INTRODUCTION
Manganese oxide (MnO) is a transitional substance with remarkable physical and chemical properties [1]. Manganese oxide is a promising material for electrochemical lithium-ion batteries [2]. Through the easy insertion and extraction of lithium, high performance in electrochromic devices has also been enhanced [3]. Applications in optoelectronics have been sparked by it. It is utilized in magnetoelectronic devices [4], electrode materials [5, 6], electrochemical capacitors [7, 8], rechargeable batteries, and a variety of other applications. MnO exhibits various electrical and magnetic features, including metal-insulator transistors and massive magnetoresistance [9,10]. A transition metal oxide is manganese oxide. Band gaps for MnO range from 2.4 to 3.6 eV [11]. By various methods, ALD [12], PLD [13], EBD [14], thermal evaporation [15], plasma-assisted MBE [16], sol-gel [17], chemical spray pyrolysis [18], and CBD  were some of the methods used to deposit manganese oxides thin films [5, 19]. Due to its ease of use, affordability, and capacity to increase the deposition area for commercial production, CBD is an excellent growth approach that has gained significant interest from the international scientific community [6]. This study describes how pure manganese via CBD produces MnO thin films. The physical characteristics of the films are established and explained in terms of those characteristics.

MATERIALS AND METHODS
CBD technology was utilized to create manganese oxide thin films with annealing variations on glass substrates. The glass slides are initially cleaned with a weak solution of hydrochloric acid (HCl, 1:5), which is followed by several rinses with distilled water and ethanol alcohol, as well as drying with special cleaning sheets. The following is the preparatory process: To achieve a concentration of 0.1 M, 1.70 g of copper (II) chloride (MnCl2.2H2O) is dissolved in 100 ml of deionized water. At room temperature, the resultant solution is a clear blue tint. The pH of the solution is then changed to 10 by adding ammonia (NH3) at a concentration of 25–30 %. The substrates should be dipped into the solution once they boil at around 90°C. It takes about 7 minutes for anything to boil or 7 minutes for the temperature to rise from room temperature to 90 degrees Celsius. After boiling for 2.5 minutes, the substrates are removed from the bath and given a distilled water rinse. This study created three sets of samples to investigate how annealing affected films. After that, the films were each annealed for two hours at (400, 450, and 500) oC in the air. Using the weighing approach, the sample’s thickness was around 230 ± 25 nm. Shimadzu model: XRD 6000 diffractometer was employed to record the XRD pattern in the diffraction angle range of 0o to 80o. The morphology was examined using AFM (AA 3000 Scanning Probe Microscope). The films underwent scanning electron microscopy (SEM) analysis using a JEOL-JSM-6360 model from Japan, operating at 20 kV. The VARIAN CARY MODEL 5000 spectrophotometer was used to acquire the absorption spectra of the MnO thin films. Gas-sensing experiments were conducted by placing the sample in a glass test chamber, coating it with MnO films, and conducting silver paste. Temperature variations were monitored using a sensitive thermocouple inside the sealed chamber. Changes in electrical resistance in response to NO2 gas were measured with a Keithley 6514 DMM setup. Controlled amounts of NO2 gas were delivered over the sample using a regulated flow meter (flow rate: 1 L per minute). All assessments of NO2 gas sensitivity were carried out at an operational temperature of 125°C.

RESULTS AND DISCUSSIONS  
Multiple peaks in Fig. 1 indicate that the films are polycrystalline materials. Manganese oxide thin films exhibit distinct peaks at diffraction angles of 34.26°, 40.73°, 58.65°, and 70°. The comparison with JCPDS card No. (01-071-1177) suggests a preferred growth along the (111) orientation, with the peak at 2θ = 34.26° displaying significantly higher intensity [20, 21]. These findings imply a specific crystallographic orientation and preferred growth direction in the manganese oxide thin films.
The average grain size (D) for the dominated reflection (111) was calculated based on Scherrer’s Eq. 1 and listed in Table 1 [22]: 

K is the assumed form factor of 0.89, the incoming beam’s wavelength (Cu Kα = 1.5406), β is FWHM, and θ is the Bragg angle. D was raised from 77.6 nm to 32.6 nm with an increase in annealing temperature (Tan) from 400 oC to 500 °C, respectively [23, 24], as shown in Table (1). The surface morphology is fine and regular with spherical nanoparticles (1).
The dislocation density (δ) for (111) was calculated using the formulae. [25]:

    
The dislocation density is 50.33, 45.35 and 42.18 at Tan, respectively.
The strain (Ԑ) for (111) was estimated using the Eq. 3 [26]:

It was discovered that strain values as Tan increased were 24.59, 23.35, and 22.51. Table 1 provides the computed structural parameters Pst. The inverse connection between grain size and other parameters is shown in Fig. 2, which shows the FWHM, D, dislocation density, and ε as functions of the produced films. 
The three-dimensional AFM images in Fig. 3 (A1, A2, A3) exhibit the properties of MnO thin films annealed at (400, 450, and 500) °C. The AFM images depict spherical-shaped grains that are uniformly distributed. Table 2 presents the Average Particle size (Pav) of MnO films, ranging from 77.6 to 32.6 nm, with root mean square (rms) values varying between 5.85 and 2.32 nm, and average surface roughness Ra ranging from 6.83 to 3.60 nm. A noteworthy observation is that as the annealing temperature increases, there is a consistent decrease in Pav, indicating a reduction in grain size. This trend is similarly observed for roughness. The AFM parameters (PAFM) against MnO annealed at (400, 450, and 500) °C thin films are graphically represented in Fig. 3, while Table 2 provides a detailed list of PAFM values. These results suggest a correlation between the annealing temperature and the surface characteristics, influencing both particle size and roughness in MnO thin films [27, 28].
The SEM images illustrated in Fig. 4 show the synthesized films’ morphological changes at different annealing temperatures. Initially, the surface is characterized by distinct, virtually flat islands. However, a transformation occurs as the annealing temperature increases, leading to a uniform surface coverage with spherical nano-grains. The decreasing size of these nano-grains with the gradual increase in annealing temperature suggests a correlation between the thermal treatment and the resultant nanostructure, highlighting the influence of annealing conditions on the film morphology [29, 30].
Fig. 4 shows the transmittance spectrum of thin MnO films that have been annealed at (400, 450, and 500) °C as a function of wavelength. It shows that the high transmittance region is at wavelengths between 450 and 900 nm for ultraviolet and visible light, while the low transmittance region is at wavelengths between 300 and 450 nm for visible light, which means that only about 10% of the transmittance occurs in the visible spectrum and 90% occurs in the ultraviolet spectrum [31, 32]. As the temperature annealing increases, the transmittance spectra of the films decrease, indicating satisfactory crystallinity of the achieved thicknesses. [33, 34].
The measured absorbance (A) of MnO thin films is related to transmittance (T) by [35]:

  
where (I) stands for incident light and (Io) for transmitted light. Fig. 6 plots the absorbance for the MnO thin films deposited in our study against the wavelength. A thorough examination of Fig. 6 reveals that the film is very absorbent in the visible region of the solar spectrum and that this absorbance progressively diminishes as the wavelength increases. The film annealed at 400 oC has weak infrared spectrum absorption [36, 37]. Fig. 6 demonstrates how absorbance rises as the annealing temperature rises. 
The absorption coefficient (α) of MnO thin films was determined by using the Eq. 5 [38]: 

             
Where d is the film thickness. Fig. 7 illustrates the fluctuation of α with photon energy for various annealing temperatures (400, 450, and 500) °C (6). It should be noted that α of MnO thin films is on the order of α > (104) cm-1, supporting the semiconductor’s straight band gap nature [39]. Our findings support [40]. It is also evident that the films’ absorption coefficient increases when Tan rises. This is explained by the fact that as the annealing temperature rises, film absorbance rises, increasing the absorption coefficient.
Fig. 8 represents the relationship between (αhυ)2 and (hv) according to Eq. 6 [41]:          

Where Eg  is the band gap,  (hv) is the photon energy, A is the energy-independent constant, and the exponent n describes the characteristics of the band transitions. Eg values were obtained by extending the linear component to the energy basis at n =0. The optical band gap was estimated and compared to the band gap energy derived from the optical transition occurring in the material and dropped from 3.42 to 3.30 eV, which are in excellent agreement with the reported values vis other methods. [42, 43]. 
Extinction coefficient (K) was evaluated according to Eq. 7 [44]: 

Refractive index (n) was evaluated according to Eq. 8 [45]: 

Figs. 9 and 10) show that these two factors—k and n—decreased as the annealing temperature increased, showing that the extinction coefficient was related to absorbance and influenced by the annealing temperature (8). At the same time, all films exposed in Fig.10 had refractive indices that ranged in value from 2.72 to 2.89 and were dependent on reflectivity (9). The composite is an absorbent material if its refractive index is increased to a high value. [46, 47].
In Fig. 11, the depicted relationship between resistance over time of MnO annealed at (400, 450, and 500) °C for 300 ppm of NO2 and an operating temperature of 125 °C, which reveals the impact of NO2 molecules, initiating an oxidation process on the surface. This process involves the liberation of bonded electrons to the surface by specific O2+ ions, leading to the drifting of electrons back to the conduction band [48, 49]. Consequently, this electron drift increases resistance and the potential wall’s reinforcement under these conditions [50, 51]. Notably, the MnO film annealed at 500 °C displays the highest resistance (R), demonstrating a direct correlation with film sensitivity and a robust resistance to gas flow [52].
The detection sensitivity, or sensor response, can be calculated using the Eq. 9 [53]:

The sensitivity plots in Fig. 12 demonstrate how the sensitivity of the sensor changes with varying annealing temperatures (400, 450, and 500) °C after exposure to NO2 gas. The observed reduction in sensitivity with increasing annealing temperature is attributed to the recombination process between the charge carriers of holes and electrons released from oxygen. This recombination process corresponds to the rising electrical resistance of the film [8]. In specific terms, for different annealing temperatures (400, 450, and 500) °C, the sensitivity dropped from 32.3% to 12.5% for 100 ppm, from 35.9% to 13.8% for 200 ppm, and from 38.5% to 15.4% for 300 ppm [54-56]. These trends suggest a complex interplay between annealing temperature, film sensitivity, and response to different concentrations of NO2 gas.

CONCLUSION
On a glass substrate, MnO thin films were created utilizing a straightforward and inexpensive chemical bath deposition process combined with annealing. They were thermally annealed to find out how thermal annealing affected the deposited films’ structural, morphological, and optical features. The X-ray diffractogram analysis results show that MnO films have a cubic structure. The grain sizes increased as the annealing temperature rose from (400 to 500) oC, but strain dislocation and density values decreased as the temperature rose. AFM examinations showed a smooth surface morphology with root mean square roughness values decreasing from 5.85 nm to 2.32 nm for the annealed films. Due to annealing at 500°C, the average particle size exhibited the same trend. It reduced from 77.6 nm to 32.6 nm. From SEM images, Uniform spherical nano-grains emerge with annealing, reshaping film morphology. Nano-grain size diminishes as temperature rises. Calculations have been made for n and k and the absorbance, transmittance, and absorption coefficient. With a rise in annealing temperature, the optical constants fall. The annealed samples’ permitted direct band gap shrank from 3.42 to 3.30 eV. NO2-induced oxidation in MnO film (400, 450 and 500 °C) increases resistance due to electron drift, correlating with sensitivity and strong resistance at 500 °C. Sensitivity variation in NO2-exposed films at different annealing temperatures, indicating a complex interplay between annealing temperature, film sensitivity, and response to varying concentrations of NO2 gas.

ACKNOWLEDGEMENTS
University of Diyala provided financial support for this project, for which the authors are grateful.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.