Goktas A, Aslan F, Tumbul A, Gunduz SH. Tuning of structural, optical and dielectric constants by various transition metal doping in ZnO:TM (TM=Mn, Co, Fe) nanostructured thin films: A comparative study. Ceramics International. 2016;43(1 Pt A):704-713. DOI: 10.1016/j.ceramint.2016.09.217
https://doi.org/10.1016/j.ceramint.2016.... ^,2828 Goktas A, Mutlu IH. Structural, Optical, and Magnetic Properties of Solution-Processed Co-Doped ZnS Thin Films. Journal of Electronic Materials. 2016;45(11):5709-5720. DOI: 10.1007/s11664-016-4771-3
https://doi.org/10.1007/s11664-016-4771-... .

3.2. Structural analysis

The structural characterization of the powders was analyzed using Rigakuminiflex 600 Xray diffractometer with CuKα radiation in the 2θ range of 20-80°. Fig.6 shows the X-ray diffraction (XRD) pattern of Ni-doped Co₃O₄ powders after calcination in muffle furnace at 400°C for 3 hours. All obtained powders show multiple diffraction peaks coincided well with the cubic spinel type structure (Fd3m space group). The presence of reflection peaks associated to (220), (311), (222), (400), (422), (511) and (440) planes at 2θ = 31.27°, 36.88°, 38.62° 44.81°, 55.75°, 59.45°and 65.38° respectively, knowing that 〈311〉 as preferential orientation. No parasitic phase of nickel clusters, nickel oxides (NiO) or Ni-Co, (NiCoO3) oxide phases has been observed in the detection limit of the apparatus, which indicate a high purity of the samples. These results have been previously confirmed by several authors²⁰20 Jacobs JP, Maltha A, Reintjes JGH, Drimal J, Ponec V, Brongersma HH. The Surface of Catalytically Active Spinels. Journal of Catalysis. 1994;147(1):294-300. DOI: 10.1006/jcat.1994.1140
https://doi.org/10.1006/jcat.1994.1140...

21 Shelef M, Wheeler MAZ, Yao HC. Ion scattering spectra from spinel surfaces. Surface Science. 1975;47(2):697-703. DOI: 10.1016/0039-6028(75)90218-6
https://doi.org/10.1016/0039-6028(75)902...

22 Pankove JI. Optical Processes in Semiconductors. Englewood Cliffs: Prentice-Hall; 1971.

23 Ali GAM, Fouad OA, Makhlouf SA. Structural, optical and electrical properties of sol-gel prepared mesoporous Co3O4/SiO2 nanocomposites. Journal of Alloys and Compounds. 2013;579:606-611. DOI: 10.1016/j.jallcom.2013.07.095
https://doi.org/10.1016/j.jallcom.2013.0...

24 Louardi A, Rmili A, Ouachtari F, Bouaoud A, Elidrissi B, Erguig H. Characterization of cobalt oxide thin films prepared by a facile spray pyrolysis technique using perfume atomizer. Journal of Alloys and Compounds. 2011;509(37):9183-9189. DOI: 10.1016/j.jallcom.2011.06.106
https://doi.org/10.1016/j.jallcom.2011.0...

25 Ravindra NM. Energy gap-refractive index relation - some observations. Infrared Physics. 1981;21(5):283-285. DOI: 10.1016/0020-0891(81)90033-6
https://doi.org/10.1016/0020-0891(81)900...

26 Tareen JAK, Malecki A, Doumerc JP, Launay JC, Dordor P, Pouchard M, et al. Growth and electrical properties of pure and Ni-doped Co3O4 single crystals. Materials Research Bulletin. 1984;19(8):989-997. DOI: 10.1016/0025-5408(84)90212-5
https://doi.org/10.1016/0025-5408(84)902...

27 Goktas A, Aslan F, Tumbul A, Gunduz SH. Tuning of structural, optical and dielectric constants by various transition metal doping in ZnO:TM (TM=Mn, Co, Fe) nanostructured thin films: A comparative study. Ceramics International. 2016;43(1 Pt A):704-713. DOI: 10.1016/j.ceramint.2016.09.217
https://doi.org/10.1016/j.ceramint.2016....

28 Goktas A, Mutlu IH. Structural, Optical, and Magnetic Properties of Solution-Processed Co-Doped ZnS Thin Films. Journal of Electronic Materials. 2016;45(11):5709-5720. DOI: 10.1007/s11664-016-4771-3
https://doi.org/10.1007/s11664-016-4771-... ^-2929 Santos GA, Santos CM, da Silva SW, Urquieta-González EA, Sartoratto PPC. Sol-gel synthesis of silica-cobalt composites by employing Co3O4 colloidal dispersions. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2012;395:217-224. DOI: 10.1016/j.colsurfa.2011.12.033
https://doi.org/10.1016/j.colsurfa.2011.... . The crystalline phase of Ni-doped Co₃O₄ is identical to the Co₃O₄cubic spinel phase. When the Ni atoms introduced into the matrix it can either "substitute" or "interstice" in the lattice. The lattice spacing was calculated from the Bragg's cubic system formula²⁰20 Jacobs JP, Maltha A, Reintjes JGH, Drimal J, Ponec V, Brongersma HH. The Surface of Catalytically Active Spinels. Journal of Catalysis. 1994;147(1):294-300. DOI: 10.1006/jcat.1994.1140
https://doi.org/10.1006/jcat.1994.1140... :

The particle sizes of Ni-doped Co₃O₄ samples were calculated using the full width at half maximum (FWHM) of (311) peak from the Debye-Scherrer formula³⁰30 Benrabah B, Bouaza A, Kadari A, Maaref MA. Impedance studies of Sb doped SnO2 thin film prepared by sol gel process. Superlattices and Microstructures. 2011;50(6):591-600. DOI: 10.1016/j.spmi.2011.08.009
https://doi.org/10.1016/j.spmi.2011.08.0... , knowing that the width increases as the particle size decreases

Where D is the crystallite size, λ is the wavelength of the CuKα radiation (1.5406Å), β the full-width half maximum (FWHM) of Bragg peak observed at Bragg angle θ. It was found that the crystallite size of the samples was in the range of [214-279] Ǻ. The values of D and β obtained are given in Table 3.

3.3. Infrared Spectroscopy

Fig.7, shows the FT-IR transmission spectra of Pure and Ni-doped Co₃O₄ films at different doping levels (3wt%, 5wt%, 7wt% and 9wt%), deposited by dip coating technique on silicon substrates and annealed at 400°C. In the investigated region 400-4000 cm^-1, all obtained spectra have two absorption bands υ₁, υ₂ at 565 cm^-1 and 662 cm^-1 assigned to the stretching vibration of metal-oxygen bond (Co-O or Ni-O) in Co₃O₄ spinel oxide with (Fd3m space group), Gomaa A et al.²³23 Ali GAM, Fouad OA, Makhlouf SA. Structural, optical and electrical properties of sol-gel prepared mesoporous Co3O4/SiO2 nanocomposites. Journal of Alloys and Compounds. 2013;579:606-611. DOI: 10.1016/j.jallcom.2013.07.095
https://doi.org/10.1016/j.jallcom.2013.0... have obtained the same results. The absorption band υ₁ at 565 cm^-1 is associated with the OB₃ vibrations in the spinel lattice where B denotes the Co³⁺ ions in an octahedral hole. The second band υ₂ (662 cm^-1) is the ABO₃ vibration, where A denotes Co²⁺ ions in a tetrahedral hole. The absorption peak υ₄ at (2350 cm^-1), is assigned to the vibration of C=O bond³¹31 Ammari A, Bellal B, Zebbar N, Benrabah B, Trari M. Thermal-frequency dependence study of the sub-band localized states effect in Sb-doped SnO2 based sol-gel thin films. Thin Solid Films. 2017;632:66-72. DOI: 10.1016/j.tsf.2017.02.060
https://doi.org/10.1016/j.tsf.2017.02.06... . The experimental values of absorption bands have been collected from different sources¹²12 Švegl F, Orel B, Grabec-Švegl I, Kaučič V. Characterization of spinel Co3O4 and Li-doped Co3O4 thin film electrocatalysts prepared by the sol-gel route. Electrochimica Acta. 2000;45(25-26):4359-4371. DOI: 10.1016/S0013-4686(00)00543-0
https://doi.org/DOI: 10.1016/S0013-4686(... ^,1717 Kandalkar SG, Gunjakar JL, Lokhande CD, Joo OS. Synthesis of cobalt oxide interconnected flacks and nano-worms structures using low temperature chemical bath deposition. Journal of Alloys and Compounds. 2009;478(1-2):594-598. DOI: 10.1016/j.jallcom.2008.11.095
https://doi.org/10.1016/j.jallcom.2008.1... ^,3232 Makhlouf MT, Abu-Zied BM, Mansoure TH. Direct Fabrication of Cobalt Oxide Nanoparticles Employing Sucrose as a Combustion Fuel. Journal of Nanoparticles. 2013;2013:384350. DOI: 10.1155/2013/384350
https://doi.org/10.1155/2013/384350... . These observations are in good agreement with the XRD results (Fig.6).

3.4. 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 circuit³³33 Barsoukov E, Macdonald JR, eds. Impedance Spectroscopy, Theory, Experiment, and Applications. 2nd ed. New York: John Wiley & Sons; 2005.. 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, Pores, defects)¹⁶16 Kharoubi A, Bouaza A, Benrabah B, Ammari A, Khiali A. Characterization of Ni-doped TiO2 thin films deposited by dip-coating technique. The European Physical Journal Applied Physics. 2015;72(3):30301. DOI: 10.1051/epjap/2015150282
https://doi.org/10.1051/epjap/2015150282... ^,3030 Benrabah B, Bouaza A, Kadari A, Maaref MA. Impedance studies of Sb doped SnO2 thin film prepared by sol gel process. Superlattices and Microstructures. 2011;50(6):591-600. DOI: 10.1016/j.spmi.2011.08.009
https://doi.org/10.1016/j.spmi.2011.08.0... ^,3434 Göktas A, Tumbul A, Aslan F. Grain size-induced structural, magnetic and magnetoresistance properties of Nd0.67Ca0.33MnO3 nanocrystalline thin films. Journal of Sol-Gel Science and Technology. 2016;78(8):262-269. DOI: 10.1007/s10971-016-3960-0
https://doi.org/10.1007/s10971-016-3960-... It also characterizes the different electrically active regions in the material and demonstrates their existence by their individual electrical properties. The electrical behavior of our films described in terms of one of the four complex expressions³⁵35 McDevitt NT, Baun WL. Infrared absorption study of metal oxides in the low frequency region (700-240 cm-1). Spectrochimica Acta. 1964;20(5):799-808. DOI: 10.1016/0371-1951(64)80079-5
https://doi.org/10.1016/0371-1951(64)800... , each consists of real and imaginary component.

Complex impedance:

Complex admittance:

Complex permittivity:

Complex modulus:

These expressions are interrelated as:

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, ω=2πυ, where υ is the applied frequency and j²=-1. Previous formalisms provide the opportunity to expand the scope to highlight a particular aspect of the electrical response of the sample. The idealized plot (Z'' versus Z') that describes the electrical behavior of a polycrystalline oxide has three components, each of these components corresponds to a particular relaxation frequency. At higher frequencies, the component corresponds to the bulk properties (ν_b). At intermediate frequencies, the electrical behavior 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)³⁶36 Mechiakh R, Meriche F, Kremer R, Bensaha R, Boudine B, Boudrioua A. TiO2 thin films prepared by sol-gel method for waveguiding applications: Correlation between the structural and optical properties. Optical Materials. 2007;30(4):645-651. DOI: 10.1016/j.optmat.2007.02.047
https://doi.org/10.1016/j.optmat.2007.02... . Several factors influence the electrical behavior of materials as chemical composition, impurities, ageing and conditions of preparation. The volume and grain boundary properties, chemical composition, impurities, ageing and preparation conditions make the actual oxide system rather complicated. The electrical characteristic of a material is shown by the appearance of semicircular arcs in the Nyquist plots. Fig.8 is the Nyquist representation of pure and Ni-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 thin films was calculated using the following equation:

The variation of the resistance and capacitance as a function of Ni doping level are listed in Table 4 and shown in Fig.9. 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 nickel 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 conduction through the grain boundaries. The effect of grain boundaries in samples becomes more dominant with respect to the contribution of the grains in the conduction mechanism.

4. Conclusion

In conclusion, we have successfully synthesised Ni-doped cobalt oxide (Co₃O₄) thin films using a sol gel technique in order to investigate their optical, structural and electrical properties. X-ray diffraction patterns revealed that Ni-doped Co₃O₄ samples were crystallized in cubic spinel structure knowing that the crystallite size was found to be from 214 to 279 Å. The as synthesis films, exhibit a high transmission ∼ 60-85% in the visible region. Optical studies concluded that Co₃O₄ has multiple band gap energies with direct transitions 2.20 eV (O^2-→Co²⁺) and 1.50 eV (O^2-→Co³⁺). 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 acceptor level within the band gap. The FT-IR spectra of pure and Ni-doped Co₃O₄ films revealed two distinct bands that arise due to the stretching vibrations of the metal Co-O or Ni-O bonds in the investigated region. The FT-IR spectra were typical of a cubic spinel structure with space group Fd-3m and served as a clear evidence for the presence of cubic Co₃O₄ in agreement with X-ray diffraction results. The complex impedance spectroscopy indicates that the physical concurrent processes of Ni-doped Co₃O₄ are connected in parallel RC. The conduction mechanism of all samples is highly due to the grain boundaries.

5. References

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