Introduction

Nickel oxide has a cubic structure, and it falls under the category of p-type semiconductors and has an energy band gap of 3.6-4 eV [1]. Nickel oxide is used across a broad spectrum of applications, including solar cells, photocatalysis, sensors, and optical devices. CuO is a p-type semiconductor with an energy gap of 1.2 eV and a monoclinic structure [2]. Material scientists are paying a lot of attention to using combined composite semiconductor materials. Various applications for doped mineral oxide nanoparticles include magnetic semiconductors, photodetectors, and optoelectronics [3]. Excellent chemical stability and optical and electrical properties define this material. Two profitable materials in the field of semiconductors are nickel oxide and copper oxide. Photovoltaic applications consider them for their exceptional performance and selectivity in reduction and oxidation reactions [4]. This material, which is a p-type semiconductor, comprises inexpensive and non-toxic elements.  NiO is considered a leading p-type semiconductor of interest [5]. Nanostructured NiO material are widely used in electrochromic, energy storage, and photo electro-catalysis because of their stable electrochemical properties and easy processing [6]. Its appropriate band gap and energy level alignment make it a favored photocathode in Photo electrochemical water splitting applications. NiO is attracting interest in PEC cells because it can act as a proton reduction site and aid in charge carrier transport. NiO has limitations that should be addressed for efficient use as a photo-cathode in water splitting, despite its advantages [7]. The performance of the Photo electrochemical device is restricted by the small hole mobility and high charge carrier recombination rate at the NiO/electrolyte interface. Nanostructured materials are crucial for energy-storage devices because of their large surface area that aids in reactions [8]. The active material's crystal structure determines the speed of charging and discharging the electrode by affecting ion diffusion resistance. Shortening the diffusion path can reduce the resistance to diffusion in the solid phase. Thus, nano-sized materials effectively reduce diffusion resistance [9]. ErCuNiO₃ composites are important materials for energy storage, photovoltaic, and solar cell fabrication.

NiO-CuO films with a thickness of 200 nm were deposited on substrates at 400 °C using the chemical spray pyrolysis method [2]. The band gap decreases as the copper oxide mixing ratio increases. Optical properties maintain a constant absorption coefficient despite the variation in photon energy. The extinction coefficient was determined by wavelength for all films, and the refractive index increased with higher CuO content. The XRD results showed a polycrystalline structure with orientation in the (111) plane. Imran Khan et al. [10] present the synthesis of nickel oxide nanoparticles and Sr-doped nickel oxide nanoparticles using hydrothermal methods. Structural characterization techniques, such as XRD and SEM are used to analyze as-synthesized samples. Samples with low Sr dopant levels (1 to 2)% produce a cubic phase for the NiO-NPs. Sr doped NiO nanoparticles range in size from 50 to 100 nm, with decreased uniformity at higher dopant concentrations. By examining the electrical and dielectric properties of the synthesized material, they explore the influence of Sr dopant levels. Electrochemical studies have revealed that the chosen material have exceptional catalytic activity for glucose sensing, because of their unique structure and shape. Tuba Cayir Taşdemirc [3] used SILAR to make nanostructured undoped and Cu doped NiO thin films on glass. The optical analysis of undoped and Cu-doped NiO films showed a decrease in bandgap from 3.34 -2.01 eV with increasing Cu concentration. With the increase in Cu concentration, SEM analysis demonstrated changes in the shape of the nanostructures. The AFM analysis showed a decrease in surface roughness when copper was coated on the surface.

Different techniques can be used to deposit films, such as chemical vapor deposition, thermal evaporation, spin coating, and spray pyrolysis [4]. The report describes the synthesis of ErCuNiO₃ composite materials using a hydrothermal method. Synthesizing nanomaterials can be done through different methods. Pawar and Deshmukh emphasize the importance of the hydrothermal method in synthesizing thin films [11]. The hydrothermal method, a heterogeneous reaction, is performed using the autoclave. Place insoluble solutes in the autoclave and keep it in the oven below the supercritical point of water. Crystallization has led to the formation of thin films on the substrate, each with different shapes and sizes. Water serves as the primary solvent during the reaction. Variations in temperature and pressure unveil remarkable properties. Water's high density allows non-polar compounds to completely dissolve. This method can create different structures, such as three-dimensional nonospheres, two-dimensional nanosheets, and one-dimensional nanowires, by adjusting process parameters. Consistent composition and uniform shapes characterize these structures. Dissolving insoluble materials at high temperature and pressure is the main purpose of this method. Advanced nanostructure preparation methods necessitate expensive instruments and surfactants. The byproduct produced by these methods has a negative impact on the environment. Pawar and Deshmukh claim that the hydrothermal method is environmentally friendly because it operates at low temperatures and does not need harmful catalysts [12]. Using this technique, material shape can be controlled and coatings can adhere well to various surfaces.

This paper assesses the impact of structure and optical properties on ErCuNiO₃ composites. The molar concentration of erbium was introduced to improve the material's photovoltaic properties.

Experimental

Materials

The materials used in this study are erbium (III) nitrate hexahydrate Sigma-Aldrich 99.9%, (Er(NO₃)₃.6H₂O), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) Sigma-Aldrich 99.9%, Nickel (II) nitrate hexahydrate. Ni(NO₃)₂·6H₂O Sigma-Aldrich 99.9%, sodium hydroxide (NaOH), polyethylene glycol, α-terpineol, deionized water, heating mantle, and FTO-fluorine-doped tin oxide substrate, and also an oven that has a temperature range of 50 to 1000 °C.

ErCuNiO₃ synthesis

To synthesize ErCuNiO₃, a 0.1 M solution of copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), nickel (II) nitrate hexahydrate Ni(NO₃)₂·6H₂O and sodium hydroxide (NaOH) were used, and sodium hydroxide (NaOH) from Sigma-Aldrich, which was 99.0% pure, to 25 mL of deionized  water and stirred it for 30 min at room temperature. Erbium (III) nitrate hexahydrate, (Er(NO₃)₃·6H₂O) Sigma-Aldrich 99.9%, at a concentration of (0.5-0.9) M. To create a uniform solution, 1 g of polyethylene glycol and 1 g of α-terpineol were added to the mixed water and stirred it for 30 min at room temperature. Erbium (III) nitrate hexahydrate, (Er(NO₃)₃·6H₂O) Sigma-Aldrich 99.9%, at a concentration of (0.5-0.9) M. To create a uniform solution, 1 g of polyethylene glycol and 1 g of α-terpineol were added to the mixed solutions and stirred for 30 minutes at room temperature. The FTO glass and solution were placed in a 100 ml Teflon-lined, stainless-steel autoclave for hydrothermal processing. The solution's temperature remained constant at 220 °C for 4 hours. The deposited ErCuNiO₃ on FTO substrate was vacuum-dried at 60 °C for 30 min after the autoclave cooled down naturally to room temperature (Figure 1). Various characterization techniques were employed to analyze the films, determining their optical, electrical, structural, morphological, and elemental compositions. The NPUFEI-NNS45 SEM was used to analyze structural and elemental compositions. The presence and types of functional groups in the ErCuNiO₃ films were examined using the JASCO-FTIR (FT/IR-6600). The films' absorbance wavelength was determined using a 756S UV-Visible spectrophotometer in the optical spectral range of 300 to 900 nm. We used the Jandel four-point probes method to analyze the electrical properties of the films.

Figure 1. Schematic diagram of synthesis procedure

Results and Discussion

Optical properties of ErCuNiO₃

Figure 2 (a) displays the absorbance spectra of the synthesized ErCuNiO₃. The absorption spectra showed a significant absorption rate between 300 and 900 nm. The presence of erbium in the CuNiO₃ lattice improves the absorption spectra of the synthesized ErCuNiO₃. The synthesis with 0.5 M has the highest absorption rate among all films. The films that have been synthesized with a 0.7 molar concentration of erbium show an improvement in performance compared to those with a lower concentration of erbium [2]. The absorbance rate of the synthesized material for photovoltaic use decreases with an increase in erbium concentration. A higher concentration of erbium results in a decreased absorption rate in the material. Introducing erbium may have caused a growth in crystallite size, as suggested by the decrease in crystallite peak observed in the XRD pattern. Decreasing the crystallite size can lead to a higher specific surface area and increased optical absorbance. The films are highly absorbent, making them ideal for solar cell technology and energy production. Figure 2 (b) consistently exhibited a high rate of transmittance, with values even reaching 85%, particularly at a wavelength of 900 nm. In the infrared spectral region, ErCuNiO₃ demonstrates a high level of transmittance. The increase of the erbium concentration leads to a rise in the transmittance spectra. The films exhibited a rise in electrical resistivity, suggesting that this could be the cause. By enhancing the thickness of the film, there is a possibility that the specific surface area will increase, leading to potential improvement in optical transmittance. Because of their exceptionally high transmittance rate, these films are an ideal choice for various applications such as photovoltaics, energy production, and solar cell systems. Figure 2 (c) revealed that the UV region had the highest reflectance. Upon evaluation, it was discovered that the films exhibited a low level of reflectance in both regions, which consequently makes them an ideal choice for both solar and photovoltaic cells. The negative reflectance observed in the material is caused by the interaction of light with the surface of the synthesized material, resulting in the effects interference and, ultimately, the cancellation of reflected light waves. They occur when the optical thickness of the material is overestimated. The estimate mentioned earlier may not fully account for the errors in the surface reflectance, as they can be larger than expected. This is mainly attributed to the influence of adjacency effects and the residuals that persist even after correction. The representation of the energy bandgap of ErCuNiO₃ can be observed in Figure 2 (d) through the graph of (αhv)² Vs hv. The utilization of the graph, which displayed the relationship between the absorption coefficient square and photon energy [13-20], allowed for the determination of indirect bandgap of the produced films. As the molar concentration of the material increases, the synthesized ErCuNiO₃ film exhibits a decrease in its indirect bandgap energy, which shifts from 1.50 eV to a range of 1.35-1.18 eV.

Figure 2. (a) absorbance, (b) transmittance, (c) reflectance, and (d) bandgap energy

Structural properties of (ErCuNiO₃)

The XRD pattern in Figure 3 (a and b) displays the polycrystalline phase (cubic structure) of CuNiO₃ in the thin film. The (111), (112), and (211) diffraction peaks are observed at 2theta angles of 26.512^o, 33.775^o, and 37.726^o, respectively. The (111) diffraction peak reveals the crystallization of the synthesized material. The presence of erbium in the CuNiO₃ lattice enhances the structure of the synthesized material, resulting in increased diffraction peaks. The synthesized ErCuNiO₃ reveals a cubic structure with diffraction peaks of (111), (101), (104) (112), (211), (222), and (311) [13-20] corresponding to 2theta angle of 26.512^o, 30.778^o, 32.054^o, 33.775^o, 37.726^o, 43.966, and 44.989^o, respectively. In Table 1, the crystallite size of ErCuNiO3 decreases as the erbium concentration increases. The erbium concentration of 0.5, 0.7, and 0.9 M resulted in crystallite sizes of 2.3276, 2.2574, and 2.1913 nm, respectively. Different structural characteristics were determined by performing calculations using Equations (1-4) [7,9,21-29].

Where, D is the crystallite size,  is the wavelength,  is the full width at half maximum, d is the d-spacing, a is the lattice constant, and is the dislocation density. Table 1 presents a correlation between the decrease in average crystallite size and the strain induced during hydrothermal process.

Figure 3. (a) XRD pattern and (b) magnified XRD pattern of the synthesized material

Table 1. Structural data of the synthesized material

Table 1. Continued

Surface morphology of the synthesized material

The surface morphology of ErCuNiO₃ is depicted in Figure 4. Small nanoparticles and significant nano wax particles are both observable in the CuNiO₃ material. Including erbium in the material causes the nanoparticles to grow and expand the surface area. This increase in surface area enhances the material's photovoltaic activities. More erbium concentration leads to higher surface energy because of nanoparticle clusters. The surface area of the synthesized films for energy storage activities is increased by a high molar concentration of erbium. Because of the strain, there was a change in the lattice's orientation, which consequently led to a reduction in the size of the material's crystallite [13-20]. Figure 5 demonstrates the elemental composition of ErCuNiO₃. All essential elements for the ErCuNiO₃ formation are clearly depicted in the spectrum. The composition of FTO substrate used in the synthesis procedure was also considered. The details of the percentage weight of the element present in the EDX analysis can be observed in Table 2. 

Figure 4. (A) Cu_0.1Ni_0.1O_3, (B) Er_0.5Cu_0.1Ni_0.1O₃, (C) Er_0.7Cu_0.1Ni_0.1O₃, and (D) Er_0.9Cu_0.1Ni_0.1O₃

Table 2. EDX spectra atomic weight percentages of the constituent elements

Figure 5. EDX spectrum of (a) Cu_0.1Ni_0.1O₃ and (b) Er_0.5Cu_0.1Ni_0.1O₃

Electrical properties of the synthesized ErCu_0.1Ni_0.1O₃

Figure 6 reveals the relationship between film thickness and electrical resistivity or conductivity. When the film thickness is increased, it leads to an increase in electrical resistivity and a simultaneous decrease in the conductivity of the films. The alignment occurs because erbium is introduced, resulting in an increase in carrier concentration and ultimately enhancing electrical conductivity. The beneficial aspect of the material lies in its ability to handle a greater amount of electric current, which is helpful for various optoelectronic applications [13-20]. The analysis of resistivity and conductivity for ErCu_0.1Ni_0.1O₃ is presented in Table 3, providing valuable insights into the electrical properties of this material. The film exhibited a decrease in electrical conductivity, from 9.77 to 5.78 S/m, as its thickness increased from 107.00 to 115.35 nm, leading to an increase in resistivity from 10.23 to 17.35 ῼ.cm. The erbium lattice has a direct impact on the electron-hole pairs found in ErCu_0.1Ni_0.1O, resulting in an alteration in the spacing between them because of the crystallite size. The variation in crystallite sizes between the Erbium molar concentration and Cu_0.1Ni_0.1O led to a reduction in the electrical conductivity of the films. The increase in resistivity and film thickness of ErCu_0.1Ni_0.1O is correlated with the concentration of erbium. The potential value of this film lies in its ability to enhance solar cell efficiency. ErCu_0.1Ni_0.1O₃, exhibits a resistivity that is highly suitable for use as buffer layers in photovoltaic systems.

Figure 6. Plot of resistivity and conductivity of ErCu_0.1Ni_0.1O₃

Table 3. Electrical properties of ErCuNiO₃

FTIR analysis of ErCu_0.1Ni_0.1O₃

In this study, the FTIR spectra of ErCu_0.1Ni_0.1O₃ are being investigated to determine the unique properties of the synthesize films in Figure 7. By increasing the erbium molar concentration in the films, Er²⁺, Cu²⁺ ions can be detected, causing a symmetrical peak to appear at 779 cm^-1. The binding of O-H to nickel causes this peak. A common trend can be observed in all films, whereby the 779 cm^-1 band in the asymmetrically stretched carbonates ion at 1354 cm^-1 decreases as the concentration increases. Atmospheric CO₂ handles the band observed at approximately 1582 cm^-1 in the spectra. The presence of a peak at approximately 1005 cm^-1 shows the Ni-O stretching vibrations in the compound ErCu_0.1Ni_0.1O₃. The material went through this change because of an increase in the volume of its unit cell. The observed strong absorption band, which is at approximately 1005 cm^-1, is the caused by the bending vibration of the oxygen octahedral deformation mode.

Figure 7. Plot of FTIR of ErCu_0.1Ni_0.1O₃

Conclusion

Using a hydrothermal approach, we have successfully synthesized ErCuNiO₃. The absorption spectra showed a significant absorption rate between 300 and 900 nm. Erbium in the CuNiO₃ lattice enhances the absorption spectra of ErCuNiO₃. The synthesis with 0.5 M exhibits the highest absorption rate compared to other films. The (111) diffraction peak reveals the crystallization of the synthesized material. The presence of erbium in the CuNiO₃ lattice enhances the structure of the synthesized material, resulting in increased diffraction peaks. The synthesized ErCuNiO₃ reveals a cubic structure with diffraction peaks of (111), (101), (104) (112), (211), (222), and (311) corresponding to 2theta angle of 26.512^o, 30.778^o, 32.054^o, 33.775^o, 37.726^o, 43.966^o, and 44.989^o, respectively. Introducing erbium into the lattice of the synthesized material results in an increase in the nanoparticle's size, increasing the surface area of the material. This increase in surface area enhances the material's photovoltaic activities. As the molar concentration of the material increases, the synthesized ErCuNiO₃ film exhibits a decrease in its indirect bandgap energy, which shifts from 1.50 eV to a range of 1.35-1.18 eV. The film exhibited a decrease in electrical conductivity, from 9.77 to 5.78 S/m, as its thickness increased from 107.00 to 115.35 nm, leading to an increase in resistivity from 10.23 to 17.35 ῼ.cm.

Disclosure Statement

The authors state they have no personal or financial conflicts that could have influenced the research in this article.

Acknowledgments

We extend our gratitude to all the authors for their significant contributions to the success of the paper.

Orcid

Imosobomeh L. Ikhioya: https://orcid.org/0000-0002-5959-4427

Citation: F. Ali, I. Ahmad, A. Hussain, S. Ahmad, M.Z. Ansar, A. Yahaya, I.L. Ikhioya*. Improved Morphological, Structural, and Optical Features of Er_xCuNiO₃ {x= 0, 0.5, 0.7, 0.9}.‎ J. Appl. Organomet. Chem., 2023, 3(4), 308-320.