Fabrication of h-MoO3 Nanorods and the Properties of the MoO3/WEP Composite Coatings Research

In this study, we prepared a novel coating composed of hexagonal molybdenum oxide (h-MoO3) nanofiller and waterborne epoxy resin (WEP) to provide corrosion protection. We optimized the h-MoO3 nanorod synthesis methodology first by changing different parameters (pH, temperature, etc.). Furthermore, the as-prepared h-MoO3 rods were characterized using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Finally, the electrochemical impedance spectroscopy (EIS) test results verified that the anticorrosive performance of the composite coatings was improved by incorporation of low content of MoO3 nanofiller (0.5 wt.%) compared to pure WEP sample. This developed composite will provide a new insight for the design and fabrication of one-dimensional (1D) nanomaterial (e.g., nanorod) reinforced epoxy coating and other polymeric coating processes.


Introduction
The economy is significantly impacted by corrosion in various industries. According to the World Corrosion Organization, the cost of corrosion annually in 2010 was over USD 2 trillion, which is more than 3% of global domestic product [1]. Therefore, novel anticorrosion materials are developed continuously to save total domestic costs. Epoxy resin is typically used to protect the surface of the materials since it has excellent adhesion, high mechanical strength, chemical stability, low shrinkage, etc. [2][3][4][5]. However, the pure epoxy coating has weak anticorrosion capability due to the inherent cross-linked structure, brittleness, and micropores easily forming during the solvent evaporation process, resulting in it not being able to meet the requirements in practical industry applications [6][7][8]. In recent years, researchers have been trying to develop nanofillers to improve the intrinsic properties of epoxy coating [9,10]. For example, the permeability and adhesion of the epoxy coating can be improved by adding nanometal Zn, SiO 2, etc. [11,12]. In this context, MoO 3 (h-MoO 3 ) is one of the promising nanofillers among the various metal oxides or metals due to its good chemical stability, excellent wear resistance, high hardness, and ecofriendliness [13,14]. So far, no studies have reported the preparation of h-MoOx composites for corrosion-resistant coatings.
Furthermore, the increasing demand for low carbon coating is attracting more attention due to the discharging of volatile organic compounds (VOCs) during the conventional solvent coating process. Hence, here we developed the coating composites at room temperature without any solvent addition. In this work, a simple coating prepared by incorporating MoO 3 into the WEP matrix was coated on the steel surface. Firstly, MoO 3 nanorods were optimized and obtained via a modified precipitation method. Subsequently, composite WEP coatings were prepared with the addition of low content of MoO 3 (0.5%). With the MoO3 (0.5%). With the synthesized MoO3 nanofiller, our work aims to investigate the enhanced anticorrosion properties of the new MoO3/WEP composites.

Materials
The materials ammonium molybdate tetrahydrate (H24Mo7N6O24·4H2O), high-temperature resistant dimethyl silicone oil, sodium chloride (NaCl), and potassium chloride (KCl) were purchased from Aladdin Industrial Corporation. All reagents were analytical grade without further purification. Epoxy resin and curing agent were purchased from Zanxi Resin Co., Ltd (Shanghai, China).

Synthesis of MoO3 Composite Coating
MoO3 nanoparticles were synthesized using a modified precipitation method [15]. Briefly, 10.0 g of ammonium molybdate was slowly added to a round-bottom flask with 100 g deionized water. The solution was magnetically stirred at room temperature for 2.5 h using a ceramic magnetic stirrer (Ronhua HJ-3, Changzhou, China), and then 2 M of HNO3 solution was added continuously until the pH of the mixture reached 1 ± 0.1. The above mixture was further heated at 90 ± 1 °C in a silica oil-bath for 12 h. After that, the white precipitations ( Figure 1a) were filtered with 0.45 µm PTFE filter membrane and washed with deionized water and ethanol several times. The obtained product was dried in the vacuum condition at 60 °C for 12 h, and the product was collected for the subsequent characterization and analysis. To obtain the optimal MoO3 rods with the desired particle size and morphology, pH (1, 1.5, 2, and 2.5), reaction temperature (80 and 90 °C), and stirring rate (250, 350, and 450 rpm) were adjusted during the synthesis process.
For the preparation of anticorrosion coating, the WEP resin was first obtained by mixing the epoxy resin, deionized water, and the curing agent. Then, the 0.5 wt.% of MoO3 was added to the epoxy matrix to obtain the homogeneous mixture. The nanocomposite mixture was then coated on the polished steel panel (Q-lab, Westlake, OH, US) via a film applicator (bar coater) to control the coating film thickness (Figure 1b). The wet film thickness was about 100 µm. The dry film thickness was 35 µm, tested by a thickness gauge (BEVS1703, Shenhua Industrial Co. Ltd., Shanghai, China).   For the preparation of anticorrosion coating, the WEP resin was first obtained by mixing the epoxy resin, deionized water, and the curing agent. Then, the 0.5 wt.% of MoO 3 was added to the epoxy matrix to obtain the homogeneous mixture. The nanocomposite mixture was then coated on the polished steel panel (Q-lab, Westlake, OH, US) via a film applicator (bar coater) to control the coating film thickness (Figure 1b). The wet film thickness was about 100 µm. The dry film thickness was 35 µm, tested by a thickness gauge (BEVS1703, Shenhua Industrial Co. Ltd., Shanghai, China).

Characterization of MoO 3
X-ray diffraction (XRD) measurements were conducted with Cu Ka radiation (36 kV, 20 mA) in an XD-6 X-ray diffractometer (Purkinje General Instrument Co., Ltd. Beijing, China). The morphology of MoO 3 nanoparticles was analyzed using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan), which operated in secondary electron yield mode and with a beam energy of 10 keV.

Electrochemistry Anticorrosion Testing of MoO 3 /WEP Coating
The anticorrosion experiments were performed in a CHI660D Electrochemistry Workstation (Chenhua Instrument Co., Ltd., Shanghai, China). The setup was equipped with three electrodes, namely the coating sample as the working electrode, saturated calomel electrode as the reference one, and platinum slide as the counter electrode. EIS testing was carried out at open circuit potential with the frequency range of 100 kHz to 0.01 Hz. The potential fluctuation was not beyond 5 mV prior to the test. For all the measurements, the EIS spectra were repeated to ensure the data is reliable. Figure 2 shows the morphologies of the synthesized MoO 3 . Generally, our SEM results indicated that the optimal conditions for the synthesis of MoO 3 nanorods were pH = 1, 90 • C reaction temperature, and medium stirring rate (350 rpm). It is worth noting that pH significantly influenced the morphology of MoO 3 . Specifically, when pH was 2.5, SEM results showed that the microstructure corresponded to isostructural belts with a diameter of >1 µm (Figure 2c). When pH was reduced to 1, the particle diameter was shortened to around 200 nm (Figure 2b). For the MoO 3 synthesized with different stirring rates, the best morphology was achieved only under the medium rate (350 rpm), as characterized by SEM. This result is consistent with the crystal nucleation theory during the precipitation [16]. The synthesis operating at a high mixing rate (>350 rpm) could result in a very high nucleation rate, and the inhomogeneous particle morphologies were observed accordingly. A very high crystal growth rate could be obtained at the low mixing rate (<350 rpm); thus, the desired particle size and the ideal morphologies could not be achieved in this case.

Optimization of MoO 3 Synthesis: XRD Characterization
The XRD patterns of MoO 3 were further investigated under the different conditions of pH, stirring rate and reaction temperature. As shown in Figure  Our result was consistent with a previous study showing that H + concentration was crucial for MoO 3 grain growth and crystal structure [18]. The as-prepared α-MoO 3 was a microscale belt with a diameter of~1 µm (Figure 2c, Section 2.1). The microscale particle size is too large to be incorporated into this coating for a film thickness of 35 µm, so α-MoO 3 was not employed for the subsequent coating studies. The XRD spectra of samples synthesized under other reaction conditions (temperature and mixing rate) are not presented here due to them having relatively more impurity peaks or lower intensity.

Optimization of MoO3 Synthesis: XRD Characterization
The XRD patterns of MoO3 were further investigated under the different conditions of pH, stirring rate and reaction temperature. As shown in Figure  , which are consistent with the standard spectrum (JCPDF:21-0569), and these peaks were in accordance with the previously reported hexagonal crystal h-MoO3 [17]. The results also suggested that the sample was MoO3 nanorod, with no other impurities. Interestingly, higher pH (pH > 2) could lead to the different crystalline phase structures (Figure 4). The spectra presented the different reflection angles located at 2θ = 13.95, 23.09, 26.95, 28.08, 29.73, 47.21, and 53.37°, which corresponded to (200), (020), (311), (202), (211), (040), and (513) crystal planes of MoO3 and identified the sample as orthorhombic crystal α-MoO3 according to the standard spectrum (JCPDF 46-1048). Our result was consistent with a previous study showing that H + concentration was crucial for MoO3 grain growth and crystal structure [18]. The as-prepared α-MoO3 was a microscale belt with a diameter of ~1 µm ( Figure  2c, Section 2.1). The microscale particle size is too large to be incorporated into this coating for a film thickness of 35 µm, so α-MoO3 was not employed for the subsequent coating studies. The XRD spectra of samples synthesized under other reaction conditions (temperature and mixing rate) are not presented here due to them having relatively more impurity peaks or lower intensity. Intensity(a.u.)

Electrochemical Impedance Spectroscopy (EIS) Testing
In this experiment, electrochemical corrosion properties were evaluated according to the composite coating. Figure 5 presents the Bode plots of WEP only and MoO3/WEP containing 0.5 wt.% h-MoO3 nanocomposite coatings on steel in 3.5 wt.% NaCl solution. Based on the Bode plots, the magnitude of the impedance in the low-frequency zone (0.01 Hz) can be applied to evaluate the corrosion resistance performance [19,20]. For the control WEP coating, the Bode impedance remained at a constant value initially and then decreased after immersion on the 30th day (Figure 5d). That of the MoO3/WEP sample was almost constant for up to 30 days. The results indicated that the polarization resistance increased with the addition of h-MoO3 nanoparticles in the epoxy matrix during

Electrochemical Impedance Spectroscopy (EIS) Testing
In this experiment, electrochemical corrosion properties were evaluated according to the composite coating. Figure 5 presents the Bode plots of WEP only and MoO 3 /WEP containing 0.5 wt.% h-MoO 3 nanocomposite coatings on steel in 3.5 wt.% NaCl solution. Based on the Bode plots, the magnitude of the impedance in the low-frequency zone (0.01 Hz) can be applied to evaluate the corrosion resistance performance [19,20]. For the control WEP coating, the Bode impedance remained at a constant value initially and then decreased after immersion on the 30th day (Figure 5d). That of the MoO 3 /WEP sample was almost constant for up to 30 days. The results indicated that the polarization resistance increased with the addition of h-MoO 3 nanoparticles in the epoxy matrix during the testing period. For both short-term (1 day, Figure 5a) and long-term (30 days, Figure 5d) immersion, the MoO 3 /WEP composite coatings had a relatively higher impedance modulus at low-frequency zone (0.01 Hz), which suggested that the composites could promote the corrosion protection of epoxy coating by incorporating h-MoO 3 nanofillers. This was mainly attributed to the nanorods extending the permeation of moisture and ions into the coatings, which is consistent with previous reports [11,12].
Our results showed that with 30 days of immersion, the MoO 3 /WEP composite coating could maintain stable anticorrosion enhancement. Therefore, the nanocomposite has a beneficial effect on the anticorrosion performance of the tested materials. Our results clearly show that the h-MoO 3 nanorods can be a promising candidate nanofiller for epoxy or similar polymeric coatings compared with the coatings without nanoparticle addition. the testing period. For both short-term (1 day, Figure 5a) and long-term (30 days, Figure  5d) immersion, the MoO3/WEP composite coatings had a relatively higher impedance modulus at low-frequency zone (0.01 Hz), which suggested that the composites could promote the corrosion protection of epoxy coating by incorporating h-MoO3 nanofillers. This was mainly attributed to the nanorods extending the permeation of moisture and ions into the coatings, which is consistent with previous reports [11,12]. Our results showed that with 30 days of immersion, the MoO3/WEP composite coating could maintain stable anticorrosion enhancement. Therefore, the nanocomposite has a beneficial effect on the anticorrosion performance of the tested materials. Our results clearly show that the h-MoO3 nanorods can be a promising candidate nanofiller for epoxy or similar polymeric coatings compared with the coatings without nanoparticle addition.

Conclusions
In this study, a novel approach was developed by incorporating 1D h-MoO3 nanofillers into a WEP matrix to promote the corrosion resistance of the composite coatings. The MoO3 nanoparticles were first synthesized based on the modified precipitation method. The h-MoO3 nanorod synthesis parameters were optimized as pH = 1, T = 90 °C, and stirring rate = 350 rpm based on XRD and SEM investigation. Subsequently, the anticorrosive performance of the MoO3/WEP coating was evaluated by EIS studies in 3.5 wt.%

Conclusions
In this study, a novel approach was developed by incorporating 1D h-MoO 3 nanofillers into a WEP matrix to promote the corrosion resistance of the composite coatings. The MoO 3 nanoparticles were first synthesized based on the modified precipitation method. The h-MoO 3 nanorod synthesis parameters were optimized as pH = 1, T = 90 • C, and stirring rate = 350 rpm based on XRD and SEM investigation. Subsequently, the anticorrosive performance of the MoO 3 /WEP coating was evaluated by EIS studies in 3.5 wt.% NaCl solution. The anticorrosion performance of the composite coating was improved significantly by the incorporation of 0.5 wt.% MoO 3 nanorods. Therefore, h-MoO 3 nanorods have the potential to be further applied to commercially available coatings to improve corrosion resistance.
Author Contributions: Project administration, supervision, conceptualization, methodology, writing, formal analysis, Y.Z. and Q.Z.; investigation, formal analysis, data curation, draft preparation, C.S. and Z.C. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.