Eminently Enhanced Anticorrosion Performance and Mechanisms of

For another example, Al-doped ZnO protective thin films fabricated by Teucher G. et al. ... The corrosion inhibition property was evaluated by electro...
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Eminently Enhanced Anticorrosion Performance and Mechanisms of X‑ZnO (X = C, N, and P) Solid Solutions Jing-Yu Zhang,†,‡ Xi-Zi Xue,†,‡ and Jin-Ku Liu*,†,‡ †

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Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China ‡ Material Corrosion and Protection, Key Laboratory of Sichuan Province, Sichuan 643000, P. R. China ABSTRACT: Nonmetal (C, N, P) doped zinc oxide solid solutions (ZnO SSs) prepared through one-step calcination method exhibited novel anticorrosion capability. The anticorrosion property was identified by electrochemical impedance spectroscopy and polarization curve technique. The maximal impedance belonged to C-ZnO SS, which was 12 times higher than pure ZnO materials. Then, a synergistic anticorrosion mechanism was proposed: the photoelectron flow suppression effect and the self-cohering process formed by doping. The tiny particle size as well as the minor zeta-potential of X-ZnO SSs eminently promoted the intermolecular cohesion and the formation of the compact surface automatically. Moreover, the photocatalytic experiments successfully verified the product and the corrosion inhibition effectiveness of the photoelectrons. Additionally, a positive correlation conclusion between the anticorrosion performance and the photocatalytic performance of the X-ZnO SSs was obtained. Consequently, developing the self-cohering anticorrosion materials is of crucial industrial application prospect and value. reports of nonmetal doped zinc oxide in the anticorrosion field were only few articles,20,23−28 not the systematic mechanism research. Derived from this, further exploring the mechanism of ZnO materials modified with different nonmetal elements and the rules of anticorrosion ability makes sense. Considering the unique physical and chemical properties of solid solution, we successfully prepared X-ZnO solid solutions (SSs) by doping nonmetal elements (X = C, N, and P) through one-step calcination method. This work also showed an interesting positive correlation outcome of the anticorrosion and the photocatalytic performance of X-ZnO SSs. Moreover, the properties of smaller size, lager specific surface area, and minor zeta-potential all enable X-ZnO SSs to become inorganic green anticorrosive pigments.

1. INTRODUCTION Currently, there are many ways of metal anticorrosion, to name but a few, the sacrificing anodes,1 impressed current protection,2 improvement of the metal material,3 and photocatalytic anticorrosion.4 Among the methods above, photocatalytic anticorrosion emerged with its strong abilities of nonsacrificial.5 Photocatalytic layer was applied as the protective layer of the metal, providing semipermanent corrosion protection6,7 by means of the transformation of solar energy. For a better efficiency of corrosion protection, the key point is to design and construct active semiconductors, among which ZnO remains an attractive one. Because of the nanosized structure,8 bond activation promotion,9 environmental sustainability,10 wide energy absorbance,11 high electron mobility,12 and room-temperature ferromagnetism,13 doped ZnO has been used at various fields14,15 such as antibiotic application,16 building materials,17 water quality improvement,18 gas sensing,19 and corrosion inhibition.20 To develop the potential of zinc oxide in the field of anticorrosion, the ZnO was modified with TiO221 and aromatic organic compounds.22 On top of that, one effective approach to enhance the anticorrosion property of the ZnO was elements doping. Rostami et al.23 synthesized Co-ZnO by combustion method, enhancing the corrosion resistance of the epoxy coating. Wu B. et al.24 manufactured Ir-ZnO/Zn superhydrophobic surface with great catalytic and anticorrosive property on zinc substrates. For another example, Al-doped ZnO protective thin films fabricated by Teucher G. et al.25 exhibited prominent corrosion resistance. Nevertheless, the © 2017 American Chemical Society

2. EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate analytical reagent (AR) and glycine (AR) were from Shanghai Titanchem Corporation. Urea (AR) and sodium phosphate dibasic (AR) were from Shanghai Lingfeng Chemical Reagent Corporation. Rhodamine B (AR) was from Shanghai Macklin Biochemical Corporation. Oil-based epoxy resin (E20) and curing agent (polyamides) were provided by Kukdo Chemical Corporation. Preparation of X-ZnO SSs. The X-ZnO SSs were synthesized by one-step combustion method. Glycine (0.02 mol), used as the leavening agents and the carbon source, was mixed with the zinc Received: July 6, 2017 Published: October 5, 2017 12260

DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Inorganic Chemistry

Figure 1. Schematic of macroscopic quantity preparation of X-ZnO SSs, the preparation of anticorrosion coatings and the mechanism research by continuous photodegradation react.

Table 1. Chemical Composition of the Mild Steel elements

Fe

C

Si

Mn

P

S

Cu

others

wt %

98.00

0.20

0.30

0.50

0.06

0.04

0.30

0.60

Figure 2. TEM patterns of (a) ZnO; (b) C-ZnO SS; (c) N-ZnO SS; (d) P-ZnO SS. prepared in the same way (Figure 1), among which the H2NCONH2 (0.02 mol) and NH4H2PO4 (2 × 10−4 mol) were used as the nitrogen and phosphorus sources, respectively. Epoxy Nanocomposites Preparation. The solid solution coatings were prepared by dispersing X-ZnO SSs and dispersants

nitrate hexahydrate (0.01 mol) in a mortar. After grinding sufficiently, transparent liquid was obtained. Then, the resultant solution was heated at 140 °C for 2 h in a drying oven. Afterward the intermediate was calcined at 600 °C in muffle furnace for another 2 h. Finally, the C-ZnO SS was prepared. The N-ZnO SS and P-ZnO SS were 12261

DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Inorganic Chemistry agent in the epoxy resin for intensive mixing. After it was stirred for 3 h, the polyamine hardener was added into the suitable fineness solution for another 10 min. The epoxy composites were applied on the carbon-steel specimens by a film applicator. Samples were then kept at room temperature for 24 h. The dry thickness of the films was ∼50 ± 5 μm measured by micrometer. The coating preparation process was illustrated in Figure 1. The chemical composition of the mild steel specimens was shown in Table 1. Electrochemical Experiment. The corrosion inhibition property was evaluated by electrochemical impedance spectroscopy (EIS) measurements. The mild steel specimens with X-ZnO SSs (2.25 cm2) coatings were exposed to the 3.50 wt % NaCl solutions for different immersion time. A three-electrode cell including mild steel specimen used as working electrode, saturated Hg2Cl2/Hg as reference electrode, and platinum electrode as auxiliary electrode. The frequency range and perturbation of the measurements were 100 kHz−100 MHz and 20 mV. Polarization test was done by employing AUTOLAB G1 at the sweep rate of 50 mV/s from −0.30 to −1.20 V. All experiments were conducted at different immersion time of 2, 6, 12, 24, 48, and 72 h on the samples. Photocatalytic Performance Examination. The photodegradation process schematic diagram was displayed in Figure 1. X-ZnO SSs catalysts (0.2 g) were separately added into 50 mL of 2 × 10−5 g L−1 Rhodamine B (Rh. B) solution and then exposed under the 350 W Xe lamp with a 420 nm filter. The reaction system was stirred for 0.5 h in dark to meet the adsorption−desorption equilibrium before irradiation. The temperature of the experimental solution was maintained at 25 °C. Samples were taken to test the absorbance of Rh. B solution until the dye became colorless as time progressed. The UV−vis absorption spectra of samples were tested at room temperature (25 °C). Characterization Sections. X-ray diffraction (XRD) was measured by a Shimadzu XD-3A diffractometer to determine the crystal structure. The microstructures and morphologies were analyzed by transmission electron microscopy (TEM) with an acceleration voltage of 200 kV (Hitachi-800). X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II) was utilized to study the surface composition. UV−vis spectroscopy (Shimadzu, UV-2600) was applied to explore the absorption properties. The isothermal nitrogen adsorption−desorption analysis by Micromeritics ASAP 2400 was tested to investigate the specific Brunauer−Emmett−Teller (BET) surface area. EIS tests were conducted by IM6e electrochemical workstation (Zahner-Electrik, Germany). Inductively coupled plasma (ICP) tests (Agilent 725ES) were made to confirmed the compositions of X-ZnO SSs.

Figure 3. XRD patterns of X-ZnO SSs modified by different elements: (a) ZnO; (b) C-ZnO SS; (c) N-ZnO SS; (d) P-ZnO SS.

and X-ZnO SSs was calculated to be 99.21%, 96.65%, 98.80%, and 98.12%, respectively. 1 XC = I 1 + k Ia (1) c

where Ia and Ic represented the amorphous phase and the cumulative diffraction intensity of the crystalline phase, the constant k here is usually equal to 1. Speculated from this, solid solution doping reduced the crystallinity of the particles, forming defect energy level in the energy gap.30 When dispersed in epoxy resin, abundant dangling bonds and unsaturated bonds in X-ZnO SSs facilitated the superior tightness of the modified corrosion coating. For the purpose of investigating the surface compositions and chemical status of X-ZnO SSs, XPS measurements were performed (Figure 4). O and Zn elements were contained in all samples. After Gaussian fitting of the curves, the O 1s spectra displayed two peaks centered at 529.5 and 530.8 eV, respectively, which corresponded to diverse states of oxygen within X-ZnO SSs. The shoulder peak on low binding energy (BE = 529.5 eV) side was assigned to the lattice O2− ions of Zn−O bonds in wurtzite structure.31 The component at higher BE (530.8 eV) was attributed to the presence of dissociated or chemisorbed oxygen or some loosely bound oxygen species (H2O, OH group, or O2).32 The minor shifts of O 1s of X-ZnO SSs owed to the formation of the solid solution.33 In addition, two different peaks at 1021.5 eV of Zn 2p3/2 and 1044.6 eV of Zn 2p1/2 were fitted in the Zn 2p (Figure 4b) spectra illustrated the existence of ZnO moiety in the solid solutions.34 However, the slight shift of peaks of Zn 2p of X-ZnO SSs toward the lower BE may indicate that the larger-sized Zn atom was more likely to be substituted by the C, N, and P atoms.35 Furthermore, by fitting the experimental line profile, the C 1s spectra of C-ZnO SS were divided into three peaks at 284.7, 286.1, and 289.1 eV (Figure 4c). The peak on the low BE was ascribed to the residual carbon and the adventitious hydrocarbon from the XPS instrument itself. The BE at 286.1 eV indicated that the carbon was doped into the interstitial positions of ZnO lattice and formed the band of C−O−Zn.36 Besides, the higher BE at 289.1 eV corresponded to the adsorbed carbonate or carbon dioxide on the surface.37 Seen from Figure 4d, the BE of N 1s of N-ZnO SS was very sensitive to the chemical environment of the nitrogen atom.38 The peak at 397.1 eV was originated from the N−Zn−N bonds,

3. RESULT AND DISCUSSION Structures and Morphologies. The morphologies and structures of the X-ZnO SSs were studied by TEM technique (Figure 2). The nonmetal doping did not change the original morphologies of ZnO materials but made the surfaces of the spherelike products much smoother and more compact. When dispersing the X-ZnO SSs in the epoxy to prepare the coating, the airtight quality of coating film improved. Moreover, the solid solution doping with C, N, and P elements decreased the particle sizes to 60, 90, and 80 nm, respectively, compared to ZnO materials of 120 nm. Hence, the diameter reduction of particles led to larger specific surface areas and denser films, which covered on the metal surface to enhance anticorrosion efficiency. As shown in the XRD patterns of X-ZnO SSs (Figure 3), all diffraction peaks were in good agreement with the standard XRD patterns of ZnO (JCPDS card No. 36−1451) of P63mc wurtzite structure,29 indicating the successful formation of the solid solution. Seen from the diffraction peak pattern, intensity of peaks of the X-ZnO SSs were less and broader than that of pure ZnO. Followed by the eq 1, the crystallinity of pure ZnO 12262

DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Inorganic Chemistry

Figure 4. XPS of O 1s, Zn 2p, C 1s, N 1s, and P 2p for ZnO and X-ZnO SSs.

Figure 5. Nitrogen absorption−desorption isotherms of different elements modified ZnO SSs: (a) ZnO materials; (b) C-ZnO SS; (c) N-ZnO SS; (d) P-ZnO SS.

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DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Figure 6. Nyquist plots of the steel panel immersed in the 3.50 wt % NaCl solution containing X-ZnO SSs extract for 2 h (a), 6 h (b), 12 h (c), 24 h (d), 48 h (e), and 72 h (f).

Table 2. Enhanced Percentage of Impedance Efficiency time percentage

2h

6h

12 h

24 h

48 h

72 h

ZnO/blank C-ZnO/blank N-ZnO/blank P-ZnO/blank

54.50% 1672.76% 81.13% 391.72%

38.67% 804.40% 76.82% 328.96%

36.09% 435.89% 52.44% 313.70%

76.82% 519.17% 120.92% 405.88%

108.28% 499.62% 133.56% 304.42%

83.24% 469.60% 150.02% 270.93%

surface areas of ZnO and C-ZnO, N-ZnO, and P-ZnO SSs were 0.22, 15.23, 12.11, and 14.04 m2 g−1, respectively. Besides, the particle sizes of X-ZnO SSs were counted by the “Nano Measurer” software based on the TEM images. The average particle sizes of ZnO and X-ZnO were 120, 50, 80, and 70 nm, respectively. The C-ZnO had the largest specific surface area and the least particle size,42 reducing the slits of the films of the pigment. Because of the largest specific surface area, the adsorption capacity of C-ZnO was stronger than other pigments, and the least particle size made the particles distribute on the surface evenly and more tightly. The block property of the coatings was enhanced by decreasing the porosities and lengthening the electrolyte pathways, which benefited for the anticorrosion performance. Anticorrosion Performance. The anticorrosion behaviors of the steel specimens coated with epoxy resin coating, epoxy with pure ZnO coating, and with different X-ZnO SSs coatings

indicating the oxygen was substituted by nitrogen in the initial O−Zn−O structure. In addition, the peaks at 398.7 and 399.7 eV could be ascribed to N− in the O−Zn−N linkages.39 The highest BE at 401.6 eV was attributed to the surface chemisorbed nitrogen species.40 These results certified that N dopant was incorporated successfully into the N-ZnO SS, which matches with the XRD results. Given in Figure 4e, the peak of P 2p at 138.5 eV was ascribed to P−O bond, which showed the perfect substitution of P in ZnO lattice through the present route without any improvement. The results above all demonstrated the successful formation of the solid solution, leading to the highly steady electrochemical performance.41 To further confirm the effect of the X-ZnO SSs, the isothermal nitrogen adsorption−desorption analysis measurement was investigated (Figure 5). All the graphics belonged to type IV, showing a rapidly increased isotherms under moderate pressure on account of the capillary condensation. The BET 12264

DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Inorganic Chemistry

Figure 7. (a) Bode plots and (b) polarization plots of the steel panel immersed in the 3.50 wt % NaCl solution containing X-ZnO pigments for 72 h.

were evaluated by EIS for different immersion time (Figure 6). The phase, real impedance (Z′), and imaginary impedance (Z″) was used to optimize the Nyquist plots.43 The diameter of semicircle portion at the high frequencies of the X-ZnO SSs pigments, especially C-ZnO SSs, was much larger than pure ZnO materials. Simultaneously, the regularity of impedance in descending sequence of C-ZnO, P-ZnO, N-ZnO, ZnO, and pure epoxy resin coatings kept all the same as time went by (Figure 6 and Table 2). Most notably, the impedance of CZnO reached as high as 121 161 ohm cm2 at the initial immersion for 2 h, enhanced by 1672.76% compared to the epoxy resin. The results derived from Nyquist plots might demonstrate that the solid solution doping X-ZnO coatings could reduce the porosity of the epoxy coating and prevent oxygen and water molecules going through coatings.44 Thus, the corrosion properties of X-ZnO SSs coatings improved a lot due to the formation of the solid shielding layer. It is difficult to make out the frequency of each point in highfrequency area of the Nyquist plots due to its cryptic frequency values. However, the Bode diagram provided a method for a description of electrochemical system features related to the frequency, expressing the impedance data more distinctly. After they were immersed in 3.50 wt % NaCl solution for 3 d, the C-, N-, and P-doped ZnO solid solution pigments performed with higher impedance values of 3.16, 1.38, and 2.19 kΩ cm−2, respectively, compared to ZnO pigments and pure epoxy resin at low-frequency end (Figure 7a). The best solid solution doping belonged to C-doped ZnO pigment, the impedance value of which enhanced almost by 281.45%. The superior anticorrosion behavior attributed to blocking the ionically conducting path through the coating caused by the diffusion inhibition of electrolytes through the coating.45 Moreover, another indicator for anticorrosion performance is the phase angle. Seen from Figure 7a, the sequence of the declined phase angles θmax of 58.72°, 54.17°, 50.84°, 41.00°, and 36.09°, respectively, belonged to C-ZnO, N-ZnO, P-ZnO, ZnO, and blank sample. The maximal phase angle belonged to the C-ZnO SS, indicating the film integrity of the pigment was the best of all the samples. The results were consistent with the Nyquist curves. Figure 7b exhibited polarization plots for the steel panel with absence and presence of anticorrosive pigments immersed in the 3.50 wt % NaCl solution. Polarization resistance (Rp) was calculated by software through anodic and cathodic current density and potential (Table 3), where Icorr, βa, and βc are the corrosion current density, anodic Tafel slopes, and cathodic Tafel slopes, respectively.46 When the coating contained anticorrosive pigments, the corrosion potential (Ecorr) became more positive, and the

Table 3. Electrochemical Parameters of the Polarization Curves sample

Ecorr (mV)

Icorr (μA cm−2)

βa (mV/dec)

βc (mV/dec)

blank ZnO C-ZnO N-ZnO P-ZnO

−841 −826 −754 −824 −768

224 219 46 174 112

752 971 272 589 1052

182 242 161 233 316

corrosion current density (Icorr) decreased. From corrosion thermodynamic perspective, Ecorr represents how facile it is to corrode. From corrosion kinetics viewpoint, Icorr stands for the rate of corrosion. The materials, which had the more positive Ecorr, squint toward forming airtight barrier for oxygen diffusion to the metal surface, thus delaying corrosion.47 The Ecorr of CZnO pigments was the most positive, then the P-ZnO, N-ZnO, ZnO, and blank sample, indicating that the solid solution doped ZnO coatings offered more effective guard to the steel panel in 3.50 wt % NaCl solution through inhibiting the anodic current of the corrosion reaction.48 In the meantime, the lower Icorr (46 μA cm−2) for C-ZnO pigments compared to ZnO pigments (219 μA cm−2) and epoxy resin (224 μA cm−2) indicated that the solid solution doped coatings benefited for the formation of a compact film and for the protection to the substrate film. The X-ZnO SSs coatings displayed superior anticorrosion efficiency owing to their extraordinary barrier effect and tight nanostructure. According to Figure 8, the equivalent circuits and the bode curves of ZnO materials and X-ZnO SSs (take C-ZnO for example) pigments were presented after immersed in 3.50 wt % NaCl solutions for 72 h. The Rs, Rct, CPEdl, Rf, and CPEf in the equivalent circuits represented for the solution resistance, charge transfer resistance, constant phase element of double layer, film resistance, and constant phase of the film, respectively.49 When using ZnO pigments as anticorrosion coating, the electrolyte was saturated with the coating and then penetrated into the interface between the coating and the base metal, thus producing two time constants. While using C-ZnO SS, the electrolyte could not reach the interface between the coating and the base metal, preventing metal from corroding as well as protecting the combination of coating and metal. The results may demonstrate that the coatings modified by X-ZnO SSs were inclined to generate better insulation to keep electrolyte from seeping into the metal surface. All the EIS parameter values were calculated according to the following equations and were listed in Table 4. The interface between the electrode and the capacitor is generally equivalent to a capacitor, called electric double layer capacitor, thus 12265

DOI: 10.1021/acs.inorgchem.7b01716 Inorg. Chem. 2017, 56, 12260−12271

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Inorganic Chemistry

Figure 8. Bode diagrams of the epoxy coatings and the electrochemical equivalent circuits of (a) ZnO pigments, (b) C-ZnO SS pigments in 3.50 wt % NaCl solutions at 72 h.

Table 4. Electrochemical Parameters Extracted from Impedance Data for the Steel Samples Immersed in the 3.5 wt % NaCl Solutions after Different Immersion Times CPEdl RP (kΩ cm2)

sample blank (2 h) ZnO (2 h) C-ZnO (2 h) N-ZnO (2 h) P-ZnO (2 h) Blank (24h) ZnO (24 h) C-ZnO (24h) N-ZnO (24 h) P-ZnO (24 h) Blank (72 h) ZnO (72 h) C-ZnO (72 h) N-ZnO (72 h) P-ZnO (72 h)

Y0 (ohm−1 cm−2 s−n)

6.76 10.52 117.31 12.24 33.10 1.83 3.28 11.42 4.03 7.98 1.08 2.00 6.17 2.67 3.95

2.19 1.07 2.48 4.50 5.43 9.90 2.11 6.69 1.03 6.24 1.44 3.97 4.50 1.61 8.94

× × × × × × × × × × × × × × ×

n

−6

0.51 0.43 0.54 0.41 0.24 0.57 0.55 0.64 0.49 0.16 0.58 0.63 0.71 0.46 0.40

10 10−7 10−11 10−6 10−9 10−5 10−5 10−6 10−5 10−7 10−4 10−5 10−5 10−5 10−6

× × × ×

10−6 10−6 10−7 10−7

7.99 1.52 4.64 1.28

× × × ×

10−5 10−5 10−5 10−5

1.94 2.53 2.64 7.67

× × × ×

10−4 10−5 10−5 10−5

forming an equivalent element Q, the impedance and admittance of which were calculated by eqs 2 and 3:37 ⎛ nπ ⎞ ω−n ω−n ⎜⎛ nπ ⎞⎟ 1 ·(jω)−n , Z′Q = cos⎜ ⎟, Z′′Q = sin , ⎝2 ⎠ ⎝2 ⎠ Y0 Y0 Y0

ZQ =

(2)

0