Synthesis, Characterization, and Electrochemical Properties of La-Doped α-Fe2O3 Nanoparticles

La-doped α-Fe2O3 nanoparticles were synthesized by a hydrothermal method. The effects of pH value on the morphology, structure, and electrochemical stability of the La-doped α-Fe2O3 nanoparticles were investigated by X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectrum, and electrochemical methods. The results show that the La-doped α-Fe2O3 nanoparticles exhibit a uniform spherical morphology at pH = 6, and are agglomerated with a poor dispersion at pH = 4 and 8. The iron oxide lattice is distorted by the La-doping, which increases the Fe–O bond strength. The decreased Fe–O bond length and the increased Fe–O bond energy at pH = 6 improve the electrochemical stability of α-Fe2O3. The waterborne coating modified with La-doped α-Fe2O3 nanoparticles exhibits a steady corrosion resistance.


Introduction
Metal corrosion affects economic development and production safety [1]. Coating is an effective strategy to protect metals from corrosion, which has been widely used in the metal protection field [2,3]. Iron oxide is regarded as an environmentally friendly inorganic coating material due to its chemical stability, covering, and coloring properties [4][5][6][7][8][9]. However, restricted by its corrosion-prone behavior, the application of iron oxide as a coating is limited [10]. Nanotechnology can modulate the structure and properties of iron oxide, which plays an important role in improving the corrosion resistance [11,12]. However, relevant research is still limited.
Rare earth La has a special electronic layer, which can be used to inhibit corrosion. Qin et al. [13] found that La and Ce elements with a special 4f electronic structure could easily release the trapped electrons and form shallow traps, thereby prolonging the lifetime of photogenerated electron-hole pairs and improving the photocatalytic activity of TiO 2 . Ning et al. [14] synthesized Ce-doped α-Fe 2 O 3 nanoparticles using a hydrothermal method. It was found that Ce-doping leads to the lattice distortion of α-Fe 2 O 3 , which not only enhances the Fe-O bond energy, but also increases the chemical stability of α-Fe 2 O 3 . Currently, research mainly focuses on the dielectric properties [15,16], magnetic properties [17][18][19][20][21], adsorption properties [22], and magnetothermal properties [23] of La-doped ferrite or other composites. However, La-doped iron oxide is not widely reported. Shan et al. [24] synthesized La-doped α-Fe 2 O 3 nanotubes by electrospinning. The sensing performance of acetone is higher on La-doped α-Fe 2 O 3 nanotubes than that on α-Fe 2 O 3 nanotubes. Aghazadeh et al. [25] investigated the electrochemical performance of undoped and La-doped magnetite nanoparticles by cyclic voltammetry and galvanostatic charge-discharge test. The results showed that La doping improved the capacitance of iron oxide. Melo et al. [26] synthesized La-doped Fe 2 O 3 pigment from a polymer precursor using the Pechini method. Ravinder et al. [27] adopted the sol-gel method to synthesize nano α-Fe 2 O 3 . It was found that the crystal size decreases with an increase in the La content, and 2 of 14 the average magnetization intensity decreases when the Fe 3+ is replaced by La 3+ . Raj [28] found that La 3+ completely replaced Fe 3+ by substitution in La-doped Fe 2 O 3 particles, which influenced the shape, the size, the distribution, and the band gap of the crystals. Zan et al. [29][30][31] found that the prepared super-foldable C-web/FeOOH-nanocone and super-foldable composite electrode can enhance the functional requirements of flexible electronic materials and also expand the application prospects of iron-based materials, providing a new research direction for the application of La-doped α-Fe 2 O 3 . To our best knowledge, the corrosion resistance properties of La-doped iron oxide have not been reported.
In this work, La-doped α-Fe 2 O 3 nanoparticles were synthesized by a hydrothermal method. The effects of pH value on the structure and morphology of α-Fe 2 O 3 were studied. The structure of the La-doped α-Fe 2 O 3 lattice was simulated by Materials Studio (MS) software, and the electrochemical stability of La-doped α-Fe 2 O 3 nanoparticles was investigated. In addition, the La-doped α-Fe 2 O 3 nanoparticles were applied to the waterborne coatings, and the corrosion resistance of the coatings was studied. This research was helpful to explore the structure and electrochemical properties of La-doped α-Fe 2 O 3 , and provided technical support for the development of corrosion-resistant waterborne coatings.

Preparation of La-Doped α-Fe 2 O 3 Nanoparticles
The La-doped α-Fe 2 O 3 nanoparticles were synthesized using a hydrothermal method. In a typical synthesis process, 1.7 g La 2 (SO 4 ) 3 and 120.0 g Fe 2 (SO 4 ) 3 were dissolved in 600 mL deionized water in a flask. The solution pH values were adjusted to 4, 6, and 8, respectively, with NaOH. The mixture was placed in a titanium alloy autoclave and treated at 160 • C for 1 h with a stirring speed of 200 rpm. Then, the autoclave was naturally cooled to room temperature. The products were washed with deionized water and ethanol several times. After drying at 120 • C for 6 h, La-doped α-Fe 2 O 3 nanoparticles were obtained. The preparation of undoped α-Fe 2 O 3 nanoparticles was similar to that of La-doped α-Fe 2 O 3 nanoparticles, except that no La 2 (SO 4 ) 3 was added, which was presented as α-Fe 2 O 3 (u). The La-doped α-Fe 2 O 3 nanoparticles synthesized at various pH values were presented as La-doped α-Fe 2 O 3 (x), where x denotes the pH value.

Preparation of La-Doped α-Fe 2 O 3 Modified Waterborne Coatings
The synthesis parameters of the La-doped α-Fe 2 O 3 -modified waterborne coatings are shown in Table 1. Firstly, the H 2 O, dispersing agent, defoamer, coalescent, ethylene glycol, acrylic resin, and thickening agent were mixed, and stirred at 300 rpm for 10 min. Then, the solution pH value was adjusted to 8-9 with ammonia. Subsequently, the La-doped α-Fe 2 O 3 nanoparticles were added to the solution. The pH value was maintained at 8-9 with ammonia. The mixture was stirred at 300 rpm for another 20 min. The viscosity range of the mixture was adjusted to 0.2-0.35 Pa·s by a viscometer (RTW−16, NEU Shenyang, China). The α-Fe 2 O 3 -modified waterborne coatings were presented as coating(u). The La-doped α-Fe 2 O 3 -modified waterborne coatings were presented as coating(x), where x denotes the pH values.
To test the electrochemical properties of the coatings, the polished 1 cm × 1 cm × 1 cm Q235 steel block was connected with a copper wire, which was encapsulated in a PVC ferrule with epoxy resin. Then, the steel block was coated with the prepared slurry by a high-pressure spray gun (w-71, ANEST IWATA Company, Shanghai, China) at room temperature, and dried in an oven at 50 • C for 1 h. The coating thickness was controlled at 50 ± 5 µm with a digital laser thickness gauge (QNIX4200, Rosengarten, Germany).

Characterization
(1) X-ray diffraction (XRD) XRD patterns were collected on an X-ray diffractometer (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany) with Cu K α (λ = 1.5418 Å) to identify the phase structures. All XRD spectra were measured in the range 2θ = 20-70 • at a scanning speed of 2 • /min. The average crystal size was calculated by the Scherrer equation.
(2) Transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) The morphology, the element distribution, and the crystal structure were observed by a TEM (FEI Tecnai G2 F20, FEI Company, Portland, OR, USA) equipped with EDS at 200 kV.

(4) Electrochemical properties
Electrochemical property detections were performed on an electrochemical workstation (Metrohm, Autolab, Utrecht, Switzerland) at room temperature. A platinum sheet (Pt) was used as the counter electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and 3.5 wt.% NaCl aqueous solution was used as the electrolyte.
The polarization curve of α-Fe 2 O 3 nanoparticles was detected as follows. The nanoparticles, liquid paraffin, and carbon powder were uniformly mixed with a ratio of 1:4:5 and worked as the working electrode. The measurements were performed at a scanning speed of 1 mV·s −1 within the range of −0.5 V-+2 V.
The polarization curve and electrochemical impedance (EIS) of the α-Fe 2 O 3 -modified waterborne coating are tested as follows. The coated steel block was selected as the working electrode, and the polarization curve was obtained in the range of −0.5 V-+1 V. The electrochemical impedance spectrum was scanned with 10mVRMS in the range of 0.01 Hz~100,000 Hz, which was further analyzed using NOVA 2.0 software (Metrohm, Beijing, China) to obtain the charge transfer resistance.   (6), and La-doped α-Fe 2 O 3 (8), the diffraction peaks at 2q = 33.152 • are shifted 0.024 • , 0.044 • , and 0.003 • lower relative to JCPDS33-0664, respectively. In addition, the diffraction peaks of La-doped α-Fe 2 O 3 (4) are widened. These results illustrate that the large La 3+ (radius = 0.106 nm) [32] is doped into the α-Fe 2 O 3 lattice (Fe 3+ radius = 0.064 nm) [33].
The average crystal size and the lattice constant of α-Fe2O3(u) and La-doped α-Fe2O3(x) (x = 4,6,8) are calculated by the Scherrer formula, as presented in Table 2. Obviously, with La doping at different pH values, the crystal size of α-Fe2O3 decreases and the lattice constant increases, of which La-doped α-Fe2O3(6) has the largest lattice constant and the largest lattice distortion.     Table 2. Obviously, with La doping at different pH values, the crystal size of α-Fe 2 O 3 decreases and the lattice constant increases, of which La-doped α-Fe 2 O 3 (6) has the largest lattice constant and the largest lattice distortion.  Clearly, with an increase in pH value, the average particle size first increases and then decreases, consistent with the change rule of the XRD results (Table 2). Compared to each other, La-doped α-Fe 2 O 3 (6) has the weakest agglomeration and better dispersion. Nanomaterials 2022, 12, x FOR PEER REVIEW 5 polygon morphology, which is not uniform, and the average size is 83.2 nm (Figur Clearly, with an increase in pH value, the average particle size first increases and decreases, consistent with the change rule of the XRD results (Table 2). Compared to other, La-doped α-Fe2O3(6) has the weakest agglomeration and better dispersion.  Moreover, as shown in Figure 3b,d,f,h, the α-Fe2O3(u) and La-doped α-Fe2O3(x)(x = 4,6,8) all have a polycrystalline structure, and the crystal planes of La2O3 are not observed, indicating that the La element is substituted in the α-Fe2O3 lattice.     Figure 5 shows the simulated crystal lattice of α-Fe2O3(u) and La-doped α-Fe2O3. Table 3 shows the simulated lattice constants, Fe-O bond length, and energy in α-Fe2O3(u) and La-       Figure 5 shows the simulated crystal lattice of α-Fe2O3(u) and La-doped α-Fe2O3 shows the simulated lattice constants, Fe-O bond length, and energy in α-Fe2O3(u) doped α-Fe2O3. When the La element is doped in the α-Fe2O3 lattice, the lattice con increased by 0.170 Å, the average Fe-O bond length is decreased by 0.044 Å, and t  Figure 5 shows the simulated crystal lattice of α-Fe 2 O 3 (u) and La-doped α-Fe 2 O 3 . Table 3 shows the simulated lattice constants, Fe-O bond length, and energy in α-Fe 2 O 3 (u) and La-doped α-Fe 2 O 3 . When the La element is doped in the α-Fe 2 O 3 lattice, the lattice constant is increased by 0.170 Å, the average Fe-O bond length is decreased by 0.044 Å, and the Fe-O bond energy is increased by 185.699 kcal/mol. A La atom replaces the steric Fe sites and bonds to O, reducing the Fe-O bond length and enlarging the Fe-O bond energy, which improves the crystal stability [14,34].   Figure 6 shows the FT-IR spectrum of α-Fe2O3(u) and La-doped α-Fe2O3(x) (x = 4, 8). As presented in Figure 6a, the band at 3400 cm −1 is assigned to the stretching vibratio of the hydroxyl group absorbed on the iron oxide surface [35]. Figure 6b shows the ma nified view of the FTIR spectrum. As reported by Miah [36], the characteristic absorptio peak of La2O3 appears at 574 cm −1 . For La-doped α-Fe2O3(x) (x = 4, 6,8), the bands at ~5 cm −1 have a redshift of 1.68 cm −1 , 0.83 cm −1 , and 0.90 cm −1 , respectively. As reported by W [37], the characteristic absorption peak of Fe2O3 is around 464 cm −1 .  [38,39]. In addition, the blueshift in th FT-IR spectra of La-doped α-Fe2O3(6) is more obvious, indicating that the Fe-O bond more distorted as the degree of lattice distortion increases.   Figure 6 shows the FT-IR spectrum of α-Fe 2 O 3 (u) and La-doped α-Fe 2 O 3 (x) (x = 4, 6, 8). As presented in Figure 6a, the band at 3400 cm −1 is assigned to the stretching vibration of the hydroxyl group absorbed on the iron oxide surface [35]. Figure 6b shows the magnified view of the FTIR spectrum. As reported by Miah [36], the characteristic absorption peak of  [38,39]. In addition, the blueshift in the FT-IR spectra of La-doped α-Fe 2 O 3 (6) is more obvious, indicating that the Fe-O bond is more distorted as the degree of lattice distortion increases.   (6), and La-doped α-Fe2O3(8) are 0.013 μA/cm −2 , 0.0024 μA/cm −2 , 0.0013 μA/cm −2 , and 0.0076 μA/cm −2 , respectively. La-doped α-Fe2O3(6) has a relatively positive corrosion potential and a lower corrosion current density, indicating the higher corrosion resistance. The more difficult the bond destruction, the stronger the corrosion resistance [40]. Therefore, the higher Fe-O bond energy in La-doped α-Fe2O3(6) leads to its better corrosion resistance.   La-doped α-Fe 2 O 3 (6) has a relatively positive corrosion potential and a lower corrosion current density, indicating the higher corrosion resistance. The more difficult the bond destruction, the stronger the corrosion resistance [40]. Therefore, the higher Fe-O bond energy in La-doped α-Fe 2 O 3 (6) leads to its better corrosion resistance.   (6) has a relatively positive corrosion potential and a lower corrosion current density, indicating the higher corrosion resistance. The more difficult the bond destruction, the stronger the corrosion resistance [40]. Therefore, the higher Fe-O bond energy in La-doped α-Fe2O3(6) leads to its better corrosion resistance.   Figure 8 shows the potentiodynamic polarization curve of α-Fe 2 O 3 and La-doped α-Fe 2 O 3 -modified waterborne coatings. The corrosion potentials of the coating(u), coating(4), coating (6), and coating(8) are −693.46 mV, −686.10 mV, −482.14 mV, and −706.65 mV, respectively. The corrosion current densities of the coating(u), coating(4), coating (6), and coating(8) are 0.28 µA/cm −2 , 0.06 µA/cm −2 , 0.028 µA/cm −2 , and 1.26 µA/cm −2 , respectively. Coating(6) has a relatively positive corrosion potential and a lower corrosion current density. Therefore, coating(6) has higher corrosion resistance.

La-Doped α-Fe 2 O 3 Modified Waterborne Coating
Nanomaterials 2022, 12, x FOR PEER REVIEW Figure 8 shows the potentiodynamic polarization curve of α-Fe2O3 and La-d Fe2O3-modified waterborne coatings. The corrosion potentials of the coating( ing(4), coating (6), and coating (8) are −693.46 mV, −686.10 mV, −482.14 mV, and mV, respectively. The corrosion current densities of the coating(u), coating(4), co and coating (8) are 0.28 μA/cm −2 , 0.06 μA/cm −2 , 0.028 μA/cm −2 , and 1.26 μA/cm −2 tively. Coating(6) has a relatively positive corrosion potential and a lower corro rent density. Therefore, coating(6) has higher corrosion resistance.   (6) is the largest. Based on the Nyquist plots, t sponding impedance frequency diagram ( Figure 9c) and phase frequency diagram (F are obtained. According to the literature [41], the impedance increases with an in the capacitive arc radius or the modulus; when the phase angle is close to 90°, th tive reactance is similar to the perfect capacitance, leading to a smaller capacitan larger impedance. Therefore, coating(6) has better corrosion resistance. Moreo equivalent circuit and Rc values are obtained by fitting the Nyquist plots, as s Figure 10. Rs, Rc, Q, and W represent the electrolyte resistance, the coating charge resistance, the coating double-layer capacitance, and the Warburg impedance tively. Rc reflects the corrosion resistance. Coating (6) gives the largest Rc value kΩ·cm 2 ), followed by coating(4) (818.9 kΩ·cm 2 ), coating(u) (69.8 kΩ·cm 2 ), and co (28.4 kΩ·cm 2 ), also indicating that the corrosion resistance of coating(6) is better.   [41], the impedance increases with an increase in the capacitive arc radius or the modulus; when the phase angle is close to 90 • , the capacitive reactance is similar to the perfect capacitance, leading to a smaller capacitance and a larger impedance. Therefore, coating(6) has better corrosion resistance. Moreover, the equivalent circuit and R c values are obtained by fitting the Nyquist plots, as shown in Figure 10. R s , R c , Q, and W represent the electrolyte resistance, the coating charge transfer resistance, the coating double-layer capacitance, and the Warburg impedance, respectively. R c reflects the corrosion resistance. Coating (6) gives the largest R c value (1951.6 kΩ·cm 2 ), followed by coating(4) (818.9 kΩ·cm 2 ), coating(u) (69.8 kΩ·cm 2 ), and coating(8) (28.4 kΩ·cm 2 ), also indicating that the corrosion resistance of coating(6) is better. Figure 11 shows the intersection Bode diagram (IBP) of coating(u) and coating(x) (x = 4, 6, 8). When the Bode phase plot and Bode impedance plot tend to be intersected at the upper left corner, the corrosion resistance is better. The corresponding phase angle and impedance of IBP show a similar increasing tendency, and the corresponding frequency shows a decreasing tendency [42]. Compared with these of coating(u) and coating(x) (x = 4, 8), the IBP value of coating(6) is the highest, indicating better corrosion resistance. resistance, the coating double-layer capacitance, and the Warburg impedance, respectively. Rc reflects the corrosion resistance. Coating (6) gives the largest Rc value (1951.6 kΩ·cm 2 ), followed by coating(4) (818.9 kΩ·cm 2 ), coating(u) (69.8 kΩ·cm 2 ), and coating(8) (28.4 kΩ·cm 2 ), also indicating that the corrosion resistance of coating(6) is better.   Figure 11 shows the intersection Bode diagram (IBP) of coating(u) and coating(x) (x = 4, 6, 8). When the Bode phase plot and Bode impedance plot tend to be intersected at the    Figure 11 shows the intersection Bode diagram (IBP) of coating(u) and = 4, 6, 8). When the Bode phase plot and Bode impedance plot tend to be inter upper left corner, the corrosion resistance is better. The corresponding pha impedance of IBP show a similar increasing tendency, and the correspondin shows a decreasing tendency [42]. Compared with these of coating(u) and co

Conclusions
α-Fe2O3 and La-doped α-Fe2O3 were prepared using a hydrothermal process synthetic pH value affects the morphology and structure of La-doped α-Fe2O3. At p and 8, the La-doped α-Fe2O3 nanoparticles show irregular morphology with agglom tion. At pH = 6, the La-doped α-Fe2O3 nanoparticles are well dispersed and show sm nanospheres.
The α-Fe2O3 lattice is distorted by the La-doping. When the synthetic pH value the La doping leads to a larger lattice distortion, which strengthens the Fe-O bond Fe2O3. The increase in binding energy improves the electrochemical stability of α-F As a result, the waterborne coating modified with La-doped α-Fe2O3 nanoparticles thesized at pH = 6 has better corrosion resistance.