ENHANCEMENTS OF STRUCTURAL AND OPTICAL PROPERTIES OF MgO: SnO 2 NANOSTRUCTURE FILMS †

This study uses the chemical precipitation method to investigate the structural and optical properties of MgO:SnO 2 nanoparticles. The thin films were deposited by the spin coating technique on glass substrates. X-ray diffraction analysis proved the crystalline structure of prepared thin films, with the peaks corresponding to the (110), (101), (200), (211), and (220) planes, with the tetragonal SnO 2 crystal structure. Fourier transforms infrared (FTIR), and scanning electron microscope (SEM) are used to characterize the functional groups, shape, and dimensions of synthesized metal oxide nanoparticles. The optical properties of the films were studied by UV-Vis spectroscopy. The bandgap energy was estimated to be in the range of (3.9-3.4 eV). The refractive index and extinction coefficient of the films were also determined, and the results indicated that the films had good transparency in the visible region. The study concludes that MgO:SnO 2 thin films obtained by the spin coating technique have potential applications in optoelectronics and gas sensors.


INTRODUCTION
MgO:SnO 2 nanoparticles are composite particles consisting of magnesium oxide (MgO) and tin dioxide (SnO 2 ) in the form of nanoparticles.These nanoparticles can be synthesized using various methods, including sol-gel [1,2], coprecipitation [3], and hydrothermal methods [4].MgO:SnO 2 nanoparticles have attracted significant attention due to their unique properties, such as high surface area, high reactivity, and good stability [5].These properties make them suitable for various applications, including gas sensing, catalysis, energy storage, and biomedical applications.One of the main applications of MgO:SnO 2 nanoparticles is in gas sensing [6].These nanoparticles have been shown to exhibit excellent sensitivity and selectivity towards various gases, such as CO, NO 2 , and H 2 [7].The high surface area of the nanoparticles provides a large surface area for gas adsorption, while the SnO 2 component provides a high catalytic activity for gas oxidation .In addition, MgO:SnO 2 nanoparticles have been investigated for their catalytic activity in various reactions, such as CO oxidation, methanol synthesis, and photocatalysis [8].The unique properties of the nanoparticles, such as their size and composition, can be tuned to optimize their catalytic activity [9].MgO:SnO 2 nanoparticles also show promise in energy storage applications, such as in lithium-ion batteries [10,11].The nanoparticles can be used as anode materials due to their high lithium-ion storage capacity and good cycling stability .Furthermore, MgO:SnO 2 nanoparticles have potential biomedical applications, such as in drug delivery and cancer therapy [12].The nanoparticles can be functionalized with various ligands and drugs to target specific cells or tissues, and their high stability and biocompatibility make them suitable for in vivo applications .Overall, MgO:SnO 2 nanoparticles have a wide range of potential applications due to their unique properties and can be synthesized using various methods [13,14] .
Spin coating is a popular method for depositing thin films, especially for research and development purposes, due to its ease of use, low cost, and compatibility with various materials.In this method, a liquid solution containing the desired precursor materials is first applied onto a spinning substrate, such as a glass or silicon wafer [15,16].The centrifugal force generated by the spinning causes the solution to spread uniformly across the substrate and evaporate, leaving behind a thin film on the surface [17] .MgO:SnO 2 thin films obtained by spin coating have potential applications in various fields, including optoelectronics [18], and gas sensors [19].The properties of the thin films, such as their thickness, composition, and morphology, can be tuned by adjusting the concentration and ratio of the precursor materials in the solution and the spin-coating parameters, such as spin speed, spin time, and temperature [20].
The study aimed to characterize MgO:SnO 2 by Co-Precipitation method and prepares thin films by spin coating and study the structural properties of the nanoparticles using XRD analysis, determine the surface morphology using SEM, investigate the optical properties of the thin films using UV-Vis spectroscopy, and study the properties of MgO:SnO 2 thin films obtained by spin coating.3. Procedure .Weigh out the desired amounts of Magnesium chloride and tin (IV) chloride and dissolve them in deionized water separately, Slowly add the Magnesium chloride solution to the tin chloride solution while stirring continuously with a magnetic stirrer with percentages MgO 1-x : SnO 2 x (x=0.2,0.3,0.4,0.5),adding Ammonium hydroxide solution to the mixture until the pH reaches around 9, The mixture will turn white and a precipitate will form, Continue stirring the mixture for another 2-3 hours to ensure complete precipitation of the nanoparticles, Centrifuge the mixture at a high speed (around 10,000 rpm) for 15-20 minutes to collect the precipitate, Washing the precipitate with deionized water to remove any impurities and residual reactants.Finally Drying the nanoparticles in an oven at a low temperature (around 80°C) for several hours until all the solvent is evaporated and the nanoparticles are fully dried and Calcination at 800°C.

EXPERIMENTAL PART
The MgO 1-x : SnO 2 x (x=0.2,0.3,0.4,0.5),thin films were prepared by the addition (the MgO 1-x : SnO 2 nanoparticles dissolved in ethylene glycol with a mass ratio (0.1:10) at room temperature), the MgO, SnO 2 , and MgO:SnO 2 concentration was 0.1 g.Glass substrates were successively cleaned with acetone, ethanol, and deionized water.MgO, SnO 2 , and MgO:SnO 2 thin films were deposited on the glass substrates by spin coating at room temperature with a rate of 2000 rpm for 30 s, the spin-coating step, the films were heated on a hot plate at 30-50 °C in the air for 10 min to remove organic contaminations.
The crystalline sizes of the pure MgO, SnO 2 and different ratio (50, 60, 70 and 80 wt.%) of Mg ions doped SnO 2 were calculated depending on the highest XRD peak using Scherrer's equation [23], to be (24.5 nm), (35.42 nm), (19.89 nm), (18.97 nm), (14.55 nm), (14.11 nm) respectively.The results indicated a clear decrease in crystalline size with the doping Mg ions ratio, as a result of the difference in the ion size of Sn and Mg ions [24].
The samples have been investigated using the FTIR test to determine the functional groups within the prepared materials.The FTIR spectra of MgO, SnO 2 , and MgO:SnO 2 powder in the wavenumber range (500-4000 cm -1 ) are shown in Fig. 8  Figure 3 presents the results of Scanning Electronic Microscopy images and particle size distributions of prepared samples.The synthesized nanoparticles exhibited a semi-spherical nanoparticle of all samples, as shown in Fig. 3 (a,b,c,d,e,f).The results revealed that the average particle size of pure MgO is about (117.93 nm), while the smaller average particle size (90.07 mn) for pure SnO 2 nanoparticles.As presented in Fig. 3(a, b), the SEM images clearly show that the SnO 2 nanoparticles are approximately identical in size.Fig. 3(c,d,e,f) showed the morphology of MgO nanoparticles doped with different ratios of SnO 2 .The SEM images revealed the agglomeration of doped nanoparticles with semi-spherical shapes, and the MgO:SnO 2 nanoparticles gathered as clusters.Finally, the SEM image demonstrated that the average particle size significantly decreased with the doping with the SnO 2 , to be (48.71nm), (48.5 nm), (50.05 nm) and (50.3 nm) of the SnO 2 ratio (50, 60, 70 and 80 wt.%) respectively, which can be attributed to the smaller ionic radius of Sn ions compared with Mg ions [24,25].UV-Vis absorption spectroscopy is a useful technique to investigate the optical properties of prepared films.The films were deposited on glass substrates using the spin coating technique.Optical measurements are conducted in the range of 200-1200 nm to determine the optical parameters.The optical absorption and transmission spectra of the MgO nanoparticles revealed a change in the band gap transition as the concentration of SnO 2 is increased, with a higher percentage of dopant, there is a small shift toward the longer wavelength region.The sharp increase in the spectra at the absorption edge demonstrates highly crystalline nanoparticles with few surface defects within the films [26,27] .Transmittance and reflectance were calculated separately, the transmittance spectra of the films are plotted as a function of the wavelength of incident light of all MgO:SnO 2 samples.The transmittance spectra is directly related to the concentration of SnO 2 , It is worth noting that the maximum value of reflectance occurs at a wavelength of 350 nm and decreases with the visible wavelength as shown in Fig. 4 .The absorption coefficient (α) was determined using the following equation, which was derived from the absorptance spectrum [28] : The absorbance (A) is used in the calculation of the absorption coefficient (α), As depicted in Fig. 5, the absorption coefficient (α) decreases as the concentration of MgO increases.This is attributed to the increase in the energy gap that occurs with higher concentrations of MgO .
The absorption coefficient (α) describes how well a material can absorb light at a particular wavelength.In the case of MgO: SnO 2 film, an increase in the concentration of MgO leads to a widening of the bandgap [29], which is the energy range where electrons are not available to absorb photons [30,31].This means that fewer photons are absorbed by the film, resulting in a decrease in the absorption coefficient (α).Therefore, as the concentration of MgO increases, the film becomes less efficient in absorbing light, causing a decrease in the absorption coefficient, which agreed with the results [32,33] .
Assuming a constant value (C) for the absorption coefficient (α), the bandgap energies of MgO: SnO 2 nanoparticles were approximately 3.9 eV, while those of tin oxides were around 3.6 eV.The bandgap energy of pure MgO was approximately 3.5 eV.These values were obtained for (MgO) (SnO 2 ) (MgO: SnO 2 ) films.When these two materials are combined in the form of a composite film, their bandgap energies are affected by several factors, including the concentration of the materials and the degree of crystallinity [36].The addition of SnO 2 to MgO can cause the bandgap energy to decrease, as observed in the case of the MgO:SnO 2 films.This is due to the change in the electronic structure of the composite film, which arises from the interaction between the two materials [37,38] .By using relation (3), the refractive index of both pure MgO, SnO 2 films, and (MgO: SnO 2 ) films has been calculated [39].The absorption spectra can be used to calculate the extinction coefficient from the equation ( 4) [40,41]: The fundamental dielectric constant is shown in Figure 8.We can see that the value of 60% MgO:40%SnO 2 is maximum (5) at the photon Energy (3.5eV), while the value of 70% MgO:30% SnO 2 decreases to (3) at the wavelength.(3.5eV).The values of (n o ) and (k o ) are connected to the real (ε r ) and imaginary (ε i ) components of the dielectric constant, The formulae ( 5) and ( 6) were utilized to calculate the values of (ε r ) and (ε i ), respectively [42,43]: ε = 2n °k°( 6) Figure 8(a) presents the real dielectric constant with the highest value of 5.21 observed at a photon energy of 3.5 eV of the 60%MgO:40%SnO 2 film, while the lowest value of 3.18 is observed at the photon energy of 3.25 eV of the 70%MgO:30%SnO 2 film.Figure 8(b) illustrates the imaginary dielectric constant, with the highest value of 0.384 observed at a photon energy of 1.3 eV for the Pure MgO film, and the lowest value of 0.023 observed at the photon energy of 0.384 eV for the 70%MgO:30%SnO 2 film.In general, both the real and imaginary dielectric constants exhibited a change in the behavior with an increase in the doping ratio [44,45] .

CONCLUSIONS
The co-precipitation method proved successful in synthesizing MgO, SnO 2 , and MgO:SnO 2 nanoparticles with distinct characteristics.X-ray diffraction analysis revealed an average crystallite size of 33 nm, while particle size analyzer results indicated an average particle size of 22 nm.The samples exhibited different crystal structures, with MgO having a cubic structure, SnO 2 nanoparticles showing a tetragonal structure, and MgO:SnO 2 displaying a tetragonal structure as well.SEM images provided further evidence of spherical and aggregated particles with a granular crystalline structure.The nanoparticles were further characterized using UV-Vis spectroscopy.It was observed ), (002) , and (231) attributed to the Cassiterite tetragonal SnO2 structure, space group (P42/mnm no.136), with lattice parameters (a = b = 4.7360 Å and c = 3.1850 Å) and (α=β=γ=90ᵒ) corresponded to the standard data (JCPDS 98-003-9173).As shown in Fig. 1(a,b).

Figure 5 .
Figure 5. Absorption coefficient (α) of prepared MgO:SnO2 films Fig. 6 shows the bandgap energy of pure MgO film, pure SnO 2 film, and MgO:SnO 2 films.The bandgap energy of the sample can be determined using the formula [34,35] :

Figure 7 .
Figure 7. (a) Refractive index and (b) extinction coefficient curves of prepared films

Figure 8 .
Figure 8.(a) Real part and (b) imaginary part of dielectric constant curves of prepared films