Influence of concentration of different chemical precursors on the physical properties of Cr 2 O 3 thin films elaborated via pneumatic spray

Abstract

In this paper, Cr₂O₃ thin films were synthesized successfully on glass substrates at 450 °C using a simple and low-cost homemade pneumatic spray system (SP) using two different precursors: chromium chloride and chromium nitrate. A systematic study of the influence of concentration of each precursor used for deposition on the structural, morphological, optical and electrical properties has investigated.

The XRD results show that the Cr₂O₃ films prepared with chromium chloride are polycrystalline with rhombohedral structure and those prepared with low concentration of chromium nitrate have a poor crystallinity. Peaks associated with Cr and O elements are present in EDS analysis that confirm the composition of the films and SEM images revealed a uniform, homogeneous and well covered surface. The measured electrical conductivity was found in the order of 5(Ω.cm)^–1. The average transmittance of the films deposited from chromium nitrate is in the range of 60 % and for the films deposited from chromium chloride, it reaches75 % in the visible region. These electrical and optical properties of Cr₂O₃ thin film ascribed to its wide band gap, are indeed required for optoelectronic devices especially for solar cell window.

Keywords

Pneumatic spray thin film molarity properties

1 Introduction

Transition metal oxides (TMOs) are interesting materials owing to their multi oxidation states of metal ions making them good candidates to a many of technological applications in the fields of materials science and engineering, such as catalysis, electrochemistry and in the vast field of electronics [1].

Because it is cheap, available, different oxidation states, good electrical conductivity, large optical band gap of 3.1 eV and diversified of morphologies, chromium oxide is one of the promising material as transparent conductor oxide (TCO).

In general, CrO_x can exist in several phases (mainly CrO, Cr₂O₃, CrO₂ and CrO₃). Among the various chromium oxides, the most stable phase under normal conditions is Cr₂O₃ [2].

It has been widely studied in the domain of corrosion protection, wear resistance [3, 4], gas sensor [5], rechargeable lithium ion batteries [6] and optical applications that include electrochromic coatings and infrared (IR) transmitting coatings [7].

Many methods have been mentioned in the literature for deposition of Cr₂O₃ films among them sputtering [8, 9], pulsed laser deposition technique [10, 11], chemical vapour deposition (CVD) method [12 –16], and spray pyrolysis [17 –19]. Among of these methods, spray pyrolysis found to be attractive because of its simplicity and low cost. Naturally, the properties of the deposited Cr₂O₃ thin films are dependent to the solvents, the nature of used precursors, and the processing conditions. Therefore, in the present investigation, Cr₂O₃ thin films have obtained using spray pneumatic and the main target of this work is to explore the effect of nature and the concentration of the precursor, on the structural, morphological, optical and electrical properties of the prepared films.

2 Experimental section

Elaborations of chromium oxide films on cleaned preheated glass substrates were carried out by means of homemade spray pyrolysis experimental setup [20].

In this work the spray solutions; hydrated chromium nitrate Cr(NO₃)₃·9H₂O and hydrated chromium chloride CrCl₃·6H₂O, dissolved in distilled water after that were sprayed via atomization processes then condensed onto the heated substrates. The solutions are prepared with 0.02, 0.04, 0.06 and 0.08 mol/l as molar concentrations.

We used Glass substrates of the type (TLC Silica gel 60 F254) and we already detailed their procedures of cleaning in [21]. Furthermore, we fixed the temperature of deposition at 450°C for 3 min.

The films structural properties were studied by XRD measurements using Rigaku-Type MiniFlex600 diffractometer with Cu Kα radiation (λ= 1.5418 Å) operated at 40 kV and 30 mA. Regarding the morphological properties examination and chemical composition determination of thin films two associated devices TESCAN VEGA3 SEM and Energy Dispersive Spectroscopy (EDS) were used respectively. We utilized UV-Vis-NIR spectrophotometer (JASCO V-770) in the wavelength domain 300–1500 nm to investigate the optical properties (transmittance, gap energy, and Urbach energy) of our specimens. While we measured the electrical conductivity of the films at room temperature through a KEITHLEY 2400 Source Meter four-point probe technique.

3 Results and discussion

3.1 Thickness measurements and thin film formation mechanism

Under the above-mentioned conditions, we elaborated Cr₂O₃ films from chromium chloride and chromium nitrate solutions. A thermal decomposition occurs to the fine droplets of the solutions after their falling over the hot substrate surface resulting in the formation of uniform and well adherent (hardly peeled with glue tape test) thin films. With the help of weight difference method using an electronic high-accuracy balance, we calculated the thickness of the deposited films. If we know the amounts of the area of film (A) and the density of Cr₂O₃ (ρ= 5.22 g/cm³) [2], the film thickness d should be determined as follows [22]: $d = m / ρ A$ (1)

From Table 1, we can notice that the thickness have a similar trend of variation with the rise in molar concentration. The continuous increase in thickness with precursor concentration indicates that the thickness may be governed by Cr-containing species. It is difficult to control the insertion of O^2– into the film during the growth process using the spray pyrolysis technique. We cannot control the atmospheric conditions where the film growth takes place accurately; therefore, the stoichiometry is only controlled by Cr species.

Table 1

Values of Crystallites size and strain (ɛ) of Cr₂O₃ thin films

	Chromium Chloride				Chromium nitrate
M	(hkl)	Crystallites	strain	Thickness	(hkl)	Crystallites	strain	Thickness
(mol/L)		size (nm)	(ɛ)^*10^–3	(nm)		size (nm)	(ɛ)^*10^–3	(nm)
0.02	110	33.20	1.044	274	116	/	/	270
0.04		56.69	0.611	334		/	/	307
0.06		47.24	0.734	513		37.97	0.913	455
0.08		35.42	0.979	666		18.99	1.826	632

The specimens obtained by the nitrate solution are thinner than those obtained by the chloride solution. This is surely owing to the contribution of several physico-chemical factors entering into the film’s growth mechanisms. Such as the type of the solution (viscosity, density, surface tension, reactivity...), the aerodynamic effect and the volatility of the solution.

3.2 Structural properties

Figure 1 plots the X-ray diffraction pattern of Cr₂O₃ thin films deposited with different precursor solutions molarities with (a) Cr chloride and (b) Cr nitrate as precursors. The XRD patterns show that all the films are polycrystalline, with rhombohedral structure (using JCPDS 00-001-1294 as reference).

Fig. 1

XRD diffraction patterns of Cr₂O₃ thin film with different concentrations for (a) Chromium chloride and (b) Chromium nitrate precursors.

It is evident from Fig. 1(a) that the films exhibit a strong orientation along (110) at 36°. The increase in concentrations leads to a continuous increase in (110) peak intensity, which confirms the improvement of the film crystallinity [23], also we observed some other peaks such as (104), (113) and (116). Figure 1 (b) shows the X-ray diffraction patterns of chromium nitrate films. For low concentrations (0.02M and 0.04M) we can see the poor crystallinity of the structure. By increasing the molarity, a peak appears at 55°, which is attributed to the (116) diffraction peak, other peaks assigned as (012), (104), (110) and (113) were also observed.

However, we remarked the presence of a broad peak situated between 10 and 30° for low concentrations (0.02M and 0.04M) for films prepared with chloride precursor and for all samples obtained by using nitrate precursors. This denotes of the presence of an amorphous phase in the film network. From this, we inferred that films deposited with nitrate are composed of small crystallites embedded in an amorphous phase.

As can be seen there is a different behaviors between samples obtained by using chloride and those obtained using nitrate precursors, the chloride crystallization is better than nitrate, and of course, this is due to the nature of each source.

The difference in the precursor properties alters the films growth mechanism from which originates the difference in the pyrolytic reaction on the surface of the substrate produces the formation of the films with different thickness. The high Cr-containing species in chloride solution produces the nucleation through the plane (110) since the latter requires less formation energy. However, when using the chromium nitrate as starting solution, decomposition may proceed through a series of oxide nitrates [24, 25]; and this may explain the obtained poor crystalline film structure.

We calculated the crystallite size (D) and strain (ɛ) of the Cr₂O₃ films from the peaks of the highest intensity. Since the intensity along the (110) plane for chromium chloride and (116) for chromium nitrate increases sharply as the precursor concentration increases, using the following formulas [26]: $D = \frac{0.9 λ}{β cos θ}$ (2) $ɛ = \frac{β cos θ}{4}$ (3) Where λ is the applied X-ray wavelength (0.15418 nm for Cu Kα radiation), θ is the diffraction peak angle and β is the full-width at half-maximum (FWHM) in radian.

We found that that the crystallite size of the films decreases with the increase in concentration for both precursors, which is probably due to the increased nucleation centers. We note that there is an inverse relationship between the crystal size and microstrain. Similar results have been reported by T. Larbi et al. [27] and slightly greater than the obtained by Wang et al. [28].

It is obvious from Table 1 that the film deposited at 0.04 M with chromium chloride characterized with the larger crystallite size, that is probably a consequent of the crystallites packed dominantly through the (110) plane, thereby improving the crystallinity of the film. Nevertheless, the latter is less dense than those deposited at higher concentrations as suggested by the broad pick at the beginning of DRX pattern.

3.3 Elemental analysis and morphological study

The EDS spectra and SEM surface images of Cr₂O₃ thin films deposited with the tow studied salts at different concentrations are represented in Figs. 2, 3 respectively.

Fig. 2

EDS spectra of Cr₂O₃ films (a) Chromium chloride, (b) Chromium nitrate precursors.

Fig. 3

SEM surface images of the Cr₂O₃ thin films deposited at various concentration for (a) Chromium chloride, (b) Chromium nitrate precursors.

To affirm the composition of Cr₂O₃ films, EDS compositional analysis was employed. Figure 2 shows an EDS spectra of Cr₂O₃ thin film deposited with different concentrations. These spectra confirm the presence of Cr and O elements. The presence of silicon peak may be due the glass substrate, this peak gets smaller when the Cr peak becomes more prominent as the molar concentration in both of chloride and nitrate precursors increases. This is due to increase in films growth on the glass substrate leading to the increase in the quantity of the elements in the films; in Table 2 we summarized the atomic percentage of elements.

Table 2

Atomic percentage of chemical composition in Cr₂O₃ films

	Chrome chloride [at. % ]				Chromium nitrate [at. % ]
	0.02M	0.04M	0.06M	0.08M	0.02M	0.04M	0.06M	0.08M
O	65.70	67.28	64.55	63.69	68.10	67.01	69.25	67.29
Cr	1.82	6.90	21.29	27.70	2.78	4.97	9.89	11.33
Si	32.40	25.64	14.08	8.53	26.37	24.51	18.03	18.06

Figure 3 denotes the top view SEM images of the Cr₂O₃ taken using magnifications of about 2,000 and indicates the polycrystalline nature of the films. These images show that the surface morphology relevant clearly on the nature of the used precursor.

The surface morphologies of the Cr₂O₃ films prepared with chromium nitrate as starting solution (see Fig. 3b) have approximately the same density and smooth surface morphology at low concentrations. When the concentrations are elevated, the film became rougher and with more sintered structure.

The micrographs of the films deposited with chromium chloride solution, clearly illustrate the formation of sub-micrometre crystallites distributed in a uniform shape over the surface. Although the films are well recovered and no cracks could be detected.

3.4 Optical properties

Figure 4 shows the optical transmission of Cr₂O₃ thin films prepared with chromium chloride and chromium nitrate precursors as a function of the wavelength in the 300–1500 nm range for different concentrations. As observed, the average transmittance of these films overall decreased gradually versus the increase in the precursor concentration. The prepared film with chrome chloride has a transparency of 75% to 15%, it is higher than that of the prepared with chromium nitrate of 60% to 15%. The reduction of transmittance at higher molar concentration is owing to the increase in the thickness of the films, and roughness of thin film can also affect its optical properties, i.e., more roughness causes the light scattering at the surface and degrade its transparency, which consistent with SEM images.

Fig. 4

Transmission spectra of Cr₂O₃ films prepared by different molar concentration (a) Chromium chloride, (b) Chromium nitrate precursors.

The 0.01M thin film reveals sufficient transparency for applications in thin film solar cells and electro chromic windows.

In addition, it can be seen in Fig. 4, a fall in T% for wavelengths around 350 nm which is the absorption edge in the Cr₂O₃ films because of the transition electronic between the valence and the conduction bands. As well as two broad peaks, they appeared at 455 and 600 nm, depicting two regions of d-d optical transitions. We assigned absorption bands to the charge transfer of ⁴A_2g ⟶ ⁴T_1g at higher energy region and ⁴A_2g ⟶ ⁴T_2g at lower energy region of Cr^3 + ions [11, 29]. The absorption edge of Cr₂O₃ thin films shifted towards a higher wavelength region (red shift) with the increase in the precursor concentration.

The optical band gaps (E_g) of the films is based on Tauc formula for direct band gap semiconductors and Urbach energy E_u that characterizes the disorder in the film, determination details of the previous two parameters are reported elsewhere [22].

From Table 3 and Fig. 5, we can conclude that the calculated optical band gaps energy of Cr₂O₃ films prepared with chloride are larger than those prepared with nitrate. We remarked as well a slight decrease in optical band gap with the increase of solution concentration for both of them.

Table 3

The electrical and optical parameters values of the deposited Cr₂O₃ thin films

M (mol/L)	Chrome chloride				Chromium nitrate
	Eg (eV)	E_U (eV)	R_sh^*10² (Ω)	σ (Ω.cm)^–1	Eg (eV)	E _U (eV)	R_sh^*10² (Ω)	σ (Ω.cm)^–1
0.02	3.53	0.51	73	4.99	3.48	0.68	67	5.53
0.04	3.49	0.54	58	5.16	3.47	0.86	66	4.94
0.06	3.42	0.58	65	2.99	3.40	0.90	65	3.38
0.08	3.40	0.67	64	2.35	3.38	0.97	59	2.68

Fig. 5

Variation of band gap and band tail width versus solution concentration of the thin films for (a) Chrome chloride, (b) Chromium nitrate precursors.

We attributed the shrinking of band gap width to the increase in the band tail width (Urbach energy). Which is due to the formation of crystal defects leading to create lattice strain in the film due to their preparation conditions (as discussed in XRD analysis); this leads to the formation of allowed states inside the forbidden gap [26, 30].

Analogous results of E_g values are reported by Jarnail Singh [11] (3.49–3.64 eV), T. Larbi [27] (3.38 eV). The values of Urbach energy (Table 3) for the films elaborated with chromium chloride are lower than the values of the films prepared with chromium nitrate; this is attributed to the poor crystallinity of the latter (presence of amorphous phase), as deduced from the XRD analysis. Our values of E_u are higher than the values obtained by Jarnail Singh [31] (0.28 eV) for the epitaxial Cr₂O₃ thin film prepared by PLD.

3.5 Electrical properties

We measured the sheet resistance (R_sh) of Cr₂O₃ thin films by means of four-point probe technique in dark and at room temperature. The conductivity (σ) of films is determined from the following formula [20]: $σ = 1 / (R_{sh} * d)$ (4) Where d is the film thickness and R_sh is the sheet resistance.

As we can see from Fig. 6, the electrical conductivity (σ) shows a decreases behavior with the increase of concentration from 0.02 M to 0.08 M. It is decreased from 5.16 (Ω.cm)^–1 up to 2.35 (Ω.cm)^–1 for chromium chloride and from 5.53 (Ω.cm)^–1 up to 2.68 (Ω.cm)^–1 for chromium nitrate. We attributed the decrease of conductivity to the decrease of carrier’s mobility. The XRD and SEM analyses proved the decrease of crystallite size, this leads to an increase in the trapping states at grain boundaries and in the other hand the increase in band tail width (Urbach energy) that characterize the disorder in the film. For sheet resistance values in Table 3 we can see that the values are decreasing but the values are close to each other.

Fig. 6

Electrical conductivity variations of Cr₂O₃ as a function of precursor molarity of thin films for (a) Chrome chloride, (b) Chromium nitrate precursors.

However, the obtained values of conductivity are much higher than the reported in the literature [11] where the work of Jarnail Singh (of the order 10^–2(Ω.cm)^–1) deals with Cr₂O₃ thin film prepared using the PLD technique and Elisabetta Arca [19] (of the order 10^–3(Ω.cm)^–1) for Cr₂O₃ thin film deposited by spray-pyrolysis.

4 Conclusion

In this study Cr₂O₃ thin films were grown on glass substrates using chromium chloride and chromium nitrate with different concentrations of precursor (0.02, 0.04, 0.06 and 0.08 mol/L) using homemade pneumatic spray pyrolysis system.

The samples obtained from the nitrate solution are thinner than those deposited using the chloride solution; this is certainly due to the contribution of several physico-chemical factors contributing into the film’s growth mechanisms. X-ray diffraction reveals a polycrystalline nature for all films prepared with chromium chloride with a preferred grain orientation through to (110) plane, whereas those prepared with low concentration of chromium nitrate have a poor crystallinity. Crystallites sizes of the films have found to be decreasing from 56 to 33 nm and from 38 to 19 nm for chromium chloride and chromium nitrate respectively. SEM images revealed that the films were uniform, pores and cracks–free. The films prepared with chromium nitrate as starting solution have approximately the same density and smooth surface morphology at low concentrations. Whereas films obtained from chromium chloride were rough and consist of sub-micrometre crystallites distributed in a uniform manner over the surface. The optical study confirms that the transmittance of Cr₂O₃ films decreases with the increase of precursor concentration as well as is enormously affected by the nature of the starting solution. The average transmittance of the films deposited from chromium nitrate is around of 60 % (Eg varies from 3.38 to 3.48 eV) and for the films deposited from chromium chloride, it attains 75 % in the visible region (Eg varies from 3.40 to 3.53 eV). The electrical conductivity of the films found in the order of 5 (Ω.cm) ^–1.

Finally, the present contribution clearly indicated that the molarity and nature of the starting solution are important parameters and affects significantly the physical properties of Cr₂O₃ films. The combination of high visible transmittance, high conductivity, and good structural and morphological properties makes that the Cr₂O₃ films that are deposited especially using chromium chloride at 0.04 mol/l are a convenient p-type transparent conducting oxide for usage in many optoelectronic devices and solar cells applications.

References

Kung

H.H.

, Transition Metal Oxides: Surface Chemistry and Catalysis, Elsevier, Amsterdam 1989.

Gago

, Vinnichenko

, Hübner

and Redondo-Cubero

, Bonding structure and morphology of chromium oxide films grown by pulsed-DC reactive magnetron sputter deposition, Journal of Alloys and Compounds 672 (2016), 529–535.

Wang

T.-G.

, Liu

, Sina

, Shi

, Iyengar

, Melin

and Kim

K.H.

, High-temperature thermal stability of nanocrystalline Cr₂O₃ films deposited on silicon wafers by arc ion plating, Surface and Coatings Technology 228 (2013), 140–147.

Sousa

P.M.

, Silvestre

A.J.

and Conde

, Cr₂O₃ thin films grown at room temperature by low pressure laser chemical vapour deposition, Thin Solid Films 519 (2011), 3653–3657.

Balouria

, Kumar

, Singha

, Samanta

, Debnath

A.K.

, Mahajan

, Bedi

R.K.

, Aswal

D.K.

, Gupta

S.K.

and Yakhmi

J.V.

, Sens. Temperature dependent H₂S and Cl₂ sensing selectivity of Cr₂O₃ thin films, Sensors and Actuators B 157 (2011), 466–472.

Boldyrev

Y.I.

, Ivanova

N.D.

, Sokolsky

G.V.

, Ivanov

S.V.

and Stadnik

O.A.

, Thin film nonstoichiometric chromium oxide-based cathode material for rechargeable and primary lithium batteries, J Solid State Electrochem 17 (2013), 2213–2221.

Al-Kuhaili

M.F.

and Durrani

S.M.A.

, Optical properties of chromium oxide thin films deposited by electron-beam evaporation, Opt Mater 29 (2007), 709–713.

Pang

, Gao

, Luo

, Yang

, Qiao

, Wang

and Volinsky

A.A.

, Annealing effects on microstructure and mechanical properties of chromium oxide coatings, Thin Solid Films 516 (2008), 4685–4689.

Hones

, Diserens

and Lévy

, Characterization of sputter-deposited chromium oxide thin films, Surf Coat Tech 120-121 (1999), 277–283.

10.

Tabbal

, Kahwaji

, Christidis

T.C.

, Nsouli

and Zahraman

, Pulsed laser deposition of nanostructured dichromium trioxide thin films, Thin Solid Films 515 (2006), 1976–1984.

11.

Singh

, Kumar

, Verma

and Kumar

, Structural and optoelectronic properties of epitaxial Ni-substituted Cr₂O₃ thin films for p-type TCO applications, Materials Science in Semiconductor Processing 123 (2021), 105483.

12.

Maruyama

and Akagi

, Chromium Oxide Thin Films Prepared by Chemical Vapor Deposition from Chromium Acelylacetonate and Chromium Hexacarbonyl, J Electrochem Soc 143 (1996), 1955–1958.

13.

Wang

, Gupta

and Klein

T.M.

, Plasma enhanced chemical vapor deposition of Cr₂O₃ thin films using chromium hexacarbonyl (Cr(CO)₆) precursor, Thin Solid Films 516 (2008), 7366–7372.

14.

Cheng

C.S.

, Gomi

and Sakata

, Electrical and Optical Properties of Cr₂0₃ Films Prepared by Chemical Vapour Deposition, Phys Stat Sol (a) 155 (1996), 417–425.

15.

Ivanova

, Surtchev

and Gesheva

, Characterization of CVD Chromium Oxide Thin Films, Phys Stat Sol (a) 184 (2001), 507–513.

16.

Sousa

P.M.

, Silvestre

A.J.

, Popovici

and Conde

, Morphological and structural characterization of CrO₂/Cr₂O₃ films grown by laser-CVD, Appl Surf Sci 247 (2005), 423–428.

17.

Misho

R.H.

, Murad

W.A.

and Fattahallah

G.H.

, Preparation and optical properties of thin films of CrO₃ and Cr₂0₃ prepared by the method of chemical spray pyrolysis, Thin Solid Films 169 (1989), 235–239.

18.

Amir

, AL-Niaimi

and Mahmood

N.A.

, Effect of thickness on optical properties of (Cr2O3) thin films prepared by chemical spray pyrolysis technique, Diyala Journal for pure Sciences 12(1) (2016), 154–16.

19.

Arca

, Fleischer

, Krasnikov

S.A.

and Shvets

, Effect of chemical precursors on the optical and electrical properties of p-type transparent conducting Cr₂O₃:(Mg,N), J Phys Chem C 117 (2013), 21901–21907.

20.

Abdelkrim

, Rahmane

, Abdelouahab

, Abdelmalek

and Brahim

, Effect of solution concentration on the structural, optical andelectrical properties of SnO₂ thin films prepared by spray pyrolysis, Optik 127 (2016), 2653–2658.

21.

Nabila

, Rahmane

and Abdelkrim

, Substrate temperature-dependent properties of sprayed cobalt oxide thin films, J Mater Sci: Mater Electron 30 (2019), 1153–1160.

22.

Abdelkrim

, Rahmane

, Nabila

, Hafida

and Abdelouahab

, Polycrystalline SnO₂ thin films grown at different substrate temperature by pneumatic spray, J Mater Sci: Mater Electron 28 (2017), 4772–4779.

23.

Kouidri

and Rahmane

, Effect of cobalt chloride concentration on structural, optical and electrical properties of Co3O4 thin films deposited by pneumatic spray, J New Technol Mater 10(1) (2020), 56–62.

24.

Stern

Kurt H.

, High temperature properties and decomposition of inorganic salts part 3, nitrates and nitrites, J Phys Chem Ref Data 1(3) (1972), 747–762.

25.

Malecki

, Malecka

, Gajerski

and Łabus

, Thermal decomposition of chromium(III) nitrate(V) nanohydrate different chromium oxides CrO_1.5+y formation, J Therm Anal Cal 72 (2003), 135–144.

26.

Abdelkrim

, Rahmane

, Abdelouahab

, Hafida

and Nabila

, Optoelectronic properties of SnO₂ thin films sprayed at different deposition times, Chin Phys B 25(4) (2016), 046801.

27.

Larbi

, Ouni

, Gantassi

, Doll

, Amlouk

and Manoubi

, Structural, Optical and Vibrational Properties of Cr₂O₃ With Ferromagnetic and Antiferromagnetic Order: A Combined Experimental and Density Functional Theory Study, Journal of Magnetism and Magnetic Materials 444 (2017), 16–22.

28.

Wang

T.-G.

, Jeong

, Liu

, Wang

, Iyengar

, Melin

and Kim

K.H.

, Study on nanocrystalline Cr₂O₃ films deposited by arc ion plating: I. composition, morphology, and microstructure analysis, Surf Coat Technol 206 (2012), 2629–2637.

29.

Roy

, Ghosh

and Naskar

M.K.

, Solvothermal synthesis of Cr₂O₃ nanocubes via template-free route, Mater Chem Phys 159 (2015), 101–106.

30.

Attouche

, Rahmane

, Hettal

and Kouidri

, Precursor nature and molarities effect on the optical, structural, morphological, and electrical properties of TiO₂ thin films deposited by spray pyrolysis, Optik - International Journal for Light and Electron Optics 203 (2020), 163985.

31.

Singh

, Verma

, Kumar

and Kumar

, Structural, optical and electrical characterization of epitaxial Cr₂O₃ thin film deposited by PLD, Mater Res Express 6 (2019), 106406.