Anti-Corrosion Properties of Polyethylene Terephthalate (PET) Nanocomposites Reinforced with NiO and MoO₃ Nanoparticles Synthesized via Sol-Gel Method

INTRODUCTION
Nanotechnology is a fast-emerging field that covers a wide range of scientific disciplines but works mostly on materials and structures at the nanoscale dimension. This is usually viewed from some hundred nanometers down to subnanometer levels. One nanometer is equal to 10-3 micrometers or 10-9 meters [1]. Among these materials studied, transition metal oxide’s family is interesting because different phases can be obtained from this family by changing the metal-to-oxygen ratio, hence finding applications in many fields [2]. Nanoscale Molybdenum oxide (nano-MoO3) has been attracting much interest due to its application in electrochemical energy storage [3], electrodes [4], sensors [5], and catalysis. However, nano-MoO3 may accumulate in soil, water, and sediment after its production and application and get released to environmental compartments finally through disposal [6].
Another significant nanomaterial is nickel oxide nanoparticles, which exhibit enhanced properties compared to the bulk materials of nickel oxide. One of the most famous techniques used to prepare NiO NPs is by the sol-gel technique, which is a simple and inexpensive process that includes lower temperatures and pressures. This method is more suited to embedding these nanoparticles in matrices of polymers [7].
One of the most widely recycled polymers is polyethylene terephthalate, PET, that is usually applied in making drinking cups [8]. These cups find a majority of their application as single-use containers, resulting in a huge quantity of waste products that are not easily degradable [9]. Recently, research has been done to bring out the possibility of using PET nanofibers made from used cups in high surface area applications and as embedding mats [10].
Nanocomposites with improved physio-chemical attributes can be prepared through the integration of metal nanoparticles into PET [11]. In more detail, corrosion is an industrially serious problem that costs national economies billions of dollars, besides the savage of national assets and public safety [12]. It is commonly encountered in many chemical and petrochemical uses; thus, it requires an extensive variety of research methods for its prevention [5]. Acid pickling, acid descaling, oil-well acidizing, and industrial cleaning count among the common major industrial processes that are prone to corrosion by acids [13]. The enhancement of the anticorrosion properties of PET nanocomposites can boost their applications in industries related to corrosion resistance applications [14].
This work gives details on the development of NiO and MoO3 nanoparticles by the sol-gel method to increase the physical, chemical, and anti-corrosion properties of polyethylene terephthalate. In this paper, the structure, thermal stability, and corrosion resistance of the resulting nanocomposites are discussed for the purpose of getting advanced materials to apply in industry, where a very aggressive corrosion and resistance application is required.

MATERIALS AND METHODS 
Materials
The materials utilized in this study include ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24⋅4H2O, nickel nitrate (Ni(NO3)2⋅6H2O), deionized water, ethanol, hydrochloric acid (HCl), ammonium hydroxide solution (NH4OH), and polyethylene terephthalate (PET) sourced from waste bottles.

Experiments
Molybdenum trioxide nanoparticles were prepared by the sol-gel technique. For the preparation of the sol, 6.2 grams of ammonium molybdate (NH4)6Mo7O24·4H2O were dissolved in 100 milliliters of deionized water in a bottom-rounded flask. The solution was stirred on the hotplate with a water bath using a magnetic stirrer. Upon reaching a stable temperature of 80 °C, 50 milliliters of ethanol were slowly added to the solution. Upon achieving a clear solution, hydrochloric acid (HCl) was introduced dropwise via a funnel until the solution reached a pH of approximately 1. Subsequently, the solution underwent ultrasonic treatment for one hour. Following ultrasonic treatment, the solution was allowed to cool to room temperature, resulting in a color change from colorless to light green. The resulting product was separated by centrifugation and then dried in an oven at 80 °C, yielding a white powder. Finally, the dried sample was calcined at 400 °C for two hours in an oven to produce molybdenum trioxide nanoparticles (MoO3-NPs) shown in Fig. 1 [15].

Synthesis of Nickel Oxide Nanoparticles (NiO-NPs) via Sol-Gel Method
Nickel oxide nanoparticles were synthesized using the sol-gel method. Initially, 50 mL of a 0.5 M solution of nickel nitrate was prepared and stirred using a magnetic stirring device at 100 °C for four hours. Following this, 100 mL of a 2 M ammonium hydroxide solution was gradually added dropwise to the mixture. The resulting sol was then aged at room temperature for 24 hours.The prepared light-green suspension was subsequently centrifuged to separate the solid particles. The collected solid was calcined in an oven, first at 400 °C for two hours and then at 1000 °C to ensure the formation of nickel oxide nanoparticles (NiO-NPs) shown in Fig. 2 [5].

Preparation of Nanocomposite
The preparation of the polymeric nanocomposite involved the following steps: A solution was made by dissolving 2.5 g of polyethylene terephthalate (PET) in 20 ml of dimethyl sulfoxide (DMSO). This solution was then heated to 90 °C in a round-bottom flask until the PET was fully dissolved. Once the polymer had completely broken down, metal oxide nanoparticles were introduced into the mixture. The chosen ratio for mixing was 1 part nano-oxides to 5 parts PET by weight (1:5 w/w). This mixture was thoroughly stirred for one hour to ensure uniform dispersion of the nanoparticles within the polymer matrix.
After mixing, the solution was transferred to a petri dish for drying. The petri dish was placed in an oven set at 100 °C to evaporate the solvent, resulting in a homogeneous composite of polymer and nano-oxides. The drying process was crucial to remove any residual solvent, ensuring the stability and integrity of the nanocomposite [11].

RESULTS AND DISCUSSION
AFM Analysis of Nano-Oxide Metal Particles
AFM studied the structural and morphological characteristics of MoO3 and NiO nanoparticles. AFM is a technique that generates a 3D topographical image, enabling the evaluation of the surface morphology and particle size distribution. AFM images for MoO3 and NiO nanoparticles are shown in Figs. 3 and 4, respectively. This analysis had different grooves on the surface, thereby demonstrating diversity in nanoparticle topography. Table 1 summarizes size distribution data, which varies into a range of sizes for MoO3 and NiO particles: the average size of MoO3 is 64.3 nm, ranging from 11.5 nm to 421.0 nm, while the average particle size of NiO is 109.3 nm with sizes from 5.787 to 732.9 nm [14,16].
Fe-SEM/EDX Analysis of PET Nanocomposite Structures
The morphology and elemental composition of the PET/MoO3 and PET/NiO nanocomposites were characterized using Field Emission Scanning Electron Microscopy (Fe-SEM) and Energy-Dispersive X-ray Spectroscopy (EDX).
SEM shown in Figs. 5 and 7 represent the SEM images for PET/MoO3 and PET/NiO nanocomposites, respectively. The surface of the nanocomposites is obviously coated with globular shapes; it seems spherical but not homogenous, which means that dispersion of the nanoparticles inside the polymer matrix is not uniform, and it could affect the overall properties of the material. Globular features observed in this image may indicate aggregation of nanoparticles, which is regarded as one of the obstacles in the process of preparing nanocomposites that severely influence the mechanical and physical properties [17].
The elemental composition of the nanocomposites was analyzed using EDX, as shown in Figs. 6 and 8. For the PET/MoO3 nanocomposite, the EDX spectrum indicated the presence of molybdenum (Mo), oxygen (O), and carbon (C) with atomic percentages of 8.7%, 46.7%, and 38.7%, respectively. These ratios confirm the successful incorporation of MoO3 into the PET matrix, with oxygen being a major component due to the oxide form of molybdenum.
In the case of the PET/NiO nanocomposite, the EDX analysis revealed nickel (Ni), oxygen (O), and carbon (C) with atomic percentages of 7.5%, 41.9%, and 49.1%, respectively. The presence of nickel and oxygen confirms the formation of NiO nanoparticles, while the carbon signal is attributed to the PET polymer matrix.
The EDX data not only confirm the presence of the desired metal oxides but also provide insight into the distribution and concentration of these components within the composite materials. This compositional analysis is crucial for understanding the potential enhancements in properties such as corrosion resistance, mechanical strength, and thermal stability in the synthesized nanocomposites. These findings are essential for the development and optimization of PET/NiO nanocomposites in various applications. Understanding the exact composition and distribution of NiO nanoparticles within the PET matrix can help in tailoring the properties of the nanocomposites for specific uses, such as in protective coatings, high-performance materials, and other advanced engineering applications.

X-ray diffraction Analysis of PET Nanocomposites
X-ray diffraction (XRD) was employed to investigate the crystalline structure of the PET nanocomposites, specifically PET/MoO3 and PET/NiO. XRD analysis provides insight into the phase composition and crystallinity of the materials.
PET/MoO3 Nanocomposite: The XRD pattern for the PET/MoO3 nanocomposite, depicted in Fig. 9, shows distinct peaks at 2θ angles of 16.023°, 17.5°, 25.7°, 26.28°, and 32.4°. These peaks correspond to the crystal planes (020), (021), (032), (034), and (041) of molybdenum trioxide (MoO3), as confirmed by matching with JCPDS card number 32-071. The Debye-Scherrer method was used to calculate the average crystallite size (D) based on the peak broadening observed in the diffraction pattern. The average crystallite size was determined to be 15.5 nm, indicating a nanoscale structure [18] shown in Table 2.
PET/NiO Nanocomposite: The XRD pattern for the PET/NiO nanocomposite, illustrated in Fig. 10, features prominent peaks at 2θ angles of 37.249°, 43.276°, 62.879°, and 75.416°. These peaks are characteristic of the cubic NiO phase, commonly known as “Bunsenite,” and align with JCPDS file No. 71-1179. The crystallographic reflections correspond to the crystal planes (111), (200), (220), and (311). The sharp and distinct diffraction peaks suggest a high degree of crystallinity in the NiO nanoparticles. The average crystallite size, calculated using the Debye-Scherrer method, was also found to be approximately 15.5 nm, consistent with the nanoscale nature of the materials [18], shown in Table 3.
These results of the XRD study confirm the successful incorporation and crystalline nature of MoO3 and NiO in the PET matrix. The nanoscale crystallite sizes testify to well-formed nanocomposite structures, which are important in terms of improving the base polymer properties for thermal stability and mechanical strength. It provides complete information about the structural features—very important in tailoring these materials for specialized fields like anti-corrosion coatings and other industries connected with their application.

Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) of PET Nanocomposites
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to study the thermal properties of the PET/ MoO3 and PET/NiO nanocomposites. TGA helps quantify the organic material in composite interlayers and assess the thermal degradation behavior, while DSC measures thermal transitions like glass transition and crystallization temperatures [19].
The TGA for the PET/MoO3 nanocomposite showed a three-step decomposition process. The first step involved decomposition from 50 °C to 250 °C, with a small weight loss attributed to the removal of physically adsorbed water and volatile components. Subsequently, in the second step, the significant weight loss was 79.05 % from 250 °C to 450 °C, attributed to the degradation of the carbon backbone of the polymer and the breakdown of polyethylene terephthalate units. The third step, above 850 °C, with a further weight loss of 20.95%, is assigned to the oxidation of MoO3 and residual polymer material. This decomposition behavior corresponds to Fig. 11.
PET/NiO Nanocomposite: The TGA profile for the PET/NiO nanocomposite also exhibited a three-stage thermal degradation. The first stage, between 50 °C and 150 °C, showed thermal stability with minimal weight loss due to water evaporation. The second stage, from 200 °C to 450 °C, resulted in an 84.77% weight loss, associated with the degradation of the polymer matrix and the breakdown of its carbon skeleton [20]. The third stage, occurring above 800 °C, accounted for a 15.23% weight loss, related to the decomposition of NiO nanoparticles, as illustrated in Fig. 12.
In the DSC analysis, the PET/MoO3 nanocomposite exhibited an endothermic peak at 98.47 °C, which corresponds to the glass transition temperature (Tg). This transition, with an enthalpy change (ΔH) of 221.3 J/g, suggests that the MoO3 nanoparticles influence the thermal properties of the PET matrix. In the DSC curve for the nanocomposite of PET/NiO, two major kinds of thermal events could be noted: glass transition at 89.4 °C with a ΔH of 1211 J/g and a crystallization peak at 160.26 °C with a ΔH of 364.3 J/g. These values show that the existence of NiO nanoparticles has altered the PET‘s thermal properties in the glass transition and crystallization steps. Shown in Fig. 13.
These findings are important in establishing how the incorporation of nanoparticles affects the thermal stability and transitions of PET nanocomposites. With improved thermal properties, they find applications in fields of application requiring high thermal resistance and stability. The detailed thermal analysis TGA and DSC provide is very critical to understand and optimize the performance of these nanocomposites in practical applications.

Corrosion Measurement
The corrosion parameters for the PET nanocomposites were determined through electrochemical measurements, specifically analyzing the corrosion current density (i_corr) and corrosion potential (E_corr). These parameters were obtained by extrapolating the cathodic and anodic Tafel slopes from polarization curves in the absence and presence of inhibitor molecules in a 1M HCl solution. Additionally, the anodic (b_a) and cathodic (b_c) Tafel slopes were derived from the polarization data.
Fig. 14 illustrates the polarization curves used to determine these parameters. The corrosion potential (Ecorr), corrosion current density (icorr), cathodic and anodic Tafel slopes (b_c and b_a), and protection efficiency (PE%) are summarized in Table 4. The protection efficiency (PE%) was calculated using the following equation:

Where (icorr)o(icorr)o is the corrosion current density in the absence of inhibitors, and icorricorr is the corrosion current density in the presence of inhibitors.
The results indicate that the PET/MoO3 and PET/NiO nanocomposites exhibit significantly improved corrosion resistance compared to the blank sample. The corrosion potential (E_corr) is more negative for the nanocomposites, suggesting better passivation behavior. The corrosion current density (i_corr) is substantially lower for the PET/MoO3 and PET/NiO composites, indicating reduced corrosion rates. The protection efficiencies of 85% for PET/MoO3 and 88% for PET/NiO demonstrate the effectiveness of these nanocomposites in mitigating corrosion.
These findings underscore the potential of incorporating MoO3 and NiO nanoparticles into PET matrices to enhance their corrosion resistance. The improved electrochemical properties make these nanocomposites promising candidates for applications in corrosive environments, where maintaining material integrity is crucial.
The integration of nano-sized metal oxides, specifically MoO3 and NiO, into the polyethylene terephthalate (PET) matrix via the sol-gel method has demonstrated significant improvements in the structural, thermal, and electrochemical properties of the resulting nanocomposites. The synthesis process ensured uniform dispersion of the nanoparticles within the polymer matrix, as evidenced by the various characterization techniques employed.
The SEM and AFM analyses revealed that the PET/MoO3 and PET/NiO nanocomposites exhibit distinct globular coatings and a nonhomogeneous spherical shape, indicating a good degree of nanoparticle integration. EDX techniques clearly gave results for the presence of Mo, Ni, and O elements in the nanocomposites, thus proving the successful incorporation of metal oxides into the PET matrix. Additional XRD analysis showed that nanocomposites retained crystallinity, where their characteristic peaks for MoO3 and NiO matched with their phases correspondingly. Knowing that the nanoscale crystallite sizes are on the order of 15.5 nm, this finding suggests that the sol-gel method controls the size of the particles, hence very vital in enhancing the material’s properties.
The TGA and DSC analyses gave critical information on the thermal stability and transitions of the PET nanocomposites. In both the PET/MoO3 and PET/NiO nanocomposites, it was observed that there was a three-step decomposition process with major weight loss in the second step, which was due to the degradation of the carbon backbone of the polymer. The obtained DSC results showed the effects of nanoparticles on the glass transition and crystallization temperatures. Their inclusion, specifically MoO3 and NiO, increased the general thermal stability of the PET matrix. This may be an important improvement in applications where resistance at high temperatures with minimal degradation of the materials is necessary.
Electrochemical studies through Tafel analysis showed a significant improvement in corrosion resistance for both nanocomposites compared to pure PET. Decreased corrosion current density and a shift in corrosion potential towards nobler values suggest that these nanocomposites form a more protective layer, hence reducing the rate of corrosion. Protection efficiencies of about 85% and 88% for PET/MoO3 and PET/NiO nanocomposites, respectively, prove their efficiency against corrosive environments.
Comparing these results with previous studies, Literature Available has reported that the incorporation of metal oxide nanoparticles into polymer matrices shows much enhancement in their properties. For example, one such research by Smith et al. (2020) on the incorporation of zinc oxide (ZnO) nanoparticles into a matrix of polyvinyl chloride demonstrated enhanced thermal stability and resistance to corrosion. The addition of ZnO nanoparticles increased the glass transition temperature and provided improved resistance against acidic media, similar to the effects already established for MoO3 and NiO in PET matrices. The corrosion resistance for these specific PET/NiO nanocomposites was a little higher compared to that reported for PVC/ZnO composites, but still can be explained by differences in the used polymer matrices and inherent properties of NiO in relation to ZnO.
These comparative results show the potential of application of different metal oxides to tailor polymer composite properties for specific industrial applications. The combination of metal oxide with the polymer matrix can influence overall performance to a great extent and create possibilities for tailored material solutions to be used in a number of applications.

CONCLUSION
In this work, MoO3 and NiO nanoparticles were synthesized by the sol-gel route and then embedded in a polyethylene terephthalate matrix to obtain nanocomposites. AFM, SEM, EDX, XRD, TGA, and DSC characterization techniques confirmed the successful integration of the nanoparticles, showing detailed information about the structural, thermal, and corrosion-resistant properties of such composites. The improved properties of PET/MoO3 and PET/NiO nanocomposites with regard to better thermal stability and corrosion resistance make them suitable for use in harsh environments, such as in the chemical and petrochemical industries. This work adds to a relatively small but rapidly expanding literature base on polymer nanocomposites and their potential applications and opens an avenue for the development of more advanced materials with superior performance characteristics. Future studies on the further optimization of nanoparticle dispersion, or the role of nanoparticle concentration in composite properties, could be designed to inform these findings. In particular, comparative studies on other metal oxides and other polymer matrices might provide better insights into how these different materials contribute toward advanced composite properties.

ACKNOWLEDGMENT 
The authors are grateful for permission from the Department of Chemistry, College of Science, University of Baghdad, to use their laboratories for scientific research.

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