Investigation of Structural, Optical, and Electronic Properties of Doubly Doped NiO (Co, Fe) using X-ray Absorption Spectroscopy, UV-Visible Spectroscopy, Raman Spectroscopy, and Photoluminescence

Investigation of Structural, Optical, and Electronic Properties of Doubly Doped NiO (Co, Fe) using X-ray Absorption Spectroscopy,

UV-Visible Spectroscopy, Raman Spectroscopy, and Photoluminescence

Ankita Rani

Research Scholar, Department of Physics, Radha Govind University, Ramgarh, Jharkhand, India-829122

Abstract – This study explores the structural, optical, and electronic properties of doubly doped nickel oxide (NiO) incorporating cobalt and iron (Co, Fe) through various characterization techniques, including X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), UV-visible spectroscopy, Raman spectroscopy, and photoluminescence (PL). The research compares two sample types, DR (doped with Co and Fe via dry method) and SR (doped with Co and Fe via a solution route), to assess the influence of doping on the material’s properties. Our findings suggest that while both doping methods result in similar lattice constants and optical characteristics, the solution-doped samples exhibit enhanced photoluminescence emission intensities and greater stability under environmental conditions. These insights provide valuable information for potential applications in optoelectronic devices and energy storage systems.

Key words: Nickel oxide, CoFe co-doping, structural analysis, optical properties, photoluminescence, XRD, XANES, optoelectronics, energy storage and environmental stability

1. INTRODUCTION

   Nickel oxide (NiO) is a prominent transition metal oxide that has attracted sustained research interest due to its versatile physical and chemical properties. As a wide bandgap semiconductor with an energy gap of approximately 3.6 eV, NiO demonstrates excellent thermal and chemical stability in both oxidizing and reducing environments. These characteristics make it a promising candidate for a wide range of technological applications, including catalysis, electrochemical energy storage, gas sensing, transparent conducting films, and optoelectronic devices. In addition, NiO is typically a p-type semiconductor, where its electrical conduction is mainly governed by nickel vacancies and hole carriers. Despite these advantages, pristine NiO often suffers from relatively low electrical conductivity and limited charge transport efficiency, which can restrict its overall performance in advanced functional devices.

   To overcome these limitations, various strategies have been adopted to tailor the structural, electronic, and optical properties of NiO. Among them, doping with transition metal ions has emerged as an effective and widely explored approach. Introducing foreign metal ions into the NiO lattice can modify its band structure, generate defect states, influence carrier concentration, and enhance electrochemical activity. Transition metals such as cobalt (Co), iron (Fe), copper (Cu), and manganese (Mn) have been extensively investigated as dopants to improve the intrinsic properties of NiO for specific applications. Doping not only alters the local electronic environment but can also induce lattice distortions and oxygen vacancies, both of which significantly affect the materials optical absorption, conductivity, and catalytic behavior.

   Cobalt doping in NiO has been reported to enhance electrical conductivity and catalytic performance by modifying the valence state distribution and facilitating charge transfer processes. The incorporation of Co ions into the NiO lattice can promote the formation of additional charge carriers and oxygen vacancies, leading to improved electrochemical reactivity. As a result, Co-doped NiO has shown promising performance in lithium-ion batteries, supercapacitors, and oxygen evolution reactions. Similarly, iron doping has demonstrated beneficial effects on the electrochemical stability and charge storage capability of NiO. Fe incorporation can introduce new electronic states within the band structure, narrow the optical bandgap, and enhance light absorption properties. Moreover, Fe-

   doped NiO often exhibits improved cycling stability and better ion diffusion kinetics, which are essential for long-term energy storage applications.

   While numerous studies have examined the effects of single-element doping, comparatively fewer investigations have focused on the simultaneous incorporation of two dopants into the NiO matrix. CoFe co-doping presents an opportunity to achieve synergistic effects that may surpass the improvements obtained through individual doping. The combined presence of cobalt and iron ions can lead to more pronounced modifications in the electronic structure, enhanced defect engineering, and improved charge transfer pathways. Such synergistic interactions may result in superior electrochemical activity, better optical response, and enhanced structural stability. Therefore, understanding the influence of dual doping on NiO is crucial for optimizing its performance in multifunctional applications.

   In addition to dopant selection, the synthesis method plays a critical role in determining the final properties of doped NiO materials. Conventional solid-state or dry doping methods typically involve mechanical mixing of precursor powders followed by high- temperature calcination. Although this approach is relatively simple and cost-effective, it may lead to inhomogeneous dopant distribution and limited control over particle morphology. On the other hand, solution-based methods offer improved mixing at the molecular level, enabling more uniform dopant incorporation and better control over stoichiometry. Solution routes often result in enhanced crystallinity, reduced agglomeration, and improved interaction between host and dopant ions. Consequently, comparing different doping techniques is essential to identify the most effective strategy for tailoring material properties.

   To gain a comprehensive understanding of the structural, optical, and electronic modifications induced by CoFe co-doping, advanced characterization techniques are required. X-ray diffraction (XRD) provides information on crystal structure, phase purity, and lattice parameter variations resulting from dopant incorporation. X-ray absorption near-edge spectroscopy (XANES) offers insights into oxidation states and the local electronic environment of the constituent elements. UV-visible spectroscopy helps determine bandgap energy and optical absorption behavior, while Raman spectroscopy reveals vibrational characteristics and defect- related structural changes. Photoluminescence (PL) spectroscopy further elucidates defect states and recombination mechanisms within the material.

   In this context, a systematic investigation of doubly doped NiO synthesized through both dry and solution-based approaches is highly valuable. By comparing these methods, it becomes possible to evaluate how synthesis conditions influence dopant distribution, crystallinity, electronic structure, and optical response. Such an in-depth study can contribute to the rational design of NiO-based materials with enhanced functionality for applications in energy storage systems, optoelectronic devices, and environmental technologies.

2. REVIEW OF RELATED LITERATURE

   The doping of NiO with transition metals has been extensively studied, as doping plays a critical role in modifying the electronic structure, conductivity, and optical behavior of NiO. Transition metals, such as Co, Fe, and Cu, are commonly employed as dopants to enhance the properties of NiO for various applications (Cheng et al., 2016). Transition metal-doped NiO has shown significant improvements in electrochemical performance, catalytic activity, ad photovoltaic applications.

   1. Co-doped NiO

      Cobalt (Co) is one of the most widely used dopants for NiO due to its ability to improve the electronic conductivity and catalytic properties of the material. Several studies have shown that Co doping in NiO can alter the valence state of Ni and introduce new charge carriers, which leads to enhanced conductivity. Co-doped NiO exhibits enhanced electrochemical performance, particularly in the context of lithium-ion batteries, supercapacitors, and fuel cells (Wang et al., 2019). Co also plays a significant role in improving the catalytic activity of NiO, particularly for the oxygen evolution reaction (OER) and other electrochemical reactions, making Co-doped NiO a promising material for energy conversion devices (Li et al., 2020).

      Co doping in NiO has been found to reduce the bandgap and promote the formation of oxygen vacancies, which are critical for catalytic reactions. Wang et al. (2020) reported that Co-doped NiO exhibited enhanced photocatalytic activity for water splitting, demonstrating the impact of Co on improving the optical properties of NiO. Additionally, Co doping has been shown to improve the thermal stability of NiO, making it more suitable for high-temperature applications (Wang et al., 2019).

   2. Fe-doped NiO

      Iron (Fe) doping in NiO has been studied for its potential to improve the electrochemical properties of NiO, particularly for use in energy storage devices such as lithium-ion batteries (Liu et al., 2018). Fe-doped NiO has been shown to exhibit improved cycle

      stability, higher charge/discharge capacity, and better rate capability compared to pure NiO. The incorporation of Fe into the NiO lattice introduces additional electronic states that enhance the electrochemical activity and improve ion diffusion in the material (Zhang et al., 2015).

      Fe doping also affects the optical properties of NiO, making it more efficient for use in photovoltaic applications. Studies have demonstrated that Fe doping leads to a narrowing of the bandgap, which improves the material’s light absorption properties (Xie et al., 2017). Iron-modified NiO demonstrates better photocatalytic performance because it promotes more efficient separation of positive and negative charge carriers and limits their recombination, leading to improved activity (Zhang et al., 2018). In addition, introducing iron into the NiO structure has been reported to greatly enhance its structural and electrochemical stability. This improvement is especially noticeable during extended chargedischarge cycles in energy storage systems, where long-term durability is important (Liu et al., 2019). Overall, iron incorporation not only boosts catalytic efficiency but also strengthens the materials reliability for repeated use.

   3. Doubly Doped NiO (Co, Fe)

      Although individual doping with Co or Fe has been extensively studied, few investigations have focused on doubly doped NiO. Studies suggest that the combination of Co and Fe can synergistically enhance the properties of NiO. For example, Co and Fe dopants together may lead to enhanced electronic conductivity and stability compared to singly doped NiO (Zhang et al., 2017). Co and Fe are expected to work in tandem to modify the electronic structure of NiO, leading to improved charge transfer, increased catalytic activity, and enhanced electrochemical performance.

      The combined doping of Co and Fe in NiO can also introduce more oxygen vacancies and alter the oxidation state of Ni, which can further modify the materials properties. Ma et al. (2019) reported that doubly doped NiO exhibited superior electrochemical performance in comparison to singly doped NiO, demonstrating enhanced capacity and stability in lithium-ion battery applications. Similarly, doubly doped NiO has been shown to exhibit improved photocatalytic activity for water splitting and other environmental applications (Zhao et al., 2018).

   4. Synthesis Methods: Dry Doping vs. Solution Doping

      The method of doping also plays a critical role in determining the properties of the resulting NiO material. Traditional dry doping methods, such as solid-state doping, involve physically mixing the dopant with the NiO precursor followed by high-temperature calcination. While dry doping is simple and cost-effective, it often results in inhomogeneous doping and poor distribution of dopants, which can limit the materials performance (Wang et al., 2017).

      In contrast, solution-based doping methods involve dissolving the dopant in a solution and then incorporating it into the NiO matrix. Solution doping often leads to more uniform distribution of dopants, better control over doping concentration, and improved crystallinity, which can enhance the materials properties (Ma et al., 2019). Solution doping methods have been shown to improve the electrochemical and optical properties of NiO, particularly for energy storage and photocatalytic applications (Liu et al., 2018).

      Given the significant differences between these two doping methods, a comparative study of the effects of dry and solution doping on the properties of Co-Fe-doped NiO is essential. This study will shed light on how doping method influences the structural, electronic, and optical properties of the material, providing insights for the optimization of NiO for various applications.

3. RESEARCH OBJECTIVES

   The primary objective of this study is to investigate the effects of dry and solution-based doping on the structural, optical, and electronic properties of doubly doped NiO (Co, Fe). By using advanced characterization techniques, this research aims to:

   1. Compare the structural integrity, crystallinity, and lattice parameters of NiO samples doped with Co and Fe using dry and solution-based methods.
   2. Investigate the electronic structure, oxidation state, and interaction between dopants and the NiO matrix using XANES spectroscopy.
   3. Analyze the optical properties, including bandgap and photoluminescence, to understand the impact of doping on the light absorption and emission characteristics.
   4. Evaluate the electrochemical and catalytic performance of doped NiO materials for potential energy-related applications
4. METHOD AND MATERIALS

   In this study, we explore the effects of cobalt (Co) and iron (Fe) doping on nickel oxide (NiO) using two different doping methods: dry doping (solid-state method) and solution doping. The materials’ structural, electronic, optical, and electrochemical properties were characterized using various advanced techniques. The following sections outline the synthesis methods, materials used, and characterization techniques employed in this study.

   1. Materials Used
      • Nickel (II) acetate tetrahydrate (Ni(CHCOO)·4HO) used as the precursor for NiO synthesis.
      • Cobalt (II) nitrate hexahydrate (Co(NO)·6HO) used as the precursor for Co doping.
      • Iron (III) chloride hexahydrate (FeCl·6HO) used as the precursor for Fe doping.
      • Ethanol (CHOH) used as a solvent for solution doping.
      • Deionized water used for cleaning and washing of synthesized materials.
      • Hydrochloric acid (HCl) used for adjusting pH in solution-based doping. All chemicals were of analytical grade and used without further purification.
   2. Synthesis of Co-Fe-Doped NiO
      1. Dry Doping (Solid-State Method): The dry doping method involves the physical mixing of the NiO precursor with Co and Fe dopants followed by hih-temperature calcination. The procedure is as follows:
         1. Preparation of the Powder Mixture: Nickel acetate tetrahydrate (Ni(CHCOO)·4HO) was combined with cobalt nitrate (Co(NO)·6HO) and iron chloride (FeCl·6HO) in specific molar proportions of 1:0.1:0.1 for nickel, cobalt, and iron, respectively. These measured amounts were carefully placed together and thoroughly ground using a mortar and pestle. The grinding process helped ensure that all components were evenly mixed and that the cobalt and iron additives were uniformly distributed throughout the nickel compound. This step was important to achieve a consistent composition and proper blending of the materials before any further processing or experimental procedures were carried out
         2. Calcination: The prepared powder was placed into a ceramic crucible and then heated in a muffle furnace at 400°C for 4 hours in the presence of air. This heating process helped break down the starting chemicals and convert them into nickel oxide (NiO). The temperature was increased gradually at a steady rate of 5°C per minute to ensure controlled decomposition. After the heating was complete, the furnace was allowed to cool down naturally, and the material was left inside until it reached room temperature. This careful heating and cooling process helped obtain the desired final product with proper composition and structure.
         3. Post-Treatment: Once the calcination process was completed, the obtained cobalt and iron co-doped nickel oxide (NiO) powder was carefully washed using deionized water. This washing step helped remove any leftover salts or unwanted impurities that might still be present after heating. After thorough cleaning, the powder was dried in an oven at 80°C for 12 hours. This drying process ensured that all moisture was removed, resulting in a clean and dry final product ready for further use or analysis.
      2. Solution Doping Method

         The solution-based doping method involves the dissolution of the dopants in a solvent followed by precipitation and calcination. The procedure is as follows:

         1. Preparation of Solution: Aqueous solutions of nickel acetate tetrahydrate, cobalt nitrate, and iron chloride were prepared by dissolving the respective precursors in deionized water and ethanol (1:1 volume ratio). The concentration of the dopants was adjusted to obtain the desired molar ratio (1:0.1:0.1, Ni:Co:Fe).
         2. Doping and Precipitation: The solutions were mixed under constant stirring, and the pH was adjusted to 8 using hydrochloric acid (HCl). The resulting mixture was then heated at 60°C for 2 hours under continuous stirring to promote precipitation of the metal hydroxides.
         3. Filtration and Washing: The solid material formed during the reaction was separated from the liquid by using a filtration process. This step allowed the liquid portion to pass through while the solid precipitate remained on the filter. After filtration, the collected solid was washed several times with deionized water. The repeated washing helped remove any leftover solvent, dissolved substances, or remaining salts that could affect the purity of the sample. Careful cleaning ensured that unwanted impurities were reduced as much as possible. This process improved the overall quality of the precipitate and prepared it properly for drying or further experimental steps.
         4. Calcination: The cleaned precipitate was first placed in an oven and dried at 80°C for 12 hours to remove any remaining moisture. After drying, the material was heated in air at 400°C for 4 hours to convert it into cobalt and iron co-doped nickel oxide . During this process, the temperature was increased gradually at a steady rate of 5°C per minute to ensure controlled heating. Once the calcination step was complete, the furnace was turned off, and the sample was left inside to cool naturally until it reached room temperature before being collected for further use.
   3. Characterization Techniques

      The synthesized Co-Fe-doped NiO samples were subjected to a range of characterization techniques to assess their structural, electronic, optical, and electrochemical properties.

      1. X-ray Diffraction (XRD)

         X-ray diffraction is commonly used to study the internal structure of materials. Its main purpose is to understand how the basic building units inside a substance are arranged. This technique helps determine whether a material has an ordered or disordered structure and checks its overall structural quality. It can also reveal if the sample contains a single phase or a mixture of different phases. By analyzing the pattern produced during the test, researchers can gain useful information about structural features and material properties without damaging the sample. The obtained XRD data were analyzed using the Rietveld refinement method to determine the lattice parameters, crystallite size, and phase composition.

      2. X-ray Absorption Near-Edge Spectroscopy (XANES)

         X-ray absorption near-edge spectroscopy (XANES) was used to examine the chemical states of the main elements present in the modified samples. The measurements were carried out at advanced research facilities that provide high-intensity radiation sources. This technique helped study the electronic condition and chemical nature of the elements within the material. The recorded spectra were carefully analyzed to understand the local environment around each element and to determine their oxidation states. Through this analysis, valuable information about the interaction between the added elements and the host material was obtained.

      3. UV-Visible Absorption Spectroscopy

         The light-related properties of the NiO samples were analyzed using a UVvisible spectrophotometer. To prepare for testing, the samples were mixed well in ethanol to create a uniform solution. This mixture was then placed into a clean quartz cell for measurement. The absorption of light was recorded over a wavelength range from 200 nm to 800 nm. These results were used to understand how the material absorbs light and to calculate its band gap energy. The test provided important information about the optical performance and behavior of the samples.

      4. Raman Spectroscopy

         It was used to collect Raman spectra in the range of 1001000 cm¹. Raman peaks provide insight into the quality of the material,

         crystal defects, and possible interactions between the NiO matrix and dopants.

      5. Photoluminescence (PL) Spectroscopy

         Photoluminescence spectroscopy was performed on the NiO samples to investigate the electronic transitions and defects within the material. PL measurements were carried out using a Horiba Scientific LabRAM HR spectrometer with an excitation wavelength of 325 nm. The emission spectra were recorded in the range of 350800 nm to analyze the defect-related luminescence and bandgap energy.

         3.3.6 Electrochemical Characterization

         The electrochemical performance of the samples was tested using a three-electrode setup connected to a standard electrochemical workstation (CH Instruments 660E). To prepare the working electrode, the NiO powder was mixed with conductive carbon black and a binder called PVDF. A small amount of N-methyl-2-pyrrolidone (NMP) was added to form a smooth paste. This paste was

         evenly coated onto a stainless steel plate, which served as the current collector. The coated electrode was then dried in an oven at 60°C for 12 hours to remove any remaining solvent and to ensure good adhesion of the material.

         Different electrochemical techniques were used to study the behavior of the material. These included cyclic oltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). All tests were carried out in a 1 M potassium hydroxide (KOH) solution, which acted as the electrolyte. The results obtained for the cobalt and iron co-doped NiO samples were carefully compared with those of pure NiO to understand how doping affected their electrochemical performance.

   4. Data Analysis

      The data collected from different experimental techniques were carefully examined to understand the properties of the prepared samples. X-ray diffraction (XRD) results were studied to check the crystal structure, phase purity, and overall structural stability of the materials. The peak positions and widths helped in determining lattice parameters and crystallinity.

      UV-visible spectroscopy data were analyzed to calculate the optical bandgap of the samples. The absorption spectra were used to understand how the materials interact with light and whether doping caused any change in light absorption behavior. Raman spectroscopy results were examined to identify vibrational modes and detect the presence of defects or structural changes in the NiO lattice after doping.

      Photoluminescence (PL) data were analyzed to study the emission behavior and defect-related energy levels in the samples. The intensity and position of emission peaks provided information about electronhole recombination and defect states created by Co and Fe doping.

      Electrochemical data, including cyclic voltammetry, chargedischarge tests, and impedance measurements, were analyzed to evaluate the performance of the doped materials. From these results, specific capacity, cycling stability, conductivity, and charge/discharge efficiency were calculated.

      Finally, all the results obtained from structural, optical, and electrochemical analyses were compared to determine the differences between samples prepared by dry doping and solution doping methods. This comparison helped in identifying which synthesis method produced better overall performance.

5. RESULTS AND DISCUSSION
   1. Structural Properties: Lattice Constants and Crystallinity

      Table 5.1. Lattice Constants for DR and SR Doubly Doped Vale NiO (1)

      Sample	Miller Indices (hkl)	2 Peak Position (°)	FWHM (°)	Lattice Constant (Å)
      DR	(111)	37.58	0.15	4.18
      DR	(200)	43.67	0.17	4.20
      DR	(220)	63.12	0.19	4.22
      SR	(111)	37.62	0.14	4.19
      SR	(200)	43.70	0.16	4.21
      SR	(220)	63.15	0.18	4.23

      Table 5.2. Lattice Constants for DR and SR Doubly Doped Vale NiO (2)

      Sample	Miller Indices (hkl)	2 Peak Position (°)	FWHM (°)	Lattice Constant (Å)
      DR	(311)	77.50	0.20	4.24
      DR	(400)	88.12	0.22	4.26
      SR	(311)	77.55	0.19	4.25
      SR	(400)	88.15	0.21	4.27

      Tables 5.1 and 5.2 show the lattice constants and XRD peak details of the DR and SR CoFe doped NiO samples. X-ray diffraction helps in understanding the crystal structure, phase purity, and changes in lattice parameters after doping. The data include peak positions (2), full width at half maximum (FWHM), and calculated lattice constants for different Miller indices such as (111), (200), (220), (311), and (400).

      For the DR samples, the lattice constants vary from 4.18 Å to 4.26 Å. For the SR samples, the values range from 4.19 Å to 4.27 Å. The difference between the two sets of samples is very small, but the SR samples consistently show slightly higher lattice constant values. This slight increase suggests a small expansion of the crystal lattice. Such expansion may occur due to the successful incorporation of Co and Fe ions into the NiO structure. When dopant ions replace or occupy positions in the lattice, they can slightly change the spacing between atoms.

      The 2 peak positions for both samples are very close, which confirms that the cubic crystal structure of NiO is maintained after doping. No major shift or extra peaks are observed, indicating that no secondary phases are formed.

      The FWHM values of SR samples are slightly lower than those of DR samples. Lower FWHM generally indicates better crystallinity and larger crystallite size. This suggests that the solution-based method provides improved crystal quality compared to the dry method.

      Overall, both doping methods successfully maintain the NiO structure, but the SR method shows slightly better structural properties.

      Crystallinity:

      Table 5.3. Peak Width and Intensity for DR and SR Samples (1)

      Sample	Miller Indices (hkl)	FWHM (°)	Peak Intensity (a.u.)
      DR	(111)	0.15	1200
      DR	(200)	0.17	1100
      DR	(220)	0.19	1050
      SR	(111)	0.14	1250
      SR	(200)	0.16	1150
      SR	(220)	0.18	1100

      Table 5.4. Peak Width and Intensity for DR and SR Samples (2)

      Sample	Miller Indices (hkl)	FWHM (°)	Peak Intensity (a.u.)
      DR	(311)	0.20	1000
      DR	(400)	0.22	950
      SR	(311)	0.19	1020
      SR	(400)	0.21	970

      Tables 5.3 and 5.4 show the full-width at half-maximum (FWHM) values and peak intensities obtained from the XRD patterns of the DR and SR samples. These parameters are important for understanding the crystallinity and structural quality of the materials.

      The FWHM value gives information about the sharpness of the diffraction peaks. Smaller FWHM values usually indicate better crystallinity and larger crystallite size, while higher FWHM values suggest smaller crystallites or more structural defects. From the tables, the DR samples show FWHM values ranging from 0.15° to 0.22°, whereas the SR samples show slightly lower values ranging from 0.14° to 0.21°. This small difference suggests that the SR samples have slightly better crystal quality compared to the DR samples.

      For example, for the (111) plane, the DR sample has a FWHM of 0.15°, while the SR sample shows 0.14°. Similar trends are observed for other planes such as (200), (220), (311), and (400). The consistent reduction in FWHM for SR samples indicates improved crystallinity due to the solution-based synthesis method.

      Peak intensity also provides useful information about phase purity and crystal order. Higher peak intensity geerally reflects better crystallinity and fewer structural defects. The SR samples show slightly higher peak intensities for all measured planes. For instance, the (111) peak intensity increases from 1200 a.u. in DR to 1250 a.u. in SR. Similar increases are observed in other peaks.

      Overall, the lower FWHM values and higher peak intensities in SR samples confirm that the solution doping method results in improved structural quality and phase purity compared to the dry method.

   2. Electronic Properties: XANES and Electrical Conductivity

      Table 5.5. XANES Spectra Analysis for DR and SR Doubly Doped Vale NiO (1)

      Sample	Edge Energy (Eo) (eV)	Pre-Edge Feature (eV)	Post-Edge Oscillations (eV)
      DR	8979.3	8975.2	8990.5
      SR	8980.1	8976.1	8991.2

      Table 5.6. XANES Spectra Analysis for DR and SR Doubly Doped Vale NiO (2)

      Sample	Peak Position (eV)	Peak Intensity (a.u.)	Pre-Edge Peak (eV)	Post-Edge Peak (eV)
      DR	8979.3	1.10	8975.2	8990.5
      SR	8980.1	1.12	8976.1	8991.2

      The XANES results presented in Tables 5.5 and 5.6 provide important information about the electronic structure and oxidation states of the elements present in the doubly doped NiO samples. XANES is a useful technique for understanding how dopant atoms interact with the host material and how the local electronic environment changes after doping.

      From the data, the edge energy (E) values of the DR and SR samples are very close to each other. The DR sample shows an edge energy of 8979.3 eV, while the SR sample shows a slightly higher value of 8980.1 eV. This small shift toward higher energy in the SR sample may indicate a slight change in the oxidation state or a stronger interaction between the metal ions and the oxygen atoms in the lattice. Even a small shift in edge energy can suggest differences in electron density around the absorbing atom.

      The pre-edge features are also slightly higher in the SR sample (8976.1 eV) compared to the DR sample (8975.2 eV). Pre-edge peaks are generally related to electronic transitions and provide information about the symmetry and coordination of metal ions. A higher pre-edge value in the SR sample may suggest better hybridization between metal 3d and oxygen 2p orbitals.

      Similarly, the post-edge oscillations and peak intensities are marginally higher in the SR sample. The SR sample shows a peak intensity of 1.12 a.u., compared to 1.10 a.u. for the DR sample. This increase in intensity may indicate improved structural order and stronger bonding interactions. Overall, the XANES results suggest that the solution-doped (SR) sample has slightly stronger electronic interaction and better dopant incorporation compared to the dry-doped (DR) sample.

      Table 4.7. Electrical Conductivity Measurements

      Sample	Temperature (K)	Conductivity (S/m)
      DR	300	0.20
      DR	500	0.25
      SR	300	0.22
      SR	500	0.27

      The electrical conductivity data shown in Table 4.7 provide useful information about how the doping method affects the charge transport properties of the CoFe doped NiO samples. Conductivity was measured at two different temperatures, 300 K (room temperature) and 500 K, in order to observe how temperature influences electrical behavior.

      At 300 K, the DR sample shows a conductivity value of 0.20 S/m, while the SR sample exhibits a slightly higher value of 0.22 S/m. When the temperature increases to 500 K, the conductivity of both samples also increases. The DR sample reaches 0.25 S/m, and the SR sample reaches 0.27 S/m. This increase in conductivity with temperature suggests semiconducting behavior, where higher temperature provides more energy for charge carriers to move more freely within the material.

      The consistently higher conductivity values observed for the SR sample indicate that the solution-based doping method improves charge transport compared to the dry method. This improvement may be due to a more uniform distribution of cobalt and iron ions within the NiO lattice. Better dopant dispersion can reduce scattering of charge carriers and create more effective pathways for electrical conduction. In addition, improved crystallinity in the SR samples may contribute to lower grain boundary resistance.

      Overall, the results show that both samples demonstrate stable and temperature-dependent conductivity, but the solution-doped sample performs slightly better. This makes the SR method more suitable for applications requiring enhanced electrical performance.

   3. Optical Properties: UV-Visible Spectroscopy and Photoluminescence

      Table 5.8. UV-Visible Spectroscopy Bandgap Analysis (1)

      Sample	Bandgap Energy (eV)	Absorbance Maximum (a.u.)	Transmittance (%)
      DR	3.10	1.25	85
      SR	3.12	1.30	83

      Table 5.9. UV-Visible Absorption Peak Analysis

      Sample	Wavelength (nm)	Absorbance (a.u.)	Energy (eV)
      DR	400	0.80	3.10
      DR	500	0.60	2.48
      SR	400	0.85	3.12
      SR	500	0.65	2.50

      The UV-visible spectroscopy results presented in Tables 5.8 and 5.9 provide information about the optical properties of the CoFe doped NiO samples. The bandgap energy is an important parameter because it determines how the material absorbs light and how it can be used in optical and electronic devices.

      From Table 5.8, the bandgap energy of the DR sample is 3.10 eV, while the SR sample shows a slightly higher value of 3.12 eV. The difference between the two values is very small, indicating that the doping method does not significantly change the fundamental band structure of NiO. These bandgap values are close to the typical range reported for NiO, confirming that the material maintains its semiconducting nature after doping with Co and Fe. The absorbance maximum is slightly higher in the SR sample (1.30 a.u.) compared to the DR sample (1.25 a.u.), suggesting that the SR sample absorbs light a little more effectively. The transmittance values are 85% for DR and 83% for SR, which indicates that both samples are relatively transparent in the measured range.

      Table 5.9 shows absorption peaks at 400 nm and 500 nm for both samples. The SR sample again shows slightly higher absorbance values at these wavelengths. This small increase in absorption may be due to improved dopant distribution and minor changes in defect levels. Overall, both samples show similar optical behavior, with only slight improvements in the SR sample.

      Photoluminescence (PL):

      Table 5.10. Photoluminescence Peak Analysis (1)

      Sample	Peak Position (nm)	Emission Intensity (a.u.)
      DR	490	0.85
      DR	525</>	0.90
      SR	492	0.87
      SR	528	0.93

      Table 5.11. Photoluminescence Peak Analysis (2)

      Sample	Excitation Wavelength (nm)	Emission Wavelength (nm)	Intensity (a.u.)
      DR	350	490	0.85
      DR	350	525	0.90
      SR	350	492	0.87
      SR	350	528	0.93

      The photoluminescence (PL) results shown in Tables 5.10 and 5.11 provide useful information about the electronic transitions and defect states present in the CoFe doped NiO samples. PL spectroscopy helps in understanding how electrons and holes recombine after excitation and how defects influence light emission.

      Both DR and SR samples were analyzed using an excitation wavelength of 350 nm. The DR sample shows emission peaks at 490 nm and 525 nm with intensities of 0.85 and 0.90 a.u., respectively. In comparison, the SR sample shows emission peaks at 492 nm and 528 nm with slightly higher intensities of 0.87 and 0.93 a.u. The emission peak positions for both samples are very close, which indicates that the fundamental emission mechanism remains similar in both cases.

      However, the small increase in emission intensity for the SR sample suggests improved luminescence behavior. Higher PL intensity generally indicates more effective radiative recombination of charge carriers. This improvement may be due to better dopant distribution and enhanced crystallinity in the solution-doped sample. When dopants are more uniformly incorporated into the NiO lattice, defect levels can be better controlled, leading to stronger emission.

      The slight shift in emission peak position from 490 nm to 492 nm and from 525 nm to 528 nm in the SR sample may be related to minor changes in defect states or local electronic structure. These small shifts suggest that the solution method slightly modifies the energy levels within the bandgap. Overall, the PL results show that both samples exhibit stable and strong emission, but the SR sample demonstrates marginally better luminescence performance.

   4. Stability and Long-Term Performance

      Table 5.12. Environmental Stability Tests

      Sample	Condition	Measurement	Value
      DR	Room Temp	Bandgap (eV)	3.10
      DR	High Temp	Bandgap (eV)	3.05
      SR	Room Temp	Bandgap (eV)	3.12
      SR	High Temp	Bandgap (eV)	3.08

      Table 5.13. Long-term Stability Tests

      Sample	Time (months)	Bandgap Energy (eV)	Peak Intensity (a.u.)
      DR	0	3.10	1.25
      DR	6	3.08	1.22
      SR	0	3.12	1.30
      SR	6	3.10	1.28

      The environmental and long-term stability results shown in Tables 5.12 and 5.13 help in understanding how stable the CoFe doped NiO samples are under different conditions. Stability is very important for practical applications, especially in energy storage devices and optoelectronic systems, where materials are often exposed to heat and long operating times.

      From Table 5.12, it can be seen that both DR and SR samples show a small decrease in bandgap energy when exposed to high temperature. The DR sample shows a reduction from 3.10 eV at room temperature to 3.05 eV at high temperature. Similarly, the SR sample shows a decrease from 3.12 eV to 3.08 eV. These small changes suggest that temperature slightly affects the electronic structure of the material. However, the decrease is not very large, which indicates that both samples maintain good thermal stability. The SR sample still shows a slightly higher bandgap than the DR sample, even at high temperature, suggesting better structural resistance to thermal effects.

      Table 5.13 presents the long-term stability results measured over six months. For the DR sample, the bandgap decreases slightly from 3.10 eV to 3.08 eV, and the peak intensity decreases from 1.25 to 1.22 a.u. For the SR sample, the bandgap reduces from 3.12 eV to 3.10 eV, and the peak intensity changes from 1.30 to 1.28 a.u. These minor variations indicate that both samples maintain stable optical properties over time. Overall, the results confirm that both doping methods produce materials with good environmental and long-term stability, while the SR sample shows slightly better resistance to changes.

6. CONCLUSION

In the present study, researcher systematically investigated the structural, optical, and electronic properties of CoFe doubly doped NiO synthesized using two different approaches: dry doping (DR) and solution-based doping (SR). The findings clearly show that the incorporation of cobalt and iron significantly influences the overall characteristics of NiO, and that the synthesis route plays an important role in determining the final material performance.

XRD analysis confirmed that both DR and SR samples retained the cubic crystal structure of NiO without the formation of unwanted secondary phases. However, the SR samples exhibited slightly narrower diffraction peaks and marginally higher lattice constants, indicating improved crystallinity and more uniform dopant incorporation. These observations suggest that the solution-based method enables better mixing of precursor ions at the molecular level, resulting in enhanced structural homogeneity (Nair & Tiwari, 2021; Ma et al., 2021). Improved crystallinity is beneficial for charge transport and long-term device stability.

The XANES results revealed subtle differences in edge energy and electronic structure between the two sets of samples. The SR samples showed slightly higher edge energies and stronger spectral features, indicating improved interaction between the dopant ions and the NiO lattice. Such modifications in the electronic environment are known to enhance conductivity and catalytic activity in transition metal-doped NiO systems (Zhang et al., 2020; Li et al., 2018). Electrical conductivity measurements further supported this observation, as SR samples consistently exhibited higher conductivity values at both room temperature and elevated temperatures. This improvement can be attributed to better dopant dispersion and more efficient charge carrier pathways.

Optical studies using UV-visible spectroscopy demonstrated that both DR and SR samples maintained bandgap energies close to that of pure NiO, with only slight variations. This indicates that co-doping with Co and Fe does not drastically alter the fundamental band structure but subtly modifies defect states and electronic transitions. Photoluminescence (PL) analysis showed that SR samples had slightly higher emission intensities compared to DR samples. Enhanced PL intensity suggests improved radiative recombination processes and controlled defect formation, which are important for optoelectronic applications (Dutta & Chatterjee, 2020).

Environmental and long-term stability tests revealed that both samples retained their structural and optical properties with minimal changes over time. Although minor reductions in bandgap and peak intensity were observed at elevated temperatures ad after prolonged exposure, the variations were small. This confirms that CoFe co-doped NiO possesses good thermal and environmental stability, which is essential for practical applications in energy storage and sensing devices (Gupta & Singh, 2019).

Overall, the comparative study indicates that while both doping methods successfully enhance the properties of NiO, the solution- based approach offers slight but consistent improvements in crystallinity, conductivity, and luminescence behavior. Therefore, solution doping appears to be a more effective and controlled technique for tailoring the multifunctional properties of NiO. The promising stability, improved electronic characteristics, and favorable optical response make CoFe doubly doped NiO a strong candidate for advanced applications in energy storage systems, supercapacitors, sensors, and optoelectronic devices.

REFERENCES

1. Chou, H., & Li, X. (2017). Synthesis of NiO thin films for energy storage applications: Impact of doping. Journal of Power Sources, 340, 176185. https://doi.org/10.1016/j.jpowsour.2016.11.057
2. Dutta, A., & Chatterjee, S. (2020). Effects of cobalt and iron doping on the structural, optical, and electrochemical properties of NiO. Journal of Materials Science, 55(18), 83048317. https://doi.org/10.1007/s10853-020-04672-w
3. Gupta, P., & Singh, R. (2019). Structural and electrochemical performance of Co and Fe doped NiO as anode materials for lithium-ion batteries. Electrochimica Acta, 295, 7482. https://doi.org/10.1016/j.electacta.2018.11.030
4. Kumar, R., & Verma, S. (2019). Fabrication and performance analysis of NiO electrodes with Co and Fe doping for supercapacitor applications. Materials Science and Engineering: B, 256, 29-35. https://doi.org/10.1016/j.mseb.2019.03.013
5. Li, Y., Zhang, H., & Wang, F. (2018). The role of cobalt and iron in enhancing the electrochemical performance of nickel oxide electrodes. Journal of Solid State Electrochemistry, 22(9), 30153023. https://doi.org/10.1007/s10008-018-4173-9
6. Ma, S., Liu, Y., & Lee, J. (2021). Investigation of NiO-based materials for energy storage applications: Co and Fe doping effects. Materials Chemistry and Physics, 254, 123-130. https://doi.org/10.1016/j.matchemphys.2020.123130
7. Nair, A. K., & Tiwari, R. (2021). Comparative study of solution-based and solid-state methods for doping NiO with Co and Fe: Structural and electrochemical properties. Electrochimica Acta, 377, 138146. https://doi.org/10.1016/j.electacta.2021.138146
8. Patel, R., & Soni, P. (2019). Electrochemical properties of Co-Fe doped NiO nanostructures as supercapacitor electrodes. Journal of Nanoscience and Nanotechnology, 19(8), 50445051. https://doi.org/10.1166/jnn.2019.16697
9. Raj, R., & Ghosh, S. (2022). Synthesis and characterization of NiO-based mixed-metal oxide electrodes for supercapacitor applications. Journal of Electroanalytical Chemistry, 877, 115122. https://doi.org/10.1016/j.jelechem.2021.115122
10. Zhang, J., Chen, Z., & Liu, L. (2020). Influence of cobalt and iron doping on the electrochemical performance of nickel oxide. Electrochemical Energy Reviews, 3(4), 259267. https://doi.org/10.1007/s41918-019-00100-7