The physical properties of spinel cubic Co 3 O 4 thin films prepared by a PSM

Abstract

Undoped and Mn-doped Co₃O₄ films were deposited on heated glasses substrates (T_S = 400°C) using a homemade pneumatic spray method (PSM). The solution concentration and deposition time are 0.1 M and 4 min respectively. The effect of manganese doping concentration on structural, optical and electrical properties of cobalt oxide were investigated. The elaborated films were characterized by X-ray diffraction, UV-Vis spectroscopy, atomic force microscopy (AFM) the three-dimensional (3D), energy dispersive spectroscopy (EDS), and four points probe measurements. The XRD study showed that all films were polycrystalline consisting with spinel cubic phase orientated along to (111) plane. The lattice strain and crystallite size were estimated by Williamson-Hall method. The morphology of Mn-doped Co₃O₄ thin films shows a homogeneous surface with straight acicular nanorods (SANRs). EDS analysis showed the presence of peaks associated with Co, O and Mn elements which confirm the composition of the thin films. The optical band gaps varies from 1.42±0.07 to 1.47±0.07 eV of E_gop1and E_gop2 varies from 1.87±0.10 to 2.11±0.11 eV. In addition, the electrical measurement show a maximum electrical conductivity (σ= 15.54±0.78 (Ω.cm)^-1) at 6% wt of Mn.

Keywords

Manganese doping pneumatic spray method (PSM)weight percentages of Manganese (WPM)spinel cubic structure (SCS)X-ray diffraction (XRD)

1 Introduction

Absorbent conducting oxide (ACO) materials such as Fe₂O₃, Cu₂O, NiO and Co₃O₄ have attracted a lot of research. The development of ACO are importance to their interesting optoelectronic, chemical optical and physical properties [1]. Among the ACO is cobalt oxide, which is one of the most remarkable material in experimental examination of scientific and technological importance [1]. Cobalt oxide has a three phases different crystalline are: Co₂O₃ (cubic structure), CoO (cubic structure) and Co₃O₄ (cubic spinel structure of the type AB₂O₄; where Co^3 + ions occupy the octahedral sites and Co²⁺ occupy the tetrahedral sites) is high stability compared to other phases [2]. It has a high optical absorbance in the spectral range of UV-Vis and electrical conduction.

Co₃O₄ thin films suitable for many applications such as catalyst for gaz sensors [3] and catalysers[4, 5], super-capacitors [6], solar selective absorber due to the p-type semiconducting [7], energy storage [8], photocatalytic[9], etc. Co₃O₄ can be prodused by several techniques such as magnetron sputtering (MST), pulsed laser deposition (PLD), chemical vapor deposition (CVD), sol-gel process (SGP), reactive evaporation (RE), electrochemical deposition (ECD) and 400 °C molecular beam epitaxy (MBE). Previous studies indicate that doping of the transition metal elements (Fe, Mn, Cu, Cr, Ni) in Co₃O₄ can enhance electrocatalytic activity [10 –14]. A. Lakehal et al. reported the optical absorption studies have shown they pure Co₃O₄ and Mn doped Co₃O₄ (9wt%) prepared by ultrasonic bath exhibits multiple band gap energies (Eg₁ = 1.33 eV and Eg₂ = 1.87 eV) for 45 min [7]. R. Venkatesha et al. have prepared Cu-doped Co₃O₄ films by nebulized spray pyrolysis method by varying the Cu dopant concentration, found band gap energie Eg₂ = 1.72 eV of 6% Cu [15].

In this work, undoped and Mn-doped Co₃O₄ films are deposited using a homemade pneumatic spray method (PSM). The used technique has many advantages, such as it can produce homogeneous thin films with very regular crystallites sizes [16, 17]. It facilitates the control of the solution concentration, the control of temperature stabilization for glass substrates, the control of time for spraying, large-area spraying, and control of composition. In addition, the morphology of films can be controlled; therefore, it is one of the most important methods for the preparation of Co₃O₄ thin films. The effect of manganese doping on Co₃O₄ thin films crystallinity, transmittance, bandgap energy, and electrical conductivity of Co₃O₄ was investigated.

2 Experimental procedure

Mn-doped Co₃O₄ films thin films prepared using a simple homemade PSM on highly cleaning glass substrates. Cobalt chloride dehydrate (CoCl₂.6H₂O) with 0,1 M and manganese (II) chloride tetrahydrate (MnCl₂.4H₂O) were used as chemicals sources in the solvent which contained equal volumes of distilled water (H₂O) with a volume of 50 ml too and two drops of chloride acid (HCl) also used as a stabilizer for the all samples in this work. Then use manganese doping with different PWM (Mn/Co = 0; 2; 4; 6; 8 and 10 weight %).The produced mixture was stirred for 1 h at temperature between 50 °C –60°C to yield a transparency and homogenous solutions. The glass substrates were cleaned using acetone and methanol solutions for 15 min respectively and rinsed by distilled water. The normalized distance of 20 cm between the spray nozzle (0,5 mm) and the substrates was maintained. The solution quantity of 50 ml was fixed during the preparation of thin films. The spray rate of 3 (ml/min) was maintained by using filtered compressed air (1bar) as a gas carrier. The deposition time was fixed to 4 min at 400 °C as substrate temperature for each film. The structural and crystalline properties of Mn-doped Co₃O₄ films were analyzed by X-Ray diffraction (Rigaku-Type MiniFlex600) with Cu K_α radiation (λ= 0,1541874 nm) in the scanning range of (2θ) was between 10° and 80°.

The optical transmission of thin films was obtained at room temperature in the range of 300 –1000 nm by using a JASCO V-770 spectrophotometer. The morphological properties were studied using (KLA-Tencor Stylus P7) atomic force microscopy (AFM) with three-dimensional (3D). EDS, associated with SEM (TESCAN-VEGA3 scanning electron microscope) was used to determine the films chemical composition. As for electrical conductivity of thin films was measured by four probe points.

3 Results and discussions

3.1 Thin films thickness

The thickness (t_h) of Mn-doped Co₃O₄ thin films as a function of the PWM was calculated from UV-visible data using the following equation [18, 19]. $t_{h} = \frac{λ_{1} λ_{2}}{2 n (λ_{1} - λ_{2})}$ (1)

Where, n is the refractive index at two adjacent maxima or minima at wavelengths λ₁ and λ₂. The variation of thickness shown in Fig. 1. It is clear that thickness is not a function of the proportion of manganese (WPM). This can be explained that manganese doping has no effect on the thickening of the thin films; this is due to the low percent of manganese (WPM).

Fig. 1

VariationCo₃O₄ thin films thickness as a function of the WPM.

3.2 Structural properties of Mn-doped Co₃O₄ thin films

The XRD spectra of thin films of undoped and Mn-doped Co₃O₄ deposited at temperature of 400 ° C and 0.1 M as a solution concentration are shown in Fig. 2. It can be noted that all thin layers exhibit several diffraction peaks around 2θ= 19.01 °; 31.196 °; 36.834 °; 38. 482 ° and 59.393 ° which correspond respectively to the planes (111), (220), (311), (222) and (511). These peak reflections indicate the phase of Co₃O₄ according to (JCPDS 42-1467) [19]. The peaks position (θ values) were used to determine the d-spacing according to Bragg’s Iaw; λ = 2d * sinθ. The lattice parameter (a) of Co₃O₄ thin films as cubic system is determined from $a = d_{hkl} \times \sqrt{h^{2} + k^{2} + l^{2}}$ , where (hkl) are the Miller indices the diffraction peaks. The value of a was determined for each film considering the three main peaks (111), (311) and (222) can be noted in Table 1.

Fig. 2

DRX patterns of Co₃O₄: Mn thin films deposited by different PWM.

Table 1

Structural parameters of Co₃O₄:Mn thin films

Mn-doped	θ (deg)			d_hkl (nm)			a (nm)
Co₃O₄ (wt%)	(111)	(311)	(222)	(111)	(311)	(222)	(111)	(311)	(222)
0	9.51	18.41	19.25	0.46	0.24	0.234	0.80855	0.8093	0.8097
2	9.512	18.42	19.26	0.46	0.24	0.234	0.80816	0.8091	0.8093
4	9.501	18.38	19.24	0.47	0.24	0.234	0.808963	0.8109	0.8102
6	9.49	18.35	19.27	0.46	0.24	0.235	0.8099	0.8119	0.8089
8	9.492	18.38	19.27	0.46	0.24	0.236	0.8096	0.8106	0.8090
10	9.492	18.39	19.22	0.46	0.24	0.237	0.8096	0.8101	0.8109

Mn-doped Co₃O₄ thin films are oriented preferentially along to the plane (111) in substrate temperature 400°C, Manickam et al. [20], Manogowri et al. [21], Louardi et al. [22], and Kouidri et al. [23] have obtained the same result for the Co₃O₄ thin films prepared by spray method. This direction has no diffraction relation with another oxide (CoO or Co₂O₃), this indicates that the thin films of Co₃O₄: Mn are more stable [24], and have minimum energy of the growth. The disappearance of the plane (220) and the decrease in the intensity of the plane (311) in the sample of 8% Mn may be due to the removal of crystal orientations with a preference for growth in the direction of the lowest energy, which is the plane (111).

This due to the increase of stress caused by the rise of Mn doping rate. However, good crystallization was obtained in the low doping. This is attributed to the increase in nucleation sites [25, 26].

The crystallite size (D) and lattice strain (ɛ), was analyzed using the williamson-hall method [26], being based on the following equation:

$\frac{β * \cos θ}{λ} = \frac{0, 9}{D} + \frac{4 ɛ * \sin θ}{λ}$ (2)

Where β: measured full width at half maximum (in radians), θ: Bragg angle (in degree), and λ: wavelength of X-rays.

Using the Williamson-Hall plot of Mn-doped Co₃O₄, the lattice strain and crystallite sizes were calculated respectively from the slope of the linear fit and y-intercept in Fig. 3. This analysis was applied to (111), (311), (222), and (511) corresponding peaks of the Co₃O₄:Mn thin films.

Fig. 3

Williamson-Hall plots of Mn-doped Co₃O₄ for cubic spinel structure.

Table 2 shows the results obtained of W-H analysis for the cubic spinel phase. As can be seen, increasing the WPM(for⩽ 4% Mn) is accompanied with the crystallite sizes and lattice strain decrease (see Fig. 4). This results demonstrate that the substitution of Mn atom in cubic spinal lattice develops a lattice distortion and an intrinsic stress leading to the growth of smaller grains (up to 4 wt% Mn WPM) [27], a smiling correlation between the compressive strain and crystallite size has been reported by Mahdjoub et all in their study of ZnMgO thin films [7]. After this value, the crystallite sizes and lattice strain increased, and that is called coalescence step [28, 29].

Table 2

Williamson-Hall XRD analysis of Mn-doped Co₃O₄ for cubic spinel structure

(hkl)		% 0	% 2	% 4	% 6	% 8	% 10
111	4 sin θ/λ	4,284334	4,286396	4,282140	4,277017	4,278158	4,278534
	β cos θ/λ	0,0341223	0,041662	0,044905	0,054003	0,046282	0,049922
220	4 sin θ/λ	6,975272	–	–	7,122787	–	–
	β cos θ/λ	0,0496311	–	–	0,038893	–	–
311	4 sin θ/λ	8,195901	8,197707	8,180243	8,1564149	8,183030	8,188084
	β cos θ/λ	0,04034	0,05256	0,031759	0,049207	0,039036	0,032692
222	4 sin θ/λ	8,555871	8,560096	8,551267	8,5641499	8,562878	8,543538
	β cos θ/λ	0,0378099	0,045747	0,056475	0,049207	0,051553	0,050556
511	4 sin θ/λ	12,851565	12,83506	12,841683	12,827709	12,834534	12,834636
	β cos θ/λ	0,0492499	0,065345	0,063590	0,0568862	0,047879	0,0512542
Crystallite size D (nm)		32,42	35,47	32,10	24,13	25,82	22, 90
Lattice strainɛ (x 10^-3)		1,39	2,75	2,38	0,737	0,754	0,288

Fig. 4

Lattice strain and crystallite size of Mn-doped Co₃O₄ from Williamson-Hall plots.

Fig. 5

3DAFM images of Co₃O₄: Mn thin films with different WPM.

3.3 Surface morphology of Mn-doped Co₃O₄ thin films

The morphological arrangement of the Mn-doped Co₃O₄ thin films deposited at 400°C with different weight percentage of manganese (WPM) was analyzed using atomic force microscopy(AFM) with three dimensional (3D)and is presented in Fig. 6. From AFM micrographs it is shown that the straight acicular nanorods (SANRs) is consistent with the results reported elsewhere [30], whereas, thin films of lower WPM, it shows fewer the SANR compared to the films witha higher WPM. A quantitative analysis of surface roughness upon small-scale image using the roughness function of the weigth percentage of Mn led to Rms values nm of 217 and 566 nm for lower WPM and higher WPM, respectively. The evolution of film surface as a function of the WPM as a function of the deposition rate can be explained as follows; at low percentage rat, this means that a few amounts of the Mn-doped Co₃O₄ have been deposited; therefore, fewer SANRs appear, so the surface is less rough. Increasing the WPM leads to an increase in the number of SANRs, and thus increased roughness.

Fig. 6

EDS spectra of Mn-doped Co₃O₄ thin films with different WPM.

The chemical composition of elements present in the Mn-doped Co₃O₄ thin films was determined using EDS spectra with the scanning electron microscopy(SEM). Figure 6 shows the atomic percentages of elements present in the Mn-doped Co₃O₄. The oxygen and manganese density increases, but the cobalt density decreases in thin films. From the EDS spectra, from percentages for chemical compositions of the films obtained near to the stoichiometric in nature. Such surface morphology may offer expanded surface area, beneficial for gas sensing [15] and electrochromic applications [31].

3.4 Optical properties

Figure 7 shows the optical transmittance spectra of Mn-doped Co₃O₄ thin films deposited on glasses substrates at 400°C under a function of WPM. It can be seen, the Mn-doped Co₃O₄ thin films have exhibit a similar optical behavior over the wavelength of 300–2000 nm, but there is noticeable high transmittance of 16% for 6 WPM in the range visible (UV-Vis). The optical transmittance of the thin films deposited at low WPM values with a spinel phase was 5% because this thin films of spinel phase have low crystalline size (D = 24, 39 nm) which increasing light scattering. This leads to minimum transmittance in the films. These films are allowed to be use in solar cells and the PN junction [32 –34].

Fig. 7

Transmittance spectra of Co₃O₄: Mn thin films with different WPM.

The absorption edges correspond to the electronic excitation of the valence band at the conduction band. The direct band gap value is determined by the valence level related to the strong O^2- (2p⁶) properties, and the conduction level at which the main contribution is given by the Co²⁺(3d⁷) orbital’s. The direct band gap value is determined by the valence level related to the strong Op properties, and the conduction level at which the main contribution is given by the Co orbitals. The presence of Co^3 +(3d⁶) in Co₃O₄ gives rise to a sub-band located within the energy gap. So, that coincide with E_gop1(is corresponds to the onset of O^2-(2p) to the Co^3 +(t2g)) excitation as highlighted in the enlarged (αhν)² versus (hν) plots in the Fig. 8 and E_gop2(is corresponds to the onset of O^2-(2p) to the Co²⁺(t2g)), which determines the valence to conduction band excitation. The optical band gap E_gop1 and E_gop2 values for all samples are summarized in Table 3 with the WPM.

Fig. 8

Plots of (αhυ)² against hυ of Co₃O₄: Mn thin films with different WPM.

Table 3

Optical and electrical parameters values of the deposited Mn-doped Co₃O₄ thin films

WPM (wt%)	Band gap energy (eV)		Urbach energy Eu (eV)		Thickness t_h(nm)	Refractive index n	Electrical conductivity σ (Ω.cm)^-1
	E_gop1	E_gop2	Eu₁	Eu₂
0	1.47 ± 0, 07	2, 016 ± 0, 10	0, 24 ± 0, 0117	0, 894 ± 0, 05	365, 64 ± 18, 30	2,71	1, 623 ± 0, 10
2	1.45 ± 0, 07	2, 056 ± 0, 10	0, 28 ± 0, 0137	0, 993 ± 0, 05	376, 38 ± 18, 82	2,68	2, 074 ± 0, 104
4	1.47 ± 0, 07	2, 11 ± 0, 11	0, 23 ± 0, 014	0, 995 ± 0, 05	297, 47 ± 14, 87	2,64	2, 633 ± 0, 132
6	1, 44 ± 0, 07	1, 873 ± 0, 1	0, 25 ± 0, 011	0, 917 ± 0, 05	291, 14 ± 14, 56	2,81	15, 536 ± 0, 78
8	1, 43 ± 0, 08	2, 051 ± 0, 10	0, 28 ± 0, 014	1 ± 0, 05	328, 81 ± 16, 44	2,69	0, 992 ± 0, 05
10	1, 42 ± 0, 07	2, 01 ± 0, 10	0, 34 ± 0, 020	1, 12 ± 0, 06	346, 45 ± 17, 32	2,72	0, 631 ± 0, 032

Co₃O₄ is known to possess strong photo absorption in the visible region, which is easily evidenced by its dark black color. To calculate the energy gap E_gop associated with the incident photon energy (hν) and the absorption coefficient(α), we use the classical equation [24]. ${(α h υ)}^{2} = A (h υ - E_{gop})$ (3)

Where A is the proportionality constant and E_gop is the band gap. Two linear regions appear indicating the existence of two different band gap values, which may be ascribed to the spin-orbit fragmenting of the valence band. The extrapolation of two straight lines to (αhυ) ² = 0 (eV) ² give values of the direct band gaps of Mn-doped Co₃O₄ thin films (see Table 3). It is found of E_gop1 values of the energy band gap varies between 1.42 ± 0, 07 and 1.46 ± 0.07 eV while E_gop2 values, which are obtained from the extrapolation of the first linear region, found to be varying between 1.873 ± 0.10 and 2.11 ± 0, 11eV (see Fig. 8). Increased film thickness could explain the increase in the optical band gap E_gop2 and vice versa [35]. The marginal variation or shift observed in E_gop2 values might be due to formation of impurity energy levels or localized energy states created between conduction band and valance band upon Mn doping. Co⁺³ substituted by Mn⁺³ leads to the formation of impurity energy levels created between CB band and VB band. This increase the amount of Co⁺³ vacancy. The observed marginal difference or decrease in the E_gop2 values is also due to the presence of a high amount of acceptor states (cation vacancies) near the top edge of the range, which belong to Co³⁺ or Co²⁺(3dt2g). This can occur as a result of overlapping upper-edge states and donor-edge states upon doping of manganese observed from EDS, resulting in a narrowing of the range of E_gop2 [3, 15]. After 6 wt% of Mn, the band gap energy increased; This can be explained by the increase in Urbach energies (see Fig. 9), that is, as the Urbach energy increases, the random energy increases, and thus the absence of crystal structural organization. The refractive index is related to optical band gap (E_gop2) of a semiconductor by the following relationship [19, 36]:

Fig. 9

Plots of (hυ) against lnα of Co₃O₄: Mn thin films with different WPM

$n = 4, 084 - 0, 68 E_{gop 2}$ (4)

The observed marginal difference or shift in the E_gop2 values is further due to the formation of impurity energy levels or localized energy states created between the conduction domain and the straightness band upon doping of manganese observed from EDS [3, 15].

3.5 Electrical properties

For measuring the sheet resistance (R_sheet) by the four-point probe technique, the current (I = 0.01μA) is applied between the outer two wires and the value of potential difference (V) is read across the two internal probes. Thus, the sheet resistance is calculated from the following relation:

$R_{sheet} = \frac{π}{In 2} * \frac{V}{I}$ (5)

The conductivity (σ) values of films are calculated from the following formula: $σ = 1 / (R_{sheet} * t_{h})$ (6)

Where t_h: is thickness of thin films.

The values of electrical conductivity (σ) of Mn-doped Co₃O₄ thin films are listed in Table 2. The variation of conductivity (σ) of thin films as a function of the WPMis illustrated in Figure 10. It can be observed that the conductivity (σ) increases with the increase of the WPM upto 6% Mn. After this value, the conductivity decreases. This increase is mainly due to an increase in the carrier concentration (holes) of the valence band due to electronic shifts in the conduction band and holes resulting from the non-stoichiometry of the oxygen network in the p-Co₃O₄ grain [37]. On the other hand, the reduction of the size of the grains and increasing of manganese gives rise to the appearance of the separate energy levels.

Fig. 10

Electrical conductivity of Mn-doped Co₃O₄ thin films as a function of the WPM.

These levels manifest in the gap as electronic defects, which probably work like donors inducing the increase in conductivity. After the value of 6% Mn, the electrical conductivity (σ) decreases can be explained as follows; the increase in resistance is attributed to the reduced crystallization of thin films, resulting in a decrease in the electron scattering centers due to the increase of the WPM [38].

4 Conclusion

Undoped and Mn-doped Co₃O₄ thin films were deposited using a homemade pneumatic spray method (PSM) by varying manganese doping weight percentages (0, 2,4,6,8 and 10 wt %) on glass substrate at temperature 400°C. The X-ray diffraction patterns revealed that the crystalline quality of pristine and Mn-doped cobalt oxide (Co₃O₄) thin films self cubic spinel structure oriented along to (111) plane. The crystal size (D) and lattice strain (ɛ) were calculated based on X-ray diffraction and Williamson-Hall relationship. The morphology of pristine and Mn-doped Co₃O₄ thin films is almost homogeneous and with the forms straight acicular nanorods (SANRs).The optical studies showed tow direct band gaps E_gop1 and E_gop2 varied between 1,416±0,0708 to 1,466±0,0733 eV and from 1,873±0,0936 to 2,11±0,1055 eV respectively. The electrical conductivity increased get to 15, 536 ± 0, 7768 (Ω.cm)^-1 with increasing percentages weight of Manganese doping (in 6 wt% of Mn) determined from four point probe technique. Co⁺³ substituted by Mn⁺³ leads to an increase in the amount of oxygen vacancy. The latter engenders the increase of electrical conductivity at 6% wt% of Mn.

It can be concluded, Manganese doping has a significant impact on the structural, morphological, optical, and electrical properties of Co₃O₄ thin films. Through the obtained results, it can be said that the pneumatic spray method (PSM) is important in obtaining homogeneous films with good structural, optical, and electrical properties. It can also be said that the Mn-doped cobalt oxide thin films prepared by the pneumatic spraying method (PSM) are applicable in photovoltaics, photocatalysis, and gas sensing applications.

References

Louardi

, El Bachiri

, Soussi

, Erguig

, Elidrissi

, Khaidar

and Monkade

, Optical Properties of Mg Cobalt Oxide (Mg _x Co₃_–_x O₄) Thin Films, Materials Focus 7 (2018), 459–463.

Alema

, Osinsky

, Mukhopadhyay

and Winston Schoenfeld

, Epitaxial growth of Co₃O₄ thin films using Co(dpm)₃ by MOCVD, Journal of Crystal Growth 525 (2019), 125207.

Perekrestov

, Spesyvyi

, Maixner

, Mašek

, Leiko

, Khalakhan

, Maňák

, Kšírová

, Hubička

and Čada

, The comparative study of electrical, optical and catalytic properties of Co₃O₄ thin nanocrystalline films prepared by reactive high-power impulse and radio frequency magnetron sputtering, Thin Solid Films 686 (2019), 137427.

Kilo

, Jabbour

, Habchi

, Abboud

, Brouche

, Khoury

and Zaouk

, Electrospray deposition and characterization of cobalt oxide thin films, Materials Science in Semiconductor Processing 24 (2014), 57–61.

Shia

, Sua

, Zhua

, Li

and Xu

, Supported cobalt oxide on graphene oxide: Highly efficient catalysts for the removal of Orange II from water, Journal of Hazardous Materials 229–230 (2012), 331–339.

Kandalkar

S.G.

, Gunjakar

J.L.

and Lokhande

C.D.

, Preparation of cobalt oxide thin films and its use in supercapacitor application, Applied Surface Science 254 (2008), 5540–5544.

Lakehal

, Benrabah

, Bouaza

, Dalache

and Hadj

, Tuning of the physical properties by various transition metal doping in Co₃O₄: TM (TM = Ni, Mn, Cu) thin films: A comparative study, Chinese Journal of Physics 56 (2018), 1845–1852.

Yao

, Xi

and Recycling

, and synthesis of LiNi 1/3 Co 1/3 Mn 1/3 O 2 from waste lithium ion batteries using d, l-malic acid. Feng, J Alloys Compd 680 (2016), 73–79.

Vennela

A.B.

, Mangalaraj

, Muthukumarasamy

, Agilan

and Hemalatha

K.V.

, Structural and Optical Properties of Co3O4 Nanoparticles Prepared by Sol-gel Technique for Photocatalytic Application, International Journal of Electrochemical Science 14 (2019), 3535–3552.

10.

Nguyen

A.I.

, Suess

D.L.

, Darago

L.E.

, Oyala

P.H.

, Levine

D.S.

, Ziegler

M.S.

, Britt

R.D.

and Tilley

T.D.

, Manganese-Cobalt Oxido Cubanes Relevant to Manganese-Doped Water Oxidation Catalysts, Journal of the American Chemical Society 139 (2017), 5579–5587.

11.

Liu

, Hui

and Hui

, ACS Appl. Vertically stacked bilayer CuCo 2 O 4/MnCo 2 O 4 heterostructures on functionalized graphite paper for high-performance electrochemical capacitors,, Interfaces 8 (2016), 3258–3267.

12.

Zou

, Su

, Silva

, Goswami

, Sathe

B.R.

and Asefa

, Efficient oxygen evolution reaction catalyzed by low-density Ni-doped Co3O4 nanomaterials derived from metal-embedded graphitic C3N4, Chem Commun 49 (2013), 7522–7524.

13.

Baidya

, Murayama

, Bera

, Safonova

O.V.

, Steiger

, Katiyar

N.K.

, Biswas

and Haruta

, Low-Temperature CO Oxidation over Combustion Made Fe- and Cr-Doped Co₃O₄ Catalysts: Role of Dopant’s Nature toward Achieving Superior Catalytic Activity and Stability, Journal of Physical Chemistry 121 (2017), 15256–15265.

14.

Grewe

, Deng

and Tüysüz

, Influence of Fe Doping on Structure and Water Oxidation Activity of Nanocast Co3O4, Chemistry of Materials 26 (2014), 3162–3168.

15.

Venkatesha

, Ravi Dhasa

, Sivakumarb

, Dhandayuthapanib

, Subramanianc

, Sanjeevirajad

and Moses Ezhil Raje

, Tailoring the physical properties and electrochromic performance of nebulizer spray coated Co3O4 films through copper doping, Solid State Ionics 334 (2019), 5–13.

16.

Daranfed

, Guermat

and Mirouh

, Experimental Study of Precursor Concentration the Co3O4 Thin Films Used as Solar Absorbers, Annales de Chimie - Science des Matériaux 44 (2020), 121–126.

17.

Salman Chiad

, Ali Noor

, Mazin Abdulmunem

and Fadhil Habubi

, Optical and Structural properties of Ni-doped Co3O4Nanostructure Thin films Via CSPM, Journal of Physics: Conference Series 1362 (2019), 012115.

18.

Tareen

J.A.K.

, Malecki

, Doumerc

J.P.

, Launay

J.C.

, Dordor

, Pouchard

and Hagenmuller

, Growth and electrical properties of pure and Ni-doped Co₃O₄ single crystalsMaterials research bulletin, Materials Research Bulletin 19 (1984), 989–997.

19.

Ambare

R.C.

, Bharadwaj

S.R.

and Lokhande

B.J.

, Electrochemical characterization of Mn: Co₃O₄ thin films prepared by spray pyrolysis via aqueous route, Current Applied Physics 14 (2014), 1582–1590.

20.

Manickam

, Ponnuswamy

, Sankar

, Mariappan

and Suresh

, Influence of Substrate Temperature on the Properties of Cobalt Oxide Thin Films Prepared by Nebulizer Spray Pyrolysis (NSP) Technique, Silicon 8 (2016), 351–360.

21.

Manogowri

, Mary Mathelane

, Valanarasu

, Kulandaisamy

, Benazir Fathima

and Kathalingam

, Effect of annealing temperature on the structural, morphological, optical and electrical properties of Co₃O₄ thin film by nebulizer spray pyrolysis technique, Journal of Materials Science: Materials in Electronics 27 (2016), 3860–3866.

22.

Louardi

, Rmili

, Ouachtari

, Bouaoud

, Elidrissi

and Erguig

, Characterization of cobalt oxide thin films prepared by a facile spray pyrolysis technique using perfume atomizer, Journal of Alloys and Compounds 509 (2011), 9183–9189.

23.

Kouidri

, Rahmane

and Allag

, Substrate temperature-dependent properties of sprayed cobalt oxide thin films, Journal of Materials Science: Materials in Electronics 30 (2019), 1153–1160.

24.

Richter

J.H.

, Electronics Properties of Metal Oxide Films Studied by Core Level Spectroscopy, Digital Comprehensive Summaries of Uppsala Faculty of Science and Technology 228 (1969).

25.

Bouhdjer

, Attaf

, Saidi

, Bendjedidi

, Benkhetta

and Bouhaf

, Correlation between the structural, morphological, optical, and electrical properties of In2O3 thin films obtained by an ultrasonic spray CVD process, J Semiconduct 36 (2015), 082002–1.

26.

Messemeche

, Saidi

, Attaf

, Benkhetta

, Chala

, Azizi

and Nouadji

, Elaboration and characterization of nano-crystalline layers of transparent titanium dioxide (Anatase-TiO₂) deposited by a sol-gel (spin coating) process, Surfaces and Interfaces 19 (2020), 100482.

27.

Venkatesh

, RaviDhas

, Sivakumar

, Dhandayuthapani

, Sudhagar

, Sanjeeviraja

and Moses EzhilRaj

, Analysis of optical dispersion parameters and electrochromic properties of manganese-doped Co₃O₄ dendrite structured thin films, Journal of Physics and Chemistry of Solids 122 (2018), 118–129.

28.

Yan

, Wu

, Dai

, Guan

and Li

, Synthetic Design of Gold Nanoparticles on Anatase TiO2 001 for Enhanced Visible Light Harvesting ACS Sustainable Chem, Eng 2 (2014).

29.

Georgescu

, Baia

, Ersen

, Baia

and Simon

, Experimental assessment of the phonon confinement in TiO2 anatase nanocrystallites by Raman spectroscopy, J Raman Spectrosc 43(7) (2012), 876–883.

30.

Herzallah

, Temam

H.B.

, Ababsa

and Gana

, Electrodeposited Ni-Co Films from Electrolytes with Different Co Contents, Defect and Diffusion Forum 406 (2021), 219–228.

31.

RaviDhas

, Venkatesh

, Sivakumar

, Moses EzhilRaj

and Sanjeeviraja

, Effect of solution molarity on optical dispersion energy parameters and electrochromic performance of Co₃O₄ films, Optical Materials 72 (2017), 717–729.

32.

Shaban

and El Sayed

A.M.

, Influence of the spin deposition parameters and La/Sn double doping on the structural, optical, and photoelectrocatalytic properties of CoCo₂O₄ photoelectrodes, Solar Energy Materials and Solar Cells 217 (2020), 110705.

33.

Bayram

, İgman

, Guney

and Simsek

, The role of cobalt doping on the optical and structural properties of Mn₃O₄ nanostructured thin films obtained by SILAR technique, Superlattices and Microstructures 128 (2019), 212–220.

34.

Huang

, Liu

, Wang

, Zhang

, Wang

, She

and Wang

, Facile loading of cobalt oxide on bismuth vanadate: Proved construction of p-n junction for efficient photoelectrochemical water oxidation, Journal of Colloid and Interface Science 570 (2020), 89–98.

35.

Benkhetta

, Attaf

, Saidi

, Bouhdjar

, Bendjedidi

, Kherkhachi

I.B.

, Nouadji

and Lehraki

, Influence of the solution flow rate on the properties of zinc oxide (ZnO) nano-crystalline films synthesized by ultrasonic spray process, Optik 127 (2016), 3005–3008.

36.

Ravindra

N.M.

, Energy gap-refractive index relation—some observations, Infrared Physics 21 (1981), 283–285.

37.

Rahmane

and Kouidri

, effect Of Cobalt Chloride Concentration On Structural, Optical And Electrical Properties Of Co3o4 Thin Films Deposited By Pneumatic Spray, Journal of New Technology and Materials 10(01) (2020), 56–62.

38.

Bouhdjer

, Attaf

, Saidi

, Benkhetta

, Aida

M.S.

, Bouhaf

and Rhil

, Influence of annealing temperature on In2O3 properties grown by an ultrasonic spray CVD process, Optik 127 (2016), 6329–6333.