Structural, Morphological and Optical Properties of (NiO)1-x(Co3O4)x Composite Thin Films Prepared by Chemical Spray Pyrolysis Technique

A mixed valence oxide of CoO and Co₂O₃ with a high oxygen content, spinel cobalt oxide Co₃O₄ has p-type semiconducting properties. It has been investigated as a possible material for a variety of applications, such as sensors [1], electrochemistry [2], energy storage [3], catalysis [4], and magnetism [5]. The catalytic characteristics of its surface are often the basis for the sensing of Co₃O₄'s gas sensing properties, and the sensors' operating temperature is typically higher than 200℃ [6].

A number of chemical sensing, photocatalysis, battery, and electrochemical applications find nickel oxide (NiO) to be a suitable semiconductor due to its features [7]. Nickel cermets find use in textiles, electrochemical coatings, anode layers in solid oxide fuel cells, lithium nickel oxide cathodes for lithium-ion batteries, nanowires, nanofibers, and special alloy and catalyst applications [7].

Additionally, nickel oxide is utilized as an antiferromagnetic layer and catalyst, in lightweight aircraft structural components, active optical filters, alkaline battery cathode materials, and materials for temperature or gas sensors, including hydrogen and carbon monoxide sensors [8].

In this study, the spray pyrolysis technique was used to prepare thin films of the composite (NiO)_1-x(Co₃O₄)_x. These two materials NiO and Co₃O₄ were used due to their good optical and structural properties, and this method was also used to obtain homogeneous nano-films.

3.1 X-Ray diffraction (XRD)

XRD of the (NiO)_1-x(Co₃O₄)_x thin films prepared by spray pyrolysis at substrate temperature of (400℃) have been shown in Figure 1. These films showed a crystal structure of the films was identified to be polycry- stalline [10], with cubic structure for nickel oxide in the planes (111), (200), (202) and (222) this is agreement with the study [10, 11], and the optimum peak was observed at the (111) plane, corresponding to the anatase phase of the Co₃O₄.

According to the data, there is a noticeable impact on the crystal structure during mixing, which causes the peak location to slightly change and the Table 1 shows peaks position for all pure and mixing films and including inter planar spacing (d), Crystallite size (D), Micro strain (ε) and dislocation density (δ), We notice from the Table 1 the crystal size increased with decreasing the dislocation density and micro strain. The average crystal size upon mixing increased from (10.02) for the mixing ratio of 75% to (27.18) for the ratio of 25%. The presence of the two phases of NiO and Co₃O₄ in the films confirms the presence of the composite nature of the two oxides [12]. The results of XRD examinations of the prepared films are consistent with the results of other researches [10, 13].

Figure 1 represents the XRD results of the prepared films, where the purple shows CO₃O₄ pure, the black shows NiO pure, the red shows (NiO)_75%(Co₃O₄)_25%, the blue shows (NiO)_50%(Co₃O₄)_50%, and the green shows (NiO)_25%(Co₃O₄)_75%.

1.png

Figure 1. XRD for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

3.2 Atomic force microscope (AFM)

AFM images have been used to observe the surface morphology and roughness of the films and the scanning method of film surfaces for all samples. Figures 2 (a, b, c, d, e) displays the 3D surface structure and granularity distribution of (NiO)_1-x(Co₃O₄)_x thin films with different Co₃O₄ concentrations (0,25,50,75 and 100)%, The results of afm of the prepared films are consistent with the results of other researches [14].

Figure 2 represents the afm results of the prepared films, where, (a) shows NiO pure, the (b) shows (NiO)_75%(Co₃O₄)_25%, the (c) shows (NiO)_50%(Co₃O₄)_50%, the (d) shows (NiO)_25%(Co₃O₄)_75%, and (e) shows Co₃O₄ pure.

_x(Co₃O₄)_x films and (RMS) root mean square of same ratios ranged of (7.81-10.3)nm. At the same time, the average diameter of grain size ranged of (20.04-29.62)nm. Figure 2 shows the AFM images of (NiO)_1-x(Co₃O₄)_x thin films synthesized at different molarities. The average roughness and root mean square (RMS) and average diameter for (NiO)_1-x(Co₃O₄)_x, estimated from AFM, are given in Table 2. The sample prepared at (NiO)₅₀(Co₃O₄)₅₀ has highest average roughness of the film. By observing the roughness values, we notice that the surface is homogeneous and nanoscopic.

21.png

22.png

23.png

24.png

25.png

31.png

32.png

33.png

34.png

35.png

Figure 2. AFM 3D images and granularity distribution for(NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

Table 2. Morphology parameters for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

3.3 Field emission scanning electron microscopy

Figure 3 (a-e) shows FESEM images of (NiO)_1-x(Co₃O₄)_x thin films at different Co₃O₄ volume ratios (0,25, 50,75 and 100)%.

3a.png

3b.png

3c.png

3d.png

3e.png

Figure 3. FESEM images for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

It can be noted thin films have a common surface shape comprising many randomly placed chunks or aggregates of Co₃O₄ on the top surface of the films and the images shows a cubic nanocrystal. The findings suggest that there was a tendency for Co₃O₄ to cluster and diffuse well in the (NiO)_1-x(Co₃O₄)_x thin films. As the concentration of cobalt oxide increased, it formed a continuous network with (NiO)_1-x(Co₃O₄)_x. The findings presented herein are consistent with the conclusions reached [15].

From the Figure 3, we notice that the size of the particles in the membranes is nano-sized, which means that the membranes can be used as a sensing application because the nano-sized size provides a high adsorption area, and this is one of the conditions for high sensitivity, this is consistent with the work of researchers [16].

3.4 Energy dispersive X-Ray spectroscopy (EDX)

The energy-dispersive X-ray analysis (EDX) spectra of the (NiO)_1-x(Co₃O₄)_x thin films deposited on glass substrate are given in Figures 4 (a-e), The energy-dispersive X-ray analysis (EDX) spectrum of the (NiO)_1-x(Co₃O₄)_x thin films with different Co₃O₄ concentrations (0,25, 50, 75 and 100)% deposited on glass substrate using the spray pyrolysis technique are given in Figure 4. The analysis revealed the prominent presence of the components Ni, O, and Co, Furthermore, as indicated in Table 3.

41.png

42.png

43.png

44.png

45.png

Figure 4. EDX images for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

Table 3. EDX analysis for prepared films elements compositions

3.5 Optical properties

In the UV-visible range (300-1100)nm, the optical properties of thin films (NiO)_1-x(Co₃O₄)_x with variation ratios of Co₃O₄ and deposited on glass substrates were recorded. In addition to the direct band gap, the absorption coefficient (α), extinction coefficient (k), refractive index (n), dielectric constants (ɛ), and optical conductivity, the variation of the absorbance spectrum, transmittance, and reflectance was also examined. The chemical composition, crystal structure, incident photon energy, thin film thickness, and thin film surface morphology all affect optical transmission and absorption spectra [16, 17].

The optical properties of (NiO)_1-x(Co₃O₄)_x thin films as shown in Figure 5 and Figure 6. The results from Figure 5 the absorption was high and then decreased with increasing wavelength. In contrast, the behavior is opposite for transmittance and reflectance. It can be seen that with increasing the concentration of Co₃O₄, the films showed higher ability to absorb electromagnetic radiation at longer wavelengths than NiO, which means that Co₃O₄ can reduce the band gap of NiO. The results showed that the unmodified NiO had a band gap value of (3.6-4)eV [18, 10].

5.png

Figure 5. Absorbance images for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

6.png

Figure 6. Energy gap images for(NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

Table 4. Optical parameters for (NiO)_1-x(Co₃O₄)_x composite thin films at different vol.% of (x)

One can observe that the energy band gap of (NiO)_1-x(Co₃O₄)_x thin films decreased from (3.75 to 2.1)eV with the increase of Co₃O₄ concentration from 25% to 100%, as shown in Figure 6 and Table 4, where some of the unpaired electrons combine with the unpaired electrons on the surface of NiO, forming a structure. This decrease in the band gap is caused by the shift of the valence band edge upward [19-21]. We notice from the figure that with increasing the concentration of Co₃O₄, the energy gap decreases. The rise in electronic transitions can be attributed to the doping process, which produced new levels near the valence band inside the energy gap. This resulted in the formation of bridges that allowed electrons to move between the valence band and the conduction band. Thus, the process of adding the concentration of Co₃O₄ led to improving the structural properties of the prepared film.

Since the energy of the falling photon is less than the energy value of the semiconductor, the electron could not be evaporated and moved from the valence pack to the conduction beam. As a result, the absorbance is decreased by increasing the wavelength. We note that the absorption increased with the concentration of the Co₃O₄, which led to a decrease in the energy gap values of the prepared films [22, 23].