Structural, optical and magnetic properties of Cu doped CeO2 nano powders synthesized by solid state reaction

In this research work, the structural, optical, and magnetic properties of Ce1−xCuxO nanoparticles have been reported. Solid state reaction technique was employed to synthesize the nanoparticle samples. The synthesized materials were analysed structural properties from x-ray diffraction (XRD) observed that the crystallite size increased from16.9 nm to 40.43 nm and the lattice constant increased with increase of Cu doping concentration, and the structural properties after doping confirm the absence of other phase and synthesized samples had a cubic structure. Using SEM and EDAX, the nanoparticles surface morphology and elemental composition in investigated. The nanoparticles have a spherical shape and approximately the expected stoichiometric ratio. The optical band gap of copper doped cerium oxide nanoparticles is calculated and found the band gap decreased with increased dopant concentration. Ce1−xCuxO nanoparticles using photoluminescence (PL) analysis at excitation wavelengths of 350 nm revealed emissions of the violet, blue, green, and red regions within the range of 350–700 nm. Vibrating sample magnetometer was found that the samples were room temperature ferromagnetic in nature and strength of magnetic moment increased with increase of doping concentration.


Introduction
Dilute magnetic semiconductors (DMS) are traditional devices that exhibit magnetic properties at room temperature as a result of inherent imperfections or the incorporation of foreign impurities.These materials function by leveraging the magnetic attribute of the charge carrier, which is principally determined by the spin of the electrons.The integration of electron manipulation and revolution plays a pivotal role in the functionality of spintronic devices.Proposed the existence of an additional degree of freedom associated with the electron spin in electrical systems.The field of spintronic holds promise in the development of future device applications that include the manipulation and control of electron spin.The remarkable physical characteristics of DMS materials have sparked significant interest in the development of novel multifunctional technologies.There has been significant interest in the potential utilization of high Curie temperature (Tc) diluted magnetic semiconductors (DMSs) materials in forthcoming spintronic devices.The utilization of DMS materials allows for the incorporation of the responsibility property in material treatment, hence facilitating instantaneous information storage through the rotating state.The scientific community has shown great interest in semiconductors with nano form due to their unique applications, which are attributed to their innovative assets and significant surface-to-volume ratio compared to their bulk counterparts.These applications include LED technology, bioelectronics, and anti-reflective coatings, as well as the development of laser and nonlinear optical devices [1][2][3][4].Several oxide materials, such as zinc oxide (ZnO) [5], tin dioxide (SnO 2 ) [6], and titanium dioxide (TiO 2 ) [7].CeO 2 for photo catalysts, pigments, sensors, refractories, electrode materials, and lubricant additives [8].Due to its high oxygen ion impedance, which is determined by the oxidation state parameter, CeO 2 has found extensive applications in various fields such as photo catalysis [9][10][11][12], optoelectronic devices [13][14][15][16], air filtration [17], fuel cells [18], solid electrolyte layers [19], electrical and chemical oxygen pumps [20], ultraviolet equipment [21], hydrogen storage materials [22], reduction of nitrogen oxides [23], fluorescence materials [24], and aerometric oxygen ion monitors.This is attributed to the significant conductance of oxide ions in ceria with different transition metals in the +3 and +4 oxidation states [25][26][27].The photocatalytic activity of cerium oxide is limited to UV light energy due to its indirect and wide band gap of 3.2 eV [13].To enhance the photocatalytic performance of materials under illumination, it is necessary to consider various factors.One significant approach involves modifying materials such as N, Sm, In, Y, Fe, Yb, Er, and Cu [28,29].Consequently, the introduction of Cu-doped CeO 2 leads to a notable alteration in its behaviour, the which can be achieved through different synthesis methods including hydrothermal [30,31], precipitation [32], and microwave [33].Sumalin Phokha et al [34] reported, Cr doped CeO 2 nanoparticles for room temperature ferromagnetism at 300 K. Al-Agel et al [35] reported that Mn doped CeO 2 , at x-0.08 concentration was found ferromagnetism behaviour.Alla et al [36] reported that, Cu doped CeO 2 nanostructures prepared by microwave refluxing technique and found ferromagnetism at room temperature.Al-Age et al [37] reported that ferromagnetism for Cu doped CeO 2 nano powders by using solvothermal method.
The present work, this is the first time to synthesize Cu doped CeO 2 (x = 0.02.0.05, 0.07, 009 and 0.11) nanopowder samples by solid state reaction method.Investigates the structural, optical and magnetic properties at room temperature.Each of these investigation was systematically arranged, physically displayed using graphs and further elaborated upon in the following sections and these substances would be suitable for use in spintronic applications.

Experimental details
The copper substituted cerium oxide nanoparticles (Ce 1−x Cu x O) at X = 0.02, 0.05, 0.07, 0.09 and 0.11 were prepared by a solid state reaction.High purity CeO 2 and CuO nanoparticles were procured from (M/S Sigma-Aldrich, 99.9999%) company.The powders were used as starting material for synthesis.According to the solid state reaction process, first clean agate mortar and pestle with help of acetone.Then weigh the powder and they are thoroughly grounded in an agate mortar and pestle to create extremely fine powders.For each Cu dopedCeO 2 powder sample, the mixture was grinded for 17 h.The prepared samples were heated in the furnace at a temperature of 950 °C for a period of 10 h.After sintering the samples were again thoroughly grinded for three hours each sample before characterizations.The samples can be acquired for analysis of their structural, optical, and magnetic properties.This analysis can be conducted using x-ray diffraction (XRD) with the utilization of the Rigaku-Miniflex-600 instrument.The surface morphology was investigated by the utilization of a scanning electron microscope (SEM) model JSM-IT 500.The reflectance and optical band gap were carried out by the UVvis NIR spectrometer (Perkin Lambda 365).The HORIBA FL-1000 photoluminescence technique is employed for the identification and characterization of material defects and impurities.The magnetic measurements were conducted using a Vibrating Sample Magnetometer (VSM), namely the Lake Shore-7410 model.

XRD studies
The x-ray diffraction technique was employed to analyse Ce 1−x Cu x O nanoparticles with varying concentrations of Cu (x = 0.02, 0.05, 0.07, 0.09, and 0.11).From figure 1 in the range of 20°to 90°, all the diffraction peaks were exactly matched with the cubic structure of cerium oxide [JCPDS Card No: 34-0394].From the CuO having JCPDS Card No 01-80-1916, some of the Cu elemental peaks like 32.70°, 47.12°, 56.01°, 58.73, 69.13°, 76.38°, 78.78°etc are matched with prepared copped doped cerium oxide samples.To substantiate this hypothesis, the average size of the crystallites in Ce 1−x Cu x O nanoparticles was determined using the utilization of the Debye-Scherrer formula.The study observed that the introduction of Cu doping led to a slight increase in the crystallite size, ranging from 16.96 nm to 40.43 nm.This phenomenon can be attributed to the expansion of the lattice constant resulting from the substitution of copper ions into the sites occupied by cerium ions.This behaviour is consistent with previous reports on the growth of CeO 2 when doped with Ni, suggesting a similar mechanism for Cu-doped CeO 2 [38,39].The equation provided was utilized to determine the lattice parameters of the given sample.
Where, d-Interplanar spacing, λ -wavelength of the radiation (λ = 1.5406Å), θ -angle of diffraction, and alattice parameters  Figure 3 illustrates the relationship between the concentration of Cu in CeO 2 , ranging from 0.02 to 0.11, and the corresponding variations in crystallite size (measured in nanometre) and full width at half maximum (FWHM).The presence of Cu [40] increased the crystallite size of pure CeO 2 , with values ranging from 16.96 nm  to 40.43 nm.Moreover, following Vegard's law [41], the alteration of lattice characteristics exhibits a linear relationship with the augmentation of Cu concentrations.Dislocation density, as an additional crystallographic parameter, is a contributing factor to the characterization of crystal structure [42].It is incorporated into the relevant equation for analysis.
where, δ is a dislocation density, D is a crystallite size.The parameters are lattice constant, crystallite size, dislocation density and optical band gap, as mentioned in table 1.

Surface morphological studies
Figure 4 displays the surface morphology spectrum of the materials examined using scanning electron microscopy (SEM).The scanning electron microscopy (SEM) image reveals the porous and uneven morphology of Ce 1−x Cu x O, which presents challenges in accurately estimating crystal size due to the presence of particle aggregation.The study investigates the incorporation of copper (Cu) into cerium dioxide (ceria) at varying concentrations of 2, 5, 7, 9, and 11 at.%).The wavelengths of interest for this investigation are 34.423nm, 34.145 nm, 38.423 nm, 40.256 nm, and 40.437 nm.

Chemical composition studies
The chemical composition of the nanopowder obtained is depicted in figure 5, as demonstrated in the inset.Based on the data presented in the figure, it can be observed that the dopant CeO 2 exhibited a larger crystallite size, as confirmed through x-ray diffraction (XRD) analysis.The dimensions of the sample powder crystallites exhibit a positive correlation with the level of doping concentration.The Ce 1−x Cu x O powders exhibit a nearly spherical morphology and display a uniform distribution.The x-ray diffraction (XRD) investigation revealed that the constituents of cerium (Ce), copper (Cu), and oxygen (O) were present in appropriate proportions, and additionally, novel elements were detected.

Optical studies
Where 'R' is the observed diffuse reflectance in UV-vis NIR spectra.Figure 7 displays the optical spectrum gap (Eg) of Cu-doped CeO 2 powders, illustrating the variations observed at different degrees of Cu concentration.The value was obtained by plotting the square of the product of the fine structure constant (α) and the Planck constant (h) multiplied by the frequency (υ) of the photons, against the energy (hυ) of the photons.This was done after extrapolating the linear region where the fine structure constant (α) is equal to zero.The optical band gap was computed using Tauc's figure, as described by equation [44].

Photoluminescence studies
The photoluminescence property of CeO 2 doped with copper (Cu) is of particular significance due to its utilization in laser light-emitting diodes, optical sensors, and various other applications.Figure 8 shows that the  photoluminescence spectra of cerium oxide nanoparticles doped with copper were acquired within the wavelength range of 350 to 700 nm.The image illustrates the photoluminescence spectra of cerium oxide nanoparticles doped with copper.The nanoparticles of CeO 2 doped with a foreign element and the Cu-doped CeO 2 exhibit significant emission peaks at wavelengths of 396 nm and 496 nm, respectively.The peak intensity of Cu-doped CeO 2 nanoparticles is moderated.The emission peak observed at a wavelength of 350 nm, which is near the band gap of CeO 2 , is believed to be caused by radiative transitions near the band edge that are formed by light.This finding has been documented in reference [45].The photoluminescence (PL) spectra indicate that the undoped indium tin oxide (ITO) nanoparticles exhibited significantly higher emission peaks at wavelengths of 396 and 496 nm, where the Cu doped CeO 2 exhibited defect related photoluminescence emission peaks at 396.68 nm and 496.30nm with reduced intensities.Hence, the emission bands violet, blue, green, and red regions are attributed to crystalline defect such as surface defect and oxygen vacancies [46,47].In contrast, the Ce 1−x Cu x O material had photoluminescence radiation peaks associated with defects, characterized by reduced intensities was observed at wavelengths of 425 nm to 700 nm.The presence of defects has been seen to impact  the structural and functional aspects of Ceria nanoparticles, including their ability to facilitate oxygen transport, catalytic activity, energy storage capabilities, and physical characteristics, among others, author reports the Mndoped CeO 2 samples exhibit a notable emission of fluorescence with peak wavelengths at 490 and 520 nm, which can be attributed to defects mediated by oxygen.Defects such as oxygen vacancies are frequently ascribed as the underlying cause of deep-level emissions observed in various oxide materials [48].The observed phenomenon can perhaps be attributed to the presence of oxygen vacancies inside films with electrical energy levels that are situated below the Ce 4 f band.Alternatively, it may arise from a transition occurring between certain localized states located between the band gap and the valence band.

Magnetic properties
The investigation focuses on the magnetic characteristics of samples x = 0.02, 0.05, 0.07, 0.09, and 0.11, and pure CeO 2 under the influence of a suitable magnetic field (H) of 1.5 T. The Vibrating sample magnetometer (VSM) approach is employed in ambient conditions.Figure 9(a) depicts the magnetization against applied magnetic field (M-H) curves for all the samples.Diamagnetism is exhibited by pure CeO 2 .All synthesized samples have a display of ferromagnetism that is rather weak.Copper (Cu) can exist in two distinct oxidation states, namely Cu + and Cu 2+ .The Cu + ion possesses a substantial 3d 10 electron configuration with no unpaired electrons, whereas the Cu 2+ ion exhibits an electronic arrangement of 3d 9 , characterized by the presence of one unpaired electron per Cu 2+ ion.The introduction of copper (Cu) doping leads to the creation of oxygen vacancies inside the samples.These vacancies then interact with unpaired electron spins, ultimately resulting in the manifestation of paramagnetic activity [49].[50].
The presence of surface imperfections and oxygen vacancies that occur during the synthesis procedure can be attributed to the observed weak ferromagnetic properties in Cu-doped CeO 2 when exposed to an external magnetic field [51].This suggests that the presence of Cu inclusions leads to a greater alignment of particles along the direction of the applied field.The presence of a modest ferromagnetic component is shown by the linear increase in magnetization of doped CeO 2 nanoparticles [52].Prior studies have indicated that an elevated concentration of dopants or significant strain can induce ferromagnetism in nanomaterials [53].The saturation of magnetic properties can be impeded by nanoparticles that experience strain at their surface, leading to an increase in magnetic anisotropy and spin canting [54].The nanopowder samples, namely x = 0.02, x = 0.05, x = 0.07, x = 0.09, and x = 0.11, exhibit a progressive increase in both coercivity and retentivity.There may be a correlation between the exchange coupling of electron spins confined within oxygen vacancies, predominantly located on nanoparticle surfaces.The potential relationship between the obtained coercivity of a remnant and exchange coupling is worth investigating.The values for y and retentivity can be found in table 2.  Figure 9(b) shows the field dependent magnetization of Ce 1−x Cu x O nanoparticles x = 0.00, 0.02, 0.05, 0.07, 0.09 and 0.11.Under the influence of magnetic field range of +2000 Oe.The enhancement of magnetic moment on increasing the dopant concentration has been observed from the figure shown above.Remarkably, all the samples report high quality of ferromagnetism.Also the rate of increase in the enhancement of different magnetic parameters varies linearly in all Cu-doped CeO 2 with concentration of Cu.When compared to the findings published in earlier papers for both transition metal-doped and pure CeO 2 nanoparticles, even the observed Ms Values are improved.The coercivity of the undoped CeO 2 matrix was found to be low at 55.99 Oe, whereas the coercivity of all synthesised samples increased from x = 0.02 to x = 0.11 were mentioned in table 2.

Conclusions
Ce 1−x Cu x O (x = 0.02, 0.05, 0.07, 0.09, and 0.11) nanoparticles were synthesized solid state reaction method and studied the impact structural, optical, and magnetic properties of the prepared samples.The x-ray diffraction studies revealed that samples of all compositions were crystallized in cerium oxide cubic structure.The surface morphological images confirm that the increase in the dopant concentration leads to an increase in the diameter of the nanoparticles.EDAX spectra confirmed that Ce 1−x Cu x O were properly doped in pure lattice and expected near atomic ratio was formed.From optical studies it was found that the optical band gap increased with increase of Cu content.The photoluminescence spectra of copper doped cerium oxide samples exhibited the same emission peak position and PL emission intensities increased due to the dopant concentration.The Cu doped CeO 2 samples exhibit room temperature ferromagnetism.An increase in saturation magnetic moment was observed in the samples with increase of Cu doping concentration which was confirmed by magnetic measurements.

Figure 1
Figure 1 shows that the x-ray diffraction (XRD) spectra of Ce 1−x Cu x O do not exhibit distinct metallic peaks, suggesting the formation of a homogeneous compound.Figure 2 shwos the XRD patterns of pure and Cu doped CeO2 nanocrystalline powder in the angle of 26°to 31.5°.When depicted graphically, it was observed that with an increase in doping concentration, the diffraction peak of the powder sample migrated towards a lower angle.The x-ray diffraction (XRD) pattern of the annealed sample of Ce 1−x Cu x O nanoparticles revealed peaks at specific angles, namely 28.24°, 32.77°, 47.19°, 56.06°, 58.81°, 69.15°, 76.45°, 78.82°, and 88.21°.These peaks corresponded to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0), and (4 2 2), respectively.The increased intensity of the diffraction peaks may be attributed to the hindrance of CeO 2 nanoparticle development.The intensity of the diffraction peaks could potentially be influenced by the incorporation of CeO 2 nanoparticles during growth.Figure3illustrates the relationship between the concentration of Cu in CeO 2 , ranging from 0.02 to 0.11, and the corresponding variations in crystallite size (measured in nanometre) and full width at half maximum (FWHM).The presence of Cu[40] increased the crystallite size of pure CeO 2 , with values ranging from 16.96 nm Figure 1 shows that the x-ray diffraction (XRD) spectra of Ce 1−x Cu x O do not exhibit distinct metallic peaks, suggesting the formation of a homogeneous compound.Figure 2 shwos the XRD patterns of pure and Cu doped CeO2 nanocrystalline powder in the angle of 26°to 31.5°.When depicted graphically, it was observed that with an increase in doping concentration, the diffraction peak of the powder sample migrated towards a lower angle.The x-ray diffraction (XRD) pattern of the annealed sample of Ce 1−x Cu x O nanoparticles revealed peaks at specific angles, namely 28.24°, 32.77°, 47.19°, 56.06°, 58.81°, 69.15°, 76.45°, 78.82°, and 88.21°.These peaks corresponded to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0), and (4 2 2), respectively.The increased intensity of the diffraction peaks may be attributed to the hindrance of CeO 2 nanoparticle development.The intensity of the diffraction peaks could potentially be influenced by the incorporation of CeO 2 nanoparticles during growth.Figure3illustrates the relationship between the concentration of Cu in CeO 2 , ranging from 0.02 to 0.11, and the corresponding variations in crystallite size (measured in nanometre) and full width at half maximum (FWHM).The presence of Cu[40] increased the crystallite size of pure CeO 2 , with values ranging from 16.96 nm

Figure 6
Figure6illustrates the diffused reflectance spectra of undoped and copper-doped cerium oxide (CeO 2 ) nanoparticles within the wavelength range of 300 to 800 nm.The optical reflectance of Ce 1-x Cu x O decreased as the dopant concentration was increased from 0.02 to 0.11.The reflectance for the band gap can be determined through the utilization of the Kubelka-Munk function, as stated in equation[43]

Figure 8 .
Figure 8. PL spectra of pure and Cu-doped CeO 2 nanocrystalline powder samples with different dopant content.

Figure 9 .
Figure 9. (a) M-H plots of pure and Cu doped CeO 2 nanoparticles in the field range of +15000 Oe to −15000 Oe (b) in the field range of −2000 to +2000 Oe.