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Effect of organic compounds and rough inorganic layer formed by plasma electrolytic oxidation on photocatalytic performance
Journal Pre-proof Effect of organic compounds and rough inorganic layer formed by plasma electrolytic oxidation on photocatalytic performance Wail Al Zoubi, Min Jun Kim, Dong Keun Yoon, Abbas Ali Salih Al-Hamdani, Yang Gon Kim, Young Gun Ko PII:
S0925-8388(20)30150-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153787
Reference:
JALCOM 153787
To appear in:
Journal of Alloys and Compounds
Received Date: 23 November 2019 Revised Date:
8 January 2020
Accepted Date: 9 January 2020
Please cite this article as: W. Al Zoubi, M.J. Kim, D.K. Yoon, A.A. Salih Al-Hamdani, Y.G. Kim, Y.G. Ko, Effect of organic compounds and rough inorganic layer formed by plasma electrolytic oxidation on photocatalytic performance, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153787. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Contributions W.A.Z. performed the experiments and wrote the manuscript. M. J. K, D. K. Y and A. A. A , analysed and interpreted the data. Y.G.K conceived and designed the concept of experiments and revised the manuscript. All authors contributed to the discussion of the results.
Effect of Organic Compounds and Rough Inorganic Layer Formed by Plasma Electrolytic Oxidation on Photocatalytic Performance Wail Al Zoubia, Min Jun Kima, Dong Keun Yoona, Abbas Ali Salih Al-Hamdanib, Yang Gon Kimc, Young Gun Koa* a
Materials Electrochemistry Group, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea b Chemistry Department, College of Science for Women, University of Baghdad, Baghdad, Iraq c Extreme Fabrication Technology Group, Korea Institute of Industrial Technology, Daegu 42994, Republic of Korea Abstract Plasma electrolytic oxidation and dip chemical coating as fabrication techniques are performed in ambient temperature and pressure, giving a competitive-edge in the commercial applicability. Thus, we proposed new compositions of flower-like hybrid materials, MgO-TiO2-HQ(8-hydroxyquinoline), MgO-TiO2-HQ-APY(2-aminopyridine) and MgO-TiO2-HQ-APH(2-aminophenol), as photocatalysis with high corrosion resistance via a combination of plasma electrolytic oxidation and dip chemical coating in which 8-HQ, 2-APY, and 2-APH were used as the organic components. TiO2-HQ-APY and MgO-TiO2-HQ-APH
exhibited
improved
simultaneous
electrochemical
and
photocatalytic
performance on photodecomposition of methylene blue (MB) in an aqueous solution in the presence of ultraviolet-visible irradiation comparing to that of MgO-TiO2-HQ. The synergistic effects of organic components anchored on TiO2 and MgO containing inorganic layer were the main source for enhanced anticorrosion resistance. The MgO-TiO2-HQ and MgO-TiO2-HQ-APH could be recovered simply by a magnet and reused several times with no significant diminution in high stability photocatalytic performance during catalytic oxidation. The work indicated that synergistic effects of aromatic heterocycles on TiO2 and MgO containing inorganic layer played key role in enhancing photocatalytic activity and electrochemical behavior. To support the experimental section, quantum chemical parameters such as highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELOMO), and gap energy (∆E) were calculated using density functional theory.
Keywords: Organic layer, Organic layer, quinoline, photocatalysis, Corrosion.
1
1. Introduction Lightweight metals have low anti-corrosion resistance, particularly in corrosive media, on account of their negative electric potential and high chemical activity. Thus, one of the most actual methods to improve electrochemical performance was the use of multilayer (hybrid) coating which could decrease or inhibit corrosion reactions between a metal weight surface and its surroundings. The coating layers were regularly composed of polymer-organic, inorganic-inorganic, organic-inorganic, organic-organic or inorganic compounds [1-3]. Thus, coating layers may be produced via one treatment, (e.g., sol-gel [4], plasma electrolytic oxidation treatments (PEO) [5,6]), two treatments (e.g., PEO and post-treatment [7], PEO and ED (electrophoretic deposition) [8], or pack cementation and sol-gel [9]), or a mixture of numerous treatments (e.g., PVD (physical vapor deposition) of organic compounds, PEO, hydrothermal treatment, and electrodeposition [10]). Therefore, coatings were fabricated to add particular functional groups to the substrate surface or to alter its catalytic, or biological, electrochemical properties. Thus, this work is focused on how to enhance both the absorption of light and corrosion resistance of TiO2-containing PEO coating on AZ31 Mg to achieve outstanding photocatalytic performance. For photocatalytic performance, the scientists employed various surface alteration methods among which doping with non-metal (N, C, S, F etc.) [11,12] as well as transition metal doping (Mn, Zn, Cr, Fe, Cu, Ni etc.) [13] appear to be the most favorable for titanium dioxide band-gab narrowing, but TiO2 alteration with graphene [14-16], metal [17] or semiconductors coupling are generally used in order to prevent hole/electron pairs recombination rate. Furthermore, several attempts to extend adsorption of TiO2 to visible light, for instance, Fe(III)-8-hydroxyquinoline-5-sulfonic acid, Er(III)-8hydroxyquinoline-5-sulfonic acid [18], 8-hydroyquinloine [19], polymers [20,21], and derivatives of ruthenium bipyridly complexes [18] have been often used to adjust TiO2 so as to proficiently use the solar light-driven photocatalysis. Thus, it has been documented that the applications of ruthenium complexes and conjugated organic compounds [22] are restricted by the low photochemical stability and poor charge transfer of former, and toxicity and cost of the ultimate. Subsequently, it is extremely 2
sought-after to grow novel donor organic compounds which absorb robustly in the ultraviolet visible (UV-vis) area of the solar light with the different levels of energy competent of injecting excited electron into TiO2 conduction bands. Non-toxicity and low cost are also significant factors to consider in the development of these new metal complexes. 8-Hydroxyquinoline and its compounds had been well-known for their outstanding properties to transfer electrons from aromatic compounds to the conduction band of TiO2 depends on absorption of visible light. 8-Hydroxyquinoine complexes with broad range UV-vis absorption can be facilely obtained. For instance, Chen et al. found that the Fe-quinoline complexes (HQSI) can transfer excited electrons to the conduction band of TiO2 and be renovated by the electrons injection from organic contaminants through the photodecomposition in aqueous media [23]. Other researchers using density functional theory investigated the photo-sensitization mechanism of Fe(III)/tris(8-hydroquinline-5carboxylicacid) on TiO2 surface. As a consequence, they reported that organic complexes have a very small LUMO (lowest unoccupied molecular orbital)-HOMO (highest occupied molecular orbital) energy gap which permits for easy electron irritation from HOMO to LUMO [24]. The HOMO electron is directly injected into the TiO2 conduction band once the organic ligands, that include the dihyroxy [25-28], dicyanomthylene [29,30], and carboxylic [31] compounds, were adsorbed on TiO2 surface. Although these organic ligands have been used to prepare visible-light-driven TiO2 photocatalyst, the interaction between TiO2-doped inorganic layer and synergistic heterocyclic compounds on weight-light metal surface has been rarely investigated, especially the direct chemical bonding. In this paper, we report the experimental and theoretical study of the impacts of electronic chemical structure and interfacial physicochemical interactions between synergistic heterocyclic compounds and TiO2-MgO containing inorganic materials, formed on AZ31 Mg with an assistance of plasmaassisted electrochemical reactions, on the photocatalytic activity of TiO2-MgO containing inorganic materials. We have revealed that the electronic structures and the synergistic effects of the aromatic heterocycles determine their energy gap between HOMO and LUMO, and additional affected electron 3
transfer from ligands and their complexes to TiO2-MgO containing inorganic material surface, which mainly determined the photocatalytic activity of inorganic layer. The organic-inorganic hybrid materials were prepared by conducting PEO on weight alloy to form inorganic layer (IL), which is pursued by dip chemical coating (DCC) to prepare organic ligand (8-hydroxyquinoline (HQ), 8hydroxyquinoline/2-amninopyridine (HQ-APY) and 8-hydroxyquinoline/2-aminophenol (HQ-APH) respectively) as organic coating on porous surface of inorganic layer (IL). After linked with synergistic organic ligands, TiO2-MgO containing inorganic materials surface showed higher photochemical stability and photocatalytic activity on the decomposition of the methylene blue under both UV-vis irradiation as well as extraordinary electrochemical stability. The results were provided by the theoretical studies, elucidated photocatalytic activities of TiO2-HQ, TO2-HQ-APY and TO2HQ-APH using two probable interfacial interactions. This paper described has provided significant insight on drawing new photocatalysts with enhanced corrosion resistance which will remarkable widen the application of inorganic coatings in photocatalytic degradation field of organic contaminants.
2. Experimental 2.1. Preparation of organic-inorganic hybrid materials 8-HQ), 2-APY, 2-APH, MB, NaAlO2, KOH, TiO2 and SnO2 were, purchased from Merck and Fluka, and used as received. Besides, AZ31 Mg alloy with the nominal composition (wt%) of 2.89 Al, 0.96 Zn, 0.31 Mn, 0.15 Fe, 0.12 Si, and balanced Mg (in wt%) were polished using SiC papers up to 2400 grit and then cleaned with deionized (DI) water and pure ethanol. TiO2-doped inorganic layer was prepared in an alkaline electrolyte composed of 4 g L-1 NaAlO2, 4 g L-1 KOH, 4 g.L-1 TiO2 and 4 g L-1 SnO2 using glass vessel equipped with Mg alloy, stainless steel net, as anode and cathode respectively, and a magnetic stirrer at temperature below 288 K. PEO treatment were carried out for 6 min under substitutional current with a current density around 100 mA.cm-2 and a frequency of 60 Hz. Following the PEO process, prepared samples were washed in DI (Distilled) water and dehydrated in warm air. 4
Subsequently, to form OL on inorganic layer, DCC was carried out by immersing the specimens after PEO in an ethanolic solution of 0.05 M 8-hydroxyhydroquinone (MgO-TiO2-HQ), 8hydroxyhydroquinone/0.01
M
2-aminopyridine
(MgO-TiO2-HQ-APY),
and
0.05
M
8-
hydroxyhydroquinone/0.01 M 2-aminophenol (MgO-TiO2-HQ-APH), respectively, for 1 day at 298 K to yield self-assembled hybrid organic-inorganic coatings. 2.2. Microstructural characterization The microstructural observation was performed using a scanning electron microscope (SEM, Hitachi, S-4800) provided with X-ray spectroscopy (EDS, HRIBA EMAX) and transmission electron microscope (TEM; Philips, CM 200) operating at 200 kV for detailed observations. The structural results were documented using Fourier-transform infrared (FT-IR; Bio-Rad, Excalibur Series FTS 3000) spectroscopy was used to define the chemical structure of the organic-inorganic materials. Xray photoelectron spectroscopy. (XPS, VG Microtech, ESCA 2000) was used to observe the surface components of the samples. Regarding the optical properties, the UV-vis spectra of the coatings were obtained by using a Varian Cary 5000 UV/VIS/NIR spectrophotometer in a range of 200-800 nm and the photoluminescence (PL) (Kimon, 1 K, Japan) of the coatings was documented in the domain 200800 nm using an irritation wavelength of 325 nm 2.3. Photocatalytic activity performance The photocatalytic valuations of the coatings are assessed by the photocatalytic removal of MB under UV-vis light irradiation. The degradations were performed in a photoreactor at a temperature of around 25 ˚C. Photocatalyst (500 mm2) were immersed in 50 ml of an aqueous MB solution (concentration = 20 mg L-1) and then stimulated in the without light for 35 min to supplement desorption and adsorption equilibrium. Uv-vis light irradiation of an aqueous solution was carried out using a 400-W UV lamp (λ>300 nm). The degradation rate of MB was checked by withdrawal 2 ml from the reaction solution. The concentration of MB in the aqueous solution was defined by UV-vis electronic spectroscopy at 663 nm. To study the photocatalytic stability and reusability of the prepared samples, hybrid materials were repeated for three cycles under the similar conditions. After each 5
experiment, the samples were divided from the MB solution, cleaned with DI water, dried in warm air, and used once more for the following cycle [32]. The kinetics of MB degradation followed pseudofirst-order model.
ln(C0 / Ct ) = kt
(1)
where k is the first-order reaction rate constant, Ct is residual concentration of reaction solution at regular time intervals under UV-vis irradiations, C0 is the initial concentration of MB, after the adsorption and desorption equilibrium. Every experiment was carried out in triplicate to confirm the photocatalytic performance of the prepared samples. 2.4. Electrochemical Measurements The corrosion measurements were evaluated in an aqueous solution (3.5% wt NaCl) at 25 °C using three different electrodes: a treated specimen with an unprotected part of 1 cm2 as working electrode, Ag/AgCl electrode as the reference, and a platinum mesh as the counter electrode. The electrochemical performance was carried out by electrochemical impedance tests (Gamry Instruments, Interface 1000). Electrochemical impedance tests were performed for 106 to 0.1 Hz at interval of 10 points/decade with a 10 mV rems. 2.5. Calculation details The geometries of 8-HQ, 2-APY and 2-APH were optimized using density functional theory (DFT) using parameterization method 3 (PM3) in the Hyper Chem-8 program [33].
3. Results and discussion 3.1. Characterization of structure and morphology The XRD patterns of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH are exhibited in Fig. 1. The peaks of TiO2 particles “rutile phase” can be observed at 36.79˚(103), 38.35˚(112), 47.95˚(200), 63.31˚(204) and 68.63˚(116) (JCPDS No.00-021-1272) which confirmed the crystal structure of TiO2. For MgAl2O4, the XRD peaks can be observed at 18.88˚(111), 36.77˚(311), 38.35˚(222), 44.61˚(400) and 68.94˚(531) (JCPDS No.00-021-1272). For SnO2, the XRD peaks can observed at 57.62˚(002), 64.98˚(112), 68.94˚(311) (JCPDS No.00-041-1445). On the other hand, 6
main peaks at the 2θ range of 5-30˚ indicated that the 8-HQ-inorganic coatings is mostly collected of MgQ2 and AlQ3 complexes which were in agreement with new reports (Ref. [33,34]). Moreover, additional XRD peaks at the 2θ range of 5-30˚ were assigned to the formation of new complexes (Scheme S1) (MgL2 and AlqL3, where L is Q, QAPY, or QAPH). These peaks were due to organic ligands on the coating surface which results in 2-aminopyridine and 2-aminophenol, respectively. Fig. 2 presents SEM and TEM images of the coatings. The organic-inorganic hybrid materials (MgOTiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH) exhibited flowerlike microstructure morphologies with a diameter of 2~5 nm, and few 2D petals adhere together (Fig.2a-c). From the high magnification SEM image in Fig. 2, hydrogen bonds between hydroxyl groups on the coating surface as well as π-electrons of the aromatic rings in the organic molecules may construct this 3D flower structure by interconnected dozen 2D sheets with a thickness of ~100 nm to form flower-like cluster, while the interaction between amino groups of complexes and hydroxyl group of TiO2 may another cause for aggregation (Fig 2d,e and Fig 2f). The availability of free electrons, aromatic rings (π-electron systems), and planar molecular in HQ facilitates electron transfer from the organic layer to the TiO2-doped inorganic layer. In other words, an electron-donor-acceptor complex results from electrostatic interaction among the charged centers of molecules and the charged inorganic materials surface, which causes in electrostatic interaction between the chemical molecules and inorganic materials surface [33]. These results indicated that porous inorganic layer may enable the heterocycles to penetrate and be simply adsorbed on the pore inorganic matrices. In order to validate the surface morphologies, the chemical composition of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH was further determined by SEM-EDS and TEM-EDS (Fig S1). Elemental analysis shown in Fig. S1 propose that a flowerlike organic-inorganic hybrid material consists solely of Mg, Al, C, N and O elements. However, significant differences in the atomic distribution of Mg, Al, C, N and O were attributed to the large aggregations and the reactions of heterocyclic compounds with inorganic compounds in the porous surface.
7
The chemical bonding state of the MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH can be seen on the XPS spectra (Fig. 3a). The wide-scan XPS spectra show the presence of Mg 1s, Al 2p, C 1s, N 1s, and O 1S energy region, as shown in Fig. 3. The high-resolution Mg 1s spectrum if Figure 3b showed one peak centered at 1302.6 eV which was attributed to the Mg 1s, demonstrating the Mg(II) oxidation state in formed complexes [35]. In terms of the Al 2p spectrum (as shown in Figure 3c), the binding energies for Al 2p at 74.7 eV correspond to the AlQ3 and AlQ2APY. Moreover, the peaks for the C 1s of the heterocycles were deconvoluted into several peaks, representing (C-C) bonds at 284.7 eV, C-O (phenol) bond at 285.1 eV, N-C (pyridine and amino) bond at 287.4 eV [36]. The peak intensities of C atom increased remarkably compared to PEO coating. For the N 1s spectrum (Fig. 3e), the N 1s peak at 399 eV 3e show that the broad N 1s spectra are composed of 398.4 eV MgN bonding and 399.2 eV Al-N bonding, respectively [37]. The O 1s spectrum (Fig. 3f) can be deconvoluted into two different peaks at 530.2 and 531.2 eV. The O 1s binding energy of 530.2 eV was assigned to the C-O bonds, while the binding energy 531.2 eV may be attributed to the O-M (Mg and Al) [38]. To confirm the quantitative results, the elemental analysis achieved via XPS indicated that increasing C and N and decreasing metallic elements (Mg, Al, Ti and Sn) were assigned to the organic layer formed on the coating surface (as shown in Table S1) [39]. Evidence for the complexation reactions on coating surface was gotten from FT-IR spectra of HQ, HQ-APY and HQ-APH, as shown in Fig. 4. In the HQ spectrum, the bands at 1666, 1576, 1467, 1375 and 1317 cm-1 are attributed to the aromatic heterocycles (quinoline group) of MgL2 and AlL3 complexes on the surface [40]. The band at 1498 cm-1 is attributed to the vibrations of the phenyl and pyridine groups in formed complexes [41]. In addition compared with the original organic compounds in Fig S2, the shift in the formation energy of the intermolecular hydrogen bonds towards higher region from about 3100 to 3350 cm-1 indicates the self-assembly association between 8HQ molecules triggered by the formation of complexes with metal ions (Mg2+ and Al3+) [42]. Therefore, the shift of the bands in the resulted complexes on the surface coating towards higher region to extent of 20-90 cm−1 as well as the disappearance of the strong band to the OH and NH2 groups in the spectra of the 8
prepared materials, indicates coordination via the nitrogen and oxygen atoms, as shown in Scheme S1. Moreover, the bands at 788 and 738 cm−1 are assigned to C-H deformation, which additional demonstrate the formation of complexes as suggested in Scheme S1. The noticed differences between the hybrid materials and the free ligand indicate that the ligand chelate with Mg2+ and Al3+ in coating surface, forming a stable MgL2 and AlL3 phase on AZ31 Mg substrate The UV-vis absorption spectra of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH were measured, as shown in Fig. 5a,b and Fig. 3S. Figure 5 shows the comparison of UV-vis absorption spectra of as the synthesized MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH with MgO-TiO2-doped coating. After reaction of organic compounds with inorganic surface, it is obviously exposed that the coatings shows increased absorption intensity in the region 300-480 nm (n→π*) due to the formation of organic complexes (Fig. 5a). The absorption wavelength edge of all hybrid photocatalysts is determined by the optical band gap (Eg) [44]. Based on transformed KubelkaMunk functions [45], Eg can be determined by the tangents to the liner portion of the plot between (αhν)1/2 vs. hν (i.e. the Tauc plot), where α is absorbance, h is Planck constant and ν is the associated frequency, as shown in Fig. 5. It is apparent that all of the MgO-TiO2-HQ (1.71 eV), MgO-TiO2-HQAPY (1.6 eV) and MgO-TiO2-HQ-APH (1.59 eV) show a narrower band gap compared to the 2.05 eV bandgap of MgAl2O4-TiO2-SnO2 (Fig 5b and Fig S3). Such reduction of the bandgap may lead to the enhanced photocatalytic activity of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH under visible light irradiation [46,47]. This enhancement is mostly assigned to the prepared organic complexes (MgL2 and AlL3). In our study, the higher visible light absorption ability of the prepared samples makes the selective photocatalytic oxidation of pollutants to be performed under the visiblelight irradiation. To address the effect of synergistic effect of organic components on the electron-hole couples separation in TiO2 based coating, PL spectra of the MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH were studied to additional comprehend of photo-generated charge carriers transformation and recombination likelihood in the hybrid materials of MgO-TiO2/organic coatings. The MgO-TiO2-HQ was showed two main peaks located at 482 and 556 nm, which attributed to the 9
recombination of free electrons of the TiO2-doped inorganic coating and restricted surface oxygen vacancies and porous, respectively [48]. As shown in figure, the composite of the MgO-TiO2-HQ affected its PL intensity, by which the PL spectra of the MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH coating was important lower than that of TiO2.This mentions to the repression of electron-hole recombination likelihood in the MgO-TiO2-HQ sample. The photoexcited electrons from TiO2 are moved into carbon atoms on aromatic heterocycles, and then the rate of the charge carriers’ recombination was decreased [49]. Therefore, the PL further confirmed the formation of hybrid chemical structure between TiO2 and bidentate absorbents hindered or almost avoids the electron-hole pair recombination. 3.2. Photocatalytic and electrochemical performances. In the current work, the photocatalytic dissolution of MB under Vis irradiation using the MgO-TiO2HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH as catalysts was studied. MB is a model pollutant for photocatalytic degeneration and a representative organic dye molecule in textile waste water. From Figs 6 and 7, a mixture of catalyst and MB solution was irradiated for around 30 min in the dark. A very tiny amount of MB was absorbed onto the hybrid material’s surface after time t. After that, MB solution was irradiated with visible light for t time in the presence of MgO-TiO2-HQ, MgO-TiO2-HQAPY, and MgO-TiO2-HQ-APH respectively. From Figs 6(a,b,c) and 7a, MgO-TiO2-HQ-APY (76%) and MgO-TiO2-HQ-APH (74%) exhibited the highest efficiency among all the samples studied. Both 2-APY and 2-APH present improved remarkably photocatalytic activity to over HQ. Therefore, it is worth to indicate that photocatalytic activity of HQ is higher than of MgO-TiO2-SnO2 under the similar condition, but weaker than that of MgO-TiO2-HQ-APY or MgO-TiO2-HQ-APH. The results indicate that efficient sensitization of MgAl2O4-TiO2-SnO2 from MgL2 and AlL3 complexes, and HL (8-HQ, 2-APY or 2-APH) alone, are recognized. The sensitization is supported via the effective electron injection from metal-organic complex (MgL2 and AlL3), or HL, to the conduction band of mixture MgAl2O4-TiO2-SnO2. In addition to the photocatalytic activity, stability of the photocatalysts is another significant factor for limiting the use of the catalysts in feasible applications. The 10
photocatalytic stability of MgO-TiO2-HQ and MgO-TiO2-HQ-APH was carried out by recycled photocatalytic experiments using and the results are presented in Fig. 7b. The photocatalytic degradation efficiency is not obviously decreased after three cycles. This shows the opportunity of using the prepared catalysts for longer operation time. The feature of prepared MgL2 and AlL3 over ligands may be elucidated by the difference of their anchoring modes to the surface of the inorganic coating containing TiO2. It has been explained that the chemical structure and attachment groups of MgL2 and AlL3 or HL on inorganic coating surface are critical to the light harvesting and the charge injection at the organic complexes-inorganic coating or ligand-inorganic coating interface, because they decide the amount of MgL2 and AlL3 or ligands molecules attached to their orientation on the inorganic coating surface. The impedance spectra of AZ31 Mg alloy in the absence and presence of MgO-TiO2-HQ, MgO-TiO2-HQ-APY, and MgO-TiO2-HQ-APH were recorded in Figure. 7c supplementary Table S2. The impedance diagrams show one capacitive loop characterized by somewhat depressed semicircle for all studied samples. This capacitive loop indicates that the degradation of magnesium in aggressive solution is mostly measured by charge transfer process, i.e., the resistance to the electron transfer of faradic process on the AZ31 Mg alloy, and formation of a protective layer on the magnesium surface [50,51]. However, the capacitive loop progressively increases with increasing synergistic effect of as-prepared samples, representing the adsorption of organic molecules and exhibition a barrier effect that would defend the AZ31 Mg from destructive attack by the solution. As seen from the EIS plots (Figure. 7c,d and supplementary Table S2), maximum corrosion prevention is observed in TiO2-HQ-APH. 3.3. Quantum chemical study To explore the relationship between the molecular electron donor properties of the ligands and coating surface, the energy gap ∆E (LUMO-HOMO) relates with the visible light response was determined (Fig. 8). From Fig.8 (a,b,c) it is exhibited that the energy gap of 8-HQ is 8.06 eV, 2-APY 7.97 eV and 2-APH 7.31 eV, indicating that smaller ∆E is the higher reaction activity and electron donor of compound [52]. Moreover, the lower value of ELUMO indicates that synergistic heterocycles could 11
simply donate electron to the unoccupied orbital of MgAl2O4-TiO2-SnO2. Therefore, it is realized that active areas of organic molecules are mostly distrusted around the functional groups containing O and N atoms. This explains that the 8-HQ, 2-PAY and 2-APH interact with the Mg and Al atoms by those functional groups, and provide electrons for metal atoms with empty orbitals. The advantage of electrons propagating from central Mg2+ and Al3+ to synergistic heterocycles after complexation process is that photoelectrons are softly to inject to TiO2 through ligand, which will enhance the photocatalytic property of TiO2. 3.4. Mechanism for photocatalytic activity of synergistic heterocycles and MgAl2O4-TiO2-SnO2 Based on the above photocatalytic analysis, a tentative charge transfer for the synergistic heterocycles and MgAl2O4-TiO2-SnO2 was suggested and presented in Fig. 9. Synergistic heterocycles upon bonding with inorganic surface containing TiO2 particles, which favorites photogenerated electron move from conduction band (CB) of TiO2 to Fermi level (π orbital) of the synergistic aromatic compounds, act as electron donor or electron acceptor under visible and UV excitation, respectively. In this case, it was usually deduced that the function of synergistic heterocycles as photogenerated electrons acceptor is critical in the repression of charge carrier’s recombination of photocatalyst hybrid materials resulting in increased effectiveness of the water treatment. In this regard, Bhatia et al. [53] reported the combination of reduced graphene with TiO2 facilitated more adsorption sites and restrained the recombination rate of electron-hole pairs. The visible-light activity of prepared MgOTiO2-HQ, MgO-TiO2-HQ-APY, and MgO-TiO2-HQ-APH was due to the red-shift of the absorption edge. It was found that synergistic effects of heterocycles, such as 2-APH, strongly increased photocatalytic activity. In the aqueous systems, the negative charge then activates pre-adsorbed O2 on the surface of TiO2 to form superoxide anion radicals which attack the aromatic rings of MB because of strong oxidizing ability, mineralize the MB to carbon dioxide and water. Therefore, the holes can react with adsorbed H2O to produce hydroxyl radicals (OH·) [54].This behavior may be due to the fact that, first, vis-light is an excitation light source for MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgOTiO2-HQ-APH due to their small energy gaps. Second, the CB energy of MgAl2O4-TiO2-SnO2 is 12
lower than that of HOMO of 8-HQ, 2-APY and 2-APH. Consequently, the photoelectrons on LUMO of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH transfer to the CB of MgAl2O4TiO2-SnO2 to decrease whole energy of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH and increase the ability of system.
4. Conclusions Finally, the flowerlike organic structure grown on the PEO coating has been developed successfully to enhance visible-light photocatalytic activity after sensitized with both either 8-HQ/2-APY or 8-HQ/2APH. PEO coating showed improved visible-light photocatalytic activity after sensitized with either of 8-HQ/2-APY or 8-HQ/2-APH. About 76% of methylene blue was decomposed after 6 h in the presence of TiO2-HQ-APH under visible light irradiation, while amount was around 55% of methylene blue when TiO2-HQ was used as photocatalyst. The difference in photocatalytic performance of these three systems may be assigned to the synergistic effects of electron-donating of heterocycles and electron transfer features of formed metal-ligands complexes, and the extent of energy level matching between metal-formed complex and TiO2. The study in this paper explains that effective electron transfer at the interface of prepared metal-ligands complex and MgAl2O4-TiO2-SnO2 self-possessed with the suitable energy level alignment between the gab energy (HOMO-LUMO) of the metal complex and the band gap of MgAl2O4-TiO2-SnO2 is critical for TiO2-MgAl2O4-SnO2 to achieve high visible light photocatalytic performance. Moreover, the high electrochemical performance of the present hybrid as-prepared photocatalysis suggests a synergistic combination of heterocycles and TiO2-doped inorganic materials in enhancing structural and chemical stabilities Author contributions All authors contributed to writing the manuscript and approved the final version of the manuscript.
Notes The authors declare no competing financial interests. 13
Acknowledgement This research was supported by the Research Grant funded by the National Research Foundation of Korea (2017R1D1A1A09000921).
14
References 1.
L. Guo, W. Feng, X. Liu, C. Lin, B. Li, Y. Qiang, Sol-gel synthesis of antibacterial hybrid coatings on titanium, Mater. Lett. 160 (2015) 448-451.
2.
V. Oldani, R. del Negro, C.L. Bianchi, R. Suriano, S. Turri, C. Pirola, B. Sacchi, Surface properties and anti-fouling assessment of coatings obtained from perfluoropolyethers and ceramic oxides nanopowders deposited on stainless steel, J. Fluorine. Chem. 180 (2015) 7-14.
3.
M. Catauro, F. Papale, F. Bollino, Characterization and biological properties of TiO2/PCL hybrid layers prepared via sol–gel dip coating for surface modification of titanium implants, J. NonCryst. Solids. 415 (2015) 9-15.
4.
I. Kheirollahi, M. Abdellahi, M. Emamalizadeh, H. Sharif, Preparation and characterization of multilayer mesoporous alumina nano membrane via sol-gel method using new precursors, Ceram. Internat. 41 (2015) 15083.
5.
X. Guan, Y. Wang, Q. Xue, L. Wang, Toward high load bearing capacity and corrosion resistance Cr/Cr2N nano-multilayer coatings against seawater attack, Surf. Coat. Technol. 282 (2015) 78-85.
6.
F. Ge, X. Zhou, F. Meng, Q. Xue, F. Huang, Tribological behavior of VC/Ni multilayer coatings prepared by non-reactive magnetron sputtering, Tribiology. Internat. 99 (2016) 140-150.
7.
A. Castellanos, A. Altube, J.M. Vega, E. García-Lecina, J.A. Díez, H.J. Grande, Effect of different post-treatments on the corrosion resistance and tribological properties of AZ91D magnesium alloy coated PEO, Surf. Coat. Technol. 278 (2015) 99-107.
8.
Y. Xiong, C. Lu, C. Wang, R. Song, Degradation behavior of n-MAO/EPD bioceramic composite coatings on magnesium alloy in simulated body fluid, J. Alloy. Compd. 625 (2015) 258-265.
9.
H. Zheng, L. Xiong, Q. Luo, S. Lu, Development of multilayer oxidation resistant coatings on Cr-50Nb alloy, Appl. Surf. Sci. 359 (2015) 515-520.
10. Z. Wu, L. He, Z. Chen, Composite biocoating of hydroxyapatite/Al2O3 on titanium formed by Al anodization and electrodeposition, Mater. Lett. 61 (2007) 2952-2955. 11. A. Fujishima, N.R. Rao, D.A. Tryk, titanium dioxide photocatalysis. J. Photochem. Photobiol., C: Photochem. Rev. 1 (2001) 1-21. 12. S.H. Cheung, P. Nachimuthu, A.G. Joly, M.H. Engelhard, M.K. Bowman, S.A. N Chambers, Incorporation and electronic structrure in N-doped TiO2 (110) rutile. Surf. Sci. 601 (2007) 17541762. 13. J.A. Navio, M. Macias, M. Gonzales-Catalan, A. Justo, Bulk and surface characterization of power iron-doped titania photocatalysts. J. Mat. Sci. 27 (1992) 3036-3042. 14. Y. Wang, R. Shi, J. Lin, Y.F Zhu, Significant photocatalytic enhancement in methylene blue degradation of TiO2 photocatalytsts via graphene-like carbon in situ hybridization. Catal. B: Environ. 100 (2010) 179-183. 15
15. Z. Liu, D. He, Y. Wang, H. Wu, J. Wang graphene doping of P3HT:PCBM photovoltaic devices. Synth. Met.160 (2010) 1036-1039. 16. M.E. Khan, M.M. Khan, M.H. Cho, CdS-Graphene nanocomposite for efficient visible-lightdriven photocatalytic and photoelectrochemical applications. J. Colloid Interface Sci. 482 (2016) 221-232. 17. A. Heciak, A.W. Morawski, B. Grzmil, S. Mozia., Cu-Modified TiO2 photocatalysts for decomposition of acetic acid with simultaneous formation of C1-C3 Hydrocarbons and Hydrogen. Appl. Catal., B: Environ. 140-141 (2013) 108-114. 18. A. Suligoj, U.L. Stangar, A. Ristic, M. Mazaj, D. Verhovsek, N.N. Tusar. TiO2-SiO2 Films from organic-free colloidal TiO2 anatase nanoparticles as photocatalyst for removal of volatile organic compounds from indoor air. Appl. Catal., B: Environ. 184 (2016) 119-131. 19. H. Nishikiori, K. Todoroki, R.A. Setiawan, K. Teshima, T. Fujii, H. Staozono, Titanium complex formation of organic ligands in titania gels. Langmuir. 31 (2015) 964-969. 20. H. Yang, J. Zhang, Y Song, S. Xu, L. Jiang, Y. Dan, Visible light photo-catalytic activity of CPVA/TiO2 composite for degrading rhodamine B. Applied Surface Science. 324 (2015) 645-651. 21. E. Filippo, C. Carlucci, A.L. Capodilupo, P. Perulli, F. Conciauro, G.A. Corrente, G. Gigli, G. Ciccarella, Facile preparation of TiO2-polyvinyl alcohol hybrid nanoparticles with improved visible light photocatalytic activity. Applied Surface Science. 331 (2015) 292-298. 22. P. Lei, F. Wang, S. Zhang, Y. Ding, J. Zhao, M. Yang, Conjugation-grafted-TiO2 nanohybrid for high photocatalytic efficiency under visible light. ACS App. Mater. Interfaces. 6 (2014) 23702376. 23. S. Chen, B. Lv, Y. Xu. Fe-quinoline complexes sensitized Si-doped TiO2 with enhanced visible light photocatalytic activity. Mater. Lett. 77 (2012) 32-34. 24. Nishikiori, H.; Natori, D.; Ebara, H.; Teshima, K.; Fujii, T. Zinc Complex Formation of Organic Ligands on Zinc Oxide and Titanium Dioxide. J. photochem. Photobiol. 2016, 327, 51-57. 25. E.L. Tae, S.H. Lee, J.K. Lee, S.S. Yoo, E.J. Kang, K.B. Yoon, A strategy to increase the efficiency of the dye-Sensitized TiO2 solar cells operated by photoexcitation of dye-to-TiO2 charge-transfer bands. J. Phys. Chem. B 109 (2005) 22513-22522. 26. S. Varaganti, G. Ramakrishna, Dynamics of interfacial charge transfer emission in small molecule sensitized TiO2 nanoparticles: Is it localized or delocalized. J. Phys. Chem. C 114 (2010) 13917-13925. 27. B.K. An, W. Hu, P.L. Burn, P. Meredith, New type II catechol-thiophene sensitizers for dyesensitized solar cells. J. Phys. Chem. C 114 (2010) 17964-17974. 28. T. Kamegawa, H. Seto, S. Matsuura, H. Yamashita, Preparation of hydroxynaphthalene-modified TiO2 via formation of surface complexes and their applications in the photocatalytic reduction of nitrobenzene under visible-light irradiation. ACS Appl. Mater. Interfaces 4 (2012) 6635-6639. 16
29. R. Jono, J. Fujisawa, H. Segawa, K. Yamashita, Theoretical study of the surface between TiO2 and TCNO showing interfacial charge-transfer transitions. J. Phys. Chem. Lett. 2 (2011) 11671170. 30. R. Jono, J. Fujisawa, H. Segawa, K. Yamashita, The origin of the strong interfacial chargetransfer absorption in the surface complex between TiO2 and dicyanmethylene compounds. Phys. Chem. Chem. Phys. 15 (2013)18584-18588. 31. W. Al Zoubi, Y.G. Ko. A novel strategy to modify the surface of plasma electrolysis produced inorganic coatings for fabricating organic@functional binding agents@inorganic materials J. All. Comps. 776 (2019) 1025-1028. 32. A.H. Mady, M.L. Baynosa, D. Tuma, J.J. Shim, Facile microwave-assisted green synthesis of Ag-ZnFe2O4@rGO nanocomposites for efficient removal of organic dyes under UV- and visiblelight irradiation. Appl. Catal. B: Environ. 203 (2017) 416-427. 33. W. Al Zoubi, Y.G. Ko, Self-assembly of hierarchical N-heterocycles inorganic materials into three-dimensional structure for superior corrosion protection. Chem Eng J. 356 (2019) 850-856. 34. Q. Zong, L. Wang, W. Sun, G. Liu, Active deposition of I (8-hydroxyquinoline) magnesium coating for enhanced corrosion resistance of AZ91D Alloy. Corros Sci. 89 (2014) 127-136. 35. J. Bennet, R. Tholkappiyan, K. Vishista, N. Victor Jaya, F. Hamed Attestation, Attestation in selfpropagating combustion approach of spinel AFe2O4 (A = Co, Mg and Mn) complexes bearing mixed oxidation states: Magneto structural properties. Applied Surface Science. 383 (2016) 113125. 36. D. Zheng, H. Li, Y. Wang, F. Zhang, Surface and interface analysis of tris-(8-hydroxyquinoline) aluminum and indium-tin-oxide using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Appl. Surf. Sci. 183 (2001) 165-172. 37. W. Al Zoubi, M.J. Kim, Y.G. Kim, Y.G. Ko. Dual-functional crosslinked polyer-inorganic materials for robust electrochemical performance and antibacterial activity. Chem Eng J. doi.org/10.1016/j.cej.2019.123654 38. M. Finsgar, D.K. Merl, An electrochemical, long-term immersion, and XPS study of 2mercaptobenzothiazole as a copper corrosion inhibitor in chloride solution. Corros. Sci. 83 (2014) 164-175. 39. W. Al Zoubi, M.P Kamil, Y.G. Ko, Synergistic influence of inorganic oxides (ZrO2 and SiO2) with N2H4 to protect composite coatings obtained via plasma electrolyte oxidation on Mg alloy. Phys. Chem. Chem. Phys. 19 (2017) 2372. 40. R.F. Zhang, S.F. Zhang, N. Yang, L.J. Yao, F.X. He, Y.P. Zhou, X. Xu, L. Chang, S.J. Bai Influence of 8-hydroxyquinoline on properties of anodic coatings obtained by micro arc oxidation on AZ91 magnesium alloys. J. Alloys Compd. 539 (2012) 249-255. 41. S.Y. Shen, Y. Zuo, X.H. Zhao, The effects of 8-hydroxyquinoline on corrosion performance of a Mg-Rich coating on AZ91D magnesium alloy. Corros. Sci. 76 (2013) 275-283. 17
42. H. Gao, Q. Li, Y. Dai, F. Luo, H.X. Zhang high efficiency corrosion inhibitor 8-hydroxyquinoline and its synergistic effect with sodium dodecylbenzenesulfonate on AZ91D magnesium alloy. Corros. Sci. 52 (2010) 1603-1609. 43. Q. Zong, L. Wang, W. Sun, G. Liu active deposition of bis(8-hydroxyquinoline) magnesium coating for enhanced corrosion resistance of AZ91D alloy. Corros. Sci. 89 (2014) 127-136. 44. H. Yu, H. Irie, K. Hashimoto, Conduction band energy level control of titanium dioxide: toward an efficient visible-light-sensitive photocatalyst. J. Am. Chem. Soc. 132 (2010) 6898-6899. 45. F. Dong, S. Guo, H. Wang, X. Li, Z. Wu, Enhancement of the visible light photocatalytic activity of C-doped TiO2 nanomaterials prepared by a green synthetic approach. J. Phys. Chem. C 115 (2011) 13285-13292. 46. S. Shaikhutdinov, H.J. Freund, Ultrathin oxide films on metal supports: structure-reactivity relations. Rev. Phys. Chem. 63 (2012) 619-633. 47. T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, Photocatalytic one-electron oxidation of biphenyl derivatives strongly coupled with the TiO2 surface. Langmuir. 20 (2004) 2753-2759. 48. M.E. Khan, M.M. Khan, M.H. Cho, Cds-graphene nanocomposite for efficient visible-lightdriven photocatalytic and photoelectrochemical applications. J. Colloid Interface Sci. 482 (2016) 221-232. 49. S. Ebrahimi, A. Bordbar-Khiabani, B. Yarmand, M.A. Asghari, Improving optoelectrical properties of photoactive anatase TiO2 coating using rGO incorporation during plasma electrolytic oxidation. Ceramic International. 45 (2019) 1746-1754. 50. W. Al Zoubi, J.H. Min, Y.G. Ko, Hybrid organic-inorganic coating via electron transfer behaviour. Sci. Rep. 7 (2017) 7063. 51. W. Al Zoubi, M.P. Kamil, H.W. Yang, Y.G. Ko. Self-assembly of basket-weave organic layer formed on defective inorganic surface. Chem. Eng J. 360 (2019) 385-392. 52. M.A. Amin, K.F. Khaled, Q. Mohsen, H.A. Arida, A study of the inhibition of iron corrosion in HCl solutions by some amino acids. Corros. Sci. 52 (2010) 1684-1695. 53. V. Bhatia, G. Malekshoar, A. Dhir, A.K. Ray, Enhanced photocatalytic degradation of atenolol using graphene TiO2 composite. J. Photochem. Photobiol. A: Chem. 332 (2017) 182-187. 54. E. Kusiak-Nejman, A. W. Morawski, TiO2/graphene-based nanocomposites for water treatment: a brief overview of charge carrier transfer, antimicrobial and photocatalytic performance. Appl. Catal. B: Environ. 253 (2019) 179-186.
18
Figure captions Figure 1. XRD patterns of as-prepared TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH. (TiO2 donates to MgO and TiO2) Figure 2. FE-SEM images of XRD patterns of (a) TiO2-HQ, (b) TiO2-HQ-APY and (c) TiO2-HQAPH. High-resolution SEM represent of the (d) TiO2-HQ-APH and (e) TiO2-HQ. (f) R-TEM images of TiO2-HQ-APH showing flower-like structure of formed complexes on the inorganic surface. Figure 3. (a) Wide XPS spectra of TiO2-HQ, TiO2-HQ-APY, and TiO2-HQ-APH. High-resolution scan Mg 1s (b), Al 2p (c), C 1s (d), N1s (e), O1s (f) are presented of TiO2-HQ-APH. Figure 4. FT-IR spectra of TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH Figure 5. (a) Absorption spectra and (b) plot of modified Kubelka-Munk functional (αhν)2(eVcm-1)2 versus photo energy of the TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH. (c) Photoluminescence spectra of the as-prepared samples. Figure 6. Comparison of the photocatalytic degradation of MB using (a) TiO2-HQ, (b) TiO2-HQ-APY and (c) TiO2-HQ-APH under UV-vis light. Figure 7. (a) Degradation MB in aqueous solution using the TiO2-HQ, TiO2-HQ-APY, TiO2-HQ-PAH catalysts under visible-light. (b) Reusability test of the TiO2-HQ and TiO2-HQ-PAH catalysts for the photodegradation of MB (20 ppm) under visible-light. (c), Nyquist plots from electrochemical impedance spectroscopy (EIS) performed after 5 hours of immersion in 3.5 wt % NaCl on PEO, TiO2HQ, TiO2-HQ-APY and TiO2-HQ-PAH. (d) Equivalent circuit used fit the obtained impedance spectra for AZ31 magnesium in the PEO, TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-PAH. Rs describes the corrosion resistance between the prepared sample and counter electrode; R1 is the outward part resistance, and CPE1 is the constant phase element; R2 represents the contribution of inward part to 19
the overall corrosion resistance and the corresponding capacitance was represented by CPE2. Rct and L represent the inductive impedance behavior, where Rct denotes the charge transfer resistance and L showed the corrosion behavior at low frequencies. Figure 8. Diagrams of crucial molecular orbital energy levels of 8-HQ (a), 2APY (b) and 2-APH (c). Figure 9. Schematic presentation of the electron transfer and charge separation in the TiO2-QH under visible light irradiation.
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We proposed a novel composition to prepare the flower hybrid. Electrochemical performance is delayed by hybrid materials compounds with compact structure. An optimum combination of protective and catalytic properties together is accomplished. No significant decrease in photocatalytic performance during catalytic oxidation.
The authors declare no competing financial interests.
S0925-8388(20)30150-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153787
Reference:
JALCOM 153787
To appear in:
Journal of Alloys and Compounds
Received Date: 23 November 2019 Revised Date:
8 January 2020
Accepted Date: 9 January 2020
Please cite this article as: W. Al Zoubi, M.J. Kim, D.K. Yoon, A.A. Salih Al-Hamdani, Y.G. Kim, Y.G. Ko, Effect of organic compounds and rough inorganic layer formed by plasma electrolytic oxidation on photocatalytic performance, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153787. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Contributions W.A.Z. performed the experiments and wrote the manuscript. M. J. K, D. K. Y and A. A. A , analysed and interpreted the data. Y.G.K conceived and designed the concept of experiments and revised the manuscript. All authors contributed to the discussion of the results.
Effect of Organic Compounds and Rough Inorganic Layer Formed by Plasma Electrolytic Oxidation on Photocatalytic Performance Wail Al Zoubia, Min Jun Kima, Dong Keun Yoona, Abbas Ali Salih Al-Hamdanib, Yang Gon Kimc, Young Gun Koa* a
Materials Electrochemistry Group, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea b Chemistry Department, College of Science for Women, University of Baghdad, Baghdad, Iraq c Extreme Fabrication Technology Group, Korea Institute of Industrial Technology, Daegu 42994, Republic of Korea Abstract Plasma electrolytic oxidation and dip chemical coating as fabrication techniques are performed in ambient temperature and pressure, giving a competitive-edge in the commercial applicability. Thus, we proposed new compositions of flower-like hybrid materials, MgO-TiO2-HQ(8-hydroxyquinoline), MgO-TiO2-HQ-APY(2-aminopyridine) and MgO-TiO2-HQ-APH(2-aminophenol), as photocatalysis with high corrosion resistance via a combination of plasma electrolytic oxidation and dip chemical coating in which 8-HQ, 2-APY, and 2-APH were used as the organic components. TiO2-HQ-APY and MgO-TiO2-HQ-APH
exhibited
improved
simultaneous
electrochemical
and
photocatalytic
performance on photodecomposition of methylene blue (MB) in an aqueous solution in the presence of ultraviolet-visible irradiation comparing to that of MgO-TiO2-HQ. The synergistic effects of organic components anchored on TiO2 and MgO containing inorganic layer were the main source for enhanced anticorrosion resistance. The MgO-TiO2-HQ and MgO-TiO2-HQ-APH could be recovered simply by a magnet and reused several times with no significant diminution in high stability photocatalytic performance during catalytic oxidation. The work indicated that synergistic effects of aromatic heterocycles on TiO2 and MgO containing inorganic layer played key role in enhancing photocatalytic activity and electrochemical behavior. To support the experimental section, quantum chemical parameters such as highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELOMO), and gap energy (∆E) were calculated using density functional theory.
Keywords: Organic layer, Organic layer, quinoline, photocatalysis, Corrosion.
1
1. Introduction Lightweight metals have low anti-corrosion resistance, particularly in corrosive media, on account of their negative electric potential and high chemical activity. Thus, one of the most actual methods to improve electrochemical performance was the use of multilayer (hybrid) coating which could decrease or inhibit corrosion reactions between a metal weight surface and its surroundings. The coating layers were regularly composed of polymer-organic, inorganic-inorganic, organic-inorganic, organic-organic or inorganic compounds [1-3]. Thus, coating layers may be produced via one treatment, (e.g., sol-gel [4], plasma electrolytic oxidation treatments (PEO) [5,6]), two treatments (e.g., PEO and post-treatment [7], PEO and ED (electrophoretic deposition) [8], or pack cementation and sol-gel [9]), or a mixture of numerous treatments (e.g., PVD (physical vapor deposition) of organic compounds, PEO, hydrothermal treatment, and electrodeposition [10]). Therefore, coatings were fabricated to add particular functional groups to the substrate surface or to alter its catalytic, or biological, electrochemical properties. Thus, this work is focused on how to enhance both the absorption of light and corrosion resistance of TiO2-containing PEO coating on AZ31 Mg to achieve outstanding photocatalytic performance. For photocatalytic performance, the scientists employed various surface alteration methods among which doping with non-metal (N, C, S, F etc.) [11,12] as well as transition metal doping (Mn, Zn, Cr, Fe, Cu, Ni etc.) [13] appear to be the most favorable for titanium dioxide band-gab narrowing, but TiO2 alteration with graphene [14-16], metal [17] or semiconductors coupling are generally used in order to prevent hole/electron pairs recombination rate. Furthermore, several attempts to extend adsorption of TiO2 to visible light, for instance, Fe(III)-8-hydroxyquinoline-5-sulfonic acid, Er(III)-8hydroxyquinoline-5-sulfonic acid [18], 8-hydroyquinloine [19], polymers [20,21], and derivatives of ruthenium bipyridly complexes [18] have been often used to adjust TiO2 so as to proficiently use the solar light-driven photocatalysis. Thus, it has been documented that the applications of ruthenium complexes and conjugated organic compounds [22] are restricted by the low photochemical stability and poor charge transfer of former, and toxicity and cost of the ultimate. Subsequently, it is extremely 2
sought-after to grow novel donor organic compounds which absorb robustly in the ultraviolet visible (UV-vis) area of the solar light with the different levels of energy competent of injecting excited electron into TiO2 conduction bands. Non-toxicity and low cost are also significant factors to consider in the development of these new metal complexes. 8-Hydroxyquinoline and its compounds had been well-known for their outstanding properties to transfer electrons from aromatic compounds to the conduction band of TiO2 depends on absorption of visible light. 8-Hydroxyquinoine complexes with broad range UV-vis absorption can be facilely obtained. For instance, Chen et al. found that the Fe-quinoline complexes (HQSI) can transfer excited electrons to the conduction band of TiO2 and be renovated by the electrons injection from organic contaminants through the photodecomposition in aqueous media [23]. Other researchers using density functional theory investigated the photo-sensitization mechanism of Fe(III)/tris(8-hydroquinline-5carboxylicacid) on TiO2 surface. As a consequence, they reported that organic complexes have a very small LUMO (lowest unoccupied molecular orbital)-HOMO (highest occupied molecular orbital) energy gap which permits for easy electron irritation from HOMO to LUMO [24]. The HOMO electron is directly injected into the TiO2 conduction band once the organic ligands, that include the dihyroxy [25-28], dicyanomthylene [29,30], and carboxylic [31] compounds, were adsorbed on TiO2 surface. Although these organic ligands have been used to prepare visible-light-driven TiO2 photocatalyst, the interaction between TiO2-doped inorganic layer and synergistic heterocyclic compounds on weight-light metal surface has been rarely investigated, especially the direct chemical bonding. In this paper, we report the experimental and theoretical study of the impacts of electronic chemical structure and interfacial physicochemical interactions between synergistic heterocyclic compounds and TiO2-MgO containing inorganic materials, formed on AZ31 Mg with an assistance of plasmaassisted electrochemical reactions, on the photocatalytic activity of TiO2-MgO containing inorganic materials. We have revealed that the electronic structures and the synergistic effects of the aromatic heterocycles determine their energy gap between HOMO and LUMO, and additional affected electron 3
transfer from ligands and their complexes to TiO2-MgO containing inorganic material surface, which mainly determined the photocatalytic activity of inorganic layer. The organic-inorganic hybrid materials were prepared by conducting PEO on weight alloy to form inorganic layer (IL), which is pursued by dip chemical coating (DCC) to prepare organic ligand (8-hydroxyquinoline (HQ), 8hydroxyquinoline/2-amninopyridine (HQ-APY) and 8-hydroxyquinoline/2-aminophenol (HQ-APH) respectively) as organic coating on porous surface of inorganic layer (IL). After linked with synergistic organic ligands, TiO2-MgO containing inorganic materials surface showed higher photochemical stability and photocatalytic activity on the decomposition of the methylene blue under both UV-vis irradiation as well as extraordinary electrochemical stability. The results were provided by the theoretical studies, elucidated photocatalytic activities of TiO2-HQ, TO2-HQ-APY and TO2HQ-APH using two probable interfacial interactions. This paper described has provided significant insight on drawing new photocatalysts with enhanced corrosion resistance which will remarkable widen the application of inorganic coatings in photocatalytic degradation field of organic contaminants.
2. Experimental 2.1. Preparation of organic-inorganic hybrid materials 8-HQ), 2-APY, 2-APH, MB, NaAlO2, KOH, TiO2 and SnO2 were, purchased from Merck and Fluka, and used as received. Besides, AZ31 Mg alloy with the nominal composition (wt%) of 2.89 Al, 0.96 Zn, 0.31 Mn, 0.15 Fe, 0.12 Si, and balanced Mg (in wt%) were polished using SiC papers up to 2400 grit and then cleaned with deionized (DI) water and pure ethanol. TiO2-doped inorganic layer was prepared in an alkaline electrolyte composed of 4 g L-1 NaAlO2, 4 g L-1 KOH, 4 g.L-1 TiO2 and 4 g L-1 SnO2 using glass vessel equipped with Mg alloy, stainless steel net, as anode and cathode respectively, and a magnetic stirrer at temperature below 288 K. PEO treatment were carried out for 6 min under substitutional current with a current density around 100 mA.cm-2 and a frequency of 60 Hz. Following the PEO process, prepared samples were washed in DI (Distilled) water and dehydrated in warm air. 4
Subsequently, to form OL on inorganic layer, DCC was carried out by immersing the specimens after PEO in an ethanolic solution of 0.05 M 8-hydroxyhydroquinone (MgO-TiO2-HQ), 8hydroxyhydroquinone/0.01
M
2-aminopyridine
(MgO-TiO2-HQ-APY),
and
0.05
M
8-
hydroxyhydroquinone/0.01 M 2-aminophenol (MgO-TiO2-HQ-APH), respectively, for 1 day at 298 K to yield self-assembled hybrid organic-inorganic coatings. 2.2. Microstructural characterization The microstructural observation was performed using a scanning electron microscope (SEM, Hitachi, S-4800) provided with X-ray spectroscopy (EDS, HRIBA EMAX) and transmission electron microscope (TEM; Philips, CM 200) operating at 200 kV for detailed observations. The structural results were documented using Fourier-transform infrared (FT-IR; Bio-Rad, Excalibur Series FTS 3000) spectroscopy was used to define the chemical structure of the organic-inorganic materials. Xray photoelectron spectroscopy. (XPS, VG Microtech, ESCA 2000) was used to observe the surface components of the samples. Regarding the optical properties, the UV-vis spectra of the coatings were obtained by using a Varian Cary 5000 UV/VIS/NIR spectrophotometer in a range of 200-800 nm and the photoluminescence (PL) (Kimon, 1 K, Japan) of the coatings was documented in the domain 200800 nm using an irritation wavelength of 325 nm 2.3. Photocatalytic activity performance The photocatalytic valuations of the coatings are assessed by the photocatalytic removal of MB under UV-vis light irradiation. The degradations were performed in a photoreactor at a temperature of around 25 ˚C. Photocatalyst (500 mm2) were immersed in 50 ml of an aqueous MB solution (concentration = 20 mg L-1) and then stimulated in the without light for 35 min to supplement desorption and adsorption equilibrium. Uv-vis light irradiation of an aqueous solution was carried out using a 400-W UV lamp (λ>300 nm). The degradation rate of MB was checked by withdrawal 2 ml from the reaction solution. The concentration of MB in the aqueous solution was defined by UV-vis electronic spectroscopy at 663 nm. To study the photocatalytic stability and reusability of the prepared samples, hybrid materials were repeated for three cycles under the similar conditions. After each 5
experiment, the samples were divided from the MB solution, cleaned with DI water, dried in warm air, and used once more for the following cycle [32]. The kinetics of MB degradation followed pseudofirst-order model.
ln(C0 / Ct ) = kt
(1)
where k is the first-order reaction rate constant, Ct is residual concentration of reaction solution at regular time intervals under UV-vis irradiations, C0 is the initial concentration of MB, after the adsorption and desorption equilibrium. Every experiment was carried out in triplicate to confirm the photocatalytic performance of the prepared samples. 2.4. Electrochemical Measurements The corrosion measurements were evaluated in an aqueous solution (3.5% wt NaCl) at 25 °C using three different electrodes: a treated specimen with an unprotected part of 1 cm2 as working electrode, Ag/AgCl electrode as the reference, and a platinum mesh as the counter electrode. The electrochemical performance was carried out by electrochemical impedance tests (Gamry Instruments, Interface 1000). Electrochemical impedance tests were performed for 106 to 0.1 Hz at interval of 10 points/decade with a 10 mV rems. 2.5. Calculation details The geometries of 8-HQ, 2-APY and 2-APH were optimized using density functional theory (DFT) using parameterization method 3 (PM3) in the Hyper Chem-8 program [33].
3. Results and discussion 3.1. Characterization of structure and morphology The XRD patterns of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH are exhibited in Fig. 1. The peaks of TiO2 particles “rutile phase” can be observed at 36.79˚(103), 38.35˚(112), 47.95˚(200), 63.31˚(204) and 68.63˚(116) (JCPDS No.00-021-1272) which confirmed the crystal structure of TiO2. For MgAl2O4, the XRD peaks can be observed at 18.88˚(111), 36.77˚(311), 38.35˚(222), 44.61˚(400) and 68.94˚(531) (JCPDS No.00-021-1272). For SnO2, the XRD peaks can observed at 57.62˚(002), 64.98˚(112), 68.94˚(311) (JCPDS No.00-041-1445). On the other hand, 6
main peaks at the 2θ range of 5-30˚ indicated that the 8-HQ-inorganic coatings is mostly collected of MgQ2 and AlQ3 complexes which were in agreement with new reports (Ref. [33,34]). Moreover, additional XRD peaks at the 2θ range of 5-30˚ were assigned to the formation of new complexes (Scheme S1) (MgL2 and AlqL3, where L is Q, QAPY, or QAPH). These peaks were due to organic ligands on the coating surface which results in 2-aminopyridine and 2-aminophenol, respectively. Fig. 2 presents SEM and TEM images of the coatings. The organic-inorganic hybrid materials (MgOTiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH) exhibited flowerlike microstructure morphologies with a diameter of 2~5 nm, and few 2D petals adhere together (Fig.2a-c). From the high magnification SEM image in Fig. 2, hydrogen bonds between hydroxyl groups on the coating surface as well as π-electrons of the aromatic rings in the organic molecules may construct this 3D flower structure by interconnected dozen 2D sheets with a thickness of ~100 nm to form flower-like cluster, while the interaction between amino groups of complexes and hydroxyl group of TiO2 may another cause for aggregation (Fig 2d,e and Fig 2f). The availability of free electrons, aromatic rings (π-electron systems), and planar molecular in HQ facilitates electron transfer from the organic layer to the TiO2-doped inorganic layer. In other words, an electron-donor-acceptor complex results from electrostatic interaction among the charged centers of molecules and the charged inorganic materials surface, which causes in electrostatic interaction between the chemical molecules and inorganic materials surface [33]. These results indicated that porous inorganic layer may enable the heterocycles to penetrate and be simply adsorbed on the pore inorganic matrices. In order to validate the surface morphologies, the chemical composition of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH was further determined by SEM-EDS and TEM-EDS (Fig S1). Elemental analysis shown in Fig. S1 propose that a flowerlike organic-inorganic hybrid material consists solely of Mg, Al, C, N and O elements. However, significant differences in the atomic distribution of Mg, Al, C, N and O were attributed to the large aggregations and the reactions of heterocyclic compounds with inorganic compounds in the porous surface.
7
The chemical bonding state of the MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH can be seen on the XPS spectra (Fig. 3a). The wide-scan XPS spectra show the presence of Mg 1s, Al 2p, C 1s, N 1s, and O 1S energy region, as shown in Fig. 3. The high-resolution Mg 1s spectrum if Figure 3b showed one peak centered at 1302.6 eV which was attributed to the Mg 1s, demonstrating the Mg(II) oxidation state in formed complexes [35]. In terms of the Al 2p spectrum (as shown in Figure 3c), the binding energies for Al 2p at 74.7 eV correspond to the AlQ3 and AlQ2APY. Moreover, the peaks for the C 1s of the heterocycles were deconvoluted into several peaks, representing (C-C) bonds at 284.7 eV, C-O (phenol) bond at 285.1 eV, N-C (pyridine and amino) bond at 287.4 eV [36]. The peak intensities of C atom increased remarkably compared to PEO coating. For the N 1s spectrum (Fig. 3e), the N 1s peak at 399 eV 3e show that the broad N 1s spectra are composed of 398.4 eV MgN bonding and 399.2 eV Al-N bonding, respectively [37]. The O 1s spectrum (Fig. 3f) can be deconvoluted into two different peaks at 530.2 and 531.2 eV. The O 1s binding energy of 530.2 eV was assigned to the C-O bonds, while the binding energy 531.2 eV may be attributed to the O-M (Mg and Al) [38]. To confirm the quantitative results, the elemental analysis achieved via XPS indicated that increasing C and N and decreasing metallic elements (Mg, Al, Ti and Sn) were assigned to the organic layer formed on the coating surface (as shown in Table S1) [39]. Evidence for the complexation reactions on coating surface was gotten from FT-IR spectra of HQ, HQ-APY and HQ-APH, as shown in Fig. 4. In the HQ spectrum, the bands at 1666, 1576, 1467, 1375 and 1317 cm-1 are attributed to the aromatic heterocycles (quinoline group) of MgL2 and AlL3 complexes on the surface [40]. The band at 1498 cm-1 is attributed to the vibrations of the phenyl and pyridine groups in formed complexes [41]. In addition compared with the original organic compounds in Fig S2, the shift in the formation energy of the intermolecular hydrogen bonds towards higher region from about 3100 to 3350 cm-1 indicates the self-assembly association between 8HQ molecules triggered by the formation of complexes with metal ions (Mg2+ and Al3+) [42]. Therefore, the shift of the bands in the resulted complexes on the surface coating towards higher region to extent of 20-90 cm−1 as well as the disappearance of the strong band to the OH and NH2 groups in the spectra of the 8
prepared materials, indicates coordination via the nitrogen and oxygen atoms, as shown in Scheme S1. Moreover, the bands at 788 and 738 cm−1 are assigned to C-H deformation, which additional demonstrate the formation of complexes as suggested in Scheme S1. The noticed differences between the hybrid materials and the free ligand indicate that the ligand chelate with Mg2+ and Al3+ in coating surface, forming a stable MgL2 and AlL3 phase on AZ31 Mg substrate The UV-vis absorption spectra of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH were measured, as shown in Fig. 5a,b and Fig. 3S. Figure 5 shows the comparison of UV-vis absorption spectra of as the synthesized MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH with MgO-TiO2-doped coating. After reaction of organic compounds with inorganic surface, it is obviously exposed that the coatings shows increased absorption intensity in the region 300-480 nm (n→π*) due to the formation of organic complexes (Fig. 5a). The absorption wavelength edge of all hybrid photocatalysts is determined by the optical band gap (Eg) [44]. Based on transformed KubelkaMunk functions [45], Eg can be determined by the tangents to the liner portion of the plot between (αhν)1/2 vs. hν (i.e. the Tauc plot), where α is absorbance, h is Planck constant and ν is the associated frequency, as shown in Fig. 5. It is apparent that all of the MgO-TiO2-HQ (1.71 eV), MgO-TiO2-HQAPY (1.6 eV) and MgO-TiO2-HQ-APH (1.59 eV) show a narrower band gap compared to the 2.05 eV bandgap of MgAl2O4-TiO2-SnO2 (Fig 5b and Fig S3). Such reduction of the bandgap may lead to the enhanced photocatalytic activity of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH under visible light irradiation [46,47]. This enhancement is mostly assigned to the prepared organic complexes (MgL2 and AlL3). In our study, the higher visible light absorption ability of the prepared samples makes the selective photocatalytic oxidation of pollutants to be performed under the visiblelight irradiation. To address the effect of synergistic effect of organic components on the electron-hole couples separation in TiO2 based coating, PL spectra of the MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH were studied to additional comprehend of photo-generated charge carriers transformation and recombination likelihood in the hybrid materials of MgO-TiO2/organic coatings. The MgO-TiO2-HQ was showed two main peaks located at 482 and 556 nm, which attributed to the 9
recombination of free electrons of the TiO2-doped inorganic coating and restricted surface oxygen vacancies and porous, respectively [48]. As shown in figure, the composite of the MgO-TiO2-HQ affected its PL intensity, by which the PL spectra of the MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH coating was important lower than that of TiO2.This mentions to the repression of electron-hole recombination likelihood in the MgO-TiO2-HQ sample. The photoexcited electrons from TiO2 are moved into carbon atoms on aromatic heterocycles, and then the rate of the charge carriers’ recombination was decreased [49]. Therefore, the PL further confirmed the formation of hybrid chemical structure between TiO2 and bidentate absorbents hindered or almost avoids the electron-hole pair recombination. 3.2. Photocatalytic and electrochemical performances. In the current work, the photocatalytic dissolution of MB under Vis irradiation using the MgO-TiO2HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH as catalysts was studied. MB is a model pollutant for photocatalytic degeneration and a representative organic dye molecule in textile waste water. From Figs 6 and 7, a mixture of catalyst and MB solution was irradiated for around 30 min in the dark. A very tiny amount of MB was absorbed onto the hybrid material’s surface after time t. After that, MB solution was irradiated with visible light for t time in the presence of MgO-TiO2-HQ, MgO-TiO2-HQAPY, and MgO-TiO2-HQ-APH respectively. From Figs 6(a,b,c) and 7a, MgO-TiO2-HQ-APY (76%) and MgO-TiO2-HQ-APH (74%) exhibited the highest efficiency among all the samples studied. Both 2-APY and 2-APH present improved remarkably photocatalytic activity to over HQ. Therefore, it is worth to indicate that photocatalytic activity of HQ is higher than of MgO-TiO2-SnO2 under the similar condition, but weaker than that of MgO-TiO2-HQ-APY or MgO-TiO2-HQ-APH. The results indicate that efficient sensitization of MgAl2O4-TiO2-SnO2 from MgL2 and AlL3 complexes, and HL (8-HQ, 2-APY or 2-APH) alone, are recognized. The sensitization is supported via the effective electron injection from metal-organic complex (MgL2 and AlL3), or HL, to the conduction band of mixture MgAl2O4-TiO2-SnO2. In addition to the photocatalytic activity, stability of the photocatalysts is another significant factor for limiting the use of the catalysts in feasible applications. The 10
photocatalytic stability of MgO-TiO2-HQ and MgO-TiO2-HQ-APH was carried out by recycled photocatalytic experiments using and the results are presented in Fig. 7b. The photocatalytic degradation efficiency is not obviously decreased after three cycles. This shows the opportunity of using the prepared catalysts for longer operation time. The feature of prepared MgL2 and AlL3 over ligands may be elucidated by the difference of their anchoring modes to the surface of the inorganic coating containing TiO2. It has been explained that the chemical structure and attachment groups of MgL2 and AlL3 or HL on inorganic coating surface are critical to the light harvesting and the charge injection at the organic complexes-inorganic coating or ligand-inorganic coating interface, because they decide the amount of MgL2 and AlL3 or ligands molecules attached to their orientation on the inorganic coating surface. The impedance spectra of AZ31 Mg alloy in the absence and presence of MgO-TiO2-HQ, MgO-TiO2-HQ-APY, and MgO-TiO2-HQ-APH were recorded in Figure. 7c supplementary Table S2. The impedance diagrams show one capacitive loop characterized by somewhat depressed semicircle for all studied samples. This capacitive loop indicates that the degradation of magnesium in aggressive solution is mostly measured by charge transfer process, i.e., the resistance to the electron transfer of faradic process on the AZ31 Mg alloy, and formation of a protective layer on the magnesium surface [50,51]. However, the capacitive loop progressively increases with increasing synergistic effect of as-prepared samples, representing the adsorption of organic molecules and exhibition a barrier effect that would defend the AZ31 Mg from destructive attack by the solution. As seen from the EIS plots (Figure. 7c,d and supplementary Table S2), maximum corrosion prevention is observed in TiO2-HQ-APH. 3.3. Quantum chemical study To explore the relationship between the molecular electron donor properties of the ligands and coating surface, the energy gap ∆E (LUMO-HOMO) relates with the visible light response was determined (Fig. 8). From Fig.8 (a,b,c) it is exhibited that the energy gap of 8-HQ is 8.06 eV, 2-APY 7.97 eV and 2-APH 7.31 eV, indicating that smaller ∆E is the higher reaction activity and electron donor of compound [52]. Moreover, the lower value of ELUMO indicates that synergistic heterocycles could 11
simply donate electron to the unoccupied orbital of MgAl2O4-TiO2-SnO2. Therefore, it is realized that active areas of organic molecules are mostly distrusted around the functional groups containing O and N atoms. This explains that the 8-HQ, 2-PAY and 2-APH interact with the Mg and Al atoms by those functional groups, and provide electrons for metal atoms with empty orbitals. The advantage of electrons propagating from central Mg2+ and Al3+ to synergistic heterocycles after complexation process is that photoelectrons are softly to inject to TiO2 through ligand, which will enhance the photocatalytic property of TiO2. 3.4. Mechanism for photocatalytic activity of synergistic heterocycles and MgAl2O4-TiO2-SnO2 Based on the above photocatalytic analysis, a tentative charge transfer for the synergistic heterocycles and MgAl2O4-TiO2-SnO2 was suggested and presented in Fig. 9. Synergistic heterocycles upon bonding with inorganic surface containing TiO2 particles, which favorites photogenerated electron move from conduction band (CB) of TiO2 to Fermi level (π orbital) of the synergistic aromatic compounds, act as electron donor or electron acceptor under visible and UV excitation, respectively. In this case, it was usually deduced that the function of synergistic heterocycles as photogenerated electrons acceptor is critical in the repression of charge carrier’s recombination of photocatalyst hybrid materials resulting in increased effectiveness of the water treatment. In this regard, Bhatia et al. [53] reported the combination of reduced graphene with TiO2 facilitated more adsorption sites and restrained the recombination rate of electron-hole pairs. The visible-light activity of prepared MgOTiO2-HQ, MgO-TiO2-HQ-APY, and MgO-TiO2-HQ-APH was due to the red-shift of the absorption edge. It was found that synergistic effects of heterocycles, such as 2-APH, strongly increased photocatalytic activity. In the aqueous systems, the negative charge then activates pre-adsorbed O2 on the surface of TiO2 to form superoxide anion radicals which attack the aromatic rings of MB because of strong oxidizing ability, mineralize the MB to carbon dioxide and water. Therefore, the holes can react with adsorbed H2O to produce hydroxyl radicals (OH·) [54].This behavior may be due to the fact that, first, vis-light is an excitation light source for MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgOTiO2-HQ-APH due to their small energy gaps. Second, the CB energy of MgAl2O4-TiO2-SnO2 is 12
lower than that of HOMO of 8-HQ, 2-APY and 2-APH. Consequently, the photoelectrons on LUMO of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQ-APH transfer to the CB of MgAl2O4TiO2-SnO2 to decrease whole energy of MgO-TiO2-HQ, MgO-TiO2-HQ-APY and MgO-TiO2-HQAPH and increase the ability of system.
4. Conclusions Finally, the flowerlike organic structure grown on the PEO coating has been developed successfully to enhance visible-light photocatalytic activity after sensitized with both either 8-HQ/2-APY or 8-HQ/2APH. PEO coating showed improved visible-light photocatalytic activity after sensitized with either of 8-HQ/2-APY or 8-HQ/2-APH. About 76% of methylene blue was decomposed after 6 h in the presence of TiO2-HQ-APH under visible light irradiation, while amount was around 55% of methylene blue when TiO2-HQ was used as photocatalyst. The difference in photocatalytic performance of these three systems may be assigned to the synergistic effects of electron-donating of heterocycles and electron transfer features of formed metal-ligands complexes, and the extent of energy level matching between metal-formed complex and TiO2. The study in this paper explains that effective electron transfer at the interface of prepared metal-ligands complex and MgAl2O4-TiO2-SnO2 self-possessed with the suitable energy level alignment between the gab energy (HOMO-LUMO) of the metal complex and the band gap of MgAl2O4-TiO2-SnO2 is critical for TiO2-MgAl2O4-SnO2 to achieve high visible light photocatalytic performance. Moreover, the high electrochemical performance of the present hybrid as-prepared photocatalysis suggests a synergistic combination of heterocycles and TiO2-doped inorganic materials in enhancing structural and chemical stabilities Author contributions All authors contributed to writing the manuscript and approved the final version of the manuscript.
Notes The authors declare no competing financial interests. 13
Acknowledgement This research was supported by the Research Grant funded by the National Research Foundation of Korea (2017R1D1A1A09000921).
14
References 1.
L. Guo, W. Feng, X. Liu, C. Lin, B. Li, Y. Qiang, Sol-gel synthesis of antibacterial hybrid coatings on titanium, Mater. Lett. 160 (2015) 448-451.
2.
V. Oldani, R. del Negro, C.L. Bianchi, R. Suriano, S. Turri, C. Pirola, B. Sacchi, Surface properties and anti-fouling assessment of coatings obtained from perfluoropolyethers and ceramic oxides nanopowders deposited on stainless steel, J. Fluorine. Chem. 180 (2015) 7-14.
3.
M. Catauro, F. Papale, F. Bollino, Characterization and biological properties of TiO2/PCL hybrid layers prepared via sol–gel dip coating for surface modification of titanium implants, J. NonCryst. Solids. 415 (2015) 9-15.
4.
I. Kheirollahi, M. Abdellahi, M. Emamalizadeh, H. Sharif, Preparation and characterization of multilayer mesoporous alumina nano membrane via sol-gel method using new precursors, Ceram. Internat. 41 (2015) 15083.
5.
X. Guan, Y. Wang, Q. Xue, L. Wang, Toward high load bearing capacity and corrosion resistance Cr/Cr2N nano-multilayer coatings against seawater attack, Surf. Coat. Technol. 282 (2015) 78-85.
6.
F. Ge, X. Zhou, F. Meng, Q. Xue, F. Huang, Tribological behavior of VC/Ni multilayer coatings prepared by non-reactive magnetron sputtering, Tribiology. Internat. 99 (2016) 140-150.
7.
A. Castellanos, A. Altube, J.M. Vega, E. García-Lecina, J.A. Díez, H.J. Grande, Effect of different post-treatments on the corrosion resistance and tribological properties of AZ91D magnesium alloy coated PEO, Surf. Coat. Technol. 278 (2015) 99-107.
8.
Y. Xiong, C. Lu, C. Wang, R. Song, Degradation behavior of n-MAO/EPD bioceramic composite coatings on magnesium alloy in simulated body fluid, J. Alloy. Compd. 625 (2015) 258-265.
9.
H. Zheng, L. Xiong, Q. Luo, S. Lu, Development of multilayer oxidation resistant coatings on Cr-50Nb alloy, Appl. Surf. Sci. 359 (2015) 515-520.
10. Z. Wu, L. He, Z. Chen, Composite biocoating of hydroxyapatite/Al2O3 on titanium formed by Al anodization and electrodeposition, Mater. Lett. 61 (2007) 2952-2955. 11. A. Fujishima, N.R. Rao, D.A. Tryk, titanium dioxide photocatalysis. J. Photochem. Photobiol., C: Photochem. Rev. 1 (2001) 1-21. 12. S.H. Cheung, P. Nachimuthu, A.G. Joly, M.H. Engelhard, M.K. Bowman, S.A. N Chambers, Incorporation and electronic structrure in N-doped TiO2 (110) rutile. Surf. Sci. 601 (2007) 17541762. 13. J.A. Navio, M. Macias, M. Gonzales-Catalan, A. Justo, Bulk and surface characterization of power iron-doped titania photocatalysts. J. Mat. Sci. 27 (1992) 3036-3042. 14. Y. Wang, R. Shi, J. Lin, Y.F Zhu, Significant photocatalytic enhancement in methylene blue degradation of TiO2 photocatalytsts via graphene-like carbon in situ hybridization. Catal. B: Environ. 100 (2010) 179-183. 15
15. Z. Liu, D. He, Y. Wang, H. Wu, J. Wang graphene doping of P3HT:PCBM photovoltaic devices. Synth. Met.160 (2010) 1036-1039. 16. M.E. Khan, M.M. Khan, M.H. Cho, CdS-Graphene nanocomposite for efficient visible-lightdriven photocatalytic and photoelectrochemical applications. J. Colloid Interface Sci. 482 (2016) 221-232. 17. A. Heciak, A.W. Morawski, B. Grzmil, S. Mozia., Cu-Modified TiO2 photocatalysts for decomposition of acetic acid with simultaneous formation of C1-C3 Hydrocarbons and Hydrogen. Appl. Catal., B: Environ. 140-141 (2013) 108-114. 18. A. Suligoj, U.L. Stangar, A. Ristic, M. Mazaj, D. Verhovsek, N.N. Tusar. TiO2-SiO2 Films from organic-free colloidal TiO2 anatase nanoparticles as photocatalyst for removal of volatile organic compounds from indoor air. Appl. Catal., B: Environ. 184 (2016) 119-131. 19. H. Nishikiori, K. Todoroki, R.A. Setiawan, K. Teshima, T. Fujii, H. Staozono, Titanium complex formation of organic ligands in titania gels. Langmuir. 31 (2015) 964-969. 20. H. Yang, J. Zhang, Y Song, S. Xu, L. Jiang, Y. Dan, Visible light photo-catalytic activity of CPVA/TiO2 composite for degrading rhodamine B. Applied Surface Science. 324 (2015) 645-651. 21. E. Filippo, C. Carlucci, A.L. Capodilupo, P. Perulli, F. Conciauro, G.A. Corrente, G. Gigli, G. Ciccarella, Facile preparation of TiO2-polyvinyl alcohol hybrid nanoparticles with improved visible light photocatalytic activity. Applied Surface Science. 331 (2015) 292-298. 22. P. Lei, F. Wang, S. Zhang, Y. Ding, J. Zhao, M. Yang, Conjugation-grafted-TiO2 nanohybrid for high photocatalytic efficiency under visible light. ACS App. Mater. Interfaces. 6 (2014) 23702376. 23. S. Chen, B. Lv, Y. Xu. Fe-quinoline complexes sensitized Si-doped TiO2 with enhanced visible light photocatalytic activity. Mater. Lett. 77 (2012) 32-34. 24. Nishikiori, H.; Natori, D.; Ebara, H.; Teshima, K.; Fujii, T. Zinc Complex Formation of Organic Ligands on Zinc Oxide and Titanium Dioxide. J. photochem. Photobiol. 2016, 327, 51-57. 25. E.L. Tae, S.H. Lee, J.K. Lee, S.S. Yoo, E.J. Kang, K.B. Yoon, A strategy to increase the efficiency of the dye-Sensitized TiO2 solar cells operated by photoexcitation of dye-to-TiO2 charge-transfer bands. J. Phys. Chem. B 109 (2005) 22513-22522. 26. S. Varaganti, G. Ramakrishna, Dynamics of interfacial charge transfer emission in small molecule sensitized TiO2 nanoparticles: Is it localized or delocalized. J. Phys. Chem. C 114 (2010) 13917-13925. 27. B.K. An, W. Hu, P.L. Burn, P. Meredith, New type II catechol-thiophene sensitizers for dyesensitized solar cells. J. Phys. Chem. C 114 (2010) 17964-17974. 28. T. Kamegawa, H. Seto, S. Matsuura, H. Yamashita, Preparation of hydroxynaphthalene-modified TiO2 via formation of surface complexes and their applications in the photocatalytic reduction of nitrobenzene under visible-light irradiation. ACS Appl. Mater. Interfaces 4 (2012) 6635-6639. 16
29. R. Jono, J. Fujisawa, H. Segawa, K. Yamashita, Theoretical study of the surface between TiO2 and TCNO showing interfacial charge-transfer transitions. J. Phys. Chem. Lett. 2 (2011) 11671170. 30. R. Jono, J. Fujisawa, H. Segawa, K. Yamashita, The origin of the strong interfacial chargetransfer absorption in the surface complex between TiO2 and dicyanmethylene compounds. Phys. Chem. Chem. Phys. 15 (2013)18584-18588. 31. W. Al Zoubi, Y.G. Ko. A novel strategy to modify the surface of plasma electrolysis produced inorganic coatings for fabricating organic@functional binding agents@inorganic materials J. All. Comps. 776 (2019) 1025-1028. 32. A.H. Mady, M.L. Baynosa, D. Tuma, J.J. Shim, Facile microwave-assisted green synthesis of Ag-ZnFe2O4@rGO nanocomposites for efficient removal of organic dyes under UV- and visiblelight irradiation. Appl. Catal. B: Environ. 203 (2017) 416-427. 33. W. Al Zoubi, Y.G. Ko, Self-assembly of hierarchical N-heterocycles inorganic materials into three-dimensional structure for superior corrosion protection. Chem Eng J. 356 (2019) 850-856. 34. Q. Zong, L. Wang, W. Sun, G. Liu, Active deposition of I (8-hydroxyquinoline) magnesium coating for enhanced corrosion resistance of AZ91D Alloy. Corros Sci. 89 (2014) 127-136. 35. J. Bennet, R. Tholkappiyan, K. Vishista, N. Victor Jaya, F. Hamed Attestation, Attestation in selfpropagating combustion approach of spinel AFe2O4 (A = Co, Mg and Mn) complexes bearing mixed oxidation states: Magneto structural properties. Applied Surface Science. 383 (2016) 113125. 36. D. Zheng, H. Li, Y. Wang, F. Zhang, Surface and interface analysis of tris-(8-hydroxyquinoline) aluminum and indium-tin-oxide using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Appl. Surf. Sci. 183 (2001) 165-172. 37. W. Al Zoubi, M.J. Kim, Y.G. Kim, Y.G. Ko. Dual-functional crosslinked polyer-inorganic materials for robust electrochemical performance and antibacterial activity. Chem Eng J. doi.org/10.1016/j.cej.2019.123654 38. M. Finsgar, D.K. Merl, An electrochemical, long-term immersion, and XPS study of 2mercaptobenzothiazole as a copper corrosion inhibitor in chloride solution. Corros. Sci. 83 (2014) 164-175. 39. W. Al Zoubi, M.P Kamil, Y.G. Ko, Synergistic influence of inorganic oxides (ZrO2 and SiO2) with N2H4 to protect composite coatings obtained via plasma electrolyte oxidation on Mg alloy. Phys. Chem. Chem. Phys. 19 (2017) 2372. 40. R.F. Zhang, S.F. Zhang, N. Yang, L.J. Yao, F.X. He, Y.P. Zhou, X. Xu, L. Chang, S.J. Bai Influence of 8-hydroxyquinoline on properties of anodic coatings obtained by micro arc oxidation on AZ91 magnesium alloys. J. Alloys Compd. 539 (2012) 249-255. 41. S.Y. Shen, Y. Zuo, X.H. Zhao, The effects of 8-hydroxyquinoline on corrosion performance of a Mg-Rich coating on AZ91D magnesium alloy. Corros. Sci. 76 (2013) 275-283. 17
42. H. Gao, Q. Li, Y. Dai, F. Luo, H.X. Zhang high efficiency corrosion inhibitor 8-hydroxyquinoline and its synergistic effect with sodium dodecylbenzenesulfonate on AZ91D magnesium alloy. Corros. Sci. 52 (2010) 1603-1609. 43. Q. Zong, L. Wang, W. Sun, G. Liu active deposition of bis(8-hydroxyquinoline) magnesium coating for enhanced corrosion resistance of AZ91D alloy. Corros. Sci. 89 (2014) 127-136. 44. H. Yu, H. Irie, K. Hashimoto, Conduction band energy level control of titanium dioxide: toward an efficient visible-light-sensitive photocatalyst. J. Am. Chem. Soc. 132 (2010) 6898-6899. 45. F. Dong, S. Guo, H. Wang, X. Li, Z. Wu, Enhancement of the visible light photocatalytic activity of C-doped TiO2 nanomaterials prepared by a green synthetic approach. J. Phys. Chem. C 115 (2011) 13285-13292. 46. S. Shaikhutdinov, H.J. Freund, Ultrathin oxide films on metal supports: structure-reactivity relations. Rev. Phys. Chem. 63 (2012) 619-633. 47. T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, Photocatalytic one-electron oxidation of biphenyl derivatives strongly coupled with the TiO2 surface. Langmuir. 20 (2004) 2753-2759. 48. M.E. Khan, M.M. Khan, M.H. Cho, Cds-graphene nanocomposite for efficient visible-lightdriven photocatalytic and photoelectrochemical applications. J. Colloid Interface Sci. 482 (2016) 221-232. 49. S. Ebrahimi, A. Bordbar-Khiabani, B. Yarmand, M.A. Asghari, Improving optoelectrical properties of photoactive anatase TiO2 coating using rGO incorporation during plasma electrolytic oxidation. Ceramic International. 45 (2019) 1746-1754. 50. W. Al Zoubi, J.H. Min, Y.G. Ko, Hybrid organic-inorganic coating via electron transfer behaviour. Sci. Rep. 7 (2017) 7063. 51. W. Al Zoubi, M.P. Kamil, H.W. Yang, Y.G. Ko. Self-assembly of basket-weave organic layer formed on defective inorganic surface. Chem. Eng J. 360 (2019) 385-392. 52. M.A. Amin, K.F. Khaled, Q. Mohsen, H.A. Arida, A study of the inhibition of iron corrosion in HCl solutions by some amino acids. Corros. Sci. 52 (2010) 1684-1695. 53. V. Bhatia, G. Malekshoar, A. Dhir, A.K. Ray, Enhanced photocatalytic degradation of atenolol using graphene TiO2 composite. J. Photochem. Photobiol. A: Chem. 332 (2017) 182-187. 54. E. Kusiak-Nejman, A. W. Morawski, TiO2/graphene-based nanocomposites for water treatment: a brief overview of charge carrier transfer, antimicrobial and photocatalytic performance. Appl. Catal. B: Environ. 253 (2019) 179-186.
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Figure captions Figure 1. XRD patterns of as-prepared TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH. (TiO2 donates to MgO and TiO2) Figure 2. FE-SEM images of XRD patterns of (a) TiO2-HQ, (b) TiO2-HQ-APY and (c) TiO2-HQAPH. High-resolution SEM represent of the (d) TiO2-HQ-APH and (e) TiO2-HQ. (f) R-TEM images of TiO2-HQ-APH showing flower-like structure of formed complexes on the inorganic surface. Figure 3. (a) Wide XPS spectra of TiO2-HQ, TiO2-HQ-APY, and TiO2-HQ-APH. High-resolution scan Mg 1s (b), Al 2p (c), C 1s (d), N1s (e), O1s (f) are presented of TiO2-HQ-APH. Figure 4. FT-IR spectra of TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH Figure 5. (a) Absorption spectra and (b) plot of modified Kubelka-Munk functional (αhν)2(eVcm-1)2 versus photo energy of the TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-APH. (c) Photoluminescence spectra of the as-prepared samples. Figure 6. Comparison of the photocatalytic degradation of MB using (a) TiO2-HQ, (b) TiO2-HQ-APY and (c) TiO2-HQ-APH under UV-vis light. Figure 7. (a) Degradation MB in aqueous solution using the TiO2-HQ, TiO2-HQ-APY, TiO2-HQ-PAH catalysts under visible-light. (b) Reusability test of the TiO2-HQ and TiO2-HQ-PAH catalysts for the photodegradation of MB (20 ppm) under visible-light. (c), Nyquist plots from electrochemical impedance spectroscopy (EIS) performed after 5 hours of immersion in 3.5 wt % NaCl on PEO, TiO2HQ, TiO2-HQ-APY and TiO2-HQ-PAH. (d) Equivalent circuit used fit the obtained impedance spectra for AZ31 magnesium in the PEO, TiO2-HQ, TiO2-HQ-APY and TiO2-HQ-PAH. Rs describes the corrosion resistance between the prepared sample and counter electrode; R1 is the outward part resistance, and CPE1 is the constant phase element; R2 represents the contribution of inward part to 19
the overall corrosion resistance and the corresponding capacitance was represented by CPE2. Rct and L represent the inductive impedance behavior, where Rct denotes the charge transfer resistance and L showed the corrosion behavior at low frequencies. Figure 8. Diagrams of crucial molecular orbital energy levels of 8-HQ (a), 2APY (b) and 2-APH (c). Figure 9. Schematic presentation of the electron transfer and charge separation in the TiO2-QH under visible light irradiation.
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We proposed a novel composition to prepare the flower hybrid. Electrochemical performance is delayed by hybrid materials compounds with compact structure. An optimum combination of protective and catalytic properties together is accomplished. No significant decrease in photocatalytic performance during catalytic oxidation.
The authors declare no competing financial interests.