INTRODUCTION
Molybdenum oxide is a transition metal oxide that has many applications such Molybdenum oxide is a transition metal oxide that has many applications such as imaging devices, smart windows and rechargeable battery electrodes, gas sensors [1]. MoO3 is known as one of the most widely used photocatalysts with a wide gap of 3.15eV, which its catalytic efficiency has long been known [2, 3]. Molybdenum oxide has three common phases:
The orthorhombic phase (𝛼-MoO3), the hexagonal phase (h-MoO3) and the highly stable monoclinic phase (𝛽-MoO3). MoO6 octahedrons are the basic building units of all MoO3 phases. The structure of h-MoO3 is stable up to 436°C and it converts to orthorhombic 𝛼-MoO3 phase at this temperature. Various methods have been used by researchers to synthesize molybdenum oxide nanostructures [4, 5], such as Liquid phase deposition [1], ultrasonic-assisted synthesis [3], solution precipitation [4, 6-8], hydrothermal synthesis [9-12], solvothermal treatment [13], solution combustion [14], spray pyrolysis [15], microwave plasma process [16], and oil bath heating and sintering combination [17]. 
Graphene, a flat network with sp2 hybrid, has attracted the attention of researchers (has been considered by researchers) due to high specific surface area, high electrical conductivity, high mechanical strength and high flexibility [18-19]. Graphene is one of the newest carbon nanostructures that, a short time after its discovery and synthesis, has found many applications in various fields such as medicine, aviation Industries, oil and petrochemical industries and smart drug carrier engines [20-22]. In addition to the lower cost of synthesizing the graphene than other carbon nanostructures, it is a suitable option for using in many industries due to two-dimensional structure and more adsorption sites [23]. 
Yang et al. prepared the MoO3–rGO composite using hydrothermal method, however, they did not introduce any application for this compound [24]. Noerochim et al. synthesized the rGO-MoO3 using a two-stage hydrothermal method and used it in the manufacture of lithium batteries [25]. 
As mentioned above, metal oxides play an essential role in the applications of various technologies. In the study, thin films of molybdenum oxide and rGO-MoO3 composite were synthesized by spray pyrolysis method and the plasma treatment effect on their structural, morphological, optical and self-cleaning properties was investigated.

MATERIALS AND METHODS
Graphene oxide colloid was purchased from Borhan Nanomeghyas Company, Iran. The other materials used in the research are from Merck brand.

Synthesis of Molybdenum Oxide (MoO3)
The mixing of 0.3 g of ammonium heptamolybdate and 100 milliliter of water was done under stirring. Stirring the solution was performed for 2 hours at 50°C.

Synthesis of Reduced Graphene Oxide 
Mix 300 ml of graphene oxide at 1 g/l concentration with 3 ml of ammonia and a few drops of hydrazine hydrate under stirring, and then adjust the temperature up to 100°C. The resulting solution was kept for 1 hour under stirring at this temperature.

Synthesis of the MoO3-rGO Nanocomposite
100 ml of reduced graphene oxide solution obtained from the previous stage placed in the ultrasonic for 30 minutes. Then it was mixed with 0.3 g of ammonium heptamolybdate and stirred for 2 hours at 50°C.

Deposition by Spray Pyrolysis Method
Washing the glass substrates were performed in an ultrasonic bath for 30 minutes. MoO3 and MoO3-rGo solutions were sprayed onto glass slides. Spray rate of 5 ml/min and nozzle height of 30cm was selected. Carrier gas used was the compressed air. Substrate temperature was set at 120°C for all samples.

Plasma treatment 
The activation of the surface of the thin layers prepared by spray pyrolysis is performed by plasma treatment and the surface roughness was created on them that prepared the surfaces for the next operation. The distance of about 4 cm was selected for the plasma head from the thin layer. The power of the plasma was 1kW, operating frequency was 20 kHz and the air was selected as the applied gas. The treated thin layers were placed in the radiant dryer at 200°C.

Characterizations
In order to investigate the formation of thin layers synthesized by spray pyrolysis method, we used the X-ray diffraction (XRD) technique, D8 Advance Bruker YT model, under CuKα radiation. For investigating the surface morphology and chemical analysis of the synthesized layers, there was used the scanning electron microscope (FE-SEM), MIRA3 TESCAN, XMU model equipped with map and EDX analysis. In order to evaluate the surface topology of the layers, we used the images of AFM prepared by atomic force microscope (AFM) device, manufactured by Ara-Research Company. Fourier transform infrared spectrometer, Nicolet AVATAR 370 equipment model was used in the range of 400-4000 Cm-1 to evaluate the chemical bonds in the obtained films. Photoluminescence features were investigated using the PL device, Perkin Elmer LS 45 model. The optical adsorption spectrum from U-3500 UV/Vis device, in the 200-700 nm was used for studying the optical properties of these films. For measure the self-cleaning features of the layers, the contact angle of water droplets with 1 microliter volume on the layers of molybdenum oxide and molybdenum oxide- reduced graphene oxide composite was measured at room temperature at 25° by Dino‐Lite digital camera (AM‐7013 MZT model), Taiwan.

RESULTS AND DISCUSSIONS
XRD Analysis
Fig. 1a shows the X-ray diffraction (XRD) pattern of MoO3 thin films deposited by sol-gel method in the stage of before plasma. Indexing the XRD peaks at 2θ∼12.82°, 25.73°, 33.66°, 35.59°, 39.16°, 46.04°, 49.21° correspond to planes (020), (040), (111), (121), (060), (200) and (002) were matched with pdf card number 00-001-0706, for confirming the formation of orthorhombic α-MoO3 phase [26]. In addition, XRD peaks at 2θ∼23.38° and 27.35° indicate the presence of a distinct secondary phase reflection from the transformed phase, which can be related to (101) and (210) planes with pdf card number 03-065-2421) [27]. 
Fig. 1b also shows the X-ray diffraction (XRD) pattern of the thin film of MoO3/rGO nanocomposite in the position before plasma treatment in the range 2θ = 10-70°. In MoO3/rGO nanocomposite, the diffraction peaks are sharp according to (020), (040) and (060) planes, indicating the highly anisotropic growth of the angular chains of the common (shared) edges of MoO6 octahedron with two similar chains, and creating the stoichiometric films in ac-plane MoO3. These layers accumulate in b-axis and are joined (hold) together by VdW forces. The preferred orientation of α-MoO3 structure shows the high crystalline nature of the synthesized samples [28, 29]. 
The lack of diffraction peak at 2θ=11° in spectrum of Molybdenum Oxide/ reduced graphene oxide nanocomposite shows the complete reduction of graphene oxide to reduced graphene oxide during thermal reaction procedure associated with spray pyrolysis deposition [30]. 
Fig. 2 shows the X-ray diffraction pattern for the thin layers of Molybdenum Oxide and Molybdenum Oxide/ reduced graphene oxide composite after plasma treatment. From the spectrum of MoO3 thin film, the spectra have amorphous behavior before 2θ=30 after plasma. After plasma treatment, in this case, the layer consists of β-MoO3 phase. In fact, the peaks recorded at 2θ=39.60, 53.16, 55.30, 58.94 correspond to planes (103), (004), (120) and (014) of monoclinic β-MoO3 phase with JCPDS–ICDD card 01-085-2405 [31]. 
In X-ray diffraction spectrum of MoO3-rGO nanocomposites, amorphous behavior is observed after plasma treatment compared to before plasma. The peak observed at 2θ=25 is a broad spectrum. Peak intensities show a sharp decrease compared to before plasma.

Morphological and topological properties
The morphology of MoO3 and MoO3–rGO nanocomposite thin films in the presence and absence of plasma were studied by field emission scanning electron microscope (FE-SEM) which is shown in Fig. 3.
As seen in Fig. 3a, the thin film of molybdenum oxide is almost uniform and consists of grains with sizes less than 25 nm. These tiny grains cling tightly to each other, creating pores inside the layer. Plasma treatment of molybdenum oxide thin films in O2 atmosphere has led to an increase in the average grain size above 40 nm and a decrease in the porosity between the grains, which is clearly seen in Fig. 3c. 
MoO3-rGO nanocomposite is composed of MoO3 nanoparticles with a number of corrugated rGO thin sheets, that MoO3 nanoparticles are scattered on rGO sheets. According the Fig. 4, after plasma treatment, the size of molybdenum oxide nanoparticles also increased on rGO sheets. The chemical composition of the thin layers synthesized before and after the plasma application was analyzed using Energy dispersive X-ray (EDS) spectrum and the results are shown in Fig. 4. The voltage that used to perform the tests was between zero and 10 kV. The presence of O and Mo and the lack of other elements confirm the formation of the thin film of molybdenum oxide. In the thin film of MoO3-rGO composite, carbon element is also observed in addition to Mo and O, which indicates the presence of carbon structure of reduced graphene oxide. No changes were observed in EDS spectrum of the samples after applying the plasma. 
The frequency distribution of elements in a material was showed using MAP analysis. The map results of these samples are plotted in Fig. 5, in order to evaluate the frequency distribution of molybdenum, oxygen and carbon elements in MoO3 and MoO3-rGO thin films. According to the Fig. 5, all these elements have a uniform distribution in the total surface of the prepared layers. In MoO3 layer, only molybdenum and oxygen elements are present, and in the MoO3-rGO composite thin film, molybdenum, oxygen and carbon elements were seen, which is consistent with EDX reports and shows the lack of impurities in the thin films.
Figs. 6 and 7 show 2D and 3D AFM images of MoO3 and MoO3–rGO composites films in the presence and absence of plasma. Comparing topology image of MoO3 and MoO3– rGO composite thin layers, it can be seen from Fig. 6 that the surface of molybdenum oxide thin film is composed of relatively small grains, but in its composite, reduced graphene oxide plates have covered these grains. The surface is relatively smooth with slight roughness and peak-valleys.
AFM images also show that the surface morphology of the thin layers has changed after plasma treatment. Comparing Figs. 6 and 7, it can be seen that the surface of MoO3– rGO composite thin layers without plasma treatment is almost smooth compared to after plasma treatment, and the slight peak and troughs is observed. After plasma treatment, the degree of surface roughness and its peak and troughs have clearly increased. It is obvious that the surface morphology of the film synthesized by spray pyrolysis method has become rougher by applying the plasma.

FTIR
Figs. 8a and 8b show the FT-IR spectrum of the thin films of MoO3 and MoO3– rGO composite before plasma treatment. The extreme peaks at 605, 874, and 992 cm-1 in IR spectrum are related to tensile mode the oxygen (O) associated with 3 metal atoms, the tensile mode of O in Mo-O-Mo units, and the tensile mode of Mo=O, respectively. The peak shown in 1610 cm-1 (Fig. 8b) related to C-C tensile vibrations shows the formation of reduced-GO. Meantime, the lack of a carbonyl peak at 1700 cm-1 shows replacement of CO by MoO3 in the surface of reduced-GO sheets [38]. 
Figs. 9a and 9b show the FT-IR spectrum of the thin film of Molybdenum Oxide and Molybdenum Oxide – reduced graphene oxide composite in the 300-3800cm-1 after plasma treatment. In Fig. 9a, as can be seen in MoO3 thin film, there are three strong vibrations at 621, 874 and 993 cm-1, that are related to the tensile mode of oxygen associated with the three metal atoms, the tensile mode of oxygen in Mo-O-Mo units and Mo=O tensile mode of α-MoO3 thin film with orthorhombic phase, respectively [39]. Two weak vibrations were also observed in 1384 and 1643 cm-1, respectively, which are related to the vibrational mode of Mo-OH band and the bending mode of absorbed water, respectively [39,40]. 
In FT-IR spectrum of Molybdenum Oxide–reduced graphene oxide nanocomposites, after plasma, tensile vibrational modes shifted to 870 and 990-1 cm due to the reduction of graphene oxide. Due to the flexural vibrational mode, ∼ 537 cm-1 band appeared in the composite material where the oxygen atom bonds to three Mo metals. In Molybdenum Oxide–reduced graphene oxide nanocomposite, the carbonyl tensile bond (C=O) disappears, which can generally be seen in about 1700 cm-1 because the carbonyl groups of reduced graphene oxide nanosheets were replaced by Molybdenum Oxide [41-44]. The broad adsorption band emerged in 1610 cm-1, which is related to C-C tensile vibrational mode. The broad adsorption band observed in 3418 cm-1 is related to H-O tensile vibrational mode. This functional group is useful for hydrogen bonding between Molybdenum Oxide and reduced graphene oxide nanoplates.

Photoluminescence
Fig. 10 shows the room-temperature PL emission at the excitation wavelength of 380 nm for thin films of MoO3 and MoO3–rGO nanocomposites before plasma treatment. The PL spectrum in MoO3 thin film shows emission peaks in 400-550 nm region. The emission rate at 430 nm is related to the band-to-band transition. The very fine shoulder peak observed at 512 nm is resulted from the e-h recombination between the oxygen receptors absorbed in the bottom level and conduction band [32]. 
In the MoO3–rGO composite thin film, an electronic transfer from electron of conduction band to deep surfaces leads to green emission. Fig. 10a shows a wide emission with peaks located at 388, 415, 455, and 533 nm. This phenomenon indicates the simultaneous emission of all transitions from different defect states. At 3.76 eV excitation, most emissions in the region of Ultra-Violet and region blue subside, and overcomes violet emissions with weak green emissions. Subsiding the PL was seen in Molybdenum Oxide/reduced graphene oxide composites, showing that a large number of excited electrons are transferred from the conduction band of Molybdenum Oxide to Fermi level of reduced graphene oxide [33, 34]. 
The PL emission was studied for α-MoO3 thin films with plasma-treated orthorhombic phase at 380 nm excitation wavelength at ambient temperature. According to report of Song et al., the PL spectrum in Fig. 11a presents a broad peak in the range of 400-600 nm with strong indigo emission located at 430-440 nm [35]. These emissions are resulted from band-to-band transition. In this study, very weak shoulders were also identified, resulting from e-h recombination between the sublevel of the absorbed oxygen receptors and the conduction band [35]. 
The photoluminescence spectrum for the thin film of MoO3–rGO composite after plasma treatment is indicated in Fig. 11b. From this Fig. 11, a main peak is seen at 453 nm. The intensity of photoluminescence peak of MoO3–rGO shows a significant decrease as a function of reduced graphene oxide content relative to the PL peak in the spectrum of MoO3. The result shows that the recombination velocity of excited electron holes in MoO3 is high. But in MoO3– reduced graphene oxide composite, the synergistic effect of the electron donor (MoO3) and an electron receptor (reduced graphene oxide) reduces facilitation of electron transfer at bond line, that effectively reduces the electron-hole recombination and results to an increase in the separation of the load carrier [36-37].

Optical Adsorption (Absorption) Studies
Fig. 12a, b indicates the data form UV–Vis reflectance spectrophotometric for the thin films of Molybdenum Oxide and MoO3– rGO composite before plasma treatment. Adsorption detected in UV wavelength region is observed at < 420 nm for α-MoO3 thin film. This result is because of the electronic transfer between 4d electrons in the conduction band of Molybdenum and 2p electrons in the valence band of Oxygen. The adsorption edge gradually expanded up to 520 nm with the addition of rGO in MoO3– rGO composite. This expansion of adsorption in the visible region is caused by the synergistic effect between reduced graphene oxide and Molybdenum Oxide nanostructures, that improves efficiency of electron-hole separation in MoO3/reduced graphene oxide nanocomposites [45]. In addition, the adsorption is increased due to the insertion of Graphene Oxide, which enables light adsorption in the region with higher wavelengths. 
Fig. 13a, b shows the data from UltraViolet–Visible reflectance spectrophotometric for the thin films of Molybdenum Oxide and MoO3–rGO composite after plasma treatment. Adsorption in MoO3 layers after plasma treatment is still in the >400 nm range and has not changed much. After plasma, adsorption also increases to some extent with the presence of reduced graphene oxide in MoO3– rGO composite, but plasma treatment does not change the shape of the peaks and adsorption region.

Investigating the Contact Angle
Measuring the contact angle is conducted to evaluate the interaction of the surface of the thin layer and water droplet. The contact angle (CA) parameter determined the wetting behavior of the surface. When CA is <90°, the wetting of a surface is occurred and non-wetting is happened when CA is larger than 90°, and consequently, they are named hydrophile and hydrophobe, respectively [41, 42].
Fig. 14 shows the shape of water droplet on the surface of thin films prepared by spray pyrolysis process in the presence and absence of plasma treatment. For investigating the self-cleaning features of the layers, water droplets with 1μl volume were placed on the thin layers of MoO3 and MoO3 - rGO composite under environmental conditions. In each sample, water droplets were located in 5 various positions and its mean was considered as the contact angle. 
According to the Fig. 14, MoO3 thin film before plasma treatment has an average contact angle of 32.06 °, which shows the surface hydrophilicity of this thin film. As observed in Fig. 14c, after applying plasma, the surface of MoO3 thin layer becomes hydrophobic and the CA of the plasma-exposed layer reaches 112.98°. The contact angle between the surface and the water droplet is influenced by two important factors: the chemical variations in surface and degree of surface roughness. According to the AFM images, it was observed that plasma treatment increases the surface roughness, therefore, it changes the surface from hydrophilicity to hydrophobicity. 
Measuring contact angle between the surface and water droplet before and after plasma treatment in MoO3-rGO thin layer is shown in Fig. 14b and 14d. As can be seen from this Fig. 14, the contact angle that droplet makes with MoO3-rGO thin layer surface before plasma treatment is 56.8°, and as shown in Fig. 14d, after plasma treatment, the contact angle becomes 145.34°. Plasma treatment has completely changed the hydrophilic into hydrophobic behavior.

CONCLUSION
MoO3 and MoO3-rGO thin layers were synthesized by spray pyrolysis technique and then treated by plasma. Structural, morphological, chemical, optical and self-cleaning properties of these layers were conducted in the presence and absence of plasma treatment. XRD pattern confirmed orthorhombic phase formation of α-MoO3. The lack of diffraction peak at 2θ=11° shows the complete reduction of Graphene Oxide to reduced GO during deposition procedure. In XRD spectrum of Molybdenum Oxide/reduced graphene oxide nanocomposite, after plasma treatment, a broad peak is observed, which indicates the amorphous behavior of the sample. According to the results of FESEM, after plasma treatment, the size of molybdenum oxide nanoparticles has increased in the pure sample and in the nanocomposite. The chemical bonds of the samples were completely studied using FTIR. The optical features of the synthesized samples were examined by photoluminescence and UV-Vis spectra. The results show that the adsorption of the samples decreases to some extent after applying the plasma and in the presence of reduced graphene oxide, but plasma treatment does not change the shape of the peaks and the adsorption region. By calculating the CA between the surface and the water droplet, it is specified that contact angle increased after plasma treatment. This result is consistent with AFM images that plasma treatment has increased the degree of surface roughness. Therefore, applying the plasma changes the surface from hydrophilicity to hydrophobicity.

ACKNOWLEDGMENTS
We would like to thank Mustansiriyah University for providing the opportunity to conduct and work on this project. We also grateful for those who supported us throughout this process and our colleagues in the department of science for believing in this work. 

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